Abstract:
The present invention provides a method of forming a planar optical waveguide comprising the steps of forming a silica-based waveguide at a first temperature which is below a melting temperature of material from which the waveguide is formed; and annealing a region of the waveguide at a second temperature which is greater than the formation temperature and less than a melting temperature of material from which the waveguide is formed, so as to alter an effective refractive index of the region. In one embodiment the step of annealing is preceded by the step of forming a thin film heater over the region of the waveguide, the heater being capable of heating the region to the second temperature. The first temperature is preferably low (below 400° C.) to maximize the range of annealing temperatures.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method of forming a planar optical waveguide, to a waveguide formed by such a method and to a method for tuning or “trimming” the waveguide. 
     BACKGROUND OF THE INVENTION 
     It is known to manufacture silica-based optical waveguides at a temperature above the melting point of the waveguide material by deposition techniques such as flame hydrolysis of a suitable precursor powder. It has been proposed, for example in U.S. Pat. No. 5,117,470 to Inoue et al., to thermally process a waveguide which is initially formed at a high temperature (about 1200° C.), so as to adjust the effective refractive index. The thermal processing involves heating the waveguide to a temperature which is below its formation temperature and melting point, followed by rapidly cooling the waveguide to room temperature. The process of post-deposition adjustment of the effective refractive index of a waveguide is sometimes referred to as “trimming”. 
     One disadvantage of this prior art trimming process is that the initial formation temperature is quite high. Where the waveguide is integrated with other components such as active optical device structures or electronic elements, those structures typically cannot withstand temperatures in excess of 1000° C. A further disadvantage is that the process described in U.S. Pat. No. 5,117,470, which is dependent on a rapid cooling after the heating step, is only effective in regions of the waveguide which contain particular dopants, such as in the cladding layers of the waveguide. 
     It is thus an object of the present invention to provide a method of forming a waveguide that at least partially ameliorates a disadvantage of the prior art. 
     SUMMARY OF THE INVENTION 
     In a first aspect, the invention resides in a method of forming a planar waveguide comprising the steps of: forming a silica-based waveguide at a first temperature which is below a melting temperature of material from which the waveguide is formed; and annealing a region of the waveguide at a second temperature which is greater than the formation temperature and less than a melting temperature of material from which the waveguide is formed, so as to alter an effective refractive index of the region. 
     Preferably the step of annealing is preceded by the step of forming a thin film heater over the region of the waveguide, the heater being capable of heating the region to the second temperature. 
     Preferably the step of annealing is preceded by the step of analysing the formed waveguide to determine the refractive index profile of the waveguide. 
     In a second aspect, the invention resides in a method of forming a planar waveguide comprising the steps of selecting a first temperature for forming a silica based waveguide which is below a melting temperature of material from which the waveguide is to be formed; forming said silica based waveguide at said first temperature from said material such that said waveguide is trimmable by a post formation annealing process on a region of said waveguide at a temperature between said first temperature and said melting temperature so as to alter the effective refractive index of the region. 
     Preferably the first temperature is selected so as to allow the refractive index of the region to be altered within an optimal range. It is thus preferred that the waveguide is formed at a temperature below 600° C., more preferably below 500° C. and still more preferably below 400° C. 
     Preferably the waveguide comprises a core formed between a buffer layer and a cladding layer, and at least one of the core, the buffer layer and the cladding layer is deposited by plasma-enhanced chemical vapour deposition (PECVD). 
     Preferably the method further comprises the step of annealing said region of the waveguide at said second temperature which is greater than the selected formation temperature and less than the melting temperature of material from which the waveguide is formed, so as to alter the effective refractive index of the region. 
