Step-index Optical Fiber Research Articles

Step-Index Optical Fibers With 0.88 Numerical Aperture

In this work, step-index optical fibers with up to 0.88 numerical aperture (NA) were designed, modeled, fabricated, and tested. Both ray-tracing and wave-optics propagation analysis were utilized to model the NAs that can be achieved for a step-index optical fiber by modifying the cone angle of the fiber tip. Cone-shaped optical fibers with conical semiangles up to 29°, which renders to NAs around 0.88 in air, were fabricated by static etching and tested when utilized as illumination sources.

Theoretical simulations of the soliton self-frequency shift of mid-infrared femtosecond pulses in step-index tellurite optical fibers: broadband tunability and high efficiency

Broadband tunable ultrafast lasers using mid-infrared (MIR) fibers operating at a 3-5 µm atmospheric transmission window are attractive sources because of their numerous applications. Tellurite fibers possess the merits of large linear and nonlinear refractive indices, sufficient chemical stability, and wide transparency range up to ∼5 µm; also, they are highly suitable for high efficiency MIR ultrafast fiber laser sources based on soliton self-frequency shift (SSFS). We numerically simulate SSFS of MIR femtosecond pulses in step-index tellurite optical fibers. A femtosecond erbium-doped fluoride fiber laser at 3.5 µm is employed as the pump source. Parameters including the peak power of the input pulse and nonlinear fiber length are optimized for high efficiency broadband tunable MIR ultrafast laser performance. Our results show that a high-efficiency 3.5-6 µm wavelength-tunable femtosecond laser can be realized by employing SSFS in a 22-cm-long segment of tellurite step-index fiber pumped by femtosecond pulses with 10-300 kW peak powers at 3.5 µm. Ultra-high energy ratios of the most redshifted solitons to the input pulses of >50% are obtained across the 3.5-5 µm tuning range. The presented numerical study provides valuable guidance for SSFS of MIR femtosecond pulses in step-index tellurite fibers and is valuable for future high efficiency wavelength-tunable MIR ultrafast fiber laser development.

Focusing light in biological tissue through a multimode optical fiber: refractive index matching.

Controlling light propagation through a step-index multimode optical fiber (MMF) has several important applications, including biological imaging. However, little consideration has been given to the coupling of fiber and tissue optics. In this Letter, we characterized the effects of tissue-induced light distortions, in particular those arising from a mismatch in the refractive index of the pre-imaging calibration and biological media. By performing the calibration in a medium matching the refractive index of the brain, optimal focusing ability was achieved, as well as a gain in focus uniformity within the field-of-view. These changes in illumination resulted in a 30% improvement in spatial resolution and intensity in fluorescence images of beads and live brain tissue. Beyond refractive index matching, our results demonstrate that sample-induced aberrations can severely deteriorate images from MMF-based systems.

Analytical study of the numerical aperture of cone-shaped optical fibers: A tool for tailored designs

In this work, a set of analytical expressions to tailor the numerical aperture of cone-shaped step optical fibers is proposed. The expressions are derived from the geometrical study of light-ray tracing at the cone-shaped tip of a regular step-index optical fiber. Analysis of the different physical phenomena that can take place at the tip of the fiber led to numerical apertures ranging from 0 to 1.5 which can be achieved via a variety of cone angles, providing great versatility in the design of light sources or light collecting devices based on step-index optical fibers.

Tapered tellurite step-index optical fiber for coherent near-to-mid-IR supercontinuum generation: experiment and modeling.

We demonstrate broadband highly coherent near-to-mid-IR supercontinuum generation using a short length of tapered tellurite step-index fiber pumped with an ultrafast laser in normal dispersion regime. The tapered tellurite fiber possesses all-normal dispersion characteristics within the whole range of the generated supercontinuum spectrum. A highly coherent near-to-mid-IR supercontinuum spectrum spanning 1.28 to 3.31μm at a -40 dB intensity level is obtained using a 3.2cm long tapered tellurite fiber when it is pumped with a 200fs laser pulse with a peak power of 19.8kW at 2μm. To obtain the supercontinuum spectrum, we also carried out numerical modeling for the tapered tellurite step-index fiber with the same geometrical parameters and pump conditions used in the experiment. The numerical observation supports the experimentally obtained result. The findings of this work show that the fabricated tapered tellurite step-index fiber is a promising nonlinear medium to obtain a coherent near-to-mid-IR supercontinuum spectrum in a short length of the fiber.

