U.S. patent application number 14/608022 was filed with the patent office on 2015-06-18 for ring cavity laser device.
This patent application is currently assigned to The Government of the United States of America, as represented by the Secretary of the Navy. The applicant listed for this patent is Marc Currie, Francis J. Kub. Invention is credited to Marc Currie, Francis J. Kub.
Application Number | 20150171588 14/608022 |
Document ID | / |
Family ID | 48280609 |
Filed Date | 2015-06-18 |
United States Patent
Application |
20150171588 |
Kind Code |
A1 |
Kub; Francis J. ; et
al. |
June 18, 2015 |
Ring Cavity Laser Device
Abstract
Laser devices are presented in which a graphene saturable
absorber and an optical amplifier are disposed in a resonant
optical cavity with an optical or electrical pump providing energy
to the optical amplifier.
Inventors: |
Kub; Francis J.; (Arnold,
MD) ; Currie; Marc; (Gaithersburg, MD) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kub; Francis J.
Currie; Marc |
Arnold
Gaithersburg |
MD
MD |
US
US |
|
|
Assignee: |
The Government of the United States
of America, as represented by the Secretary of the Navy
Washington
DC
|
Family ID: |
48280609 |
Appl. No.: |
14/608022 |
Filed: |
January 28, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13676652 |
Nov 14, 2012 |
9014221 |
|
|
14608022 |
|
|
|
|
Current U.S.
Class: |
372/11 ;
372/18 |
Current CPC
Class: |
H01S 3/1106 20130101;
H01S 3/081 20130101; H01S 3/091 20130101; H01S 3/1112 20130101;
H01S 5/0601 20130101; H01S 3/06 20130101; H01S 5/028 20130101; H01S
5/14 20130101; H01S 5/50 20130101; H01S 5/10 20130101; H01S 5/1838
20130101; H01S 3/1115 20130101; H01S 3/0906 20130101; H01S 3/10
20130101; H01S 3/113 20130101; H01S 3/1118 20130101; H01S 3/083
20130101; H01S 5/3401 20130101; H01S 5/1032 20130101; H01S 3/11
20130101; H01S 3/0815 20130101; H01S 3/0813 20130101; B82Y 20/00
20130101 |
International
Class: |
H01S 3/083 20060101
H01S003/083; H01S 3/09 20060101 H01S003/09; H01S 3/091 20060101
H01S003/091; H01S 3/11 20060101 H01S003/11; H01S 3/113 20060101
H01S003/113 |
Claims
1. A ring cavity laser device, comprising: a first mirror with a
first reflective surface; a second mirror with a second reflective
surface at least partially facing the first reflective surface of
the first mirror; a third mirror with a third reflective surface at
least partially facing the first reflective surface of the first
mirror and the second reflective surface of the second mirror, the
first, second and third mirrors defining an optical ring cavity to
establish a resonant optical cavity in which electromagnetic energy
is amplified by stimulated emission of coherent radiation that is
partially transmitted through the second mirror; an optical gain
medium disposed at least partially between the first and second
mirrors; an electrical or optical pump providing energy to the
optical gain medium; and a graphene saturable absorber disposed at
least partially between the second and third reflective
surfaces.
2. The laser device of claim 1, further comprising a laser disposed
on a second side of the first reflective surface and operative to
direct light at least partially toward the first reflective
surface.
3. The laser device of claim 2, wherein optical ring cavity is an
injection locked optical ring cavity and wherein the laser device
is a mode-locked laser or a Q-switched laser configured to emit
stimulated coherent radiation through the second mirror with center
wavelengths of about 1,800 nm or more and about 25,000 nm or
less.
4. The laser device of claim 1, wherein the laser device is a
mode-locked laser or a Q-switched laser configured to emit
stimulated coherent radiation through the second mirror with center
wavelengths of from about 280 nm to about 1,800 nm.
5. A linear cavity laser device, comprising: a first mirror with a
first surface; a second mirror with a second surface at least
partially facing the first surface of the first mirror, the first
and second mirrors defining a resonant optical cavity in which
electromagnetic energy is amplified by stimulated emission of
coherent radiation that is partially transmitted through the second
mirror; a solid-state optically pumped optical gain medium disposed
at least partially between the first and second surfaces; an
optical pump providing light to the solid-state optically pumped
optical gain medium; a graphene saturable absorber with single
layer graphene disposed at least partially between the solid-state
optically pumped optical gain medium and the first surface; and a
laser disposed on a second side of the first mirror and operative
to direct light at least partially toward the first mirror.
6. The linear cavity laser device of claim 5, wherein the
solid-state optically pumped optical gain medium is a rare earth
doped fiber amplifier operable at wavelengths of about 1.4 .mu.m or
more.
7. The linear cavity laser device of claim 5, wherein the
solid-state optically pumped optical gain medium is a solid state
optically pumped infrared optical gain medium.
8. A laser device, comprising: a first mirror with a first surface;
a second mirror with a second surface at least partially facing the
first surface of the first mirror, the first and second mirrors
defining a resonant optical cavity to establish a resonant optical
cavity in which electromagnetic energy is amplified by stimulated
emission of coherent radiation that is partially transmitted
through the second mirror; a fiber or bulk crystal optical
amplifier disposed at least partially between the first and second
surfaces; an electrical or optical pump providing energy to the
optical amplifier; a graphene saturable absorber disposed at least
partially between the first and second surfaces; and a laser
disposed on a second side of the first mirror and operative to
direct light at least partially toward the first mirror.
Description
REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to and the benefit of U.S.
Provisional Patent Application Ser. No. 61/559,308, filed Nov. 14,
2011, and entitled "Method for Implementing Infrared Laser", and
U.S. patent application Ser. No. 13/676,652, filed on Nov. 14,
2012, entitled Infrared Lasers, the entirety of each is hereby
incorporated by reference.
BACKGROUND
[0002] Laser devices provide output from an optical cavity when an
included gain medium overcomes the cavity losses so that
amplification via stimulated emission occurs. Lasers produce light
over some natural bandwidth or range of frequencies, sometimes
referred to as the gain bandwidth, which is largely determined by
the laser gain medium and the optical cavity or resonant cavity of
the laser. Saturable absorbers can be introduced into the optical
cavity to provide variability in the cavity loss, which is
typically nonlinear such that when a small number of photons are
present there is a larger cavity loss than when a large number of
photons are present. This allows Q-switching to occur if the
excited upper state lifetime of the gain medium is sufficiently
long to accumulate enough energy to overcome the cavity losses by
forming a pulse. Alternatively, if the gain bandwidth and laser
cavity simultaneously support a large optical bandwidth, a
saturable absorber can initiate mode-locking or phase-locking of
the laser by providing lower loss to a multiple longitudinal
optical modes with a specific phase relationship within the laser
resonant cavity. Mode-locked lasers can produce extremely short
duration light pulses, on the order of picoseconds or femtoseconds.
Moreover, passive, hybrid, or active mode-locking can be produced
in a laser. Active mode-locking can be accomplished using an
external signal to induce intra-cavity light modulation. Passive
mode-locking, in contrast, involves incorporation of a structural
element, such as a saturable absorber into the laser cavity to
cause self-modulation of the light. In this manner, passive
mode-locked lasers use the light in the cavity to cause a change in
some intra-cavity element, which will then itself produce a change
in the intra-cavity light. Saturable absorbers exhibit
intensity-dependent transmission or reflection, and thus behave
differently depending on the intensity of the light. For passive
mode-locking in a laser, an ideal saturable absorber selectively
absorbs low-intensity light, while transmitting or reflecting light
which is of sufficiently high intensity, thereby acting as an
optical gate. Thus far, implementing passive mode-locked or
Q-switched infrared lasers has been difficult, especially at
infrared wavelengths, and a need remains for improved laser devices
and implementation approaches, particularly for compact laser
device structures.
SUMMARY OF DISCLOSURE
[0003] Various details of the present disclosure are hereinafter
summarized to facilitate a basic understanding, where this summary
is not an extensive overview of the disclosure, and is intended
neither to identify certain elements of the disclosure, nor to
delineate the scope thereof. Rather, the primary purpose of this
summary is to present some concepts of the disclosure in a
simplified form prior to the more detailed description that is
presented hereinafter.
[0004] Linear cavity, external cavity, extended cavity, and ring
cavity laser devices and implementation approaches are provided in
which a fixed or position-adjustable graphene saturable absorber
structure, operating in the reflective or transmissive mode, is
situated in the laser optical cavity, and various embodiments
facilitate mode-locked or Q-switched operation at ultraviolet,
visible, and/or infrared wavelengths. In various implementations,
mode-locked lasers or Q-switched lasers employ semiconductor
optical amplifiers (SOA) as the gain medium in combination with
graphene saturable absorbers in a linear cavity or external cavity
arrangement. In various implementations, mode-locked lasers or
Q-switched lasers employ fiber or bulk crystal optical amplifiers
(OA) as the gain medium in combination with graphene saturable
absorbers in a linear cavity or external cavity arrangement. In
certain embodiments, the graphene saturable absorber is integrated
with the semiconductor optical amplifier or fiber or bulk crystal
optical amplifier. Certain embodiments, moreover, provide laser
implementations by integration of a graphene saturable absorber
into a vertical cavity surface emitting laser (VCSEL) structure,
integration of graphene saturable absorber into a vertical external
cavity surface emitting (VECSEL) structure, and the combination of
a graphene saturable absorber into an external cavity laser.
