Abstract:
A waveguide laser is formed by starting with a glass disc doped with a rare earth element to define a lasant material. The disc is etched or machined to define an elongated waveguide channel having a spiral configuration. The open area between the walls of the waveguide channel is filled with a cladding material. An end reflector is formed on the radial inner end of the spiral waveguide. First cladding layers are formed on both sides of the spiral waveguide. A second cladding layer is deposited on at least one of the first cladding layers. A heat sink is connected to the second cladding layer. A plurality of optical pump sources are positioned about the side walls of the structure to excite the lasant material and generate a laser beam. In one preferred embodiment, the side walls of the structure are provided with a convex configuration to enhance pump coupling.

Description:
PRIORITY  
       [0001]     This application claims priority from provisional application Ser. No. 60/542,112, filed Feb. 4, 2004, the disclosure of which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to lasers employed in material processing, optical telecommunications, projection display, and optical fabrication technology. The invention relates in particular to a monolithic wafer-scale waveguide laser.  
       DISCUSSION OF BACKGROUND ART  
       [0003]     Double clad (DC) optical fiber technology has given rise to a new class of infrared lasers that are more compact, energy efficient, and reliable than solid-state lasers based on rod, slab, and disk laser architectures. DC fiber lasers generally rely on at least one (often many) discrete, near-infrared (NIR) semiconductor diode pump laser to provide excitation energy to a resonant cavity of the laser. The resonant cavity is formed in a rare earth (Nd, Yb, Pr, Er, etc.) ion-doped core region of the DC fiber.  
         [0004]     The pump laser output energy is typically fiber coupled and then spliced to an inner cladding of the double clad fiber. The refractive index profile of a DC fiber confines the pump laser energy to the inner cladding and core regions of the fiber. The active ions in the central core region are excited by the pump energy and, by stimulated emission and the waveguiding action of the higher index host core region, radiate laser light along the axis of the DC fiber core. A reflector and an output coupler (often fiber Bragg gratings) form the ends of the fiber laser cavity. A better mode quality (and longer wavelength) energy is emitted from the output coupler than is emitted by the pump lasers. In essence, the DC fiber laser converts relatively poor mode quality, i.e. low brightness, energy from the pump laser to superior mode quality, i.e., higher brightness, radiation. The output radiation wavelength depends upon the detailed spectroscopy of the active ions in the core region, the pump laser wavelength, and the optical path length of the fiber laser cavity.  
         [0005]     U.S. Pat. No. 6,052,392, to Ueda et al, discloses a laser including an optical guide with active lasing substance, wherein laser oscillation is provided by supplying excitation light to the active lasing substances. The optical guide is continuous and is relatively long over an area containing the optical guide. It is arranged in a conglomerate form by being repeatedly folded or wound. Excitation light is radiated to the optical guide at its outer periphery. The conglomerate form may be a disc shape, a cone shape, a regular polyhedron shape, a truncated polyhedron shape, an ellipse shape, a cocoon shape, an ellipsoid of revolution shape, a spiral shape, a sphere shape, a donut or ring shape, a torus shape, a fabric shape, a shape linearly converted from one of the aforementioned shapes, or a shape in combination of all or part of those shapes. The optical guide is preferably made of an optical fiber and has at least one optical waveguide. The optical fiber in the conglomerate form is made immobile by covering all or a part of the optical fiber with a setting substance which transmits the excitation light. The setting substance can be selected from a setting organic resin or glass, or a setting inorganic medium. The optical guide is either a double clad type optical fiber or an optical waveguide, formed with a clad, and with a second clad placed outside the clad.  
         [0006]     The apparatus of Ueda et al has several disadvantages. It requires careful winding/spooling of at least one discrete segment of ion-doped glass fiber to form a resonant cavity within a cylindrical or circular space. The optical fiber has a relatively small diameter of between about 100 micrometers (μm) and 1,000 μm. Given this relatively small diameter, the optical fiber is exposed to a danger of scratching the fiber cladding during handling. This can make winding and spooling the optical fiber a very tedious operation. Further, the excitation of the active ions in the optical fiber core is achieved by side-pumping the spooled fiber laser cavity with one or more semiconductor diode lasers. This requires the use of discrete lenses (or mirrors) to efficiently couple the pump laser output energy into the fiber laser cavity, thereby adding labor and cost to the manufacturing process. Finally, suppression of damage due to mechanical vibration of the fiber laser cavity and efficient coupling of pump laser energy into the cavity requires “potting” of the fiber in a binding matrix. The binding matrix must be transparent to the pump laser energy, it must fill in the gaps between the windings of the fiber laser cavity to minimize Fresnel reflection and optical scattering losses, and it should not inhibit the conduction of unwanted heat out of the fiber laser cavity. It is difficult (if not impossible) to find a binding matrix that satisfies all of these requirements.  
