Abstract:
A tunable optical dispersion compensator (TODC) having a silica arrayed-waveguide grating (AWG) directly coupled at its input to a Mach-Zehnder interferometer device and at its output to a polymer thermo-optic lens.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is related to U.S. patent application Ser. No. 11/308,045 filed on Mar. 3, 2006 and U.S. patent application Ser. No. 11/164,644 filed Nov. 30, 2005. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates generally to the field optical communications and in particular to a tunable optical dispersion compensator. 
       BACKGROUND OF THE INVENTION 
       [0003]    In long-distance transmission of optical signals, the accumulation of chromatic dispersion in optical fibers presents serious problems. These problems intensify with an increase in bit rate and the distance traveled by the optical signals. Efforts to date that compensate for dispersion have primarily involved the use of dispersion compensating optical fibers (DCF). 
         [0004]    Dispersion compensating efforts that employ DCF—while well-proven—are not particularly amenable to integration in existing network elements. This is due—in part—because DCF is employed as a large spool of fiber which occupies significant space in a network office and is not adjustable. In addition, service providers that utilize DCF in their networks must accurately characterize their fiber, deploy more expensive optical amplifiers and accept additional latency added to links employing the DCF [˜20% additional latency for a fully compensated standard-single-mode fiber (SSMF) link]. Finally, DCF cannot satisfy all of the dispersion compensation requirements of many 40-Gb/s links, consequently a tunable optical dispersion compensator (TODC) having a small tuning range is often required in addition to the DCF. 
         [0005]    A TODC employing an arrayed waveguide grating (AWG) and thermo-optic lens was described in U.S. Pat. No. 7,006,730 directed to a “Multichannel Integrated Tunable Thermo-Optic Lens and Dispersion Compensator the entire contents of which are hereby incorporated by reference. The TODC described therein appeared to be an attractive alternative/supplement to DCF. 
       SUMMARY OF THE INVENTION 
       [0006]    I have developed according to the present invention a tunable optical dispersion compensator (TODC) including a Mach-Zehnder interferometer structure coupled to selected inputs of a silica arrayed-waveguide grating (AWG) apparatus, the output of which is coupled to an adjustable lens. Advantageously, the Mach-Zehnder interferometer structure broadens the transmissivity passband of the TODC at high dispersion settings in a low-loss manner. 
         [0007]    According to an aspect of the invention, the Mach-Zehnder interferometer structure has two optical paths of different length namely L 1  and L 1 +ΔL 1  which is coupled to central input waveguides of an AWG apparatus having a number of unequal length waveguides that differ in length by an integer multiple of ΔL 1 . 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0008]    A more complete understanding of the present invention may be realized by reference to the accompanying drawings in which: 
           [0009]      FIG. 1  is a schematic of an arrayed waveguide grating coupled to a Mach-Zehnder interferometer structure according to the present invention; 
           [0010]      FIG. 2  is a schematic of an arrayed waveguide grating coupled to a Mach-Zehnder interferometer integrated on a single optical chip according to the present invention; 
           [0011]      FIG. 3  is a schematic of an integrated TODC according to the present invention; and 
           [0012]      FIG. 4  is a series of schematics showing various coupling configurations of the Mach-Zehnder interferometer structure according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    The following merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. 
         [0014]    Furthermore, all examples and conditional language recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. 
         [0015]    Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure. 
         [0016]    Thus, for example, it will be appreciated by those skilled in the art that the diagrams herein represent conceptual views of illustrative structures embodying the principles of the invention. 
         [0017]    With initial reference to  FIG. 1 , those skilled in the art will quickly recognize the well-known frequency routing device  110  which may operate as a multiplexer and demultiplexer of optical frequencies. Such frequency routing devices, are well known in the art and are described in more detail in U.S. Pat. No. 5,002,350 which issued to Dragone on Jan. 30, 1996, the entire contents of which are hereby incorporated by reference. As further known by those skilled in the art, such devices are often referred to as arrayed waveguide gratings (AWGs). 
         [0018]    Star coupler(s)  101 ,  102  each include a plurality of input ports connected to a plurality of output ports via a free space region. The plurality (N) of output ports (for coupler  101 ) are connected to N waveguides  110 [ 1 ] . . .  110 [ n ], which provide a pre-determined amount of path length difference to a corresponding plurality (N) of input ports of star coupler  102 . Preferably, these devices are formed from waveguides and integrated onto an optical “chip” (not specifically shown) and each of the couplers may include one or more input and/or output waveguides e.g.,  106 . 
