Common knowledge of optical fiber, optical modules and optical interfaces

This article for you to share optical fiber, optical module and optical interface common knowledge, I hope to help you.

The commonly used optical modules in Ethernet switches are SFP, GBIC, XFP, and XENPAK.

Their full English name is:

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

GBIC: GigaBit Interface Converter, Gigabit Ethernet interface converter

XFP: 10-Gigabit small Form-factorPluggable transceiver 100 Gigabit Ethernet interface

Small package-pluggable transceiver

XENPAK: 10 Gigabit EtherNet TransceiverPAcKage Ethernet interface transceiver collection package

Fiber connector

The fiber connector consists of the fiber and the fiber, which consists of the pin and the peripheral locking structure. According to the different locking mechanisms, the optical fiber connectors can be divided into FC type, SC type, LC type, ST type and KTRJ type.

FC connector uses thread locking mechanism, which is the most used fiber active connector.

SC is a rectangular joint, developed by NTT, without thread connection, and can be directly inserted and pulled out. Compared with FC connector, it has small operation space and easy to use. Low-end Ethernet products are very common.

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

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

The needle of KTRJ is plastic, which is located through the steel needle. With the increase of the number of insertion and extraction, the mating surface will wear out, and the long-term stability is not as good as that of the ceramic needle connector.

Optical fiber knowledge

Optical fiber is the conductor for transmitting light waves. Optical fiber can be divided into single mode fiber and multimode fiber from the optical transmission mode.

In a single-mode fiber, there is only one base mode mode, which means that light is transmitted only along the inner core of the fiber. Because of the complete avoidance of mode scattering, the transmission band of single mode fiber is very wide, so it is suitable for high speed, long distance fiber communication.

There are multiple modes of optical transmission in multimode fibers, with poor transmission performance, narrow frequency band, small transmission rate and short distance due to dispersion or aberration.

Characteristic parameters of the optical fiber

The structure of optical fiber is prefabricated quartz optical fiber rod is pulled, and the outer diameter of multimode fiber and single mode fiber for communication is 125 μ m.

The body is divided into two regions: core (Core) and cladding (Cladding layer). The core diameter of single mode fiber is 8~10 μ m, and there are two standard fiber core specifications, the core diameter is 62.5 μ m (American standard) and 50 μ m (European standard) respectively.

Interface optical fiber specifications are described as follows: 62.5 μ m / 125 μ m multimode optical fiber, of which 62.5 μ m refers to the core diameter of the optical fiber, and 125 μ m refers to the outer diameter of the optical fiber.

The single-mode fiber uses 1310nm or 1550 nm.

The multimode fiber uses mostly 850 nm.

Single-mode fibers and multimode fibers can be distinguished in color. Single-mode fiber exterior is yellow and multi-mode fiber exterior is orange.

Gigabit light mouth

Gigabit light can work in both mandatory and self-negotiation modes.802.3 In the specification, the Gigabit light port only supports 1000M rate, and supports two duplex modes: full-duplex (Full) and half-duplex (Half).

The most fundamental difference between self-negotiation and coercion is that the code stream is different when the physical link is established. Self-negotiation mode sends / C / code, that is, configuration (Configuration) code stream, while forced mode sends / I / code, that is, dle code stream.

Gigabit light mouth from the negotiation process

1. Both ends are set to the self-negotiation mode

Both parties send / C / code streams to each other. If they receive 3 same / C / code and the received code stream matches the working mode of the terminal, they will return to the other a / C / code with the Ack response. After receiving the Ack information, the two can communicate and set the port to UP state.

2. One end is set to self-negotiation, and one end is set to force

Send / C / code stream from the negotiation end, and send / I / code stream from the forced end. The forced end cannot provide the negotiation information to the opposite end, and cannot return the Ack response to the opposite end, so the DOWN from the negotiation end. However, the forced end itself can recognize / C / code, and think that the opposite end is a matching port, so set the local port directly to UP state.

