Application of maximum length sequences to photoacoustic chemical analysis Ralph T. Muehleisen and Arash Soleimani Civil, Architectural, and Environmental Engineering Illinois Institute of Technology, Chicago, IL 60616 muehleisen@iit.edu 1
Outline Basics of Photoacoustic Analysis (PAS) Problems with current technology Maximal Length Sequences Proposed Systems Possible Implementations MATLAB Simulink Flow Diagram 2
Basics of Photoacoustic Analysis A sample (gas, liquid or solid) is illuminated by a time varying light source (usually a visible or near-ir wavelength laser with modulated output) If the light is absorbed the substance under study it heats and expands. When the light is removed the substance cools and contracts. The expansion and contraction gives rise to an acoustic signal proportional to the optically absorbed energy. This acoustic signal is measured with an acoustic sensor. The result is an acoustically measured optical absorption spectrum and hence the method is called photoacoustic spectroscopy (PAS) or optoacoustic spectroscopy (OAS) 3
Some simple PAS systems At left is a commercially available gas analysis system that uses a white light source and color filters to determine an absorption spectrum. Sensitivity, noise immunity and number of target chemicals are somewhat limited. At right is a pulse laser system used to analyze the chemical properties of thin films by sensing the acoustic signals with a PZT mounted in a backing solid. 4
Benefits of PAS PAS can be used for solids, liquids or gases Gases and liquids are usually pumped into cells and the acoustic signal in the fluid is measured The acoustic signals radiated by the solids are measured in the air or the vibration of the solid itself can be measured FTIR systems are used to profile the chemical composition of solids at different depths PAS can have very high sensitivity (approaching partsper-trillion) with good selectivity (some instruments can differentiate between thousands of chemicals) PAS analysis is fairly quick with measurement times of seconds to minutes 5
Applications to Harbor Security PAS is a possible solution for real-time, or near real time, monitoring of trace amounts of chemical contaminants: In the air near harbor, cargo containers and transport vessels In the water near transport vessels and in harbor On the surface of cargo containers and transport vessels 6
Two Standard PAS Methods Modulated CW PAS CW laser is mechanically modulated by an optical chopper External lock-in amplifier is used for real-time cross correlation of modulation signal and mic output Pulse PAS Higher power pulsed laser is pulsed in/through sample Might use a parametric modulation to increase S/N 7
Problems with conventional PAS Most conventional pas systems have some problems for use in homeland security including: Portability: Systems using lasers for high resolution and sensitivity are too large to be handheld and often need vibration isolating optical tables. Ruggedness: The conventional optical components and higher power pulsed lasers are usually not suitable for field use. Power: Conventional systems often utilize resonant cells to increase S/N which means power hungry gas samplers and pumps are required 8
Possible Solution: MLS Modulation One possible solution to the problem is to utilize low power laser diodes that are amplitude modulated by a maximum-length sequence (MLS), either by modulation of the laser power or modulated optical deflection Provides the increased S/N of modulated CW laser with the optical/mechanical simplicity of pulsed laser Note: This basic idea was implemented for liquid and solid PAS in the mid 80 s using a PRSB spaced optical choppers but the short MLS size and slow speed of the available computers limited its usefulness 9
Maximum length sequences (MLS) Maximum length sequences (MLS) are a pseudo-random binary sequences (PRBS) that have the following properties: Separately derived MLS sequences are orthogonal. Autocorrelation is The autocorrelation is unity at t=0 and -1/N for all other values of t where N is the length of the sequence so it. Noiselike: The cross correlation of an MLS sequence and any other random noise signal is essentially zero MLS signals have been used for decades in acoustic pulse reflectrometry as well as for the measurement of acoustic impulse responses of rooms and electronic equipment. The Galios fields that are the basis of MLS signals are also the basis of modern spectrum communication and the error correcting codes on CDs 10
Advantages and Disadvantages of MLS Advantages of MLS over pulse and MLS has much better noise rejection than pulse impulse excitation and long MLS signals can provide a high enough S/N that the resonance cells can be removed Simultaneous measurement of different optical frequencies is possible (frequency multiplexing) Disadvantages: Some nonlinearities in the system response show up as correlated noise which reduces S/N We still need to research how best to apply the MLS modulation 11
Multiple Laser Implementation One possible implementation utilizes multiple LED fixed frequency lasers (for low cost and ruggedness). The laser output is combined with a fiberoptic coupler and the mixed signal is used to excite the sample. The output from the mixed signal input is measured and orthogonality is used to separate the responses at different frequencies 12
White light source implementation Another implementation could use a white LED, a prismatic lens, and an MLS modulated micromirror array to provide a multiple wavelength excitation of the sample. The micro-mirrors are used to direct the different wavelengths to a single point in space near the microphone. MLS orthogonality is used to find the response at different wavelengths 13
Proposed Research Analytic We are currently searching the literature to find the most complete analytic models for modulated LED laser response, micro-mirror response and photoacoustic energy conversion. These models will be studied to determine how the nonlinearities will affect the proposed MLS system and determine some practical limits for MLS modulation Simulation A MATLAB Simulink simulation will be developed which will model the LED laser or LED/micro-mirror system, external and system noise and light and acoustic reflections. The simulation will be used to determine realistic detection sensitivity and chemical selectability bounds and develop preliminary results to pursue funding to build a prototype 14
MATLAB/SimuLink Model MLS Signal Laser Model + Microphone Response Data Acquisition Optical Noise Gas Properties + Acoustic Noise Detection Algorithm Thermal Noise Photoacoustic Effect Acoustic Propagation Acoustic Reflection Absorption Coefficient Error Estimates The MATLAB/Simulink model will implement a time domain simulation of the optical absorption, sample heating, acoustic propagation, and sensor measurement Simulation will include the effects of non-stationary source, sensor and environmental noise, reflection from nearby objects Monte-Carlo methods will be use determine error bounds and expected performance 15
Summary An photoacoustic chemical analysis system utilizing maximum-length sequence optical modulation is proposed Analytic and MATLAB/Simulink models are under development It is expected that the new modulation scheme will allow for the use of inexpensive, rugged, low power lasers or prismatic lenses with micromirror sytsems to allow low power, portable PAS systems to be developed for homeland security and environmental 16 monitoring