Abstract:
Inverting optics are used to invert, with respect to the dispersion plane, the wavefront of a monochromator employing a beam making more than one pass through the dispersing medium. Further, the inverting functionality can be turned-on or turned-off, thereby reversibly converting between additive and subtractive monochromator architectures. Inversion reversal is accomplished by reorienting the inverting optics orthogonally about an axis coaxial with the beam, either back and forth or monotonically, or by displacing portions or all of the inverting optics into and out of the beam. Examples of inverting optics include Dove prisms and equivalent multiple all-reflective surfaces. The system and method can be applied to two-pass and other multi-pass monochromators and to dual and other multiple serial monochromator configurations using diffraction gratings or other dispersing elements.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a Continuation-in-Part claiming priority benefit of commonly assigned U.S. patent application Ser. No. 10/349,898 entitled “DYNAMIC METHOD FOR CHANGING THE OPERATION OF A MULTIPLE-PASS MONOCHROMATOR BETWEEN ADDITIVE AND SUBTRACTIVE MODES,” filed Jan. 23, 2003, the disclosure of which is hereby incorporated herein by reference. 

   TECHNICAL FIELD 
   The invention relates to optical spectral analysis and particularly to dynamic systems and methods for changing the operation of a multiple-pass monochromator between additive and subtractive modes. 
   BACKGROUND OF THE INVENTION 
   It is possible for a light beam to make multiple dispersive passes with respect to a dispersive medium, but for simplicity, a double-pass case illustrates the relevant principles. 
     FIG. 1  illustrates schematically a current optical spectrum analyzer (OSA) instrument (see Agilent Technologies data sheet 8614xB Optical Spectrum Analyzer Family Technical Specifications), incorporating a double-pass monochromator  10  having diffraction grating  17  as a dispersive element. An input beam  12   a  typically entering through input fiber  11 , which can serve as an input aperture, is directed by first mirror  13   a  and second mirror  13   b  through collimating element  16  in two passes  12   a  and  12   c  onto diffraction grating  17 . Between the two passes, the beam is directed through an intermediate resolution defining aperture that is normally incorporated into slit wheel  14  to provide a range of aperture sizes. Collimating element  16 , for example a lens, refocuses diffracted light from the surface of grating  17  in each pass  12   b  and  12   d  back into the optical plane of slit wheel  14 . Output beam  12   e  is deflected by output mirror  13   c  into output fiber  18 , which can function as an output aperture of monochromator  10 , such that it reduces stray light in the system and, due to diffraction and coupling effects, improves resolution bandwidth compared to a single-pass monochromator. 
   Referring to the coordinate axes in  FIG. 1 , the grating dispersion direction is parallel to the y-axis, which is perpendicular to the plane of the figure, whereas the non-dispersion direction is parallel to the x-axis, pointing upward parallel to the plane of the figure. Both x and y axes are mutually perpendicular to the z-axis, which is essentially parallel to the effective propagation direction of light beam passes  12   a ,  12   b ,  12   c , and  12   d . There are two varieties of double-pass monochromator, the additive or double-dispersive monochromator, and the subtractive or re-condensing monochromator. Each of these architectures provides specific advantages depending on the application. In an additive monochromator, the orientation of the wavefront of light impinging on dispersing component  17  is the same, for example plus y, in both first pass and second pass. Because the light beam is focused between two passes  12   a  and  12   c  through intermediate slit or aperture system  14  (where the wavefront both in and perpendicular to the plane of dispersion is naturally inverted to −y, −x), the additive architecture requires introduction of an additional top-to-bottom inversion (−y to plus y) of the wavefront in the plane of dispersion between the first contact and the second contact with dispersing element  17 . In current instruments, component  15  typically represents a half-wave plate that rotates the polarization of the light beam by 90 degrees between first pass  12   b  and second pass  12   c . Alternatively (or additionally), component  15  illustratively represents a top-to-bottom inverter (−y to plus y) of the wavefront, which, when inserted in second pass  12   c  immediately after reflection from second mirror  13   b , converts monochromator  10  from subtractive to additive architecture. This results in the dispersion in second pass  12   c  being additive to the dispersion in first pass  12   a  (both plus y or both −y), producing a distinct narrowing in system resolution of a double-pass monochromator compared with that of a single-pass monochromator. 
