JP2898813B2 - Semiconductor laser controller - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a semiconductor laser control device,
More specifically, the present invention relates to a semiconductor laser control device that controls a light output of a semiconductor laser used as a light source in a laser printer, an optical disk device, a digital copying machine, an optical communication device, and the like.

[0002]

2. Description of the Related Art Semiconductor lasers are extremely small and can be directly modulated at a high speed by a drive current. Therefore, in recent years, semiconductor lasers have been widely used as light sources for optical disk devices, laser printers and the like. However, the relationship between the drive current and the light output of the semiconductor laser changes remarkably depending on the temperature, so that there is a problem when trying to set the light intensity of the semiconductor laser to a desired value. In order to solve this problem and utilize the advantages of semiconductor lasers, various APCs (Automatic
Power Control) circuits have been proposed.

The APC circuit can be roughly classified into the following three systems. The light output of the semiconductor laser is monitored by a light-receiving element, and a signal proportional to a light-receiving current (proportional to the light output of the semiconductor laser) generated in the light-receiving element and a light emission level command signal are always equalized. The optical output of a semiconductor laser is controlled to a desired value by an optical / electrical negative feedback loop for controlling the forward current.

During the power setting time, the light output of the semiconductor laser is monitored by a light receiving element, and a signal proportional to a light receiving current (proportional to the light output of the semiconductor laser) generated in the light receiving element and a light emission level command signal are obtained. The forward current of the semiconductor laser is controlled to be equal, and the optical output of the semiconductor laser is controlled to a desired value by holding the value of the forward current of the semiconductor laser set in the power setting period outside the power setting period. At the same time, outside the power setting period, information is loaded on the optical output of the semiconductor laser by modulating the forward current of the semiconductor laser set in the power setting period based on the information. The semiconductor laser temperature is measured, and the forward current of the semiconductor laser is controlled by the measured temperature signal, or the temperature of the semiconductor laser is controlled to be constant, and the optical output of the semiconductor laser is set to a desired value. Control method.

In order to make the optical output of the semiconductor laser a desired value, the method (1) is desirable. However, the operating speed of the light-receiving element, the operating speed of the amplifying element forming the optical / electrical negative feedback loop, and the like are determined. The limit places a limit on the control speed. For example, when considering the cross frequency of the open loop of the optoelectronic negative feedback loop as a measure of the control speed, step response characteristics of the optical output of the semiconductor laser when the crossover frequency is f ₀ is approximated as: it can. Pout = P ₀ {1-exp (−2πf ₀ t)} Pout: light output of the semiconductor laser P ₀ : set light intensity of the semiconductor laser t: time In many applications of the semiconductor laser, the light output of the semiconductor laser is used. It is necessary that the total amount of light (integrated value of light output ∫Pout) be a predetermined value from immediately after changing the time t until the set time τ ₀ elapses.

[0006]

(Equation 1)

If τ ₀ = 50 ns and the allowable range of error is 0.4%, f ₀ > 800 MHz must be satisfied, which is extremely difficult. The method (2) does not have the above-mentioned problem of the method (1) and can modulate a semiconductor laser at a high speed. However, in the method (2), the light output of the semiconductor laser is not always controlled, so that the light quantity of the semiconductor laser easily fluctuates due to disturbance or the like. The disturbance includes, for example, a draw loop characteristic of a semiconductor laser, and the light quantity of the semiconductor laser easily causes an error of about several percent due to the draw loop characteristic. As an attempt to suppress the droop loop characteristic of a semiconductor laser, there has been proposed a method of compensating the thermal time constant of the semiconductor laser by adjusting the frequency characteristic of the semiconductor laser drive current.The thermal time constant of the semiconductor laser is different for each semiconductor laser. There are problems such as individual variations and differences depending on the surrounding environment of the semiconductor laser. In addition, there is a problem such as a change in the amount of light due to the influence of the return light of the semiconductor laser, which is a problem in an optical disk device or the like.

In order to solve this problem, for example, Japanese Unexamined Patent Publication No. Hei 2-205086 has been proposed. This publication discloses an optical / electrical negative feedback loop for monitoring the optical output of a semiconductor laser with a light receiving element and controlling the forward current of the semiconductor laser so that the output is equal to the emission level command signal; Conversion means for converting a command signal into a forward current of the semiconductor laser, and the semiconductor laser is controlled by a sum or difference current between the control current of the optical / electrical negative feedback loop and the current generated by the conversion means. To control. However, the design of the optical / electrical negative feedback loop is not easy, and it is insufficient as a high-speed, high-accuracy, high-resolution semiconductor laser control device.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-described problems, and has been made in consideration of the problems caused by the use of an IC using bipolar transistors.
An object of the present invention is to provide a semiconductor laser control device that facilitates the design of an electric negative feedback loop and enables high-speed control.

[0010]

