U.S. patent application number 09/788747 was filed with the patent office on 2002-09-12 for system and method for improving laser power and stabilization using high duty cycle radio frequency injection.
This patent application is currently assigned to Eastman Kodak Company. Invention is credited to Markis, William R., Roddy, James E..
Application Number | 20020125406 09/788747 |
Document ID | / |
Family ID | 25145423 |
Filed Date | 2002-09-12 |
United States Patent
Application |
20020125406 |
Kind Code |
A1 |
Roddy, James E. ; et
al. |
September 12, 2002 |
System and method for improving laser power and stabilization using
high duty cycle radio frequency injection
Abstract
A system and method of stabilizing laser output levels includes
a laser (10), an injection circuit for injecting a radio frequency
waveform, and a control circuit for energizing and stabilizing the
laser. The radio frequency waveform injected by the injection
circuit has a high duty cycle to maintain high output power while
providing a stable multimode spectrum. A back facet photodiode
sensor (102) detects radiation emitted from a back facet
semiconductor laser (101) and provides a feedback signal to the
control circuit (41) for maintaining the laser output power. The
response of the photodiode is not fast enough to track intensity
variations due to the RF waveform, and thus provides feedback to
the control circuit (41) only when there is a substantial need to
adjust laser power.
Inventors: |
Roddy, James E.; (Rochester,
NY) ; Markis, William R.; (Spencerport, NY) |
Correspondence
Address: |
Milton S. Sales
Patent Legal Staff
Eastman Kodak Company
343 State Street
Rochester
NY
14650-2201
US
|
Assignee: |
Eastman Kodak Company
|
Family ID: |
25145423 |
Appl. No.: |
09/788747 |
Filed: |
February 20, 2001 |
Current U.S.
Class: |
250/205 |
Current CPC
Class: |
H01S 5/0427 20130101;
H01S 5/02212 20130101; H01S 5/06832 20130101 |
Class at
Publication: |
250/205 |
International
Class: |
G01J 001/32; H01S
003/13 |
Claims
What is claimed is:
1. A system for stabilizing laser output levels comprising: a laser
responsive to a control signal for generating a radiation beam; a
control circuit connected to said laser for providing said control
signal to said laser; an injection circuit connected to said
control circuit for injecting a high duty cycle radio frequency
waveform into said laser; a back facet photodiode sensor for
detecting radiation from a back facet of said laser and for
providing a feedback signal to said control circuit for maintaining
a power level of said laser constant; wherein said radio frequency
waveform causes said laser to oscillate above and below a DC bias
point between a lasing threshold level and an asymmetrical level
above the DC bias point; and wherein said injection circuit injects
said radio frequency waveform with a duty cycle greater than
50%.
2. A system as in claim 1, wherein said back facet photodiode and
said control circuit respond to significant drifts in laser output
power.
3. A system as in claim 1, wherein said high duty cycle radio
frequency waveform creates a stable output spectrum from said
laser.
4. A system as in claim 1, wherein said high duty cycle radio
frequency waveform injected into said laser confines inherent laser
noise within each pixel of an image
5. A system as in claim 1, wherein a thermoelectric cooler is
affixed to said laser and a controller for said thermoelectric
cooler controls a temperature of said thermoelectric cooler such
that said laser has additional output stability.
6. A system as in claim 1, wherein said injection circuit is
comprised of: a sine wave oscillator with excess feedback and
altered bias to generate an asymmetrical waveform; a capacitor,
wherein said asymmetrical waveform is directed through said
capacitor; a direct current source; and an inductor, wherein said
DC current source is directed through said inductor and drives said
laser.
7. A system as in claim 1, wherein said injection circuit is
comprised of: a direct current source capable of providing current;
a signal generator capable of generating a pulsed waveform; and an
active electrical component, wherein said component is capable of
shunting said current away from said laser when driven by said
pulsed waveform.
