Alexandre A. P. Pohl

Gratings in Structured Optical Fibres

Grating writing in structured optical fibres and their properties and applications are reviewed. To date, most gratings have been written in a straightforward manner into structured fibres containing a photosensitive germanosilicate step-index core. However, gratings have also been written directly into single material, structured silica fibres and into air-clad cores using two and higher-photon processes with both UV and near IR pulsed (nanosecond-femtosecond) light. Given the intrinsic-added functionality possible within a structured optical fibre, structured fibre gratings offer further capabilities for sensors, diagnostics, lasers, and devices.

Detailed analysis of the longitudinal acousto-optical resonances in a fiber Bragg modulator

The interaction frequencies between longitudinal acoustic waves and fiber Bragg grating are numerically and experimentally assessed. Since the grating modulation depends on the acoustic drive, the combined analysis provides a more efficient operation. In this paper, 3-D finite element and transfer matrix methods allow investigating the electrical, mechanical and optical resonances of an acousto-optical device. The frequency response allows locating the resonances and characterizing their mechanical displacements. Measurements of the grating response under resonant excitation are compared to simulated results. A smaller than <1.5% average difference between simulated-measured resonances indicates that the method is useful for the design and characterization of optical modulators.

Bragg grating writing in acoustically excited optical fiber

Direct grating writing was modified by adding an acousto optic modulator in the optical fibre assembly. The acoustic wave changes the properties of the fibre while the beam is scanned along the fiber, creating a complex FBG profile that depends on the acoustic driver parameters.

Control of the long period grating spectrum through low frequency flexural acoustic waves

We have shown experimental results of the excitation of long period fiber gratings by means of flexural acoustic waves with a wavelength larger than the grating period, validated by numerical simulations. The effect of the acoustic wave on the grating is modeled with the method of assumed modes, which delivers the strain field inside the grating, then used as the input to the transfer matrix method, needed for calculating the grating spectrum. The experimental and numerical results are found to be in good agreement, even though only the strain-optic effects are taken into account.

Complex Bragg grating writing using direct modulation of the optical fiber with flexural waves

A flexural wave is applied to an optical fiber during the process of Bragg grating inscription using the direct writing method through a phase mask. Using this approach, we can dither the writing process to allow complex grating writing. Examples we demonstrate are tunable sampled gratings and phase-shifted gratings.

Control of the properties of fiber Bragg gratings based on the acousto-optic effect

A method for a fast and fine control of a notch filter and the group delay of a fiber Bragg grating (FBG) is presented. The method is based on the acousto-optic effect in fibers.

Acousto-optic modulation of a fiber Bragg grating in suspended core fiber for mode-locked all-fiber lasers

The interaction of a fiber Bragg grating and longitudinal acoustic waves in a three-air-holes suspended core fiber is experimentally investigated and employed to mode-lock an ytterbium-doped fiber laser. An optimized design of an acousto-optic modulator based on two piezoelectric transducers and 1 cm grating length is also proposed. For an electrical signal strength of 10 V applied to the modulator, the results indicate output pulses with a width of less than 550 ps at a repetition rate of 10 MHz. The reduction of the grating length and the power consumed by the transducer, when compared to previous studies, points out to more efficient, compact and fast acousto-optic modulators for mode-locked all-fiber lasers.

Numerical characterization of piezoelectric resonant transducer modes for acoustic wave excitation in optical fibers

The control of light in optical fibers by acoustic waves often requires a piezoelectric transducer as acoustic driver. As the fiber acoustic excitation depends on the deformation and frequency of the transducer, the analysis of the resonances and piezo mechanical behavior provides more efficient operation for acousto-optic devices. In this paper, an acousto-optic modulator is modeled using a three-dimensional finite element method in order to investigate the effect of the vibration modes of a piezoelectric disc in the excitation of acoustic waves in a fiber segment. The results indicate that in specific resonances, transducer radial and thickness modes are useful for the excitation of flexural and longitudinal acoustic waves in optical fiber.

Characterization of flexural acoustic waves in optical fibers using an extrinsic Fabry–Perot interferometer

A setup for the characterization of flexural acoustic modes in optical fibers is reported. An optical system based on an extrinsic Fabry–Perot interferometer is developed in order to measure the mechanical deformations of the fiber caused by a standing acoustic wave. Measurement of the excitation frequency and period allow estimation of the acoustic phase velocity propagating in a standard single mode and in a photonic crystal fiber.

Tunable acoustic bursts for customized tapered fiber Bragg structures

The inscription of customized fiber Bragg structures under acoustic excitation is proposed and demonstrated. By setting the proper acoustic excitation function, the desired spectrum of the grating can be achieved. In this paper, the effect of applying a burst acoustic wave during the process of Bragg grating inscription using the direct writing method through a phase mask is investigated. Among all the characteristics that can be changed due to a periodic defect insertion in the uniform Bragg structure, the control of phase is used to demonstrate this assertion. The results of numerical simulations are shown to be in excellent agreement with experiments. It can be further extended to generate multiple phase shifts in optical fiber and/or waveguides, if the defects are inserted at different positions, as is shown.