JP4799595B2 - Extinction ratio compensation laser drive circuit and optical communication device - Google Patents

Description

  The present invention relates to an APC (Automatic Power Control) type laser drive circuit having an extinction ratio compensation function and an optical communication apparatus using the same.

  In the field of optical communication, a laser module including a light emitting circuit made of a laser diode (LD) and a light receiving circuit made of a photodiode (PD) is known. The LD of the light emitting circuit outputs a predetermined light output by adding a bias current to a pulse current corresponding to input data, and outputs a monitor light for APC. The PD of the light receiving circuit receives the monitor light output from the LD and performs light-current conversion. Based on the current obtained by this conversion, the magnitudes of the PD bias current and pulse current are controlled so as to obtain a constant light output and extinction ratio.

  As is well known, the threshold current of the LD and the conversion efficiency fluctuate due to temperature fluctuations, process fluctuations, deterioration due to long-term use, and the like. In addition, the LD characteristics have completely different threshold currents, conversion efficiencies, variations due to their temperatures, etc. The coupling efficiency of LD-PD also varies. Therefore, in order to always obtain a constant light output and extinction ratio, it is necessary to appropriately initialize the magnitudes of the bias current and the pulse current and always optimize them according to the use state.

  Here, fluctuations in the LD threshold current and conversion efficiency due to temperature fluctuations will be described with reference to FIGS.

  FIG. 1 shows a conventional LD driving example at a normal temperature (T2) among a low temperature (T1), a normal temperature (T2), and a high temperature (T3) as a solid line current-light conversion characteristic (IP characteristic) 12. Is. Here, I is an injection current (drive current) to the LD, P is an optical output of the LD, and the slope of the IP characteristic represents the conversion efficiency. The threshold current of the LD at normal temperature is Ith2, the bias current Ib is set to be equal to the threshold current Ith2 (Ib = Ith2), and the pulse current Ip according to the input data is superimposed on the bias current Ib. Here, when a pulse current Ip having a duty ratio of 1: 1 (High period: Low period) is applied to the LD, a desired high extinction ratio (Pmax / Pmin) as shown in FIG. The maximum light output Pmax and the minimum light output Pmin showing the duty ratio of the above are obtained.

  FIG. 2 shows a conventional LD driving example at a high temperature (T3) with a solid line IP characteristic 13. At a high temperature, the LD threshold current changes to Ith3 (> Ith2), and the conversion efficiency is lower than that at room temperature. However, when the same bias current Ib (= Ith2) and pulse current Ip are applied to the LD as at room temperature (T2), the optical output P is equal to the maximum optical output Pmax3 and minimum optical output Pmin3 shown in the figure. The maximum value becomes smaller and the extinction ratio deteriorates. In addition, the duty ratio of the light output P is greatly deteriorated.

  FIG. 3 shows a conventional LD driving example at a low temperature (T1) with a solid line IP characteristic 11. At a low temperature, the LD threshold current changes to Ith1 (> Ith2), and the conversion efficiency is higher than that at room temperature. However, when the same bias current Ib (= Ith2) and pulse current Ip are applied to the LD as at room temperature (T2), the optical output P is as shown in the maximum optical output Pmax1 and minimum optical output Pmin1 shown in the figure. Both the maximum value and the minimum value become large, and the extinction ratio is deteriorated.

  As described above, when the bias current Ib is lower than the threshold current Ith3 as shown in FIG. 2, the maximum light output, the extinction ratio, and the duty ratio are rapidly deteriorated, and the bias current Ib is changed to the threshold current Ith1 as shown in FIG. When the value exceeds the extinction ratio, the extinction ratio is greatly deteriorated, which causes a problem that both of them interfere with communication.

  FIG. 4 shows an ideal LD driving example in which a constant maximum light output, extinction ratio, and duty ratio can be obtained regardless of the ambient temperature. That is, at high temperature (T3), the bias current is increased to Ib3 (= Ith3) in response to the increase in threshold current from Ith2 to Ith3, and the pulse current is increased to Ip3 in response to a decrease in conversion efficiency. Let At low temperature (T1), the bias current is reduced to Ib1 (= Ith1) corresponding to the decrease of the threshold current from Ith2 to Ith1, and the pulse current is decreased to Ip1 corresponding to the increase in conversion efficiency. To make it happen. As a result, the same maximum light output Pmax and minimum light output Pmin as in normal temperature (T2) are always obtained regardless of the ambient temperature.