Simulation Study of Waveguide and Chromatic Dispersion of a Single Mode Step Index Optical Fiber

Simulation Study of Waveguide and Chromatic Dispersion of a Single Mode Step Index Optical Fiber

Supercontinuum generation by intermodal four-wave mixing in a step-index few-mode fibre

We demonstrate broadband supercontinuum generation from 560 nm up to 2350 nm by coupling a simple Q-switched picosecond laser at 1064 nm into a normally dispersive step-index few-mode optical fiber designed to support five modes. It is further shown that multiple cascaded intermodal four-wave mixing and Raman processes occur in the fiber leading to the generation of new frequency components with far detuning up to 165 THz. The multimode properties of this fiber yield a number of intermodal nonlinear coupling terms, and we compare the generated parametric sideband wavelengths from the experiment with calculations from phase-matching conditions for intermodal four-wave mixing.

Experimental Demonstration of Subband Index Techniques for <inline-formula> <tex-math notation="LaTeX">$m$ </tex-math> </inline-formula>-CAP in Short-Range SI-POF Links

In this letter, novel spectrally efficient subband index (SI) techniques have been developed for multiband carrierless amplitude and phase modulation ( $m$ -CAP). The SI techniques transmit additional information bit on the index of the conventional $m$ -CAP subbands. Experimental demonstrations are performed in a short-range optical data link using step-index plastic optical fiber. The demonstrations show that the proposed techniques enable flexibility in design and significantly improve the power/spectral efficiency of the conventional $m$ -CAP.

Doubly periodic waves, bright and dark solitons for a coupled monomode step-index optical fiber system

In this paper, a coupled monomode step-index optical fiber system is under investigation, which can be governed by a coupled Schrodinger equation. Though extending the Jacobi elliptic function method in complex form, a series of Jacobi elliptic function solitons are obtained for the equation. These solutions can be expressed in the form of doubly periodic waves. When the modulus of Jacobi elliptic function tends to 1, the bright and dark optical solitons are observed. Furthermore, the modulus of Jacobi elliptic function can control the periodic wave density. The wave width and direction of the solitons are effected by parameters.

Modeling of transmission characteristics in step-index polymer optical fiber using the matrix exponential method.

In this paper, we propose to calculate the impulse and frequency response of step-index polymer optical fiber in the presence of mode coupling using the matrix exponential method. The computation time in this method is independent of the fiber length and is typically 2 orders of magnitude shortened compared with the standard numerical procedures involving finite difference methods. Results obtained with our method are compared with those of known cases, where an analytical solution for the time-dependent power flow equation exists, and with a finite difference method. We also investigate the impact of mode coupling and differential modal attenuation on impulse response duration and shape.

Bragg Gratings Inscription in TS-Doped PMMA POF by Using 248-nm KrF Pulses

We demonstrate the Bragg gratings inscription in the 850-nm spectral region by using trans-4-stilbenemethanol-doped poly methyl methacrylate step-index optical fiber and a 248-nm krypton fluoride (KrF) excimer laser system. The gratings inscription only takes 0.4 s. A detailed study of the grating fabrication process using different pulse repetition rates is also reported. In addition, temperature, humidity, and strain sensitivities are measured to demonstrate the potentiality of these components for different sensing applications.

Optical Strain Measurement with Step-Index Polymer Optical Fiber Based on the Phase Measurement of an Intensity-Modulated Signal.

