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The goal of this project is to develop a highly sensitive method to simultaneously monitor the population of a range of species in a flame or other combustion environment. The method depends upon combining recently developed supercontinuum sources with a novel low loss optical cavity formed by two Brewster-Angle based Prism Retroreflectors. Starting the project, we had demonstrated that the prism cavity could be used with a home built supercontinuum source that is pumped by a 30 kHz Q-switched laser. The power generated by this system is limited by optical damage to the photonic bandgap fiber used. We have worked to develop a higher average power system based upon a mode lock laser system that runs at ~80 MHz. Such a system will immediately improve by several times the available supercontinuum power. Ultimately, such a system could be used as stabilized frequency comb, which should allow several order of magnitude improvement in the optical transmission of the cavity. Since we are presently limited by shot-noise, this will translate into a dramatic improvement in sensitivity.

We purchased on the used market a low power (~1 W) mode locked laser, but this proved to have insufficient peak power to generate a supercontinuum. Considerable effort was made to amplify this laser using a pair of high power diode pumped Nd:YAG gain heads we had previously had. We tried to first build a regenerative amplifier, and failing that, a multiple pass amplifier with birefringence compensation. Though we were able to get sufficient gain (~15 times), the degradation in mode quality prevented us from efficiently coupling this light into the optical fiber. After much effort, this approach was abandoned, and we purchased (again used) a high power VANGARD laser from Spectra physics. This laser generates ~15 W of 1.06 mm light and has been used to generate ~3 W of supercontinuum radiation, almost an order improvement over what we previously had. We have recently received a polarization preserving supercontinuum fiber and this should allow us to effectively double our useable light since we must use p-polarized light in our optical cavity.

We have developed a large vacuum chamber that will hold the prism cavity and the low-pressure flame. In order to be able to control the precise prism alignment, which could shift due to the thermal load created by the flame, we have purchased motorized six-axis stages to position the mirrors and have developed the software to allow external alignment of the prisms to maximize the ring-down time of the cavity. We have also developed a photon-counting system, based upon a multichannel scalar, which allows us to rapidly determine the cavity ring-down time to high accuracy, which is needed to convert changes in the cavity transmission to absolute absorption strengths. We are presently working on the coupling the output of the prism cavity to a FTIR spectrometer, which will allow us to monitor the entire spectrum of the supercontinuum simultaneously. Up to present, we have used a CCD camera mounted on a 1 meter grating spectrograph, which gave excellent spectral resolution but limited use to monitoring only a few tens of nm simultaneously.

In a separately funded project, we are attempting to develop a similar cavity based upon prisms made of CaF2, which should allow us to extend the useful range of our spectrometer, currently (~500-2000 nm) down to ~250 nm.