WAVEGUIDE PAIR WITH CLADDING

Abstract: A planar waveguide structure has, supported on a lower refractive index buffer layer (5), a pair of optical cores (1, 2) that, over at least a portion of their length, are closely spaced. These cores are covered with a layer (6) of cladding material comprising boron and phosphorus doped silica glass deposited by PECVD as a succession of individually annealed layers in order to minimise the incidence of voids in the deposit between the cores.

Description: WAVEGUIDE PAIR WITH CLADDING Background to the Invention The present invention relates to dielectric waveguide pairs made by plasma enhanced chemical vapour deposition (PECVD). This type of waveguide is frequently referred to as a planar waveguide, where the dielectric constant of the waveguide or core material differs slightly from that of a substrate material known as the buffer. The wavelengths transmitted by this type of waveguide will generally lie in the optical bands, e.g. 1 -2μm for optical communication systems. Summary of the Invention The invention is particularly concerned with cases in which the relative refractive indices of the core and the buffer are such as to allow a significant amount of cross-coupling either from the cores, or between the cores, without undue losses. Such losses can arise from irregularities or discontinuities in the intervening medium. Embodiments of the invention aim to provide cladding for the cores produced by PECVD which maintain good and low-loss optical coupling between the two waveguide cores. According to the invention there is provided a waveguide device comprising in cross-section a buffer of dielectric material, elongated dielectric cores deposited on the buffer and of a different refractive index from that of the buffer, and a cladding arranged over the cores and over the buffer, at least in the neighbourhood of the cores, wherein the material of the cladding has a lower flowing temperature than that of the materials of the cores and buffer, and has a refractive index that approximates to that of the buffer. Preferably, the cladding material consists essentially of a boron and phosphorus doped silica glass (BPSG). When the buffer is pure silica, its refractive index is matched by the cladding and its flowing temperature satisfactorily exceeds that of the cladding. A feature of preferred embodiments is that the cladding is in a layer constructed in stages It is believed that, in combination with the choice of material for the cladding, namely BPSG, the use of a multi-deposit process minimises the creation of air voids or other physical imperfections that may cause discontinuities and accordingly reduces the undesirable lossy wave coupling between the cores which otherwise tends to occur The use of twin dielectric cores on a buffer of silica or other material, and enveloped in dielectric cladding, is known in planar waveguide techniques. Brief Description of the Drawing The invention will be further explained with reference to an embodiment by way of example, in conjunction with the accompanying drawing, which shows a transverse section across a planar waveguide illustrating schematically an undesirable fault which is largely eradicated by the use of the invention. Detailed Description of a Preferred Embodiment Referring to the drawing, the planar waveguide seen in cross-section is assumed to extend an appreciable distance into and out of the plane of the drawing, for transmitting the optical energy longitudinally and for coupling some energy across to the intervening medium The coupling devices may be a Y-junction, a directional coupler or a radiative star and the like, where parallel le. i μhs of waveguide are as close as less than one micron (1μm) to each other The figure hc.;b two waveguide cores 1 , 2 which have oeen deposited on a substrate 4, suitably of silicon, with an intervening uf+ r layer 5 Buffer layei 5, which is preferably of silica inhibits the optical fields arising during operation from interacting with substrate 4. The cores and the buffer, at least in surrounding regions, are covered with a layer of cladding 6 which may be 20μm or more thick, and which plays protective and optical roles. If the heights of the waveguide cores shown schematically at 1 , 2 are 5μm it is very difficult to apply a cladding 6 to the region 3 between the waveguides. The cladding 6 in this region 3 tends to have overhangs and voids, such as that shown schematically at 7, due to significantly less material being deposited. Overhangs and voids cause abrupt changes in refractive index, which leads to undesirable energy losses and reflections, which tend moreover to be unpredictable. In this embodiment a process for applying cladding to closely spaced cores has been developed which reduces the tendency of these undesirable defects to form in the region 3 between the cores. According to this embodiment firstly the buffer 5 is deposited on the substrate. The next stage is to deposit core material by PECVD, and then to selectively etch the deposited core material to leave a pattern of two cores 1 , 2 with an intervening volume 3 in the form of a channel of rectangular cross-section. The cores are typically 5μm deep and about 6μm in width with a separation channel between the cores having a width typically in the range from 1 to 8 μm. It is believed that, with two such closely spaced cores, the amount of cladding 6 material that can be deposited as a final step by PECVD in the region 3 depends upon the range of angles from which material can reach the region. Hence the occurrence of the formation of overhangs and voids, such as that shown schematically at 7, which may be 3μm wide, may be minimised. Such an undesirable void would prejudice the coupling action of the device, and this embodiment includes a special cladding process, as the final deposition step, incorporating boron phosphorus silica glass as the composition of the material of cladding 6. The cladding includes four stages of deposition and annealing, as follows. Stage 1 Deposit 1.0μm of BPSG, then flow at 1000°C for 1 hour. Stage 2 Deposit 3.2μm of BPSG, then flow at 1000°C for 1 hour. Stage 3 Deposit 6.6μm of BPSG, then flow at 1000°C for 1 hour. Stage 4 Deposit 6.6μm of BPSG, then flow at 1000°C for 1 hour. The cladding composition is silica glass containing 2.4 weight % Phosphorus and 3.4 weight % Boron. The composition is suitable for the deposition, and also has a lower flowing temperature than the cores 1 , 2 and has a refractive index close to that of the pure silica buffer 5. Optical performance has improved performance in comparison with previously produced cladding.

Claims: Claims: 1. A cladded waveguide pair produced by plasma enhanced chemical vapour deposition (PECVD) on a buffer layer (5) on a substrate (4), with a cladding layer (6) of boron and phosphorus doped (BPSG) produced by multiple stages on the waveguide pair and adjoining regions of the buffer wherein each stage consists of a deposition step and a subsequent annealing step by which the cladding is flowed. 2. A cladded waveguide pair according to claim 1 in which the BPSG cladding layer has a lower flowing temperature than the core material and a refractive index close to that of the buffer. 3. A cladded waveguide pair according to claim 2 in which the BPSG includes silica glass containing 2.4 weight % phosphorus and 3.4 weight % boron. 4. A cladded waveguide pair according to claim 1 , 2 or 3 wherein the buffer is pure silica. 5. A method of producing a cladded waveguide pair by PECVD comprising depositing a buffer layer (5) on a substrate (4), depositing a pair of spaced waveguide cores (2) on the buffer layer, and producing a layer of cladding (6) on and near the core pair including the interval (3) between the pair, wherein the cladding is produced in multiple stages, each stage including a deposition step followed by step of annealing in which the deposited material is flowed. 6. A method according to claim 5 wherein the layer of cladding includes BPSG silica glass consisting of 2.4% phosphorus and 3.4% boron. 7. A method according to claim 5 or 6 wherein each annealing step includes flowing 1000°C for one hour.

Inventors: DAY STEPHEN, GRANT MICHAEL FRANCIS, DAY, STEPHEN, GRANT, MICHAEL, FRANCIS
Applicants: DAY STEPHEN, GRANT MICHAEL FRANCIS, NORTHERN TELECOM LTD, DAY, STEPHEN, GRANT, MICHAEL, FRANCIS, NORTHERN TELECOM LIMITED
Application year: 1997
Main classification: G02B6
IPC codes: G02B6