U.S. patent number RE37,524 [Application Number 09/452,769] was granted by the patent office on 2002-01-22 for device for driving a semiconductor laser by a plurality of driving elements.
This patent grant is currently assigned to Kabushiki Kaisha Toshiba. Invention is credited to Toyoki Taguchi.
United States Patent |
RE37,524 |
Taguchi |
January 22, 2002 |
**Please see images for:
( Certificate of Correction ) ** |
Device for driving a semiconductor laser by a plurality of driving
elements
Abstract
A semiconductor laser apparatus comprises a semiconductor laser,
a photodetector, first and second variable gain units, a
controllable amplifying unit a driving unit, and a compensation
unit. A control signal is supplied to one of input terminals of
each of the first and second variable gain units and an output
signal of the photodetector is negative-fed back to the other of
the input terminals of each of the first and second variable gain
units. A varied gain of an error between the control signal and the
output signal is output independently of each of the first and
second variable gain units. The controllable amplifying unit
amplifies the error signal from the first variable gain unit with
desired frequency characteristics. The driving unit includes a
plurality of driving elements connected in parallel and supplies a
driving current to the semiconductor laser in accordance with a
drive control signal from the controllable amplifying unit. The
compensation unit negatively feeds back a compensation current to
the negative feedback terminals to compensate a phase delay on the
basis of the error signal output from the second variable gain
unit.
Inventors: |
Taguchi; Toyoki (Yokohama,
JP) |
Assignee: |
Kabushiki Kaisha Toshiba
(Kawasaki, JP)
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Family
ID: |
27322021 |
Appl.
No.: |
09/452,769 |
Filed: |
December 1, 1999 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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465413 |
Jun 5, 1995 |
5579329 |
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Reissue of: |
712835 |
Sep 12, 1996 |
05694409 |
Dec 2, 1997 |
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Foreign Application Priority Data
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Jul 15, 1994 [JP] |
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6-162580 |
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Current U.S.
Class: |
372/29.011;
372/38.01; 372/38.02; 372/38.08; 372/50.1 |
Current CPC
Class: |
G11B
7/126 (20130101); H01S 5/06832 (20130101) |
Current International
Class: |
H01S
5/0683 (20060101); H01S 5/00 (20060101); G11B
7/125 (20060101); H01S 003/10 () |
Field of
Search: |
;372/38.1,38.01,38.02,38.07,38.08,29.011,50 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2-205087 |
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Aug 1990 |
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JP |
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4-208581 |
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Jul 1992 |
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JP |
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6-215403 |
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Aug 1994 |
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JP |
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Other References
"Laser Drive IC Realizing Real-Time APC Control, TA8546FN," Takashi
Kimura et al., Toshiba Review, 49 (11):790-793 (1994). .
Taguchi, et al., "High-Precision Laser Control System (11)" Optical
Disk Apparatuses of Electronics, Information and Communications
Engineering, C-372, 1991. (No month)..
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Primary Examiner: Davie; James W.
Attorney, Agent or Firm: Finnegan, Henderson, Farabow,
Garrett & Dunner, L.L.P.
Parent Case Text
CROSS-REFERENCE TO THE RELATED APPLICATION
This application is a continuation-in-part of U.S. patent
application Ser. No. 08/465,413 filed Jun. 5, 1995 now U.S. Pat No.
5,579,329.
Claims
What is claimed is:
1. A semiconductor laser apparatus comprising:
a semiconductor laser;
a photodetector for detecting an output beam of said semiconductors
laser;
a driving unit having a plurality of driving elements connected in
parallel, for driving said semiconductor laser;
a controllable amplifying unit for receiving an externally supplied
control signal and a monitor signal of said photodetector and
supplying a drive signal determined by said control signal and said
monitor signal of said photodetector to said driving unit, thereby
feedback-controlling the output beam of the semiconductor laser;
and
a compensation unit for actively controlling a phase of a feedback
control loop constituted by said photodetector, said driving unit
and said controllable amplifying unit in accordance with temporal
and electrical behaviors of the feedback control loop.
2. The semiconductor laser apparatus according to claim 1, wherein
said compensation unit comprises a variable gain amplifier and a CR
circuit including a capacitance and a resistor,
the control signal and the output beam of said photodetector are
delivered to input terminals of said variable gain amplifier,
an output terminal of said variable gain amplifier is connected to
one terminal of said CR circuit, and
the other terminal of the CR circuit is connected to an input of
said controllable amplifying unit.
3. The semiconductor laser apparatus according to claim 2, wherein
said variable gain amplifier has flat frequency characteristics up
to a frequency band of at least 1.5 times the control band of said
variable gain amplifier.
4. The semiconductor laser apparatus according to claim 1, wherein
said controllable amplifying unit includes at least a variable gain
differential amplifier functioning as a first-stage amplifier, and
a rear-stage amplifier having a higher gain than said first-stage
amplifier.
5. The semiconductor laser apparatus according to claim 4, wherein
said controllable amplifying unit includes a high-frequency
addition unit provided between said first-stage amplifier and said
rear-stage amplifier, said high-frequency addition unit adding a
high-frequency signal to an output of said first-stage
amplifier.
