U.S. patent application number 14/622414 was filed with the patent office on 2015-06-25 for light source drive circuit, optical scanning apparatus, semiconductor drive circuit, and image forming apparatus.
The applicant listed for this patent is Hayato FUJITA, Masaaki ISHIDA, Muneaki IWATA, Atsufumi OMORI. Invention is credited to Hayato FUJITA, Masaaki ISHIDA, Muneaki IWATA, Atsufumi OMORI.
Application Number | 20150180200 14/622414 |
Document ID | / |
Family ID | 50727537 |
Filed Date | 2015-06-25 |
United States Patent
Application |
20150180200 |
Kind Code |
A1 |
FUJITA; Hayato ; et
al. |
June 25, 2015 |
LIGHT SOURCE DRIVE CIRCUIT, OPTICAL SCANNING APPARATUS,
SEMICONDUCTOR DRIVE CIRCUIT, AND IMAGE FORMING APPARATUS
Abstract
A light source drive circuit which drives a light source is
disclosed, including a drive current generating unit which
generates a drive current, the driving current including a
predetermined current for obtaining a predetermined light amount
from the light source; and first and second overshoot currents
which are applied to the predetermined current in synchronization
thereto; and a control unit which sets, to the drive current
generating unit, a value of the first overshoot current to a fixed
value, and a value of the second overshoot current in accordance
with a light amount output from the light source.
Inventors: |
FUJITA; Hayato; (Kanagawa,
JP) ; OMORI; Atsufumi; (Kanagawa, JP) ; IWATA;
Muneaki; (Kanagawa, JP) ; ISHIDA; Masaaki;
(Kanagawa, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
FUJITA; Hayato
OMORI; Atsufumi
IWATA; Muneaki
ISHIDA; Masaaki |
Kanagawa
Kanagawa
Kanagawa
Kanagawa |
|
JP
JP
JP
JP |
|
|
Family ID: |
50727537 |
Appl. No.: |
14/622414 |
Filed: |
February 13, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14072946 |
Nov 6, 2013 |
8957934 |
|
|
14622414 |
|
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|
Current U.S.
Class: |
347/247 ;
372/38.02 |
Current CPC
Class: |
H01S 5/0427 20130101;
H01S 5/068 20130101; H01S 5/183 20130101; H05B 47/10 20200101; H01S
5/06 20130101; B41J 2/471 20130101; B41J 2/435 20130101; H01S
5/0261 20130101; H01S 5/06216 20130101; Y02B 20/00 20130101; B41J
2/455 20130101; G06K 15/1209 20130101 |
International
Class: |
H01S 5/042 20060101
H01S005/042; B41J 2/455 20060101 B41J002/455; H01S 5/026 20060101
H01S005/026; H01S 5/183 20060101 H01S005/183; H01S 5/068 20060101
H01S005/068 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 21, 2012 |
JP |
2012-255470 |
Nov 29, 2012 |
JP |
2012-260529 |
Claims
1. A semiconductor laser drive circuit which drives and modulates a
semiconductor laser, comprising: a current source which supplies a
drive current for driving and lighting the semiconductor laser; a
first overshoot current source which supplies a current at a time
of a rise of the drive current; and a second overshoot current
source which supplies a current at the time of the rise of the
drive current, wherein a time in which the first overshoot current
source supplies the current is a time which is shorter than a time
in which the semiconductor laser responds to the drive current; and
a time in which the second overshoot current source supplies the
current is longer than the time in which the first overshoot
current source supplies the current and is shorter than the time in
which the semiconductor laser responds to the drive current.
2. The semiconductor laser drive circuit as claimed in claim 1,
wherein the first overshoot current source and the second overshoot
current source provide a power supply simultaneously with a timing
at which the current source supplies the drive current.
3. The semiconductor laser drive circuit as claimed in claim 2,
wherein a signal of an addition current in which the current
supplied by the first overshoot current source and the current
supplied by the second overshoot current source are added is a
signal in which a signal of the drive current or a delayed signal
of the drive current is differentiated at a predetermined change
point.
4. The semiconductor laser drive circuit as claimed in claim 1,
wherein the current source includes a transistor which performs a
voltage-to-current conversion; and, to an emitter or a source of
the transistor, a first capacitor as the first overshoot current
source is connected and a resistor and a second capacitor are
serially connected as the second overshoot current source.
5. The semiconductor laser drive circuit as claimed in claim 4,
wherein the resistor is a variable resistor, and the first
capacitor and the second capacitors are variable capacitors.
6. The semiconductor laser drive circuit as claimed in claim 1,
wherein the semiconductor laser is a VCSEL.
7. The semiconductor laser drive circuit as claimed in claim 1,
wherein the semiconductor laser is a red laser or a red laser
array.
8. An image forming apparatus, comprising: a semiconductor laser
whose output is modulated by an image modulation signal; and a
scanning unit which scans a rotating photosensitive body with a
light of the semiconductor laser, wherein an electrostatic latent
image is formed onto the rotating photosensitive body in accordance
with the image modulation signal, and wherein the semiconductor
laser is driven by the semiconductor laser drive circuit as claimed
in claim 1.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a divisional of and is based upon
and claims the benefit of priority of U.S. application Ser. No.
14/072,946, filed Nov. 6, 2013 which claims the benefit of Japanese
Priority Application No. 2012-255470 filed on Nov. 21, 2012 and on
Japanese Priority Application No. 2012-260529 filed on Nov. 29,
2012, the entire contents of which are hereby incorporated by
reference.
TECHNICAL FIELD
[0002] The present invention relates to a light source drive
circuit which generates a drive current including a predetermined
current for obtaining a predetermined light amount from a light
source and first and second overshoot currents which are added in
synchronization with the predetermined current; an optical scanning
apparatus; and an image forming apparatus.
[0003] The present invention also relates to a semiconductor laser
drive circuit which controls an optical output of a semiconductor
laser light source in a laser printer, an optical disk apparatus, a
digital copier, an optical communication apparatus, etc.; and the
image forming apparatus which includes the semiconductor laser
drive circuit.
BACKGROUND ART
[0004] In a related-art image forming apparatus for use in product
printing, etc., a predetermined optical output is obtained from a
light source such as an LD (laser diode), etc., to expose a
photosensitive body therewith and express a density of an
image.
[0005] Now, it is known that, in the related art, a light emission
delay time which depends on a response characteristic of a light
source before obtaining a predetermined optical output from the
light source occurs. Moreover, in the related art, it is known, for
example, that, from a time at which a drive current is supplied to
a light source to a time at which an optical output is detected, a
light emission delay time occurs which depends on a parasitic
capacitance of a circuit, etc., in which the light source is
mounted.
[0006] Therefore, in the related-art image forming apparatus, when
a time to cause an optical output is set to a short time of less
than or equal to a few ns, for example, the optical output becomes
less than a predetermined light amount, and the density of an image
decreases, possibly causing unevenness in the image.
[0007] Thus, in the related art, schemes are provided to solve the
above-described problems. For example, Patent document 1 discloses
providing a charge and discharge circuit, wherein an overshoot
current is generated by discharging at a time of a rise of an
output of the LD (laser diode) to reduce a light emission delay
time which depends on a response characteristic of a light source.
Moreover, Patent document 2 discloses initially superposing a
threshold current at a start time of lighting the LD and
controlling a light emission amount thereof.
[0008] However, the overshoot current in Patent document 1 is
generated primarily for reducing a delay time which depends on the
response characteristic of the light source, so that it is
difficult to improve the light emission delay time which depends on
a parasitic capacitance. Moreover, in Patent document 2, while the
threshold current is initially superimposed at the start time of
lighting the LD, the threshold current is insufficient for charging
a parasitic capacitance, so that it is difficult to sufficiently
reduce a light emission delay time which depends on the parasitic
capacitance. In particular, it becomes more difficult to reduce a
delay time which depends on the parasitic capacitance in a circuit
with a large parasitic capacitance and a light source with a large
differential resistance.
[0009] A related-art semiconductor laser drive circuit is broadly
classified into a non-bias technique and a bias technique.
[0010] In the non-bias technique, a bias current of a semiconductor
laser is set to 0 and the semiconductor laser is driven by a pulse
current which corresponds to an input signal.
[0011] Here, when the semiconductor laser with a large threshold
current is driven by the non-bias technique, it requires some time
before a carrier having a density which allows laser light emission
is generated even when a drive current which corresponds to an
input signal is applied to the semiconductor laser, leading to a
light emission delay. The light emission delay does not become
problematic when an input signal is sufficiently longer than a
light emission delay time (when a light emission delay amount is
negligible).
[0012] However, when it is necessary to drive the semiconductor
laser at high speed as the laser printer, the optical disk
apparatus, the digital copier, etc., increases in speed, only a
pulse with a pulse width smaller than a desired pulse width may be
obtained with the non-bias technique.
[0013] In order to solve the problems in the non-bias technique as
described above, the bias technique is being proposed.
[0014] In the bias technique, the bias current of the semiconductor
laser is set to a threshold current of the semiconductor laser and
a pulse current which corresponds to the input signal is added to
the bias current while applying the bias current continuously to
drive the semiconductor laser.
[0015] For the bias technique, a current corresponding to a light
emission threshold (a light emission threshold current) is applied
to the semiconductor laser in advance, eliminating the light
emission delay time.
[0016] However, for the bias technique, electricity is being turned
on continuously around the light emission threshold even when the
semiconductor laser does not emit light (normally 200 .mu.Ws to 300
.mu.Ws). Therefore, for optical communications using the
semiconductor laser which is driven by the bias technique, an
extinction ratio becomes small. The small extinction ratio of the
semiconductor laser causes surface staining of an image in the
laser printer, the optical disk apparatus, digital copier, etc.,
that uses the semiconductor laser for the light source.
[0017] In the field of optical communications, in order to solve
the above-described problems, a configuration is being proposed of
basically using a non-bias technique and applying a light emission
threshold current immediately before causing light to be emitted
(see Patent documents 3 and 4, for example).
[0018] However, recently, image forming apparatuses which use a 650
nm red semiconductor laser, a 400 nm ultraviolet semiconductor
laser, etc., in the quest for an increased resolution in the laser
printer, the optical disk apparatus, the digital copier, etc., are
being put to practical use.
[0019] Moreover, for an increased speed in processing and an
increased resolution of images, semiconductor lasers such as a
VCSEL (vertical cavity surface emitting laser) in which it is easy
to integrate multiple light sources are also being put to practical
use.
[0020] These semiconductor lasers have a characteristic that more
time, relative to the related-art 1.3 .mu.m-band, 1.5 .mu.m-band,
and 780 .mu.m-band semiconductor lasers, is required before a
carrier having a density which allows laser light emission is
generated due to reasons such as a large differential resistance
thereof.
[0021] Moreover, these semiconductor lasers are able to yield only
a pulse width which is smaller than a desired pulse width even with
the bias technique. Therefore, a semiconductor laser driving method
in the light of these characteristics is needed.
[0022] Furthermore, in a case of seeking to cause a low density to
be manifested by an optical output of a short time (for example,
less than or equal to several ns), a light emission output does not
reach a peak strength of a beam spot. Therefore, there is a problem
that, in the above-described case, the density becomes
unnecessarily low, so as not to be able to cause the density to be
manifested correctly.
[0023] There is also known a technique of superimposing a
differential pulse at a time of a rise of a laser drive signal
applied to the semiconductor laser in order to solve the
above-described problem (see Patent document 5, for example).
[0024] However, with this method, a peak of the differential pulse
cannot be controlled, so there is a high risk of destroying the
semiconductor laser. Moreover, the time in which the differential
pulse is superimposed also depends on a differential waveform.
Thus, with this method, there is a problem that it is not
necessarily the case that a subsequent tone manifestation increases
linearly even when an initial ultra low density may be corrected
for.
[0025] A technique is being proposed of providing a high-speed and
high-accuracy semiconductor laser drive control and conducting a
correction using four currents of a bias current; a light emission
threshold current; a light emission current; and a drive auxiliary
current (see Patent document 2, for example).
[0026] With the technique proposed in Patent document 2, an ideal
shape of an almost square wave as an optical waveform may be
obtained without question.
[0027] However, with the technique proposed in Patent document 2, a
waveform of a pulse of an output signal may become narrower than a
waveform of a pulse of an input signal depending on set values of
the bias current and the light emission threshold current, or, in
other words, a pulse narrowing phenomenon may occur.
[0028] Now, as the semiconductor lasers for use in the image
forming apparatus, etc., a semiconductor laser array, the VCSEL,
etc., are often used. The semiconductor lasers have various
characteristics depending on the structure, wavelength
characteristics, output characteristics, etc.
[0029] For example, the 650 nm-band red semiconductor laser
generally has a differential resistance which is larger than that
of the 780 nm-band infrared semiconductor laser. Therefore, with
the red 650 nm-band red semiconductor laser, a square wave may not
be obtained at high speed, so that waveform dullness may occur
depending on a configuration of a driving circuit, substrate,
etc.
[0030] Moreover, even with a semiconductor laser which emits an
infrared light, the VCSEL, for example, has a differential
resistance of a few hundreds of Qs, which is very large relative to
that of an edge type laser due to differences in structure.
Therefore, using the VCSEL results in a CR time constant by a
terminal capacitance of the VCSEL itself; a parasitic capacitance
of a substrate; and a terminal capacitance of a driver. In other
words, the VCSEL itself may not yield a predetermined response
waveform even when mounted to a substrate even though it has a
device characteristic of being able to modulate at high speed or a
cutoff frequency of Ft.
