Abstract:
MOPA laser apparatus includes a master oscillator and a preamplifier providing a train of optical pulses. The pulse train is input to a fiber optic device arranged to multiply the pulse-repetition frequency (PRF) of the input pulse train. The PRF multiplying device divides each pulse in the into train into two pulses and delays one pulse relative to the other, some portion of one of the pulses and a portion of the other are delivered by the fiber optic PRF multiplying device as an output pulse-train having a PRF equal to twice the PRF of the input pulse train.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention relates in general to pulsed master oscillator power-amplifier (MOPA) lasers. The invention relates in particular to methods of providing high-average power with low peak power in such lasers. 
       DISCUSSION OF BACKGROUND ART 
       [0002]    Modelocked pulsed (quasi CW) MOPA fiber lasers are preferred for laser material processing applications in microelectronics and other fields because they are very efficient, compact, and rugged compared, for example, to modelocked solid-state lasers. Many of the laser applications require the lowest possible peak power, or pulse energy, in order to minimize heat effects in materials being processed and damage to optics of a processing system. A high average power is preferred, however, for maximizing throughput. In order to achieve this, a high pulse-repetition frequency is required. 
         [0003]    Maximum repetition rates of modelocked master oscillators in MOPA fiber-lasers are limited however to about 100 megahertz (MHz). This limitation is imposed by one or more factors, particularly, a minimum fiber-leads length required by fiber splicing procedure (about 5 centimeters per splice), and a minimum doped-fiber length required for pump absorption and gain in the cavity. The minimum fiber leads length is about 5 centimeters (cm) and the minimum doped-fiber length is about 10 cm. There is a need for a method of increasing the PRF of modelocked MOPA fiber-lasers beyond this limit, preferably to a PRF greater than about 200 MHz. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention is directed to apparatus for increasing the pulse-repetition frequency of a pulse train from a laser or MOPA laser. In one embodiment of apparatus in accordance with the present invention comprises a laser device arranged to deliver a first train of pulses at a first pulse-repetition frequency (PRF). A fiber optic PRF multiplier having an input port and an output port, is arranged to receive the train of pulses at the input port thereof, divide each pulse in the first train of pulses into first and second pulses, delay the first pulse relative to the second pulse and deliver a first portion of each of the second and delayed first pulses from the first output port thereof. This provides that a second train of pulses is delivered from the first output port of the PRF multiplier, the second train of pulses having a second PRF equal to twice the first PRF. 
         [0005]    Preferably an arrangement is provided for detecting any differences between subsequent pulses in the second train of pulses. An amplitude adjusting arrangement is provided cooperative in a closed loop with the difference detecting arrangement for adjusting the amplitude of either the first or second pulses such that differences between the amplitude of pulses in the second train is minimized. 
         [0006]    In another embodiment of an apparatus in accordance with the present invention the fiber optic PRF multiplier is arranged to receive the train of pulses at the input port thereof, divide each pulse in the first train of pulses into first and second pulses, deliver the second pulse from the output port thereof, amplify the first pulse and delay the amplified first pulse relative to the second pulse and deliver a first portion of the amplified delayed first pulse from the output port thereof. This also provides that a second train of pulses is delivered from the output port of the PRF multiplier, the second train of pulses having a second PRF equal to twice the first PRF. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    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. 
           [0008]      FIG. 1  schematically illustrates, in block diagram form a preferred embodiment of a MOPA fiber laser in accordance with the resent invention including a master oscillator, a fiber-preamplifier, a fiber amplifier, and a pulse-repetition frequency (PRF) multiplier located between the preamplifier and the amplifier. 
           [0009]      FIG. 2  schematically illustrates a first preferred example of a PRF multiplier in accordance with the present invention, including a fiber Mach-Zehnder interferometer having first and second arms, with an amplitude control module located in the second arm. 
           [0010]      FIG. 2A  schematically illustrates one preferred example in accordance with the present invention of the amplitude control module of  FIG. 2 , including a periodically driven semiconductor optical amplifier. 
           [0011]      FIG. 3  schematically illustrates a second preferred example of a PRF multiplier in accordance with the present invention, a fiber Michelson interferometer having first and second arms with an amplitude control module located in the second arm. 
           [0012]      FIG. 4  schematically illustrates a third preferred example of a PRF multiplier in accordance with the present invention, including a three-port circulator having an input fiber attached to the first port, a multiplier fiber attached to the second port and an output fiber attached to the third port, with the multiplier fiber having a partially reflective fiber Bragg grating (FBG) therein and being terminated by a semiconductor amplifier. 
