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Tunable filter relies on liquid crystals

08 Feb 2006

Meadowlark Optics says its tunable liquid-crystal based filter can switch its transmission characteristics in around 100 milliseconds.

Applications as diverse as solar astronomy, hyperspectral imaging and fluorescence microscopy could all benefit from a tunable filter developed by US polarization expert Meadowlark Optics. Based on liquid crystal technology, the filter's transmission characteristics can be changed in real time by simply varying the applied voltage.

"You have the option to scroll around the whole spectrum or zoom in on a narrow part of the spectrum depending on the model of filter," Paul Searcy, Meadowlark's director of R&D told Optics.org. "The user just sends one command to the filter and it hops to the new wavelength. The transmission pass band of the filter is not fixed at a centre wavelength."

Using a low voltage signal to change the filter's pass band also reduces mechanical disturbances. As Searcy explains, the product essentially replaces large sets of interference filters and removes the need for a rotating filter wheel that generates unwanted vibration on an optical bench.

Searcy says Meadowlark's key breakthrough is its proprietary combination of liquid crystals, polarisers and waveplates. "By designing and stacking different stages, made from a polariser, a waveplate and then a second polariser, and carefully choosing the liquid crystals, we can extend the filter's range over 700 nm or more while still maintaining a narrow pass band," he said.

For example, Meadowlark offers a standard filter with a wavelength range from 420 to 1000 nm, a bandwidth of around 5 nm, a tuning resolution of 0.1 nm and a switching speed of less than 100 milliseconds.

One area which could benefit from the device is hyperspectral imaging. Controlled by a USB or serial interface, the filter would sit in front of a detector and switch to new wavelengths when required. "Rather than having multiple CCDs and filters, you can have a tunable filter and a single CCD," said Searcy. "The filter has a transmission at the peak wavelength of up to 50% for polarised light."

Meadowlark launched this product at Photonics West, which was held in San Jose last month.

Jacqueline Hewett is technology editor on Optics.org and Opto & Laser Europe magazine.

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