Frequency Chirping In 10 Atmosphere CO 2 Lasers

Abstract

ABSTRACT The frequency chirping of high pressure CO2 lasers operated at 10 atmospheres was inves­ tigated experimentally. When the high pressure lasers were not placed in optical cavities, significant changes in the index of refraction of the laser medium were observed. A rapid phase perturbation associated to the discharge was then detected; the phase shift could attain 100 degrees. This rapid phase perturbation was followed by a much slower phase drift. The insertion of the high pressure CO2 lasers in optical cavities led to single mode emission when seeded with continuous CO2 lasers. Absolute chirp rates reaching 200 MHZ/us were measured during the 100 nanosecond pulses. 1. INTRODUCTION CO2 lasers operated at pressures of 10 atmospheres or above offer the possibility of continuous tunability over four emission bands comprized between 9 and 11 um. At such pressures, collisional broadening causes an overlap between neighbouring rotational- vibrational transitions. Although the gain spectrum is weakly modulated due to a residual rotational substructure1, the gain coefficients are nevertheless high enough to allow laser oscillation at any frequency in between most of the emission lines of low pressure CO2 lasers.A tunable, high power source in the mid-infrared, such as a high pressure CO2 laser, can find applications in laser radar technology, remote sensing and nonlinear spectroscopy. For such applications, the spectral content of the laser pulses must be restricted to a single mode. Continuously tunable single mode emission from high pressure CO2 lasers was achieved using a three mirror resonator^^ or an interferometric device^. Single mode pulses have also been obtained^ by injection of the beam from a frequency stabilized CW-CO2 laser operated at low pressure; the tunability of the scheme is limited by the linewidth of the injection source. In principle, continuous tunability can be achieved with the latter method using PbSnTe lasers as the injection source or by shifting the frequency of the CW- CO2 laser with an electro-optic device6.For many applications, it is not only essential that the spectral content of the pulses emitted by high pressure CO2 lasers is limited to a single mode, but also that the instan­ taneous frequency does not change during the pulse. If frequency chirping is present, it must be minimized, or at least be predictable. Hence, measurements must be made to detect if frequency chirping exists in high pressure CO2 lasers and, if it does exist, determine the laser parameters that affect it. We recall that frequency chirping'^ has been observed in atmospheric pressure CO? lasers; the use of variable reflectivity mirrors11 has virtually eliminated any frequency chirping in such lasers. One major difference between high pressure and atmospheric pressure CO? lasers is the duration of the emitted pulses, the pulses being much shorter at a pressure of 10 atmospheres. Lovold1^ has recently charac­ terized the frequency chirping of RF-excited, high pressure CO2 lasers with a waveguide structure where mode selectivity is achieved with a three mirror resonator. In this paper, we are concerned with measurements of frequency chirping in transversely excited 10 atmosphere CO? lasers equipped with an open resonator and where mode filtering is made by injection locking.The paper is divided as follows. The experimental set-up for the measurement of phase drift in the absence or in the presence of laser action is described in the next section; the characteristics of the two high pressure CO2 lasers used for the experiments are also given. The experimental results are presented in section 3. The results are briefly discussed in section 4.