JP3295029B2 - Optical transmission device and system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an analog optical transmission device based on an optical SCM system, and more particularly, to an optical SC transmission system.
Apparatus that can suppress noise or distortion that increases due to reflected light and noise or distortion that increases due to reflected return light to a light source and instability of the light source, which are problems when transmitted in the M system. About. Further, the present invention relates to an analog optical transmission system capable of reducing beat noise which is a problem when optical signals from a plurality of light sources are collectively received.

[0002]

2. Description of the Related Art Optical SCM (SubCarrier Mu)
The ltiplexing transmission method is a method of converting a frequency-multiplexed electric signal to be transmitted into a laser light intensity-modulated by the signal and transmitting the converted laser signal. Unlike digital transmission by on / off of light, A / D, D A / A converter is not required, and it is characterized by a very wide band and low loss as compared with a conventional transmission system using a coaxial cable. For this reason, in recent years, its practical use is greatly expected.

[0003] It is known that such an optical SCM transmission system has the following problems. First, when reflected light is generated in an optical fiber, noise and distortion increase and transmission characteristics deteriorate. Second, when reflected light is coupled to the light source, the state of the light source becomes unstable and noise and distortion increase. Third, when collectively receiving optical signals emitted from a plurality of light sources, if the wavelengths of the optical signals are close to each other, beat noise is generated, which degrades transmission characteristics. The optical signal is reflected on the end face of the optical component or the end face of the connector of the optical fiber. Or,
Reflected light also occurs due to Rayleigh scattering or the like in the optical fiber.

[0004] Among the above three problems, as an example of an apparatus for suppressing the first degradation of transmission characteristics, Japanese Patent Laid-Open No. 5-29 is disclosed.
No. 1671 discloses an analog optical transmission device and an optical fiber amplifier. FIG.
FIG. 67 is a block diagram illustrating a configuration of a conventional analog optical transmission device disclosed in No. 671. 17 includes a multiplexing unit 501, an adding unit 502, a video signal input terminal 503,
Amplifiers 504 and 508, Signal Laser Diode 5
05, an optical fiber 506, a light receiving element 507, a video signal output terminal 509, and optical connectors 510 and 511.

[0005] The analog electric signals of the N channels (ch 1 to N) are RF-multiplexed by a multiplexing section 501, added with a pilot signal by an adding section 502, and then input to a video signal input terminal 503. Video signal input terminal 5
The electrical signal input to the signal 03 is amplified by the amplifier 504 and then converted into an optical signal by the signal semiconductor laser element 505. The optical signal obtained by the conversion by the signal semiconductor laser element 505 is transmitted to the receiving side via the optical fiber 506 and the optical connectors 510 and 511. The transmitted optical signal is converted into an electric signal again by the light receiving element 507, further amplified by the amplifier 508, and output from the video signal output terminal 509.

In the above operation, a part of the optical signal output from the signal semiconductor laser element 505 is reflected by the optical connectors 510 and 511 or the optical fiber 506
Rayleigh scattering at. Further, part of the reflected light is re-reflected, and multiple reflected light traveling in the same direction as the optical signal is generated. In general, in a semiconductor laser, there is a wavelength fluctuation accompanying electric-optical conversion, so that at the time of photoelectric conversion, reflected light traveling in the same direction interferes with an optical signal to generate electric intensity modulation, resulting in interference noise. Therefore, it is expected that noise or distortion will occur in the electric signal output from the video signal output terminal 509 as it is. Therefore, the apparatus shown in FIG. 17 disperses the power of interference noise over a wide frequency band by adding a pilot signal to an electric signal to be transmitted, converting the added electric signal into an optical signal, and transmitting the optical signal. Let me.
As a result, the power of the interference noise in the transmission frequency band is reduced, and as a result, noise or distortion due to reflected light is reduced.

Japanese Patent Application Laid-Open No. Hei 5-291671 also discloses a condition that the frequency of a pilot signal is set to be equal to or lower than the frequency corresponding to the spectral line width of the semiconductor laser. The apparatus shown in FIG. By adding the signals, noise or distortion due to reflected light generated in the optical fiber 506 and the optical connectors 510 and 511 can be sufficiently reduced. However, in the device shown in FIG.
2 of multiplexed analog electric signal and pilot signal
The next intermodulation distortion (hereinafter, IM2) is newly generated.

[0008] US Pat. No. 5,373,385 uses a configuration similar to that of JP-A-5-291167. In this patent, the frequency of the additional signal is specified to be outside the band of the signal to be transmitted. Therefore, although the additional signal does not directly affect the signal to be transmitted, IM2 is newly generated as in Japanese Patent Laid-Open No. Hei 5-291671, which adversely affects the signal to be transmitted.

On the other hand, US Pat. No. 5,430,569 discloses a configuration capable of reducing IM2 newly generated by adding an additional signal as described above. In this patent, when IM2 occurs in a band allocated for transmission of a signal to be transmitted, the effect is reduced by using a predistortion compensator. If IM2 occurs outside the band, the predistortion compensator is omitted.

As a second method for suppressing the deterioration of the transmission characteristics, a method of inserting an optical isolator between the light source and the optical fiber is generally adopted so that the reflected light does not couple to the light source.

As a third method for suppressing the deterioration of transmission characteristics, Japanese Patent Application Laid-Open No. 6-177840 discloses an optical communication system for suppressing beat interference. FIG.
It is a block diagram which shows the structure of the conventional optical transmission system which used the optical communication system of -177840. FIG.
Includes transmission ends 600 to 602, reception ends 603 and 604, optical waveguides 605 and 606, and an optical star coupler 607. Sending end 600-602
Each oscillator 608 _{1-608 3,} electrical modulator 6
09 containing _{1-609 3} and the optical modulator 610 _{1-610 3.} The receiving end 603 includes an optical demodulator 611, frequency selection filters 612 _{1 to} 612 ₃ , and electric demodulators 613 _{1 to} 613.
3 including ₃ and the oscillator 614 _{1-614 3.} Receiving end 6
04 is an optical demodulator 615, a frequency selection filter 616 ₁
～616 _3, including an electrical demodulator 617 _{1-617 3} and the oscillator 618 _{1-618 3.}

Oscillator (shown as f1 to f3 in the figure) 608
_{1 to} 608 ₃ , 614 _{1 to} 614 ₃ , 618 _{1 to} 618 ₃
Output subcarriers (electric signals) having frequencies f1 to f3, respectively. Each electrical modulator 609 _{1-609 3} modulates the sub-carrier at the input signal. Each of the optical modulators 610 _{1 to} 610 ₃ is a subcarrier having a wavelength λ.
The main carriers (optical signals) of 1 to λ3 are modulated. The optical star coupler 607 multiplexes / demultiplexes the main carrier. The optical demodulators 611 and 615 each demodulate the main carrier. Frequency selection filter 612 _{1 to} 61
2 ₃ , 616 _{1 to} 616 ₃ are, respectively,
Select subcarriers of frequencies f1 to f3. The electric demodulators 613 _{1 to} 613 ₃ and 617 _{1 to} 617 ₃ demodulate the subcarriers, respectively.

The operation of the system shown in FIG.
Will be explained. Oscillator 608₁ ~ 608_Three Are each frequency
When the sub-carriers of numbers f1 to f3 are output, the electric modulator
609₁ ~ 609_Three Is the sub signal of the input signals (1) to (3)
Modulate the carrier. Next, the optical modulator 610₁ ~ 610
_Three Are the modulated subcarriers and have wavelengths λ1 to λ, respectively.
The intensity of the main carrier (optical signal) of No. 3 is modulated. Modulated
Each of the main carriers is connected via an optical waveguide 605.
Transmitted to the optical star coupler 607 to be multiplexed / demultiplexed.
Thereafter, the receiving ends 603 and 604 are transmitted through the optical waveguide 606.
Is transmitted to At the receiving ends 603 and 604,
The optical demodulators 611 and 615 transmit the transmitted main key.
Carrier, and a frequency selection filter 612₁ ~ 612
_Three , 616₁ ~ 616_Three Respectively, from each demodulated output,
Select subcarriers of frequencies f1 to f3. And
Air demodulator 613₁ ~ 613_Three , 617₁ ~ 617_Three 
Are respectively oscillators 614₁ ~ 614_Three , 618₁ ~ 6
18 _Three Using the subcarriers output by
When the carrier is demodulated, input signals (1) to (3) are obtained.
Can be

In the above operation, at the time of optical demodulation, beat noise is generated due to beat waves of two main carriers whose wavelengths are close to each other, and the reception state at the receiving ends 603 and 604 is deteriorated. Therefore, the optical modulators 610 _{1 to} 610
By changing the temperature of the LD included in ₃ or changing the bias current, the transmitting ends 600 to 602 are changed.
Independently every time, the wavelengths λ1 to λ3 of the main carrier are periodically changed. This allows the time for the beat frequency to match the subcarrier frequency to be very short,
It is said that the influence of the beat noise on the reception state can be reduced. For example, when transmitting an analog signal for CATV, if the time during which the beat frequency coincides with the subcarrier frequency is shortened as described above, the reception state is hardly affected. In particular, when the modulation signal is an analog television signal in the cable television system, even if beat noise is momentarily mixed, it cannot be recognized on the screen.

An optical communication system which can suppress beat interference by using another method is disclosed in US Pat. No. 5,532,865. The system is configured such that an input optical signal and light from a light source are multiplexed, and the obtained optical signal is once branched and then output. One of the branched optical signals is converted into an electric signal, and it is detected whether or not the beat frequency is close to the subcarrier frequency. If so, the beat frequency is changed by changing the wavelength of the light source. To remove the effect of beat noise.

[0016]

However, in each of the above-mentioned conventional examples, the following problems remain.
That is, regarding the first problem, Japanese Patent Application Laid-Open No. 5-291
No. 671 and US Pat. No. 5,430,569, both
IM2 may occur in the transmission band and the transmission characteristics may be degraded. In US Pat. No. 5,373,385, although IM2 in the transmission band can be reduced, a predistortion compensator for that purpose is newly required. With respect to the distortion compensator, it is necessary to accurately adjust the distortion power and the phase amount for compensation, and this adjustment operation is not easy.

Regarding the second problem, the conventional method increases the price of the device by providing an expensive optical isolator. In other words, a distributed feedback semiconductor laser (DF
In general, an optical isolator is built in a (B-LD) module. Conventionally, an optical SCM transmission device often employs this expensive DFB-LD as a light source.
On the other hand, Fabry-Perot type semiconductor laser (FP-LD) modules are generally used for digital optical communication and are very inexpensive but do not include an optical isolator. If the second transmission characteristic degradation can be suppressed without providing an optical isolator, the price of the device can be significantly reduced.

Regarding the third problem, JP-A-6-17 / 1994
The method disclosed in Japanese Patent No. 7840 may reduce the influence of beat noise as long as a video signal is transmitted. However, for example, when the modulation signal is a digital modulation signal, instantaneous noise contamination causes a burst error of the digital signal. In that case, even if error correction is performed, burst errors may not be sufficiently corrected. Also, when transmitting a video signal, intermodulation distortion occurs between a frequency component that varies the wavelength of a main carrier, which is an optical signal, and a frequency component of a subcarrier that transmits the video signal.

In Japanese Patent Application Laid-Open No. Hei 6-177840, beat noise cannot always be sufficiently reduced. It is for the following reasons. That is, the effect of reducing the beat noise is due to the chirping characteristic of the semiconductor laser, and largely depends not only on the frequency of the additional signal applied to the semiconductor laser, but also on the degree of light modulation and the bias current value.
Therefore, in order to sufficiently suppress beat noise, it is necessary to integrate such parameters. But,
In JP-A-6-177840, the amount of reduction of the beat noise is not quantitatively evaluated in such a manner, and thus the beat noise cannot always be sufficiently reduced.

Further, in US Pat. No. 5,532,865, in order to change the wavelength of the light source, for example, it is necessary to change the temperature of the light source or change the bias current. If the bias current is changed, the degree of optical modulation, which is an important design parameter of the optical SCM transmission system, also changes, which may lead to deterioration of distortion characteristics other than beat interference and deterioration of C / N characteristics. On the other hand, in order to change the temperature, it is necessary to provide parts for that purpose. In addition to what has been said regarding the second problem, the FP-LD module generally does not have a temperature control function, and if it is used as a light source for optical SCM transmission, components for temperature control are required. Must be provided.

Therefore, the first object of the present invention is to provide the light S
An object of the present invention is to provide an optical transmission device capable of sufficiently suppressing transmission characteristic deterioration caused by reflected light when performing transmission by the CM system with a simple and inexpensive configuration. A second object of the present invention is to provide an optical transmission device that can sufficiently suppress transmission characteristic deterioration caused by reflected return light to a light source when performing transmission by the optical SCM system with a simple and inexpensive configuration. is there. Further, a third object of the present invention is to provide a point-to-point transmission in which optical signals transmitted from a plurality of slave stations are collectively received by the master station when performing a many-to-one transmission by the optical SCM method. It is an object of the present invention to provide an optical transmission system capable of sufficiently suppressing transmission characteristic deterioration caused by a simple and inexpensive configuration.

