Abstract:
Achromatic phase-matching (APM) is used for efficiently multiplying the frequency of broad bandwidth light by using a nonlinear optical medium comprising a second-harmonic generation (SHG) crystal. Stationary optical elements whose configuration, properties, and arrangement have been optimized to match the dispersion characteristics of the SHG crystal to at least the second order. These elements include a plurality of prismatic elements for directing an input light beam onto the SHG crystal such that each ray wavelength is aligned to match the phase-matching angle for the crystal at each wavelength of light to at least the second order and such that every ray wavelength overlap within the crystal.

Description:
STATEMENT OF PRIORITY 
     This following application for U.S. patent claims the benefits of U.S. Provisional Patent Application Serial No. 60/065,264, filed on Nov. 6, 1997. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     This invention was made with Government support under contract DE-AC04-94AL85000 awarded by the U.S. Department of Energy to Sandia Corporation. The Government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates in general to a system and method for nonlinear frequency conversion tunable laser light using achromatic phase-matching, and in particular to an achromatic phase-matching optical system and method which exactly matches the high order dispersion characteristics of nonlinear optical materials. 
     Many applications require broadly tunable UV light. No such laser source exists, however, so tunable UV is usually obtained by frequency-doubling a tunable laser in the visible and near-IR by using nonlinear optical effects such as a second harmonic generation process. Such processes are phenomenon which derive from nonlinear polarization effects of certain material media. The effect depends upon crystal structure, particularly anisotropic structure. Commonly used crystals are β-barium borate (“BBO”), potassium dihydrogen phosphate (“KDP”) and lithium triborate (“LBO”). 
     Because frequency-doubling, therefore, involves passing light through a nonlinear crystal, and the effects of its wavelength-dependent refractive index must be taken into account. In particular, in order for frequency doubling to take place in the crystal, the refractive index of the incident light alt the “fundamental” wavelength must equal the refractive index of the frequency doubled light to be produced. Since the refractive index of the crystal varies botlr with the angle of incidence and with the frequency of the input beam it is apparent the that absent extraordinary precaution only a very narrow range of frequencies of a broadband beam can enter a crystal at the appropriate incident angle for efficient frequency doubling. Unfortunately, this procedure is sensitive to vibrations and can be unreliable, despite the use of feedback. In addition, it produces undesirable beam walk as the laser tunes, which must be corrected with yet another moving part. 
     Second-harmonic generation of light (hereafter referred to as “SHG”) the generation of light of twice the optical frequency of input laser light, has been an essential tool of laser research for many years. It is used widely to generate ultraviolet light because such wavelengths are difficult to generate directly from a laser. Indeed, this technique is often used to generate visible light from a near-infrared laser because it is easier to generate near-infrared laser light than it is to generate visible light. In general, however, it is possible to frequency-double light from virtually all visible and near-infrared lasers. 
     A particular type of laser light which is important to frequency-double is broadband light. However, the use of SHG processes to frequency-double broadband light which is incoherent has proved to be difficult and inefficient. (In general, ultrashort pulses generated by lasers can be considered broadband light whose frequencies are in phase while incoherent light can be considered broadband light whose frequencies are randomly phased.) These two types of light are difficult to frequency-double due to their respective large bandwidths. As a result of the large bandwidths, efficient methods for frequency-doubling both of these types of light have not been developed. 
     The efficiency η of a SHG process depends on several factors. A first factor is the nonlinear coefficient of a SHG crystal used. This factor depends on internal properties of the crystal and can only be improved by manipulating the composition of the crystal. 
     Second, η is proportional to the square of the length of the crystal, L, the distant through which light ray propagate through the crystal. Thus, thick crystals yield much higher efficiency than thin ones. 
     Third, η depends on the laser intensity and is, typically, directly proportional to the laser intensity. Consequently, continuous-beam lasers, which have relatively low intensity, frequency-double inefficiently while pulsed lasers, which generally achieve higher intensity, frequency-double more efficiently. In general, the shorter the pulse the more efficiently it frequency-doubles, given a fixed energy per pulse. 
