The Dispersion Reduction Frequency Upconversion System at 1550 nm With Tightly Focused Beam

The upconversion detection is a promising method to detect infrared radiation. It is proved that the focused beam makes a contribution to improving the upconversion efficiency. A near-infrared detector at room-temperature based on sum frequency generation(SFG) with focused beams is demonstrated. To enhance the upconversion efficiency, the signal and pump beams are tightly focused by the same focal lens. To reduce that the dispersion of the focal lens has a negative effect on upconversion efficiency in a tightly focusing system, the relation between the upconversion efficiency and the focused beams in our system is investigated. In the setup, the signal and pump beams are combined by a polarization maintaining wavelength division multiplexer (PMWDM) that is connected on a collimator. The mixed beam is focused into a periodically poled lithium niobate (PPLN) bulk by a lens of 35 mm focal length. The signal pulse at 1550 nm is converted to 863 nm with a 172 mW continuous-wave pump beam at 1950 nm. The converted signal is measured by a photomultiplier tube(PMT) and results shows the maximum upconversion efficiency is
$5.23 \times 10^{-4}$
while the pump power is 172 mW.