U.S. patent application number 12/915162 was filed with the patent office on 2012-05-03 for systems and methods for visible light source evaluation.
Invention is credited to Steven Joseph Gregorski.
Application Number | 20120105834 12/915162 |
Document ID | / |
Family ID | 45994391 |
Filed Date | 2012-05-03 |
United States Patent
Application |
20120105834 |
Kind Code |
A1 |
Gregorski; Steven Joseph |
May 3, 2012 |
SYSTEMS AND METHODS FOR VISIBLE LIGHT SOURCE EVALUATION
Abstract
Particular embodiments of the present disclosure relate systems
and methods for evaluating visible light sources. According to one
embodiment, a method of evaluating a visible light source including
a semiconductor laser having a gain section, a wavelength selective
section, and a phase section includes applying a gain drive signal
to the gain section of the semiconductor laser at a gain modulation
frequency, and applying a triangular wave drive signal to the
wavelength selective section of the semiconductor laser at a
wavelength selective modulation frequency that is greater than the
gain modulation frequency. The light source emits a plurality of
optical output pulses. Output power values of the optical output
pulses at a selected wavelength are detected. The output power
value of one or more selected output pulses is compared with an
output power threshold value to generate an indication of whether
the visible light source satisfies an output power
specification.
Inventors: |
Gregorski; Steven Joseph;
(Painted Post, NY) |
Family ID: |
45994391 |
Appl. No.: |
12/915162 |
Filed: |
October 29, 2010 |
Current U.S.
Class: |
356/229 |
Current CPC
Class: |
H01S 5/06256 20130101;
H01S 5/0092 20130101; H01S 5/06251 20130101; H01S 5/0014
20130101 |
Class at
Publication: |
356/229 |
International
Class: |
G01J 1/08 20060101
G01J001/08 |
Claims
1. A method of evaluating a visible light source comprising a
semiconductor laser optically coupled to a wavelength conversion
device, the semiconductor laser comprising a gain section, a
wavelength selective section, and a phase section, and the method
comprising: applying a gain drive signal to the gain section of the
semiconductor laser at a gain modulation frequency; applying a
triangular wave drive signal to the wavelength selective section of
the semiconductor laser at a wavelength selective modulation
frequency that is greater than the gain modulation frequency,
wherein the visible light source emits a plurality of optical
output pulses in response to the gain drive signal and the
triangular wave drive signal; detecting an output power value of
individual ones of the plurality of optical output pulses at a
selected wavelength; and comparing an output power value of one or
more selected output pulses with an output power threshold value to
generate an indication of whether the visible light source
satisfies an output power specification.
2. The method as claimed in claim 1 further comprising: determining
a maximum optical output pulse having a maximum output power value;
and comparing the maximum output power value with the output power
threshold value.
3. The method as claimed in claim 1 further comprising: determining
an average output power value of two or more of individual ones of
the plurality of optical output pulses; and comparing the average
output power value with the output power threshold value.
4. The method as claimed in claim 1 wherein the wavelength
conversion device frequency-doubles an output beam emitted by the
semiconductor laser such that the plurality of optical output
pulses are in a green spectral range.
5. The method as claimed in claim 4 wherein: the triangular wave
drive signal varies a wavelength of the output of the semiconductor
laser; and the wavelength of the output emitted by the
semiconductor laser matches a phase matching wavelength of the
wavelength conversion device twice during each period of the
triangular wave drive signal while the gain drive signal provides a
current to the gain section.
6. The method as claimed in claim 5 wherein: the method further
comprises determining a wavelength matching current for the
wavelength conversion device; and the triangular wave drive signal
is characterized by a peak-to-peak amplitude of 20 mA that is
centered on the wavelength matching current.
7. The method as claimed in claim 1 wherein: the gain drive signal
comprises a gain duty cycle; and the wavelength selective
modulation frequency is greater than the gain modulation frequency
divided by the gain duty cycle.
8. The method as claimed in claim 1 wherein the gain modulation
frequency is within a range of about 0 Hz to about 1.0 kHz.
9. The method as claimed in claim 1 wherein the gain drive signal
and the triangular wave drive signal are asynchronous with respect
to one another.
10. The method as claimed in claim 1 further comprising: varying a
gain duty cycle of the gain drive signal across a duty cycle range;
comparing output power values of optical output pulses associated
with a low gain duty cycle with optical output power values of
optical output pulses associated with a high gain duty cycle to
determine an optical output power value variation; and comparing
the optical output power value variation with a variation threshold
value to generate an indication of whether the visible light source
satisfies a thermal impedance specification.
11. The method as claimed in claim 1 further comprising applying a
phase drive signal to the phase section of the semiconductor laser,
wherein the phase drive signal and the triangular wave drive signal
applied to the wavelength selective section are
anti-correlated.
12. A system for evaluating a visible light source comprising a
semiconductor laser and a wavelength conversion device optically
coupled to the semiconductor laser, the semiconductor laser
comprising a gain section, a wavelength selective section, and a
phase section, wherein: the system comprises a drive electronics
controller and an optical power detection and analysis module; the
drive electronics controller comprises a gain section driver and a
wavelength selective section driver; and the system is programmed
such that: the gain section driver applies a gain drive signal to
the gain section of the semiconductor laser at a gain modulation
frequency, the wavelength selective section driver applies a
triangular wave drive signal to the wavelength selective section of
the semiconductor laser at a wavelength selective modulation
frequency that is greater than the gain modulation frequency, the
visible light source emits a plurality of optical output pulses in
response to the gain drive signal and the triangular wave drive
signal, and the plurality of optical output pulses is detected by
the optical power detection and analysis module, and an output
power value of one or more selected optical output pulses at a
selected wavelength is compared with an output power threshold
value to generate an indication of whether the visible light source
satisfies an output power specification.
