Coherent Optics: Modulations, DSP, Applications & FAQ
- Coherent Optics: The challenge?
- What is Coherent Optics?
- Coherent Optics Modulation vs. PAM4 vs. NRZ
- Coherent DSP (Digital Signal Processor)
- Coherent Optics Pros and Cons?
- Coherent Optics: Frequently Asked Questions
Coherent Optics: The challenge?
Between 2021 and 2026, global internet traffic is anticipated to increase at a Compound Annual Growth Rate (CAGR) of 26% (source: Cisco’s Annual Internet Report), putting pressure on existing telecommunication infrastructure. While current widely used NRZ modulated (light On/Off) signals have 10-25G baud rate limitations and for long distance links we are used to deploy EDFA (Erbium-doped optical fiber amplifiers) and DCM (Dispersion Compensation Modules) which have their own limitations. With data rate increases aggregation and long-haul transmission networks will be pushed towards 400G/800G Coherent Optics, while Access and Metro area networks towards 25G, 100G and 400G levels.
What is Coherent Optics?
Coherent Optics refers to optical transceivers that use coherent modulation (QPSK/QAM) instead of amplitude modulation (NRZ/PAM4) for establishing high bandwidth (400G/800G Ethernet), long distance interconnection lines. Initially, the technical specifications of Coherent Optical transceivers were held proprietary by suppliers of optical transmission equipment. However, in recent years, standardization groups such as MSA (with it’s Open ZR+ MSA) and OIF (with it’s: implementation agreement for a 400G ZR coherent optical interface) have made progress in establishing standardized specifications for these transceivers.
An important additional thing we need to keep in mind when discussing Coherent Optics is the requirement to operate within a legacy DWDM C-Band grid where all telecom transport networks operate. Let’s go through the coherent optics fundamentals and look at the most crucial components that make coherent optics “tick”. Here, we will discuss: Modulation technique, DSP (digital signal processor), Coherent Optics Pros and Cons, answers to the most frequently asked questions regarding Coherent Optics, such as Analogue Coherent Optics versus Digital Coherent Optics, typical Coherent Optics form factors, Coherent Optics main standards, do you need to use EDFA & DCM with Coherent Optics and more.
Coherent Optics Modulation vs. PAM4 vs. NRZ
Radio engineers have benefited from more sophisticated modulation schemes for many years, and they are now being used more frequently in the optical industry. Let’s look at the underlying concepts of coherent modulation and how these compare to today’s most common modulation schemes in the optical world: NRZ and PAM4.
NRZ (Non-Return to Zero)
To begin, NRZ, also known as On/Off Keying (OOK), is a modulation technique that has long been used in the optical industry. It consists of optical laser light flashing on and off, which corresponds to binary 1 and 0 signals. NRZ modulation allows us to obtain a maximum baud rate of 25G.
PAM4 (Pulse Amplitude Modulation 4-level)
Next comes PAM4 which uses four different signal levels for signal transmission, with each symbol representing two bits of information. PAM4 outperforms NRZ in terms of data transfer capacity. At the same baud rate, PAM4 doubles the amount of information that may be transmitted. In other words, although NRZ can only convey one bit every baud unit, PAM4 can broadcast two bits per unit.
DP-QPSK Coherent Optics Modulation
Dual Polarization Quadrature Phase-Shift Keying (DP-QPSK) is a modulation technique used in optical communication that can encode four bits per symbol. It uses four different phases as well as two polarizations (vertical and horizontal), so when a DP-QPSK symbol is sent, information for four bits is transmitted. DP-QPSK is commonly used in long-distance 100G coherent optics lines and typically requires a coherent optical receiver.
16-QAM, 64-QAM Coherent Optics Modulation
16-QAM is a kind of Quadrature Modulation (QAM) in which a carrier wave with a given frequency can exist in one of sixteen different states, each represented by a symbol containing one of sixteen alternative amplitude and phase values. The 64-QAM modulation carrier wave with a given frequency can exist in one of sixty-four distinct and measurable states in the constellation plot. The QAM technology allows you to alter both the amplitude and phase of the carrier signal. In 16-QAM, each symbol represents four bits, whereas in 64-QAM, each symbol represents six bits. 16-QAM modulation technique is commonly used in 400G Coherent optics lines, while 64-QAM modulation in 800G Coherent optics lines.
Coherent DSP (Digital Signal Processor)
A Coherent DSP (Digital Signal Processor) chip placed on a coherent optical transceiver PCB is an essential component of coherent communication systems. The DSP chip consumes around 50% of the power from a coherent optical transceiver. In coherent optics, DSP is responsible for:
- analog-digital conversation, fiber transmission is analog optical signals, but data processing is digital
- encoding and decoding data into three light signal properties: amplitude, phase, and polarization
- use pilot signals to help the receiver decode. The pilot serves as a reference for phase and polarization
- adaptive equalization compensates for signal spectrum distortion
- ethernet framing and conversion from Ethernet to Optical Transport Network (OTN) formats
- FEC improves tolerance to noise and distortion
- compensation for chromatic dispersion and non-linear distortion
Coherent Optics Pros and Cons?
