The pursuit of faster and more cost-effective optical transport networks is a never-ending quest for both optical-system vendors and network operators in the telecommunications industry. In much the same manner as the transition from 2.5G (gigabits) to 10G in the late 1990s, the industry is now experiencing similar challenges with the next capacity-quadrupling technology step from 10G to 40G.
The pace at which this transition will occur is largely governed by the access to suitable technology at a reasonable cost. This article explains how dispersion compensation based on Fiber Bragg Gratings (FBGs) can deliver cost savings as well as meet the technical requirement needed to facilitate higher-bit-rate optical transport networks.
During the past couple of years, FBG-based dispersion compensators have become a real alternative to the incumbent technology of dispersion compensating fibers (DCFs). With DCF technology maturing to the point where changes can only be evolutionary rather than revolutionary, the field has now opened up to the disruptive and cost-effective technology of FBGs.
Initially faced with the skepticism received by any disruptive technology, the advantages of utilizing FBGs for dispersion management have eventually become too obvious to ignore. This is evident from the thousands of FBG-DCMs deployed in various systems worldwide over the past several years.
FBG-based dispersion compensation
Chromatic dispersion, i.e. temporal distortion (spreading or smearing) of short optical pulses as they traverse optical fibers, is a fundamental problem in optical transport. The distortions of the signal will, if not properly compensated for, lead to inter-symbol interference that eventually results in data loss and/or traffic interruption.
The traditional means of overcoming the issue of dispersion has been to incorporate bundles of DCF throughout the optical network. DCF-based compensation is a quite straightforward technique, based on optical fibers having a dispersion coefficient with an opposite sign compared to standard single-mode fiber used for the actual transport.
Typically, DCFs have a dispersion coefficient four to eight times that of standard single-mode fiber. However, this level of dispersion is achieved by reducing the diameter of the fiber core. This, in turn, increases the fiber transmission loss as well as limits the levels of optical power that can effectively be transmitted through the fiber without inducing other distortions, so-called “nonlinear” effects.
Chromatic dispersion compensation using highly efficient reflective FBGs is significantly different from DCF compensation. It proves to have, as described later, some obvious benefits with regard to addressing both the technical as well as the cost-related issues of current and future dispersion compensation.
Dispersion compensation utilizing FBGs is based on the introduction of wavelength-specific time delays through the use of a precisely chirped FBG. By combining such a FBG with a standard optical circulator, a highly effective dispersion compensation module (DCM) can be realized.
A graphical illustration of a FBG and the dispersion compensating principle is shown in Figures 1 and 2.
The re-compression of a dispersion-broadened pulse is accomplished by letting the “fast” wavelengths of the pulse reflect farther away in the FBG than the “slow” wavelengths reflected closer to the circulator. The exact reflection position for each wavelength is governed by the precise, photo-induced, refractive index changes within the fiber, which are controlled, down to single nanometers, by a highly sophisticated manufacturing method.
Accurate control over the FBG chirp is the key for precise dispersion compensation. By utilizing state-of-the art direct-write FBG manufacturing techniques, dispersion characteristics can be made to precisely mimic the dispersion properties of the fiber or span intended for compensation.
Two main types of FBG-based dispersion compensators are commercially available today: multi-channel (or channelized) and continuous. The channelized version provides channel-spacing-specific, or grid-specific, compensation. The continuous type provides, in much the same manner as a DCF, continuous compensation throughout the C or L band. The continuous type thereby offers total channel plan independence, a feature especially interesting when considering higher bit rates, dense channel spacing, and future upgradeability.
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