U.S. patent application number 12/166027 was filed with the patent office on 2008-11-20 for microchip-yb fiber hybrid optical amplifier for micro-machining and marking.
This patent application is currently assigned to IMRA AMERICA, INC.. Invention is credited to Martin E. FERMANN, Almantas GALVANAUSKAS, Donald J. HARTER, Ferenc RAKSI.
Application Number | 20080285117 12/166027 |
Document ID | / |
Family ID | 35186785 |
Filed Date | 2008-11-20 |
United States Patent
Application |
20080285117 |
Kind Code |
A1 |
GALVANAUSKAS; Almantas ; et
al. |
November 20, 2008 |
MICROCHIP-Yb FIBER HYBRID OPTICAL AMPLIFIER FOR MICRO-MACHINING AND
MARKING
Abstract
The invention describes techniques for the control of the
spatial as well as spectral beam quality of multi-mode fiber
amplification of high peak power pulses as well as using such a
configuration to replace the present diode-pumped, Neodynium based
sources. Perfect spatial beam-quality can be ensured by exciting
the fundamental mode in the multi-mode fibers with appropriate
mode-matching optics and techniques. The loss of spatial
beam-quality in the multi-mode fibers along the fiber length can be
minimized by using multi-mode fibers with large cladding diameters.
Near diffraction-limited coherent multi-mode amplifiers can be
conveniently cladding pumped, allowing for the generation of high
average power. Moreover, the polarization state in the multi-mode
fiber amplifiers can be preserved by implementing multi-mode fibers
with stress producing regions or elliptical fiber cores These
lasers find application as a general replacement of Nd: based
lasers, especially Nd:YAG lasers. Particularly utility is disclosed
for applications in the marking, micro-machining and drilling
areas.
Inventors: |
GALVANAUSKAS; Almantas; (Ann
Arbor, MI) ; HARTER; Donald J.; (Ann Arbor, MI)
; FERMANN; Martin E.; (Ann Arbor, MI) ; RAKSI;
Ferenc; (Ann Arbor, MI) |
Correspondence
Address: |
SUGHRUE, MION, ZINN, MACPEAK & SEAS, PLLC
2100 Pennsylvania Avenue, N.W.
Washington
DC
20037-3213
US
|
Assignee: |
IMRA AMERICA, INC.
|
Family ID: |
35186785 |
Appl. No.: |
12/166027 |
Filed: |
July 1, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11643760 |
Dec 22, 2006 |
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12166027 |
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11339679 |
Jan 26, 2006 |
7190511 |
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11643760 |
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11141704 |
Jun 1, 2005 |
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11339679 |
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10645662 |
Aug 22, 2003 |
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11141704 |
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09317221 |
May 24, 1999 |
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10645662 |
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09116241 |
Jul 16, 1998 |
6208458 |
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09317221 |
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08822967 |
Mar 21, 1997 |
6181463 |
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09116241 |
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Current U.S.
Class: |
359/341.1 |
Current CPC
Class: |
H01S 3/094069 20130101;
H01S 3/1618 20130101; G02B 6/262 20130101; H01S 3/09415 20130101;
H01S 3/06754 20130101; H01S 3/1611 20130101; H01S 3/0615 20130101;
H01S 3/0627 20130101; H01S 3/094007 20130101; G02F 1/3525 20130101;
H01S 3/06708 20130101; H01S 3/0606 20130101; H01S 3/1673 20130101;
H01S 3/094003 20130101; H01S 3/302 20130101; H01S 3/115
20130101 |
Class at
Publication: |
359/341.1 |
International
Class: |
H01S 3/00 20060101
H01S003/00 |
Claims
1-5. (canceled)
6. A fiber amplifier comprising; a transverse cross section that
comprises at least one core region, said amplifier further
containing at least one coreless end used for the extraction of
optical energy from said fiber amplifier, said optical energy being
contained in a beam of a certain diameter; said coreless end
further having a transverse cross section which differs from the
transverse cross section at fiber locations remote from said end,
the diameter of said extracted optical beam at said at least one
coreless end being larger than the diameter of said fiber core.
7. A method of increasing the optical damage threshold of a fiber
amplifier having a fiber with a core, comprising: forming a
coreless end on said fiber which is used for extraction of optical
energy from said amplifier to increase the diameter of optical
energy at said end to exceed the diameter of said fiber core.
8. A fiber amplifier comprising; a transverse cross section that
comprises at least one core, said amplifier further containing at
least one coreless end used for the input of optical energy into
said fiber amplifier, said optical energy being contained in a beam
of a certain diameter; said end further having a transverse cross
section which differs from the transverse cross section at fiber
locations remote from said end, the diameter of said input optical
beam at said at least one end being larger than the diameter of
said fiber core.
9. A buffer pigtail, comprising; a light input end; a light output
end coupled to a fiber amplifier in a manner as to eliminate or
minimize any interface therebetween; said pigtail having a
composition matching that of the fiber of said fiber amplifier, and
functioning to increase an optical damage threshold of said fiber
amplifier.