         [0007]     U.S. Pat. No. 4,782,491 to Snitzer, teaches an optical fiber laser comprising a nearly pure fused silica glass, neodymium doped active core within a cavity in the form of a single mode optical fiber. The gain cavity is end pumped at a nominal wavelength of 0.8 μm and its length and neodymium concentration are adjusted to maximize pump absorption and minimize concentration quenching. Dichroic mirrors are preferably integrally formed on ends of the cavity and have reflection characteristics selected so that the laser has an output at a nominal wavelength of 1.06 μm.  
         [0008]     U.S. Pat. No. 4,780,877 to Snitzer, depicts an optical fiber laser comprising a gain cavity in the form of a single-mode optical fiber with integrally formed dichroic mirror end sections to provide feedback. The fiber core comprises a host material of silicate glass preferably doped with 0.01 to 1 weight percent of just erbium oxide as a lasing medium. The laser is end pumped at approximately 1.49 μm with a laser diode, preferably indium gallium arsenide phosphide (InGaAsP), and has an output at 1.54 μm.  
         [0009]     U.S. Pat. No. 4,680,767 to Hakimi, et al., discloses an optical fiber laser comprising a gain cavity in the form of a single-mode optical fiber with integrally formed reflective end sections for provision of feedback. One end-section is an etalon for modifying the gain cavity resonant characteristics and intensity modulation, and the other end-section is used to alter gain cavity effective length to tune and frequency modulate. The emission spectrum of the laser gain material (which is preferably neodymium oxide incorporated in a silicate glass core), along with the etalon section reflection, pump energy level, and gain cavity length, all cooperate such that lasing takes place over just a single line of narrow width or over more than one line within a narrow band. Electro-optic material in the end sections permit output frequency and amplitude to be selectively activated in response to the application of applied voltages.  
         [0010]     U.S. Pat. No. 4,015,217 to Snitzer, teaches laserable material with a host material of non-gaseous, non-periodic atomic structures. The host material is plastic dispersed in solid solution within the plastic and is a chelate of a rate earth metal.  
         [0011]     All of the Snitzer designs, as well as the Hakimi design, require the handling of at least one discrete segment of ion-doped glass fiber to form a resonant cavity. Given the small diameter of such optical fiber and the danger of scratching the fiber cladding or fracturing the fiber during handling, as discussed above, this can be a tedious operation. Further, in each of the Snitzer and Hakimi designs, excitation of the active ions is achieved by end-pumping or side-pumping the fiber laser cavity with one or more semiconductor diode lasers. This requires the splicing of additional segments of (undoped) fiber to couple the pump laser output energy into the fiber laser cavity. Accordingly, labor requirements can be high and manufacturing yields can be challenging.  
         [0012]     Finally, in the Snitzer and Hakimi designs, suppression of damage due to mechanical vibration of the fiber laser cavity typically requires spooling and “potting” of the fiber in a binding matrix (usually an organic material). The binding matrix should not inhibit the conduction of unwanted heat out of the fiber. It is difficult to find a suitably compliant and robust binding matrix that is also a good thermal conductor. The above cited patents are incorporated herein by reference.  
         [0013]     There remain several technical problems in need of resolution. Because the optical conversion efficiency of an ion-doped DC fiber core is less than 100% (typically between 50% and 70%), the remaining pump laser energy must be dissipated as heat along the length of the DC fiber. Some provision must be made to conduct this unwanted heat away from the DC fiber. In a high output power, for example, greater than 100 Watts (W), fiber laser, thermal management is a significant challenge.  
         [0014]     The state of the art in semiconductor pump lasers is such that multiple pump lasers must be fiber coupled and then spliced to the DC fiber&#39;s inner cladding. In high output power designs, splicing of the fiber-coupled emitter pumps or multiple emitter bars is a major factor in the cost, manufacturing yield, and reliability of the fiber laser.  
         [0015]     In order to achieve sufficient optical gain, the fiber laser cavity is typically very long, for example between 1 meter (m) and 100 m, necessitating winding and/or spooling of the double clad fiber to save space. The DC fiber must be handled with care during such winding to avoid scratches or fractures, and it must be protected from mechanical damage (vibration, etc.) during use. Therefore, the DC fiber is usually potted in some kind of binder matrix (often an organic material) after it has been spooled.  
