Joseph Gazalet

Acousto-optic couplings in two-dimensional phoxonic crystal cavities

We investigate the acousto-optic coupling, based on both photo-elastic and opto-mechanical mechanisms, in periodic structures with simultaneous photonic and phononic band gaps. The investigations are focused on a cavity defect in which the strong confinement of acoustic and optic waves enhances the interaction. We calculate the modulation of each photonic mode frequency by each phononic mode confined in the cavity. We compare the strength for the photo-elastic and opto-mechanical effects in the different cases. Both mechanisms can be in phase or out of phase and produce additive or subtractive effects in the total acousto-optic coupling.

Optomechanical interactions in two-dimensional Si and GaAs phoXonic cavities

We investigate theoretically the optomechanical interactions inside cavities created in two-dimensional infinite phoXonic crystals constituted by a square array of holes in silicon (Si) and gallium arsenide (GaAs) matrices. The cavity is simply obtained by removing one hole in the perfect crystal. Our calculations take into account two mechanisms that contribute to the optomechanical coupling, namely the bulk photoelastic effect and the deformations of the interfaces due to the acoustic strain. The coupling strength is estimated by two different methods, modulation of the photonic mode frequency during one period of the acoustic oscillations and calculation of the optomechanical coupling rate. We discuss the important roles of the symmetry and degeneracy of the modes to discriminate which ones are not able to interact efficiently. Calculations in Si and GaAs crystals at different optical wavelengths emphasize the dependence of the photoelastic contribution to the optomechanical interaction as a function of material and wavelength, especially owing to the significant variation of the photoelastic coefficients near the semiconductor band gap.

Frequency and angular bandwidth of acousto-optic deflectors with ultrasonic walk-off

In the paper, bandwidth parameters of acousto-optical deflectors (AODs) are analyzed from the point of view of acoustical anisotropy. Equations for bandwidth and central frequency of AOD are derived for arbitrary propagation direction of ultrasound in optically uniaxial crystals. The phenomenon of bandwidth shift due to phase mismatch at the central frequency is studied theoretically and verified experimentally.

Simultaneous bandgaps in LiNbO3 phoxonic crystal slab

We study simultaneous photonic and phononic crystal slabs created in Z-cut lithium niobate membranes. Bandgaps for guided waves are identified using the three-dimensional finite element method (FEM). Three lattices are considered: the square, the hexagonal, and the honeycomb lattices. We investigate the evolution of band gaps as a function of geometrical parameters such as hole radius and membrane thickness. We show the existence of dual photonic and phononic bandgaps in the triangular lattice for suitable geometrical parameters and specific modal symmetries for both the elastic and the electromagnetic fields.

Acousto-optic wide band optical low-frequency shifter

A wide bandwidth tunable optical low-frequency shifter is presented in this work. It is based on two acousto-optic devices operating in tandem. The relevant parameters of the specific configurations of acousto-optic interactions in paratellurite material are detailed. Results from numerical computations leading to the practical design parameters are given. The low-frequency shifter has been experimentally tested at a visible wavelength λ₀=514  nm and a high efficiency (>60%) has been measured. The tuning capability of the optical shifter covers a bandwidth Δf=26  MHz.

Acousto-optic couplings in a phoXonic crystal slab L1 cavity

Acousto-optic coupling mechanisms within simultaneous photonic and phononic crystal cavities are theoretically studied. The structure considered is a silicon slab drilled periodically with air holes arranged on a square lattice; the cavity is obtained by removing one hole from the ideal crystal structure. Two acousto-optic coupling mechanisms are taken into account: the classical photo-elastic effect and the opto-mechanic effect which arises from the moving boundaries. The coupling mechanisms are computed according to the finite element method. The results reveal that the mechanisms can sustain each other or act with counteracted effects.

Double acousto-optic deflector system for increased scanning range of laser beams

HIGHLIGHTSA cascaded acousto‐optic deflectors system is proposed.The interaction is based on high and low frequency wide band interaction in paratellurite.The scanning range is increased by a factor 2. ABSTRACT A new laser scanning system is presented based on two wide‐band acousto‐optic deflectors. The interaction medium is tellurium dioxide. Anisotropic interactions take place under two different tangential phase matching configurations in such a way that the acousto‐optic bandwidths add up. We demonstrate the feasibility of such a cascade deflection system for the wavelength of &lgr; = 514 nm. The total frequency bandwidth is &Dgr;f = 100 MHz, equally distributed between the two acousto‐optic deflectors. The total angular scan at the output is &Dgr;&THgr; = 4.4° leading to 125 resolvable spots for a 1 mm truncated Gaussian beam.

Acousto-optical tunable transmissive grating beam splitter

We present a tunable transmissive grating beam splitter for multiple laser line separation based on acousto-optic interaction in tellurium dioxide. Acousto-optic devices are well known for light modulation, frequency shifting, filtration or deflection. For a deflector, the incident light beam is monochromatic and the angular deviation is proportional to the ultrasonic frequency excursion. For a tunable filter, the selected wavelength is determined by the ultrasonic frequency. Several wavelengths may be simultaneously diffracted using several associated ultrasonic frequencies and all the diffracted beams have the same angular deviation. Unlike the classical operating modes of acousto-optic devices, we consider the simultaneous diffraction of several optical wavelengths by a single ultrasonic frequency. The device is based on Bragg anisotropic interaction in the specific “Tangent Phase Matching” configuration. The acousto-optic interaction takes place with a single ultrasonic frequency and the diffraction efficiency remains high over a wide optical spectral range. The different diffracted beams are then angularly well separated, due to the slow velocity of the ultrasonic wave propagating in tellurium dioxide. The optical bandwidth is directly related to the operating ultrasonic frequency. Numerical calculations were carried out to determine the main parameters of the device: operating ultrasonic frequency, optical bandwidth, tunability range, crystalline cut and transducer length. A practical device has been designed for visible spectrum. Experimental results will be presented as for example a spectral bandwidth from 450 nm to 550 nm with a RF carrier frequency f = 125 MHz.

340 nm Bandwidth Automatic Dynamic Optical Equalizer for CWDM Networks

This paper presents an automatic optical equalizer based on a pair of in-house developed high efficiency, acousto-optic variable optical attenuators (AO-VOAs). The system is polarization insensitive and presents a wide bandwidth compatible with coarse wavelength division multiplexing (CWDM) requirements: 340 nm. The system operation is automatic and bidirectional and equalization is obtained in one single iteration of algorithm loop.

Aoustic modulation of TM and TE waves in two-dimensional silicon phoxonic cavitiy

We investigate the photon-phonon coupling, based on both photo-elastic and opto-mechanical mechanisms, in a two-dimensional periodic structure exhibiting simultaneous photonic and phononic band gaps. The coupling takes place in a cavity defect in which the strong confinement of acoustic and optic waves enhances the acousto-optic interaction. We calculate the modulation of each photonic mode frequency by each phononic mode confined in the cavity and compare the acousto-optic interactions for both TM and TE waves. We also compare the strength of the photo-elastic and opto-mechanical effects in the different cases. Both mechanisms can be in phase or out of phase and produce additive or subtractive effects in the total acoustooptic coupling.