What is GPON?

Passive Optical Network (PON) technology has become one of the mainstream technologies for Fiber-to-the-X (FTTx) network construction. As users' demand for high bandwidth continues to grow, especially with the popularization of high-traffic applications such as OTT video and 4K TV, operators have included 10G GPON technology in their schedules to meet users' urgent need for faster and more reliable network connections. GPON is generally divided into GPON, XG-PON and XGS-PON.

Gigabit Passive Optical Network (GPON) is an optical fiber transmission technology that uses a single optical fiber line to transmit data to achieve high-speed, high-bandwidth network connections. The basic principles of GPON involve light transmission and the use of optical splitters. In the GPON network, an optical fiber line connects multiple users and distributes signals to different end users through optical splitters to achieve data transmission.

The architecture of GPON includes optical line terminal (OLT) and optical network unit (ONU). The OLT is responsible for communicating with the ONU on the user side, and the ONU is responsible for communicating with the user equipment. This distributed structure enables the GPON system to support a large number of users and be widely used in different fields.

1. GPON Technical Specifications

Among the technical specifications of GPON, one of the most prominent features is its high bandwidth requirements. GPON is typically capable of providing transmission rates of 1.25 Gbps (downstream direction) and 2.5 Gbps (upstream direction). This high bandwidth makes GPON excellent in supporting high-traffic applications such as high-definition video and large-capacity file transfer.

In addition, GPON also has certain advantages in distance. Fiber optic transmission allows signal transmission distances to reach tens of kilometers, which enables GPON to meet a wide range of network topology needs.

Since the uplink rate of GPON is relatively low, the cost of ONU's sending components (such as lasers) is also low, so the total price of the equipment is low.

2. GPON Features

High bandwidth: GPON can provide transmission rates of up to 2.5 Gbps (uplink) and 1.25 Gbps (downlink), which enables it to meet users' needs for high-speed broadband connections.

Point-to-multipoint architecture: GPON uses a point-to-multipoint optical fiber transmission architecture to connect an optical line terminal (OLT) and multiple optical network units (ONU) through an optical fiber line. This distributed architecture allows multiple users to share the same optical fiber, improving network resource utilization.

Symmetric and asymmetric transmission: GPON supports symmetric and asymmetric transmission, that is, the uplink and downlink transmission rates can be different. This enables the network to better adapt to the needs of different users and applications.

ITU-T standards: The technical specifications of GPON are formulated by the Telecommunications Sector of the International Telecommunications (ITU-T) and are specifically defined in the G.984.x series of recommendations. This provides a unified standard for equipment from different manufacturers and increases the interoperability of equipment.

3. GPON Advantages and Limitations

One of the advantages of GPON is its relatively low cost. Fiber optic networks are often more cost-effective than traditional copper cable networks, especially in large-scale deployments. In addition, GPON supports symmetric and asymmetric transmission, making it suitable for different application scenarios.

However, GPON also has some limitations. Due to its limitations in transmission rate and bandwidth, the network may face bandwidth bottlenecks when user demands continue to increase. Upgrading the GPON system to meet higher requirements may face some technical and economic challenges.

4. GPON Application Scenarios

Home broadband network: GPON provides home users with high-speed and stable broadband connections, supporting high-definition video streaming, online games and other needs.

Enterprise network: In an enterprise environment, GPON can provide reliable communication infrastructure to meet the needs of enterprises for daily office work and large-scale data transmission.

Government and campus networks: GPON is also widely used in government agencies and school networks, meeting the needs of these institutions for high-bandwidth and high-stability networks.

What Are the Key Components of Optical Transceiver Module?

The function of optical transceiver module is to perform photoelectric conversion, and its internal TOSA, ROSA and BOSA are the key components to realize the photoelectric conversion function. The optical device is composed of transmitter and receiver to complete the optical-electrical or electrical-optical conversion of optical signals.

The interior is composed of optical devices, functional circuits and optical interfaces. The optical device is the main component of the optical transceiver module.

