Impulse Response Of Fibers Research Articles

Impulse Response Measurement of Spooled and Twisted Few-Mode Multi-Core Fiber for Short-Range Optical Links

The impact of bending and twisting on the impulse response matrix is investigated using optical vector network analysis. The heterogeneous 36-core 3-mode fiber under test is spooled at bending radii of 8 cm to 50 cm and additionally up to 4 twists per meter along the longitudinal direction are applied. The by-design large differential mode group delay of the fiber enables comprehensive measurement of the inter-mode-group cross-talk of the fiber impulse response matrix, which is directly measured with an optical vector network analyzer. A maximal inter-mode-group cross-talk of −28.1 dB is observed over all 36 cores. Furthermore, no significant changes related to the bending radii or additional strain by twisting the fiber were detected. These results demonstrate that the transmission characteristics of few-mode fibers with large differential mode group delay are fairly unaffected by bend- and twist-related perturbations which may prove beneficial for short-range mode-group multiplexed transmission systems using partial multiple-input multiple-output equalization in conjunction with mode-selective multiplexers.

Estimation of angle-dependent mode coupling and attenuation in step-index plastic optical fibers from impulse responses

We report on a method for estimation of angle-dependent mode coupling and attenuation in step-index plastic optical fibers (SI-POFs) from the shapes of impulse responses at two different fiber lengths. While alternating the fiber lengths, deviations between simulated and reference impulse responses are minimized by optimizing both mode coupling and attenuation parameters using pattern-search routines. Applying a matrix-based finite-difference approach to Gloge's time-dependent power flow equation fast computation of simulated impulse responses is enabled. We demonstrate that mode-dependent coupling and attenuation parameters converge to values that reconstruct fiber characteristics reported by other authors. We show that our results can be used for prediction of impulse responses, yielding determination of frequency responses, fiber bandwidths and coupling lengths. We conclude that our method enables characterization of SI-POFs from fiber impulse response measurements.

Spatial resolution enhancement in fiber Raman distributed temperature sensor by employing ForWaRD deconvolution algorithm

Governing equations of a Raman fiber distributed temperature sensor supported by a counter propagating pumped Raman amplifier are written in the form of integral equations. Forward pump and backward going probe are in the pulsed and continuous wave modes of operation respectively. It is shown that powers of the forward and backward Raman signals (stokes and anti stokes) approximately are the convolution of the input forward pump pulse and corresponding optical fiber impulse responses. The Fourier Wavelet Regularized Deconvolution (ForWaRD) method is employed to improve the spatial resolution in fiber Raman distributed temperature sensor without reducing the pulse width of the laser source.

Principal Modes in Graded-Index Multimode Fiber in Presence of Spatial- and Polarization-Mode Coupling

Power-coupling models are inherently unable to describe certain mode coupling effects in multimode fiber (MMF) when using coherent sources at high bit rates, such as polarization dependence of the impulse response. We develop a field-coupling model for propagation in graded-index MMF, analogous to the principal-states model for polarization-mode dispersion in single-mode fiber. Our model allows computation of the fiber impulse response, given a launched electric-field profile and polarization. In order to model both spatial- and polarization-mode coupling, we divide a MMF into numerous short sections, each having random curvature and random angular orientation. The model can be described using only a few parameters, including fiber length, number of sections, and curvature variance. For each random realization of a MMF, we compute a propagation matrix, the principal modes (PMs), and corresponding group delays (GDs). When the curvature variance and fiber length are small (low-coupling regime), the GDs are close to their uncoupled values, and scale linearly with fiber length, while the PMs remain highly polarized. In this regime, our model reproduces the polarization dependence of the impulse response that is observed in silica MMF. When the curvature variance and fiber length are sufficiently large (high-coupling regime), the GD spread is reduced, and the GDs scale with the square root of the fiber length, while the PMs become depolarized. In this regime, our model is consistent with the reduced GD spread observed in plastic MMF.

