Common knowledge of optical fibers, optical modules and optical interfaces


In this article, ZR Fiber shares the common knowledge of optical fibers, optical modules and optical interfaces, and hopes to be helpful to everyone.

Optical modules commonly used in Ethernet switches include SFP, GBIC, XFP, and XENPAK.


Their full English names:

SFP: Small Form-factor Pluggabletransceiver, small package pluggable transceiver

GBIC: GigaBit Interface Converter, Gigabit Ethernet Interface Converter

XFP: 10-Gigabit small Form-factorPluggable transceiver 10 Gigabit Ethernet interface

Small Package Pluggable Transceivers

XENPAK: 10 Gigabit EtherNet TransceiverPAcKage 10 Gigabit Ethernet Interface Transceiver Collection Package


The optical fiber connector

The optical fiber connector is composed of an optical fiber and a plug at both ends of the optical fiber, and the plug is composed of a pin and a peripheral locking structure. According to different locking mechanisms, fiber optic connectors can be divided into FC type, SC type, LC type, ST type and KTRJ type.


The FC connector adopts a threaded locking mechanism, which is an early and most used optical fiber active connector.


SC is a rectangular connector developed by NTT. It does not need screw connection and can be plugged directly. Compared with the FC connector, it has a small operating space and is easy to use. Low-end Ethernet products are very common.


LC is a Mini-type SC connector developed by LUCENT. It has a smaller volume and has been widely used in the system. It is a direction for the development of optical fiber active connectors in the future. Low-end Ethernet products are very common.


ST connector is developed by AT&T company, with bayonet locking mechanism, the main parameters and indicators are equivalent to FC and SC connectors, but it is not widely used in the company, usually used in multi-mode device connection, and other manufacturers equipment Used more when docking.


The pins of KTRJ are made of plastic and are positioned by steel pins. With the increase of insertion and extraction times, each mating surface will wear out, and the long-term stability is not as good as that of ceramic pin connectors.


Fiber knowledge

Optical fibers are conductors that transmit light waves. Optical fibers can be divided into single-mode fibers and multi-mode fibers from the mode of optical transmission.


There is only one fundamental mode of light transmission in a single-mode fiber, which means that light travels only along the inner core of the fiber. Due to the complete avoidance of modal scattering, the transmission frequency band of single-mode fiber is very wide, so it is suitable for high-speed, long-distance fiber communication.


There are multiple modes of light transmission in multimode fiber. Due to dispersion or aberration, the transmission performance of this fiber is poor, the frequency band is narrow, the transmission rate is small, and the distance is short.


Characteristics of optical fibers

The structure of the optical fiber is drawn from a prefabricated silica optical fiber rod, and the outer diameter of the multi-mode optical fiber and single-mode optical fiber for communication is 125 μm.

FTTH Cable

The fiber body is divided into two regions: the core (Core) and the cladding layer (Cladding layer). The core diameter of single-mode fiber is 8~10μm, and the core diameter of multi-mode fiber has two standard specifications, the core diameter is 62.5μm (American standard) and 50μm (European standard).


The interface fiber specification has the following description: 62.5μm/125μm multimode fiber, where 62.5μm refers to the core diameter of the fiber, and 125μm refers to the outer diameter of the fiber.


Single-mode fibers use light wavelengths of 1310 nm or 1550 nm.


The wavelength of light used in multimode fibers is mostly 850 nm.


Single-mode fiber and multi-mode fiber can be distinguished by color. The outer body of single-mode fiber is yellow, and the outer body of multi-mode fiber is orange-red.


Gigabit optical port

The Gigabit optical port can work in two modes: forced and auto-negotiation. In the 802.3 specification, the Gigabit optical port only supports 1000M speed, and supports two duplex modes: Full and Half.


The most fundamental difference between auto-negotiation and forcing is that the code streams sent when the two establish a physical link are different. The auto-negotiation mode sends the /C/ code, that is, the configuration code stream, while the mandatory mode sends the /C/ code. I/code, that is, the idle code stream.


Gigabit optical port auto-negotiation process

1. Both ends are set to auto-negotiation mode

The two parties send /C/ code streams to each other. If three consecutive /C/ codes are received and the received code streams match the working mode of the local end, a /C/ code with an Ack response will be returned to the other party. After receiving the Ack information, the peer end believes that the two can communicate with each other, and sets the port to the UP state.