10. A buffer pigtail, comprising; a light output end; a light input
end coupled to a fiber amplifier in a manner as to eliminate or
minimize any interface therebetween; said pigtail having a
composition matching that of the fiber of said fiber amplifier, and
functioning to increase an optical damage threshold of said fiber
amplifier.
Description
[0001] This is a continuation of application Ser. No. 11/643,760
filed Dec. 22, 2006 which is a continuation of Ser. No. 11/339,679
filed Jan. 26, 2006, which is a continuation of U.S. application
Ser. Nos. 11/141,704 filed Jun. 1, 2005 and 10/645,662 filed Aug.
22, 2003, which is a continuation of Ser. No. 09/317,221 filed May
24, 1999, which is a continuation in part of Ser. No. 09/116,241
filed Jul. 16, 1998, U.S. Pat. No. 6,208,458 which is a
continuation in part of Ser. No. 08/822,967, filed Mar. 21, 1997,
U.S. Pat. No. 6,181,463. The entire disclosures of the prior
applications are considered part of the disclosure of the
accompanying continuation application and are hereby incorporated
by reference.
FIELD OF THE INVENTION
[0002] This invention relates generally to laser systems having
application to such fields as micro-machining, drilling and
marking. A primary characteristic of these lasers is their
high-powered short-pulsed output, which in, for example, an
industrial application, preferably machines the surface of a target
or workpiece by an ablation technique. The invention also relates
generally to laser systems which can serve in replacement of more
expensive Nd based lasers, such as diode-pumped Q-switched Nd:YAG
lasers and other lasers using Nd-based materials.
BACKGROUND OF THE INVENTION
[0003] It has been known in the prior art to use pulsed laser
systems to effect such processes as diverse as metal machining and
biological tissue removal. Of chief concern in these systems is the
amount of "collateral damage" to the surrounding regions of the
workpiece, or, in the case of biological uses, surrounding tissues.
In the case of the machining of metallic workpieces, for example,
laser pulses greater than 100 microseconds in duration will machine
the workpiece at the cost of creating a significant pool of molten
liquid which is ejected from the beam impact site. Cleanly machined
features cannot be obtained with this machining technique owing to
the tendency of the molten material to spatter the workpiece and/or
freeze and harden on the workpiece itself. This effect is due, of
course, to the transfer of a significant amount of heat into the
workpiece material at the target zone and at surrounding areas as
well. In the case of biological procedures, this heat transfer
effect typically causes unacceptable collateral damage to the
surrounding tissues.
[0004] A general but partial solution to this problem resides in
the use of shorter pulse durations. With shorter pulses the target
is heated more quickly and thus reaches the evaporation point
before significant liquid is permitted to form. Thus, in this
arena, the shorter Q-switched temporal pulse may find advantage in
certain applications. The pulse widths of conventional Q-switched,
solid state lasers used in micro machining is approximately 50-200
nanoseconds. This pulse width has for many cases proven to provide
a reasonable balance between laser cost, machining accuracy and
collateral effects such as the size of the heat-affected zone
(HAZ), it being generally understood that the cost of laser systems
of significant power increases greatly with the shortness of the
period of the output pulse.
[0005] However, even in the above mentioned pulse width range, the
degree of heat transfer into the material is unacceptable for many
applications. Recently developed lasers reported at OE/LASE SPIE
vol. 2380 pp 138-143 (1995) which generate pulses in the 8-20 ns
range abate this problem to a degree, however since the threshold
for ablation in the nanosecond range decreases as the reciprocal of
the square root of the laser temporal pulse width, it is apparent
that as the pulsewidth is further reduced, the range of potential
applications broadens considerably.
[0006] With advances in pulsed laser systems, lasers having pulse
widths well into the femtosecond regime have become available. At
these ultrashort pulse widths, collateral damage to surrounding
regions becomes almost negligible, because of the lack of
significant heat transfer into zones outside of the immediate
target area. Essentially, the material at the target is
substantially instantaneously vaporized while the fleeting duration
of the impact of the laser energy substantially eliminates the
possibility of heat transfer into surrounding areas. In general, it
is known that the heat penetration depth L is proportional to the
square root of the product of the heat diffusion coefficient
(specific to the material) and the pulse width t. Consequently, as
the pulse width becomes shorter, the heat penetration depth
decreases proportionately. With femtosecond pulses, ablation thus
takes place before significant heat can be transferred into the
material, so that little or no heat effected zone (HAZ) is created.
U.S. Pat. Nos. 5,656,186 and 5,720,894, incorporated herein by
reference, discuss the above effects generally, and disclose laser
systems operating well into the femtosecond regime in some
instances.
[0007] However, as previously mentioned, the costs associated with
femtosecond-regime micro-machining lasers are not insignificant;
they presently cost five to fifteen times more than the present
nanosecond-regime micro-machining sources. Thus, there is a need in
the industrial and medical fields for a micro-machining or marking
laser which reduces the collateral damage problems of the prior
art, yet has a cost comparable to the present sources. This goal
has been achieved through the present invention, which, through the
use of a novel and highly efficient combination of Q-switching and
Yb fiber laser techniques, provides a source operating in the short
nanosecond or sub-nanosecond regime which is less expensive than
the micro-machining sources now conventionally used, generating
pulses as much as 4 orders of magnitude smaller than that in the
known micromachining arts, and thus producing a greatly decreased
heat affected zone which is practical for a wide variety of
applications while avoiding the greatly increased cost of present
femtosecond systems.