     As earlier noted, in order for frequency-doubling to take place in a SHG crystals, the refractive index of the input laser light (again, the “fundamental” wavelength) must equal the refractive index of the frequency-doubled light to be produced. Since the refractive index of a crystal is a function of both the incidence angle and frequency of the input beam different incidence angles must be used to obtain maximum efficiency η for different wavelengths. The requirement that a wavelength enter the crystal at the appropriate angle necessary to frequency-double most efficiently will be referred to hereinafter as the “phase-matching condition,” or simply “phase-matching” for short. The angle will be referred to as the “phase-matching angle,” and is a function of wavelength. 
     Because the efficiency η of the SHG process is strongly “peaked” with respect to the entrance angle for a given wavelength and also with respect to wavelength for a given angle only a small very narrow range of wavelengths near the exact phase-matching wavelength can still yield highly efficient SHG process. The range of wavelengths that achieves high-efficiency frequency-doubling for a single angle is called the crystal&#39;s “phase-matching bandwidth” for that angle. If the input laser light contains frequencies outside this bandwidth, such frequencies will not produce their corresponding second harmonic (i.e., will not be frequency-doubled and the efficiency of the overall process is reduced. 
     When the crystal bandwidth is greater than the input light bandwidth, the above effect can be neglected. However, when the crystal bandwidth is less than the bandwidth of the input light, the SHG efficiency is proportional to the crystal bandwidth, yielding a fourth factor. In this case, the efficiency can be written approximately as:        η   ∝       d   2     /       L   2          (       Δ                   λ   cr         Δ                   λ   l         )                                
     where d is the nonlinear coefficient of the crystal, I is the intensity of the light, L is the length of the crystal through which the light propagates, Δλ cr  is the bandwidth of the crystal, and Δλ I  is the bandwidth of the incident light. Furthermore, the bandwidth, Δλ cr  of an SHG crystal is given by:          Δ                   λ   cr       =       λ     4      l             (          n          λ       )     t     -       (          n          λ       )     s                                
     where λ is the wavelength of light and dn/dλ is the derivative cof the refractive index n with respect to wavelength at the appropriate polarization of the fundamental wavelength and second harmonic wavelength, indicated by the subscripts, f and s, respectively. 
     Thus, the bandwidth of an SHG crystal is a function of the crystal&#39;s refractive-index vs. wavelength curve: a fundamental property of the crystal. Furthermore, the bandwidth is inversely proportional to the crystal length. Hence, if one attempts to increase the conversion efficiency by increasing the crystal length, one must also increase the precision of the phase-matching thereby reducing the tolerance for error in the entrance angle of the incoming beam. 
     Various attempts to improve the efficiency of the SHG process have been and continue to be made. Several researchers have introduced achromatic phase-matching (APM) devices that use angular dispersion so that each wavelength enters the nonlinear crystal at its appropriate phase-matching angle as a way of increasing the bandwidth of the crystal and therefore, increase its efficiency. The crystal and all dispersing optics remain fixed. Because such systems have no moving parts, they are inherently instantaneously tunable, and can be used for nonlinear conversion of tunable or broadband (such as ultrashort) radiation. Most of these devices have used gratings or prisms in combination with lenses which are sensitive to translational misalignment. Also, previous work has considered only the lowest order (linear) term of the media-created dispersion and the phase-matching angle tuning function. Bandwidths of about 10 times the natural bandwidth of the crystal were achieved; larger bandwidths were only obtained by using a divergent beam at the expense of conversion efficiency. 
     The relationship between the phase-matching angle and the wavelength λ is best approximated by a high order polynomial. By modeling an angularly dispersive optical system such that the dispersion angle(s) of the light propagating through that system, as a function of the wavelength(s), match the phase-matching angle(s) of the SHG crystal, again as a function of wavelength, in both the first and the second order terms of the polynomial, it is possible to bring a much broader band of light wavelengths into the SHGI crystal at the optimum angles for frequency doubling (see FIG.  7 ). The instant invention seeks to implement this process for increasing the efficiency of frequency doubling through the application of this technique. 
     The crystal dispersion and phase-matching-angle tuning functions have now been modeled exactly using Sellmeier equations. A “grism” (a prism having a transmission or reflection grating on one surface), which combines the high dispersion property of gratings with the optimum first and second-order behavior of the dispersion angle tuning function has been used in combination with other elements in order to achieve phase-matching over a broader range of incident light wavelengths. Unfortunately, grisms with high diffraction efficiency are not yet available. Indeed, no previous APM device has simultaneously achieved high efficiency and a tuning range greater than approximately 10 times the crystal bandwidth. Furthermore, previous attempts to coalign the otherwise divergent and dispersed second-harmonic beams after passage through the SHG crystal were only partially successful, due in part, to the insufficient precision in matching the second-harmonic dispersion function to the phase-matching function. 
