Optical waveguides – Planar optical waveguide – Thin film optical waveguide
Reexamination Certificate
2002-08-02
2004-04-06
Bruce, David V. (Department: 2882)
Optical waveguides
Planar optical waveguide
Thin film optical waveguide
C065S386000
Reexamination Certificate
active
06718109
ABSTRACT:
This invention relates to an optical waveguide with a multi-layer core and in particular to an optical waveguide with a composite core in which the consolidation temperature of a first core layer is above the softening temperature of a second core layer deposited thereon.
Planar waveguides are fabricated by forming several layers on top of a substrate, usually a silicon wafer. In the case of a FHD fabrication process, the layers which make up the waveguide are first deposited as a layer of fine glass particles or “soot”. Alternatively the glass can be deposited by a variety of other techniques, for example, plasma enhanced chemical vapour deposition (PECVD), low pressure chemical vapour deposition (LPCVD), which may be done in isolation or combination and further may be in combination with flame hydrolysis deposition (FHD).
In the case of the FHD process the soot layers are consolidated into denser glass layers, either individually immediately after each layer is deposited or several layers may be consolidated together. In the case of the other processes, although deposited as a glass a densification and/or desiccation procedure is often also employed. If a layer is heated to a sufficiently high temperature in excess of its consolidation temperature, the viscosity of the consolidated layer is reduced until eventually the glass is able to flow. When this occurs, surface irregularities can be removed as the surface of the layer is smoothed.
During fabrication of an optical planar waveguide, it is know to consolidate a core layer using a temperature cycle in which at one stage the layer is heated to the “softening” temperature, which is significantly higher than the actual consolidation temperature. This enhanced temperature stage ensures that the glass forming the core layer is sufficiently softened at its top surface for the consolidated core layer to flow and form a relatively smooth and level layer.
The smoother the surface of a waveguide the less light is scattered at the surface; heating a layer to its softening temperature for a period of time is therefore desirable if a high-quality waveguide is to be fabricated. However, to ensure that the underlying layers are not deformed during the consolidation and/or softening of subsequent layers, the consolidation and softening temperatures of each subsequent layer are usually less than the softening temperature of the underlying layer.
In order to achieve a suitably smooth core layer upper surface, without reaching temperatures which exceed the consolidation temperatures of the underlying layers and/or which could cause thermal deformation of the waveguide's substrate, it is usually desirable to introduce selected dopants into the core layer during the deposition stage.
In the present invention the composition of the glass forming the lowest core layer is thus selected so that its refractive index is close to that of the overlying core layer(s) whilst its consolidation temperature is greater than the softening temperature of the topmost overlying core layer. Similarly, the cladding formed around the core layers and under the core layers must have the correct thermal characteristics to ensure that the core is not deformed during fabrication of the waveguide.
As a consequence all layers (buffer if employed, core and cladding) must be deposited with decreasing consolidation temperature and sufficient buffer in between each. In addition, the maximum consolidation temperature allowed, typically for the core layer, is limited to ~1360° C. by the onset of striations and implosions due to the silicon substrate.
The selected core dopants lower the temperature at which the top surface of the core layer begins to f low. For example, dopants such as boron, phosphorous and/or titanium ion species may be introduced into germano silicate glass during the deposition stage in selected quantities to give the desired properties, for example; the right thermal characteristics, refractive index and coefficient of expansion. Other co-dopants could include tantalum, aluminium, lanthanum, niobium and zirconium. Germano silica based core glass is the preferred example but germania may not be necessary in all cases.
The invention seeks to provide several advantages in the fabrication of an optical waveguide. The waveguide according to the invention has a composite core in which a first layer comprises a glass whose consolidation temperature is close to the maximum allowed (~13600° C.). A “skinning” layer is then deposited on top of the underlying core layer(s) whose thickness is only of the order of ten percent of the thickness of the underlying core layer(s). Generally, the “skinning” layer has a much increased dopant concentration but match the refractive index of the underlying core layer(s). This uppermost “skinning” layer typically has a consolidation temperature ~50° C. less than the consolidation temperature(s) of the underlying core layer(s). The uppermost “skinning” layer fully consolidates and, due to its softening temperature being lower than the consolidation temperature(s) of the underlying core layer(s), is further softened. This promotes a surface “skinning effect” which gives rise to a low surface roughness. The region of increased dopant is thus minimised, and is located, for example, at the edge of the waveguide core where the optical field of the guided mode is minimised: the impact of any density fluctuations is thus reduced.
In order to ensure that both the consolidation and the softening temperatures of the core layer are sufficiently low, the core layer needs to be quite heavily doped. At such high levels of concentration, the dopants are more susceptible to non-uniform distribution within the core layer, and this results in the core layer exhibiting an undesirably high level of density fluctuations. The presence of density fluctuations affects the consistency of the refractive index across the layer, which should be as uniform as possible if the waveguide is to be used in large scale applications, for example, such as an array waveguide grating. The minimisation of such density fluctuations is particularly desirable in the fabrication of large-scale waveguides, for example, waveguides whose dimensions are in excess of 2×2 &mgr;m
2
.
Furthermore, when cladding the core, since the volume of the softer core glass is minimised a closer match in consolidation temperature between the clad and core layers can be employed before significant deformation of the core layer is observed.
During the consolidation phase, there is a reduction in surface area whilst at the same time an increase in density of the deposited layer. Necking between the deposited soot particles forms an open network with pores, which subsequently densifies with closure of the pores. Thus, it is essential that the consolidation conditions employed ensure that the lower viscosity uppermost (or “skinning” layer) does not consolidate prematurely.
Poor consolidation conditions may give rise to gas trapping problems which would damage the consolidating layer(s). To mitigate this, a suitable consolidation ramp temperature rate, such as for example 5° C./min, may be used which enables the consolidating layer to be formed bubble free. He gas can also be used as it aids sintering by promoting core collapse.
The present invention seeks to obviate or mitigate the aforementioned disadvantages by providing a waveguide with a multi-layer core which has a uniform refractive index and a smooth uppermost surface.
A first aspect of the invention seeks to provide an optical waveguide with a multi-layer waveguide core, the waveguide comprising:
a substrate;
a waveguide core formed on the substrate; and
at least one upper cladding layer embedding said waveguide core, the waveguide core having a composite core layer comprising:
a first core layer with a softening temperature T
1S
formed on the substrate; and
at least one other core layer formed on the first core layer, wherein the softening temperature T
2S
of at least one of said at least one other core layers is less than the
Bonar James Ronald
Ian Laming Richard
Alcatel Optronics UK Limited
Artman Thomas R
Bruce David V.
Sughrue & Mion, PLLC
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