     The invention further resides in a planar waveguide formed by any of the above-described methods. Furthermore, the invention resides in an optical device incorporating a waveguide formed according to the methods of the invention. An optical device may be, without limitation, any one of a group comprising an arrayed-waveguide grating (AWG), a Mach-Zehnder interferometer, a directional coupler, a polarization beam splitter, an N×M optical switch matrix, an optical modulator, an optical attenuator, a variable optical attenuator, an add/drop multiplexer and a variable add/drop multiplexer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described by way of non-limiting example only with reference to preferred embodiments and to the accompanying figures in which: 
     FIG. 1 shows a schematic cross section of a waveguide structure deposited at a temperature T 1 ; 
     FIG. 2 shows a schematic cross section of the waveguide structure of FIG. 1 on which has been deposited a thin film heater; 
     FIG. 3 shows a schematic plan view of a waveguide structure with a series of heating elements; 
     FIG. 4 shows an embodiment of the invention in a Mach-Zehnder interferometer; and 
     FIG. 5 shows an embodiment of the invention in an arrayed waveguide grating. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The following embodiments describe a planar waveguide and a method for its fabrication, as well as optical devices incorporating the waveguide. 
     The present invention recognises the need to have a waveguide that can be tuned or “trimmed” by post-formation processes to adjust its effective refractive index and more particularly that the limits of the effective refractive index range in which the waveguide can be trimmed is optimised by the careful selection of the formation temperature of the waveguide. 
     In FIG. 1 there is shown a substrate  10  on which a silica-based waveguide  20  has been deposited using plasma-enhanced chemical vapour deposition (PECVD). The waveguide comprises a core  21 , formed between a buffer layer  22  and a cladding layer  23 , as is known in the art. It is preferred that the PECVD process, in particular the core layer deposition, is carried out using a liquid silicon-containing source material such as TEOS for the precursor vapour, and is conducted in the absence of nitrogen or nitrogen-containing source materials. During the PECVD process, the substrate  10  is maintained at a formation temperature T 1  which is below the melting temperature of the waveguide material. The effective refractive index of each layer of the waveguide is influenced by, amongst other deposition parameters, frequency of input power, level of input power, precursor flow rate, flux and energy of ion bombardment, overall pressure during the deposition, type and concentration of dopants in the various layers of the waveguide  20 , and the temperature T 1 . Further details of the PECVD process are contained in Australian provisional patent application number PR1782, entitled “Silica-based Optical Device Fabrication” and filed in the name of Redfern Integrated Optics Pty. Ltd., the contents of which are incorporated herein by reference. The temperature T 1  is minimised so as to allow a maximum range of refractive index adjustment in the formed waveguide structure  20  through subsequent annealing at a temperature below the melting temperature of the waveguide  20 . It is preferred that T 1  is approx. 350° C. to allow the resultant waveguide structure to be trimmed by a post-formation treatment with a maximum range of refractive indices available. Annealing the waveguide at temperatures above the deposition temperature will lower the refractive index of the materials in the waveguide, and thereby lower the effective refractive index of the waveguide. 
     To enable trimming of the waveguide structure  20 , a thin film heater  30  is deposited on the top of waveguide structure  20  as illustrated in FIG.  2 . The thin film heater is capable of producing a power density sufficient to beat the waveguide structure  20  above the formation temperature T 1  and preferably up to just below the melting temperature of the waveguide structure. The heater component is connected to contact pads (not shown) for connection to a power supply. 
     The device illustrated in FIG. 2 is a generic waveguide structure with an effective refractive index (as deposited) that has been selected to allow maximum trimming depending on the purpose for which the waveguide will ultimately be employed by an end user. 
     To trim the effective refractive index to requirements, the existing effective refractive index can first be confirmed using known optical diagnostic techniques. Current is then applied to the heating element  30  to heat a region beneath the element  30  to a temperature above the formation temperature but below the melting temperature of the waveguide. Heating continues until a temperature required to attain the desired effective refractive index is reached. 
     As is apparent from the waveguide plan view of FIG. 3, a series of heating elements  33 ,  34 ,  35  may be formed over the waveguide  20  along its length. This arrangement of heaters allows a non-uniform heating profile to be provided, either to account for inhomogenities in the waveguide to produce a uniform refractive index, or to produce a non-uniform refractive index profile n 2 , n 3  and n 4  where the waveguide is covered by heating elements  33 ,  34 ,  35 , respectively. Non-uniform heating may be achieved by supplying a different heating current to each of the heating elements  33 ,  34 ,  35  or by having elements of different dimensions and/or materials. 
