Graded-index Optical Fiber Research Articles

Splitting of levels in a cylindrical dielectric waveguide

A splitting of modes in a cylindrical graded-index optical fiber is demonstrated by solving the full Maxwell equations using the perturbation analysis. It is shown that the degeneracy of vortex Laguerre-Gauss modes with distinct orbital angular momentum and polarization (spin) but the same total angular momentum is lifted due to the spin-orbit (vector) and tensor forces. Numerical estimations of group delays of modes in optical fiber and frequency splitting in Fabry-Perot and ring resonators are presented.

Modal dispersion, pulse broadening and maximum transmission rate in GRIN optical fibers encompass a central dip in the core index profile

Intermodal and intramodal dispersions signify one of the problems in graded-index multi-mode optical fibers (GRIN) used for LAN communication systems and for sensing applications. A central index dip (depression) in the profile of core refractive-index may occur due to the CVD fabrication processes. The index dip may also be intentionally designed to broaden the fundamental mode field profile toward a plateau-like distribution, which have advantages for fiber–source connections, fiber amplifiers and self-imaging applications. Effect of core central index dip on the propagation parameters of GRIN fiber, such as intermodal dispersion, intramodal dispersion and root-mean-square broadening, is investigated. The conventional methods usually study optical signal propagation in optical fiber in terms of mode characteristics and the number of modes, but in this work multiple-beam Fizeau interferometry is proposed as an inductive but alternative methodology to afford a radial approach to determine dispersion, pulse broadening and maximum transmission rate in GRIN optical fiber having a central index dip.

Stimulated Raman scattering cascade spanning the wavelength range of 523 to 1750 nm using a graded-index multimode optical fiber

We report on the generation of a Raman cascade spanning the wavelength range of 523 to 1750 nm wavelength range in a standard telecommunication graded-index multimode optical fiber. Despite the highly multimode nature of the pump, the Raman peaks are generated in specific modes of the fiber, confirming substantial beam cleanup during the stimulated Raman scattering process.

Observation of stimulated Brillouin scattering in silica graded‐index multimode optical fibre based on pump‐probe technique

Based on the pump-probe technique (PPT), stimulated Brillouin scattering (SBS) was observed in a 100 m-long silica graded-index multimode optical fibre with the same core diameter as that of a perfluorinated graded-index polymer optical fibre (PFGI-POF) (=50 µm). The SBS threshold was measured to be ∼50 mW, which was much lower than 5.08 W, the SBS threshold obtained for the same fibre without the PPT. This result indicates that the PPT is useful even under multimode conditions and that the difficulty in observing SBS in PFGI-POFs consists not in their physical problems but in their material problems (high propagation loss etc.).

An Antenna Measurement System Based on Optical Feeding

A radiation measurement system by using optical feeding is proposed. The system replaces conventional electrical feeding to antennas by the optical feeding which is composed of an electrical/optical (E/O) converter, a graded-index (GI) optical fiber, and an optical/electrical (O/E) converter. The GI fiber is used so as the O/E converter becomes very compact by using a simple means of coupling between the fiber and the photo-diode in the converter. A vertical surface emitting laser (VCSEL) is used in the E/O converter to make the system available till 6 GHz. This combination also makes the system cost-effective. The validity as well as the advantage of the system is demonstrated by measuring an ultra-wideband (UWB) antenna both by the optical and electrical feeding systems and comparing with a calculated result. Ripples in radiation pattern due to the electrical feeding are successfully suppressed by the optical feeding. For example, in a radiation measurement on the azimuth plane at 3 GHz, ripple amplitude of 1.0 dB that appeared in the electrical feeding is reduced to 0.3 dB. In addition, a circularly polarized (CP) antenna is successfully measured by the proposed system to show that the system is available not only for amplitude but also phase measurements.

