Patent Publication Number: US-2022236546-A1

Title: Laser beam circulator

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 63/142,883, filed on Jan. 28, 2021, and to U.S. Provisional Patent Application No. 63/148,719, filed on Feb. 12, 2021. The entirety of both applications is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to the field of laser beam circulators, and more particularly to asymmetric confocal laser beam circulators. 
     BACKGROUND OF THE INVENTION 
     In conventional optical systems, a laser beam may pass through a sample, at the same location on the sample, multiple times with minimal beam distortion. The conventional systems include an unstable cavity constructed from lenses and/or curved mirrors. The conventional systems also include a shifter, which may be a piece of transparent material or a combination of flat mirrors. The shifter may ensure that the pump beam does not return to its original launching point, which increases the number of times that the pump beam may pass through the sample. 
     SUMMARY 
     A laser beam circulator includes a first mirror and a second mirror. The first and second mirrors are symmetric with respect to an axis therebetween. The circulator also includes a sample that is substantially planar. The axis extends through the sample. The sample is oriented at an angle with respect to a plane that is perpendicular to the axis. The angle is from about 0.1° to about 10°. The circulator also includes a laser configured to emit a laser beam that circulates multiple times from the second mirror to the sample to the first mirror and back to the second mirror. The laser beam passes through the sample during each circulation. The sample absorbs a portion of the laser beam each time the laser beam passes through the sample. 
     In another embodiment, the circulator includes a first mirror and a second mirror. The first and second mirrors are substantially parabolic. The first and second mirrors are symmetric with respect to an axis therebetween. The circulator also includes a sample that is substantially planar. The axis extends through the sample. The circulator also includes a laser configured to emit a laser beam that circulates multiple times from the second mirror to the sample to the first mirror and back to the second mirror. The laser beam reflects off of the second mirror at a first angle. The laser beam contacts a focal point on the sample. The sample is tilted around the focal point such that the sample is oriented at a second angle with respect to a plane that is perpendicular to the axis. The second angle is from about 0.1° to about 10°. The laser beam passes through the sample during each circulation. The sample absorbs a portion of the laser beam each time the laser beam passes through the sample. 
     In yet another embodiment, the circulator includes a first mirror and a second mirror. The first and second mirrors are substantially planar. The circulator also includes a sample that is substantially planar. The circulator also includes a first lens positioned at least partially between the first mirror and the sample. The circulator also includes a second lens positioned at least partially between the second mirror and the sample. The circulator also includes a laser configured to emit a laser beam that circulates multiple times from the second mirror through the first lens to the sample, from the sample through the second lens to the first mirror, and from the first mirror back to the second mirror. The laser beam contacts a focal point on the sample. The sample is tilted around the focal point such that the sample is oriented at an angle with respect to a plane that is perpendicular to the axis. The angle is from about 0.1° to about 10°. The laser beam passes through the sample during each circulation. The sample absorbs a portion of the laser beam each time the laser beam passes through the sample. 
     Advantages of the embodiments will be set forth in part in the description which follows, and in part will be understood from the description, or may be learned by practice of the invention. The advantages will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description, serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a curved mirror, according to an embodiment. 
         FIG. 2  illustrates a circulator, according to an embodiment. 
         FIG. 3  illustrates the circulator with a controlled (e.g., larger) spot size on the sample, according to an embodiment. 
         FIG. 4  illustrates another circulator, according to an embodiment. 
         FIG. 5  illustrates the circulator with a controlled (e.g., larger) spot size on the sample  210 , according to an embodiment. 
         FIG. 6  illustrates the circulator and a laser cavity for the sample, according to an embodiment. 
         FIG. 7  illustrates the aspheric lens-based ring circulator with a plurality of beams, according to an embodiment. 
         FIG. 8  illustrates another circulator, according to an embodiment. 
         FIG. 9  illustrates another circulator, according to an embodiment. 
         FIG. 10A  illustrates a side view of a circulator with an on-axis PACC geometry, and  FIG. 10B  illustrates an end view of the circulator, according to an embodiment. 
         FIG. 11A  illustrates a side view of another circulator with an on-axis PACC geometry, and  FIG. 11B  illustrates an end view of the circulator, according to an embodiment. 
         FIG. 12A  illustrates a side view of another circulator, and  FIG. 12B  illustrates an end view of the circulator, according to an embodiment. 
         FIG. 13  illustrates a side view of another circulator, according to an embodiment. 
         FIG. 14A  illustrates a side view of another circulator,  FIG. 14B  illustrates an end view of the circulator in a single-beam configuration, and  FIG. 14C  illustrates an end view of the circulator in a multi-beam configuration, according to an embodiment. 
         FIG. 15  illustrates a side view of another circulator and a gain module (e.g., a heatsink and/or heat spreader), according to an embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Reference will now be made in detail to the present embodiments, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements. Moreover, all ranges disclosed herein are to be understood to encompass any and all sub-ranges subsumed therein. For example, a range of “less than 10” can include any and all sub-ranges between (and including) the minimum value of zero and the maximum value of 10, that is, any and all sub-ranges having a minimum value of equal to or greater than zero and a maximum value of equal to or less than 10, e.g., 1 to 5. In certain cases, the numerical values as stated for the parameter can take on negative values. In this case, the example value of range stated as “less than 10” can assume negative values, e.g., −1, −2, −3, −10, −20, −30, etc. 
     The following embodiments are described for illustrative purposes only with reference to the Figures. Those of skill in the art will appreciate that the following description is exemplary in nature, and that various modifications to the parameters set forth herein could be made without departing from the scope of the present invention. It is intended that the specification and examples be considered as examples only. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments. 
     Tilt-Induced Asymmetric Confocal Reflective Ring Circulator 
     The system and method described herein may include multi-pass circulator cavities that use an asymmetric confocal concept in transmission geometry and/or reflection geometry. 
       FIG. 1  illustrates a curved mirror  100 , according to an embodiment. The curved mirror  100  may be or include a parabolic mirror. More particularly, the mirror  100  may be part of a parabola. When the part of the mirror  100  is the central part of the parabola, this is referred to as an on-axis parabolic mirror. However, when the part of the mirror  100  is not the central part of the parabola, this is referred to as an OAPM. In one embodiment, the mirror  100  may be or include an off-axis parabolic mirror (OAPM). 
     The effective focal length of the curved mirror  100  may depend on the angle of incidence (θ) with respect to the symmetric axis  102  of the mirror  100 . For example, for the parabolic mirror  100 , the effective focal length versus angle (θ) can be written as: 
     
