Coherent optics typically increase the receiver sensitivity and the link budget by 15dB over a conventional optical receiver. However, for some applications, such as higher link budgets for XG(S)-PON and NG-PON2 systems, only 6dB separates the smallest link budget (N1) from the largest (E2). Conventional optics can already achieve the N1 budget so only a 6dB improvement is needed for the other budget classes. Therefore, simplification and cost reduction can be applied that reduces the theoretical sensitivity to an extent that would be unacceptable in a long-haul optical system.
Let us call this ‘quasi-coherent’ optical communication.
The following design concepts and diagrams have been supplied by Jesper B Jensen of Bifrost Communications
As illustrated in Figure 1, a quasi-coherent system can be obtained with as little as 1 polarizing beam splitter (PBS), 1 half-mirror (HM) and 2 conventional single-ended photodetectors (PDs) compared to the 4 PBS, 4 HMs and 4 pairs of balanced PDs (8 PDs in total) needed for the “full” homodyne receiver. In addition, the complex and power-hungry DSP required for phase locking the local oscillator (LO) to the signal can be avoided. By deliberately introducing a frequency offset between the signal and the LO, the received signal after the PDs is moved away from baseband and into passband, and the requirement of a well-defined phase relation between signal and LO goes away. In fact, if the PD bandwidth allows it, some minor frequency drift can even be tolerated. Since the signal out of the PDs are moved into baseband they can no longer be detected by a conventional non-return-to-zero (NRZ) receiver. Simple analog signal processing in the form of envelope detection (i.e. rectification followed by low-pass filtering) is all the signal processing that is needed.
This is illustrated in Figures 2a and 2b, where the signal out of the PD is shown on the left, and the signal after envelope detection is shown on the right. In this case, the bit pattern  has been successfully recovered. Looking closer at the signal on the left, i.e. the signal out of the PD, the frequency offset between signal and LO can be observed as an oscillation with an unstable frequency. The reason for this instability is that the frequency offset is unstable. In this case, the signal was generated by a directly modulated laser (DML), which has chirp meaning the ones and zeros are at slightly different optical frequencies. The quasi coherent receiver has no problem “absorbing” this frequency variation, as can be seen by the perfectly recovered signal on the right. In contrast, for homodyne detection, a chirp free laser with very low phase noise in generally required. That the simplified quasi-coherent receiver can operate with low-cost DMLs makes it particularly attractive for access applications such as NG-PON2.
The performance of this type of receiver has been verified in laboratory experiments using an early-stage prototype analog envelope detector. The result for 10Gbps data rate is shown above in Figure 3. The different curves represent bit error rate (BER) measurement with different LO power ranging from 8 dBm to 16 dBm. The measurements were performed using PIN photodetectors, i.e. not APDs.
A receiver sensitivity of -33.5 dBm from this early demonstration shows the potential of achieving Class E1 performance or even E2 with further maturity of the technology.
While the above tests show 10Gbps performance recent results using the same technology operating at 25Gbps has been shown to achieve -30dBm sensitivity.