JPH09148661A - Optical noise suppression circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To restrain signal fluctuation without performing recognition regeneration on an electric stage, by converting an optical signal which is intensity- modulated by a digital signal into an electric signal having the state of signs corresponding to ON OFF, and inputting the signal of the form that the signs of the electric signal are inverted via a converting means, in a semiconductor laser in the state of oscillation. SOLUTION: A signal light IM which is intensity-modulated by a digital signal is inputted in a photoelectric converting means 11, which outputs an electric signal whose voltage is -Vm and +Vm corresponding to ON and OFF of the signal light IM. A modulation means 12 adds the electric signal and the bias voltage Vb of a semiconductor laser 13, and converts the electric signal into an electric signal E whose voltage is (Vb-Vm) or (Vb+Vm) when the signal light IM is ON or OFF, respectively. The modulation means 12 applies the electric signal E to the anode side of the semiconductor laser 13. The laser oscillation turns off when the signal light IM is ON, and turns on when the signal light IM is OFF, so that a signal light IM' which is complementarily modulated to the signal light IM is outputted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical noise suppression circuit that suppresses signal fluctuations due to noise light superimposed on signal light while propagating in an optical fiber transmission line, for example.

[0002]

2. Description of the Related Art In optical fiber communication, noise light such as spontaneous emission light of an optical amplifier inserted into an optical fiber transmission line and four-wave mixing light generated in the optical fiber transmission line during wavelength division multiplex transmission is generated. The signal fluctuation caused by the superposition of the noise light on the signal light is a factor limiting the transmission distance. Therefore, when performing long-distance transmission, it is necessary to remove signal fluctuation due to noise light for each predetermined section.

A first conventional method for eliminating signal fluctuations is
The optical signal is once converted into an electrical signal, and the electrical stage identifies the signal.
This is a method of converting the optical signal again after reproducing. A second conventional method for removing signal fluctuation is a method for removing signal fluctuation without converting an optical signal into an electric signal. This is a method to suppress the oscillation of the semiconductor laser by injecting signal light with different wavelengths into the oscillating semiconductor laser from the outside (K.Inoue and K.Oda, "Nois.
e reduction in wavelength conversion using a light i
njected laser diode ", IEEE Photonics Technology Le
tters, vol.7, pp.500-501, 1995). The operating principle of this method will be described with reference to FIG.

When light is injected into the oscillating semiconductor laser from the outside, carrier electrons are consumed for stimulated emission of the injected light, and the output of the oscillated light is reduced accordingly. As shown in FIG. 14, when the injected light power is increased,
Along with that, the oscillation light power decreases, and when the level exceeds a predetermined level (hereinafter referred to as “suppression threshold”), oscillation stops.
Therefore, the intensity-modulated signal light IM fluctuating due to noise light
Is injected into the semiconductor laser at a level above the suppression threshold,
The oscillating light IM 'is complementarily modulated such that it is turned off when the signal light is on and turned on when the signal light is off.
This is equivalent to effectively converting the transmission signal from the injection light into the oscillation light of the semiconductor laser. Signal fluctuations are suppressed during this signal conversion process. The reason will be described below.

Here, consider a signal fluctuation induced by noise light such as spontaneous emission light or four-wave mixing light. Signal fluctuations due to noise light mainly occur when the signal light is on. This is because the fluctuation when the signal light is on is caused by the interference with the noise light, the fluctuation when the signal light is off is caused by the interference between the noise lights, and the interference fluctuation is proportional to the product of the two components causing the interference. However, since the signal light level is higher than the noise light level, the signal fluctuation when the signal light is on becomes larger than when it is off.

As described above, the level of the signal light fluctuates mainly when it is on. However, since the on level is set to be equal to or higher than the suppression threshold of the semiconductor laser, the oscillation light is turned off even if the signal light fluctuates. It does not become a fluctuation just by becoming. That is, in the process of converting the injected light into the oscillated light, the fluctuation of the injected light is removed.

[0007]

In the first prior art,
The structure is complicated because the signal is identified and reproduced at the electric stage, and when processing a high-speed signal, an identification / reproduction circuit that operates at high speed is required. The second prior art is
There is a problem that frequency chirping occurs in the signal conversion process. When the semiconductor laser is switched between the oscillating state and the non-oscillating state, the carrier density in the active layer changes greatly, and the refractive index changes accordingly. When the refractive index changes, the effective resonator length of the semiconductor laser changes, which changes the oscillation frequency. This phenomenon is called frequency chirping, and the signal light in which the frequency chirping has occurred distorts the signal waveform due to the difference in the propagation speed corresponding to the frequency in the optical fiber. This is not desirable in optical fiber communication.