   In the prior art, depicted by subtractive monochromator  10 , simply by focusing the light through the intermediate slit in slit wheel  14 , the wavefront is inverted top to bottom (−y) relative to the orientation of first pass wavefront orientation (plus y) at dispersing element  17 . In general, it is not necessary to further invert the wavefront, as the required wavefront inversion is already accomplished, such that a separate wavefront-inverting component illustratively represented by component  15  is not needed. The spectrally dispersed and filtered (by intermediate slit at slit wheel  14 ) light from first pass  12   b  is spectrally recombined by dispersing element  17  in second pass  12   c . The result is a spectrally uniform and highly compact focal spot at the monochromator exit aperture or slit  18 . Since the light is already spectrally dispersed and prefiltered at intermediate slit  14  in the first pass, system resolution for a two-pass subtractive monochromator is substantially the same as system resolution of a corresponding single pass monochromator. 
   Additionally, all path lengths of the light transmitted through a subtractive monochromator are made equal by having inverted the wavefront top-to-bottom. For example, in a reflective diffraction grating implementation, diffraction grating  17  is tilted to align the desired wavelength to intermediate slit  14 , thereby introducing a wavelength-dependent path length. For example, the beam of light impinging on the grating surface encounters path lengths varying monotonically with position along the length of the grating in the plane of dispersion, resulting in a range of path lengths corresponding to the range of wavelengths transmitted through output slit  18 . Inverting the wavefront between passes cancels this path length difference and hence the temporal dispersion that results from path length difference. The additive monochromator architecture does not cancel this path length variation, but rather effectively doubles it in the second pass. 
   In an optical spectrum analyzer (OSA) employing a monochromator as its optical engine, a conventional operational procedure is to sweep a reflective diffraction grating (or other dispersive component) in angle in order to obtain the spectra of various optical wavelengths. The spectrally dispersed light is propagated onto a variable width output slit and is sensed in an analog fashion by a single finite area photodiode or similar detector. The speed at which an OSA can determine spectral information derived from a photodetector signal is contingent on many factors, including the physical size (active area) of the photodiode. Small area photodiodes are generally faster and have a direct impact on the speed of an OSA. The subtractive monochromator engine will provide an output spot of filtered and re-combined spectra that can be focused within a small detector active area. An additive monochromator produces a highly spread or dispersed spot on the output slit. This requires a large area detector to collect the exiting light. In addition to being slow, the large area of the detector is inclined to collect unwanted or “stray” light. This necessitates some sort of noise suppression, for example a chopping system, to minimize optical noise. Such accessories add both expense and complexity. On the other hand, an additive monochromator has a significantly better (narrower wavelength spread) resolution, which is desirable in certain applications. 
   A narrower resolution in a multiple-pass monochromator is generally useful only when the slits (both internal and exit) are positioned near their narrow settings. In such circumstances, even an additive monochromator would function well with a small area photodiode. It is when the slits are enlarged that the small area detector will not capture the entire incident cone of output light from an additive monochromator. Given these considerations, if a component could be added to a monochromator architecture that would allow the architecture to switch reversibly between additive and subtractive, then the functionality could be additive and achieve the narrowest resolution bandwidth on the smallest slit, and could revert to a subtractive configuration for all wider slits in order to increase speed. This maximizes OSA measurement speeds at all slit widths, while providing the opportunity for the narrowest resolution bandwidth on the smallest slit, which is the only filter setting where an improvement in resolution bandwidth offers substantial value. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention is directed to a system and method in which inverting optics are used to invert, with respect to the dispersion plane, the wavefront of a monochromator beam making more than one pass through the dispersing medium. Further, the inverting functionality can be turned-on or turned-off, thereby reversibly converting between additive and subtractive monochromator architectures. In various embodiments, reversal is accomplished by reorienting the inverting optics orthogonally about an axis coaxial with the beam, either back and forth or monotonically, or by displacing portions or all of the inverting optics into and out of the beam. Examples of inverting optics include Dove prisms, other inverting prisms, and equivalent multiple all-reflective surfaces. The system and method can be applied to two-pass and other multi-pass monochromators and to dual and other multiple serial monochromator configurations using diffraction gratings or other dispersing elements. 
