Abstract:
A DWDM add/drop system for use in optical communication system is disclosed. Using semiconductor fabrication techniques, a plurality of waveguide arrays and signal carriers are substantially symmetrically arranged about an optical axis of the system. Electrode heaters are provided proximate junctions created at the intersections of selected waveguides. Using the heaters, portions of optical signals may be redirected to other waveguides. In addition, the heaters may be used to attenuate or otherwise modify signals in the waveguides. The waveguide arrays are arranged such that a plurality of signal processing operations may be performed substantially simultaneously. In a preferred embodiment, the switches and waveguide arrays are coupled with a light focusing device and a dispersion apparatus to form a switched, combined multiplexer/demultiplexer having signal attenuation and modification capabilities.

Description:
TECHNICAL FIELD  
         [0001]    The present invention is related to optical communications systems for wavelength division demultiplexing, optical signal switching and wavelength division multiplexing and, more particularly, to an optical communication system having a waveguide array for multiplexing and demultiplexing multiple wavelength signals in combination with an array of optical switches which also functions as a variable optical attenuator.  
         BACKGROUND OF THE INVENTION  
         [0002]    The increased demand for data communication and the remarkable growth of the Internet have resulted in increased demand for communication capability within metropolitan areas. There has also been an equally large increase in demand for communication capability between large metropolitan areas. Optical communication systems using a network of fiber optic cables are being developed and installed to meet the increased demand.  
           [0003]    The data transmission capacity of fiber optic cables and fiber optic networks has been substantially increased as a result of wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM). Within WDM and DWDM systems, optical signals assigned to different wavelengths are combined (multiplexed) into a multiple wavelength signal for transmission over a single fiber optic cable or other suitable waveguide. A typical DWDM system modulates multiple data streams on to different portions of the light spectrum. For example, one data stream may have an assigned wavelength of 1534 nanometers (nm) and the next data stream may have an assigned wavelength of 1543.8 nm. The required spacing between assigned wavelengths is generally established by International Telecommunications Union (ITU) specifications. These spacings include 0.4 μm and 0.8 μm.  
           [0004]    Demultiplexing, the reverse process of multiplexing, typically refers to separation of a multiple wavelength or multi-wavelength signal transmitted by a single fiber optic cable or other suitable waveguide into constituent optical signals for each wavelength. Each optical signal may be further processed to obtain the associated data stream or other information. Both multiplexing and demultiplexing are required for satisfactory operation of WDM and DWDM systems. Multiplexing and demultiplexing of optical signals in conventional DWDM systems are typically performed by two separate relatively expensive and often difficult to manufacture optical devices.  
           [0005]    Various types of optical switches and techniques are currently used in optical communication systems. Many currently available optical switches are based upon optoelectric and electrooptic conversion of light signals and electrical signals within the associated optical switch. One type of presently available optical switch includes a matrix of thermooptic switching elements interconnected by waveguides formed on a silica substrate. Switching of light signals is accomplished by the use of thin film heaters to vary the temperature of the switching elements. Electrical circuits are also provided to supply switching current to the heaters. A heat sink may be provided to dissipate heat caused by the switching operations. One example of such switches is shown in U.S. Pat. No. 5,653,008.  
           [0006]    Various types of optical signal amplifiers, wavelength division demultiplexers, optical switches, wavelength division multiplexers and techniques are currently used in optical communication systems. Optical signal amplifiers, wavelength division multiplexers and demultiplexers and other components associated with optical communication systems that transmit multiple wavelength light signals typically function best when respective signal levels for the multiple wavelength optical signals are substantially equal with each other. A substantial variation in signal level of multiple wavelength optical signals can result in an undesirable signal to noise ratio and resulting poor performance.  
           [0007]    Multiple wavelength optical signals are normally collectively amplified by a light amplifier. The amplification factor of many light amplifiers is dependent upon the wavelength of each optical signal. Therefore, the amplification factors for multiple wavelength optical signals varies depending on the specific wavelength of each signal. The resulting difference between signal levels for multiple wavelength optical signals amplified by a single amplifier is often relatively small. However, when a large number of light amplifiers (ten or more) are used in a fiber optic communication system, the variation in signal levels becomes cumulative and may result in unsatisfactory lowering of associated signal to noise ratios. Therefore, variable optical attenuators are often provided at the input stage and/or output stage of light amplifiers in both large metropolitan communication systems and long distance fiber optic communication systems to adjust signal levels or intensity of multiple wavelength light signals to maintain a desired signal to noise ratio.  
