Abstract:
The invention provides a dispersion compensation system and method formed by cascading a series of GT cavities with three setting parameters, reflectivity, resonant wavelength, and free-spectral-range. In one aspect of the invention, the GT cavities can synthesize any shape of combined dispersion compensation, including positive, negative, slope dispersion compensation. In another aspect of the invention, the GT cavities are tunable or dynamic to accommodate various types of dispersion compensation. Advantageously, the present invention provides an effective cost solution for a more precise dispersion compensation tuning.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 09/750,933, filed Dec. 29, 2000, entitled “Synthesis of Optical Dispersion Compensators and Methods Using A Series of GT Cavities,” which is a continuation of U.S. patent application Ser. No. 09/718,644, filed on Nov. 22, 2000, now U.S. Pat. No. 6,487,342, entitled “Method, System and Apparatus for Chromatic Dispersion Compensation Utilizing a Gires-Tournoise Interferometer,” all of which are assigned to the assignee of this application and incorporated herein by reference. 
    
    
     BACKGROUND INFORMATION 
     1. Field of the Invention 
     The present invention relates to the field of fiber optic networks, and more particularly to chromatic dispersion compensators in optical and photonic networks which carry wavelength division multiplexed signals. 
     2. Description of Related Art 
     Fiber optic communication systems are becoming increasingly popular for data transmission due to their high speed and high data capacity capabilities. Wavelength division multiplexing is used in such fiber optic communication systems to transfer a relatively large amount of data at a high speed. In wavelength division multiplexing, multiple information-carrying signals, each signal having light of a specific restricted wavelength range, may be transmitted along the same optical fiber. 
     Each individual information-carrying light is referred to as either “signal” or “channel”. The totality of multiple combined signals in a wavelength-division multiplexed optical fiber, optical line, or optical system, where each signal is of a different wavelength range, is referred to as a “composite optical signal”. 
     The term “wavelength”, denoted by the Greek letter λ (lambda) is used synonymously with the terms “signal” or “channel”. Although each information-carrying channel may include light of a certain range of physical wavelengths, for simplicity, a single channel is referred to as a single wavelength, λ, and a plurality of n such channels are referred to as “n wavelengths” denoted as λ 1 , λ 2 , . . . λ n . Used in this sense, the term “wavelength” may be understood to refer to “the channel nominally comprised of light of a range of physical wavelength centered at the particular wavelength λ”. 
     Chromatic dispersion is a common well-know problems in high-speed transmission of optical signals. Chromatic dispersion refers to the effect where the various physical wavelengths having an individual channel either travel through an optical fiber or component at different speeds—for instance, longer wavelengths travel faster than shorter wavelengths, or vice versa—or else travel different length paths through a component. This particular problem becomes more acute for data transmission speeds higher than 2.5 gigabytes per second. The resulting pulses of the signal will be stretched, will possibly overlap, and will cause increased difficulty for optical receivers to distinguish where one pulse begins and another ends. This effect seriously compromises the integrity of a signal. Therefore, for fiber optic communication system that provides a high transmission capacity, the system must be equipped to compensate for chromatic dispersion. 
     Conventional techniques in dealing with chromatic dispersion compensation have been proposed or implemented, such as spectral shaping, interferometers, negative dispersion fiber, and spectral inversion. The objective is to make longer wavelengths travel faster, or make shorter wavelengths travel slower, so that a composite optical signal arrives to a receiver location at the same time. It is also known that Gires-Tournois interferometers (GT cavity) can be used for dispersion compensation. However, a significant shortcoming in GT cavity is that the compensation bandwidth is too narrow for real applications. 
     Accordingly, there is a need to have a system and method for synthesis of dispersion compensation utilizing GT cavities, which synthesizes any desired compensation functions and provides dispersion compensators on demand. 
     SUMMARY OF THE INVENTION 
     The invention provides dispersion compensation systems and methods formed by cascading a series of GT cavities for compensating different chromatic dispersion. In one aspect of the invention, the GT cavities can synthesize any shape of combined dispersion compensation, including positive, negative, and slope dispersion compensation. In another aspect of the invention, the GT cavities are tunable or dynamic to accommodate various types of dispersion compensation. Advantageously, the present invention provides an effective cost solution for easy dispersion compensation tuning. 
     Other structures and methods are disclosed in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graphical diagram illustrating a dispersion compensator of a group delay function in a single GT cavity in accordance with the present invention. 
     FIG. 2 is a graphical diagram illustrating a group delay function of a synthesized constant dispersion compensator in accordance with the present invention. 
     FIG. 3 is a graphical diagram illustrating the dispersion function of the synthesized compensator as shown in FIG.  2 . 
     FIG. 4 is a general architectural diagram illustrating a first embodiment of a dispersion compensation synthesis using GT cavities in accordance—with the present invention. 
     FIG. 5 is an architectural diagram illustrating a tunable compensator formed by compensators and optical switches as shown in the first embodiment in accordance with the present invention. 
     FIG. 6 is an architectural diagram illustrating a second embodiment of a dispersion compensation synthesis using a series of GT cavities in accordance with the present invention. 
     FIG. 7 is a block diagram illustrating an cavity wavelength tuning by air pressure control in accordance with the present invention. 
     FIG. 8 is a block diagram illustrating a sealed cavity in accordance with the present invention. 
     FIG. 9 is a flow chart illustrating the process of performing a dispersion compensation synthesis using GT cavities in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 is a graphical diagram illustrating a dispersion compensator of a group delay function  10  in a single GT cavity. The group delay is represented as a function of wavelength GD(R, λ), where GD stands for group delay and R denotes as the reflectivity of the cavity front mirror. The higher the value of R, the higher the peak group delay and the narrower the group delay function. It is analogous to a Dirac function within a limited region. Any desired group delay function can be synthesized by a summation of a set of group delay functions with various R i  and wavelength shift, Δλ i , as shown in equation 1 below:                F        (   λ   )       =       ∑     i   =   1     N            GD        (     Ri   ,     Δ                 λ                 i       )       i               Eq   .                (   1   )                                  
     where N represents the number of cavities. 
