CN102186066A - Optical fiber wired television super-trunk line transmission system - Google Patents

Abstract

The invention discloses an optical fiber cable TV super-trunk line transmission system. The signal transmission relationship among the modules of the system is as follows: the radio frequency television signal is modulated into the first optical signal and the second optical signal through the front-end module 1, and the first The optical signal passes through the forward optical fiber 8 through the first sub-front-end module 2, the second sub-front-end module 3, the third sub-front-end module 4, the fourth sub-front-end module 5, the fifth sub-front-end module 6, and the sixth sub-front-end module 7, the second optical signal reverse optical fiber 9 passes through the sixth front-end module 7, the fifth front-end module 6, the fourth front-end module 5, the third front-end module 4, the second front-end module 3, the second front-end module One sub-front-end module 2 transmits to realize high-quality digital signal transmission over long distances. The system can improve the reliability of the system; it can compensate the combined second-order intermodulation index and improve the compensation effect; the Raman amplifier is used to improve the CNR index.

Description

A super-trunk transmission system for optical fiber cable television

technical field

The present invention relates to an optical fiber cable TV ultra-trunk transmission system, which is used for the long-distance and large-scale networking project of the subcarrier multiplexing optical fiber cable TV network, and integrates the large-scale networking of the radio and TV cable TV network and the conversion of analog TV to digital TV. Translation plays an important role.

Background technique

Optical fiber cable television ultra-trunk line transmission system uses 1550nm wavelength, and the dispersion constant of conventional single-mode optical fiber at 1550nm wavelength is as high as 17ps/km.nm. The combination of the self-phase modulation effect of the optical fiber at high power and the strong dispersion of the optical fiber over a long distance results in the conversion from the parasitic phase modulation to the parasitic amplitude modulation of the electric field of the light wave, which leads to the nonlinear index of the subcarrier multiplexing optical fiber transmission system—combined second-order The intermodulation is greatly deteriorated, and the carrier-to-noise ratio is restricted. The combined effect of combined second-order intermodulation and carrier-to-noise ratio determines the modulation error ratio of digital TV. How to reduce the degradation of combined second-order intermodulation and carrier-to-noise ratio is the biggest challenge for the realization of fiber-optic cable television super-trunk transmission system.

One of the ways to overcome the combined second-order intermodulation degradation is to perform dispersion compensation on the optical fiber. Since the self-phase modulation in the fiber can be converted into additional light wave intensity modulation through fiber dispersion, people hope to overcome the influence of fiber nonlinearity from the perspective of controlling fiber dispersion. Dispersion compensation can be realized with dispersion compensating fiber or optical fiber. The advantages of dispersion compensating fiber are large bandwidth, good linearity of delay characteristics (small delay ripple), and less sensitivity to the position of the compensator than chirped fiber gratings. The disadvantage is that the insertion loss is large and the nonlinearity is also large.

The fiber-optic CATV ultra-trunk line transmission system requires the use of erbium-doped fiber amplifier chains. The fiber span between two adjacent EDFAs in the chain is determined by the input optical power tolerated by the system index and the input optical power of the next erbium-doped fiber amplifier. The optical power entering the fiber is subject to the stimulated Brillouin scattering threshold and the second-order intermodulation index of the system combination, and generally does not exceed 16dBm in the ultra-trunk system. The input optical power of the erbium-doped fiber amplifier is subject to the system carrier-to-noise ratio index. Due to the existence of amplified spontaneous emission noise (ASE), the system output carrier-to-noise ratio impairment caused by the erbium-doped fiber amplifier depends on its input optical power. The greater the input optical power, the less the carrier-to-noise ratio drops.

The original fiber-optic CATV ultra-trunk line transmission system is shown in Figure 1. It consists of an optical transmitter (31), multiple segments of optical fiber and multiple erbium-doped optical fiber amplifiers (32, 33, 34, 35) cascaded. The dispersion compensation fiber (36) is inserted in the place, and the length of the dispersion compensation fiber is calculated according to D*L=D _d *L _d , D is the dispersion constant of the transmission fiber, D _d is the dispersion of the dispersion compensation fiber, and L is the transmission fiber to be compensated length, L _d is the length of the dispersion compensating fiber.

