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Twisted optical fibres: it's time to think chiral

03 Jun 2009

Taking inspiration from the helical-shaped designs found in nature, Chiral Photonics is adding a new twist to optical technologies. Dan Neugroschl, the company's president, tells Marie Freebody about the unique ways in which light interacts with chiral structures.

How did the company begin?

The company was formed in 1999 when physicists Azriel Genack and Victor Kopp at City University of New York, US, discovered and explained lasing in cholesteric liquid-crystal (CLC) films. CLCs are thin-film materials that are often used to fabricate fish-tank thermometers or mood rings, which change colour according to temperature. The molecules in CLCs are arranged in a helical or chiral arrangement and, as temperature varies, so does the pitch of the helical structure, which thus reflects different wavelengths of light.

Genack and Kopp decided to mimic the self-assembled structure of CLCs using tiny lengths of twisted fibre. The company now produces sensors, filters and interconnects and has accumulated more than 30 patents based on chiral photonics.

What is chiral photonics?

Chiral photonics stems from the unique way in which light interacts with chiral or helical structures such as twisted optical fibres. At Chiral Photonics we have built microforming towers that allow us to fabricate devices based on fibres that can be twisted through more than 25,000 revolutions over a 1 inch length. The density of twists per inch (or periodicity) determines the extent of filtering, polarizing or scattering of light and results in three categories of chiral fibres.

Chiral gratings with a pitch of the order of 100 µm (known as chiral long-period gratings) couple light into the cladding of the optical fibre and are the basis for our sensor technology.

Chiral gratings with a pitch of the order of 10 µm (known as chiral intermediate-period gratings) scatter a portion of the light out of the fibre and are the basis for our polarizer and isolator technology.

Chiral gratings with a pitch of the order of 1 µm (known as chiral short-period gratings) reflect light within the fibre core and are useful for filtering and lasing.

The gratings can target specific wavelengths determined by their helical pitch. The gratings also have a handedness, much like a screw. This handedness can be used to selectively determine how they interact with a specific polarization of light. Unpolarized light contains a mixture of left and right circularly polarized light. Only circularly polarized light with the same handedness as the chiral structure will "feel" the structure and be scattered backwards, into the cladding or out of the fibre.

For example, a right-handed intermediate-period grating will scatter only right-handed circularly polarized light. This will leave left-handed light to be transmitted. This polarization selectivity, and the ability to simply convert circular to linear polarizations via the same chiral structures, is the basis for non-absorbing polarizers and polarization-selective lasing.

What are the advantages of chiral photonic structures?

Chiral structures enable in-fibre devices that can displace discrete optical elements such as lasers, filters and sensors. These devices improve system transmission efficiency and robustness and ease integration. What's more, the devices are manufactured using an automated and scalable process, which, for example, promises lasers that are a fraction of the cost and three times more efficient than today's semiconductor lasers.

What are the main applications?

Our spot-size-converting interconnects are being used by leading telecoms and datacoms companies to directly couple sub-micron-sized waveguides to standard optical fibres. These interconnects exhibit low loss (less than 0.5 dB), preservation of polarization and channel spacing below 25 µm and obviate the need for on-chip spot-size conversion, which takes up precious space. They also eliminate the air gap required for typical lens-fibre coupling, which compromises package stability, especially for waveguides with small mode-field sizes.

The company also offers a temperature sensor that can be used at temperatures up to 1100 °C without degradation. The sensors are based on pure silica fibres and are being used for turbine and combustion engine development and monitoring; geothermal, oil and gas applications (both borehole and refining processes); materials processing, such as metal smelting and welding; and solar collection.

Our in-fibre chiral polarizers exhibit excellent broadband performance, bend-insensitive stability across a broad temperature range and an ability to address a wide range of wavelengths. These polarizers are used for sensing applications – including current, biomedical, gyroscope and pressure sensors – as well as telecoms applications.

Chiral Photonics also offers twisted capillary tubes, which can be used for the study of protein molecules. Passing the protein through a twisted channel allows it to unfold and be imaged. What's more, the rotation of the protein as it passes through the channel enables 360° imaging. In addition, the capillary tubes have other microfluidic applications, including mixing and uniform heat exchange.

What can we expect to see from Chiral Photonics in the future?

One of the most troublesome and expensive components needed in the construction of high-power fibre lasers is the optical isolator. The current state-of-the-art is an in-line isolator that is composed of bulk optical components. The drawback of these isolators is that free-space facets in high-power applications can be easily burned – even with the slightest misalignment. These misalignments are especially hard to avoid where high optical power is present, as even minimal absorption can result in slight thermal excursions.

Chiral Photonics is developing an in-fibre chiral isolator that has no exposed facets. Although this is work in progress, we want to involve fibre laser manufacturers at an early stage as isolator pigtails are integral to the design and to that of the fibre laser into which it will be integrated.

We are also developing our first active component, a chiral fibre laser based on chiral short-period gratings, which will exploit the very efficient chiral feedback structure to enable a low-cost, single-polarization, single-frequency laser that can be used for sensing and high power, seed-laser applications.

• This article originally appeared in the June 2009 issue of Optics & Laser Europe magazine.

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