[0022]

Means for Solving the Problems and Effects of the Invention A first invention is an optical transmission device for converting an electric signal into an optical signal directly intensity-modulated by the signal and transmitting the signal, and outputs an additional signal. Oscillating means, synthesizing means for synthesizing an electric signal to be transmitted and an additional signal output by the oscillating means, a DC current source for outputting a DC bias current, and an electric signal and DC source obtained by synthesizing the synthesizing means A semiconductor laser that outputs an optical signal that is directly intensity-modulated with a signal obtained by combining a DC bias current output by the semiconductor laser, an optical transmission path for transmitting an optical signal output by the semiconductor laser, and an optical transmission path. Photoelectric conversion means for converting an optical signal transmitted through the optical signal into an electric signal, wherein the oscillating means transmits a signal higher than the frequency corresponding to the bandwidth of the frequency band allocated to the electric signal to be transmitted and transmits the signal. Power telegraph It is characterized by outputting a low frequency additional signal than half the lowest frequency of the frequency band allocated to.

As described above, according to the first aspect, the semiconductor laser is modulated by the additional signal. When the semiconductor laser is directly intensity-modulated, it is simultaneously frequency-modulated, so that the semiconductor laser is subjected to frequency modulation by an additional signal.
Since the spectrum of the frequency-modulated optical signal is expanded over a wide band, even if reflected light or reflected return light to the semiconductor laser is generated, deterioration of transmission characteristics due to the reflected light or reflected light is suppressed.
At this time, by setting the frequency range of the additional signal to be higher than the frequency corresponding to the bandwidth of the frequency band allocated to the electric signal to be transmitted, the transmission of the additional signal is temporarily performed due to the nonlinearity of the semiconductor laser or the like. Even if the secondary intermodulation distortion with the power signal should occur, the frequency at which the distortion occurs is outside the frequency band assigned to the electrical signal to be transmitted, and therefore the electrical signal to be transmitted due to the secondary intermodulation distortion. Will not be affected. Further, by setting the frequency range of the additional signal to be lower than half of the lowest frequency of the frequency band allocated to the electric signal to be transmitted, the additional signal may be temporarily reduced due to the nonlinearity of the semiconductor laser or the like.
Even if the second harmonic distortion occurs, the frequency at which the distortion occurs is out of the frequency band allocated to the electric signal to be transmitted, so that the second harmonic distortion does not affect the electric signal to be transmitted. . In addition, since transmission characteristic degradation caused by reflected return light can be suppressed, a semiconductor laser module without a built-in optical isolator can be used as a light source.

According to a second aspect, in the first aspect, the semiconductor laser is a Fabry-Perot semiconductor laser.

As described above, according to the second aspect of the invention, the use of a Fabry-Perot type semiconductor laser makes it possible to reduce the price of the apparatus.

According to a third aspect, in the second aspect, the additional signal output from the oscillating means is modulated by predetermined data.

As described above, according to the third aspect, for example, monitoring data can be transmitted by the additional signal.

In a fourth aspect based on the third aspect, the Fabry-Perot type semiconductor laser comprises an active layer having an optical signal amplifying function, and a narrower emission angle of the optical signal output from the active layer. It has a chip structure in which the spot size conversion means and the spot size conversion means are formed on the same substrate.

As described above, according to the fourth invention, the LD
The optical signal emitted from the chip can be efficiently coupled to the optical fiber.

In a fifth aspect based on the fourth aspect, the electric signal to be transmitted is a signal obtained by frequency division multiplexing one or more radio signals for mobile communication, and should be transmitted.
There are a plurality of electrical signals, and each of the electrical signals to be transmitted is assigned a unique frequency band,
Oscillating means outputs the broadest higher than the frequency corresponding to the bandwidth of the frequency band and lower frequency addition signal than half the lowest frequency among a plurality of specific frequency bands among a plurality of specific frequency bands It is characterized by doing.

As described above, according to the fifth aspect, high quality transmission of a radio signal for mobile communication in a plurality of bands can be performed.

In a sixth aspect based on the fifth aspect, the electric signal transmitted using at least one of the plurality of unique frequency bands is a code division multiplexed signal, and the oscillating means comprises a code division multiplexed signal. A frequency higher than the frequency corresponding to the widest frequency band among a plurality of unique frequency bands excluding the frequency band assigned to the multiplexed signal, and a plurality of unique
And outputting an additional signal having a frequency lower than half of the lowest frequency of the frequency band.

As described above, according to the sixth aspect, both the second-order intermodulation distortion and the second-order harmonic distortion occur in a band other than the frequency band allocated to the code-division multiplexed electric signal. Will not be done.

According to a seventh aspect, in the sixth aspect, the optical transmission line includes one or more optical fibers, and the emitting end face of the Fabry-Perot type semiconductor laser and the end face of the optical fiber coupled thereto are parallel to each other. It is characterized by being placed out of position.

As described above, according to the seventh aspect, the amount of reflected light returning to the semiconductor laser can be reduced, and as a result, the minimum level value required for the additional signal can be reduced.

According to an eighth aspect based on the seventh aspect, the apparatus further comprises an additional signal level adjusting means for adjusting the level of the additional signal output by the oscillation means, wherein the additional signal level adjusting means comprises an optical signal output by the semiconductor laser. Frequency modulation index β
Is a conditional expression β ≧ (2 / π) · 10 for reducing at least P decibels of noise or distortion generated in the optical transmission line.
It is characterized in that the level of the additional signal output by the oscillating means is adjusted so as to satisfy ^{P / 10} (where π is the pi).

As described above, according to the eighth aspect, distortion or noise caused by reflected light and reflected return light can be reduced by a desired amount.

In a ninth aspect based on the seventh aspect , the additional signal level adjusting means is arranged such that the frequency modulation index β is a conditional expression β ≧
It is characterized in that the level of the additional signal is adjusted so as to satisfy 1.7.

As described above, according to the ninth aspect, the noise or distortion caused by the reflected light and the reflected return light is at least Pmin (Pmin is the minimum value among the plurality of minimum values of the distortion reduction amount P). ) Can be reduced.

In a tenth aspect based on the seventh aspect, the degree of light modulation assigned to each of the plurality of electric signals to be transmitted and the additional signal obtained by level adjustment by the level adjusting means is mi (i = 1). ,..., N), is characterized in that the total light modulation degree {(mi) ² } does not exceed 0.3.

As described above, according to the tenth aspect, clipping distortion hardly occurs, and high quality transmission can be realized.

According to an eleventh aspect based on the third aspect, the Fabry-Perot type semiconductor laser has a chip structure in which an active layer having an optical signal amplifying action is tapered.

As described above, according to the eleventh aspect, L
The optical signal emitted from the D chip can be efficiently coupled to the optical fiber.

According to a twelfth aspect, in the eleventh aspect,
The electrical signal to be transmitted is a signal obtained by frequency division multiplexing one or more radio signals for mobile communications, transmission
There are a plurality of electric signals to be transmitted , and each of the electric signals to be transmitted is assigned a unique frequency band, and the oscillating means sets the frequency band to the widest one of the plurality of unique frequency bands. It is characterized in that an additional signal having a frequency higher than the corresponding frequency and lower than half of the lowest frequency among a plurality of unique frequency bands is output.

As described above, according to the twelfth aspect, high quality transmission of a radio signal for mobile communication in a plurality of bands can be performed.

According to a thirteenth aspect, in the twelfth aspect,
The electric signal transmitted using at least one frequency band among the plurality of unique frequency bands is a code division multiplexed signal, and the oscillating means includes a plurality of unique frequencies excluding the frequency band assigned to the code division multiplexed signal. higher than the frequency corresponding to the widest frequency band among the bands, and a plurality of solid
It is characterized by outputting a low frequency additional signal than half the lowest frequency of the frequency band of the organic.

As described above, according to the thirteenth aspect, both the second-order intermodulation distortion and the second-order harmonic distortion occur in bands other than the frequency band allocated to the code-division multiplexed electric signal. Will not be done.

According to a fourteenth aspect, in the thirteenth aspect,
The optical transmission path includes one or more optical fibers, and the emission end face of the Fabry-Perot type semiconductor laser and the end face of the optical fiber coupled thereto are set so as to be shifted from a parallel positional relationship with each other.

As described above, according to the fourteenth aspect, the amount of reflected light returning to the semiconductor laser can be reduced, and as a result, the minimum level value required for the additional signal can be reduced.

According to a fifteenth aspect, in the fourteenth aspect,
And an additional signal level adjusting means for adjusting the level of the additional signal output from the oscillation means, wherein the additional signal level adjusting means determines whether the frequency modulation index β of the optical signal output by the semiconductor laser is noise or distortion generated in the optical transmission line. Β ≧ (2 / π) · 10 ^{P / 10} for reducing P by at least P decibels
(However, π is the pi), the level of the additional signal output by the oscillation means is adjusted.

As described above, according to the fifteenth aspect, distortion or noise caused by reflected light and reflected return light can be reduced by a desired amount.

According to a sixteenth aspect, in the fourteenth aspect ,
The additional signal level adjusting means adjusts the level of the additional signal so that the frequency modulation index β satisfies the conditional expression β ≧ 1.7.

As described above, according to the sixteenth aspect, at least Pmin (Pmin is the minimum value among the plurality of minimum values of the distortion reduction amount P) ) Can be reduced.

According to a seventeenth aspect, in the fourteenth aspect,
When the light modulation degrees assigned to the plurality of electric signals to be transmitted and the additional signal obtained by level adjustment by the level adjustment means are mi (i = 1, 2,..., N), respectively, the total light modulation degree {(Mi) ² } is not more than 0.3.

As described above, according to the seventeenth aspect, clipping distortion hardly occurs, and high quality transmission can be realized.

An eighteenth aspect of the present invention is an optical transmission system for converting an electric signal into an optical signal directly intensity-modulated by the signal and transmitting the signal bidirectionally.
Device, a second device that transmits a second electrical signal, and an optical transmission line that interconnects the first device and the second device, wherein the first device includes a first device and a second device. An oscillating means for outputting an additional signal having a frequency lower than half of the lowest frequency of the frequency band allocated for transmission of the second electric signal; and a second synthesizing means for combining the first electric signal with the additional signal output from the oscillating means. 1, a first DC current source for outputting a DC bias current, and a signal obtained by combining the first combining means and a DC bias current output by the first DC current source. A first semiconductor laser that outputs an optical signal that is directly intensity-modulated with the obtained signal, and a first photoelectric conversion unit that converts the optical signal transmitted from the second device into an electric signal, The second device converts an optical signal transmitted from the first device into an electric signal A second photoelectric converting means, a band separating means for separating an electric signal obtained by the conversion by the second photoelectric converting means into a first electric signal and an additional signal output from the oscillating means, Second combining means for combining the additional signal obtained by the band separating means and the second electric signal,
A second DC current source for outputting a DC bias current;
A second semiconductor laser that outputs an optical signal that is directly intensity-modulated with a signal obtained by combining the signal obtained by the combining means and the DC bias current output by the second DC current source; Wherein the optical transmission line includes a first optical fiber for transmitting an optical signal output from the first semiconductor laser to the second device, and an optical signal output from the second semiconductor laser for the first optical fiber. A second optical fiber for transmission to the other device.

As described above, according to the eighteenth aspect, only when the oscillating means for outputting the additional signal is provided only in the first device and the signal is transmitted from the first device to the second device. However, even when a signal is transmitted from the second device to the first device, noise or distortion due to reflected light and reflected return light can be reduced.

The nineteenth invention is an optical transmission system for converting an electric signal into an optical signal directly intensity-modulated by the signal and transmitting the optical signal from a plurality of slave stations to a master station. A plurality of slave stations are classified into first to nth (where n
Is called an arbitrary even number of two or more slave stations, and among the plurality of slave stations, a second k (where k = 1, 2,..., N /
Each of the slave stations 2) includes an oscillating means for outputting an additional signal, a synthesizing means for synthesizing an electric signal to be transmitted by the slave station and the additional signal output from the oscillating means, and a DC current for outputting a DC bias current. And a semiconductor laser that outputs an optical signal that is directly intensity-modulated with a signal obtained by combining the signal obtained by the combining means and the DC bias current output by the DC current source, and Each of the 2k-1 slave stations directly outputs a DC current source that outputs a DC bias current, a signal obtained by combining an electric signal to be transmitted by the slave station and a DC bias current output by the DC current source. A semiconductor laser that outputs an intensity-modulated optical signal.

As described above, according to the nineteenth aspect, the semiconductor laser is modulated by the additional signal. When the semiconductor laser is directly intensity-modulated, it is simultaneously frequency-modulated, so that the semiconductor laser is subjected to frequency modulation by an additional signal.
Since the spectrum of the frequency-modulated optical signal is expanded over a wide band, even if reflected light or reflected return light to the semiconductor laser is generated, deterioration of transmission characteristics due to the reflected light or reflected light is suppressed.
In addition, even when the master station collectively receives optical signals transmitted from a plurality of slave stations and generates beat noise, deterioration of transmission characteristics due to the beat noise is suppressed. In addition, since the additional signal is added only to the even-numbered slave stations, the system configuration is simpler than the case where the additional signal is added to all the slave stations.

