Optical injection method for the measurement of small linewidth enhancement factor in solid-state lasers
Résumé
Summary form only given. The linewidth enhancement factor, also referred to as Henry factor or α factor, quantifies the intrinsic phase-amplitude coupling in a laser gain medium. Very extensive literature exists on measurement methods for semiconductor lasers, including direct estimation of the gain asymmetry, pump induced phase modulation (“AM/FM” method) and optical injection. Unfortunately none of these methods is well-suited for solid-state lasers, which feature slower dynamics and small values of α. Few measures are available in the literature: a value of α = 0.25 ± 0.13 was found in a Nd:YVO 4 laser, using injection and AM/FM modulation methods [1], and a surprisingly large α ~ 1 was reported in Nd:YAG microchip lasers.Here, we propose a new "FM/AM" method based on the amplitude response of the laser to a phasemodulated injection. We show that for an injected laser, this response is minimal for a particular value Δm of the frequency detuning Δ between the injected field and the laser field, and that this value is directly related to a. Furthermore, unlike standard methods that often use pump modulation, we operate at a constant intracavity power, so the measurement is free from any thermal considerations [2]. Our method can greatly benefit of operating the laser in a dual frequency regime, that is, when the laser produces two orthogonal polarization modes with a tunable frequency difference. Indeed, this, combined with a frequency-shifted optical feedback, allow to avoid the critical need of an external stable master laser, as one mode of the laser can be used to inject the other one.
Domaines
Optique [physics.optics]
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