JPH08116307A - Optical fiber transmission system - Google Patents

Abstract

PURPOSE: To perform the long distance transmission of small waveform distortion by suppressing the influence of the wavelength dispersion and self-phase modulation effect of a transmission line. CONSTITUTION: A single mode optical fiber 2a of negative dispersion between a transmitter and a repeater, and a single mode optical fiber 2b of positive dispersion between the repeaters, and a single mode optical fiber 2c of negative dispersion between the repeater and a receiver, are connected alternately. Assuming the products of each distance and each dispersion value of the optical fibers 2a, 2b and 2c as A, B and C, the optical fibers are connected so as to set the sum of A and C equal to B, and signal light is transmitted in phase modulated light of constant light intensity. At this time, the self-phase modulation effect can be eliminated since the light intensity is constant. Actually, the small waveform dispersion occurs due to alternately appearing positive and negative dispersion, however, positive and negative dispersion areas of the waveform dispersion can be reversed in a primary range by reducing dispersion, and phase change by the self-phase modulation effect can be offset in the primary range.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention generally relates to an optical fiber transmission system, and more particularly to a long-distance optical fiber transmission system in which signal waveform distortion is small.

[0002]

2. Description of the Related Art When modulated light is propagated in a long-distance single-mode optical fiber transmission line, first, chromatic dispersion of the optical fiber causes waveform distortion, which greatly deteriorates transmission characteristics. To prevent this, there is a method of making the chromatic dispersion value of the fiber zero by using a zero dispersion fiber or the like in the transmission line.

However, when the chromatic dispersion in the optical fiber transmission line is zero, the transmission characteristics are deteriorated due to four-photon mixing. Therefore, as an alternative, an optical fiber having a small negative dispersion and a positive fiber for compensating for it are used. There is known a method in which optical fibers having dispersion are alternately connected to each other so that the dispersion becomes zero locally while avoiding the dispersion becoming zero locally.

By this method, when the light intensity of the signal light is low, it is possible to prevent the deterioration of the transmission characteristics due to wavelength dispersion. However, since the light intensity of the signal light is low, there is a problem that many repeaters are required and the cost of the transmission system becomes high.

[0005]

Even when an optical fiber having a positive dispersion and an optical fiber having a negative dispersion are alternately connected to form a transmission line, the transmission distance is long and the light intensity is sufficiently high. If not so low, the self-phase modulation effect in the optical fiber spreads the optical spectrum, and as a result, the influence of chromatic dispersion is expanded and the transmission characteristics deteriorate.

As a method of solving this problem and transmitting a signal over a long distance, there are optical soliton transmission and cancellation of distortion by phase conjugate conversion. However, all of these require very precise control of the characteristics of the optical fiber transmission line, and the current technology cannot immediately cope with them.

The present invention has been made in view of the above circumstances, and an object thereof is to suppress the influence of chromatic dispersion and self-phase modulation effect of a transmission line in a long-distance optical fiber transmission line. Then, it is an object of the present invention to provide an optical fiber transmission system capable of transmitting a signal with little deterioration in transmission characteristics.

[0008]

The present invention provides an optical fiber transmission system in which a transmitter and a receiver are connected to both ends of a single mode optical fiber which constitutes a transmission line, and a signal light is transmitted through the optical fiber. , An optical fiber having positive dispersion and an optical fiber having negative dispersion are connected so that the total dispersion is zero and the self-phase modulation effect is approximately the same as the self-phase modulation effect that occurs in the zero-dispersion optical fiber. Provided is an optical fiber transmission system characterized by comprising a transmission line.

Preferably, the phase-modulated light having a constant intensity is transmitted as the signal light. Since there is no fluctuation in the light intensity of the signal light,
Basically, the self-phase modulation effect can be eliminated.
Also in this case, since waveform distortion appears in the case of long-distance transmission, transmittance control means is inserted in the transmission line to control the light intensity after transmission to be constant.

In the case of transmitting intensity-modulated light as signal light, a phase modulator is inserted in the transmission line, and the signal light is subjected to phase modulation whose magnitude is equal and opposite to the integrated phase amount due to the self-phase modulation effect. To drive the phase modulator.

[0011]

According to the present invention, since the optical fiber having positive dispersion and the optical fiber having negative dispersion are connected as described above, the dispersion is completely canceled and the influence of the self-phase modulation effect is eliminated. Since it is possible to approach the self-phase modulation effect of, the deterioration of the transmission characteristics can be effectively prevented.