13. The system as claimed in claim 12 wherein the system is further
programmed such that a maximum optical output pulse having a
maximum output power value is determined and compared with the
output power threshold value.
14. The system as claimed in claim 12 wherein the system is further
programmed such that an average output power value of two or more
of individual ones of the plurality of optical output pulses is
determined and compared with the output power threshold value.
15. The system as claimed in claim 12 wherein the wavelength
conversion device frequency-doubles an output beam emitted by the
semiconductor laser such that the plurality of optical output
pulses are in a green spectral range.
16. The system as claimed in claim 15 wherein: the triangular wave
drive signal varies a wavelength of the output emitted by the
semiconductor laser; and the wavelength of the output emitted by
the semiconductor laser matches a phase matching wavelength of the
wavelength conversion device twice during each period of the
triangular wave drive signal while the gain drive signal provides a
current to the gain section.
17. The system as claimed in claim 16 wherein: the system is
further programmed to detect a wavelength matching current for the
wavelength conversion device; and the triangular wave drive signal
is characterized by a peak-to-peak amplitude that is centered on
the wavelength matching current.
18. The system as claimed in claim 12 wherein: the gain section
driver applies the gain drive signal at a gain duty cycle; and the
wavelength selective modulation frequency is greater than the gain
modulation frequency divided by the gain duty cycle.
19. The system as claimed in claim 11 wherein the system is further
programmed such that: the gain section driver applies the gain
drive signal to the gain section of the semiconductor laser at a
gain duty cycle that varies across a duty cycle range; output power
values of optical output pulses associated with a low gain duty
cycle are compared with optical output power values of optical
output pulses associated with a high gain duty cycle to determine
an optical output power value variation; and the optical output
power value variation is compared with a variation threshold value
to generate an indication of whether the semiconductor laser
satisfies a thermal impedance specification.
20. A method of evaluating a visible light source comprising a
semiconductor laser optically coupled to a wavelength conversion
device, the semiconductor laser comprising a gain section, a
wavelength selective section, and a phase section, and the method
comprising: applying a gain drive signal to the gain section of the
semiconductor laser at a gain modulation frequency and a gain duty
cycle; applying a triangular wave drive signal to the wavelength
selective section of the semiconductor laser at a wavelength
selective modulation frequency, wherein the wavelength selective
modulation frequency is greater than the gain modulation frequency
divided by the gain duty cycle, and the visible light source emits
a plurality of optical output pulses in response to the gain drive
signal and the triangular wave drive signal; detecting an output
power value of individual ones of the plurality of optical output
pulses at a desired wavelength; comparing an output power value of
one or more selected output pulses with an output power threshold
value to generate an indication of whether the visible light source
satisfies an output power specification; varying the gain duty
cycle of the gain drive signal across a duty cycle range; comparing
output power values of optical output pulses associated with a low
gain duty cycle with optical output power values of optical output
pulses associated with a high gain duty cycle to determine an
optical output power value variation; and comparing the optical
output power value variation with a variation threshold value to
generate an indication of whether the visible light source
satisfies a thermal impedance specification.
Description
BACKGROUND
[0001] 1. FIELD
[0002] Embodiments of the present disclosure generally relate to
the evaluation of semiconductor lasers. More specifically, the
embodiments relate to systems and methods of evaluating the optical
output of visible light sources comprising a wavelength conversion
device and a semiconductor laser having a gain section, a
wavelength selective section, and a phase section.
[0003] 2. Technical Background
[0004] Semiconductor lasers, such as distributed Bragg reflector
(DBR) lasers, are being utilized in an increasing number of
applications. For example, semiconductor lasers capable of
producing optical radiation in the red, blue and green optical
spectrums may be incorporated into a laser projector to produce
scanned laser images. However, wavelength thermal drift and cavity
mode-hopping in semiconductor lasers may adversely affect optical
output power and create noticeable defects in the scanned laser
image produced by laser scanner system (particularly those emitting
wavelength-converted optical radiation in the green spectral
range). Therefore, some semiconductor laser packages are driven by
a high frequency modulated gain current to minimize the effect of
wavelength thermal drift and cavity mode-hopping in the image.
However, the complicated electronics necessary to produce such high
frequency gain currents are not cost effective or easily
implemented into testing procedures of high-volume production
environments. For example, high-frequency, high-gain current gain
drive signals may require that the test equipment be located very
close to the laser under test and may also produce significant
electrical noise. Further, the high-frequency, high-gain current
gain drive signals may necessitate soldered connections of the
laser under test to the evaluation equipment, which may be
unacceptable in a high-volume production environment.
[0005] Accordingly, a need exists for alternative systems and
methods for evaluating visible light sources.
SUMMARY
[0006] Generally, embodiments described herein are directed to
systems and methods for evaluating a visible light source in a
production environment. By way of example and not limitation, a
visible light source may comprises one or more semiconductor lasers
that are configured to emit radiation at one or more wavelengths
that are converted by a wavelength conversion device. The visible
light source may be implemented in a laser system, such as a laser
projection system, for example. During production of the visible
light sources, it may be desirable to perform an evaluation to
determine if the visible light sources produced meet particular
specifications. Evaluation processes should be efficient so that
visible light sources may be rapidly tested to reduce the overall
cost of the fabrication process. Those visible light sources
satisfying the specifications, such as optical output power and
thermal impedance, for example, may be marked as satisfactory and
passed on to the next step in the fabrication process. Those not
meeting the specifications may be marked as scrap and
discarded.
