NIR improvements near

Spectroscopy, forensic science and even a future generation of quantum communication devices could benefit from the latest research at the National Institute of Standards and Technology (NIST) where a new, highly sensitive, and low-cost approach has been developed for measuring light in the near-infrared range.

Writing in the August issue of Optics Express, the NIST team explains that their approach could be used to measure, with high precision, specific wavelengths of near infrared light for telecommunications applications as well as the rather weak infrared signals emitted by fragile biomaterials and nanomaterials to the single-photon level in spectroscopy.

Lijun Ma, Oliver Slattery, and Xiao Tang wanted to develop a way to use existing detectors such as avalanche photodiode detectors (APD) in the NIR. APDs work very well for detecting visible light and are widely used, but are ineffective for the detection of near infrared. Nevertheless, a single photon detector could be a key device in the development of highly sensitive instruments for measuring NIR spectra.

Researchers in the wider field have made steady progress over the last three decades. They have increased the efficiency and sensitivity of visible and ultraviolet photon detectors but methods for detecting single photons in the near-infrared (NIR) range have generally faltered, NIST says. Moreover, the current methods available to NIR investigators are inefficient, slow, overwhelmed with static noise. Moreover, they often rely on expensive superconducting detectors that need low-temperature operating environments.

Tang and colleagues have now adapted a technique developed previously by NIST scientists for the field of quantum cryptography. The original technique uses a pump laser and a periodically poled lithium niobate waveguide as the conversion medium to "up convert" photons at one frequency to a higher frequency. In other words, the effect shifts near infrared photons up to the visible range. Once converted to visible light, the signal photons are easily detected by commercially available APDs.

The team studied the system's characteristics, monitoring sensitivity, dark count rate, spectral scan speed, signal transfer function of the waveguide, and polarization sensitivity. The overall single photon detection efficiency of the up-conversion detector is about 32%, they explain.

"Due to the laws of energy conservation and momentum conservation in the conversion process, only those photons that have certain polarization and wavelength, determined by the pump laser, can be converted to visible wavelengths," Tang told SpectroscopyNOW, "If a narrow-band tunable pump laser is used in this frequency-conversion process, the up-conversion detector becomes a spectrometer."

According to Tang, the new frequency-conversion approach to NIR detection lets them measure spectra with a sensitivity of more than a thousand times that of common commercial optical spectrometers. "With its ultra high sensitivity the spectrometer can measure spectra for signals at a level as low as -126 dBm," the researchers say, "We have demonstrated the spectrometers high sensitivity by measuring the spectrum of a greatly attenuated multimode emission from a laser diode at the 1310 nm band."

"Our key achievement here was to reduce the noise," explains Tang, "but our success would not have been possible without the many years of work by others in this field." He adds that the NIST team hopes that the discovery will lead to new methods for studying diseases, pharmaceuticals, secure communications and forensic science. "We are very excited to make this technology available to the larger scientific community," he says.