To achieve the above object, the present invention provides (1)
A semiconductor laser control device that monitors a part of the optical output of a driven semiconductor laser and controls a forward current of the semiconductor laser so that a monitor signal proportional to an output light intensity of the semiconductor laser is equal to a light emission command signal. An optical / electrical negative feedback loop for amplifying a difference current between the light emission command signal and the monitor signal by an error current amplifier to control the forward current of the semiconductor laser; and the light emission command signal for the error current amplifier. , A first D / A converter, and a comparison between an output of the current detection circuit and an output of the first D / A converter. A comparator, a memory function for holding an output result of the comparator at a predetermined timing, a timing generation circuit for generating the predetermined timing, and an output from the memory. Second D performing D / A conversion in accordance with
/ A converter, and a current adding circuit that outputs a current proportional to the light emission command signal, and determines a proportional coefficient based on an output of the second D / A converter. The current detection circuit is constituted by a high-pass filter; and (3) the minimum value of the light emission command signal is a predetermined value, and the change of the light emission command signal is changed from a minimum value to a maximum value. (4) In (3), the current detection circuit is configured by a high-pass filter. (5) In (1), the timing generation is configured by a delay circuit. (6) In the above (5), the minimum value of the delay time of the delay circuit is set longer than the control time of the optical / electrical negative feedback loop. Is constituted by a high-pass filter, and the delay time per bit of the conversion timing is made shorter than the time constant of the high-pass filter. (8) In the above (1), the error current amplifier is controlled by an input current. An input unit that converts the charge of the capacitance into an error voltage by charging and discharging, and that changes the error voltage through a high impedance input circuit to change the emitter current difference of the first pair transistor by a current proportional to the error voltage; The base of the first pair transistor
By giving the emitter voltage difference as the base-emitter voltage difference of the second pair transistor, the collector current of the second pair transistor is changed, and the current proportional to the collector current of the second pair transistor is defined as the output current. having an output section for, or the light-(9)
An A / D conversion circuit for A / D converting a forward current of the semiconductor laser according to a change in the light emission command signal via an electric negative feedback loop; The minimum value of the light emission command signal is a predetermined value, and the change of the light emission command signal is changed from the minimum value to the maximum value. Further, (11) in the above (9),
The conversion timing of each bit of the A / D conversion is constituted by a delay circuit. Further, (12) in the above (9), the error current amplifier charges and discharges an electric charge of a capacitance by an input current to generate an error. An input unit for converting the error voltage into a voltage and changing the emitter current difference of the first pair transistor by a current proportional to the error voltage via a high impedance input circuit; and a base / emitter of the first pair transistor. By giving the voltage difference as the base-emitter voltage difference of the second pair transistor, the collector current of the second pair transistor is changed, and the current proportional to the collector current of the second pair transistor is set as the output current. And an output unit. Hereinafter, a description will be given based on examples of the present invention.

FIG. 1 is a block diagram for explaining an embodiment of an optical / electrical negative feedback loop used in a semiconductor laser control device according to the present invention. In FIG. 1, reference numeral 1 denotes a high impedance circuit, and 2 denotes a voltage / current. The conversion circuit 3 is a forward current conversion circuit. Note that the configuration of FIG. 1 corresponds to a portion I in FIG. 8 described later.

[0012] emitting a command signal Isignal, to monitor the portion of the light output P ₀ of the driven laser diode (LD), connected to the photovoltaic current Im proportional to the input light intensity is the same terminal of the capacitance Cf Therefore, the current flowing in the capacitance Cf is Isign
al-Im. Terminal voltage V ₁ of the capacitor Cf is changed by the difference current Isignal-Im. Terminal voltage V ₁ of the capacitance Cf via a high impedance circuit 1 is input to the voltage-current conversion circuit 2, the emitter current of the transistor Q1 I _E1, changing the emitter current of transistor Q2 -I _E1. Now the bias current for operating the Shiranjisuta Q1, Q2 and I _1, the emitter current of the transistor Q1 is I ₁ + I _E1
And the emitter current of the transistor Q2 is I ₁ −I _E1
Becomes Transistor Q1 base and transistor Q2
Are biased to the same potential as the base. The base-emitter voltages of the transistors Q1 and Q2 are as follows.

V _BE1 = V _T ln (I ₁ + I _E1 ) −V _T ln (Is ₁ ) V _BE2 = V _T ln (I ₁ −I _E1 ) −V _T ln (Is ₁ ) The base-emitter voltages of the transistors Q3 and Q4 are as follows. _{V BE3 = V T · ln (} I 0 -I E3) -V T · ln (Is 2) V BE4 = V T · ln (I 0 + I E3) -V T · ln (Is 2) here, the transistor Q3 When Q4 emitter current was _{I 0 + I E3, I 0} -I E3 respectively. On the other hand, the emitter of transistor Q1 is connected to the base of transistor Q4, the emitter of transistor Q2 is connected to the base of transistor Q3, and the emitters of transistors Q3 and Q4 are connected.

Therefore, the transistor Q1 and the transistor
The base-emitter potential difference of the
4 and the base-emitter potential difference between transistor Q3 and
It becomes. V_BE1-V_BE2= V_BE4-V_BE3  From the base-emitter voltage formula of each transistor, I_E3= (I₀/ I₁) ・ I_E1  Becomes Here, the terminal voltage V of the capacitance Cf₁
And emitter current I_E1High-in relation to
The voltage / current conversion circuit 2 operates via the impedance circuit 1.
It is supposed to work. This proportionality factor is represented by A₀Tomorrow
If I_E3= (I₀/ I₁) ・ A₀・ V₁  Becomes

Therefore, the change in the emitter currents of the transistors Q3 and Q4 is proportional to the voltage between the terminals of the capacitance Cf. If the current amplification factors of the transistors Q3 and Q4 are sufficiently large, the emitter currents of the transistors Q3 and Q4 are equal to the collector currents. Thus, the capacitance Cf
The current proportional to the inter-terminal voltage V ₁ of the transistor Q3
Transistors Q3 and Q4 are provided via a conversion circuit 3 which becomes a collector current of Q4 and converts the current into a forward current of the semiconductor laser.
Current proportional to the collector current of a semiconductor laser (LD)
Drive current. The proportionality factor conversion circuit 3 for converting the forward current of the semiconductor laser and A _1, Ith the threshold current of the semiconductor laser, the differential quantum efficiency eta, the optical output P _0,
Assuming that the coupling efficiency between the semiconductor laser (LD) and the photodiode (PD) that monitors the optical output is α and the radiation sensitivity of the photodiode (PD) is S, the photovoltaic current Im of the photodiode (PD) is Semiconductor laser (LD)
Of the light output P ₀ is as follows. P ₀ = η · {(I ₀ / I ₁ ) · A ₁ · A ₀ · V ₁ −Ith} Im = α · S · η · {(I ₀ / I ₁ ) · A ₁ · A ₀ · V ₁ −Ith}