[0005] In accordance with one or more aspects of the present
disclosure, a laser device is provided which includes first and
second mirrors (flat or curved) with corresponding reflective
surfaces that face one another to define a resonant linear optical
cavity in which electromagnetic energy is amplified by stimulated
emission of coherent radiation that is partially transmitted
through the partially reflective second mirror (output coupler). A
semiconductor optical amplifier and a graphene saturable absorber
are disposed between the reflective surfaces and an electrical or
optical pump provides current and/or light to the semiconductor
optical amplifier. In certain embodiments, the device is a
Q-switched laser or a mode-locked laser operable at center
wavelengths of 1,800 nm or more and about 200 .mu.m (1.5 terahertz)
or less, and in the mode-locked case with a spectral width
(spectral bandwidth) of as much as 10% or more of the center
wavelength. In certain embodiments, the spectral widths can be more
than 20% of the center wavelength. In various embodiments,
moreover, a mode-locked or Q-switched laser device is provided
which is operable at center wavelengths of about 280 nm or more and
about 1,800 nm. Certain non-limiting embodiments provide a
semiconductor optical amplifier comprising a periodic structure or
chirped structure or graded structure of alternating superlattice
with a periodic series of semiconductor layers of at least two
different material compositions. Certain non-limiting embodiments
provide a semiconductor optical amplifier comprising a double
quantum well superlattice with two types of superlattice
structures.
[0006] In certain non-limiting embodiments, the graphene saturable
absorber is disposed between the reflective surface of the first
mirror and the semiconductor optical amplifier. In certain
non-limiting embodiments, the graphene saturable absorber is
integral with the first mirror and is between the reflective
surface of first mirror and the semiconductor optical amplifier. In
certain non-limiting embodiments, the graphene saturable absorber
is disposed between the semiconductor optical amplifier and the
reflective surface of the second mirror. In certain non-limiting
embodiments, the graphene saturable absorber is integral with the
second mirror and is between the semiconductor optical amplifiers
and reflective surface of the second mirror. The graphene saturable
absorber may be integral to the semiconductor optical amplifier in
certain embodiments, and the second mirror (the output coupler) may
be integral to the semiconductor optical amplifier or may be spaced
therefrom. In some embodiments, the second mirror (output coupler)
can be a partially reflective facet on a second end of the
semiconductor optical amplifier, or the second mirror may be a
partially reflective coating on the second end of the semiconductor
optical amplifier. In certain embodiments, the second mirror
(output coupler) is spaced from the semiconductor optical
amplifier, and may be a Bragg mirror. One or more optical
components, such as a gas cell may be longitudinally disposed
between the second end of the semiconductor optical amplifier and
the second mirror in certain non-limiting embodiments.
[0007] In some embodiments, moreover, the graphene saturable
absorber may be integral to the first mirror, where the first
mirror can be a highly reflective coating on the graphene saturable
absorber, or the first mirror can be a Bragg mirror (distributed
Bragg reflector) or a highly reflective coating on a substrate or a
metal substrate in certain non-limiting embodiments.
[0008] Certain embodiments of the laser device provide a graphene
saturable absorber which is spaced from the semiconductor optical
amplifier, and may include a gas cell, bandpass and other spectral
filters, an etalon, a microelectromechanical (MEM) controlled
etalon, polarizer, prisms, dispersive elements, lenses, or other
optical component or components between the first end of the
semiconductor optical amplifier and the graphene saturable
absorber. For example, the graphene saturable absorber may be
integral to the first mirror. In other embodiments, one or more
optical components may be longitudinally disposed between the
second end of the semiconductor optical amplifier and the second
mirror.
[0009] In certain embodiments, the graphene saturable absorber is
spaced from the first mirror, may be integral to the semiconductor
optical amplifier, may be between the semiconductor optical
amplifier and the second mirror (output coupler), or may be
integral to the second mirror. One or more optical components, such
as a gas cell may be located within the optical cavity and for
example, may be disposed between the graphene saturable absorber
and the first mirror.
[0010] In certain embodiments mirrors may not be discrete objects,
but still confine the light within a resonant cavity, e.g.
whispering-gallery waves, and the graphene saturable is integral
with the cavity by direct or evanescent coupling with the resonant
cavity.
[0011] In accordance with further aspects of the present
disclosure, a linear cavity laser device is provided, which
includes a first mirror with a first reflective surface, as well as
a second mirror with a second reflective surface facing the
reflective surface of the first mirror. The mirrors define a linear
(longitudinal) resonant optical cavity, with a semiconductor
optical amplifier disposed at least partially between the first and
second reflective surfaces and an electrical or optical pump
providing current or light to the semiconductor optical amplifier.
In addition, a graphene saturable absorber is integral with the
second mirror at least partially between the semiconductor optical
amplifier and the second reflective surface. In certain
embodiments, the graphene saturable absorber is spaced from the
semiconductor optical amplifier. The semiconductor optical
amplifier, moreover, can be a vertical cavity surface emitter laser
(VCSEL) in certain non-limiting embodiments. The semiconductor
optical amplifier in certain embodiments has first and second ends,
with the graphene saturable absorber bonded to the second end, and
the second mirror can be integral with the graphene saturable
absorber to provide a monolithic laser structure.
[0012] A linear cavity laser device is provided in accordance with
further aspects of the present disclosure, including first and
second mirrors with corresponding reflective surfaces, along with a
solid state optically pumped optical gain medium disposed at least
partially between the reflective surfaces and an optical pump
providing light to the solid-state optically pumped optical gain
medium. In certain embodiments, the solid-state optically pumped
optical gain medium can be an infrared (including near infrared or
mid infrared) solid-state optically pumped optical gain medium. A
graphene saturable absorber is disposed at least partially between
the solid-state optically pumped optical gain medium and the first
reflective surface. In certain embodiments, the optical gain medium
is a rare earth doped fiber amplifier operable at wavelengths of
about 1.4 .mu.m or more and about 3.0 .mu.m or less. In other
embodiments, the solid-state optical gain medium includes at least
one II-VI family doped crystal. In further embodiments, the
solid-state optical gain medium includes at least one transition
metal doped zinc chalcogenide, or may include at least one lead
salt. In certain embodiments, moreover, at least one optical
component, such as a gas cell, may be longitudinally disposed
between the solid-state optical gain medium and the second
mirror.
[0013] In accordance with further aspects of the disclosure, a
linear cavity laser device is provided, which includes first and
second partially reflective mirrors with corresponding reflective
surfaces that define a longitudinal injection locked optical
cavity, together with an electrically or optically pumped optical
gain medium between the reflective surfaces, and a graphene
saturable absorber disposed at least partially between the optical
gain medium and the first reflective surface. A laser is disposed
on a second side of the first partially reflective mirror to direct
light at least partially toward the first partially reflective
mirror. The linear cavity laser device may further include one or
more gas cells or other optical components longitudinally disposed
between the optical gain medium and the second mirror.
[0014] A ring cavity laser device is provided in accordance with
further aspects of the disclosure, including first and second
mirrors with corresponding reflective surfaces, as well as a third
mirror with a third reflective surface, where the mirrors define a
resonant optical cavity in which electromagnetic energy is
amplified by stimulated emission of coherent radiation that is
partially transmitted through the second mirror. An electrically or
optically pumped optical gain medium is disposed at least partially
between the first and second mirrors, and a graphene saturable
absorber is disposed at least partially between the second and
third reflective surfaces.
