Abstract:
An optical multiplexer that adjusts the wavelength response and compensates for temperature effects by using rotatable mirror. The wavelength response of the device is adjusted by aligning the mirror at a correct angle with respect to the surface terminating the optical waveguide grating. The temperature dependence of the index of refraction of the material comprising the waveguides is compensated for by rotating a reflecting surface of the mirror, the rotation based on differential thermal expansion. Some exemplary embodiments may comprise a slab waveguide on a substrate (the slab waveguide having a first and second arcuate end surfaces) attached to a submount, a mirror assembly rigidly attached to the submount (the mirror assembly comprising a first and second materials having different coefficients of thermal expansion), and an optical waveguide grating (upon the substrate attached to the submount) optically coupled between the second arcuate surface and the mirror assembly. A portion of the mirror assembly between the reflector surface and where the mirror assembly is rigidly attached to the submount deforms as a function of temperature to change an angle between the optical waveguide grating and the reflecting surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of provisional application Ser. No. 60/540,941 filed Jan. 30, 2004 and titled, “Temperature compensated optical multiplexer,” which application is incorporated by reference herein as if reproduced in full below. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not applicable. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   Various embodiments of the invention are directed to temperature compensated optical multiplexers. More particularly, embodiments of the invention are directed to a reflective arrayed waveguide grating multiplexer having a mirror that rotates based on differential thermal expansion to compensate for differences in index of refraction caused by temperature changes. 
   2. Discussion of the Related Art 
   Optical multiplexers may comprise waveguides within which light of varying frequencies is allowed to propagate. However, the index of refraction within the waveguides changes with operating temperature, which therefore changes the optical path lengths and adversely affects operation. In order to obviate the adverse effects of temperature, some related art systems attempt to precisely control the temperature of optical multiplexers. Precise temperature control may be difficult and costly, particularly in remote locations. 
   Other related art devices may attempt to compensate for temperature changes rather than perform temperature control. Published United States Patent Application No. 2002/0097961A1 to Kazarinov discloses such a system. In the Kazarinov system, a rigid mirror is rotationally fixed to the substrate upon which the waveguides are formed. The mirror is rotated as a function of temperature by a thermally conductive body, e.g. a copper block, pushing on the reflective surface. However, it is difficult to rotationally mount the mirror on the substrate, and further the thermally conductive body pushing on the reflective surface and the substrate tends to distort the mirror and produce stress in the grating, degrading performance. 
   SUMMARY OF SOME OF THE PREFERRED EMBODIMENTS 
   The problems noted above are solved in large part by an optical multiplexer that compensates for temperature effects by rotating a reflector surface optically coupled to grating waveguides of the multiplexer. The rotation of the reflector surface is based on differential thermal expansion. Some exemplary embodiments may comprise a waveguide sections and a mirror assembly rigidly coupled to a common silicon submount. A portion of the mirror assembly between the reflector surface and where the mirror assembly is rigidly attached to the submount deforms as a function of temperature to change an angle between the optical waveguide grating and the reflector surface. 
   The disclosed devices and methods comprise a combination of features and advantages which enable it to overcome the deficiencies of the prior art devices. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a detailed description of embodiments of the invention, reference will now be made to the accompanying drawings in which: 
       FIG. 1  illustrates a portion of an optical multiplexer in accordance with embodiments of the invention; 
       FIG. 2  illustrates a portion of the optical multiplexer of  FIG. 1  in greater detail; 
       FIG. 3  illustrates an optical multiplexer comprising a mirror assembly that rotates as a function of temperature; 
       FIG. 4  illustrates an elevational side view of the optical multiplexer of  FIG. 3 ; and 
       FIG. 5  illustrates a mirror assembly in accordance with embodiments of the invention. 
   

   NOTATION AND NOMENCLATURE 
   Certain terms are used throughout the following description and claims to refer to particular system components. This document does not intend to distinguish between components that differ in name but not function. 