[0031] Furthermore, with the semiconductor laser, there is a large
fluctuation in a light emission strength relative to a current
amount between an LED (light emitting diode) region up to the
threshold current and an LD region on and above the threshold
current. Here, when driving the image forming apparatus by
increasing a current to a light emission strength from a state in
which the bias current of less than or equal to the threshold
current is applied, the light emission strength in the LED region
is low. In other words, this case causes a light emission delay
relative to a drive signal.
[0032] Patent Documents
[0033] Patent document 1: JP4349470B;
[0034] Patent document 2: JP3466599B;
[0035] Patent document 3: JP4-283978A;
[0036] Patent document 4: JP9-83050A; and
[0037] Patent document 5: JP5-328071A.
DISCLOSURE OF THE INVENTION
[0038] In order to solve the above-described problems, an object of
the present invention is to provide a light source drive circuit,
an optical scanning apparatus, and an image forming apparatus that
make it possible to reduce a light emission delay time of an
optical output and improve a response characteristic.
[0039] Another object of the present invention is to provide a
semiconductor laser drive circuit which eliminates a light emission
delay while obtaining a predetermined response waveform for driving
a semiconductor laser.
[0040] According to an embodiment of the present invention, a light
source drive circuit which drives a light source is provided,
including a drive current generating unit which generates a drive
current, the driving current including a predetermined current for
obtaining a predetermined light amount from the light source; and
first and second overshoot currents which are applied to the
predetermined current in synchronization thereto; and a control
unit which sets, to the drive current generating unit, a value of
the first overshoot current to a fixed value, and a value of the
second overshoot current in accordance with a light amount output
from the light source.
[0041] According to another embodiment of the present invention, a
semiconductor laser drive circuit which drives and modulates a
semiconductor laser is provided, including a current source which
supplies a drive current for driving and lighting the semiconductor
laser; a first overshoot current source which supplies a current at
a time of a rise of the drive current; and a second overshoot
current source which supplies a current at the time of the rise of
the drive current, wherein a time in which the first overshoot
current source supplies the current is a time which is shorter than
a time in which the semiconductor laser responds to the drive
current; and a time in which the second overshoot current source
supplies the current is longer than the time in which the first
overshoot current source supplies the current and is shorter than
the time in which the semiconductor laser responds to the drive
current.
[0042] The present invention makes it possible to reduce a light
emission delay time of an optical output and to improve a response
characteristic.
[0043] The present invention also makes it possible to eliminate a
light emission delay while obtaining a predetermined response
waveform for driving the semiconductor laser.
BRIEF DESCRIPTION OF THE DRAWINGS
[0044] Other objects, features, and advantages of the present
invention will become more apparent from the following detailed
descriptions when read in conjunction with the accompanying
drawings, in which:
[0045] FIG. 1 is a view for explaining a light emission delay time
of a light source;
[0046] FIG. 2 is a view for explaining a parasitic capacitance of
the light source;
[0047] FIG. 3 is a view for explaining a drive current which is
supplied to the light source from a light source drive circuit;
[0048] FIG. 4 is a diagram for explaining a schematic configuration
of an image forming apparatus according to a first embodiment;
[0049] FIG. 5 is a diagram for explaining the light source drive
circuit according to the first embodiment;
[0050] FIG. 6 is a diagram for explaining values stored in a memory
and a functional configuration of a CPU;
[0051] FIG. 7 is a diagram for explaining generation of first and
second overshoot generating signals;
[0052] FIG. 8 is a flowchart for explaining a process of an Iov2
value setting unit according to the first embodiment;
[0053] FIG. 9 is a diagram for explaining a second overshoot
current Iov2.
[0054] FIGS. 10A and 10B are diagrams illustrating an example of a
drive current waveform and an optical output waveform when a light
amount of the light source changes;
[0055] FIGS. 11A and 11B are diagrams illustrating an example of
the drive current waveform and the optical output waveform when a
second overshoot period Tov2 is changed;
[0056] FIG. 12 is a diagram illustrating an example of a functional
configuration of an evaluation apparatus connected to the light
source drive circuit;
[0057] FIG. 13 is a flowchart for explaining setting of values of a
first overshoot period Tov1 and a first overshoot current Iov1 by
the evaluation apparatus;
[0058] FIG. 14 is a flowchart for explaining a process of a Tov1
value setting unit in the evaluation apparatus;
[0059] FIGS. 15A to 15D are diagrams for explaining the first
overshoot period Tov1;
[0060] FIG. 16 is a diagram illustrating a current-optical output
characteristic of an LD;
[0061] FIG. 17 is a flowchart for explaining a process of an Iov1
value setting unit in the evaluation apparatus;
[0062] FIGS. 18A to 18C are first diagrams for explaining an
advantageous effect of the first embodiment;
[0063] FIGS. 19A and 19B are second diagrams for explaining the
advantageous effect of the first embodiment;
[0064] FIG. 20 is a diagram for explaining the light source drive
circuit of a second embodiment;
[0065] FIG. 21 is a diagram for explaining a drive current waveform
of the second embodiment;
[0066] FIG. 22 is a diagram for explaining the light source drive
circuit of a third embodiment;
[0067] FIG. 23 is a diagram for explaining a functional
configuration of the CPU of the third embodiment;
[0068] FIG. 24 is a diagram for explaining a drive current waveform
of the third embodiment;
[0069] FIG. 25 is a conceptual diagram illustrating an embodiment
of a semiconductor laser drive circuit according to the present
invention;
[0070] FIG. 26 is a diagram which compares a waveform diagram of a
current generated by the above-mentioned semiconductor laser drive
circuit with a waveform diagram of a current generated by a related
art semiconductor laser drive circuit;
[0071] FIG. 27 is a circuit diagram illustrating one example of the
related art semiconductor laser drive circuit;
[0072] FIG. 28 is a circuit diagram illustrating an embodiment of
the above-mentioned semiconductor laser drive circuit;
[0073] FIG. 29 is a waveform diagram illustrating a relationship
between a waveform of a semiconductor laser drive current generated
by the above-mentioned semiconductor laser drive circuit and an
overshoot current;
[0074] FIG. 30 is a diagram illustrating a change in output when a
weak current is applied to a semiconductor laser;
[0075] FIG. 31 is a diagram illustrating a change in a drop voltage
when the weak current is applied to the semiconductor laser;
[0076] FIG. 32 is a table illustrating an output and a drop voltage
when the weak current is applied to the semiconductor laser;
[0077] FIG. 33 is a diagram illustrating a drive current waveform
and an IL characteristic of the semiconductor laser;
[0078] FIG. 34 is a conceptual diagram illustrating a relationship
between a current flowing within a circuit and a driver supplying a
drive current to the semiconductor laser;
[0079] FIG. 35 is a diagram which illustrates a drive current
waveform of the semiconductor laser when an optical output is
different from the drive current waveform in FIG. 33;
[0080] FIG. 36 is a diagram illustrating the drive current waveform
of the semiconductor laser in the present semiconductor laser drive
circuit;
[0081] FIG. 37 is a diagram illustrating a drive current waveform
of a VCSEL in the present semiconductor laser drive circuit;
and
[0082] FIG. 38 is a central sectional diagram illustrating an
embodiment of the image forming apparatus which includes the
present semiconductor laser drive circuit.
BEST MODE FOR CARRYING OUT THE INVENTION
[0083] Below, embodiments of the present invention are described
using the drawings.
[0084] FIG. 1 is a view for explaining a light emission delay time
of a light source. FIG. 1 shows a drive current waveform supplied
to a light source; and an optical output waveform of the light
source to which the drive current is supplied. In FIG. 1, an output
of the light source is shown in a light amount.
[0085] A light emission delay time t shown in FIG. 1 shows a time
from when supplying of the drive current to the light source is
started to when the light source outputs a predetermined light
amount Po. The predetermined light amount Po is a target light
amount set in advance. The light emission delay time t is a sum of
a parasitic delay time ta and a response delay time tb. The
parasitic delay time ta is a charging time to a parasitic
capacitance produced in parallel with a light source that is
present in a wiring which connects the light source and a circuit,
or an in-package wiring of the light source. Details of the
parasitic delay time ta are described below. As a charge amount and
the charging time increase as the parasitic capacitance increases,
the parasitic delay time ta tends to increase accordingly.
[0086] The response delay time tb is a response time from when the
light source starts light emission with a predetermined current Top
being supplied to the light source to when the predetermined light
amount Po is output. The predetermined current Iop is a current
whose value is set in advance for obtaining the predetermined light
amount Po. The response delay time tb results from a characteristic
of the light source and has an impact due to a differential
resistance, for example. The larger the differential resistance the
more difficult it becomes for a current to flow into the light
source, so that the response delay time tb tends to increase
accordingly.
[0087] In practice, the light emission delay time t, which is up to
when the drive current is supplied to the light source, includes a
wiring delay time, etc., on a circuit board, other than the
parasitic delay time ta and the response relay time tb. However, in
the description of the present specification, the wiring delay
time, etc., is ignored, so that the light emission delay time t is
set to be a sum of the parasitic delay time ta and the response
delay time tb. Moreover, in the description of the present
specification, respective falls of the drive current waveform and
the optical output waveform are shown to be aligned.
[0088] Below the parasitic capacitance is explained with reference
to FIG. 2. FIG. 2 is a view for explaining the parasitic
capacitance of the light source.
[0089] In the present embodiment, the light source is set to be an
LD (laser diode), for example. In the LD shown in FIG. 2, when a
predetermined current Iop is supplied, a predetermined light amount
Po is output. C shown in FIG. 2 is a parasitic capacitance. The
parasitic capacitance C includes a parasitic capacitance which is
produced, when the LD is mounted on a circuit substrate, etc.,
together with a circuit such as an LD driver, etc., in a wiring
which connects the LD and the circuit such as the LD driver, etc.
Moreover, when the LD and the circuit including the LD driver,
etc., are integrated into a package, the parasitic capacitance C
also includes a parasitic capacitance of the package, etc.
[0090] When the predetermined current Iop is supplied to the LD, a
current Ic, which is a portion of the predetermined current Iop, is
supplied to the parasitic capacitance C to charge the parasitic
capacitance C. While the parasitic capacitance C is being charged
by the predetermined current Iop, a current (Iop-Ic), which is a
portion of the predetermined current Top, is supplied to the LD.
Then, when charging of the parasitic capacitance C is completed,
the predetermined current Iop is supplied to the LD. In other
words, during the time of charging the parasitic capacitance C by
the current Ic, only the portion (Iop-Ic) of the predetermined
current Top is supplied, so that it becomes a time during which no
optical output is obtained. This time during which no optical
output is obtained becomes a parasitic delay time.
[0091] Next, a drive current which is supplied to the light source
from a light source drive circuit according to the present
invention is described with reference to FIG. 3. FIG. 3 is a view
for explaining the drive current which is supplied to the light
source from the light source drive circuit. FIG. 3 shows a drive
current waveform supplied to the light source from the light source
drive circuit.
[0092] A drive current Ik which is supplied to the light source
includes the predetermined current Iop for obtaining the
predetermined light amount Po from the light source; and a first
overshoot current Iov1 and a second overshoot current Iov2 that are
superposed to the predetermined current Iop in synchronization with
a rise of the predetermined current Iop. The predetermined current
Iop includes a switching current Ih and a bias current Ib.
[0093] The first overshoot current Iov1 is a current set based on
the parasitic delay time ta. The parasitic delay time ta may be
predetermined from a circuit substrate, etc., on which the light
source is mounted. Thus, a value of the first overshoot current
Iov1 and an applying period (below-called a "first overshoot
period" Tov1) may be arranged to be a fixed value which is set in
alignment with the predetermined parasitic delay time ta. According
to the present invention, the first overshoot period Tov1 is set to
be a period which is shorter than the parasitic delay time ta, or
in other words, the first overshoot period Tov1 is set to be a
period which is shorter than a period from when the predetermined
current Top is supplied to the light source to when the light
source starts light emission. Details of a method of setting the
value of the first overshoot current Iov1 and the first overshoot
period Tov1 are described below.
[0094] The second overshoot current Iov2 is a current whose value
is set based on the response delay time tb. The response delay time
tb varies with a response characteristic of the light source. For
example, the response characteristic may differ when the light
source deteriorates. Moreover, the response characteristic may also
differ depending on variations at a time of manufacturing for each
light source, etc. Then, a value of the second overshoot current
Iov2 is set to be a varying value which is adjusted in accordance
with a response characteristic of the light source. An applying
period of the second overshoot current Iov2 (below-called a "second
overshoot period" Tov2) is a fixed value.
[0095] According to the present invention, the drive current Ik as
shown in FIG. 3 is generated and supplied to the light source,
making it possible to reduce the parasitic delay time to with the
first overshoot current Iov and to shorten the response delay time
tb with the second overshoot current Iov2. Moreover, a value of the
first overshoot current Iov1 is arranged to be a fixed value and
only a value of the second overshoot current Iov2 is adjusted,
facilitating control with respect to reducing of the light emission
delay time t and making it possible to improve the response
characteristic of the light source in a short time.
First Embodiment
[0096] A description is given below with regard to a first
embodiment of the present invention with reference to the drawings.
FIG. 4 is a diagram for explaining a schematic configuration of an
image forming apparatus according to the first embodiment.
[0097] An image forming apparatus 10 according to the present
embodiment includes an optical scanning apparatus 20; a
photosensitive body 30; a write control unit 40; and a clock
generation circuit 50.
