JP2000082993A - Optical analog transmission device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical analog transmission device which secures a wide dynamic range and can suppress optical beat noise at the time of SCM(sub- carrier multiplex)-multiplexing analog signals from a plurality of slave stations and transmitting them to a master station. SOLUTION: In slave stations 1a, 1b,-1n, optical signals obtained by directly modulating Fabry-Perot semiconductor laser elements 8a, 8b,-8n with an optical modulation degree larger than '1' by a frequency modulation signal modulated by an information signal being an analog signal are transmitted through an optical fiber 2 and are SCM-multiplexed with an optical signal from another slave station on a transmission line. In a master station, the optical signals which are SCM-multiplexed are collectively received by one photodetector 9. The signal of the desired slave station is extracted from the reception signal. A bias-tee 6 adds DC bias, a demodulation circuit 14 demodulates a frequency and an information signal is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmission apparatus for performing SCM multiplex transmission of analog signals such as radio signals from a plurality of slave stations to a master station via an optical fiber.

[0001]

2. Description of the Related Art In recent years, mobile communication base stations or IT
2. Description of the Related Art There is known an optical network in which slave stations corresponding to V (Industrial Televisions) monitors are arranged in various places and information is transmitted from the slave stations to a master station via optical fibers. Among them, an optical SCM in which signals transmitted from each slave station are optical signals modulated by different subcarrier signals, and these are multiplexed and received collectively.
(Sub-Carrier Multiplex) networks have received much attention because they can transmit many electrical signals simultaneously.

However, since optical signals are multiplexed, generation of beat noise is inevitable, and this is a major problem. That is, there is a problem of beat noise generated by interference of optical signals from a plurality of slave stations.

[0003] Here, the beat noise means that an optical signal B from another slave station is located at a distance of Δλ from an optical signal A from one slave station, and these optical signals are received by one receiver. Is a noise component that occurs in the frequency band of Δλ of the received signal when collectively received by the receiver.

If the wavelengths of the optical signal A and the optical signal B are not sufficiently separated, that is, if Δλ is small, beat noise falls within the information signal band, and the receiving sensitivity deteriorates. become unable.

In response to the problem of beat noise, a method of reducing the coherence of optical signals from each slave station is described in the document "Operation.
of a subcarrier multiple-access passive optical ne
twork with multimode lasers in the presence of opt
ical beat interference ”, TuQ5, pp. 90-91, in OFC'9
5].

FIG. 18 shows a system disclosed in this document. FIG.
8, an information signal, which is a digital signal of “1” or “0”, is input to a VCO (Voltage Controlled Oscillator).
Directly modulates the laser element with the frequency modulation signal output from the laser element. At that time, clipping occurs in the laser element to spread the optical spectrum and reduce the coherency of the optical signal.

However, in this method, since the laser element causes clipping, the transmitted optical signals 51, 5
As shown in FIG. 19, the waveform amplitudes 2 and 53 are vertically asymmetric. Therefore, if the received frequency modulation signal is demodulated, distortion occurs in the information signal.

In particular, when the information signal is a multi-channel analog signal, as shown in FIG.
(3 ^rd Interference Mudulation; third harmonic), I
Since distortion such as M5 (fifth IM; fifth harmonic) overlaps with an adjacent channel, CNR (Carrier-to-Noise Ratio;
(Carrier / noise ratio), and the dynamic range is significantly suppressed.

[0009]

The optical SCM network has the advantage that a large number of electrical signals can be transmitted simultaneously, but on the other hand, since optical signals are multiplexed, the generation of beat noise is inevitable, which is a major problem. ing.

In order to reduce the coherence of the optical signal from each slave station, a digital information signal is input to a VCO to control the oscillation frequency corresponding to the information signal. The laser device is directly modulated by the frequency modulation signal output from the VCO, and at that time, clipping is caused by the laser device.
Thus, a technique has been proposed in which the optical spectrum is spread to reduce the coherency of the optical signal so that the influence of interference does not occur.

However, in this method, since clipping is caused by the laser element, the waveform of the transmitted optical signal is vertically asymmetric due to the effect of clipping. Therefore, if the received frequency modulation signal is demodulated, distortion occurs in the information signal.

In particular, when the information signal is a multi-channel analog signal, distortions such as IM3 and IM5 overlap adjacent channels, thereby deteriorating the CNR and significantly suppressing the dynamic range.

[0013] Therefore, the restrictions on the power of the information signal that can be transmitted and the optical transmission distance are strict, resulting in a serious problem of deteriorating the communication quality.

Therefore, the object of the present invention is to
Information signals are transmitted from multiple slave stations to the master station via optical fibers.
Provided is an optical transmission apparatus for performing SCM multiplexing, which has an effect of reducing optical beat noise due to clipping, and which can secure a wide dynamic range even for an analog signal of a multi-channel information signal. It is in.

[0015] In the radio base station, an interference wave may be mixed in a frequency band close to a received desired wave. In some cases, the power of the interference wave is higher than the power of the desired wave. Particularly, in an optical analog transmission device that mainly configures a wireless base station only with an antenna unit and an electrical-optical conversion unit and optically transmits a received wireless signal to a master station,
This interference wave causes pressure on the optical modulation degree of the desired wave.

If the degree of optical modulation of the desired wave is limited to a small value, the CNR of the desired wave is reduced in the master station, so that the specified error rate may not be achieved.

Therefore, another object of the present invention is to provide a technique for suppressing an interference wave.

Further, the optical modulation degree of the optical signal transmitted from the slave station to the master station is set to “1” or more, and the CN of the desired wave in the master station is changed.
There is a method of increasing R. However, if the desired wave is a radio signal that is angle-modulated, the reception sensitivity deteriorates due to the amplitude limitation. Therefore, the required CNR value becomes large, and the specifications for the slave station become strict.

Therefore, still another object of the present invention is to prevent the deterioration of the receiving sensitivity due to the amplitude limitation even when the optical modulation degree of the optical signal transmitted from the slave station to the master station is "1" or more. It is to provide the technology to make.

[0020]

In order to achieve the above object, the present invention is configured as follows. In the present invention,
In at least two or more slave stations, an optical signal having an amplitude waveform with clipping obtained by directly modulating a laser element with an optical modulation degree larger than "1" by a frequency modulation signal modulated by an information signal is used. It is intended for a system that multiplexes and transmits to a master station via an optical fiber which is a transmission path. As the laser element, a Fabry-Perot type semiconductor laser element is used. Then, the vertically asymmetric signal waveform that has been clipped is complemented by a vertically symmetric signal waveform in the master station or the slave station.

First, the optical modulation degree OMI (Op
Here is the definition of tical modulation index). When the slave station modulates the semiconductor laser device with a sine wave of the frequency fs, the intensity of the transmitted optical signal is expressed as follows. P (t) = P0 + P1cos (2pfst) + P2cos (4pfst) + (1) At this time, the optical modulation degree OMI is defined by OMI = P1 / P0. Even when clipping occurs in the semiconductor laser device, as shown in FIG. 21, assuming that there is no waveform change due to clipping, OMI is defined in the same manner as described above, and OMI> 1. [1] The optical analog transmission device according to the first invention is:
Having a plurality of slave stations each having a semiconductor laser element as a light source, these slave stations, by a frequency modulation signal modulated by an information signal, the semiconductor laser element,
By directly modulating with an optical modulation factor larger than 1, the optical signals are converted into optical signals having vertically asymmetric waveforms, and these optical signals are transmitted to a transmission line which is an optical fiber. Optical signal from the station and SCM (Sub-Carrier Mu
ltiplex) In the optical transmission device in which the multiplexed and SCM-multiplexed optical signal is transmitted to the master station and received by the photoelectric conversion means, the information signal is an analog signal;
The master station extracts a frequency modulation signal of a desired slave station from a reception signal obtained by converting the signal into an electric signal by the photoelectric conversion unit, a unit for adding a DC bias voltage to the extraction signal of the extraction unit, Amplitude limiting means for subjecting a signal to which a DC bias voltage has been added to amplitude limitation for making the amplitude of a waveform vertically symmetric, and demodulation means for frequency-demodulating the signal after the amplitude limitation processing to obtain an information signal. It is characterized by having.

This system has a plurality of slave stations, each of which is provided with a semiconductor laser element (for example, a Fabry-Perot type semiconductor laser element) as a light source.
The semiconductor laser device is directly modulated with an optical modulation factor larger than 1 to be converted into an optical signal by a frequency modulation signal modulated by an information signal of an analog signal, thereby clipping a waveform, reducing coherency, and reducing these optical signals. (Sub-Carrier Multiplex) multiplexed and transmitted to a master station via an optical transmission line, and the master station targets an optical transmission apparatus that receives the SCM multiplexed optical signal by a photoelectric conversion unit. In such an optical transmission device,
The master station extracts a frequency modulation signal of a desired slave station from the received signal obtained by converting the signal into an electric signal by the photoelectric conversion means, adds a DC bias voltage to the extracted signal, and symmetrically adjusts the amplitude of the waveform. In other words, the information signal is obtained by subjecting the signal to an amplitude limitation in order to make the amplitude of the waveform vertically symmetric, and then demodulating the frequency.

Normally, in frequency demodulation, after the amplitude fluctuation of the received signal is suppressed by a limiter or AGC (Auto Gain Control), a discriminator, delay detection, PL
The signal is demodulated into an information signal by synchronous detection using L or the like. According to the present invention, what has been transmitted as a signal that has become vertically asymmetrical due to clipping on the slave station side, and then, at the master station, a DC bias voltage is added to the received signal to make it get rid of the frequency modulation signal. The amplitude limit for returning the average level to a proper position is corrected to a waveform having a symmetrical amplitude, and the demodulated signal is subjected to frequency demodulation.

Therefore, distortion due to a shift in the average level of the received signal at the time of frequency demodulation can be suppressed, the effect of improving the input dynamic range with respect to the optical modulation degree by the FM premodulation of the information signal can be obtained, and conversion with high communication quality can be obtained. Can be guaranteed. In addition, even if a Fabry-Perot type semiconductor laser device is used, it is not affected by distortion and optical beat noise.
A low-cost optical SCM multiplex network can be provided. [2] The optical analog transmission device according to the second invention is:
Having a plurality of slave stations each having a semiconductor laser element as a light source, these slave stations, by a frequency modulation signal modulated by an information signal, the semiconductor laser element,
The optical signal is directly modulated at an optical modulation factor greater than 1 to form an optical signal, these optical signals are SCM-multiplexed, and transmitted to a master station via an optical transmission line. The master station converts the SCM-multiplexed optical signal into photoelectric conversion means. In the optical transmission device configured to receive the information signal, the information signal provided is an analog signal, and the slave station has an amplitude limit for matching the average level of the amplitude of the transmitted optical signal to the average level of the frequency modulation signal. Is provided to the frequency modulation signal.

This system has a plurality of slave stations, each of which is provided with a semiconductor laser element (for example, a Fabry-Perot semiconductor laser element) as a light source.
By directly modulating the semiconductor laser device with a frequency modulation signal modulated by an information signal of an analog signal at an optical modulation factor greater than 1 to form an optical signal, the waveform is clipped to reduce the coherency, and the When a signal is SCM-multiplexed and transmitted to a master station via an optical transmission path, occurrence of beat noise can be suppressed, and the master station
Although the SCM multiplexed optical signal is received by the photoelectric conversion means, in this optical transmission device,
Amplitude limitation for matching the average level of the amplitude of the optical signal to be transmitted to the average level of the frequency modulation signal is performed on the frequency modulation signal.

According to the present invention, the slave station limits the amplitude of the frequency-modulated signal modulated by the information signal in advance so that the average level of the optical signal can be adjusted together with the clipping during direct modulation of the semiconductor laser device. The average level of the frequency modulation signal is made to coincide with the position. Therefore, on the receiving side of the master station, normal frequency demodulation may be performed, and no distortion occurs in the demodulated information signal.

[3] The optical analog transmission apparatus according to the third invention is the optical analog transmission apparatus according to the first and second inventions, wherein the information signal is a multi-channel analog signal. It is characterized by.

When a plurality of optical signals which are asymmetric due to clipping are multiplexed, harmonics such as IM3 and IM5 are generated. Distortion due to such harmonics is caused by multi-channel analog signals such as radio signals. In this case, the dynamic range is significantly reduced. By adopting the configuration of the first and second aspects of the present invention, even if the information signal is a multi-channel analog signal, it is possible to reduce distortion due to the effect of clipping during frequency demodulation.
A wide dynamic range can be guaranteed. That is, the effect of expanding the dynamic range by pre-modulating the information signal into the frequency modulation signal is not impaired. [4] The optical analog transmission device according to the fourth invention comprises:
Having a plurality of slave stations each having a semiconductor laser element as a light source, these slave stations, by a frequency modulation signal modulated by an information signal, the semiconductor laser element,
The optical signal is directly modulated at an optical modulation factor greater than 1 to form an optical signal, these optical signals are SCM-multiplexed, and transmitted to a master station via an optical transmission line. The master station converts the SCM-multiplexed optical signal into photoelectric conversion means. Wherein the information signal is a signal subjected to π / 4 shift QPSK modulation.

Although the frequency modulation signal does not deteriorate the demodulation characteristics with respect to the signal amplitude limit, the π / 4 shift QPSK modulation signal has a large tolerance even though the demodulation characteristics slightly deteriorate. Therefore, clipping may occur when directly modulating the laser element with a π / 4 shift QPSK modulation signal instead of a frequency modulation signal. Since the π / 4 shift QPSK modulation signal is used as a radio signal for mobile communication, when the present invention is applied to a base station for mobile communication, there is no need for premodulation for converting an information signal into a frequency modulation signal. The device scale of the slave station and the master station can be reduced. [5] The optical analog transmission device according to the fifth invention is:
An optical analog transmission device according to the first invention,
The master station detects a distortion amount of the information signal after the frequency demodulation,
It is characterized in that the added DC bias voltage value is controlled so that the distortion is suppressed.

At the time of frequency demodulation at the master station, the amount of distortion generated in the information signal depends on the DC bias voltage value added to the received signal. Therefore, it is necessary to add an appropriate DC bias voltage. According to the present invention, a high-quality and high-reliability analog optical transmission device is measured by measuring a distortion amount included in an information signal after frequency demodulation and constantly controlling a DC bias voltage so as to suppress the distortion amount. Can be provided. [6] The optical analog transmission device according to the sixth invention is
An optical analog transmission device according to the second invention,
The slave station includes means for receiving a part of the optical signal transmitted to the master station, means for frequency demodulating the received signal, and detecting means for detecting the amount of distortion generated in the frequency demodulated signal. And controlling the amplitude limiting amount so that the distortion amount is reduced.

The clipping state of the optical signal depends on the bias condition of the laser device. The bias condition of the laser element is considered to fluctuate due to the influence of temperature change, aging, and the like, and it is necessary to appropriately control the clipping amount. Therefore, the optical signal of the laser output is received, and the amount of distortion included in the information signal after frequency demodulation is measured.
By controlling the DC bias voltage and the DC bias current so as to suppress the distortion amount and to make the amplitude control amount appropriate, a high-quality and highly reliable analog optical transmission device can be provided. [7] The optical analog transmission apparatus according to the seventh aspect of the present invention includes:
The optical analog transmission device according to the first to sixth, wherein:
The light modulation degree of the semiconductor laser device is 2.0 or more.

The state of diffusion of the optical spectrum due to clipping depends on the degree of optical modulation OMI.
By setting the value to 0 ”or more, the optical spectrum can be stably diffused even when the bias condition of the laser is changed. [8] The optical analog transmission apparatus according to the eighth aspect of the present invention provides:
An optical analog transmission device according to the first to seventh aspects,
The semiconductor laser device is a Fabry-Perot semiconductor laser device.

