Abstract:
A microcrystal laser assembly including a gain-crystal includes a frame having a high thermal conductivity. The frame has a base with two spaced apart portions extending from the base. The gain-crystal has a resonator output mirror on one surface thereof. The gain-crystal is supported on the spaced-apart portions of the frame in the space therebetween. Another resonator minor is supported in that space, spaced apart from the output mirror, on a pedestal attached to the base of the frame. The pedestal and the frame have different CTE. Varying the frame temperature varies the spacing between the resonator mirrors depending on the CTE difference between the pedestal and the frame.

Description:
PRIORITY INFORMATION 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 13/718,795, filed Dec. 18, 2012, the disclosure of which is herein incorporated by reference in its entirety. 
     
    
     TECHNICAL FIELD OF THE INVENTION 
       [0002]    The present invention relates in general to single-frequency (single-wavelength) microcrystal lasers. The invention relates in particular to stabilizing the single wavelength by temperature control of the lasers. 
       DISCUSSION OF BACKGROUND ART 
       [0003]    A microcrystal laser (micro-laser) is a laser having a solid-state gain-element in the form of a very thin crystal of a gain-medium, for example, neodymium-doped yttrium aluminum garnet (Nd:YAG) or neodymium-doped yttrium orthovandate (Nd:YVO 4 ). A microcrystal laser can be of a monolithic form, in which a laser resonator (resonant cavity) is formed by applying reflective coatings (resonator mirrors) to opposite faces of the crystal. Here, the optical length of the resonator is determined by the thickness and the refractive index, of the crystal, and the temperature of the crystal. 
         [0004]    A microcrystal laser can also have a so-called semi-monolithic form in which one resonator minor is coated on one face of the crystal and the other resonator minor is spaced apart from the crystal. This semi-monolithic form has an advantage over the monolithic form in that the resonator length can be selected independent of the thickness and material of the crystal. This semi-monolithic form also allows inclusion in the resonator of an active or passive Q-switch for providing pulsed operation of the laser. 
         [0005]    Any laser-resonator has a number of resonant frequencies (wavelengths) determined by the optical length of the resonator. Any of these resonant wavelengths that fit within a gain-bandwidth of the gain-element can be lasing wavelengths (modes) of the laser resonator. The above referenced Nd:YAG and Nd:YVO 4  gain media have a gain bandwidth of the order of about 1 nanometer (nm). In a “conventional” laser-resonator wherein the resonator length is a few centimeters or more, many modes will fit even within this 1 nm-bandwidth. Various relatively complex resonator arrangements are known for selecting one lasing mode from those possible lasing modes. 
         [0006]    As the optical path length of a laser-resonator is reduced, the possible number of resonant wavelengths is reduced, and the wavelength-separation (free spectral range or FSR) of those resonant wavelengths within the gain-bandwidth of the gain-element is increased. In a micro-laser the resonator optical length is decreased until there are few enough resonant wavelengths within the gain-bandwidth of the gain-element (gain-crystal) that only one wavelength will be above a threshold gain-level, and, accordingly only that wavelength will “lase” (oscillate). The lasing efficiency, and accordingly the laser output power, will be determined, inter alia, by the wavelength of that lasing mode relative to the peak-gain wavelength in the gain-bandwidth. This lasing mode wavelength is dependent on the temperature of the micro-laser. It is taught that control of the temperature can provide for tuning of stability of the output wavelength of a micro-laser. 
         [0007]    U.S. Pre-grant Publication No. 2011/0243158, assigned to the assignee of the present invention, and the complete disclosure of which is hereby incorporated by reference, describes a semi-monolithic micro-laser including a saturable semiconductor minor (SESAM). The SESAM is used as a resonator mirror and provides for passive Q-switched operation of the micro-laser. 
