U.S. patent application number 17/327508 was filed with the patent office on 2021-09-09 for broadband arbitrary wavelength multichannel laser source.
The applicant listed for this patent is ROCKLEY PHOTONICS LIMITED. Invention is credited to Pradeep Srinivasan, Aaron John Zilkie.
Application Number | 20210281051 17/327508 |
Document ID | / |
Family ID | 1000005611521 |
Filed Date | 2021-09-09 |
United States Patent
Application |
20210281051 |
Kind Code |
A1 |
Zilkie; Aaron John ; et
al. |
September 9, 2021 |
BROADBAND ARBITRARY WAVELENGTH MULTICHANNEL LASER SOURCE
Abstract
A multi-channel laser source, including: a bus waveguide
coupled, at an output end of the bus waveguide, to an output of the
multi-channel laser source; a first semiconductor optical
amplifier; a first back mirror; a first wavelength-dependent
coupler, having a first resonant wavelength, on the bus waveguide;
a second semiconductor optical amplifier; a second back mirror; and
a second wavelength-dependent coupler, on the bus waveguide, having
a second resonant wavelength, different from the first resonant
wavelength. In some embodiments the first semiconductor optical
amplifier is coupled to the bus waveguide by the first
wavelength-dependent coupler, which is nearer to the output end of
the bus waveguide than the second wavelength-dependent coupler, the
second semiconductor optical amplifier is coupled to the bus
waveguide by the second wavelength-dependent coupler, and the first
wavelength-dependent coupler is configured to transmit light, at
the second resonant wavelength, along the bus waveguide.
Inventors: |
Zilkie; Aaron John;
(Pasadena, CA) ; Srinivasan; Pradeep; (Fremont,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ROCKLEY PHOTONICS LIMITED |
Altrincham |
|
GB |
|
|
Family ID: |
1000005611521 |
Appl. No.: |
17/327508 |
Filed: |
May 21, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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17172033 |
Feb 9, 2021 |
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17327508 |
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17104929 |
Nov 25, 2020 |
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17172033 |
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17022901 |
Sep 16, 2020 |
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17104929 |
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16007896 |
Jun 13, 2018 |
10811848 |
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17022901 |
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62519754 |
Jun 14, 2017 |
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62548917 |
Aug 22, 2017 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01S 5/4062 20130101;
H01S 5/4087 20130101; H01S 5/4068 20130101; H01S 5/142 20130101;
G02B 6/29343 20130101; H01S 5/06837 20130101; G02B 6/29344
20130101; G02B 6/29329 20130101; G02B 6/29395 20130101 |
International
Class: |
H01S 5/40 20060101
H01S005/40; H01S 5/14 20060101 H01S005/14; G02B 6/293 20060101
G02B006/293; H01S 5/0683 20060101 H01S005/0683 |
Claims
1. A multi-channel laser source, comprising: a bus waveguide
coupled, at an output end of the bus waveguide, to an output of the
multi-channel laser source; a first semiconductor optical
amplifier; a first back mirror; a first wavelength-dependent
coupler having a first resonant wavelength; a second semiconductor
optical amplifier; a second back mirror; and a second
wavelength-dependent coupler having a second resonant wavelength,
different from the first resonant wavelength; the first
semiconductor optical amplifier comprising: a first end coupled to
the first back mirror, and a second end, the first
wavelength-dependent coupler comprising: a channel port connected
to the second end of the first semiconductor optical amplifier; a
bus output connected to a first portion of the bus waveguide; and a
bus input, connected to a second portion of the bus waveguide more
distant from the output end of the bus waveguide than the first
portion of the bus waveguide; the second semiconductor optical
amplifier being coupled to the bus waveguide through the second
wavelength-dependent coupler, the first wavelength-dependent
coupler being nearer to the output end of the bus waveguide than
the second wavelength-dependent coupler, the first
wavelength-dependent coupler being configured to transmit light, at
the second resonant wavelength, from the bus input of the first
wavelength-dependent coupler to the bus output of the first
wavelength-dependent coupler.
2. The multi-channel laser source of claim 1, further comprising an
output coupler at the output end of the bus waveguide, wherein the
first wavelength-dependent coupler is configured to transmit light
at the first resonant wavelength from the channel port of the first
wavelength-dependent coupler to the bus output of the first
wavelength-dependent coupler.
3. The multi-channel laser source of claim 1, wherein the first
wavelength-dependent coupler is configured to reflect a first
portion of light received at the first resonant wavelength at the
channel port of the first wavelength-dependent coupler, and to
transmit, to the bus output of the first wavelength-dependent
coupler, a second portion of light received at the first resonant
wavelength at the channel port of the first wavelength-dependent
coupler.
4. The multi-channel laser source of claim 3, wherein the first
portion is at least 10% of the light received, and the second
portion is at least 40% of the light received.
5. The multi-channel laser source of claim 1, wherein the first
wavelength-dependent coupler is configured to transmit, to a fourth
port of the first wavelength-dependent coupler, light received at
the channel port at the second resonant wavelength.
6. The multi-channel laser source of claim 5, wherein the fourth
port of the first wavelength-dependent coupler is connected to an
optical absorber.
7. The multi-channel laser source of claim 1, wherein the first
back mirror and the first semiconductor optical amplifier are
configured as a reflective semiconductor optical amplifier.
8. The multi-channel laser source of claim 1, wherein the first
wavelength-dependent coupler comprises a first ring resonator.
9. The multi-channel laser source of claim 8, wherein the first
wavelength-dependent coupler further comprises a second ring
resonator, the first ring resonator and the second ring resonator
being configured to operate as a vernier ring resonator filter.
10. The multi-channel laser source of claim 1, wherein the first
wavelength-dependent coupler comprises a grating assisted
co-directional coupler.
