A. D. May

Stability of polarized modes in a quasi-isotropic laser

The polarization states of quasi-isotropic lasers are highly sensitive to residual cavity anisotropics and to weak (parasitic) anisotropic feedback. Internal anisotropics are usually constant, whereas the effective mirror anisotropy produced by feedback is strongly frequency (phase) dependent. In this paper we develop a model for such a laser and obtain analytic steady-state solutions for the case when both anisotropics are parallel. The linear stability analysis is also analytic. For the 3.39-μm He–Ne laser the theory explains the previously observed variations with frequency of the intensity of the laser, the regions of monostable linear polarization, and in the bistable region the inverse dependence of the width of the hysteresis loop on the low-signal net gain. In contrast to Lamb’s theory the calculations show that the polarization flip arises from an instability in the relative phase between the vector components of a mode.

Stability of polarized modes in a quasi-isotropic laser: experimental confirmation

The analysis of the stability of the polarization modes of a quasi-isotropic laser is extended. The theory predicts the existence of catastrophes in the hysteresis loop of polarization versus frequency. Allowance is made for a weak cavity birefringence. Experiments with a He–Ne laser that operates at 3.39 μm quantitatively confirm the dependence of the hysteresis on gain, including the previously unidentified catastrophes.

Polarization modes in a quasi-isotropic laser: a general anisotropy model with applications

We present a model for the polarization states of a quasi-isotropic laser. The model includes the polarization competition among the gain medium, general cavity anisotropies, and the anisotropy arising from weak but arbitrarily polarized feedback. Three examples of linearly polarized feedback are given, one with the axes of the internal and external anisotropies parallel to each other and two other examples with the axes inclined at 45°. The new calculations are in agreement both with earlier calculations based on a more restricted model and with existing experimental results for a He–Ne laser operating at 3.39 μm. An important feature of the calculations is a method of finding all the stationary solutions, even in the general case. For the three examples considered we find many stationary polarization states. A linear stability analysis shows that only two are stable and permits us to relate our calculations to additive pulse mode locking, to Casperson instabilities, and to Hopf bifurcations.

Propulsion and Angular Stabilization of Dust Particles in a Laser Cavity. II

Certain dust particles exhibit steady rectilinear motion when situated in the standing light wave inside a laser cavity. Some observations and a simple theory are presented which show that these particles are oriented by the laser beam. In addition, semiquantitative calculations are given which indicate that the driving force and stabilizing torque are radiometric in origin.

Anomalous broadening and shifting in D2 and D2–He mixtures

Abstract Precise and accurate measurements of the Q branch lines of D2 in D2 and D2–He mixtures at several temperatures reveal an anomalous dependence of the widths and shifts on density. At high densities the widths and shifts are linear in density but with a constant off-set when extrapolated back to zero density. At low densities the shifts exhibit normal behaviour, being proportional to the density. Low density is identified with the free streaming or Voigt region. High density is identified as the usual collision dominated regime where the Dicke width is much smaller than the collisional width. It does not appear as if any existing models or theories of isolated lines can explain the present results.

Design of a difference-frequency infrared laser spectrometer for absorption line-shape studies

An infrared laser spectrometer based on difference-frequency generation in LiIO(3) is described. The spectrometer has a frequency uncertainty of less than 1 MHz and a signal-to-noise ratio between 3000:1 and 10,000:1. These properties allow the spectrometer to be used for studies of the non-Lorentzian and non-Voigt character of absorption line shapes in atmospheric trace gases.

Patterns at the onset of electroconvection in freely suspended smectic films

We report the results of experiments on electrically driven convection that occurs in a thin, freely suspended film of smectic A liquid crystal when an electric field is applied in the plane of the film. Convection in a vortex pattern is found above a well-defined critical voltage. The film behaves as a two-dimensional isotropic liquid: neither its thickness nor the director field are modified by the flow. We present measurements of the critical voltage at the onset of convection in two experimental configurations—one which allows the injection of charges into the film from the electrodes, and one which does not. When injection is present, the critical voltage for the onset of flow increases monotonically with increasing frequency of applied field. With no injection, there is no instability at DC and the critical voltage diverges there. The nature of the flow pattern observed at onset changes with frequency. Below a certain frequency the film flows in vortices that extend over the width of the film; above this frequency the flow is confined to two lines of smaller vortices localized along the electrodes. We present a simple discussion of the mechanisms which drive the convection.

Intracavity dispersion measurements with a 3.39-μm He–Ne laser

The beat frequency between a (0,0) and a (0,1) mode of a 3.39-μm He–Ne laser is measured as a function of the position of the modes with respect to the line center and for a fixed position as a function of the pressure of CH4 contained inside the cavity. The results suggest that the use of intracavity dispersion is a viable detection technique and furthermore that the dispersion of the gain medium in a dual-polarization laser will not pose a serious limitation to the measurement of intracavity anisotropies.

Principles of an intracavity polarimeter for measuring small optical anisotropies with an application to diffractive dichroism

An intracavity polarimeter is described that is capable of measuring both weak birefringence and dichroism simultaneously. It uses, for the first time to our knowledge, polarization flips in a quasi-isotropic He-Ne laser. The laser operates at 3.39 µm. Calibration of the polarimeter is carried out by using the anisotropic properties of a tilted plate. As an illustration of the sensitivity, the diffractive anisotropy of an intracavity slit is measured.

Dicke-narrowed line shapes in CO-Ar: Measurements, calculations, and a revised interpretation

New line shape calculations for CO buffered by Ar are compared to high‐resolution measurements from a difference‐frequency laser spectrometer, over a range of thermodynamic conditions relevant to the atmosphere. The calculations are based on solving the quantum kinetic (i.e. transport/relaxation) equation for the molecules within the impact approximation, and rely on the commonly used MOLSCAT and MOLCOL codes to determine the speed‐dependent collisional relaxation rate. Velocity‐changing effects are treated classically using a rigid sphere potential. The comparison initially reveals that the experimental profiles exhibit only 10% to 30% of the expected Dicke narrowing, which leads us to reevaluate our understanding of the narrowing process. A more subtle aspect of the disagreement between theory and experiment draws our attention to an assumption implicit in the calculation of the collisional relaxation rate: the assumption of a Maxwellian form for the velocity dependence of the off‐diagonal elements of the density matrix (i.e. the optical coherences). This assumption allows for an analytical simplification of the problem, but eliminates velocity‐changing effects (so that they must be added back in using a supplementary classical calculation, which is based here on a rigid sphere interaction). We find that the removal of the above‐mentioned assumption should allow for accurate and fully quantum mechanical (but numerical) line shape calculations for systems like CO‐Ar on existing computers.