U.S. patent application number 14/199638 was filed with the patent office on 2016-09-29 for ultra-high power single mode fiber laser system with non-uniformly configured fiber-to-fiber rod multimode amplifier.
This patent application is currently assigned to IPG Photonics Corporation. The applicant listed for this patent is Valentin Gapontsev, Igor Samartsev, Dimitri Yagodkin. Invention is credited to Valentin Gapontsev, Igor Samartsev, Dimitri Yagodkin.
Application Number | 20160285228 14/199638 |
Document ID | / |
Family ID | 51491965 |
Filed Date | 2016-09-29 |
United States Patent
Application |
20160285228 |
Kind Code |
A1 |
Gapontsev; Valentin ; et
al. |
September 29, 2016 |
Ultra-High Power Single Mode Fiber Laser System With Non-Uniformly
Configured Fiber-to-Fiber Rod Multimode Amplifier
Abstract
A high power single mode ("SM") laser system includes an
amplifier configured with a monolithic fiber to rod fiber waveguide
which is structured with a multimode ("MM") core and at least one
cladding surrounding the core. The MM core is configured with a
small diameter uniform input region receiving and guiding a SM
signal light, a mode-transforming frustoconical core region
expanding outwards from the input region and a relatively large
diameter uniform output portion. The high power laser system is
further structured with a MM pump light delivery fiber having a
numerical aperture NA.sub.2, which is at most equal to that one of
the output core portion. The amplifier and pump light output fiber
traverse an unconfined delivery cable and terminate upstream from a
mirror which is configured to focus the incident pump light into
the core of the amplifier in a counter-propagating direction. The
mirror is further structured with an opening aligned with the
optical axis of the amplifier and configured to provide a lossless
passage of amplified signal light in a propagating direction.
Inventors: |
Gapontsev; Valentin;
(Worcester, MA) ; Samartsev; Igor; (Oxford,
MA) ; Yagodkin; Dimitri; (Burbach, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Gapontsev; Valentin
Samartsev; Igor
Yagodkin; Dimitri |
Worcester
Oxford
Burbach |
MA
MA |
US
US
DE |
|
|
Assignee: |
IPG Photonics Corporation
Oxford
MA
|
Family ID: |
51491965 |
Appl. No.: |
14/199638 |
Filed: |
March 6, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61773370 |
Mar 6, 2013 |
|
|
|
61773365 |
Mar 6, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01S 3/0407 20130101;
H01S 3/094069 20130101; H01S 3/094038 20130101; H01S 3/0941
20130101; G02B 6/0288 20130101; H01S 3/094049 20130101; H01S
3/06783 20130101; H01S 3/06733 20130101; H01S 3/06745 20130101;
H01S 3/094007 20130101; G02B 6/4296 20130101; H01S 3/005 20130101;
H01S 3/06704 20130101; H01S 3/06729 20130101; H01S 3/094003
20130101; H01S 3/06754 20130101 |
International
Class: |
H01S 3/067 20060101
H01S003/067; H01S 3/094 20060101 H01S003/094; H01S 3/04 20060101
H01S003/04; H01S 3/0941 20060101 H01S003/0941 |
Claims
1. An ultra-high power fiber laser system, comprising: a base laser
console enclosing: a single mode ("SM") seed source emitting SM
signal, a SM passive fiber receiving and guiding the SM signal
light in a propagating direction, a fiber pigtailed laser diode
pump outputting pump light, and a utility assembly configured to
support a laser system operation including control and safety
electronics; an optical laser head spaced from the base laser
console; at least one flexible delivery cable extending between the
console and laser head; a fiber to fiber rod booster amplifier
having a major length traversing the delivery cable and provided
with an output end which is directly coupled to the laser head, the
booster amplifier being configured with an all doped multimode
(MM'') monolithic core configured with at least: a uniformly
dimensioned input core region coupled to a downstream end of a core
of the SM passive fiber, the cores of respective input core region
and SM fiber being configured with respective mode field diameters
("MFD") which substantially match one another, a mode transforming
core region expanding from the input core section and configured to
expand the MFD of the SM while preventing an excitation of high
order modes, an output amplifying uniformly dimensioned core region
extending from the mode transforming core region and having a
diameter larger than that of the input core region, wherein the
booster fiber amplifier is operative to emit system output light in
substantially the SM with a power varying in a kW-MW range; a
mirror mounted in the laser head and spaced downstream from the
booster amplifier; and a least one MM pump light delivery fiber
extending through the delivery cable and guiding the pump light so
that the pump light is incident on the mirror configured to
redirect the pump light in a counter-propagating direction to
end-pump the MM core of the booster amplifier.
