JP2007053672A - Optical transmission apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical transmission apparatus, capable of effectively suppressing SBS (stimulated Brillouin scattering) generated at optical fiber transmission. <P>SOLUTION: A signal level detector 40 detects the level of an input signal 10 and outputs an input signal level indicative of the level of the input signal 10. A signal frequency detector 50 detects the frequency of the input signal 10 and outputs an input signal frequency indicative of the frequency of the input signal 10. In response to the input signal level and the input signal frequency, a control unit 30 controls the bias current supplied to an optical source 20 so as to make the FM modulation index for optical signal outputted from the optical source 20 substantially constant. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical transmission apparatus that transmits an optical signal, and more particularly to an optical transmission apparatus that suppresses SBS (stimulated Brillouin scattering) that occurs during transmission of a long-distance optical fiber.

  In recent years, long-distance transmission using an optical fiber has become widespread. In the future, it is considered that an optical transmission device using a light source with higher output is required for further long-distance transmission. However, it is known that when a high-power optical signal is input to an optical fiber, SBS (stimulated Brillouin scattering) is generated by reflected light reflected from the optical fiber, and the transmission characteristics of the optical signal are rapidly deteriorated. In addition, since SBS is more likely to occur as the line width of modulated light is narrower, the possibility of occurrence of SBS increases when the degree of light modulation (input signal level to the light source) is small.

  In order to suppress such SBS, for example, Patent Document 1 discloses an optical transmission device that adjusts the output of a light source based on the level of reflected light reflected from an optical fiber. FIG. 11 is a block diagram illustrating an example of a configuration of a conventional optical transmission apparatus. In FIG. 11, the conventional optical transmission device includes a light source 920, a control unit 930, a demultiplexing unit 970, a light detection unit 980, and an optical fiber 990. The light source 920 converts an input signal 910 that is an electrical signal into an optical signal and outputs the optical signal. The demultiplexing unit 970 inputs the optical signal output from the light source 920 to the optical fiber 990, and inputs the reflected light reflected from the optical fiber 990 to the light detection unit 980.

The light detection unit 980 detects the level of reflected light from the optical fiber 990 input via the demultiplexing unit 970. The control unit 930 controls the bias current of the light source 920 based on the level of reflected light detected by the light detection unit 980. Specifically, the control unit 930 reduces the bias current of the light source 920 when the level of reflected light detected by the light detection unit 980 increases. As a result, the conventional optical transmission device reduces the level of light incident on the optical fiber 990 and suppresses the occurrence of SBS.
JP-A-5-316045 JP-A-9-069814

  However, in the conventional optical transmission apparatus, even if the fluctuation of the reflected light level from the optical fiber 990 is a very small amount, the generated SBS varies greatly. There is a problem that it is difficult to control the bias current supplied to the 920. FIG. 12 is a diagram for explaining a problem in the conventional optical transmission apparatus. In FIG. 12, the solid line indicates the level of the signal (input signal 910) input to the light source 920, and the broken line indicates the third-order distortion characteristic (measured value) of the optical signal transmitted by the optical fiber 990. Referring to FIG. 12, in the conventional optical transmission apparatus, SBS starts to occur when the level of the input signal 910 is less than −4 dB, the reflected light level increases as the level of the input signal 910 decreases, and distortion occurs. The characteristics are degraded.

  It can also be seen that in the vicinity where SBS begins to occur, the reflected light level fluctuates only by about 0.2 dBm while the level of the input signal 910 changes from −4 dBm to −5 dBm. For this reason, in the conventional optical transmission apparatus, it is necessary to accurately detect the fluctuation of the reflected light level of about 0.2 dB and control the bias current to the light source 920. However, the reflected light level is about 30 dB or more smaller than the incident light level, and it is very difficult to detect a very small reflected light level fluctuation of about 0.2 dB, and the occurrence of SBS is effectively suppressed. It was difficult to do.

  Therefore, an object of the present invention is to provide an optical transmission apparatus that can effectively suppress the occurrence of SBS.

  The present invention is directed to an optical transmission apparatus that converts an input signal, which is an electrical signal, into a light intensity signal and transmits the signal. And in order to achieve the said objective, the optical transmission apparatus of this invention is provided with a light source and a bias current control part. The light source converts the input signal into a light intensity signal based on a predetermined bias current and outputs the light signal as an optical signal. The bias current control unit controls the bias current supplied to the light source based on the input signal level indicating the level of the input signal and the input signal frequency indicating the frequency of the input signal.

