Abstract:
An apparatus for adjusting the wavelength of a laser capable of lasing at multiple wavelengths by using a single acousto-optical modulator and a pair of optical reflectors inside a laser cavity. By adjusting the frequency and amplitude of the radio-frequency source to the acousto-optical modulator, undesired wavelengths are suppressed in the laser cavity, leaving appreciable gain only at the desired wavelength.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Many laser gain media are capable of lasing at different wavelengths. By designing a laser cavity in a particular manner, a desired wavelength can be selected from the multiple possible lasing wavelengths by suppressing the gain at undesired wavelengths. Typically, a laser design is fixed for only one wavelength, and once the laser cavity is built, it is not a simple matter to adjust the laser to lase at a different wavelength. 
         [0002]    For example, a simple method for selecting a particular wavelength is to use mirrors that only efficiently reflect the desired wavelength. For instance, dielectrically coated mirrors can be used that reflect only a narrow range of wavelengths. While this method allows one to tune a laser to a single wavelength or a narrow range of wavelengths, the method is not ideal, because it does not allow the operator to adjust or tune the wavelength once the laser has been built. In order to change the operating wavelength of the laser, the entire system must be rebuilt. In addition, if the gain medium supports lasing at wavelengths that are closely spaced, the mirrors may reflect more than the desired wavelength. 
         [0003]    Another option for selecting a single wavelength is to use diffractive optics, such as diffraction gratings within the laser cavity, often in a Littrow configuration. The undesired wavelengths are spatially separated and blocked, thereby introducing substantial losses in the laser cavity at the undesired wavelengths and allowing gain only at the desired wavelength. Lasers using diffractive optics in this manner can be tuned by rotating the diffraction grating relative to the incoming beam. This solution is not without its problems, however. Alignment of the grating can be problematic, and the stability of the system can be lacking because the optical components must physically move to tune the laser. 
         [0004]    This invention provides an electronically-adjustable system for selecting different wavelengths from the same laser cavity configuration without the need to change the laser cavity configuration by using a single acousto-optical device and a pair of optical reflectors inside the laser cavity, thereby creating a stable, wavelength-adjustable laser. 
       SUMMARY OF THE INVENTION 
       [0005]    A wavelength-adjustable laser of the present invention comprises a gain medium for amplifying a laser beam in a wavelength range; a first optical reflector; an acousto-optical modulator located between the gain medium and the first optical reflector such that the acousto-optical modulator diffracts a portion of the laser beam into a plurality of diffracted laser beams wherein each diffracted laser beam diffracts at a different angle from an optical axis formed by the laser beam based on the wavelength of the diffracted laser beam; a second optical reflector and a third optical reflector positioned about the optical axis such that only a selected diffracted laser beam from the plurality of diffracted laser beams is reflected off both the second optical reflector and the third optical reflector into the acousto-optical modulator; and an adjustable radio-frequency source coupled to the acousto-optical modulator wherein the wavelength of the selected diffracted laser beam is changed by adjusting the frequency of a radio signal emitted from the adjustable radio-frequency source. 
         [0006]    A method for adjusting the wavelength of a laser of the present invention comprises the steps of positioning an acousto-optical modulator between a gain medium and a first optical reflector; coupling an adjustable radio-frequency source to the acousto-optical modulator; and positioning a second optical reflector and a third optical reflector about an optical axis formed by an undiffracted laser beam coming from the acousto-optical modulator. 
         [0007]    The features, functions, and advantages can be achieved independently in various embodiments of the present inventions or may be combined in yet other embodiments. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a schematic representation of a preferred form of the invention. 
           [0009]      FIG. 2  is a schematic representation of an acousto-optical modulator. 
           [0010]      FIG. 3  is a schematic representation of the laser passing through the acousto-optical modulator. 
           [0011]      FIG. 4  is a schematic representation of the laser passing through the acousto-optical modulator. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    The present invention, as shown in  FIG. 1 , uses a single acousto-optical modulator  10  inside a laser cavity  12 , along with two additional optical reflectors  14  and  16 , to cause the laser cavity  12  to lase at a one of a number of possible wavelengths. The laser cavity  12  consists of a partially-reflecting mirror (or output coupler)  18 , a gain medium  20  (such as an Argon-ion gas cell), and a high-reflecting mirror  22 . Alternatively, another mirror, such as mirror  22 , could act as the output coupler. The type of gain medium is not critical to this invention, and any of a number of common gain media can be used, such as dyes, gas cells, solid state crystals, glass, chemicals, or semiconductors. In the example of an Argon-ion gas cell gain medium, the gain medium can support lasing at a number of different wavelengths: 351 nm, 454.6 nm, 457.9 nm, 465.8 nm, 476.5 nm, 488.0 nm, 496.5 nm, 501.7 nm, 514.5 nm, and 528.7 nm. 
         [0013]    In order to select one desired wavelength and suppress any undesired ones, the acousto-optical modulator  10  (for example, a Bragg cell) and two highly-reflecting mirrors  14  and  16  are inserted inside laser cavity  12 . Bragg cell  10  is coupled to a radio-frequency source  24  that emits a radio signal whose frequency and amplitude can be varied. Such Bragg cells are common optical components that are available from a variety of sources. Bragg cells are comprised of a solid material, such as a crystal, quartz, or glass, with an piezoelectric transducer located on one end. A radio-frequency electrical signal, whose frequency and amplitude can be adjusted, drives the piezoelectric transducer, thereby creating a traveling acoustic wave inside the Bragg material. This acoustic wave creates regions of higher- and lower indices of refraction inside the Bragg material, which can diffract light according to the Bragg equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     sin 
                      