   The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
       FIG. 1  illustrates schematically a current optical spectrum analyzer (OSA) instrument incorporating a double-pass monochromator; 
       FIGS. 2A–2B  illustrate the wavefront-inverting properties of a Dove refractive prism; 
       FIGS. 3A and 3B  represent schematically a mirror assembly functionally equivalent to a Dove prism; 
       FIGS. 4A and 4B  illustrate further embodiments for accomplishing reversible inversion of a beam wavefront, involving substantially linear insertion into and retraction from the input beam of a pick-off prism; and 
       FIGS. 5A–5C  are schematic diagrams depicting a further reflective Dove assembly implementation between two Czerny-Turner monochromator systems in a serial dual monochromator. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In accordance with embodiments of the present invention, a Dove refractive prism (see for example M. Born and E. Wolf, “Principles of Optics,” 6 th  Edition, Pergamon Press, 1980, p. 244) or equivalent mirror system added to a monochromator architecture allows the architecture to switch reversibly between additive and subtractive, such that the functionality can for example be additive and achieve the narrowest resolution bandwidth with the narrowest slit and can revert to a subtractive functionality and achieve increased speed for all wider slits.  FIGS. 2A–2B  illustrate the well-known wavefront-inverting properties of a Dove refractive prism  20 . When Dove prism  20  is oriented as illustrated in  FIG. 2A  in input light beam  201  propagating along the z-axis, input wavefront  203  aligned with respective horizontal and vertical x- and y-axes  210  and  209   a  when passed through Dove prism  20  is inverted 180 degrees relative to the y-axis to produce output wavefront  204  aligned with inverted vertical y-axis  209   b  in output light beam  202 . No wavefront inversion occurs relative to horizontal x-axis  210  when passing input beam  201  through Dove prism  20 . For ease of understanding the wavefront-inverting properties of Dove prism  20 , the extreme rays of input beam  201  in the top and bottom y-axis positions are labeled respectively top ray  205   a  and bottom ray  205   b . At input face  21  of Dove prism  20 , both top ray  205   a  and bottom ray  205   b  are refracted downward in respective rays  206   a  and  206   b , and then are reflected at reflective face  22  (out of view within Dove prism  20  in  FIG. 2A ) to produce respective rays  207   a  and  207   b , where ray  207   b  is now the top ray and ray  207   a  is now the bottom ray, as depicted in  FIG. 2A . Rays  207   a ,  207   b  are then refracted at output face  23  of Dove prism  20  to propagate as respective output rays  208   a ,  208   b  within output beam  202  in the same z-axis direction as original input beam  201 , forming wavefront  204  inverted relative to the vertical y-axis as depicted by the diamond symbols on the vertical wavefront axes in  FIG. 2A . 
     FIG. 2B  illustrates Dove prism  20  reoriented orthogonally about the z-axis propagation direction of input beam  201  relative to the orientation depicted in  FIG. 2A . Through the same processes as described in connection with  FIG. 2A , in this configuration output wavefront  204  is inverted relative to the x-axis, such that horizontal axis  210   a  of input wavefront  203  is inverted to become horizontal axis  210   b  at output wavefront  204 . The wavefront inversion is horizontal only, as depicted by the circular symbols on the horizontal wavefront axes in  FIG. 2B , leaving wavefront vertical axis orientation  209  unaltered. 
   If for example Dove prism  20  is positioned in a beam propagating through a two-pass monochromator, and if the dispersion direction of the diffraction grating or other dispersing component is parallel to the vertical y-axis, then the configuration depicted in  FIG. 2A  will invert the orientation of the wavefront relative to the vertical plane of dispersion. If the monochromator was in a subtractive configuration without Dove prism  20 , it will be converted to an additive state if Dove prism  20  is positioned between the first and second encounters of the beam with the diffraction grating or other dispersing component. To nullify the switching effect, Dove prism  20  is further reoriented orthogonally about the z-axis propagation direction relative to the orientation in  FIG. 2A , inverting the wavefront orientation in a direction orthogonal to the plane of dispersion and thereby imparting no effective change to the subtractive monochromator. Because it is a monolithic component, Dove prism  20  is easy to fabricate and align; however, because of aberration problems recognized in the art to be caused by dispersion accompanying refraction at surfaces  21  and  23 , it is a viable solution only in a collimated beam environment.  FIGS. 3A and 3B  represent schematically mirror assembly  30  functionally equivalent to Dove prism  20 . Such an all-reflective configuration does not introduce unwanted dispersion, but is generally more expensive and difficult to build, align and rotate. 
   In  FIGS. 3A and 3B , reflective input and output surfaces  31  and  33  respectively perform beam deflecting functions in mirror assembly  30  analogous to refractive input and output faces  21  and  23  of Dove prism  20 , and reflective surface  32  of mirror assembly  30  performs a function analogous to reflective face  22  of Dove prism  20 . In  FIG. 3A , input wavefront  203  is inverted relative to the vertical y-axis, producing output wavefront  204 , such that the direction of input wavefront vertical axis  209   a  is inverted to become output wavefront vertical axis  209   b , and horizontal wavefront axis  210  is unaltered. Conversely, in  FIG. 3B  in which mirror assembly  30  is reoriented orthogonally about the z-axis propagation direction relative to its orientation in  FIG. 3A , output wavefront  204  is inverted relative to the x-axis, such that horizontal axis  210   a  of input wavefront  203  is inverted to become horizontal axis  210   b  at output wavefront  204 . The wavefront inversion is horizontal, leaving wavefront vertical axis orientation  209  unaltered. In a manner similar to that described in connection with  FIG. 2A , outer rays  205   a ,  205   b ,  225   a ,  225   b  of input beam  201  can be traced through mirror assembly  30  to obtain the respective wavefront inversions. 
   To achieve reversible switching, configurations such as those depicted in  FIGS. 2A–3B  do not necessarily have to be reoriented back and forth about the z-axis propagation direction. They can simply be rotationally reoriented in orthogonal steps in the same monotonic direction for each transition between additive and subtractive architecture. They can also be inserted into and then retracted from the beam. As used herein, “orthogonally” denotes orientation at substantially right angles. In the present embodiments, an angle of 90 degrees is functionally ideal, although other angles ranging from approximately 80 degrees to approximately 100 degrees provide satisfactory performance. Similarly, although “coaxial” denotes ideal alignment, the term as used herein is intended to extend to a tolerance range of approximately plus and minus five degrees of misalignment. 
     FIGS. 4A and 4B  illustrate further embodiments for accomplishing reversible inversion of beam wavefront  203  having respective vertical and horizontal axes  209   a  and  210 , involving insertion into and retraction from input beam  201 , for example along horizontal direction  401   a , of pick-off prism  41   a  having reflectively coated wedge surfaces  42  and  43 , which deflect input beam  201  onto a substantially stationary reflective component  44  and then realign the reflected and wavefront-inverted beam  202  along its original z-axis direction. Referring to  FIG. 4A , when pick-off prism  41   a  is retracted, there is no effect on input beam  201 . Alternatively to pick-off prism  41   a , pick-off prism  41   b  can be inserted into and retracted from input beam  201  along vertical direction  401   b . Referring to  FIG. 4B , when pick-off prism  41   a  (or equivalently  41   b ) is inserted into input beam  201  in the illustrated orientation, the input beam is processed by the assembly incorporating pick-off prism  41   a  and stationary reflective component  44 , such that input wavefront  203  is inverted relative to the vertical y-axis to produce output wavefront  204  having inverted vertical wavefront axis  209   b . Output wavefront  204  is unaltered relative to input wavefront  203  relative to horizontal wavefront axis  210 . Input beam  201  is first deflected by reflective wedge surface  42  to reflective stationary surface  45 , where it is again reflected onto reflective stationary surface  46 . The reflected beam from reflective stationary surface  46  is then reflected from reflective stationary surface  47  onto reflective wedge surface  43 , from which it is deflected to form output beam  202  propagating in the original z-axis direction. Once again, in a manner similar to that described in connection with  FIG. 2A , top and bottom individual rays  405   a – 410   a  and  405   b – 410   b  of input beam  201  can be traced through the assembly of pick-off prism  41   a  and stationary reflective component  44  to obtain the described wavefront inversion in output beam  202 , such that the top output ray is ray  410   b  and bottom output ray is ray  410   a.    
   In accordance with the above-described embodiments, a Dove prism or equivalent mirror assembly can be implemented to invert the wavefront in a collimated beam after the light has been filtered by the internal slit of a four-pass monochromator, for example a Littman-Metcalf monochromator (M. Littman and H. Metcalf, “Spectrally Narrow Pulsed Dye Laser without Beam Expander,” Appl. Opt. 17, 1978, pp. 2224–2227). The prism or equivalent mirror assembly is positioned such as to invert the wavefront for the third and fourth passes from the diffraction grating (not shown) relative to the first and second passes, thereby creating an additive architecture. If the prism were reoriented orthogonally about an axis coaxial with the beam, the architecture would revert to subtractive, with wavefront inversion produced by the slit alone in both horizontal and vertical axes. 
