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Basic Knowledge of Optical Transceiver Modules


Optical module is an optical transceiver integrated module.

The optical transceiver integrated module is composed of optoelectronic devices, functional circuits and optical interfaces, etc. The optoelectronic devices include the two parts of transmitting and receiving.

The transmitting part is the electrical signal of a certain code rate is input and processed by the internal driver chip to drive the semiconductor laser (LD) or light-emitting diode (LED) to emit a modulated optical signal at a corresponding rate. It has an optical power automatic control circuit inside, so that The output optical signal power remains stable.

The receiving part is an optical signal with a certain code rate is input into the module and then converted into an electrical signal by a light detection diode. The electrical signal of corresponding code rate is output after passing through the preamplifier, and the output signal is generally at the PECL level. At the same time, an alarm signal will be sent when the input optical power is less than a certain value.

There are many important photoelectric technical parameters for optical transceiver module, but for a hot plug optical module, the following three parameters are the most concerned when selecting:

Center Wavelength
850nm(MM, multi-mode, low cost but short transmission distance, generally only 500M);
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 above 40KM, the farthest can be up to 120KM without any repeater)
In addition to the above conventional wavelengths, CWDM wavelength (SM, single-mode, color optical module) and DWDM wavelength (SM, single-mode, color optical module) are also used in multiplex transmission.

Data Rates
The number of bits of data transmitted per second (bit), in bps.
There are 7 types in common use currently: 155Mbps, 1.25Gbps, 2.5Gbps, 10Gbps, 25Gbps, 40Gbps, 100Gbps, etc. The transmission rate is generally downward compatible, so 155M optical modules are also called FE (100M) optical modules, 1.25G optical modules are also called GE (Gigabit) optical modules, and 10G optical modules are also called 10GE (10 Gigabit) optical modules. 10G optical modules are the most widely used module in optical transmission equipment currently. In addition, in the optical storage system (SAN), its transmission rate is 2Gbps, 4Gbps and 8Gbps.

Transmission Distance
The distance in kilometers (km) that optical signals can be transmitted directly without relay amplification. The transmission distance of the optical module is generally 550m for multi-mode, 20km, 40km, 80km and 120km for single-mode, etc.

Laser Types
The laser is a key device in the optical transceiver 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 that the different semiconductor materials and different resonant cavity structures. The price of DFB lasers is much more expensive than that of FP lasers. Optical modules with transmission distances within 40KM generally use FP lasers; optical modules with transmission distances ≥ 40KM generally use DFB lasers.

Loss and Dispersion
Loss refers to the loss of light energy due to the 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 as the transmission distance increases. Dispersion is mainly due to the fact that electromagnetic waves of different wavelengths propagate at different speeds in the same medium, resulting in different wavelength components of the optical signal arriving at the receiving end at different times due to the accumulation of transmission distances, resulting in pulse broadening, which makes it impossible to distinguish signals 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, the link loss of the 1550nm optical module is generally calculated at 0.20dBm/km. But the calculation of the dispersion value is very complicated and generally only for reference.

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 receiving optical power of the optical module under a certain rate and bit error rate. Both are in dBm and are important parameters affecting transmission distance. The transmission distance of optical modules is mainly limited by loss and dispersion. The loss limit can be estimated according to the formula: loss limited distance = (transmitted optical power - receiving sensitivity) / fiber attenuation. The fiber attenuation is related to the actual selected fiber. Generally, the G.652 optical fiber can achieve 0.5dB/km in the 1310nm band, and 0.3dB/km in the 1550nm band or even better. The 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. The 10GE optical module complies with the 802.3ae standard, and the transmission distance is related to the type of optical fiber and the optical performance of the optical transceiver module.

Service Life
International unified standard, 7x24 hours of uninterrupted work for 50,000 hours (equivalent to 5 years).

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