Abstract:
Frequency-multiplied fiber-MOPA apparatus includes one enclosure containing a master oscillator and fiber amplifier stages and another enclosure containing frequency-multiplying stages. Radiation is transmitted between the enclosures by a transport fiber in a flexible jacket or enclosure. The transport fiber functions additionally as a power amplifier fiber, and amplifies the radiation while transporting the radiation between the enclosures. The amplifying transport fiber is energized by diode-lasers in the enclosure containing the master oscillator and fiber amplifiers.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention relates in general to master oscillator power amplifier (MOPA) laser systems including multiple fiber-amplifier stages. The invention relates in particular to pulsed fiber MOPA laser systems including one or more stages of frequency conversion. 
       DISCUSSION OF BACKGROUND ART 
       [0002]    Frequency converted fiber MOPAs are increasingly being used in applications where frequency-converted solid lasers were previously used. Such applications include micromachining/materials processing and wafer inspection. Fiber laser and fiber amplifier systems have certain advantages over solid state lasers. These advantages include more efficient use of pump power, permanence of alignment, and in many instances a convenience of packaging which is due to the fact that amplifier fibers can be coiled in an enclosure. 
         [0003]    In a relatively low power frequency-converted MOPA system, for example having an average power for fundamental radiation of less than about 50-100 Watts (W), the master oscillator, fiber amplifier stages, diode-laser arrays for providing optical pump radiation, and one or two stages of harmonic conversion can usually be packaged in a single enclosure having a “footprint” of about 60 centimeters (cm)×20 cm. Power for powering the diode-lasers and other components can be supplied to the enclosure from a separate power supply, via a suitable cable and electrical connectors. 
         [0004]    For a MOPA having higher average fundamental power, packing all MOPA and harmonic generating components in a single enclosure is impractical because of the heat-load created by less than 100% efficient pumping of the diode-lasers and MOPA components. One arrangement for packing such a MOPA is to package the power supply master oscillator and low power fiber amplifiers in a first enclosure, and to package a final power amplifier stage and harmonic generating stages in a second enclosure. A transport fiber arranged between the enclosures connects the amplified signal from the first enclosure to the power amplifier in the second enclosure. A diode-laser array for pumping the power amplifier can be located in the first or the second enclosure. If the diode-laser array for the power amplifier is in the first enclosure, a fiber will be required to transport pump radiation to the second enclosure. In either case, there will need to be an electrical connection between the enclosures as power will be required in the second enclosure for providing temperature control of the harmonic generating stages. 
         [0005]    Amplifier fibers typically have a core diameter directly related to the peak power to be generated in the fiber. This is required to prevent the peak radiation intensity from reaching levels that could cause nonlinear optical effects, or even catastrophic optical damage. Certain types of amplifier fiber, such as PCF (photonic crystal fibers), used for such high power have low numerical aperture (NA) which makes them vulnerable to bending losses. Further, some photonic crystal fibers are not flexible and must be mounted in a rigid holder. 
         [0006]    A large core diameter or a low numerical aperture will increase the minimum possible bending radius of an amplifier fiber to a level where it is not possible to package (coil) the amplifier fiber in an enclosure of the convenient dimensions possible in lower power MOPAs. There is a need for a method of packing a high-power fiber-amplifier in a MOPA that does not require scaling the dimensions of MOPA enclosures to accommodate the high-power fiber-amplifier. 
       SUMMARY OF THE INVENTION 
       [0007]    In one aspect of the present invention laser apparatus comprises an enclosure having a master oscillator located therein for generating signal radiation. One or more fiber amplifiers are located in the enclosure for amplifying the signal radiation. A transport fiber extends from the first enclosure. The transport fiber is arranged to further amplify the amplified signal radiation and transport the further-amplified signal radiation to either a device wherein the further-amplified radiation will be used, or a location where the further amplified radiation will be used. 
         [0008]    In one preferred embodiment of the invention, the device is a harmonic-generator including one or more optically nonlinear crystals for frequency-multiplying the further-amplified radiation. The harmonic-generator is in another enclosure remote from that in which the master oscillator is located. The transport fiber can be selectively connected or disconnected from the enclosure in which the harmonic-generator is enclosed. The transport fiber is housed in a flexible jacket and is fluid-cooled. The amplifying transport fiber is energized (optically pumped) by diode-lasers in the enclosure in which the master oscillator is located. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain principles of the present invention. 
