Tunable lasers handbook phần 2 pot

34 R. C. Sze and D. G. Harris originate by field emission from a cathode (frequently carbon felt), which has been negatively pulsed with respect to the anode, generally maintained at ground. The vacuum diode (generally operating at 10-5 to 10-7 Torr) is separated from the high-pressure laser gases by a thin foil. The emitted electrons pass through the foil, though losing some energy, and enter the lasing media, creating ions. Although large and expensive. these devices are easily scaled to meter dimensions and allow long-pulse (1 psec or greater) pumping. They are therefore generally used as amplifiers rather than oscillators. Preionized avalanche discharges have been utilized to produce a uniform plasma. The low-energy electrons in the plasma acquire sufficient energy to excite the rare gas atoms to a metastable state, thus allowing the reaction kinetics to proceed along the neutral reaction channel. The relative ease and low cost of this approach has led to the rapid development of high-average-power lasers. Discharge excimer lasers are discussed in Section 4. Table 1 lists some of the best known excimer lasers with their respective electronic transitions and approximate emission bandwidth andlor tuning ranges. In addition to tunability, an important characteristic in pulsed gas lasers. including excimer lasers, is narrow-linewidth emission. Some of the early work on tunable narrow-linewidth excimer lasers was reported by Loree et al. [3] who uti- lized isosceles prisms to provide intracavity dispersion and wavelength tuning in excimer lasers. These authors report linewidths of 0.1 to 0.2 nm and 0.05 nm for KrF and ArF lasers, respectively [3]. Additional and alternative methods to yield narrow-linewidth emission include the use of intracavity etalons [9] and grazing- incidence (GI) configurations [4]. During this period. circa 1981. multiple-prism TABLE 1 Excimer Laser Transitions0 Laser Transition h (nm) - Bandwidth Reference .AIF B+X 193 17000 GHzh KrF B+X 218 10500 GHzh 2583 GHz XZCl B+X 308 374 GHz 201 GHz 308.2 397 GHz 223 GHz XeF B-1X 35 1 187 GHzc 353 330 GHzr C+A 466-514 nmhc OAdapted from Duarte [2]. hTuning range. ‘Elecuon beam excitation. 3 Tunable Excimer lasers 35 TABLE 2 Narrow-Linewidth Gas Laser Oscillatorsa Laser Cavity A (nm) Av Eo Reference ArF KrF X?Cl XeCl XeCl XeCl XeF CO, CO, CO, C02 MPL GI GIh GE' GI 3 etalons MGId GIh GIh MPL HMPGP 193 218 308 308 308 308 35 1 10,591 10,591 10.591 10.591 10 GHz 59 GHz -31 GHz -1.5 GHz -1 GHz 5150 MHz -40 MHz 117 MHz 100-700 MHz 5130 MHz 107 MHz 150 pJ 15 pJ 50 mT -1 mT 3mT 2-5 pJ -0.1 pJ 140 mT 230 mJ 200 mJ 85 mT 3From Dume [l?]. "pen-cavity configuration. 'Incorporates Michelson interferometer. dhhltipass grating interferometer. eHybrid multiple-prism grazing-incidence cavity. grating configurations were also introduced to pulsed gas lasers [10,11]. In this regard, note that multiple-prism Littrow (MPL) grating configurations were subse- quently incorporated in commercially available gas lasers. Table 2 provides a use- ful summary of different types of cavities available for narrow-linewidth gas laser oscillators. including excimer lasers, with their respective emission performance. The performance of some oscillatorlamplifier and master oscillator/forced oscillator excimer laser systems is summarized in Table 3. Applications for tunable narrow-linewidth excimer lasers include spec- troscopy, selective photoionization processes, laser radar. and lidar. In this chapter first we survey the basic spectroscopic characteristics of excimer laser emission. and then follow up with a review of tuning methods for discharge and electron beam pumped excimer lasers. For a historical perspective on excimer lasers the reader should consult [ 11. 