[0015] In various implementations, linear and/or ring cavities can
be employed with more than two mirrors, for instance, to fold the
cavity. Also, other optical elements (e.g., prisms, polarizers,
dispersive elements, bandpass and other spectral filters, etc.) can
be placed within the laser cavity.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The following description and drawings set forth certain
illustrative implementations of the disclosure in detail, which are
indicative of several exemplary ways in which the various
principles of the disclosure may be carried out. The illustrated
examples, however, are not exhaustive of the many possible
embodiments of the disclosure. Other objects, advantages and novel
features of the disclosure will be set forth in the following
detailed description when considered in conjunction with the
drawings, in which:
[0017] FIG. 1 is a partial sectional side elevation view
illustrating a linear cavity laser device with a graphene saturable
absorber and reflective mirror attached to a first facet end of a
semiconductor optical amplifier;
[0018] FIG. 2A is a partial sectional side elevation view
illustrating a P-N quantum cascade superlattice semiconductor
optical amplifier;
[0019] FIG. 2B is a partial sectional side elevation view
illustrating an N-N quantum cascade superlattice semiconductor
optical amplifier;
[0020] FIG. 3 is a partial sectional side elevation view
illustrating a linear cavity laser device with a graphene saturable
absorber and reflective mirror attached to a first facet end of a
semiconductor optical amplifier, as well as a partially reflective
mirror spaced from the second facet end of the semiconductor
optical amplifier;
[0021] FIG. 4 is a partial sectional side elevation view
illustrating a linear cavity laser device with a graphene saturable
absorber and a gas cell or other optical component disposed between
the second end of the semiconductor optical amplifier and the
partially reflective mirror;
[0022] FIG. 5 is a partial sectional side elevation view
illustrating a linear cavity laser device with a graphene saturable
absorber integral with a highly reflective first mirror and
longitudinally spaced from the semiconductor optical amplifier, as
well as a partially reflective mirror integral with the second end
of the semiconductor optical amplifier;
[0023] FIG. 6 is a partial sectional side elevation view
illustrating a linear cavity laser device with a graphene saturable
absorber integral with a highly reflective first mirror and a gas
cell or other optical component disposed between the graphene
saturable absorber and the semiconductor optical amplifier, with a
partially reflective mirror integral with the second end of the
semiconductor optical amplifier;
[0024] FIG. 7 is a partial sectional side elevation view
illustrating a linear cavity laser device with a graphene saturable
absorber integral with a highly reflective first mirror and
longitudinally spaced from the semiconductor optical amplifier,
with a separate partially reflective mirror facing the second end
of the semiconductor optical amplifier;
[0025] FIG. 8 is a partial sectional side elevation view
illustrating a linear cavity laser device with a graphene saturable
absorber integral with a highly reflective first mirror and an
optical component disposed between the graphene saturable absorber
and the semiconductor optical amplifier, including a separate
partially reflective mirror facing the second end of the
semiconductor optical amplifier;
[0026] FIG. 9 is a partial sectional side elevation view
illustrating a linear cavity laser device with a graphene saturable
absorber and a gas cell or other optical component located between
the second end of the semiconductor optical amplifier and the
partially reflective second mirror;
[0027] FIG. 10 is a partial sectional side elevation view
illustrating a linear cavity laser device with a graphene saturable
absorber deposited or bonded onto a first facet end of the
semiconductor optical amplifier, as well as an optical component
disposed between the graphene saturable absorber and the highly
reflective first mirror;
[0028] FIG. 11 is a partial sectional side elevation view
illustrating a linear cavity laser device including a graphene
saturable absorber and an integrated partially reflective second
mirror;
[0029] FIG. 12 is a partial sectional side elevation view
illustrating a linear cavity laser device with a graphene saturable
absorber bonded to the second facet end of the semiconductor
optical amplifier and having an integrated partially reflective
second mirror;
[0030] FIG. 13 is a partial sectional side elevation view
illustrating a linear cavity laser device with a graphene saturable
absorber and a solid state optically pumped infrared optical
amplifier or gain medium;
[0031] FIG. 14 is a partial sectional side elevation view
illustrating a linear cavity laser device with a graphene saturable
absorber and two partially reflective mirrors forming an injection
locked (coupled) optical cavity with a laser to direct light at
least partially toward a second side of the first partially
reflective mirror;
[0032] FIG. 15 is a partial sectional side elevation view
illustrating a ring cavity laser device with two highly reflective
mirrors and a partially reflective mirror, including an optical
amplifier or gain medium disposed between a first one of the highly
reflective mirrors and the partially reflective mirror, and a
graphene saturable absorber disposed at least partially between the
second highly reflective mirror and the partially reflective
mirror;
[0033] FIG. 16 is a partial sectional side elevation view
illustrating a linear cavity laser device with four mirrors and two
optical elements, in which at least one of the optical elements is
a graphene saturable absorber;
[0034] FIG. 17 is a partial sectional side elevation view
illustrating a ring cavity laser device with five mirrors and four
optical elements, with at least one of the optical elements being a
graphene saturable absorber; and
[0035] FIGS. 18-20 are partial side elevation views illustrating
further details of vertical cavity embodiments.
DETAILED DESCRIPTION OF THE DISCLOSURE
[0036] One or more embodiments or implementations are set forth in
conjunction with the drawings, where like reference numerals refer
to like elements throughout, and where the various features are not
necessarily drawn to scale.
[0037] Semiconductor optical amplifiers for ultraviolet, visible,
and near infrared wavelengths typically have an interband design in
which photons are emitted by the recombination of electron-hole
pairs across the material bandgap. Semiconductor optical amplifiers
for infrared wavelength can have interband cascade semiconductor
optical amplifier design or quantum cascade semiconductor amplifier
design. The interband cascade semiconductor optical amplifier laser
typically operate in the wavelength range of 3-5 .mu.m and quantum
cascade semiconductor optical amplifier typically operate in the
wavelength range of in the 5-18 .mu.m but terahertz lasers to 200
micron wavelength have been experimentally demonstrated. The
semiconductor optical amplifier designs typically have
antireflective coatings on their facets and also can have tilted
facets to reduce the refection of light back into the semiconductor
optical amplifier.
[0038] Single mode or multimode laser devices are disclosed herein
which include a graphene saturable absorber and at least two
mirrors, wherein flat mirrors may be used, but alignment to a
required precision may be difficult. Transmissive saturable
absorbers may be employed, and extended cavities may be provided.
One or more mirror positions may be adjustable to facilitate
mode-locking, and quantum cascade laser (QCL) master-oscillator
power-amplifiers (MOPAs) may be included. In certain embodiments,
moreover, polarization optics, and dispersive optics, spectral
filters or other additional optical elements may be provided within
the laser cavity, for instance, a MEMS controlled bandpass filter
within the cavity. An output coupler may be provided, such as a one
dimensional distributed feedback (1D-DFB), photonic crystal
distributed feedback (PCDFB), or a ring cavity surface emitting
(RCSE) QCLs. Substrate emitting DFB QCL configurations are
contemplated in which the functions of the distributed feedback and
the surface emission are separated. Certain embodiments provide
substrate emitting quantum cascade ring lasers with high power
single transverse mode QCLs and applications of plasmonics to
achieving high beam quality (low divergence, multibeam operation
etc.). Two section devices are contemplated where one section acts
as the pumping section (referred to as master oscillator (MO)
section) and the other section serves as a power amplifier (PA).
The MO section features a DFB grating to ensure single-mode
emission. Moreover, single mode quantum cascade lasers and
plasmonic collimators are contemplated.
[0039] Mid-IR mode-locked laser may be fabricated from an interband
cascade laser (ICL) in the 3-5 .mu.m range and quantum cascade
lasers (QCL) are provided in the 5-12 .mu.m region. Short pulsed
operation of these lasers previously has not been achieved due to
the poor energy storage capabilities of the gain media. While this
hinders pulsed operation of these materials via Q-switching, it
does not affect the mode-locked operation. Producing a burst of
photons by switching the gain or loss in a laser cavity requires
storing energy in the laser's excited state and then suddenly
modulating this gain/loss within the cavity to produce a pulse.
This, however, is not the only way to produce optical pulses.
Mode-locking a laser produces optical pulses by "locking" the phase
of a large number of longitudinal modes within the laser, similar
in concept to phasing a series of harmonic sine waves to produce a
step. This theory exploits the operation of many continuous wave
(cw) optical modes in the cavity by locking their phases, thereby
creating an optical pulse without energy storage.
[0040] Referring initially to FIGS. 1-4, various non-limiting laser
device embodiments are hereinafter illustrated and described, in
which a graphene saturable absorber 130 is provided in the laser
optical cavity or resonator. The type of linear cavity in certain
embodiments may be a "Fabry-Perot" optical cavity. Graphene is
substantially an sp2 bonded carbon layer which forms in a
two-dimensional honeycomb crystal lattice layer or sheet with a
carbon-carbon bond length of about 0.142 nm. Multiple stacked
sheets may form with an interplanar spacing of about 0.335 nm, and
may be stacked in certain arrangements such as A-B (Bernal)
ordering. One exemplary linear cavity laser device 100 with a first
end 101 and a second end 102 is shown in FIG. 1, including a first
highly reflective mirror 110 with a first reflective surface 112,
and a second partially reflective mirror 120 with a second
reflective surface 122 at least partially facing the first
reflective surface 112 of the first mirror 110. The mirrors in
various embodiments can be flat or curved. The mirrors 110 and 120
define a laser optical resonator or optical cavity with a cavity
length 103 extending along a longitudinal axis 104 in which
electromagnetic energy is amplified by stimulated emission of
coherent radiation that is partially transmitted through the second
mirror 120. Within the optical cavity is a semiconductor optical
amplifier (SOA) 140 disposed at least partially between the first
and second reflective surfaces 112 and 122, which is provided or
"pumped" with energy (e.g., electrical current or light) 152 via an
electrical or optical pump 150. The semiconductor optical amplifier
in certain embodiments may have anti-reflection coatings on the
facets and can have titled facets to reduce the stimulated emission
light back into semiconductor optical amplifier. In addition, a
graphene saturable absorber 130 is disposed in the optical cavity
at least partially between the mirrors 110 and 120. In certain
embodiments, the graphene saturable absorber structure 130 can be
an antiresonant design, or a resonant design, or a no resonant
design.
[0041] Any type of semiconductor optical amplifier 140 can be used.