[0007] In some laser applications, the gain section of one or more
of the semiconductor lasers included in a visible light source may
be driven by the application of a high-frequency current (e.g.,
>25 MHz at 650 mA) to a gain section. The high-frequency gain
current may be utilized to minimize wavelength thermal drift and
cavity mode-hopping within the semiconductor laser. However,
testing each visible light source/semiconductor laser with such
high-frequency and high gain current presents significant problems
in fully-automated, high-volume manufacturing environments. For
example, test methods utilizing high-frequency, high-current drive
signals require expensive circuitry, prevents the drive signal from
being located in relatively close proximity to the light source
under test, and produce undesirable electrical noise.
[0008] Generally, embodiments utilize a low frequency gain drive
signal to drive a gain section of a semiconductor laser
incorporated into a visible light source package. A triangular wave
drive signal is applied to a wavelength selective section of the
semiconductor laser at a frequency that is greater than the
frequency of the gain drive signal to produce a plurality of output
pulses that are detected and analyzed to evaluate various
properties of the visible light source under test.
[0009] In one embodiment, a method of evaluating a visible light
source including a wavelength conversion device and a semiconductor
laser having a gain section, a wavelength selective section, and a
phase section includes applying a gain drive signal to the gain
section of the semiconductor laser at a gain modulation frequency,
and applying a triangular wave drive signal to the wavelength
selective section of the semiconductor laser at a wavelength
selective modulation frequency that is greater than the gain
modulation frequency. The visible light source emits a plurality of
optical output pulses in response to the gain drive signal and the
triangular wave drive signal. The method further includes detecting
an output power value of individual ones of the plurality of
optical output pulses at a selected wavelength, and comparing the
output power value of one or more selected output pulses with an
output power threshold value to generate an indication of whether
the visible light source satisfies an output power
specification.
[0010] In another embodiment, a system for evaluating a visible
light source including a semiconductor laser and a wavelength
conversion device optical coupled to the semiconductor laser, the
semiconductor laser having a gain section, a wavelength selective
section, and a phase section includes a drive electronics
controller and an optical power detection and analysis module. The
drive electronics controller has a gain section driver and a
wavelength selective section driver. The system is programmed such
that the gain section driver applies a gain drive signal to the
gain section of the semiconductor laser at a gain modulation
frequency, and the wavelength selective section driver applies a
triangular wave drive signal to the wavelength selective section of
the semiconductor laser at a wavelength selective modulation
frequency that is greater than the gain modulation frequency. The
visible light source emits a plurality of optical output pulses in
response to the gain drive signal and the triangular wave drive
signal. The system is further programmed such that the plurality of
optical output pulses is detected by the optical power detection
and analysis module, and an output power value of one or more
selected optical output pulses at a selected wavelength are
compared with an output power threshold value to generate an
indication of whether the visible light source satisfies an output
power specification.
[0011] In yet another embodiment, a method of evaluating a visible
light source including a wavelength conversion device and a
semiconductor laser having a gain section, a wavelength selective
section, and a phase section includes applying a gain drive signal
to the gain section of the semiconductor laser at a gain modulation
frequency, and applying a triangular wave drive signal to the
wavelength selective section of the semiconductor laser at a
wavelength selective modulation frequency that is greater than the
gain modulation frequency. The visible light source emits a
plurality of optical output pulses in response to the gain drive
signal and the triangular wave drive signal. The method further
includes detecting an output power value of individual ones of the
plurality of optical output pulses at a selected wavelength, and
comparing the output power value of one or more selected output
pulses with an output power threshold value to generate an
indication of whether the visible light source satisfies an output
power specification. A gain duty cycle of the gain drive signal may
be varied across a duty cycle range, and output power values of
optical output pulses associated with a low gain duty cycle are
compared with optical output power values of optical output pulses
associated with a high gain duty cycle to determine an optical
output power value variation. The optical output power value
variation is compared with a variation threshold value to generate
an indication of whether the visible light source satisfies a
thermal impedance specification.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a schematic illustration of a system for
evaluating a visible light source according to one or more
embodiments described and illustrated herein;
[0013] FIG. 2 is a graph illustrating an exemplary gain drive
signal and an exemplary triangular wave drive signal according to
one or more embodiments described and illustrated herein;
[0014] FIG. 3 is a graph illustrating an exemplary gain drive
signal, an exemplary triangular wave drive signal and instances of
phase matching between a semiconductor laser and a wavelength
conversion device according to one or more embodiments described
and illustrated herein;
[0015] FIG. 4 is a graph illustrating an exemplary CW gain drive
signal and an exemplary triangular wave drive signal according to
one or more embodiments described and illustrated herein;
[0016] FIG. 5 is a graph illustrating an exemplary output response
of a semiconductor laser driven by the gain drive signal and
triangular wave drive signal illustrated in FIG. 2; and
[0017] FIG. 6 is a graph illustrating an exemplary output response
of a semiconductor laser driven by the gain drive signal and
triangular wave drive signal illustrated in FIG. 4.
