Abstract:
A higher intensity eye safe laser is provided for long range target designation or illumination as well as long range eye safe communications by providing a single beam line combination of an optical parametric oscillator and optical parametric amplifier which are used to double the output of the optical parametric oscillator while limiting beam spread to less than 1.2 milliradians assuming a 20 mm clear aperture. The OPO/OPA combination requires no conditioning, isolation or synchronization optics and provides a factor of two improvements in beam quality as compared to an equivalent optical parametric oscillator, with the subject system providing a compact robust configuration. The high conversion is provided by the use of a simple optical parametric oscillator seeding an optical parametric amplifier without double passing the pump pulse in the optical parametric oscillator. Low beam divergence operation is provided with minimum optics in a compact space with the optical parametric oscillator operating at a reduced signal intercavity flux which provides increased damaged threshold margins.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This utility patent application relates to U.S. Provisional Application No. 60/390,952, entitled: Efficient, High Brightness KTP Optical Oscillator—Amplifier In Single Beamline and filed Jun. 24, 2002. 

   FIELD OF INVENTION 
   This invention relates to eye safe lasers and more particularly to the utilization of a combined optical parametric oscillator and an optical parametric amplifier to significantly increase power without increasing beam divergence. 
   BACKGROUND OF THE INVENTION 
   High-energy “eye safe” laser illumination is needed for various ranging and illumination applications most specifically with respect LIDAR and gated viewing systems. Of specific interest is single pulse identification at ranges exceeding 25 km using a 1.5 micron source packaged for airborne operation. The desired combination of high pulse energy, low beam divergence within the system packaging and operating constraints has proved challenging as reported by Larry R. Marshall, Alex Kax and Orhan Aytur, in an article entitled “ Multimode Pumping of Optical Parametric Oscillators ”, IEEE Journal of Quantum Electronics 32 177-182 (1996). It will be appreciated that airborne systems require and in fact mandate compactness, low weight and reliability. 
   In the past laser target designators have used Nd:YAG pumped KTP crystals utilized in an optical parametric oscillator which produces 60 mJ of energy when pumping the optical parametric oscillator with a 1 micron pumping source. 
   The task for military and other purposes requires a doubling of the output energy so as to provide a long-range eye safe laser for use in long range laser ranging and laser illumination applications. In order to provide the required energy on target, between 100 mJ and 200 mJ must be produced from the laser transmitter with a beam quality of less than 25 mm-milliradian. For this application, such performance must be obtained using a designator class laser, 300 mJ, 20 Hz, in a compact configuration, with a transmitting aperture less than 20 mm. 
   One of the first attempts to increase power for such eye safe lasers included simply adding additional KTP crystals in the oscillator cavity, with the suggestion of using three such crystals to boost the 60 mJ prior output power to a 150 mJ level. 
   However, this approach while generating the required output power, resulting in a beam spread of 80 milliradians, clearly an order of magnitude greater than that which is desirable. It is noted that the cross section of such a beam at 25 kilometers puts very little of the projected energy on target. Moreover, for target illumination purposes requiring high resolution, such beam widths are not readily unusable. 
   In an effort to reduce the beam divergence, it was suggested to pump an optical parametric oscillator followed by an optical parametric amplifier by dividing the pumping pulse into two pulses. The first pulse was to be redirected through the optical parametric amplifier, whereas the second pulse is directed through a pulse timing delay unit so as to appropriately pump the follow on optical parametric amplifier. 
   However, providing an optical delay takes up a significant amount of space and requires an increased parts count which can in some cases deleteriously affect the efficiency of the system due to alignment problems. 
   Also synchronizing the pulse time delay is non-trivial problem so that the pumping pulse from the pumping laser arrives at the optical parametric amplifier so that it is not pumped too soon. It would therefore be desirable to have a single beam line system which avoids alignment problems while at the same time eliminating beam redirecting optics and yet still have an intense output beam with a less than 1.2 milliradian beam divergence. 
   As will be appreciated, the above proposed optical parametric oscillator, optical parametric amplifier combination was initially ruled out due to the optical complexity and space needed to split and synchronize the pump beam. 
