Abstract

A number of visible upconversion lasers have been demonstrated using near-IR sources to pump short monolithic laser crystals doped with Er³⁺, Nd³⁺, or Tm³⁺ ions at low temperatures.¹,² In many cases the spectral widths of the emission lines were comparable with the frequency spacing of the cavity modes, and the upconversion lasers operated in a single longitudinal mode. Recently, blue laser operation at 469.7 nm was achieved by using red or near-IR dye laser radiation for upconversion pumping of a 3.6-mm-long laser crystal of Er:YLiF4 with mirror coatings directly applied to the spherically (r = 2 cm) polished end faces.² This laser operates on the ²P_3/2 → ⁴I_11/2 transition of Er³⁺, which has a homogeneous linewidth of 16 GHz at low temperatures. The free spectral range (FSR) of the short monolithic laser crystal was 29 GHz. To achieve laser oscillation, the laser mode frequency must overlap with the narrow emission line and, consequently, the laser operates in the TEMjk mode that exhibits the lowest pump threshold considering the frequency match of the cavity modes and the gain spectrum and the spatial overlap of the laser and pump profiles. Systematic studies of the laser mode selection were performed by applying a magnetic field to the Er:YLiF₄ crystal and observing the far field of the laser output and measuring the laser frequencies with a scanning Fabry-Perot interferometer. With appropriate adjustment of the magnetic field, Zeeman tuning of the blue Er³⁺ laser transition could be used to obtain laser operation in the TEM₀₀ mode. Sufficiently large detuning of the magnetic field resulted in TEM₀₁ mode operation, and further changes of the field led to a switching of the modes from TEMok to TEM_0k+1 modes every 2.5 kG. The Zeeman tuning of the ²P_3/2 → ⁴I_11/2 emission line was 2.1 GHz/kG. The measured 5.2 GHz spacing of adjacent TEM_0k modes is in good agreement with theoretical calculations for higher-order Gaussian modes in this geometry. The blue upconversion laser operated between 1.5 and 35 K, and in this range the laser mode frequency was measured to exhibit a temperature tuning rate of 140 MHz/K.

© 1990 Optical Society of America