Coherent optical module refers to a typically hot-pluggable coherent optical transceiver that uses coherent modulation (BPSK/QPSK/QAM) rather than amplitude modulation (RZ/NRZ/PAM4) and is typically used in high-bandwidth data communications applications. Optical modules typically have an electrical interface on the side that connects to the inside of the system and an optical interface on the side that connects to the outside world through a fiber optic cable. The technical details of coherent optical modules were proprietary for many years, but have recently attracted efforts by multi-source agreement (MSA) groups and a standards development organizations such as the Optical Internetworking Forum. Coherent optical modules can either plug into a front panel socket or an on-board socket. Coherent optical modules form a smaller piece of a much larger optical module industry.
There are multiple variants of the electrical interface of coherent optical modules use.
The Optical Internetworking Forum in 2016 published the CFP2-ACO or CFP2 - Analog Coherent Optics Module Interoperability Agreement (IA). This IA supports a configuration where the digital signal processor (DSP) is on the main board and analog optical components are on the module. This IA is useful in the case when the DSP exceeds the module power envelope. [1] The ACO interface can be used in coherent optics applications when the link delivers a flexible amount of bandwidth to the system, for example when combined with FlexE. The initial ACO IA is for the CFP2 module. The typical optical modulations that are used include Dual Polarization Quadrature Phase Shift Keying (DP-QPSK) and Quadrature Amplitude Modulation QAM-16.
These modules put the DSP on the module and use a conventional retimed digital interface. These modules can use the same optical modulation techniques as the ACO interfaces do.
Many different forms of optical modulation and multiplexing have been employed in coherent optical modules.
Some coherent optical modules can fall back to older, simpler modulation techniques such as on-off keying (NRZ) and/or Pulse-amplitude modulation with 4 levels (PAM-4) when appropriate. This is used, for example, when it is discovered that the module on the other end of the link does not support coherent modulation.
Techniques include Dual Polarization Quadrature Phase Shift Keying (DP-QPSK) and QAM-16.
Tunable lasers are sometimes used in combination with coherent modulation to allow a module to support various forms of network-based optical switching such as needed in certain cases by an optical mesh networks or a Reconfigurable optical add-drop multiplexer (ROADM). In these, the transmit laser can be tuned to a different optical frequency/wavelength. Similarly the receiver is able to receive different optical frequencies.
Different optical wavelengths, also referred to as lambdas, of light are multiplexed within some coherent optical modules using wavelength-division multiplexing (WDM). Variants include Coarse WDM (CWDM), Dense WDM (DWDM).
Coherent optical modules have a series of components inside, some of which have received attention from standards development organizations.
In many cases, the baud rate of the coherent optical interface does not equal the baud rate of the electrical interface. In these cases, a "gearbox" may be used within the module to convert between the rates. Because coherent modules typically have Digital Signal Processors, these gearbox functions sometimes are implemented in firmware.
Particularly in the long-reach module market, in-module Forward Error Correction (FEC) has been included. This has been in both proprietary and standards-based forms. Coherent optical modules sometimes have used soft-decision decoder FEC algorithms
The OIF has created interoperability agreements to create multi-vendor interoperability for a series of in-module components, particularly focused on coherent transmission. These have included
The OIF has created interoperability agreements to create multi-vendor interoperability for the tunable lasers that are sometimes used in optical modules. These have included
Several Multi-source agreements (MSAs) have coherent optical modules.
The C form-factor pluggable (CFP) is an MSA among competing manufacturers for a common form-factor for the transmission of high-speed digital signals.
Some pluggable modules fit on top of the printed circuit board instead of on the front panel.
Long-haul Optical Transport Network (OTN) networks were the traditional users of coherent modulation. Cloud scale data centers have become an important consumer of coherent optical modules, particularly in the Ethernet connectivity space for reaches greater than 10 km where the advantages of coherent modulation can outweigh the increased cost.
The main trade show for the coherent optical module industry is the Optical Fiber Conference (OFC), that is held annually in southern California. Other prominent shows for the industry include ECOC in Europe and FOE in Japan.
Modulation is defined as the process by which some characteristics like amplitude, frequency, and phase of a carrier signal are varied in accordance with a modulating wave.
Quadrature amplitude modulation (QAM) is the name of a family of digital modulation methods and a related family of analog modulation methods widely used in modern telecommunications to transmit information. It conveys two analog message signals, or two digital bit streams, by changing (modulating) the amplitudes of two carrier waves, using the amplitude-shift keying (ASK) digital modulation scheme or amplitude modulation (AM) analog modulation scheme. The two carrier waves are of the same frequency and are out of phase with each other by 90°, a condition known as orthogonality or quadrature. The transmitted signal is created by adding the two carrier waves together. At the receiver, the two waves can be coherently separated (demodulated) because of their orthogonality. Another key property is that the modulations are low-frequency/low-bandwidth waveforms compared to the carrier frequency, which is known as the narrowband assumption.
Phase-shift keying (PSK) is a digital modulation process which conveys data by changing (modulating) the phase of a constant frequency carrier wave. The modulation is accomplished by varying the sine and cosine inputs at a precise time. It is widely used for wireless LANs, RFID and Bluetooth communication.
