Abstract:
A thermo-optical device may use a heater to tune an optical device such as an optical switch, a Mach-Zehnder interferometer, or a variable optical attenuator, to mention a few examples. In some embodiments, polarization-dependent losses caused by the heating and power efficiency may be improved by defining a clad core including an optical core and cladding material on a substrate and covering the clad core on three sides with a heater.

Description:
This is a divisional of prior application Ser. No. 10/351,828, filed Jan. 27, 2003 is now a U.S. Pat. No. 6,961,495. 

   BACKGROUND 
   This invention relates generally to optical networks that convey optical signals. 
   Optical networks may use wavelength division multiplexing so that a plurality of channels, each of a different wavelength, may be multiplexed over the same cable. At a desired termination point for any one of the multiplexed channels, an optical add/drop multiplexer allows light of a given wavelength to be extracted from a plurality of multiplexed light channels. Similarly, a light channel of a given wavelength may be added to the network by an add/drop multiplexer. 
   One technique for forming an optical add/drop multiplexer is to use the Mach-Zehnder configuration. The Mach-Zehnder interferometer may include two spaced arms, at least one of which may be tuned using a heater. A Mach-Zehnder interferometer may be tuned by changing the refractive index of one of the two arms of the Mach-Zehnder interferometer by heating one arm using an electrical heater. 
   However, existing heaters have relatively large power consumption when used for purposes of tuning a Mach-Zehnder interferometer. These devices may also exhibit relatively high polarization-dependent losses. 
   Polarization-dependent losses are losses incurred by various network optical components that are contingent upon the state of polarization of the light interacting with those components. A network component may attenuate light selectively, depending on its state of polarization, changing the intensity of the propagating signal in a random fashion. 
   Examples of thermal optical devices that exhibit polarization-dependent losses include optical switches, splitters, and variable optical attenuators. A variable optical attenuator is a device that changes the applied attenuation to compensate for example, for the aging of a transmitter or amplifier or to respond to a network&#39;s operating conditions. A splitter is a device that splits light into different channels. Optical switches route an optical signal without electro-optical or optoelectrical conversions. Thermal optical devices generally require the application of external power. 
   Thus, there is a need for better ways to heat thermal optical devices. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic depiction of one embodiment of the present invention; 
       FIG. 2  is a schematic depiction of another embodiment of the present invention; 
       FIG. 3  is an enlarged cross-sectional view taken generally along the line  3 — 3  in  FIGS. 1  or  2 ; and 
       FIG. 4  is an enlarged cross-sectional view taken through another embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 1 , a thermo-optical device  10  may be in the form of a Mach-Zehnder interferometer. Other thermo-optical devices  10  include variable optical attenuators, splitters, and optical switches. While an example follows which considers a Mach-Zehnder thermo-optical device, the present invention is not limited to any particular type of thermo-optical device. 
   In one embodiment, the interferometer may be implemented as a planar light circuit that is formed using integrated circuit processing techniques in a substrate  26 . The various components may be formed as integrated waveguides formed on the substrate  26  in one embodiment. 
   A pair of Bragg gratings  12   a  and  12   b  may be formed on the substrate  26 . In one embodiment, the gratings may be formed on a substrate  26  that is a planar waveguide. The thermo-optical device  10  also includes a pair of 3 deciBel (dB) (50-50% coupling) couplers  14   a  and  14   b , The input coupler  14   a  is coupled to an input port  16  that receives one or more input wavelengths of light. The coupler  14   a  is also coupled to a port  18 . A second coupler  14   b  is coupled to a port  20  and an express port  22  to output a passed wavelength. Each coupler  14  includes a bar side and a cross bar side as indicated in  FIG. 1 . 
   Each grating  12  constitutes one of two arms of the Mach-Zehnder or Michelson interferometer in accordance with some embodiments of the present invention. Input lights that are Bragg matched to the gratings  12  propagate backwardly along the Mach-Zehnder arms and interfere with one another in the first coupler  14   a , Once the optical paths of both reflective lights are balanced, all the lights over the wavelength span of interest are phase-matched and all optical energy is transferred into the cross path of the first coupler  14   a  with little energy returning back to the bar path. 