Characteristics comparison of the ZDSFs based on the triangular graded index and step index single-mode optical fibers

In this paper, the zero-dispersion shifted single-mode optical fibers based on the triangular graded index and step index profiles are successfully designed by applying the method outlined by Oskoui et al. (Journal of Applied Optics, 46 (2007) 6330) and the optimal values for the structural parameters are evaluated. Simulation results indicate that in order to shift the zero-dispersion wavelength near to 1.55μm, the larger core radius is required in the triangular graded index optical fiber compared to the step index one which is useful from fiber fabrication view points. Also, the dispersion and dispersion slope values in the triangular graded index structure are smaller than ones in the step index type and this fact make a great enlargement in the dispersion and higher order dispersion lengths. However, the nonlinear behavior of the step index ZDSF is superior due to larger effective area, but owing to smaller higher order dispersion length, the transmission distance will be restricted with dispersion slope influence instead of nonlinear effects. Meanwhile, the thermal coefficients of the designed graded index are lower than the structure based on the step index and as a result this kind of fiber has better sustainable behavior against temperature variances.

Semi Analytical Formulation of Dispersion Characteristics for a Step and ParabolicIndex Optical Fiber: Introduction of Nelder-Mead Simplex Method for Nonlinear Unconstrained Minimization

Variational principle has been incorporated in the proposed semi analytical formulation to characterize graded index optical fiber and optimization process is being carried out by NelderMead Simplex method for nonlinear unconstrained minimization. By introducing increased optimizing parameters in the formulation of fundamental modal field, more flexibility to refine a design and thus achieving a greater accuracy for a particular requirement, has been presented. Employing variational technique, effective index, normalized propagation constant, normalized group delay and modal dispersion parameter of an optical fiber have been evaluated analytically, by using the proposed formulation of fundamental field. The calculations show excellent agreement to the exact results, over a wide range of normalized frequency. Optimized values of three parameters, incorporated in the fundamental modal field have been presented for various values of normalized frequencies. This approximation can be used in the analysis of doped and nonlinear fiber amplifier, or in the case of dispersion–shifted fiber with large effective area.

Improved Semi Analytical Model of Effective Area for a Step and ParabolicIndex Optical Fiber: Introduction of Increased Parameters in Fundamental Modal Field

An accurate three parameter approximate solution of fundamental mode, to describe different propagation parameter of graded-index optical fiber has been presented. Due to three parameter optimization of the fundamental modal solution, the results are much more accurate for a wide range of optical fiber specifications. Employing variational technique, an estimation of effective area and effective index with proposed fundamental modal field are carried out. It has been shown that the results match identically with the exact available results for step and parabolic refractive index profile optical fiber. The accurate analytical expressions for the evaluation of effective area and effective index are formulated. Optimization of three parameters has been carried out for a wide range of normalized frequencies. Data for these parameters are also provided, which can be used directly while calculating different design parameters of an optical fiber.

Poly(styrene)-based graded-index plastic optical fiber for home networks

We investigated poly(styrene) (PSt)-based graded-index plastic optical fiber (GI-POF) with low loss and high bandwidth for home networks. To install the GI-POF in home networks, the attenuation must be below 200 dB/km at a 670-680 nm wavelength, and the bandwidth must be over 2.0 GHz for the 50 m fiber. In this study, we selected a dibenzothiophene (DBT) as a dopant to PSt, and we fabricated PSt-DBT-based GI-POF. We confirmed that the PSt-DBT-based GI-POF has high bandwidth (4.4 GHz) for 50 m fiber and low loss (166-193 dB/km) at a 670-680 nm wavelength and obtained the GI-POF that satisfied the requirements for home networks.

Multiple-beam interference fringes applied to investigate the index profile of the optical fiber dip

A numerical-based phase shift method is presented to study optical fibers having a dip in the refractive index profile at the center of the core. Mathematical expressions for the shape of multiple-beam Fizeau fringes crossing a graded-index optical fiber (GRIN) comprising a central graded-index dip or GRIN fiber with a constant refractive index dip, immersed in a silvered liquid wedge, are derived. The index profile parameters of the central dip are determined by comparing the theoretical fringe shape across the fiber cross-section and the experimental fringe shape obtained from micro-interferograms of the wedge interferometer.

Low-loss coupling between two single-mode optical fibers with different mode-field diameters using a graded-index multimode optical fiber

We present a method for ultra-low-loss coupling between two single-mode optical fibers with different mode-field diameters using multimode interference in a graded-index multimode optical fiber. We perform a detailed analysis of the interference effects and show that the graded-index fiber can also be used as a beam expander or condenser. The results are important for devices in which optical fibers with different mode-field diameters are coupled in series, such as in ultra-short-pulse fiber ring lasers, or in optical fiber communication links.

