JP2842369B2 - Laser diode drive circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a laser diode driving circuit, and more particularly to a laser diode (LD) driving circuit used in an optical communication device.

[0002]

2. Description of the Related Art Conventionally, in a laser diode drive circuit (hereinafter referred to as an LD drive circuit), as shown in FIG. 5, a drive circuit 23 controls a laser diode 24 based on input data input from an input terminal 21. It is driving.

[0003] The photodiode 25 monitors the back light of the driven laser diode 24, converts the monitored light into an electric signal, and outputs the electric signal to the current-voltage conversion circuit 30. The current-voltage conversion circuit 30 converts the current from the photodiode 25 into a voltage and sends the voltage to the peak value detector 26 and the first average value detector 27.

The peak value detector 26 is a current / voltage conversion circuit 3
The peak value of the voltage from 0 is monitored, and the detected peak value is supplied to the drive circuit 23 to control the drive current of the laser diode 24. Further, the first average value detector 27 monitors the average value of the voltage from the current / voltage conversion circuit 30, and sends the detected voltage to the amplifier 29.

[0005] A second average detector 28 monitors the average of the input data input from the input terminal 21, and sends the detected voltage to the amplifier 29. The amplifier 29 is connected to the voltage detected by the first average detector 27 and the second average detector 28.
Then, the output voltage is supplied to the bias circuit 22 as a control voltage to control the pulse width of the light output waveform of the laser diode 24.

As a result, an optical transmitter that outputs stable optical output power without pulse width distortion without depending on the characteristics of the laser diode 24 and temperature fluctuation can be obtained. This technique is described in detail in JP-A-5-90673.

As another LD driving circuit, as shown in FIG. 7, a rear light output of a laser diode 31 is taken out by a rear light monitor circuit 32, and the light output is taken by a low-pass filter (LPF) 38. To the comparator 35 via the
The data is compared with input data input through a low-pass filter (LPF) 37.

In this case, the output current of the DC bias current source 34 is controlled by the output Δv of the comparator 35, and the output current of the pulse current source 33 is controlled via the change ratio setting circuit 36. Give feedback so that it is constant.

If the change ratio setting circuit 36 sets the ratio between the pulse current change .DELTA.IP and the DC bias current change .DELTA.IB to a predetermined value, the threshold current and the quantum efficiency can be maintained even when the optical output is stabilized against a temperature change. , A DC bias current and a pulse current that follow the change of the optical output pulse width are obtained, and the light output pulse width does not change without lowering the extinction ratio. This technique is described in detail in Japanese Patent Publication No. 2-62954.

[0010]

In the conventional LD driving circuit described above, the driving current of the laser diode is controlled by monitoring the peak value of the optical output power, and the average value of the optical output power and the average value of the input data are calculated. In the case of the technique of controlling the bias current by comparing the bias current, the equilibrium state is established even when the bias current is lower than the threshold current value of the laser diode, as shown in FIG. This is because the peak value in the optical output waveform is a desired value, and the average value of the optical output waveform is equal to the average value of the input data.

Accordingly, the optical output waveform may be balanced in a state where the optical spectrum is spread (chirping), and the chirping deteriorates the transmission waveform due to dispersion of the fiber.

The output of the DC bias current source and the output current of the pulse current source are controlled by the output of the comparator. In the case of the technique of applying feedback so that the constant becomes constant, the constant of the temperature change must be obtained in advance from the characteristic data of each laser diode, and the resistance value of the change ratio setting circuit must be individually set so as to be this constant. . That is, when the laser diode is replaced, the resistance value must be reset to a value corresponding to the laser diode.

In the case of this technique, in the characteristic data, the relationship between the current flowing through the laser diode and the optical output power is deteriorated due to aging, and the efficiency is degraded and changed.

Accordingly, an object of the present invention is to solve the above-mentioned problems, to prevent the spread of the optical spectrum and the deterioration of the extinction ratio due to chirping, and to stably supply a high-quality optical signal. It is to provide a diode drive circuit.

[0015]

A first laser diode driving circuit according to the present invention is a laser diode driving circuit including a laser diode element and a monitoring photodiode element in a laser diode module for performing intensity modulation. First peak detecting means for detecting the maximum value of only the AC component of the output of the photodiode element; second peak detecting means for detecting the maximum value of the output of the photodiode element; First and second conversion means for converting the output of each of the two peak detection means from analog to digital, and extracting a DC component of the output of the photodiode element from the output of each of the first and second conversion means Extracting means for comparing the output of the first peak detecting means with a preset predetermined value; Bias current control means for controlling a bias current of the laser diode based on the DC component extracted in step (a), and pulse current control means for controlling a modulation signal amplitude of the laser diode based on a comparison result of the comparison means. Have.

