Note: Descriptions are shown in the official language in which they were submitted.
STABLE LINEWIDTH NARROWING OF A COHERENT COMB LASER
Field of the Invention
[0001] The
present invention relates in general to devices for stable narrowing of
linewidths of quantum dot or dash mode-locked coherent comb lasers (CCLs), and
in
particular to a technique for concurrently narrowing a plurality of mode-
locked modes using a
self-injection external cavity with high stability.
Background of the Invention
[0002]
Communication networks need to keep up with the growth of today's Internet
data
traffic. The telecommunications industry needs new photonics equipment to
improve current
optical networks and for deployment in next generation optical networks.
Semiconductor
lasers are among the most important generation components in optical
telecommunication
systems.
Optical linewidth of semiconductor lasers is important because linewidth
determines the laser's coherence length and phase noise. The maximum data rate
in an
optical fiber communications link is determined by the ratio of signal power
to noise power as
per the Shannon-Hartley equation. Narrow linewidth is an essential requirement
for lasers
used in high data rate coherent communications, since phase noise impacts
signal noise by
the coherent detection process. Even non-coherent modulation schemes can
suffer from a
reduction in signal quality when phase noise is translated into amplitude
noise. Thus, lasers
for modern optical communications systems now require linewidths of a few
hundred KHz or
less.
[0003]
Unfortunately semiconductor lasers typically have linewidths on the order of
several to tens of MHz. Consequently, techniques for reducing optical
linewidth of
semiconductor lasers have been of growing importance since the move to
coherent optical
communications that has been building over the last decade.
[0004]
Accordingly, there has been a significant amount of interest in optical
coherent
comb lasers (CCLs) and their benefits as a source of multiple spectral lines
(also known as
"tones") in coherent optical fiber communications, because CCLs have been used
to create
the carrier frequencies in dense wavelength division multiplexing (DWDM)
optical systems
with net data rates exceeding Terabit/s transmission rates and high spectral
efficiency [1-3].
Different techniques have been used to generate multi-wavelength lasers:
modulator-based
comb sources [3], spatial mode beating within a multimode fiber section [4],
multi-cavity
oscillation [5], comprising highly nonlinear fibers for spectral broadening
[6], or high-Q
microresonators [7]. However, these techniques either require complex setups
with discrete
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CA 3012938 2018-07-31
components, high pump powers with delicate operating procedures, or they
provide only a
limited number of spectral carriers.
[0005] For practical systems, a compact, low-cost, energy-efficient CCL is
desired.
Applicant developed nanostructured InAs/InP quantum dot (QD) multi-band (multi-
colour)
multiwavelength mode locked laser, and has demonstrated intra-band and inter-
band mode-
locking (US patent 7,991,023). Its use as a coherence comb laser (QD-CCL) over
a large
wavelength range covering C- or L-band has been demonstrated [8-18]. Unlike
uniform
semiconductor layers in most telecommunication lasers, in the QD CCL, light is
emitted and
amplified by millions of semiconductor QDs (typically less than 50 nm lateral
diameter).
Each QD acts like an isolated light source acting independently of its
neighbours, and each
QD emits light at its own respective wavelength. By providing high efficiency
QDs with a
desired emission frequency distribution, the CCL is more stable and has much
better
performance compared to other multi-wavelength lasers. Importantly, a single
CCL has
been shown to simultaneously produce 50 or more separate lines at spatially
distributed
wavelengths over the telecommunications C-band or L-band. To achieve these
properties
we have put considerable effort to design, grow and fabricate InAs/InP QD gain
materials
and produce CCLs.
[0006] More recently Applicant has demonstrated CCLs with repetition rates
from 10 to
437 GHz and a total output power up to 50 mW, at room temperature [8-18].
Applicant has
investigated relative intensity noises (RINs), phase noises, RF beating
signals and other
parameters of both filtered individual channels and the whole CCL's output [17-
18].
Unfortunately, the single filtered channels of QD CCLs generally exhibit
strong phase noise
and broad optical linewidths, typically of the order of MHz [17-21]. As a
consequence,
wavelength-division multiplexing (WDM) data transmission using these CCLs has
been
restricted to direct detection schemes [22] or differential quadrature phase
shift keying
(DQPSK), which only uses relatively few (4) symbols. While these CCLs have
high symbol
rates, their aggregate data rates (up to 504 Gbit/s [23]) are limited by the
symbol sets.
Coherent transmission can use many more than 4 symbols to achieve higher baud
rates,
where linewidth allows. The CCLs are not satisfactory for TbiVs (and higher)
coherence
optical networking systems.
[0007] Furthermore, other uses for CCLs, such as in high precision optical
measurement
devices or high resolution spectral analysis, are limited by this phase noise.
[0008] In order to improve net data transmission rates and spectral
efficiency in optical
coherent communication systems, researchers have put significant efforts to
simultaneously
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reduce optical linewidth of each individual channel of CCLs. For example, a
feed-forward
heterodyne scheme has been used to simultaneously reduce the optical linewidth
of many
comb lines from mode-locked lasers [24-25]. Both [25], and [26] use a local
oscillator (LO)
and a Mach-Zender Modulator (MZM). The LOs have a narrow linewidth (narrower
than the
narrowest linewidth achieved by the feedforward system). These references show
the
difficulty of producing a large set of comb lines (more than 20)
simultaneously narrowed to a
high degree (below a few hundred kHz), even when resorting to the relatively
complex
setups.
