JP2007166678A - Optical analog transmission equipment - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide optical analog equipment, capable of suppressing optical beat noise, by securing a broad dynamic range in transmitting SCM-multiplexed analog signals from multiple slave stations to their master station. <P>SOLUTION: In slave stations 1a, 1b, ... and 1n, optical signals produced by directly modulating Fabry-Perot semiconductor laser elements 8a, 8b, ... and 8n with a frequency-modulated signal, modulated by an analog information signal at the degree of optical modulation, whose value is greater than or equal to 1 are transmitted via an optical fiber 2 and SCM-multiplexed on a transmission path with optical signals from other substations. Regarding the master station, SCM-multiplexed optical signals are received in batch by a single PD9, signals from the desired substation are extracted, a dc bias is applied via a bias tee 6, and then the frequency is demodulated by a demodulation circuit 14 and an information signal is obtained. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical transmission apparatus that performs SCM multiplex transmission of an analog signal such as a radio signal from a plurality of slave stations to a master station via an optical fiber.

  In recent years, a base station for mobile communications or a slave station equivalent to an ITV (Industrial Televisions) monitor has been placed in various locations, and light is transmitted from the slave station to the master station via an optical fiber. The network is known. In particular, an optical SCM (Sub-Carrier Multiplex) network in which a signal transmitted from each slave station is an optical signal modulated by a different subcarrier signal and multiplexed and received together is a large number of electrical signals. Can be transmitted at the same time.

  However, since optical signals are multiplexed, generation of beat noise is unavoidable, which is a big problem. That is, it is a problem of beat noise generated by interference of optical signals from a plurality of slave stations.

  Here, the beat noise refers to an optical signal B from another slave station at a distance of Δλ with respect to the optical signal A from a certain slave station, and these optical signals are collectively received by one receiver. The noise component generated in the frequency band of Δλ of the received signal.

  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 reception sensitivity deteriorates. In the worst case, reception is not possible at all.

  Non-Patent Document 1 below introduces a method for reducing the coherence of an optical signal from each slave station with respect to the problem of beat noise.

  The method disclosed in this document is shown in FIG. In FIG. 18, an information signal which is a digital signal of “1” and “0” is input to a VCO (voltage controlled oscillator), and the laser element is directly modulated by the frequency modulation signal output from this VCO. In other words, clipping is caused by the laser element, the optical spectrum is diffused, and the coherency of the optical signal is reduced.

  However, since this method causes clipping by the laser element, the transmitted optical signals 51, 52, and 53 have asymmetric waveform amplitudes as shown in FIG. Therefore, when the received frequency modulation signal is demodulated, the information signal is distorted.

In particular, when the information signal is a multi-channel analog signal, as shown in FIG. 20, distortion such as IM3 (3rd Interference Modulation; third harmonic), IM5 (fifth IM; fifth harmonic) is adjacent. Since it overlaps the channel, the CNR (Carrier-to-Noise Ratio) is reduced and the dynamic range is significantly suppressed.
“Operation of a subcarrier multiple-access passive optical network with multimode lasers in the presence of optical beat interference”, TuQ5, pp..90-91, in OFC'95

  The optical SCM network has an advantage that a large number of electrical signals can be transmitted simultaneously. However, since the optical signals are multiplexed, the generation of beat noise is unavoidable, which is a big problem.

  As a method of reducing the coherence of the optical signal from each slave station with respect to the problem of beat noise, a digital information signal is input to the VCO and the oscillation frequency is controlled in correspondence with the information signal. As a result, from this VCO The laser element is directly modulated with the output frequency modulation signal, and at that time, the laser element is caused to cause clipping, thereby spreading the optical spectrum and reducing the coherency of the optical signal, and the influence of interference. There has been proposed a technique for preventing the occurrence of the problem.

  However, since this system causes clipping by the laser element, the waveform of the transmitted optical signal is asymmetrical due to the effect of clipping. Therefore, when the received frequency modulation signal is demodulated, the information signal is distorted.

  In particular, when the information signal is a multi-channel analog signal, distortion such as IM3 and IM5 overlaps with an adjacent channel, so that the CNR is deteriorated and the dynamic range is significantly suppressed.

  For this reason, restrictions on the power of information signals that can be transmitted, the optical transmission distance, and the like become severe, and this is a major problem that degrades communication quality.

  Accordingly, an object of the present invention is to provide an information signal having an effect of reducing optical beat noise due to clipping in an optical transmission apparatus that performs SCM multiplexing of an information signal from a plurality of slave stations to a master station via an optical fiber. However, an object of the present invention is to provide an analog optical transmission apparatus that can ensure a wide dynamic range even for multi-channel analog signals.

  In the radio base station, interference waves may be mixed in a frequency band close to the received desired wave. In some cases, the power of the interference wave is greater 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, this interference wave has an optical modulation degree of a desired wave. Cause pressure.

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

  Accordingly, another object of the present invention is to provide a technique for suppressing interference waves.

  Further, 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 at the master station. However, when the desired wave is a radio signal that is angle-modulated, the reception sensitivity is degraded due to the amplitude limitation. For this reason, the required CNR value becomes large, and the specifications for the slave stations become strict.

  Therefore, still another object of the present invention is to provide a technique capable of preventing 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. Is to provide.

  In order to achieve the above object, according to one aspect of the present invention, an optical signal obtained by directly modulating a semiconductor laser element with an information signal received at a slave station is transmitted to the master station via an optical transmission line. In the optical analog transmission apparatus, the slave station includes receiving means for receiving an information signal transmitted as a radio signal, and a band-pass filter, and the information signal received by the receiving means is subjected to frequency conversion signal processing. An optical analog transmission device characterized in that the semiconductor laser element is directly modulated by passing through the band-pass filter without being applied.

  According to the present invention, in an optical transmission apparatus that generates a more clipped optical signal for use in transmission when directly modulating a laser element with a frequency modulation signal modulated with an analog signal that is transmission information, According to the invention, it is possible to suppress the occurrence of distortion such as IM3 and IM5 due to the asymmetry of the signal waveform due to clipping during frequency demodulation. Especially for mobile communication system base stations using multiple mobile terminals that handle multi-channel analog signals and ITV that handles video signals, the input dynamic range improvement effect on the optical modulation degree by FM pre-modulation It is possible to provide an optical transmission device that can be obtained to the maximum and can guarantee higher communication quality.

  In addition, since the optical transmission apparatus according to the present invention also has a beat noise suppression effect due to clipping, two major problems of distortion and beat noise can be solved in analog optical transmission. In addition, a multi-channel analog signal can be transmitted by SCM multiplexing while ensuring a wide dynamic range. Moreover, each slave station can use an inexpensive laser element without using an expensive laser element with excellent distortion characteristics, and it is not necessary to perform wavelength selection or wavelength stability control. Only one optical receiving system is required. Therefore, it is possible to reduce the scale of the apparatus and to simplify the configuration, reduce the size, and reduce the cost.

  The present invention is also characterized in that the slave station directly modulates the laser with the roll-off shaped information signal. If the slave station is a radio base station and an information signal is a received radio signal, the intensity of interference waves other than the desired wave may be high. At this 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 the semiconductor laser element is directly modulated, it is possible to avoid the optical modulation degree of the desired wave from being pressed by the interference wave. That is, it is possible to transmit a stable signal strength of a desired wave to the master station side regardless of the presence or absence of an interference wave. In addition, even if the amplitude-limited information signal is subjected to roll-off shaping, reception sensitivity does not deteriorate.

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

  Setting the optical modulation degree to “1” or more can reduce the coherence of the optical signal, and can suppress the influence of beat noise that occurs when the optical signal from another slave station is subjected to the inverse multiplexing. That is, the signal strength can be increased without deteriorating the reception sensitivity, and the noise level can be suppressed, so that the dynamic range in the optical transmission system can be expanded.

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

  <Technique relating to optical beat noise reduction and dynamic range ensuring> (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 device is made to have an optical modulation degree greater than 1 by a frequency modulation signal modulated by an information signal. An optical signal obtained by direct modulation is transmitted to a master station via an optical fiber as a transmission path, and the optical signal is transmitted with an optical signal transmitted from another slave station on the transmission path and SCM (Sub-Carrier Multiplex ) Multiplexing, the master station is an optical transmission device that collectively receives the SCM multiplexed optical signal with one photoelectric conversion element (PD; Photo-Detector), the information signal is an analog signal, The master station extracts the frequency modulation signal of the desired slave station from the received signal of the PD, adds a DC bias voltage to the extracted signal, limits the amplitude to make it vertically symmetrical, and then demodulates the frequency. And obtaining the information signal Yes.

  Normally, in frequency demodulation, amplitude fluctuations of a received signal are suppressed by a limiter or AGC (Auto Gain Control), and then demodulated into an information signal by a discriminator, delay detection, synchronous detection using a PLL, or the like.

  However, in the present invention, the master station applies an amplitude limitation to return the average level of the frequency modulation signal to an appropriate position by adding a DC bias voltage to the reception signal that has become asymmetrical due to clipping. Demodulate.

  This makes it possible to suppress distortion due to deviations in the average level of the received signal during frequency demodulation, ensuring an input dynamic range for the optical modulation degree by FM pre-modulation of the information signal, and guaranteeing conversion with high communication quality In addition, even if a Fabry-Perot type semiconductor laser element is used, it is not affected by distortion and optical beat noise, so that an effect that a low-cost optical SCM multiplex network can be provided can be expected.

  Details will be described.