           [0008]    Variable optical attenuators are often included in optical communication systems to maintain a desired signal level for each optical signal or wavelength. Examples of variable optical attenuators (VOA) include natural density filters that are often used to suppress the amount of light depending on wavelength characteristics. Other variable optical attenuators include mechanical devices that position a glass substrate so that light signals may be attenuated by varying the position of the glass substrate. Still other variable optical attenuators attenuate light signals by rotating the polarization of each light signal as it passes through a Faraday element.  
         SUMMARY OF THE INVENTION  
         [0009]    In accordance with teachings of the present invention, a system is provided for demultiplexing, switching, attenuating and multiplexing multiple wavelength optical signals using a waveguide array, an array of optical switches and a diffraction grating. An array of waveguides formed in accordance with teachings of the present invention in combination with a lens assembly and a diffraction grating may function simultaneously as both a wavelength division multiplexer and a wavelength division demultiplexer. Optical switches formed in accordance with teachings of the present invention may be used to both switch optical signals and attenuate the respective signal level of optical signals flowing through the switches.  
           [0010]    In another aspect, the present invention provides a communication system for multiple wavelength signals including a combined multiplexer/demultiplexer operable to substantially simultaneously multiplex and demultiplex multiple wavelength optical signals. The communication system preferably also includes an array of optical switches operable to.  
           [0011]    In a further aspect, the present invention provides a signal processing system having a plurality of waveguide arrays substantially symmetrically disposed relative to an optical axis of the system and an array of optical switches operable to selectively direct optical signal carried by the waveguides. The plurality of waveguides are preferably operable to perform at least one signal processing operation on optical signal reflected from a dispersion apparatus.  
           [0012]    Technical benefits of the present invention include providing a communication system or network with substantially reduced insertion loss and substantially reduced manufacturing costs as compared to a conventional communication system or network having a wavelength division demultiplexer, an array of optical switches, a corresponding array of variable optical attenuators, and a wavelength division multiplexer. A single device having an array of waveguides formed in accordance with teachings of the present invention may function simultaneously as a multiple wavelength optical signal multiplexer and demultiplexer that reduces the number of multiplexers and demultiplexers required for a given number of optical signals by one-half.  
           [0013]    An array of optical switches formed in accordance with teachings of the present invention may function to switch and attenuate multiple wavelength optical signals as well as eliminate the requirement for separate variable optical attenuators. Therefore, a communication system or network formed in accordance with teachings of the present invention will have approximately one-half the number of components associated with a conventional communication system or network having the same performance characteristics with respect to multiple wavelength optical signals. For example, a conventional communication system or network capable of switching forty channels would normally require a DWDM multiplexer, a DWDM demultiplexer, forty (40) 2×2 optical switches and forty (40) variable optical attenuators. A communication system capable of switching forty (40) optical signals formed in accordance with the teachings of the present invention will require only one (1) bi-directional DWDM multiplexer/demultiplexer and an array of forty (40) optical switches.  
           [0014]    The quality of a light signal is determined by the ratio between the signal level and the intensity of noise associated with the light signal. This ratio is commonly referred to as the signal to noise ratio (SNR). Optical switches formed in accordance with teachings of the present invention may be satisfactorily used to adjust the intensity or signal level of multiple wavelength light signals communicated through a fiber optic system to establish the desired signal to noise ratio for optimum performance of amplifiers, bi-directional wavelength division multiplexers/demultiplexers and other components of an optical communication system.  
           [0015]    A communication system or network formed in accordance with teachings of the present invention may be satisfactorily used with single mode, multiple mode, or a combination of single mode and multiple mode fibers as input and output fibers and to form fiber or waveguide arrays. A combined multiplexer/demultiplexer formed in accordance with teachings of the present invention may use the same imaging and beam optics, diffraction grating and mechanical packaging to both multiplex and demultiplex multiple wavelength optical signals. Technical benefits of the present invention include substantial savings of cost, space and weight. The present invention is particularly advantageous when more than one multiplexer or demultiplexer is required at the same location in an optical communication system or network. For some applications multiple wavelength optical signals from different channels of a principle fiber line may be separated, dropped, added or cross connected and then recombined into a multiple wavelength optical signal without requiring the use of separate multiple wavelength multiplexers and multiple wavelength demultiplexers.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    A more complete and thorough understanding of the present invention and its advantages may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which like reference numbers indicate like features, and wherein:  
         [0017]    [0017]FIG. 1 is a schematic drawing showing an optical communication system formed in accordance with teachings of the present invention including a bi-directional wavelength division multiplexer/demultiplexer and an array of optical switches which also function as variable optical attenuators;  
         [0018]    [0018]FIG. 2 is a schematic drawing showing a plan view with portions broken away of a 2×2 optical switch satisfactory for use with the optical communication system of FIG. 1;  
         [0019]    [0019]FIG. 3 is a graph showing optical output or signal level versus time measured at the drop port of the optical switch of FIG. 2 for a given current flow through the electrode heater;  
         [0020]    [0020]FIG. 4 is a graph showing attenuation of an optical signal measured at the drop port associated with the optical switch of FIG. 2;  
         [0021]    [0021]FIG. 5 is a schematic drawing showing a plan view of another embodiment of an optical switch satisfactory for use with the optical communication system of FIG. 1;  
         [0022]    [0022]FIG. 6 is a schematic drawing in section taken along line  6 - 6  of FIG. 5;  
         [0023]    [0023]FIG. 7A is a schematic drawing showing an isometric view of a bi-directional wavelength division multiplexer/demultiplexer satisfactory for use with the optical communication system of FIG. 1;  
         [0024]    [0024]FIG. 7B is a schematic drawing showing an isometric view of an alternate embodiment of the bi-directional wavelength division multiplexer/demultiplexer depicted in FIG. 7A; and  
         [0025]    [0025]FIG. 8 is a schematic drawing showing another embodiment of an optical communication system formed in accordance with teachings of the present invention including a bi-directional wavelength division multiplexer/demultiplexer and an array of optical switches which also function as variable optical attenuators.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]    Preferred embodiments of the present invention and its advantages are best understood by referring to FIGS. 1 through 8 of the drawings, like numerals being used for like and corresponding parts of the various drawings.  