Here, in the case of DC operation, if a resistance equivalently in parallel with the capacitance Cf is R, then V ₁ = R · (I signal −Im), so P ₀ = η · (I ₀ / I ₁ ) ・ A ₁・ A ₀・ R ・ I signal / [1 + α ・ S ・ η ・ (I ₀ / I ₁ ) ・ A ₁・ A ₀・ R] −η ・ Ith / [1 + α ・ S ・ η ・ (I ₀ / I ₁ ) · A ₁ · A ₀ · R]. R = 50 (kΩ), η = 0.15 (mW / mA), α · S = 0.133 (mA / mW) A ₀ = 2, A ₁ = 1 / 6.8 (Ω), I ₀ / When I ₁ = 10 and Ith = 50 mA, α · S · η · (I ₀ / I ₁ ) · A ₁ · A ₀ · R 03000≫1 Ith (I ₀ / I ₁ ) · A ₁ · A ₀ · R = _0.34μA ≒ case 3μW the above standard output level P ₀ of about _{1mW P 0 = Isignal / α ·} S becomes P ₀ is proportional to Isignal.

Next, in the case of the AC operation, since V ₁ = (Isignal−Im) / jωCf, the gain Gv in the open loop of the optical / electrical negative feedback loop is as follows. Gv = α · S · η · (I ₀ / I ₁ ) · A ₁ · A ₀ / (jωCf) Here, the phase delay in the circuit operation is greatly reduced for the following reason.

Light emission command signal and photodiode (PD)
Is compared by charging / discharging the capacitance Cf, so that there is almost no occurrence other than the design phase delay factor expressed by the term jωCf. In addition, the high-impedance circuit is formed of, for example, a transistor, and if a common-collector circuit is used, the high-impedance circuit operates up to a frequency close to the cutoff frequency of the transistor, so that the speed can be easily increased. Furthermore, the voltage / current conversion circuit 2 can be easily operated at high speed by adding a resistor to the emitter of the transistor to extract the collector current. When a bias current of about 100 μA flows through the transistors Q1 and Q2, the emitter resistance value becomes 300.
Since it is on the order of Ω, the effect of capacitance inserted in parallel equivalently is greatly reduced.

Therefore, if the frequency at which the open loop gain of the optical / electrical negative feedback loop is 1 is about 200,
The expression of Gv is established as it is. Now, when α · S · η · (I ₀ / I ₁ ) · A ₀ is a constant value, the frequency at which the open loop gain of the optical / electrical negative feedback loop is 1 is set by the value of the proportional coefficient A ₁ and the value of the capacitance Cf. can do. However, the values of α, S, and η in semiconductor lasers vary greatly among individual semiconductor lasers. For this reason, when the semiconductor laser control circuit is integrated into an IC, it is necessary that this variation can be set by an external component for each individual semiconductor laser. Moreover, due to the large absolute value dispersion of the individual elements in the IC, it is impossible to set the value of the proportional coefficient A ₁ and the capacitance Cf with the IC. Therefore, the above problem can be eliminated by allowing the values of the proportional coefficient A ₁ and the capacitance Cf to be determined by external components.

FIG. 2 is a circuit diagram of the optical / electrical negative feedback loop shown in FIG. The block indicated by a dashed line in the figure corresponds to FIG. The high-impedance circuit 1 described with reference to FIG.
9. The voltage / current conversion circuit 2 of FIG. 1 is realized by the resistor R1. The transistors corresponding to the transistors Q1 and Q2 in FIG. 1 are Q21 and Q20 in FIG.
In FIG. 2, the base-emitter voltage difference between Q21 and Q20 is applied via transistors Q14 and Q13 to a transistor Q1 corresponding to transistors Q3 and Q4 in FIG.
0, Q9. In this case, a bias current flows through Q14 and Q13 so that the voltage drops at the transistors Q14 and Q13 are the same. The amount of change in the collector current of the transistor Q9 is converted into a voltage by the resistor R2 and becomes the base voltage of the transistor of Q1. Here, voltage / current conversion is performed while performing DC shift by the transistors Q6, Q7, Q3, the resistor R4, and the capacitance C0. The base voltage of the transistor of Q1 is DC-shifted to become the emitter voltage of transistor Q0.

As a result, the transistor R0 is connected to the resistor Rf.
The current determined by the emitter voltage of the transistor and the value of the resistor Rf flows, and the current amplification factor of the transistor Q0 is sufficiently large.
The emitter current of the transistor Q0 is
Equal to the collector current of Thus, the forward current of the semiconductor laser (LD) is controlled. Here, A ₁ described in the description of FIG. 1 is 1 / Rf, and when the optical / electrical negative feedback loop circuit according to the present invention is formed into an IC, the resistor Rf is provided as an external component, so that the circuit shown in FIG. light·
What has been described in the setting of the frequency characteristic of the electric negative feedback loop is realized.

FIG. 3 shows the bias voltage generating circuit of FIG. 2. The operation of FIG. 2 is guaranteed by applying the bias shown in FIG. Hereinafter, the operation will be described. A reference voltage stable at a temperature is generated at a portion I in the drawing. That is, the reference voltage is generated between the resistor R38 and the emitter of the transistor Q37. Therefore, the current flowing through the resistor R38 becomes very stable. A current mirror circuit composed of transistors Q38, Q39, and Q40 is connected to the collector of the transistor Q37, and a current flows in the direction indicated by the arrow.

The transistors Q41, Q42 and Q43 are also current mirror circuits, and flow current in the directions indicated by arrows. Voltage V ₂ is intended to apply a constant voltage to the transistor Q43, Vcc voltage V ₂ is the voltage drop across the resistor R43
Stable voltage can be obtained. Also, the transistor Q43
Since the same base voltage is applied to the transistor Q48, a stable current flows through the collector of the transistor Q48 in the direction indicated by the arrow. Transistor Q4
7, a stable base voltage V ₄ is generated by the current mirror circuit 7.

