A Dye Laser Pumped by an Air Laser

A modified dye laser system operating at UNARC is discussed. The dye laser (peak wavelengths440 nm) is pumped by an air/Ng_-laser. The dye is a new one, Tinopal, provided by CIBA Co. in Egypt. The dye laser/air laser characteristics are presented.


INTRODUCTION
The possibility of achieving optical gain (negative absorption) in complex molecules was noticed by Ivanov (1960) even before the development of lasers. Dye lasers have subsequently been pumped by flashlamps (Snavely, 1969), nitrogen lasers, and many other types of lasers (Schafer, 1973). N2 lasers are very attractive lasers for excitation of a variety of dyes. The high efficiency of the N2 laser pumping and its pulse characteristics allow efficient excitation of dyes with significant output. The continuously tunable wavelength output of a dye laser has made it possible to carry out experiments in high resolution spectroscopy. In these experiments radiation could be tuned to excite, selectively, any given transition within the wavelength range of the dye.
In this article, we report preliminary detection and gain analysis of the radiation from a new dye (Tinopal) pumped by an air/N2 laser. A solution of the radiative transfer equation in the dye allows experimental measurements of both absorption and emission coefficients. The pumping power of the system is large enough to exceed the threshold power required to lase dyes placed inside a Littrow cavity. Details of the pumping system and the dye laser arrangement are described. A schematic diagram of the dye laser and measuring system is shown in Figure 1. The pumping laser is an AVCO EVERETT N2-1aser.
The gain of nitrogen lasers is so high that only one mirror is required to produce a pulse of power over 20 kWatt and duration less than 15 nanoseconds. This pulse duration is shorter than that necessary to cause significant excitation of the triplet states of dyes and hence triplet states losses are considered insignificant. The pulse shapes for two adjustments of the back reflection mirror are shown in Figure 2.
A detailed account of calibration and characterisation apparatus is given by E1-Raey and Amer (1982). Due to a small air leakage into the system and use of commercial nitrogen, the system produces low power. A power of near 10 kwatt/pulse is estimated. The relative average pumping power of the N2 enriched air laser is monitored by a radiometer having a pyroelectric detector of flat spectral response.
The N2/air laser spectrum contains, in addition to the prominant 3371 A line of the second positive system of N2(C 31Iu--B 3IIg), several ultraviolet lines of longer wavelengths associated with other components of air. Figure  profile represents the convolution of the real profile I(v) with monochromator response function A (v) of a bandwidth -3 A, according to This is the main factor responsible for the observed low amplitude and large bandwidth of the 3371 A line as compared to other lines.
The output of the air/N2-1aser is incident on a dye cell placed inside a cavity in a Littrow arrangement. A shutter is introduced inside the cavity so as to block the beam from reaching the grating. The amplified spontaneous emission (ASE) from the dye is synchronously detected by using a lock-in amplifier tuned to the pulse repetition frequency of the pumping laser. The gain spectra of several dyes of industrial interest are measured by the length dependence of ASE (Dienes and  To find the effective pump power density and hence obtain a measure of the excitation transfer distance, it is necessary to plot the variations of gain with pump power. Such a plot is shown in Figure 5, and is a straight line. The extrapolated y-axis intercept should equal the ground state loss (Dienes and Madden, 1973).

ANALYSIS OF THE DYE LASER
If the shutter inside the cavity is removed and the grating angle is adjusted, feedback and hence stimulated emission occurs with consequent spectral narrowing of the output. Figure 6 shows the dye laser output spectrum for three different pumping powers, superimposed on the amplified spontaneous emission of the dye (Tinopal of concentration 0.8 gm/one litre of water of pH 7  6 Dye laser output spectrum (convoluted with instrumental response).
Amplified spontaneous emission of the same dye is shown for comparison.
increasing the rate ot flow of the N2 gas. Reduction ot bandwidths is then possible for higher diffraction orders by using internal and/or external Fabry-Perot etalons.

CONCLUSION
A dye laser beam ot tunable wavelength is detected from organic Tinopal solution, pumped by an air/N2-1aser. Preliminary measurements ot gain spectra and pumping power variations are carried out. Study ot polarisation effects, bandwidth narrowing and gain enhancements are in progress. Applications in detection ot trace pollutants, high resolution spectroscopy and lifetime measurements are planned.