  Preferably, the bias current control unit detects a level of the input signal and outputs the input signal level, a signal level detection unit which detects the frequency of the input signal and outputs the input signal frequency, And a control unit that controls a bias current supplied to the light source based on the input signal level and the input signal frequency.

  The bias current control unit may include a control unit that receives an input signal level and an input signal frequency and controls a bias current supplied to the light source based on the input signal level and the input signal frequency. Good.

  The optical transmission apparatus may further include a signal level adjustment unit that adjusts the level of the input signal. In this case, the control unit controls the signal level adjusting unit based on the input signal level and the input signal frequency.

  Preferably, the signal frequency detector is a highest signal frequency detector that detects the highest frequency of the input signals.

  The control unit calculates the optical modulation degree of the optical signal output from the light source from the bias current supplied to the light source and the input signal level, and the quotient of the calculated optical modulation degree and the input signal frequency is maintained substantially constant. As described above, the degree of light modulation is adjusted by controlling the bias current supplied to the light source.

  The output light wavelength of the light source is different from the zero dispersion wavelength of the optical fiber connected to the light source.

  As described above, according to the present invention, SBS can be generated by detecting an input signal level to a light source having a larger absolute level and fluctuation level than reflected light compared to a conventional method of detecting reflected light. It becomes possible to suppress effectively.

  Further, paying attention to the fact that the SBS size corresponds to the FM modulation index of the optical signal output from the light source, the FM modulation index is input to the light source and the light modulation degree of the optical signal output from the light source. Since it is proportional to the quotient with the signal frequency, the bias current supplied to the light source is controlled so that the FM modulation index becomes constant according to the change in the input signal level (corresponding to the optical modulation degree) and the input signal frequency. To adjust the light modulation degree. As a result, the optical transmission apparatus can effectively suppress the occurrence of SBS while maintaining the CNR of the optical signal.

  Furthermore, in long-distance optical transmission, when the output light wavelength of the light source is different from the zero dispersion wavelength of the optical fiber connected to the light source, it is possible to maintain dispersion distortion when suppressing SBS. In actual operation, since the (initial) value of the FM modulation index to be maintained is always determined from the set transmission parameters, the input signal level and the frequency arrangement of the input signal after the initial operation are changed. Accordingly, SBS can be automatically suppressed.

  In addition to controlling the bias current supplied to the light source, by adjusting the input signal level (light modulation degree) to the light source, the distortion characteristics and dispersion distortion of the light source are improved while maintaining the initial CNR value. It becomes possible to do.

  Embodiments of the present invention will be described below with reference to the drawings. In addition, embodiment of this invention is not limited to what is shown below.

(First embodiment)
FIG. 1 is a block diagram showing an example of the configuration of the optical transmission apparatus according to the first embodiment of the present invention. In FIG. 1, the optical transmission apparatus according to the first embodiment of the present invention includes a light source 20, a control unit 30, a signal level detection unit 40, and a signal frequency detection unit 50. The optical transmission device is a device for converting an arbitrary electric signal into a light intensity signal (hereinafter referred to as an optical signal) and transmitting the light. An input signal 10 that is an electrical signal is input to the optical transmission apparatus. The light source 20 converts the input signal 10 into an optical signal and outputs it. The signal level detection unit 40 detects the input signal level to the light source 20. The signal frequency detection unit 50 detects an input signal frequency to the light source 20. The control unit 30 controls the bias current supplied to the light source 20 in accordance with the input signal level detected by the signal level detection unit 40 and the change of the input signal frequency detected by the signal frequency detection unit 50. The control unit 30, the signal level detection unit 40, and the signal frequency detection unit 50 can be a bias current control unit.

  FIG. 2 is a diagram showing the relationship between the bias current applied to a general light source and the light emission power. In FIG. 2, the light source outputs an optical signal whose intensity is modulated around the average light emission power by applying a bias current and an input signal. That is, the light emission power of the light source can be adjusted by changing the bias current supplied to the light source.

  FIG. 3 is a schematic diagram showing the spectrum of an optical signal output when the intensity of the light source is directly modulated. In FIG. 3, SBS occurs when the peak power of the spectrum of the optical signal exceeds the SBS threshold. In the optical transmission apparatus, conditions regarding the degree of optical modulation and the light emission power are determined so that SBS does not occur at the start of operation. The spectrum of the optical signal under this condition is shown in FIG.

