Abstract:
A device that compensates for focal point drift of transmitted optical beams caused by ambient temperature changes by using a position adjuster to adjust the alignment of channels of a target planar lightwave circuit (“PLC”). The channels of the target PLC may taper open. In one implementation, the device may also use a birefringence plate to compensate for polarization dependent focal point drift.

Description:
FIELD 
   The subject matter disclosed herein generally relates to the field of optical circuits and in particular relates to techniques to transfer optical signals. 
   DESCRIPTION OF RELATED ART 
   Planar lightwave circuits (PLCs) are systems that include, but are not limited to, waveguides, light sources, and/or detectors in the plane of the circuit. One well known use of a PLC is as a waveguide for optical signals in optical networks. PLCs are often developed using silica-on-silicon (SOS) technology.  FIG. 1  is a cross sectional schematic diagram that shows a typical SOS architecture. A layer of lower cladding  12  is typically formed onto a substrate  10 . A waveguide core layer  20  is formed over the lower cladding  12 , and an upper cladding  24  is formed over the waveguide core layer  20 . Waveguide core layer  20  can be used to guide beams, such as light beams. In one example, the substrate  10  may be silicon, the lower cladding  12  may be SiO 2 , the core layer  20  may be SiO 2  doped with Germanium, and the upper cladding  24  may be a borophosphosilicate glass (BPSG). 
   For example,  FIG. 2  depicts an example of a system  200  having an arrayed waveguide  205  with multiple channels that each can transmit optical signals through a free propagation region (FPR)  210  to a target channel  220 . Ambient temperature variations can change the refractive index of the channels of arrayed waveguide  205  that can cause variations in their dispersive properties and thereby change a focal point of light beams emitted from the channels of arrayed waveguide  205  and through the FPR  210 . When a target of beams from channels of arrayed waveguide  205  is a target channel  220 , ambient temperature variations might cause the beam to not focus onto the expected location (for the design wavelength) Such out-of-focus beam transmission can cause degradation of transmitted signal quality and lost signal power as well as interference with beams transmitted by adjacent waveguide channels or neighboring wavelength channels. 
   In this system  200 , a material controls the position of the target channel  220 . The material has thermal expansion properties so that when the ambient temperature changes, the material moves the target channel  220  along the surface  230  of the FPR  210 , in the directions of the arrow, so that the beams transferred from the arrayed waveguide  205  focus onto the channel  220 . 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross sectional schematic diagram that shows a typical SOS architecture. 
       FIG. 2  depicts an example of an arrayed waveguide that transmits signals through a free propagation region to a target channel. 
       FIG. 3  depicts a system in which some embodiments of the present invention can be used. 
       FIG. 4  schematically depicts an embodiment of the present invention in a beam transfer system. 
       FIGS. 5 and 6  depict examples of a beam transfer system in accordance with embodiments of the present invention. 
       FIG. 7  depicts a manner in which first and second PLCs can be formed in accordance with an embodiment of the present invention. 
       FIGS. 8A ,  8 B,  9 A, and  9 B depict examples that show tapered channel openings in accordance with embodiments of the present invention. 
       FIGS. 10-14  depict examples of a plate coupling connector channels to complementary connector channels in accordance with embodiments of the present invention. 
   

   Note that use of the same reference numbers in different figures indicates the same or like elements. 
   DETAILED DESCRIPTION 
   One embodiment of the present invention may include a first planar lightwave circuit having a channel to guide signals; a second planar lightwave circuit having a channel to receive the signals; and a position adjuster to adjust the location of the channel of the second planar lightwave circuit relative to the channel of the first planar lightwave circuit in response to changes in ambient temperature. These and other features and advantages of the present invention will become more apparent from the following detailed description taken together with the accompanying drawings. 
     FIG. 3  depicts a system in which some embodiments of the present invention can be used. An input system  300  may provide optical signals to a beam transfer system, which may multiplex or de-multiplex such optical signals according to wavelength and transfer select optical signals to an output system  350 . The input system  300  can include a semiconductor optical amplifier (SOA) that can selectively block or amplify and transfer optical signals to the beam transfer system. One example implementation of an output system  350  is an optical-to-electrical signal converter. Such optical-to-electrical signal converters can provide the electrical signals to a SONET/SDH receiver and/or Gigabit Ethernet receiver. 
