Patent Publication Number: US-2015077853-A1

Title: Single-Longitudinal Mode Laser with High Resolution Filter

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a non-provisional application claiming the benefit of U.S. Application No. 61/702,332, with a priority date of Sep. 18, 2012. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISC APPENDIX 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     This invention relates generally to single-longitudinal mode (SLM) lasers and microchip lasers. More particularly the invention relates to a single-longitudinal mode laser implementation which utilizes a high resolution filter to select a single-longitudinal mode output from a microchip laser having multiple longitudinal modes. 
     Single-longitudinal mode (SLM) laser has a wide range of applications such as holography, interferometers, precision measurement, high-resolution spectroscopy, coherent optical communications, and laser trapping or cooling. In the prior art, a variety of single-longitudinal mode lasers have been developed. One approach in achieving SLM operation is through the use of ring laser geometry which is disclosed in U.S. Pat. No. 5,052,815, issued on Oct. 1, 1991 to Nightingale et al. A twisted-mode technique for producing an SLM laser is disclosed by Lukas et al in U.S. Pat. No. 5,164,947, issued on Nov. 17, 1992. Another SLM laser technique utilizes a Brewster polarizer and a birefringent material to form a Lyot filter which narrows the frequency bandwidth for single longitudinal mode operation (U.S. Pat. No.  5 , 381 , 427 , issued to Wedekind et al. on Jan.  10 ,  1995 ). Recently an orthogonal-polarization traveling-wave mode technique for producing SLM laser is disclosed by Ma et al in U.S. Pat. No. 7,742,509, issued on Jun. 22, 2010. More recently key techniques for single-mode and frequency doubling laser have been disclosed by Zhang in U.S. Publication No. USRE43421 E1, published on May 29, 2012. 
     In the prior art, all the SLM lasers have utilized intra-cavity frequency selecting methods to realize single longitudinal mode performance. The drawback of the prior art of SLM lasers is that they are relatively complicated, bulky, and expensive. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention presents a compact and easy-to-use implementation of an SLM laser via a microchip laser, comprising a pump light source and a laser cavity. A microchip laser normally has a cavity length in the range of 1-10 mm or typically 2.5 mm for a 532 nm green microchip laser. Most microchip lasers have multiple-longitudinal mode output because the gain band width is larger than the frequency separation of two adjacent longitudinal modes. For example, the wavelength difference of two adjacent longitudinal modes is about 0.03 nm for a typical 532 nm green microchip laser having 2.5 mm total cavity length. 
     A single longitudinal mode may be generated from a microchip laser if the multiple longitudinal modes can be separated and selected by an external filter. But it is difficult to make such a filter because separating two adjacent longitudinal modes with narrow wavelength differences requires very high resolution. Therefore a high resolution filter (hereafter referred to as HR filter) for making an SLM laser with a microchip is desirable. The HR filter should be able to efficiently select a single longitudinal mode as the output from a microchip laser having multiple longitudinal modes. An HR filter most likely utilizes diffraction gratings as the mode-selection component for high resolution and high efficiency. 
     In the present invention, the microchip laser includes a pump source, a laser cavity, and a plurality of collimating lenses. There are two types of laser cavities: the single lasing material cavity with coatings for fundamental frequency operation and the two-component cavity with lasing and frequency doubling materials with coatings for intra-cavity frequency doubling operation. The present invention provides broad wavelength selection from visible to near infrared. The advantages of the present invention include compact size, low cost, and ease-of-use. 
     One aspect of the present invention is the apparatus and methods in which the HR filter comprises a grating and plurality of optic components with unique structure and configurations. The HR filters may have a single, double, triple, or quadruple-pass construction in which the laser beam is diffracted by the grating once, twice, three, or four times respectively to achieve the required high resolution. One of the keys to enabling the HR filter is that the grating has to be configured to the up-limit diffraction angle of 80-90 degrees for the single-pass structure and to a near up-limit diffraction angle of 73-90 degrees for the double, triple, or quadruple-pass structure. 
     The major object of this invention is to develop a compact SLM laser by utilizing a microchip laser with multiple longitudinal modes and an external HR filter to select a single longitudinal mode as the output. 
