Rapid scan absorption spectroscopy using a waveform-driven electro-optic phase modulator in the 1.6-1.65 μm region

Abstract

A method is reported for performing fast optical frequency scans over a bandwidth of 36.9 GHz and at a sweep rate of 40 kHz using a single second-order sideband from an electro-optic phase modulator driven by an arbitrary waveform generator. Single sideband selection is accomplished using the resonator modes of a Fabry–Perot filter cavity having a finesse of ≈44 and a free-spectral range of 300 MHz. The finesse is sufficiently high to give <2% total transmission of the laser frequency carrier and all other nonresonant sidebands while sufficiently low to ensure on-resonance switching times as short as 100 ns. A frequency offset component of a diode laser is used for active stabilization of the laser to the filter cavity at all times eliminating frequency drift of the filter cavity transmission comb used for single sideband selection and scanning. The method is demonstrated for the detection of CO2 near 1602 nm and for CH4 lines near 1643 nm. Detection of ambient level concentrations of each of these gases is demonstrated in a 25 μs scan over a path length of 50 m at a sensitivity of ≈3×10−9 cm−1 Hz−1/2. The corresponding measurement uncertainties (k=1 or 1σ) in a (2–3) ms time period and a 1 km path length are <±2 μmol/mol (ppm) for CO2 and <±5 nmol/mol (ppb) for CH4. The arbitrary waveform control of the pulse sequence, repetition rate, and duty cycle provides for optimization of the light source for a variety of application areas that include path integrated differential absorption and differential absorption light detection and ranging.