U.S. patent application number 11/525385 was filed with the patent office on 2008-12-04 for compact external cavity mid-ir optical lasers.
This patent application is currently assigned to DAYLIGHT SOLUTIONS INC.. Invention is credited to David F. Arnone, Timothy Day.
Application Number | 20080298406 11/525385 |
Document ID | / |
Family ID | 39201305 |
Filed Date | 2008-12-04 |
United States Patent
Application |
20080298406 |
Kind Code |
A1 |
Day; Timothy ; et
al. |
December 4, 2008 |
COMPACT EXTERNAL CAVITY MID-IR OPTICAL LASERS
Abstract
Highly compact quantum well based laser systems with external
cavity configurations are tightly integrated in a very small
mounting system having high thermal and vibrational stability. The
mounting systems may include adjustability and alignment features
specifically designed to account for the particular nature of the
micro components used. The laser systems may provide for wavelength
selection, including dynamic wavelength selection. The laser
systems may also provide special output couplers.
Inventors: |
Day; Timothy; (Poway,
CA) ; Arnone; David F.; (Mountain View, CA) |
Correspondence
Address: |
Roeder & Broder LLP
5560 Chelsea Avenue
La Jolla
CA
92037
US
|
Assignee: |
DAYLIGHT SOLUTIONS INC.
|
Family ID: |
39201305 |
Appl. No.: |
11/525385 |
Filed: |
September 22, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11154264 |
Jun 15, 2005 |
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11525385 |
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Current U.S.
Class: |
372/39 |
Current CPC
Class: |
H01S 5/02253 20210101;
H01S 5/02484 20130101; H01S 5/3401 20130101; G02B 7/023 20130101;
H01S 5/02415 20130101; H01S 5/143 20130101; H01S 5/028 20130101;
H01S 5/141 20130101; B82Y 20/00 20130101 |
Class at
Publication: |
372/39 |
International
Class: |
H01S 3/14 20060101
H01S003/14 |
Claims
1. A laser comprising: a gain medium; and an optical resonator
cavity, wherein the gain medium comprises a unipolar quantum well
stack semiconductor structure, and wherein the optical resonator
cavity is not more than about 30 millimeters in length and
comprises at least two feedback elements arranged to couple a
feedback beam to the gain medium, and at least one of said feedback
elements is spatially removed from said gain medium.
2. A laser as claimed in claim 1, comprising a collimated output
beam having a beam waist defined by 1/e.sup.2 intensity, wherein
the diameter of the waist is no more than about 7 millimeters at an
exit aperture.
3. A laser as claimed in claim 1, wherein said output beam
comprises substantially only longitudinal modes.
4. A laser as claimed in claim 3, wherein said unipolar quantum
well stack semiconductor structure is arranged in a narrow stripe
which increases resonator losses for transverse oscillation modes
such that losses exceed amplifier gain for transverse modes.
5. A laser as claimed in claim 4, wherein said unipolar quantum
well stack semiconductor structure is arranged as a narrow stripe
of about less than 25 microns.
6. A laser as claimed in claim 1, said gain medium further
comprising: two end facets; a waveguide system; a current path; and
a base substrate, wherein said end facets comprise planar surfaces,
wherein said waveguide system formed about the quantum well stack
semiconductor structure is comprised of optically conductive layers
and optically absorbing material, wherein said current path a
comprises a plurality of electrically conductive elements arranged
to form a serial electrical circuit through which electric current
can pass, and wherein said base substrate is a layer of bulk
crystal material having a matched crystalline structure with
respect to said quantum well stack semiconductor.
7. A laser as claimed in claim 6, wherein at least one feedback
element is integrated with the gain medium.
8. A laser as claimed in claim 7, wherein said feedback element is
a high reflector thin film deposited on an end facet of the gain
medium.
9. A laser as claimed in claim 8, wherein said feedback element is
a high reflector thin film wavelength notch filter deposited on an
end facet of the gain medium.
10. A laser as claimed in claim 6, wherein a spatially removed
feedback element is a high reflector arranged to return an optical
beam in free space back to the gain medium via one of said end
facets.
11. A laser as claimed in claim 10, wherein said end facet is
prepared with an anti-reflection coating.
12. A laser as claimed in claim 11, wherein said anti-reflection
coating is arranged as a bandpass filter.
13. A laser as claimed in claim 10, wherein said high reflector is
combined with a wavelength select element, the wavelength select
element being arranged to couple light of a particular wavelength
band to said gain medium.
14. A laser as claimed in claim 13, said wavelength select element
is selected from the group consisting of: acousto-optic light
modulator; electro-optic light modulator; a grid; a thin-film
filter; a prism; a grating; a holographic optical element;
kinoform; binary optical element; a Fabry-Perot system arranged as
a wavelength filter; and a Fresnel surface relief optical
element.
15. A laser as claimed in claim 13, wherein said combined high
reflector and wavelength select element is coupled to an end facet
of said gain medium by a lens.
16. A laser as claimed in claim 15, wherein said lens has a
numerical aperture greater than about 0.7 and an effective focal
length less than about 8 millimeters.
17. A laser as claimed in claim 16, wherein said lens is a
plano-convex lens having a planar side coupled to said end facet,
and a convex side coupled to said combined high reflector and
wavelength select element.
18. A laser as claimed in claim 17, wherein said lens is further
characterized as a near diffraction limited, aspheric, thick lens
having a working distance less than about 3 millimeters.
19. A laser as claimed in claim 18, wherein said thick lens has a
center thickness CT=2.08 and the aspheric surface is described by
the equation: Z = Y 2 R ( 1 + 1 - ( 1 + K ) Y 2 / R 2 ) + A 4 Y 4 +
A 6 Y 6 + A 8 Y 8 ##EQU00002## where R=2.2015; K=-0.8285;
A4=-6.6119; A6=-2.7837; A8=2.6921; and X, Y and Z are spatial
axes.
20. A laser as claimed in claim 16, wherein said combined high
reflector and wavelength select element is arranged as a
grating.
21. A laser as claimed in claim 16, wherein said combined high
reflector and wavelength select element is arranged as a prism.
22. A laser as claimed in claim 13, wherein said wavelength select
element is arranged as a two part system, each part spatially
removed with respect to the other.
23. A laser as claimed in claim 20, wherein said grating is
rotatably mounted on a pivot axis.