The Fabry-Perot type semiconductor laser device is
Since the cost is low, the device cost of the entire optical network in which a large number of slave stations are arranged can be reduced. Further, the optical spectrum width of the Fabry-Perot semiconductor laser device is wider than that of the distributed feedback semiconductor laser device, and the coherence of the light source can be easily suppressed by combining with the clipping technique. According to the present invention described in the above items [1] to [8], when a laser element is directly modulated with a frequency modulation signal modulated with an analog signal as transmission information, a more clipped optical signal is generated. Thus, in an optical transmission device adapted to be used for transmission, distortion such as IM3 (third harmonic) and IM5 (fifth harmonic) due to signal waveform asymmetry due to clipping during frequency demodulation can be suppressed. In particular, for a base station of a mobile communication system using a plurality of mobile terminals, such as an analog signal of a multi-channel, and an ITV that handles a video signal, the input dynamic range improvement effect on the optical modulation degree by the FM premodulation is improved. It is possible to provide an optical transmission device that can obtain a maximum value and can guarantee higher communication quality.

Further, the optical transmission apparatus according to the present invention described in the above items [1] to [8] also has a beat noise suppressing effect due to clipping, and thus there are two major problems of distortion and beat noise in analog optical transmission. Of a multi-channel analog signal from a plurality of slave stations to a master station while securing a wide dynamic range.
M multiplex transmission is possible. Moreover, in each slave station, an inexpensive laser element can be used without using an expensive laser element with excellent distortion characteristics, and there is no need to perform wavelength selection and wavelength stabilization control. Only one optical receiving system is required. Therefore, it is possible to reduce the size of the apparatus and realize simplification of the configuration, downsizing, and cost reduction. [9] Further, the optical analog transmission apparatus according to the ninth invention transmits an optical signal obtained by directly modulating a semiconductor laser element with an information signal received by a slave station to a master station via an optical transmission line. In the optical analog transmission apparatus, the slave station includes a receiving unit for receiving an information signal transmitted as a radio signal, and a band-pass filter, and the information signal received by the receiving unit is subjected to frequency conversion signal processing. Without applying
The semiconductor laser device is directly modulated by passing through a band-pass filter.

That is, in the optical analog transmission apparatus according to the ninth aspect, the semiconductor laser element is directly modulated with the information signal, and the optical signal output from the semiconductor laser element is transmitted through the transmission line which is an optical fiber. It is intended for an optical analog transmission device for transmitting from a slave station to a master station, wherein the slave station is a radio base station, the information signal is a received radio signal, and the radio signal is not subjected to frequency conversion. The laser beam passes through a band-pass filter and is applied to a semiconductor laser device, so that the semiconductor laser device is directly modulated.

The optical analog transmission apparatus according to the ninth invention is a technique relating to interference wave suppression. In the slave station, an information signal is passed through a band-pass filter so that an interference wave received together with the information signal is increased. Can be suppressed. ADVANTAGE OF THE INVENTION According to this invention, when converting an information signal into an optical signal, it becomes possible to provide a stable optical modulation degree. [10] An optical analog transmission apparatus according to the tenth aspect of the present invention transmits an optical signal obtained by directly modulating a semiconductor laser element with an information signal received by a slave station to a master station via an optical transmission path. In the optical analog transmission apparatus, the slave station receives an information signal transmitted as a radio signal angle-modulated with a digital signal, and a channel selection and waveform for transmitting the information signal received by the reception means. A filter for equalizing, wherein the semiconductor laser device is directly modulated by an information signal passed through the filter.

That is, in the optical analog transmission apparatus according to the tenth aspect, the semiconductor laser device is directly modulated with the information signal, and the optical signal output from the semiconductor laser device is transmitted through the transmission line, which is an optical fiber, to the slave. It is intended for an optical analog transmission device for transmitting from a station to a master station, the slave station is a radio base station, and the information signal is a radio signal angle-modulated with a digital signal, and this information signal is channel-selected. And a filter for waveform equalization.

The optical analog transmission apparatus according to the tenth aspect is a technique relating to interference wave suppression, in which a slave station selects a desired channel from an information signal and equalizes the waveform of the information signal as the desired channel. This has the following effects. The channel selection suppresses the interference wave included in the information signal. For this reason, the intensity of the information signal does not depend on the interference wave, and the degree of light modulation for the information signal can be stabilized when the information signal is converted into an optical signal. Further, when the information signal is subjected to waveform equalization, the reception sensitivity of the information signal does not deteriorate even if the transmission system has nonlinear signal processing such as amplitude limitation. Therefore, the components used in the transmission system from the slave station to the master station or the degree of freedom of the transmission system are expanded, and the configuration of the optical analog transmission apparatus can be diversified. [11] An optical analog transmission apparatus according to the eleventh aspect of the present invention provides an optical signal obtained by directly modulating a semiconductor laser element using an information signal received by a slave station to a master station via an optical transmission path. In the optical analog transmission device to be transmitted, the slave station receives an information signal transmitted as a radio signal angle-modulated with a digital signal,
Waveform equalizing means for equalizing the information signal received by the receiving means to obtain a modulated signal of the semiconductor laser device is provided.

That is, in the optical analog transmission apparatus according to the eleventh aspect, the semiconductor laser element is directly modulated with the information signal, and the optical signal output from the semiconductor laser element is transmitted through the transmission line which is an optical fiber. An optical analog transmission device for transmitting from a slave station to a master station, wherein the slave station is a radio base station, the information signal is a radio signal angularly modulated with a digital signal, and the slave station has the information The signal is a signal angle-modulated with a digital signal, and the information signal is waveform-equalized.

The present invention takes advantage of the fact that the information signal is equalized in waveform so that even if the information signal is subjected to non-linear signal processing such as amplitude limitation, the reception sensitivity does not deteriorate. Therefore, after equalizing the waveform of the information signal, the information signal is subjected to non-linear signal processing such as amplitude limitation, but since the reception sensitivity does not deteriorate, in order to stabilize the optical modulation degree of the optical signal, It becomes possible to use a limiter amplifier instead of an AGC amplifier for the laser driving amplifier.

Since the limiter amplifier does not require an additional configuration such as a power detector, the configuration of the slave station can be simplified. Further, an information signal composed of a plurality of channels is limited in optical modulation degree per channel by waveform distortion due to clipping. However, according to the present invention, since the waveform is equalized, the tolerance against waveform distortion is increased, and the upper limit of the degree of light modulation can be increased. [12] An optical analog transmission apparatus according to a twelfth aspect of the present invention is the optical analog transmission apparatus according to any one of the above items [9] to [11], wherein the slave station has a route roll-off filter for performing roll-off shaping of an information signal. The configuration is provided with.

That is, an optical analog transmission apparatus according to the twelfth aspect is directed to the optical analog transmission apparatus described in the above items [9] to [11]. It is characterized by being transmitted through a roll-off filter and subjected to roll-off shaping.

The optical analog transmission apparatus according to the twelfth aspect relates to a technique relating to interference wave suppression, in which a received radio signal is arranged with a narrow band-pass filter of a root roll-off filter to thereby obtain a signal other than a desired wave. Since the interference wave is largely suppressed, it is possible to prevent the optical modulation degree of the desired wave from being suppressed by the interference wave when directly modulating the semiconductor laser device. Therefore, it is possible to transmit the stable signal strength of the desired wave to the master station regardless of the presence or absence of the interference wave. [13] The optical analog transmission apparatus according to the thirteenth invention is directed to the invention described in the above items [10] to [12], has a plurality of slave stations, and transmits from each of these slave stations. Optical signals obtained by directly modulating the semiconductor laser device with an optical modulation factor greater than “1” by the information signals arranged in different frequency bands, and the optical signals are transmitted to the other slave stations on the optical transmission line. Light signal from SC and SC
It is characterized in that M (Sub-Carrier Multiplex) is configured to be multiplexed and received collectively at the master station.

That is, the optical analog transmission apparatus according to the thirteenth invention is directed to the invention described in the above items [10] to [12].
The optical signal transmitted from each of the slave stations causes the semiconductor laser element to have an optical modulation degree larger than “1” (that is, 100) by the information signals arranged in different frequency bands.
[%] Or more, and the optical signal is directly modulated and output. The optical signal is SCM (sub-carrier multiplexed) multiplexed with the optical signal from the other slave station on the optical transmission line, and is then multiplexed at the master station. Make sure you receive them all at once.

The optical analog transmission apparatus according to the thirteenth aspect is a technique for increasing the degree of optical modulation. In ordinary analog transmission, if the optical modulation degree is “1” or more, the amplitude is limited, waveform distortion occurs, and the reception sensitivity of the information signal is deteriorated. However, for a radio signal such as a QPSK signal, after roll-off shaping, the reception sensitivity does not deteriorate even if the amplitude is limited. Further, by setting the optical modulation degree to “1” or more, the coherence of the optical signal can be reduced, and the effect of beat noise generated when passively multiplexing with an optical signal from another slave station can be suppressed. Become. [14] The optical analog transmission device according to the fourteenth invention is the optical analog transmission device according to the ninth to thirteenth inventions, wherein the information signal is modulated in amplitude and angle by a digital signal. It is characterized by.

According to the present invention, the information signal is waveform-equalized in the slave station, thereby reducing the deterioration of the receiving sensitivity with respect to non-linear signal processing such as amplitude limitation in the transmission system from the slave station to the moon. be able to. [15] The optical analog transmission apparatus according to the fifteenth invention is the optical analog transmission apparatus according to any one of the above items [9] to [14], wherein the band-pass filter for waveform equalization is 3 [dB]. ] The transmission bandwidth Δf is characterized by a relation of Bsym ≦ Δf ≦ 2Bsym with respect to the transmission rate Bsym [symbol / second] of the symbol of the information signal.

That is, the optical analog transmission apparatus according to the fifteenth invention is directed to the inventions described in the above items [9] to [14], and in the slave station, band transmission for waveform equalization is performed. 3 [dB] transmission bandwidth Δf of the filter
Is the transmission rate of the information signal symbol Bsym [symbol / s
econd], Bsym ≦ Δf ≦ 2Bsym.

Normally, roll-off shaping is performed by band limiting filters on the transmitting side and the receiving side. According to the present invention, if the band-pass filter included in the slave station satisfies the above condition, it is possible to perform ideal roll-off shaping on the information signal on the transmission side of the wireless signal and the slave station. . [16] The optical analog transmission apparatus according to the sixteenth aspect of the present invention has a plurality of slave stations, and each slave station converts an optical signal obtained by directly modulating a semiconductor laser device using a received information signal into an optical signal. In an optical analog transmission apparatus configured to transmit to a master station via a transmission line, the slave station includes a receiving unit that receives an information signal transmitted as a radio signal that is angle-modulated with a digital signal. The semiconductor laser device is directly modulated with an optical modulation factor larger than "1" by using the information signal received at (1), and the optical signal transmitted from each slave station is SCM (sub--
Carrier Multiplex) is characterized in that it is configured to multiplex and transmit to the master station.

That is, the optical analog transmission apparatus according to the sixteenth invention has a plurality of slave stations each having a semiconductor laser element as a light source, and each slave station uses its information signal to control its own semiconductor laser element. , The information signal is converted into an optical signal by directly modulating with an optical modulation degree larger than “1”,
The converted optical signal is transmitted to a transmission line, which is an optical fiber, to be sent to the master station. The optical signal from each slave station is transmitted along with an optical signal from another slave station to the SCM (Sub-Carrier) on the transmission line.
Multiplex) An optical analog transmission apparatus in which the multiplexed signal is transmitted to the master station, so that the master station collectively receives the SCM multiplexed optical signals, wherein the information signal is a digital signal. The signal is an angle-modulated signal.

When the slave station is a wireless base station and the information signal being wirelessly transmitted is angle-modulated with a digital signal, the reception sensitivity deteriorates if the amplitude is limited.

However, if the conditions of radio propagation are good and the CNR of the information signal can be made large, it is possible to satisfy the error rate as a specification even if there is some deterioration in reception sensitivity. Therefore, the filter that passes the radio signal need not be a roll-off filter, and the flexibility of the specifications of the slave station can be secured.

In the inventions described in the above items [9] to [15], the information signal is a radio signal, and the waveform equalization of the radio signal received by the slave station by the analog filter is performed as follows. The following effects are also obtained for the master station.

Conventionally, a receiver of a master station converts a radio signal transmitted from a slave station into a digital signal and performs waveform equalization with a digital filter. However, this function of waveform equalization requires a high sampling frequency and high-speed signal processing for an information signal.

Therefore, when the transmission speed of the information signal is increased, a higher speed is required for the digital circuit, so that the scale becomes large and the power consumption increases.

In particular, the number of receivers is large in a master station that handles information signals from a plurality of slave stations. Therefore, the increase in the scale and power consumption of each receiver imposes a heavy load on the master station.
However, according to the present invention, since the received signal optically transmitted to the master station is equalized in waveform, the digital filter does not need the function of equalizing waveform. Therefore, the power consumption of the master station can be reduced, and the device scale can be reduced.

[0056]

Embodiments of the present invention will be described below with reference to the drawings. First, a technique related to optical beat noise reduction and dynamic range securing will be described.

<Technique Related to Reduction of Optical Beat Noise and Ensuring Dynamic Range> (First Embodiment) A first embodiment of the present invention will be described. In the optical analog transmission apparatus shown as the first embodiment, in at least two or more slave stations, a Fabry-Perot type semiconductor laser element is modulated with an optical modulation degree larger than 1 by a frequency modulation signal modulated by an information signal. The optical signal obtained by the direct modulation is transmitted to the master station via an optical fiber which is a transmission line, and the optical signal is combined with the optical signal transmitted from another slave station on the transmission line and the SCM (Sub-Carrier Multiplex). ) Multiplex, the master station,
The SCM-multiplexed optical signal is converted into one photoelectric conversion element (P
D: Photo-Detector), which is an optical transmission device that collectively receives the information signal, wherein the information signal is an analog signal, and the master station extracts a frequency modulation signal of a desired slave station from the reception signal of the PD. A DC bias voltage is added to the extracted signal, an amplitude limit for vertical symmetry is applied, and then the information signal is obtained by frequency demodulation.

Normally, in frequency demodulation, after the amplitude fluctuation of the received signal is suppressed by a limiter or AGC (Auto Gain Control), a discriminator, delay detection, and PL
The signal is demodulated into an information signal by synchronous detection using L or the like.

However, according to the present invention, the master station applies a DC bias voltage to the received signal which has become vertically asymmetrical due to clipping, thereby limiting the amplitude of the average level of the frequency-modulated signal to an appropriate position. Before performing frequency demodulation.

As a result, it is possible to suppress the distortion due to the deviation of the average level of the received signal at the time of frequency demodulation, to secure the input dynamic range for the optical modulation degree by the FM pre-modulation of the information signal, and to realize the conversion with high communication quality. In addition, the use of a Fabry-Perot type semiconductor laser device is not affected by distortion and optical beat noise, so that a low-cost optical SCM multiplexing network can be provided. it can.

Details will be described.

FIG. 1 is a block diagram showing the configuration of the optical transmission device of this embodiment. In the figure, 1a, 1b, to 1n
Are slave stations, 2 is an optical transmission path, 3 is a master station, 4 is an optical coupler, 5a, 5b,..., 5n are FM (Frequency Modulatio).
n) modulation circuit, 6a, 6b,.
Reference numerals a, 7b, to 7n denote current sources, 8a, 8b, to 8n denote laser elements, 9 denotes a PD (Photo Detector), 10 denotes a bandpass filter, 11 denotes a voltage source, 12 denotes a limiter, 13 denotes a bandpass filter, 14 Is an FM demodulation circuit, and 6 is a bias tee.