[0024] Upon application of the base voltage V ₄ to the base of the transistor Q _46, a stable from the base voltage of the transistor Q45. A voltage V ₃ obtained between the collector of the emitter and the transistor Q44 of the transistors Q45. This voltage V ₃ has the same potential as the point a in FIG. 2, and a stable voltage can be obtained. In Part II of the figure, it is obtained as a stable voltage to which the voltage V ₅ is not affected by the supply voltage. That is, a voltage having the same temperature characteristic as the base voltage of transistor Q1 in FIG. 2 is obtained. The same applies to the voltage V ₆ , and FIG.
A voltage which is not affected by the temperature characteristics of the transistor is obtained at the point b and is stable.

FIG. 4 is a diagram showing another embodiment of the optical / electrical negative feedback loop. FIG. 4 is the same as FIG. 2, but the connection of the resistor R2 in FIG. 2 is the emitter terminal of the driving transistor of the semiconductor laser. By doing so, the linearity when the emitter current of the drive transistor of the semiconductor laser is small as compared with FIG. 2 is improved.

FIG. 5 shows the bias voltage generation circuit of FIG. However, the bias of V ₂ in Figure 4 is similar to FIG. 3, are omitted in the description of FIG. Hereinafter, the operation will be described. In the section I in FIG.
₉ is a stable voltage which is not affected by the voltage Vcc.
Further, the voltage V ₈ is created so that the manner equivalent to connecting to the base of the transistor Q6 in FIG. 4, the variation of temperature changes and the characteristics of the transistors in the II portion is configured to be the same I have. Voltage V _{7 in} section III
Is for obtaining the emitter voltage of the transistor Q0 in FIG.

FIG. 6 is a diagram showing still another embodiment of the optical / electrical negative feedback loop. 6, the conversion circuit 3 for converting the forward current of the semiconductor laser in FIG. 1 is omitted, and the semiconductor laser (LD) is directly driven by the collector currents of the transistors Q3 and Q4 in FIG. By doing so, an error current amplifier that can configure an optical / electrical negative feedback loop very quickly can be realized.
With the configuration of FIG. 6, a control circuit having the highest speed can be obtained. However, transistors Q9,
There is a problem that a large-sized transistor must be used as Q10.

FIG. 7 is a diagram showing still another embodiment of the optical / electrical negative feedback loop. FIG. 7 shows the transistor Q of FIG.
This is a case where the base voltages of the transistors Q16 and Q17 supplying the bias currents of the transistors Q18 and Q19 can be set independently. In this way, the DC gain in the open loop of the optical / electrical negative feedback loop is determined. The ratio between I ₁ and I ₀ can be adjusted, and the frequency at which the gain becomes 1 in the open loop of the optical / electrical negative feedback loop can be changed by an external voltage. Voltage V _10, as shown in FIG. 3
And corresponds to the voltage V ₁₀ between the resistors R47 and the resistor 48 in.

FIGS. 8A and 8B are views showing a semiconductor laser control circuit according to the present invention. In the figure, 11 is a level shift circuit, 12 is a 5 BIT D / A converter (1), and 13 is a main circuit. Amplifier, 14 is a 5-bit D / A converter (2),
15 is a current detection circuit, 16 is a 3 BIT D / A converter (3), 17 is a comparator, 18 is a DF / F (D-flip-flop), 19 is a 3 BIT D / A converter (4), 20 Is a timing generation circuit. The I section including the main amplifier 13 corresponds to the configuration in FIG.
The part B is a drive circuit, and the part C is an automatic setting circuit. In FIG. 8, since the high-speed control of the semiconductor laser is limited only by the optical / electrical negative feedback loop, the current proportional to the light emission command signal is independent of the optical / electrical negative feedback loop so that high-speed modulation is possible. Is added to the driving circuit II for driving the semiconductor laser (LD). In this case,
An automatic setting circuit III for automatically setting a conversion ratio for converting a light emission command signal into a drive current of a semiconductor laser (LD) is added.

FIG. 9 shows an example of a timing signal for explaining the operation of FIG. Hereinafter, a method of setting the conversion efficiency will be described with reference to FIGS. When TC is low, regardless of the input data D0 to D4, all data is forcibly set to low internally, and the outputs of 5BIT D / A12 and 5BIT D / A14 are set to the lowest level. When TC is low, T0 to T3 are all in a low state. After TC changes from low to high, T
0 changes from low to high, and the DF / F 18 enters a state of receiving an input clock from the clear mode. During this period, the inputs of the 3BIT D / A 19 are all low, and the output O (D / A (4)) is at the lowest level. Also, when T0 is low, the output of the 5-bit D / A 12 (in the embodiment of the present invention, this signal is a light emission command signal) is at the lowest level, but since it has an offset, it has a semiconductor. The light output of the laser (LD) is not zero. Therefore, the main amplifier outputs a current corresponding to the offset of the light emission command signal by the optical / electrical negative feedback loop. The current value at this time is Iout1
And

Next, after T0 goes high, the input data D
Force all data internally high regardless of 0-D4, 5BIT D / A12 and 5BIT D / A1
Set the output of 4 to the highest level. Then, the output current of the main amplifier 13 outputs a current Iout2 corresponding to the highest level of the light emission command signal by the optical / electrical negative feedback loop. The current detection circuit 15 outputs a difference current A between Iout and Iout1. On the other hand, 3BITD / A16 outputs a current B corresponding to the maximum value. The currents A and B are compared by a comparator, and the result is output to C, which is input to the data (D) of the DF / F 18. Above period (T1 is low period)
In, since the output of the DF / F18 is low, 3 BIT
5 BIT D / controlled by D / A19
A14 outputs a current corresponding to the lowest output of 3BIT D / A19. After the above operation, T1 changes from low to high, and the comparison result C of the comparator is first converted to DF / F1.
8a.

