Experimental demonstration of an optical differentiator based on a fiber Bragg grating in transmission

Phase-Inserted Fiber Gratings and Their Applications to Optical Filtering, Optical Signal Processing, and Optical Sensing: Review

Phase-inserted fiber gratings (PI-FGs) refer to those gratings where there exist a number of the phase-shifts (spatial spacing) among different sections (or local periods) of the gratings themselves. All the PI-FGs developed to date can mainly be divided into three categories: phase-shifted gratings, phase-only sampled gratings, and phase-modulated gratings, of which the utilized gratings could be either the Bragg ones (FBGs) or the long-period ones (LPGs). As results of the proposed the PI-FGs where the numbers, quantities, and positions of the inserted phases along the fiber direction are optimally selected, PI-FGs have already been designed and used as various complex filters such as the ultra-narrow filters, the triangular (edge) filters, the high channel-count filters, and the flat-top band-pass/band-stop filters, which, however, are extremely difficult or even impossible to be realized by using the ordinary fiber gratings. In this paper, we have briefly but fully reviewed the past and recent advances on PI-FGs, in which the principles and design methods, the corresponding fabrication techniques, and applications of the different PI-FGs to the fields of optical filtering, optical signal processing, and optical sensing, etc., have been highlighted.

Reconfigurable photonic temporal differentiator based on a dual-drive Mach-Zehnder modulator

We propose and demonstrate a reconfigurable photonic temporal differentiator based on a dual drive Mach-Zehnder modulator (DDMZM). The differentiator can be reconfigured to different differentiation types by simply adjusting the bias voltage of DDMZM. Both simulations and experiments are carried out to verify the proposed scheme. In the experiment, a pair of polarity-reversed field differentiation and a pair of polarity-reversed intensity differentiation are successfully generated. The differentiation accuracy and conversion efficiency versus the time delay are also investigated.

Cancellation of the zeroth order by a low-contrast grating

The cancellation of the 0th order is described by a low-contrast fused-silica grating. In reported works, the high-contrast grating and complicated structure were applied with reasonable and excellent performance. However, the low-contrast grating is proved that it can also cancel the 0th order with the period more than 2λ in this paper. Grating parameters are optimized by using rigorous coupled-wave analysis, whose physical essence for cancellation of the 0th order can be well explained by modal method. The fabrication tolerance is investigated for production of the low-contrast grating for cancellation of the 0th order, which can be potentially used for writing the fiber Bragg grating.

Spatiotemporal pulse shaping using resonant diffraction gratings

We propose a new theoretical model describing spatiotemporal transformations of two-dimensional optical pulses by resonant diffraction gratings. The diffraction of the pulse is described in terms of a linear system. Simple analytical approximations for the transfer function and the impulse response of the system are derived. The derived approximations contain five independent parameters, which can be estimated using the rigorous coupled-wave analysis. The presented numerical simulation results demonstrate that the resonant grating can perform complex pulse transformations, such as the simultaneous spatial and temporal differentiation of the optical pulse envelope.

THz-bandwidth photonic Hilbert transformers based on fiber Bragg gratings in transmission

THz-bandwidth photonic Hilbert transformers (PHTs) are implemented for the first time, to the best of our knowledge, based on fiber Bragg grating (FBG) technology. To increase the practical bandwidth limitation of FBGs (typically <200  GHz), a superstructure based on two superimposed linearly-chirped FBGs operating in transmission has been employed. The use of a transmission FBG involves first a conversion of the non-minimum phase response of the PHT into a minimum-phase response by adding an anticipated instantaneous component to the desired system temporal impulse response. Using this methodology, a 3-THz-bandwidth integer PHT and a fractional (order 0.81) PHT are designed, fabricated, and successfully characterized.

Optical computation of the Laplace operator using phase-shifted Bragg grating

Diffraction of a 3D optical beam on a multilayer phase-shifted Bragg grating (PSBG) is considered. It is shown that the PSBG enables optical computation of the spatial Laplace operator of the electromagnetic field components of the incident beam. The computation of the Laplacian is performed in reflection at normal incidence. As a special case, the parameters of the PSBG transforming the incident Gaussian beam into a Laguerre-Gaussian mode of order (1,0) are obtained. Presented numerical results demonstrate high quality of the Laplace operator computation and confirm the possibility of the formation of Laguerre-Gaussian mode. We expect the proposed applications to be useful for all-optical data processing.

Compact in-line optical notch filter based on an asymmetric microfiber coupler

Novel compact in-line optical filters with narrow rejection bandwidths are proposed, and one is experimentally demonstrated based on asymmetric microfiber (MF) couplers. It is composed of silica and bismuth-oxide-glass MFs and less than 500 μm long. Single transmission dips in a wavelength range of up to 300 nm are obtained, and the transmission extinction ratio is more than 30 dB. Additionally, a -20  dB bandwidth of 0.88 nm is achieved. This narrow bandwidth of the transmission notch benefits from the huge refractive index difference between the two MFs. These experimental results agree well with the theoretical predictions. With the advantages of having a simple structure, being fiberized, and having a small footprint, this device can be an attractive element for micro/nanophotonics, optical sensing, optical signal processing, and optical fiber communications.