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Journal of Physics D Applied Physics

Atomic absorption of thermally dissociated iodine for laser applications

D R Gray, H. J. Baker, Terence A. King

Published Feb 1, 1977• 17 citations
Publisher

Abstract

An absorption cell for the 1.315 mu m iodine laser wavelength has been constructed using thermal dissociation of molecular iodine vapour to produce ground-state iodine atoms. Measurements of the small-signal absorption coefficient for iodine pressures up to 5 Torr and temperatures up to 750 degrees C have been compared with the predictions of a simple thermodynamic model of the iodine dissociation equilibrium, and satisfactory agreement obtained. The model readily allows calculation of operating conditions required for the use of the iodine absorber as a saturable absorber, for free-induction decay pulse generation and for coherent pulse propagation experiments.

DOI
10.1088/0022-3727/10/2/002

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This thesis looks into the aspects of molecular rot-vibrational energy level relaxation of iodine when it is excited by a resonant laser. In addition, the effects of Doppler detuning of molecular levels are explored. The proposed Active Hyperspectral Imaging Project [1] will detect in the atmosphere, low concentrations of a specific element's molecules by using remote sensing optical techniques. In order to improve the signal to noise ratio, a 'high field' coherent excitation technique is proposed. However such coherent excitation should take less time than the typical relaxation time of the system to avoid de-coherence effects. The purpose of the present study is to assess the values of the de-population and de-phasing relaxation times in iodine. The result of this experiment will be used for the design of an intra-cavity coherent excitation experiment in which the power needed for excitation would be obtained by a resonant enhancement inside a low-loss optical storage cavity. We can estimate the minimum power required for coherent excitation based on our findings of the relaxation times. This high field experiment would gain information regarding the coupling between the molecule and the laser field. In the present 'low field' experiment, output from a low-powered tunable diode laser was sent through a Pockels cell to create a top hat pulse which was used to excite gaseous iodine, located in an evacuated cell. The time dependence of a spectrally resolved fluorescence signal was used to observe the relaxation behavior of some targeted excited state and the behavior was fitted to a model resulting in estimates of de-phasing and depopulation relaxation parameters. The experiment was also set-up to observe the effects of Doppler broadening due to molecular velocities. Fluorescence was collected from a narrow strip inside the iodine cell (see figure 3.3) whose short axis was parallel to the laser beam and its length was altered to assess the Doppler contribution. The information of de-phasing and de-population collected from the low field experiment would be useful for the high field experiment in which enough intensity will be created to induce Rabi oscillations [2] by the placement of a custom iodine cell inside a short near-confocal cavity. These oscillations would aid in the estimation of the dipole matrix element (which quantifies the coupling between the molecule and the laser field). The time duration of de-phasing and de-population will determine the minimum field intensity required to produce a Rabi oscillation whose period is less than the de-phasing and de-population times. The situation for the low field experiment was modeled by a two-level energy system interacting with a single mode field. In chapter 2, equation of motion is developed which includes the detuning effect. An output intensity model designed to fit the data is presented that would be used to calculate the rise and decay times helping us to ascertain the relaxation effects. The practical setup is presented in chapter 3. Details regarding the beam line, stabilization of the diode laser, selection of the iodine absorption line by tuning the laser and selection of a fluorescence line of iodine are all described in that chapter. The procedure of placement of the Pockels cell, production of a 1 µs laser pulse and data collection are also given. The chapter concludes with a representative data-set. For the Analysis, the model from the second chapter is used to fit to the experimental data. Values of rise and fall times are presented and the relationship between these values is explored. The average value of de-phasing due to molecular collision is calculated and the power requirements for the Rabi oscillations are discussed in this chapter. The thesis concludes with a summary and suggestions for further work.

