Noise Reduction by Differential Detection for Mid-infrared Laser Spectroscopy

Differential detection technique was applied to mid-infrared laser spectroscopy with use of MOEMS-based wavelength swept/tunable quantum cascade laser. It was found that the noise reduction effect was 3.8 times higher than the method without differential detection. We demonstrated transmission spectrum measurement of methane (CH4) over 200 cm^-1 (7.14 μm to 8.34 μm).

1. Introduction

Mid-infrared laser spectroscopy has been applied various kind of gas sensing applications due to unique feature of transmission spectra called fingerprint region [1]–[2]. Recently, MOEMS-based compact external cavity quantum-cascade lasers (EC-QCLs) enabling broadband wavelength-swept exceeding 200 cm^-1 was developed [3]. Appearance of the developed EC-QCL was shown in Fig. 1. Thanks to this compactness, spectroscopy system can be integrated in small footprint to be a portable, and it is expected to be adapted to broad range of applications more than gas sensing such as healthcare application [4]. However, as a light source it is said that the EC-QCLs are inferior in stability of light output intensity compared to conventionally used light bulb of glowbar. In this paper, differential detection technique is applied to suppress noise generated from intensity fluctuation of EC-QCLs, and demonstration of wideband transmission spectrum measurement of methane over 200 cm^-1 is reported. Fig. 1.

Appearance of the developed MOMES-based EC-QCL packaged in a butterfly package. Its size is 12.7 × 30 × 13.1 mm excluded pins.