  FIG. 1 is a block diagram illustrating the configuration of the optical transmission apparatus according to the present embodiment. In the figure, 1a, 1b, and 1n are slave stations, 2 is an optical transmission line, 3 is a master station, 4 is an optical coupler, 5a, 5b, and 5n are FM (Frequency Modulation) modulation circuits, 6a, 6b, and so on. 6n is a bias tee, 7a, 7b and -7n are current sources, 8a, 8b and -8n are laser elements, 9 is a PD (Photo Detector), 10 is a bandpass filter, 11 is a voltage source, 12 is a limiter, 13 is a limiter A band pass fill, 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 element 8b. The slave station 1n includes an FM modulation circuit 5n, a bias tee 6n, a current source 7n, and a laser element 8n.

  The master station 3 includes a PD 9, band pass filters 10 and 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 FM-modulate the input analog signals and output them, and the current sources 7a, 7b, .about.7n supply predetermined DC currents. is there. The bias tees 6a, 6b, .about.6n superimpose and output the FM modulation signals output from the FM modulation circuits 5a, 5b, .about.5n with the DC bias signals from the current sources 7a, 7b, .about.7n. The laser elements 8a, 8b, to 8n are elements that are directly modulated by the output signals of the bias tees 6a, 6b, to 6n to oscillate laser light, and output this as an optical signal to the optical fiber 2 that is an optical transmission path. It is.

  The FM modulation signals from the FM modulation circuits 5a, 5b, .about.5n are superimposed on the DC bias signals from the current sources 7a, 7b, .about.7n by the bias tees 6a, 6b, .about.6n, and the corresponding laser elements 8a, 8b,. If the laser elements 8a, 8b, and 8n are directly modulated by applying to 8n, the output generates clipping if the optical modulation index OMI (Optical Modulation Index) is set to exceed 100%. Therefore, here, the light modulation degree OMI is set to 100 [%] or more, that is, the light modulation degree OMI is set to “1” or more so that the output is actively clipped.

  The optical coupler 4 multiplexes and outputs optical signals from the laser elements 8a, 8b, .about.8n, and the PD 9 on the master station 3 side photoelectrically converts the multiplexed optical signal to produce an SCM multiplexed signal. The band 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, and the bias tee 6 adds the DC bias voltage from the voltage source 11 to the output signal of the bandpass filter 10 to obtain the average level of the output signal. Is adjusted to the average level of the output signal when there is no clipping.

  That is, since the optical signal transmitted from the slave station is output in a state where clipping occurs on the laser element side in the slave station, it differs from the average level of the signal when there is no clipping. By adding a DC bias voltage from the voltage source 11 with the bias tee 6 to the reception signal transmitted through the band-pass filter 10 for band extraction, the reception signal is adjusted to the average level when there is no clipping. It is to be corrected.

  The limiter 12 is an amplitude limiting circuit having a vertically symmetric limiter characteristic, and an output signal having a vertically symmetric amplitude level is obtained by giving a limit to the signal output from the bias tee 6 by a vertically symmetric limiter characteristic. Is. The bandpass filter 13 is a circuit that removes harmonic components generated by the amplitude limiter from the output signal from the limiter 12, and extracts received signals from desired base stations 1a, 1b, to 1n.

  The FM demodulating circuit 14 is a circuit that obtains an original information signal by FM demodulating the signal extracted through the bandpass filter 13, and includes a discriminator, delay detection, synchronous detection using a PLL (Phase Locked Loop), and the like. This is a circuit for demodulating the information signal.

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

  In each of the slave stations 1a, 1b, to 1n in the present system, analog signals, that is, analog signals 61a, 61b, to 61n such as video signals and radio signals that are not binary digital signals are converted into FM (Frequency Modulation). By inputting to the modulation circuits 5a, 5b, .about.5n, FM modulation is performed to obtain FM modulation signals 62a, 62b, .about.62n.

  The FM modulation signals 62a, 62b, .about.62n are superimposed with a DC bias current that determines the degree of optical modulation, that is, superimposed with the DC bias currents from the current sources 7a, 7b, .about.7n at the bias tees 6a, 6b, .about.6n. The laser elements 8a, 8b, .about.8n are directly modulated by applying to the laser elements 8a, 8b, .about.8n.

  That is, the magnitude of the DC bias current value determines the degree of optical modulation when directly modulating the laser elements 8a, 8b, to 8n, and the ratio of clipping of the waveform varies depending on the level of the optical modulation degree.

  Here, in the present invention, the optical signals output from the laser elements 8a, 8b, to 8n are set so that the optical modulation index OMI (Optical Modulation Index) exceeds 100 [%]. Clipping is generated in the waveform of the signal to be transmitted by direct modulation under the condition of [%] or more. As the laser elements 8a, 8b, to 8n, Fabry-Perot type semiconductor laser elements are used.

  As a result, as shown in FIG. 2, the optical spectrum of the optical signals 52a, 52b, .about.52n, which are the outputs of the laser elements 8a, 8b, .about.8n, is widened to reduce the coherency and interfere with other optical signals. Is reduced.

  The optical signals 52a, 52b, to 52n output from the laser elements 8a, 8b, to 8n in the slave stations 1a, 1b, to 1n are transmitted through the optical fiber 2 that is an optical transmission path. After being multiplexed, it is transmitted to the master station 3.

  In this way, the optical signal output from the slave station is multiplexed with the optical signal from the other slave station by the optical coupler 4 and then transmitted to the master station 3.

  The FM modulation signals 62a, 62b, and 62n of the respective slave stations 1a, 1b, and 1n are arranged in different frequency bands, and the master station 3 receives them at one PD9 at a time and receives the SCM multiplexed signal. Obtained electrical signal. The received electrical signal is transmitted through the band-pass filter 10 to extract the received signal 63 from the desired base stations (slave stations) 1a, 1b, to 1n.

  As shown in FIG. 3A, the average level of the waveform of the received signal 63 is different from the average level of the signal when there is no clipping because clipping occurs on the slave stations 1a, 1b, to 1n side. The difference level depends on OMI.

  Therefore, a DC bias voltage from the voltage source 11 is added to the signal after passing through the band-pass filter 10 by the bias tee 6. As a result, the received signal 63 is set to the received signal 64 in the form of matching the average level with the average level when there is no clipping as shown in FIG.

  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 to be supplied to the slave stations 1a, 1b,... 1n are sine wave 2-tone signals, and a DC bias signal is applied to the information signal to give a modulation of an optical modulation degree of 100% or more. FIG. 3D shows the numerical calculation result of the frequency spectrum of the frequency demodulated signal (information signal 66) when the laser element 8 is caused to cause clipping.

  As a technique for suppressing beat noise associated with optical multiplexing, on the slave station side, an information signal is multiplexed and sent as an optical signal in a form in which the amplitude of the waveform becomes asymmetrical due to clipping. This vertical asymmetry calls for the occurrence of distortion such as IM3 on the demodulation side, which becomes a problem. However, in the system of the present invention, as can be seen from FIG. 3 (d), in the master station on the receiving side, the received signal is the bias tee 6 and the appropriate DC bias voltage is set so that the waveform amplitude of the limiter output signal 65 is vertically symmetrical. Was added. This suppresses the generation of harmonic distortion such as IM3 in the demodulated information signal and eliminates the problem of vertical asymmetry due to clipping.

  On the other hand, when the signal processing for changing the average level is not performed as in the conventional apparatus, that is, when the received signal 63 is input to the limiter 12 as it is, the limiter output signal 67 has a waveform amplitude as shown in FIG. Becomes an asymmetrically limited frequency modulation signal. When this output signal 67 is input to the FM demodulation circuit 14 and demodulated, the influence of the waveform asymmetry due to clipping appears as distortion.

  FIG. 4 (c) shows the numerical calculation result of the frequency spectrum of the FM demodulated signal in the conventional apparatus when clipping occurs under the same conditions as in FIG. 3 (d). As can be seen from FIG. 4C, in the case of the prior art, the third harmonic IM3 is greatly generated, and the dynamic range is remarkably suppressed.

  As described above, according to the present invention, the master station adds a DC bias voltage to the received signal whose waveform amplitude is asymmetric in the vertical direction due to clipping at the slave station, so that the average level of the frequency modulation signal is set appropriately. After the adjustment to return to the position, the amplitude is limited so that the amplitude of the waveform is vertically symmetrical, and then the frequency demodulation is performed. Therefore, distortion due to the deviation of the average level of the received signal at the time of frequency demodulation can be suppressed, and information can be suppressed. It can be obtained without impairing the input dynamic range improvement effect on the optical modulation degree by the FM pre-modulation of the signal, and the conversion with high communication quality can be guaranteed. Even if a Fabry-Perot type semiconductor laser element is used, it is not affected by distortion and optical beat noise, so that a low-cost optical SCM multiplex network can be provided.

  By the way, for example, an operational amplifier is used as the limiter 12 having the configuration shown in FIG. 1, but there is an amplitude limitation using a diode. Therefore, an example in which a diode is used as a limiter will be described next as a second embodiment.

(Second Embodiment) FIGS. 5A and 5B are block diagrams showing a configuration example of the master station 3 in the second embodiment in which a diode is used as a limiter. In (b) and (b), the diode connection is changed.

  In FIG. 5, 9 is a PD (Photo Detector), 10 is a band pass filter, 11 is a voltage source, 13 is a band pass filter, 14 is an FM demodulation circuit, 19 is a diode, 20 is a bias coil, 21 is a resistor, 22 is It is a capacitor.

  Here, the PD 9 on the master station 3 side photoelectrically converts the multiplexed optical signal and obtains it as an SCM multiplexed signal and outputs it as a received signal. The bandpass filter 10 Is for extracting a signal band of a desired slave station from the electrical signal from the PD 9.

