Wavelength dispersing device

Abstract

A compact wavelength dispersing device and a wavelength selective optical switch based on the wavelength dispersing device is described. The wavelength dispersing device has a folding mirror that folds the optical path at least three times. A focal length of a focusing coupler of the device is reduced and the NA is increased, while the increased optical aberrations are mitigated by using an optional coma-compensating wedge. A double-pass arrangement for a transmission diffraction grating allows further focal length and overall size reduction due to increased angular dispersion.

1 . A wavelength dispersing device for dispersing a light beam into narrowband sub-beams having focal spots spaced apart along a line of dispersion, the wavelength dispersing device comprising:
 an input port for inputting the light beam;   a dispersive unit optically coupled to the input port, for dispersing the light beam into the narrowband sub-beams;   a focusing coupler having optical power in a plane containing the line of dispersion, for receiving the spatially separated narrowband sub-beams from the dispersive unit and for focusing them onto the line of dispersion, so that the focal spots of the narrowband sub-beams are disposed along the line of dispersion; and   a folding mirror disposed in optical paths between:
 the input port and the dispersive unit; 
 the dispersive unit and the focusing coupler; and 
 the focusing coupler and the line of dispersion. 
   
   
   
       2 . A wavelength dispersing device of  claim 1 , wherein the folding mirror is disposed in an optical path between the input port and the focusing coupler, and wherein the focusing coupler is disposed in an optical path between the folding mirror and the dispersive unit. 
   
   
       3 . A wavelength dispersing device of  claim 1 , wherein the folding mirror has a flat refractive surface and a flat reflective surface, so that in operation, the spatially separated narrowband sub-beams first impinge on the flat refractive surface, then on the flat reflective surface, and then again on the flat refractive surface, whereby coma of the wavelength dispersive device is lessened, so that an average width of the focal spots along the line of dispersion can be reduced. 
   
   
       4 . A wavelength dispersing device of  claim 3 , wherein the flat reflective surface has a dielectric reflector. 
   
   
       5 . A wavelength dispersing device of  claim 3 , wherein the folding mirror is an optical wedge having such an index of refraction that in operation, the spatially separated narrowband sub-beams impinging upon the flat reflective surface are reflected by total internal reflection. 
   
   
       6 . A wavelength dispersing device of  claim 3 , wherein the flat refractive surface is tilted relative to the flat reflective surface. 
   
   
       7 . A wavelength dispersing device of  claim 1 ,
 wherein the dispersive unit has a transmission diffraction grating and a retroreflector, wherein in operation, the narrowband sub-beams impinge first on the transmission diffraction grating, then on the retroreflector, and then back on the transmission diffraction grating;   wherein the transmission diffraction grating has parallel grating lines, and wherein the transmission diffraction grating and the retroreflector are disposed so that at least one of the narrowband sub-beams is not perpendicular to the grating lines, whereby focal spots originating from stray reflections of the narrowband sub-beams are offset relative to the line of dispersion.   
   
   
       8 . A wavelength dispersing device of  claim 1 , wherein the dispersive unit has a transmission diffraction grating optically coupled to a retroreflector, so that in operation, the narrowband sub-beams first impinge on the transmission diffraction grating, then on the retroreflector, and then back on the transmission diffraction grating. 
   
   
       9 . A wavelength dispersing device of  claim 1 , wherein the focusing coupler is selected from a group consisting of a concave mirror and a lens. 
   
   
       10 . A wavelength selective optical switch module for wavelength selective switching of individual wavelength channels between an input port thereof and a plurality of output ports thereof, the wavelength selective optical switch module comprising:
 a wavelength dispersing device of  claim 1 , wherein the input port of the wavelength selective optical switch module is the input port of the wavelength dispersing device, and wherein in operation, the narrowband sub-beams carry the individual wavelength channels; and   an array of directors disposed along the line of dispersion, for redirecting at least one of the narrowband sub-beams to propagate back through the wavelength dispersive device towards the output ports so as to couple into a particular of the output ports.   
   
   
       11 . A wavelength dispersing device for dispersing a light beam into narrowband sub-beams in a plane of dispersion and for focusing the narrowband sub-beams into focal spots in a focal plane, the wavelength dispersing device comprising:
 an input port for inputting the light beam;   a folding mirror for folding optical paths of the light beam and of the narrowband sub-beams;   a dispersive unit for dispersing the light beam into the narrowband sub-beams in the plane of dispersion; and   a concave mirror having optical power in the plane of dispersion, for focusing the narrowband sub-beams into the focal spots in the focal plane;   wherein in operation, the light beam from the input port is coupled to the folding mirror; from the folding mirror to the dispersive unit that disperses the light beam into narrowband sub-beams that are coupled back to the folding mirror; from the folding mirror to the concave mirror for focusing the narrowband sub-beams; from the concave mirror back to the folding mirror; and from the folding mirror to the focal plane, wherein the narrowband sub-beams are focused into the focal spots.   
   
