JP2000277847A - Semiconductor laser drive device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain stable light modulation output in a semiconductor laser drive circuit that modulates a laser beam corresponding to an image signal of multi-gradation. SOLUTION: Since a control loop 1 controlling an offset current and a control loop 2 controlling a gain to control a modulation current from an image signal are synthesized using a differential amplifier 11, a semiconductor laser drive device is constituted of a smallest DA converter 2, and the effects of errors produced by the DA converter 2 is minimized. Furthermore, even if characteristics of a semiconductor laser LD is varied, a drive current Ild of the semiconductor laser LD is compensated, and the differential amplifier 11 has a function for suppressing noise to a low level, the semiconductor laser LD is modulated stably.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser driving device for modulating an optical output of a semiconductor laser according to a multi-tone image signal.

[0002]

2. Description of the Related Art FIG. 4 shows a relationship between a forward current and a light output of a semiconductor laser.

When a forward current is gradually supplied to a semiconductor laser, laser oscillation starts at a threshold current (Ith), and when a forward current is further supplied, an optical output increases linearly. The relationship between the forward current and the light output changes depending on the case temperature and aging of the semiconductor laser.

In a conventional semiconductor laser driving device,
A first current source for controlling an offset current equal to or greater than a threshold current Ith of the semiconductor laser when the optical output of the semiconductor laser is at a minimum value for use; and a driving current for the semiconductor laser based on a multi-gradation image signal. And the second current source
Or, the semiconductor laser is driven by the sum of the currents of the first current source and the second current source.

FIG. 2 shows an example of a conventional semiconductor laser driving device.

1, 2 are DA converters, 3 is a current switch, 4
Is the first current source, 5 is the second current source, LD is the semiconductor laser, PD
Denotes a photodiode, 21 denotes an Offset current control circuit, 7 denotes a first comparator, 22 denotes a gain control circuit, and 9 denotes a second comparator.

More specifically, a DA converter 1 for converting the output of the Offset current control circuit into an analog voltage,
A first current source 4 for converting the output voltage of the converter 1 into a V-I to generate a current, a semiconductor laser LD, a photodiode PD for monitoring the output of the semiconductor laser LD, and a photodiode PD
The first comparator 7 compares the output of the comparator with the reference voltage 1 (Vref1).
And an up / down counter. The counter counts up or down by the output of the first comparator 7, and turns off when the threshold current Ith of the semiconductor laser LD exceeds the threshold current Ith.
Offset current control circuit 6 that controls the set current to flow
A DA converter 2 that generates an analog voltage according to an image signal, a second current source 5 that V-I converts an output voltage of the DA converter 2 to generate a current, Semiconductor laser LD, photodiode PD, and photodiode
Second comparator 9 for comparing the output of PD with reference voltage 2 (Vref2)
And an up / down counter which counts up or down by the output of the second comparator 9 and controls the gain of the DA converter so that the optical output of the semiconductor laser LD becomes the set output. A second control loop comprising a gain control circuit 8 and a threshold current Ith of the semiconductor laser when the optical output of the semiconductor laser is at a minimum use value.
The first current source that controls the above Offset current to flow and the second current source that generates the drive current of the semiconductor laser by a multi-gradation image signal are wired-or coupled, and the first current source and the first current source are connected. The semiconductor laser is driven by the sum of the current with the second current source.

[0008]

However, a DA converter has a difference between an actual analog output and a theoretical value expected when a digital code is given to the DA converter. The difference is a gain. Errors (full-scale errors), zero errors (offset errors), non-linear errors and noise.
The gain error is the error between the ideal D / A converter output and the actual device output at full scale input. The zero error causes a small offset current or voltage due to switch leakage or offset. Therefore, when the input is zero, the output curve does not pass through zero (0 V). The nonlinearity error is
This is the difference between the analog output and the theoretical value.

The conventional semiconductor laser driving device comprises a first current source for controlling an offset current equal to or more than a threshold current Ith of the semiconductor laser when the optical output of the semiconductor laser is at a minimum use value, and a multi-gradation image signal. And a second current source for controlling the drive current of the semiconductor laser by a wired-or,
The semiconductor laser is driven by the sum of the currents of the current source and the second current source. Therefore, the errors of the two DA converters are added and affect the drive current of the semiconductor laser.

It is an object of the present invention to minimize errors in a semiconductor laser driving device and to stably modulate light output of a semiconductor laser.