       SUMMARY OF THE INVENTION  
       [0016]     A waveguide laser is formed by starting with a glass disc doped with a rare earth element to define a laser gain medium material. Using semiconductor type manufacturing techniques, the disc is etched or machined to define an elongated waveguide channel having a spiral configuration. The open area between the walls of the waveguide channel is filled with a material having a lower index of refraction. An end reflector is formed on the radial inner end of the spiral waveguide.  
         [0017]     First cladding layers are formed on both sides of the spiral waveguide. The index of refraction of the cladding layers preferably matches the index of refraction of the material located between the waveguide walls. In the preferred embodiment, a pair of second cladding layers are deposited on the first cladding layers. Each second cladding layer has an index of refraction less than the index of refraction of the first cladding layers. At least one heat sink is connected to one of the second cladding layers.  
         [0018]     A plurality of optical pump sources are positioned about the side walls of the structure. Preferably, the optical pump sources are semiconductor diode lasers. In one preferred embodiment, the side walls of the structure are provided with a convex configuration to enhance coupling.  
         [0019]     In a preferred fabrication method, the glass disc is first bonded to a glass substrate. Then the spiral waveguide is formed by etching or machining. A capping layer is then conformally deposited or flowed over the spiral structure to fill the voids. After planarization, a top substrate is bonded onto the capping layer. The top and bottom substrates can then be ground and polished to define the first cladding layers. The second cladding layers can then be deposited onto the first cladding layers. Finally, the heat sinks can be bonded to the second layers.  
         [0020]     Further features of the subject invention will be apparent in view of the following detailed description and drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     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 the principles of the present invention.  
         [0022]      FIGS. 1A and 1B  are respectively plan and elevation cross-section views, with  FIG. 1B  seen generally in the direction  1 B- 1 B of  FIG. 1A , and  FIG. 1A  seen generally in the direction  1 A- 1 A of  FIG. 1B , schematically illustrating one preferred embodiment of a monolithic wafer-scale waveguide laser in accordance with the present invention including a spiral waveguide of a laser material immersed in a cladding material in the form of a disk, the periphery of which forms an anamorphic lens.  
         [0023]      FIGS. 2A and 2B  are respectively plan and elevation cross section views, with  FIG. 2B  seen generally in the direction  2 B- 2 B of  FIG. 2A , and  FIG. 2A  seen generally in the direction  2 A- 2 A of  FIG. 2B , schematically illustrating the laser of  FIGS. 1A and 1B , further including a plurality of diode pump lasers disposed around the periphery of the cladding material and delivering pump energy to the spiral waveguide via the lens formed on the periphery of the cladding material.  
         [0024]     FIGS.  3 A-L are elevation cross-section views schematically illustrating steps in one preferred method for fabricating the monolithic wafer-scale waveguide-laser of  FIGS. 1A and 1B . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]     Referring now to the drawings, wherein like features are designated by like reference numerals,  FIG. 1A  and  FIG. 1B  schematically illustrate a preferred embodiment  10  of a monolithic, wafer-scale waveguide-laser in accordance with the present invention. Laser  10  includes a wafer body  12 , preferably disc-shaped and having a diameter (D). The wafer body includes an ion-doped spiral waveguide  14  preferably having a rectangular cross-section. The waveguide is formed from a material having a refractive index n 1 . The rectangular cross-section is characterized by a thickness (height) t 1  and a width w 1 . The waveguide spirals are separated center-to-center by a distance Λ 1 .  
         [0026]     The waveguide layer is immersed in an inner cladding layer  16 . Cladding layer  16  is formed from a material having a refractive index n 2 , where n 2  is less than n 1 . Inner cladding layer  16  has a thickness t 2 . The inner cladding layer is sandwiched between first and second outer cladding layers  18  and  20 . Outer cladding layers are formed from a material having a refractive index n 3 , where n 3  is less than n 2 , and have a thickness designated generally t 3 . A heat sink  22  is attached to cladding layer  18 , and a similar heat sink  24  is attached to cladding layer  20 .  
         [0027]     Periphery  16 P of the cladding layer may be provided with a convex surface curvature perpendicular to the plane of layer. This curvature together with the circular form of the periphery in the plane of the layer gives the periphery the form of an anamorphic lens. This is convenient for coupling optical pump energy into the cladding layer as described further hereinbelow. In alternate embodiment, the periphery of the cladding layer is planar. In this case, it may be desirable to use diode pump lasers with focusing lenses (see for example, U.S. Pat. No. 5,949,932, incorporated herein by reference).  
