Thermal lens spectrometry is a highly sensitive method for detecting very small quantities of material by the absorption of light from a laser source. It takes advantage of the thermal lens effect to measure the absorption spectrum of a sample. This invention utilizes an optical parametric oscillator (OPO) to obtain a continuously tunable laser source over a wide spectral range. A pump beam from the OPO and a probe beam from a laser are directed coaxially through a flow cell containing a sample. Sensors measure the intensity of the light from the probe beam after it has passed through the sample, while the OPO is tuned through a range of frequencies. Changes in the probe beam intensity measurement result from the wavelength-dependent heating of the sample by absorption of the pump beam. A plot of these intensity changes therefore provides a characteristic signature of the substance in the flow cell.
For many spectroscopic and spectrometric applications, such as nonlinear, photothermal, and fluorescence spectrometry, widely and continuously tunable laser sources are required. Until recently dye lasers generally have been used for tunable laser spectroscopy. However, the tuning range of dye lasers tends to be severely limited: each dye can cover only a few hundred angstroms, and the total range is limited to approximately 400 nm to 1 micrometer. In contrast, a spectrometer using a single OPO system could have a tuning range of 200 to 2500 nm.
The thermal lens system is extremely sensitive. In comparison to conventional absorption spectrometry, for example, it is almost 1000 times more sensitive for probing NO2 diluted in air. Because of this sensitivity, only a small sample volume is needed and excellent spatial resolution can be obtained. As a demonstration experiment, the system was used successfully in measuring the entire visible thermal lens spectrum of NO2 in a single scan with adequate resolution to resolve the important peaks. As long as a line of sight to the sample is maintained, the spectrometer is capable of operating at a distance.
The compact OPO system provides high output power and efficiency as compared to conventional sources, making highly sensitive measurements possible. The OPO system is completely solid-state, except for the pump laser, and consists of commercially available components. It is therefore a cost-effective alternative to conventional systems. Because of the compactness of the design and the simplicity of operation, it could be operated as a remote sensor. It would be possible, for example, to design a mobile system that could monitor smokestack emissions by aiming the beams from the spectrometer at retroreflectors mounted at the smokestack tops. Similarly, auto emissions could be monitored by spectrometers mounted at highway toll booths.
An experimental system using a beta barium borate OPO invented at Cornell and exclusively licensed to Spectra-Physics Lasers, Inc., Mountain View, CA, was built and tested. Data on the detection of NO2 was collected and is available. The OPO is commercially available, and Spectra-Physics has expressed willingness to cooperate in the commercialization of this invention.