[0011]

In order to solve the above-mentioned problems, the present invention provides a current source for converting a set analog voltage into a VI, a semiconductor laser driven by the current source, and a semiconductor laser. A photodiode for monitoring the light output, and the output of the photodiode and the reference voltage 1 (V
ref1), and an output of the first comparator is controlled so that an offset current equal to or more than the threshold current Ith of the semiconductor laser flows to the current source when the optical output of the semiconductor laser is at the minimum use value. A first control loop including an offset current control circuit, a DA converter that converts an image signal into an analog voltage, a current source, a semiconductor laser, a photodiode, an output of the photodiode, and a reference voltage 2 (Vref2). And a gain control circuit that controls the gain of the DA converter so that the optical output of the semiconductor laser becomes an output according to the image signal based on the output of the second comparator.
Of the first control loop in the configuration with the control loop of
A differential amplifier for inputting and calculating an output of the fset current control circuit and an output of the DA converter of the second control loop to combine the first control loop and the second control loop. And Note that an addition amplifier can be used instead of the differential amplifier.

As described above, the control loop 1 including the semiconductor laser, the photodiode, the first comparator, and the Offset current control circuit, the DA converter, the semiconductor laser, the photodiode, the second comparator, and the gain control A control loop 2 composed of a circuit and an output of the Offset current control circuit and an output of the D / A converter are coupled by being operated by the amplifier, and a minimum D / A converter and a current source are used. Can be configured. Therefore, errors in the semiconductor laser driving device can be minimized.

Further, a characteristic of the differential amplifier is a function of removing a common-mode component. In other words, utilizing the property that there is a large gain for the differential input component and a small gain for the in-phase component, it is useful to discriminate the signal from the noise and reduce the noise. As a result, even if the characteristics of the semiconductor laser change due to temperature or aging, a stable optical output can be modulated.

[0014]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows an embodiment of a semiconductor laser driving apparatus using the present invention, wherein 2 is a DA converter, 3 is a current switch, 4 is a current source, 6 is an Offset current control circuit, and 7 is a 1
A comparator, 8 is a gain control circuit, 9 is a second comparator, 11 is a differential amplifier, and 12 is an OP amplifier.

FIG. 1 shows a differential amplifier 11, a current source 4, a semiconductor laser LD, a photodiode PD, a first comparator 7, Offs
et a control loop (1) including the current control circuit 6, and a control loop including the DA converter 2, the differential amplifier 11, the current source 4, the semiconductor laser LD, the photodiode PD, the second comparator 9, and the gain control circuit 8. (2), and the two control loops (1) and (2) are combined by the differential amplifier 11.

The multi-tone image signal is input to the DA converter 2 and is converted to an analog voltage -Vsw in proportion to the gain voltage Vgain output from the gain control circuit 8. The output -Vsw of the DA converter 2 is input to the differential amplifier 11 and operated together with the output Vos from the Offset current control circuit 6 for setting the optical output of the minimum use value of the semiconductor laser LD.
Is converted into the set voltage Vs of the current source 4 for driving the current. The set voltage Vs is converted into a drive current Ild by the current source 4, and the semiconductor laser LD emits light by the drive current flowing through the current switch 3, and a part of the light output is converted into a current by the photodiode PD. Then, it is converted to the monitor voltage Vpd by the resistor R5. The monitor voltage Vpd is compared by a first comparator 7 with a reference voltage Vref1 set to obtain a minimum light output, and the output of the first comparator 7 is output together with a sample-and-hold signal 1SH1 together with an offset current control circuit. 6 and sets the Offset current. The monitor voltage Vpd is compared with a reference voltage 2Vref2 set to obtain an optical output according to the image signal by the second comparator 9, and the output of the second comparator 9 is gain controlled together with the sample hold signal 2SH2. Acts on the circuit 8 to set the gain. When the image signal is not modulated, the current switch 3 is turned to the resistor R6 to prevent the current source 4 from malfunctioning.

FIG. 5 shows an example of the Offset current control circuit 6, in which Tr1 and Tr2 are transistors, R7 and R8 are resistors, Csh1 is a capacitor, and 13 is a sample-hold signal.
A control circuit for turning on / off the transistor Tr1 or the transistor Tr2 by the output of the first comparator 6 when 1SH1 is at "H". The OP amplifier forms a non-inverting amplifier. A constant current source may be used instead of the resistors R7 and R8. An FET may be used instead of the transistors Tr1 and Tr2.

FIG. 6 shows an example of the gain control circuit 8. Tr3 and Tr4 are transistors, R9 and R10 are resistors, Csh2 is a capacitor, and 15 is a second comparator when the sample and hold signal 2SH2 is "H". A control circuit 16 for turning on / off the transistor Tr3 or the transistor Tr4 by the output of 8, and an OP amplifier 16 forms a non-inverting amplifier. A constant current source may be used instead of the resistors R9 and R10.
An FET may be used instead of the transistors Tr3 and Tr4.

