Coherent Detection

There are three basic technologies behind CoreOptics’ 40G CP-QPSK: dual polarization, DQPSK modulation, and coherent detection with DSP.

Firstly, there is polarization multiplexing, commonly known as dual polarization. This enables the transmission of a signal by decomposing it into two polarization signals of the same frequency at 90º from each other in order to avoid mutual interaction as they are being launched into the fiber. One signal (X) is transmitted in the horizontal polarization and the other (Y) in the vertical polarization.

These two signals can be combined or split up using an optical polarization beam splitter. Each polarization allows the signal to have one bit per symbol, and combining these polarization signals effectively allows for the modulation of two bits per symbol.

Secondly, there is the Differential Quadrature Phase-Shift Keying (DQPSK) modulation format. In contrast to On-Off Keying (OOK) where the signal is either on or off, resulting in only one bit per interval or symbol, QPSK modulation supports two bits per interval or symbol. This is because the signal can exist in one of four different phases that allows it to be encoded by two bits per symbol. Hence, the symbol rate is only half the bit rate.

By combining the effects of dual polarization and DQPSK, we now have four bits per symbol: one from each polarization and two from QPSK. Hence, polarization-multiplexed QPSK has a symbol rate that is one-fourth the bit rate (for example: at 40 Gb/s, the symbol rate is only at 10 Gb/s). This lower rate eases the bandwidth requirements on optoelectronic components. Also, it results in a narrower optical spectrum and higher tolerance to CD and PMD.

Finally, there are the great benefits of coherent detection at the receiver. As known from applications in the wireless world, coherent detection improves receiver sensitivity, compensates for optical dispersion by operating on signals proportional to the optical field, and uses digital carrier recovery techniques. In this approach, a local phase reference or oscillator is mixed with the incoming signal for the two independent polarizations, X and Y. The signal is further decomposed into its I and Q states, yielding four independent signals: XI, YI, XQ, and YQ.

In this way, these optical analogue signals contain all the amplitude, phase, and polarization information before they are received by the four photo-detectors and converted into digital streams. Subsequently, the data is recovered by means of DSP where the functions of clock recovery, equalization, Carrier Phase Estimation (CPE) and recovery are performed.

All of the components in the core of today's networks – ROADMs, switches, filters, amplifiers, etc. – were provisioned for 10 Gb/s networks. The demand for higher bandwidth rates is driving a new requirement of at least 40 Gb/s and many carriers are already considering the right migration path for their networks. Many important considerations await these carriers, but ease of migration and CAPEX costs are high on the list.

Coherent detection and polarization-multiplexed DQPSK technology is superior to direct-detection (DPSK, RZ-DQPSK) technologies in cost savings and optical performance for migration to 40 Gb/s. To get to 100 Gb/s, coherent technology is the only choice. Using coherent electronic technologies will reduce complexity, cost, and support for 40 Gb/s data rates on existing 10 Gb/s networks, resulting in a viable migration path for 100 Gb/s. With its flexibility and universal attributes born by enhanced DSP, coherent detection provides carriers with the building blocks for today's and tomorrow's bandwidth demands.

Please contact a CoreOptics’ representative to see our extensive network study for a 30-hub regional network with varying system reach, utilizing CP-QPSK. Learn first-hand about CAPEX savings ranging from 20 - 50 % over direct-detection technologies.