   The cross path of the first coupler  14   a  becomes the drop wavelength port  18  at which signals at the Bragg wavelength of the Bragg gratings  12  get filtered out from other channels. Signals at wavelengths other than the Bragg wavelength transmit through the Bragg gratings  12  and merge in the second coupler  14   b.    
   All transmitted lights of the wavelength span of interest are phase matched using a balanced Mach-Zehnder interferometer. All energy is transferred into the cross path of the second coupler  14   b  with little leakage to the bar path. As a result, the cross path of the coupler  14   b  becomes the pass wavelength port  22  through which signals outside the Bragg grating reflection band are transmitted. 
   The bar path of the second coupler  14   b  may be used as an add port into which signals that carry the Bragg wavelength are launched. These added signals are reflected by Bragg gratings  12 , carried through the cross path of the second coupler  14   b  and join the pass signals at the pass wavelength port  22  without interfering with each other. 
   An optical add/drop multiplexer may use the Mach-Zehnder interferometer, which may be tuned by heating both of the gratings  12  using heaters  24  associated with each grating  12  in one embodiment. Such heating may be used to initially tune the Mach-Zehnder interferometer. As a result of heating, the thermo-optical device  10  may be controllably operated. Thus, the heater  24 , in one embodiment of the present invention, encloses the upper surfaces of each grating  12 . 
   In connection with a variable optical attenuator  10   a,  one arm  23  may have a heater  24  in one embodiment of the present invention as shown in  FIG. 2 . 
   Referring to  FIG. 3 , the grating  12   a  may be defined within a region  38  in the substrate  26 . In the embodiment of  FIG. 1 , a cross-section through the grating  12   b  would be the same as  FIG. 3 . The substrate  26  may include a silicon substrate  30 , a SiO 2  layer  26  over the substrate  30  and a boron phosphate silicon glass layer  34  over the top. A trench  36  is formed through the layers  32  and  34  down to the substrate  30 . Formed within the boron phosphate glass layer  34 , in the region within the trench  36 , is a core  12   a  which corresponds to the grating  12   a  and which carries the signal. 
   A metal resistance heater  24  may be formed over the portions of the layers  34  and  32  within the trench  36 . Thus, the grating  12   a  may be heated from its top and sides. The grating  12   a  may also be effectively heated from below because the heater  24  also contacts and heats the substrate  30  which underlies the grating  12   a.    
   By enabling the grating  12   a  to be heated within the trench  36 , power consumption may be reduced, in some embodiments, by removing unnecessary cladding material such that heat from the heater flows mainly towards the core  12   a , Also, quarter wavelength optical path difference Mach-Zehnder interferometers may be used as well for the same purpose. 
   Referring to  FIG. 4 , in accordance with another embodiment of the present invention, the structure may represent a core  38  of an optical switch  26   a  in accordance with one embodiment of the present invention. In such case no gratings  12  may be used. 
   In some embodiments, the polarization-dependent losses and power consumption of thermo-optical devices may be reduced. One possible explanation for this effect is that with conventional devices, the grating  12  or core  38  is only heated from above. This may result in a mis-match in thermal expansion coefficients of the heater and that of the cladding material such as boron phosphate silicon glass. This mis-match may generate mechanical stresses at the heater/cladding interface. Since this induced mechanical stress may only appear on one side, namely the top side, the induced refractive index of the core due to the stress optical effect is mainly in the stress direction, causing induced birefringence, which ultimately appears as polarization-dependent losses. 
   These induced polarization-dependent losses may be reduced by using the surrounding heater configuration. Since the heater  24  surrounds the grating  12   a  or  12   b  or core  38  on three sides, the stress induced by the larger thermal expansion coefficients of the heater and silicon substrate may have cubic symmetry since higher thermal expansion materials surround the core on all four sides. Unique stress-axes may not exist and, thus, birefringence may be reduced. As a result, power consumption and phase dependent losses may be reduced in some embodiments. 
   While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.