NITROGEN LASER

Nitrogen laser (N2) has been served as an ultraviolet radiation source due to its low cost and the easiness of operation as well as maintenance. It has been used extensively as a pumping source for dye laser and the in the photochemistry.

The N2 laser is a very special type of gas laser. First, the lifetime of the upper laser level is shorter than the lower laser level. This property does not allow the N2 laser to be operates in the continuous mode. Therefore, the N2 laser can only be operates in pulsed mode where the maximum width of the pulse is limited the upper laser level lifetime.

Besides this, the gain coefficient of the N2 laser is very high. Nitrogen laser is also known as "super-radiant" laser. This type of laser is operated without any optical resonator. The output pulse has a larger divergence and shorter coherence length. This limits the application of N2 lasers. However, this shortcoming can be overcome by oscillator-amplifier configuration.

PRINCIPLES OF NITROGEN LASER
The N₂ lasers are operated in pulsed mode. Nitrogen molecules are excited by gas discharge produced from a short but fast rise time electrical pulse. The excited molecules will relax to the lower state by radiation. There are a few different transitions involve in this relaxation process. However, the dominant transition is from the lowest vibrational level of the C³Pu electronic state to various vibrational levels of the B³Pg electronic state. This gives the output radiation in the ultraviolet region and the most intense wavelength is 337.1nm.

Unlike most of the lasers, the lifetime of C³Pu is shorter compared to the lifetime of the B³Pg state. Therefore, the population inversion and also the laser action can only be obtained in for a short period of time. This is known as self-terminating lasers.

Due to the short lifetime in the upper laser level, the population inversion can only be obtained by a rapid pulse discharge from fast circuit, such as Blumlein circuit or capacitor transfer circuit. In this short period of time when the population inversion occurs, the process of stimulated emission happens. However, the gain is sufficiently large that the laser radiation becomes very intense in only one pass. In the same time, the upper laser level is largely depopulated through the stimulated emission. Thus, the optical resonator is not necessary in the N2 laser. As a result, the output beam quality is always poor in spatial coherency and has high divergence compared to other laser systems.

To overcome partially these shortcomings, oscillator-amplifier system is introduced. A master oscillator is used to produce a high quality output pulse. This pulse is usually low in its power and it will be amplified in the power amplifier. With this configuration, the applications of the N2 laser can be further extended.

OUR RESEARCH IN NITROGREN LASER
Nitrogen laser research is one of the oldest laser researches in our laboratory. Various generation of N2 laser has been constructed in our laboratory. Presently, the N2 laser research can be divided into three categories.

The first category is with the aim of producing high power N2 laser. This high power N2 laser is designed as a pumping source where its beam quality is not emphasized. The influence of the circuit parameters on the output power is studied. Different electrode configurations to produce different pumping density are also used in our research.

Another group of N2 research involves the simulation of the nitrogen laser. A computer model based on Fitzimmons approach has been developed. Two integro-differential equations can be obtained to model the driving circuit. The gas discharge model is based on the accurate and simple Townsend equation.

The third group of N2 research is working on a master-oscillator-power-amplifier system (MOPA). This system is with the objective of producing high power high quality of ultraviolet pulse. A compact MOPA system has been constructed and parametrically studied.