24. A laser as claimed in claim 23, further comprising an actuator
coupled to said grating such that the grating may be rotated about
said pivot axis in response to application of a drive signal.
25. A laser as claimed in claim 21, said prism is rotatably mounted
on a pivot axis.
26. A laser as claimed in claim 25, further comprising an actuator
coupled to said prism such that it may be rotated about said pivot
axis in response to application of a drive signal.
27. A laser as claimed in claim 14, wherein said wavelength select
element is dynamic in response to an applied signal.
28. A laser as claimed in claim 24, wherein said actuator is an
electromechanical transducer selected from the group consisting of:
piezoelectric crystal; voice coil; stepper motor; micro
electromechanical system; and linear motor.
29. A laser as claimed in claim 26, wherein said actuator is an
electromechanical transducer selected from the group consisting of:
piezoelectric crystal; voice coil; stepper motor; and linear
motor.
30. A laser as claimed in claim 17, wherein said gain medium, said
high reflector, said wavelength select element, and said lens are
firmly affixed and held in a positionally relative manner by a
mounting system.
31. A laser as claimed in claim 30, wherein said mounting system
comprises: a base element; a lens mount; a feedback element mount;
and a system axis, wherein said base element is arranged to receive
a semiconductor gain medium therein at a receiving seat and provide
mechanical and mounting support thereto such that a system axis is
coincident with a symmetry axis of a seated gain medium, wherein
said lens mount is arranged to receive a lens therein and provide
mechanical and mounting support thereto such that the system axis
is coincident with a lens axis, and wherein said feedback element
mount is arranged to receive a feedback element and to provide
mechanical and mounting support thereto such that the system axis
intersects an optical surface of the feedback element.
32. A laser as claimed in claim 31, wherein said mounting system is
comprised of a plurality of elements, each of said elements having
a similar thermal coefficient of expansion.
33. A laser as claimed in claim 32, wherein said plurality of
elements are formed from a similar metal.
34. A laser as claimed in claim 33, wherein said mounting system
has a mass of no more than about 200 g.
35. A laser as claimed in claim 33, wherein said mounting system
has no linear dimension greater than about 30 millimeters.
36. A laser as claimed in claim 31, wherein said mounting system
further comprises a surface arranged as a thermoelectric cooler
interface.
37. A laser as claimed in claim 36, wherein said surface is
thermally coupled to each of said gain medium seat, said lens
mount, and said feedback element mount.
38. A laser as claimed in claim 31, wherein said mounting system
further comprises a diamond submount element suitable for receipt
at the base element receiving seat, said diamond submount element
being thermally coupled to said gain medium.
39. A laser as claimed in claim 31, wherein said base element
further comprises a lapped planar surface which forms a first
portion of a sliding interface.
40. A laser as claimed in claim 31, wherein said base element
further comprises electronic leads coupled to said gain medium.
41. A laser as claimed in claim 31, wherein said lens mount further
comprises: a lens holder; and a lens plate, wherein said lens
holder is a cylindrical element having a receiving cavity and lens
seat therein, and further having a threaded outer surface, and
wherein said lens plate has a complementary threaded hole with a
thread axis perpendicular to a lapped planar surface forming a
second portion of a sliding interface such that the lens holder may
be coupled to the lens plate via said threaded interface to provide
a linearly adjustable mechanism along the system axis between the
lens holder and lens plate.
42. A laser as claimed in claim 31, said feedback element mount
further comprising: a stationary portion; and a rotatable portion,
wherein said rotatable portion is arranged to receive a feedback
element therein at a seat, the rotatable portion being rotatably
mounted about a pivot axis provided by the stationary portion.
43. A laser as claimed in claim 31, wherein said mounting system
further comprises a lens mount comprising: a lens holder; and a
lens plate, wherein said lens holder is a cylindrical element
having a receiving cavity and lens and seat therein, and further
having a threaded outer surface, and wherein said lens plate has a
complementary threaded hole with a thread axis perpendicular to a
lapped planar surface forming a second portion of a sliding
interface such that the lens holder may be coupled to the lens
plate via said threaded interface to provide a linearly adjustable
mechanism along the system axis between the lens holder and lens
plate.
44. A laser as claimed in claim 15, further comprising an output
coupler lens arranged to collimate an output beam from said quantum
well gain medium.
45. A laser as claimed in claim 44, said lens having a numerical
aperture greater than about 0.7 and effective focal length less
than about 4 millimeters.
46. A laser as claimed in claim 45, wherein said lens is a
plano-convex aspherical thick lens arranged in an infinite
conjugate ratio configuration.
47. A laser as claimed in claim 46, wherein said aspheric surface
is defined by the relationship: Z = Y 2 R ( 1 + 1 - ( 1 + K ) Y 2 /
R 2 ) + A 4 Y 4 + A 6 Y 6 + A 8 Y 8 ##EQU00003## where R=2.2015;
K=-0.8285; A4=-6.6119; A6=-2.7837; A8=2.6921; and X, Y and Z are
spatial axes.
Description
RELATED APPLICATIONS
[0001] This application is a continuation in part of copending U.S.
application Ser. No. 11/154,264 filed Jun. 15, 2005, the entirety
of which is incorporated by reference.
BACKGROUND
[0002] 1. Field
[0003] The following disclosure is generally concerned with mid-IR
laser systems based upon quantum well semiconductors, and
specifically concerned with short external cavity, highly compact
quantum well lasers.
[0004] 2. Related Technology
[0005] Unipolar Quantum well lasers, sometimes referred to by a
name coined by early pioneers "quantum cascade lasers" or QCLs,
were first suggested in the 70s and finally reduced to practice in
a laboratory late in the 80s. These lasers enjoy remarkably unique
properties quite unlike other semiconductor laser systems. Quite
unlike their predecessor cousins the diode laser, these lasers are
formed from a single semiconductor type (either N-type or P-type)
and do not include lasing at a band gap of a semiconductor
junction. These devices which lase in the valuable mid-IR spectrum
can be fashioned to support extremely wide gain bandwidths, they
are suitable for high-power output, they are very small,
inexpensive, efficient and durable. Despite these, quantum well
lasers have not yet made an appreciable commercial impact. Their
use remains almost exclusively restricted to professional research
laboratories having highly specialized supporting equipment. Some
versions of the systems require complex cooling apparatus,
sophisticated electronic drive and detection means and other
specialized optical support. Recently, quantum well gain systems
have been arranged in conjunction with optical resonators which
include a free space portion. Sometimes called in the art "external
cavity QCLs" or "ECQCLs", these arrangements permit valuable access
to the resonator cavity which was not otherwise available in
devices with gain medium having integrated end mirrors. By way of
the cavity access, advanced wavelength tuning schemes are just
being suggested at the time of this writing.
