Abstract:
A Q-switched all-fiber laser utilizes a long period fiber grating (LPFG) modulator. The LPFG modulator is characterized by optical spectral characteristics that are controlled by application of stress via an actuator. In particular, the actuator applies stress to selected sections of the LPFG in order to modulate a light signal at a specified wavelength. Further, a controller is utilized to control the application of stress in the time domain, and thereby switch the Q-factor of the fiber laser cavity. In addition to the LPFG, the laser cavity comprises a pair of fiber Bragg gratings (FBGs) and a fiber gain medium.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional of commonly owned U.S. patent application Ser. No. 12/022,255, filed Jan. 30, 2008, entitled Q-SWITCH ALL-FIBER LASER which claims priority to U.S. Provisional Patent Application 60/901,255, filed Feb. 13, 2007, entitled Q SWITCHED FIBER LASER WITH ALL FIBER CONFIGURATIONS, which is incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention is generally related to the field of lasers, and more particularly to a Q-switched all-fibre laser. 
     BACKGROUND OF THE INVENTION 
     Characteristic features of fiber lasers include high output beam quality, compact size, ease-of-use, and low running cost. Fiber lasers can generate either continuous-wave (CW) radiation or pulse radiation. Pulsed operation can be achieved via Q-switching techniques. Q-switched fiber lasers are preferred for applications such as micro-machining, marking, and scientific research due to their high peak power and excellent beam quality. Q-switching is achieved by inserting an optical modulator in the laser resonance cavity to control optical loss in the cavity. In particular, the modulator functions as an optical loss switch. Initially, cavity loss is kept on a high level (low Q factor state). Laser oscillation cannot occur at this initial period, but energy from a pump source accumulates in the gain medium. Subsequently, cavity loss is switched to a low loss level (high Q factor state), so that laser oscillation builds up quickly in the cavity and generates a high peak power laser pulse. When the laser cavity is switched between low Q and high Q by the optical modulator, sequenced laser pulses are produced. 
     Optical modulation for Q-switching can be achieved by either active or passive means. Examples of active Q-switching modulation means include acousto-optic modulators (AOMs) and electro-optic modulators (EOMs). The AOM comprises optical crystals such as tellurium dioxide, crystalline quartz, and fused silica. The EOM comprises optical materials such as potassium di-deuterium phosphate (KD*P), beta barium borate (BBO), lithium niobate (LiNbO 3 ), as well as NH 4 H 2 PO 4  (ADP), and other materials. One drawback of known AOM and EOM devices is that they are relatively bulky. This is a drawback because the fibre core has a relatively small diameter, the difference of which relative to the size of the modulator complicates light coupling between the device and an optical fiber. Further, AOM and EOM devices are relatively expensive. 
     A typical configuration of a Q-switched fiber laser is illustrated in  FIG. 1 . The laser cavity comprises a pair of fiber Bragg grating (FBG) reflectors ( 15 ,  35 ) having the same center wavelength, a gain fiber ( 18 ) which provides optical gain, and an optical modulator ( 90 ) coupled to an optical fiber pigtail ( 20 ) for coupling a light signal between the fiber and the modulator. The optical modulator may be either an AOM or EOM type. A pump source ( 1 ) provides pump light ( 5 ) which is coupled to the fiber laser cavity to excite the gain fiber ( 18 ). The FBG reflectors provide optical feedback for laser oscillation. The optical modulator ( 90 ) is employed as a switch to control optical loss within the laser cavity, and thereby provide Q-switching. Initially, the cavity loss is kept on a high level with the modulator switch “off” (low Q factor state of the laser cavity), at which time no light signal passes through the modulator ( 90 ). As discussed above, laser oscillation does not occur at this time, but energy from pump light source ( 5 ) accumulates in the gain fiber ( 18 ). Subsequently, the cavity loss is reduced over a relatively short time by “switching on” the optical modulator to a low loss level (high Q factor state of the laser cavity), at which time the light signal passes through optical modulator ( 90 ). Consequently, laser oscillation builds up quickly in the cavity and generates a high peak power laser pulse. The FBG pair ( 15 ,  35 ) have the same center wavelength and function as narrow band reflective mirrors which provide optical feedback to the laser cavity and confine the laser oscillation wavelength to the FBG wavelength. Since the FBG has a relatively narrow reflective bandwidth, the laser oscillates only at this wavelength and the output has a narrow wavelength spectrum. When the laser cavity is switched sequentially between the low Q factor state and the high Q factor state by means of the optical modulator ( 90 ), sequenced laser pulses are produced. Modulator switch control is achieved by means of a signal ( 95 ) from an external controller ( 96 ). One device of the FBG pair ( 15 ,  35 ) is partially transparent and has relatively lower reflectivity, resulting in a percentage of the generated laser light being permitted to leave laser cavity and deliver the laser output ( 38  or  42 ). 
