Working principle of optical module

As an important part of optical fiber communication, optical modules are optoelectronic devices that realize the functions of photoelectric conversion and electro-optic conversion in the process of optical signal transmission.

Optical module works in the physical layer of OSI model and is one of the core devices in optical fiber communication system. It is mainly composed of optoelectronic devices (optical transmitter, optical receiver), functional circuits and optical interfaces. Its main function is to realize the photoelectric conversion and electro-optic conversion functions in optical fiber communication. The working principle of the optical module is shown in the working principle diagram of the optical module.

The transmitting interface inputs an electrical signal with a certain code rate. After being processed by the internal driving chip, the driving semiconductor laser (LD) or light emitting diode (LED) transmits the modulated optical signal with the corresponding rate. After being transmitted through the optical fiber, the receiving interface converts the optical signal from the optical detection diode into an electrical signal, and outputs the electrical signal with the corresponding code rate after passing through the preamplifier.

Appearance structure of optical module

There are many kinds of optical modules and different appearance structures, but their basic composition structure includes the following parts, as shown in the appearance structure of figure optical module (illustrated by SFP package).

Table 1-1 description of each structure of optical module

What are the key performance indicators of the optical module

How to measure the performance index of optical module? We can understand the performance index of optical module from the following aspects.

Optical module transmitter

Average transmitted optical power

Average transmitted light power refers to the light 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. The transmitted optical power is related to the proportion of "1" in the transmitted data signal. The more "1", the greater the optical power. When the transmitter sends pseudo-random sequence signal, "1" and "0" account for about half respectively. At this time, the power obtained from the test is the average transmitted optical power, and the unit is w or MW or DBM. Where w or MW is the linear unit and DBM is the logarithmic unit. In communication, we usually use DBM to represent optical power.

extinction ratio

Extinction ratio refers to the minimum value of the ratio of the average optical power when the laser emits all "1" codes and the average optical power when the laser emits all "0" codes under the condition of full modulation, and the unit is dB. As shown in Figure 1-3, when we convert an electrical signal into an optical signal, the laser of the transmitting part of the optical module converts it into an optical signal according to the code rate of the input electrical signal. The average optical power at full "1" code means the average power of laser light emission, the average optical power at full "0" code means the average power of laser non light emission, and the extinction ratio means 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 value range of extinction ratio is 8.2db to 10dB.

Central wavelength of optical signal

In the emission spectrum, the wavelength corresponding to the midpoint of the line segment connecting the 50% maximum amplitude value. Different kinds of lasers or two lasers of the same kind will have different central wavelengths due to process, production and other reasons. Even the same laser may have different central wavelengths under different conditions. Generally, manufacturers of optical devices and optical modules provide users with a parameter, that is, the central wavelength (such as 850nm), which is generally a range. At present, there are three kinds of central wavelengths of commonly used optical modules: 850nm band, 1310nm band and 1550nm band.

Why is it defined in these three bands? This is related to the transmission medium and optical fiber loss 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 at 850nm is less, and the loss at 900 ~ 1300nm is higher; The loss at 1310nm is lower, the loss at 1550nm is the lowest, and the loss above 1650nm tends to increase. Therefore, 850nm is the so-called short wavelength window, and 1310nm and 1550nm are the long wavelength window.

Optical receiving module

Overload optical power

Also known as saturated optical power, it refers to the maximum input average optical power that the optical module can receive under the condition of a certain bit error rate (BER = 10-12). The unit is DBM.

It should be noted that photocurrent saturation will occur in the photodetector 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, resulting in bit error. In short, if the input optical power exceeds the overload optical power, it may cause damage to the equipment. In use and operation, strong light irradiation should be avoided as far as possible to prevent exceeding the overload optical power.

Receiving sensitivity

Receiving sensitivity refers to the minimum average input optical power that the optical module can receive under the condition of a certain bit error rate (BER = 10-12). If the transmitted light power refers to the light intensity at the transmitting end, the reception 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, and the higher the requirements for the receiving devices of the optical module.

Received optical power

The received optical power refers to the average optical power range that the optical module can receive under the condition of a certain bit error rate (BER = 10-12). 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.

Generally speaking, when the received optical power is less 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, the signal may not be received normally because of bit error.

