Abstract:
The disclosure provides an optical apparatus including at least one optical element including glass, at least one support including silicon and a housing including glass. Furthermore, the at least one optical element and the at least one support can be anodically bonded together, and the at least one support and the housing can be anodically bonded together. The disclosure further provides a method for fabricating optical components with durable bonds and incorporates active alignment.

Description:
PRIORITY 
       [0001]    This application claims the benefit of priority from U.S. Provisional Application No. 61/883,222, filed Sep. 27, 2013, which is herein incorporated by reference in its entirety. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    This invention was made with U.S. Government support under Grant #FA8650-09-1-7943 awarded by the United States Air Force/Air Force Research Laboratory. The U.S. Government has certain rights in the invention. 
     
    
     FIELD 
       [0003]    This disclosure relates to systems using aligned optical elements, and methods directed to the alignment of optical elements. 
       BACKGROUND 
       [0004]    In many applications it is desirable to create optical assemblies that are stable, easy to fabricate, and are capable of operating in harsh chemical and thermal environments. For example, it is desirable to have optical assemblies that can operate in the presence of alkali-metal vapor and can be heated up to 200° C. without alignment changes. It is further desirable to have optical assemblies that are vacuum-compatible and have low outgassing properties.  FIG. 1  depicts a prior art multi-pass cell  100 . Multi-pass cell  100  can be hermetically sealed and filled with alkali-metal vapor  102 . The multi-pass cell  100  can be made using two cylindrical mirrors  130  and  140 . One mirror (generally a front mirror  130 ) can have a hole or aperture  132  at a center of the mirror in order to allow for the entrance and exit of beams (such as laser beam  195  from lasing source  190 ). The second mirror can include back mirror  140 . Curvature axes of the two mirrors  130  and  140  can be oriented at a specific angle relative to each other, and a distance between the mirrors  130  and  140  can be accurately set to allow for a multi-pass beam pattern between the mirrors  130 . It can be advantageous to place both mirrors  130  and  140  inside a vacuum-sealed enclosure to eliminate losses associated with light passing through optical windows. 
         [0005]    However, consequences of using alkali-metal vapor over the above-described temperature range can largely preclude the use of adhesives to mount optical elements  130  and  140 . Moreover, while various optical contacting techniques can be used to mount optical elements  130  and  140 , such optical contacting techniques can require atomic level polishing and flatness, and so can be expensive to implement. 
       SUMMARY 
       [0006]    Systems and methods using anodic bonding are provided. Anodic binding can be a fairly robust technique, and does not place stringent requirements on surface preparation. 
         [0007]    Consistent with an embodiment, this disclosure describes glass cells that can contain internal optical elements, such as mirrors, lens, prisms, etc., and processes of fabricating the same. Consistent with an embodiment, the glass cells can be configured to satisfy several requirements, such as being hermetically sealed, contain optical elements with precision alignment, and configured to withstand chemically reactive substances (gas or liquid), such as alkali-metal vapor, at high temperature for prolonged periods of time. In addition, or alternatively, the glass cells can be configured to contain any substance (gas or liquid) that is desired to be free of contamination. Certain embodiments directed to methods of fabricating such cells and/or elements also do not require particularly high quality optical surfaces in order to operate in such environments (such as, without limitation, an alkali-metal or other chemically reactive environment). 
         [0008]    As discussed herein, anodic bonding is a process of bonding silicon to glass (such as PYREX or other suitable borosilicate glass having a coefficient of thermal expansion matching that of silicon) by applying an electric field at an elevated temperature. Disclosed is a procedure for fabricating optical structures, systems, and apparatus with active alignment—such as by using a laser—and then fixing certain optical components and other structures using anodic bonding. The processes and structures disclosed herein can allow one to make hermetic structures that can withstand high-temperature alkali-metal vapor or other chemically reactive environments (which may include gas or liquid phases). Moreover, the hermetic structures can be used to contain substances (gas or liquid) desired to be free of contamination. 
