Abstract:
An all-fiber saturable absorber Q-switched laser and the method for producing saturable absorber Q-switched pulses are provided. By locating a saturable absorber fiber in the intensity-enhanced section of a ring resonator, the Q-switched pulses are produced and enhanced. The present application is advantageous in the simple design and effective cost, and is applicable for a variety of fiber-type laser materials.

Description:
BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to a Q-switched laser, and more particularly, to a passively Q-switched laser. 
         [0003]    2. Background of the Invention 
         [0004]    Q-switched operation is a useful technique employed in laser systems to produce short and high-intensity laser pulses. Q-switched lasers can be realized using active or passive Q-switches. The passive Q-switched laser is also called the saturable absorber Q-switched laser because of using a saturable absorber material in the resonator to modulate the Q factor and produce laser pulses. Compared to the active Q-switched laser, the passive Q-switched laser has the advantages of high efficiency, flexibility, compactness and low cost. The saturable absorber medium, however, is hard to acquire. 
         [0005]    The traditional Q-switched fiber lasers generally employ bulk Q-switches. These fiber lasers contain free-space sections in the resonators, and require sophisticated techniques of alignment for in-and-out light coupling between fibers and the Q-switches. The disadvantages of these fiber lasers are high cavity loss, low Q-switching efficiency and difficulties of packaging. 
         [0006]    All-fiber actively Q-switched lasers have been realized using active Q-switches as acousto-optic modulators, piezoelectric (PZT) actuators and magnetostrictive transducers. These active Q-switches require additional electronically driven apparatuses that increase the cost of laser systems. 
         [0007]    To solve the problems above, applicants previously presented an idea of an all-fiber passive Q-switched laser (TW patent application No. 96144909). The idea is that the photon density passing through a saturable absorber fiber is increased by adjusting the area ratio of the cores between the saturable absorber fiber and the gain fiber. The enhanced power density in the saturable absorber fiber results in a fast saturation rate of absorption population and then activated the Q-switching mechanism. To improve the Q-switching efficiency and simplify the laser scheme, applicants provide the present invention of All-Fiber Saturable Absorber Q-switched Laser and Method for Producing Saturable Absorber Q-switched Pulse. 
       SUMMARY 
       [0008]    The present invention provides an all-fiber saturable absorber Q-switched laser which is simple and suitable for various laser materials. 
         [0009]    The present invention also provides an all-fiber saturable absorber Q-switched laser which is composed of fiber-type components and capable of producing high-intensity laser pulses after being pumped. 
         [0010]    According to one aspect of the present invention, the all-fiber saturable absorber Q-switched laser of the present invention includes a gain fiber, an optical circulator, a fiber grating, and a saturable absorber fiber. The gain fiber is pumped by a pump source and then emits a laser beam. The optical circulator is configured at an output side of the gain fiber to exclude a light beam emitted by the pump source and to output the laser beam. The fiber grating is configured at a first output side of the optical circulator to reflect the laser beam; and the fiber grating and the optical circulator define an intensity-enhanced section. The saturable absorber fiber is configured in the intensity-enhanced section to absorb the laser beam and to produce a laser pulse when the saturable absorber fiber is saturated. 
         [0011]    Preferably, the gain fiber, the optical circulator and the fiber grating form a ring resonator. 
         [0012]    Preferably, the fiber grating is configured to reflect a part of the laser beam back to the ring resonator and to output a remaining part of the laser beam which is not reflected thereby. 
         [0013]    Preferably, the all-fiber saturable absorber Q-switched laser further includes a power splitter configured at a second output side of the optical circulator to output a certain part of laser beam from the ring resonator. 
         [0014]    Preferably, the optical circulator is configured to control the resonant direction of the ring resonator. 
         [0015]    Preferably, the wavelength of the saturable absorber Q-switched laser is determined by the reflected spectrum of the fiber grating. 