11. The multi-channel laser source of claim 10, wherein the first
wavelength-dependent coupler further comprises a distributed Bragg
reflector connected in cascade with the grating assisted
co-directional coupler.
12. The multi-channel laser source of claim 1, wherein the first
wavelength-dependent coupler comprises a wavelength actuator for
adjusting the first resonant wavelength.
13. The multi-channel laser source of claim 1, further comprising a
phase shifter between the first back mirror and the first
wavelength-dependent coupler.
14. The multi-channel laser source of claim 1, further comprising
an amplitude modulator between the first back mirror and the first
wavelength-dependent coupler.
15. The multi-channel laser source of claim 1, wherein the first
semiconductor optical amplifier is the same semiconductor optical
amplifier as the second semiconductor optical amplifier.
16. The multi-channel laser source of claim 1, wherein the first
semiconductor optical amplifier comprises a first waveguide in a
first semiconductor chip and the second semiconductor optical
amplifier comprises a second waveguide in the first semiconductor
chip.
17. The multi-channel laser source of claim 1, wherein the first
semiconductor optical amplifier comprises a waveguide in a first
semiconductor chip, and the second semiconductor optical amplifier
comprises a waveguide in a second semiconductor chip, different
from the first semiconductor chip.
18. The multi-channel laser source of claim 1, further comprising:
a wavelength sensor configured to receive a portion of, and to
sense a wavelength of, light emitted by the first semiconductor
optical amplifier; and a control system configured: to receive a
wavelength sensing signal from the wavelength sensor, to calculate
a difference between the wavelength sensing signal and a wavelength
setpoint, and to apply a wavelength correction signal to a
wavelength actuator, to reduce the difference between the
wavelength sensing signal and the wavelength setpoint.
19. The multi-channel laser source of claim 18, further comprising
a phase shifter between the first back mirror and the first
wavelength-dependent coupler, wherein the wavelength actuator
comprises the phase shifter.
20. The multi-channel laser source of claim 18, wherein the first
wavelength-dependent coupler comprises a coupler wavelength
actuator for adjusting the first resonant wavelength, wherein the
wavelength actuator comprises the coupler wavelength actuator.
21. The multi-channel laser source of claim 18, wherein the
wavelength sensor is configured to receive light from a fourth port
of the first wavelength-dependent coupler.
22. The multi-channel laser source of claim 18, wherein the
wavelength sensor comprises a Mach-Zehnder interferometer having a
first arm and a second arm, longer than the first arm, and a
temperature control system configured to control the temperature of
a portion of the second arm.
23. The multi-channel laser source of claim 18, wherein: the first
semiconductor optical amplifier comprises a waveguide in a first
semiconductor chip; and the wavelength sensor comprises a
photodiode, the photodiode being in the first semiconductor
chip.
24. A multiplexed multi-channel laser source comprising: a first
multi-channel laser source according to claim 1, a second first
multi-channel laser source according to claim 1, and a multiplexer,
the multiplexer comprising: a first input, a second input, and an
output, the multiplexer being configured: to transmit light from
first input to the output, and to transmit light from second input
to the output.
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
[0001] The present application is a continuation of U.S. patent
application Ser. No. 17/172,033, filed Feb. 9, 2021, entitled
"BROADBAND ARBITRARY WAVELENGTH MULTICHANNEL LASER SOURCE", which
is a continuation of U.S. patent application Ser. No. 17/104,929,
filed Nov. 25, 2020, entitled "BROADBAND ARBITRARY WAVELENGTH
MULTICHANNEL LASER SOURCE", which is a continuation of U.S. patent
application Ser. No. 17/022,901, filed Sep. 16, 2020, entitled
"BROADBAND ARBITRARY WAVELENGTH MULTICHANNEL LASER SOURCE", which
is a continuation of U.S. patent application Ser. No. 16/007,896,
filed Jun. 13, 2018, issued as U.S. Pat. No. 10,811,848, issued on
Oct. 20, 2020, entitled "BROADBAND ARBITRARY WAVELENGTH
MULTICHANNEL LASER SOURCE", which claims the benefit of U.S.
Provisional Application No. 62/519,754, filed Jun. 14, 2017,
entitled "BROADBAND ARBITRARY WAVELENGTH MULTICHANNEL LASER
SOURCE", and of U.S. Provisional Application No. 62/548,917, filed
Aug. 22, 2017, entitled "BROADBAND ARBITRARY WAVELENGTH
MULTICHANNEL LASER SOURCE". The entire contents of all of the
applications identified in this paragraph are incorporated herein
by reference.
FIELD
[0002] One or more aspects of embodiments according to the present
disclosure relate to light sources, and more particularly to a
multichannel light source.
BACKGROUND
[0003] Some systems may use a silicon photonics integrated
multichannel tunable laser source which has arbitrary wavelength
channels covering a wavelength span of multiple hundreds to one or
more thousands of nanometers, a span which is much larger than the
gain bandwidth of a single reflective semiconductor optical
amplifier (RSOA) die, or RSOA "chip".
[0004] Such a laser source may be constructed using a single laser
cavity for each channel, with, e.g., each laser constructed
according to U.S. Pat. No. 9,270,078 (the "'078 patent"), which is
incorporated herein by reference in its entirety. The channels may
be combined externally to the cavity using a channel combiner such
as an optical multiplexer (MUX), or additional ring resonator
tunable filters. The use of a MUX external to the laser cavities
for combining may have the disadvantages of (i) imposing a minimum
channel spacing due to the periodicity of the passband response,
(ii) introducing stop-bands where channels cannot exist, and (iii)
for small channel spacing (and a large number of channels),
incurring relatively high MUX losses. In an embodiment with
multiple MUXs of different designs and channel spacings connected
to one or more additional MUX to combine the outputs of the
multiple MUXs to one common output, the optical loss may also be
relatively high.
