DSP and Silicon Photonics in Coherent Systems
By Atul Srivastava

In this tech info, we will provide background and features of digital coherent DSP and silicon photonics technology, which are key building blocks of the next-generation digital coherent transceivers for the present day optical communication networks described previously.

DSP in Coherent Systems

The digital coherent DSP is a digital signal processing LSI for large-capacity communication using intensity-phase modulation, and is the core device of digital coherent communication systems.
Figure1 shows the block diagram of a low-power DSP, which has three-chip functions successfully integrated into a single chip and still has less than half power consumption as compared to the previous generation 16nm chip set. In order to achieve the same functionality, the earlier generation DSPs required external OTN Framer and transmitter MUX chips. The new DSP outputs 4 x 28/32G/64G analog waveforms including QPSK and 16QAM for 100G, 200G and 400G, respectively from the Digital-to-Analog Converter (DAC). It includes a framer to accommodate 100GE signal into OTU4 frame, and a spectral shaping filter that can realize spectrally efficient Nyquist pulse.

Fig. 1. Block diagram of a low-power DSP. OTN Framer and 4ch DAC are integrated in the chip.
Lower part shows examples of Flexible DSP operation for 100km and 4000km transmission reach
with optimized performance and power dissipation.

Functional optimization with regard to power consumption has been realized with selectable FEC and CD compensation options for trade-offs on performance requiring higher computational complexity. State of the art 7nm CMOS technology can achieve DSP for higher data rates such as 600 Gb/s (64Gbaud and 64QAM) while reducing the power consumption even further. A 7nm DSP for 400ZR application needs to have less than 10W power dissipation in order to fit into a 15W transceiver module. The DSP has been designed to provide flexibility where the performance and power dissipation can be optimized for the application. Figure 1 includes an example of such optimization for application range 100km and 4000km. The same transceiver can be used for metro and short reach applications by suitably selecting the options of FEC elements, equalizers for the CD compensation and Nyquist filter to optimize the power dissipation by 20-25%^{* 1}.

Silicon Photonics in Integrated Transceiver

As discussed in previous blog, higher level of photonic integration is key to the development of next generation smaller size 400G transceiver modules. State-of-the-art Silicon Photonics is a promising platform for lower cost integrated photonic devices which can leverage mass production capability of CMOS technology^{* 2}. Silicon photonics based Modulators, Ge Photodetectors (PDs), polarization beam splitter/combiners, polarization rotators, grating couplers and coherent mixers have been demonstrated for 64Gbaud applications in 400ZR module. Insensitivity to temperature and humidity in silicon photonic devices such as modulators eliminate the need of TEC and for temperature control and shrink the package size and eliminate hermetic sealing. Recently, silicon photonics based COSA devices have been reported^{* 2}.

Fig. 2. Silicon Photonics Coherent Optical Sub-assembly (COSA) device schematic

Figure 2 shows a schematic of the Silicon Photonics COSA device. All elements of the optical transceiver except the laser are integrated into a single package. These include two in-phase/quadrature (IQ) modulators, a polarization beam combiner and rotator (PBCR), a polarization beam splitter and rotator (PBSR), couplers, a coherent mixer, and photo diodes are integrated into a single Si Photonics chip. The Si Photonics chip is co-packaged with a 4-channel modulator driver IC and 4-channel trans-impedance amplifier IC. Three edge-coupled fibers are used as the optical interfaces for the input port from the laser, signal output port from the modulator, and signal input port to the receiver. The input port from the laser is a polarization maintaining fiber, and the other ports are non-polarization maintaining fibers. Light output from the laser is shared between the modulator and the coherent receiver. The package is non-hermetic low profile and does not need TEC. The non-hermetic packaging of the device also allows optical edge coupling without a lens. It also includes a flexible printed circuits (FPC) for DC and RF connections which makes it suitable for the next generation small size transceiver modules.