3. Marco conceptual
3.6 La producción de monogástricos en Colombia
Rare gas halide excimer lasers are pulsed lasers that use a mixture
of a noble gas (Ar, Kr, Xe) and a halogen (F, Cl, Br, I) as the active amplifying medium. The term excimer refers to molecules that exist only in the excited state, and dissociate in the ground state. Therefore, excimers are ideal laser gain media because of the absence of a ground state population. The laser gas mixture (usually at P ^ 1 atm.) contains a small concentration of the halogen and the rare gas in a buffer gas (He, Ne, or Ar). Upon excitation, the halogen and rare gas atoms combine to form a rare gas halide (excimer) molecule. The excited molecules
returning to the ground state emit discrete short-wavelength ultraviolet radiation and dissociate into single atoms. The principal interest in
excimer lasers is because their emission wavelengths lie in the ultraviolet and vacuum ultraviolet (from 193nm to 351nm) regions of the spectrum, where other laser sources are not readily available, and also because of the relatively efficient production of their excited states, which results in high
overall wall-plug to optical output efficiencies (typically ^ 1%).
A variety of pumping techniques have been developed and utilised to achieve efficient excitation of excimer lasers. These include direct electron
Chapter 3: The Pump Laser 54 beam pumping, electron beam sustained discharges, and fast avalanche
self-sustained electric discharges. Of these different approaches, fast avalanche electric discharges offer a relatively simple and efficient method for pumping excimer lasers when low to medium output powers are required. Discharges of this type have been used in the past for pulsed N2 (337nm) and COg (1 0pm) lasers. In this pumping scheme, the input
electrical energy is stored in a capacitor bank and then rapidly transferred, by closure of a switch, into the laser gas mix through an avalanche discharge, initiated between two transverse electrodes which are mounted within the laser chamber. The simplest circuit configuration
regularly used in fast transverse discharge excimer lasers is the c-c
transfer circuit, shown schematically in Fig. 3.1. In the diagram, ci denotes the primary energy storage capacitor, s is a high-voltage, high- current switch (spark gap or thyratron), and C2 is the secondary energy storage or “peaking” capacitor. The electrical energy at high voltage is stored in c^, and C2 is initially uncharged. Upon closure of the switch s,
voltage gain is achieved across the electrode gap, as charge is rapidly transferred from ci to C2, until the point where gas breakdown voltage is
reached and C2 discharges its stored energy into the laser medium. The
inductor L4 is used to maintain the voltage across C2 (and hence the
electrodes) zero while is charged, but otherwise does not participate in
the circuit dynamics once the switch s is closed. The inductor L i serves to
isolate the H.V. power supply from the rest of the circuit. The intrinsic self inductances of the pidmary and secondary circuit loops are represented by
L2 and L3, respectively. The inductance L2 determines the voltage rise time
in the first loop and hence the voltage at breakdown, while L3 determines
the current rise time in the second loop and hence the rate of energy deposition into the gas [1].
Chapter 3: The Pump Laser 55
c
Fig. 3.1. The c-c transfer circuit.
The discharge circuits of all excimer lasers are characterised by very low inductance and impedance, very fast voltage and current rise times, and high peak powers. The primary reason for this property is the inherent instability of the electrical discharge in the gas mixes used in
these lasers. When electrical breakdown occurs, free electrons which are present in the gas, begin to multiply exponentially through the avalanche process. In low-pressure discharges, the secondary electrons can spread rapidly by diffusion, and effectively homogenise the ionisation within the discharge volume, thus leading to the formation of a uniform glow dis
charge. At high pressures (such as those used in excimer lasers),
however, the ability of these species to diffuse rapidly is significantly reduced. This results in the formation of spatial non-uniformities in the electron distribution which can grow with time and develop quickly into several unstable and constricted arcs, or streamers. The most serious
consequence of discharge filamentation is that the resulting inhomoge-
neous distribution of refractive index can make lasing impossible, or at best, give rise to an output beam of poor optical quality. In this case, dis
Chapter 3: The Pump Laser 66 charge pumping becomes very inefficient, due to the large regions of low
current density. Even when spatial homogenisation is established, the discharge stability will only last until the formation of arcs and streamers
caused by some secondary discharge phenomena, such as hot spots on the electrodes or thermal instabilities, which usually develop over longer time
scales (> 40 ns). Because of this phenomenon, the usefial electrical pulse length of lasers with no external stabilisation is usually limited to ^ 30 ns at low pressures 0.5 atm.) and proportionally less at higher pressures. For these reasons, it is essential to minimise the inductance of the dis
charge circuit and to keep the current and voltage pulse fast-rising and short in duration 30 ns at P 0.5 atm.), so that more of the stored elec trical energy can be dumped into the laser medium before the inherent in stabilities in the discharge set in and terminate the lasing process. In practice, this may be achieved by, for example, replacing the lumped inductive circuit elements by distributed versions, by careful component arrangement, and by compact physical design of the laser assembly itself. All these provisions can lead to major reductions in the various loop
inductances and result in a more efficient excitation of the gas and a superior optical quality in the output beam. The formation of arcs and streamers in the discharge may in turn be significantly reduced or fore
stalled by utilising electrodes of smooth profiles to eliminate any strong localised electric field gradients at the surfaces, and by preionisation of the gas prior to the initiation of the discharge. Here, some form of volume ion
isation is used to “flood” the inter-electrode region with electrons, from which the main discharge can avalanche uniformly into a glow discharge rather than degenerating prematurely into arcs and streamers. Several methods of preionisation have been successfully applied to excimer lasers. A simple and yet effective technique frequently employed, is to irradiate the laser gas mix by a flux of ultraviolet radiation, immediately before the
Chapter 3: The Pump Laser 57 initiation of the main discharge (commonly known as UV preionisation). The flux of UV radiation can be generated by means of, for example, a
short corona discharge pulse [3], [4], or a spark array distributed along the length of the laser [5]-[8], In any preionising configuration, however, it is
important to use a geometry that produces uniform preionisation through the discharge volume and not only in the regions close to the preionising
elements. Because of the small penetration depth of UV radiation in the gas mixes used in these lasers, it is advantageous to locate the preioniser as close to the discharge volume as practically possible and to produce the UV radiation within the laser chamber. When large discharge volumes are involved, the more penetrating effect of X-rays may be utilised to preionise the gas (X-my preionisation). This technique has been success fully applied to large-bore excimer lasers [9], although the size and
complexity of the overall system is considerably increased.