JPH06164049A - Tabilization circuit of optical output waveform - Google Patents

Abstract

(57) [Abstract] [Object] An optical transmitter having a laser module having a light emitting circuit, a light receiving circuit for monitor light output from the light emitting circuit, and a drive circuit for driving the light emitting circuit of the laser module. The present invention relates to an optical output waveform stabilizing circuit for stabilizing the output waveform of a light emitting circuit, and an object thereof is to provide a circuit for simultaneously stabilizing the amplitude, vertical symmetry, and extinction ratio of the optical output waveform. [Configuration] An average value output voltage output from the two detection circuits by inputting an electrical signal output from a photodiode by electrically converting monitor light output from the light emitting circuit to an average value detection circuit and a peak value detection circuit. And a peak value output voltage are applied to an arithmetic circuit, and in the arithmetic circuit, a voltage proportional to a difference voltage between a voltage twice the average value output voltage and the peak value output voltage is generated,
The generated differential voltage is fed back to the laser drive circuit.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical output waveform stabilizing circuit applied to an optical transmitter, and more particularly to an optical output waveform capable of stabilizing output amplitude, vertical symmetry of waveform, and extinction ratio. Regarding the stabilizing circuit.

Optical communication is based on the low loss / broad band property of optical fibers and the non-interference property of optical signals and electric signals, such as a backbone communication network which is a communication infrastructure of society, a corporate premises communication network, and a computer. It is used in a very wide range, such as wiring between devices in a system.

As a modulation method in optical communication, a direct modulation method in which the level of an input data signal is converted into an optical output level of a light emitting element, and an optical output of a constant level is taken out from the light emitting element, and an input data signal is output in an optical waveguide. There is an external modulation method that controls the transmission level by. It is expected that these will be selectively used in fields suitable for each, and will be applied, but it is expected that the direct modulation method, which was previously developed and introduced in large quantities, will continue to be the mainstream. In this sense, technical development related to the direct modulation system will continue to be important.

Regarding the light emitting element, a light emitting diode was initially used, but in particular, it is replaced with a laser diode in terms of high-speed response, and the share of the laser diode is almost 100% in the communication field at present. Therefore, the technical development by the direct modulation method using the laser diode is actively continued, especially, the input waveform fluctuation,
It is strongly desired to develop a technique for keeping the amplitude of the optical output, the vertical symmetry of the waveform, and the extinction ratio constant even when the power supply changes or the characteristics of the laser diode itself change.

[0005]

2. Description of the Related Art FIG. 8 is a block diagram of an optical transmitter for explaining the operation of a conventional optical output waveform stabilizing circuit.

In FIG. 8, 1 is a laser drive circuit, and 1
1 is an input circuit, 12 is a first current source circuit, 13 is a second current source circuit, 14 is a reference voltage setting circuit, 2 is a laser module, 21 is a light emitting circuit, 22 is a light receiving circuit, and 23 is a temperature detector. 3b is an optical output waveform stabilizing circuit, 31 is an average value detecting circuit, 35 is a second error amplifier, and 36 is a third error amplifier.

The input circuit generates a voltage corresponding to the difference between the input data signal and the output voltage of the reference voltage setting circuit and applies it to the first current source circuit. The first current source circuit supplies the laser diode of the light emitting circuit with a pulse current that determines the output light amplitude corresponding to the amplitude of the input data signal.
The second current source circuit is connected to the connection point between the first current source circuit and the light emitting circuit and supplies a bias current, which is a current when the input data signal is at a low level, to the laser diode.

Most of the light emission output of the laser diode is input to the optical fiber and is transmitted. At the same time, monitor light, which is a part of the light emission output of the laser diode, is incident on the photodiode of the light receiving circuit, and the output obtained by electrically converting it is applied to the average value detection circuit 31.

The output voltage of the average value detection circuit is applied to the second error amplifier 35, and the error voltage with the second reference voltage, which is the output voltage of the average value detection circuit when the optical output waveform is normal, is calculated. The generated error voltage is applied to the pulse current control terminal of the first current source circuit 12 to keep the amplitude of the optical output waveform constant.

