U.S. patent application number 16/886530 was filed with the patent office on 2021-09-09 for broadened spectrum laser diode for display device.
This patent application is currently assigned to Microsoft Technology Licensing, LLC. The applicant listed for this patent is Microsoft Technology Licensing, LLC. Invention is credited to Solomon Shlomo MEHARI, Shiva SHAHIN, Dale Eugene ZIMMERMAN.
Application Number | 20210281047 16/886530 |
Document ID | / |
Family ID | 1000004900645 |
Filed Date | 2021-09-09 |
United States Patent
Application |
20210281047 |
Kind Code |
A1 |
SHAHIN; Shiva ; et
al. |
September 9, 2021 |
BROADENED SPECTRUM LASER DIODE FOR DISPLAY DEVICE
Abstract
A broad-spectrum laser for use in a MEMS laser scanning display
device is provided. In one example, the broad-spectrum laser
includes a laser diode emitter with plural quantum wells each
having a different spectral peak. In another example, the
broad-spectrum laser includes a laser diode emitter with a tunable
absorber to achieve a broadened emissions spectrum. In another
example, the broad-spectrum laser includes a laser diode emitter
array having plural individual emitters with different spectral
peaks.
Inventors: |
SHAHIN; Shiva; (Seattle,
WA) ; ZIMMERMAN; Dale Eugene; (Redmond, WA) ;
MEHARI; Solomon Shlomo; (Redmond, WA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Microsoft Technology Licensing, LLC |
Redmond |
WA |
US |
|
|
Assignee: |
Microsoft Technology Licensing,
LLC
Redmond
WA
|
Family ID: |
1000004900645 |
Appl. No.: |
16/886530 |
Filed: |
May 28, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62987110 |
Mar 9, 2020 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01S 5/062 20130101;
G02B 27/0172 20130101; G02B 2027/0178 20130101; H01S 5/1096
20130101; H01S 5/34326 20130101; H01S 5/3403 20130101; H01S 5/0601
20130101; H01S 5/34333 20130101; H01S 5/026 20130101; H01S 5/125
20130101 |
International
Class: |
H01S 5/10 20060101
H01S005/10; G02B 27/01 20060101 G02B027/01; H01S 5/125 20060101
H01S005/125; H01S 5/343 20060101 H01S005/343; H01S 5/06 20060101
H01S005/06; H01S 5/062 20060101 H01S005/062; H01S 5/34 20060101
H01S005/34; H01S 5/026 20060101 H01S005/026 |
Claims
1. A laser diode emitter array comprising: a substrate; and a first
semiconductor layer deposited directly or indirectly on the
substrate; a plurality of quantum well layer sections in a quantum
well layer deposited directly or indirectly on the first
semiconductor layer and having a substantially uniform epitaxial
structure, each of the quantum well layer sections having a
respective peak wavelength in emitted light when energized, each
respective peak wavelength being different; a second semiconductor
layer deposited directly or indirectly on the quantum well layer,
wherein when the laser diode emitter array is energized, the
plurality of quantum well layer sections collectively lase emitted
light having a broader bandwidth than is emitted by any individual
quantum well layer section of the plurality of quantum well layer
sections, and the emitted light includes the plurality of
respective peak wavelengths across an emitted light frequency
spectrum.
2. The laser diode emitter array of claim 1, wherein the different
respective peak wavelengths of each quantum well layer section are
due to quantum well intermixing by interdiffusion of constituent
atoms between each quantum well layer section in the quantum well
layer and the first and/or second semiconducting layers.
3. The laser diode emitter array of claim 2, wherein the
interdiffusion is a thermal interdiffusion that has been achieved
at least in part by application of a strain inducing thin film
layer having a plurality of segments of varying thickness,
annealing at a predetermined temperature, and removal of the strain
inducing thin film layer.
4. The laser diode emitter array of claim 3, wherein the substrate
defines a horizontal plane, and the plurality of quantum well layer
sections are spatially distributed horizontally and parallel to the
plane.
5. The laser diode emitter array of claim 4, wherein each of the
plurality of segments of the strain inducing layer was positioned
before annealing and prior to removal above a corresponding one of
the plurality of quantum well layer sections, to thereby vary the
interdiffusion within each quantum well layer section based on the
respective thickness of the corresponding segment positioned above
each quantum well layer section.
6. A laser diode emitter comprising: a substrate; and a first
semiconductor layer deposited directly or indirectly on the
substrate; a plurality of quantum well layers, each of the quantum
well layers having a respective material composition to achieve a
respective peak wavelength in emitted light when energized, each
respective peak wavelength being different; and a second
semiconductor layer deposited directly or indirectly on a topmost
one of the quantum well layers, wherein when the laser diode
emitter is energized, the plurality of quantum well layers
collectively lase emitted light having a broader bandwidth than is
emitted by any individual quantum well layer of the plurality of
quantum well layers, and the emitted light includes the plurality
of respective peak wavelengths across an emitted light frequency
spectrum.
7. The laser diode emitter of claim 6, wherein each respective
material composition of the quantum well layers follows the
formula: In.sub.x[Al.sub.yGa.sub.1-y].sub.1-xP, where
0.4<x<0.6 and 0<y<0.55.
8. The laser diode emitter of claim 6, wherein each respective
material composition of the quantum well layers follows the
formula: In.sub.xGa.sub.1-xN, where 0.26<x<0.4.
9. The laser diode emitter of claim 6, wherein each respective
material composition of the quantum well layers follows the
formula: In.sub.xGa.sub.1-xN, where 0.15<x<0.25.
10. The laser diode emitter of claim 6, wherein the substrate
defines a horizontal plane and wherein each of the plurality of
quantum wells is positioned at a different respective vertical
displacement relative to the substrate; and the plurality of
quantum wells overlap each other as viewed from above along a
vertical axis passing through each of the plurality of quantum
wells, wherein growth of each of the plurality of quantum well
layers is varied by altering epitaxial growth conditions of the
respective plurality of quantum well layers to thereby cause each
of the plurality of quantum well layers to lase at a different peak
energy when energized.
11. A method for use in manufacturing a laser diode emitter, the
method comprising: forming a substrate; forming a first
semiconducting layer of semiconducting material of a first type
directly or indirectly on the substrate; depositing a quantum well
layer directly or indirectly on the first semiconducting layer by
epitaxial growth, the quantum well layer including at least first
and second quantum well layer sections; forming a second
semiconducting layer of a second type of semiconducting material
directly or indirectly on the quantum well layer; depositing a
strain inducing thin film layer having varying thickness directly
or indirectly on the second semiconducting layer; annealing the
substrate, first semiconductor layer, second semiconducting layer,
quantum well layer and strain inducing thin film layer at a
predetermined annealing temperature, to thereby achieve varied
interdiffusion of constituent atoms between each respective quantum
well layer section in the quantum well layer and the first and/or
second semiconducting layers during annealing, the interdiffusion
of each quantum layer well section varying based on the thickness
of the thin film layer positioned above the quantum layer well
section; and removing the strain inducing thin film layer to leave
the quantum well layer sections having the varied interdiffusion,
wherein the varied interdiffusion in each of the quantum well layer
sections causes each of the quantum well layer sections to lase at
a different peak energy when energized.
12. The method of claim 11, wherein the quantum well layer has a
substantially uniform epitaxial structure.
13. The method of claim 11, further comprising: depositing by
epitaxial growth, one or more of a cladding layer and/or a
waveguide layer between the first semiconductor layer and the
quantum well layer.
14. The method of claim 11, further comprising: depositing by
epitaxial growth, one or more of a cladding layer and/or a
waveguide layer between the quantum well layer and the second
semiconducting layer.
15. The method of claim 11, wherein the first type of
semiconducting material is an n-type material, and the second type
of semiconducting material is a p-type material; or the first type
of semiconducting material is a p-type material, and the second
type of semiconducting material is an n-type material.
16. A method for use in manufacturing a laser diode emitter,
comprising: forming a substrate; forming a first semiconducting
layer of semiconducting material of a first type directly or
indirectly on the substrate; depositing a first quantum well layer
directly or indirectly on the first semiconducting layer by
epitaxial growth, under first epitaxial growth conditions to
achieve a first epitaxial structure in the first quantum well
layer; depositing a first intervening layer directly or indirectly
on the first quantum well layer by epitaxial growth; depositing a
second quantum well layer directly or indirectly on the first
separating layer by epitaxial growth under second epitaxial growth
conditions to achieve a second epitaxial structure in the second
quantum well layer; and forming a second semiconducting layer of a
second type of semiconducting material directly or indirectly on
the second quantum well layer, wherein both the first quantum well
layer and the second quantum well layer include In.sub.xGa.sub.1-xN
or both the first quantum well layer and second quantum well layer
include In.sub.x[Al.sub.yGa.sub.1-y].sub.1-xP.
17. The method of claim 16, wherein growth of each of the first
quantum well layer and the second quantum well layer is varied by
altering epitaxial growth conditions of the respective first and
second quantum well layers to thereby cause each of the first and
second quantum well layers to lase at a different peak energy when
energized.
18. The method of claim 17, wherein the first epitaxial structure
and the second epitaxial structure are varied by changing a growth
temperature of In.sub.xGa.sub.1-xN at which each of the first and
second quantum well layers is grown.
19. The method of claim 17, wherein the first epitaxial structure
and the second epitaxial structure are varied by changing a growth
temperature of In.sub.x[Al.sub.yGa.sub.1-y].sub.1-xP at which each
of the first and second quantum well layers is grown.
20. The method of claim 17, wherein the first epitaxial structure
and the second epitaxial structure are varied by changing an indium
flow rate during the epitaxial growth of the first quantum well
layer and the second quantum well layer.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional Patent
Application Ser. No. 62/987,110, filed Mar. 9, 2020, the entirety
of which is hereby incorporated herein by reference for all
purposes.
BACKGROUND
[0002] Laser scanning micro-electro-mechanical systems (MEMS) based
displays have several advantages over liquid crystal displays
(LCDs) and liquid crystal on silicon (LCOS) displays. Such
advantages include smaller size, lighter weight, lower power
consumption, and higher brightness and contrast. In a
waveguide-based laser scanning display, image light beams emitted
from the laser are coupled into one or more waveguides. These light
beams propagate through the waveguide(s), while being replicated,
and then are coupled out of the waveguide(s) into a user's
eyes.
SUMMARY
[0003] A laser diode emitter for use in a MEMS laser scanning
display device is provided. The laser diode emitter may comprise a
gain section coupled with a first current or voltage source, and a
tunable absorber section coupled with a second current or voltage
source. The second current or voltage source may be configured to
supply a drive current or voltage that sweeps through a range of
values over a period of time to cause a shifting of a wavelength of
the laser diode emitter at which an overall gain is at maximum, to
thereby cause the laser diode emitter to emit spectrally broadened
light.
[0004] Additionally, a MEMS laser scanning display device is also
provided. The MEMS laser scanning display may comprise a display, a
laser light source, and a MEMS scanning mirror. The laser light
source may include an emitter array including a plurality of laser
diode emitters. Each laser diode emitter may be configured to emit
a respective different wavelength of light, to thereby form a
broadband light beam with a broadband emission spectrum. The
broadband light beam may have an emission spectrum that includes a
plurality of peaks. The MEMS scanning mirror may be configured to
guide the light beam, via a wave guide with an in-coupling grating
and out-coupling grating, to achieve a scanning pattern across the
display and thereby form a displayed image.
[0005] Additionally, a laser diode emitter is provided. The laser
diode emitter may comprise a substrate, and a plurality of quantum
wells formed on the substrate. Each of the quantum wells may have a
respective composition to achieve a respective peak wavelength in
emitted light when energized, each respective peak wavelength being
different. When the laser diode emitter is energized, the plurality
of quantum wells may collectively lase emitted light having a
broader bandwidth than is emitted by any individual quantum well of
the plurality of quantum wells. The emitted light may have a
plurality of respective peak wavelengths across an emitted light
frequency spectrum.
