Abstract:
Methods of optical communication and optical communication systems are described. According to a first aspect, a method of optical communication includes providing an optical signal and providing a plurality of data signals. This aspect also includes passing a plurality of desired portions of the optical signal using a plurality of respective optical modulators, the desired portions individually having at least one predefined wavelength. The method also includes optically modulating the desired portions of the optical signal using the respective optical modulators and responsive to respective ones of the data signals and outputting the desired portions of the optical signal to an optical communication medium after the modulating.

Description:
TECHNICAL FIELD 
     The invention relates to methods of optical communication and optical communication systems. 
     BACKGROUND OF THE INVENTION 
     The amount of information communicated within networks, such as voice and data networks, has increased dramatically in recent years. Accordingly, such has resulted in demands for increased bandwidth in networks to communicate more information at increased rates of data transfer. As the demands for bandwidth of data communications continues to increase, improved devices and methodologies to accommodate the demands are desired. 
     One example of data transmission technology uses low power, high data rate and wavelength division multiplexing to achieve high bit rate data transmission. An exemplary implementation utilizes a relatively large number of optical sources at different wavelengths. However, such configurations can be relatively difficult to fabricate and relatively expensive to package. 
     Another solution has been to directly modulate light sources, such as laser diodes. However, the rate of modulation within such systems is less than desirable to accommodate the increasing bandwidth demands. 
     More specifically, conventional fiber optic communications systems typically rely on a separate source for each optical wavelength used in a wavelength division multiplexed system. However, as more and more optical wavelengths are used, larger numbers of active devices must be packaged in transmitter modules. Removing the heat from these devices constrains the package design and complicates the ability to inject high speed data signals into the devices. Also, since the optical sources are typically laser diodes, the performance of the sources varies significantly over temperature. In addition, data is encoded on each optical signal by modulation of the optical intensity at that wavelength. 
     Accordingly, there exists a need for an improved approach to generating frequency multiplexed optical signals. 
     SUMMARY OF THE INVENTION 
     Aspects of the present invention provide for multiplexing individually modulated components of a source light. A broad-spectrum light source provides the source light; an optical divider divides the source light into plural carrier beams. A multi-channel modulator modulates each carrier beam responsive to a respective data signal to yield a respective encoded beam. An optical combiner multiplexes the encoded beams. The optical combiner can also inject the multiplexed signal into a communication medium for reception elsewhere. 
     According to a realization of the present invention, the optical combiner frequency multiplexes the encoded beams. To this end, the encoded beams can have different wavelengths. The differences in wavelengths can be imposed originally by an optical divider as it generates carrier beams having different wavelengths. Alternatively, the carrier wavelengths need not differ; instead, the modulator itself causes the encoded beams to have different wavelengths. 
     According to additional exemplary aspects, optical modulators pass a desired portion of a received optical signal having at least one predefined wavelength. The modulators optically modulate the desired portion of the optical signal having the at least one predefined wavelength responsive to a respective data signal. 
     Additional aspects of the invention disclose methods which include passing a plurality of desired portions of an optical signal using a plurality of respective optical modulators. The desired portions of the signal individually have at least one predefined wavelength. The method also includes optically modulating the desired portions of the optical signal using the respective optical modulators responsive to data signals. In one exemplary implementation, the optically modulating is implemented using frequency modulation. 
     As is apparent from the foregoing, the present invention has both method and structural aspects. By using a single broadband light source for the multiple components of a multiplexed signal, the present invention overcomes many of the problems faced by prior art systems that use multiple light sources. It much easier and more cost-effective to manufacture a single broad-spectrum light source than multiple single-frequency light sources. Furthermore, more channels can be implemented without encountering heat-dissipation limits. Also, since the light source is not modulated, switching speed limitations associated with modulating a light source directly are not encountered. More specifically, aspects of the invention disclose arrangements and methodologies wherein signal bandwidths are limited by the response of the modulator which can be much faster than the bandwidth of a laser. Certain embodiments of the invention provide other advantages in addition to or in lieu of the advantages described above, as is apparent from the description below with reference to the following drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the invention are described below with reference to the following accompanying drawings depicting examples embodying the best mode for practicing the invention. 
         FIG. 1  is a functional block diagram of an exemplary optical communication system. 
         FIG. 2  is an illustrative representation of one exemplary implementation of the optical communication system depicted in  FIG. 1 . 