DMD Measurements and Equalization Simulations for 62.5- and 50-<tex>$muhboxm$</tex>Legacy Multimode Fibers at 1300 nm

We present the results from the differential modal delay (DMD) measurements at 1310 nm performed over a large sample of 62.5- and 50-μm fiber distributed data interface grade multimode fibers (MMF). We have found that the typical DMD values are in the range 0.5-1.5 ps/m, but a significant portion of the measured fibers have DMD values exceeding the commonly used value of 2 ps/m. Since the expected yield at moderate (150-300 m) distances of 10 Gb/s links employing legacy fibers is very poor, we have simulated equalization techniques in the receiver for the measured fiber impulse responses. We show here that both the yield and the achievable distance are improved when equalization is used.

Categorization of Gerbil auditory fiber responses

This ongoing work is concerned with what the gerbil's ear tells the gerbil's brain. The responses of over 600 auditory nerve fibers to band-limited Gaussian white noise were recorded, and a smoothed, linear estimate of each fiber's impulse response was derived using a reverse correlation procedure and a state space model. In this paper, we investigate the impulse responses’ dependence on animal, stimulus amplitude, and tuning frequency. Despite great fiber response variability, we conclude that their responses tend to be briefer and more asymmetric for moderate to high frequencies.

Optical time domain reflectometry using an M-ary FSK probe and coherent detection

This paper proposes a new approach to enhancing the performance of optical time domain reflectometry (OTDR). This approach launches a probe signal modulated in the M-ary FSK format into a test fiber and detects the backscattering by coherent detection, which offers excellent frequency discrimination, followed by simple signal processing to recover the fiber impulse response. Fault location experiments verify the theoretical prediction of that the proposed approach reduces the measurement time to 1/M without sacrificing measurable dynamic range or spatial resolution. They also show that the approach is effective in reducing fluctuations in the OTDR trace. It is experimentally confirmed that OTDR with the proposed approach is feasible for long haul transmission systems.

Correct relation between the impulse response of GRIN fibers and the excitation by a laser diode

In the case of optimal excitation of a multimode graded-index fiber by a laser diode, the coupling efficiency KO between the Gaussian laser beam and the fundamental mode of the fiber describes the input mode power distribution uniquely. Using this result, fiber impulse responses are calculated for two realistic excitations, first, by a gain-guided laser diode and second, by an index-guided laser diode. Also, the commonly used uniform and stationary excitations are given for comparison. The calculations yielding these results are based on the power diffusion equations and include profile dispersion, material dispersion, mode-dependent attenuation, and mode coupling. The mode group delay times have been computed for a parabolic profile using the scalar wave equation. However, delay times for other profiles can be introduced in the calculations. As K0 characterizes laser fiber coupling uniquely and because it can simply be measured, we propose the use of K0 as a characteristic launching parameter which allows easy comparison of measured impulse responses.

Multilevel Signaling over Step-Index Fibers

This paper investigates the effects of a multilevel signal applied in an optical communication system with a step-index fiber. The model for description of the fiber impulse response used here is more accurate than previously used models, which influenced conclusions significantly. It is found that multilevel signals can improve performances of a digital optical system when intersymbol interference is significant.

On the Phase of the Modulation Transfer Function of a Multimode Optical-Fiber Guide

We consider the range of validity of a Hilbert-transform approach in which the measured magnitude of the modulation-transfer-function of an optical fiber is used to compute the fiber's impulse response. It is argued that a key “minimum-phase assumption” can fail to be satisfied in important cases, and a few closely related experimental and analytical results are presented.

Propagation characteristics of multimode fibers with graded core index

Bending loss and impulse response of multimode fibers with graded-index core have been studied. A multimode fiber which has a large index difference is profitable because of low loss in bends. Impulse response of the fiber in which all of the possible modes are excited with the same intensity depends remarkably on the fourth-order term of the index profile. A small value of mode delay of 0.15 ns/km is estimated to be obtainable in a fiber having an optimum index profile and index difference ratio of 0.05 using incoherent light.

Impulse Response of Fibers With Ring-Shaped Parabolic Index Distribution

The index distribution in the cross section of a multimode fiber has an important influence on the modal group velocities and, hence, on the fiber impulse response. In this paper we derive a method for the evaluation of arbitrary circular symmetric index profiles. In particular, we compute the impulse response of a fiber with a ring-shaped parabolic index profile which exhibits useful equalizing properties. The pulse spread is found to be nearly one order of magnitude smaller than that of a fiber with an equal, but abrupt, index decline from core to cladding.