U.S. patent application number 16/438572 was filed with the patent office on 2020-12-17 for reducing back reflection in hybrid lasers.
The applicant listed for this patent is Elenion Technologies, LLC. Invention is credited to Ran Ding, Hang Guan, Michael J. Hochberg, Yang Liu, Andreas Weirich.
Application Number | 20200395728 16/438572 |
Document ID | / |
Family ID | 1000004260756 |
Filed Date | 2020-12-17 |
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United States Patent
Application |
20200395728 |
Kind Code |
A1 |
Guan; Hang ; et al. |
December 17, 2020 |
REDUCING BACK REFLECTION IN HYBRID LASERS
Abstract
In conventional hybrid lasers large back refection may lead to a
degradation of relative intensity noise (RIN), linewidth
broadening, mode hopping, etc. To solve the aforementioned problem
a hybrid laser includes a mode converter for converting a
higher-back-reflection mode of the light to a mode providing less
back reflection to the gain chip. The mode converter may comprise a
polarization rotator, a waveguide converter, or high-order mode
converter. A routing waveguide may be provided including a phase
shifter, e.g. a doped waveguide, for adjusting a cavity length of
the laser cavity.
Inventors: |
Guan; Hang; (New York,
NY) ; Liu; Yang; (Elmhurst, NY) ; Ding;
Ran; (New York, NY) ; Weirich; Andreas;
(Ottawa, CA) ; Hochberg; Michael J.; (New York,
NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Elenion Technologies, LLC |
New York |
NY |
US |
|
|
Family ID: |
1000004260756 |
Appl. No.: |
16/438572 |
Filed: |
June 12, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01S 3/2375 20130101;
H01S 3/063 20130101; G02B 6/126 20130101; G02B 6/30 20130101; G02B
2006/12195 20130101; G02B 6/1228 20130101; G02B 6/14 20130101; G02B
2006/12152 20130101 |
International
Class: |
H01S 3/23 20060101
H01S003/23; G02B 6/14 20060101 G02B006/14; G02B 6/126 20060101
G02B006/126; G02B 6/122 20060101 G02B006/122; G02B 6/30 20060101
G02B006/30; H01S 3/063 20060101 H01S003/063 |
Claims
1. A hybrid laser comprising: a gain chip including a gain medium
for generating and amplifying light; a first reflector optically
coupled to the gain medium for reflecting at least a first portion
of the light back through the gain medium; a photonic integrated
circuit (PIC) chip comprising: an edge coupler for transmitting the
light between the gain chip and a device layer on the PIC chip; a
mode converter for converting a mode of the light to a mode
providing less back reflection for light traveling back and forth
within the PIC chip; a routing waveguide extending from the mode
converter; and a second reflector coupled to the routing waveguide
for reflecting at least a second portion of the light back to the
gain medium forming a laser cavity with the first reflector; and an
output port coupled to the first or second reflector for outputting
laser light; wherein the mode converter comprises a waveguide
converter for converting a ridge waveguide to a rib waveguide; and
wherein the routing waveguide comprises the rib waveguide,
comprising a wider lower slab region and a narrower upper ridge
region.
2. The hybrid laser according to claim 1, wherein the routing
waveguide includes a phase shifter for adjusting a laser cavity
length of the laser cavity.
3. The hybrid laser according to claim 2, wherein the phase shifter
comprises a doped waveguide; and further comprising a controller
capable of applying a bias voltage to the doped waveguide.
4. The hybrid laser according to claim 1, wherein the edge coupler
comprises a spot-size converter extending from the gain medium for
reducing a mode size of the light exiting the gain medium.
5. The hybrid laser according to claim 4, wherein the edge coupler
extends at an acute angled from a normal from the gain chip to
reduce back reflection into the gain medium.
6. A hybrid laser comprising: a gain chip including a gain medium
for generating and amplifying light a first reflector optically
coupled to the gain medium for reflecting at least a first portion
of the light back through the gain medium; a photonic integrated
circuit (PIC) chip comprising: an edge coupler for transmitting the
light between the gain chip and a device layer on the PIC chip; a
mode converter for converting a mode of the light to a mode
providing less back reflection for light traveling back and forth
within the PIC chip; a routing waveguide extending from the mode
converter; a second reflector coupled to the routing waveguide for
reflecting at least a second portion of the light back to the gain
medium forming a laser cavity with the first reflector; and an
output port coupled to the first or second reflector for outputting
laser light wherein the mode converter comprises a polarization
rotator comprising a bent and tapered optical waveguide
polarization rotator.
