Abstract

One method of improving the poor spectral characteristics of free-running laser diodes is to use the optical self-locking to an external confocal Fabry-Perot resonator.¹ Locking only occurs when the diode-cavity light path is an integral multiple of the resonant wavelength. This can be achieved by a piezo mounted mirror located between laser diode and resonator. We describe a simple, new technique for generating this phase mirror locking signal which is based on the observation that the relative intensities of higher order transverse modes excited in the confocal resonator change when the phase mirror is moved. The intensity difference between the spatial mode patterns leaving the resonator can then be used directly to generate the locking signal. The main theoretical aspects and the experimental realization of this new technique will be described. Two independent, identical systems employing a 674nm diode laser and resonator have been built and their frequency stability was measured. The laser linewidth is reduced to less than 100 kHz. The long-term stability was limited by the drift of the confocal resonators which were locked to polarization stabilized HeNe-lasers.

© 1993 Optical Society of America