Optical waveguides – Planar optical waveguide
Reexamination Certificate
2002-01-15
2004-05-11
Ullah, Akm Enayet (Department: 2874)
Optical waveguides
Planar optical waveguide
C065S386000
Reexamination Certificate
active
06735370
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a waveguide for an optical circuit, and a method of fabrication thereof.
The method relates in particular to the fabrication of a waveguide for an optical circuit with smoothed waveguide core boundaries. More specifically, the method relates to the fabrication of a rounded, elliptical or circular waveguide core by the isotropic diffusion of dopants in a core layer of a phosphosilicate waveguide wafer, such that the diffused core layer forms the circular waveguide core. In this manner, a core may be formed which is symmetric about the core axis.
This diffusion is thermally promoted either during the deposition of an uppercladding layer or by subsequent thermal processing of the waveguide wafer.
BACKGROUND OF THE INVENTION
The general process of fabricating a glass waveguide for optical circuits comprises forming at least one buffer layer, e.g. a thermal oxide layer, on a silicon wafer substrate. Additional buffer layers and/or at least one lower cladding layers may then be formed on top of the buffer layer. A core layer composed of a doped silica film is then formed on top of the buffer layer or lower cladding layer.
The core layer is then etched, for example, by reactive ion techniques, to form a square or rectangular waveguide or other suitable cross-sectional profile. The etched core is then embedded by an upper cladding layer. The core layer refractive index is usually higher than that of the surrounding layers. This concentrates the propagation of light in the core layer.
Planar channel waveguides are usually formed using dry etch methods to produce waveguides with square or rectangular cross-sections. Such angular waveguides have several disadvantages, in particular the geometrical mismatch between the waveguides and optical fibres in an optical circuit. The production of channel waveguides with a circular cross-section is particularly advantageous in that this increases the transmission efficiency between the waveguide and the rest of an optical circuit.
Channel waveguides are also susceptible to scatter loss (Mie scattering) due to imperfections in their sidewalls. This is reduced by smoothing the profile of the waveguide and this provides low propagation loss in the waveguides.
Circular optical waveguides are known in principle (for example, see Sun et al., “Silica-based circular cross-sectioned channel waveguides”, IEEE Photonics Technology Letters, 3, p.p. 238-240, 1991). Sun et al., disclose large dimension (~50 &mgr;m) GeO
2
doped silica waveguides which are reactive ion etched to form rectangular channel cross-sections. This method involves depositing a lower cladding layer with a reduced amount of Germanium doped silicon on to the wafer substrate prior to the deposition of a core layer. When the wafer is placed in the selective wet etch, the lower cladding layer is etched at a much faster rate to form a pedestal underneath the core region.
According to Sun et al., the waveguide can then be heated above the core softening temperature so that the surface tension of the glass functions to round the waveguide core. Such wet etching techniques are time consuming and moreover, do not offer truly circular cross sections as the core cannot be rounded at the interface between the core layer and the pedestal (i.e., the lower cladding layer lying directly beneath the core).
The current invention in contrast relies on the mobility of dopant ions in a square or rectangular etched core to migrate outwards into both upper and lower cladding layers. This forms waveguides which have substantially smoothed boundary walls, in particular the side walls are smoothed.
Further diffusion rounds the core region, and providing the diffusion is sufficiently isotropic the core region becomes sufficiently rounded to form a circular waveguide. This diffusion is thermally promoted either during the consolidation of the upper cladding layer or during subsequent thermal processing. By selecting the composition of the upper and lower cladding layers, the refractive indexes and consolidation temperatures can be chosen to modify the rate at which the core dopant ions diffuse into each layer and the elipticity of the resulting waveguide core accordingly adjusted.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, there is provided a waveguide for an optical circuit comprising:
a substrate;
a doped lower cladding layer;
a doped waveguide core formed on the lower cladding layer; and
a doped upper cladding layer embedding the waveguide core;
wherein the waveguide core includes mobile dopant ions which have diffused into the upper cladding layer and the lower cladding layer to form an ion diffusion region around said waveguide core such that the waveguide core boundary walls are substantially smooth.
Preferably, the waveguide further includes a buffer layer formed on the substrate and wherein the lower cladding layer is formed on the buffer layer. The substrate may comprise silicon and/or silica and/or sapphire. The buffer layer may include a thermally oxidised layer of the substrate.
Preferably, the buffer layer comprises doped silica.
Preferably, the thickness of the buffer layer is in the range 0.2 &mgr;m to 20 &mgr;m.
The lower cladding layer may comprise doped silica. The lower cladding layer may include at least one Phosphorus oxide and/or at least one Boron oxide.
Preferably, the lower cladding layer includes at least one Phosphorus oxide and at least one Boron oxide, wherein the Phosphorus oxide to Boron oxide ratio is such that the lower cladding layer refractive index is substantially equal to the refractive index of the buffer layer.
The lower cladding layer may include doped silica, at least one Phosphorus oxide and at least one Boron oxide, wherein the silica:Phosphorus oxide:Boron oxide ratio is in the range of 75 to 95 wt % silica:1 to 7 wt % Phosphorus oxide:4 to 18 wt % Boron oxide.
Preferably, the lower cladding layer has a silica:Phosphorus oxide:Boron oxide ratio in the range of 80 to 90 wt % silica:2.5 to 6 wt % Phosphorus oxide:7.5 to 14 wt % Boron oxide.
More preferably, the lower cladding layer has a silica; to Phosphorus oxide; to Boron oxide ratio of 82 wt % silica; to 5 wt % Phosphorus oxide; to 13 wt % Boron oxide.
Preferably, the thickness of the lower cladding layer is 1 &mgr;m to 20 &mgr;m.
The waveguide core may comprise doped silica. The mobile dopant ions of the waveguide core may include Phosphorus and/or Fluorine and/or compounds of these elements. Dopant ions of the waveguide core may include Phosphorus and/or Fluorine and/or Aluminium and/or Boron and/or Germanium and/or Tin and/or Titanium and/or compounds of these elements.
Preferably, the waveguide core includes Phosphorus oxide and/or Boron oxide. More preferably, the waveguide core comprises P
2
O
5
—SiO
2
.
Preferably, the refractive index of the waveguide core differs from that of the lower cladding layer by at least 0.05%.
Preferably, the waveguide core includes silica, and at least one Phosphorus oxide, wherein the silica to Phosphorus oxide ratio is in the range of 75 to 95 wt % silica to 5 to 25 wt % Phosphorus oxide.
More preferably, the waveguide core has a silica to Phosphorus oxide ratio of 80 wt % silica to 20 wt % Phosphorus oxide.
Preferably, the thickness of the waveguide core is in the range 2 &mgr;m to 60 &mgr;m.
More preferably, the thickness of the waveguide core is 6 &mgr;m.
Preferably, the lower cladding layer and the upper cladding layer refractive indices are substantially equal. The lower cladding layer and the upper cladding layer may comprise the same material.
Preferably, the waveguide core has a mobile ion dopant concentration higher than the mobile ion dopant concentration of the lower cladding layer or the upper cladding layer.
Preferably, the ion diffusion region is isotropic with respect to the waveguide core.
Preferably, the ion diffusion region surrounding the waveguide core forms a substantially rounded waveguide core.
More preferably, the rounded waveguide core is elliptical or circular in cr
Aitchison James Stewart
Bonar James Ronald
Da Silva Marques Paulo Vicente
Connelly-Cushwa Michelle R.
Lahive & Cockfield LLP
The University Court of the University of Glasgow
Ullah Akm Enayet
LandOfFree
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