Application Description

Tunable diode laser absorption spectroscopy (TDLAS) is one of the most common techniques to analyze the properties and constituents of gases such as concentration, temperature, pressure, and flow velocity. TDLAS measures the wavelength-dependent absorption of light through a gas medium. As the name implies, the technique usually employs a tunable-wavelength diode laser as the light source.  When the wavelength of light matches one absorption line of a gas species present in the sample, the photodetector records a reduction in the light intensity. When the gas concentration is ultra-low, the induced change in transmission becomes extremely small and difficult to detect sufficiently fast. The sensitivity of TDLAS is significantly enhanced by modulating the current of the laser, which leads to a modulation of the wavelength and of the light intensity. The information about the absorption response is then recovered by demodulating the signal from the photodetector at the modulating frequency and at its second-order harmonic.

Measurement Strategies

The diode laser is driven by a DC signal with a superimposed small AC component at a frequency between a few kHz and a few MHz. When the gas absorption lines are unknown or several absorption lines are under study, the laser frequency is swept across a wide range over a few seconds. Traditionally, two lock-in amplifiers are needed to acquire the signal from the photodetector and demodulate it at the modulation frequency or at its second-order harmonic; to enhance the signal-to-noise ratio (SNR), the amplitude of the AC signal – i.e., the demodulation depth – is optimized by sweeping the frequency when the DC signal is fixed. A data acquisition (DAQ) card is used to analyze the measurement signals from the lock-in amplifiers and from the driving signal.