Grantee Research Project Results
Final Report: Cavity Ring-Down Spectroscopy for Measurement of Criteria Pollutants
EPA Contract Number: 68D00242Title: Cavity Ring-Down Spectroscopy for Measurement of Criteria Pollutants
Investigators: Baer, D. S.
Small Business: Informed Diagnostics Inc.
EPA Contact:
Phase: I
Project Period: September 1, 2000 through March 1, 2001
Project Amount: $69,965
RFA: Small Business Innovation Research (SBIR) - Phase I (2000) RFA Text | Recipients Lists
Research Category: Ecological Indicators/Assessment/Restoration , SBIR - Monitoring , Small Business Innovation Research (SBIR)
Description:
The Clean Air Act names carbon monoxide (CO) and NOx as criteria pollutants that contribute to the formation of urban smog. CO and NOx also affect the global climate by interacting with other chemical compounds in the atmosphere to form greenhouse gases. The conventional methods of measuring these criteria pollutants do not meet the needs for sensitivity, reliability, cost and/or ease of use. In this Phase I program, Informed Diagnostics Inc. (ID) evaluated the feasibility of cavity ringdown spectroscopy (CRDS) instruments for sensitive detection of CO and NO in mobile emission sources.ID designed and demonstrated a CRDS instrument that measures the absorption lineshapes of these gases in the near-infrared spectral region and determines their concentration in air with high sensitivity. The instrument employed telecommunications-grade distributed feedback (DFB) lasers operating at 1795 nm (NO; 3n band) and 1564 nm (CO; 3n band). For NO measurements, custom designed DFB diode lasers with high power output (6 mW) and narrow linewidth (2 MHz) were fabricated and characterized and then demonstrated in the CRDS instrument. For CO measurements, a commercially available DFB diode laser was used to record absorption lineshapes (3n band) for sensitive concentration measurements. Custom fabricated cavity mirrors with extremely high reflectivity over a very wide wavelength range were designed and incorporated into the instrument to allow simultaneous measurements of both target species. Custom electronics boards, designed and fabricated at ID, controlled and operated the optical and electronic components of the system. The results of the Phase I program have shown that CRDS instruments based on near-infrared diode lasers may be used for highly sensitive concentration measurements of multiple gases.
Summary/Accomplishments (Outputs/Outcomes):
A CRDS instrument was developed and evaluated for measurements of CO and NO concentrations using a pair of semiconductor diode lasers operating at 1564 nm and 1795 nm, respectively. The performance of each of the optical and electrical subsystems (including the diode lasers, detectors, optics, cavity mirrors, and control systems) was evaluated and benchmarked at ID. Each of the subsystems performed at the necessary levels for sensitive concentration measurements. The custom fabricated diode laser (near 1795 nm) and the commercially available diode laser (near 1564 nm) operated with sufficient output power (>3 mW delivered to the cavity), linewidth (2 MHz), and spatial beam quality necessary for effective CRDS measurements. In addition, these lasers could be readily current tuned over the necessary wavelength range (~30 GHz) for high-resolution absorption measurements of NO and CO transitions in the respective 3n bands near 1795 nm and 1564 nm. Thus these lasers satisfied the requirements of rugged, room temperature, tunable sources for the CRDS instrument.Detector modules based on both InGaAs (for measurements near 1565 nm) and extended-wavelength InGaAs (for measurements near 1795 nm) photodetectors, fabricated at ID, demonstrated the necessary gain (>3x104 V/A), electrical bandwidth (>20 MHz) and technical noise necessary to effectively record fast, repetitive ringdown traces with peak signals greater than 5 mV.
Compact laser modules that included the thermoelectric cooler to control the laser temperature and necessary optics (aspheric lens, cylindrical lens, anamorphic prisms) to produce circular collimated beam to the ringdown cavity were designed at ID, fabricated by an outside vendor, and evaluated at ID. Custom antireflection coatings on the optical components maximized transmission of the laser beam power to the cavity and yielded an overall transmission of 40-50% at both 1565 nm and 1795 nm.
Cavity mirrors for the Informed Diagnostics 3-mirror "ring" cavity were custom designed for high reflectivity over the entire wavelength range from 1530 nm to 1810 nm. The cavity ring configuration includes two mirrors oriented at 45-degrees angle of incidence (the so-called input/output mirrors) and a single curved mirror for 0-degrees angle of incidence with respect to the laser beam. The measured transmission through the input/output mirrors was less than 15 ppm (for s-polarized light), which corresponds to a mirror reflectivity of about 99.9985% (neglecting losses due to absorption). We are not aware of any near-infrared mirrors with greater reflectivity for non-normal angles of incidence. The measured reflectivity of the curved mirror was greater than 99.999%.
These mirrors were used to create an enclosed cavity with an especially narrow linewidth (Dncavity ~ 7 kHz) that corresponds to an extremely long ringdown lifetime of ~40 microseconds for the 40-cm long cavity. Unfortunately, the corresponding cavity linewidth was much narrower than the measured DFB laser linewidths (Dnlaser ~ 2 MHz). The very small ratio of cavity linewidth and laser linewidth resulted in inefficient coupling of s-polarized laser light into the cavity. The overall cavity reflectivity for p-polarized light was relatively small (~99.7%) and resulted in very short ringdown times (~0.2 microseconds) that were unacceptable for absorption lineshape measurements.