8. A system as in claim 1 wherein said injection circuit is
comprised of: a pulse forming circuit; a direct current source
capable of producing current; a transformer, wherein said
transformer is connected to said pulse forming circuit and direct
current source; and a fast clamping diode, wherein said diode
shunts said current away from said laser.
9. A system as in claim 1 wherein said radio frequency is at least
twice the pixel clock frequency.
10. A method of stabilizing laser output levels comprising the
steps of: forming front and back facets on a laser element of a
laser; injecting said laser with current and a radio frequency
signal, wherein said radio frequency signal is a high duty cycle
waveform; and inducing multimode operation of said laser.
11. A method as in claim 10, wherein said duty cycle is greater
than 50%.
12. A method as in claim 10, wherein said multimode operation of
said laser confines inherent laser noise within each pixel of an
image
13. A method as in claim 10, wherein said radio frequency signal
with said high duty cycle waveform is generated by a sine wave
oscillator with excess feedback and altered bias.
14. A method as in claim 10, wherein said radio frequency signal
with said high duty cycle waveform is generated by inputting a
pulsed waveform into an active electrical component to shunt said
current away from said laser.
15. A method as in claim 10, wherein said radio frequency signal
with said high duty cycle waveform is generated by a pulse forming
circuit connected to a transformer and a fast clamping diode,
wherein said current is shunted away from said laser.
16. A system for stabilizing laser output levels comprising: a
laser responsive to a control signal for generating a radiation
beam; a control circuit connected to said laser for providing said
control signal to said laser; an injection circuit connected to
said control circuit for injecting a high duty cycle radio
frequency waveform into said laser; a back facet photodiode sensor
for detecting radiation from a back facet of said laser and for
providing a feedback signal to said control circuit for maintaining
a power level of said laser constant; wherein said radio frequency
waveform causes said laser to oscillate between a DC bias point
between a lasing threshold level; and wherein said injection
circuit injects said radio frequency waveform with a duty cycle
greater than 50%.
Description
FIELD OF THE INVENTION
[0001] This invention relates in general to stabilized
semiconductor lasers for imaging applications and in particular, to
a high duty cycle radio frequency waveform injected into a
semiconductor laser with a back facet closed loop control
circuit.
BACKGROUND OF THE INVENTION
[0002] In many imaging applications, it is often desirable to have
an inexpensive semiconductor laser device that provides constant
wavelength and power output, as well as low noise. In one type of
laser raster printing system, a photosensitive media is placed on a
drum and is written to by a semiconductor laser. A light beam from
the laser is typically deflected from a polygon or galvanometer,
and focused through an imaging lens. The image is written pixel by
pixel using a raster scan technique onto a photosensitive
media.
[0003] Controlling the amount of laser energy delivered is
important in achieving quality images. Unwanted variations in laser
energy delivered to a photosensitive media can introduce
objectionable artifacts, such as dark and light streaks or spots in
the image printed on the media. In many image writing applications,
laser optical power must be controlled to better than 0.5% accuracy
in order to obtain a reasonable image quality.
[0004] Optical power is affected by many parameters, such as
semiconductor laser driving current and operating temperature. In
order to ensure that a laser operates at a stable condition, an
operating temperature is chosen in which the laser operates at a
wavelength which is relatively constant. For example, assume a
particular laser has a relatively stable operating wavelength of
685 nm only over a narrow temperature range of 3.degree. C. Outside
this range, there would be variations in intensity of the laser
output power as the laser hops from one mode to another. A
thermoelectric cooler must be used to keep the laser in its stable
range of operation.
[0005] Another problem which may cause variations in laser power
output is caused by optical feedback, which is unwanted light
reflected back into a laser by optical elements external to the
laser. Optical feedback can disturb laser operation and cause
intensity fluctuations which may amount to as much as 10% or 20%.
As more components are added, such as in a collimator lens and beam
forming optics, the stable temperature range in which the laser can
operate may be decreased significantly from the 3.degree. C. noted
above, to only a few tenths of a degree.
