Basic Knowledge of Optical Fiber, Optical Transceiver and Optical Interface
Optical Transceiver
Optical modules commonly adopted in Ethernet switches are SFP, GBIC, XFP, and XENPAK. Their full name is that:
SFP: Small Form-factor Pluggable transceiver
GBIC：GigaBit Interface Converter
XFP:10-Gigabit small Form-factor Pluggable transceiver
XENPAK:10 Gigabit Ethernet Transceiver Package
 What is GBIC ?
GBIC is short for Giga Bitrate Interface Converter, which is an interface device for converting gigabit electrical signals into optical signals. The GBIC can be designed for hot swapping, which is an interchangeable product meeting international standards. Gigabit switches designed with GBIC interface have a large market share in the market due to flexible interchangeability.
What is SFP ?
SFP is short for SMALL FORM PLUGGABLE, which can be simply understood as an upgraded version of GBIC. The SFP module is half the size of the GBIC module and can be configured with more than double the number of ports on the same panel. The other functions of the SFP module are basically the same as the GBIC. Some switch manufacturers call the SFP module a miniaturized GBIC (MINI-GBIC). Future optical modules must support hot swapping, that is, modules can be connected or disconnected from the device without powering off. Since the optical modules are hot-swappable, network administrators can upgrade and expand the system without shutting down the network. Users will not have any effect. Hotpluggingalso 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 plan the total cost of transmission and reception, link distance, and all network topologies based on network upgrade requirements without having to replace the system board. The hot-swappable optical modules currently have GBIC and SFP. Since the SFP and SFF are similar in size, they can be directly inserted on the circuit board, which saves space and time on the package, and has a wide application range. Its future development is worth looking forward to, and may even threaten the SFF market.
What is SFF ?
The SFF (Small Form Factor) small package optical module adopts advanced precision optics and circuit integration technology. It is half the size of the ordinary duplex SC (1X9) type optical transceiver module, and 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 uses a MT-RJ interface similar to the copper network, the size is the same as the common computer network copper interface, which is beneficial to the transition of the existing copper-based network equipment to a higher-speed optical network. To meet the dramatic increase in network bandwidth demand.
Fiber Optical Connector
The fiber connector is composed of a fiber and a plug at both ends of the fiber, and the plug is composed of a ceramic sleeve and a peripheral locking structure. According to different locking mechanisms, fiber connectors can be classified into FC type, SC type, LC type, ST type and KTRJ type.
The FC connector adopts a thread locking mechanism and is an optical fiber movable connector which is the earliest and most used invention.
SC is a rectangular joint developed by NTT. It can be directly inserted and removed without thread connection, which is commonly used at low-end ethernet cabling. Compared with FC connector.
LC is a Mini type SC connector developed by LUCENT. It has a smaller size and has been widely used in the system, which is commonly used at low-end ethernet cabling. It is a direction for the development of fiber optic connectors in the future.
The ST connector is developed by AT&T. It uses a bayonet locking mechanism. The main parameters are equivalent to FC and SC connectors, but they are not common in the company. They are usually used in multimode devices to connect with other manufacturers. Use more when docking.
MTRJ’s ferrule are made of plastic and are positioned by steel pins. As the number of insertions and removals increases, the mating surfaces wear and wear, and the long-term stability is not as good as the ceramic pin connectors.
Fiber Optic Basic Knowledge
An optical fiber is a conductor that transmits light waves. Optical fiber can be divided into singlemode fiber and multimode fiber according to the mode of optical transmission.
In single-mode fiber, the light transmission has only one fundamental mode, which means that light is transmitted only along the inner core of the fiber. Since the mode dispersion is completely avoided, the single-mode fiber has a wide transmission band and is suitable for high-speed, long-distance fiber communication.
In multimode fiber, there are multiple modes of optical transmission. Due to dispersion or aberration, transmission performance of such an optical fiber is poor, the frequency band is narrow, the transmission rate is small, and the distance is short.
