SPECTROMETERS: MEMS bring new order to diffraction gratings

Spectrometers using diffraction gratings have a built-in limitation. If the gratings produce diffraction orders that are close enough together, it is difficult to tell where one ends and the next begins. Device builders avoid this by limiting the free spectral range of the device.

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Spectrometers using diffraction gratings have a built-in limitation. If the gratings produce diffraction orders that are close enough together, it is difficult to tell where one ends and the next begins. Device builders avoid this by limiting the free spectral range of the device.

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In a grating with no voltage applied (top), three diffraction orders are apparent. When voltage is used to shift the grating, all the power is directed into a single order.

New microelectromechanical systems (MEMS) technology may overcome that limitation, says James Castracane, director of technology at the Center for Advanced Thin Film Technology at the State University of New York, Albany (Albany, NY). At Photonics West (San Jose, CA) in January, Castracane and his colleagues, Mikhail and Olga Gutin of InterScience (Troy, NY), discussed micromechanically controlled diffraction, which could prove useful for spectrometers, optical pressure sensors, and similar devices.

The researchers fabricated a MEMS compound grating (MCG), a device with an architecture similar to a conventional diffraction grating. Such a device can have one or more levels of polysilicon rulings, each 4 to 15 µm wide and 100 to 500 µm long. The rulings are over an array of electrodes, which, when switched on or off, adjust the position of the rulings to alter the diffractive effects of the grating (see figure). The rulings usually move over a space of 100 to 500 nm, depending on the wavelength the device is studying.

Voltage the key

"Just by applying a voltage to this device, you can change how the optical grating is placed," Castracane said. "Since you can redirect power into different orders, you can look at both a low- and high-dispersion spectrum without having to add gratings."

Normally, if diffraction orders overlapped, it would be difficult to distinguish the first order at 800 nm, say, from the second order at 400 nm. But with an MCG, "you can watch how the intensity in these orders changes as a function of voltage," Castracane said. "The first order will change twice as fast as the other one in intensity as you change the gratings."

Another advantage of the system is that the input and output angles could be set up so that the change in intensity would be out of phase in two different orders, even to the point that the power in one order would increase as the power in the second order decreased. Such a setup makes the sensor insensitive to fluctuations in source power, which means that fiber can be used for input and output without causing problems.

The gratings are being built and tested in the laboratory, with researchers examining various architectures. They are also working on integrating MCGs into commercial miniature spectrometers from Ocean Optics (Dunedin, FL). Castracane said the gratings are beyond the prototype stage but not yet at the commercial stage. He hopes to improve the number of gratings per wafer fabricated, for instance.

Neil Savage

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