Development of video analog optical fiber technology

AddTime£º13-07-2016   Author£ºadmin

     There are three kinds of signals in the monitoring system: image, audio, data, how to put these three kinds of signals under effective control is one of the factors to be considered. Before the application of fiber copper because of low cost and has been used in a large amount (but in the long-distance transmission using optical fiber transmission cost is lower than that of the copper wires), but copper cable transmission is more and more exposure the drawback, short transmission distance, confidentiality poor, vulnerable to electromagnetic interference, maintenance high cost, and so on. After the emergence of optical fiber, optical fiber communication applications get rapid development, has become a long / short distance transmission over 500/800 meters of distance preferred, can be expected when a further decline in the cost of optical fiber, the fiber will replace copper, a large number of applications. HYG optical communication equipment Co., Ltd. to explain about the development of video simulation of optical fiber technology, showing the inevitable development of optical fiber.

     In the transmission of optical fiber monitoring system, the mode of transmission signal can be divided into two ways: one is analog optical fiber transmission, the other is digital optical fiber transmission. At present, analog optical fiber transmission is widely used because of its mature technology guarantee. Commonly used analog optical fiber transmission, can be broadly divided into the following categories: VIDEO, DATA, AUDIO, VIDEO+DATA, VIDEO+AUDIO, VIDEO+DATA+AUDIO, etc..

In this article, mainly discusses the analog optical fiber transmission technology, process, equipment type, video signal of several important parameters of nouns explain, test and design (selection of equipment) to consider the safety, maintain and cost factors.

    1 technology and technology of optical fiber transmission equipment

     The traditional analog optical transceiver technology used in two ways: FM and AM. The early optical fiber transmission equipment mostly uses the AM technology, and with the passage of time, the FM technology has become the market mainstream, table 1 the characteristics of AM and FM for the qualitative comparison: by Table 1 shows, the FM technology is am technology more reliable: strong anti-interference ability, high fidelity, online form good medium transmission of non linear distortion requirements is not high, greatly improve the sensitivity of the optical receiver.

     Early optical transmission equipment, the welding process for plug-in and plug welding technology has its inherent in the side, such as plate electromagnetic interference, high power consumption of equipment, product volume and so on, thus the transmission system caused a certain impact, due to the plate between electromagnetic perturbation is larger, system is introduced for the noise is large, thus affecting the system signal to noise ratio and the video index of the system. Now products are mostly used SMT technology, reduce the system's electromagnetic noise effect, can better reflect the design intent.

     2, the type of optical fiber transmission equipment

      Transmission mode of optical fiber transmission equipment can be divided into two parts: multimode fiber transmission equipment and single mode optical fiber transmission equipment.

       Multimode optical fiber transmission equipment used by the optical device is LED, usually by wavelength can be divided into 850nm and 1300nm two wavelength, according to the output power can be divided into ordinary LED and enhance LED - ELED. 62.5mm and 50mm two kinds of fiber used in multimode optical fiber transmission.

      In multimode optical fiber transmission decided transmission distance is a major factor in the operating wavelength of the optical fiber bandwidth and led, for example, if the operating wavelength of 1300nm led and 50 micron fiber, the transmission bandwidth is 400MHz.km link attenuation 0.7dB/km. If baseband transmission frequency f is 150MHz, for a fiber power -18dBm, receiver sensitivity is - 25 DBM in optical fiber transmission system, the maximum link loss of 7 dB, then can be calculated:

     ST connector loss:

     2dB (two ST connectors)

     Optical loss margin: 2

     Transmission distance of theory:

      L= (7 dB-2 dB dB-2) km /0.7dB/km=4.2

      L is the transmission distance, which is calculated according to the bandwidth of the optical fiber:

      L=B/F=400MHz.km/150MHz=2.6km

     Where B is the optical fiber bandwidth, f for baseband transmission frequency, then the actual transmission test, l u 2.6km, therefore, decided to transmission distance is the main factor of multimode fiber bandwidth.

      The optical device used in the single mode transmission device is LD, which can be divided into 1300nm and 850nm according to the wavelength, the output power can be divided into normal LD, high power LD, DFB-LD (distributed feedback optical device). The most common fiber used for single-mode optical fiber transmission is G.652, and its diameter is 9 microns.

      Light of wavelength of 1310nm in G.652 fiber upload to lose, decided to limit the transmission distance of is attenuation factor, because under the wavelength of 1310nm and fiber material dispersion and the structure of the dispersion cancel each other out the total dispersion is zero. In 1310 nm with small amplitude of the optical signal to achieve wide band transmission.

      Wavelength of 1550nm optical transmission G.652 fiber attenuation factor is small, considered purely from the attenuation factor and 1550nm wavelength of light in the same optical power transmission distance is greater than the wavelength of 1310nm optical transmission distance, but the actual situation is not the case, the relationship between single-mode optical fiber bandwidth B and dispersion factor D:

B=132.5/ (DlxDxL) GHz

     Where L is the length of the optical fiber, Dl for the spectral width, for the 1550nm wavelength of light, its dispersion factor as shown in Table 3 for 20 ps/ (nm.km), assuming its spectral width is equal to 1nm, the transmission distance of L=50 km, there are:

B=132.5/ (DxL) GHz=132.5MHz

      That is to say, for waveform simulation, using 1550nm light, when the transmission distance is 50 kilometers, transmission bandwidth is less than that of the 132.5 MHz, if baseband transmission frequency f is 150MHz. So the transmission distance is less than 50km. Moreover, the actual application, light source spectrum line width to more than 1 nm.

     It can be seen from the above formula, the 1550nm wavelength of light transmission in the G.652 optical fiber determines the transmission distance limit is mainly the dispersion factor.