The present invention is an optical noise suppression circuit capable of suppressing signal fluctuations without performing identification / reproduction at an electric stage in comparison with the first prior art, and frequency chirping with respect to the second prior art. It is an object of the present invention to provide an optical noise suppression circuit that can suppress signal fluctuations by performing signal conversion without causing this.

[0009]

The optical noise suppression circuit according to claim 1 solves the problem of the first prior art. The signal light intensity-modulated by the digital signal is turned on
The electric signal is converted into an electric signal having a code state corresponding to OFF, and the electric signal is inputted to the semiconductor laser in the oscillating state in a form in which the sign of the electric signal is inverted via the modulation means. At this time, the semiconductor laser
The oscillation is stopped for the electric signal when the signal light is on. As a result, the ON state of the signal light is converted into the OFF state of the output light of the semiconductor laser, so that the signal fluctuation superimposed on the ON level of the signal light can be suppressed without performing the identification / reproduction at the electric stage.

The optical noise suppression circuit according to claim 2 solves the problems of the first and second prior arts. The signal light intensity-modulated by the digital signal is converted into a frequency-modulated signal corresponding to its on / off. The optical frequency selection means inputs the frequency-modulated signal light, blocks the frequency component when the intensity-modulated signal light is on, and passes the frequency component when the intensity-modulated signal light is off. As a result, the ON / OFF of the input signal light is converted into the OFF / ON of the output light of the optical frequency selection means, and the output light in which the signal fluctuation superimposed on the ON level of the input signal light is suppressed can be obtained.

The optical noise suppression circuit according to claim 3 solves the problems of the first and second prior arts. The signal light intensity-modulated by the digital signal is converted into an electric signal having a binary state corresponding to ON / OFF, and the electric signal is input to the oscillating semiconductor laser. At this time, the oscillation light of the semiconductor laser is frequency-modulated according to the binary state of the electric signal without causing frequency chirping. The optical frequency selection means inputs the frequency-modulated oscillation light, blocks the frequency component when the signal light is on, and passes the frequency component when the signal light is off. This turns on the input signal light.
The off state is converted into the off state and the on state of the output light of the optical frequency selection means, and the output light in which the signal fluctuation superimposed on the on level of the signal light is suppressed can be obtained.

An optical noise suppression circuit according to claim 4 solves the problems of the first and second prior arts. The signal light intensity-modulated by the digital signal is input to the oscillating semiconductor laser. At this time, the oscillation light of the semiconductor laser is
Frequency modulation is performed by turning on / off the signal light without causing frequency chirping. The optical frequency selection means inputs the frequency-modulated oscillation light, blocks the frequency component when the signal light is on, and passes the frequency component when the signal light is off. As a result, the ON / OFF of the input signal light is converted into the OFF / ON of the output light of the optical frequency selection means, and the output light in which the signal fluctuation superimposed on the ON level of the signal light is suppressed can be obtained.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION

(Embodiment of claim 1) FIG. 1 shows an embodiment of the optical noise suppression circuit of claim 1. In the figure, photoelectric conversion means 1
1 is a signal light IM intensity-modulated by a digital signal
, And outputs an electric signal corresponding to ON / OFF of the intensity modulated signal light IM. For example, the intensity modulated signal light IM
It is assumed that an electric signal having a voltage of −Vm is output when the switch is turned on and a voltage of + Vm is output when the switch is turned off. The modulation means 12 adds this electric signal and the bias voltage Vb of the semiconductor laser 13, and when the intensity-modulated signal light IM is on, the voltage (Vb-V).
m), the electric signal E of voltage (Vb + Vm) is applied to the anode side of the semiconductor laser 13 when it is off.

Here, as shown in FIG. 2, the oscillation threshold voltage of the semiconductor laser 13 is set to Vth so that 0 <(Vb-Vm) <Vth and Vth <(Vb + Vm). The "inverted form of the sign" in claim 1 means the relationship between the applied voltage and the semiconductor laser 13. As a result, the laser oscillation is turned off when the intensity modulation signal light IM is on, and the laser oscillation is turned on when the intensity modulation signal light IM is off.
That is, the intensity-modulated signal light IM 'that is modulated complementarily to the intensity-modulated signal light IM is output.

