Abstract:
A pulsed fiber laser including fiber preamplifier and power amplifier stages is disclosed. A fiber preamplifier includes first and second preamplifier stages that receive and amplify a seed pulse. A filter isolator placed between the preamplifier stages suppresses noise from the first preamplifier stage. An acoustic optical modulator located in the second preamplifier stage eliminates unwanted wavelengths from the amplified seed pulse received from the first preamplifier stage. The pulsed fiber laser is rugged and lightweight.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a Continuation of U.S. application Ser. No. 12/815,057, filed Jun. 14, 2010 (now U.S. Pat. No. 8,270,441), which is a Continuation of U.S. application Ser. No. 10/581,416, filed Jun. 2, 2006 (now U.S. Pat. No. 7,738,514), which is the National Phase Entry of International Application No. PCT/US2004/040572 filed on Dec. 6, 2004, which claims benefit of U.S. Provisional Application No. 60/526,613, filed Dec. 4, 2003. All of these applications are incorporated by reference in their entireties. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention relates to the art of high-power fiber lasers. In particular, the invention relates to the art of high-power, fully integrated fiber laser systems. 
       BACKGROUND ART 
       [0003]    Optical fiber amplifiers that receive coherent light of relatively low power from a seed laser and amplify that light with fiber laser amplifiers are known. When the systems are to be used for such applications as target marking, target ranging, imaging, and tracking, and LIDAR, among others, a primary objective has been to obtain a high-power, single mode output, or output with relatively low multimode content. This is particular difficult because of the necessity of controlling amplified spontaneous emission (ASE), controlling the excitation of unwanted modes, and reducing the effects of non-linearity. One technique that has been proposed includes that of cascaded, pulse-pumped amplifiers where pumping is synchronized with the pulse to be amplified. Such a system is shown in U.S. Pat. No. 5,933,271. This is however limited to relatively low pulse peak powers when compared to conventional solid state lasers that are capable of peak powers in the hundreds of kilowatts to megawatts. 
       SUMMARY 
       [0004]    Increase in the power of a near diffraction-limited CW beam generated from doped (Yb, Er, Yb:Er, Nd etc.) fiber lasers constitutes an important advancement, because this fiber technology is uniquely efficient and providing fully integrated fiber laser systems. Achieving high pulse energies with pulsed fiber lasers is a much more formidable problem, and the successful solution as described herein leads to a number of practically important applications. Difficulty in scaling pulse energies arises from the limited size of the fiber core and the relatively long pulse propagation length necessary to achieve high gain. Peak powers within fiber-based amplifier systems are further limited by non-linear phenomena within the fiber. Increasing the size of the core appears to be one of the main directions of the technological advancement towards higher energies. This scaling, however, can result in a highly multimode core and, consequently, to significant degradation of the beam quality. 
         [0005]    The present invention relates to the generation of greater than 50-mJ, 10-ns pulses, with a total peak power of 5 MWatts. Another aspect of the invention is that the mode quality of a highly multimode, large-core fiber can be significantly improved by using the mode-filtering effect of a coiled, low-NA core. The invention uses a coiled fiber of about 115 μm diameter and low numerical aperture core, which supports a large number of transverse modes, to produce low divergence output beam with M 2  between 6 and 8 and preferably 6.5, thus effectively reducing number of modes at the output of the fiber to a small number of modes. The numerical aperture of the fiber is preferably between 0.06 and 0.08 and is more preferably about 0.07. The diameter of the coil is preferably about nine to eleven inches and more preferably about 10 inches. Effective numerical apertures of 0.04 for the beam can be achieved with such fiber amplifiers. 