DETAILED DESCRIPTION
[0018] Embodiments described herein are directed to systems and
methods for evaluating visible light sources and associated
semiconductor lasers and wavelength conversion devices using a low
frequency periodic gain drive signal applied to the gain section
and a triangular wave drive signal to a wavelength selective
section of the semiconductor laser. The semiconductor laser emits a
plurality of optical output pulses in response to the periodic gain
drive signal and the triangular wave drive signal that are
converted into higher harmonics by a wavelength conversion device.
These pulses are detected and analyzed to determine if the visible
light source satisfies particular specification requirements.
[0019] A system 100 for evaluating a visible light source 110 under
test is illustrated in FIG. 1. The system 100 generally comprises a
drive electronics controller 140 and an optical power detection and
analysis module 130. As described in more detail below, components
of the drive electronics controller 140 and the optical power
detection and analysis module 130 may reside in a computing device
132, such as a computer, with a data acquisition module 135 and an
analog voltage waveform generator.
[0020] The visible light source 110 illustrated in FIG. 1 comprises
a semiconductor laser 111 and a wavelength conversion device 118.
Although the specific structure of the various types of
semiconductor lasers in which the concepts of particular
embodiments of the present invention can be incorporated is taught
in readily available technical literature relating to the design
and fabrication of semiconductor lasers, the concepts of particular
embodiments of the present disclosure may be conveniently
illustrated with general reference to a three-section DBR-type
semiconductor laser 111 illustrated schematically in FIG. 1. In
FIG. 1, the DBR laser 111 is optically coupled to the wavelength
conversion device 118. The light beam output 117 emitted by the
semiconductor laser 111 can be either directly coupled into a
waveguide 119 of the wavelength conversion device 180 or can be
coupled through collimating and focusing optics, or some other type
of suitable optical element or optical system. The wavelength
conversion device 118, which may be configured as a non-linear
optical crystal, converts the incident light into higher harmonic
waves using the non-linear properties of the crystal and outputs
the converted signal (e.g., by second harmonic generation (SHG)).
This type of configuration is particularly useful in generating
shorter wavelength laser beams from longer wavelength semiconductor
lasers and can be used, for example, as a visible laser source for
laser projection systems.
[0021] The DBR laser 111 illustrated schematically in FIG. 1
comprises a wavelength selective section 112, a phase section 114,
and a gain section 116. The wavelength selective section 112, which
can also be referred to as the DBR section of the laser 111,
typically comprises a first order or second order Bragg grating
positioned outside the active region of the laser cavity. This
section provides wavelength selection, as the grating acts as a
mirror whose reflection coefficient depends on the wavelength. The
gain section 116 of the DBR laser 111 provides the major optical
gain of the laser and the phase section 114 creates an adjustable
phase shift between the gain material of the gain section 116 and
the reflective material of the wavelength selective section 112.
The wavelength selective section 112 may be provided in a number of
suitable alternative configurations that may or may not employ a
Bragg grating.
[0022] Respective control electrodes 102, 104, 106, are
incorporated in the wavelength selective section 112, the phase
section 114, the gain section 116, or combinations thereof, and are
merely illustrated schematically in FIG. 1. It is contemplated that
the electrodes 102, 104, 106 may take a variety of forms. For
example, the control electrodes 102, 104, 106 are illustrated in
FIG. 1 as respective electrode pairs but it is contemplated that
single electrode elements 102, 104, 106 in one or more of the
sections 112, 114, 116 will also be suitable for practicing
particular embodiments of the present invention. The control
electrodes 102, 104, 106 can be used to inject electrical current
into the corresponding sections 112, 114, 116 of the laser 111. The
injected current can be used to alter the operating properties of
the laser by, for example, controlling the temperature of one or
more of the laser sections, injecting electrical current into a
conductively doped semiconductor region defined in the laser
substrate, controlling the index of refraction of the wavelength
selective and phase sections 112, 114 of the laser 111, controlling
optical gain in the gain section 116 of the laser, etc.
[0023] The wavelength conversion efficiency of the wavelength
conversion device 118 illustrated in FIG. 1 is dependent on the
wavelength matching between the semiconductor laser 111 and the
wavelength conversion device 118. In one embodiment, the wavelength
conversion device 118 is a MgO-doped periodically poled lithium
niobate (PPLN) crystal. The wavelength at which the wavelength
conversion device 118 converts most efficiently is referred to as
the phasematching wavelength. The output power of the higher
harmonic light wave 120 emitted from the wavelength conversion
device 118 drops drastically when the output wavelength of the
laser 111 deviates from the optimal conversion wavelength of the
wavelength conversion device 118.
[0024] The operational specifications of the visual light may
depend on the particular applications that the visible light
sources are to be implemented in. For example, the visual light
source 110 illustrated in FIG. 1 may be configured such that it
emits optical radiation in the green spectral range, and may be
used in conjunction with a red and blue semiconductor laser in a
laser projection system. The green visible light source 110 should
emit a green output laser beam at a output power level that is
above an output power level threshold value so that the laser
projection system may produce satisfactory scanned laser
images.
[0025] Still referring to FIG. 1, the drive electronics controller
140 may be configured as an analog voltage waveform generator
capable of generating waveforms described herein. The drive
electronics controller 140 may comprise a gain section driver that
provides a gain drive signal and a wavelength selective section
driver that provides a triangular wave drive signal. The drive
electronics controller 140 may be configured as an electronics card
that is provided in a computer 132, as illustrated in FIG. 1. The
drive electronics controller 140 may also be configured as a
stand-alone device separate from a computer, such as a function
generator.
