Development of a Phase Shift- , Fourier Transform- Cavity Ring Down Spectrometer Jose P. P. Barros, and R. A. H. Engeln, D. C. Schram (July 2000) Eindhoven University of Technology, Dpt. Applied Physics, Group Equilibrium and Transport in Plasmas Abstract The possibilities for combining two Cavity Ring Down Spectroscopy (CRDS) techniques were studied. The objective is to lay the grounds for the development of a sensitive broad band, gas phase, cw, optical absorption spectrometer. CRDS is a type of multi-pass absorption spectroscopy. In CRD the “lifetime” of light intensity in an optical resonant cavity is related to the loss in the cavity [5]. This loss is due to the mirrors’ non-ideal reflectance and to the absorption of the gas inside the cavity. Therefore CRD can be used to determine with high sensitivity the reflectance of mirrors as well as densities, temperature, pressure of the medium between the mirrors PS-CRD is a cw based technique, in which the light being coupled to the cavity is modulated and a Phase Shift is detected, which is related to the loss in the cavity. This has been demonstrated experimentally with a ring-dye laser [ 2] . Here we demonstrate this same technique with a low power diode laser, detecting loss due to absorption in part of a weak O2 band (A-band) of up to few ppm/cm. The Signal to Noise Ratio (SNR) was studied has a function of modulation frequency and depth. A LabView program was developed in order to control the experiment from the computer via GPIB [1]. FT-CRD has been demonstrated with a broad band pulsed dye laser as source [ 3] . In that work the light exiting the cavity was coupled into a Michelson-type interferometer and the time dependence acquired for a range of path differences. Loss at every wavelength could be deduced from this. In this work is demonstrated in theory how PS - and FT -CRD can be combined, creating an absorption spectrometer that can be used with a broad band (e. g. white light) cw source, that could measure loss in a resonant cavity for the different wavelengths by detecting a phase. The possibility of coupling the light from a modulated optical cascaded arc into a confocal optical cavity was also tested. The arc could be modulated up to 20kHz, 10% depth. For lower frequencies higher depths are possible. The results strongly indicate that the light can be coupled, but they are not yet conclusive. Detector R R 5 ns d L Time I n t e n s i t y absorptie [Fig. 1]- Schematic representation of a pulsed CRD experiment. d is the distance between the mirrors, and L the path over which absorption is considered. Theory of the PS- and FT- CRD Symbiosis Conventional Cavity Ring Down (CRD) Spectroscopy The multi-pass absorption technique CRD can be implemented by measuring the rate of decay in intensity of a light pulse in an optical cavity. The most elementary way to understand this is to consider an optical cavity consisting of two highly reflective mirrors and a light pulse that is coupled into this cavity [5]. If the reflectance R of the mirrors is very close to unity, then only a small fraction of the intensity of the incoming beam is coupled into the cavity, (1-R), an aspect intrinsic to the CRD technique. Consider then this pulse that has entered the cavity. It will be reflected back and forth inside the cavity, and if there is not any absorbing medium between the mirrors, then the only losses will occur at the mirrors, due to their finite transmittance . In approximation we can go from these discrete changes in intensity to a continuous change, that is, considering the intensity as a function of time. We then have that (in approximation), the intensity for a given frequency v decreases exponentially in time: ) ( 0 0 ). , ( ) , ( v t e t v I t v I t - = [1] in which τ 0 is given by: [ ] )) ( ln( ) ( 0 v R c d v = t [2] If there are absorbing species in the region L between the mirrors, (see fig. 1), there will be losses due to absorption and scattering. In gas phase, scattering can usually be neglected as compared to absorption (scattering cross-sections are much smaller), and so a further term due to absorption is introduced in the denominator for τ . ( [ ] L N R c d p ) ( ) ( 1 ) ( n s n n t - = [3] considering homogeneous absorption at v over a distance L, by a density N p . N p can then be calculated after measuring τ : - = ) ( 1 ) ( 1 ) ( 0 v v c d L v N p t t s [4] an expression most useful when analysing data in CRD measurements. It also implies that, in order to determine N(v), there must be a measure or estimate/calculation for τ 0 , that is, the ring down time without absorption 1 . This can be done by measuring τ without any absorbing species in the cavity, or at a wavelength on which we are sure to have no absorption (that is, one could measure against the baseline). This is a simple approach to CRD, one can find more detail by looking at the electromagnetic solutions 1 in the case of a spectrum, it will in general correspond to τ measured from the baseline.