[0006] This Summary is provided to introduce a selection of
concepts in a simplified form that are further described below in
the Detailed Description. This Summary is not intended to identify
key features or essential features of the claimed subject matter,
nor is it intended to be used to limit the scope of the claimed
subject matter. Furthermore, the claimed subject matter is not
limited to implementations that solve any or all disadvantages
noted in any part of this disclosure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 shows aspects of an example implementation
environment for a near-eye display system.
[0008] FIG. 2 schematically shows an example configuration of the
near-eye display system of FIG. 1.
[0009] FIG. 3 shows a graph indicating that light output from a
laser diode emitter, such as the laser diode emitter of FIG. 2, has
coherence peaks that overlap with interference fringes of a
waveguide.
[0010] FIG. 4 shows an example image having interference fringe
artifacts, due to the coherence peak overlaps of FIG. 3.
[0011] FIGS. 5A-5E illustrate the relationship between the breadth
of the incoming light spectrum and the efficiency and angles of
light transmission through in-coupling and out-coupling gratings of
a waveguide, which in turn can affect the color uniformity and
spatial overlap of the emitted light in a configuration such as
that of FIG. 2.
[0012] FIG. 6 is a cross-sectional view of a laser diode emitter
that may be used in the system of FIG. 1, featuring a tunable
absorber section and a gain section.
[0013] FIG. 7 is a graph of emitted light spectrum of the laser
diode emitter of FIG. 6, illustrating a relationship between gain
and wavelength for each of several tunable absorber drive
currents.
[0014] FIGS. 8A-8B illustrate emission spectra from a conventional
laser and from a laser diode emitter shown in FIG. 6 with a tunable
absorber section without a flattened gain profile.
[0015] FIGS. 9A-9C illustrate flattening of the gain spectrum of a
laser diode emitter such as the laser diode emitter of FIG. 6,
using the varying indium incorporation techniques described below
in relation to FIGS. 11A-B.
[0016] FIG. 10 is a perspective view of a laser diode array that
may be used in the system of FIG. 1, incorporating laser diode
emitters formed of a semiconductor material on surfaces of varying
surface normal angles to thereby vary the composition of the
semiconductor material, which in turn causes the emitted spectrum
from each emitter to vary.
[0017] FIGS. 11A-11B illustrate broadening of the emission spectrum
of a laser diode array of the configuration shown in FIG. 10 or 12,
for example.
[0018] FIG. 12 illustrates a laser diode array that may be used in
the system of FIG. 1 that includes a plurality of laser diode
emitters configured to have the same gain material but different
reflection or loss profile to make the emitters lase at different
wavelengths.
[0019] FIG. 13 is a cross-sectional side view of an example
configuration of a laser diode emitter that may be used in the
system of FIG. 1, having a plurality of layers deposited on a
substrate.
[0020] FIG. 14 illustrates emission spectra of a conventional laser
diode emitter and of a laser diode emitter of the configuration
shown in FIG. 13.
[0021] FIG. 15 is a schematic drawing showing modification of a
laser diode stack through annealing with strain inducing layers
deposited thereon, to manufacture the laser diode emitter of FIG.
13.
[0022] FIG. 16 illustrates a process flow for fabricating an
example configuration of the laser diode emitter of FIG. 13.
[0023] FIG. 17 shows perspective and side views of an example
configuration of the laser diode emitter of FIG. 13.
[0024] FIG. 18 shows an example configuration of the laser diode
emitter of FIG. 6, which features spatially varied substrate
surface vicinal angles prior to epitaxial growth along a
waveguide.
[0025] FIGS. 19A-19C illustrate a configuration of the laser diode
emitter of FIG. 6, which features quantum well intermixing along
the wave guide, to vary the gain material bandgap along the
waveguide and thereby achieve a laser diode emitter with spectrally
broadened gain.
[0026] FIGS. 20A-20C illustrate an example configuration of the
laser diode emitter of FIG. 6, which features a patterned substrate
prior to epitaxial growth along the waveguide of the laser diode
emitter.
[0027] FIG. 21 illustrates a flowchart of a method, according to
one embodiment of the present disclosure.
[0028] FIG. 22 illustrates a flowchart of a method, according to
another embodiment of the present disclosure.
[0029] FIG. 23 illustrates a flowchart of a method, according to
another embodiment of the present disclosure.
[0030] FIG. 24 shows a schematic view of an example computing
environment according to which the near-eye display system of FIG.
1 may be implemented.
DETAILED DESCRIPTION
[0031] Compared to other types of light sources such as light
emitting diodes (LEDs) that work based on a light emission process
such as spontaneous emission, lasers produce light through
stimulated emission. Typically, the stimulated emission of lasers
creates photons of identical phase, frequency, and direction of
travel in a cavity of the laser device, the photons being at a
wavelength with gain near a maximum. Thus, the stimulated emission
process typically results in a narrower spectral bandwidth in
lasers compared to other types of light sources such as LEDs.
[0032] The narrower spectra in lasers may potentially cause several
challenges in display applications. For example, the narrower
spectra may potentially cause high contrast fringe artifacts in
waveguide-based displays. In a waveguide-based laser scanning
display, there are myriad (e.g., millions of) light paths resulting
from multiple interactions of image light beams propagating through
a waveguide and grating structures (e.g., surface relief grating
SRG). As such, there are myriad possible interactions between light
beams with different optical path lengths (OPLs). It will be
appreciated that a laser is a coherent light source where certain
OPLs that are proportional to an optical cavity length of the laser
form temporal coherence peaks. If an OPL of the waveguide matches
any of the coherence OPLs of the laser, an interference fringe may
be formed. An interference fringe may create an artifact in an
image presented by the waveguide-based laser scanning display. Such
an artifact may degrade the perceived image quality of the
waveguide-based laser scanning display.
[0033] Another potential image artifact that may occur due to the
narrower spectra of lasers is color nonuniformity. Due to the
typically small bandwidths of lasers and the dispersive nature of
surface relief gratings (SRGs), the incoming light couples inside
the waveguides only at a small range of angles (diffraction's
dependence on wavelength: .theta.(.lamda.)). Considering that the
SRG's coupling efficiency is angle dependent, the out-coupled beams
from the SRGs have different coupling efficiencies resulting in an
amplitude modulation artifact in the field of view (FOV). On the
other hand, due to a small range of in-coupled angles, the output
beam from each of these angles may not have a complete spatial
overlap. These artifacts may potentially degrade the perceived
image quality produced by a laser-based display application.
[0034] To address these issues and to mitigate image artifacts such
as interference fringes and color nonuniformity, FIG. 1 illustrates
an example near-eye display system 100 that implements a laser
light source having a larger range of wavelengths than typical
laser devices. As will be discussed in more detail below, the
larger range of wavelengths may potentially result in a larger
range of diffraction angles which leads to a smoother spatial
variation of the grating efficiency as well as an improved spatial
overlap of the out-coupled light.
[0035] As illustrated herein, near-eye display system 100 is a
component of a head-mounted electronic device 102, which is worn
and operated by a user 104. The near-eye display system 100 is
configured to present virtual imagery in the user's field of view.
In some implementations, user-input componentry of the wearable
electronic device 104 may enable the user to interact with the
virtual imagery. The wearable electronic device 102 takes the form
of eyeglasses in the example of FIG. 1. In other examples, the
wearable electronic device 102 may take the form of goggles, a
helmet, or a visor. In still other examples, the near-eye display
system 100 may be a component of a non-wearable electronic device,
such as a heads-up display. However, it should be appreciated that
the broad emission spectrum of the example laser light source
implementations discussed herein may also benefit other
applications such as, for example, fiber optic gyroscopes (FOG) and
optical coherence tomography (OCT).
[0036] The near-eye display system 100 may be configured to cover
one or both eyes of the user 104 and may be adapted for monocular
or binocular image display. In examples in which the near-eye
display system 100 covers only one eye, but binocular image display
is desired, a complementary near-eye display system may be arranged
over the other eye. In examples in which the near-eye display
system covers both eyes and binocular image display is desired, the
virtual imagery presented by near-eye display system 100 may be
divided into right and left portions directed to the right and left
eyes, respectively. In scenarios in which stereoscopic image
display is desired, the virtual imagery from the right and left
portions, or complementary near-eye display systems, may be
configured with appropriate stereo disparity so as to present a
three-dimensional subject or scene.
[0037] FIG. 2 shows an example micro-electro-mechanical system
(MEMS) laser scanning display device 200 that may be incorporated
into the near-eye display system 100 of FIG. 1, or other near-eye
display system. The MEMS laser scanning display device 200 uses a
laser assembly 201 as a laser light source. The laser assembly 201
comprises a plurality of individual laser light sources 201L which
may each emit an associated color of light. The laser light sources
201L may each be comprised of one or more laser diode emitters 202.
For example, red laser light source 201LR emits red light, green
laser light source 201LG emits green light, and blue laser light
source 202LB emits blue light. Although only three laser light
sources 201L are shown, it will be appreciated that the laser
assembly 201 may include any suitable number of laser light sources
201L. For example, the laser assembly 201 may include 0, 1, 2, 3,
or more than 3 red laser light sources 201LR; 0, 1, 2, 3, or more
than 3 green laser lights sources 201LG; 0, 1, 2, 3, or more than 3
blue laser light sources 201LB; and 0, 1, 2, 3, or more than 3
laser diode emitters of other colors. Any combination or
modification in the number of laser diode emitters may also be
available (e.g., 2 red, 2 green, 2 blue, or 1 red, 1 green, 2 blue,
etc.). Accordingly, any suitable number of laser light sources may
be used to irradiate/illuminate pixels for generating image
content. Further, each laser light source may have a suitable
number of laser diode emitters 202, which may be singular or
arranged in an array for example.
[0038] In some, though not all, configurations, the laser assembly
201 also includes a collimating lens assembly 204 (or other
diffractive optical element) that is structured to direct light to
another location or otherwise operate on the light in some manner.
In this example, each of the laser light sources 201LR, 201LG, and
201LB has a corresponding collimating lens 204, individually
labeled 204R, 204G, 204B. In some implementations, however, a
single collimating lens may be used for more than one laser light
source.
[0039] The MEMS laser scanning display device 200 includes
combination optics 205 configured to spatially combine the light
beams lased from the plurality of laser light sources 201LR, 201LG,
and 201LB into a single light beam.
[0040] The MEMS laser scanning display device 200 includes a MEMS
mirror system 206, though the principles disclosed herein are
applicable to any type of laser-based display unit and not only to
architectures with the MEMS mirror system 206. The MEMS mirror
system 206 is configured to collect laser light from the
combination optics 205, which combines light lased from three
different sources (i.e., the laser light sources 201LR, 201LG, and
201LB) into a single light beam. Additionally, the MEMS mirror
system 206 is configured to direct laser light 208A (which, in this
example includes red laser light, green laser light, and blue laser
light) to a waveguide 210. Furthermore, the MEMS mirror system 206
is configured to redirect its mirrors/mirror array so that the
laser light 208A is aimed at different locations at the waveguide
210. As shown, laser lights 208B and 208C are aimed at different
locations on the waveguide 210. In this manner, the MEMS mirror
system 206 is able to route light to different locations by
adjusting the aim of its corresponding mirror array. It will be
appreciated that the laser lights 208A-C may be modulated to
include varying degrees or intensities (or even an absence of any
one or more) of red, green, blue, or other color, laser light.
[0041] The waveguide 210 is configured to redirect or propagate the
laser light 208A-C to a desired location which is viewable by a
user's eye 212. It will be appreciated that waveguide 210 may be
any type of waveguide display (e.g., a surface relief grating
waveguide).
[0042] The laser light 208A-C enters the waveguide 210 via an entry
grating 214. The laser light 208A-C then propagates (e.g., via
total internal reflection) through the waveguide 210 until it
reaches an exit grating 216. It will be appreciated that the angles
with which the laser light 208A-C enters the waveguide 210 are
preserved as the laser light 208A-C propagates through the
waveguide 210. This condition is shown by the different angles that
each of the respective laser lights 208A-C propagate through the
waveguide 210. By configuring the entry grating 214 and the exit
grating 216 to meet certain design parameters, the MEMS mirror
system 206 is able to use waveguide 210 to propagate light towards
the user's eye 212.