7. The hybrid laser according to claim 6, wherein the a
polarization rotator is for converting between a TE0 mode and a TM0
mode.
8. The hybrid laser according to claim 6, wherein the polarization
rotator comprises a high-order mode converter selected from the
group consisting of a TM0-to-TE1 mode converter, and a TE0-to-TE1
mode converter.
9. A hybrid laser comprising: a gain chip including a gain medium
for generating and amplifying light a first reflector optically
coupled to the gain medium for reflecting at least a first portion
of the light back through the gain medium; a photonic integrated
circuit (PIC) chip comprising: an edge coupler for transmitting the
light between the gain chip and a device layer on the PIC chip; a
mode converter for converting a mode of the light to a mode
providing less back reflection for light traveling back and forth
within the PIC chip; a routing waveguide extending from the mode
converter; a second reflector coupled to the routing waveguide for
reflecting at least a second portion of the light back to the gain
medium forming a laser cavity with the first reflector; and an
output port coupled to the first or second reflector for outputting
laser light wherein the mode converter comprises a waveguide
converter for converting a ridge waveguide to a bus waveguide; and
wherein the routing waveguide comprises the a bus waveguide.
10. The hybrid laser according to claim 9, wherein the waveguide
converter expands in width from a first end having a first width a
same as the ridge waveguide to a second end having a second width a
same as the bus waveguide, which is at least twice the first width
of the first end.
11. The hybrid laser according to claim 9, wherein the waveguide
converter comprises a gradually tapering waveguide which expands
from 400 .mu.m to 500 .mu.m wide to between 800 .mu.m to 1200 .mu.m
wide.
12. (canceled)
13. The hybrid laser according to claim 1, wherein the waveguide
converter includes a slab region expanding in width from a first
end having a first width a same as the ridge waveguide to a second
end having a second width a same as the slab region of the routing
waveguide, which is at least twice the first width of the first
end.
14. The hybrid laser according to claim 1, wherein the waveguide
converter comprises a tapering slab region, which gradually expands
from 400 .mu.m to 500 .mu.m wide to between 800 .mu.m to 1200 .mu.m
wide.
15. The hybrid laser according to claim 1, wherein the PIC chip
includes a trench for receiving the gain chip.
16. The hybrid laser according to claim 1, wherein the first
reflector comprises a reflective facet of the gain chip.
17. The hybrid laser according to claim 1, wherein the first
reflector is <95% reflective to selected wavelengths in the
light.
18. The hybrid laser according to claim 1, wherein the second
reflector is between 30% and 90% reflective to selected wavelengths
in the light.
19. The hybrid laser according to claim 18, wherein the second
reflector comprises a reflector selected from the group consisting
of a single ring reflector, a Sagnac loop mirror, a Vernier ring
reflector, a distributed Bragg reflector, and a distributed
feedback reflector.
20. The hybrid laser according to claim 18, wherein the output port
is coupled to the second reflector.
Description
TECHNICAL FIELD
[0001] The present invention relates to a hybrid laser, and in
particular to a hybrid laser with reduced back reflection.
BACKGROUND
[0002] Conventional silicon photonics-based hybrid lasers are
sensitive to external feedback. Large back refection leads to a
degradation of relative intensity noise (RIN), linewidth
broadening, mode hopping, etc.
[0003] Previous attempts at reducing back reflection include the
use of an integrated isolator; however, integrated isolators on a
chip are extremely difficult and expensive to implement and have
large insertion loss and low isolation. Alternatively, the use of
an angled coupling interface between the III-V gain chip and the
silicon chip has been tried, but this increases the difficulty in
aligning the III-V gain chip and the silicon chip, and the back
reflection reduction is limited. Another solution includes the use
of index-matching materials between the gain chip and the silicon
chip, but the shortcomings include an increase in the packaging
complexity and cost.
[0004] An object of the present invention is to overcome the
shortcomings of the prior art by implementing an on-chip mode
converter to reduce the accumulated back reflection as light
travels back-and-forth within the PIC chip.