6. The semiconductor laser apparatus according to claim 4, wherein
said controllable amplifying unit includes a D/A conversion unit
for converting an externally supplied digital signal to an analog
control signal and supplying the analog control signal to said
first-stage amplifier in order to control the output beam in
accordance with the externally supplied digital signal.
7. The semiconductor laser apparatus according to claim 1, further
comprising a transient compensation unit for preventing an
overshoot from occurring in the feedback control loop.
8. The semiconductor laser apparatus according to claim 1, further
comprising an equalizing compensation unit for compensating for a
control signal input to the feedback control loop to prevent a sag
from occurring in pulse response characteristics of the output beam
of said semiconductor laser.
9. The semiconductor laser apparatus according to claim 1, wherein
said semiconductor laser, said photodetector, said driving unit,
said controllable amplifying unit and said compensation unit are
formed on one chip.
10. The semiconductor laser apparatus according to claim 1, wherein
said semiconductor laser, said photodetector, said driving unit,
said controllable amplifying unit and said compensation unit are
formed on one chip, and said apparatus is applied to an information
recording apparatus.
11. The semiconductor laser apparatus according to claim 1, wherein
said semiconductor laser, said photodetector, said driving unit,
said controllable amplifying unit and said compensation unit are
formed on one chip, and said apparatus is applied to an information
recording/reproduction apparatus..Iadd.
12. A semiconductor laser control device for supplying a
high-frequency superimposed drive current to a semiconductor laser,
comprising:
a control circuit which receives a control signal and processes the
control signal to obtain a signal output corresponding to an amount
of emission light from semiconductor laser;
a high-frequency signal generating circuit which generates a
high-frequency signal for reducing noise caused by light returning
to the semiconductor laser; and
an addition unit which adds the high-frequency signal to the signal
output from the control circuit,
wherein the control circuit, the high-frequency signal generating
circuit, and the addition unit are integrally fabricated on one
chip..Iaddend..Iadd.
13. A semiconductor laser control device according to claim 12,
wherein the control circuit includes an amplifier having a gain
control terminal..Iaddend..Iadd.
14. A semiconductor laser control device according to claim 12,
wherein the control circuit receives a detection signal, other than
the control signal, obtained by detecting an output beam of the
semiconductor laser..Iaddend..Iadd.
15. A semiconductor laser control device according to claim 12,
further comprising a driver transistor which outputs the
high-frequency superimposed drive current to the semiconductor
laser..Iaddend..Iadd.
16. A semiconductor laser control device for supplying a
high-frequency superimposed drive current to a semiconductor laser,
comprising:
a control circuit which receives a control signal and processes the
control signal;
a high-frequency signal generating circuit which generates a
high-frequency signal for reducing noise caused by light returning
to the semiconductor laser;
a terminal disposed to couple the high-frequency signal generating
circuit with an external resistor; and
an addition unit which adds the high-frequency signal to an output
signal from the control circuit,
wherein the control circuit, the high-frequency signal generating
circuit, the terminal, and the addition unit are integrally
fabricated on one chip..Iaddend..Iadd.
17. A semiconductor laser control device according to claim 16,
wherein the control circuit includes an amplifier having a gain
control terminal..Iaddend..Iadd.
18. A semiconductor laser control device according to claim 16,
wherein the control circuit outputs a signal for adjusting an
amount of emission light from the semiconductor
laser..Iaddend..Iadd.
19. A semiconductor laser control device according to claim 16,
wherein the control circuit receives a detection signal, other than
the control signal, obtained by detecting an output beam of the
semiconductor laser..Iaddend..Iadd.
20. A semiconductor laser control device according to claim 16,
further comprising a driver transistor which outputs the
high-frequency superimposed drive current to the semiconductor
laser..Iaddend..Iadd.
21. A semiconductor laser control device including an
auto-power-control circuit for negatively feeding back a detection
signal obtained by detecting an output beam of a semiconductor
laser to control an amount of emission light from the semiconductor
laser, wherein the improvement comprises:
a high-frequency signal generating circuit integrated with the
auto-power control circuit within one chip, the high-frequency
signal generating circuit generating a high-frequency signal for
reducing noise caused by light returning to the semiconductor
laser..Iaddend..Iadd.
22. A semiconductor laser control device according to claim 21,
wherein the auto-power-control includes an amplifier having an
inverted input terminal supplied with the detection
signal..Iaddend..Iadd.
23. A semiconductor laser control device according to claim 21,
further comprising a driver transistor which outputs a
high-frequency superimposed drive current to the semiconductor
laser..Iaddend.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser apparatus
used in an optical disk apparatus, a laser printer, an optical data
communication system, and the like.
2. Description of the Related Art
Semiconductor lasers are widely used in systems of optical disk
apparatuses, which are large capacity memory apparatuses, or laser
printers, because of advantages of easy modulation of direct light
intensity, a small size, low power consumption and high
efficiency.