     An attempt to improve the efficiency of the SHG process is to carry out achromatic phase-matching of the laser pulse incident upon the SHG crystal. FIGS. 1 and 2 show two such conventional approaches. As illustrated, the input light beam  102  is dispersed into its individual frequency components  104  using a diffraction grating  106 . As a result, the frequency components  104  of the input light will each propagate at a different angle, with adjacent frequencies having adjacent angles of propagation. Then, using a single lens  108  (FIG. 1) or a two-lens telescope  208  (FIG.  2 ), these light rays are recombined at the SHG crystal  110 . In this manner, all frequencies overlap at the same point and each frequency enters the crystal  110  at its optimal phase-matching angle. Thus, each frequency component of the laser pulse efficiently frequency doubles. In other words, each frequency component essentially acts as an independent and narrowband process, each of which can be quite efficient when a relatively thick crystal is used. Since each frequency component can be treated as a narrowband beam that does not require an SHG crystal with a large bandwidth, a relatively thick crystal can be used. 
     It is important to note that, because the second harmonic beam produced will be dispersed at an angle, an analogous optical apparatus must be used on the output side of the crystal to reconstruct, i.e., to coalign onto a single path all of the second-harmonic rays/beams within the converted bandwidth. 
     While these designs potentially achieve improved efficiency in the SHG process itself, they introduce a new inefficiency associated with the diffraction grating. Diffraction gratings are not particularly efficient, and since an additional diffraction grating is required to coalign the second harmonic rays/beams on the other side of the SHG crystal, efficiency is reduced even more. This is especially true if the diffraction grating must operate on ultraviolet light, which will be the most common case in SHG processes. When the inefficiencies of the diffraction gratings  106  are considered, the overall efficiency of the SHG process is reduced by roughly a factor of 4. While the overall efficiency of these designs is still greater than that typically obtainable without achromatic phase-matching using standard crystals, the efficiency is not sufficiently improved that achromatic phase-matching has found practical use. 
     An alternative approach uses prisms instead of diffraction gratings to disperse the input beam. Both disperse light into its frequency components, but prisms can be anti-reflection-coated or used at Brewster&#39;s angle and hence, can result in insignificant loss of efficiency. However, prisms typically have about one tenth the dispersion, which is required for a typical achromatic: phase-matching situation. 
     As illustrated in FIGS. 3 and 4, in such designs the prisms have been used in conjunction with lens devices to amplify the prism dispersion to appropriate values. In FIG. 3, the input light  301  is incident on a single prism  303  and a two-lens telescope  305  is used to amplify the dispersion of the prism  303  and to focus the light onto the SHG crystal  307 . In FIG. 4, light  401  is passed through two appositely oriented prisms  403 ,  404  and then directed through a single lens  407  to recombine the various frequencies in the SHG crystal  409 . 
     The device depicted in FIG. 3 achieves sufficient dispersion because the two-lens telescope  305  amplifies the dispersion of the prism by 1/M, where M is the magnification of the telescope. A problem associated with such a design is that the group velocity dispersion (the tendency for red wavelengths to travel faster than blue wavelengths) in the system is always positive. Thus, the pulse spreads in time greatly reducing the efficiency of the overall systems for most types of ultrashort light pulses as more fully described below. 
     The device of FIG. 4 achieves sufficient dispersion because a sufficiently short-focal-length lens  407  can be used to recombine the spatially dispersed rays out of the two-prism assembly to achieve the desired dispersion. While this design can achieve zero (or negative) group-velocity dispersion, it suffers from a different flaw. The angle at which the light rays are incident at the crystal is dependent upon the input position of the input light beam  401  relative to the lens, and in fact, any system which uses lenses is sensitive to the exact position of the lenses. 
     Finally, as with second-harmonic conversion, other nonlinear optical conversion processes also require angular phase-matching. such processes are mathematically more complex than SHG, but the angles of all input beams must still be precisely controlled to provided efficient phase-matching. Similarly to SHG, the phase-matching angles of all input beams and the resultant angles of all output beams are each a function of all their wavelengths and, in some cases, of the input angles, as well. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide an optical system for performing achromatic phase-matching by matching the dispersion angles of input rays/beams to the phase-matching angles of those rays/beams to a high order, and to similarly coalign the converted output rays/beams. 