The slave station 1a includes an FM modulation circuit 5a, a bias tee 6a, a current source 7a, and a laser element 8a. The slave station 1b includes an FM modulation circuit 5b, a bias tee 6b, a current source 7b, and a laser. The slave station 1n includes an FM modulation circuit 5n and a bias tee 6.
n, a current source 7n, and a laser element 8n.

The master station 3 includes a PD 9, band pass filters 10, 13, a voltage source 11, a bias tee 6, a limiter 12, and an FM demodulation circuit.

Of these, the FM modulation circuits 5a, 5b,
.About.5n are for FM-modulating and outputting the input analog signal, and current sources 7a, 7b, .about.7n are for supplying a predetermined DC current. Also, the bias tees 6a, 6b, to 6n are provided with FM modulation circuits 5a, 5b, to
n from the current sources 7a, 7b,.
The laser elements 8a, 8b,..., 8n are bias tees 6a, 8b,.
This is an element that is directly modulated by the output signals a, 6b, to 6n to oscillate laser light, and outputs this as an optical signal to the optical fiber 2, which is an optical transmission line.

The F signals from the FM modulation circuits 5a, 5b,.
The M modulation signal is superimposed on the DC bias signals from the current sources 7a, 7b, to 7n by the bias tees 6a, 6b, to 6n, and is applied to the corresponding laser elements 8a, 8b, to 8n, thereby providing the laser element 8a. , 8b,..., 8n, if the optical modulation index OMI (Optical Modulation Index) is set to exceed 100%, the output will cause clipping. [%] Or more, that is, the optical modulation degree OMI is “1”
With the above settings, the output is positively clipped.

The optical coupler 4 is connected to each of the laser elements 8a, 8
b, .about.8n are multiplexed and output, and the PD 9 of the master station 3 photoelectrically converts the multiplexed optical signal to obtain an SCM multiplexed electric signal and outputs it. , And the band-pass filter 10
This is for extracting a signal band of a desired slave station from the electric signal from D9.

The voltage source 11 supplies a required DC bias voltage. The bias tee 6 adds the DC bias voltage from the voltage source 11 to the output signal of the band-pass filter 10 and outputs the output signal. Is adjusted to the average level of the output signal when clipping does not exist.

That is, since the optical signal transmitted from the slave station is output in a state where clipping has occurred on the laser element side in the slave station, it differs from the average level of the signal without clipping. By adding the DC bias voltage from the voltage source 11 to the reception signal transmitted through the band-pass filter 10 for extracting the desired signal band by the bias tee 6, the average level is adjusted to the average level in the case where there is no clipping. The received signal is corrected.

The limiter 12 is an amplitude limiting circuit having vertically symmetrical limiter characteristics. The limiter 12 limits the signal output from the bias tee 6 by an upper and lower symmetrical limiter characteristic to provide an output having a vertically symmetrical amplitude level. Get the signal. The band-pass filter 13 is a circuit that removes a harmonic component generated by the amplitude limiter from the output signal from the limiter 12.
a, 1b, to 1n.

The FM demodulation circuit 14 is a circuit for FM-demodulating the signal extracted through the band-pass filter 13 to obtain an original information signal, and uses a discriminator, delay detection, and PLL (Phase Locked Loop). This is a circuit for demodulating an information signal by synchronous detection or the like.

Next, the operation of the present system having such a configuration will be described.

Each of the slave stations 1a, 1b,.
In 1n, analog signals, that is, analog signals 61a, 61b, to 61n such as video signals and wireless signals, which are not binary digital signals, are converted into FM (Frequency Modula) signals.
) FM modulation is performed by inputting the signals to the modulation circuits 5a, 5b, to 5n to generate FM modulation signals 62a, 62b, to
62n is obtained.

The FM modulated signals 62a, 62b, to 62
n superimposes a DC bias current that determines the degree of light modulation, that is, the current source 7 is controlled by the bias tees 6a, 6b,.
The laser elements 8a, 8b, to 8n are directly modulated by being superimposed on the DC bias currents from a, 7b, to 7n and applied to the laser elements 8a, 8b, to 8n.

That is, the magnitude of the DC bias current value determines the degree of light modulation when directly modulating the laser elements 8a, 8b, to 8n, and the level of the degree of light modulation determines the rate at which the waveform is clipped. different.

Here, in the present invention, the laser element 8
The optical signals output from a, 8b,..., 8n are set so that the optical modulation index OMI (Optical Modulation Index) exceeds 100 [%], and this OMI is 100 [%].
By performing direct modulation under the above conditions, clipping occurs in the waveform of the signal to be transmitted. Laser elements 8a, 8
For b, .about.8n, a Fabry-Perot semiconductor laser device is used.

As a result, the laser elements 8a, 8b,.
As shown in FIG. 2, the optical spectra of the optical signals 52a, 52b, to 52n, which are the outputs, have a wider band, lower coherency, and reduced coherence with other optical signals.

The optical signal 52 output from each of the laser elements 8a, 8b, to 8n in each of the slave stations 1a, 1b, to 1n.
a, 52b, to 52n are optical fibers 2 which are optical transmission lines.
, Multiplexed by the optical coupler 4, and then transmitted to the master station 3.

In this manner, the optical signal output from the slave station is multiplexed with the optical signal from another slave station by the optical coupler 4 and then multiplexed into the master station 3.
Transmitted to

The FM modulated signals 62a, 62b, to 62n of the slave stations 1a, 1b, to 1n are arranged in different frequency bands, and the master station 3 collectively receives the signals by one PD9. An electric signal multiplexed with the SCM is obtained. The received electric signal is transmitted through the bandpass filter 10,
A received signal 63 from a desired base station (slave station) 1a, 1b, to 1n is extracted.

The average level of the waveform of the received signal 63 is shown in FIG.
As shown in (a), since clipping occurs on the slave stations 1a, 1b, to 1n, the average level of the signal in the absence of clipping differs from that of the signal, and the difference level depends on the OMI. .

Therefore, a DC bias voltage from the voltage source 11 is added to the signal transmitted through the band pass filter 10 by the bias tee 6. As a result, the received signal 63 is
A received signal 6 whose average level is adjusted to the average level in the absence of clipping, as shown in FIG.
4 is assumed.

The received signal 64 is input to the limiter 12 to obtain a vertically symmetrical limiter output signal 65, which is transmitted through the bandpass filter 13, and then input to the FM demodulation circuit 14 to obtain an information signal 66. An example of such a limiter is an operational amplifier.

The information signals 61a, 61b,... 61n given to the slave stations 1a, 1b,... 1n are sine-wave two-tone signals. Applying a bias signal to the laser element 8
FIG. 3D shows the result of numerical calculation of the frequency spectrum of the frequency demodulated signal (information signal 66) when clipping is caused to occur.

As a technique for suppressing beat noise due to optical multiplexing, an information signal is multiplexed and transmitted as an optical signal in a form in which the amplitude of a waveform becomes vertically asymmetrical due to clipping on the slave station side. Such asymmetry in the up / down direction causes a problem such as generation of distortion such as IM3 on the demodulation side, which is a problem. However, in the system of the present invention, FIG.
As can be seen from (d), the master station on the receiving side applies an appropriate DC bias voltage to the received signal at the bias tee 6 so that the waveform amplitude of the limiter output signal 65 is vertically symmetrical. As a result, the occurrence of harmonic distortion such as IM3 in the demodulated information signal is suppressed, and the problem of vertical asymmetry due to clipping is eliminated.

On the other hand, when the signal processing for changing the average level is not performed as in the conventional device, that is, when the received signal 63
Is input to the limiter 12 as it is, the limiter output signal 67 becomes a frequency modulation signal in which the amplitude of the waveform is asymmetrically limited as shown in FIG. This output signal 67
Is input to the FM demodulation circuit 14 and demodulated, the effect of waveform asymmetry due to clipping appears as distortion.

FIG. 4C shows clipping in FIG. 3D.
Numerical calculation results of the frequency spectrum of the FM demodulated and demodulated signal in the conventional apparatus when the same conditions were caused are shown.
As can be seen from FIG. 4C, in the case of the related art, the third harmonic IM3 is large, and the dynamic range is significantly suppressed.

As described above, according to the present invention, in the master station,
A DC bias voltage is added to the received signal whose waveform amplitude has become vertically asymmetrical due to clipping on the slave station to adjust the average level of the frequency modulation signal to an appropriate position. Since the frequency demodulation is performed after limiting the amplitude so as to satisfy the following condition, distortion due to a shift in the average level of the received signal at the time of frequency demodulation can be suppressed, and the input dynamic range with respect to the optical modulation degree by the FM premodulation of the information signal is improved. It is possible to obtain a conversion with high communication quality, without loss of effect. Further, even if a Fabry-Perot type semiconductor laser device is used, it is not affected by distortion and optical beat noise, so that a low-cost optical SCM multiplexed network can be provided.

Incidentally, the limiter 12 having the structure shown in FIG.
For example, although an operational amplifier is used, there is also an amplitude limitation using a diode. Therefore, an example in which a diode is used as a limiter will be described below as a second embodiment. (Second Embodiment) FIGS. 5A and 5B are block diagrams showing a configuration example of a master station 3 according to a second embodiment in which a diode is used as a limiter. )
In (b), the connection form of the diode is changed.

In FIG. 5, reference numeral 9 denotes a PD (Photo Detector).
r), 10 is a bandpass filter, 11 is a voltage source, 13 is a bandpass filter, 14 is an FM demodulation circuit, 19 is a diode, 20 is a bias coil, 21 is a resistor, and 22 is a capacitor.

Here, the PD 9 on the master station 3 side photoelectrically converts the multiplexed and transmitted optical signal to obtain an SCM multiplexed electric signal, and outputs this as a received signal. The pass filter 10 is for extracting a signal band of a desired slave station from the electric signal from the PD 9.

The voltage source 11 is for supplying a required DC bias voltage.
Is for adding a DC bias voltage from the voltage source 11 to the output of the bandpass filter 10.

In the case of the configuration shown in FIG.
The cathode side is connected to the output side of the bandpass filter 10 and the connection end side between the bias coil 20 and the anode side is connected to the input side of the bandpass filter 13 via the capacitor 22. The anode side of the diode 19 is grounded via the resistor 21.

In the case of the configuration shown in FIG. 5B, there is no bias coil, and the diode 19 has its cathode connected to the voltage source 11.
, And the anode side is connected to the output side of the band-pass filter 10. The output side of the band pass filter 10 is grounded via a resistor 21.

The output side of the band pass filter 10 is connected to the input side of the band pass filter 13 via the capacitor 22.

The anode side of the diode 19 is grounded via the resistor 21. The diode 19 is for clipping the reception signal obtained by adding the DC bias voltage from the voltage source 11 to the output of the bandpass filter 10, and the cathode side is connected to the bias coil 20 side.

The capacitor 22 is for cutting a DC component. By obtaining only an AC component through the capacitor 22 and inputting the AC component to the band-pass filter 13, a harmonic generated by the amplitude limitation by the diode is removed. By removing the signal, a signal of a desired frequency component is extracted. The signal extracted by the band pass filter 13 is demodulated by the FM demodulation circuit 14 to obtain an information signal.

The operation of the present system having the above configuration will be described.

In the second embodiment, as in the first embodiment, the base station 3 uses the bandpass filter 10 to convert the SCM multiplexed signals received by the PD 9 collectively into the desired base station (child). Station 1a, 1b,... 1n
1 is extracted. Then, by limiting the amplitude of one end of the reception signal 71 by the diode 19, a vertically symmetric reception signal is obtained.

That is, in order to suppress beat noise, the optical signal transmitted from the slave station is clipped at one end with the FM modulation signal of the transmission information, and thereby directly modulates the laser element. The spectrum is spread to reduce the coherency of the optical signal. Therefore, since the waveform of the signal is vertically asymmetric, the diode 19 limits the amplitude of the other end of the reception signal 71 which is not clipped, thereby obtaining a vertically symmetric reception signal.

As an insertion configuration of the diode 19, for example, FIG. 5A or FIG. 5B can be considered.

Here, as shown in FIG.
A description will now be given of how the amplitude is limited to 1. The DC bias voltage from the voltage source 11 is added to the reception signal 71 via the bias coil 20. Optimum selection of diode characteristics and DC bias voltage, diode 19, DC cut capacitor 2
2 is transmitted to obtain a vertically symmetrical reception signal 72 as shown in FIGS. 6 (a) to 6 (c).

The reception signal 72 is further passed through the band-pass filter 13 to extract a required frequency component, which is then input to the frequency demodulation circuit 14 and demodulated to obtain an information signal.

This embodiment describes an embodiment in which the limiter is constituted by a diode. The same effect as that of the first embodiment can be obtained, and the distortion generated in the information signal can be suppressed.

In the first embodiment, the FM demodulation circuit 14
Is a differential circuit such as a discriminator or a frequency demodulation circuit based on synchronous detection, it is effective to add a DC bias voltage as described above.

On the other hand, a frequency demodulation circuit (FM demodulation circuit 1)
In 4), there is a circuit based on differential detection. FIG. 7 shows an example of the master station 3 using an FM demodulation circuit by delay detection.

In the configuration of master station 3 shown in FIG.
Reference numeral 9 denotes an FM demodulation circuit that performs frequency demodulation by delay detection. The FM demodulation circuit 29 includes a branch circuit 25 for distributing an input signal, a delay circuit 26 for delaying the input signal by a delay time τ, and an exclusive OR of one branch output of the branch circuit 25 and an output of the delay circuit 26. ExOR circuit 2
7. It is configured to extract a signal component in a required frequency band from the output of the ExOR circuit 27.

The master station 3 is also provided with a comparator 24 at a stage preceding the FM demodulation circuit 29. The comparator 24 converts the frequency modulation signal 63, which is a received signal, into a threshold voltage signal (Vth) 72 from the voltage source 11 and By comparing the magnitudes of the signals, the signals are converted into digital signals of “1” and “0”, and the signals are supplied to the FM demodulation circuit 29.

In such a configuration, the multiplexed optical signal from the slave station is photoelectrically converted by the PD 9 in the master station 3 into an electric signal. After the extraction, this electric signal (frequency modulation signal 63) is input to the comparator 24.

In the comparator 24, the frequency modulation signal 6
3 is connected to a threshold voltage signal (Vt
h) By comparing 72 with the magnitude, the signal is extracted as a digital signal waveform 73 of "1" and "0". Extracted signal 7
3 is branched into two by a branch circuit 25, and one of them is input to an ExOR circuit 27 with a time difference τ by a delay circuit 26.

The output signal of the ExOR circuit 27 is passed through a low-pass filter 28 to remove high-frequency components, and a signal waveform 73
By extracting the rising / falling phase information, frequency demodulation can be performed and an information signal can be extracted.

Conventional frequency demodulation circuit (FM demodulation circuit) 2
9, as shown in the upper part of FIG.
Is adjusted to the average level of the received frequency modulation signal 63.

However, in the present invention, since clipping occurs on the slave station side, as shown in the lower part of FIG. 8, it is assumed that clipping does not occur, instead of the conventional setting of Vth 72. Frequency modulation signal 6
Adjust to an average level of 3.

This difference in the set value of Vth 72 appears as a shift in the phase information of the comparator output signal 73.

FIG. 8 shows both comparator output signals 73.
Is shown. It can be seen that the pulse width as the phase information is shifted.

When the conventional comparator output signal 73 is demodulated, distortion occurs in the information signal, but in the comparator output signal 73 according to the present invention, no distortion occurs in the information signal when demodulated. (Third Embodiment) A third embodiment of the present invention will be described.

