Coherent light generators – Optical fiber laser
Reexamination Certificate
2003-05-30
2004-10-05
Wong, Don (Department: 2828)
Coherent light generators
Optical fiber laser
C372S069000, C385S027000
Reexamination Certificate
active
06801550
ABSTRACT:
FIELD OF INVENTION
This invention relates to high power fiber lasers and more particularly to a method and apparatus for pumping the fiber laser.
BACKGROUND OF THE INVENTION
Fiber lasers exhibit great potential for applications as high power directed energy sources. Fiber lasers offer the advantages of high efficiency, minimal cooling requirements, and good beam quality. However, the main problem with fiber lasers is obtaining high power out of a fiber because it requires a significant amount of pumping power. Even with the larger cores available through the use of a double clad system, it is still only with difficulty that one can get a few hundred watts of pump light into the end of a fiber. For 1-KW applications one needs at least 2 to 4 KW of laser pump power. How to obtain such high pump power for solid-state lasers has proved to be a problem.
Recent laser demonstrations of Yb doped fibers have shown the power scaling capability of fiber lasers with >100W single mode output and beam quality of M
2
≅1.1. However power scaling beyond the 200W level is severely limited by current fiber and diode pump coupling technology of end-pumped, single-mode double clad Yb:fiber lasers. Step-index, single mode Yb:fiber lasers operating around 1 &mgr;m are limited to about a 200 W power level due to the onset of non-linear optical effects that lead to damage of the fiber core as well as defect related damage at the high CW intensities encountered (>>100 MW/cm
2
). Pump power coupled into the two ends of a double clad fiber laser is limited to about 500 W, employing the brightest currently available laser diode arrays. Even with the high efficiency (50-60%) of an Yb:fiber laser this would result in a maximum fiber laser output of only 250-300 W.
Advances in side coupling to fibers now permit the coupling of multiple discrete emitters to a fiber. However at the pump powers required for a 1-KW laser this would represent greater than 1000 individual diode packages and pump fibers, resulting in a very complex system.
What is required is a 1-KW fiber laser generating a single mode, polarizing preserving output using a longer operating wavelength to achieve a diffraction-limited output. Furthermore, one needs to dramatically reduce the complexity of ultra-high-power fiber lasers.
By way of further background, fiber lasers show great promise as efficient high power continuous wave laser sources. They are highly efficient due a combination of low loss and long interaction length. They can produce diffraction-limited single mode outputs, have a very high surface to volume ratio to efficiently dissipate heat and can use all-fiber couplers and reflectors developed for telecom applications to achieve monolithic, alignment-free resonant cavities.
The highest power yet reported is 110 W from a CW Yb:fiber laser operating in a near-diffraction limited spatial mode with an optical efficiency of 58% and a wall plug electrical-to-optical efficiency of >20%. Work in progress has demonstrated greater than 200 W single-mode CW output with prospects for 300-500 W in multimode operation. In pulsed operation, 64-KW peak power with 51.2 W average power was generated by amplifying 10 ps 1064 nm pulses from a mode-locked Nd:YVO
4
laser in a large mode area (LMA) Yb:fiber amplifier.
However, several important limitations exist to scaling the output power of fiber lasers. These include most notables limited pump coupling to the fiber.
Thus, the principal limitation to date has been the coupling of large pump powers into the active regions of a single mode fiber. The advent of double-clad geometry fiber lasers, where the pump light is guided in a second multimode core, led to greatly increased output power. However, the amount of pump power that may be coupled into the end of a double clad fiber is limited by the brightness of the laser diode and the two entry ports. Newer, highly collimated laser diode arrays have achieved coupling of 250 W into a 400 &mgr;m, 0.22 NA fiber which could permit the end-coupling of up to 500 W of pump into the fiber. However this is still several times less power than that required for a 1-KW output.
The other primary limitation to producing high power fiber lasers is the onset of non-linear optical effects. More benign non-linear affects such as Stimulated Raman Scattering (SRS) and Self-phase modulation (SPM) result in spectral broadening. However Stimulated Brillouin Scattering (SBS) can lead to self-Q switching of the fiber which in turn can damage the fiber due to high intensity. The role of thermal effects has not yet been shown theoretically beyond tuning of the laser output with temperature. However, at high average powers, thermal effects may significantly affect laser performance through thermally induced changes in the refractive index profile and thermal population of the terminal laser level in quasi-three lasers such as Yb, Er and Tm. Imperfections in the fiber surface can also lead to damage due to the high CW intensities (>100 MW/cm
2
for 100 W output).
Although fiber lasers have numerous advantages over bulk lasers, one of their primary disadvantages is the difficulty of pumping them due to their small dimensions. Hence, to date fiber laser CW power output has been limited by the amount of pump power that can be coupled into the ends of the fiber.
A significant advance in high power fibers lasers was the advent of the double clad fiber structure. The central core region of radius, r, and an index, n
2
contains an active laser ion (such as Nd, Yb, Er, or Tm) and is usually sized to support only the fundamental mode. It is surrounded by an inner cladding region of index, no and usually has a polygonal cross-section. This is in turn surrounded by the outer cladding with index, n
0
. For guiding in each region the indices must meet the condition n
0
<n
1
<n
2
. The numerical aperture, NA of each region is given by:
NA=(n
i
2
−n
j
2
)
1/2
The principle of operation of this structure is that light from low brightness pump sources can be guided in the inner cladding region and can be absorbed by the active core, eliminating the need for high brightness sources to pump directly into the active core. Since the overlap area between the inner cladding and the core is small, then the effective pump absorption, &agr;
eff
is the core absorption coefficient, &agr; reduced by the ratio of the areas:
&agr;hd eff=&agr;A
core
/A
clad
Furthermore if the inner cladding region is circularly symmetric and the core is centered, the absorption is further reduced since only the meridonal rays intersect the core region. The use of a polygonal cladding eliminates skew rays, whereas offsetting the core allows skewed rays to access the core region.
The amount of power that may be coupled into the ends of a double clad fiber laser is determined by the brightness of the pump source and the size and numerical aperture of the inner cladding. The radiance theorem states that in radiance (diameter-NA product) is constant in an optical system expressed as the following relation:
D
out
NA
out
=D
in
NA
in
Currently the best results for fiber coupling of laser diodes is 250 W from a 400 &mgr;m, 0.22 NA fiber. This would be sufficient for coupling into the inner cladding region of a typical double clad fiber (200 &mgr;m, 0.44 NA). Using both ends, up-to 500 W of pump could be coupled. However this is still several times less pump than that required for a 1-KW laser source. Given these limitations, it is clear that in order to scale fiber lasers to kilowatt output powers, a scalable side-coupled pump technique is required.
As to side coupled pumping schemes, there are three potential scalable coupling techniques, presented here in order of increasing risk (i.e., difficulty related to fabrication/implementation). As the risk increases, however, the final system complexity generally decreases. Note that for each technique presented below, each is capable of delivering 20-30W of pump power per tap into a double clad fiber (DCF).
The first method is a side c
Setzler Scott D.
Snell Kevin J.
BAE Systems Information and Electronic Systems Integration Inc.
Long Daniel J.
Nguyen Phillip
Tendler Robert K.
LandOfFree
Multiple emitter side pumping method and apparatus for fiber... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Multiple emitter side pumping method and apparatus for fiber..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Multiple emitter side pumping method and apparatus for fiber... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3266676