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Teflon reduces loss in microfiber resonators

07 Sep 2007

optics.org discovers why researchers in the UK are embedding microfiber coil resonators in a thin layer of Teflon - and it’s nothing to do with cooking.

Embedding a microfiber coil resonator (MCR) in Teflon overcomes the problems of stability and reliability seen when such structures operate in air, say researchers at the UK's Optoelectronics Research Centre in Southampton. Thanks to the unique optical and mechanical properties of the Teflon-coated devices, they have the potential to be crucial elements in future micro- and nano-photonic devices. (Optics Letters 32 2164)

"A considerable fraction of the transmitted power can propagate in the evanescent field outside of a nanowire," researcher Fei Xu told optics.org. "Light can be confined to a very small area over a long length to observe nonlinear interactions. Nanowires can also be bent and manipulated whilst remaining relatively strong mechanically."

An MCR is a coiled singlemode optical microfiber where the diameter of the microfiber and the distance between the adjacent turns are comparable with the wavelength of the transmitted radiation. It also benefits from low bend loss and low losses at the fiber's input and output.

Previous MCRs, such as a knot or self-coupling design where the coils of the resonator are in contact, have not been stable in air. "The loss of these microfibers would increase with time in air," said Xu. "This entire problem can be solved by coating the MCR in a low-loss low-index material like Teflon."

Xu and colleagues make their microfibers using the flame brushing technique. This involves moving a small flame under an optical fiber while it is being stretched by two translation stages. "We use a microfiber with a diameter of approximately 2 microns and the length of the uniform waist is around 3.5 mm," commented Xu.

The next step is to wind the microfiber around a silica-based rod three times. The outer surface of the rod is pre-coated with a thin Teflon fluoropolymer resin. To embed the resonator, the rod is covered with an additional layer of Teflon, which takes around 20 minutes depending on the volume and concentration of the Teflon solution.

Each end of the microfiber was terminated in a pigtail that in turn was connected to an erbium-doped fiber amplifier and an optical spectrum analyser. This allowed the team to monitor the resonator properties during fabrication and embedding in real time over wavelength interval of 1525 to 1535 nm.

"A resonance extinction ratio greater than 9dB, a free-spectral-range of 0.8 nm and a Q factor in excess of 6000 were observed for the embedded resonator," concluded Xu. "We are now trying to produce a higher Q factor resonator. If a disposable material is used for the rod, it can be dissolved leaving a sensor with an intrinsic channel."

Jacqueline Hewett is editor of Optics & Laser Europe magazine.

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