The transmit signal strength is limited by laser heat dissipation and power consumption. Consequently, significant research and development efforts have been undertaken to improve the sensitivity of the receiver. Two technology alternatives exist: direct detection or coherent detection.
Predominantly suited to trans-oceanic submarine or terrestrial long haul applications, the performance of the coherent detection is undeniably superior to that of direct detection. In the case of coherent detection, the opto-electric conversion process is linear. Thus the phase information embedded in the optical signal is preserved permitting the straight forward electrical compensation of fiber linear effects including chromatic dispersion (CD) and polarization mode dispersion (PMD). However, the hardware required to perform coherent detection is somewhat more elaborate and comprises a local oscillator, a 90° Hybrid module necessary to discriminate the phase quadrature’s of the received optical signal, and four balanced photodiodes to detect the signal from a single polarization as seen in Fig. 2.
A direct detection technique, on the other hand, requires only a delay interferometer and two single photodiode translating into a device of much lower cost and complexity. The advantage of longer transmission distances (in the range of several 1000 km) enabled using a coherent approach is overkill for deployments of much shorter reaches - ranges of several 100 km.
The direct detection technique is thus seen as an attractive alternative for metro networks, enterprise and datacenters where economic viability plays a vital role.
Taking the minimalist approach, direct detection works without additional equipment in most cases. When the application range approaches the limit of direct detection technology, the simple addition of dispersion compensation and amplification may be used to augment signal detection and extend reach.