According to a twentieth aspect, in the nineteenth aspect,
Each semiconductor laser provided in the plurality of slave stations is a Fabry-Perot semiconductor laser.

As described above, according to the twentieth aspect, the price of the system can be reduced. Further, since the Fabry-Perot type semiconductor laser also has an effect of reducing beat noise due to multi-mode oscillation, the transmission characteristics are further improved accordingly.

According to a twenty-first aspect, in the twentieth aspect,
The connection form between the master station and the plurality of slave stations is a bus type.

As described above, according to the twenty-first aspect, since the number of optical transmission lines is apparently one, the optical transmission lines can be used more effectively than in the case of a tree-type connection mode.

A twenty-second invention is an optical transmission system for converting an electric signal into an optical signal directly intensity-modulated by the signal and transmitting the optical signal from a plurality of slave stations to a master station. A plurality of slave stations are classified into first to nth (where n
Is called an arbitrary odd number of three or more, and the second k (where k = 1, 2,..., (N−
Each of the slave stations 1) and 2) outputs oscillating means for outputting an additional signal, synthesizing means for synthesizing an electric signal to be transmitted by the slave station with the additional signal output from the oscillating means, and outputting a DC bias current. And a semiconductor laser that outputs an optical signal that is directly intensity-modulated with a signal obtained by synthesizing a signal obtained by synthesizing the synthesizing means and a DC bias current output by the DC current source. The 2k-1 slave stations were obtained by combining a DC current source for outputting a DC bias current, an electric signal to be transmitted by the slave station, and a DC bias current output from the DC current source. A semiconductor laser that outputs an optical signal directly intensity-modulated by a signal.

As described above, according to the twenty-second aspect, the semiconductor laser is modulated by the additional signal. When the semiconductor laser is directly intensity-modulated, it is simultaneously frequency-modulated, so that the semiconductor laser is subjected to frequency modulation by an additional signal.
Since the spectrum of the frequency-modulated optical signal is expanded over a wide band, even if reflected light or reflected return light to the semiconductor laser is generated, deterioration of transmission characteristics due to the reflected light or reflected light is suppressed.
In addition, even when the master station collectively receives optical signals transmitted from a plurality of slave stations and generates beat noise, deterioration of transmission characteristics due to the beat noise is suppressed. In addition, since the additional signal is added only to the even-numbered slave stations, the system configuration is simpler than the case where the additional signal is added to all the slave stations. Furthermore, since the total number of slave stations is odd, the system configuration is simpler than adding an additional signal only at odd-numbered slave stations.

According to a twenty-third aspect, in the twenty-second aspect,
Each semiconductor laser provided in the plurality of slave stations is a Fabry-Perot semiconductor laser.

As described above, according to the twenty-third aspect, the price of the system can be reduced. Further, since the Fabry-Perot type semiconductor laser also has an effect of reducing beat noise due to multi-mode oscillation, the transmission characteristics are further improved accordingly.

According to a twenty-fourth aspect, in the twenty-third aspect,
The connection form between the master station and the plurality of slave stations is a bus type.

As described above, according to the twenty-fourth aspect, since the number of optical transmission lines is apparently one, the optical transmission lines can be used more effectively than in the case of the tree-type connection mode.

A twenty-fifth invention is an optical transmission apparatus for converting an electric signal into an optical signal directly modulated in intensity by the signal and transmitting the electric signal from a plurality of slave stations to a master station via an optical transmission path, Oscillating means for outputting the additional signal,
Synthesizing means for synthesizing an electric signal to be transmitted by the slave station and an additional signal output by the oscillating means; a DC current source for outputting a DC bias current; a signal obtained by synthesizing the synthesizing means and a DC current source And a semiconductor laser that outputs an optical signal that is directly intensity-modulated with a signal obtained by combining the output DC bias current with the DC bias current.

As described above, according to the twenty-fifth aspect, the semiconductor laser is modulated by the additional signal. When the semiconductor laser is directly intensity-modulated, it is simultaneously frequency-modulated, so that the semiconductor laser is subjected to frequency modulation by an additional signal.
Since the spectrum of the frequency-modulated optical signal is expanded over a wide band, even if reflected light or reflected return light to the semiconductor laser is generated, deterioration of transmission characteristics due to the reflected light or reflected light is suppressed.
In addition, even if the master station collectively receives optical signals transmitted from a plurality of slave stations and generates beat noise, deterioration of transmission characteristics due to the beat noise is suppressed.

According to a twenty-sixth invention, in the twenty-fifth invention,
Each semiconductor laser provided in the plurality of slave stations is a Fabry-Perot semiconductor laser.

As described above, according to the twenty-sixth aspect, the price of the system can be reduced. Further, since the Fabry-Perot type semiconductor laser also has an effect of reducing beat noise due to multi-mode oscillation, the transmission characteristics are further improved accordingly.

The twenty-seventh invention is the twenty-sixth invention, wherein
The connection form between the master station and the plurality of slave stations is a bus type.

As described above, according to the twenty-seventh aspect, since the number of optical transmission lines is apparently one, the optical transmission lines can be used more effectively than in the case of a tree-type connection mode.

According to a twenty-eighth aspect, in the twenty-seventh aspect,
Each oscillator provided in the plurality of slave stations is characterized by outputting additional signals having different frequencies.

As described above, according to the twenty-eighth aspect, by judging the frequency of the additional signal, the master station can know which slave station the signal is from.

According to a twenty-ninth aspect, in the twenty-eighth aspect,
The additional signal output from each of the oscillating means is modulated by data.

As described above, according to the twenty-ninth aspect, for example, data for monitoring can be transmitted by the additional signal.

A thirtieth aspect of the present invention is based on the twenty-ninth aspect,
The master station converts the optical signals transmitted from the plurality of slave stations into electrical signals in order to detect a system failure, and converts the electrical signals obtained by the photoelectric converter into electrical signals. Separating means for separating an electric signal to be transmitted and an additional signal, and signal detecting means for detecting, from the additional signal obtained by separating the separating means, additional signals output by each of the oscillating means of the plurality of slave stations, respectively And further comprising:

As described above, according to the thirtieth aspect, the master station can successively estimate the failure of the slave station, the location where the optical transmission line is cut, and the like.

According to a thirty-first aspect, in the thirtieth aspect,
The electrical signal to be transmitted is a signal obtained by frequency division multiplexing one or more radio signals for mobile communications, transmission
There are a plurality of electric signals to be transmitted , each of the electric signals to be transmitted is assigned a unique frequency band, and each of the oscillating means has a bandwidth of a widest frequency band among the plurality of unique frequency bands. Higher than the frequency corresponding to
Further, an additional signal having a frequency lower than half of the lowest frequency among a plurality of unique frequency bands is output.

As described above, according to the thirty-first aspect, high quality transmission of a radio signal for mobile communication in a plurality of bands can be performed.

According to a thirty-second aspect, in the thirty-first aspect,
An electric signal transmitted using at least one frequency band among the plurality of unique frequency bands is a code division multiplexed signal, and each of the oscillating units includes a plurality of unique frequency bands other than the frequency band assigned to the code division multiplexed signal . Higher than the frequency corresponding to the widest frequency band among the frequency bands, and
It is characterized in that an additional signal having a frequency lower than half of the lowest frequency of the unique frequency band is output.

As described above, according to the thirty-second aspect, both the second-order intermodulation distortion and the second-order harmonic distortion occur in a band other than the frequency band allocated to the code-division multiplexed electric signal. Will not be done.

A thirty-third invention is a method according to the thirty-first invention, wherein
Each Fabry-Perot type semiconductor laser has an active layer having an optical signal amplifying action, and a spot size conversion means for narrowing a radiation angle of an optical signal output from the active layer formed on the same substrate. It has a chip structure.

As described above, according to the thirty-third aspect, L
The optical signal emitted from the D chip can be efficiently coupled to the optical fiber.

A thirty-fourth aspect is the invention according to the thirty-third aspect,
Each of the plurality of slave stations further includes additional signal level adjusting means for adjusting the level of the additional signal output from each oscillation means, and each additional signal level adjusting means includes a frequency modulation index β of the optical signal output from each semiconductor laser. Is a conditional expression β <1 / (2) for making the beat noise Q times (where Q <1).
It is characterized in that the level of the additional signal output by each oscillation means is adjusted so that the value satisfies Q).

As described above, according to the thirty-fourth aspect, a desired amount of beat noise can be reduced.

The thirty-fifth invention is the thirty-third invention, wherein
Each of the plurality of slave stations further includes additional signal level adjusting means for adjusting the level of the additional signal output from each oscillation means, and each additional signal level adjusting means includes a frequency modulation index β of the optical signal output from each semiconductor laser. Is a conditional expression β <1 / (2) for making the beat noise Q times (where Q <1).
Q), and a conditional expression β ≧ (2 / π) · 1 for reducing at least P decibels of noise or distortion generated in the optical transmission line.
It is characterized in that the level of the additional signal output from each oscillation means is adjusted so that the value satisfies all of 0 ^{P / 10} (where π is the pi).

As described above, according to the thirty-fifth aspect, beat noise and distortion or noise caused by reflected light and reflected return light can be reduced by a desired amount.

[0092] A thirty-sixth invention is a method according to the thirty-fifth invention, wherein
The optical transmission path includes one or more optical fibers, and the emission end face of each Fabry-Perot type semiconductor laser and the end face of the optical fiber coupled thereto are set so as to be shifted from a positional relationship parallel to each other.

As described above, according to the thirty-sixth aspect, the amount of reflected light returning to the semiconductor laser can be reduced, and as a result, the minimum level value required for the additional signal can be reduced.

A thirty-seventh invention is based on the thirty-fifth invention, wherein:
When the degree of optical modulation assigned to each electric signal to be transmitted and the additional signal obtained by level adjustment by each additional signal level adjusting means is mi (i = 1, 2,..., N), the total light The modulation degree {(mi) ² } does not exceed 0.3.

As described above, according to the thirty-seventh aspect, clipping distortion hardly occurs, and high quality transmission can be realized.

A thirty-eighth aspect of the present invention is the thirty-first aspect, wherein:
Each Fabry-Perot type semiconductor laser is characterized in that it has a chip structure in which the active layer having the function of amplifying an optical signal is tapered.

As described above, according to the thirty-eighth aspect, L
The optical signal emitted from the D chip can be efficiently coupled to the optical fiber.

A thirty-ninth aspect is the thirty-eighth aspect, wherein
Each of the plurality of slave stations further includes additional signal level adjusting means for adjusting the level of the additional signal output from each oscillation means, and each additional signal level adjusting means includes a frequency modulation index β of the optical signal output from each semiconductor laser. Is a conditional expression β <1 / (2) for making the beat noise Q times (where Q <1).
It is characterized in that the level of the additional signal output by each oscillation means is adjusted so that the value satisfies Q).

As described above, according to the thirty-ninth aspect, a desired amount of beat noise can be reduced.

A fortieth aspect of the present invention is the thirty-eighth aspect of the present invention,
Each of the plurality of slave stations further includes additional signal level adjusting means for adjusting the level of the additional signal output from each oscillation means, and each additional signal level adjusting means includes a frequency modulation index β of the optical signal output from each semiconductor laser. Is a conditional expression β <1 / (2) for making the beat noise Q times (where Q <1).
Q), and a conditional expression β ≧ (2 / π) · 1 for reducing at least P decibels of noise or distortion generated in the optical transmission line.
It is characterized in that the level of the additional signal output from each oscillation means is adjusted so that the value satisfies all of 0 ^{P / 10} (where π is the pi).

As described above, according to the fortieth aspect, a desired amount of distortion or noise caused by beat noise and reflected light and reflected return light can be reduced.

A forty-first invention is based on the fortieth invention, wherein
The optical transmission path includes one or more optical fibers, and the emission end face of each Fabry-Perot type semiconductor laser and the end face of the optical fiber coupled thereto are set so as to be shifted from a positional relationship parallel to each other.

As described above, according to the forty-first aspect, the amount of reflected light returning to the semiconductor laser can be reduced, and as a result, the minimum level value required for the additional signal can be reduced.

[0104] A forty-second aspect is based on the fortieth aspect, wherein:
When the degree of optical modulation assigned to each electric signal to be transmitted and the additional signal obtained by level adjustment by each additional signal level adjusting means is mi (i = 1, 2,..., N), the total light The modulation degree {(mi) ² } does not exceed 0.3.

As described above, according to the forty-second aspect, clipping distortion hardly occurs, and high quality transmission can be realized.

A forty-third invention is directed to the thirtieth invention, wherein:
Each of the Fabry-Perot type semiconductor lasers is characterized in that the center wavelengths of optical signals output from the respective lasers are separated from each other by a predetermined wavelength interval.

As described above, according to the forty-third aspect, the beat noise that causes the deterioration of the transmission characteristics is only the beat noise between the side modes of the FP-LD. Deterioration of transmission characteristics is smaller than in a certain case.

