Mode Optical Fiber Research Articles

Orthogonal frequency division multiplexing for mitigation of optical fiber channel impairments

Optical communication either in free space wireless mode or optical fiber mode is much preferred for high data transmission rate in the range of 100 GHz or few THz. Implementation of OFDM in fiber helps to simplify the dispersion compensation but nonlinearity in fiber material will become a major challenge. This Chapter is review and literature survey about the development and possibilities for OFDM in optical fiber communication systems. It presents the basic features of OFDM and its impact on the mitigation of optical fiber impairments.

Significance of modulator extinction ratio in long distance coherent optical time domain reflectometry

In coherent optical time-domain reflectometry, external modulation is used to maintain the coherence of laser probe pulses launched into optical fibers. However, the residual continuous wave (CW) component produced by modulation may considerably degrade the system sensitivity. The backscattered signal from the pulse must be dominant compared to the CW signal. We discuss the effects of the finite extinction ratio (ER) on the instrument’s sensing range. A model analyzing the impact of the CW component on the backscattered signal as a function of the ER, fiber length, and pulse widths is proposed. It is also shown that acousto-optic modulation is more suitable than electro-optic modulation in optical fiber for longer distances. The results are confirmed experimentally in a 31–km-long fiber. A 1-Hz vibration was applied at 25.5 km, and the resulting signal-to-noise ratio of ∼13 dB was measured using 75-ns pulses. This result is in agreement with the performance predicted by the model.

What optical fiber modes reveal: group velocity and effective index for external perturbations

Precise control of fiber modes and their dispersion is essential, particularly for fields such as nonlinear frequency conversion or biosensing, both of which often require extensive and time-consuming simulations for design optimization. Here, we develop a first-order perturbation theory for predicting the effective index of bound and leaky fiber modes that is applicable for arbitrary global perturbations as long as the perturbations in the external surrounding are constantly homogeneous and isotropic deviations from the unperturbed fiber. This includes changes not only in permittivity and permeability, but also in wavelength. Thus, we are able to calculate the group velocity solely from the field distributions of the fiber modes at a single wavelength, which therefore allows for large-scale parameter sweeps for accurately managing dispersion. We demonstrate the capabilities of our theory for various trial systems such as step index fibers, photonic crystal fibers, and light cages.

Progress on mode field distribution and characterization technology of the optical fiber laser

In fields of practical applications such as the optical fiber communications, optical fiber lasers, and optical fiber sensinIn fields of practical applications such as the optical fiber communication, the optical fiber laser, and the optical fiber sensing, it is necessary to focus on the mode problems in optical fibers. Mode division multiplexing is an effective method to improve the information capacity of optical communication. Interference between modes is the basic method for the most of optical fiber sensing. Mode controlling technique is one of the key technologies for beam quality control of high-power fiber lasers. Therefore, research on the theory of optical fiber modes, modes’ generation, modes’ conversion, and mode characterization technology are of great significance and practical application value. In this paper, we discuss the mode and beam quality of the optical fiber, analyze the methods of multiple modes’ generation and conversion, and summarize the mode characterization methods by means of incoherent, coherent and low-coherence measurement. Currently, the fiber mode characterization is a hot research topic. Among a variety of characterization methods, the spatial and spectral imaging method (S2) and the spatial and spectral double Fourier transform method (F2) have significant advantages. Without knowing the geometric parameters of the fiber in advance, we can obtain characteristics such as the mode field distribution, the mode power ratio, and the group time delay. Results indicate that the F2 method is better for characterizing mode field distributions of high-power fiber lasers.

High-Order Fiber Mode Beam Parameter Optimization for Transport and Rotation of Single Cells.

Optical tweezers are becoming increasingly important in biomedical applications for the trapping, propelling, binding, and controlled rotation of biological particles. These capabilities enable applications such as cell surgery, microinjections, organelle extraction and modification, and preimplantation genetic diagnosis. In particular, optical fiber-based tweezers are compact, highly flexible, and can be readily integrated into lab-on-a-chip devices. Taking advantage of the beam structure inherent in high-order modes of propagation in optical fiber, LP11, LP21, and LP31 fiber modes can generate structured radial light fields with two or more concentrations in the cross-section of a beam, forming multiple traps for bioparticles with a single optical fiber. In this paper, we report the dynamic modeling and optimization of single cell manipulation with two to six optical traps formed by a single fiber, generated by either spatial light modulation (SLM) or slanted incidence in laser-fiber coupling. In particular, we focus on beam size optimization for arbitrary target cell sizes to enable trapped transport and controlled rotation of a single cell, using a point matching method (PMM) of the T-matrix to compute trapping forces and rotation torque. Finally, we validated these optimized beam sizes experimentally for the LP21 mode. This work provides a new understanding of optimal optical manipulation using high-order fiber modes at the single-cell level.

