Abstract:
Feedback from a power monitor sampling a portion of the output beam of an optical resonator is used to control the position of a pump beam relative to a second laser. The pump beam position or orientation is adjusted in response to a dither signal imposed on the position or tilt of an external optic or mirror in order to maximize the efficiency of the second laser in converting pump power to output power. Feedback based on the response of the power monitor is used to control the position or tilt of the mirror or optic to which the dither was applied.

Description:
CROSS-REFERENCE TO APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Ser. No. 60/331,967, filed Nov. 20, 2001, which application is fully incorporated by reference herein. 
     
    
     
       BACKGROUND  
         [0002]    1 . Field of the Invention  
           [0003]    The present invention is directed to an optical system with a cavity pumped by a pump source, and more particularly to an optical system where an efficiency of the cavity is maximized by adjusting a position of the pump beam relative to the cavity.  
           [0004]    2. Description of the Related Art  
           [0005]    In recent years, medical and industrial applications using laser systems have proliferated. As the lasers have become more reliable and commonplace, there has also been a greater emphasis on improving control of the laser parameters in order to improve outcomes in practical settings. Providing the required controls presents a greater challenge as increasingly complex laser systems are being introduced into applications which place stringent demands on performance and operating lifetime even though the preferred devices are required to be more compact and cost effective. Ultra-fast lasers, build-up cavities involving resonant frequency doubling, systems including optical parametric conversion devices and multiple harmonic modules and high power fiber lasers are all examples of complex laser systems requiring sophisticated controls to perform their intended functions.  
           [0006]    Ultrashort pulse lasers have, in particular, been promoted as an effective new tool for a variety of medical and industrial applications, and especially where small interactions zones, fine feature sizes and limited collateral damage are considered highly beneficial. Examples include metrology measurements, two-photon microscopy, material processing, stereolithography and corneal sculpting procedures. In the case of material processing applications ultrafast lasers exploit localized laser induced breakdown mechanisms to provide submicron processing capability. Some applications exploit the ability of ultrafast lasers to ablate surface regions that are even smaller than their minimum, diffraction-limited, spot size. Many micro-machining, inscription, and hole drilling procedures have been proposed that take advantage of the high degree of precision provided by ultrafast interactions. Examples include drilling holes with sub-wavelength pitch to produce photonic crystals as described in U.S. Pat. No. 6,433,305, removal of biological and other types of material while incurrring minimal collateral damage and attaining greatly increased cut quality, as taught in U.S. Pat. No. 5,720,894, precise surface ablation in either opaque or transparent materials as described in U.S. Pat. Nos. 5,656,186 and 6,333,485, and inscription of micro patterns in various materials.  
           [0007]    Note is taken of the fact that the efficacy of micro-machining procedures carried out with ultrashort pulse lasers depends in a large measure on the precision of controls provided by the system of the key output laser parameters including power, pulse energy and/or pulse width. In particular, controlling and stabilizing the output power are essential to the precision with which micro-holes can be drilled, micro-patterns can be inscribed or clean repeatable cuts can be made. Procedure repeatability and high throughputs are especially important considerations for virtually all industrial, biological and surgical applications which contemplate the use of ultrafast lasers.  
           [0008]    Another especially good example of an application requiring a high degree of control is provided by emerging metrology applications such as the ultrasonic short pulse technique successfully developed into a semiconductor inspection tool. The technique, described in U.S. Pat. Nos. 5,959,735 (Optical stress generator and detector) and 5,748,317 (Apparatus and method for characterizing thin film and interfaces using an optical heat generator and detector), both by Maris et al, uses femtosecond laser pulses to produce ultrasonic echoes which are analysed to derive the thickness of single or multi-layer metal films used in integrated circuit manufacturing. With metal layers ranging from under 20Å to over 5 μm, high precisions with better than 1-2% repeatability are required along with high throughput rates. Precise control of key laser parameters is therefore essential for this application. In particular, variations in power can contribute to nonuniformities in thickness measurements which can compromise the measurements.  
           [0009]    In many of foregoing applications, it is required that the laser be capable of hands-off reliable operation for prolonged periods of time in an industrial or medical setting. At the same time during the time the output laser beam is coupled to a work piece, the laser must provide power levels and other operational characteristics that are as constant as possible and be free of long term drift or unpredictable power instabilities. Generally, it is known that uncontrolled fluctuations in power or other laser parameters such as the pulse width, wavelength or beam divergence lower the accuracy of the laser interactions with a target material and compromise the system performance. Whereas methods of stabilizing operating laser parameters are known in the art, many such techniques require numerous additional components and are too complex to implement in an industrial setting especially where reliable throughputs and space considerations are paramount. It is therefore highly desirable to provide a laser system with improved reliability and stabilized output control features on a fine scale using the most expedient and cost effective means.  