[0009] Prior
art for reducing linewidth of single mode lasers are also known. For
example, US Patent 8,804,787 to Coleman et al. claims a particular arrangement
for tapping
a laser signal from a single mode laser cavity, attenuating the laser signal,
and feeding the
attenuated (-30 to -80 dB) laser signal back into the laser cavity, where the
laser driver
provides sufficient drive stability so that a frequency variation of the laser
is less than a free
spectral range (FSR) of the secondary cavity. This patent specifically
identifies as an
unexpected result: "that an uncontrolled OPL[Optical Path Length] to the back
reflection
provided by the first branch provides significant spectral narrowing, which
can be several
orders of magnitude narrowing". A reduction of linewidth from 118 kHz to 2 kHz
was
demonstrated for a single wavelength QD laser. "Polarization Maintaining (PM)
fiber or non-
Polarization Maintaining SM fiber" can be used.
[0010] Recent
papers [29,30] associated with a European Commission EC-FP7 Big
Pipes project demonstrate simultaneous linewidth narrowing of 60 lines in a
Quantum Dash
mode-locked laser diode using resonant feedback from a secondary cavity,
without any LO.
The secondary cavity is provided with a freespace optical setup from a
backside facet of the
mode-locked laser diode that is barely disclosed. Freespace optical waveguides
are
typically polarization maintaining. Stability of the linewidth is not
discussed in any of the prior
art references, including these recent papers.
Stability is particularly important for
commercial deployment of lasers used in telecommunications applications. Given
the highly
schematic description of the optical system provided in these papers, it is
unclear what kind
of stability could be provided with their system. Given that "the external
cavity length is
adjusted to be near a multiple [M] of the optical length of the laser"[30],
and a known variably
of the laser optical length in operation, it is a safe assumption that the
stability is poor
outside of highly controlled lab settings. It should be noted that a large OPL
for the external
cavity (which would be desirable for a large linewidth reduction factor) will
require this
multiple M to be large. However if M is large, a small variation in the
laser's effective OPL
(S) will generate a difference MxS in the distance of the reflector from the
intended position.
The ability to predict or adapt the OPL of the external cavity is not trivial,
if possible, and both
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the OPL of the external cavity and the attenuation have cumulative effects in
terms of
varying output, leading to a further source of instablility.
[0011] Accordingly there is a need for a technique for concurrently
narrowing linewidths
of a plurality of mode-locked comb lines in a CCL, without relying on a narrow
linewidth LO
and MZMs, without reducing a number of lines of the CCL, while retaining
stability of the
narrowed linewidth. Furthermore, there is a need for stably narrowing more
linewidths of a
CCL, to a greater extent, without complicated and expensive equipment to setup
and
maintain.
Summary of the Invention
[0012] Applicant has discovered a low-cost and efficient technique for
simultaneously
narrowing linewidths of coherent comb lasers (CCLs) with improved stability
using a
polarization maintaining fiber-based secondary cavity. The technique does not
rely on
narrower linewidth local oscillators (L0s), and Mach-Zender Modulators (MZMs),
and can be
achieved with less equipment and cost than such techniques. The technique has
demonstrably simultaneously reduced the optical linewidth of each of 39
individual channels
of a 25GHz OD CCLs from a few of MHz down to less than 200 kHz, without
reducing the
number of lines, and has stability far higher than what is possible with with
long OPL
freespace optics, and secondary cavities composed of non-polarization
maintaining single
mode fibre.
[0013] Accordingly a method for narrowing a linewidth of a coherent comb
laser (CCL) is
provided. The method involves: providing a mode-locked semiconductor coherent
comb
laser (CCL) with a laser cavity defined by an active gain material in a
waveguide between
two facets, the CCL adapted to output of at least 4 mode-locked lines, each
with an original
linewidth of less than 100 MHz; tapping a fraction of a power from the CCL
from the laser
cavity to form a tapped beam; propagating the tapped beam to an attenuator to
produce an
attenuated beam and propagating the attenuated beam back to the laser cavity,
on a solid
waveguide; and reinserting the attenuated beam into the laser cavity, where
the reinserted
beam has a power less than 10% of a power of the tapped beam. The reinsertion
allows the
CCL to be operated to output the mode-locked lines, each with a linewidth of
less than 80%
of the original linewidth, and an optical path between tapping and reinsertion
is polarization
maintaining.
[0014] The mode-locked semiconductor CCL provided, preferably: is adapted
to output
at least 10 mode-locked lines; is adapted to output at least 4 mode-locked
lines with original
linewidths between 10 and 80 MHz; is adapted to output at least 10 mode-locked
lines in an
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CA 3012938 2018-07-31
optical networking telecommunications band; is electrically pumped; is a ridge
waveguide
laser with edge facets forming a Fabry-Perot cavity; is one of: a small edge-
emitting laser, an
external cavity laser, a monolithic (internal-cavity) laser, a diode bar
laser, a stacked diode
bar laser, a surface-emitting laser (VCSEL), such as an optically pumped
surface-emitting
external-cavity semiconductor laser (VECSEL), or a quantum cascade laser; or
has an
active gain material comprising quantum wells, dots, dashes or rods formed of
GaAs,
AIGaAs, InGaAs, InAs, GaInNAs, GaN, GaP, InGaP, InP, GaInP, or a combination
thereof.
More specifically, the CCL preferably is adapted to output at least 10 mode-
locked lines with
original linewidths between 10 and 80 MHz, or between 1 and 30 MHz; is adapted
to output
at least 25 mode-locked lines in an optical networking telecommunications "C"
band; is
electrically pumped, controlled by a low noise laser driver, and temperature
controlled; or
has an active gain material comprising quantum dots, and/or dashes formed of
GaAs,
AIGaAs, InGaAs, InAs, GaInNAs, GaN, GaP, InGaP, InP, GaInP or a combination
thereof.
