U.S. patent application number 12/335692 was filed with the patent office on 2010-06-17 for multi-variable control methods for optical packages.
Invention is credited to Jacques Gollier, Garrett Andrew Piech, Dragan Pikula, Daniel Ohen Ricketts.
Application Number | 20100150185 12/335692 |
Document ID | / |
Family ID | 42200234 |
Filed Date | 2010-06-17 |
United States Patent
Application |
20100150185 |
Kind Code |
A1 |
Gollier; Jacques ; et
al. |
June 17, 2010 |
MULTI-VARIABLE CONTROL METHODS FOR OPTICAL PACKAGES
Abstract
According to one embodiment of the present invention, an optical
package comprises one or more semiconductor lasers coupled to a
wavelength conversion device with adaptive optics. The optical
package also comprises a package controller programmed to operate
the semiconductor laser and the adaptive optics based on modulated
feedback control signals supplied to the wavelength selective
section of the semiconductor laser and the adaptive optics. The
wavelength control signal supplied to the wavelength selective
section of the semiconductor laser may be adjusted based on the
modulated wavelength feedback control signal such that the response
parameter of the wavelength conversion device is optimized.
Similarly, the position control signals supplied to the adaptive
optics may be adjusted based on the modulated feedback position
control signals such that the response parameter of the wavelength
conversion device is optimized.
Inventors: |
Gollier; Jacques; (Painted
Post, NY) ; Piech; Garrett Andrew; (Horseheads,
NY) ; Pikula; Dragan; (Horseheads, NY) ;
Ricketts; Daniel Ohen; (Corning, NY) |
Correspondence
Address: |
CORNING INCORPORATED
SP-TI-3-1
CORNING
NY
14831
US
|
Family ID: |
42200234 |
Appl. No.: |
12/335692 |
Filed: |
December 16, 2008 |
Current U.S.
Class: |
372/20 |
Current CPC
Class: |
G02F 1/35 20130101; H01S
5/0092 20130101; H01S 5/0683 20130101; H01S 5/06256 20130101; G02F
1/3775 20130101; H01S 5/0687 20130101 |
Class at
Publication: |
372/20 |
International
Class: |
H01S 3/10 20060101
H01S003/10 |
Claims
1. A method for controlling an optical package comprising a
semiconductor laser, a wavelength conversion device, adaptive
optics configured to optically couple an output beam of the
semiconductor laser into a waveguide portion of an input facet of a
wavelength conversion device, an optical detector, and a package
controller programmed to operate the semiconductor laser and at
least one adjustable optical component of the adaptive optics, the
method comprising: controlling the periodic lasing intensity of the
semiconductor laser by controlling an amount of gain current
I.sub.GAIN injected into a gain section of the semiconductor laser,
wherein the gain current I.sub.GAIN has a periodic frequency
v.sub.DATA; supplying a wavelength control signal to a wavelength
selective section of the semiconductor laser to control the lasing
wavelength .lamda..sub.1 of the semiconductor laser; positioning
the output beam on the input facet of the wavelength conversion
device by supplying a position control signal to the adjustable
optical component; supplying a modulated wavelength feedback
control signal to the wavelength selective section to modulate the
lasing wavelength .lamda..sub.1 of the semiconductor laser by
wherein: v.sub..lamda.=(N+1/P)v.sub.DATA where v.sub..lamda. is the
frequency of the modulated wavelength feedback control signal, N is
an integer and P is an integer greater than one; supplying a
modulated position feedback control signal to the adjustable
optical component to modulate the position of the output beam on
the input facet of the wavelength conversion device, wherein the
modulated position feedback control signal is out of phase with the
modulated wavelength feedback control signal and a frequency
v.sub..theta. of the modulated position feedback control signal is
the same as the frequency v.sub..lamda. of the modulated wavelength
feedback control signal; measuring a response parameter of the
wavelength conversion device; analyzing the measured response
parameter of the wavelength conversion device to identify spectral
and positional components attributable to the modulated wavelength
feedback control signal and modulated position control signal;
adjusting the wavelength control signal based on the spectral
component of the measured response parameter to optimize the
response parameter of the wavelength conversion device; and
adjusting the position control signal based on the positional
component attributable to the modulated position control signal to
optimize the response parameter of the wavelength conversion
device.
2. The method of claim 1 wherein the response parameter is the
output intensity of the wavelength conversion device.
3. The method of claim 1 wherein the response parameter is the gain
current I.sub.GAIN injected into the gain section of the
semiconductor laser.
4. The method of claim 1 wherein the wavelength control signal
controls a temperature T.sub..lamda. of the wavelength selective
section, an amount of current I.sub..lamda. injected into the
wavelength selective section, or both.
5. The method of claim 1 wherein the adjustable optical component
is adjustable in a first direction and a second direction.
6. The method of claim 5 wherein the modulated position feedback
control signal is periodically alternated between oscillating the
adjustable optical component in the first direction and oscillating
the adjustable optical component in the second direction.
7. The method of claim 1 wherein the modulated position feedback
control signal and the modulated wavelength feedback control signal
are out of phase by 90 degrees.
8. The method of claim 1 wherein the spectral component and the
positional component are isolated by multiplying the measured
response parameter by one of sin(.omega.t) or cos(.omega.t), where
.omega.=2.lamda.v.sub..theta.=2.pi.v.sub..lamda. and t is a period
of modulation, and integrating the product over T periods of
modulation, wherein the result of the integrand is proportional to
a direction and magnitude of the adjustment to the wavelength
control signal or the position control signal.
9. The method of claim 1 wherein: the adjustable optical component
of the adaptive optics is an adjustable mirror; and the adaptive
optics, the wavelength conversion device, and the semiconductor
laser are positioned to define a folded optical pathway between an
output of the semiconductor laser and an input of the wavelength
conversion device.
10. The method of claim 9 wherein the adjustable optical component
comprises a MEMS mirror.
11. A method of controlling an optical package comprising a
semiconductor laser, a wavelength conversion device, adaptive
optics configured to optically couple an output beam of the
semiconductor laser into a waveguide portion of an input facet of a
wavelength conversion device, an optical detector, and a package
controller programmed to operate the semiconductor laser and at
least one adjustable optical component of the adaptive optics, the
method comprising: controlling the periodic lasing intensity of the
semiconductor laser by controlling an amount of gain current
I.sub.GAIN injected into a gain section of the semiconductor laser,
wherein the gain current I.sub.GAIN has a periodic frequency
v.sub.DATA; supplying a first position control signal to the
adjustable optical component to control the position of the output
beam on the input facet of the wavelength conversion device in a
first direction; supplying a second position control signal to the
adjustable optical component to control the position of the output
beam on the input facet of the wavelength conversion device in a
second direction; supplying a first modulated position feedback
control signal to the adjustable optical component to modulate the
position of the output beam on the input facet of the wavelength
conversion device in the first direction, wherein:
v.sub..theta.1=(N+1/P)v.sub.DATA where v.sub..theta.1 is the
frequency of the first modulated wavelength feedback control
signal, N is an integer and P is an integer greater than 1;
supplying a second modulated position feedback control signal to
the adjustable optical component to modulate the position of the
output beam on the input facet of the wavelength conversion device
in the second direction, wherein the second modulated position
feedback control signal is out of phase with the first modulated
position feedback control signal and a frequency v.sub..theta.1 of
the modulated position feedback control signal is the same as the
frequency v.sub..theta.2 of the modulated wavelength feedback
control signal; measuring a response parameter of the wavelength
conversion device; analyzing the measured response parameter of the
wavelength conversion device to identify positional components
attributable to the first modulated position feedback control
signal and the second modulated position feedback control signal;
adjusting the first position control signal based on the positional
component attributable to the first modulated position feedback
control signal to optimize the response parameter of the wavelength
conversion device; and adjusting the second position control signal
based on the positional component attributable to the second
modulated position feedback control signal to optimize the response
parameter of the wavelength conversion device.