         [0009]    In one embodiment, optical elements can be held in a fixture device allowing fine position adjustment and can be aligned relative to each other using a laser. The optical elements can then be held in a custom chuck with wax on one side. In this way, a portion of the optical elements can be cut-off or removed by a diamond wheel and polished. The portion removed can create a pre-aligned surface for anodic bonding. The optical elements can then be removed from the custom chuck by melting the wax. One of the optical elements can then be anodically bonded to a substrate of silicon (e.g., a silicon wafer). The optical elements can then be placed again in the fixture device and the second optical element can be aligned relative to the first using a laser. The second optical element can then be mechanically clamped to the silicon substrate and anodically bonded. The resulting construction of the silicon substrate and glass optical elements can then be anodically bonded into a glass cell to make a hermetic structure. 
         [0010]    In another embodiment, a back surface of an optical element can be bonded to a silicon disk that is larger (i.e., that has a larger cross-section, for example). The edges of the silicon disk (such as a silicon wafer) that extend beyond the optical element can then be anodically bonded to exposed edges of a glass tube so that the optical element is inside the glass tube. A second optical element can be actively aligned (for example, using a laser), anodically bonded to a second silicon disk (such as a silicon wafer with a larger cross-section, again), and the resulting structure anodically bonded to an opposing exposed end of the glass tube, thus providing a hermetic structure with two aligned optical elements inside. 
         [0011]    Suitable glass structures for anodically bonding to silicon structures can include PYREX glass, or other suitable borosilicate glass having a coefficient of thermal expansion matching that of silicon. Preferably, coatings that may be present on the glass optical elements should be configured to withstand the temperature of 250 C during the anodic bonding. 
         [0012]    Consistent with this disclosure, the process can be commercialized for making sealed glass cells with internal optical components. Applications can include magnetometry with multi-pass cells. Other applications, such as “on-chip” applications, can be fabricated consistent with the current disclosure with suitable scaling. 
         [0013]    In one aspect, the present disclosure is directed to an optical apparatus including at least one optical element including glass, at least one support including silicon and a housing including glass. Furthermore, the at least one optical element and the at least one support can be anodically bonded together. Further still, the at least one support and the housing can be anodically bonded together. 
         [0014]    In a further aspect, the present disclosure is directed to a method providing optical components with durable bonds. The method can include providing at least one optical element including glass, providing at least one support including silicon, and providing a housing including glass. Furthermore, the at least one optical element and the at least one support can be anodically bonded together, and the at least one support and the housing can be anodically bonded together. 
         [0015]    In further aspects consistent with the disclosure, the glass can borosilicate glass, and the at least one optical element can be a mirror. Further still, the glass can include PYREX. Moreover, the at least one optical element is selected from a set consisting of: a lens and a prism. 
         [0016]    Consistent with further embodiments, an apparatus or method can include providing a second optical element including glass. Further still, the second optical element and the at least one support can be anodically bonded to one another. 
         [0017]    In further embodiments, the at least one optical element can be a mirror with an aperture for entrance and exit of a laser beam, and the second optical element can be a window with anti-reflective coating. Further still, the at least one optical element can be a mirror with an aperture for entrance and exit of a laser beam, and the second optical element can be a mirror, a lens or a prism. 
         [0018]    In further embodiments, the at least one optical element and the second optical element can define a multi-pass cavity. Further still, the housing can include a cell, and the housing can be configured to couple to a vacuum system for evacuating and filling the cell with a chemically reactive substance (liquid or gas), such as, without limitation, alkali-metal vapor. 
         [0019]    Further still, the support can be anodically bonded to the housing on a side of the support and about a perimeter of the side, and an inner portion of the side can be bonded to the at least one optical element. Moreover, the support can be anodically bonded to the housing on a side of the support, and the at least one optical element can be anodically bonded to an opposite side of the support. 