[0008] As mentioned above, Q-switching is currently a common
technique for generating nanosecond optical pulses. It is known
that the main parameter which determines the duration of a
Q-switched laser pulse is the laser cavity round-trip time
T.sub.round-trip=2L.sub.cavity/c, where c is the speed of light and
L.sub.cavity is the laser cavity length. Therefore, shorter laser
cavity length is generally required for generating shorter
Q-switched pulses. However, it is known that this shortening of the
cavity length normally reduces the mode volume which makes if more
difficult to achieve suitable pulse energies. Further amplification
in a solid-state amplifier is usually not a practical solution due
to the very low gain characteristic of solid-state amplifiers.
Moreover, pushing the energies from a short pulse microchip laser
sufficient for micromachining, reduces the microchip laser
efficiencies to around 5%.
[0009] Here we demonstrate that by using a low energy microchip
laser in conjunction with a highly efficient large core Yb fiber
amplifier these problems can be overcome and subnanosecond optical
pulses can be achieved at high pulse energies.
[0010] Known Nd: based lasers, in addition to being expensive, are
less efficient compared to Yb-doped fiber amplifiers. For example,
Nd:YAG lasers transform the diode pump power to optical output at
approximately 50% efficiency. In contrast, Yb fiber amplifiers
transform laser diode pump power to optical output with about 90%
efficiency. This better efficiency leads to certain cost savings,
especially when the comparison is based on cost per unit of output
power.
[0011] The amplification of high peak-power and high-energy pulses
in a diffraction-limited optical beam in single-mode (SM) optical
fiber amplifiers is generally limited by the small fiber core size
that needs to be employed to ensure SM operation of the fiber. To
overcome the energy and peak power limitations, recently the use of
multi-mode (MM) fiber amplifiers has been suggested (U.S. Pat. No.
5,818,630 to Fermann and Harter, herein incorporated by reference).
In this work the loss of spatial beam quality in MM fiber
amplifiers is prevented by excitation of the fundamental mode via
the use of appropriate mode-matching bulk optics or fiber tapers as
suggested in U.S. Ser. No. 09/199,728 to Fermann et al., herein
incorporated by reference.
[0012] Particularly interesting are MM fiber amplifiers that are
double-clad since they can be conveniently pumped with high-power
diode lasers to produce high average powers. Moreover, the
achievable small cladding/core ratio in double-clad MM fibers also
allows the efficient operation of fiber lasers with small
absorption cross sections, as suggested in the aforementioned U.S.
Pat. No. 5,818,630 to Fermann and Harter.
[0013] Cladding-pumped fiber amplifiers and lasers have been known
for many years. See U.S. Pat. No. 4,829,529 to J. D. Kafka, U.S.
Pat. No. 4,815,079 to Snitzer et al., U.S. Pat. No. 5,854,865 to
Goldberg, U.S. Pat. No. 5,864,644 to DiGiovanni et al., and U.S.
Pat. No. 5,867,305 to Waarts et al. In the early work in this area
(Kafka and Snitzer) only double-clad fiber amplifiers comprising a
SM core were considered for cladding-pumping, resulting in obvious
limitations for the amplification of high peak power pulses.
Moreover, Snitzer et al. only considered double clad fibers with
approximately rectangular-shaped or non-centrosymmetric cladding
cross sections to optimize the absorption efficiency of such
fibers. The use of relatively small cladding/core area ratios
enabled by double-clad fibers with a large multi-mode core,
however, allows for the efficient implementation of any arbitrary
cladding cross section, i.e. circular, circular with an offset
core, rectangular, hexagonal, gear-shaped, octagonal etc. The work
by Kafka was equally restrictive in that it only considered
double-clad fibers with a single-mode core pumped with coherent
pump diode lasers. Again the use of relatively small cladding/core
area ratios enabled by double-clad fibers with a large multi-mode
core enables the efficient implementation of pump diode lasers with
any degree of coherence.
[0014] The later work of Goldberg and DiGiovanni was not
necessarily restricted to the use of double-clad fibers with SM
fiber cores. However, none of the work by Goldberg and DiGiovanni
(or Kafka, Snitzer or Waarts et al.) considered any technique for
the effective use of multi-mode double-clad fibers as
diffraction-limited or near diffraction-limited high-power
amplifiers. No methods were described for exciting the fundamental
mode in multi-mode amplifiers, no methods were described for
minimizing mode-coupling in multi-mode amplifiers and no methods
were described for controlling the excitation and the size of the
fundamental mode by gain-guiding or by the implementation of an
optimized distribution of the dopant ions inside the multi-mode
fiber core.
[0015] Moreover, the specific pump injection technique suggested by
DiGiovanni comprises built-in limitations for the efficiency of
fundamental-mode excitation in multi-mode fiber amplifiers.