FIG. 9 is a block diagram showing the configuration of the optical transmission device of this embodiment.

In FIG. 9, the same components as those in the first embodiment are denoted by the same reference numerals. That is, in the figure, 1
, 1n are slave stations, 2 is an optical transmission path, 3 is a master station, 4 is an optical coupler, 5a, 5b,... 5n is an FM modulator circuit, 6a, 6b, to 6n are bias tees, 7a, 7b,
.. 7n are current sources, 8a, 8b,.
Is PD (Photo Detector), 10, 10a, 10b, ...
10n is a bandpass filter, 11, 11a, 11b,
11n is a voltage source, 12, 12a, 12b, and 12n are limiters, 13 is a band pass fill, and 14 is an FM demodulation circuit.

The slave station 1a includes an FM demodulation circuit 5a, two bias tees 6a, a current source 7a, a voltage source 11a, a band-pass filter 10a, and a laser element 8a. The slave station 1b includes an FM demodulation circuit. 5b, two bias tees 6b, a current source 7b, a voltage source 11b, a bandpass filter 10b, and a laser element 8b.
The slave station 1n has an FM demodulation circuit 5n and two bias tees 6.
n, current source 7n, voltage source 11n, bandpass filter 1
0n and a laser element 8n.

The master station 3 includes the PD 9, the band pass filters 10, 13, the limiter 12, and the FM demodulation circuit.

In this embodiment, in contrast to the configuration of the first embodiment shown in FIG. 1, in the slave stations 1a, 1b,... 1n, the bias tees 6a, 6b,.
b, to 11n, bandpass filters 10a, 10b, to
10n is newly provided, and the voltage source 11 and the bias tee 6 are deleted in the master station 3.

The voltage sources 11a, 11b,..., 11n are for supplying required DC bias voltages, respectively, and the preceding-stage bias tees 6a, 6b,. DC bias voltages from the voltage sources 11a, 11b, and 11n are added to the outputs of the signals .about.5n to make the average level a received signal that matches the average level in the absence of clipping.

The limiters 12a, 12b, to 12n are amplitude limiting circuits having vertically symmetric limiter characteristics, and limit the signals output from the corresponding bias tees 6a, 6b, to 6n by the vertically symmetric limiter characteristics. Thus, an output signal having a vertically symmetrical amplitude level is obtained.

The band pass filters 10a, 10a
b and 10n are circuits for extracting components of a desired frequency band from the output signals from the limiters 12a, 12b, and 12n, and the bias tees 6a, 6b,.
n denotes the FM modulation signals output from the band-pass filters 10a, 10b, to 10n and the current sources 7a, 7b, to 7n.
The laser elements 8a, 8b,..., 8n are directly modulated by the output signals of the bias tees 6a, 6b,. Is output as an optical signal to the optical fiber 2 which is an optical transmission line.

The optical coupler 4 multiplexes and outputs optical signals from the laser elements 8a, 8b, to 8n transmitted through the optical fiber 2, and outputs the multiplexed optical signals.
Reference numeral 9 denotes a photoelectric conversion unit that obtains an electric signal obtained by performing an SCM multiplex signal by photoelectrically converting the multiplexed optical signal, and outputs the electric signal. To extract the received signals from the desired base stations 1a, 1b, to 1n.

The limiter 12 is an amplitude limiting circuit having vertically symmetrical limiter characteristics.
A signal output from 0 is restricted by a vertically symmetric limiter characteristic to obtain an output signal having a vertically symmetric peak level. The band-pass filter 13 is a circuit for extracting a component of a desired frequency band from the output signal from the limiter 12, and the FM demodulation circuit 14 FM-demodulates the signal passed through the band-pass filter 13 to convert the original information signal. It is a circuit to get.

In the third embodiment having such a configuration, the asymmetry of the waveform shape of the optical signal due to clipping is determined by each slave station 1.
The correction is performed on the sides a, 1b,.

That is, in each of the slave stations 1a, 1b,..., 1n, the information signals 61a, 61b,.
n is an FM (Frequency Modulation) modulation circuit 5a, 5
b, .about.5n, and performs FM modulation.
2a, 62b, to 62n are obtained ([State A] in FIG. 10).

FM modulation signals 62a, 62b, -62n
Are applied with DC bias voltages from the voltage sources 11a, 11b, to 11n at the bias tees 6a, 6b, to 6n on the preceding stage, and limiters 12a, 12b, to 12n, and band-pass filters for removing harmonics. 10a, 10b,
〜１０10 n are transmitted.

As a result, one side of the waveform is clipped, and the modulated signals 69a, 69 from which higher harmonic components have been removed.
9b and -69n are obtained ([State B] in FIG. 10). The modulation signals 69a, 69b, to 69n are vertically asymmetric.

The modulation signals 69a, 69 having asymmetrical upper and lower sides
9b, -69n are bias tees 6a, 6b,
6n are superimposed on the DC bias currents from the current sources 7a, 7b, 7n, and the laser elements 8a, 8b,
Direct modulation at MI> 100 [%].

Here, the limit of the laser elements 8a, 8b,...
b, the optical signal 53 to be transmitted with the limit of ~ 12n
The DC bias voltage and the OMI are adjusted so that a, 53b, to 53n are symmetrical up and down, that is, so that the average level does not change ([State C] in FIG. 10).

In the third embodiment, the transmitted optical signal 5
Since the signal waveforms of 3a, 53b, to 53n are already vertically symmetrical, the master station 3 may be provided with a normal FM demodulation circuit.

Next, an embodiment in which the position of the average level of the transmitted optical signal is not changed, that is, the vertical amplitude of the FM modulation signal waveform is not changed, thereby simplifying the configuration of the master station 3 will be described. Will be described as a fourth embodiment. (Fourth Embodiment) In a fourth embodiment, a method of not changing the position of the average level of the transmitted optical signal will be described. FIG. 11 shows the configuration of the optical transmission device according to the fourth embodiment. The main configuration is the same as in the first to third embodiments, and the same components are denoted by the same reference numerals.

In the configuration shown here, the master station 3 supplies the voltage source 1
1 and the bias tee 6 are eliminated, and the amplitudes of the frequency modulation signals 62a, 62b, to 62n input to the laser element are increased, and the level of the laser elements 8a, 8b, to 8n is lower than the threshold value Ith to the saturation region. This is realized by clipping the upper and lower portions of the signal by the E / O characteristics of the laser elements 8a, 8b,.

Normally, the characteristics of the laser element have a modulation current-light output characteristic (E / O characteristic) as shown in FIG. As can be seen from the figure, when the value of the modulation current applied to the laser element is equal to or less than the threshold value Ith, the laser element does not oscillate, so that the light output is small. On the other hand, when the value of the modulation current becomes too large, the light output is saturated. I do.

Therefore, the amplitude of the frequency modulation signals 62a, 62b, to 62n applied to the laser elements 8a, 8b, to 8n is increased, and the amplitude from the level below the threshold Ith of the laser elements 8a, 8b, to 8n to the saturation region is increased. By making the frequency modulation signal oscillate in the range, the laser element 8
The upper and lower portions of the signal are clipped by the E / O characteristics of a, 8b, and 8n.

By adjusting the bias current levels of the frequency modulation signals 62a, 62b, to 62n applied to the laser elements 8a, 8b, to 8n to set the upper and lower clip amounts to be the same as in FIG. The average level of the optical signal 54 can be equal to the position of the average level of the frequency modulation signal 62 as shown in FIG.

On the master station 3 side, since there is no position variation of the average level of the optical signal 54, the output signal of the limiter 12 can be output without performing the signal processing for adding the bias as described in the first embodiment. As a result, a vertically symmetrical reception signal can be obtained, and an information signal with suppressed distortion can be obtained in the frequency demodulation circuit 13.

Further, as shown in FIG. 13A, the amplitude may be limited by an additional circuit in which a diode 23 is connected in parallel with the laser element 8. In this case, when the voltage applied to the diode 23 does not exceed the rising voltage VF when the modulation current 71 is applied to the laser element 8, the laser element 8
All the modulation current 71 flows, and the modulation of the modulation current 71 is applied to the laser element 8.

However, when the voltage exceeds the rising voltage VF of the diode 23, the modulation current 71 corresponding to the excess voltage flows through the diode 23.
No modulation of 1 is applied.

Therefore, the characteristic of the modulation current-optical output of the laser as shown in FIG. 13B is obtained, and it becomes possible to clip the upper and lower portions of the optical signal 55 by adding such a simple circuit.

As described above, the first to fourth embodiments have described the configuration in which an analog information signal is pre-modulated into a frequency modulation signal. The reason is that, when demodulating the frequency-modulated signal, the fluctuation of the amplitude component can be removed by a limiter or the like, and the transmission characteristics do not deteriorate with respect to the amplitude clipping.
Because it is practical because it is resistant. However, besides this, the π / 4 shift QPS used for wireless communication is also used.
Since the same effect can be obtained even with K modulation, an example thereof will be described as a fifth embodiment. (Fifth Embodiment) FIG. 14 is a block diagram showing the configuration of an optical transmission apparatus according to the fifth embodiment. A wireless base station capable of receiving a wireless signal of π / 4 shift QPSK modulation is provided for a slave station. This is an example of the case where The main configuration is the same as that of the first embodiment, and the same components are denoted by the same reference numerals.

In FIG. 14, 1a, 1b, to 1n are slave stations, 2 is an optical transmission line, 3 is a master station, 4 is an optical coupler,
6a, 6b, ６6n are bias tees, 7a, 7b, ７7
n is a current source, 8a, 8b, to 8n are laser elements, 9 is P
D, 10 are bandpass filters, 11 is a voltage source, 12 is a limiter, 13 is a bandpass filter, 15a, 15b,
15n are antennas, 16a, 16b and 16n are low noise amplifiers, and 17 is a π / 4 shift QPSK demodulation circuit.

The slave station 1a includes a low noise amplifier 16a, a bias tee 6a, a current source 7a, and a laser element 8a. The slave station 1b includes a low noise amplifier 16b, a bias tee 6b, a current source 7b, and a laser element 8b. The slave station 1n includes a low noise amplifier 16n,
It comprises a bias tee 6n, a current source 7n, and a laser element 8n.

The master station 3 comprises a PD 9, band pass filters 10, 13, a voltage source 11, a bias tee 6, a limiter 12, and a π / 4 shift QPSK demodulation circuit 17.

Assuming that a radio signal transmitted from a mobile terminal or the like has been subjected to π / 4 shift QPSK modulation,
a, 1b,.
The low-noise amplifiers 16a, 16b, to 16n amplify the π / 4-shifted QPSK-modulated radio signals from the mobile terminals in the service area of the own station, which are received by a, 15b, to 15n. The amplified signal is applied to the bias tee 6a,
6b to 6n, DC bias signals from current sources 7a, 7b, to 7n are added, and laser elements 8a, 8 corresponding to the own station are added.
b, .about.8n, and these laser elements 8a, 8b,.
n is directly modulated.

At this time, similarly to the first to fourth embodiments,
The optical modulation degree OMI (Optica) of the optical signals 55a, 55b, to 55n output from the laser elements 8a, 8b, to 8n.
l Modulation Index) is set to exceed 100 [%] to generate clipping.

Here, if the radio signal transmitted from an arbitrary slave station 1L is in the same frequency band as the radio signals transmitted from the other slave stations 1a, 1b,..., 1n (n ≠ L), the low noise amplifiers 16a, 16b , .About.16n, and frequency conversion is performed. In the optical coupler 4, the other slave stations 1a, 1b,.
SCM multiplexing with an optical signal from (n ≠ L) is enabled.

In the master station 3, the optical coupler 4 uses the SCM
The multiplexed and transmitted optical signals are collectively received by one PD 9, and received signals from desired slave stations 1 a, 1 b, to 1 n are extracted by a band-pass filter 10. The signal processing after the bandpass filter 10 is the same as that of the first embodiment, and the reception signal 7 transmitted through the bandpass filter 13 is used.
0 is input to the π / 4 shift QPSK demodulation circuit 17 to obtain transmission information.

The first to fourth embodiments have described the configuration for premodulating an analog information signal into a frequency modulation signal. This is because, when demodulating a frequency-modulated signal, fluctuations in the amplitude component can be removed by a limiter or the like, and there is no deterioration in transmission characteristics with respect to amplitude clipping, and the frequency-modulated signal is resistant.

Similarly, π / 4 shift QPSK is
It has a constant envelope, is resistant to amplitude clipping, like frequency-modulated signals, and is frequently used as a radio signal in mobile communications.

Therefore, as in the fifth embodiment, a mobile station (base station) and a master station (centralized base station) for mobile communication use
If the modulation scheme of the radio signal is π / 4 shift QPSK modulation, the π / 4 shift
The laser element may be directly modulated with the shifted QPSK modulation signal, thereby causing clipping of the waveform.

As described above, in the configuration of the fifth embodiment, π
It has been shown that the present invention is applicable to / 4 shift QPSK modulation.

The main object of the present invention is to
As a technique for suppressing beat noise caused by optical multiplexing, when an information signal is multiplexed and transmitted as an optical signal in a form that is vertically asymmetric due to clipping on the slave station side, the vertical asymmetry of the signal waveform due to clipping is reduced. The present invention is directed to an improvement against calling distortion generation due to harmonics such as IM3 on the demodulation side.

As a remedy, in the system according to the first embodiment of the present invention, in the master station on the receiving side, the received signal is applied to the bias tee 6 and an appropriate DC bias voltage such that the limiter output signal 65 is vertically symmetrical. Is added to the demodulated information signal.
And so on, so that the problem of vertical asymmetry due to clipping is eliminated.

However, such processing alone cannot be said to be sufficient for a system capable of maintaining reliable reliability.

That is, in the case of the configuration of the first embodiment, in the configuration on the master station 3 side, the FM bias is applied by the magnitude of the DC bias voltage from the voltage source 11 added by the bias tee 6.
The amount of distortion generated in the information signal demodulated by the demodulation circuit 14 differs. In addition, the characteristics of the laser device on the slave station side may change due to aging and the like, and the clipping state of the transmitted optical signal may also change.

Therefore, in order to provide an optical transmission device which can cope with such a situation and is stable over a long period of time and has high reliability,
It is necessary to control the DC bias voltage to an appropriate value.

An embodiment in which such control is enabled will be described next as a sixth embodiment. (Sixth Embodiment) The influence of clipping of the transmitted signal by the laser element 8 is limited by the limit by clipping at the laser element and the limit by the limiter 12. Are symmetric, that is, the average level does not fluctuate,
In the sixth embodiment, the DC bias voltage is adjusted according to the amount of distortion of the demodulated signal.

FIG. 15 shows a configuration diagram of the master station 3 in the optical transmission device of the sixth embodiment. The configuration of the slave station is the same as that of the first embodiment, and the same components as those of the first embodiment in the configuration of the master station 3 are denoted by the same reference numerals.

In the sixth embodiment shown in FIG. 15, a distortion detector 18 is further provided in the master station configuration in the first embodiment shown in FIG. Is monitored, and the voltage value of the voltage source 11 can be adjusted according to the distortion included in the FM demodulated signal output. Note that the voltage source 11 has a configuration in which the output voltage can be changed according to the output of the distortion detector 18.

The amount of distortion generated in the information signal demodulated by the FM demodulation circuit 14 varies depending on the magnitude of the DC bias voltage from the voltage source 11 added at the bias tee 6 on the master station 3 side. In addition, the characteristics of the laser on the slave station side may change due to aging or the like, and the clipping state of the transmitted optical signal may also change.

Therefore, in order to provide an optical transmission device which can cope with such a situation and is stable over a long period of time and has a high reliability,
It is necessary to control the DC bias voltage to an appropriate value.

