2012 Progress Report: Compact Multi-Pollutant Mid-Infrared Laser Spectroscopic Trace-Gas SensorEPA Grant Number: R835137
Title: Compact Multi-Pollutant Mid-Infrared Laser Spectroscopic Trace-Gas Sensor
Investigators: Wysocki, Gerard
Institution: Princeton University
EPA Project Officer: Hunt, Sherri
Project Period: February 1, 2012 through January 31, 2016
Project Period Covered by this Report: February 1, 2012 through January 31,2013
Project Amount: $250,000
RFA: Developing the Next Generation of Air Quality Measurement Technology (2011) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Air
This project will focus on development of a compact, cryogen-free trace gas sensor node targeting the spectral range containing the absorption band of benzene, which is a highly toxic atmospheric pollutant. The proposed proof-of-concept instrumentation will also provide multi-species sensing capabilities. Two major innovations will be addressed: multi-component chemical analysis will be enabled thorough application of novel mid-IR widely tunable external cavity quantum cascade lasers, and an ultra-compact sensor system will be developed to address deployments in wireless sensor networks.
In this project we focus on development of a compact spectroscopic sensor node for quantitative measurements of multiple chemical compounds simultaneously. We proposed application of broadly tunable mid-IR semiconductor lasers (with a particular focus on quantum cascade lasers, QCLs) to meet the requirements for wide electromagnetic frequency coverage required for multi-species detection, as well as we plan to develop an ultra-compact sensor system with a dedicated control and data acquisition electronics for applications in wireless sensor network (WSN) configurations.
The project is carried out accordingly to the originally proposed research plan that is divided into 4 major research tasks: (1) design and build a laboratory breadboard prototype of an EC-QCL based sensor system operating at 9.6 μm, (2) laboratory tests and calibration of the breadboard prototype, (3) develop EC-QCL based sensor node with WSN capabilities, and (4) field test and instrument inter-comparison. In addition to the approaches included in the research plan submitted to EPA in 2011, we have carried out evaluation of the most recent laser technologies that are potentially more suitable to successfully accomplish the proposed work.
To date the efforts were primarily focused on development and evaluation of broadly tunable mid-IR laser technology and early proof-of-concept spectroscopic experiments needed to assess the feasibility of laser direct absorption spectroscopy to detection of the target compounds. For these studies a broadband absorber, Benzene (C6H6), and a small molecule with well-resolved fundamental ro-vibrational structure, Ammonia (NH3), have been proposed as test molecules. In this initial phase carried out as task #1 we have developed a laboratory breadboard system (as shown in Fig. 10 of the original proposal). This spectroscopic prototype is currently under studies to provide a benchmark for further development of a compact broadly tunable WSN node. We have also carried out an extensive spectroscopic analysis of C6H6 band at 9.6 μm to determine the best conditions for quantitative analysis of unknown samples using both direct as well as wavelength modulation spectroscopy (WMS) approaches. Optimum WMS conditions including the laser modulation parameters and potential spectral interferences have been identified.
We have initially evaluated two laser source technologies as potential spectroscopic engines of the WSN node: (1) external cavity QCL (EC-QCL) and (2) single frequency distributed feedback QCL (DFB-QCL). The conclusions from the initial tests suggest that although the EC-QCL provides necessary tuning range, its limited opto-mechanical stability might represent an important drawback if applied in field deployable WSNs. The DFB-QCL tested in this project allowed for initial spectroscopic evaluation of C6H6 detection and showed both capability of sensitive detection as well as outperform EC-QCL in terms of optomechanical stability. Nevertheless the spectral coverage offered by DFB-QCL technology is not sufficient for the proposed multi-species detection. Following this initial findings we have identified another recently emerging broadly tunable QCL technology based on distributed Bragg reflectors (DBR-QCLs). We have partnered with Nanoplus GMBH in Germany, a potential supplier of DBR-QCLs, to carry out feasibility tests of this technology to perform the proposed tasks. A dedicated optical/electrical test setup has been developed and initial tests of this new laser technology have been carried out with promising first results.
We have also carried out theoretical and experimental research on calibration of direct laser absorption spectroscopy systems planned in the task #2 of this project. A new calibration technology of WSN nodes has been fully developed and characterized (publication in a peer-reviewed journal is underway).
The main goals of the project that include development of a widely tunable sensor node with WSN capabilities, and instrument field test and inter-comparison with other established technologies, have not changed from the original grant application.
In the year 2, we plan to continue work on tasks #1 (Design and build a laboratory breadboard prototype of an EC-QCL based sensor system operating at 9.6 μm), and task #2 (Laboratory tests and calibration of the breadboard prototype). Within the task #1 we specifically plant to continue the performance optimization and experimental verification of the spectroscopic wavelength modulation spectra (WMS) modeled in the year 1. WMS detection of C6H6 and NH3 will be performed using the prototype system developed in year 1. In addition to the main stream research within task #1 we also plan testing of a new DBR-QCL technology for spectroscopic applications. We will perform: (1) full characterization of the DBR laser source, (2) demonstration of all-electrical high resolution tuning, and (3) spectroscopy of trace-gases. Within the task #2 we plan to focus on extension of the calibration method studied in year 1 to molecules that require long optical paths for sensitive detection of environmentally relevant concentrations (specifically C6H6 and NH3). We also plan so start developments originally planned as task #3 (Develop EC-QCL based sensor node with WSN capabilities), but after full evaluation of DBR-QCL this task might be carried out using this new broadly tunable QCL technology in place of EC-QCL.
1. P. Fuchs, J. Friedl, S. Höfling, J. Koeth, A. Forchel, L. Worschech, and M. Kamp, "Single mode quantum cascade lasers with shallow‐etched distributed Bragg reflector," Opt. Express 20, 3890‐3897 (2012).
2. T. Tsai, and G. Wysocki, "External‐cavity quantum cascade lasers with fast wavelength scanning," Appl Phys B 100, 243‐251 (2010).