JP3366130B2 - Semiconductor laser control method and device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser control method and apparatus, and more particularly to a semiconductor laser control method and apparatus for controlling a bias current required to drive a semiconductor laser.

At present, optical communication technology is widely applied,
Further, the development of optical technology in various fields such as introduction of optical communication to subscriber systems, optical switching, and optical connection is expected. In the optical communication technology, a semiconductor laser is generally used to convert an input digital signal which is an electric signal into an optical signal. Since speeding up of this semiconductor laser is very important in speeding up optical transmission, there is a demand for development of a semiconductor laser capable of responding to an input signal at a higher speed.

[0003]

2. Description of the Related Art In order to perform a high speed operation on a semiconductor laser, the bias current applied to the semiconductor laser is made equal to its threshold current or at a current value smaller than the threshold current. It is important to control the current value as close to the threshold current as possible. By doing so, when the drive current that changes corresponding to the input digital signal flows, the semiconductor laser immediately enters the oscillation state, and high-speed response of the semiconductor laser to the input digital signal can be realized. In other words, controlling the bias current to the above value is also a condition for minimizing the laser oscillation delay time.

By the way, in optical communication using an optical fiber, the transmission loss in the optical fiber is 1.3 μ, which is small.
InGaAsP / InP semiconductor lasers, which are semiconductor lasers that oscillate light with a wavelength of m to 1.6 μm, are widely used. However, in this semiconductor laser, the temperature dependence of the threshold current is large, so in order to ensure high-speed response, the bias current is controlled so as to follow the threshold current that changes with temperature. It is important to.

Bias current control of the prior art will be described with reference to FIGS. 7 to 9. First, the control of the bias current using the Peltier device, which is the first conventional technique, will be described with reference to FIG.

In the control of the bias current according to the first conventional technique, the Peltier element is used to keep the semiconductor laser at a constant temperature, the threshold current thereof is kept constant, and the corresponding bias current is also kept constant. Is.

In the bias current control device for controlling the bias current of the semiconductor laser 100 shown in FIG. 7, the drive circuit 101 generates a drive current I _d for driving the semiconductor laser 100 in response to an input digital signal, and the semiconductor laser 100. And a predetermined bias current I _b1 is generated and output to the semiconductor laser 100. The Peltier element 102 is arranged between the semiconductor laser 100 and the heat sink 103, and is driven by the Peltier element drive current I _P to keep the temperature of the semiconductor laser 100 at a predetermined constant temperature. The temperature sensor 104 detects the temperature of the surface of the Peltier element 102 on which the semiconductor laser 100 is arranged, and outputs a sensor signal S _S to the temperature control circuit 105. The temperature control circuit 105 receives the sensor signal S _S from the temperature sensor 104.
Based on the above, the temperature of the semiconductor laser 100 is detected, and a Peltier element drive current I _P for controlling the Peltier element 102 so that the semiconductor laser 100 reaches a predetermined constant temperature is output to the Peltier element 102.

According to the bias current control device of the first prior art, the semiconductor laser 100 is always controlled to a constant temperature by the operation of the Peltier element 102, so that the threshold current of the semiconductor laser 100 also becomes a substantially constant value. An optimum bias current I _b1 can be obtained by causing a predetermined bias current set in advance to flow against this constant threshold current.

Next, the bias current control using the second conventional technique will be described with reference to FIG. In the control of the bias current according to the second conventional technique, the temperature of the semiconductor laser is detected, and the bias current is set to a preset bias current corresponding to the temperature.

Bias of the semiconductor laser 100 shown in FIG.
In the bias current controller that controls the current, the temperature
The sensor 104 is the semiconductor laser 10 of the heat sink 103.
0 detects the temperature of the surface on which the sensor signal S_S
Is output to the temperature monitor circuit 106. Temperature monitor circuit 1
06 is a sensor signal S from the temperature sensor 104._SBased on
Detects the temperature of the semiconductor laser 100 and responds to that temperature
The bias current to a preset bias current.
Control signal S_BIs output to the drive circuit 101. Drive circuit 1
01 is the bias control signal S_BBased on
Bias current I_b2To generate and output
Digital signal S_INDrive current I based on _dTo generate
Ias current I_b2And output to the semiconductor laser 100.
It

According to the bias current control device of the second prior art, the temperature of the semiconductor laser 100 is detected, and the bias current I _b2 preset corresponding to the temperature flows, so that the temperature changes as the temperature changes. Therefore, even if the threshold current of the semiconductor laser 100 changes, the bias current I _b2 corresponding thereto can be passed.

Next, the control of the bias current using the third conventional technique will be described with reference to FIG. In the control of the bias current according to the third conventional technique, a part of the laser light output from the semiconductor laser is detected by a photodetector, and the bias current is controlled based on the detected output, so-called APC (Auto Power Control). ) Is done.

Bias of the semiconductor laser 100 shown in FIG.
In the bias current control device that controls the current, the light detection
The laser 107 outputs the laser light L output from the semiconductor laser 100.
Part of the light is received and the detector output S_LOptical output detection circuit 10
Output to 8. The optical output detection circuit 108 has a detector output S
_LBased on that, the average value of the output of the semiconductor laser 100 is constant.
Control to control the bias current so that
Signal S_BIs output to the drive circuit 101. Drive circuit 101
Is the bias control signal S_BBased on the bias current I_b3
Generates and outputs the input digital signal S_INBased on
Drive current I _dTo generate the bias current I_b3Superimposed on
Output to the semiconductor laser 100.

According to the bias current control device of the third prior art, the bias current I _b3 is controlled based on the detector output S _L of the photodetector 107 so that the average value of the output of the semiconductor laser 100 becomes constant. Therefore, the bias current I _b3 corresponding to the threshold current that changes depending on the temperature can be passed.

[0015]

However, according to the first conventional technique shown in FIG. 7, since a circuit such as the temperature control circuit 105 for controlling the temperature of the Peltier element 102 is required, the entire apparatus is required. However, there is a problem in that it becomes expensive and the circuit mounting scale becomes large. Further, when the threshold current increases due to the deterioration of the semiconductor laser 100, the threshold current at the same temperature changes, so that the threshold current and the bias current deviate. there were.

