Periodic Microbends Research Articles

High-efficiency broadband fiber-optic mechanical intermodal converter

We demonstrate a highly efficient, broadband fiber-optic intermodal converter. The technique relies on a long period grating mechanically induced in a two-mode fiber. A compact, portable apparatus was designed and fabricated, where period-variable metallic corrugation is implemented to form periodic micro-bends along the fiber. The coupling strength between the interacting fiber modes and the grating period can be tuned continuously and individually using two control knobs in the apparatus. Experimental results show that the complete coupling between the LP01 and LP11 modes is achieved, which is confirmed by an observed over-coupling while increasing the grating strength. For the short fiber length of <1.9 cm (33 grating periods), large band-rejection of −32.5 dB was obtained at resonance. The band rejection efficiency over 98.6% have been achieved in the entire communication C-band. As the grating strength increased, two over-couplings were observed at resonance, which indicates the high efficiency of the device. Experimental results are confirmed by our numerical simulations.

Designing and Constructing the Strain Sensor Using Microbend Multimode Fiber

The microbend sensor is designed to experience a light loss when force is applied to the sensor. The periodic microbends cause propagating light to couple into higher order modes, the existing higher order modes become unguided modes. Three models of deform cells are fabricated at (3, 5, 8) mm pitchand tested by using MMF and laser source at 850 nm. The maximum output power of (8, 5, 3)mm model is (3, 2.7, 2.55)nW respectively at applied force 5N and the minimum value is (1.9, 1.65, 1.5)nW respectively at 60N.The strain is calculated at different microbend cells ,and the best sensitivity of this sensor for cell 8mm is equal to 0.6nW/N.

Designing and Constructing the Strain Sensor Using Microbend Multimode Fiber

The microbend sensor is designed to experience a light loss when force is applied to the sensor. The periodic microbends cause propagating light to couple into higher order modes, the existing higher order modes become unguided modes. Three models of deform cells are fabricated at (3, 5, 8) mm pitchand tested by using MMF and laser source at 850 nm. The maximum output power of (8, 5, 3)mm model is (3, 2.7, 2.55)nW respectively at applied force 5N and the minimum value is (1.9, 1.65, 1.5)nW respectively at 60N.The strain is calculated at different microbend cells ,and the best sensitivity of this sensor for cell 8mm is equal to 0.6nW/N.

Design and development of lab model of piezo-optic sensor for Structural Health Monitoring

This work deals with fibre optic pressure sensors for Structural Health Monitoring (SHM) based on the piezo-optic principle. Various samples were prepared and investigated under room ambient conditions. In one of the samples, periodic microbends of 1 cm spatial periodicity were introduced in the PCS600 optical fibre, which was then embedded in an Araldite matrix with the composition of 60% hardener and 40% resin. This sample was marked as S1. In another sample, the same fibre without microbending was embedded in the same Araldite matrix, and was marked as S2. In another sample, the fibre without microbending was embedded in a cement matrix composite, and was marked as S3. Sample S3 was an effort to develop a pressure sensor for real working environment. Here conventional portland cement was used for the fabrication of the sample. High pressure up to 60 KPa was applied to the sample under cyclic loading and unloading, and the corresponding output power noted. Sensitivities of fibre optic pressure sensor for samples S1, S2 and S3 are 2.304, 0.906 and 2.218 nW KPa–1, with reproducibilities 96.61%, 97.95% and 87.24% respectively. The sensitivities obtained indicate that the sensor has excellent potential to replace existing bulky electrical sensors.

Mechanism and characteristics of long period fiber gratings in simplified hollow-core photonic crystal fibers

We demonstrate the fabrication of high-quality LPFGs in simplified hollow-core photonic crystal fibers, composed of a hollow hexagonal core and six crown-like air holes, using CO2-laser-irradiation method. Theoretical and experimental investigations indicate that the LPFGs are originated from the strong mode-coupling between the LP01 and LP11 core modes. And a dominant physical mechanism for the mode-coupling is experimentally confirmed to be the periodic microbends rather than the deformations of the cross-section or other common factors. In addition, the LPFGs are highly sensitive to strain and nearly insensitive to temperature, and are promising candidates for gas sensors and nonlinear optical devices.