         FIG. 3  is a top view of an array of exemplary optical modulators. 
         FIG. 4  is a cross-sectional view of the array shown in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , an exemplary optical communication system  10  is illustrated. The depicted optical communication system  10  includes one or more data source  12 , a light source  20 , an optical divider  22 , an optical modulator array  24  and an optical combiner  26 . Light source  20 , optical divider  22 , optical modulator array  24 , optical combiner  26  and optical communication medium  28  are optically coupled with one another. 
     Light source  20  is configured as a broad spectrum optical source in the described exemplary embodiment. For example, light source  20  is configured as an edge emitting light emitting diode (EELED) configured to emit a broad spectrum optical signal  21 , also referred to as a source-light beam, having a plurality of wavelengths. In the described exemplary arrangement, broad spectrum optical signal  21  is approximately 200 nm wide. Other configurations of light source  20  configured to emit other optical signals are possible. 
     The light of broad spectrum optical signal  21  is provided to an optical divider  22 . Optical divider  22  divides the light of the broad spectrum optical signal  21  into a plurality of optical signals  23 , also referred to as carrier light-beams. In exemplary arrangements, optical divider  22  is configured as a beam splitter, array waveguide (AWG), prism or other wavelength dispersive element. 
     Optical signals  23  outputted from optical divider  22  have respective different portions or segments of the wavelength spectrum of broad spectrum optical signal  21 . In one exemplary arrangement, optical signals  23  individually comprise a portion of optical signal  21  having one or more respective different wavelength than the other optical signals  23  as determined by optical divider  22 . 
     According to aspects of the invention, the number of channels within optical communication system  10  is determined by the number of optical signals  23  outputted from optical divider  22 . According to one exemplary embodiment of the present invention, individual channels correspond to respective different wavelengths of optical signal  21 . The number of optical signals  23  generated by optical divider  22  may be varied according to the desired implementation of optical communication system  10 . 
     Optical modulator array  24  comprises a plurality of optical modulators (exemplary optical modulators are described below with respect to references  34 ,  34   a,    34   b,    34   c  illustrated in  FIGS. 3 and 4 ). Such optical modulators within array  24  are individually configured to pass a desired portion of the optical signal  21  and to optically modulate the desired portion of the optical signal  21  for encoding data thereon. In the embodiment illustrated in  FIG. 1 , desired portions of optical signal  21  correspond to respective optical signals  23  as described in further detail below. 
     According to aspects of the present invention, optical modulators of array  24  are configured to implement frequency modulation of the respective desired portions of optical signal  21 . Alternatively, amplitude modulation or other modulation schemes may be utilized to encode data upon the portions  23  of optical signal  21 . 
     Depending upon the configuration of optical communication system  10  shown in  FIG. 1  or provided in other arrangements, optical modulators of array  24  also operate to filter undesired portions of optical signal  21  or optical signals  23 . Individual optical modulators of optical modulator array  24  have passbands configured to pass and to modulate light within a desired portion (passing light having one or more predefined wavelength) and to filter light within undesired portions (at other wavelengths outside of the respective passbands of the optical modulators). For example, optical modulators of optical modulator array  24  are individually configured to pass and modulate a portion of optical signal  21  within the respective passband and to not pass or modulate portions of optical signal  21  outside of the respective passband. In an exemplary arrangement, one or more of the optical modulators is configured to pass one or more wavelength of light different than at least one wavelength of light passed by the others of the optical modulators. 
     The optical modulators of array  24  are configured to provide appropriate spacing of the desired portions of optical signal  21  from one another. Passbands of the optical modulators are separated by appropriate guard bands to avoid cross-talk or other interference between channels in one embodiment. 
     In the described optical communication system  10  depicted in  FIG. 1 , optical signals  23  are provided to respective optical modulators of modulator array  24 . Desired portions of the optical signal  21  to be passed and modulated within the respective optical modulators may be substantially provided as respective optical signals  23  as determined within divider  22  and corresponding to the respective passbands of the optical modulators leaving minimal or no filtering of light of the optical signals  23 . In such an arrangement, optical divider  22  is configured to divide the optical signal  21  into optical signals  23  substantially comprising the desired portions having wavelengths of light corresponding to the passbands of the respective optical modulators. Alternatively, filtering of light from individual optical signals  23  is implemented by the optical modulators to remove undesired light from optical signals  23 . Optical signals modulated and outputted from modulator array  24  have reference  25  in  FIG. 1 . 