           [0010]      FIG. 1  schematically illustrates a preferred embodiment of a fiber MOPA in accordance with the present invention, including a first enclosure housing a master oscillator, two stages of fiber-amplification and diode-lasers for providing pump radiation, the first enclosure being demountably connected by a water-cooled power amplifying fiber in a flexible housing to a second enclosure including one or more stages of harmonic conversion. 
           [0011]      FIG. 2  schematically illustrates one arrangement of amplifiers and diode-lasers in the first enclosure of  FIG. 1  including four diode-lasers delivering pump radiation to the water-cooled power amplifying fiber and an arrangement including re-circulating chiller for providing cooling water to the power-amplifier fiber. 
           [0012]      FIG. 3  schematically illustrates one arrangement of harmonic conversion stages and a fiber connector for the amplifying fiber in the second enclosure of  FIG. 1   
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    Referring now to the drawings, wherein like components are designated by like reference numerals,  FIG. 1  schematically illustrates a fiber MOPA apparatus  10  in accordance with the present invention. MOPA  10  includes an enclosure  12  housing a master oscillator (MO), low power fiber amplifier stages, diode lasers for proving optical pump radiation for fiber amplifier stages and a power supply for providing power for the diode lasers and the master oscillator. Another enclosure  22  includes harmonic generation stages including optically nonlinear crystals. Pulses generated by the master oscillator in enclosure  12  and amplified by the low power fiber-amplifier stages in the enclosure are transported by a power-amplifier fiber  16  to enclosure  22  to be delivered to the harmonic generating stages. The harmonic radiation provided by the generators is delivered from enclosure  22  via a window  24  therein. 
         [0014]    Power-amplifier fiber  16  receives pump-radiation for diode-lasers within enclosure  12 . The power amplifier fiber is housed in a flexible jacket  20  and the fiber and jacket assembly (fiber assembly)  14  are connected by a connector arrangement  18  to enclosure  22 . The connector arrangement allows the fiber assembly to be disconnected from enclosure  22 , for example for convenience of transporting apparatus  10 . The rigidity of the jacket is preferably selected such that assembly  14  can not be bent in a radius less than a bending-loss determined minimum bending radius for power-amplifier fiber  16 . 
         [0015]    An arrangement within enclosure  12  re-circulates cooling water (or some other fluid) through space  26  between power-amplifier  16  and jacket  20 . An electrical lead  30  is connected by an electrical connector  32  to enclosure  22  and provides power from the power supply in enclosure  12  to thermo electric temperature controllers (TECs) for maintaining selected phase-matching temperatures for optically nonlinear crystals in enclosure  22 . Although electrical lead  30  is depicted as being separate from fiber/jacket assembly  14  in  FIG. 1 , the lead can be integrated into assembly  14 , for example, by winding the electrical lead helically around the jacket. 
         [0016]      FIG. 2  schematically illustrates one example of an arrangement of amplifiers and diode-lasers in the enclosure  12  of apparatus  10  of  FIG. 1 . Here a master oscillator  34  in the form of a directly modulated diode-laser provides seed pulses of radiation to be amplified by the apparatus. The seed pulses can be generally defined as signal radiation. Optical fibers for low power amplification and for delivering pump radiation are designated by bold, solid lines. Electrical connections are designated by dashed, bold solid lines, to avoid confusion with lines designating optical fibers and lead lines of reference numerals. 
         [0017]    Seed pulses (signal radiation) from master oscillator  34  are delivered via an isolator  36  to an amplifier-fiber  38  providing a first stage of amplification. Amplified pulses from fiber  38  are delivered via an isolator  40  to an amplifier-fiber  42  providing a second stage of amplification. Amplifier-fiber  38  is optically pumped by radiation from a diode-laser  44  fiber-coupled to fiber  38  via a wavelength division multiplex coupler  36 . Amplifier-fiber  42  is optically pumped by radiation from a diode-laser  48  fiber-coupled to fiber  42  via a wavelength division multiplex coupler  50 . 
         [0018]    Twice-amplified pulses from amplifier-fiber  42  are delivered via an isolator  52  and a tapered coupler  54  to fiber  16 . Fiber  16  may be a large-mode-area (LMA) fiber having a solid core and claddings or a photonic crystal fiber (PCF). In this example, pump-radiation from four diode-lasers  56  is fiber-coupled into cladding (not explicitly shown) via fibers fused-coupled to the cladding. A power supply  64  provides current for the pump diode-lasers and the master oscillator. A separate power supply  62  provides power via lead  30  to TECs in enclosure  22  as discussed above. 
