Key Parameters Interpretation of Optical Modules

The optical module works at the physical layer of the OSI model and is an important part of optical fiber communication. Its main function is to realize the photoelectric conversion and electro-optical conversion functions in optical fiber communication. The key performance indicators of the optical module can be measured from two aspects: the optical module transmitting end and the optical module receiving end.

Transmitter of Optical Module
The key performance indicators of the transmitting end of the optical module mainly include: the average transmitted optical power, the extinction ratio, and the central wavelength of the optical signal.

Average Transmit Optical Power
The average transmitted optical power refers to the optical power output by the light source at the transmitting end of the optical module under normal working conditions, which can be understood as the intensity of light. In communication, we usually use dBm to represent optical power. The transmitted optical power is related to the proportion of “1” in the transmitted data signal. The more “1”, the greater the optical power. The transmitted optical power of the optical module is an important parameter that affect the transmission distance of the optical module.

The methods for detecting the optical power emitted by the optical module include: reading DDM information by the switch, eye diagram test, spectrometer test, optical power meter or optical power instrument test.

In the figure above, you can see Alarms, Warnings, where the warning alarm belongs to the first-level alarm, and the alarm is the warning/warning range, which belongs to the second-level alarm. Generally speaking, if it exceeds the first level, it can be used, but the bit error rate will increase. If it exceeds the second level, the optical module will not be able to be used, so it is a normal value within the first level.

In the figure above, the transmitted optical power of the optical module is -3.55 dBm, which is within the warning range of -3 dBm to -9.5 dBm, and the data is normal.

Extinction Ratio
Extinction ratio is one of the important parameters used to measure the quality of optical modules.

The extinction ratio refers to the minimum value of the ratio of the average optical power of the laser when emitting all “1” codes to the average optical power emitted when all “0” codes are emitted under full modulation conditions, and the unit is dB. Extinction ratio can be regarded as a measure of laser operating efficiency. The value of the extinction ratio is not that the larger the optical module is, the better it is, but the optical module whose extinction ratio meets the 802.3 standard is better. Typical minimum values for the extinction ratio range from 8.2dB to 10dB.

The Center Wavelength of the Optical Signal
Currently, there are mainly three central wavelengths of optical modules commonly used: 850nm band, 1310nm band and 1550nm band. Optical fiber loss usually decreases with wavelength lengthening, 850nm loss is less, 900~1300nm loss becomes higher; and 1310nm becomes lower, 1550nm loss is the lowest, and loss above 1650nm tends to increase. So 850nm is the so-called short wavelength window, and 1310nm and 1550nm are long wavelength windows.

Receiver of Optical Module
The key performance indicators of the receiving end of the optical module mainly include: overload optical power, receiving sensitivity, and receiving optical power.
Overload Optical Power

Overload optical power, also known as saturated optical power, refers to the maximum input average optical power that the receiving end components can receive under a certain bit error rate of the optical module. The unit is dBm.

It should be noted that the photodetector will appear photocurrent saturation phenomenon under strong light irradiation. When this phenomenon occurs, the detector needs a certain period of time to recover. At this time, the receiving sensitivity decreases, and the received signal may be misjudged. cause code errors. To put it simply, if the input optical power exceeds this overload optical power, it may cause damage to the equipment. During use and operation, try to avoid strong light exposure to prevent exceeding the overload optical power.

Receiver Sensitivity
Receiving sensitivity refers to the minimum average input optical power that the receiving end components can receive under a certain bit error rate condition of the optical module, and the unit is dBm. If the transmit optical power refers to the light intensity at the sending end, then the receive sensitivity refers to the light intensity that can be detected by the optical module.

In general, the higher the rate, the worse the receiving sensitivity, that is, the greater the minimum received optical power, the higher the requirements for the receiving end components of the optical module.

Received Optical Power
The received optical power refers to the average optical power range that the receiving end components can receive under a certain bit error rate condition of the optical module. The unit is dBm. The upper limit of the received optical power is the overload optical power, and the lower limit is the receiving sensitivity.

Generally speaking, when the received optical power is lower than the receiving sensitivity, the signal may not be received normally because the optical power is too weak. When the received optical power is greater than the overload optical power, signals may not be received normally because of bit errors.