   In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ”. Also, the term “couple” or “couples” is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection, or through an indirect connection via other devices and connections. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The optical waveguide device depicted in  FIGS. 1 and 2  comprises three main parts, an input/output optical waveguide structure A; a slab waveguide region B; and a reflective optical waveguide grating C. The input/output waveguide structure A comprises an input optical waveguide Wi and a plurality of output optical waveguides Wo. The waveguides Wo are spaced apart on one side of the waveguide Wi. The waveguides Wi and Wo interface with and extend radially from an arcuate first end surface  12  of a slab waveguide  10  of region B. The waveguides Wi and Wo provide ports for communicating light waves to and from the slab waveguide  10 . 
   The slab waveguide  10  has a second arcuate end surface  14 , disposed opposite from the first end surface  12 . The second arcuate surface  14  interfaces with a plurality of laterally spaced tapered optical waveguide sections  16  which extend radially from the second arcuate surface  14 . The waveguide sections  16  taper from wider ends at the interface with the second surface  14  to narrower ends more remote from the second surface  14 . At the interface between the wider ends of the tapered sections  16  and the slab end surface  14 , light is confined within the tapered sections and throughout the lengths of the respective tapers. 
   The optical reflective waveguide grating structure C comprises the tapered waveguide sections  16 , the narrower ends of which are continued as an array of laterally spaced apart waveguide sections  18 . Each waveguide section  18  terminates, and the termination points of the waveguide sections  18  define a termination surface, as illustrated by dashed line  20  in  FIGS. 1 and 2 . The lateral spacing (labed P 1  in  FIG. 1 ) of adjacent waveguide sections at their termination points should be constant. As shown in  FIG. 2 , the waveguide sections  18  include straight portions  18   a , having the same width as each other and different lengths, which are continuations of the narrower ends of the tapered waveguide sections  16 . The waveguide sections  18  also comprise curved portions  18   b  which have different radii of curvature. Portions  18   c  of the waveguide sections  18  extend from and tangentially to the curved portions  18   b , each terminating proximate to the reflector surface  20 . 
   Consider the device illustrated in  FIG. 1  operating as a demultiplexer. A plurality of optical signals, each having a different wavelength and which have been multiplexed together are communicated by an optical fiber to the input waveguide Wi, and are diffracted as they travel across the slab waveguide  10 . After crossing the slab waveguide  10  the light impinges on the second arcuate end surface  14 . The optical signals are then propagated along the respective optically isolated grating waveguide sections  18 , reflected from a reflector surface  26  of a mirror assembly  22  ( FIG. 3 ), and returned to the slab waveguide  10  along the waveguide sections  16  and  18 . Because of the different lengths of the optical paths, wavefronts of the light are shifted causing constructive and destructive interference such that substantially only one wavelength of light impinges on each output optical waveguide Wo. Operating conversely, a plurality individual single wavelength optical signals could be fed to the waveguides Wo, and after propagation to and from the reflector surface  20 , emerge at the waveguide Wi as a multiplexed set of optical signals. 
   The waveguide sections Wi, Wo, the slab waveguide  10 , the tapered waveguide sections  16 , and the grating waveguide sections  18  conveniently may be constructed as an integrated structure on a substrate  32  ( FIG. 4 ). Each of the waveguides may comprise a propagation core of high refractive index material sandwiched between cladding layers of low refractive index material. In some embodiments, a silicon substrate may be used with the cladding and core layers defined by differently doped silica layers. 
   Light waves transmitted through the slab waveguide  10  are propagated in two dimensions with light signals confined in the core layer of the dielectric material, the vertical dimension (thickness) of which (perpendicular to the plane of  FIG. 1 ) is such that single mode waveguide transmission of light waves is ensured. Using a silica-on-silicon slab waveguide structure, the thickness is on the order of about 5 microns. The difference in the index of refraction between the core (doped silica) and cladding (undoped or differently doped silica) materials may be greater than about 0.5%. In the lateral dimension (essentially bound by the periphery of the waveguide slab) there is no confinement. The interface between the input/output waveguide structure Wi, Wo and the slab waveguide end surface  12  (interface arc I), as well as the interface between the slab waveguide end surface  14  and the tapered waveguide sections  16 , (interface arc II), should each form an arc of a circle. The two circles preferably have the same radius R. The center of the interface arc I is located on the interface arc II, and vice versa. U.S. Pat. No. 6,493,487 to Temkin, which is incorporated by reference herein as if reproduced in full below, discusses in greater detail the relationships and sizes of the various components. 
