Abstract:
An arrangement for dispersing light comprises a blazed diffraction grating and a mirror. The blazed diffraction grating comprises a grating plane and a multiplicity of blazed facets. Each blazed facet is oriented at a blaze angle to the grating plane. The mirror couples to the blazed diffraction grating and is oriented parallel to the blazed facets. The arrangement permits a highly dispersive optical function in a very compact structure with low polarization dependent loss.

Description:
FIELD OF THE INVENTION 
   This invention relates to the field of optics. More particularly, this invention relates to the field of optics where light is dispersed. 
   BACKGROUND OF THE INVENTION 
   Diffraction gratings disperse light. A blazed diffraction grating disperses light into a single order. The diffraction gratings are used in devices such as spectrometers and scanning monochromators. The diffraction gratings are also used in WDM (wavelength division multiplex) optical communication. In the WDM optical communication, multiple wavelengths of light each carry a communication signal. Each of the multiple wavelengths of light forms a WDM channel. In DWDM (dense WDM) optical communication, a subset of the WDM optical communication, the WDM channels are spaced close together. A typical DWDM application operates at a wavelength band about 1,550 nm, has 90 channels, and has spacing of about 0.4 nm between adjacent channels. In the WDM optical communication, the diffraction gratings are used to demultiplex and to multiplex the WDM channels. 
   The diffraction gratings used in the WDM optical communication suffer from several deficiencies. A first deficiency is that the diffraction gratings produce a relatively small angular dispersion for adjacent WDM channels. In order to distinctly separate the WDM channels, the relatively small angular dispersion leads to a need for a long optical path. A second deficiency is that reflective diffraction gratings having a fine ruling density tend to exhibit polarization dependent loss, which can exceed 1 dB. 
   What is needed is a method of dispersing light that disperses light with a dispersion which is greater than what is available with a blazed diffraction grating. 
   What is needed is a method of dispersing light that disperses light with a dispersion which is greater than what is available with a blazed diffraction grating and which also reduces polarization dependent loss. 
   SUMMARY OF THE INVENTION 
   Embodiments of the present invention include an arrangement for dispersing light. The arrangement for dispersing light comprises a blazed diffraction grating and a mirror. The blazed diffraction grating comprises a grating plane and a multiplicity of blazed facets. Each blazed facet is oriented at a blaze angle to the grating plane. The mirror couples to the blazed diffraction grating. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates the preferred light dispersion arrangement of the present invention. 
       FIG. 2  illustrates blazed facets of a blazed diffraction grating of the preferred light dispersion arrangement of the present invention. 
       FIG. 3  illustrates the preferred light dispersion arrangement of the present invention demultiplexing multiple WDM channels. 
       FIG. 4  illustrates the preferred light dispersion arrangement of the present invention multiplexing the multiple WDM channels. 
       FIG. 5  illustrates an alternative light dispersion arrangement of the present invention. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   The preferred light dispersion arrangement of the present invention is illustrated in  FIG. 1 . The preferred light dispersion arrangement  10  comprises a blazed diffraction grating  12  and a mirror  14 . A blaze arrow  15  indicates a blaze direction for the blazed diffraction grating  12 . The mirror  14  couples to the blazed diffraction grating  12 . A mirror normal  16  and a grating normal  18  are coplanar. The angel between the mirror normal  16  and the grating normal  18  is γ. 
   A portion of the blazed diffraction grating  12  is further illustrated in  FIG. 2 . The portion  20  of the blazed diffraction grating  12  comprises a grating plane  22  and blazed facets  24 . The blazed facets  24  are at a blaze angle ψ relative to the grating plane  22 . 
   Referring to  FIG. 1 , in operation, collimated light  26  is directed onto the blazed diffraction grating  12  at a first incidence angle α 1 . The blazed diffraction grating  12  diffracts the collimated light  26  into a first diffraction angle β 1 , which forms first diffracted light  28 . The mirror  14  reflects the first diffracted light  28  back to the blazed diffraction grating  12 . The first diffracted light  28  is incident upon the blazed diffraction grating  12  at a second incidence angle α 2 . The blazed diffraction grating  12  diffracts the first diffracted light  18  into a second diffraction angle β 2 , which forms second diffracted light  30 . 
   It will be readily apparent to one skilled in the art that the first diffracted light  28  and the second diffracted light  30  depict light of a particular wavelength and that light of other wavelengths will diffract at angles other than the first and second diffraction angles, β 1  and β 2 . 