[0043] The laser assembly 201 and the MEMS mirror system 206 may be
controlled by a controller 220. The controller 220 may be
configured to control the MEMS mirror system 206, in conjunction
with the laser assembly 201 to progressively scan a set of pixels
218 to a target display area for a user's eye 212 to view (e.g., by
adjusting the mirror array so that the combined RGB laser beam or
light is aimed at different locations) individual pixels of that
image in such a rapid manner that the entirety of the image appears
before the user's eye 212 without the user realizing that the image
was progressively scanned pixel by pixel and line by line. In this
way, the MEMS laser scanning display device 200 may project or
render image content for a user to view.
[0044] The MEMS mirror system 206 may be able to scan an image
(i.e., pixels of that image) at any image resolution or range of
image resolutions (e.g., in cases where foveated rendering is
used). For example, in some implementations, the MEMS mirror system
206 is configured to scan RGB light from the laser assembly 201
with a resolution of 2,000 pixels by 1,200 pixels, or any other
resolution.
[0045] There are various instances in which, due to the wave
properties of laser light, fringe interference can occur. Laser
light output from the waveguide is a composite of multiple separate
waveforms with different optical path lengths (OPLs). Such a
composite waveform may have multiple peaks (i.e., coherence peaks
where OPLs of different light beams have temporal coherence) and
valleys as a result of combining each of the multiple waveforms.
FIG. 3 shows a graph 300 of an example composite waveform of laser
light output from a laser diode emitter. The composite waveform has
a plurality of coherence peaks spaced apart by valleys. The
distance between the coherence peaks is proportional to the
effective length of the optical cavity of the laser diode emitter.
The shape of the coherence peaks is defined by the laser gain
spectrum of the laser diode emitter. In this example, the active
gain section occupies the entire optical cavity and has a length of
300 .mu.m.
[0046] Additionally, FIG. 3 shows a graph 302 of interference
fringes of the waveguide. In instances where the interference
fringes do not overlap with the coherence peaks of the laser light,
the interference fringes do not create artifacts in an image output
from the waveguide. In the depicted example, interference fringes
304, 306, and 308 do not overlap with coherence peaks of the laser
light and thus do not create artifacts in the image. However,
interference fringe 310 overlaps with coherence peak 312 and
interference fringe 314 overlaps with coherence peak 316. These
overlapping interference fringes create artifacts in the image
output from the waveguide. For example, these interference fringes
may result from input laser light into the waveguide bouncing back
and forth a few times and then combining with itself exiting the
waveguide. As a result of these interference fringes, various
different bright and dark rings/spots are created. FIG. 4 shows an
example image 400 including artifacts 402 and 404 caused by such
interference fringes. The artifacts 402, 404 include irregularities
in terms of brightness. The presence of such artifacts in an image
can lower the perceived quality of the image and thereby negatively
affect the user experience. In the depicted example, the artifact
402 distorts the appearance of a dragon in image 400. Further, the
artifact 404 distorts the appearance of a fireball shot by a wizard
at the dragon.
[0047] As discussed above, another potential image artifact that
may occur due to the narrower spectra of lasers is color
nonuniformity. The cause of this artifact is generally illustrated
in FIG. 5A and FIG. 5C. FIG. 5C illustrates a typically narrow
bandwidth of laser light, while FIG. 5A illustrates the dispersive
nature of SRGs. The incoming light of narrow bandwidth such as
shown in FIG. 5C couples inside the waveguides only at a small
range of angles, due to the dependence of the diffraction on
wavelength: .theta.(.lamda.). Considering that the SRG's coupling
efficiency is angle dependent, the out-coupled beams from the SRGs
have different coupling efficiencies (see E1-E3 in FIG. 5E),
resulting in an amplitude modulation artifact in the field of view
(FOV). Thus, a narrow lasing spectrum (FIG. 5C) due to the small
range of in-coupled angles when passed through the SRG (FIG. 5A)
can form an output beam (E1-E3 of FIG. 5A and FIG. 5E) from
component light paths have been coupled at varied coupling
efficiencies by the SRG, negatively affecting the color
uniformity.
[0048] On the other hand, FIG. 5B illustrates an example
configuration of a waveguide receiving light from a spectrally
broad laser diode emitter that may be used in near-eye display
system 100 described herein. As shown, due to the spectrally broad
light source, the bandwidth of which is illustrated in FIG. 5D, the
light inside the waveguide couples in via an SRG and propagates
through the wave guide at a larger range of angles as compared to
the example shown in FIG. 5A. The larger range of angles may
provide several potential benefits. For example, as illustrated in
FIG. 5B, the larger range of angles may provide a correspondingly
larger spatial coverage of the out-coupled beam, which may result
in a more uniform image. As another example, the broad spectrum
light source may provide a relatively smoother change of SRG
out-coupling efficiency from E1 to E3 as shown in FIG. 5B as
compared with the abrupt efficiency points in the narrower spectrum
light source example of FIG. 5A. FIG. 5E is a graph showing that
the SRG grating's efficiency dependence on the incoming light
angle. It will be appreciated that the solid dots representing
outcoupled beams from the spectrally broad light source of FIG. 5D
are densely and evenly spaced along the efficiency/angle line, as
compared to the white dots representing the outcoupled light from
the spectrally narrower light source of FIG. 5C. As a result of the
more uniform spatial distribution of the light paths within the
waveguide, greater color uniformity is achieved by use of the
spectrally broad light source.
[0049] Several configurations for laser light sources 201L
including laser diode emitters 202 that achieve emission of
spectrally broad light will now be described. FIG. 6 illustrates
one embodiment for a laser diode emitter 202A that emits spectrally
broadened light and can be incorporated as a laser diode emitter
202 of any color of laser light source including 201LR, 201LG, and
201LB in near-eye display system 100 described above. Laser diode
emitter 202A can thus take the form of a laser diode emitter for
use in a MEMS laser scanning display device having a laser cavity
that includes a gain section 600 and a tunable absorber section
602. FIG. 6 illustrates a schematic view of a vertical
cross-section along a Y-Z plane of a laser diode emitter. As shown,
the laser diode emitter 202A may include a substrate 612 that
extends across the X-Y plane shown in FIG. 6. The various layers of
laser diode emitter 202A may be directly or indirectly deposited on
top of the substrate 612 in a series of layers. The gain section
600 and tunable absorber section 602 are sections of the laser
cavity of the laser diode emitter 202 that extend vertically along
the Z-axis from the substrate 612. A dividing line 614 is shown to
schematically illustrate a boundary between the gain section 600
and the tunable absorber section 602. However, it should be
appreciated that the various layers of the laser diode emitter 202A
may extend through both the gain section 600 and the tunable
absorber section 602. For example, at least a quantum well layer
616 may extend through each of the gain section 600 and the tunable
absorber section 602 within the laser cavity of the laser diode
emitter 202A.
[0050] As illustrated in FIG. 6, the gain section 600 is coupled
with a first current or voltage source 604, and the tunable
absorber section 602 is coupled with a second current or voltage
source 606. The first current or voltage source 604 is coupled to
the gain section 600 via a first electrode 608, and the second
current or voltage source 606 is coupled with the tunable absorber
section 602 via a second electrode 610. In one example, at least a
portion of the gain section 600 is electrically isolated from at
least a portion of the tunable absorber section 602. For example,
the first electrode 608 coupling the gain section 600 to the first
current or voltage source 604 may be separated from the second
electrode 610 coupling the tunable absorber section 602 to the
second current or voltage source 606. As the first electrode 608
and the second electrode 610 are separated, the drive currents
supplied by the first and second current or voltage sources will be
injected to the respective portions of the gain section 600 and
tunable absorber section 602 that lie underneath the first and
second electrodes.
[0051] In the example illustrated in FIG. 6, a portion 614 of the
gain section located under the first electrode 608 will be supplied
with a voltage or current from the first voltage or current source
604, while the portion 616 of the tunable absorber section located
under the second electrode 610 will be supplied with a voltage or
current from the second voltage or current source 606. As the first
electrode 608 and second electrode 610 are electrically separated,
separate drive voltages or currents may be supplied to the portion
614 of the gain section and the portion 616 of the tunable absorber
section. In one example, the portion 614 of the gain section and
the portion 616 of the tunable absorber section 602 are further
electrically isolated from each other via a notch 618 that extends
downward from the first and second electrodes into the top layers
of the laser diode emitter 202A. In the illustrated example, the
notch 618 electrically isolates the two terminals for receiving the
drive currents Ia and Ip for each of the gain section 600 and
tunable absorber section 602, respectively, as well as the first
few layers of the laser diode emitter diode (e.g. portions 614 and
616).
[0052] In the example laser diode emitter 202A illustrated in FIG.
6, the gain of the gain section 600 may be controllably varied by a
gain drive current supplied by the first voltage or current source
604. The tunable absorber section 602 is drivable by the second
voltage or current source 606 to adjust a bandwidth of the emitted
laser light. The amount of current delivered to the tunable
absorber section 602 is inversely related to the capability of the
tunable absorber section 602 to absorb photons at certain
wavelengths. Thus, as more drive current is supplied to the tunable
absorber section 602, the absorber experiences a reduction in its
ability to absorb photons at certain wavelengths. Wavelengths of
light not absorbed by the tunable absorber section 602 are
available for amplification by the gain section 600. Thus, by
changing the current applied to the tunable absorber section 602,
the spectrum of laser light can also be changed. By applying a
pulse absorber drive current, also referred to as "chirping" the
tunable absorber section 602 herein, at modulated current levels, a
target gain spectrum may be achieved. The effects of tuning the
semiconductor laser diode emitter 202A in this manner, using the
described combination of a chirped tunable absorber section and
integrated gain section, may be measured using Hakki-Paoli
measurements on the laser gain spectrum (which indicates loss at
negative values), for example.
[0053] It should be appreciated that a laser is composed of a gain
material in a cavity, and lasing starts once the gain equals the
loss inside the cavity. Hence, laser spectral bandwidth is
determined by the wavelength range where the gain spectrum
(.GAMMA.g-ai) exceeds the reflector loss am(.lamda.). By shifting
the wavelength at which the loss is minimum or has the maximum
overall gain inside the cavity, the lasing wavelength will shift.
However, for a larger shift in the lasing wavelength, the gain
spectrum may also be flattened over a larger wavelength range to
mitigate potential problems due to the lasing wavelength being
potentially limited by the gain spectral peak. By chirping the
tunable absorber section 602 fast enough (faster than one-pixel
time and faster than the overall response time of the laser to the
input currents driving the gain and absorber sections before
getting to a steady state regime response), the laser spectrum of
laser diode emitter 202A may be broadened.
[0054] Accordingly, the second voltage or current source 606 is
configured to supply a drive current or voltage that sweeps through
a range of values over a period of time to cause a shifting of a
wavelength of the laser diode emitter 202A at which an overall gain
is at maximum, to thereby cause the laser diode emitter 202A to
emit spectrally broadened light. As used herein, the term
"spectrally broadened light" is used to describe a laser light
spectrum that has been broadened as compared to a laser light
spectrum emitted by a conventional laser diode emitter that emits
light with a single gain peak and narrow bandwidth.
[0055] As illustrated in FIG. 7, an absorption spectral edge inside
the tunable absorber section 602 is shifted by 1 to 20 nanometers
over the range of values supplied by the second current or voltage
source 606. More specifically, the absorption spectral edge shifts
by 18 nm when changing the injected current into the tunable
absorber section 602 within the range of 0.25 mA to 11.5 mA. In one
example, the second current or voltage source 606 may be configured
to sweep through the range of values (e.g. 0.25 mA to 11.5 mA) over
a period of time between 2 nanoseconds and 20 nanoseconds. More
particularly, the second current or voltage source 606 may be
configured to sweep through the range of values in a time period
defined by a single-pixel display time, which is typically 4
nanoseconds. As another example, the second current or voltage
source 606 may be configured to sweep through the range of values
over other time periods, such as, for example, four-pixel display
times, which is typically 16 nanoseconds. It should be appreciated
that these time periods are merely exemplary, and that any suitable
time period may be selected for sweeping through the range of
values.