[0006] QCLs have been described in extensive patent publications
from an early day as pioneers recognized their immense future
value. In particular, Bell Laboratories now Lucent Technologies,
produced at least the following patents related to early quantum
cascade laser systems. These include U.S. Pat. Nos. 5,311,009;
5,457,709; 5,502,787; 5,509,025; 5,570,386; 5,727,010; 5,745,516;
5,901,168; 5,936,989; 5,978,397; 6,023,482; 6,055,254; 6,055,257;
6,091,753; 6,134,257; 6,137,817; 6,144,681; and 6,148,012. Of
course, many others also have since made interesting inventions
around the QCL foundation.
[0007] Of particular importance for this disclosure is quantum well
lasers configured in external cavity configurations.
[0008] Recently, new patent publications have just started to
suggest these combinations. In particular, US Patent Application
Publication 2003/0043877, titled "multiple wavelength broad
bandwidth optically pumped semiconductor laser" teaches quantum
well based system having tunability taken up outside the gain
medium. Inventor Kaspi further suggests an optically pumped version
of this systems which requires special optical coupling between a
pump source and the gain medium device--i.e. addition ex-cavity
cooperation.
[0009] Another important patent related publication is teaching by
Masselink et al published Sep. 29, 2005 as patent application
publication numbered US 2005/0213627. This invention includes a
quantum cascade laser structure coupled to an external cavity to
effect wavelength tuning. Masselink et al' systems are particularly
distinguished in that they employ a mechanical stress or strain on
the device crystal to impart a preferred output.
[0010] Non-patent publications have also now started to suggest
interesting arrangements of QCLs in combination with wavelength
tuning performed in free space or externally with respect to the
gain medium.
[0011] A description of a tunable ECQCL is presented as "Broadly
tunable external cavity quantum-cascade lasers" by Maulini, R. et
al. from the Institute of Physics, University of Neuchatel,
Switzerland. Broadly tunable (300 cm.sup.-1) TEC cooled external
cavity systems have been demonstrated. So called bound to continuum
device designs support very wide gain bandwidth in these very
useful systems.
[0012] Hildebrandt et al also teach of external cavity wavelength
tuned QCLS. Hildebrandt points out that a QCL gain medium must be
coupled to an external cavity via carefully prepared
anti-reflection coatings at one device emission facet. Hildebrandt
arranged his systems in a Littrow configuration to achieve selected
wavelength feedback via a grating element. In another disclosure,
one titled: "Quantum cascade external cavity laser systems in the
mid-infrared spectral range" Hildebrandt et al similarly describe
QCL gain systems coupled with external cavities. They indicate
compact devices can be made via use of ZnSe collimation lenses in
Littrow feedback arrangements.
[0013] Hensley, et al demonstrate on behalf of Physical Sciences,
Inc. long wave (THz) QCLs in an external cavity. In a professional
laboratory environment, a cryostatically cooled system is coupled
on an optical bench to a movable grating via an off-axis parabolic
collimation optic. Liquid helium and significant supporting
apparatus are required to realize the device. The total cavity
volume is greater than 100's of cubic centimeters. The same Hensley
also publishes a "QCL breath analysis system" shown on an optical
bench in a collection of high precision mounting and alignment
optics maintained on a rigid optical bench. Again, this system
suitable for laboratory use may be characterize as another very
large cavity system.
[0014] While systems and inventions of the art are designed to
achieve particular goals and objectives, some of those being no
less than remarkable, the art has limitations which prevent laser
use in new ways now possible. Inventions of the art are not used
and cannot be used to realize the advantages and objectives of the
inventions taught herefollowing.
SUMMARY
[0015] A compact, external cavity mid-IR optical laser system may
utilize unipolar quantum well semiconductors in conjunction with
unique structural arrangements of component parts which operate
together as a unipolar quantum well external cavity laser. An
external cavity including a free space portion permits cavity
access whereby wavelength tuning may be effected. Both static and
dynamic wavelength tuning systems may be arranged in conjunction
with these lasers.
[0016] A particular distinction of these devices relates to their
overall size. The cavities presented here are arranged in
configurations permitting an overall cavity length less than 30
millimeters. Collimating optics and wavelength select elements are
very tightly integrated and placed in close proximity with one
another. As such, a high-performance laser package of very small
size is made durable and rugged and is suitable for use as a
stand-alone commercial product.
[0017] A special base preferably supports integration of
semiconductor gain medium, collimation lens holder, and a
wavelength select element to form a short external cavity. In
preferred versions, a cavity may be made as small as 8 millimeters.
This includes a first cavity mirror, a gain medium coupled to a
free space portion, a diverging beam incident on a collimation
lens, a collimated beam portion, and a wavelength select element.
Further, the base may additionally accommodate a specialized output
coupler lens, an electronic drive assembly, and active cooling
systems. The component parts are fashioned to cooperate with each
other in a very small volume of just a few cubic centimeters. These
parts are specifically designed to perform well together in view of
both thermal and vibrational considerations. In addition, the
mounting system provides accurate alignment mechanisms suitable for
very close proximity, precise positioning. In this way, durable,
miniature mid-IR lasers based upon quantum well semiconductors may
be provided with physical characteristics and structural properties
that make them suitable for wide scale commercial use.
[0018] A special intra-cavity collimating lens may be used to
couple light emitted by the gain medium to the wavelength select
element. These lenses are mid-IR, high numerical aperture, short
focal length aspheric singlets of plano-convex configuration. The
lenses provide special cooperation between short cavities and
integrated mounting systems and gain media having highly divergent
output, enabling systems that are highly compact,
BRIEF DESCRIPTION OF THE DRAWING FIGURES
[0019] These and other features, aspects, and advantages will
become better understood with regard to the following description,
appended claims and drawings where:
[0020] FIG. 1 is an exploded perspective view of one preferred
version of these devices;
[0021] A basic cross sectional block diagram a laser and associated
nomenclature is presented as FIG. 2;
[0022] FIG. 3 illustrates in a similar diagram showing a laser with
a static wavelength tuning element;
[0023] FIG. 4 depicts an alternative version of a resonator
including a two part wavelength selection system.