Coherent optics’ main advantage is its ability to construct high-speed (100G/400G/800G) long-distance interconnection lines (up to 1000km) in our well-known DWDM C-band channel grid.
The disadvantage of Coherent optical modules is that they are notably more expensive than standard NRZ/PAM4 modules and need significantly more power (15-25W for 400G QSFP-DD ZR/ZR+ Coherent optics vs. 5.5W for 100G QSFP28 DWDM 100G PAM4 module).
To put it briefly, coherent optical modules have advantages over 100km reach, where building long haul links with NRZ/PAM4 modules would require a lot of amplification and dispersion compensation, whereas in shorter networks below 100km, NRZ/PAM4 are much simpler, cost efficient and require less power consumption.
Coherent Optics: Frequently Asked Questions
What is Coherent Optics?
Coherent Optics are optical transceivers that use coherent modulation (DP-QSFP, 16-QAM, 64-QAM) and DSP (Digital Signal Processor) to transport high data rate signals (100G/400G/800G) over long distances (from a few hundred kilometers to 1000 km).
Where Coherent Optics are used?
Coherent optics are used in long-haul and ultra-long-haul, high-capacity DWDM telecommunications networks, often spanning several hundred kilometers to 1000 km, with 100G/400G/800G capacity and DWDM C-Band channel spectrum.
Coherent Optics vs DWDM?
DWDM is a type of wavelength division multiplexing that allows many wavelengths (colors of light) to pass over the same dark fiber at once. DWDM technology accepts both coherent and non-coherent optics.
What is the difference between coherent vs non coherent optics?
Coherent optics use coherent modulation (DP-QSFP, 16-QAM, 64-QAM) and DSP (Digital Signal Processor), while non coherent optics use amplitude modulation (NRZ/PAM4).
Digital Coherent Optics vs Analog Coherent Optics (DCO vs ACO)?
For Digital Coherent Optics (DCO), the DSP chip is directly soldered to the optical transceiver PCB, whereas Analogue Coherent Optics (ACO) uses analogue communication between module and host, with the DSP located externally on the host transponder card.
Coherent Optics Form Factors
Typical Coherent Optics form factors are CFP-DCO (100G/200G), CFP2-DCO (400G), OSFP-DCO (100G/400G), QSFP-DD DCO (400G), where later ones are smaller in size, less expensive and require lower power consumption.
Coherent Optics Main Standards - OIF 400ZR vs OpenZR+?
Open ZR+, OIF 400G ZR and are among the main Coherent Optics standards which enable carriers to match coherent optics from different vendors in their network. In below table is main comparison between OIF 400G ZR vs Open ZR+ MSA.
| Parameter | 400ZR | OpenZR+ |
|---|---|---|
| Optical Reach | <120 km | ~400 km (400G) |
| Line Capacity | 400G | 100G – 400G |
| Modulation | 16QAM | QPSK, 8QAM, 16QAM |
| Baud Rate | ~60 GBaud | 100G: 30 Gbaud200G – 400G: 60 Gbaud |
| Tx Launch Power | -10 dBm | -10 dBm |
| Client Interface | 100GE, 400GE | 100GE, 400GE |
| Power | 15 – 20W | 18 – 20W |
| Typical Modules | QSFP-DD, OSFP, CFP2-DCO | QSFP-DD, OSFP, CFP2-DCO |
Will I need EDFA & DCM if I use Coherent Optics?
There is no need for DCM (Dispersion Compensation Module) in Coherent Optics because the DSP (Digital Signal Processor) takes care of that part. However, there is a need for EDFA (Erbium-doped optical fibre amplifier) because the typical Coherent Optics Tx Min Launch Power is -10 dBm and Max Receiver Sensitivity is -20 dBm, which leaves us with 10 dBm for DWDM Mux/Demux, where 40ch AAWG type DWDM mux would add 3.7 dBm on both ends, leaving us with (10 dBm-3.7*2=) 2.6 dBm, and we would need some 2-3 dBm safety margin. There is almost no dBm remaining for connectivity distance. The OIF 400ZR and OpenZR+ standards continue to work towards coherent optics with TX output power at ~0 dBm instead of -10 dBm. This is achieved by incorporating internal EDFA in the transceiver, while typical transmission networks and typical external EDFA devices require input signals at 0 to +5 dBm.
What is the difference between Baud rate vs Bit rate?
Bit rate refers to the total bits transmitted per unit of time, typically second, while baud rate refers to the total symbols transmitted per unit of time. For binary transmission NRZ modulated signal (sometimes referred to On/Off Keying (OOK) bit rate and baud rate is the same, but once we use more sophisticated modulation techniques they differ. For example, for 800G we use 64-QAM modulation which allows to transport 6 bits per symbol and additionally we use polarization which doubles the capacity, which in the end gives us close to 1 terabyte of traffic per second, from whom we need to deduct portion, which is required for internal use, like FEC, framing, overhead. In the end we get 800G bit rate at 100G baud rate.
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