Abstract:
A communication system is disclosed that includes a modulator and a collection unit. The modulator modulates a first electromagnetic signal having a first frequency when the modulator has a first grating period and produces a first modulated electromagnetic signal. The modulator modulates a second electromagnetic signal having a second frequency when the modulator has a second grating period that is different than the first grating period and produces a second modulated electromagnetic signal. The modulator modulates a electromagnetic signal having a frequency when the modulator has a first grating period and produces a first modulated electromagnetic signal which is directed in first direction. The modulator modulates a electromagnetic signal having the same frequency when the modulator has a second grating period and produces a second modulated electromagnetic signal which is directed in second direction. In various embodiments, the system may electromagnetic signals having different frequencies, modulators having different grating periods, and/or collection units for collecting modulated signals at different angular directions with respect to the modulator.

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 communication systems in which information is transmitted via optical fibers, and relates in particular to signal switching systems for such communication systems.  
           [0003]    Although fiber optic communication systems are able to transmit a large volume of information at a very high rate and are able to simultaneously transmit a plurality of signals in a single optical fiber, communication systems employing fiber optics must process the signal at transmitter and receiver ends of a optical fiber transmission path or at any detection system. The processing of such signals by transmitters and receivers is typically very slow compared to the speed at which the communication signals travel along the optical fibers. The processing of the signals by transmitters and receivers involves not only modulating/demodulating the signals (typically from/to another format such as analog or digital electronic signals), but also combining/separating different signals that are transmitted along a common optical fiber.  
           [0004]    Devices for processing signals for fiber optic communication include transmitters that separately modulate different signals and then combine the separately modulated different signals for transmission along an optical fiber. For example, U.S. Pat. No. 6,342,960 discloses a system that divides a broadband wavefront into a plurality of signals of different frequencies using a diffraction grating and a plurality of independent grating light valve (GLV) modulators for separately modulating each different signal. Such a system, however, has a static variation of diffraction period and requires very precise calibration of the diffraction grating and reflectors to ensure that each different frequency signal contacts the appropriate GLV from exactly the correct angle. In fact, the system further discloses that a calibration detector array may be used to detect misalignment of the reflector module assembly.  
           [0005]    There is a need, therefore, for an efficient and economical system and method for processing communication signals that are transferred with optical fibers.  
         SUMMARY OF THE INVENTION  
         [0006]    The invention provides a communication system that includes a modulator and a collection unit. The modulator modulates a first electromagnetic signal having a first frequency when the modulator has a first grating period and produces a first modulated electromagnetic signal. The modulator modulates a second electromagnetic signal having a second frequency when the modulator has a second grating period that is different than the first grating period and produces a second modulated electromagnetic signal. The collection unit is for collecting the first and second modulated electromagnetic signals for transmission. In various embodiments, the system may include electromagnetic signals having different frequencies, modulators having different grating periods, and/or collection units for collecting modulated signals at different angular directions with respect to the modulator. For example, in accordance with an embodiment, the invention provides a communication system that includes a modulator and a collection unit. The modulator modulates a electromagnetic signal having a narrow bandwidth frequency when the modulator has a first grating period and produces a first modulated electromagnetic signal that is directed in first direction. The modulator modulates an electromagnetic signal having the same narrow bandwidth frequency when the modulator has a second grating period and produces a second modulated electromagnetic signal that is directed in a second direction. Both multiplexing techniques may be performed by the same communication system device, or may be employed separately. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    The following description may be further understood with reference to the accompanying drawings in which:  
         [0008]    [0008]FIG. 1 shows an illustrative diagrammatic view of a communication system in accordance with an embodiment of the invention;  
         [0009]    [0009]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;  
         [0010]    [0010]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;  
         [0011]    [0011]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;  
         [0012]    [0012]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;  
         [0013]    [0013]FIG. 6 shows an illustrative diagrammatic view of a timing chart for a communication system in accordance with an embodiment of the invention;  
         [0014]    [0014]FIG. 7 shows an illustrative diagrammatic view of a communication system in accordance with another embodiment of the invention; and  
         [0015]    [0015]FIG. 8 shows an illustrative diagrammatic view of a communication system in accordance with a further embodiment of the invention. 
     
