Optical Module

The optical module works in the physical layer of the OSI model and is one of the core devices in the optical fiber communication system. It's mostly photoelectrons

The device (optical transmitter, optical receiver), functional circuit, and optical interface parts play the main role in achieving photoelectric conversion and electro-optical conversion functions in optical fiber communication. The working principle of the optical module is shown in the figure.

The sending interface injects an electrical signal of a certain bit rate, and after processing by the internal driver chip, the driving semiconductor laser (LD) or light-emitting diode (LED) emits a modulated optical signal of the corresponding rate. After transmission through the optical fiber, the receiving interface converts the optical signal into an electrical signal by the optical detection diode and outputs the electrical signal of the corresponding bit rate after passing through the preamplifier.

v Appearance of an optical module

There are various types of optical modules and different appearance structures. However, the basic structure of optical modules consists of the following parts, as shown in the figure for the appearance of optical modules (SFP package as an example).

Figure 2

Description of each optical module structure:

v light module of key performance indicators (kpis)

How to measure the performance indicators of optical modules? You can understand the performance indicators of optical modules from the following aspects.

Optical transceiver sending end

² Average transmitted light 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 light intensity. The transmitted optical power is related to the proportion of "1" in the data signal sent. The more "1," the greater the optical power. When the transmitter sends the pseudo-random sequence signal, "1" and "0" are roughly equal to each other, and then the power obtained by the test is the average transmitted optical power, in W or mW or dBm. Where W or mW is a linear unit and dBm is a logarithmic unit. In communication, we usually use dBm to represent optical power.

² extinction ratio

Extinction ratio refers to the minimum ratio of the average optical power transmitted by the laser at full "1" code to the average optical power transmitted at full "0" code under the condition of full modulation, in dB. As shown in Figure 3-1, when we convert electrical signals into optical signals, the laser in the transmitting part of the optical module converts them into optical signals according to the bit rate of the input electrical signals. The average optical power of all "1" codes refers to the average power of laser luminescence, and the average optical power of all "0" codes refers to the average power of laser non-luminescence, and the extinction ratio refers to the distinguishing ability of 0 and 1 signals. Therefore, the extinction ratio can be regarded as a measure of laser operation efficiency. The typical minimum extinction ratio ranges from 8.2 dB to 10 dB.

Figure 3. Schematic diagram of laser operation

² The central wavelength of an optical signal

In the emission spectrum, the connection 50% of the midpoint of the highest value line segment corresponds to the wavelength. Different kinds of lasers or two lasers of the same kind will have different central wavelengths due to the process, production, and other reasons, even if the same laser under different conditions may also have different central wavelengths. Generally, manufacturers of optical devices and optical modules provide the user with a parameter, namely the central wavelength (such as 850 nm), which is usually in a range. Currently, there are three main central wavelengths of optical modules in common use: the 850 nm band, the 1310 nm band, and the 1550 nm band. Why is it defined in these three bands? This is related to the loss of optical fiber in the transmission medium of optical signal. Through continuous research and experiments, it is found that the optical fiber loss usually decreases with the increase of wavelength, the loss of 850 nm is less, and the loss of 900-1300 nm becomes higher. The loss at 1310 nm is lower, the loss at 1550 nm is the lowest, and the loss above 1650 nm tends to increase. So 850 nm is what's called the short wavelength window, and 1310 nm and 1550 nm are the long wavelength windows.

Optical module receiving end

² Overload optical power

Also known as saturated optical power, refers to the optical module under a certain bit error rate (BER=10-12) conditions, receiving components
The maximum average optical input power that can be received. The unit is dBm.
It should be noted that the photocurrent saturation phenomenon will occur in the light detector under the strong light irradiation. when this phenomenon occurs,
After that, the detector needs a certain amount of time to recover. At this time, the receiving sensitivity decreases, and the received signal may be misjudged and cause the error code phenomenon. Simply put, if the input optical power exceeds the overload optical power, it may cause damage to the equipment. In use and operation, strong light should be avoided as far as possible to prevent exceeding the overload optical power.

² Receive sensitivity

Receiving sensitivity refers to the minimum average input optical power that can be received by the receiving component under a certain bit error rate (BER=10-12) of the optical module. If the transmitted light power refers to the intensity of light at the sending end, then the received sensitivity refers to the intensity of light that can be detected by the optical module. The unit is 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 on the receiving end of the optical module.

² Received optical power

The received optical power refers to the average optical power range that can be received by the receiving component under a certain bit error rate (BER=10-12) 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 maximum of the received sensitivity.

Comprehensive performance index

² interface rate

The maximum error-free transmission rate of electrical signals that an optical device can carry is defined by Ethernet standards as follows:

155 Mbit/s, 1.25 Gbit/s, 10.3125 Gbit/s, 25 Gbit/s

41.25 Gbit/s, 100 Gbit/s, 200 Gbit/s, and 400 Gbit/s.。

² transmission distance

The transmission distance of the optical module is limited by loss and dispersion. Loss is the loss of light energy caused by absorption, scattering, and leakage of the medium when the light is transmitted in the fiber. This part of energy dissipates at a certain rate with the increase of the transmission distance. The generation of dispersion is mainly because the electromagnetic waves of different wavelengths in the same medium propagation speed are not equal, resulting in different wavelength components of the optical signal due to the accumulation of transmission distance arriving at the receiving end at different times, resulting in pulse widening, and then the signal value can not be distinguished.
In the aspect of dispersion limitation of the optical module, the limited distance is much larger than the limited distance of loss, so it cannot be considered. The loss limit can be estimated by the formula: loss limit distance = (transmitted light power - accepted sensitivity)/fiber attenuation. The attenuation of optical fiber is strongly related to the actual optical fiber selected.

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