           [0010]    Typically, the more complex laser systems that are the subject of the present invention comprise at least two or more key subsystems, each of which may be a laser cavity or optical system. In this case changing parameters of an output beam which is the one delivered to the target requires controlling an existing input system or subsystem with its own fully designed control electronics and drivers.  
           [0011]    For example, the pump laser may comprise a commercially designed diode pumped green laser used to drive a tunable IR laser such as a Ti:sapphire laser designed to provide ultrashort pulses. Alternatively the tunable laser may comprise an optical parametric converter or a Raman shifter to provide a fixed set of wavelengths. In still other examples the optical system may include build up cavities for resonant harmonic conversion or an injection seeded amplifier in a MOPA configuration.  
           [0012]    In all of these cases, controlling and adjusting the output power of a laser consisting of one or more complex subsystems can be a major issue.  
           [0013]    There is therefore a need for techniques that can provide a high degree of control of selected properties of the output from optical systems that may include one or more laser subsystems. There is a particular need for cost effective techniques that provide a means of compensating for misalignment that can degrade system performance in an industrial environment. There is further need to be able to make these adjustments in a way that enhances system reliability and extends system lifetime in medical and industrial applications. This invention not only provides an important tool for meeting these criteria, but also makes possible extended operation of a complex optical system without need for frequent maintenance. The system can remain enclosed for greatly extended periods of operation, meaning that it is less likely to be adversely affected by an industrial environment.  
           [0014]    Control of a pump beam into a second laser is the subject of U.S. Pat. No. 4,514,849 (Dye Laser with Rotating Wedge Alignment Servo), by Witte et al. They describe a servo system in which a rotating wedge is used to produce a movement of a pump beam into a second laser, with the movement defining a conical surface. The resultant modulation of the power of the second laser is used to create a feedback control to a motor-driven mirror to direct the beam to a spot in the second laser that maximizes the output power. The use of the rotating wedge approach has several disadvantages. The wedge is fixed, such that the amplitude of the dither cannot be adjusted. The final alignment can only be an average of the positions of the pump beam as it traces a circular path in the gain medium while it is driven by the feedback loop toward the position that produces maximum power. As a result, the pump beam can never pass through the position of best alignment while the dither is in process, because the best it can do is to continue to circle it. Witte et al used the error signal induced by rotating the wedge to control the angular tilt of a motor-driven mirror. This method requires that at least two optical elements must undergo mechanical motion. The Witte system neither teaches nor suggests dithering the positioning mirror to be aligned, which would reduce the number of moving optical elements to one.  
           [0015]    In U.S. Pat. No. 5,033,061 (Laser alignment servo method and apparatus), Hobart et al apply the concept of dithering the angular alignment of an intracavity mirror to optimizing the performance of the laser using feedback to adjust the alignment of the same mirror. In this case the dithered optic is part of the laser for which power is being maximized rather than being an external optic that is optimizing the position or orientation of a pump beam. There will always be some resultant modulation of the output power, which means that there will be some induced noise at the dither frequency. One way to minimize the noise contribution is to use the error signal to maximize pumping efficiency while holding the output power fixed.  
           [0016]    This invention utilizes movement of a mirror or other suitable optical element external to the second laser to achieve efficient pumping of the second laser by using feedback from an external power monitor that samples a portion of the output beam from the second laser. In this case a dither is applied to the mirror or optic for which alignment is being adjusted. The dither can be applied in each of two orthogonal directions and the amplitude of the dither can be adjusted electronically to minimize the introduction of noise in the output of the pumped laser. The movement of the optic as well as the applied dither motion can be made using a variety of transducers, such as piezoelectric devices, stepper motors, DC motors, and electromagnetic transducers. The movement of the optic in response to the feedback can be made using the same transducers that apply the dither motion or by a different transducer. Movement of the optic along orthogonal directions can be made by using the same or different transducers. Furthermore, the dither motion applied to the optic along orthogonal directions can be made with the same or different transducers. Simplicity is best served by using the same transducer type for all movements of the optic, such as an arrangement of two or more piezoelectric stacks that can supply many microns of motion with an applied voltage of below one hundred volts.  
           [0017]    Use of this invention provides advantages for system stability against misalignment that might result from changes of environmental conditions such as temperature changes or from drifts in alignment of optical components. In addition, changes of beam pointing in the pump laser that might result from thermal effects can be compensated by the automated adjustment of the controlled mirror or optic. As a result, use of the technique results in extended reliability and lifetime enhancement for the second laser.  
