Abstract:
The invention relates to a tunable optical add/drop module (TOADM) monolithically integrated on a single planar lightwave circuit (PLC). The present invention overcomes the shortcomings of the prior art by providing virtual pupils at the interface between the channel waveguides and the slab waveguide on the PLC for focusing each wavelength channel, and additional on-chip lenses on the PLC for transforming the focal plane of the spatially dispersive demultiplexer into a substantially flat plane at the edge of the PLC. On-chip lenses are realized as reflective surfaces within slab waveguiding regions having a surface curvature to provide optical power.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of U.S. Provisional Patent Application No. 60/889,439 filed Feb. 12, 2007, entitled “PLC-BASED TUNABLE 3-PORT FILTER” which is incorporated herein by reference for all purposes. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a wavelength selective device based on planar lightwave circuit (PLC) technology. In particular, the present invention relates to a tunable optical add/drop module (TOADM) monolithically integrated on a single PLC. 
       BACKGROUND OF THE INVENTION 
       [0003]    Prior art wavelength selective devices are constructed with discrete structures coupled through free space. This type of assembly suffers a number of limitations. Optical alignment of the elements is highly sensitive and costly to produce. In the interests of reliability and robustness to environmental factors, it is desirable to perform as many of the required functions as possible on a monolithically integrated planar lightwave circuit. (PLC). 
         [0004]    Planar lightwave circuits (PLC) are constructed as lithographically formed waveguides on a planar substrate. The waveguides are surrounded by a lower index cladding material for confining optical signal within the waveguide circuit. PLCs are frequently constructed as silica on silicon assemblies in which a silicon substrate is deposited with a silica lower cladding, waveguide cores of germanium doped silica are formed and an upper cladding of borophosphosilicate glass is deposited over the waveguide cores. Alternatively, PLCs are formed in InGaAsP or optically transmissive polymer or glass. Structures formed in the PCL include channel waveguides which confine the signal in two dimensions orthogonal to the direction of light propagation, and slab regions which confine the optical signal in one dimension and allow a wavefront to spread over a defined region in the orthogonal dimension. In optical communications, the dimensions are typically confined to single mode transmission. 
         [0005]    A typical demultiplexer for separating the multiplexed optical beam is an arrayed waveguide diffraction grating (AWG) constructed as a PLC. The AWG was invented by Dragone by combining a dispersive array of waveguides with input and output “star couplers” on a planar lightwave circuit chip. The AWG can work both as a DWDM demultiplexer and as a DWDM multiplexer, as taught by Dragone in U.S. Pat. No. 5,002,350 (March 1991). Other dispersion devices such as echelle gratings can also be realized in PLC for the multiplexing/ demultiplexing functions. 
         [0006]    U.S. Pat. No. 7,027,684 issued Apr. 11, 2006 to Ducellier et al, and United States Patent Publication No. 2004/0252938 published Dec. 16, 2004 to Ducellier et al relate to single and multi-layer planar lightwave circuit (PLC) wavelength selective switches (WSS), respectively, which are illustrated in  FIGS. 1 and 2 . A single level device  1 , illustrated in  FIG. 1 , includes a PLC  2  with an input AWG in the middle, and a plurality of output AWG&#39;s on either side of the input AWG. An input optical signal launched into the input AWG is dispersed into constituent wavelengths, which are directed at different angles through lensing  3  to an array of tiltable mirrors  4 . The light is collimated in one direction, e.g. vertically, by a first cylindrical lens  5  adjacent to the PLC  2 , while a cylindrical switching lens  6  focuses the output light in the horizontal direction onto the tiltable mirrors  4 . Each wavelength channels falls onto a different one of the tiltable mirrors  4 , which redirect the individual wavelength channels back through the lensing  3  to whichever output AWG is desired for recombination, and output an output port. For the single level device the tiltable mirrors  4  rotate about a single axis to redirect the wavelength channels within the dispersion plane, i.e. the plane of the PLC  2 . 