  The voltage source 11 is for supplying a required DC bias voltage, and the bias coil 20 is for adding the DC bias voltage from the voltage source 11 to the output of the band pass filter 10.

  In the case of the configuration of FIG. 5A, the diode 19 has a cathode side connected to the output side of the bandpass filter 10 and the connection end side of the bias coil 20, and the anode side is connected to the bandpass filter 13 via the capacitor 22. Connected to the input side. The anode side of the diode 19 is grounded via a resistor 21.

  5B, there is no bias coil, the diode 19 has its cathode side connected to the voltage source 11 and its anode side connected to the output side of the bandpass filter 10. The output side of the bandpass filter 10 is grounded via a resistor 21.

  The output side of the bandpass filter 10 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 a resistor 21. The diode 19 is used to clip the received signal obtained by adding the DC bias voltage from the voltage source 11 to the output of the band pass filter 10, and the cathode side is connected to the bias coil 20 side.

  The capacitor 22 is used for cutting the DC component. By obtaining only the AC component via the capacitor 22 and inputting it to the bandpass filter 13, the harmonics generated by the amplitude limitation by the diode are removed. A signal having a desired frequency component is extracted. The signal extracted by the bandpass filter 13 is demodulated by the FM demodulation circuit 14 to obtain an information signal.

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

  Also in the second embodiment, similar to the first embodiment, a desired base station (slave station) 1a is obtained by a band pass filter 10 from SCM multiplexed signals obtained by the PD 9 in the master station 3 collectively. , 1b,..., 1n are extracted. The diode 19 limits the amplitude of one end of the received signal 71 to obtain a vertically symmetric received signal.

  In other words, the optical signal transmitted from the slave station side spreads the optical spectrum by clipping the FM modulation signal of transmission information at one end and directly modulating the laser element with this in order to suppress beat noise. This weakens the coherency of the optical signal. Accordingly, since the waveform of the signal is asymmetrical in the vertical direction, the diode 19 limits the amplitude of the other end of the reception signal 71 that is not clipped, thereby obtaining a reception signal that is symmetric in the vertical direction.

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

  Here, the state of amplitude limitation of the received signal 71 will be described with reference to FIG. A DC bias voltage from the voltage source 11 is added to the reception signal 71 via the bias coil 20. The diode characteristics and the DC bias voltage are optimally selected and transmitted through the diode 19 and the DC cut capacitor 22 to obtain a vertically symmetrical reception signal 72 as shown in FIGS. 6 (a) to 6 (c).

  The received signal 72 is further transmitted through the bandpass 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.

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

  In the first embodiment, if the FM demodulating circuit 14 is a differentiating circuit such as a discriminator or a frequency demodulating circuit using synchronous detection, the addition of the DC bias voltage as described above is effective.

  On the other hand, the frequency demodulation circuit (FM demodulation circuit 14) includes a circuit based on delay detection. FIG. 7 shows an example of the master station 3 using the FM demodulation circuit based on delay detection.

  In the configuration of the master station 3 shown in FIG. 7, reference numeral 29 denotes an FM demodulator circuit that performs frequency demodulation by delay detection. This FM demodulating circuit 29 includes a branch circuit 25 that distributes an input signal, a delay circuit 26 that delays the input signal by a delay time τ, and an exclusive OR of one branch output of the branch circuit 25 and the output of the delay circuit 26. The ExOR circuit 27 is configured to extract a signal component of a required frequency band from the output of the ExOR circuit 27.

  The master station 3 is also provided with a comparator 24 in front of the FM demodulating circuit 29. The comparator 24 compares the magnitude of the frequency modulation signal 63, which is a received signal, with the threshold voltage signal (Vth) 72 from the voltage source 11. Thus, a digital format signal of “1” and “0” is provided and provided to the FM demodulation circuit 29.

  In such a configuration, the multiplexed optical signal from the slave station side is photoelectrically converted into an electrical signal by the PD 9 in the master station 3, and after passing through the band pass filter 10, a desired signal band is extracted. The electric signal (frequency modulation signal 63) is input to the comparator 24.

  The comparator 24 compares the magnitude of the frequency modulation signal 63 with the threshold voltage signal (Vth) 72 from the voltage source 11 and extracts it as a signal waveform 73 in a digital format of “1” and “0”. The extracted signal 73 is branched into two by the branch circuit 25, and one of them is given a time difference τ by the delay circuit 26 and input to the ExOR circuit 27.

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

  In the conventional frequency demodulator circuit (FM demodulator circuit) 29, as shown in the upper part of FIG. 8, Vth 72 is set to match the average level of the received frequency modulation signal 63.

  However, since clipping occurs on the slave station side in the present invention, as shown in the lower part of FIG. 8, the frequency modulation signal is assumed not to cause clipping but to set Vth 72 as usual. Adjust to an average level of 63.

  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. It can be seen that there is a deviation in the pulse width which is the phase information.

  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 illustrating the configuration of the optical transmission apparatus according to the present 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, 1a, 1b,... 1n are slave stations, 2 is an optical transmission line, 3 is a master station, 4 is an optical coupler, 5a, 5b, ... 5n are FM modulation circuits, and 6a, 6b,. Bias tee, 7a, 7b,... 7n are current sources, 8a, 8b, .about.8n are laser elements, 9 is a PD (Photo Detector), 10, 10a, 10b, .about.10n are bandpass filters, 11, 11a, 11b,. ˜11n is a voltage source, 12, 12a, 12b, ˜12n are limiters, 13 is a band pass fill, and 14 is an FM demodulation circuit.

  The slave station 1a includes an FM demodulator 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 FM demodulator circuits 5b and 2b. It comprises a bias tee 6b, a current source 7b, a voltage source 11b, a band pass filter 10b, and a laser element 8b. The slave station 1n has an FM demodulator circuit 5n, two bias tees 6n, a current source 7n, a voltage A source 11n, a bandpass filter 10n, and a laser element 8n are provided.

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

  In this embodiment, the slave stations 1a, 1b,... 1n have bias tees 6a, 6b, .about.6n, voltage sources 11a, 11b, .about.11n, bands, as compared with the configuration of the first embodiment shown in FIG. Pass filters 10a, 10b, and 10n are newly provided. In the master station 3, the voltage source 11 and the bias tee 6 are deleted.

  The voltage sources 11a, 11b, .about.11n are for supplying a required DC bias voltage, respectively, and the bias tees 6a, 6b, .about.6n on the front stage side are the FM modulation circuits 5a, 5b,. A DC bias voltage from the voltage sources 11a, 11b, .about.11n is added to the output so as to obtain a received signal in which the average level matches the average level when there is no clipping.

  The limiters 12a, 12b, .about.12n are amplitude limiting circuits having vertically symmetrical limiter characteristics, and the signals output from the corresponding bias tees 6a, 6b, .about.6n are vertically controlled by giving restrictions based on the vertically symmetrical limiter characteristics. An output signal having a symmetrical amplitude level is obtained.

  The bandpass filters 10a, 10b, and 10n are circuits that extract components of a desired frequency band from the output signals from the limiters 12a, 12b, and 12n, and the bias ties 6a, 6b, and 6n on the rear stage side. , Which outputs the FM modulation signals output from the bandpass filters 10a, 10b, and 10n superimposed on the DC bias signals from the current sources 7a, 7b, and 7n, and outputs the laser elements 8a, 8b, and 8n. Is an element that is directly modulated by the output signals of the bias tees 6a, 6b, .about.6n on the rear stage side, oscillates laser light, and outputs it as an optical signal to the optical fiber 2 that is an optical transmission line.

  The optical coupler 4 multiplexes and outputs the optical signals from the laser elements 8a, 8b, .about.8n transmitted through the optical fiber 2, and the PD 9 on the master station 3 side is multiplexed. The optical signal is photoelectrically converted to obtain an SCM multiplexed signal as an electrical signal, which is output, and the bandpass filter 10 extracts a desired frequency band component from the electrical signal from the PD 9, The received signals from the desired base stations 1a, 1b, to 1n are extracted.

  The limiter 12 is an amplitude limiting circuit having a vertically symmetric limiter characteristic, and an output signal having a wave height level that is symmetric in the vertical direction is obtained by giving the signal output from the bandpass filter 10 a restriction by the vertically symmetric limiter characteristic. To get. The bandpass filter 13 is a circuit that extracts 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 that has passed through the bandpass filter 13 to obtain the original information signal. It is a circuit to obtain.

  In the third embodiment having such a configuration, the asymmetry of the waveform shape of the optical signal due to clipping is corrected on the side of each of the slave stations 1a, 1b, to 1n.

  That is, in each of the slave stations 1a, 1b, ˜1n, analog information signals 61a, 61b, ˜61n are input to FM (Frequency Modulation) modulation circuits 5a, 5b, ˜5n, FM modulation is performed, and FM modulated signals 62a, 62b, to 62n are obtained ([State A] in FIG. 10).

  The FM modulation signals 62a, 62b, .about.62n are added with DC bias voltages from the voltage sources 11a, 11b, .about.11n at the preceding stage bias tees 6a, 6b, .about.6n, and limiters 12a, 12b, .about.12n, harmonics. The band-pass filters 10a, 10b, and 10n for removing waves are transmitted.

  As a result, modulated signals 69a, 69b, to 69n from which one side of the waveform is clipped and from which the harmonic components are removed are obtained ([State B] in FIG. 10). The modulation signals 69a, 69b, to 69n are asymmetrical in the vertical direction.