  However, when the target of optical transmission is, for example, a broadcast signal, some channels may stop in the time zone such as midnight and the number of channels may decrease. In such a case, the input signal level to the light source decreases, the sideband energy transitions to the carrier as shown in FIG. 3B, and the peak power of the spectrum of the optical signal may exceed the SBS threshold. Under such circumstances, SBS occurs, and the transmission characteristics of the optical transmission apparatus are greatly degraded.

  As a method of suppressing the occurrence of SBS, there is a method of increasing the input signal level to the light source by amplifying the input signal. However, in this method, since the optical modulation degree of each channel is also increased, the distortion characteristics of the optical signal after transmission are deteriorated. As another method, there is a method of reducing the peak power of the spectrum of the optical signal by reducing the bias current supplied to the light source. In this method, while maintaining the CNR and distortion characteristics of the optical signal after transmission (details will be described later), the peak power of the optical signal can be lowered below the SBS threshold as in the spectrum shown in FIG. it can. Therefore, when the signal level detection unit 40 detects a decrease in the input signal level to the light source 20, the optical transmission apparatus according to the present embodiment reduces the bias current supplied to the light source 20 by the control unit 30.

  The occurrence of SBS is closely related to the FM modulation index of the optical signal. That is, it is known that SBS is more likely to occur as the FM modulation index of an optical signal is smaller (see, for example, Patent Document 2). The FM modulation index is proportional to the quotient of the light modulation degree and the modulation frequency (input signal frequency) of the light source 20 (that is, “FM modulation index / light modulation degree / modulation frequency”). FIG. 4 is a diagram showing the relationship (measurement result) between the degree of light modulation and the reflected light power. FIG. 4A shows the measurement result when the modulation frequency is 50 MHz or more, and FIG. 4B shows the measurement result when the modulation frequency is 50 MHz or less. In FIG. 4, the horizontal axis indicates the light modulation degree, and the vertical axis indicates the reflected light power. The magnitude of the reflected light power represents the amount of SBS generated. The smaller the reflected light power is, the more SBS is suppressed.

  Here, since the light modulation degree is proportional to the input signal level to the light source 20, it can be calculated based on the input signal level detected by the signal level detector 40. Further, the modulation frequency (input signal frequency) of the light source 20 can be detected by the signal frequency detection unit 50. For this reason, the optical transmission apparatus can calculate the FM modulation index of the optical signal using the signal level detection unit 40 and the signal frequency detection unit 50.

  In the optical transmission device, no SBS is generated in the initial state, and the optical modulation degree and the bias current satisfying the required transmission performance are set. Therefore, from the input signal level and the input signal frequency to the light source 20 in the initial state. , The initial value of the FM modulation index can be determined. The initial value of the FM modulation index is stored in, for example, a memory mounted on the control unit 30.

  Subsequently, after the start of the operation of the optical transmission apparatus, when the FM modulation index decreases due to, for example, a change in the input signal level (light modulation degree) or the input signal frequency to the light source 20, the control unit 30 causes the light source 20 to By reducing the bias current supplied to the optical modulation, the optical modulation degree is increased, and the FM modulation index is controlled so as to substantially match the initial value. On the contrary, when the FM modulation index increases, the control unit 30 increases the bias current supplied to the light source 20, thereby reducing the light modulation degree and controlling the FM modulation index to substantially match the initial value. . In this manner, the optical transmission apparatus can always avoid the occurrence of SBS by controlling the optical modulation degree by changing the bias current so as to maintain the FM modulation index substantially constant.

  The optical transmission apparatus according to the present embodiment is effective when the input signal 10 to the light source 20 is multiplexed with a plurality of different frequency signals such as an SCM (Sub-Carrier Multiplexing) signal. It is possible to effectively suppress the occurrence of SBS by controlling the bias current supplied to the light source 20 so that the FM modulation index becomes substantially constant after deriving a proper modulation frequency. However, this effective frequency determination may be difficult in consideration of matching with the SBS generation condition.

