Abstract:
A pulsed coherent fiber array laser system that includes a beam generating sub-system that provides a signal pulse beam having pulses of the desired duration that is split into several fiber channels. Optical leakage between the pulses in each split beam is measured and locked to a reference beam by a phase sensing circuit and phase adjusters so that the phase of each fiber pulsed beam is aligned with the phase of the reference beam. A pulse clipper or filter is employed to remove the pulses in the fiber beams so that they do not saturate the phase sensing circuit. The beam generating sub-system can employ any suitable combination of devices to generate the signal beam and the reference beam, including continuous wave master oscillators, amplitude modulators, frequency shifters, injection seed oscillators, Q-switched lasers, reference oscillators, frequency lockers, wavelength division multiplexers, time gated switches, etc.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates generally to a pulsed fiber array laser and, more particularly, to a pulsed fiber array laser that compares the phase of optical leakage between beam pulses in each fiber to a reference beam so that the phase of each fiber beam pulse can be phase-locked to the reference beam to provide a coherent laser array output beam without having to measure the phase of the pulses themselves. 
         [0003]    2. Discussion of the Related Art 
         [0004]    Coherent fiber array laser systems are known in the art for generating a good beam-quality laser beam from an array of fiber lasers. In recent years, there has been a rapid development of array laser architecture for high power laser weapon systems to destroy or “kill” distant targets, such as ballistic missiles, cruise missiles, bombers or the like. Known coherent array laser systems developed thus far have sometimes been tailored for these applications, where the laser runs in a continuous-wave (CW) mode or quasi-CW mode. Short pulsed (&lt;10 nsec) fiber array lasers have been less developed in the art. Applications for such a pulsed array laser system include target range finding, target speed detection, remote chemical sensing, and remote target illumination. In particular, the scaling of pulsed fiber sources to pulse energies in the tens of milli-Joule (mJ) levels is highly desirable, the more so if the high efficiency, waveform characteristics and beam quality of the fiber source can be maintained. 
         [0005]    It appears that coherent beam combining is the most promising method for scaling pulse energy beyond that achievable from a single fiber, which is currently limited to a few mJ. Depending on the intensity of the beam desired for a particular application, the fiber array may include 10-100 fibers. Perceived advantages of a pulsed coherent fiber array laser system include a factor of two to three in the improvement of overall system efficiency, significant packaging benefits and waveform flexibility. 
         [0006]    Known fiber array laser systems employ some type of phase control that aligns the phase of each of the beams in the individual fibers to provide the coherent beam. Typically, the phase of each beam in each fiber is adjusted in order to phase-lock each beam to a common reference beam. Known coherent fiber array lasers are generally CW lasers where each of the individual fiber beams is on for a period of time that is long enough to measure the phase of the fiber beams, and to adjust the phase of each beam to phase-lock to the reference beam. 
         [0007]    There is an interest in the art to develop a pulsed coherent fiber array laser system for various applications, where each pulse in the beam may be only on for less than 10 nano-second, less than the required time needed for the known coherent beam combining phase-locking techniques. A pulsed coherent fiber array laser needs to control the phase of the beam pulses in each fiber in a very short period of time in order to provide the coherent beam. Currently, no pulsed coherent fiber array laser system is able to provide phase control of the individual beams fast enough. 
       SUMMARY OF THE INVENTION 
       [0008]    In accordance with the teachings of the present invention, a pulsed coherent fiber array laser system is disclosed. The laser system includes a beam generating sub-system that provides a signal pulse beam having pulses of the desired duration that is split into several fiber channels. Each split channel beam then passes through a phase adjuster and a chain of amplifiers. The amplified beams are recombined and a small portion of the output beam is sampled for phase measurement relative to a reference beam. Optical leakage between the pulses in each split beam is used to measure the phase and to phase-lock each beam to a reference beam by a phase sensing circuit and phase adjusters. A pulse clipper or filter is employed to remove the pulses in the fiber beams so that they do not saturate the phase sensing circuit. The beam generating sub-system can employ any suitable combination of devices to generate the pulsed signal beam and the reference beam, including continuous wave laser master oscillators, amplitude modulators, frequency shifters, injection seeded laser oscillators, Q-switched lasers, reference laser oscillators, frequency lockers, wavelength division multiplexers, time gated switches, etc. 