  The modulation signals 69a, 69b, .about.69n having asymmetric top and bottom are superimposed on the DC bias currents from the current sources 7a, 7b, .about.7n by the bias tees 6a, 6b,. 8b and 8n are directly modulated with OMI> 100 [%].

  Here, the upper and lower sides of the transmitted optical signals 53a, 53b, and 53n are symmetrical with each other by the limit due to clipping of the laser elements 8a, 8b, and 8n and the limit due to the limiters 12a, 12b, and 12n, that is, an average. The DC bias voltage and the OMI are adjusted so that the level does not fluctuate ([State C] in FIG. 10).

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

  Next, a fourth embodiment in which the configuration of the master station 3 is simplified by not changing the average level position of the transmitted optical signal, that is, by changing the vertical amplitude of the FM modulated signal waveform. This will be described as an example.

(Fourth Embodiment) In the fourth embodiment, a method for changing the position of the average level of the transmitted optical signal will be described. The configuration of the optical transmission apparatus of the fourth embodiment is shown in FIG. The main configuration is the same as in the first to third embodiments, and the same components are denoted by the same reference numerals.

  The configuration shown here is a configuration in which the voltage source 11 and the bias tee 6 are eliminated from the master station 3, and the amplitude of the frequency modulation signals 62a, 62b,... , .About.8n, the frequency modulation signal is oscillated in the range from the level below the threshold Ith to the saturation region, so that the upper and lower sides of the signal are clipped by the E / O characteristics of the laser elements 8a, 8b, .about.8n. By doing so, this is realized.

  Normally, the laser element has 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 less than or equal to the threshold value Ith, the laser element does not oscillate, so the light output is small. On the other hand, if the value of the modulation current is too large, the light output is saturated. To do.

  Therefore, the amplitude of the frequency modulation signals 62a, 62b, .about.62n applied to the laser elements 8a, 8b, .about.8n is increased, and the frequency ranges from the level below the threshold Ith of the laser elements 8a, 8b, .about.8n to the saturation region. By making the modulation signal oscillate, the top and bottom of the signal are clipped by the E / O characteristics of the laser elements 8a, 8b, .about.8n.

  FIG. 12 shows the adjustment of the bias current levels of the frequency modulation signals 62a, 62b, .about.62n applied to the laser elements 8a, 8b, .about.8n and setting the upper and lower clip amounts to be the same amount. Thus, the average level of the optical signal 54 can be made equal to the position of the average level of the frequency modulation signal 62.

  On the master station 3 side, there is no position fluctuation of the average level of the optical signal 54. Therefore, as the output signal of the limiter 12 does not perform signal processing to add a bias as described in the first embodiment, A symmetrical reception signal is obtained, and an information signal in which distortion is suppressed 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 modulation current 71 is applied to the laser element 8, if the voltage applied to the diode 23 does not exceed the rising voltage VF, all the modulation current 71 flows through the laser element 8, and the modulation current 71 is supplied to the laser element 8. Modulation is applied.

  However, when the rising voltage VF of the diode 23 is exceeded, the modulation current 71 corresponding to the excess voltage flows to the diode 23, so that the modulation current 71 is not modulated on the laser element 8.

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

  As described above, the first to fourth embodiments have described the configuration in which the analog information signal is premodulated into the frequency modulation signal. The reason is that the frequency-modulated signal is practical because it can remove fluctuations in the amplitude component with a limiter or the like when demodulating, has no deterioration in transmission characteristics with respect to amplitude clipping, and has tolerance. However, in addition to this, the same effect can be obtained even with π / 4 shift QPSK modulation used in wireless communication, so an example thereof will be described as a fifth embodiment.

(Fifth Embodiment) FIG. 14 is a block diagram showing a configuration of an optical transmission apparatus according to the fifth embodiment, in which a slave station can receive a radio signal of π / 4 shift QPSK modulation. This is an example of the case. 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, and 1n are slave stations, 2 is an optical transmission line, 3 is a master station, 4 is an optical coupler, 6a, 6b, and 6n are bias tees, and 7a, 7b, and 7n are current sources. , 8a, 8b, ˜8n are laser elements, 9 is a PD, 10 is a band pass filter, 11 is a voltage source, 12 is a limiter, 13 is a band pass fill, 15a, 15b, ˜15n are antennas, 16a, 16b, ˜. Reference numeral 16n denotes a low noise amplifier, and 17 denotes a π / 4 shift QPSK demodulating 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, a bias tee 6n, a current source 7n, and a laser element 8n.

  The master station 3 includes a PD 9, band pass filters 10 and 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 is subjected to π / 4 shift QPSK modulation, each of the slave stations 1a, 1b, ˜1n has its own station service received by its own antenna 15a, 15b, ˜15n. The π / 4 shift QPSK modulated radio signal from the mobile terminal in the area is amplified by the low noise amplifiers 16a, 16b, to 16n. Then, the amplified signals are supplied with direct current bias signals from the current sources 7a, 7b, ˜7n by the bias tees 6a, 6b, ˜6n, and given to the laser elements 8a, 8b, ˜8n corresponding to the local station. The laser elements 8a, 8b, to 8n are directly modulated.

  At this time, as in the first to fourth embodiments, the optical modulation index OMI (Optical Modulation Index) of the optical signals 55a, 55b,... 55n output from the laser elements 8a, 8b,. ] Is set so that clipping is exceeded.

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

  In the master station 3, the optical signals sent after being SCM multiplexed by the optical coupler 4 are collectively received by one PD 9, and the received signals from the desired slave stations 1a, 1b,... 10 to extract. The signal processing after the band-pass filter 10 is the same as that of the first embodiment, and the received signal 70 transmitted through the band-pass filter 13 is input to the demodulation circuit 17 for π / 4 shift QPSK, and the transmission information is input. obtain.

  Up to the first to fourth embodiments, the configuration for premodulating an information signal as an analog signal into a frequency modulation signal has been described. This is because fluctuation of the amplitude component can be removed by a limiter or the like when demodulating the frequency modulation signal, and there is no deterioration in transmission characteristics with respect to amplitude clipping, and there is strength.

  Similarly, the π / 4 shift QPSK has a constant envelope, and is resistant to amplitude clipping like the frequency modulation signal, and is frequently used as a radio signal for mobile communication.

  Therefore, as in the fifth embodiment, if the radio signal modulation method is π / 4 shift QPSK modulation in the mobile communication slave station (base station) and the master station (centralized base station), the frequency modulation signal Alternatively, the laser element may be directly modulated with the π / 4 shift QPSK modulation signal without pre-modulating the waveform, thereby causing clipping in the waveform.

  As described above, in the configuration of the fifth embodiment, it has been shown that the present invention can be applied to π / 4 shift QPSK modulation.

  By the way, the main purpose of the present invention is as a technique for suppressing beat noise associated with optical multiplexing when the slave station side multiplexes and sends an information signal as an optical signal in a form that is vertically asymmetric due to clipping. In addition, the vertical asymmetry of the signal waveform due to clipping is an improvement over what is called distortion generation due to harmonics such as IM3 on the demodulation side.

  As a measure for improvement, in the system according to the first embodiment of the present invention, the master station on the receiving side adds an appropriate DC bias voltage so that the received signal is biased at 6 and the limiter output signal 65 is vertically symmetrical. As a result, the demodulated information signal is subjected to distortion suppression such as IM3 to eliminate the up-down asymmetry problem due to clipping.

  However, such a process is not yet sufficient for a system that can maintain reliable reliability.

  That is, in the case of the configuration of the first embodiment, in the configuration on the master station 3 side, the information signal after demodulation by the FM demodulation circuit 14 depends on the magnitude of the DC bias voltage from the voltage source 11 added by the bias tee 6. The amount of distortion that occurs inside is different. In addition, the characteristics of the laser element on the slave station side may change due to the influence of secular change or the like, and the clipping state of the transmitted optical signal may also change.

  Therefore, in order to provide such an optical transmission apparatus that can cope with such a situation and is stable in the long term, it is necessary to control the DC bias voltage to an appropriate value.

  Accordingly, an embodiment that enables such control will be described as a sixth embodiment.

(Sixth Embodiment) The effect of clipping by the laser element 8 on the transmitted signal is determined by the upper limit and the lower limit of the waveform of the transmitted signal by the limit due to clipping by the laser element and the limit by the limiter 12. In the sixth embodiment, the DC bias voltage is adjusted so as to correspond to the distortion amount of the demodulated signal so as to be symmetrical, that is, the average level does not fluctuate.

  FIG. 15 shows a configuration diagram of the master station 3 in the optical transmission apparatus of the sixth embodiment. The configuration on the slave station side is the same as in the first embodiment, and the same reference numerals are assigned to the same configurations as in the first embodiment in the configuration of the master station 3.

  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. 1, and the output of the FM demodulation circuit 14 is output by this distortion detector 18. Monitoring is performed so that the voltage value of the voltage source 11 can be adjusted in response to distortion included in the FM demodulated signal output. The voltage source 11 has a configuration in which the output voltage can be varied according to the output of the strain 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 the influence of aging, etc., and the clipping state of the transmitted optical signal may also change.

  Therefore, in order to provide such an optical transmission apparatus that can cope with such a situation and is stable in the long term, it is necessary to control the DC bias voltage to an appropriate value.

  Therefore, 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. Based on the detection result, the distortion detector 18 has a function of controlling the DC bias voltage value to be added to the voltage source 11 so that the distortion amount can be suppressed.

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

  The above is a configuration in which adjustment of the DC bias voltage by the bias tee 6 is performed in accordance with the distortion detection amount of the distortion detector on the master station side with respect to a change in the clipping state of the transmitted optical signal. The distortion amount generated in the information signal demodulated by the FM demodulation circuit 14 is improved, and the long-term stability and reliability can be improved. Since this can be configured to be performed on the slave station side, an example thereof will be described as a seventh embodiment.

