Optical waveguides – Optical fiber waveguide with cladding – With graded index core or cladding
Reexamination Certificate
2002-03-07
2004-06-15
Font, Frank G. (Department: 2877)
Optical waveguides
Optical fiber waveguide with cladding
With graded index core or cladding
C372S006000
Reexamination Certificate
active
06751388
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to coherent light sources. More particularly, it relates to high power optical fiber lasers with doping profiles selected to produce complex-valued V
c
-parameters to support gain-guiding of radiation.
BACKGROUND OF THE INVENTION
Optical fiber lasers have a core doped with ions capable of providing laser amplification when pumped with optical energy. In conventional fiber lasers, the core has a higher index of refraction than the surrounding cladding. Fiber lasers have a number of inherent advantages compared to solid state and gas lasers. Fiber lasers are simple, rugged, and inexpensive devices with a minimum of complex-valued optical and mechanical components. Optical fiber materials are compatible with a very wide range of solid state laser ions operating at many different wavelengths. These laser ions can be distributed along a substantial length of fiber, up to many meters long, with the pumping light fully trapped within the fiber over the full distance. At the same time the unwanted optical losses for either pump or signal waves can be very small in modern fibers. For additional background information on fiber lasers the reader is referred to U.S. Pat. No. 3,808,549 to Maurer.
In many applications, both for telecommunications and for fiber laser devices, it is desirable to have a single mode fiber, i.e., a fiber that can propagate only one single lowest-order mode, with no higher-order modes being able to propagate or be trapped by the fiber. For general teaching on single-mode fiber lasers the reader is referred to Poole et al., “Fabrication of Low-Loss Optical Fibers Containing Rare-Earth Ions”, Optics Letters, Vol. 22, 1985, pp. 737-738. Now, specifically, achieving single-mode behavior requires a combination of a small enough index step &Dgr;n between the core and cladding regions of the fiber and a small enough diameter d for the core region of the fiber.
Since optical fibers have a relatively small diameter (e.g. <1 mm, which is small compared to solid state lasers), the optical power densities are large even for small total optical powers. This leads to both efficient pumping and efficient signal extraction in a wide variety of laser ions. All of the incident pumping radiation can be absorbed by the laser ions even on very weakly absorbing pump transitions, and the overall conversion of pumping light to laser output can be extremely efficient. Further, the small outer diameter of the optical fiber permits efficient heat extraction.
In recent years, high power fiber lasers have been manufactured with ever increasing optical powers. A major advance has been the development of ‘cladding pumped’ fiber lasers as disclosed in U.S. Pat. No. 4,815,079 to Snitzer et al. In cladding pumped fiber lasers, the laser light is confined to a small core (usually single mode) while the pump light propagates in a much larger cladding surrounding the core. The laser light in the core retains its desired single mode characteristic while the pump light is gradually absorbed by the lasing ions in the core. The large cladding permits high pump energies to be injected into the ends of the fiber, and permits this pump energy to be supplied by spatially incoherent pump sources such as spatially incoherent diode laser arrays. This increases the attainable power output of fiber lasers. Fiber lasers providing tens of watts of optical power output are now possible with cladding pumped designs. This has made possible new applications for fiber lasers including material processing and other high-power applications.
An important objective in the design of many optical fiber lasers is to obtain amplification of only a single transverse mode of the fiber core. This severely limits the size of the fiber core. The diameter of the core in conventional index-guided fibers must be limited to less than about 10 microns if the laser output is to have only a single transverse mode. Cores larger than this will propagate multiple higher-order transverse modes. This size limitation results in a ceiling on the achievable output power of the fiber laser due to a maximum power intensity that the core can carry. When the laser power intensity (watts/mm
2
) in a single mode fiber exceeds a certain maximum value, stimulated Raman scattering occurs which converts the laser light to other wavelengths. The Raman scattering is inherent in the fiber material itself and places an absolute limit on the maximum power intensity the core can carry. The threshold for the onset of stimulated Raman scattering is a few tens of watts for single mode cores of typical size. Increasing the size of the core reduces the power intensity, thereby preventing Raman scattering, but invariably allows unwanted high-order transverse modes to be produced.
In U.S. Pat. No. 5,818,630 Fermann et al. teach single-mode amplifiers based on multi-mode fibers. The problem of multi-mode propagation and mode conversion is partially avoided by using relatively short fiber lengths together with careful shaping or mode-matching of the injected light so as to launch only the preferred fundamental or lowest-order mode at the input end of the fiber, and with the entire length of the fiber maintained in a very straight line so as to minimize conversion of the light into higher-order modes as the light travels along the fiber. In addition, Fermann et al. teach confinement of the doping to the center of the fiber core in order to preferentially amplify the fundamental mode, to reduce amplified spontaneous emission and to allow gain-guiding of the fundamental mode, which is centered on the fiber axis. In addition, Fermann et al. propose that mode-filters be integrated into the laser cavity to promote a single near-diffraction limited mode. The fibers used by Fermann et al. have a V-parameter higher than 2.5 and a relatively high index of refraction difference between the fiber core and cladding.
The term gain-guiding as used by Fermann et al. defines a gain confinement or preferential amplification achieved by the doping profile. The fibers do not actually gain-guide any modes, rather, the modes are guided because of the refractive index profile. In other words, the doping profile does not support any guided modes.
In U.S. Pat. No. 5,712,941 to Imoto et al. teach the use of single-mode fiber with multiple cores and consolidated cores exhibiting various doping profiles. In this case the doping profiles also do not support any gain-guiding and a refractive index profile is used to define the guided modes.
In U.S. Pat. No. 5,187,759 DiGiovanni et al. teach a high gain multi-mode optical amplifier which attempts to prevent excitation of the numerous higher order modes. DiGiovanni et al. teach to carefully launch the radiation substantially along the center axis of the multi-mode fiber within a predetermined launch angle. Thus, rather than exciting all modes, only lower order modes are affected. They also teach that the doping profile can be adjusted to further reduce mode coupling.
Unfortunately, none of the above solutions can be used to produce a long and stable multi-mode fiber operating in just the fundamental mode and yielding high output power. In fact, due to optical aberrations, even well corrected optics used to carefully launch radiation into multi-mode fibers typically allow the excitation of the fundamental mode only with maximum efficiency of about 95%. Therefore, to date, it has been considered that mode-locking of a multi-mode fiber is impossible and no stable operation of a mode-locked multi-mode fiber laser has yet been demonstrated.
In U.S. Pat. No. 6,275,512 Fermann teaches a mode-locked multi-mode fiber laser pulse source and suggests that the above-mentioned problems be overcome by suitable cavity design. Specifically, Fermann teaches the use of a saturable absorber in the laser cavity to achieve mode locking in multi-mode fibers. His objective is to achieve stable generation of high peak power pulses from mode-locked multi-mode fibers. Unfortunately, such mode locking cannot be easily e
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The Board of Trustees of the Leland Stanford Junior University
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