     Another object of this invention is to provide a method of generating an SLM laser via an HR filter to select a single longitudinal mode from the multiple longitudinal modes of a microchip laser. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A clear understanding of the key features of the invention summarized above may be had by reference to the appended drawings, which illustrate the method and system of the invention, although it will be understood that such drawings depict preferred embodiments of the invention and, therefore, are not to be considered as limiting its scope with regard to other embodiments which the invention is capable of contemplating. 
         FIG. 1  is an illustration of the method and system of this invention showing a SLM laser comprised of a microchip laser having an output of multiple longitudinal modes and a HR filter for selecting a single longitudinal mode as the final output. 
         FIG. 2  is an illustration of the method and system of this invention showing (a) a grating at standard Littrow configuration; (b) a grating at the up-limit configuration in which the diffraction angle is in the up limit of 80 to 90 degree to achieve the required high resolution. 
         FIG. 3  is an illustration of the method and system of this invention showing two longitudinal modes using the grating at the up-limit configuration (a) is unable to be separated spatially in the case of diversion beam; (b) needs large distance to be separated spatially in case of collimated beam; (c) is able to be separated at the focal point in a conversion beam. 
         FIG. 4  is an illustration of the microchip laser (a) with the laser cavity of single material for fundament frequency output; (b) with the laser cavity of two materials for intra-cavity frequency doubling output. 
         FIG. 5  is an illustration of a preferred embodiment of the SLM laser of this invention having a microchip laser with the multiple longitudinal modes and a single-pass structured HR filter which spatially separates the multiple longitudinal modes and selects a single longitudinal mode as the output. 
         FIG. 6  is an illustration of an alternate preferred embodiment of the SLM laser of this invention the same as illustrated in  FIG. 5  but having two additional collimating lenses to collimate the single longitudinal mode output. 
         FIG. 7  is an illustration of a preferred embodiment of the SLM laser of this invention having a microchip laser and a double-pass HR filter. 
         FIG. 8  is an illustration of an alternate preferred embodiment of the SLM laser of this invention having a microchip laser and a double-pass HR filter. 
         FIG. 9  is an illustration of a preferred embodiment of the SLM laser of this invention having a microchip laser and a triple-pass HR filter. 
         FIG. 10  is an illustration of a preferred embodiment of the SLM laser of this invention having a microchip laser and a quadruple-pass HR filter. 
         FIG. 11  is an illustration of an alternate preferred embodiment of the SLM laser of this invention having a microchip laser and a quadruple-pass HR filter. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Although specific embodiments of the present invention will now be described with reference to the drawings, it should be understood that such embodiments are by way example only merely illustrative of but a small number of the many possible specific embodiments which can represent applications of the present invention. Various changes and modifications obvious to one skilled in the art the present invention pertains are deemed to be within the spirit, scope and contemplation of the present invention. 
     Referring to  FIG. 1 , the single longitudinal mode (also SLM) laser in the present invention is illustrated by means of a microchip laser  101 , which outputs multiple longitudinal modes  103 ; a high resolution filter  102  (also HR filter), which blocks other adjacent longitudinal modes and selects a single longitudinal mode as the output  104  from the multiple longitudinal modes  103 . The SLM laser in the present invention, different from the prior art, utilizes a HR filter as the extra-cavity frequency selecting methods to realize single longitude mode performance. 
     The output from a microchip laser comprises two or more longitudinal modes. The wavelength difference of two adjacent longitudinal modes depends on the length of laser cavity. A typical 532 nm green microchip laser having 2.5 mm cavity length has, for example, a wavelength difference of about 0.03 nm. The desired HR filter should have enough resolution power to separate the two adjacent longitudinal modes. A grating having high groove density should be utilized to construct the HR filter to have the required high resolution. However, the highest groove density of a grating is limited by the wavelength of the laser beam. For example, the highest groove density is limited to 3760 and 1880 lines per mm for 532 nm and 1064 nm wavelength, respectively. The available grating is 3600 and 1800 lines per mm for 532 nm and 1064 nm, respectively. Even with a high density grating available an up-limit grating configuration must be utilized in constructing the HR filter  102  in order to achieve the required resolution of separating adjacent longitudinal modes. 