         [0027]    The terms “optical signal or signals” and “light signal or signals” are used in this application to include the full range of all electromagnetic radiation which may be satisfactorily used to communicate information using a waveguide and/or fiber optic cable. A bi-directional wavelength division multiplexer/demultiplexer incorporating teachings of the present invention may be satisfactorily used with such optical signals. An array of optical switches incorporating teachings of the present invention may be satisfactorily used to both switch and attenuate or reduce the intensity or signal level of such optical signals. Signal level or intensity may also be referred to as “optical power.” 
         [0028]    The terms “bi-directional wavelength division multiplexer/demultiplexer” and “combined multiplexer/demultiplexer” are used in this application to refer to an optical device that may be satisfactorily used to simultaneously multiplex multiple wavelength optical signals and demultiplex multiple wavelength optical signals. A bi-directional wavelength division multiplexer/demultiplexer formed in accordance with teachings of the present invention may also be used to only demultiplex multiple wavelength optical signals or multiplex multiple wavelength optical signals as desired for a specific communication system.  
         [0029]    The term “waveguide ” is used in this application to include the full range of optical devices that may be satisfactorily used to communicate optical signals. A waveguide typically includes a core formed from a first optical material and disposed in a channel formed in a second optical material. A fiber optic cable is one example of a specific type of waveguide. However, waveguides satisfactory for use with the present invention may have various configurations other than fiber optic cables and cores disposed in a channel.  
         [0030]    Various features of the present invention will be described with respect to an optical communication system or network such as communication system  20  shown in FIG. 1 and communication system  120  shown in FIG. 8. An optical communication system or network formed in accordance with teachings of the present invention may be satisfactorily used in long distance fiber optic communication systems (not expressly shown) or large metropolitan area optical communication systems (not expressly shown). Various features of the present invention will be described with respect to a multiple wavelength signal having four spectral components (λ 1 , λ 2 , λ 3  and λ 4 ) . However, the present invention may be used with multiple wavelength optical signals having any number of spectral components or wavelengths.  
         [0031]    For the embodiment of the present invention shown in FIG. 1, communication system  20  preferably includes a plurality of signal carriers, such as input fiber optic cable  21  and output fiber optic cable  22 . Wavelength division multiplexing (WDM) techniques may be used to allow signal carriers or fiber optic cables  21  and  22  to carry multiple optical signals at various wavelengths, substantially increasing the efficiency of signal carriers or fiber optic cables  21  and  22 . Dense wavelength division multiplexing (DWDM) techniques have been developed to allow existing fiber optic networks to better satisfy increased demand for communication capabilities.  
         [0032]    Various features of the present invention will be described with respect to communicating an optical signal having at least four wavelengths (λ 1 , λ 2 , λ 3  and λ 4 ) Respective amplifiers (not expressly shown) may be coupled with input signal carrier or fiber optic cable  21  and output signal carrier or fiber optic cable  22 . Communication system  20  preferably includes bi-directional wavelength division multiplexer/demultiplexer  40  and at least one array of optical switches  100 . For purposes of illustrating various features of the present invention optical switches  100  have been designated  100   a ,  100   b ,  100   c  and  100   d . A plurality of fiber optic cables and/or waveguides  31 ,  32 ,  33  and  34  are preferably coupled with bi-directional wavelength division multiplexer/demultiplexer  40  and respective optical switches  100 .  