     3, video signal of DG (differential gain), DP (differential phase) s / N (signal to noise ratio) DG (differential gain): in the PAL television signal, chrominance signal is modulation in frequency 4.43mhz color subcarrier, and color subcarrier is superposition in the luminance signal, color subcarrier wave amplitude determines the degree of saturation of the color signal. The DG distortion of the video signal is that the gain characteristic of the system varies with the level of the input signal.

Popular said, due to the brightness fire hidden levels are changed to white electricity at ordinary times, at the end of the video channel output produced the chroma signal amplitude changes, so that in bright and dark parts, the color saturation hue (especially saturation) had different changes. DP (differential phase): in the PAL television signal, the chrominance signal is modulation in frequency 4.43mhz color subcarrier, and color subcarrier is superimposed on the luminance signal and color subcarrier phase determines the tone of the color signal. The DG distortion of the video signal is the change of the phase shift characteristic of the system with the input video signal. The phase shift of the transmission line changes with different brightness levels, the color synchronization and color subcarrier phase shift between the changes, so the screen is bright and dark parts of the tone is different.

S/N (signal to noise ratio): in the transmission of TV signal, the ratio of the peak value of the signal power and the effective value of the noise is expressed as the ratio of the signal power.

4, optical fiber transmission equipment, video index detection and commonly used instrument

(1) in the industrial control field, due to the analog FM signal demodulation noise spectrum is triangular in shape, with the increase of the baseband frequency, demodulation noise is more and more big, with the decline in the S / N, image quality is also declining, rules of fine twill patterns on the monitor screen, fluttering like interference pattern, snow and so on. When the modulation waveform is an analog signal, postdetection signal level with increasing signal frequency decreased the representations of are the distortion of the non linear, increase the harmonics of the fundamental component, which affects the DG (differential gain), DP (differential phase). DG differential gain does not meet the requirements. The amplitude of the chroma signal changes at different luminance levels. The change of the amplitude of the chroma signal leads to the change of color saturation. So at the time of the change of the brightness of the screen, the image color saturation to change, the light electricity at ordinary times of red in the eye power usually may become reddish or dark red, causing image distortion. DP differential phase does not meet the requirements. The phase of the chroma signal changes at different luminance levels, and the phase change of the chroma signal causes the color change. In this way, when the brightness level changes, the color of the image should be changed, resulting in distortion.

(2) well known optical attenuator is usually used air attenuation or polarizer attenuation in order to increase the transmission loss and digital signal optical fiber transmission and available BER said the transmission quality is good or bad, and can be used to increase the optical attenuator method to test the receiver sensitivity. But the multi-channel video analog signal transmission in the optical fiber, more to consider noise effect and system of non linear distortion (including optical devices and optical fiber nonlinear distortion), so if by adding optical power of the optical attenuator are testing out is only the amount of power, which the system signal to noise, S / n, the system of non linear distortion is not through adding optical attenuator simulation method. The best way is to use the actual distance of the optical fiber to detect.

(3) there are five main indicators of the video: reflection loss, gain and stability of the intervention, video noise, nonlinear video and video linear distortion, which can reflect the channel quality of analog signals. The optical power, the ratio of noise to noise and the receiving sensitivity of optical fiber transmission are reflected in the quality of optical fiber transmission.

There are several test parameters:

Fiber optical power

Signal to noise ratio

Differential gain

Differential phase

Video signal amplitude

Video waveform monitoring and chroma phase monitoring

(4) test instrument:

Spectrum analyzer

Test signal generator

Vector oscilloscope

Waveform monitor

Video integrated test instrument

Optical power meter

The following instruments can be used:

Tek 2715 cable TV spectrum analyzer

TSG-271PAL TV test signal generator

VM700T full automatic video integrated testing instrument

Tek1711B TV waveform monitor

Tek1721 vector oscilloscope

TOP-200 optical power meter

   5, design program (select equipment) to consider the safety, effective maintenance and cost factors

First, telecom optical fiber transmission equipment, maintenance system security generally have loop system, to ensure reliable transmission; closed-circuit TV optical fiber transmission system, the basic point to point transmission, generally do not have the switching system. If the transmission of the optical fiber or transmission equipment failure, will affect the operation of the entire system. Therefore, should try to avoid using large capacity (such as 64 Road, 32 video / audio equipment) multiplexing optical fiber transmission equipment, while the use of small capacity (e.g., 8 Road, even 4 visual audio equipment) transmission equipment to ensure the security of the whole system.

Secondly, once the device itself out of trouble, the whole monitoring system will be paralyzed, and this device is usually very expensive, owners will not consider on spare parts, manufacturers are not equipped with spot (make to order). At the same time, maintenance cycle is very long (usually more than 1 month). So, from the point of view of security and effective maintenance can not be considered.

In addition, in terms of cost, although the use of large capacity multiplexing transmission to achieve the purpose of saving fiber resources, but its cost is much higher than the use of small capacity optical transmission. And with the fiber prices further down, optical transmission system in optical fiber and cable cost accounts for the proportion is more and more small, and optical procurement costs accounted for proportion is becoming bigger and bigger.

So, in addition to limited by the optical fiber resources, the general design is not the use of large capacity optical transmission equipment. In order to ensure the quality of signal transmission, it is necessary to further discuss whether large capacity multiplex transmission of current monitoring video signal can meet the requirements of monitoring video signal.

From the perspective of the development trend of the monitoring industry, optical fiber digital transmission equipment is the direction of future development. However, due to the factors of technology, the optical fiber digital transmission equipment used in the monitoring industry is still a conceptual transition product. Because of its mature technology, low price, real-time and non - destructive transmission, optical fiber analog transmission equipment is still an excellent means of transmission, and it will be widely used in recent years.http://www.hygoptics.com