Conventional semiconductor lasers, however, have a disadvantage in
that the amount of emission light varies due to the following
characteristics:
(1) A variation in differential quantitative due to a temperature
variation and a variation with the passing of time,
(2) A variation in threshold current due to a temperature variation
and reflection light (return light), and
(3) Occurrence of mode hopping noise due to reflection light
(return light).
In order to drive the semiconductor laser, a control circuit for
monitoring and stabilizing the amount of output light of the
semiconductor laser is indispensable. In particular, in an optical
disk apparatus, in order to increase the memory capacity and data
transfer rate, light intensity modulation with higher precision and
reduction in noise at the time of reproduction are required.
A wide band front APC method is known as a method for reducing
laser noise with a currency available semiconductor laser used as a
light source of an optical disk apparatus (e.g., TAGUCHI, HOSHINO:
"High-Precision Laser Control System (11)" in Optical Disk
Apparatuses, General Meeting in Spring 1991 of the institute of
Electronics, Information and Communications Engineering, C-372,
etc.).
In the wide band front APC method, a light beam actually radiated
on an optical disk in a recording/reproducing mode, i.e., part of a
front beam of a semiconductor laser is guided to, and detected by,
a photodetector. A detection signal from the photodetector is used
for light output control of the semiconductor laser. According to
this method, the control band is increased and thus the laser noise
is reduced.
Regarding the wide band front APC, it is important how the control
band is increased in relation to the reproduction signal band. A
technique for making the control band of the wide band front APC
wider than the reproduction signal band is disclosed in, e.g., Jpn.
Pat. Appln. KOKAI No. 4-209581 (the title of the invention:
"Semiconductor Laser Apparatus").
The semiconductor laser apparatus is provided with an error
detection circuit for outputting an error signal representing an
error between an output signal negatively fed bank from a
photodetector for detecting an output beam of the semiconductor
laser and an external control signal. A feedback loop is formed
which controls a drive current for the semiconductor laser on the
basis of the error signal. In addition, the semiconductor laser
apparatus is provided with a compensation circuit for negatively
feeding back a compensation current for compensating a phase delay
of the feedback loop to an input terminal of the error detection
circuit.
In the prior art, however, no consideration has been paid to the
variation in feedback amount of the feedback loop due to the
aforementioned variation in differential quantitative efficiency or
the variation with the passing of time of the semiconductor laser,
or the variance in adjustment of optical systems among
apparatuses.
Nor has consideration been paid to the variation in frequency
characteristics due to a delay in the semiconductor laser or a
variation in junction capacitance in the photodetector.
If the aforementioned variation feedback amount or frequency
characteristics occurs, laser noise cannot fully be reduced, in
particular, in an information recording/reproducing apparatus such
as an optical disk apparatus. Furthermore, unnecessary noise occurs
due to degradation in transient response characteristics to
noise.
Moreover, the degradation in transient pulse response
characteristics at the time of recording poses a more serious
problem, since it result in a recording mark variation and greatly
loses a reproduction margin.
On the other hand, in a laser printer, etc., there is a demand for
a much higher light turn/off ratio, i.e., a light turn on/off
ratio. However, in a turn-off region, i.e., a threshold lower than
laser oscillation, the feedback efficiency is greatly lowered and
high light turn-off ratio cannot be obtained. This problem will now
be described with reference to simulation results shown in FIGS. 1
and 2.
FIG. 1 shows a response waveform of a monitor PD current at the
time of turn on/off. It is understood that the control system is
deteriorated because the light is not fully turned off at the time
instant of turn off, although the light has high-speed
responsiveness and stable at the time of turn on.
FIG. 2 shows a response waveform of an LD (laser diode) driving
current at the time of turn on/off. It is understood that the
driving current can be controlled to only the level of threshold
current at the time instant of turn off.
As has been mentioned above, in the conventional semiconductor
laser apparatus, no consideration is paid to the variation in
feedback amount of the feedback loop due to the variation in
differential quantitative efficiency or the variation with the
passing of time of the semiconductor laser, or the variance in
frequency characteristics due to a delay in the semiconductor laser
or a variation in junction capacitance in the photodetector.
Thus, in the case of the information recording/reproducing
apparatus such as an optical disk apparatus, the effect of laser
noise reduction is not sufficiently and unnecessary noise is caused
by the degradation in transient response characteristics to
noise.
In the case where the performance in light turn-off ratio is
required, the feedback efficiency decreases greatly at a level
lower than the laser oscillation threshold and a high light
turn-off ratio cannot be obtained.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a high-powered
semiconductor laser apparatus wherein the feedback amount of a
feedback system for negatively feeding back a driving current of a
semiconductor laser is automatically compensated without damaging
the dynamic range of a controllable amplifier, the variation in
characteristics due to a delay in a semiconductor laser or a
variation in junction capacitance in the photodetector, and a high
turn-off ratio is obtained.
To attain the above object, there is provided a semiconductor laser
apparatus comprising:
a semiconductor laser;
a photodetector for detecting an output beam of the semiconductor
laser;
a driving unit having a plurality of driving elements connected in
parallel, for driving the semiconductor laser;
a controllable amplifying unit for receiving an externally supplied
control signal and a monitor signal of the photodetector and
supplying a drive signal determined by the control signal and the
monitor signal of the photodetector to the driving unit, thereby
feedback-controlling the output beam of the semiconductor laser;
and
a compensation unit for actively controlling a phase of a feedback
control loop constituted by the photodetector, the driving unit and
the controllable amplifying unit in accordance with temporal and
electrical behaviors of the feedback control loop.