This system has the following problems:

1. The calculation of the length of the dispersion compensation fiber only considers the influence of dispersion, but does not take into account the influence of the self-phase modulation effect caused by the large input fiber power, which makes the compensation inaccurate and needs to be adjusted repeatedly in the project.

2. When there is an ultra-long-span optical relay section greater than 100km, the system carrier-to-noise ratio index is seriously deteriorated due to the small input optical power of the subsequent erbium-doped fiber amplifier. If an optical relay point is added in the middle, the Under some conditions, it will be limited by power supply, environment and other conditions.

3. Due to the large coverage and many users of the optical fiber cable TV super trunk line transmission system, if equipment failure or optical fiber breakage occurs at a certain point in Figure 1, it will cause a large-scale viewing interruption, and the repair time will be long. This is the radio and television safety broadcast control center not allowed.

Due to the above reasons, in recent years, the experiments and engineering practice of fiber-optic cable TV ultra-trunk transmission system in China have basically adopted the method of trial and error, lacking theoretical guidance, resulting in large differences in experimental results and unsatisfactory results. In engineering, various equipment and parameters need to be debugged repeatedly, which makes the construction period long and the performance unstable.

Contents of the invention

The purpose of the present invention is to provide an optical fiber cable television super-trunk transmission system, which can not only improve the reliability of TV super-trunk transmission, but also reduce the debugging time of the trunk transmission system.

To achieve the above object, the present invention adopts the following technical solutions:

An optical fiber cable television super-trunk line transmission system of the present invention, the system includes a front-end module 1, a first sub-front-end module 2, a second sub-front-end module 3, a third sub-front-end module 4, a fourth sub-front-end module 5, a fifth sub-front-end module Sub-front-end module 6, sixth sub-front-end module 7, forward optical fiber 8, reverse optical fiber 9, wherein:

The front-end module is used for modulating the radio frequency television signal into the first optical signal and the second optical signal;

The first branch front-end module 2, the second branch front-end module 3, the third branch front-end module 4, the fourth branch front-end module 5, the fifth branch front-end module 6, and the sixth branch front-end module 7, each module is used for Dispersion compensation, amplifying optical signals, splitting optical signals, selecting optical signals, distributing optical power, and compensating the power of optical signals.

The signal transmission relationship between the above modules is as follows: the radio frequency television signal is modulated into the first optical signal and the second optical signal through the front-end module 1, and the first optical signal passes through the forward optical fiber 8 through the first sub-front-end module in turn 2. The second branch front-end module 3, the third branch front-end module 4, the fourth branch front-end module 5, the fifth branch front-end module 6, and the sixth branch front-end module 7 are transmitted, and the second optical signal reverse optical fiber 9 passes through the first Six-point front-end module 7, fifth-point front-end module 6, fourth-point front-end module 5, third-point front-end module 4, second-point front-end module 3, and first-point front-end module 2 to transmit digital signals over long distances transmission.

The front-end module 1 includes a forward optical transmitter 11, a reverse optical transmitter 12, a forward switching optical switch 13, a reverse switching optical switch 14, a forward erbium-doped fiber amplifier 15, and a reverse erbium-doped fiber amplifier 16 ,in:,

The forward optical transmitter 11 and the reverse optical transmitter 12 respectively modulate the radio frequency television signal to two output optical ports;

The forward switching optical switch 13 is used to switch out the forward optical signal, the normally closed port is connected to the first output optical port of the optical transmitter 17, and the normally open port is connected to the first output optical port of the optical transmitter 18 Mouth, common port is connected with erbium-doped fiber amplifier 21;

The reverse switching optical switch 14 is used to switch out the reverse optical signal, the normally closed port is connected with the second output optical port of the optical transmitter 17, and the second output optical port of the optical transmitter 18 of the normally open port is The port is connected, and the common port is connected with the erbium-doped fiber amplifier 22;

A forward erbium-doped fiber amplifier 15 is used to amplify the forward optical signal and output it to the forward optical fiber 8;

The reverse erbium-doped fiber amplifier 16 is used to amplify the reverse optical signal and output it to the reverse optical fiber 9 .