[0108] A forty-fourth invention is characterized in that, in the forty-third invention,
It is characterized in that the number of slave stations connected to one optical fiber is up to three.

As described above, according to the forty-fourth aspect, F
When selecting the wavelength of the P-LD, three wavelength regions may be secured. In this case, the wavelength selection can be easily performed since the region is limited to only one side except the center wavelength region.

[0110]

Embodiments of the present invention will be described below with reference to the drawings. (First Embodiment) FIG. 1 is a block diagram showing a configuration of an optical transmission device according to a first embodiment of the present invention. 1 includes an electric signal level adjusting units 101 and 109, an oscillating unit 102, an additional signal level adjusting unit 103, and a combining unit 10.
4, a semiconductor laser 105, a DC current source 106, an optical fiber 107, and a photoelectric conversion unit 108 are provided.

Electric signal level adjusting units 101 and 109
Respectively adjust the level of the electrical signal. Oscillator 10
2 outputs an additional signal (here, a sine wave). Additional signal level adjustment section 103 adjusts the level of the additional signal. The combining unit 104 combines the electric signal and the additional signal. The DC current source 106 outputs a DC bias current for adding a DC bias current component to the electric signal.
The semiconductor laser 105 outputs an optical signal that is directly intensity-modulated by an electric signal and a DC bias current. Optical fiber 1
07 transmits an optical signal. The photoelectric conversion unit 108 converts an optical signal into an electric signal.

Hereinafter, the operation of the apparatus shown in FIG. 1 for transmitting an electric signal will be described. Here, it is assumed that the electric signal to be transmitted is a signal obtained by multiplexing a predetermined signal, particularly a signal obtained by frequency division multiplexing a wireless signal for mobile communication. The input electric signal is adjusted to a predetermined level in the electric signal level adjustment unit 101. On the other hand, the additional signal output from the oscillator 102 is adjusted to a predetermined level by the additional signal level adjuster 103. The combining unit 104 combines the electric signal and the additional signal whose levels have been adjusted. When the DC current source 106 outputs a DC bias current for adding a predetermined DC bias component to the electric signal to be transmitted, the semiconductor laser 105 combines the DC bias current output from the DC current source 106 with the combining unit 1.
04 outputs an optical signal that is directly intensity-modulated with the electric signal obtained by synthesis. The output optical signal is transmitted to the receiving side via the optical fiber 107. The transmitted optical signal is converted into an electric signal by an optical-electrical conversion unit 108, further adjusted to a predetermined level by an electric signal level adjustment unit 109, and then output.

In the above operation, when an optical signal is transmitted to the receiving side, reflection occurs at the connection surface of the optical fiber 107, the end surface of the semiconductor laser 105 or the photoelectric conversion unit 108, and reflected light is generated. Further, reflected light is also generated due to Rayleigh scattering in the optical fiber 107 or the like. Further, when the reflected light is re-reflected and re-reflected, multiple reflected light is also generated. Noise or distortion due to reflected light caused by these various causes deteriorates the transmission characteristics of the device in FIG. 1 when only the signal input to the electric signal level adjustment unit 101 is transmitted. However, in the apparatus of FIG. 1, the additional signal level adjuster 103 adjusts the level of the additional signal output from the oscillator 102 to reduce noise or distortion due to reflected light. The details will be described below.

The optical signal output from the semiconductor laser 105 is
It is frequency-modulated simultaneously with direct intensity modulation. The frequency modulation index is generally represented by the ratio between the maximum frequency shift and the modulation frequency. The frequency modulation index β due to the additional signal output from the oscillator 102 indicates the frequency of the additional signal.
f, the DC bias current output from the DC current source 106 is I
b, given by the following equation (1), where Ith is the emission threshold current of the semiconductor laser 105, m is the light modulation degree, and dF / dI is the frequency modulation efficiency. β = {m · (Ib−Ith) · (dF / dI)} / f (1)

On the other hand, the reduction amount P (unit is decibel, hereinafter, dB) of noise or distortion due to reflected light generated during optical transmission is:
Using the zero-order Bessel function J ₀ , it can be expressed by the following equation (2). P = 10 · log {J ₀ (β) ² } (2)

FIG. 2 is a graph showing the relationship between the frequency modulation index β and the noise or distortion reduction amount PdB due to reflected light generated during optical transmission. In FIG. 2, the horizontal axis is logarithmic. As is clear from FIG. 2, the reduction amount P is β
In <1, the value is at most several decibels, while in β> 1, the value repeatedly increases and decreases greatly as β changes. Have a characteristic that they monotonically increase as the frequency modulation index β increases (that is, Pmin1 <Pmin2 <...).

Further, the minimum values Pmin1, Pmin2,.
Are arranged on a straight line as represented by a dotted line in FIG. We consider this straight line to be approximated by the following equation (3) by considering the amount of dispersion of the power spectrum due to the frequency modulation of the oscillation frequency of the laser beam and the limit of the zero-order Bessel function J0. That was newly derived. P = 10 · log (πβ / 2) (3)

From the above expression (3), a conditional expression for at least reducing the noise or distortion due to the reflected light by PdB, that is, the following expression (4) is obtained. β = (2 / π) · 10 ^{P / 10} … (4)

In the above equation (1), the emission threshold Ith and the frequency modulation efficiency dF / dI are
And the degree of light modulation m is determined by the level of the additional signal. Therefore, if the frequency of the additional signal and the DC bias current are set to values in appropriate ranges, the frequency modulation index β can be adjusted by adjusting the level of the additional signal.
Can be set to a value that satisfies the above equation (4). That is, the additional signal level adjustment unit 103 sets the oscillation unit 10 such that the frequency modulation index β satisfies the above expression (4).
By adjusting the level of the additional signal output from 2, the noise or distortion due to the reflected light can be reduced by at least PdB.

As can be seen from FIG. 2, β = 1.7.
Is the minimum β at which a reduction amount equal to the first minimum value Pmin1 can be obtained. Therefore, the additional signal level adjusting unit 103 sets the oscillation unit 1 to a value that satisfies β ≧ 1.7.
02, the noise or distortion due to the reflected light is reduced by at least Pmin1d.
B can be reduced.

As is apparent from the above equation (1), the bias current may be adjusted instead of adjusting the level of the additional signal. Alternatively, the level of the additional signal and the bias current may be adjusted simultaneously.

Here, in general, when an electric signal obtained by frequency-multiplexing a plurality of signals is converted into an optical signal directly intensity-modulated by the electric signal and transmitted, the number of signals to be multiplexed is set to N, Assuming that the light modulation degree assigned to the signal is m ₁ and the light modulation degree assigned to the additional signal is m ₂ , the total light modulation degree is represented by {(m ₁ ) ² · N + (m ₂ ) ² }. .
This total light modulation degree needs to be set to a certain value or less due to the limitation of the distortion (third-order intermodulation distortion) characteristic generated during the electric-to-optical conversion in the semiconductor laser 105. For example, CAT
In the case of V signal transmission, the value is generally said to be 0.45.

However, in the case of transmitting a mobile phone signal, since ultra-low distortion characteristics (about 70 dB depending on the application location) are required, the total optical modulation degree needs to be 0.3 or less.
Hereinafter, the reason will be described with reference to FIG. FIG. 3 shows the overall optical modulation factor dependency of the third-order intermodulation distortion generated during the electro-optical conversion in the semiconductor laser 105 (FIG. 3 shows the average value and the worst value in the case of 32 and 64 carriers. This will be described later). FIG. 3 shows that the third-order intermodulation distortion sharply increases when the total light modulation degree is 0.3 or more. This is the semiconductor laser 105
This is a phenomenon that has occurred because the electric signal input to the semiconductor laser 105 has become equal to or less than the threshold current of the semiconductor laser 105. This sharply increasing distortion is called clipping distortion and is a factor that greatly degrades transmission characteristics. It is generally known that clipping distortion causes a burst error when transmitting a digitally modulated signal, so that even if an error correction code is used, the code error rate cannot be reduced. By setting the total optical modulation factor to 0.3 or less, the above-described clipping distortion can hardly occur, and high-quality optical transmission can be realized.

Also, as β increases, the degree of light modulation m ₂ assigned to the additional signal increases, so that the degree of light modulation that can be assigned to the plurality of signals decreases accordingly.
Signal transmission quality is degraded. From this, when attention is paid to the transmission quality of a signal, it can be said that β is preferably as small as possible. That is, the additional signal level adjuster 103 adjusts the level of the additional signal output from the oscillator 102 so that the frequency modulation index β satisfies the conditional expression β ≧ 1.7, so that noise due to reflected light or noise It can be seen that the effect of reducing the distortion and balancing the signal transmission quality is also obtained.

As described above, by adding the additional signal having the above level to the electric signal to be transmitted, noise or distortion due to reflected light can be reduced by at least a predetermined amount. However, on the other hand, a second-order intermodulation distortion (IM2) between the additional signal and the electric signal to be transmitted is newly generated, which deteriorates transmission characteristics. Therefore, the operation of the apparatus of FIG. 1 for suppressing IM2 will be described below.

If the frequency of the additional signal output from the oscillating unit 102 is lower than the frequency corresponding to the bandwidth of the frequency band of the electric signal to be transmitted, IM2 is generated in the transmission frequency band of the electric signal. Further, when the frequency of the additional signal output from the oscillation unit 102 is equal to or more than の of the lowest frequency of the transmission frequency band of the electric signal to be transmitted, the largest second harmonic distortion among the harmonic distortions of the electric signal ( Hereafter, HD)
Occur in the transmission frequency band.

In addition, considering that the frequency modulation index β can be increased as the frequency of the additional signal output from the oscillating unit 102 decreases, the frequency of the additional signal is changed to the bandwidth of the transmission frequency band of the electric signal to be transmitted. By setting the frequency higher than the frequency corresponding to and lower than 最低 of the lowest frequency of the transmission frequency band of the electric signal to be transmitted, neither IM2 nor HD can be generated in the transmission frequency band.

However, when the electric signal to be transmitted is a radio signal for mobile communication, there is a concern that IM2 or HD outside the transmission frequency band of the electric signal may be radiated into space and become unnecessary radiation. However, this can be solved by selecting only signals within the transmission frequency band with a bandpass filter.

For example, it is assumed that an electric signal to be transmitted is a signal obtained by frequency multiplexing a radio signal for digital mobile telephone in Japan. The transmission frequency band of digital mobile phones in Japan is 800 MHz
There are two bands, a band and a 1.5 GHz band. 8 of these
Consider a case where optical transmission is performed using only the 00 MHz band.
In this case, the transmission frequency band of the downlink signal is 810 to 830.
MHz, the bandwidth of the transmission frequency band is 20 MHz.
z, 1/2 of the lowest frequency of the transmission frequency band is 405 MH
z. Therefore, 405 MHz larger than 20 MHz
The oscillating unit 10 can obtain an additional signal having a smaller frequency.
By setting the transmission frequency of No. 2, neither IM2 nor HD occurs in the transmission frequency band.

Considering that the frequency modulation efficiency is inversely proportional to the modulation frequency, the frequency of the additional signal is preferably as low as possible. That is, in the above example, the frequency of the additional signal is preferably a value close to 20 MHz. However, when only signals within the transmission frequency band are transmitted by the band-pass filter and unnecessary radiation outside the band is cut off, if the IM2 is generated in the vicinity of the transmission frequency band, the band-pass filter can sufficiently reduce the level of IM2. May not be able to lower. However, this can be solved by setting the frequency of the additional signal slightly higher than 20 MHz and moving the IM2 frequency away from the maximum frequency of the transmission frequency band.

Here, the main points of the above description will be summarized with reference to the example of the above-mentioned portable telephone. For example, if the frequency of the additional signal output from the oscillating unit 102 is about 22 MHz, the frequency is higher than the bandwidth of the transmission frequency band of 20 MHz and lower than １ ／ of the lowest frequency 810 MHz of the transmission frequency band. Neither IM2 nor HD occurs in the band. Regarding noise or distortion due to reflected light, the DC bias current output from the DC current source 106 is 40 mA, the threshold current of the semiconductor laser 105 is 10 mA, and the frequency modulation efficiency is 220 MHz / m.
In the case of A, if the degree of light modulation m2 is 0.57% or more, the frequency modulation index β becomes a value satisfying the conditional expression β ≧ 1.7. Therefore, by adjusting the level of the additional signal so that the light modulation degree becomes 0.57% or more, noise or distortion due to reflected light can be reduced by at least Pmin1 dB (about 6 dB in FIG. 2).

Further, in the 800 MHz band and 1.5 GHz
Consider a case where a signal is transmitted using two bands.
In this case, the wider bandwidth is in the 1.5 GHz band, which has a bandwidth of 36 MHz. Therefore, the frequency of the additional signal output from the oscillator 102 is set to 3
What is necessary is just to make it larger than 6 MHz (for example, about 40 MHz). In this case, in order to satisfy β ≧ 1.7 under the same conditions as described above, the light modulation degree may be set to 1.03% or more. As described above, transmission in a plurality of bands can be performed while maintaining the same effect as transmission in one band. The transmission frequency band actually allocated to each mobile phone service provider has a narrower bandwidth, but when transmitting a wireless signal through such a narrow transmission frequency band, the bandwidth is reduced. The oscillation frequency and the degree of light modulation may be set based on this.