         [0016]    Preferably, the all-fiber saturable absorber Q-switched laser further includes a wavelength-division multiplexer configured at an input side of the gain fiber to couple the light beam emitted by the pump source into the gain fiber. 
         [0017]    Preferably, the gain fiber is an erbium-doped fiber. 
         [0018]    Preferably, the saturable absorber fiber is an erbium-doped fiber. 
         [0019]    According to another aspect of the present invention, a method for producing a saturable absorber Q-switched laser pulse is provided. The method includes steps of pumping a gain fiber by a pump source to emit a laser beam; guiding the laser beam to an optical circulator to control the resonant direction of the laser beam; and absorbing the laser beam by a saturable absorber fiber and producing a laser pulse after the saturable absorber fiber is saturated 
         [0020]    Preferably, the method further includes a step of coupling the light beam emitted by the pump source into the gain fiber by a wavelength division multiplexer to pump the gain fiber and generate the laser beam. 
         [0021]    Preferably, the method further includes a step of determining the wavelength of the saturable absorber Q-switched laser and reflecting a part of the laser beam back to the ring resonator. 
         [0022]    Preferably, the method further includes a step of outputting a certain part of the laser beam from the ring resonator. 
         [0023]    Preferably, the saturable absorber fiber absorbs the laser beam until the saturable absorber fiber is saturated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The foregoing aspects described herein will become more readily apparent by reference to the following Description when taken in conjunction with the accompanying drawings wherein: 
           [0025]      FIG. 1  is a schematic diagram of an all-fiber saturable absorber Q-switched laser according to the first embodiment; 
           [0026]      FIG. 2  is the output measurement of the all-fiber saturable absorber Q-switched laser with accordance to the first embodiment; and 
           [0027]      FIG. 3  is a schematic diagram of the all-fiber saturable absorber Q-switched laser according to the second embodiment. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0028]      FIG. 1  is a schematic diagram of an all-fiber saturable absorber Q-switched laser  10  according to the first embodiment. The all-fiber saturable absorber Q-switched laser  10  and method for producing saturable absorber Q-switched laser are understood with reference to  FIG. 1 . According to this embodiment, the all-fiber saturable absorber Q-switched laser  10  with a ring resonator of the present invention includes a gain fiber  11 , a saturable absorber fiber  12 , an optical circulator  13 , a fiber grating  14  and other optical components. The ring resonator is formed by the gain fiber  11 , the optical circulator  13 , and the fiber grating  14 . 
         [0029]    The gain fiber  11  configured in a gain region R G  of the all-fiber saturable absorber Q-switched laser  10  is excited by a pump source 15  and emits a laser beam to the optical circulator  13 . 
         [0030]    The optical circulator  13  is configured at an output side of the gain fiber to receive the laser beam emitted from the gain fiber  11  and to determine the paths of the laser beam from point A to point B and point B to point C in the optical circulator  13 . The optical circulator  13  is also configured to exclude a light beam emitted by the pump source 15 , thereby preventing the saturable absorber fiber  12  from absorbing the light beam emitted by the pump source 15  since the light beam may pass through the gain fiber  12 . 
         [0031]    The fiber grating  14  is configured at a first output side (port B) of the optical circulator  13 . Moreover, the fiber grating  14  and the optical circulator  13  define an intensity-enhanced section R E . A certain part of the laser beam passing through the optical circulator  13  (from port A to port B) is reflected back into the ring resonator (from port B to port C) by the fiber grating  14 . The laser beam that is not reflected by the fiber grating  14  is outputted. The wavelength of the laser beam is decided by the reflection spectrum of the fiber grating  14 . 
         [0032]    The saturable absorber fiber  12  is configured in the intensity-enhanced section R E  of the all-fiber saturable absorber Q-switched laser  10  to absorb the laser beam (from port A to port B) and to produce a laser pulse when the saturable absorber fiber is saturated. 