As seen in FIGS. 2A and 2B, certain implementations of the SOA 140
include a superlattice with a periodic series of semiconductor
layers of at least two different material compositions in a
waveguide configuration. In the example of FIG. 2A, the SOA 140
extends between first and second facets or ends 141 and 142,
respectively, and includes a P-N quantum cascade superlattice
configuration, with an uppermost layer of a first semiconductor
type (SEMICONDUCTOR 1) being heavily doped with a P type dopant
(P+) and a lower most semiconductor layer of a second type
(SEMICONDUCTOR 2) being N-doped (N+). The facets 141, 142 can be
coated for antiflectivity and can be titled. In the case that the
graphene saturable absorber 130 is bonded to the first end of the
SOA 140, the facet is generally not titled in certain nonlimiting
embodiments. The facet 141 and/or 142 can be titled for the case
that the graphene saturable absorber 130 is bonded to the second
end of the SOA 140. Ohmic contacts 144 are provided at the top and
bottom of the SOA superlattice structure 140 in the illustrated
embodiment, for connection to an electrical pump 150 and an
electrical reference or ground by which the SOA 140 is pumped with
electrical current 152 in certain implementations. In other
embodiments, the SOA 140 can be optically pumped via an optical
pump 150 providing light 152 to the SOA structure 140. FIG. 2B
shows another embodiment of the SOA 140, in which the uppermost and
lower most semiconductor layers of the superlattice structure are
both doped with dopants of the same conductivity type (N+). Quantum
Cascade Laser embodiments can be implemented as a waveguide on a
semiconductor.
[0042] In the example of FIG. 1 (as well as the examples of FIGS.
3-10), the graphene saturable absorber 130 is longitudinally
disposed between the first reflective surface 112 and the SOA 140.
In addition, the graphene saturable absorber 130 in FIG. 1 is
integral with the first end or facet 141 of the SOA 140, and the
second mirror 120 is integral with the second SOA end or facet 142.
In certain non-limiting implementations, the second (partially
reflective) mirror 120 can be a partially reflective facet on the
second end 142 of the SOA 140. In other possible implementations,
the second mirror 120 is formed as a partially reflective coating
on the second end 142 of the SOA 140. In practice, the laser device
100 can be a Q-switched laser or a mode-locked laser configured to
emit stimulated coherent radiation with center wavelengths of about
1,800 nm or more and about 25,000 nm or less and having a spectral
width as much as 10% or more of the center wavelength through the
second mirror 120. Mode-locked laser devices 100 have been created
with 0.006% spectral widths to 15% spectral widths. In certain
embodiments, the spectral width can be greater than 0.5%, and as
much as 10% or more. In further nonlimiting embodiments, the
spectral width is greater than 2%. In certain embodiments, the
graphene saturable absorber can be bonded to the first end of the
SOA. In other possible implementations, the laser 100 is a
Q-switched or mode-locked device, operable to emit stimulated
coherent radiation with center wavelengths of about 280 nm or more
and about 1,800 nm or less and having a spectral width as much as
10% or more of the center wavelength through the second mirror
120.
[0043] Passive mode-locked or Q-switched laser operation involves
an optical gain media 140, 170, 180 that operates at the operation
wavelength, as well as an optical cavity for feedback, and a
saturable absorber 130 that is effective at the operational
wavelength. Various non-limiting embodiments are illustrated and
described below in connection with FIGS. 3-15, each employing
graphene saturable absorber 130 in combination with an optical gain
medium such as an SOA 140 (FIGS. 1-10), a semiconductor optical
amplifier or vertical cavity surface emitter laser (VCSEL) (FIGS.
11 and 12) or other optical amplifier medium (FIGS. 13-15).
[0044] Graphene saturable absorbers 130 facilitate broadband
operation through gapless linear dispersion of Dirac electrons. The
optical absorption, ultrafast carrier relaxation time, and
controllable modulation depth of graphene have been actively
measured, and the thermal conductivity of monolayer graphene was
measured to be approximately 5000 Wm.sup.-1 K.sup.-1 at room
temperature, which is the highest value ever found among any
natural material. In addition, several graphene mode-locked fiber
lasers have been demonstrated at wavelengths near 1000 nm and 1550
nm. Wide-range wavelength tunability was also realized in both
mode-locked and Q-switched fiber lasers. In particular, the use of
graphene saturable absorbers 130 for mode-locked or Q-switched
lasers 100 facilitates operation at infrared wavelengths. One
passive approach to make a mode-locked laser 100 is to combine a
saturable absorber with optical gain medium in an external cavity.
However, conventional semiconductor saturable absorbers do not
operate in the infrared wavelengths. The inventors have appreciated
the graphene performs well as a saturable absorber 130 for all
infrared wavelengths, and thus a mode-locked or Q-switched laser
100 can be implemented at infrared wavelengths by combining the
graphene saturable absorber 130 with an optical gain medium (e.g.,
SOA 140 in this embodiment). Graphene's performance as saturable
absorber is different from conventional semiconductor saturable
absorbers in that conventional semiconductor saturable absorbers
are engineered for each individual wavelength of operation, whereas
a graphene saturable absorber does not need any additional
engineering and operates in a similar fashion at all wavelengths
from the visible through the infrared.
[0045] The combination of the SOA 140 with the graphene saturable
absorber 130 in a longitudinal or linear cavity arrangement also
facilitates mode-locking or Q-switching operation of the laser 100.
The use of an SOA 140 as the gain medium for infrared wavelength
advantageously facilitates provision of optical gain at wavelengths
ranging from approximately 230 nm to 14,000 nm by appropriate
design of the quantum well structure in certain embodiments of an
SOA 140. Other optical gain mediums such as rare earth doped fiber
amplifiers are not readily available for operation at wavelengths
longer than approximately 5,000 nm, but a quantum cascade
semiconductor optical amplifier (QCSOA) 140 operates as an optical
gain medium from approximately near infrared to far infrared. In
addition, the SOA 140 can be combined with the graphene saturable
absorber 130 in an external cavity arrangement to implement a
compact mode-locked or Q-switched laser 100, wherein compact
monolithic designs are also possible (e.g., FIGS. 1 and 12). In
certain embodiments, for example, the graphene saturable absorber
130 can be integrated with the SOA 140 to make an especially
compact laser 100. In addition, the SOA 140 can be electrically
pumped in certain embodiments to facilitate creation of a compact
laser device 100. Other embodiments (e.g., FIGS. 11 and 12)
integrate the graphene saturable absorber 130 with a VCSEL
structure 170 to likewise facilitate implementation of a compact
laser structure. Moreover, other optical gain mediums can be used,
such as a rare earth doped fiber optical amplifier in combination
with a graphene saturable absorber 130 to implement infrared
mode-locked or Q-switched laser device 100. In addition, while
primarily illustrated in connection with electrically pumped SOAs
140 and other optical gain mediums 170, 180, graphene saturable
absorbers 130 can also be used in various embodiments in
combination with optically pumped SOAs 140 and VCSELs 170. In
certain embodiments, the laser 100 can be mode-locked or Q-switched
for wavelengths within a wavelength band from 2.0 .mu.m to 12
.mu.m.
[0046] The optical gain medium (whether an SOA 140, a semiconductor
diode laser or vertical cavity semiconductor optical amplifier
VCSOA 170, or other optical amplifier 180) can in certain
embodiments be pumped by a second light source, such as another
laser (e.g., 190 in FIG. 14 below). For example, infrared light can
be amplified in the optical gain medium 140, 170, 180 using an
electrically driven solid-state laser. Alternatively, the gain
media can be an optically pumped fiber, an optically pumped
crystal, an optically pumped semiconductor optical amplifier in
various embodiments. A mode-locked laser can be implemented in a
variety of laser arrangements, for example a linear cavity (e.g.,
FIGS. 1-13), a ring cavity (e.g., FIG. 15), and an extended
semiconductor laser cavity (e.g., FIG. 14), including the optical
gain medium within the optical cavity. In each of these
arrangements, the graphene saturable absorber 130 can be used in
transmission or reflection mode. The graphene saturable absorber
130, due to its atomic layer thickness and its ability to be grown
on one substrate then transferred to different substrates, can be
integral with one of the mirrors 110, 120 in certain embodiments
for double pass use in reflection.
[0047] One factor in implementing a Q-switched laser device 100 is
the storage time for optically excited carriers in the optical
amplifier medium 140, 170, 180. In particular, if the optical
amplifier storage time is short it is difficult to make a
Q-switched laser, however, it may be possible to make a mode-locked
laser. Semiconductor optical amplifiers designed for infrared
wavelengths such as Quantum Cascade SOA devices 140 or intersubband
SOAs 140 typically only have a short storage time which would allow
mode-locked laser operation but may not facilitate Q-switched laser
operation. Semiconductor optical amplifiers 140 designed for
wavelengths less than 1,700 nm can have a storage time that is
sufficiently long to enable Q-switched operation. Mode-locked
operation is also possible.
[0048] The embodiments of FIGS. 1-3 provide an integrated graphene
saturable absorber 130 and the SOA 140 in an optical cavity, where
the cavity includes a highly reflective mirror 110 on a first side
101, a graphene saturable absorber 130 between the highly
reflective mirror 110 and a first facet end 141 of the SOA 140 with
the graphene saturable absorber 130 preferably monolithically
integrated with the first facet 141. An electrically (or optionally
optically) pumped SOA 140 optical gain medium is operable at the
selected wavelength and can also have narrow band or broad band
operation, and a partially reflective mirror 120 can be formed
either on the second facet surface 142 of the SOA 140, or an
optional partially reflective mirror 120 can be spaced from the
second facet 142 (FIG. 3) that performs as the output coupler of
the laser device 100 on the second side 102.