A second laser diode driving circuit according to the present invention further comprises a means for extracting only the AC signal from the output of the photodiode element, in addition to the above configuration.

A third laser diode driving circuit according to the present invention is a laser diode driving circuit including a laser diode element and a monitoring photodiode element in a laser diode module for performing intensity modulation, wherein an output of the photodiode element is output. First peak detecting means for detecting the maximum value of only the AC signal, second peak detecting means for detecting the maximum value of the output of the photodiode element, and the first and second peak detecting means, respectively. First and second conversion means for performing an analog / digital conversion of the output of the first, second and third conversion means, a calculation means for performing a predetermined calculation on the output of each of the first and second conversion means, and an output of the first peak detection means Comparing means for comparing the bias voltage of the laser diode with a predetermined value set in advance. It comprises a bias current control means for controlling, a pulse current control means for controlling the modulation signal amplitude of the laser diode on the basis of a comparison result of the comparing means.

A fourth laser diode driving circuit according to the present invention further comprises a means for extracting only the AC signal from the output of the photodiode element, in addition to the above configuration.

In a fifth laser diode drive circuit according to the present invention, in the above configuration, the arithmetic means calculates the difference between the output of the first conversion means and the output of the second conversion means. The bias current control means is configured to control the bias current such that the calculation result of the calculation means falls between the first and second predetermined values.

In a sixth laser diode driving circuit according to the present invention, in the above-mentioned configuration, the arithmetic means is configured to output a signal b-b to the output a of the first conversion means and the output b of the second conversion means. 2a, and the bias current control means is configured to control the bias current such that the calculation result of the calculation means falls between the first and second set values. doing.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the operation of the present invention will be described below.

The maximum value MAC of only the AC component SAC of the output of the laser diode detected by the first peak detection circuit,
The first DC / DC component of the output of the laser diode detected by the peak detection circuit + the maximum value ACDC of the AC component SACDC is referred to as the first A / D.
The conversion result is converted into digital values a and b by the converter and the second A / D converter, respectively, and the calculation result c obtained by performing the calculation of the calculation expression b-2a by the calculation circuit satisfies the conditional expression Y ≧ c ≧ X. While adjusting the bias current IB with the bias current control circuit,
Difference value PCO between maximum value MAC obtained by comparator and set value VREF
The pulse current IP is adjusted by the pulse current control circuit based on NT.

This makes it possible to cause the laser diode to emit light under the best conditions balanced with respect to the extinction ratio and the chirping, and with a stable power. It becomes possible to supply IP and bias current IB to the laser diode.

Therefore, it is possible to prevent the quality of the optical signal to be transmitted from being deteriorated without being affected by the temperature fluctuation and the aging deterioration, and to obtain the performance not depending on the individual difference of the characteristics of the laser diode. Thus, productivity can be improved.

Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing the configuration of one embodiment of the present invention. In the figure, a laser diode drive circuit (hereinafter, referred to as an LD drive circuit) according to one embodiment of the present invention includes a laser diode module (hereinafter, referred to as an LD module) 1, a capacitor 4, a first peak detection circuit 5, The second peak detection circuit 6, the comparator 7, and the first A
/ D (analog / digital) converter 8 and second A / D
Converter 9, arithmetic circuit 10, and bias current control circuit 1
1 and a pulse current control circuit 12.
In the LD module 1, a laser diode (LD) 2 and a photodiode (PD) 3 are provided.

Laser diode 2 in LD module 1
Are the pulse current IP output from the pulse current control circuit 12 and the bias current IB output from the bias current control circuit 11.
And outputs an optical signal OPT-OUT.

Photodiode 3 in LD module 1
Monitors the optical signal OPT-OUT from the laser diode 2 and outputs a monitor signal M. The capacitor 4 cuts off the DC component of the monitor signal M from the photodiode 3 and outputs only the AC component SAC of the monitor signal M to the first peak detection circuit 5.