         [0019]    Fiber  16  is cooled by passing a cooling fluid, such as water, from a recirculating chiller (cooler)  58  via an input conduit  60  outward between an inner flexible jacket (tube)  21  and the fiber. The fluid returns between inner jacket  21  and outer jacket (tube)  20  then via an output conduit  62  to the chiller. 
         [0020]    It should be noted that the subject invention is not intended to be limited to the any particular method of initially generating the laser pulses. For example, light from a CW laser diode can be externally modulated. In addition, a mode-locked laser can be used as a source of laser pulses. In the latter case, it may be desirable to include a pulse picker within enclosure  12  to reduce the repetition rate of the pulses to be amplified. 
         [0021]    It should also be noted that some photonic crystal fibers are essentially rigid and would be supported in a rigid mount between the two enclosures. 
         [0022]      FIG. 3  schematically illustrates one arrangement of harmonic-conversion stages and fiber connector  18  in enclosure  22  of apparatus  10  of  FIG. 1 . Here fiber connector arrangement  18  includes a receiver member  17  which is attached to wall  23  of enclosure  22 . A connector member  19  is attached to fiber assembly  14  and is removeably (demountably) coupled to receiver member  18 , for connecting (or disconnecting) the fiber assembly from enclosure  22 . Electrical connection to and within the enclosure and TECs within the enclosure are not shown in  FIG. 3  for simplicity of illustration. 
         [0023]    Fiber  16  delivers a diverging beam of radiation  70  into enclosure  22 . The radiation has a fundamental wavelength of the master oscillator and amplifier fibers. Beam  70  is collimated by a lens  72  and directed by a turning mirror  74  to a lens  76 . Lens  76  focuses the fundamental wavelength radiation to a beam waist in an optically nonlinear crystal  78  arranged to frequency-double the fundamental radiation to provide second-harmonic (2H) radiation. The 2H-radiation and residual fundamental radiation from the frequency-doubling process are collimated by a lens  80  then re-focused by a lens  82  into an optically nonlinear crystal  84  arranged to sum-frequency mix the 2H-radiation and residual fundamental radiation to provide third-harmonic (3H) radiation. The 3H-radiation and residual 2H and fundamental radiation from the sum-frequency mixing process are collimated by a lens  86 . A dichroic beamsplitter  88  separates the residual 2H and fundamental radiation from the 3H radiation, and sent to a beam dump (not shown). The 3H-radiation is delivered from enclosure  22  via window  24  therein as output radiation. 
         [0024]    It should be noted here that the harmonic conversion example described above is but one example of frequency conversion that can be carried out in the enclosure. More or less stages of conversion may be included for generating second or higher harmonic radiation. One or more crystals may by arranged for optical parametric generation wherein the fundamental wavelength radiation delivered from fiber  16  is frequency divided into parametric signal radiation and parametric idler radiation each having a wavelength longer than the wavelength of the fundamental wavelength radiation. These and any other frequency conversions may be carried out without departing from spirit and scope of the present invention. 
         [0025]    Further it should be noted here that the multi-stage amplifier arrangement of enclosure  12  is one example provided to illustrate principles of the present invention and should not be construed as limiting. By way of example, more or less stages of low-power amplification may be included and different methods of coupling optical pump radiation to the amplifier fibers may be used. It is also possible to provide a separate power supply outside of the enclosure but electrically connected thereto. Different methods of circulating cooling fluid through fiber assembly  14  may also be used. It should also be noted that fiber assembly  14  may also be used simply to amplify and transport fundamental radiation from enclosure  22  to a location or device where, or in which, the radiation may be used. By way of example, one such device may be a device for scanning and focusing beam  70  for laser-drilling, laser-engraving or laser-machining operations. 
         [0026]    In addition, while enclosure  22  is illustrated with optics for changing the frequency of the laser pulses, alternative laser pulse modification techniques can be employed in enclosure  22  other than (or in conjunction with) frequency conversion. For example, an additional amplifier stage or stages can be provided for further increasing the energy of the pulses. Alternatively, optics for changing the width of the pulse, such as stretchers or compressors, can be provided in enclosure  22 . It should also be noted that the concept of using an amplifying transport fiber might also be of interest in continuous wave (CW) systems. One of the key advantages of the subject invention is that by combining the amplifying and transport functions into one fiber, the overall package size can be reduced in cases where the amplifying fiber is of the type that cannot be bent or has a limited bend radius. These and other variations of the present invention may be practiced without departing from the sprit and scope of the present invention as defined by the claims appended hereto.