Additional objects and advantages of the invention will be set
forth in the description which follows, and in part will be obvious
from the description, or may be learned by practice of the
invention. The objects and advantages of the invention may be
realized and obtained by means of the instrumentalities and
combinations particularly pointed out in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and constitute
a part of the specification, illustrate presently preferred
embodiments of the invention and, together with the general
description given above and the detailed description of the
preferred embodiments given below, serve to explain the principles
of the invention.
FIG. 1 is a graph showing a monitor PD current waveform at the time
of on/off control of a semiconductor laser in a conventional
semiconductor laser apparatus;
FIG. 2 is a graph showing an LD (laser diode) driving current
waveform at the time of on/off control of the semiconductor laser
in the conventional semiconductor laser apparatus;
FIG. 3 is a block diagram schematically showing a semiconductor
laser apparatus according to a first principle of the present
invention;
FIG. 4 is a detailed circuit diagram of a semiconductor laser
apparatus according to a first embodiment of the present
invention;
FIG. 5 is a circuit diagram showing a loop constituted by a
differential variable amplifier and a compensation circuit shown in
FIG. 4;
FIG. 6 is a graph showing a phase progress effect obtained when the
gain of the differential variable amplifier shown in FIG. 4 is
varied;
FIG. 7 is a detailed circuit diagram of a semiconductor laser
apparatus according to a second embodiment of the present
invention;
FIG. 8 is a detailed circuit diagram of a semiconductor laser
apparatus according to a third embodiment of the present
invention;
FIG. 9 is a block diagram showing the principle of parallel driving
of a semiconductor laser;
FIG. 10 is a graph showing closed-loop frequency characteristics in
the parallel driving;
FIG. 11 is a graph showing pulse response characteristics of a
laser diode of an optical system;
FIG. 12 is a block diagram of an equalizing compensation
system;
FIG. 13 is a graph showing simulation results of closed-loop
frequency characteristics in the equalizing compensation system;
and
FIG. 14 is a graph showing simulation results of pulse response
characteristics of a laser diode of an optical system in the
equalizing compensation system.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A semiconductor laser apparatus according to a first principle of
the present invention will now be described with reference to FIG.
3.
As is shown in FIG. 3, the semiconductor laser apparatus according
to the first principle comprises a semiconductor laser 510, a
photodetector 520, a driving unit 530, a controllable amplifying
unit 540 and a compensation unit 550. The photodetector 520 detects
an output light beam from the semiconductor laser 510. The driving
unit 530 supplies a driving current to the semiconductor laser 510.
The controllable amplifying unit 540 APC-controls the output of the
semiconductor laser by feedback control. For this purpose, the
controllable amplifying unit 540 receives a control signal from
terminals .[.531.]. .Iadd.541 .Iaddend.and an output light signal
of the photodetector 520 and supplies a driving signal determined
by the control signal and output light signal to the driving unit
530. The compensation unit 550 actively controls the phase, in
particular, a phase delay, of a feedback loop constituted by the
photodetector 520, driving unit 530 and controllable amplifying
unit 540, in accordance with temporal and electrical behaviors of
the feedback loop.
FIG. 4 shows a first embodiment of the semiconductor laser
apparatus according to the first principle shown in FIG. 3. As is
shown in FIG. 4, the semiconductor laser apparatus of the first
embodiment comprises an integrated semiconductor laser control
circuit 1, a semiconductor laser 23 connected to the laser control
circuit 1, a photodetector 24 for detecting an output light beam of
the semiconductor laser 23 and a plurality of resistors connected
externally to the semiconductor laser control circuit 1.
The structure of the semiconductor laser control circuit 1 will now
be described. A control current signal is input to an input
terminal 30 from the outside. The control current signal is
converted to a control voltage signal by a current/voltage
converter 2. The control voltage signal is supplied to each of a
controllable amplifying unit 10 and a variable gain type
differential amplifier s functioning as a second variable gain
means. The controllable amplifying unit 10 comprises a variable,
gain type differential amplifier 6 functioning as a first variable
gain means, an operational amplifier 7 and a differential amplifier
s for subjecting outputs of both amplifiers 6 and 7 to subtraction.
The controllable amplifying unit 10 outputs a driving control
signal.
The driving control signal output from the differential amplifier 8
is shifted to a desired potential level by a level shift 11. The
level-shifted driving control signal is input to each of a buffer
18, a transistor 12 for detecting a driving current of the
semiconductor laser 23, and a transistor 13 for supplying a driving
current to the semiconductor laser 23.
The damping capacity of the transistor 13 is about 100 times that
of the transistor 12. About 1% of the driving current flows through
the transistor 12, and the driving current is monitored on the
basis of a voltage drop in an external resistor R1. The comparator
14 compares the voltage drop with a reference voltage Ref3 and
outputs a comparison result to the outside via a buffer 16. In
addition, the comparator 14 limits the driving current by lowering
the output potential of the level shift 11.
