Astigmatically compensated, high gain cooperative upconversion laser

In recent years a variety of schemes have been explored for compact, short-wavelength laser sources in solids. These include wide gap semiconductor diode lasers,* nonlinear generation of optical harmonics by phase matching,2 quasiphase matching,3 and upconversion lasers in bulk media4 and fibers.5 In practical terms, while all these approaches have shown promise, all currently exhibit important limitations barring practical applications. Wide gap semiconductors present growth and doping problems and to date have not attained room-temperature operation by current injection. Nonlinear conversion techniques require critical alignment in bulk crystals and suffer reduced efficiencies in fibers and slab waveguides. Upconversion lasers operate by virtue of complex internal dynamics in which the basic mechanisms responsible for the upgrading of photon energy are still the subject of intense inquiry. Hence there are many questions, both practical and fundamental, which remain to be answered before limitations of the various approaches becomes fully evident. In past upconversion laser research, low efficiencies were generally reported and liquid helium cryogenics were typically required. However many different mechanisms of upconversion exist and high efficiency, high-temperature cw operation is undoubtedly achievable. Here we report characteristics of an Er:LiYF4 cooperative upconversion laser which attains 20% efficiency and operates at temperatures as high as 200 K in an open cavity configuration. Upconversion output at nearly twice the pump photon energy is achieved in our device uniquely through cooperative upconversion, one of three known types of upconversion which may be broadly categorized by their reliance on multiphoton, avalanche,’ or cooperatives’ processes. The laser inversion itself is shown in the present case to be due entirely to a cooperative energy sharing process involving three atoms, very similar in nature to the monolithic Er:CaF2 trio laser we reported previously.’ By introducing a 3-mirror, astigmatically compensated cavity with Er:LiYF4 as the gain medium however, we have been able to study cavity losses, make precise assignments of excitation and emission wavelengths and investigate details of our nonlinear dynamics model in a fashion not possible in the original trio laser. In particular we show that cw operation occurs even when a single Stark level of the 4113,2 state of Er3+ is simultaneously the pumped level and the terminal laser level. Pumping of the lowest Stark Ievel of the 4Il3/2 state, in which the predominant trio interaction takes place, also results in cw laser action. This substantiates the simple model and analysis of cooperative dynamics presented in our earlier paper. The experimental setup is indicated in Fig. 1. A continuous-wave NaCl color center laser which was tunable in the region of 1.55 ,um was used to provide resonant excitation of the 41,3,, level of trivalent erbium, the laser active species. The gain medium consisted of a 3 mm thick crystal of 5% Er:LiYF4 oriented at Brewster’s angle (0s =55.5”) with its optic axis in the plane of incidence parallel to the crystal surface. This orientation permits gain extraction on either r or (T polarized transitions. The laser crystal was mounted in vacuum on the cold finger of a liquid helium open cycle cryostat capable of operation down to 6 K. Using a value for the extraordinary refractive index” of n,( 0) = 1.453, an interarm angle of 8=26.2” was calculated for compensation of the astigmatism at the trio laser wavelength. To minimize internal losses while retaining experimental flexibility, all focusing and cavity optics except the output coupler were placed inside the dewar housing. External feedthroughs were used to permit cavity adjustments in vacuum, and independent XYZ positioning of the crystal was made possible by flexible bellows connecting the dewar head to the vacuum chamber. To verify that inversion in this system arises strictly from energy-sharing interactions of trios of Er ions initially excited to the 4I,3,, level, we measured the time and power dependencies of upconversion emission at two wavelengths, as shown in Figs. 2(a) and 2(b) respectively. The Fig. 2(b) inset indicates the dynamics schematically. The time-domain measurements revealed the evolution of upconversion population in the 4S3,2 and 41,1,2 levels following impulse excitation of the 4113,2 level. They were made with an s-1 photomultiplier terminated in 50 ohms and amplified with a transimpedance amplifier of DC-13 MHz bandwidth. Results in Fig. 2 (a) clearly show that no population is observed in either the ‘1i1,2 or the 4S3,2 state during the excitation pulse of 100 ps duration. Upconversion emission grows and peaks on a timescale much longer than the pulsewidth. This is far too long for any process involving the absorption of more than one photon from the incident pulse by a single atom, yet Fig. 2 (b) indicates that upconversion fluorescence intensity from the ‘S3/2 level varies as the incident intensity cubed, when saturation of the pump transition is carefully avoided. These observations are similar to those’ in CaF, except that the small prompt fluorescence signal observed in the earlier work is not observed at all in LiYF4. Therefore all multiphoton excitation processes are ruled out by the