The Sensitive Infrared Signal Detection by Sum Frequency Generation

Teh-Hwa Wong, Jirong Yu and Yingxin Bai 1 Science System and Applications Inc. One Enterprise Parkway, Hampton, VA 23666 2 NASA Langley Research Center, MS. 468, Hampton, VA 23681 Abstract An up-conversion device that converts 2.05-μm light to 700 nm signal by sum frequency generation using a periodically poled lithium niobate crystal is demonstrated. The achieved 92% up-conversion efficiency paves the path to detect extremely weak 2.05-μm signal with well established silicon avalanche photodiode detector for sensitive lidar applications. Introduction The near-infrared and mid-infrared wavelengths offer intrinsic advantages in remote sensing and lidar operations. The overtone and fundamental rovibrational spectrum of gas molecules are in this region, offering the opportunity to optically measure the concentration of these trace gases. Unfortunately, sensitive infrared photodetectors in this region are limited. Currently, the detector for the 2.05-μm signal is InGaAs PIN detector with extended wavelength capability that has no signal gain and limited NEP. In contrast, silicon avalanche photodiode detectors and single photon counting modules can operate at room temperature with much better detection efficiency and lower dark current in the visible region [1]. Our approach is to use high efficient frequency up-conversion device to convert the infrared light at 2.05-μm to visible/near-infrared signal at 700nm, and then to detect the 700nm signal by Si APDs. Thus, the weak 2.05-μm signal can be detected by well established high performance Si APD detectors. Experiment The schematic of the experimental setup for intra-cavity up-conversion is shown in Figure 1. One periodically poled 5 mol% MgO-doped congruent lithium niobate (PPLN) with 16.14 m grating space was used in this study. The 50 mm long PPLN crystal is located inside a Teflon oven, which is mounted on top of a multi-axis stage. A CW 808 nm diode laser is used to pump a Nd:YAG rod inside the cavity to generate the 1064 nm beam. The leakage of 1064 nm light through M2 was used to monitor the circulating intra-cavity pump power. The 150 mm radius-of curvature mirrors M3 and M4 serve as the input (output) for the probe (signal) laser. A twomicron DFB laser was also aligned through the PPLN and the power was measured at the back of the M4 mirror. A CaF2 dispersion prism and a laser line filter were used to separate the 700 nm and block the other wavelengths for accurate measurement.