[0015] An optical path length of the secondary cavity is preferably between
5 and 50 m,
and the attenuation level is preferably between 15 and 60 dB.
[0016] Tapping the CCL preferably comprises: collecting output of a
backside facet of
the CCL, or providing a coupler to tap a fraction of an output of the CCL.
[0017] Reinserting the attenuated beam preferably comprises reinjecting the
attenuated
beam into the laser cavity via the backside facet, or the coupler.
[0018] Propagating the tapped beam to an attenuator preferably comprises:
coupling the
tapped beam from a bidirectional waveguide path to a unidirectional waveguide
circuit
including the attenuator; coupling the tapped beam from a bidirectional
waveguide path,
which includes the attenuator, to a unidirectional waveguide circuit;
providing the attenuator
on a bidirectional waveguide path that includes a reflector; or providing a
partial reflector on
the bidirectional waveguide path that serves to both attenuate and reflect the
tapped beam.
[0019] Coupling the tapped beam is preferably provided by an optical
circulator.
[0020] The attenuator is preferably a variable optical attenuator.
[0021] The attenuator preferably has an attenuation range of at least 10
dB; avoids
creating spurious reflections; attenuates each of the lines to somewhat the
same degree;
and does not vary an OPL of the secondary cavity while changing the degree of
attenuation.
The attenuator preferably controls light transmission by an aperture
variation, with partial
occlusion of the beam.
CA 3012938 2018-07-31
[0022] The solid waveguide of the optical path between tapping and
reinsertion is
preferably provided by single mode optical fibres, a microphotonic chip, a
photonic crystal
arrangement, or an integrated optical system.
[0023] One of the mode-locked lines output preferably has a stability such
that over a
one hour period, the linewidth does not vary by more than 100 kHz.
[0024] Also accordingly, a narrow linewidth multi-wavelength laser (MWL) is
provided,
comprising: a mode-locked semiconductor coherent comb laser (CCL) with a laser
cavity
defined by an active gain material in a waveguide between two facets, the CCL
adapted to
output of at least 4 mode-locked lines, each with an original linewidth of
less than 100 MHz;
and a secondary cavity coupled to the laser cavity for tapping a beam of the
CCL and
propagating the tapped beam to an attenuator and reinserting the attenuated
beam into the
cavity at a power less than 10% of a power of the tapped beam, the secondary
cavity
consisting of polarization maintaining solid waveguides between polarization
maintaining
components. A linewidth of each of the at least 4 lines is reduced in
proportion to a
difference in optical path length between the feedback cavity and the laser
cavity.
[0025] Preferably the CCL: is adapted to output at least 10 mode-locked
lines; is
adapted to output at least 4 mode-locked lines with original linewidths
between 10 and 80
MHz; is adapted to output at least 10 mode-locked lines in an optical
networking
telecommunications band; is electrically pumped; is a ridge waveguide laser
with edge
facets forming a Fabry-Perot cavity; is one of: a small edge-emitting laser,
an external cavity
laser, a monolithic (internal-cavity) laser, a diode bar laser, a stacked
diode bar laser, a
surface-emitting laser (VCSEL), such as an optically pumped surface-emitting
external-
cavity semiconductor laser (VECSEL), or a quantum cascade laser; or has an
active gain
material comprising quantum wells, dots, dashes or rods formed of GaAs,
AIGaAs, InGaAs,
InAs, GaInNAs, GaN, Gap, InGaP, InP, GaInP, or a combination thereof. More
specifically,
the CCL preferably: is adapted to output at least 10 mode-locked lines with
original
linewidths between 10 and 80 MHz, or between 1 and 30 MHz; is adapted to
output at least
25 mode-locked lines in an optical networking telecommunications "C" band; is
electrically
pumped, controlled by a low noise laser driver, and temperature controlled; or
has an active
gain material comprising quantum dots, and/or dashes formed of GaAs, AlGaAs,
InGaAs,
InAs, GaInNAs, GaN, Gap, InGaP, InP, GaInP or a combination thereof.
[0026] An optical path length of the secondary cavity is preferably between
5 and 50 m,
and the attenuation level is between 15 and 60 dB.
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[0027] The secondary cavity preferably comprises an optical coupling from
one of a
backside facet of the CCL, and/or a tap of an output of the CCL via which the
beam is
tapped and/or reinserted. The secondary cavity preferably comprises: a
bidirectional
waveguide path coupled to a unidirectional waveguide circuit including the
attenuator; a
bidirectional waveguide path, including the attenuator, coupled to a
unidirectional waveguide
circuit; a bidirectional waveguide path that includes a reflector; or a
partial reflector on the
bidirectional waveguide path that serves to both attenuate and reflect the
tapped beam.
[0028] The coupling of the tapped beam may be provided by an optical
circulator.
[0029] The attenuator may be a variable optical attenuator with an
attenuation range of
at least 10 dB, provisioned to avoid creating spurious reflections, to
attenuate each of the
lines to somewhat the same degree, and to not vary an OPL of the secondary
cavity while
changing the degree of attenuation. The attenuator preferably controls light
transmission by
an aperture variation, with partial occlusion of the beam.
[0030] The secondary cavity preferably comprises an optical path between
tapping and
reinsertion provided by: single mode optical fibres; a free-space optical
system; a
nnicrophotonic chip; a photonic crystal arrangement; or an integrated optical
system.
[0031] One of the at least 4 lines preferably has a stability such that
over a one hour
period, the linewidth does not vary by more than 100 kHz.
[0032] Further features of the invention will be described or will become
apparent in the
course of the following detailed description.
