OPTICAL PLANAR WAVEGUIDE DEVICE AND METHOD OF FABRICATION

Abstract: A device for processing an optical signal, the device comprising a processing element monolithically integrated with a planar silica-based waveguide structure in which the optical signal propagates.

Description: OPTICAL PLANAR WAVEGUIDE DEVICE AND METHOD OF FABRICATION Field of the invention The present invention relates to a planar optical waveguide device for the processing of optical signals and its construction. Background of the invention The utilisation of optical circuits is becoming increasingly important in the transmission of high bandwidth telecommunication signals. Further, fast switching and wavelength selection and filtering are rapidly becoming indispensable components of all optical Dense Wavelength Division Multiplexed (DWDM) networks. Such devices tend to be exceedingly complex and expensive to construct. Another very important optical device at the heart of the DWDM systems is a multi wavelength transmitter. Among available candidates for multi wavelength transmitters, are those based on external cavity silica-based laser modules which offer low chirp and greater wavelength stability and control. Unfortunately, such devices often have minimal modulation capabilities . The desirable inclusion of optical signal modulation functions in such modules would enhance their performance and increase the cost effectiveness of fabrication. Known carrier injection/absorption effects in semiconductors permit switching/modulation to be performed on the required time scale of nanoseconds or less. Existing devices incorporating semi-conductor functionality are typically epitaxially grown, which involves high growth temperatures, or fabricated in monocrystalline semiconductor substrates. Whilst having high modulation/switching performance such devices are relatively expensive to fabricate, suffer high insertion losses because of the utilisation of the semi-conductor waveguide material, suffer cross talk and are limited in the number of wavelength channels . Recently, a hybrid solution has been proposed where integrated semiconductor optical amplifier (SOA) gates which perform switching are integrated on a platform with silica-based waveguides used for signal routing and fibre interconnects . Although this solution combines advantages of fast switching in semiconductors with the versatility of silica-based waveguides, it is still not ideal from point of view of mass anufacturability and cost, since the SOA gates still need to be manufactured on separate chips and then assembled in modules, requiring alignment and wire bonding. Summary of the Invention The present invention provides a device for processing an optical signal, the device comprising a processing element monolithically integrated with a planar silica- based waveguide structure in which the optical signal propagates . The processing element may be arranged, in use, in a manner such that a refractive index of a portion of the processing element is alterable to effect modulation of the optical signal. The processing element may be arranged, in use, in a manner such that an effective refractive index for at least one optical mode of the optical signal in a portion of the waveguide structure is dependent on the refractive index of the portion of the processing element. The processing element may be arranged, in use, to perform optical signal processing on the optical signal. The processing element may comprise a semiconductor component. The semiconductor component can incorporate silicon. The semiconductor component may incorporate hydrogenated amorphous silicon. Alternatively, the semiconductor component incorporates polycrystalline silicon. Alternatively, the processing element may comprise an electro-optic material for modulating the optical signal. The material may have a high X2. In one embodiment, the material comprises barium titanate. A geometry and spatial relationship of the processing element and the waveguide structure may be chosen such that, in use, optical signal losses associated with optical mode transmission between the processing element and the waveguide structure are reduced. The processing element may be arranged, in use, to convert at least a portion of the optical signal into a corresponding electrical signal. The processing element may incorporate a semiconductor component. The semiconductor component may have a selected bandgap to, in use, exhibit photosensitivity at a wavelength of the optical signal. The semiconductor component may incorporates hydrogenated silicon and hydrogenated germanium. The waveguide structure and the processing element may be monolithically integrated on a substrate comprising electrical circuits for the processing element. The substrate may incorporate a silicon wafer. The present invention may alternatively be defined as providing a method of monolithically integrating a processing element with a planar silica-based waveguide structure . The method may comprise utilising hollow cathode plasma enhanced chemical