     It is another object to provide an achromatic phase-matching optical system, for use in a SHG process, having elements configured to very closely match the first and second order terms of the angular dispersion to the first and second order terms of the phase-matching angle of the SHG crystal such that the angular dispersion at all incident frequencies of a broadband pulse of light (or tunable) passing through the elements is such that each given frequency enters the SHG crystal sufficiently close to its exact phase-matching angle that nonlinear conversion is efficient. 
     It is yet another object of this invention to provide an optical parametric amplifier wherein the instant invention is used to align a “seed” signal beam into the nonlinear crystal at appropriate angle for phase matching and is then “pumped” by another beam of greater power, transferring that power to the seed beam and amplifying it at the expense of the pump beam. 
     It is still another object of this invention to provide an optical parametric oscillator wherein the nonlinear crystal is contained with in the resonator cavity, and one which arranged with APM elements on either side of the nonlinear crystal to maintain the phase matched condition in both direction in the resonator cavity. 
     Yet another object of this invention is to provide an optical parametric oscillator using APM as described herein wherein a mode-locI(ing device is introduced into the oscillator optical path and the path length adjusted for any of a plurality of resonant beat frequencies. 
     To achieve these and other objects, there is provided an apparatus and method for efficiently converting a large bandwidth light pulse into a similarly large bandwidth pulse of light whose frequency is a multiple of its original incoming frequency by means of a nonlinear optical medium. If the nonlinear responding medium is a SHG crystal, the light beam incident upon the crystal is passed though the crystal and is frequency-doubled. It should be noted, however, that the instant invention is not limited solely to frequency doubling nonlinear responses. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The instant invention will be better understood by reference to the detailed description of the preferred embodiments of the invention with reference to the following drawings in which: 
     FIG. 1 illustrates a conventional achromatic phase-matching system; 
     FIG. 2 illustrates another conventional achromatic phase-matching system; 
     FIG. 3 illustrates a conventional achromatic phase-matching system using a single prism; 
     FIG. 4 illustrates a conventional achromatic phase-matching system using two prisms and a single lens; 
     FIG. 5 illustrates the preferred embodiment of the invention; an achromatic phase-matching system designed to match the first and second orders of dispersion to those orders of the phase-matching argle of the SHG crystal using a series of fixed prisms before the crystal and another series of prisms after the SHG crystal to coalign the second harmonic rays/beams emanating from the crystal. 
     FIG. 6 illustrates a contour plot of the experimentally measured small-signal relative second-harmonic conversion efficiency vs. wavelength and absolute crystal angle. The solid curve is the theoretically predicted difference between the dispersion and exact phase-matching angles for the SHG crystal. Bounding the solid line are model-derived±50% conversion efficiency limits. 
     FIG. 7 illustrates another view of FIG. 2 shown as the relative conversion efficiency taken along a fixed absolute crystal angle with respect to an optical table. The figure compares the theoretically predicted relative conversion efficiency (solid curve) with the experimentally measured data. Prior art is shown by the dashed curve. 
     FIG. 8 illustrates the experimentally measured second-harmonic beam position (triangles) and angle (circles) vs. wavelength, in the plane of dispersion, at the output of the preferred embodiment. The solid curve is the model-predicted angle. 
     FIG. 9 illustrates embodiment 2; the use of a grism to provide the appropriate first and second order dispersion angle. 
     FIG. 10 shows a simplified view of the process for realigning signal or idler beams emanating from an optical parametric generator. 
     FIG. 11 illustrates embodiment 3 and 4 using the preferred embodiment as an optical parametric amplifier or oscillator. 
     FIG. 12 illustrates a schematic of embodiment 4 wherein the preferred embodiment is configured as an optical parametric oscillator with a mode-locking device for generating ultrashort pulses. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     PREFERRED EMBODIMENT 
     The apparatus and method according to the instant invention is illustrated schematically in FIG.  5 . It is noted that throughout this application the term “optical” is used in its broadest sense as pertaining to light. Moreover, the term “light” is used in its broadest sense and includes all forms of electromagnetic radiation and shall not be construed to be limited solely to visible light. 
