Optical hybrid

A 90° optical hybrid is a six-port device that is used for coherent signal demodulation for either homodyne or heterodyne detection. It would mix the incoming signal with the four quadratural states associated with the reference signal in the complex-field space. The optical hybrid would then deliver the four light signals to two pairs of balanced detectors.

Why optical hybrids are needed

Since the late 1990s, the transport capacities of long haul and ultra-long haul fiber-optic communications systems have been significantly increased by the introduction of EDFA, DWDM, dispersion compensation, and FEC technologies. For fiber-optic communications systems utilizing such technologies, the universal on/off-keying (OOK) modulation format in conjunction with direct detection methods have been sufficient to address data rates up to 10 Gbit/s per channel.

In order to economically extend the reach and data capacity beyond such legacy systems and into next-generation networks, several technological advancements must take place, including but not limited to, 1) adoption of a differential phase-shift keying (DPSK) modulation format, as opposed to OOK; 2) developments in optical coherent detection; and 3) progress in adaptive electrical equalization technology. In combination, these technologies will boost a signal’s robustness and spectral efficiency against noise and transmission impairments.

Such crucial strides in optical signal technology are no longer theoretical possibilities but are feasible solutions in present-day optical networking technology. The path for an optical coherent system has already been paved by 1) the deployment of DPSK modulated systems by Tier-1 network providers; and 2) the increased computational capacity and speed of electronic digital signal processing circuits in receivers, which provides an efficient adaptive electrical equalization solution to the costly and difficult optical phase-lock loop. These advances coupled with recent introduction of six-port optical hybrid solution should give pause to Tier-1 providers and carriers to reassess their earlier rationales for not adopting and implementing an optical coherent detection scheme. Perhaps with such advances, optical networks will begin to realize the benefits already recognized in microwave and RF transmission systems for extending capacity and repeaterless transmission distances through coherent detection.

September 30, 2011

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