           [0013]      FIG. 5  schematically illustrates a fourth preferred example of a PRF multiplier in accordance with the present invention, a fiber loop interferometer having an amplitude control module located in the fiber loop. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    Referring now to the drawings, wherein like components are designated by like reference numerals,  FIG. 1  schematically illustrates a preferred embodiment  20  of a MOPA fiber-laser in accordance with the present invention. Laser  20  includes a master oscillator (MO)  22 , for example a modelocked fiber-laser. Such a MO can deliver pulse at a PRF up to about 100 MHz. MO  22  is connected to a fiber pre-amplifier  24 , which performs a preliminary amplification of pulses from the MO at the delivered frequency. Pre-amplified pulses are then delivered to a pulse-repetition frequency multiplier (PRF multiplier)  26  in accordance with the present invention, preferred examples of which are described in detail further hereinbelow. 
         [0015]    Two, temporally spaced apart pulses are delivered by the PRF-multiplier for every pre-amplified pulse delivered to the PRF-multiplier such that the output PRF of the PRF-multiplier is twice the PRF of the MO. Output pulses from PRF-multiplier  26  are amplified by a fiber power-amplifier  28  having one or more amplification stages. 
         [0016]    The amplified pulses are pulses of radiation have a fundamental wavelength of the MO. Optionally, the amplified pulses can be delivered to a harmonic-generator including one or more optically nonlinear crystals (not shown) for converting the fundamental radiation pulses to pulses of radiation having a wavelength that is an integer sub-multiple of the fundamental wavelength 
         [0017]    A description of several arrangements of PRF-multipliers in accordance with the present invention is set forth below with reference to  FIGS. 2 through 5 . The arrangements of  FIGS. 2 through 4  are, in general, fiber devices including effectively two optically-connected arms. An input pulse is divided into two portions with each portion being sent into a separate one of the arms, portions of the pulse portions are then coupled back into a single output fiber to form a pulse train having a PRF twice that of the input-pulse PRF. 
         [0018]      FIG. 2  schematically illustrates a first example  26 A of such an arrangement, configured similar to a fiber Mach-Zehnder interferometer. In example  26 A an input pulse is directed into a first port of a PM, four-port fiber-coupler  40 . Radiation coupled into the slow- or fast-axes of the first port of such a coupler splits between two output ports (port- 3  and port  4 ) with a coupling-ratio α. Optical power is split in proportion of α:1−α between the two output ports. Such a coupler may be referred to as a proportional or divisional coupler. Here, α is preferably 0.50, such that the input pulse is split in a 50:50 ratio. The output radiation is linearly polarized in both output ports. A simple modification of the four-port coupler, when one port is removed because no light is coupled into it, is called three-port PM coupler. 
         [0019]    About 50% of the input pulse propagates along arm A of the PRF-multiplier and the remaining portion propagates along arm B of the PRF multiplier. Arm A has an optical path length different from that of arm B. Preferably an optical path length difference in the two arms is selected to be equal to L=T/2v g  (where v g  is the group velocity of light). In this case, radiation in each of the arms arrives at another 50:50 PM coupler  42  at different times to effectively form two separate pulses temporally spaced apart by one-half of the input pulse spacing. For a typical group velocity value of 205,000 km/s, a path difference of about 1.025 meters (m) is required to provide a delay of 5 ns. 
         [0020]    Radiation arriving at port  1  of coupler  42  is divided about equally between port- 3  and port  4  of coupler  42 . Radiation arriving at port- 2  of coupler  42  is similarly divided equally between port  3  and port  4  of the coupler. This provides that the output of each of ports  3  and  4  is a pulse train having a PRF that is twice the PRF of the MO with pulses in each of the trains having an amplitude about one-fourth that of the input pulses. The output of any one of ports  3  and  4  is directed to amplifier  28  (see  FIG. 1 ) for further amplification, the other is discarded. This may seem, on first consideration, to be prohibitively wasteful of energy. Optical amplification in fibers, however, is a sufficiently efficient, high-gain process that half of the pre-amplified pulse energy can be discarded without a significant cost penalty. 
         [0021]    PRF-multiplier  26 A is depicted in  FIG. 2  as including what is referred to as an “amplitude control module”  46 . While not a required part of the inventive MOPA fiber-laser, amplitude control module  46  can be useful in cases, inter alia, where the couplers do not provide an exact 50:50 division between output ports. This can result in pulses in an output train from any one of the output ports have different peak power, which differences may be increased in any subsequent harmonic generation operations. 