[0024] FIG. 5 is another exploded perspective of a system having an
added output coupler portion; and
[0025] FIG. 6 illustrates in cross section a dynamic wavelength
tunable system.
DETAILED DESCRIPTION
[0026] Throughout this disclosure, reference is made to some terms
which may or may not be exactly defined in popular dictionaries as
they are defined here. To provide a more precise disclosure, the
following terms are presented with a view to clarity so that the
true breadth and scope may be more readily appreciated. Although
every attempt is made to be precise and thorough, it is a necessary
condition that not all meanings associated with each term can be
completely set forth. Accordingly, each term is intended to also
include its common meaning which may be derived from general usage
within the pertinent arts or by dictionary meaning. Where the
presented definition is in conflict with a dictionary or arts
definition, one must consider context of use and provide liberal
discretion to arrive at an intended meaning. One will be well
advised to error on the side of attaching broader meanings to terms
used in order to fully appreciate the entire depth of the teaching
and to understand all intended variations.
[0027] Quantum Well Stack. A quantum well stack is a semiconductor
structure having a plurality of thin layers of highly regulated
thickness. These thin layers define quantum well and barrier
systems which support particular electron energy states and energy
state transitions in accordance with a particular design. By
careful selection of these energy state transitions, a quantum well
stack may be fashioned and operate as the core of a laser gain
medium. When a quantum well stack semiconductor is combined with a
suitable optical resonator system and energizing or `pumping`
system, a device which supports stimulated emission or lasing
action is achieved. For this paragraph, `thin` means on the order
of the de Broglie wavelength of an associated electron.
[0028] Unipolar Quantum Well Gain Medium. A unipolar quantum well
gain medium is a system of elements including pump mechanism,
waveguide system, unipolar quantum well stack, emission facets, and
a base substrate, which operate together to provide optical
amplification by stimulated emission. For purposes of this
disclosure, a unipolar quantum well gain medium is distinct from a
laser in that it does not include an optical resonator. Gain media
described herein are always unipolar. That is, gain media are
comprised of semiconductor materials of a declared semiconductor
type either `P` or `N`. Thus, unipolar devices are certainly
distinct from diodes which include both N-type and P-type and
particularly a junction between. Conversely, unipolar systems are
comprised of exclusively one semiconductor type.
[0029] High Numerical Aperture. High numerical aperture is herein
defined to mean numerical apertures greater than about 0.7. While
common optical systems typically include those having a numerical
aperture less than 0.5; optical systems having a numerical aperture
of 0.7 or greater are exceptional and considered as having a "high
numerical aperture". While a numerical aperture of 1 has
theoretical meaning, it remains quite difficult to realize in
practical systems a numerical aperture greater than about 0.95.
Thus, `high numerical aperture` is best considered as those systems
having a numerical aperture with any value between about 0.7 and
0.95.
[0030] Short Focal Length. Short focal length refers to lenses and
focusing mirrors which have a focal length less than about 8
millimeters. As there is no certain meaning associated with a lens
having f=0, we declare a lower limit on our `short focal length` to
be about 0.5 millimeters. Any value between about 0.5 and 8
millimeters is herein considered a short focal length.
[0031] In accordance with preferred embodiments, highly compact,
high performance tunable mid-IR laser systems are provided. In
particular, quantum well based semiconductor lasers arranged with
external resonator cavities. It will be appreciated that each of
the embodiments described include an apparatus and that the
apparatus of one preferred embodiment may be different than the
apparatus of another embodiment.
[0032] While practitioners of the art have devised remarkable
quantum cascade lasers in external cavity configurations, those
systems are exclusively arranged with component parts which are
quite large and bulky--indeed hundreds of times larger than the
miniature systems which are first presented in this disclosure.
Lasers of these embodiments are highly compact systems of very
tightly integrated component parts. Further, these laser systems
include certain optical components of a nature heretofore never
attempted. System mechanical designs account for two major
objectives including: provision of excellent thermal coupling
between all component parts to reduce misalignment due to thermal
expansion, and provision of extremely short distances between
component parts to enhance vibrational resistance. Further, it is a
feature of great importance that these systems so arranged in
tightly integrated packages are highly mobile, ruggedized light
source assemblies.
[0033] An important physical structure which contributes to these
new designs is an optical resonator fashioned to support a cavity
length no more than about 30 millimeters. In best versions a cavity
length can be made between about 8-12 millimeters. A cavity is
necessarily greater than 5 millimeters to contain all required
component parts. Accordingly, cavities of these embodiments are
particularly distinct as including those having a length of any
value between 5 and 30 millimeters. Special new optical elements
from which a resonant cavity is comprised have been created to
cooperate with these very short cavity lengths. Namely, a highly
unique lens suitable for use in the mid-IR spectrum is a near
diffraction limited, aspherical plano-convex singlet of numerical
aperture greater than about 0.7. The lens cooperates particularly
well with and is suited to the characteristic high divergent output
of a quantum well gain medium. Such a lens is used as an in-cavity
collimation lens to couple output from a gain medium to a cavity
feedback element.
[0034] Because the lens is used as an intracavity element, various
aspects of its performance greatly influence the efficiency of
these lenses. For example, lenses considered `acceptable` by others
suffer from diffraction complexities. That is, lenses having
numerical aperture less than 0.7 tend to introduce and couple light
of high spatial frequency back to the gain medium and result in
very `noisy` outputs. Further, spherical lenses used by others
cannot achieve diffraction limited spot sizes and tend to be very
lossy as well as tending to excite higher order or transverse
cavity modes. Thus spherical lenses used intracavity severely
diminish laser performance. Finally, lenses having very short focal
lengths of 8 millimeters or less permit small cavity configurations
and stable mounting systems not achievable in systems built upon
conventional optics.
[0035] In addition, a second similar lens may be used to condition
a laser output beam. When used as an output coupler, the lens
produces distinct output beams of very high quality not found in
any existing semiconductor devices today.
