Novel Nd:YVO4 Lasers

Classical Nd:YVO₄ lasers suffer from thermo-optical aberrations that restrict TEM₀₀ operation to a certain pumping power range. Objective of the project is to build compact lasers with cw output powers of up to 30 W exhibiting no thermal lensing. Additionally, only "naked" bars without any pump beam shaping optics are used for pumping. Figure 1 shows such a concept in a multipass amplifier geometry used to amplify a frequency-modulated low power beam for coherent inter-satellite communications. Neglecting a weak Zernike defocus term, the peak-to-valley aberrations of the crystal are just 0.06 λ at 40 W pump power.

Frequency-doubled Nd-based ground-state lasers are commercially available with up to 1 W of output power. Nd:YVO₄ offers a transition at 914 nm, which is very attractive for frequency-doubling into the blue. In principal, a 914-nm Nd:YVO₄ laser can be pumped with laser diodes at 880 nm, resulting in an extremely low quantum defect. The difficulties to build an efficient 914-nm laser are:

1. Cooling: Due to the quasi-three-level nature of the laser, the thermal occupation of the lower laser level needs to be kept as low as possible.
2. Pumping: Compared to 4-level lasers, a very large fraction of the Neodymium ions needs to be excited. This requires a high pump intensity that can only be achieved by a multi-pass pumping scheme.
3. Amplified spontaneous emission and parasitic oscillation: Due to the high gain at 1064 nm, the laser is very sensitive to amplified spontaneous emission and parasitic oscillation.