[0098] The optical scanning apparatus 20 according to the present
embodiment includes a polygon mirror 21; a scanning lens 22; a
light source drive circuit 100, an LD (laser diode; a semiconductor
laser) which is a light emitting element (light source); and a PD
(photo detector) to be a light receiving element. While the light
source is set to be the LD according to the present embodiment, it
is not limited thereto. The light source may be a semiconductor
laser array (LDA; laser diode array), a VCSEL (vertical cavity
surface emitting laser), etc.
[0099] A laser light which is emitted from the LD is scanned by the
rotating polygon mirror 21 and irradiated onto the photosensitive
body 30, which is a medium to be scanned, via the scanning lens 22.
A laser light which is irradiated becomes a light spot on the
photosensitive body 30, thereby forming an electrostatic latent
image on the photosensitive body 30. Moreover, the polygon mirror
21 irradiates, onto the PD, the laser light each time scanning of
one line is completed. When the laser light is irradiated thereon,
the PD converts the irradiated laser light into an electrical
signal and inputs this electrical signal into a phase
synchronization circuit 41 included in the write control unit 40.
When the electrical signal is input therein, the phase
synchronization circuit 41 generates a pixel clock corresponding to
the following one line. Moreover, a high-frequency clock signal is
input from the clock generation circuit 50 into the phase
synchronization circuit 41, thereby achieving phase synchronization
of the pixel clock.
[0100] The write control unit 40 supplies a reference pulse signal
to the light source drive circuit 100 in accordance with the pixel
clock generated. Moreover, the write control unit 40 supplies a
target light amount setting signal to the light source drive
circuit 100, driving the LD. In this way, an electrostatic latent
image of image data is formed on the photosensitive body 30.
[0101] Below the light source drive circuit 100 of the present
embodiment is explained with reference to FIG. 5. FIG. 5 is a
diagram for explaining a light source drive circuit according to
the first embodiment.
[0102] The light source drive circuit 100 according to the present
embodiment includes a CPU (central processing unit) 110; a memory
120; a DAC (digital to analog converter) 130; an LPF (low-pass
filter) 140; an ADC (analog to digital converter) 150; an LD driver
200; and a resistor R1. The resistor R1 does not have to be
included in the light source drive circuit 100. In this case, the
resistor R1 is provided outside the light source drive circuit
100.
[0103] The light source drive circuit 100 according to the present
embodiment that is connected between the LD and the PD, controls
driving of the LD based on the electrical signal output from the PD
in accordance with a light amount of the PD.
[0104] The CPU 110 controls various operations of the light source
drive circuit 100. The memory 120 stores various values, etc., for
use in the operations of the light source drive circuit 100.
Details of the values stored in the memory 120 and functions of the
CPU 110 are described below.
[0105] The DAC 130 converts a signal output from the CPU 110 into
analog values. The LPF 140 passes a signal of a predetermined band
out of electrical signals output from the PD. The ADC 150 converts
the electrical signal output from the LPF 140 into digital
values.
[0106] The LD driver 200 generates a drive current to be supplied
to the LD based on the reference pulse signal and the target light
amount setting signal and controls a light emission timing of the
LD. The LD driver 200 according to the present embodiment causes
the drive current of the LD to overshoot.
[0107] The light source drive circuit 100 according to the present
embodiment performs control of the drive current Ik with the CPU
110 and the LD driver 200. More specifically, the light source
drive circuit 100 calculates a value of the second overshoot
current Iov2 in accordance with an optical output of the LD. Then,
the light source drive circuit 100 generates the drive current
waveform in which the first overshoot current Iov1, which is set in
advance; and the second overshoot current Iov2 are synchronized to
the reference pulse signal.
[0108] Below, the LD driver 200 according to the present embodiment
is explained. The LD driver 200 according to the present embodiment
includes a switching current source 210; a bias current source 220;
a first overshoot current source 230; a second overshoot current
source 240; and switches 211, 221, 231, and 241.
[0109] The switching current source 210, the bias current source
220, the first overshoot current source 230, and the second
overshoot current source 240 generate a drive current Ik for the
LD. The drive current Ik according to the present embodiment is a
current in which current values output from the respective current
sources are added.
[0110] The switching current source 210 generates a predetermined
switching current Ih based on a lighting control signal from the
CPU 110. The switching current source 210 is connected to the LD
via the switch 211. With the switch 211, which includes a
transistor, etc., for example, on/off is controlled based on a
lighting control signal supplied from the CPU 110. Moreover, a
value of the switching current Ih is set in accordance with
instructions from the CPU 110.
[0111] The bias current source 220 generates a predetermined bias
current Ib based on a bias generating signal from the CPU 110. The
bias current source 220 is connected to the LD via the switch 221.
With the switch 221, which includes a transistor, etc., for
example, on/off is controlled based on the bias generating signal
supplied from the CPU 110. Moreover, a value of the bias current Ib
is set in accordance with instructions from the CPU 110.
[0112] The first overshoot current source 230 generates the first
overshoot current Iov1 as a first auxiliary drive current as
auxiliary to the switching current Ih at a rise of the lighting
control signal. The first overshoot current source 230 is connected
to the LD via the switch 231. With the switch 231, which includes a
transistor, etc., for example, on/off is controlled based on a
first overshoot generating signal supplied from the CPU 110.
According to the present embodiment, a period during which the
first overshoot generating signal is on is the first overshoot
period Tov1. More specifically, the switch 231 according to the
present embodiment is set to be on from a rise of the lighting
control signal to the first overshoot period Tov1.
[0113] The second overshoot current source 240 generates the second
overshoot current Iov2 as a second auxiliary drive current as
auxiliary to the switching current Ih at a rise of the lighting
control signal. The second overshoot current source 240 is
connected to the LD via the switch 241. With the switch 241, which
includes a transistor, etc., for example, on/off is controlled
based on a second overshoot generating signal supplied from the CPU
110. According to the present embodiment, a period during which the
second overshoot generating signal is on is the second overshoot
period Tov2. More specifically, the switch 241 according to the
present embodiment is set to be on from a rise of the lighting
control signal to the second overshoot period Tov2.
[0114] Below functions of the CPU 110 according to the present
embodiment and the values stored in the memory 120 according to the
present embodiment are explained with reference to FIG. 6. FIG. 6
is a diagram for explaining the values stored in the memory and a
functional configuration of the CPU.
[0115] The CPU 110 according to the present embodiment includes a
current control unit 111; a pulse generating unit 112; and an Iov2
value setting unit 113.
[0116] The memory 120 includes a current value storage unit 121; a
delay time storage unit 122; and a lighting pattern storage unit
123. Setting values in various current sources included in the
light source drive circuit 100 are stored in the current value
storage unit 121. More specifically, values of the switching
current Ih; the bias current Ib; and the first overshoot current
Iov1 are set in the current value storage unit 121.
[0117] A delay time for determining the first overshoot period Tov1
and the second overshoot period Tov2 is stored in the delay time
storage unit 122. A lighting pattern signal for the LD that is to
be used when calculating the second overshoot current Iov2 by the
below-described Iov2 value setting unit 113 is stored in the
lighting pattern storage unit 123.
[0118] In the CPU 110, the current control unit 111 obtains setting
values of various current sources that are stored in the current
value storage unit 122 and causes currents corresponding to the
setting values to be output via the DAC 130 to the various current
sources.
[0119] The pulse generating unit 112 is a signal generating unit
which generates a first overshoot generating signal and a second
overshoot generating signal based on the reference signal and the
delay time stored in the delay time storage unit 122. Moreover, the
pulse generating unit 112 may generate a bias generating signal and
a lighting pattern signal. The lighting pattern signal according to
the present embodiment is a signal which is supplied to the switch
211 only when calculating the second overshoot current Iov2 by the
Iov2 value setting unit 113. With respect to the switch 211, when
the image forming apparatus 10 performs an image forming operation,
on/off thereof is controlled by a lighting control signal based on
image data supplied from the write control unit 40.
[0120] The Iov2 value setting unit 113 calculates and sets the
second overshoot current Iov2 based on an output of the PD. The
Iov2 value setting unit 113 according to the present embodiment
includes a current value selecting unit 114; an integrated light
amount obtaining unit 115; and a determining unit 116. Details of
the Iov2 value setting unit 113 are described below.
[0121] Next, with reference to FIG. 7, generation of the first and
second overshoot generating signals by the pulse generating unit
112 according to the present embodiment is explained. FIG. 7 is a
diagram for explaining the generation of the first and second
overshoot generating signals.
[0122] The pulse generating unit 112 according to the present
embodiment obtains, for example, a delay time t1 which corresponds
to the first overshoot period Tov1 and a delay time t2 which
corresponds to the second overshoot period Tov2 from a delay time
storage unit 122. Then, the pulse generating unit 112 generates a
pulse signal S1, which is the reference pulse signal delayed by the
delay time of t1 in seconds; and a pulse signal S2, which is the
reference pulse signal delayed by the delay time of t2 in seconds.
For example, when the reference pulse signal is at a high level and
the pulse signal S1 is at a low level, the pulse generating unit
112 generates a first overshoot signal which is on (which is at a
high level) for the first overshoot period Tov1. Moreover, when the
reference pulse signal is at the high level and the pulse signal S2
is at the low level, the pulse generating unit 112 generates a
second overshoot generating signal which is on (which is at a high
level) for the second overshoot period Tov2.
[0123] While the delay times t1 and t2 are stored in the memory 120
for the pulse generating unit 112 according to the present
embodiment, it is not limited thereto. The delay times t1 and t2
according to the present embodiment may be obtained in a method
different from those described in the above. For example, the pulse
generating unit 112 according to the present embodiment may simply
generate pulse signals S1 and S2 by an inverter sequence or a
buffer sequence. Moreover, according to the present embodiment, a
reference pulse signal may be delayed by a low pass filter
including a resistor, a capacitor, etc., after which
waveform-rectified signals may be used as the pulse signals S1 and
S2. In either case, changing a delay amount may be easily realized
by changing the number of steps or a filter coefficient.
[0124] Next, with reference to FIG. 8, setting of the second
overshoot current Iov2 by the Iov2 value setting unit 113 according
to the present embodiment is explained. FIG. 8 is a flowchart for
explaining a process of the Iov2 value setting unit according to
the first embodiment.
[0125] When an integrated light amount ratio of an output waveform
of the PD when a light is caused to be emitted from the LD based on
a lighting pattern signal falls within a predetermined range, the
Iov2 value setting unit 113 according to the present embodiment
sets a current value as a value of the second overshoot current
Iov2. The integrated light amount ratio is a value indicating a
proportion of an integrated light amount of an output waveform of
the PD relative to an integrated light amount corresponding to one
period of the lighting pattern signal.
[0126] First, the CPU 110 initially accepts an instruction for
setting the second overshoot current Iov2 (step S801). According to
the present embodiment, the setting instruction is accepted when
starting supplying of a drive current Ik to the LD again after
supplying of the drive current Ik from the light source drive
circuit 100 to the LD is stopped. This setting instruction may be
provided from a main CPU (not shown) which controls the whole
operation of the image forming apparatus 10 to the CPU 110, for
example. More specifically, according to the present embodiment,
the setting instruction is accepted, for example, when the image
forming apparatus 10 is activated from a sleep mode, or when a door
provided in a housing of the image forming apparatus 10 is opened
and then closed, etc.
[0127] Next, the Iov2 value setting unit 113 reads a value of a
predetermined current Iop from the current value storage unit 121
(step S802). Next, the Iov2 value setting unit 113 reads a lighting
pattern signal from the lighting pattern signal storage unit 123
(step S803). The lighting pattern signal according to the present
embodiment is a signal which is generated in advance such as to
turn on the LD corresponding to one pixel and turn off the LD
corresponding to one pixel. Next, the Iov2 value setting unit 113
reads the delay time t1, or, in other words, the first overshoot
period Tov1 from the delay time storage unit 122 (step S804). Next,
the Iov2 value setting unit 113 reads a value of the first
overshoot current Iov1 from the current value storage unit 121
(step S805). Next, the Iov2 value setting unit 113 read the delay
time t2, or, in other words, the second overshoot period Tov2 from
the delay time storage unit 122 (step S806).
[0128] Next, the Iov2 value setting unit 113 outputs, to the DAC
130, a current value selecting signal for selecting a current value
by the current value selecting unit 114 (step S807). The current
value selecting unit 114 selects in an ascending order of a current
value out of current values which can be output in the DAC 130.
[0129] When the current value selecting signal is received from the
CPU 110, the DAC 130 converts the selected current value to an
analog value to output the converted analog value to the second
overshoot current source 240. The second overshoot current source
240 supplies a selected current value to the LD. Then, the pulse
generating unit 112 supplies a second overshoot generating signal
which is synchronized with a rise of the lighting pattern signal.
The second overshoot generating signal turns on the switch 241 only
for the second overshoot period Tov2 read in step S806.
[0130] Next, the Iov2 value setting value 113 calculates an
integrated light amount ratio of an output waveform of the PD by
the integrated light amount calculating unit 115 (step S808). Next,
the Iov2 value setting unit 113 determines whether the calculated
integrated light amount ratio is within a predetermined range by
the determining unit 116 (step S809). When the integrated light
amount ratio is within a predetermined range in step S809, the Iov2
value setting unit 113 sets the then selected current value as a
value of the second overshoot current Iov2. In step S809, when the
integrated light amount ratio is not within the predetermined
range, the Iov2 value setting unit 113 returns to step S807, and
selects the second largest current value.