     The up-limit configuration is defined as a grating configured to have a diffraction angle in the 80-90 degree range. Referring to  FIG. 2(   b ), the up-limit grating configuration is illustrated. For comparison,  FIG. 2(   a ) illustrates a grating in a conventional Littrow configuration in which the incident angle (θ)  13  of the incident beam  11  and the diffraction angle of the diffracted beam  12  are the same, relative to the surface normal  14  of the grating  10 . However, a grating at Littrow configuration does not have the needed resolution power to separate the adjacent longitudinal modes because the angle dispersion  9  of multiple longitudinal modes after diffracted by the grating is relatively small as illustrated in  FIG. 2(   a ). For example, a grating with 3600 grooves per mm at Littrow configuration cannot separate the adjacent longitudinal modes from a 532 nm green microchip laser by about 0.03 nm. However an up-limit configuration, where the diffraction angle is in the 80-90 degree range, can greatly improve the grating dispersion power since the angle dispersion is proportional to 1/cos(θ), where θ is the diffraction angle. As the diffraction angle θ approaches 90 degrees, the angle dispersion becomes very large. 
     As illustrated in  FIG. 2(   b ), the incident angle  18  of the incident beam  15  relative to the grating  10  is configured such that the diffraction angle (θ)  17  of the diffracted beam  16  is in the range of 80-90 degrees. The incident angle can be determined according to grating equation d(sin(θ1)+sin(θ2))=λ, where d is the grating line distance, λ is the wavelength, θ1 and θ2 are the incident and diffraction angle, respectively. For example the incident angle can be calculated to be 66.8 and 66.3 degree for the diffraction angle of 85 and 88 degrees, respectively, for wavelength 532 nm and a grating with 3600 groove per mm. The angle dispersion  19  of multiple longitudinal modes is increased greatly in the up-limit configuration as illustrated in  FIG. 2(   b ). For example, the resolution (angle dispersion) of the grating at the up-limit diffraction angle  17  θ=85 and 88 degrees is 3.3 and 8.4 times, respectively, of that at Littrow configuration (θ=73 degrees) for a same grating. The greatly increased resolution (angle dispersion) in the up-limit configuration allows us to construct a high resolution (HR) filter with needed resolution and compact size. 
     However, the construction of the HR filter is hindered by another problem even with a high resolution grating at the up-limit configuration. Most microchip lasers, with or without collimation lenses, have a beam diversion larger than 1 mRad (0.001 radian or 0.057 degree). The beam diversion is also greatly increased by a grating in the up-limit configuration. The increased beam diversion makes it impossible to separate a single longitudinal mode from the adjacent longitudinal modes. As illustrated in  FIG. 3(   a ) two adjacent longitudinal modes with a diversion beam shape  23  are sent to the grating  10  at the up-limit configuration. The two adjacent longitudinal modes  24  (solid line) and  25  (dashed line) are dispersed with a dispersion angle  20  after the grating  10 . Since the beam diversion of the two longitudinal modes after the grating is larger than the dispersion angle  20  the two adjacent longitudinal modes cannot be separated in this case. 
     To solve the beam diversion problem we have utilized a collimating lens unit, which comprises at least two lenses  21  and  22  separated by a separation distance  33 , in front of the HR filter to produce a collimated beam or a conversion beam as illustrated in  FIGS. 3(   b ) and  3 ( c ), respectively.  FIG. 3(   b ) illustrates the case of collimated beam shape  26 , in which the two adjacent longitudinal modes are collimated by the lens  21  and  22  and then sent to the grating  10 . After the grating  10  the two modes are dispersed with a dispersion angle  20 . After certain distance the two adjacent longitudinal modes  27  and  28  will be separated since the beam shape of the two modes keeps collimated after the grating  10 . The drawback of the collimated beam shape is that the distance to separate adjacent longitudinal modes could be very large. 
     For a compact HR filter, a short separation distance is desired. A conversion beam shape is utilized to separate adjacent longitudinal modes more efficiently. As illustrated in  FIG. 3(   c ) the two adjacent longitudinal modes are shaped to a conversion beam  29  by adjusting the distance between the beam shaping lens  21  and  22  and are sent to the grating  10 . After the grating  10  the two longitudinal modes  30  and  31  are also dispersed with a dispersion angle  20 . However the two modes are focused after the grating  20  in this case. Therefore the two adjacent modes  30  and  31  are completely separated at the focal point position  32  as illustrated in  FIG. 3(   c ). An aperture can be placed at the focal point to select only one longitudinal mode to pass through as the output. The distance of the focal point  32  from the grating  10  depends on the beam conversion and is adjustable by the distance between the collimating lenses  21  and  22 . A very compact HR filter can be constructed with a conversion beam shaping and a grating at up-limit configuration. 