         SUMMARY  
         [0018]    Accordingly, an object of the present invention is to provide an improved optical system that includes a cavity pumped by a pump source.  
           [0019]    Another object of the present invention is to provide an optical system with a cavity pumped by a pump source with improved efficiency of the cavity.  
           [0020]    A further object of the present invention is to provide an optical system with a cavity pumped by a pump source that maximizes the efficiency of the cavity.  
           [0021]    These and other objects of the present invention are achieved in an optical system with a pump source that produces a first output beam. A cavity is pumped by the first output beam and produces a second output beam. A power monitor is positioned to receive at least a portion of the second output beam. In response to a signal from the power monitor an efficiency of the cavity is maximized by adjusting a position of the first output beam relative to the cavity.  
           [0022]    In another embodiment of the present invention, an optical system has a pump source that produces a first output beam. A cavity is pumped by the first output beam and produces a second output beam. A first power monitor is positioned to receive at least a portion of the second output beam. The first power monitor provides an input to a summing junction coupled to the pump source. In response to a signal from the power monitor, an efficiency of the cavity is maximized by adjusting a position of the first output beam relative to the cavity.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    [0023]FIG. 1 illustrates one embodiment of the optical system of the present invention.  
         [0024]    [0024]FIG. 2 illustrates one embodiment of the cavity device of FIG. 1.  
         [0025]    [0025]FIG. 3 illustrates another embodiment of the cavity device of FIG. 1.  
         [0026]    [0026]FIG. 4 illustrates another embodiment of the cavity device of FIG. 1. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]    In one embodiment of the present invention, illustrated in FIG. 1, an optical system  10  has a pump source  12  that produces a first output beam  14 . A cavity  16  is pumped by first output beam  14  and produces a second output beam  18 . A power monitor  20  is positioned to receive at least a portion of second output beam  18 . In response to a signal  19  from power monitor  20 , an efficiency of cavity  16  is maximized by adjusting a position of first output beam  14  relative to cavity  16 . Pump source can include a second harmonic generator such as one made of LBO.  
         [0028]    A reflector  22  can be positioned between pump source  12  and cavity  16  in order to directed first output beam  14  into cavity  16 . Reflector  22  is preferably movably mounted, and can be mounted to be dithered. A response of second output beam  18  to this dithering can be used to determine an orientation of reflector  22  which maximizes power of second output beam  18 . The response of second output beam  18  can also be used to minimize power of first output beam  14  while maintaining the same power of second beam  18 .  
         [0029]    A beam splitter  24  can be included and can be positioned along a beam path of second output beam  18 . Beam splitter  24  directs at least a portion of second output beam  18  to power monitor  20 .  
         [0030]    Pump source  12  can be an optically pumped laser including but not limited to a diode pump source and can be fiber coupled. Pump source  12  can include a gain medium 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 and YbAG. In one embodiment, the gain medium is Nd:YVO 4  with a doping level of less than 0.5%.  
         [0031]    Cavity  16  can be a variety of devices including but not limited to an OPO, a build-up cavity, a Ti:sapphire laser, a non-linear device, a frequency doubler and the like. The build up cavity can include non-linear optical components.  
         [0032]    One or both of pump source  12  or cavity  16  can include a modelocking device. Suitable mode-locking devices include but are not limited to, a multiple quantum well saturable absorber, a non-linear mirror mode locker, a polarization coupled mode locker, an acousto-optic modulator, and the like.  
         [0033]    Referring now to FIG. 2, one embodiment of cavity  16 , denoted as  100 , includes an end mirror  112  and an output coupler  114  that generally define a resonator cavity  116 . -Output coupler  114  can be curved or flat. Resonator cavity  116  produces an output beam with selected spectral components.  
         [0034]    A gain medium  118  is positioned in resonator cavity  116 . A dispersion member  120  is positioned in resonator cavity  116 . Dispersion member  120  creates a spread of spectral components of the intracavity beam in a lateral direction. Dispersion member  120  can be a variety of optical elements including but not limited to a grating pair, and the like.  
         [0035]    An aperture member  126  is positioned in resonator cavity  116  in a path of the intracavity beam. Aperture member  126  defines an aperture that provides a low loss intracavity beam path for a range of spectral components. At first position  122 , the range of spectral components of the intracavity beam follows a single beam path. When the intracavity beam travels from position  122  to position  124 , dispersion member  120  creates a spatial spread of the range of spectral components. When the intracavity beam travels from position  124  to position  122 , the reverse process occurs.  