Further, according to the second conventional technique shown in FIG. 8, when the threshold current increases due to deterioration of the semiconductor laser 100, the threshold current at the same temperature changes. However, there is a problem that the threshold current and the bias current deviate from each other.

Further, according to the third conventional technique shown in FIG. 9, since it is necessary to provide the photodetector and its detection circuit in the bias current control device, the circuit mounting scale becomes large, and the entire device becomes large. There was a problem that it became expensive.

In view of the above problems, it is an object of the present invention to make the bias current equal to the threshold current even when the threshold current changes due to deterioration of the semiconductor laser and changes in the operating environment. Another object of the present invention is to provide a small-sized and inexpensive semiconductor laser control method and device capable of controlling the current value as close to the threshold current as possible with a current value lower than the threshold current.

[0019]

In order to solve the above problems, the invention according to claim 1 is obtained by superimposing a bias current on a drive current corresponding to an input digital signal, and a semiconductor. A differential resistance value detection step of detecting and outputting a differential resistance value of the semiconductor laser that changes based on an operating current supplied to the laser, the detected differential resistance value, and the differential resistance value of the semiconductor laser in a predetermined operating environment. A predetermined reference resistance value preset corresponding to the threshold current, and a comparison step of outputting a comparison signal,
A bias current control step of controlling the bias current so that the differential resistance value becomes equal to the reference resistance value based on the comparison signal.

A second aspect of the present invention is the semiconductor laser control method according to the first aspect, wherein the differential resistance value detecting step is a constant sufficiently lower than a bit rate of the input digital signal. A step of supplying a minute alternating current having a frequency and a minute alternating current having a constant current amplitude value that is sufficiently smaller than the input digital signal current to the operating current, and measuring the voltage applied to the semiconductor laser. Voltage measuring step, and extracting a minute AC voltage corresponding to the minute AC current from the measured voltage, converting the minute AC voltage into a DC voltage proportional to the differential resistance value based on its amplitude, and outputting And a step of comparing the reference voltage corresponding to the reference resistance value with the DC voltage, and outputting a comparison signal. Configured to be a degree.

According to a third aspect of the present invention, there is provided the semiconductor laser control method according to the first aspect, wherein the differential resistance value detecting step has a constant value sufficiently lower than a bit rate of the input digital signal. A step of supplying a minute alternating current having a frequency and a minute alternating current having a constant current amplitude value that is sufficiently smaller than the input digital signal current to the operating current, and measuring the voltage applied to the semiconductor laser. Voltage measuring step, and extracting a minute AC voltage corresponding to the minute AC current from the measured voltage, converting the minute AC voltage into a DC voltage proportional to the differential resistance value based on its amplitude, and outputting And a bit ratio for detecting a bit ratio obtained by dividing the bit number of one logic of the input digital signal in one of the predetermined periods by the total number of bits in the predetermined period. A detection step, the said DC voltage corrected based on the change of the bit ratio, it is configured to include a correction step of outputting a corrected DC voltage, the comparing step,
The voltage comparison step is configured to compare a reference voltage corresponding to the reference resistance value with the corrected DC voltage and output a comparison signal.

According to a fourth aspect of the present invention, a drive means for generating and outputting a drive current corresponding to an input digital signal, a bias means for generating and outputting a bias current based on a bias current control signal, and the drive means. A differential resistance value detection means for detecting and outputting a differential resistance value of the semiconductor laser, which is obtained by superimposing the bias current on a current and changes based on an operating current supplied to the semiconductor laser. Comparing means for comparing the differential resistance value with a predetermined reference resistance value preset corresponding to the threshold current of the semiconductor laser in a predetermined operating environment, and outputting a comparison signal; Based on the comparison signal,
Bias current control means for outputting the bias current control signal for controlling the bias current so that the differential resistance value becomes equal to the reference resistance value.

According to a fifth aspect of the present invention, in the semiconductor laser control device according to the fourth aspect, the differential resistance value detecting means has a constant value that is sufficiently lower than a bit rate of the input digital signal. A minute AC current supply means for superimposing a minute AC current having a frequency and a constant current amplitude value that is sufficiently smaller than the input digital signal current on the operating current, and measures the voltage applied to the semiconductor laser. Voltage measuring means for extracting a minute AC voltage corresponding to the minute AC current from the measured voltage, converting the minute AC voltage into a DC voltage proportional to the differential resistance value based on its amplitude, and outputting the DC voltage. And a comparing means for comparing the DC voltage with a reference voltage corresponding to the reference resistance value and outputting a comparison signal. Configured to be in the stage.

According to a sixth aspect of the present invention, in the semiconductor laser control device according to the fourth aspect, the differential resistance value detecting means has a constant value that is sufficiently lower than a bit rate of the input digital signal. A minute AC current supply means for superimposing a minute AC current having a frequency and a constant current amplitude value that is sufficiently smaller than the input digital signal current on the operating current, and measures the voltage applied to the semiconductor laser. Voltage measuring means for extracting a minute AC voltage corresponding to the minute AC current from the measured voltage, converting the minute AC voltage into a DC voltage proportional to the differential resistance value based on its amplitude, and outputting the DC voltage. And a bit ratio for detecting a bit ratio obtained by dividing the number of bits of one logic in one of the predetermined periods of the input digital signal by the total number of bits in the predetermined period. Detection means, the said DC voltage corrected based on the change of the bit ratio, is configured to include a correction means for outputting a corrected DC voltage, said comparison means,
The voltage comparison means is configured to compare the corrected DC voltage with a reference voltage corresponding to the reference resistance value and output a comparison signal.

[0025]

According to the invention described in claim 1, in the differential resistance value detecting step, the differential resistance value of the semiconductor laser, which changes based on the operating current, is detected and output.

