Abstract:
A solid state zig-zag slab laser amplifier in which depolarization occurring at total internal reflection from opposed lateral faces of the amplifier slab is controlled by selecting a complex evanescent coating that provides a selected phase retardance that results in minimization of depolarization. Without use of the complex coating, small changes in incidence angles can result in phase retardance changes large enough to increase depolarization significantly, especially when the amplifier is operated at higher powers. Appropriate selection of the complex evanescent coating allows a desired phase retardance angle to be maintained relatively constant over a small range of angles of incidence, at a given wavelength, and therefore permits minimization of depolarization and birefringence effects.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to laser amplifiers and, more particularly, to end-pumped zig-zag solid state lasers. A zig-zag solid state laser includes an elongated slab of rare earth doped lasing material, such as yttrium-aluminum-garnet (YAG). An input beam generated by a master oscillator is launched into one end facet of the slab, at an angle selected to result in multiple internal reflections from the internal faces of the slab. A pump beam is also input at one end of the slab and amplification of the input beam takes place as the input beam is reflected back and forth along the slab. The doped region of the slab in which amplification takes place is cooled by external means.  
         [0002]     The laser structure briefly described above is disclosed in detail in U.S. Pat. No. 6,094,297, referred to in this document as the Injeyan &#39;297 patent, issued to Hagop Injeyan et al. and assigned to the same assignee as the present invention.  
         [0003]     Although the amplifier described in the Injeyan &#39;297 patent is efficient and produces a beam of good quality and polarization properties, improvement is called for when many such amplifiers are combined to produce higher power beams. For such a configuration, it is highly desirable that each amplifier output should have linear polarization. Linear polarization is also required by applications that utilize frequency conversion of the output. As this zig-zag slab architecture has been extended to higher powers, it has been found that the polarization properties of the amplified output can degrade significantly, such that linear polarization can no longer be maintained effectively.  
         [0004]     Accordingly, there is a need for an improved zig-zag slab laser amplifier having significantly improved linear output polarization properties. The present invention is directed to this end.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention resides in an optical amplifier comprising an elongated slab of a solid state lasing material having a predetermined cross-section defining opposing end faces, a plurality of lateral faces, a longitudinal axis defined between the opposing end faces generally parallel to the lateral faces; means for enabling pumped light to be directed along an axis generally parallel to the longitudinal axis; and one or more sources of pump beams. An input optical beam is repeatedly reflected between two opposed lateral faces as it progresses through and is amplified in the elongated slab. In accordance with the invention, the slab includes a complex evanescent coating disposed on the two opposed lateral faces, to reduce depolarization arising from birefringence accumulated upon propagation of a beam through the slab.  
         [0006]     More specifically, the complex evanescent coating is selected to provide a selected phase retardance at each total internal reflection of the input optical beam as it zig-zags through the slab, the selected phase retardance resulting in minimization of the total accumulated depolarization. For example, the complex evanescent coating may be selected to provide a phase retardance of approximately zero at total internal reflection for the input optical beam. Ideally, the complex coating is selected to provide approximately the same selected phase retardance over a range of angles of incidence of the input optical beam with respect to the opposed lateral faces.  
         [0007]     The amplifier may further comprise a polarization rotator, positioned to receive an amplified optical beam output from the elongated slab and to rotate the angle of polarization of the output beam; and optical means for reflecting the polarization-rotated output beam back into the elongated slab along a different path to provide a second amplification pass. As disclosed, in the optical amplifier of the invention the elongated slab of a solid state lasing material comprises two end sections of undoped solid state lasing material designed to limit absorption of pump light, and a central section of doped solid state lasing material diffusion bonded to the two end sections.  
         [0008]     It will be appreciated from the foregoing that the present invention represents a significant advance in solid state laser amplifiers. Specifically, the use of a complex coating on opposed lateral faces of a zig-zag slab laser provides for polarization control and eliminates a serious drawback of optical amplifiers of this type when used at higher powers. Other aspects and advantages of the invention will become apparent from the following more detailed description, taken in conjunction with the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is cross-sectional diagrammatic view of a solid state zig-zag laser of the prior art.  
         [0010]      FIG. 2  is a graph showing variation of the total accumulated depolarization for a zig-zag slab versus total internal reflection (TIR) phase retardance.  
         [0011]      FIG. 3  is a graph showing variation of TIR phase retardance with internal angle of incidence, for a slab using a conventional thick silica evanescent coating.  
         [0012]      FIG. 4  is a view similar to  FIG. 1 , but showing a dual-pass architecture with a 90° polarization rotation and angular separation between first and second passes through the amplifier slab.  
