U.S. patent application number 16/226478 was filed with the patent office on 2019-11-28 for rgb combiner using mems alignment and plc.
The applicant listed for this patent is Kaiam Corp.. Invention is credited to Henk Bulthuis, John Heanue, Bardia Pezeshki.
Application Number | 20190361172 16/226478 |
Document ID | / |
Family ID | 57248619 |
Filed Date | 2019-11-28 |
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United States Patent
Application |
20190361172 |
Kind Code |
A1 |
Heanue; John ; et
al. |
November 28, 2019 |
RGB COMBINER USING MEMS ALIGNMENT AND PLC
Abstract
Light from discrete red, blue, and green lasers are combined
into a single output using a planar lightwave circuit (PLC). In
some embodiments some light from an output of the PLC is reflected
back to the lasers, and in some embodiments the reflected light is
primarily of one of the red, green, or blue wavelengths. In some
embodiments multiple lasers of slightly differing wavelengths are
provided as light sources for some or all of the red, blue, and
green light.
Inventors: |
Heanue; John; (Boston,
MA) ; Pezeshki; Bardia; (Menlo Park, CA) ;
Bulthuis; Henk; (Newark, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kaiam Corp. |
Newark |
CA |
US |
|
|
Family ID: |
57248619 |
Appl. No.: |
16/226478 |
Filed: |
December 19, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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15573633 |
Nov 13, 2017 |
10180537 |
|
|
PCT/US2016/032215 |
May 12, 2016 |
|
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16226478 |
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62160492 |
May 12, 2015 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 6/4204 20130101;
G02B 6/4287 20130101; G02B 6/12019 20130101; H01S 5/02284 20130101;
G02B 6/12026 20130101 |
International
Class: |
G02B 6/12 20060101
G02B006/12; H01S 5/022 20060101 H01S005/022; G02B 6/42 20060101
G02B006/42 |
Claims
1. A device useful as a color display light source, comprising: a
plurality of laser diodes, outputs of the laser diodes combinable
to provide different colors in a spectrum of visible light; and a
planar lightwave circuit (PLC) configured to receive light of the
laser diodes and combine at least some of the received light into
an output, the planar lightwave circuit also configured to reflect
some of the light about a particular wavelength in the visible
spectrum back to at least one of the laser diodes.
2. The device of claim 1, wherein a first of the laser diodes
comprises a first laser diode configured to generate a red light, a
second of the laser diodes comprises a second laser diode
configured to generate a green light, and a third of the laser
diodes comprises a third laser diode configured to generate a blue
light.
3. The device of claim 2, wherein the particular wavelength in the
visible spectrum is a wavelength for red light.
4. The device of claim 3, wherein the first of the laser diodes
includes an anti-reflective coating on a front output facet.
5. The device of claim 4, wherein the second and third of the laser
diodes are at least partially reflective on their front output
facets.
6. The device of claim 5, wherein the first laser diode provide a
gain medium for an external cavity laser.
7. The device of claims 3, wherein the PLC is configured to reflect
the at, least some of the light using a partly reflecting surface
on an output of the PLC.
8. The device of claim 2, further comprising a plurality of lenses
positioned to couple light from the laser diodes to the PLC.
9. The device of claim 8, wherein each of the plurality of lenses
are on a respective lever that magnifies motion of the lens.
10. The device of claim 8, wherein each lens is a ZnS lens.
11.-15. (canceled)
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates generally to color display
light sources, and more particularly to a light source combiner
with laser light sources.
[0002] Color displays are often implemented using discrete red,
green, and blue (RGB) light sources. These can be lamps, LEDs, or
lasers. Lasers are particularly advantageous because they can be
combined into a single-mode optical fiber, allowing an effective
point RGB source for high-resolution applications. Often the lasers
are combined using bulk optics that require complicated alignments
and take up a lot of space. These assemblies can be difficult to
keep aligned over a wide range of environmental conditions and over
life of the assembly.