[0005] Ring resonator tunable filters may be used to combine the
light from multiple lasers externally to the laser cavities, but
such an embodiment includes further ring resonator tunable filters
in addition to the ones used inside each laser cavity. As a result,
more tunable elements may be included, requiring more stabilization
circuits. Moreover, locking the external filter wavelength to the
internal laser filter wavelength may increase the complexity of the
system.
[0006] Thus, there is a need for an improved multichannel laser
source.
SUMMARY
[0007] According to an embodiment of the present disclosure there
is provided a multi-channel laser source, including: a bus
waveguide coupled, at an output end of the bus waveguide, to an
output of the multi-channel laser source; a first semiconductor
optical amplifier; a first back mirror; a first
wavelength-dependent coupler having a first resonant wavelength; a
second semiconductor optical amplifier; a second back mirror; and a
second wavelength-dependent coupler having a second resonant
wavelength, different from the first resonant wavelength; the first
semiconductor optical amplifier including: a first end coupled to
the first back mirror, and a second end, the first
wavelength-dependent coupler including: a channel port connected to
the second end of the first semiconductor optical amplifier; a bus
output connected to a first portion of the bus waveguide; and a bus
input, connected to a second portion of the bus waveguide more
distant from the output end of the bus waveguide than the first
portion of the bus waveguide; the second semiconductor optical
amplifier being coupled to the bus waveguide through the second
wavelength-dependent coupler, the first wavelength-dependent
coupler being nearer to the output end of the bus waveguide than
the second wavelength-dependent coupler, the first
wavelength-dependent coupler being configured to transmit light, at
the second resonant wavelength, from the bus input of the first
wavelength-dependent coupler to the bus output of the first
wavelength-dependent coupler.
[0008] In one embodiment, the multi-channel laser source includes
an output coupler at the output end of the bus waveguide, wherein
the first wavelength-dependent coupler is configured to transmit
light at the first resonant wavelength from the channel port of the
first wavelength-dependent coupler to the bus output of the first
wavelength-dependent coupler.
[0009] In one embodiment, the first wavelength-dependent coupler is
configured to reflect a first portion of light received at the
first resonant wavelength at the channel port of the first
wavelength-dependent coupler, and to transmit, to the bus output of
the first wavelength-dependent coupler, a second portion of light
received at the first resonant wavelength at the channel port of
the first wavelength-dependent coupler.
[0010] In one embodiment, the first portion is at least 10% of the
light received, and the second portion is at least 40% of the light
received.
[0011] In one embodiment, the first wavelength-dependent coupler is
configured to transmit, to a fourth port of the first
wavelength-dependent coupler, light received at the channel port at
the second resonant wavelength.
[0012] In one embodiment, the fourth port of the first
wavelength-dependent coupler is connected to an optical
absorber.
[0013] In one embodiment, the first back mirror and the first
semiconductor optical amplifier are configured as a reflective
semiconductor optical amplifier.
[0014] In one embodiment, the first wavelength-dependent coupler
includes a first ring resonator.
[0015] In one embodiment, the first wavelength-dependent coupler
further includes a second ring resonator, the first ring resonator
and the second ring resonator being configured to operate as a
vernier ring resonator filter.
[0016] In one embodiment, the first wavelength-dependent coupler
includes a grating assisted co-directional coupler.
[0017] In one embodiment, the first wavelength-dependent coupler
further includes a distributed Bragg reflector connected in cascade
with the grating assisted co-directional coupler.
[0018] In one embodiment, the first wavelength-dependent coupler
includes a wavelength actuator for adjusting the first resonant
wavelength.
[0019] In one embodiment, the multi-channel laser source includes a
phase shifter between the first back mirror and the first
wavelength-dependent coupler.
[0020] In one embodiment, the multi-channel laser source includes
an amplitude modulator between the first back mirror and the first
wavelength-dependent coupler.
[0021] In one embodiment, the first semiconductor optical amplifier
is the same semiconductor optical amplifier as the second
semiconductor optical amplifier.
[0022] In one embodiment, the first semiconductor optical amplifier
includes a first waveguide in a first semiconductor chip and the
second semiconductor optical amplifier includes a second waveguide
in the first semiconductor chip.
[0023] In one embodiment, the first semiconductor optical amplifier
includes a waveguide in a first semiconductor chip, and the second
semiconductor optical amplifier includes a waveguide in a second
semiconductor chip, different from the first semiconductor
chip.
[0024] In one embodiment, the multi-channel laser source includes:
a wavelength sensor configured to receive a portion of, and to
sense a wavelength of, light emitted by the first semiconductor
optical amplifier; and a control system configured: to receive a
wavelength sensing signal from the wavelength sensor, to calculate
a difference between the wavelength sensing signal and a wavelength
setpoint, and to apply a wavelength correction signal to a
wavelength actuator, to reduce the difference between the
wavelength sensing signal and the wavelength setpoint.
[0025] In one embodiment, the multi-channel laser source includes a
phase shifter between the first back mirror and the first
wavelength-dependent coupler, wherein the wavelength actuator
includes the phase shifter.
[0026] In one embodiment, the first wavelength-dependent coupler
includes a coupler wavelength actuator for adjusting the first
resonant wavelength, wherein the wavelength actuator includes the
coupler wavelength actuator.
[0027] In one embodiment, the wavelength sensor is configured to
receive light from a fourth port of the first wavelength-dependent
coupler.
[0028] In one embodiment, the wavelength sensor includes a
Mach-Zehnder interferometer having a first arm and a second arm,
longer than the first arm, and a temperature control system
configured to control the temperature of a portion of the second
arm.
[0029] In one embodiment, the first semiconductor optical amplifier
includes a waveguide in a first semiconductor chip; and the
wavelength sensor includes a photodiode, the photodiode being in
the first semiconductor chip.
