John Reid

Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)




Professor B.K. Garside


Professor E.A. Ballik


The work described in this thesis concerns the investigation of the properties of transversely excited (TE) CO2 lasers. The introduction of these high pressure CO2 lasers in 1970 represented a very significant advance in high power laser technology. However, despite concentrated research on this CO2 laser, many aspects of the laser dynamics are not clearly understood. This is particularly true of the processes, taking place on a time-scale of a microsecond or less, which control the dynamics of TE CO2 lasers. The work reported here was initiated in 1970, and was aimed at the development of a better understanding of the dynamics of pulsed TE lasers.

The initial investigations were concerned with the small-signal gain in typical pulsed TE CO2 amplifiers. Two differential experimental techniques were used to measure gain; (1) A direct method employing a low pressure cw CO2 laser as a probe, and (2) An in-cavity technique which enabled very accurate measurement to be made of relative gain. A theoretical model was developed to explain the observed time-dependence of the gain profiles. Particular attention was paid to the behaviour of the lower laser level during the risetime of the gain, as this has been the subject of some recent controversy. The results obtained here unambiguously demonstrate that the lower laser level empties rapidly during the risetime of the gain.

Once the correct gain mechanism was determined, the pumping processes which take place during the discharge current pulse were investigated. The small-signal gain in typical gas mixtures was measured as a function of increasing discharge energy, and a gain saturation effect was observed. Careful experimental investigation revealed that this saturation of gain with increasing energy is not caused by secondary effects such as temperature increases or discharge deterioration, but is a fundamental property of CO2 discharges. At low input energies, the pumping efficiency of the upper laser level was found to agree with conventional models, but the measured efficiency falls drastically at high input energies. This has important consequences for all high energy TE CO2 lasers.

In the final section of this thesis, the interaction between the CO2 gain medium and intense laser radiation is studied. Experimental laser pulses are observed under carefully controlled conditions in a Q-switched cavity. These pulses are compared with the predictions of a theoretical model which explicitly includes rotational relaxation within the sublevels of the 10.4 μ band. It is shown that the effect of rotational coupling dominates the laser dynamics in moderately low pressure CO2 lasers, and its inclusion in the laser dynamical equations accounts for most of the difficulties encountered with previous models. The excellent fits obtained between theory and experiment enable careful studies to be made of slowly decaying laser pulse "tails". It is shown that these tails are controlled by the various relxation rates of the lower laser level. The relaxation rates are determined under conditions which are relevant to the laser dynamics.

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