[0131] Below, with reference to FIG. 9, the second overshoot
current Iov2 is further explained. FIG. 9 is a diagram for
explaining the second overshoot current Iov2.
[0132] FIG. 9 shows a case in which a predetermined range is set to
.+-.5% with 50% as a center value, for example, in order to make
the output waveform of the PD close to an ideal waveform without a
light emission delay. The predetermined range, which is a preset
value, may be arbitrarily set.
[0133] FIG. 9, in (1), shows an output waveform of the PD in case a
current value has not been selected by the current value selecting
unit 114 and the drive current Ik becomes the predetermined current
Iop which is synchronized with a lighting pattern signal. In this
case, an integrated light amount ratio of the output waveform of
the PD in a time period H corresponding to one period of the
lighting pattern signal becomes less than 45%.
[0134] FIG. 9, in (2), shows an output waveform of the PD when a
smallest current value Iv' is selected by the current value
selecting unit 114. Then, the drive current Ik is overshot by an
amount corresponding to a current value Iv' from a rise to the end
of the second overshoot period Tov2. In this case as well, the
integrated light amount ratio of the output waveform of the PD in
the time period H corresponding to one period of the lighting
pattern signal becomes less than 45%.
[0135] Next, FIG. 9 in (3) shows an output waveform of a PD when a
current value Iv larger than the current value Iv' is selected by
the current value selecting unit 114. Then, the drive current Ik is
overshot by an amount corresponding to a current value Iv' from a
rise to the second overshoot period Tov2. In this case, an
integrated light amount ratio of the output waveform of the PD in a
time period H corresponding to one period of the lighting pattern
signal becomes between 50% to 55%. Therefore, the Iov2 value
setting unit 113 sets the current value Iv as a value of the second
overshoot current Iov2.
[0136] As described above, according to the present embodiment, the
value of the first overshoot current Iov1 and the first overshoot
period Tov1, which are stored in advance in the memory 120, are
used without changing them even when a predetermined light amount
Po to be targeted and a predetermined current Iop change. As for
the value of the second overshoot current Iov2, it is adjusted and
used every time an amount of light emitted from the light source
changes. Thus, according to the present embodiment, the first
overshoot current Iov1 may be set only once, so that a value may be
adjusted only for the second overshoot current Iov2. Therefore, the
present embodiment makes it possible to reduce an adjustment time
and to reduce the circuit scale.
[0137] FIGS. 10A and 10B are diagrams illustrating an example of a
drive current waveform and an optical output waveform when a light
amount of the light source changes. FIG. 10A shows the drive
current waveform and the optical output waveform at a time of a low
light amount (Pa), while FIG. 10B shows a drive current waveform
and an optical output waveform at a time of a high light amount
(Pb). Moreover, the relative magnitude of a value of a
predetermined current Iopa in FIG. 10A and a value of a
predetermined current Iopb in FIG. 10B becomes such that
Iopa<Iopb. This is because a light amount changes in accordance
with a magnitude of the value of the predetermined current Iop.
[0138] Focusing on the second overshoot current Iov2 in FIGS. 10A
and 10B, it is seen that the current amount changes with a change
in the light amount. The current amount is determined by
multiplying a value of the second overshoot current Iov2 and the
second overshoot time Tov2.
[0139] In the examples in FIGS. 10A and 10B, a value of the second
overshoot current Iov2 increases with an increase in the light
amount. Thus, the relative magnitude of the value of the second
overshoot current Iov2a in FIG. 10A and the value of the second
overshoot current Iov2b in FIG. 10B becomes such that
Iov2a<Iov2b. On the other hand, the value of the first overshoot
current Iov1 is primarily determined based on the whole system such
as a wiring of a board, a packaging of a light source, etc., so
that the current value does not change even when the magnitude of
the predetermined light amount Po and the magnitude of the
predetermined current Iop are changed, yielding a relationship of
Iov1a=Iov1b. In this way, the present embodiment makes it possible
to obtain a stable optical output waveform with a simple setting of
adjusting only a value of the second overshoot current Iov2 when
the light amount is changed.
[0140] While the second overshoot period Tov2 is set to be a fixed
value which corresponds to a delay time t2 stored in the memory in
the present embodiment, a value of the delay time t2 may be changed
to change the second overshoot period Tov2.
[0141] FIGS. 11A and 11B are diagrams illustrating examples of the
drive current waveform and the optical output waveform when the
second overshoot period Tov2 is changed. FIG. 11A shows a case in
which the second overshoot period Tov2 is short, while FIG. 11B
shows a case in which the second overshoot period Tov2 is long.
[0142] Comparing optical output waveforms in FIGS. 11A and 11B, the
response delay time ta2 in FIG. 11A is shorter than the response
delay time ta1 in FIG. 11B (ta1<ta2), an optical output waveform
in FIG. 11A has a shorter rise time, so that it rises more steeply.
In other words, according to the present embodiment, the second
overshoot period Tov2 may be set longer to increase dullness in a
rise of the optical output waveform and it may be set shorter to
decrease the dullness in the rise thereof. In this way, the present
embodiment makes it possible to adjust a response delay time ta of
an optical output waveform with the second overshoot period Tov2
and makes it possible to set an arbitrary response delay time ta in
accordance with a usage condition, environment, etc., of the image
forming apparatus 10.
[0143] Next, setting of the value of the first overshoot current
Iov1 and the first overshoot period Tov1 according to the present
embodiment is explained. According to the present embodiment, the
value of the first overshoot current Iov1 and the first overshoot
period Tov1 are fixed values to be preset.
[0144] The value of the first overshoot current Iov1 and the first
overshoot period Tov1 according to the present embodiment may be
calculated by an evaluation apparatus, etc., which are connected to
the light source drive circuit 100 in a manufacturing process of
the image forming apparatus 10, for example, and the calculated
value may be stored in the memory 120.
[0145] Below, calculation of the value of the first overshoot
current Iov1 and the first overshoot period Tov1 by the evaluation
apparatus connected to the light source drive circuit 100 is
explained.
[0146] FIG. 12 is a diagram illustrating an example of a functional
configuration of the evaluation apparatus connected to the light
source drive circuit. According to the present embodiment, the
evaluation apparatus 300 may be connected between the output side
of the ADC 150 and the input side of the CPU 110, for example.
[0147] The evaluation apparatus 300 is a computer which includes an
arithmetic processing unit and a storage unit, for example. The
evaluation apparatus 300 includes an instruction accepting unit
310, a Tov1 value setting unit 320, an Iov1 value setting unit 330,
and a threshold storage unit 340.
[0148] The instruction accepting unit 310 accepts setting
instructions for the first overshoot period Tov1 and the first
overshoot current Iov1. According to the present embodiment, it may
be assumed that, for example, the evaluation apparatus 300 has
accepted the setting instructions when it is connected to the light
source drive circuit 100. Moreover, when the setting instructions
are input by an evaluator who uses the evaluation apparatus 300,
for example, the evaluation apparatus 300 may accept it.
[0149] The Tov1 value setting unit 320, which includes a pulse
selecting unit 321, an integrated light amount obtaining unit 322,
and a Tov1 determining unit 323, calculates and sets the first
overshoot period Tov1.
[0150] The Iov1 value setting unit 330, which includes a current
value selecting unit 331, an integrated light amount obtaining unit
332, and an Iov1 determining unit 333, calculates and sets a value
of the first overshoot current Iov1.
[0151] In the threshold storage unit 340 are stored a Tov1
threshold 341 which is used in the Tov1 value setting unit 320 and
an Iov1 threshold 342 which is used in the Iov1 value setting unit
330.
[0152] The Tov1 threshold 341 is a threshold for determining
whether light emission of the LD has been detected. The Iov1
threshold 342 is a threshold for determining whether a light amount
of the LD has reached a predetermined light amount. Details of
processes of the Tov1 value setting unit 320 and the Iov1 value
setting unit 330 will be described below.
[0153] Below, setting of the value of the first overshoot current
Iov1 and the first overshoot period Tov1 by the evaluation
apparatus 300 according to the present embodiment is explained with
reference to FIG. 13. FIG. 13 is a flowchart for explaining setting
of the value of the first overshoot current Iov1 and the first
overshoot period Tov1 by the evaluation apparatus.
[0154] When setting instructions are accepted by the instruction
accepting unit 310 (step S131), first the first overshoot period
Tov1 is set by the Tov1 value setting unit 320 (step S132). Then,
the evaluation apparatus 300 sets a value of the first overshoot
current Iov1 by the Iov1 value setting unit 330 (step S133).
[0155] In this way, in the present embodiment, first the first
overshoot period Tov1 is set and then the value of the first
overshoot current Iov1 is set.
[0156] Next, setting of the first overshoot period Tov1 according
to the Tov1 value setting unit 320 is explained with reference to
FIG. 14. FIG. 14 is a flowchart for explaining a process of a Tov1
value setting unit in the evaluation apparatus.
[0157] According to the present embodiment, a time from when
supplying to the LD of a current larger than the predetermined
current Iop is started to when light emission of the LD is detected
is set as the first overshoot period Tov1.
[0158] According to the present embodiment, it is preferable to set
the first overshoot period Tov1 to a shorter period. The first
overshoot period Tov1 may be shortened to make it possible to
charge a parasitic capacitance C in a short time and to reduce a
parasitic delay time.
[0159] According to the present embodiment, a current larger than
the predetermined current Iop supplied to the LD when the first
overshoot period Tov1 is set is called a "Tov1 setting current" Is.
The Tov1 setting current Is is stored within a storage apparatus
(not shown) included by the evaluation apparatus 300, for example.
Moreover, the Tov1 setting current Is may be stored in the memory
120 of the light source drive circuit 100, for example.
[0160] When the instruction accepting unit 310 in the evaluation
apparatus 300 according to the present embodiment accepts setting
instructions, the Tov1 value setting unit 320 reads a Tov1 setting
current Is (Step S141). Next, the evaluation apparatus 300 causes
the Tov1 setting current Is to be output to the first overshoot
current source 230 via the CPU 110 and the DAC 130 (step S142).
Then, the bias current Ib and the switching current Ih are turned
off, so that a current supplied to the LD is to be only a Tov1
setting current Is which is output from the first overshoot current
source 230.
[0161] Next, the Tov1 value setting unit 320 outputs an instruction
signal for selecting a pulse signal to the CPU 110 by the pulse
selecting unit 321 (step S143). Then, the pulse selecting unit 321
causes selection of a pulse signal in an ascending order of a pulse
width of the pulse signal to the CPU 110 in order to determine
whether light emission of the LD has been detected.
[0162] When the pulse signal is selected in the CPU 110, the pulse
signal selected is supplied to the switch 231 as the first
overshoot generating signal via the LD driver 200. The switch 231
according to the present embodiment is to be turned on during a
period in which the first overshoot generating signal is supplied.
Therefore, when the switch 231 is turned on, the Tov1 setting
current is supplied to the LD.
[0163] Next, the integrated light amount obtaining unit 322
obtains, as a digital value via the ADC 150, an integrated light
amount obtained by integrating a waveform of an electrical signal
output from the PD based on a light amount of the LD by the LPF 140
(step S143).
[0164] Next, the Tov1 determining unit 323 refers to the Tov1
threshold 341 stored in the threshold storage unit 340 and
determines whether the integrated light amount is greater than or
equal to the Tov1 threshold (step S144). In step S144, when the
integrated light amount is greater than or equal to the Tov1
threshold, the Tov1 determining unit 323 determines that light
emission of the LD has been detected, and sets a pulse width of a
pulse signal selected just one previous to the pulse signal
selected in step S142 is set as the first overshoot period Tov1.
Here, the Tov1 determining unit 323 saves the first overshoot
period Tov1 in the memory 120 via the CPU 110 (step S145).
[0165] When the integrated light amount is less than the Tov1
threshold in step S144, the Tov1 value setting unit 320 returns to
step S142 and selects the next narrowest pulse signal.
[0166] Below, with reference to FIGS. 15A to 15D, the first
overshoot period Tov1 is further explained. FIGS. 15A to 15D are
diagrams for explaining the first overshoot period Tov1. FIGS. 15A
to 15D show output waveforms of the PD when the Tov1 setting
current Is is provided to the light source in synchronization with
pulse signals with different pulse widths.
[0167] FIGS. 15A to 15D show output waveforms of the PD when pulse
signals are selected in an ascending order of the pulse width
thereof. The electrical signal output from the PD is converted to a
voltage value by a resistor R1 so as to be supplied to the LPF
140.
[0168] FIG. 15A shows an output waveform of the PD when a pulse
signal P10 which is first selected in the CPU 110 is supplied to
the LD. The pulse signal P10 is a signal with a narrowest pulse
width out of pulse signals which can be selected by the CPU 110 and
the pulse width is set to be P1. Then, an output of the PD is not
manifested, so that the integrated light amount is 0. Therefore, it
is seen that no light emission occurs in the LD.
[0169] FIG. 15B shows an output waveform of the PD when a pulse
signal P20 with a pulse width P2 is selected. Then, an output of
the PD is slightly manifested, so that the integrated light amount
is S1.
[0170] FIG. 15C shows an output waveform of the PD when a pulse
signal P30 with a pulse width P3 is selected. Then, an output of
the PD is slightly manifested, so that the integrated light amount
is S2.
[0171] According to the present embodiment, it is assumed that
light emission of the LD is detected when the pulse width of the
pulse signal is gradually increased in this way, so that the
integrated light amount of the output waveform of the PD becomes
greater than or equal to a Tov1 threshold.