         [0013]      FIG. 5  is a view similar to  FIG. 1 , but including a complex evanescent coating in accordance with the present invention.  
         [0014]      FIG. 6  is a graph showing phase retardance versus wavelength and angle of incidence, to indicate the relative constant magnitude of phase retardance over a small range of incident angles at a give wavelength, in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]     As shown in the drawings for purposes of illustration, the present invention is concerned with techniques for controlling polarization in a zig-zag slab laser amplifier.  FIG. 1  is a cross-sectional view of a zig-zag slab laser amplifier of the prior art, as illustrated in U.S. Pat. No. 6,094,297, referred to in this document as the Injeyan &#39;297 patent, which is hereby incorporated by reference into this document.  
         [0016]     In the Injeyan &#39;297 device, and in the present invention, an optical amplifier, generally identified by reference numeral  20 , utilizes end pumping. With such a configuration, pumped light is generally co-aligned with amplified light along a longitudinal axis of a slab  22 , resulting in a relatively long absorption length and providing relatively higher overall efficiencies. This configuration is particularly suitable for optical amplifiers that utilize solid state lasing material with relatively low absorption coefficients, such as those materials using ytterbium (Yb) and tellurium (Tm) as dopants. The absorption of the pumped light may be confined to a central region of the slab  22  to reduce heating at the opposing ends of the slab, which are known to be susceptible to warping.  
         [0017]     The optical amplifier  20  includes the elongated slab  22  and a pair of pump beam sources  21  and  26 . The elongated slab  22  is formed with a generally rectangular or square cross section defining a pair of opposing end faces  28  and  30  and four lateral faces  32 . As used in this description, a longitudinal or lasing axis  33  is defined as an axis generally parallel to the lateral surfaces  32  between the opposing end faces  28  and  30 . A major axis is defined as a horizontal axis in the direction of the zig-zag pattern, while a minor axis is defined to be a vertical axis generally perpendicular to the major axis. Both the major and minor axes are perpendicular to the longitudinal axis.  
         [0018]     The slab  22  may be formed from a solid state lasing material with a relatively high index of refraction to cause internal reflection of the input beam in a generally zig-zag pattern as illustrated in  FIG. 1 , forming a so called zig-zag amplifier. Such zig-zag amplifiers are known to effect brightness scaling by allowing the input beam to average thermal gradients in the slab, effectively providing a homogeneous gain medium. In order to reduce heating of the ends of the slab  22 , the slab  22  may be formed as a diffusion bonded composite material. More particularly, along the longitudinal axis  33  of the slab  22 , the opposing end portions  34  and  36  of the slab  22  can be formed from undoped host materials, such as yttrium-aluminum-garnet (YAG). These end portions  34  and  36  can be diffusion bonded to a central portion  38  of the slab  22  formed from a doped host material, such as Yb doped YAG (Yb:YAG) forming two diffusion bond interfaces  40  and  42 . Such diffusion bonding techniques are known in the art, for example, as described in detail in U.S. Pat. No. 5,441,803 hereby incorporated by reference. Such a configuration limits the absorption length to the center portion  38  of the slab  22 . By limiting the absorption length to the center portion  38  of the slab  22 , heat generated by optical pumping is mainly limited to the center portion  38  and away from the end portions  34  and  36 , which are susceptible to warping. As mentioned above, the pump beams  21  and  26  are reflected through the slab  22  from the opposed end faces  30  and  28 , respectively. The pump beams  21  and  26  may enter opposing lateral faces  32  of the slab  22  at opposing end portions  34  and  36 , respectively, as generally shown in  FIG. 1 . In order to enable launching of pump beams into the slab  22 , one or more footprints or windows  41  and  43  may be formed on opposing end portions  36  and  34 , respectively. The windows  41  and  43  may be formed by way of a coating, such as an antireflection coating selected for the wavelength of the pump beams  21  and  26 . As also shown in  FIG. 1 , the antireflection coating is disposed on the lateral face  32  as well as the opposing end faces  28  and  30 , thereby reducing losses of both the input beam and the pump beams. The pump beams  21  and  26  are directed to opposing lateral faces  32  at opposing end portions  34  and  36  of the slab  22 . The pump beams  21  and  26  are totally reflected from the opposing end faces  28  and  30  so that they are co-aligned with the longitudinal axis  33 . By utilizing the composite slab  22  as discussed above, the absorption length of the slab  22  is limited to the central portion  38 .  