When transmitting the phase-modulated light having a constant intensity as the signal light, the self-phase modulation effect can be basically removed. Further, in the case of long-distance transmission, deterioration of transmission characteristics can be effectively prevented by inserting a transmittance control means into the transmission path and controlling the light intensity after transmission to be constant.

When transmitting intensity-modulated light as signal light, a phase modulator is inserted in the transmission line, and phase modulation is performed on the signal light in the same direction and opposite direction as the integrated phase amount due to the self-phase modulation effect. By applying it, the influence of the self-phase modulation effect can be removed, and the deterioration of the transmission characteristics can be effectively prevented.

[0014]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 shows the configuration of an optical fiber transmission line to which the present invention is applied. A transmitter 4 for transmitting signal light and a receiver 6 for receiving the signal light are connected to both ends of the single mode optical fiber transmission line 2.

Then, an optical fiber transmission system is constructed by inserting a plurality of repeaters 8 into the transmission path 2 at predetermined intervals. The distance of the optical fiber transmission line 2 to which the present invention is applied is, for example, a long distance of 5,000 km or more, and the distance between the repeaters is, for example, about 100 km.

Referring to FIG. 2, there is shown a signal waveform diagram between one repeater when the signal light is transmitted through the zero dispersion optical fiber. Here, x = 0 is the start between repeaters. When the dispersion of the optical fiber transmission line is always zero, and the attenuation from x = 0 to the transmission distance x due to the transmission loss of the optical fiber is T (x), the light intensity I (x, t) at time t is transmitted. It changes like T (x) I (0, t) according to the distance x, and the phase of light φ (x, t) is at the point of x = L by the self-phase modulation effect compared to the initial stage of x = 0.

[0017]

[Equation 1]

Only changes. However, K is a coefficient of the self-phase modulation effect. On the other hand, when there is a small dispersion D (x) in the optical fiber transmission line, the waveform I (x, t) at the distance x
Is distorted compared to I (0, t), but if this is ignored because the self-phase modulation effect is small, the distortion is the amount of dispersion received by the signal light.

[0019]

[Equation 2]

It is determined by If we think that this distortion is small and express it with a first-order approximation of variance, the distortion is

[0021]

(Equation 3)

It becomes Here, C (t) is a proportional coefficient. Therefore, the light intensity at x is

[0023]

[Equation 4]

It is Due to this light intensity, the phase φ (x, t) of the signal light is KI (x,
t) changes. Therefore, the phase changes are added and x =
At the point of L,

[0025]

(Equation 5)

And the value when the variance is always zero

[0027]

(Equation 6)

Compared with

[0029]

(Equation 7)

There is a phase shift of. Therefore, as a transmission line

[0031]

(Equation 8)

It is designed so that this value becomes zero in the next repeater while keeping the value of [S] small. By designing in this way, the phase shift due to the self-phase modulation effect can be made the same as in the case of using the zero-dispersion fiber, and the deterioration of the transmission characteristics can be prevented.

Next, an example of such a transmission line will be described with reference to FIG. In the present invention, between the repeaters, that is, FIG.
3 between the repeaters 8 or between the transmitters / receivers 4, 6 and the repeater 8, as shown in FIG. 3, for example, a single mode optical fiber 2a having a negative dispersion, a single mode having a positive dispersion. The optical fiber 2b and the single mode optical fiber 2c having negative dispersion are alternately connected.

The product of the distance and the dispersion value of the optical fiber 2a is A,
The product of the distance and the dispersion value of the optical fiber 2b is B, and the optical fiber 2
When the product of the distance of c and the dispersion value is C, the optical fibers are connected so as to satisfy the relationship of A + C = B.

In FIG. 3, the starting point of the optical fiber transmission line is a, the connecting point of the optical fibers 2a and 2b is b,
Assuming that the connection point between the optical fibers 2b and 2c is c and the end point of the optical fiber transmission line is d, the relationship between the integrated amount of dispersion and the light intensity (that is, the light transmittance from x = 0) and the transmission distance is As shown in FIG. 4, a to d correspond to a to d in FIG.

