Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2002-02-13
2004-07-20
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S126000, C385S127000, C385S128000, C372S006000
Reexamination Certificate
active
06766075
ABSTRACT:
TECHNICAL FIELD
This invention relates to side pumping a dual clad fiber laser through delivery fibers disposed along the length of the laser fiber, the delivery fibers having a numerical aperture of not more than half the numerical aperture of the laser fiber, having a core with a refractive index equal to the refractive index of the inner cladding (the multimode core) of the laser fiber.
BACKGOUND ART
High power fiber lasers, including fiber laser amplifiers, are most effectively pumped by semiconductor laser diodes. It is commonplace in the art to use diode laser pumps which are pigtailed, i.e., coupled into a multimode fiber, for ease of power delivery. In a multi-core, laser fiber, the diode pump's power, at a preselected wavelength, is absorbed by a dopant, or combination of dopants, in a single-mode core of the laser fiber, which emit at a different wavelength, dependent on the dopant. The typical beam shape of an edge emitting semiconductor laser is elliptical and is best matched to the cross section of a rectangular shaped inner cladding. Heretofore, although there are patents and papers describing various schemes to feed the pump light through the side of the laser fiber, all practical systems thus far pump the laser through the end of the laser fiber.
End pumping has its drawbacks. There exist only two ports. The size of the end facets impose a severe limitation on the amount of pump power that can be injected into the fiber laser, which often suffers damage when the pump power exceeds 60W. It has been shown in R. Waarts, “Fiber Laser Technology for Commercial and DoD Applications”, Thirteenth Annual Solid-State and Diode Transfer Laser Technology Review, 2000, that the power damage threshold for cladding materials is approximately 1.9×10
9
W/m
2
by assuming a 60W of pump power launched into a cladding with a diameter of 100 &mgr;m. Finally, the absorption of pump power, and therefore the gain of the end pumped system, along the length of fiber, is very non-uniform; thus the system suffers a significant loss in efficiency.
In U.S. Pat. No. 5,864,644, fused tapered couplers are used to combine power, by placing the fibers together, twisting and pulling when heat is applied. However, the dimensions of the fibers are altered and the waveguiding properties of the fibers are consequently changed. Nonetheless, the method allows the power in the fused area to combine and be coupled into the fiber end facet. However, the power must be launched within the numerical aperture of the receiving fiber; power residing outside of this cone will be lost.
U.S. Pat. No. 5,127,068 discloses the use of a cylindrical micro-lens for collimating the output of a diode laser bar so that the pump radiation can be coupled into a double clad fiber laser. Such an arrangement requires a precisely aligned narrow spacing between the lens and the fiber facet. This requirement creates difficulty in making a proper alignment without causing physical damage to the optical components.
Much effort has been made to develop high power fiber lasers by clad pumping, but such effort has only reached limited success.
In a clad pumped laser system, the diode pump power is introduced into an inner cladding region, where it propagates due to internal reflection along its length. While pump power is propagating within the inner cladding region, it undergoes multiple reflections and repetitively passes through a doped core, where it is absorbed.
Side pumping has the ability for use of multiple ports, which has two major advantages: (1) while the pump power density at each port can be maintained well below the material damage thresholds, the sum over multiple inputs can provide extremely large amounts of pump power for the system, and (2) the pump power being distributed along the length of the fiber flattens the spatial gain in the system, thereby increasing the efficiency of the system per unit length. Side pumping can be accomplished by evanescent means or by direct coupling.
One method for side pumping of a clad pumped fiber laser system by evanescent means, disclosed in U.S. Pat. No. 5,999,673, uses fused tapered coupler techniques in which the delivery fiber is heated and pulled at its output end, thereby creating a tapered portion. Tapering the delivery fiber inherently causes a significant loss of pump power, which leaks out of the fiber. The tapered portion of the fiber is placed in contact with the double clad system by twisting the two fibers together, and then heated and pulled, thus causing that portion of the double clad system to be deformed; such deformation on a multicore system may destroy the phase locking of the system.
Another side pumping method, disclosed in U.S. Pat. No. 5,854,865, employs a deep V-groove cut into the inner cladding of a double clad fiber laser. By focusing the diode laser beam onto the V-groove with a lens, the pump light can be coupled into the inner cladding with a coupling efficiency greater than 80%. However, fabrication of V-grooves using an abrasive process to remove a significant amount of material (nearly half) can lead to structural damage to the clad pumped system where localized stresses become a major issue and increase the chance of fiber breakage. Also, V-grooves compromise the waveguiding capabilities of the system; therefore, if multiple V-grooves are used they must be placed far apart, thereby limiting the number of available ports.
Other means of side pumping by direct injection are disclosed in: E. F. Stephens, D. L. Wise, “A Ruggedized Fiber Laser for Force Protection Capabilities”, Thirteenth Annual Solid State and Diode Laser Technology Review, 2000, T. Weber, W. Luthy, H. P. Weber, “Side-Pumped Fiber Laser”, Applied Physics B, Vol. 63, pp. 131-134, 1996, and I. K. IIev, R. W. Waynan, “Grazing-lncidence-Based Hollow Taper for infrared Laser-to-Fiber Coupling”, Applied Physics Letters, Vol. 74, pp. 2921-2923, 1999. These either use various external micro optics, such as prisms, lenses and tapering components, or inject the pump power into a closed system with a highly reflecting inner wall. These efforts have not provided a satisfactory solution due to low coupling efficiencies or difficulties in maintaining perfect optical alignments.
U.S. Pat. No. 4,815,079 discloses a clad pumped laser system in which the diode pump power is introduced through side-coupled delivery fibers. However, the laser fiber has a rectangular inner cladding (outer core), and the delivery fibers also have a similar, rectangular core. By making certain assumptions, it is claimed that coupling efficiencies as high as 89% are achievable;
however, in the practical world, that configuration cannot yield efficiencies much over 30%. Further, the rectangular-cored delivery fibers are not well suited to easy interconnection with commercially available diode lasers and their pigtails or connectors.
The various clad pumping schemes for fiber double clad laser fibers known to the prior art are not effective in producing very high power without incorporating additional, necessary features. Very high power cannot be obtained from fiber lasers by end pumping alone, because the high pump power can lead to power damage at the two end facets of the fiber. Side pumping of a fiber laser has heretofore been very inefficient.
DISCLOSURE OF INVENTION
Objects of the present invention include: provision of high power fiber lasers; provision of an efficient and robust clad-pumped laser; efficient coupling of pump power from a delivery fiber by directly injecting the power into a laser fiber, through the sidewall, without twisting, tapering or cutting grooves in the fiber; delivering pump power to a laser fiber in a manner satisfying total internal reflection, without using external micro optics, such as lenses and prisms, at the interface; delivering pump power to an optical fiber without the need for subsequent adjustment; providing a more uniform gain profile in fiber lasers including fiber laser amplifiers; allowing large amounts of pump power to be injected into a fiber laser system; inje
Cheo Peter K.
King George G.
Doan Jennifer
Palmer Phan T. H.
PC Photonics Corporation
Williams M. P.
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