12. The method of claim 11 wherein the response parameter is the
output intensity of the wavelength conversion device.
13. The method of claim 11 wherein the response parameter is the
gain current I.sub.GAIN injected into the gain section of the
semiconductor laser.
14. The method of claim 11 further comprising: supplying a
wavelength control signal to a wavelength selective section of the
semiconductor laser to control the lasing wavelength .lamda..sub.1
of the semiconductor laser; supplying a modulated wavelength
feedback control signal to the wavelength selective section to
modulate the lasing wavelength .lamda..sub.1 of the semiconductor
laser, wherein: v.sub..lamda.=(K+1/L)v.sub.DATA where v.sub..lamda.
is the frequency of the modulated wavelength feedback control
signal, K is an integer and K is not equal to N and L is an integer
greater than one; analyzing the measured response parameter of the
wavelength conversion device to identify a spectral component
attributable to the modulated wavelength feedback control signal
and modulated position control signal; and adjusting the wavelength
control signal based on the spectral component of the measured
response parameter to optimize the response parameter of the
wavelength conversion device.
15. The method of claim 11 wherein the wavelength control signal
controls a temperature T.sub..lamda. of the wavelength selective
section, an amount of current I.sub..lamda. injected into the
wavelength selective section, or both.
16. The method of claim 11 wherein the first modulated position
feedback control signal and the second modulated position feedback
control signal are out of phase by 90 degrees.
17. The method of claim 16 wherein the positional components are
isolated by multiplying the measured response parameter by one of
sin(.omega.t) or cos(.omega.t), where
.omega.=2.pi.v.sub..theta.1=2.pi.v.sub..theta.2 and t is a period
of modulation, and integrating the product over T periods of
modulation, wherein the result of the integrand is proportional to
a direction and magnitude of the adjustment to the first position
control signal or the second position control signal.
18. The method of claim 14 wherein the spectral components are
isolated by sin(.omega.t) or cos(.omega.t), where
.omega.=2.pi.v.sub..lamda. and t is a period of modulation, and
integrating the product over an integer multiple of 1/v.sub.DATA,
1/v.sub..lamda. and 1/v.sub..theta.1.
19. The method of claim 11 wherein: the adjustable optical
component of the adaptive optics is an adjustable mirror; and the
adaptive optics, the wavelength conversion device, and the
semiconductor laser are positioned to define a folded optical
pathway between an output of the semiconductor laser and an input
of the wavelength conversion device.
20. The method of claim 11 wherein the adjustable optical component
is a MEMS mirror.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention generally relates to semiconductor
lasers, laser controllers, optical packages, and other optical
systems incorporating semiconductor lasers. More specifically, the
present invention relates to methods for optimizing the output of
optical packages that include, inter alia, a semiconductor laser
optically coupled to a second harmonic generation (SHG) crystal, or
another type of wavelength conversion device, with adaptive
optics.
[0003] 2. Technical Background
[0004] Short wavelength light sources can be formed by combining a
single-wavelength semiconductor laser, such as an infrared or
near-infrared distributed feedback (DFB) laser, distributed Bragg
reflector (DBR) laser, or Fabry-Perot laser, with a light
wavelength conversion device, such as a second harmonic generation
(SHG) crystal. Typically, the SHG crystal is used to generate
higher harmonic waves of the fundamental laser signal. To do so,
the lasing wavelength is preferably tuned to the spectral center of
the wavelength converting SHG crystal and the output of the laser
is preferably aligned with the waveguide portion at the input facet
of the wavelength converting crystal.
[0005] Waveguide optical mode field diameters of typical SHG
crystals, such as MgO-doped periodically poled lithium niobate
(PPLN) crystals, can be in the range of a few microns. As a result,
properly aligning the beam from the semiconductor laser with the
waveguide of the SHG crystal such that the output of the SHG
crystal is optimized may be a difficult task. More specifically,
optimizing the output of the SHG crystal requires that the position
of the beam of the semiconductor laser be precisely controlled
along two axes on the input face of the SHG crystal. Accordingly,
at least two variables must be monitored and controlled to position
the beam of the semiconductor laser such that the output of the
laser is maximized.
[0006] Similarly, the phase matching bandwidth of SHG crystals are
typically narrow, generally less than 1 nm. For example, for a 12
mm long PPLN crystal, the phase matching bandwidth may be about
0.16 nm. As such, the wavelength of the semiconductor laser must be
precisely controlled to optimize the second harmonic output of the
SHG crystal. This may be accomplished by the application of heat to
the wavelength control section of the semiconductor laser or by
injecting current into the wavelength control section of the
semiconductor laser.
[0007] Accordingly, to maximize the output efficiency of the
wavelength conversion device, at least three variables must be
controlled. Therefore, multi-variable control techniques for
optical packages comprising a semiconductor laser optically coupled
to a wavelength conversion device, such as a second harmonic
generation (SHG) crystal, are needed.
SUMMARY OF THE INVENTION
[0008] One aspect of the present invention includes a
multi-variable control method for controlling an optical package in
which a single output of the optical package is monitored to
determine how to adjust multiple control variables. Specifically,
the multi-variable control method uses multiple phase, frequency
and/or time shifted modulation signals to optimize the output of
the optical package. The optical package may include a
semiconductor laser, a wavelength conversion device, adaptive
optics configured to optically couple an output beam of the
semiconductor laser into a waveguide portion of an input facet of a
wavelength conversion device, an optical detector, and a package
controller programmed to operate the semiconductor laser and at
least one adjustable optical component of the adaptive optics.
[0009] According to one embodiment shown and described herein, the
method of controlling the optical package may include controlling
the periodic lasing intensity of the semiconductor laser by
controlling an amount of gain current I.sub.GAIN injected into a
gain section of the semiconductor laser. The periodic frequency
V.sub.DATA of the gain current I.sub.GAIN may represent an encoded
data signal which contains data for generating an image in a laser
projection system. A wavelength control signal may be supplied to a
wavelength selective section of the semiconductor laser to control
the lasing wavelength .lamda..sub.1 of the semiconductor laser. The
output beam of the semiconductor laser may be positioned on the
input facet of the wavelength conversion device by supplying a
position control signal to the adjustable optical component which,
in turn, varies the position of the adjustable optical component. A
modulated wavelength feedback control signal may be supplied to the
wavelength selective section to modulate the lasing wavelength
.lamda..sub.1 of the semiconductor laser. The modulated wavelength
feedback control signal may have a frequency v.sub..lamda. such
that:
v.sub..lamda.=(N+1/P)v.sub.DATA
where, N is an integer and P is an integer greater than one. In
addition, a modulated position feedback control signal may be
supplied to the adjustable optical component to modulate the
position of the output beam on the input facet of the wavelength
conversion device. The modulated position feedback control signal
may be out of phase with the modulated wavelength feedback control
signal while the frequency v.sub..theta. of the modulated position
feedback control signal may be the same as the frequency
v.sub..lamda. of the modulated wavelength feedback control signal.