For this purpose, in this embodiment, the output signal from the FM demodulation circuit 14 is branched and input to the distortion detector 18, and the distortion detector 18 detects the amount of distortion. The distortion detector 18 has a function of controlling a DC bias voltage value to be added to the voltage source 11 based on the detection result so that the distortion amount is suppressed.

As a result, the voltage of the voltage source 11 is adjusted and controlled in response to the distortion, and a long-term stable and reliable optical transmission device can be obtained.

As described above, the master station adjusts the DC bias voltage by the bias tee 6 in response to a change in the clipping state of the transmitted optical signal or the like in accordance with the amount of distortion detected by the distortion detector. Thus, the amount of distortion generated in the information signal demodulated by the FM demodulation circuit 14 is improved, and the configuration is such that long-term stability and reliability can be improved. Since this can be performed on the slave station side, an example will be described below as a seventh embodiment. (Seventh Embodiment) The effect of clipping by the laser element 8 of a signal to be transmitted is limited by the limit by the clipping by the laser element and the limit by the limiter 12, so that the waveform of the transmitted signal is vertically symmetrical. In other words, the DC bias voltage is adjusted according to the amount of distortion of the optical signal to be transmitted so that the average level does not fluctuate. By adjusting the amount of DC bias current superimposition,
In the seventh embodiment, the light modulation degree OMI of the laser element 8 is adjusted.

FIG. 16 is a configuration diagram showing a slave station of the optical transmission device according to the seventh embodiment. The configuration of the master station is the same as that of the third embodiment, and the same components as those of the first to sixth embodiments are denoted by the same reference numerals.

In FIG. 16, the slave station 1 includes an FM modulation circuit 5, a bias tee 6, a voltage source 11, a laser element 8, a PD
9, a band pass filter 13, a bias tee 6, a current source 7, a limiter 12, an FM demodulation circuit 14, and a distortion detector 18.

Among these, the FM demodulation circuit 5 performs FM modulation on an input analog signal and outputs the same. The voltage source 11 is a variable voltage generator and has a required level of DC bias voltage. And the bias tee 6 is connected to the output of the FM modulation circuit 5 in the preceding stage.
A DC bias voltage from the voltage source 11 is added to make the average level of the optical signal 56 a reception signal that matches the average level in the case where clipping does not exist. That is, the DC bias voltage 1 is adjusted by putting on the getter so that the zero level of the waveform becomes the zero cross point of the original FM modulation waveform and the zero cross point of the optical signal 56 to be transmitted.
It is one.

The limiter 12 is an amplitude limiting circuit having vertically symmetrical limiter characteristics. The signal output from the bias tee 6 is limited by the vertically symmetrical limiter characteristics so that the waveform becomes vertically asymmetric at this time. To obtain an output signal. The band-pass filter 13 is a circuit for extracting a component in a desired frequency band from an output signal from the limiter 12, and the current source 7 is a variable current generator and has a required level of DC bias current. Is to supply.

The bias tee 6 at the subsequent stage is connected to the FM modulation signal output from the band-pass filter 13 by the current source 7.
The superimposed DC bias current is output from the superimposed DC bias current and supplied to the laser element 8.

The laser element 8 is an element which is directly modulated by the output signal of the bias tee 6 at the subsequent stage, oscillates laser light, and outputs this as an optical signal to the optical fiber 2 which is an optical transmission line.

The DC bias current and the amplitude of the FM modulation signal, which are combined by the bias tee 6 at the subsequent stage, determine the degree of optical modulation when directly modulating the laser element 8, and the clipping ratio of the waveform is determined.

The PD 9 is for photoelectrically converting the optical signal back into an electric signal.
The frequency detector demodulates the frequency of the electric signal output from the FM demodulation circuit 14 and returns it to the original signal.
The amount of distortion is detected from the demodulated output of the above, and the voltage value of the voltage source 11 and the current value of the current source 7 are adjusted and controlled according to the detected distortion.

In the present system, a bias tee 11 for applying a bias voltage for adjusting a zero level position of a signal waveform by applying a getter to a signal in a slave station and a bias tee 6 for applying a bias current for determining an optical modulation degree are provided. In addition, a distortion detector 18 is provided in the slave station for monitoring distortion in an optical signal sent from the slave station to the master station, and a voltage value and a current of the voltage source 11 corresponding to the distortion detected by the distortion detector 18 are provided. The current value of the source 7 can be adjusted. The slave station in the third embodiment is provided with a distortion monitoring function so that the voltage value and the current of the voltage source 11 corresponding to the distortion included in the FM modulation signal output can be adjusted. The current value of the source 7 can be adjusted in response to distortion.

In the third embodiment, the asymmetry of the optical signal due to clipping is corrected in the slave station 1 in the slave station.
A distortion monitoring function is provided so that the voltage value of the voltage source 11 and the current value of the current source 7 can be adjusted corresponding to the distortion included in the FM modulation signal output.

In such a configuration, the slave station 1 inputs an analog information signal 61 to the FM modulation circuit 5 and performs FM modulation to obtain an FM modulation signal 62. The FM modulation signal 62 Then, the DC bias voltage from the voltage source 11 is added thereto, and the DC bias voltage is transmitted through the limiter 12 and the band-pass filter 10 for removing harmonics.

As a result, a modulated signal 69 in which one side of the waveform is clipped and harmonic components are removed is obtained. This modulation signal 69 is vertically asymmetric.

The modulation signal 69 whose top and bottom are asymmetrical
Is superimposed on the DC bias current from the current source 7 by the bias tee 6 provided for determining the optical modulation degree OMI, and directly modulates the laser element 8 at OMI> 100 [%].

Here, the upper limit of the transmitted optical signal 53 is symmetrical with the limit by the clipping of the laser element 8 and the limit by the limiter 12, that is, the average level position of the transmitted optical signal 56, and The DC bias voltage and the OMI are adjusted so that the position of the average level of the output signal of the FM modulation circuit 5 matches.

As described above, the modulation signal 69 which is vertically asymmetric is superimposed on the DC bias current via the bias tee 6, and the laser element 8 is directly modulated with OMI> 100 [%], thereby achieving coherency. The optical signal is converted into an optical signal having a low optical noise and, therefore, hardly generating beat noise, and transmitted to an optical fiber transmission line.

Here, in the present system, the distortion caused by the limit by the clipping of the laser element 8 and the limit by the limiter 12 is adjusted so that the transmitted optical signal 53 is symmetrical up and down, that is, so that the average level does not fluctuate. Detector 1
The DC bias voltage and the optical modulation OMI are adjusted in accordance with the detected distortion amount of No. 8.

That is, the output light of the laser element 8 is
And demodulate it by the FM demodulation circuit 14 to return to the original signal. The distortion detector 18 monitors the output of the FM demodulation circuit 14 and adjusts the voltage value of the voltage source 11 and the current value of the current source 7 according to the distortion included in the FM demodulated signal output.

The distortion detector 18 sets the upper and lower portions of the optical signal 53 to be transmitted symmetrically, that is, transmits the signal by the limit by the clipping of the laser element 8 and the limit by the limiter 12 according to the amount of distortion detected by itself. Optical signal 5
6 and a control output for adjusting and controlling the DC bias voltage value and the optical modulation factor OMI so that the position of the average level position of the FM modulation circuit 6 coincides with the position of the average level of the output signal of the FM modulation circuit 5. The output levels of the voltage source 11 and the current source 7 are adjusted in accordance with the output of the distortion detector 18. As a result, the laser element 8 is an optical signal having low coherency, and hence hard to generate beat noise, and has no distortion. It becomes possible to convert an FM signal into an optical signal and transmit the signal to a transmission line using an optical fiber.

The amount of distortion is monitored to control the bias voltage and the OMI so as to reduce the amount of distortion. As a result, the laser element 8 is affected by aging, temperature change, and the like.
Even if the bias characteristic of the laser element 8 changes, the change in the clipping state of the optical signal 56 transmitted from the slave station 1 is corrected to a normal state, and the demodulation on the master station side due to the change in the clipping state. The amount of distortion included in the information signal can be suppressed, the deterioration of the demodulated signal can be prevented, and the communication quality can be maintained.

Therefore, an optical transmission device which is stable for a long period and has high reliability can be provided.

As described above, in the sixth and seventh embodiments, the change in the clipping state due to the aging or temperature change of the laser element can be corrected and controlled by the master station or the slave station.

In these embodiments, the restored FM
The distortion amount included in the demodulated signal is detected by the distortion detector 18,
Based on the detection result, a DC bias voltage value added by the voltage source 11 and a DC bias current added by the current source 7 are controlled in a direction in which the amount of distortion is suppressed. By
The optical modulation degree OMI changes. The degree of light modulation changes the amount of spread of the light spectrum, so that the beat noise suppression effect becomes unstable. So, let me touch on this next. (Eighth Embodiment) The configuration of the optical transmission device of the eighth embodiment is as follows.
The configuration is the same as that of the first embodiment.

In the laser element 8, the manner in which the optical spectrum spreads greatly depends on the OMI. FIG. 17 shows the relationship between the OMI [%] of the Fabry-Perot laser device and the 3 dB band [nm] of the optical spectrum.

As shown in FIG. 17, the light modulation degree OMI
However, when clipping starts at OMI> 100 [%], the optical spectrum rapidly expands, and OMI = about 17%.
At 5%, the spread of the optical spectrum is almost saturated. As the spread of the optical spectrum is larger, the influence of beat noise is reduced as the spread of the optical spectrum is larger.
It is better to be ≧ 175 [%].

However, there is a possibility that the bias condition changes due to the temperature state of the laser element 8, aging, etc., and the OMI changes. At this time, if OMI <175 [%] or less, the amount of spread of the optical spectrum greatly depends on the OMI, so that the influence of beat noise changes and stable communication quality as an optical transmission device cannot be guaranteed.

As described above, in the OMI, the optical spectrum is sufficiently diffused, and it is necessary to select a saturation region in which the state of diffusion does not change even with a slight change in the bias condition. Therefore, according to FIG. 17, OMI ≧ 200 [%]
It is considered that the above conditions are satisfied. Therefore, the DC bias current value is set to a value that can control the OMI within the range of OMI ≧ 200 [%].

Further, the light spectrum spread of the laser element 8 also depends on the return light to the laser output surface. When the reflection at the connection destination of the optical fiber 2 connected to the laser element 8 is large, the oscillation of the laser element 8 is increased by the influence of the return light due to the reflection, and the optical spectrum tends to be narrowed.

Inserting an isolator at the output end of the laser element 8 in order to suppress such return light due to reflection also has an effect on optical spectrum diffusion.

The clipping modulation of the laser element also has the effect of reducing the range in which beat noise occurs at a predetermined value or more in the wavelength positional relationship between slave stations. That is, until now, the center wavelength of the light source between arbitrary slave stations has been set to 0.5 [nm].
The beat noise could not be reduced to a predetermined value or less if the distance was not more than the above value. However, when clipping modulation is used, the effect can be improved only by separating the noise by 0.2 [nm] or more.

The clipping modulation has an advantage that the effect of suppressing beat noise is large, but it cannot be completely removed. Therefore, using the present embodiment in combination with the wavelength control is effective because higher communication quality can be ensured. At this time, by applying the clipping modulation to each slave station as compared with the wavelength accuracy and the wavelength interval of the conventional wavelength multiplexing, it is possible to greatly ease the demand for the wavelength accuracy and the wavelength interval. Therefore, the combined use of clipping modulation and wavelength multiplexing is effective for suppressing and avoiding beat noise.

The present invention is not limited to the above-described embodiment, but can be implemented with various modifications.

As described above, in the first to eighth embodiments, the techniques related to the reduction of the optical beat noise and the securing of the dynamic range have been described. Next, a technique related to interference wave suppression and a technique to prevent deterioration of reception sensitivity due to amplitude limitation even when the optical modulation degree of an optical signal transmitted from a slave station to a master station is set to “1” or more. Will be described.

A technique relating to interference wave suppression will be described.

<Technique Related to Interference Wave Suppression> In a radio base station, an interference wave may be mixed in a frequency band close to a received desired wave. In some cases, the power of the interference wave is higher than the power of the desired wave. In particular, in an optical analog transmission apparatus in which a radio base station mainly includes only an antenna unit and an electro-optical conversion unit and optically transmits a received radio signal to a master station, the interference wave has a light modulation degree of a desired wave. Cause pressure.

If the degree of optical modulation of the desired wave is limited to a small value, the CNR of the desired wave is reduced in the master station, so that the specified error rate may not be achieved.

Therefore, a technique for coping with this will be described below.

[0204] First, here, a technique for enabling a slave station to greatly suppress an interference wave received together with an information signal and for ensuring a stable optical modulation factor when converting an information signal into an optical signal. A ninth embodiment will be described. (Ninth Embodiment) In the ninth embodiment, a band-pass filter is provided in the slave station so that the received information signal is transmitted to the slave station in order to greatly suppress the interference wave received together with the information signal in the slave station. By passing through a band-pass filter, an interference wave received together with the information signal is suppressed.

The ninth embodiment of the present invention will be described. FIG.
FIG. 19 is a block diagram illustrating a configuration of an optical transmission device according to a ninth embodiment. Here, the modulation method of the radio signal 81 is QPS
The configuration of the wireless transmitter 29 in the case of K (quadrature phase-shift keying) is shown.

In FIG. 22, 1 is a slave station, 29 is a radio transmitter owned by a user who is a mobile terminal, 3 is a master station, 2 is an optical fiber, and an optical fiber connecting the slave station 1 and the master station 3. It is a transmission path.

[0207] Of these, the radio transmitter 29 is the signal source 3
0, a quadrature modulator 31, a local oscillator 33, and a multiplier 3
4, a bandpass filter 35, a power amplifier 36, and a transmission antenna 37.

When the modulation method is QPSK, there are an I channel signal 82 and a Q channel signal 83. The signal source 30 in the radio transmitter 29 generates an I channel signal 82 and a Q channel signal 83, and the quadrature modulator 31 quadrature modulates these signals 82 and 83 to convert them into a QPSK signal 84. is there. The local oscillator 33 generates a local oscillation frequency signal, and the multiplier 34 multiplies the local oscillation frequency signal by the QPSK signal 84 and converts the signal into a signal in a high frequency band for wireless transmission. . The band-pass filter 35 limits the band of the radio transmission high frequency band signal converted by the multiplier 34. The power amplifier 36 converts the band-limited radio transmission high frequency band signal. The transmission antenna 37 transmits the amplified radio signal 81 to the air.

[0209] The slave station 1 includes a bias tee 6, a current source 7, a laser element 8, an antenna 15, a low noise amplifier 16, and a band-pass filter 91.

[0210] The antenna 15 in the slave station 1 receives a radio signal, and the low noise amplifier 16 amplifies the radio signal 81 received by the antenna 15 together with the radio signal 89 as an interference wave. Bandpass filter 9
1 is for extracting a frequency component of a required channel from the amplified signal, and the bias tee 6 applies a current to the received signal 87 of the specific channel component extracted by the band-pass filter 91. The DC bias signal from the source 7 is added and output. The laser element 8 receives the received signal 87 of the specific channel component to which the DC bias signal is added by the bias tee 6, and outputs the level of the signal. A corresponding laser beam is generated. The laser beam (optical signal 57) is transmitted to the master station 3 via the optical fiber 3.

The master station 3 comprises a PD (light receiving element for photoelectric conversion) 9, a bandpass filter 10, and a preamplifier 39.
And a quadrature demodulator 40.