In this signal conversion process, signal fluctuations induced by noise light such as spontaneous emission light and four-wave mixing light are suppressed. That is, as described above, the on-level of the signal light fluctuates mainly, but the oscillation of the semiconductor laser is turned off when the input signal light is on, so that the output light does not fluctuate. On the other hand, the fluctuation of the off level of the input signal light is the fluctuation of the on level of the output signal light, but as described above, the fluctuation of the input signal light due to the noise light is smaller when it is off than when it is on. The fluctuation of the signal light is smaller than that of the input signal light. Therefore, the configuration of FIG. 1 makes it possible to obtain output light with reduced signal fluctuation.

Further, the photoelectric conversion means 11 may be so constructed as to output an electric signal of voltage + Vm when the intensity modulated signal light IM is turned on and voltage -Vm when turned off. In this case, the bias voltage of the semiconductor laser 13 is set to -Vb, and the modulation means 12 applies the voltage when the intensity modulation signal light IM is on.
An electric signal E having a voltage (-Vb-Vm) and a voltage (-Vb-Vm) when off is applied to the cathode side of the semiconductor laser 13. Further, assuming that the oscillation threshold voltage of the semiconductor laser 13 is −Vth, 0> (− Vb + Vm)> − Vth and −Vth> (− Vb−V
m). The "inverted form of the sign" in claim 1 means the relationship between the applied voltage and the semiconductor laser 13. Thereby, the laser oscillation is turned off when the intensity-modulated signal light IM is on, and the laser oscillation is turned on when the intensity-modulated signal light IM is off,
Output light with reduced signal fluctuation can be obtained by the same principle.

If the polarities of the output voltages of the photoelectric conversion means 11 are opposite in the above two examples, an inverter,
Alternatively, the polarity may be inverted through the inverting amplifier and applied to the semiconductor laser 13. Such a configuration is also one form of the "inverted sign form" in claim 1. (Embodiments of Claims 2 and 3) FIG. 3 shows an embodiment of the optical noise suppression circuit of Claims 2 and 3.

In the figure, the photoelectric conversion means 11 receives the signal light IM intensity-modulated by a digital signal,
An electric signal corresponding to ON / OFF of the intensity modulated signal light IM is output. This electric signal is applied to the semiconductor laser 13 together with a constant bias current. Here, unlike the embodiment of claim 1, the grounded state of the semiconductor laser 13 is not limited. Further, the applied electric signal is set so that the semiconductor laser 13 is always oscillated in any of the two states. The semiconductor laser 13 frequency-modulates the oscillation light by this electric signal and outputs it. For example, the intensity modulation signal light IM is frequency-modulated between f ₂ + Δf and f ₂ by turning on and off.

The optical frequency selecting means 14 is the semiconductor laser 1
The frequency-modulated signal light FM output from 3 is input, the frequency component when the intensity-modulated signal light IM is turned on is blocked from the frequency-modulated signal light FM, and the frequency component when the intensity-modulated signal light IM is turned off. Let it pass. For example, as shown in FIG. 5, the peak of the transmission frequency of the optical frequency selection means 14 is made to coincide with the oscillation frequency f ₂ of the semiconductor laser 13. Also,
The transmission band is made narrower than the frequency shift of the oscillation frequency of the semiconductor laser 13 due to ON / OFF of the intensity modulated signal light IM. When the optical frequency selection means 14 is set in this manner, the transparent output is turned on when the intensity modulated signal light IM is off, and the transparent output is turned off when the intensity modulated signal light IM is on. That is, the frequency modulation signal light FM output from the semiconductor laser 13 is converted into the intensity modulation signal light IM ′ by the optical frequency selection means 14.

With the above configuration, the intensity-modulated signal light IM input to the photoelectric conversion means 11 is converted into the frequency-modulated signal light FM by the semiconductor laser 13, and the on / off state is inverted by the optical frequency selection means 14. The intensity-modulated signal light IM 'is converted and output. Here, the principle that the signal fluctuation is suppressed by this configuration will be described.