In the comparison step, the differential resistance value is compared with a predetermined reference resistance value preset corresponding to the threshold current of the semiconductor laser in a predetermined operating environment, and a comparison signal is output.

By the way, the above-mentioned differential resistance value is a series resistance value (determined by the structure, material, etc. of the semiconductor laser from the resistance value equal to or higher than the reference resistance value when the threshold current flows through the semiconductor laser). It changes discontinuously up to a resistance value lower than the reference resistance value), and changes in response to a change in the bias current when the drive current does not flow.

Therefore, in the bias current control step,
Based on the comparison signal, the bias current is controlled so that the differential resistance value becomes equal to the reference resistance value. Therefore, since the bias current is controlled so that the differential resistance value approaches the higher value of the value corresponding to the discontinuity point or the value corresponding to the discontinuity point, it is possible to respond to changes in the operating environment, etc. With respect to the threshold current of the semiconductor laser that changes by, the bias current becomes equal to the threshold current, or is a value less than the threshold current, from the threshold current,
The value is within a predetermined range corresponding to the oscillation delay time of the semiconductor laser allowed by design.

According to the invention described in claim 2, claim 1
In addition to the function of the invention described in (1), in the step of supplying a minute alternating current, the constant frequency is sufficiently lower than the bit rate of the input digital signal, and the constant frequency is sufficiently smaller than the input digital signal current. A small alternating current having a current amplitude value is superimposed on the operating current.

In the voltage measuring step, the voltage applied to the semiconductor laser is measured. In the minute AC voltage extraction step, a minute AC voltage corresponding to the minute AC current is extracted from the measured voltage, and the minute AC voltage is converted into a DC voltage proportional to the differential resistance value based on its amplitude and output. To be done.

Therefore, a minute AC voltage that changes in response to a change in the differential resistance value is extracted, and a DC voltage proportional to its amplitude is output. Therefore, a DC voltage proportional to the differential resistance value is obtained, and this DC voltage is obtained. In the voltage comparison step, the reference voltage corresponding to the reference resistance value is compared, and a comparison signal is output based on the comparison result.

According to the invention of claim 3, claim 1
In addition to the function of the invention described in (1), in the step of supplying a minute alternating current, the constant frequency is sufficiently lower than the bit rate of the input digital signal, and the constant frequency is sufficiently smaller than the input digital signal current. A small alternating current having a current amplitude value is superimposed on the operating current.

In the voltage measuring step, the voltage applied to the semiconductor laser is measured. In the minute AC voltage extraction step, a minute AC voltage corresponding to the minute AC current is extracted from the measured voltage, and the minute AC voltage is converted into a DC voltage proportional to the differential resistance value based on its amplitude and output. It

In the bit ratio detecting step, the bit ratio obtained by dividing the number of bits of one logic in the predetermined period of the input digital signal by the total number of bits in the predetermined period is detected. In the correction step, the DC voltage is corrected based on the change in the bit ratio, and the corrected DC voltage is output.

Therefore, the minute AC voltage that changes in accordance with the change in the differential resistance value is extracted, and the DC voltage proportional to the amplitude thereof is output, so that the DC voltage proportional to the differential resistance value is obtained. This DC voltage changes in accordance with the change in the bit ratio, but this change is corrected in the correction step, and a stable corrected DC voltage is obtained. Then, the corrected DC voltage is compared with the reference voltage corresponding to the reference resistance value in the voltage comparison step, and the comparison signal is output based on the comparison result.

According to the invention described in claim 4, the driving means generates the driving current and outputs it to the semiconductor laser and the differential resistance value detecting means. Further, the bias means generates a bias current based on the bias current control signal and outputs it to the semiconductor laser and the differential resistance value detecting means.

As a result, the driving current and the bias current are superposed and supplied as the operating current to the semiconductor laser and the differential resistance value detecting means. On the other hand, the differential resistance value detecting means detects the differential resistance value of the semiconductor laser based on the change of the operating current and outputs it to the comparing means.

The comparing means compares the differential resistance value with a predetermined reference resistance value preset corresponding to the threshold current of the semiconductor laser in a predetermined operating environment, and outputs a comparison signal to the bias current control means. Output to.

By the way, the above-mentioned differential resistance value is a series resistance value (determined by the structure, material, etc. of the semiconductor laser from the resistance value equal to or higher than the reference resistance value when the threshold current flows through the semiconductor laser). It changes discontinuously up to a resistance value lower than the reference resistance value), and changes in response to a change in the bias current when the drive current does not flow.

Therefore, the bias current control means outputs to the bias means a bias current control signal for controlling the bias current so that the differential resistance value becomes equal to the reference resistance value based on the comparison signal.

Therefore, since the bias current is controlled so that the differential resistance value approaches the higher value of the value corresponding to the discontinuity point or the value corresponding to the discontinuity point, changes in the operating environment, etc. The threshold current of the semiconductor laser that changes corresponding to, the bias current becomes equal to the threshold current, or is a value less than the threshold current, from the threshold current , A value within a predetermined range corresponding to the semiconductor laser oscillation delay time allowed by design.

According to the invention of claim 5, claim 4
In addition to the operation of the invention described in (1), the minute alternating current supply means has a constant frequency that is sufficiently lower than the bit rate of the input digital signal, and a current amplitude value that is sufficiently smaller than the input digital signal current. Superimposing a small AC current on the operating current.

The voltage measuring means measures the voltage applied to the semiconductor laser. The minute AC voltage extracting means extracts a minute AC voltage corresponding to the minute AC current from the measured voltage,
The minute AC voltage is converted into a DC voltage proportional to the differential resistance value based on its amplitude and output.

Therefore, since the minute AC voltage that changes in response to the change in the differential resistance value is extracted and the DC voltage that is proportional to the amplitude thereof is output, the DC voltage that is proportional to the differential resistance value is obtained. In the voltage comparison step, the reference voltage corresponding to the reference resistance value is compared, and a comparison signal is output based on the comparison result.