The response parameter of the wavelength conversion device may be
measured with the optical detector as the modulated signals are
supplied to the adjustable optical component and the semiconductor
lasers. Thereafter, the measured response parameter of the
wavelength conversion device may be analyzed to identify spectral
and positional components attributable to the modulated wavelength
feedback control signal and modulated position control signal.
After the measured response parameter is analyzed, the wavelength
control signal and the position control signal may be adjusted
based on the spectral and positional components of the measured
response parameter, respectively, to optimize the response
parameter of the wavelength conversion device.
[0010] In another embodiment shown and described herein, the method
of controlling the optical package may include controlling the
periodic lasing intensity of the semiconductor laser by controlling
an amount of gain current I.sub.GAIN injected into a gain section
of the semiconductor laser, wherein a periodic frequency V.sub.DATA
of the gain current I.sub.GAIN represents an encoded data signal. A
first position control signal may be supplied to the adjustable
optical component to control the position of the output beam on the
input facet of the wavelength conversion device in a first
direction. In addition, a second position control signal may be
supplied to the adjustable optical component to control the
position of the output beam on the input facet of the wavelength
conversion device in a second direction. A first modulated position
feedback control signal may be supplied to the adjustable optical
component to modulate the position of the output beam on the input
facet of the wavelength conversion device in the first direction.
The first modulated position feedback control signal may have a
frequency v.sub..theta.1 such that:
v.sub..theta.1=(n+1P)v.sub.DATA
where N is an integer and P is an integer greater than 1. A second
modulated position feedback control signal is supplied to the
adjustable optical component to modulate the position of the output
beam on the input facet of the wavelength conversion device in the
second direction. The second modulated position feedback control
signal may be out of phase with the first modulated position
feedback control signal. The frequency v.sub..theta.1 of the
modulated position feedback control signal may be the same as the
frequency v.sub..theta.2 of the second modulated position feedback
control signal. The response parameter of the wavelength conversion
device may be measured with the optical detector. Thereafter, the
measured response parameter of the wavelength conversion device is
analyzed to identify positional components attributable to the
first modulated position feedback control signal and the second
modulated position feedback control signal. The first position
control signal and the second position control signal are then
adjusted based on the positional component attributable to the
first modulated position feedback control signal and the second
modulated position feedback control signal, respectively, to
optimize the response parameter of the wavelength conversion
device.
[0011] Additional features and advantages of the invention will be
set forth in the detailed description which follows, and in part
will be readily apparent to those skilled in the art from that
description or recognized by practicing the invention as described
herein, including the detailed description which follows, the
claims, as well as the appended drawings.
[0012] It is to be understood that both the foregoing general
description and the following detailed description present
embodiments of the invention, and are intended to provide an
overview or framework for understanding the nature and character of
the invention as it is claimed. The accompanying drawings are
included to provide a further understanding of the invention, and
are incorporated into and constitute a part of this specification.
The drawings illustrate various embodiments of the invention, and
together with the description serve to explain the principles and
operations of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a schematic diagram of one embodiment of an
optical package shown and described herein;
[0014] FIG. 2 is a schematic diagram of one embodiment of an
optical package shown and described herein;
[0015] FIG. 3 depicts a semiconductor laser for use in conjunction
with one or more embodiments of the optical packages shown and
described herein;
[0016] FIG. 4 depicts a wavelength conversion device for use in
conjunction with one or more embodiments of the optical packages
shown and described herein; and
[0017] FIG. 5 is a simplified graphical illustration of the
frequency content of an encoded data signal and a modulated control
signal.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0018] Reference will now be made in detail to embodiments of the
invention, examples of which are illustrated in the accompanying
drawing. Whenever possible, the same reference numerals will be
used throughout the drawings to refer to the same or like parts.
One embodiment of an optical package for use in conjunction with
the control methods described herein is shown in FIG. 1. The
optical package generally comprises a semiconductor laser, adaptive
optics, a wavelength conversion device and a package controller.
The output of the semiconductor laser is optically coupled into the
input facet of the wavelength conversion device with the adaptive
optics. The package controller is electrically coupled to the
semiconductor laser and the adaptive optics and configured to
control the output of the semiconductor laser and the alignment of
the semiconductor laser with the wavelength conversions device.
Various components and configurations of the optical package and
multivariable control methods for optimizing the output of the
optical package will be further described herein.
[0019] FIGS. 1 and 2 generally depict two embodiments of optical
packages 100, 200 described herein. It should be understood that
the solid lines and arrows indicate the electrical
interconnectivity of various components of the optical packages.
These solid lines and arrows are also indicative of electrical
signals propagated between the various components including,
without limitation, electronic control signals, data signals and
the like. Further, it should also be understood that the dashed
lines and arrows indicate infrared light beams and visible light
beams emitted by the semiconductor laser and the wavelength
conversion device, respectively.
[0020] Referring initially to FIGS. 1 and 2, although the general
structure of the various types of optical packages in which the
concepts of particular embodiments of the present invention can be
incorporated are taught in readily available technical literature
relating to the design and fabrication of frequency or
wavelength-converted semiconductor laser sources, the concepts of
particular embodiments of the present invention may be conveniently
illustrated with general reference to the optical packages 100, 200
which include, for example, a semiconductor laser 110 optically
coupled to a wavelength conversion device 120. The infrared light
beam 119 emitted by the semiconductor laser 110 may be either
directly coupled into the waveguide portion of the wavelength
conversion device 120 or can be coupled into the waveguide portion
of wavelength conversion device 120 using adaptive optics 140. The
wavelength conversion device 120 converts the infrared light beam
119 into higher harmonic waves and outputs a visible light beam
128. This type of optical package 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 semiconductor laser 110, which is schematically
illustrated in FIG. 3, may generally comprise a wavelength
selective section 112, a phase matching section 114, and a gain
section 116. The wavelength selective section 112, which may also
be referred to as the distributed Bragg reflector or DBR section of
the semiconductor laser 110, 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 semiconductor laser
110 provides the major optical gain of the laser and the phase
matching section 114 creates an adjustable optical path length or
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 113, 115, 117 are incorporated
in the wavelength selective section 112, the phase matching section
114, the gain section 116, or combinations thereof, and are merely
illustrated schematically in FIG. 3. It is contemplated that the
electrodes 113, 115, 117 may take a variety of forms. For example,
the control electrodes 113, 115, 117 are illustrated in FIG. 3 as
respective electrode pairs but it is contemplated that single
electrode elements 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 113, 115, 117 can be used
to inject electrical current into the corresponding sections 112,
114, 116 of the semiconductor laser 110. For example, in one
embodiment, current injected in to the wavelength selective section
112 of the semiconductor laser 110 can be used to control the
wavelength .lamda..sub.1 of the infrared light beam 119 emitted
from the output facet 118 of the semiconductor laser 110 by
altering the operating properties of the laser. The injected
current may be used to control the temperature of the wavelength
selective section 112 or the index of refraction of the wavelength
selective section. Accordingly, by adjusting the amount of current
injected into the wavelength selective section, the wavelength of
the infrared light beam 119 emitted by the semiconductor laser may
be varied. Current injected into the phase matching section 114 or
gain section 116 may be similarly used to control the output of the
semiconductor laser 110.