Intensity-modulated signal light I on which noise light is superimposed
As described above, M has fluctuation in the on-level. As shown in FIG. 5, this signal fluctuation is directly converted into the frequency fluctuation of the optical frequency (f ₂ + Δf) component of the frequency modulation signal light FM corresponding to the turning on of the intensity modulation signal light IM. The frequency-modulated signal light FM is further converted into the intensity-modulated signal light IM ′ by the optical frequency selection means 14, but the frequency fluctuation (fluctuation in signal strength) is suppressed by turning off the intensity-modulated signal light IM ′. Intensity modulated signal light I
There is no fluctuation in the on level of M '. Therefore, with this configuration, it is possible to output the intensity-modulated signal light IM ′ with suppressed signal fluctuation from the intensity-modulated signal light IM on which noise light is superimposed.

Further, since the optical frequency selection means 14 is configured to selectively output only the frequency component of the free oscillation of the semiconductor laser 13, frequency chirping, which is a problem of the second prior art, may occur. It is possible to obtain the noise suppression effect. (Embodiments of claims 2 and 4) FIG. 4 shows an embodiment of the optical noise suppression circuit of claims 2 and 4.

In the figure, a semiconductor laser 13 inputs a signal light IM intensity-modulated by a digital signal, and frequency-modulates the oscillating light by turning on / off the intensity-modulated signal light IM, and outputs it. The injection level of the intensity modulated signal light IM input to the semiconductor laser 13 is set to a level at which the oscillation of the semiconductor laser 13 does not stop, that is, a value smaller than the suppression threshold value shown in FIG. In this case, the carrier density in the semiconductor laser 13 is minutely modulated by turning on / off the intensity-modulated signal light IM, whereby the oscillation frequency is modulated by the intensity-modulated signal light IM. That is, the intensity modulation signal light IM injected into the semiconductor laser 13 is converted into the frequency modulation signal light FM.

The optical frequency of the intensity modulated signal light IM is f
₁ , the frequency of free oscillation of the semiconductor laser 13, that is, the oscillation frequency when the intensity-modulated signal light IM is off is f.
_{If 2} (≠ f ₁ ), the output light of the semiconductor laser 13 is frequency-modulated between f ₂ + Δf and f ₂ by turning on / off the intensity-modulated signal light IM. When f ₁ and f ₂ are close enough to each other for injection locking, the intensity-modulated signal light IM
The frequency modulation is performed between f ₁ and f ₂ by turning on and off.

The optical frequency selecting means 14 is similarly constructed,
Frequency modulated signal light FM output from the semiconductor laser 13
However, the intensity modulation signal light IM ′ is generated by the optical frequency selection means 14.
Is converted to With the above configuration, the intensity-modulated signal light IM input to the semiconductor laser 13 is the frequency-modulated signal light FM.
Is converted into intensity modulated signal light IM 'in which the on / off state is inverted and output from the optical frequency selection means 14.

In this configuration, the signal fluctuation is shown in FIG. 5 when the optical frequency f ₁ of the intensity-modulated signal light IM and the oscillation frequency f ₂ of the semiconductor laser 13 are separated to such an extent that injection locking is not achieved. As described above, the optical frequency (f ₂ + Δ of the frequency-modulated signal light FM corresponding to the turning on of the intensity-modulated signal light IM
f) It is directly converted into the frequency fluctuation of the component. on the other hand,
When the oscillation frequency f ₂ of the semiconductor laser 13 is the intensity modulated signal light I
When injection-locked by M, the intensity-modulated signal light I
Optical frequency f of the frequency-modulated signal light FM corresponding to the turning on of M
Converted to fluctuations in signal strength of _one component. Similarly, the signal fluctuation can be suppressed without causing the frequency chirping through the optical frequency selection means 14.

[0027]

【Example】

(First Example for the Embodiment of Claim 1) FIG. 6 shows a configuration of a first example for the embodiment (FIG. 1) of claim 1. In the figure, the intensity-modulated signal light IM is input to the photodiode 41, and is converted into an electric signal of a voltage −Vm when the intensity-modulated signal light IM is on and a voltage + Vm when it is off. This electric signal is applied to the semiconductor laser 13 from the RF terminal of the bias T42 via the resistor 43. The cathode of the semiconductor laser 13 is grounded, and an electric signal is applied to the anode. A bias voltage + Vb is applied to the anode side via the DC terminal of the bias T42. As a result, the voltage (Vb-Vm) is generated when the intensity-modulated signal light IM is on, and the voltage (Vb + Vm) is generated when it is off.
The electric signal E of m) is applied to the anode side of the semiconductor laser 13.