2. EXCIMER ACTIVE MEDIA Excimers are an important active media for lasers operating in the ultravio- let and vacuum ultraviolet (VUV) spectral regions. Although a comprehensive understanding of excimers can involve quite a complex modeling of kinetic reactions and absorbing species, these molecules do share some common features. Consequently, a few simple models and concepts 36 R. C. Sze and D. G. Harris TABLE 3 Escimer Lasers Oscillator/Amplifier and Master Oscillator/Forced Oscillator Oscillator Output Laser medium configuration Secondary stage Linewidth energy (mJj Reference KrF GI XeCl Double etalon XeCl GI XeCl IVPL XeF Dye laser (C+N KrF 3 etalons AIF Prism expander grating KrF XeCl Amplifier 1 GHz Amplifier 599 MHz AmplifieP 4.5 GHz Amplifier 15 GHz Amplifier 6 GHz Forced oscillator 3 GHz Forced oscillator 9 GHz 6 GHz 9 GHz 50 310 300 450-750 UK) 100 200 120 oRegenerative. can be used to explain their spectroscopic features with regard to frequency nar- rowing and tunability of the lasing spectrum. Excimers are a class of molecules in which an electronically excited molec- ular state is formed by one atom in an electronically excited state associating with a second atom in its ground state. The molecular ground state is unbound or only weakly bound (by van der Waals forces). Consequently. a population inver- sion is automatically established when the excited state is formed. A photon is emitted and the resulting ground state molecule dissociates. along the lower potential curve, in a time comparable to one vibrational period (-10-12 sec) (Fig. 1). The practical advantage of such a system is that one photon can be extracted from each excited molecule produced. rather than the situation in conventional laser media in which only enough photons can be extracted to equalize the popu- lations in the upper and lower levels. The emission from the bound repulsive transition is typically a broad coritinuum resulting from the lack of vibrational structure and the steepness of the unbound ground state. Emissions from excimers with a weakly bound ground state. most notably XeCl and XeF, show a more conventional vibrational and rotational structure. Using laser rate equations and semiclassical theory, one can go quite far with elementary derivations toward describing the behavior of excimers. Indeed calcula- tions of the gain coefficient, saturation intensity, stimulated emission cross sections and even modeling of the ground state can be quite easily accomplished [27, 27aI. Care must be taken not to rely completely on these models, because these parame- ters can vary quite differently depending on the experimental conditions. For instance, the saturation parameter may vary bj7 a factor of 2 or more depending on 3 Tunable Excimer Lasers 37 > a, c w P I\ Other excited states \ r=*tomic AB’ Excimer upper level excitation Excirner emission A+B Weak Van Der Waals Bonding Internuclear Separation FIGURE 1 Energy level diagram for excimer lasers showing relevant electronic states. the pumping rate and the plasma conditions. Predicting the lasing spectra, or even fluorescence. can involve more than 100 kinetic reactions and loss processes. The most developed of this class of molecules as laser media are the rare gas halides, which show strong lasing on the B+X transitions of ArF (193 nm). KrF (248 nm), XeCl(308 nm), and XeF (351 and 353 nm). The C+A transition of XeF (490 nm) has also emerged as a potential high-power tunable laser source in the visible spectrum. The rare gas excimers are important sources of WV radiation: Ar, (126 nm), Krz (146 nm), and Xe, (172 nm). The requirement that the pump source be a relativistic electron beam has limited their availability and development. 2.1 Rare Gas Halide Excirners The most developed of the excimer lasers are the rare gas halides, which have shown high single pulse energy, high average power, and high efficiency. The most important of these are ArF, KrF, XeC1, and XeF. The former two, with an unbound ground state, exhibit continuous homogeneously broadened spectra. The latter two excimers, with weakly bound ground states, exhibit the highly structured spectra of overlapping rovibrational transitions. 2.1. I ArF (793 nm) The ArF spectrum is a continuum similar to that of KrF. The B+X emission is a *x-?X transition. The reaction kinetics are also similar to KrF. However, 38 R. C. Sze and D. G. Harris there are features in the spectrum due to the absorption of molecular oxygen (Schurnann-Runge band) within the resonator cavity. Interest in line narrowing and tuning of ArF has grown as applications for shorter wavelength sources developed in the area of microfabrication. Ochi et al. [28] has built an oscillator with a 1.6-pm linewidth at 350 Hz with 7.4 mJ per pulse. 