Christian Cuadrado-Laborde

In-fiber all-optical fractional differentiator

We demonstrate that an asymmetrical pi phase-shifted fiber Bragg grating operated in reflection can provide the required spectral response for implementing an all-optical fractional differentiator. There are different (but equivalent) ways to design it, e.g., by using different gratings lengths and keeping the same index modulation depth at both sides of the pi phase shift, or vice versa. Analytical expressions were found relating the fractional differentiator order with the grating parameters. The device shows a good accuracy calculating the fractional time derivatives of the complex field of an arbitrary input optical waveform. The introduced concept is supported by numerical simulations.

Mode locking of an all-fiber laser by acousto-optic superlattice modulation

Active mode locking of an erbium-doped all-fiber laser with a Bragg-grating-based acousto-optic modulator is demonstrated. The fiber Bragg grating was acoustically modulated by a standing longitudinal elastic wave, which periodically modulates the sidebands at twice the acoustic frequency. The laser has a Fabry-Perot configuration in which cavity loss modulation is achieved by tuning the output fiber Bragg grating to one of the acoustically induced sidebands. Optical pulses at 9 MHz repetition rate, 120 mW peak power, and 780 ps temporal width were obtained. The output results to be stable and has a timing jitter below 40 ps. The measured linewidth, 2.8 pm, demonstrates that these pulses are transform limited.

Doubly active Q switching and mode locking of an all-fiber laser

Simultaneous and independent active Q switching and active mode locking of an erbium-doped fiber laser is demonstrated using all-fiber modulation techniques. A magnetostrictive rod attached to the output fiber Bragg grating modulates the Q factor of the Fabry-Perot cavity, whereas active mode locking is achieved by amplitude modulation with a Bragg-grating-based acousto-optic device. Fully modulated Q-switched mode-locked trains of optical pulses were obtained for a wide range of pump powers and repetition rates. For a Q-switched repetition rate of 500 Hz and a pump power of 100 mW, the laser generates trains of 12-14 mode-locked pulses of about 1 ns each, within an envelope of 550 ns, an overall energy of 0.65 microJ, and a peak power higher than 250 W for the central pulses of the train.

Actively mode-locked fiber ring laser by intermodal acousto-optic modulation.

We report an actively mode-locked fiber ring laser. A simple and low-insertion-loss acousto-optic modulator driven by standing flexural waves, which couples core-to-cladding modes in a standard single-mode optical fiber, is used as an active mechanism for mode locking. Among the remarkable features of the modulator, we mention its high modulation depth (72%), broad bandwidth (187 GHz), easy tunability in the optical wavelength, and low insertion losses (0.7 dB). The narrowest optical pulses obtained were of 95 ps time width, 21 mW peak power, repetition rate of 4.758 MHz, and 110 mW of pump power.

Transform-limited pulses generated by an actively Q-switched distributed fiber laser

A single-mode, transform-limited, actively Q-switched distributed-feedback fiber laser is presented, based on a new in-line acoustic pulse generator. Our technique permits a continuous adjustment of the repetition rate that modulates the Q factor of the cavity. Optical pulses of 800 mW peak power, 32 ns temporal width, and up to 20 kHz repetition rates were obtained. The measured linewidth demonstrates that these pulses are transform limited: 6 MHz for a train of pulses of 10 kHz repetition rate, 80 ns temporal width, and 60 mW peak power. Efficient excitation of spontaneous Brillouin scattering is demonstrated.

Proposal and Design of a Photonic In-Fiber Fractional Hilbert Transformer

We numerically demonstrate that an asymmetrical fiber Bragg grating of uniform-period operated in reflection can provide the required spectral response for implementing an all-optical fractional Hilbert transformer. The device shows a good accuracy calculating the fractional Hilbert transform of the complex field of some typical input optical waveforms. The sensibility of the proposed device under different conditions was also analyzed.

Theoretical treatment of a self-sustained, ball milling induced, mechanochemical reaction in the Fe2O3–Al system

Abstract A theoretical approach to self-sustained reactions (SSR) is established based on self-propagating high temperature synthesis. The corresponding heat conduction equation has been solved numerically and conclusions about the occurrence of the reaction through the whole sample have been obtained from the relationship between the dissipation mechanisms. These results strongly suggest that radiation heat flux plays a dominant role in the propagation of the reaction in reactive ball milling processes. The influence of different parameters on the occurrence of the SSR or its development were studied. Data for the formation of iron–alumina composites by reducing hematite with aluminum were used, in order to obtain realistic values. However the results are sufficiently generic to extrapolate the conclusions to other high enthalpy reactions.

Mechanochemical Reactions in Fe2O3–M (M: Al, Ti)

The production of metal–ceramic nanodispersion by mechanical milling of powders through the displacement reaction Fe2O3+M→Fe+M-oxide (with M: Al, Ti) was studied. The reaction progress with milling time was followed by recording the temperature and pressure during the process. The samples were characterized by X-ray diffraction and Mössbauer spectroscopy at the intermediate and final stages. In both cases self-sustained reactions were observed with different activation times. The results confirm that mechanical work at room temperature yields the reduction of hematite by Ti and Al. The final oxides were identified as Ti2O3 and Al2O3, respectively. The dependence of the intermediate and final stages on the milling conditions and the starting composition will be discussed.

Phase recovery by using optical fiber dispersion

We propose a simple and fast procedure to retrieve the phase profile of arbitrary light pulses. It combines a first experimental stage, followed by a one-step numerical stage. To this end, it is necessary to perform a Fresnel transform, which is obtained just by propagating the light pulses through an optical fiber. We experimentally test this proposal recovering the phase profile in the light pulses provided by a passively mode-locked laser. The proposal is then compared with a temporal variation of the Gerchberg-Saxton recursive algorithm, which is specially modified for this purpose.

Dissipative soliton resonance in a full polarization-maintaining fiber ring laser at different values of dispersion.

We investigated the dissipative solitons resonance in an ytterbium-doped fiber ring laser in which all the elements are polarization maintaining (PM). A semiconductor saturable absorber mirror was used as a mode-locker. The cavity included a normal dispersion single-mode fiber (SMF) and an anomalous dispersion photonic crystal fiber. The change of the length of the PM SMF allows the variation of the net-normal dispersion of the cavity in the range from 0.022 ps2 to 0.262 ps2. As the absolute value of the net-normal dispersion increases from 0.022 ps2 to 0.21 ps2, a square-shaped single pulse transformed to a single right-angle trapezoid-shaped pulse, and, at the dispersion of 0.262 ps2, to multiple right-angle trapezoid-shaped pulses, per round-trip.