Brief Description of the Drawings
[0033] In order that the invention may be more clearly understood,
embodiments thereof
will now be described in detail by way of example, with reference to the
accompanying
drawings, in which:
FIG. 1 is a flowchart illustrating principal steps of a method in accordance
with an
embodiment of the present invention;
FIG. 2a is a schematic illustration of apparatus accordance with an embodiment
of the
present invention;
FIG. 2b is a schematic illustration of apparatus accordance with another
embodiment of the
present invention;
FIG. 2c is a schematic illustration of apparatus accordance with another
embodiment of the
present invention;
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FIG. 2d is a schematic illustration of apparatus accordance with another
embodiment of the
present invention;
FIG. 2e is a schematic illustration of apparatus accordance with another
embodiment of the
present invention;
FIG. 2f is a schematic illustration of apparatus accordance with another
embodiment of the
present invention;
FIG. 3A pertains to a CCL used to demonstrate the present invention, in
particular 3A is a
schematic illustration of a mode-locked InAs/InP Quantum Dash CCL used for the
demonstration of the present invention with a micrograph inset;
FIG. 3B also pertains to a CCL used to demonstrate the present invention: 3B
shows laser
output spectrum showing the characteristic comb output of the same CCL;
FIG. 4A is a graph, in conjunction with 4b, comparing single line optical
noise spectra for
optimized driven CCL in comparison with the same CCL with secondary cavity
self-feedback
respectively for the 1st and 15th lines of the CCL;
FIG. 4B is also a graph, in conjunction with 4A, comparing single line optical
noise spectra
for optimized driven CCL in comparison with the same CCL with secondary cavity
self-
feedback respectively for the 15t and 15th lines of the CCL;
FIG. 5 is a linear graph showing linewidth of a CCL with optimized driving
without and with a
secondary cavity self-feedback;
FIG. 6 is a graph showing normalized RF beating signal spectra for optimized
driven CCL in
comparison with the same CCL with secondary cavity self-feedback; and
FIG. 7 is a graph showing optical linewidth as a function of time of an
individual channel from
a self-locked 25 GHz QD CCL comparing a polarization maintaining secondary
cavity with a
non-polarization maintaining secondary cavity.
Description of Preferred Embodiments
[0034] Herein a cost-effective technique for reducing linewidth of a
coherent comb laser
(CCL) is described that provides higher stability of the narrowed lines. The
technique avoids
use of narrow linewidth Local Oscillators and Mach-Zender Modulators, and does
not reduce
a bandwidth, or number of lines of the CCL, and simultaneously reduces
linewidth of a
number of mode-locked lines. The technique uses polarization maintaining solid
waveguide
between at least a multimode laser and attenuator to produce a secondary
cavity that is
substantially polarization maintaining.
[0035] FIG. 1 is a schematic illustration of a method for reducing
linewidth of mode-
locked lines of a CCL, in accordance with the present invention. The method
begins at
step 10 by providing a CCL that has at least a few, such as 4 mode-locked
emission lines
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CA 3012938 2018-07-31
(also referred to as emission frequencies, or wavelengths). For
telecommunications
purposes, preferably the CCL has as many lines as possible within the
telecommunication
bands, and these are preferably evenly spaced apart. The CCL is preferably
electrically
pumped, and is a semiconductor laser. The CCL may be a ridge waveguide laser
with edge
facets forming a Fabry-Perot cavity. The type of semiconductor laser may be: a
small edge-
emitting laser; an external cavity laser; a monolithic (internal-cavity)
laser; a diode bar laser;
a stacked diode bar laser; a surface-emitting laser (VCSEL); such as an
optically pumped
surface-emitting external-cavity semiconductor laser (VECSEL); or a quantum
cascade
laser. The CCL may have an active material of quantum wells, dots, dashes or
rods formed
of GaAs, AIGaAs, InGaAs, InAs, GaInNAs, GaN, GaP, InGaP, InP, or GaInP, and
more
preferably quantum dots or dashes.
[0036] A set of the lines of the CCL are mode-locked if driven by a
suitable low noise
laser driver. Linewidths of these lines are principally determined by phase
noise: a variation
of the instantaneous line frequency over time. The property of mode-locked
lines is that this
phase noise varies similarly as a function of time at each of the lines, but
the frequency
variations at the different lines may vary in amplitude, thus it is common for
linewidths to vary
gradually, often monotonically, across the spectrum of a mode-locked laser. A
range of
these linewidths, are important indicators of how much improvement to the
modelocking, and
to the linewidth (or phase noise, or frequency variation), the present
invention can produce,
and what Optical Path Length (OPL) the secondary cavity should have, as
explained
hereinbelow.
[0037] At step 14, a CCL output is tapped uniformly across at least the
gain spectrum of
the mode-locked lines to form a beam. This may be done by either facets of the
CCL, or by
a beam splitter on the CCL output. The CCL may be a symmetric laser, with
identical facet
coatings on both ends of the FP cavity, and the tapped beam can be drawn from
either laser
facet. The power of the tapped beam, relative to the CCL output may be
determined by a
transmittance of the facet by which the tapped beam is drawn, or by a coupler.
Care is
taken to ensure that the tapped power forms a beam without reflecting power
back into the
cavity at different distances from the facet. The tapped beam is transmitted
through a
polarization maintaining (PM) single mode fiber (SMF) as shown in the system
embodiments
of the invention herein, although a microphotonic chip with suitable high
quality coupling and
optical path length could alternatively provide an advantageously integrated
optical
arrangement. Both PM-SMF and PM microphotonic chips are solid waveguide
technology
that provide a desired optical path length for the secondary cavity. While it
will be
appreciated that some part of the secondary cavity may be provided by
freespace optics,
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CA 3012938 2018-07-31
such as within a preferred variable optical attenuator, or within certain
optical circulators, the
whole secondary cavity remains PM, and the waveguides are nearly exclusively
solid. It
should be noted that photonic crystal arrangements may be provided in the PM-
SMF or
nnicrophotonic chip.