       
         
           
             
               f 
               ⁡ 
               
                 ( 
                 θ 
                 ) 
               
             
             = 
             
               
                 2 
                 ⁢ 
                 F 
               
               
                 1 
                 + 
                 
                   cos 
                   ⁡ 
                   
                     ( 
                     θ 
                     ) 
                   
                 
               
             
           
         
       
     
     where f(θ) represents the effective off-axis focal length, and F represents the on-axis (θ=0) focal length of the entire parabolic mirror. Thus, the effective focal length may be tuned by adjusting the incident angle (θ) rather than acquiring (or crafting) two nearly mismatched mirrors. The angle θ may be from about 10° to about 80°, about 20° to about 70°, or about 30° to about 60°. 
     OAPM-Based Ring Circulator 
       FIG. 2  illustrates a circulator  200 , according to an embodiment. The circulator  200  may also or instead be referred to as a circulator cavity or an OAPM-based ring circulator cavity. The circulator  200  may include two mirrors  100 A,  100 B that are symmetric with respect to an axis  202 . In an embodiment, the mirrors  100 A,  100 B may be or include curved mirrors (e.g., OAPMs). 
     The circulator  200  may also include a sample  210 . In one embodiment, the sample  210  may be planar (e.g., flat) and reflective (e.g., include a mirror). In another embodiment, a third mirror (not shown) may be positioned behind the sample  210  such that the sample  210  is positioned between the first mirror  100 A and the third mirror, and between the second mirror  100 B and the third mirror. 
     The circulator  200  may also include a laser  220  that is configured to emit a laser beam  222 . In the embodiment shown, the laser beam  222  initially passes through an opening  102  in the first mirror  100 A. The laser beam  222  then reflects off of the second mirror  100 B at the incident angle (θ). The laser beam  222  then contacts (e.g., passes through) a spot  212  on the sample  210 . The spot  212  may have a cross-sectional length (e.g., radius) from about 10 μm to about 10 mm. The radius may determine the spot size (e.g., area) of the beam  222  on the sample  210 . As mentioned above, the sample  210  may be reflective, which causes the laser beam  222  to reflect off of the sample  210  toward the first mirror  100 A. The foregoing has described one circulation of the laser beam  222  which is substantially triangular in this embodiment. The laser beam  222  may continue to circulate more times (three circulations/loops are shown), contacting (e.g., passing through) the spot  212  during each circulation. The sample  210  may be or include a weak absorber or an amplifier that may absorb a portion of a laser beam  222  each time the laser beam  222  passes through the sample  210 . Thus, the circulator  200  may be referred to as a multi-pass circulator. 
     As may be seen, the axis  202  may extend through the sample  210 . For example, the axis  202  may extend through the spot  212 . However, the sample  210  may not be perpendicular to the axis  202 . Rather, the sample  210  may be oriented at an angle α with respect to a line or plane that is perpendicular/normal to the axis  202 . The angle α may be from about 0.1° to about 10°, about 0.3° to about 5°, or about 1° to about 3°. For example, the angle α may be just large enough so that the reflected beam on the mirror  100 A (in its first pass) does not overlap with the incident beam, which is fed through the opening  102 . Tilting the sample  210  around the spot  212  by the angle α may result in an asymmetric confocal cavity in a reflection geometry. 
     The configuration in  FIG. 2  creates a tightly focused spot  212  at the sample  210 . Ray tracing simulation shows that greater than about 40 passes into/through the sample  210  can be achieved. 