The above-mentioned first sub-front-end module 2, second sub-front-end module 3, third sub-front-end module 4, fourth sub-front-end module 5, fifth sub-front-end module 6, sixth sub-front-end module 7, wherein,

The first branch front-end module 2, the second branch front-end module 3, the third branch front-end module 4, the fourth branch front-end module 5, the fifth branch front-end module 6, and the sixth branch front-end module 7 respectively include:

Forward dispersion compensation module 21 is used to carry out dispersion compensation to forward optical fiber 8, and its output is connected with forward erbium-doped fiber amplifier 23;

The reverse dispersion compensation module 22 is used to carry out dispersion compensation to the previous relay section of the reverse optical fiber 9, and its output is connected to the erbium-doped fiber amplifier 24;

Forward erbium-doped fiber amplifier 23, for amplifying forward optical signal, its output is connected with forward optical splitter 25;

Reverse erbium-doped fiber amplifier 24 is used to amplify the reverse optical signal, and its output is connected with reverse optical splitter 26;

The forward optical splitter 25 is used to split the forward optical signal, one output is connected to the forward optical fiber, and the other output is connected to the normally closed port of the optical selection switch 27;

The reverse optical splitter 26 is used to split the reverse optical signal, one output is connected to the next relay section of the reverse optical fiber 9, and the other output is connected to the normally open port of the selection optical switch 27;

Optical selection switch 27 is used to select forward or reverse optical signal, and the output is connected with distribution erbium-doped fiber amplifier 28;

Distribute the erbium-doped fiber amplifier 28, for providing enough optical power to the optical distribution network behind the sub-front-end module;

Forward pumping Raman amplifier 29, when the length of the previous optical fiber section of forward optical fiber 8 exceeds 100km, it is used to compensate the power of the forward optical signal, and the output is connected to the forward dispersion compensation module 21;

The reverse pump Raman amplifier 30 is used to compensate the power of the reverse optical signal when the length of the previous hop of the reverse optical fiber 9 exceeds 100 km, and the output is connected to the reverse dispersion compensation module 22 .

The above-mentioned forward dispersion compensation module 21 includes an erbium-doped fiber amplifier 201 and a dispersion compensation fiber 202, and the reverse dispersion compensation module 22 includes an erbium-doped fiber amplifier 203 and a dispersion compensation fiber 204, wherein:

Erbium-doped fiber amplifier 201, used to compensate the loss of the dispersion compensation fiber, the output is connected to the dispersion compensation fiber;

Dispersion compensating fiber 202, used to improve the second-order intermodulation index of the system combination caused by fiber dispersion and self-phase modulation, the length of the dispersion compensating fiber According to the following calculation formula, its calculation formula:

为DCF的色散常数，D为传输光纤的色散常数，

为入纤光功率，

为传输光纤的非线性系数，L为补偿段的光纤长度，

为色散补偿光纤的输入光功率，

为色散补偿光纤的非线性系数，

为色散补偿光纤的长度。α为传输光纤的损耗常数，

为DCF的损耗常数。In the formula,

is the dispersion constant of DCF, D is the dispersion constant of the transmission fiber,

is the optical power into the fiber,

is the nonlinear coefficient of the transmission fiber, L is the fiber length of the compensation section,

is the input optical power of the dispersion compensating fiber,

is the nonlinear coefficient of the dispersion compensating fiber,

The length of the fiber for dispersion compensation. α is the loss constant of the transmission fiber,

is the loss constant of DCF.