Numerical analysis and optimization of optical spectral characteristics of fiber Bragg gratings modulated by a transverse acoustic wave

Acoustic waves that impinge transversely on a fiber Bragg grating (FBG) induce periodic microbends of the fiber, which modulate the phase index and lead to the changes of optical spectral characteristics of the FBG. We investigated the spectral characteristics of a FBG modulated by a transverse acoustic wave. The corresponding theoretical model is presented by modifying the multimode coupled equations. A fast algorithm based on the Newton-Raphson method is proposed to simulate numerically the spectral characteristics of such a FBG. Our numerical results are in excellent agreement with the known experimental results. For the first time, to our knowledge, the known experimental results have been reproduced by numerical simulations. Moreover, the optimization of the reflective spectra of such a FBG is also discussed. From the perspective of inherent physical mechanisms, the exceptional spectral characteristics of such a FBG are discussed as well.

Asymmetric long period fiber gratings fabricated by use of CO2 laser to carve periodic grooves on the optical fiber

An asymmetric long period fiber grating (LPFG) with a large attenuation of −47.39dB and a low insertion loss of 0.34dB is fabricated by use of focused CO2 laser beam to carve periodic grooves on one side of the optical fiber. Such periodic grooves and the stretch-induced periodic microbends can effectively enhance the refractive index modulation and increase the average strain sensitivity of the resonant wavelength of the LPFG to −102.89nm∕mε. The resonant wavelength and the peak attenuation of the LPFG can be tuned by ∼12nm and ∼20dB, respectively, by the application of a stretching force.

Embedded fibre optic microbend sensor for measurement of high pressure and crack detection

This paper reports fibre optic pressure sensor based on periodic microbending loss phenomenon in embedded optical fibres. Periodic microbends of spatial periodicity 5 mm has been introduced in the optical fibre PCS600 embedded in the sample of epoxy matrix araldite. High pressure has been applied on the sample in cyclic operations and output power recorded in different cycles of operations. It has been possible to calibrate output power in terms of pressure up to 3.0 × 10 6 Pa for epoxy matrix araldite.

Long-period fiber gratings based on periodic microbends

We demonstrate a new type of high-performance long-period fiber grating based on arc-induced periodic microbends. The fabrication method is simple and does not require special fibers. Flexibility in controlling the filter parameters makes it possible to produce arbitrary filter profiles by use of a simple apodization technique, which is difficult to do with conventional long-period gratings.

Analysis of intermodal coupling in a two-mode fiber with periodic microbends

Simple expressions for the coupling between the LP(01) and LP(11) modes of a two-mode optical fiber with a periodic microbending structure are developed. Implementation of the microbend structure using a flexural acoustic wave is described. The dependences of the acoustic frequency and power requirements on the pertinent fiber parameters are presented.

Fiber-optic modal coupler using periodic microbending

An efficient LP01 to LP11 modal coupler using periodic microbends, spaced by a beat length between the modes, was built and tested. The optical-wavelength dependence of the device is investigated, as are the polarization characteristics and the LP mode approximation. Finally, the loss of the modal coupler was measured as a function of wavelength.

Experimental investigation of mode coupling in a multimode graded-index fiber caused by periodic microbends using computer-generated spatial filters

The results are described of an experimental investigation of mode coupling in a multimode graded-index fiber as a function of the periodic microbend amplitude. Computer-generated holograms were used as mode selecting spatial filters for measurements of the modal power distribution. The experimental data are in a good qualitative agreement with theoretical predictions. The results obtained may be helpful in the development of amplitude-type graded-index fiber sensors with high sensitivity.