[0056] However, as described above, the lasing spectrum is also
determined by the gain spectral shape. Thus, the overall bandwidth
of the emitted laser light may potentially not reach the full 18 nm
shift shown in FIG. 7. Rather, as shown in FIGS. 8A-B, for an
absorber drive current driven in a pulsed manner to vary between
0.25 mA and 11.5 mA, supplied to the tunable absorber section 602,
the wavelength at which the overall gain is at maximum shifts, and
hence the overall spectrum changes from approximately 1 nm in
bandwidth as shown at FIG. 8A to a broader range of approximately 3
nm in bandwidth as shown at FIG. 8B.
[0057] To take advantage of the whole absorption tuning bandwidth
of 18 nm in this example, flattening the gain spectrum over a
bandwidth equal to or larger than 18 nm may provide advantageous
benefits. Thus, in one example, the gain section 600 of the laser
diode emitter 202A may be configured to have a flattened gain
spectrum. As used herein, the term "flattened gain spectrum" may be
used to describe a gain spectrum that has a substantially flattened
peak, such as by including a plurality of peaks at different
wavelengths. A flattened gain spectrum may have a flattened peak as
compared to a typical gain spectrum of a conventional laser diode
emitter that has a sharper peak.
[0058] One proposed method discussed thus far has been to use the
structure shown in FIG. 6 to increase the lasing bandwidth. In
order to increase the bandwidth of emitted light from this
embodiment even further, the gain spectrum may be flattened through
various approaches for flattening the gain spectrum of single
emitters, such as growing non-uniform multiple quantum wells for a
laser diode emitter to achieve a single emitter with a flat and
broad gain spectrum that is uniform across the wafer and along the
waveguide of the laser diode emitter, as shown in FIG. 17. This
approach achieves an emitter structure in which quantum well
sections of the multiple quantum well layers each have different
composition, thickness, and/or strain in the quantum well layer,
which causes them to lase at different peak wavelengths. Another
approach for flattening the gain spectrum of single emitters that
may be used with the embodiment of FIG. 6 is shown in FIG. 18, and
includes an emitter structure that features spatially varied
substrate surface vicinal angles prior to epitaxial growth along a
waveguide of a single laser diode emitter to achieve a structure
with different vicinal angles along the waveguide of the laser
diode emitter, which results in a laser diode emitter with a
flattened gain spectrum. In FIG. 18, along the laser diode
waveguide, the substrate vicinal angle is changed. FIG. 18 depicts
one particular example in which the change is a parabolic change;
however, it will be appreciated that other changes in vicinal angle
along the waveguide are also contemplated. For every angle along
the waveguide, the indium incorporation, and hence the lasing
wavelength, is different. In this specific example, along the
waveguide the substrate angle gradually changes from 0 to .theta.,
hence the lasing wavelengths gradually changes from .lamda..sub.1
to .lamda..sub.2 covering a range between the two. Typically,
.theta. is a small angle that is less than 2.degree. (although
other ranges of values are possible), and thus it will be
appreciated that the FIG. 18 is not to scale and the change in
.theta. is exaggerated for illustrative purposes.
[0059] Another approach may include quantum well intermixing in the
quantum well layer along the waveguide after epitaxial growth to
achieve a single laser diode emitter with a flattened gain
spectrum, as shown in FIGS. 19A-19C. As shown in FIG. 19A, a
thickness of a quantum well intermixing mask layer formed along the
waveguide and on the laser diode stack structure is varied. The
laser diode stack with mask layer is subject to annealing at high
annealing temperatures, thereby causing quantum well intermixing
along the waveguide, and then removed. Due to the differences in
thickness of the mask layer, indium incorporation along the
waveguide due to the quantum well intermixing, and hence the gain
material bandgap along the waveguide, will gradually change as
shown in FIG. 19B, resulting in a flatter gain and a broader range
of the lasing spectrum, as shown in FIG. 19C.
[0060] Another approach may include using a patterned substrate
prior to epitaxial growth along the waveguide of the laser diode
emitter, as shown in FIGS. 20A-20C. As shown in FIG. 20A,
patterning the substrate may be accomplished by, along the laser
waveguide, changing growth factors such as the growth rate (and
hence the quantum well thickness) depending on the distance from
the etched patterned area. This results in a change of lasing
wavelength along the waveguide as shown in FIG. 20B from
.lamda..sub.1 to .lamda..sub.4. Growing the epitaxial layers and
processing the waveguide in this manner produces a laser that lases
at a broader range of wavelengths in the emitted lasing spectrum,
as illustrated in FIG. 20C. It should be noted that other forms of
patterns on the substrate prior to the growth of the epitaxial
layers could be used to achieve a similar result.
[0061] Turning now to FIGS. 9A-C, an example flattened spectrum
achievable through one of the above discussed approaches (spatially
varied substrate surface vicinal angles prior to epitaxial growth
along a waveguide of a laser diode emitter) is shown, where a
combination of spectra centered at different wavelengths will
result in an overall broader spectrum which is substantially flat
over a larger range of wavelengths. FIG. 9A shows the gain spectrum
of a laser with single emitter in which there is a maximum at
approximately 515 nm. FIG. 9B shows an overlay of the spectrum of
FIG. 9A and three additional spectra similarly shaped, but with
local maxima at approximately 520 nm, 525, and 530 nm. When viewed
from left to right, each of these individual spectra may be
described as a concave waveform with a leading edge, a local
maximum, and a trailing edge. The gain in these individual spectra
decreases as the wavelength deviates from the local maximum.
However, in FIG. 9C, an overlay of the four spectra shows that
between the maxima at approximately 515 nm and 530 nm, the overlay
remains relatively flat when compared to any of the individual
spectra. Thus, as shown in FIG. 9C, the flattened gain spectrum has
a substantially flatter top as compared to a typical gain spectrum
of a conventional laser shown in FIG. 9A.
[0062] As an alternative to the approach shown in FIG. 6 using a
tunable absorber, and as an alternative to the single laser diode
emitter with multiple quantum wells of FIG. 17, FIGS. 10-16
illustrate implementations of laser emitter arrays configured for
broad spectrum outputs by spatially distributing individual
emitters that have been tuned through various processes to emit
light of slightly different peak wavelengths. By incorporating the
emitter arrays of FIGS. 10-16 into the laser diode emitters 202 of
the MEMS laser scanning display device 200 of FIG. 2, the image
artifacts such as fringe artifacts and color uniformity artifacts
of FIG. 4 can be inhibited. It will be appreciated that embodiments
of FIGS. 10-16 are merely exemplary and that other techniques for
spatial distribution of the spectral bandwidth may also be adopted
within the MEMS laser scanning display device 200.
[0063] FIG. 10 illustrates another embodiment of the broad-spectrum
laser light source 201L, including a light emitter array 202B of a
plurality of laser diode emitters 202B1-4. The laser diode array
202B may take the form of a light-emitting device having a gallium
nitride (GaN) (in the case of green or blue laser diode emitters)
or gallium arsenide (GaAs) (in the case of red laser diode
emitters) substrate layer 1000 and a plurality of laser diode
emitters 202B1-4 formed by deposition of indium containing thin
layers, such as indium gallium nitride (InGaN). The GaN or GaAS
substrate 1000 has a plurality of substrate angles, which may be
identical or varied, affecting the indium content in the laser
diode emitters, which in turn affects the wavelength of emitted
light. Thus, the effect of the plurality of laser diode emitters
each with a different emission spectrum, is an overall broadened
emission spectrum. While in the depicted embodiment the substrate
angle of each laser diode emitter 202B1-4 is different from the
others, it will be appreciated that in other embodiments some of
the laser diode emitters may share a same substrate angle, so long
as at least two, and typically at least multiple laser diode
emitters have different substrate angles in the array.
[0064] The example broad-spectrum laser light source 201L of FIG.
10 may be incorporated in the MEMS laser scanning display device
200 shown in FIG. 2. As discussed above, the MEMS laser scanning
display device may include a MEMS scanning mirror 206 configured to
guide the light beam, via a waveguide 210 with an in-coupling
grating 214 and out-coupling grating 216, to achieve a scanning
pattern across the display and thereby form a displayed image. As
discussed above with respect to FIGS. 5A-E, as compared to a laser
light source that emits a light beam with an emission spectrum
having a single peak, the broadband light beam increases color
uniformity in the displayed image by having a larger range of
wavelengths in the laser light source, which results in a larger
range of diffraction angles at the in-coupling grating and
out-coupling grating, which in turn causes a smoother spatial
variation of the grating efficiency as well as an increased spatial
overlap of the out-coupled light.
[0065] To achieve the broadband light beam described above, the
MEMS laser scanning display device 200 further may include a
plurality of laser light sources 201L, and each of those laser
light sources may take the form of the example broad-spectrum laser
light source 201L with varying substrate angles shown in FIG. 10.
As illustrated, the broad-spectrum laser light source 201L includes
an emitter array 202B including a plurality of laser diode emitters
202B1-4. Each laser diode emitter 202B1-4 is configured to emit
respective different wavelengths of light, to thereby form a
broadband light beam with a broadband emission spectrum. That is,
the example broad-spectrum laser diode emitter of FIG. 10 spatially
distributes a larger range of wavelengths by use of a plurality of
laser diode emitters 202B1-4.
[0066] For example as shown in FIG. 11A, a typical laser diode
emitter at green wavelengths has a maximum full width half maximum
(FWHM) bandwidth of .about.2.5 nm after pulsing the laser. If a
light source with a bandwidth of .about.10 nm potentially mitigates
artifacts such as the ones mentioned above, four laser diode
emitters each with a FWHM bandwidth of .about.2.5 nm and an offset
of 2.5 nm in their central wavelength may be used to achieve the
potential advantages described herein.
[0067] In contrast, FIG. 11B shows a superposition of the offset
spectra of the plurality of laser diode emitters 202B1-4 of FIG.
10. That is, the example broad-spectrum laser light source of FIG.
10 may be configured to emit a broadband light beam that has an
emission spectrum that includes a plurality of peaks, as shown in
FIG. 11B. The number and the wavelength offset of the laser diode
emitters may be selected based on the overall lasing bandwidth that
is needed and the system level considerations and limitations of
MEMS scanners with a plurality of laser diode emitters.
[0068] Turning back to FIG. 10, the laser light source may include
a substrate 1000 having a plurality of deposition regions 1006 on a
top surface 1008 thereof. Each deposition region 1006 has a
different surface normal 1010 formed at orthogonally relative to a
surface of the deposition region 1006. Each surface normal 1010 is
also formed at an angle relative to a planar bottom surface 1012 of
the substrate 1000. Each laser diode emitter 202B1-4 may include a
semiconductor material 1014 deposited on each of the deposition
regions 1006 with surface normal 1010 of different angles in
semiconductor regions 1006.
[0069] The semiconductor region of each laser diode emitter 202B1-4
may be configured to emit a respective different wavelength of
light, to thereby form the light beam with a broadband emission
spectrum with a plurality of peaks. In one example, the
semiconductor material may be InGaN, and indium incorporation in
each of the deposition regions may vary due to different vicinal
angles in each deposition region. The varying vicinal angles in
each deposition region may also change the multiple quantum wells
(MQW) thicknesses and/or the strain in the MQWs, which will result
in different peak wavelengths. Further, the varying of the
incorporation of indium in each of the deposition regions may also
cause variation in the wavelengths of each laser diode emitter. In
this manner, the substrate angle may affect the indium content,
thicknesses, and/or strain in the MQWs, which determines the lasing
wavelength. The substrate angle may be picked based on the
wavelength offset that is needed. The physical distance between
laser diode emitters may be selected based upon the following
considerations: (1) maximum electrical, optical, and thermal
isolation between laser diode emitters, (2) MEMS system level
requirement for minimum visibility of scan lines, and (3) maximum
image artifact mitigation. That is, each laser diode emitter
supports a certain range of diffracted angles (.theta.) in the SRGs
and the vicinity of laser diode emitters will result in a smoothing
out of the color uniformity and other bandwidth dependent
artifacts.