[0026] Also schematically illustrated in FIG. 1 is the gain section
and wavelength selective section amplifier drive electronics 144
that are electrically coupled to the drive electronics controller
140 and electrodes 102, 106, of the wavelength selective and gain
sections 112, 116, respectively, as shown by arrows 142, 143. The
drive electronics controller 140 is configured to provide the
respective drive waveforms to the amplifier drive electronics 144
to control the gain and wavelength of the semiconductor laser 111.
The amplifier drive electronics 144 amplifies the waveforms
provided by the drive electronics controller 140. In one
embodiment, the amplifier drive electronics 144 may be a component
of the semiconductor laser 111. Alternatively, in another
embodiment, the amplifier drive electronics 144 may be a component
of the evaluation system 100 and incorporated into the drive
electronics controller 140. The drive waveforms cause the
semiconductor laser 111 to emit an output beam 117. As an example
and not a limitation, the output beam emitted by the semiconductor
laser has a wavelength of approximately 1060 nm that is then
inputted into the wavelength conversion device 118, where it is
frequency-doubled and exits as converted output beam 120 having a
converted wavelength of approximately 530 nm.
[0027] It is noted that the visible light source 110 should be
coupled to the evaluation system 100 such that it is easily
connected and disconnected to reduce overall testing time. Any
number of electrical connections may be utilized to removably
connect the semiconductor laser 111 of the visible light source to
the drive electronics controller 140 (and the amplifier drive
electronics 144 if a component separate from the semiconductor
laser 111).
[0028] The optical power detection and analysis module 130 may be
configured as one or more components. In the embodiment illustrated
in FIG. 1, the optical power detection and analysis module 130
comprises an optical power detector 133 and an analog voltage data
acquisition module 135. The optical power detector 133 should be
capable of detecting the energy of an optical signal at the desired
wavelength and/or range of wavelengths (e.g., wavelengths in the
green spectral range). As an example and not a limitation, the
optical power detector 133 may include a photodiode configured to
detect optical energy at about 560 nm. The optical power detector
133 converts the optical energy into an analog voltage that
corresponds to the amount of optical energy detected. The analog
signal generated by the optical power detector 133 may then be
provided to the analog voltage data acquisition module 135, which
may be configured as a data acquisition card that is installed on a
computing device 132. The optical power detection and analysis
module 130 may also utilize a processor and software residing on
the computing device 132. In another embodiment, the analog voltage
data acquisition module 135 may be a stand-alone device and not
incorporated into the computing device. The analog voltage data
acquisition module 135 receives the analog signal from the optical
power detector 133 and converts it into digital data. The computing
device 132 may perform additional analysis functionality described
herein, such as selecting maximum optical output pulses, averaging
output power, and other functions as described below. In an
alternative embodiment, the optical power detector 133 and the
analog voltage data acquisition module 135 may be configured as a
single component that detects optical energy and converts an analog
signal into digital data that is provided to a computing device
132.
[0029] Embodiments described herein drive the gain section 116 with
a relatively slow (e.g., a 100 Hz gain modulation frequency) square
wave with a maximum gain current and gain duty cycle. By modulating
the gain section 116 at a specified gain duty cycle, the maximum
gain current and the time-averaged thermal loading of the
semiconductor laser 111 may be independently controlled.
Additionally, a time-varying triangular wave drive signal is
applied to the wavelength selective section 112 to ensure
wavelength matching between the semiconductor laser 111 and the
wavelength conversion device 118 occurs on a regular, repeating
basis. As described in more detail below, the choice of frequency
for the triangular wave drive signal applied to the wavelength
selective section 112 may be based on the gain drive signal applied
to the gain section 116, which may typically be ten times greater
than the gain modulation frequency. The two drive signals applied
to the gain and wavelength selective sections may be the same for
every visible light source that is evaluated, and, in one
embodiment, are not adjusted or otherwise altered during the
measurement process. In one embodiment, there is no feedback
control loop on the gain or wavelength selective sections.
[0030] The sampling rate of the optical power detection analysis
module 130 should be chosen such that it is greater than the
wavelength selective modulation frequency. In one embodiment, the
sampling rate of the analog voltage data acquisition module 135 of
the optical power detection and analysis module 130 is one hundred
times greater than the wavelength selective modulation
frequency.
[0031] Referring now to FIG. 2, a graph of an exemplary gain drive
signal 210 applied to the gain section 116 and an exemplary
triangular wave drive signal applied to the wavelength selective
section 112 are illustrated. The vertical axis is arbitrary units
(A.U.) and corresponds to the electrical current being provided to
the gain and wavelength selective sections. The horizontal axis is
time. The wavelength selective modulation frequency chosen should
be greater than the gain modulation frequency. Because the
wavelength selective modulation frequency is greater than the gain
modulation frequency, there is a high probability that wavelength
matching between the semiconductor laser 111 and the wavelength
conversion device 118 will occur during the times when the gain
section is powered by the gain drive signal. In one embodiment, the
modulation frequency of the triangular wave drive signal is chosen
such that it is greater than the gain modulation frequency divided
by the gain drive signal duty cycle. For example, the wavelength
selective modulation frequency may be about ten times greater than
the gain modulation frequency.
