Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2000-11-25
2001-08-14
Healy, Brian (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S024000, C385S123000, C385S126000, C385S127000, C385S141000, C385S011000, C385S053000, C385S055000, C372S006000, C372S043010, C359S199200, C359S199200, C359S199200, C359S199200, C359S341430
Reexamination Certificate
active
06275632
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to scaling power of fiber light sources for coupling high optical output power to optical devices and applications and, more particularly, to high power fiber gain media system having tens to hundreds of watts, such as 50 and 100 watts or more, of output power achieved through WDM, TDM and/or PBM combining of multiple semiconductor gain medium sources or multiple fiber gain medium sources.
BACKGROUND OF THE INVENTION
Due to the development of reliable high power laser diodes and diode arrays, it is now possible to achieve higher power from all types of solid state lasers. Typical solid state lasers, such as Nd:YAG lasers, typically operate over a fairly narrow wavelength determined by a narrow band of atomic transitions. They are also limited in their temporal operation, e.g., their pulse modulation is limited. On the other hand, fiber gain media, such as rare earth doped fiber gain sources, can be operated comparatively over a wide wavelength band. As an example, Yb doped fiber sources are operative over a wavelength range of about 1060 nm to 1150 nm depending on a number of design parameters including fiber length and the application of wavelength-selective feedback. Also, because of the high gain of the fibers, they may be operated as amplifiers providing precise control over the temporal output of the laser source. Thus, rare-earth doped fiber gain sources, versus pumped solid state lasers, may be controlled in their temporal output over a wide range of pulse lengths and modulation rates.
The use of a single mode fiber for linear power scaling, i.e., to increase or upscale the optical power, is better because of forced laser oscillation in single transverse mode. Also, fiber lasers offer a low cost, easily produce power source at selected wavelength operation for telecommunications, printing, signal detection and medical applications. The upper limits of power scaling in conventional single clad, rare earth doped monomode fibers is limited because of the numerical aperture (NA) and core size incompatibility of these single mode fibers with the beam parameters and NA of high power laser diodes and laser diode arrays. As outlined in the paper of H. Zellmer at al., entitled, “High-Power CW Neodymium Doped Fiber Laser Operating at 9.2 W With High Beam Quality”,
OPTICS LETTERS
, Vol. 20(6), pp. 578-580, Mar. 15, 1995, to scale the pump power, a larger fiber core diameter that is adapted to the emitter dimensions of the high power laser diode or diode array. However, reduced beam quality results because an increase fiber diameter permits multimode operation.
To overcome this problem, specially configured double clad fibers have been developed where the pump radiation is launched directly into a multimode waveguide having an inner cladding surrounding a single mode core, i.e., a pump core or inner cladding which has a larger NA and large cross area which is compatible with the beam parameters of high power laser diodes or arrays. A double clad fiber, for example, comprises a single mode core, doped with a rare earth such as Yb, Nd, Er or other rare earth dopants, or combination of such dopants, such as, Er:Yb, surrounded by an inner cladding of lower refractive index material compared to the core. High output power of the fiber gain source is achieved by launching multimode pump light into the pump core of the fiber having a wavelength corresponding to the pump absorption bandwidth of the rare earth dopant in the fiber core. The pump light propagates in the multimode inner cladding and is absorbed into the active monomode core over the length of the fiber. The multimode inner cladding permits the multi-traversing of the core by the light with a wavelength corresponding to excited emission state of the doping atoms in the pump core for bring about stimulated transition of the excited atoms to a lower energy level resulting in gain for signal amplification. As a result, multimode pump light from a high power diode laser array is converted into single transverse mode power output of several watts from a single mode core of the double clad fiber. For example, for a typical 100 &mgr;m by 300 &mgr;m multimode pump light with a 100 &mgr;m by 300 &mgr;m diameter beam and a 0.47 NA, the beam may be efficiently coupled to the inner pump cladding of the fiber. The output from the fiber is a single mode beam with a 10 &mgr;m diameter and a 0.1 NA. This is about a three order increase in coupled brightness.
These double clad fiber gain sources can be operated as fiber amplifiers or fiber lasers. The laser configuration employs feedback means such as in the form of a pair of reflectors or fiber Bragg gratings making it a relatively simple structure, but is limited in most cases to cw operation. The amplifier configuration has the advantage of accurate control of the temporal output of the fiber source. The output optical power of either the laser or amplifier configuration is limited by the amount of pump light that may be injected into the fiber, the optical-to-optical conversion efficiency, and the maximum power achievable before the onset of fiber degradation. For a given rare earth dopant, the theoretical conversion efficiency is around 40% to 70% and, in actual practice, similar levels of conversion efficiency have been achieved.
The output power from a single fiber gain source can be increased by pumping the fiber source from both ends or at multiple points along the length of the fiber. Also, the output power from a single fiber gain source can be increased by increasing the size of the fiber and its numerical aperture (NA). In practice, however, the size of the fiber is limited to a diameter of several 100 &mgr;m and its NA is approximately 0.45. The NA is limited by availability of suitable polymers that can be employed for the outer cladding of the fiber. Of course, the output power of the fiber gain source with a given input aperture and NA can be increased by increasing the output power and brightness of the pump source for pumping the fiber medium. Typically, this can be accomplished by the use of multimode and multi-emitter semiconductor laser diode arrays or laser bars as pumping sources. The format and brightness of the array or bar should be optimally matched to the etendue of the inner cladding of the fiber. Theoretically, the brightness of the source should not be detrimentally affected in accomplishing this reformatting but, in practice, the brightness is significantly lower. As an example, a typical fiber coupled laser bar providing 17 W of cw power may be coupled to a fiber pump core having 170 &mgr;m by 330 &mgr;m rectangular cross-section aperture and an NA of 0.45.
It is known to scale power in a fiber source by injecting pump light in both ends of the fiber source, such as exemplified in the patent to Huber U.S. Pat. No. 5,268,910. Also, it is known to scale to even higher output powers in fiber sources by increasing either the pump power, such as higher power semiconductor pump sources or using multiple semiconductor pump sources, or by increasing the pump efficiency, such as by decentering the active core of a double clad fiber relative to the surrounding pump (inner cladding) core or use longer fibers with periodic fiber bends to convert, in both of these cases, more of the multimodes in the pump core. See, for example, H. Zellmer at al., supra; the patent to Chirravuri et al. U.S. Pat. No. 5,287,216; and the patent to Delavaux U.S. Pat. No. 5,185,826. Moreover, it has been previously disclosed in work published by Lew Goldberg at al. to scale output power by connecting in series a plurality of fiber gain source stages, such as double clad fiber amplifiers. In this case, multiple fiber sources are coupled in series, and the power from the first fiber source is coupled into the second fiber source and so. Each fiber source may be pumped from one or both ends, such as through the employment of dichroic mirrors which separate between pump light, which is typically around 808 nm or 915 nm, and the output waveleng
Archambault Jean-Luc
Grubb Stephen G.
Sanders Steven
Scifres Donald R.
Waarts Robert G.
Carothers, Jr. W. Douglas
Healy Brian
SDL Inc.
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