Core Alignment of Butt Coupling Between Single-Mode and Multimode Optical Fibers by Monitoring Brillouin Scattering Signal

We propose a new method to optimize the core alignment of the butt coupling between a single-mode optical fiber (SMF) and a graded-index multimode optical fiber (GI-MMF) by monitoring the spectrum of Brillouin scattered light. This method is stable, free from costly or elaborate equipments, applicable to the coupling of optical fibers with different core diameters or materials, and available even when the power of transmitted light cannot be measured. We experimentally show the usefulness of this method in the following three cases: where a silica-based SMF is butt-coupled to 1) a silica-based GI-MMF with 50-μm core diameter; 2) a perfluorinated GI polymer optical fiber (PFGI-POF) with 62.5-μm core diameter; and 3) a PFGI-POF with 120-μm core diameter. Depending on the relative core position, not only the Stokes power but also the Brillouin frequency shift changed, which probably originates from the excitation of higher order modes.

CWDM based HDMI interconnect incorporating passively aligned POF linked optical subassembly modules.

A four-channel transmitter OSA (TOSA) and a receiver optical sub-assembly (ROSA) module were presented. They take advantage of a coarse WDM (CWDM) scheme, employing two types of VCSELs at 780 and 850 nm, where no wavelength filters are involved in the TOSA. The ROSA and TOSA were constructed through a fully passive alignment process using components produced by virtue of a cost effective plastic injection molding technique. In order to build a high quality optical HDMI interconnect, four channel optical links between these modules ware established via two graded-index plastic optical fibers (GI-POFs). The HDMI interconnect was thoroughly evaluated in terms of the alignment tolerance, the light beam propagation, and the data transmission capability. For the ROSA, the measured tolerance, as affected by the photodiode alignment, was ~45 μm and over 200 μm for the transverse and longitudinal directions, respectively. For the TOSA, the tolerance, which is mostly dependent upon the VCSEL alignment, was ~20 μm and more than 200 μm for the transverse and longitudinal directions, respectively. The beam profiles for the TOSA and ROSA were monitored to confirm their feasibility from the optical coupling perspective. A digital signal at 2.5 Gb/s was efficiently transmitted through the HDMI interconnect with a bit error ratio of below 10-16. A 1080p HDMI signal from a Blu-ray player was delivered through the interconnect to an LCD monitor and successfully displayed a high quality video.

High-Bandwidth Graded-Index Plastic Optical Fiber With Low-Attenuation, High-Bending Ability, and High-Thermal Stability for Home-Networks

The graded-index plastic optical fiber (GI-POF) is expected to be a communication medium for the next-generation optical home network because of its simple-to-use connection, installation, and high bandwidth. In spite of the expectation, we had a problem that a typical GI-POF using poly (methyl methacrylate) (PMMA) had high transmission loss in the expected communication wavelength band (VCSEL: 670-680 nm) for home networks; the required values of being below 200 dB/km could not be achieved. We, therefore, propose poly (2, 2, 2-trichloroethyl methacrylate) (PTCEMA) as a base material for the GI-POF. A PTCEMA-based GI-POF was fabricated, and its characteristics were evaluated. The PTCEMA is a prominent material in terms of its transparency and heat-resistant property. Our results demonstrated that the fabricated fiber surpassed the desired characteristics for the home network pertaining to attenuation and heat resistance. Specifically, the attenuation in the wavelength band (670-680 nm) was 104-136 dB/km, and the glass transition temperature (Tg) was 102°C in the core center where the Tg was at its lowest. Moreover, we confirmed that our PTCEMA-based GI-POF had sufficient mechanical strength and low bending loss. These results indicate that our novel GI-POF can be a candidate for home networks.

Investigation of refractive index profile and mode field distribution of optical fibers using digital holographic phase shifting interferometric method

In this paper a digital holographic phase shifting interferometric method is used to characterize the graded index (GR-IN) optical fibers. Off-axis digital holographic Mach–Zehnder like arrangement in aid of phase shifting tool is applied. Numerical reconstruction of phase shifted holograms is applied to calculate the optical phase difference distribution in the image plane, and then the optical phase differences across GR-IN fibers are extracted considering image enhancement and noise reduction steps. Also, a simple algorithm is presented to modify the position of the extracted optical phase differences across the fibers to be perpendicular to x-axis, so that it becomes easy to calculate the mean optical phase differences along the fiber. The mean optical phase differences in combination with multilayer model are applied to calculate the refractive index profiles across GR-IN optical fibers. An analytical model is presented to predict the mode distribution associated with the effective indices and the propagation coefficient of the parabolic refractive index profile GR-IN fiber.