[0036] In general, laser systems produce optical output in a light
beam which can be quantified in various important ways to reflect
the quality of the system. One important aspect of a laser beam is
its intensity. For a prescribed gain medium, laser resonators might
be arranged in various ways to produce preferred outputs. If the
beam diameter of a collimated laser beam is reduced, the beam's
intensity is increased. Thus it is sometimes desirable to arrange a
laser device to have as narrow of a beam as possible to achieve the
maximum output intensity. Laser system arrangements presented here
employ two important new configurations to increase the intensity
of quantum well based mid-IR laser outputs. A first includes a
special output coupler lens. A lens with a very high numerical
aperture and short focal length yields a collimated output beam on
the order of about 4 millimeters. As such, the beam intensity is
greatly improved when compared to common systems using 1 inch
collimation optics standard in the art.
[0037] In addition, the systems are specially arranged to improve
the on-axis intensity of an output beam. By careful design and
configuration of a gain medium active region or `stripe`, off-axial
or transverse oscillation modes are suppressed. Lasers operating
solely in longitudinal modes have higher on-axis intensity. This is
particularly important in systems having optical elements
susceptible to aperture clipping of high spatial frequency
components. When a quantum well gain medium is arranged as a narrow
stripe of about 25 or less micrometers, transverse modes are lost
to absorption at an opaque or otherwise absorbing material on the
gain medium side extremities.
[0038] Accordingly these systems include quantum well based mid-IR
lasers having very high beam intensities, and in particular on-axis
intensity. Output beams characterized as having a beam waist
diameter of 4 millimeters (at 1/e.sup.2 points) and comprising only
longitudinal modes, are of exceptionally high quality and
usefulness and cannot be realized without the optical arrangements
taught herein.
[0039] Another major feature of some preferred versions relates to
wavelength tuning. Because a quantum well gain medium can be
configured to support a very wide gain bandwidth, these devices are
ideal for wideband wavelength tuning. Very small wavelength select
elements are used in place of or in conjunction with a cavity
mirror to force cavity resonance on a single narrow wavelength
band. Wavelength select elements used in these devices are distinct
because their clear apertures are very small; sometimes on the
order of 25-50 square millimeters. In some versions where a
wavelength select element is tuned by rotation about an axis, their
small size and more specifically light mass, makes them easy to
drive at high speed with micro-electromechanical actuators with
moderate power consumption.
[0040] These systems are further particularly characterized by
their advanced monolithic mounting systems. An integrated system of
cooperating elements provides positioning, alignment, and mounting
function for the various compact optical elements from which these
optical sources are comprised. In preferred versions, the mounting
system can be machined from metal having high thermal conductivity.
In best versions, the entire mass of the mounting system is very
small and of the order of the few hundred grams and occupies a
volume less than about 10 cubic centimeters. However some versions
may be implemented in very small mounting systems of just 1 cubic
centimeter; while other systems remain useful and enjoy the
functionality brought by these teachings and still be as large as
30 cubic centimeters. Thus, these mounting systems include all
those having a volume with any value between about 1 and 30 cubic
centimeters.
[0041] One preferred embodiment is expressed in FIG. 1. A gain
medium semiconductor device 1, cooperates with chip carrier or sub
mount 2, to form a highly conductive thermal coupling therebetween.
In some preferred versions, a chip carrier is fashioned from
diamond which has a very high thermal conductivity and quickly
spreads heat generated at the gain medium and further passes that
heat into the bulk material of the primary base element or `sled`
3. A diamond chip carrier is also coupled to a receiving seat in
the sled to form a bond which is highly conductive with respect to
heat. This sled base element is preferably fashioned from a forged
block of metal and sometimes a metal like copper or copper alloy
having good thermal conduction properties. As such, the sled has
high thermal momentum as well as high thermal conductivity which
further enhances thermal stability of these systems. The sled is
further coupled to other system components in a manner which
operates to keep the entire system at the same relative
temperature. Since alignment stability is improved when all parts
of the system are maintained at the same temperature, it is useful
to provide good thermal coupling between the sled and the lens
plate 4, and further a feedback element holder 5. When the system
operates at various temperatures, all parts expand and contract in
conjunction with each other as one piece to preserve alignment.
[0042] The sled additionally provides means for disposing heat
generated at the gain medium. The bottom surface 6 of the sled is
configured for thermal coupling with a heat sink. Any heat
generated quickly passes through the sled bulk material and exits
to the bottom surface where heat may be further carried away. In
some advanced versions, a heatsink is prepared as an active system,
a thermoelectric cooler TEC, and those systems may be directly
coupled to the bottom surfaces of these sleds. Accordingly, use of
advanced heatsink facility is fully anticipated for demanding high
performance applications. However, in some systems the bottom
surface of the base may operate to dissipate heat merely by
radiative emission and an active heat sink is not necessary.
[0043] The lens plate is prepared with a lapped planar surface 7
which cooperates with a similar lapped planar surface 8 to form a
sliding planar interface. This sliding interface permits positional
adjustments of the lens plate in two orthogonal directions, i.e.
those in the plane surface. Set screws 9 mate with oversized holes
10. When a desired relative position between the lens plate and the
sled is achieved, set screws may be applied to disable further
movement at the sliding interface and hold fast the lens plate to
the sled. It is easy to appreciate in this adjustment/lockout
scheme, good thermal coupling is maintained by way of contact over
the large surface area. The lens plate also includes a threaded
hole 11, having a thread axis perpendicular to the lens plate
surface. These threads are preferably quite fine in pitch to permit
more precise adjustability. The threaded hole is arranged to
cooperate with mating threads on the outside surface of a
cylindrical lens holder element 12. The lens holder element
includes a lens receiving cavity and seat into which a lens 13 may
be inserted and securely seated and affixed. Thereafter, the
lens/lens holder may be inserted together into the threaded hole.
Position adjustments along an optical axis are effected by
advancing or retracting the thread coupling between the lens holder
and the lens plate. In this way, it is easy to accurately place the
lens surface quite precisely near an emission facet of the quantum
well gain medium. Indeed, this system is effective for placing the
lens surface about 600 microns from the emission surface with fine
adjustment which easily supports increase or decrease in increments
of about 1 micron. Again, one will appreciate that the very
intimate mounting arrangement of the lens guarantees good thermal
exchange and the lens, lens holder, and lens plate all stay at the
same temperature as the sled as they are very tightly integrated
and thermally coupled thereto in a unique fashion. Temperature
variations between parts are essentially eliminated.