2. One end is set to auto-negotiation, and one end is set to mandatory

The auto-negotiation end sends the /C/ code stream, and the forced end sends the /I/ code stream. The forced end cannot provide the peer end with the negotiation information of the local end, and cannot return the Ack response to the opposite end, so the auto-negotiation end is Down. However, the forced end itself can recognize the /C/ code and think that the opposite end is the port that matches itself, so directly set the local port to the UP state


3. Both ends are set to mandatory mode

Both parties send /I/ stream to each other. After one end receives the /I/ stream, it considers that the other end is the port that matches itself, and directly sets the local port to the UP state.


How does fiber optics work?

Optical fibers for communication are composed of hair-thin glass filaments covered with a plastic protective layer. The glass filament consists essentially of two parts: a core with a diameter of 9 to 62.5 μm and an outer cladding of a low refractive index glass material with a diameter of 125 μm. Although there are other types of fibers depending on the materials used and different sizes, the most common ones are mentioned here. Light is transmitted in the core part of the fiber by "total internal reflection", that is, after entering one end of the fiber, light is reflected back and forth between the interface between the core and the cladding, and then transmitted to the other end of the fiber. A fiber with a core diameter of 62.5 μm and a cladding outer diameter of 125 μm is called 62.5/125 μm light.


What is the difference between multimode and singlemode fiber?


Fibers that can propagate hundreds to thousands of modes are called multimode (MM) fibers. According to the radial distribution of refractive index in the core and cladding, it can be divided into step multimode fiber and graded multimode fiber. Almost all multimode fibers are 50/125μm or 62.5/125μm in size, and the bandwidth (the amount of information transmitted by the fiber) is typically 200MHz to 2GHz. Multimode optical transceivers can transmit up to 5 kilometers through multimode optical fibers. Light-emitting diodes or lasers are used as light sources.


Single mode:

Fibers that can only propagate one mode are called single-mode fibers. Standard single-mode (SM) fiber has a refractive index profile similar to that of step-type fiber, except that the core diameter is much smaller than that of multimode fiber.


Single-mode fiber is 9-10/125 μm in size and has unlimited bandwidth and lower loss than multimode fiber. The single-mode optical transceiver is mostly used for long-distance transmission, sometimes reaching 150 to 200 kilometers. LDs or LEDs with narrow spectral lines are used as light sources.


Differences and connections:

Single-mode equipment typically operates on both single-mode fiber and multi-mode fiber, while multi-mode equipment is limited to operation on multi-mode fiber.


How about transmission loss when using fiber optic cable?

It depends on the wavelength of the transmitted light and the type of fiber used.


When 850nm wavelength is used for multimode fiber: 3.0dB/km


When 1310nm wavelength is used for multimode fiber: 1.0dB/km


When 1310nm wavelength is used for single mode fiber: 0.4dB/km


When 1550nm wavelength is used for single mode fiber: 0.2dB/km


What is GBIC?

GBIC is the abbreviation of Giga Bitrate Interface Converter, which is an interface device that converts gigabit electrical signals into optical signals. The GBIC is designed to be hot-swappable. GBIC is an interchangeable product that complies with international standards. Gigabit switches designed with GBIC interface occupy a large market share in the market due to their flexible interchangeability.


What is SFP?

SFP is the abbreviation of SMALL FORM PLUGGABLE, which can be simply understood as an upgraded version of GBIC. The volume of the SFP module is reduced by half compared to the GBIC module, and more than double the number of ports can be configured on the same panel. The other functions of the SFP module are basically the same as those of the GBIC. Some switch manufacturers call the SFP module a miniaturized GBIC (MINI-GBIC).


Future optical modules must support hot swap, that is, the module can be connected or disconnected from the device without cutting off the power supply. Since the optical module is hot-swappable, network managers can upgrade and expand the system without shutting down the network. Users will have no impact. Hot swappability also simplifies overall maintenance and enables end users to better manage their transceiver modules. At the same time, due to this hot-swappable performance, the module enables network managers to make overall plans for transceiver costs, link distances, and all network topologies based on network upgrade requirements without having to replace the entire system board. The optical modules that support this hot swap currently include GBIC and SFP. Since SFP and SFF are similar in size, they can be directly inserted on the circuit board, saving space and time in packaging, and have a wide range of applications. Therefore, Its future development is worth looking forward to, and may even threaten the SFF market.