   A demultiplexing application of the preferred light dispersion arrangement  10  of the present invention is illustrated in  FIG. 3 . In the demultiplexing application  40 , a range of WDM channels ranging from a zeroth wavelength channel λ 0  to an ith wavelength channel λ i  are incident upon the blazed diffraction grating  12  as a collimated beam  42 . The blazed diffraction grating  12  diffracts the range of WDM channels, which disperses the range of WDM channels into a first range of diffracted angles Δβ 1 . The mirror  14  reflects the range of WDM channels back to the blazed diffraction grating  12 . The blazed diffraction grating  12  further disperses the range of WDM channels into a second range of diffracted angles Δβ 2 . Effectively, the second range of diffracted angles Δβ 2  appear to be originating from a virtual point  44 . 
   For a 1,550 nm WDM wavelength band, a maximum ruling density for the blazed diffraction grating  10  is about 1290 lines/mm. Because of a small width of the blazed facets  24  ( FIG. 2 ), polarization loss at the maximum ruling density of 1290 lines/mm is at a maxima. The preferred light dispersion arrangement  10  allows use of a more modest ruling density of 600 lines/mm, which because of a double pass of the blazed diffraction grating  12  produces an effective ruling density of about 1,200 lines/mm. This is close to the maximum ruling density. Also, because a width of the blazed facets  24  is larger than the small width associated with the maximum ruling density, the effective ruling density of 1,200 lines/mm is accompanied by less polarization dependent loss than the polarization dependent loss found with the maximum ruling density. A quarter-wave plate can be used to compensate for polarization dependent loss of an originally large polarization dependent loss grating; however, this results in reduced throughput. 
   A multiplexing application of the preferred light dispersion arrangement  10  of the present invention is illustrated in  FIG. 4 . In the multiplexing application  50 , the range of WDM channels ranging from the zeroth wavelength channel λ 0  to the ith wavelength channel λ i  are incident upon the blazed diffraction grating  12  at the second range of dispersion angles Δβ 2 . The blazed diffraction grating  12  reduces the dispersion of the range of WDM channels, which places the range of WDM channels into the first range of diffracted angles Δβ 1 . The mirror  14  reflects the range of WDM channels back to the blazed diffraction grating  12 . The blazed diffraction grating  12  then collimates the range of WDM channels into a output beam  52 . 
   An alternative light dispersion arrangement of the present invention is illustrated in  FIG. 5 . The alternative light dispersion arrangement  60  comprises an alternative blazed diffraction grating  62 , an alternative mirror  64 , and a quarter wave plate  66 . In operation, collimated light  68  is directed onto the alternative blaze grating  62 , which diffracts the collimated light  68  forming first diffracted light  70 . The alternative mirror  64  reflects the first diffracted light  70  back to the alternative blaze diffraction grating  62 . The alternative blazed diffraction  62  grating diffracts the first diffracted light  70  forming second diffracted light  72 . The alternative mirror  64  reflects the second diffracted light  72  back to the alternative blazed diffraction grating  62 . The alternative blazed diffraction grating  62  diffracts the second diffracted light  72  forming third diffracted light  74 . The alternative mirror  64  reflects the third diffracted light  74  back to the alternative blazed diffraction grating  62 . The alternative blazed diffraction  62  grating diffracts the third diffracted light  74  forming fourth diffracted light  76 . 
   In the alternative light dispersion arrangement  60 , the quarter wave plate  66  rotates a polarization of the second diffracted light  72  on its way from the alternative blazed diffraction grating  62  to the alternative mirror  64  and also on its way from the alternative mirror  64  back to the alternative blazed diffraction grating  62 . This double rotation of the polarization by the quarter wave plate  66  produces an orthogonal rotation of a particular polarization of the second diffracted light  72 . By orienting an optical axis of the quarter wave plate to orthogonally rotate the polarization which experiences a worst polarization dependent loss, the polarization dependent loss is minimized. 
   Similarly, in an alternative embodiment to the demultiplexing and multiplexing applications described in relation to  FIGS. 3 and 4 , respectively, a quarter wave plate can be introduced between the blazed diffraction grating  12  and the mirror  14  to intersect the range of WDM channels as they are diffracted from blazed diffraction grating  12  to the mirror  14  and also as they are reflected from the mirror  14  to the diffraction grating  12 . By intersecting the range of WDM channels twice, the quarter wave plate provides an orthogonal rotation of the polarization of each channel, which minimizes polarization dependent loss. Alternatively, a half wave plate can be introduced between the blazed diffraction grating  12  and the mirror  14  to either intersect the range of WDM channels as they are diffracted from blazed diffraction grating  12  to the mirror  14  or as they are reflected from the mirror  14  to the diffraction grating  12 . 
   It will be readily apparent to one skilled in the art that other various modifications may be made to the embodiments without departing from the spirit and scope of the invention as defined by the appended claims.