         [0020]    Further still, providing the second optical element including glass can include actively aligning the at least one optical element and the second optical element. Additional features and advantages will be set forth in part in the description which follows, being apparent from the description of or learned by practice of the disclosed embodiments. The features and advantages will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. 
         [0021]    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 scope of the embodiments, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments and, together with the description, serve to explain the features, advantages, and principles of the disclosed embodiments. 
           [0023]      FIG. 1  depicts a prior art multi-pass cell; 
           [0024]      FIG. 2  depicts an exploded view of an embodiment consistent with the disclosure; 
           [0025]      FIGS. 3-5  depict various processes associated with fabricating an embodiment consistent with  FIG. 2 ; 
           [0026]      FIGS. 6-12  depict various processes associated with fabricating the embodiment of  FIG. 13 ; 
           [0027]      FIG. 13  another embodiment consistent with the disclosure; and 
           [0028]    FIGS. 14 - 15  depict fabrication processes and a resulting further embodiment ( FIG. 15 ) consistent with the disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    Reference will now be made in detail to the one or more embodiments, characteristics 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. 
         [0030]      FIG. 2  depicts an exploded view of one embodiment of a multi-pass cell  200  consistent with the current disclosure. Shown in  FIG. 2  are front mirror  230  with aperture  232 , front silicon disk  235 , entrance window  250 , cylindrical glass cell  210 , back mirror  240  and back silicon disk  245 . Consistent with this disclosure, the back surfaces of front mirror  230  and back mirror  240  (i.e., the sides of front mirror  230  and back mirror  240  that face away from the inside of glass cell  210 ), can be polished lambda/2 or better at 632 nm and scratch-dig can be 40-20 or better. 
         [0031]    Front mirror  230  can generally have a hole or aperture  232  for entrance and exit beams. Consistent with the disclosure, each of the mirrors  230  and  240  can have a silicon wafer bonded to its back, such that the respective silicon wafer has the same or larger diameter than the diameter of cylindrical glass cell  210 . As disclosed herein, each silicon wafer (such as silicon disk  235  and silicon disk  245 ) can be prepared differently for front mirror  230  and back mirror  240 . For example, a hole can be opened on front silicon disk  235 , where the hole is available for the entrance and exit beams. A size of the hole on front silicon disk  235  can be as large as a surface antireflection (“AR”) coating area on entrance window  250 . An entrance window  250  can include AR coating on both sides in the central area of window  250  associated with the hole in front disk  250  and aperture  232 . Surface flatness of AR coated entrance window  250  can be lambda/2 at 632 nm and scratch-dig can be 40-20. Consistent with the embodiment of  FIG. 2 , entrance window  250  is not coated in an annular area from an outside edge (or perimeter) associated with front silicon disk  235  (which can be an annular area 2-3 mm from edge). Such a perimeter region of entrance window  250  can remain uncoated so that the annulus bare glass region is available for an anodic bond with front silicon disk  235 . Front mirror  230 , back mirror  240 , and cylindrical glass cell  210  can include PYREX glass (or other suitable borosilicate glass), and the coefficient of thermal expansion (“CTE”) of each of silicon disk  235  and silicon disk  245  can match the CTE of PYREX glass (or other suitable borosilicate glass), so there is no stress on the anodic bond components at high temperature. 
         [0032]    Before anodic bonding, the back surfaces of front mirror  230  and back mirror  240 , the portions of the cylindrical glass cell  210  intended to bond with the silicon disks, and the annular region of entrance window  250  can be cleaned with RCA acid bath or PIRANHA solution. 
         [0033]    As discussed above, the front mirror  230  can have aperture  232  for an entrance and exit beam. Moreover, consistent with the disclosure, the opening of aperture  232  on the back side of front mirror  230  can be much larger than a corresponding opening on the curvature side of front mirror  230 . Such a configuration can allow for easy injection input beam at a large angle. After cleaning front mirror  230  and front silicon disk  235 , an anodic bond can be applied on the components in a clean room or a dust free environment, so that the front mirror  230  and front silicon disk  235  are bonded as shown in  FIG. 3 . The back silicon disk  245  need not have central hole. After front silicon disk  235  is anodically bonded to back surface of front mirror  230 , the non-central annular region of entrance window  250  can be anodically bonded to the front silicon disk  235  in order to hold entrance window  250  and front mirror  230  together. 
