Patent Application
20100260210
The present invention relates in general to fiber-lasers and fiber-amplifiers. The invention relates in particular optically pumping fiber-lasers and fiber-amplifiers with radiation from an array of diode-lasers.
Fiber-lasers, including fiber oscillator/amplifier combinations (MOPAs) are gradually replacing conventional solid-state lasers in several laser applications. Fiber-lasers and amplifiers have advantages over solid-state lasers in ruggedness and optical efficiency. CW fiber-lasers having a very simple architecture are capable of delivering a very high-powered beam, for example, a beam having a power in excess of 1 kilowatt (kW), in a single mode. Pulsed fiber-lasers can deliver peak-power as high as 10 kW or greater. Fiber-lasers can have an optical efficiency, for example between about 60% and 90%.
High-power CW fiber-lasers are extremely useful in material processing applications, such as cutting of complex 3D shapes found in hydro-formed automotive parts, and long-offset welding of complex shaped parts. High peak-power pulsed fiber-lasers with single mode output can be used for scribing of solar cell panels. Advantageously, high peak power enables efficient frequency conversion into visible and UV wavelength ranges.
In theory at least the output power of a fiber-laser is limited only by how much optical pumping power can be delivered into an optical gain-fiber for energizing a doped-core of the gain-fiber. In practice there are limits due, inter alia, to non-linear effects which can broaden the spectrum of pump radiation resulting in reduction of absorption efficiency, and photo-darkening of the fiber material which can lead to reduction of efficiency, excessive heating, and even catastrophic failure. The non-linear effects become increasingly problematical as the gain-fiber is longer. Long gain-fibers are necessary with low brightness diode-laser pump sources currently available.
Prior-art fiber-lasers use primarily one of two different pumping arrangements. These arrangements are schematically illustrated in
In arrangement 10 of
Relatively low-power, for example between about 10 Watts (W) and 60 W, pump modules 18 are coupled to small-diameter fibers 20. By way of example, fibers 20 can be about 100 micrometers in (core) diameter. Fibers 20 are spliced to the gain-fiber in such a manner that the fiber-core carrying the signal being amplified is not affected, but the pump energy is coupled into the cladding of the gain-fiber. Outputs of several modules can be aggregated in each amplifier stage. Additional amplifier stages can be connected in series to increase total gain. However, adding stages of amplification does require optical isolators such as isolator 22. It is also evident that for pump modules having a power of only 10 W, 100 pump-modules and 100 fiber-splices would be required to couple 1 KW of pump power into the amplifier chain.
In arrangement 24 of
In both of the above described approaches optical pumping is limited by limitations of coupling the output of a plurality of diode-laser emitters into an optical fiber. An optical fiber has a fixed maximum cone of acceptance (NA) for radiation. Coupling is optimal when this cone is exactly filled (neither over-filled nor under-filled) with radiation. The power optically coupled depends on the brightness of the radiation exactly filling the cone.
One usual method of providing more radiation power than can be provided by a single diode-laser emitter, is to provide the radiation from a one-dimensional or two-dimensional array of such emitters. A one-dimensional array of diode-laser emitters is typically referred to as a diode-laser bar. The emitters have an emitting aperture about 1 micrometer (μm) high (in what is referred to as the fast-axis of the emitter) and a width from about 10 μm to over 100 μm (in what is referred to as the slow-axis of the emitter). The bars are usually about 1 centimeter (cm) long and between about 1 and 4 millimeters (mm) wide, with the emitters having a length in the width-direction of the bar and emitting apertures aligned in the slow-axis direction. There can be as many as 50 or more emitters in a one-centimeter long bar. The ration of the total width of emitter apertures to the distance between opposite end ones of the emitters is referred to as the fill-factor of the bar. The fill-factor can practically be as high a 90%. Two dimensional arrays of emitters can be formed by stacking a plurality of diode-laser arrays, one above, the other in the fast-axis direction.
As far as raw power is concerned, a diode-laser bar having a high fill-factor, for example equal to or greater than about 50% offers the lowest cost per watt ($/W) available for diode-laser output power. A problem, however, as far as brightness is concerned, is that the higher the fill-factor of a diode-laser bar the less bright the aggregate output of the bar will be.
Various optical arrangements, having various degrees of success, have been proposed or implemented for overcoming this problem. Most of these involve complicated combinations of prisms, lenses or polarization sensitive devices, and are relatively expensive and space consuming compared with a simple optical arrangement of a fast-axis collimating lens and a focusing lens that can be used to focus the output of a single emitter. This expense difference becomes increasingly burdensome when a plurality of such arrangements is required. There is a need for an alternate method and apparatus for using high-fill-factor diode-laser bars for optically pumping a fiber-laser or fiber-amplifier.
