Application of maximum length sequences to photoacoustic chemical analysis

Abstract

There is a great need for inexpensive, rugged, portable, and versatile chemical detectors for use in both security and environmental measurement. Current sensors that meet the inexpensive, portable, and rugged criterion are usually limited in sensitivity and dedicated to detecting a limited number of chemicals. Sensors designed for airborne measurement typically cannot be used to measure surface chemicals. The photoacoustic method is one of the most sensitive techniques for trace analysis in both the air and on surfaces. Unfortunately, photoacoustic systems are typically expensive, usually not portable, and usually cannot be used for both airborne and surface chemical analysis. A new design for a photoacoustic measurement system is proposed that utilizes maximum length sequences (MLS) to modulate multiple, inexpensive, fixed frequency, laser diodes. The MLS modulation and post-acquisition correlation processing should provide a very high signal-to-noise measurement without need for lock-in amplifiers, high power pulsed lasers, optical modulators, or resonant measurement cells. These changes should allow for the design of a rugged, portable, inexpensive photoacoustic measurement system that can be used for both airborne and surface trace chemical detection.