Optical Module Knowledge | What are the key performance indicators of optical modules?

What are the key performance indicators of optical modules

How do you measure the performance metrics of an optical module? We can read the performance indicators of optical modules from the following aspects.

• Optical module transmitter side

• Average transmit optical power

Average transmit optical power refers to the optical power output by the light source at the transmitting end of the optical module under normal operating conditions, which can be understood as the intensity of light. The transmitting optical power is related to the proportion of "1" in the transmitted data signal, the more "1", the higher the optical power. When the transmitter sends a pseudo-random sequence of signals, "1" and "0" accounted for roughly half, when the test power is the average transmitted optical power in W or mW or dBm. where W or mW is a linear unit, dBm is a logarithmic unit. In communications, we usually use dBm to indicate the optical power.

Extinction Ratio

Extinction ratio is the minimum value of the average optical power of the laser in full modulation conditions when transmitting full "1" code and the average optical power ratio in dB when transmitting full "0" code. The laser of the transmitting part of the optical module is converted into an optical signal according to the code rate of the input electrical signal. The average optical power of the full "1" code means the average power of the laser light, the average optical power of the full "0" code means the average power of the laser does not light, the extinction ratio that characterizes the difference between 0, 1 signal, so the extinction ratio can be seen as a Laser operating efficiency measurement. Typical minimum values of extinction ratio range from 8.2dB to 10dB.

The center wavelength of the optical signal

In the emission spectrum, the wavelength corresponding to the midpoint of the line segment connecting 50 ℅ of the maximum amplitude. Different kinds of lasers or the same kind of two lasers, due to the process, production and other reasons will have differences in the center wavelength, even the same laser in different conditions may have different center wavelength. Generally, manufacturers of optical devices and optical modules, provide users with a parameter, the central wavelength (e.g. 850nm), which will generally be a range. There are three main center wavelengths of optical modules commonly used today: 850nm band, 1310nm band, and 1550nm band.

Why is it defined in these three bands? This is related to the optical signal transmission medium fiber loss. Through continuous research experiments, people found that fiber loss usually decreases with the wavelength, 850nm loss less, 900 ~ 1300nm loss and become higher; and 1310nm and become lower, 1550nm loss the lowest, 1650nm loss tends to increase. So 850nm is the so-called short wavelength window, 1310nm and 1550nm is the long wavelength window.

• Receiving side of optical module

• Overload optical power

Also known as saturation optical power, refers to the optical module in a certain BER (BER = 10-12) conditions, the maximum input average optical power that can be received by the components at the receiving end. The unit is dBm.

It should be noted that the photodetector in the strong light irradiation will appear photocurrent saturation phenomenon, when this phenomenon, the detector needs a certain amount of time to recover, at this time, the reception sensitivity decreases, the received signal may be misjudged and cause false code phenomenon. Simply put, the input optical power exceeds the overload optical power, may cause damage to the equipment, in the use of operation should try to avoid strong light exposure to prevent the overload optical power.

Receiving sensitivity

Receiving sensitivity refers to the minimum average input optical power that can be received by the receiver component under a certain BER (BER=10-12) condition of the optical module. If the transmit optical power refers to the light intensity at the transmitter, then the receive sensitivity refers to the light intensity that can be detected by the optical module. The unit is dBm.

In general, the higher the rate the worse the reception sensitivity, that is, the greater the minimum received optical power, the higher the requirements for the optical module receiver side devices.

Receive optical power

Receive optical power refers to the optical module in a certain BER (BER = 10-12) conditions, the average optical power received by the receiver component range. The unit is dBm. the upper limit of the received optical power is the overload optical power, and the lower limit is the maximum value of the received sensitivity.

Collectively, it means that when the received optical power is less than the received sensitivity, the signal may not be received properly, because the optical power is too weak. When the received optical power is greater than the overload optical power, may also not be able to properly receive the signal, because there is a false code phenomenon.

Comprehensive performance indicators

Interface Rate

The maximum electrical signal rate that can be carried by optical devices without error code transmission is specified in the Ethernet standard: 125Mbit/s, 1.25Gbit/s, 10.3125Gbit/s, 41.25Gbit/s.

Transmission Distance

The distance that can be transmitted by optical modules is mainly limited by both loss and dispersion. Loss is the loss of light energy due to absorption, scattering and leakage in the optical fiber, which is dissipated at a certain rate as the transmission distance increases. Dispersion occurs mainly because electromagnetic waves of different wavelengths propagate at different speeds in the same medium, resulting in different wavelength components of the optical signal reaching the receiver at different times due to the accumulation of transmission distances, resulting in pulse spreading and thus inability to distinguish signal values.

In the optical module dispersion limited, the limited distance is much larger than the loss limited distance, which can be disregarded. The loss limit can be estimated according to the formula: loss limited distance = (transmit optical power - receive sensitivity) / fiber attenuation. The attenuation of the fiber and the actual choice of the fiber strong correlation.