Various attempts have been made to realize such ideal LD driving. One conventional technique is that an average value output from both detection circuits is input to an average value detection circuit and a peak value detection circuit by inputting an electrical signal output by the PD from the photoelectric conversion of the monitor light output by the LD. The output voltage and the peak value output voltage are applied to the arithmetic circuit, and the arithmetic circuit generates a voltage proportional to the difference between the voltage twice the average value output voltage and the peak value output voltage, and the generated voltage Is fed back to the reference voltage setting terminal or the bias current control terminal of the laser drive circuit, and the average output voltage is fed back to the pulse current control terminal, so that the amplitude, vertical symmetry and extinction ratio of the optical output waveform can be reduced. It stabilizes (Patent Document 1).
JP-A-6-164049

  In the above prior art, the average light output is constant, but the minimum light output is assumed to be equal to 0 so that the difference between the voltage twice the average value output voltage and the peak value output voltage becomes 0. Since the control is performed, there is a problem that an offset effect corresponding to the size of the existing minimum light output (≠ 0) occurs.

  Further, in the above prior art, when the bias current exceeds the threshold current, the average optical output is equal to the reference value, but the duty ratio of the optical output is not 1: 1 and the voltage is twice the average output voltage. If the difference from the peak output voltage is zero, there is a possibility that an equilibrium state is reached, and the desired maximum light output, extinction ratio, and duty ratio may not be obtained.

  The purpose of the present invention is to optimize the bias current and pulse current applied to the LD even when fluctuations occur in the threshold current and conversion efficiency of the LD due to temperature fluctuation, process fluctuation, deterioration due to long-term use, etc. Thus, an object of the present invention is to provide a laser drive circuit that can always control the maximum light output, the extinction ratio, and the duty ratio to be constant.

  In order to achieve the above object, a first extinction ratio compensating laser driving circuit according to the present invention is a laser driving circuit that controls the light emitting circuit using a light receiving circuit that receives output light of the light emitting circuit. A drive circuit for driving the signal, a bias circuit for adding a bias current to the pulse current output from the drive circuit, an I / V conversion circuit for current-voltage conversion of the output of the light receiving circuit, and the I / V conversion circuit A first maximum value detection circuit for detecting a maximum value of the output voltage of the first I / V converter, a first average value detection circuit for detecting an average value of the output voltage of the I / V conversion circuit, and the maximum value and the first reference. A first comparison circuit that compares a value and feeds back a result to the drive circuit; a second comparison circuit that compares the average value and a second reference value and feeds back the result to the bias circuit; Detects the maximum drive current of the light emitting circuit The second maximum value detection circuit, the second average value detection circuit for detecting the average value of the drive current of the light emitting circuit, and the maximum value of the output voltage of the I / V conversion circuit is the first reference value. A threshold current detection circuit that receives a signal from the first comparison circuit if larger than that, calculates a threshold current from the two maximum values and the average of the two, and feeds back to the bias circuit; Is adopted.

  The second extinction ratio compensating laser driving circuit according to the present invention is the first extinction ratio compensating laser driving circuit according to the present invention, wherein the light receiving circuit is arranged so as to improve detection accuracy in the threshold current detecting circuit. The configuration further including an amplifier circuit for amplifying the output current is adopted.

  An optical communication apparatus according to the present invention receives the first or second extinction ratio compensating laser driving circuit according to the present invention, a light emitting circuit driven by the laser driving circuit, and light output from the light emitting circuit. A configuration including a light receiving circuit is employed.

  As described above, according to the present invention, the bias applied to the LD even when the threshold current and the conversion efficiency of the LD vary due to temperature variation, process variation, deterioration due to long-term use, and the like. By optimizing the current and the pulse current, it is possible to provide a laser drive circuit that can always control the maximum light output, the extinction ratio, and the duty ratio to be constant.

  FIG. 5 is a block diagram of an extinction ratio compensating laser drive circuit according to the first embodiment of the present invention. In FIG. 5, reference numeral 21 denotes a laser module, which emits a predetermined light output, and a light emitting circuit 21a composed of a laser diode (LD) that performs current-light conversion so as to output APC monitor light, and this light emission. The light receiving circuit 21b includes a photodiode (PD) for receiving the monitor light output from the circuit 21a. 22 is an LD drive circuit, 23 is a bias circuit, 24 is an I / V conversion circuit, 25 is a maximum value detection circuit, 26 is an average value detection circuit, 27 is a first comparison circuit, 28 is a reference value creation circuit, and 29 is This is a second comparison circuit.