         [0034]    Consistent with the disclosure, back mirror  240  and back silicon disk  245  can be anodically bonded together in the manner described above for front mirror  230  and front silicon disk  235 . Moreover, an additional uncoated PYREX window (not shown) or other suitable borosilicate glass window can be bonded to the exposed (back) side of silicon disk  245 . Such an additional window can provide (for example, and without limitation) structural support to multi-pass cell  200 . 
         [0035]    The bonded combination of front mirror  230 , entrance window  250 , and front silicon disk  235 , and the bonded combination of back mirror  240  and back silicon disk  245  can be placed on optical alignment stages in order to set a rotation angle and distance necessary for operation of a multi-pass cavity. For example, front mirror  230  can be configured with a rotation stage with tilt mechanism and back mirror  240  can be configured with a rotation stage with tilt mechanism, all configured on a translation stage so as to allow for the necessary degrees of freedom to position both mirrors (i.e., front mirror  230  and back mirror  240 ). After configuring front mirror  230  and back mirror  240  to have a desired beam pattern and number of beam passes between the mirrors on an optical alignment stage, a distance can be measured from back surface of front mirror  230  to the back surface of the back mirror  240 . That distance will be the length of cylindrical glass cell  210  from one end to other end. The distance measurement can be done with tolerance of +/−3 micron. In addition, for purposes of later ensuring that the front mirror  230  and the back mirror  240  are oriented correctly in the assembled multi-pass cell  200 , orientation marks can be placed on the edges of front mirror  230  and back mirror  240  while they are fixed on the optical alignment stage. 
         [0036]    Cylindrical glass cell  210  can be configured with stem  220  to connect to a vacuum system (not shown) for evacuating and filling a completed multi-pass cell  200  with alkali-metal vapor. The stem  220  can be attached to cylindrical glass cell  210  using glass blowing techniques. After configuring cylindrical glass cell  210  with stem  220 , cylindrical glass cell  210  can be cut from both ends so that it exhibits the distance required of the multi-pass cell  200  (i.e., the distance measured above). 
         [0037]    In order to take the subsequent grinding and polishing into account, both ends of cylindrical glass cell  210  can be configured to have an additional extra 0.5 mm length. Moreover, both ends of cylindrical glass cell  210  can be polished so that they can be anodically bonded to the front silicon disk  235  and the back silicon disk  245  (where both the front and back disks  235  and  245  are already anodically bonded to front mirror  230  and back mirror  240 ). Surface finish can be better than lambda/2 at 632 nm. Scratch-dig can be 10-5 on both end surfaces after polishing. 
         [0038]    After the grinding and polishing processes are completed, cylindrical glass cell  210  can exhibit the previously measured distance with a tolerance of +/−10 micron. Moreover, parallelism between both ends can preferably exhibit a tolerance of +/−1 min or less. 
         [0039]    To complete assembly of multi-pass cell  200 , the combination of entrance window  250 , front silicon disk  235 , and front mirror  230  can be anodically bonded to one end of cylindrical glass cell  210 . Specifically, because the diameter of front mirror  230  is smaller than the diameter of front silicon disk  235 , there can be sufficient annulus area on front silicon disk  235  to anodically bond front silicon disk  235  to the edge of cylindrical glass cell  210 . Moreover, the relative orientation of cylindrical glass cell  210  to front mirror  230  can be determined and/or fixed by examining the mark (described earlier) previously placed on the edge of front mirror  230 . After orienting and centering the front mirror  230  with cylindrical glass cell  210 , an anodic bond can be applied to bond the front silicon disk  235  and the cylindrical glass cell  210  and vacuum-seal one end of multi-pass cell  200 . This is depicted in  FIG. 3 . 
