• Utpal Nundy

      Articles written in Pramana – Journal of Physics

    • Generation of 13.9 𝜇m radiation from CO2 by cascade lasing or externally applied CO2 laser

      Utpal Nundy

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      13.9 𝜇m radiation from the $10^{0}0–01^{1}0$ transition can be obtained from a CO2 laser by saturating the 00$^{0}1–10^{0}0$, 10.6 𝜇m transition with an internally generated q-switched pulse or by the application of an external 10.6 𝜇m pulse. Because of Fermi resonance between the symmetric stretch and the bending modes, decay of population from the $10^{0}0$ level is fast, and such lasers operate at low power and energies. A theoretical model was developed to study such lasers. The results of the calculations indicate that a large-aperture E-beam-sustained discharge is effective for excitation of the cryogenically cooled gain medium, which uses He rich mixture at low pressure. The system is scalable and capable of generating large powers and energies.

    • Generation of tunable 16 𝜇m radiation from CO2 by cascade lasing

      Utpal Nundy Manoj Kumar

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      In this paper we propose a scheme to generate tunable 16 𝜇m radiation from CO2 molecules by cascade lasing. The stimulating 9.5 𝜇m radiation is generated internally by the fast rotating mirror Q-switching technique. The optical scheme proposed by us uses an intracavity prism to separate the 9.5 𝜇m and the 16 𝜇m beams. This facilitates independent tuning of the two beams if required. In the present configuration, only the 16 𝜇m cavity is dispersive. The 9.5 𝜇m beam grows spontaneously in a stable semiconfocal resonator. We have developed a theoretical model to simulate the proposed scheme. The model predicts the energy and power of 16 𝜇m radiation. The calculated values are much higher than the previously obtained experimental values. The results point out the feasibility of developing a laser system based on the theoretical design parameters presented in this paper. Such laser systems can find application in uranium isotope separation studies.

    • Benefits of cryogenic cooling on the operation of a pulsed CO2 laser

      Utpal Nundy

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      The paper presents results of a theoretical model of a pulsed electron beam controlled CO2 laser (EBCL) to investigate the effect of cooling on the laser gas mixture. It is shown that cryogenic cooling can significantly improve the performance of the laser. The efficiency of an EBCL improved from 20% to 25.3% by cooling it to 200 K. The improvement is mainly due to the decrease of thermal population of the CO2 (0 1 0) vibration level.

    • A new approach to model CW CO$_2$ laser using rate equations


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      Two popular methods to analyse the operation of CW CO$_2$ lasers use the temperature model and the rate equation model. Among the two, the latter model directly calculates the population densities in the various vibrational levels connected with the lasing action, and provides a clearer illustration of the processes involved. Rate equation models used earlier grouped a number of vibration levels together, on the basis of normal modes of vibrations of CO$_2$. However, such grouping has an inherent disadvantage as it requires that theselevels be in thermal equilibrium. Here we report a new approach for modelling CW CO$_2$ lasers wherein the relevant vibration levels are identified and independently treated. They are connected with each other through theprocesses of excitation, relaxation and radiative transitions. We use the universally accepted rate coefficients to describe these processes. The other distinguishing feature of our model is the methodology adopted for carryingout the calculations. For instance, the CW case being a steady state, all the rate equations are thus equated to zero. In the prior works, researchers derived analytical expressions for the vibration level population densities, thatbecomes quite a tedious task with increasing number of levels. Grouping of the vibration levels helped in restricting the number of equations and this facilitated the derivation of these analytical expressions. We show that insteady state, these rate equations form a set of linear algebric equations. Instead of deriving analytical expressions, these can be elegantly solved using the matrix method. The population inversion calculated in this manner alongwith the relaxation rate of the upper laser level determines the output power of the laser. We have applied the model to an experimental CW laser reported in literature. Our results match the experimentally reported power.

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