         [0022]      FIG. 2A  schematically illustrates one example  46 A of an amplitude control module  46 , together with necessary control elements for the module. Amplitude control module  46 A includes a length  48  of a doped-core optical gain fiber. The gain fiber is energized (optically pumped) by radiation from a semiconductor laser (diode-laser)  50  directed into the gain fiber via a wavelength division multiplex (WDM) coupler  52 . A sampling coupler  54  having a relatively small coupling coefficient α, for example, about 0.01, directs a portion of the PRF-multiplied pulse-train being sent for further amplification (or further multiplication) to a fast photo-detector  56 , the output of which is connected to an error signal generator  58 . Error signal generator  58  provides an error signal proportional to the difference between consecutive pulses in the train. That error signal is used to adjust the output of the energizing diode-laser  50  in a closed-loop arrangement to adjust the gain of the amplifier fiber and drive the error signal to zero. The length of the gain fiber must be taken into account as being a part of the total length of arm B of the PRF-multiplier. 
         [0023]    It should be noted here that the fiber amplifier arrangement in arm B is only one possible amplifier arrangement. Another optical amplifier such as a semiconductor optical amplifier (SOA) may be substituted without departing from the spirit and scope of the present invention. A variable attenuator may also be substituted for a variable gain amplifier. A semiconductor optical amplifier variably driven below threshold can function as a variable amplifier. Another example of a variable attenuator is a mechanical attenuator known as a gap-loss attenuator. In such an attenuator, the variable loss is induced by variably pulling fiber connector end-faces apart, i.e., varying a gap between the fiber end-faces. As radiation exits an end face in an expanding cone, the receiving end face will capture less radiation as the gap becomes larger. Such an attenuator is commercially available, for example, from Metrotek Inc. of St. Petersburg, Fla. 
         [0024]    It should also be noted that once an optical amplifier or attenuator is placed in one arm of an interferometer a fiber-coupler split ratio of 50:50 used in passive interferometers (an interferometer without an amplitude control module) is no longer required. A coupler split-ratio can be arbitrarily determined by available gain or loss from an optical amplifier or attenuator. It is possible that if amplification of the PRF-multiplied pulse train is carried out with amplifier  28  operated in a saturated mode, the degree of saturation of the amplifier can be selected to even out variations in pulse power with an active amplitude control arrangement. 
         [0025]      FIG. 3  schematically illustrates a second example  26 B of a PRF-multiplier in accordance with the present invention based on a fiber Michelson interferometer. Here an input pulse is directed into port  2  of a 50:50 PM fused-coupler  40  and is divided about equally into arms A and B of the multiplier attached to ports  4  and  3 , respectively of the coupler. One of the arms may include an amplitude control module  46  as described in detail above with reference to  FIG. 2A . Here, module  46  is optionally included in arm A. 
         [0026]    Each arm is terminated by a reflective device  60 , which can be a fiber Bragg grating or a multilayer dielectric mirror. The mirrors reflect the pulse portions from the coupler back to the coupler where each pulse portion is split into two further portions that exit the coupler via ports  1  and  2  thereof. A result is that a pulse train is delivered from each of ports  1  and  2  of the coupler, with each pulse train having a PRF twice the PRF of the MO (the input pulse PRF) with pulses in the trains having a peak amplitude having an amplitude about one-fourth of the peak amplitude of an input pulse. The PRF-multiplied pulse train delivered from port  1  of coupler  40  is passed to amplifier  28  of  FIG. 1  for further amplification. As the pulse train delivered from port  2  of the coupler is sent back along the optical fiber used to deliver the input pulse it is advisable to include an isolator between pre-amplifier  24  and the PRF multiplier to prevent the PRF-multiplied pulse-train from being fed back into the pre-amplifier. 
         [0027]    It should be noted that in PRF multiplier  26 B, the difference in fiber length required to provide a given optical path difference in arms A and B is only one-half that of the Mach-Zehnder based arrangement of PRF-multiplier  26 A of  FIG. 2 . This is because the pulse portions in the two arms of PRF-multiplier  26 B make a double-pass through the arms, which are in effect “folded” by reflectors  60 . 
         [0028]      FIG. 4  schematically illustrates a third example  26 C of a PRF-multiplier in accordance with the present invention. Here, an input pulse is directed into port  1  of a three-port circulator. A three port circulator is a fiber optic device having first second and third ports numbered consecutively and arranged such that radiation entering the first port is delivered from the second port and radiation entering the second port is delivered from the third port. 
         [0029]    The input pulse entering port  1  of the circulator is directed by the circulator  64  into a length  66  of optical fiber terminated by a diode-laser  67 , periodically operated in an amplification mode, and having a partially reflective, partially transmissive FBG  68  located between the circulator and the diode-laser. In accordance with the general description given above, the FBG  68  can be described as dividing fiber  66  into an arm A between FBG  68  and diodes  67  and an arm B between the FBG and the circulator. The FBG can be described as forming a type of fiber Fabry-Perot interferometer in arm B with the diode-laser, more specifically, with a highly reflective facet mirror of the diode-laser stripe. 