     Referring to  FIG. 2   a , the FBG is formed by introducing a refractive index change in discreet areas of the fiber core. The modified area ( 151 ) within the fiber core has a smaller refractive index difference of period Λ B  relative to the adjacent unmodified area ( 152 ). Several techniques are known for changing the refractive index of discreet areas of the fibre core. One technique is to expose the area to a UV laser beam, e.g., area ( 151 ) is altered by exposure to UV light, but area ( 152 ) is neither exposed nor altered. 
     The principle characteristic parameters of a FBG are center wavelength λ B , bandwidth Δλ B , and reflectivity. The condition for high reflection, known as the Bragg condition, relates the reflected wavelength, or Bragg wavelength, λ B  to the grating period Λ B  and the effective refractive index of the fiber core n via:
 
λ B =2 nΛ   B .
 
 FIGS. 2   b ,  2   c , and  2   d  illustrate the spectral characteristics of a FBG. When broad band light ( 110 ,  FIG. 2   a ) having spectrum ( 120 ,  FIG. 2   b ) is input into the FBG as shown, the reflected light ( 112 ,  FIG. 2   a ) has a corresponding spectrum ( 122 ,  FIG. 2   c ), and the transmitted light ( 111 ,  FIG. 2   a ) has a corresponding spectrum ( 121 ,  FIG. 2   d ).
 
     Somewhat similar to the FBG in terms of physical configuration, a Long Period Fiber Grating (LPFG) has a grating period Λ L  which is considerably longer than the period Λ B  of the FBG, i.e., typically Λ L  is 200˜2000 times longer than Λ B . The LPFG couples the fundamental mode in the fiber core with the cladding modes of the fiber and propagates them in the same direction. The excited cladding modes are attenuated, resulting in the appearance of resonance loss in the transmission spectrum. However, in contrast with the FBG, the LPFG does not produce reflected light.  FIGS. 3   a ,  3   b  and  3   c  illustrate the physical configuration and the spectral transmission characteristics of a LPFG. The periodic grating structure ( 22 ,  FIG. 3   a ) can be made by using a UV laser beam to “burn” discreet, periodically spaced areas in the fiber core in a manner which is similar to that described above with reference to the FBG, where the modified area ( 251 ) exhibits a refractive index change in comparison with unmodified area ( 252 ). Recent research suggests that the modified areas can be also formed by using a high voltage electric arc discharge or CO 2  laser to “burn” the fiber, i.e., introducing structural changes and slight geometrical deformation in the irradiated area of the fibre. Alternatively, mechanical stress can be used, i.e., by applying static stress to the areas of the fibre to be modified through a corrugated plate. The refractive index at the areas subjected to stress is changed in accordance with the photo-elastic effect, but the adjacent areas which are not subjected to stress are unmodified. 
     When a broad band light ( 210 ,  FIG. 3   a ) having spectrum ( 220 ,  FIG. 3   b ) is input into the LPFG, the transmitted light ( 211 ,  FIG. 3   a ) has a corresponding spectral characteristic ( 221 ,  FIG. 3   c ), several resonance loss peaks ( 222 ,  223 ), including the fundamental mode coupling with different cladding modes of the fiber. However, there is no light reflection. Considering resonance loss peak ( 222 ,  FIG. 3   c ), having a center wavelength λ L , and bandwidth Δλ L , the resonance loss of the LPFG is due to the coupling of the fundamental mode in the fiber core with the cladding modes of the fiber. The phase matching between the fundamental mode and cladding modes at wavelength λ mL  can be expressed as:
 
λ mL =( n   core   −n   cl   m )Λ L ,
 
where n core  is the effective refractive index of the fundamental mode, n cl   m  is the effective refractive index of the m th  cladding mode, and Λ L  is the period of the LPFG. Since several cladding modes can satisfy this condition, each one is at a different center wavelength λ mL , and thus the transmission spectrum of the LPFG exhibits a series of transmission loss notch peaks ( 222 ,  223 ,  FIG. 3C ).
 
       FIGS. 4   a - 4   c  illustrate the physical configuration and the spectral transmission characteristics of a phase shifted LPFG. In the phase shifted LPFG, a part of the grating period is shifted at the grating center by Λp. As a result, a phase shift is introduced into the LPFG. For example, by introducing a π-phase shift at the center of the LPFG, the notch peak (See  FIG. 3   c ) is changed to a reverse peak ( 232 ,  FIG. 4   c ). For a broad band input ( 220 ,  FIG. 4   b ), a corresponding transmission spectrum ( 231 ,  FIG. 4   c ) of the phase shifted LPFG is produced, enabling transmission at wavelength λ L . 