[0030] In one embodiment, a multiplexed multi-channel laser source
includes: a first multi-channel laser source, a second first
multi-channel laser source, and a multiplexer, the multiplexer
including: a first input, a second input, and an output, the
multiplexer being configured: to transmit light from first input to
the output, and to transmit light from second input to the
output.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] These and other features and advantages of the present
invention will be appreciated and understood with reference to the
specification, claims, and appended drawings wherein:
[0032] FIG. 1A is a schematic illustration of a multichannel laser,
according to an embodiment of the present invention;
[0033] FIG. 1B is a schematic illustration of an optical spectrum,
according to an embodiment of the present invention;
[0034] FIG. 2A is a schematic illustration of a vernier ring
resonator filter, according to an embodiment of the present
invention;
[0035] FIG. 2B is a schematic illustration of a vernier ring
resonator filter, according to an embodiment of the present
invention;
[0036] FIG. 2C is a schematic illustration of a vernier ring
resonator filter, according to an embodiment of the present
invention;
[0037] FIG. 3A is a schematic illustration of a multichannel laser
source, according to an embodiment of the present invention;
[0038] FIG. 3B is a schematic illustration of an optical spectrum,
according to an embodiment of the present invention;
[0039] FIG. 4 is a schematic illustration of a multichannel laser
source, according to an embodiment of the present invention;
[0040] FIG. 5 is a schematic illustration of a system for
wavelength sensing, according to an embodiment of the present
invention;
[0041] FIG. 6 is a schematic illustration of a multichannel laser
source, according to an embodiment of the present invention;
[0042] FIG. 7 is a schematic illustration of a multichannel laser
source, according to an embodiment of the present invention;
[0043] FIG. 8 is a schematic illustration of a multichannel laser
source, according to an embodiment of the present invention;
[0044] FIG. 9A is a schematic illustration of a tunable
grating-assisted co-directional coupler of a first type, according
to an embodiment of the present invention;
[0045] FIG. 9B is a schematic illustration of a tunable
grating-assisted co-directional coupler of a second type, according
to an embodiment of the present invention; and
[0046] FIG. 9C is a schematic illustration of a tunable
grating-assisted co-directional coupler of a third type, according
to an embodiment of the present invention.
DETAILED DESCRIPTION
[0047] The detailed description set forth below in connection with
the appended drawings is intended as a description of exemplary
embodiments of a multichannel laser source provided in accordance
with the present invention and is not intended to represent the
only forms in which the present invention may be constructed or
utilized. The description sets forth the features of the present
invention in connection with the illustrated embodiments. It is to
be understood, however, that the same or equivalent functions and
structures may be accomplished by different embodiments that are
also intended to be encompassed within the spirit and scope of the
invention. As denoted elsewhere herein, like element numbers are
intended to indicate like elements or features.
[0048] Referring to FIG. 1A, in a first embodiment, multiple RSOAs
100 of different band-gaps and material compositions are integrated
to construct a multichannel tunable laser. Each RSOA provides
optical gain to one or more of a plurality of laser channels, each
generating light at a respective wavelength, and all channels are
combined into a single output waveguide, or "bus waveguide" 115. In
this embodiment, a single vernier ring resonator tunable filter set
(or simply "vernier ring resonator filter") is employed, for each
channel, to perform gain selection inside the laser cavity to
select the lasing wavelength, and to combine the lasing light with
that of other channels into the single bus waveguide 115, all
inside a laser cavity having a shared output coupler (or "output
mirror") 120. Each vernier ring resonator filter operates as a
wavelength-dependent coupler, as discussed in further detail below.
Only one vernier ring resonator filter is needed for each channel,
and as many channels can be added as desired, at the expense of
making the laser cavity longer. Each RSOA chip may be separately
fabricated (e.g., out of a III-V semiconductor material) and
aligned with (and bonded to) a silicon photonics chip which
includes the other optical components shown (e.g., the vernier ring
resonator filters 110, the bus waveguide 115, and the output
coupler 120). The reflective surface of the RSOA may operate as the
back mirror of the laser cavity. In some embodiments, the back
mirror is a separate element from the semiconductor optical
amplifier. FIG. 1B shows an example of an optical spectrum that may
be generated by the embodiment of FIG. 1A. A fourth port of each
vernier ring resonator filter may be connected to an optical
absorber 145 (or used for wavelength sensing), as discussed in
further detail below.
[0049] Referring to FIG. 2A, in some embodiments, each vernier ring
resonator filter is a four-port device including three multimode
interference couplers (MMIs) 210A-C, connected together with
waveguide paths referred to as ring-halves 220A-220D. Each MMI has
four ports that may be referred to as an input port 230A, an output
port 230B, a coupled port 230C, and an isolated port 230D. The
ratio of (i) power transmitted to the coupled port to (ii) power
supplied to the input port may be referred to as the coupling
factor. If the output port 230B is defined to be the port opposite
the input port 230A, as in FIG. 2A, then the coupling factor may
also be referred to as the "cross coupling" and the ratio of (i)
the power transmitted to the coupled port to (ii) power supplied to
the input port may be referred to as the "bar coupling" (the
terminology employed in the '078 patent). The input port and the
isolated port may be referred to (e.g., in the '078 patent) as
"back end" ports and the output port and the coupled port may be
referred to (e.g., in the '078 patent) as "front end" ports. Other
examples of vernier ring resonator filters are disclosed in the
'078 patent.
