Dimitris Varelas

Postfabrication resonance peak positioning of long-period cladding-mode-coupled gratings

A simple, flexible method of postfabrication positioning of resonance wavelengths of long-period cladding-modecoupled gratings is proposed. This method is based on changing the outer fiber diameter. Reducing the diameter by etching the fiber in HF acid shifts the loss peaks to higher wavelengths. A shift as large as 130 nm after 5 min of HF etching was observed for our strongest grating peak, corresponding to the cladding mode HE19. The experimental results are in excellent agreement with model calculations.

Mechanical degradation of optical fibers induced by UV light

The mechanical resistance of UV irradiated optical fibers has been investigated by dynamic fatigue tests. The intrinsic Weibull distribution of the pristine and chemically stripped fiber were almost identical. The high germanium doped fiber was homogeneously irradiate using a KrF excimer with different pulse energy densities and different total doses, both corresponding to typical Bragg grating fabrication conditions. The Weibull distribution of the irradiated fibers depends on the pulse energy density and the total irradiation dose. Both a strong broadening and a reduction of the fiber strength were observed. With increasing total irradiation dose, the median breaking stress of the fiber decreases from 4.8 GPa for the pristine fiber to 2.5, 2.2, and 1.8 GPa for pulse energy densities of 50, 100 and 200 mJ/cm2, respectively, for a total dose of 1 kJ/cm2. At higher total irradiation levels the median breaking stress and hence the strain were found to be independent of laser fluence with a value of 2.7 GPa for 200 mJ/cm2.

Reversibility of photoinduced birefringence in ultralow-birefringence fibers.

New measurements of the birefringence induced in ultralow-birefringence fibers by a succession of orthogonally polarized UV side exposures show the reversibility of the process. Moreover, it is shown that the birefringence growth depends on two parameters, the total irradiation dose and the nature of the successive polarized UV side exposures. These observations indicate that geometric asymmetry is not the major cause of the birefringence and confirm a model of birefringence growth based on dipole alignment.

Advances in fiber gratings: technology, applications, and reliability

Basic components and devices for optical fiber sensor and telecommunication networks should exhibit survival times of 20 to 40 years. Fiber gratings are important basic devices for such networks. This demands good thermal and mechanical reliability of the grating itself. However, as it will be shown in this paper, both reliability aspects depend strongly on factors like grating fabrication conditions, the fiber used, a possible photosensitization process, etc. The thermal stability of fiber gratings is strongly linked to the fiber, its co-dopants and possible photosensitization by hydrogen loading. Determining the electron distribution of the involved defects can assess the reliability. To a first approximation the decay is determined by the decay frequency v0. Good thermal stability over 25 years at 40 °C can be predicted for germanosilicate fibers if the gratings are annealed for 110 hours at T≈120 °C. Higher operating temperatures demand higher annealing temperatures for reasonable annealing times and do need therefore special protections like polyimide coatings. The mechanical breaking strength of fiber gratings is related to the grating fabrication process and fiber preparation. For telecommunication applications, gratings are in general fabricated off-line. In this case, careful chemical fiber stripping and handling is needed. When gratings are written using pulsed KrF excimer laser irradiation a considerable mechanical degradation is observed, which depends on the irradiation conditions. In contrary, frequency doubled CW-Ar+ laser irradiation almost preserves the median breaking stress. Gratings fabricated by one single laser shot or written through the fiber coating show almost no degradation. However, due to their low reflectivity the use of single shot gratings is mostly restricted to sensor applications. The fabrication of grating devices by irradiation through the coating is still limited to index modulations of 10-4 due to coating degradation.

Optical low-coherence reflectometry characterization of cladding modes excited by long-period fiber gratings

The group refractive-index difference of cladding modes excited by a long-period fiber grating is characterized by use of the technique of optical low-coherence reflectometry, with a precision of <10(-4) . Very good agreement between theoretical and experimental results is demonstrated.

Birefringence writing and erasing in ultra-low-birefringence fibers by polarized ultraviolet side-exposure: Origin and applications

This paper presents a new method for writing and erasing a birefringence in optical fibers by polarized ultra violet side-exposure. This experiment gives new insight into the origin of the induced birefringence and some applications are outlined.