SUMMARY OF THE INVENTION
[0005] Accordingly, the present invention relates to a hybrid laser
comprising:
[0006] a gain chip including a gain medium for generating and
amplifying light;
[0007] a first reflector optically coupled to the gain medium for
reflecting at least a portion of the light back through the gain
medium;
[0008] a photonic integrated circuit (PIC) chip comprising:
[0009] an edge coupler for transmitting the light between the gain
chip and a device layer on the PIC chip;
[0010] a mode converter for converting a mode of the light to a
mode providing less back reflection to the gain chip;
[0011] a routing waveguide extending from the mode converter;
[0012] a second reflector coupled to the routing waveguide for
reflecting at least a portion of the light back to the gain medium
forming a laser cavity with the first reflector; and
[0013] an output port coupled to the first or second reflector for
outputting a portion of the light.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The invention will be described in greater detail with
reference to the accompanying drawings which represent preferred
embodiments thereof, wherein:
[0015] FIG. 1A is a schematic diagram of a hybrid laser in
accordance with an embodiment of the present invention;
[0016] FIG. 1B is a schematic diagram of a hybrid laser in
accordance with another embodiment of the present invention;
[0017] FIGS. 2A and 2B are mode profiles for ridge waveguides for
different polarizations;
[0018] FIG. 3A is a schematic diagram of a middle section of the
hybrid laser of FIG. 1A or 1B according to an embodiment of the
present invention;
[0019] FIG. 3B is a schematic diagram of a middle section of the
hybrid laser of FIG. 1A or 1B according to an embodiment of the
present invention;
[0020] FIGS. 4A and 4B are mode profiles for bus and rib
waveguides, respectively;
[0021] FIG. 5 is a schematic diagram of a middle section of the
hybrid laser of FIG. 1A or 1B according to an embodiment of the
present invention; and
[0022] FIG. 6 is a schematic diagram of a middle section of the
hybrid laser of FIG. 1A or 1B according to an embodiment of the
present invention.
DETAILED DESCRIPTION
[0023] While the present teachings are described in conjunction
with various embodiments and examples, it is not intended that the
present teachings be limited to such embodiments. On the contrary,
the present teachings encompass various alternatives and
equivalents, as will be appreciated by those of skill in the
art.
[0024] FIGS. 1A and 1B illustrate schematic views of hybrid lasers
1a and 1b, respectively, in accordance with embodiments of the
present invention. Each of the hybrid lasers 1a and 1b is comprised
of: a gain chip 2, an edge coupler 3, a mode converter 4, a routing
waveguide 5 with or without a phase shifter, a first reflector 6,
and a second reflector 7.
[0025] The gain chip 2 includes a gain medium, which may comprise
any suitable amplification material, e.g. a suitable group III-V
gain material, such as InP, GaAs and GaN based materials, in
particular a reflective semiconductor optical amplifier (RSOA),
which may be based on bulk, quantum well or quantum dot material.
The gain chip 2 may be provided on a photonic integrated circuit
(PIC) chip 8 with the other elements of the hybrid laser 1b (FIG.
1B) or the gain chip 2 may be provided on a separate gain chip
fixed to the side of the PIC chip 8 with the remaining elements of
the hybrid laser la provided thereon (FIG. 1A). The gain chip 2 may
be a reflective semiconductor optical amplifier (RSOA), which may
be placed, e.g. flip-chip bonded, into a trench 10 on the PIC chip
8 (FIG. 1B). The gain chip 2 may include a highly reflective, e.g.
<95% of light in a desired wavelength range, surface on an outer
facet thereof forming the second reflector 7 (FIG. 1A).
Alternatively, an independent reflector, e.g. on a facet of the PIC
chip 8 (FIG. 1B) or on a separate chip (not shown), may be provided
to form the second reflector 7.
[0026] The first reflector 6 may comprise, but not limited to the
following variations: a) single ring reflector, (b) a Sagnac loop
mirror, (c) a Vernier ring reflector, (d) a distributed Bragg
reflector, and (e) a distributed feedback reflector. The first
reflector 6 may be a partial reflector with a reflectance of
between 30% and 90% of the light in the desired wavelength range
forming an output port 11 for amplified light at the desired
wavelength. The reflectance of the first and second reflectors 6
and 7 may be reversed providing the output port 11 at the second
reflector 7, i.e. proximate the gain chip 2.
[0027] The edge coupler 3 may include an angled mode converter,
which may include be angled at a small acute angle to a normal from
the output facet of the gain medium 2, e.g. by 5.degree. to
15.degree., and may include an anti-reflection coating to reduce
the back reflection at the output facet. The edge coupler 3 may
also include an mode spot-size converter, which may include a
tapering width and or height for expanding the mode reentering the
gain chip 2 and for contracting the mode leaving the gain chip
2.
