Lloyd W. Hillman

Observation of a spectral hole due to population oscillations in a homogeneously broadened optical absorption line

Abstract We have observed a dip in the absorption profile of the homogeneously broadened green absorption band of ruby for frequencies near that of a saturating, cw laser beam. The shape and depth of the hole thus produced have been measured using weak amplitude modulation sidebands as a probe, and are found to be in good agreement with theoretical predictions. These predictions are based on a model that ascribes the origin of the hole to the periodic modulation of the ground state population at the beat frequency between the pump and probe beams. The theory includes the effect of pump depletion. Holes as narrow as 37 Hz (HWHM) have been observed.

Natural modes for the analysis of optical bistability and laser instability

A self-consistent description is given for the propagation of a weak optical probe beam through a homogeneously broadened two-level medium in the presence of a strong, collinear, near-resonant pump beam. The propagation of the probe beam is given in terms of bichromatic natural modes, which contain frequency components symmetrically displaced from that of the pump beam. The two frequency components have a definite relative amplitude and phase, which are determined solely by the frequency detunings of the optical fields from the atomic resonance and by the intensity of the pump field. A simple method for calculating these natural modes is derived, and their importance in describing nearly degenerate four-wave mixing, optical bistability, and the stability of homogeneously broadened ring lasers is discussed.

Dynamics of homogeneously broadened lasers: higher-order bichromatic states of operation

A theory is presented that describes a series of bichromatic states of an ideal homogeneously broadened laser. These states are produced when the laser is pumped far above threshold so that the intracavity field is strong enough to overcome damping and produce Rabi oscillations of the atomic inversion. The two-frequency laser field is amplitude modulated at a frequency that may be either the Rabi frequency or a subharmonic of it. The theoretical predictions are compared qualitatively with recent experimental results.

Multimode Instabilities of Homogeneously Broadened Lasers

The cw homogeneously broadened ring laser appears at first glance to be an ideal laser system in which simple rate equation theory applies. A series of experiments in several different laboratories has shown that this is not-the case, and that in fact such lasers exhibit a number of instabilities. HILLMAN et al. [1] have demonstrated a multimode instability in which a dye laser discontinuously switches from single frequency to two-frequency operation. The separation between the two frequency components is the Rabi frequency or a subharmonic of it, and is sometimes as large as 50 THz. RAYMER et al. [2] have observed many-mode operation of a dye laser with strong intermode competition. MANDEL et al. [3] and KUHLKE et al. [4] have observed a different sort of instability in which the ring laser operates in a single longitudinal cavity mode, but switches between clockwise and counterclockwise direction of propagation around the ring. In the present paper we will describe another dynamical instability in which the laser operates in two longitudinal cavity modes, but switches between the two directions of propagation.

Multimode Instabilities In Cw Dye Lasers

New experimental results are presented which show the behavior of a homogeneously broadened ring dye laser both below and above the threshold for the onset of the two-frequency instability. It is shown that the spectrum of the output of the laser is extremely sensitive to the detuning of the initial lasing frequency from the center of the gain curve. Spectral splittings as large as 340 A are reported. The close connection between this instability and the production of femtosecond mode-locked laser pulses is discussed.

Intrinsic Instabilities in Homogeneously Broadened Lasers

We have studied the behavior of a cw homogeneously broadened ring dye laser operading far above threshold. We have found that although the laser operates in a single mode when it is near threshold, it is intrinsically unstable when pumped far above threshold. The onset of the instability is characterized by an abrupt increase in the power circulating in the cavity. In addition, the spectral output of the laser changes. Below the onset of the instability lasing occurs at gain center, while above this threshold, lasing occurs simultaneously at two frequencies symmetrically displaced from gain center. The splitting of the two lasing frequencies follows a parabolic power dependence. At still greater pump powers, higher order splittings and instabilities were observed. These instabilities exhibit hysteresis about the critical pump powers.