Dispersion management is becoming paramount in high-speed wavelength-division-multiplexed lightwave systems, that operate at per-channel rates of 40 Gb/s and higher. The dispersion tolerances, in these systems, are small enough that sources of dispersion variation, that are negligible in slower systems, become critically important to network performance. At these high-bit rates, active dispersion compensation modules may be required to respond dynamically to changes occurring in the network, such as variations in the per-channel power, reconfigurations of the channel's path that are caused by add-drop operations, and environmental changes, such as changes in ambient temperature. We present a comprehensive discussion of an emerging tunable dispersion compensating device, based on thermally actuated fiber gratings. These per-channel devices rely on a distributed on-fiber thin film heater, deposited onto the outer surface of a fiber Bragg grating. Current flowing through the thin film generates resistive heating at rates that are governed by the thickness profile of the metal film. A chirp in the grating is obtained by using a thin-film, whose thickness varies with position along the length of the grating in a prescribed manner; the chirp rate is adjusted by varying the applied current. The paper reviews some of the basic characteristics of these devices and their implementation, in a range of different applications, including the mitigation of power penalties associated with optical power variations. We present detailed analysis of the impact of group-delay ripple and polarization-mode dispersion on systems performance, and present results from systems experiments, that demonstrate the performance of these devices at bit rates of 10, 20, 40 and 160 Gb/s. We also discuss advantages and disadvantages of this technology, and compare to other devices.

Integrated tunable fiber gratings for dispersion management in high-bit rate systems

The temperature dependence of chromatic dispersion is examined for various types of fiber. Its coefficient is found to depend strongly on the dispersion slope. Dispersion-flattened fiber has a significantly low coefficient of -0.0005ps/nm/km/°C, compared with -0.0038ps/nm/km/°C for large-core nonzero dispersion-shifted fiber. Transmission lines with low dispersion slopes consisting of pure silica core fiber and dispersion-compensating fiber also exhibit low coefficients of less than -0.001ps/nm/km/°C because of their compensating effects.

Temperature dependence of chromatic dispersion in various types of optical fiber

In this letter, we demonstrate a simple technique for dispersion monitoring by adding a single inband subcarrier tone to the transmitted data signal. A measurable dispersion of up to 1200 ps/nm is demonstrated in a 10-Gb/s channel using a 7-9 GHz subcarrier and the addition of the subcarrier induced a power penalty of <0.5 dB. Dynamic ranges exceeding 20 dB and resolution sensitivities better than 10 (ps/nm)/dB are shown. With an 8-GHz tone and a 15% modulation depth for 10-Gb/s signals, we show a measurement range of 975 ps/nm with a 22-dB dynamic range. We used the monitor output signal to achieve accurate tunable dispersion compensation.

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Online chromatic dispersion monitoring and compensation using a single inband subcarrier tone

In the paper we analyze the fundamental accuracy limits of the time/frequency transfer in fiber-optic transmission systems based on intensity modulation and direct detection (IM-DD) of the light signal The unidirectional and bidirectional time/frequency transfer schemes are considered, and their main limitations are pointed out In particular, the impact of the fiber backscattering and the temperature dependence of the chromatic dispersion are examined in the context of bidirectional transfer Finally, experimental results are presented and related to the preceding considerations The experiments performed with a 60 km long fiber demonstrate single-picosecond accuracy of the time transfer Our measurements suggest that it should be possible to obtain better accuracy of time/frequency transfer than that reported in the literature for systems based on the IM-DD principle

https://iopscience.iop.org/article/10.1088/0957-0233/21/7/075302/pdf

Optical fibers in time and frequency transfer

A rate 0.964 forward error correcting (FEC) code is integrated into the low-speed tributaries of a 565-Mb/s lightwave system as an exploratory system design approach toward relaxing requirements on laser sources in dispersion-limited operation. By virtually removing error rate floors, regardless of their cause, FEC is shown to provide an increasing advantage in conditions of greater degradation and to be effective against mode partition noise (MPN), mode jumping, and reflection impairments.The experimental FEC code is implemented in a standard gate array. The FEC code is described and its performance is analyzed. A new system design strategy is suggested for low-cost gigabit lightwave systems using FEC. >

Forward error correction in dispersion-limited lightwave systems

This paper describes the temperature dependence of chromatic dispersion in single-mode fibers for a temperature range of -60°- 250°C. Highly accurate dispersion measurements demonstrated a linear relationship (0.025 nm/°C) between the zero dispersion wavelength and temperature for tests conducted below 150°C. Further tests in the high-temperature range showed an increase in this dependence. For both temperature ranges, the results were independent of fiber coating and structure.

M. Nishimura

Papers

Temperature dependence of chromatic dispersion in single mode fibers