         [0033]    Bi-directional wavelength division multiplexer/demultiplexer  40  may include two or more waveguide arrays or sets of waveguides. For the embodiment of the present invention shown in FIGS. 7A, 7B and  8 , the first waveguide array has been designated  51 . The second waveguide array has been designated  52 . For purposes of describing various features of the present invention, the portion of fiber optic cables and/or waveguides  31 - 34  used to couple optical switches  100   a ,  100   b ,  100   c , and  100   d  with first waveguide array  51  are designated  31   a ,  32   a ,  33   a  and  34   a . The portion of fiber optic cables and/or waveguides  31 - 34  which couple optical switches  100   a ,  100   b ,  100   d  and  100   c  with second waveguide array  52  are designated  31   b ,  32   b ,  33   b  and  34   b . Arrows have been added to show the direction of optical signal travel through fiber optic cables and/or waveguides  31 ,  32 ,  33  and  34 . See FIGS. 1 and 8.  
         [0034]    Input signal carrier or fiber optic cable  21  preferably provides multiple wavelength signal (λ 1 , λ 2 , λ 3 , λ 4 ) to bi-directional wavelength division multiplexer/demultiplexer  40 . Bi-directional wavelength division multiplexer/demultiplexer  40  is preferably operable to demultiplex the multiple wavelength signal into its individual spectral components. During the demultiplexing process, the optical signal corresponding with λ 1  is preferably directed through fiber optic cable and/or waveguide  31   b  to optical switch  100   a . In a similar manner optical signals corresponding with wavelengths λ 2 , λ 3  and λ 4  are preferably directed by fiber optic cables and/or waveguides  32   b ,  33   b  and  34   b  to respective optical switches  100   b ,  100   c  and  100   d.    
         [0035]    As discussed later in more detail, optical switches  100  may direct the respective optical signals (λ 1 , λ 2 , λ 3  or λ 4 ) to respective drop ports (not expressly shown) coupled with fiber optic cables designated  71 ,  72 ,  73  and  74  or to respective fiber optic cables and/or waveguides  31   a ,  32   a ,  33   a  and  34   a . Fiber optic cables  61 ,  62 ,  63  and  64  may supply optical signals having respective wavelengths λ 1 ,  80   2 , λ 3  and λ 4  to add ports (not expressly shown) at respective optical switches  100   a ,  100   b ,  100   c  and  100   d.    
         [0036]    The portions of communication system  20  shown in FIG. 1 and communication system  120  shown in FIG. 8 include respective input and output fiber optic cables, four add ports and four drop ports. However, various communication systems may be formed in accordance with teachings of the present invention using bi-directional wavelength division multiplexer/demultiplexers and multiple arrays of optical switches having any number of input and output channels, add ports and drop ports.  
         [0037]    [0037]FIG. 2 is a schematic drawing showing a plan view of one example of an optical switch incorporating teachings of the present invention. Optical switches  100   a ,  100   b ,  100   c  and  100   d  have substantially the same design as the optical switch depicted in FIG. 2. For the embodiment shown in FIG. 2, optical switch  111  preferably includes first waveguide  101  and second waveguide  102 . Each waveguide  101  and  102  also includes respective input ends “a” and output ends “b”. Although various features of the present invention will be described with respect to an optical signal traveling from input end “a” to output end “b” of a waveguide, an optical switching device formed in accordance with teachings of the present invention may be satisfactorily used to switch or redirect optical signals traveling in either direction through the waveguide.  
         [0038]    For the embodiments of the present invention represented by communication systems  20  and  120 , input end  101   a  of each optical switch  100   a ,  100   b ,  100   c  and  100   d  is preferably coupled with respective fiber optic cables and/or waveguides  31   b ,  32   b ,  33   b  and  34   b . Output end  102   a  of each optical switch  100   a ,  100   b ,  100   c  and  100   d  is preferably coupled with respective fiber optic cables and/or waveguides  31   a ,  32   a ,  33   a  and  34   a . Input end  102   a  of each optical switch  100   a ,  100   b ,  100   c  and  100   d  is preferably coupled with a respective add port and fiber optic cable  61 ,  62 ,  63  and  64 . Output end  101   b  of each optical switch  100   a ,  100   b ,  100   c  and  100   d  is preferably coupled with a respective drop port and fiber optic cables  71 ,  72 ,  73  and  74 .  
         [0039]    For embodiments of the present invention represented by communication systems  20  and  120 , optical signals may travel from input end  101   a  through first waveguide  101  to output end  101   b  or may be directed by switch  111  to travel through second waveguide  102  to an output port (not expressly shown) coupled with output end  102   b . Except for insertion losses and other minor losses associated with an optical signal traveling through a waveguide, the optical power level of an optical signal entering input end  101   a  is approximately equal to the total optical power level exiting from output end  101   b  plus output end  102   b . Except for insertion losses and other minor losses associated with transmission of an optical signal through a waveguide, the total optical energy level or power level of optical signals communicated through optical switch  111  remains substantially constant.  