[0006] Other factors may affect the stability of laser operating
systems. For example, characteristics of some components change
with age, and small contaminants may accumulate on the surfaces of
the optical elements. This change can cause variations in
reflectivity which results in optical feedback to the laser.
[0007] Past attempts to stabilize laser performance have met with
mixed results. For example, thermoelectric coolers have been used
to prevent drift with ambient temperature. However, over the
operating life of the equipment, lasers still may change modes
because laser characteristic changes or external optical elements
shift, causing optical feedback. Furthermore, thermoelectric
coolers add additional cost and complexity.
[0008] Another method of stabilizing laser is using back facet
photosensors which detect a portion of the light leaving a back
facet of the laser to control laser output. This has not been
entirely successful because the layers of dielectric mirror coating
on the back facet of the laser are wavelength specific. Therefore,
small changes in the wavelength of the light leaving the back facet
can result in large changes in power to the back facet sensor,
while the actual laser output is essentially unchanged. The power
control loop on the laser ends up making a light level correction
where none should be made.
[0009] Another attempt at stabilization of lasers has used radio
frequencies (RF) to stabilize low power level lasers, for example,
laser printing in the range of 1 to 2 mW. These low power RF
stabilization schemes, however, are not suitable for high power
laser stabilization because of intensity control problems. This
type of RF stabilization in a high power laser has a possibility of
back-biasing the laser diode and destroying it. See U.S. Pat. Nos.
5,197,059; 5,386,409; and 5,495,464. Other undesirable features in
RF control are decreased lifetime and overdriving of the laser. See
U.S. Pat. Nos. 5,495,464 and 5,175,722.
[0010] A further attempt at stabilization of low power lasers has
used radio frequencies with low duty cycle waveforms. U.S. Pat. No.
5,386,409 discloses the use of low duty cycle radio frequency
waveform to stabilize a semiconductor laser for reading and writing
to an optical disk.
[0011] In addition, attempts have been made to stabilize high power
semiconductor lasers at approximately 20 to 100 mW using RF
injection. U.S. Pat. No. 6,049,073 discloses a system and method
for high power semiconductor laser stabilization using RF
injection, where the RF waveform is a sine wave. This method of
stabilization, however, only allows half the laser's rated output
power to be available as stabilized power because 50% duty cycle
sine wave is used as the RF drive. Driving the laser at higher
current levels to increase power results in overdriving the laser
and decreasing lifetime. Increasing the RF drive to the laser can
result in back biasing the laser and destroying it.
[0012] It is, therefore, desirable to stabilize a high power
semiconductor laser at or near its rated maximum power against
changes in temperature, current variations, effects of aging, and
optical feedback.
SUMMARY OF THE INVENTION
[0013] It is an object of the present invention to provide a high
power output radio frequency injected stabilized semiconductor
laser. It is another object of the invention to provide a laser
with a stable spectrum output that allows for accurate back facet
photodiode control. It is a further object of this invention to
eliminate the need for thermoelectric cooling to control the output
of the laser. It is an additional object of the invention to
confine any inherent laser noise within each pixel of an image when
the stabilized semiconductor laser is used as part of a raster scan
image writing system, thus rendering the resultant spatial noise
invisible to the eye.
[0014] The present invention is directed to overcoming one or more
of the problems set forth above. According to one aspect of the
present invention, a system for stabilizing laser output levels
comprises a laser responsive to a control signal for generating a
radiation beam. A control circuit connects to the laser providing
the control signal to the laser. An injection circuit connects to
the control circuit injecting a high duty cycle radio frequency
waveform into the laser. A back facet photodiode sensor detects
radiation from a back facet of the laser and provides a feedback
signal to the control circuit to maintain a power level of the
laser constant. A radio frequency waveform causes the laser to
oscillate above and below a DC bias point between the levels of a
lasing threshold level and an asymmetrical level above a DC bias
point. An injection circuit injects the radio frequency waveform
with a duty cycle greater than 50%.