         [0028]     Spiral waveguide  14  has an inner terminal end  14 A having a highly reflective cap, preferably a Bragg grating reflector  26 . An output beam coupling notch  28  in the periphery of the cladding layer is disposed on an outer terminal end  14 B of waveguide  14  and provides an output route for an output beam as indicated in  FIG. 1A .  
         [0029]     Referring now to  FIGS. 2A and 2B , in one preferred arrangement for delivering optical pump energy to spiral waveguide  14  a plurality of semiconductor diode pump lasers  30  are arrayed outside periphery  16 P of the wafer body at the level of the inner cladding layer  16 . The diode pump lasers provide excitation energy designated by rays  32 . The excitation energy is free-space coupled through the edge of the wafer body  12 , i.e., through periphery  16 P of inner cladding layer  16 , and into the inner cladding layer. The pump energy is confined between the outer cladding layers and, due to multiple reflections between the outer cladding layers, activates ions in the ion-doped spiral waveguide  14  to stimulate a laser light emission along the longitudinal axis of the waveguide (not shown). Accordingly, as with prior art DC devices, the apparatus converts the low brightness energy from the discrete pump diode lasers in the array to a higher brightness output radiation. The wavelength of the output radiation is dependent on the characteristics of the ions in the waveguide material, the pump laser wavelength λ pump , and the optical path length of the fiber laser cavity, i.e., of waveguide  14 .  
         [0030]     The following relationships are important for the design of a laser in accordance with the present invention. Refractive indices of the cladding follow a relationship n 1 &gt;n 2 &gt;n 3 . This provides for efficient waveguiding of laser radiation in waveguide  14  and pump energy in cladding layer  16 . Regarding dimensions of waveguide  14 , t 1  is preferably on the order of w 1  and t 1  is equal to w 1 , for a square cross-section waveguide. The values of n 1 , t 1 , and w 1  are determined by the desired transverse mode structure and polarization state of the output laser beam. Spacing Λ 1  between spirals is greater than w 1 , and is chosen to be large enough to avoid evanescent wave coupling between spirals. Regarding thickness of the cladding layers, t 2  should be greater than twice t 1  and preferably much greater than twice t 1  for practical wafer fabrication. Thickness t 3  of outer cladding layers  18  and  20  is greater than λ pump , and is chosen to avoid evanescent wave coupling of pump laser energy into heat sinks  22  and  24 . Diameter D of disk body  12 , is chosen to be large enough to accommodate the desired number of spirals of waveguide  14 , i.e., the desired laser cavity length and gain, and large enough to avoid bending losses in the inner most spirals.  
         [0031]     Other relationships obtain that are consistent with previously published laser physics and laser engineering principles. See for example: O. Svelto and D. C. Hanna,  Principles of Lasers , (Plenum Press, NY, 1989); M. J. Weber,  CRC Handbook of Laser Science and Technology , Vol. III, (CRC Press, Boca Raton, Fla., 1986); and S. Sudo,  Optical Fiber Amplifiers , (Artech House, Norwood, Mass., 1997), all of which are incorporated in their entirety by reference herein.  
         [0032]     FIGS.  3 A-L schematically illustrate steps, in seriatim, in one preferred method of fabricating disc body  12  of above-described laser  10 . In a first step (see  FIG. 3A ) a glass wafer  60  doped with a rare earth element is provided, the ion-doped wafer having a refractive index (n 1 ). The wafer provides the material from which spiral waveguide  14  will be made.  
         [0033]     In a second step (see  FIG. 3B ) doped glass wafer  60  is bonded to a glass block  64  having a diameter D. The bonding is effected either by optical contact or diffusion bonding. The glass of the block has a refractive index (n 2 ) and will provide a part of inner cladding layer  16 .  
         [0034]     Next, wafer  60  is ground and polished to a thickness t 1  (see  FIG. 3C ). This is the thickness of the spiral waveguide  14 .  
         [0035]     In a fourth step, the waveguide layer is patterned and etched (or micro-machined) into the spiral configuration of waveguide  14  (see  FIG. 3D ). Waveguide  14  provides the laser cavity as discussed above. The area from which the material was removed to create the spiral waveguide defines a spiral spacer channel.  
         [0036]     Next, a capping layer  64  having a refractive index as closely matched to n 2  as practicable is deposited onto spiral waveguide  14  (see  FIG. 3E ). The deposited capping layer has an uneven surface  64 S.  
         [0037]     Next, surface  64 S of capping layer  64  is planarized (see  FIG. 3F ) and Bragg reflector  26  is written on the inner terminal end of the waveguide spiral. Procedures for writing a Bragg grating in a waveguide are well-known in the art and accordingly are not described or illustrated herein.  