         [0040]    An add port (not expressly shown) may be coupled with input end  102   a  of second waveguide  102 . Add signals will generally travel from input end  102   a  through second waveguide  102  to the output port (not expressly shown) coupled with output end  102   b . A drop port (not expressly shown) may be coupled with output end  101   b  of each of first waveguide  101 . Fiber optic cable  71 ,  72 ,  73  and  74  are preferably coupled with respective drop ports.  
         [0041]    Angle α defined by intersection or junction  103  between first waveguide  101  and second waveguide  102  is preferably selected to be in the range of approximately two degrees (2°) and eight degrees (8°). For at least one application, for example using Silicon Oxide (SiO), angle α is preferably equal to approximately three degrees (3°). For some polymers, angle α is preferably equal to approximately six degrees (6°). By forming optical switch  111  with angle α having a value between approximately two degrees (2°) and eight degrees (8°), an optical signal may travel through first waveguide  101  from input end  101   a  to output end  101   b  without any significant perturbation or reflection at intersection or junction  103  unless the index of refraction at junction  103  is changed in accordance with teachings of the present invention. The index of refraction at junction  103  may be changed by thermooptic, electrooptic, magnetooptic, or acoustooptic effects.  
         [0042]    Electrode heater  104  is preferably disposed adjacent to junction or intersection  103  to produce desired thermooptic effects. Electrode heater  104  may be formed from various types of materials including nickel chrome alloys (NiCr) and chromium gold (Cr/Au), and other metal and alloys. Electrode heater  104  may be used to apply a desired amount of heat to junction or intersection  103  to direct or deflect optical signals from first waveguide  101  to second waveguide  102  or from second waveguide  102  to first waveguide  101 . As discussed later in more detail, electrode heater  104  may also be used to attenuate the signal level of optical signals in waveguides  101  and  102 .  
         [0043]    When electrical current is supplied to electrode heater  104 , heating will occur in a cladding layer  114  disposed between electrode heater  104  and junction  103 , see FIG. 6,to produce a desired thermooptic effect such as attenuation of an optical signal and/or switching of an optical signal. For example, an optical signal corresponding with wavelength λ 1  may be directed to input end  101   a  of waveguide  101 . An appropriate amount of electrical current may be supplied to electrode heater  104  to provide a desired amount of heating at intersection or junction  103  to direct at least a portion of the optical signal corresponding with wavelength λ 1  from waveguide  101  to waveguide  102  and output end  101   b . Attenuation of optical signals will be discussed in more detail with respect to the graphs shown in FIGS. 3 and 4.  
         [0044]    The configuration and location of electrode heater  104  allows selected heating of portions of waveguides  101  and  102  to form what may be considered an imaginary mirror disposed along a longitudinal center line of intersection  103 . Heating cladding layer  114  and portions of waveguides  101  and  102  at intersection  103  will permit a change in the refractive index such that total internal refraction may be achieved. In effect, heating caused by electrode heater  104  at or above a selected value will provide an imaginary mirror at intersection  103  that reflects or deflects light signals from waveguide  101  to waveguide  102 . The same total internal refraction or imaginary mirror effect will also cause optical signals traveling through second waveguide  102  to be reflected or deflected into first waveguide  101 .  
         [0045]    For the embodiment of the present invention shown in FIG. 2, current may flow from variable current source  106  through lead  108  to electrode heater  104  and return through electrical lead  110  to ground  112 . The current flow through electrode heater  104  may be varied in accordance with teachings of the present invention to allow switch  100  to function as a variable optical attenuator. Waveguides  101  and  102 , electrode heater  104 , current source  106 , electrical leads  108  and  110  and ground  112  may be formed on a substrate using conventional semiconductor fabrication techniques.  
         [0046]    [0046]FIG. 3 is a graph showing optical signal level versus time for a given current flow. For one example of an optical switch  111 , optical signal level was measured at output end  101   b  of first guide  101  versus time in seconds at a substantially constant current flow through electrode heater  104 . The current supplied to electrode heater  104  was maintained at approximately forty (40) milliamps for thirty six hundred (3600) seconds or sixty (60) minutes.  