[0070] The example laser diode emitters act like a spectrally
broadened laser diode emitter for mitigating the image artifacts
discussed above in two ways. By shifting the vertical locations of
scan lines in each frame, within a few frames, each pixel or line
will be rendered by all laser diode emitters. Hence, the user
perceives the combined frames similarly to the case of a broadband
laser diode emitter for a fast-enough frame rate. Additionally, the
spatial vicinity of laser diode emitters with wavelength offset
where each laser diode emitter excites a different range of
diffracted angles, may help reduce the artifacts visibility
locally.
[0071] FIG. 12 illustrates another example laser light source 201L
for emitting spectrally broadened laser light. The example laser
light source 201L may include a laser diode emitter array 202C that
includes a plurality of laser diode emitters 202C1-4. Each of the
plurality of laser diode emitters 202C1-4 in this configuration is
configured to have the same gain material (and thus same gain peak
in its emission spectra) but different reflection or loss profile
to make the laser diode emitters emit light at different peak
wavelengths. One way to tune the reflection profile could be
realized through Bragg gratings with different designs, as
shown.
[0072] As shown in FIG. 12, the example laser light source 201L
includes a plurality of cavities filled with a gain material for
each laser diode emitter 202C1-4. The cavities are resonant and
defined by a highly reflective Distributed Bragg Reflection (DBR)
mirror on one end and a different, low reflectivity exit grating
1200 for each of the plurality of laser diode emitters 202C1-C4 on
another end. When current is injected to the cavities, each laser
diode emitter 202C1-C4 emits light of a different, respective
wavelength .lamda.1-.lamda.4 out of the exit grating. In one
example configuration, this can result in a shift of the lasing
wavelength or the effective gain peak by +/-5 nm. It will be
appreciated that shifts of other magnitudes may also be implemented
as desired.
[0073] Another configuration of the broad-spectrum laser diode
emitter is shown at 202D in FIG. 13 and described below. As shown,
the laser diode emitter 202D includes a substrate 1300. Substrate
1300 typically includes a first contact layer 1308A formed directly
or indirectly on an underside thereof, to which an electrode for
electrically coupling a driving or voltage source may be provided.
The laser diode emitter 202D may further include a first cladding
layer 1304A deposited directly or indirectly onto the substrate
1300, a first waveguide layer 1306A deposited directly or
indirectly onto the first cladding layer 1304A, and a plurality of
quantum wells 1302 formed in a quantum well layer. The plurality of
quantum wells is depicted as including four quantum wells 1302;
however, it will be appreciated that other suitable numbers of
quantum well layers may be included.
[0074] Each of the quantum wells 1302 has a respective material
composition to achieve a respective peak wavelength in emitted
light when energized. The respective material composition of each
quantum well 1302 is different. Because the material composition of
the quantum wells 1302, along with factors such as thickness and
strain, determines the peak wavelength of emitted light, each
respective peak wavelength is different. Therefore, when the laser
diode emitter 202D is energized, the plurality of quantum wells
1302 collectively lase emitted light having a broader bandwidth
than is emitted by any individual quantum well 1302 of the
plurality of quantum wells 1302, and the emitted light includes the
plurality of respective peak wavelengths across an emitted light
frequency spectrum. As used herein, "collectively lase" means lase
as a group to emit a spectrum of light together as a group. Thus,
while individual quantum wells 1302 within the laser diode emitter
202D may each have a respective narrow (e.g. 2 nm) peak bandwidth
of emitted light, the peak bandwidth of emitted light of the laser
diode emitter as a whole will be greater (e.g. 10 nm or more) than
any of the individual quantum wells 1302 therein. The emitted light
having a broader bandwidth is the result of superposition of waves
of the emitted light from each individual quantum well 1302. In
other words, each individual quantum well 1302 of the laser diode
emitter 202D emits light with a respective peak wavelength and
associated emission spectrum, and the emission spectrum of the
laser diode emitter 202D is the sum of the emission spectra of the
individual quantum wells 1302. Example spectra of individual
quantum wells 1302 collectively lasing are provided in FIG. 14 and
described in detail below.
[0075] Laser diode emitter 202D further includes a second waveguide
layer 1306B deposited directly or indirectly onto the quantum well
layers 1302 deposited directly or indirectly on the topmost quantum
well 1302, a second cladding layer 1304B deposited directly or
indirectly onto the second waveguide layer 1306B, and a second
contact layer 1308 deposited directly or indirectly onto the second
cladding layer 1304B. The second contact layer 1308 includes an
electrode for coupling to the driving voltage or current source,
similar to the first contact layer. The depicted layers, with the
possible exception of the substrate, are formed by epitaxial growth
of semiconducting materials and may be referred to collectively as
epitaxial layers. Examples of semiconducting materials include
GaAs, InAlGaP, GaN, AlGaN, and InGaN. However, it should be
appreciated that other semiconducting materials may be used.
[0076] As described above, the material composition of the quantum
wells 1302 determines the peak wavelength of the emitted light. In
one example, the laser diode emitter 202D may be configured to emit
red light, and the material composition may follow the formula:
In.sub.x[Al.sub.yGa.sub.1-y].sub.1-xP, where 0.4<x<0.6 and
0<y<0.55. It will be appreciated that in other aspects, the
material composition may follow any suitable formula for a laser
diode emitter emitting red light.
[0077] In another example, the laser diode emitter 202D may be
configured to emit green light, and the material composition of the
quantum wells 1302 may follow the formula: In.sub.xGa.sub.1-xN,
where 0.26<x<0.4. It will be appreciated that in other
aspects, the material composition may follow any suitable formula
for a laser diode emitter emitting green light.
[0078] In another example, the laser diode emitter 202D may be
configured to emit blue light, and the material composition of the
quantum wells 1302 may follow the formula: In.sub.xGa.sub.1-xN,
where 0.15<x<0.25. It will be appreciated that in other
aspects, the material composition may follow any suitable formula
for a laser diode emitter emitting blue light.
[0079] Turning now to FIG. 14, a simulated gain spectrum of a
conventional laser diode emitter comprising four quantum wells of
uniform composition is shown along with the simulated gain spectra
of the four constituent quantum wells of the conventional laser
diode emitter. Uniform quantum wells may be used to the provide
potential benefit of a narrow bandwidth that may be suitable for
laser applications. In this configuration, a narrow (ca. 2 nm) peak
bandwidth is obtained.
[0080] FIG. 14 further illustrates plot of wavelength vs. gain for
each of laser diode emitter 202D described above, and a
conventional laser diode. The configuration of laser diode emitter
202D that has been graphed includes two quantum wells having a
first material composition and two quantum wells having a second
material composition that is different from the first. For each of
the conventional laser diode and laser diode emitter 202D, an
output of the individual quantum well spectra and a combined
spectrum of lased light from all quantum wells has been plotted. As
can be observed, the peak of the combined spectrum of the
conventional laser is narrow (around 2 nm) whereas the peak of the
combined spectrum of the laser diode emitter 202D has a broad
(about 10 nm) peak bandwidth.
[0081] Turning now to FIGS. 15 and 16, a method of manufacturing a
laser diode emitter 202D1 according to another configuration of the
present disclosure is illustrated. A wafer comprising a substrate
1500 and a plurality of epitaxial layers 1501 is shown. Epitaxial
layers 1501 include a plurality of quantum wells that are organized
in a plurality of quantum well layer sections 1502 (i.e., quantum
well layer sections arranged along the horizontal axis in FIG. 15).
In this configuration, the material composition of each of the
quantum well layer sections 1502 in the plurality of quantum well
layer sections 1502 includes an epitaxial structure, the epitaxial
structure of each composition is substantially uniform across the
wafer when deposited, and the quantum wells formed therein are
intermixed through thermal interdiffusion with adjacent
semiconductor layers to thereby achieve a different peak in the
corresponding gain spectrum for each quantum well layer section
1502. As shown, epitaxial layers 1501 are uniformly deposited onto
a substrate 1500 (top left of FIG. 15) and the resulting quantum
well layer sections 1502 (bottom left of FIG. 15) are uniform and
therefore would emit light of a same energy and peak wavelength
without further processing, but the quantum well layer sections
1502 are further processed to have different peak wavelengths, as
described below.
[0082] Further processing, with the goal of producing quantum well
layer sections 1502 each emitting a different peak wavelength, may
be achieved using the following techniques. Continuing with FIG. 15
at left, the emission spectrum of each quantum well layer section
can be modified by application of a strain inducing thin film layer
1504 followed first by an annealing process and second by removal
of the strain inducing thin film layer, both of which are described
in more detail below. The right side of FIG. 15 illustrates the
stack after annealing but prior to the removal of the strain
inducing film layer 1504.
[0083] Although the use of a strain inducing thin film layer 1504
is described herein as a way to produce a wafer having quantum well
layer sections 1502 each emitting a different peak wavelength due
to different epitaxial structures, other suitable techniques may
also be used. For example, one alternative technique is regrowth of
a modified epitaxial structure at different sections of the wafer,
either on native substrate 1500 or on etched sections of substrate
1500. Another alternative technique involves etching each of the
quantum well layer sections 1502 before depositing additional
layers by epitaxial growth.
[0084] Continuing with FIG. 15, the material for the strain
inducing thin film layer 1504 is chosen such that the lattice
constants of the strain inducing thin film layer 1504 are
mismatched with those of the material on which the strain inducing
thin film layer 1504 is deposited. In this configuration, the
strain inducing thin film layer 1504 does not cover the entire
surface on which it is deposited, but instead has a plurality of
segments of varying thickness. In this configuration, the plurality
of segments are separated by gaps of 3 .mu.m, and each segment has
a respective predetermined uniform thickness. In another
configuration, the segments may be adjacent to one another without
gaps, or gaps of another dimension may be provided. For example, in
other configurations, the gaps may be 1, 2, 4, or 5 .mu.m. In other
configurations, the gaps may even be 100, 200, 300, 400, or 500
.mu.m. Typically, a same number of quantum well layer sections 1502
and segments of thin film layer 1504 are provided. Annealing at a
predetermined temperature serves to trigger diffusion and
intermixing between each quantum well layer sections 1502 and
regions of the semiconductor layers adjacent the quantum well layer
sections 1502, as illustrated in FIG. 15 (bottom right) and FIG.
16. The different respective peak wavelengths of each quantum well
layer section 1502 are due to quantum well intermixing by
interdiffusion of constituent atoms between each of the quantum
well layer sections 1502 in the quantum well layer and respective
regions of the first and/or second semiconducting layers vertically
adjacent the corresponding quantum well layer sections. It will be
appreciated that each of the plurality of segments of the strain
inducing layer 1504 was positioned before annealing and prior to
removal above a corresponding one of the plurality of quantum well
layer sections 1502, to thereby vary the interdiffusion within each
quantum well layer section based on the respective thickness of the
corresponding segment positioned above each quantum well layer
section. Removal of the strain inducing thin film layer 1504
results in the plurality of quantum well layer sections 1502 being
spatially distributed horizontally and parallel to a horizontal
plane defined by the substrate 1500, each section 1502 having
varied diffusion therewithin, and each section 1502 being
configured by this varied diffusion to lase at a different
corresponding peak wavelength.
[0085] Energy states of the pre-annealing quantum well and
post-annealing intermixed quantum well layer sections 1502 are
depicted in energy graph 1506 of idealized quantum well structure
sections 1502 at bottom of FIG. 15. As shown, at left in energy
graph 1506, the energy function is depicted as having sharp corners
and being the same for each quantum well layer section 1502. As
depicted at right, the energy function is shown to be gradually
less sharp cornered from left to right, which has the effect of
shifting the emitted energy profile in a gradually more pronounced
manner from left to right FIG. 15.