[0050] Each pair of ring-halves forms, with two respective MMIs, a
closed optical path that may be referred to as a ring resonator (or
as a "simple" or "single" resonator, to distinguish it from a
vernier ring resonator filter, which may include two or more
(coupled) simple ring resonators). Light may be coupled into or out
of this closed optical path through the MMIs. For example, for a
first resonator including the first MMI 210A, the first and second
ring halves 220A, 220B, and the second MMI 210B, light propagating
in the forward direction (from the RSOA to the output coupler) may
enter the input port 230A of the first MMI 210A, and propagate
around a first ring, formed by the first and second ring halves
220A, 220B and the first and second MMIs 210A, 210B. A portion of
the light propagating around this ring may be coupled out of the
first ring and into a second ring formed by the third and fourth
ring halves 220C, 220D and the second and third MMIs 210B, 210C. At
a wavelength for which the round-trip phase around the closed path
is a multiple of 2.pi., the ring resonator may be said to be
resonant, and light coupled into the ring resonator interferes
constructively with light circulating in the ring resonator,
resulting in greater circulating power than is the case for
wavelengths at which the ring resonator is not resonant. The
wavelength (or frequency) separation between consecutive resonant
wavelengths of the ring resonator is referred to as a the free
spectral range (FSR) of the ring resonator. The second ring
resonator, formed by the third and fourth ring halves 220C, 220D
and the second and third MMIs 210B, 210C, may operate in a similar
manner.
[0051] The vernier ring resonator filter (which includes the first
ring resonator and the second ring resonator) may, as mentioned
above, act as a wavelength-dependent coupler, with (for example,
for forward travelling light) the input port 230A of the first MMI
210A being the input port of the vernier ring resonator filter, and
the coupled port 230C of the third MMI 210C being the coupled port
of the vernier ring resonator filter. At a wavelength at which both
the first resonator and the second resonator are resonant, the
vernier ring resonator filter is resonant, and the coupling ratio
of the vernier ring resonator filter is high. The vernier ring
resonator filter may be a reciprocal device, so that when it is
resonant, light returning, on the bus waveguide 115, from the
output coupler 120, may be coupled, through the vernier ring
resonator filter 110, back to the RSOA. When a resonant wavelength
of the first vernier ring resonator filter is nearly equal to a
resonant wavelength of the second vernier ring resonator filter,
the vernier ring resonator filter may be resonant at a wavelength
that is between the two wavelengths. When the vernier ring
resonator filter is not resonant, the coupling ratio of the vernier
ring resonator filter is low. The first ring resonator and the
second ring resonator may have slightly different free spectral
ranges, so that the wavelengths at which the vernier ring resonator
filter is resonant are relatively widely separated, and so that
only one wavelength at which the vernier ring resonator filter is
resonant falls within the gain bandwidth of the RSOA. This
(relatively wide) separation between consecutive resonant
wavelengths of the vernier ring resonator filter may be referred to
as the free spectral range (FSR) of the vernier ring resonator
filter. The width of a resonant peak of the vernier ring resonator
filter (e.g., the wavelength range over which the coupling ratio is
within 3 dB of the maximum coupling ratio in the peak) may be
referred to as the "bandwidth" of the vernier ring resonator
filter, and it may be expressed in units of wavelength or
frequency. In some embodiments, a two-ring vernier ring resonator
filter such as that of FIG. 2B, or a three-ring vernier ring
resonator filter such as that of FIG. 2C, may be used instead of
the two-ring vernier ring resonator filter of FIG. 2A in the
embodiment of FIG. 1A (or in the embodiments of FIGS. 3A, 4, and 5,
discussed below). The resonant wavelength of a vernier ring
resonator filter may be tuned using phase shifters 135 (FIG. 1A)
which may use heaters, as shown for example in FIGS. 2B and 2C, or
which may use p-n or p-i-n junctions, as described, for example, in
the '078 patent. In some embodiments each of the MMIs shown in
FIGS. 2A-2C has a cross coupling ratio equal to its bar coupling
ratio.
[0052] Referring again to FIG. 1A, a plurality of vernier ring
resonator filters 110 according to FIG. 2A (or alternate
embodiments of the vernier resonator, such as those disclosed in
the '078 patent) may be used to couple a respective plurality of
RSOAs to a bus waveguide, at the output end of which a broadband
partially reflective element acts as the output coupler 120 for all
of the channels. Each RSOA chip 100 may provide optical gain in a
respective wavelength band. The RSOA chips 100 may include
different respective epitaxial ("epi") designs or material systems,
and have different respective gain spectrum center wavelengths,
spanning, together, a large spectral range. The number of channels
and the bandwidth in each band may be limited by the gain bandwidth
of the RSOA chip 100 for that band. Each RSOA chip may include an
array of RSOAs 105, each RSOA 105 being formed as a separate
waveguide in the RSOA chip 100, and each providing optical gain for
a respective one of the channels using the RSOA chip 100. As used
herein, a "band" or "wavelength band" refers to a range of
wavelengths over which an RSOA chip has appreciable gain.
[0053] In some embodiments, each channel includes a phase shifter
130 (.DELTA..phi.) and an amplitude modulator 140 (.DELTA.T). Phase
shifters 130 may be included to enable accurate control of lasing
wavelengths, and amplitude modulators 140 may be included to enable
modulation of the laser power. The bandwidth over which amplitude
modulation inside laser cavity may be performed is inversely
proportional to the cavity length. In some embodiments, amplitude
modulation at rates of a few kHz, or a few MHz, may be used for
channel identification or for homodyne/heterodyne detection at a
receiver; modulation at GHz frequencies may be impractical, in some
embodiments, because of the length of the cavity.
[0054] In some embodiments, as an alternative to the use of
amplitude modulators, the RSOA bias is modulated with the desired
amplitude modulation pattern. This eliminates the need for separate
amplitude modulators inside the laser cavity which add loss, but
increases the complexity of the RSOA drive circuitry. The length of
the laser cavity may be roughly the same for all channels, and
increases for all channels as more channels are added. The cavity
length may be selected so that the wavelength separation between
cavity modes (the free spectral range of the cavity) is greater
than the bandwidth of any of the vernier ring resonator filters, so
that only one mode at a time will lase in any channel. For example,
for a cavity free spectral range of 10 picometers (pm), a cavity
length of about 5 cm may be used; this cavity length may
accommodate 100 channels or more.
