Abstract:
In accordance with the present invention, a waveguide grating comprises a core and a first cladding material adjacent the core. The first cladding is configured, as by etching, to provide a periodic grating, and a second cladding material having a controllable refractive index overlies the first cladding material. If the index of the second cladding is different from that of the first cladding, the configuration of the first cladding provides an optical grating. If, however, the controllable index of the second cladding is adjusted to equal that of the first cladding, the grating becomes essentially transparent. This grating is particularly useful as a reconfigurable add/drop filter in a WDM optical communication system. It is also useful in grating-assisted couplers and variable optical delay lines.

Description:
FIELD OF THE INVENTION 
     This invention relates to optical waveguide gratings and, in particular, to etched waveguide gratings particularly useful in add/drop filters, grating-assisted couplers and variable delay lines for optical communication systems. 
     BACKGROUND OF THE INVENTION 
     Optical gratings are important elements for selectively controlling specific wavelengths of light within optical communication systems. Such gratings include Bragg gratings and long period gratings. A grating typically comprises a body of material and a plurality of substantially equally spaced optical grating elements such as index perturbations, slits or grooves. 
     A typical Bragg grating comprises a length of optical waveguide, including a plurality of perturbations in the index of refraction substantially equally spaced along the waveguide length. These perturbations selectively reflect light of wavelength λ equal to twice the spacing Λ between successive perturbations times the effective refractive index, i.e. λ=2n eff Λ, where λ is the vacuum wavelength and n eff  is the effective refractive index of the fundamental mode. The remaining wavelengths pass essentially unimpeded. Such Bragg gratings have found use in a variety of applications including filtering, adding and dropping optical signal channels, stabilization of semiconductor lasers, reflection of fiber amplifier pump energy, and dispersion compensation. 
     A difficulty with conventional Bragg gratings is that they filter only a fixed wavelength. Each grating selectively reflects only light in a narrow bandwidth centered around λ=2n eff Λ. However in many applications, such as wavelength division multiplexing (WDM), it is desirable to have a reconfigurable grating whose wavelength response can be controllably altered. 
     Long-period grating devices provide wavelength dependent loss and may be used for spectral shaping. A long-period grating couples optical power between two copropagating modes with very low back reflections. A long-period grating typically comprises a length of optical waveguide wherein a plurality of refractive index perturbations are spaced along the waveguide by a periodic distance Λ′ which is large compared to the wavelength λ of the transmitted light. In contrast with conventional Bragg gratings, long-period use a periodic spacing Λ′ which is typically at least 10 times larger than the transmitted wavelength, i.e. Λ′≧10λ. Typically Λ′ is in the range 15-1500 micrometers, and the width of a perturbation is in the range ⅕Λ′ to ⅘Λ′. In some applications, such as chirped gratings, the spacing A′ can vary along the length of the grating. 
     Long-period grating devices selectively remove light at specific wavelengths by mode conversion. In contrast with conventional Bragg gratings in which light is reflected and stays in the waveguide core, long-period gratings remove light without reflection, as by converting it from a guided mode to a non-guided mode. (A non-guided mode is a mode which is not confined to the core, but rather, is defined by the entire waveguide structure. Often, the non-guided is a cladding mode). The spacing Λ′ of the perturbations is chosen to shift transmitted light in the region of a selected peak wavelength λ p  into a non-guided mode, thereby reducing in intensity a band of light centered about λ p . Alternatively, the spacing Λ′ can be chosen to shift light from one guided mode to a second guided mode (typically a higher order mode), which is stripped off. 
     A shortcoming of conventional long-period gratings, however, is their limited ability to dynamically equalize gain. They filter only a fixed wavelength. Each long-period grating with a given periodicity (Λ′) selectively filters light in a narrow bandwidth centered around. λ p =(n g −n ng ).Λ′, where n g  and n ng  are the effective indices of the core and the cladding modes, respectively. The value of n g  is dependent on the core and cladding refractive index while n ng  is dependent on core, cladding and air indices. 