•Jan 1, 2008•0 citations
An absorption cell for the 1.315 mu m iodine laser wavelength has been constructed using thermal dissociation of molecular iodine vapour to produce ground-state iodine atoms. Measurements of the small-signal absorption coefficient for iodine pressures up to 5 Torr and temperatures up to 750 degrees C have been compared with the predictions of a simple thermodynamic model of the iodine dissociation equilibrium, and satisfactory agreement obtained. The model readily allows calculation of operating conditions required for the use of the iodine absorber as a saturable absorber, for free-induction decay pulse generation and for coherent pulse propagation experiments.
The physical basis of the operation of an atomic iodine passive switch (for iodine photodissociation lasers) is considered. Atomic iodine is obtained by thermal dissociation of I 2 . An experimental dependence of the switch transmission on the radiation intensity is reported; it is in satisfactory agreement with the theory. A weak-signal attenuation by a factor of ~ 100 is achieved for 70–80 % transmission of a saturating signal ( I ~5 × 10 8 W/cm 2 ). It is shown that a switch of this kind can be used as a decoupler in multistage amplifying systems.
Coherent propagation of intense short laser pulses through degenerate absorbing media is investigated with the use of an atomic-iodine laser-absorber self-resonant combination. Four degenerate systems, the $^{2}P_{\frac{1}{2}}\ensuremath{-}^{2}P_{\frac{3}{2}}$, $F=3\ensuremath{\leftrightarrow}{F}^{\ensuremath{'}}=4$, $\ensuremath{\Delta}{M}_{F}=0,\ifmmode\pm\else\textpm\fi{}1$ and $F=2\ensuremath{\leftrightarrow}{F}^{\ensuremath{'}}=2$, $\ensuremath{\Delta}{M}_{F}=0,\ifmmode\pm\else\textpm\fi{}1$ transitions, are studied under various conditions. Theoretical analysis based on the "pulse-area---pulse-energy" approach shows different pulse propagation behaviors for three typical types of degeneracy. Experimental results give good agreement with theoretical predictions. It is concluded that self-induced transparency exists in any degenerate two-level system, provided that suitable polarization of radiation is used. The usefulness of self-induced-transparency phenomena for measurements of transitional dipole moment and homogeneous relaxation time is also demonstrated.
We introduce two approaches to the synthesis of the optimized schemes of dissociation control of diatomic molecules. The first one bases on the early introduced speed-gradient algorithms of the quantum control. We use the semi classical model of molecular dynamics onto the ground state. The introduced algorithm is a robust and it has only one tuning parameter. The numerical results of the control of the predissociation of iodine molecule illustrate the high possibility of the dissociation for 100 fs laser pulse. The second approach bases on quantum model of dissociation through B excited electronic state. This model doesn't assume the solving of any time-depended equations. The optimized scheme of iodine dissociation with different nuclear isotopes is obtained. The best isotope separation achieves up to 300 fs with 80% efficiency.
We have investigated degenerate four-wave mixing (DFWM) in a strongly absorbing molecular gas. A theory taking into account saturable absorption of the interacting beams is used to explain the dependence of the DFWM reflectivity on laser intensity and on the molecular number density. These parameters have a great influence on the observed spectra, owing to linear absorption that suppresses the DFWM signal in the line center of the molecular transitions. The great sensitivity of the spectra to the experimental conditions limits the applicability of DFWM for spectroscopic studies in absorbing media.
This thesis looks into the aspects of molecular rot-vibrational energy level relaxation of iodine when it is excited by a resonant laser. In addition, the effects of Doppler detuning of molecular levels are explored. The proposed Active Hyperspectral Imaging Project [1] will detect in the atmosphere, low concentrations of a specific element's molecules by using remote sensing optical techniques. In order to improve the signal to noise ratio, a 'high field' coherent excitation technique is proposed. However such coherent excitation should take less time than the typical relaxation time of the system to avoid de-coherence effects. The purpose of the present study is to assess the values of the de-population and de-phasing relaxation times in iodine. The result of this experiment will be used for the design of an intra-cavity coherent excitation experiment in which the power needed for excitation would be obtained by a resonant enhancement inside a low-loss optical storage cavity. We can estimate the minimum power required for coherent excitation based on our findings of the relaxation times. This high field experiment would gain information regarding the coupling between the molecule and the laser field. In the present 'low field' experiment, output from a low-powered tunable diode laser was sent through a Pockels cell to create a top hat pulse which was used to excite gaseous iodine, located in an evacuated cell. The time dependence of a spectrally resolved fluorescence signal was used to observe the relaxation behavior of some targeted excited state and the behavior was fitted to a model resulting in estimates of de-phasing and depopulation relaxation parameters. The experiment was also set-up to observe the effects of Doppler broadening due to molecular velocities. Fluorescence was collected from a narrow strip inside the iodine cell (see figure 3.3) whose short axis was parallel to the laser beam and its length was altered to assess the Doppler contribution. The information of de-phasing and de-population collected from the low field experiment would be useful for the high field experiment in which enough intensity will be created to induce Rabi oscillations [2] by the placement of a custom iodine cell inside a short near-confocal cavity. These oscillations would aid in the estimation of the dipole matrix element (which quantifies the coupling between the molecule and the laser field). The time duration of de-phasing and de-population will determine the minimum field intensity required to produce a Rabi oscillation whose period is less than the de-phasing and de-population times. The situation for the low field experiment was modeled by a two-level energy system interacting with a single mode field. In chapter 2, equation of motion is developed which includes the detuning effect. An output intensity model designed to fit the data is presented that would be used to calculate the rise and decay times helping us to ascertain the relaxation effects. The practical setup is presented in chapter 3. Details regarding the beam line, stabilization of the diode laser, selection of the iodine absorption line by tuning the laser and selection of a fluorescence line of iodine are all described in that chapter. The procedure of placement of the Pockels cell, production of a 1 µs laser pulse and data collection are also given. The chapter concludes with a representative data-set. For the Analysis, the model from the second chapter is used to fit to the experimental data. Values of rise and fall times are presented and the relationship between these values is explored. The average value of de-phasing due to molecular collision is calculated and the power requirements for the Rabi oscillations are discussed in this chapter. The thesis concludes with a summary and suggestions for further work.