3. Both ends are set to mandatory mode

Both parties send / I / code stream to each other. After receiving / I / code stream at one end, the opposite end is the port matching with them, and directly set the terminal port as UP state.

How does the optical fiber work?

The optical fiber for communication is composed of a thin glass wire covered with a plastic protective layer. The glass wire essentially consists of two parts: a glass material with a core diameter of 9 to 62.5 μ m and a diameter of 125 μ m. Although there are some other types of optical fiber according to the materials used and different sizes, the most common ones are mentioned here. Light is transmitted in the form of "full internal reflection" in the core layer of the fiber, that is, after the light enters one end of the fiber, it reflects back and forth between the core layer and the cladding interface, and then transmits to the other end of the fiber. An optical fiber with core diameter of 62.5 μ m and outer diameter of 125 μ m is called 62.5/125μ m light.

The difference between multi-mode and single-mode fiber?

Multi-mode:

Fiber fibers that can spread hundreds to thousands of modes, called multimode (MM) fibers. According to the radial distribution of refractive index in fiber core and cladding, it can be divided into step multimode fiber and gradient multimode fiber. Almost all multimode fibers are sized at 50 / 125 μ m or 62.5/125μ m, and the bandwidth (information transmission of the fiber) is typically 200 MHz to 2GHz. Multimode optical terminal machine can transmit for up to 5 kilometers through multimode optical fiber. A light-emitting diode or laser is used as the light source.

Single-mode:

A fiber that can propagate only one mode is called a single-mode fiber. The standard single-mode (SM) fiber refractive index distribution is similar to the step fiber, but the fiber core diameter is much smaller than the multimode fiber.

The size of the single-mode fiber is 9-10 / 125 μ m, and it has the characteristics of infinite bandwidth and lower loss than the multimode fiber. The single-mode optical terminal machine is mostly used for long-distance transmission, sometimes reaching 150 to 200 kilometers. LD or LED with narrow spectral lines was used as the light source.

Difference and connection:

Single-mode equipment can usually operate on both single-mode fibers, while multi-mode equipment is limited to operating on multimode fibers.

What is the transmission loss when using the optical cable?

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

850nm wavelength when used for multi-mode fiber: 3.0 dB / km

When 1310nm wavelength is used for multi-mode fiber: 1.0 dB / km

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

1,550 nm wavelength when used for single-mode optical fiber: 0.2 dB / km

What is a GBIC?

GBIC is short for Giga Bitrate Interface Converter, an interface device that transforms gigabit electrical signals into optical signals. The GBIC can be designed for hot plug use. GBIC is an interchangeable product that meets international standards. The gigabit switch designed with GBIC interface occupies a large market share in the market due to its flexible exchange.

What is SFP?

SFP is short for SMALL FORM PLUGGABLE and can be simply understood as an upgraded version of GBIC. The SFP module volume is reduced by half compared to the GBIC module, and more than double more ports can be configured on the same panel. Other functions of the SFP module are essentially consistent with the GBIC. Some switch vendors call the SFP module a miniaturized GBIC (MINI-GBIC).

The future optical module must support hot plug, that is, without the need to cut off the power supply, the module can be connected or disconnected from the device. Because the optical module is hot plug, the network manager can upgrade and expand the system without shutting down the network, which will have no impact on online users. Hot swappability also simplifies overall maintenance efforts and allows end-users to better manage their transceiver modules. At the same time, due to this heat exchange performance, the module enables the network managers to overplan the delivery cost, link distance, and all the network topologies according to the network upgrade requirements, without the full replacement of the system board. The optical module supporting this hot plug currently has GBIC and SFP. Since the appearance of SFP and SFF is similar in size, it can be directly inserted into the circuit board, which saves space and time in packaging, and has a wide application range. 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 optical and circuit integration technology, which is only half the size of ordinary duplex SC (1X9) optical fiber transceiver module. Double the number of optical ports can be increased in the same space, which can increase the density of line ports, and reduce the system cost per port. In addition, the SFF small package module adopts the KT-RJ interface similar to the copper wire network, which is the same size as the common copper wire interface of the computer network, which is conducive to the transition of the existing copper cable network equipment to the faster optical fiber network to meet the rapid growth of network bandwidth demand.