[0015] According to one embodiment of the present invention a radio
frequency signal is injected into a semiconductor laser, wherein
the waveform has a duty cycle greater than 50%. A control circuit
connected to the laser provides the control signal and an injection
circuit injects a radio frequency signal into the laser. A back
facet photodiode sensor detects radiation emitted from a back facet
of the laser diode and provides a feedback signal to the control
circuit for adjusting laser output power.
[0016] The advantage of injecting a radio frequency waveform with a
high duty cycle into a semiconductor laser is that the laser will
have both high output power and stability without exceeding the
maximum rated parameters of current or power. For example, a 50 mW
laser with an RF waveform with a 90% duty cycle will allow 45 mW of
stabilized power without driving the current above I.sub.op, the
maximum rated current. In order to achieve high power, the laser is
operated predominantly above laser threshold, and will only operate
near the lasing threshold for short intervals of the duty
cycle.
[0017] To achieve stability, the injection of the radio frequency
waveform will force the laser to mode hop at high frequency,
essentially forcing the laser to have a stable multimode spectrum.
This result is accomplished by driving the laser down to or
slightly below threshold, forcing it out of lasing and then
allowing it to re-establish lasing at a rate of millions of times
per second. Because the spectral output is stable over time, the
current from the photodiode is truly representative of the laser
output power. A shift in current represents a drift in laser output
power, not a hop in laser wavelength. The rate of intensity
fluctuation will be greater than that which a back facet photodiode
detects because of the photodiode's response characteristics. From
the low frequency perspective of the photodiode and feedback
circuit, the laser intensity is stable. Since the spectrum detected
by the photodiode is stable, historical problems associated with
using a back facet photodiode and control circuit as a means of
stabilizing a laser will be solved. Only significant slow drifts in
the laser output power, not wavelength, will be detected, and the
control circuit will make appropriate adjustments to the current
supplied to the semiconductor laser.
[0018] The added complexity and cost associated with thermoelectric
cooling can be eliminated. Because radio frequency injection
creates laser stability, it eliminates the need to have a
thermoelectric cooler control the temperature of the laser. Any
changes in the output wavelength of the laser will be very minor,
and it is unnecessary to introduce the expense and complexity of a
thermoelectric cooler to control the laser.
[0019] Laser noise associated with mode hop that may normally
appear in an image can be shifted to higher frequencies where it is
not noticeable by the human eye. The present invention uses a
circuit to inject a high duty cycle radio frequency waveform to
force the laser to a stable multimode spectral output. Any mode
hopping that occur will be at the injected radio frequency.
Increasing the mode hopping frequency of a laser shifts the noise
spectrum of the laser such that the intensity noise is averaged
within each pixel, thus making the noise less visible in images
that are written with lasers.
[0020] The invention and its objects and advantages will become
more apparent in the detailed description of the preferred
embodiment presented below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a partial cut away perspective view of a
semiconductor laser;
[0022] FIG. 2 is a schematic view of a radio frequency (RF)
stabilized laser according to the present invention;
[0023] FIG. 3 is a graph of power versus input current for a
stabilized semiconductor laser with a RF injected sine wave;
[0024] FIG. 4 is a graph of power versus input current for a
stabilized semiconductor laser with a RF injected waveform having a
90% duty cycle;
[0025] FIG. 5 is a graph of input current versus time for a
stabilized semiconductor laser with a high duty cycle RF injected
waveform;
[0026] FIG. 6 is a schematic of a control circuit and RF injection
circuit;
[0027] FIG. 7a is a schematic of a distorted sine wave oscillator
circuit used to generate a high duty cycle RF waveform to be
injected into a semiconductor laser;
[0028] FIG. 7b is a graph of the semiconductor laser drive current
showing laser operating current I.sub.op and lasing threshold
current I.sub.th;
[0029] FIG. 8a is a schematic of a shunt modulator circuit used to
generate a high duty cycle RF waveform used to generate a high duty
cycle RF waveform to be injected into a semiconductor laser;
[0030] FIG. 8b is a graph of a pulsed input signal to the shunt
modulator circuit;
[0031] FIG. 8c is a graph of the semiconductor laser drive current
showing laser operating current I.sub.op and lasing threshold
current I.sub.th.
