Tunable, fiber coupled spectrometer based on difference-frequency generation in periodically poled lithium niobate

A Yb-fiber amplifier seeded by a 1083 nm DBR diode laser is fiber coupled with an external cavity diode laser (814 nm to 870 nm) and differencefrequency mixed in a periodically poled lithium niobate crystal (3.25 μm to 4.4 μm). Phase matching characteristics and the spectroscopic detection of several trace gases using this compact sensor will be reported. OCIS code: (190.2620) Frequency conversion; (300.6340) Spectroscopy, infrared; Introduction The recent development of compact mid-IR sources using frequency-converted near infrared diode laser using periodically-poled lithium niobate has been demonstrated to be applicable for the sensitive, selective and real time detection of many trace gas species in the mid-IR spectroscopic "fingerprint" region [1-3]. The optical architecture however has been based on discrete optical elements and is therefore susceptible to vibrations and temperature drifts if operated in a non-laboratory environment. In this paper we describe the development of a fiber coupled difference-frequency mixing mid-IR source with inherent robustness, high conversion efficiency and wide tunablity, packaged in a portable suitcase. Sensor configuration This spectroscopic mid-IR source reported here is tunable from 3.25 μm to 4.4 μm. The device uses as pump sources an external cavity (814 nm to 870 nm) and a distributed Bragg reflector (DBR, P=50mW, λ=1083 nm) diode laser seeding a Ytterbium doped fiber amplifier pumped by a 975 nm, P=2 W diode laser (Fig.1)[4,5]. Both lasers are coupled into a single mode fiber and combined by a wavelength division multiplexer (WDM). The linear polarization output from the fiber for a e+e→e nonlinear mixing process in the PPLN crystal is maintained by using two polarization controllers in the fiber delivery system. Coarse frequency tuning of the ECDL is achieved by rotation of its diffraction grating relative to the optical axis of the laser cavity. Fine tuning and scanning of single or multi-component absorption lines of up to ~25 GHz is accomplished by current modulation of the DBR diode laser. The fiber amplifier boosts the seed pump power of 10 mW at 1083 nm to 540 mW. Quasi-phase matching and DFG tuning characteristics Firstly, an aspheric lens (f=8 mm; 0.5NA) was used for imaging the fiber output into the 19 mm