         [0038]     Following the planarizing and grating writing steps, a glass superstrate  66  is contact or diffusion bonded to planarized capping layer  64  (see  FIG. 3G ). The superstrate has a refractive index matched to n 2 . With the superstrate in place, physical elements for providing the inner cladding layer  16  are present.  
         [0039]     Next, substrate  62  and superstrate  66  are ground and polished to a total thickness t 2  (see  FIG. 3H ). This thickness is distributed around waveguide  14  as required to provide inner cladding layer  16  in which the waveguide is immersed. If desired, (see  FIG. 31 ) the periphery  16 P of inner cladding layer  16  can be ground and polished to provide a convex surface suitable for focusing the excitation energy from the semiconductor diode pump lasers into the inner cladding layer. Following that polishing step, output beam coupling notch  28  is cut, ground, and polished on the perimeter  16 P of the inner cladding (see  FIG. 3J ). The outer cladding layers  18  and  20  are then deposited on opposite sides of inner cladding layer  16  (see  FIG. 3K ). After these outer cladding layers are deposited the heat sinks are attached to the outer cladding layers to complete the disc body  12 . The complete laser can then be completed by adding pump diode lasers as depicted in  FIGS. 2A and 2B .  
         [0040]     The invention can be fabricated using planar processing techniques that are widely used in the production of integrated circuits, opto-electronic semiconductor devices, and optical components, for example, thin-film deposition, photolithographic patterning, etching, contact bonding, and polishing. The resulting monolithic wafer structure preserves all of the good features of fiber laser technology such as compactness, high optical conversion efficiency, and excellent output beam quality. The invention allows for very effective heat sinking through both flat large area wafer surfaces and, including the pump lasers around the wafer perimeter, it consists of fewer piece parts than current fiber lasers. Therefore, the invention is intrinsically more reliable and less expensive to manufacture than the existing fiber lasers and other solid state lasers.  
         [0041]     The present invention eliminates the need to handle discrete fiber in the formation of the ion-doped waveguide laser cavity, eliminates the need for a fiber binding matrix (to suppress damage due to mechanical vibration), and eliminates the need for any pump laser fiber coupling (and all associated fiber splices). The invention integrates a self-aligned anamorphic lensing function at the edge of the wafer to efficiently couple pump laser energy from single emitter pumps or from multiple emitter bar pumps into the laser cavity. The monolithic nature of the invention lends itself to the cost-saving benefits of wafer scale planar processing techniques.  
         [0042]     Applications and possible uses of the invention are manifold. For example, the present invention could be employed to provide fiber delivered IR laser energy for material processing, such as laser engraving, micro-bending, soldering, heat treating, drilling, cutting, welding, and the like. The invention is particularly attractive in the high power domain because it can use relatively inexpensive multiple emitter semiconductor pump laser bars without any discrete free-space or fiber-optic coupling components.  
         [0043]     It is contemplated that the present invention be employed to provide fiber delivery of tightly focused IR laser energy onto gas clusters or metal targets to induce plasma generation of soft x-rays. This is one of the most promising approaches to the reliable generation of soft x-rays for next generation high resolution integrated circuit photolithographic patterning.  
         [0044]     It is further contemplated that the present invention be employed to provide fiber delivery of frequency upconverted IR laser energy for visible wavelength projection display or high speed reprographic applications. The invention readily lends itself to the integration of suitable upconversion materials, for example, ion-doped fluoride glass, in the wafer structure.  
         [0045]     Further, the present invention could be utilized in multiple output single wavelength applications, for example, laser marking and reprographics. Multiple independent laser cavities can be formed within a single ion-doped layer by interleaving spiral waveguides in the layer.  
         [0046]     Moreover, the present invention may be employed in multiple wavelength applications, including, for example, red/green/blue wavelengths for projection display. Multiple independent spiral laser cavities can be formed by stacking multiple ion-doped layers within the monolithic wafer structure. Thus, one wafer can be designed to incorporate multiple waveguide lasers emitting at different wavelengths.  
         [0047]     Finally, among the many presently contemplated uses, the present invention can be employed in multiple output/multiple wavelength applications for example, color sensitive laser marking and reprographics. Multiple independent laser cavities can be formed within a single ion-doped layer by interleaving spiral waveguides in the layer, and multiple ion-doped layers can be then formed by stacking multiple ion-doped layers within the monolithic wafer structure.  
         [0048]     The present invention is described above in terms of a 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.