2. An ultra-high power single mode ("SM") booster stage,
comprising: a fiber to fiber rod amplifier extending through free
space and configured with a multimode ("MM") non-uniformly
dimensioned core, which guides and amplifies signal light, and a
cladding coextending with and surrounding the core; a MM pump light
output fiber extending through free space and including a terminal
region coextending with a terminal region of the amplifier; a laser
head receiving the terminal regions of respective amplifier and
pump light output fibers; and a mirror provided in the laser head
and having a central opening, which is dimensioned to be traversed
by the amplified signal light in the propagating direction, and the
mirror being configured to redirect the pump light incident thereon
in a counter-propagating direction so that the pump light is
coupled into the core of the amplifier.
3. The booster stage of claim 2 further comprising an unconfined
delivery cable traversed by the amplifier and pump light output
fiber, the terminal regions of respective output fiber and
amplifier extending substantially parallel to one and projecting
over a downstream end of the delivery cable into the laser
head.
4. The booster stage of claim 2, wherein the MM core is doped with
light emitters and structured with: a small diameter uniform input
region configured to guide a single mode ("SM") signal light along
a fiber part of the amplifier in a propagating direction, and a
mode transforming region bridging the input and output
portions.
5. The booster stage of claims 4, wherein the MM core further
includes a large diameter uniform output region extending from the
mode transforming region and guiding the SM signal light along the
terminal region of the amplifier,
6. The booster stage of claims 2 through 4 further comprising: a
buffer fused to the terminal regions of respective amplifier and
output fiber and mounted to the laser head, and a sleeve enclosing
at least a part of the terminal regions of respective amplifier and
pump fiber, buffer and mirror and coupled thereto, wherein the
mirror and terminal regions of respective amplifier and output
fiber are adjustable relative to one another so as to couple the
pump light into the MM core of the amplifier while outputting the
amplified SM signal light through the opening in the propagating
direction
7. The booster stage of claims 2 and 3, wherein a numerical
aperture of the pump light is at most equal to a numerical aperture
of the output core region of the amplifier.
8. The booster stage of claim 2, wherein the mirror is spherical or
aspherical.
9. The booster stage of claims 2 through 5 further comprising an
air supply system operative to introduce an air stream into the
sleeve so as to carry out impurities out of the sleeve as the air
stream exits through the hole of the mirror.
10. The booster stage of claim 2 further comprising an actuator
operative displace the mirror and the terminal regions of
respective amplifier and delivery fiber in XYZ planes relative to
one another.
11. The booster stage of claim 2, wherein the fiber to fiber rod
amplifier is continuous between input and output terminal regions
thereof.
12. The booster stage of claim 2, wherein the fiber to fiber rod
amplifier includes fiber and fiber rod parts fused together.
13. The booster stage of claims 2 through 5, wherein the sleeve
includes two cup-shaped parts insertable one into another and
coupled to one another so as to provide an impurities-fee interior
of the laser head.
14. An ultra-high power fiber laser system comprising: a seed laser
source configured to deliver a signal; the booster stage of any one
of claims 2-12.
15. The ultra-high power fiber laser system of claim 14 further
comprising a housing enclosing a system operative to generate a
high harmonic of a fundamental frequency of the amplified SM signal
light, the system being configured with a housing optically and
mechanically coupled to the laser head at a distance from the
terminal regions of respective amplifier and pump light output
fiber.
16. The ultra-high power fiber laser system of claim 15, wherein
the laser head and housing are configured with respective passages
aligned with the opening of the mirror and guiding the amplified
signal light in the propagating direction from the laser head into
the housing.