[0032] FIG. 9a is a schematic of a fast pulse network modulator
circuit used to generate a high duty cycle RF waveform to be
injected into a semiconductor laser;
[0033] FIG. 9b is a graph of the semiconductor laser drive current
showing laser operating current I.sub.op and lasing threshold
current I.sub.th.
DETAILED DESCRIPTION OF THE INVENTION
[0034] The present invention will be directed in particular to
elements forming part of, or in cooperation more directly with, the
apparatus in accordance with the present invention. It is
understood that elements not specifically shown or described may
take various forms well known to those skilled in the art.
[0035] FIG. 1 shows a semiconductor laser 12. Laser 12 is in a
container defined by a cap 104 having an aperture 103 in a stem 106
and terminal 107. A semiconductor laser element 101 is mounted on a
heatsink 105 with a light-emitting face on the side of aperture
103. A back facet photodiode 102 is fixed to stem 106 with a light
receiving surface facing the semiconductor laser element 101. A
laser beam 110 and a light power output (P.sub.o) is emitted from
the semiconductor laser element 110 through aperture 103. At the
same time, a monitor beam 120 with a light power output (P.sub.m),
at usually about 3% of P.sub.o, is emitted from the semiconductor
laser element 101 toward the photodiode 102. The laser beam 110 is
directed through writer optics, not shown.
[0036] FIG. 2 shows a RF stabilized laser configuration 10. A laser
diode 12 and laser drive assembly 40 are attached to an aluminum
block 16 which is screwed to a heatsink plate 18. The heatsink
plate 18 is attached to a collimator mount 24, which in turn is
attached to mounting bracket 20. Collimator mount 24 also holds a
collimator lens 22. The stabilized laser 10 is aligned to writer
optics, not shown. In an alternate embodiment, the stabilized laser
10 is coupled to an optical fiber allowing the stabilized laser 10
to be mounted at a remote location.
[0037] FIG. 3 shows a graph of power versus input current for a 50
mW Mitsubishi 1413 R01 semiconductor laser, with a threshold just
above 30 mA and a maximum current of 90 mA. When the DC level is
set to 60 mA, the laser provides 25 mW of output, which is half of
the rated value. The AC signal, provided by a Colpitts RF
oscillator, is added to the applied DC level to swing the laser
current from 30 mA (I.sub.th, laser threshold) to 90 mA (I.sub.op,
the maximum rated optical power out). Laser power I.sub.op is 50 mW
for this laser and at I.sub.th it is approximately 0.1 mW optical
power. The semiconductor laser is turned on to maximum power and
essentially turned off during each cycle of the RF. The RF
frequency is typically about 200 MHz for a writer system with a
pixel clock of approximately 20 MHz, thereby turning the laser on
and off about 10 times during each pixel. The sine wave generated
by the Colpitts oscillator has a 50% duty cycle, because it is easy
to generate but has little or no higher harmonic content. Only the
fundamental 200 MHz signal is generated, making it easier to
deliver the signal to the laser diode.
[0038] FIG. 4 shows a graph of power versus input current with a
90% duty cycle waveform. When the duty cycle of the injection
waveform is increased, the average power level of the stabilized
power will be increased. For example, if the waveform has a duty
cycle of 90%, then 45 mW out of a possible 50 mW would be
stabilized output power.
[0039] FIG. 5 shows a waveform where the drive signal is
predominantly at a high level, and only occasionally goes low in a
very short duration spike. The spike must be low enough to take the
laser below threshold and just long enough to disrupt lasing.