[0044] Feedback element 14 is preferably coupled with the feedback
element holder on a pivot axis 15 whereby the device remains
rotatably affixed. While the pivot axis is sometimes arranged as
shown intersecting the system axis, it is also possible in some
versions to arrange a pivot axis which does not intersect the optic
axis nor even intersect the feedback element itself. The precise
location and nature of the pivot axis is configured differently in
various versions. Adjustments to the feedback element are freely
made while the holder remains secure and held fast at its large
area mounting pads 16 on the sled. The large area assures good
thermal transfer between the holder and the sled. Thus, the
feedback element holder is also strongly coupled to the sled with
respect to thermal considerations assuring further that all
components remain at constant or otherwise stable temperature.
Finally, electrical leads 17 and electronics sub-assembly 18 used
to energize the gain media may additionally be coupled to a sled
surface.
[0045] FIG. 1 illustrates a preferred embodiment. However as it
includes a great amount of detail, it might seem to suggest a
narrow definition for some component parts. This is not intended
and the reader is reminded that the true scope of these systems and
individual components can only be appreciated by the entire
teaching rather than by a single specific embodiment.
[0046] These embodiments are further distinct from the art in many
ways. Primary important features of these embodiments not found in
the art relate to improvements in stability, ruggedness, mobility
and miniaturization. Further, improved beam output quality is also
achieved. These improvements are realized because highly
specialized components are used in unique arrangements to form new
optical sources. In particular a very short external resonator is
combined with a unipolar quantum well gain medium. The art is
replete with quantum cascade lasers having short cavities (about
1-3 millimeters), however these cavities are always directly
integrated with the gain medium. That is, the cavity mirrors are
formed on the gain medium emission facets. These arrangements
severely restrict access to the cavity and foreclose on many useful
possibilities. The art additionally includes many external cavity
QCLs systems. However these all have been arranged with large
precision multi-purpose optical elements, separated by great
distances, in highly controlled and regulated laboratories. Except
for the expensive precision instruments which provide for accurate
alignment, and highly controlled temperature regulation facilities
such as cryonics systems, and the vibration and mechanical
stability provided by huge optical tables, these systems are
susceptible to failure and were otherwise impossible to realize.
Only by means of the teachings here can compact mid-IR external
cavity sources be realized. These exceptional arrangements do not
require large precision instruments, cumbersome cooling apparatus,
nor stable optical benches.
[0047] A first unique structural feature readily appreciated in
these systems is the very short cavity length CL. Cavities of these
embodiments are arranged to be no more than about 30 millimeters.
In some exceptional versions, a cavity is made between 8-12
millimeters in length. While still including a free space portion,
the total cavity volume may be many thousands of times smaller than
external cavities of QCLs in the published arts. With reference to
FIG. 2, one will appreciate that a quantum well gain medium 21
having a first feedback element, an end mirror 22 integrated
therewith and a second feedback element 23 forms a optical resonant
cavity of length CL having free space portion FS. Accordingly, by
"external cavity" it is herein meant that the cavity includes at
least one free space portion. That is, at least some portion of the
resonant cavity is external with respect to the gain medium.
Optical element 24 may be an anti-reflection system which permits
good coupling of optical beams into and out of the gain medium
device.
[0048] The diagram of FIG. 2 supports several important variations
each of which is considered a viable subset of these embodiments. A
resonant cavity can be formed where both feedback elements 22 and
23 are arranged as mirrors. Because a beam leaving the gain medium
is highly divergent, feedback element 23 may be arranged as a
mirror having a curved surface. A high reflector HR mirror with a
radius of curvature which approximates the wavefronts will return
an incident beam back to the gain medium for further amplification
while at the same time permitting a small portion of the light to
pass as laser output. While the mirror is shown in the diagram as a
simple block, it can be easily appreciated that such end mirrors
for lasers may have a high degree of curvature; the drawing is not
intended to be an engineering drawing. A first surface spherical
mirror such as gold on germanium is sufficient for this
application. This configuration is particularly interesting because
the reflectivity of the mirror can be finely tuned to a precise
value which cooperates with any particular resonator demanding a
feedback of certain strength. The second mirror 22 of the cavity
may be arranged as a multi-layer thin film deposit on a crystal
cleaved facet to form a complete resonator cavity.
[0049] One first alternative includes a special wavelength
selection means which discriminates against wavelengths not in a
prescribed wavelength band. Anti-reflection coating 24 may be
arranged as a thin-film wavelength filter which passes light only
of prescribed wavelength. By careful selection of the thin-film
filter layers, one effectively tunes the laser to oscillate in a
particular wavelength band. This is one example of a static
wavelength tuned system. Thus, the performance of the cavity can be
manipulated by various of the optical components from which it is
formed.
[0050] It is similarly possible to prepare feedback element 22 as a
notch filter mirror to similarly cause the system to lase only on a
prescribed wavelength band. This is a first example of how an end
mirror may be combined with a wavelength select function as a
single element.
[0051] In all of these suggested alternatives, a short cavity of 30
millimeters or less including a free space portion is formed in
conjunction with a quantum well gain medium to produce highly
compact lasers. While systems described in conjunction with FIG. 2
are believed to be most useful, there are other systems with even
higher performance and those are illustrated herefollowing in
connection with FIG. 3.
[0052] In some advanced versions of these preferred embodiments, a
wavelength select element of high precision is deployed. For
example, a grating element, or specifically a blazed grating
element may be arranged to select a very narrow wavelength band for
feedback to the amplification stage or gain medium. In this way,
very precise wavelength tuning is enabled. While a grating is very
useful, a prism or other dispersion element may be used in its
stead.
[0053] In most cases, use of such a dispersion element for fine
wavelength selection demands an incident beam having planar
wavefronts; i.e. a collimated input. Accordingly, versions of these
external cavity quantum well gain medium lasers include one
additional optical component as an integral part of the cavity. A
collimation lens is provided between the gain medium and
high-performance wavelength select element. The lens converts a
highly divergent light beam from the gain medium to collimated
light suitable for interaction with a blazed grating for
example.
[0054] FIG. 3 illustrates a quantum well gain medium 31 having a
feedback element 32 an antireflection coating 33 and second
feedback element 34, a wavelength select element of specialized
nature. In addition, these cavities also include a high-performance
lens 35. A near diffraction limited aspherical mid-IR lens of
numerical aperture greater than about 0.7 and focal length less
than about 8 millimeters is preferably arranged as a plano-convex
thick lens. When properly coupled with the emission facets of the
quantum well gain medium, the lens forms a collimated beam 36
having a beam waist less than 5 millimeters. Beams with small
cross-sections permit use of a second feedback element having a
correspondingly small clear aperture. This is a great benefit for
stability and system compactness.