  The LD driving circuit 22 supplies a pulse current Ip so as to drive the light emitting circuit 21a according to input data (DATA). The bias circuit 23 adds a bias current Ib to the pulse current Ip output from the LD drive circuit 22. That is, the injection current (drive current) I to the light emitting circuit 21a is equal to Ib + Ip. The I / V conversion circuit 24 performs current-voltage conversion on the output of the light receiving circuit 21b. The maximum value detection circuit 25 detects the maximum value Vmax of the output voltage of the I / V conversion circuit 24. The average value detection circuit 26 detects the average value Vave of the output voltage of the I / V conversion circuit 24. The first comparison circuit 27 compares the maximum value Vmax output from the maximum value detection circuit 25 with a first reference value (first reference voltage) Vref1 set in advance, and performs comparison so that the difference becomes zero. The result is fed back to the LD drive circuit 22 to charge and discharge the pulse current Ip. The reference value creating circuit 28 generates a second reference value (second reference voltage) Vref2 from the first reference value Vref1. The second comparison circuit 29 compares the average value Vave output from the average value detection circuit 26 with the second reference value Vref2, and feeds back the comparison result to the bias circuit 23 so that the difference becomes zero. The bias current Ib is charged and discharged.

  According to the configuration of FIG. 5, by making the maximum value Vmax coincide with the reference value Vref1, the threshold current and conversion efficiency of the LD are increased and decreased as shown in FIG. By keeping the maximum light output Pmax constant and matching the maximum value Vmax and the average value Vave with the reference values Vref1 and Vref2 in the two feedback loops respectively, the extinction ratio and the duty ratio are always set as shown in FIG. Can be kept constant. In addition, since the duty ratio of the input data (DATA) is not changed in the configuration of FIG. 5, the duty ratio of the optical output is unlikely to deteriorate even when the conversion efficiency of the LD varies greatly.

  FIG. 6 shows a configuration example of the reference value creation circuit 28 in FIG. That is, a specific method for generating the second reference value Vref2 is shown. In order to keep the extinction ratio constant, the ratio between Vmax and Vmin should be kept constant when the maximum value of the output voltage of the I / V conversion circuit 24 is Vmax and the minimum value is Vmin. Therefore, when Vmax is made to coincide with Vref1 as described above, Vmin is determined from Vmax, and Vave is determined to be an intermediate value between Vmax and Vmin. Therefore, it can be defined as “Vmax: Vmin = R + r: r = constant”, and the reference value of the average value Vave, that is, the second reference value Vref2 is resistance-divided as Vmax × {(R / 2) + r} / (R + r). Can be set. Note that any method other than resistance division may be used as long as the method can generate the reference value.

  The second reference value Vref2 may be generated based on the first reference value Vref1 as shown by the solid line arrow of the first reference value Vref1 entering the reference value generating circuit 28 in FIG. As shown by the broken line arrow from the maximum value detection circuit 25 to the reference value generation circuit 28, the maximum value Vmax detected may be used as a reference. However, if the feedback of the maximum value Vmax and the feedback of the average value Vave are performed at the same time, it takes a long convergence time until the equilibrium state is reached. Therefore, the second reference from the first reference value Vref1 set in advance. It is better to create the value Vref2. The optimization is for the first reference value Vref1 in which both the maximum value Vmax and the average value Vave are fixed, and the convergence time is expected to be increased.

  FIG. 7 shows a first modification of the configuration of FIG. The laser drive circuit of FIG. 7 is obtained by adding an initial bias determination circuit 31 to the configuration of FIG. The initial bias determination circuit 31 automatically sets an optimal initial bias value of the bias circuit 23 according to the initial state. Specifically, arbitrary three currents Ib−α, Ib, and Ib + α are input to the light emitting circuit 21a, and the initial bias determining circuit 31 monitors the output current of the light receiving circuit 21b in each case. Since the IP characteristic (for example, characteristic 12 in FIG. 1) has a bending point, the amount of change between the output current in the case of Ib-α and the output current in the case of Ib is changed by continuously changing the bias current value. (First change amount) and a change amount (second change amount) between the output current in the case of Ib and the output current in the case of Ib + α (second change amount) are detected, and the first change amount and the second change amount are detected. Are different from each other, the current at this time is recognized as a current substantially equal to the threshold current Ith of the LD, and this is set as the initial bias current Ib. Thereby, the initial bias current Ib can be made substantially equal to the threshold current Ith. Therefore, the initial bias current setting suitable for each LD can be automatically performed, and the inspection process can be simplified and the cost of the product can be reduced.