                     
                         
                     
                      
                     θ 
                   
                   = 
                   
                     
                       m 
                        
                       
                           
                       
                        
                       λ 
                     
                     Λ 
                   
                 
               
               
                 
                   Eq. 1 
                 
               
             
           
         
       
     
         [0000]    where θ is the angle that the diffracted beam emerges from the Bragg cell with respect to the undiffracted beam, λ is the wavelength of the laser beam, Λ is the acoustic wavelength inside the Bragg cell, and m is the integral order of diffraction (−2, −1, 0, 1, 2, etc.), as shown in  FIG. 2 . Thus, the diffracted angle of the laser beam passing through the Bragg cell depends on the ratio of the wavelength of the laser beam A relative to the wavelength of the acoustic wave A in the Bragg cell. 
         [0014]    In addition to being diffracted, the laser beam will experience a frequency (or wavelength) shift in the amount of the frequency of the acoustic wave in the Bragg cell: 
         [0000]      ν out =ν in   +mν   acous    Eq. 2 
         [0000]    where ν out  is the frequency-shifted frequency of the laser beam, ν in  is the original, unshifted frequency of the laser beam, m is the integral order of diffraction, and ν acous  is the acoustic frequency of the acoustic wave in the Bragg cell (ν acous =V acous /Λ, where V acous  is the speed of sound in the Bragg material). Because the frequency of the laser beam is directly related to its wavelength (c=λν, where c is the speed of light), the laser beam also shifts in wavelength: 
         [0000]    
       
         
           
             
               
                 
                   
                     Δ 
                      
                     
                         
                     
                      
                     λ 
                   
                   = 
                   
                     
                       
                         λ 
                         out 
                       
                       - 
                       
                         λ 
                         in 
                       
                     
                     = 
                     
                       
                         λ 
                         in 
                       
                       ( 
                       
                         
                           1 
                           
                             1 
                             + 
                             
                               
                                 m 
                                  
                                 
                                     
                                 
                                  
                                 
                                   λ 
                                   in 
                                 
                                  
                                 
                                   v 
                                   acous 
                                 
                               
                               c 
                             
                           
                         
                         - 
                         1 
                       
                       ) 
                     
                   
                 
               
               
                 
                   Eq. 3 
                 
               
             
           
         
       
     