    
       [0016]    The drawings are shown for illustrative purposes only.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    As shown in FIG. 1, a communication system  10  in accordance with an embodiment of the invention includes a light modulator  12  in communication with a transmitter controller  14 . A multi-frequency carrier signal is received at input  16  and directed toward the modulator  12  via reflectors  18  and  20 . The modulator  12  is, for example, a GLV having a time varying grating period. In various embodiments, the modulator may be reflective or transmissive, e.g., by using a transmission LCD. The diffraction relationship between the grating period and the diffraction angle is defined as:  
         sin                   α   MAX       =     k        λ   Δ                             
 
         [0018]    where kεN, α MAX  is the diffraction angle, λ is the wavelength of the carrier signal, and Δ is the grating period. If the grating period is variable in time, the diffractive light modulator can switch or modulate the diffraction angle. When using a grating light valve all ribbons should be controlled to realize variable grating periods. Controlled formation of groups of activated and non-activated ribbons (ribbon patterns) results in different grating periods. In further embodiments, the angle of incidence of the carrier signal or signals onto the modulator may be varied by, for example, adjusting the positions of the mirrors  18  and  20  or using other adjustable optics. The dynamic variability of grating period can be either used for wavelength separation or combination or for direction multiplexing (variation of diffraction angle due to variation of grating period) of a single wavelength and parallel to this for time multiplexed modulation.  
         [0019]    The modulated light  22  from modulator  12  is directed by reflectors  24  (or lenses) toward fiber optic coupling optics  26  into an optical fiber  28 . The optical fiber  28  carries the time division multiplexed multi-frequency signals to a receiver that may include a detector  30  and a receiver controller  32 . The receiver controller  32  and the transmitter controller  14  are commonly coupled to a timing controller  34  as shown in FIG. 1. The receiver output signal is provided at the output port  36 .  
         [0020]    The operation of the system may be characterized by the following relationship  
         θ     ra                 d       =       λ   j       2        Δ   j                               
 
         [0021]    where θ rad  is the spectral angle of the signal from the modulator  12 , λ j  is the wavelength of the carrier signal at each frequency and Δ j  is the grating period for each wavelength λ j . Generally, different frequency carrier signals λ j  may be designed to provide first order spectral reflection at the same angle θ rad  by adjusting the period of the grating Δ j . The carrier signals λ j  are modulated by the modulator  12  to produce blocks of digital information that is time division multiplexed among the different carrier signals.  
         [0022]    In particular, the modulator  12  may provide a grating period of Δ 1  and be switchable as shown at  40  and  42  in FIGS. 2A and 2B to provide the responses  44  and  46  shown in FIGS. 3A and 3B respectively. Specifically, when the grating appears as shown at  40  in FIG. 2A the response to a carrier signal λ 1  may be as shown at  44  in FIG. 3A including virtually no response in the first order, whereas when the grating appears as shown at  42  in FIG. 2B the response maybe as shown at  46  in FIG. 3B including a strong first order response. If the first order response is detected, the system may produce digital information (with comparatively low modulation speed) by switching the grating back and forth between the states as shown in FIGS. 2A and 2B using the grating period of Δ 1 .  
         [0023]    As shown at  48  and  50  in FIGS. 4A and 4B, the grating period may be changed to be Δ 2 =2Δ 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 carrier 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 may be changed to be Δ 3 =3Δ 1  as shown at  52  and  54  in FIGS. 4A and 4B, and with properly chosen values for θ rad , λ j  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 . This permits each carrier signal λ j  to provide a modulated first order response at the same angle θ rad . These modulated signals may be time division multiplexed by timing the modulator 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  60  in FIG. 6, the modulated λ 1  signal includes digital information during times t 1 , t 4 , t 7  etc. As shown at  62  in FIG. 6, the modulated λ 2  signal includes digital information during times t 2 , t 5 , t 8  etc. As shown at  64  in FIG. 6, the modulated λ 3  signal includes digital information during times t 3 , t 6  etc. The system, therefore, permits multiple signals to be modulated and combined at high speeds using the above relationship between θ rad , λ j  and Δ j .  
         [0024]    As shown in FIG. 7, a system  70  in accordance with a further embodiment of the invention includes a light modulator  72  in communication with a transmitter controller  74 . A carrier signal is received at input  76  and directed toward the modulator  72  via reflectors  78  and  80 . The modulator  72  may be a GLV having a time varying grating period. The diffraction relationship between the grating period and the diffraction angle may be as defined above.  
         [0025]    The modulated light  82   a - 82   c  from modulator  72  is directed by reflectors  84   a - 84   c  toward fiber optic coupling optics  86   a - 86   c  respectively where the modulated light is coupled into each of optical fibers  88   a - 88   c  respectively. The optical fibers carry the signals to receivers that may include detectors  90   a - 90   c  and output ports  96   a - 96   c  respectively as shown. The signals may or may not be time-division multiplexed as required.  
         [0026]    The operation of this system may be characterized by the following relationship  
         θ     ra                   d   j         =     λ     2        Δ   j                               
 