[0172] The Tov1 threshold is a proportion of an integrated light
amount of an output waveform of the PD corresponding to light
emission of the LD by the pulse signal relative to the integrated
light amount (below-called the "total integrated light amount") of
the output waveform of the PD when the LD outputs a predetermined
light amount Po. According to the present embodiment, the Tov
threshold may be set to around a few % of the total integrated
light amount. According to the present embodiment, the Tov1
threshold is set to 5%, for example. In this case, when the
integrated light amount of the output waveform of the PD reaches at
least 5% of the total integrated light amount, it is determined
that light emission occurs in the LD.
[0173] In FIG. 15A to 15D, assuming that the integrated light
amount S1 is around 3% of the total integrated light amount; and
the integrated light amount S2 is around 10% of the total
integrated light amount, the Tov1 value setting unit 320 sets the
pulse width P2 to the first overshoot period Tov1.
[0174] Now, a Tov1 setting current Is which is supplied to the LD
when setting the first overshoot period Tov1 is explained with
reference to FIG. 16.
[0175] FIG. 16 is a diagram illustrating a current-optical output
characteristic of the LD. In FIG. 16, a region S1 is a linear
region in the current-optical output characteristic, while a region
S2 is a non-linear region in the current-optical output
characteristic.
[0176] A value of the Tov1 setting current Is according to the
present embodiment may be a value which is larger than a
predetermined current Iop within the linear region S1 shown in FIG.
16 and less than or equal to a current Imax which corresponds to a
maximum optical output Pmax in the non-linear region S2.
[0177] It is preferable in the present embodiment that light
emission of the LD may be detected in a shorter time the larger the
Tov1 setting current Is in the above-mentioned range and the first
overshoot period Tov1 may be set in a short time. Thus, for
example, in the present embodiment, the Tov1 setting current Is may
be set to be the current Imax. More specifically, for example, in
the present embodiment, a value of the Tov1 setting current Is may
be determined such that the first overshoot period Tov1 becomes
around 1 nano second.
[0178] In this way, according to the present embodiment, a current
which is larger than a predetermined current Iop for obtaining a
predetermined light amount Po which is set as a target light amount
from the LD when setting the first overshoot period Tov1 is
supplied to the LD, causing light emission of the LD to be detected
in a short time. According to the present embodiment, even when a
value of the Tov1 setting current Is is set to be a value exceeding
a rated current of the LD, the Tov1 setting current Is is supplied
to the LD during a period over which the LD does not break
down.
[0179] Next, setting of the first overshoot current Iov1 by the
Iov1 value setting unit 330 of the evaluation apparatus 300 is
explained with reference to FIG. 17. FIG. 17 is a flowchart for
explaining a process of the Iov1 value setting unit in the
evaluation apparatus.
[0180] In the evaluation apparatus 300 of the present embodiment,
when the instruction accepting unit 310 accepts a setting
instruction (step S1701), the Iov1 value setting unit 330 reads the
predetermined current Iop (step S1702). The Iov1 value setting unit
330 reads a lighting pattern signal from the memory 120 via the CPU
110 (step S1703). Next, the Iov1 value setting unit 330 reads the
first overshoot period Tov1 (step S1704).
[0181] When the first overshoot period Tov1 is read, the Iov1 value
setting unit 330 outputs a current value selecting signal for
selecting a current value by the current value selecting unit 331
to the DAC 130 via the CPU 110 (step S1705). The current value
selecting unit 331 selects a current value in an ascending order of
the value thereof out of current values which can be output in the
DAC 130.
[0182] When the current value selecting signal is received via the
CPU 110, the DAC 130 converts the selected current value to an
analog value to output the analog value to the first overshoot
current source 230. The first overshoot current source 230 supplies
the selected current value to the LD. Here, the pulse generating
unit 112 supplies a first overshoot generating signal which is
synchronized with a rise of the lighting pattern signal to the
switch 231. The first overshoot generating signal turns on the
switch 231 only for the first overshoot period Tov1 read in step
S1704.
[0183] Next, the Iov1 value setting unit 330 obtains, by the
integrated light amount obtaining unit 332, an integrated light
amount of an output waveform of the PD that is output from the ADC
150 (step S1706). Next, the Iov1 value setting unit 330, by the
Iov1 determining unit 333, refers to the threshold storage unit 340
and determines whether the obtained integrated light amount is
greater than or equal to the Iov1 threshold (step S1707).
[0184] In step S1707, when the integrated light amount is greater
than or equal to the Iov1 threshold, the Iov1 value setting unit
330 sets the current value selected then to be a value of the
second overshoot current Iov2 and saves the set current value in
the memory 120 via the CPU 110 (step S1708). In step S1707, when
the integrated light amount is not greater than or equal to the
Iov1 threshold, the Iov1 value setting unit 330 returns to step
S1705, and selects the next largest current value.
[0185] As described above, according to the present embodiment, the
evaluation apparatus 300 which is connected to the light source
drive circuit 100 sets the first overshoot period Tov1 and a value
of the first overshoot current Iov1 as fixed values to the light
source drive circuit 100.
[0186] While the evaluation apparatus 300 is arranged to be
connected to the outside of the light source drive circuit 100, it
is not limited thereto. For example, in the light source drive
circuit 100 according to the present embodiment, the CPU 110 may
include functions included in the evaluation apparatus 300, for
example. In this case, the light source drive circuit 100 may save
the first overshoot period Tov1 and the value of the first
overshoot current Iov1 in the memory 120 without using the
evaluation apparatus 300.
[0187] FIGS. 18A to 18C are first diagrams for explaining
advantageous effects of the first embodiment.
[0188] FIG. 18A shows a drive current waveform and an optical
output waveform when only a predetermined current Iop is applied to
the LD. FIG. 18B shows a drive current waveform and an optical
output waveform when the predetermined current Top and the first
overshoot current Iov1 are applied to the LD. FIG. 18C shows a
drive current waveform and an optical output waveform according to
the present embodiment.
[0189] In FIG. 18A, the drive current waveform is a rectangular
wave, while there is dullness in a rise of the optical output
waveform. Moreover, in FIG. 18A, a light emission delay time T1
from when supplying of the drive current to the light source is
started to when the light source outputs a predetermined light
amount Po occurs.
[0190] In FIG. 18B, the first overshoot current Iov1 which is
applied in synchronization with the predetermined current Iop
charges a parasitic capacitance in advance, so that the light
emission delay time T2 of the optical output waveform is shorter
than the light emission delay time T1. However, for the rise of the
optical output waveform, there is dullness which is equivalent to
that as in FIG. 18A.
[0191] In the drive current waveform of the present embodiment
shown in FIG. 18C, the first overshoot current Iov1 and the second
overshoot current Iov2 are applied in synchronization with the
predetermined current Iop. According to the present embodiment, the
parasitic capacitance is charged by the first overshoot current
Iov1 and dullness in the rise of the optical output waveform is
improved by the second overshoot current Iov2. Thus, the present
embodiment makes it possible to obtain an optical output waveform
which is close to a rectangular wave. Moreover, it is seen that the
light emission delay time T3 is shorter than the light emission
delay times T1 and T2.
[0192] FIGS. 19A and 19B are second diagrams for explaining the
advantageous effects of the first embodiment.
[0193] FIG. 19A is a diagram showing a waveform of a drive current
including a overshoot current in a related art and a corresponding
optical output waveform. FIG. 19B is a diagram showing a drive
current waveform and an optical output waveform according to the
present embodiment.
[0194] Compared to the optical output waveform shown in FIG. 18A,
the optical output waveform shown in FIG. 19A has dullness in the
rise thereof reduced, so that the response delay time tb is
shortened. However, in the optical output waveform shown in FIG.
19A, there remains a large parasitic delay time ta. Thus, an
optical output waveform pulse is narrowed, making it not possible
to compensate for a light amount.
[0195] On the other hand, as shown in FIG. 19B, according to the
present embodiment, the parasitic capacitance may be charged at
high speed by the first overshoot current Iov1 to shorten the
parasitic delay time ta. Moreover, according to the present
embodiment, the response delay time tb may also be reduced due to
an effect of the second overshoot current Iov2. Thus, the present
embodiment makes it possible to reduce the final light emission
delay time t and compensate for the required light amount.
[0196] Now, in the related art, an LD with a large package, in
particular, has various varying factors for the response
characteristics such as a resistance component increasing depending
on a wavelength band or an increase in the parasitic capacitance.
For example, compared to a 780 nm band infrared semiconductor laser
in a wavelength, a 650 nm band red light semiconductor laser
generally has a large differential resistance; thus, it is not
always the case that an optical output response is obtained at high
speed, so that dullness of the waveform may occur. Moreover, even
with the infrared semiconductor laser, a VCSEL (vertical cavity
surface emitting laser), etc., has a differential resistance of
around a few hundred Os due to a structural difference, which
differential resistance is very large compared to that of an edge
type laser. Thus, a CR time constant results from a terminal
capacitance of a VCSEL itself; a parasitic capacitance of a
substrate having mounted thereon the VCSEL; a terminal capacitance
of a driver, etc., and a differential resistance of the VCSEL.
Therefore, there is a problem that, even when the VCSEL itself has
a cutoff frequency Ft or a device property of being able to
modulate at high speed, when it is mounted on the substrate, the
desired high-speed optical output response is not obtained, causing
an increased light emission delay time.
[0197] The present embodiment corrects for an optical output
waveform in accordance with a parasitic waveform, a differential
resistance, etc., regardless of a type of light source, making it
possible to reduce the light emission delay time and obtain the
light output waveform close to a current waveform of the
predetermined current Iop. In other words, according to the present
embodiment, even when there are multiple light sources such as the
VCSEL and the light sources have a large differential resistance,
the first overshoot current Iov1 and the second overshoot current
Iov2 that are optimum for each light source may be set. Thus, the
present embodiment makes it possible to reduce light emission
variations among the light sources and to reduce color drift and
density variations of images in the image forming apparatus 10, for
example. Moreover, according to the present embodiment, the second
overshoot current Iov2 is adjusted in accordance with a light
amount and a magnitude of the predetermined current Iop, making it
possible to obtain a desired optical output waveform even when the
light amount of the light source is changed.
[0198] In this way, the present embodiment makes it possible to
reduce a light emission delay time of an optical output and to
improve a response characteristic.
Second Embodiment
[0199] Below a second embodiment of the present invention is
explained with reference to the drawings. The second embodiment of
the present invention is different from the first embodiment in
that a bias current Ib is not included in the drive current Ik.
Thus, in the explanation of the second embodiment below, only
differences from the first embodiment are explained and the same
letters/numerals used in the explanations of the first embodiment
are given to those having the same functional configuration as the
first embodiment, so that the explanations thereof are omitted.
[0200] FIG. 20 is a diagram for explaining a light source drive
circuit according to the second embodiment. A light source drive
circuit 100A according to the present embodiment has an LD driver
200A. The LD driver 200A includes a switching current source 210; a
first overshoot current source 230; a second overshoot current
source 240; and switches 211, 231, and 241. The LD driver 200A
according to the present embodiment includes output currents from
three current sources based on an analog value supplied from the
DAC 130 and generates a drive current Ik. More specifically, the LD
driver 200A according to the present embodiment generates the drive
current Ik to which the first overshoot current Iov1 and the second
overshoot current Iov2 are applied in synchronization with a rise
of a switching current Ih and supplies the generated drive current
Ik to the LD.
[0201] FIG. 21 is a diagram illustrating a drive current waveform
according to the second embodiment.
[0202] In the drive current Ik according to the present embodiment,
the bias current Ib is not used, so that a value of the switching
current Ih and a value of the predetermined current Ip become
equal.
Third Embodiment
[0203] With reference to the drawings, a third embodiment of the
present invention is explained with reference to the drawings. The
third embodiment of the present invention is different from the
first embodiment in that the drive current Ik includes an
undershoot current. Thus, for the third embodiment of the present
invention, only differences from the first embodiment are explained
and the same letters/numerals used in the explanations of the first
embodiment are given to those having the same functional
configuration as the first embodiment, so that the explanations
thereof are omitted.
[0204] FIG. 22 is a diagram for explaining a light source drive
circuit according to the third embodiment.
[0205] A light source drive circuit 100B according to the present
embodiment includes an LD driver 200B. The light source drive
circuit 100B includes a CPU 110A and the LD driver 200B. The LD
driver 200B includes a switching current source 210; a bias current
source 220; a first overshoot current source 230; a second
overshoot current source 240; an undershoot current source 250; and
switches 211, 221, 231, 241, 251. A connection of the undershoot
current source 250 with the LD is controlled by on/off of the
switch 251. When the switch 251 is turned on, the undershoot
current source 250 supplies an undershoot current Iud to the LD in
synchronization with a fall of a switching current Ih.
[0206] On/off of the switch 251 is controlled by an undershoot
generating signal supplied from the CPU 110. More specifically, the
switch 251 is turned on during a period (below called an
"undershoot period" Tud) in which the undershoot generating signal
is at a high level.
[0207] FIG. 23 is a diagram for explaining a functional
configuration of a CPU according to the third embodiment. The CPU
110A according to the present embodiment includes an Iud value
setting unit 117 in addition to the respective units included in
the CPU 110 according to the first embodiment.
[0208] The Iud value setting unit 117 according to the present
embodiment refers to the memory 120 and calculates a sum of a
current amount of a first overshoot current Iov1 and a current
amount of a second overshoot current Iov2. The current amount is
defined by a current value.times.on time. More specifically, for
example, the current amount of the first overshoot current Iov1 is
a product of the value of the first overshoot current Iov1 and the
second overshoot period Tov1. The current amount of the second
overshoot current Iov2 is a product of the value of the second
overshoot current Iov2 and the value of the second overshoot period
Tov2.