         [0009]    Additional features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic block diagram of a pulsed laser array system that employs a continuous wave master oscillator and an amplitude modulator, according to an embodiment of the present invention; 
           [0011]      FIG. 2  is a detailed schematic diagram of the pulsed laser array system shown in  FIG. 1 ; 
           [0012]      FIG. 3  is a schematic block diagram of a pulsed laser array system that employs a Q-switched laser, a seed oscillator that also provides a reference beam and a cavity locker, according to another embodiment of the present invention; 
           [0013]      FIG. 4  is a schematic block diagram of a pulsed laser array system that employs frequency locking of a master oscillator to a reference oscillator, according to another embodiment of the present invention; 
           [0014]      FIG. 5  is a schematic block diagram of a pulsed laser array system that employs a reference oscillator and wavelength division multiplexer, according to another embodiment of the present invention; and 
           [0015]      FIG. 6  is a schematic block diagram of a pulsed laser array system that employs a reference oscillator and a time gated switch, according to another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0016]    The following discussion of the embodiments of the invention directed to a pulsed coherent fiber array laser system is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. 
         [0017]    The present invention proposes a technique for locking the phase of the pulses produced by a pulsed laser array system to the phase of a reference beam. The technique includes generating optical leakage between pulses that is phase correlated with the pulses, measuring the phase of the optical leakage in each fiber signal beam, and locking the phase of the leakage to the reference beam so that the pulses themselves will be in phase with each other. Various techniques are known in the art for phase locking a signal beam to a reference beam. According to the invention, the phase of each beam pulse must be detected prior to arrival of the pulse. For pulse repetition rates less than 20 kHz, as determined by the typical phase noise spectrum in the fiber amplifiers, there must be leakage between the pulses to provide the phase determination. 
         [0018]      FIG. 1  is a schematic block diagram of a pulsed coherent fiber laser array system  10 , according to an embodiment of the present invention. The system  10  includes a continuous wave (CW) master oscillator  12 , whose output is split into a signal beam  14  and a reference beam  16 . The signal beam  14  is amplitude modulated by an amplitude modulator  18  to generate signal beam pulses. The amplitude modulator  18  has a finite contrast ratio so that a small amount of signal beam is leaked through between pulses. Since the optical leakage and the signal beam pulses are derived from the same signal beam, the phase of the optical leakage is highly correlated, in fact phase-locked, with the signal beam pulses. The pulses will typically be on the order of 1-10 nano-seconds in duration with an appropriate time between the pulses for the purposes described below. The pulsed signal beam from the amplitude modulator  18  is split into a plurality of fiber beam channels, where a separate channel is provided for each fiber in the fiber array. As will be discussed in detail below, the optical leakage between the beam pulses in each fiber channel is locked to a reference beam by applying appropriate feedback to a phase adjuster  20  in an array  22  of phase adjusters. The coherent signal beam pulses in each fiber channel are then amplified by a chain of amplifiers  24  in an array of amplifier chains  26 . The amplified signal beam pulses from each channel are then emitted and combined as a single coherent main beam  28  from the system  10 . 
         [0019]    The reference beam  16  is frequency shifted by a frequency shifter  30 , and is combined with a portion of the main beam  28  by a coupler  32 . The frequency shifter  30  shifts the frequency of the reference beam  16  for heterodyne signal generation. The coupled reference beam and main beam are then sent to a hederodyne array circuit  34  that converts the optical signal to an electrical signal. Any suitable detector array for this purpose can be used as the heterodyne array circuit  34 , as will be appreciated by those skilled in the art. 
         [0020]    As discussed above, the system  10  measures the phase difference between the frequency shifted reference beam  16  and the optical leakage between the pulses. However, the pulses also propagate through the heterodyne array circuit  34 , and act to saturate the system electronics. Therefore, the pulses that are not used to measure the phase difference between the reference beam and the optical leakage are clipped by a pulse clipper circuit  36  to remove the pulses so that the only thing left in the signal is the optical leakage between the pulses and the reference beam. In one embodiment, the optical leakage between the pulses is about 45 dB down from the pulses. As is well known in the art of heterodyne detection, the optical leakage can be even smaller by having a more intense reference beam. 
