Video monitoring optical transceiver application principle and mode analysis

With the rapid development of broadband digital optical transmission device technology and digital video technology, digital video optical transceivers have begun to move toward the market and gradually replaced the analog optical transceiver market, ushering in the era of high-speed development of digital video optical transceivers, and showing mature technology and superior performance. Wide application and strong demand.

Optical principle and coding
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.

Optical Mode Analysis

The difference between a single mode optical transceiver and a multimode optical transceiver lies in the fact that the refraction angle of the light wave in the core is different from the fiber diameter of the core. The single mode is used for transmitting data over long distances and the multimode is used for data transmission within 2 kilometers. There is a gradual reduction in the equipment price gap between the single-mode optical transceiver and the multi-mode optical transceiver. The following will analyze the difference between the single-mode optical transceiver and the multi-mode optical transceiver and purchase the optical transceiver.

The commonly used optical fibers are divided into two types according to modes: multimode optical fibers and single-mode optical fibers. The optical fiber cable made of multi-mode optical fiber is the earliest commercially available fiber optic cable. Due to the problems of serious dispersion, large attenuation, narrow available bandwidth, and high cost, it has already withdrawn from the telecommunication operator's optical network platform, which is the main battlefield of optical fiber cable, but it is only smart. The fiber optic cabling of buildings also retains a certain market share.

When the multi-mode optical fiber cable is used for video image transmission, it can only meet the transmission distance of about 3 to 5 km, and has a greater limitation on the bandwidth (for analog modulation) and transmission rate (for digital) of the video optical transceiver. Suitable for short-distance, small-capacity, simple applications. In particular, the effect of dispersion greatly limits the application of high-rate transmission of non-compressed digital optical transceivers in terms of transmission distance and capacity.

The single-mode optical fiber made of single-mode optical fiber (mainly G.652 optical fiber) has become the mainstream of current optical communication transmission due to its excellent characteristics and low price. The emerging new technologies in the field of optical fiber communications, whether it is long-distance, large-capacity or multi-service, are all developed for single-mode optical fibers. The service life of the optical cable is more than 20 years, which is much longer than the actual service life of the optical transceiver. It is necessary to consider the laying of optical cables from the perspective of strategic resources. It must take into account future capacity expansion and equipment upgrades. The number of optical fiber cores is sufficient, and new services can continue to be carried.

China Telecom’s optical fiber transmission equipment has been replaced for several generations, but the normal use of optical cables laid in more than a decade or two years ago is a prime example. It is also worth noting that the price gap between the single-mode optical transceivers and the multi-mode optical transceivers is gradually decreasing, and the price difference between multi-channel video equipment is even zero. Moreover, single-mode optical cables are also cheaper than multi-mode optical cables. The old view that the cost of multi-mode equipment must be low must be updated. It seems that single-mode optical fiber cable is currently the recommended choice, of course, optical transceivers must use optical transceivers that support single-mode optical fiber. In simple terms, single-mode/multi-mode optical fiber/optical fiber is generally selected based on the number of kilometers and cost, and the corresponding type of optical transceiver is selected.

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