DiGiovanni considers a fused taper bundle with a single-mode fiber
pig-tail in the center of the bundle, which is then spliced to the
double-clad amplifier fiber to simultaneously deliver both the pump
light (via the outside fibers of the fused taper bundle) and the
signal light (via the single-mode fiber pig-tail) to the amplifier
fiber. Due to the limited packing ability of circular structures,
air gaps remain in the fiber bundle before tapering. Once tapered,
surface tension pulls all the fibers in the fiber bundle together,
essentially eliminating the air gaps (as discussed by DiGiovanni et
al.). As a result the outside cladding of the taper bundle becomes
distorted (resulting in a non-circular shape with ridges where the
fibers were touching and with valleys where there were air-gaps).
Hence the central core region and the fundamental mode also become
distorted which limits the excitation efficiency of the fundamental
mode in a MM fiber when splicing the fiber bundle to the
double-clad fiber. In fact any geometric differences in the
cladding shape of the fiber bundle or the double-clad fiber will
lead to a limited excitation efficiency of the fundamental mode in
the MM fiber in the process of splicing.
[0016] For reducing size and cost of the system as well as for
increasing efficiency of the amplification side-pumping (as
described in aforementioned U.S. Pat. No. 5,818,630) rather than
end-pumping might be advantageous. For the benefits of fiber
reliability the use of fiber couplers is preferred. The use of
fiber couplers for pump light injection into MM fibers is discussed
in aforementioned U.S. Ser. No. 09/199,728.
[0017] Normally for many applications a single polarization is
desirable, so the use of polarization preserving fiber is
desirable. There are several means of making polarization
preserving fiber. However, for multimode fiber, elliptical core
fiber is the easiest to manufacture and to obtain at this time.
[0018] Another attractive feature would be ease of fiber coupling
the laser to the application, by using the amplifier fiber as the
fiber delivery system, or a multimode undoped fiber spliced to the
end of the amplifier fiber. This is similar to the fiber delivery
system described in U.S. Pat. No. 5,867,304 and its progeny, herein
incorporated by reference, where a multimode fiber is used for
delivery of a single mode beam. The purpose is to lower the
intensity in the fiber by using the larger effective mode-field
diameter. This allows higher peak powers; >1 KW pulses can be
transmitted without the onset of nonlinear processes. In U.S. Pat.
No. 5,867,304, this fiber is used with ultrashort pulses where the
fiber dispersion distorts the pulses. However, with nanosecond
pulses, dispersion has a negligible effect on the pulse width so
dispersion compensation is not necessary.
SUMMARY OF THE INVENTION
[0019] According to the invention, the goals set out in the
foregoing are achieved through the use of a miniature Q-switched
pulse source which is coupled to a doped Yb fiber laser which
obtains single mode amplification in a multi mode fiber. Short
pulse duration, efficiency, high power, high energy, cost
efficiency and compactness are essentially achieved through the use
of the combination of a compact diode-pumped microchip laser and a
specially designed diode-pumped fiber amplifier. Short duration is
achieved through the short cavity length of a microchip laser,
whereas high efficiency is achieved through the use of a Yb-doped
fiber amplifier pumped at 980 nm. High power is achieved through
cladding pumping geometry, and large fiber core (high core to
cladding ratio).
[0020] High energy is achieved through a number of design features:
the large core, with single mode excitation and propagation, allows
a large cross-sectional area and, consequently, permits relatively
low peak intensities and high saturation energies. Further, the
large core provides a good core-to-cladding ratio, which in
conjunction with the high doping level available for Yb
significantly reduces the pump absorption length and allows for
short amplifier lengths (0.1 to .about.2 m), thus reducing
detrimental nonlinear effects in the fiber without compromising
power and energy extraction efficiencies. For very large cores,
direct in-core pumping can be used. Side pumping provides higher
power extraction efficiency and shorter interaction length compared
to copropagating geometries (along with pump diode protection).
Pigtailing of the fiber ends increases the surface damage threshold
and allows a significant increase in output pulse energies and
powers, while a composite core allows the robust coupling of the
microchip seed pump into a fundamental mode of the fiber core. This
also permits use of a non-perfectly-gaussian input beam from the
microchip laser.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a schematic layout of the laser system of the
invention;
[0022] FIG. 2 illustrates schematically one actively Q-switched
micro-laser according to the invention;
[0023] FIG. 2a illustrates a typical layout of the actively
Q-switched micro chip laser;
[0024] FIG. 3 illustrates the temporal profile of the output of the
lasers of FIGS. 2 and 2a; and
[0025] FIGS. 4 and 4a, where FIG. 4 is presented as an inset in
FIG. 1, illustrate a fiber-end coupling and optical damage
avoidance technique.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0026] FIG. 1 illustrates the system configuration of the laser
according to the present invention. In this Figure, reference
numeral 101 indicates a microchip laser source, illustrated in
greater detail in FIGS. 2 and 2(a). It should be noted that, as
used herein, the term "micro chip laser" refers to a laser of small
device size, where at least some of the components, such as the
gain medium and the end mirror, are monolithic. In this
specification, the terms "microchip laser" and "microlaser" are
used interchangeably to refer to a laser having these
characteristics. As described in detail below, the micro chip laser
101 according to the invention is an actively Q-switched laser
which is typically diode pumped.
