4.6 [micro sign]m-band difference frequency generation and CO isotopologue detection

Abstract

There are many strong absorption lines of environmental gases in the mid-infrared region in atmospheric air, including those of greenhouse effect gases. Reported is difference frequency generation in a directbonded periodically poled LiNbO 3 ridge waveguide in the 4.6 mm band, which is the most appropriate wavelength range for observing carbon monoxide absorption spectra. It is also shown that isotopologue absorption lines can be detected using the output light generated in a waveguide with mature telecommunication band laser diode light sources. The direct-bonded ridge waveguide consisted of a 50 mmlong chip and its wavelength conversion efficiency was approximately 4%/W. Introduction:The mid-infrared region and the 2 -5 mm bands are particularly useful for environmental gas detection. The strong absorption lines of various gases can be observed in these wavelength regions. These absorption lines are fundamental and well-known ro-vibration transitions based on the normal vibrations of individual gas molecules. These lines have been observed with quantum cascaded lasers [1] as well as difference frequency generation (DFG) laser sources [2 -5]. DFG light sources using periodically poled LiNbO 3 (PPLN) ridge waveguides (WGs) can be constructed with mature telecommunication band laser diodes (LDs). This makes them compact, having highly quantum conversion efficiency, capable of CW-operation at room temperature. Moreover, their wavelengths are easy to control precisely, and their outputs simple to amplify. Since the idler beam of the DFG output in the mid-infrared range can be generated with near-infrared pump and signal beams, the input near-infrared beams can be used as mid-infrared optical axis adjustment guides. This Letter describes 4.6 mm-band DFG in a direct-bonded 50 mm-long PPLN-WG with a 1.03 mm LD as a pump and a 1.3 mm external cavity laser diode source (ECLD) as a tunable signal. The idler output is utilised as a mid-infrared light source to observe the absorption spectrum of carbon monoxide isotopologues, which in this case are 12 CO and 13 CO. The result implies that it is easy to apply this approach to the detection of ambient CO and the sensing of CO partial combustion resulting from oxygen deficiency in the atmosphere.

48 μm difference-frequency generation using a waveguide-PPLN crystal and its application to mid-infrared Lamb-dip spectroscopy

“…DFG conversion efficiencies of 35%∕W at 3.4 μm [17] and 69%∕W at 3.8 μm [18] were obtained with WG-PPLN, where the absorption coefficient of PPLN is less than 0.03 cm −1 . Recently, 300 nW output was achieved at 4.6 μm with ∼4%∕W conversion efficiency [19].In this Letter, we report DFG of 4.8 μm using WG-PPLN and its application to Lamb-dip sub-Doppler rovibrational spectroscopy of carbonyl sulfide (OCS). Our WG-PPLN DFG system, which consists of two external cavity diode lasers (ECDLs) and tapered amplifiers (TAs) as pump sources, shows a conversion efficiency of up to 2%∕W at 4.8 μm and a linewidth of ∼2 MHz without any extra stabilization nor cavities by virtue of DFG process.…”

“…DFG conversion efficiencies of 35%∕W at 3.4 μm [17] and 69%∕W at 3.8 μm [18] were obtained with WG-PPLN, where the absorption coefficient of PPLN is less than 0.03 cm −1 . Recently, 300 nW output was achieved at 4.6 μm with ∼4%∕W conversion efficiency [19].…”