[0209] Then, the Iud value setting unit 117 sets the value of the
undershoot current Iud and the undershoot period Tud such that a
sum of the two current amounts and the current amount of the
undershoot current Iud become equal.
[0210] For the undershoot current Iud, roles are played of
correcting for dullness in a fall of the optical output waveform
and discharging of a parasitic capacitance charged by the first
overshoot current Iov1, etc. Therefore, the current amount of the
undershoot current needed for improving the optical output waveform
becomes almost equal to the current amount of the second overshoot
current Iov2 and the current amount of the undershoot current
needed for discharging the parasitic capacitance becomes almost
equal to the current amount of the first overshoot current
Iov1.
[0211] Thus, according to the present embodiment, the current
amount of the undershoot current Iud may be set such that it
becomes equal to the sum of the two current amounts to further
improve a response characteristic of the optical output
waveform.
[0212] Moreover, according to the present embodiment, the
undershoot current Iud is set using the first and the second
overshoot currents Iov1 and Iov2, so that complex operations, etc.,
are not needed, making it possible to speedily set the undershoot
current Iud. Moreover, according to the present embodiment, the
undershoot period Tud may be set equal to the second overshoot
period Tov2. In this case, a response at the time of a rise of the
optical output waveform and a response at the time of a fall may be
set to be almost equal.
[0213] FIG. 24 is a diagram illustrating a drive current waveform
according to the third embodiment.
[0214] In the drive current Ik according to the present embodiment,
an undershoot current Iud is applied in synchronization with a fall
of the predetermined current Iop according to the first embodiment.
The undershoot current Iud may reduce dullness in the fall of the
optical output waveform and further speedily discharge the
parasitic capacitance charged.
[0215] Below embodiments of a semiconductor laser drive circuit and
the image forming apparatus which includes the semiconductor drive
circuit is described with reference to the drawings.
(Semiconductor Laser Drive Circuit)
[0216] First, embodiments of the semiconductor laser drive circuit
according to the present invention are described.
Configuration of Semiconductor Laser Drive Circuit
[0217] FIG. 25 is a conceptual diagram illustrating an embodiment
of the semiconductor laser drive circuit according to the present
invention.
[0218] The semiconductor laser drive circuit according to the
present invention includes a light emission current generating unit
1; a first differential current generating unit 2; and a second
differential current generating unit 3.
[0219] The light generation current generating unit 1 is a unit
which steadily converts a light emission signal (voltage) at a time
of lighting a semiconductor laser LD to generate a light emission
current as a semiconductor laser drive current. The light
generation current generating unit 1 corresponds to a current
source according to the present invention.
[0220] The first differential current generating unit 2 is a unit
which differentiates the light emission signal to generate a first
overshoot current which is injected for a very short initial time
(for example, 0.5 ns to 1.0 ns) in which the light emission
generating unit 1 is started. The first differential current
generating unit 2 corresponds to the first overshoot current source
according to the present invention.
[0221] The second differential current generating unit 3 is a unit
which differentiates a light emission signal with a differential
value different from that in the first differential current
generating unit 2 to generate a second overshoot current which is
injected for a short initial time (for example, 1.0 ns to 5.0 ns)
in which the light emission generating unit 1 is started. The
second differential current generating unit 3 corresponds to the
second overshoot current source according to the present invention.
The second differential current generating unit 3 injects the
second overshoot current with a time constant (an overshoot time
and an overshoot amount) that is different from that for the first
overshoot current.
[0222] Here, the semiconductor laser drive circuit drives the
semiconductor laser LD by a sum current of three currents of the
light emission current; the first overshoot current; and the second
overshoot current that are generated in the above-described units
for generating three types of currents. The above-described
configuration makes it possible for the semiconductor laser drive
circuit to properly and stably supply an integrated light amount
which is most necessary for forming an image.
[0223] As the semiconductor laser LD, an infrared laser diode which
emits an infrared light; various laser diodes or laser diode arrays
that emit a red light, a blue light, etc., may be used, for
example.
(Operation of the Semiconductor Laser Drive Circuit)
[0224] Next, an operation of the semiconductor laser drive circuit
is described with reference to a semiconductor laser drive current
waveform which is generated by the semiconductor laser drive
circuit.
[0225] FIG. 26 is a diagram which compares a waveform diagram of a
current generated by the semiconductor laser drive circuit
according to the present invention with a waveform diagram of a
current generated by a related art semiconductor laser drive
circuit.
[0226] Here, (a) in FIG. 26 is a waveform diagram illustrating a
light emission signal which causes the semiconductor laser LD to
emit light, while (b) in FIG. 26 is a diagram for an ideal waveform
which causes the semiconductor laser LD to emit light.
[0227] (c) in FIG. 26 is a waveform diagram of a drive current
generated by the related art semiconductor laser drive circuit.
[0228] (d) in FIG. 26 is a waveform diagram of a modulation current
which causes the semiconductor laser LD to emit light.
[0229] (e) in FIG. 26 is a waveform diagram of the first overshoot
current generated by the first differential current generating unit
2.
[0230] (f) in FIG. 26 is a waveform diagram of the second overshoot
current generated by the second differential current generating
unit 3.
[0231] (g) in FIG. 26 is a waveform diagram of a drive current
generated by the present circuit.
[0232] Next, a configuration of one example of a related art
semiconductor laser drive circuit and an operation thereof are
described in order to compare with the configuration of the present
circuit and an operation thereof.
[0233] FIG. 27 is a circuit diagram illustrating one example of a
related art semiconductor laser drive circuit.
[0234] A related art semiconductor laser drive circuit includes a
drive transistor Tr, into whose base (gate) a light emission signal
is input. Moreover, the related art semiconductor laser drive
circuit includes a semiconductor laser LD which is connected to a
collector of a drive transistor Tr and a resistor R which is
connected to an emitter of the drive transistor Tr.
[0235] In the related semiconductor laser drive circuit, when a
light emission signal flows into a base of the drive transistor Tr,
a semiconductor laser drive current flows into the semiconductor
laser Ld and an emitter current flows into the resistor R. In FIG.
27, a waveform of the semiconductor laser drive current is W1; a
waveform of the light emission signal is W2; and a waveform of the
emitter current is W3.
[0236] Dullness in a rise current of the drive transistor Tr, a
light emission delay, etc., occur in a path leading to the
semiconductor laser LD from the drive transistor Tr, so that a
waveform W1 of the semiconductor laser drive current turns into a
waveform such as what is shown in (c) in FIG. 26.
[0237] Here, factors for the waveform W1 of the semiconductor laser
drive current according to the related-art semiconductor laser
drive circuit to turn into a waveform such as what is shown in (c)
in FIG. 26 in a related-art semiconductor laser drive circuit
possibly includes influences of an output impedance of a drive
transistor Tr, a parasitic capacitance of wiring of a
printed-circuit board, etc. Moreover, other factors possibly
include influences of an input capacitance of the semiconductor
laser LD, an impedance of the semiconductor laser LD, etc.
[0238] These influences may cause failures such as dullness in a
rise of the drive transistor Tr, a light emission delay, etc., to
occur in the waveform W1 of the semiconductor laser drive current
according to the related art semiconductor laser drive circuit.
When the above-described failures occur in the waveform W1 of the
semiconductor laser drive current, a desired output (a light
emission amount, a light emission time, etc.) by the semiconductor
laser LD is not obtained in an initial period of light emission
(for example, between T1 and T3 as shown in FIG. 26, for
example).
[0239] On the other hand, the semiconductor laser drive circuit
according to the present invention corrects for the semiconductor
laser drive current by the below-described configuration.
[0240] FIG. 28 is a circuit diagram which shows an embodiment of
the semiconductor laser drive circuit according to the present
circuit.
[0241] The semiconductor laser drive circuit includes a drive
transistor Tr; and a semiconductor laser LD which is connected to a
collector of the drive transistor Tr, which is the same as a
related art semiconductor laser drive circuit shown in FIG. 27.
[0242] While a bipolar transistor is shown as one example in FIGS.
27 and 28 for the drive transistor Tr, it is not limited thereto in
the present circuit. For example, as the drive transistor Tr, a
CMOS (complementary metal oxide semiconductor) may also be
used.
[0243] In the semiconductor laser drive circuit, a resistor R1
which corresponds to the light emission current generating unit 1
is connected to an emitter of the drive transistor Tr. Moreover, a
capacitor C1 which corresponds to the first differential current
generating unit 2 is connected to an emitter of the drive
transistor Tr. Furthermore, a capacitor C2 and a resistor R2
corresponding to the second differential current generating unit 3
are serially connected to the emitter of the drive transistor Tr.
In other words, in the semiconductor laser drive circuit, the
capacitor C2 and the resistor R2; the capacitor C1; and the
resistor R1 are connected in parallel to the emitter of the drive
transistor Tr. The capacitor C2 and the resistor R2; the capacitor
C1; and the resistor R1 may be connected to a source of the drive
transistor Tr.
[0244] In the semiconductor laser drive circuit as described above,
the first overshoot current is generated by the capacitor C1. Here,
a current which flows through the capacitor C1 becomes a simple
differential waveform corresponding to the first overshoot current,
so that a differential wave of only a change point of the
modulation current is generated.
[0245] Moreover, in the semiconductor laser drive circuit as
described above, the second overshoot current is generated by the
capacitor C2 and the resistor R2. Here, a current which flows
through the capacitor C2 becomes a simple differential waveform
corresponding to the second overshoot current, so that a
differential wave of only a change point of the modulation current
is generated.
[0246] Next, a semiconductor laser drive current is described. A
light emission signal is set by a signal input as an ON/OFF signal
to an LD driver (not shown) from outside the semiconductor laser
drive circuit.
[0247] The light emission signal is input into a base (a gate) of
the drive transistor Tr shown in FIGS. 27 and 28 after being set to
an appropriate level as a binary voltage signal by an LD
driver.
[0248] A waveform W2 of the light emission signal is essentially
identical to an ideal waveform shown in (b) in FIG. 26. Therefore,
it is desirable that a height and a width of the pulse are
identical to those of the waveform of the light emission signal
even if some slight delay occurs.
[0249] Here, in the semiconductor laser drive circuit, it is
assumed that a certain desired quantitative relationship applies
between conversion from a voltage of the light emission signal
shown in (a) in FIG. 26 to a modulation current shown in (d) in
FIG. 26; and a drive current generated by the semiconductor laser
drive circuit that is shown in (g) in FIG. 26 from the modulation
current and that it is met.
[0250] Now, various failures such as a light emission delay, etc.,
occur in a path leading to the semiconductor laser LD from the
drive transistor Tr as described previously even when the waveform
of the modulation current is a square wave whose waveform is
identical to the waveform of the light emission signal.
[0251] In order to improve such failures, in the first differential
current generating unit 2, a first overshoot current for outputting
in quite a short time (for example, less than or equal to 1 ns) is
generated in which light emission does not actually occur even when
the semiconductor laser LD shown in T1-T2 in (e) in FIG. 26
responds. In the first differential current generating unit 2,
charging of the capacitor C1 occurs with a capacitance which
corresponds to the influences which may cause the failures
described previously to shorten a light emission delay time of the
semiconductor laser LD.
[0252] Moreover, the second differential current generating unit 3
generates a second overshoot current for outputting in a time
longer than a time for a first overshoot current that is shown in
T1-T3 in (f) in FIG. 26, or more specifically, an initial light
emission time (for example, less than or equal to 5 ns) in which
the semiconductor laser LD responds. As described above, the
present circuit outputs the first overshoot current and the second
overshoot current for times determined respectively such that
dullness in a rise of the drive transistor Tr is corrected for. In
this way, according to the present circuit, a waveform in times of
T1-T3 is corrected for such as for a waveform of the semiconductor
laser drive current shown in (g) in FIG. 26. In other words, the
present circuit makes it possible to generate a waveform of a
semiconductor laser drive current that is almost identical to a
waveform of a light emission signal input from the LD driver.
[0253] While only the first overshoot current and the second
overshoot current that are generated at a time of a rise of the
modulation current are illustrated in (e) and (f) in FIG. 26, an
undershoot current may be generated at a time of a fall of the
modulation current. Generating the undershoot current at the time
of the fall makes it possible for the semiconductor laser drive
circuit to also perform an operation of turning OFF the
semiconductor laser LD at high speed.
[0254] Moreover, as shown in (a), (e), and (f) in FIG. 26, it may
be arranged to set, as the same time, a timing of supplying the
second overshoot current, the first overshoot current waveform, and
the light emission signal waveform. As a result, it is made
possible for the semiconductor laser drive circuit to eliminate a
light emission delay while obtaining a predetermined response
waveform.
[0255] Moreover, it is made possible for the semiconductor laser
drive circuit to change a relative magnitude relationship between a
capacitance of the capacitor C1 and a resistance value of the
resistor R1; and a capacitance of the capacitor C2 and a resistance
value of the resistor R2 to change a current value of a current
generated. In other words, when seeking to generate a corrected
waveform shown in (g) in FIG. 26, it suffices to set R1=R2 and
C1<C2, for example.
[0256] In other words, it is made possible for the present circuit
to realize the corrected waveform shown in (g) in FIG. 26 in a
simple configuration.