     Here, off-axis means that the angle θ is not zero (θ≈0 is on-axis). Off-axis may be used with variable θ because the sample  210  may be titled by a small angle α (different from θ) so that the reflecting the beam on the mirror  100 A will be at an angle (θ+2α), which in turn makes the effective focal length of the two mirrors  100 A,  100 B different. 
     OAPM-Based Ring Circulator (Large Spot Size) 
       FIG. 3  illustrates the circulator  200  with a controlled (e.g., larger) spot size  212 , according to an embodiment. Ray tracing models show that by pre-focusing the beam  222  (e.g., using a lens  230 ), a collimated beam may be generated at the sample  210  in greater than about 30 passes with negligible distortion. More particularly, when a larger spot size is desired, the laser beam  222  may be initially focused by the lens  230  to an intermediate point  232  on the axis  202  before passing through the opening  102  in the mirror  100 A. The laser beam  222  may then be recollimated by the mirror  100 B. In subsequent circulations (e.g., round trips), the laser beam  222  may be focused by the mirror  100 A and then recollimated by the mirror  100 B. This may result in a larger spot size at the spot  212 . While this geometry may not produce a perfectly re-collimated beam in every pass (i.e., the beam radius slightly changes in each roundtrip), it is surprisingly effective for multi-pass pumping where the spot size on the sample  210  is greater than a predetermined size (e.g., radius). 
     Aspheric Lens-Based Ring Circulator 
       FIG. 4  illustrates another circulator  400 , according to an embodiment. The circulator  400  may be referred to as an aspheric lens-based ring circulator. The circulator  400  may include the mirrors  100 A,  100 B; however, in this embodiment, the mirrors  100 A,  100 B may be planar instead of curved. The circulator  400  may also include the sample  210 , which may be oriented at the angle α with respect to normal to the axis  202 . 
     The circulator  400  may also include one or more (e.g., two) lenses  410 A,  410 B. The lenses  410 A,  410 B may be aspheric lenses, meaning that the surface profiles are not portions of a sphere or cylinder. This surface profile can reduce or eliminate spherical aberration and also reduce other optical aberrations such as astigmatism, compared to a simple lens. 
     The combination of the planar mirrors  100 A,  100 B and the lenses  410 A,  410 B may replace the curved (e.g., parabolic) mirrors in  FIG. 2 . This may be a more convenient and less costly option to generate a multi-pass cavity. Although three reflective surfaces are shown (e.g., mirror  100 A, mirror  100 B, and sample  210 ), this configuration can be implemented with any odd number of reflective surfaces. 
     4f Aspheric Lens-Based Ring Circulator 
       FIG. 5  illustrates the circulator  400  with a controlled (e.g., larger) spot size on the sample  210 , according to an embodiment. The spot size may or may not be tightly focused. To achieve this, the circulator  400  may be configured to implement an intermediate focus and re-collimation process. More particularly, when a larger spot size is desired, the laser beam  222  may be initially focused by the lens  410 C to an intermediate point  432  on the axis  202  before passing through the opening  102  in the mirror  100 A. The laser beam  222  may then be recollimated by the lens  410 B. In subsequent circulations (e.g., round trips), the laser beam  222  may be focused by the lens  410 A and then recollimated by the mirror  410 B. This may result in a larger spot size at the spot  212 . In contrast to the 2f embodiment shown in  FIG. 4 , the embodiment in  FIG. 5  is 4f, where f represents the focal length of the aspheric lens. In addition, f p  represents the distance between the lens  410 C and the focal spot  432 , which is also the focal distance of the focusing lens  410 C. The lenses  410 A- 410 C are positioned such that the intermediate focal point  432  is situated at a distance f from both lenses  410 A and  410 B. 