         [0021]    The combined reference beam and optical leakage is then sent to a phase sensor circuit  38  that measures the phase difference between the optical leakage and the reference beam for each fiber beam. The phase sensor circuit  38  provides a phase adjusting signal indicative of the phase difference between the optical leakage between each pulse in each pulsed fiber beam to each phase adjuster  20  to adjust the phase of the leakage between the pulses so that it is in phase with the reference beam. Therefore, the main beam  28  will be coherent in that all of the fiber beams will be at the same phase. The phase sensor circuit  38  can be any phase sensor circuit suitable for the purposes described herein, such as those used for continuous wave laser array systems. Suitable non-limiting examples can be found in U.S. Pat. No. 6,167,024, titled Multiple Channel Control Using Orthogonally Modulated Coded Drive Signals, issued Dec. 26, 2000 to Upton et al.; U.S. Pat. No. 6,366,356, titled High Average Power Fiber Laser System with High-Speed, Parallel Wavefront Sensor, issued Apr. 2, 2004 to Brosnan et al.; and U.S. Pat. No. 6,813,069, titled Method and Apparatus for Controlling a Fiber Optic Phased Array Utilizing Frequency Shifting, issued Nov. 2, 2004 to Rice et al. 
         [0022]      FIG. 2  is a schematic diagram of a pulsed coherent fiber laser array system  40 , according to an embodiment of the present invention. The system  40  is a detailed variation of the system  10 . The system  40  includes a master oscillator  42  that can be any single frequency continuous wave laser suitable for the purposes described herein. The master oscillator  42  generates a continuous laser beam that is separated into two fibers  44  and  46  by a coupler  48  to provide a signal beam and a reference beam. The signal beam propagating on the fiber  44  is sent to an amplitude modulator  50  that amplitude modulates the beam to create a pulsed signal beam having pulses of a suitable duration. A small fraction of the signal beam is leaked through the modulator  50  between the pulsed signal. The pulsed signal beams are amplified by an amplifier  52  and split into a plurality of fiber channels by a beam splitter  54 . The number of fiber channels would depend on the application and typically would be between 10 and 100. The beam splitter  54  also includes phase modulators  56  that control the phase of the beams in each fiber channel so that all of the beams are in phase, as will be discussed in more detail below. The beam splitter  54  can be any commercially available device suitable for the purposes discussed herein, many of which are known by those skilled in the art. 
         [0023]    Each pulsed signal fiber beam in each channel is then sent to a path-length adjuster  60  that provides path-length matching for the fiber channels. Particularly, in order to couple the phase all of the fiber beams together, all of the path lengths of the fiber channels need to be the same. The path-length adjusters  60  are set at manufacture so that the manufacturing tolerances are overcome to provide the path-length matching in each fiber channel. Each fiber beam in each channel is then amplified by a pre-amplifier  62  and a power amplifier  64 . An optical isolator  66  is provided between the pre-amplifiers  62  and the power amplifiers  64  to provide power isolation for the high intensity pulsed fiber beams provided by the power amplifiers  64 . The pulsed fiber beams are amplified by the amplifiers  64  using a diode pump source  70 . The diode pump source  70  provides pump light that is coupled into each of the fiber channels by pump couplers  72 . The pump light travels in an opposite direction to the fiber beams through the amplifiers  64 . The amplified pulsed fiber beams are then sent to a collimator array  78  that includes a plurality of tightly bunched lenses  80  that collimate the pulsed fiber beams into a collimated output beam  84 . The collimated output beam  84  is emitted from the collimator array  78  as the output beam of the system  40 . 
         [0024]    The reference beam on the fiber  44  is sent through a frequency shifter  86  that shifts the frequency of the reference beam, an amplifier  88  that amplifies the frequency shifted reference beam and a collimator  90  that collimates the frequency shifted and amplified reference beam. The output beam  84  is split by a beam splitter  92  so that a small portion of the output beam  84  is split off and combined with the reference beam. The combined beam is focused by a lens  94 , and then re-collimated by a lens  96 . The combined beam is then received by a detector  98 , representing the heterodyne array circuit  34 , that converts the combined reference beam and output beam to electrical signals. The electrical signals are then sent to a pulse clipper circuit  100  that clips the pulses, as discussed above, to remove them from the reference beam and the optical leakage in the output beam  84 . The electrical signals are then sent to a phase sensor circuit  102  that measures the difference of the phase between the reference beam and the optical leakage in the fiber beams, and provides an electrical signal to each of the phase modulators  56  so that the phase of each fiber beam is controlled to be in phase with the reference beam. 
