Abstract:
An illumination system is disclosed for producing polychromatic illumination. The illumination system includes a plurality of monochromatic illumination sources for producing a plurality of monochromatic illumination fields, and a modulator for receiving each of the plurality of monochromatic illumination fields and for producing a polychromatic illumination field along an output direction that includes at least a portion of each of the monochromatic illumination fields. In accordance with an embodiment, the system uses time division multiplexing for color mixing.

Description:
[0001]    This Application claims priority to U.S. Provisional Application Ser. No. 60/394,129 filed Jul. 3, 2002. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The invention generally relates to illumination systems, and relates in particular to systems for producing polychromatic illumination of selected frequencies.  
           [0003]    Illumination sources that provide polychromatic illumination of selected frequencies may be used for a wide variety of purposes, including for example, imaging and communication.  
           [0004]    For example, U.S. Pat. No. 6,219,015 discloses an imaging system and method for providing a visual display by combining grating light valves (GLVs) and exploiting the grating dispersion of white light to isolate the three primary color components of each pixel in a color display system. This patent also discloses the use of separate monochromatic light sources.  
           [0005]    U.S. Pat. No. 6,342,960 discloses a communication system that divides a broadband wavefront into a plurality of signals of different frequencies using a diffraction grating and a plurality of GLVs for separately modulating each different signal.  
           [0006]    Although white light or broadband sources include illumination of a wide range of frequencies, these sources in either of the above systems may require that the light that is outside the selected frequencies be filtered to avoid noise or high intensity background light.  
           [0007]    There is a need, therefore, for an efficient and economical system and method for producing polychromatic illumination of selected frequencies.  
         SUMMARY OF THE INVENTION  
         [0008]    The invention provides an illumination system for producing polychromatic illumination. The illumination system includes a plurality of monochromatic illumination sources for producing a plurality of monochromatic illumination fields, and a modulator for receiving each of the plurality of monochromatic illumination fields and for producing a polychromatic illumination field along an output direction that includes at least a portion of each of the monochromatic illumination fields. In an embodiment, the illumination system includes three monochromatic illumination fields and the modulator produces a polychromatic illumination field that is time division multiplexed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The following description may be further understood with reference to the accompanying drawings in which:  
         [0010]    [0010]FIG. 1 shows an illustrative diagrammatic view of an illumination system in accordance with an embodiment of the invention;  
         [0011]    [0011]FIGS. 2A and 2B show illustrative diagrammatic views of the ribbons of a gradient light valve modulator having a spacing of Δ 1  in accordance with an embodiment of the invention;  
         [0012]    [0012]FIGS. 3A and 3B show illustrative graphical views of the intensity distribution in the Fourier plane for a non-activated grating and an activated grating;  
         [0013]    [0013]FIGS. 4A and 4B show illustrative diagrammatic views of the ribbons of a gradient light valve modulator having a spacing of Δ 2  in accordance with an embodiment of the invention;  
         [0014]    [0014]FIGS. 5A and 5B show illustrative diagrammatic views of the ribbons of a gradient light valve modulator having a spacing of Δ 3  in accordance with an embodiment of the invention;  
         [0015]    [0015]FIG. 6 shows an illustrative diagrammatic view of a timing chart for an illumination system in accordance with an embodiment of the invention;  
         [0016]    [0016]FIG. 7 shows an illustrative diagrammatic view of an imaging system incorporating a polychromatic illumination system in accordance with an embodiment of the invention;  
         [0017]    [0017]FIG. 8 shows an illustrative diagrammatic view of a communication system incorporating a polychromatic illumination system in accordance with an embodiment of the invention; and  
         [0018]    [0018]FIG. 9 shows an illustrative diagrammatic view of a timing chart for an communication system in accordance with an embodiment of the invention. 
     
    
       [0019]    The drawings are shown for illustrative purposes only.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    As shown in FIG. 1, an illumination system  10  in accordance with an embodiment of the invention includes a light modulator  12 , which may, for example be a GLV. The GLV  12  receives monochromatic illumination signals from each of three monochromatic sources  14 ,  16  and  18  and produces a polychromatic illumination field  20 . The monochromatic sources  14 ,  16  and  18  produce illumination having wavelengths of λ 1 , λ 2  and λ 3  respectively. The diffraction relationship between the grating period and the diffraction angle is defined by:  
         sin                   α   MAX       =     k        λ   Δ                             
 
         [0021]    where kεN, α MAX  is the diffraction angle, λ is the wavelength of the carrier signal, and Δ is the grating period. In an embodiment, the angle of incidence of each monochromatic illumination field onto the GLV  12  is determined such that the polychromatic illumination is directed in the same direction (shown perpendicular to the GLV  12  in FIG. 1) without adjusting the modulator. The polychromatic illumination is produced by the (e.g., first order) reflection of the monochromatic illumination off of the GLV  12 . In this embodiment, the intensity of each of the monochromatic illumination fields within the combined polychromatic illumination is determined by the intensity of each of the monochromatic sources  14 ,  16  and  18 .  
         [0022]    In another embodiment, the intensity of each monochromatic illumination field within the polychromatic illumination may be varied with respect to one another. In this embodiment, the angles of incidence of each monochromatic illumination beam on the GLV  12  is selected such that the first order reflected signals are coincident upon one another for grating period spacing that are readily achievable by the GLV  12 , e.g., Δ 1 , Δ 2  and Δ 3  where Δ 2 =2Δ 1  and Δ 3 =3Δ 1 . The operation of the system may be characterized by the following relationship:  
         θ   rad     =       λ   j       2        Δ   j                               
 
