Abstract:
In accordance with the invention, a high speed thermo-optic phase shifter comprises a length of optical waveguide including a waveguiding core of a first material having an index of refraction n 1  and a first order temperature dependence |dn 1 /dT| and, optically coupled to the core, a length of a second material having an index n 2  preferably greater than the core (n 2 &gt;n 1 ) and a first order temperature dependence |dn 2 /dT| than the core (|dn 2 /dT|&gt;|dn 1 /dT|). Advantageously, the length of second material is adiabatically tapered at both ends. Upon heating, as by a resistance heater, the second material changes the optical pathlength by an amount predominantly determined by |dn 2 /dT| providing faster switching speed. In a preferred embodiment, the core comprises silica, and the second material comprises silicon to produce switching speeds up to a few hundred MHz.

Description:
FIELD OF THE INVENTION 
   This invention relates to devices for processing optical signals and, in particular, to a high speed thermo-optic phase shifter for controllably changing the optical path length for light passing through the shifter and thus the phase of the light. 
   BACKGROUND 
   Thermo-optic phase-shifting devices are essential components of optical communication systems. By thermally changing the refractive index of material in an optical pathway, they can control switching, attenuation or modulation of an optical signal. The principle of operation is that by heating a waveguide, the lightwave in the waveguide can be delayed enough to cause a change from constructive to destructive interference (or vice versa) with an undelayed lightwave, resulting in switching. 
   A typical thermo-optic phase shifter comprises a resistance heater thermally coupled to the high index core of a silica waveguide. Heat changes the temperature of the core and thereby the refractive index since it is temperature dependent. This changes the integrated product of index and distance (optical pathlength) and hence changes the time required for the passage of the light. 
   While such phase shifters are simple to fabricate and operate, they are unfortunately slow and consume too much power for many applications. Typically their switching frequencies are limited to a few kHz and they consume about 50-350 mW of electrical power. Phase shifters that could provide faster switching at comparable or lower power would be highly desirable. 
   SUMMARY OF THE INVENTION 
   In accordance with the invention, a high speed thermo-optic phase shifter comprises a length of optical waveguide including a waveguiding core of a first material having an index of refraction n 1  and a first order temperature dependence |dn 1 /dT| and, optically coupled to the core, a length of a second material having an index n 2  preferably greater than the core (n 2 &gt;n 1 ) and a first order temperature dependence |dn 2 /dT| greater than the core (|dn 2 /dT|&gt;|dn 1 /dT|). Advantageously, the length of second material is shaped at each end for adiabatically coupling to the waveguiding core. Upon heating, as by a resistance heater, the second material changes the optical pathlength by an amount predominantly determined by |dn 2 /dT|, thus providing faster switching speed. In a preferred embodiment, the core comprises silica, and the second material comprises silicon to produce switching speeds up to a few hundred MHz. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The advantages, nature and various additional features of the invention will appear more fully upon consideration of the illustrative embodiments now to be described in detail in connection with the accompanying drawings. In the drawings: 
       FIGS. 1A ,  1 B,  1 C and  1 D are schematic side, top and transverse cross sectional views of a high-speed thermo-optic phase shifter in accordance with the invention; 
       FIGS. 2A ,  2 B,  2 C and  2 D illustrate alternative forms of the phase shifter; and 
       FIG. 3  is a schematic top view of a switch or modulator employing the phase shifter of  FIG. 1  or FIG.  2 . 
   

   It is to be understood that these drawings are for purposes of illustrating the concepts of the invention and are not to scale. 
   DETAILED DESCRIPTION 
   Referring to the drawings,  FIG. 1A  is a schematic side view of a thermo-optic phase shifter  11  comprising an optical waveguide structure  12  composed of a waveguiding core  13  and upper and lower cladding layers  14 ,  15 . The core  13  is composed of a material having an index of refraction n 1  greater than the index of refraction of the cladding layers  14 ,  15 . Typically the index of refraction of the core will also exhibit a temperature dependence |dn 1 /dT|. A heater  16 , which can be a length of resistive metal, is thermally coupled to the core, as through upper cladding layer  14 . 
   In accordance with the invention, a secondary core  17  having an index n 2  with a greater temperature dependence than the core (|dn 2 /dT|&gt;|dn 1 /dT|) is both optically coupled to the core and thermally coupled to the heater  16 . In practice, the secondary core  17  is closely spaced along a length of core  13  along a region thermally coupled to the heater. Closely spaced, in this context, means that the secondary core is within the exponential intensity tail of light transmitted in core  13 . The index of the secondary core  17  is greater than the index of the cladding and advantageously greater than the index of core  13  (n 2 &gt;n 1 ). 