[0055] Other important features of these feedback elements include
their wavelength selection ability. Because the beam is collimated
by the lens, a grating can be used to cause a single narrow
wavelength band to be fed back to the gain medium for
amplification. Thus, the resonator has an output beam 37 of very
narrow linewidth. While a grating is a preferred type of wavelength
select element, similar arrangements of alternative wavelength
select elements are possible. In laser systems it is possible to
achieve wavelength selection or wavelength tuning by way of a
wavelength select element from the group including: an
acousto-optic light modulator; electro-optic light modulator; a
grid; a thin-film filter; a prism; a grating; a holographic optical
element; kinoform; binary optical element; a Fabry-Perot system
arranged as a wavelength filter; and a Fresnel surface relief
optical element. Sometimes, the wavelength is selected by rotating
a dispersion element about a rotation axis. In other systems, it is
not necessary to apply a mechanical displacement to effect a
wavelength change. For example, an acousto-optic device may
diffract light in response to a change in an applied driving
signal. Similarly, an electrooptical system may be used to couple
light of a specific wavelength to a gain medium in response to an
applied electronic signal. A Fabry-Perot wavelength filter may be
driven by an applied heating system to tune the pass frequency in
another wavelength selection scheme. In yet another, a micro
electromechanical system, or MEMS, might be coupled to cooperating
devices to form a wavelength selection system. Either of these or
another may have particular advantage connected therewith and it is
noted that compound laser systems benefiting from such advantage
while also achieving the other structures first presented here are
fully anticipated.
[0056] It is to be further appreciated that many various
alternative arrangements of these devices are possible; any of
which may be combined with the arrangements presented herein to
gain the advantages suggested. Thus, slight deviations as to cavity
configuration should not be considered outside the scope of these
embodiments. The following illustration suggests some alternative
cavity configurations considered equally part of these
teachings.
[0057] Cavities illustrated in FIG. 3 are sometimes referred to as
a `Littrow` configuration. More particularly, a double-ended
Littrow. It is `double-ended` because both ends of the gain medium
are used--one as an output and the other in conjunction with a
wavelength tuning system. It is possible in Littrow systems to take
an output beam off the grating (i.e. the zero order or reflected
beam) in a single-end Littrow arrangement. Both single-end and
double-end Littrow configurations are considered viable cavity
arrangements which cooperate with these embodiments and both fit
equally well within the scope thereof.
[0058] Important alternative cavity configurations additionally
include those having a two-part wavelength select element. A
`Littman` or sometimes Littman/Metcalf arrangement includes
wavelength selection means taken up via a combination of a
dispersion element and spatially displaced mirror. FIG. 4 shows one
possible arrangement of the Littman/Metcalf cavity which may be
used as a resonator cavity in these systems.
[0059] A double ended Littman/Metcalf is realized when a gain
medium 41 is prepared with an output emission facet 42 and an
anti-reflection facet 43. Collimation lens 44 couples a beam to
grating 45 which is arranged in a grazing incidence orientation. A
diffracted beam is incident on mirror 46 and returned to the
grating where a second diffraction occurs and a beam of very narrow
linewidth is returned as feedback to the gain medium. For geometric
convenience, the mirror is represented in phantom 47 to `unfold`
the cavity and provide consistency with respect to the free space
FS and cavity length CL indicators.
[0060] It is further possible to provide these Littman/Metcalf
cavities in single end versions. If end mirror 42 is arranged as a
high reflector without output, then a laser output beam may be
taken from the zero-order or glancing reflection 48 of the grating.
Accordingly, both Littrow and Littman/Metcalf are various versions
thereof are all considered useful resonator cavity arrangements in
view of these embodiments. One does not traverse the claims merely
be selecting some cavity system not explicitly set forth and
presented in detail here. It is recognized that many possible
resonators not mentioned will gain great benefit from the new
configurations defined in the appended claims.
[0061] Since these optical sources are designed for use in
conjunction with other systems and optical set-up and experiment,
it is quite convenient that they provide a final output of
preferred conditioned nature. Commercially available quantum
cascade lasers have all been provided with a highly divergent
output and it was left to the end-user scientist to provide
coupling optics suitable for his particular system. Those provided
optics might be sufficient for collimating a beam of those devices,
however the optic provided necessarily has no mechanical
relationship with the laser cavity and component mounting
systems.
[0062] In contrast, most preferred versions of these systems are
arranged with pre-conditioned output beams. In particular, highly
collimated beams of very small cross-section are provided by way of
an integrated output coupler. A very special micro-lens is tightly
coupled to the laser and more particularly to the sled. The
micro-lens is carefully positioned to collimate the laser emission
and provide this narrow beam as system output.
[0063] FIG. 5 illustrates. Sled base element 51, directly receives
quantum well gain medium semiconductor 52 thereon at a provided
mounting seat fashioned to align and thermally connect the
semiconductor to the base. Cavity side lens plate 53 couples lens
holder 54 to hold lens 55. Wavelength select type feedback element
56 is held in feedback element holder 57 to complete the wavelength
tuned resonator cavity. Output coupler side lens plate 58 couples
with lens holder 59 to receive therein and hold output coupler lens
510. The output coupler lens is positioned and aligned to receive
laser output from an emission surface of the gain medium and to
collimate that output into a very narrow beam of planar wavefronts.
The output coupler lens, quite similar or identical in size and
specification as the cavity lens, is preferably a mid-IR,
aspherical singlet having focal length less than 8 millimeters and
numerical aperture between about 0.7 and 0.95.
[0064] Finally, electronic subassembly 511 is disposed near the
semiconductor to provide drive current as necessary. Since the
output coupler lens is also integrated with the system package, it
remains temperature and alignment stable without requiring
sophisticated laboratory type systems to provide advance coupling,
re-alignment, and conditioning.
[0065] FIG. 6 is a cross section diagram which presents these
systems in clear schematic. Quantum well gain medium 61 is joined
by thin-film mirror or high reflector element 62 and grating type
feedback element 63 to form a cavity; the cavity length being less
than about 30 millimeters but preferably about 8-12 millimeters.