The PD 9 in the master station 3 receives the optical signal 57 transmitted through the optical fiber 2 and converts it into an electric signal. The preamplifier 39 amplifies this electric signal, and The pass filter 10 suppresses a noise component from the amplified electric signal and extracts a signal component. A quadrature demodulator 40 demodulates the signal transmitted through the band pass filter 10 and restores the original information signal. Is what you do.

In such a configuration, the radio transmitter 29
Then, an I channel signal 82 and a Q channel signal 83 are generated from the signal source 30. These signals 82 and 83 are converted into a QPSK signal 84 by the quadrature modulator 31, and further converted into a high frequency band for wireless transmission by the local oscillator 33 and the multiplier 34. Then, the band pass filter 35
After being band-limited by passing through, the signal is amplified via the power amplifier 36 and converted into a radio signal 81 as the transmission antenna 3.
7 to the slave station 1 side.

Radio signal 81 transmitted from radio transmitter 29 of the subscriber is received by antenna 15 of slave station 1. At this time, the radio signal 89 used by another subscriber or another radio service is also received by the antenna 15.

In the mobile station 1, the radio signal 81 received by the antenna 15 is sent to the low noise amplifier 16 together with the radio signal 89 as an interference wave, and is amplified there. Then, the signal is further passed through a band-pass filter 91 to extract a specific channel component. And this band transmission filter 9
1 is applied to the DC signal from the current source 7 by the bias tee 6, and
To directly modulate the laser element 8.

By this modulation, an optical signal 57 is output from the laser element 8. Then, the optical signal 57 output from the laser element 8 is transmitted to the master station 3 via the optical fiber 2. In the master station 3, the transmitted optical signal 57
Is received by the PD 9, converted into an electric signal, and then amplified by the preamplifier 39. Then, the noise component is suppressed by passing through the band-pass filter 10 and input to the quadrature demodulator 40 to obtain a transmission signal.

In the slave station 1, as shown in FIG. 23A, the radio signal 89 as an interference wave may be larger than the radio signal 81 as a desired wave.

In such a case, the optical modulation degree of the optical signal 57 is determined by the intensity of the interference wave, and the optical modulation degree of the desired wave is limited. Then, C of the received signal in the master station 3
The NR is limited.

Therefore, in the slave station 1, before the electrical-optical conversion (that is, before the optical conversion by the laser element 8), a band-pass filter having a transmission characteristic as shown by a dotted line in FIG. It is necessary to pass the reception signal including the radio signals 81 and 89 through 91.

The band-pass filter 91 has a transmission characteristic as shown by a dotted line in FIG. 23 (b). As a result, the radio signal 81 as the desired wave is transmitted, but the band of the radio signal 89 as the interference wave is passed. Since there is no interference wave, the interference wave is suppressed. Therefore, regardless of the signal strength of the interference wave, the degree of light modulation of the desired wave can be stabilized, and the upper limit of the degree of light modulation can be increased.

If the frequency interval between the desired wave and the interference wave is not sufficiently separated, the degree of suppression of the interference wave is increased by narrowing the transmission band of the band-pass filter 23.

However, if the transmission band of the band-pass filter 91 is too narrow, intersymbol interference occurs in a desired wave. At this time, the narrowest pass band filter 91 that can be selected
Is a filter for waveform equalization.

In the ideal waveform equalization performed in the transmission system, the inter-symbol interference of the received signal is made zero at the time of code judgment. Set the symbol rate of the received signal to Bsym [symbol / second],
The Nyquist frequency that is 1/2 of the symbol rate Bsym is fN [Hz]. As shown in FIG. 24, an ideal band equalizing filter for waveform equalization has a cutoff frequency (the cutoff frequency here is a frequency at which the transmittance is ideally “0”) is a transmission center. For the frequency fc [Hz], fc ± fN. In addition, the 3 [dB] transmission bandwidth Δf is as follows: Δf = 2fN = Bsym

However, if there is a time deviation at the time of code determination, there is a disadvantage that intersymbol interference becomes extremely large. Therefore, the cutoff frequency is actually fc ± (1 + α)
A transmission system free of intersymbol interference even if the transmission band is widened as fN and the filter characteristics within the band are moderated.

This α is called a roll-off rate, and 0 ≦ α ≦ 1
(Where α = 0 is an ideal filter).

The waveform shaping passed through the filter having the cutoff frequency up to (1 + α) times is called roll-off shaping.

The transmission characteristic g (x) of the roll-off shaping is expressed as follows.

G (x) = 1 (where | x | <1−α) g (x) = (1/2) · [1-sin１− (π / 2α) ·
(X-1)｝] (where 1−α ≦ | x | ≦ 1−α) g (x) = 0 (where 1 + α <| x |) where x is a frequency difference from the center frequency fc, This is normalized by the Nyquist frequency fN.

In the roll-off shaping, the convergence of the impulse response is faster than that of the ideal filter, so that the influence of the time deviation at the judgment time is small. Normally, in wireless communication, equal filters are inserted on the transmitting side and the receiving side, and roll-off shaping is performed using the pass characteristics of both filters. This filter is called a root roll-off filter.

FIG. 25 shows the 3 [dB] transmission bandwidth and the symbol rate Bsym of the band transmission filter 91 provided in the slave station 1.
Shows the relationship.

The band-pass filter 91 changes the 3 [dB] pass band width by Δf [H] around the center frequency fc of the information signal.
z]. When roll-off shaping is performed on the information signal only on the receiving side of the slave station 1, Δf = Bsym. For an information signal that has already been rolled off on the transmitting side, Δf = 2Bsym is set so that the signal component of the information signal is not suppressed.

When waveform equalization is performed on the transmitting side and the receiving side by a root roll-off filter having a roll-off coefficient α, ideally, Δf = (α + 3) Bsym / 3. Bsym ≦ Δf ≦ 2Bsym in consideration of the fluctuation of

From the above, the condition of the 3 [dB] pass bandwidth Δf [Hz] of the band-pass filter 23 provided in the slave station 1 is as follows.
It can be seen that the symbol rate Bsym [symbol / second] is optimal if it is within the range of Bsym ≦ Δf ≦ 2Bsym.

Since the bit rate Bsym [bit / second] and the symbol rate Bsym differ depending on the modulation method, the 3 [dB] pass bandwidth Δf of the band-pass filter cannot be represented by the bit rate Bm.

As an example, when the information signal is QPSK modulation, if the bit rate is Bbit, the symbol rate Bsym is Bsym = Bbit / 2. Further, the Nyquist frequency fN [Hz] has the following relationship: fN = Bsym / 2 = Bbit / 4.

As described above, according to the ninth embodiment, the band-pass filter 38 is provided in the slave station 1, and the information signal received in the slave station 1 is passed through the band-pass filter 38. Since the signal is extracted and the laser element 8 is driven by the extracted information signal to transmit light to the master station 3, the slave station 1 filters the interference wave received together with the information signal into the band-pass filter 91. Can be greatly suppressed by passing through. Therefore, according to the present invention, it is possible to provide a stable optical modulation when converting an information signal into an optical signal.

The above configuration is such that, at the slave station, an information signal of a specific channel is extracted by a band-pass filter and then photoelectrically converted and transmitted to the master station.
Although it used to be able to suppress the interference wave included in the information signal, since the information signal is not a single channel but a plurality of channels in the slave station, we want to be able to select any desired channel. By the way. Therefore, an example in which an information signal of a desired channel is selected from information signals to enable waveform equalization will be described next as a tenth embodiment. (Tenth Embodiment) FIG. 26 is a block diagram showing the configuration of the optical transmission apparatus according to the tenth embodiment. In FIG. 26, the same reference numerals are assigned to the slave station 1 and the master station 3 for the same configurations as in the ninth embodiment. Note that, similarly to the ninth embodiment, the modulation method of the radio signal 81 is QPSK (quad
rature phase-shift keying).

In FIG. 26, 1 is a slave station, 29 is a radio transmitter owned by a user who is a mobile terminal, 2 is an optical fiber, and 3 is a master station. The radio transmitter 29 includes a signal source 30, a quadrature modulator 31, and a local oscillator 3 as in the ninth embodiment.
3, a multiplier 34, a band-pass filter 35, a power amplifier 36, and a transmission antenna 37. And
In the optical transmission apparatus according to the tenth embodiment, a transmission-side route roll-off filter 32 is further provided so that the output of the quadrature modulator 31 is band-limited.

That is, since the modulation scheme is QPSK, the signal source 30 in the radio transmitter 29 generates an I-channel signal 82 and a Q-channel signal 83, and the quadrature modulator 31 Modulate and QPS
The QPSK signal 84 is converted to a K signal 84, and the QPSK signal 84 is band-limited by passing through a transmission-side route roll-off filter 32 for waveform roll-off shaping. In this configuration, the signal is multiplied by a local oscillation frequency signal from the oscillator 33 and converted into a signal in a high frequency band for wireless transmission.

The band-pass filter 35 limits the band of the signal in the high frequency band for radio transmission converted by the multiplier 34. The power amplifier 36 controls the band of the high frequency band for radio transmission in the band. , And the transmitting antenna 37 uses the amplified radio signal 81.
Is transmitted in the air.

The slave station 1 includes a bias tee 6, a current source 7, a laser element 8, an antenna 15, a low noise amplifier 16, a local oscillator 33a, a multiplier 34a,
A receiving-side route roll-off filter 38 is provided.

That is, here, the local oscillator 33a,
A multiplier 34a is newly provided to provide a function of selecting a desired channel, and a channel selection,
It differs from the ninth embodiment in that it has a function of removing an image generated at the time of waveform equalization and frequency conversion.

The low noise amplifier 16 in the slave station 1
The wireless signal 81 received by the antenna 15 of the slave station 1 is amplified together with the wireless signal 89 as an interference wave. The multiplier 34a multiplies the local oscillation frequency signal from the local oscillator 33a by the output of the low-noise amplifier 16 to downconvert the frequency, and the receiving-side root roll-off filter 38 generates a waveform from the downconverted received signal. The received signal 87 of the desired specific channel component is extracted by roll-off shaping, and the bias tee 6 adds the DC bias signal from the current source 7 to the extracted signal and outputs the signal. The element 8 receives the signal 87 of the specific channel component to which the DC bias signal is added by the bias tee 6, and generates a laser beam corresponding to the level of the signal. The laser beam (optical signal 57) is transmitted to the master station 3 via the optical fiber 3.

The master station 3 includes a PD (light receiving element for photoelectric conversion) 9, a bandpass filter 10, and a preamplifier 39.
And a quadrature demodulator 40.

The PD 9 in the master station 3 receives the optical signal 57 transmitted through the optical fiber 2 and converts it into an electric signal. The preamplifier 39 amplifies this electric signal, and The pass filter 10 suppresses a noise component from the amplified electric signal and extracts a signal component. A quadrature demodulator 40 demodulates the signal transmitted through the band pass filter 10 and restores the original information signal. Is what you do.

The operation of the optical transmission apparatus according to the tenth embodiment having such a configuration will be described. Here, as in the ninth embodiment, the modulation method of the radio signal 81 is QPSK (quadrature ph).
ase-shift keying).

At the subscriber's radio transmitter 29, the I-channel signal 82 and the Q-channel signal 8
Generates 3. Each of the signals 82 and 83 is converted into a QPSK signal 84 by the quadrature modulator 31, passed through the transmission-side route roll-off filter 32, and subjected to band-limited QPSK.
It becomes a K signal 85. The QPSK signal 85 is output from the local oscillator 3
The signal is converted into a high frequency band for wireless transmission by the multiplier 3 and the multiplier 34.

Then, the signal is transmitted as a radio signal 81 from the transmitting antenna 37 to the slave station 1 via the band pass filter 35 and the power amplifier 36. The wireless signal 81 transmitted from the wireless transmitter 29 of the subscriber is received by the antenna 15 of the slave station 1. At this time, the radio signal 89 used by another subscriber or another radio service is also received by the antenna 15. The received radio signal 81 is amplified by the low noise amplifier 16 together with the radio signal 89 as an interference wave, and is converted into a low frequency band QPSK signal 86 by the local oscillator 33a and the multiplier 34a.

The band of the QPSK signal 86 is limited by the receiving-side route roll-off filter 38, so that the radio signal 89, which is an interference wave, is largely suppressed. Further, by down-converting the frequency bands of the radio signals 81 and 89, it is possible to suppress an interference wave having a higher intensity adjacent thereto.

The reception signal 87 transmitted through the reception-side root roll-off filter 38 is added with the DC bias signal from the current source 7 by the bias tee 6, and directly modulates the laser element 8. The optical signal 57 output from the laser element 8 is
As in the ninth embodiment, the signal is transmitted to the master station 3 via the optical fiber 2.

In the master station 3, the transmitted optical signal 5
7 is received by the PD 9, converted into an electric signal, and then amplified by the preamplifier 39. Then, the noise component is suppressed by passing through the bandpass filter 10 and input to the quadrature demodulator 40 to obtain a transmission signal.

In this embodiment, a root roll-off filter is provided on the transmission side and the reception side, and the transmission side suppresses unnecessary frequency components by shaping the waveform through the root roll-off filter, thereby reducing the frequency of a specific channel. By narrowing down the transmission, the receiving side suppresses unnecessary frequency components by shaping the waveform of the received signal through a route roll-off filter, thereby reducing the frequency of the desired specific channel so that it can be received.
An arbitrary desired channel can be selected from the information signal to perform waveform equalization.

Then, in the slave station 1, selecting a desired channel from the received information signals and equalizing the waveform of the information signal as the desired channel has the following effects.

That is, the channel selection suppresses the interference wave included in the information signal. For this reason, the intensity of the information signal does not depend on the interference wave, and the degree of light modulation for the information signal can be stabilized when the information signal is converted into an optical signal.

When the information signal is subjected to waveform equalization, the reception sensitivity of the information signal is not deteriorated even if the transmission system has nonlinear signal processing such as amplitude limitation. Therefore, the components used in the transmission system from the slave station to the master station or the degree of freedom of the transmission system are expanded, and the configuration of the optical analog transmission apparatus can be diversified.

In the tenth embodiment, unnecessary signals are suppressed by shaping the waveform of a received signal through a route roll-off filter at a slave station on the receiving side, thereby narrowing down the frequency to a desired specific channel frequency. By enabling reception, an arbitrary desired channel can be selected from the information signal and transmitted to the master station.

In a radio base station, an interference wave may be mixed in a frequency band close to a received desired wave, but sometimes the power of the interference wave is larger than the power of the desired wave. In an optical analog transmission device in which a base station mainly includes only an antenna unit and an electro-optical conversion unit and optically transmits a received radio signal to a master station, this interference wave suppresses the optical modulation degree of a desired wave. Cause you to

If the degree of optical modulation of the desired wave is limited to a small value, the CNR of the desired wave is reduced in the master station, so that the specified error rate may not be achieved. However, this can be solved by the ninth and tenth embodiments described above. <Technique for increasing the degree of optical modulation of a transmission optical signal> The technique for suppressing the interference wave has been described above. Next, a technique for increasing the optical modulation degree of the optical signal transmitted from the slave station to the master station will be described.

There is a method in which the optical modulation degree of the optical signal transmitted from the slave station to the master station is set to “1” or more to increase the CNR of the desired wave in the master station. However, if the desired wave is a radio signal that is angle-modulated, the reception sensitivity deteriorates due to the amplitude limitation. Therefore, the required CNR value becomes large, and the specifications for the slave station become strict.

An optical signal is transmitted from the slave station to the master station. The strength of the optical signal received by the master station is preferably higher. However, in order to secure the intensity of the optical signal received by the master station 3 which is the receiving side of the optical signal, if the optical modulation degree per channel on the transmitting side is increased, the optical modulation degree OMI of the optical signal is increased. (Optical Modulation Index) is 100
[%] Easily, clipping (amplitude limitation), C
There is a concern that the NR (carrier noise ratio) will deteriorate.