[0032] In the illustrated example, the gain section is modulated by
a 100 Hz square wave with an 85% gain duty cycle. The wavelength
selective section is modulated with a 1 KHz triangular wave. It
should be understood that the 100 Hz square wave and 1 KHz
triangular wave are illustrated and described herein as exemplary
frequency values. In one embodiment, the square gain drive signal
210 may have a gain modulation frequency between about 0 Hz and
about 1 KHz, while the triangular wave drive signal may have a
wavelength selective modulation frequency between about 500 Hz and
about 10 KHz. As described in more detail below, a triangular
waveform may be chosen to drive the wavelength selective section
112 so that wavelength tuning between the semiconductor laser 111
and the wavelength conversion device 118 is ensured. Although other
waveforms may be used, a triangular wave is preferred over a sine
wave for the DBR section modulation because the slew rate for a
triangular wave is constant. Additionally, a triangular wave is
preferred over a sawtooth wave for DBR section modulation because a
triangular wave is symmetric on rising and falling edges, and does
not require the very high slew rates that a sawtooth wave may
require.
[0033] FIG. 3 graphically illustrates how the DBR current to obtain
maximum green conversion efficiency (i.e., the current value
applied to the wavelength selection section such that the
semiconductor laser produces an output beam at the phase matching
wavelength of the wavelength conversion device) is obtained twice
per cycle of the triangular wave drive signal. As illustrated in
FIG. 3, the gain drive signal transitions from a low value 211 to a
high value 213 at rising edge 212. The high value portion 213 of
the gain drive signal is the maximum applied gain current, while
the low value portion 211 may be ground, for example. Dashed line
230 illustrates a hypothetical DBR phase matching current. The
semiconductor device will produce an output pulse at instances when
the triangular wave drive signal 220 intersects the DBR phase
matching current line 230 and the gain drive signal 210 is high and
supplying a current to the gain section of the semiconductor laser.
For example, the semiconductor laser will produce output pulses at
instances 232b, 232c, and 232d. The semiconductor laser does not
produce an output pulse at instance 232a because no current is
provided to the gain section of the semiconductor laser at that
instance.
[0034] The timing between the rising edge 212 of the gain drive
signal and the next instance of DBR phase matching current may be
referred to as a phase delay (illustrated by arrow 234). The phase
delay will remain fixed if the gain drive signal 210 and triangular
wave drive signal 220 are synchronous with respect to one another,
but will vary if they are asynchronous (i.e., the two frequencies
are not integer multiples of one another). An example of
synchronous modulation would be a wavelength selective modulation
frequency of 1 KHz and a gain modulation frequency of 100 Hz, as
illustrated in FIG. 2. An example of asynchronous modulation would
be a wavelength selective modulation frequency of 1 KHz and a gain
modulation frequency of 99 Hz. It may be advantageous to have a
varying phase delay between the gain and DBR modulation patterns
due to the extra degree of randomness which it provides to the
modulation technique.
[0035] FIG. 4 illustrates an exemplary gain drive signal 210'
having a gain modulation frequency of 0 Hz such that it is driven
in a continuous wave (CW) mode. Therefore, the gain section is
driven with an unmodulated gain current during the evaluation. The
wavelength selection section is modulated with a 1 KHz triangular
wave as depicted in FIG. 2.
[0036] FIG. 5 graphically illustrates an exemplary green output
response from the modulation pattern illustrated in FIG. 2 using a
visible light source comprising a semiconductor laser and
wavelength conversion device as illustrated in FIG. 1. Shown is the
data from the high speed (100 KHz) voltage logging of the optical
power meter 133. The vertical axis is the voltage (V) of the analog
signal provide by the optical power meter 133, and the horizontal
axis is the data samples (#). The voltage corresponds to the
optical output power detected by the optical power meter. A block
of 500 data points is illustrated. Therefore, individual blocks of
500 data points are processed at a frequency of 100,000/500=200 Hz.
A plurality of optical output pulses (400a-400e) are illustrated.
Maximum green output occurs whenever the DBR current provided by
the triangular wave drive signal is at the appropriate value to
obtain phase matching, as described above. No output is present
when the gain is not being driven.
[0037] The gain current and gain duty cycle of the gain drive
signal 210 are adjusted to desired values. As an example and not a
limitation, the maximum gain current may be set to 650 mA and the
gain duty cycle may be set to 75%. The values chosen may depend on
the specific parameters of the semiconductor laser under test. In
one embodiment, the peak-to-peak amplitude of the triangular wave
drive signal 220 applied to the wavelength selective section may be
selected to span a maximum permissible range (e.g., 0 to 150 mA) to
ensure that a DBR current operable to emit a pulse at the phase
matching wavelength will be obtained.
[0038] In another embodiment, the peak-to-peak amplitude of the
triangular wave drive signal 220 may be modified on a per
unit-under-test basis. The triangular wave drive signal 220 may be
tailored to have a peak-to-peak amplitude range that is smaller
than the maximum permissible range and centered on the phase
matching wavelength of the visible light source 110 under test.
This technique may provide a faster result when measuring the
optical output power of the visible light source 110, while still
employing a modulation scheme that does not require real-time
feedback control. Determining an appropriate sub-range of current
for the wavelength selective section/DBR heater on a particular
semiconductor laser may be achieved in a relatively short time,
e.g., under one second. One method of determining the sub-range of
current may comprise driving the wavelength selective section 112
over the full permissible range (e.g., 0 to 150 mA, which is the
same as a midpoint DBR current of 75 mA with a range of +/-75 mA).