         [0006]    The preferred arrangement comprises an all-fiber, cascaded four amplifier system seeded with an electric-pulse-driven, single-longitudinal-mode diode laser emitting at 1064 nm. This arrangement allows for a very high power, pulsed, laser source tunable from 1030 nm to 1085 nm. Such seeding enables control of both the shape of the seed pulse and its repetition rate, which is selectable by the electric-pulse generator in the range from a single shot to 1 MHz. (It may not be possible to use pulse pumping at seed pulse frequencies approaching 1 MHz. At the higher frequencies, the pumping is preferably continuous.) Seed pulses as low as 10-30 nJ are amplified in a single-mode, core-pumped Yb-doped fiber pre-amplifier, having standard optical components and pumped with telecom-grade 980-nm single-mode diodes. For pulse repetition rates in the range from 10 Hz to 100 Hz, up to 500 nJ has been obtained in the preamplifier stage. These pulses are then launched into a cladding-pumped 10-μm diameter core Yb-doped fiber amplifier with a 125 1-1 m cladding to produce up to 50 μJ per pulse. Isolation from ASE is achieved by the use of optical isolators, electro-optical time gates, and narrow bandpass filters at 1064 nm to suppress 1039-nm peak ASE emission between the stages. ASE is also limited by the use of pulse pumping that is timed with the pulses to be amplified. Previous systems have relied on the ability to use large average powers of the seed signal to overcome issues of spontaneous emissions with the amplifiers. This, however, has limited the ability to develop fiber laser amplifiers at low pulse repetition frequencies. 
         [0007]    The output from the second preamplifier stage is then divided into a plurality of channels. As many as seven channels have been demonstrated. This preferably is accomplished by directing the output from the preamplifier into a series of splitters. Each of the outputs from the splitters is directed to a mode field adaptor that couples the light pulse to the first stage of a clad pumped fiber laser power amplifier. The first stage of the power amplifier preferably comprises a coiled gain fiber having a 30 μm core and a 250 μm cladding. 
         [0008]    The fiber amplifiers of the first stage of the power amplifier are pulse pumped. The pulsed pumping light is directed into the amplifier cladding by the use of a tapered fiber bundle. Tapered fiber bundles are known, and those used in the preferred embodiment of the invention are manufactured to minimize loss and suppression of unwanted modes. Tapered fiber bundles can be effectively used as a means of stripping off unwanted higher order modes generated within the gain medium. 
         [0009]    The output from each of the first stage power amplifier fibers is directed to a fiber laser amplifier of the second stage of the power amplifier. The fiber laser amplifiers of the second stage of the power amplifier are also pulse pumped by directing the pumping light into a tapered fiber bundle, which couples the pump light into the cladding of the fiber of the second stage of the power amplifier. 
         [0010]    The second power amplifier stage utilizes a clad gain fiber having a 115 μm core diameter and a 350 μm cladding. The final power amplifier stage is based on a large core, double-clad 3-5 m long Yb-doped fiber with 115 μm diameter, low numerical aperture core as defined above, and 600-μm diameter, 0.46 NA inner pump cladding. The amplifier was end-pumped with 915-nm diode laser. 
         [0011]    Amplified signals generated within the various channels of the system can be re-combined with one another to further enhance the peak power of the amplifier. This is achieved by controlling the signal and pump pulse timings within the various parallel legs of the system. It will be appreciated that the invention provides nanosecond pulse energies in the tens of millijoules range with very large core fibers. Large core dimensions ensure significant extractable pulse energies as well as increased susceptibility to detrimental nonlinear and bulk damage effects. Mode quality can be significantly improved by using coiled, low NA fibers to ensure loss for higher order transversal modes. 
         [0012]    It is an object of this invention to provide a high power fiber laser amplifier. 
         [0013]    It is a further object of this invention to provide a fully integrated high power fiber laser amplifier. 
         [0014]    It is yet another object of this invention to provide a high power fiber laser system having one or more fiber amplifier stages using coiled low numerical aperture clad fiber amplifiers and tapered fiber bundles to provide pump energy to the amplifier cladding. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a block diagram of the overall design of the preferred embodiment. 
           [0016]      FIGS. 2-5  are schematic diagrams of a four stage integrated laser fiber amplifier in accordance with the invention. 