17. The ultra-high power fiber laser system of claim 14 further
comprising a main console spaced from the laser head and housing
the seed laser source, the seed source being provided with a SM
passive seed output fiber fused to an upstream region of the
amplifier within the main console, the seed output fiber being
provided with a core configured with a mode field diameter which
substantially matches that one of the input region of the MM core
of the amplifier.
18. The ultra-high power fiber laser system of claim 17 further
comprising a utility assembly enclosed within the main console and
configured to support a laser system operation, the utility
assembly including control and safety electronics.
19. The ultra-high power fiber laser system of claim 17 further
comprising a cooling system configured to reduce thereto-dynamic
stresses produced by the signal and pump lights within the
amplifier, the cooling system being configured with an outer layer
of polymeric material coated upon an outer surface of the amplifier
and capable of withstanding a temperature of up to a several
hundred .degree. C.
20. The ultra-high power fiber laser system of claim 19, wherein
the cooling system includes a source of coolant housed within the
main console, at least one flexible pipe having an input, which is
coupled to the source, and output, the flexible pipe having a major
length thereof extending either within the delivery cable or
outside the delivery cable; and a cooling fluid traversing the
flexible pipe.
21. (canceled)
Description
BACKGROUND OF THE DISCLOSURE
Field of the Disclosure
[0001] The disclosure relates to an ultra-high power fiber laser
system provided with a monolithic fiber-to-rod fiber amplifier
directly delivering signal light in substantially a fundamental
mode to a laser head over free space.
BACKGROUND OF THE DISCLOSURE
Prior Art
[0002] The dramatic rise in output power from rare-earth-doped
fiber sources over the past decade, via the use of double clad
fibers led to a range of fiber-laser system with outstanding
performance in terms of output power, beam quality, overall
efficiency, and wavelength flexibility. Yet the power scaling of
modern high power fiber laser systems is far from satisfying ever
increasing industry demands.
[0003] As well understood by one of ordinary skill, the premise,
underlying an efficient high power, single or low mode ("SM/LM")
fiber amplifier, is rather simple: maximally enlarge the core
diameter of SM/LM active fibers and minimize the length of the MM
active core guiding light. The reason for a large core diameter and
short length can be easily understood by the necessity of having
high power peak and average-power levels and substantially
diffraction-limited laser outputs.
[0004] However, increasing the core diameter of waveguides leads to
increasing the number of guided high order modes ("HOM") which
degrade the beam quality. This can be mitigated by the core's
greatly reduced numerical aperture ("NA"), but doing so critically
limits the amount of pump light that can be coupled into the core.
In this case, the only viable option in the prior art for reaching
high powers is clad pumping. Using clad pumping requires increasing
the necessary length of active fibers since absorption of pump
light coupled into the cladding is about eight times less efficient
than that coupled into the core. Hence a threshold for the onset of
NLEs radically lowers. Numerous techniques for improving the
scalability of high power laser systems of emitting SM/LM outputs
have been developed and are briefly discussed immediately
below.
[0005] One critical development greatly affecting the scalability
includes a double clad fiber which is well known to an artisan in
the fiber laser arts. For example, U.S. Pat. No. 5,818,630 and its
extended family of patents disclose a high power fiber laser system
including a double clad MM active fiber amplifier. A good quality
of output beam is realized by a mode matching element--so called
mode converter well known to one of ordinary skill in the optics as
a beam expander--located between SM passive and MM active fibers.
The converter is configured to expand the SM to a size of
fundamental mode of the active fiber which, as one of ordinary
skill knows, may approximately be described as a Gaussian
shape.
[0006] This design does not come without certain liabilities.
Obviously, it is bulky and not rugged limiting thus its use to a
relatively stress-free environment, which is not easy to create and
maintain in the field. If a tapered fiber is used as a mode
matching element, it is fused to ends of respective SM and MM
fibers. In this configuration, the power losses at splices between
fibers and distortion of a diffraction limited beam are sharply
increased. Also, because of the double-clad configuration, a
pumping technique includes coupling pump light into an inner
cladding which increases the length of the amplifier and rises a
thresholds for NLEs.