         [0007]    A two level device  11 , illustrated in  FIG. 2 , includes a second PLC  12 , similar to the PLC  2 , superposed above the PLC  2  with a plurality of input or output AWG&#39;s and ports. A second cylindrical lens  15  is superposed above the first cylindrical lens  5  for focusing the beams of light onto the output AWG&#39;s provided on the second PLC  12 . For the two-level device, tiltable mirrors  14  rotate about two perpendicular axes to redirect the wavelength channels within the dispersion plane (as above) and at an acute angle to the dispersion plane into a plane parallel to the dispersion plane, i.e. the plane of the PLC  12 . 
         [0008]    In the aforementioned Ducellier devices, the AWG&#39;s terminate in straight linear arrays at the edge of the chip, whereby without the curvature at the AWG outputs, the “foci” occur at infinity. Accordingly, an external, bulk-optic lens is required to function as more than simply a field lens, but as a full (spatially) Fourier transforming lens. Consequently, not only is the external lens required to be extremely well aligned, i.e. relatively expensive and extremely sensitive to misalignments, but the optical path is necessarily mostly in air. 
         [0009]    An object of the present invention is to provide a tunable add/drop filter in a monolithic PLC. 
         [0010]    A further object of the present invention is to overcome the shortcomings of the prior art by providing virtual pupils at the interface between the channel waveguides and the slab waveguide on the PLC for focusing each wavelength channel. A further lens system comprises a plurality of on-chip lenses on the PLC for transforming the focal plane of the spatially dispersive demultiplexer into a substantially flat plane at the edge of the PLC. On-chip lenses are realized as reflective surfaces within slab waveguiding regions having a surface curvature to provide optical power. 
       SUMMARY OF THE INVENTION 
       [0011]    Accordingly, the present invention relates to a tunable add/drop filter in a planar lightwave circuit (PLC) comprising:
   an input port for launching an input optical signal comprising a plurality of wavelength channels into the PLC;   a first demultiplexer with an input pupil and an output pupil for spatially separating the input optical signal into the wavelength channels;   a slab waveguide region comprising a high-index core layer, surrounded above and below by lower-index cladding layers for transmitting the separated wavelength channels from the first demultiplexer to an add/drop port;   a lens system in the slab waveguide region for focusing the plurality of separated wavelength channels from the output pupil of the first demultiplexer onto straight line at an edge of the PLC,   a separate reflector adapted for transverse movement along the edge of the PLC for reflecting the separated wavelength channels;   an add/drop port within a transmissive window within the reflector for receiving one or more of the separated wavelength channels as a drop port, or for inputting a new wavelength channel as an add port, the add/drop port being tunable as a function of transverse motion of the transmissive window along the separated wavelength channels focused at the edge of the PLC;   a second demultiplexer with an input pupil optically coupled with the reflector and the add/drop port for receiving and multiplexing the one or more wavelength channels reflected by the reflector and any new wavelength channel input at the add port from the lens system in the slab waveguide region, and an output pupil; and   an output port optically coupled to the output pupil of the second demultiplexer for outputting the multiplexed wavelength channels;   wherein the output pupil of the first demultiplexer is positioned with respect to the input pupil of the second demultiplexer, such that at least one wavelength channel focused on the reflector is optically coupled into the input pupil of the second demultiplexer.   
 