         [0025]    Different techniques are known in the art for generating and frequency locking a pulsed signal beam and a reference beam.  FIG. 3  is a schematic block diagram of a pulsed array laser system  110  similar to the pulsed array laser system  10 , where like elements are identified by the same reference numeral. In the system  110 , the signal pulse beam is generated directly by a Q-switched laser  112  without the need for an amplitude modulator. A separate seed continuous wave laser oscillator  114  provides the reference beam and an injection seed beam for the Q-switched laser  112 . The operation of a Q-switched laser, an injection seed oscillator and a cavity locker in this combination are well known to those skilled in the art. The cavity locker  116  locks the frequency and phase of the pulsed signal beam from the Q-switched laser  112  to the frequency and phase of the injection seed beam. The seed beam between the Q-switched pulses in the signal beam from the Q-switched laser  112  serves as the optical leakage that is used for phase determination. The Q-switched laser output beam is split into a plurality of pulsed fiber beams. 
         [0026]      FIG. 4  is a schematic block diagram of a pulsed array laser system  120  similar to the laser array system  10 , where like elements are identified by the same reference numeral. In this design, a continuous wave reference oscillator  122  is used to provide the reference beam. Because the signal beam and the reference beam are generated by different devices, the signal beam and the reference beam need to be frequency and phase locked to provide the phase probing. The frequency of the signal beam from the master oscillator  12  is locked relative to the frequency of the reference beam from the reference oscillator  122  by a frequency locking circuit. A portion of the signal beam from the master oscillator  12  is coupled off by a coupler  124  and mixed with the reference beam from the reference oscillator  122  in a mixer  126  that subtracts and adds the frequency of the beams. The mixed reference beam and pulsed signal beam are frequency locked by a frequency locker circuit  128 . If the beams from the master oscillator  12  and the reference oscillator  22  are at the same frequency, then the frequency shifter  30  is necessary to generate the heterodyne signal. However, if the frequency of the reference beam from the reference oscillator  122  is locked to a frequency that is different from the frequency of the signal beam, then the frequency shifter  30  is not required for heterodyne generation. 
         [0027]      FIG. 5  is a schematic block diagram of a pulsed laser array system  130  similar to the laser systems discussed above, where like elements are identified by the same reference numeral. The pulsed signal beam is provided by a Q-switched laser or amplitude modulated master oscillator  132 . In this embodiment, the wavelength of the reference beam is different than the wavelength of the signal beam. A wavelength division multiplexer (WDM)  134  combines the reference beam from the reference oscillator  122  and the pulsed signal beam from the Q-switched laser or amplitude modulated master oscillator  132  to probe the phase change through the amplifier chain  26  between the signal beam and the reference beam. An assumption is made that the phase change of the signal beam and the reference beam are well correlated through the amplifier chain  20 ,which can be achieved, for example, the making the signal beam and the reference beam close in wavelength. The reference beam from the reference oscillator  122  is frequency shifted by the frequency shifter  30  to generate the heterodyne signal. The high peak power pulses can be optically filtered by an optical band-pass filter  136  instead of being electronically clipped by the pulse clipper circuit  36 . The system  130  offers a number of advantages including the wavelength that the reference beam can be provided so it does not extract energy form the amplifier chain  20 , the reference beam does not interfere with the signal pulse and active sensing and ladar applications, and the pulse clipper circuit is not necessary. 
         [0028]      FIG. 6  is a schematic block diagram of a pulse laser array system  140  similar to the system  130 , where like elements are identified by the same reference numeral. In the system  140 , the WDM  136  is replaced with a time-gated switch  142 . The reference oscillator  122  can be a continuous wave oscillator or a pulsed oscillator. The reference beam is combined with the signal beam at the switch  142  prior to the arrival of the signal pulse. The phase is probed by the reference beam through the amplifier chain  20 . As long as the gated reference pulse is longer than the response time of the electronics, the phase locking electronics is the same, as discussed above. The system  140  offers a number of advantages including that the reference pulses can be made small and timed so as to not interfere with the signal pulse in active sensing in ladar applications. 
         [0029]    The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.