     Referring to  FIG. 4 , two types of microchip lasers having multiple longitudinal modes output in the present invention are illustrated.  FIG. 4  ( a ) illustrates a fundamental microchip laser  105  consisting of a laser pump source  40 , a laser cavity  41  comprising a lasing material with coatings for fundamental frequency output, and collimating lenses  21  and  22  separated by a separation distance  33  for beam shape control.  FIG. 4  ( b ) illustrates a frequency-doubling microchip laser  106  consisting of a laser pump source  40 , a laser cavity comprising a lasing material  41  and a frequency doubling material  42  with coatings for frequency doubling output, and the collimating lens  21  and  22  separated by a separation distance  33 . The lasing material can be Nd:YAG, Yb:YAG, Nd:YVO 4 , Nd:GdVO 4 , Nd:YLF and Nd:KGW crystals. The frequency doubling materials can be Beta-Barium Borate (BBO), Lithium Borate (LBO), and numerous other materials such as BiBO, KDP, KTP, and KTA crystals. 
     Referring to  FIG. 5 , a preferred embodiment of the SLM laser in the present invention having the HR filter  102  with a single-pass structure is illustrated. The SLM laser comprises a microchip laser  101  and a HR filter  102 . The laser beam is shaped to a conversion beam by the collimating lenses inside the microchip laser. The multiple longitudinal modes  103  from the microchip laser  101  are sent to the HR filter  102  for selecting a single longitudinal mode as the output  104 . The HR filter  102  comprises a grating  10  having the up-limit configuration, a reflection mirror  51 , and an aperture  54  for spatial filtering. The multiple longitudinal modes  103  with conversion beam shape are dispersed by the grating  10  with an up-limit diffraction angle of 80-90 degrees. The dispersed longitudinal modes  50  from the grating  10  are redirected by the mirror  51  to the aperture  54 . The aperture  54  blocks any other longitudinal modes  52  and  53  and allows only one longitudinal mode to pass through as the single longitudinal mode output  104 . 
     Referring to  FIG. 6 , an alternate preferred embodiment of the SLM laser in the present invention having the HR filter  102  with a single-pass structure is illustrated. The differences of this SLM laser from the one shown in  FIG. 5  are the two collimation lenses  55  and  56  being added into the HR filter. The first collimation lens  55  is a diversion lens to expand the beam size. The second collimation lens  56  is a conversion lens to collimate the single longitudinal mode output  104 . 
     Referring to  FIG. 7 , an alternate preferred embodiment of the SLM laser in the present invention having the HR filter  102  with a double-pass structure is illustrated. The SLM laser comprises a microchip laser  101  and a HR filter  102  having a double-pass structure for increased wavelength separation power. The laser beam is shaped to a conversion beam by the collimation lenses inside the microchip laser. The multiple longitudinal modes  103  from the microchip laser  101  are sent to the HR filter  102  for selecting a single longitudinal mode as the output  104 . The HR filter  102  comprises a grating  10 , mirrors  71 ,  72 , and  73 , and an aperture  76 . For the increased resolution of the double-pass HR filter the grating may be configured at the near up-limit configuration of somewhat smaller diffraction angle. The laser beam of the multiple longitudinal modes  103  with a conversion beam shape is diffracted a first time by the grating  10  to mirror  71  and then is reflected back to the grating  10  by mirror  71  and is diffracted a second time by the grating to mirror  72 . The dispersed laser beam then is redirected by the mirror  72  to the mirror  73  and then to the aperture  76  which selects a single longitudinal mode to pass through as the final output  104 , while other adjacent longitudinal modes  74  and  75  are blocked by the aperture  76 . One of the features of the double-pass HR filter is that the first-time diffraction angle  77  formed when the beam diffracts from grating  10  to mirror  71 , is within the near up-limit configuration range of 73-90 degrees. The first-time diffraction angle is larger than the second-time diffraction angle  78  formed when the beam diffracts from grating  10  to mirror  72 . This double-pass HR filter has a relative lower resolution power compared to another double-pass HR filter discussed below. 
