Active control of long-period fiber-grating-based filters made in erbium-doped optical fibers.

Integrating an active medium with a filtering component can bring new functionalities compared with configurations where the two stages (units) are placed in series. We exploit this phenomenon and demonstrate all-optical tunable filters based on long-period fiber gratings written in an active optical fiber. The tuning is obtained via optical pumping at 980 nm.

Picosecond and sub-picosecond flat-top pulse generation using uniform long-period fiber gratings.

We propose a novel linear filtering scheme based on ultrafast all-optical differentiation for re-shaping of ultrashort pulses generated from a mode-locked laser into flat-top pulses. The technique is demonstrated using simple all-fiber optical filters, more specifically uniform long period fiber gratings (LPGs) operated in transmission. The large bandwidth typical for these fiber filters allows scaling the technique to the sub-picosecond regime. In the experiments reported here, 600-fs and 1.8-ps Gaussian-like optical pulses (@ 1535 nm) have been re-shaped into 1-ps and 3.2-ps flat-top pulses, respectively, using a single 9-cm long uniform LPG.

Switchable optical element with Bragg mode diffraction.

A theoretical model of a new electronically switchable grating design that uses a multilayer structure of an electro-optic (EO) material with an interdigitated-electrode type of array is proposed as an original technique for calculating the induced refractive index. It is shown that asymmetrical distribution of the electric field induces a slanted Bragg grating, which allows the slant angle to be switched electronically among more than two switching states. Parameters of the suggested design are calculated for a number of EO materials. A special case of frequency-based switching is anticipated for some polymer-dispersed liquid-crystal materials.

Electro-optically reconfigurable waveguide superimposed gratings.

A new concept of electro-optically reconfigurable waveguide superimposed gratings with their widely controllable transmission characteristics are present. It is based on two types of superimposed refractive index gratings, electronically induced in an electro-optically active core. These gratings can be independently switched ON and OFF or both simultaneously activated with the controllable weighting factor. In this case its transmission characteristics represent double-dip rejection band spectrum with independent control of the dip positions and bandwidth. This simple concept opens opportunities for developing a number of tunable devices for integrated optics by use of the proposed design as a building block.

Tunable electro-optic microlens array. I. Planar geometry.

A description is given of a microlens array design, believed to be new, with tunable focal length. Converging or diverging periodic refractive-index distribution is induced in a linear electro-optic wafer through the application of electric field profiles on both sides of the wafer. The transparent electrodes on both sides of the wafer are positioned such that the electrodes on the opposite side compensate the phase delay from the electrodes on the front side of the wafer for a normally incident plane wave, suppressing the intrinsic electrode diffraction for the device without applied voltage. The original technique of the electric field calculation was developed to analyze accurately the induced refractive index inside the wafer. Its focal length changes from 7 mm to infinity at a 1-mum wavelength with an external voltage range of 0-100 V.

Tunable electro-optic Microlens array. II. Cylindrical geometry.

Although the concept of an artificial compound eye has been discussed in the literature, its optical arrangement has never been widely adopted for optical design. A design is presented for a tunable gradient-index microlens array, believed to be new, induced electro-optically inside a cylindrical shell. The transparent electrodes on the both sides of the shell are positioned such that the electrodes on the opposite side compensate the phase delay from the electrodes on the front side for a normally incident plane wave, thus suppressing the intrinsic electrode diffraction for the device without applied voltage. The original technique of the electric field calculation was developed to analyze the induced refractive index inside the shell for two types of electro-optic (EO) ceramics: with linear and with quadratic EO effects. For the linear effect it was shown that for given EO coefficients, electric field strength and intrinsic refractive index, the electrode number should exceed a certain amount to make the focal distance less than the cylinder radius. The quadratic effect provides higher sensitivity to the type of the diffracted wave polarization. It was shown how the quadratic coefficient ratio R(12)/R(11) affects the focal-length difference between TE and TM light polarization.

Interdigitated electrode-induced phase grating with an electrically switchable and tunable period.

A new design for an adjustable electro-optic phase grating inside a waveguide is proposed. The electric field and the refractive-index distribution induced inside a waveguide by voltage applied to double-sided periodic interdigitated electrode arrays are calculated rigorously on the basis of an original analytical technique. The modeling was carried out with the Mathcad software. It is shown that the fundamental periodicity of the induced grating inside the waveguide can be switched between l and 2l by application of the appropriate voltage, where l is the spatial periodicity of the interdigitated electrodes. One can also fine tune the peak grating reflectivity by changing the constant component of the induced refractive index with the help of the constant component of the electric field inside the waveguide. The suggested design can be used as a basic idea for a variety of optical communication networking applications, including switching, modulation, deflection, and data processing.

Adjustable electro-optic microlens with two concentric ring electrodes.

A theoretical model of a new variable-focal-length lens design that uses an electro-optic wafer with two double-sided concentric ring electrodes is proposed on the basis of an original technique for calculating the induced refractive index. It is shown that the relationship between ring radii and wafer thickness produces the required distribution of the refractive index to create converging focusing elements.

Modeling of a converging gradient-index lens with variable focal length in a lanthanum-modified lead zirconate titanate ceramic cylinder with a lateral multielectrode structure.

A mathematical model is presented to characterize the performance of a lanthanum-modified lead zirconate titanate electro-optical cylindrical structure with an arbitrary number N of lateral electrodes switched with a series of voltages V(0) and V(0) + DV. The method yields a solution that can be written as a sum of numerical and analytical components, and the contribution of numerical components becomes increasingly smaller as the electrode wrapping angle is decreasing below pi/3N. The effects of the electrode wrapping angle and the switching voltages on the induced refractive-index distribution and electrode capacitance are analyzed. It is shown that the design offers a simple and compact structure that performs dynamic light diverging with a focal length that can be controlled by the applied voltage. The possibility of light steering is considered for a two-electrode structure.