Basic knowledge of optical modules
1. Definition:
Optical module: that is, an integrated optical transceiver module.
2. Structure:
The integrated optical transceiver module is composed of optoelectronic devices, functional circuits and optical interfaces. The optoelectronic devices include two parts: transmitting and receiving.
The transmitting part is: the input electrical signal with a certain code rate is processed by the internal driver chip to drive the semiconductor laser (LD) or light emitting diode (LED) to emit a modulated optical signal of the corresponding rate. The output optical signal power remains stable.
The receiving part is: the optical signal with a certain code rate is converted into an electrical signal by the light detection diode after being input into the module. After the preamplifier, the electrical signal of the corresponding code rate is output, and the output signal is generally PECL level. At the same time, when the input optical power is less than a certain value, an alarm signal will be output.
3. Parameters and significance of optical modules
Optical modules have many important optoelectronic technical parameters, but for the two types of hot-swappable optical modules, GBIC and SFP, the following three parameters are most concerned when selecting:
1) Center wavelength
The unit is nanometer (nm), and there are currently three main types:
850nm (MM, multimode, low cost but short transmission distance, generally only 500M can be transmitted); 1310nm (SM, single mode, large loss but small dispersion during transmission, generally used for transmission within 40KM);
1550nm (SM, single-mode, small loss but large dispersion during transmission, generally used for long-distance transmission over 40KM, up to 120KM can be directly transmitted without relay);
2) Transmission rate
The number of bits (bits) of data transmitted per second, in bps.
There are currently four commonly used: 155Mbps, 1.25Gbps, 2.5Gbps, 10Gbps and so on. The transmission rate is generally backward compatible, so the 155M optical module is also called FE (100M) optical module, and the 1.25G optical module is also called GE (Gigabit) optical module, which are the most widely used modules in optical transmission equipment at present. In addition, it has 2Gbps, 4Gbps and 8Gbps transmission rates in the optical storage system (SAN).
3) Transmission distance
The distance that an optical signal can be directly transmitted without repeater amplification, in kilometers (also known as kilometers, km). Optical modules generally have the following specifications: multi-mode 550m, single-mode 15km, 40km, 80km and 120km and so on.
In addition to the above three main technical parameters (wavelength, speed, distance), the optical module has the following basic concepts, which only need a simple understanding.
a. Laser category
The laser is the core device in the optical module. It injects current into the semiconductor material and emits laser light through the photon oscillation and gain of the resonator. At present, the most commonly used lasers are FP and DFB lasers. The difference between them is the semiconductor material and cavity structure. The price of DFB lasers is much more expensive than that of FP lasers. Optical modules with a transmission distance of less than 40KM generally use FP lasers; optical modules with a transmission distance of ≥40KM generally use DFB lasers.
b, loss and dispersion
Loss is the loss of light energy due to absorption, scattering and leakage of the medium when light is transmitted in the optical fiber. This part of the energy is dissipated at a certain rate with the increase of the transmission distance. Dispersion is mainly caused by the fact that electromagnetic waves of different wavelengths travel in the same medium at different speeds, resulting in different wavelength components of the optical signal reaching the receiving end at different times due to the accumulation of transmission distances, resulting in pulse broadening and inability to distinguish the signal. value. These two parameters mainly affect the transmission distance of the optical module. In the actual application process, the link loss of the 1310nm optical module is generally calculated at 0.35dBm/km, and the link loss of the 1550nm optical module is generally calculated at .20dBm/km. The calculation of the dispersion value Very complex, generally for reference only.
c. Transmitting optical power and receiving sensitivity
Transmitting optical power refers to the output optical power of the light source at the transmitting end of the optical module, and receiving sensitivity refers to the minimum received optical power of the optical module at a certain rate and bit error rate. The units of these two parameters are both dBm (meaning decibel milliwatts, the logarithmic form of the power unit mw, the calculation formula is 10lg, and 1mw is converted to 0dBm), which are mainly used to define the transmission distance of the product, different wavelengths, transmission rates and The optical transmission power and receiving sensitivity of the optical module of the transmission distance will be different, as long as the transmission distance can be ensured.
d. The service life of the optical module
Internationally unified standard, 7Х24 hours of uninterrupted work for 50,000 hours (equivalent to 5 years). e. Optical fiber interface
SFP optical modules are all LC interfaces, GBIC optical modules are SC interfaces, and other interfaces include FC and ST.
ZR Cable Optical Module
Optical module classification
1. Classification by application
Ethernet application rate: 100Base (100M), 1000Base (Gigabit), 10GE. SDH application rate: 155M, 622M, 2.5G, 10G.
2. Classification by package
According to the package: 1&TImes;9, SFF, SFP, GBIC, XENPAK, XFP.
1&TImes;9 package——welding optical module, the general speed is not higher than gigabit, and SC interface is mostly used.
SFF package--soldering small package optical modules, the general speed is not higher than gigabit, and LC interfaces are mostly used.
GBIC package - hot-swappable Gigabit interface optical module, using SC interface.
SFP package - hot-swappable small package module, currently the highest data rate can reach 4G, mostly using LC interface.
XENPAK package - used in 10 Gigabit Ethernet, using SC interface.