Here, as shown in FIG. 6B, the oscillation threshold voltage of the semiconductor laser 13 is set to Vth so that 0 <(Vb-Vm) <Vth and Vth <(Vb + Vm). As a result, the intensity-modulated signal light I
When M is on, laser oscillation is off, and when the intensity-modulated signal light IM is off, laser oscillation is on.
An intensity-modulated signal light IM ′ is output which is complementarily modulated with respect to the intensity-modulated signal light IM and in which the signal fluctuation is suppressed.

(Second Example for the Embodiment of Claim 1) FIG. 7 shows a second example for the embodiment of Claim 1 (FIG. 1).
The structure of an Example is shown. In the figure, the intensity modulated signal light IM
Is input to the photodiode 41, and the intensity-modulated signal light I
Voltage + Vm when M is on, voltage -Vm when off
Is converted into an electric signal. This electrical signal is the bias T
It is applied to the semiconductor laser 13 via the resistor 43 from the RF terminal of 42. The semiconductor laser 13 has its anode grounded and an electric signal applied to its cathode. A bias voltage -Vb is applied to the cathode side via the DC terminal of the bias T42. As a result, when the intensity-modulated signal light IM is turned on, the electric signal E having a voltage (-Vb + Vm) and a voltage (-Vb-Vm) when the light is off is supplied to the semiconductor laser 13.
Applied to the cathode side of.

Here, as shown in FIG. 7B, the oscillation threshold voltage of the semiconductor laser 13 is -Vth, and 0> (-Vb + Vm)>-Vth and -Vth> (-Vb-V).
m). As a result, the intensity-modulated signal light I
When M is on, laser oscillation is off, and when the intensity-modulated signal light IM is off, laser oscillation is on.
An intensity-modulated signal light IM ′ is output which is complementarily modulated with respect to the intensity-modulated signal light IM and in which the signal fluctuation is suppressed.

Examples of the embodiments (FIG. 4) of claims 2 and 4 will be described below, but the optical frequency selecting means in the examples of the embodiments (claim 3) of claims 2 and 3 are similarly configured. It (First Example for Embodiments of Claims 2 and 4) FIG.
Shows a configuration of a first example for the embodiment (FIG. 4) of claims 2 and 4.

This embodiment is characterized in that the Fabry-Perot type optical filter 21 is used as the optical frequency selecting means. In (1), the intensity-modulated signal light IM is input from one end surface of the semiconductor laser 13, and the frequency-modulated signal light FM output from the other end surface is input to the Fabry-Perot type optical filter 21 and the intensity-modulated signal light IM is input. It is a structure that is converted into ′.
Since the Fabry-Perot optical filter 21 has a transmission characteristic having periodic peaks, one peak thereof is matched with the frequency of the semiconductor laser 13 during free oscillation.

(2) is characterized in that the intensity modulated signal light IM is input from one end face of the semiconductor laser 13 and the frequency modulated signal light FM output from the same end face is used.
Therefore, the optical circulator 31 that separates the intensity-modulated signal light IM input to the semiconductor laser 13 and the output frequency-modulated signal light FM is used. The frequency-modulated signal light FM output from the optical circulator 31 is converted into the intensity-modulated signal light IM ′ via the Fabry-Perot optical filter 21. An optical coupler may be used instead of the optical circulator 31.

(Second Example for Embodiments of Claims 2 and 4) FIG. 9 shows the configuration of a second example for the embodiment of Claims 2 and 4 (FIG. 4). In this embodiment, a grating (diffraction grating) type optical filter 2 is used as an optical frequency selecting means.
2 is used. That is, the intensity-modulated signal light IM is input from one end face of the semiconductor laser 13, the frequency-modulated signal light FM output from the other end face is input to the grating type optical filter 22, and the reflected light in a predetermined angle direction is intensity-reduced. The modulated signal light IM 'is extracted. In addition,
In the grating type optical filter 22, since the angle of the reflected light and the optical frequency correspond to each other, the frequency light at the free oscillation of the semiconductor laser 13 is set to be extracted as the reflected light.

