Measuring acetylene concentrations using a frequency chirped continuous wave diode laser operating in the near infrared

Abstract

Two frequency chirped continuous wave diode lasers operating in the near infrared (IR) at wavelengths of λ ∼ 1.535 µm and λ ∼ 1.520 µm have been used to measure acetylene concentrations using the P(17) and R(9) rotational lines of the (ν₁ + ν₃) vibrational combination band. The diode lasers were frequency chirped by applying an electrical current pulse to the laser driver at a repetition rate of greater than 1 kHz. As the laser is operated at high repetition rates, more than 1000 spectra per second can, in principle, be acquired and summed, allowing fast accumulation of data, rapid averaging and consequent improvement of the signal to noise ratio and detection limit. Experiments were performed using a single-pass cell with a path length of 16.4 cm, and also an astigmatic multi-pass absorption cell aligned to give a path length of 56 m. Detection limits corresponding to minimum detectable absorption coefficients, α_min, of 5.6 × 10⁻⁵ and 7.8 × 10⁻⁸ cm⁻¹, respectively, were obtained over a 4 s detection bandwidth. These detection limits would correspond to mixing ratios of 21 parts per million by volume (ppmv) and 59 parts per billion by volume (ppbv) of acetylene at 1 atm in air, with the deleterious effects of pressure broadening accounted for. The single-pass cell was used to perform breakthrough volume (BTV) experiments for the low volume adsorbent traps used to pre-concentrate organic compounds in air, taking advantage of the capability of the system to measure concentrations in real time.