[0007] Recently, the fiber laser industry has turned to crystal
fiber rods typically used in output stages of amplifier chains to
address the scalability of fiber amplifiers. Based on air hole clad
technology, a crystal fiber rod includes a double clad structure
with a doped core, large diameter pump core or inner cladding and
outer cladding.
[0008] The SM doped core of the rod fiber has a very small NA, and
is made from fused silica/quartz which typically hosts a low dopant
concentration. The low numerical aperture limits the amount of high
power pump light which may be coupled into the core in amounts
necessary for reaching ultrahigh powers in a kW-MW range depending
on whether a laser system operates in CW or pulsed regime.
Accordingly, a sufficient amount of pump light can be coupled only
into the pump core or inner cladding. To fully utilize clad-coupled
pump light, thus, a fiber rod should have a length varying between
several tens (typically exceeding 50) of centimeters and meters.
Even the shortest available fiber rod thus is detrimentally
affected by the presence of NLEs. The latter, of course, critically
limits the laser's power scalability.
[0009] A low concentration of ions in fiber rods, such as ytterbium
("Yb"), is typically about 7xx ppm. With such a low dopant
concentration, absorption of pump light is also low. To obtain the
desired kW-MW powers, the pump light should be emitted at very high
powers. To provide adequate absorption of the pump light, the
overall length of a fiber rod should be increased. As discussed
above, increasing the overall length lowers a threshold for NLEs
which, in turn, limits power scalability an amplifier.
[0010] An open-end structure of fiber rods is another area of
concern. Typically, launching an input signal throughout air holes
can be realized only by micro-optics. The latter, of course,
complicates the entire system configuration and makes the latter
cumbersome and expensive. The presence of air in holes lowers
thermal conductivity properties. In particular, the air holes slow
dissipation of heat which, in turn, may damage the rod itself and
cause an environmental hazard.
[0011] The use of crystal fiber rods and amplifying fiber devices
based on the rod is disclosed in U.S. Pat. No. 7,813,603 ("603").
The structure as taught by the '603 includes an amplifying medium,
at least one pump light delivery fiber, and a reflective element
directing the pump light into the amplifying medium in a direction
which is counter to the direction of signal light propagation. The
amplifying medium is configured as a multi-clad photonic crystal
fiber rod with art inner cladding, referred to as a multimode pump
core which receives the reflected pump light in a direction counter
to a signal propagating direction. As discussed above, because of a
small SM doped core, the absorption of the reflected pump light
occurs along a substantial fiber length to avoid the onset of NLEs
at a low power level. The disclosed structure operates at no more
than a 150 W output power to prevent damage to the doped core.
[0012] Summarizing the above, the design of high power fiber
systems faces difficult challenges because of the following
factors: nonlinear effects in fibers in general and fiber rods in
particular, loss of fundamental mode power to high order modes
("HOM"); pump brightness and, of course, excessive heat generation.
Although each factor limits power scaling independently, they are
also interrelated.
[0013] A need, therefore, exists for an ultra-high power SM fiber
laser system substantially overcoming the above-discussed
disadvantages of the known systems.
[0014] Another need exists for a compact, portable SM ultra-high
power fiber laser system capable of outputting kW level average and
MW level peak powers.
BRIEF DESCRIPTION OF THE DISCLOSURE
[0015] The disclosed high power SM laser system is configured with
a booster stage including an unconfined monolithic fiber-to-rod
fiber booster which is defined by consecutive fiber input,
transforming and fiber rod output regions. The monolithic MM
waveguide is structured with a continuous MM core and at least one
cladding which coextends and surrounds the core. The input region
of the core extending through the input fiber region of the
waveguide is small and configured to support SM signal light which
is received from a seed source.
[0016] Somewhere along the length of the waveguide, the core
expands assuming a bottleneck-shaped cross-section, which defines
the transforming region running into the output amplifying region.
The output region is structured with a uniform diameter larger than
the uniform diameter of the input core portion. Despite the
possibility of supporting multiple high order modes ("HOM"), it is
a single, fundamental mode that is greatly amplified compared with
the amplification of HOMs which are thus reduced to noise. As a
consequence, the amplifier emits light in substantially a single,
fundamental mode. The booster has no splices and hence neither
splice losses nor the possibility of coupling between the
fundamental and HOMs.