[0257] Moreover, the capacitance of the capacitor C1; the
resistance value of the resistor R2; and the capacitance of the
capacitor C2 may be made variable. In this way, in the
semiconductor laser drive circuit, a semiconductor laser drive
current may be corrected for regardless of a light amount (output)
or a type of the semiconductor laser LD.
[0258] Here, configurations which make the resistance value and the
capacitance variable include a configuration such that a number of
combinations of a capacitance and a resistance value of the
capacitor C2; the resistor R2; and the capacitor C1 is determined
and a selection thereof is set to a register to obtain a desired
characteristic.
[0259] Next, a reason is explained for generating two types of
overshoot currents of a first overshoot current and a second
overshoot current in order to correct for a current at a time of a
rise of the semiconductor laser LD.
[0260] FIG. 29 shows a set of waveform diagrams illustrating a
relationship between an overshoot current and a waveform of a
semiconductor laser drive current generated by the present
circuit.
[0261] (a) in FIG. 29 is a waveform diagram of a semiconductor
laser drive current which is corrected for only by the first
overshoot current.
[0262] (b) in FIG. 29 is a waveform diagram of the semiconductor
laser drive current which is corrected for only by the second
overshoot current.
[0263] (c) in FIG. 29 is a waveform diagram of the semiconductor
laser drive current which is corrected for by increasing the
current value of the second overshoot current.
[0264] (d) in FIG. 29 is a waveform diagram of the semiconductor
laser drive current which is corrected for by the second overshoot
current in (c) in FIG. 29.
[0265] As shown in (a) in FIG. 29, when the semiconductor laser
drive current is corrected for only by the first overshoot current,
a light emission delay amount at a time of a rise is corrected for.
However, overshooting or undershooting (ringing) occurs in the
subsequent waveform. Therefore, with the correction only by the
first overshoot current, it is not possible for the semiconductor
laser LD to obtain a favorable and stable integrated light
amount.
[0266] On the other hand, when the semiconductor laser drive
current is corrected for only by the second overshoot current,
there is a certain correction effect for the light emission delay
amount as shown in (b) in FIG. 29. However, in order to correct for
the light emission delay amount at an initial stage of a rise in
this case, it is possible to increase a current value of the second
overshoot current as shown in (c). However, increasing the current
value of the second overshoot current could cause the optical
waveform to overshoot as shown in (d) in FIG. 29.
[0267] As described above, in order to stably generate a desired
optical waveform regardless of a characteristic such as a type of
the semiconductor laser LD, it is effective to conduct correction
by two types of overshoot currents as in the semiconductor laser
drive circuit.
[0268] Next, a relationship between an output and a drop voltage
and a semiconductor laser drive current in the semiconductor laser
drive circuit is described.
[0269] FIG. 30 is a diagram illustrating a change in output when a
weak current is applied to the semiconductor laser LD. Moreover,
FIG. 31 is a diagram illustrating a change in a drop voltage when
the weak current is applied to the semiconductor laser LD.
Furthermore, FIG. 32 is a table illustrating an output and a drop
voltage when the weak current is applied to the semiconductor laser
LD.
[0270] As shown in FIGS. 30 to 32, in the semiconductor laser drive
circuit, a drive current LD of the semiconductor laser LD and a
drop voltage VLDDOWN of the semiconductor laser LD rise, and an
output also rises.
[0271] More specifically, as shown in FIG. 31, when the drive
current LD of the semiconductor laser LD is 250 .mu.A, there is
already the drop voltage VLDDOWN of around 1.4V in the
semiconductor laser LD. As the semiconductor laser LD includes a
direct current resistance component, when LD increases, the VLDDOWN
also gradually increases.
[0272] Here, the reason that there is some VLDDOWN even for a
slight current value of LD is possibly that LD causes an impedance
of the semiconductor laser LD to decrease, so that a response
characteristic of the semiconductor laser LD when a threshold
current is applied improves.
[0273] In other words, using the first overshoot current,
electricity is supplied to the capacitor C1 in a range such that
light emission does not occur due to the semiconductor laser LD
responding and for a very short time (for example, less than or
equal to 1 ns), so that a parasitic capacitance of the
semiconductor laser drive circuit such as a semiconductor laser LD,
an LD driver, etc., becomes around 1.4V.
[0274] In this way, the semiconductor laser drive circuit causes
the semiconductor laser LD to be operable quickly by the first
overshoot current and the second overshoot current. Therefore, it
is made possible for the semiconductor laser drive circuit to cause
the semiconductor laser LD to be able to be speedily turned on
instantaneously.
[0275] While an example using a predetermined semiconductor laser
LD is described in FIGS. 30 to 32, a similar characteristic is also
demonstrated in a different semiconductor laser LD. In other words,
the semiconductor laser drive circuit may eliminate a light
emission delay while obtaining a predetermined response waveform
for driving the semiconductor laser for various types of
semiconductor lasers.
(Principles of light emission delay of semiconductor laser LD)
[0276] Next, principles of waveform dullness and a light emission
delay of the semiconductor laser LD are described.
[0277] FIG. 33 is a diagram illustrating a drive current waveform
and an IL characteristic (injection current-Light output
characteristic) of the semiconductor laser. In FIG. 33, symbols for
the respective currents represent the following:
Iop: laser operation current Ith: laser threshold current Ib: bias
current Idrv: Iop-Ib=Ild+Iled: laser drive current for driving at
high speed Ild: current corresponding to LD region of laser Iled:
current corresponding to LED region of laser Vop: operating voltage
at time of light emission Vth: voltage at time of applying
threshold current Vb: voltage at time of applying bias current
.DELTA.V: Vth-Vb: differential voltage when changing from operating
voltage at time of applying bias current to operating voltage at
time of light emission.
[0278] When driving the semiconductor laser in a LBP (LaserBeam
Printer), it is desired to apply a bias current up to Ith,
considering an operation of the semiconductor laser.
[0279] However, such a case may cause an LED light emission in the
semiconductor laser, possibly causing surface staining of a
photosensitive body. Thus, when driving the semiconductor laser, a
current value is normally set to less than or equal to Ith.
[0280] Moreover, as for Ib, assuming Ib=0, a rise characteristic of
light deteriorates. Therefore, a certain amount of Ib normally
needs to be applied. In other words, Ib is set in a tradeoff of
reducing current consumption and a rise characteristic of light of
the semiconductor laser LD.
[0281] In a drive current waveform for lighting the semiconductor
laser LD shown in FIG. 33, it is assumed that a rise of a current
operates in a straight-line approximation for brevity of
explanations.
[0282] It is assumed that the drive current rises from Ib at time
t1; becomes Ib+Iled at time t2; and becomes Iop=Ib+Iled+Ild at time
t3.
[0283] FIG. 34 is a conceptual diagram illustrating a relationship
between a current flowing within a circuit and a driver which
supplies a drive current to the semiconductor laser.
[0284] In a semiconductor laser drive circuit in which a
semiconductor laser LD and an LD driver 4 are mounted, when
connecting between the semiconductor laser LD and the LD driver 4
by wiring, there are multiple parasitic capacitances such as a
parasitic capacitance of a semiconductor laser package, etc.; a
parasitic capacitance of a LD driver 4 package, etc.; a parasitic
capacitance due to wiring, etc. Here, in FIG. 34, multiple
parasitic capacitances are denoted as C.
[0285] In other words, in the semiconductor laser LD, after Ic,
which is a current for charging a parasitic capacitance C, flows,
Iled+Ild, which is a current to be inherently applied to the
semiconductor laser LD flows. Then, there occurs a time difference
before Iled+Ild flows after Ic flows into the semiconductor laser
LD.
[0286] Here, it is assumed that a total of parasitic capacitances
in the semiconductor laser drive circuit be C. As shown in FIG. 34,
in a transient operation from t0 to t1, out of a current 1 which is
output from the semiconductor laser drive circuit, a transient
current Ic flows into the parasitic capacitance C. Thereafter,
current (Iled+Ild) flows in the semiconductor laser LD.
[0287] FIG. 35 is a diagram which illustrates a drive current
waveform of the semiconductor laser when an optical output is
different from the drive current waveform in FIG. 33.
[0288] It is assumed that, in the semiconductor laser drive
circuit, a rise occurs in the same time t3-t1 regardless of a drive
current amount.
[0289] For brevity of explanations, FIG. 35 shows a hypothetical
current waveform diagram in which it is assumed that a time from
time t0 to t1 be Ic, after which (Iled+Ild) flows, and Ic becomes 0
at time t3.
[0290] Moreover, in FIG. 35, a case of a current Iop2 and a case of
a current Iop1 with a current value smaller than. Iop is described,
assuming that desired currents differ.
[0291] More specifically, in the case of Iop2, (Iled+Ild) becomes
Ib at time t0; Ib at time t12 (with Ic flowing between t0 and t11);
Iled at time t22; and Iop2 at time t3.
[0292] On the other hand, in the case of Iop1, it becomes Ib at
time t0; Ib at time t11 (with Ic flowing between t0 and t11); Iled
at time t21; and Iop1 at time t3.
[0293] Moreover, a pulse light emission delay time (a time in which
pulse narrowing of optical pulse and drive current pulse occurs) is
considered to be able to be calculated by a sum of a time to reach
Iled; and a difference of a charge time for a parasitic capacitance
C.
[0294] In other words, when the current is Iop1, the pulse light
emission delay time is
(t11-t0)+(t21-t11)-(t21-t0).
[0295] On the other hand, when the current is Iop2, the pulse light
emission delay time is
(t12-t0)+(t22-t12)=(t22-t0).
[0296] Next, the parasitic capacitance C of the semiconductor laser
drive circuit is described. As described previously, the parasitic
capacitance C of the semiconductor laser drive circuit is a certain
value determined by an output capacitance of a driver, a board, and
an input capacitance of an LD element. Thus, a required charge Q is
Q=C.times..DELTA.V.
[0297] In other words, a charge Q1 in the case of the current Iop1
and a charge Q2 in the case of Iop2 are respectively
Q1=1/2.times.Iop1.times.{(t11-t0)/(t3-t0)}.times.(t11-t0);
and
Q2=1/2.times.Iop2.times.{(t12-t0)/(t3-t0)}.times.(t12-t0).
[0298] Then, assuming a current ratio Iop1/Iop2=K,
(t12-t0)/(t11-t0)
becomes a square root of K.
[0299] Therefore, as for the charge time for the parasitic
capacitance C, determining a ratio K of Iop1 and Iop2 leads to
determining a ratio of the delay time. Thus, a difference in the
charge time for the parasitic capacitance C due to a difference in
the drive current may be corrected for by expanding a pulse such
that the above-mentioned delay time is subjected to reverse
correction.
[0300] Here, when Iled is sufficiently small, .DELTA.V is also
sufficiently small, so that correcting for pulse narrowing due to
the drive current difference may not be needed. In such a case,
when Iled/(Iled+Ild)<0.1, or, in other words, when the parasitic
capacitance C is sufficiently charged, there is no need for the
correcting. On the other hand, when Iled/(Iled+Ild).gtoreq.0.1,
charging into the parasitic capacitance C cannot be ignored, so
that the correcting becomes necessary.
[0301] Next, a difference is considered for Iop1 and Iop2 of a time
for a current which flows into the semiconductor laser LD to reach
Iled.
[0302] The time to reach Iled results in a current ratio of Top.
Thus, the difference of the time for the current which flows into
the semiconductor laser LD to reach Iled becomes
Iop1/Iop2=(t21-t11)/(t22-t12)=K.
[0303] FIG. 36 is a diagram illustrating the drive current waveform
of the semiconductor laser in the semiconductor laser drive
circuit. A pulse narrowing time which occurs in the semiconductor
laser drive circuit is described using FIG. 36.
[0304] It is assumed that a rise time of the drive current applied
to the semiconductor laser is 2 ns; Ith=10 mA; Ib=7 mA; and a
difference .DELTA.V (in the LD operating voltage at
Ith-Ib)=0.25V.
[0305] Here, a behavior is considered of the semiconductor laser LD
when Iled=Ith-Ib=3 mA; Ild=2 mA; and a total current It1=5 mA are
applied.
[0306] First, an amount of correction due to charging a parasitic
capacitance is considered. Assuming the parasitic capacitance
between the semiconductor laser and the LD driver 4 of 5 pF,
Q=5.times.10.sup.-12.times.0.25.times.10.sup.-12 from
Q=C.times.V.
[0307] Moreover, an applied current amount Itr at a rise portion is
shown by the following equation as a function of time t.
Itr=2.5.times.10.sup.-3.times.10.sup.9.times.t
Here,
Q=1/2.times.Itr.times.t=1/2.times.2.5.times.10.sup.-3.times.10.sup.9.tim-
es.t.sup.2
[0308] Moreover, Q=CV yields
Q=1.25.times.10.sup.-12=1/2.times.2.5.times.10.sup.-3.times.10.sup.9.tim-
es.t.sup.2
With t=1.times.10.sup.-9, a delay of 1 ns in a rise occurs for the
semiconductor laser LD.
[0309] Here, assuming Ild=20 mA, with a total current It2=23 mA
yielding
t=0.466.times.10.sup.-9,
a delay of 0.466 ns in a rise occurs for the semiconductor laser
LD. A delay time tled1 due to Iled is a time corresponding to a
square root of a total current ratio of It1/It2.
[0310] Next, correction of a delay time of Iled is considered.
It1=5 mA yields It1=Iled(3 mA)+Ild(2 mA).
