Selection of the laser transition at 1064 nm, choice of dielectric mirrors.

The section “Spectroscopy of the neodymium ion” demonstrated that the Nd:YAG can function over several laser transitions, specifically three bands: 1064 nm, 946 nm and 1320 nm. The transition corresponding to 1064 nm has by far the highest effective gain so the laser naturally tends to operate at 1064 nm.

However, to avoid unwanted oscillations, it is better to monitor the reflection coefficients of the mirrors so that is greater than G02 at the undesirable wavelength, . This means the lasing threshold will not be reached.

The dielectric treatment used for the mirrors is based on the principle of interference: it consists of a deposit of thin layers (for example, alternating layers of SiO2 and TiO2) so that certain wavelengths interfere constructively and others do not. Dielectric mirrors have a band of reflectivity that usually stretches over tens of nanometres in wavelength. On either side of this band, the mirror is transparent. Figure E5 shows the classic reflectivity curve for a mirror reflecting at 1064 nm and a photo of such a mirror placed on a white sheet. The sheet is clearly visible through the mirror, proof that the mirror is transparent in the visible spectrum while it is completely reflecting in the near infrared.