[0023] The wavelength conversion device 120, which is schematically
illustrated in FIG. 4, generally comprises a non-linear optical
crystal, such as a second harmonic generation (SHG) crystal. In one
embodiment, the wavelength conversion device 120 may comprise an
MgO-doped, periodically polled lithium niobate (PPLN) crystal
although other, similar non-linear optical crystals may be used.
Further, it should be understood that the wavelength conversion
device may be a second harmonic generation (SHG) crystal or a
non-linear optical crystal capable of converting light to higher
order harmonics.
[0024] The wavelength conversion device 120 generally comprises an
input facet 122 and an output facet 126. A waveguide portion 124 of
the wavelength conversion device 120 extends between the input
facet 122 to the output facet 128. When the wavelength conversion
device 120 is a PPLN crystal, the waveguide portion of the PPLN
crystal may have dimensions (e.g., height and width) on the order
of 5 microns. An infrared light beam 119 directed into the
waveguide portion 124 of the wavelength conversion device 120 is
propagated through the wavelength conversion device 120 where it is
converted to a visible light beam 128 which is emitted from the
output facet 126 of the wavelength conversion device. In one
embodiment, the infrared light beam 119 produced by the
semiconductor laser 110 and directed into the waveguide portion 124
of the wavelength conversion device 120 has a wavelength of about
1060 nm. In this embodiment, the wavelength conversion device 120
converts the infrared light beam 119 to green light such that the
visible light beam 128 has a wavelength of about 530 nm.
[0025] Referring now to FIG. 1, one embodiment of an optical
package 100 is depicted in which the semiconductor laser 110 and
the wavelength conversion device 120 have a substantially linear
configuration. More specifically, the output of the semiconductor
laser 110 and the input of the wavelength conversion device 120 are
substantially aligned along a single optical axis. As shown in FIG.
1, the near infrared light 119 emitted by the semiconductor laser
110 is coupled into a waveguide portion of the wavelength
conversion device 120 with adaptive optics 140.
[0026] In the embodiment shown in FIG. 1, the adaptive optics 140
generally comprises an adjustable optical component, specifically a
lens 142. The lens 142 collimates and focuses the near infrared
light 119 emitted by the semiconductor laser 110 into the waveguide
portion of the wavelength conversion device 120. However, it should
be understood that other types of lenses, multiple lenses, or other
optical elements may be used. The lens 142 may be coupled to an
actuator (not shown) for adjusting the position of the lens 142 in
the x- and y-directions such that the lens 142 is an adjustable
optical component. Adjusting the position of the lens in the x- and
y-directions may facilitate positioning the infrared light beam 119
along the input face of the wavelength conversion device 120 and,
more specifically, on the waveguide portion of the wavelength
conversion device, such that the infrared light beam 119 is aligned
with the waveguide portion and the output of the wavelength
conversion device 120 is optimized.
[0027] Referring now to FIG. 2, another embodiment of an optical
package 200 is shown in which the semiconductor laser 110, the
wavelength conversion device 120 and the adaptive optics 140 are
oriented in a folded configuration. More specifically, the output
of the semiconductor laser 110 and the input of the wavelength
conversion device 120 are positioned on substantially parallel
optical axes. As with the embodiment shown in FIG. 1, the infrared
light beam 119 emitted by the semiconductor laser 110 is coupled
into the waveguide portion of the wavelength conversion device 120
with adaptive optics 140. However, in this embodiment, the infrared
light beam 119 must be redirected from its initial pathway in order
to order to facilitate coupling the infrared light beam 119 into
the waveguide portion of the wavelength conversion device 120.
Accordingly, in this embodiment, the adaptive optics 140 may
comprise an adjustable optical component, specifically an
adjustable mirror 144, and a lens 142.
[0028] As described hereinabove, the lens 142 of the adaptive
optics 140 may collimate and focus the infrared light beam 119
emitted by the semiconductor laser 110 into the waveguide portion
of the wavelength conversion device 120.
[0029] The adjustable mirror 144 may be rotated about an axis of
rotation substantially parallel to the x-axis depicted in FIG. 2
and about an axis of rotation substantially parallel to the y-axis
to introduce angular deviation in the infrared light beam 119. The
adjustable mirror 144 may comprise a mirror portion and an actuator
portion and the adjustable mirror 144 may be rotated about either
axis of rotation by adjusting the actuator portion of the
adjustable optical component.
[0030] For example, in one embodiment, the adjustable mirror 144
may comprise one or more movable micro-opto-electromechanical
systems (MOEMS) or micro-electro-mechanical system (MEMS)
operatively coupled to a mirror. The MEMS or MOEMS devices may be
configured and arranged to vary the position of the infrared light
beam 119 on the input facet of the wavelength conversion device
120. Use of MEMS or MOEMS devices enables adjustment of the
infrared light beam 119 to be done extremely rapidly over large
ranges. For example, a MEMS mirror with a .+-.1 degree mechanical
deflection, when used in conjunction with a 3 mm focal length lens,
may allow the beam spot to be angularly displaced .+-.100 .mu.m on
the input face of the wavelength conversion device 120. The
adjustment of the beam spot may be done at frequencies on the order
of 100 Hz to 10 kHz due to the fast response time of the MEMS or
MOEMS device.
[0031] Alternatively or additionally, the adjustable optical
component may comprise one or more liquid lens components
configured for beam steering and/or beam focusing. Still further,
it is contemplated that the adjustable optical component may
comprise one or more mirrors and/or lenses mounted to
micro-actuators. In one contemplated embodiment, the adjustable
optical component may be a movable or adjustable lens, as described
with respect to FIG. 1, used in conjunction with a fixed mirror to
form a folded optical pathway between the semiconductor laser 110
and the wavelength conversion device 120.
[0032] In the optical package 200 illustrated in FIG. 2, the
adjustable mirror 144 is a micro-opto-electromechanical mirror
incorporated in a relatively compact, folded-path optical system.
In the illustrated configuration, the adjustable mirror 144 is
configured to fold the optical path such that the optical path
initially passes through the lens 142 to reach the adjustable
mirror 144 as a collimated or nearly collimated beam and
subsequently returns through the same lens 142 to be focused on the
wavelength conversion device 120. This type of optical
configuration is particularly applicable to wavelength converted
laser sources where the cross-sectional size of the laser beam
generated by the semiconductor laser 110 is close to the size of
the waveguide on the input face of the wavelength conversion device
120, in which case a magnification close to one would yield optimum
coupling in focusing the beam spot on the input face of the
wavelength conversion device 120. For the purposes of defining and
describing this embodiment of the optical package 200, it is noted
that reference herein to a "collimated or nearly collimated" beam
is intended to cover any beam configuration where the degree of
beam divergence or convergence is reduced, directing the beam
towards a more collimated state.
[0033] Referring now to both FIGS. 1 and 2, the optical packages
100, 200 may also comprise a beam splitter 180 positioned proximate
the output of the wavelength conversion device 120. The beam
splitter 180 is used to redirect a portion of the visible light
beam 128 emitted from the wavelength conversion device 120 into an
optical detector 170 which is used to measure the intensity of the
emitted visible light beam 128 and output an electrical signal
proportional to the measured intensity.