[0049] The graphene saturable absorber 130 can be constructed so as
to advantageously provide optical absorption over a wide range of
wavelengths from ultraviolet (UV) to terahertz wavelengths, along
with a high damage threshold and tunable response set in certain
embodiments by selecting the number of graphene layers used in
forming the saturable absorber 130. In this regard, graphene films
may be formed as sheets as thin as one atom thick, and the
saturable absorption response can be tuned/modified by increasing
the number of layers of graphene. Graphene has a high optical
damage threshold, which can be particularly advantageous for
Q-switched and mode-locked lasers 100 that produce peak irradiances
greater than 10 GW/cm.sup.2. Moreover, graphene material 130 has a
very high thermal conductivity, which is useful for removing
absorbed heat in both the saturated and unsaturated cases.
[0050] In practice, the graphene saturable absorber 130 can operate
in either a transmission mode or a reflection mode. In certain
embodiments, the graphene saturable absorber structure can be an
antiresonant or resonant or no resonant design. A graphene
saturable absorber 130 designed for the transmission mode can
optionally be combined with a partially reflective mirror 120 in
certain embodiments. One design for a partially reflective second
mirror 120 may include a Bragg mirror (also referred to as a
dielectric mirror or distributed Bragg reflector) which can be any
structure having an alternating sequence of layers of two or more
different optical materials, typically dielectric or semiconductor
layers, designed for specified reflectivity at different
wavelengths of light. A dielectric distributed Bragg reflector
typically has alternating layers of dielectric materials and a
semiconductor distributed Bragg reflector typically has alternating
layers of semiconductor materials. In other embodiments, a thin
film coating or coatings on a substrate can also be used to
implement the partially reflective mirror 120, which may be
integral with the second end 142 of the SOA 140 in certain
implementations. The Bragg Reflector (or Distributed Bragg
Reflector) may have reflectivity from a "periodic structure" and
not strictly reflection from a first reflective surface.
[0051] In one non-limiting example, the graphene saturable absorber
130 can include graphene material epitaxially grown on a silicon
face of a silicon carbide (SiC) substrate. The graphene material
typically grows approximately one or several graphene sheet thick
on the on-axis silicon-face of SiC. A Bragg material design or thin
film deposited on the SiC material or bonded to the SiC can
implement a partially or highly reflective mirror 110, where the
example of FIG. 1 may be implemented as a highly reflective first
mirror 110 integrated with the graphene saturable absorber 130. In
other embodiments (e.g., FIGS. 11 and 12 below), the graphene
saturable absorber 130 can be integrated with the second end 142 of
the optical gain medium 140, 170, 180 and/or with the partially
reflective second mirror 120 by a variety of fabrication
techniques.
[0052] In other non-limiting embodiments, the graphene material 130
can be epitaxially grown on the carbon face of a SiC substrate,
where such graphene material typically grows 10 graphene sheets to
40 graphene sheets thick. A partially or highly reflective mirror
110, 120 can be implemented as discussed above for such
embodiments.
[0053] In other implementations, the graphene saturable absorber
material 130 can be grown on a metal material and can then be
transferred and bonded to a transparent substrate such as diamond
substrate, sapphire substrate, quartz substrate, silicon substrate,
SiC substrate, etc. A Bragg mirror material design or thin film can
be deposited on the SiC material or bonded to the SiC in order to
implement a partially reflective or highly reflective mirror 110,
120.
[0054] In further non-limiting embodiments, graphene flakes,
graphene nanoplatelets, graphene nanosheets, graphene oxide flakes,
graphene oxide nanoplatelets, graphene oxide nanosheets, graphene
material, or fluorographene material, etc can be deposited on a
transparent substrate such as a diamond substrate, a sapphire
substrate, a quartz substrate, a silicon substrate, a partially
reflective mirror substrate, reflective mirror substrate, a
substrate with a Bragg mirror, SOA first end, SOA second end, fiber
optical amplifier first end, fiber optical amplifiers second end,
bulk crystal optical amplifier first end, bulk crystal optical
amplifier second end, etc., to provide the saturable absorber 130.
A Bragg material design or thin film deposited on the SiC material
or bonded to the SiC can implement a partially reflective or highly
reflective mirror 110, 120.
[0055] Still other possible non-limiting embodiments of the
graphene saturable absorber 130 can be created by incorporating the
graphene material (including graphene flakes, graphene
nanoplatelets, graphene nanosheets, graphene oxide flakes, graphene
oxide nanoplatelets, graphene oxide nanosheets, graphene material,
fluorographene material, etc.) into a polymer and then depositing
or coating these on a transparent substrate or partially or highly
reflective mirror 110, 120. A Bragg material design or thin film
deposited on the SiC material or bonded to the SiC can implement a
partially or highly reflective mirror 110, 120. In this regard,
diamond, boron nitride and SiC substrates advantageously provide
high thermal conductivity, and are thus attractive for applications
that would require high power absorption in the graphene saturable
absorber 130. The silicon substrate, moreover, can be advantageous
for infrared and terahertz application because of good transmission
properties.
[0056] Using these and other fabrication techniques, the graphene
saturable absorber 130 can be designed to operate in the reflection
mode by growing, transferring, bonding, depositing graphene or
graphene oxide material on a substrate or on a first facet 141 of a
SOA 140 or by depositing a highly reflective coating on the
graphene material 130. One design for a highly reflective mirror
110 is to use a Bragg material design mirror with a selected number
of pairs. Thin film coating can also be used to implement highly
reflective mirrors. In addition, a partially reflective mirror
(e.g., the second mirror 120 in FIG. 1) can be implemented using
the above-described or other techniques.
[0057] Referring now to FIGS. 2A and 2B, the semiconductor optical
amplifier (SOA) in certain embodiments is preferably electrically
pumped but can be optically pumped via a pump 150 providing current
or light 152 to the SOA structure 140 (e.g., FIG. 1). The SOA 140
provides the optical gain medium that operates at the selected
optical wavelength. The SOA 140 can have narrow band or broadband
operation. For example, broadband operation is preferred for
operation of the optical cavity for frequency comb spectroscopy for
molecular sensing applications. The SOA 140 in certain embodiments
may be a direct recombination SOA, quantum cascade SOA, an
intersubband SOA, ridge waveguide SOA designs, a vertical cavity
surface emitting SOA, and/or other SOA known to those skilled in
the art. A quantum cascade or intersubband SOA design 140 can
advantageously generate optical amplified light at infrared
wavelengths between approximately 3 .mu.m and approximately 24
.mu.m wavelengths, and can generate terahertz wavelengths for
special quantum cascade SOA designs. A direct recombination SOA 140
in certain embodiments can operate between approximately 240 nm and
approximately 3 .mu.m wavelengths. Lead salt material can also be
used as semiconductor optical amplifier material.
[0058] In one non-limiting implementation, the highly reflective
mirror 110 and the graphene saturable absorber 130 can be integral
with the first facet surface 141 of the SOA 140. In certain
embodiments, the first facet 141 of the SOA 140 can be cleaved
and/or coated for maximum transmission. The light from the first
facet 141 is coupled out of the SOA 140. A highly reflective mirror
110 on a graphene material 130 can be used in certain embodiments
to reflect the light back into the SOA 140, with the graphene
material 130 operating as a saturable absorber. The graphene
saturable absorber 130 can be attached, bonded or deposited onto
the SOA first facet or end 141. There are several approaches by
which the graphene saturable absorber 130 can be integrated with
the SOA 140. One approach is to grow the graphene material 130 on a
metal surface such as copper or nickel (not shown). The front
surface of the graphene material 130 can be attached (bonded,
glued, etc.) to the first facet 141 of the SOA 140. The metal
material can then be optionally etched away leaving the graphene
material 130 on the SOA first facet 141. A highly reflective mirror
material 110 can then be deposited on the opposite side of the
graphene material 130. One option for implementing the highly
reflective material 110 on the graphene material 130 is to not etch
away the metal material as discussed above, and simply use the
metal as the reflective material 110. The graphene material 130 can
also be deposited on the SOA facet 141 as graphene material flakes
or graphene nanoplatelets. The graphene material 130 can also be
incorporated into a polymer and then deposited or coated on the SOA
facet 141. There is a minimal or no separation between the graphene
material surface 130 and the SOA facet surface 141 in certain
embodiments.
[0059] In the embodiment of FIG. 1, the second facet 142 or end of
the SOA 140 is cleaved and/or coated to provide the output coupler
for the laser device 100, thus constituting the partially
reflective second mirror 120 with a partially reflective surface
122. As seen in FIG. 3, a separate partially reflective mirror 120
can be used instead of relying on the second facet 142 to implement
a partially reflective mirror of the resonant cavity. The linear
cavity laser in the illustrated embodiments can optionally include
a region for sensing within the optical cavity, for example, for
insertion of gas samples to be measured to determine the gas
molecules within the region for sensing. The linear cavity laser
100 can be operated in certain embodiments in an optical frequency
spectroscopy mode (e.g. frequency-comb spectroscopy) to permit
enhanced sensing of the gas molecules within such region for
sensing.
