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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.  Unfortunately, this laser proved to be unstable – turning itself off after a period of use, and requiring us to go through a complete restart cycle.  The operating time required for this reset drifted downward, from about once per day to about once or twice per hour.  We tried to diagnosis and fix the problem ourselves, including borrowing spare electronic components, but these did not resolve the problem.   We shipped the laser back to Spectra Physics for repair.  After having the laser for more than nine months, Spectra Physics abandoned the repair, claiming they no longer had the expertise on staff to fix their own laser product (which was less than a decade old).  This was a loss, and we have returned to using the 30 kHz, Q-switched laser to pump the fiber.

We have purchased a polarization preserving supercontinuum fiber which allows us to produce supercontinuum with a high degree of linear polarization (>100:1 extinction coefficient).  This has increased our usable power by a factor of two since only “P” polarization has low loss in our prism cavity.  We had to develop a method to determine what the proper input polarization pump laser is into the fiber, but that is now solved.

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.  For this we have had to upgrade the mid-IR FTIR to work in the near-IR/Visible.   For this, we changed out the beam splitter and also purchased a new detector specifically designed for this region.   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.   We have also obtained an InGaAs camera that will allow us to use the grating spectrograph (which has higher resolution and signal to noise than the FTIR). 

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.

We have also worked on the theory of cavity-enhanced spectroscopy.  We completed and submitted for publication a paper that analyzes the optimal trigger threshold for cw-CRDS measurements and predicts the resulting sensitivity limit in terms of the input laser power and spectral width, the mirror reflectivity, and the noise equivalent power of the detector.   We have also analyzed the predicted sensitivity of a variant to cw-CRDS where the laser is rapidly swept through resonance with the cavity instead of turned off when the resonance condition is met.  It is found that for a very narrow bandwidth laser (<1 kHz short term linewidth), this method is only a few times less sensitivity than traditional cw-CRDS.  However, when used a DFB diode laser, as is the most attractive option in the near-IR, the rapid phase jitter of the laser translates into a Lorentzian lineshape that results in light entering the cavity even when the laser is far off resonance.  Since this light has random phase relative that already inside the cavity, this results in considerable noise and thus limited sensitivity of the frequency swept CRDS approach.