Gustavo C. Amaral

WDM-PON Monitoring With Tunable Photon Counting OTDR

We explore the performance of a tunable optical time domain reflectometer based on single photon detection for fault location and quantification in wavelength division multiplexing (WDM)-passive optical networks based on a 32-channel cyclic arrayed waveguide grating using the S- and C-bands for upstream (156 Mb/s) and downstream (2.5 Gb/s) transmission and the L-band for monitoring. Downstream Rayleigh and Raman scattering contributions to the dark count noise were evaluated and properly filtered out with a total WDM + filter rejection ratio of 90 dB. Upstream power was found to generate the greatest contribution to the noise floor and is the limiting factor to the upstream transmission data rate. In-service monitoring experiments with full upstream and downstream power show that the proposed scheme is able to achieve dynamic range levels of up to 32 dB with 5-m spatial resolution and no measurable penalty in data transmission.

Automatic Fault Detection in WDM-PON With Tunable Photon Counting OTDR

An automatic fiber-optical fault analysis system making use of a tunable photon counting optical time-domain reflectometry (OTDR) (ν-OTDR) is proposed and demonstrated in a passive optical network testbed. The employment of the ℓ trend filter as a signal processing tool enables the minimization of the intrinsic coherent random noise impact on the acquired data and also an automatic identification of fiber faults. A feedback loop between an FPGA-based acquisition unit and the filters selections yields highly accurate automatic results and minute monitoring periods with ITU-T grid DWDM tunability, spatial resolution of 6 m and a measured 32 dB dynamic range.

Investigation of bend loss in single mode fibers with ultra-high-resolution photon-counting optical time domain reflectometer

An intensity peak associated with fiber bending could be detected thanks to the use of an ultra-high-resolution photon-counting optical time domain reflectometer (OTDR) setup. The peak intensity is shown to be dependent on the curvature radius and angular distance of the bend. To account for such peaks, we propose a model based on modal mismatching and coupling inside the bend region and show that the model is highly consistent with the acquired data. Combining the information of the bend peak and bend loss, and taking advantage of the high dependence of the peak value with the local modal field parameter, the technique could be employed as an optical fiber local-parameter characterization method.

Spectral characterization of weak coherent state sources based on two-photon interference

We demonstrate a method for characterizing the coherence function of coherent states based on two-photon interference. Two states from frequency mismatched faint laser sources are fed into a Hong–Ou–Mandel interferometer, and the interference pattern is fitted with the presented theoretical model for the quantum beat. The fitting parameters are compared to the classical optical beat when bright versions of the sources are used. The results show the equivalence between both techniques.

Fiber Monitoring Using a Sub-Carrier Band in a Sub-Carrier Multiplexed Radio-Over-Fiber Transmission System for Applications in Analog Mobile Fronthaul

In this paper, we describe an efficient method for monitoring fiber links utilizing sub-carrier multiplexing. By assigning a void sub-carrier frequency band for monitoring purposes, the method reuses the data transmitter without any impact on data transmission and provides capability of in-service reflectometry measurements of fiber optic lines with 10-m spatial resolution and 1.0-dB fault detection sensitivity. Its promising properties and performance enable potential application in emerging networks such as relatively short distance analogue mobile fronthaul.

Few-photon heterodyne spectroscopy.

We perform a high-resolution Fourier transform spectroscopy of optical sources in the few-photon regime based on the phenomenon of two-photon interference in a beam splitter. From the heterodyne interferogram, between test and reference sources, it is possible to obtain the spectrum of the test source relative to that of the reference. The method proves to be a useful asset for spectral characterization of faint optical sources below the range covered by classical heterodyne beating techniques.

Adaptive Filter for Automatic Identification of Multiple Faults in a Noisy OTDR Profile

We present a novel methodology that is able to distinguish meaningful level shifts from typical signal fluctuations which, in a fiber monitoring context, is associated with the problem of identifying small losses within a noisy optical time-domain reflectometer (OTDR) profile. A two-stage regularization filtering can accurately identify the location of the significant level shifts with an efficient parameter-free algorithm. The developed methodology demands low computational effort and can easily be embedded in a dedicated processing unit. Our case studies compare the new methodology with the current available ones and show that it is the most adequate technique for fast detection of multiple unknown level shifts in a noisy OTDR profile.

Linear-optic heralded photon source

We present the first Heralded Single-Photon Source, to our knowledge, based only on linear optics and weak coherent states. By time-tuning a Hong-Ou-Mandel interferometer fed with frequencydisplaced coherent states, the output photons can be synchronously heralded following sub-Poisson statistics, which is indicated by the second-order correlation function (g (0) = 0.556). When compared to state-of-the-art asynchronous spontaneous parametric down-conversion-based sources, the technique presents comparable yield and at least one order of magnitude higher coherence time.

Single-ended in-service hybrid monitoring of fiber-extended copper lines

We report on the simultaneous monitoring of both fiber and copper links in the context of analog radio signal transmission over fiber-extended copper mobile fronthaul networks. The same monitoring signal is used to determine both the fibers and coppers characteristics, which translates into a low-complexity monitoring structure readily embeddable in the transmitter. A copper-tofiber converter unit is devised so that both monitoring and data signals can coexist even when the control is centralized at the central office. In-service fiber monitoring is achieved with a spatial resolution of 10 m and >7 dB dynamic range, while in-service copper monitoring is achieved with a maximum reach of 100 m and an error in fault location smaller than 1 m. Low-cost centralized monitoring of hybrid networks represents a step toward the deployment of fiber-extended copper lines, which is a promising candidate for next-generation communication architectures.

An optimal polarization tracking algorithm for Lithium-Niobate-based polarization controllers

We present an optimal algorithm for the three-stage arbitrary polarization tracking using Lithium-Niobate-based Polarization Controllers: device calibration, polarization state rotation, and stabilization. The theoretical model representing the lithium-niobate-based polarization controller is derived and the methodology is successfully applied. Results are numerically simulated in the MATLAB environment.