     Techniques have been devised for tuning gratings, and an important application of tunable gratings is in the fabrication of tunable add/drop filters in optical communication systems. Tunable filters are widely used in WDM systems to add or drop a channel at the terminals or at an intermediate point in the system. Such filters must have flat passbands and good stopband rejection. UV-photoinduced Bragg gratings written in optical fibers or planar waveguides are typically employed because of their excellent spectral characteristics. Many techniques for fabricating tunable Bragg gratings in fibers have been introduced such as temperature and stretching. When tunable filters are reconfigured in a system, the operation should be transparent to the other channels on the system, i.e. hitless reconfiguration is desirable. This is a limitation for current tunable gratings unless a switch is used to bypass the grating during the reconfiguration period. Alternatively, the grating must have a sufficiently narrow bandwidth to fit in between channels. This requires gratings with extremely good characteristics so that the grating is transparent to adjacent channels. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a waveguide grating comprises a core and a first cladding material adjacent the core. The first cladding is configured, as by etching, to provide a periodic grating, and a second cladding material having a controllable refractive index overlies the first cladding material. If the index of the second cladding is different from that of the first cladding, the configuration of the first cladding provides an optical grating. If, however, the controllable index of the second cladding is adjusted to equal that of the first cladding, the grating becomes essentially transparent. This grating is particularly useful as a reconfigurable add/drop filter in a WDM optical communication system. It is also useful in grating-assisted couplers and variable optical delay lines. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The advantages, nature and various additional features of the invention will appear more fully upon consideration of the illustrative embodiment now to be described in detail. In the drawings: 
     FIG. 1 is a schematic cross section of a tunable waveguide grating; 
     FIG. 2 is a graphical illustration of the refractive indices of components of the FIG. 1 device useful in understanding the operation of the device; 
     FIGS. 3A and 3B illustrate grating-assisted couplers using the tunable grating of FIG. 1; 
     FIG. 4 schematically illustrates a tunable all-pass filter using the grating of FIG. 1; 
     FIG. 5 schematically illustrates a tunable drop filter using the gratings of FIG. 1; 
     FIG. 6 illustrates an alternative drop filter using FIG. 1 gratings; and 
     FIG. 7 illustrates a variable optical delay line using the gratings of FIG.  1 . 
    
    
     It is to be understood that these drawings are for purposes of illustrating the concepts of the invention and, except for the graphs, are not to scale. 
     DETAILED DESCRIPTION 
     Referring to the drawings, FIG. 1 is a schematic cross section of a tunable waveguide grating  10  comprising a core  11  having an index of refraction n 1  peripherally surrounded by cladding layers  12 A,  12 B having indices of refraction e.g. n 2 , n 3  less than n 1 . A portion of one of the cladding layers  12 B is configured, as by etching, to provide spatial variations forming a periodic grating  13 . A second cladding  14  having an index of refraction n 3 &lt;n 1  overlies the first cladding  12 B and fills the grating  13 . While the device can be either a planar waveguide grating or a fiber grating, in the planar waveguide form shown in FIG. 1, it is conveniently fabricated on a supporting substrate  15 . The grating  13  can be a Bragg grating or a long period grating depending on the period spacing in relation to the wavelength of transmitted light. 
     In accordance with the invention, the claddings  12 A,  12 B and  14  are made of materials such that n 2  or n 3  can be independently controlled. Specifically, the device can be tuned between at least two states: 1) n 2 ≠n 3  and 2) n 2 =n 3 . In the first state, n 2 ≠n 3 , the periodic structure acts as an optical grating. In the second state, n 2 =n 3 , the grating becomes optically transparent and has negligible effect on transmitted light. 
     The preferred mechanism for tuning the grating between the two states is to make one of the claddings of a material whose refractive index varies more with temperature than the other cladding. For example, the cladding  12 B can be silica and the second cladding  14  can be a polymer with a more temperature sensitive refractive index. A resistance heating element  16  thermally coupled to the second cladding  14  permits the desired tuning. 
     Alternatively, the grating could be tuned between the two states by making one of the claddings of electrooptic or liquid crystal material and applying an electric field for control. One of the claddings can be made of semiconductor material and the injection or depletion of charges can tune the index. One of the claddings can be a magnetooptic material tuned by a magnetic field; or it can be a glass with an index tuned by strain. 
     The invention can now be better understood by consideration of the following specific example. 
     EXAMPLE 
     The substrate  16  can be silicon. The claddings  12 A,  12 B can be Ge-doped silica, and the core  11  can be formed of a higher index doped silica. The grating  13  can be defined as an etched, crenelated region within cladding  12 B. The second cladding  14  can be a polymer such as a halogenated acrylate. 
     The refractive indices for the silica-based layers at a wavelength of 1.55 micrometers are: cladding  12 A, 1.445; core  11 , 1.46234, cladding  12 B, 1.44512. For a Bragg grating, the grating layer is 1.1 micrometers thick with a 1.0 micrometer etch depth and a period of Λ=λ/2n e =530 nanometers where n e =1.4511 is the effective index. The polymer upper cladding has a refractive index temperature dependence of −4×10 −4 /°C. 
     FIG. 2, which is useful in understanding the operation of the exemplary device, plots the refractive indices of the claddings as a function of temperature. When the temperature is set so that the refractive index of the polymer  14  matches that of the grating layer cladding  12 B (as indicated by the circle in FIG.  2 ), the grating is optically transparent. As the temperature increases, the index of the polymer decreases, and the grating strength increases. For a 73° C. change in temperature, the refractive index has changed so that the grating strength is proportional to δn e /n e =1.×10 −3 . The reflection bandwidth is related to the grating strength by Δλ≈λδn e /n e , so bandwidths up to ˜1 nm can be achieved with these parameters. The grating strength is therefore sufficient for filters in WDM communication systems. 