According to a sixth aspect of the invention, in addition to the operation of the fourth aspect of the invention, the minute alternating current supply means has a constant frequency sufficiently lower than the bit rate of the input digital signal. , A small alternating current having a constant current amplitude value that is sufficiently smaller than the input digital signal current is superimposed on the operating current.

The voltage measuring means measures the voltage applied to the semiconductor laser. The minute AC voltage extracting means extracts a minute AC voltage corresponding to the smiling AC current from the measured voltage, converts the minute AC voltage into a DC voltage proportional to the differential resistance value based on its amplitude, and outputs the DC voltage.

The bit ratio detecting means detects a bit ratio obtained by dividing the number of logical bits in one of the predetermined periods of the input digital signal by the total number of bits in the predetermined period. The correction means is
The DC voltage is corrected based on the bit ratio, and the corrected DC voltage is output.

Therefore, the minute AC voltage that changes in accordance with the change in the differential resistance value is extracted, and the DC voltage proportional to the amplitude thereof is output, so that the DC voltage proportional to the differential resistance value is obtained. This DC voltage changes in accordance with the change in the bit ratio, but this change is corrected by the correction means, and a stable corrected DC voltage is obtained. Then, the corrected DC voltage is compared with the reference voltage corresponding to the reference resistance value in the voltage comparison step, and the comparison signal is output based on the comparison result.

[0049]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, referring to FIG.
It will be described with reference to FIGS. (I) in principle the beginning of the present invention, the principle of the present invention will be described with reference to FIGS.

First, the relationship between the current I and the voltage V in a general semiconductor laser will be described with reference to FIG. When the current I of the semiconductor laser is smaller than the oscillation threshold current I _TH peculiar to the semiconductor laser, the semiconductor laser exhibits characteristics similar to those of a normal semiconductor diode. The relationship between the current I and the voltage V at that time is as follows.

I = I ₀ exp (q (V−R _S I) / nkT) (1) where I ₀ is a constant determined by the material properties and structure of the semiconductor laser (semiconductor diode), and q is Electric charge, n is a constant (usually a value between 1 and 2) determined by a semiconductor laser (semiconductor diode), k is Boltzmann's constant, T
Is an absolute temperature, and R _S is a series resistance determined by layers other than the active layer of the semiconductor laser.

The relationship represented by the equation (1) corresponds to the range indicated by "α" in FIG. Next, when the current I exceeds the oscillation threshold current I _TH , the semiconductor laser oscillates and emits the laser light L, but since the carrier density of the active layer does not increase during laser oscillation, the voltage is clamped and the active The layer voltage V is approximately constant.

The relationship between the current I and the voltage V at this time is V = V _TH + _RS (I−I _TH ) ... (2), which corresponds to the range indicated by “β” in FIG. . The graph in this range is almost linear and its slope is RS. Here, V _TH is the voltage of the semiconductor laser when a threshold current flows in the semiconductor laser.

From the above description, the current I of the semiconductor laser I
In the graph showing the relationship between the voltage V and the voltage V, the current I is the threshold current I
_The shape has discontinuities at _TH . Further, as another characteristic of the semiconductor laser, the above-mentioned threshold current I _TH has a characteristic of increasing when the temperature of the semiconductor laser rises or when the semiconductor laser itself deteriorates.

Next, the relationship between the differential resistance value of the semiconductor laser and the current I based on the above-mentioned relationship between the current I and the voltage V will be described with reference to FIG. The differential resistance value R _d of the semiconductor laser is defined by R _d = dV / dI.
From the relationship between the current I and the voltage V shown in (a), a discontinuous value is taken before and after the threshold current. The value of the differential resistance value R _d is
When the current I is smaller than the threshold current I _TH , R _d = _RS + (nkT) / (qI) (3), and when the current I is equal to or more than the threshold current I _TH , R _d = _RS. . Therefore, when the current I is the threshold current I _TH , the differential resistance value R _d has a discontinuity of ΔR _d = (nkT) / (qI _TH ) ... (4).

The relationship between the differential resistance value R _{d and} the current I is shown in FIG. 2 (b). Here, the discontinuity point of the differential resistance value R _d has a characteristic that it shifts to the direction in which the current I increases as the temperature of the semiconductor laser rises. In FIG. 2B, three types of temperatures Ta, Tb and Tc The relationship between the current I and the differential resistance value R _{d at} (Ta <Tb <Tc) is shown. As shown in FIG. 2B, the differential resistance values R _d are discontinuous at the threshold currents I _THa , I _{TH b,} and I _THc corresponding to the respective temperatures.

By the way, in order to oscillate the semiconductor laser in response to a change in the input signal and emit the laser beam,
It is necessary to apply a predetermined bias current I _b in advance and add a drive current I _d that changes corresponding to a change in the input signal so as to be superposed on it. Then, the bias current I _b
When the value of the operating current I _D , which is the sum of the driving current I _d and the driving current I _d , exceeds the above-mentioned threshold current I _TH , the semiconductor laser oscillates and laser light is emitted. At this time, in order to oscillate the semiconductor laser so as to respond to the input signal at a high speed, as described above, the bias current I _b is made equal to the threshold current I _{TH at} the temperature at that time. Or
A value less than the threshold current I _TH, preferably set to as close as possible value to the threshold current I _TH. The present invention, control of the bias current I _b for this, detecting the differential resistance value mentioned above, it is an attempt to correspond to the change.

That is, the current I on the horizontal axis in FIG. 2A or FIG. 2B is the drive current I when the drive current I _d for driving the semiconductor laser is applied.
is the operating current I _D that is the sum of _d and the bias current I _b ,
When the drive current I _d is not applied, it can be considered as the bias current I _b itself. Therefore, it can be considered that the differential resistance value R _d changes depending on the bias current I _b .

Here, the temperature T _C in FIG. 2B is the maximum value of the practical temperature as the predetermined operating environment of the semiconductor laser, and the differential resistance value at the discontinuous point at that time is the reference resistance value R _F. . Then, by controlling the bias current I _b to equal the differential resistance value R _d in the reference resistance value R _F, threshold current bias current I _b at that time I _THC
Can be controlled equally. Further, if the reference resistance value R _F is set to a value in a certain range larger than the differential resistance value at the discontinuity point, it is a value smaller than the threshold current I _THC, and is a semiconductor allowed from the threshold current I _{TH C} by design. It can be set within a predetermined range corresponding to the oscillation delay time of the laser.

