CW  LASING  ACTION

     Not all lasers can operate in CW mode; many lasers operate strictly in pulsed mode. Most of these lasers never became commercially feasible because industry demand is considerable higher for CW lasers. However, nitrogen and excimer pulsed lasers are popular. For example, molecular nitrogen, with a UV output at 337.1 nm, is a self -terminating laser transition that cannot operate in CW mode.

          In any laser, the probability that a particular system will lase in CW mode (i.e., produce continuous laser output) is determined in large part by the relative lifetime of the levels involved in the laser transition. If the lower level has a relatively long lifetime, atoms in that lower energy state stay there longer, giving them a good chance of absorbing photons as well as of violating the population inversion criteria. In this situation a pulsed laser may still be possible where the upper level is filled quickly and preferentially over the lower level, but eventually the population of the lower level will exceed that of the upper level and lasing will cease. On the other hand, if the lower level has a short lifetime, atoms in that state decay quickly to another state (ground state in a model four-level laser, but quite possibly an intervening level), where they will not be available to absorb the newly emitted photons in the laser. CW gas lasers invariably possess the latter characteristic.

        The lifetime, or to be more specific, the spontaneous radiative lifetime of a lasing species is the time that an atom will stay at a particular energy level before spontaneously losing energy. An atom at an upper lasing level, for example, can lose energy by emitting a photon, either spontaneously or through stimulated emission. The rate of spontaneous emission is defined by the product of the Einstein coefficient and the number of atoms at the upper energy level N₂:

 r_spontaneous = A₂₁N₂

where A₂₁ is the Einstein coefficient for spontaneous emission and N₂ is the number of atoms at the upper energy state. It might be noted that the A coefficient for absorption is related to the spontaneous lifetime as

where τ is the spontaneous radiative lifetime of the upper state. So if the upper level has a long spontaneous lifetime, the A coefficient will be small, as will be the rate of spontaneous emission. With a low spontaneous emission rate, stimulated emission is given a better chance of occurring. There are also implications here regarding then storage of energy in a particular energy level, which allows the production of giant pulses using a technique called Q-switching.

          In a four-level laser the ability to lase in CW mode is not only defined by the lifetime of the upper lasing level but also by the lifetime of the lower lasing level. If the lower level has a relatively long lifetime, atoms in that (lower) energy state stay there longer, giving them a good chance of absorbing photons. This, of course, would serve to hinder laser action since (1) the laser medium will strongly absorb photons produced by the laser, and (2) this allows the atomic population of the lower level to build violating the required inversion criteria. On the other hand, if the lower level has a short lifetime, atoms in that state decay quickly to another state (the ground state in a model four-level laser system but possibly to an intermediate level), where they will not absorb the newly emitted photons in the laser. CW gas lasers (those that produce continuous lasing output) invariably possess the latter characteristic since a long lower lifetime (i.e., longer than the lifetime of the upper state) would probably not allow a population inversion to be maintained.

          A non-favorable condition (for CW lasing action) exists in the nitrogen laser where the upper lasing level is very short compared to the lower level. The upper level has a lifetime of about 10 ns, whereas the lower level has a lifetime of about 10 ms. Assuming that a mechanism exists to pump energy quickly into the upper level (and it does, in the form of a high-current transverse discharge), the laser can operate for a maximum of 10 ns before the lower level is populated, inversion is violated, and lasing ceases. The dynamics of the nitrogen laser are depicted in Figure 1.1, with important energy levels at various times in the lasing process highlighted. At t = 0 s, pumping begins and the upper lasing level fills. Laser action ensues until at about 10 ns, the population of the lower lasing level exceeds that of the upper level and lasing action ceases since population inversion is no longer maintained. This illustrates the self-terminating nature of the species. About 10 ms later the lower level depopulates via a transition to a metastable energy state that has a longer lifetime yet. The situation also illustrates that it is not possible to operate such a laser in CW mode, since once the lower level has filled, these molecules of nitrogen are no longer available to be pumped to the upper lasing level.

       The only reason the nitrogen laser operates at all is that a fast pump mechanism exists in which a current of thousands of amperes is made to pass through a small volume of gas. With fast rise times on the current, inversion can be achieved and lasing ensues (it is short-lived, however, due to the short ULL lifetime). Achieving a fast discharge such as this requires careful laser design. Another example of a pulsed laser system with unfavorable lifetimes

Figure 1.1. Nitrogen laser dynamics.

is the copper-vapor laser. This laser has an upper-level lifetime of about 10 ns and a lower-level lifetime of 270 μs. An effect called resonance trapping can be used to increase this upper-level lifetime. In this effect the ground-state population of copper atoms influences population inversion by trapping radiation emitted from a transition between the upper level and the ground state by spontaneous emission. This radiation is emitted spontaneously by atoms at the ULL as opposed to the lasing transition to the LLL, which is brought about by stimulated emission. The spontaneously emitted radiation is reabsorbed by ground-state atoms (or trapped) and serves to pump copper atoms at the ground state back into the upper level. This serves to extend the effective upper-level lifetime from 10 ns to 370 ns. This improves the situation and greatly reduces the discharge pumping rate required, which simplifies the design of the laser since the discharge may be much slower. (Nitrogen lasers, which have an ULL lifetime of about 20 ns, utilize extremely fast discharge electronics.) Still, the upper-level lifetime of 370 ns is much shorter than the lower-level lifetime, so this laser operates strictly in pulsed mode.