A window comparator 15 determines the control state when the output
voltage of the operational amplifier 7 is in a range between Ref1
and Ref2, and outputs a determination result to the outside via a
buffer 17.
In addition, the semiconductor laser control circuit 1 includes a
power supply monitor 21 for detecting a decrease in power supply
voltage and automatically halting the control operation, and a
power save circuit 22 for starting and stopping the control
operation by external control.
The control voltage signal from the current/voltage converter 2 is
a modulated signal voltage which is intensity-modulated so as to
represent, e.g., digital information. The modulated signal voltage
is supplied to an inverted input terminal of the controllable
amplifying unit 10 as a control current via a resistor element Ri.
A reference voltage generated by a current/voltage converter 3
having the same structure as the current/voltage converter 2 is
input to the non-inverted input terminal of the controllable
amplifying unit 10.
An anode of the photodetector 24 is connected to a terminal 39. A
monitor current from the photodetector 24 is negative-fed back to
the inverted input terminal of the controllable amplifying unit 10
via the terminal 39. Thus, the output beam from the semiconductor
laser 23 is intensity-modulated in proportion to the modulated
signal voltage which is the control voltage signal from the
current/voltage converter 2, That is, the semiconductor laser 23 is
auto-power-controlled.
The gain of the operational amplifier 7 ran be varied by an
external resistor Rz, and the zero-point frequency for avoiding the
influence of junction capacitance of the photodetector 24 can be
optimized.
Voltage/current converters 19 and 20 pull up potentials provided
from input terminals 44 and 43 in their insides on the basis of
reference voltage and resistance. The pull-up levels of the
voltage/current converters 19 and 20 can be set by voltage input or
external resistors Rg1 and Rg2. Output currents from the
voltage/current converters 19 and 20 are input to the variable gain
type differential amplifiers 6 and 5, respectively.
In general, the dynamic range of an output from a variable gain
type differential amplifier varies greatly due to a variation of
gain. Thus, if the variable gain type differential amplifier is
provided at the rear stage of the circuit (controllable amplifying
unit), the dynamic range of the control system lowers greatly. It
is therefore desirable that the variable gain type differential
amplifier be provided at the first stage of the circuit
(controllable amplifying unit). That the differential amplifier has
a gain varying function is indispensable to compensate a variance
among devices and optical systems.
A description will now be given of the effect of separation between
the variable gain type differential amplifier 5 and controllable
amplifying unit 10, which is one of the features of the present
invention. An output terminal of the variable gain type
differential amplifier 5 is connected to a terminal 33. The
terminal 33 is connected to a terminal 32 on the outside of the
semiconductor laser control circuit 1. Thus, a phase delay
compensation current, i.e., a compensation current to compensate
degradation of response characteristics due to an influence of
junction capacitance of the semiconductor laser 24, is generated by
a compensation circuit 9 comprising an RC series circuit. In this
case, a loop constituted by the variable gain type differential
amplifier 5 and compensation 9 can be regarded as an independent
feedback loop, as shown in FIG. 5. Specifically, the phase
characteristics of Vout alone, i.e., an input to the controllable
amplifying unit 10, may be considered with respect to the operation
of the semiconductor laser 23.
FIG. 6 shows the phase progress effect obtained at the time the
gain of the variable gain type differential amplifier 5 has been
varied. As is shown in FIG. 6, sufficient phase progress effect is
obtained only by varying the gain of the differential amplifier 5.
There is no need to alter the constant of the compensation circuit
9. Thus, the apparatus of this embodiment can be built in an
integrated circuit and is cost-effective.
On the other hand, a necessary gain in the control system can be
set without adjustment, if the control gain cross frequency, i.e.,
control band is determined at first. In order to achieve this
effect, it is necessary to flatten the frequency characteristics of
the variable gain type differential amplifier 5 up to 1.5 times the
target control band.
As has been described above, in the present embodiment, even if the
gain of the first variable gain amplifying means provided at the
first stage is varied, the dynamic range of the rear-stage
controllable amplifying means with a higher gain is not degraded.
In particular, high-precision control less vulnerable to a variance
in threshold current can be achieved.
Furthermore, since the second variable gain type amplifying means
is independent of the feedback control loop, the phase compensation
of the feedback can be adjusted independently. Since the gain can
be varied, the constant of the compensation means need not be
altered. Thus, the cost for parts and adjustment can be
reduced.
A semiconductor laser apparatus according to a second embodiment of
the present invention will now be described with reference to FIG.
7. According to the second embodiment, when the semiconductor laser
apparatus is used at a low power supply voltage, a driving current
can be amplified without degrading pulse characteristics since a
semiconductor laser 22 can be driven by a plurality of drive
transistors arranged in parallel. In this parallel driving, a
transient compensation for preventing a feedback control loop from
overshooting is adopted, as is a high-frequency addition for adding
a high-frequency oscillation signal to an output signal of a
variable gain amplifier GCAMP1. The parallel driving and transient
compensation will be described in detail with reference to FIGS. 9
to 11.