By the way, in an analog optical transmission device using a semiconductor laser, in addition to the noise or distortion due to the reflected light described above (noise or distortion caused directly by the reflected light), the analog light transmission device generally enters the laser chip. The influence of reflected light (hereinafter, reflected return light) may be a problem. Although the effect of the reflected return light has not been completely elucidated, it is known that, for example, laser oscillation becomes unstable and mode hopping occurs. We have experimentally confirmed that when a Fabry-Perot type semiconductor laser (hereinafter, FP-LD) is used, mode hopping occurs due to reflected return light generated by Rayleigh scattering, and as a result, noise or distortion increases. .

As described in the section of the prior art, semiconductor lasers used for optical transmission are mainly distributed feedback semiconductor lasers (hereinafter, DFB-LD) and FP-LDs.
It is. Among them, D used for analog optical transmission
FB-LD. The DFB-LD module has a built-in optical isolator, and this optical isolator reduces the amount of reflected light that enters the laser chip to 1 / 10,000 or less. For this reason, in the conventional analog optical transmission device provided with the DFB-LD, the price of the DFB-LD is about 10 times the price of the FP-LD instead of being substantially unaffected by the reflected return light. Was expensive.

On the other hand, the FP-LD is generally used for optical digital transmission, and does not have an optical isolator. For this reason, although FP-LD is inexpensive,
Conventionally, it has not been used for analog optical transmission. However, the apparatus of FIG. 1 can reduce noise or distortion due to reflected return light in the same manner as noise or distortion due to reflected light is reduced. That is, in the apparatus shown in FIG. 1, the noise or distortion due to reflected light generated during transmission is guaranteed to be reduced by at least a predetermined amount, and the noise or distortion due to reflected return light is also reduced. An FP-LD can be employed as 105, and as a result, the price of the apparatus can be significantly reduced.

The signal output from the oscillating section 102 is not limited to a sine wave, but may be a signal modulated by analog modulation / digital modulation. In this case, monitoring data and the like can be transmitted by the additional signal.

In recent years, FP-LDs capable of narrowing the emission angle of an optical signal have been developed in order to efficiently couple an optical signal emitted from an LD chip to an optical fiber. This type of FP-LD is roughly classified into two types according to its chip structure. One is a type in which a tapered waveguide for narrowing the radiation angle is formed on the same substrate as the active layer outside the active layer of the conventional structure.
The other is a type in which the active layer itself is formed in a tapered shape. It is expected that the optical power coupled to the optical fiber of each type of FP-LD is about two to three times that of the conventional type. However, on the contrary, it also means that the reflected return light from the optical fiber transmission line is easily coupled to the LD chip. Therefore, in an FP-LD capable of narrowing the emission angle of an optical signal as described above, it is essential to add an additional signal as described above. In this case, the amount of reflected return light can be reduced by displacing the exit end face of the FP-LD chip and the end face of the optical fiber coupled thereto from a positional relationship parallel to each other, and as a result, an additional signal is obtained. The minimum level can be lower.

A plurality of continuous transmission frequency bands (for example, the 800 MHz band and 1.5 GHz
Let us consider a case where one of the bands is assigned for transmission of a code division multiplexed signal. In the transmission of a code division multiplexed signal, signals assigned to each code are usually transmitted with substantially the same power, so that a wide dynamic range characteristic is not required. Therefore, the frequency of the additional signal is higher than the frequency corresponding to the bandwidth of the widest frequency band among a plurality of continuous frequency bands other than the frequency band assigned to the code division multiplexed signal, and the By setting it lower than half of the lowest frequency,
Both IM2 and HD can be prevented from occurring in a plurality of continuous frequency bands other than the frequency band allocated to the code division multiplexed signal.

It is known that when a large number of frequency-division multiplexed signals or code division multiplexed signals are optically transmitted collectively, distortion due to clipping in a semiconductor laser occurs. This clipping distortion has already been described with reference to FIG. FIG. 3 shows an optical modulation factor mi = m assigned to each carrier component and a composite triple distortion (Composite Triple Beam) which is one of the third-order intermodulation distortions.
t, hereinafter, CTB) is shown. We have a total of 3 to perform 16 and 32 carrier transmissions in the 800 MHz band and 1.5 GHz band, respectively.
The average value and the maximum hold value (maximum value) of CTB were measured for the cases of 2 and 64 carriers. The measurement frequency is in the 800 MHz band. In FIG. 3, when the total optical modulation degree {(mi) ² } exceeds 0.3, both the average value and the maximum value of CTB are independent of the number of transmission carriers.
It turns out that it deteriorates greatly. From this measurement result, in order to maintain low distortion characteristics without being affected by distortion due to clipping, conditional expression {(mi) ² } is applied to each carrier component.
It can be seen that the light modulation degree mi satisfying <0.3 may be assigned.

FIGS. 4 to 11 show the manner in which the effect of the present invention on the above-mentioned total light modulation is actually confirmed. 4 and 5 show the results of measuring the temperature dependence of the third-order intermodulation distortion (IM3). FIG. 4 shows the case without an additional signal, and FIG. 5 shows the case with an additional signal. 6 and 7 show the results of measuring the temperature dependence of noise. FIG. 6 shows the case where there is an additional signal, and FIG. 7 shows the case where there is no additional signal. 4 to 7, an FP-LD is used as a light source, and a frequency of 1485.9 MHz and 149
A 2.1 MHz sine wave (optical modulation factor 10%) is used, and the additional signal has a 25.8 MHz frequency sine wave (optical modulation factor 1).
0%) was used. The length of the optical fiber is 2 km.
The measured frequency of IM3 is 1479.7 MHz, and the frequency of noise is 1489 MHz.

4 and 5, when there is no additional signal, 20 to 21 degrees, 22 to 24.5 degrees, and 25 to 27.5 degrees
Degrees, IM3 greatly increases around 28.5-29 degrees,
It can be seen that the IM3 characteristic has deteriorated to about -45 dB. On the other hand, when there is an additional signal, IM3 is about -85 dB or less at any temperature, and it can be seen that the IM3 characteristic is improved by about 40 dB compared to the case where there is no additional signal.

6 and 7, when there is no additional signal, 20 to 21 degrees, 22 to 24.5 degrees, and 25 to 27.5 degrees
It can be seen that the noise greatly increases around 28.5 to 29 degrees. The vertical axis is the relative intensity noise (RIN),
This RIN increases to about -137 dB / Hz around each of the above temperatures. On the other hand, if there is an additional signal,
At any temperature, the RIN is about -152 dB / Hz or less, and it can be seen that the noise characteristic is improved by about 15 dB as compared with the case where there is no additional signal.

It is considered that the cause of the deterioration of the IM3 characteristics and the noise characteristics is mainly Rayleigh scattered light generated in the optical fiber, but it cannot explain the fact that IM3 and RIN have temperature dependence. So we have IM3
And RIN at around 25 to 27.5 degrees where the deterioration is large, and at around 21 to 22 degrees where there is almost no deterioration, the FP-
The spectral line width of LD was measured by the self-delay homodyne method. The measurement results are shown in FIGS. FIG.
Is the spectrum of FP-LD around 25-27.5 degrees,
FIG. 9 is a spectrum of the FP-LD around 21 to 22 degrees. 8 and 9 show the average value and the max hold value.

In FIGS. 8 and 9, the spectrum at the temperature where the deterioration is large (FIG. 8) has a 3 dB bandwidth and a spectral line width of 1 MHz or less. Note that 7
The spurious around 4 MHz is due to the influence of the reflected light from the optical connector at the tip of the pigtail fiber of the FP-LD. On the other hand, the spectrum at the temperature where there is almost no deterioration (the one in FIG. 9) is 3 dB.
It can be seen that the spectral line width is considerably wide at about 236 MHz in the bandwidth. Considering also that the spectral line width indicates the coherence of the optical signal, when the spectral line width is narrow, reflected return light due to Rayleigh scattering is LD.
IM3 and RIN characteristics are degraded due to interference with the optical signal within the optical path, but when the spectral line width is wide, even if there is reflected return light due to Rayleigh scattering, the coherence of the optical signal is low,
It is considered that M3 and RIN characteristics hardly deteriorate. This experimentally demonstrated that IM3 and RIN have temperature dependence.

Further, when a 2-km optical fiber is connected to the FP-LD, the FP-L
The spectral line width of D was measured. FIG. 10 shows the measurement results.
And 11. FIG. 10 shows the F around 25 to 27.5 degrees when a 2 km fiber is connected to the FP-LD.
FIG. 11 shows the spectrum of P-LD at 2 km.
Around 21 to 22 degrees when the fiber of
It is a spectrum of P-LD. 10 and 11 show:
The average value and the max hold value are shown. FIG.
10 and 11, it can be seen that the spectrum near 25 to 27.5 degrees where both IM3 and RIN greatly deteriorate (FIG. 10) is very unstable due to the influence of reflected return light. On the other hand, there is almost no deterioration 21 to 2
It can be seen that the spectrum around 2 degrees (FIG. 11) is almost the same as the spectrum without the optical fiber connected (FIG. 9) and is stable. Thus, we can see that expanding the spectrum of an optical signal by adding an additional signal can improve the IM3 characteristics and RIN
It was confirmed that the effect of suppressing the deterioration of the characteristics was also obtained.

(Second Embodiment) FIG. 12 is a block diagram showing a configuration of an optical transmission system according to a second embodiment of the present invention. The system shown in FIG.
Base station device 21 and optical fibers 207 and 216
It has. The center-side device 20 includes a first electric signal level adjustment unit 201, an oscillation unit 202, an additional signal level adjustment unit 203, a first synthesis unit 204, and a first semiconductor laser 2.
05, first DC current source 206, first photoelectric conversion unit 2
17 and a second electric signal level adjuster 218.
The base station device 21 includes a second optical-electrical conversion unit 208, a third electric signal level adjustment unit 209, a fourth electric signal level adjustment unit 210, a band separation unit 211, a fifth electric signal level adjustment unit 212, It includes a second synthesizing unit 213, a second semiconductor laser 214, and a second DC current source 215.

In FIG. 12, the first radio signal to be transmitted from the center device 20 to the base station device 21 is the first radio signal.
, And is adjusted to a predetermined level there. Further, a sine wave (additional signal) is output from the oscillation unit 202 and the additional signal level adjustment unit 203
Is adjusted to a predetermined level. Outputs from the first electric signal level adjustment unit 201 and the additional signal level adjustment unit 203 are combined by a first combining unit 204 by frequency division multiplexing. Then, the signal output from the first synthesis unit 204 and the DC bias current output from the first DC current source 206 directly modulate the intensity of the first semiconductor laser 205. The modulated optical signal output from the first semiconductor laser 205 is transmitted to the base station device 21 via the optical fiber 207. The transmitted optical signal is converted into an electric signal by the second opto-electrical converter 208 and then converted into an electric signal.
At 11, the radio signal and the additional signal are separated, and the radio signal is supplied to the third electric signal level adjusting unit 209 and the additional signal is supplied to the fifth electric signal level adjusting unit 212. The wireless signal provided to the third electric signal level adjusting unit 209 is adjusted to a predetermined level there, and is output as a first wireless signal.

The transmission operation from the center device 20 to the base station device 21 in the system shown in FIG. In this case, the level of the additional signal from the oscillating unit 202 was adjusted by the additional signal level adjusting unit 203 so that the frequency modulation index of the optical signal output from the first semiconductor laser 205 due to the additional signal was 1.7 or more. After that, by performing frequency division multiplexing with the first wireless signal in the first combining unit 204, the amount of noise or distortion generated by converting the wavelength fluctuation due to reflection in the optical fiber 207 into intensity modulation can be reduced. this is,
The effect is the same as that described in the first embodiment.

On the other hand, from the base station apparatus 21 to the center side apparatus 2
The second radio signal to be transmitted to 0 is input to the fourth electric signal level adjustment unit 210, where the second radio signal is adjusted to a predetermined level. The additional signal separated by the band separation unit 211 and given to the fifth electric signal level adjustment unit 212 is adjusted to a predetermined level there. After that, the output from the fourth electric signal level adjusting section 210 and the output from the fifth electric signal level adjusting section 212 are combined by the second combining section 213 by frequency division multiplexing. Then, the signal output from the second synthesis unit 213 and the second DC current source 21
5 directly modulates the intensity of the second semiconductor laser 214. Second semiconductor laser 2
The modulated optical signal output from the optical fiber 2
The data is transmitted to the center-side device 20 via the communication device 16. The transmitted optical signal is converted into an electric signal by a first optical-electrical conversion unit 217 and further adjusted to a predetermined level by a second electric signal level adjustment unit 218, and then output as a second wireless signal. You.

