**2. Tests facilities**

#### **2.1 Small beam facilities**

Measurements are currently performed on Q-switched Nd:YAG lasers supplying three wavelengths, the fundamental (1.064µm, 1), the second (0.532µm, 2) and third (0.355µm, 3) harmonics. Laser injection seeding ensures a longitudinal monomode beam and a stable temporal profile. The lasers deliver approximately 1 J at a nominal repetition rate of 10Hz at 1. Beam is focused into the sample by a convex lens which focal length is few meters. It induces a depth of focus (DOF) bigger than the sample thickness, ensuring the beam shape to be constant along the DOF.

Two parameters are essential in damage tests: the beam profile and the energy of each shot. Equivalent surface *Seq* (i.e. defined as the surface given at 1/e for a Gaussian beam) of the beam is determined at the equivalent plane corresponding to distance between the focusing lens and the sample (see Fig. 1). At the focus point, beam is millimetric Gaussian shaped and diameter (given at 1/e) is around 0.5 to 1 mm. Shot-to-shot laser fluences fluctuations (about 15% at one standard deviation) are mainly due to fluctuations of the equivalent surface of the beam. This is the reason why fluence has to be determined for each shot:


Fig. 1. Typical experimental setup for laser damage testing (Nd:Yag facility) with small beam.