The forward dispersion compensation module 21 and the reverse dispersion compensation module 22 respectively include a low-power erbium-doped optical fiber amplifier and a dispersion compensation optical fiber.

Compared with the prior art, a fiber-optic cable television ultra-trunk line transmission system of the present invention has the following prominent substantive features and significant advantages:

1. The system adopts a dual-transmitter dual-fiber bidirectional transmission structure, and redundantly backs up the signal, so that the system recovery time is shortened to the switching time of the optical selection switch, which reaches the millisecond level, and the system reliability is improved;

2. The system pre-calculates the length of the dispersion compensation fiber according to the input optical power and fiber length of the compensation section, and customizes the dispersion compensator, so that the combined second-order intermodulation index can be accurately compensated, so that the compensation effect is better;

3. The system uses a Raman amplifier to solve the problem of high-quality signal transmission with an ultra-long span of more than 100km without optical relay, which improves the input optical power of the subsequent EDFA and improves the CNR index.

Description of drawings

Fig. 1 is the structural schematic diagram of existing optical fiber cable television ultra-trunk transmission system;

Fig. 2 is the structural representation of a kind of optical fiber cable television ultra-trunk line transmission system of the present invention;

FIG. 3 is a schematic diagram of forward optical fiber line configuration according to an embodiment of the present invention.

Detailed ways

The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments. As shown in Figure 2, an optical fiber cable television super-trunk line transmission system of the present invention, the system includes a front-end module (1), a first sub-front-end module (2), a second sub-front-end module (3), a third sub-front-end Module (4), fourth-point front-end module (5), fifth-point front-end module (6), sixth-point front-end module (7), forward optical fiber (8), reverse optical fiber (9), of which:

The front-end module is used to modulate the radio frequency television signal into the first optical signal and the second optical signal,

The first branch front-end module (2), the second branch front-end module (3), the third branch front-end module (4), the fourth branch front-end module (5), the fifth branch front-end module (6), the sixth branch It is divided into front-end modules (7), and each module is used for dispersion compensation, amplifying optical signals, splitting optical signals, selecting optical signals, distributing optical power, and compensating the power of optical signals;

The signal transmission relationship between the above modules is as follows: the radio frequency television signal is modulated into the first optical signal and the second optical signal through the front-end module (1), and the first optical signal passes through the forward optical fiber (8) in turn through the second optical signal. One point front-end module (2), the second point front-end module (3), the third point front-end module (4), the fourth point front-end module (5), the fifth point front-end module (6), the sixth point front-end module ( 7) Transmission, the second optical signal reverse optical fiber (9) passes through the sixth sub-front-end module (7), the fifth sub-front-end module (6), the fourth sub-front-end module (5), the third sub-front-end module ( 4), the second sub-front-end module (3) and the first sub-front-end module (2) are transmitted to realize digital signal transmission over a long distance.

The front-end module (1) includes: forward optical transmitter (11), reverse optical transmitter (12), forward switching optical switch (13), reverse switching optical switch (14), forward erbium-doped Optical fiber amplifier (15), reverse erbium-doped optical fiber amplifier (16), wherein:,

The forward optical transmitter (11) and the reverse optical transmitter (12) respectively modulate radio frequency television signals to two output optical ports;

The forward switching optical switch (13) is used to switch out the forward optical signal, the normally closed port is connected to the first output optical port of the optical transmitter (17), and the normally open port is connected to the optical transmitter (18) The first output optical port of the common port is connected with the erbium-doped fiber amplifier (21);

Reverse switching optical switch (14), used to switch out the reverse optical signal, the normally closed port is connected to the second output optical port of the optical transmitter (17), and the normally open port of the optical transmitter (18) The second output optical port is connected, and the common port is connected with the erbium-doped fiber amplifier (22);

A forward erbium-doped fiber amplifier (15), used to amplify the forward optical signal and output it to the forward optical fiber (8);