         [0019]     An input light beam  44  is directed to one end face  28  at a relatively small angle, for example, less than 10° relative to the normal (perpendicular) direction of the end face. By confining the angle of incidence of the input beam  44  and selecting a material having a relatively high index of refraction, the input light beam is totally reflected along the slab  22  in a generally zig-zag pattern as shown and is out coupled as an amplified beam  46  from the opposing end face  30 . The zig-zag pattern across the slab temperature gradients combined with uniform pumping by the guided diode light and insulated slab edge results in relatively low thermal lensing with limited birefringence.  
         [0020]     It is known in the art that pumping of the slab  22  results in increased temperature in the area where the pump light is absorbed. As mentioned above, pump beams, for example, from diode arrays, are directed generally perpendicular to the lateral faces  32  through the windows or footprints  41  and  43 , and reflected from the opposing end faces  28  and  30  to cause the pump beams to be directed along the longitudinal axis  33 . In order to cool the slab  22 , various cooling methods can be used. Both conduction and convection cooling systems are suitable. An example of a conduction cooling system is to attach the slab  22  to a high intensity impingement cooler, for example, as manufactured by Thermal Electron in San Diego, Calif. or SDL, Inc. in San Jose Calif.  
         [0021]     To minimize the thermal resistance between the slab  22  and the coolers, a thin layer of a thermally conductive material such as a soft metal, such as indium or gold, may be used. During assembly, the cooler/indium/slab assembly may be held under pressure at elevated temperatures, approximately 150° C. to flow the indium and eliminate contact resistance. For direct or convective cooling, the slab  22  may sealed in the dead zones with a thin layer of turbulent coolant flowing over the slab faces to remove heat as discussed in detail in U.S. Pat. No. 5,646,773, which is hereby incorporated by reference. An exemplary convection cooling system is disclosed for example, in commonly owned U.S. Pat. No. 5,646,773, which is also hereby incorporated by reference.  
         [0022]     In the case of convection and conduction cooling, the lateral faces  32  of the slab  22  are coated with a dielectric material which serves as an evanescent wave coating  48  to preserve total internal reflection. As shown in  FIG. 1 , the evanescent wave coating  48  may extend from one end face  28 ,  30  to a region slightly beyond the diffusion bond interface  42 , adjacent to the opposing end face. The evanescent wave coating  48  allows the slab  22  to be directly adhered to the impingement cooler. A thick layer (2-4 μm) of MgF 2  or SiO 2  may be used as the evanescent wave coating  48 .  
         [0023]     The coating  48  is a uniform film deposited on the slab surface and having an index of refraction lower than that of the slab material. The amplified beam&#39;s field decays exponentially in the evanescent coating  48  such that there is a negligible field present at the coating surface. Cooling applied to the coated surfaces of the slab  22  removes excess heat from the slab without impacting the optical performance. Temperature gradients that form within the slab can induce index non-uniformities and birefringence. However, the alternate transversals (zig-zags) of the beam tend to average out these effects and maintain both good beam quality and polarization purity. In particular, shear stress develops within the slab  22  and can lead to polarization rotation, but these polarization effects can be cancelled out by nearly identical adjacent traversals in opposite directions (zigs and zags) along the slab width, at least for low and moderate optical powers.  
         [0024]     At high powers, it has recently been observed that devices of this general type can display severe depolarization. A further discovery is that the effect of total internal reflections (TIR) upon the polarization state of the propagating beam can play a very significant role in the amount of observed amplifier output polarization. The two polarization components of the amplified beam are the S-field, in the plane of the slab surface, and the P-field, in a plane perpendicular to the surface plane. These components experience a phase retardance upon total internal reflection, and the phase retardance can cause undesirable interference between the two polarizations as the amplified beam bounces back and forth down the slab, which can impede cancellation of the induced birefringence. In particular, it has been found that the polarization purity of the amplifier output could vary by a factor 5-10 times depending on the value of the TIR phase retardance. This phenomenon is illustrated in  FIG. 2 .  
         [0025]      FIG. 2  shows the variation of depolarization (plotted along the vertical axis) for a 2×20×125 mm zig-zag slab amplifier with  40  internal reflections (bounces), versus the TIR phase retardance, plotted along the horizontal axis and measured in radians. For each of the two wavelengths shown, 633 nm (curve A) and 1064 nm (curve B), there is a periodic variation in depolarization. Specifically, the figure shows a phase retardance spacing between depolarization maxima of approximately 10°. Therefore, the depolarization value can change from a minimum to a maximum value over a span of approximately only 5° of TIR phase retardance.  