The transmission operation from the base station apparatus 21 to the center apparatus 20 in the system shown in FIG. In this case, the level of the additional signal obtained by the band separation unit 211 is changed to the fifth level so that the frequency modulation index of the optical signal output from the second semiconductor laser 214 by the additional signal becomes 1.7 or more. Is adjusted by the electric signal level adjustment unit 212, and then frequency-division multiplexed with the second radio signal by the second combining unit 213, so that the wavelength fluctuation due to the reflection in the optical fiber 216 is converted into intensity modulation. The amount of noise or distortion can be reduced. This is the same effect as that described in the first embodiment.

As described above, in the system shown in FIG. 12, a case where a radio signal is transmitted from center device 20 to base station device 21 and a case where a radio signal is transmitted from base station device 21 to center device 20 are respectively described. When individually noted, both cases have the same configuration as the first embodiment. Therefore, the same effects as in the first embodiment can be obtained in both forward and reverse transmissions.

In addition, the oscillation unit 202 is connected to the center-side device 20.
By installing the optical fibers (207 and 207) not only when transmitting a wireless signal from the center apparatus 20 to the base station apparatus 21 but also when transmitting a wireless signal from the base station apparatus 21 to the center apparatus 20. The amount of noise or distortion caused by the reflection in 216) can be reduced.

If different oscillation wavelengths are assigned to the first semiconductor laser 205 and the second semiconductor laser 214, a two-core optical fiber (207 and 216)
Instead, bidirectional communication can be performed using a single optical fiber, and the optical transmission path can be used efficiently. In this case, an optical multiplexing component for multiplexing / demultiplexing the optical signal output from the first semiconductor laser 205 and the optical signal output from the second semiconductor laser 214 at both ends of the single-core optical fiber. It is necessary to provide a wave part.

(Third Embodiment) FIG. 13 is a block diagram showing a configuration of an optical transmission system according to a third embodiment of the present invention. The system shown in FIG.
01 _{1 to} 301 _n (where n is any even number of 2 or more)
And an optical fiber 302. The above slave station 301
Out of _{1 to} 301 _n , the slave station 301 _(2k) (where k =
1, 2,..., N / 2) include an optical multiplexing unit 303, an oscillating unit 304, a level adjusting unit 305, an adding unit 306, a driving unit 307, and a semiconductor laser 308, respectively. Slave station 301
_(2k-1) includes the optical multiplexing unit 303, the driving unit 307, and the semiconductor laser 308, respectively. However, the numbers 1 to n assigned to the respective slave stations match the order when all the slave stations are arranged in the order of the wavelengths of the optical signals emitted from the respective slave stations.

Each of the slave stations 301 _{1 to} 301 _n belongs to the same wavelength band and emits optical signals having different wavelengths. The optical fiber 302 transmits an optical signal. Master station 30
0 receives the transmitted optical signal. The oscillation unit 304
Outputs additional signal. Level adjustment section 305 adjusts the level of the additional signal. Adder 306 adds the electric signal and the additional signal. The drive unit 307 adds a DC bias component to the electric signal. The semiconductor laser 308 outputs an optical signal directly intensity-modulated with an electric signal. The optical multiplexing unit 303 multiplexes the optical signals.

The operation of the system shown in FIG. 13 for multiplexing and transmitting an optical signal directly intensity-modulated with an electric signal to be transmitted will be described below. In each of the slave stations 301 _(2k) , the oscillation section 304 outputs an additional signal, and the level adjustment section 3
05 adjusts the level of the additional signal. Adder 30
6 is an electric signal to be transmitted by the own station (electric signal 2k);
The level adjustment unit 305 adds the additional signal obtained by level adjustment. Next, the drive unit 307 adds a DC bias component to the electric signal obtained by the addition unit 306, and the semiconductor laser 308 is directly intensity-modulated by the electric signal obtained by adding the DC bias component. The optical signal is output.

On the other hand, in each of the slave stations 301 _(2k-1) , the driving section 307 sends an electric signal (electric signal 2k-
1) A DC bias component is added to the semiconductor laser 308.
Outputs an optical signal directly intensity-modulated with an electric signal obtained by adding a DC bias component. Thus the child station 30
1 ₁ optical signal emitted from to 301 _n, the optical multiplexing section 30
3 and multiplexed by the master station 30 via the optical fiber 302.
0 is transmitted. Master station 300 receives the transmitted optical signal. Note that the received optical signal is subjected to photoelectric conversion, and then separated into electrical signals 1 to n as necessary.

[0158] In the above operation, when the optical-electrical conversion, optical signal wavelength is closest to each other, the beat wave of the frequency of the optical signal and the slave station 301 optical _{signal 2} is emitted to emit example slave station 301 _1, the environmental temperature If the frequency matches or becomes very close to any one of the frequencies of the electric signals 1 to n due to the influence of the change of the beat signal, the beat wave becomes beat noise and adversely affects the optical transmission. Optical signal and the slave station 301 ₃ child station 301 ₂ emitted
The same applies to the optical signal emitted by. In addition, the child station 30
1 ₃ for even optical signal and the slave station 301 optical _{signal 4} emitted that is emitted, ..., slave station 301 _(n-1) is an optical signal and the slave station 3 that emits
The same applies to the optical signal emitted from 01 _n . These beat noise is equivalent both, below, the system of FIG. 13, suppressing the beat noise by the beat wave between the optical signal and the slave station 301 optical _{signal 2} is emitted to the slave station 301 ₁ emitted The operation to be performed will be described.

[0159] In the second slave station 301 _2, the semiconductor laser 3
08 is simultaneously frequency-modulated when intensity-modulated. Therefore, the spectrum of the optical signal is dispersed into a plurality of modes by adding an additional signal. At this time, if the frequency modulation index is sufficiently large, the power of each mode of the optical signal is approximately 1 / the number of modes when the power is not dispersed. Thus, the slave station 301 ₁ and the slave stations 3
01 even beat noise due to match or beat wave in close proximity with any of the frequency of the beat wave frequency electrical signals 1~n of the optical signal output from the ₂ occurs can be reduced the power of the resulting beat noise . When the spectral line width of the optical signal is larger than the band of the electric signal, the power of the beat noise can be reduced to approximately one-half the number of modes.

Here, the number of modes is a maximum integer not exceeding a value obtained by adding 1 to a value (= β) obtained by dividing the maximum frequency shift due to the additional signal by the frequency of the additional signal. The power of each mode of beat noise is Q times (where Q <
In order to satisfy 1), β needs to be larger than 1 / Q. The above β is generally called a frequency modulation index, and the frequency of the additional signal is f, the optical modulation degree of the additional signal is m,
Assuming that the emission threshold current of the semiconductor laser 308 is Ith, the DC bias current is Ib, and the frequency modulation efficiency is dF / dI, this is given by the above equation (1) (see the first embodiment).

On the other hand, the degree of light modulation m is determined by the level of the additional signal. Therefore, in the system of FIG. 13, when the emission threshold current, the DC bias current, the frequency modulation efficiency, and the additional signal frequency of the semiconductor laser 308 are values within predetermined ranges, the frequency modulation index β is larger than 1 / Q. Adjust the level of the additional signal so that it becomes larger. Thereby, the power of the beat noise can be reduced by Q times.

For example, each of the above parameters is represented by (Ib-It)
h) = 50 mA, dF / dI = 200 MHz / mA, f
= 20 MHz, the level adjuster 305 adjusts the level of the additional signal so that the optical modulation degree m of the additional signal becomes 0.2, so that β = 100, and the power of the beat noise is reduced by about 100 minutes. Can be reduced to 1. In the system shown in FIG.
It goes without saying that the same effect as above can be obtained even if the component of _{(2k) and} the component of the slave station 301 _(2k-1) are interchanged.

FIGS. 14 and 15 show how the beat noise reduction effect of the present invention is actually confirmed. We used an FP-LD and a variable wavelength light source as the light source, and observed the spectrum of beat noise with a spectrum analyzer. FIG. 14 shows the spectrum of the beat noise when both the FP-LD and the tunable light source are not modulated. FIG. 15 shows that the tunable light source is not modulated and the FP-LD has a frequency modulation index of 27.8. 4 shows the spectrum of beat noise when an additional signal is added such that 14 and 15, it can be seen that when no additional signal is added, a very high-power beat noise is generated at the frequency difference between the two optical frequencies (FIG. 14). Therefore, when the frequency of the beat noise coincides with or is close to the frequency of the electric signal to be transmitted, the influence becomes very large. On the other hand, when the additional signal is added, the FP-LD shows that the spectrum of the beat noise is spread over a wide band because of the frequency modulation received by the additional signal simultaneously with the direct intensity modulation (FIG. 15). In this case, it can also be seen that the frequency modulation index β is β >> 1 and that the modes are almost equal.
The bandwidth of the transmission frequency band of the electric signal to be transmitted is
It is about several tens of kHz for mobile phone signals and about 6 MHz for cable television signals, which is narrower than the spectral line width of a light source whose bandwidth is originally about tens of MHz. Therefore, it can be seen that the maximum value of the noise component that affects the transmission band of the electric signal to be transmitted can be reduced by adding the additional signal and expanding the noise band to a wide frequency range as shown in FIG.

It is needless to say that the same effect as described above can be obtained even when the total number n of the slave stations is an odd number of 3 or more. However, in this case, the slave stations 301 _{1 to} 301 _n
Of the slave stations 301 _(2k) (where k = 1, 2,...,
(N-1) / 2) are the optical multiplexing unit 303 and the oscillating unit 3 respectively.
04, level adjustment unit 305, addition unit 306, drive unit 30
7 and the semiconductor laser 308, and the slave station 301 _(2k-1)
Has a configuration including the optical multiplexing unit 303, the driving unit 307, and the semiconductor laser 308, respectively, and the slave station 301 _{(2k- 1)} has the optical multiplexing unit 303, the oscillation unit 304, and the level adjustment unit 3 respectively.
05, an adder 306, a driver 307, and a semiconductor laser 308, and each of the slave stations 301 _(2k)
3. The size of the device is smaller than that of the configuration including the driving unit 307 and the semiconductor laser 308.

As can be understood from the above description, in the system shown in FIG. 13, the basic operation relating to the driving of the semiconductor laser is the same as that described in the first embodiment. Therefore, the system shown in FIG.
D can be used, and the price of the system can be significantly reduced as compared with the case where a DFB-LD is used as in the related art. Further, when an FP-LD is used as the semiconductor laser 308, the effect of reducing beat noise due to multi-mode oscillation is added, so that a further reduction effect can be obtained.

In the present embodiment, the case where the connection form between the master station 300 and each of the slave stations 301 _{1 to} 301 _n is of the bus type has been described. However, the same effect can be obtained in the case of the tree form. Needless to say. In that case, each slave station 30
The optical multiplexing unit 303 provided in 11 _{1 to} 301 _n becomes unnecessary, and instead, an optical signal transmitted from each of the slave stations 301 _{1 to} 301 _n is multiplexed in the vicinity of the master station 300 or in the master station 300. An optical multiplexer is required. In the bus format, the number of necessary optical transmission lines is apparently one, so that the optical transmission lines can be used more effectively than in the tree format. In addition, it is needless to say that the same effect can be obtained with respect to the reduction of beat noise even in a connection configuration in which the tree format and the bus format are mixed, thereby giving flexibility to the location of the slave station. it can.

(Fourth Embodiment) FIG. 16 is a block diagram showing a configuration of an optical transmission system according to a fourth embodiment of the present invention. The system shown in FIG.
01 _{1 to} 401 _n (where n is an arbitrary integer of 2 or more)
And an optical fiber 402. Slave station 401 _1-
401 _n is an optical multiplexing unit 403, an oscillation unit 404,
A level adjusting unit 405, an adding unit 406, a driving unit 407, and a semiconductor laser 408 are included. The master station 400 includes a photoelectric conversion unit 409, a separation unit 410, and a plurality of signal detection units 41.
Including 1.

The photoelectric conversion unit 409 converts an optical signal into an electric signal. The separation unit 410 separates the electric signal obtained by the conversion by the photoelectric conversion unit 409 into an electric signal to be transmitted and an additional signal. Each signal detection unit 411 includes a separation unit 410
Are separated from the slave stations 401 ₁ to 40 ₁
1 each oscillation unit 404 _n detects the output signal. Other components perform similar operations as corresponding components of the system of FIG.

The operation of the system shown in FIG. 16 for multiplexing and transmitting an optical signal directly intensity-modulated with an electric signal to be transmitted will be described below. In each of the slave stations 401 _{1 to} 401 _n , the oscillation section 404 outputs an additional signal, and the level adjustment section 405 adjusts the level of the additional signal. The adding unit 406 adds the electric signal to be transmitted (the first to n-th electric signals) and the additional signal. Next, the driving unit 40
7, a DC bias component is added to the electric signal obtained by the adding unit 406, and the semiconductor laser 408 outputs an optical signal directly intensity-modulated by the electric signal obtained by adding the DC bias component. I do.

