Lasers are known which depend for their operation on lasing transitions between upper and lower lasing levels of ions in a lasing medium each of which levels lies above the ions' ground state. If the average lifetime of ions in the upper lasing level (ULL) is longer than the lifetime of ions in the lower lasing level (LLL) then pumping of the laser medium to populate the ULL by excitation of ions from the ground state, perhaps via a level above the ULL, can be employed to maintain the population inversion between the ULL and LLL to allow lasing action to proceed. If, however, the lifetime of the ULL is shorter than the LLL, the lasing transition is known as self-terminating because the build up of ions in the LLL destroys the population inversion required for lasing. Consequently such self-terminating lasers normally only operate in pulsed form the LLL being given time to empty between pumping pulses.
One example of such a transition is the .sup.4 I.sub.11/2 to .sup.4 I.sub.13/2 transition of the erbium ion which is self-terminating in the majority of investigated host media. This self-limiting transition of the erbium ion produces 2.7 to 3 .mu.m laser output which, if obtainable in extended continuous wave operation, would have considerable applications to future long-haul optical communications, medical and sensing systems.
International patent applications having International publication numbers W087/07447 and W087/07448 teach one method of providing at least quasi-continuous lasing operation using of this normally self-limiting transition. The erbium ions are of sufficient density that their close proximity permits energy transfer between ions in the LLL. Ions are excited to the LLL from the ground state where they interact, some ions being elevated to a higher energy level, one or more other ions dropping through a non-radiative process to the ground state for each one so elevated. This produces a population inversion between the levels to produce lasing. A pulse of 1.5 .mu.m light provides the initial populating of the LLL which is then followed by the ion interaction to produce a self-terminating lasing operation between the ULL and LLL. Quasi-continuous lasing is obtainable by sequentially flashing flash lamps having a flash duration of 15 to 20 ms to maintain the co-operative ion interaction.
In an article by B. M. Antipenko, U. B. Raba, K. B. Seiranyan and L. K. Sukhareva entitled "Quasi-continuous lasing of an LiYF.sub.4 :Er:Pr crystal at 0.85 .mu." Sov. J. Quantum Electron. 13(9) Sep 1983 pp 1237-1239 it was opined that the relaxation of the .sup.4 I.sub.13/2 level of Erbium in LiYFu:Er:Pr crystal (by selective quenching of the level by the Pr ions) is accelerated by the flash lamp radiation itself. However, to achieve true continuous wave operation it is necessary to provide continuous high pump intensity which is impracticable. The short lifetime of the pump lamps would prevent extended operations and there would be a requirement to cool the laser medium.