An embodiment in which such a concern is eliminated and the limit of the optical modulation degree of the optical signal can be made 100% or more will be described as an eleventh embodiment. (Eleventh embodiment) In the present invention, the slave station 1 performs waveform equalization and then optically modulates the light to transmit the light to the master station, thereby preventing the reception sensitivity from deteriorating due to clipping. In this way, it is possible to limit the optical modulation degree of the optical signal on the slave station side to 100% or more. Then, in the master station 3, a received signal of a larger CNR is obtained.

An eleventh embodiment of the present invention will be described. FIG.
FIG. 7 is a block diagram showing the configuration of the optical transmission device according to the eleventh embodiment. The same code | symbol is attached | subjected to the slave station 1 and the master station 3 about the structure same as 9th Example.

The slave station 1 has a bias tee 6 and a current source 7
, A laser element 8, an antenna 15, a low-noise amplifier 16, local oscillators 33A, 33B, to 33N, multipliers 34A, 34B, to 34N, and filters 50A, 5N.
0B, .about.50N and an adder 43 are provided.

That is, here, the local oscillators 33A, 3
3B, ~ 33N and multipliers 34A, 34B, ~ 34N
, Filters 50A, 50B, to 50N, and adder 43
Are newly provided, and waveform equalization is performed on a plurality of channels, which is different from the tenth embodiment.

The low noise amplifier 16 in the slave station 1
The wireless signal 81 received by the antenna 15 of the slave station 1 is amplified together with the wireless signal 89 as an interference wave. The multiplier 34A multiplies the local oscillation frequency signal from the local oscillator 33A by the output of the low noise amplifier 16 to perform frequency down-conversion, and the filter 50A outputs a desired specific channel component from the down-converted received signal. The multiplier 34B multiplies the local oscillation frequency signal from the local oscillator 33B by the output of the low-noise amplifier 16 to perform frequency down-conversion, and the filter 50B performs the down-conversion. A multiplier 34N multiplies the local oscillation frequency signal from the local oscillator 33N by an output of the low noise amplifier 16 to down-convert the frequency of the received signal 87B of a desired specific channel component from the received signal. The filter 50N is a down converter The reception signal 87N of a desired specific channel component is extracted from the received reception signal.

The adder 43 multiplexes the outputs (received signals 87A, 87N) of the filters 50A, 50B, 50N, which are the outputs of the respective systems A, N.
The bias tee 6 adds the DC bias signal from the current source 7 to the combined signal and outputs the combined signal. The laser element 8 outputs the combined signal to which the DC bias signal is added by the bias tee 6. The laser beam is applied and modulated according to the signal. The laser beam (optical signal 70) is transmitted to the master station 3 via the optical fiber 3.

The master station 3 comprises a PD (light receiving element for photoelectric conversion) 9, a bandpass filter 10, and a preamplifier 39.
And a quadrature demodulator 40.

The PD 9 in the master station 3 receives the optical signal 57 transmitted through the optical fiber 2 and converts it into an electric signal. The preamplifier 39 amplifies this electric signal, and The pass filter 10 suppresses a noise component from the amplified electric signal and extracts a signal component. A quadrature demodulator 40 demodulates the signal transmitted through the band pass filter 10 and restores the original information signal. Is what you do.

The operation of the optical transmission apparatus according to the tenth embodiment having such a configuration will be described. Here, as in the ninth embodiment, the modulation method of the radio signal 81 is QPSK (quadrature ph).
ase-shift keying).

In the slave station 1, first, the antenna 15 receives a plurality of radio signals 81A, 81B to 81N. The received radio signals 81A, 81B, to 81N are amplified by the low noise amplifier 16 and divided into a desired number N of channels. The radio signals 81A, 81B, to 81N are output from local oscillators 33A, 33B, to 33N and multipliers 34A, 34B, to
34N, the frequency is converted by the filters 50A and 50N.
B, passing through 50N and receiving signals 87A, 87B,
87N is obtained.

The received signals 87A, 87B, to 87N have different frequencies, and are arranged in a frequency arrangement in which third-order intermodulation distortion does not decrease. Also, the received signals 81A, 81A
B, .about.81N are equalized in waveform by filters 50A, 50B, .about.50N.

The received signals 87A, 87B,..., 87N are multiplexed by the adder 48, and the bias tee 6 adds a DC bias signal from the current source 7 to directly modulate the laser element 8.

Optical signal 90 output from laser element 8
Is transmitted to the master station 3 via the optical fiber 2. Master station 3
Then, the transmitted optical signal 90 is received by the PD 9, and is amplified by the preamplifier 39. Then, the bandpass filter 10 extracts the reception signal 88 of a desired channel among the plurality of channels A, B to N. Bandpass filter 10
Is wider than the modulation band of the received signal 88 so as not to deteriorate the signal waveform. The received signal 88 is transmitted to the demodulator 49
And demodulated to obtain transmission information.

In this embodiment, the local oscillators (33A, 33B, -33N), multipliers (34A, 34B, -34N), filters (50A, 5N)
0B, .about.50N), and the adders 43 multiplex the signals for the plurality of channels A, B,.
Is modulated. Therefore, the optical signal 90 output from the laser element 8 of the slave station 1 has a plurality of channels A, B,
To N configurations.

In the master station 3, the transmitted optical signal 90 is received by the PD 9, amplified by the preamplifier 39, and received by the band-pass filter 10 into a received signal of a desired channel among a plurality of channels A, B to N. Extract 88.

Therefore, the master station 3 wants to increase the degree of optical modulation per channel in order to obtain the signal strength to be received. However, if the degree of optical modulation per channel is increased, the optical signal Modulation degree OMI (Optical M
odulation index) easily exceeds 100 [%], and the possibility of clipping (amplitude limitation) increases.

However, in the present invention, since the waveforms are equalized for each channel by the filters 50A, 50B, to 50N in the slave station, the amplitude limit has a tolerance and the receiving sensitivity does not deteriorate due to clipping. Therefore, the limit of the optical modulation degree of the optical signal can be set to 100 [%] or more, and the master station 3 has a larger CN.
An R received signal 88 can be obtained.

The information signal from the subscriber is received by the slave station as the remote station, and the received information signal is sent from the slave station to the master station. Since the operation is performed, the intensity of the optical signal received by the master station is preferably higher. However, if an interference wave is present in the received signal at the slave station and the optical signal strength of the desired wave fluctuates, it becomes difficult to transmit a desired signal having a stable CNR to the master station.

[0279] Next, a twelfth embodiment will be described in which an optical signal of a desired wave can be transmitted to the master station with a stable signal intensity even when there is an interference wave. (Twelfth Embodiment) By passing a received radio signal through a narrow band-pass filter of a root roll-off filter, it is possible to largely suppress interference waves other than the desired wave. Therefore, a narrow band-pass filter of a root roll-off filter is provided in the slave station, and a received radio signal is extracted through the narrow band-pass filter of the root roll-off filter.

A twelfth embodiment of the present invention will be described. FIG.
FIG. 8 is a block diagram showing the configuration of the optical transmission apparatus according to the twelfth embodiment. The configurations of the subscriber's wireless transmitter 29, slave station 1 and master station 3 are the same as in the tenth embodiment described with reference to FIG.

However, a plurality of slave stations 1a, 1b,
.About.1n are passively multiplexed (SCM multiplexed) by the optical coupler 4 and accommodated. In FIG. 28, the same components as those in FIG. 26 are denoted by the same reference numerals.

As in the tenth embodiment, the modulation method of the radio signal 81 is QPSK (Quadture phase / shift keyin).
g), and the slave stations 1a, 1b,.
Transmit to the n side. The wireless signal 81 transmitted from the wireless transmitter 29 of the subscriber is received by a certain one of the plurality of slave stations 1a, 1b, to 1n, for example, the antenna 15a of the slave station 1a.

In the slave station 1a, the received radio signal 81
Is amplified by the low noise amplifier 16a, and is converted into a QPSK signal 86 in a low frequency band by the local oscillator 33 and the multiplier 34. Then, the band of the QPSK signal 86 is limited by the receiving-side route roll-off filter 38a.
Here, as the root roll-off filter 38a, a narrow band-pass filter for performing waveform low-off shaping is used, so that interference waves other than the desired wave can be largely suppressed.

Therefore, when directly modulating the semiconductor laser device, it is possible to prevent the optical modulation degree of the desired wave from being suppressed by the interference wave, and to obtain a stable signal intensity of the desired wave regardless of the presence or absence of the interference wave. This can be transmitted to the master station.

The received signal 87 transmitted through the receiving-side root roll-off filter 38a is supplied to the current source 7 by the bias tee 6a.
a to the laser element 8
a is directly modulated. Here, the optical modulation degree OMI (Optical Modulat) of the optical signal 57a output from the laser element 8a.
ion Index) is set to exceed 100%. Further, the radio signal 87L transmitted from an arbitrary slave station 1L (1 ≦ L ≦ N) is set to a different frequency band from the radio signal 87 transmitted from the other slave stations 1a, to 1n (≠ L). , SC with the optical signal from the other slave stations 1a, to 1n (≠ L)
Enables M multiplexing.

In the master station 3, the SCM-multiplexed optical signals are collectively received by one PD 9 and amplified by the preamplifier 39. Then, by passing through the band-pass filter 10, a reception signal 88 from a desired slave station among the slave stations 1a, 1b, to 1n is extracted.

The band-pass filter 10 has a width wider than the modulation band of the received signal 88 so as not to deteriorate the signal components. The received signal 88 is input to the quadrature demodulator 40, where it is demodulated to obtain transmission information. Further, as the laser element 8, a Fabry-Perot type semiconductor laser element or a distributed feedback type semiconductor laser element may be used, but it is preferable to use a Fabry-Perot type semiconductor laser element having smaller coherence.

Normally, the optical modulation degree of the optical signal 57 is set to 100
[%], The C of the received signal 88 in the master station 3
The NR becomes larger and the noise component becomes smaller, but on the other hand, the amplitude of the transmission signal is limited, so that the waveform of the received signal 88 is distorted and the receiving sensitivity is deteriorated.

However, in the slave station 1, when the transmission signal is subjected to roll-off shaping and then the amplitude is limited, the reception sensitivity does not deteriorate regardless of the waveform distortion. In FIG.
9 shows an error rate characteristic of the QPSK modulated wireless signal 88 with respect to the CNR.

[0290] The root roll-off filter 38
The error rate between the case where the filter is provided and the case where a narrow band filter other than the root roll-off filter (here, the Patterworth filter) is provided is defined as “with roll-off shaping” and “without roll-off shaping”. Shown.

The light modulation was set to 250 [%]. When there is no roll-off shaping, the receiving sensitivity is 1
[DB] Deterioration is weak. The deterioration of the receiving sensitivity is caused by the
Will increase the required CNR,
It narrows the dynamic range.

On the other hand, when roll-off shaping is performed, the error rate is equal to the theoretical limit, and there is no deterioration in reception sensitivity. Therefore, the degree of light modulation can be increased for the wireless signal 88 without strict specification requirements.

After the QPSK signal is roll-off shaped, the reception sensitivity does not deteriorate even if the soft limit or the hard limit is performed. A limiter amplifier may be used as a driver amplifier used when modulating the LD (laser element) 8 of the slave station 1.

In the present embodiment, the slave station 1 performs roll-off shaping on the transmission signal and then limits the amplitude.
As a result, the reception sensitivity does not deteriorate regardless of the waveform distortion.

FIG. 30 shows the details of the configuration of the drive circuit portion of the LD 8 used in the present invention.

As shown in FIG. 30, LD8 used in the present invention is
Is composed of a driver amplifier 41, a coupler 42, a power detector 43, a reference voltage generator 44, a comparator 45, and a loop filter 46.

The driver amplifier 41 is a variable gain type amplifier to which a DC bias signal from the current source 7 is added to amplify a reception signal output from the bias tee 6 and output the amplified signal to the laser element 8. The laser element 8 receives the received signal 87 to which the DC bias signal is added by the bias tee 6, and generates a laser beam modulated corresponding to the signal.

When driving the laser element 8 by amplifying a signal with the driver amplifier 41, normally, in order to keep the optical modulation degree of the optical signal output from the laser element 8 at a constant value, the AGC ( Auto Gain control) needs to be performed.

Therefore, in the case of the AGC shown in FIG. 30, first, the output signal of the driver amplifier 41 is partially branched by the coupler 42, the signal intensity is detected by the power detector 43, and the detected signal intensity is detected. Is a signal obtained by comparing the reference voltage value from the reference voltage generator 44 with a comparator 45 to obtain a difference signal, and extracting the difference signal output from the comparator 45 through a loop filter 46, and Control the gain of

As described above, when the received signal 87 transmitted through the receiving-side root roll-off filter 38 is added to the DC bias signal from the current source 7 by the bias tee 6 to directly modulate the laser element 8, The signal is amplified through the driver amplifier 41 so as to have a signal level sufficient to drive the laser element 8, and the amplified signal is applied to the laser element 8. In order to maintain the optical modulation degree of the optical signal output from the laser element at a constant value. The automatic gain control is performed by an AGC circuit of a driver amplifier 41 including a coupler 42, a power detector 43, a reference voltage generator 44, a comparator 45, and a loop filter 46.

However, the laser element 8 can be driven with a simpler configuration without using a driver amplifier requiring an AGC circuit. An example thereof will be described next with reference to FIG. In order to drive the laser element 8 without using a driver amplifier, it is preferable to use a limiter amplifier.

FIG. 31 shows an example of a drive circuit of the LD 8 when a limiter amplifier is used. That is, the reception signal 87 transmitted through the reception-side root roll-off filter 38a is added with the DC bias signal from the current source 7a by the bias tee 6a, and then amplified through the limiter amplifier 47 before being applied to the laser element 8. Configuration.

The limiter amplifier 47 does not require a power detector, a comparator, and the like, and can simplify the drive circuit configuration of the laser device (LD) 8. Therefore, it is possible to promote further miniaturization of the slave station 1 as a wireless base station.

Normally, when transmitting an analog signal, the upper and lower amplitudes are clipped and the receiving sensitivity is deteriorated, so that the limiter amplifier is not used. However, the reception sensitivity of the roll-off shaped QPSK signal does not deteriorate even if the amplitude is clipped. Therefore, in such a case, a configuration in which the signal is amplified by the limiter amplifier 47 and then applied to the laser element 8 can be used.

In the above, the embodiment has been described in the case where the radio signal is a QPSK signal.
A signal or an angle modulation method such as an offset QPSK signal may be used. In addition, the transmission-side filter 32 and the reception-side filter 38 are both root roll-off filters.

On the other hand, in the slave station 1 that accommodates a radio signal that transmits a radio signal that has undergone full roll-off shaping on the transmitting side, a filter that does not perform roll-off shaping may be used as the filter 38.

As described above, in the twelfth embodiment, in the slave station,
By causing a received radio signal including an interference wave to pass through a narrow band-pass filter of a root roll-off filter, an interference wave other than a desired wave is largely suppressed.
Therefore, when optically transmitting a received signal from the slave station to the master station, when directly modulating the semiconductor laser element for optical conversion in the slave station, the optical modulation degree of the desired wave is suppressed by the interference wave. It is something that can be avoided. Therefore, according to the twelfth embodiment, it is possible to transmit an optical signal to a master station with a stable signal strength even if a received signal contains an interference wave, even if it is not present.