[0055] The output mirror may, for example, be implemented with a
1.times.2 power splitter with a broadband high reflector on one
output arm, where the split ratio of the 1.times.2 power splitter
determines the reflectance of the output mirror, and where the
splitter is implemented with a broadband MMI or a directional
coupler, and the broadband high reflector is implemented with a
metal coating, or a Sagnac loop. The total spectral span of the
multichannel laser may ultimately be limited by the characteristics
of the broadband MMI or coupler used in the output mirror. In some
embodiments, the broadband output mirror is implemented with an
advanced thin film coating integrated in the output waveguide, or
with a broadband (e.g., chirped) DBR grating included in the output
waveguide.
[0056] Referring to FIG. 3A, in a second embodiment a multichannel
laser source includes a plurality of multichannel lasers each
having a respective bus waveguide. A respective broadband partially
reflective element acts, in each multichannel laser, as a
respective output coupler 120 for all of the channels of the
multichannel laser. The outputs of all of the multichannel lasers
are combined in a band multiplexer (MUX), to form the output of the
multichannel laser source. The embodiment of FIG. 3A may be used if
the bandwidth of a readily available output coupler 120 (as used,
for example, in the embodiment of FIG. 1A) is not sufficient to
cover a desired spectral span, or if the cavity length would be too
long if the multichannel laser source were constructed according to
FIG. 1A. The embodiment of FIG. 3A may be more compact than the
embodiment of FIG. 1A, if the band MUX is compact. FIG. 3B shows an
example of an optical spectrum that may be generated by the
embodiment of FIG. 3A.
[0057] As used herein, a multichannel laser refers to a laser
having a plurality of channels, such as the laser of FIG. 1A, and
being capable of producing light at more than one wavelength
simultaneously, the channels sharing at least one element (e.g.,
the output coupler 120 of FIG. 1A). As used herein, a multichannel
laser source refers to a light source that is capable of producing
light at more than one wavelength simultaneously, and that includes
one or more lasers (such as the embodiment illustrated in FIG. 3A,
which includes three multichannel lasers).
[0058] FIG. 3A shows one bus waveguide for each band, i.e., for
each of the RSOA chips 100, but this correspondence is not
required. Each bus waveguide may collect light from a plurality of
channels using a single RSOA chip 100 as illustrated, or it may
collect light from a plurality of channels using more than one
different RSOA chip 100 (e.g., it may collect light from one or
more channels using a first RSOA chip 100 and from one or more
channels using a second RSOA chip 100), or each of a plurality of
bus waveguides may collect light from a respective subset of a
plurality of channels using a single RSOA chip 100.
[0059] Referring to FIG. 4, a third embodiment may be more compact
than the embodiments of FIGS. 1A and 3A, if an inhomogeneously
broadened RSOA material is used (such as quantum dot (QD) or
quantum dash (QDASH) material) in one or more of the RSOA chips 100
instead of a quantum well (QW) heterostructure. Inhomogeneously
broadened gain materials can support lasing of multiple modes in a
single waveguide (i.e., in a single RSOA) in the RSOA chip 100. The
wavelength spacing between channels sharing an RSOA in the QD or
QDASH RSOA chip may be larger than the homogeneous broadening width
of the RSOA material, so that the channels do not compete for gain
to a significant extent. QD or QDASH materials may not be readily
available for all bands of the multichannel laser source;
accordingly some of the RSOA chips 100 may use such materials, and
some others may use other materials. The RSOA chips that are
inhomogeneously broadened may then have more than one channel per
RSOA (i.e., per waveguide in the RSOA chip). In the embodiment of
FIG. 4, the top-most RSOA chip (used for the "p" wavelength band)
is inhomogeneously broadened and includes only one RSOA 105, which
supports multiple channels (of which three are shown)
simultaneously; the other two RSOA chips 100 are configured to have
only one channel per RSOA. FIG. 3B shows an example of an optical
spectrum that may be generated by the embodiment of FIG. 4.
[0060] It will be understood that in some embodiments, single ring
resonators, or composite ring resonators including more than two
ring resonators, may be used in place of one or more of the vernier
ring resonator filters 110 of the embodiments of FIGS. 1A, 3A and
4. It will be understood that although only three RSOA chips and
three bands are illustrated in FIGS. 1A, 3A, and 4, a multichannel
laser or multichannel laser source may include more or fewer RSOA
chips and bands.
[0061] Referring to FIG. 5, in some embodiments wavelength sensing
may be performed using an unequal-arm Mach-Zehnder interferometer
510 and two photodetectors 520. Each of the photodetectors 520 may
include a reverse-biased junction on or coupled to a waveguide, and
may be fabricated on an RSOA chip 100 operating in the same
wavelength band. The Mach-Zehnder interferometer 510 includes a
first MMI 530 that acts as a splitter, and a second MMI 530 that
acts as a combiner. The waveguide of a first arm 540 of the
Mach-Zehnder interferometer 510 may lie alongside the waveguide of
the second arm 550 of the Mach-Zehnder interferometer 510, except
for a portion 560 that may be longer than a corresponding portion
of the second arm 550 and may result in the length difference
between the two arms 540, 550. The temperature of the Mach-Zehnder
interferometer 510 may be sensed and controlled, to reduce the
differential phase change in the Mach-Zehnder interferometer 510
that otherwise may occur if the temperature (and, as a result, the
differential optical path delay) of the Mach-Zehnder interferometer
510 were to change. FIG. 5 shows two channels of a multichannel
laser source which has more than two channels (the remainder of
which are not shown in FIG. 5).