Thereafter, the output B of the 3BIT D / A 16 becomes a half of the level when T1 is low, and according to the output result of the DF / F 18a, 3BITD / A
19 output O (D / A (4)) changes, and 5 BIT D
/ A14 changes. Since the changing speed of this current is set to be lower than the control speed of the optical / electrical negative feedback loop, the main amplifier 13 is controlled so that the optical output of the semiconductor laser does not change according to the output change of the 5 BIT D / A 14. The current changes. Hereinafter, the same operation is performed up to the timings of T2 and T3. After the DF / F 18 fetches the input data, the mode is such that the input data D0 to D4 are valid. The above operation is performed by performing the sequential conversion type A / D conversion of the forward current of the semiconductor laser for obtaining the optical output corresponding to the change of the emission command signal through the optical / electrical negative feedback loop. No. In this way 5
BIT D / A14 full scale is main amplifier 13
Are set so that the change in the output current of the output is minimized. Therefore, 5BI is set so that the light level corresponds to the light emission command signal.
Since the TD / A 14 drives the semiconductor laser, the control amount of the optical / electrical negative feedback loop is reduced, and the high-speed modulation characteristics are improved.

In the above description, DF / F is 3B
In the case of IT, any number of BITs may be used. Further, in the embodiment of the present invention, the DF / F is used.
It need not be F. Further, in the embodiment of the present invention, the D / A converter is used for the light emission command signal, but this may not be the D / A converter. Further, the light emission command signal is changed from the minimum value to the maximum value, but the change is the same at any level from any level. In addition, 5BIT
Although the D / A 14 drives a current proportional to the emission command signal to the semiconductor laser, the D / A converter needs to be a D / A converter because the full scale can be changed by an external voltage by using a multiplication circuit or the like. Absent.

FIG. 10 is a diagram showing another embodiment of the semiconductor laser control circuit according to the present invention. In FIG. 10, reference numeral 21 denotes a level shift circuit, 22 denotes a light emission level generation circuit D / A unit, and 23 denotes a current addition circuit D. / A section, 24 is an error detection circuit, 25 is a control current detection circuit, 26 is a current setting circuit, 27 is a current addition circuit output section, 28 is a main amplifier, 29 is a light emission level generation circuit reference current generation section, and 30 is a delay. A circuit 31 is a reference voltage generation circuit.

The operation will be described below step by step. The input data D0 to D4 are input to the level shift circuit 21, and
It is converted to an internal logic level by the slice level set by _BB . At this time, when the control signal T0 of the delay circuit 30 is high and T6 is low, a high-level internal logic signal is output regardless of the input data. When the control signal T0 is low and T6 is low, Outputs low-level signal regardless of input data. Further, it outputs a slice voltage _VA which is a middle point of the output signal.

The output data DA0 of the level shift circuit 21
To DA4 and the slice level signal _VA are equal to D of the current adding circuit.
/ A unit 23 and the light emission level generation circuit D / A unit 22. The D / A both input data DA0~DA4 outputs the maximum current I ₀ and Is at a low level, respectively. The full scale of the output current of the current adding circuit 23 is determined by _VCA which is the output of the current setting circuit. On the other hand, the full scale of the output Is of the light emission level generation circuit D / A unit 22 is determined by the signal I _R from the reference current generation unit 29 of the light emission level generation circuit.

The current adding circuit output section 27 is provided with a current adding circuit D.
/ Amplifies the output current I ₀ of the A 23, the amplification factor is set by the value of the external resistor R _E, and drives the semiconductor laser (LD). Thus it is possible to set the maximum drive current through an external resistor R _E be the characteristics of the semiconductor laser is changed R _E
Can be dealt with simply by changing the value of.

The main amplifier 28 includes a light emission level generation circuit D
The output current Is of the / A section 23 and a part of the optical output of the semiconductor laser are monitored, and a difference current between the photocurrent and the photocurrent proportional to the optical output is input to Iin, and Iin is amplified to drive the external transistor Q1. This controls the forward current of the semiconductor laser (LD). The main amplifier 28, the semiconductor laser (LD), and the photodiode (PD) form an optical / electrical negative feedback loop. Also, the main amplifier 28
Is the control signal V _GC from the light emission level generation circuit 29
Allows the gain to be changed. In addition, a monitor output V _MON for monitoring the control current of the semiconductor laser and a protection circuit when the control current exceeds a predetermined current are built in. When the protection circuit operates, an error can be detected. The output _{VER is} output to the error detection circuit. An external component includes a transistor for driving the semiconductor laser at the last stage of the main amplifier 28, an emitter resistor Rf of the transistor, and a capacitance Cf for integrating a difference current between the output current of the photodiode (PD) and the light emission command signal Is. This stabilizes the control speed of the optical / electrical negative feedback loop and reduces the power consumption of the block in FIG. 10 (excluding the transistor Q1).

The control current detection circuit 20 includes the current detection circuit 15, the 3-bit D / A 16 and the comparator 17 shown in FIG. DF0, DF1 of comparison result
Is output to the current setting circuit 26. Error detection circuit 24
Compares the input of V _ER with the reference voltage V _R1 and outputs an ERROR signal if an error has occurred. The current setting circuit 26 receives the outputs DF0 and DF1 of the control current detection circuit 25, and receives the timing T set by the delay circuit 30.
1, T3, and T5 to hold the data of DF0 and DF1, and according to the held data, the current addition circuit D
/ The full scale of the A section 22 outputs an output V _CA to be set. After holding the data at the timing of T5, the timing signal T6 is output to the level shift circuit 21.