[0027] In order to achieve excitation of only the fundamental mode
in a multimode-core fiber amplifier, the beam waist
.omega..sub.input of a mode coupled into the amplifier from a
microchip laser has to approximately match the beam waist
.omega..sub.mode of this fundamental mode:
.omega..sub.input.apprxeq..omega..sub.mode. Note, that for the
step-index fiber .omega..sub.mode=0.7 r.sub.core, where r.sub.core
is the radius of a fiber core. Therefore, the output of the
microchip laser 101 has to be directed into the fiber amplifier
input (FIG. 4) through properly designed mode-matching optics 102.
The essential function of this mode-matching optical arrangement is
to transform the mode size of an optical beam at the output of a
microchip laser .omega..sub.output into the proper beam size
.omega..sub.input at the input of the fiber amplifier. This imaging
function can be achieved by a variety of optical arrangements, one
example of which is schematically represented in FIG. 1. Note that
the focusing lens in this arrangement is also used to focus the
pump light from a laser diode, and that it is essential for our
invention to achieve focusing of these two input beams at two
different planes, as described below.
[0028] The inventors have determined experimentally that
limitations on the maximum extractable energies in a fiber
amplifier originate from a number of effects, two significant ones
being the Raman gain and surface damage at the input and output
facets of the fiber core.
[0029] The optical damage threshold at the surface of a glass is
characterized by the optical intensity I.sub.th.sup.damage of an
optical beam at this surface. Generally, this threshold intensity
is determined by the type of material used and by its surface
quality. It also depends on the duration of the pulse and average
power (repetition rate) of the pulse train. As is known, the
threshold intensity for optical damage in the nanosecond range
decreases as the reciprocal of the square root of the laser
temporal pulsewidth: I.sub.th.sup.damage.varies.1/ T.sub.pulse.
[0030] The inventors have demonstrated that the optical surface
damage threshold can be significantly increased by using a beam
expansion technique, as shown schematically in FIG. 4 and in
greater detail in FIG. 4a. Here, the fiber-end is bonded to a
buffer of the same material as the fiber. At the end surface, the
optical beam will be expanded to .omega..sub.expanded according
to:
.omega..sub.expanded=.omega..sub.mode {square root over
((1+2L/.omega..sub.mode.sup.2k))}
[0031] Here, k=2.pi.n/.lamda., n is the glass refractive index,
.lamda. is the wavelength of the amplified signal and L is the
thickness of the buffer. It is critical that the quality of the
bond between the surfaces of the fiber and the buffer be
sufficiently high to eliminate any optical interface, and, thus, to
eliminate surface damage at this surface. Various known bonding
techniques can be used to achieve this quality. In the present
case, a silica-glass rod of the same diameter as the outer diameter
of the pump-cladding was spliced to the end of the fiber. The
maximum improvement .eta. of the damage threshold is determined by
the square of the ratio between the radius of the buffer rod
R.sub.buffer and the size of the core mode
.omega..sub.mode:.eta.=(R.sub.buffer/.omega..sub.mode).sup.2. In
the case of a 50 micron core and a 300 micron buffer pigtail as
used in our experimental configuration the improvement was found to
be .about.70 times. Such buffer-pigtail protection is required for
both input and output ends of an amplifier. In the case signal and
pump beams are entering the same end of a fiber (copropagating
configuration) the incoming laser beam has to be focused on the end
of the fiber, as shown in FIG. 4a, inside the bonded buffer, where
there is no interface. If the bonded buffer is a coreless rod of
the same diameter as fiber-amplifier inner cladding (pump
cladding), as shown in FIG. 4a, the pump beam should be focused at
the entrance facet of this silica rod. Note, that generally this
buffer can be a slab with transverse dimension much larger than the
pump cladding. In this case pump beam could be directly focused
into the pump cladding. In the case side pumping is used via a
V-groove or a fiber pigtail the corresponding element can be either
placed directly in the fiber amplifier after the buffer bonding
point, or (if a silica rod is used as a buffer) in this coreless
pigtail.
[0032] The Raman effect causes the spectrum of the amplified pulse
to shift towards the longer wavelengths and outside the
amplification bandwidth of the Yb-fiber amplifier. Raman effect
onset is characterized by a threshold intensity I.sub.th.sup.Raman
in the fiber core which, as is known in the prior art, is inversely
proportional to the effective propagation length L.sub.eff of an
amplified pulse and the Raman gain coefficient:
I.sub.th.sup.Raman.varies.1/L.sub.eff g.sub.Raman. Since the Raman
gain coefficient is determined by the fiber glass properties, in
order to maximize extractable peak powers and, hence, pulse
energies, one has to increase the core size and decrease the
interaction length. The interaction length can be reduced by using
fibers with high doping level which lowers the fiber length,
propagating amplified pulses opposite to the direction of the pump
beam which lowers the pulse energy until the end of the fiber where
the gain will be higher. Also, use of multimode large core fibers
in the double clad configuration facilitates pump absorption and
allows shorter amplifier lengths.
