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 ($1e 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 trigger and synchronization jitter in the electronic detection system, and the accuracy of available H2O absorption databases are discussed.