     When silica is heated, the change in refractive index typically has a reversible component and an irreversible component. It is therefore necessary when trimming the waveguide to allow the waveguide to cool to room temperature before measuring the irreversible component of the refractive index. It is also important not to heat the waveguide to a temperature that results in an irreversible refractive index beyond that required. A typical trimming process will thus include the sequence of heating the waveguide, allowing the waveguide to cool and measuring the irreversible component of the effective refractive index. This process can be repeated with progressively greater heating currents (or progressively more pulses of current) until the waveguide has the desired effective refractive index. 
     Of course, if the thermal properties of the waveguide and heating properties of the thin film heater are known, the waveguide can be trimmed using a predetermined heating time and temperature which are known to produce the required irreversible refractive index profile. 
     When a core is to be trimmed close to other waveguides, the core can be thermally isolated by forming air-filled grooves either side of the core. The grooves can be formed in the cladding layer and buffer layer, for example by etching. 
     An application of the present invention is in a Mach-Zehnder interferometer described herein with reference to FIG.  4 . The interferometer  40  includes a pair of waveguides  60 ,  70  each having a region  61 ,  71  respectively on which is formed respective heating elements  62 ,  72  connected to contact pads  80  for supplying a heating current to the elements  62 ,  72 . The interferometer can be manufactured using known PECVD processes modified in accordance with the present invention. The arms  61 ,  71  can be annealed post formation at a temperature above the formation temperature but below the melting point to permanently set the effective refractive index in the arms to that required for the interferometer. 
     The interferometer can then be operated in a switching mode by utilising the thermo-optic effect in the arms  61 ,  71  to reversibly adjust the refractive index in one arm relative to the other, thereby introducing a phase shift between the two waveguides  61 ,  71 . The effective refractive index can be adjusted during operation of the interferometer by employing the heating elements  62 ,  72  at much lower temperatures than during the annealing process, typically at temperatures of approximately 100° C. Because the switching temperatures are below the formation temperature of the waveguides, the refractive index change is reversible during the switching mode. 
     A further application of the present invention is in an arrayed-waveguide grating (AWG) described schematically with reference to FIG.  5 . An AWG  90  comprises an array  91  of waveguides disposed between a first star coupler  92  and a second star coupler  93 . Ideally, the optical path length of the waveguides  91  increases monotonically by a constant value across the waveguide array. If the path lengths of the waveguides  91  do not increase by a constant value across the array, there will be cross-talk between output channels  94  when the AWG is operated as a demultiplexer. An AWG can be manufactured according to the present invention by first depositing a series of adjacent waveguides on a single substrate at a formation temperature below the melting point of the waveguide material. Post-formation optical analysis of the AWG can confirm the effective refractive index profile of the array  91  and detect any inhomogenities in the array that could potentially lead to signal cross-talk. It sometimes happens that non-uniformities in the deposition process produce a systematic variation in effective refractive index across the array of waveguides  91 . In this case, a tapered heater can be deposited across the width of the array such that the narrowest regions of the heater cover the waveguides which need to be trimmed to the greatest extent, and vice versa. FIG. 5 illustrates an example of such a heater in which a series of tapered heating elements  96  are deposited along the length of the waveguide array  91 , each element covering the width of the array  91 . The heating elements  96  are tapered to provide greater heating in the narrower sections. A heating current is then applied through contact pads  95  to the heating elements  96  to remove the inhomogenities by altering the effective refractive index profile of the waveguide array  91 . An alternative technique (not shown) of using heating elements to trim the waveguides  91  is to deposit separate heating elements over one or more selected waveguides within the array  91  and to anneal those waveguides separately without affecting the remaining waveguides in the array. 
     As will be apparent to the skilled addressee, the present invention allows waveguide structures to be produced in a more flexible and thus more cost effective manner by providing a post-formation process allowing the structures to be trimmed to requirements. An important advantage of the invention is that the methods can be used for the production of generic components that can be trimmed within broad parameters to suit specific requirements with little consideration of the specific use to which those components will ultimately be put. 
     It will be appreciated by the person skilled in the art that numerous modifications and/or variations can be made to the present invention as described herein without departing from the spirit or scope of the invention as broadly described and it is thus intended that any or all such modifications and/or variations are embraced herein.