(Third Example for Embodiments of Claims 2 and 4) FIG. 10 shows the configuration of a third example for the embodiment of Claims 2 and 4 (FIG. 4). The present embodiment is characterized in that an arrayed waveguide diffraction grating type optical filter 23 is used as an optical frequency selection means.

In (1), the intensity modulated signal light IM is input from one end surface of the semiconductor laser 13, and the frequency modulated signal light FM output from the other end surface is input to the arrayed waveguide diffraction grating type optical filter 23. The intensity-modulated signal light IM 'is converted into the intensity modulated signal light IM'. In the arrayed waveguide diffraction grating type optical filter 23, since the input / output port and the optical frequency have a predetermined relationship, the frequency light at the free oscillation of the semiconductor laser 13 is set to be extracted to the predetermined output port.

(2) is characterized in that the intensity modulated signal light IM is inputted from one end face of the semiconductor laser 13 and the frequency modulated signal light FM outputted from the same end face is used.
Therefore, the optical circulator 31 that separates the intensity-modulated signal light IM input to the semiconductor laser 13 and the output frequency-modulated signal light FM is used. The frequency-modulated signal light FM output from the optical circulator 31 is intensity-modulated signal light IM ′ through the arrayed waveguide diffraction grating type optical filter 23.
Is converted to An optical coupler may be used instead of the optical circulator 31.

(3) utilizes the fact that the input / output ports of the arrayed-waveguide diffraction grating type optical filter 23 and the optical frequency have a predetermined relationship, and thus the arrayed-waveguide diffraction grating type optical filter 23 is used.
It has the function of an optical circulator. That is, the intensity-modulated signal light IM having the optical frequency f ₁ is transmitted from the input port a of the arrayed-waveguide diffraction grating optical filter 23 to the output port b, and input to the semiconductor laser 13. Frequency modulated signal light FM output from the semiconductor laser 13
Is the optical frequency f ₂ corresponding to the turning off of the intensity-modulated signal light IM.
Is transmitted from the output port b of the arrayed waveguide diffraction grating type optical filter 23 to the input port c. With this configuration, the oscillated light can be extracted from the signal injection surface of the semiconductor laser 13 without using the optical circulator.

(Fourth Example for Embodiments of Claims 2 and 4) FIG. 11 shows the configuration of a fourth example for the embodiment of Claims 2 and 4 (FIG. 4). The present embodiment is characterized in that a distributed feedback type semiconductor optical filter 24 is used as the optical frequency selecting means.

In (1), the intensity-modulated signal light IM is input from one end face of the semiconductor laser 13, and the frequency-modulated signal light FM output from the other end face is input to the distributed feedback type semiconductor optical filter 24. This is a configuration for converting into modulated signal light IM ′. Since the distributed feedback semiconductor optical filter 24 has a characteristic of transmitting only a predetermined optical frequency, its transmission frequency is matched with the frequency of the semiconductor laser 13 during free oscillation.

(2) is characterized in that the intensity modulated signal light IM is inputted from one end face of the semiconductor laser 13 and the frequency modulated signal light FM outputted from the same end face is used.
Therefore, the optical circulator 31 that separates the intensity-modulated signal light IM input to the semiconductor laser 13 and the output frequency-modulated signal light FM is used. The frequency modulation signal light FM output from the optical circulator 31 is converted into the intensity modulation signal light IM ′ via the distributed feedback semiconductor optical filter 24. An optical coupler may be used instead of the optical circulator 31.

(Fifth Example for the Embodiments of Claims 2 and 4) FIG. 12 shows the configuration of a fifth example for the embodiment of Claims 2 and 4 (FIG. 4). In this embodiment, a fiber grating type optical filter 25 is used as an optical frequency selecting means.
Is used. That is, the semiconductor laser 1
The intensity-modulated signal light IM is input from one end face of No. 3, the frequency-modulated signal light FM output from the other end face is input to the fiber grating type optical filter 25 via the optical circulator 31, and the reflected light thereof is an optical circulator. 31
Is taken out as intensity-modulated signal light IM '. In addition,
Since the fiber grating type optical filter 25 has a characteristic of reflecting only a predetermined optical frequency, its reflection frequency is matched with the frequency of the semiconductor laser 13 during free oscillation.

