Introduction to the knowledge and principle of optical modules

Any optical module has two functions of sending and receiving, performing photoelectric conversion and electro-optical conversion, so that the optical modules are inseparable from the devices at both ends of the network. Nowadays, there are often tens of thousands of devices in a data center. To realize the interconnection of these devices, optical modules are indispensable. Today, optical modules have become a segment of the data center.

Knowledge of optical modules in weak current engineering

Optical module classification

By package: 1*9, GBIC, SFF, SFP, XFP, SFP+, X2, XENPARK, 300pin, etc.

By rate: 155M, 622M, 1.25G, 2.5G, 4.25G, 10G, 40G, etc.

By wavelength: conventional wavelength, CWDM, DWDM, etc.

By mode: single-mode fiber (yellow), multi-mode fiber (orange-red).

By Usability: Hot-pluggable (GBIC, SFP, XFP, XENPAK) and non-hot-pluggable (1*9, SFF).

Basic principles of packaged optical modules

Optical Transceiver

The integrated optical transceiver module is the core device of optical communication, which completes the optical-electrical/electrical-optical conversion of optical signals.

It consists of two parts: the receiving part and the transmitting part. The receiving part realizes the photo-electric conversion, and the transmitting part realizes the electric-optic conversion.

Launch part:

The electrical signal input with a certain code rate is processed by the internal driver chip to drive the semiconductor laser (LD) or light emitting diode (LED)

It emits a modulated optical signal with a corresponding rate, and it has an automatic optical power control circuit (APC) inside to keep the output optical signal power stable.

Receive part:

After the optical signal of a certain code rate is input to the module, it is converted into an electrical signal by the light detection diode, and the electrical signal of the corresponding code rate is output after the preamplifier, and the output signal is generally PECL level. At the same time, when the input optical power is less than a certain value, an alarm signal will be output.

Weak current engineering optical module

The main parameters of the optical module

1. Transmission rate

Transmission rate refers to the number of transmitted bits per second, in Mb/s or Gb/s. Main rates: 100M, 1000M, 2.5G, 4.25G and 10G.

2. Transmission distance

The transmission distance of the optical module is divided into three types: short distance, medium distance and long distance. It is generally considered that 2km and below are short distances, 10-20km are medium distances, and 30km, 40km and above are long distances.

The transmission distance of the optical module is limited, mainly because the optical signal will have a certain loss and dispersion when it is transmitted in the optical fiber.

Notice:

Loss is the loss of light energy due to 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 with the increase of the transmission distance.

Dispersion is mainly caused by the fact that electromagnetic waves of different wavelengths travel in the same medium at different speeds, resulting in different wavelength components of the optical signal reaching the receiving end at different times due to the accumulation of transmission distances, resulting in pulse broadening and inability to distinguish the signal. value.

Therefore, users need to select appropriate optical modules according to their actual networking conditions to meet different transmission distance requirements.

3. Center wavelength

The central wavelength refers to the optical band used for optical signal transmission. At present, there are three main central wavelengths of optical modules: 850nm band, 1310nm band and 1550nm band.

850nm band: mostly used for ≤2km short-distance transmission

1310nm and 1550nm bands: mostly used for medium and long distance transmission, more than 2km transmission.

Fiber Type

1. Fiber Mode

According to the transmission mode of light in the optical fiber, the optical fiber can be divided into two types: single-mode optical fiber and multi-mode optical fiber.

Multimode fiber (MMF, Multi Mode Fiber) has a thicker core and can transmit light in multiple modes. However, the intermodal dispersion is relatively large, and the intermodal dispersion will gradually increase with the increase of the transmission distance. The transmission distance of a multimode fiber is also related to its transmission rate, core diameter, and mode bandwidth. For details, see the table below.

Single-mode fiber (SMF, Single Mode Fiber) has a thin core and can only transmit light in one mode. Therefore, its intermodal dispersion is very small, which is suitable for long-distance communication.