2. 7.2 KrF (248 nrn) Much research has been done on KrF lasers because of their use as high- power lasers for laser fusion research as well as their use in the microelectronics industry. The KrF spectrum is a broad continuum (Fig. 2), which is considered to be homogeneously broadened owing to its repulsive ground state. Narrow absorp- tion lines have been observed that are attributed to the excited states of rare gas ions. Spectral tuning has been observed over a continuous range of 355 cm-1. 2.7.3 XeF (BEXJ The structure of the XeF molecule is significantly different from that of the other rare gas halides and consequently its spectral properties also differ. The X state is bound by 1065 cm-1 and therefore has vibrational levels. Additionally, the C state lies about 700 cm-1 below the B state. The spectra of the B+X tran- sition show emissions at 353 and 351 nm [30-331. Early investigators also noted that as the temperature was increased, the lasing efficiency of the B+X transi- tion improved significantly [35.36] (Fig. 3). Several explanations exist to explain this improved efficiency: (1) increased vibrational relaxation of the B state, (2) increased dissociation of the X state, and (3) decreased narrowband absorption at 351 nm. The complexity of the molecular structure implies that energy is not IIIIIIIIIII II’IIIIII I I I I I I I ’ I I I 260 250 240 230 Wavelength (nm) FIGURE 2 Ewing [29]). Fluorescent spectrum from the B’E,,2-X2Z,:2 transition in KrF (from Brau and 3 Tunable Excirner Lasers 39 transferred rapidly between the states and therefore the spectrum is not homoge- neously broadened. The 353-nm band emission comes primarily from the XeF (B, 1“ = 0) -+ XeF (X. I]’’ = 3) transition. whereas the 351-nm band is composed of radiation from the XeF (B, ?’ = 1) -+ XeF (X, Y’’ = 3) and XeF (B, 1,’ = 0) -+ XeF (X, Y”= 2) transi- tions. Each vibrational transition has four rotational branches: Pe. Re. Pf. and Rf where e andfrepresent spin “up” and spin ”down” for the transitions. Both bands have considerable structure, which is attributed to overlapping rovibronic transi- tions. As the temperature is increased. the spectra and efficiency of the 353-nrn a I I I I 351 .O 351 5 352.0 352 5 353 0 353.5 Wavelength (nrn) I I I I 1- I 351.0 351.5 352.0 352.5 353.0 353.5 Wavelength (nrn) FIGURE 3 Inhomogeneous characteristics ar2 evident (from Harris et al. [34]i. Fre2 running lasers spectrum of X2F (B+X transition) at (ai 300’K and (b) li@K 40 R. C. Sze and D. G. Harris band remain virtually unchanged, whereas the 351-nm band shows marked changes in both. The energy stored in XeF resides in a multitude of rotational states, which must be collisionally coupled on time scales that are short compared to the stim- ulated emission rate in order to achieve narrowband lasing. The appearance of clusters of rotational lines lasing relatively independently suggests that the rota- tional relaxation rates in the B and/or X states may be too slow to allow narrow- band lasing. Indeed, it is difficult to achieve efficient injection locking when the small signal gain is much greater than the threshold gain [37.38]. 2.7.4 XeF (C-+A) The XeF molecule also emits a broad continuum between 470 and 500 nm from the C+A transition (l rL2n). The A state is repulsive, without a potential well, so the emission is a true continuum, allowing narrowband lasing as well as continuous tuning across the emission spectrum. The excitation sources have been both short-pulse and long-pulse electron beams. Under short-pulse excita- tion (10 MW/cm; for 10 ns) the media has optical absorption during the electron beam deposition time and then gain (3Wcm) in the plasma afterglow. Narrow- band tuning as well as injection seeding has been used to tune across the gain profile [39-43]. The media show gain throughout the energy deposition pulse under low-power long-pulse electron beam excitation (250 kW/cm3 for 700 ns). However strong lasing is reached only after 300 ns [44]. 