That is, in the case of temperature T _C , the detected differential resistance value is smaller than the reference resistance value R _F (specifically, if the reference resistance value R _F is set as the differential resistance value at the discontinuity point, Since there are continuous points, when the resistance value is smaller than the reference resistance value R _F , the value is always the series resistance R _S. ) At that time, the bias current I _b is decreased and the differential resistance value becomes the reference resistance value R _F. Control to be.

The detected differential resistance value is the reference resistance value R.
When it is larger than _F , the bias current I _b is increased so that the differential resistance value becomes the reference resistance value R _F.

As a result, by setting the reference resistance value R _F to a value between the differential resistance value at the discontinuity point and the series resistance R _S or at the discontinuity point, the bias current I _b and the threshold current I
_THC can be made equal to each other, or the reference resistance value R _F can be set to a value in a certain range larger than the differential resistance value at the discontinuous point, so that the bias current I _b can be a threshold value smaller than the threshold current I _THC. The value current I _THC can be set within a predetermined range.

This control is also effective for a temperature T _b different from the temperature T _C. That is, in FIG. _2B , when the threshold current changes from I _THC to I _THb due to the temperature of the semiconductor laser _dropping from T _C to T _b , the differential resistance value becomes the reference resistance as described above. or a bias current I _b by controlling to a value R _F is equal to the threshold current I _THb, or with less than the threshold current I _THb, within a predetermined range from the threshold current I _THb can do.

That is, at the temperature T _b , when the detected differential resistance value is smaller than the reference resistance value R _F , the bias current I _b is decreased and the differential resistance value is increased so that the reference resistance value R _F is reached. To do. When the detected differential resistance value is larger than the reference resistance value R _F , the bias current I
_b is increased, the differential resistance value is decreased, and the reference resistance value R
Control to become _F. As a result, the bias current I
or equal to the threshold current I _{TH b} a _b, or with less than the threshold current I _THb, it can be within a predetermined range from the threshold current I _{TH b.}

Even when the temperature of the semiconductor laser changes from T _b to T _a , the bias current I is controlled by the same control.
or equal to the threshold current I _THa the _b, or with less than the threshold current I _THa, it can be within a predetermined range from the threshold current I _THa.

Further, when the temperature of the semiconductor laser rises from T _a to T _b and T _C , the bias current I _b is equal to the threshold current at each temperature or the threshold value is controlled by the same control. It can be within a predetermined range from the threshold current with a value smaller than the current.

The actual value of the series resistance R _S is about 5Ω, and the differential resistance value is 80 ° C. and the threshold current is 10 mA (value in the strained quantum well active layer semiconductor laser). Discontinuity of differential resistance value at threshold current Δ
Since R _d is about 6Ω, it can be sufficiently controlled.

Next, the drive current I is actually_dIs the bias current
I_bVoltage V and differential resistance value R when superimposed on_dof
The change with time will be described with reference to FIG. Is
First, the current I is the drive current I_dIs the input digital signal
Since it is modulated corresponding to, as shown in FIG.
Changes to. In FIG. 3A, the current is (I_d＋ I
_b), The semiconductor laser oscillates when
Ias current I _bIs the threshold current I_THThe following values are
It Corresponding to this, the voltage V is as shown in FIG.
Changes to. And the differential resistance value R_dIs the drive current I_d
Corresponding to the modulation of, the change occurs as shown in FIG.
That is, the drive current I_dIs applied and the semiconductor laser emits
When shaking, the differential resistance value R_dIs the series resistance R_SWhen
Become equal and drive current I_dIs not applied and the semiconductor laser
When not oscillating, differential resistance value R_dIs the bias voltage
Resistance value R when flowing only_CHas become.

Next, the basic structure of the present invention for controlling the bias current I _b described above will be described with reference to FIG. As shown in FIG. 1, the semiconductor laser control device by the bias current control of the present invention uses an input digital signal S
Bias means for generating and outputting a driving current I _d that changes corresponding to _IN and that oscillates the semiconductor laser 1 and generates and outputs a bias current I _b under the control of the bias current control means. 3, the differential resistance value detecting means 4 for detecting and outputting the differential resistance value R _d of the semiconductor laser from the operating current I _{D, the} value of the differential resistance value R _d , and the semiconductor in a predetermined operating environment (temperature T _C ). Comparison means 5 for comparing with a predetermined reference resistance value R _F preset corresponding to the threshold current (I _THC ) of the laser 1 and outputting a comparison signal S _C.
And a bias current control means 6 that outputs a bias control signal S _B that controls the bias current I _b so that the value of the differential resistance value R _d becomes equal to the reference resistance value R _F based on the comparison signal S _C. It is equipped with. Then, based on the differential resistance value R _d detected by the differential resistance value detecting means 4, the bias current I _b is controlled by the bias current controlling means 6 as described above.

Further, in the differential resistance value detecting means 4,
More specifically, the differential resistance value R _d is detected by detecting the voltage V shown in FIG. (II) First Embodiment Next, a first embodiment corresponding to the invention described in claims 1, 2, 4 and 5 will be described with reference to FIGS. 4 and 6.

The semiconductor laser bias current control device 10 shown in FIG. 4 drives the semiconductor laser 1 and changes the drive current I corresponding to the change of the input digital signal S _IN.
Drive circuit 11 as drive means for generating and outputting _d
And a bias circuit 12 as bias means for generating and outputting a bias current I _b which is superimposed on the drive current I _d based on the control of the bias current control circuit 17 as the bias current control means, and the drive current I _d . Bias current I _b
Detecting the differential resistance value R _d of the semiconductor laser 1 which changes based on the change in the operating current I _D combined bets, and the differential resistance value detecting means 4 to output a DC voltage V _D which is proportional to the differential resistance value R _d , A predetermined reference resistance value R preset corresponding to the value of the DC voltage V _D and the threshold current I _TH of the semiconductor laser 1 at the maximum practical temperature as a predetermined operating environment.
_The comparison signal S _C is compared with the reference voltage V _S corresponding to _F.
A voltage comparator 16 as a comparing means for outputting a comparison on the basis of the signal S _C, if the value of the DC voltage V _D becomes equal to the reference voltage V _S, or a value higher than the reference voltage V _S,
In addition, the bias current control circuit 17 serves as a bias current control unit that controls the bias current I _b so as to approach the reference voltage V _S.