[0060] The disclosed laser devices 100 (and 200 in FIG. 15) in
various embodiments provide certain advantages by use of a graphene
saturable absorber 130, particularly for mode-locked or Q-switched
operation. For example, the disclosed devices 100, 200 facilitate
mode-locked operation at infrared wavelengths with center
wavelengths in the range of 1,800 nm to 25,000 nm, with a spectral
width as much as 10% or more of the center wavelength. In addition,
certain non-limiting embodiments facilitate compact Q-switched or
mode-locked laser structures operable with center wavelengths in
the range of 280 nm to 1,800 nm and having a spectral width as much
as 10% or more of the center wavelength. Various embodiments
facilitate implementation of a high repetition, femtosecond laser
at infrared wavelengths. Moreover, certain embodiments facilitate
implementation of Frequency Comb Spectroscopy (which is a highly
sensitive technique for performing molecular sensing) at infrared
wavelengths, with comb frequency spread over a >1000 nm spectral
width. Also, the disclosed devices 100, 200 can provide a compact
laser by combining a graphene saturable absorber 130 with a
semiconductor optical amplifier 140 in an external cavity. In this
regard, the disclosed laser devices 100, 200 can be implemented as
a compact laser by integrating a graphene saturable absorber 130
with a semiconductor optical amplifier 140 in an external cavity.
Certain embodiments, moreover, provide a compact laser 100, 200 by
integrating a graphene saturable absorber into a VCSEL structure
(e.g., FIGS. 11 and 12 below).
[0061] FIG. 4 illustrates another embodiment in which the second
facet 142 of the SOA 140 can be cleaved and/or coated for maximum
transmission, and a partially reflective mirror 120 performs as the
output coupler of the laser 100. In addition, an optional gas cell
or other optical component 160 can be located between the second
facet 142 of the SOA 140 and the partially reflective mirror 120.
For operation at infrared wavelengths longer than approximately
1,700 nm, one particularly advantageous optical cavity arrangement
is as a linear optical cavity, particularly for situations in which
optical isolators are not available to implement a ring cavity
design. The optical cavity arrangement is also appropriate for
lasers 100 that operate at center wavelengths less than 1,700
nm.
[0062] Referring also to FIGS. 5-9, certain embodiments may employ
the graphene saturable absorber 130 spaced from the first end 141
of the SOA 140, where some embodiments (e.g., FIGS. 6 and 8) may
also provide one or more optical components 160, such as a gas cell
in certain implementations, disposed between the graphene saturable
absorber 130 and the first SOA facet end 141.
[0063] Some embodiments, moreover, may employ one or more optical
components 160 (e.g., a gas cell in certain implementations)
disposed at least partially between the second SOA facet end 142
and the second mirror 120, as seen in FIG. 9. In certain
implementations, moreover, the graphene saturable absorber 130 may
be integral with the first mirror 110. As seen in FIG. 5, for
example, this provides a structure including a graphene saturable
absorber 130 in combination with an SOA 140 in an extended optical
cavity of the device 100. As seen in FIGS. 5-9, moreover, the
optical cavity includes a highly reflective mirror 110 on a first
side 101, a graphene saturable absorber 130 between the highly
reflective mirror 110 and a first facet 141 of the SOA 140 (in one
arrangement, the graphene saturable absorber 130 can be integrated
with the mirror 110).
[0064] In addition, as seen in FIGS. 6 and 8, an optional gas cell
or other optical component 160 can be disposed at least partially
between the highly reflective mirror 110 and the SOA first facet
141. The SOA 140 can be electrically or optically pumped, and can
operate at the selected wavelength for narrow band or broadband
operation. In addition, a partially reflective mirror 120 can be
formed either on the SOA second facet surface 142 or as a separate
partially reflective mirror structure 120 separated from the second
facet 142 that operates as the output coupler of the resulting
laser device 100 on the second side of the optical cavity 102.
[0065] FIG. 10 illustrates another non-limiting embodiment of the
laser device 100, in which the graphene saturable absorber material
130 is integrated with the first facet 141 of the SOA 140. In
certain implementations, at least one optical component 160 is
longitudinally disposed between the graphene saturable absorber 130
and the first mirror 110 as seen in FIG. 10, and other
implementations are possible in which no optical component 160 is
provided in the device 100. In certain embodiments, moreover, the
graphene material 130 can be integrated with the SOA 140, for
example, with graphene being deposited or bonded to the first facet
surface 141 in certain implementations.
[0066] Referring also to FIGS. 11, 12 and 18-20, FIG. 11
illustrates a linear cavity (external cavity) laser device 100
including a graphene saturable absorber 130 and an integrated
partially reflective second mirror 120, where the graphene
saturable absorber 130 may be spaced from a second facet end 172 of
the optical gain medium 170 as shown in FIG. 11, or may instead be
integrated with the optical gain medium 170 as shown in FIG. 12. As
with the above embodiments, the first and second mirrors 110 and
120 define a resonant optical cavity to establish a standing wave
(or a traveling wave) of electromagnetic energy and to facilitate
emission of stimulated coherent radiation through the second mirror
120. The mirror 110 can be a Distributed Bragg Reflector (DBR), a
DBR in combination with a metal reflector, or a metal reflector.
The optical gain medium in the embodiments of FIGS. 11 and 12 is a
semiconductor optical amplifier 170 with a first facet end 171 and
a second facet end 172 disposed at least partially between the
first and second reflective surfaces 112, 122 of the mirrors 110,
120, respectively. The second facet end 172 can be coated with
antireflecting material or antireflection materials to reduce the
reflection of light back into the semiconductor optical amplifier
170. The second facet end can also be tilted to reduce the
reflection of light back into the semiconductor optical amplifier.
An electrical or optical pump 150 is operable to provide electrical
current and/or light to the semiconductor optical amplifier
170.
[0067] In these embodiments, the graphene saturable absorber 130 is
integral with the second mirror at least partially between the
semiconductor optical amplifier 170 and the second reflective
mirror surface 122. Any form of semiconductor optical amplifier 170
can be used in these embodiments. For instance, the semiconductor
optical amplifier can be a vertical cavity surface emitter optical
amplifier (VCSOA) 170 disposed at least partially between the first
and second reflective surfaces 112, 122. In the embodiment of FIG.
12, moreover, the graphene saturable absorber 130 can be bonded to
or deposited on the second end 172 of the semiconductor optical
amplifier 170 with optional spacer layer between the graphene
saturable absorber and the second end 172. The process of bonding
the graphene saturable absorber to the second end 172 can be a
process or direct bonding, adhesive bonding, or other bonding
approach. The second end 172 can be the surface of the vertical
cavity semiconductor optical amplifiers gain region and can also be
the surface of the semiconductor. Additional material such as
semiconductor material or dielectric material can be deposited or
bonded to the surface of the graphene saturable absorber 130 to act
as a phase shift layer or spacer layer. The second mirror 120 can
be integral with or onto the graphene saturable absorber 130 to
provide a monolithic laser structure.
[0068] In certain embodiments, the second mirror 120 can be a
partially reflecting distributed Bragg reflector. In certain
embodiments, the graphene saturable absorber can be integral with
the second mirror 120 with an optical spacer material, and then the
graphene saturable absorber and second mirror can be bonded to the
second end 172 or to a spacer material layer on the surface of the
second end 172. In certain embodiments, the graphene saturable
absorber 130 can be bonded to the second end 172 or to a spacer
material on the surface of the second end and then a distributed
Bragg reflector can be grown on the surface of the graphene
saturable absorber or on the surface of a spacer material deposited
on the surface of the graphene saturable absorber 130. The
separation between the second mirror 120 and the second end 172 can
be controlled by a MEMS device to change the length of the cavity.
The mirror 110 can be a Distributed Bragg Reflector (DBR), a DBR in
combination with a metal reflector, or a metal reflector.
[0069] As seen in FIGS. 18-20, a graphene saturable absorber 130
can optionally function as, and/or form a part of, a top electrode
for the VCSOA, and may be a transparent electrode for the VCSOA. A
metal layer used to form mirror 110 or a metal layer on the bottom
surface of the DBR that forms mirror 110 can function as the bottom
electrode for the VCSOA. In FIG. 18, a graphene layer 130 is formed
on the top side of the SOA 170 to function as a transparent top
electrode for the VCSOA. In FIG. 19, an optional spacer material
layer 131 can be formed on the top side of the SOA 170, with the
graphene layer 130 being formed above the spacer material layer
131. As seen in FIG. 20, moreover, the vertical cavity device can
include a metal structure or layer 132 formed over the graphene 130
(with or without the optional spacer layer 131). In the example of
FIG. 20, moreover, the top metal electrode structure 132 includes
an opening 133 exposing at least a portion of the graphene layer
130. Various different implementations are possible in which a top
electrode is formed above (directly or with intervening structures
or materials) a top surface of the SOA 170, where the top electrode
can include a graphene material layer, preferably transparent. In
this regard, the graphene material layer 130 forming at least a
part of the top electrode structure can operate as a saturable
absorber. In certain implementations (e.g., FIGS. 19 and 20), a
spacer material layer 131 is provided between the upper surface of
the SOA 170 and the graphene material layer 130. Certain
embodiments of the top electrode, moreover, can include a metal
structure 132 formed (directly or indirectly) over the graphene
130. In addition, such a metal structure 132 can be a ring
structure with a cylindrical opening 133 as shown in FIG. 20, or
can be a solid layer, or can include an opening or aperture 133 of
any suitable size and shape which exposes at least a portion of the
graphene material layer 130. The vertical current flow in the
VCSOA, in this regard, can be confined by the design of the top
metal electrode 132 and/or by oxygen ion implant or by other
approaches known to those skilled in the art. The top metal
electrode 132 can be a ring shape with an opening 133 in the center
for light emission. The top metal electrode 132 can contact the
graphene saturable absorber 130 which can function as a transparent
electrode to facilitate a uniform vertical current flow and/or
larger light emission area. Current flow confinement approaches
such as oxygen ion implantation or proton ion implantation can be
used to aid in confining the vertical current flow to the active
region of the vertical cavity semiconductor optical amplifier
(VCSOA) or vertical cavity surface emitting laser (VCSEL) in the
case that the second mirror is integral to the VCSOA with integral
graphene saturable absorber or VECSEL for the case that the second
mirror is separated from the VCSOA.