     In this application, an APM device is described which is made entirely of prisms operating near Brewster&#39;s angle or anti-reflective(“AR”)-coated for normal incidence. This device also includes dispersion after the SHO crystal to coalign all of the second-harmonic beams  525 . It has a full-width-hall-maximum bandwidth of about 110 nm fundamental wavelength centered at 660 nm using a 4 mm-long type 1 BBO crystal for second-harmonic generation. This experimentally derived bandwidth is about 150 times the natural bandwidth of a 4 mm-long BBO crystal cut for type 1 phase-matching. 
     A single glass prism has only ˜{fraction (1/10)} the dispersion necessary at the crystal, so ten prisms in series could be used to achieve the required dispersion. Instead, one equilateral prism  503  is used followed by three Littrow prisms (30° apex angle)  504 ,  505 , and  506 , each of which not only adds to the dispersion, but also magnifies it as demonstrated in U.S. Pat. No. 5,648,866, (herein incorporated by reference). The broadband input beams  500  enter Littrow prisms  504  through  506  near normal incidence and exit near Brewster&#39;s angle (˜60°). Each Littrow prism spatially compresses the now dispersed beam  520  in the plane of refraction, which introduces a magnification of the upstream dispersion angle by a factor of about 1.8. Two additional prisms  501  and  502  are used on the input side of the equilateral prism to spatially (but not angularly) disperse the incoming beam  500 , so that the angular dispersion introduced by the remaining prisms causes all frequencies to overlap spatially in the crystal. These first two prisms ( 501  and  502 ) solve another problem: the magnification of prisms  504 ,  505 , and  506 , increases the divergence of beam  520  at each frequency, potentially beyond the acceptance angle of BBO crystal  517 . Prisms  501  and  502  are also Littrow, but are oriented with respect to each other, and with the remaining prisms, in order to demagnify the beam divergence, partially compensating for the magnification of the other Littrow prisms. All of the input prisms are made of SF11 glass with the exception of prism  502  which is configured in F 2  glass. The material is so chosen so that its index of refraction is smaller than that of prism  501 . This must be so in order that the net angular dispersion of prisms  501  and  502  is zero. 
     The long path between the prisms  501  and  502  is folded twice by two high-reflectivity mirrors  513  and  514 . Furthermore, the polarization through the prisms is chosen to be p (in the plane of dispersion) since most optical faces in the APM device are near Brewster&#39;s angle and the remaining faces are anti-reflection coated. A zero-order half-wave plate is then required just before the BBO crystal in order to rotate the polarization to s (out of the plane of dispersion) for type 1 phase-matching. 
     At a nominal wavelength of 650 nm the six prisms on the input side of the APM device shown in FIG. 5 are arranged to constrain light entering and exiting each prism as follows. Prisms  501  and  506  each has its apex angle oriented to the left of the input beam  500  and dispersed beam  520 , respectively. Each is rotated so that 650 nm wavelength light is incident at 59.5° and at 0.5°, respectively. Light exits prism  501  at 17.6° and prism  506  at 61.73°. Prisms  502 ,  503 ,  504 , and  505  each has its apex oriented to the right of the dispersed input beam  520 . Furthermore, prisms  502 ,  503 ,  504 , and  505  are rotated such that light is incident upon them at angles of 59.6°, 60.68°, 1.0° and 1.0° respectively, and exits at angles of −3.67°, 64.78°, 60.83°, and 60.83°, respectively. A zero-order half-wave plate  518  is attached to SHG crystal  517 , here a crystal of β barium borate (“BBO”). The purpose of the wave plate is to rotate the incoming light rays by 90° out of the plane of dispersion, i.e., from a polarization of p to a polarization of s in order to match the polarization of crystal  517 . 
     The output side of the device, that is, after BBO crystal  517 , is qualitatively the reverse of the input side, but all the prisms are of fused silica. The apex and incident angles are also different from the input. The prisms  511  and  512  do not remagnify the divergence because they have the same index, and hence cannot be arranged analogously to prisms  501  and  502  of the input. No wave plate is needed since the converted light emanating from crystal  517  is again rotated by 90° to a polarization of p. Because of the different magnification from the input, the collinearly aligned output beam  530  is wider than the input by a factor of about 4, which can be compensated for by adding a cylindrical telescope (not shown) after prism  512 . 