[0034] The optical packages 100, 200 may also comprise a package
controller 150. The package controller 150 may comprise one or more
micro-controllers or programmable logic controllers used to store
and execute a programmed instruction set for operating the optical
package 100, 200. Alternatively, the micro-controllers or
programmable logic controllers may directly execute an instruction
set. The package controller 150 may be electrically coupled to the
semiconductor laser 120, the adaptive optics 140 and the optical
detector 170 and programmed to operate both the semiconductor laser
110 and the adaptive optics 140. More specifically, in one
embodiment, the package controller 150 may comprise drivers 152,
154 for controlling the adaptive optics and the wavelength
selective section of the semiconductor laser, respectively.
[0035] The adaptive optics driver 152 may be coupled to the
adaptive optics 140 with leads 152, 158 and supplies the adaptive
optics 140 with x- and y-position control signals through the leads
152, 158, respectively. The x- and y-position control signals
facilitate positioning the adjustable optical component of the
adaptive optics in the x- and y-directions which, in turn,
facilitates positioning the infrared light beam 119 of the
semiconductor laser 110 on the input facet of the wavelength
conversion device 120. For example, when the adjustable optical
component of the adaptive optics 140 is an adjustable lens 142, as
shown in FIG. 1, the x- and y-position control signals may be used
to position the lens 142 in the x- and y-directions. Alternatively,
when the adjustable optical component of the adaptive optics 140 is
an adjustable mirror 144, as shown in FIG. 2, the x- and y-position
control signals may be used to rotate the adjustable mirror 144
about an axis of rotation parallel to the x-axis and about an axis
of rotation parallel to the y-axis.
[0036] The wavelength selective section driver 154 may be coupled
to the semiconductor laser 110 with lead 155. The wavelength
selective section driver 154 may supply the wavelength selective
section 112 of the semiconductor laser 110 with wavelength control
signal(s) which facilitates adjusting the wavelength .lamda..sub.1
of the infrared light beam 119 emitted from the output facet of the
semiconductor laser 110.
[0037] Further, the output of the optical detector 170 may be
electrically coupled to an input of the package controller 150 with
lead 172 such that the output signal of the optical detector 170 is
passed to the package controller 150.
[0038] The optical package 100, 200 shown in FIGS. 1 and 2 may be
coupled to a data source 160, such a programmable logic controller,
which supplies the optical package 100, 200 with an encoded data
signal which may be representative of a video image, still image or
the like. More specifically, the data source 160 may be coupled to
the gain section of the semiconductor laser 110 via lead 162. The
data source 160 may control the periodic lasing intensity of the
semiconductor laser 110 such that the output of the optical package
100, 200 forms an image when projected. To control the periodic
lasing intensity of the semiconductor laser 110, the encoded data
signal injects a gain current I.sub.GAIN into the gain section of
the semiconductor laser 110. Typically, the periodic frequency
V.sub.DATA of the gain current I.sub.GAIN is representative of the
video image or still image of the encoded data signal such that,
when the output of the optical package is projected, as modulated
by the periodic frequency V.sub.DATA of the gain current
I.sub.GAIN, the projected image is the video image or still image
of the encoded data signal. Typically, the periodic frequency
V.sub.DATA of the encoded data signal is about 60 Hz which
generally corresponds to the video frame rate of a projected image.
FIG. 5 graphically illustrates the simplified frequency content of
the encoded data signal for a still image. The x-axis of the graph
is expressed in terms of the periodic frequency V.sub.DATA of the
encoded data signal while the y-axis depicts the magnitude of the
output of the optical package. The encoded data signal produces
content at the periodic frequency V.sub.DATA of the encoded data
signal and at harmonics of the periodic frequency V.sub.DATA. As
FIG. 5 is representative of a still image, the frequency content of
the image is only DC content (e.g., the content at 0) and content
at multiples of the periodic data frequency V.sub.DATA (e.g.,
1*v.sub.DATA, 2*v.sub.DATA, etc.) However, in the case of a video
image, additional low frequencies will be present in the signal in
between DC and 1*v.sub.DATA, in between 1*v.sub.DATA and
2*v.sub.DATA, etc.
[0039] Multi-variable control methods for optimizing the output of
the optical packages 100, 200 will now be discussed with specific
reference to the optical package 200 shown in FIG. 2. However, it
should be understood that such control techniques are also
applicable to the optical package 100 shown in FIG. 1 and generally
applicable to optical packages have similar components and
configurations.
[0040] The multi-variable control methods described herein utilize
the package controller 150 to apply control signals to the
semiconductor laser 110 and the adaptive optics 140 in order to
optimize the output of the optical package 200. More specifically,
the control signals applied to the semiconductor laser 110 and the
adaptive optics 140 may be applied to the semiconductor laser 110
and adaptive optics 140 using a single modulation frequency, whose
frequency is different than the periodic frequency V.sub.DATA of
the encoded data signal supplied by the data source 160, and is
also different from the frequency of the harmonics of the data
signal, to avoid any interference with the encoded data signal.
[0041] In one embodiment, the periodic lasing intensity of the
semiconductor laser is controlled by supplying the gain section of
the semiconductor laser with a gain current I.sub.GAIN having a
periodic frequency V.sub.DATA, as described herein. The package
controller 150 controls the lasing wavelength .lamda..sub.1 of the
semiconductor laser 110 by supplying a wavelength control signal to
the wavelength selective section of the semiconductor laser 110.
Similarly, the package controller 150 supplies the adaptive optics
140, specifically the adjustable optical component of the adaptive
optics 140, with a position control signal to position the IR light
beam 119 on the input facet of the wavelength conversion device
120.
[0042] In order to optimize the output of the optical package 200,
specifically the properties of the visible light beam 128 emitted
by the wavelength conversion device 120, the package controller 150
may be used to supply the wavelength selective section of the
semiconductor laser 110 with a modulated or dithered wavelength
feedback control signal to modulate or dither the lasing wavelength
.lamda..sub.1 of the semiconductor laser 110. As described
hereinabove, to optimize the output of the wavelength conversion
device, the wavelength of the IR light beam 119 of the
semiconductor laser must be close to the phase matching bandwidth
of the wavelength conversion device. When the lasing wavelength
does not correspond to the phase matching bandwidth of the
wavelength conversion device, the intensity of the visible light
beam 128 may be reduced. Accordingly, by modulating the lasing
wavelength .lamda..sub.1 of the semiconductor laser 110 while
monitoring the output intensity of the wavelength conversion device
120 with the optical detector 170, a specific lasing wavelength
.lamda..sub.1 may be determined based on fluctuations in the output
intensity of the wavelength conversion device such that the output
intensity of the wavelength conversion device 120 is optimized. In
one embodiment, the output intensity of the wavelength conversion
device 120 may be optimized when the output intensity is at a
maximum. In another embodiment, the output intensity of the
wavelength conversion device 120 may be optimized when the output
intensity reaches a predetermined value other than the maximum
output intensity.
