The design and fabrication of a integrated symmetric directional coupler dependent o the pumping power and operating at a 1534 nm wavelength is reported. The twin-core waveguide was inscribed into Er3+/Yb3+ co-doped phosphate glass by a femtosecond laser direct writing technique. By optical pumping, the coupling ratio can be modulated due to the changes induced in the refractive index of the material. The experimental results demonstrated that the coupling ratio can be tuned continuously from 100/0 to 50/50 by increasing the pump's power from 0 to 350 mW. The developed twin-core coupler has promising applications for on-chip all-optical signal processing and communication systems.

Bragg gratings inscribed in active waveguides combine very efficient reflective properties with the amplifying capability of rare-earths, which may lead to large amplification and lasing performance. However, the response of these photonic structures highly depends on the grating parameters and working conditions, so modeling their behavior and dependences becomes fundamental. In this work, a numerical method has been implemented to simulate the optical power propagation along an Er/Yb-codoped integrated waveguide Bragg grating as a function of its most relevant operational parameters. The results obtained show the optimal conditions to maximize its performance as a highly amplifying reflector, but also its capability as a monolithic laser. In addition, the modeling results adequately match experimental values measured in fs-laser written structures in Er/Yb-codoped phosphate glass, supporting the accuracy of the numerical method developed and its usefulness for further optimizing these promising photonic structures.

Modal decomposition of light is essential to study its propagation properties in waveguides and photonic devices. Modal analysis can be carried out by implementing a computer-generated hologram acting as a match filter in a spatial light modulator. In this work, a series of aspects to be taken into account in order to get the most out of this method are presented, aiming to provide useful operational procedures. First of all, a method for filter size adjustment based on the standard fiber LP-mode symmetry is presented. The influence of the mode normalization in the complex amplitude encoding-inherent noise is then investigated. Finally, a robust method to measure the phase difference between modes is proposed. These procedures are tested by wavefront reconstruction in a conventional few-mode fiber.

We present a modal analysis of coupled two-core integrated waveguides fabricated by femtosecond laser writing as a function of the core-to-core distance, illuminating position and input light wavelength. In order to do that we use the correlation filter method, implementing the computer generated holograms in a phase-only spatial light modulator. Due to the two-core waveguide symmetry, we prove it is not necessary to encode the complex amplitude in a phase-only device as long as the cores are not strongly coupled. A comparison between experimental and numerical modal weights is presented, showing that simple phase-only match filters allow the modal decomposition of two-core waveguides output beams.

The output signal intensity distribution of several fs-laser written double-core active waveguides was registered. The ratio between cores output powers exhibited a strong dependence on the input pump power when both input pump and signal powers were directly coupled to one of the cores. A model based on the coupled-mode theory which incorporates non-identical cores and longitudinally variable signal gain coefficients was implemented. Simulations based on this model are compared to experiments. A good agreement is achievable if small differences between cores’ parameters (within the manufacturing precision limits) are allowed.

A new methodology to completely characterize active multicore waveguides is proposed. The method is applied to two-core waveguides written in an Er 3+ /Yb 3+ co-doped phosphate glass by femtosecond laser inscription. Determination of the core diameter and refractive index variation is based on asymmetric excitation of the dual core waveguide and measurements of the intensity output distribution. Moreover, a single core waveguide has also been written for characterization purposes and signal enhancement measurements carried out on it have allowed the determination of the glass active properties. Finally, enhancement measurements on the two-core waveguides for different pump powers and core-to-core separations have been performed, together with simulations of the experiments using the parameters determined in the characterization procedure. The good agreement between measurements and simulations supports the validity of the characterization method presented. The proposed characterization technique is suitable for any multicore symmetric structure regardless the fabrication method and the number of cores, allowing the benefits of multi-core fibre designs to be transferred to integrated active structures.

OPTOEL 2019, Zaragoza, 3 al 5 de julio de 2019. -- http://optoel2019.unizar.es/23820/detail/optoel-2019.html

This study includes the experimental both passive and active characterization of multicore waveguides written in phosphate glass codoped with erbium and ytterbium, with the aim of designing and optimizing photonic devices based on this type of structures.

Erbium-doped antiresonant reflecting optical wave-guides (ARROWs) allow combining wavelength selective guiding due to their attractive spectral versatility with an active operation. In this letter, the analysis and design of a ring-type erbium-doped ARROW amplifier is presented. The influence of the involved passive and active parameters (ring thickness and diameter, refractive index variation, pump and signal wavelengths, Er3+-ion concentration, and input pump power) on the spectral response of the structure and the optical power propagation losses and on the amplifier performance is numerically analysed. The opposite influence of the ring diameter on the optical power confinement and on the pump power density causes the existence of a diameter value that maximizes the amplifier net gain. For a cladding refractive index of 1.4 and moderate index variations, Delta n = 0.2 - 0.4, the optimum ring diameter is in the range of 20 mu m. To compensate signal confinement losses (a few dB/cm), high erbium concentrations (similar to 1 x 10(26) ion/m(3)) are required.

We present a study of the feasibility of transforming the multicore fiber in a supermode-interference based bending sensor into an active stage. By compensating the antisymmetric supermode losses for high curvatures, fringe visibility is improved and the sensor operating range can be extended. For a given multicore fibre length and Er3+-ion concentration sensor calibration curves allow assessing the bending radius by measuring the visibility as a function of the input pump power. The Er3+-ion concentration, available input pump power and multicore fiber length set the minimum bending radius that could be measured.

Active integrated multicore waveguide structures (AIMWS) with high refractive index contrast values and controlled local composition can be fabricated inside a bulk doped glass as a result of recent developments in high repetition femtosecond laser writing. The modeling of these structures presents some distinct characteristics when compared to that of corresponding multicore fiber structures. In this paper we present a study of the influence of two of these features in the signal amplification: the glass homogeneous doping and the existence of a zone of depressed refractive index. In order to do that, we use a home-made computer code that solves the optical power propagation equations of the structure supermodes coupled to the rate equations of the active ions.

Las estructuras multinúcleo integradas son buenas candidatas para mejorar las prestaciones de los dispositivos ópticos activos [1]. Estamos trabajando en la caracterización de estas estructuras fabricadas mediante escritura por láser de femtosegundo [2]. Se presentan las dos técnicas de caracterización modal implementadas por nuestro grupo.

Multicore fibers (MCFs) are expected as a good candidate for overcoming the capacity limit of current optical communication systems. This paper describes the modal analysis of a 3-core MCF taking into account some geometrical and material degrees of freedom. A reasonable tailoring of the modal distribution is shown to be achievable.