DOI: 10.1007/s00340-003-1336-y Appl. Phys. B 78, 229–233 (2004) Lasers and Optics Applied Physics B m. takahashi s. ohara t. tezuka h. ashizawa m. endo s. yamaguchi ✉ k. nanri t. fujioka Trace gas monitor based on difference frequency generation at 4 μm using mass-production laser diodes as pump and signal light sources Department of Physics, School of Science, Tokai University, 1117 Kitakaname Hiratsuka, Kanagawa 259-1292 Japan Received: 29 October 2002/ Revised version: 16 September 2003 Published online: 10 December 2003 • © Springer-Verlag 2003 ABSTRACT New pump and signal laser sources for difference frequency generation (DFG) at 4 μ m are described. A laser diode with a 980 nm fiber Bragg grating and a 785 nm Fabry- Perot (FP) laser diode were coupled into an optical fiber and mixed in a periodically poled Mg-doped lithium niobate (PPMgLN) crystal, resulting in efficient mid-IR DFG. The DFG power was measured to be 0.23 μ W for a pump power of 5 mW and a signal power of 50 mW with a slope efficiency of 0.92 mW/W 2 . A Doppler-broadened absorption spectrum of N 2 O at 2485.2 cm −1 (3.927 μ m) was observed in a 0.1 m- long gas cell at a pressure of 133 Pa. The spectral linewidth of the DFG source was estimated to be 161 MHz (FWHM) for an averaging time of 700 ms. Real-time monitoring of N 2 O in a multipass cell with an optical path length of 36 m at a concen- tration level of 1 ppm was demonstrated. PACS 42.62.Fi; 42.72.Ai; 07.57.Hm 1 Introduction Highly sensitive and selective gas sensing tech- niques are required for various fields, which include envi- ronmental monitoring, industrial plant surveillance and in- dustrial process control. Infrared absorption spectroscopy is a convenient and useful technique since it can be applied to many gas species and is capable of fast response. Absorption spectroscopy using narrowband, near infrared (1.4–1.6 μ m) distributed feedback (DFB) diode lasers for telecommunica- tion which can generate power levels of tens of milliwatts, were investigated [1–3]. In the mid-infrared region, quantum- cascade lasers which can generate several hundreds of milli- watts (4.2–24 μ m) [4, 5], and laser frequency conversion in a periodically poled lithium niobate (PPLN) transparency re- gion (2 –5 μ m) have advanced greatly for the spectroscopic applications [6, 7]. Hence, absorption measurements of trace species presented in a multi-component gas mixture such as polluted air have been made using such mid-IR spectroscopic sources. In particular, the quasi-phase matched (QPM) dif- ference frequency generation (DFG) in PPLN is a convenient ✉ Fax: +81-463-50-20/3, E-mail: Shigeru@keyaki.cc.u-tokai.ac.jp spectroscopic source that can generate coherent light in the 2 –5 μ m wavelength region where fundamental ro-vibrational absorption spectra of many species of gases exist at room temperature. Moreover, the large absorption cross sections (20 –200 times larger than overtone absorption cross sections) in such a “finger-print region” are inherently advantageous for direct laser absorption spectroscopy. Excellent sensitiv- ity (∼ sub-ppb) and spectral selectivity (∼ 40 MHz) without active frequency control of QPM-DFG-based trace gas sen- sor have been demonstrated [8]. High DFG power levels of several hundred microwatts [9] were achieved with pump powers at the Watt level realized by injection-seeded, narrow- band DFB, distributed Bragg reflector (DBR) diode laser, or Yb and/or Er fiber amplifiers. Long-term detection perform- ance of the DFG-based gas sensor has also been investigated using an optical single mode fiber delivery system from the pump/signal laser sources to the PPLN crystal [10, 11]. The objective of this study has been to develop a DFG trace gas sensor suitable for the detection of six important gas species, namely N 2 O, NO, NO 2 , SO 2 , CO, and CO 2 , relevant to temporal concentration change of either automobile ex- haust emission or gas emissions from waste incinerators and furnaces, taking full advantage of the quick response of opti- cal monitors [12]. The absorption lines of the gas species are located between 3.46 μ m and 5.30 μ m and the range of these gas concentrations to be monitored is between ∼ ppm and 500 ppm with a response time of approximately 10 seconds. Spectral resolution of such monitors should be ∼ 100 MHz, which is better than those with conventional FTIRs (Fourier Transform Infrared Spectroscopy). In order to provide a prac- ticable and inexpensive DFG scheme, however, one of the critical issues is the choice of pump and signal laser source combination in terms of DFG wavelengths, frequency and power stability, spectroscopic resolution, and laser cost. If one considers using a signal laser at a wavelength of 980 nm, the system will be conveniently accessible in the desirable mid- infrared region by switching only pump lasers whose wave- lengths are from 764 nm to 827 nm. Such design consideration can permit the use of mass-produced lasers applied in com- mercial electronic devices for both pump and signal lasers of DFG spectroscopic systems. In this experiment, a high-power 980 nm LD with a fiber Bragg grating (FBG) is used as a sig- nal laser, whereas a 785 nm LD is selected as a pump laser for DFG source. However, these mass-produced lasers have