Fiber-Optic Surface Waveguide Modes Excited by Inter/Intra Mode Transition for Refractometric Sensitivity Enhancement

The generation of surface waveguide modes in telecommunication-grade optical fibers is highly desirable, since it would enable the development of novel all-fiber devices with high performance. Herein, we carry out a comprehensive theoretical investigation on the intrinsic excitation mechanism and sensing performance of fiber-optic surface waveguide modes supported by a tilted fiber grating (TFG) with nanometric-scale gold and TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films. The results reveal that both P-and S-polarized surface waveguide modes originate from the mode transition tuned by the Au-TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> composite, including inter mode transition associated with two adjacent modes and intra mode transition only related to the mode itself. During the mode transition, the power carried by cladding modes is absorbed by the Au-TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> composite, thereby yielding a strong surface-localized evanescent field that is sensitive to external medium. It is found that for both polarizations the surface waveguide modes induced by the inter/intra mode transition present almost the same refractometric sensitivity, but with the P-polarization (605.6 nm/RIU) being more sensitive than the S-counterpart (339.2 nm/RIU), the state-of-the-art surface plasmon resonance (539.7 nm/RIU), and also the approach enhanced by high-refractive-index film (7.4 nm/RIU). The results attained here represent a breakthrough since they would open new pathways toward single molecular detection.

Graphene-coated double D-type low loss optical fiber modulator.

A graphene-coated double D-type low loss all-fiber modulator is proposed. The modulator is improved on the basis of standard fiber. Only the cladding is processed without grinding the original core structure. The upper and lower cladding are cut same distance. This can ensure that the mode field does not deviate in one direction, so that most of the mode field is still tied to the core, which greatly reduces the device loss. The existence of the double graphene layer can also ensure a very excellent modulation efficiency. The calculation results show that the mode loss of our proposed dual-D modulator under X polarization is 0.125 dB/mm, and the mode field mismatch loss is 0.25%. The mode loss in Y polarization is 0.033 dB/mm, and the mode field mismatch loss is 0.32%. When the modulation voltage is 5 V, the modulation depth is 78.4% under the condition of five-layer graphene, while the modulation speed can reach 15.38 GHz. Besides maintaining low modulation voltage and higher modulation efficiency, this structure makes full use of the advantages of good fiber coupling, and will be widely used in future fiber communications and all-fiber systems.

W-Shaped Profile and Breather-Like Soliton of the Fractional Nonlinear Schrödinger Equation Describing the Polarization Mode in Optical Fibers

W-Shaped Profile and Breather-Like Soliton of the Fractional Nonlinear Schrödinger Equation Describing the Polarization Mode in Optical Fibers

Research on the Optimization Design Method of Reliability Enhancement Test of Fiber Optic Acousto-optic Modulator Based on Fuzzy Theory

The main purpose of the reliability enhancement test is to expose the weakness of the components in order to improve the process structure design. The current domestic test standards are Accelerated stress test procedures for electric and electronic products, Guidance on the definition and application of high-acceleration tests for avionics and so on. However, these standards have relatively broad provisions on test conditions. The feasible test schemes are not unique that lead to different required test conditions and test costs. The emerging device fiber optical acousto-optic modulator is expensive and the test cost is high. How to choose an efficient enhanced test program is particularly important. Based on the fuzzy theory, an optimization design method for the enhancement test of the fiber-optic acousto-optic modulator is established in this paper. This paper takes the test cost-efficiency ratio as the optimization target, and uses the reliability estimation deviation and cost as the fuzzy evaluation index for optimizing and designing of the enhancement test plan. Finally, the feasibility of the method is verified by case analysis.

Multiwavelength confocal fiber optic microscopy

We present a multiwavelength fiber optic confocal microscope (MCFOM) based on the use of a wide spectral emission light source and some commercially available fiber optic components to make a reliable and compact system. We use single mode fiber optics SMF-28 to illuminate and collect the reflected light from the sample avoiding the use of a pinhole as a filter for the unfocused planes; as well as two lenses to collimate the light. The chromatic aberration produced by the lenses is used to interrogate several planes simultaneously, eliminating the confocal scanning movement obtaining an average confocal resolution of 4 μm and a surface scanning resolution of 1 μm. We demonstrated enough sensitivity to scan biological samples with a depth of penetration of 26 μm showing the potential use of this proposal in research laboratories as an alternative low cost confocal microscopy.