The optical signals thus emitted from the slave stations 401 _{1 to} 401 _n are multiplexed by the optical multiplexing section 403 and transmitted to the master station 400 via the optical fiber 402. In the master station 400, the photoelectric conversion unit 409 converts the transmitted optical signal into an electric signal. Then, separation section 410 separates the converted electric signal into an electric signal to be transmitted (electric signals 1 to n) and an additional signal. Signal detector 41
1 denotes the slave stations 401 ₁ to 401 ₁ from the additional signals obtained by separation.
The signal output from each oscillation unit 404 of 401 _n is detected.
For example, the detection is performed by using the frequency of the separated additional signal,
This can be performed by comparing the signals output from the oscillators 404 of the slave stations 401 _{1 to} 401 _n with each other.

In the above operation, as in the system shown in FIG. 13, at the time of photoelectric conversion, beat noise is generated due to the beat waves of the optical signals whose wavelengths are closest to each other. The operation of suppressing the beat noise is the same as that described in the third embodiment. That is, when the semiconductor laser 408 is directly intensity-modulated, it is simultaneously frequency-modulated.
By adding the additional signals, the spectrum of the optical signal is dispersed into a plurality of modes. At this time, the power of each mode of the optical signal is approximately 1 / mode number of the power in the case of not dispersing.

However, in the system shown in FIG. 13, one of the optical signals whose wavelengths are closest to each other is mode-dispersed.
In the system of FIG. 16, in order to disperse both modes, the power of each mode of the beat noise is multiplied by Q times (where Q <
In order to make 1), β may be made larger than 1 / (2Q). Therefore, in the system of FIG.
08, the DC bias current, the frequency modulation efficiency, and the frequency of the additional signal are within a predetermined range, the level of the additional signal is set so that the frequency modulation index β becomes larger than 1 / (2Q). adjust. Thereby, the power of the beat noise can be reduced by Q times.

In the system shown in FIG.
The master station 400 can sequentially recognize a failure of _{11 to} 401 _n or a cut portion of the optical fiber 402 as follows. That is, for example, when the additional signal outputted from the slave station 401 ₁ of the oscillator 404 signal detector 411 can not detect, the main station 400 can recognize the possibility that the slave station 401 ₁ has failed. Further, when the signal detection unit 411 can not detect the additional signal outputted from the slave station 401 ₂ of the oscillator has been added signal detected despite of the slave station 401 ₁ output from the 404 oscillator portion 404, Slave station 401
_In addition to the possibility that ₁ has failed, the slave stations 401 ₁ and 40
It is possible to recognize the possibility that the optical fiber 402 between ₁₂ and 12 has been cut.

As described above, in the system of FIG. 16, it is possible to determine whether or not the additional signal output from each of the oscillators 404 of the slave stations 401 _{1 to} 401 _n has been transmitted to the master station 400. The master station 400 can sequentially estimate the failure of the slave stations 401 _{1 to} 401 _n , the cut position of the optical fiber 402, and the like. Note that, unlike the third embodiment, this is the first effect obtained by adopting a configuration in which the additional signal is added to the electric signal to be transmitted for all the slave stations 401 _{1 to} 401 _n .

When this embodiment is compared with the first embodiment, the drive of the semiconductor laser 408 is basically the same as that of the first embodiment. Therefore, an FP-LD can be used as the semiconductor laser 408, and the price of the system can be significantly reduced as compared with the case where a DFB-LD is used as in the related art. Further, the semiconductor laser 40
When the FP-LD is adopted as 8, the effect of reducing the beat noise due to the multi-mode oscillation is added, so that a further reduction effect can be obtained. Further, other effects obtained in the first embodiment can be obtained similarly.

When beat noise generated from the largest modes of the FP-LD affects subcarriers,
In a transmission system that requires high-performance transmission characteristics, the beat noise may seriously degrade the transmission characteristics. In such a case, it is conceivable to perform wavelength selection in advance to use the FP-LDs of the slave stations 401 _{1 to} 401 _n whose center wavelengths are separated to some extent from each other. FP- with center wavelengths separated from each other to some extent
By using the LD, the beat noise that causes the deterioration of the transmission characteristics is only the beat noise between the side modes of the FP-LD, so that the deterioration of the transmission characteristics is smaller than when there is the beat noise from the largest modes. Becomes smaller. Further, in this case, if the total number of slave stations (connected to one optical fiber 402) is set to 3, it is sufficient to secure three wavelength regions when performing wavelength selection of the FP-LD. For example,
It is assumed that the center wavelength needs to be separated by 10 nm or more. First,
If the wavelength region in the center is determined and a wavelength interval of 10 nm is taken on both sides, the FP-LD whose central wavelength is further away,
All can be used. In this case, the wavelength selection can be easily performed since the region is limited to only one side except the center wavelength region.

Also, in the present embodiment, the case where the connection form between the master station 400 and each of the slave stations 401 _{1 to} 401 _n is a bus form has been described. However, the same effect can be obtained even when the connection form is a tree form. Needless to say. In that case, each slave station 40
The optical multiplexing unit 403 provided in 11 _{1 to} 401 _n becomes unnecessary, and instead, an optical signal transmitted from each of the slave stations 401 _{1 to} 401 _n is multiplexed in the vicinity of the master station 400 or in the master station 400. An optical multiplexer is required. In the bus format, the number of necessary optical transmission lines is apparently one, so that the optical transmission lines can be used more effectively than in the tree format. In addition, it is needless to say that the same effect can be obtained with respect to the reduction of beat noise even in a connection configuration in which the tree format and the bus format are mixed, thereby giving flexibility to the location of the slave station. it can.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of an optical transmission device according to a first embodiment of the present invention.

FIG. 2 is a graph showing a relationship between a frequency modulation index β and a reduction amount PdB of noise or distortion due to reflected light generated during optical transmission.

FIG. 3 is a diagram showing the dependence of third-order intermodulation distortion generated at the time of electro-optical conversion on the total optical modulation factor.

FIG. 4 is a diagram illustrating a result of measuring the temperature dependency of third-order intermodulation distortion (IM3) (without an additional signal).

FIG. 5 is a diagram showing a result of measuring the temperature dependency of third-order intermodulation distortion (IM3) (in the case where an additional signal is present).

FIG. 6 is a diagram showing a result of measuring the temperature dependency of noise (in the case where an additional signal is present).

FIG. 7 is a diagram illustrating a result of measuring temperature dependency of noise (in the case where there is no additional signal).

FIG. 8 is a diagram showing a spectrum of FP-LD around 25 to 27.5 degrees.

FIG. 9 is a diagram showing a spectrum of FP-LD around 21 to 22 degrees.

FIG. 10 is a diagram showing a spectrum of the FP-LD around 25 to 27.5 degrees when a 2 km fiber is connected to the FP-LD.

FIG. 11 is a diagram showing a spectrum of the FP-LD around 21 to 22 degrees when a 2 km fiber is connected to the FP-LD.

FIG. 12 is a block diagram illustrating a configuration of an optical transmission system according to a second embodiment of the present invention.

FIG. 13 is a block diagram illustrating a configuration of an optical transmission system according to a third embodiment of the present invention.

FIG. 14 is a diagram showing a spectrum of beat noise when both the FP-LD and the variable wavelength light source are not modulated.

FIG. 15 is a diagram showing a spectrum of beat noise when the wavelength variable light source is non-modulated and an additional signal is added to the FP-LD so that the frequency modulation index becomes 27.8.

FIG. 16 is a block diagram illustrating a configuration of an optical transmission system according to a fourth embodiment of the present invention.

FIG. 17 is a block diagram illustrating an example of a configuration of a conventional optical transmission device.

FIG. 18 is a block diagram illustrating an example of a configuration of a conventional optical transmission system.

[Explanation of symbols]

 Reference Signs List 20 center device 21 base station device 101, 109 electric signal level adjustment unit 102, 202, 304, 404 oscillation unit 103, 203 additional signal level adjustment unit 104 synthesis unit 105, 308, 408 semiconductor laser 106 DC current source 107, 207, 216, 302, 402 Optical fiber 108, 409 Photoelectric conversion unit 201 First electric signal level adjustment unit 204 First synthesis unit 205 First semiconductor laser 206 .., First DC current source 208, second photoelectric conversion unit 209, third electric signal level adjustment unit 210, fourth electric signal level adjustment unit 211, band separation unit 212, fifth electric signal level adjustment Unit 213 second combining unit 214 second semiconductor laser 215 second DC current source 217 first photoelectric conversion unit 218 second Air signal level adjustment units 300, 400 ... master stations 301, 401 ... slave stations 303, 403 ... optical multiplexing units 305, 405 ... level adjustment units 306, 406 ... addition units 307, 407 ... drive units 410 ... separation units 411 ... signals Detection unit

──────────────────────────────────────────────────の Continued on the front page (72) Inventor Hiroaki Yamamoto 1006 Kazuma Kadoma, Osaka Prefecture Inside Matsushita Electric Industrial Co., Ltd. (72) Inventor Katsuyuki Fujito 1006 Kazuma Kadoma, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (72) Inventor Yutaka Fukuya 2-1-1 Toranomon, Minato-ku, Tokyo NTTI Mobile Communication Network (56) References JP-A-7-193537 (JP, A) JP-A-6-104843 (JP, A) JP-A-4-139924 (JP, A) JP-A-2-26425 (JP, A) A) JP-A-4-280521 (JP, A) JP-A-2-226921 (JP, A) JP-A-2-148929 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) ) H04B 10/00-10/28 H04J 14/00-14 / 08 H04Q 7/36

Claims (44)