         [0036]    A movably mounted mirror  128  is provided. In response to a feedback signal movably mounted mirror  128  maintains the output beam at a same position at output coupler  114 . Movably mounted mirror  128  can be rotatably mounted. A variety of different mechanisms can be used to mount mounted mirror  128  including but not limited to the use of a piezoelectric device, and the like. Movably mounted mirror  128  holds the intracavity beam at a fixed position relative to the aperture to maintain a stable wavelength of the output beam. Movably mounted mirror  128  can be positioned between the aperture member  126  and end mirror  112 .  
         [0037]    Aperture member  126  blocks non-selected spectral components of the intracavity beam that are incident on gain medium  118 . Aperture member  126  has an aperture that passes the selected spectral components that are reflected from end mirror  112 , and oscillate in resonator cavity  116 . The non-selected spectral components do not pass through the aperture and do not oscillate in resonator cavity  116 .  
         [0038]    A beam splitter  130 , or other suitable device, can be positioned at an exterior of resonator cavity  116  along a beam path  132  of the output beam, and creates first and second beams  134  and  136 . A detector  138  is positioned along a beam path of beam  134 . In response to the detection of beam  134 , detector  138  produces a feedback signal  139  for movably mounted mirror  128 . A variety of different detectors  138  can be utilized including but not limited to a position sensitive detector, a quad-cell detector, bi-cell detector, and the like.  
         [0039]    Oscillator system  100  can also include a non-linear device  140  including but not limited to a frequency doubler. Additional fold mirrors and other optical components can be included.  
         [0040]    With reference now to FIG. 3, another embodiment of cavity  16  is an optical oscillator system  210  with an end mirror  212  and an output coupler  214  that define a resonator cavity  216  for an intracavity beam that produces an output beam  217  of selected spectral components. A gain medium  220  is positioned in resonator cavity  216 . An aperture member  218  is positioned in resonator cavity  216  in a path of the intracavity beam. Aperture member  218  has an aperture that provides a low loss intracavity beam path for a range of spectral components. A first prism pair  222  is positioned between aperture member  218  and output coupler  214 . A movably mounted mirror  224  is provided. In response to a feedback signal  223 , movably mounted mirror  224  maintains the output beam at a same position at output coupler  214 .  
         [0041]    At a first position  226 , the range of spectral components of the intracavity beam follows a single beam path. When the intracavity beam travels from position  226  to position  228 , first prism pair  222  creates a spatial spread of the range of spectral components. When the intracavity beam travels from position  228  to position  226 , the reverse process occurs. Oscillator system  210  can include a retro-reflector  230 , or suitable optical device.  
         [0042]    A beam splitter  232  and a detector  234  can be positioned at the exterior of resonator cavity  216 . Beam splitter  232  splits output beam  217  into beams  236  and  238 . Detector  234  is positioned along a path of beam  236 . In response to beam  236 , detector  234  produces the feedback signal  223  to movably mounted mirror  224 . A non-linear device  242 , including but not limited to a frequency doubler, can be included in optical oscillator system  210 . Oscillator system  210  can include additional optical components  
         [0043]    In another embodiment of the present invention, illustrated in FIG. 4, cavity  316  is an optical oscillator system  310  and includes an end mirror  312  and an output coupler  314  that define a resonator cavity  316  for an intracavity beam. Resonator cavity  316  produces an output beam  318  with selected spectral components. A gain medium  320  is positioned in resonator cavity  316 . A first prism pair  322  is positioned in resonator cavity  316 . A second prism pair  324  is positioned between first prism pair  322  and output coupler  314 . An aperture member  326  is positioned between first and second prism pairs  322  and  324  in a path  328  of the intracavity beam. Aperture member  326  defines an aperture that provides a low loss intracavity beam path for a range of spectral components. A movably mounted mirror  330  is provided. In response to a feedback signal  331 , movably mounted mirror  330  maintains output beam  318  at a same position at output coupler  314 . First prism pair  322  has first and second sides  330  and  332 , and second prism pair  324  has first and second sides  336  and  338 .  
         [0044]    When the intracavity beam travels from first side  336  to second side  338 , second prism pair  324  creates a spatial spread of the spectral components. When traveling from first side  330  to second side  332 , first prism pair  322  reverses the process. A retro reflector  339 , or other suitable optical device, can be included.  
         [0045]    A beam splitter  340  and a detector  342  can be positioned at the exterior of resonator cavity  316 . Beam splitter  340  and detector  342  provide the some functions as beam splitters  130 ,  232  and detectors  138 ,  234  respectively. A nonlinear device  344  can be included. Oscillator system  310  can include additional optical elements.  
         [0046]    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.