(Seventh Embodiment) The effect of clipping by a laser element 8 on a signal to be transmitted is symmetrical with respect to the limit due to clipping by the laser element and the limit due to the limiter 12, and the waveform of the transmitted signal is symmetrical. So that the average level does not fluctuate, the DC bias voltage is adjusted in response to the distortion amount of the optical signal to be transmitted, and the current from the current source 7 by the bias tee 6 in response to the distortion amount of the optical signal to be transmitted. In the seventh embodiment, the amount of superimposing of the DC bias current is adjusted to adjust the optical modulation degree OMI of the laser element 8.

  FIG. 16 is a configuration diagram illustrating a slave station of the optical transmission apparatus according to the seventh embodiment. The configuration on the master station side is the same as that of the third embodiment, and the same reference numerals are given to the same components as those of the first to sixth embodiments.

  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 And a strain detector 18.

  Among these, the FM demodulator circuit 5 performs FM modulation on the input analog signal and outputs it, and the voltage source 11 is a variable voltage generator and supplies a DC bias voltage of a required level. The bias tee 6 adds a DC bias voltage from the voltage source 11 to the output of the FM modulation circuit 5 in the previous stage, so that the average level of the optical signal 56 becomes the average level when there is no clipping. The combined received signal is used. That is, the DC bias voltage 11 is adjusted by using the getter so that the zero level of the waveform becomes the zero cross point of the original FM modulation waveform.

  The limiter 12 is an amplitude limiting circuit having a vertically symmetrical limiter characteristic, and the signal output from the bias tee 6 is limited by the vertically symmetrical limiter characteristic, so that the output signal whose waveform becomes asymmetrical vertically at this point in time. Is what you get. The band-pass filter 13 is a circuit that extracts a component in a desired frequency band from the output signal from the limiter 12, and the current source 7 is a variable current generator that has a required level of DC bias current. It is for supplying.

  The bias tee 6 in the subsequent stage outputs a DC bias current from the current source 7 superimposed on the FM modulation signal output from the band-pass filter 13 and outputs it to the laser element 8.

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

  The direct current bias current combined with the bias tee 6 at the subsequent stage and the amplitude of the FM modulation signal determine the degree of optical modulation when the laser element 8 is directly modulated, and the ratio of clipping of the waveform is determined.

  The PD 9 is for converting an optical signal into an electrical signal by photoelectric conversion, and the FM demodulating circuit 14 is for demodulating the frequency of the electrical signal output from the PD 9 and returning it to the original signal. Reference numeral 18 detects a distortion amount from the demodulated output of the FM demodulation circuit 14 and adjusts and controls the voltage value of the voltage source 11 and the current value of the current source 7 in accordance with the detected distortion.

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

  In the third embodiment, in the slave station, the asymmetry of the optical signal due to clipping is corrected on the slave station 1 side in the slave station. The voltage value of the voltage source 11 and the current value of the current source 7 can be adjusted to cope with the distortion corresponding to the distortion included in the 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 is a voltage source with a bias tee 6. The DC bias voltage from 11 is added to transmit the limiter 12 and the band-pass filter 10 for removing harmonics.

  Thereby, one side of the waveform is clipped, and a modulated signal 69 from which harmonic components are removed is obtained. The modulation signal 69 is asymmetric in the vertical direction.

  The modulation signal 69 whose top and bottom are asymmetric is superimposed on the DC bias current from the current source 7 by the bias tee 6 provided to determine the optical modulation degree OMI, and causes the laser element 8 to have OMI> 100 [%]. Modulate directly with.

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

  In this way, the modulation signal 69 whose top and bottom are asymmetric is superimposed on the DC bias current via the bias tee 6, and thus the laser element 8 is directly modulated with OMI> 100 [%], so that the coherency is low. Therefore, it is converted into an optical signal in which beat noise is unlikely to be generated and transmitted to a transmission line using an optical fiber.

  Here, in the present system, the distortion detector 18 is configured so that the upper and lower sides of the transmitted optical signal 53 are symmetrical, that is, the average level does not fluctuate between the limit due to clipping of the laser element 8 and the limit due to the limiter 12. The DC bias voltage and the optical modulation degree OMI are adjusted to correspond to the detected distortion amount.

  That is, the output light of the laser element 8 is returned to an electric signal by the PD 9 and demodulated by the FM demodulation circuit 14 to be restored 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 in accordance with the distortion included in the FM demodulated signal output.

  The distortion detector 18 is configured so that the upper and lower sides of the transmitted optical signal 53 are symmetric with each other by the limit due to clipping of the laser element 8 and the limit due to the limiter 12 in accordance with the amount of distortion detected by itself. A control output for adjusting and controlling the DC bias voltage value and the optical modulation degree OMI may be generated so that the average level position of the signal 56 and the position of the average level of the output signal of the FM modulation circuit 5 coincide. For example, the output levels of the voltage source 11 and the current source 7 are adjusted to correspond to the output of the distortion detector 18. As a result, the laser element 8 is an optical signal with low coherency, and thus is less likely to generate beat noise, and distortion is not generated. It becomes possible to convert the FM modulation signal into a non-optical signal and transmit it to the transmission line by the optical fiber.

  As a result of monitoring the amount of distortion and controlling the bias voltage and OMI so as to reduce the amount of distortion, the bias characteristic of the laser element 8 changes due to the influence of the aging of the laser element 8, the temperature change, and the like. However, 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 amount of distortion included in the information signal at the time of demodulation on the parent station side due to the fluctuation of the clipping state is suppressed. As a result, the demodulated signal can be prevented from deteriorating and the communication quality can be maintained.

  Therefore, it is possible to provide an optical transmission device that is stable and reliable over the long term.

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

  In these embodiments, the distortion amount included in the restored FM demodulated signal is detected by the distortion detector 18, and the DC bias applied by the voltage source 11 in the direction in which the distortion amount is suppressed based on the detection result. The voltage value and the DC bias current applied by the current source 7 are controlled. The light modulation degree OMI changes depending on the DC bias voltage value and the DC bias current. Since the amount of diffusion of the optical spectrum changes depending on the degree of light modulation, the effect of suppressing beat noise becomes unstable. I will touch on this next.

(Eighth Embodiment) The configuration of the optical transmission apparatus of the eighth embodiment is the same as that of the first embodiment.

  In the laser element 8, the state of the optical spectrum spread largely depends on the OMI. FIG. 17 shows the relationship between the OMI [%] of the Fabry-Perot laser element and the 3 dB band [nm] of the optical spectrum.

  As shown in FIG. 17, when clipping starts when the optical modulation degree OMI is OMI> 100 [%], the optical spectrum suddenly spreads, and when OMI = about 175 [%], the optical spectrum spread is almost saturated. . The greater the spread of the optical spectrum, the smaller the influence of beat noise, so the degree of optical modulation OMI is preferably OMI ≧ 175 [%].

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

  From the above, the OMI should be selected in a saturated region where the light spectrum is sufficiently diffused and the state of diffusion does not change even if a slight change in bias conditions occurs. Therefore, according to FIG. 17, it is considered that OMI ≧ 200 [%] satisfies the above-described condition. Therefore, the DC bias current value is set to a value that allows the OMI to be controlled in the range of OMI ≧ 200 [%].

  Further, the optical spectrum diffusion 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 strengthened due to 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 to suppress the return light due to such reflection is also effective for optical spectrum diffusion.

  The clipping modulation of the laser element also has an effect of reducing the range in which beat noise is generated above a predetermined value in the positional relationship of wavelengths between slave stations. In other words, until now, if the center wavelength of the light source between arbitrary slave stations is not separated by 0.5 [nm] or more, beat noise cannot be reduced to a predetermined value or less when using clipping modulation. It is an effect that it is improved only by separating by 0.2 [nm] or more.

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

  In addition, this invention is not limited to the Example mentioned above, A various deformation | transformation can be implemented.

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

  A technique related to interference wave suppression will be described.

  <Technique Regarding Interference Wave Suppression> In a radio base station, an interference wave may be mixed in a frequency band close to the received desired wave. In some cases, the power of the interference wave is greater 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, this interference wave has an optical modulation degree of a desired wave. Cause pressure.

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

  A technique for dealing with this will be described next.

  Here, as a ninth embodiment, a technique for enabling a slave station to largely suppress an interference wave received together with an information signal and ensuring a stable degree of optical modulation when converting the information signal into an optical signal is described as a ninth embodiment. explain.

(Ninth Embodiment) In order to be able to largely suppress the interference wave received together with the information signal in the slave station, in the ninth embodiment, a band transmission filter is provided in the slave station, and the received information signal is By passing the band-pass filter, the interference wave received together with the information signal is suppressed.

  A ninth embodiment of the present invention will be described. FIG. 22 is a block diagram illustrating the configuration of the optical transmission apparatus according to the ninth embodiment. Here, the configuration of the wireless transmitter 29 when the modulation method of the wireless signal 81 is QPSK (quadrature phase-shift keying) is shown.

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

  Among these, the wireless transmitter 29 includes a signal source 30, a quadrature modulator 31, a local oscillator 33, a multiplier 34, a band pass 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 wireless transmitter 29 generates an I channel signal 82 and a Q channel signal 83, and the quadrature modulator 31 performs quadrature modulation on these signals 82 and 83 and converts 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 and the QPSK signal 84 to convert the signal into a high frequency band signal for wireless transmission. . The band pass filter 35 band-limits the signal in the high frequency band for radio transmission converted by the multiplier 34, and the power amplifier 36 applies the signal in the high frequency band for radio transmission limited in band. The transmission antenna 37 is for transmitting the amplified radio signal 81 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, and a band transmission filter 91.