         [0040]    The bonded combination of back mirror  240  and back silicon disk  245  can be aligned and oriented relative to front mirror  230  (which has been anodically bonded to the cylindrical glass cell  210 ). For example, the bonded combination of front mirror  230  and cylindrical glass cell  210  can be positioned and held with any suitable mechanical assembly tool, and which can also be used to clamp the bonded combination of back mirror  240  and back silicon disk  245 . For example, the bonded combination of back mirror  240  and back silicon disk  245  can be mounted on a rotation mount and then mounted on XY translation stage. Moreover, back silicon disk  245  can be positioned against the open end of cylindrical glass tube  210 . Accordingly, the bonded combination of back mirror  240  and back silicon disk  245  can be centered and rotated relative to the front mirror  230  in order to configure the system as a whole to provide the designed beam pattern and number of beam passes on an active optical alignment stage. Such active alignment is depicted in  FIG. 4 . Because cylindrical glass tube  210  is already configured to provide the total length of the designed multi-pass cavity (through the length of multi-pass cell  200 )—including the thicknesses of both front mirror  230  and back mirror  240 , a designed beam pattern and number of beam passes can be controlled by rotating and centering the back mirror  240 . After completing back mirror  240  alignment in order to provide the designed multi-pass beam pattern, a clamping mechanism can be used to hold back mirror  240  in position as it is aligned relative to front mirror  230 . Accordingly, back silicon disk  245  can then be anodically bonded to cylindrical glass cell  210  without releasing the clamping mechanism. 
         [0041]    After completing the anodic bond between back silicon disk  245  and cylindrical glass cell  210 , the back mirror  240  (and the multi-pass cell  200 ) can be released from the clamping mechanism. Completed multi-pass cell  200  is depicted in  FIG. 5 . 
         [0042]    The anodic bond between cylindrical glass cell  210 , front silicon disk  235  (including entrance window  250 ), and back silicon disk  345  provide a vacuum-tight enclosed cell. A vacuum leak test can be applied from stem  220  to verify proper anodic bonding. Multi-pass cell  200  can be evacuated and filled with alkali metal vapor (or other chemically reactive substance, or substance desired to be free of contamination) through the stem  220 . 
         [0043]    Another embodiment consistent with this disclosure can begin with the alignment process depicted in  FIG. 6 . For example, front mirror  630  (including aperture  632 ) and back mirror  640  can be mounted on optical alignment stage  660  in order to provide a designed number of beam passes and to provide a beam pattern in cavity. Rotation and tilt stage  660 - 1  can provide rotation and tilt to front mirror  630 , and rotation and tilt stage  660 - 2  can provide rotation and tilt to back mirror  640 . Moreover, a translation stage (not shown) can provide translation control to back mirror  640  along direction  662 . 
         [0044]    After alignment is complete, front mirror  630  and back mirror  640  can be fixed on a V-grove aluminum block  770  as shown in  FIGS. 7 and 8  without disturbing the designed beam pattern and measured output power of the designed cavity. As described further below, hot wax (not shown) can be used to maintain the cavity configuration without disturbing output beam power. 
         [0045]    For example, V-block  770  can be heated to wax melting temperature in order to melt wax on the V-block  770 . When the melted wax is ready, V-block  770  can be moved (such as in direction  735 ) to touch the mirrors—as from the bottom of cavity. Preferably, the amount of wax can be just enough to cover a bottom edge of the mirrors by about 1-2 mm. The sides of the groove in the V-block  770  can also have wax in order to hold the designed cavity from the edge of front mirror  630  and back mirror  640 . As the wax cools, front mirror  630  and back mirror  640  can be held in V-block  770 . When wax and V-block  770  get to room temperature, the front mirror  630  and back mirror  640  can be released from their mounts on optical alignment stage  660 . At this point, V-block  770  will have a multi-pass cavity setup on it. 