BRIEF SUMMARY OF THE INVENTION
[0003] Some embodiments in accordance with various aspects of the
invention include a plurality of laser diodes, outputs of the laser
diodes combinable to provide different colors in a spectrum of
visible light; and a planar lightwave circuit configured to receive
light of the laser diodes and combine at least some of the received
light into an output, the planar lightwave circuit also configured
to reflect some of the light about a particular wavelength in the
visible spectrum back to at least one of the laser diodes.
[0004] In some embodiments in accordance with various aspects of
the invention include a plurality of groups of lasers, each group
of lasers including a plurality of lasers, with lasers of a first
group of lasers configured to generate red light, lasers of a
second group of lasers configured to generate green light, and
lasers of a third group of lasers configured to generate blue
light, with each of the plurality of lasers configured to generate
light at different wavelengths, and with each of the plurality of
laser separately activatable.
[0005] These and other aspects of the invention are more fully
comprehended upon review of this disclosure.
BRIEF DESCRIPTION OF THE FIGURES
[0006] FIG. 1 shows an optical subassembly that combines four
discrete laser diodes into a single-mode output using a Planar
Light Circuit (PLC) combiner.
[0007] FIG. 2 illustrates a MEMs packaging platform providing an
RGB combiner in accordance with aspects of the invention.
[0008] FIG. 3 illustrates an example lens design useful in
embodiments in accordance with aspects of the invention.
[0009] FIG. 4 illustrates a further embodiment of a MEMs packaging
platform providing an RGB combiner in which laser diodes act as
gain mediums for extended-cavity lasers, accordance with aspects of
the invention.
[0010] FIG. 5 illustrates an example PLC design in accordance with
aspects of the invention.
[0011] FIG. 6 illustrates an example spectrum for a channel of the
PLC of FIG. 5.
[0012] FIG. 7 illustrates a further embodiment of a MEMs packaging
platform providing an RGB combiner in accordance with aspects of
the invention.
DETAILED DESCRIPTION
[0013] FIG. 1 shows an optical subassembly that combines four
discrete laser diodes into a single-mode output using a Planar
Light Circuit (PLC) combiner. The optical subassembly may be part
of a MEMs packaging technology for combining lasers into a single
fiber output. A PLC chip 201 has four input waveguides (not shown
in FIG. 1) and contains a wavelength multiplexer such as an AWG
(not shown) with a single output on the other side of the chip (not
shown). The assembly contains four lasers 204 that emit light into
four lenses 203, one lens per laser. The lens focuses the light and
matches the mode to the input waveguides of the PLC 201. The lenses
are mounted on a corresponding movable stage built on a silicon
chip 202 using silicon MEMS (micro-electro-mechanical systems)
techniques. Each movable stage is connected to a lever 205 that
magnifies the motion of the lens. At the end of the lever is a
heater 206 used to lock down the lever with the lens in the optimal
position. The assembly process starts with bonding all the
components on the MEMS chips 202. Each lens is separately aligned
using the lever and the levers are locked with the heaters. This
process can be a simple and high yield technique for aligning
lasers to PLCs.
[0014] The MEMs packaging platform may be used to provide a
compact, stable RGB combiner. The concept is illustrated in FIG. 2.
Discrete red, green, and blue laser diodes (LDs) 213 are mounted on
a substrate 211. Light from the discrete red, green, and blue laser
diodes (LDs) are coupled into input waveguides 219 of a PLC 217
using microlenses 215. The microlenses may be mounted and adjusted
as discussed with respect to FIG. 1. The PLC combines the light
into a single output 221. The combiner can be based on an Arrayed
Waveguide Grating (AWG) design or use directional couplers, for
example. Optional tap waveguides can be included to direct a small
fraction of the light in each input to monitor photodiodes 223. The
output of the photodiodes can be used in a feedback loop to adjust
the laser currents in order to maintain the desired output power
for each color.