An optical coupler may be used instead of the optical circulator 31, but in this case, the semiconductor laser 13 is used.
An optical isolator is inserted between the semiconductor laser 13 and the optical coupler in order to avoid the unstable operation of the laser oscillation due to the returning light to the laser. (Sixth Example for Embodiments of Claims 2 and 4) FIG.
Shows a configuration of a sixth example for the embodiment (FIG. 4) of claims 2 and 4.

The present embodiment is characterized in that the interference film type optical filter 26 is used as the optical frequency selecting means. (1)
Receives the intensity-modulated signal light IM from one end face of the semiconductor laser 13 and inputs the frequency-modulated signal light FM output from the other end face into the interference film type optical filter 26 to convert it into the intensity-modulated signal light IM '. This is the configuration. Since the interference film type optical filter 26 can be designed to transmit only a predetermined optical frequency, the transmission frequency is set to the semiconductor laser 13.
Match the frequency of free oscillation of.

(2) is characterized in that the intensity modulated signal light IM is inputted from one end face of the semiconductor laser 13 and the frequency modulated signal light FM outputted from the same end face is used.
Therefore, the optical circulator 31 that separates the intensity-modulated signal light IM input to the semiconductor laser 13 and the output frequency-modulated signal light FM is used. The frequency-modulated signal light FM output from the optical circulator 31 is converted into the intensity-modulated signal light IM ′ via the interference film type optical filter 26.
An optical coupler may be used instead of the optical circulator 31.

(3) utilizes the characteristic of the interference film type optical filter 26 that it transmits at a predetermined optical frequency and reflects it at other optical frequencies, and the interference film type optical filter 26 is provided with an optical circulator. It has a function. That is, the intensity-modulated signal light IM having the optical frequency f ₁ is reflected by the interference film type optical filter 26 and input to the semiconductor laser 13. In the frequency-modulated signal light FM output from the semiconductor laser 13, the component of the optical frequency f ₂ corresponding to the turning off of the intensity-modulated signal light IM passes through the interference film type optical filter 26. With this configuration, the oscillated light can be extracted from the signal injection surface of the semiconductor laser 13 without using the optical circulator.

(Embodiment corresponding to claims 5, 6 and 7) The photoelectric conversion means 11 using the photodiode 41, the semiconductor laser 13, and the bias T42 and the like are included to show the connection relationship between the photodiode 41 and the semiconductor laser 13. The defining modulation means 12 can be manufactured on the same semiconductor substrate by a known technique. Further, the optical frequency selecting means 14 using the Fabry-Perot type optical filter 21 and the like can be manufactured on the same semiconductor substrate together with other components. When using a single mode semiconductor laser,
Since the grating is formed on the same semiconductor substrate, it can also be used as the optical frequency selecting means 14. As a result, stability and mass productivity can be improved, and economic efficiency and reliability can be improved.

(Example Corresponding to Claim 8) In the configurations of the above-described embodiments and examples, the fluctuation of the on-level of the input signal light can be suppressed, but the fluctuation of the off-level changes the fluctuation of the on-level of the output signal light. Is converted to. Further, although the output signal light and the input signal light have a complementary relationship, it may be preferable that the input signal light and the output signal light have the same sign in configuring the system.

To solve these problems, an even number (for example, two) of the above-mentioned optical noise suppressing circuits may be connected in series. As a result, the fluctuation of the on-level of the output signal light of the first stage is suppressed by the circuit of the second stage, and it is possible to obtain the output signal light in which the fluctuations of the on-level and the off-level of the input signal light are suppressed. Further, since the on / off code is inverted at the first stage and further inverted at the second stage, output signal light having the same code as the input signal light can be obtained.

[0050]

As described above, the optical noise suppression circuit of the present invention converts the input signal light into an electric signal and inputs the signal to the oscillating semiconductor laser in the form of an inverted sign, so that the electric stage With, it is possible to obtain the output signal light in which the fluctuation of the input signal light is suppressed without performing the identification / reproduction.

Further, the input signal light is converted into an electric signal and inputted to the semiconductor laser, or directly injected into the semiconductor laser to frequency-modulate the oscillation light, and only the frequency component when the signal light is off is selected. As a result, it is possible to obtain output signal light in which fluctuations of the input signal light are suppressed without performing identification / reproduction in the electrical stage and without causing frequency chirping.