The present invention is directed to providing multi-kilowatt average power and high peak power fiber-lasers and amplifiers powered by radiation from relatively inexpensive diode-laser bars. In one aspect, apparatus in accordance with the present invention comprises an optical gain-fiber having a doped-core surrounded by a cladding and a plurality of external-cavity optically-pumped semiconductor lasers (OPS-lasers). Each of the OPS-lasers is optically pumped by at least one diode-laser bar. An arrangement is provided for optically coupling the radiation from the output beams of the OPS-lasers into the cladding of the gain-fiber for energizing the doped-core of the gain-fiber.
In one embodiment of the invention, the optical coupling arrangement includes a lens arranged to focus the radiation from the plurality of OPS laser output beams into the cladding of the gain-fiber at one end thereof. In another embodiment of the invention, the optical coupling arrangement includes a lens and a delivery optical fiber having a core surrounded by a cladding. The lens is arranged to focus the radiation from the OPS-laser output-beams into the core of the delivery-fiber at one end thereof. An opposite end of the delivery fiber is arranged to couple the OPS laser radiation from the core thereof into the cladding of the gain-fiber.
In another aspect of the present invention the diode-laser bars can be high fill-factor diode-laser bars which have low brightness, but are relatively inexpensive. Only a simple single-element optic is required to concentrate the diode-laser radiation onto a gain structure of the OPS-laser. The OPS-laser converts this low-brightness pump-radiation from the diode-laser bar into single-mode, very high brightness pump-radiation for the gain-fiber.
The high brightness of the OPS-laser pump-radiation enables pumping double-clad gain-fibers having a relatively small cladding diameter compared with that of gain-fibers that are pumped directly with diode-laser radiation. This is very important for achieving average output power greater than 1 kW, or peak power greater than 10 kW, in a single mode fiber-laser.
Small cladding diameter provides that that the cladding-to-core area ratio in the gain-fiber cam be correspondingly reduced. This advantageously leads to short pump-radiation absorption length, thus mitigating above discussed nonlinear effects that set the limit to the average and peak power of a prior-art single mode fiber-laser. Fibers having a relatively small core-diameter, for example about 15 μm diameter, and made of phosphor-silicate glass can be used instead commonly used alumino-silicate fibers having a 25 μm core-diameter. Phosphor-silicate fibers are more resistant to “photo-darkening” which typically limits the lifetime of fiber-lasers. Additionally, the small clad-core area ratio provides that that ytterbium (Yb) doped fibers can be pumped “resonantly”, that is at a wavelength that is close to the generated wavelength. An example could be pumping in a 990 nanometers (nm) to 1020 nm wavelength band while emitting at a wavelength between about 1060 and 1090 nm. Low absorption relative to absorption at 915 nm or 976 nm radiation bands in Yb doped cores makes pumping essentially impossible with lower brightness pump beams. This is due to increased length required due to increased length of fibers and onset of above discussed nonlinear effects.
Resonant pumping minimizes quantum defect and, thus, heat released in the fiber. Such heat release leads to another fundamental limitation of power output possibility in prior-art single mode fiber-lasers. OPS-lasers have sufficient wavelength flexibility to facilitate resonant pumping. Because of the above discussed advantages, the inventive use of diode-pumped OPS-laser radiation for pumping fiber-lasers and fiber-amplifiers can provide fiber-lasers having CW or peak pulse-power levels well in excess of those achievable with prior-art direct diode-laser radiation pumped fiber-lasers to be provided in a cost efficient manner. Other advantages and embodiments of the present invention will be evident to those skilled in the art from the detailed of the present invention provided hereinbelow.
The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain principles of the present invention.
Referring again to the drawings, wherein like components are designated by like reference numerals,
Optical pump radiation is provided by a pump module 36 including plurality of external-cavity, surface-emitting, semiconductor lasers (OPS-lasers) 38. Each laser delivers a beam of radiation 40 preferably in a single lateral mode or at least a “low-M2” (for example M2<2) mode. The beams are directed parallel to each other, here, by an arrangement of turning mirrors 41, to a positive lens 42. Radiation from all of the beams is focused by lens 42, as indicated by converging rays 40, into inner cladding 19 of gain-fiber 16, with a small portion, of course, directed into core 17. The beams are preferably collimated and in fact a single lateral mode OPS-laser beam can be collimated to close to the diffraction limit using a relatively simple commercial catalog lens element. Alternatively “as-delivered” OPS-laser beams have sufficiently low divergence that a collimating lens may be omitted. In the arrangement of laser 30 FBG 32 would be transparent to pump radiation and fully reflective for laser radiation. FBG 34 would be partially reflective and partially transmissive for laser radiation.
