High speed engine gas thermometry by Fourier-domain mode-locked laser absorption spectroscopy.

We present a novel method for low noise, high-speed, real-time spectroscopy to monitor molecular absorption spectra. The system is based on a rapidly swept, narrowband CW Fourier-domain mode-locked (FDML) laser source for spectral encoding in time and an optically time-multiplexed split-pulse data acquisition system for improved noise performance and sensitivity. An acquisition speed of ∼100 kHz, a spectral resolution better than 0.1 nm over a wavelength range of ∼1335–1373 nm and a relative noise level of ∼5 mOD (∼1% minimum detectable base-e absorbance) are achieved. The system is applied for crank-angle-resolved gas thermometry by H2O absorption spectroscopy in an engine motoring at 600 and 900 rpm with a precision of ∼1%. Influences of various noise sources such as laser phase and intensity noise, trigger and synchronization jitter in the electronic detection system, and the accuracy of available H2O absorption databases are discussed.