A 551-nm diode laser pumped upconversion laser

Abstract

There have recently been several reports of visible upconversion lasers that are pumped by cw dye lasers.1,2 We report the first operation to our knowledge of an upconversion laser pumped by a semiconductor diode laser. Using a GaAlAs diode laser array as the pump source, upconversion lasing at 551 nm was observed in LiYF4:Er3+ (5%) at 41 K. The ~791-nm pump light populates the long-lived (~5 ms) intermediate 4I11/2 state of Er3+ and an efficient energy transfer upconversion process involving two ions in 4I11/2 results in excitation of the 4S3/2 upper laser level. When the near infrared pump laser was turned off rapidly the green laser continued to oscillate for several hundred microseconds, demonstrating that upconversion pumping occurs predominantly by energy transfer rather than sequential two-photon absorption. The relatively poor spectral overlap of the multimode output spectrum (~3-nm band-width) of the laser array and the 0.8-nm wide Er3+ absorption lines resulted in only 15% absorption of the pump light in the 3mm long laser crystal which had mirror coatings directly deposited on spherically polished faces. As a consequence, the pump threshold of 225 mW of incident diode laser power was relatively high; with 340 mW of pump power the green output was 100 µW. A much lower pump threshold of 13 mW of incident power was observed with a cw near infrared narrowband dye laser. With 130 mW of dye laser power a 551-nm output of 4.2 mW was achieved. This latter result suggests the use of index guided single-mode GaAlAs diode lasers for efficient pumping of erbium upconversion lasers.