[0070] In one possible implementation, the partially reflective
mirror 120 and the graphene saturable absorber 130 are integral
with the same substrate. The second facet 172 of the solid-state
gain medium 170 is cleaved or ion milled (optionally at a tilt
angle) and/or coated for maximum transmission and/or minimum
reflection in certain nonlimiting embodiments. The light from the
second facet 172 is coupled out of the gain medium 170, and a
partially reflective mirror 120 on a graphene material 130 is used
to reflect the light back into the optical gain medium 170. The
graphene material 130 performs as a saturable absorber, and can be
attached, bonded or deposited onto the second facet 172 of the
optical gain medium 170, and/or to the partially reflective mirror
substrate 120 or onto a separate transparent substrate (not shown).
The partially reflective mirror 120 can be implemented in certain
embodiments as a Bragg design or maybe implemented as thin film
material.
[0071] There are several approaches by which the graphene saturable
absorber 130 can be integrated with the partially reflective mirror
120 or the gain medium second facet 172. One non-limiting approach
is that the graphene material 130 can be first grown on a metal
surface (not shown) such as copper or nickel. The front surface of
the graphene material 130 can be attached (e.g., bonded, glued,
etc.) to the mirror substrate 120 and/or to the second facet 172.
The metal material can then be optionally etched away leaving the
graphene material as a transparent material on the mirror
substrate. A partially reflective mirror material 120 can then be
deposited on the other side of the graphene material 130. The
graphene material 130 can also be deposited on the partially
reflective mirror substrate 120 or second optical gain medium facet
172 as graphene material flakes or graphene nanoplatelets and
certain non-limiting implementations. The graphene material 130 can
also be incorporated into a polymer and then deposited or coated on
the partially reflective mirror 120 or second facet 172 of the
optical gain medium 170. The linear cavity laser 100 in certain
embodiments can optionally include a region for sensing within the
optical cavity for insertion of gas samples to be measured to
determine the gas molecules within the sensing region. In addition,
the linear cavity laser 100 can be operated in the Frequency Comb
Spectroscopy mode to permit enhanced sensing of the gas molecules
within such sensing region.
[0072] In these embodiments of FIGS. 11 and 12, moreover, the first
mirror 110 is integrated with the first SOA facet 171, or can be a
Bragg mirror integrated within the optical gain medium 170.
Moreover, an electrically or optically pumped gain medium 170
operates in certain embodiments at the selected wavelength and can
also have narrow band or broadband operation. In the implementation
of FIG. 12, the graphene saturable absorber material 130 can be
bonded to the second facet 172 of the gain medium 170, and the
partially reflective mirror 120 can be integrated with the graphene
saturable absorber 130 to implement a monolithic or near monolithic
laser structure 100. In one non-limiting implementation, the
partially reflective mirror 120 and the graphene saturable absorber
130 can be integral with the same substrate. The second facet 172
of the solid-state gain medium 170 may be cleaved and/or coated for
maximum transmission in certain embodiments, such that the light
from the second facet 172 is coupled out of the gain medium 170.
The partially reflective mirror 120 may be integrated with the
graphene material 130 so as to reflect the light back into the gain
medium 170. The graphene material 130 performs as a saturable
absorber, and can be attached, bonded or deposited onto the second
facet 172, or onto the partially reflective mirror substrate 120 or
onto a separate substrate. The partially reflective mirror 120 in
certain embodiments can be implemented as a Bragg design or may be
implemented by thin film material.
[0073] There are several approaches by which the graphene saturable
absorber 130 can be integrated with the partially reflective mirror
120 or the second facet 172. For example, the graphene material 130
can be first grown on a metal surface such as copper or nickel, and
the front surface of the graphene material 130 can be attached
(bonded, glued, etc.) to the mirror substrate 120 or the second
facet 172. The metal material can then be optionally etched away
leaving the graphene material 130 on the facet 172. A highly
reflective mirror material can then be deposited on the other
surface of the graphene 130. The graphene material 130 can also be
deposited on the partially reflective mirror substrate 120 or the
second facet 172 as graphene material flakes or graphene
nanoplatelets. The graphene material 130 can also be incorporated
into a polymer and then deposited or coated on the partially
reflective mirror substrate 120 or second facet 172. The linear
cavity laser 100 can also include a region for sensing within the
optical cavity for inserting gas samples to be measured to
determine the gas molecules within the sensing region. In addition,
the linear cavity laser can be operated in the Frequency Comb
Spectroscopy mode to permit enhanced sensing of the gas molecules
within the sensing region.
[0074] Referring also to FIGS. 13 and 14, a linear cavity laser
device 100 is provided with a first mirror 110 with a first
reflective surface 112 and a second mirror 120 with a second
reflective surface 122 which at least partially faces the first
reflective surface 112. The first and second mirrors 110 and 120
define a resonant optical cavity in which electromagnetic energy is
amplified by stimulated emission of coherent radiation that is
partially transmitted through the second mirror 120. A solid-state
optically pumped infrared optical gain medium 180 is disposed at
least partially between the first and second reflective surfaces
112 and 122, and is pumped via an optical pump 150 providing light
to the optical gain medium 180. In addition, a graphene saturable
absorber 130 is disposed at least partially between the solid-state
optically pumped infrared optical gain medium 180 and the first
reflective surface 112. The gain medium 180 in certain embodiments
can be a rare earth doped fiber amplifier operable at wavelengths
of about 1400 nm or more and about 3000 nm or less. In other
embodiments, the solid-state gain medium 180 includes at least one
II-VI family doped crystal. Further embodiments provide an optical
gain medium 180 that includes at least one transition metal doped
zinc chalcogenide, or at least one lead salt. Moreover, various
embodiments may include one or more optical components 160 (e.g., a
gas cell) longitudinally disposed between the solid-state optical
gain medium 180 and the second mirror 120 as shown in FIGS. 13 and
14, or such may be omitted in other implementations.
[0075] In these embodiments, the resonator or laser optical cavity
is created with a solid-state optically pumped gain medium 180
within the optical cavity. In certain embodiments, the solid-state
optically pumped optical gain medium can be an infrared (including
near infrared or mid infrared) solid-state optically pumped optical
gain medium. Typical types of solid-state optical gain mediums 180
include crystals, glasses, ceramics, or composite laser material
doped with rare earth ions, doped with transition metal ions, or
semiconductor gain medium. The ion doped solid-state laser 100 can
be made in the form of bulk laser, fiber laser or other types of
waveguide lasers. The solid-state laser 100 may generate output
power at levels between a few milliwatts and many kilowatts.
[0076] The solid-state optically pumped laser embodiments 100 may
be either end pumped or side pumped in certain embodiments. The
solid-state optically pumped gain medium 180 may have the geometry
of a fiber, rod, slab, microchip, etc. Many solid-state lasers are
optically pumped with flash lamps, arc lamps, or laser diodes. The
laser transitions of rare-earth doped or transition metal doped
solid-state optically pumped gain mediums 180 are normally weakly
allowed transitions; i.e. transitions with very low oscillator
strength, which leads to long radiative upper-state lifetime and
consequently yields good energy storage with long upper-state
lifetimes of microseconds to milliseconds and is thus advantageous
for Q-switched lasers 100. The laser crystal can easily store an
amount of energy which, when released in the form of a nanosecond
pulse, leads to a peak power which is orders of magnitude above the
average power.
[0077] In mode-locked operation, the solid-state laser 100 can
generate ultra-short pulses with durations on the order of
picoseconds or femtoseconds. Passive mode-locking can have
Q-switching instabilities which are suppressed via suitable design
and operational control, including parameters such pump energy,
energy storage lifetime, cavity length, etc. Most rare-earth doped
laser crystals, such as Nd:YAG and NdYVO.sub.4 have small gain
bandwidths so that tuning is possibly only within a rather limited
range. Tuning ranges of tens of nms and more are possible with
rare-earth-doped glasses, and particularly with transition-metal
doped crystals such as Ti:sapphire, Cr:LiSAF and Cr:ZnSe (vibronic
lasers).