[0017] The booster may be unconfined extending over free space and
delivering signal light to a working zone without the use of
customary SM passive delivery fibers. The increased core diameter
of the booster allows greater pump light powers to be absorbed in a
short core. In particular, at least one MM pump light fiber is
provided in close proximity to the output end of the booster. The
disclosed configuration of a pump mechanism allows pump light to be
coupled into the core of the booster in a direction counter to as
signal propagating direction which, as known to the artisan,
intensifies absorption of pump light.
[0018] The counter-propagating coupling of pump light is provided
by a reflective element spaced from the output ends of respective
booster and delivery fiber. Configuring the pump light delivery
fiber with a numerical aperture ("NA") smaller than that of the
booster and the desired curvature of the reflective element help
coupling the reflected pump light into the output core end of the
booster.
[0019] The large core diameter of the output region and pumping
mechanism are important parameters allowing the booster to be
relatively short. The length is selected to provide absorption of
coupled pump light mainly along the output core region of the rod
fiber region of the booster. Despite great pump light powers, the
reduced length minimizes the onset of NLEs.
[0020] The reflective element is configured with an opening aligned
with the optical axis of the booster. The dimensions of the opening
are selected to prevent meaningful pump light losses and provide no
losses of amplified signal light traversing the opening in a
propagating direction.
[0021] High power density of the amplified signal light is
hazardous to fiber surfaces. To reduce it, the disclosed system is
configured with a coreless termination block typically made from
quartz. The block is positioned between the ends of respective
booster and pump light delivery fiber and reflective element. The
upstream face of the block is fused to fiber ends of respective
booster and pump fiber.
[0022] The compactness of the disclosed booster is further improved
by providing a sleeve enclosing the rod fiber portion, termination
block and reflective element. The sleeve is coupled to the enclosed
optical components to define an end package also known as a laser
head.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] The above and other features and advantages of the disclosed
system will become readily apparent from the following specific
description accompanied by the drawings, in which:
[0024] FIG. 1 shows an optical scheme of the disclose booster
stage;
[0025] FIG. 1A is a cross-sectional view of booster stage along
lines A-A in FIG. 1;
[0026] FIG. 2 illustrates an amplifier of the booster stage of FIG.
1;
[0027] FIG. 3 illustrates a laser head receiving the booster stage
of FIG. 1;
[0028] FIG. 4 is an exemplary schematic of the terminal package of
FIG. 3.
SPECIFIC DESCRIPTION
[0029] Reference will now be made in detail to embodiments of the
invention. Wherever possible, same or similar numerals are used in
the drawings and the description to refer to the same or like parts
or steps. The drawings are in simplified form and are not to
precise scale. Unless specifically noted, it is intended that the
words and phrases in the specification and claims be given the
ordinary and accustomed meaning to those of ordinary skill in the
fiber laser arts. The word "couple" and similar terms do not
necessarily denote direct and immediate connections, but also
include mechanical optical connections through free space or
intermediate elements.
[0030] Referring to FIGS. 1 and 1A, an exemplary ultra-high power
fiber laser system 10 is capable of emitting multi-kW and higher
signal light in substantially a fundamental mode and MW peak-power
output. The system 10 may be configured with a main console which
includes one or a plurality of cabinets 11 housing one or more pump
sources 13, seed laser 14, optional pre-amplifying cascade(s),
electronics, cooling systems and all other devices and components
which are cumulatively denoted as 35 and configured to assist in
generating an ultra-high power SM system output.
[0031] The SM signal light emitted by seed laser 14 is further
guided along and amplified in a fiber booster stage 12 configured
with a flexible delivery cable 25 which extends over free space
between the console and a laser head 15. The booster stage 12
further includes a fiber booster 18 traversing cable 25 and
configured as an active double clad fiber with a MM core which is
doped with one or more light emitters, such as rare-earth elements.
