  This paper proposes a novel and rapid calibration-free wavelength modulation
spectroscopy algorithm based on even-order harmonics. The proposed algorithm,
analytically deduced from Voigt line-shape function, only involves simple
algebraic operations to describe the actual gas absorption spectra, thus
eliminating the time-consuming simulations and line-shape fitting procedures
adopted in traditional algorithms. Instead of acquiring the entirely scanned
absorption line-shape, the proposed technique only requires extraction of the
peak values of the harmonics. This characteristic significantly benefits gas
diagnosis at elevated pressure and/or temperature, in which the entirely
scanned absorption is very difficult to be obtained due to the broadened
line-shapes. The proposed algorithm is validated by both numerical simulation
and condition-controlled experiment, indicating millisecond-level calculation
of gas parameters with the relative error less than 4% in the experiments.
