In this paper, we report the first laser-excited optical double-resonance experiments in rare-earth-doped crystals. Infrared upconversion in LaCl3 : Pr3+ and LaF3 : Pr3+ has been achieved by means of a tunable cw dye laser pump. The quantum efficiency of the upconversion process at 2.03 μ in LaCl3 : Pr3+ is 1.2×10−3, which is an improvement of 4×105 over the highest efficiency found previously with incoherent sources. It is shown that an efficiency of 1 can be easily obtained with higher fluxes and improved crystal quality. Direct excitation of the output fluorescence from the 3P0 level is observed in both LaCl3 : Pr3+ and LaF3 : Pr3+ when the dye laser frequency is resonant with the states of the lower-lying 1D2 manifold in the absence of the infrared input. Detailed assignments of the observed double-resonance transitions have been made from wavelength and polarization measurements.
Here we describe a class of cw upconversion lasers in which inversion is due exclusively to cooperative interactions of rare earth ions in highly doped solids, resulting in unique opportunities for output wavelength agility, large upconversion factors, and high quantum efficiencies through nonlinear dynamics. We have isolated capabilities of the cooperative mechanism by performing experiments in Er:CaF2 and Er:LiYF4 with infrared pumping of the first Er3+ resonance at 1.53 μm. At liquid helium temperatures, laser action at 544 nm was achieved in an astigmatically compensated, three-mirror cavity with 5% efficiency in LiYF4. The upconversion factor (ratio of pump to laser wavelength) was 2.81, to our knowledge the highest ever reported for cw upconversion lasers. At 77 K, laser action at 551 nm was observed. Surprisingly, output at both 544 and 551 nm was truly cw with cooperative pumping, unlike earlier green erbium lasers, which exhibited self-pulsing. Also at 77 K, near infrared laser action was obtained at 0.855 pm with exceptionally high efficiency in CaF2(28%) and LiYF4 (20%) on self-quenching transitions. Successful demonstration of cw laser action on such transitions also suggests a means whereby quantum efficiencies may exceed unity. The lower laser level in such cases is the origin of cooperative pumping to the upper laser level, but also receives population as the result of stimulated emission. Hence population can be recycled for enhanced quantum efficiency. Additionally, population pulsations can occur and sustained oscillations in laser output have been observed in a cooperative erbium upconversion laser at 2.8 μm.
We have measured the pump-probe spectrum for CaF2:Sm2+ over a wide (360–1020 nm) spectral range. We observed gain throughout most of the 730-nm emission band, although there is some evidence of excited-state absorption (ESA) in this region. A much stronger ESA was observed near 500 nm. The temperature dependence of the ESA signal clearly indicates that the absorption is associated with divalent samarium. Because of the width and strength of this transition, we assign the ESA to a 4f55d → conduction band transition. We calculated the oscillator strength for this ESA band to be 0.10 and rationalized its magnitude based on an intensity-borrowing mechanism that involves the transition from the valence band to the conduction band.
The optical-absorption transitions originating from the lowest 4f5d excited state of
Pr3+ in
Y3Al5O12 have been studied. The experimental arrangement uses a 7-ns 266-nm light pulse to populate the lowest 4f5d state, and a spatially overlapped, time-sequenced pulse from a frequency-tunable dye laser to induce transitions from that excited state. The excited-state-absorption (ESA) spectrum covering the 900--300-nm probe wavelength range is characterized by two broad peaks centered at 700 and 350 nm on a slowly rising background. The peak ESA cross section at 355 nm is (1\ifmmode\pm\else\textpm\fi{}0.1)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}17}$
cm2. The position, shape, and strength of the spectrum suggest that the terminal states of the ESA transitions are the second 4f5d state of
Pr3+ and the conduction band of the host lattice. The ESA measurements have been extended to transitions from the
3P0 and
1D2 excited states of the 4
f2 configuration of
Pr3+ as well. The ESA cross sections at a probe wavelength of 532 nm are (8\ifmmode\pm\else\textpm\fi{}1)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}19}$
cm2 and (5.7\ifmmode\pm\else\textpm\fi{}0.6)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}19}$
cm2 for transitions from the
3P0 and
1D2 excited states, respectively. The terminal state of ESA transitions from these two levels is the lowest 4f5d state. The strong ESA completely inhibits the potential laser action based on emission from these states in this crystal.
Erbium-Doped Fiber Amplifiers (EDFAs) can very efficiently convert pump to signal energy. Quantum efficiencies higher than 80 % with 1.48 pm pump [1], and 93% when pumped at 980 nm [2] are reported. However, even higher values should be theoretically possible for an ideal EDFA. Various physical effects may lead to quantum efficiency (QE) reduction. Not all of them are quite well understood yet. At Erbium concentrations above 100 ppm by weight the pump loss is partly explained by energy transfer between two excited ions resulting in upcon- version process [3]. At low concentrations Excited-State Absorption (ESA) could be a more significant effect reducing QE. ESA of photons from 1.5 |im band have been predicted from absorption measurements [4] and also observed [5]. However, to our knowledge, no quantitative analysis has been made which would distinguish between the contributions of ESA and two-particle upconversion when measurements of short wavelength luminescence were performed.