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Advances in the structures and designs of thin optical films known as waveguides, essential components of photonic devices such as lasers, modulators, couplers, and optical sensors, are being made by researchers in Hong Kong.

At The Chinese University of Hong Kong, properties of sol-gel materials used in waveguides have been enhanced for possible applications in a number of fields, ranging from clinical medicine to wide bandwidth optical communications.
The introduction of high purity, organic and inorganic dopants into sol-gel material led to the development of a fibre optic oxygen sensor capable of operating in temperatures from -175OC to 200OC. The sensor takes advantage of the behaviour of an organic dye which phosphoresces strongly in the absence of oxygen, but with phosphorescence reducing in the progressive presence of oxygen.
It was also found that by doping sol-gel film with a dye of nonlinear optical characteristics, a substantial change in the refractive index occurred with the application of low-power continuous-wave argon ion laser.
The material could be attractive for use in optical signal processing components. Another enhancement was the preparation of high quality dye-doped titania-silica thin film distributed feedback waveguide lasers of variable refractive index.
Principal Investigator, Dr Dennis Lo, said: “The distributed feedback waveguide lasers appear ideal as transmitters for wideband communication, and communication requiring a large number of connections as in local area networks.”

A dye-doped sol-gel slab glows when excited by a blue laser

Researchers at the City University of Hong Kong, meanwhile, have produced a range of novel optical waveguide designs which solve a major problem caused by the polarization of light.
While light transmission characteristics of optical waveguides depend, in general, on the polarization state of light, the polarization state of output light from an optical fibre is random.
This means that when a photonic integrated circuit (PIC) such as a modulator is connected to an optical fibre, as in most practical applications, the output of the PIC may become unstable. Principal Investigator Prof Kin-seng Chiang and his team approached the problem by producing waveguide designs that are polarization insensitive.
For devices based on phase matching or optical interference, polarization independence is achieved when polarization modes of the waveguide propagate with the same phase velocity; with a so-called zero-birefringence waveguide. With an anisotropic crystal like lithium niobate, the optic axis of the crystal must be oriented precisely at a specific angle to the physical axis of the device, which brings difficulties in fabrication.
Prof Chiang believes he is the first scientist to point out that zero-birefringence waveguides can be designed with isotropic materials like glass and semiconductor material. Work is continuing with experimental waveguide designs and verifying their polarization-insensitive properties.

Principal Investigators
Dr Dennis Lo:
Prof Kin-seng Chiang: