BACKGROUND OF THE INVENTION 
     Optical monochromators characterize spectral content of optical signals, such as optical channels within dense wavelength division multiplexed (DWDM) optical communication systems. High signal selectivity and low insertion loss are increasingly important performance parameters of a monochromator as the optical channels within DWDM systems become more closely spaced. For example, signal selectivity of at least 35 dB at 0.4 nanometer offsets from the optical channel&#39;s center wavelength is desirable to sufficiently characterize wavelength, power and signal-to-noise ratio (SNR) of optical signals within a DWDM system having a channel spacing of 100 GHz, whereas higher signal selectivity, at least 35 dB at 0.2 nanometer offsets, is desirable for a channel spacing of 50 GHz. Low insertion loss is important for measuring low amplitude noise in SNR measurements of a DWDM system. 
     Grating-based optical monochromators that use multipass configurations have high signal selectivity. However, grating-based monochromators that are physically compact, such as those using a Littman-Metcalf configuration, typically have high insertion loss which degrades measurement sensitivity and may render the monochromators unsuitable for measuring SNR. Walk-off crystals separate polarization states of optical signals into separate optical beams which enables the polarization states to be aligned to minimize insertion loss, thereby improving the measurement sensitivity of the monochromator. However, optical beams having large diameter are not readily accommodated by presently available walk-off crystals, which reduces illumination area of the grating, in turn decreasing the signal selectivity of the optical monochromator. Accordingly, there is a need for an optical monochromator that has both high signal selectivity and low insertion loss. 
     SUMMARY OF THE INVENTION 
     According to the preferred embodiment of the present invention an optical monochromator has high signal selectivity and low insertion loss, and is well-suited for characterizing a variety of optical signals, including closely-spaced optical channels within DWDM systems. The optical monochromator is physically compact and includes a bulk-optic polarization beam splitter having a polarizing interface and an angled surface that separate orthogonal polarization states of an applied optical signal into separate optical beams. Low insertion loss is achieved by reconciling the polarization states of the separate optical beams to an optimum polarization state that minimizes insertion loss when the optical beams are applied to a dispersive element. High signal selectivity is achieved using a multipass configuration and by illuminating large areas of the dispersive element, since large beam diameters are accommodated by the bulk-optic polarization beam splitter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an optical monochromator constructed according to a preferred embodiment of the present invention; and 
     FIGS. 2A and 2B show detailed views of alternative types of bulk-optic polarization beam splitters included in the optical monochromator shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows an optical monochromator  10  constructed according to a preferred embodiment of the present invention. The monochromator  10  is useful for characterizing spectral content of applied optical signals and is typically included within an optical spectrum analyzer or other measurement instrument, or alternatively, within an optical communication system monitor. The monochromator  10  includes a bulk-optic polarization beam splitter  12 , a polarization rotator  14 , a dispersive element  16 , such as a diffraction grating, and a reflector  18 . An optical input beam  11  is applied to the bulk-optic polarization beam splitter  12  from an optical fiber  24  or other source. The light within the input beam  11  is separated into two orthogonal polarization components or polarization states, designated as S and P, by the bulk-optic polarization beam splitter  12 . Detailed views of the bulk-optic polarization beam splitter  12  are shown in FIGS. 2A and 2B. A polarizing interface  21  within the bulk-optic polarization beam splitter  12  transmits P-polarized light to the back surface  20  where it emerges as a P-polarized optical beam P 1 . The interface  21  of the bulk-optic polarization beam splitter  12  reflects S-polarized light toward an angled surface  22  of the bulk-optic polarization beam splitter  12  where the S-polarized light is reflected and directed toward the back surface  20 . The S-polarized light also emerges from the back surface  20  as an S-polarized optical beam S 1 . 
     The optical beam P 1  propagates through polarization rotator  14  which rotates the polarization state of the optical beam P 1  to the S-polarization state, forming an S-polarized optical beam S 2 . 
     The S-polarized optical beams S 1 , S 2  are incident on the dispersive element  16 . The dispersive element  16  is a diffraction grating, prism or other device that spatially separates applied optical beams according to the wavelength components or wavelength segments of the optical beams. Optical beam S 3  and optical beam S 4  emerge from the dispersive element  16  and correspond to a preselected optical wavelength segment of the applied optical input beam  11 . Other optical beams corresponding to optical wavelengths of the input beam  11  emerge from the dispersive element  16  at various dispersion angles relative to the surface of the dispersive element, however, only optical beam S 3  and optical beam S 4  are shown in FIG.  1 . The optical beams S 3 , S 4  which correspond to a predetermined optical wavelength segment of the applied optical input beam  11  are spatially separated from the other optical beams (not shown) that correspond to other optical wavelength components of the input beam  11 . 
     The optical beams S 3 , S 4  are incident on the reflector  18 , which is positioned to receive these output beams S 3 , S 4  and which redirects the reflected S-polarized optical beams S 5 , S 6  back toward the dispersive element  16 . In this example, the reflector  18  is a retro-reflector which directs the optical beam S 5  offset from optical beam S 3 , and directs the optical beam S 6  offset from optical beam S 4 . 
     The S-polarized optical beams S 7 , S 8  emerge from the dispersive element  16 . The optical beam S 7  propagates through polarization rotator  14  which changes the polarization state of the optical beam S 7  to the P-polarization state, forming P-polarized optical beam P 7 . The optical beam P 7  and optical beam S 8  are incident on the bulk-optic polarization beam splitter  12  which combines the optical beam P 7  and optical beam S 8  into output beam  23 . The P-polarized optical beam P 7  propagates through the interface  21  of the bulk-optic polarization beam splitter  12  while the S-polarized optical beam S 8  is incident on the angled surface  22  of the bulk-optic polarization beam splitter  12  where it is reflected and directed toward the interface  21 . The interface  21  then directs this S-polarized optical beam S 8  co-linear with the P-polarized beam P 7  to form the output beam  23 . 