         [0033]    The all-fiber saturable absorber Q-switched laser  10  of the present invention further comprises a wavelength division multiplexer (WDM)  16  which is configured at an output side of the pump source 15  or an input side of the gain fiber  12  to couple the light beam emitted by the pump source 15  into the gain fiber  11 , combined with the reflected part of the laser beam. The laser beam is absorbed twice when double passing through the saturable absorber fiber  12 . When the saturable absorber fiber  12  is saturated, the laser beam is not absorbed by the saturable absorber fiber  12 , thus giving rise to the laser pulse. 
         [0034]    It is understood that the laser beam passes through the intensity-enhanced section R E  twice in one roundtrip of the all-fiber saturable absorber Q-switched laser  10  of the present invention. The photon density in the saturable absorber fiber  12  is therefore on average twice (or more than twice) that in the gain fiber  11 . The higher photon intensity results in a fast bleaching of the saturable absorber fiber  12 , and then a passive Q-switching performance. 
         [0035]    In one preferred embodiment of the present invention, the gain fiber  11  and the saturable absorber fiber  12  are erbium-doped fibers. 
         [0036]    The material of the gain fiber  11  and the saturable absorber fiber  12  may be the same in the present invention. The gain fiber  11  and the saturable absorber fiber  12 , in one preferred embodiment of the present invention, are the same erbium-doped fibers having an absorption loss of 110 dB/m at 1530 nm, and a core diameter of 4 μm as example. The length of the gain fiber 11 is 50 cm and that of the saturable absorber fiber 12 is 15 cm. The reflectivity of the fiber grating  14  is 10% with a bandwidth less than 0.2 nm. The total roundtrip length of the ring resonator is 400 cm. 
         [0037]      FIG. 2  shows the output measurement of the all-fiber saturable absorber Q-switched laser  10  according to the preferred embodiment above. The pulse energy and the pulse width of the output laser pulse shown in  FIG. 2  are 2.4 μJ and 40 ns. Moreover, for the ring resonator having a roundtrip length of 100 cm, the pulse width of the output laser pulse can be narrowed down to 10 ns and the peak pulse power of the all-fiber saturable absorber Q-switched laser  10  is about 240 W. 
         [0038]    Referring now to  FIG. 3 , it is a schematic diagram of an all-fiber saturable absorber Q-switched laser according to a second embodiment of the present invention. According to this embodiment, the all-fiber saturable absorber Q-switched laser  30  with a ring resonator comprises a gain fiber  31 , a saturable absorber fiber  32 , an optical circulator  33 , a fiber grating  34 ; a wavelength division multiplexer  36  configured at the output side of a pump source 35 , a power splitter  38 , and other optical components. The ring resonator is formed by the gain fiber  31 , the optical circulator  33 , and the fiber grating  34 . The difference between this embodiment and the first embodiment is that the laser beam is 100% reflected back to the ring resonator by the fiber grating  34 . 
         [0039]    The power splitter  38  is configured at a second output side (port C) of the optical circulator  33  to output a certain part of laser beam from the resonator. As discussed above, after the saturable absorber fiber  32  is saturated, the laser beam from the optical circulator  33  is not absorbed by the saturable absorber fiber  32 . Then the output part of the laser beam is Q-switched, thereby giving rise to a high-intensity Q-switched laser pulse. The laser beam passes intensity-enhanced section R E  twice in one roundtrip of the all-fiber saturable absorber Q-switched laser  30 . The photon density in the saturable absorber fiber  32  is therefore on average twice (or more than twice) that in the gain fiber  31 . The higher photon intensity results in a fast bleaching of the saturable absorber fiber  32 , and then a passive Q-switching performance. 
         [0040]    It is apparent from the discussion above that by the present invention, the structure of a passive Q-switched laser is simplified, the efficiency of the saturable absorber Q-switching is increased, and the choice of Q-switch materials is more flexible. Moreover, since the present invention is an all-fiber design which has low cavity loss, high Q-switching efficiency, ease of packaging and operation, and low fabrication cost.