A reverse erbium-doped fiber amplifier (16), used to amplify the reverse optical signal and output it to the reverse optical fiber (9);

The first branch front-end module (2), the second branch front-end module (3), the third branch front-end module (4), the fourth branch front-end module (5), the fifth branch front-end module (6), the sixth branch sub-front-end module (7),

The first branch front-end module (2), the second branch front-end module (3), the third branch front-end module (4), the fourth branch front-end module (5), the fifth branch front-end module (6), the sixth branch Sub-front-end modules (7) include:

The forward dispersion compensation module (21) is used to perform dispersion compensation on the forward optical fiber (8), and its output is connected to the forward erbium-doped fiber amplifier (23);

The reverse dispersion compensation module (22) is used to perform dispersion compensation on the previous relay section of the reverse optical fiber (9), and its output is connected to the erbium-doped fiber amplifier (24);

The forward erbium-doped fiber amplifier (23) is used to amplify the forward optical signal, and its output is connected to the forward optical splitter (25);

The reverse erbium-doped fiber amplifier (24) is used to amplify the reverse optical signal, and its output is connected to the reverse optical splitter (26);

The forward optical splitter (25) is used to split the forward optical signal, one output is connected to the forward optical fiber, and the other output is connected to the normally closed port of the optical selection switch (27);

The reverse optical splitter (26) is used to split the reverse optical signal, one output is connected to the next relay section of the reverse optical fiber (9), and the other output is connected to the normal optical selector switch (27). open port connection;

Optical selector switch (27), used to select forward or reverse optical signal, the output is connected with distribution erbium-doped fiber amplifier (28);

Distribution of erbium-doped fiber amplifiers (28), used to provide sufficient optical power to the optical distribution network after the sub-head-end module;

A forward-pumped Raman amplifier (29), used to compensate the power of the forward optical signal when the length of the previous fiber section of the forward optical fiber (8) exceeds 100 km, and the output is connected to the forward dispersion compensation module (21);

Reversely pumped Raman amplifier (30), used to compensate the power of the reverse optical signal when the length of the previous trunk section of the reverse optical fiber (9) exceeds 100km, the output is connected to the reverse dispersion compensation module (22) .

按如下计算式，其计算式：The above-mentioned forward dispersion compensation module (21) includes an erbium-doped fiber amplifier (201) and a dispersion compensation fiber (202), and the reverse dispersion compensation module (22) includes an erbium-doped fiber amplifier (203) and a dispersion compensation fiber (204), Among them: erbium-doped fiber amplifier (201) (203), used to compensate the loss of dispersion compensation fiber, the output is connected with dispersion compensation fiber; dispersion compensation fiber (202) (204), used to improve fiber dispersion and self-phase modulation caused by The second-order intermodulation index of the system combination, the length of the dispersion compensating fiber

According to the following calculation formula, its calculation formula:

为DCF的色散常数，D为传输光纤的色散常数，

为入纤光功率，

为传输光纤的非线性系数，L为补偿段的光纤长度，

为色散补偿光纤的输入光功率，

为色散补偿光纤的非线性系数，为色散补偿光纤的长度。α为传输光纤的损耗常数，

为DCF的损耗常数。In the formula,

is the dispersion constant of DCF, D is the dispersion constant of the transmission fiber,

is the optical power into the fiber,

is the nonlinear coefficient of the transmission fiber, L is the fiber length of the compensation section,

is the input optical power of the dispersion compensating fiber,

is the nonlinear coefficient of the dispersion compensating fiber, The length of the fiber for dispersion compensation. α is the loss constant of the transmission fiber,

is the loss constant of DCF.

The forward dispersion compensation module (21) and the reverse dispersion compensation module (22) respectively include: a low-power erbium-doped optical fiber amplifier and a dispersion compensation optical fiber.

