Abstract:
Wavelength stability of an optical oscillator has been enhanced by feedback from an external position-sensing detector to control the position or tilt of an intracavity optical element, such as a mirror. The wavelength stability results from stabilization of the intracavity beam position relative to an aperture in the oscillator. The wavelength selectivity of the aperture results from incorporation of a dispersive element in the oscillator cavity that produces a mapping of wavelength to beam position at the aperture.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Ser. No. 60/331,967, filed Nov. 20, 2001, which application is fully incorporated herein by reference. 

   BACKGROUND 
   1. Field of the Invention 
   This invention relates generally to optical oscillators, and more particularly to optical oscillators where the intracavity beam is maintained at a fixed position within an intracavity optical system to maintain wavelength stability of the output beam. 
   2. Description of the Related Art 
   Accordingly, what is needed is a system and method for providing a cost-effective-wavelength locker to stabilize the wavelength of an optical oscillator, such as a tunable laser or optical parametric oscillator. The wavelength locker produced should be reliable and stable over a range of temperatures. 
   One way of accomplishing wavelength stabilization is by use of feedback from a reference to control a wavelength-tuning element inside the optical oscillator. This tuning may be accomplished by temperature control of a gain medium, by adjustment of temperature or angular tilt or spacing of an intracavity etalon, by adjusting the angle of a prism, a grating, a mirror, or a birefringent filter, by adjusting the position of a coated tuning wedge to control the spacing of the equivalent etalon seen by the cavity, or by adjustment of the cavity length, or other suitable means. All of these approaches can be used in combination with feedback from an external reference spectrometer, an external reference interferometer or etalon, an atomic or molecular absorption line, or other suitable means. In this invention, the wavelength reference is already built into the optical oscillator, and stabilization of the wavelength is achieved by stabilization of the beam path relative to the internal reference. This internal reference includes wavelength dispersive optical elements and a slit, components that could also be used to create an external spectrometer. 
   Another way to accomplish wavelength stabilization is to injection-seed the oscillator with a beam from another oscillator that operates with a stable wavelength. This is commonly done in pulsed lasers to achieve narrow linewidth wavelength stabilized operation through injection seeding by a low power cw laser beam. The laser pulse is initiated by amplification of the narrow band light provided by the seed laser rather than being initiated by spontaneous emission within the gain medium. U.S. Pat. No. 4,955,027 (Wavelength Locked Laser Light Source) by Piper at el, describes a system in which laser output wavelength stability is enhanced through such an injection-seeding process. 
   In U.S. Pat. No. 5,809,048 (Wavelength Stabilized Light Source) by Shichijyo et al, a means of providing wavelength filtered optical feedback (using a birefringent Lyot filter) to a semiconductor laser is described as producing improved wavelength stability. This is an example of direct feedback from an external wavelength sensitive optical device to lock the wavelength of the oscillator, a type of injection seeding in which the injected signal is derived from the filtered output of the laser to be controlled. 
   In U.S. Pat. No. 4,583,228 (Frequency Stabilization of Lasers) by Brown et al, the wavelength stabilization of a semiconductor laser is based on a feedback signals derived from an external Fabry-Perot interferometer that were used to control both the drive current and the laser temperature. This is an example of electronic feedback that is derived from an external wavelength-sensing optical device. Electonic control is applied to critical laser parameters to control the wavelength. 
   In U.S. Pat. No. 6,393,037 (Wavelength Selector for Laser with Adjustable Angular Dispersion) by Basting et al, the wavelength and linewidth of a laser are controlled by use of signals generated in a linewidth and wavelength-monitoring unit, which samples the laser output beam. Control is provided through use of a signal processor that can direct prisms to rotate within the laser to change refraction angles to change laser wavelength and linewidth. This has some similarity to the invention described here in that both rely on stabilization or control by means of movement of an optical element. 
   In U.S. Pat. No. 5,017,806 (Broadly Tunable High Repetition Rate Femtosecond Optical Parametric Oscillator) by Edelstein et al, a synchronously pumped OPO (optical parametric oscillator) that can produce femtosecond light pulses is described. The output wavelength of the OPO is held stable by a feedback loop that controls the length of the OPO cavity. The feedback is derived from detectors that monitor the direction of shift of the output spectrum. Because the performance of such an OPO also depends on the wavelength stability of the mode-locked pump laser, it can clearly benefit from application of the present invention to stability of the pump laser wavelength. 
   In U.S. Pat. No. 4,932,030 (Frequency Stabilization of Long Wavelength Semiconductor Laser via Optogalvanic Effect) by Chung, wavelength stabilization is achieved by locking the output wavelength of a laser to a transition in an atomic absorber excited in an electrical discharge. In this case, the feedback loop used to control the laser was responding to an optogalvanic signal derived by means of dithering lock-in techniques. 
   The present invention differs from the techniques mentioned above in that the wavelength reference is located within the oscillator rather than externally, and the wavelength stability derives from feedback from an external position-sensing detector to maintain the intracavity beam position relative to the reference. 