The first peak detection circuit 5 detects the maximum value VAC of the AC component SAC of the monitor signal M from the capacitor 4 and outputs the same to the comparator 7 and the first A / D converter 8. The second peak detection circuit 6 receives the monitor signal M from the photodiode 3
Of the DC component + AC component SACDC is detected and output to the second A / D converter 9.

The first A / D converter 8 converts the maximum value MAC from the first peak detection circuit 5, which is an analog value, into a digital value a.
And outputs the digital value a to the arithmetic circuit 10. The second A / D converter 9 converts the maximum value MACDC from the second peak detection circuit 6 which is an analog value into a digital value b, and outputs the digital value b to the arithmetic circuit 10.

The arithmetic circuit 10 performs the operation of the operation expression b-2a on the basis of the digital value a from the first A / D converter 8 and the digital value b from the second A / D converter 9, and the operation result c To the bias current control circuit 11. The bias current control circuit 11 outputs a bias current IB to the laser diode 2 based on the operation result c of the operation circuit 10.

The comparator 7 compares the maximum value MAC from the first peak detection circuit 5 with the set value VREF, and outputs the difference value PCONT to the pulse current control circuit 12. The pulse current control circuit 12 compares the input data D with the differential value PCO from the comparator 7.
The pulse current is output to the laser diode 2 with a pulse amplitude based on NT.

The operation of the embodiment of the present invention will be described with reference to FIG. The photodiode 3 in the LD module 1 receives an optical signal OPT-OU from the laser diode 2.
Monitor T and output monitor signal M. This monitor signal M is branched into two, one is input to the capacitor 4, and the other is input to the second peak detection circuit 6.

The DC component of the monitor signal M input to the capacitor 4 is cut off, and only the AC component SAC of the monitor signal M is input to the first peak detection circuit 5. The maximum value MAC of the AC component SAC input to the first peak detection circuit 5 is detected.

The maximum value MAC detected by the first peak detection circuit 5 is branched into two, one of which is input to the comparator 7 and the other is input to the first A / D converter 8. First A / D converter 8
Is converted from analog to digital, and the digital value a is input to the arithmetic circuit 10.

The monitor signal M input to the second peak detection circuit 6, ie, the DC component + AC component S of the monitor signal M
For ACDC, the maximum value MADCC is detected, and the maximum value MADCC is input to the second A / D converter 9. The maximum value MACC input to the second A / D converter 9 is converted from analog to digital, and the digital value b is input to the arithmetic circuit 10.

The arithmetic circuit 10 performs an arithmetic operation b-2a on the digital value a input from the first A / D converter 8 and the digital value b input from the second A / D converter 9. The calculation result c of the calculation formula b-2a is a value of a DC component superimposed on the monitor signal M, and is input to the bias current control circuit 11.

The bias current control circuit 11 calculates the bias current based on the calculation result c and the predetermined set values X and Y (Y>X> 0).
Adjustment of the bias current IB to the laser diode 2 is performed by a feedback operation so that Y ≧ c ≧ X is satisfied (IB → IB ′).

If the above set values X and Y are made sufficiently close to zero, the DC component superimposed on the output of the laser diode 2 becomes equal to zero, so that the extinction ratio can be prevented from deteriorating. When the calculation result c becomes a negative value, it indicates that the bias current IB is insufficient with respect to the threshold current Ith of the laser diode 2, which also indicates that the calculation result c has a positive value sufficiently close to zero. Feedback works so that
The bias current IB becomes substantially equal to the threshold current Ith.

Accordingly, it is possible to prevent the optical spectrum from being spread due to chirping. Here, the first A / D
The reason why the converter 8, the second A / D converter 9, and the arithmetic circuit 10 are used is that high-precision control is required for the set values X and Y.

Also, the first peak detection circuit 5 outputs a signal from the comparator 7
Is compared with the set value VREF and output to the pulse current control circuit 12 as the difference value PCINT. The pulse current control circuit 12 adjusts the pulse current IP to the laser diode 2 by a feedback operation so that the output amplitude ALD of the laser diode 2 is set in advance based on the difference value PCONT (IP → IP).
').

In the above configuration, for example, the pulse current control circuit 12 operates at a pulse current IP of 560 MHz and 50 mA.
And the laser diode 2 outputs an optical signal OPT-OUT of 1 mW when the bias current control circuit 11 outputs a bias current IB of 20 mA.

The photodiode 3 is a laser diode 2
Monitor the optical signal OPT-OUT from the
Is output. The 0.1 μF capacitor 4 is connected to the monitor signal M
, And outputs only the AC component SAC to the first peak detection circuit 5.

