Rectangular pulse generation based on pulse reshaping using a superstructured fiber Bragg grating

Abstract

Recent advances in ultrafast laser technology call for new all-optical methods for precisely manipulating and controlling the shape of short pulses. The most widely known pulse-shaping technique involves filtering of the spatially dispersed frequency components of a short pulse using bulk gratings and appropriate phase and amplitude masks. Alternative coherent pulse shaping techniques have also been developed. For example, in Ref [2] pulse shaping was achieved using second-harmonic generation in aperiodic quasi-phase-matching structures. The use of arrayed-waveguide gratings for optical processing applications has also been proposed. Fiber Bragg Gratings (FBGs) can also be viewed, and used, as spectral filters of controllable phase and amplitude. FBGs offer all the advantages associated with fiber components, such as ready integration into fiber systems, minimal coupling losses, and in addition offer tunability through control of the grating's strain and temperature. For a weak FBG, i.e. one in which light penetrates through the full grating, its frequency response is given by the Fourier transform of the index modulation profile along the grating length. This principle has been used in the past, in conjunction with conventional uniform FBG designs, to generate a train of dark pulses, and to obtain a matched filter for the detection of loops square pulses. However, advances in the fabrication of FBGs now allow the fabrication of gratings with almost arbitrary amplitude and phase characteristics, greatly extending the range of applications of the approach. Control of the grating's frequency response is obtained by spatially modulating a uniform grating's refractive index profile with a sampling function that corresponds to the desired impulse response of the grating. Recently, we demonstrated the use of relatively simple superstructured gratings for pattern generation and recognition, as required for optical code division multiple access applications (OCDMA), and have also demonstrated the use of an FBG to perform pulse repetition rate multiplication from 10 to 40 GHz. In this work, we have progressed the approach, and successfully demonstrated the fabrication and use of a truly complex superstructure grating designed to transform short optical pulses (2.5 ps at 10 GHz) into a corresponding train of 20 ps rectangular pulses. Such pulse forms are suited to nonlinear optical switching applications, in which a square switching window is required.

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Identifiers

ORCID for P. Petropoulos: orcid.org/0000-0002-1576-8034

ORCID for D.J. Richardson: orcid.org/0000-0002-7751-1058

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