[0043] As noted hereinabove, the frequency of the modulated or
dithered wavelength feedback control signal is selected to avoid
interference with the encoded data signal or harmonics of the
encoded data signal. Accordingly, in the embodiments described
herein, the frequency of the modulated or dithered wavelength
feedback control signal is:
v.sub..lamda.=(n+1/P)v.sub.DATA
where v.sub..lamda. is the frequency of the modulated wavelength
feedback control signal, N is an integer and P is an integer
greater than one. In embodiments where the optical package 200 is
used to display video images, the integer N may be greater than
zero so as to avoid interference with lower frequencies which may
be present in the video signal. For example, in one embodiment, N
may be 1 and P may be 2 such that the frequency v.sub..lamda. of
the modulated wavelength feedback control signal is 1.5*v.sub.DATA.
In another embodiment, N may be 2 and P may be 2 such that the
frequency v.sub..lamda. of the modulated wavelength feedback
control signal is 2.5*v.sub.DATA. This embodiment is graphically
illustrated in FIG. 5 which shows the frequency content of a
modulated feedback control signal having a frequency of
2.5*v.sub.DATA.
[0044] As the wavelength selective section of the semiconductor
laser 110 is supplied with a modulated wavelength feedback control
signal, the package controller 150 may simultaneously provide the
adjustable optical component of the adaptive optics with a
modulated or dithered position feedback control signal to modulate
or dither the position of the adjustable optical component in one
direction and, therefore, modulate the position of the IR light
beam 119 of the semiconductor laser on the input facet of the
wavelength conversion device 120. As discussed hereinabove, the
dimensions of the waveguide portion of the wavelength conversion
device, and the mode field diameter of the wavelength conversion
device, may be small, on the order of several microns. If the beam
119 of the semiconductor laser 110 is misaligned with the waveguide
portion of the wavelength conversion device 120, the output
intensity of the wavelength conversion device 120 may be less than
when the output beam is precisely aligned with the waveguide
portion.
[0045] Accordingly, by modulating the position of the adjustable
optical component while measuring the output intensity of the
wavelength conversion device, the output beam 119 of the
semiconductor laser 110 may be scanned over the surface of the
input facet of the wavelength conversion device 120 and any
measured changes in the output intensity of the wavelength
conversion device may be correlated to the position of the
adjustable optical component as it is modulated. Using this
technique, a position of the adjustable optical component may be
determined such that the output of the wavelength device is
optimized.
[0046] As described herein, modulating the adjustable optical
component comprises moving the adjustable optical component in
either a first direction or a second direction. For example, in one
embodiment, when the adjustable optical component is an adjustable
mirror, such as a MEMS or MOEMS mirror, as shown in FIG. 2, moving
the adjustable optical component in a first direction comprises
oscillating the optical component about a first axis, such as the
x-axis. Similarly, moving the adjustable optical component in the
second direction comprises oscillating the adjustable optical
component about a second axis, such as the y-axis.
[0047] In another embodiment, when the adjustable optical component
is an adjustable lens, the position of the lens may be moved in a
first direction by oscillating the position of the lens along a
first axis, such as the x-axis, while the position of the lens may
be moved in a second direction by oscillating the position of the
lens along a second axis, such as the y-axis.
[0048] As described hereinabove with respect to the modulated
wavelength control signal, the modulated position feedback control
signal has a frequency different than that of the encoded data
signal of the data source 160. More specifically, the frequency
v.sub..theta. of the modulated position feedback control signal is
the same as the frequency v.sub..lamda. of the modulated wavelength
feedback control signal. However, to facilitate differentiation of
changes in the output intensity of the wavelength conversion device
due to the modulated position feedback control signal from changes
due to the modulated wavelength feedback control signal, the
modulated position feedback control signal and the modulated
wavelength feedback control signal may be made out of phase with
one another. In one embodiment, the modulated feedback position
control signal is 90 degrees out of phase with the modulated
wavelength feedback control signal.
[0049] The modulated position feedback control signal facilitates
modulating the position of the optical component in one direction
(e.g., either a first direction or a second direction). However, to
optimize the output of the wavelength conversion device, the
adjustable optical component must be positioned in at least two
directions (e.g., the x- and y-directions) in order to optimize the
output of the wavelength conversion device.
[0050] Accordingly, in one embodiment, the package controller 150
may be programmed to periodically alternate between modulating the
adjustable optical component in a first direction and modulating
the adjustable optical component in a second direction. For
example, for a given period of time, the modulated position
feedback control signal may be applied by the package controller to
rotate the adjustable mirror 144 shown in FIG. 2 about the x-axis
such that the IR light beam 119 is scanned over the input facet of
the wavelength conversion device in the y-direction. After the
given time period has elapsed, the modulated position feedback
control signal may be applied to rotate the adjustable mirror 144
shown in FIG. 2 about the y-axis such that the IR light beam 119 is
scanned over the input facet of the wavelength conversion device in
the x-direction. Therefore, by alternating between applying the
modulated position feedback control signal to two different axes at
a given modulation or dither frequency, the modulated position
feedback control signal may be used to position the output beam 119
of the semiconductor laser along both axes thereby optimizing the
output intensity of the wavelength conversion device 120 for both
axes. Alternatively, in another embodiment, the package controller
150 maybe programmed to periodically alternate between modulating
the wavelength control signal and modulating the adjustable optical
component in a second direction.
[0051] As the modulated wavelength feedback control signal is
applied to the semiconductor laser 110 and the modulated position
feedback control signal is applied to the adjustable optical
component of the adaptive optics 140, the output intensity of the
wavelength conversion device 120 may be measured with the optical
detector 170. The optical detector 170 provides the package
controller 150 with an electrical signal which is indicative of the
output intensity and the changes in the output intensity of the
wavelength conversion device. Accordingly, it should now be
understood that the changes in the output intensity of the
wavelength conversion device may be attributed to both the
modulated position feedback control signal and the modulated
wavelength feedback control signal.
[0052] The package controller 150 may be operable to analyze the
measured output intensity of the wavelength conversion device to
identify and isolate spectral components (e.g., changes in the
output intensity due to the modulated wavelength feedback control
signal) and positional components (e.g., changes in the output
intensity due to the time shifted application of the modulated
position feedback control signal) of the measured output intensity
of the wavelength conversion device 120. Based on the identified
spectral and positional components of the measured output
intensity, the package controller may also determine the required
change in the wavelength control signal and the positional control
signals such that the output of the wavelength conversion device is
optimized. Methods for analyzing the measured output intensity of
the wavelength conversion device 120 to isolate the spectral and
positional components and to determine the required change in the
control signals will now be describe in more detail.