       FIGS. 5   a - 5   c  illustrate the physical configuration and the spectral transmission characteristics of cascaded LPFGs. Cascaded LPFGs are formed by connecting a pair of LPFGs ( 25 ,  26 ) in series. Each of the LPFGs has a grating length d 1  and d 2 , and together define a separation distance of L. When broad band light ( 210 ) having spectrum ( 220 ,  FIG. 5   b ) is input into the cascaded LPFGs, the corresponding transmitted light ( 211 ) has a corresponding spectral transmission response ( 241 ,  FIG. 5   c ). It can be seen from  FIGS. 5   b  and  5   c  that the spectrum of the transmitted light has several spectral transparent peaks ( 242 ,  244  and  246 ) and several spectral loss peaks ( 245 ,  243 ). This is due to interference between the fundamental mode and cladding modes. The first LPFG couples part of the fundamental mode to the cladding modes, and then the coupled cladding modes and fundamental mode travel along the fiber simultaneously to the second LPFG. At the second LPFG, the two modes interact with each other and generate spectral interference fringe patterns. The fringe spacing Δλ PL  is related to the grating length d 1 , d 2 , d and the separation distance L between the two LPFGs. An increase in L corresponds with a decrease in the fringe spacing Δλ PL . For multi-channel filter applications the distance L is typically less than 600 mm. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment of the present invention, a Q-switched fibre laser apparatus comprises at least one reflector, a gain fibre, and a long period fiber grating modulator employed to Q-switch introduced light applied to the reflector and gain medium. 
     In accordance with another embodiment of the invention, a method for producing laser light comprises introducing pump light to at least one reflector, a gain fibre, and a long period fiber grating modulator, and Q-switching the light with the long period fiber grating modulator, whereby a Q-switched fibre laser is provided. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  illustrates the physical configuration of a Q-switched fiber laser. 
         FIGS. 2   a ,  2   b ,  2   c  and  2   d  illustrate the physical configuration and spectral transmission characteristics of a FBG. 
         FIGS. 3   a ,  3   b , and  3   c  illustrate the physical configuration and spectral transmission characteristics of a LPFG. 
         FIGS. 4   a ,  4   b , and  4   c  illustrate the physical configuration and spectral transmission characteristics of a phase shifted LPFG. 
         FIGS. 5   a ,  5   b , and  5   c  illustrate the physical configuration and spectral transmission characteristics of a cascaded LPFG pair. 
         FIGS. 6   a ,  6   b , and  6   c  illustrate a LFPG optical modulator. 
         FIG. 6   d  and  FIG. 6   e  illustrate the spectral transmission behavior of the innovative LPFG modulator. 
         FIG. 7  illustrates use of the LPFG modulator as a component of an all-fiber in-line device such as an all-fiber Q-switched laser. 
         FIGS. 8   a ,  8   b , and  8   c  illustrate an alternative LPFG modulator formed by employing a phase shifted LPFG. 
         FIG. 9   a  illustrates an embodiment of the modulator based on cascaded LPFGs. 
         FIGS. 9   b ,  9   c ,  9   d  and  9   e  illustrate the spectral transmission characteristics of the embodiment of  FIG. 9   a.    
         FIG. 10  illustrates a Q-switched fiber laser employing two LPFG modulators in the fiber laser cavity. 
         FIGS. 11   a ,  11   b , and  11   c  illustrate an LPFG modulator assembly using two LPFGs. 
         FIG. 12  illustrates a Q-switched fiber laser employing a LPFG modulator in which the pump light is coupled into the laser cavity from the middle of the laser cavity. 
         FIG. 13  illustrates a Q-switched fiber laser employing a LPFG modulator in which a ring laser cavity is used and the LPFG modulator is placed outside of the fiber loop. 
         FIG. 14  illustrates a Q-switched fiber laser employing a LPFG modulator in which a ring laser cavity is used and the LPFG modulator is placed inside of the fiber loop. 
         FIG. 15  illustrates a Q-switched fiber laser in which a LPFG modulator is employed in a ring laser cavity and an optical isolator is employed to achieve unidirectional laser oscillation. 