The semiconductor laser apparatus of the second embodiment can
favorably be incorporated into an information
recording/reproduction apparatus such as an optical disk apparatus.
Since the currently-used optical disk apparatus is equipped with a
monitoring photodetector having good characteristics, it does not
need any equivalent for the equalizing function of a semiconductor
laser apparatus according to a third embodiment of the present
invention, which is shown in FIG. 8 and will be described
later.
In FIG. 7, the structural elements common to those shown in FIG. 4
are denoted by like reference numerals and their descriptions are
omitted.
Referring to FIG. 7, an output signal of an input I/V converter
circuit 2 is inverted by an inverting buffer AMP101, and the
inverted signal is supplied to a drive transistor 105 via a level
shift 103. An output signal of the drive transistor 105 is added to
that of a controlling drive transistor 13 at a terminal (LDK) 37.
The added signal is supplied to the semiconductor laser 22. The
gain of the drive transistor 105 is determined by a resistor (Rb)
externally connected to a terminal (IB) 106 (open-loop) control).
An output signal of the inverting buffer AMP101 is led to a
compensation circuit 102 constituted by a series circuit of both a
resistor Rtr and a capacitor ctr. A high-frequency compensation
current output from the circuit 102, which enables the transient
compensation, is supplied to each of inverting input terminals of
the variable gain amplifiers GCAMP1 and GCAMP2.
In the semiconductor laser apparatus shown in FIG. 7, the open-loop
control is adopted in addition to the feedback control system shown
in FIG. 4 and, in this case, a feedback current of the open-loop
control, corresponding to an input signal, has to be exactly fed
back to a feedback point, i.e., an inverted input terminal point of
the variable gain amplifier GCAMP1, in order to prevent an
influence upon the feedback control system. If inexact feedback is
performed, the feedback control system operates so as to compensate
for a component corresponding to the inexact feedback, with the
result that an overshoot of output beam occurs and the
characteristics are degraded. The high-frequency compensation
current functions to compensate a decrease in feedback current in a
high-frequency band of the semiconductor laser 22 and photodetector
23, prevent an overshoot from occurring, and avoid an influence
upon the feedback control system.
Furthermore, in the apparatus shown in FIG. 7, a balance output
signal of the variable gain amplifier GCAMP1 and those of a
high-frequency oscillator 112 having terminals 110 and 111 are
added by adders 113 and 114, respectively. Since the added
high-frequency oscillation signals are supplied to the
semiconductor laser 22 through controllable amplifier HGAMP 7,
level shift 11, and drive transistor 13, a feedback loop is not
influenced by variations in loop gain. Since, moreover, moreover,
the gain of the variable gain amplifier GCAMP1 is decreased (to
smaller than 6 dB), the feedback loop cannot be influenced by the
variations in loop gain. It is usually when a high gain amplifier
is arranged ahead of a point where the high-frequency oscillation
signals are added that the variations in loop gain exercise an
influence upon the feedback loop. In this case, the amplifier
generates undesirable components to attenuate the added
signals.
If the high-frequency oscillation signals are added to the output
of the variable gain amplifier GCAMP1, superimposition of
high-frequency oscillation signals, which is not influenced by
variations in loop gain or does not influence the feedback loop,
can be achieved.
As has been described above, in the second embodiment, two driving
elements are arranged in parallel so as to supply a driving current
in response to an input signal Furthermore, an overshoot occurring
duo to parallel driving of the two driving elements is
transient-compensated by delivering a high-frequency component of
the input signal to the feedback point.
As a result, the open-loop control is performed and the
characteristics of the semiconductor laser can be fully exhibited.
Therefore, pulse characteristics such as a decrease in pulse rising
time can be improved.
In the vicinity of a light turn-off region, the controllable
amplifier HGAMP 7 comes into a saturated region and the pulse
characteristics deteriorate greatly. According to the parallel
driving, the output amplitude of the controllable amplifier is
reduced; thus, the amplifier performs the control without coming
into the saturated state, and the light turn-off ratio is
remarkably enhanced. For this reason, the load on the controllable
amplifier is reduced and the light turn-off ratio is improved.
If the high-frequency oscillation signals are added to the output
of the variable gain amplifier GCAMP1, superimposition of
high-frequency oscillation signals, which is not influenced by
variations in loop gain or does not influence the feedback loop,
can be achieved.
A semiconductor laser apparatus according to a third embodiment of
the present invention Will now be described with reference to FIG.
8. Like the apparatus of the second embodiment, that of the third
embodiment adopts parallel driving and transient compensation. It
also adopts equalizing compensation for correcting a control signal
input to a feedback control loop in order to prevent a sag of pulse
response characteristics of an output beam of a semiconductor laser
22 from occurring. The equalizing compensation will be described in
detail with reference to FIGS. 12 to 14.
The semiconductor laser apparatus of the third embodiment can
favorably be incorporated into an information recording apparatus
such as a laser beam printer. An equalizing function is essential
to the laser beam printer since a photodetector is incorporated
into a semiconductor laser.