[0015] In some embodiments, the microlenses are made in wafer form
in silicon. For an RGB combiner, an alternate material may be
preferred. While glass is used in some embodiments, it is
advantageous to use a material that allows similar wafer-scale
fabrication, and preferably has a high refractive index. One
example is ZnS, which is transparent through the visible spectrum.
An example lens 311 design is shown in FIG. 3, with optical paths
313 through the lens also illustrated. In some embodiments the lens
has a 0.25 mm center thickness and 0.2 mm clear aperture. The sag
of the lens surface is such that it can be readily manufactured
using wafer-level etching processes.
[0016] One problem with a laser-based RGB combiner is that the
laser wavelength drifts over temperature. Maintaining accurate
color rendering may require temperature stabilization. Temperature
stabilization may be provided by mounting the MEMs assembly on a
thermoelectric cooler (TEC). A TEC, however, adds cost and consumes
additional power, which can be a big disadvantage for portable
consumer applications. An alternative approach is shown in FIG.
4.
[0017] FIG. 4 illustrates a further embodiment of a MEMs packaging
platform used to provide an RGB combiner in accordance with aspects
of the invention. Discrete red, green, and blue laser diodes (LDs)
413 are mounted on a substrate 411. Light from the discrete red,
green, and blue laser diodes (LDs) are coupled into input
waveguides 419 of a PLC 417 using microlenses 415. The microlenses
may be mounted and adjusted as discussed with respect to FIG. 1.
The PLC combines the light into a single output 421. The combiner
can be based on an Arrayed Waveguide Grating (AWG) design or use
directional couplers, for example. Optional tap waveguides can be
included to direct a small fraction of the light in each input to
monitor photodiodes 423. The output of the photodiodes can be used
in a feedback loop to adjust the laser currents in order to
maintain the desired output power for each color.
[0018] In the embodiment of FIG. 4, the laser diodes (LDs) are
anti-reflection coated on their front (output) facets, with the LDs
acting as the gain medium for an extended-cavity laser. Lasing is
achieved by providing feedback from a partly-reflecting surface on
the PLC, for example the output of the PLC. The lasing wavelength
for each of the lasers is determined by the passband of the PLC.
The PLC material is silica, which has low temperature dependence.
The change in PLC passband wavelength is on the order of 0.01
nm/deg, an order of magnitude lower than that of the LDs, so that a
TEC is not required.
[0019] An example PLC design is shown in FIG. 5. The design is
AWG-based, with each color operating on a different order of the
AWG. The Free Spectral Range (FSR) of the AWG is 25 THz. The
overall size of the chip is less than 3 mm.times.10 mm. Waveguides
are approximately 1 um.times.1 um with an index contrast of 1.5%
giving a mode field diameter of about 2 um. In some embodiments
segmented spotsize converters are used to facilitate coupling of
light into and/or out of the PLC. In some such embodiments the mode
field diameter may be larger, for example 4-6 um. As means of
illustration, the spectrum of the filter for the blue channel is
given in FIG. 6. The design of the filter is such that only one
peak falls within the gain bandwidth of the lasing medium. This
prevents excitation of the laser outside the target wavelength
range. In practice, it may be desirable to use gain chips with
broader gain spectra. In this case, the FSR of the AWG can be
increased; however, the resulting footprint will increase. An
alternative implementation is to incorporate a post-filter in the
PLC design. This secondary filter can be implemented as a
wavelength-dependent direction coupler or any other suitable
broadband filter.
[0020] Though lasing through the PLC stabilizes the wavelength, it
does reduce the output power. This is because any coupling loss
between the semiconductor and PLC and any loss due to the PLC
becomes an intra-cavity loss, amplified by the Q of the cavity,
rather than simply a proportional loss on the output power.