     FIGS. 5A–5C  are schematic diagrams depicting a further reflective Dove assembly implementation between two Czerny-Turner monochromator systems  51   a ,  51   b  (see M. Czerny and F. Turner, “Über den Astigmatismus bei Spiegelspectrometern,” Z. Physik 61, 1930, pp. 792–797) in serial dual monochromator  50 . An input light beam is focused into first monochromator  51   a  through entrance slit  501   a , collimated by first parabolic mirror  502   a , spectrally dispersed by first diffraction grating  503   a  having grating lines  504   a  oriented parallel to the y-axis, and refocused by second parabolic mirror  502   b  onto internal slit spectral filter  501   b  between first monochromator  51   a  and second monochromator  51   b  of dual monochromator  50 . Dove reflective inversion assembly  52  is positioned in the diverging beam just after output focus  50   b  of first monochromator  51   a . In second monochromator  51   b , the dispersed and filtered beam diverging from internal focus slit  501   b  is recollimated at third parabolic mirror  502   c , further dispersed at second diffraction grating  503   b  having grating lines  504   b  parallel to grating lines  504   a , and refocused by fourth parabolic mirror  502   d  onto exit slit  501   c . Optional fold mirrors  505   a ,  505   b  are disposed to facilitate a compact overall configuration of dual monochromator  50 . A non-collimated architecture such as that depicted at the diverging beam location of Dove inversion assembly  52  in  FIG. 5A  requires the use of an all-reflective Dove embodiment. 
   In dual monochromator  50 , since grating lines  504   a  and  504   b  of both diffraction gratings  503   a  and  503   b  are oriented parallel to the y-axis, dispersion direction  510  is consequently parallel to the x-axis in accordance with diffraction grating theory and practice well known in the art.  FIG. 5B  is a schematic diagram illustrating in more detail a portion of  FIG. 5A  including Dove inversion assembly  52 . From fold mirror  505   a  the beam converges onto internal focus slit  501   b . Wavefront  511  with vertical wavefront axis  531  and horizontal wavefront axis  532  in the converging beam from fold mirror  505   a  is inverted in both transverse axes as indicated by the arrow directions at wavefront  512  in the diverging beam after passing through internal focus slit  501   b  and prior to impinging on Dove inversion assembly  52 , incorporating reflective surfaces  521 – 523 . Dove inversion assembly  52  is oriented to invert wavefront  512  in the y-axis direction only, which is orthogonal to dispersion direction  510 . At fold mirror  505   b  this results in wavefront  513  having inverted y-axis  533  and non-inverted x-axis  534  relative to wavefront  512 . The aggregate wavefront inversion between first diffraction grating  503   a  and second diffraction grating  503   b  is produced by passing the beam through combined internal focus slit  501   b  and Dove inversion assembly  52 . Internal focus slit  501   b  produces wavefront inversion in both dispersion and non-dispersion axes, whereas Dove inversion assembly  52  produces wavefront inversion in the non-dispersion direction only. Since there is only a single wavefront inversion in the dispersion direction, the architecture of dual monochromator  50  is subtractive. 
     FIG. 5C  is a detailed schematic diagram illustrating the same components as in  FIG. 5B , except that all reflective surfaces  521 – 523  of Dove inversion assembly  52  are reoriented orthogonally about the beam propagation axis. In this case, Dove inversion assembly  52  is oriented to invert wavefront  512  in the x-axis direction only, which is parallel to dispersion direction  510 . At fold mirror  505   b  this results in wavefront  513  having inverted x-axis  536  and non-inverted y-axis  535  relative to wavefront  512 . The aggregate wavefront inversion between first diffraction grating  503   a  and second diffraction grating  503   b  is then that internal focus slit  501   b  produces wavefront inversion in both dispersion and non-dispersion axes, whereas Dove inversion assembly  52  produces wavefront inversion in the dispersion direction only. Since there are now two successive wavefront inversions in the dispersion direction, the architecture of dual monochromator  50  is additive. In accordance with the teachings of the embodiments, the architecture of dual monochromator  50  is reversibly convertible between subtractive and additive by reorienting Dove assembly  52  orthogonally about the propagation axis of the beam. 
   Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.