As a substitute, another set of mirrors with slightly lower reflectivity
(total cavity loss = 100 ppm) and designed for the 1500 nm to 1700 nm wavelength
range was used for measurements of CO concentration. The measurement sensitivity
(1 x 10-9 cm-1) corresponded to a CO detection limit of ~450 ppbv. Since the
absorption strengths of the probed CO and target NO absorption lines in the
respective 3n bands are similar, we believe that if the reflectivity of the
custom-coated mirrors were appropriately reduced, a relatively straightforward
task, similar detection limits for NO may be obtained.
Unfortunately, due to
time and budget constraints, another set of cavity mirrors with slightly lower
reflectivity near 1800 nm for NO absorption measurements could not be designed
and fabricated before the end of the 6-month Phase I program. Nevertheless the
successful CRDS measurements of CO concentrations suggest that cavity mirrors
with slightly reduced reflectivity should result in similarly effective NO
measurements and result in a compact, rugged instrument capable of sensitive
simultaneous measurements of multiple species.
Informed Diagnostics Inc. also completed the Phase I commercialization analysis. Foresight Science and Technology, Inc. determined that ID's cavity ringdown spectroscopy technique has the potential to provide a technically and economically effective instrument for sensitive, accurate gas measurements for industrial process monitoring and control applications that could perform well in the commercial marketplace. During the Phase I program, ID has also received strong encouragement from several industrial end users to continue development of portable compact CRDS instruments for accurate measurements of trace gases. ID is currently evaluating the attractiveness of these market opportunities.
Conclusions:
Several important conclusions may be drawn from the results of the Phase I program to determine the feasibility of cavity ringdown spectroscopy instruments for measurements of trace gases in the near-infrared spectral region.- A CRDS instrument was designed and fabricated for measurements of multiple gas species from absorption measurements at wavelengths ranging from 1530 nm to 1810 nm using multiple diode lasers, specially designed cavity mirrors, and specially designed control electronics and detectors.
- Telecommunications-grade distributed feedback diode lasers operating near 1564 nm and 1795 nm may be fabricated with sufficiently high power, narrow spectral linewidth and beam quality for CRDS measurements of CO and NO absorption, respectively. Prospective users however may encounter difficulties appropriating suitable diode lasers outside the wavelength range from 1510 nm to 1650 nm (the ITU C-band and L-bands) since many laser manufacturers are pursuing solely telecommunications applications.
- The reflectivity of the cavity mirrors must be tailored to match the output power and linewidth of available lasers to allow sufficient optical power into the cavity. Generally the performance of CRDS systems improves as the reflectivity of the cavity mirrors increases. If the mirror reflectivity is too high, however, it may be difficult to inject sufficient optical power into the cavity to yield appreciable "build up" (in the cavity). The easy and direct solution is to use mirrors with slightly lower reflectivity to allow sufficient optical coupling into the cavity.
- Cavity mirrors may be fabricated with sufficiently high reflectivity over a very wide wavelength range to allow effective CRDS measurements of both CO (at 1564 nm) and NO (1795 nm) simultaneously. The reflectivity of the mirrors delivered to ID was extremely high and uniform over the wavelength range from 1530 nm to 1810 nm. In the next-generation design, it will be a straightforward task to reduce the reflectivity of the mirrors while maintaining uniformity over the wide wavelength range.
- A prototype CRDS instrument was demonstrated for measurements of CO
concentration using mirrors with slightly lower reflectivity over the range from
1500 nm to 1700 nm. The instrument yielded a measurement sensitivity (~1x10-9
cm-1) that corresponded to CO detection sensitivity better than 450 ppbv. This
instrument was identical to that which may be used for measurements of both NO
and CO simultaneously except for the cavity mirrors and diode laser. Similar
detection sensitivity should be obtainable for NO measurements near 1795 nm
provided cavity mirrors with slightly lower reflectivity may be designed. Since
this requirement is limited by time and not the technical capability of our
coating vendor, the present SBIR Phase I program has essentially demonstrated
the feasibility of a CRDS instrument for concentration measurements of multiple gases including CO and NO.
Supplemental Keywords:
Emissions, pollution, cavity ringdown spectroscopy., RFA, Scientific Discipline, Air, Toxics, Waste, Ecosystem Protection/Environmental Exposure & Risk, Chemical Engineering, air toxics, Environmental Chemistry, HAPS, Chemistry, Monitoring/Modeling, mobile sources, Environmental Monitoring, Atmospheric Sciences, tropospheric ozone, Incineration/Combustion, EPCRA, Engineering, Environmental Engineering, monitoring, Nitrogen Oxides, Nox, Nitrogen dioxide, ambient air quality, criteria air pollutants, stratospheric ozone, vehicle emissions, air pollutants, automobile combustion, carbon monoxide (CO), cavity ring-down spectroscopy, criteria pollutants, motor vehicle exhaust, nitrogen dioxide (NO2), ambient air, ozone, spectroscopic studies, carbon monoxide, NO, Nitric oxide, combustion engines, measurement, vehicular exhaust, nitrogen oxides (Nox), measurement methods , NO2The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.