         [0008]      FIG. 1  schematically depicts one practical disclosed structure  2  of the &#39;158 micro-laser. The laser is assembled on a base  17 . A thin gain-crystal  4  has a partially reflective and partially transmissive mirror  6  on one face thereof. The coated crystal is supported mirror-side-down on a transparent support  10 . An anti-reflection coating  13  is provided on the opposite face of the crystal. A SESAM  8  is supported on base  17 . The SESAM is spaced apart from the gain-crystal by spacers  16  and  16 ′ leaving an air gap  12  between the SESAM and the gain-crystal. A laser-resonator is formed between the SESAM and mirror  6 . The optical length of the resonator provided by the optical thickness of the gain crystal and the thickness of the air is fine-tuned by thermal expansion or contraction of spacers  16  and  16 ′. While not disputing the practicality of the prior-art structure of  FIG. 1 , it has been determined that there is significant room for improvement, particularly with regard to providing tuning and temperature stability of the lasing wavelength. 
       SUMMARY OF THE INVENTION 
       [0009]    In one aspect laser apparatus in accordance with the present invention comprises a frame having a base-portion and two spaced-apart support-portions extending from the base-portion. The support-portions are of a first thermally conductive material having a first coefficient of thermal expansion (CTE). A pedestal is attached to the base-portion in thermal communication therewith and in the space between the support-portions. The pedestal is of a second thermally conductive material having a second CTE different from the first CTE. A saturable absorption minor supported on the pedestal. A thin crystal of a solid-state gain-medium is attached to a crystal-support element in the space between the support-portions of the frame, and is spaced apart from the saturable absorption mirror leaving an air-gap between the crystal and the saturable absorption mirror. The crystal-support element is attached to the support-portions of the frame. A partially transmissive minor is located between the crystal and the crystal-support element. The partially transmissive mirror and the saturable absorption mirror form a laser-resonator including the crystal, and the laser-resonator has an optical length determined by the thickness of the crystal and the air-gap. Varying the temperature of the frame and the pedestal varies the air-gap, and accordingly the optical length of the laser-resonator, dependent on the difference between the first CTE and the second CTE. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain principles of the present invention. 
           [0011]      FIG. 1  is a side-elevation view schematically illustrating an above-referenced prior-art structural arrangement for a passively Q-switched microcrystal laser. 
           [0012]      FIG. 2  is a side elevation view schematically illustrating a preferred embodiment of a passively Q-switched microcrystal laser in accordance with the present invention. 
           [0013]      FIG. 3  is a three-dimensional view schematically illustrating further detail of the microcrystal laser of  FIG. 2  including a heating or cooling element for adjusting and controlling the temperature of the laser. 
           [0014]      FIG. 4  schematically illustrates a closed-loop arrangement for controlling the operating wavelength of the microcrystal laser of  FIG. 3  using the heating and cooling element. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    Referring now to the drawings, wherein like components are designated by like reference numerals,  FIG. 2  schematically illustrates a preferred embodiment of a solid-state microcrystal laser  20  in accordance with the present invention. Components of the laser assembly are depicted relatively approximately to scale, but expanded in actual size. Exemplary dimensions are provided further hereinbelow. Additional perspective of the assembly is provided in  FIG. 3 . 
         [0016]    Laser  20  includes a generally U-shaped unitary frame  22  which is massive compared to the actual laser components. Frame  22  includes a base portion  24  and upwardly extending side portions  26 A and  26 B. Frame  22  is made from a material having a high-thermal conductivity, for example, copper (Cu) or an alloy thereof such as an alloy of copper and tungsten (W). It is possible make the base-portion and the support-portions as separate units, but this may not be any more convenient than the unitary structure. In this description the terminology “high thermal conductivity” refers to a conductivity about equal to or greater than 5 Watts per meter per degree Kelvin (W/m.K). The gain-element of laser  20  is a thin crystal  40 , for example a crystal of a host material such as YAG or YVO 4  doped with laser-active Nd ions. A crystal-support element  28  is made from an optically transparent material, preferably also having a high thermal conductivity. A preferred material for support element  28  is the same as that of the crystal but without the laser-ion doping. YAG and YVO 4  have the required high thermal conductivity and also have a CTE comparable to copper-tungsten alloy. One alternative material is optical sapphire (aluminum oxide, Al 2 O 3 ). 
         [0017]    Crystal support element  28  is bonded to members  26 A and  26 B of frame  22  via tabs  36 A and  36 B respectively. These tabs are also preferably made from a material having a high thermal conductivity, and can be conveniently made from the same material as support member  28 . 
