CHAPTER 2. THE DYE LASER •••: » •1 THE DYE OSCILLATOR AND WAVELENGTH MONITORING UNIT [THE DYE OSCILLATOR (CLOSE UP) 2.1 INTRODUCTION It is a wellknown fact, that the lasers have played a key- role in the foundation and development of modern spectroscopic techniques. The organic dye lasers in particular, have captured a great deal of attention due to their continuous wavelength tunability, indeed, many visible wavelength lasers are known but have been eclipsed by this important advantage of organic dye lasers. In other words, this laser is the spectroscopist's dream, in that its wavelength can be precisely controlled in every applicable parameter space. Both, the output wavelength and the bandwidth can be accurately determined. In addition, the time duration of the output is variable from continuous wave to subpicosecond pulse regime. Moreover these lasers operate with competitive working efficiencies with other lasers and cover not only the entire visible region but also the ultra-violet and infra-red region of the electro-magnetic spectrum. No wonder, dye lasers have found various diversified field applications. The complete design and fabricational details of a narrow linewidth, multiple prism pulsed dye oscillator is presented in this chapter. The technological development in the design modification of the dye resonators, their comparative study, the design and fabrication considerations and the working of the dye oscillator along with the laser parametric measurements are thoroughly discussed. A brief description of the nitrogen laser, used to pump the dye laser, is also incorporated in this chapter. 2.2 SPECTROSCOPY OF TEE DYE MOLECULE The laser action in the solution containing organic dye was realized, first, by P.P. Sorokin and J.R. Lankard [1] in 1966. 10 The organic dyes absorb and emit radiation from ultraviolet to infrared wavelengths. Eventhough thousands of dyes are known •' today, only few fluoresce in the solution. The laser dyes are charcterised by separate absorption and emission bands. The emission band is at longer wavelength than the principal absorption band. Generally these bands are approximately 1000 cm wide on energy scale. The energy level diagram of a typical dye molecule is illustrated in Fig. 2.1. The laser action in a dye molecule can be explained by means of five electronic energy levels in it and the transitions between these levels. Each of the electronic energy levels is very wide in energy and broadened by a continuum of vibrational, rotational, and solvent states. These levels are divided into two manifold of states, the singlet manifold and the triplet manifold. The transitions in the singlet manifold are responsible for the laser action in the dye molecule. The singlet states are labled S0, 'h\, and S2 for ground, first excited, and second excited states respectively. The triplet states are labled T0 and T^. The triplet manifold is connected to the singlet V manifold by means of spin-forbidden transitions. The pump optical energy { ^^) is absorbed by molecules in the ground singlet state S0 and they get excited to the first excited singlet state S^• Once the molecules are excited, they are subjected to various radiative and non-radiative de-excitation -> processes. In Fig. 2.1, the radiative transitions are shown by straight solid arrows, while the non-radiative, wavy arrows. The molecules relax to the lower state of the vibronic manifold in S^^ 11 followed by a return to S0 by spontaneous emission of fluorescence photons (/^FL) which contribute to the laser action. They undergo various types of non-radiative relaxation processes also, resulting in lowering of the efficiency of the laser. They can relax non-radiatively to the ground state S0 or can populate triplet manifold T0 via Inter System Crossing (ISC). The time taken for all these radiationless relaxations is of the order of few pico-seconds. The lifetime of the first excited singlet state S^ is of particular importance, to the operation of the laser, especially when operated in the pulsed mode. Normally it is of the order of 1 to 5 nanosecond times the quantum yield (QY) for fluorescence for the dye, i.e. Ts = (QY) Tr (2.1) where fr is the radiative lifetime and tg is the observable lifetime of the state. The quantum yield (QY) is always less than one, due to the radiationless transitions, which compete with the spontaneous fluorescence for depleting the first excited singlet state population. There are two types of transitions, that significantly affect the performance of the dye laser. The transition between S^ state and higher lying singlet state S2 can occur by partial internal absorption of fluorescence light. Although this absorption affects the laser gain relation in a significant manner, it does not constitute an energy loss, since it repopulates the excited state. In contrast, the non-radiative transitions, between the states of equal multiplicity i.e. internal conversion (IC) and the transition to a triplet manifold 12 T0 via inter