â… . How does fiber work?
The optical fiber for communication consists of a thin, hair-like glass filament covered with a plastic protective layer. The glass filament consists essentially of two parts: a core diameter of 9 to 62.5 μm and a low refractive index glass material having a diameter of 125 μm. Although there are some other types of fibers depending on the materials used and the different sizes, the most common ones are mentioned here. Light is transmitted in the “total internal reflection” mode of the core portion of the optical fiber, that is, after the light enters one end of the optical fiber, it is reflected back and forth between the core layer and the cladding interface, and then transmitted to the other end of the optical fiber. The fiber with a core diameter of 62.5 μm and a cladding outer diameter of 125 μm is called 62.5/125 μm.
â…¡. What is the difference between multimode and singlemode fiber?
Multimode fiber: Fibers that can travel from hundreds to thousands of modes are called multimode (MM) fibers. According to the radial distribution of the refractive index in the core and the cladding, it can be further divided into step multimode fiber and gradual multimode fiber. Almost all multimode fibers are 50/125 μm or 62.5/125 μm in size, and the bandwidth (the amount of information transmitted by the fiber) is usually 200 MHz to 2 GHz. Multimode optical transceivers can carry up to 5 kilometers of transmission over multimode fiber. Use a light-emitting diode or laser as a light source.
Singlemode fiber: A fiber that can only propagate one mode is called a single mode fiber. The standard single mode (SM) fiber refractive index profile is similar to the step fiber, except that the core diameter is much smaller than the multimode fiber.     The size of the single mode fiber is 9-10/125 μm and has an infinite bandwidth and lower loss characteristics than the multimode fiber. Single-mode optical transceivers are often used for long-distance transmission, sometimes reaching 150 to 200 kilometers. LEDs with narrower LD or spectral lines are used as the light source.
Difference and connection: Single-mode devices typically operate on both single-mode fibers and multimode fibers, while multimode devices are limited to operation on multimode fibers.
â…¢. What is the transmission loss when using fiber optic cable?
This depends on the wavelength of the transmitted light and the type of fiber used.
When the 850 nm wavelength is used for multimode fiber: 3.0 dB/km    When the 1310 nm wavelength is used for multimode fiber: 1.0 dB/km    When the 1310nm wavelength is used for single mode fiber: 0.4 dB / km    When the 1550 nm wavelength is used for single mode fiber: 0.2 dB/km
â…£. Optical fiber characteristic parameters
The structure of the optical fiber is prefabricated by a quartz fiber rod, and the outer diameter of the multimode fiber and the single mode fiber for communication are both 125 μm.
The slimming is divided into two areas: the core and the Cladding layer. The single-mode fiber core has a core diameter of 8~10μm. The multimode fiber core diameter has two standard specifications, and the core diameter is 62.5μm (US standard) and 50μm (European standard).
The fiber interface specification has such a description: 62.5 μm / 125 μm multimode fiber, of which 62.5 μm refers to the core diameter of the fiber, and 125 μm refers to the outer diameter of the fiber. The single mode fiber uses a light wavelength of 1310 nm or 1550 nm. Multimode fibers use a wavelength of 850 nm. Single mode fiber and multimode fiber can be distinguished in color. The single-mode fiber outer body is yellow, and the multimode fiber outer body is orange.
Gigabit Optical Auto-Negotiation Port
Gigabit optical ports can work in both forced and auto-negotiated modes. In the 802.3 specifications, the Gigabit optical port supports only 1000M speed and supports full-duplex (Full) and half-duplex (Half) duplex modes. The most fundamental difference between auto-negotiation and coercion is that the code stream sent when the two establish a physical link is different. The auto-negotiation mode sends the /C/ code, which is the configuration code stream, and the forced mode sends / I / code, which is the idle stream.
Gigabit Optical Port Auto-Negotiation Process
â… Both ends are set to auto-negotiation mode
The two parties send each other/C/code stream. If three identical /C/codes are received consecutively and the received code stream matches the working mode of the local end, the other party returns a /C/ code with an Ack response. After receiving the Ack information, the peer considers that the two can communicate with each other and set the port to the UP state.