         [0030]    Each input pulse (P) in a train thereof is partially reflected (pulse P 1 ) and partially transmitted (pulse P 2 ) by the FBG. Reflected pulses P 1  re-enter the circulator via port  2  and leaves through port  3  of circulator  64  at the input-pulse PRF. Arm B provides a resonator or a cavity for radiation partially transmitted through FBG  68 . The length of arm B is preferably selected to provide a cavity round-trip length of one half of a pulse-period. Pulse P 2  is amplified (pulse P 2A ) by passage into and out of the diode-laser cavity. A first portion of amplified pulse P 2A  (pulse-portion P 3 ) will be transmitted by the FBG one-half of a pulse-period after pulse-portion P 1  is reflected by the FBG. Pulse Portion P 3  enters the circulator via port  2  thereof and leaves the circulator via port  3  thereof one-half of a pulse period after Pulse P 1 . The output from port  3  of the circulator is a train of pulse-portions, alternating P 1  and P 3 , and having a PRF twice the PRF of the input pulse. 
         [0031]    A portion of pulse P 2A  (pulse-portion P 4 ) is reflected by FBG  68  back toward the diode-laser. It is desirable to suppress this pulse portion to prevent the pulse portion from leaving arm B and interfering with a reflected pulse P 1  from a subsequent input pulse P. This is achieved by operating the diode-laser using a train (drive signal) of short current pulses such that the diode-laser amplifies (is “on” or active) during passage of pulse portion P 2  and is “off” or inactive during the period when pulse-portion P 4  arrives at the diode-laser. The diode-laser is active and inactive once during every pulse-period of the input pulse train. When the diode laser is “off” the diode-laser becomes an absorber, and effectively suppresses pulse-portion P 4 . A result of this is that the output from port  3  of circulator  64  is a train of pulse-portions alternating P 1  and P 3  and having a PRF twice the PRF of the input pulse. Diode-laser  66  may be operated in a closed-loop mode with variable gain during on periods (as described above with reference to  FIG. 2A ) such that pulse portions in the output train thereof have equal amplitude. 
         [0032]      FIG. 5  schematically illustrates a third example  26 D of a PRF-multiplier in accordance with the present invention. Here, each input pulse (P) in a train thereof is directed into port  2  of a fiber coupler  43 . A portion (pulse) P 1  of the pulse exits the coupler via port  4  thereof, and another portion (pulse) P 2  is transmitted from port  3  of the circulator into a fiber loop  70  including a semiconductor optical amplifier (SOA)  72 . Fiber loop  70  reconnects to the circulator via port  1  thereof. SOA  72  is periodically exited in the manner of diode-laser  66  of PRF-multiplier  26 C. The length of loop  70  is selected to provide a round-trip time therein that is of one-half of a pulse-period of the input pulse-train. 
         [0033]    Pulse P 2  is amplified (pulse P 2A ) by passage through SOA  72 . A first portion of amplified pulse P 2A  (pulse-portion P 3 ) will be transmitted out of port  4  of circulator  43  one-half of a pulse-period after pulse-portion P 1  was transmitted. Part of this pulse-portion P 2A  (pulse-portion P 4 ) is transmitted from port  3  of coupler  43  back into loop  70 . Here again it is desirable to suppress this pulse portion to prevent the pulse-portion from leaving the loop and interfering with a pulse-portion P 1  of a subsequent input pulse P. 
         [0034]    This suppression is achieved by periodically operating the SOA in the manner described above with reference to diode-laser (amplifier)  66  of PRF multiplier  26 C. The output from port  4  of coupler  43  is a train of pulse-portions, alternating P 1  and P 3 , and having a PRF twice the PRF of the input pulse. SOA  66  may be operated in a closed-loop mode with variable gain during “on” periods (as described above with reference to  FIG. 2A ) such that pulse-portions in the output train thereof have equal amplitude. 
         [0035]    All of the above described PRF multipliers optimally multiply the PRF of an input pulse train by a factor of two. Those skilled in the art will recognize without further illustration or detailed description that if a greater multiplication factor is desired, the output of two or more of the inventive multipliers, not necessarily of the same type, may be cascaded together in series to increase the PRF input to the first by a factor of four, eight (using three cascaded multipliers), or more. 
         [0036]    In summary the present invention is described above in terms of a preferred and other embodiments. The invention is not limited, however, to the embodiments described and depicted. Rather, the invention is limited only by the claims appended hereto.