2. The end face and diameter of the optical fiber

According to the end face of the inner pin of the fiber optic connector connector: PC, SPC, UPC, APC

According to the diameter of the optical fiber connector: Φ3, ​​Φ2, Φ0.9

3. Optical fiber interface connector type

Interface connectors are used to connect pluggable modules and corresponding transmission media. The optical fiber connector is an indispensable passive device in the optical fiber communication system. Its use makes it possible to detachably connect the optical channels, which not only facilitates the commissioning and maintenance of the optical system, but also makes the switching scheduling of the optical system more convenient. flexible.

According to the type of fiber:

Single-mode fiber connector (generally G.652 fiber: fiber inner diameter 9um, outer diameter 125um);

Multimode fiber connector (one is G.651 fiber with an inner diameter of 50um and an outer diameter of 125um; the other is an inner diameter of 62.5um and an outer diameter of 125um);

According to the connector form of optical fiber connectors: FC, SC, ST, LC, MU, MTRJ, etc., currently commonly used are FC, SC, ST, LC

SC (Subscriber Connector Standard Connector, standard fiber optic connector), a molded plug-in coupling connector developed by Japan's NTT company. The shell adopts the molding process and is made of molded glass fiber plastic, which is rectangular; the pin is made of precision ceramics, and the coupling sleeve is a metal slotted sleeve structure. The fastening method adopts the plug-in type and does not need to be rotated. The appearance diagram is as follows:

Note: To protect the cleanliness of the optical fiber connector, be sure to cover the dust cap when the optical fiber is not connected.

Interface metrics

output optical power

The output optical power refers to the output optical power of the light source at the transmitting end of the optical module.

It can be understood as the intensity of light, 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 express optical power.

Formula: P(dBm)=10Log(P/1mW)

The optical power is attenuated by half and reduced by 3dB. The optical power of 0dBm corresponds to 1mW and is measured by an optical power meter. For PON products, because the ONU end adopts the burst mode, it needs to use a dedicated optical power meter for measurement, which can be connected in series in the line to give the current upstream and downstream optical power in real time.

Receive sensitivity

The receiving sensitivity refers to the minimum received optical power of the optical module under the condition of a certain rate and bit error rate, unit: 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 for the device at the receiving end of the optical module.

Taking into account the increase in link loss caused by fiber aging or other unforeseen factors, the optimal received optical power range is controlled from 2-3dB above the receiving sensitivity to 2-3dB below the overload point, which is the white area in the above figure.

Compression Sensitivity

Compression sensitivity refers to the sensitivity value of the input signal after adding jitter and vertical eye closure deterioration conditions, unit: dBm. This concept is only for 10G interface modules (XENPAK modules and XFP modules).

Transmitting optical power and receiving sensitivity of optical modules

Transmitting optical power refers to the light intensity at the transmitting end, and receiving sensitivity refers to the detectable light intensity. Both are in dBm and are important parameters that affect the transmission distance. The distance that an optical module can transmit is limited mainly by loss and dispersion. The loss limit can be based on the formula:

Loss limited distance = (transmitting optical power - receiving sensitivity) / fiber attenuation to estimate.

The fiber attenuation is related to the actual selected fiber. Generally, the current G.652 fiber can achieve 0.5dB/km in the 1310nm band and 0.3dB/km in the 1550nm band or even better. 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.

Saturation optical power value

Refers to the maximum optical power that can be detected at the receiving end of the optical module, generally -3dBm. When the received optical power is greater than the saturated optical power, bit errors will also occur. Therefore, there will be bit errors in the loopback test without attenuation for the optical module with high transmit optical power.

light saturation

Also known as saturated optical power, it refers to the maximum input optical power when maintaining a certain bit error rate (10-10 to 10-12) at a certain transmission rate, unit: dBm.

It should be noted that the photodetector will have a photocurrent saturation phenomenon under strong light irradiation. When this phenomenon occurs, the detector needs a certain time to recover. At this time, the receiving sensitivity is reduced, and the received signal may be misjudged and may be misjudged. It will cause bit error phenomenon, and it is also very easy to damage the detector at the receiving end, so try to avoid exceeding its saturated optical power during operation. Note that for a long-distance optical module, since its average output optical power is generally greater than its maximum input optical power (ie optical saturation), users are advised to pay attention to the length of the optical fiber when using it to ensure that the actual received optical power reaching the optical module is less than its optical power. Saturation, otherwise it may cause damage to the optical module.