The optical output power of the visible light source (which may be
a green optical output in one embodiment) may be analyzed as a
function of DBR current provided by the triangular wave drive
signal that provides the best DBR current for the semiconductor
laser under test, i.e., the wavelength matching current. After the
wavelength matching current is determined, the triangular wave
drive signal may be reset such that the midpoint current for the
modulation is set to that wavelength matching current and with a
smaller range (e.g., +/-10 mA, or other ranges).
[0039] Similarly, FIG. 6 graphically illustrates an exemplary green
output response from the modulation pattern illustrated in FIG. 4
wherein the gain is driven CW and the laser is always on (i.e., a
frequency of 0 Hz). A plurality of optical output pulses
(500a-500j) are illustrated. Maximum green output occurs whenever
the DBR current is at the appropriate value to obtain phase
matching.
[0040] Referring to both FIGS. 5 and 6, the analog voltage data
provided by the optical power meter 133 may be captured and
analyzed. In one embodiment, data is analyzed in blocks of 500
acquired data points. It should be understood that data may be
analyzed in blocks of more or fewer data points, and that the
number of data points in each block may depend on the capabilities
of the software and the processor analyzing incoming data. In one
embodiment, a peak output power value is determined from every
block of 500 data points and are stored for a specific period of
time. The number of data blocks and the actual time duration of the
full evaluation is related to any thermal time constants of the
laser, and may vary for different semiconductor laser designs.
Measurement of optical output power should be performed for a
sufficient period of time to statistically ensure that the maximum
optical output power that the visible light source is capable of
producing is sampled and recorded.
[0041] The maximum optical output pulse having the maximum optical
output power value from each data block is determined and recorded
or otherwise selected as the optical output power of the
semiconductor laser and visible light source. Using the plurality
of pulses shown in FIG. 6 as an example, optical output pulse 500e
has the highest optical output power value in the data block
illustrated. The optical output power value of optical output pulse
500e may be compared with the maximum output power value of the
other data blocks of the evaluation. The optical output pulse
having the highest optical output power value of all of the data
blocks is selected as the representative optical output pulse.
[0042] The selected optical output value having the maximum optical
power may then be compared with an output power threshold value to
determine if the visible light source under test meets output power
specifications. The output power threshold value may be dependent
on the type of visible light source package under test and the
application for which it is to be utilized. Those visible light
sources where the selected optical output value is less than the
output power threshold value may be labeled as faulty and
discarded, while those where the selected optical output value is
greater than the output power threshold value may be labeled as in
compliance with the output power specification and selected for
further processing.
[0043] In another embodiment, the output power values of all (or a
selected few) of the optical output pulses of each data block may
be averaged to generate an average output power value for each data
block that is then compared with the average output power value for
the remaining data blocks. The averaged output power value having
the highest output power value may be selected as representative of
the visible light source. Alternatively, a maximum output power
value for each data block obtained over a given measurement time
period may be determined and recorded as described above and then
averaged. The averaged maximum output power values of each data
block may then be compared with the output power threshold value.
In this manner, an atypical optical output pulse may be discounted
if one did occur.
[0044] In an alternative embodiment, the phase section 114 may also
be driven by a time-varying drive signal in addition to the
wavelength selective section 112. Driving the phase section 114 is
such a manner may further scramble the wavelength of the output
beam emitted by the semiconductor laser 111. By providing a
time-varying phase drive signal to the phase section 114 that is
anti-correlated with respect to the triangular wave drive signal
applied to the wavelength selective section 112, an additional
degree of randomness to the instantaneous laser wavelength tuning
may be achieved, which may also improve the overall measurement
technique. Anti-correlated means that the frequency of the
triangular wave drive signal applied to the wavelength selective
section 112 and the phase drive signal are not integer multiples of
one another. The phase drive signal may be configured as a
triangular wave or other periodic signals. Alternatively, the phase
drive signal may be configured as white noise that is applied to
the phase section 114. The frequency of the phase drive signal
should be greater than the frequency of the drive signal applied to
the wavelength selective section 112. As an example and not a
limitation, the frequency of the phase drive signal may be about
ten times greater than triangular wave drive signal applied to the
wavelength selective section 112.
[0045] It is also possible to evaluate the thermal properties of
the visible light source under test. As an example and not a
limitation, the temperature of a 1060 nm DBR semiconductor laser
directly influences the infrared output power for a given gain
current applied to the gain section. Higher temperatures result in
lower IR output, and by extension, lower green optical output power
in a frequency doubling device. The temperature of the DBR
semiconductor laser is influenced by the electrical power
dissipated in the gain section. The electrical power dissipation,
in turn, is determined by the gain current and the duty cycle of
the gain modulation. Therefore, the measured green optical output
of a visible light source is a function of both the gain current
and the duty cycle of the gain modulation. Embodiments of the
present disclosure may independently control both of these
parameters during an evaluation to independently control both the
peak gain current and the time-averaged thermal loading of the
semiconductor laser. In this manner, drive parameters that mimic
how end-users might be expected to operate the visible light source
may be duplicated so that the most relevant measured values may be
obtained.
[0046] The manipulation of gain current and/or gain duty cycle can
be used to identify high thermal impedance issues in the visible
light source (e.g., solder voiding). By fixing the modulated gain
current maximum value of the gain drive signal while performing
optical output power measurements at a varying gain duty cycle,
embodiments of the present disclosure may be used as a diagnostic
tool to also identify visible light sources with
out-of-specification thermal impedance. Hypothetically, a visible
light source with near zero thermal impedance should measure the
same optical output power, regardless of the gain duty cycle of the
gain drive signal, as long as the maximum gain current is
maintained at a constant value. A visible light source driven with
a variable gain duty cycle that measured a large decrease in
optical output power as the gain duty cycle was increased (thereby
increasing the time-averaged thermal loading) could be rejected as
scrap since it likely has a thermal impedance issue due to a
manufacturing defect.