At this time, for example, when the first peak detection circuit 5 detects 20 mV as the maximum value VAC of the AC component SAC, and the second peak detection circuit 6 detects 41 mV as the maximum value MICDC of the DC component + AC component SACDC, “010100” is output from the first A / D converter 8 and “101001” is output from the second A / D converter 9 to the arithmetic circuit 10.

The arithmetic circuit 10 is "101001" -2.times.
“010100” is calculated, and the calculation result c = 1 is output to the bias current control circuit 11. The bias current control circuit 11 determines the conditional expression 0.3 ≧ c ≧ based on the operation result c = 1.
0.1 (analog value) to satisfy the bias current I
Adjust B and output.

When 20 mV is input as the maximum value MAC from the first peak detection circuit 5, the comparator 7 compares it with the set value VREF = 22 mV, for example, and the difference value PCONT = -2 m
V is output to the pulse current control circuit 12. The pulse current control circuit 12 converts the data D into a pulse current I with a pulse amplitude of 48 mA such that the difference value PCONT = -2 mV is set to 0 mV.
Output as P.

FIGS. 2 and 3 are diagrams showing input / output examples of each part of the LD drive circuit shown in FIG. FIG. 2A shows a waveform example of the data D input to the pulse current control circuit 12 of FIG. 1, and FIG. 2B shows a waveform example of the pulse current IP output from the pulse current control circuit 12 of FIG. And FIG. 2 (c)
Is an optical output OP output from the laser diode 2 in FIG.
4 shows a waveform example of T-OUT.

FIG. 2D shows an example of the waveform of the monitor signal M output from the photodiode 3 of FIG. 1, and FIG.
2 shows a waveform example of the AC component SAC of the monitor signal M output from the photodiode 3 in FIG. 1, and FIG. 2F shows the DC component + AC component SACDC of the monitor signal M output from the photodiode 3 in FIG. 5 shows an example of the waveform.

FIG. 3A shows the first peak detection circuit 5 of FIG.
FIG. 3 (b) shows an example of the waveform of the maximum value MAC output from FIG.
Is the maximum value M output from the second peak detection circuit 6 in FIG.
FIG. 3C shows an example of a waveform of an ACDC waveform, and FIG. 3C shows an example of a waveform of a digital value a output from the first A / D converter 8 in FIG.

FIG. 3D shows a waveform example of the digital value b output from the second A / D converter 9 of FIG.
FIG. 3E shows a waveform example of a value (2a) obtained by multiplying the digital value a output from the first A / D converter 8 in FIG. 1 by 2, and FIG. 3F shows the output from the arithmetic circuit 10 in FIG. 5 shows an example of a waveform of a calculation result c to be performed.

The operation of the embodiment of the present invention will be described in detail with reference to FIGS. The data D input to the LD drive circuit has a pattern with a mark ratio of 1/2 (see FIG. 2A). The pulse current for driving the laser diode 2 by being input to the pulse current control circuit 12 It becomes IP [see FIG. 2 (b)], and LD module 1
Supplied to

The LD module 1 is supplied with an appropriate bias current IB. The pulse current IP is superimposed on the proper bias current IB, the laser diode 2 emits light, and the optical output OPT-OU
T is output [see FIG. 2 (c)].

The optical output OPT-OUT is received by the photodiode 3, and the monitor signal M is output from the photodiode 3 (see FIG. 2D). The monitor signal M is 2
One is supplied to the first peak detection circuit 5 via the capacitor 4 as only the AC component SAC, and the other is supplied to the second peak detection circuit 6 as a DC component + AC component SACDC.

Here, since the AC component SAC has passed through the capacitor 4, it has a waveform from which the DC component has been removed, and the center of the power becomes the ground (GND) level (see FIG. 2 (e)). . On the other hand, the DC component + AC component SACDC has a waveform from which the DC component has not been removed, and the waveform is above the ground level [FIG.
(F)].

The first peak detection circuit 5 and the second peak detection circuit 6 output the maximum values MAC and MADCC of the amplitudes of the AC component SAC and the DC component + AC component SACDC, respectively (FIG. 3A).
And FIG. 3 (b)]. These maximum values MAC and MACDC are used for the first A / D converter 8 and the second A /
It is converted into digital values a and b by the D converter 9 [FIG.
(C) and FIG. 3 (d)].