[0028] The routing waveguide 5 may comprise a wide ridge waveguide,
a strip-loaded waveguide, or other low back-scattering waveguide,
and may be comprised of any suitable material, for example
semiconductor waveguides, such as silicon and silicon nitride,
etc.
[0029] Light generated or amplified in the gain chip 2 is coupled
through the edge coupler 3 onto a device layer on the PIC chip 8.
The mode converter 4 formed in the device layer transforms the
shape of the mode of the light to a mode shape providing less back
reflection to the gain medium. For example, the mode converter 4
may comprise a polarization rotator, which rotates the TE0 mode
leaving the gain chip 2 into the TM0 mode, or the mode converter 4
may comprise a waveguide converter, which converts the shape of the
edge coupler 3, e.g. a ridge waveguide, into a less back-reflective
form of waveguide, e.g. a bus waveguide or a rib waveguide. The
adjusted mode travels through the routing waveguide 5, which may
comprise a structure corresponding to the selected mode converter
4, e.g. ridge for polarization rotator, bus for ridge-to-bus
waveguide converter, and rib for ridge-to-rib waveguide converter.
Then at least a portion of the adjusted mode gets reflected by the
first reflector 6 back through the routing waveguide 5 to the mode
converter 4, which converts the mode shape back to the original
mode shape, e.g. TE0 in a ridge waveguide, for reentry into the
gain medium of the gain chip 2, via the edge coupler 3. Light
bounces back and forth between the second reflector 7 and the first
reflector 6, and becomes the wanted laser output at an output port
11 provided at the first or second reflector 6 or 7. To ensure only
selective wavelength gets amplified in the gain medium a highly
reflective reflector 6 or 7 may include a wavelength selective
filter coating for passing unwanted wavelengths out of the laser
cavity. Alternatively or in addition, an optical filter may be
provided within the laser 1A or 1B to pass the desired wavelengths
and filter out unwanted wavelengths.
[0030] The routing waveguide 5 may also include a phase shifter for
adjusting and/or selecting the optical distance between the first
and second reflectors 6 and 7, i.e. the laser cavity length, and
therefore the wavelength of the output light. The phase shifter may
comprise a special type of routing waveguide 5, because the phase
shifter may be comprised of a doped waveguide, as opposed to the
normal routing waveguide 5, which is not necessarily doped. Since
the phase shifter is doped, the phase may be controlled by adding
different biasing voltages via a control system to automatically
maintain the same wavelength over time or to adjust to a different
wavelength. A light detector, e.g. a <5% tap and a
photodetector, positioned prior to the output port 11 may be used
to provide information about wavelength and power of the output
light to the control system. The phase shifter, or doped waveguide,
may be important to the hybrid laser 1a or 1b, for wavelength
selection and to avoid mode-hopping during the operation of
laser.
[0031] FIGS. 2(A) and 2(B) illustrate the geometric dimensions,
mode profiles, and measured back-reflection of silicon photonic
ridge waveguides. With reference to FIG. 2A, a ridge waveguide with
a TE0 mode has the largest back-reflection, i.e. -21 dB for 1 cm
long waveguide. Accordingly, when a ridge waveguide with a TE0 mode
combination is used as the routing waveguide 5 and mode, a
tremendous amount of reflected power is introduced into the gain
chip 2, which degrades the performance of the laser 1a or 1b. To
solve this problem, the mode converter 4 is provided between the
edge coupler 3 and the routing waveguide 5. Accordingly, if a ridge
waveguide and TM0 mode is used for the routing waveguide 5 in the
PIC chip 8, the back-reflection caused by the routing waveguide 5
may be reduced by -19 dB (from -21 dB/cm to -40 dB/cm), which will
significantly reduce the back-reflection into the gain chip 2, and
thus improve the laser performance.
[0032] According to an exemplary embodiment, illustrated in FIG. 3,
the middle section of the hybrid laser 1a or 1b may include the
edge coupler 3, which may include a ridge waveguide for receiving a
TE0 mode from the laser chip 2. The mode converter 4', e.g.
polarization rotator, may comprise a bent and tapered optical
waveguide polarization rotator, such as one disclosed in U.S. Pat.