[0047] The time-average thermal loading test may be performed after
the maximum optical output test described above. In one embodiment,
the gain section of the semiconductor laser may be driven by a gain
drive signal having a low gain duty cycle, such as 50%, for
example. Optical output power values may be measured and
temporarily stored by the optical power detection and analysis
module while the semiconductor laser is driven at the low gain duty
cycle. The gain duty cycle may be gradually increased and optical
output power values continuously measured and stored until a high,
maximum gain duty cycle is reached (e.g., 90% or other value). The
optical output power values associated with the low gain duty
cycle(s) may be compared with optical output power values
associated with the high gain duty cycle(s) to determine an optical
output power value variation. The optical output power value
variation represents the difference in optical output power between
low and high thermal loading. For example, selected measured
optical output power values at a low gain duty cycle may be
averaged to determine an average low gain duty cycle power value,
and selected measured optical output power values at a high gain
duty cycle may be averaged to determine an average high gain duty
cycle power value. The high gain duty cycle power value may be
subtracted from the low gain duty cycle power value to calculate
the optical output power value variation. The calculated optical
output power value variation may then be compared with a variation
threshold value to determine if the visible light source under test
meets appropriate thermal impedance specifications, which may
depend on the particular design of the visible light source or the
application in which it is incorporated.
[0048] In embodiments where the gain section is driven at CW gain
current, the thermal loading on the semiconductor laser is
completely determined by the gain current that is chosen. To test
the thermal characteristics of the visible light source driven at
CW gain current, the gain current applied to the gain section may
be adjusted from a low value to a high value, and optical output
power values measured. A difference between the optical output
power values at low gain current and the optical output power
values at a high gain current is calculated to determine if the
visible light source under test satisfies the thermal impedance
specifications.
[0049] Referring generally to FIG. 1, it is noted that some visible
light source designs may use adaptive optics (not shown) to
accurately couple the output beam 117 of the semiconductor laser to
the waveguide portion 119 of the wavelength conversion device 118.
The adaptive optics may be implemented in a visible light source
110 in a variety of configurations, such as a tunable lens or an
adjustable mirror. A feedback loop is utilized to continuously
adjust the adaptive optics such that the output beam 117 is aligned
with the waveguide 119 to maximize green output power. As described
above with reference to FIGS. 5 and 6, the analog voltage stream
received from the optical power meter 133 may be sampled at a high
rate and analyzed in sequential blocks of data (e.g., blocks of 500
data points). If the sample rate is 100 KHz and the data block size
is 500 points, then blocks of data may be processed at a frequency
of 200 Hz. The peak optical power value from each data block can be
numerically determined and made available in software to the
adaptive optics control loops such that the output beam 117 may be
continuously aligned with the waveguide 119 of the wavelength
conversion device 118.
[0050] It should now be understood that the embodiments described
herein may be utilized to efficiently evaluate optical output
capabilities and thermal characteristics of visible light sources
comprising semiconductor lasers and wavelength conversion devices.
A low frequency periodic gain drive signal is applied to the gain
section of the laser and a higher frequency triangular wave drive
signal is applied to the wavelength selective section of the
semiconductor laser. Both drive signals cause the semiconductor
laser to emit a plurality of optical output pulses. One or more
optical output pulses having a maximum optical output value is
determined and compared with an optical output power threshold
value to determine if the visible light source under test is in
compliance with an output power specification. In some embodiments,
thermal characteristics of the visible light source may be tested
by varying a gain duty cycle and/or a gain current to determine if
the visible light source is in compliance with a thermal impedance
specification. The low frequency signals used to evaluate the
visible light sources under test enable the system equipment to be
positioned at distances of more than a meter away from the light
source, and also produce lower noise levels than the
high-frequency, high current drive signals used to drive the
semiconductor lasers to produce video-quality images.
[0051] It is to be understood that the preceding detailed
description is intended to provide an overview or framework for
understanding the nature and character of the subject matter as it
is claimed. It will be apparent to those skilled in the art that
various modifications and variations can be made to the embodiments
described herein without departing from the spirit and scope of the
disclosure. Thus, it is intended that the present disclosure cover
the modifications and variations provided they come within the
scope of the appended claims and their equivalents.
[0052] It is noted that terms like "preferably," "commonly," and
"typically," when utilized herein, are not intended to limit the
scope of the claimed invention or to imply that certain features
are critical, essential, or even important to the structure or
function of the claimed invention. Rather, these terms are merely
intended to highlight alternative or additional features that may
or may not be utilized in a particular embodiment of the present
invention.
[0053] For the purposes of describing and defining the present
invention it is noted that the terms "substantially,"
"approximately" and "about" are utilized herein to represent the
inherent degree of uncertainty that may be attributed to any
quantitative comparison, value, measurement, or other
representation. The term "substantially" is also utilized herein to
represent the degree by which a quantitative representation. e.g.,
"substantially above zero," varies from a stated reference, e.g.,
"zero," and should be interpreted to require that the quantitative
representation varies from the stated reference by a readily
discernable amount.
* * * * *