Rotational Relaxation Studies in Co2 Molecules by Means of a Multiline Tea Co2 Laser System

One of the most serious problems in high-power, short-pulse, 10.6#m CO2 laser systems is the efficiency. When the short-pulse (pulse duration _~ 1 ns) amplification efficiency is compared to the efficiency of normal "long-pulse" (pulse duration "-lOOns) CO2 amplifying systems, the former is rather poor. As is well-known, energy extraction from a laser amplifier only occurs at the wavelengths which are injected. Therefore, conventional oscillators that usually operate on one transition are limited in their capability to extract energy from an amplifier. For example, at room temperature the population in the J = 19 level of the 001 vibrational band is approx. 7 ~0 of the total population in this band. If the injected pulse of a single transition laser is very short, rotational effects can be ignored and the efficiency is about 7 ~,,, of the "long-pulse" efficiency. It is therefore advantageous to inject the amplifier with a laser pulse that oscillates on many transitions. In the past several methods have been used to construct a multiline oscillator, 11'2) but they all lack the ability to control the various lines independently. We developed a controllable multiline laser oscillator with up to six adjacent lines oscillating simultaneously on the P-branch of the 001 100 vibrational transition/3~ These lines are independently adjustable which makes it an ideal oscillator for studying the saturation parameter as a function of the number of rotational lines injected as well as a function of the separation between the lines. The results can provide new information about the collisional processes that accomplish rotational relaxation. This information is expected to be important for efficient short-pulse energy extraction from CO2 laser amplifiers. The oscillator configuration is shown in Fig. 1. The setup is slightly modified with respect to Ref. (3). The result is a decreased intensity of radiation at the grating, mode-locker and output coupling mirror. With the diaphragms in the dispersed arm adjusted properly, the output pulse has a FWHM of 300 ns and consists of a train of pulses. These pulses have a FW HM of 1.2 ns, as measured with a photon-drag detector and a Tektronix R7912 transient digitizer. 'Fhe shape of the total output pulse is shown in Fig. 2(a), the corresponding energy spectrum is shown in Fig. 2(b). This spectrum appears to be reproducible.