The differential resistance value detecting means 4 is an input digital
Signal S_INA constant frequency that is sufficiently low compared to the bit rate of
Has a number and the input digital signal S_INSufficient compared to the current of
Operating a small alternating current with a small constant current amplitude value.
Flow I_DAs a means for supplying minute alternating current
The current supply circuit 13 and the voltage applied to the semiconductor laser 1
Pressure (hereinafter referred to as operating voltage V) is measured and smoothed.
A voltage measuring circuit 14 as a voltage measuring means, and an operating voltage V
From the minute AC voltage V corresponding to the minute AC current ₁Extract
DC voltage V proportional to its amplitude_DAnd output
Micro AC voltage extraction circuit 1 as micro AC voltage extraction means
5 and 5.

[0073] Here, the input digital signal S constant frequency sufficiently lower than the frequency band corresponding to the bit rate of the _IN, it refers to a frequency smaller than the lower limit of the signal band of the input digital signal S _IN, the input digital The constant current amplitude value that is sufficiently smaller than the current of the signal S _IN means that the laser light L generated by a minute AC current is smaller than the output of the laser light L of the semiconductor laser 1 based on the input digital signal S _IN. Is a constant current amplitude value that is small enough not to affect the operation of the photodetector as noise in the photodetector that receives the laser light L.

In the above structure, the voltage measuring circuit 14 is
For example, it is configured by an AC buffer amplifier for low frequency. The minute AC voltage extracting circuit 15 is composed of, for example, a bandpass filter type amplifier, a detector and a rectifier. Further, the voltage comparison circuit 16 is composed of, for example, a differential amplifier, and the bias current control circuit 17 is composed of, for example, a differential pair of transistors or the like.

Next, the operation will be described with reference to FIGS. 4 and 6. The drive current I _d generated by the drive circuit 11 is superposed with the bias current I _b generated by the bias circuit 12, and becomes an operating current I _D for driving the semiconductor laser 1, which is applied to the semiconductor laser 1. This operating current I _D
The minute alternating current supply circuit 13 superimposes the minute alternating current. The frequency of this minute alternating current is, for example, 100 kHz when the bit rate of the input digital signal S _IN is 100 Mb / s or more, and its current amplitude value is, for example, when the modulation current of the input digital signal S _IN is 20 mA. If there is, it will be 0.2mA. The waveform of the operating current _ID superposed with this minute alternating current is as shown in FIG. The operating voltage V generated in the semiconductor laser 1 when the operating current _ID is applied to the semiconductor laser 1 is shown in FIG.
It has a waveform as shown in.

As shown in FIG. 6B, since the differential resistance value R _d is equal to the series resistance R _S and is constant, when the semiconductor laser 1 is oscillating (the operating current _ID is the threshold value). The change B of the voltage due to the minute alternating current of the operating voltage V (when it is larger than the value current I _TH ) is small.

On the other hand, when the semiconductor laser 1 is not oscillating (when the operating current I _D is less than the threshold current I _TH ), the change A of the operating voltage V caused by the minute AC current is shown in FIG. As shown in FIG. 2 (b), since the differential resistance value R _d exceeds the discontinuity point and becomes larger than the series resistance R _S (because the operating current _ID is less than or equal to the threshold current), a small alternating current Corresponding to the change in the semiconductor laser 1 is oscillating (see reference numeral B in FIG. 6B), the change becomes larger as the differential resistance value R _d becomes larger. The change in the operating voltage V is caused by the voltage measuring circuit 14
When smoothed by, the output voltage V ₁ has a waveform shown in FIG. This change in the output voltage V ₁ corresponds to the superposed minute alternating current, and the change (amplitude) A of the voltage is the differential resistance value if the frequency and the amplitude of the minute alternating current are constant as described above. The value is proportional to the magnitude of R _d .

Thereafter, this output voltage V₁Is a small AC voltage
The extraction circuit 15 detects and rectifies the AC signal
The change A of the voltage as the amplitude is the DC voltage V_DIs converted to
It Micro AC current supply circuit 13 and voltage measurement circuit 1 described above
4 and the operation of the minute AC voltage extraction circuit 15 cause the differential resistance
Resistance value R_dDC voltage V proportional to the magnitude of_DIs obtained.
This DC voltage V_DIs a predetermined value in the voltage comparison circuit 16.
Reference resistance value R_FReference voltage V corresponding to_SCompared to
Comparison signal S based on the result_CIs output and the bias voltage is
It is input to the flow control circuit 17. And bias current control
The control circuit 17 has been described in the principle of the present invention.
Bias control signal S for controlling the bias current_BBut
It is output, and based on this, in the bias circuit, the differential resistance is
Resistance value R _dCorresponding to the threshold current at that time based on the change of
Bias current I_bIs generated and output.

According to the first embodiment described above, the differential resistance value R _d of the semiconductor laser 1 is detected, and the bias current is controlled based on the change, so that no other device is required and the signal processing is performed. The bias current can be always equal to the threshold current, or can be a value less than or equal to the threshold current and within a predetermined range from the threshold current by only Since all the circuits constituting the embodiment can be integrated into a circuit, a semiconductor laser capable of high-speed response can be obtained with a small size, an inexpensive and simple circuit.