  Therefore, the optical transmission apparatus supplies the light source 20 by paying attention to the tendency that SBS is likely to occur when the FM modulation index is small as described above (that is, when the input signal frequency is high at the same optical modulation degree). The bias current to be controlled may be controlled. FIG. 5 is a schematic diagram showing how SBS generation conditions are defined at the highest frequency during SCM transmission. When the input signal 10 is an SCM signal, for example, the optical modulation degree of each frequency signal included in the SCM signal (total effective modulation degree: m√N) is m, and the number of channels is N (see FIG. 5A). ). In such a case, the optical transmission device assumes that all signal frequencies are aggregated to the highest frequency (fN) of the SCM signal as shown in FIG. The bias current supplied to 20 is controlled.

  As a result, the optical transmission apparatus can effectively avoid the occurrence of SBS without being affected by the level and frequency arrangement of the SCM signal that is the input signal 10. That is, when a plurality of different frequency signals are multiplexed on the input signal 10, the optical transmission apparatus detects the highest frequency among the signals input to the light source 20 by the signal frequency detection unit 50, and the control unit 30 If the FM modulation index is derived using the highest frequency and the input signal level, the occurrence of SBS can be effectively avoided. In such a case, since the signal frequency detection unit 50 detects the highest frequency among the input signals 10, it can be a maximum frequency detection unit.

  As an example, FIG. 6 shows the amount of change in the light modulation degree when the light emission power of the light source 20 is reduced by 1 dB. FIG. 6 is a diagram assuming that the current above the bias threshold is reduced from ΔIb to ΔIba in order to reduce the light emission power of the light source 20 by 1 dB. In general, it is known that the current above the bias threshold and the light emission power are in a proportional relationship, and ΔIb / ΔIba = 1.259 (corresponding to 1 dB of power) between the current above the bias threshold and the light emission power. A relationship is established. Here, if the 0 to Peak value of the modulation current is Im, the amount of change in the optical modulation degree is (Im / ΔIba) because Im does not change when only the bias current is reduced. /(Im/ΔIb)=ΔIb/ΔIba=1.259 times. That is, the control unit 30 can control the degree of light modulation by changing the bias current. Further, when the degree of optical modulation becomes 1.259 times, the CNR of the optical signal increases by 2 dB (= 20 log 10 (1.259)).

ｍ：光変調度（５％）
Ｉ₀：ＰＤの受光電力（変換効率０．８７Ａ／Ｗとして計算）
Ｚ₀：インピーダンス（５０Ω）
ＲＩＮ：ＲＩＮ（−１５０ｄＢ／Ｈｚ）
ｅ：電気素量（１．６０２×１０^-19Ｃ）
Ｉ_n：入力換算雑音電流密度（６ｐＡ／√Ｈｚ）
Ｂ：信号帯域幅（４．２ＭＨｚ） FIG. 7 is a diagram showing the relationship between the light receiving power of an optical signal and the CNR. Moreover, the formula (1) shows the calculation formula of CNR. As can be seen from FIG. 7, when the received light power is reduced by 1 dB, the CNR deteriorates by about 2 dB at the maximum. From this, when the light transmission power of the light source 20 is reduced by, for example, 1 dB, the optical transmission device has a CNR degradation (up to about 2 dB) due to a decrease in the light reception power and an improvement in CNR due to an increase in the degree of optical modulation ( 2 dB) is offset, and it can be seen that the CNR before the emission power is reduced can be maintained or improved.

m: Light modulation degree (5%)
I ₀ : PD received power (calculated as conversion efficiency 0.87 A / W)
Z ₀ : Impedance (50Ω)
RIN: RIN (-150 dB / Hz)
e: Elementary electric quantity (1.602 × 10 ⁻¹⁹ C)
I _n : Input equivalent noise current density (6 pA / √Hz)
B: Signal bandwidth (4.2 MHz)

Ｄ：分散量
Ｌ：伝送距離
ｄｖ／ｄｌ：チャープ量（Ｈｚ／Ａ）
λ：光源波長
ｃ：光速
Ｉ_b：バイアス電流
Ｉ_th：バイアス閾値電流
ｍ：光変調度 Further, in the optical transmission device of the present embodiment, when the emission wavelength of the light source 20 and the zero dispersion wavelength of the optical fiber are different, chromatic dispersion distortion occurs in the optical fiber. Equation (2) shows the relationship between each transmission parameter and the amount of chromatic dispersion distortion. Since the optical transmission device of this embodiment controls only the bias current supplied to the light source 20, the modulation current is constant, and the amount of chromatic dispersion distortion can be reduced even when the emission wavelength of the light source 20 is different from the zero dispersion wavelength of the optical fiber. Can be constant. For this reason, the optical transmission apparatus of this embodiment has a problem in a system that optically transmits using a 1.55 μm band wavelength light source to a 1.3 μm band zero-dispersion fiber that has already been laid to realize long-distance transmission. SBS can be effectively suppressed without increasing the dispersion distortion at all.