[0040] FIG. 6 shows a laser drive system 30, a power level adjust
42, and a control circuit 41 to provide constant laser power output
by utilizing the feedback signal 50 from the photodiode 102. A high
duty cycle RF source 44, commonly called an injection circuit, is
injected into the semiconductor laser 101 to induce a stable
multimode spectrum.
[0041] FIG. 7a shows a schematic of a distorted sine wave
oscillator circuit used to generate a high duty cycle RF waveform
to be injected into a semiconductor laser. A sine wave oscillator
with excess feedback and altered bias is used to create an
asymmetrical sine wave. When injected into the semiconductor laser,
the asymmetrical radio frequency sine waveform is capable of
stabilizing the output spectrum of the laser and increasing the
laser's output power. FIG. 7b is a graph of the semiconductor laser
drive current showing laser operating current I.sub.op and lasing
threshold current I.sub.th. For example, if a 200 MHz RF distorted
sine waveform is injected, the semiconductor laser is driven down
to or slightly below threshold and forced to come back up into
lasing at 200 million times a second. Based on the DC level, the RF
is adjusted to drive the laser to operate at threshold or slightly
below threshold. However, the drive current should stay above 0 mA.
If the drive current is below zero, the laser could become back
biased and be destroyed. Likewise, driving the laser above its
rated I.sub.op can cause damage or reduce the lifetime of the
laser. Moreover, the multimode operation of the semiconductor laser
will transfer the intrinsic noise of said laser to higher
frequencies, thus substantially reducing their visibility when such
a laser is integrated into a laser raster system capable of writing
images. Furthermore, the back facet photodiode, which is used in
combination with the control circuit to monitor and control the
output of the laser, is not responsive to the fast switching at the
radio frequency . The back facet photodiode cannot detect the rapid
changes in the output of the laser, and therefore continues to
supply the same amount of current. Changes in laser output are
therefore only detected within the response characteristics of the
photodiode. Because the laser spectral output is stable over time,
the current from the photodiode is truly representative of the
laser output power. A shift in current now represents a drift in
laser output power, not a hop in laser wavelength. Thus, the
historical unreliability of back-facet photodiodes to control laser
output power is remedied.
[0042] FIG. 8a is a schematic of a shunt modulator circuit used to
generate a high duty cycle RF waveform to be injected into a
semiconductor laser. The shunt modulator circuit is comprised of a
DC current source and an active device. The active device in FIG.
8a is a single NPN bipolar transistor. However, other active
components could be combined to produce the same effect in the
shunt modulator circuit. The DC current is momentarily shunted by
an active device connected in parallel with the ground or a
suitable alternate load for a brief period of time. FIG. 8b is a
graph of the semiconductor laser drive current showing laser
operating current I.sub.op and lasing threshold current I.sub.th.
When the pulsed input of the active device briefly shunts the
current from the semiconductor laser, the laser operates at or
below lasing threshold. While the current is not being shunted, the
semiconductor laser operates above lasing threshold. Frequent
switching between operation near lasing threshold and above lasing
threshold will induce multimode operation in the laser. Adjusting
the pulsed input signal to the active element of the circuit will
affect the duration that the laser is lasing above threshold, and a
stable laser with a high power will result. In addition, the
multimode operation of the semiconductor laser will transfer the
intrinsic noise of said laser to higher frequencies, thus
substantially reducing their visibility when such a laser is
integrated into a laser raster system capable of writing image.
Furthermore, the back facet photodiode, which is used in
combination with the control circuit to monitor and control the
output of the laser, is not responsive enough to the fast
switching. The back facet photodiode cannot detect the changes in
the output of the laser, and therefore continues to supply the same
amount of current. Changes in laser output are therefore only
detected within the response characteristics of the photodiode.
Because the laser spectral output is stable over time, the current
from the photodiode is truly representative of the laser output
power. A shift in current now represents a drift in laser output
power, not a hop in laser wavelength. Thus, the historical
unreliability of backfacet photodiodes to control laser output
power is remedied.