In the first embodiment described above, the voltage measuring circuit 14 is composed of the low frequency AC buffer amplifier, but instead of this, it is composed of the sample hold circuit triggered by the input digital signal S _IN. It is also possible to do so. At this time, the operating voltage V is selectively sampled and held only when the bias current _Ib is in the logical "0" state (off state) of the input digital signal S _IN . With this configuration, the differential resistance value R _d can be detected more accurately. (III) Second Embodiment Next, a second embodiment corresponding to the invention described in claims 1, 3, 4 and 6 will be described with reference to FIGS. 5 and 6.

In the second embodiment shown in FIG. 5, the same components as those in the first embodiment shown in FIG. 4 are designated by the same reference numerals, and the detailed description thereof will be omitted. In the second embodiment, the input digital signal at the mark ratio (the predetermined period as a bit ratio of the input digital signal S _IN S _IN
DC voltage V corresponding to the fluctuation of the number of bits indicating logic “1” divided by the total number of bits in the predetermined period)
The fluctuation of _D is corrected by the correction circuit, and the bias current I _b is controlled based on the stable corrected DC voltage V _DR .

As shown in FIG. 5, the semiconductor laser bias current control device 20 of the second embodiment has the mark ratio of the input digital signal S _IN in addition to the configuration of the semiconductor laser bias current control device 10 of the first embodiment. A mark ratio detection circuit 21 including an integrator that detects and outputs M, and an AGC (Auto) that corrects the DC voltage V _D based on the detected mark ratio M.
Correction circuit 22 including a gain control circuit, etc.
It consists of and.

Next, the operation will be described. The same components as those in the first embodiment operate in the same manner as in the first embodiment, and thus the description thereof will be omitted.

The mark ratio detection circuit 21 detects the mark ratio M from the average voltage by integrating the input digital signal S _IN . Then, the correction circuit 22 corrects the DC voltage V _D based on the mark ratio M and outputs the corrected DC voltage V _DR . More specifically, when the mark ratio M is high, the number of logic "1" bits of the input digital signal S _IN is large, so that the amplitude A of FIG. 6B or 6C fluctuates to be small. At this time, the DC voltage V _D becomes smaller than the correct value, so that the AG forming the correction circuit 22 is configured.
Correct so as to increase the amplification factor of C. Also, the mark rate M
Is low, the amplitude A fluctuates so as to become large, and therefore the DC voltage V _D becomes larger than a correct value. Therefore, the correction is performed so that the amplification factor of the AGC forming the correction circuit 22 is lowered.

After that, the voltage comparison circuit 16 compares the corrected DC voltage V _DR with the reference voltage V _S, so that the comparison signal S _C in which the variation caused by the mark ratio M is corrected is output. The bias current I _b is controlled by the control.

According to the second embodiment described above, in addition to the effect of the first embodiment, even if the DC voltage V _D fluctuates due to the fluctuation of the mark ratio M, it can be corrected and the bias can be more accurately. The current can be controlled to the desired value.

In the above-described second embodiment, the mark ratio obtained by dividing the number of bits indicating the logic "1" in the input digital signal S _{IN in} a predetermined period by the total number of bits in the predetermined period. However, the present invention is not limited to this, and the input digital signal S _{IN in} the predetermined period may be divided by the total number of bits in the predetermined period by the number of bits indicating logic “0”.

[0088]

As described above, according to the invention described in claim 1 or 4, the differential resistance value which changes corresponding to the change of the bias current is detected from the operating voltage of the semiconductor laser, and The bias current is controlled so that the value of the differential resistance value becomes equal to the reference resistance value by comparing with a predetermined reference resistance value.

Therefore, even if the threshold current changes due to a change in operating temperature or deterioration of the semiconductor laser, the bias current is always made equal to the threshold current only by signal processing without the need for any other device. Or a value smaller than the threshold current and within a predetermined range corresponding to the oscillation delay time of the semiconductor laser allowed by the design from the threshold current, and further the entire circuit is integrated. Since the circuit can be formed, a semiconductor laser capable of high-speed response can be obtained in a small size and at a low cost.

According to the invention described in claim 2 or 5, in addition to the effect of the invention described in claim 1 or 4, a minute alternating current is superposed on the operating current, and a change in the voltage is converted into a minute alternating current. Since a corresponding minute AC voltage is extracted and the DC voltage proportional to the differential resistance value obtained based on its amplitude is compared with the reference voltage corresponding to the reference resistance value, high-speed response is possible with a simpler circuit. It is possible to obtain various semiconductor lasers.

According to the invention of claim 3 or 6, in addition to the effect of the invention of claim 2 or 5, the bit ratio of the input digital signal is detected and proportional to the differential resistance value correspondingly. Since the DC voltage to be applied is corrected, a stable DC voltage can be obtained even if the bit ratio of the input digital signal changes, and more stable bias current control becomes possible.

[Brief description of drawings]

FIG. 1 is a diagram showing a basic configuration of the present invention.

2A and 2B are diagrams showing a relationship between a current and a voltage or a differential resistance value in a semiconductor laser, FIG. 2A is a diagram showing a current-voltage characteristic, and FIG. 2B is a diagram showing a current-differential resistance value characteristic. .

3A and 3B are diagrams showing a relationship between each parameter and time in the semiconductor laser, FIG. 3A is a view showing a relationship between time and current, FIG. 3B is a view showing a relationship between time and voltage, and FIG. It is a figure which shows the relationship between time and a differential resistance value.

FIG. 4 is a diagram showing a configuration of a first exemplary embodiment of the present invention.

FIG. 5 is a diagram showing a configuration of a second exemplary embodiment of the present invention.

FIG. 6 is a diagram showing a waveform of each part in the first and second embodiments of the present invention, (a) is a diagram showing a relationship between time and an operating current I _D , and (b) is a time and an operating voltage. is a diagram showing the relationship and V, is a diagram showing a (c) the time and the output voltages V ₁ relationship.

FIG. 7 is a diagram showing a configuration of a first conventional technique.

FIG. 8 is a diagram showing a configuration of a second conventional technique.

FIG. 9 is a diagram showing a configuration of a third conventional technique.