     The arrangement of prisms elements is as follows: prisms  507  and  512  have their apex angles oriented to the right of the outgoing beam  525 . The remaining prisms, prisms  508  through  511  have apex angles to the left of beam  525 . Apex angles of prisms  507  through  509  are 30° while apex angles for prisms  510  through  512  are 68°. For nominal 325 nm output light (½ of the “fundamental” input light) prisms  507  and  512  are rotates so that light is incident on each at angles of 62.5° and 56°, respectively. Light exits each of these elements at angles of −10.12° and 55.89°, respectively. Prisms  508 ,  509 ,  510 , and  511  similarly are rotated so that 325 nm wavelength light is incident at angles of 64.1°, 64.1°, 64.1°, and 56° respectively, while light exits at an angle of −10.98°, −10.98°, 48.99°, and 55.89°, respectively. The reader should appreciate that many similar arrangements of entrance and exit angles are possible depending upon the chose of prism type and of equivalent structures. The arrangement described above is for illustrative purposes only and should not be construed as in any way limiting or restricting the invention described and disclosed herein. 
     The instant invention was characterized with a tunable commercial optical parametric oscillator (OPO) pumped with the third harmonic of a Q-switched Nd:YAG laser. FIG. 6 shows a number of density contours of the experimentally measured relative second-harmonic conversion efficiency as a function of wavelength and absolute crystal angle. Each point is an average of the second-harmonic pulse energy divided by the square of the fundamental energy averaged over several laser shots, and then normalized to the maximum efficiency value at each wavelength to remove the wavelength dependence of the detector and transmission of optical elements. The plot should consist of a sinc 2  angle tuning curve at each wavelength. Shown for comparison is the computed difference between the predicted dispersion angle for the preferred embodiment and the exact phase-matching angle of BBO crystal  517 . It follows the experimental maxima, as it should. Once the input prisms of the device were pre-aligned to the computed optimum orientations using a red HeNe laser, only one degree of freedom was needed to optimize the dispersion experimentally. This optimization was accomplished by the adjustment of the angles of prisms  503  and  505 , so that the angular positions of the maxima of the sinc 2  angle tuning curves at two well-separated wavelengths matched the computed difference curve. 
     FIG. 7 is a slice of FIG. 6 at fixed (zero) absolute crystal angle measured with respect to the optical bench. FIG. 7 shows clearly a full-wvidth-half-maximum fundamental bandwidth of approximately 110 nm. The experimental points agree with the relative conversion efficiency computed from the predicted angle difference curve in FIG.  6 . Shown for comparison is the predicted relative conversion efficiency of a grating operating at the Littrow condition (diffracted angle=−incident angle), with the correct linear dispersion to imatch the BBO angle tuning curve. Its bandwidth is only 20 nm since it does rot match the BBO angle tuning curve beyond the first order. 
     The output of the device was also pre-aligned with a red HeNe laser. We measured and coaligned the second-harmonic beam positions and angles precisely using lenses to image them onto a CCD array (after the telescope mentioned above). The output prisms were experimentally optimized by adjusting the angle of prism  505  to provide nearly constant output position and the angle of prism  506  to center the experimental output angle curve with respect to the predicted curve. 
     FIG. 8 shows the measured position and angle (in the dispersion plane) after the last prism  512  as functions of wavelength, and the angle predicted from the computed optimum prism orientations. The position has been normalized to the spot diameter at the exit of prism  512 . Each point is the average of 40 laser shots of the centroid of the beam spots on the CCD, taking into account the magnification of the imaging lenses and the telescope. Since the collinearity is quadratically limited (as the theoretical curve is nearly a parabola), the computed parabolic curvature can be achieved only with perfect alignment of all of the output elements. With even slightly imperfect alignment, the achieved parabola will be sharper, as observed. 
     The position and angle out of the dispersion plane (vertical) should remain constant over all wavelengths. However, small tilting of a prism can introduce its own vertical dispersion, and couple dispersion from upstream into the vertical plane. A mostly linear dependence was observed of both vertical position and angle on wavelength (slopes of 40 μm/nm and 15 μr/nm respectively). One prism is believed responsible for most or all of this dispersion. In theory, this minor problem is easily compensated by tilting other prisms. 
     We believe that this is the first complete and practical broadband frequency-doubling device. 