         [0046]    To prepare the front mirror  630  and the back mirror  840  for bonding to a silicon substrate, front mirror  630  and back mirror  640  can be cut with a diamond rotating saw from a top edge of the mirrors. For example, each of the front mirror  630  and the back mirror  640  can be cut so as to provide a 2-3 mm 2  flat surface area for grinding and polishing as shown in  FIG. 8 . Front mirror  830 , after cutting, provides a flat portion  885  for bonding to a silicon substrate. Similarly, back mirror  840 , after cutting, provides a flat portion  886  for bonding to a silicon substrate. Accordingly, portion  885  of front mirror  830  can stand on and be bonded to a flat silicon substrate surface, and portion  886  of back mirror  840  can stand on and be bonded to a flat silicon substrate surface. 
         [0047]    The coated surfaces of front mirror  830  and back mirror  840  can be protected during grinding and polishing. Surface flatness of portions  885  and  886  and quality of the flattened edge surface can be better than lambda/2 and 60-40 scratch-dig or better respectively. 
         [0048]    After grinding, polishing and cleaning as previously described, front mirror  830  and back mirror  840  can be anodically bonded to a flat silicon substrate. Silicon substrate  935 , shown in  FIG. 9 , can be configured to hold both front mirror  830  and back mirror  840 , can be configured to be 2-3 mm longer than the designed cavity length, and a width of silicon substrate  935  can be 2-3 mm shorter than the diameters of front mirror  830  and back mirror  840 . 
         [0049]    Preferably, an anodic bond can be applied to one mirror at a time. And preferably, the front mirror  830  can be bonded on silicon substrate  935  first as depicted in  FIG. 9 . After front mirror  830  is bonded to silicon substrate  935 , bonded front mirror  830  and silicon substrate  935  can be mounted on a mechanical assembly  1000 , which can include a clamping mechanism  1074  for back mirror  840 . Preferably, back mirror  840  can be held with an extension post (such as with vacuum holder  1076 ) to a goniometer to be tilted left or right on its flattened portion  886 . Goniometer can be mounted on X Y Z stages to place back mirror  840  on the silicon substrate  935 . Accordingly, all three degrees of freedom indicated with arrows  1077  are available. A designed multi-pass beam pattern and number of beam passes are already determined by the orientation of the flattened portions  885  and  886  of front mirror  830  and back mirror  840 , respectively. Because front mirror  830  is already anodically bonded to silicon substrate  935 , a multi-pass pattern can be realigned by tilting back mirror  840  at a distance corresponding to cavity length. Cavity length can be adjusted by using a Z (horizontal) transition stage (not shown). When an output beam power and a multi-pass pattern are aligned as previously set (see  FIG. 10 ), back mirror  840  can be clamped on silicon substrate  935  using clamping mechanism  1074  so that back mirror  840  is held together with front mirror  830  (which is held by clamping mechanism  1072 ). 
         [0050]    Mechanical assembly  1000  can include a metal base  1070  and electrically isolated posts (which are portions of clamping mechanisms  1072  and  1074 ). Spring-loaded screws  1073  and  1075  can be used to fix front mirror  830  and back mirror  840  within the electrically isolated posts. The electrically isolated posts can include (for example) ceramic material. 
         [0051]    Back mirror  840  can now preferably be taken off the extension post  1076 . Mechanical assembly  1000  now carries front mirror  830  and clamped back mirror  840  on silicon substrate  935 . An anodic bond now can be applied on clamped back mirror  840  while the configuration is on the mechanical assembly tool. After the anodic bond is applied between the back mirror  840  and the silicon substrate  935 , the designed multi-pass cavity is fixed and bonded on silicon substrate  935 . This is depicted in  FIG. 11 . 