         [0000]    where λ out  is the wavelength-shifted wavelength of the laser beam, λ in  is the original, unshifted wavelength of the laser beam, m is the integral order of diffraction, ν acous  is the acoustic frequency of the acoustic wave in the Bragg cell, and c is the speed-of light. 
         [0015]    Thus, for the positive first-order diffracted beam (m=1), the outgoing laser beam will be frequency upshifted by the acoustic frequency (ν acous ) and wavelength downshifted by the amount in Equation 3. Similarly, for the negative first-order diffracted beam (m=−1), the outgoing laser beam will be frequency downshifted by the acoustic frequency (ν acous ) and wavelength upshifted by the amount in Equation 3. Notably, the zero-order undiffracted beam (m=0) will not experience a frequency or wavelength shift. 
         [0016]    In order to avoid a continual frequency (or wavelength) shift as the laser beam passes through the Bragg cell, the positive first-order diffracted beam can be returned through the Bragg cell along the path of the negative first-order diffracted beam, thereby exactly canceling the frequency (wavelength) shift. The laser will be frequency upshifted by ν acous  and then frequency downshifted by ν acous , for a zero net frequency shift. 
         [0017]    Thus, in the preferred embodiment, the invention comprises adding a Bragg cell  10  with two highly-reflecting mirrors  14  and  16  inside the laser cavity  12 . In order to preserve the symmetry and efficiency of the Bragg cell, the Bragg cell  10  is placed with its face nominally perpendicular to the incoming laser beam  26  from the gain medium (from the left in  FIG. 1 ). As the laser beam passes through the Bragg cell  10 , the positive first-order diffracted beam  28  (m=1) diffracts in one angle away (towards the top in  FIG. 1 ) from the undiffracted beam  30  (m=0), while the negative first-order diffracted beam  32  (m=−1) diffracts in the same angle but in the opposite direction (towards the bottom in  FIG. 1 ). The positive first-order diffracted beam will be frequency upshifted by the acoustic frequency in the Bragg cell, while the negative first-order diffracted beam will be frequency downshifted by the acoustic frequency. Whether the positive first-order beam emerges to the left or to the right of the undiffracted beam is not important for purposes of this invention. 
         [0018]    The two highly-reflecting mirror  14  and  16  are placed symmetrically on either side of the undiffracted beam  30  such that the positive and negative first-order diffracted beams  28  and  32  will reflect onto each other and pass back through the Bragg cell  10  at the same angle they emerged from the Bragg cell  10 . In this way, the optical path lengths for the positive and negative first-order diffracted beams  28  and  32  are identical and each frequency shift will be exactly canceled as the beams pass back through the Bragg cell  10 . 
         [0019]    By adjusting the frequency of the radio-frequency source  24  coupled to the Bragg cell  10 , the angle of the first-order diffracted beams for a particular wavelength can be adjusted according to Equation 1, above. Mirrors  14  and  16  can be placed so that only the desired wavelength falls on them in such a way as to be reflected back into the Bragg cell  10  to form a closed optical path with the laser cavity  12 . Undesired wavelengths will be diffracted at an angle such that they will entirely miss mirrors  14  and  16  or will strike mirrors  14  and  16  in such a way as not to be reflected back through Bragg cell  10 , or will reflect back through Bragg cell  10  at an angle that deviates from the optical axis of the laser cavity  12 . For example, in  FIG. 3 , the frequency of the radio-frequency source  24  to the Bragg cell  10  is adjusted such that desired wavelength is diffracted onto the middle of mirrors  14  and  16  (laser beams  28  and  32 ), while the undesired wavelengths are diffracted entirely off mirrors  14  and  16  (laser beams  34 ). The laser will only lase at those wavelengths whose gain for one round trip through the laser cavity  12  is equal to or greater than the round trip losses for that wavelength. In this way, the Bragg cell  10  and mirrors  14  and  16  introduce significant loss in the laser cavity  12  at undesired wavelengths, while introducing little or no loss at the desired wavelength. Alternatively, mirrors  14  and  16  can be positioned with additional mirrors or optical elements to accomplish the same effect of only allowing the desired wavelength to return through the Bragg cell  10  in a closed optical path within laser cavity  12 . 