The protection switching process of a kind of optical fiber cable television ultra-trunk line transmission system of the present invention is described as follows:

The switching process of the front-end module (1) in the above system:

The radio frequency signals input by the optical transmitter (11) and the optical transmitter (12) in the front-end module (1) in the above system are the same. When the system is working normally, the optical signal output by the optical transmitter (11) enters the normally closed port of the optical switch (13), is connected to the erbium-doped fiber amplifier (15) through the optical switch (13), and is input to the forward optical fiber after being amplified . The optical signal output by the optical transmitter (12) enters the normally closed port of the optical switch (14), is connected to the erbium-doped optical fiber amplifier (16) through the optical switch (14), and is input to the reverse optical fiber after being amplified.

When the optical transmitter (11) is damaged and there is no optical output, the optical switch (13) detects that the normally closed port has no optical input, then switches to the normally open port, and connects the optical transmitter (12) with the erbium-doped fiber amplifier (15) , restore the downlink forward optical signal, when the optical transmitter (12) is damaged and there is no optical output, the optical switch (14) detects that the normally closed port has no optical input, then switches to the normally open port and connects to the optical transmitter (11) With the erbium-doped fiber amplifier (16), the downlink reverse optical signal is recovered. Therefore, it is ensured that the forward and reverse optical signals can be generated since only one transmitter works.

The switching process of the second sub-front-end module (3) in the above system:

When the above system works normally, the forward optical signal from the last fiber section of the forward optical fiber (8) is input into the reverse pump Raman amplifier (29), and after passing through the erbium-doped optical fiber amplifier (201), it is input into the dispersion compensation optical fiber ( 202), and then input the optical splitter (25) through the forward erbium-doped fiber amplifier (23), the output of the optical splitter (25) is connected to the next forward fiber section, and the other output is connected to the optical selection switch (27) The normally closed port is output to the optical distribution network after distribution of the erbium-doped optical fiber amplifier (28). The reverse optical signal from the last fiber section of the reverse optical fiber (9) is input into the reverse pumped Raman amplifier (30), after passing through the erbium-doped optical fiber amplifier (203), it is input into the dispersion compensation optical fiber (204), and then passed through the doped The erbium fiber amplifier (24) is input to the optical splitter (26), and one output of the reverse optical splitter (26) is connected to the next reverse optical relay section, and the other output is connected to the normally open port of the optical selection switch (27).

When the forward optical fiber breaks or a certain device on the forward optical fiber path fails, the optical selection switch (27) detects that there is no optical input at the normally closed port, then switches to the normally open port and connects to the reverse optical splitter (26) And distribute the erbium-doped optical amplifier (28), make the reverse optical path communicate with the optical distribution network, restore the optical signal.

In order to verify the transmission reliability of the optical fiber cable TV ultra-trunk transmission system of the present invention and the debugging time of the trunk transmission system, the dispersion compensation optical fiber length, carrier-to-noise ratio, and modulation error ratio index of the system are verified and tested:

As shown in Figure 3, the configuration diagram of the forward optical fiber line in the transmission system of the present invention, the total length of the line of this configuration diagram is 560km, and the optical selection switch and the optical splitter that do not have an impact on the system index are omitted in the figure .

Dispersion compensating optical fibers (39, 42, 46, 49, 52) are respectively arranged in the optical fiber section (61), optical fiber section (62), optical fiber section (63), optical fiber section (64), and optical fiber section (65), and the optical fiber section (60) Dispersion compensation is not used, and erbium-doped fiber amplifiers (41), erbium-doped fiber amplifiers (45), erbium-doped fiber amplifiers (48), and erbium-doped fiber amplifiers (51) are used to compensate the attenuation of dispersion-compensated optical fibers respectively. The long-span section (61) and the ultra-long-span section (63) use 14dB reverse-pumped Raman amplifiers (38) and reverse-pumped Raman amplifiers (44) to increase output power and improve system carrier-to-noise ratio .