[0038] The tapped beam is attenuated in transit through the secondary
cavity (step 14)
exclusive of the CCL's (primary or FP) cavity. Herein the secondary cavity
subsumes the FP
cavity and further extends from tap to reinsertion. The attenuation preferably
includes at
least one controlled attenuator, that allows for varying a degree of
attenuation. The
controlled attenuator preferably operates in a manner that does not reflect
the beam (i.e.
avoids creating spurious reflections); attenuates each frequency to somewhat
the same
degree; and does not vary an OPL of the secondary cavity while changing the
degree of
attenuation. An aperture-based variable optical attenuator may accomplish this
effectively.
However, a well-controlled optical path with a fixed attenuation at the right
level and the
correct optical path length can equally function. Also, as can be gleaned by
the equation
hereinbelow, control over OPL of the secondary cavity is an equivalent for
control over
attenuation (in the limiting case of optimized feed-back), although precise
control over OPL
is more technically challenging than attenuation.
[0039] In step 16, the attenuated beam is reinserted into the laser cavity,
with at least a
decimated power (i.e. at most 1/10th of the power tapped). Herein, all
physical ranges and
half-ranges for parameters are intended to equally support every subrange
thereof. The
reinsertion may be via the first or second laser facet.
[0040] To design such a system for a given CCL, one must choose the OPL
(generally
1-1000m; more preferably 5-300m; more preferably 10 to 50m) and the
attenuation level
(generally 10-80dB, more preferably 30-60dB) of the secondary cavity.
[0041] Specifically, selection of Lõõ the OPL of the secondary cavity, can
be made with
a CCL once the following parameters of the CCL are known: L. , the OPL of the
effective
laser cavity; rexõ an amplitude reflection coefficient of the external cavity
(square root of
laser power reflection coefficient across the secondary cavity); rea, , an
amplitude reflection
coefficient of the laser cavity where it joins the secondary cavity; and a, a
linewidth
enhancement (Henry) factor. It is common knowledge how to measure these
parameters.
Assuming resonance, the equation relating F, the linewidth narrowing ratio
(linewidth
optimized with the secondary cavity / linewidth optimized without), with
1,,,,the OPL of the
secondary cavity, is:
CA 3012938 2018-07-31
F =[1+ Lex, . rex' + a2 - r2)]-2
rcõ,,
[0042] This equation, though simplified for optimized conditions, allows
for estimation of
the maximum linewidth narrowing factor. The problem of extending the OPL and
attenuation
of the secondary cavity across a largest range of lines is non-trivial and
depends on many
factors that are known to those of skill in the art. In general, an RF beating
signal spectrum
of the CCL is observed, and if its linewidth is less than a few hundred kHz it
is sufficiently
mode-locked. Furthermore the optical phase noise of each (or a representative
number of
individual lines is assayed to determine the total phase noise. If the optical
phase noise of
each line is less than 100 MHz, and a variation in optical phase noise across
the lines is less
than 100 times, the present invention is expected to narrow linewidth by a
factor of at least
20%. In general the lower the optical phase noise of the lines, the higher the
OPL can be
chosen, and the higher the gain factor achievable, subject to the ability to
achieve resonant
conditions.
[0043] FIG. 2a is a schematic illustration of a first embodiment of the
present invention.
The first embodiment includes a secondary cavity defined at a backside of a
CCL 20. Herein
frontside of CCL 20 is an end of the laser from which the output is emitted,
and backside is
opposite the frontside. Herein like features are identified by like reference
numerals, and
their descriptions are not repeated in each embodiment of the invention,
unless to point out a
different aspect of the invention.
[0044] The CCL 20 is a semiconductor laser controlled and electrically
excited by a laser
driver 21, and a thermoelectric cooler (TEC) 22. It will be appreciated that
the laser driver 21
is optimized for controlling laser output of the CCL 20 with the secondary
cavity feedback.
The backside is coupled to a first segment of polarization maintaining single
mode fibre (PM-
SMF) 23a, in a manner well known in the art for avoiding back reflections. The
first segment
is coupled to port 1 of a polarization maintaining optical circulator (PM-0C)
24. The signal
from port 1 is emitted from port 2 of the PM-DC 24 coupled to a second segment
of the PM-
SMF 23b. The second segment 22b is coupled to a variable optical attenuator
(VOA) 25
which attenuates the beam, and forwards the attenuated beam along a third
segment of the
PM-SMF 23c, which couples to port 3 of the PM-OC 24. One advantage of using a
PM-OC
is that back reflections to the first segment 23a are essentially precluded.
Any reflections
entering port 2, whether back reflected from the VOA 25, or cycling through
port 3, will
substantially exit port 3 in an indefinite, highly attenuated, loop. The
attenuated beam
received at port 3 is output to port 1 for reinsertion into the CCL cavity,
via the first
segment 23a. A fixed optical path length of the secondary cavity is provided
with a spool 26.
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An advantage of the location of the spool 26 on the second or third segments
is that any
coupling (Fresnel) reflection losses are not propagated back to the CCL cavity
except via the
port 3-port 1 path. An advantage of locating the spool 26 in the first
segment, assuming no
coupling reflection, is that only half the spool length is needed to provide
the OPL.