What is SFF?

SFF (Small Form Factor) small package optical module adopts advanced precision optics and circuit integration technology, and its size is only half of that of ordinary duplex SC (1X9) optical fiber transceiver module. It can double the number of optical ports in the same space. Increase line port density and reduce system cost per port. And because the SFF small package module adopts the KT-RJ interface similar to the copper network, the size is the same as that of the common computer network copper interface, which is conducive to the transition of the existing copper-based network equipment to the higher-speed optical fiber network. To meet the dramatic increase in network bandwidth demand.


Network connection device interface type

BNC interface

The BNC interface refers to the coaxial cable interface. The BNC interface is used for 75 ohm coaxial cable connection. It provides two channels of receiving (RX) and transmitting (TX), which are used for the connection of unbalanced signals.


Fiber interface

A fiber optic interface is a physical interface used to connect fiber optic cables. There are usually SC, ST, LC, FC and other types. For 10Base-F connection, the connector is usually ST type, and the other end of the FC is connected to the fiber optic cable rack. FC is the abbreviation of FerruleConnector. Its external reinforcement method is a metal sleeve, and the fastening method is a turnbuckle. ST interface is usually used for 10Base-F, SC interface is usually used for 100Base-FX and GBIC, LC is usually used for SFP.


RJ-45 interface

The RJ-45 interface is the most commonly used interface for Ethernet. RJ-45 is a common name, which refers to the standardization by IEC (60) 603-7 and uses 8 positions (8 pins) defined by the international connector standard. modular jacks or plugs.


RS-232 interface

RS-232-C interface (also known as EIA RS-232-C) is the most commonly used serial communication interface. It is a standard for serial communication jointly developed by the Electronic Industries Association (EIA) of the United States in 1970 in conjunction with Bell Systems, modem manufacturers and computer terminal manufacturers. Its full name is "Serial Binary Data Exchange Interface Technical Standard between Data Terminal Equipment (DTE) and Data Communication Equipment (DCE)". The standard stipulates that a 25-pin DB25 connector is used, and the signal content of each pin of the connector is specified, and the level of various signals is also specified.


RJ-11 interface

The RJ-11 interface is what we usually call the telephone line interface. RJ-11 is the generic name used for connectors developed by Western Electric. Its shape is defined as a 6-pin connection device. Formerly known as WExW, the x here means "active", a contact or a needle. For example, WE6W has all 6 contacts, numbered 1 to 6, WE4W interface only uses 4 pins, the outermost two contacts (1 and 6) are not used, WE2W only uses the middle two pins (that is, the telephone line interface).



With the rapid growth of Internet IP data services, the demand for bandwidth of transmission lines is increasing. Although DWDM (Dense Wavelength Division Multiplexing) technology is the most effective way to solve line bandwidth expansion, CWDM (Coarse Wavelength Division Multiplexing) technology has advantages over DWDM in terms of system cost and maintainability.


Both CWDM and DWDM belong to wavelength division multiplexing technologies, which can couple light of different wavelengths into a single-core fiber and transmit them together.


The latest ITU standard for CWDM is G.695, which specifies 18 wavelength channels with an interval of 20nm from 1271nm to 1611nm. Considering the influence of the water peak of ordinary G.652 fibers, 16 channels are generally used. Because the channel interval is large, the multiplexer and the laser are cheaper than DWDM devices.


The channel spacing of DWDM has different spacings such as 0.4nm, 0.8nm, and 1.6nm according to the needs. The spacing is small and additional wavelength control devices are required, so the equipment based on DWDM technology is more expensive than equipment based on CWDM technology.


The PIN photodiode is a layer of lightly doped N-type material, called I (Intrinsic, intrinsic) layer, between the highly doped P-type and N-type semiconductors. Because it is lightly doped, the electron concentration is very low, and a wide depletion layer is formed after diffusion, which can improve its response speed and conversion efficiency.


The APD avalanche photodiode not only has the function of light/electrical conversion, but also has an internal amplification effect, and its amplification effect is completed by the avalanche multiplication effect inside the tube. APD is a photodiode with gain. In the case where the sensitivity of the optical receiver is high, the use of APD is beneficial to prolong the transmission distance of the system.


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