     EMBODIMENT 2 
     An alternate embodiment which utilizes a prism/grating combination, or “grism,” to provide the necessary beam dispersion is shown in FIG.  9 . This design utilizes four prisms one if which is the prism/grating combination. The grism combination is used because the dispersion of a simple grating cannot simultaneously match both the first and second order terms of the SHG crystal phase-matching angle tuning function whereas the grism can. This is possible because the grating equation for a grating on the exit face of the prism is, 
     
       
         sin θ o =nθ i −bλ 
       
     
     where n is the refractive index of the grism and b is the groove density of the grating. θ o  and θ I  are the diffracted and incident angles, respectively. The linear and quadratic dispersion are,              ∂     θ   o         ∂   λ       =       -              b                   sec                   θ   o         ,           ∂   2          θ   o         ∂     λ   2         =       +       (       ∂     θ   o         ∂   λ       )     2          tan                   θ   o                                
     When the diffracted angle is negative (as with the Littrow or Bragg conditions), the linear and quadratic terms have the same sign. The beam  910  must cross the normal to the grating for the quadratic term to have the correct sign, but the quadratic term is still of insufficient magnitude relative to the linear term in a normal grating. Because of this property prior art designs that use gratings have limited the achievable phase-matching bandwidth at the crystal to only about 10 times the natural bandwidth of the crystal (˜10 nm at 650 nm fundamental wavelength). 
     Because of the factor n, the grating incident angle θ I  (inside the grism) can be greater than the critical angle of the grism substrate. With the grism the diffracted angle θ o , and hence the second order dispersion, can be larger and still of the correct sign than in a normal grating. 
     FIG. 9 illustrates a schematic of this alternate embodiment. The doubling crystal is a BBO crystal  907  cut for type 1 phase-matching of (nominally) 650 nm light. Crystal  907  is about 4 mm long (i.e., the crystal presents a nominal 4 mm propagation path for the incoming light rays/beams  910 ) and has a natural phase-matching bandwidth of &lt;1 nm and an acceptance angle of 1 mrad. The first two prisms  901  and  902  serve to disperse the different wavelengths laterally but not angularly. Grism  903  and the Littrow prism  905  together introduce the appropriate higher order dispersion and cause the different wavelengths to converge in SHG crystal  907  at the phase-matched angle appropriate for each. 
     Prisms  901  and  902  are equilateral (apex angle of 60°) and are constructed of SF10 glass, having an index of refraction of about 1.72. Incidence angles for both are 62.9° and 54.9° respectively. The beam  910  is bent by 56° in both. The optical path length is determined by the size of several elements and the requirement that all wavelengths converge in the crystal. The apex angles of these two prisms are oriented opposite one another, while the apex angles of elements  902 ,  903  and  905  are all oriented in the same direction. 
     Grism  903  is an equilateral prism constructed from BK7 glass and having 600-grooves/mm grating  904  on the exit face. 650 nm wavelength light enters the face of grism  903  at an incident angle of 11° and on the grating at 58°. The first diffracted order exits grism  903  at 58° and enters the Littrow prism  905  (apex angle of 30°) at an incident angle of 2°. The Littrow prism  905  is constructed of SF11 glass its apex angle is oriented in the same direction as the two preceding prism elements. The 650 nm wavelength light leaves the Littrow prism  905  at a 63° exit angle. 
     Again, most of the optical interfaces are near Brewster&#39;s angle, necessitating the use of p polarization to eliminate reflective losses. A zero-order half-wave plate  906  is, therefore, placed in contact with, or just prior to, the SHG crystal  907  entrance face in order to rotate the incoming light into a polarization to s so that the crystal phase matching plane would be aligned with the beam dispersion plane. 
     The ray/beam reconstruction arrangement can be designed similarly with another grism and other prisms, qualitatively in the opposite order as above, but with materials and other properties appropriately chosen such that the net first and second order dispersion of the second-harmonic light matches those terms of the crystal phase-matching angle. Alternatively, the above-described embodiment may also includes an all-prism beam reconstruction arrangement identical to that shown in FIG. 5 as elements  507  through  512 . This portion of the embodiment has not been shown for the sake of brevity as it has already been illustrated once. It should not be assumed that because it is not expressly illustrated it is not part of this embodiment. Furthermore, the written description and drawings are provided for illustrative purpose as sufficiently descriptive of the instant invention as to allow one skilled in the art to practice said invention. The foregoing is not and should not be considered to be an exhaustive. Many modifications will be suggested to the skilled artisan upon review of the above disclosure, including embodiments which use only two or one prism in combination with a grism, both for dispersing light before the SHG crystal and for reconstructing the converted beam  920 . Accordingly, the invention is only limited by the fair scope of the appended claims. 