In FIG. 8, the structural elements common to those shown in FIG. 7
are denoted by like reference numerals and their descriptions are
omitted. The semiconductor laser apparatus incorporates a D/A
converter section for converting a digital signal into an analog
control signal in order to control an output beam in accordance
with external digital data so as to easily perform the gradation
control. The D/A converter section includes input terminals 120 to
128, a latch 129 and an A/D converter 130. According to the third
embodiment, the equalizing compensation is executed by adding an
output signal of a buffer 101 in a current mode at a feedback point
through an externally-arranged compensation circuit 131 including a
resistor Req and a capacitor Ceq. The equalizing compensation is
intended to compensate for a control signal input to a feedback
control loop in order to prevent a sag of pulse response
characteristics of an output beam of the semiconductor laser 22
from occurring.
The parallel driving and transient compensation in the second and
third embodiments will now be described in detail. The parallel
driving contributes to an improvement in driving response
characteristics of a semiconductor laser. Conventionally there is a
problem that only the response characteristics corresponding to a
feedback control band are exhibited in order to increase the data
transfer speed of an optical disk apparatus. To resolve this
problem, the semiconductor laser is parallelly driven by a
plurality of driving elements, thereby obtaining response
characteristics corresponding to an LD (laser diode) band.
The parallel driving also contributes to an increase in driving
capacity of a semiconductor laser. When the semiconductor laser is
driven by a large amount of current, a transistor formed in an IC
has to increase in size; however, there occurs a problem that the
frequency characteristics of the transistor is degraded. To resolve
the problem the semiconductor laser is parallelly driven by two
driving transistors, thereby enhancing the driving capacity without
degrading the frequency characteristics.
FIG. 9 is a block diagram showing the principle of the parallel
driving, Referring to FIG. 9, the output signals of a controllable
amplifier 203 and a buffer 206 are added to each other at an
addition point 264 to drive an optical system (LD: laser diode)
205. The addition point 202, controllable amplifier 203, addition
point 204, and optical system 205 form a feedback loop, and its
feedback band is about half the frequency band of the single laser
diode of the optical system 205 even by the use of wideband
technology, since the feedback band is restricted by the frequency
band of all the laser diodes of the optical system 205.
If an input control signal is supplied through the buffer 206 and
addition point 204 to drive the laser diode of the optical system
205, the frequency characteristics of the system (laser diode) 205
can be exhibited sufficiently. However, when a wideband signal is
input to the addition point 204, the components more than those in
the frequency band of the optical system 205 causes a disturbance
in the control loop, and an overshoot occurs in the response
characteristics. To avoid the overshoot, an output signal of a
compensator 207 having an inverse characteristic of transmittance
of the optical system 205, is input to an adder 201, and the output
signal and the input signal of the control loop are subjected to
subtraction, thereby restricting the control band.
The foregoing parallel driving will be described with reference to
FIGS. 10 and 11.
In FIG. 10, line 302a shows a closed-loop frequency characteristic
obtained by the feedback control only, while line 302b does a
closed-loop frequency characteristic obtained by the parallel
driving. It is seen from FIG. 10 that the control band of the
latter is two or more times as wide as that of the former.
FIG. 11 is a graph representing pulse response characteristics of
outputs of a laser diode of the optical system 205. In FIG. 11,
curve 303a shows a pulse response characteristic obtained by the
feedback control only, curve 303b indicates a pulse response
characteristic obtained when no transient compensation is performed
in the parallel driving, and curve 303c shows a pulse response
characteristic obtained when transient compensation is performed in
the parallel driving. It is apparent from curve 303b that an
over-shoot occurs and from curve 303c that an overshoot is greatly
decreased. It is also apparent from curve 303c that the pulse
rising time (Tr) and pulse falling time (Tf) are considerably
improved, and the parallel driving increases the data transfer
speed further.
The above-described parallel driving is featured by driving an LD
(laser diode) in parallel by directly inputting a control signal to
a conventional feedback system. The feedback system has a drawback
of causing an overshoot since it is rendered in a noise-applied
state. To resolve this drawback, the control signal input to the
feedback system is corrected by a transient compensation means to
avoid the noise-applied state.
The equalizing compensation adopted in the third embodiment will
now be described in detail. The equalizing compensation aims at
compensating for a distortion of frequency characteristics of a
photo-detector (PD) incorporated in a semiconductor laser diode
(LD). When a semiconductor laser apparatus is applied to an
information recording apparatus such as a laser beam printer, the
gradation control of the intensity of laser beams output from a
high-speed semiconductor laser diode is required for high-precision
printing (gradation printing) and high-speed printing. Since,
however, the photodetector is used to detect a feedback output in
order to decrease in cost, its frequency characteristics we not
generally flat because of a delay in scattering (which is caused by
a delay in laser beam emitted outside a beam-receiving area), and
such frequency characteristics have an influence upon the response
characteristics of the semiconductor laser diode. Avoiding the
influence due to the delay in scattering improves the response
characteristics of the semiconductor laser diode. It is thus
possible to greatly improve in both printing quality and printing
speed without causing a sag or the like in the response
characteristics of the semiconductor laser diode.
