Experimental setup

Pumping at 808 nm is carried out by a diode laser, which emits a power of 500 mW over a rectangular emitting surface ( by ). The pumping radiation is collected by a lens, which returns the image of the emitting surface to infinity (collimation). It is then focused in the Nd:YAG crystal.

The magnification of the optical system (collimation and focusing) is equal to 1. The beam emitted is very divergent (50°), so the objective lens used has to be very wide to collect all the flux emitted by the source. This is why a collimating lens with a numerical aperture of 0.5 is used here.

The crystal is 10 mm in length. The optical axis of the cavity continues along the optical axis of the pumping and the process is known as longitudinal pumping.

The crystal has a dielectric coating on its entry side. This consists of a mirror coating at wavelength 1064 nm and an anti-reflection at 808 nm so that the beam from the pump travels through the crystal while the beam from the laser is reflected.

The focal point in the laser crystal is about by (optical aberrations mean that the narrowest rectangular section of the diode ( ) is not imaged correctly ). The focal point may seem very small, but the pump beam must be focused in the crystal for the pumping to be efficient and for the effective gain to be high enough so that laser oscillation can occur. In fact, when the intensity of the laser beam is small, the effective gain G0 is linked to the irradiance of the pump Ip on the crystal by the following formula (following the hypothesis that the pump beam and the laser beam have the same section):

where Cste is a constant defined by the spectroscopic parameters of the crystal and the size of the beams. The gain in the system shown in Figure E3 can easily be measured experimentally. For a 500 mW pump focused onto a surface about hundred microns wide, G0 is roughly 1.5 at 1064 nm for the Nd:YAG crystal.