Design and realization of fiber integrated grating taps for performance monitoring applications

EPFL, 2007
Thèse EPFL

Performance monitoring is one of the key features to create self-managed optical networks. Monitoring the characteristics of an optical channel is important for designing robust and reliable optical telecommunication system. This thesis examined the use of highly integrated and low loss in-fiber taps for monitoring the power and the wavelength of an optical signal that propagates in the core of an optical fiber. The taps are tilted fiber Bragg gratings (TFBG) that are induced in the fiber core by UV-laser light. These gratings have the ability to diffract light outside the fiber in a dispersive and very directional manner. The scattered light may be measured and serve as a monitoring signal. Specifically, the grating length can be reduced and the index of modulation increased such that the spectral properties of the scattered light are conserved. Novel power and wavelength monitors have been designed and realized by exploiting this key characteristic. The power monitor uses one single TFBG and can measure the power of a single wavelength signal over a bandwidth that can be adapted by the grating tilt angle. The wavelength monitor uses a pair of TFBG that spectra are shifted in wavelength and overlap. This allows for decoupling the power and the wavelength of the propagating signal. Simulations showed that it is possible to extend the operating wavelength window of this device while keeping a high wavelength selectivity. The impact of an undesirable variation of the polarization state of the incident light on the device performances has been analyzed and calculated. As a result, it is possible to compensate the polarization dependency of the monitoring signal by adapting either the length or the index modulation of one of the gratings. These devices have been fabricated in standard telecommunication fiber using 500 µm-long tilted fiber Bragg gratings. The power monitor operates over a wavelength range larger that 60 nm. That makes it suitable for fast power monitoring of a tunable laser that operates over a entire communication band. The wavelength monitor has a low insertion loss (30 nm). Using a scattering model, it has been highlighted that the spatial distribution of the scattered light outside the fiber depends on the wavelength and on the polarization state of the incident light. A new characterization setup was built for measuring the azimuthal scattering distribution for various polarization states. A good agreement was found with the calculations.

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