At least one pump light output fiber 24 also extends over free
space within delivery cable 25 between cabinet 11 and laser head
15, as shown in FIG. 1A. The laser head 15 is configured with a
reflective element structured to couple pump light into the
amplifier's output end in a counter-propagating direction. Due to
the structural specifics of booster stage 12 along with pumps and
laser head 15, system 10 is operative to emit a substantially
diffraction limited output beam in a signal light propagating
direction.
[0032] The high power SM laser system 10 may have multiple
amplifying stages or a single one as shown, which is referred to as
a final amplifying stage known as booster stage 12 to one of
ordinary skill in the art. The seed source 14 is preferably
configured as a single frequency, SM fiber laser with a SM output
passive fiber 16 delivering signal light at the desired wavelength
to booster stage 12.
[0033] The output fiber 16 is spliced to booster 18 (FIG. 2)
traversing delivery cable 25 and having a MM core which is doped
with one or multiple different types of light emitters selected
from known rare earth elements. The booster 18 may include
separately manufactured fiber and fiber rod parts fused together,
but preferably is manufactured as a monolithic, one piece
component. The signal light is amplified to the desired level as it
is emitted from booster 18.
[0034] Preferably, system 10 operates in the pulsed regime and is
capable of emitting MW SM signal light beam. If system 10 operates
in the continuous regime, average output powers may reach high kW
levels. Regardless of the operational regime, an M.sup.2 beam
quality parameter of the emitted signal light varies between 1.1
and 1.5.
[0035] The high power output of booster stage 12 is coupled into
laser head 15 which, as known to one of ordinary skill in the art,
is provided with beam-guiding optics and located close to the
workpiece to be laser treated. In particular, laser head 15
encloses a terminal block or buffer 20 configured to prevent the
optical surface damage, as known to the artisan. The output
diffraction limited signal beam is further guided through a central
opening of mirror 22 in the signal propagating direction, as will
be disclosed in detail herein below.
[0036] Referring FIG. 2, booster 18 which may have a double clad
configuration with a doped MM core 30 and coextending therewith
claddings. A uniformly shaped input fiber portion 36 is coupled to
passive fiber 16 which guides SM signal light from seed 14 (FIG.
1). An output rod fiber portion 40 of amplifier 18 is also
uniformly shaped and has respective diameters of core and cladding
parts larger than those of the input fiber region. A frustoconical
mode transforming portion 38 bridges input and output fiber
portions 36 and 40, respectively.
[0037] The continuous core MM 30 is configured, in a preferred
embodiment, with at least three portions: an input uniformly
dimensioned region 42, a frustoconical mode transforming region 44
and output amplifying region 46. The excitation of only the
fundamental mode in doped MM core 30 is realized by initially
matching a mode field diameter ("MFD") of the fundamental mode of
MM core 30 with that of passive fiber 16. It is also desirable that
the shapes, i.e., intensity profiles, of respective single and
fundamental modes also substantially match one another. Since the
MFDs of respective SM delivery fiber 16 and amplifier 18
substantially match, there is no need for a mode-matching
optics--the faucets of respective fibers are directly spliced to
one another.
[0038] The excited fundamental mode is guided along input core
region 42 with a relatively small diameter d.sub.1. Entering mode
transforming core region 44, the fundamental mode adiabatically
expends to have a second diameter d.sub.2 which is larger than the
diameter of input core region 42. As the fundamental mode expands
and propagates along respective transforming and amplifying core
portions 44 and 46, respectively, practically no HOMs are excited
which allows the amplified signal light exits booster 18 in the
fundamental mode.
[0039] The core 30 may have two regions instead of the above
disclosed three. In particular, core 30 may be manufactured only
with an input and mode transforming core regions. The cladding of
amplifier 18 may be configured with an inner surface extending
complementary to the outer surface of MM core 30 and thus have the
same two or three differently dimensioned and shaped regions.
Alternatively, the cladding may have a uniform cross-section.
[0040] Turning to FIG. 3, optionally, booster stage 12 (FIG. 1) may
be provided with buffer 20. The upstream, input face of buffer 20
is coupled to the output ends of respective pump fiber and
amplifier 24, 18, respectively. The buffer 20 may be configured as
a silica-glass coreless rod and operative to prevent the damage to
fiber ends due to the reduced power density of the output beam.