  Note that the smaller the value of α is, the more accurate the initial bias current Ib can be set. Further, the number of input points may be three or more, or may be obtained by calculation from two or more input points. Any method may be used as long as the threshold current Ith is obtained. Also, arbitrary three points of current Ib-α, Ib, Ib + α are input to the light emitting circuit 21a and the output of the light receiving circuit 21b is monitored, but the monitored signal is the I / V conversion even with the output current of the light receiving circuit 21b. The output voltage of the circuit 24 may be used.

  FIG. 8 shows a second modification of the configuration of FIG. The laser drive circuit of FIG. 8 is obtained by adding an adaptive drive circuit 32 and an adaptive bias circuit 33 to the configuration of FIG. The LD threshold current and the conversion efficiency may change abruptly at the initial setting, when the ambient temperature changes abruptly, or when the deterioration of the LD and each component progresses. In such a case, optimization can be performed with the configuration of FIG. 5, but it takes time to converge to the optimum value. Therefore, when a change of a certain amount or more occurs, the first comparison circuit 27 outputs a signal that a sudden change has occurred, and the adaptive drive circuit 32 receiving the signal suddenly changes the pulse current Ip. The LD drive circuit 22 is acted on to charge and discharge. In addition, the signal indicating that a sudden change has occurred from the second comparison circuit 29 is output, and the adaptive bias circuit 33 that receives the signal acts on the bias circuit 23 to rapidly charge and discharge the bias current Ib. As described above, by adopting the configuration of FIG. 8, high-precision and high-speed optimization is possible even when the LD characteristics change suddenly. Note that both the adaptive circuits 32 and 33 may be used simultaneously, or only one of them may be used.

  FIG. 9 shows a third modification of the configuration of FIG. The laser drive circuit of FIG. 9 includes a threshold value in addition to the maximum value detection circuit 41 for detecting the maximum value Imax of the LD drive current and the average value detection circuit 42 for detecting the average value Iave of the LD drive current. A current detection circuit 43 and an amplifier circuit 44 are added to the configuration of FIG. The threshold current detection circuit 43 receives a signal from the first comparison circuit 27 when the maximum value Vmax of the output voltage of the I / V conversion circuit 24 is larger than the first reference value Vref1, and receives the two maximum values Vmax and Imax. Then, a calculation for obtaining the threshold current Ith from the two average values Vave and Iave is performed, and feedback to the bias circuit 23 is performed. In order to improve the detection accuracy in the threshold current detection circuit 43, when the output current of the light receiving circuit 21b is small, the amplifier circuit 44 amplifies the output current.

  When the threshold current Ith is small and the conversion efficiency is large, such as when the ambient temperature becomes low, the duty ratio of the output current of the light receiving circuit 21b remains 1: 1, and the maximum light output Pmax and the minimum light As a result of the output Pmin both increasing, the extinction ratio deteriorates (see FIG. 3). In such a case, the maximum value detection circuit 25 adjusts Vmax to the first reference value Vref1, and the threshold current detection circuit 43 determines the threshold current from the two points of the relationship between Vmax and Imax and the relationship between Vave and Iave. By calculating Ith and feeding back the threshold current Ith to the bias current Ib of the bias circuit 23, it can be converged to the optimum value at a higher speed. When the injection current (drive current) I to the LD is equal to or greater than the threshold current Ith, the IP characteristic can be approximated by a linear expression, and therefore the threshold current Ith is obtained from two points on the IP characteristic. By adopting the configuration of FIG. 9, the bias current Ib can be optimized at a higher speed than the configuration of FIG. Note that, in the calculation method of the threshold current Ith, it may be approximated by a linear expression as described above, may be approximated by a multi-order expression of second or higher order, or any other calculation method may be used.