Further, the voltage corresponding to the temperature change output from the temperature detector 23 of the laser module is applied to the third error amplifier 36, and the third reference which is the output voltage of the temperature detector at room temperature is applied. An error voltage with respect to the voltage is generated, and the error voltage is applied to the bias current control terminal of the second current source circuit 13 to suppress the change in the current-optical output characteristic of the laser diode due to the temperature change.

FIG. 9 is a diagram for explaining the operation of the conventional optical output waveform stabilizing circuit, and is a diagram for explaining the operation when the conversion efficiency of the laser diode changes. In FIG. 9, the vertical axis represents the optical output power and the horizontal axis represents the current of the laser diode. The diagonal line a is a normal current-optical output power characteristic (this slope is the conversion efficiency), and the diagonal line b is a current-optical output power characteristic when the conversion efficiency is changed. Now, set the bias current to Ib in FIG. 9 and set the pulse current to Ip in FIG.
When set to, the optical output signal has an amplitude indicated by c in the figure corresponding to the characteristic a. When the characteristic changes to b, the optical output signal changes to the amplitude shown in d of FIG. By applying the average value of the changed signal to the second error amplifier and feeding back the error voltage to the pulse current control terminal, the pulse current changes to Ip ′, the amplitude of the optical output signal becomes e, and the characteristic The amplitude is controlled to be equal to the amplitude c corresponding to a.

FIG. 10 is a diagram for explaining the operation of the conventional optical output waveform stabilizing circuit, and is a diagram for explaining the operation when the threshold current of the laser diode fluctuates due to temperature fluctuation.

In FIG. 10, the vertical axis and the horizontal axis are the same as in FIG. 9, and the diagonal line a is also the same as in FIG. The diagonal line f is the current-light output characteristic of the laser diode that has changed due to temperature fluctuations. The voltage output from the temperature detector is applied to the second error amplifier in response to the change in temperature to generate an error voltage with respect to the second reference voltage, and the error voltage is biased by the second current source circuit. By applying it to the current control terminal, the bias current changes from Ib to Ib '. That is, the amplitude of the optical signal output, which was initially c ′, is once reduced to d ′, but finally becomes e ′ by the control of the bias current, and becomes equal to the initial amplitude of the optical signal output.

However, it is not caused by the temperature fluctuation, and for example,
When the bias current fluctuates due to fluctuations in the element characteristics of the second current source circuit or fluctuations in the power supply, the bias current cannot be controlled because there is no corresponding sensor, and as a result, the output waveform changes. . Although not shown, especially when the bias current is lowered, the optical output waveform is cut at the lower side to be vertically asymmetrical and the amplitude is reduced. At this time, the error voltage based on the output voltage of the average value detection circuit changes to control the current of the first current source circuit, so the average value of the optical output waveform can be controlled to a normal value, but Asymmetry is not corrected. Further, since only the average value is returned to the normal value, the laser diode emits light (usually called zero emission) even when the input signal is at a low level, which causes deterioration of the error rate on the receiving side.

Further, the optical output waveform cannot be kept constant even when the vertical symmetry of the input data signal changes. FIG. 11 is a diagram showing a change in the optical output waveform when the vertical symmetry of the input data signal is lost. FIG. 11A is an eye diagram of the data signal in which the vertical symmetry is lost, in which the intersection of the waveforms transitioning between the high level and the low level is lower than the center point of the high level and the low level. Is expressed in accordance with the level of a normal signal. In this eye diagram, since the intersection of the waveforms is lower than the center value of the high level and the low level, the average value of the optical output waveform decreases, and when the output of the first error amplifier is applied to the pulse current control terminal, the average value The effect of feedback occurs in the direction of increasing, and as shown in (b), the average value is equal to the original value, but the optical output amplitude becomes larger than the original amplitude and becomes stable. Conversely, when the intersection of the waveforms is higher than the center value of the high level and the low level, the average value voltage rises, so if the output voltage of the first error amplifier is applied to the pulse current control terminal, the average value The effect of feedback that decreases the output is generated, the output amplitude becomes smaller than that in the normal state, and the state of zero emission becomes stable.

