Fiber Bragg grating (FBG) is an important element in many applications including filters and dispersion compensators in fiber communication systems. With recently developed inverse scattering algorithm, FBGs with desired reflection spectrum and/or dispersion properties can now be designed. However, most of these designs require arbitrary grating amplitude and phase control. Previously, fabrication of such FBGs relies on the accurate control of the temporal variation of the intensity pattern using a piezo electric translation stage. The precision of this fabrication method is limited by the noise in the control voltage, which is usually larger than 1%. The distortion in piezo response also affects the performance. In this paper, we develop and demonstrate a novel writing technique for arbitrary FBG fabrication. Our technique is based on a translate-and-write configuration. The incorporation of a precisely controlled shutter allows the apodization and phase of the FBG to be continuously changed at each grating line. The shutter error mainly results from the control signal's timing jitter, which is normally lower than 0.1%. Using this writing technique, we demonstrate a Hamming apodized grating with 20mm length, -22 dB minimum transmission, and < -25 dB reflection side lobe suppression. Furthermore, phase-shift in a grating can be fabricated by a simple delay in the control signal. We also demonstrate FBGs with π, π/2, 3π/2 phase-shifts, respectively. Our experimental results are in excellent agreement with theoretical predictions. To show the capability to fabricate a FBG with arbitrary structure, we demonstrate a 35 mm long zero dispersion grating.
The photorefractive effect is a phenomenon in which the local index of refraction is changed by the spatial variation of the light intensity. Although the phrase 'photorefractive effect' has been traditionally used for such effects in electro-optic materials, new materials, including photopolymers and photosensitive glasses, have been developd in recent years and are playing increasingly important roles in optical fiber communication systems. Photopolymers in combination with liquid crystals are ideal materials for wavelength selective tunable devices. The improved optical quality and large dynamic range of photopolymers make them promising materials for holographic recording. Holographic gratings recorded in photopolymers can be employed as distributed Bragg reflectors. The large birefringence of liquid crystals can be used to tune the index of refraction to cover a large wavelength range. In addition, birefringence of liquid crystals can be used to tune the index of refraction to cover a large wavelength range. In addition, the combination of photopolymer and liquid crystal also leads to a new material known as holographic polymer dispersed liquid crystal (H-PDLC) which provides a medium for switchable holograms. Photonic devices made of these materials can be easily incorporated into an optical fiber system because of the low index of refraction of polymers and liquid crystals, and their relatively easy processing techniques. Besides photopolymers, photosensitive glasses are also promising for applications in fiber optic systems. Fiber Bragg gratings (FBGs) have been used as bandpass filters and dispersion compensators. In this paper, we describe the applications of photopolymers, H-PDLCs, and FBGs in fiber optic devices. Specifically, we will describe our recent works on photonic devices such as filters, switches, and dispersion compensators for WDM systems.
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