[0078] In some solid-state gain mediums 180, particularly in those
doped with transition metal ions, there is a strong interaction of
the electronic states with lattice vibrations (phonons). This
vibrational-electronic (vibronic) interaction leads to a strong
homogeneous broadening and thus a large gain bandwidth. Examples of
solid-state gain medium material include rare earth doped fiber
amplifier such as erbium-doped fiber amplifiers that operate at
wavelengths from 1500 to 1600 nm. Other examples include doped
crystals of the II-VI family, for example, Cr.sub.2+:ZnSe chromium
doped II-VI compound based lasers as the most successful
room-temperature diode-pumpable sources in the wave-length range
between about 2 .mu.m and about 3.5 .mu.m; Cr:CdSe; Fe.sub.2+:ZnSe
lasers can emit at wavelengths of about 3.7 .mu.m to 5.1 .mu.m; and
Cr.sub.2+:CdMnTe. Other possible implementations can employ
transition metal-doped zinc chalcogenides, such as Tm:YAL, Tm:YAG,
Rare earth doped or transition metal doped laser ceramics, and/or
lead salt such as PbSSe, PbSnTe, PbSnSeTe and/or PbSnS.
[0079] The infrared gain material 180 preferably absorbs light at
one or more wavelengths (e.g., from the optical pump 150) and
amplifies light in the desired infrared 2000 to 6000 nm optical
band. The optically pumped infrared gain material 180 performs the
functions of a gain medium that operates at the operational
wavelength, and creates infrared light within the optical cavity
which can be amplified. In this linear laser cavity, the optically
pumped infrared gain material 180 is located between two mirrors
110 and 120 which provide the feedback for the laser device 100. In
addition a graphene saturable absorber 130 is placed within the
cavity. The location of the graphene saturable absorber 130 in
certain embodiments will modify the performance of the laser device
100 to produce the desired mode-locked laser pulses.
[0080] Optional optical devices 160 such as lenses, folding
mirrors, spectral filters, polarization and dispersive optics can
be inserted between the mirrors 110, 120 in order to improve the
device performance. The laser output can be transmitted through one
or more of the mirrors 110, 120, e.g., from a beam splitter, or
from an intra-cavity reflection. A further variation can include a
highly reflective mirror (e.g., mirror 110) on a first side of the
optical cavity, a graphene saturable absorber 130 between the
highly reflective mirror 110 and a first facet 181 of the optical
amplifier material 180 (in one arrangement, the graphene saturable
absorber 130 can be integrated with the mirror 110), an optional
gas cell 160 located between the mirror 110 and the facet 181 of
the optical gain medium 180, an optically pumped optical gain
medium 180 that operates at the selected wavelength and can also
have narrow band or broad band operation, and a partially
reflective mirror 120 separated from the second facet 182 of the
gain medium 180 which performs as the output coupler of laser
device 100 on the second side 102 of the optical cavity. The linear
cavity laser 100 may include a sensing region within the optical
cavity for inserting gas samples to be measured to determine the
gas molecules within the sensing region, and the laser device 100
can be operated in the Frequency Comb Spectroscopy mode to permit
enhanced sensing of the gas molecules within sensing region.
[0081] FIG. 14 illustrates a linear cavity laser device 100 with a
graphene saturable absorber 130 and two partially reflective
mirrors 120A and 120B, respectively, forming an injection locked
(coupled) optical cavity with a laser 190 to direct light at least
partially toward a second side of the first partially reflective
mirror 120A. An optical gain medium 180 is disposed at least
partially between the first and second reflective surfaces 112, 122
of the respective mirrors 120A and 120B, and is pumped by an
electrical or optical pump 150 that provides electrical current or
light to the gain medium 180. A graphene saturable absorber 130 is
disposed at least partially between the optical gain medium 180 and
the first reflective surface 112, and the laser 190 directs light
at least partially toward the first partially reflective mirror
120A. In certain embodiments, one or more optical components 160
can be longitudinally disposed between the optical gain medium 180
and the second mirror 120, for example, a gas cell. The saturable
absorber 130 in this embodiment, as with the other embodiments
illustrated and described herein, can be a graphene material or a
graphene-containing material, and can be integrated with the
partially reflective mirror 110 as shown. The mirror 110, moreover,
can include a mirror coating on the graphene material 130, or the
graphene material 130 can be formed on a mirror structure 110, or
the partially reflective mirror 110 can be formed on a separate
substrate in various embodiments.
[0082] FIG. 15 shows a ring cavity laser device 200 in accordance
with further aspects of the present disclosure, which includes two
highly reflective mirrors 110A and 110B with reflective surfaces
112A and 112B, respectively, along with a partially reflective
mirror 120 providing an output coupling for the device 200. An
optical amplifier or gain medium 180 is disposed between the mirror
110A and the partially reflective mirror 120, and a graphene
saturable absorber 130 is disposed at least partially between the
second highly reflective mirror 110B and the partially reflective
mirror 120. In this configuration, a reflective surface 112B of the
mirror 110B at least partially faces the reflective surfaces 112A
and 122 of the other mirrors, and the same is true of the other
mirrors, whereby the reflective surface of each mirror at least
partially faces that of the other mirrors. Moreover, in this
configuration, the mirrors 110A, 120 and 110B define an optical
ring cavity to establish a standing wave of electromagnetic energy
and to facilitate emission of stimulated coherent radiation through
the second mirror 120. An electrical or optical pump 150 provides
current or light to the optical gain medium 180, and the graphene
material 130 operates as a graphene saturable absorber 130 disposed
at least partially between the second and third reflective surfaces
122 and 112B. Various embodiments are possible, wherein the
graphene saturable absorber 130 can operate in the transmission
mode in certain embodiments. The gain medium 180 in certain
embodiments can be an optical pumped fiber or crystal gain medium,
or an electrically biased (electrically pumped) semiconductor
optical amplifier (SOA).
[0083] In various nonlimiting implementations, the optical gain
medium 180 can be contacted by one or more of the reflective
surfaces 112, 122 and/or by an optical element 160.
[0084] Referring now to FIGS. 16 and 17, a linear cavity laser
device and/or a ring cavity laser device may include more than two
mirrors and at least one graphene saturable absorber. In this
regard, the device shown in FIG. 16 is linear cavity laser device
with four mirrors M1-M4 and two optical elements OE1 and OE2, in
which at least one of the optical elements OE1 and/or OE2 is a
graphene saturable absorber. FIG. 17 shows an exemplary ring cavity
laser device with five mirrors M1-M5 and four optical elements
OE1-OE4, where at least one of the optical elements OE1-OE4 is a
graphene saturable absorber.
[0085] In certain nonlimiting implementations, a bandpass filter
component can be provided within the cavity. In certain
embodiments, an etalon can be provided in combination with a
spectral filter to establish multi wavelength operation. In
addition, the facet on the output end may be an anti-reflection
coating, and the facet may be titled in certain embodiments to
prevent or mitigate feedback (e.g., minimize or reduce Fabry-Perot
resonances) due to the residual facet reflectivity. Moreover, the
output coupler in certain embodiments can be an external mirror
with partial reflectivity. In certain embodiments, the mode-locked
laser can be implemented using passive mode locking, but
embodiments are contemplated in which hybrid mode locking is used
(takes advantage of both the stability offered by an actively
mode-locked system and the pulse shortening mechanisms provided by
the saturable absorber (SA)). For instance, a hybrid mode-locked
(HML) monolithically integrated indium phosphide (InP) laser can be
used, where the hybrid mode-locking scheme takes advantage of both
the stability offered by an actively mode-locked system and the
pulse shortening mechanisms provided by the graphene saturable
absorber (SA). In other embodiments, a hybrid SOA-Raman amplifier
can be used in conjunction with a graphene saturable absorber.
[0086] In certain embodiments, moreover, the emission can be out of
the bottom of the substrate (e.g., the light goes horizontal and
then vertical near the end). Quantum Cascade Amplifiers, for
example, can have an emission out of the bottom substrate, and the
graphene saturable absorber may be bonded to the back surface of
the substrate. In a substrate emitting DFB QCL configuration, the
functions of the distributed feedback and the surface emission may
be separated.
[0087] Moreover, various embodiments can employ frequency tuning by
varying the current (e.g., electrically pumped embodiments). In
various embodiments, a Quantum Cascade Amplifier can be combined
with a Bragg region. Strong feedback needed for single-mode
operation may be obtained by a first-order Bragg grating. A
dispersive element can be used in certain embodiments, for example
a diffraction grating in a laser with an external cavity in order
to spectrally filter the radiation reflected in the laser amplifier
in order to produce globally single-mode laser radiation. Another
example of a dispersive element implementation involves replacing
one or more of the Bragg mirrors with a chirped Bragg mirror,
thereby altering the spectral phase of the light as it is reflected
by the applied chirp. One possible design for a Quantum Cascade
Amplifier is a Master Oscillator Power Amplifier (MOPA) in which a
Bragg region is used to select out a signal mode. For such
embodiments, the graphene saturable absorber can be located between
the Quantum Cascade output facet and the external mirror.
[0088] The above examples are merely illustrative of several
possible embodiments of various aspects of the present disclosure,
wherein equivalent alterations and/or modifications will occur to
others skilled in the art upon reading and understanding this
specification and the annexed drawings. In addition, although a
particular feature of the disclosure may have been illustrated
and/or described with respect to only one of several
implementations, such feature may be combined with one or more
other features of the other implementations as may be desired and
advantageous for any given or particular application. Also, to the
extent that the terms "including", "includes", "having", "has",
"with", or variants thereof are used in the detailed description
and/or in the claims, such terms are intended to be inclusive in a
manner similar to the term "comprising".
* * * * *