system crossing (ISC) reduce the quantum efficiency for the fluorescence and the energy is totally lost into heating of the dye solution. However, for a short pulse dye laser, this absorption can often be ignored, since the quantum yield of production of triplet state molecules from excited singlet state via inter system crossing (ISC) is approximately 0.5 % [2]. It i*s important to note here, that the dye laser is a true four level system. The excitation is well separated in energy from the emission by essentially infinitely fast radiationless transitions. Therefore, the device can reach laser threshold at very small population inversions. The laser action involves stimulated emission between low lying levels of excited singlet manifold and higher levels of the ground singlet state manifold. The stimulated emission occurs over a continuous range, because the levels involved in each of these manifold are so close to each other, that the collisionai broadening mixes them completely. 2.3 TECHNOLOGICAL DEVELOPMENTS The importance of the dye laser became evident soon after its introduction in 1966 [1]. Indeed in the period 1966-1971 few hundreds of journal papers were published on dye lasers and their applications, however, during this period only the tunability aspect of the dye laser as a different more versatile laser source was apparent. The most fundamental laser characteristics, narrow linewidth oscillation, was absent from the dye laser until 1972. In that year, T.W. Hansch [3] developed and introduced the first narrow band, broadly tunable pulsed dye laser. Subsequent 13 development in the dye laser resonator designs was based on the following fundamental grating principles [4]. i The angular dispersion is governed solely by grating angle and not by the number of grooves, ii The spectral resolution is equal to the number of grooves illuminated times the grating order. Hansch's approach of intracavity expansion of dye beam, with the help of beam expanding telescope, in order to cover more number of grooves, reduced the efficiency because of the increase in length of resonator [see Fig. 2.2(a)]. In the latter design [5-7] [See-^ig. 2.2(b)] the telescope was replaced by a single prism, used as a beam expander, at a grazing incident angle. However this high incidence angle, introduced equivalently high loss due the reflection off the prism surface. Thus the gain in efficiency by shortening the resonator, in this manner, nearly compensated for the loss introduced by the surface. A further significant development in the area of cavity compactness and narrow linewidth emission was the grazing incidence pulsed dye laser oscillator [8-9] [See Fig.2.2(c)]. However this design introduced a serious problem of Amplified Spontaneous Emission (ASE) background, getting coupled out with the dye laser beam. In spite of the disadvantages of the prism resonator designs, the advantages and inherent simplicity of the prism expanders compared to refracting telescopes were irltriguing. More importantly was the one-dimensional nature of the expansion maintaining alignment simplicity- a tremendous advantage over 14 Xp oc " V DC (a) DG FRM (c) Fig.2.2 : Dye laser resonator designs, Ap : Pump laser beam CL : Cylindrical Lens DC : Dye cell OC : Output coupler BET : Beam Expanding Telescope DG : Diffraction Grating BEP Beam Expanding Prism FRM : Fully Reflecting Mirror spherical lenses telescopes, in which element spacings are critical or in which unnecessary two-dimensional expansion occurs. Also, inside the high gain laser cavity, this pre- expansion significantly narrowed the laser linewidth. Realizing these facts, in 1977 G.K. Klauminser [10] reported a novel idea of using multiple prisms, instead of one, for the expansion of the beam. He employed four prisms in tandem and achieved magnification of 40x at lower losses. The reflection loss can greatly be reduced by using multiple prisms at smaller incidence angle, which together yield the desired magnifcation. Indeed, with a Multiple Prism Littrow (MPL) design, better linewidths at competitive efficiencies were demonstrated [11]. The level of Amplified Spontaneous Emission (ASE) was reduced by more than two orders of magnitude compared to the open cavity single prism design. In addition these oscillators offered very good beam quality with the divergence close to the diffraction limit. Various novel resonator designs, [12-16] incorporating multiple prisms as one of the intraresonator elements, have appeared in the recent years with an interesting change in the choice of the grating. The high groove density grating was replaced by a coarse grating due to the realization of the fact, that the resolution can not be increased by using a grating with very high groove density, since the grating order for a given wavelength will be correspondingly reduced. In fact a coarse grating used in higher orders would be a better choice than using a fine grating, as long as one dye could not operate on two 15 adjacent grating orders.