，采用的DCF参数为：

，

，

，

。采用7dBm输出的小功率掺铒光纤放大器补偿色散补偿光纤的衰减，经计算得到的色散补偿光纤长度如表1：Calculate the optimal dispersion compensation fiber length according to the above formula (1)

, the DCF parameters used are:

,

,

,

. A low-power erbium-doped fiber amplifier with a 7dBm output is used to compensate the attenuation of the dispersion compensation fiber. The calculated length of the dispersion compensation fiber is shown in Table 1:

Table 1 Dispersion compensation fiber length

Dispersion compensation fiber number Length (km) 39 13 42 7 46 11 49 6 52 7

As shown in Figure 3, the system of the present invention is tested according to the forward optical fiber line configuration in the figure, and the obtained indexes are carrier-to-noise ratio CNR>40dB, combined second-order intermodulation index CSO>55dB, digital TV modulation error ratio MER> 35dB, which meets the transmission requirements of high-quality digital TV transmission over long distances.

As can be seen from the above narration, a kind of fiber-optic cable TV ultra-trunk line transmission system of the present invention adopts a dual-transmitter dual-fiber bidirectional redundant transmission structure, which can improve system reliability, and improve the dispersion compensation effect by calculating the length of the dispersion-compensating optical fiber. The combined second-order intermodulation index is obtained; the reverse-pumped distributed Raman amplifier is used to solve the problem of high-quality signal transmission with ultra-long span without optical relay, and the carrier-to-noise ratio index is improved. And the calculation method of the system index is given, which can configure the transmission equipment according to the actual topology in the system design stage, thereby shortening the project debugging time.

Claims (5)