   SUMMARY OF THE INVENTION 
   Rather than attempting to amplify the output of an eye safe laser by merely inserting more crystals in the oscillator cavity, and rather than providing a system in which an optical parametric oscillator, OPO, was followed by an optical parametric amplifier, OPA, provided with separate pumping pulses, in the subject invention an optical parametric oscillator, optical parametric amplifier combination was placed on a single beam line starting with the pump laser, followed by the optical parametric oscillator and followed in turn by the optical parametric amplifier outside of the oscillator cavity. Surprisingly, in a first test of the subject system the signal output energy was within 5% of the multi-crystal optical parametric oscillator system but had significant improvement in beam divergence. 
   Two factors prompted moving two of the three OPO crystals directly outside the optical cavity of the optical parametric oscillator. The first factor is alignment insensitivity in the non-critically phased matched OPO crystal orientation, and secondly the long operating pump pulse width of 18 nanoseconds. 
   To further reduce signal beam divergence, the optical parametric oscillator was reconfigured to an unstable resonator having a magnification of 1.25 and with a cavity length of 8 cm. With a 1064 nanometer pumping laser, a fused silica input mirror was 75 cm concave providing high transmission at 1064 nanometers and high reflectivity at 1570 nanometers. The out coupler, also fused silica, was a 60 cm convex device, with a 45% reflective at 1570 nanometers, with less than 5% reflective at the pump and idler wavelength. This optical parametric oscillator, combined with the direct pumped dual crystal optical parametric amplifier provided required brightness and operated with slope efficiency approaching 60%. In one embodiment, signal OPO-OPA combination the increased OPO threshold to 110 mJ versus 50 mJ, with both configurations producing approximately equal signal outputs. Interestingly, up to 75 mJ of Amplified Parametric Emission was obtained with OPO cavity mirrors removed. 
   More specifically, in one embodiment a 300 mJ pump laser operating at 1 micron was used to pump the optical parametric oscillator which was configured so as to be a singly resonant oscillator having an output only at 1.5 microns. The output power from optical parametric oscillator was measured to be 75 mJ. Moreover, the pump power left over after pumping the optical parametric oscillator was measured to be 225 mJ which was injected into the optical parametric amplifier. 
   It was interesting to note that there was no optical delay timing necessary due to the inline, single beam line configuration, and synchronization of pumping pulses was completely eliminated with the subject configuration. 
   While the OPO-OPA combination did produce 3.0 micron outputs, these were suppressed through appropriate coating on the mirrors forming the cavity. 
   The result is an eye safe laser having double the power currently provided which permits long range laser ranging up to 25 kilometers as well as utility in laser illuminating systems. With a 20 mm clear aperture and a less than a 1.2 milliradian beam divergence, the subject laser can be utilized for single pulse applications in which objects can be identified exceeding 25 kilometers. Due to the narrowness of the beam angle, resolution of target recognition systems is greatly improved over the long distances. 