   Light waves entering the slab waveguide  10  through input optical waveguide Wi propagate across the slabe waveguide  10  and impinge on the grating region C. Since the grating waveguide sections  18  are unequal in length, the optical round trip path from the input waveguide Wi to the reflector surface  20  and back to each output waveguide Wo is different for each waveguide section  18 , resulting in a phase shift along each round trip path. The phase shift between neighboring waveguide sections  18  depends on the light wavelength in the respective channels, the difference in the physical path lengths, and the index of refraction of the optical waveguide. The phase shift increment is constant across the grating region C and, for each light wave returning back to the interface arc I (surface  12 ), results in the rotation of the wavefront. 
   However, changes in temperature result in changes in the index of refraction, and therefore changes the optical path length (product of the round trip physical path length and the index of refraction). As illustrated in  FIG. 3 , a multiplexer in accordance with at least some embodiments of the invention comprises a mirror assembly  22  forming the reflector surface  26 . In particular, the mirror assembly  22  comprises a first material  24 , which may be polished on one end to create the reflector surface  26 . The mirror assembly  22  may also comprise a second material  28 . In accordance with embodiments of the invention, the first material  24  and the second material  28  may have different coefficients of thermal expansion, such that as the overall temperature changes, differential thermal expansion between the first material  24  and the second material  28  creates a deformation zone  27 . Deformation within the deformation zone  27  results in a rotation of the reflector surface  26 , as illustrated by dashed line  30 . Thus, the path length associated with each one of the waveguides  18  changes as a function of the temperature. As the temperature of the device changes, the wavefront is adjusted by the differing path lengths (caused by rotation of the mirror) to keep each channel substantially focused on its output waveguide Wo. 
   The distance between the surface  20  where the waveguides  18  terminate and the reflecting surface  26  is exaggerated in  FIG. 3  to illustrate rotation of the reflecting surface  26 . In operation, the reflecting surface may be approximately 10 micrometers (microns) from the termination surface  20  of the waveguides  18 . 
     FIG. 4  shows an elevational side view of the temperature compensated multiplexer/demultiplexer in accordance with embodiments of the invention. In particular,  FIG. 4  illustrates the input/output waveguide structure A, free space region B, and grating waveguide region C, possibly fabricated directly on substrate  32 . The substrate, in turn, may be affixed to a submount  36 . Likewise, the mirror assembly  22  may be rigidly coupled to the submount  36  on one end. In accordance with alternative embodiments, the substrate  36  may be extended in the direction of the grating waveguides, and the mirror assembly rigidly coupled to the substrate. In order for the reflector surface  26  to rotate, an attachment zone or fixed end  34  of the mirror assembly  22  may be rigidly coupled to the submount  36 , such as by epoxy  38 . Thus, while the fixed end  34  remains fixed, differential expansion caused by differences in the coefficient of thermal expansion of the materials of the mirror assembly  22  allows the reflector surface  26  to rotate (in a direction perpendicular to the page as illustrated in  FIG. 4 ).  FIG. 4  also illustrates that there may be a gap  40  between the composite mirror structure  22  and the submount  36 , which may be on the order of a few microns. In accordance with at least some embodiments the gap between the termination surface  20  of the grating waveguides  18  and the mirror assembly  22  may be filled with an index matching material  42  that improves the optical coupling between the grating waveguide region C of the multiplexer and the mirror assembly  22 . This index matching material  42  also suppresses undesirable optical reflections at the termination points of the grating waveguide region along termination surface  20 . However, the presence of material  42 , with its own coefficient of thermal expansion, may require adjustment in the rate of rotation of the mirror assembly  22 . 