That is, the conventional optical output stabilizing circuit may not prevent the deterioration of the waveform, which is not desirable.

[0017]

SUMMARY OF THE INVENTION The present invention addresses the above problems and provides an optical output waveform stabilizing circuit that keeps the optical output waveform constant even when there are power supply fluctuations, element characteristic fluctuations, input waveform fluctuations, and the like. The present invention aims to improve the stability and reliability of the optical communication device.

[0018]

FIG. 1 is a main block diagram of an optical transmitter for explaining the principle of the present invention. In FIG. 1, 1 is a laser driving circuit, 2 is a laser module, 21 is a light emitting circuit, 22 is a light receiving circuit, 3 is a light output waveform stabilizing circuit, 31 is an average value detection circuit, 32 is a peak value detection circuit, 33 Is an arithmetic circuit, and 34a is an error amplifier.

The feature of the present invention is that the optical output waveform stabilizing circuit 3 is provided.
The output voltage of the average value detection circuit 31 and the peak value detection circuit 32 provided in the above is applied to the arithmetic circuit 33, and the output is calculated by using the output voltage of the average value detection circuit and the output voltage of the peak value detection circuit. Is provided to the laser drive circuit via the error amplifier 34a.

Here, the arithmetic circuit is constructed according to the following principle. FIG. 2 is a diagram showing the output voltage of the photodiode included in the light receiving circuit in relation to the current-light output characteristics of the laser diode.

In the output voltage of the photodiode of FIG. 2, V _H is the peak voltage, V _M is the average voltage, and V _L is the minimum voltage. Therefore, from the above definition, V _M = (V _H + V _L ) / 2 holds. Obtaining a relational expression _{V L = 2 · V M -V} H if deformed it.

From this, if V _L = 0 as shown in FIG. 2, the optical output waveform becomes vertically symmetrical and zero emission is eliminated. Therefore, it is not necessary to correct the waveform. The error rate performance is the best.

On the other hand, when the vertical symmetry of the optical output waveform is lost, it is necessary to correct the waveform, but in this case V
_{Since L} ≠ 0, V _L can be used for waveform correction. That is, the arithmetic circuit is configured to generate a voltage n · _VL that is proportional to the difference voltage between twice the average value voltage of the optical output waveform and the peak value voltage.

[0024]

Before explaining the function of the present invention, the effect of the reference voltage on the vertical asymmetry of the eye pattern of the input data signal waveform will be described.

First, when the eye pattern is vertically symmetrical, the intersection of the waveforms is naturally at the center of the high level and the low level. This means that the reference voltage matches the center value of the logic level.

Now, assuming that the reference voltage has risen, the input data signal does not go to a high level unless it goes to a level higher than usual, and it can go to a low level at a level higher than usual, so that the output eye is In the pattern, the center of gravity is shifted to the low level side, that is, the average value of the output waveform decreases. On the contrary, when the reference voltage is lowered, the output eye pattern has a shape in which the center of gravity is moved to the high level side, and the average value of the output waveform becomes high.

Therefore, the above nV _L is converted to the error amplifier 34a.
And the error voltage with respect to the reference voltage of the error amplifier is fed back to the reference voltage setting terminal of the laser drive circuit, it is possible to maintain the vertical symmetry of the output waveform.

That is, the vertical symmetry of the output waveform is lost,
When the average value voltage decreases, the above n · _VL becomes a negative voltage. On the other hand, when the reference voltage decreases, the average value of the output waveform increases. Therefore, the vertical symmetry of the output waveform can be changed by applying the change of n · _VL to the reference voltage setting terminal, and as a result, the vertical symmetry can be maintained. On the contrary, when the average value of the output waveform becomes high, the above n · _VL becomes a positive voltage. On the other hand, when the reference voltage rises, the average value of the output waveform decreases, so n · V _L
By applying the change of (4) to the reference voltage setting terminal, the up-down symmetry of the output waveform can be maintained.

