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CRC Press eBooks

Laser Operating Schemes with Strong Depopulation of the Ground States of Generating Ln3+ Ions

Alexander A. Kaminskii

Published Aug 27, 2020• 0 citations
Publisher

Abstract

Among the many crystalline lasers based on insulating compounds doped with Ln 3+ ions, 1 only one laser is known to generate according to a classical three-level operating scheme, this being the “blue-green” lasing of LiYF 4 :Pr 3+ (its stimulated emission [SE] transition terminates at the lower [zero] Stark level of the ground 3 H 4 state of Pr 3+ ions). 2 For all others the terminal laser level usually has an energy E term ≫ kT. In these cases the number of excited Ln 3+ ions distributed over the working levels, mainly, the initial laser ones, is small and amounts to a few percent of the total number of generating ions in a crystal. This indicates that SE in those multilevel Ln 3+ systems proceeds with practically fully populated levels of the ground state (their usual population is N 0 = 10 19 to 10 21 cm −3 , which corresponds to Ln 3+ ion concentration amounting to C Ln = 0.1 to 10 at.%).

DOI
10.1201/9781003067962-12

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In the late 1960s, investigators specializing in the field of physics and spectroscopy of crystals doped with Ln 3+ ions turned their attention to a new class of the phenomena caused by noncoherent interactions between activator ions in excited states, which were referred to in the literature as cooperative phenomena. 1–4 From the viewpoint of the use of these phenomena for pumping Ln 3+ -activated laser crystals, apparently the processes of cooperative sensitization and stepwise energy transfer are of the greatest value.

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CW lasing in Yb3+: GGG crystals pumped at 0.925 μm

A M Belovolov, Mikhail I. Belovolov, Evgenii M Dianov +2 more

Single-mode cw lasing was obtained for the first time in Yb 3+ : GGG crystals at room temperature at 1.030 and 1.037 μm upon pumping Yb 3+ ions by a 0.925-μm neodymium-doped fibre laser. Lasing was observed at the transitions between the lower Stark sublevel of the 2 F 5/2 level and the upper Stark sublevels of the 2 F 7/2 ground state of Yb 3+ ions. The study of lasing characteristic showed that lasing occurs according to the four-level scheme. The lasing threshold with respect to the absorbed power was 85 mW and the slope lasing efficiency was 20%. Upon 1.7-W pumping, the output power of 310 mW was achieved with the output resonator mirror with the transmission coefficient of 1%. The effective cross section of the lasing transition was 0.9 × 10 -20 cm 2 .

Quantum Electronics•Jul 31, 2006•1 citation
Among the many crystalline lasers based on insulating compounds doped with Ln 3+ ions, 1 only one laser is known to generate according to a classical three-level operating scheme, this being the “blue-green” lasing of LiYF 4 :Pr 3+ (its stimulated emission [SE] transition terminates at the lower [zero] Stark level of the ground 3 H 4 state of Pr 3+ ions). 2 For all others the terminal laser level usually has an energy E term ≫ kT. In these cases the number of excited Ln 3+ ions distributed over the working levels, mainly, the initial laser ones, is small and amounts to a few percent of the total number of generating ions in a crystal. This indicates that SE in those multilevel Ln 3+ systems proceeds with practically fully populated levels of the ground state (their usual population is N 0 = 10 19 to 10 21 cm −3 , which corresponds to Ln 3+ ion concentration amounting to C Ln = 0.1 to 10 at.%).
In the late 1960s, investigators specializing in the field of physics and spectroscopy of crystals doped with Ln 3+ ions turned their attention to a new class of the phenomena caused by noncoherent interactions between activator ions in excited states, which were referred to in the literature as cooperative phenomena. 1–4 From the viewpoint of the use of these phenomena for pumping Ln 3+ -activated laser crystals, apparently the processes of cooperative sensitization and stepwise energy transfer are of the greatest value.
New ways are proposed of increasing broadband-lamp pumping effectiveness of laser crystals with trivalent lanthanides. The method is based on the broadening of lasing-ion absorption spectrum by the remove (excitation) of their considerable part from the ground state into one (some) of the upper states with the help of additional resonant bleaching laser add-pumping Advantages of the new pumping schemes are illustrated by the results, obtained during quantitative analysis of the necessary conditions of stimulated-emission excitation for four laser crystals (LaF3, LiYF4, BaY2F8, and YA103) with Pr3+ ions, which can lase at 3Po → 3F4,3,2, 3H6,5 channels at 300 K with lamp pumping. The method proposed is also suitable for laser effectiveness increase in crystals with other lanthanides activators.
We report on an optically pumped laser where photons are simultaneously generated by population inversion and by stimulated Raman scattering in the same active medium, namely crystalline silicon doped by bismuth (Si∶Bi).The medium utilizes three electronic levels: ground state [j1i: 1sðA 1 Þ in Si∶Bi], upper [j3i: 2p Æ ] and lower [j2i: 1sðEÞ] laser levels.The j1i ↔ j3i and j2i ↔ j3i transitions are optically allowed and the j1i ↔ j2i transition is Raman active.Lasing based on population inversion occurs between the states j3i and j2i, while Raman scattering benefits from the Raman-active transition.At high pump power the inversion-based stimulated emission j3i → j2i disappears, because electronic scattering from j1i to j2i via a virtual state dominates and the electrons are excited into j2i rather than into j3i.Starting as population inversion-based lasing, it ends as stimulated Raman scattering.Our model shows that such a competition occurs on the timescale of the 10-ps-long pump pulse.
This report summarizes research carried out to investigate energy transfer in an activator-activator system; that is a system for which energy transfer is reversible between certain metastable levels and for which stimulated emission from each of these levels can be generated. The specific system investigated is Er-Ho:YLF. Laser operation from two different ions in the same solid state host has been demonstrated. The 750nm (5S(2) - 5I(7):Ho) and 850nm (4S(3/2) - 4I(13/2:Er) transitions were observed simultaneously and separately at 77K. Cross transfer between the S levels was also observed and the cross transfer rates measured in crystals of five different compositions. Cross transfer between the S levels is mitigated by other cross relaxation transfer schemes which quench the S level storage time of each ion. Only about 25% of the initial population of one ion is transferred to the other in the optimum concentration, 2%Ho-2%Er, at room temperature. The cross transfer rate is only weakly temperature and concentration dependent; at low temperatures cross transfer is somewhat more effective as the decay rates of each ion are longer.
Single-mode cw lasing was obtained for the first time in Yb 3+ : GGG crystals at room temperature at 1.030 and 1.037 μm upon pumping Yb 3+ ions by a 0.925-μm neodymium-doped fibre laser. Lasing was observed at the transitions between the lower Stark sublevel of the 2 F 5/2 level and the upper Stark sublevels of the 2 F 7/2 ground state of Yb 3+ ions. The study of lasing characteristic showed that lasing occurs according to the four-level scheme. The lasing threshold with respect to the absorbed power was 85 mW and the slope lasing efficiency was 20%. Upon 1.7-W pumping, the output power of 310 mW was achieved with the output resonator mirror with the transmission coefficient of 1%. The effective cross section of the lasing transition was 0.9 × 10 -20 cm 2 .