In the optical fiber transmission line shown in FIG.
Assuming that the loss of the optical fiber is 0.21 dB / km, the length is 32 km having a negative dispersion value of −0.5 ps / nm · km.
Optical fiber 2a with a positive dispersion value of 16.8 ps / nm
When the optical fiber 2b having a length of 3 km having a km of 3 km and the optical fiber 2c having a length of 69 km having a negative dispersion value of −0.5 ps / nm · km are connected, the above equation (8) is set to zero. Therefore, the self-phase modulation effect can be made the same as when the zero dispersion fiber is adopted.

Preferably, the signal light to be transmitted is phase-modulated light having a constant light intensity. Since the light intensity is constant,
Basically, the self-phase modulation effect can be eliminated.
In fact, alternating positive and negative dispersion causes a small waveform distortion, but if the dispersion is small, this distortion becomes opposite in the positive dispersion region and the negative dispersion region in the first-order range. , In the first order range, the phase change due to the self-phase modulation effect is canceled.

However, even when the phase-modulated light having a constant light intensity is used as the signal light, in reality, due to the higher-order effect combined with the self-phase modulation effect and the deviation of the dispersion value of the fiber from the design value. Therefore, the waveform distortion increases little by little, which becomes an obstacle in long-distance transmission.

Therefore, when waveform distortion appears after a certain distance, a transmittance control means such as a light intensity modulator is passed through so as to remove the distortion. The light intensity modulator can be provided in the repeater 8 shown in FIG.

A method of driving the light intensity modulator will be described with reference to FIG. A beam splitter 10 and a light intensity modulator 12 are inserted in the optical fiber transmission line 2. The intensity of the signal light split by the beam splitter 10 is measured by the photodetector 14 and converted into an electric signal.

A low frequency component is removed by passing this electric signal through a high-pass filter 16 such as a capacitor, and the light intensity modulator 12 is driven based on this signal. That is, in the portion where the light intensity is strong, the light intensity modulator 12 is driven so as to have a low transmittance, and the signal light after passing through the light intensity modulator 12 is driven so that the intensity thereof is substantially constant. This makes it possible to correct intensity distortion (waveform distortion) and prevent self-phase modulation effects from accumulating and affecting.

In this case, the fact that there is a small amount of distortion before correcting the optical intensity distortion implies that the phase has already been modulated in accordance with the intensity distortion, so the transmission line after distortion correction is In some cases, it may be desirable to overcorrect the intensity distortion in order to cancel the phase modulation.

That is, the light intensity modulator 12 is driven so that the input intensity and the output intensity have a negative correlation. In other words, the light intensity modulator 12 is driven so that the output intensity is low when the input intensity is high with respect to a certain standard intensity, and the output intensity is high when the input intensity is low.

In the case of a linear transmission line that does not consider the self-phase modulation effect, it suffices to set only the dispersion value of the entire transmission line to zero, but the self-phase modulation effect is a non-linear phenomenon, so It is necessary to keep the distortion at each place small.

The amount of distortion at each place is proportional to the integral of the dispersion value from the point where the strain is zero to that place, and the phase change due to the self-phase modulation effect at that point is the amount of distortion and the average light at that point. It is the product of the intensity and the self-phase modulation effect, which is the product of the self-phase modulation coefficient.

Therefore, the integrated phase change from the point where the distortion is zero becomes the modulated phase at that point, so that the phase change does not become large, so that the change is centered around zero. . By configuring the optical fiber transmission line in this way, it becomes possible to transmit the signal light with a small waveform distortion over a long distance.

Next, a 1-bit delay demodulation method of phase-modulated light will be described with reference to FIG. It is assumed that the signal light is modulated by the phase modulator between phases 0 and π. The signal light propagating through the transmission path 2 is split by the half mirror 18,
The branched light is propagated through the detour transmission path 19 and delayed by 1 bit before being input to the half mirror 20.

When the phase difference between the straight light and the branched light is π, the half mirror 20 interferes with each other and all the light travels downward and is detected by the photodetector 24. On the other hand, when there is no phase difference between the straight traveling light and the branched light, the straight traveling light passes through the half mirror 20, the branched light is reflected by the half mirror 20, and all the light is detected by the photodetector 22.

Therefore, by subtracting the output of the photodetector 24 from the output of the photodetector 22 by the subtractor 26, the signal I or -I is output from the subtractor 26 according to the phase difference between the straight light and the branched light. Is output, and the signal light can be demodulated.