1. An optical fiber cable TV ultra-trunk line transmission system, characterized in that the system includes a front-end module (1), a first sub-front-end module (2), a second sub-front-end module (3), a third sub-front-end module (4 ), the fourth sub-front-end module (5), the fifth sub-front-end module (6), the sixth sub-front-end module (7), forward optical fiber (8), reverse optical fiber (9), where: The front-end module is used for modulating the radio frequency television signal into the first optical signal and the second optical signal; The first branch front-end module (2), the second branch front-end module (3), the third branch front-end module (4), the fourth branch front-end module (5), the fifth branch front-end module (6), the sixth branch It is divided into front-end modules (7), and each module is used for dispersion compensation, amplifying optical signals, splitting optical signals, selecting optical signals, distributing optical power, and compensating the power of optical signals; The signal transmission relationship between the above modules is as follows: the radio frequency television signal is modulated into the first optical signal and the second optical signal through the front-end module (1), and the first optical signal passes through the forward optical fiber (8) in turn through the second optical signal. One point front-end module (2), the second point front-end module (3), the third point front-end module (4), the fourth point front-end module (5), the fifth point front-end module (6), the sixth point front-end module ( 7) Transmission, the second optical signal reverse optical fiber (9) passes through the sixth sub-front-end module (7), the fifth sub-front-end module (6), the fourth sub-front-end module (5), the third sub-front-end module ( 4), the second sub-front-end module (3) and the first sub-front-end module (2) are transmitted to realize digital signal transmission over a long distance. 2. An optical fiber cable TV ultra-trunk transmission system according to claim (1), characterized in that: the above-mentioned front-end module, the front-end module (1) includes a forward optical transmitter (11), a reverse Optical transmitter (12), forward switch (13), reverse switch optical switch (14), forward erbium-doped fiber amplifier (15), reverse erbium-doped fiber amplifier (16), wherein: The forward optical transmitter (11) and the reverse optical transmitter (12) respectively modulate radio frequency television signals to two output optical ports; The forward switching optical switch (13) is used to switch out the forward optical signal, the normally closed port is connected to the first output optical port of the optical transmitter (11), and the normally open port is connected to the optical transmitter (12) The first output optical port of the common port is connected with the erbium-doped fiber amplifier (15); Reverse switching optical switch (14), used to switch out the reverse optical signal, the normally closed port is connected to the second output optical port of the optical transmitter (12), and the normally open port is connected to the optical transmitter (11 ) is connected to the second output optical port, and the common port is connected to the erbium-doped fiber amplifier (16); A forward erbium-doped fiber amplifier (15), used to amplify the forward optical signal and output it to the forward optical fiber (8); The reverse erbium-doped fiber amplifier (16) is used to amplify the reverse optical signal and output it to the reverse optical fiber (9). 3. An optical fiber cable television super-trunk line transmission system according to claim (1), characterized in that: the first sub-front-end module (2), the second sub-front-end module (3), and the third sub-front-end module (4), the fourth sub-front-end module (5), the fifth sub-front-end module (6), the sixth sub-front-end module (7), wherein, The first branch front-end module (2), the second branch front-end module (3), the third branch front-end module (4), the fourth branch front-end module (5), the fifth branch front-end module (6), the sixth branch Sub-front-end modules (7) include: The forward dispersion compensation module (21) is used to perform dispersion compensation on the forward optical fiber (8), and its output is connected to the forward erbium-doped fiber amplifier (23); The reverse dispersion compensation module (22) is used to perform dispersion compensation on the previous relay section of the reverse optical fiber (9), and its output is connected to the erbium-doped fiber amplifier (24); The forward erbium-doped fiber amplifier (23) is used to amplify the forward optical signal, and its output is connected to the forward optical splitter (25); The reverse erbium-doped fiber amplifier (24) is used to amplify the reverse optical signal, and its output is connected to the reverse optical splitter (26); Forward optical splitter (25), used for splitting the forward optical signal, one output is connected to the forward optical fiber, and the other output is connected to the normally closed port of the optical switch (27); The reverse optical splitter (26) is used to split the reverse optical signal, one output is connected to the next relay section of the reverse optical fiber (9), and the other output is connected to the normally open optical switch (27) port connection; The optical switch (27) is used to select the forward or reverse optical signal, and the output is connected to the distributed erbium-doped fiber amplifier (28); Distribution of erbium-doped fiber amplifiers (28), used to provide sufficient optical power to the optical distribution network after the sub-head-end module; A forward-pumped Raman amplifier (29), used to compensate the power of the forward optical signal when the length of the previous fiber section of the forward optical fiber (8) exceeds 100 km, and the output is connected to the forward dispersion compensation module (21); Reversely pumped Raman amplifier (30), used to compensate the power of the reverse optical signal when the length of the previous trunk section of the reverse optical fiber (9) exceeds 100km, the output is connected to the reverse dispersion compensation module (22) . 4. The fiber-optic CATV ultra-trunk line transmission system according to claim (1), characterized in that the above-mentioned forward dispersion compensation module (21) includes an erbium-doped optical fiber amplifier (201) and a dispersion-compensating optical fiber (202) , the reverse dispersion compensation module (22) includes an erbium-doped fiber amplifier (203) and a dispersion compensation fiber (204), wherein: Erbium-doped fiber amplifier (201) (203), used to compensate the loss of the dispersion compensation fiber, the output is connected to the dispersion compensation fiber; Dispersion compensation fiber (202) (204), used to improve the second-order intermodulation index of the system combination caused by fiber dispersion and self-phase modulation, the length of the dispersion compensation fiber According to the following calculation formula, its calculation formula:

 

In the formula,

is the dispersion constant of DCF, D is the dispersion constant of the transmission fiber, is the optical power into the fiber,

is the nonlinear coefficient of the transmission fiber, L is the fiber length of the compensation section,

is the input optical power of the dispersion compensating fiber,

is the nonlinear coefficient of the dispersion compensating fiber, is the length of the dispersion compensating fiber, α is the loss constant of the transmission fiber, is the loss constant of DCF. 5. The fiber-optic CATV ultra-trunk line transmission system according to claim (1), characterized in that: the forward dispersion compensation module (21) and the reverse dispersion compensation module (22) respectively include doped Erbium fiber amplifier, dispersion compensation fiber.

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