At the same time, the fluctuation of the reference voltage can be suppressed.
it can. Because when the reference voltage fluctuates,
The vertical symmetry of the output waveform fluctuates, and as a result n is detected.
・ V _LReference voltage setting via the first error amplifier
This is because it is returned to the terminal.

[0030] Also, the laser diode current when the input data signal is low, i.e. terminal for controlling the bias current, i.e. the bias current control terminal, if feedback the change of the n · V _L, the laser diode bias It is possible to suppress fluctuations in output waveform due to fluctuations in current.

That is, when the bias current increases, the output waveform shifts to the higher level. For this reason, the above-mentioned n · _VL becomes a positive voltage by the amount of level shift, and therefore the bias current can be kept constant by giving a change opposite to this change to the bias current control terminal. It goes without saying that the same effect is exerted on the decrease of the bias current.

At the same time, even if the threshold current of the laser diode changes, the fluctuation of the output waveform can be suppressed by feeding back V _L to the bias current control terminal.

[0033]

FIG. 3 is a block diagram of an optical transmitter for explaining an embodiment of the present invention. In FIG. 3, 1 is a laser drive circuit, 11 is an input circuit, 12 is a first current source circuit,
13 is a second current source circuit, 14 is a reference voltage setting circuit, 2
Is a laser module, 21 is a light emitting circuit, 22 is a light receiving circuit, and 23 is a temperature detector. Further, 3 is an optical output waveform stabilization circuit, 31 is an average value detection circuit, 32 is a peak value detection circuit, 33 is an arithmetic circuit, 34 is a first error amplifier, 35 is a second error amplifier, and 36 is a second error amplifier. It is the third error amplifier.

[0034] That is, the voltage n · V _L which is proportional to the differential voltage of twice the peak value voltage of the average value voltage, the first error amplifier 34
Through the reference voltage setting circuit 14 and returns the average value voltage to the second error amplifier 3
The output voltage of the temperature detector 23 to the pulse current control terminal of the first current source circuit 12 via the third error amplifier 3
It is fed back via 6 to the bias current control terminal of the second current source.

Since the voltage at the reference voltage setting terminal is originally the voltage V ₁ at the center of the logic level of the input data signal, the first error amplifier generates a voltage corresponding to the difference between n · V _L and V _1. To do. Also, originally, the voltage of the pulse current control terminal is
Since it is set to be equal to the average value voltage V ₂ of the signal output from the input circuit when operating normally, the second error amplifier corresponds to the difference between the average value voltage and V ₂ when there is a fluctuation factor. To generate a voltage. Further, originally, the voltage of the bias current control terminal is the output voltage V of the temperature detector at room temperature.
_Since it is set equal to ₃ , in the third error amplifier 36,
A voltage corresponding to the difference between the output voltage of the temperature detector and V ₃ when the temperature changes is generated.

The change in the output voltage of the average value detection circuit 31 is applied to the pulse current control terminal of the first current source circuit via the second error amplifier 35 to control the average value of the output waveform. To do. Further, the change in the output voltage of the temperature detector 23 is applied to the bias current control terminal of the second current source circuit 13 via the third error amplifier 36, and the current-optical power of the laser diode due to temperature fluctuations. Suppress fluctuations in characteristics. However, this alone will keep the output waveform constant against variations in the output waveform's vertical symmetry due to variations in the input waveform's vertical symmetry / reference voltage variations, and variations in the laser diode characteristics due to causes other than temperature variations. You can't do that already.

The feature of the embodiment of the present invention is that a voltage proportional to the difference voltage between the peak value voltage and twice the average value voltage of the output waveform is generated in the arithmetic circuit 33, and the change of the generated voltage. Is fed back to the reference voltage setting terminal of the reference voltage setting circuit 14 via the first error amplifier 34. Hereinafter, this point will be mainly described.

First, a configuration example of a circuit for generating a difference voltage between twice the average value voltage and the peak value voltage will be described. FIG. 4 is a configuration example of a main part of an arithmetic circuit for generating a difference voltage between twice the average value voltage and the peak value voltage.