Polymer optical fibers (POFs) have been proposed for optical strain sensors due to their large elastic strain range compared to glass optical fibers (GOFs). The phase response of a single-mode polymer optical fiber (SM-POF) is well-known in the literature, and depends on the physical deformation of the fiber as well as the impact on the refractive index of the core. In this paper, we investigate the impact of strain on a step-index polymer optical fiber (SI-POF). In particular, we discuss the responsivity of an optical strain sensor which is based on the phase measurement of an intensity-modulated signal. In comparison to the phase response of an SM-POF, we must take additional influences into account. Firstly, the SI-POF is a multi-mode fiber (MMF). Consequently, we not only consider the strain dependence of the refractive index, but also its dependency on the propagation angle . Second, we investigate the phase of an intensity-modulated signal. The development of this modulation phase along the fiber is influenced by modal dispersion, scattering, and attenuation. The modulation phase therefore has no linear dependency on the length of the fiber, even in the unstrained state. For the proper consideration of these effects, we rely on a novel model for step-index multi-mode fibers (SI-MMFs). We expand the model to consider the strain-induced effects, simulate the strain responsivity of the sensor, and compare it to experimental results. This led to the conclusion that the scattering behavior of a SI-POF is strain-dependent, which was further proven by measuring the far field at the end of a SI-POF under different strain conditions.

10-Gb/s Transmission Over 10-m SI-POF With <inline-formula> <tex-math notation="LaTeX">${M}$ </tex-math> </inline-formula>-PAM and Multilayer Perceptron Equalizer

We demonstrate the gigabit-per-second transmission over a step-index plastic optical fiber (SI-POF) of 10-m length with a pulse-amplitude modulation (PAM). A multilayer perceptron-based equalizer is used to mitigate an intersymbol interference and non-linearity in the system. Using this equalizer with 32-PAM, a data rate of 10 Gb/s is achieved over the 10-m SI-POF at a bit error rate of 10−2, which is below the 20% forward error correction limit.

Influence of mode coupling on three, four and five spatially multiplexed channels in multimode step-index plastic optical fibers

We investigate the influence of mode coupling on space division multiplexing capability of three multimode step-index plastic optical fibers with different strengths of mode coupling. Results show that mode coupling significantly limits the fiber length at which the space division multiplexing can be realized with a minimal crosstalk between the neighbor optical channels. Three, four and five spatially multiplexed channels in the investigated multimode step-index plastic optical fibers can be employed with a minimal crosstalk up to the fiber lengths which are about 7%, 5% and 3% of the corresponding coupling lengths (fiber length where equilibrium mode distribution is achieved), respectively. Such characterization of optical fibers should be considered in designing an optical fiber transmission system for space division multiplexing.

Dynamics of Nonlinear Excitation of the High-Order Mode in a Single-Mode Step-Index Optical Fiber

This work is concerned with approximate model of higher-order mode nonlinear excitation in a singlemode silica optical fiber. We present some results of simulation for step-index optical fiber under femtosecond optical pulse launching, which confirm ability of relatively stable higher-order mode excitation in such singlemode optical fiber over sufficiently narrow range of launched optical power variation.

Thermo-optic characteristics of hybrid polymer/silica microstructured optical fiber: An analytical approach

Microstructured optical fibers (MOFs) allow a variety of advanced materials to be infiltrated in their air-voids for obtaining the increased fiber functionality, and offering a new versatile platform for developing the compact sensors devices. We aim to investigate the thermal characteristics of high-index core triangular hybrid polymer/silica MOFs with circular air-voids infused with polymer by using the analytical field model [1]. We demonstrate that infiltration of air-voids with polymer, e.g., polydimethylsiloxane (PDMS) can facilitate to tune the fundamental modal properties of MOF such as effective index of the mode, near and the far-field profiles, effective mode area and the numerical aperture over the temperature ranging from 0 °C to 100 °C, for different values of relative air-void ratios. The evolution of the mode shape for a given temperature has been investigated in transition from near-field to far-field regime. We have studied the thermal dependence of splice losses between hybrid MOF and the standard step-index single-mode optical fiber in combination with Fresnel losses. For enhancing the evanescent field interactions, we have evaluated fraction of power associated with fundamental mode of hybrid MOF. We have compared the accuracy of our results with those based on full-vector finite-difference (FD) method, as available in the literature.