By using the optical noise suppression circuit of the present invention for optical fiber transmission, the transmission distance limited by the spontaneous emission light of the optical amplifier or the four-wave mixing light in the optical fiber transmission line can be further expanded.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of an optical noise suppression circuit according to claim 1.

FIG. 2 is a diagram for explaining the operation principle of the optical noise suppression circuit according to claim 1;

FIG. 3 is a block diagram showing an embodiment of the optical noise suppression circuit according to claims 2 and 3.

FIG. 4 is a block diagram showing an embodiment of the optical noise suppression circuit according to claims 2 and 4.

FIG. 5 is a diagram for explaining the operation principle of the optical noise suppression circuit according to claims 2, 3 and 4;

FIG. 6 is a diagram showing a configuration of a first example with respect to the embodiment (FIG. 1) of claim 1;

FIG. 7 is a diagram showing a configuration of a second example with respect to the embodiment (FIG. 1) of claim 1;

FIG. 8: First to the embodiment (FIG. 4) of claims 2 and 4
The figure which shows the structure of an Example.

9 a second to the embodiment of claims 2 and 4 (FIG. 4)
The figure which shows the structure of an Example.

FIG. 10 is a diagram showing the configuration of a third example with respect to the embodiment (FIG. 4) of claims 2 and 4;

FIG. 11 is a diagram showing a configuration of a fourth example with respect to the embodiment (FIG. 4) of claims 2 and 4;

FIG. 12 is a diagram showing the configuration of a fifth example with respect to the embodiment (FIG. 4) of claims 2 and 4;

FIG. 13 is a diagram showing the configuration of a sixth example with respect to the embodiments (FIG. 4) of claims 2 and 4;

FIG. 14 is a diagram for explaining the operation principle of a conventional optical noise suppression circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 photoelectric conversion means 12 modulation means 13 semiconductor laser 14 optical frequency selection means 21 Fabry-Perot type optical filter 22 grating type optical filter 23 array waveguide diffraction grating type optical filter 24 distributed feedback type semiconductor optical filter 25 fiber grating type optical filter 26 Interference film type optical filter 31 Optical circulator 41 Photodiode 42 Bias T 43 Resistor

Claims (8)

[Claims] 1. An opto-electric conversion means for inputting signal light intensity-modulated by a digital signal and converting it into an electric signal having a code state corresponding to ON / OFF of the signal light, and oscillating by applying a predetermined bias voltage. A semiconductor laser set to a state, and a modulation for inputting the electric signal to the semiconductor laser in a form in which its sign is inverted, and for making the semiconductor laser stop oscillating with respect to the electric signal when the signal light is on. An optical noise suppression circuit comprising: 2. A means for converting signal light intensity-modulated by a digital signal into frequency-modulated signal light and outputting the frequency-modulated signal light, and inputting the frequency-modulated signal light to block a frequency component when the signal light is on. An optical noise suppression circuit comprising: an optical frequency selection unit that allows a frequency component when the signal light is off. 3. A photoelectric conversion means for inputting signal light intensity-modulated by a digital signal and converting it into an electric signal having a binary state corresponding to ON / OFF of the signal light, and by applying a predetermined bias voltage. A semiconductor laser which is set to an oscillating state and frequency-modulates and outputs oscillated light in accordance with the binary state of the electric signal; and the frequency-modulated oscillated light is input to block the frequency component when the signal light is on. And an optical frequency selecting means for allowing a frequency component when the signal light is off to pass therethrough. 4. A semiconductor laser which inputs signal light intensity-modulated by a digital signal, frequency-modulates and outputs oscillation light by turning the signal light on and off, and inputs the frequency-modulated oscillation light, An optical noise suppression circuit, comprising: an optical frequency selection unit that blocks a frequency component when the signal light is on and passes a frequency component when the signal light is off. 5. The optical noise suppression circuit according to claim 1, wherein the photoelectric conversion means, the modulation means and the semiconductor laser are formed on the same semiconductor substrate. . 6. The optical noise suppressing circuit according to claim 3, wherein the photoelectric conversion means, the semiconductor laser, and the optical frequency selecting means are formed on the same semiconductor substrate. Suppression circuit. 7. The optical noise suppression circuit according to claim 4, wherein the semiconductor laser and the optical frequency selection means are formed on the same semiconductor substrate. 8. An optical noise suppression circuit having a configuration in which an even number of the optical noise suppression circuits according to any one of claims 1 to 7 are connected in series.