The arithmetic circuit 10 calculates the amount of light emitted by the DC component of the output of the laser diode 2 (calculation result c). In this case, the digital value b indicates the amount of light emission due to the DC component + the AC component SACDC, and the digital value a indicates half the amount of light emission due to the AC component SAC. Therefore, the amount of light emission due to the DC component is obtained from b-2a. [See FIGS. 3E and 3F].

The calculation result c is used as the bias current supply circuit 1
1 from the bias current supply circuit 11,
A bias current IB that satisfies .gtoreq.c.gtoreq.X is output. The set values X and Y are sufficiently close to zero, and Y>X>
0 relationship. That is, the calculation result c is a value sufficiently close to zero, and the light emission amount due to the DC component is slight,
To a negligible extent, the bias current IB is equal to the threshold current Ith
It can be said that.

Here, as shown in FIG. 4, consider a case where the LD characteristic curve has deteriorated due to a change from the initial LD characteristic curve. Then, since the optical output OPT-OUT decreases due to the fluctuation, the pulse current control circuit 12 increases the pulse current IP by the feedback function.

On the other hand, the bias current IB is insufficient.
Light emission due to the DC component is zero. At this time, the operation result c is equal to zero, and the conditional expression Y ≧ c ≧ X
Will not be satisfied. Thus, the bias current control circuit 11 has a feedback function to increase the bias current IB.

Next, consider the case where the efficiency has increased due to fluctuations from the initial LD characteristic curve. Then, the optical output OPT
Since −OUT is larger than the set value due to the fluctuation, the pulse current control circuit 12 reduces the pulse current IP by the feedback function.

On the other hand, the bias current IB is too large, and light is emitted due to the DC component. At this time, the calculation result c exceeds the set value Y, and the conditional expression Y ≧ c ≧ X is not satisfied. As a result, the bias current IB
The feedback function works to reduce

Therefore, even if the LD characteristic changes, the pulse current IP changes and the optical output OPT-OUT becomes constant, and the bias current IB changes to change the threshold current It.
h, it is possible to prevent the extinction ratio from deteriorating and the optical spectrum from spreading due to chirping.

As described above, the maximum value MAC of only the AC component SAC of the output of the laser diode 2 detected by the first peak detection circuit 5 and the DC component of the output of the laser diode 2 detected by the second peak detection circuit 6 The first and second A / D converters 8 and 9 convert the maximum value of the AC component SACDC to the digital values a and b, respectively, and the arithmetic circuit 10 calculates the arithmetic expression b-2a. Calculated result c is conditional expression Y ≧
The bias current IB is adjusted by the bias current control circuit 11 so as to satisfy c ≧ X, and the pulse current control circuit 12 adjusts the pulse current IB based on the difference value PCONT between the maximum value MAC obtained by the comparator 7 and the set value VREF. By adjusting IP, the laser diode 2 can emit light under the best conditions balanced in terms of extinction ratio and chirping, and with stable power.

Accordingly, appropriate pulse currents IP and bias currents IB can be automatically supplied to the laser diode 2 in accordance with the set conditions, so that transmission is effected without being affected by temperature fluctuations and aging. It is possible to prevent the deterioration of the quality of the optical signal and to obtain the performance independent of the individual difference of the characteristics of the laser diode 2. In addition, since performance that does not depend on individual differences in characteristics of the laser diode 2 can be obtained, productivity can be improved.

[0064]

As described above, according to the present invention, the maximum value of only the AC component of the output of the photodiode element is detected, the maximum value of the output of the photodiode element is detected, and these maximum values are detected. The bias current to the laser diode is controlled based on the DC component of the output of the photodiode element extracted from the digitally converted value, and based on the comparison result between the maximum value of only the AC component and a preset predetermined value. By controlling the pulse current to the laser diode, it is possible to prevent the spread of the optical spectrum and the deterioration of the extinction ratio due to the chirping, and it is possible to stably supply a high-quality optical signal.

[Brief description of the drawings]

FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention.

2A is a diagram illustrating a waveform example of data input to the pulse current control circuit of FIG. 1, and FIG. 2B is a diagram illustrating a waveform example of a pulse current output from the pulse current control circuit of FIG. ,
1C is a diagram showing an example of a waveform of an optical output output from the laser diode of FIG. 1, FIG. 2D is a diagram showing an example of a waveform of a monitor signal output from the photodiode of FIG. 1, and FIG. FIG. 3F is a diagram showing a waveform example of an AC component of a monitor signal output from the photodiode of FIG. 1, and FIG. 2F is a diagram showing a waveform example of a DC component + AC component of the monitor signal output from the photodiode of FIG.