vapour deposition (PECVD) for forming the silica-based waveguide structure. At least a portion of the silica-based waveguide structure may be formed after forming the processing element . The method may comprise the step of solid phase crystallisation of amorphous silicon to form the semiconductor processing element. The method may comprise the step of utilising PECVD for depositing hydrogenated amorphous silicon for the formation of the semiconductor processing element. The method may comprise removing a portion of a cladding layer of the waveguide structure as part of monolithically integrating the processing element. Brief Description of Drawings Notwithstanding any other forms which may fall within the scope of the present invention, preferred forms of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Fig. 1 illustrates a device constructed in the course of the principles of the present invention; Fig. 2 illustrates the process of incorporation of the device Fig. 1 in a Mach-Zehnder arrangement. Description preferred and other embodiments In the preferred embodiment, silica-based waveguides and semiconductor carrier injection/absorption elements are integrated monolithically. This results in substantial cost reductions compared to a hybrid integration alternative since batch processing techniques can be utilised throughout the fabrication process. Turning now to Fig. 1, there is illustrated a scheme with a sectional view of an example device 1 which is formed on a substrate 2 and utilises a semiconductor component 7 which, in this embodiment, comprises thin film amorphous silicon, polycrystalline silicon or a silicon/germanium p-i-n structure. It is incorporated into a silica based channel waveguide structure 40 so that large and fast phase/amplitude modulation of optical signals in the waveguide 4 can be achieved via a free carrier in ection/absorption mechanism or in case of silicon- germanium structure to convert an optical signal into an electrical signal. Through the utilisation of amorphous silicon structures, the speed of operation can be increased due to the intrinsically small carrier life time in amorphous material. Also, the use of polycrystalline silicon with variable grain size gives an additional tool to find a viable compromise between semiconductor quality (i.e. defect density and recombination rate) to lower the operating current and carrier life times to achieve high speed modulation. Generally with increase in the grain size the polycrystalline silicon properties move closer towards that of crystalline silicon and vice versa. Overall, this allows for an extremely attractive technique for construction of DWDM modulated sources since the speed of modulation of semiconducted materials is combined with the high functionality and performance of silica-based devices . It has been recognised that, in one embodiment of the invention, the incorporation of amorphous semiconductor into a silica based waveguide can be achieved through the use of a low temperature silica deposition technique. High temperature would destroy both semiconductor properties of the material and p-i-n carrier injecting structures. One form of suitable deposition technique identified is that outlined in PCT application PCT/AU/96/00563 entitled "A Method and Apparatus for the Construction of Photosensitive waveguides" and in CM Horwitz, S. Boronkay, R. Gross and K.E. Davies, "Hollow Cathode Etching and Deposition" J. Vac. Sci technology AG, at pages 1837 to 1844 (1998) which discloses a low temperature Hollow Cathode-Plasma Enhanced Chemical Vapour Deposition (HS-PECVD) process. The semiconductor component can be formed as a p- i-n structure in amorphous silicon for example by the method described in S. Guha, X. Xu, J. Yang and A. Banerjee "High deposition rate amorphous silicon-based multijunction solar cell" Appl . Phys . Lett. 66(5), PP 595-597, 1995. In Fig. 1 there is shown one such constructed device, where light propagating in the core 4 of the silica-based channel waveguide structure 40 interacts with the semiconductor component 7 , which is either incorporated in or located sufficiently close to the core 4, so that the effective refractive index of the fundamental optical mode of the waveguide is determined by both the refractive index of silica layers 3, 4, 5 and the refractive index of the semiconductor component 7. When the refractive index of the semiconductor is changed by carrier injection by applying electrical signal to the electrodes 8, 9, the effective refractive index of the fundamental mode propagating in silica-based waveguide in vicinity of the semiconductor is also changed accordingly. This can be utilized for very fast (on the time scale of minority carrier lifetime in the semiconductor) modulation or switching e.g. by using the induced phase shift in a Mach-Zehnder configuration. Fig. 2 illustrates an example Mach-Zehnder arrangement wherein a semiconductor layer 47 is formed over a core 42 of one arm of the two arm Mach-Zehnder in the system. By using the semiconductor 