         [0026]     As shown in  FIG. 3 , for a slab with a conventional evanescent coating the TIR phase retardance can vary as much as 5° or more if the internal beam angle (the angle of incidence) changes by only 1° or less. For example, a change in internal angle from 54° to 55° results in a change in phase retardance from approximately 10° to more than 15°. A prior art amplifier technique involves the use of two passes through the slab  22 , as shown in  FIG. 4 . The output beam from the first pass, indicated at  50 , is reflected through a quartz polarization rotator  52  and then reflected back into the slab  22  as a second-pass input beam  54 . In the second pass, traverses a slightly different path from the first-pass beam, and emerges from the end face  28  as an output beam  56 . Implementation of this technique requires significant angular separation between the beams of the first and second passes. Clearly, if this angular separation results in a different internal angle, then, as discussed above with reference to  FIG. 3 , even a small change in internal angle can result in a change in a phase retardance of several degrees. Further, as seen from  FIG. 2 , this change in phase retardance can result in a significant depolarization effect.  
         [0027]     In accordance with the present invention, and as shown in  FIG. 5 , a complex evanescent coating  48 ′ is included in the slab architecture, such that the TIR phase retardance has a specific value chosen advantageously to minimize the amplifier depolarization. For example, an S-P phase retardance near zero or near 0.23 radians (13°) results in minimum amplifier depolarization, as shown in  FIG. 2 .  
         [0028]     Design of such complex coatings has been described in the past by, for example, J. H. Apfel in “Graphical method to design internal reflection phase retarders,” Applied Optics, vol. 23, pages 1178-1183 (1984). For this particular application, a multi-layer coating with an overcoat of a thick silica (or other material) layer can be designed with great flexibility to achieve the desired phase shift, while ensuring that the field at the surface of the thick overcoat is negligible. In addition, since the amplified light does not penetrate the thick layer, an additional (multi-layer) coating may be added on top of the thick layer without adversely affecting the amplified light. These additional coating layers may be used to tailor the properties for incident angles smaller the TIR critical angle, e.g., to provide high transmission of the pump light as it enters near the end of the slab  22  at near-normal incidence.  
         [0029]     Furthermore, the design may be optimized such that the TIR phase retardance is nearly constant over a modest range of incident angles. This feature has two additional advantages. First, it allows more tolerance in the other elements of the slab design, such as small variations in the slab dimensions, and in the indices of refraction of both the slab and coating materials. Secondly, it allows the slab to be double-passed, as shown in  FIG. 4 , wherein the first pass output beam is returned back through the slab, after rotating the polarization 90° by an appropriate device such as a quartz crystal or a Faraday rotator. In order to separate this backward propagating second pass from the input beam, it may be required to launch it at a slightly different angle (and perhaps use a different number of bounces) from the first pass beam. If these two passes have nearly equal depolarization effects, then the dual pass combination with a 90° polarization rotation interposed can effectively cancel any residual depolarization present in a single pass. Designing the TIR coating  48 ′ to be relatively insensitive to the incident angle enables the use of this enhanced dual pass architecture for depolarization reduction.  
         [0030]     An example of the performance of a multi-layer TIR coating designed for near-zero phase retardance over a small range of angles is shown in  FIG. 6 . At a wavelength of approximately 1.07 μm, the phase retardance is approximately zero over a range of incident angles from 55° to 58°, as indicated by the multiple curves in the region of the square  60 . This range is adequate to accommodate two passes with an adjacent even number of bounces. For example, in a zig-zag slab design of interest (slab width of 2 mm and length of 125 mm, and nominally with 40 bounces), the difference between the internal angle of modes with consecutive even number of bounces is only approximately 1.3°. This multi-layer coating would allow two passes (using the architecture of  FIG. 4 ) to propagate through the slab with up to about a 3° difference in internal angle while both passes experience near zero TIR phase retardance.  
         [0031]     It will be appreciated from the foregoing that the present invention represents a significant advance in the field of high power solid-state lasers. In particular, the invention provides a technique for minimizing depolarization and resultant birefringence in a zig-zag slab laser. The invention may be usefully employed in scaled arrays of zig-zag laser amplifiers with higher power and beam quality. Such devices have both military and commercial applications, such as in directed energy weapon systems, remote sensing, and material processing. Any application requiring very high laser power would significantly benefit from the increased power that the invention provides in a slab laser structure. It will also be appreciated that, although a specific embodiment of the invention has been described for purposes of illustration, various modifications may be made without departing from the spirit and scope of the invention. Therefore, the invention should not be limited except as by the appended claims.