LNA laser for helium experiments

The authors designed, built, and operated an LNA laser end-pumped by a diode laser array for the purpose of exciting the He transition 2³S → 2³P near λ= 1.083 μm. To discriminate against the strong LNA peak at λ= 1.054 μm in favor of the weak one at λ= 1.083 μm, they use a dichroic coating on the output coupler that reflects approx.98% at 1.083 μm but only 60% at 1.054 μm. Since this makes the cavity more compact, it both eases alignment and separates the modes. The authors have been studying the properties of LNA crystals and coatings on them for the purpose of designing the best laser. Typical good operation has a threshold of approx. 90 mW of approx 794-nm pump light and slope efficiency of approx. 15%, but the six crystals studied have very different values of these parameters. The amount of pump light admitted to the crystal and the loss at 1.083 μm have been combined into a single figure of merit (FM) for a particular crystal. The correlation between the figure of merit and the measured laser threshold and efficiency of four of the six was examined and found to be quite good.