Application of a continuous-wave tunable erbium-doped fiber laser to molecular spectroscopy in the near infrared

Development of a continuous-wave tunable fiber laser-based spectrometer for applied spectroscopy is reported. Wide wavelength tunability of an erbium-doped fiber laser (EDFL) was investigated in the near-infrared region of 1543-1601 nm. Continuous mode-hop free fine frequency tuning has been accomplished by temperature tuning in conjunction with mechanical tuning. The overall spectroscopic performance of the EDFL was evaluated in terms of frequency tunability along with its suitability for molecular spectroscopy. High-resolution absorption spectra of acetylene (C2H2) were recorded near 1544 nm with a minimum measurable absorption coefficient of about 3.5x10(-7) cm(-1)/Hz(1/2) for direct absorption spectroscopy associated with a 100-m long multipass cell. Detections of C2H2 at different concentration levels were performed as well with high dynamic detection range varying from 100% purity to sub ppmv using cavity ring down spectroscopy. A 3 sigma-detection-limited minimum detectable concentration (MDC) of 400 ppbv has been obtained by using the transition line P-e(22) of the nu(1)+nu(3)+nu(1/5) (Pi(g))-nu(1/5)(Pi(u)) hot band near 1543.92 nm with a detection bandwidth of 2.3 Hz. This corresponds to a minimum detectable absorption coefficient of 6.6x10(-11) cm(-1)/Hz(1/2). The sensitivity limit could be further improved by almost one order of magnitude (down to similar to 60 ppbv) by use of the P-e(27) line of the nu(1)+nu(3) (Sigma(+)(u))-0(Sigma g(+)) combination band near 1543.68 nm.