     While the tunable grating of FIG. 1 is used in connection with but a single core-defined waveguide, it is also possible to use the FIG. 1 gratings with more complex waveguide structures. For example, FIGS. 3A and 3B are schematic top views showing grating assisted couplers using tunable gratings  10 A,  10 B. FIG. 3A shows a Bragg grating assisted coupler  30  comprising a pair of optically. coupled waveguide cores  31 A and  31 B having a tunable Bragg grating  10 A overlying the coupling region. Output of wavelength λ c  corresponding to the grating resonance is reflected as shown. The coupling strength can be varied by tuning the grating refractive index contrast. 
     FIG. 3B shows a long-period grating assisted coupler  33  comprising a pair of optically coupled waveguide cores  31 A and  31 B having a tunable long-period grating  10 B overlying the coupling region. Output of a wavelength λ c  corresponding to the forward-shifted mode is transmitted as shown. Again the coupling strength can be varied by tuning the cladding index contrast. 
     FIG. 4 schematically illustrates a tunable all-pass filter  40  using a tunable waveguide grating  10  as described in connection with FIG.  1 . The filter  40  comprises a length of waveguide  41  passing adjacent a waveguiding ring resonator  42 . The waveguide  41  and ring resonator  42  are sufficiently close together in a coupling region  43  that there is overlap of the exponential tails of the light they carry. The tunable waveguide grating  10  overlies the coupling region  43 . Grating  10  can be either a Bragg grating or a long-period grating. Advantageously, a phase shifter  44 , such as a resistive heater, is coupled to the ring resonator  42 . 
     In operation, the device acts as a tunable all-pass filter. If grating  10  is a Bragg grating, light from the input couples by reflection from the grating  10  clockwise around the ring  42  back to the grating  10  where it is reflected toward the output. If grating  10  is a long-period grating, the input couples forward into the ring  42 . It travels counterclockwise around the ring and forward couples into the waveguide  41  toward the output. 
     Control of the grating  10  via its associated heater  16  permit the strength of the coupling between the waveguide and the ring. Control of the phase shifter  44  permits tuning the resonant wavelength of ring  42 . 
     FIG. 5 schematically illustrates a tunable drop filter  50  using a pair of tunable waveguide gratings  10 A and  10 B as described in FIG.  1 . The drop filter  50  comprises a pair of waveguides  41 A and  41 B passing adjacent a waveguiding ring resonator  42  at respective coupling regions  43 A and  43 B. Tunable waveguide gratings  10 A and  10 B are disposed overlying the respective coupling regions  43 A,  43 B. The gratings  10 A and  10 B can both be Bragg gratings or both be long-period gratings. Advantageously, a phase shifter  44 , such as resistive heater, is coupled to the ring resonator  42 . 
     In operation, the device acts as a tunable drop filter. If gratings  10 A and  10 B are both Bragg gratings tuned to a wavelength λ to be dropped, then λ from the input waveguide  41 A reaching grating  10 A is coupled by reflection into ring  42  in the clockwise direction. Upon reaching grating  10 B, it is coupled by reflection into output waveguide  41 B. If the gratings are both long-period gratings a similar result is achieved by forward coupling. 
     FIG. 6 schematically illustrates an. alternative drop filter  60  using a plurality of tunable waveguide gratings  10 A,  10 B,  10 C as described in FIG.  1 . The drop filter  60  comprises a pair of generally parallel waveguides  61 ,  62  having a plurality of coupling regions  43 A,  43 B,  43 C. With a respective plurality of overlying gratings  10 A,  10 B,  10 C. Advantageously, phase shifters  44 A,  44 B are coupled to waveguide  62  between each pair of gratings. The gratings  10 A,  10 B and  10 C are Bragg gratings, and the waveguides  61 ,  62  advantageously have significantly different-effective indices of refraction (e.g. 2.0 and 1.4). 
     In operation, a broadband signal can enter input waveguide  61 . If gratings  10 A,  10 B,  10 C are tuned to the drop wavelength λ, then when the signal reaches a grating, the drop wavelength will be reflectively coupled into drop waveguide  62 . Phase shifters  44 A and  44 B can tune the phase relation between the successive reflections, and the different effective indices of the waveguides minimizes back reflection. The non-dropped components on waveguide  61  continue to its output. 
     FIG. 7 is a schematic cross section of a variable optical delay line using a variation of the FIG. 1 grating. The device of FIG. 7 is similar to the device of FIG. 1 except that instead of a single index control element overlying the grating  13 , a succession of control elements  16 A- 16 P are disposed overlying the length of the grating  13 . 
     At the nominal temperature where the grating is transparent, the FIG. 7 device acts as a waveguide. By locally heating a section of the cladding  14 , the grating appears and an incoming signal at the grating resonance is reflected. The grating can be effectively moved down the waveguide by turning off upstream heaters and turning on downstream heaters. This increases the delay experienced by the signal before reflection. 
     It is to be understood that the above-described embodiments are illustrative of only a few of the many possible specific embodiments which can represent applications of the principles of the invention. Numerous and varied other arrangements can be readily devised by those skilled in the art without departing from the spirit and scope of the invention.