         FIG. 16  illustrates a Q-switched fiber laser having a ring laser cavity in which the pump light is coupled into the laser cavity from the middle of the cavity. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS. 6   a - 6   c , a LFPG optical modulator is provided via the controlled (time, area and force) application of stress to an optical material to introduce refractive index changes in the material in accordance with the photo-elastic effect. As illustrated, a small section ( 253 ) of the LPFG ( 22 ) is subjected to stress ( 203 ) through force applied by an actuator ( 202 ). The stress may be applied by mechanical, acoustic or other means. The actuator ( 202 ) may include a piezo actuator that operates in response to a modulating voltage ( 205 ) from a controller ( 206 ). The applied stress ( 203 ) causes a temporary deformation of the material at section ( 253 ) and a corresponding refractive index change at section ( 253 ). The periodic structure and spectral transmission behavior of the LPFG are changed in a corresponding manner. In particular, the magnitude of the refractive index change is related to the magnitude of applied force, the periodic structure and spectral transmission behavior is related to (a) which areas are subjected to stress and (b) the period and frequency at which stress is applied. 
       FIGS. 6   b  and  6   c  are cross-sectional views of the LFPG of  FIG. 6   a  that illustrate different configurations for applying stress to the fibre ( 20 ). In  FIG. 6   b  the LPFG fiber ( 20 ) is disposed between actuator ( 202 ) and a plate ( 215 ). The fiber can be fixed in place with glue ( 207 ).  FIG. 6   c  shows an alternative embodiment in which a V-groove plate ( 216 ) is employed in lieu of the flat plate ( 215 ,  FIG. 6   b ) for enhanced fiber fixing and enhanced stress distribution. 
       FIG. 6   d  and  FIG. 6   e  illustrate the spectral transmission behavior of the innovative LPFG modulator. The transmission spectrum of the LPFG when no stress is applied is shown by a first section ( 221 ,  FIG. 6   d ), i.e., a narrow band input light ( 122 ) with center wavelength λ L  is blocked since the resonance loss peak ( 222 ) of the LPFG is just at this wavelength. This corresponds to the “switch off” state of the LPFG modulator. The bandwidth of the signal light is narrower than the bandwidth Δλ L  of the LPFG. When stress is applied to section ( 253 ,  FIG. 6   a ), the transmission spectrum is changed as shown in  FIG. 6   e , with the resonance loss peak ( 222 ,  FIG. 6   d ) becoming peak ( 222   a ,  FIG. 6   e ). The narrow band input light ( 122 ) can now pass through the LPFG. This corresponds to the “switch on” state of the LPFG modulator. Thus, the input light ( 122 ) with center wavelength λ L  can be modulated in response to the control signal applied to the actuator. 
       FIG. 7  illustrates use of the LPFG modulator as a component of an all-fiber in-line device such as an all-fiber Q-switched laser. The illustrated laser cavity has a Fabry-Perot configuration and includes a pair of FBG reflectors ( 15 ,  35 ) having the same center wavelength λ B , a gain fiber ( 18 ), and an LPFG modulator ( 201 ). The resonance loss peak λ L  of the LPFG is matched with center wavelength λ B  of the FBGs. The bandwidth Δλ B  of the FBGs is narrower than bandwidth Δλ L  of the LPFG, i.e., Δλ B &lt;&lt;Δλ L . The laser oscillation wavelength is confined by the FBGs at wavelength λ B . The LPFG modulator is employed to switch the Q factor of the laser cavity, i.e., control optical loss in the time domain. Switching is provided in response to a modulating voltage ( 205 ) applied to the actuator by a controller ( 206 ). Pump source ( 1 ) couples pump light ( 5 ) into the laser cavity to pump gain fiber ( 18 ). The LPFG modulator is transparent at the pump wavelength. One or both of the FBG reflectors ( 15 ,  35 ) are partially transparent at its wavelength. Consequently, the laser output ( 38  or  42 ) can be provided from either fiber end ( 37 ) or fibre end ( 9 ), or both fiber ends. 
     An alternative LPFG modulator can be formed by employing a phase shifted LPFG as shown in  FIG. 8   a . In this embodiment the stress ( 203 ) is applied to the phase shift section on the LPFG through actuator ( 202 ). The transmission spectrum of the phase shifted LPFG with and without applied stress is shown in  FIG. 8   b  and  FIG. 8   c . When no stress is applied to the LPFG, a narrow band signal light ( 122 ) can pass through the area ( 232 ) of the phase shifted LPFG, i.e., in the “switch on” state. When stress is applied to area ( 253 ) the LPFG has resonance loss ( 232   a ) at wavelength λ L , i.e., in the “switch off” state. As with the previous embodiment, the bandwidth of the signal light is narrower than bandwidth Δλ L  of the LPFG. 