         [0019]    With these preliminary structures described, we may now describe more particularly an apparatus according to the present invention. With continued reference to  FIG. 1 , there is shown a Mach-Zehnder interferometer device  103  having two arms  104 ,  105  which are optically communicating with and providing input to the first star coupler  101  at a location of the star coupler normally associated with the central two input ports (not specifically shown). Accordingly, an input optical signal entering the Mach-Zehnder interferometer device  103  is split such that portions of the split signal traverse the two arms  104 ,  105  and are subsequently introduced into the first star coupler  101 . As shown in this  FIG. 1 , one of the arms  104  of the Mach-Zehnder interferometer device  103  exhibits a path length L 1  while the other (longer) arm  105  exhibits a path length of L 1 +ΔL 1  Note that  103  is a special Mach-Zehnder interferometer in that the right-hand side coupler is the star coupler  101  itself. There is no 2×1 or 2×2 50/50 coupler in  103  in this design. 
         [0020]    The first star coupler  101  is optically connected to the second star coupler  102  via an array of waveguides  110 [ 1 ] . . .  110 [ n ] having a predetermined length. According to an aspect of the present invention, the lengths of the waveguides in the array increase by an amount substantially equal to ΔL 2 , where ΔL 2  is the length difference between the two paths ( 104 ,  105 ) of the Mach-Zehnder interferometer structure  103 . More particularly, the first waveguide  110 [ 1 ] exhibits a path length of L 2  and each successive waveguide in the array is increased in length by an amount substantially equal to ΔL 2 . There may be a small deviation in ΔL 2 , i.e., a chirp, without departing from the spirit of the invention. Accordingly, the second waveguide in the array  110 [ 2 ] will exhibit a length of L 2 +ΔL 2 ; the third waveguide  110 [ 3 ] will exhibit a length of L 2 +2ΔL 2 . Accordingly, the last waveguide  110 [ n ] will exhibit a length of L 2 +(n−1)ΔL 2  where n is the number of the waveguide in the array. The combined structure  101 ,  110 , and  102  is known as an AWG. 
         [0021]    Turning now to  FIG. 2 , there is shown an integrated device constructed according to the teachings of the present invention which may advantageously serve as a foundation for a TODC. With particular reference to  FIG. 2 , there is shown an integrated optical chip  100  including an AWG having a pair of star couplers  101 ,  102  optically interconnected by a plurality of waveguides  110  comprising a number of unequal length individual waveguides  110 [ 1 ],  110 [ 2 ] . . .  110 [ n ]. Coupled to a central pair of input waveguides of the first star coupler  101  is a Mach-Zehnder interferometer device comprising a pair of optical waveguides  104 ,  105  which are optically coupled by optical couplers  120  and  130 . Optical chip  100  is preferably a planar lightwave circuit (PLC). 
         [0022]    As shown in  FIG. 2  and discussed with respect to  FIG. 1 , one of the two waveguides comprising the Mach-Zehnder interferometer device exhibits a length of L 1 , while the other waveguide  105  exhibits a length that is substantially L+ΔL in length. As shown further in  FIG. 2 , the lengths of the waveguides comprising the AWG are related to ΔL 2 , wherein each of the individual waveguides  110 [ 1 ],  110 [ 2 ] . . .  110 [ n ] comprising the AWG  110  exhibit a path length of substantially L 2 , L 2 +ΔL 2 , . . . L 2 +(n−1)ΔL 2 , respectively. 
         [0023]    Those skilled in the art will now observe that the second star coupler  102  is positioned at the edge of optical chip  100 . More particularly, it is positioned such that an edge of the chip is located where output waveguides (not specifically shown) would normally be found in a pure frequency routing device. As will be shown, this structure permits the advantageous construction of a TODC. 
         [0024]    Turning now to  FIG. 3 , a representative layout of an integrated TODC is shown. More particularly, optical signals are input to the optical chip  300  through input optical waveguide  310  which is coupled to Mach-Zehnder interferometer structure  320 . The Mach-Zehnder interferometer structure is coupled to inputs of a first star coupler  333  of a frequency routing device comprising the first star coupler  333 , and a second star coupler  335 , which are optically interconnected by a number of unequal length waveguides  337 [ 1 ] . . .  337 [ n ] which comprise an AWG  330 . 