In this embodiment, each of the slave stations 1a, 1b,
By setting the optical modulation degree of the laser element 8 at 1n to 100% or more, the output optical signals 57a, 57b,.
7n can be suppressed. Thus, when a plurality of slave stations 1a, 1b,... 1n are optically SCM multiplexed and collectively received by the PD 9 of the master station 3, the amount of beat noise generated can be reduced. That is, it is possible to avoid the influence of the interference wave received by the slave station 1 and the beat noise caused by multiplexing the plurality of slave stations 1, and to obtain the received signal 88 having a stable CNR.

As described above, in the ninth to twelfth embodiments, an example has been described in which a signal which is QPSK-modulated with a digital signal is used as an information signal. Amplitud
APSK (Amplitude Phase-s) including e Modulation)
This embodiment can be applied to a configuration using modulation.

[0310] Conventionally, roll-off shaping of an information signal, which is a received signal, has been performed by a digital filter in the demodulator of the master station. However, when an information signal to be handled becomes faster, a digital circuit having a higher sampling frequency and a faster signal processing function is required. In particular, in a master station that handles information signals from a plurality of slave stations, since the number of receivers is large, such a digital circuit increases in scale and power consumption. However, according to the present invention, the master station
Since each slave station handles the received signal that has been rolled off in the intermediate wave band, the digital circuit does not need a waveform equalization function. Therefore, the power consumption of the master station and the system scale can be reduced.

[0311]

As described above, when a laser element is directly modulated by a frequency modulation signal modulated by an analog signal as transmission information, a more clipped optical signal is generated and transmitted together. According to the present invention, in an optical transmission device, it is possible to suppress occurrence of distortion such as IM3 and IM5 due to asymmetry of a signal waveform due to clipping during frequency demodulation.
In particular, for a base station of a mobile communication system using a plurality of mobile terminals, such as an analog signal of a multi-channel, and an ITV that handles a video signal, the input dynamic range improvement effect on the optical modulation degree by the FM premodulation is improved. It is possible to provide an optical transmission device that can obtain a maximum value and can guarantee higher communication quality.

The optical transmission apparatus according to the present invention also has the effect of suppressing beat noise due to clipping, so that two major problems of distortion and beat noise can be solved in analog optical transmission. Thus, multi-channel analog signals can be SCM multiplex-transmitted to the master station while securing a wide dynamic range. Moreover, in each slave station, an inexpensive laser element can be used without using an expensive laser element with excellent distortion characteristics, and there is no need to perform wavelength selection and wavelength stabilization control. Only one optical receiving system is required. Therefore, it is possible to reduce the size of the apparatus and realize simplification of the configuration, downsizing, and cost reduction.

Also, the present invention is characterized in that a laser is directly modulated by a roll-off shaped information signal in a slave station. If the slave station is a wireless base station and the information signal is a received wireless signal, the intensity of an interference wave other than the desired wave may be large. At that time, by passing the radio signal through the root roll-off filter, it is possible to suppress the interference wave to the maximum without distorting the waveform of the desired wave.

Therefore, when directly modulating the semiconductor laser element, it is possible to prevent the optical modulation degree of the desired wave from being suppressed by the interference wave. In other words, regardless of the presence or absence of interference waves,
The stable signal strength of the desired wave can be transmitted to the master station. In addition, even if the information signal subjected to the roll-off shaping is subjected to the amplitude limitation, the reception sensitivity does not deteriorate.

Therefore, in order to stabilize the degree of optical modulation of an optical signal, a limiter amplifier can be applied to an amplifier for driving a laser. The limiter amplifier has no additional configuration compared to the AGC-controlled amplifier, so that the configuration of the slave station can be simplified. Further, by setting the optical modulation degree of the optical signal to “1” or more, the signal strength of the received signal at the master station can be increased.

When the optical modulation degree is set to “1” or more, the coherence of the optical signal can be reduced, and the influence of beat noise generated when the optical signal from another station is multiplexed with the base station can be suppressed.
That is, the signal strength can be increased and the noise level can be suppressed without deteriorating the receiving sensitivity, so that the dynamic range in the optical transmission system can be expanded.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining the present invention, and is a configuration diagram showing a first embodiment of the present invention.

FIG. 2 is a diagram for explaining the present invention, and is a diagram showing an optical spectrum of an optical signal output from a slave station in the first embodiment of the present invention.

FIG. 3 is a diagram for explaining the present invention, and is a diagram showing a demodulation process in a master station according to the first embodiment of the present invention.

FIG. 4 is a diagram showing a demodulation process in a master station in a conventional device.

FIG. 5 is a diagram for explaining the present invention, and is a configuration diagram showing a second embodiment of the present invention.

FIG. 6 is a diagram illustrating an amplitude limitation in a master station according to the second embodiment of the present invention.

FIG. 7 is a diagram illustrating a frequency demodulation circuit based on differential detection.

FIG. 8 is a diagram showing an influence of a clipping on a waveform in differential detection.

FIG. 9 is a diagram for explaining the present invention, and is a configuration diagram showing a third embodiment of the present invention.

FIG. 10 is a diagram for explaining the present invention, showing a light modulation process in a slave station according to the third embodiment of the present invention.

FIG. 11 is a diagram for explaining the present invention, and is a configuration diagram showing a fourth embodiment of the present invention.

FIG. 12 is a diagram for explaining the present invention and is a diagram showing an optical modulation process in a slave station according to the fourth embodiment of the present invention.

FIG. 13 is a diagram illustrating a circuit added to a laser according to a fourth embodiment of the present invention.

FIG. 14 is a diagram for explaining the present invention, and is a configuration diagram showing a fifth embodiment of the present invention.

FIG. 15 is a diagram for explaining the present invention, and is a block diagram of a master station showing a sixth embodiment of the present invention.

FIG. 16 is a diagram for explaining the present invention, and is a configuration diagram of a slave station showing a seventh embodiment of the present invention.

FIG. 17 is a diagram for explaining the present invention, and is a diagram showing a relationship between an OMI and an optical spectrum according to an eighth embodiment of the present invention.

FIG. 18 is a configuration diagram of a conventional optical transmission device to which clipping by an FM modulation signal is applied.

FIG. 19 is a diagram showing an optical signal waveform when clipping occurs.

FIG. 20 is a diagram showing a frequency spectrum of a conventional multi-channel analog signal after FM demodulation.

FIG. 21 is a diagram showing a definition of a light modulation degree OMI.

FIG. 22 is a diagram for explaining the present invention, and is a configuration diagram showing a ninth embodiment of the present invention;

FIG. 23 is a diagram for explaining the present invention, and is a diagram showing a relationship between a desired wave and an interference wave in the ninth embodiment;

FIG. 24 is a diagram for explaining the present invention and is a diagram illustrating transmission characteristics of a waveform equalization filter.

FIG. 25 is a diagram for explaining the present invention, and is a diagram illustrating a relationship between a 3 [dB] transmission bandwidth of a band transmission filter and a symbol rate.

FIG. 26 is a diagram for explaining the present invention, and is a configuration diagram showing a tenth embodiment of the present invention.

FIG. 27 is a diagram for explaining the present invention, and is a configuration diagram showing an eleventh embodiment of the present invention.

FIG. 28 is a diagram for explaining the present invention, and is a configuration diagram showing a twelfth embodiment of the present invention.

FIG. 29 is a diagram for explaining the present invention, and shows a twelfth embodiment.
FIG. 9 is a diagram illustrating reception sensitivity in the example of FIG.

FIG. 30 is a diagram for explaining the present invention, wherein AGC
FIG. 3 is a diagram illustrating a laser drive circuit using a controlled amplifier.

FIG. 31 is a diagram for explaining the present invention, and is a diagram showing a laser drive circuit using a limiter amplifier.

[Explanation of symbols]

1a, 1b, ~ 1n ... slave station 2 ... optical fiber 3 ... master station 4 ... optical coupler 5a, 5b, ~ 5n ... frequency modulation circuit 6, 6a, 6b, ~ 6n ... bias tee 7a, 7b, ~ 7n ... current Sources 8, 8a, 8b, to 8n Laser element (LD) 9 Photodetector (PD) 10, 13, 11a, 11b, to 11n Bandpass filter 11, 11a, 11b, to 11n Voltage source 12, 12a , 12b, ~ 12n ... limiter 14 ... frequency demodulation circuit 15a, 15b, ~ 15n ... antennas 16a, 16b, ~ 16n ... low noise amplifier 17 ... π / 4 shift QPSK demodulation circuit 18 ... distortion detector 19 ... diode 20 ... bias addition Coil 21 ... resistor 22 ... DC cut capacitor 24 ... comparator 25 ... branch circuit 26 ... delay circuit 27 ... ExOR circuit 28 ... low-pass filter 30 ... signal source 31 Quadrature modulator 32: transmission-side root roll-off filter 33, 33a: local oscillator 34, 34a: multiplier 35: band-pass filter 36: power amplifier 37: antenna 38: reception-side root roll-off filter 39: preamplifier 40: quadrature demodulation Device 41 Driver amplifier 42 Coupler 43 Power detector 44 Reference voltage 45 Comparator 46 Loop filter 47 Limiter amplifier 48 Adder 49 Demodulator 50A, 50B,-50N Waveform equalizing filter 81 Wireless Signal 82 ... I-channel signal 83 ... Q-channel signal 84 ... QPSK signal 85 ... Root roll-off shaped QPSK signal 86 ... Received signal 87 ... Roll-off shaped QPSK signal 88 ... Received signal 89 ... A radio signal that is an interference wave 90 ... optical signal

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04B 10/04 10/06 10/24 15/00

Claims (16)

[Claims] A plurality of slave stations each having a semiconductor laser element as a light source, and these slave stations use a frequency modulation signal modulated by an information signal to control the semiconductor laser element with an optical modulation degree larger than one. By directly modulating the optical signals, the optical signals are converted into optical signals whose waveforms are vertically asymmetric, and these optical signals are transmitted to a transmission line, which is an optical fiber. SCM (Sub
-Carrier Multiplex), the optical signal multiplexed and SCM-multiplexed is transmitted to a master station, and the optical signal is received by a photoelectric conversion unit. The information signal is an analog signal. Means for extracting a frequency modulation signal of a desired slave station from a reception signal obtained by converting the signal into an electric signal by the conversion means; means for adding a DC bias voltage to the extraction signal of the extraction means; and addition of the DC bias voltage It is characterized by comprising amplitude limiting means for performing amplitude limitation on the signal to make the amplitude of the waveform vertically symmetric, and demodulating means for frequency-demodulating the signal after the amplitude limitation processing to obtain an information signal. Optical analog transmission equipment. And a plurality of slave stations each including a semiconductor laser element as a light source, and these slave stations use a frequency modulation signal modulated by an information signal to control the semiconductor laser element by an optical modulation degree greater than one. By directly modulating the optical signals, the optical signals are converted into optical signals whose waveforms are vertically asymmetric, and these optical signals are transmitted to a transmission line, which is an optical fiber. SCM (Sub
-Carrier Multiplex), the optical signal multiplexed and SCM-multiplexed is transmitted to a master station, and in an optical transmission device configured to be received by photoelectric conversion means, the information signal given is an analog signal, and the slave station An optical analog transmission apparatus, further comprising means for imposing an amplitude limit on the frequency-modulated signal to make the average level of the amplitude of the transmitted optical signal coincide with the average level of the frequency-modulated signal. 3. An optical analog transmission apparatus according to claim 1, wherein said information signal is a multi-channel analog signal. 4. A plurality of slave stations each including a semiconductor laser element as a light source, and these slave stations use a frequency modulation signal modulated by an information signal to control the semiconductor laser element by an optical modulation degree larger than 1. By directly modulating the optical signals, the optical signals are converted into optical signals whose waveforms are vertically asymmetric, and these optical signals are transmitted to a transmission line, which is an optical fiber. SCM (Sub
-Carrier Multiplex) The optical signal multiplexed and SCM-multiplexed is transmitted to a master station, and is received by a photoelectric conversion means. In the optical transmission device, a signal in which the information signal is π / 4 shifted QPSK modulated An optical analog transmission device, characterized in that: 5. The optical analog transmission device according to claim 1, wherein the master station detects a distortion amount of the information signal after the frequency demodulation, and sets a DC bias voltage value to be added so that the distortion is suppressed. An optical analog transmission device characterized by controlling. 6. The optical analog transmission apparatus according to claim 2, wherein the slave station receives a part of the optical signal transmitted to the master station, demodulates the received signal, and demodulates the received signal. An optical analog transmission apparatus, comprising: a detection unit configured to detect a distortion amount of a distortion generated in a demodulated signal; and a unit configured to control the amplitude limiting amount so that the distortion amount is reduced. 7. The optical analog transmission device according to claim 1, wherein the degree of light modulation in the semiconductor laser device is 2.0 or more. 8. The optical analog transmission device according to claim 1, wherein the semiconductor laser device is a Fabry-Perot type semiconductor laser device. 9. An optical analog transmission apparatus wherein an optical signal obtained by directly modulating a semiconductor laser element with an information signal received by a slave station is transmitted to a master station via an optical transmission line. Comprises a receiving unit for receiving an information signal transmitted as a wireless signal, and a band-pass filter, the information signal received by the receiving unit passes through the band-pass filter without performing signal processing of frequency conversion. An optical analog transmission device having a configuration for directly modulating the semiconductor laser element. 10. An optical analog transmission apparatus wherein an optical signal obtained by directly modulating a semiconductor laser element with an information signal received by a slave station is transmitted to a master station via an optical transmission line. Comprises a receiving means for receiving an information signal transmitted as a radio signal angle-modulated by a digital signal, and a filter for channel selection and waveform equalization for transmitting the information signal received by the receiving means, An optical analog transmission device, wherein the semiconductor laser element is directly modulated by an information signal passed through a filter. 11. An optical analog transmission apparatus for transmitting an optical signal obtained by directly modulating a semiconductor laser device using an information signal received by a slave station to a master station via an optical transmission line. A receiving unit for receiving an information signal transmitted as a radio signal angle-modulated by a digital signal; and a waveform equalization of the information signal received by the receiving unit to obtain a modulation signal of the semiconductor laser device. An optical analog transmission device comprising: a waveform equalization unit. 12. The optical analog transmission apparatus according to claim 9, wherein said slave station includes a root roll-off filter for performing roll-off shaping of an information signal. . 13. The optical analog transmission apparatus according to claim 10, wherein the plurality of slave stations are provided, and optical signals transmitted from each of the slave stations are arranged in different frequency bands. The information signal is used to directly modulate a semiconductor laser device with an optical modulation factor greater than "1", and the optical signal is transmitted over the optical transmission line with an optical signal from another slave station and an SCM (Sub-Carrier).
Multiplex) An optical analog transmission apparatus characterized in that it is multiplexed and received collectively at a master station. 14. The optical analog transmission apparatus according to claim 9, wherein said information signal is modulated in digital amplitude and angle by a digital signal. 15. The optical analog transmission apparatus according to claim 9, wherein the band-pass filter for waveform equalization has its 3 [dB].
The transmission bandwidth Δf is the transmission rate B of the symbol of the information signal.
An optical analog transmission device characterized by having a relationship of Bsym ≦ Δf ≦ 2Bsym with respect to sym [symbol / second]. 16. A mobile station having a plurality of slave stations, wherein each slave station transmits an optical signal obtained by directly modulating a semiconductor laser element using a received information signal to a master station via an optical transmission line. In the optical analog transmission device, the slave station receives an information signal transmitted as a radio signal angle-modulated with a digital signal, and the semiconductor laser using the information signal received by the reception unit. A configuration in which the elements are directly modulated with an optical modulation factor greater than "1", and an optical signal transmitted from each slave station is multiplexed by SCM (sub-carrier multiplex) and transmitted to the master station. An optical analog transmission device, characterized in that: