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Anatomy of a Coherent Optical Engine

headshot of Paul Momtahan

November 6, 2023
By Paul Momtahan
Director, Solutions Marketing

Coherent optical engines have revolutionized optical transport, delivering huge gains in wavelength capacity-reach and spectral efficiency, along with significantly lower cost per bit and power consumption. But have you ever wondered what is inside these miraculous devices that can now deliver 400 Gb/s in a compact pluggable and 800 Gb/s, evolving to 1.2 Tb/s, in embedded form factors?

Generic coherent optical engine: high levelFigure 1: Generic coherent optical engine: high level

Well, at a high level they comprise three basic building blocks: a digital ASIC, analog electronics, and photonics, as shown in Figure 1, with the analog electronics and photonics often packaged together as a transmit-receive optical sub-assembly (TROSA). Together with the radio frequency (RF) interconnects and the packaging, these three building blocks constitute a coherent engine. However, each of these blocks consists of multiple functions.

Inside the Digital ASIC

The digital ASIC, often referred to simply as “the DSP,” shown in Figure 2, includes digital signal processing (DSP) functions for the receive and transmit directions. Common transmit DSP functions include modulation encoding, spectral shaping, and pre-compensation. Common receive DSP functions include chromatic dispersion (CD) compensation, clock recovery, polarization recovery, polarization mode dispersion (PMD) and polarization-dependent loss (PDL) compensation, carrier recovery, and modulation decoding.

Digital ASIC building blocksFigure 2: Digital ASIC building blocks

In addition, some digital ASICs include further value-added DSP functions. For example, Infinera’s ICE6 also includes nonlinear compensation and state-of-polarization tracking circuitry, as well as long-codeword probabilistic constellation shaping within the modulation encoding/decoding. Another differentiating feature in Infinera’s ICE4, ICE6, and ICE-X engines is digital subcarriers that provide benefits such as reducing the noise that comes from compensating for chromatic dispersion and, in the case of ICE-X pluggables, also enabling point-to-multipoint connectivity. DSPs for digital subcarriers perform each function individually on each subcarrier, as shown in Figure 3.

Subcarrier-based DSP example with four subcarriersFigure 3: Subcarrier-based DSP example with four subcarriers

Modern digital ASICs also include the digital-to-analog converter (DAC) and the analog-to-digital converter (ADC). The DAC converts the digital signal from the transmit DSP to an analog voltage that will ultimately be used to drive the photonics. The ADC takes the analog signal from the analog electronics and converts it to a digital signal that can be understood by the receive DSP.

Over time, more and more functions have been integrated into the digital ASIC in order to reduce costs, space, and power, leveraging the increased processing power enabled by each new CMOS generation. Today, common non-DSP functions integrated into the digital ASIC include forward error correction (FEC), framing, multiplexing, encryption, and performance monitoring. In addition, ICE-X pluggables also integrate multiple intelligent management functions. Furthermore, ICE6’s digital ASIC supports two independent wavelengths, which enables value-added features like SD-FEC gain sharing and bandwidth virtualization over two wavelengths – for example, three 400 GbE over two 600 Gb/s wavelengths. 

Inside the Analog Electronics

Inside the RF ASICFigure 4: Inside the RF ASIC

The analog electronics play a critical role sitting in between the digital ASIC and the photonics. They consist of drivers and transimpedance amplifiers (TIAs), with four drivers and four TIAs required for a single coherent interface. In the transmit direction, the drivers take the low voltages from the DAC and convert them to the higher voltages required by the modulator. In the receive direction, the TIAs take the currents from the photodetectors and convert them to the voltages required by the ADC. The analog electronics are typically packaged as a single RF analog ASIC and made from a material other than the CMOS silicon used for the digital ASIC. For example, the RF analog ASICs in Infinera’s coherent optical engines and TROSAs are made from silicon germanium (SiGe). 

Inside the Photonics

Photonics building blocksFigure 5: Photonics building blocks

Inside the photonics, key transmit functions include the laser and the modulator. The laser generates light with the required frequency. As with all DWDM lasers, this laser is made from indium phosphide (InP). The modulator then takes the light from the laser, and by changing the phase and amplitude, encodes the data. It does this by using an electric field to change the refractive index of the material the light is passing through. This material is InP, silicon (i.e., silicon photonics), or possibly lithium niobate. Infinera’s modulators are InP, which has superior modulator performance relative to silicon photonics and also provides the ability to integrate the entire photonic block, including lasers and amplification, in a single photonic integrated circuit (PIC) for both transmit and receive. These PICs are manufactured at Infinera’s fab in California.

Mach-Zehnder modulatorFigure 6: Mach-Zehnder modulator

The coherent modulator actually leverages four Mach-Zehnder modulators (MZMs). Each MZM splits the light into two arms, changes the phase in one arm or more typically both arms, as shown in Figure 6, and then combines these two arms, letting them interfere to control the amplitude. A pair of phase-shifted MZMs can control amplitude and phase, with the four MZMs required for the two polarizations used in coherent transmission. Additional components inside the coherent modulator include splitters, combiners, phase shifters, a polarization rotator, and a polarization beam combiner, as shown in Figure 7.

Coherent modulator with four MZMsFigure 7: Coherent modulator with four MZMs

In addition, some coherent engines might include an amplifier, as shown previously in Figure 5, in the transmit direction in order to boost the wavelength power. If the photonics are based on InP, this amplifier can be a semiconductor optical amplifier (SOA) integrated in the PIC, as is the case with Infinera’s coherent engines. However, if the photonics are based on silicon photonics, this would have to be a micro erbium-doped fiber amplifier (EDFA), and due to the high out-of-band noise of this EDFA, a tuneable optical filter (TOF) would probably also be required.

Coherent receive photonicsFigure 8: Coherent receive photonics

The receive direction consists of some passive photonics and photodetectors, as shown in Figure 8. The photodetectors detect the light and convert it to electrical current. They are made from InP, or in the case of silicon photonics, germanium. The passive photonics include a polarization beam splitter that separates the two polarizations of the coherent signal and two 90º hybrids that extract the phase and amplitude from each polarization in the form of the in-phase (I) and quadrature (Q) components. A laser is also required to enable the phase information to be extracted from the received modulated wavelength. In many devices this is the same laser used for transmit. However, sometimes independent lasers are used for transmit and receive, enabling transmit and receive on different frequencies or, as is the case with ICE6, delivering more power from the transmit laser.

Putting It All Together

Generic coherent optical engine: detailedFigure 9: Generic coherent optical engine: detailed

Figure 9 provides a more detailed view of the coherent optical engine building blocks. Beyond the digital ASIC, the RF analog ASIC, and the photonics, RF interconnects and overall packaging also play an important role in optical engine performance by minimizing the noise generated inside the optical engine.

So, there you have it – the anatomy of a coherent optical engine, which you can also see in Infinera’s new infographic below. The next time you surf the internet or stream a movie with your family, you might think about the amazing optical engine technology that underpins the network – or not, and just pass the popcorn.

Infographic: Anatomy of a Coherent Optical Engine

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