         [0018]    A partially transmitting (at the lasing wavelength) mirror-coating is deposited on surface  34  of crystal-support element  28 . The mirror-coating  42  is not visible on the scale of the drawing but can be assumed to be synonymous with surface  34 . An antireflection coating is applied to a surface of crystal  40  which is placed on the support member then attached to attached support element  28  by a band of adhesive (not shown) around the edge of the crystal. 
         [0019]    The minor-coating on the support member provides an output-coupling end-minor of laser  20 . In this embodiment, the mirror-coating is made highly transmissive for optical pump radiation, which is directed into crystal  40  through support element  28  and the minor-coating. Surface  32  of the crystal-support element  28  is preferably antireflection coated for both the laser-radiation and pump-radiation wavelengths. 
         [0020]    An alternative method of assembling thin crystal  40  to support-element  28  could be to begin with a block of laser-crystal material; apply the mirror-coating to one surface of the block; optically contact the mirror-coated surface of the block to the support element; then grind and polish the block to a required thickness for the crystal. Those skilled in the optical fabrication art my devise other methods for forming the crystal and required coating on the support element without departing from the spirit and scope of the present invention. 
         [0021]    A saturable absorption minor  50  is aligned parallel to the minor-coating on the support element and provides the other end of the laser-resonator of laser  20 . The saturable absorbing minor is for providing passive Q-switched operation of laser  20 . In this preferred embodiment, minor  50  is a SESAM including multiple layers of semiconductor materials in the gallium aluminum arsenide (GaAlAs) system epitaxially grown on a gallium arsenide (GaAs) substrate. The SESAM is supported, in channel  27  between frame portions  26 A and  26 B of frame  22 , via an optional strain-compensating element  52 , on a pedestal  54  attached to base-portion  24  of frame  22 . Here it should be noted that semiconductor saturable absorption minors are often referred to as saturable Bragg reflectors (SBRs). 
         [0022]    Pedestal  54  preferably has a high thermal conductivity, but must be of material having a coefficient of thermal expansion (CTE) different from that of the material of frame  22 . In this preferred embodiment, pedestal  54  is made from aluminum (Al) and the frame is made from a copper-tungsten alloy. A cut-out portion  25  of base portion  24  of frame  22  provides for adjusting alignment of the pedestal during attachment thereof to the frame. 
         [0023]    The CTE of the aluminum pedestal is higher than that of the copper tungsten frame. A W80Cu20 copper-tungsten allow has a CTE of about 9*10 −6 . Aluminum has a CTE of about 24*10 −6 . Strain-compensating element  52  in this embodiment is formed from aluminum nitride, which has a relatively high thermal conductivity and a CTE which is comparable to the GaAs of the SESAM. The SESAM is attached to the strain compensating element, and the strain compensating element is attached to the pedestal by solder bonding. Providing the strain-compensating element reduces strain on the relatively fragile SESAM, which results from CTE differential as the solder-bond solidifies and cools. 
         [0024]    The dimensions of components of the assembly of laser  20  are selected such that the optical length of the laser-resonator is less than about 1.0 millimeters (mm) to minimize the number resonant wavelengths within the gain-bandwidth of the gain-crystal. Here, the resonator optical length is provided by the thickness of crystal  40  multiplied by the refractive index of the crystal, and the thickness (distance) of an air-gap  45  between SESAM  50  and crystal  40 . 
         [0025]    It is useful to add a coating to the SESAM which allows transmission of the laser wavelength but reflects the pump radiation wavelength. This prevents absorption of pump radiation into the SESAM which could damage or impair the passive Q-switching performance of the SESAM. Alternatively such a coating could be provided on surface  44  of crystal  40 . 
         [0026]    In an example of the preferred embodiment here-described, crystal  40  is an Nd:YAG crystal having a thickness of 150 μm and a refractive index of about 1.8. Air-gap  44  is has a thickness (length) of about 50 μm. This provides that the laser resonator has a FSR of about 1.5 nm. This provides that only one resonant wavelength within the gain-bandwidth can have a gain above the threshold for lasing. 