[0311] When a rise time of the drive current is 2 ns and the delay
time due to Iled is tled1, a delay of tled1=2 ns.times.(3 mA/5
mA)=1.2 ns occurs. Here, It1=23 mA yields It1=Iled(3 mA)+Ild(20
mA).
[0312] When a rise time of the drive current is 2 ns, a delay time
tled2 due to Iled becomes tled2=2 ns.times.(3 mA/23 mA)=0.26 ns. In
other words, the delay time tled2 due to Iled is a time
corresponding to a total current ratio of It1/It2.
[0313] As described above, when a total current it1=5 mA is
applied, a total 2.2 ns of a delay of 1 ns that is an amount of
correction due to charging the parasitic capacitance C; and 1.2 ns
that is a correction amount of a delay time of Iled becomes a delay
time, or, in other words, a pulse narrowing time in the whole
semiconductor laser drive circuit.
[0314] FIG. 37 is a diagram illustrating a drive current waveform
of a VCSEL in the present circuit. The pulse narrowing time in a
surface emitting laser (VCSEL) is described using FIG. 37.
[0315] It is assumed that a rise time of the drive current applied
to the VCSEL is 2 ns; Ith=0.6 mA; Ib=0.3 mA; and a difference
.DELTA.V (in LD operating voltage at Ith-Ib)=0.34V.
[0316] Here, a behavior is considered when Iled=Ith-Ib=0.3 mA;
Ild=0.2 mA; and It3=0.5 mA are applied.
[0317] First, an amount of correction due to charging a parasitic
capacitance is considered. Here, assuming the parasitic capacitance
between the semiconductor laser and the LD driver 4 of 5 pF,
Q=5.times.10.sup.-12.times.0.34=1.7.times.10.sup.-12 from
Q=C.times.V.
[0318] Moreover, an applied current amount Itr at a rise portion is
shown by the following equation as a function of time t:
Itr=2.5.times.10.sup.-3.times.10.sup.9.times.t
Here,
Q=1/2.times.Itr.times.t=1/2.times.2.5.times.10.sup.-3.times.10.sup.9.tim-
es.t.sup.2
From Q=CV,
[0319]
Q=1.7.times.10.sup.-12=1/2.times.2.5.times.10.sup.-3.times.10.sup.-
9.times.t.sup.2
t=3.69.times.10.sup.-9, causing a delay in a rise of 3.69 ns.
[0320] Here, Ild=2 mA and a total current It4=2.3 mA yields
t=0.466.times.10.sup.-9, causing a delay of 0.466 ns in a rise.
This is a time corresponding to a square root of a total current
Ith ratio It1/It2.
[0321] Next, correction of a delay time of Iled is considered.
It4=0.5 mA yields It4=Iled (0.3 mA)+Ild (0.2 mA).
[0322] Here, a delay time tled3 due to Iled and a rise time 2 ns of
the drive current causes a delay of
tled3=2 ns.times.(0.3 mA/0.5 mA)=1.2 ns.
It1=2.3 mA yields It4=Iled(0.3 mA)+Ild(0.2 mA).
[0323] A delay time tled4 due to Iled and a rise time 2 ns of the
drive current causes a delay of tled4=2 ns.times.(0.3 mA/2.3 mA).
This is a time corresponding to a total current Ith ratio of
It1/It2.
[0324] As described above, when a total current It3=0.5 mA is
applied, a total 4.88 ns of a delay of 3.68 ns that is an amount of
correction due to charging the parasitic capacitance C; and 1.2 ns
that is a correction amount of a delay time of Iled becomes a delay
time, or, in other words, a pulse narrowing time in the whole
semiconductor laser drive circuit.
[0325] In a case of the VCSEL, a drive current is smaller than that
of the semiconductor laser, causing a larger delay and requiring
more correction for pulse delay narrowing.
[0326] Moreover, even for the semiconductor laser, when Ith or the
drive current is small, the above-described correction becomes
effective when used in a low light amount.
(Operational Advantages Related to the Embodiment of the
Semiconductor Laser Drive Circuit)
[0327] According to the above-described embodiment of the
semiconductor laser drive circuit, the first differential current
generating unit 3 and the second differential current generating
unit 3 generate two types of overshoots to thereby reduce a light
emission delay time which is a lighting time difference between
laser light emission and a drive current of a semiconductor laser
due to a parasitic capacitance which is parasitic on a portion from
a driver to a semiconductor laser.
[0328] In other words, according to the above-described embodiment
of the semiconductor laser drive circuit, a semiconductor laser
drive circuit having a superior pulse width reproducibility may be
realized without narrowing a pulse width of a light emission
signal.
[0329] Moreover, according to the above-described embodiment of the
semiconductor laser drive circuit, a high-speed and high-accuracy
semiconductor laser drive circuit may be provided with a superior
tone reproduction in low density without depending on a
characteristic of the semiconductor laser.
[0330] Furthermore, according to the above-described embodiment of
the semiconductor laser drive circuit, timings for supplying the
first overshoot current and the second overshoot current and a
timing for supplying the semiconductor laser drive current may be
set to be the same time to eliminate a light emission delay while
obtaining a predetermined response waveform.
[0331] Moreover, according to the above-described embodiment of the
semiconductor laser drive circuit, the first overshoot current and
the second overshoot current are generated by a signal which
differentiates a delay signal of a semiconductor laser drive
current or the semiconductor laser drive current, making it
possible to eliminate a light emission delay while obtaining a
predetermined response waveform.
[0332] Furthermore, according to the above-described embodiment of
the semiconductor laser drive circuit, the first differential
current generating unit 2 may be configured with the capacitor C1
and the second differential current generating unit 3 may be
configured with the resistor R2 and the capacitor C2 to eliminate a
light emission delay while obtaining a predetermined response
waveform with a simple configuration.
[0333] Moreover, according to the above-described embodiment of the
semiconductor laser drive circuit, a high-speed and high-accuracy
semiconductor laser drive circuit may be provided with a superior
tone reproduction in low density without depending on a
characteristic of the semiconductor laser using a variable resistor
and a variable capacitor.
[0334] Furthermore, according to the above-described embodiment of
the semiconductor laser drive circuit, a superior high-speed and
high-accuracy semiconductor laser drive circuit may be provided
without depending on a characteristic of the semiconductor laser as
a VCSEL, a red laser, a red laser array, etc., may be used as a
semiconductor laser.
(Image Forming Apparatus Including the Semiconductor Laser Drive
Circuit)
[0335] Next, an image forming apparatus including the semiconductor
laser drive circuit is described.
[0336] FIG. 38 is a central sectional diagram illustrating an
embodiment of the image forming apparatus including the
semiconductor laser drive circuit. An image forming apparatus 2000,
which is a multi-functional machine including respective functions
of a copier, a printer, and a facsimile machine, includes a main
body apparatus 1001; a reading apparatus 1002; an automatic
document feeding apparatus 1003, etc.
[0337] The main body apparatus 1001, which is a tandem multi-color
printer which forms a full-color image by overlaying four colors
(black, cyan, magenta, yellow), includes an optical scanning
apparatus 2010; photosensitive drums 2030 (2030a, 2030b, 2030c,
2030d); a transfer belt 2040; a transfer roller 2042; a fixing
roller 2050; a paper-feeding roller 2054; a regist roller pair
2056; a paper-discharging roller 2058; a paper-feeding tray 2060; a
paper-discharging tray 2070; a communication control apparatus
2080; and a printer control apparatus 2090.
[0338] The communication control apparatus 2080 controls two-way
communications with a host apparatus such as a personal computer,
etc., via a communications network, etc. The printer control
apparatus 2090 integrally controls respective units included in the
image forming apparatus 2000.
[0339] Below a paper face of the transfer belt 2040 are arranged,
in an order of one for yellow 2032d; one for magenta 2030c; one for
cyan 2030b; and one for black 2030a from the upstream side in a
moving direction of the transfer belt 2040 (counterclockwise on a
paper face in FIG. 38), photoconductive photosensitive drums 2030
formed in a cylindrical shape as image bearing bodies which are
exposed by the optical scanning apparatus 2010 and on which
electrostatic latent images are formed.
[0340] Surrounding the respective photosensitive bodies 2030 are
arranged, in order in a rotating direction of the photosensitive
drum, process members which follow an electrophotographic technique
(an electrophotographic process), charging apparatuses 2032 (2032a,
2032b, 2032c, 2032d); developing rollers 2033 (2033a, 2033b, 2033c,
2033d); toner cartridges 2034 (2034a, 2034b, 2034c, 2034d);
cleaning units 2031 (2031a, 2031b, 2031c, 2031d), etc.
[0341] As a charging unit, a corona changer may also be used.
[0342] The photosensitive drum 2030a; the charging apparatus 2032a;
the developing roller 2033a; the toner cartridge 2034a; and the
cleaning unit 2031a are used as a set, configuring an image forming
station which forms a black (K) image.
[0343] The photosensitive drum 2030b; the charging apparatus 2032b;
the developing roller 2033b; the toner cartridge 2034b; and the
cleaning unit 2031b are used as a set, configuring an image forming
station which forms a cyan (C) image.
[0344] The photosensitive drum 2030c; the charging apparatus 2032c;
the developing roller 2033c; the toner cartridge 2034c; and the
cleaning unit 2031c are used as a set, configuring an image forming
station which forms a magenta (M) image.
[0345] The photosensitive drum 2030d; the charging apparatus 2032d;
the developing roller 2033d; the toner cartridge 2034d; and the
cleaning unit 2031d are used as a set, configuring an image forming
station which forms a yellow (Y) image.
[0346] The optical scanning apparatus 2010 which corresponds to a
scanning unit according to the present invention is an optical
writing apparatus which optically writes onto the photosensitive
drum 2030 and executes an exposing process of the
electrophotographic process. The optical scanning apparatus 2010
irradiates, onto a surface of a charged photosensitive drum 2030,
light beams modulated to respective colors (image modulation
signals) based on multi-color image information (black image
information, cyan image information, magenta image information,
yellow image information) from a host apparatus connected to the
communications control apparatus 2080. On the surface of the
photosensitive drum (rotating photosensitive body) 2030, electric
charges disappear only on a portion onto which the light beams are
irradiated, so that an electrostatic latent image corresponding to
the image information is formed. The formed electrostatic latent
image, which is a negative latent image, moves in a direction of a
corresponding developing roller 2033 with a rotation of the
photosensitive drum 2030.
[0347] Here, into the optical scanning apparatus 2010 is embedded
the above-described semiconductor laser drive circuit. Then, a
semiconductor laser of the optical scanning apparatus 2010 is
driven by the semiconductor laser drive circuit to irradiate the
light beams onto the photosensitive drum 2030.
[0348] In the toner cartridge 2034a is stored black toner; in the
toner cartridge 2034b is stored cyan toner; in the toner cartridge
2034c is stored magenta toner; and in the toner cartridge 2034d is
stored yellow toner. Toners of respective colors that are stored in
the toner cartridges 2034 are supplied to the corresponding
developing roller 2033.
[0349] With a rotation of the developing roller 2033, toner from
the corresponding toner cartridge 2034 is thinly and uniformly
applied onto a surface of the developing roller 2033. When in
contact with a surface of the photosensitive drum 2030
corresponding to respective colors, as the toner applied to the
surface of the developing roller 2033 adheres to the electrostatic
latent image formed on a surface of the photosensitive drum 2030,
the electrostatic latent image is visualized, so that the toner
image is formed. The formed toner image moves in a direction of the
transfer belt 2040 with a rotation of the photosensitive drum
2030.
[0350] Toner images of each color of yellow, magenta, cyan, and
black are successively transferred onto the transfer belt 2040 at
predetermined timings, so that a color image is formed.
[0351] A transfer paper sheet which is a recording medium is stored
in a paper-feeding tray 2060. Near the paper-feeding tray 2060, a
paper-feeding roller 2054 is arranged. The uppermost transfer paper
sheet stored in the paper-feeding tray 2060 is fed to the
paper-feeding roller 2054 and a tip of the fed transfer paper sheet
is caught by the regist roller pair 2056. The regist roller pair
2056 sends out, toward a gap between the transfer belt 2040 and the
transfer roller 2042, the transfer paper sheet in alignment with a
timing at which a toner image on the photosensitive drum 2030 moves
to a transfer position. Onto the sent out transfer paper sheet is
transferred a color image on the transfer belt 2040. The transfer
paper sheet onto which the color image is transferred is sent out
to the fixing roller 2050.
[0352] To the transfer paper sheet sent out to the fixing roller
2050 is applied heat and pressure, so that toner is fixed onto the
transfer paper sheet. The transfer paper sheet onto which the toner
is fixed is sent out to the paper-discharging tray 2070 via the
paper-discharging roller 2058, so that it is successively stacked
onto the paper-discharging tray 2070.
[0353] The cleaning unit 2031 removes toner (residual toner) which
remained on a surface of the photosensitive drum 2030 after the
toner image is transferred. A surface of the photosensitive drum
2030 from which the residual toner is removed returns again to a
position opposing the corresponding charging apparatus 2032.
(Operational Advantages Related to the Embodiment of the
Semiconductor Laser Drive Circuit)
[0354] According to the above-described embodiment of the
semiconductor laser drive circuit, the semiconductor laser drive
circuit may be used to eliminate a light emission delay while
obtaining a predetermined response waveform with a simple
configuration.
[0355] While the present invention has been described in the above
based on the respective embodiments, the present invention is not
to be limited to requirements shown in the above-described
embodiments. These points may be changed within a range which does
not impair the gist of the invention and may be appropriately
determined in accordance with applications thereof.
* * * * *