[0045] FIG. 2b is a schematic illustration of a second embodiment of the
present
invention. The second embodiment differs from the first embodiment in that the
PM-OC 24
is replaced with a PM coupler 27, and an isolator 28 is placed in the
secondary cavity. The
PM coupler 27 receives the tapped beam on segment 23a, and may couple with 90%
efficiency to segment 23b, or may be a balanced 50-50 1:2 splitter/combiner.
Preferably
there are no back-reflections. Even a small (0.01%) reflection risks
multiplexing multiple
OPL feedback signals to the CCL, may spoil the feedback and make the secondary
cavity
uncontrollable. To the extent that the tapped beam is divided and goes into a
back side of
the isolator 28, the isolator 28 serves as a secondary attenuator. It will be
appreciated that
the isolator 28 is described as preventing light coupling from segment 23d to
23a, but it
would be equivalent if the isolator operated in the opposite direction. The
isolator 28 may be
a multi-stage isolator with a high isolation factor. Alternatively, to provide
equivalent
unidirectivity, a plurality of single stage isolators may be distributed
before and after the
spool 26, and/or before and after the VOA 25.
[0046] FIG. 2c is a variant of the first embodiment, with an added PM
coupler 27 is used
to permit the secondary cavity to branch from a laser output on the frontside
of the CCL
cavity. The coupler 27 is preferably a 90:10 coupler that sends 90% of the
light to an isolator
28 for emission of the laser. The isolator 28, preferably a multi-stage
isolator, is provided on
the frontside to prevent back reflections from the laser as used, from
affecting laser stability,
as is conventional on such lasers. The PM-OC 24 of FIG. 2c is a 4-port
circulator, in which
any beam that is not transmitted from port a to port a+1, exits at port a+2.
By skipping port 3
in this setup, any component of the tapped beam that is not sent to the VOA
25, is withdrawn
via port 3. Any attenuated beam that is not coupled back to port 1, is re-
attenuated at the
VOA 25 and will have negligible effect on the CCL 20 with twice the
attenuation.
[0047] While FIG. 2c subsumes essentially the embodiment of FIG. 2a, the
ring could
alternatively be provided as per the embodiment of FIG. 2b, if a 1 by 3
coupler is used
instead of the 1 by 2 coupler, or two 1 by 2 couplers are used in series from
the laser output.
[0048] Furthermore, as shown in FIG. 2d, the tapped beam may be from the
frontside of
the CCL 20, and the reinsertion can be into the backside of the CCL 20. By
switching a
direction of the isolator, the opposite is provided, and is equally feasible.
12
CA 3012938 2018-07-31
[0049] FIG. 2e schematically illustrates a full duplex embodiment of the
secondary
cavity. No isolator or circulator is used, but retroreflection is provided by
a mirror 29. The
mirror 29 may be a partially reflective, or highly reflective coated end of
the fibre with suitable
attention to preventing other light from entering the coated end.
[0050] FIG. 2f is similar to FIG. 2e but uses a partial reflector 30 with a
known and
controlled fractional reflectivity and fractional transmissivity. The
transmitted beam is lost to
the secondary cavity, and isolator 28 prevents any further reflections from
entrance into the
system. This set up requires a controlled but fixed OPL opposed to the VOA 25,
to induce
retroreflection, which has the disadvantage of not permitting any
reconfiguration after initial
set up and calibration of the CCL, other than what can be achieved by varying
the laser
driver 21's injection. After this retroreflection, an isolator 28 is provided
to prevent further
reflections from entering into the secondary cavity.
[0051] In the previous embodiments, the features used in one embodiment can
generally
be added or replaced with those of other embodiments without departing from
the intended
range of embodiments illustrative of the present invention.
Examples
[0052] FIG. 3A is a schematic illustration of the InAs/InP Quantum Dash CCL
used for
the demonstration of the linewidth narrowing with a secondary cavity. The
InAs/InP QD CCL
was grown by chemical beam epitaxy (CBE) on exactly (100) oriented n-type InP
substrates.
The undoped active region of the QD sample consisted of five stacked layers of
InAs QDs
with In0.816Ga0.184As0.392P0.608 (1.150) barriers. The QDs could be tuned to
operate in
the C- or L-band using a QD double cap growth procedure and a GaAs sublayer
[27-28]. In
the double cap process the QDs are partially capped with a thin layer of InP,
followed by a
30 second growth interruption and then complete capping with the 1.15Q barrier
material. A
thickness of the partial cap controls a height of the QDs, and hence their
emission
wavelength, and is also narrows the height distribution of the QDs, resulting
in a narrower 3-
dB gain spectrum. The thin GaAs sublayer promotes dash rather than dot growth.
This
active layer was embedded in a 355 nm thick 1.150 waveguiding core, providing
both carrier
and optical confinement. The waveguiding core was surrounded by p-doped (top)
and n-
doped (bottom) layers of InP and capped with a heavily doped thin InGaAs layer
to facilitate
the fabrication of low resistance Ohmic contacts.
[0053] This sample was fabricated into a single lateral mode ridge
waveguide laser with
a ridge width of 1.8 pm, and then cleaved to form a F-P laser cavity for the
CCL. A laser
13
CA 3012938 2018-07-31
cavity with length 1693 pm was produced for the CCL. The output of this CCL
was coupled
to an anti-reflection (AR) coated lensed fiber followed by a two-stage C-band
optical isolator
to reduce any back-reflection to the QD CCL. The laser was driven with a DC
injection
current using a low noise laser driver (ILX Lightwave model LDX-3620B), and
tested with a
heat sink maintained at 20 C using a thermoelectric cooler (MeIcor).