No. 9,829,632 issued Nov. 28, 2017 in name of Ma et al, which is
incorporated herein by reference, for rotating the polarization of
the light from the TE0 mode to the TM0 mode, and then from the TM0
mode back to the TE0 mode upon return from the first reflector 6.
The routing waveguide 5 may also be comprised of a ridge
waveguide.
[0033] With reference to FIG. 3B, the mode converter 4' may also
comprise a higher-order mode converter, e.g. a TM0-to-TE1 mode
converter, and a TE0-to-TE1 mode converter, such as disclosed in
U.S. Pat. No. 9,746,609 issued Aug. 29, 2017 to Ma et al, and U.S.
Pat. No. 9,829,632 issued Nov. 28, 2017 in name of Ma et al, which
are incorporated herein by reference. Any other suitable
polarization rotator is also within the scope of the invention. As
illustrated in FIG. 3B, to reduce the back-reflection to the gain
chip 2, the mode converter 4', e.g. a TM0-to-TE1 mode converter,
expands the ridge waveguide forming the edge coupler 3 into a wide
bus routing waveguide 5, changing the TM0 mode into a high-order
TE1 mode. The mode converter 4' may comprises a bi-layer taper
having an input porta at the output of the edge coupler 3 and an
output port at the input of the routing waveguide. The bi-layer
taper includes a first lower slab layer with a width that
continuously expands from the input port to the output port; and a
second upper ridge layer with a width that initially tapers and
then widens from the input port to the output port.
[0034] FIGS. 4(A) and 4(B) illustrate the geometric dimensions,
mode profiles, and measured back-reflection of silicon photonic bus
and rib waveguides, respectively.
[0035] Accordingly, in an alternative exemplary embodiment
illustrated in FIG. 5, to reduce the back-reflection to the gain
chip 2, the mode converter 4'', e.g. a tapering ridge-to-bus
waveguide converter, expands the ridge waveguide forming the edge
coupler 3 into a bus waveguide, while maintaining the TE0 mode. The
edge coupler 3 may be comprised of a ridge waveguide, e.g. with a
width of 400 nm to 600 nm, and a height of about 150 nm to 300 nm,
and the routing waveguide 5' may be comprised of a bus waveguide,
e.g. with a width of 800 nm to 1200 nm and a height of about 150 nm
to 300 nm. Accordingly, the waveguide converter 4'' comprises a
gradually, e.g. adiabatically, expanding waveguide with a width
(and/or height) that expands by at least twice the original width,
e.g. from the width (and/or height) of the ridge waveguide 3 to the
width (and/or height) of the bus waveguide 5'. As a result, the
back reflection of the routing waveguide 5 is reduced by 16 dB/cm
(from -21 dB/cm to -37 dB/cm) as in FIGS. 4(A) and 4(B).
[0036] According to another alternative exemplary embodiment
illustrated in FIG. 6, to reduce the back-reflection to the gain
chip 2, the mode converter 4''', e.g. a tapering ridge-to-rib
waveguide converter, expands the ridge waveguide forming the edge
coupler 3 into a rib waveguide, while maintaining the TE0 mode. The
edge coupler 3 may be comprised of a ridge waveguide, e.g. with a
width of 400 nm to 600 nm, and a height of about 150 nm to 300 nm,
and the routing waveguide 5'' may be comprised of a rib waveguide,
e.g. with a lower slab region 12 with a width of 1200 nm to 1800 nm
and a height of 50 nm to 150 nm, and an upper ridge region 13 with
a width of 400 nm to 600 nm and a height of 50 nm to 150 nm.
Accordingly, the waveguide converter 4''' comprises a gradually,
e.g. adiabatically, expanding, e.g. linearly or exponentially,
lower slab waveguide region 14 with a width that expands by at
least twice the original width, e.g. from the width of the ridge
waveguide 3 to about the width of the slab region 12 of the rib
waveguide 5. As a result, the back reflection of the routing (rib)
waveguide 5 is reduced by 13 dB/cm (from -21 dB/cm to -34
dB/cm).
[0037] The foregoing description of one or more embodiments of the
invention has been presented for the purposes of illustration and
description. It is not intended to be exhaustive or to limit the
invention to the precise form disclosed. Many modifications and
variations are possible in light of the above teaching. It is
intended that the scope of the invention be limited not by this
detailed description, but rather by the claims appended hereto.
* * * * *