  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 a radio signal 89 that is an interference wave. The band pass filter 91 is for extracting a frequency component of a required channel from the amplified signal, and the bias tee 6 is a received signal 87 of a specific channel component extracted by the band pass filter 91. In addition, a DC bias signal from the current source 7 is added and output, and the laser element 8 is applied with a reception signal 87 of a specific channel component to which the DC bias signal is added by the bias tee 6, A laser beam corresponding to the signal level is generated. This laser beam (optical signal 57) is transmitted to the master station 3 through the optical fiber 3.

  The master station 3 includes a PD (light receiving element for photoelectric conversion) 9, a band pass filter 10, 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 electrical signal. The preamplifier 39 amplifies this electrical signal. The bandpass filter 10 The signal component is extracted by suppressing the noise component from the amplified electric signal, and the quadrature demodulator 40 demodulates the signal that has passed through the bandpass filter 10 to restore the original information signal. is there.

  In such a configuration, the wireless transmitter 29 generates an I channel signal 82 and a Q channel signal 83 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, after being band-limited by passing through the band-pass filter 35, it is amplified via the power amplifier 36 and transmitted as a radio signal 81 from the transmitting antenna 37 to the slave station 1 side.

  A radio signal 81 transmitted from the subscriber's radio transmitter 29 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.

  In the slave 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 which is an interference wave, and is amplified here. Then, it is further passed through a band transmission filter 91 to extract a specific channel component. The received signal 87 that has passed through the band-pass filter 91 is applied with a DC bias signal from the current source 7 by the bias tee 6 and applied to the laser element 8 to directly modulate the laser element 8.

  By this modulation, an optical signal 57 is output from the laser element 8. 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 electrical 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 that is an interference wave may be larger than the radio signal 81 that is 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. And the CNR of the received signal in the master station 3 is limited.

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

  As a result of the transmission characteristics as shown by the dotted line in FIG. 23B, the band transmission filter 91 transmits the radio signal 81 that is the desired wave, but does not pass the band of the radio signal 89 that is the interference wave. Interference waves are suppressed. Therefore, the optical modulation degree of the desired wave can be stabilized regardless of the signal intensity of the interference wave, and the upper limit of the optical modulation degree can be increased.

  When 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 transmission filter 23.

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

  Ideal waveform equalization performed in the transmission system sets the intersymbol interference to zero at the time of code determination for the received signal. The symbol rate of the received signal is Bsym [symbol / second], and the Nyquist frequency that is ½ of the symbol rate Bsym is fN [Hz]. As shown in FIG. 24, an ideal band isostatic filter for waveform equalization has a cut-off frequency (the cut-off frequency here is a frequency at which the transmittance is ideally “0”). For frequency fc [Hz], fc ± fN. The 3 [dB] transmission bandwidth Δf is Δf = 2fN = Bsym.

  However, if a time deviation occurs at the time of code determination, there is a disadvantage that intersymbol interference becomes extremely large. Therefore, in practice, a transmission system having no intersymbol interference is used even when the cut-off frequency is fc ± (1 + α) fN, the transmission band is widened, 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).

  Waveform shaping through a filter having a cutoff frequency up to (1 + α) times is called roll-off shaping.

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

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

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

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

  The band-pass filter 91 sets the 3 [dB] pass bandwidth to Δf [Hz] around the center frequency fc of the information signal. When the information signal is subjected to roll-off shaping only on the receiving side of the slave station 1, Δf = Bsym. For an information signal that has already been roll-off shaped on the transmission side, Δf = 2Bsym so that the signal component of the information signal is not suppressed.

  When the waveform equalization is performed by the root roll-off filter having the roll-off coefficient α on the transmission side and the reception side, ideally, Δf = (α + 3) Bsym / 3. In consideration, Bsym ≦ Δf ≦ 2Bsym.

  From the above, the condition of 3 [dB] pass bandwidth Δf [Hz] of the band transmission filter 23 provided in the slave station 1 is in the range of Bsym ≦ Δf ≦ 2Bsym with respect to the symbol rate Bsym [symbol / second]. It turns out that it is optimal.

  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 expressed by the bit rate Bm.

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

  As described above, according to the ninth embodiment, the slave station 1 is provided with the band transmission filter 38, and the information signal received by the slave station 1 is passed through the band transmission filter 38, thereby extracting the information signal of the desired channel. Since the laser element 8 is driven by the extracted information signal and optically transmitted to the master station 3, the slave station 1 passes the interference wave received together with the information signal through the band transmission filter 91. Can be greatly suppressed. Therefore, according to the present invention, it is possible to provide a stable degree of optical modulation when converting an information signal into an optical signal.

  The above is a configuration in which an information signal of a specific channel is extracted by a band transmission filter in a slave station, and then photoelectrically converted and transmitted to the master station, and interference signals included in the information signal can be suppressed by channel selection. However, since the information signal is not a single channel but a plurality of channels in the slave station, it is desired to be able to select any desired channel. Accordingly, an example in which the information signal of the desired channel is selected from the information signals and waveform equalization can be selected next will be described as a tenth embodiment.

(Tenth Embodiment) FIG. 26 is a block diagram showing a configuration of an optical transmission apparatus according to the tenth embodiment. In FIG. 26, for the slave station 1 and the master station 3, the same components as those in the ninth embodiment are denoted by the same reference numerals. Note that, as in the ninth embodiment, the modulation method of the radio signal 81 is QPSK (quadrature phase-shift keying).

  In FIG. 26, 1 is a slave station, 29 is a wireless transmitter possessed by a user who is a mobile terminal, 2 is an optical fiber, and 3 is a master station. As in the ninth embodiment, the wireless transmitter 29 includes a signal source 30, a quadrature modulator 31, a local oscillator 33, a multiplier 34, a bandpass filter 35, a power amplifier 36, and a transmission antenna 37. . In the optical transmission apparatus of the tenth embodiment, a transmission-side root roll-off filter 32 is further provided to limit the band of the output of the quadrature modulator 31.

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

  The band pass filter 35 band-limits the signal in the high frequency band for radio transmission converted by the multiplier 34, and the power amplifier 36 applies the signal in the high frequency band for radio transmission limited in band. The transmission antenna 37 is for transmitting the amplified radio signal 81 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, and a reception side root roll-off filter 38. It is provided and configured.

  That is, here, a local oscillator 33a and a multiplier 34a are newly provided to provide a function for selecting a desired channel, and the reception side root roll-off filter 38 is used in place of the band-pass filter 91 to select the channel and equalize the waveform. The difference from the ninth embodiment is that it has a function of removing an image generated during frequency conversion.

  The low noise amplifier 16 in the slave station 1 amplifies the radio signal 81 received by the antenna 15 of the slave station 1 together with a radio signal 89 that is 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, and performs frequency down-conversion. The reception-side root roll-off filter 38 generates a waveform from the down-converted reception signal. A received signal 87 of a desired specific channel component is extracted by roll-off shaping, and a bias tee 6 adds a DC bias signal from the current source 7 to the extracted signal and outputs it. The element 8 receives a specific channel component received signal 87 to which a DC bias signal is added by the bias tee 6 and generates a laser beam corresponding to the level of the signal. This laser beam (optical signal 57) is transmitted to the master station 3 through the optical fiber 3.

  The master station 3 includes a PD (light receiving element for photoelectric conversion) 9, a band pass filter 10, 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 electrical signal. The preamplifier 39 amplifies this electrical signal. The bandpass filter 10 The signal component is extracted by suppressing the noise component from the amplified electric signal, and the quadrature demodulator 40 demodulates the signal that has passed through the bandpass filter 10 to restore the original information signal. is there.

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

  In the subscriber's radio transmitter 29, an I channel signal 82 and a Q channel signal 83 are generated from the signal source 30. The signals 82 and 83 are converted into a QPSK signal 84 by the quadrature modulator 31, passed through the transmission-side root roll-off filter 32, and become a band-limited QPSK signal 85. The QPSK signal 85 is converted into a high frequency band for wireless transmission by the local oscillator 33 and the multiplier 34.

  Then, the signal is transmitted from the transmission antenna 37 to the slave station 1 side as a radio signal 81 via the band pass filter 35 and the power amplifier 36. The radio signal 81 transmitted from the subscriber's radio transmitter 29 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 which is an interference wave, and is converted into a low frequency band QPSK signal 86 by the local oscillator 33a and the multiplier 34a.

  The QPSK signal 86 is band-limited by the reception-side root roll-off filter 38, thereby greatly suppressing the radio signal 89 that is an interference wave. 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 and more adjacent to each other.

  The received signal 87 transmitted through the receiving side root roll-off filter 38 is added with a DC bias signal from the current source 7 by the bias tee 6 to directly modulate the laser element 8. The optical signal 57 output from the laser element 8 is transmitted to the master station 3 through the optical fiber 2 as in the ninth embodiment.

  In the master station 3, the transmitted optical signal 57 is received by the PD 9, converted into an electrical 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 this embodiment, a root roll-off filter is provided on the transmission side and the reception side, and on the transmission side, unnecessary frequency components are suppressed by performing waveform roll-off shaping through the root roll-off filter, and transmission is performed by narrowing down to a specific channel frequency. On the receiving side, the received signal is waveform-rolled off through a root roll-off filter to suppress unnecessary frequency components and receive the signal with the desired specific channel frequency. The desired channel can be selected and waveform equalization can be performed.