2.1.5 XeCl(308 nrn) The C state of XeCl molecule lies approximately 230 cm-1 below the B state. Additionally, the ground state is bound by 255 cm-1, lasing in the B+X bands occurs predominantly on the 0-1 band but also weakly on the 0-2 and e3 bands [45]. Although XeCl lasers have been made to operate narrow band, attempts to injection seed amplifiers have shown a strong wavelength dependence [46], which has been attributed to saturation of the lower vibrational levels [47]. Owing to the long gas lifetime and ability to use inexpensive nonquartz optics, XeCl has been the preferred excimer to test line-narrowing techniques and novel resonators. 2.7.6 Other Rare Gas Halide Excirners Lasing has been observed in several other rare gas halides, and although these systems have not been developed to the extent of those already discussed they do offer potentially tunable radiation. Excimer emission has been observed at 175.0 nm in ArCl [27], 222 nm in KrCl [48,49], and 281.8 nm in XeBr [50], which are believed to be excimers with repulsive ground states. A short operat- ing lifetime for XeBr has not yet been thoroughly addressed [51]. There has been renewed interest in KrCl because it offers potentially higher efficiency than XeCl [52.53]. The pulse lengths have been extended to 185 ns, but nothing has been pursued in the area of spectral control [54,55]. 3 Tunable Excirner Lasers 41 2.2 Rare Gas Excjmer Lasers The IC: -12; transitions in the noble gases (Ar,, Kr,, Xe,) provide VUV laser radiation. They all exhibit continuum emission The low stimulated emis- sion cross sections and short lifetimes of the upper states require high pump rates. which necessitates an electron beam generator as a pumping source. The expense and cumbersome nature of such systems have unfortunately limited their availability to relatively few laboratories. Despite the dearth of low-loss and damage-resistant optical materials in the VUV, there has been considerable progress in line narrowing and tunability of these three laser media. The perfor- mance of these lasers is listed in Table 4. 3. TUNING OF DISCHARGE AN5 ELECTRON BEAM PUMPED EXCIMER LASERS The avalanche discharge excimer laser is the most common format that is readily available to the researcher. These devices are relatively compact and occupy a fraction of the space of an optical table. In terms of frequency tunabil- ity. they can potentially access the full bandwidth of the excimer laser transi- tions, which, as we have seen in the previous sections, vary from molecule to molecule. For a typicall homogeneously broadened single broadband transition the full-width half-maximum bandwidth is of the order of 200 cm-1. Typically a narrowband tunable oscillator is developed that is then amplified in single-pass. multiple-pass, or regenerative amplifier configurations to obtain high powers (Fig. 4), Often the amplifier may be an electron beam pumped or electron beam sustained discharge laser. These lasers are generally low-gain, large-volume devices with temporal gain times of a factor of 10 to 20 longer than the commercially available avalanche dischxge lasers. TABLE 4 Performance of Rare Gas Excimer Lasersa Wavelength Linewidth output Laser (nm) (nmJ Tuning elements poiier t MW) Reference Ar2* 124.5-127.5 0.3 Prism 2 [57] 123.2-1 27.4 0.6 Grating 0.001 tjgl 126 16 [W Kr,' 115.7 0.8 [6@1 xe2- 170-175 0.13 Prism 0.7 t611 JAdapted from Hooker and Webb [56] 42 R. C. Sze and D. G. Harris DISPERSIVE OSCILLATOR AMPLIFIER ELEMENTS GAIN MEDIUM GAIN MEDIUM I I NARROWBAND TUNED OSCILLATOR AND SINGLE PASS AMPLIFIER DISPERSIVE OSCILLATOR AMPLIFIER ELEMENTS GAIN MEDIUM GAIN MEDIUM NARROWBAND TUNED OSCILLATOR WITH REGENERATIVE AMPLIFIER FIGURE 4 Generalized oscillator-amplifier configurations. Amplifier stages incorporating unstable resonator optics can also be known as forced oscillators. The temporal characteristics of the oscillator must meet a number of requirements in terms of obtainable linewidths and in terms of compatibility with the temporal characteristics of the amplifier. The narrowness of the line- width using a dispersive element, such as