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