         [0023]    where θ rad  is the angle of incidence of each monochromatic field onto the GLV  12 , λ j  is the wavelength of the carrier signal at each frequency and Δ 1  is the grating period for each wavelength λ j . In particular, the GLV  12  may provide a grating period of Δ 1  and be switchable as shown at  26  and  28  in FIGS. 2A and 2B to provide the responses  30  and  32  shown in FIGS. 3A and 3B respectively. Specifically, when the grating appears as shown at  26  in FIG. 2A the response to a carrier signal λ 1  may be as shown at  30  in FIG. 3A including virtually no response in the first order, whereas when the grating appears as shown at  28  in FIG. 2B the response may be as shown at  32  in FIG. 3B including a strong first order response. The intensity of the first order response may be adjusted between the levels shown in FIGS. 3A and 3B by adjusting the GLV to a position intermediate the positions shown in FIGS. 2A and 2B.  
         [0024]    As shown at  34  and  36  in FIGS. 4A and 4B, the grating period may be changed to be A 2    2  Δ 1  by pairing adjacent ribbons. If the values of θ rad , λ j  and Δ j  are properly chosen, the first order response angle for the signal λ 2  using a grating period of Δ 2  will be the same as for λ 1  using the grating period Δ 1  (of, for example, 3-5 microns). Similarly, the grating period maybe changed to be Δ 3 =3 Δ 1  as shown at  38  and  40  in FIGS. 4A and 4B, and with properly chosen values for θ rad , λ 1  and Δ j  the first order response angle for the carrier signal λ 3  using a grating period of Δ 3  will be the same as for λ 1  using the grating period of Δ 1 .  
         [0025]    These first order signals may be time division multiplexed by timing the modulator (coupled to a controller  22 ) to provide the grating period Δ 1  at times t 1 , t 4 , t 7  etc., to provide the grating period Δ 2  at times t 2 , t 5 , t 8  etc., and to provide the grating period Δ 3  at times t 3 , t 6  etc. In particular, as shown at  42  in FIG. 6, the first order reflected λ 1  signal includes a high intensity during times t 1 , t 4 , t 7  etc. As shown at  44  in FIG. 6, the first order reflected λ 2  signal includes a high intensity during times t 2 , t 5 , t 8  etc. As shown at  46  in FIG. 6, the first order reflected λ 3  signal includes a high intensity during times t 3 , t 6  etc. The system, therefore, permits multiple monochromatic signal to be selectively combined at high speeds using the above relationship between θ rad , λ j  and α j .  
         [0026]    As shown in FIG. 7, an illumination system of the invention may be used with an imaging system for viewing a diffractive image such as a hologram. In particular the system may include a modulator  52  such as a GLV, three monochromatic light sources  56 ,  58  and  60  and a modulation controller  54 . The illumination system produces a time division multiplexed polychromatic illumination  62  that is directed toward a previously recorded holographic recording  64 . Preferably, the holographic recording has been recorded using monochromatic coherent light sources that are the same frequencies as the three monochromatic sources  56 ,  58  and  60 . The resulting image may then be view as indicated at  66 . Such an illumination system may also be used for viewing systems in which it is desirable to selectively view or exclude one or more of the recorded images by selectively including or excluding one of the light sources. In other embodiments, the system may be used for holographic memory.  
         [0027]    As shown in FIG. 8, an illumination system in accordance with an embodiment of the invention may be used in connection with a communication system for transmitting digital data via optical fibers. The illumination system may include a modulator  72  such as a GLV, three monochromatic light sources  76 ,  78  and  80  that provide carrier signals, and a transmitter controller  74 . The illumination system produces time division multiplexed modulated signals by switching the modulator on and off during the on period for each monochromatic source shown in FIG. 6. In particular, as shown in FIG. 9, illumination from the first source  76  is modulated during time intervals t 1 , t 4 , t 7  etc. as shown at  100 , illumination from the second source  78  is modulated during time intervals t 2 , t 5 , t 8  etc. as shown at  102 , and illumination from the third source  80  is modulated during time intervals t 3 , t 6  etc. as shown at  104 .  
         [0028]    The time divisional multiplexed modulated signals  82  are direct via optics  84  into an optical fiber  86  for transmission to a receiver that includes a detector  88 , a receiver controller  90  and an output  94 . The receiver controller  90  and the transmitter controller  74  are commonly coupled to a timing controller  92 .  
         [0029]    In further embodiments, each of the values θ rad ,λ and Δ maybe variable to achieve further systems of increased flexibility and functionality. Those skilled in the art will appreciate that numerous modifications and variations may be made to the above disclosed embodiments without departing from the spirit and scope of the present invention.