Quality parameters analysis of optical imaging systems with enhanced focal depth using the Wigner distribution function

An analysis of the Strehl ratio and the optical transfer function as imaging quality parameters of optical elements with enhanced focal length is carried out by employing the Wigner distribution function. To this end, we use four different pupil functions: a full circular aperture, a hyper-Gaussian aperture, a quartic phase plate, and a logarithmic phase mask. A comparison is performed between the quality parameters and test images formed by these pupil functions at different defocus distances.     

Temporal transformation of periodic incoherent ultrashort light pulses by chirped fiber gratings

The analogy between free-space propagation of optical beams and light-pulse reflection from linearly chirped fiber gratings is used to analyze the Lau effect in the temporal domain. The coherence conditions that are satisfied in the spatial domain for obtaining, at certain fixed locations, periodic fringes patterns are reformulated for guided light propagation. In this analogy, spatial periodic irradiance distributions are transformed in periodic sequences of light pulses. An optical setup is proposed to produce sharp pulse trains, with minimal distortion effects, that have repetition frequencies that are different from those associated with the input periodic optical signal. Some numerical results are given to illustrate this approach.     

Phase mask for spatial and temporal control of ultrashort light pulses focused by lenses

The field amplitude associated with ultrashort light pulses was analyzed by using the phase-space formalism of the Wigner distribution function (WDF). The diffraction integral was properly modified to take into account the dispersion effects (up to second order). A two-dimensional WDF associated with a reduced pupil function was derived, from which the on-axis irradiance was obtained for varying times. A two-dimensional and rotationally symmetric quartic-phase mask to control the temporal stretching of femtosecond light pulses passing through optical systems was proposed and analyzed. A Gaussian spatial and temporal pulse passing through a single lens with and without the phase mask was investigated.     

Wavelength multiplexed hydrogen sensor based on palladium-coated fibre-taper and Bragg grating

A novel configuration of a wavelength multiplexed hydrogen sensor based on a palladium-coated tapered fibre and a fibre Bragg grating is presented. This scheme allows cascading several sensors along a single fibre, which increases the capability of implementing multipoint sensor networks for volumetric detection. Moreover, in this configuration the light interacts twice with the palladium layer, thus enhancing the sensitivity of the sensor.     

Optical systems under broad-band illumination: Analysis based on a wigner distribution function approach

Abstract

The irradiance produced by an optical imaging system suffering from aberrations under broad-band illumination is analysed in the phase-space domain of the Wigner distribution function associated with a modified one-dimensional pupil aperture. To this end, a coordinate transformation is employed in such a way that the varying irradiance along different paths in the image space can be derived from different slices of the Wigner distribution function, which involve both the defocus distance and the dispersion properties of the optical system. The method is illustrated by comparing the axial irradiance distribution for different primary aberrations. Also, the irradiance along parabolic paths is obtained in order to analyse the coma effect.     

Phase pupil functions for focal-depth enhancement derived from a Wigner distribution function

A method for obtaining phase-retardation functions, which give rise to an increase of the image focal depth, is proposed. To this end, the Wigner distribution function corresponding to a specific aperture that has an associated small depth of focus in image space is conveniently sheared in the phase-space domain to generate a new Wigner distribution function. From this new function a more uniform on-axis image irradiance can be accomplished. This approach is illustrated by comparison of the imaging performance of both the derived phase function and a previously reported logarithmic phase distribution.     

Simultaneous Switching of the Q -Value and Operation Wavelength in an Erbium-Doped Fiber Laser

We report a pulsed erbium-doped fiber laser with simultaneous switching of the laser cavity Q-value and generation wavelength. The laser generates a train of short pulses with adjacent pulses alternately being on one of two fixed wavelengths. This regime is realized at periodical modulation of physical length of overwritten fiber Bragg gratings composing the laser cavity