In FIG. 4, 331 is a differential amplifier, 33
2 to 334 are resistors. When the negative-phase gain when negative feedback is applied to the differential amplifier is (-G), the common-mode gain is (G +
By making use of the fact that 1), G given by (value of resistance 333) ÷ (value of resistance 332) is made equal to 1, in-phase gain becomes 2 and differential gain becomes (−1). Therefore, if the average value voltage is applied to the in-phase input terminal and the peak value voltage is applied to the anti-phase input terminal of the circuit of FIG. It is possible to obtain a voltage difference between the doubled voltage and the peak value voltage. In addition, here
Voltage source drive is implicitly assumed. Also, the resistor 33
Since 4 is irrelevant to the gain of this circuit, it does not need to be primary, but it is desirable to insert it in series with the common mode input terminal in order to reduce the offset voltage.

However, the configuration of the circuit for generating V _L is not limited to the above, and the average value voltage and the peak value voltage are input to the amplifiers of gain 2 and gain (−1) respectively, and their outputs are output. There are unlimited variations such as connecting terminals. If a circuit having a required gain is connected to the output terminal of such a circuit, an arithmetic circuit for generating a voltage n · _VL that is proportional to the difference voltage between twice the average voltage and the peak voltage is configured. it can.

Now, the vertical symmetry of the output waveform is lost, and
When the average voltage decreases, the output voltage n · V of the arithmetic circuit_LIs
Become negative. On the other hand, when the reference voltage drops, the output waveform averages.
Since the value has a relationship of shifting to the high level side, the n.
V_LVoltage that changes in the same direction as
If applied to the child, it can be controlled to maintain the symmetry of the output waveform.
Wear. Therefore, n · V_LIs the common mode input of the first error amplifier
Applied to the terminal and the first reference voltage voltage to the negative phase input terminal
V₁Is applied to the reference voltage setting terminal, V ₁To V_L
It is possible to give a voltage in which the variation of Now
In that case, feedback is applied in the direction of decreasing the reference voltage.
The input voltage applied to the first current source circuit 12 from the input circuit.
The average value of the data waveform rises and eventually rises above the optical output waveform.
The lower symmetry can be maintained. Conversely, the average value of the output waveform is
When rising, V_LIs positive, the third error amplifier
Output voltage is V₁Higher, which causes the input circuit to
The average value of the data waveform applied to the first current source circuit is low.
It is controlled in the downward direction, and finally the upper and lower pairs of the optical output waveform are paired.
The name is maintained.

The reason why the vertical symmetry of the optical output waveform is lost is that when the symmetry of the waveform of the input data signal is lost, the reference voltage is relative to the center value of the logical amplitude even if the waveform of the input data signal is normal. Error occurs, the threshold current of the laser diode changes irrespective of temperature (if it is due to temperature change, the circuit that feeds back the voltage change from the temperature detector can suppress it) changes the bias current. Then, it may become smaller than the threshold current. According to the optical output waveform stabilizing circuit of the embodiment of the present invention, the change in the vertical symmetry of the waveform can be suppressed regardless of the cause.

As an example, a case where the threshold current of the laser diode becomes larger than the bias current will be described with reference to the drawings. FIG. 5 is a diagram for explaining the operation of keeping the vertical symmetry of the waveform and the amplitude constant when the threshold current of the laser diode becomes larger than the bias current. In this figure, only the part where the signal crosses the high level and the low level in the eye pattern of the optical output waveform is shown.

FIG. 5a shows the current-light output characteristics of the laser diode. As in the case shown in FIG. 2, if the bias current is equal to the threshold current, the optical output waveform is also symmetrical with respect to the symmetrical input waveform (indicated by g).
However, when the bias current decreases, the laser diode does not emit light on the low level side of the input data signal. Therefore, the vertical symmetry of the optical output waveform is broken (indicated by h), the output amplitude is reduced, and at the same time, the average value voltage is reduced and the output voltage n · _{VL of the} arithmetic circuit is also a negative voltage. . Since the circuit configuration of FIG. 3 has a function of feeding back the change of the average value of the output signal to the pulse current control terminal, it acts to cancel the change of the average value voltage, and at the same time, outputs the output of the arithmetic circuit. Since the action of canceling the change in the voltage n · V _L and returning it to 0 V works, the amplitude and the vertical symmetry of the optical output waveform are finally maintained (indicated by i). In this case, while the bias current remains unchanged, the pulse current changes from Ip to Ip ′, and although not shown, the reference voltage changes and the waveform is maintained.