     Referring to  FIG. 8 , an alternate preferred embodiment of the SLM laser in the present invention having the HR filter  102  with a double-pass structure is illustrated. The differences of this SLM laser from the one shown in  FIG. 7  are the different position of the reflection mirror  71 . This double-pass HR filter has the second-time diffraction angle  78  within the near up-limit configuration range and has the second-time diffraction angle  77  larger than the first-time diffraction angle. The resolution power of this double-pass HR filter is relatively higher than that of the double-pass HR filter discussed above where the second-time diffraction angle is smaller than the first-time diffraction angle. The first and second-time diffraction angles will be same in the case of the incident angle and the diffraction angle are the same. Importantly, the first-time diffraction angle may be greater than, less than, or equal to the second-time diffraction angle depending on the resolution power desired. 
     Referring to  FIG. 9 , an alternate preferred embodiment of the SLM laser in the present invention having the HR filter  102  with a triple-pass structure is illustrated. The SLM laser comprises a microchip laser  101  and a HR filter  102  having a triple pass structure with further increased wavelength separation power. The HR filter  102  includes a grating unit  10  with the near up-limit configuration, the reflection mirrors  81 - 84 , and an aperture  87 . The laser beam of the multiple longitudinal modes  103  is diffracted for a first time by the grating  10  to the mirror  81 ; then the laser beam is reflected back to the grating  10  by the mirror  81  and is diffracted a second time by grating  10  to mirror  82 ; then the laser beam is reflected back to grating  10  again by mirror  82  and is diffracted a third time by grating  10  to mirror  83 , where the third-time diffraction angle is same as the first time-diffraction angle  77 ; then the laser beam is redirected from mirror  83  to mirror  84 , and then to aperture  87 ; finally a single longitudinal mode is selected to pass through the aperture  87  as the output  104  and other adjacent longitudinal modes  85  and  86  are blocked by the aperture  87 . The first-time, second-time, and third-time diffraction angle may be configured to achieve the resolution power desired. 
     Referring to  FIG. 10 , an alternate preferred embodiment of the SLM laser in the present invention having the HR filter  102  with a quadruple-pass structure is illustrated. The SLM laser comprises a microchip laser  101  and a HR filter  102  having a quadruple-pass structure with further increased wavelength separation power. The HR filter  102  comprises a grating unit  10  with the near up-limit configuration, the reflection mirrors  81 - 84 , and an aperture  87 . The laser beam of the multiple longitudinal modes  103  is diffracted first time by the grating  10  to the mirror  81 ; then the laser beam is reflected back to the grating  10  by the mirror  81  and is diffracted second time by the grating  10  to the mirror  82 ; then the laser beam is reflected back to the grating  10  again by the mirror  82  and is diffracted third time by the grating  10  to the mirror  81 , where the third-time diffraction angle  79  is the same as the first time-diffraction angle  77 ; then the laser beam is reflected back to the grating  10  again by the mirror  81 ; then the laser beam is diffracted fourth time by the grating to the mirror  83 , where the fourth-time diffraction angle  80  is the same as the second time-diffraction angle  78 ; Then the laser beam goes to the mirror  84 , and to the aperture  87 ; finally a single longitudinal mode is selected to pass through the aperture as the output  104  and other adjacent longitudinal modes  85  and  86  are blocked by the aperture  87 . The quadruple-pass HR filter has the first and third-time diffraction angles in the near up-limit configuration range and has the second and fourth-time diffraction angles smaller. This quadruple-pass HR filter has a relatively lower resolution power compared to another quadruple-pass HR filter discussed below. 
     Referring to  FIG. 11 , an alternate preferred embodiment of the SLM laser in the present invention having the HR filter  102  with a quadruple-pass structure is illustrated. The SLM laser comprises a microchip laser  101  and a HR filter  102  having a quadruple- pass structure same as that shown in  FIG. 10  but with one difference. The difference is that the reflection mirror  81  is located above the incident beam  103  in this HR filter while the reflection mirror  81  is located below the incident beam  103  in that shown in  FIG. 10 . Similarly, the multiple longitudinal modes  103  are diffracted four times by the grating  10  and finally a single longitudinal mode is selected as the output  104 . The quadruple-pass HR filter has the second and fourth time diffraction angles in the near up-limit configuration range and has the first and third time diffraction angles smaller, where the third-time diffraction angles  79  is same as the first-time diffraction angle  77  and the fourth-diffraction angle  80  is same as the second-time diffraction angle  78 . This quadruple-pass HR filter has a relatively higher resolution power compared to another quadruple-pass HR filter discussed above. 
     Although the invention has been explained in relation to its preferred embodiment, it is to be understood that many other possible modifications and variations can be made without departing from the spirit and scope of the invention as herein described.