     In an alternative implementation of optical communication system  10 , divider  22  provides no wavelength division but rather divides optical signal  21  into optical signals  23  which individually have substantially the same wavelength spectrum as signal  21 . Accordingly, optical signals  23  comprise broad spectrum signals in such an embodiment. Optical modulators of array  24  filter and modulate the broad spectrum signals  23  providing optical signals  25  as described above. In such an arrangement, the optical modulators are configured to filter undesired portions of optical signals  23  outside of the respective passbands of the optical modulators and to pass and to modulate the respective desired portions of optical signals  23 . 
     Data source(s)  12  are configured to provide a plurality of data signals  13  containing information to be communicated within optical communication system  10 . Data source(s)  12  are arranged in the described embodiment to provide a plurality of data signals  13  corresponding to the channels within optical communication system  10 . For example, the number of data signals  13  corresponds to the number of optical signals  23 ,  25  within optical communication system  10 . The data signals  13  are utilized to modulate the desired portions of optical signal  21  to form optical signals  25 . At any given time, one or more of the channels may not be utilized. Other embodiments are possible. 
     Data source  12  outputs the data signals  13  comprising electrical signals. Exemplary data signals  13  individually have a frequency utilized to control modulation of desired portions of the optical signal  21  using optical modulators of array  24 . Exemplary data signals  13  have MHz or GHz frequencies, with the higher frequencies, such as 1–100 GHz for example, providing increased bandwidth compared with the lower frequency signals. 
     The optical modulators of array  24  have respective filter frequencies. The filter frequencies of the optical modulators of array  24  are different in one exemplary embodiment to provide different communication channels of optical communication systems  10 . The passbands of the respective optical modulators of array  24  are designed to be electronically tunable as described below. Accordingly, data signals  13  are utilized to control the electronic tuning of the respective optical modulators  34  to encode the data upon the respective desired portions of optical signal  21  by modulating the filter frequencies and passbands of the respective optical modulators  34  at the data rates of data signals  13 . 
     Modulator array  24  outputs the plurality of modulated desired portions as optical signals  25 , also referred to as encoded light-beams, to combiner  26 . Combiner  26  is configured to receive the desired modulated optical signals  25  and to combine such signals  25  into an optical signal  27 , also referred to as a multiplexed-light beam in at least one embodiment, for communication using optical communication medium  28 . In one configuration, combiner  26  is configured to frequency multiplex signals  25  to combine signals  25 . 
     Optical communication medium  28  is implemented in any desired configuration configured to communicate one or more optical signal. Exemplary optical communication media include an optical waveguide comprising one or more optical fiber, air or other appropriate optical transmission medium. 
     Other arrangements of optical communication system are possible in addition to those described with reference to  FIG. 1 . 
     Referring to  FIG. 2 , one exemplary implementation of optical communication system  10  of  FIG. 1  is depicted. Light source  20  is configured as an edge emitting light emitting diode  30  coupled with optical divider  22  implemented as an array waveguide  32 . Optical modulator array  24  is coupled with array waveguide  30 . Although not shown in  FIG. 2 , data source  12  supplies desired data signals to optical modulator array  24 . Optical combiner  26  is coupled intermediate optical modulator array  24  and optical communication medium  28 . Optical communication medium  28  is implemented as a single optical fiber  28  configured to communicate the modulated desired portions of optical signal  21  outputted from array  24  and combined in combiner  26 . 
     Decoding of communication data can be accomplished by one or more standard technique. For example, one decoding technique includes demultiplexing the optical signals at different wavelengths into separate channels and then converting frequency modulation to intensity modulation which can be monitored with an optical detector. Other decoding arrangements may be used. 
     Referring to  FIGS. 3 and 4 , an exemplary configuration of optical modulator array  24  is depicted. Modulator  24  comprises a plurality of modulators  34 ,  34   a,    34   b,    34   c  in the depicted embodiment corresponding to four communication channels within optical communication system  10 . More or less channels are provided according to other optical communication systems and methodologies of the present invention. 
     The depicted modulators  34 ,  34   a,    34   b,    34   c  are configured as Fabry-Perot cavities in the described embodiment. Modulators  34 ,  34   a,    34   b,    34   c  are tuned to one or more respective wavelength (i.e., passbands) and are configured to modulate desired portions of optical signal  21  having the respective wavelengths. As described above, modulators  34 ,  34   a,    34   b,    34   c  pass and modulate portions of optical signal  21  within the respective passbands of the modulators. If wavelengths of light outside of the respective pass bands are provided to modulators  34 ,  34   a,    34   b,    34   c , such light is filtered and not passed according to the exemplary arrangement. 