[0062] The lengths of the two arms 540, 550 may be selected so that
when the wavelength of light received by the Mach-Zehnder
interferometer 510 is the desired wavelength, the respective
photocurrents generated by the two photodetectors 520 are equal.
Accordingly, a feedback circuit may form an error signal by
calculating (e.g., using a differential amplifier) the difference
between two photocurrents, and the error signal may be amplified
and filtered and fed back to one or more elements (or "wavelength
actuators") for adjusting the wavelength. Such a wavelength
actuator may be part of a wavelength-dependent coupler (and may be
referred to as a "coupler wavelength actuator") and may be, for
example, a phase shifter (e.g., a heater, or a p-i-n junction) on
one or more of (e.g., on all of) the half-rings, on a tunable
grating-assisted co-directional coupler (discussed in further
detail below) and/or on a distributed Bragg reflector (discussed in
further detail below). In some embodiments, if the free spectral
range of the laser cavity of a channel is greater than the resonant
bandwidth of the wavelength-dependent coupler, the phase shifter
130 may be controlled so as to keep a resonant frequency of the
laser cavity within the resonant bandwidth of the
wavelength-dependent coupler. In such an embodiment, the phase
shifter 130 acts as an additional wavelength actuator that may
simply follow the center wavelength of the wavelength-dependent
coupler, or that may provide finer (or faster) wavelength control
than the phase shifter of the wavelength-dependent coupler. In this
manner each output wavelength may be controlled. Each of the output
wavelengths may also be tunable, for example by adding an offset
signal to the error signal before it is amplified and filtered by
the feedback circuit. The Mach-Zehnder interferometer 510 may be
fed a portion of the light emitted by the RSOA of the channel for
which the wavelength is to be measured, e.g., it may be fed by
light from the output port 230B of the first MMI 210A of a
respective vernier ring resonator filter of the channel for which
the wavelength is to be measured (and controlled), as shown in FIG.
5.
[0063] FIG. 6 shows an embodiment of a multichannel laser that is
analogous to the embodiment of FIG. 1A, in which the vernier ring
resonator filters 110 of FIG. 1A have been replaced with
grating-assisted co-directional couplers (e.g., tunable
grating-assisted co-directional couplers (TGACDCs)) of a first
type. These grating-assisted co-directional couplers operate as
wavelength-dependent couplers, as discussed in further detail
below. In a manner analogous to that of the embodiment of FIG. 1A,
in the embodiment of FIG. 6 a TGACDC is employed, for each channel,
to perform gain selection inside the laser cavity to select the
lasing wavelength, and to combine the lasing light with that of
other channels into the single bus waveguide 115, all inside a
laser cavity having a shared output coupler 120. FIG. 1B shows an
example of an optical spectrum that may be generated by the
embodiment of FIG. 6.
[0064] FIGS. 7 and 8 show embodiments of multichannel laser sources
each including a plurality of channels, using TGACDCs as both
wavelength-selective output couplers and as couplers for combining
the light generated by the plurality of channels. FIGS. 7 and 8 use
TGACDCs of a second and third type, respectively, as discussed in
further detail below.
[0065] Three different types of TGACDCs may be used: i) a first
type (as described in Z.-M. Chuang and L. A. Coldren, IEEE JQE 29
(4) 1993 p. 1071) designed to 100% transmit distributed Bragg
reflector (DBR) resonant wavelengths to the drop T port (FIG. 9A,
"TGACDC1"), ii) a second type designed to partially reflect DBR
resonant wavelengths back to the RSOA (FIG. 9B, "TGACDC2"), and
partially transmit resonant wavelengths to the drop T port, and
iii) a third, composite type, consisting of a regular DBR (as
described in A. J. Zilkie et al., "Power-efficient III-V/Silicon
external cavity DBR lasers," Opt. Express 20, 23456-23462 (2012))
combined with a TGACDC of the first type, to form a composite
device (FIG. 9C, "TGACDC3"), also designed to partially reflect DBR
resonant wavelengths (wavelengths that are resonant both in the DBR
and in the included TGACDC of the first type) back to the RSOA,
partially transmit resonant wavelengths to the following TGACDC of
the first type, and further 100% transmit (by the TGACDC of the
first type) the resonant wavelengths to the drop T port. A
composite TGACDC of the third type may behave qualitatively like a
TGACDC of the second type, and may be used instead of a TGACDC of
the second type to avoid difficult design constraints that may be
present in a TGACDC of the second type. Off resonance (i.e., for
wavelengths for which the TGACDC is not resonant) each of the three
types TGACDC may behave as two substantially independent, parallel
waveguides, with little or no coupling between them (i.e., light
passing straight through the top waveguide with little or no
coupling to the drop waveguide), and little or no reflection from
the TGACDC.
[0066] In some embodiments all grating DBR wavelengths are made
tunable by adding a waveguide integrated heater to the grating
(e.g., using a metal on waveguide heater or a Si-doped integrated
heater, possibly with an undercut to make it more efficient).
[0067] As mentioned above, if a TGACDC of the first type (the type
of FIG. 9A) is used (as illustrated in FIG. 6, for example), each
TGACDC performs the same function as one of the vernier ring
resonator filters 110 of FIG. 1A, and all of the lasers (at
different respective wavelengths) share an output mirror, the
output mirror for all of the lasers being the common output mirror
120 shown.
[0068] Referring to FIG. 7, if a TGACDC of the second type 710 (the
type of FIG. 9B) is used, the TGACDC 710 acts like a regular DBR
mirror but transmits laser output light out to the bus waveguide.
In this case no common output mirror is needed as each wavelength's
output mirror is the respective TGACDC 710. This also provides the
advantage that the cavity length for the respective laser at each
wavelength is much shorter, close to a traditional single channel
DBR, meaning that the cavity FSR is much larger, and each laser may
therefore have a much larger tuning range between mode hops, and
the number of lasers that can be added is not limited. A suitable
TGACDC 710 of the type of FIG. 9B may however be more difficult to
design and may have more design constraints and/or be more
difficult to fabricate. FIG. 1B shows an example of an optical
spectrum that may be generated by the embodiment of FIG. 7.
