A detailed experimental study of laser signature as a function of laser mirror distance variations is reported. This was done for small sealed low-power CO 2 lasers (≈ 1 watt) of the type useful in laser communication. It is shown that most laser-output profiles change drastically from half-wavelength to half-wavelength except the strong P(20) line, which seems to remain relatively constant. Frequency width is given for many of the laser output profiles appearing in the various signatures observed. Line competition is also discussed.

Signature variations with mirror separation for small sealed CO 2 lasers

The optical feedback interferometry phenomenon occurs when a portion of the output optical power is back-scattered from a remote target and coupled into the laser cavity to vary the laser’s emission properties (frequency and power mostly) Thus, this scheme results in a compact, self-aligned and contact-less interferometric sensor Recent applications of optical feedback interferometer in the domains of microfluidics or acoustics have shown promising results and open new fields of researches However in these applications, the amplitude of the sensing signal is extremely small due to the weakness of the backscattered power changes that are measured In this thesis, an analytical model that describes the laser injection current and temperature dependence of the optical feedback interferometry signal strength for a single-mode laser diode has been derived from the Lang and Kobayashi rate equations The model has been developed for all the known signal acquisition methods of the optical feedback interferometry scheme: from the package included monitoring photodiode, by collection of the laser power with an external photodetector and by amplification of the variations in the laser junction voltage The model shows that both the photodiodes and the voltage signals strengths are related to the laser slope efficiency, which itself is a function of the laser injection current and of the temperature Moreover, the model predicts different behaviors of the photodiodes and the voltage signal strengths with the change of the laser injection current and the temperature; an important result that has been proven by conducting measurements on all three signals for a wide range of injection current and temperature Therefore, this simple model provides important insights into the radically different biasing strategies required to achieve optimal sensor sensitivity for the different interferometric signal acquisition schemes In addition, the phase and amplitude relationships between the external and the in-package photodiode signals have been investigated theoretically and experimentally demonstrating unexpected results Based on our model and 
experimental observations, a critical study has been performed on the impact of the combination of the three signals in the signal processing strategy in order to improve the sensor sensibility to low amplitude optical feedback

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Analysis of the different signal acquisition schemes of an optical feedback based laser diode interferometer

Frequency stabilized lasers as absolute standards

On decrit une methode d'asservissement de la frequence d'un laser de puissance (CO2 ou N2O) a partir d'un second laser stabilise sur un « Lamp dip ». Le laser de puissance n'est pas module et l'asservissement est realisable en tout point de la raie d'emission avec une stabilite moyenne a long terme de 10-9.

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A. Waksberg

Papers

Stabilzation of a CO 2 laser using a three-mirror laser system