[0053] In one embodiment, where two modulation or dither signals
(e.g., a modulated wavelength feedback control signal and modulated
position control signal) which are 90 degrees out of phase, the
combined modulation signal C(t) may be written as:
C(t)=.epsilon..sub..theta.cos(.omega.t)+.epsilon..sub..lamda.sin(.omega.-
t),
where the cosine term is the modulated position feedback control
signal, the sine term is the modulated wavelength control signal
and .epsilon..sub..theta. and .epsilon..sub..lamda. are the
amplitudes of the modulated position feedback control signal and
the modulated wavelength control signal, respectively. C(t) may be
rewritten as
C(t)=Msin(.omega.t+.alpha.)
where .omega.=2.pi.v.sub..theta.=2.pi.v.sub..lamda. and t is a
period of modulation. M and .alpha. are functions of
.epsilon..sub..theta. and .epsilon..sub..lamda. and may be written
as:
M = ( .theta. ) 2 + ( .lamda. ) 2 and ##EQU00001## .alpha. = arctan
( .theta. .lamda. ) . ##EQU00001.2##
[0054] The encoded data signal DATA(t) provided by the data source,
including any higher order harmonics of the encoded data signal,
may be represented as a Fourier series such that:
DATA(t)=A.sub.0+A.sub.1cos(2.pi.v.sub.DATAt)+B.sub.1sin(2.pi.v.sub.DATAt-
)+A.sub.2cos(2.pi.v.sub.DATAt)+B.sub.2sin(2.pi.v.sub.DATAt)+
[0055] Accordingly, the modulated measured output intensity
S.sub.out(t) of the wavelength conversion device may be expressed
as the sum of the combined modulation signal C(t) and the encoded
data signal DATA(t) such that:
S.sub.out(t)=C(t)+DATA(t)=[Msin(.omega.t+.alpha.)+DATA(t)]
[0056] In order to isolate the position and wavelength modulation
feedback control signals from the measured output intensity, the
package controller 150 may be programmed to multiply the measured
modulated output intensity (e.g., S.sub.out(t)) by one of
sin(.omega.t) or cos(.omega.t) and integrate the product over T
periods of modulation where T=P/v.sub.DATA. This operation
eliminates the modulation inherent in the measured modulated output
intensity due to the modulation of the encoded data signal at a
frequency v.sub.DATA. Depending on which multiplier is used (e.g.,
sin(.omega.t) or cos(.omega.t)), one of the spectral component of
the measured output intensity or the positional component of the
measured output intensity will be eliminated through the
multiplication/integration step. The results of the integration
will be proportional to the change which must be applied to the
respective control signal to optimize the output intensity of the
wavelength conversion device for that particular control signal.
More specifically, the sign and magnitude of the result of the
integration will indicate the direction and amount by which the
control signal must be adjusted to optimize the output of the
wavelength conversion device.
[0057] For example, when the modulated wavelength control signal is
a cosine function and the modulated position feedback control
signal is a sine function, as shown above, the following scheme may
be applied to isolate the spectral and positional components of the
measured output intensity of the wavelength conversion device. To
isolate the spectral component (e.g., to eliminate the positional
component)the modulated measured output intensity S.sub.out(t) is
multiplied by sin(.omega.t) and integrated over T periods of the
encoded data signal such that:
.intg. 0 T [ M sin ( .omega. t + .alpha. ) + DATA ( t ) ] ( sin
.omega. t ) t ##EQU00002##
where T=P/v.sub.DATA. Since the modulation frequency of the
wavelength control feedback signal and the position control
feedback signal (e.g.,
.omega.=2.pi.v.sub..theta.=2.pi.v.sub..lamda.) to the frequency of
the encoded data signal (e.g., 2.pi.v.sub.DATA), or multiples
thereof, the integration yields the following:
.intg. 0 T [ M sin ( .omega. t + .alpha. ) + DATA ( t ) ] ( sin
.omega. t ) t = M T 2 cos ( .alpha. ) . ##EQU00003##
When the modulated wavelength feedback control signal is a sine
function, the result of the integrand is proportional to the
magnitude and direction by which the wavelength control signal to
the wavelength selective section of the semiconductor laser 110
should be adjusted to optimize the output of the wavelength
conversion device.
[0058] Similarly, to isolate the positional component (e.g., to
eliminate the spectral component) the measured modulated output
intensity may be multiplied by cos(.omega.t) and integrated over T
periods such that:
.intg. 0 T [ M sin ( .omega. t + .alpha. ) + DATA ( t ) ] ( cos
.omega. t ) t = M T 2 sin ( .alpha. ) ##EQU00004##
When the modulated position control signal is a cosine function, as
described above, the result of the integration yields the magnitude
and direction by which the wavelength control signal to the
adjustable optical component should be adjusted to optimize the
output of the wavelength conversion device.
[0059] While the example calculation shown and described herein is
for the situation where the modulated wavelength control signal is
a sine function and the modulated position feedback control signal
is a cosine function, it should be understood that similar
operations may be performed when the modulated wavelength control
signal is a cosine function and the modulated position feedback
control signal is a sine function. However, in those circumstances,
different multipliers may be used. More specifically, to isolate a
spectral or positional component based on a control signal having a
sine modulation, the measured output intensity should be multiplied
by sin(.omega.t) and then integrated while, to isolate a spectral
or positional component based on a control signal having a cosine
modulation, the measured output intensity should be multiplied by
cos (.omega.t) and then integrated.
[0060] Further, it should be understood that the isolated
positional component may be indicative of the adjustment that
should be made to the adjustable optical component on one of two
axes at any given time. Accordingly, the package controller may be
operable to correlate the magnitude and direction of the adjustment
to the axis of the adjustable component that was being modulated
when the measurement of the output intensity of the wavelength
conversion device was made.
[0061] After the package controller has identified and isolated the
spectral and positional component of the measured output intensity
that are attributable to the modulated wavelength control signal
and the modulated position feedback control signal, the package
controller adjusts the wavelength control signal applied to the
wavelength selective section of the laser based on the isolated
spectral component of the measured output intensity such that the
output intensity of the wavelength conversion device is optimized.
The package controller also adjusts the position control signals
applied to the adjustable optical component based on the isolated
positional component of the measured output intensity such that the
measured output intensity of the wavelength conversion device is
optimized.
[0062] Changes in the output intensity of the wavelength conversion
device due to the application of the modulated feedback control
signals are a fraction of the average output intensity of the
wavelength conversion device. Accordingly, the magnitude of the
adjustment integral will depend on the average output intensity of
the wavelength conversion device. Similarly, the convergence speed
is also dependent on the output intensity. However, it is desirable
to have the same convergence speed regardless of the output
intensity of the wavelength conversion device. As such, in one
embodiment, the result of the adjustment integral may be normalized
by dividing the integrand by the average output intensity of the
wavelength conversion device over the period of integration.
[0063] In another embodiment of the multi-variable control method
described herein, the package controller modulates both the x- and
y-position control signals simultaneously (as opposed to
simultaneously modulating the wavelength control signal and the
position control signal). In this embodiment, the package
controller 150 supplies the adjustable optical component with a
first modulated feedback position control signal to modulate the
position of the adjustable optical component on a first axis and
thereby modulate the position of the output beam on the input facet
of the wavelength conversion device in a first direction. To avoid
interference with the encoded data signal, the first modulated
feedback position control signal has a frequency v.sub..theta.1
such that
v.sub..theta.1=(N+1/P)v.sub.DATA
where v.sub..theta.1 is the frequency of the first modulated
wavelength feedback control signal, N is an integer and P is an
integer greater than 1. In one embodiment, where the encoded data
signal is a video signal, the integer N may be greater than zero to
avoid interference with the low frequency components of the video
signal.
[0064] Simultaneously, the package controller 150 supplies the
adjustable optical component with a second modulated feedback
position control signal to modulate the position of the adjustable
optical component on a second axis and thereby modulate the
position of the output beam on the input facet of the wavelength
conversion device in a second direction. To avoid interference with
the encoded data signal, the second modulated position feedback
control signal has frequency v.sub..theta.2 which is the same as
the frequency v.theta.1 of the first modulated position feedback
control signal. However, to facilitate differentiation of changes
in the output intensity due to the first modulated position
feedback control signal and the second modulated position feedback
control signal, the two signals are phase shifted such that the
first modulated position feedback control signal and the second
modulated position feedback control signal are out of phase with
one another. In one embodiment, the two signals are 90 degrees out
of phase with one another.