Performance Comparison of Equalization Techniques for SI-POF Multi-Gigabit Communication With PAM- M and Device Non-Linearities

In this paper, Gigabit per second transmission over short-range step-index plastic optical fibre (SI-POF) is implemented with multi-level pulse amplitude modulation (PAM- M ) scheme using a laser diode (LD) as the optical source. In particular, the POF channel distortion and the non-linear distortion from the LD are considered. To mitigate these distortions under various PAM- M levels, the bit error rate (BER) performances of the transversal, Volterra and multi-layer perceptron (MLP) based decision feedback equalizers are evaluated and compared. Furthermore, the inverse polynomial technique for non-linear mitigation is also considered. MLP is shown to offer the best BER performance for PAM scheme using more than eight levels especially for a system with high non-linearities. With the MLP equalizer, we record the highest data rate of 3, 7.8, and 18 Gbps for POF lengths of 60, 30, and 10 m, respectively.

Low-Complexity PAPR Reduction Scheme Combining Multi-Band Hadamard Precoding and Clipping in OFDM-Based Optical Communications

In this paper, we propose a low-complexity peak-to-average power ratio (PAPR) reduction scheme that combines both multi-band (MB)-Hadamard precoding and clipping for the optical orthogonal frequency division multiplexing (OFDM) systems. Approximations of PAPR distribution for baseband OFDM signals are analyzed and the effective signal-to-noise ratio (SNR) of the whole transmission link considering both the clipping and quantization noise are presented. After that, the MB-Hadamard precoding is adopted to compresses the peak signals, minimizing the contaminating influence of signal distortions in subsequent clipping operations. In addition, the received SNRs and bit error rate (BER) are calculated theoretically for each split sub-band. The 50-m step-index polymer optical fiber (POF) transmission is adopted as a special case to both evaluate the system performance and then compare the proposed scheme with other well-known PAPR reduction techniques. With this scheme, the PAPR is reduced effectively and the system’s BER performance is improved significantly. The results show that the proposed scheme with appropriate number of sub-bands precoding provides favorable trade-offs among PAPR reduction, power spectral density, transmission rate, BER, and computational complexity, which demonstrates its feasibility and validity.

New method for calculating the coupling coefficient in graded index optical fibers

A simple method is proposed for determining the mode coupling coefficient D in graded index multimode optical fibers. It only requires observation of the output modal power distribution P(m, z) for one fiber length z as the Gaussian launching modal power distribution changes, with the Gaussian input light distribution centered along the graded index optical fiber axis (θ0=0) without radial offset (r0 = 0). A similar method we previously proposed for calculating the coupling coefficient D in a step-index multimode optical fibers where the output angular power distributions P(θ, z) for one fiber length z with the Gaussian input light distribution launched centrally along the step-index optical fiber axis (θ0=0) is needed to be known.

Novel optical solitary waves and modulation instability analysis for the coupled nonlinear Schrödinger equation in monomode step-index optical fibers

This paper addresses the coupled nonlinear Schrödinger equation (CNLSE) in monomode step-index in optical fibers which describes the nonlinear modulations of two monochromatic waves, whose group velocities are almost equal. A class of dark, bright, dark-bright and dark-singular optical solitary wave solutions of the model are constructed using the complex envelope function ansatz. Singular solitary waves are also retrieved as bye products of the in integration scheme. This naturally lead to some constraint conditions placed on the solitary wave parameters which must hold for the solitary waves to exist. The modulation instability (MI) analysis of the model is studied based on the standard linear-stability analysis. Numerical simulation and physical interpretations of the obtained results are demonstrated. It is hoped that the results reported in this paper can enrich the nonlinear dynamical behaviors of the CNLSE.