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Waveguide boosts spectrometer sensitivity

01 Dec 2008

A waveguide that converts light from the near-infrared part of the spectrum to the visible is allowing scientists to improve the sensitivity of an infrared spectrometer.

Researchers at Stanford University, US, and Japan's National Institute of Informatics have created the first waveguide-based infrared spectrometer. According to the group, the spectrometer's increased sensitivity, room temperature operation and integration potential make it interesting for industrial applications (Optics Express 16 19557).

"Our waveguide-based spectrometer is two orders of magnitude more sensitive compared with commercial grating-based optical spectrometers at telecoms wavelengths," Qiang Zhang, a researcher at Stanford's Ginzton Laboratory, told optics.org. "What's more, unlike conventional grating-based spectrometers, our instrument does not require cryogenic cooling and can be made very compact."

Infrared spectrometers are typically composed of a diffraction grating and an InGaAs linear array for photon detection. The sensitivity of such spectrometers is limited to around -90 dBm at telecommunication wavelengths, which is caused by the large dark current of InGaAs arrays.

The approach adopted by Zhang and colleagues was to convert photons from the near infrared range (NIR) to the visible, where more sensitive optical detectors can be used. By up-converting the photons, the team is able to use a silicon single photon detector in its setup instead of an array of detectors. This not only increases sensitivity but also reduces the cost and complexity of the system.

Up-conversion was achieved using sum-frequency generation in a periodically poled lithium niobate (PPLN) waveguide. In this process, two low-energy photons are combined to get one high-energy photon.

"We used sum frequency generation in a PPLN nonlinear waveguide to up-convert photons from the NIR to the visible range," explained Zhang. "We add together the frequencies of a 1550 nm pump beam and a 1310 nm signal beam to achieve visible light at 702 nm. This up-converted light is then detected by a single photon counting module."

The team used a 1.3 µm Fabry-Perot laser diode as the signal beam. A C-band external-cavity tunable diode laser with a 1 pm linewidth, amplified by an erbium-doped fibre amplifier, was used as the pump source.

Although the group has no current plans to commercialize its spectrometer, it hopes to continue to improve the resolution and reduce the cost of the device.

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