Hybrid CATV/16-QAM OFDM in-building networks over SMF and GI-POF transport

A hybrid CATV/16-QAM OFDM in-building network over a combination of single-mode fiber (SMF) and graded-index plastic optical fiber (GI-POF) transport is proposed and experimentally demonstrated with good qualities of service. In this system, a 1556 nm optical signal is directly transmitted along with two fiber spans (20-km SMF + 25-m GI-POF). Without using any wavelength conversion or bridge circuit between SMF and POF connection, error free transmissions with sufficient low bit error rate (BER) values are achieved for 2.5 Gbps/2.5 GHz and 5 Gbps/2.5 GHz OFDM signals; as well as good performances of carrier-to-noise ratio (CNR), composite second-order (CSO), and composite triple beat (CTB) are obtained for CATV one. This proposed network reveals an outstanding one with economy and convenience to be installed.

Integration of FTTH and GI-POF in-house networks based on injection locking and direct-detection techniques

An integration of fiber-to-the-home (FTTH) and graded-index plastic optical fiber (GI-POF) in-house networks based on injection-locked vertical cavity surface emitting lasers (VCSELs) and direct-detection technique is proposed and experimentally demonstrated. Sufficient low bit error rate (BER) values were obtained over a combination of 20-km single-mode fiber (SMF) and 50-m GI-POF links. Signal qualities satisfy the worldwide interoperability for microwave access (WiMAX) requirement with data signals of 20 Mbps/5.8 GHz and 70 Mbps/10 GHz, respectively. Since our proposed network does not use sophisticated and expensive RF devices in premises, it reveals a prominent one with simpler and more economic advantages. Our proposed architecture is suitable for the SMF-based primary and GI-POF-based in-house networks.

Lorentzian approximation of the fundamental mode in single mode linear and nonlinear fiber

In this paper, we approximate the fundamental mode in single mode step and graded index optical fiber by a single parameter Lorentzian function within scalar variational framework. We show that it works in a low V region, a region important for evanescent wave coupling, more accurately in terms of propagation constant in comparison to single parameter Gaussian for linear and nonlinear fibers. Furthermore, we show that the low V variations of normalized propagation constant b ( V ), group delay d ( V ), and waveguide dispersion g ( V ) with V based on Lorentzian approximation match more accurately with the results for linear optical fiber in comparison to Gaussian approximation in the low V region.

Recent Results From the EU POF-PLUS Project: Multi-Gigabit Transmission Over 1 mm Core Diameter Plastic Optical Fibers

Recent activity to achieve multi-gigabit transmission over 1 mm core diameter graded-index and step-index plastic optical fibers for distances up to 50 meters is reported in this paper. By employing a simple intensity-modulated direct-detection system with pulse amplitude or digital multi-tone modulation techniques, low-cost transceivers and easy to install large-core POFs, it is demonstrated that multi-gigabit transmission up to 10 Gbit/s over 1-mm core diameter POF infrastructure is feasible. The results presented in this paper were obtained in the EU FP7 POF-PLUS project, which focused on applications in different scenarios, such as in next-generation in-building residential networks and in datacom applications.

Wavelength and beam launching effects on silica optical fiber in local area networks

In this paper, we report the modal dispersion of silica graded-index optical fibers as a function of the input mode parameters and lunching conditions in local area network (LAN) context. In that, we examine the mode-depending parameters, namely, modal delay, modal attenuation and mode-coupling effects as a function of wavelength. We show that the number of excited mode groups depends strongly on the spot beam radius when the fiber is excited with an axial Gaussian beam where we find an optimal axial diameter exciting only two mode groups. We present a comparison of the number of excited mode groups, the optimal spot radius beam, the signal penalty and the 3-dB baseband bandwidth enhancement for the optimal axial launching compared with full mode excitation, offset launching and mode-field matched axial launching.