     FIG. 4  schematically depicts an embodiment of the present invention in beam transfer system  400 . Beam transfer system  400  can include a first PLC  410  and second PLC  420 . First PLC  410  can include first signal guides  412 , free propagation region  414 , and connector channels  416 . Second PLC  420  can include second signal guides  422 , free propagation region  424 , and connector channels  426 . Each of first signal guides  412 , connector channels  416  and  426 , and second signal guides  422  can transfer light beams. Although second signal guides  422  are depicted as having a single channel, second signal guides  422  can include multiple channels, each of which can guide optical signals. A position adjuster  430  can be used to move the second PLC  420  along, for example, the Y axis so that connector channels  416  transfer signals to complementary channels of connector channels  426  with little dispersion and little power loss. 
   Beam transfer system  400  can be used to multiplex optical signals transmitted by first signal guides  412 . Conversely, beam transfer system  400  can be used to de-multiplex optical signals transmitted by second signal guides  422 . 
   Position adjuster  430  can be implemented as a surface to which second PLC  420  is mounted and that has thermal expansion properties such that when the ambient temperature changes, the surface moves the second PLC  420  along the surface  440  of the first PLC  410 , along, for example, the Y axis, so that first PLC  410  transfers signals to the complementary channels of the second PLC  420  with little dispersion and little power loss. For example,  FIG. 5  depicts an example of beam transfer system  400  that shows second PLC  420  coupled to first PLC  410  using this example implementation of position adjuster  430  (shown as position adjuster  430 A). 
   As shown in  FIG. 5 , first PLC  410  can be mounted to base surface  510 . A glue can be used to mount first PLC  410  to base surface  510 . Base surface  510  can have thermal expansion properties similar to those of the layer of first PLC  410  that contacts base surface  510  (e.g., silicon). One side of elevated base  520  can be mounted to base surface  510  and the opposite side of elevated base  520  can be mounted to position adjuster  430 A. Position adjuster  430 A in this implementation can but does not have to be rectangular shaped. Position adjuster  430 A can be mounted to a side of second PLC  420  that is opposite to the side of second PLC  420  that contacts base surface  510 . An adhesive can be used to affix position adjuster  430 A to elevated base  520  and affix position adjuster  430 A to second PLC  420 . Second PLC  420  can be mounted to position adjuster  430 A but second PLC  420  may be allowed to move along the Y axis so that the connector channels  426  can move relative to stationary connector channels  416 . Supports  530  can hold second PLC  420  into contact with base surface  510  but allow connector channels  426  to move along the base surface  510 . In one implementation, supports  530  can use a spring to hold second PLC  420  into contact with base surface  510 . In one implementation, a sheet in the X-Y plane can be provided between surface  510  and the region in which first PLC  410  and second PLC  420  contact each other. This sheet may improve proper coupling of channels of the first PLC  410  with those of second PLC  420  along the Z-axis by making the coupling to be less sensitive to ridges in the Z-axis on base surface  510 . 
   In another embodiment, position adjuster  430  can be implemented as a micromechanical device to which the second PLC  420  is mounted and that continuously moves the second PLC  420  along the surface  440  of the first PLC  410  and along, for example, the Y axis based on changes in ambient temperature. In another implementation of position adjuster  430  as a micromechanical device, rather than move the second PLC  420  in a continuous manner, position adjuster  430  moves the second PLC  420  according to a step relationship so that second PLC  420  moves incremental distances based on incremental changes in the ambient temperature. For example, this implementation of the position adjuster  430  may move the second PLC  420  along the Y axis a distance of ±X microns each ±Z degrees Celsius change. The micromechanical implementation of the position adjuster  430  can be powered by a battery or photovoltaic device (e.g., solar power). The micromechanical implementation of the position adjuster  430  may use power only when moving the second PLC  420 . For example,  FIG. 6  depicts an example of beam transfer system  400  that shows second PLC  420  coupled to first PLC  410  using this example implementation of position adjuster  430  (shown as position adjuster  430 B). 
   As shown in  FIG. 6 , first PLC  410  can be mounted to a base surface  610 . A glue can be used to mount first PLC  410  to base surface  610 . Base surface  610  can have thermal expansion properties similar to those of the layer of first PLC  410  that contacts base surface  610  (e.g., silicon). Supports  630  can hold second PLC  420  into contact with base surface  610  but allow second PLC  420  to move along the Y axis so that the connector channels  426  can move relative to stationary connector channels  416 . In one implementation, supports  630  can use a spring to hold second PLC  420  into contact with base surface  610 . Position adjuster  430 B of this implementation can be positioned between the base surface  610  and the second PLC  420 . Position adjuster  430 B can include a first portion  650 A and second portion  650 B. First portion  650 A can be affixed to second PLC  420  and second portion  650 B can be affixed to base surface  610 . Position adjuster  650 B may move second PLC  420  by moving first portion  650 A relative to second portion  650 B in a continuous or step-like manner in response to changes in ambient temperature. In one implementation, a sheet in the X-Y plane can be provided between surface  510  and the region in which first PLC  410  and second PLC  420  contact each other. This sheet may improve proper coupling of channels of the first PLC  410  with those of second PLC  420  along the Z-axis by making the coupling to be less sensitive to ridges in the Z-axis on base surface  510 . 