  FIG. 10 shows a fourth modification of the configuration of FIG. The laser drive circuit of FIG. 10 includes a rise detection circuit 51 for detecting the rise of the output voltage of the I / V conversion circuit 24, and a fall detection for detecting the fall of the output voltage of the I / V conversion circuit 24. A circuit 52, a first arithmetic circuit 53 for calculating the time difference between the rise and fall of these output voltages, a rise detection circuit 54 for detecting the rise of the LD drive current I, and the LD drive current I A fall detection circuit 55 for detecting the fall, a second calculation circuit 56 for calculating the time difference between the rise and fall of the LD drive current I, the output of the first calculation circuit 53 and the first output 2 is compared with the output of the second arithmetic circuit 56 and the result is fed back to the bias circuit 23 so that an optical output with a constant duty ratio can be obtained. And a third comparison circuit 57 for controlling is obtained by adding to the configuration of FIG.

  In the configuration of FIG. 10, the rising and falling of each of the input current of the light emitting circuit 21a and the output current of the light receiving circuit 21b are detected, and the time from the rising to the falling is calculated. By feeding back the time obtained by these calculations to the bias circuit 23, it becomes possible to obtain an optical output with a constant duty ratio. In addition, by using the configuration of FIG. 10, when the duty ratio is deteriorated, double feedback is applied, and it is possible to converge to the optimum value faster than the configuration of FIG. Become. Note that the rising and falling edges may be detected using a high-precision latch circuit, may be processed by software, or any configuration that can detect time.

  FIG. 11 is a block diagram of an extinction ratio compensating laser driving circuit according to the second embodiment of the present invention. In FIG. 11, reference numeral 21 denotes a laser module, which emits a predetermined light output, and a light emitting circuit 21a composed of a laser diode (LD) that performs current-light conversion so as to output APC monitor light, and this light emission. The light receiving circuit 21b includes a photodiode (PD) for receiving the monitor light output from the circuit 21a. 22 is an LD drive circuit, 23 is a bias circuit, 24 is an I / V conversion circuit, 25 is a maximum value detection circuit, 27 is a comparison circuit, and 61 is a duty detection circuit. The duty detection circuit 61 includes a reference value creation circuit 62 and a charge pump circuit 63.

  The LD driving circuit 22 supplies a pulse current Ip so as to drive the light emitting circuit 21a according to input data (DATA). The bias circuit 23 adds a bias current Ib to the pulse current Ip output from the LD drive circuit 22. That is, the injection current (drive current) I to the light emitting circuit 21a is equal to Ib + Ip. The I / V conversion circuit 24 performs current-voltage conversion on the output of the light receiving circuit 21b. The maximum value detection circuit 25 detects the maximum value Vmax of the output voltage of the I / V conversion circuit 24. The comparison circuit 27 compares the maximum value Vmax output from the maximum value detection circuit 25 with a first reference value (first reference voltage) Vref1 set in advance, and sets the comparison result to LD so that the difference becomes zero. By returning to the drive circuit 22, the pulse current Ip is charged and discharged.

  The duty detection circuit 61 detects the duty ratio of the output voltage of the I / V conversion circuit 24 and feeds it back to the bias circuit 23. In the configuration of FIG. 11, the input voltage value of the charge pump circuit 63 rises and falls according to the High period and Low period of the output voltage of the I / V conversion circuit 24, and the voltage fluctuation is fed back to the bias circuit 23. Specifically, the characteristic that the duty ratio of the light output converges to a constant value is used. A reference value (reference voltage) Vref3 is generated from Vref1 by the reference value generation circuit 62 and used for the thresholds of the High period and Low period.

  In the present embodiment, if the duty ratio detected by the duty detection circuit 61 is not 1: 1, feedback is performed to the bias circuit 23 according to the ratio. That is, as shown in FIG. 2, since the ratio of the Low period is high at high temperatures, the bias current Ib is increased at high temperatures to control the optical output duty ratio to be 1: 1.

  As described above, by adopting the configuration of FIG. 11, the maximum light output, the extinction ratio, and the duty ratio can always be made constant with a simple configuration as compared with the case of FIG. The duty ratio may be detected using an A / D converter circuit, a low-pass filter (LPF) circuit, or any circuit configuration that can detect the duty ratio. It doesn't matter.

  FIG. 12 shows one modification of the configuration of FIG. In the laser drive circuit of FIG. 12, the duty detection circuit 61 includes two average value detection circuits 64 and 66, an inversion circuit 65, and a comparison circuit 67. One average value detection circuit 64 represents the average value of the positive phase output voltage of the I / V conversion circuit 24, and the other average value detection circuit 66 represents the average value of the negative phase output voltage of the I / V conversion circuit 24. To detect. The comparison circuit 67 compares the outputs of both average value detection circuits 64 and 66 and feeds back the result to the bias circuit 23. In this configuration, if the duty ratio is 1: 1, it is used that the average value of the positive phase output voltage is equal to the average value of the negative phase output voltage. Both average value detection circuits 64 and 66 can be easily configured by using an LPF circuit. Note that any circuit configuration may be used as long as the average value can be detected.