         [0027]    where θ rad     j    is the spectral angle of the signal from the modulator  72  when the modulator has a grating period j, λ is the wavelength of the carrier signal and Δ j  is each grating period j. Generally, the carrier signal λ provides first order spectral reflection at the angle θ rad     j    when the grating period is Δ j . The carrier signal λ is modulated by the modulator  72  to produce blocks of digital information that is time division multiplexed along each of the different optical fibers  88   a - 88   c.    
         [0028]    Specifically, the modulator  72  may provide a grating period of Δ 1  and be switchable to provide digital data using the first order response  82   a  at an angle of θ rad     1    that is directed via reflectors  84   a  toward fiber optic coupler  86   a . When the grating period is set to Δ 2  digital data is provided using the first order response  82   b  at an angle of θ rad     2    that is directed via reflector  84   b  toward fiber optic coupler  86   b . When the grating period is set to Δ 3  digital data is provided using the first order response  82   c  at an angle of θ rad     3    that is directed via reflectors  84   c  toward fiber optic coupler  86   c . The different signals may be time division multiplexed as discussed above with reference to FIG. 1.  
         [0029]    As shown in FIG. 8, a system  100  in accordance with a further embodiment of the invention includes a light modulator  102  in communication with a transmitter controller  104 . A multi-frequency carrier signal is received at input  106  and directed toward the modulator  102  via reflectors  108  and  110 . The modulator  102  may be a GLV having a fixed grating period A. The diffraction relationship between the carrier frequency, grating period and diffraction angle may be as defined above.  
         [0030]    The modulated light  112   a - 112   c  from modulator  102  is directed by reflectors  114   a - 114   c  toward fiber optic coupling optics  116   a - 116   c  respectively where the modulated light is coupled into each of optical fibers  118   a - 118   c  respectively. The optical fibers carry the time division multiplexed signals to receivers that may include detectors  120   a - 120   c  and a receiver controller  122 . The receiver controller  122  and the transmitter controller  104  are commonly coupled to a timing controller  124  as shown in FIG. 8. The receiver output signal is provided at the output ports  126   a - 126   c  respectively as shown.  
         [0031]    The operation of this system may be characterized by the following relationship  
         θ     ra                   d   j         =       λ   j       2      Δ                             
 
         [0032]    where θ rad      j    is the spectral angle of the signal from the modulator  72  when the wavelength of the carrier signal is λ j . Generally, each carrier signal λ j  provides first order spectral reflection at the angle θ rad     j    when the grating period is fixed at Δ. Each carrier signal λ j  is modulated by the modulator  102  to produce blocks of digital information that is time division multiplexed along each of the different optical fibers  118   a - 118   c.    
         [0033]    Specifically, the modulator  102  having a grating period of Δ may be switchable to provide digital data using the first order response  112   a  of a first carrier signal having a wavelength λ 1  at an angle of θ rad     1    that is directed via reflectors  114   a  toward fiber optic coupler  116   a . For the input carrier signal having a wavelength λ 2  the digital data is provided using the first order response  112   b  at an angle of θ rad     2    that is directed via reflector  114   b  toward fiber optic coupler  116   b . For the input carrier signal having a wavelength λ 3  digital data is provided using the first order response  112   c  at an angle of θ rad     3    that is directed via reflectors  114   c  toward fiber optic coupler  116   c . The different signals may be time division multiplexed as discussed above with reference to FIG. 1.  
         [0034]    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.