Efficient broadband difference frequency generation in a direct-bonded periodically-poled LiNbO 3 waveguide and the observation of CO isotopomer absorption from 2.3 to 2.45 μm

Abstract

Efficient difference frequency generation (DFG) is obtained in the 2-μm region by using a direct-bonded periodically-poled LiNbO 3 (PPLN) ridge waveguide. The direct-bonding technique can utilize the bulk LN characteristics, which provide certain advantages including no additional absorption, precise device design and reproducible device fabrication. We achieved a conversion efficiency of 100%/W in the fabricated waveguide with a 0.94-μm pump laser diode (LD) and a 1.5-μm band tunable signal LD source. We also achieved a wide tunable range of over 0.1 μm in a 50-mm-long waveguide with a single-pitch PPLN at a constant temperature. This is because the DFG bandwidth is decided by the phase mismatch Δk . Generally, Δk=0 is only obtained at a certain wavelength, however, the Sellmeier equation shows that Δk~0 is easy to realize in the 2-μm region when the pump is set at 0.90-0.96 μm. Subtle Δk changes around 0 realized group velocity matching and a broadband output could be obtained. Compact and broadband tunable light sources are expected to be used for trace gas sensing in the near to medium infrared regions. This report also describes the bundle observation of carbon monoxide isotopomer absorption lines. The DFG output bandwidth is sufficient to observe 12 CO and 13 CO simultaneously. The absorption lines of the P and R branches for each gas are clearly observed between 2.30 and 2.45 μm. DFG in the 2-μm region using direct-bonded PPLN ridge waveguides is a promising approach for opening up new broadband applications.