[0054] The performance of the QD CCL was characterized using an optical
spectrum
analyzer (Anritsu MS9740A), a 50GHz (max.) PXA signal analyzer (Keysight
Technologies
Model N9030A), a 45 GHz IR photodetector (New Focus Model-1014), an optical
autocorrelator (Femtochrome Research Inc. FR-103HS), a delayed self-heterodyne
interferometer (Advantest 07332 and R3361A), an 0E4000 automated laser
linewidth/phase
noise measurement system (OEWaves Inc.) and power meters (Newport 840, ILX
Lightwave
FPM-8210H and OMM-6810B). FIG. 3B shows 58 lines output by the laser with the
optical
spectrum analyzer.
[0055] L-I-V curves were measured for the CCL, and the lasing threshold
current was
found to be 48 mA, with a slope efficiency of 0.13 mW/mA. The following
properties were
obtained for the CCL in its original state: active length: 1693 pm; frequency
spacing:
25 GHz; injection current: 380 mA; temperature: 20 C; center wavelength:
1537.7 nm; 3-dB
bandwidth: -10.46 nm; and channels with the optical signal-to-noise ratio
(OSNR) of more
than 35 dB: at least 39. The laser's series resistance is 1.46 Ohm. The
optical average
output power measured by a large area detector is 42 mW in these conditions.
The optical
linewidth of each individual channel is from -1 MHz to 4.5 MHz between 1542.92
nm to
1532.46 nm over the 53 channels, as graphically shown in FIG. 4.
[0056] While this CCL is excellent - all laser channels with excellent OSNR
are very
stable because of highly inhomogeneous QD gain broadening due to statistically
distributed
sizes / geometries, composition and environment of self-assembled ODs -
experimental
results have clearly shown that the optical linewidth of the single filtered
channels of 1-
4.5 MHz is not good enough for: terabit/s (or better) coherence optical
networking systems;
high precision optical measurement; or high resolution spectral analysis. In
order to narrow
the optical linewidth of every individual channel of the QD CCLs, the simple
external cavity,
self-feedback system was invented.
[0057] The secondary cavity substantially as shown in FIG. 2c was used to
demonstrate
this invention, but with a 3 port PM OC. The frontside laser output of the QD
MWL was
optically coupled to an anti-reflection (AR) coated lensed polarization-
maintaining (PM)
single-mode fiber (SMF), followed by a two-stage optical isolator to prevent
reflection back to
14
CA 3012938 2018-07-31
laser cavity from the measurement system. Naturally the TE alignment of the
laser output
was aligned with the polarization angle of the PM-SMF. The backside facet of
the OD CCL
was optically coupled to another AR-coated lensed PM-SMF (23a) connected with
port 1 of
the PM optical circulator (OC) (Lightstar Inc. Model: PM0C-1550-B-900-5-0-0.8
5.5x35mm
FC/APC X3). The VOA 25 was a PM VOA based on a mechanical aperture to occlude
part
of the tapped beam without inducing any optical path change while adjusting
attenuation.
The VOA 25 is (Lightstar Inc. Model: PMV0A-15501-900-5-0-0.8 26x18x8 FC/APC
X2) is
adapted to attenuate 1%-99% of a power of the tapped beam.
[0058] The secondary ring optical PM fiber cavity thus produced a self-
injected optical
feedback cavity that is weakly coupled to the laser cavity for tapping a
fraction of a power via
a backside of the OD CCL and reinserting it with an estimated power of 1V to
10-5 of that of
the tapped beam.
[0059] The schematic in FIG. 2c shows a spool 26 of a fixed length chosen
to provide an
OPL for the secondary cavity, however, the three lengths of PM-SMF extended
from the PM-
OC, along with the PM OC between the CCL and coupler 27 were sufficient to
produce the
11 m OPL required. Thus the spool 26 was distributed in this instance. The
secondary ring
PM fiber cavity had an OPL of a few thousand times longer than the -1.7 mm
cavity length
of the QD CCL was found to produce a very strong linewidth narrowing function
of an ultra-
narrow filter. In this case the laser power fed back via the secondary cavity
is used to
improve laser locking, bringing about significant narrowing of the laser
emitting spectrum.
[0060] After the CCL was coupled to the secondary cavity, it's power
characteristics
were altered, and the ultra-low noise driver was re-optimized for the new
laser
characteristics. In a manner known in the art the OPL of the laser cavity was
varied to
rematch a phase of the cavity for the secondary cavity feedback.
[0061] FIGs. 4A,B are both graphical comparisons of single line's phase
noise for an
optimized driven laser with and without the secondary cavity self-feedback.
FIG. 4A shows
the first line, which is around 1545 nm (see FIG. 3B) and already has a
narrowest linewidth
without secondary cavity self-feedback (labeled C#1 OLw). This noise profile
shows a
many-peaked, generally higher amplitude phase noise below 1 MHz, with a
general
decrease in amplitude as frequency increases, but after about 1 MHz, the phase
noise
amplitude flattens out. In contrast the phase noise of the same channel with
secondary
cavity self-feedback (labelled C#1 OLw/o) has a substantially worse noise
profile below 1
MHz, but an improved noise profile above 1 MHz that more than offsets the
losses below 1
MHz. It will be appreciated by those of ordinary skill, that low frequency
phase noise is
. 15
CA 3012938 2018-07-31
readily compensated by low noise drivers of current semiconductor lasers. It
therefore is
clear from this graphical comparison that the 1st channel will be improved by
a suitable
driver.
[0062] FIG. 4B shows again that for the 15th channel, that above 1 MHz, the
noise
amplitude of the line with the secondary cavity self-feedback (C#15 OLw/o) is
appreciably
lower for a typical line.
