Abstract:
The specification describes thermo-optically controlled waveguides where the thermo-optic control is a heater strip precisely aligned to the waveguide core by using the heater strip as an etch mask to define at least part of the core/upper cladding structure. In one embodiment the heater strip mask is larger than the underlying waveguide core so as to produce, on etching, a core surrounded with a minimum of cladding material on the sidewalls of the core, and a confined heat channel between the waveguide core and the heater strip. In a second embodiment the heater strip is used as an etch mask to define a core cladding stack, and a third cladding layer is deposited to clad the sidewalls of the core. In this embodiment, the cladding material in the stack can be optimized for thermal conductivity, and the third cladding material can be optimized for thermo-optic sensitivity.

Description:
FIELD OF THE INVENTION 
     This invention relates to optical waveguides tuned by thermo-optic control. 
     BACKGROUND OF THE INVENTION 
     The ability to effect localized changes in refractive index (η) in planar lightguide circuits (PLC s) is an adjustment tool that is useful in a variety of devices. For example, this mechanism can be used to tune the phase in Mach-Zehnder Interferometers (MZI). See e.g. B. Zheng and C. Zheng, “A study of directional coupler modulator with thermo-controlled coupling coefficient, Integrated Optoelectronics, Beijing, China: SPIE Vol. 2891, 1996, pp. 178-182. Directional couplers having a 50% power splitting ratio are needed in Mach-Zehnder Interferometers (MZI). Directional couplers where coupling ratios vary from zero to one are useful in Fourier filters and ring filters. Couplers are widely used in optical transmission systems. 
     In these applications, the refractive index in the device as fabricated must be very close to the design value so that the waveguide has the desired spectral response. An important evolution for silica based PLCs is toward more compact devices, which require a higher refractive index difference (Δ) between the core and cladding. A disadvantage of using high (Δ) is that fabrication variations in refractive index increase. In tunable couplers, normal fabrication tolerances cause variations in the coupling ratios of ±5% for 3 dB couplers in silica with low Δs (≦0.7%). A method for actively tuning the refractive index in local regions of a planar waveguide after fabrication would be beneficial by: 1) increasing device yield, and 2) providing additional functionality by allowing the refractive index in a local region of the waveguide to be modulated. 
     For silica waveguides the most effective post-fabrication technique to actively tune the PLC is to use thermal heaters and rely on the thermal dependence of the refractive index (dη/dT). Heaters can be deposited on the surface of the waveguide and used to tune the refractive index. However, the refractive index of silica is relatively insensitive to temperature, thus requiring a substantial temperature change to effect the desired adjustment. This requires both higher power and longer heating times. This is especially the case where the device structure has high thermal inertia between the heater and the core of the waveguide, and where the heater and the waveguide are not well aligned. A η tunable waveguide with more sensitive and more rapid thermal response would represent a significant advance in the art. 
     SUMMARY OF THE INVENTION 
     We have developed a device structure and fabrication technique for improving the thermal efficiency of tunable waveguide heaters, thereby yielding devices with improved tuning sensitivity and response. The technique involves forming the heater strip on the upper cladding of a dual clad planar waveguide, then using the heater as a mask for forming a heat channel between the heater and the waveguide core. In an alternative embodiment, the heater strip can also used to define the core itself. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a sectional view of a portion of a PLC showing the basic structure of a conventional η tunable waveguide; 
     FIG. 2 is a sectional view similar to that of FIG. 1 showing a n tunable waveguide with the aligned heat channel of the invention; 
     FIGS. 3 to  11  are schematic representations of a sequence of fabrication steps for the manufacture of the device of the invention; 
     FIG. 12 is a schematic diagram of a directional coupler that may be fabricated using the principles of the invention; and 
     FIGS. 12 to  19  are schematic representations of an alternative sequence of fabrication steps for the manufacture of a self-aligned heater and core device. 
    
    
     DETAILED DESCRIPTION 
     With reference to FIG. 1, the substrate for the PLC is shown at  11 . The substrate may be glass or other suitable rigid support. The preferred substrate material is silicon which is used in so-called optical bench technology for high quality optical integrated circuits. The technology for processing state of the art PLCs follows, in some respects, silicon IC wafer fabrication. The lower cladding layer of the waveguide is shown at  13  and the upper cladding layer at  14 . The waveguide core is designated  15 . In the conventional η tunable waveguide the heater strip  16  is located on the surface of the upper cladding layer as shown. The heater strip comprises an electro-resistive material such as chrome, or nickel chrome. Electrodes, not shown in this view, are typically gold or copper contact pads at the ends of the strip wire, and are connected to a power source. 