The optical devices used for optical signal conversion are called TOSA and ROSA.

TOSA (Transmitting Optical Sub-Assembly) mainly completes the conversion of electrical signals into optical signals. With the light source (semiconductor light-emitting diode or laser diode) as the core, LD chip, monitor photodiode (MD) and other components are packaged in a TO coaxial package or butterfly package, which constitutes TOSA.

In TOSA, laser diode is the most commonly used semiconductor emitting device for optical transceiver modules. Threshold current (Ith) and slope efficiency (S) are the two main parameters. In order to make the LD work quickly, a DC bias current slightly greater than the threshold current must be provided to the LD, that is, the laser is emitted only when the forward current exceeds the threshold current.

ROSA (Resceiving Optical Sub-Assembly) optical receiving assembly, in the high data rate optical fiber module, PIN or APD photodiode and TIA are usually assembled in a sealed metal casing to form an optical receiving assembly.

The figure below is the schematic diagram of the optical module ROSA, which is composed of a photodetector (PIN/APD), a TIA pre-amplifier, and a limiting amplifier.

Photodetector, the main device of ROSA, is mainly used to convert optical signals into electronic signals through the photoelectric effect. The common photodetectors in optical communications are PIN photodiodes and avalanche photodiodes (APDs). APDs are high-sensitivity photodetectors that use the avalanche multiplication effect to double the photocurrent. Compared with PIN photodiode, the receiving sensitivity of APD can be improved by 6~10dB.

The weak signal current generated by the photodetector is converted into a signal voltage of sufficient magnitude by the preamplifier TIA, and then output. TIA is actually a voltage converter, which converts electro-optic current into voltage.

At this time, the voltage signal output by the TIA is still an analog signal, which needs to be converted into a digital signal before the signal processing circuit can recognize it. The function of the Poster Amplifier behind the TIA is to convert signals of different amplitudes into digital signals with the same amplitude.

After introducing TOSA and ROSA, let's take a look at what is BOSA?

With the development of process level technology, the modules can be made smaller. TOSA and ROSA integrate the transmission and reception of light (LD and PIN/APD) through the coaxial coupling process, plus splitters, optical fibers and other components, called BOSA (Bi-Directional Optical Sub-Assembly).

Nowadays, the high-speed optical transceiver module integrates high-performance DSP at the receiving end, and its performance in terms of dispersion and noise processing is really good.

Application of Magneto Optical Switch in Wind LiDAR

Basic principles of magneto-optical switch:

The magneto-optical switch is an optical switch that utilizes the Faraday magneto-optical effect. It changes the effect of the magneto-optical crystal on the polarization plane of the incident polarized light by changing the external magnetic field, thereby achieving the effect of optical path switching.

Compared with traditional mechanical optical switches, magneto-optical switches have obvious advantages. The switching speed of the magneto-optical switch reaches μs level and has the following advantages: low polarization sensitivity, no moving parts, small insertion loss, fast response speed, high degree of integration, small crosstalk, small size, etc.

It can meet the requirements of strong anti-interference ability, low driving voltage, high stability, simple and reliable circuit design, and long-term continuous operation.

Application in Wind LiDAR:

The laser emitted by the laser light source system is coupled to the optical antenna by an optical fiber. The radar is equipped with four light-emitting lenses, with an angle of 30° horizontally and an angle of 25° vertically. The four laser beams are switched at a frequency of 50HZ.

The lens in the optical antenna is focused to a fixed distance, where the energy is most concentrated and scatters with the aerosol at that point. The scattered returned light is coupled back into the optical fiber through the lens.

It is mixed with the local oscillator light generated by the laser light source system in the coupler, and then the optical signal is converted into an electrical signal by the photoelectric conversion module and amplified.

The data acquisition system collects electrical signals and performs spectrum processing, and then the data processing system calculates the spectrum to obtain the wind speed information of the single channel.