In fiber-optic communications, wavelength-division multiplexing (WDM) is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths of laser light. This technique enables bidirectional communications over a single strand of fiber as well as multiplication of capacity.
Fibre Channel (FC) is a high-speed data transfer protocol providing in-order, lossless delivery of raw block data. Fibre Channel is primarily used to connect computer data storage to servers in storage area networks (SAN) in commercial data centers.
Small Form-factor Pluggable (SFP) is a compact, hot-pluggable network interface module format used for both telecommunication and data communications applications. An SFP interface on networking hardware is a modular slot for a media-specific transceiver, such as for a fiber-optic cable or a copper cable. The advantage of using SFPs compared to fixed interfaces is that individual ports can be equipped with different types of transceivers as required, with the majority including optical line terminals, network cards, switches and routers.
A cable modem termination system is a piece of equipment, typically located in a cable company's headend or hubsite, which is used to provide data services, such as cable Internet or Voice over IP, to cable subscribers. A CMTS provides many of the same functions provided by the DSLAM in a DSL system.
The Optical Internetworking Forum (OIF) is a prominent non-profit consortium that was founded in 1998. It promotes the development and deployment of interoperable computer networking products and services through implementation agreements (IAs) for optical networking products and component technologies including SerDes devices.
Multi-mode optical fiber is a type of optical fiber mostly used for communication over short distances, such as within a building or on a campus. Multi-mode links can be used for data rates up to 800 Gbit/s. Multi-mode fiber has a fairly large core diameter that enables multiple light modes to be propagated and limits the maximum length of a transmission link because of modal dispersion. The standard G.651.1 defines the most widely used forms of multi-mode optical fiber.
Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of infrared or visible light through an optical fiber. The light is a form of carrier wave that is modulated to carry information. Fiber is preferred over electrical cabling when high bandwidth, long distance, or immunity to electromagnetic interference is required. This type of communication can transmit voice, video, and telemetry through local area networks or across long distances.
40 Gigabit Ethernet (40GbE) and 100 Gigabit Ethernet (100GbE) are groups of computer networking technologies for transmitting Ethernet frames at rates of 40 and 100 gigabits per second (Gbit/s), respectively. These technologies offer significantly higher speeds than 10 Gigabit Ethernet. The technology was first defined by the IEEE 802.3ba-2010 standard and later by the 802.3bg-2011, 802.3bj-2014, 802.3bm-2015, and 802.3cd-2018 standards. The first succeeding Terabit Ethernet specifications were approved in 2017.
ETSI Satellite Digital Radio describes a standard of satellite digital radio. It is an activity of the European standardisation organisation ETSI.
10 Gigabit Ethernet is a group of computer networking technologies for transmitting Ethernet frames at a rate of 10 gigabits per second. It was first defined by the IEEE 802.3ae-2002 standard. Unlike previous Ethernet standards, 10GbE defines only full-duplex point-to-point links which are generally connected by network switches; shared-medium CSMA/CD operation has not been carried over from the previous generations of Ethernet standards so half-duplex operation and repeater hubs do not exist in 10GbE. The first standard for faster 100 Gigabit Ethernet links was approved in 2010.
The C form-factor pluggable is a multi-source agreement to produce a common form-factor for the transmission of high-speed digital signals. The c stands for the Latin letter C used to express the number 100 (centum), since the standard was primarily developed for 100 Gigabit Ethernet systems.
The XFP is a standard for transceivers for high-speed computer network and telecommunication links that use optical fiber. It was defined by an industry group in 2002, along with its interface to other electrical components, which is called XFI.
Polarization-division multiplexing (PDM) is a physical layer method for multiplexing signals carried on electromagnetic waves, allowing two channels of information to be transmitted on the same carrier frequency by using waves of two orthogonal polarization states. It is used in microwave links such as satellite television downlinks to double the bandwidth by using two orthogonally polarized feed antennas in satellite dishes. It is also used in fiber optic communication by transmitting separate left and right circularly polarized light beams through the same optical fiber.
A super-channel is an evolution in dense wavelength-division multiplexing (DWDM) in which multiple, coherent optical carriers are combined to create a unified channel of a higher data rate, and which is brought into service in a single operational cycle.
FlexE, short for Flexible Ethernet, is a communications protocol published by the Optical Internetworking Forum (OIF).
The Common Electrical I/O (CEI) refers to a series of influential Interoperability Agreements (IAs) that have been published by the Optical Internetworking Forum (OIF). CEI defines the electrical and jitter requirements for 3.125, 6, 11, 25-28, and 56 Gbit/s electrical interfaces.
An optical module is a typically hot-pluggable optical transceiver used in high-bandwidth data communications applications. Optical modules typically have an electrical interface on the side that connects to the inside of the system and an optical interface on the side that connects to the outside world through a fiber optic cable. The form factor and electrical interface are often specified by an interested group using a multi-source agreement (MSA). Optical modules can either plug into a front panel socket or an on-board socket. Sometimes the optical module is replaced by an electrical interface module that implements either an active or passive electrical connection to the outside world. A large industry supports the manufacturing and use of optical modules.
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