     EMBODIMENT 3 
     Illustrated in FIG. 10 is a schematic for an optical parametric generator (“OPG”) when mirror elements are not present, and illustrated in FIG. 11 is an optical parametric oscillator (“OPO”) when mirror elements are present. Those skilled in the art will appreciate that such a device can be constructed by adding suitably reflective members  1101  on either side of the instant invention described in the preferred embodiment (and illustrated in FIG.  5 ). Previous work by others describes the construction and operation of broadband optical parametric generators such that the signal (output) beams from the generator are collinear over a large spectral bandwidth. However, in order to achieve this condition, non-collinear phase-matching among the pump (input), signal (primriary output), and idler (secondary output) beams must be used, which sacrifices efficiency. 
     Modifying the previous design by incorporating APM reduces the non-collinearity of the required phase-matching condition by removing the collinearity condition of the signal beam,  1020  (FIG.  10 ). This not only increases the conversion efficiency but also broadens the available signal bandwidth. The pump beam  1010  and SHG crystal  1005  remain fixed, but the signal and idler beams  1015  and  1020  respectively, are allowed to change direction as a function of their wavelengths. The fixed pump angle is chosen to optimize phase-matching cumulatively over a desired spectral band using APIVI as described in the foregoing. half-wave plates  1006  may be necessary also necessary on both sides of SHG crystal  1005  in order to properly rotate light entering into the SHG crystal into the s polarization orientation depending on the polarization type of phase-matching being used. APM after the nonlinear crystal then realigns the signal beams  1020  so that all are about parallel (collinear) over the entire signal bandwidth. APM may also be used to align the idler beams  1015  in the same manner. By adding APM before the SHG crystal to align a “seed” signal (or idler) beam to the appropriate phase-match angle it is possible to construct an optical parametric amplifier (“OPA”). Unlike current commercially available designs this embodiment would not require rotating the SHG crystal in order to align the signal (or idler) beams. Elements  1001  through  1003 , taken together, schematically represent the APM design practiced in the preferred embodiment; these elements should not be construed as single elements. 
     EMBODIMENT 4 
     Finally, as shown in FIG. 11, by adding mirror resonator elements  1101  to create a resonator cavity  1100  (defined as the optical path between reflective resonator elements  1101 ) and by adding a second tuning element (shown schematically as element  1203  in FIG.  12 ), such as a grating, the instant invention can be made to operate as a narrowband, tunable optical parametric oscillator (“OPO). The APM optics may align either the signal beams  1115 , or the idler beams  1120  for singly resonant behavior or both set of beams may be simultaneously aligned using possible separate optical pathways for doubly resonant operation. 
     Lastly, as shown schematically in FIG. 12, a mode-locking mechanism selected from any known to the art, may be placed into the optical path of the OPO described in FIG. 11, to mode-lock the broadband output of the SHG crystal and allow the OPO to operate as an ultrashort pulse generator. The pump beam  1100  is permitted to be either synchronous or continuous wave. The APM optics also provide temporal dispersion compensation for the resonator cavity, so that the optical pulse length is minimized at the mode-locking mechanism and maximized at the SHG crystal. Using the pulse stretch/compress ratio of the temporal dispersion between the mode-lock and the crystal which are large enough will allow the pulse length at the crystal will approach the round-trip time. This allows a continuous wave pump to be converted efficiently because the pulse duty cycle at the crystal would be high. This device then can operate at wavelengths not directly or easily attainable with existing pulse generating systems. 
     The above-described embodiments are provided as illustrative of the instant invention and are not considered exhaustive. Many modifications will be suggested to the skilled artisan upon review of the above disclosure. Furthermore, the written description and drawings as provided herein are for illustrative purposes and are sufficiently descriptive as to allow one skilled in the art to practice said invention. The foregoing is not and should not be considered to be an exhaustive. Accordingly, the invention is only limited by the fair scope of the appended claims.