[0063] FIG. 5 is a graphical representation of linewidths of measured lines
of the CCL
with optimized drivers, showing lines without the secondary cavity self-
feedback having
linewidths of 0.9-4.5 MHz corresponding with lines with the secondary cavity
self-feedback
with linewidths of less than 200 kHz.
[0064] Table 1 lists measured channel numbers (C#), wavelength (in nm),
optical
linewidth without secondary cavity self-feedback (0Lw/o) in MHz, optical
linewidth with
secondary cavity self-feedback (0Lw), in MHz, and the reduction ratio (Ratio).
This is the
data graphed in FIG. 5. It clearly shows the improvement to mode-locking
produced by the
secondary cavity self-feedback which has its greatest effect for lowest line
numbers.
C# Wavelength 0 Lw/o OLw Ratio
1 1545.14 0.92 0.012 76.67
2 1544.94 0.93 0.013 71.54
3 1544.741 0.95 0.015 63.33
4 1544.54 0.97 0.017 57.06
1544.3395 1 0.024 41.67
6 1544.139 1.1 0.03 36.67
7 1543.94 1.16 0.035 33.14
8 1543.741 1.25 0.044 28.41
11 1543.142 1.52 0.056 27.14
1542.343 2 0.08 25.00
19 1541.55 2.34 0.095 24.63
23 1540.75 2.89 0.121 23.88
27 1539.951 3.21 0.136 23.60
31 1539.151 3.79 0.162 23.40
35 1538.352 4.22 0.182 23.19
39 1537.552 4.51 0.198 22.78
[0065] Linewidths of individual channels of the CCL as a function of
wavelength, for both
the original CCL, and the CCL with the secondary cavity show the remarkable
reduction in
linewidth, especially for higher wavelength lines. Reduction of the laser
linewidths is
dramatic: for example the line near 1538.5 nm shows a reduction factor of
about 35 (the
16
CA 3012938 2018-07-31
linewidth with secondary cavity is about 3% the linewidth without). All of the
lines from
1537.5-1545 originally had linewidths above about 0.9 MHz, are now well less
than 200kHz,
varying from about 1.2% to 4.4% of the original (without secondary cavity)
feedback.
[0066] Normalized RF beating signal spectra, with and without self-
injection feedback, is
further plotted in FIG. 6. The RF beating signal spectra particularly
illustrate the non-
common mode noise properties of the lines, in that any co-variation of the
lines are not
represented, as the lines beat against each other, (if they co-vary, this
variation is filtered
out). The RF beating signals show a substantially narrower peak, and lower
baseline (-
50 dB) with the self-injection feedback, as opposed to the original output of
the CCL, which
has a baseline of about -27 dB. The RF beating signals with small RF full
width at half
maximum (fwhm: -300 Hz), clearly shows that the phase fluctuations of their
longitudinal
modes are synchronized and correlated, as expected in a phase-mode-locked
laser.
[0067] While the foregoing improvements to linewidth are in line with prior
art linewidth
improvements using secondary cavity self-feedback, the present invention
provides that
these linewidths are highly stable. High speed measurement of phase noise
using
heterodyne detectors can show similar phase noise improvements, without
purporting to
provide stability of the linewidth.
[0068] In order to show the variability, FIG. 7 graphs 8 measurements using
homodyne
detection (OEWaves Inc. OE 4000 TM). Each measurement produced a scan as shown
in
FIG. 4A,B over a period of about 15 minutes. The same secondary cavity and CCL
were
used, except for the components being replaced with polarization maintaining
solid
waveguides (bottom plot identified with squares) vs. non-polarization
maintaining solid
waveguides (top plot triangles). As mentioned above, if a heterodyne or self-
homodyne
measurement is performed, a very small sample time is required, and a measured
linewidth
approaching the square plot can be obtained, however subsequent measurements,
even
within minutes, will not show the same value.
[0069] In general, the experiments were performed for both PM and non-PM
solid
waveguide secondary cavities with monitored feedback power to keep the same
intensity
(some measurements involved realigning the current unpackaged CCL prior to
measurement). The operation conditions are 330 mA and 20 C, as before. The
tested
individual channel's wavelength is 1540 nm.
[0070] The PM solid waveguide secondary cavity clearly shows both a far
lower
linewidth, and far less variation. The specific 8 data points provided for
both PM solid
17
CA 3012938 2018-07-31
waveguide secondary cavities show a range of 0.9-8.3 kHz difference between
measurements separated by 1 hour (start time to start time). The mean
difference between
measurements is 4.9kHz, and the standard deviation of the 8 values is 5.9 kHz.
The non-
PM data points show a range of 350-1395 kHz between successive measurements.
The
mean difference is 820 kHz, and the standard deviation of the 8 values is 570
kHz.
Accordingly, it is observed that using PM solid waveguide secondary cavity
self-feedback, a
linewidth of the laser cavity can be improved by at least 20%, and have a
stability such that
over one hour the linewidth does not vary by more than 100 kHz, more
preferably by more
than 80 kHz, more preferably by more than 40 kHz, 20 kHz, or about 5 kHz on
average.
[0071] Applicant notes that stability over larger periods have been
performed, and the
PM-SMF secondary cavity self-feedback has been shown to be very stable over
even longer
periods.
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[0073] Other
advantages that are inherent to the structure are obvious to one skilled in
the art. The embodiments are described herein illustratively and are not meant
to limit the
scope of the invention as claimed. Variations of the foregoing embodiments
will be evident
to a person of ordinary skill and are intended by the inventor to be
encompassed by the
following claims.
21
CA 3012938 2018-07-31
