Applied optics - Tampere University of Technology

Department of Physics

Optics research

Applied optics | Nonlinear optics | Nonlinear fiber optics
 

Applied Optics

We study and develop advanced optical spectroscopy techniques to monitor e.g. atomic and molecular concentrations. We work mostly with gas phase detection, and utilizing the latest photonic components, we are introducing new online monitoring techniques that have industrial potential.

Optical diagnostics of combustion gases

Combustion is an important process in the production of heat and electricity. In a combustion process the chemical energy of the fuel is converted to heat that can be further exploited to produce electricity. Conditions in a combustion boiler set challenges for monitoring techniques because the temperature and the pressure may be anything between 600–1800 °C and 1–50 bar. Moreover, the corrosive gases released in combustion complicate the sampling of the flue gases.

Optical methods offer reliable techniques for monitoring gases, emitted from combustion processes, with good sensitivity at wide range of concentrations and the possibility of remote sensing. During last two decades the Applied Optics group has involved in two large combustion projects (LIEKKI and ChemCom). The most of our research is done in a laboratory where we study fundamental spectroscopic properties of the combustion gases and their interactions. According to the interests of the collaborators, we have also developed novel optical techniques for full-scale industrial use in power plant boilers.

Principle of optical monitoring of gases, released in a power plant boiler.

 

Optical detection of alpha radiation

The optical detection of radioactive alpha sources is possible by measuring the collision-induced fluorescence of nitrogen molecules in the air (see picture below). The benefit of the optical detection method is the long range of fluorescence photons which enables radioactivity measurements from a safe distance. On the contrary, conventional detectors require direct interaction with an alpha particle, and, therefore, the detection is possible only at distances shorter than alpha’s range. The optical method is feasible in environments where the level of ultraviolet lighting is low, preferably nonexistent. We focus on various alpha monitoring techniques which can be utilized in metal recycling and decommissioning as well as in nonproliferation inspection.

Alpha radiation excites atmospheric molecules via secondary effects. The process is similar to northern lights and the optical alpha detection is possible by measuring these “micro” northern lights around the radiation source.

 

Photoacoustic spectroscopy of gases

Photoacoustic spectroscopy (PAS) is a sensitive technique for trace gas analysis. The PA technique is based on the detection of sound waves that are generated due to the absorption of modulated light. The amplitude of the acoustic wave is directly proportional to the product of laser power, the concentration of the molecules in the gas sample, and the sensitivity of the PA detector. Usually, the quantitative analysis of molecular concentrations is of interest. In the Applied Optics group various PA detectors have been developed and applied to measuring different gases, including for example CO2, NO2, NO, O2. Also the detection of different gaseous vapors that are relevant in bio-mass combustion and human breath analysis, has been demonstrated.

Photoacoustic measurement of gas concentration in high-temperature photoacoustic cell.

 

Supercontinuum spectroscopy for sensitive multi-species analytics

In real-time emission and process monitoring there exists needs for methods that can track multiple trace gas species simultaneously with high sensitivity. Spectrally broadband optical methods are excellent candidates for this type of sensing. In typical broadband techniques, however, speed and sensitivity of the measurement are often limited by the low spectral brightness of the broadband light source.

Supercontinuum generation is a highly nonlinear process where laser light is converted into light with a very broad spectral bandwidth, resulting in an intense “white” laser beam. This is usually accomplished by coupling ultrashort laser pulses from a mode-locked titanium-sapphire laser into a highly nonlinear optical fiber, such as a photonic crystal fiber (see picture below). We study cost-effective methods for generating supercontinua specially tailored for use in ultra sensitive multi-component gas sensing applications. We demonstrate the possible benefits from using such a source experimentally in Cavity-Enhanced Absorption Spectroscopy (CEAS) and compare the results to other broadband techniques. This work is a part of Cluster for Energy and Environment (CLEEN) Measurement, Monitoring and Environmental Assessment (MMEA) research program.

Supercontinuum laser beam, dispersed spectrally by a diffraction grating. In gas sensing applications different molecules absorb different colors of the spectrum, which allows a rapid and selective multi-gas analysis.

 

Monitoring heavy metals in water

Toxic heavy metals can be released into the water from industrial processes. Due to the environmental threat, the concentrations of various metals must be monitored. Usually this is carried out by analyzing water samples in a laboratory, which is a laborious task. In our group laser-induced breakdown spectroscopy (LIBS) has been applied for qualitative and quantitative analysis of waste waters where heavy metals are found in small concentrations (< 1 mg/l).

LIBS is a rapid and selective spectroscopic method for measuring elemental concentrations of matter. A high-power laser pulse, usually a nanosecond pulse from a Q-switched Nd:YAG laser, is focused onto the sample which is being analyzed. Due to the light absorption and rapid heating of the matter, the sample is vaporized and a very hot plasma is formed. The atoms and ions in the plasma are thermally excited to upper electronic states. After a short time of cooling, the fluorescence light, emitted by the atoms and ions, is measured. Each element has narrow, characteristic emission lines in the visible and UV region of the electromagnetic spectrum which allows the detection of multiple elements simultaneously.

Basic set-up for laser- induced breakdown spectroscopy. Intense laser pulses are focused onto a sample. The fluorescence spectrum, emitted by the plasma, is measured and analyzed.

 

Updated by: Toivonen Juha, 27.02.2013 18:18.
Keywords: science and research