     FIG. 5  shows a perspective exploded view of the mirror assembly  22  in accordance with embodiments of the invention. In particular, the first material  24  may be fashioned into a plate as illustrated in  FIG. 5 . The reflector surface  26  may then be created by polishing the edge of the first material  24 . In accordance with embodiments of the invention, the first material  24  may comprise a material that has a coefficient of thermal expansion approximately equal to that of silicon. Thus, the first material  24  may be silicon, or the first material  24  may be a metal alloy with a coefficient of thermal expansion similar to that of silicon such as an alloy of steel and nickel known as Invar. In accordance with embodiments of the invention, an aperture  44  is cut through the first material  24 , and in some embodiments the aperture is circular. Additionally, a channel  46  may be cut into the first material  24 . The second material  28 , preferably in the form of a cylindrical plug, is placed within the aperture  44 . The second material may be any suitable material having a coefficient of thermal expansion greater than that of the first material  24 , for example, aluminum, copper, brass, steel or silver. Once the second material  28  in the form of a plug is placed within the aperture  44 , differences in coefficients of thermal expansion result in deformation within the deformation zone  2  ( FIG. 3 ), which in turn rotates the reflector surface  26 , as illustrated by dashed line  30  in  FIG. 3 . This method of assembly eliminates the need for any adhesives, resulting in a highly reliable mirror. 
   Referring again to  FIG. 3 , for a mirror assembly  22  having a long dimension length L of approximately 27.7 millimeters (mm), a width W of approximately 15.4 mm, and a thickness of approximately 1.1 mm, the circular aperture  44  preferably has a diameter of approximately 3.2 mm. Prior to placement within the aperture, the second material  28  in the form of a cylindrical plug may have an outside diameter of approximately 3.3 mm at room temperature. Installation of the plug of second material  28  may take place by cooling the second material in liquid nitrogen, and placing the plug within the aperture  44  while the second material is at or near the temperature of liquid nitrogen. As the second material  28  warms, it is held in place friction coupling. The mirror assembly  22  constructed in accordance with embodiments of the invention preferably rotates with temperature at a rate of 1.8×10 −4  degrees/degree C. When placed in the assembly illustrated in  FIG. 3  the multiplexer operates independently of the ambient temperature in the range of 0-85° C. The rate of rotation may be adjusted, for example to accommodate the presence of index matching material  42 , by changing the diameter of the opening  44  or by changing the material  28 . 
   In optical transmission systems all the channels coincide with the predetermined set of wavelengths defined by the International Telecommunication Union (ITU). This set of wavelengths is known as the ITU grid. The response channels of multiplexing devices should match the ITU grid to within ¼ of the channel passband width. For example, with multiplexers operating on a 100 GHz grid, each channel has a passband width of 0.2 nm (nanometer) and each channel should be within 0.05 nm of the nearest ITU wavelengths. Practical manufacturing tolerances encountered in the fabrication multiplexers make these tolerances very difficult to meet. This is because neither the index of refraction of the materials used nor the precision of forming the required waveguide structures can be controlled with the required precision. As discussed in the Background section, temperature tuning is used in the related art to shift the channel response to the required wavelengths. However, temperature tuning requires heating or cooling elements and provision of electrical power to the package. 
   The mirror assembly described addresses this problem. The transmission spectra of a device as illustrated in  FIG. 3  shift with the angular position of the reflector surface. As the angle between the device and the reflector surface  26  is increased the spectra of all channels shift linearly to longer wavelengths. By varying the mirror angle by ˜0.03°, a wavelength shift as high as 2.0 nm may be obtained. Since the intended channel-to-channel separation is approximately 0.8 nm, this shift is sufficient to move the response wavelength to the ITU grid. It should be also pointed out that this process does not alter the overall performance of the device. In the test devices that form the basis of this specification, a loss penalty of less than 0.15 dB (decibels) was observed for the angular tilt corresponding to the wavelength shift equal to the channel-to-channel separation (˜0.8 nm). 
   Operation of the multiplexer illustrated in  FIGS. 1 and 2  with the mirror illustrated in  FIG. 3  satisfies two problems. The initial placement of the mirror assembly  22  at a correct angle with respect to the reflecting surface  20  assures wavelength response match with the ITU grid. Once the mirror assembly  22  is attached to the submount  36  the wavelength response is fixed. At this point any variation in the ambient temperature will be compensated for by rotation of the surface  26  of the external mirror. 
   The above discussion is meant to be illustrative of the principles and various embodiments of the present invention. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.