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What are the optical module parameters?

In the summary of modern information networks, optical fiber communication occupies a dominant position. As network coverage becomes more and more extensive and communication capacity continues to increase, the improvement of communication links is also an inevitable development. Optical modules realize photoelectric signals in optical communication networks. The conversion is one of the main components of optical fiber communication. However, we usually talk about optical modules. So, what are the parameters of optical modules? do you know? In this article, FS will give you a detailed introduction.


1. Center wavelength

The working wavelength of the optical module is actually a range, and the parameter of the center wavelength is used for the convenience of description. The unit of the center wavelength in nanometers (nm). The general center wavelengths are 850nm, 1310nm and 1550nm, as well as 1270nm-1610nm of the CWDM series (20nm interval) and 1528nm-1623nm (0.8nm or 0.4nm interval) of the DWDM series.


1) 850nm (MM, multi-mode, low cost but short transmission distance, generally only 500m transmission);


2) 1310nm (SM, single-mode, large loss but small dispersion during transmission, generally used for transmission within 40km);


3) 1550nm (SM, single-mode, low loss but large dispersion during transmission, generally used for long-distance transmission above 40km, and the farthest can be directly transmitted without relay 120km).


2. Transmission distance

Because the fiber itself has side effects such as dispersion and loss on the optical signal. Therefore, different types of light sources can transmit different distances. When connecting an optical interface, select the optical module and optical fiber according to the farthest signal transmission distance. The transmission distance of the optical module is divided into three types: short distance, medium distance and long-distance. Generally speaking, 2km and below are short distances, 10-20km are medium distances, and 30km, 40km and above are long distances.


3. Transmission rate

The transmission rate refers to the number of bits (bits) of data transmitted per second, in bps. The transmission rate is as low as 100M and as high as 400Gbps. There are five commonly used rates: 1Gbps, 10Gbps, 25Gbps, 40Gbps, and 100Gbps. In addition, the optical module in the optical storage system (SAN) has three rates of 2Gbps, 4Gbps and 8Gbps.


After understanding the above three optical module parameters, do you have a preliminary understanding of the optical module? If you want a further understanding, let's take a look at the other parameters of the optical module!


1. Loss and dispersion

Loss is the loss of light energy due to absorption, scattering and leakage of the medium when light is transmitted in an optical fiber. This part of the energy is dissipated at a certain rate as the transmission distance increases.


The generation of chromatic dispersion is mainly due to the unequal speed of electromagnetic waves of different wavelengths when propagating in the same medium, which causes the different wavelength components of the optical signal to arrive at the receiving end at different times due to the accumulation of transmission distance, resulting in pulse broadening and inability to distinguish the signal value.


In general, the link loss is calculated at 0.35dBm/km for 1310nm optical modules, and the link loss is calculated at 0.20dBm/km for 1550nm optical modules. The calculation of chromatic dispersion value is very complicated and is generally only for reference. These two parameters are mainly used to define the transmission distance of the product. Optical modules with different wavelengths, transmission rates and transmission distances will have different optical transmit power and receiving sensitivity.


2. Laser category:

At present, the most commonly used lasers are FP and DFB. The semiconductor materials and cavity structure of the two are different. DFB lasers are expensive and are mostly used for optical modules with a transmission distance of more than 40km. FP lasers are cheap and generally used for transmission. Optical modules within 40km.


3. Optical fiber interface:

Refers to the interface where the optical module is connected to the fiber jumper. Generally, there are several types of MPO, duplex LC, simplex LC and simplex SC. MPO optical ports can be subdivided into MPO 12 (for 8 or 12 fibers) and MPO24 (for 16 or 24 fibers) according to the number of optical fibers used for optical module transmission. SFP optical modules all have LC interfaces, GBIC optical modules all have SC interfaces, and other interfaces include FC and ST.


4. Output optical power:

The output optical power refers to the output optical power of the light source at the transmitting end of the optical module. It can be understood as the intensity of light in W or mW or dBm. Among them, W or mW is a linear unit, and dBm is a logarithmic unit. In communication, we usually use dBm to represent optical power.


Formula: P(dBm)=10Log(P/1mW)


Under the normal working conditions of the module, the output optical power and emitted optical power (light intensity at the transmitting end) of the optical module are important parameters that affect the transmission distance. When two optical modules are connected, the transmit optical power should meet the range of received optical power.


5. Maximum receiving sensitivity:

Receiving sensitivity refers to the minimum received optical power of an optical module at a certain rate and bit error rate, in dBm. In general, the higher the rate, the worse the receiving sensitivity, that is, the greater the minimum received optical power, and the higher the requirements for the receiving end of the optical module. Taking into account the increase in link loss caused by fiber aging or other unforeseen factors, the optimal received optical power range is controlled from 2-3dB above the receiving sensitivity to 2-3dB below the overload point.


6. Extinction ratio:

The extinction ratio is one of the parameters used to measure the quality of the optical module. The minimum value of the ratio of the average optical power of the signal to the average optical power of the space signal under the full modulation condition, which represents the distinguishing ability of 0 and 1 signals. The two factors that affect the extinction ratio in the optical module: bias current (bias) and modulation current (Mod), tentatively regard them as ER=Bias/Mod. The value of the extinction ratio is not that the larger the optical module, the better, but the optical module whose extinction ratio meets the 802.3 standard.


7. Light saturation:

Also known as saturated optical power, it refers to the maximum input optical power when a certain bit error rate (10-10~10-12) is maintained at a certain transmission rate, in dBm.


It should be noted that the photodetector will experience photocurrent saturation under strong light irradiation. When this phenomenon occurs, the detector needs a certain time to recover. At this time, the receiving sensitivity decreases, and the received signal may be misjudged. It can cause bit errors, and it is also very easy to damage the receiver detector. During the operation, try to avoid exceeding its saturated optical power.


8. Maximum power consumption:

The power consumption of the module with different model parameters is different, and the brands of the same model are also slightly different. Gigabit is generally about 1W; SFP+ 10 Gigabit is generally 1.2-1.5W; XFP 10 Gigabit short-distance 1.5-2W, long-distance 3.5W; 100G depends on the package, generally 3.5-9W.


9. The service life of the optical module:

Internationally unified standards, 7×24 hours of uninterrupted work 50,000 hours (equivalent to 5 years).


10. Environment:

The temperature of the commercial optical module: 0~+70℃; the temperature of the stretched optical module: -20~85℃; the temperature of the industrial optical module: -40~85℃; working voltage: 3.3V; working level: TTL.

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