           [0017]      FIG. 6  is a graph illustrating pulse timing in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0018]    In the preferred embodiment, the invention is an all-fiber, integrated laser system that is capable of producing very high peak power. The system is rugged and lightweight, which means that it is particularly useful for use in portable instruments used in severe environments, such as military high vibration and shock applications. One such use is that of laser targeting. Other potential applications include aircraft systems, space based systems, as well as commercial platforms (material processing, welding, laser surgery) where precise control over pulse widths, pulse shapes, pulse repetition frequencies, peak powers, and high electrical to optical conversion efficiencies can provide the user with immense advantages. Typical solid state laser systems lack such abilities of wavelength tenability, pulse control, as well as precision pointing which are possible with a fiber amplifier demonstrating comparable peak powers and mode content. This design for fiber amplifiers is not limited to this wavelength of 1064 nm, and holds true for fiber amplifier systems ranging from the near ultraviolet to the infrared. 
         [0019]      FIG. 1  is a block diagram showing the overall design of an all-fiber, integrated laser system in accordance with the invention. The system includes a pulsed laser  4  that providing a seed signal to a cascaded set of fiber laser amplifiers, illustrated at I, II, III, and IV. The laser amplifiers are preferably pulse pumped, and the timing of the pumping is controlled by control circuit  24 . The individual amplifier stages will be described in more detail in connection with  FIGS. 2 through 5 . 
         [0020]      FIG. 2  is a schematic diagram of the first stage of a preamplifier in accordance with a preferred embodiment of the invention. The embodiment shown in  FIG. 2  comprises a first stage preamplifier that is generally of MOPA configuration and uses a coiled, single mode fiber amplifier  2  to amplifier the seed pulse from a pulsed laser diode  4 . The seed laser is a known diode laser capable of operating at a wavelength of 1064 nm. The output pulse from the seed laser  4  is fiber coupled and directed to an optical isolator  6 , such as a polarization dependent isolator known in the art. Light from the isolator is coupled to the fiber amplifier by a wavefront division multiplexer (WDM)  8 . The WDM  8  also couples pump light from a pump laser  10  into the fiber amplifier  2  in a first direction. A second WDM  12  directs light from a second pump laser  10  into the fiber amplifier  2  from the opposite direction. The WDM&#39;s also prevent backward traveling ASE from the amplifier to the 980 nm pumps and avoid terminal damage. The pump laser preferably operates at 980 nm and 200 mW and is a single mode solid state laser controlled by a timing circuit  24 , as will be described in more detail below. 
         [0021]    Each of the components to be described herein is optically connected to one or more other components by coupler fibers to provide the fully integrated laser fiber system. The coupler fibers are shown in the drawings by solid or broken lines as is conventional, and splices between individual fibers are indicated by squares. 
         [0022]    To provide measurement of the power in the system, a tap  14  is used to direct a small amount of the seed laser energy to a photodetector  16 . Additional taps may also be provided as will be described. 
         [0023]    Fiber amplifier  2  is preferably 61 . . . 1 m in core diameter, Yb doped single mode fiber of 15-20 meters in length. 
         [0024]    The amplified light pulse is directed to the second stage of the system ( FIG. 3 ) through a filter isolator  18  containing a narrow band filter to suppress ASE noise to transmit into the amplifier. 
         [0025]      FIG. 3  shows the second stage of a preamplifier in accordance with the invention. The amplified light obtained from the first stage of the preamplifier shown in  FIG. 2  is directed to the input of an acoustic optical modulator (AOM)  20 , which acts as a time gated filter to eliminate unwanted wavelengths. AOM  20  is preferably tuned to the pulse frequency of the seed laser. The AOM  20  is operated by a RF driver  22 , which is in turn controlled by control circuit  24 . The control circuit controls the operation of the several elements by providing control signals to the seed laser, the pump lasers, and other components in the system. A tap  26  and photodetector  28  may be provided in this stage also. 
         [0026]    The second stage of the preamplifier comprises a coiled, clad-pumped fiber amplifier  30 . This fiber amplifier is preferably of 10 μm core diameter and 125 μm cladding. Light from the first preamplifier stage is transmitted from the AOM filter to second stage of the preamplifier by a mode field adaptor (MFA)  32 , which matches the modes passed through the AOM to the fiber amplifier  30  for further amplification. 