[0069] A TGACDC of the third type (the type of FIG. 9C) is an
alternative implementation to the TGACDC of the type of FIG. 9B,
but would have design constraints and/or fabrication difficulties
relaxed because the DBR that acts as the laser mirror is separated
from the GACDC functionality. FIG. 8 shows a multichannel laser
source using TGACDCs of the third type 810. In FIG. 8 each
composite TGACDC of the third type 810 is illustrated for
simplicity as a simple TGACDC. FIG. 1B shows an example of an
optical spectrum that may be generated by the embodiment of FIG. 8.
Although the grating-assisted co-directional couplers are, in the
descriptions of some embodiments, referred to as TGACDCs ("tunable
grating-assisted co-directional couplers"), in some embodiments the
grating-assisted co-directional couplers are not tunable.
[0070] In each of FIGS. 6, 7, and 8, wavelength sending and control
may be performed by feeding a portion of the light emitted by the
RSOA of the channel for which the wavelength is to be measured to a
suitable wavelength sensor (as described, for example, with
reference to FIG. 5), e.g., the wavelength sensor may be fed a
portion of the power out of the "out1 through" port of the TGACDC.
In other embodiments this port of the TGACDC is coupled to an
optical absorber to prevent parasitic cavities or light from this
port being back-reflected into the laser cavity or the bus
waveguide.
[0071] As used herein, a "wavelength-dependent coupler" is an
optical device with at least three ports, including a channel port,
a bus input, and a bus output, and in which the coupling between
ports, or the reflectance of one or more ports, depends on the
wavelength of light fed to the wavelength-dependent coupler. The
ports of the wavelength-dependent coupler may also be referred to
by other names, as, for example, in the descriptions above of
vernier ring resonator filters and of grating-assisted
co-directional couplers. In some embodiments (e.g., those of FIG.
1A and FIG. 6), light fed to the channel port at a resonant
wavelength of the wavelength-dependent coupler may be largely
transmitted to the bus output of the wavelength-dependent coupler
(e.g., with loss of between 0.1 dB and 1.0 dB, or of less than 0.1
dB), and light fed to the bus input at a wavelength different from
the resonant wavelength of the wavelength-dependent coupler (or
different from every resonant wavelength of the
wavelength-dependent coupler, if it has more than one resonant
wavelength) may be largely transmitted (e.g., with loss of between
0.1 dB and 1.0 dB, or of less than 0.1 dB) to the bus output of the
wavelength-dependent coupler. The reflectance back to the channel
port and the reflectance back to the bus input may be between 10%
and 1%, e.g., less than 5% or even less than 1%. In other
embodiments (e.g., those of FIGS. 7 and 8), light fed to the
channel port at a resonant wavelength of the wavelength-dependent
coupler may be partially reflected (e.g., with a reflectance of
between 10% and 50%), and partially transmitted (e.g., with a
transmittance of between 40% and 90%) to the bus output of the
wavelength-dependent coupler, and light fed to the bus input at a
wavelength different from the resonant wavelength of the
wavelength-dependent coupler (or different from every resonant
wavelength of the wavelength-dependent coupler, if it has more than
one resonant wavelength) may be largely transmitted (e.g., with
loss of between 0.1 dB and 1.0 dB, or of less than 0.1 dB) to the
bus output of the wavelength-dependent coupler. The reflectance (i)
back to the channel port, at a wavelength different from the
resonant wavelength of the wavelength-dependent coupler (or
different from every resonant wavelength of the
wavelength-dependent coupler, if it has more than one resonant
wavelength) and (ii) back to the bus input, may be between 10% and
1%, e.g., less than 5% or even less than 1%. A wavelength-dependent
coupler may have a fourth port that may (as mentioned above) be
connected to an optical absorber or that may be used as a source of
a portion of the light generated in one of the channels, for use in
a wavelength sensing and control system for the channel. Both
possible uses of the fourth port are illustrated in FIGS. 6, 7, and
8, with arrows at the fourth ports of two of the
wavelength-dependent couplers denoting light sent to a wavelength
sensor. In some embodiments, the 3 dB bandwidth of the resonant
characteristic of a wavelength-dependent coupler is between 0.01 nm
and 1.00 nm.
[0072] As mentioned above, the vernier ring resonator filters
(e.g., of FIG. 1A) and the grating-assisted co-directional couplers
(e.g., TGACDCs) described herein are examples of
wavelength-dependent couplers. For example, in FIG. 1A the channel
port of the each of the vernier ring resonator filters is the upper
left port, the bus input is the lower left port, the bus output is
the lower right port, and the fourth port is the upper right
port.
[0073] Any numerical range recited herein is intended to include
all sub-ranges of the same numerical precision subsumed within the
recited range. For example, a range of "1.0 to 10.0" is intended to
include all subranges between (and including) the recited minimum
value of 1.0 and the recited maximum value of 10.0, that is, having
a minimum value equal to or greater than 1.0 and a maximum value
equal to or less than 10.0, such as, for example, 2.4 to 7.6. Any
maximum numerical limitation recited herein is intended to include
all lower numerical limitations subsumed therein and any minimum
numerical limitation recited in this specification is intended to
include all higher numerical limitations subsumed therein.
[0074] Although exemplary embodiments of a multichannel laser
source have been specifically described and illustrated herein,
many modifications and variations will be apparent to those skilled
in the art. Accordingly, it is to be understood that a multichannel
laser source constructed according to principles of this invention
may be embodied other than as specifically described herein. The
invention is also defined in the following claims, and equivalents
thereof.
* * * * *