   In an advantageous embodiment, the waveguide is a planar waveguide overlying a supporting substrate  18 . A local trench  19  is advantageously formed in upper cladding  15 , as by etching, to bring the heater  16  closer to the core  13  for thermal efficiency and speed. The high index secondary core provides sufficient optical mode confinement that the upper cladding thickness in the trench can be reduced as low as about two micrometers. The secondary core  17  preferably intervenes in the thermal path between the heater and the core  13 .  FIG. 1B  schematically illustrates the device in transverse cross section. 
   As can be better seen from the top view of  FIG. 1C , the secondary core  17  is of limited length, roughly co-extensive with the heater  16  whose length, in turn, is chosen to produce a desired phase shift. The ends  17 A and  17 B of secondary core  17  are advantageously shaped, as by tapering, to adiabatically couple to core  13 . Light traveling along core  13  is gradually coupled into core  17  at one end e.g.  17 A, without significant loss due to non-guided modes and similarly coupled back into core  13  at the other end  17 B. This adiabatic coupling avoids the need for complex mode converters. 
   In an advantageous embodiment, the substrate  18  can be a silicon wafer, the core  13  can be silica doped to increase its refractive index and the cladding layers  14 ,  15  can be silica or air. The secondary core  17  can be polysilicon. For adiabatic low-loss coupling to a standard—Δ waveguide, it is important to taper the ends  9 A,  9 B to a very fine dimension (e.g. on the order of 60 nm at the tips). Alternatively, long period gratings  19  can be etched in the ends of the secondary core as shown in  FIG. 1D  (top view). Choice of the grating period permits excitation of a particular mode of the silicon waveguide. 
   The device can be made using a modified form of the silicon optical bench process described by C. H. Henry et al. in “Glass Waveguides on Silicon for Hybrid Optical Packaging,” 7  J. Lightwave Technol ., pp. 1530-39 (1989). In essence a silicon substrate is provided with a base layer of SiO 2 , and thin core layers of doped silica glass and polysilicon are deposited on the oxide. The polysilicon is configured to form secondary core  17  (with tapered or grating ends), and the underlying doped silica is configured to form core  13 , all using standard photolithographic techniques. A layer of doped silica glass is deposited on the core to act as upper cladding  14 . The upper cladding can be optionally trenched to receive the heater  16 , and the heater can be deposited as by sputtering or vacuum evaporation and can be patterned by photolithography. In typical applications, the core  13  has a thickness of a few micrometers. The secondary core  17  has a thickness of a few tenths of a micrometer and a length of a few centimeters. 
   In operation, light traveling along core  13  begins coupling into secondary core  17  at upstream end  17 A. Coupling is facilitated by the secondary core  17  having a higher refractive index than core  13 , and low-loss coupling is obtained by the tapered or grating formation of end  17 A. A controlled phase shift (delay) is introduced by the application of heat from heater  16 . The heat changes the index of the temperature sensitive secondary core  17  more rapidly than the core  13  (Recall that |dn 2 /dT|&gt;|dn 1 /dT|). Polysilicon, for example, produces about 20 times more phase delay per degree of temperature rise than a standard silica core. After incurring the delay along core  17 , at downstream end  17 B the delayed light couples back into core  13 . The result is phase shifting at a high speed as compared to standard silica cores. 
   For a π phase change in silica, a temperature change of about 77.5 degrees Celsius is required; however, for a silicon or polysilicon waveguide with almost 100% mode confinement in the core, a change of only about 4.2 degrees is needed. 
     FIG. 2A  illustrates a longitudinal cross section of an alternative embodiment of a phase shifter in accordance with the invention wherein a silicon or polysilicon waveguide  20  optically couples light between segments  21 A and  21 B of conventional silica waveguide. Long period gratings or adiabatic tapers at the ends  20 A,  20 B are used to couple the silicon waveguide with the lower-index-contrast conventional waveguide. A locally etched trough  22  in the cladding  23  can bring the heating electrode  24  close to or actually on the silicon core. Because the evanescent (exponential) tail of the lower-contrast waveguides  21 A,  21 B will extend into the cladding  23 , the substrate  25  is advantageously recessed under the segments  21 A,  21 B and under the overlap regions  20 A,  20 B. In general, the thinner the silica between the electrode and the silicon core, the faster the response of the phase shifter. This structure can be readily fabricated using the well known SOI (silicon-oxide-insulator) fabrication process. 