   In summary, a higher intensity eye safe laser is provided for long range target designation or illumination as well as long range eye safe communications by providing a single beam line combination of an optical parametric oscillator and optical parametric amplifier which are used to double the output of the optical parametric oscillator while limiting beam spread to less than 1.2 milliradians with a 20 mm aperture. The OPO/OPA combination requires no conditioning, isolation or synchronization optics and provides a factor of two improvements in beam quality as compared to an equivalent optical parametric oscillator, with the subject system providing a compact robust configuration. The high conversion is provided by the use of a simple optical parametric oscillator seeding an optical parametric amplifier without double passing the pump pulse in the optical parametric oscillator. Low beam divergence operation is provided with minimum optics in a compact space with the optical parametric oscillator operating at a reduced signal intercavity flux which provides increased damaged threshold margins. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features of the subject invention will be better understood in connection with the Detailed Description in conjunction with the Drawings, of which: 
       FIG. 1  is a diagrammatic illustration of a tactical situation in which an airborne laser target designator produces a single pulse for illuminating a target at a substantial distance in which the laser is an eye safe laser operating in the 1.5 micron region of the electromagnetic spectrum; 
       FIG. 2  is a block diagram of a prior art laser for use in a laser target designating system in which an optical parametric oscillator utilizes a non-linear crystal such as KTP pumped by a 1 micron pumping laser to produce a 60 mJ output having a beam divergence less than 1.2 milliradians with a 20 mm aperture; 
       FIG. 3  is a block diagram of a proposed multicrystal optical parametric oscillator for increasing the output over the system depicted in  FIG. 2  but resulting in an unacceptable beam spread; 
       FIG. 4  is a block diagram of a combined optical parametric oscillator, optical parametric amplifier system in which pumping pulses are split apart, with one pumping pulse used to pump the optical parametric oscillator and with the other pumping pulse delayed and used to pump the optical parametric amplifier, illustrating the number of optical elements used; 
       FIGS. 5A ,  5 B, and  5 C are graphs showing the generation of pumping pulses and the resultant output from respectively the optical parametric oscillator and optical parametric amplifier of  FIG. 4 , indicating the delay necessary in the pumping pulse to the optical parametric amplifier; 
       FIG. 6  is a block diagram of the subject combined optical parametric oscillator and optical parametric amplifier located in a single beam line in which the optical parametric oscillator is pumped with the output from a pump laser, the output of which is utilized to pump the optical parametric amplifier without the requirement of synchronization, thus to produce a 150 mJ output at 1.5 microns with a less than 1.2 milliradian beam divergence when using a 20 mm aperture, with half of the power from the optical parametric oscillator and with the other half of the power from the optical parametric amplifier; and, 
       FIG. 7  is a pair of graphs showing the pumping pulse for the optical parametric oscillator and the reduced power of the optical parametric amplifier pumping pulse, with the original pumping pulse being reduced by the amount of energy converted by the optical parametric oscillator to produce its 75 mJ output which is coupled to the optical parametric amplifier. 
   

   DETAILED DESCRIPTION 
   Referring now to  FIG. 1 , for eye safe laser target designation or target illumination, in an airborne application an aircraft  10  is provided with a laser target designator  12  which provides a beam  14  directed to a target  16  in which the target is illuminated by a single pulse  18 . 
   It will be appreciated that it is important to provide an eye safe laser for such laser target designation or illumination due to the fact that human beings may be in the area and if the radiation impinges upon the eye of a human being, damage can occur unless the radiation is in the 1.5 micron range. 
   For long distance eye safe laser ranging and illumination, for instance up to and exceeding 25 km, it is only with difficulty that one can provide sufficient power on target to operate at such long ranges. 
   Referring to  FIG. 2 , as has been attempted, a pump laser  20  has been utilized to pump an optical parametric oscillator  22  having an input mirror  24  and an output mirror  26  into which is disposed a non-linear crystal  28  usually of KTP. The result is a output beam  30  having an output power of 60 mJ and a beam divergence of less than 1.2 milliradians with a 20 mm aperture, clearly usable for close in laser target designation and illumination applications, but not enough power for robust long range work. 
   Note that beam divergence depends on aperture size. When one multiplies the aperture size with beam divergence, one gets a beam quality characteristic measured in terms of mm-milliradians. Thus for a 20 mm clear aperture and a 1.2 milliradian beam spread, one has a beam quality of 24 mm-milliradians. 
   In typical military designator/range finder laser transmitters, the KTP optical parametric oscillator is pumped using the 300 mJ to 400 mJ output from the 1.06 μm designator laser. In one embodiment, the pumping laser is a diode-pumped, conduction cooled, Nd:YAG zig-zag slab oscillator-amplifier which produces 300 mJ at 1064 nm with a pulse repetition frequency of 20 hertz, and with each of the pulses being 15 nanoseconds in duration. 
   As mentioned hereinbefore, 60 mJ is insufficient for long range operation. In an effort to increase the laser output, the pumping laser was used to pump a multi-crystal optical parametric oscillator  32  having three KTP crystals  34 ,  36  and  38 , with the result that an output beam  40  did indeed deliver the 150 mJ. However, the output beam had significant divergence or beam spread measured at one point at 80 milliradians for a 20 mm aperture. It will be appreciated that such a widely diverging beam has no ready application for long range laser target designators or laser target illuminators. 
