A synchronously pumped Laser Without Inversion
F.B. de Jong, A. Mavromanolakis, R.J.C. Spreeuw, and H.B. van Linden van den Heuvell
Van der Waals-Zeeman Laboratorium, Universiteit van Amsterdam
The study of atomic coherence in laser schemes has led to many suggestions for Lasing Without Inversion (LWI) schemes. Two kinds of schemes can be identified: 1) Schemes based on coherent population trapping where a hidden inversion can be revealed by a basis transformation. So far, only amplification has been reported. 2) Schemes that are inversionless in the sense that there is no atomic basis for which inversion can be found. Laser action was reported in a V- and L-type scheme in Rb  and Na . The V-scheme has been discussed in terms of the quantum Zeno effect .
We report the first laser oscillation experiment in a coherent population trapping scheme, realized in low-pressure cadmium vapor. The experiment is divided in three steps (left picture): The first linearly-polarized excitation pulse prepares a coherent superposition of the |m=1> and |m=-1> Zeeman sublevels of the 3P1 state. The superposition states are the |fC > and |fN > state. Initially only the |fN > state is populated but the 3P1 states start to perform a Larmor precession in a magnetic field (middle picture). After a delay time a second pulse with linear, orthogonal polarization excites part of the available |fN > population to the upper |u > state. Immediately after the second pulse a third pulse arrives with polarization parallel to the first pulse. This third pulse probes the population difference between the |u > and |fC > state, and can be either attenuated or amplified. Single pass gain of more than a factor of 10 have been measured. Using a simple model to fit the gain/absorption curves as a function of magnetic field, we have obtained values for the populations fractions in the system, showing that there is no inversion in the atomic basis. Combined with a simple optical method to measure absolute densities of a gas, this model results in a completely optical method to determine the absolute populations and density in the system. Without the third probe pulse but with a cavity around the medium the amplifier is transformed to a laser. The right figure shows the output pulse train. The measured gain per pass and saturation are in agreement with the amplifier results.
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