         [0047]    [0047]FIG. 4 is a graph showing output power or signal level in decibels (dB) measured at output end  101   b  of waveguide  101  versus electrical current flow through electrode heater  104 . An optical signal with constant power or signal level was supplied to input end  101   b  of waveguide  101  while the electric current flow to electrode heater  104  was varied in accordance with teachings of the present invention. As previously noted, the power level or signal level of an optical signal entering input end  101   b  of waveguide  101  is generally equal to the combined power level or signal level of optical signals exiting from output end  102   b  of waveguide  102  and output end  101   b  of waveguide  101 . Therefore, a similar measurement of output power or signal level measured at output end  102   b  of waveguide  102  would be approximately the inverse of the graph shown in FIG. 4.  
         [0048]    The graphical information shown in FIG. 4 demonstrates that for an optical switch such as optical switch  111 , increasing current flow through electrode heater  104  may be used to attenuate or decrease the output power or signal level of an optical signal traveling between input end  101   a  to output end  101   b  of waveguide  100 . In the same manner, the output power or signal level of the portion of the optical signal directed to output end  102   b  of waveguide  102  may be selectively increased. For this embodiment of the present invention, the attenuation of an optical signal at output end  101   b  or increase in optical signal at output end  102   b  is particularly significant between approximately twenty-two (22) milliamps and forty (40) milliamps.  
         [0049]    As the current flow through electrode heater  104  is increased from zero to forty (40) milliamps, an increasing portion of an optical signal that enters input end  101   a  of waveguide  100  will be directed towards output end  102   b  of waveguide  102 . The total sum of the optical signals exiting from output end  101   b  and output end  102   b  is generally equal to the signal level of the optical signal entering input end  101   a . Therefore, an array of optical switches formed in accordance with teachings of the present invention may be used to control the signal level of each optical signal exiting from respective output end  102   b  and directed to respective fiber optic cables and/or waveguides  31   a ,  32   a ,  33   a  and  34   a . By varying the current supplied to respective electrode heaters  104  of optical switches  100   a ,  100   b ,  100   c  and  100   d , the output of each optical signal corresponding with wavelength λ 1 , λ 2 , λ 3  and λ 4  may be adjusted to approximately the same value. Thus, the signal levels of the optical signals returned to bi-directional multiple wavelength multiplexer/demultiplexer  40  are maintained substantially equal with each other.  
         [0050]    [0050]FIG. 5 is a schematic drawing showing additional details associated with one embodiment of optical switch  111 . Examples of low resistance electrical leads  108  and  110  are shown in more detail. For the embodiment of the present invention as shown in FIG. 5, electrode heater  104  has a generally rectangular configuration defined in part by a pair of longitudinal edges  104   a  and  104   b  and a pair of lateral edges  104   c  and  104   d . Longitudinal edges  104   a  and  104   b  may have a length of approximately two hundred fifty micrometers (250 μm). Lateral edges  104   c  and  104   d  may have a length of approximately ten micrometers (10 μm). The thickness of electrode heater  104  is preferably very small, almost zero, as compared with the thickness of first waveguide  101  and second waveguide  102 .  
         [0051]    Also depicted in FIG. 5 is that longitudinal edge  104   b  of electrode heater  104  is preferably disposed on a line that corresponds generally with the longitudinal center line of junction or intersection  103  between first waveguide  101  and second waveguide  102 . For some applications, the vertical spacing or distance between electrode heater  104  and the corresponding junction or intersection  103  is approximately five micrometers (5 μm) and preferably within a range of plus or minus 0.5 μm. The lateral offset between longitudinal edge  104   b  of electrode heater  104  and the corresponding longitudinal center line of intersection  103  is preferably less than 9.5 μm. When the offset between electrode heater  104  and the respective intersection  103  exceeds these limits, desired heating of intersection or junction  103  and resulting internal reflection of an optical signal traveling therethrough may not occur as desired.  
         [0052]    [0052]FIG. 6 shows one example of waveguides formed on a substrate using semiconductor fabrication techniques to produce an optical switching device incorporating teachings of the present invention. For the embodiment of the present invention shown in FIG. 6, substrate  116  may be part of a typical silicon wafer used in semiconductor fabrication. However, an optical switching device may be formed in accordance with teachings of the present invention on a wide variety of substrates and is not limited to use with only conventional silicon substrates.  
         [0053]    For the embodiment of the present invention shown in FIG. 6, optical switch  100  preferably includes layer  118  disposed immediately adjacent to substrate  116 . Layer  118  may be formed from various types of material such as silicon dioxide (SiO 2 ), or other materials such as Teflon AF 240. First waveguide  101  and second waveguide  102  may be formed from various types of material such as a combination of silicon dioxide and germanium oxide (SiO 2 :GeO 2 ) with an index of refraction of approximately 1.4538. For some applications, layer  118  may have a thickness of approximately fifteen micrometers (15 μm) with an index of refraction of approximately 1.445.  