[0033] It is important to note that for certain applications the
presence of strong Raman components in the amplified pulses does
not reduce the usability of these pulses. One example is laser
marking. The inventors demonstrated experimentally that surface
marking is not sensitive to the Raman spectral shift and there is
no degradation in the marking quality even for pulses with only a
small fraction of the total energy in non-Raman shifted spectral
components. In one specific example, this allowed use of .about.150
.mu.J of total pulse energy vs .about.40 .mu.J that was available
without Raman shifting. Thus, for this type of application
significantly higher energies are available from this particular
fiber amplifier.
[0034] However, many applications are sensitive to the presence of
the Raman shift. For example, when wavelength shift is required
prior to end use, via second-harmonic or other frequency conversion
methods, the Raman component would significantly reduce the
efficiency of this conversion and would produce large amplitude
fluctuations. For such applications, a number of existing
techniques currently employed in fiber telecommunication systems
(See, OFC'95 Tutorial Session) could be used for Raman-effect
reduction in the fiber amplifiers, in addition to the methods
described in this invention for optimizing fiber amplifiers in
order to minimize their susceptibility to Raman effect.
[0035] The fiber amplifier 103 is a Yb-doped large-core
cladding-pumped fiber amplifier. The core diameter of this fiber is
approximately 10 micrometers-1 mm in diameter and thus is a true
multimode fiber. However, this multimode fiber performs single mode
amplification using the techniques described in U.S. Pat. No.
5,818,630, herewith incorporated by reference.
[0036] Reference numeral 104 illustrates the pump for the Yb
multimode fiber laser. The pump is advantageously configured as a
side-pumping broad area laser diode, the details of which are well
known in the art. The Yb fiber amplifier can transform the pump
power into an optical output with an extremely high efficiency of
90%. In addition, the multimode Yb amplifier fiber produces an
output which is higher by more than an order of magnitude over that
obtainable with a corresponding conventional single mode fiber
amplifier. The combination of extremely high efficiency and high
gain allows the source microchip laser to operate in a relatively
low energy, higher efficiency regime with little input power.
[0037] FIGS. 2 and 2(a) illustrate two preferred embodiments of the
micro-laser or microchip laser used according to the invention.
These devices are extremely compact, simple, inexpensive and have
low power requirements, yet produce extremely short high peak power
pulses. According to the invention, the microlasers employed are
diode pumped lasers which are actively Q-switched. A primary
advantage of these miniature lasers is that they readily provide
output laser pulses of very short duration as a consequence of
their short laser cavities. Active Q-switching gives good control
over the repetition rate and the number of pulses delivered at a
time, which is useful in marking and micromachining
applications.
[0038] The microchip laser is a solid-state device designed to
provide nanosecond laser pulses at 1064 nm wavelength. Diode
pumping enables high pump-to-laser efficiency, compact design, and
reduced thermal problems in the gain material. The cavity is
designed to provide the shortest possible pulse duration achievable
with active Q-switching with moderate (3 micro J) pulse energy.
[0039] Two representative laser cavity designs are shown in the
Figures. The gain material is Nd doped Yttrium Orthovanadate
(Nd:YVO.sub.4) at 1% doping level. It is cut and oriented in a way
(a-cut) to provide maximum absorption at the pump wavelength. In
addition, the crystal is wedge shaped in FIG. 2a, which allows the
laser to operate only in one linear polarization. The crystal is
pumped longitudinally through its coated dichroic dielectric mirror
surface 201. The pump laser 203 is a 100 micron wide laser diode
with 1 Watt cw pump power. The coating 201 provides passage of pump
light at 808 nm and reflection of laser light at 1064 nm. This
surface acts also as a laser cavity mirror. The laser has a flat
output coupler. Some thermal focusing in the cavity tends to
stabilize the laser cavity mode, but it is basically an unstable
resonator.
[0040] A Pockels cell 207 and a quarter-wave plate 209 inside the
cavity form an electro-optic Q-switch. The Pockels cell is made of
LiNbO.sub.3, in the transversal field configuration. The Pockels
cell at the off state has zero retardation. The quarter-wave plate
provides a static half wave retardation of light in a round trip,
which means changing the polarization of light inside the cavity.
This opposite polarization is then deflected out of the cavity
(FIG. 2a) by the wedge shaped gain material acting as a polarizer,
or a polarizer is placed inside the cavity (FIG. 2). The laser is
in the static off state with the voltage off at the Pockels cell.
When the gain material is pumped continuously, the pump energy is
stored in the gain material for approximately 100 microseconds, the
fluorescence lifetime of the gain material. To Q-switch the laser,
a fast, 2.5 ns rise time high voltage pulse (1200 V) is applied to
the Pockels cell. The voltage on the Pockels cell introduces a
quarter-wave retardation, which compensates the retardation of the
wave plate. The intra-cavity laser field then builds up unimpeded
until it finally reaches saturation by depleting the gain. The
laser pulse leaves the cavity through the output coupler 211, which
has 70% reflectivity and 30% transmission. The resulting laser
pulse has 750 ps pulse duration and 3 micro J energy (FIG. 3). A
solid-state driving electronics circuit provides the fast, high
voltage switching pulses for the Pockels cell with a repetition
rate up to 15 kHz. To operate the laser as a cw source a static
voltage can be applied to the Pockels cell.