     Individual modulators  34 ,  34   a,    34   b,    34   c  include a respective one of cavities  42 ,  42   a,    42   b,    42   c,  electrodes  44 ,  46 , insulators  48  and mirrors  50  as shown. Modulators  34 ,  34   a,    34   b,    34   c  are provided upon a substrate  40  which is transparent to wavelengths of light to be communicated within optical communication system  10  in the described exemplary embodiment. An exemplary substrate  40  comprises silicon. Insulators  48  are provided intermediate electrodes  46  and cavities  42 ,  42   a,    42   b,    42   c  as illustrated and comprise silicon in one example. 
     Referring specifically to  FIG. 4 , mirrors  50  are provided upon upper and lower portions of respective cavities  42 ,  42   a,    42   b,    42   c.  Exemplary mirrors  50  in one instance comprise high reflectivity mirrors, such as Bragg mirrors, comprising two or more even number of layers of transparent material having different refractive indices, such as silicon dioxide, titanium oxide or silicon nitride, for example. 
     In the described embodiment, light from optical signal  21  is received within the upper surfaces of cavities  42 ,  42   a,    42   b,    42   c  and passed through the lower surfaces of the respective cavities and through substrate  40  for application to combiner  26  illustrated in  FIG. 1 . 
     Respective data signals  13  (not shown in  FIGS. 3 and 4 ) are provided to electrodes  44 ,  46  to electronically tune respective cavities  42 ,  42   a,    42   b,    42   c.  Optical path lengths of the modulators  34 ,  34   a,    34   b,    34   c  dictate the frequencies of the respective passbands of the respective modulators. The optical path lengths of modulators  34 ,  34   a,    34   b,    34   c,  are defined by the physical length and refractive indices of cavities  42 ,  42   a,    42   b,    42   c.  Varying the physical length and/or refractive indices varies the passband of the respective modulator  34 ,  34   a,    34   b,    34   c.    
     In the described exemplary embodiment, the respective cavities  42 ,  42   a,    42   b,    42   c  have different physical lengths, as illustrated, tuned to the desired portions of optical signal  21  to be passed and modulated. In the described embodiment, cavities  42 ,  42   a,    42   b,    42   c  contain a material having a relatively high electro-optic coefficient. Exemplary materials include electrically controllable birefringent material, such as lithiumniobate, barbarium titanate or other materials including polymer materials having high electro-optic coefficients. Cavities  42 ,  42   a,    42   b,    42   c  contain the same or different birefringent material depending upon the configuration of array  24  and frequencies of light to be modulated. 
     The material(s) within cavities  42 ,  42   a,    42   b,    42   c  may be varied to further tune optical modulators  32 ,  32   a,    32   b,    32   c  to the desired passbands. In such an arrangement, the physical length of cavities  42 ,  42   a,    42   b,    42  may be held constant or varied depending upon the desired configuration and desired passbands. In general, the effective cavity length may be shorter if distributed Bragg mirrors are utilized as mirrors  50  inasmuch as mirror thickness can be a reasonable fraction of overall cavity length. 
     Data signals  13  applied to the electrodes  44 ,  46  vary the refractive indices of the birefringent material in cavities  42 ,  42   a,    42   b,    42   c  providing modulation of the filter frequencies of modulators  34 ,  34   a,    34   b,    34   c  and modulation of the desired portions of optical signal  21  passing therethrough. The wavelengths or frequencies of the desired portions of the optical signal  21  are modulated within modulators  34 ,  34   a,    34   b,    34   c  responsive to the varying of the refractive indices of materials within cavities  42 ,  42   a,    42   b,    42   c.    
     As described, the present invention provides improved devices and methods for encoding data on an optical signal. In one example of the invention, frequency modulation obtained by modulation of a filter illuminated with a broadband source provides signal bandwidths which are limited by the response of the tunable filter which can be much faster than the bandwidth of a laser which is limited by capacitance and carrier dynamics. Accordingly, aspects of the invention provide usage of a bright, broad spectrum incoherent optical source together with high speed tunable filters to achieve high data rate transmission over a broad range of operating temperatures. Other aspects are provided as described above.