   In an effort to minimize beam spread, referring now to  FIG. 4 , a combined optical parametric oscillator and optical parametric amplifier was designed in which pump laser  20  had its output divided by beam splitting mirror  42  into two pumping pulses P 1  along path  44  and P 2  along path  46 . The first pumping pulse was delivered to optical parametric oscillator  22  identical to that of  FIG. 2 , the output of which on line  46  was passed through a beam splitting mirror  48  to an optical parametric amplifier  50  having two additional crystals  52  and  54  identical to the KTP non-linear crystal  28 . 
   The second pumping pulse, P 2 , was to be passed through an optical pulse timing delay unit  60  which was to be redirected by a mirror  62  to beam splitting mirror  48  where the delayed pumping pulse was injected into optical parametric amplifier  50 . 
   However, the timing, synchronization and duration of these two pulses is critical in the generation of the higher amplitude output. Referring to  FIG. 5A , the pumping pulse  61  is illustrated as having a waveform  63  with a peak at point P 1  and a temporal duration of ΔTP 1 . This pumping pulse is split at mirror  42  into pulses  65  and  67 , respectively, P 2  and P 3 , which only reduces the peak amplitude but does not alter the waveform shape or temporal duration. Note that ΔTP 1 =ΔTP 2 =ΔTP 3 . 
   Referring to  FIG. 5B , pumping pulse P 2  is directed into the OPO, which generates an OPO signal output  69 . The generated OPO signal waveform is of shorter duration than the pumping pulse due to the non-linear nature of the device. Note that a portion of the pump pulse is needed to achieve threshold, ΔT thresh , in the device and once that threshold is exceeded the OPO signal is rapidly extracted. 
   Referring to  FIG. 5C , an optical delay  67  is introduced for P 3  to insure P 3  arrives at the OPA at the beginning  71  of the OPO signal, and increases the peak OPO signal. This addresses the fact that there is a delay between the time that the OPO lasers and when the OPO needs to be pumped when using the dual pulse pumping system. 
   As can be seen from  FIGS. 5A ,  5 B, and  5 C, there are significant synchronization issues introduced due to the fact that the optimal pumping pulse for the optical parametric amplifier must be delayed with respect with respect to the optimal pumping pulse for the optical parametric oscillator. Moreover, for the delayed pulse pumping there are at least five additional optical elements, namely the four mirrors involved in the pump pulse beam splitting and the optical pulse timing delay unit  60 . Not only are the parts count increased, but alignment problems are exacerbated in terms of the mirrors involved. Moreover, the laser target designator/illuminator of  FIG. 4  is not at all compact as there has to be significant space allotted for the separation of the two pumping pulses and the pumping pulse timing delay unit. 
   Referring now to  FIG. 6 , in one embodiment of the subject invention two pumping pulses are avoided. Rather a single pumping pulse is first delivered to the oscillator with enough of the pumping pulse left over from the output of the oscillator to pump the amplifier. Moreover, since the left over pumping pulse comes out of the oscillator with the 1.5 micron signal output, it arrives at exactly the right time to pump the amplifier, just as the amplifier is receiving the output of the oscillator. 
   To illustrate this, in  FIG. 6  a pumping laser  80  has its output  82  directed though an input mirror  84  into a singly resonant oscillator having a crystal  86  interposed between input mirror  84  and output mirror  88 . The combination of input mirror  84  crystal  86  and output mirror  88  constitutes optical parametric oscillator  90  as illustrated. 
   In one embodiment, the pumping laser is a diode laser producing a 300 mJ output at 1.0 microns, with the non-linear crystal  86  being a KTP crystal. When this crystal is pumped with 1.0 micron energy, its response is an output at 1.5 microns as well as 3.3 microns. However, coatings on mirrors  84  and  88  suppress the 3.3 micron output such that upon pumping optical parametric oscillator  90  produces a 75 mJ output at 1.5 microns coupled into an optical parametric amplifier  92  composed of non-linear crystals  94  and  96  identical in one embodiment to the crystal in optical parametric oscillator. 