         [0052]    Fixed multi-pass cavity  1100  of  FIG. 11  can next be bonded onto the base of a rectangular glass cell, such as rectangular glass cell  1210  of  FIG. 12 . As above, rectangular glass cell  1210  can have a stem  1220 . And again, both ends of rectangular glass cell  1210  can polished as previously described in connection with  FIGS. 2-5 . 
         [0053]    After both ends of rectangular glass cell  1210  are prepared for anodic bonding, fixed multi-pass cavity  1100  can be bonded on one of the inner surfaces of rectangular glass cell  1210 , such as the bottom inner surface  1210 - 2 . 
         [0054]    After such anodic bonding, rectangular glass cell  1210  can be closed and sealed with PYREX windows (or other suitable borosilicate glass) for holding vacuum and alkali-metal vapor gas. As previously described in connection with multi-pass cell  200 , completed multi-pass cell  1200  can be configured to withstand chemically reactive substances (gas or liquid), such as alkali-metal vapor, at high temperature for prolonged periods of time. In addition, or alternatively, the multi-pass cell  1200  can be configured to contain any substance (gas or liquid) that is desired to be free of contamination. A process for bonding an entrance window  1250  on an entrance side of rectangular glass cell  1210  has been already described in connection with  FIGS. 2-5 . A similar anodic bonding method can apply the other end of rectangular tube cell with uncoated PYREX window (not shown), or other suitable borosilicate gas, onto silicon wafer  1215 . After front and back windows are sealed, rectangular glass cell  1210  provides an internal fixed multi-pass cavity. This is depicted in  FIG. 13  as multi-pass cell  1200 . 
         [0055]    Consistent with yet another embodiment, a glass-blown vapor cell  1410  can be configured to provide an optically high quality vapor cell. Consistent with this disclosure, this embodiment can be implemented with any PYREX vapor cell or any cell using suitable borosilicate glass (such as cell  1410  of  FIG. 14 ), and replaces an entrance side  1410 - 3  of the cell  1410  (which may otherwise provide wave front distortion) with an optical quality and AR coated window  1440 . This embodiment also allows one to easily insert a fixed multi-pass cavity into the cell  1410 , and allows one to seal the cell  1410 . 
         [0056]    Consistent with this embodiment, front mirror  830  and back mirror  840  can be bonded on a silicon substrate  935  as has been described above in connection with  FIGS. 6-11 . Accordingly, the front mirror  830  and the back mirror  840  can be provided as anodically bonded on silicon substrate  935 . Preferably, however, the dimension of the silicon substrate in this embodiment can be 4-8 mm longer than the length of the fixed multi-pass cavity, and also larger (in width) than the diameters of front mirror  830  and back mirror  840 . 
         [0057]    Further consistent with this embodiment, a rectangular glass cell  1410  is provided using glass blowing techniques. The rectangular glass cell can be core drilled from entrance side  1410 - 3  and can have a rectangular opening cut off from side  1410 - 2  as shown in  FIG. 14 . 
         [0058]    Accordingly, any distorted glass surface on an entrance side  1410 - 3  of the rectangular glass cell  1410  can be removed by drilling a hole from the entrance side  1410 - 3 . Moreover, polished glass on the edges of hole  1410 - 3  can be used for anodic bonding to bond a double-sided AR coated window  1450  to silicon disk  1455  (such as a silicon wafer). 
         [0059]    The rectangular opening  1410 - 2  provides an opening to insert the fixed multi-pass cavity including front mirror  830 , back mirror  840 , and silicon substrate  935 . The edges of the rectangular opening  1410 - 2  can be used for anodic bonding to substrate  935  as shown in  FIG. 15  to seal the multi-pass cell  1400 . The surfaces which will be anodically bonded need to be polished before anodic bonding as described previously. 
         [0060]    Because the entrance side  1410 - 3  is bonded to an AR coated window  1450  and rectangular opening  1410 - 2  is bonded to silicon substrate  935 , the multi-pass cell  1400  can be completely sealed to hold vacuum. 
         [0061]    Other embodiments 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.