When the reference voltage fluctuates due to fluctuations in the power supply, the vertical symmetry of the data signal waveform applied from the input circuit to the first current source circuit changes. Similar to the above, this change can be detected as a change in the output voltage n · _VL of the arithmetic circuit, and the voltage changing in the same direction as this change is fed back to the reference voltage setting terminal, so that the change in the reference voltage is suppressed. can do.

FIG. 6 is a block diagram of an optical transmitter for explaining the second embodiment of the present invention. 6, all reference numerals are the same as those in FIG. In FIG. 6, the temperature detector included in the laser module is not used. Therefore,
There is an advantage that it can be applied to a laser module that does not have a temperature detector. The configuration in which the output voltage of the average value detection circuit is applied to the pulse current control terminal that determines the pulse current in the second current source circuit via the second error amplifier is shown in FIG.
The circuit configuration is the same as that of the above, except that the output voltage of the arithmetic circuit is applied to the bias current control terminal of the second current source circuit via the third error amplifier.

Now, it is assumed that the bias current has decreased due to power supply fluctuation, element characteristic fluctuation, and the like. As a result, the optical output waveform has a shape in which the low level side is cut, and the amplitude decreases, and at the same time, the average value voltage of the output waveform decreases. At this time, the decrease of the average value voltage is applied to the pulse current control terminal of the first current source circuit via the second error amplifier, so that the action of restoring the average value of the output waveform to the original works. However, with this alone, the symmetry of the output waveform remains unchanged and the waveform emits zero light.

Here, the output voltage n · V _L of the arithmetic circuit is fed back to the bias current control terminal of the second current source circuit via the third error amplifier to maintain the symmetry of the output waveform. You can That is, the average value of the output waveform is reduced, n · V _L is negative, a higher voltage than V ₃ by applying the voltage to the inverting input terminal of the third error amplifier to the bias current control terminal It can be applied, and as a result, the bias current, which once tends to decrease, can be returned to the original value.

At the same time, since the change in the average value voltage is fed back to the pulse current control terminal via the second error amplifier, a vertically symmetrical optical output waveform with the amplitude kept constant can be obtained. .

In the circuit configuration of FIG. 6, when the feedback is applied to the first and second current source circuits and the input circuit is outside the feedback loop, the vertical symmetry of the input data signal waveform is broken. It is impossible to suppress the fluctuation of the reference voltage and the fluctuation of the signal waveform due to the fluctuation of the characteristics of the elements constituting the input circuit. However, if it is configured as described below, it is possible to suppress variations in the output waveform with respect to all variations.

FIG. 7 is a block diagram of an optical transmitter for explaining the third embodiment of the present invention. In FIG. 7, all reference numerals are the same as those in FIG. In the circuit configuration of FIG.
The output voltage n · V _L of the arithmetic circuit is fed back to the bias current control terminal of the second current source circuit via the third error amplifier, and is also fed to the reference voltage setting circuit via the first error amplifier. It is fed back to the reference voltage setting terminal of. The effect of each feedback is as already explained in the section of the description of the first and second embodiments. Here, to explain again, the effect of returning to the reference voltage setting terminal is as follows.

Even if the input waveform is vertically asymmetrical, the output waveform is vertically symmetrical. It is possible to suppress the fluctuation of the waveform due to the fluctuation of the reference voltage. The effect of returning to the bias current control terminal is as follows.

Even if the current-light output characteristics of the laser diode fluctuate, the bias current is changed to suppress the fluctuation of the output waveform. The output waveform fluctuation due to the bias current fluctuation is suppressed. That is, by feedback of both, it is possible to suppress all fluctuations that can be considered in the circuit, such as input fluctuations and element characteristic fluctuations. Moreover, the causes of the above fluctuation are power fluctuation,
Considering that environmental fluctuations such as temperature fluctuations may be involved,
It is possible to suppress the effects of all changes, including environmental changes.