47, the refractive index of part of arm 42 is modulated relative to the arm 10 so as to produce a phase shift which in turn results in an intensity modulation between the output arms 11, 13. Further, alternative applications for utilising the change in the effective refractive index of the fundamental mode can be undertaken. For example, the change in refractive index can be utilised to alter the peak spectral position of a Bragg grating formed in the region of a silica based waveguide where mode refractive index changes are induced by the presence of a semiconductor layer. The shifting of the peak spectral position can in turn provide for the large modulation of a signal or selection between different wavelengths. In yet another application the refractive index changes induced by the semiconductor in the multimode region of a multimode interference (MMI) device or waveguide array gratings (WAG) , can be used for fast wavelength switching between different output ports. In addition to the carrier injection effect in semiconductors which provides fast changes in refractive index, free carrier absorption effects can also be used for optical signal modulation or attenuation. In this case the driving electric signal determines the intensity (not phase as in carrier injection) of light at the output of the semiconductor interaction region. In practical realisation of such devices the issues of adiabatic transition between pure silica and silica/semiconductor region of the channel waveguide are preferably manipulated in order to minimise the loss associated with this transition. In practice, the most suitable type of modulation (carrier injection or absorption) should be experimentally determined for amorphous silicon material produced with e.g. Ultra-High-Vacuum PECVD machine. Carrier injection, as opposed to carrier absorption, has smaller power requirements but at the same time can be off-set by simultaneous thermo-optic effect (which drives refractive index in the opposite direction) unless an adequate heat sink is provided and/or operating current is kept low. In an alternative embodiment of the present invention, the semiconductor component can be used to convert an optical signal into an electrical signal thus performing a detector function. For that purpose the semiconductor component is designed and fabricated in such a way that it exhibits absorption at the optical signal wavelength. The necessary absorption (determined by the semiconductor junction band gap) can be tailored by varying the composition of the semiconductor material . For example, a p-i-n structure based on pure amorphous silicon has little absorption at the 1.5 micron telecommunication wavelength, but codoping with Ge can reduce the band gap and provide the necessary photosensitivity in the infrared. In practical realisation of such integrated photodetector such issues as its size and back reflections should be addressed. The size can determine the detector capacitance, which in turn determines the maximum achievable bandwidth. High light conversion efficiency will be required to minimise the semiconductor component size (thus increasing bandwidth) . The effect of back reflections can be minimised by either using a dielectric (eg. silicon nitride) antireflection coating or by the angling the waveguide end face on which the semiconductor component is formed. The semiconductor component for the detector application can be fabricated, for example, by the method described in Hwang SB, Fang YK, Chen KH, Liu CR, Hwang JD, Chou MH "An a-SiH/a-Si, Ge : H bulk barrier phototransistor with a-SiC:H barrier enhancement layer for high-gain IR optical detector" IEEE Transactions on Electron Devices, 40(4) , pp721-726, 1993. Combined with the integrated transmitter modules based on the use of semiconductor component as a modulator, the additional detector function of the semiconductor component can provide for an important synergistic effect. This can allow the fabrication of low cost transceiver modules for optical local access networks. In another preferred embodiment a silicon substrate is used for a device structure and driver circuits for modulator or amplifier circuits for a detector are fabricated in the silicon substrate prior to the fabrication of integrated silica-based and semiconductor components. This allows for additional cost savings via increase of functionality of each single chip, mass- manufactured by the batch IC techniques. Again, it has been recognised that low temperature techniques for fabrication of the silica based planar waveguide component allows for the implementation of such an embodiment of the present invention. In another embodiment, the processing element may incorporate electro-optic materials to modulate an optical signal propagating in a silica-based waveguide. Materials having a high X2 (e.g. barium titanate) . It would be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive. E.g. other deposition techniques may be used for the silica-based waveguide including sol-gel deposition techniques .