[0043] FIG. 9a is a schematic of a fast pulse network modulator
circuit used to generate a high duty cycle RF waveform to be
injected into a semiconductor laser. The circuit consists of a DC
current source, a transformer, and a diode wherein said diode is
"fast clamping" and sensitive to large pulses that occur rapidly
over time. A fast pulse generator, such as a blocking oscillator,
is used to create narrow pulses that are superimposed onto the DC
drive current to the semiconductor laser. Additional control
circuitry is required to control the pulses, as well as to prevent
reverse polarity on the semiconductor laser. FIG. 9b is a graph of
the semiconductor laser drive current showing laser operating
current I.sub.op and lasing threshold current I.sub.th. The graph
shows that the laser current drive signal will allow the laser to
operate above threshold, and operates near threshold for short
periods. Frequent switching between operation near lasing threshold
and above lasing threshold will induce multimode operation in the
laser. Adjusting the pulsed input signal to the active element of
the circuit will affect the duration that the laser is lasing above
threshold, and a stable laser with a high power output will result.
In addition, the multimode operation of the semiconductor laser
will transfer the intrinsic noise of the laser to higher
frequencies, thus substantially reducing their visibility when such
a laser is integrated into a laser raster system capable of writing
images. Furthermore, the back facet photodiode, which is used in
combination with the control circuit to monitor and control the
output of the laser, is not responsive to the fast switching. The
back facet photodiode cannot detect the changes in the output of
the laser, and therefore continues to supply the same amount of
current. Changes in laser output are therefore only detected within
the response characteristics of the photodiode. Thus, the
historical unreliability of back-facet photodiodes to control laser
output power is remedied.
[0044] Single longitudinal mode operation is the quietest method of
laser operation. However, it is difficult to keep the laser from
mode hopping for long periods of time. Driving the laser to
multiple longitudinal mode operation with RF injection is the next
quietest method of operation. In noise level tests on a
semiconductor laser, it is believed that the laser is not
necessarily operating multimode at any instant it is turned on.
Rather, operating the laser at or slightly below lasing threshold
allows it to resume lasing in any of the approximately 4 or 5
likely longitudinal modes. Cycling between near threshold and
lasing many times during the writing of one pixel allows an
averaging effect to take place. The RF frequency should be several
times the pixel clock frequency or pixel rate, with 10 times being
a reasonable value. Thus, if all mode possibilities are not of the
same intensity, the exposure from the average of ten samples should
not vary significantly. The noise is not completely eliminated, but
it is effectively confined to each written pixel, and does not show
up as light and dark spots in an image. In addition, driving the
laser to essentially multimode operation yields a stable output,
which eliminates the cost and complexity of laser output control by
thermoelectric cooling.
[0045] Thus, it is seen that a stabilized laser according to the
present invention using radio frequency signal injection is able to
produce high power output, produce a stable output spectrum that
eliminates the need for thermoelectric cooling, and confine the
inherent laser noise within each pixel of an image.
[0046] The invention has been described in detail with particular
reference to certain preferred embodiments thereof, but it will be
understood that variations and modifications can be effected within
the scope of the invention.
PARTS LIST
[0047] 10. Laser configuration
[0048] 12. Semiconductor laser
[0049] 16. Aluminum block
[0050] 18. Heatsink plate
[0051] 20. Mounting bracket
[0052] 22. Collimator lens
[0053] 24. Collimator mount
[0054] 30. Laser drive system
[0055] 40. Laser drive assembly
[0056] 41. Control circuit
[0057] 42. Power level adjust
[0058] 44. High duty cycle RF source
[0059] 50. Feedback signal
[0060] 101. Semiconductor laser element
[0061] 102. Photodiode
[0062] 103. Aperture
[0063] 104. Cap
[0064] 105. Heatsink
[0065] 106. Stem
[0066] 107. Terminal
[0067] 110. Laser element
[0068] 120. Monitor beam
* * * * *