[0054]

As described in detail above, a voltage proportional to the difference voltage between the peak value voltage and twice the average value voltage of the signal obtained by electrically converting the monitor light of the laser diode is generated,
By feeding back the voltage change to at least one of the reference voltage setting terminal and the bias current control terminal of the laser drive circuit, the vertical symmetry of the optical output waveform and the extinction ratio are kept constant, and the amplitude is controlled to be constant. It will be possible.

Compared with the conventional circuit that feeds back the change in the average value voltage to the pulse current control terminal and is effective only for some of the fluctuation factors, the effect is remarkably excellent. it can.

[Brief description of drawings]

FIG. 1 is a block diagram of an optical transmitter for explaining the principle of the present invention.

FIG. 2 is a diagram showing the output voltage of the photodiode in relation to the current-light output characteristics of the laser diode.

FIG. 3 is a block diagram of an optical transmitter for explaining an embodiment of the invention.

FIG. 4 is a configuration example of a main part of an arithmetic circuit.

FIG. 5 is a diagram for explaining the operation of keeping the symmetry of the waveform and the amplitude constant when the threshold current of the laser diode changes.

FIG. 6 is a block diagram of an optical transmitter for explaining a second embodiment of the present invention.

FIG. 7 is a block diagram of an optical transmitter for explaining a third embodiment of the present invention.

FIG. 8 is a block diagram of an optical transmitter for explaining the operation of a conventional optical output waveform stabilizing circuit.

FIG. 9 is a diagram for explaining the operation of the conventional optical output waveform stabilizing circuit-in the case where the conversion efficiency changes.

FIG. 10 is a diagram for explaining the operation of the conventional optical output waveform stabilizing circuit-in the case where the threshold current fluctuates due to temperature fluctuation.

FIG. 11 is a diagram showing changes in the optical output waveform when the vertical symmetry of the data signal is lost.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 laser drive circuit 2 laser module 21 light emitting circuit 22 light receiving circuit 3 optical output waveform stabilizing circuit 31 average value detection circuit 32 peak value detection circuit 33 arithmetic circuit 34a error amplifier

Claims (4)

[Claims] 1. A light emitting circuit of a laser module in an optical transmitter having a light emitting circuit, a laser module having a light receiving circuit for monitor light output from the light emitting circuit, and a drive circuit for driving the light emitting circuit of the laser module. Is an optical output waveform stabilization circuit that stabilizes the output waveform of the laser module. The average value detection circuit and the peak value of the electrical signal output by the light receiving circuit that electrically converts the monitor light output by the laser module light emitting circuit The average value voltage and the peak value voltage output from the two detection circuits are input to the detection circuit, and the difference voltage between the peak value voltage and the voltage twice the average value voltage is applied by the calculation circuit. An optical output waveform stabilizing circuit is provided, which is configured to generate a voltage proportional to the voltage and to feed the generated voltage back to the laser driving circuit. 2. The optical output waveform stabilizing circuit according to claim 1, wherein a voltage proportional to a difference voltage between a voltage twice as high as an average value voltage and a peak value voltage generated by the arithmetic circuit is input data signal. An optical output waveform stabilizing circuit, which supplies the central value voltage of the logic amplitude to the reference voltage control terminal of the laser drive circuit. 3. The optical output waveform stabilizing circuit according to claim 1, wherein a voltage, which is generated by the arithmetic circuit, and which is proportional to the difference voltage between the voltage twice the average value voltage and the peak value voltage, is input data signal. An optical output waveform stabilizing circuit which determines the current of the laser diode when is at a low level, and which is fed back to the bias current control terminal of the laser drive circuit. 4. The laser output circuit according to claim 1, wherein a voltage proportional to a difference voltage between a voltage twice as high as an average value voltage and a peak value voltage, which is generated by the arithmetic circuit, is generated by the operation circuit. An optical output waveform stabilizing circuit having a configuration for feeding back to a reference voltage setting terminal and a bias current control terminal included in.