   One advantage of the position adjuster  430  that moves second PLC  420  in a step motion is less frictional wear of surfaces of materials in contact than that of the system  200 . Frictional wear may be less than that of system  200  because position adjuster  430  does not move the second PLC  420  except for incremental changes in ambient temperature. In contrast, system  200  moves the channel  220  for any changes in ambient temperature and so may cause continuous frictional wear. 
   The first PLC  410  and second PLC  420  of beam transfer system  400  can be fabricated together so that channels of connector channels  416  and connector channels  426  are continuous as shown in FIG.  7 . The first PLC  410  can be separated from the second PLC  420  by using a saw or etching technique. The channels may but do not have to be separated approximately half-way along the length of each channel and along the Y axis. The edges of the first PLC  410  and second PLC  420  that are to contact each other or a plate (described in more detail with respect to  FIGS. 10-14  below) can be polished. 
   One advantage of embodiments of beam transfer system  400  described herein is they can be easier to fabricate than the system  200  described with respect to FIG.  2 . The beam transfer system  400  may be fabricated using high precision straight-line cuts. 
   The shortest distance between the center axes of adjacent channels can be selected so that the cross talk of signals to be transmitted to adjacent channels of connector channels  426  are nearly in phase (or multiples of approximately 360 degrees out of phase). The shortest distance between the center axes of all adjacent channels along the X-Y plane can be the same but do not have to be the same. 
   In some implementations of the first PLC  410  and second PLC  420 , the grating order (i.e., number of 2π phase shifts between two neighboring channels) can be an even number and the number of channels can be less than forty (40). In some implementations, first PLC  410  and second PLC  420  can transfer light beams frequency spaced by approximately 100 GHz. 
   In another embodiment of the present invention, the cross sections (in the Y-Z plane) of connector channels  416  and/or connector channels  426  can widen towards the plane of contact between the connector channels  416  and connector channels  426 . For example,  FIGS. 8A and 8B  depict examples of this embodiment of the present invention that shows tapered channel openings. In three dimensions, the tapers can be cone shaped or parabola shaped. The tapered openings of this embodiment can improve the transfer of optical signals to channels of connector channels  426  from complementary channels of connector channels  416 . For example, the widest opening of any channel cross section along the Y-Z plane can be selected so that signal loss caused by receiving an unfocused signal is less than 3 dB. The channel cross section openings among channels of connector channels  416  and/or connector channels  426  can vary. 
   In another embodiment of the present invention, the cross sections (in the Y-Z plane) of connector channels  416  and/or connector channels  426  can decrease towards the plane of contact between the connector channels  416  and connector channels  426 . In one implementation, the cross sections can decrease in an exponential manner. For example,  FIGS. 9A and 9B  depicts examples of this embodiment of the present invention that shows tapered channel openings. In three dimensions, the tapers can be cone shaped. 
   Similar to focal point drift caused by ambient temperature variations, polarization of signals transferred by connector channels  416  may cause focal point drift that can diminish signal power of signals transferred to connector channels  426 . In accordance with an embodiment of the present invention, a plate can be provided along the Y-Z plane to couple signals transmitted by connector channels  416  to complementary channels of connector channels  426 .  FIGS. 10-14  depict examples of plate  700  coupling connector channels  416  to complementary channels of connector channels  426 . Connector channels  416  and connector channels  426  can directly contact the plate  700 . The plate  700  can be a birefringence plate that provides, for example, approximately ninety (90) degree rotation of polarization. The plate  700  can be at least as long as necessary to couple all channels of connector channels  416  to complementary channels of connector channels  426 . The plate  700  can be affixed to a first PLC  410  using a glue having an index of refraction that matches that of either the channels of connector channels  416  or the plate  700 . Connector channels  426  of second PLC  420  can contact the plate  700 . In another implementation, the plate  700  can be held stationary using a support structure and the connector channels  416  and connector channels  426  contact the plate  700 . 
   The drawings and the forgoing description gave examples of the present invention. The scope of the present invention, however, is by no means limited by these specific examples. Numerous variations, whether explicitly given in the specification or not, such as differences in structure, dimension, and use of material, are possible. For example, the techniques described herein can be applied in any scenario, and not just arrayed waveguides, to compensate for focal drift where signals are to be transmitted from one optical channel to another optical channel. The scope of the invention is given by the following claims.