FIG. 7 shows an example of the DA converter 2.
Is a register file that holds 8-bit digital data, 2R and R are resistors, S1 to S8 are current switches that turn on and off according to bits B1 to B8 in the register file, and 18 is an OP amplifier and an inverting amplifier. Has formed. OP
When the non-inverting input terminal (+) of the amplifier 18 is grounded, the inverting input terminal (-) in a virtual short circuit relationship is always at the ground potential as long as the OP amplifier operates normally. The current Iout1 is determined by the input voltage (Vref) and the combined resistance of the ladder resistor composed of R-2R and the current switches S1 to S8. The current flowing through the feedback resistor R from the output terminal of the OP amplifier to the inverting input terminal (-) is Iout1
The output voltage of the OP amplifier is negative (-Vs
w).

FIG. 8 shows an example of the current source 4, wherein R11 is a resistor, Tr5 is a transistor, 19 is an OP amplifier, and a resistor R11
By controlling the transistor Tr5 so that the current flowing through the transistor Tr becomes Ild = Vs / R11, the voltage Vs applied to the non-inverting input terminal (+) is converted into the current Ild flowing through the resistor R11.

FIG. 3 shows how the set voltage Vs of the power supply current 4 changes according to the gain of the DA converter 2. The horizontal axis is DA
At the input of the converter 2, the vertical axis is the set voltage of the current source 4.
Gain (T1) is the gain when the case temperature of the semiconductor laser LD is T1, and the gain is large when the case temperature rises to T2.
(T2), and the output of the DA converter 2 also increases. Conversely,
When the case temperature falls from T2 to T1, the gain decreases and the output of the DA converter also decreases.

The output Vs of the differential amplifier 11 includes the output Vos of the offset current control circuit 6, the output -Vsw of the DA converter 2, and the resistance R
From 1 to R4, Vs = ((R1 + R2) / (R3 + R4)) R4 / R1 × Vos-R2 / R1 (-
Vsw) Here, if R1 = R3 and R2 = R4, Vs = R2 / R1 (Vos + Vsw). In general, the differential amplifier 11 has R1 // R2 = R
If 3 // R4, the offset error due to the input bias current can be minimized. (Setting of Offset Current) When the image signal is zero (0), the drive current Ild of the semiconductor laser LD is controlled so that the optical output of the semiconductor laser LD becomes the minimum use value (for example, P1 in FIG. 4).
First, the current switch 3 is turned to the semiconductor laser LD side. Since there is no charge at the beginning of the capacitor Csh1, the output voltage Vos of the Offset current control circuit 6 is zero (0 V). Since the output Vsw of the DA converter 2 is also zero (0 V), the semiconductor laser LD
Is also zero (0A). Since the semiconductor laser LD does not emit light, the voltage monitored by the photodiode PD
Vpd is also zero (0 V), and the first comparator 7 outputs “H”. Offse
In the t current control circuit 6, when the sample-and-hold signal SH1 is "H", if the output of the first comparator 7 is "H", the transistor Tr1 is turned on and the transistor Tr2 is turned off to charge the capacitor Csh1 with electric charge. 1 If the output of comparator 7 is "L", transistor Tr1 turns off, transistor Tr2 turns on and capacitor
The charge of Csh1 is discharged, and the monitor voltage Vpd and the reference voltage 1 (Vref
Repeat until 1) is equal. Monitor voltage Vpd and reference voltage 1
When (Vref1) becomes equal, the sample and hold signal SH1
Is set to “L”, and the output voltage Vos of the Offset current control circuit 6 is held. If the case temperature and characteristics of the semiconductor laser LD change, the sample-and-hold signal SH1 is set to "H" again to
fset Set current.

(Setting of Gain) Next, after setting the Offset current, the gain of the DA converter 2 is set. First, the image signal is set to a specific value (for example, D2 in FIG. 3). Since there is no charge in the initial stage of the capacitor Csh2, the gain control circuit 8
Is zero (0 V). Since the output Vsw of the DA converter 2 is also zero (0 V), the driving current I
ld is only the Offset current. Since the semiconductor laser LD outputs a laser beam of the minimum use value (P1), the monitor voltage Vpd is lower than the reference voltage 2 (Vref2), so that the second comparator 9 outputs "H".
Is output. In the gain control circuit 8, when the sample-and-hold signal SH2 is "H", if the output of the second comparator 9 is "H", the transistor Tr3 is turned on and the transistor Tr4 is turned off to charge the capacitor Csh2 with electric charge. If the output of the comparator 9 is "L", the transistor Tr3 is turned off and the transistor Tr4 is turned on to discharge the electric charge of the capacitor Csh2 until the monitor voltage Vpd becomes equal to the reference voltage 2 (Vref2). Monitor voltage Vpd
And when the reference voltage 2 (Vref2) becomes equal, the sample-and-hold signal SH2 is set to "L", and the output voltage Vg of the gain control circuit 8 is set.
Hold ain. If the case temperature or characteristics of the semiconductor laser LD changes, the sample and hold signal 2SH2
To "H" to set the gain.