   
       12 . A wavelength dispersing device of  claim 11 , wherein on an optical path from the folding mirror to the dispersive unit, the light beam is first directed to the concave mirror, is collimated thereby, and then is directed towards the dispersive unit. 
   
   
       13 . A wavelength dispersing device of  claim 11 , further comprising a coma-correcting optical wedge shaped and disposed in an optical path of the narrowband sub-beams so as to compensate for coma of the concave mirror, thereby reducing an average width of the focal spots measurable along a line connecting the focal spots. 
   
   
       14 . A wavelength dispersing device for dispersing a light beam into narrowband sub-beams having focal spots spaced apart along a line of dispersion, comprising:
 an input port for inputting the light beam;   a dispersive unit optically coupled to the input port, for dispersing the light beam into the narrowband sub-beams;   a focusing coupler having optical power in a plane containing the line of dispersion, for receiving the spatially separated narrowband sub-beams from the dispersive unit and for focusing them onto the line of dispersion, so that the focal spots of the narrowband sub-beams are disposed along the line of dispersion; and   a coma-compensating optical element having two flat optical faces disposed in an optical path between the focusing coupler and the line of dispersion, for compensating coma of the focusing coupler and for reducing an average width of the focal spots along the line of dispersion.   
   
   
       15 . A wavelength dispersing device of  claim 14 , wherein the coma-compensating optical element is an optical wedge. 
   
   
       16 . A wavelength dispersing device of  claim 14 , wherein the coma-compensating optical element is disposed in an optical path between the input port and the dispersive unit. 
   
   
       17 . A wavelength dispersing device of  claim 14 , wherein the focusing coupler is selected from a group consisting of a concave mirror and a lens. 
   
   
       18 . A wavelength dispersing device of  claim 14 , wherein the dispersive unit has a transmission diffraction grating optically coupled to a retroreflector for reflecting the narrowband sub-beams dispersed by the transmission diffraction grating back to the transmission diffraction grating, for additional dispersing by the transmission diffraction grating. 
   
   
       19 . A dispersive unit for spatially separating an optical beam into narrowband sub-beams,
 wherein the narrowband sub-beams are co-planar in a plane of dispersion,   wherein in operation, stray optical beams resulting from stray reflections of the narrowband sub-beams in the dispersive unit form a non-zero angle with the plane of dispersion,   the dispersive unit comprising:   a flat transmission diffraction grating for receiving the optical beam and dispersing the optical beam into the narrowband sub-beams, and   a flat retroreflector for reflecting the narrowband sub-beams dispersed by the transmission diffraction grating back to the transmission diffraction grating, for additional dispersing by the transmission diffraction grating in the plane of dispersion;   wherein the transmission diffraction grating has parallel grating lines, and wherein the transmission diffraction grating and the retroreflector are disposed so that so that at least one of the narrowband sub-beams is not perpendicular to the grating lines, whereby in operation, the stray optical beams form a non-zero angle with the plane of dispersion.   
   
   
       20 . A dispersive unit of  claim 19 , wherein a plane of transmission diffraction grating is not perpendicular to a plane of incidence of the optical beam on the transmission diffraction grating. 
   
   
       21 . A dispersive unit of  claim 19 , wherein a plane of the retroreflector is not perpendicular to the plane of incidence of the optical beam on the transmission diffraction grating. 
   
   
       22 . A wavelength selective optical switch module for wavelength selective switching of individual wavelength channels between an input port thereof and a plurality of output ports thereof, the wavelength selective optical switch module comprising:
 a dispersive unit of  claim 19 , wherein the narrowband sub-beams are for carrying the individual wavelength channels;   a focusing coupler having a focal length in the plane of dispersion, for receiving the spatially separated narrowband sub-beams and for focusing them in a focal plane disposed substantially one focal length away from the focusing coupler; and   an array of directors disposed in the focal plane, for receiving the focused narrowband sub-beams and for redirecting at least one of the narrowband sub-beams to propagate back through the dispersive unit towards the output ports, so as to couple the at least one of the narrowband sub-beam into a particular of the output ports;   wherein in operation, the stray optical beams having the non-zero angle with the plane of dispersion are focused by the focusing coupler such that they are offset relative to the focused narrowband sub-beams in the focal plane.   
   
   
       23 . A wavelength selective optical switch of  claim 22 , further comprising a light baffle disposed to intercept the stray optical beams at a distance from the focal plane not exceeding 25% of the focal length of the focusing coupler.