Abstract

4fⁿ ⇒ 4fⁿ transitions of trivalent rare earth ions doped in crystals frequently give sharp spectral lines. The transitions associated with the higher levels of a multiplet can be relatively sharp, 10-1000 GHz, where the width is determined by the decay rate of the levels by spontaneous phonon emission to lower levels. The transitions and their linewidth can be determined using conventional absorption or excitation techniques. On the other hand the transitions associated with the meta-stable lower levels typically give much sharper features with linewidths of 1 - 100 GHz and the widths are determined by the inhomogeneities of the crystal field. These inhomogeneous linewdths can be obtained by conventional spectroscopic techniques but the homogeneous linewidth requires more innovative techniques such as optical holeburning or photon echoes. The time domain photon echo measurements do not requiring a high resolution source and have frequently been used to establish that the homogeneous linewidth of such transitions. The lines can be very narrow down to the kHz regime [1,2]. Should complimentary measurements be attempted in the frequency domain utilising holeburning a special frequency stabilised lasers is required and one has been developed in association with the current work [3].

© 1994 Optical Society of America