       FIG. 6  illustrates the circulator  400  with another (e.g., curved) mirror  600 A, according to an embodiment. In an example, the circulator  400  may be used to pump a thin disk laser. The position and/or orientation (e.g., angle) of one or more of the reflective surfaces (e.g., mirrors  100 A,  100 B,  600 A, and/or sample  210 ) may be modified so that the input coupling mirror  100 A is not too close to a pump focus  610  to avoid optical damage in high power pump operations. 
     Here, the laser cavity is defined at least partially by the sample  210  and the mirror  600 A. The beam between the sample  210  and the mirror  600 A will be formed if laser action occurs between the mirror  210  (e.g., mirror gain sample, pumped by this circulator  400 ) and the laser cavity mirror  600 A. This beam is referred to as the laser cavity beam, which may not be part of the circulator  400 . Rather, it is the intracavity laser beam for the resonator formed by the gain chip  210  and the external cavity mirror  600 A. 
     Multi-Pump Beam Capability 
       FIG. 7  illustrates the aspheric lens-based ring circulator  400  with a plurality of beams  222 , according to an embodiment. The beams  222  may (e.g., simultaneously) be launched onto the sample  210  through the input coupling mirror  400 A. This may be used to pump the gain medium of a disk laser (sample  210 ) with multiple pump beams to achieve high-power outputs. 
     Parabolic Asymmetric Confocal Cavities (PACC) 
     An asymmetric confocal cavity may be generated using on-axis and/or off-axis parabolic mirrors. As mentioned above, the term “off-axis” refers to when a portion of the parabolic mirror away from the axis is cut and used. This term is not used when the whole parabolic mirror is used, even though the rays incident and reflected are away from the axis and in some cases are significantly off-axis. The two parabolic mirrors in  FIG. 8  are on-axis parabolic mirrors with a hole in the center. 
     OAPM-Based Asymmetric Confocal Ring Circulator 
       FIG. 8  illustrates another circulator  800 , according to an embodiment. In this embodiment, the sample  210  is not reflective and does not have a mirror attached to it. The mirrors  810 A,  810 B may be or include parabolic mirrors that are symmetric to one another with respect to an axis  802 . In addition to the hole in the mirror  810 A through which the laser beam  222  initially passes, the mirrors  810 A,  810 B may each also have a (e.g., central) hole  812 A,  812 B formed therethrough. The central holes  812 A,  812 B may be used to allow another laser beam (e.g., formed by an external cavity similar to  FIG. 6 ) to pass through the multi-pass pumped sample  210 . The axis  802  may extend through the sample  210  and the holes  812 A,  812 B. The circulator  800  in  FIG. 8  may be used to pump the sample  210 , which may be or include a laser gain medium. 
       FIG. 9  illustrates a reflective (e.g., three mirror) ring circulator  900 , according to an embodiment. This may include two (e.g., off-axis parabolic) mirrors  910 A,  910 B that are symmetric to one another with respect to an axis  902 . However, the mirrors  910 A,  910 B may be laterally offset from one another by a distance δf in a direction that is parallel to the axis  902 . In some embodiments, a third mirror  910 C may be positioned proximate to (e.g., behind) the sample  210 . The circulator  900  may be defined at least partially by the mirrors  910 A- 910 C. A laser cavity may be defined at least partially by the mirrors  910 C and  910 D. This is similar to embodiment in  FIG. 6 . 
     Multi-Beam PACC 
       FIG. 10A  illustrates a side view of a circulator  1000  with an on-axis PACC geometry, and  FIG. 10B  illustrates an end view of the circulator  1000 , according to an embodiment. The design shown in  FIGS. 10A and 10B  allows for launching multiple pump beams  222  through holes  1012  in the mirror  1010 A. Alternatively, the multiple pump beams  222  may pass outside the periphery of the mirror  1010 A if the mirror  1010 A has a smaller size (e.g., diameter). The circulator  1000  in  FIGS. 10A and 10B  may be used for pumping the sample  210  at the focus or a gas-filled cell for spectroscopy. The number of beams N beam  that can be launched can be estimated as: 
     