     The heater strip is typically formed using a simple additive technique such as mask evaporation. The pattern of heat flow from the heater strip to the waveguide core is indicated schematically in the figure by flow lines  17 . The heater and core are intentionally shown mis-aligned to represent a common defect in the conventional process. The resultant non-uniform heating of the waveguide core is evident. Severe misalignment can result in temperature gradients in the waveguide core which cause serious degradation in performance. 
     A more efficient waveguide heating structure is shown in FIG.  2 . Here the heating strip  21  is aligned over the waveguide core by blanket depositing the electro-resistive material and defining the heater strip using photolithography. The heater strip is then used as the alignment feature, i.e. part of the mask, to etch the upper cladding layer to form heat channel  24 . The combination of alignment, and confinement of the heat flow pattern, as represented in FIG. 2, increases the efficiency of the heating mechanism, and the response and performance of the device. 
     A suitable process for making the device of FIG. 2, given by way of example and not intended as limiting the invention, is shown by the sequence of steps represented by FIGS. 3 to  11 . 
     With reference to FIG. 3, a 15 μm oxide layer  31  is grown on a 5″ silicon wafer  32  by high pressure oxidation to form the lower cladding layer for the waveguide. As shown in FIG. 4, the core layer  33  for the waveguide is deposited over lower cladding layer  31  by CVD deposition of doped SiO 2  using established CVD techniques. Typical CVD deposition processes use precursors of silane or halogenated silane and hydrogen, with hydrides or halides of phosphorus or germanium for the doping material. The level of doping is such as to create an index difference between core layer  33  and cladding layer  31  of 0.3-1.5%. The thickness of the core layer in this example is approximately 5 μm. The manufacture of waveguides for PLCs adopts many of the techniques used in optical fiber technology that are well known and widely used. The specifics of the glass technology form no part of the invention. 
     In the sequence of steps described here a single waveguide is shown for convenience. It will be recognized by those skilled in the art that at least two side-by-side waveguides are used in a typical coupling device. The waveguide is then defined by lithography. The feature sizes are relatively large, for example a 5 μm waveguide is typical. Consequently, photolithography is generally suitable, although other lithography methods, e.g. methods using electron beam or x-ray actinic radiation, may also be used. A lithographic mask layer is applied over cladding layer  33  and patterned as shown in FIG. 5 to produce mask feature  34  corresponding to the waveguide core. The core layer  33  is then etched, using mask  34 , to produce the waveguide core. The structure after etching, and after removal of the mask  34 , is shown in FIG. 6, with the waveguide core shown at  35 . The etching technique is preferably Reactive Ion Etching (RIE) which will etch through a relatively thick silica layer without excessive undercut and produce relatively steep sidewalls. The sidewalls shown in these figures, which are not necessarily to scale, are shown vertical for simplicity. 
     The upper cladding layer  38  is then deposited over the waveguide  35  as shown in FIG.  7 . This layer may be, for example, BPTEOS (silica codoped with boron and phosphorus) having the same index as the lower cladding layer  31 . 
     The heater strip is formed on the surface of the upper cladding layer by blanket depositing a thermo-electric material over the upper cladding layer  38  to form metal layer  41  as shown in FIG.  8 . The metal layer is masked with photomask feature  42 , shown in FIG. 9, and etched to produce heater strip  43  as shown in FIG.  10 . Typically, in a conventional process, the heater strip is formed by evaporating or sputtering a layer of the resistive material and patterning the layer by a standard lift-off process, or by conventional lithographic masking and wet etching. However, in the preferred embodiment of the invention the heater strip is formed by photolithography to achieve precise alignment between the heater strip  43  and the waveguide core  35 . Heat from the heater strip locally changes the refractive index of the upper cladding layer  38  and thus changes the optical containment of waveguide  35 . 
     Gold electrode pads (not shown) may be provided at the ends of the strip heater using photolithography and wet etch or a lift-off technique. 
     An important feature of the process described in this embodiment is that the width W 1  of heater strip  43  should be substantially greater than the width W 2  of waveguide core  35 . For effective waveguide function, W 1  should exceed W 2  by at least 6 μm, but for effective heat channeling should not exceed W 2  by more than 20 μm. Preferably W 1  is in the range 1.5 to 7 W 2 . 
     Using the heater strip  43  as part of the etch mask, the upper cladding layer is etched to form thermal channel  45  as shown in FIG.  11 . Prior to the etch step, mask  42  may be removed or left in place as desired. Upper cladding layer  38  is preferably etched by RIE to produce relatively vertical sidewalls. The thermal channel  45  serves also as the top and sidewall cladding for waveguide core  35 . 
     Heat from heater strip  43 , change s the refractive index of the upper cladding layer  38  and thus changes the optical containment of waveguide  35 . 