The cavity includes free space portion FS. An anti-reflection
coating 64 deposited on an emission facet of the gain medium and
provides good coupling of optical beams both from and to the free
space portion of the cavity. Cavity lens 65 is devised and aligned
to collimate the beam before falling incident on the grating type
dispersion element. The grating may be rotatably mounted on a pivot
axis 66 such that rotation about the pivot axis changes the
wavelength band which is coupled to the cavity.
[0066] A careful observer will also note that rotation on the axis
illustrated also simultaneously changes the cavity length CL. In
some versions, it is desirable to arrange special pivot axes which
permit wavelength tuning while at the same time preserve cavity
length. The precise location of all possible axes is not meant to
be included in the figure.
[0067] The grating can be moved over a range of angles 67 to
implicitly give the device a tuning range in agreement with the
design of the gain bandwidth. An electromechanical actuator 68 may
be connected to the dynamic feedback element.
Micro-electromechanical actuators may be configured from
piezoelectric crystals, micro stepper motors, linear motors, voice
coils, micro-electromechanical systems MEMS, or similar
transducers. An electronic drive signal 69 can then be applied to
dynamically select the system wavelength and effect scanning
functions. Laser output is received by output coupler lens 610 and
converted from a highly divergent beam into a highly collimated
output beam 611 having a beam waist of a few millimeters up to
about 8.
[0068] In accordance with the description presented, one will now
appreciate these embodiments are defined and distinguished as:
[0069] Lasers comprising a gain medium and optical resonator cavity
where the gain medium is a quantum well stack semiconductor
structure and the cavity is defined as two mirrors separated by
about 30 millimeters and includes a free space portion.
[0070] These laser systems may also be characterized as having a
collimated output beam with a 4-10 millimeter beam waist-1/e.sup.2
intensity points. In a special arrangement, a cavity is arranged to
lase solely in longitudinal modes--as transverse modes are quenched
by a narrow gain medium stripe--stripes about 25 micrometers or
slightly less are useful for producing this effect.
[0071] Systems where the gain medium includes: planar end facets
formed in the natural crystal planes, a waveguide formed about the
quantum well stack from optically conductive layers and optically
absorbing regrowth material, a current path through the quantum
well stack, and a base substrate having a matched crystalline
structure with respect to said quantum well stack semiconductor are
also included in these embodiments.
[0072] Preferred embodiments include lasers having at least one
feedback element integrated with the gain medium. Alternative
versions, have a feedback element arranged as a high reflector thin
film deposited on an end facet of the gain medium. A feedback
element may also be a thin film wavelength notch filter deposited
on an end facet of the gain medium.
[0073] Best versions include a gain medium having an end facet
prepared with an anti-reflection coating to facilitate beam
coupling to free space.
[0074] Preferred embodiments include arrangements where a high
reflector is combined with a wavelength select element.
[0075] While a grating is preferred in some versions, other
dispersion elements might be used to achieve wavelength selection
including: acousto-optic light modulator; electro-optic light
modulator; a wire grid; a thin-film filter; a prism; a holographic
optical element; kinoform; binary optical element; a Fabry-Perot
system arranged as a wavelength filter; and a Fresnel surface
relief optical element.
[0076] A high reflector and wavelength select element is preferably
coupled to an end facet of a gain medium by a lens with a numerical
aperture greater than about 0.7 and an effective focal length less
than about 8 millimeters. Such lenses are near diffraction limited,
aspheric plano-convex thick lenses having a working distance less
than about 3 millimeters. One particular preferred version is
characterized as a thick lens having a center thickness CT=2.08 and
the aspheric surface described by the equation:
Z = Y 2 R ( 1 + 1 - ( 1 + K ) Y 2 / R 2 ) + A 4 Y 4 + A 6 Y 6 + A 8
Y 8 ##EQU00001##
where R=2.2015; K=-0.8285; A4=-6.6119; A6=-2.7837; A8=2.6921; and
X, Y and Z are spatial axes.
[0077] Systems arranged as dynamically tunable include a grating or
prism rotatably mounted on a pivot axis coupled to actuator whereby
the grating may be rotated in response to application of an
electronic drive signal. Preferred actuators are electromechanical
transducers from the group: piezoelectric crystals; voice coils;
stepper motors; micro electromechanical systems MEMS; and linear
motors.
[0078] Preferred embodiments include a mounting system of a base
element; a lens mount; a feedback element mount arranged about a
system axis. The base element is arranged to receive a
semiconductor gain medium therein at a receiving seat in a manner
where a system axis is coincident with a symmetry axis of the
mounted gain medium. The lens mount with a lens therein provides
mechanical and mounting support whereby the system axis is
coincident with the lens axis. The feedback element mount receives
a feedback element provides mechanical and mounting support whereby
the system axis intersects an optical surface of the feedback
element.
[0079] The thermal coefficient of expansion is similar for all the
elements of the mounting system to assure even expansion and
preserved alignment during temperature changes. This can be
accomplished by forming each of the elements from a similar metal.
Together, the mounting systems mass can be made as small as a few
hundred grams and have no linear dimension greater than about 30
millimeters and occupy a volume of about 30 cubic centimeters. In
highest performance versions, a mounting system additionally
includes a surface arranged as a thermoelectric cooler interface
assuring good thermal coupling between the gain medium seat, lens
mount, and feedback element mount.
[0080] A diamond submount may be used to better couple the base
element to the gain medium in versions having best heat extraction
configurations.
[0081] A base element has a lapped planar surface which forms a
portion of a sliding interface along with the lens plate having a
similar flat surface. Lens mounts also have a cylindrical lens
holder with lens seat therein, and threaded outer surface.
[0082] A mount feedback element is a stationary portion and a
movable portion, the movable portion is rotatably mounted about a
pivot axis coupled with the stationary portion.
[0083] Best versions are lasers with a mounting system also
including an output coupler lens mount. Such output coupler lens is
arranged to collimate output from the lasers so presented.
[0084] The examples above are directed to specific embodiments
which illustrate preferred versions of devices and methods of these
embodiments. In the interests of completeness, a more general
description of devices and the elements of which they are comprised
as well as methods and the steps of which they are comprised is
presented herefollowing.
[0085] One will now fully appreciate how compact and rugged mid-IR
lasers having high intensity output beams are realized. Although
the present embodiments have been described in considerable detail
with clear and concise language and with reference to certain
preferred versions thereof including best modes anticipated by the
inventors, other versions are possible. Therefore, the spirit and
scope of the invention should not be limited by the description of
the preferred versions contained therein, but rather by the claims
appended hereto.
* * * * *