[0065] As described hereinabove, the output intensity of the
wavelength conversion device may be measured with the optical
detector 170 as the first modulated position feedback control
signal and the second modulated position feedback control signals
are applied to the adjustable optical component which, in turn,
sends an electrical signal to the package controller 150 indicative
of the changes in the output intensity of the wavelength conversion
device. Thereafter, the controller analyzes the measured output
intensity and determines first and second positional components of
the measured output intensity which are attributable to each of the
first modulated position feedback control signal and the second
modulated position feedback control signal, respectively, using the
multiplication/integration techniques described above.
Specifically, to isolate the first positional and second positional
components from the measured output intensity of the wavelength
conversion device 120, the measured output intensity is multiplied
by sin(.omega.t) or cos(.omega.t) where
.omega.=2.pi.v.sub..theta.1=2.lamda.v.sub..theta.2. Multiplying by
sin(.omega.t) will eliminate one of the first positional component
or the second positional component while multiplying by cos
(.omega.t) will eliminate the other. The product is then integrated
over T periods where T=P/v.sub.DATA and P is an integer as defined
herein. The result of the integration yields the magnitude and
direction of change which should be applied to the position control
signal of the respective component (e.g. the first positional or
second positional component) to optimize the output of the
wavelength conversion device.
[0066] Based on the first and second positional components, the
package controller adjusts the first position control signal and
the second position control signal supplied to the adjustable
optical components such that the output of the wavelength
conversion device is optimized.
[0067] In one embodiment, as the adjustable optical component is
modulated with the first modulated position feedback control signal
and the second modulated position feedback control signal, a
modulated wavelength feedback control signal is supplied to the
wavelength selective section to modulate the lasing wavelength
.lamda.1 of the semiconductor laser. In this embodiment, the
modulated wavelength feedback control signal may have a frequency
such that
v.sub..lamda.=(K+1/L)v.sub.DATA
where v.sub..lamda. is the frequency of the modulated wavelength
feedback control signal, K and L are integers greater than one and
the modulation frequencies are selected such that v.sub..lamda. and
v.sub..theta. and are sufficiently different to permit resolution
of the portion of the measured modulated output signal which is
attributable to each. In one embodiment, where the encoded digital
signal is a video signal, the integer K may be greater than zero to
avoid interference with low frequency components of the video
signal.
[0068] Spectral components of the measured output intensity of the
wavelength conversion device 120 attributable to the modulated
wavelength feedback control signal may be isolated using techniques
similar to those described hereinabove. More specifically, the
output intensity of the wavelength conversion device 120 may be
multiplied by one of sin(.omega.t) (when the modulated wavelength
feedback control signal is a sine function) or cos(.omega.t) (when
the modulated wavelength feedback control signal is a cosine
function) where .omega.=2.pi.v.sub..lamda. and t is a period of
modulation. Thereafter, the spectral component of the measured
output intensity is integrated over an integer multiple of all
three modulations periods (e.g., 1/v.sub.DATA, 1/v.sub..lamda.,
1/v.sub..theta.1) thereby eliminating unwanted frequencies and
preserving an error signal proportional to the signal change caused
by the modulation of the wavelength control signal. For example, if
the modulation periods are 18 ms, 1 ms, and 10 ms, a suitable
integration period would be 180 ms. The sign and magnitude of the
integration are indicative of a direction and amount of change
which should be applied to the wavelength control signal such that
the output of the wavelength conversion device is optimized. The
package controller utilizes the results of the integration to
adjust the wavelength control signal supplied to the wavelength
selective section of the semiconductor laser.
[0069] In another embodiment, the gain current supplied to the gain
section of the semiconductor laser may be constantly adjusted to
achieve a target output intensity from the wavelength conversion
device. Such a control routine is called automatic power control
(APC). When APC control routines are used, any change in the
optical alignment of the semiconductor laser with the wavelength
conversion device or a change in the wavelength of the
semiconductor laser may not produce an appreciable change in the
output intensity of the wavelength conversion device. Instead, the
changes in the alignment and/or the wavelength of the semiconductor
laser may cause a corresponding change in the gain current applied
to the gain section of the semiconductor laser. For example, in one
embodiment, the package controller may include circuitry to measure
and adjust the gain current supplied to the gain section of the
semiconductor laser. As described herein above, modulated
wavelength feedback control signals and modulated position control
signals may be supplied to the semiconductor laser and the adaptive
optics while the change in gain current is monitored as a function
of changes in the optical alignment of the semiconductor laser
and/or the wavelength of the semiconductor laser. Based on the
changes measured changes in the gain current, the control
algorithms discussed herein may be to determine the changes in the
gain current attributable due to the modulated wavelength feedback
control signal and the modulated position control signals.
Thereafter, the package controller may adjust the wavelength
control signal supplied to the semiconductor laser and the position
control signal supplied to the adaptive optics to optimize the gain
current and, more specifically, to minimize the gain current
supplied to the gain section of the semiconductor laser.
[0070] Accordingly, it should now be understood that the control
methods described herein may be used to adjust an operating
parameter of the optical package (e.g., the wavelength control
signal or the alignment of the adaptive optics) by modulating the
wavelength control signal and/or the position control signal(s)
while measuring a response parameter (e.g., the gain current
supplied to the gain section of the semiconductor laser or the
output intensity of the wavelength conversion device) and thereby
adjust the operating parameters such that the response parameter is
optimized.
[0071] It should now be understood that the methods described
herein facilitate multi-variable control of an optical package
using control signals applied to various components of the optical
package at a single frequency.
[0072] It is to be understood that the preceding detailed
description of the invention is intended to provide an overview or
framework for understanding the nature and character of the
invention as it is claimed. It will be apparent to those skilled in
the art that various modifications and variations can be made to
the present invention without departing from the spirit and scope
of the invention. Thus, it is intended that the present invention
cover the modifications and variations of this invention provided
they come within the scope of the appended claims and their
equivalents.
[0073] For the purposes of defining and describing the present
invention, it is noted that reference herein to values that are "on
the order of" a specified magnitude should be taken to encompass
any value that does not vary from the specified magnitude by one or
more orders of magnitude. It is also noted that one or more of the
following claims recites a controller "programmed to" execute one
or more recited acts. For the purposes of defining the present
invention, it is noted that this phrase is introduced in the claims
as an open-ended transitional phrase and should be interpreted in
like manner as the more commonly used open-ended preamble term
"comprising." In addition, it is noted that recitations herein of a
component of the present invention, such as a controller being
"programmed" to embody a particular property, function in a
particular manner, etc., are structural recitations, as opposed to
recitations of intended use. More specifically, the references
herein to the manner in which a component is "programmed" denotes
an existing physical condition of the component and, as such, is to
be taken as a definite recitation of the structural characteristics
of the component.
[0074] 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. Further, it is noted that reference to a value,
parameter, or variable being a "function of" another value,
parameter, or variable should not be taken to mean that the value,
parameter, or variable is a function of one and only one value,
parameter, or variable.
[0075] For the purposes of describing and defining the present
invention it is noted that the term "substantially" is 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.
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