What knowledge do you need to know to purchase an optical module?

First, the basic knowledge of the optical module

1.Definition of optical module:

Optical module: that is, the optical transceiver module.

2.The structure of the optical module:

The optical transceiver module is composed of an optoelectronic device, a functional circuit and an optical interface, and the optoelectronic device includes two parts: transmitting and receiving.

The transmitting part is: an electric signal inputting a certain code rate is processed by an internal driving chip to drive a semiconductor laser (LD) or a light emitting diode (LED) to emit a modulated light signal of a corresponding rate, and an optical power automatic control circuit is internally provided therein. The output optical signal power remains stable.

The receiving part is: an optical signal input module of a certain code rate is converted into an electrical signal by the photodetecting diode. After the preamplifier, the electrical signal of the corresponding code rate is output, and the output signal is generally PECL level. At the same time, an alarm signal is output after the input optical power is less than a certain value.

3.The parameters and significance of the optical module

Optical modules have many important optoelectronic technical parameters. However, for the two hot-swappable optical modules, GBIC and SFP, the following three parameters are most concerned when selecting:

Center wavelength

In nanometers (nm), there are currently three main types:

850nm (MM, multimode, low cost but short transmission distance, generally only 500M); 1310nm (SM, single mode, large loss during transmission but small dispersion, generally used for transmission within 40KM);

1550nm (SM, single mode, low loss during transmission but large dispersion, generally used for long-distance transmission above 40KM, and can directly transmit 120KM without relay);

Transmission rate

The number of bits (bits) of data transmitted per second, in bps.

There are currently four types commonly used: 155 Mbps, 1.25 Gbps, 2.5 Gbps, 10 Gbps, and the like. The transmission rate is generally backward compatible. Therefore, the 155M optical module is also called the FE (100 Mbps) optical module, and the 1.25G optical module is also called the GE (Gigabit) optical module. This is the most widely used module in optical transmission equipment. In addition, its transmission rate in fiber storage systems (SAN) is 2Gbps, 4Gbps and 8Gbps.

Transmission distance

The optical signal does not need to be relayed to a distance that can be directly transmitted, in kilometers (also called kilometers, km). Optical modules generally have the following specifications: multimode 550m, single mode 15km, 40km, 80km, and 120km, and so on.

Second, the basic concept of optical modules

1.Laser category

A laser is the most central component of an optical module that injects current into a semiconductor material and emits laser light through photon oscillations and gains in the cavity. At present, the most commonly used lasers are FP and DFB lasers. The difference is that the semiconductor material and the cavity structure are different. The price of the DFB laser is much more expensive than the FP laser.Optical modules with transmission distances up to 40KM generally use FP lasers.Optical modules with transmission distances ≥40KM generally use DFB lasers.

2.Transmitted optical power and receiving sensitivity

The transmitted optical power refers to the output optical power of the light source at the transmitting end of the optical module. The receiving sensitivity refers to the minimum received optical power of the optical module at a certain rate and bit error rate.

The units of these two parameters are dBm (meaning decibel milliwatt, the logarithm of the power unit mw, the calculation formula is 10lg, 1mw is converted to 0dBm), which is mainly used to define the transmission distance of the product, different wavelengths, transmission rate and The optical module’s optical transmit power and receive sensitivity will be different, as long as the transmission distance can be ensured.

3.Loss and dispersion

Loss is the loss of light energy due to the absorption and scattering of the medium and the leakage of light when the light is transmitted in the fiber. This part of the energy is dissipated at a certain rate as the transmission distance increases.The dispersion is mainly caused by the unequal speed of electromagnetic waves of different wavelengths propagating in the same medium, which causes different wavelength components of the optical signal to reach the receiving end at different times due to the accumulation of the transmission distance, resulting in pulse broadening and thus inability to distinguish signals. value. These two parameters mainly affect the transmission distance of the optical module. In the actual application process, the 1310nm optical module generally calculates the link loss at 0.35dBm/km, and the 1550nm optical module generally calculates the link loss at .20dBm/km, and calculates the dispersion value. Very complicated, generally only for reference.

4.The life of the optical module

International unified standards, 50,000 hours of continuous work, 50,000 hours (equivalent to 5 years).

The SFP optical modules are all LC interfaces. The GBIC optical modules are all SC interfaces. Other interfaces include FC and ST.