Optical coherent systems

The commercial feasibility of a coherent system for optical signal transmission was first investigated around 1990 as a means to improve a receiver’s sensitivity. In contrast to existing optical direct-detection system technology, an optical coherent detection scheme would detect not only an optical signal’s amplitude but phase and polarization as well. With an optical coherent detection system’s increased detection capability and spectral efficiency, more data can be transmitted within the same optical bandwidth. Moreover, because coherent detection allows an optical signal’s phase and polarization to be detected and therefore measured and processed, transmission impairments which previously presented challenges to accurate data reception, can, in theory, be mitigated electronically when an optical signal is converted into the electronic domain.

However, the technology did not soon gain commercial traction because the implementation and benefits of an optical coherent system could not be realized by existing systems and technologies. As demand for higher transmission capacity systems has evolved, a method that quadruples transmission capacity to 40 Gbit/s while maintaining the transmission properties of a 10 Gbit/s non-coherent transmission system has been introduced by Nortel in the OME 6500. It uses dual-polarization, quadrature phase-shift keyed modulation of the light source, and a coherent receiver with advanced electronic compensation of path impairments. Another approach for 10 Gbit/s optical coherent transmission uses heterodyne technique for electrical compensation of the chromatic dispersion in the IF domain was introduced by Discovery Semiconductors in 2005[1].

Implementing a coherent detection system in optical networks requires 1) a method to stabilize frequency difference between a transmitter and receiver within close tolerances; 2) he capability to minimize or mitigate frequency chirp or other signal inhibiting noise; 3) an availability of an optical mixer to properly combine the signal and the local amplifying light source or local oscillator (LO); and 4) an ability to stabilize the relative state of polarization between the transmitter and the local oscillator. These technologies were not available in the 1990s. A further setback to the adoption and commercialization of an optical coherent system was the introduction of the EDFA, an alternative low cost solution to the sensitivity issue. Notwithstanding the myriad challenges, an optical coherent system (also referred to as Coherent Light Wave) remains a holy grail of sorts to the optical community because of its advantages over traditional detection technologies:

An increase of receiver sensitivity by 15 to 20 dB compared to incoherent systems, there-fore, permitting longer transmission distances (up to an additional 100 km near 1.55 μm in fiber). This enhancement is particularly significant for space based laser communications where a fiber-based solution similar to the EDFA is not available.

Compatibility with complex modulation formats such as DPSK or DQPSK.

Concurrent detection of a light signal’s amplitude, phase and polarization allowing more detailed information to be conveyed and extracted, thereby increasing tolerance to network impairments, such as chromatic dispersion, and improving system performance.

Better rejection of interference from adjacent channels in DWDM systems, allowing more channels to be packed within the transmission band.

Linear transformation of a received optical signal to an electrical signal that can then be analyzed using modern DSP technology.

September 30, 2011

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