Non-Gaussian modes in a HeNe laser.

Single transverse mode oscillation is realized in a conventional HeNe laser outside the stability region of the optical resonator. Depending on the mirror separation different spatial modes can be generated. The mode volume of these modes is laterally limited by the diameter of the discharge capillary rather than by the beam waist of a stable Gaussian mode. Numerical solution of the Maxwell equations with appropriate boundary conditions shows good agreement with the observations. Such modes could potentially facilitate single transverse mode operation of waveguide lasers and fiber lasers.

Saturated absorption in acetylene and hydrogen cyanide in hollow-core photonic bandgap fibers.

Saturated absorption is studied in overtone transitions of C2H2 and H13CN molecules confined in the hollow core of a photonic bandgap fiber. The dynamics of filling and venting the fiber is markedly different for the two molecules owing to the presence of a permanent dipole moment in one of them. Saturation is observed for input power down to 10 mW, and well resolved Lamb dips limited by transit time broadening across the 10 ?m core diameter are observed with a counter-propagating probe beam.

Dynamics of gas flow in hollow core photonic bandgap fibers.

The dynamics of gas flow in a hollow core photonic bandgap fiber is studied over four decades of pressure covering free molecular flow as well as hydrodynamic flow. Expressions are derived that allow for determination of the pressure inside the fiber as a function of time and position in the limits of Knudsen number Kn>>1 and Kn<<1. The expressions, which are validated by using absorption lines of acetylene as probes of the pressure inside the fiber, provide a straightforward way of predicting the temporal response for gas sensors of any fiber geometry.

Saturated optical absorption by slow molecules in hollow-core photonic band-gap fibers.

We report on saturated absorption in a hollow-core photonic band-gap fiber filled with 12C2H2 molecules. We find that slow molecules provide a major contribution to the signal in the limit of low optical power and low pressure where the signal deviates significantly from the usual Lorentzian line shape. In particular, we observe a linewidth reduction of about 3 times as compared to the transit-time limited linewidth.