         [0025]    Shown in  FIG. 3 , the second star coupler  335  is positioned at an edge of the optical chip  300  at a point where output waveguides would normally be positioned. Affixed to that edge of the optical chip, is preferably is a small planar lightwave circuit PLC  340  which includes a lens element  342  and a mirror  345 . The lens element  342  is an element that provides a parabolic refractive index profile for adjusting the amount of dispersion provided by the TODC. As may be appreciated, PLC  340  may be either monolithically integrated with the main PLC  100  or may be a separate chip. Possible implementations of the lens element  342  include a thermooptic lens or electrooptic lens. In the case where it is a thermooptic lens, the lens element  342  is preferably constructed from a material that exhibits a suitable refractive index change upon heating while sufficiently dissipating the heat. 
         [0026]    When configured in this manner, portions of light input to input/output waveguide  310  traverses the first slab waveguide star coupler  333 , the grating  330 , the second slab waveguide star coupler  335 , traverses the lens PLC  340 , is reflected by the mirror  345 , and subsequently output via input/output waveguide  310  having an amount of its accumulated dispersion compensated. In a preferred embodiment, the mirror  345  length along slab  335  will only be equal to or less than the width of the Brillouin zone of grating  330 . This ensures that high diffraction orders from the grating are not reflected back into the grating. In addition, the mirror  345  is preferably flat, as it is easiest to cut and/or polish a flat surface, both for the PLC  340  and for the mirror  345 . As can be appreciated, when the mirror  345  is flat, the device provides negative dispersion when no lens element  342  is not activated which compensates the dispersion of most single-mode optical fibers. The mirror  345  may also be curved, which will adjust the non-activated-lens dispersion setting. 
         [0027]    It is explained in U.S. Pat. No. 7,006,730 (which is hereby incorporated by reference) how the TODC operates when structure  320  is replaced by a single waveguide. A fundamental issue with that TODC design is that the transmissivity passband narrows as the dispersion magnitude is increased. This narrowing is due to the fact that at the wavelengths at the edges of the passband, the lens element  342  causes the light distribution to be off-center in the waveguide array  330 . This in turn causes the light to be focused at a tilted angle into the output waveguide  310  when  320  is not present, causing high loss at the passband edges, and thus narrowing the passband. The present invention is the addition of element  320 . Element  320  accepts this tilted beam with significantly higher efficiency than a single waveguide, thus improving the loss at the passband edges. The net result of adding element  320  is a significantly wider transmissivity passband at high dispersion settings. We must change the effective orientation of element  320  when the sign of dispersion changes (i.e., the longer arm must trade places with the shorter arm), and at zero dispersion we do not want  320  at all. These adjustments are addressed by  FIG. 4 . 
         [0028]    Turning now to that  FIG. 4 , there is shown a series of schematic diagrams (a), (b) and (c) showing representative input configurations according to the present invention for − dispersion, + dispersion and 0 dispersion respectively. Note that in these diagrams, only the Mach-Zehnder interferometer structure(s) and first star coupler are shown from the configuration of  FIG. 2 . Also, it is assumed that the Mach-Zehnder interferometer structure is coupled to the two center input waveguides of that first star coupler and the optional dummy waveguides are not shown for clarity. 
         [0029]    More particularly,  FIG. 4(   a ) shows a − dispersion configuration wherein a first coupler of the Mach-Zehnder interferometer structure includes a 1×2 (50/50) coupler and a second coupler of that Mach-Zehnder interferometer structure is a 100/0 coupler.  FIG. 4(   b ) is a + dispersion configuration and the first coupler of the Mach-Zehnder interferometer structure includes a 1×2 coupler (50/50) while the second coupler is a 0/100 coupler. Finally,  FIG. 4(   c ) shows a 0 dispersion configuration where the first coupler is a 0/100 configuration while the second is a 50/50. 
         [0030]    Although  FIG. 3  shows a reflective structure for the TODC, one could create a transmissive structure by duplicating the structure and making it symmetric about the mirror  345  and removing mirror  345 . 
         [0031]    Although the previous discussion has focused on a tunable optical dispersion compensator, one may also use this invention to construct a fixed optical dispersion compensator. In such a case, lens element  342  is either non-adjustable or non-existent. 
         [0032]    At this point, while we have discussed and described our invention using some specific examples, those skilled in the art will recognize that our teachings are not so limited. Accordingly, our invention should be only limited by the scope of the claims attached hereto.