a grating or multiple-prism arrange- ment, is typically improved by an order of magnitude or more over single-pass linewidths when many round-trips are available in the oscillator [62]. Thus, the gain time in the oscillator is an important factor in the achievable linewidth of an excimer laser system. The gain time of the oscillator must also be compatible with the gain time of the amplifier system. It is, however, possible to have oscil- lator gain times that are shorter than the amplifier system and still extract energy from the amplifier for the full gain time of the amplifier. In single-pass and multiple-pass configurations, this can be done by beam- splitting the oscillator pulse and restacking the pulses with appropriate time delays so that the total pulse length matches the total gain time of the amplifier. In a regenerative amplifier configuration, a short-pulse oscillator can control the total gain time of the amplifier if the reflected field of the amplified oscillator pulse from the first pass is sufficient LO control the frequency output of the second pass and so forth. Generally, the degradation of the narrow frequency field is such that the technique is not effective when factors of 10 in gain times between the oscillator and amplifier are involved. The success of the latter method is generally based on the conservatism of the regenerative amplifier design. In general, care should be taken to ensure the magnification is large enough so that the amplifier is incapable of going into oscillation without the injected oscillator pulse. Remember that the wavelength purity of the amplified pulse cannot be better than the ratio of the injected oscillator intensity over the amplified spontaneous emis- sion (ASE) in the amplifier radiated into the solid angle of the oscillator beam. It 3 Tunable Excimer Lasers 43 is simplest to have the injected oscillator pulse length equal to or larger than the amplifier gain time. In the next subsections we briefly discuss the general techniques that are used to obtain narrow-linewidth tunable systems, including a discussion of the gain in the narrowness of the oscillator linewidths as a function of the number of cavity round-trips. The operation of unstable resonators is also discussed so that the limitations of an injection seeded regenerative amplifier can be understood. A brief discussion of avalanche discharge techniques is then given to instill a feel for the type of devices that are generally available. This includes typically short-pulse devices (25 ns) as well as techniques that allow stable discharges resulting in laser pulse lengths of hundreds of nanoseconds. A short review of electron beam and electron beam sustained discharges will be given as well. 3.1 Tuning and Line-Narrowing Methods expander chain such as that shown in Fig. 5 is discussed in detail in Chapter 2. and Piper [63] reduces to The passive spectral width for a Brewster prism, Littrow prism, and beam For the case of Brewster prisms. the generalized equation given by Duarte (1) For a multiple-prism assembly, or sequence, composed of I- prisms the overall single-pass dispersion is given by [ 121 FIGURE 5 Dispersive oscillator incorporaring a multiple-prism assembly (from Sze er a[. [~j],. [...]... 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Q6 0 w N a I a A 0 2 0.4 0 .2 0 0 5 1 5 IO 20 25 N FIGURE 12 Normalized linewidth as a function of round-cavity trips for (a) Case a and (b) Case b under uniform illuminationconditions (from Sze et al 1151) 50 R C Sze and D G Harris -UNIFORM ASE - QAUSSIANBEAM CCASE a BEAM a - - 10 I I I- 9 5 I I -GAUSSIAN 0.0 - - QAUSSIAN I BEAM CASE a BEAM CASE - b 0.6 - - 0.4 2 J - - - 0 .2 - - _- 0 W... given by M , = 2D/1 .22 hR2 (1 I > In Eqs (9), (lo), and ( l l ) , D is the large dimension of the discharge area, h is the wavelength of the laser transition, L is the cavity separation, La is the gain length, and A is the gain length product (usually between 20 to 30 for excimer laser systems) for which superradiance becomes observable The unstable resonator equations are R, +R, =2L ( 12) and where R... 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