       
         
           
             
               
                 N 
                 
                   b 
                   ⁢ 
                   e 
                   ⁢ 
                   a 
                   ⁢ 
                   m 
                 
               
               ≈ 
               
                 π 
                 ⁢ 
                 
                   
                     f 
                     2 
                   
                   
                     δ 
                     ⁢ 
                     f 
                   
                 
               
             
             = 
             
               
                 1 
                 ⁢ 
                 0 
               
               - 
               
                 3 
                 ⁢ 
                 0 
               
             
           
         
       
     
     where f 2  represents the focal length of the parabolic mirror  1010 B, and of δf=f 2 −f 1  represents the asymmetry in the focal lengths between mirrors  1010 A and  1010 B. 
       FIG. 11A  illustrates a side view of another circulator  1100  with an on-axis PACC geometry, and  FIG. 11B  illustrates an end view of the circulator  1100 , according to an embodiment. The circulator  1100  may include two (e.g., parabolic) mirrors  1110 A,  1110 B. The mirror  1110 A may have a small hole  1112  formed therethrough. The hole  1112  may be or include an optical fiber therein. The mirror  1110 B may have a small hole  1114  formed therethrough. The hole  1114  may be or include an optical fiber therein. The circulator  1100  can be used for transmission spectroscopy in gases by coupling the laser out of the cavity through the hole  1114  in the mirror  1110 B, and measuring the transmitted light with a detector  1116 , according to an embodiment. 
       FIG. 12A  illustrates a side view of another circulator  1200 , and  FIG. 12B  illustrates an end view of the circulator  1200 , according to an embodiment. The circulator PACC  1200  may include a microphone  1220  that can detect the sound generated during absorption. As a result, the circulator  1200  can be used for photo-acoustic spectroscopy in gases and/or aerosols. 
       FIG. 13  illustrates a side view of another circulator  1300 , according to an embodiment. The circulator  1300  includes a ringdown detector  1320  such as an avalanche photodiode (APD) that may be configured to construct a non-resonant circulator for cavity ring-down spectroscopy (CRDS) where a short laser pulse with nanosecond(s) duration is launched through the hole, and each pass it is attenuated by the medium (e.g., gas) inside the cavity, leaked through mirror  1310 B. The transmitted time histogram of the laser, detected by the detector  1320 , may then exhibit an exponential decay with a time constant that is inversely proportional to the absorbance of the specimen. As a result, the circulator  1300  may be used in cavity ring-down spectroscopy. A lens  1330  may be used to focus the light onto the detector  1320 . 
       FIG. 14A  illustrates a side view of another circulator  1400 ,  FIG. 14B  illustrates an end view of the circulator  1400  in a single-beam configuration, and  FIG. 14C  illustrates an end view of the circulator  1400  in a multi-beam configuration, according to an embodiment. The circulator  1400  includes a microphone  1220  as well as the ringdown detector  1320 . As a result, the circulator  1400  can be used in (e.g., simultaneous) photoacoustic and ringdown spectroscopy. This may be particularly useful in measuring the extinction coefficient in aerosols because it can differentiate between absorptive and scattering losses. More particularly, the photoacoustic signal may be sensitive to absorptive while ringdown measures total loss in the cell. 
       FIG. 15  illustrates a side view of another circulator  1500 , according to an embodiment. The circulator  1500  may be used in a multi-pass amplifier. Here, a pump beam  222  creates gain in the sample (e.g., a disk)  210 , while a signal  1520  entering through a hole  1520 A in the mirror  1510 A is amplified after multiple passes in the gain medium  210  before exiting through a hole  1520 B in the mirror  1510 B. 
     While the invention has been illustrated with respect to one or more implementations, alterations and/or modifications can be made to the illustrated examples without departing from the spirit and scope of the appended claims. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular function. 
     Furthermore, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description and the claims, such terms are intended to be inclusive in a manner similar to the term “comprising.” As used herein, the phrase “one or more of”, for example, A, B, and C means any of the following: either A, B, or C alone; or combinations of two, such as A and B, B and C, and A and C; or combinations of three A, B and C. 
     Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.