(57) [Claims] 1. An optical transmission device for converting an electric signal into an optical signal directly intensity-modulated by the signal and transmitting the converted signal, wherein an oscillating means for outputting an additional signal, an electric signal to be transmitted and the oscillating means are provided. Combining means for combining the output additional signal, a DC current source for outputting a DC bias current, and combining an electric signal obtained by the combining means with the DC bias current output from the DC current source. A semiconductor laser that outputs an optical signal that is directly intensity-modulated with the obtained signal, an optical transmission path for transmitting the optical signal output by the semiconductor laser, and an optical signal that is transmitted via the optical transmission path And an opto-electric conversion means for converting the electric signal to an electric signal, wherein the oscillation means is higher than a frequency corresponding to a bandwidth of a frequency band allocated to the electric signal to be transmitted, and And outputting a low frequency additional signal than half the lowest frequency of the hit was frequency band Ri, an optical transmission device. 2. The optical transmission device according to claim 1, wherein the semiconductor laser is a Fabry-Perot semiconductor laser. 3. An additional signal output from said oscillating means is modulated by predetermined data.
The optical transmission device according to claim 2. 4. The semiconductor laser of the Fabry-Perot type,
It has a chip structure in which an active layer having an amplifying function of an optical signal and a spot size converting means for narrowing a radiation angle of an optical signal output from the active layer are formed on the same substrate. The optical transmission device according to claim 3, wherein 5. The electric signal to be transmitted is a signal obtained by frequency-division multiplexing one or more radio signals for mobile communication, wherein the electric signal to be transmitted is plural, and into an electric signal to be transmitted, each is assigned a unique frequency band, the oscillation means is higher than the frequency corresponding to the bandwidth of the widest frequency band among a plurality of said unique frequency bands, and a plurality The optical transmission apparatus according to claim 4, wherein an additional signal having a frequency lower than half of the lowest frequency among the specific frequency bands is output. 6. An electric signal transmitted using at least one frequency band among the plurality of unique frequency bands is a code division multiplex signal, and the oscillating means includes a frequency assigned to the code division multiplex signal. higher than the frequency corresponding to the widest frequency band among a plurality of said unique frequency bands excluding the band, and double
The optical transmission apparatus according to claim 5, wherein the additional signal having a frequency lower than half of the lowest frequency of the number of the specific frequency bands is output. 7. The optical transmission line includes one or more optical fibers, and an emission end face of the Fabry-Perot type semiconductor laser and an end face of an optical fiber coupled thereto are set so as to be displaced from a positional relationship parallel to each other. The optical transmission device according to claim 6, wherein: 8. An additional signal level adjusting means for adjusting a level of the additional signal output by the oscillation means, wherein the additional signal level adjusting means has a frequency modulation index β of an optical signal output by the semiconductor laser, The oscillating means outputs an output signal so as to satisfy a conditional expression β ≧ (2 / π) · 10 ^{P / 10} (where π is a circular constant) for reducing at least P decibels of noise or distortion generated in the optical transmission line. The optical transmission apparatus according to claim 7, wherein the level of the additional signal is adjusted. Wherein said additional signal level adjusting means, so that the frequency modulation index beta satisfy the condition beta ≧ 1.7, and adjusting the level of the additional signal, the light according to claim 7 Transmission equipment. 10. An optical modulation factor assigned to a plurality of electric signals to be transmitted and an additional signal obtained by level adjustment by the level adjustment unit is defined as mi (i = 1, 2,
.., N), the total light modulation degree {(mi) ² }
The optical transmission device according to claim 7, wherein? 11. The optical transmission device according to claim 3, wherein the Fabry-Perot type semiconductor laser has a chip structure in which an active layer having an optical signal amplifying action is tapered. 12. The electric signal to be transmitted is a signal obtained by frequency-division multiplexing one or more radio signals for mobile communication, and the electric signal to be transmitted is plural, and into an electric signal to be transmitted, each is assigned a unique frequency band, the oscillation means is higher than the frequency corresponding to the bandwidth of the widest frequency band among a plurality of said unique frequency bands, and a plurality 12. The optical transmission apparatus according to claim 11, wherein an additional signal having a frequency lower than half of the lowest frequency in said specific frequency band is output. 13. An electric signal transmitted using at least one frequency band among the plurality of unique frequency bands is a code division multiplexed signal, and the oscillating means includes a frequency assigned to the code division multiplexed signal. higher than the frequency corresponding to the widest frequency band among a plurality of said unique frequency bands excluding the band, and double
13. The optical transmission apparatus according to claim 12, wherein an additional signal having a frequency lower than half of the lowest frequency of the number of the specific frequency bands is output. 14. The optical transmission line includes one or more optical fibers, and an output end face of the Fabry-Perot type semiconductor laser and an end face of an optical fiber coupled to the Fabry-Perot type semiconductor laser are displaced from a positional relationship parallel to each other. 14. The optical transmission device according to claim 13, wherein: 15. An additional signal level adjusting means for adjusting a level of an additional signal output by the oscillation means, wherein the additional signal level adjusting means has a frequency modulation index β of an optical signal output by the semiconductor laser. The oscillating means outputs an output signal so as to satisfy a conditional expression β ≧ (2 / π) · 10 ^{P / 10} (where π is a circular constant) for reducing at least P decibels of noise or distortion generated in the optical transmission line. The optical transmission apparatus according to claim 14, wherein the level of the additional signal is adjusted. 16. The additional signal level adjusting means, so that the frequency modulation index beta satisfy the condition beta ≧ 1.7, and adjusting the level of the additional signal, claim 14
An optical transmission device according to claim 1. 17. The optical modulation factors assigned to a plurality of electric signals to be transmitted and an additional signal obtained by level adjustment by the level adjustment unit are respectively mi (i = 1, 2,
.., N), the total light modulation degree {(mi) ² }
The optical transmission device according to claim 14, wherein? 18. An optical transmission system for converting an electric signal into an optical signal directly intensity-modulated by the signal and transmitting the signal bidirectionally, comprising: a first device for transmitting a first electric signal; A second device that transmits an electric signal of the first device and an optical transmission line that interconnects the first device and the second device, wherein the first device has the first and second devices . Allocated for the transmission of electrical signals
Oscillating means for outputting an additional signal having a frequency higher than the frequency corresponding to the bandwidth of the frequency band and lower than half of the lowest frequency of the frequency band allocated for transmitting the first and second electric signals; A first combining unit that combines the first electric signal and the additional signal output from the oscillation unit; a first DC current source that outputs a DC bias current; and the first combining unit combines the first electric signal and the additional signal output from the oscillation unit. And the first signal
A first semiconductor laser that outputs an optical signal that is directly intensity-modulated with a signal obtained by combining the DC bias current output by the DC current source of the first embodiment, and an optical signal that is transmitted from the second device. A first photoelectric conversion unit for converting the optical signal transmitted from the first device into an electric signal; and a second photoelectric conversion unit for converting the optical signal transmitted from the first device into an electric signal. Band separation means for separating the electric signal obtained by the second photoelectric conversion means into the first electric signal and the additional signal output from the oscillating means; Second combining means for combining the obtained additional signal and the second electric signal; a second DC current source for outputting a DC bias current; and a signal obtained by combining the second combining means. And the second
A second semiconductor laser that outputs an optical signal that is directly intensity-modulated with a signal obtained by combining a DC bias current output by the DC current source of the first semiconductor laser, and wherein the optical transmission path includes the first semiconductor A first optical fiber for transmitting an optical signal output from a laser to the second device; and a second optical fiber for transmitting an optical signal output from the second semiconductor laser to the first device. Including an optical fiber,
Optical transmission system. 19. An optical transmission system for converting an electric signal into an optical signal directly intensity-modulated by the signal and transmitting the converted signal from a plurality of slave stations to a master station, wherein the plurality of the plurality of slave stations are arranged in the order of the wavelength of the emitted optical signal. Are referred to as first to nth (where n is an arbitrary even number of 2 or more) slave stations, a second k (where k =
Each of the slave stations (1, 2,..., N / 2) has an oscillating means for outputting an additional signal, and a synthesizing means for synthesizing an electric signal to be transmitted by the slave station and the additional signal output from the oscillating means. A DC current source that outputs a DC bias current; and light that is directly intensity-modulated with a signal obtained by combining the signal obtained by the combining unit and the DC bias current output by the DC current source. A 2k-1 slave station that outputs a DC bias current; an electric signal to be transmitted by the slave station; and a DC bias output by the DC current source. and a semiconductor laser for outputting an optical signal directly intensity-modulated by a signal obtained by combining the current, the oscillation means is assigned to an electrical signal to be said transmission
Higher than the frequency corresponding to the bandwidth of the
Frequency band assigned to the electrical signal to be transmitted
Output additional signal with frequency lower than half of the lowest frequency
It characterized in that the optical transmission system. 20. The optical transmission system according to claim 19, wherein each semiconductor laser provided in said plurality of slave stations is a Fabry-Perot semiconductor laser. 21. The optical transmission system according to claim 20, wherein a connection form between said master station and said plurality of slave stations is a bus type. 22. An optical transmission system for converting an electric signal into an optical signal directly intensity-modulated by the signal and transmitting the electric signal from a plurality of slave stations to a master station, wherein the plurality of the plurality of slave stations are arranged in the order of the wavelength of the emitted optical signal. Are referred to as first to nth (where n is an arbitrary odd number of 3 or more) child stations, a second k (where k =
Each of the slave stations (1, 2,..., (N-1) / 2) synthesizes an oscillating means for outputting an additional signal, an electric signal to be transmitted by the slave station, and an additional signal output from the oscillating means. A DC current source that outputs a DC bias current; and a signal obtained by synthesizing a signal obtained by the synthesis unit and a DC bias current output by the DC current source. A semiconductor laser that outputs a modulated optical signal, the 2k-1 slave stations each include a DC current source that outputs a DC bias current, an electric signal to be transmitted by the slave station, and the DC current source. and a semiconductor laser for outputting an optical signal directly intensity-modulated by a signal obtained by combining the DC bias current outputted by said oscillating means, assigned to the electrical signal to be said transmission
Higher than the frequency corresponding to the bandwidth of the
Frequency band assigned to the electrical signal to be transmitted
Output additional signal with frequency lower than half of the lowest frequency
It characterized in that the optical transmission system. 23. The optical transmission system according to claim 22, wherein each semiconductor laser provided in said plurality of slave stations is a Fabry-Perot semiconductor laser. 24. The optical transmission system according to claim 23, wherein a connection form between said master station and said plurality of slave stations is a bus type. 25. An optical transmission device for converting an electric signal into an optical signal directly intensity-modulated by the signal and transmitting the optical signal from a plurality of slave stations to a master station via an optical transmission path, wherein the plurality of slave stations Each of the stations includes an oscillating unit that outputs an additional signal, a combining unit that combines an electric signal to be transmitted by the slave station with the additional signal output by the oscillating unit, a DC current source that outputs a DC bias current, and a semiconductor laser which said combining means outputs an optical signal directly intensity-modulated by a signal obtained by synthesizing a DC bias current, wherein the signal obtained by combining a DC current source and an output, said oscillator The means are assigned to the electrical signal to be transmitted.
Higher than the frequency corresponding to the bandwidth of the
Frequency band assigned to the electrical signal to be transmitted
Output additional signal with frequency lower than half of the lowest frequency
It characterized in that the optical transmission system. 26. The optical transmission system according to claim 25, wherein each semiconductor laser provided in said plurality of slave stations is a Fabry-Perot semiconductor laser. 27. The optical transmission system according to claim 26, wherein a connection form between said master station and said plurality of slave stations is a bus type. 28. The optical transmission system according to claim 27, wherein each of said oscillating means provided in said plurality of slave stations outputs additional signals having different frequencies. 29. The optical transmission system according to claim 28, wherein the additional signal output from each said oscillating means is modulated by data. 30. The master station, in order to detect a system failure, converts the optical signals transmitted from the plurality of slave stations into electrical signals, and converts the optical signals into electrical signals. Separating means for separating the obtained electric signal into an electric signal to be transmitted and an additional signal, and each of the oscillating means provided in the plurality of slave stations from the additional signal obtained by separating the separating means. 30. The optical transmission system according to claim 29, further comprising: signal detection means for detecting each of the output additional signals. 31. The electric signal to be transmitted is a signal obtained by frequency-division multiplexing one or more radio signals for mobile communication, and the electric signal to be transmitted is plural, and A unique frequency band is assigned to each of the electric signals to be transmitted, and each of the oscillating units is higher than a frequency corresponding to a bandwidth of a widest one of the plurality of unique frequency bands, and 31. The optical transmission system according to claim 30, wherein an additional signal having a frequency lower than half of the lowest frequency among the plurality of unique frequency bands is output. 32. An electric signal transmitted using at least one frequency band among the plurality of unique frequency bands is a code division multiplexed signal, and each of said oscillating means is assigned to said code division multiplexed signal. Higher than the frequency corresponding to the widest frequency band among the plurality of unique frequency bands excluding the frequency band, and
The optical transmission system according to claim 31, wherein an additional signal having a frequency lower than half of the lowest frequency of the plurality of unique frequency bands is output. 33. Each of the Fabry-Perot type semiconductor lasers comprises an active layer having an optical signal amplifying function and a spot size converting means for narrowing a radiation angle of an optical signal output from the active layer on the same substrate. 32. The optical transmission system according to claim 31, having a chip structure as formed above. 34. Each of the plurality of slave stations further includes additional signal level adjusting means for adjusting the level of an additional signal output by each of the oscillating means. The frequency modulation index β of the output optical signal gives the beat noise Q
Conditional expression β <1 / (2) for doubling (where Q <1)
34. The optical transmission system according to claim 33, wherein a level of the additional signal output from each of the oscillation units is adjusted so that the value satisfies Q). 35. Each of the plurality of slave stations further includes additional signal level adjusting means for adjusting the level of an additional signal output by each of the oscillating means. The frequency modulation index β of the output optical signal is equal to the conditional expression β <1/1 for making the beat noise Q times (where Q <1).
(2Q), and a conditional expression β ≧ (2 /) for reducing at least P decibels of noise or distortion generated in the optical transmission line.
34. The level of the additional signal output by each of the oscillating means is adjusted so that the value satisfies any one of (π) · 10 ^{P / 10} (where π is a pi). An optical transmission system according to item 1. 36. The optical transmission path includes one or more optical fibers, and an emission end face of each of the Fabry-Perot type semiconductor lasers and an end face of an optical fiber coupled thereto are set so as to be shifted from a parallel positional relationship with each other. Characterized by the fact that
The optical transmission system according to claim 35. 37. The degree of optical modulation assigned to each of the electric signals to be transmitted and the additional signal obtained by level adjustment by each of the additional signal level adjusting means is defined as mi (i = i
, (N, 1, 2,...)
The optical transmission system according to claim 35, wherein i) ^{2 2} does not exceed 0.3. 38. Each of the Fabry-Perot semiconductor lasers has a chip structure in which an active layer having an optical signal amplifying action is tapered.
An optical transmission system according to item 1. 39. Each of the plurality of slave stations further includes an additional signal level adjusting means for adjusting a level of an additional signal output by each of the oscillating means. The frequency modulation index β of the output optical signal gives the beat noise Q
Conditional expression β <1 / (2) for doubling (where Q <1)
39. The optical transmission system according to claim 38, wherein the level of the additional signal output by each of said oscillation units is adjusted so that the value satisfies Q). 40. Each of the plurality of slave stations further includes an additional signal level adjusting means for adjusting a level of an additional signal output by each of the oscillating means. The frequency modulation index β of the output optical signal is equal to the conditional expression β <1/1 for making the beat noise Q times (where Q <1).
(2Q), and a conditional expression β ≧ (2 /) for reducing at least P decibels of noise or distortion generated in the optical transmission line.
39. The level of the additional signal output by each of the oscillating means is adjusted so that the value satisfies any one of (π) · 10 ^{P / 10} (where π is a circular constant). An optical transmission system according to item 1. 41. The optical transmission line includes one or more optical fibers, and an emission end face of each of the Fabry-Perot type semiconductor lasers and an end face of an optical fiber coupled to the Fabry-Perot type semiconductor laser are arranged so as to be displaced from a positional relationship parallel to each other. Characterized by the fact that
The optical transmission system according to claim 40. 42. The optical modulation degree assigned to each of the electric signals to be transmitted and the additional signal obtained by level adjustment by each of the additional signal level adjusting means is represented by mi (i = i
, (N, 1, 2,...)
41. The optical transmission system according to claim 40, wherein i) ²な い does not exceed 0.3. 43. Each of the Fabry-Perot semiconductor lasers is such that the center wavelengths of optical signals outputted from the respective lasers are separated from each other by a predetermined wavelength interval. An optical transmission system according to item 1. 44. The optical transmission system according to claim 43, wherein the number of slave stations connected to one optical fiber is a maximum of three.