Optical transceiver classification and principle

Optical transceiver classification and principle

The optical transmission system consists of three parts: a light source (optical transmitter), a transmission medium, and a detector (optical receiver). The work of the light source and the detector is performed by the optical transceiver. The Optical Multiplexer is a device that transforms multiple E1s (a data transmission standard for trunk lines, typically at 2.048 MbPS, which is adopted in China and Europe) into optical signals and transmits them. Its role is mainly to realize electro-optical and optical - Electric conversion). Optical transceivers mainly include analog optical transceivers and digital optical transceivers:

1, analog optical transceiver

The analog optical transceiver adopts the PFM modulation technology to transmit the image signal in real time, which is currently used more often. The transmitting end will first perform the PFM modulation of the analog video signal (generally, there are several methods of frequency modulation, phase modulation and amplitude modulation, so that the analog optical transceiver is divided into several optical transceivers such as frequency modulation, phase modulation and amplitude modulation), and then the electro-optical conversion is performed. After transmitting to the receiver, the optical-electrical conversion is performed, and then PFM demodulation is performed to recover the video signal. Due to the adoption of PFM modulation technology, the transmission distance can easily reach around 30Km, and the transmission distance of some products can reach 60Km, or even hundreds of kilometers. Moreover, the distortion of the image signal after transmission is very small, with high signal-to-noise ratio and little nonlinear distortion. By using wavelength division multiplexing technology, it is also possible to realize bidirectional transmission of image and data signals on one optical fiber.

2, digital optical transceiver

Since digital technology has obvious advantages in many respects compared with traditional analog technologies, just as digital technology has replaced analog technology in many fields, the digitalization of optical transceivers is also an inevitable trend. At present, digital image optical transceivers mainly have two technical methods: one is an MPEGII image compression digital optical transceiver, and the other is a non-compressed digital image optical transceiver.

The image compression digital optical transceiver generally adopts MPEGII image compression technology. It can compress moving images into N×2Mbps data streams and transmit them through a standard telecommunication interface or directly through optical fibers.

Due to the use of image compression technology, it can greatly reduce the signal transmission bandwidth to facilitate the use of less resources to transmit image signals. At the same time, thanks to the N×2Mbps standard interface, it is possible to use the rich channels of existing telecommunication transmission equipment to transmit surveillance images, which brings convenience to engineering applications. However, image compression digital optical transceivers also have their inherent disadvantages. Its fatal weakness is that it cannot guarantee the real-time performance of image transmission. Because image compression and decompression require a certain amount of time, a 1-2S delay is generally generated for the transmitted image. Therefore, this kind of equipment is only suitable for places with low requirement for real-time performance. In addition, the image will produce some distortion after compression, and the price of this optical transceiver is also high.

The principle of the uncompressed digital image optical transceiver is to carry out A/D conversion of the analog video signal and multiplex the signal of voice, audio, data, etc., and then transmit it through the optical fiber. It uses a higher data rate to guarantee the transmission quality and real-time performance of the video signal. Since the bandwidth of the optical fiber is very large, this high data rate does not put too much demands on the transmission channel. The uncompressed digital image optical transceiver provides excellent image transmission quality (signal-to-noise ratio is greater than 60dB, differential phase distortion is less than 2, differential gain distortion is less than 2%), achieves broadcast-grade transmission quality, and image transmission is fully real-time. Due to the adoption of digital technology, sophisticated communication technologies such as multiplexing and optical transceiver technologies can be used in the device to improve the reliability of the device and reduce costs.