         [0020]    Because the Bragg cell  10  is not perfectly efficient, some of the laser beam will not be diffracted and will remain in the zero-order. Thus, mirror  22  must remain in the laser cavity  12  to reflect the undiffracted laser beam  30  back into Bragg cell  10  to avoid losing light at the desired wavelength. 
         [0021]    In addition, because the Bragg cell  10  is not perfectly efficient, when the positive and negative first-order diffracted beams  28  and  32  return through the Bragg cell  10 , some of these diffracted beams will not be diffracted again, but will pass straight through the Bragg cell  10  in an undiffracted order (beams  36  and  38 ), as shown in  FIG. 4 . Because the undiffracted beams  36  and  38  leave the laser cavity  12 , they will introduce loss in the laser cavity  12  at the desired wavelength. In order to minimize this unwanted loss, the position of mirror  22  can be adjusted so that returning laser beam  40  to the Bragg cell  10  from mirror  22  will be exactly out of phase (and, thus, destructively interfere) with the laser beams returning from mirrors  28  and  32  to Bragg cell  10 , thereby reducing the intensity of the undiffracted beams  36  and  38 . Adjusting the amplitude of the radio-frequency source  24  coupled to the Bragg cell  10  adjusts the efficiency of the diffraction in the Bragg cell  10 , and, therefore, the intensity of the diffracted laser beams  28  and  32 . For a particular laser system, the amplitude of the radio-frequency source  24  should be adjusted so as to create sufficient loss in the undesired wavelengths to reduce or suppress the total gain at these wavelengths, while keeping the loss in the desired wavelength low enough that the laser beam at the desired wavelength experiences a net gain in the laser system. In this way, the desired wavelength will experience the most gain, and the laser will primarily lase at the desired wavelength. 
         [0022]    By positioning mirrors  14 ,  16 , and  22  in such a way that the optical path length from Bragg cell  10  to mirror  14  to mirror  16  back to Bragg cell  10  equals the optical path length from Bragg cell  10  to mirror  22  back to Bragg cell  10 , the wavelength of the laser can be changed by simply adjusting the frequency of the radio-frequency source  24 . Alternatively, if the optical path length from Bragg cell  10  to mirror  14  to mirror  16  back to Bragg cell  10  does not equal the optical path length from Bragg cell  10  to mirror  22  back to Bragg cell  10 , the wavelength of the laser can be adjusted to only certain, supported wavelengths in the laser cavity  12 . 
         [0023]    In another embodiment of the invention, two wavelengths can be selected and amplified within the laser cavity  12  by adjusting the radio-frequency source  24  to be comprised of a combination of two different radio-frequencies signals. This combination of radio-frequency signals is then coupled to the Bragg cell  10 , such that a combination of traveling acoustic waves are created in the Bragg cell  10 . The first radio-frequency signal is adjusted such that one selected wavelength is diffracted by the Bragg cell  10  onto mirrors  14  and  16 , as described above. The second radio-frequency signal is adjusted such that a second selected wavelength is diffracted by the Bragg cell  10  onto the same mirrors  14  and  16  and back into the Bragg cell  10 . In this configuration, only the two desired wavelengths will be diffracted in such a way as to form a closed optical path in laser cavity  12 . Because part of the laser at the second selected wavelength will be diffracted out of the laser cavity  12  by the first radio-frequency signal component of the traveling acoustic wave in the Bragg cell  10  and part of the laser at the first selected wavelength will be diffracted out of the laser cavity  12  by the second radio-frequency signal component of the traveling acoustic wave in the Bragg cell  10 , the amplitude of both radio-frequency signals must be adjusted so as to allow net gain for both desired wavelengths in laser cavity  12  and net loss for every other undesired wavelength. 
         [0024]    Because there is no need to physically move any components inside the laser cavity, the laser should be extremely stable, being insensitive to vibration and thermal effects, as well as being quickly and easily adjustable between different wavelengths.