Research on technology of 1550nm all-fiber continuous wave coherent Doppler wind lidar

In order to obtain the telemetry data of wind field 80 m ahead of the wind turbine, an all-fiber continuous wave coherent Doppler lidar (CW-CDL) system is demonstrated in this paper. The CW-CDL of all-fiber structure has advantages of the compact structure, good stability and low cost. However, there is inherent crosstalk in the optical circulator of the optical fiber module, the crosstalk light and the reference light enter the photon detector together to produce phase induced intensity noise (PIIN). Aiming to effectively solve the noise, we proposed a polarization control method by combining a polarization beam splitter and a quarter wave plate to reduce the PIIN in the system. The wind speed Doppler signal at 80 meters is obtained on a sunny day, and the typical signal-to-noise ratio can reach 21.7 dB.

Photonic-assisted microwave frequency measurement towards cognitive radio

Cognitive radio frequency (RF) systems can be seen through an OODA (observe, orient, decide and act) loop, and electronic warfare (EW) can be used in some of these stages, namely the observe and the act. Instantaneous microwave frequency measurement (IFM) systems can contribute to cognitive RF technology taking the real world to the data world. In an OODA loop, IFM can be arranged as in the observe stage. Here we have proposed an IFM device that allows the adjustment of the directional couplers coupling coefficients, optical fiber lengths, modulation index and modulation formats, which makes the system tunable. The analytical model result shows good agreement with the simulation, achieving frequency differences of 130 MHz and 210 MHz for the lowest and the highest amplitude comparison function values, respectively.

Coupling of silver nanoparticle-conjugated fluorescent dyes into optical fiber modes for enhanced signal-to-noise ratio

We present the optical coupling of the silver nanoparticles (AgNPs)-conjugated dye molecule into fiber optical modes for detecting fluorescence with the enhanced signal-to-noise (S/N) ratio. This near field coupling of the excited state of organic dye (FAM) molecules into the fiber multimodes occurs by immobilizing them on the exposed surface of fiber core, permitting the coupled light to be guided along the fiber for detection. This fiber based scheme is the first attempt to single out the fluorescence using fiber modes not for carrying excitation light but only for collecting emission light via the dye-fiber coupling. The emission-selective coupling into fiber modes turns out to be effective in reducing the unwanted background noise arising from both the false detection of excitation light and bulk autofluorescence.This scheme differs from the previously reported fluorescence sensors based on waveguides where guided modes at λex excite dye molecules via their evanescent fields. In addition, the local fields enhanced by AgNPs in close proximity to FAM molecules on the fiber core surface increase the rates of dye excitation and radiative decay/AgNP supported surface plasmon coupled emission. While focusing on demonstrating the proof-of-concept of the scheme presented, we obtain the maximum of 4.2-fold enhancement of the signal-to-noise (S/N) ratio in detecting fluorescence as compared to a conventional fluorescence detection scheme. The results presented in the fiber-based scheme may find an application where high S/N ratio fluorescence based biochemical assay is required in a small-sized device with remote sensing capability.

Lossy Mode Resonance Sensors Based on Double ITO/TiO2 and Triple TiO2/ITO/TiO2 Thin Film Coated Single Mode Fiber Taper

Based on the previously developed computer program for rigorous calculation of modes in optical fibers containing layers of various materials of arbitrary, including nanometer, thickness, the calculation and optimization of single-mode double adiabatic tapers covered with one, two and three-layer films: ITO, ITO/TiO2, TiO2/ITO/TiO2 of equal thickness is performed, with the aim of their application as the environment refractive index sensors. It is shown that in the range of wavelengths 1.00–1.6 µm, two, and in particular, three-layer coatings not only can increase the sensitivity, but also significantly reduce the width of the lossy mode resonance (LMR), which, in turn, leads to a significant increase in the figure of merit (FOM) of the sensor, defined as the sensitivity/width of the LMR.

The structure of normal modes in parallel ideal optical fibers with strong coupling

In this paper, we studied an effect of strong evanescent coupling on the structure of normal modes in a system of parallel ideal multimode optical fibers. Using the formalism of the degenerate perturbation theory and a scalar waveguide equation for this system, analytical expressions of higher-order supermodes and their propagation constants have been determined. We have shown that the structure of modes in the case of strong evanescent coupling coincides with the structure of normal modes for weakly coupled parallel fibers. We have demonstrated that in the presence of strong coupling, expressions for corrections to the scalar propagation constant are modified, deducing them analytically.