   An advantageous variation of the  FIG. 2A  phase shifter uses a rib waveguide to transmit heat from an electrode removed from the waveguiding region.  FIG. 2B  is a transverse cross section of a phase shifter similar to that shown in  FIG. 2A  except that the polysilicon waveguide  20  is disposed overlying the cladding  23 , has a thickened rib  20 A and has one or more laterally extending flanges  20 B. The thickened rib  20 A is the guiding region, and rib  20 A is heated by one or more heating electrodes  24  removed from the waveguiding rib  20 A but thermally coupled to the rib by flanges  20 B. The advantages of the embodiment include 1) fast coupling of heat through silicon rather than cladding and 2) positioning of the heating electrodes on the flanges  20 B removed from the optical guiding region  20 A. Metal near the waveguiding region would produce unwanted loss. In addition the disposition of the waveguide  20  on the surface permits rapid cooling when the heating power is reduced which, in turn, enhances speed of response. The cladding can be silica, silicon nitride for high speed, or even air by bridging the rib across an air gap. 
   To roughly quantify the improved performance obtainable, applicants calculated the response time for a π-phase shift for a conventional device and the  FIG. 1  device. Two-dimensional calculations were performed using ICEPAK software. Simulations predict a response time as low as 3.3 ns at a steady state power of 3.6 mW for a π phase shift or, with air cladding, a slower response time of 15 ns but only 0.26 mW for a π phase shift. 
     FIG. 2C  is similar to the phase shifter of  FIG. 2B  except that one flange  20 B is thermally coupled to a heat sink as by a fin  20 C connected to substrate  25 . Heat from electrode  24  heats rib  20 A. When the heating power is reduced, the heat rapidly couples from the rib through the flange and fin (preferably polysilicon) to the heat sinking substrate  25  (preferably silicon). 
     FIG. 3  illustrates a thermo-optical switch or modulator  30  employing one or more high-speed thermo-optic phase shifters  11  according to  FIG. 1  or FIG.  2 . The switch  30  comprises a pair of optical waveguides  31 ,  32  interacting via couplers  33 ,  34  (typically 3 dB couplers or beam splitters and recombiners). The coupler  33  splits input light to the two waveguides, and coupler  34  recombines the light from the two waveguides. In essence, the waveguides and couplers form a Mach-Zehnder interferometer. 
   At least one of the waveguide “arms”, here upper waveguide  31 , includes a thermo-optic phase shifter  11  for controllably changing the optical pathlength through the arm as compared to the pathlength through the other arm  32 . 
   In operation, after the light beam is split at input coupler  33 , the light is recombined at output coupler  34 . The light will recombine by constructive interference if it recombines in phase. It will recombine by destructive interference if it recombines with a π phase difference. Phase shifter  11  can control this phase difference and thus determine whether the output light intensity is minimally reduced, essentially zero or modulated to some intermediate level. 
   It can now be seen that, in one aspect, the invention is a thermo-optic phase shifting device for thermally changing the phase shifting device for thermally changing the phase of light traveling therethrough. The device comprises an optical waveguiding structure comprising a first waveguiding core, a second waveguiding core, and a cladding peripherally surrounding the first and second cores. The first core has an index of refraction n 1  with a temperature dependence |dn 1 /dT|. The second core has a length less than the length of the first core, a pair of ends, an index of refraction n 2 &gt;n 1  and a temperature dependence |dn 2 /dT|&gt;|dn 1 /dT|. The second core is optically coupled to the first core so that light traveling along the first core is coupled into the second core beginning at one of the ends and from the second core to the first core at the other end. A heater is thermally coupled to the second core between the ends to thermally change the index of refraction along the second core. Thus light entering through the first core is coupled into the second core, thermally shifted in phase, and coupled back into the first core. 
   In another aspect, the invention is a thermo-optic switch or modulator. It comprises a pair of optical waveguides interacting by a pair of optical couplers so that a light beam on one of the waveguides is split to both the waveguides by the first coupler and recombined at the second coupler. At least one of the waveguides includes a thermo-optic switch as described above by which the phase difference between the beams is controlled to control the output light intensity. 
   It is understood that the above-described embodiments are illustrative of only a few of the many possible specific embodiments, which can represent applications of the invention. Numerous and varied other arrangements can be made by those skilled in the art without departing from the spirit and scope of the invention.