Raman Noise Measurement in Periodically Poled Lithium Niobate at Telecommunication Wavelengths*

Frequency up-conversion is important for long-distance quantum communication where transmitted photons need to be at telecommunication wavelengths to minimize loss in optical fibers, but sources, memories and detectors may be optimized at other wavelengths. However, Raman noise can be problematic in frequency up-conversion processes since it introduces unwanted noise that can degrade the quality of the up-converted signal. Such Raman noise can reduce the signal-to-noise ratio (SNR), and cause efficiency loss in the up-converted signal. We measure the Raman noise spectrum in periodically poled lithium niobate (PPLN) using a pump at 1590, 1595, and 1600 nm. Anti-stokes Raman peaks were observed between 1553 nm and 1563 nm. The results show that a frequency conversion pump set at 1595 nm produces minimal Raman noise at a 1550 nm signal, so that a quantum signal at 1550 nm could be converted to 786 nm while achieving a high SNR.