D: Dispersion amount L: Transmission distance dv / dl: Chirp amount (Hz / A)
λ: wavelength of light source c: speed of light I _b : bias current I _th : bias threshold current m: degree of light modulation

  In the optical transmission apparatus of the present embodiment, the input signal level to the light source 20 detected by the signal level detection unit 40 and the instantaneous fluctuation of the input signal frequency detected by the signal frequency detection unit 50 are as follows. In order to prevent the control of keeping the FM modulation index constant, it may further include an averaging unit that averages the detected input signal level and input signal frequency to some extent (see FIG. 8).

  In addition, the optical transmission apparatus according to the present embodiment does not include the signal level detection unit 40 and the signal frequency detection unit 50, and the input signal level indicating the level of the input signal 10 and the input signal frequency indicating the frequency of the input signal 10 are The configuration may be such that it is directly input to the control unit 30 (see FIG. 9). Also in this case, the optical transmission device can control the bias current supplied to the light source 20 so that the FM modulation index is substantially constant based on the input signal level and the input signal frequency. Further, if the input signal frequency is information related to the highest frequency among the signal frequencies input to the light source 20, the optical transmission apparatus effectively performs SBS regardless of the frequency arrangement other than the highest frequency as described above. It becomes possible to suppress.

  Further, in the optical transmission device of the present embodiment, the light source 20 generally has temperature characteristics, and when the driving condition of the light source 20 is changed with time as described above, the temperature is not stable, The characteristics may become unstable. In such a case, it is desirable for the optical transmission device to control the temperature of the light source 20.

  Furthermore, in the optical transmission apparatus of this embodiment, when an optical amplifier such as an EDFA is used in the transmission path, SBS may occur if the optical power amplified by the optical amplifier is too large. Therefore, it is desirable that the optical transmission device be operated after adjusting the amplification factor of the optical amplifier so that the optical power output from the optical amplifier is equal to or lower than the light emission power of the light source 20.

  As described above, according to the optical transmission apparatus according to the first embodiment of the present invention, a light source having a larger absolute level and fluctuation level than reflected light compared to a conventional method of detecting reflected light from an optical fiber. By detecting the input signal level to 20, the occurrence of SBS can be effectively suppressed.

  Further, paying attention to the fact that the magnitude of SBS corresponds to the FM modulation index of the optical signal output from the light source 20, the FM modulation index corresponds to the light modulation degree of the optical signal output from the light source 20 and the light source 20. Since it is proportional to the quotient of the input signal frequency, the bias current supplied to the light source 20 so that the FM modulation index becomes constant according to the input signal level (corresponding to the optical modulation degree) and the change of the input signal frequency. The degree of light modulation is adjusted by controlling. As a result, the optical transmission apparatus can effectively suppress the occurrence of SBS while maintaining the CNR of the optical signal.

  Furthermore, in long-distance optical transmission, when the output light wavelength of the light source 20 and the zero dispersion wavelength of the optical fiber connected to the light source 20 are different, it is possible to maintain dispersion distortion when suppressing SBS. Become. In actual operation, since the (initial) value of the FM modulation index to be maintained is always determined from the set transmission parameters, the input signal level and the frequency arrangement of the input signal after the initial operation are changed. Accordingly, SBS can be automatically suppressed.

(Second Embodiment)
As in the relationship (an example) between the received light power and the CNR shown in FIG. 7, in the region where the received light power is about −10 dB or more, the CNR deterioration amount is less than 2 dB with respect to the received light power being reduced by 1 dB. In such a case, in the optical transmission apparatus, when the light emission power of the light source 20 is reduced while maintaining the input signal level to the light source 20, the CNR improvement amount due to the increase in the degree of light modulation is the CNR degradation due to the reduction in the light reception power. As a total, the CNR on the receiving side is improved. However, this improvement in CNR corresponds to surplus performance for the optical transmission system. For this reason, the optical transmission apparatus according to the second embodiment reduces the degree of optical modulation by an amount corresponding to the improvement of the CNR, and maintains a trade-off relationship with the CNR characteristic while maintaining the CNR before the improvement. A certain distortion characteristic can be improved.

