Photonic Crystal Fibre Laser Modulator by Using LiNBO3 PZEC

In this chapter, an alternative for performing analogue modulation in amplitude and frequency has been proposed. This possibility is understandable, based on the approach to the principles of operation of our photonic crystal fibre acousto-optic modulator (AOM). By modifying the birefringence of a photonic crystal optical fibre (PCF), which is part of a loop of the Sagnac interferometer (IS) of optical fibre, it is possible to modulate its intensity in a linear regime. Initially, this modulation of the birefringence was achieved by twisting the ends of the birefringent loop of the IS, which caused a little dynamic regulation of the intensity of the laser leaving the IS. On the other hand, taking into account that a piezoelectric crystal (PZEC) is a transducer that also converts the changes in voltage applied to its ends into proportional variations in its physical dimensions (length, width, height); A cylindrical PZEC was chosen, with the purpose of being able to coil the birefringent PCF in the PZEC cylinder so that when excited, it suffers deformations proportional to the applied voltage that manifest themselves in radial expansions and contractions of the cylinder, consequently producing a controlled elongation in the birefringent PCF, which causes the modulation of the laser beam at the IS output, controlled in frequency and intensity. The characteristics of these parameters are based on the frequency range of the PZEC and the limitations imposed by the elasticity of the PCF.

The brief description of the IS to function as an AOM takes advantage of the high birefringence of PM-PCF as its low sensitivity to temperature changes. The positive or negative linear laser modulation is achieved by tuning the interferometer within the C or L bands of optical communications. The modulation frequency is practically determined by the resonance frequency of the piezoelectric transducer. The mechanical fibre-piezoelectric coupling is fundamental to reaching the shifting of the interferometer's transmittance, TSI (λ), at RF frequencies. With this work, the possibility of performing a linear laser modulation based on a tuned fibre optic interferometer with PM-PCF loop, whose nominal birefringence is modified by the induced strain with the controlled expansions/contractions of a piezoelectric transducer, 7.6x10-4±∆ni, was demonstrated. The laser modulation performed is described by a mathematical model that justifies its experimental response in the domain of time and frequency. The acousto-optic modulator has a Gaussian response with bandwidth from 50 kHz to 55 kHz, and its dynamic range is from 0.0 to 45 dB.