Comprehensive performance index

Interface rate

The maximum electrical signal rate of error free transmission that can be carried by optical devices is specified in Ethernet standard: 125mbit / s, 1.25gbit/s, 10.3125gbit/s and 41.25gbit/s.

transmission distance

The transmission distance of optical module is mainly limited by loss and dispersion. Loss is the loss of light energy caused by the absorption, scattering and leakage of medium when light is transmitted in optical fiber. This part of energy is dissipated at a certain rate with the increase of transmission distance. The main reason for dispersion is that electromagnetic waves of different wavelengths have different velocities when propagating in the same medium, resulting in different wavelength components of optical signals arriving at the receiving end at different times due to the accumulation of transmission distance, resulting in pulse broadening and unable to distinguish the signal value.

In terms of limited optical distance, the limited optical distance can not be considered. The loss limit can be estimated according to the formula: loss limited distance = (transmitted optical power - Acceptance sensitivity) / fiber attenuation. The attenuation of optical fiber is strongly related to the actual optical fiber.

What are the common types of optical modules

Classification by rate

In order to meet the requirements of various transmission rates, optical modules with different rates are produced: 400ge optical module, 100ge optical module, 40ge optical module, 25ge optical module, 10GE optical module, Ge optical module, Fe optical module, etc.

Classification by package type

The higher the transmission rate, the more complex the structure, resulting in different packaging methods. The packaging types applicable to Huawei switches include qsfp-dd, qsfp28, qsfp +, sfp28, SFP / esfp, SFP +, CXP, CFP, etc.

Classification by mode

Optical fiber is divided into single-mode optical fiber and multi-mode optical fiber. In order to use different types of optical fibers, single-mode optical modules and multi-mode optical modules are produced.

The central wavelength of single-mode optical module is generally 1310nm and 1550nm, which is used together with single-mode optical fiber. Single mode optical fiber has wide transmission frequency bandwidth and large transmission capacity, which is suitable for long-distance transmission.

The center wavelength of multimode optical module is generally 850nm, which is used together with multimode optical fiber. Multimode fiber has the defect of mode dispersion. Its transmission performance is worse than that of single-mode fiber, but the cost is low. It is suitable for small capacity and short distance transmission.

Note: when using long-distance optical module, the transmitted optical power is generally greater than the overload optical power. Therefore, it is necessary to pay attention to the length of optical fiber to ensure that the actual received optical power is less than the overload optical power. If the optical fiber length is short, the long-distance optical module needs to be used in combination with optical fiber attenuation (attenuation value per unit length of optical fiber, unit: dB / km) to avoid burning the opposite optical module.

Classification by central wavelength

The working wavelength of the optical module is a range. In order to facilitate the description, the parameter of the central wavelength is used, and the unit is nanometer (nm).

In order to support optical signal transmission, different optical bands are used to produce optical modules with different central wavelengths, such as 850nm, 1310nm and 1550nm.

Sort by color

By color, the biggest difference between color optical modules and other types of optical modules is the different central wavelength:

The central wavelength of general optical modules includes 850nm, 1310nm and 1550nm. The central wavelength is relatively single. We call this kind of light "black and white light" or "gray light".

The color light module carries several lights with different central wavelengths, so the intersection is colorful. We call this kind of light "color light".

Color light modules are divided into rough set wave light module (CWDM) and dense wave light module (DWDM). In the same band, there are more types of dense wave light modules, so the dense wave light module makes more full use of the resources of the band. The light with different central wavelengths can be transmitted without interference in the same optical fiber. Therefore, the light with different central wavelengths from the multi-channel color light module can be synthesized and transmitted together through the passive combiner, and the far end divides the light into multiple channels according to different central wavelengths through the wave splitter, which effectively saves the optical fiber line. Color optical module is mainly used in long-distance transmission lines.

Main causes of optical module failure and protective measures

The optical module must have standardized operation methods in application. Any non-standard action may cause hidden damage or permanent failure. Welcome to the official account: network engineer, Aron.

Main causes of optical module failure

The main causes of optical module failure are the deterioration of optical module performance caused by ESD damage and the failure of optical link caused by optical port pollution and damage. The main causes of optical port pollution and damage are:

      The optical port of the optical module is exposed to the environment, and the optical port is polluted by dust.

1. The end face of the used optical fiber connector has been polluted, and the optical port of the optical module has been secondary polluted.

2. The end face of optical connector with pigtail is not used properly, such as end face scratch, etc.

3. Inferior optical fiber connectors are used.

How to effectively prevent the failure of optical module is mainly divided into ESD protection and physical protection.

ESD protection

ESD damage is a major problem that leads to the deterioration of the performance of optical devices and even the loss of photoelectric function of devices. In addition, ESD damaged optical devices are not easy to test and screen, and it is difficult to locate them quickly if they fail.

Statement: This article comes from the Internet