Magnification of mask fabricated fibre Bragg gratings

Bragg gratings have been formed in optical fibres by diffraction of a phase mask. This technique is expanded by allowing for the possibility of shifting the Bragg wavelength for a fixed mask periodicity. This method uses a simple lens to introduce a curvature on to the wavefront of the diffracting light that results in a magnification of the original periodicity formed by the mask. The authors show experimentally that the Bragg wavelength is shifted to a shorter wavelength by the addition of a converging lens before the mask.<>     

Electronic technique for temperature compensation of fibre Bragg gratings sensors

Optical sensors composed of fibre Bragg gratings (FBG) are becoming a choice for a wide range of sensing applications. However, in order to manufacture temperature independent sensors, it is necessary to compensate for the intrinsic temperature dependence of the FBG. In this letter we present an extremely effective, simple and low cost novel technique for stabilising the bias point of an interrogation current amplitude detection system which uses a laser diode source and a pair of matched FBGs. An electronic feedback control circuit measures the wavelength shift due to temperature variations in a reference FBG and actuates on the laser diode temperature, to induce changes in its wavelength output and compensate for the temperature dependence of the FBG sensor.     

Optical comb filter based on spectral talbot effect in uniform fibre bragg gratings

A comb optical filter based on the spectral Talbot effect in uniform low reflecting fibre Bragg gratings is numerically and experimentally demonstrated, relaxing the need for any chirp or equivalent phase shifts between the gratings. It can be understood by the analogy between the compression and rate multiplication of phase modulated pulse trains and sampled fibre Bragg grating spectrum formation     

Bragg gratings written in all-SiO/sub 2/ and Ge-doped core fibers with 800-nm femtosecond radiation and a phase mask

Femtosecond laser pulses at 800 nm and 120 fs were used to fabricate high-quality retroreflecting fiber Bragg gratings in standard Ge-doped telecom fiber (Corning SMF-28) and all-silica-core Fluorine doped cladding single-mode fiber using a deep-etch silica zero-order ed phase mask. Induced index modulations of 1.9/spl times/10/sup -3/ were achieved with peak power intensities of 2.9/spl times/10/sup 12/ W/cm/sup 2/ without any fiber sensitization such as hydrogen loading. The fiber gratings have annealing characteristics similar to type II damage fiber gratings and demonstrate stable operation at temperatures as high as 950/spl deg/C. The grating devices exhibit low polarization dependence. The primary mechanism of induced index change results from a structural modification to the fiber core.     

Efficient grating writing through fibre coating with femtosecond IR radiation and phase mask

Bragg gratings have been written through the polymer coating into hydrogen loaded SMF-28 fibres with ultrafast 800 nm radiation and a phase mask. Index modulations of up to 3.5times10^-4 were induced with fibre strengths remaining at ~85% of the pristine fibre value     

Ultrasonic transducer configuration for producing a phase grating of nearly uniform strength

A configuration is given which produces nearly uniform phase retardation of a light beam passing through the near field of an ultrasonic transducer.     

Differential phase tracking applied to Bragg gratings in multi-core fibre for high accuracy curvature measurement

High resolution quasi-static and dynamic curvature measurements are made by differential interferometric phase measurement of fibre Bragg gratings in separate cores of a multi-core fibre. A DC curvature stability of 1/spl times/10/sup -3/m/sup -1/ and an AC curvature resolution of 1/spl times/10/sup -4/ m/sup -1//Hz/sup 1/2/ are reported.     

Generation of multiple-pitch gratings in WDM devices by using a single computer-generated phase mask

Use of a single computer-generated phase mask (CGPM) for monolithic fabrication of submicronmeter multiple-pitch gratings is proposed. The CGPM is a computer-generated Fourier hologram generating multiple diffracted beams with equal intensity from a single incident beam. The submicronmeter grating periods differing by as little as a few angstroms are estimated with basis of the multiple diffraction angles. As a result of preliminary experiment, an array of nine gratings has been achieved by using a CGPM with 2-/spl mu/m minimum feature size. Limit of the CGPM period due to increase in wavelength spacing of WDM devices is also discussed.     

Chirped fibre Bragg gratings for phased-array antennas

A variable delay line for phased-array antennas based on a chirped fibre Bragg grating is demonstrated. The time delay of a microwave modulating signal is modified by scanning in wavelength a chirped grating. In this initial experiment, time delay variations up to 556 ps have been achieved using a grating of 0.4 nm bandwidth and 6 cm length and modulating in the frequency range 390 MHz-5.20 GHz     

Mode coupling in spiral fibre gratings

The coupling properties of spiral fibre gratings are presented. It is shown that LP₀₁ modes in spiral fibre gratings couple to both LP₁₁ even and odd modes     

Fabrication and characterisation of long, narrowband fibre gratingsby phase mask scanning

Fibre Bragg gratings up to 50 mm in length have been fabricated using a phase mask scanning technique. Reflectivities of up to 70% and linewidths as narrow as 0.029 nm have been measured. The fabrication technique lends itself to writing gratings with controllable length, chirp and shading     

Novel passive phase noise cancelling technique for interferometric fibre optic sensors

A novel technique for the cancellation of phase noise in a fibre interferometer caused by laser frequency emission jitter is described. The scheme is based on the use of a reference interferometer and phase generated carrier passive homodyne phase detection. Signal processing involving cross-multiplication of the sine and cosine components of the sensor and reference interferometer outputs is shown to provide cancellation of phase noise.<>     

Fiber Bragg gratings with suppressed cladding modes made in SMF-28 with a femtosecond IR laser and a phase mask

High reflectivity fiber Bragg gratings (FBG) with strong cladding mode suppression were written in standard telecom fiber (SMF-28) with femtosecond 800-nm laser pulses and a phase mask. No special fiber photosensitization or fiber design was required. Induced index modulations were created by two-beam interference from the phase mask in both the core and cladding region, which resulted in cladding-mode coupling loss <0.1 dB for a high reflecting -30 dB in transmission FBG.     

干涉法制造椭圆环光栅

本文给出了利用透射圆锥透镜（Ａｘｉｃｏｎ）制造全息椭圆环衍射光栅的基本原理，实验方法和结果。     

两元相位光栅光调制

基于两元相位光栅的自成像效应和相位补偿的原理,本文提出了两元光学的一种新的双相位光栅光调制器。用菲涅耳衍射理论导出了任意光束的解析解,进行了高斯光束的计算机模拟。表明调制度正比于光束直径而反比于两光栅的间距,最后给出了实验。     

High birefringence fibre Bragg gratings written in tapered photonic crystal fibre with femtosecond IR radiation

High birefringence Bragg gratings were written with ultrafast 800 nm radiation in non-birefringent pure silica core photonic crystal fibres (PCF) that were tapered with an oxy-hydrogen flame. The grating spectra indicate that a birefringence above 10^-3 was created in the tapered PCF     

Electric-arc-induced long-period gratings in fluid-filled photonic bandgap fibre

Electric-arc-induced long-period gratings in a fluid-filled photonic bandgap fibre (PBGF) is demonstrated. Gratings of varying pitch were fabricated and excellent agreement between the measured resonances and numerically simulated phase matching curves is obtained. The PBGF geometry allows the detection of very small arc-induced structural deformations of the fibre and their correlation with the grating strength.     

Bend measurement using Bragg gratings in multicore fibre

The first measurements of curvature made using Bragg gratings written in separate cores of a multicore optical fibre are described. The gratings act as independent, but isothermal, strain gauges. The difference in Bragg wavelength between the gratings provides a temperature-independent measurement of the local curvature