The light emission level generation circuit reference current generation unit 29 uses the reference current set by the external resistor VR ₁ to control the gain control signal V _GC of the main amplifier 28 and the full scale set current of the light emission level generation circuit D / A unit 22. I _R
Is output in conjunction with. Further, the light emission level generation circuit D is independent of the gain control signal V _GC by the V _CONT voltage.
The full scale of the / A section 22 can be set. The delay circuit 30 outputs T0, T1, T3, and T5 whose states change with a certain delay according to the TC signal. The reference voltage generation circuit 31 generates a reference voltage for the operation of the block. In the above blocks, the level shift circuit 21 includes the reference voltage generation circuit 31, the main amplifier 2
8. It is arranged so that the physical distance from the current adding circuit output section 27 is as far as possible.

FIG. 11 is a configuration diagram of the level shift circuit. When T0 is low, the bias current of the differential switch to which D0 to D4 is input becomes 0, and DA0 to DA4 are forcibly set to the high level. When T6 is low, V
_The current is drawn from the input of _BB , forcing the slice level to low. As a result, DA0 to DA4 go low regardless of the input data D0 to D4. FIG. 12 is a configuration diagram of the light-emission level generation circuit D / A unit.
Each switching current value of the differential switch operated by _R is set.

FIG. 13 is a configuration diagram of the current addition circuit D / A unit. The transistors Q70 and Q71 are differential switches, and the transistors Q72 and Q73 and the transistor Q7
Similarly, transistors Q4, Q75, transistors Q76, Q77, and transistors Q78, Q79 are also differential switches. Assuming that the size of the transistor Q80 is 1, Q81 is 2, Q8
2 is 4, Q83 is 8, and Q84 is 16. That is, the voltage flowing in the resistor R80 and I _1,
The current flowing through the resistor R81 is I ₂ , the current flowing through the resistor R82 is I ₃ , the current flowing through the resistor R83 is I ₄ , and the resistor R
The current flowing to 84 when the _{I 5, I 2 = 2I 1} , I 3
= 4I ₁ , I ₄ = 8I ₁ , and I ₅ = 16I ₁ . DA0 Gallo - when the current I ₀ is the transistor Q7
1 and consequently a current I ₁ flows. Also,
DA1 Gallo - When the current I ₀ flows through the transistor Q73, as a result, current I ₂ flows. By sequentially operating the differential switches in this manner, an added current is output.

FIG. 14 is a configuration diagram of the error detection circuit. When the voltage V _ER is applied to the base of the transistor Q85, the voltage drop of the voltage Vcc by the resistors R85, a
A corresponding voltage is generated at the point. That is, V _ER = Vcc
When −V _B is set, a voltage of V _B is generated at the point b. The voltage V _{B at the} point b is compared with the voltage V _R1 , and if V _R1 > V _B , an error signal is generated.

FIG. 15 is a configuration diagram of the control current detection circuit. The V _MON signal is divided by the external resistors RS0 and RS1, and the division ratio can be set according to the type of the semiconductor laser. A high-pass filter is formed by an external capacitance, and the high time of T0 is equal to T0.
The control current of the optical / electrical negative feedback loop corresponding to the time when the light emission command signal changes from the minimum level to the maximum level utilizing the fact that the time from when the signal becomes low to when the signal T6 becomes high is sufficiently long. Is detected while maintaining the accuracy of the DC potential. This makes it possible to detect the amount of change in the control current with a simple configuration.

FIG. 16 is a configuration diagram of the current setting circuit.
The I part in the figure corresponds to the DF / F 18a in FIG. 8, the II part is 18c in FIG. 8, the III part is 18b in FIG. 8, and the IV part is the 3BIT D / A converter in FIG. 19 respectively. FIG. 17 is a configuration diagram of an output unit of the current addition circuit. An amplified version of the current I ₀ obtained in the current adding circuit D / A section shown in FIG. 13 flows as the arrow direction of the current in FIG.

FIG. 18 is a configuration diagram of the main amplifier.
In a semiconductor laser control circuit, since a semiconductor laser is easily damaged by an excessive current, a semiconductor laser driving power supply is usually turned on after the power supply of the control circuit is turned on. When the semiconductor laser drive current is not supplied after the control circuit is powered on, the optical / electrical negative feedback loop is operationally saturated. Due to this effect, the potential of the input section of Iin decreases, and not only the D / A section at the preceding stage but also the transistor at the next stage become saturated. Therefore, latch-up occurs in the IC. In order to prevent this, in FIG. 18, the potential of Iin does not become lower than the potential that saturates the base voltage of the transistor so that the potential of Iin does not drop too much and does not affect the normal operation. A transistor TR for applying a potential is added.

FIGS. 19A and 19B are diagrams showing the configuration of the reference current generator of the light emission level generator. In FIG. 19, the voltage V _R is input to the base of the transistor so that the emitter potential of the transistor becomes constant. Thus, the potential between the terminals of the external resistor VR is stabilized. By changing the resistance value of the resistor VR, the full scale of the light emission command signal and the gain in the open loop of the optical / electrical negative feedback loop change in conjunction. The gain of the open loop and the full scale of the light emission command signal decrease as the full scale increases. Therefore, when adjusting the resistance VR1, all the input data of the level shift circuit are set to high, and the adjustment is performed in a direction in which the full scale increases from the minimum value. This is necessary for protection of the semiconductor laser. Therefore, when adjusting the resistance VR1, it is necessary to make the frequency at which the open loop gain becomes 1 lower than an appropriate value from the viewpoint of the stability of the optical / electrical negative feedback loop. This is set by an external switch and a capacitance connected in series with the switch (this is realized by turning on the switch at the time of adjustment and turning off the switch after the adjustment is completed).