         [0054]    Waveguides  101  and  102  are preferably formed on layer  118  and disposed in respective channels  115  and  117  formed in cladding layer  114 . For one embodiment, channels  115  and  117  preferably have a generally rectangular cross section with dimensions in the range of approximately of six or seven micrometers (6 or 7 μm). Layer  114  may sometimes be referred to as “top cladding”. Layer  114  may be formed from Teflon AF 1600 having an index of refraction of approximately 1.31. The thermooptic coefficient of many polymers is generally less than zero. As a result, when the temperature of such polymers is increased, the corresponding index of refraction is reduced. Teflon AF 1600 represents one example of a polymer having the desired thermooptic coefficient.  
         [0055]    For other applications first layer  118  may be formed from silicon dioxide having a thickness of approximately 2.4 micrometers (2.4 μm). Second layer or top cladding  114  may be formed from polymeric material such as Ultradel 9021 having an index of refraction of approximately 1.526. Waveguides  101  and  102  may be formed from Ultradel 9120 having an index of refraction of approximately 1.5397.  
         [0056]    For still other applications first layer  118  may be formed from Teflon AF 240 having an index of refraction of approximately 1.29. Second layer or top cladding  114  may be formed from Teflon AF 240 having an index of refraction of 1.29. The thickness of first layer  118  may be approximately five micrometers (5 μm). Waveguides  101  and  102  may be formed from Teflon AF 160 having an index of refraction of approximately 1.31.  
         [0057]    Waveguides  101  and  102  may be formed from a wide variety of materials including polyimide, Teflon, PFCB, a mixture of silicon dioxide and polymer, ion exchange and polymer and fluorinated polyimide. Layer  114  may be formed from Ultradel polymer U 9120 having a refraction index of 1.5397 and waveguides  101  and  102  of Ultradel U 9020 having a refraction index of 1.526.  
         [0058]    [0058]FIGS. 7A and 7B are schematic drawings showing portions of combined multiplexer/demultiplexers  140   a  and  140   b , respectively, which may be satisfactorily used with communication systems  20  and/or  120 . A combined multiplexer/demultiplexer incorporating teachings of the present invention may also be used with optical sensors and spectroscopy equipment. Optical components associated with combined multiplexer/demultiplexer  140   a  or  140   b  include fiber optic input cable  121  and fiber optic output cable  122 , see FIGS. 7B and 8, which are respectively coupled with first waveguide array  51  and second waveguide array  52 . Other optical components of combined multiplexer/demultiplexer  140  include light focusing device  142  and dispersion apparatus or diffraction grating  144 .  
         [0059]    Various types of reflective diffraction gratings and transmissive diffraction gratings may be satisfactorily used with a combined multiplexer/demultiplexer formed in accordance with teachings of the present invention. For some applications diffraction grating  144  may be a Littrow assembly or a Litmann Metcalf assembly or any other diffraction grating satisfactory for separating a multiple wavelength optical signal into selected spectral components and combining individual optical signals corresponding with selected spectral components into a multiple wavelength optical signal. Various types of dispersive elements in addition to diffraction gratings may also be used.  
         [0060]    As illustrated in FIGS. 7A and 8, first waveguide array  51 , second waveguide array  52 , light focusing device  142  and diffraction grating  144  are preferably optically aligned with each other along optical axis  146 . First waveguide array  51  and second waveguide array  52  are preferably symmetrically disposed with respect to optical axis  146 . In an alternate embodiment, illustrated in FIG. 7B, substantially symmetric disposition of signal carriers or fiber cables  121 ,  122  and waveguide arrays  51 ,  52  may be achieved by aligning fiber cables  121 ,  122  and waveguide arrays  51 ,  52  on opposing sides of optical axis  146 . As a result of aligning first waveguide array  51  and second waveguide array  52  with optical axis  146  as illustrated in FIGS. 7A and 8 or by aligning fiber cables  121 ,  122  and waveguide arrays  51 ,  52  as illustrated in FIG. 7B, combined multiplexer/demultiplexers  140   a  and  140   b  may substantially simultaneously demultiplex multiple wavelength optical signals received from fiber optic input cable  121  and multiplex respective optical signals into a multiple wavelength optical signal directed to fiber optic output cable  122 . The present invention allows first waveguide array  51  and second waveguide array  52  to use common optical beam handling components and dispersion components such as light focusing device  142  and diffraction grating  144 . For some applications, light focusing device  142  may include a single bi-convex lens or any other lens assembly operable to collimate diverging light or focus collimated light as desired. For some applications, diffraction grating  144  may have a blazed surface (not expressly shown).  
         [0061]    For the embodiment of the present invention shown in FIGS. 7A and 8, input fiber  121  may be disposed at one end of first waveguide array  51  and output fiber  122  may be disposed at the same end of second waveguide array  52 . Input fiber  121  is preferably located immediately adjacent to and disposed above output fiber  122 . For the embodiment of the present invention shown in FIG. 7A, first waveguide array  51  and second waveguide array  52  may be formed by plurality of fiber optic cables disposed in respective “v” grooves. For other applications, as illustrated in FIGS. 7B and 8, first waveguide array  51  and second waveguide  52  may be formed using semiconductor fabrication techniques by placing a waveguide core in a respective waveguide channel. See also FIG. 6. The present invention allows fabricating multiple waveguide arrays, stacked relative to each other, using such semiconductor fabrication techniques.  
         [0062]    For the embodiment of the present invention shown in FIGS. 7A and 8, a portion of input fiber optic cable  121  may be used to provide waveguide  51   a . In a similar manner, a portion of output fiber optic cable  122  may be used to provide waveguide  52   a . In a similar manner, as illustrated in FIG. 8, portions of fiber optic cables  31 ,  32 ,  33  and  34  may be used to provide respective waveguides  51   b  through  51   e  and  52   b  through  52   e.    
         [0063]    During operation of combined multiplexer/demultiplexer  140   a , input fiber  121  may be used to communicate a multiple wavelength optical signal to input waveguide  51   a . Light focusing device  142  and diffraction grating  144  cooperate with each other to reflect and disperse the multiple wavelength optical signals emitted from input waveguide  51   a  into selected spectral components directed to respective waveguides  52   b  through  52   e  of second waveguide array  52 , preferably employing inverse symmetry. For the embodiment shown in FIGS. 7A and 8, second waveguide array  52  in combination with light focusing device  142  and diffraction grating  144  function as a demultiplexer.  
         [0064]    Combined multiplexer/demultiplexer  140   b  of FIG. 7B operates in a similar manner. For example, a signal may be received by light focusing device  142  from input cable  121 . Light focusing choice  142  may then direct the multiple wavelength signal to dispersion apparatus or diffraction grating  144 . At diffraction grating  144 , the optical signal is preferably diffracted into a plurality of selected spectral components and reflected back to light focusing device  142 . Light focusing device  142  then preferably directs the spectral components to one or more waveguides or waveguide arrays. The waveguides or waveguide arrays then preferably perform one or more signal processing operations, e.g., multiplexing, demultiplexing, attenuation, adding, dropping, etc., on the spectral components. In one embodiment, one or more processed signals may be communicated by cable  121  or  122 .  
         [0065]    As another operating example, input fiber  121  could be an input from a telecommunications line that contains many different wavelength channels. The signals from the different channels would be reflected and dispersed across the bottom row of receiving fibers in array  52 . The different wavelength signal of each channel would go into a different fiber in array  52  to be separated or demultiplexed. In an opposing manner, the array  51  could be simultaneously used to recombine all the different wavelengths from individual channels, one in each fiber, by the dispersion and reflection from the diffraction grating on to output fiber  122 . The recombined signals would be sent out on output fiber  122  (multiplexed); thus, making a combination multiplexer/demultiplexer in one module.  
         [0066]    Respective optical signals from other sources such as optical switches  100   a ,  100   b ,  100   c  and  100   d  may be coupled with waveguides  51   b  through  51   e  of first waveguide array  51 . Light focusing device  142  and diffraction grating  144  preferably cooperate with each other to combine or multiplex respective optical signals from waveguides  51   b  through  51   e  of first waveguide array  51  into a multiple wavelength optical signal. Light focusing device  142  directs the multiple wavelength optical signal to output waveguide  52   a  of second waveguide array  52  again employing inverse symmetry. Signals from sources other than switches  100  may be coupled with waveguides  51   b  through  51   e  of first waveguide array  51  and multiplexed by dispersion and reflection from diffraction grating  144  into a multiple wavelength optical signal that is directed to output waveguide  52   a  of second waveguide array  52 .  
         [0067]    By positioning and forming first waveguide array  51  and second waveguide array  52  based on inversion symmetry with respect to optical axis  146  or by providing inversion symmetry between signal carriers  121 ,  122  and waveguide arrays  51 ,  52 , imaging and reflection from diffraction grating  144  allows substantially simultaneous multiplexing and demultiplexing of multiple wavelength optical signals. The present invention is not limited to only a first waveguide array and a second waveguide array or a first input cable and a second input cable. Multiple multiplexing and demultiplexing functions may be satisfactorily formed by disposing any desired number of waveguide arrays and signal carriers, with appropriate inversion symmetry, relative to a light focusing device, a dispersion device and the associated optical axis.  
         [0068]    Although the present invention has been described with respect to a specific preferred embodiment thereof, various changes and modifications may be suggested to one skilled in the art and it is intended that the present invention encompass such changes and modifications fall within the scope of the appended claims.