         [0021]    Another feature of the present invention provides that the first demultiplexer and the second demultiplexer comprise arrayed waveguide gratings.
   A further feature of the present invention provides that the spatially separated wavelength channels of the first demultiplexer and the second demultiplexer are in reflective communication along a common circle.   A further feature of the present invention defines the lens system in the slab waveguide region as a telecentric lens system.   A further feature of the present invention provides that chief rays of each reflected wavelength channel are incident on the reflector at equal angles.   
 
         [0025]    Another aspect of the present invention relates to a tunable add/drop filter in a planar lightwave circuit (PLC), wherein the lens system comprises at least one reflective surface within a slab waveguiding region, the reflective surface having a curvature to impart optical power.
   A further aspect of the present invention includes the reflector comprising a separate PLC including an add/drop port waveguide disposed in the transmissive window.   In a further embodiment of the present invention, the reflector PLC includes the add/drop port waveguide comprising a Mach-Zehnder interferometer.   
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0028]    The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein: 
           [0029]      FIG. 1  is a plan view of a prior art PLC based wavelength selective switch; 
           [0030]      FIG. 2  is a side view of a prior art multi-layer PLC based wavelength selective switch; 
           [0031]      FIG. 3  is a plan view of a PLC based tunable 3 port add/drop filter in accordance with the present invention; 
           [0032]      FIG. 4  is a detailed view of a reflector at the chip edge of the device of  FIG. 3  including the add/drop port; 
           [0033]      FIG. 5  is a detailed view of an alternate reflector at the chip edge of the device of  FIG. 3 , including a Mach-Zehnder coupler in the add/drop port; 
           [0034]      FIG. 6A  is an illustration of the optical fields exiting the pupils AO 1  and AO 2  without additional lenses; 
           [0035]      FIG. 6B  is an illustration of the optical fields exiting the pupils AO 1  and AO 2  corrected by a field lens as realized in accordance with the present invention; 
           [0036]      FIG. 7  is a schematic illustration of the 3 port tunable add/drop filter as shown in  FIG. 3 , schematically illustrating the layer structure of the PLC and the curved lenses M 1 , M 2  and M 3 . 
       
    
    
     DETAILED DESCRIPTION  
       [0037]    The present invention as illustrated in  FIG. 3  relates to a 3-port tunable filter in a monolithic PLC  10 , comprising an INPUT port F 1  (i.e. single-mode fiber), an OUTPUT port F 2  and an ADD/DROP port F 3 . The functionality of the device is such that a given wavelength division multiplexed (WDM) channel can be selected, and that channel can either be
   a) Added through the ADD/DROP port F 3  to a signal launched via the input port F 1  for output the OUTPUT port F 2 , or   b) dropped through the ADD/DROP port F 3  from a signal launched via the INPUT port F 1 , while any remaining channels are output the output port F 2 .
 
It is assumed that these functions are mutually exclusive, so that one or the other function is assigned to the ADD/DROP port, but not both simultaneously. All other channels not selected are passed directly from the INPUT port F 1  to the OUTPUT port F 2  (the so-called EXPRESS configuration), with minimal attenuation.
   