Furthermore, this intracavity loss increases the laser threshold
which makes the laser less efficient. The modal structure of the
laser also changes, as the cavity length becomes much larger. So
ideally, one may want some subset of the three lasers to include
the PLC in the cavity to stabilize the wavelength, and the other
lasers to function independent of the PLC and only use the PLC to
combine the output. For example, given the human eye
characteristics and the temperature dependence of the lasers, the
red laser is the most susceptible to perceived color change with
temperature. So in some embodiments the AR coating on the output of
the PLC is to be a minimum in the blue and green wavelengths, but
partially reflecting in the red. Similarly, the red laser diode
would be AR coated to act as a gain element only, while the green
and blue laser diodes would be partially reflective on their output
facets.
[0021] The MEMs packaging technology discussed herein allows for
incorporation of additional channels. In general, additional
channels may be added, so long as the chip footprint can
accommodate the space. Furthermore, the alignment yield is high, so
it is possible to implement assemblies with 10 or more channels
while still maintaining reasonable overall yield.
[0022] An alternate way of making a wavelength-stabilized source is
shown in FIG. 7. The embodiment of FIG. 7 is similar to that of
FIG. 4. In the embodiment of FIG. 7, however, groups of lasers are
used for each color. In the embodiment of FIG. 7 groups of discrete
red LDs 7131a-c, green LDs 7132a-c, and blue LDs 7133a-c mounted on
a substrate 711. Light from the discrete red, green, and blue laser
diodes (LDs) are coupled into input waveguides 719 of a PLC 717
using microlenses 715. The microlenses may be mounted and adjusted
as discussed with respect to FIG. 1. The PLC combines the light
into a single output 721. The combiner can be based on an Arrayed
Waveguide Grating (AWG) design or use directional couplers, for
example. Although not illustrated in FIG. 7, in some embodiments
optional tap waveguides can be included to direct a small fraction
of the light in each input to monitor photodiodes, with the output
of the photodiodes used in a feedback loop to adjust the laser
currents in order to maintain the desired output power for each
color.
[0023] The lasers within each group are chosen such that, at a
nominal temperature, their wavelengths vary by a small amount.
Based on the measured temperature (for example as measured by a
circuit (not shown) with temperature dependent operation), a
different laser of a group can be turned on, for example by laser
control circuitry (not shown). For example, a low wavelength laser
within a group can be used when the temperature is at the high
range, while a mid-wavelength laser is used at mid-temperature and
a high-wavelength laser is used at low temperature. Alternatively,
a plurality, which may be all, lasers of a group can be turned on
simultaneously and their relative powers adjusted to provide a
desired color balance.
[0024] The ability to integrate more channels in a relatively
straightforward manner can also be used to achieve higher-power
sources. A given application may benefit from high output power,
but there may not be an appropriate LD available to deliver such
power. In this case multiple chips can be used to increase the
power within each color band.
[0025] In some embodiments an AWG with closely spaced transmission
wavelengths from adjacent waveguides is used in providing high
output power. Similar gain chips are coupled to these channels and
all lase within the PLC, as described previously. The output of the
PLC will be a narrow comb where each of the gain chips lase in one
of the closely spaced transmission wavelengths. But all the light
is emitted from the single output of the PLC.
[0026] Some embodiments in accordance with various aspects of the
invention include one, some or all of the foregoing:
[0027] Some embodiments in accordance with aspects of the invention
include an RGB combiner which uses MEMs coupling of light between
laser diodes and a PLC. In some embodiments an RGB combiner uses
ZnS microlenses in coupling light between laser diodes and a PLC.
In some embodiments the RGB combiner is a wavelength-stabilized RGB
combiner, with one or all of the channels (e.g. color) lasing
through the PLC, with the PLC providing at least part of a cavity
of an external-cavity laser. In some embodiments
wavelength-stabilization is provided through use of selectable
inputs, for example separately activatable. In some embodiments
high-power source is provided, using multiple laser chips, where
some or all of the lasers lase through the PLC.
[0028] Although the invention has been discussed with respect to
various embodiments, it should be recognized that the invention
comprises the novel and non-obvious claims supported by this
disclosure.
* * * * *