       FIG. 9   a  illustrates an embodiment of the modulator based on cascaded LPFGs. A pair of LPFGs ( 25 ,  26 ) are disposed in series. Actuator ( 202  or  202   b  or  202   a ) applies stress to the section of LPFG ( 25 ) or LPFG ( 26 ) or on the fiber section ( 227 ) between LPFG ( 25 ) and LPFG ( 26 ). Initially, when no stress is applied, the transmission spectrum is as shown at section ( 241 ) in  FIG. 9   b . The wavelength of the signal light ( 122 ) is matched at the wavelength λ L1 , which is at loss peak ( 243 ) on the spectrum of the cascaded LPFGs. Consequently, the signal light ( 122 ) cannot pass through and the modulator is in the “switch off” state. When the stress is applied at any of points ( 202 ,  202   b  or  202   a ), the transmission spectrum is changed as shown in  FIG. 9   c , where the signal light ( 122 ) can pass through since λ L1  at peak ( 243   a ) is transparent. 
       FIGS. 9   d  and  9   e  illustrate an alternative embodiment in which, when no stress is applied, the signal light ( 122 ) can pass through the cascaded LPFGs since the wavelength of the signal light is set to match λ L2  at ( 244 ,  FIG. 9   d ). When stress is applied, the signal light ( 122 ) is blocked since the spectrum of the cascaded LPFGs is changed as shown in  FIG. 9   e  where the signal light ( 122 ) is at the loss peak ( 244   a ) in the spectrum of the cascaded LPFGs. 
     Generally, any of the LPFG modulators described above can be utilized to provide an all-fibre Q-switched laser.  FIG. 10 , for example, illustrates an alternative embodiment of the Q-switched laser in which two LPFG modulators ( 201   a ,  201   b ) are employed in the fiber laser cavity to enhance switch extinction. Two or more LPFGs can also be packaged together as shown in  FIGS. 11   a ,  11   b  and  11   c . The fibers ( 262 ,  271 ) with LPFGs ( 265 ,  275 ) are sandwiched between actuator ( 202 ) and plate ( 215 ) or V-groove ( 216 ). Again, glue ( 207 ) may be used to protect and fix the fiber. Modulating voltage ( 209 ) is applied to actuator ( 202 ).  FIG. 12  illustrates an embodiment of the Q-switched fiber laser system in which the pump light is coupled into the fiber laser cavity from the middle of the laser cavity. In particular, the pump light ( 5 ) is coupled into laser cavity through pump coupler ( 4 ). The Q-switched fiber laser can also be implemented with ring laser cavity configurations as shown in  FIG. 13 . The laser cavity comprises FBG reflector ( 15 ), LPFG modulator ( 201 ), fiber coupler ( 60 ) and gain fiber ( 18 ). Two arms ( 62 ,  64 ) of the fiber coupler ( 60 ) are spliced with gain fiber ( 18 ) to form a fiber loop. The LPFG modulator ( 201 ) is placed outside of the fiber loop between the FBG ( 15 ) and the fiber coupler ( 60 ). The LPFG modulator is transparent at the pump wavelength, and the resulting laser output comes from the arm ( 63 ) of the fiber coupler.  FIG. 14  illustrates another possible embodiment of the ring fiber laser cavity where the LPFG modulator ( 201 ) is placed inside the fiber loop. Furthermore, the gain fiber can be placed outside of the fiber loop. In this case the fiber loop forms a fiber loop mirror.  FIG. 15  illustrates another alternative embodiment of the ring fiber laser cavity in which an isolator ( 70 ) is placed in the fiber loop in order to achieve unidirectional laser oscillation in the laser cavity. The LPFG modulator ( 201 ) can be placed either in the fiber loop or outside of the fiber loop between the FBG ( 15 ) and the fiber coupler ( 60 ).  FIG. 16  illustrates an embodiment of the LPFG modulator based Q-switched fiber laser having ring laser cavity in which the pump light is coupled into the laser cavity from the middle of the cavity through the pump coupler ( 4 ). In any of the embodiments of the Q-switched fiber laser employing an LPFG modulator, the LPFG modulator may be a simple LPFG based modulator, a phase shifted LPFG based modulator or a cascaded LPFGs based modulator. Further, one or more LPFG modulators may be used in a fiber laser cavity in order to improve switch extinction. 
     While the invention is described through the above exemplary embodiments, it will be understood by those of ordinary skill in the art that modification to and variation of the illustrated embodiments may be made without departing from the inventive concepts herein disclosed. Moreover, while the preferred embodiments are described in connection with various illustrative structures, one skilled in the art will recognize that the system may be embodied using a variety of specific structures. Accordingly, the invention should not be viewed as limited except by the scope and spirit of the appended claims.