Lightwave Logic Inc (LWLG)

[Photonics_Guy]

Long-haul Networks (>10km) are predominantly moving to the use of coherent detection (phase modulation formats) to improve economics and performance.

Here is a great article (mildly technical) that describes these benefits, so you don't have to take my word for it. ;)

From Merits of Coherent Detection Optical Transmission:

A “coherent” optical transmission system is characterized by its capability to do “coherent detection,” which means that an optical receiver can track the phase of an optical transmitter (and hence “phase coherence”) so as to extract any phase and frequency information carried by a transmitted signal.

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In contrast to coherent detection is direct detection, typically used by 10Gb/s or lower-speed systems. In a direct detection receiver, its photo-detector only responds to changes in the receiving signal optical power, and cannot extract any phase or frequency information from the optical carrier.

Coherent detection therefore offers several key advantages compared to direct detection:

(1) Greatly improved receiver sensitivity.
(2) Can extract amplitude, frequency, and phase information from an optical carrier, and consequently can achieve much higher capacity in the same bandwidth.
(3) Its DSP can compensate very large chromatic and polarization mode dispersion due to optical fibers, and eliminate the need for optical dispersion compensators and the associated optical amplifiers. This saves not only significant capex, but also simplifies optical network design tremendously, because the complicated dispersion map associated with 10G and 40G direct detection systems is no longer needed.
(4) When using balanced detectors with a high common mode noise rejection ratio (CMRR), not only signal-to-noise ratio (SNR) can be improved further, but also agile wavelength selection can be achieved by LO tuning without the use of an optical filter or demultiplexer. This feature enables the next-generation colorless and directionless (CD), or even colorless, directionless, and contentionless (CDC), reconfigurable optical add-drop multiplexers (ROADM).

It should also be noted that laser phase noise of an LO is an important impairment in coherent systems as it impacts the “phase coherence”. The “linewidth” parameter of a laser diode is directly related to its phase noise.

Although its fundamental concept is derived from wireless communications systems, coherent detection has started another paradigm shift in optical fiber communications. Its impact has become as huge as what commercial laser diodes and erbium-doped fiber amplifiers have brought to the industry. Consequently, long-haul optical networks have already become coherent-centric today, while metro optical networks are bound to become coherent-centric in the next few years.