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Frequency Converter Enables Ultra-High Sensitivity Infrared Spectrometry

Single photon level spectroscopy for the elusive infrared region has been demonstrated in our quantum communications laboratory. As part of ITL’s Quantum Information Program, we have developed and demonstrated a new technology to measure the spectra in the near infrared (IR) region at single photon levels of light. Using the frequency up-conversion technology developed previously in the lab, we successfully implemented a highly sensitive spectral measurement system from 1,300 nm to 1,320 nm. This new method can measure spectra for signals as weak as -126 dBm and is about three orders of magnitude more sensitive than existing commercial systems.

Single photon detection at near IR regions is very difficult due to the lack of high performance detectors. InGaAs and superconducting-based detectors can be operated in this region but they have some significant drawbacks. The readily available silicon-based avalanche photodiodes (Si-APDs) do not work at these wavelengths, although they are very efficient in the visible region. Two years ago, We, the ITL Quantum Key Distribution (QKD) team developed an up-conversion detector that uses a periodically poled lithium niobate (known as PPLN) waveguide to convert photons at NIR (1310 nm) to the visible region (710 nm) using a strong pump light at a fixed wavelength (1550 nm). The converted photons can then be detected by using a Si-APD with a very high efficiency and low noise. The converter was used in the QKD system for transmission of photons over telecom fibers. OPTICS EXPRESS Vol. 15, No. 12, 7247 (2007).

Based on this up-conversion technology, we have further developed a new method to measure spectra at single photon level in the near infrared region. During the frequency conversion process, the wavelength and polarization of the signal photons are strictly selected by the energy conservation condition in the PPLN waveguide. By using a narrow band, but tunable pump source, the system can scan an IR region and convert a narrow band of IR photons to visible photons to obtain a spectrum as the pump wavelength scans by.

This technology provides a new and better tool for spectral measurement in the near infrared region. In comparison with conventional spectrometers, the technology shows unique advantages, including significantly reducing insertion loss and increasing throughput efficiency; the ability to measure spectra with sensitivity of at about three orders of magnitude higher than that of commercial spectrometers; the ability to be built for both polarization-sensitive and non-polarization-sensitive operations, which can be useful and convenient in some special cases; and the system can be very compact. It is believed that this technology will be utilized in many other fields. See here for more information.