 
         [0040]    A nearly monolithic planar implementation of this device is shown schematically in  FIG. 3 . The PLC chip  10  is outlined in black. The elements within the chip  10  will be described in detail below. The fibers  101 ,  102 ,  103  representing the  3  ports F 1 , F 2 , F 3  are connected optically to the chip  10  via either fiber ferrules or other PLC chips, as will be explained below. Such fiber/chip interface structures  100  are indicated schematically by the rectangles. If the desired functionality of the device conforms to option a), as described in the last paragraph, then F 3  represents the ADD port, and for option b) F 3  represents the DROP port. 
         [0041]    The optical signals emanating from the fiber-pigtailed INPUT F 1  and OUTPUT F 2  ports are transmitted through their respective arrayed-waveguide grating (AWG) structures AWG  1   120  and AWG  2   130 , exiting the arrays at the indicated pupils, AO 1  (Array Output  1 ) and AO 2 . The AWGs  120 ,  130  separate and combine the wavelength components of these optical signals. Viewing AO 1  and AO 2  as sub-pupils of a larger, common pupil, these wavelength-separated fields are then telecentrically focused to an interface plane  12  at F 3  on the edge of the chip  10 , via the natural focusing of the AWG outputs, which lies on a common circle for both AWGs, and a series of curved mirror surfaces, indicated by M 1 , M 2  and M 3 . A preferred embodiment of the invention invokes total internal reflection (TIR) at an etched glass-air interface M 1 , M 2 , M 3 , as was assumed in the optimization for the design represented in  FIG. 3 . Similar pupil structures are disclosed in U.S. Patent Publication No. 2008/0031570 published Feb. 7, 2008, which is incorporated herein by reference. 
         [0042]    The focusing lenses M 1 , M 2  and M 3  can be lithographically defined, in the same manner and mask step as the Rowland circle and channel waveguides comprising the AWG arrays. Specifically, the regions  122 ,  124 ,  126 , in which rays appear in the figures, correspond to slab waveguide areas, terminating at the curved interfaces represented by M 1 , M 2  and M 3 . As shown schematically in  FIG. 7 , on the slab side of the interface one will find the usual slab waveguide strata, i.e. high-index core layer Cl, surrounded above and below by lower-index cladding layers CL, and on the other side of the interface, at regions  112 ,  114 ,  116  those layers are etched away. Thus, the slab regions  122 ,  124 ,  126  interface to either air (for a purely total internal reflection interface) or a metalized interface (requiring another deposition step). These etched regions  112 ,  114 ,  116  can also be filled with other low index material. 
         [0043]    The geometries for the reflective surfaces M 1 , M 2 , M 3  focusing lenses are precisely the same as those that would be considered for off-axis telescopes, subject to the constraint that all angles of incidence are greater than the critical angle for total internal reflection. If there were only a single field, e.g. collimating the output of a single waveguide, or coupling a single waveguide to another single waveguide, then only one reflecting surface would be required, and being lithographically defined, it can be as complicated as design requires for an arbitrarily large pupil. 
         [0044]    On the other hand, as in the case of the 3-port TOADM according to the present invention, to accommodate multiple field angles, a single reflector is insufficient, and—as with optical designs in general-more elements, each of which provides less bending of rays, hence lower aberrations, are required. 
         [0045]    The required functionality is then realized in the coupling to the ADD/DROP fiber  103  at F 3 , as shown in detail in  FIG. 4 , where the incident angles of the focused fields have been exaggerated. Depending upon the desired passband characteristics for the ADD/DROP path, the ADD/DROP fiber, presumably an SMF fiber, can be either:
   directly located at the focal plane  12  of the chip (if Gaussian passbands are desired, and the mode size is matched to the PLC chip outputs);   spliced to a thermally-expanded core fiber (if Gaussian passbands are desired, but the required mode size is larger than standard SMF);   coupled to a multi-mode waveguide, with the ADD/DROP reflector structure  108  representing a PLC chip (if flat passbands are desired and lowest loss is not important); or   coupled to a Mach-Zehnder coupler  104 , or some other realization of a chip interface in which the focus shifts with wavelength, with a free spectral range equivalent to a channel spacing, as shown in  FIG. 5  (if flat passbands are desired and lowest loss is important).   
 
         [0050]    Because lowest loss is desired for the EXPRESS path, the regions  106  of the reflector structure  108  of the ADD/DROP port F 3  are mirrored, while a transmissive window  105  provides coupling to the input waveguide  103 . Hence the need for telecentricity in the imaging system, as will be described in detail below. The tuning mechanism for selecting an optical channel is the process of moving the ADD/DROP reflector structure  108  transversely, so that the transmissive window  105  is aligned with the wavelength corresponding to the desired channel center of the spatially separated and focused wavelength channels. 
         [0051]    In one embodiment, the ADD/DROP reflector structure  108  of the ADD/DROP port F 3  is reciprocated by a flexure structure in which the output fiber  103  and reflective surface  106  of the reflector structure  108  at F 3  rests on a platform supported by two flexure extensions, and are pushed normal to those flexures, resulting in almost pure translation (without rotation) along the intended direction (not shown) 
         [0052]    The fields exiting the pupils AO 1  and AO 2  would come to a virtual focus within the chip  10  along a Rowland circle, as shown in  FIG. 6A . The dashed lines represent chief ray trajectories (i.e. rays from the center of a pupil to the center of the focused spot at the output) for 3 such wavelength components. For the EXPRESS paths, it is desired that what emanates from one pupil will, for all wavelengths not corresponding to the ADD/DROP channel, reflect from a plane mirror  106  at the focus  12 . However, this would require that the chief rays are incident at equal but opposite angles from that mirror normal. It is clear from inspection of  FIG. 6A  that this can only be possible for one wavelength, not all wavelengths. What is required for this to be true of all wavelengths is the presence of a lens M (roughly classifiable as a field lens), as suggested in  FIG. 6B . This lens M straightens the chief rays from each sub-pupil so that they are incident at equal but opposite angles from the mirror  106  normal for all wavelengths, as required. Thus what enables a nearly monolithic functionality is the implementation of a field lens in the form of curved mirror structures M 1 , M 2 , M 3 .