The equalizing compensation will be described with reference to
FIG. 12. FIG. 12 is a block diagram of an equalizing compensation
system for compensating for undesired characteristics of the
photodetector. An addition point 202, a control amplifier 203, and
an optical system 205 form a conventional wideband feedback loop.
If, however, the frequency characteristics of a photodetector 212
of the optical system 205 are deteriorated by a delay in
scattering, their inverse characteristics are caused in the output
of a semiconductor laser diode 211 of the optical system 205, as
described above. To compensate for the inverse characteristics, an
equalizing filter 213 processes an input signal such that a signal
input to a control loop has an inverse characteristic of transfer
function of the photodetector, and an output signal the equalizing
filter 213 is input to an adder 210, thus subjecting the signals
input to both the adder 210 and control loop to subtraction.
The effects of the equalizing compensation will now be described
with reference to FIGS. 13 and 14.
FIG. 13 shows simulation results of closed-loop frequency
characteristics at an output point of the semiconductor laser diode
211. In FIG. 13, curve 305a indicates a closed-loop frequency
characteristic obtained only in the output of the wideband feedback
loop and, in this case, the gain is increased in a wide range
because of undesired characteristics of the photodetector 212,
while curve 305b represents a closed-loop frequency characteristic
obtained when the equalizing compensation is executed and this is a
flattened, good characteristic.
FIG. 14 shows simulation results of pulse response characteristics.
In FIG. 14, curve 306a indicates a pulse response characteristic
obtained when no equalizing compensation is performed, and curve
306b does a pulse response characteristic (which can be
approximated by the primary high-pass filter characteristic)
obtained when equalizing compensation is performed. When no
equalizing compensation is performed, a sag occurs but it is
completely compensated by the equalizing compensation. The
objective of the wideband feedback loop is how the output of the
semiconductor laser diode 211 is equalized with the input control
signal, and that of the equalizing compensation is to equalize the
output point of the semiconductor laser diode 211 with the input
control signal. The equalizing compensation can be achieved by
processing the input control signal using a filter having the saw
transfer characteristic as that of the semiconductor laser diode
211. If the compensation function for the equalizing compensation
is X(s), the required condition is given by the following equation
(1):
The compensation function X(s) can thus be obtained from the
following equation (2):
However, the compensation function need not be achieved faithfully
as a filter, but can be approximated by a high-pass filter obtained
by cutting off the pole representing the transfer function P(s)
low-pass filter characteristics. For example, if the transfer
function P(s) is given by the following equation (3).
the compensation function X(s), which is expressed by the following
equation (4), shows a high-pass filter.
Conventionally, the frequency characteristics of the photodetector
incorporated in the semiconductor laser diode are not generally
constant due to a delay in scattering until the above cutoff is
executed, and the output of the photodetector is decreased several
decibels from 3 dB at frequencies ranging from about 100 kHz to 20
MHz. If such a photodetector is used for wideband feedback control,
a problem of causing an inverse characteristic of an increase of
several decibels from 3 dB at frequencies ranging from about 100
kHz to 20 MHz occurs in the feedback output frequency
characteristics of the semiconductor laser diode, as a feature of
the feedback control, in order to compensate for the decrease in
the output. In this case, the pulse response characteristics of
output laser beams cause a sag and degrades the quality of
printing. To resolve this problem, a control signal input to the
feedback system is corrected by the equalizing compensation to
prevent a sag from occurring in the pulse response
characteristics.
As has been described above, according to the present invention,
since the first variable gain amplifier is provided at the
first-stage controllable amplifying section, the dynamic range of
the rear-stage controllable amplifying section with a higher gain
is not degraded even though the gain of the first variable gain
amplifier is varied. In particular, high-precision control less
vulnerable to a variance in threshold current can be achieved.
Furthermore, since the second variable gain amplifier is
independent of the feedback control loop, the phase compensation of
the feedback can be adjusted independently. Since the gain of the
second variable gain amplifier is variable, the constant of the
compensation means need not be altered, thus lowering the costs for
parts and adjustment.
When the semiconductor laser apparatus of the present invention is
applied to an information recording/reproduction apparatus, it is
resistant to a variance in several factors and its manufacturing
yield is improved.
When the semiconductor laser apparatus is applied to an image
recording apparatus, high-speed, high-quality characteristics can
be achieved by virtue of high light turn-off ratio, high-precision
light quantity, and high-speed, stable pulse characteristics.
When a semiconductor laser apparatus wherein the driving response
characteristics of a semiconductor laser diode are improved by the
parallel driving and transient compensation, is applied to an
optical disk apparatus, the speed of data transfer can be
increased.
When a semiconductor laser apparatus wherein the distortion of
frequency characteristics of a photodetector of a semiconductor
laser diode is corrected by the equalizing compensation, is applied
to a laser printer, both printing quality and printing speed can be
greatly increased, without causing any sag in the pulse response
characteristics of output laser beams.
Additional advantages and modifications will readily occur to those
skilled in the art. Therefore, the invention in its broader aspects
is not limited to the specific details, and representative devices
shown and described herein. Accordingly, various modifications may
be made without departing from the spirit or scope of the general
inventive concept as defined by the appended claims and their
equivalents.
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