[0041] The pump light delivery fiber 24 is configured as a passive,
MM fiber. Preferably, a downstream end region 48 of delivery fiber
24 extends parallel to output region 40 of amplifier 18. The output
ends of amplifier 18 and pump fiber 24, respectively, may be
directly bonded to the upstream face of buffer 20, viewed along a
signal light propagation direction Ls. Other spatial relationships
between these two fibers also within the scope of the disclosure.
For example one, of the delivery and active fibers can be bonded to
the upstream face of the buffer at an angle relative to the optical
axis of the other. More than a single delivery fiber can be used in
combination with amplifier 18.
[0042] The reflective element 22 may be configured as a spherical
or aspherical mirror. An opening 50, provided in mirror 22 and
centered on the optical axis of system 10, is dimensioned to
prevent or minimize pump light losses in the propagating direction.
Preferably, opening 50 has a diameter twice as large as a beam
diameter, but may be somewhat larger, for example, thrice the
diameter of the beam waist. The diameter of mirror 22 is
substantially the same as a distance between the downstream facet
of booster 18 and opening 50.
[0043] Referring to FIGS. 3 and 4, the downstream end regions 40
and 48 of respective booster 18 and pump light output fiber 24
extend beyond the delivery cable 25 of FIG. 1 and are mounted
within a protective sleeve 52 adjacent to the output end of the
delivery cable. In particular, sleeve 52 may surround output region
40 of booster 18 and downstream end region 48 of pump light
delivery fiber and mirror 22. If buffer 20 is provided, it is also
enclosed within sleeve 52 which hemetically seals the enclosed
components adhered to the inner surface of sleeve 52. Any suitable
adhering means, such as epoxy, can be used as adhering material.
Thus, sleeve, 52 creates substantially impurities free environment
which may be further enforced by a stream of fluid, such as air,
periodically pumped into the sleeve by a fluid delivery means
54.
[0044] The end package of optical components enclosed within sleeve
52 is adjustable to provide a reliable coupling of pump light into
core 30 of amplifier 18 and substantially lossless passage of
signal light through an output sleeve opening or passage 62. In
particular, an adjustment mechanism 56 is operative to displace the
downstream ends of waveguide 18 and delivery fiber 24,
respectively, and mirror 22 in XYZ planes relative to one another.
The XYZ actuators are well known to one of ordinary skill in the
mechanical art and can be easily adjusted for the purposes of this
disclosure.
[0045] The sleeve 52 may include two U-shaped caps 58 and 60
overlapping one another to define a closed space. The large
diameter cup 60 is provided with passage 62 defined in the cup's
downstream bottom and aligned with opening 50 of mirror 22, which
is mounted to this bottom. The other cup 58 receives the output end
regions of respective fibers 24 and 18 and buffer 20.
[0046] The disclosed structure can be used in conjunction with a
harmonic generator to obtain wavelengths that cannot be directly
accessed with modern laser technology. Nonlinear frequency
conversion techniques allow generating laser radiation at
wavelengths in the UV, visible and IR spectral ranges. As known to
the artisan, the harmonic generation is realized by a nonlinear
crystal 62. Structurally, crystal 62 and collimating optics 64 may
be placed inside a housing 66 which is optically and mechanically
coupled to sleeve 52.
[0047] The disclosed system is subject to high thermo-dynamic
stresses due ultra-high powers. To combat deleterious effects of
thermal stresses, the disclosed system 10 is configured with a
cooling means. The cooling means may include a source of
pressurized cooling agent, such as water or any other suitable
fluid traversing one or more flexible pipes. The flexible pipe may
be provided within delivery cable 25 or outside it. Alternatively,
the cooling means include a layer of polymeric material,
temperature-resistant material coated upon the outer surface of the
amplifier.
[0048] Although shown and disclosed is what is believed to be the
most practical and preferred embodiments, it is apparent that
departures from the disclosed configurations and methods will
suggest themselves to those skilled in the art and may be used
without departing from the spirit and scope of the invention.
Accordingly, the present invention should be construed to cohere
with all modifications that may fall within the scope of the
appended claims.
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