Research
Sensors

Absorption based Sensors

TDLAS (Tunable Diode Laser Absorption Spectroscopy) is a powerful tool in many sensing applications. Tunable diode lasers are powerful, narrow linewidth light sources capable of measuring the absorption of gaseous species very accurately. Using appropriate spectroscopic models the concentration of a molecular species can be calculated from the integrated absorption when scanning the laser source across a particular transition provided that the spectral region is free of background absorption from other species.

Goals

This project started as a collaboration between Dr. Robert P. Lucht (Mechanical Engineering, Purdue University) and Dr. Jerald A. Caton (Mechanical Engineering, Texas A&M University) and my group at TU Darmstadt.

The goal is the development of real-time sensors for species in the gas phase. We are aiming at absorption based sensors in spectral regions accessible only by non-linear frequency conversion.

As an example we discuss our Opens internal link in current windowNO sensor in somewhat more detail. This work aims at a realtime sensor for NO in the exhaust of large combustion engines and turbines as used for instance in power plants in order to implement combustion control schemes to optimize the efficiency of the engines while reducing the NO emission of the exhaust. The continuous absorption technique is appropriate for such a task as we are not seeking time resolved information on the exhaust process itself, but rather the information of the entire concentration of NO in the exhaust.

Another possible application for this sensor system is a stand-alone, real-time sensor for the enforcement of the future emission standards.

  • G. J. Ray et al., Opt. Lett., 26 (2001) 1870-1872.
  • Sherif F. Hanna et al., Appl. Phys. B 75 (2002) 113-117.
  • Th. N. Anderson et al., Appl. Opt. 44 (2005) 1491.
  • Robert P. Lucht et al., US Patent 6 982 426
  • R. Barron-Jimenez, J et al., Applied Physics B 85 (2006) 185-197.

For these sensors, we generally try to set up TDLAS based systems in spectral regions not accessible directly such as the UV spectral range or the IR. We use non-linear conversion processes in single-pass setups in order to keep the experimental efforts as small as possible. Therefore, we generally use two light sources and perform sum- or difference frequency generation. Usually one light source is weak, but tunable and the other provides the powers in order to generate nW of power at the desired wavelength.

Principle setup of our sensors. The wavelength modulation is optional. The solid state laser system usually consists of two lasers whose outputs are combined by sum- or difference frequency generation.
Principle setup of our sensors. The wavelength modulation is optional. The solid state laser system usually consists of two lasers whose outputs are combined by sum- or difference frequency generation.

Due to environmental concerns and shrinking resources, combustion control is becoming increasingly more important. An important pollutant in combustion processes is NO. We are therefore developing a NO sensor based on modern laser technology. We also have a CO sensor in preparation.

The sensor is based on absorption in the UV spectral range. The UV range has the advantage that typical transition strengths are stronger such that the ultimate sensitivity can be higher than with comparable sensors in the infra-red range. Another advantage is that no background signal due to the absorption of other species are present.

Technology

The UV light is generated by frequency summing a blue diode laser in an external cavity (395 nm, 9 mW) with a diode pumped, frequency doubled Nd:YAG laser (532 nm, 100 mW) in a beta barium borate crystal (BBO). In the one-pass configuration we generate approximately 300 nW of light at around 226 nm, which corresponds to the X-A transition in NO.

The measurement is performed using two photo-multiplier, one for the reference and the other for the signal itself. So far we have achieved a sensitivity of 5 ppm per m absorption length in the laboratory.

NO Sensor Setup

Principle setup of the NO sensor.

Sample spectrum including a fit of the theoretical model to the data.

Labview integration of the sensor.
Labview integration of the sensor.

Prototype sensors for OH, NO and CO have been set up at Texas A&M University. The Texas sensor for NO has been successfully tested with practical engines.

The NO sensor here in Darmstadt has been successfully combined with a LabView interface, which automatically acquires the data, and performs the data analysis. Long term measurements of up to 6 hours have been successfully performed. Currently,

Our current goals

  • Improving the sensitivity of the sensor
  • Extending the mode-hop free tuning range of the ECDL diode laser
  • New applications for this type of sensor