   A convenient application of the present invention is in an ultra-short-pulse laser that already utilizes prisms to provide dispersion compensation that is necessary for production of the ultra short pulses, especially for pulses shorter than one picosecond in duration. In this case the prism sequence that is used for dispersion compensation can also be used as the wavelength reference needed for the stabilization of the output wavelength. Pulses as short as 100 femtoseconds in duration can only be produced if the laser spectrum has more than 6 nm of bandwidth (the full width at half maximum of the spectral spread of the laser output beam). In this case, the output wavelength can be defined as the wavelength halfway between the half-maximum wavelengths, and that is the wavelength that is kept stable against environmental changes by application of the invention. 
   Such a wavelength stabilized laser system would have applications to pumping of an optical parametric oscillator (angle-tuned, temperature-tuned, or wavelength-tuned), to creation of harmonics in non-linear crystals by processes that have phase-matching sensitivity (angle-tuned or temperature-tuned, for example), and to seeding of amplifier systems such as regenerative chirp-pulse amplifiers, for which stability of the amplified pulse is critically dependent on stability of the seed pulse wavelength. In addition, such lasers have applications to scanning microscopy systems for which enhanced wavelength stability is desirable for examination of samples that have wavelength sensitivity. 
   SUMMARY 
   Accordingly, an object of the present invention is to provide an optical oscillator system that has improved wavelength stability of the output beam. 
   Another object of the present invention is to provide an optical oscillator system where the intracavity beam is maintained at a fixed position to maintain wavelength stability of the output beam. 
   These and other objects of the present invention are achieved in an optical oscillator system with an end mirror and an output coupler that define a resonator cavity for an intracavity beam. The resonator cavity produces an output beam with selected spectral components. A gain medium is positioned in the resonator cavity. An aperture member is positioned in the resonator cavity in a path of the intracavity beam. The aperture member defines an aperture that provides a low loss intracavity beam path for a range of spectral components. A dispersion member with first and second sides is positioned in the resonator cavity. When the intracavity beam travels from the first side to the second side the dispersion member creates a spatial spread process of the range of spectral components. From the second side to the first side the process is reversed. A movably mounted mirror is included. In response to a feedback signal the movably mounted mirror maintains the output beam at a same position at the output coupler. 
   In another embodiment of the present invention, an end mirror and an output coupler define a resonator cavity for an intracavity beam that produces an output beam with selected spectral components. A gain medium is positioned in the resonator cavity. An aperture member is positioned in the resonator cavity in a path of the intracavity beam. The aperture member defines an aperture that provides a low loss intracavity beam path for a range of spectral components. A first prism pair has first and second sides and is positioned between the aperture member and the output coupler. When the intracavity beam travels from the first side to the second side the first prism pair creates a spatial spread process of the range of spectral components. From the second side to the first side the process is reversed. A movably mounted mirror is included. In response to a feedback signal the movably mounted mirror maintains the output beam at a same position at the output coupler. 
   In another embodiment of the present invention, an optical oscillator system includes an end mirror and an output coupler that define a resonator cavity for an intracavity beam. The resonator cavity produces an output beam with selected spectral components. A gain medium is positioned in the resonator cavity. A first prism pair has first and second sides and is positioned in the resonator cavity. A second prism pair has first and second sides and is positioned between the first prism pair and the output coupler. An aperture member is positioned between the first and second prism pairs in a path of the intracavity beam. The aperture member defines an aperture to create a path for the output beam. A movably mounted mirror is included. In response to a feedback signal the movably mounted mirror maintains the output beam at a same position at the output coupler. When the intracavity beam travels from the first side to the second side of the second prism pair, the second prism pair creates a spatial spread process of the spectral components. When traveling from the first side to the second side of the first prism pair, the first prism pair reverses the process. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of one embodiment of an optical oscillator system of the present invention that includes a dispersion device. 
       FIG. 2  is a schematic diagram of another embodiment of an optical oscillator system of the present invention with a prism pair. 
       FIG. 3  is a schematic diagram of another embodiment of an optical oscillator system of the present invention with two prism pairs. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In various embodiments, the present invention is an optical oscillator system, including but not limited to a laser system, build up cavity, OPO, amplifier system and the like. Examples of laser systems include but are not limited to Ti:sapphire lasers, and the like. 
   Referring to  FIG. 1 , one embodiment of an optical oscillator system  10  of the present invention includes an end mirror  12  and an output coupler  14  that generally define a resonator cavity  16 . Output coupler  14  can be curved or flat. Resonator cavity  16  produces an output beam with selected spectral components. 
   A gain medium  18  is positioned in resonator cavity  16 . A dispersion member  20  is positioned in resonator cavity  16 . Dispersion member  20  creates a spread of spectral components of the intracavity beam in a lateral direction. Dispersion member  20  can be a variety of optical elements including but not limited to a grating pair, and the like. 
   An aperture member  26  is positioned in resonator cavity  16  in a path of the intracavity beam. Aperture member  26  defines an aperture that provides a low loss intracavity beam path for a range of spectral components. At first position  22 , the range of spectral components of the intracavity beam follows a single beam path. When the intracavity beam travels from position  22  to position  24 , dispersion member  20  creates a spatial spread of the range of spectral components. When the intracavity beam travels from position  24  to position  22 , the reverse process occurs. 