â…¡One end is set to auto-negotiate and one end is set to mandatory
The auto-negotiation end sends a /C/stream, and the forced end sends the /I/stream. The forcing end cannot provide the peer with the negotiation information of the local end, and cannot return the Ack response to the peer. Therefore, the auto-negotiation terminal DOWN. However, the forcing end itself can recognize the /C/code, and consider that the peer end is a port that matches itself, so directly set the local port to the UP state.
â…¢ Both ends are set to forced modeThe two parties send each other/I/streams. After receiving the /I/stream, the peer considers that the peer is the port that matches the peer.
Network connection device interface type
The BNC interface refers to the coaxial cable interface. The BNC interface is used for the 75-ohm coaxial cable connection. It provides two channels of receiving (RX) and transmitting (TX), which are used for the connection of unbalanced signals.
The fiber interface is a physical interface used to connect fiber optic cables. There are usually several types such as SC, ST, LC, and FC. For a 10Base-F connection, the connector is usually of the ST type, and the other end of the FC is connected to the fiber optic step frame. FC is the abbreviation of Ferrule Connector. The external reinforcement method is metal sleeve and the fastening method 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 is the most commonly used interface for Ethernet. RJ-45 is a common name, which is standardized by IEC (60) 603-7 and uses 8 positions (8 pins) defined by international connector standards. Modular jack or plug.
The RS-232-C interface (also known as EIA RS-232-C) is the most commonly used serial communication interface. It was developed in 1970 by the Electronic Industries Association (EIA) in conjunction with Bell Systems, modem manufacturers, and computer terminal manufacturers for serial communication standards. Its full name is “the serial binary data exchange interface technology standard between data terminal equipment (DTE) and data communication equipment (DCE)”. The standard specifies a 25-pin DB25 connector that specifies the signal content of each pin of the connector and also specifies the level of each signal.
The RJ-11 interface is what we usually call the telephone line interface. RJ-11 is the generic name for connectors developed by Western Electric. Its shape is defined as a 6-pin connector. Formerly known as WExW, where x means “active”, contact or wire. For example, WE6W has all 6 contacts, numbered 1 to 6, the WE4W interface uses only 4 pins, the outermost two contacts (1 and 6) are not used, and WE2W uses only the middle two pins (ie for telephone line interface).
CWDM and DWDM
With the rapid growth of IP data services on the Internet, the demand for transmission line bandwidth is increasing. Although DWDM (Dense Wavelength Division Multiplexing) technology is the most effective method to solve line bandwidth expansion, CWDM (Coarse Wavelength Division Multiplexing) technology has advantages in system cost and maintainability compared to DWDM.
Both CWDM and DWDM belong to the wavelength division multiplexing technology, which can couple different wavelengths of light into a single-core fiber and transmit them together. The latest ITU standard for CWDM is G.695, which specifies 18 wavelength channels with a spacing of 20 nm from 1271 nm to 1611 nm. Considering the water peak effect of ordinary G.652 fiber, 16 channels are generally used. Because the channel spacing is large, both the multiplexer and the laser are cheaper than the DWDM device.T he channel spacing of DWDM is 0.4 nm, 0.8 nm, 1.6 nm, etc., and the interval is small, requiring additional wavelength control devices. Therefore, devices based on DWDM technology are more expensive than devices based on CWDM technology.
PIN and APD
The PIN photodiode is between a P-type and N-type semiconductor with a high doping concentration, and a lightly doped N-type material called an I (Intrinsic, intrinsic) layer. Due to the light doping, the electron concentration is very low, and after diffusion, a wide depletion layer is formed, which can improve the response speed and conversion efficiency.
APD avalanche photodiode, which not only has optical/electrical conversion, but also has internal amplification, and its amplification is achieved 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 extend the transmission distance of the system.
