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  • May 2012

Optics InfoBase > Spotlight on Optics > Characterization and modeling of Bragg gratings written in polymer fiber for use as filters in the THz region

Characterization and modeling of Bragg gratings written in polymer fiber for use as filters in the THz region

Published in Optics Express, Vol. 20 Issue 9, pp.9564-9571 (2012)
by Shu Fan Zhou, Laurence Reekie, Hau Ping Chan, Yuk Tak Chow, Po Sheun Chung, and Kwai Man Luk

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Spotlight summary: Working in the near-infrared part of the electromagnetic spectrum has allowed photonics researchers to significantly simplify their development of complex and advanced setups. Transferring spatially and temporally coherent electromagnetic radiation from the source of generation to another location is as simple as connecting the source (typically a laser) to a standard optical fiber, which can transmit the radiation over as long as one kilometer with less than 5% loss. Naturally, the fiber does not need to be kept straight, but can be bent considerably, so that the fiber can be connected to any other piece of equipment in the lab without wasting time on careful alignment of the optical components. Apart from simply moving the radiation from point A to point B, components are available that split up the radiation in, say, a 50/50 ratio between two arms, but it is also possible to combine the radiation from two arms into one fiber and thereby easily make interference experiments. Another part of the researchers’ hassle-free toolbox is spectral filtering built right into the optical fiber, by writing a Bragg grating in the fiber. This allows one to, e.g., easily remove a specific narrow part of the spectrum, or it can be used to sense changes in the surrounding environment of the fiber Bragg grating by detecting changes in the peak reflectivity wavelength.

The situation is markedly different for researchers working in other parts of the electromagnetic spectrum, e.g. terahertz radiation (~0.1-10 THz). This spectral region is interesting due to its use for spectroscopic analysis in e.g. materials research, as many systems of interest have characteristic lifetimes on the picosecond scale. Security systems are also often mentioned as a potential application by using THz-imaging to visualize metal objects through clothes, and THz spectroscopy to detect the presence of specific chemicals in a box, be it drugs or explosives, without even opening the package.

So far, researchers working with THz radiation do not yet have all the near-infrared equivalents of photonic components to simplify their development of complex setups. However, bendable “fibers” (some mm thickness) with relatively low loss have been recently demonstrated made of the polymer Topas (Nielsen et al., Opt. Express 17, p. 8592, 2009). The present work by Zhou et al. adds another essential component to the toolbox of THz-researchers: a fiber Bragg grating that can be used for selective frequency filtering or sensing.

The fiber is first fabricated by extruding the polymer Topas to a fiber of either 0.3 or 0.6 mm diameter. The grating is then written in the fiber by simply using a UV laser to ablate notches into the side of the fiber with a pitch of either 361 µm or 509 µm. The researchers demonstrate that one of their Bragg gratings has a 60 dB reflectivity at 0.265 THz.

This work allows THz-researchers to make selective frequency filtering in a relatively low loss waveguide which also tightly confines the radiation. The tight confinement allows strong interaction of the electromagnetic field with a small sample of e.g. a liquid in which the grating is immersed, thus enabling development of new exciting sensing applications in the THz-regime.

--Michael Henoch Frosz

Technical Division: Optoelectronics
ToC Category: Fiber Optics and Optical Communications
OCIS Codes: (250.5460) Optoelectronics : Polymer waveguides
(060.3735) Fiber optics and optical communications : Fiber Bragg gratings
(300.6495) Spectroscopy : Spectroscopy, teraherz

Posted on May 10, 2012

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