     Alternatively, a group delay function can be characterized by equation 2 as shown below:                g        (   x   )       =     ∫     ∑         δ   i          (       x   i     -     x   ′       )              g   i          (     x   ′     )                 x   ′                     Eq   .                (   2   )                                  
     where the term δ(x-x′) represents the position function, and g(x′) represents the weighing function. The term δ(x i -x′) is adjusted by cavity thickness, and the term g i (x′) is adjusted by a reflectivity parameter, R. 
     FIG. 2 is a graphical diagram illustrating a group delay function  20  of a synthesized constant dispersion compensator. The group delay function is the summation of the group delay functions of 7 GT cavities with various reflectivity and resonant wavelengths. The combined group delay function has a constant negative slope, which gives a constant negative dispersion. FIG. 3 is the derivative of FIG. 2, that is the synthesized dispersion function. Because the spectrum of GT cavities is a periodical function of wavelength, each channel will have the same dispersion function as long as the free-spectral-range of each cavity matches the International Telecommunications Union (ITU) grid. Another example of synthesis of dispersion compensation is dispersion slope compensation. That is, the desired dispersion function is a linear function of channel central wavelength. This dispersion slope compensation function can be synthesized by means of controlling three parameters of the GT cavities, that is reflectivity, resonant wavelength, and free-spectral-range. 
     FIG. 4 is a general architectural diagram  40  illustrating a first embodiment of a dispersion compensation synthesis using GT cavities  41 ,  42 ,  43 , and  44 . Each of the GT cavities  41 ,  42 ,  43 , and  44  has a different granularity of dispersion compensation. By setting three parameters of each GT cavity, that is, reflectivity, resonant wavelength, and free-spectral-range, any desired chromatic dispersion compensation function can be synthesized. 
     FIG. 5 is an architectural diagram illustrating a tunable compensator  50  formed by compensators and optical switches as shown in the first embodiment. In principle, any fixed dispersion compensators can be synthesized by the description given with respect to FIG.  1 . In this implementation, a combination of fixed compensators and optical switches are selected to construct the tunable dispersion compensator  50 , which includes N fixed compensators  52 ,  54 ,  56 , and  58 , N−1 2×2 switches  53 ,  55 , and  57 , and two 1×2 switches  51  and  59 . The compensators compensates at different dispersion rate, with the compensator  52  set at 50 ps/nm, the compensator  54  set at 100 ps/nm, the compensator  56  set at 200 ps/nm, and the compensator  58  set at 400 ps/nm. Depending on the precision of dispersion compensation required, a compensator among the fixed compensators  52 ,  54 ,  56 , and  58  with smallest dispersion produces the resolution of the tunable compensator  50 . 
     FIG. 6 is an architectural diagram illustrating a second embodiment of a dispersion compensation synthesis using a series of GT cavities. The tunable dispersion compensator  50  can be coupled to a generic dispersion compensator  60  for precision tuning for dispersion compensation. The tunable dispersion compensator  50  operates according to the description provided above with respect to FIG. 5, such that an appropriate dispersion compensation rate is utilized in conjunction with a generic dispersion compensator  60 . 
     FIG. 7 is a block diagram illustrating a cavity  70  with wavelength tuning by air pressure control. By controlling the air pressure inside the cavity  70  through an air outlet  71 , the resonant wavelength of the cavity can be precisely set. This can be done by putting the cavity  70  into a hermetic sealed cell  72 , whose air pressure can be precisely controlled. The cell has an optical window  73  so that the cavity  70  can be accessed optically from outside the hermetically sealed cell  72 . In one embodiment, one atmosphere pressure change gives about 0.45 nm wavelength tuning at wavelength 1550 nm. 
     FIG. 8 is a block diagram illustrating a sealed cavity  80  with wavelength tuning of the sealed cavity  80  by laser-evaporating materials inside the cavity  80 . To control the air pressure, some material is put inside the cavity before the cavity is sealed. By shining a laser beam on the material through an cavity window  81 , the material is evaporated and changes the air pressure inside the cavity  80 . The sealed cavity  80  is preferably completely or substantially sealed. 
     FIG. 9 is a flow chart illustrating the process  90  for performing a dispersion compensation synthesis using GT cavities. At the initial stage, the process  90  determines  91  whether to compensate the dispersion of an optical signal at a first rate. The switch  51  is turned ON  92  if the process  90  decides to activate the first GT cavity  41  for dispersion compensation. Otherwise, the switch  51  is turned OFF  93 . Continuing to the next stage with a finer precision of dispersion tuning, the process  90  determines  94  whether to compensation the dispersion of the optical signal at a second rate. The switch  53  is turned ON  95  if the process  90  decides to activate the second GT cavity  42  for dispersion compensation. Otherwise, the switch  53  is turned OFF  96 . Additional stages of GT cavities can be constructed to achieve greater precision of dispersion compensation in an optical signal. 
     The above embodiments are only illustrative of the principles of this invention and are not intended to limit the invention to the particular embodiments described. For example, although FIG. 5 shows dispersion compensation at 50 ps/nm, 100 ps/nm, 200 ps/nm, and 400 ps/nm, one of ordinary skill in the art should recognize that different increments of dispersion compensation can be selected without departing from the spirits in the present invention. Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments can be practiced without departing from the scope of the invention as set forth in the appended claims.