  FIG. 10 is a block diagram showing an example of the configuration of the optical transmission apparatus according to the second embodiment of the present invention. In FIG. 10, the optical transmission device according to the second embodiment further includes a signal level adjustment unit 60 as compared to the optical transmission device according to the first embodiment. The control unit 31 controls the bias current supplied to the light source 20 so that the FM modulation index of the optical signal output from the light source 20 is constant, and simultaneously controls the signal level adjustment unit 60 to control the light source 20. Adjust the input signal level to.

  Specifically, the control unit 31 calculates a post-transmission CNR (initial value) before controlling the bias current of the light source 20, for example, by substituting each transmission parameter into Equation (1). Thereafter, the control unit 31 controls the bias current supplied to the light source 20, calculates the post-transmission CNR after each transmission parameter changes, and controls the signal level adjustment unit 60 so that this becomes the CNR initial value. Then, the degree of light modulation is changed.

  As described above, according to the optical transmission device according to the second embodiment of the present invention, in addition to controlling the bias current supplied to the light source 20, the input signal level (light modulation degree) to the light source 20 is adjusted. Thus, it is possible to improve the distortion characteristics and the dispersion distortion of the light source 20 while maintaining the initial value of the CNR.

  The optical transmission apparatus of the present invention is suitable for suppressing the occurrence of SBS and the like, and is useful for transmitting an optical signal over a long distance.

1 is a block diagram showing an example of the configuration of an optical transmission apparatus according to a first embodiment of the present invention. A diagram showing the relationship between the bias current applied to a general light source and the light emission power Schematic diagram showing the spectrum of the optical signal output when the intensity of the light source is directly modulated The figure which shows the relationship (measurement result) between light modulation degree and reflected light power Schematic diagram showing how SBS generation conditions are defined at the highest frequency during SCM transmission The figure which shows the variation | change_quantity of the light modulation degree at the time of reducing the light emission power of the light source 20 1dB. The figure which shows the relationship between light reception power and CNR The block diagram which shows another structural example of the optical transmission apparatus which concerns on the 1st Embodiment of this invention. The block diagram which shows another structural example of the optical transmission apparatus which concerns on the 1st Embodiment of this invention. The block diagram which shows an example of a structure of the optical transmission apparatus which concerns on the 2nd Embodiment of this invention. Block diagram showing an example of the configuration of a conventional optical transmission apparatus The figure explaining the problem in the conventional optical transmission apparatus

Explanation of symbols

10, 910 Input signal 20, 920 Light source 30, 930 Control unit 40 Signal level detection unit 50 Signal frequency detection unit 60 Signal level adjustment unit 970 Demultiplexing unit 980 Optical detection unit 990 Optical transmission line

Claims (7)

An optical transmission device that converts an input signal, which is an electrical signal, into a light intensity signal and transmits the signal,
A light source that outputs the input signal as an optical signal that is an intensity signal of light based on a predetermined bias current;
A bias current control unit configured to control a bias current supplied to the light source based on an input signal level indicating the level of the input signal and an input signal frequency indicating the frequency of the input signal; Optical transmission device. The bias current controller is
A signal level detector that detects the level of the input signal and outputs the input signal level;
A signal frequency detector that detects the frequency of the input signal and outputs the input signal frequency;
The optical transmission apparatus according to claim 1, further comprising: a control unit that controls a bias current supplied to the light source based on the input signal level and the input signal frequency.   The bias current control unit is a controller that receives the input signal level and the input signal frequency, and controls a bias current supplied to the light source based on the input signal level and the input signal frequency. The optical transmission device according to claim 1, further comprising: A signal level adjustment unit for adjusting the level of the input signal;
4. The optical transmission device according to claim 2, wherein the control unit controls the signal level adjustment unit based on the input signal level and the input signal frequency. 5.   The optical transmission device according to claim 1, wherein the signal frequency detection unit is a highest signal frequency detection unit that detects a highest frequency among the input signals.   The control unit calculates a light modulation degree of an optical signal output from the light source from a bias current supplied to the light source and the input signal level, and a quotient of the calculated light modulation degree and the input signal frequency is calculated. 6. The optical transmission device according to claim 2, wherein the degree of optical modulation is adjusted by controlling a bias current supplied to the light source so as to be maintained substantially constant. The optical transmission apparatus according to claim 1, wherein an output light wavelength of the light source is different from a zero dispersion wavelength of an optical fiber connected to the light source.

Images