Mid-Infrared Saturated Absorption Spectroscopy Inside the Hollow Fiber
D.N. Patel1*, Tzu-Han Su2, Hsiang-Chen Chui2, Jow-Tsong Shy1,3
1Department of Physics, National Tsing Hua University, Hsinchu, Taiwan
2Department of Photonics, National Cheng Kung University, Tainan, Taiwan
3Institute of Photonics Technologies, National Tsing Hua University, Hsinchu, Taiwan
* presenting author:D N Patel, email:dnp.iitk@gmail.com
Saturation spectroscopy is recognized as a powerful tool to observe the frequency precession far below the Doppler limit [1]. We have performed the mid-infrared Doppler-free spectroscopy inside the hollow fiber using a difference frequency generation (DFG) source. We have established a widely tunable mid-infrared DFG source using a 1 W tapered amplifier boosted external cavity diode laser (ECDL) system and a 10 W YDFA (ytterbium-doped fiber amplifier) boosted Nd:YAG laser system. The two laser beams from ECDL and Nd:YAG laser systems are mixed in a 50 mm long MgO doped periodically poled lithium niobate (MgO:PPLN) crystal to generate the DFG radiation over the tuning range of 3.97 to 4.71 µm. The maximum power of DFG source is about 3 mW. The iodine stabilization scheme is used to stabilize the Nd:YAG laser frequency and saturated absorption spectroscopy is performed on R(60) transition of CO2 fundamental (0001 ← 0000) band at 4.193 µm inside a hollow fiber of diameter 300 µm and length 100 cm while keeping the CO2 pressure at 100 mTorr. The observed spectrum is fitted with a combination of Gaussian and Lorentzian profile and the full width half maximum (FWHM) of the Lamb dip is reported to 4.56±0.23 MHz and Doppler width (FWHM) to 134.13±0.47 MHz which is in close agreement with theoretical width 132.11 MHz. We have also determined the line width (4.40±0.15 MHz) of Lamb dip in well agreement with the measured dip using the dependence of peak amplitude of the third derivative signal with respect to the modulation width [2]. The third derivative locking method is used to lock the ECDL frequency to a hyperfine transition of CO2. This study may be potentially used in the generation of compact mid-IR frequency standard source.
[1] T. W. Hansch, I. S. Shahin, and A. L. Schawlow, Nature-Phys. Sci. 235, 63 (1972).
[2] H.-M. Fang, S.-C. Wang and J.-T. Shy, Opt. Commun. 257, 76 (2006).

Keywords: Nonlinear Optics, Spectroscopy, Laser