         [0027]    The fiber amplifier  30  is clad pumped by directing pulsed pump light from a pump laser  34 , to the cladding of the amplifier  30  through a 2×2 coupler  36 . The pump light is transmitted through a short wave pass filter to prevent the forward traveling ASE and signal from damaging the pump laser. In the preferred embodiment, the pump is a 915 nm, 5 W multimode fiber coupled pump source. 
         [0028]    Amplified light from the fiber amplifier  30  is directed to the power amplifier stages through a filter/isolator  40  comprising a 5 nm narrow band 1064 nm filter. 
         [0029]    Referring now to  FIG. 4 , the light signal from the second stage of the preamplifier as illustrated in  FIG. 3  is directed to the first stage of a power amplifier. The light is first directed to a number of splitters for dividing the light into a plurality of channels. In the embodiments shown, the light from the preamplifier is divided among seven channels. The first splitter  42  is a 2×2 splitter that divides the incoming light into two parts of approximately equal power. The remaining splitters  44  are preferably 2×1 splitters that divide the light into seven beams of approximately equal power and an eighth beam of about 1% for power monitoring by photodetector  46 . By this arrangement, the light from the preamplifier is divided into several parallel channels for simultaneous amplification while maintaining the desired qualities of the beam, namely low mode and high power. It will be understood that more or fewer than seven channels may be used. 
         [0030]    Each output from a splitter  44  is directed to a first-stage fiber laser power amplifier  48  through a mode field adaptor  50 . In this stage, the laser power amplifier is preferably clad pumped amplifier having a 30 μm core diameter and a 250 μm cladding diameter. The fiber is coiled to suppress unwanted modes. 
         [0031]    It will be appreciated that a feature of the invention is that the core diameter of the fiber amplifier increases in each subsequent stage. Thus, the core diameter in the preamplifier stage 2 is 10 μm, the core diameter in the first power amplifier stage is 30 μm, and the diameter in the second power amplifier stage is 115 μm. Mode field adaptors  50  are provided to match the 10 μm fibers from splitters  44  to the 30 μm core of the amplifiers  48  to provide mode control. 
         [0032]    Each of the fiber amplifiers is pulse pumped by pumping laser  52 , which is a diode laser preferably operating at 915 nm and total power of 200 watts with each fiber having 50 watts. The several fiber laser amplifiers  48  are provided with light from the pump laser by dividing the light from the pump laser among several fibers  56  by splitters  54 . Pump light from laser is directed into the cladding of the fiber amplifiers  48  through tapered fiber bundles (TFB)  58 . 
         [0033]    Amplified light output from the fiber amplifiers  48  is directed to the final stage of amplification through filter/isolators  60 . 
         [0034]      FIG. 5  illustrates stage 2 of the power amplifier, which is the final stage of amplification in the preferred embodiment. Light from the several channels shown in  FIG. 4  is coupled to a like number of laser fiber power amplifiers  64  in stage 2 by mode field adaptors  62 . The laser fiber amplifiers  64  preferably comprise 115 μm core, 350 μm cladding fibers. The mode field adaptors  62  match the 30 nm diameters of the fibers connecting the first and second power amplifier stages to the 115 μm diameters of the fiber amplifiers  64 . 
         [0035]    Pumping light from diode lasers  66  is provided to the second stage power amplifiers  64  through tapered fiber bundles  68 . Diode lasers  66  preferably operate at 915 nm and 200 W and produce a plurality of output channels that are directed to the TFB  68 . 
         [0036]    The output beams from the power amplifiers  64  are directed along output fibers  70  to a beam combiner  72 , which represents the output of the system. 
         [0037]      FIG. 6  illustrates the preferred timing for the system described above. Channel A shows the pulse provided by control circuit  24  to the seed diode driver  74  that controls the seed diode  4 . Channel B represents the signal provided to the diode driver  76  that controls the stage one preamplifier pump diodes. Channel C illustrates the signal provided to the AOM  20  in  FIG. 3 . Channel D illustrates the signal provided to the preamplifier stage 2 diode driver  78  for the pump laser  34 . Channel E represents the signal pulses provided to the diode driver  80  for the pump laser  52 . Channel F represents the signal pulses provided to the diode driver  82  for the pump lasers  66 .