[0086] FIG. 16 provides a continuing illustration of the
manufacturing process for the laser diode emitter 202D1, which will
now be described. A top view (top left of FIG. 16) and side view
(top right of FIG. 16) of the wafer that will be used to
manufacture laser diode emitter 202D1 includes the substrate 1500,
epitaxial layers 1501 and quantum well layers 1502, and the
plurality of segments of strain inducing thin film layer 1504. The
wafer undergoes annealing at a temperature and duration sufficient
to trigger diffusion and intermixing of the quantum well layer
sections 1502 and the layers adjacent the quantum well layer
sections 1502. Diffusion and intermixing during annealing may be
the result of differing thermal expansion coefficients of the
strain inducing thin film layer 1504 and the layers on which it is
deposited. During annealing, the differing thermal expansion causes
increased strain resulting in defects between the quantum well
layers and the layers adjacent the quantum well layers. At
annealing temperatures, the defects allow diffusion of atoms
between the quantum well layers and the semiconductor layers
adjacent the quantum well layers. In this example, the annealing
temperature is between 800.degree. C. and 1000.degree. C., for
example about 950 degrees, and the annealing duration is 30 to 150
seconds, however other annealing temperatures and annealing
durations may be used.
[0087] The thickness of the segments of strain inducing thin film
layer segments 1504 is proportional to the change in peak
wavelength emitted by the respective quantum well layer sections
1502 (top right and bottom left of FIG. 16). Although in FIG. 16,
three segments of strain inducing thin film are used resulting in
four peak wavelengths of emitted light, it will be appreciated that
any suitable number of segments of strain inducing thin film may be
used. For example, two, four, or five segments of strain inducing
thin film may be used resulting in three, five, or six peak
wavelengths of emitted light respectively. The three segments of
strain inducing thin film depicted in FIG. 16 are each of a
different thickness, and may be 200 nm, 400 nm, and 600 nm
respectively, although any thickness capable of inducing a change
diffusion of atoms during quantum well intermixing at annealing
temperatures in the underlying quantum well sections may be used.
In other examples two or more of the segments of strain inducing
thin film layer 1504 may be of the same thickness. The strain
inducing thin film layer 1504 may be a dielectric material and in
this example is SiO.sub.2, however Si.sub.3N.sub.4, SiO.sub.x,
SrF2, Mo/SiO.sub.2, or any other suitable material or combinations
of materials may also be used.
[0088] Another embodiment of the broad-spectrum laser diode emitter
is shown in FIG. 17 at 202D2 and described below. As shown, laser
diode emitter 202D2 may have a substrate 1700, and a plurality of
quantum wells 1702. The quantum wells 1702 are non-uniform, each
having a narrow gain bandwidth centered at different respective
wavelengths, the overall effect being a laser diode emitter 202D2
with a broader peak gain bandwidth.
[0089] Like the embodiments described above, laser diode emitter
202D2 also provides methods and device configurations for the
formation of semiconductor laser diode emitters operating in the
visible regime that have a broad emission spectrum. These laser
diode emitters, coupled with SRG waveguides, may provide an
improved image quality by mitigating potential image artifacts such
as fringe artifacts and color uniformity artifact in MEMS laser
scanning display devices 200, as described above. The approach,
detailed below, modifies the emission wavelengths of the multiple
emitting layers and generates a broad gain spectrum.
[0090] As discussed elsewhere above, laser diode emitters 202F
consist of multiple thin films grown epitaxially on a substrate
material, such as GaAs for red laser diode emitters or GaN for
green or blue laser diode emitters, as shown schematically in FIG.
17. These devices rely on engineering positive "p-type", and
negative "n-type", regions of material that come into contact to
allow opposite electrical charges collide and generate light. The
light generation occurs most efficiently inside energetically
confined region known as quantum wells 1702, which also determine
the photons wavelength. Typically, laser diode emitters consist of
multiple quantum wells 1702 and the desired emission wavelength is
chosen by engineering the composition, thickness, and strain
conditions of the multiple quantum wells 1702. That is, by changing
the indium content in the In.sub.xGa.sub.1-xP alloy for red
emitting laser diode emitters or in the In.sub.xGa.sub.1-xN alloy
for blue or green emitting laser diode emitters. Note that the
quantum wells' alloy can be generally composed of (InAlGa)P alloy
for red lasers or (InAlGa)N alloy for blue and green lasers. As an
example, typical blue emitting quantum wells 1702 can have
In.sub.0.18Ga.sub.0.82N alloy, green emitting QWs can have
In.sub.0.3Ga.sub.0.7N alloy, and red emitting QWs can have
In.sub.0.55Ga.sub.0.45P alloy.
[0091] In conventional laser diode emitters, where a narrow
spectral width has been suitable for many laser applications,
uniform MQWs design may be desired. These designs generate light
around a center wavelength as demonstrated in FIG. 14, which shows
the simulated gain spectrum of a LD that contains four uniform
In.sub.0.57Ga.sub.0.43P MQWs. The result is a peak gain bandwidth
of .about.2 nm.
[0092] The approach for the example laser diode emitter of FIG. 17
includes growing non-uniform multiple quantum wells 1702 to broaden
the emission spectrum of the laser diode emitter 202F. A plot of
the gain vs. wavelength for an example configuration of the laser
diode emitter 202D2 of FIG. 17 is shown in FIG. 14, previously
discussed. In FIG. 14. laser diode emitter 202D2 is plotted with a
configuration of two quantum wells with an indium composition of
In.sub.0.6Ga.sub.0.4P have a peak gain at .about.658 nm and another
two quantum wells with an indium composition of
In.sub.0.55Ga.sub.0.45P have a peak gain at .about.642 nm, which
result in a peak gain bandwidth of .about.10 nm. Other alternative
implementations may include changing the total number of quantum
wells, the number quantum wells emitting at each wavelength, the
quantum wells thicknesses, or the strain conditions of the quantum
wells. For instance, another implementation is a laser diode
emitter with three quantum wells emitting at one wavelength and
another quantum wells at a different wavelength, and so forth.
[0093] As one parameter to vary the emission wavelength of these
laser diode emitters is the indium composition in the multiple
quantum wells, the approach described above may be extended to the
entire visible range. The composition may be varied by changing the
growth temperature of the In.sub.xGa.sub.1-xN or
In.sub.xGa.sub.1-xP alloy or the indium flow rate during the
quantum wells' epitaxial growth.
[0094] Turning back to FIG. 17, laser diode emitter 202 includes a
substrate 1700 and a plurality of quantum wells 1702 formed in a
quantum well layer that is formed directly or indirectly on the
substrate 1700. The substrate 1700 defines a horizontal plane
wherein each of the plurality of quantum wells 1702 is positioned
at a different respective vertical displacement relative to the
substrate 1702. The relative displacements may vary in magnitude
based on the thicknesses of quantum wells 1702 and of other layers
in the laser diode emitter 202D2. The plurality of quantum wells
1702 overlap each other as viewed from above along a vertical axis
V AXIS passing through each of the plurality of quantum wells 1702.
Each of the quantum wells 1702 has a different composition to
achieve a different peak wavelength in emitted light when
energized. When the laser diode emitter 202D2 is energized, the
plurality of quantum wells 1702 collectively lase emitted light
having a broader bandwidth than is emitted by a single quantum
well. The emitted light has a plurality of different peak
wavelengths in an emitted light spectrum, as shown in FIG. 14.
[0095] In the plurality of quantum wells 1702, two or more wells
1702 may be included. As depicted, four quantum wells 1702 are
included. In other embodiments, two, three, or other greater number
of wells may be included.
[0096] In the depicted embodiment, as shown in FIG. 14, quantum
wells 3 and 4 emit a similar spectrum which is different from
quantum wells 1 and 2. It will be appreciated that in other
embodiments, each of the at least four quantum wells may emits a
different peak wavelength of light.
[0097] Returning to FIG. 17, the quantum wells 1702 may be formed
by a material, such as, for example, indium gallium nitride, or
indium aluminum gallium phosphide. In one example, the material of
the quantum wells follows one of the following formulas:
In.sub.x[Al.sub.yGa.sub.1-y].sub.1-xP and In.sub.xGa.sub.1-xN. The
substrate 1702 may be formed by a material, such as, for example,
gallium arsenide and gallium nitride. For example, the emitted
light spectrum may be blue, the substrate may be gallium nitride,
and the material for the quantum wells may be indium gallium
nitride. In one specific example, the indium gallium nitride is
In.sub.xGa.sub.1-xN, where 0.15<x<0.25. In another example
configuration, the emitted light spectrum may be green, the
substrate may be gallium nitride, and the material for the quantum
wells may be indium gallium nitride. More specifically, the indium
gallium nitride may be In.sub.xGa.sub.1-xN, where 0.26<x<0.4.
In another example, the emitted light spectrum may be red, the
substrate 1700 may be gallium arsenide, and the material for the
quantum wells 1702 may be indium aluminum gallium phosphide. In
this example, specifically the indium aluminum gallium phosphide
may be In.sub.x[Al.sub.yGa.sub.1-y].sub.1-xP, where 0.4<x<0.6
and 0<y<0.55.
[0098] In one specific configuration, the plurality of quantum
wells 1702 may include at least three quantum wells 1702, and at
least two quantum wells 1702 of the plurality of quantum wells 1702
may be configured to emit light at a same peak wavelength and at
least one of the plurality of quantum wells 1702 may be configured
to emit light at a different peak wavelength from the same peak
wavelength.
[0099] It should be appreciated that the quantum wells 1702 may be
formed by epitaxial growth of In.sub.xGa.sub.1-xN or
In.sub.x[Al.sub.yGa.sub.1-y].sub.1-xP alloy, and the composition of
the quantum wells 1702 may be varied by changing a growth
temperature of In.sub.xGa.sub.1-xN or
In.sub.x[Al.sub.yGa.sub.1-y].sub.1-xP alloy or an indium flow rate
during the epitaxial growth.
[0100] Turning now to FIG. 21, a method 2100 for producing a laser
diode stack for use in a laser diode emitter 202 according to one
embodiment of the present disclosure will now be described. A
method 2100 for producing a laser diode stack for use in a laser
diode emitter, is provided. As illustrated at 2102, the method in
one embodiment comprises forming a first semiconducting layer of
semiconducting material of a first type in the laser diode stack.
At 2104, the method further comprises depositing one or more
quantum well layers directly or indirectly on the first
semiconducting layer by epitaxial growth in the laser diode stack,
the one or more quantum well layers including at least first and
second quantum well layer sections. At 2106, the method further
comprises forming a second semiconducting layer of a second type of
semiconducting material in the laser diode stack directly or
indirectly on the one or more quantum well layers. At 2108, the
method further comprises depositing a strain inducing thin film
layer having varying thickness along the horizontal direction of
the wafer directly or indirectly on the second semiconducting layer
of the laser diode stack. At 2110, the method further comprises
annealing the laser diode stack and strain inducing thin film layer
at a predetermined annealing temperature, to thereby achieve varied
interdiffusion of constituent atoms between each respective the
quantum well layer section 1502 in the quantum well layer and the
first and/or second semiconducting layers during annealing, the
interdiffusion of each quantum well layer section 1502 varying
based in part on the thickness of the thin film layer positioned
above the quantum well layer section 1502. At 2112, the method
further comprises removing the strain inducing thin film layer to
leave the sections under the varying thin film thickness having the
varied interdiffusion, wherein the varied interdiffusion of each of
the quantum well layer sections under the varying thin film
thickness causes each of the quantum well layer sections under the
varying thin film thickness to lase at a different peak energy when
energized.
[0101] In one aspect, the method further comprises depositing by
epitaxial growth, one or more cladding layers and/or a waveguide
layer between the first semiconductor layer and the quantum well
layer.
[0102] In another aspect, the method further comprises depositing
by epitaxial growth, one or more cladding layers and/or a waveguide
layer between the quantum well layer and the second semiconducting
layer.
[0103] In another aspect, the first type of semiconducting material
is an n-type material, and the second type of semiconducting
material is a p-type material.
[0104] In another aspect, the first type of semiconducting material
is a p-type material, and the second type of semiconducting
material is an n-type material.
[0105] Turning now to FIG. 19, a method 2200 for producing a laser
diode stack for use in a laser diode emitter according to one
embodiment of the present disclosure will now be described.