[0041] Single longitudinal mode operation is often desired in
lasers. Besides the favorable spectral properties to the laser,
single-mode operation reduces the timing jitter. In single
longitudinal mode operation there is no mode competition and gain
cross-saturation between modes. As a result, the uncertainty of the
turn-on time of the laser relative to the trigger pulse, the
jitter, is reduced. Timing jitter of less than 100 ps is obtained
when the laser operates in single mode.
[0042] The laser cavity is designed for single-longitudinal-mode
operation. For long term stability it is particularly important
that the laser cavity is stabilized against temperature induced
changes. The cavity is designed so that temperature induced effects
do not cause mode-hopping in the laser. The mechanical and optical
construction of the laser is such that the thermal expansion of the
base whereon the laser in mounted compensates for the thermal
effects in the materials. In addition to thermal expansion, further
consideration was given to high thermal conductivity and good
electrical and mechanical properties of the base material, which
enables temperature stabilization of the components.
[0043] Because the length of the resonator is approximately 8 mm,
the laser can support 4 to 6 longitudinal modes at this cavity
length. To achieve single mode operation we employed a resonant
reflector etalon output coupler. The use of an resonant reflector
etalon to maintain single mode operation is described in Koechner
pp. 242-244. The output coupler is a solid Fabry-Perot etalon
working in the reflection mode. Its reflectivity R is modulated as
a function of wavelength. The maximum value of reflectivity occurs
at the resonant wavelengths given by
.delta..sub.etalon/2.pi.=m, (1)
where .delta..sub.etalon the phase difference between interfering
optical beams in the etalon at consecutive reflections m is a half
integer number (m=1/2, 3/2, 5/2, . . . ). On the other hand,
resonant wavelengths of the laser cavity are determined by the
total optical phase difference between beams of consecutive
reflections inside the cavity, .delta..sub.cav,
.delta..sub.cav=4.pi..SIGMA.(n.sub.il.sub.i)/.lamda.
The summation takes into account all the optical materials; gain
material, Pockels cell, polarizer and quarter-waveplate material
and air with their respective optical thickness n.sub.il.sub.i. The
resonant condition for the cavity is
.delta..sub.cav/2.pi.=n, (2)
where n is an integer value (n=1, 2, 3, . . . ). Lasing occurs
essentially when the resonant wavelength of the output coupler
etalon coincides with the resonant wavelength of the laser
resonator cavity. This is given by simultaneous satisfaction of the
above half-integer and integer conditions for m and n respectively.
The number of allowable modes under the gain profile can be
restricted to 1 by proper choice of the output coupler etalon. In
our embodiment of the microlaser a single uncoated LiNbO.sub.3
plate of 1 mm thickness provides sufficient mode selectivity to
allow the laser to operate in a single longitudinal mode.
[0044] The resonance conditions (1) and (2) are temperature
dependent, since the thermal expansion and the thermal change of
the refractive index changes the optical path-length in the laser
cavity and in the resonant reflector output coupler. These effects
combine to shift the resonance peaks of the resonant reflector and
the laser cavity. We have a limited choice of the optical materials
from which the laser is constructed. Their thermal expansion
constants and thermal induced refractive index coefficients
determine the thermal change of resonance conditions, which in
general results in a mismatch of resonances (1) and (2) as the
temperature changes and causes mode hopping of the laser. The
thermal expansion of the base on which the laser is constructed
also contributes to the change of the wavelength of the laser. We
have a rather free choice of the base material. By using Aluminum
Nitride ceramic as the laser base we achieved that the thermal
shift of the laser wavelength was matched to the thermal shift of
the resonance condition of the resonant reflector output coupler
and mode hopping has not occurred within a 4 degree C. temperature
interval. Temperature stabilization of the laser cavity within 1
degree C. resulted in continuous single longitudinal mode operation
of the laser.
[0045] An alternative source may be a passively Q-switched
microchip laser, which can be very inexpensive and may be preferred
in some cases for this reason. The primary reason to use a
miniature source is to keep the laser cavity short which reduces
the pulse width of the laser.
[0046] The miniature laser is coupled to a doped fiber gain medium.
In the invention this medium is a Yb:fiber.
