Abstract

We recently reported the observation of upconversion laser action in LiYF₄:Nd³⁺ at 729.6 and 413.0 nm originating from the ²P_3/2 level at 26,258 cm^–1.¹ Here we report on cascade laser action at 729.6 and 1053.0 nm using upconversion pumping with a yellow dye laser at low temperatures (T < 70 K). The upconversion excitation of the ²P_3/2 level exhibits an unusual nonlinear dependence on pump power and a delayed buildup time.¹ The 2-mm long laser crystal of LiYF₄:Nd³⁺ (1%) had spherically polished faces on which mirror coatings were directly deposited. The mirror reflectivities were high (>98%) at 730 nm and low (~15%) at 1053 nm. When pumped by a 603.6-nm cw dye laser oscillation was observed at 729.6 nm, corresponding to the ²P_3/2(1) → (⁴f_5/2, ²H_9/2)(3) transition. The ions quickly relax nonradiatively to the ⁴F_3/2 state whose lifetime is 525 µs. The resulting buildup in population brings the 1053.0-nm laser transition [⁴F_3/2(1) → ⁴F_11/2(2)] above threshold. At 20 K the pump threshold for cascade laser operation was obtained with 40 mW of incident dye laser power. Investigations of the laser dynamics showed that the 1053.0-nm laser reaches its threshold ~300 µs after the onset of 729.6-nm laser.

© 1989 Optical Society of America