V. Berdejo

In situ assessment and minimization of nonlinear propagation effects for femtosecond-laser waveguide writing in dielectrics

The effect of nonlinear propagation on the shape of the focal volume has been assessed by in situ plasma emission imaging during the subsurface processing of a commercial phosphate glass. The sample was processed with an elliptically shaped femtosecond-laser beam at 1 kHz repetition rate and scanned transversely with respect to the writing beam axis. As a consequence, optimal conditions for minimizing undesirable nonlinear propagation effects during the production of optical waveguides by direct laser writing have been determined. Under these conditions, it is possible to induce structural transformations and still preserve the focal volume shape associated with the linear propagation regime. While at low pulse energy a single scan laser-written structure does not support a guided mode, the use of multiple scans with minimized nonlinear propagation effects enables the production of optical waveguides. The latter show a significantly improved performance in terms of the refractive index change and propagation losses when compared to single scan waveguides.

Performance of ultrafast laser written active waveguides by rigorous modeling of optical gain measurements

An Er:Yb co-doped P2O5-La2O5 based glass has been synthesized and used for producing 1.6 cm-long active optical waveguides using a low repetition (1 kHz) rate Ti:Al2O3 fs-laser amplifier. Before processing, the laser energy deposition profile for an elliptically shaped beam was simulated, and the best processing conditions for optimizing the focal volume shape, minimizing non-linear propagation effects, were determined. Under these conditions, a multi-scan writing approach was used to maximize the refractive index change induced and to minimize the transmission losses. After processing, the optical powers propagating inside the waveguide (pump absorption, co- and counter-propagating ASE, low signal gain, …) were measured for uni- and bi-directional pumping schemes, and the measurements were simulated and fitted using an ad hoc developed model to describe the behavior of laser written waveguides. The measurements provide internal gain figures comparable to the best ones reported in phosphate glasses for low repetition rate writing even with larger insertion losses. The simulations provide access to key parameters of the waveguide characteristics (coupling losses and propagation losses, Yb3+ ⇔ Er3+ energy transfer rates, Er3+ upconversion coefficient), which have been used to model the expected performance of these structures in terms of length and doping level. A moderate increase of the Er3+ and Yb3+ doping level would potentially lead to net gain values up to 9.4 dB for a waveguide length of 25 mm.

Study of the use of methanol-filled Er-doped suspended-core fibres in a temperature-sensing ring laser system

We report on an experimental/numerical investigation into the use of methanol-filled Er-doped suspended-core fibres (SCFs) in temperature-sensing ring laser systems. We have adopted a ring laser configuration that includes an Er-doped SCF as a temperature-dependent attenuator (TDA) with a step-index Er-doped fibre (EDF) as the laser active medium. The laser performance dependence on the temperature was measured both in continuous wave (CW) and transient regimes. CW laser output power and build-up time values are compared with those of similar laser systems based on other types of Er-doped PCFs or using other laser configurations. A notable variation of 0.73% °C−1 was achieved in CW operation. Then, by means of parameters obtained by numerically fitting the experimental results, the potential sensing performance of the laser configuration with an SCF as a TDA is studied. Moreover, two ring cavity laser configurations (with the SCF acting basically as an attenuator or also as the active media) are compared and the influence of the position of the coupler inside the ring cavity and the contribution of the erbium doping to improve the sensor features are analysed. The longer interaction lengths compatible with laser action using the Er-doped SCF as a TDA could provide variations of laser output power up to 8.6% °C−1 for 90 mW pump power and a 1 m methanol-filled SCF.

Study of upconversion in PCFs with high erbium concentration

We report on a comparison of characterization techniques for high concentration erbium-doped photonic crystal fibres (PCFs). A highly erbium-doped-silica PCF was fabricated and an amplifier based on the PCF was built. Then, measurements on the amplifier output optical powers were carried out. To model the amplifier, three different formalisms were assumed for the Er3+-ion upconversion mechanism and the numerical results were fitted to the experimental ones. The sets of best-fit parameters are compared and the use of these techniques for active PCF characterisation is discussed.

Detailed characterization of a highly Yb-doped double-clad fiber

A characterization method based on the careful measurement of the characteristic parameters and fluorescence emission spectra of a highly Yb-doped double-clad fiber is presented. The method is successfully checked by numerically fitting experimental results of a ring laser based on highly doped double-clad Yb-doped silica fibers with different dopant concentrations, fiber lengths and ring-laser output-coupler rates.

High gain, short length optical amplifier in heavily doped phosphate glass for miniature optics

In this work we demonstrate the production of a high gain, short length waveguide amplifiers and lasers produced by state of the art femtosecond (fs) laser micromachining.

A simple theoretical model for erbium doped PCF ring lasers design

In this paper a simple theoretical model is presented where the energy conservation principle is used. The model is based on semi-analytical equations describing the behaviour of an erbium-doped photonic crystal fibre (PCF) inside a ring laser. These semi-analytical equations allow the characterisation of the erbium-doped PCF. Spectral absorption and emission coefficients can be determined through the measurement of the gain in the PCF as a function of pump power attenuation for several fibre lengths by means of a linear fitting. These coefficients are proportional to the erbium concentration and to the corresponding absorption or emission cross section. So if the concentration is known the erbium cross sections can be immediately determined. The model was successfully checked by means of two different home-made erbium doped PCFs. Once the fibres were characterised the values of the spectral absorption and emission coefficients were used to simulate the behaviour of a back propagating ring laser made of each fibre. Passive losses of the components in the cavity were previously calibrated. A good agreement was found between simulated and experimental values of efficiency, pump power threshold and output laser power for a wide set of experimental situations (several values of the input pump power, output coupling factor, laser wavelength and fibre length).

Erbium-doped photonic crystal fiber chaotic laser

An erbium-doped photonic crystal fiber laser has been designed, constructed and characterized in order to examine the feasibility of this kind of devices for secure communications applications based on two identical chaotic lasers. Inclusion of a tailored photonic crystal fiber as active medium improves considerably the security of the device because it allows customization of the mode transversal profile, very influential on the laser dynamics and virtually impossible to be cloned by undesired listeners. The laser design has been facilitated by the combination of characterization procedures and models developed by us, which allow prediction of the most suitable laser features (losses, length of active fiber, etc.) to a given purpose (in our case, a laser that emits chaotically for a wide assortment of pump modulation conditions). The chaotic signals obtained have been characterized by means of topological analysis techniques. The underlying chaotic attractors found present topological structures belonging to classes of which very scarce experimental results have been reported. This fact is interesting from the point of view of the study of nonlinear systems and, besides, it is promising for secure communications: the stranger the signals, the more difficult for an eavesdropper to synthesize another system with similar dynamics.