Design of Spatial-Mode (De)Multiplexer for Few-Mode Fibers Based on a Cyclically Used Michelson-Like Interferometer

Few mode optical fibers are a promising way to continue increasing the data rate in optical communications. However, an efficient method to launch and extract separately each mode is essential. The design of a interferometric spatial mode (de)multiplexer for few mode optical fibers is presented. It is based on a single Michelson-like interferometer which consists of standard optical elements and has a reflective image inverter in one arm. Particular care has been taken in its design so that both polarizations behave the same. Moreover, this interferometer can process several pairs of modes simultaneously. The multiplexer also consists of: a phase plate, focusing optics at both ports of the interferometer and elliptical core fibers to recirculate some outputs. It can multiplex ten spatial and polarization modes and it presents low losses and no intrinsic crosstalk between modes. Additionally, it is polarization insensitive, achromatic, compact and inexpensive. The same system can work as a demultiplexer when used in reverse. In this case, both the losses and the crosstalk remain very low. Similar designs that perform other functions, like an add-drop mode multiplexing, are also suggested.

Research on Rayleigh Scattering and Brillouin Scattering of Bidirectional Wavelength Division Multiplexing System

In order to study the influence of Rayleigh and Brillouin scattering on the performance of bidirectional Wavelength Division Multiplexing (WDM) system. On the VPI simulation platform, a two-way WDM simulation system was built. Rayleigh scattering and Brillouin scattering were generated by setting the parameters of the optical fiber module and system parameters such as Brillouin Scattering, Rayleigh scattering, Rayleigh Distortion, and Rayleigh Distortions. Then, the frequency spectrum of the received signal and the bit error rate were observed and analysed. Experimental results show that Rayleigh and Brillouin scattering will cause the performance of the two-way WDM system to degrade. By separating the back propagation channel, the impact of these two effects on the performance of the two-way WDM system can be reduced.

Propagation stability in optical fibers: role of path memory and angular momentum

AbstractWith growing interest in the spatial dimension of light, multimode fibers, which support eigenmodes with unique spatial and polarization attributes, have experienced resurgent attention. Exploiting this spatial diversity often requires robust modes during propagation, which, in realistic fibers, experience perturbations such as bends and path redirections. By isolating the effects of different perturbations an optical fiber experiences, we study the fundamental characteristics that distinguish the propagation stability of different spatial modes. Fiber perturbations can be cast in terms of the angular momentum they impart on light. Hence, the angular momentum content of eigenmodes (including their polarization states) plays a crucial role in how different modes are affected by fiber perturbations. We show that, accounting for common fiber-deployment conditions, including the more subtle effect of light’s path memory arising from geometric Pancharatnam–Berry phases, circularly polarized orbital angular momentum modes are the most stable eigenbasis for light propagation in suitably designed fibers. Aided by this stability, we show a controllable, wavelength-agnostic means of tailoring light’s phase due to its geometric phase arising from path memory effects. We expect that these findings will help inform the optimal modal basis to use in the variety of applications that envisage using higher-order modes of optical fibers.

Efficient interrogation method of forward Brillouin scattering in optical fibers using a narrow bandwidth long-period grating

A new technique, to the best of our knowledge, for the characterization of the effective refractive index modulation in optical fibers due to transverse acoustic mode resonances excited by electrostriction is reported. The resonances excited by an optical pulse are probed by a narrow bandwidth long-period grating (LPG) inscribed in the fiber, which is interrogated by a continuous wave (CW) beam. The LPG used in this experiment has a narrow bandwidth and high sensitivity to small mode index perturbations, allowing the measurement of index variations from below 10-9 to 10-6. Radial and torsional-radial acoustic modes were characterized up to 1.1 GHz. The linewidth of resonances was found to be much shorter than in previous reports in which long fiber lengths are typically required, obtaining Q factors as high as 5000.

Power scaling of dispersive-wave generation in higher-order optical fiber modes

Dispersive-wave generation in higher-order HE 1 l modes of step-index optical fibers is studied numerically. The limits to power scaling from nonlinear intermodal couplings are investigated. It is shown that single-mode operation is possible up to power levels of a few MW, whereas both pump and dispersive-wave power distributes over several modes for higher powers, making the frequency conversion process less specific and efficient. The use of a circularly polarized pump is found to be beneficial for power scaling, both due to a reduced effective nonlinearity, and a reduction in intermodal couplings to LP 2 l -like modes.