When the variable resistance VR1 has a small value, the full scale is minimized. Also,
As mentioned in the description of the operation of FIG. 1, the change of the gain of the main amplifier is varied in inverse proportion to I _0, the V _GC such that the current changes in proportion to the full-scale setting current emission command signal Has been generated. However, the difference from the case of FIG. 1 is different from the case of FIG. 1 due to the effect of the emitter resistance of the transistor at the time of voltage / current conversion of the main amplifier. In order to eliminate this, the emitter current ratio of TR1 and TR2 is changed and guaranteed. As a result, the full-scale setting current of the light emission command signal and the bias current of the pair transistor of the main amplifier are not completely proportional.

Now, the current set by the variable resistor VR1 flows to the transistor TR9 and the transistor T9
The collector current of R9 is the pair silane transistor TR3, TR4
And divert to This shunt ratio is the same as the shunt ratio of the transistors TR5 and TR6, and the shunt ratio of TR5 and TR6 is determined by setting the current of TR7 by the external input voltage V _CONT . Thus, I _R is set by obtaining a shunt ratio proportional to the external input voltage. Further, the full scale of the light emission command signal can be set independently of the gain of the main amplifier, so that the maximum value of the light emission amount of the semiconductor laser is adjusted to VR1 and then the semiconductor laser control circuit of the present invention is in the operating state. You can also set it.

FIG. 20 is a configuration diagram of the delay circuit. A slight delay in the operation timing of the transistor is generated at the portion I in the drawing. That is, the delay time Δt ₁ shown in FIG. 9 is obtained from the signal of the part a by the RC time constant of the part II. Also, R in Part III
The delay time Δt ₂ shown in FIG. 9 is obtained by the C time constant. Further IV
The delay time Δt ₃ shown in FIG. 9 is obtained from the RC time constant of the section. FIG. 21 is a configuration diagram of the reference voltage generation circuit. The circuit is configured so that the voltage at the point a of the I part in the drawing becomes a stable voltage. Section II generates a voltage Vcsp by a current mirror circuit. Also generates a voltage Vcs ₁ in the current mirror circuit similarly III portion. Voltage V _R is the voltage to the base of the transistor is applied, the emitter potential configured to be stable. As for _VR1 , the same voltage as at point a is obtained. _VR2 outputs such a current that a reverse temperature characteristic between the transistor base and the emitter is obtained.

[0051]

As apparent from the above description, the present invention has the following effects. (1) An effect corresponding to claim 1, wherein a part of the optical output of the driven semiconductor laser is monitored, and the monitor signal proportional to the output light intensity of the semiconductor laser is equal to the emission command signal. An optical / electrical negative feedback loop for controlling the forward current of the semiconductor laser by amplifying a difference current between the emission command signal and the monitor signal by an error current amplifier. A current detection circuit for detecting a change amount of an output current of the error current amplifier according to a change in the light emission command signal; a first D / A converter; an output of the current detection circuit; A comparator for comparing an output of the D / A converter with a memory function for holding an output result of the comparator at a predetermined timing; and a timing generator for generating the predetermined timing. A circuit, a second D / A converter for performing D / A conversion in accordance with the output from the memory, and a current which is proportional to the light emission command signal and outputs a proportional coefficient to the second D / A converter. Since it is configured to have the current addition circuit determined by the output, it is easy to design an optical / electrical negative feedback loop using an IC using bipolar transistors, and high-speed control is possible. In addition, the number of elements can be reduced, so that downsizing and downsizing can be achieved. (2) Effect corresponding to claim 2: Since the current detection circuit is constituted by a high-pass filter, a simple circuit configuration can be realized, and the number of elements can be reduced. (3) An effect corresponding to claim 3; in addition to the effect of claim 1, the minimum value of the light emission command signal is a predetermined value, and the change of the light emission command signal changes from the minimum value to the maximum value. This makes it easy to design an optical / electrical negative feedback loop by using an integrated circuit using the IC, and enables high-speed control. Further, the number of elements can be reduced. (4) Effect corresponding to claim 4: Since the current detection circuit is constituted by a high-pass filter, a simple circuit configuration can be realized, and the number of elements can be reduced. (5) The effect corresponding to claim 5; in addition to the effect of claim 1, since the timing generation is constituted by a delay circuit, the design of an optical / electrical negative feedback loop by using an IC using bipolar transistors is facilitated, In addition, high-speed control becomes possible. In particular, since a delay circuit is used, the circuit configuration is simple and the number of elements can be reduced. (6) Effect corresponding to claim 6: Since the minimum value of the delay time of the delay circuit is longer than the control time of the optical / electrical negative feedback loop, higher-speed control can be realized. (7) The effect corresponding to claim 7: the current detection circuit is constituted by a high-pass filter, and the delay time per bit of the conversion timing is shorter than the time constant of the high-pass filter, so that the setting of the current detection circuit is more accurate. become. (8) An effect corresponding to claim 8; in addition to the effect of claim 1, the error current amplifier converts an error voltage by charging and discharging a charge of a capacitance with an input current, and converts the error voltage into a high impedance input circuit. An input section for changing the emitter current difference of the first pair transistor by a current proportional to the error voltage, and the base-emitter voltage of the second pair transistor to the base-emitter voltage of the second pair transistor And the output section that outputs a current proportional to the collector current of the second pair transistor by changing the collector current of the second pair transistor by giving the difference as the difference. The design of an optical / electrical negative feedback loop by integration into ICs using simplifies and enables high-speed control Further, the number of elements can be reduced, and downsizing and downsizing can be realized. (9) An effect corresponding to claim 9; since there is provided an A / D conversion circuit for A / D converting a forward current of the semiconductor laser according to a change in the light emission command signal via an optical / electrical negative feedback loop, In addition to the effect of the first aspect, a more stable light output can be obtained. (10) An A / D conversion circuit for A / D converting a forward current of a semiconductor laser according to a change in a light emission command signal through an optical / electrical negative feedback loop. In addition to the effect of the third aspect, a more stable light output can be obtained. (11) An effect corresponding to claim 11: an A / D conversion circuit for A / D converting the forward current of the semiconductor laser according to the change of the emission command signal via the optical / electrical negative feedback loop. In addition to the effect of claim 5, more stable light output can be obtained. (12) An effect corresponding to the twelfth aspect: an A / D conversion circuit for A / D converting the forward current of the semiconductor laser according to a change in the light emission command signal via an optical / electrical negative feedback loop. In addition to the effect of claim 8, a more stable light output can be obtained.