[0106] A method 2200 for producing a laser diode stack for use in a
laser diode emitter is provided. As illustrated at 2202, the method
in one embodiment comprises forming a first semiconducting layer of
semiconducting material of a first type in the laser diode stack.
At 2204, the method further comprises, depositing a first quantum
well layer directly or indirectly on the first semiconducting layer
by epitaxial growth in the laser diode stack, under first epitaxial
growth conditions to achieve a first epitaxial structure in the
first quantum well layer. At 2206, the method further comprises,
depositing a first intervening layer directly or indirectly on the
first quantum well layer by epitaxial growth in the laser diode
stack. At 2208, the method further comprises, depositing a second
quantum well layer directly or indirectly on the first separating
layer by epitaxial growth in the laser diode stack under second
epitaxial growth conditions to achieve a second epitaxial structure
in the second quantum well layer. At 2210, the method further
comprises, forming a second semiconducting layer of a second type
of semiconducting material in the laser diode stack. For blue or
green laser emitters, both the first quantum well layer and second
quantum well layer include In.sub.xGa.sub.1-xN. For red laser
emitters, both the first quantum well layer and the second quantum
well layer include In.sub.x[Al.sub.yGa.sub.1-y].sub.1-xP.
[0107] In one aspect, growth of each of the first quantum well
layer and the second quantum well layer is varied by altering
epitaxial growth conditions of the respective first and second
quantum well layers to thereby cause each of the first and second
quantum well layers to lase at a different peak energy when
energized.
[0108] In another aspect, the first epitaxial structure and the
second epitaxial structure are varied by changing a growth
temperature of In.sub.xGa.sub.1-xN at which each of the first and
second quantum well layers is grown.
[0109] In another aspect, the first epitaxial structure and the
second epitaxial structure are varied by changing a growth
temperature of In.sub.x[Al.sub.yGa.sub.1-y].sub.1-xP at which each
of the first and second quantum well layers is grown.
[0110] In another aspect, the first epitaxial structure and the
second epitaxial structure are varied by changing an indium flow
rate during the epitaxial growth of the first quantum well layer
and the second quantum well layer.
[0111] Turning now to FIG. 23, a method 2300 for modulating drive
current for a tunable absorber section to sweep through a range of
values according to one embodiment of the present disclosure will
now be described.
[0112] As illustrated at 2302, the method in one embodiment
comprises supplying, via a first current or voltage source, a first
drive current or voltage to a gain section coupled with the first
current or voltage source. At 2304, the method further comprises
supplying, via a second current or voltage source, a second drive
current or voltage to a tunable absorber section coupled with the
second current or voltage source, wherein the gain section and the
tunable absorber section are included in a laser diode emitter for
use in a micro electro-mechanical system (MEMS) laser scanning
display device, this can also be used in applications other than
MEMS based displays where a broad bandwidth laser diode is needed.
At 2306, the method further comprises modulating the second drive
current or voltage to sweep through a range of values over a period
of time to cause a shifting of a wavelength of the laser diode
emitter at which an overall gain is at maximum, to thereby cause
the laser diode emitter to emit spectrally broadened light.
[0113] In one aspect, at least a portion of the gain section is
electrically isolated from at least a portion of the tunable
absorber section.
[0114] In another aspect, a first electrode coupling the gain
section to the first current or voltage source is separated from a
second electrode coupling the tunable absorber section to the
second current or voltage source.
[0115] In another aspect, the second current or voltage source is
configured to sweep through the range of values over a period of
time between 2 nanoseconds and 20 nanoseconds.
[0116] In another aspect, an absorption edge inside the tunable
absorber section is shifted by up to 20 nanometers over the range
of values supplied by the second current or voltage source.
[0117] In another aspect, at least a quantum well layer extends
through each of the gain section and the tunable absorber
section.
[0118] In another aspect, the gain section is configured to have a
flattened gain spectrum.
[0119] As an alternative to method 2300, it will be appreciated
that the first and second drive currents or voltages may be driven
similarly with the same source, rather than different current or
voltage sources. In this case a structure such as shown in FIG. 6
acts similarly to a laser diode emitter with no chirper. However,
with a broad and flattened gain spectrum provided for example by
the various laser diode emitter structures described herein, some
examples of which are illustrated in FIG. 17-20C, a broadened
lasing spectrum may be obtained even when the first and second
drive currents or voltages are driven similarly with the same
source.
[0120] It will be appreciated that the systems and methods
described above may be employed to achieve a laser diode emitter
having a broadened spectrum of emitted light, which may be
incorporated into a laser scanning display device 200 of a near eye
display system 100, with the beneficial effect of reducing
artifacts and improving the color uniformity of the image displayed
to a user of such devices.
[0121] In some embodiments, the methods and processes described
herein may be tied to a computing system of one or more computing
devices. In particular, such methods and processes may be
implemented as a computer-application program or service, an
application-programming interface (API), a library, and/or other
computer-program product.
[0122] FIG. 24 schematically shows a non-limiting embodiment of a
computing system 2400 that can enact one or more of the methods and
processes described above. Computing system 2400 is shown in
simplified form. Computing system 2400 may embody the near-eye
display system of FIG. 1 and laser scanning display device 200 of
FIG. 2, described above. Computing system 2400 may take the form of
one or more personal computers, server computers, tablet computers,
home-entertainment computers, network computing devices, gaming
devices, mobile computing devices, mobile communication devices
(e.g., smart phone), and/or other computing devices, and wearable
computing devices such as smart wristwatches and head mounted
augmented reality devices.
[0123] Computing system 2400 includes a logic processor 2402
volatile memory 2404, and a non-volatile storage device 2406.
Computing system 2400 may optionally include a display subsystem
2408, input subsystem 2410, communication subsystem 2412, and/or
other components not shown in FIG. 24.
[0124] Logic processor 2402 includes one or more physical devices
configured to execute instructions. For example, the logic
processor may be configured to execute instructions that are part
of one or more applications, programs, routines, libraries,
objects, components, data structures, or other logical constructs.
Such instructions may be implemented to perform a task, implement a
data type, transform the state of one or more components, achieve a
technical effect, or otherwise arrive at a desired result.
[0125] The logic processor may include one or more physical
processors (hardware) configured to execute software instructions.
Additionally or alternatively, the logic processor may include one
or more hardware logic circuits or firmware devices configured to
execute hardware-implemented logic or firmware instructions.
Processors of the logic processor 2402 may be single-core or
multi-core, and the instructions executed thereon may be configured
for sequential, parallel, and/or distributed processing. Individual
components of the logic processor optionally may be distributed
among two or more separate devices, which may be remotely located
and/or configured for coordinated processing. Aspects of the logic
processor may be virtualized and executed by remotely accessible,
networked computing devices configured in a cloud-computing
configuration. In such a case, these virtualized aspects are run on
different physical logic processors of various different machines,
it will be understood.
[0126] Non-volatile storage device 2406 includes one or more
physical devices configured to hold instructions executable by the
logic processors to implement the methods and processes described
herein. When such methods and processes are implemented, the state
of non-volatile storage device 2406 may be transformed--e.g., to
hold different data.
[0127] Non-volatile storage device 2406 may include physical
devices that are removable and/or built in. Non-volatile storage
device 2406 may include optical memory (e.g., CD, DVD, HD-DVD,
Blu-Ray Disc, etc.), semiconductor memory (e.g., ROM, EPROM,
EEPROM, FLASH memory, etc.), and/or magnetic memory (e.g.,
hard-disk drive, floppy-disk drive, tape drive, MRAM, etc.), or
other mass storage device technology. Non-volatile storage device
2406 may include nonvolatile, dynamic, static, read/write,
read-only, sequential-access, location-addressable,
file-addressable, and/or content-addressable devices. It will be
appreciated that non-volatile storage device 2406 is configured to
hold instructions even when power is cut to the non-volatile
storage device 2406.
[0128] Volatile memory 2404 may include physical devices that
include random access memory. Volatile memory 2404 is typically
utilized by logic processor 2402 to temporarily store information
during processing of software instructions. It will be appreciated
that volatile memory 2404 typically does not continue to store
instructions when power is cut to the volatile memory 2404.
[0129] Aspects of logic processor 2402, volatile memory 2404, and
non-volatile storage device 2406 may be integrated together into
one or more hardware-logic components. Such hardware-logic
components may include field-programmable gate arrays (FPGAs),
program- and application-specific integrated circuits
(PASIC/ASICs), program- and application-specific standard products
(PSSP/ASSPs), system-on-a-chip (SOC), and complex programmable
logic devices (CPLDs), for example.
[0130] The terms "module," "program," and "engine" may be used to
describe an aspect of computing system 2400 typically implemented
in software by a processor to perform a particular function using
portions of volatile memory, which function involves transformative
processing that specially configures the processor to perform the
function. Thus, a module, program, or engine may be instantiated
via logic processor 2402 executing instructions held by
non-volatile storage device 2406, using portions of volatile memory
2404. It will be understood that different modules, programs,
and/or engines may be instantiated from the same application,
service, code block, object, library, routine, API, function, etc.
Likewise, the same module, program, and/or engine may be
instantiated by different applications, services, code blocks,
objects, routines, APIs, functions, etc. The terms "module,"
"program," and "engine" may encompass individual or groups of
executable files, data files, libraries, drivers, scripts, database
records, etc.
[0131] When included, display subsystem 2408 may be used to present
a visual representation of data held by non-volatile storage device
2406. The visual representation may take the form of a graphical
user interface (GUI). As the herein described methods and processes
change the data held by the non-volatile storage device, and thus
transform the state of the non-volatile storage device, the state
of display subsystem 2408 may likewise be transformed to visually
represent changes in the underlying data. Display subsystem 2408
may include one or more display devices utilizing virtually any
type of technology. Such display devices may be combined with logic
processor 2402, volatile memory 2404, and/or non-volatile storage
device 2406 in a shared enclosure, or such display devices may be
peripheral display devices.
[0132] When included, input subsystem 2410 may comprise or
interface with one or more user-input devices such as a keyboard,
mouse, touch screen, or game controller. In some embodiments, the
input subsystem may comprise or interface with selected natural
user input (NUI) componentry. Such componentry may be integrated or
peripheral, and the transduction and/or processing of input actions
may be handled on- or off-board. Example NUI componentry may
include a microphone for speech and/or voice recognition; an
infrared, color, stereoscopic, and/or depth camera for machine
vision and/or gesture recognition; a head tracker, eye tracker,
accelerometer, and/or gyroscope for motion detection and/or intent
recognition; as well as electric-field sensing componentry for
assessing brain activity; and/or any other suitable sensor.
[0133] When included, communication subsystem 2412 may be
configured to communicatively couple various computing devices
described herein with each other, and with other devices.
Communication subsystem 2412 may include wired and/or wireless
communication devices compatible with one or more different
communication protocols. As non-limiting examples, the
communication subsystem may be configured for communication via a
wireless telephone network, or a wired or wireless local- or
wide-area network, such as a HDMI over Wi-Fi connection. In some
embodiments, the communication subsystem may allow computing system
2400 to send and/or receive messages to and/or from other devices
via a network such as the Internet.
[0134] The following paragraphs provide additional description of
the subject matter of the present disclosure. According to one
aspect, a laser diode emitter for use in a micro electro-mechanical
system (MEMS) laser scanning display device is provided that
comprises a gain section coupled with a first current or voltage
source, and a tunable absorber section coupled with a second
current or voltage source, wherein the second current or voltage
source is configured to supply a drive current or voltage that
sweeps through a range of values over a period of time to cause a
shifting of a wavelength of the laser diode emitter at which an
overall gain is at maximum, to thereby cause the laser diode
emitter to emit spectrally broadened light.
[0135] In this aspect, at least a portion of the gain section may
be electrically isolated from at least a portion of the tunable
absorber section.
[0136] In this aspect, a first electrode coupling the gain section
to the first current or voltage source may be separated from a
second electrode coupling the tunable absorber section to the
second current or voltage source.
[0137] In this aspect, the second current or voltage source may be
configured to sweep through the range of values over a period of
time between 2 nanoseconds and 20 nanoseconds.