     The monochromator  10  shown in FIG. 1 has the output beam  23  emerging from the bulk-optic polarization beam splitter  12  offset from the input beam  11 . Alternatively, the output beam  23  is coincident with the input beam  11 , for example, by using a reflector  18  which is a retro-reflector having an apex  28  positioned midway between optical beam S 3  and optical beam S 4 . This positioning directs optical beam SS to be co-linear with optical beam S 4  and optical beam S 6  to be co-linear with optical beam S 3 . When optical beam S 3  and optical beam S 6  are co-linear, optical beam S 1  and optical beam S 8  are co-linear. When optical beam S 4  and optical beam S 5  are co-linear, optical beam S 2  and optical beam S 7  are co-linear. With the co-linear arrangement of optical beams, the input beam  11  and the output beam  23  are coincident and the input beam  11  and output beam  23  are coupled to the optical monochromator  10  using a single fiber  24 . 
     Optical wavelength content is analyzed by detecting or otherwise processing the output beam  23 . FIG. 1 shows a single mode fiber  26  intercepting output beam  23 . Alternatively, a detector (not shown) intercepts the output beam  23 . When a single mode fiber  26  is used, low optical coupling loss is achieved through precise alignment of the output beam  23  emerging from the bulk-optic polarization beam splitter  12  and the fiber  26 . Precise alignment is provided when optical beam S 1  and optical beam P 1  from the bulk-optic polarization beam splitter  12  are parallel and when parallel arrangement of the optical beams is maintained within the monochromator  10  so that the optical beam S 8  and optical beam P 7  are parallel combined by the bulk-optic polarization beam splitter  12 . The bulk-optic polarization beam splitter  12  is constructed to provide for the parallel arrangement of the optical beams within the monochromator  10 . 
     FIGS. 2A and 2B show detailed views of alternative types of bulk-optic polarization beam splitters  12  included in the optical monochromator  10 . The bulk-optic polarization beam splitters  12  include two glass portions, a first portion  12   a  and a second portion  12   b . Optical beams S 1  and optical beam P 1  are shown emanating from the back surface  20  of the bulk-optic polarization beam splitters  12 . Two surfaces  22  and  27  of a first portion  12   a  of the bulk-optic polarization beam splitter  12  are formed parallel to provide for a parallel alignment of the optical beams S 1 , P 1 . Using known bulk-optic processing techniques for fabricating optical windows, a parallelism of 0.0006 degrees is achieved for the two surfaces  22 ,  27  of the first portion  12   a . Thin-film coating  29  on the surface  27  at the the interface  21  between the first portion  12   a  and second portion  12   b  causes polarization splitting of the applied optical beam  11 . The bulk-optic polarization beam splitter  12  shown in FIG. 2A has a continuous flat back surface  20  as a result of alignment of the first portion  12   a  and the second portion  12   b . The continuous back surface  20  maintains parallel orientation of the optical beams S 1 , P 1  at the transition between the back surface  20  and the medium surrounding the bulk-optic polarization beam splitter  12 . The bulk-optic polarization beam splitter  12  shown in FIG. 2B has a single, continuous flat back surface  20  that maintains parallel orientation of the optical beams S 1 , P 1  at the transition between the back surface  20  and the medium surrounding the bulk-optic polarization beam splitter  12 . 
     In this example, the optical monochromator is a Littman-Metcalf configuration in which uniformly polarized optical beams are incident on the dispersive element multiple times. The high signal selectivity of the monochromator  10  is attributable to the multiple passes of the optical beams on the dispersive element  16  and to the large areas A of the dispersive element  16  that are illuminated. Large illumination area is achieved because large diameter optical beams are readily accommodated by the bulk-optic polarization beam splitter  12 . Repeated selection of similar wavelength segments at each of the multiple passes results in increased signal selectivity as the number of passes increases. Alternatively, the monochromator  10  is a single-pass configuration and the uniformly polarized optical beams S 1 , S 2  are incident on the dispersive element  16 . In the single pass configuration, the reflector  18  is absent and optical beams S 3 , S 4  of the light dispersed by the dispersive element  16  are intercepted by a fiber, detector, aperture or other type of receiver. In the multipass configuration of FIG. 1 optical beam P 7  and optical beam S 8  are shown incident upon the bulk-optic polarization beam splitter  12 . Alternatively, optical beams emitted from the dispersive element  16  are intercepted by a fiber, detector, aperture or other type of receiver. 
     The insertion loss of the dispersive element  16  is minimized by assuring that the polarization state of the optical beams S 1 , S 2 , S 5 , S 6  incident on the dispersive element  16  are oriented relative to the dispersive element  16  for the maximum efficiency of the dispersive element  16 . 
     While the preferred embodiment of the present invention has been illustrated in detail, it should be apparent that modifications and adaptations to those embodiments may occur to one skilled in the art without departing from the scope of the present invention as set forth in the following claims.
An optical monochromator has high signal selectivity and low insertion loss, and is well-suited for characterizing a variety of optical signals, including closely-spaced optical channels within DWDM systems. The optical monochromator includes a bulk-optic polarization beam splitter that separates orthogonal polarization states of an applied optical signal into separate optical beams. Low insertion loss is achieved by reconciling the polarization states of the separate optical beams to an optimum polarization state that minimizes insertion loss when the optical beams are applied to a dispersive element. High signal selectivity is achieved using a multipass configuration and by illuminating large areas of the dispersive element, since large beam diameters are accommodated by the bulk-optic polarization beam splitter.