XFP package - 10G optical module, can be used in 10 Gigabit Ethernet, SONET and other systems, mostly using LC interface.
3. Classification by laser
LED, VCSEL, FP LD, DFB L.
4. Classification by wavelength
850nm, 1310nm, 1550nm, etc.
5. Classification by usage
Non-hot-pluggable (1&TImes;9, SFF), hot-pluggable (GBIC, SFP, XENPAK, XFP).
With the increasing demand for bandwidth, the network has developed rapidly. 100G is an important part of 100G network, and now several standards and package types have emerged.
Types of 100G Optical Modules
According to different packaging methods, 100G optical modules mainly include CFP/CFP2/CFP4, CXP and Q28. Among them, CFP/CFP2/CFP4 and CXP are the packaging methods of early 100G optical modules, and QSFP28 is a new generation of 100G optical modules. It has become the mainstream packaging of 100G optical modules. The principle of 100G QSFP28 optical module is similar to that of QSFP+ optical module, which transmits 100G optical signals in a 4&TImes;
Basic Principles of Optical Modules
Optical Transceiver
The integrated optical transceiver module is the core device of optical communication, which completes the optical-electrical/electrical-optical conversion of optical signals. It consists of two parts: the receiving part and the transmitting part. The receiving part realizes the photo-electric conversion, and the transmitting part realizes the electric-optic conversion.
Launch part:
The input electrical signal with a certain code rate is processed by the internal driver chip to drive the semiconductor laser (LD) or light emitting diode (LED) to emit a modulated optical signal of the corresponding rate. It has an automatic optical power control circuit (APC) inside to make the output The optical signal power remains stable.
Receive part:
After the optical signal of a certain code rate is input to the module, it is converted into an electrical signal by the light detection diode, and the electrical signal of the corresponding code rate is output after the preamplifier, and the output signal is generally PECL level. At the same time, when the input optical power is less than a certain value, an alarm signal will be output.
Selection of optical modules (main reference)
1. Classification and main specifications of optical fiber connectors
Optical fiber connectors are installed on both ends of a section of optical fiber, and are mainly used for optical wiring. According to the type of fiber: single-mode fiber connector (generally G.652 fiber: fiber inner diameter 9um, outer diameter 125um), multimode fiber connector
According to the connector form of the optical fiber connector: FC, SC, ST, LC, MU, MTRJ, etc., currently commonly used are FC, SC, ST, LC.
According to the end face of the inner pin of the optical fiber connector connector: PC, SPC, UPC, APC According to the diameter of the optical fiber connector: Φ3, Φ2, Φ0.9.
The performance of optical fiber connectors mainly includes optical performance, interchangeability, mechanical performance, environmental performance and life. The most important of these are insertion loss and return loss.
2. Transmitting optical power and receiving sensitivity of the optical module
Transmitting optical power refers to the light intensity at the transmitting end, and receiving sensitivity refers to the detectable light intensity. Both are in dBm and are important parameters that affect the transmission distance. The distance that an optical module can transmit is limited mainly by loss and dispersion. The loss limit can be estimated according to the formula: loss limited distance = (transmitting optical power - receiving sensitivity) / fiber attenuation. The fiber attenuation is related to the actual selected fiber. Generally, the current G.652 fiber can achieve 0.5dB/km in the 1310nm band and 0.3dB/km in the 1550nm band or even better. 50um multimode fiber is 4dB/km in the 850nm band and 2dB/km in the 1310nm band. For 100M and 1000M optical modules, the dispersion limitation is far greater than the loss limitation, so it can be ignored. 10GE optical modules follow the 802.3ae standard, and the transmission distance is related to the type of optical fiber selected and the optical performance of the optical module.
The main parameters of the optical module
1. Transfer rate
Transmission rate refers to the number of transmitted bits per second, in Mb/s or Gb/s. Main rates: 100M, 1000M, 2.5G, 4.25G and 10G.
2. Transmission distance
The transmission distance of the optical module is divided into three types: short distance, medium distance and long distance. It is generally considered that 2km and below are short distances, 10-20km are medium distances, and 30km, 40km and above are long distances.
The transmission distance of the optical module is limited, mainly because the optical signal will have a certain loss and dispersion when it is transmitted in the optical fiber.
Notice:
Loss is the loss of light energy due to absorption, scattering and leakage of the medium when light is transmitted in the optical fiber. This part of the energy is dissipated at a certain rate with the increase of the transmission distance.
Dispersion is mainly caused by the fact that electromagnetic waves of different wavelengths travel in the same medium at different speeds, resulting in different wavelength components of the optical signal reaching the receiving end at different times due to the accumulation of transmission distances, resulting in pulse broadening and inability to distinguish the signal. value.
Therefore, users need to select appropriate optical modules according to their actual networking conditions to meet different transmission distance requirements.
3. Center wavelength
The central wavelength refers to the optical band used for optical signal transmission. At present, there are three main central wavelengths of optical modules: 850nm band, 1310nm band and 1550nm band.
850nm band: mostly used for ≤2km short-distance transmission
1310nm and 1550nm bands: mostly used for medium and long distance transmission, more than 2km transmission.