   A movably mounted mirror  28  is provided. In response to feedback, a signal, movably mounted mirror  28  maintains the output beam at a same position at output coupler  14 . Movably mounted mirror  28  can be rotatably mounted. A variety of different mechanisms can be used to mount mounted mirror  28  including but not limited to the use of a piezoelectric device, and the like. Movably mounted mirror  28  holds the intracavity beam at a fixed position relative to the aperture to maintain a stable wavelength of the output beam. Movably mounted mirror  28  can be positioned between the aperture member  26  and end mirror  12 . 
   Aperture member  26  blocks non-selected spectral components of the intracavity beam that are incident on gain medium  18 . Aperture member  26  has an aperture that passes the selected spectral components that are reflected from end mirror  12 , and oscillate in resonator cavity  16 . The non-selected spectral components do not pass through the aperture and do not oscillate in resonator cavity  16 . 
   A beam splitter  30 , or other suitable device, can be positioned at an exterior of resonator cavity  16  along a beam path  32  of the output beam, and creates first and second beams  34  and  36 . A detector  38  is positioned along a beam path of beam  34 . In response to the detection of beam  34 , detector  38  produces a feedback signal  39  for movably mounted mirror  28 . A variety of different detectors  38  can be utilized including but not limited to a position sensitive detector such as a quad detector, bi-cell detector, and the like. 
   Oscillator system  10  can also include a non-linear device  40  including but not limited to a frequency doubler. Additional fold mirrors and other optical components can be included, as illustrated in FIG.  1 . 
   With reference now to  FIG. 2 , another embodiment of the present invention is an optical oscillator system  110  with an end mirror  112  and an output coupler  114  that define a resonator cavity  116  for an intracavity beam that produces an output beam  118  of selected spectral components. A gain medium  120  is positioned in resonator cavity  116 . An aperture member  118  is positioned in resonator cavity  118  in a path of the intracavity beam. Aperture member  118  has an aperture that provides a low loss intracavity beam path for a range of spectral components. A first prism pair  122  is positioned between aperture member  118  and output coupler  114 . A movably mounted mirror  124  is provided. In response to a feedback signal  123 , movably mounted mirror  124  maintains the output beam at a same position at output coupler  114 . 
   At a first position  126 , the range of spectral components of the intracavity beam follows a single beam path. When the intracavity beam travels from position  126  to position  128 , first prism pair  122  creates a spatial spread of the range of spectral components. When the intracavity beam travels from position  128  to position  126 , the reverse process occurs. Oscillator system  110  can include a retro-reflector  130 , or suitable optical device. 
   A beam splitter  132  and a detector  134  are positioned at the exterior of resonator cavity  116 . Again, beam splitter  132  splits the output beam  135  into beams  136  and  138 . Detector  134  is positioned to along a path of beam  136 . In response to beam  136 , detector  132  produces the feedback signal  123  to movably mounted mirror  124 . A non-linear device  138 , including but not limited to a frequency doublers, can be included in optical oscillator system  110 . Oscillator system  110  can include additional optical components. 
   In another embodiment of the present invention, illustrated in  FIG. 3 , an optical oscillator system  210  includes an end mirror  212  and an output coupler  214  that define a resonator cavity  216  for an intracavity beam. Resonator cavity  216  produces an output beam  218  with selected spectral components. A gain medium  220  is positioned in resonator cavity  216 . A first prism pair  222  is positioned in resonator cavity  216 . A second prism pair  224  is positioned between first prism pair  222  and output coupler  214 . An aperture member  226  is positioned between first and second prism pairs  222  and  224  in a path  228  of the intracavity beam. Aperture member  226  defines an aperture that provides a low loss intracavity beam path for a range of spectral components. A movably mounted mirror  230  is provided. In response to a feedback signal  231 , movably mounted mirror  230  maintains output beam  218  at a same position at output coupler  214 . First prism pair  222  has first and second sides  230  and  232 , and second prism pair  224  has first and second sides  236  and  238  respectively. 
   When the intracavity beam travels from first side  236  to second side  238 , second prism pair  224  creates a spatial spread of the spectral components. When traveling from first side  230  to second side  232 , first prism pair  222  reverses the process. A retro reflector  239 , or other suitable optical device, can be included. 
   A beam splitter  240  and a detector  242  are positioned at the exterior of resonator cavity  216 . Beam splitter  240  and detector  242  provide the some functions as beam splitters  30 ,  132  and detectors  38 ,  134 , respectively. A non-linear device  244  can be included. Oscillator system  210  can include additional optical elements. 
   Gain medium  18 ,  120  and  220  can be made of a variety of different materials including but not limited to, Nd:YVO 4 , Nd:YAG, Nd:YLF, Nd:Glass, Ti:sapphire, Cr:YAG, Cr:Forsterite, Yb:YAG, Yb:KGW, Yb:KYW, Yb:glass, KYbW, YbAG and the like. In one embodiment, the preferred gain medium is Nd:YVO 4  with a doping level of less than 0.5%. 
   While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiment, but on the contrary it is intended to cover various modifications and equivalent arrangement included within the spirit and scope of the claims which follow.