It should be noted, here that the pumping arrangement discussed above with gain-fiber 16 serving as an oscillator, can equally well be applied to a stage of fiber-amplification, for example by omitting FBGs 32 and 34 from the gain-fiber. It would be necessary, however, to direct pump light into the gain-fiber by reflection from or transmission through a dichroic beamsplitter in the manner described above with reference to
It should also be noted that the subject invention is not limited to conventional double clad fibers where there is a solid doped core and solid annular cladding material. For example, certain fibers are formed where the doped core is annular in configuration. Further, it is known to form the cladding region with air holes. The latter fibers are often referred to as photonic crystals. It is intended the references to doped cores and claddings in the claims cover these variants.
On a first consideration it would seem to be prohibitively expensive to use OPS-lasers for fiber-laser pumping instead of diode-lasers, as diode-lasers are required to optically pump the OPS-lasers and the optical efficiency of the OPS-lasers is considerably less than 100%. It has been determined, however, that an OPS-laser suitable for use in an OPS-laser pump module in accordance with the present invention can be pumped by an inexpensive high-power, high fill-factor diode-laser bar that would be totally unsuitable for prior-art diode-laser pumping arrangements, at least because of insufficient brightness. Further it has been determined that an optical arrangement for directing the pump radiation from the high fill-factor diode-laser bar onto the OPS chip can be easily produced inexpensively in volume.
The high fill-factor diode-laser bar 72 preferably has a fill-factor greater than or equal to about 50%. Diode-laser bar 72 supplies optical pump-radiation for the OPS-laser, as noted above, and is supported in thermal contact with a heat sink 74. Emitters 76 of the diode-laser bar each deliver a beam 78. Only three beams 78 are depicted in
Fast-axis collimated beam 78 is incident on a mirror 82, which has optical power only in the slow-axis of the diode-laser bar. Mirror 82 focuses each beam 38 in the slow-axis into a spot on gain-structure 64 of the OPS chip. Outer rays of the fan of rays directed to the chip can have incidence angles up to about 70°. The spot is about square in shape and in practical examples may have dimensions about 1.0 millimeters (mm) by about 1.0 mm. The spots from each beam overlap. A commercially available 50% fill-factor bar having 25 emitters each with a width of about 200 μm in the slow-axis can deliver about 100 W of total power into the 1.0 mm spot. A true cylindrical (part-circular cross-section) surface will provide effective slow-axis focusing. An example is discussed further hereinbelow.
Optical pumping of gain-structure 64 causes a beam of laser radiation 84 to circulate in resonator 61, generally along the Z-axis. Optionally a birefringent filter (BRF) 86 or some other wavelength selective element can be provided for selecting a wavelength of the circulating radiation from within the gain-bandwidth of gain-structure 64. A portion of the circulating radiation is transmitted by mirror 70 as output beam 40. Preferentially the resonator is configured such that the beam is delivered as a single-lateral-mode beam. As delivered from mirror 70 in the optical element configuration depicted the beam would have a diameter of about 1000 μm and divergence on the order of about 1.0 milliradians, dependent on the resonator length. Optionally a lens 88 is provided for collimating beam 40. The function of lens 88 could be provided to some degree by replacing plane surface 71 of element 60 with a convex surface.
It should be noted here that only a sufficient description of an external-cavity, optically-pumped, surface-emitting semiconductor laser is provided herein to enable one skilled in the art to understand principles of the present invention. A more detailed, description of an OPS laser is provided in U.S. Pat. No. 6,097,742, granted to Spinelli et al., assigned to the assignee of the present invention, and the complete disclosure of which is hereby incorporated herein by reference.
By way of example for an optimized transmission of mirrored surface 70, and a pump power of about 100.0 W delivered to gain-structure 64, output beam 40, would have a power of about 40.0 W. The brightness of beam 40 in a single lateral mode (M2 about 1.1) would be about 600 (six-hundred) times greater than the brightness of the pump radiation. This would allow the beam to be collimated to near the diffraction limit, with the aggregate of a plurality of the collimated beams being focusable to a near diffraction-limited spot size.
It should be noted here that while only one diode-laser bar is depicted for delivering pump-radiation, it is possible to deliver pump radiation from two or more diode-laser bars. Ultimately, the deliverable power will be limited by cooling limitations of the gain-structure.