     Optical devices that use the thermoelectric control described here are well known. An example of such device is given by way of example and is illustrated in FIG.  12 . With reference to FIG. 12, PLC substrate is shown at  51 , with two waveguides shown at  52  and  53 , with a coupling section where the waveguides run parallel and closely spaced to one another. The length of the coupling section is designated L. The coupling region, i.e. the space between the waveguides along the coupling section, is designated  54  in the figures. A heater strip (not shown) covers all or a substantial part of the coupling region. 
     The basic operation of a directional coupler is well known. It splits lightwaves coherently in a manner similar to a beam splitter in bulk optics. The input lightwave to waveguide  52  is P i  and the output lightwave from waveguide  53  is P o  When the waveguides are closely spaced, as in FIG. 12, the evanescent tail of the lightwave in waveguide  52  extends into waveguide  53  and induces an electric polarization. The polarization generates a lightwave in waveguide  53 , which couples back to waveguide  52 . 
     In the example given, the two waveguides are single mode and are parallel and identical in structure in the coupling region. Both waveguides bend away from each other at the ends as shown, and gradually decouple. The input lightwave P i  and the output lightwave P o  are related by: 
     
       
         P i =k P o   
       
     
     where k is the coupling ratio. The coupling ratio is strongly affected by the coupling region, and in particular by the core-to-cladding refractive index difference which is temperature dependent. As described earlier, this dependency can be utilized to adjust the refractive index after the fabrication of the waveguides has been completed. 
     An alternative technique for fabricating the thermo-optic control means used in the PLC devices discussed above will be described in conjunction with FIGS. 13 to  19 . Many of the process features are the same as those detailed earlier and will only briefly be set forth in describing this alternative sequence. In FIG. 13, the PLC substrate is shown at  61 , with lower cladding layer  62  and core layer  63 . This stage in the sequence is essentially that shown in FIG. 4 of the earlier embodiment. In that embodiment the waveguide core layer was patterned to form the waveguide core prior to depositing the second cladding material. In the approach to be described now, a multilayer stack is formed including the first and second cladding layers, the core layer and the heater strip layer. FIG. 14 shows the second cladding layer  64  formed over the core layer  63 . The metal heater strip layer  65  is deposited as shown in FIG.  15 . This multilayer stack can be produced in a single processing tool, e.g. a CVD apparatus or a cluster tool, without any intervening processing. The metal heater strip layer  65  is masked with mask  71 , as shown in FIG. 16, and the heater strip  73  defined as shown in FIG.  17 . Using the heater strip feature as an etch mask, as in the earlier embodiment, the remaining multilayer stack is etched down to the first cladding layer. An etch stop layer, as well known in the art, can be employed between the first cladding layer and the core layer for etch control. The resulting structure, shown in FIG. 18, has waveguide core  76  and heater strip  73  precisely self-aligned Sidewall cladding for waveguide core  76  is provided by depositing sidewall cladding layer  77  over the etched stack as shown in FIG.  19 . 
     In addition to the simplified processing afforded by this embodiment, it also allows selective tailoring of the thermal properties of the heat channel  75 . The thermal inertia between heating strip  73  and waveguide core  76  can be reduced by using an efficient heat transfer material for layer  64  (FIG.  17 ). 
     Another option for improving the efficiency of the coupling device shown in FIG. 12 is to use a cladding material for the space between waveguides in the coupler section which has a refractive index that is relatively more sensitive to temperature changes than conventional waveguide materials. Conventional cladding materials are SiO 2 , or a doped silica, with a temperature dependence of refractive index, dη/dT, of the order of +10 −5 /° C. Polymer cladding materials are available with dη/dT that is consistently in the range −0.5×10 −4 /° C. to −4×10 −4 /° C. Consequently, a much larger change in the effective index is possible for the same change in temperature. 
     This expedient may be implemented using a polymer cladding material for layer  77  in FIG. 19. A prepolymer may be applied by spinning, by syringe, or by suitable technique, and then curing to produce the polymer cladding material  77  that separates waveguide core  76  from an adjacent waveguide core, not shown. A wide variety of polymers are useful as the localized cladding material. Desired properties of the polymer include: low loss at wavelengths of interest (1.3-1.6 μm), adherent, thermally stable, hydrolytically stable, crack resistant, and an index in the range 1.3-1.6 μm. Preferred polymers are fluorinated polymers and silicon-based polymers (siloxanes). 
     This approach, combined with that described earlier, allows the cladding material for the coupling region to be optimized for thermo-optic response, while the cladding material between the heater strip and the waveguide core can be optimized for efficient heat transfer. 
     The waveguide structures in the devices described herein are conventional waveguides with a strip-like configuration and typically rectangular, or preferably essentially square, in cross section. 
     Various additional modifications of this invention will occur to those skilled in the art. All deviations from the specific teachings of this specification that basically rely on the principles and their equivalents through which the art has been advanced are properly considered within the scope of the invention as described and claimed.