[0138] In this aspect, an absorption edge inside the tunable
absorber section may be shifted by 1 nanometer to 20 nanometers
over the range of values supplied by the second current or voltage
source.
[0139] In this aspect, at least a quantum well layer may extend
through each of the gain section and the tunable absorber
section.
[0140] In this aspect, the gain section may be configured to have a
flattened gain spectrum.
[0141] According to another aspect, a method is provided that
comprises supplying, via a first current or voltage source, a first
drive current or voltage to a gain section coupled with the first
current or voltage source, supplying, via a second current or
voltage source, a second drive current or voltage to a tunable
absorber section coupled with the second current or voltage source.
The gain section and the tunable absorber section may be included
in a laser diode emitter for user in a micro electro-mechanical
system (MEMS) laser scanning display device.
[0142] In this aspect, at least a portion of the gain section may
be electrically isolated from at least a portion of the tunable
absorber section.
[0143] In this aspect, a first electrode coupling the gain section
to the first current or voltage source may be separated from a
second electrode coupling the tunable absorber section to the
second current or voltage source.
[0144] In this aspect, the second current or voltage source may be
configured to sweep through the range of values over a period of
time between 2 nanoseconds and 20 nanoseconds.
[0145] In this aspect, an absorption edge inside the tunable
absorber section may shifted by up to 20 nanometers over the range
of values supplied by the second current or voltage source.
[0146] In this aspect, at least a quantum well layer may extend
through each of the gain section and the tunable absorber
section.
[0147] In this aspect, the gain section may be configured to have a
flattened gain spectrum achieved by a structure selected from the
group consisting of different vicinal angles along a waveguide of
the laser diode emitter, quantum well intermixing in the quantum
well layer along the waveguide, quantum well sections with
different composition, thickness, and/or strain in the quantum well
layer, and a patterned substrate prior to epitaxial growth along
the waveguide of the laser diode emitter.
[0148] According to another aspect, a micro electro-mechanical
system (MEMS) laser scanning display device is provided that
comprises a display. The MEMS laser scanning display device may
further comprise a laser light source including an emitter array
including a plurality of laser diode emitters, each laser diode
emitter being configured to emit a respective different wavelength
of light, to thereby form a broadband light beam with a broadband
emission spectrum, wherein the broadband light beam has an emission
spectra that includes a plurality of peaks. The MEMS laser scanning
display device may further comprise a MEMS scanning mirror
configured to guide the light beam, via a wave guide with an
in-coupling grating and out-coupling grating, to achieve a scanning
pattern across the display and thereby form a displayed image.
[0149] In this aspect, as compared to a laser light source that
emits a light beam with an emission spectrum having a single peak,
the broadband light beam may increase color uniformity in the
displayed image by having a larger range of wavelengths in the
laser light source, which results in a larger range of diffraction
angles at the in-coupling grating and out-coupling grating, which
in turn causes a smoother spatial variation of the grating
efficiency as well as an increased spatial overlap of the
out-coupled light.
[0150] In this aspect, the laser light source may include a
substrate having a plurality of deposition regions on a top surface
thereof, each deposition region having a different surface normal
formed at orthogonally relative to a surface of the deposition
region and at an angle relative to a planar bottom surface of the
substrate, and each laser diode emitter may include a semiconductor
material deposited on each of the deposition regions with surface
normals of different angles in semiconductor regions, the
semiconductor region of each emitter being configured to emit a
respective different wavelength of light, to thereby form the light
beam with a broadband emission spectrum with the plurality of
peaks.
[0151] In this aspect, the semiconductor material may be InGaN and
indium incorporation, quantum well thickness, and/or strain in each
of the deposition regions may vary due to different vicinal angles
in each deposition region, and the varying incorporation of indium
in each of the deposition regions causes variation in the
wavelengths of each diode emitter.
[0152] In this aspect, the laser light source may further include a
plurality of cavities filled with a gain material for each laser
diode emitter, cavities being resonant, and defined by a
distributed Bragg reflection mirror on one end and an exit grating
for each of the plurality of laser diode emitters on another end,
such that when current is injected to the cavities, each laser
diode emitter emits light of a different, respective
wavelength.
[0153] In this aspect, the impinging light on each of the gratings
may have a same wavelength spectrum.
[0154] According to another aspect, a laser diode emitter array is
provided comprising a substrate and a first semiconductor layer
deposited directly or indirectly on the substrate. The laser diode
emitter array may further comprise a plurality of quantum well
layer sections in a quantum well layer deposited directly or
indirectly on the first semiconductor layer and having a
substantially uniform epitaxial structure, each of the quantum well
layer sections having a respective peak wavelength in emitted light
when energized, each respective peak wavelength being different.
The laser diode emitter array may further comprise a second
semiconductor layer deposited directly or indirectly on the quantum
well layer, wherein when the laser diode emitter array is
energized, the plurality of quantum well layer sections
collectively lase emitted light having a broader bandwidth than is
emitted by any individual quantum well layer section of the
plurality of quantum well layer sections, and the emitted light
includes the plurality of respective peak wavelengths across an
emitted light frequency spectrum.
[0155] In this aspect, the different respective peak wavelengths of
each quantum well layer section may be due to quantum well
intermixing by interdiffusion of constituent atoms between each
quantum well layer section in the quantum well layer and the first
and/or second semiconducting layers.
[0156] In this aspect, the interdiffusion may be a thermal
interdiffusion that has been achieved at least in part by
application of a strain inducing thin film layer having a plurality
of segments of varying thickness, annealing at a predetermined
temperature, and removal of the strain inducing thin film
layer.
[0157] In this aspect, the substrate may define a horizontal plane,
and the plurality of quantum well layer sections may be spatially
distributed horizontally and parallel to the plane.
[0158] In this aspect, each of the plurality of segments of the
strain inducing layer may have been positioned before annealing and
prior to removal above a corresponding one of the plurality of
quantum well layer sections, to thereby vary the interdiffusion
within each quantum well layer section based on the respective
thickness of the corresponding segment positioned above each
quantum well layer section.
[0159] In another aspect, a laser diode emitter is provided
comprising a substrate, a first semiconductor layer deposited
directly or indirectly on the substrate. The laser diode emitter
may further comprise a plurality of quantum well layers, each of
the quantum well layers having a respective material composition to
achieve a respective peak wavelength in emitted light when
energized, each respective peak wavelength being different. The
laser diode emitter may further comprise a second semiconductor
layer deposited directly or indirectly on a topmost one of the
quantum well layers, and when the laser diode emitter is energized,
the plurality of quantum well layers collectively lase emitted
light having a broader bandwidth than is emitted by any individual
quantum well layer of the plurality of quantum well layers, and the
emitted light includes the plurality of respective peak wavelengths
across an emitted light frequency spectrum.
[0160] In this aspect, each respective material composition of the
quantum well layers may follow the formula:
In.sub.x[Al.sub.yGa.sub.1-y].sub.1-xP, where 0.4<x<0.6 and
0<y<0.55.
[0161] In this aspect, each respective material composition of the
quantum well layers may follow the formula: In.sub.xGa.sub.1-xN,
where 0.26<x<0.4.
[0162] In this aspect, each respective material composition of the
quantum well layers may follow the formula: In.sub.xGa.sub.1-xN,
where 0.15<x<0.25.
[0163] In this aspect, the substrate may define a horizontal plane
and each of the plurality of quantum wells may be positioned at a
different respective vertical displacement relative to the
substrate, and the plurality of quantum wells may overlap each
other as viewed from above along a vertical axis passing through
each of the plurality of quantum wells, wherein growth of each of
the plurality of quantum well layers is varied by altering
epitaxial growth conditions of the respective plurality of quantum
well layers to thereby cause each of the plurality of quantum well
layers to lase at a different peak energy when energized.
[0164] In another aspect, a method for use in manufacturing a laser
diode emitter is provide comprising forming a substrate, forming a
first semiconducting layer of semiconducting material of a first
type directly or indirectly on the substrate, depositing a quantum
well layer directly or indirectly on the first semiconducting layer
by epitaxial growth, the quantum well layer including at least
first and second quantum well layer sections, forming a second
semiconducting layer of a second type of semiconducting material
directly or indirectly on the quantum well layer, depositing a
strain inducing thin film layer having varying thickness directly
or indirectly on the second semiconducting layer. The method may
further comprise annealing the substrate, first semiconductor
layer, second semiconducting layer, quantum well layer and strain
inducing thin film layer at a predetermined annealing temperature,
to thereby achieve varied interdiffusion of constituent atoms
between each respective quantum well layer section in the quantum
well layer and the first and/or second semiconducting layers during
annealing, the interdiffusion of each quantum layer well section
varying based on the thickness of the thin film layer positioned
above the quantum layer well section. The method may further
comprise removing the strain inducing thin film layer to leave the
quantum well layer sections having the varied interdiffusion,
wherein the varied interdiffusion in each of the quantum well layer
sections causes each of the quantum well layer sections to lase at
a different peak energy when energized.
[0165] In this aspect, the quantum well layer may have a
substantially uniform epitaxial structure.
[0166] In this aspect, the method may further comprise depositing
by epitaxial growth, one or more of a cladding layer and/or a
waveguide layer between the first semiconductor layer and the
quantum well layer.
[0167] In this aspect, the method may further comprise depositing
by epitaxial growth, one or more of a cladding layer and/or a
waveguide layer between the quantum well layer and the second
semiconducting layer.
[0168] In this aspect, the first type of semiconducting material
may be an n-type material, and the second type of semiconducting
material may be a p-type material, or the first type of
semiconducting material may be a p-type material, and the second
type of semiconducting material may be an n-type material.
[0169] In another aspect, a method for use in manufacturing a laser
diode emitter is provided comprising forming a substrate, forming a
first semiconducting layer of semiconducting material of a first
type directly or indirectly on the substrate, depositing a first
quantum well layer directly or indirectly on the first
semiconducting layer by epitaxial growth, under first epitaxial
growth conditions to achieve a first epitaxial structure in the
first quantum well layer. The method may further comprise
depositing a first intervening layer directly or indirectly on the
first quantum well layer by epitaxial growth, depositing a second
quantum well layer directly or indirectly on the first separating
layer by epitaxial growth under second epitaxial growth conditions
to achieve a second epitaxial structure in the second quantum well
layer. The method may further comprise forming a second
semiconducting layer of a second type of semiconducting material
directly or indirectly on the second quantum well layer, wherein
both the first quantum well layer and the second quantum well layer
include In.sub.xGa.sub.1-xN or both the first quantum well layer
and second quantum well layer include
In.sub.x[Al.sub.yGa.sub.1-y].sub.1-xP.
[0170] In this aspect, growth of each of the first quantum well
layer and the second quantum well layer may be varied by altering
epitaxial growth conditions of the respective first and second
quantum well layers to thereby cause each of the first and second
quantum well layers to lase at a different peak energy when
energized.
[0171] In this aspect, the first epitaxial structure and the second
epitaxial structure may be varied by changing a growth temperature
of In.sub.xGa.sub.1-xN at which each of the first and second
quantum well layers is grown.
[0172] In this aspect, the first epitaxial structure and the second
epitaxial structure may be varied by changing a growth temperature
of In.sub.x[Al.sub.yGa.sub.1-y].sub.1-xP at which each of the first
and second quantum well layers is grown.
[0173] In this aspect, the first epitaxial structure and the second
epitaxial structure may be varied by changing an indium flow rate
during the epitaxial growth of the first quantum well layer and the
second quantum well layer.
[0174] It will be understood that the configurations and/or
approaches described herein are exemplary in nature, and that these
specific embodiments or examples are not to be considered in a
limiting sense, because numerous variations are possible. The
specific routines or methods described herein may represent one or
more of any number of processing strategies. As such, various acts
illustrated and/or described may be performed in the sequence
illustrated and/or described, in other sequences, in parallel, or
omitted. Likewise, the order of the above-described processes may
be changed.
[0175] The subject matter of the present disclosure includes all
novel and non-obvious combinations and sub-combinations of the
various processes, systems and configurations, and other features,
functions, acts, and/or properties disclosed herein, as well as any
and all equivalents thereof.
* * * * *