An OPS-pumped laser in accordance with the present invention, because of the very high brightness of the OPS-laser beam is particularly suited to resonant pumping wherein the pump-radiation is selected to have a wavelength close to the emitting wavelength (gain-wavelength) of the gain-fiber. By way of example in Yb-doped gain-fiber, i.e., a fiber having a Yb-doped core, pump-radiation may have a wavelength between about 990 nanometers (nm) and 1020 nm and the emission wavelength could be selected between about 1060 nm and 1090 nm. The pump wavelength can be select by selecting a suitable composition for active layers of the gain-structure with fine selection using BRF 86. The emission wavelength can be selected by narrow bandwidth FBGs in the gain-fiber. This resonant pumping lowers the quantum defect of the pumping and produces less heat due to absorbed, unconverted pump radiation.
While absorption for pump radiation is low in the region between about 990 nm and 1020 nm relative to absorption peaks at 915 nm or 976 nm, this is compensated by the high brightness of the OPS-laser pump radiation. Resonant pumping in Yb-doped gain-fibers is essentially impossible with lower brightness diode-laser pump-beams.
Those skilled in the art to which the present invention pertains will recognize that the cost of fabricating a concentrator such as concentrator 92, all else being equal, will be somewhat greater than the cost of fabricating a simple true-cylinder reflector such as mirror 82 of laser 60. The cost difference may be somewhat less for a concentrator tapered only in the slow-axis (X-axis). The calculated intensity distribution in the pump spot, in the slow-axis and fast-axis, for such a one-dimensional tapered concentrator is schematically illustrated in the graphs
Y(t)=5.7576727537 t2+1.5802789316 t4−1.0400024281 t6+6.0083075238 t8−3.0265843283 t10−20.2943710586 t12+30.1437988598 t14−12.2092446403 t16 (1)
where t=X/(7.5 mm) X in mm, Y in mm and X has values between −6.5 mm and 6.5 mm. The center thickness of the lens is 5.5 mm, and the polynomial assumes that the lens is made from S-TIH53 glass available from Ohara Corporation of Branchburg, N.J. The intensity distribution on gain-structure 64 would be about the same as could be achieved with the cylindrical reflective mirror of
It should be noted, here, that the concentrator and lens arrangements for directing diode-laser radiation are discussed above primarily for completeness of description. The cylindrical lens reflector arrangement of laser 60 for directing the diode-laser radiation onto the gain-structure of the OPS-laser is the least expensive, and more than adequate for most applications.
An OPS-laser typically has somewhat limited optical conversion efficiency, for example, between about 40% and 50% in the arrangement of laser 60. This is mitigated, however, in the present invention by the simplicity of the OPS-resonator, the relatively low cost of high fill-factor, low brightness diode-laser bars, and the simplicity and low cost of optics for directing the radiation from the bars.
One option for coupling higher OPS-laser power into a gain-fiber includes using OPS-lasers that include two or more-gain chips. OPS-lasers including two, independently pumped OPS-chips are described in the above-referenced Spinelli et al. patent. Another option is to polarization-combine pairs of OPS-laser beams having different polarization orientations into a combined beam, and direct the combined beam to lens 42. Yet another option is to wavelength-combine beams having different wavelengths using dichroic combiners.
By way of example
Using wavelength-combining, more than two beams may be combined into a single beam and is not restricted to beam combining using dichroic beam-combiners. Those skilled in the art will recognize without further detailed description or illustration that wavelength-combining of beams is can be effected using diffraction gratings or prisms. Any such means may be used alone or in combination without departing from the spirit and scope of the present invention.
The cost of the inventive fiber-laser pumping scheme is believed to be at least comparable with, and possibly even be less than cost of direct diode-laser pumping. The cost of the OPS-laser resonator and the simple diode-laser bar pumping arrangement for the OPS laser compares with the cost of high brightness single emitters with multiple combiners, or diode-laser bars with complex and expensive combiner optics, that are required for prior-art direct diode-laser pumping of a gain-fiber. In a sense, the OPS-laser acts as a “brightness converter” for low quality light from the diode-bars. The brightness of the OPS-laser radiation can be greater than 500 times the brightness of radiation from a 50% fill-factor diode-laser bar. Because of this, the use of the high quality OPS-laser beams for optically pumping gain-fibers can provide fiber-lasers having CW of peak pulse-power levels well in excess of those achievable with prior-art direct diode-laser radiation pumped fiber-lasers, and with comparable or longer lifetime.
In summary, the present invention is described above in terms of preferred and other embodiments. The invention is not limited, however, to the embodiments described and depicted. Rather, the invention is limited only by the claims appended hereto.