When the setting of the Offset current and the setting of the gain are completed, the semiconductor laser driving device starts the sample-and-hold signal.
By fixing SH1 and SH2 to "L", the semiconductor laser LD outputs an optical output corresponding to a multi-gradation image signal between a minimum use value (P1) and a maximum use value (for example, P2 in FIG. 4). Modulate. Since the characteristics of the semiconductor laser LD change due to a change in case temperature and aging, the offset current and the gain are periodically adjusted.

In the example described above, the differential amplifier 1
1 was used, but an inverting amplifier was used instead of the non-inverting amplifier of the gain control circuit 8, a negative gain voltage was input to the DA converter, the output of the DA converter 2 was converted to a positive analog voltage, and added to the amplifier. An amplifier may be used.

[0027]

As is apparent from the above, according to the semiconductor laser driving apparatus of the present invention, the control loop (1) for adjusting the offset current and the control loop (for controlling the modulation current from the image signal by adjusting the gain). 2) is synthesized using the differential amplifier 11, the semiconductor laser driving device can be configured with the minimum DA converter 2, and the influence of the error generated by the DA converter 2 can be minimized. In addition, even if the characteristics of the semiconductor laser LD change, the drive current Ild of the semiconductor laser LD is corrected, and the differential amplifier 11 has a function of suppressing noise, so that the semiconductor laser LD can be modulated stably.

[Brief description of the drawings]

FIG. 1 is a diagram showing one embodiment of a semiconductor laser driving device of the present invention.

FIG. 2 is a diagram showing an example of a conventional semiconductor laser driving device.

FIG. 3 is a diagram showing a state in which a voltage Vs changes by adjusting a gain of a DA converter 2.

FIG. 4 is a diagram showing how a semiconductor laser changes with a case temperature.

FIG. 5 shows an OF used in the semiconductor laser driving device of the present invention.
FIG. 4 is a diagram illustrating an example of an fset current control circuit.

FIG. 6 is a diagram showing an example of a gain control circuit used in the semiconductor laser driving device of the present invention.

FIG. 7 shows a DA used in the semiconductor laser driving device of the present invention.
The figure which shows the example of a converter.

FIG. 8 is a diagram showing an example of a current source used in the semiconductor laser driving device of the present invention.

[Explanation of symbols]

1, 2,... DA converter, 3, current switch, 4, 5, current source, LD, semiconductor laser, PD, photodiode, 6 Of
fset current control circuit, 8: gain control circuit, 7, 9 ... comparator, 11 ... differential amplifier, 13, 14 ... control circuit, 12, 14, 16, 18, 19 ... OP amplifier, 17 ...
Register file.

Claims (3)

[Claims] 1. A current source for performing VI conversion from a set analog voltage, a semiconductor laser driven by the current source, a photodiode for monitoring an optical output of the semiconductor laser, an output of the photodiode and a reference voltage. 1 (Vref1)
And an offset current for controlling an offset current greater than or equal to a threshold current Ith of the semiconductor laser to flow to the current source when the optical output of the semiconductor laser is at a minimum use value by the output of the first comparator. A first control loop including a control circuit, a DA converter that converts an image signal into an analog voltage, a current source, a semiconductor laser, a photodiode,
A second comparator for comparing the output of the photodiode with a reference voltage 2 (Vref2), and controlling the gain of the DA converter so that the output of the semiconductor laser becomes an output corresponding to the image signal by the output of the second comparator And a second control loop including a gain control circuit that performs an output of the Offset current control circuit of the first control loop;
Input the output of the DA converter of the control loop and calculate
A semiconductor laser drive device comprising a differential amplifier for combining a first control loop and a second control loop. 2. The semiconductor laser driving device according to claim 1, wherein an addition amplifier is used instead of the differential amplifier. 3. A reference voltage 1 (Vref1) and a reference voltage 2 (Vref2)
And means for switching between the first comparator and the second comparator, and one comparator is used by sharing the first comparator and the second comparator.
3. The semiconductor laser driving device according to item 1.