   Thus it will be seen that the optical parametric amplifier is seeded with 75 mJ at 1.5 microns as well as a pumping pulse of 225 mJ of 1.0 micron energy. 
   It has been found that the output from the optical parametric amplifier is 150 mJ at 1.5 microns with a beam divergence less than 1.2 milliradians assuming a 20 mm aperture. 
   What will be appreciated is that there need be no specialized synchronization for the pumping pulses for the optical parametric oscillator and the optical parametric amplifier. The delay of the 225 mJ pumping pulse caused by the pumping of the optical parametric oscillator is exactly such as to provide the appropriate timing for the pumping pulse for the optical parametric amplifier. The result also is a 15 nanosecond pulse  98  useful for long range eye safe single pulse target illumination and designation. 
   Referring to  FIG. 7 , it will be appreciated that the pumping pulse for the optical parametric oscillator is at  300  mJ and 1.0 microns as illustrated by waveform  100 , whereas the pumping pulse  102  for the optical parametric amplifier is at 225 mJ at 1.0 microns, with this waveform being slightly flattened due to the fact of its having passed through the optical parametric oscillator. 
   As will be seen, one has therefore taken the Nd:YAG pumped KTP optical parametric oscillator of the prior art and amplified its output from 60 mJ to 150 mJ at greater than 35% efficiency with less than 1.2 milliradians beam divergence using in one embodiment the existing telescope magnification of 3.8 and a clear aperture 20 millimeter combination. In this embodiment, the prior diode pump Nd:YAG laser was configured as an oscillator amplifier using identical side diode pumped conduction cooled zig-zag slabs. The details of such a pump laser have been described in “ Multifunction Laser Radar ” J. A. Hutchinson, C. W. Trussell, S. J. Hamlin, T. H. Allik, J. C. McCarthy and M. S. Bowers, Laser Radar Technology and Applications IV, Gary W. Kamerman, Christian Werner, Editors, Proceedings of SPIE, Vol. 3707, pumping pulse, 222-233 (1999). Note that in one embodiment, the pump laser pulse width is 18 nanoseconds in one embodiment operating at 9 milliradians for repetition rates between 10 and 20 Hz. 
   Note that in the subject application the pumping laser has a simple 1064 nanometers pumped non-critically phased matched KTP OPO-OPA architecture, with the optical parametric oscillator having an unstable resonator of magnification 1.2 as described in “ Improved OPO Brightness with a GRM Non - confocal Unstable Resonator ”, S. Chandra, T. H. Allik, J. A. Hutchinson, and M. S. Bowers, “ OSA Trends in Optics and Photonics on Advanced Solid State Lasers ”, Stephen A. Payne and Clifford R. Pollock (Optical Society of America, Washington, D. C., 1996), Vol. 1, 177-178, with both generated signal and residual pump directly pumping dual 20 millimeter length KTP OPA&#39;s. The pump beam spot size input to the optical parametric oscillator was approximate; 4×4 millimeters providing a 110 MW/cm 2  of drive. 
   Ideally the optical parametric oscillator threshold is set to provide approximately half the total OPO-OPA output. A low optical parametric oscillator threshold will couple too efficiently to high order oscillator modes and degrade the beam quality, while a high optical parametric oscillator threshold will force the amplified parametric florescence to dominate, also with poor beam quality. 
   It is noted that while the subject invention has been described in connection with eye safe lasers, the same techniques can be used at other wavelengths. The subject technique is thus usable at other wavelengths with the criteria that the non-linear converter is preferably non-critically phase matched, meaning that is has reduced alignment sensitivity. Whether or not non-critically phase matched, in one embodiment the subject system is usable in the mid infrared using an OPO-OPA combination involving zinc germanium phosphide crystals, with the laser being angle tunable between 3-5 microns and using a 2 micron pump laser. 
   Having now described a few embodiments of the invention, and some modifications and variations thereto, it should be apparent to those skilled in the art that the foregoing is merely illustrative and not limiting, having been presented by the way of example only. Numerous modifications and other embodiments are within the scope of one of ordinary skill in the art and are contemplated as falling within the scope of the invention as limited only by the appended claims and equivalents thereto.