Network connection device interface type

BNC joggle

BNC interface refers to the coaxial cable interface, BNC interface is used for 75 euro coaxial cable connection, providing two channels of receiving (RX) and hair (TX), it is used for the connection of non-equilibrium signals.

Fiber interface

A fiber optic interface is a physical interface used to connect fiber optic cables. There are usually several types such as SC, ST, LC, FC. For the 10Base-F connection, the connector is usually of the ST type, and the other end FC is connected to a fiber optic cable frame. FC is short for FerruleConnector, its external reinforcement mode is metal cover, fastening mode is screw buckle. The ST interface is usually used for 10Base-F, the SC interface is usually used for 100Base-FX and GBIC, and the LC is usually used for SFP.

The RJ-45 interface

The RJ-45 interface is the most commonly used interface in Ethernet. The RJ-45 is a common name that refers to a modular jack or plug standardized by IEC (60) 603-7 using 8 positions (8 pins) defined by the international connector standard.

The RS-232 interface

The RS-232-C interface (also known as EIA RS-232-C) is the most commonly used serial communication interface. It was a standard for serial communication jointly developed by the 1970 association of the American Electronics Industry Association (EIA) and Bell Systems, modem manufacturers and computer terminal manufacturers. Its full name is the "Technical Standard for Serial Binary Data Exchange Interface between Data Terminal Devices (DTE) and Data Communication Devices (DCE)". The standard specifies a DB 25 connector with 25 feet, stipulating the signal content of each pin of the connector, and also defining the level of various signals.

The RJ-11 interface

The RJ-11 interface is what we usually call a telephone line interface. RJ-11 is the common name for the connector developed for Western Electronics (Western Electric). Its shape is defined as a 6-pin connecting device. Originally called WEx W, x means "active," contact or needle. For example, WE 6 W has all 6 contacts, number 1 to 6, WE 4 W interface only uses 4 pins, the outer two contacts (1 and 6) are not used, and WE 2 W only uses the middle two pins (i. e., the telephone line interface).

CWDM and DWDM

With the rapid growth of Internet's IP data service, the demand for transmission line bandwidth is constantly increasing. Although DWDM (dense wavelength multiplexing) technology is the most effective way to solve line bandwidth expansion, CWDM (coarse wavelength multiplexing) technology has advantages over DWDM in terms of system cost, maintainability and so on.

Both CWDM and DWDM belong to the wave division multiplexing technology, which can couple the light of different wavelengths into a single core fiber and transmit it together.

The latest ITU standard for CWDM is G.695, which specifies 18 wavelength channels of 20nm interval from 1271nm to 1611nm. 16 channels are generally used considering the water peak effect of ordinary G.652 fiber. Because of the large channel interval, split devices and lasers are cheaper than DWDM devices.

The channel interval of DWDM has different intervals such as 0.4nm,0.8nm,1.6nm according to the need, which has a small interval and requires additional wavelength control devices. Therefore, the price of the equipment based on DWDM technology is higher than the equipment based on CWDM technology.

PIN photodiode is a layer of light doping N-type material, called I (Intrinsic, intrinsic) layer. Due to the light doping, the electron concentration is very low, and a very wide depletion layer is formed after diffusion, which can improve its response speed and conversion efficiency.

APD avalanche photodiode, which not only has the light / electric conversion effect, but also has the internal amplification effect, its amplification effect is completed by the avalanche multiplication effect inside the pipe. APD is a photodiode with gain. So the APD is beneficial to extend the transmission distance of the system.