Claims: We Claim : 1. A device for processing an optical signal, the device comprising a processing element monolithically integrated with a planar silica-based waveguide structure in which the optical signal propagates. 2. A device as claimed in claim 1, wherein the processing element is arranged, in use in a manner such that a refractive index of a portion of the processing element is alterable to effect modulation of the optical signal . 3. A device as claimed in any one of the preceding claims, wherein the processing element is arranged, in use, in a manner such that an effective refractive index for at least one optical mode of the optical signal in a portion of the waveguide structure is dependent on the refractive index of the portion of the processing element. 4. A device as claimed in any one of the preceding claims, wherein the processing element is arranged, in use, to perform optical signal processing on the optical signal. 5. A device as claimed in any one of the preceding claims, wherein the processing element may comprise an electro-optic material for modulating the optical signal. 6. A device as claimed in claim 5, wherein the electro-optic material has a high X2. 7. A device as claimed in claim 6, wherein the material comprises barium titanate. 8. A device as claimed in any one of the preceding claims, wherein the processing element comprises a semiconductor component. 9. A device as claimed in claim 8, wherein the semiconductor component incorporates silicon. 10. A device as claimed in claim 9, wherein the semiconductor component incorporates hydrogenated amorphous silicon. 11. A device as claimed in claim 9, wherein the semiconductor component incorporates polycrystalline silicon . 12. A device as claimed in claim 8, wherein the grain size of polycrystalline silicon is adjusted to control carrier life time thus controlling achievable modulation frequency. 13. A device as claimed in any one of the preceding claims, wherein a geometry and spatial relationship of the processing element and the waveguide structure are chosen such that, in use, optical signal losses associated with optical mode transmission between the processing element and the waveguide structure are reduced. 14. A device as claimed in claim 1, wherein the processing element is arranged, in use, to convert at least a portion of the optical signal into a corresponding electrical signal. 15. A device as claimed in claim 14, wherein the processing element incorporates a semiconductor component. 16. A device as claimed in claim 15, wherein the semiconductor component has a selected bandgap to, in use, exhibit absorption at a wavelength of the optical signal. 17. A device as claimed in any one of claims 15 or 16, wherein the semiconductor component incorporates hydrogenated silicon and hydrogenated germanium. 18. A device as claimed in any one of the preceding claims, wherein the waveguide structure and the processing element are monolithically integrated on a substrate comprising electrical circuits for the processing element. 19. A device as claimed in clam 18, wherein the substrate incorporates a silicon wafer. 20. A method of monolithically integrating a processing element with a planar silica-based waveguide structure, the method comprising utilising hollow cathode plasma enhanced chemical vapour deposition (PECVD) for forming the silica-based waveguide structure. 21. A method as claimed in claim 20, wherein at least a portion of the silica-based waveguide structure is formed after forming the processing element. 22. A method of monolithically integrating a semiconductor processing element with a planar waveguide structure, the method comprising the step of solid phase crystallisation of amorphous silicon to form the semiconductor processing element. 23. A method of monolithically integrating a semiconductor processing element with a planar waveguide structure, the method comprising the step of utilising PECVD for depositing hydrogenated amorphous silicon for the formation of the semiconductor processing element. 24. A method as claimed in any one of claims 20 to 23, the method comprising the step of removing a portion of a cladding layer of the waveguide structure as part of monolithically integrating the processing element.

Inventors: BAZYLENKO MICHAEL, SCEATS MARK, BAZYLENKO, MICHAEL, SCEATS, MARK
Applicants: BAZYLENKO MICHAEL, SCEATS MARK, UNIV SYDNEY, BAZYLENKO, MICHAEL, SCEATS, MARK, THE UNIVERSITY OF SYDNEY
Application year: 2000
Main classification: G02B6
IPC codes: G02B6, G02F1