[Explanation of symbols]

1, 100 ... Semiconductor laser 2 ... Driving means 3 ... Bias means 4 ... Differential resistance value detection means 5 ... Comparison means 6 ... Bias current control means 7, 103 ... Heat sink 10 ... Semiconductor laser bias current control circuit 11 of the first embodiment Drive circuit 12 Bias circuit 13 Micro AC current supply circuit 14 Voltage measurement circuit 15 Micro AC voltage extraction circuit 16 Voltage comparison circuit 17 Bias current control circuit 20 Semiconductor laser bias current control circuit of the second embodiment 21 ... Mark ratio detection circuit 22 ... Correction circuit 101 ... Driving circuit 102 ... Peltier element 104 ... Temperature sensor 105 ... Temperature control circuit 106 ... Temperature monitor circuit 107 ... Photodetector 108 ... Optical output detection circuit _ID ... Operating current _Id Drive current I _b Bias current L Laser light M Mark ratio R _d Differential resistance R _F Reference resistance S _IN Input input Digital signal S _B ... bias control signal S _C ... comparison signal S _S ... sensor signal S _L ... detector output V ₁ ... output voltage V _D ... DC voltage V _DR ... correction DC voltage V _S ... reference voltage

Claims (6)

(57) [Claims] 1. A differential resistance value of the semiconductor laser, which is obtained by superimposing a bias current on a drive current corresponding to an input digital signal and changes based on an operating current supplied to the semiconductor laser, The differential resistance value detecting step of outputting, and comparing the detected differential resistance value with a predetermined reference resistance value preset corresponding to the threshold current of the semiconductor laser in a predetermined operating environment, A comparison step of outputting a comparison signal, and a bias current control step of controlling the bias current so that the differential resistance value becomes equal to the reference resistance value based on the comparison signal. Semiconductor laser control method. 2. The semiconductor laser control method according to claim 1, wherein the differential resistance value detecting step has a constant frequency sufficiently lower than a bit rate of the input digital signal,
Micro AC current supply step of superimposing a micro AC current having a constant current amplitude value which is sufficiently smaller than the input digital signal current on the operating current, and voltage measurement step of measuring a voltage applied to the semiconductor laser. And, from the measured voltage, a minute AC voltage corresponding to the minute AC current is extracted, and the minute AC voltage is converted into a DC voltage proportional to the differential resistance value based on the amplitude, and the minute AC voltage is output. An extraction step, and the comparison step is a voltage comparison step of comparing a reference voltage corresponding to the reference resistance value with the DC voltage and outputting a comparison signal. Control method. 3. The semiconductor laser control method according to claim 1, wherein the differential resistance value detecting step has a constant frequency sufficiently lower than a bit rate of the input digital signal,
Micro AC current supply step of superimposing a micro AC current having a constant current amplitude value which is sufficiently smaller than the input digital signal current on the operating current, and voltage measurement step of measuring a voltage applied to the semiconductor laser. And, from the measured voltage, a minute AC voltage corresponding to the minute AC current is extracted, and the minute AC voltage is converted into a DC voltage proportional to the differential resistance value based on the amplitude, and the minute AC voltage is output. An extraction step; a bit ratio detection step of detecting a bit ratio obtained by dividing the number of bits of one logic of a predetermined period of the input digital signal by the total number of bits within the predetermined period; And a correction step of correcting the DC voltage and outputting the corrected DC voltage, wherein the comparing step includes a reference voltage corresponding to the reference resistance value and the correction direct voltage. The semiconductor laser control method, which is a voltage comparison step for outputting a comparison signal by comparing the voltage. 4. A drive means for generating and outputting a drive current corresponding to an input digital signal, and a bias current based on a bias current control signal,
A bias means for outputting, and a differential resistance that is obtained by superimposing the bias current on the drive current and that detects and outputs a differential resistance value of the semiconductor laser that changes based on an operating current supplied to the semiconductor laser. A resistance value detecting means compares the detected differential resistance value with a predetermined reference resistance value set in advance corresponding to the threshold current of the semiconductor laser in a predetermined operating environment, and a comparison signal And a bias current control unit that outputs the bias current control signal that controls the bias current so that the differential resistance value becomes equal to the reference resistance value, based on the comparison signal. A semiconductor laser control device characterized by the above. 5. The semiconductor laser control device according to claim 4, wherein the differential resistance value detecting means has a constant frequency sufficiently lower than a bit rate of the input digital signal,
A minute alternating current supply means for superimposing a minute alternating current having a constant current amplitude value that is sufficiently smaller than the input digital signal current on the operating current, and a voltage measuring means for measuring the voltage applied to the semiconductor laser. And, from the measured voltage, a minute AC voltage corresponding to the minute AC current is extracted, and the minute AC voltage is converted into a DC voltage proportional to the differential resistance value based on the amplitude, and the minute AC voltage is output. Extraction means, and the comparison means is a voltage comparison means for comparing the DC voltage with a reference voltage corresponding to the reference resistance value and outputting a comparison signal. Control device. 6. The semiconductor laser control device according to claim 4, wherein the differential resistance value detection means has a constant frequency sufficiently lower than a bit rate of the input digital signal,
A minute alternating current supply means for superimposing a minute alternating current having a constant current amplitude value that is sufficiently smaller than the input digital signal current on the operating current, and a voltage measuring means for measuring the voltage applied to the semiconductor laser. And, from the measured voltage, a minute AC voltage corresponding to the minute AC current is extracted, and the minute AC voltage is converted into a DC voltage proportional to the differential resistance value based on the amplitude, and the minute AC voltage is output. Extracting means, bit ratio detecting means for detecting a bit ratio obtained by dividing the number of bits of one logic in a predetermined period of the input digital signal by the total number of bits in the predetermined period, and the bit ratio detecting device based on a change in the bit ratio. Compensating means for compensating the DC voltage and outputting the compensating DC voltage, and the comparing means is a base corresponding to the compensating DC voltage and the reference resistance value. The semiconductor laser control apparatus which is a voltage comparator for outputting a comparison signal by comparing the voltage.