  Then, in the slave station 1, selecting a desired channel from the received information signal and performing waveform equalization on the information signal that is the desired channel has the following effects.

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

  Further, if waveform equalization is performed on the information signal, 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 degree of freedom of components or transmission methods used in the transmission system from the slave station to the master station is increased, and the configuration of the optical analog transmission apparatus can be diversified.

  In the tenth embodiment, the received signal is subjected to waveform roll-off shaping through a root roll-off filter at a slave station on the receiving side, so that unnecessary frequency components can be suppressed, and reception can be performed by narrowing down to a desired specific channel frequency. As a result, any 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 near frequency band with respect to a received desired wave, but sometimes the power of the interference wave may be larger than the power of the desired wave. In an optical analog transmission apparatus that 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 causes the optical modulation degree of the desired wave to be compressed. Become.

  If the optical modulation degree of the desired wave is limited to a small value, the CNR of the desired wave becomes small in the master station, so that there may be a case where the specified error rate cannot be achieved. However, this can be solved by the ninth and tenth embodiments.

<Technology for Increasing the Optical Modulation of Transmission Optical Signal> The interference wave suppression technology 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 an optical signal transmitted from a slave station to a master station is set to “1” or more to increase the CNR of a desired wave in the master station. However, when the desired wave is a radio signal that is angle-modulated, the reception sensitivity is degraded due to the amplitude limitation. For this reason, the required CNR value becomes large, and the specifications for the slave stations become strict.

  An optical signal is transmitted from the slave station to the master station, but the strength of the optical signal received at the master station is preferably high. However, in order to secure the intensity of the optical signal received by the master station 3 that is the optical signal receiving side, if the optical modulation degree per channel on the transmission side is increased, the optical modulation degree OMI of the optical signal is increased. (Optical Modulation Index) is likely to exceed 100 [%], and there is a concern that clipping (amplitude limitation) causes CNR (carrier noise ratio) to deteriorate.

  Next, an eleventh embodiment will be described, in which such a concern can be solved and the limit of the optical modulation degree of the optical signal can be set to 100% or more.

(Eleventh embodiment) In the present invention, after the waveform equalization is performed in the slave station 1, it is optically modulated and optically transmitted to the master station, so that the reception sensitivity is not deteriorated due to clipping. In this way, the limit of the optical modulation degree of the optical signal on the slave station side can be set to 100 [%] or more. Then, the master station 3 can obtain a received signal having a larger CNR.

  An eleventh embodiment of the present invention will be described. FIG. 27 is a block diagram illustrating the configuration of the optical transmission apparatus according to the eleventh embodiment. In the slave station 1 and the master station 3, the same components as those in the ninth embodiment are denoted by the same reference numerals.

  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, local oscillators 33A, 33B, to 33N, multipliers 34A, 34B, to 34N, and a filter. 50A, 50B, .about.50N and an adder 43 are provided.

  That is, here, local oscillators 33A, 33B,... 33N, multipliers 34A, 34B, .about.34N, filters 50A, 50B, .about.50N, and an adder 43 are newly provided, and waveforms for a plurality of channels are provided. It differs from the tenth embodiment in that equalization is performed.

  The low noise amplifier 16 in the slave station 1 amplifies the radio signal 81 received by the antenna 15 of the slave station 1 together with a radio signal 89 that is an interference wave. The multiplier 34A multiplies the local oscillation frequency signal from the local oscillator 33A and the output of the low noise amplifier 16 to down-convert the frequency, and the filter 50A selects a desired specific channel component from the down-converted received signal. The received signal 87A is extracted, and 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 is subjected to this down-conversion. A received signal 87B having a desired specific channel component is extracted from the received signal, and so on ... ... The multiplier 34N multiplies the local oscillation frequency signal from the local oscillator 33N by the output of the low noise amplifier 16 to down-convert the frequency. The filter 50N is this down-converter The received signal 87N of the desired specific channel component is extracted from the received received signal.

  The adder 43 combines the outputs (reception signals 87A, 87N) of the filters 50A, 50B, 50N, which are outputs of the respective systems A, N, and the bias tee 6 is combined. A DC bias signal from the current source 7 is added to the output signal, and the laser element 8 is applied with a combined signal to which the DC bias signal is added by the bias tee 6 and follows the signal. It generates a laser beam that has been modulated. This laser beam (optical signal 70) 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 band pass filter 10, 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 electrical signal. The preamplifier 39 amplifies this electrical signal. The bandpass filter 10 The signal component is extracted by suppressing the noise component from the amplified electric signal, and the quadrature demodulator 40 demodulates the signal that has passed through the bandpass filter 10 to restore the original information signal. is there.

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

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

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

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

  The optical signal 90 output from the laser element 8 is transmitted to the master station 3 through the optical fiber 2. In the master station 3, the transmitted optical signal 90 is received by the PD 9 and amplified by the preamplifier 39. Then, the bandpass filter 10 extracts a reception signal 88 of a desired channel among the plurality of channels A and B to N. The band-pass filter 10 is wider than the modulation band of the received signal 88 so that the signal waveform is not deteriorated. Received signal 88 is input to demodulator 49 and demodulated to obtain transmission information.

  In this embodiment, there are local oscillators (33A, 33B,... 33N), multipliers (34A, 34B, .about.34N), filters (50A, 50B, .about.50N) for each of A, B,. Channels A, B,..., N are combined by an adder 43, thereby modulating the laser element 8. 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.

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

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

  However, in the present invention, the waveform equalization is performed for each channel by the filters 50A, 50B, to 50N in the slave stations, so that the amplitude limit is resistant and the reception sensitivity is not deteriorated due to clipping. Therefore, the limit of the optical modulation degree of the optical signal can be set to 100 [%] or more, and the received signal 88 having a larger CNR can be obtained in the master station 3.

  By the way, an information signal from a subscriber is received by a slave station as a remote station, and the received information signal is sent from the slave station to the master station, but transmission between the slave station master stations is performed by optical transmission. The intensity of the optical signal received at the master station should be high. However, if there is an interference wave in the received signal at the slave station and the optical signal signal intensity of the desired wave fluctuates, it becomes difficult to transmit the desired signal having a stable CNR to the parent station side.

  Therefore, an embodiment in which an optical signal of a desired wave can be transmitted to the master station side with a stable signal intensity even when there is an interference wave will be described as a twelfth embodiment.

(Twelfth embodiment) By passing a received radio signal through a narrow band-pass filter of a root roll-off filter, interference waves other than the desired wave can be greatly suppressed. Therefore, the slave station is provided with a narrow band-pass filter of the root roll-off filter, and the received radio signal is extracted through the narrow-band band pass filter of the root roll-off filter.

  A twelfth embodiment of the present invention will be described. FIG. 28 is a block diagram illustrating 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 those in the tenth embodiment described with reference to FIG.

  However, a plurality of slave stations 1a, 1b, to 1n are accommodated in the master station 3 by being passively multiplexed (SCM multiplexed) by the optical coupler 4. In FIG. 28, the same components as those in FIG. 26 are denoted by the same reference numerals.

  Similarly to the tenth embodiment, the modulation method of the radio signal 81 is set to QPSK (Quadrature phase / shift keying) and transmitted from the transmitting antenna 37 to the slave stations 1a, 1b, to 1n. The radio signal 81 transmitted from the subscriber's radio transmitter 29 is received by a certain slave station, for example, the antenna 15a of the slave station 1a among the plurality of slave stations 1a, 1b, to 1n.

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

  Therefore, when directly modulating the semiconductor laser element, the degree of optical modulation of the desired wave can be avoided from being pressed by the interference wave, and the stable signal intensity of the desired wave can be obtained on the master station side regardless of the presence or absence of the interference wave. Can be communicated to.

  The reception signal 87 that has passed through the reception-side root roll-off filter 38a is added with a DC bias signal from the current source 7a by the bias tee 6a to directly modulate the laser element 8a. Here, the optical modulation index OMI (Optical Modulation Index) of the optical signal 57a output from the laser element 8a is set to exceed 100 [%]. The radio signal 87L transmitted from any 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). The SCM multiplexing with the optical signals from the other slave stations 1a, .about.1n (≠ L) is made possible.

  In the master station 3, the optical signals multiplexed in SCM 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 is wider than the modulation band of the received signal 88 so as not to deteriorate the signal component. Received signal 88 is input to quadrature demodulator 40, where it is demodulated to obtain transmission information. The laser element 8 may be a Fabry-Perot type semiconductor laser element or a distributed feedback type semiconductor laser element, but it is preferable to employ a Fabry-Perot type semiconductor laser element with less interference.

  Normally, if the optical modulation degree of the optical signal 57 is 100% or more, the CNR of the received signal 88 at the master station 3 becomes larger and the noise component becomes smaller, but the amplitude of the transmission signal is limited. Therefore, the waveform of the reception signal 88 is distorted and the reception sensitivity is deteriorated.

  However, when the slave station 1 performs the roll-off shaping on the transmission signal and then limits the amplitude, the reception sensitivity does not deteriorate regardless of the waveform distortion. FIG. 29 shows an error rate characteristic with respect to the CNR of the radio signal 88 subjected to QPSK modulation.

  The error rate when the slave station 1 is provided with the root roll-off filter 38 and with a narrow band filter other than the root roll-off filter (here, the Putterworth filter) is expressed as “with roll-off shaping”, “roll-off” Shown against theoretical limits as “no off-shaping”.

  The light modulation degree was 250 [%]. When there is no roll-off shaping, the reception sensitivity deteriorates slightly by 1 [dB] from the theoretical limit. The deterioration of the reception sensitivity increases the CNR required for the radio signal 88 and 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 optical modulation can be increased with stricter specification requirements for the radio signal 88.