[Brief description of the drawings]

FIG. 1 is a configuration diagram for explaining an embodiment of an optical / electrical negative feedback loop used in a semiconductor laser control device according to the present invention.

FIG. 2 is a circuit configuration diagram of an optical / electrical negative feedback loop shown in FIG.

FIG. 3 is a configuration diagram of a bias voltage generation circuit of FIG. 2;

FIG. 4 is a diagram showing another embodiment of the optical / electrical negative feedback loop.

FIG. 5 is a configuration diagram of a bias voltage generation circuit of FIG. 4;

FIG. 6 is a diagram showing still another embodiment of the optical / electrical negative feedback loop.

FIG. 7 is a diagram showing still another embodiment of the optical / electrical negative feedback loop.

FIG. 8 is a diagram showing a control circuit of a semiconductor laser according to the present invention.

FIG. 9 is a diagram showing a tine mig chart of FIG. 8;

FIG. 10 is a diagram showing another embodiment of the semiconductor laser control circuit according to the present invention.

FIG. 11 is a configuration diagram of a level shift circuit.

FIG. 12 is a configuration diagram of a light emission level generation circuit D / A unit.

FIG. 13 is a configuration diagram of a current addition circuit D / A unit.

FIG. 14 is a configuration diagram of an error detection circuit.

FIG. 15 is a configuration diagram of a control current detection circuit.

FIG. 16 is a configuration diagram of a current setting circuit.

FIG. 17 is a configuration diagram of a current addition circuit output unit.

FIG. 18 is a configuration diagram of a main amplifier.

FIG. 19 is a configuration diagram of a light emission level generation circuit reference current generation unit.

FIG. 20 is a configuration diagram of a delay circuit.

FIG. 21 is a configuration diagram of a reference voltage generation circuit.

[Explanation of symbols]

1. High impedance circuit 2. Voltage / current conversion circuit 3. Forward current conversion circuit.

 ──────────────────────────────────────────────────続 き Continued on front page (31) Priority claim number Japanese Patent Application No. 3-25708 (32) Priority date Hei 3 (1991) January 25 (33) Priority claim country Japan (JP)

Claims (12)

(57) [Claims] 1. A part of the optical output of a driven semiconductor laser is monitored, and a forward current of the semiconductor laser is controlled so that a monitor signal proportional to an output light intensity of the semiconductor laser becomes equal to a light emission command signal. In the semiconductor laser control device, an optical / electrical negative feedback loop that controls the forward current of the semiconductor laser by amplifying a difference current between the light emission command signal and the monitor signal by an error current amplifier,
A current detection circuit for detecting a change amount of an output current according to a change of the light emission command signal of the error current amplifier;
A D / A converter, a comparator for comparing an output of the current detection circuit with an output of the first D / A converter, and a memory function for holding an output result of the comparator at a predetermined timing. A timing generation circuit for generating the predetermined timing, a second D / A converter for performing D / A conversion according to an output from the memory, and outputting a current proportional to the light emission command signal, and A current adding circuit determined by an output of the second D / A converter. 2. The semiconductor laser control device according to claim 1, wherein said current detection circuit comprises a high-pass filter. 3. The semiconductor laser control device according to claim 1, wherein a minimum value of the light emission command signal is a predetermined value, and a change of the light emission command signal is changed from a minimum value to a maximum value. 4. The semiconductor laser control device according to claim 3, wherein said current detection circuit comprises a high-pass filter. 5. The semiconductor laser control device according to claim 1, wherein said timing generation is constituted by a delay circuit. 6. The semiconductor laser control device according to claim 5, wherein a minimum value of the delay time of the delay circuit is longer than a control time of the optical / electrical negative feedback loop. 7. The semiconductor laser control device according to claim 6, wherein said current detection circuit is constituted by a high-pass filter, and a delay time per bit of said conversion timing is shorter than a time constant of said high-pass filter. . 8. The error current amplifier converts an input current into an error voltage by charging / discharging a capacitance, and converts the error voltage to a difference between emitter currents of a first pair transistor through a high impedance input circuit. An input unit that changes by a current proportional to the error voltage, and a base-emitter voltage difference of the first pair transistor is given as a base-emitter voltage difference of a second pair transistor, so that the second pair transistor 2. The semiconductor laser control device according to claim 1, further comprising: an output unit that changes a collector current and uses a current proportional to the collector current of the second pair transistor as an output current. 9. through the optical-electrical negative feedback loop, to have an A / D converter circuit for A / D conversion of the forward current of the semiconductor laser in response to changes in the light emission command signal
2. The semiconductor laser control device according to claim 1, wherein: 10. The semiconductor laser control device according to claim 9, wherein the minimum value of the light emission command signal is a predetermined value, and the change of the light emission command signal is changed from the minimum value to the maximum value. 11. The semiconductor laser control device according to claim 9, wherein the conversion timing of each bit of the A / D conversion is constituted by a delay circuit. 12. The error current amplifier converts an electric current of a capacitance into and out of a capacitance by an input current to convert the error voltage into an error voltage. The error current amplifier determines a difference between emitter currents of a first pair transistor through a high impedance input circuit. An input unit that changes by a current proportional to the error voltage, and a base-emitter voltage difference of the first pair transistor is given as a base-emitter voltage difference of a second pair transistor, so that the second pair transistor 10. The semiconductor laser control device according to claim 9, further comprising: an output unit that changes a collector current and uses a current proportional to the collector current of the second pair transistor as an output current.