It is a characteristic view which shows the example of the conventional laser diode drive at the normal temperature. It is a characteristic view which shows the example of the conventional laser diode drive at the time of high temperature. It is a characteristic view which shows the example of the conventional laser diode drive at the time of low temperature. FIG. 6 is a characteristic diagram showing an ideal laser diode drive example that can obtain a constant maximum light output, extinction ratio, and duty ratio regardless of the ambient temperature. 1 is a block diagram of a laser drive circuit according to a first embodiment of the present invention. FIG. 6 is a circuit diagram illustrating a configuration example of a reference value generating circuit in FIG. 5. It is a block diagram which shows the 1st modification of a structure of FIG. It is a block diagram which shows the 2nd modification of the structure of FIG. It is a block diagram which shows the 3rd modification of a structure of FIG. It is a block diagram which shows the 4th modification of a structure of FIG. It is a block diagram of the laser drive circuit which concerns on the 2nd Embodiment of this invention. It is a block diagram which shows one modification of the structure of FIG.

Explanation of symbols

11, 12, 13 Current-light conversion characteristics 21 Laser module 21a Light emitting circuit (laser diode: LD)
21b Light receiving circuit (photodiode: PD)
22 LD drive circuit 23 Bias circuit 24 I / V conversion circuit 25 Maximum value detection circuit 26 Average value detection circuit 27, 29 Comparison circuit 28 Reference value creation circuit 31 Initial bias determination circuit 32 Adaptive drive circuit 33 Adaptive bias circuit 41 Maximum value detection Circuit 42 Average value detection circuit 43 Threshold current detection circuit 44 Amplifier circuits 51 and 54 Rising detection circuits 52 and 55 Falling detection circuits 53 and 56 Operation circuit 57 Comparison circuit 61 Duty detection circuit 62 Reference value creation circuit 63 Charge pump circuit 64 66 Average value detection circuit 65 Inversion circuit 67 Comparison circuit DATA Input data I Injection current (drive current)
Iave average value (average current)
Ib, Ib1, Ib3 Bias current Imax Maximum value (maximum current)
Ip, Ip1, Ip3 Pulse current Ith1, Ith2, Ith3 Threshold current P Optical output Pmax, Pmax1, Pmax3 Maximum optical output Pmin, Pmin1, Pmin3 Minimum optical output T1, T2, T3 Temperature Vave average value (average voltage)
Vmax maximum value (maximum voltage)
Vref1, Vref2, Vref3 reference value (reference voltage)

Claims (3)

A laser driving circuit for controlling the light emitting circuit using a light receiving circuit for receiving output light of the light emitting circuit,
A driving circuit for driving the light emitting circuit;
A bias circuit for adding a bias current to the pulse current output from the drive circuit;
An I / V conversion circuit for current-voltage conversion of the output of the light receiving circuit;
A first maximum value detection circuit for detecting a maximum value of an output voltage of the I / V conversion circuit;
A first average value detection circuit for detecting an average value of output voltages of the I / V conversion circuit;
A first comparison circuit that compares the maximum value with a first reference value and feeds back a result to the drive circuit;
A second comparison circuit that compares the average value with a second reference value and feeds back the result to the bias circuit;
A second maximum value detection circuit for detecting a maximum value of the drive current of the light emitting circuit;
A second average value detection circuit for detecting an average value of the drive current of the light emitting circuit;
When the maximum value of the output voltage of the I / V conversion circuit is larger than the first reference value, a signal is received from the first comparison circuit, and a threshold current is calculated from the two maximum values and the two average values. A laser drive circuit comprising: a threshold current detection circuit that performs a calculation to obtain and feeds back to the bias circuit. The laser drive circuit according to claim 1, wherein
A laser driving circuit, further comprising an amplifying circuit for amplifying an output current of the light receiving circuit so as to improve detection accuracy in the threshold current detecting circuit. The laser drive circuit according to claim 1 or 2,
A light emitting circuit driven by the laser driving circuit;
An optical communication apparatus comprising: a light receiving circuit that receives output light of the light emitting circuit.

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