3A is a diagram illustrating a waveform example of a maximum value output from a first peak detection circuit in FIG. 1, and FIG. 3B is a waveform example of a maximum value output from a second peak detection circuit in FIG. Figure showing
(C) is a diagram showing a waveform example of a digital value output from the first A / D converter of FIG. 1, and (d) is a waveform example of a digital value output from the second A / D converter of FIG. Figure,
(E) is a diagram showing a waveform example of a value obtained by multiplying the digital value outputted from the first A / D converter of FIG. 1 by 2, and (f) is a diagram showing a waveform example of FIG.
FIG. 7 is a diagram showing a waveform example of a calculation result output from the calculation circuit of FIG.

FIG. 4 is a diagram showing a characteristic curve of a laser diode.

FIG. 5 is a block diagram showing a configuration of an example of a conventional example.

6A is a diagram illustrating a waveform example of data input when the optical spectrum spread is deteriorated, FIG. 6B is a diagram illustrating a waveform example of LD driving when the optical spectrum spread is deteriorated, and FIG. FIG. 5 is a diagram showing a waveform example of the light output of FIG.

FIG. 7 is a block diagram showing a configuration of another example of the conventional example.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 laser diode module 2 laser diode 3 photodiode 4 capacitor 5 first peak detection circuit 6 second peak detection circuit 7 comparator 8 first A / D converter 9 second A / D converter 10 arithmetic circuit 11 bias current control circuit 12 Pulse current control circuit

──────────────────────────────────────────────────の Continued on front page (51) Int.Cl. ⁶ Identification code FI H04B 10/06 10/14 10/26 10/28

Claims (6)

(57) [Claims] 1. A laser diode drive circuit including a laser diode element and a monitoring photodiode element in a laser diode module for performing intensity modulation, wherein a maximum value of only an AC component in an output of the photodiode element is detected. First peak detecting means for detecting the maximum value of the output of the photodiode element, second peak detecting means for detecting the maximum value of the output of the photodiode element, and first peak detecting means for converting the output of each of the first and second peak detecting means from analog to digital. And the second
Conversion means, extraction means for extracting a DC component of the output of the photodiode element from the output of each of the first and second conversion means, and an output of the first peak detection means. Comparison means for comparing a predetermined value, a bias current control means for controlling a bias current of the laser diode based on the DC component extracted by the extraction means, and a laser diode based on a comparison result of the comparison means And a pulse current control means for controlling the amplitude of the modulation signal. 2. The laser diode drive circuit according to claim 1, further comprising means for extracting only the AC signal from the output of the photodiode element. 3. A laser diode driving circuit including a laser diode element and a monitoring photodiode element in a laser diode module for performing intensity modulation, wherein a maximum value of only an AC signal among outputs of the photodiode element is detected. First peak detecting means for detecting the maximum value of the output of the photodiode element, second peak detecting means for detecting the maximum value of the output of the photodiode element, and first peak detecting means for converting the output of each of the first and second peak detecting means from analog to digital. And the second
Conversion means, calculation means for performing a predetermined calculation on the output of each of the first and second conversion means, and comparison means for comparing the output of the first peak detection means with a preset predetermined value A bias current control unit that controls a bias current of the laser diode based on a calculation result of the calculation unit; and a pulse current control unit that controls a modulation signal amplitude of the laser diode based on a comparison result of the comparison unit. A laser diode drive circuit comprising: 4. The laser diode driving circuit according to claim 3, further comprising means for extracting only the AC signal from the output of the photodiode element. 5. The calculation means is configured to calculate a difference between an output of the first conversion means and an output of the second conversion means, and the bias current control means is configured to calculate a result of the calculation by the calculation means. 5. The laser diode drive circuit according to claim 3, wherein the bias current is controlled so that the bias current is between predetermined first and second set values. 6. An arithmetic unit according to claim 1, wherein an output a of said first conversion unit and an output b of said second conversion unit are b-2
a, wherein the bias current control means controls the bias current so that the calculation result of the calculation means falls between predetermined first and second set values. The laser diode driving circuit according to claim 3, wherein: