Tunable Diode Laser Spectroscopy With Wavelength Modulation: A Phasor Decomposition Method for Calibration-Free Measurements of Gas Concentration and Pressure

The principles and implementation of a phasor decomposition method for analyzing signals in tunable diode laser spectroscopy with wavelength modulation are described. This new technique enables recovery of the isolated and normalized residual amplitude modulation (RAM) signal from measured first harmonic signals at any chosen fundamental modulation frequency. Like the previously reported RAM technique, this new approach is absolute, yielding gas absorption line shape functions, concentrations and pressures without the need for calibration, under certain defined operating conditions. It represents an advancement of the RAM technique in that it obviates the need to operate at a specific high frequency to achieve phase quadrature between the RAM and derivative signals: the signals may be recovered at their maximum levels at any frequency. Measurements of the 1650.96 nm and the 1666.2 nm rotation/vibration absorption line shape functions for 1% and 10% methane in nitrogen at various pressures are compared to theoretical predictions derived from HITRAN data. The excellent agreement confirms the validity of the new technique. Further measurements of concentration and pressure confirm the efficacy of the technique for determining concentration in industrial process environments where the pressure may be unknown and changing. With the above features this new method is particularly suited to stand alone instrumentation for on-line deployment in industrial processes where the calibration factors in the conventional approach would present significant difficulties.

I. Introduction

Tunable diode laser spectroscopy (TDLS) with wavelength modulation spectroscopy (WMS) has become the preferred choice for a number of applications requiring gas composition measurements. In the midinfrared (IR) [1]–[6], the technique takes advantage of the strong fundamental vibration/rotation gas absorption lines to realize high sensitivity, but suffers from the relatively high cost of components (lasers and detectors), which are not particularly robust and often require cooling to achieve the necessary performance. On the other hand, TDLS in the near IR [7]–[13], while addressing weaker overtone absorption lines and sacrificing sensitivity, has taken full advantage of the availability of low-cost, robust, high-performance components developed and priced for the volume telecommunications market [viz. distributed feedback (DFB) lasers, vertical cavity surface emitting lasers (VCSELS), and indium gallium arsenide photodiodes]. In addition, compatibility with single mode optical fiber provides the potential of remote access to harsh and difficult environments and also for multipoint systems interrogated by a single laser via a suitable fiber optic network [14], [15]. With such capability, flexibility, and cost structure, the near infrared approach, both with direct detection [16]–[19] and wavelength modulation [7]–[13], [20]–[28] has generated great interest for industrial process applications.