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

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

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

  As shown in FIG. 30, the driving circuit of the LD 8 used in the present invention includes 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 amplifier, which adds a DC bias signal from the current source 7 and amplifies the received signal output from the bias tee 6 and outputs the amplified signal to the laser element 8. Reference numeral 8 indicates that a reception signal 87 to which a DC bias signal is added is applied by the bias tee 6 to generate a laser beam modulated in accordance with 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) of the driver amplifier 41 is used. ) Is necessary.

  Therefore, in the case of the configuration shown in FIG. 30, as the AGC, first, the output signal of the driver amplifier 41 is partially branched by the coupler 42, the signal strength is detected by the power detector 43, and the detected signal strength is referred to. The reference voltage value from the voltage generator 44 is compared with the comparator 45 to obtain a difference signal, and the difference signal output from the comparator 45 is extracted by passing through the loop filter 46, and the gain of the driver amplifier 41 is increased. Control.

  In this way, the direct current bias signal from the current source 7 is added to the reception signal 87 transmitted through the reception side root roll-off filter 38 by the bias tee 6 to directly modulate the laser element 8, thereby driving the laser element 8. The signal is amplified through the driver amplifier 41 so as to have a signal level sufficient to be applied to the laser element 8, and a coupler is used to keep the optical modulation degree of the optical signal output from the laser element at a constant value. 42, automatic gain control is performed by an AGC circuit of a driver amplifier 41 including 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 that requires an AGC circuit. An example of this will now be described with reference to FIG. In order to drive the laser element 8 without using a driver amplifier, a limiter amplifier may be used.

  FIG. 31 shows an example of a drive circuit for the LD 8 when a limiter amplifier is used. That is, the received signal 87 that has passed through the receiving-side root roll-off filter 38a is added with a DC bias signal from the current source 7a by the bias tee 6a, amplified through the limiter amplifier 47, and then applied to the laser element 8. The configuration is as follows.

  The limiter amplifier 47 does not require a power detector or a comparator, and can simplify the drive circuit configuration of the laser element (LD) 8. Therefore, further miniaturization of the slave station 1 that is a radio base station can be promoted.

  Normally, when transmitting an analog signal, the upper and lower amplitudes are clipped and the reception 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, but an angle modulation method such as a 1/4 shift QPSK signal or an offset QPSK signal may be used. In addition, although the transmission side filter 32 and the reception side filter 38 are both root roll-off filters, the transmission characteristics including the transmission and reception side filters may be roll-off shaping.

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

  As described above, the twelfth embodiment greatly suppresses interference waves other than the desired wave by passing the received radio signal including the interference wave through the narrow band-pass filter of the root roll-off filter in the slave station. I did it. 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 can be avoided. Therefore, according to the twelfth embodiment, it is possible to transmit an optical signal to the master station side with a stable signal intensity regardless of whether the received signal contains an interference wave or not.

  Further, in this embodiment, the coherence of the output optical signals 57a, 57b,... 57n is set by setting the optical modulation degree of the laser element 8 in each of the slave stations 1a, 1b,. Can be suppressed. Therefore, 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 together 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 in which a signal QPSK-modulated with a digital signal is used as an information signal has been described. The present embodiment can also be applied to a configuration using APSK (Amplitude Phase-shift keying) modulation including the.

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

It is a figure for demonstrating this invention, Comprising: It is the block diagram which showed the 1st Example of this invention. It is a figure for demonstrating this invention, Comprising: It is the figure which showed the optical spectrum of the output optical signal from the sub_station | mobile_unit in the 1st Example of this invention. It is a figure for demonstrating this invention, Comprising: It is the figure which showed the demodulation process in the master station in the 1st Example of this invention. It is the figure which showed the demodulation process in the master station in the conventional apparatus. It is a figure for demonstrating this invention, Comprising: It is the block diagram which showed the 2nd Example of this invention. It is the figure which showed the amplitude restriction | limiting in the master station in the 2nd Example of this invention. It is the figure which showed the frequency demodulation circuit by delay detection. It is the figure which showed the influence on the waveform of clipping in a delay detection. It is a figure for demonstrating this invention, Comprising: The block diagram which showed the 3rd Example of this invention. It is a figure for demonstrating this invention, Comprising: The figure which showed the optical modulation process in the sub_station | mobile_unit in the 3rd Example of this invention. It is a figure for demonstrating this invention, Comprising: The block diagram which showed the 4th Example of this invention. It is a figure for demonstrating this invention, Comprising: It is the figure which showed the optical modulation process in the sub_station | mobile_unit in the 4th Example of this invention. It is the figure which showed the addition circuit to the laser in the 4th Example of this invention. It is a figure for demonstrating this invention, Comprising: It is a block diagram which shows the 5th Example of this invention. It is a figure for demonstrating this invention, Comprising: It is a master station block diagram which shows the 6th Example of this invention. It is a figure for demonstrating this invention, Comprising: It is a subunit | mobile_unit block diagram which shows the 7th Example of this invention. It is a figure for demonstrating this invention, Comprising: It is a figure which shows the relationship between OMI and the optical spectrum regarding the 8th Example of this invention. It is a block diagram of the optical transmission apparatus which applied the clipping by the conventional FM modulation signal. It is a figure which shows the optical signal waveform at the time of causing clipping. It is a figure which shows the frequency spectrum of the multichannel analog signal after the conventional FM demodulation. The figure which showed the definition of the light modulation degree OMI. It is a figure for demonstrating this invention, Comprising: The block diagram which showed the 9th Example of this invention. It is a figure for demonstrating this invention, Comprising: The relationship figure of the desired wave and interference wave in 9th Example. It is a figure for demonstrating this invention, Comprising: It is a figure which shows the transmission characteristic of a waveform equalization filter. It is a figure for demonstrating this invention, Comprising: It is a figure which shows the relationship between 3 [dB] transmission bandwidth of a band transmission filter, and a symbol rate. It is a figure for demonstrating this invention, Comprising: The block diagram which showed the 10th Example of this invention. It is a figure for demonstrating this invention, Comprising: It is a block diagram which shows the 11th Example of this invention. It is a figure for demonstrating this invention, Comprising: It is a block diagram which shows the 12th Example of this invention. It is a figure for demonstrating this invention, Comprising: It is a figure which shows the receiving sensitivity in a 12th Example. It is a figure for demonstrating this invention, Comprising: It is a figure which shows the laser drive circuit by the amplifier by which AGC control was carried out. It is a figure for demonstrating this invention, Comprising: It is a figure which shows the laser drive circuit by 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 source, 8, 8a, 8b, -8n ... laser element (LD), 9 ... photodetector (PD), 10, 13, 11a, 11b, -11n ... band pass filters, 11, 11a, 11b, .about.11n... Voltage source, 12, 12a, 12b, .about.12n... Limiter, 14... Frequency demodulation circuit, 15a, 15b, .about.15n. QPSK demodulation circuit, 18 ... distortion detector, 19 ... diode, 20 ... bias applying coil, 21 ... resistor, 22 ... DC cut capacitor, 24 ... comparator, 25 ... branch circuit, 26 ... delay circuit, 27 ... ExOR circuit 28 ... -Pass filter, 30 ... signal source, 31 ... quadrature modulator, 32 ... transmission-side root roll-off filter, 33,33a ... local oscillator, 34,34a ... multiplier, 35 ... bandpass filter, 36 ... power amplifier, 37 ... antenna , 38 ... reception side roll-off filter, 39 ... preamplifier, 40 ... quadrature demodulator, 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 equalization filter, 81 ... radio signal, 82 ... I channel signal, 83 ... Q channel signal, 84 ... QPSK signal, 85 ... Route roll-off shaped QPSK signal, 86... Received signal, 87... Roll-off shaped QPSK signal, 88. Signal, 89 ... radio signal is an interference wave, 90 ... optical signal

Claims (5)

In an optical analog transmission device that transmits an optical signal obtained by directly modulating a semiconductor laser element with an information signal received at a slave station to a master station via an optical transmission line,
The slave station includes a receiving means for receiving an information signal transmitted as a radio signal, and a band-pass filter.
The information signal received by the receiving means passes through the band pass filter without being subjected to frequency conversion signal processing, and directly modulates the semiconductor laser device. . In an optical analog transmission device that transmits an optical signal obtained by directly modulating a semiconductor laser element with an information signal received at a slave station to a master station via an optical transmission line,
The slave station is a receiving means for receiving an information signal transmitted as a radio signal angle-modulated with a digital signal;
A filter for transmitting the information signal received by the receiving means,
An optical analog transmission device characterized in that the semiconductor laser element is directly modulated by an information signal that has passed through the filter. In an optical analog transmission apparatus configured to transmit an optical signal obtained by directly modulating a semiconductor laser element using an information signal received at a slave station to a master station via an optical transmission line,
The slave station is a receiving means for receiving an information signal transmitted as a radio signal angle-modulated with a digital signal;
An optical analog transmission device comprising: a band transmission filter that obtains a modulation signal of a semiconductor laser element by waveform-equalizing the information signal received by the receiving means. The optical analog transmission device according to any one of claims 1 to 3,
An optical analog transmission device characterized in that the information signal is modulated with a digital signal in amplitude and angle. In the optical analog transmission device according to any one of claims 1, 3, and 4,
The band-pass filter has a 3 [dB] transmission bandwidth Δf having a relationship of Bsym ≦ Δf ≦ 2Bsym with respect to a symbol transmission rate Bsym [symbol / second] of the information signal. An optical analog transmission device.

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