| A recent reformulation of the laser field equations using the true modes of the laser cavity and including a noise polarization term to account for spontaneous emission shows that the conventional description of quantum spontaneous emission or quantum noise needs to be modified for laser amplifiers or oscillators with nonorthogonal transverse eigenmodes. Gain-guided lasers and unstable-resonator lasers in particular have nonorthogonal transverse modes, and hence are predicted to exhibit excess quantum noise. One important consequence of this theory is the appearance of an excess noise factor which multiplies the well-known Schawlow-Townes formula for the laser quantum linewidth. Analytic and numerical computations have predicted that this excess noise factor can be substantially greater than unity. Experimental confirmations of this effect have, however, been limited to date.;The main purpose of this thesis is to report on the use of the so-called Pound-Drever locking scheme for the measurement of laser frequency noise, and in particular the observation of excess quantum noise in an unstable-resonator laser. The Pound-Drever locking scheme provides a highly sensitive frequency discriminator which is immune (to first-order) to intensity fluctuations and has a large bandwidth for frequency noise measurement. In our experiment a Pound-Drever measurement tracks the slow drifts of our laser, while the discriminator signal gives a sensitive measure of the level of quantum noise in the laser oscillator. We will describe the application of this measurement technique to the experimental investigation of excess quantum noise in an unstable-resonator diode-pumped solid-state laser. We have measured the quantum noise contribution to the frequency spectra of both a stable-resonator and an unstable-resonator diode-pumped Nd:YAG laser. The stable-resonator laser shows no excess noise, as expected. The unstable-resonator laser, however, exhibits a large excess quantum noise, in reasonable agreement with theoretical predictions. |