[0047] In order to reach higher peak powers, the invention utilizes
a multi-mode fiber to propagate single mode pulses as described in
U.S. Pat. No. 5,818,630. As described above a mode converter is
used to convert the single mode input to excite the fundamental
mode of the multimode fiber. The mode converter 102 used in this
case is a combination of lenses which mode-matches the output of
the microchip laser to the beam diameter for single mode excitation
of the multimode fiber. In addition to the lenses for
mode-conversion, gain guiding in the Yb:fiber can be used to relax
the tolerances on mode matching. Without gain in the Yb fiber,
robust fundamental-mode excitation becomes increasingly difficult
to achieve for the increasing core size of a fiber amplifier. We
found experimentally that it is particularly advantageous to employ
specially designed fibers in which Yb-doping in the center of the
core has a significantly smaller diameter than the core itself. In
this case, the fundamental mode light experiences significantly
higher gain than multimode light. In our experimental
configuration, we used 50 .mu.m diameter core with 25 .mu.m
diameter doped region in the center, which exhibited a
significantly more robust performance compared to 25 .mu.m
homogeneously doped core. Besides relaxing the alignment
tolerances, the beam parameters of the source are also relaxed. As
the microchip laser may not have a perfect diffraction limited beam
output, gain guiding can be used to correct for this. Also, gain
guiding can correct the distortion expected from DiGiovanni pump
couplers.
[0048] The Yb fiber in this example had a 300 .mu.m outer diameter
and a 50 .mu.m core. The use of relatively small cladding/core area
ratios enabled by double-clad fibers, together with a large
multi-mode core, allows for the efficient absorption of the pump
with, for example, a gear-shape cladding cross section. The
resultant Yb amplifier can be as short as 1.5 M long, as compared
to 5-40 M which would be required of a typical single mode Yb
amplifier.
[0049] Another advantage of this optical source is the ease of
adding a multimode fiber delivery system which propagates a
single-mode. In many applications fiber delivery is very important,
such as in surgery, dentistry and marking in confined spaces. An
example of marking in confined spaces is the marking of assembled
automotive or other parts for antitheft purposes.
[0050] An additional advantage of the shorter pulse is that
nonlinear processes for frequency conversion are more efficient
with the higher peak powers which come from shorter pulses with
similar energies. For certain applications where wavelength
conversion is necessary, for example in UV-range radiation for via
hole drilling, the output of the laser must be frequency tripled to
create the UV radiation. This source, could, for example, replace
frequency tripled Q-switched Nd:YAG lasers and eximer lasers for
this application.
[0051] Another application where frequency conversion is important
is dentistry. For example, in U.S. Pat. No. 5,720,894, it is
described that UV radiation performs relatively damage free
material removal by hard tissue ablation primarily due to the
stronger absorption of that wavelength regime. Three preferred
wavelengths for applications in medicine and dentistry are 2.1
.mu.m, 2.9 .mu.m and 1.55 .mu.m. Like UV radiation, the preference
is due to the strong absorption coefficient of biological tissues
at these wavelengths.
[0052] The most straight forward means for generating 1.55 .mu.m
radiation is to use a laser source which emits at 1.55 .mu.m and a
doped fiber which amplifies 1.55 .mu.m radiation. A microchip laser
which emits 1.55 .mu.m radiation is known, and described in Thony
et al. It is well known that erbium fiber amplifies 1.5 .mu.m
radiation. An alternative source could be a compact erbium doped
waveguide laser as described in; H. Suche, T. Oesselke, J.
Pandavenes, R. Ricken, K. Rochhausen, W. Sohler, S. Balsamo, I.
Montrosset, and K. K. Wong "Efficient Q-switched Ti;Er:LiNbO.sub.3
waveguide laser", Electron. Lett., Vol. 34, No. 12, 11 Jun. 1998,
pp 1228-1230.
[0053] Another alternative is to use a laser source which emits a
different wavelength, such as that of the invention, and use a
frequency conversion step to generate the 1.5 .mu.m radiation.
Examples of a nonlinear conversion step at the output include
doubling, tripling, quadrupling, Raman shift, OPO, OPA or OPG. To
generate 1.55 .mu.m radiation, converting a 1.06 source in a PPLN
OPG is quite convenient.
[0054] In order to generate other wavelengths such as 2.1 and 2.9
.mu.m similar methods can be applied to this laser concept.
[0055] The multimode amplifier of the invention can also amplify a
cw source or operate as a cw source. For example, a marking laser
often has the option of being operated in a cw mode for generating
more of a heat type mark. For the design of high-power cw lasers
the use of MM fibers is advantageous as the reduced cladding/core
area ratio reduces the absorption length in such structures. For
very high cw laser powers, nonlinear effects can indeed occur and
thus MM fibers can be used for the construction of compact
ultra-high power cw fiber lasers. The MM fibers can then be
effectively used for the pumping of fiber Raman amplifiers or for
the construction of Raman lasers operating at wavelength regions
shifted away from the gain band of the doped fibers.
[0056] As previously indicated, a number of major advantages are
achieved according to the invention by employing the combination of
a Q-switched microchip laser and a Yb: fiber amplifier. Because of
the efficiency and gain of the Yb fiber amplifier, the output power
of the microchip laser need not be large. The peak power of this
amplifier is limited by nonlinear effects in the fiber and by the
optical damage thresholds primarily at the fiber ends. The delivery
fiber may be a simple multimode undoped fiber spliced to the end of
the amplifier fiber, or the amplifier 103 can itself constitute the
fiber delivery system. Thus, a simple, inexpensive laser system
suitable for a wide variety of applications can be efficiently
produced.
* * * * *