         [0027]    In this example of laser  20 , dimension A of frame  22  is about 10 mm and dimension B is about 8 mm. The height C of pedestal  54  is about 3 mm. With all parts fixedly bonded, variations in the resonator length will occur primarily due to the CTE difference between the frame and the pedestal. 
         [0028]    In this example, with frame  22  made from a W80Cu20 alloy with pedestal  54  made from aluminum, with CTE values noted above, the CTE difference will provide that air-gap  45  changes in thickness (distance) by about 0.5 μm for 10° C. change in temperature. At the nominal lasing wavelength of 1.064 μm (1064 nm) this 0.5 μm change is about one-half wavelength, so the temperature effectively scans (tunes) the resonator through one free-spectral range thereof. With the exemplary FRS of about 1.5 nm, this equates to a wavelength-sensitivity (temperature-tuning coefficient) of about 0.15 nm/° C. 
         [0029]    As seen in  FIG. 3 , a Peltier element  56  can be provided for heating the assembly. Such an element can also be operated for cooling as is known in the art. Element  56  is in contact with a block  58  preferentially of the same material frame  22 . Block  58  is in thermal contact with the base portion  24  and side portions  22 A and  22 B of frame  22 . This provides for a relatively quick response of the frame and attached components to temperature changes provided by the heater. Such a Peltier element can provide relative thermal control within ±0.05 ° C. of a nominal temperature. 
         [0030]    It is preferred to operate laser  20  at a temperature of less than about 30° C. for providing reliability and extending lifetime of the SESAM. If laser  20  were enclosed in sealed environment, it may be possible to reduce the temperature significantly below 30° C. without incurring problems due to condensation, thereby further extending the SESAM lifetime. 
         [0031]    The selection of the differential CTE of the frame and pedestal components will result from a compromise between a differential CTE which is low enough to provide thermal stability, but high enough to allow an FSR-range of wavelength tuning within a relatively small temperature range around an anticipated operating ambient temperature. This tuning is necessary, as a lasing wavelength at or near the maximum gain cannot be guaranteed from the mechanical assembly alone. 
         [0032]    The frame and support members being massive compared to the actual laser components, and having relatively much higher thermal mass, once temperature tuned to lasing wavelength it is possible that the laser could operate for relatively long periods with the heating element at a fixed temperature. It would be useful, however, to provide for a closed-loop control arrangement for stabilizing the wavelength. A brief description of one possible such arrangement is set forth below with reference to  FIG. 4 . 
         [0033]      FIG. 4  schematically illustrates a stabilized microcrystal laser arrangement  70  including a microcrystal laser assembly  20  similar to that described above with reference to  FIGS. 2 and 3 . In  FIG. 4 , Peltier element  56  is depicted as being attached to the base of frame  22  for convenience of illustration. The Peltier element is preferably arranged as discussed above with reference to  FIG. 3 . 
         [0034]    A pump radiation source  72 , such as a diode-laser, delivers pump-radiation P. The pump-radiation is transmitted through a dichroic mirror  74 , then through crystal support element  28  to gain-crystal  40  of laser  20 . Dichroic mirror  74  is transparent for the wavelength of the pump-radiation and reflective for the wavelength radiation generated by laser  20 . In response to the delivery of pump radiation, laser  20  delivers laser radiation L. This radiation is reflected by dichroic minor  74  to a beam-splitting minor  76 . Mirror  76  reflects a small fraction of the laser radiation, for example about  1 % thereof, and transmits the remainder as laser output. 
         [0035]    The reflected laser radiation from mirror  76  is incident on a detector  78 . A signal from detector  78  representative of the laser output power is transmitted to a microprocessor  80 . The microprocessor is programmed to increase or decrease electrical power supplied by a heater power supply  82  to element  56  (correspondingly increasing or decreasing the temperature of the laser assembly) until the detected laser output power is maximized. The absolute value of the maximized, stabilized, output power will be determined, inter alia, by the pump-radiation power delivered to the gain-crystal. 
         [0036]    The present invention is described above in terms of a preferred and other embodiments. The invention is not limited, however, to the embodiments described and depicted. Rather, the invention is defined by the claims appended hereto.