L'émission laser par diffusion Brillouin stimulée dans les fibres optiques

Among all scattering processes, spontaneous Brillouin scattering originates from the interaction between light and thermal acoustic waves propagating in the medium. The interference resulting from the incident light and its spontaneously backscattered fraction generates an acoustic wave through the electrostriction process, which in turns also scatter the incident light and reinforce the backscattered intensity. It appears throughout the equations of this stimulated process that this energy transfer from the incident light to the backscattered light is formally equivalent to an optical gain that can be used to obtain a laser emission. In optical fibers, the spectral characteristics of this Brillouin gain not only depend on the fiber type, but also on temperature and applied strain. Continuous and stable Brillouin laser operation is obtained using high quality fiber ring resonators. Since precise characterization of those resonators is of key importance, a new method based on the analysis of Rayleigh backscattering within the fiber ring resonator using an optical time domain reflectometer has been developed in the present work. By increasing the incident pump intensity, the Stokes wave increases and in turns act as a pump to create a second order Stokes wave. That way multiple Stokes orders can be generated. Using both Brillouin gain and ring cavity equations, a model depending only on the pump intensity has been established. It allows evaluation of the threshold and the intensity of each generated Stokes wave. Necessary criteria for obtaining lowest threshold, as well as considerations about efficiency and temperature influence are discussed and compared to the measurements performed on different Brillouin ring lasers. Since the Brillouin gain comes from the interference between pump and backscattered Stokes waves, a model based only on circulating intensities cannot be complete. The analysis of the ring polarization eigenmodes as a function of the fiber birefringence is necessary in order to compute at every location in the ring the polarization state of both pump and Stokes waves to determine their mixing efficiency. The Brillouin gain over one round trip can thus be precisely calculated. Although this analysis is only relevant on rings having well known birefringence, a linear or circular birefringence, some conclusions can be drawn for rings having an uncontrolled birefringence. Measurements performed on a circular birefringence ring resonator confirms the theoretical prediction. The emission frequency of the Brillouin laser is fixed by the ring resonance and will adapt to any environmental changes. However, the optical waves themselves also modify the emission frequency throughout Kerr effect and dispersion of the Brillouin gain expressed by Kramers-Kronig relations. The process that leads Brillouin laser light to exhibit an exceptional coherence has been studied and values of its linewidth are proposed. The measurement of the beat note between two Stokes waves asses these considerations. Increasing the length of the fiber ring decreases the laser threshold, but gain competition between the different resonances underneath the Brillouin gain curve tends to make the emission unstable. Depending on the ring configuration or pump noise, a continuous, chaotic or pulsed regime can be addressed. The study of the Brillouin laser has highlighted many of its interesting properties, such as coherence, frequency shift in the range of 10 to 12 GHz, Brillouin gain directivity, etc. Among all applications that can take advantage of these remarkable characteristics, the Brillouin gyroscope is probably the one which motivated the biggest interest. However, all applications presented in this work: the Brillouin current sensor, the coherent laser linewidth measurement setup, the phase modulated microwave generator and the PSK signal sender, bring new solutions addressed to a wide variety of domains such as sensing, metrology, instrumentation and telecommunication.

Robert, Philippe Alain
Lausanne, EPFL

 Record created 2005-03-16, last modified 2018-01-27

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