As an alternative, the phase modulated light can be demodulated by a Mach-Zehnder interferometer having an optical path difference corresponding to 1 bit. Next, a case where intensity-modulated light as signal light is propagated through the optical fiber transmission line having the configuration of FIG. 3 will be described. In this case, since the signal light propagating through the transmission line is intensity modulated light, a large phase change occurs in the optical pulse on the receiving side due to the self-phase modulation effect of the transmission line.

Therefore, as shown in FIG. 7, the beam splitter 10 and the phase modulator 28 are inserted in the transmission line 2, and the light branched by the beam splitter 10 is detected by the photodetector 14 and converted into an electric signal.

This signal is input to the phase modulator 28, and the phase modulator 28 is driven so that the signal light is subjected to phase modulation whose magnitude is equal to and the opposite direction to the integrated phase amount of the self-phase modulation effect.

That is, when the intensity of the signal light is large and the phase is relatively delayed, the phase modulator 2 advances the phase.
8 is driven, and when the intensity of the signal light is low and the phase advances relatively, the phase modulator 28 is driven so as to delay the phase. By applying such phase modulation to the signal light, it is possible to cancel the phase change of the intensity modulated light.

[0054]

According to the present invention, in a long-distance optical fiber transmission line, the effects of chromatic dispersion and self-phase modulation effect of the transmission line can be suppressed and long-distance transmission with small waveform distortion can be achieved. Play.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an optical fiber transmission line.

FIG. 2 is a signal waveform diagram in a zero dispersion fiber.

FIG. 3 is a diagram illustrating the principle of the present invention.

FIG. 4 is a diagram showing a relationship between an integrated amount of dispersion and a transmission distance.

FIG. 5 is a diagram showing a schematic configuration of a transmittance control unit.

FIG. 6 is a schematic configuration diagram of a 1-bit delay demodulator.

FIG. 7 is a schematic configuration diagram of phase compensation means.

[Explanation of symbols]

 2 optical fiber transmission path 4 transmitter 6 receiver 8 repeater 10 beam splitter 12 light intensity modulator 14, 22, 24 photodetector 28 phase modulator

Claims (10)

[Claims] 1. An optical fiber transmission system in which a transmitter and a receiver are connected to both ends of a single-mode optical fiber forming a transmission line, and a signal light is transmitted through the optical fiber, wherein each optical fiber is a repeater. For each section divided into a plurality of sections, such as an interval, an optical fiber having a positive dispersion and an optical fiber having a negative dispersion have an overall dispersion of zero and an overall self-phase modulation effect of zero. An optical fiber transmission system characterized in that a transmission line is configured so that the transmission line is configured to be approximately the same as the self-phase modulation effect that occurs when it is assumed to be composed of a distributed optical fiber. 2. The product of a value obtained by integrating the dispersion value from the start point of the section to a certain point in the traveling direction of light and the light transmittance from the start point to the point changes around zero. 2. The optical fiber transmission system according to claim 1, wherein the dispersion of the fibers is distributed so that the product of the product from the start point to the end point of the section is zero. 3. The optical fiber transmission system according to claim 1, wherein the signal light is phase modulated light or frequency modulated light. 4. The optical fiber transmission system according to claim 3, wherein a transmission rate control means for keeping the light intensity after transmission constant is provided at the starting point of each section of the transmission line. 5. The transmittance control means comprises a branching means for branching the signal light, an intensity measuring means for measuring the intensity of the branched signal light, and a light intensity modulation for controlling the transmittance based on the measured intensity. 5. It is comprised from the container.
The optical fiber transmission system described. 6. The optical fiber transmission system according to claim 5, wherein the optical intensity modulator is controlled based on the measured intensity so that the input intensity and the output intensity have a negative correlation. 7. The signal light is intensity-modulated light, a phase modulator is inserted in the transmission line, and the integrated phase amount of the self-phase modulation effect is equal to the magnitude of the self-phase modulation effect according to the intensity of the signal light. The optical fiber transmission system according to claim 1 or 2, wherein the phase modulator is driven so as to apply phase modulation to the signal light. 8. The apparatus further comprises: a branching unit for branching a part of the signal light inserted into the transmission line, and a measuring unit for measuring the intensity of the signal light branched by the branching unit. The optical fiber transmission system according to claim 7. 9. An optical fiber transmission system in which a transmitter and a receiver are connected to both ends of a single-mode optical fiber forming a transmission line, and an intensity-modulated signal light is transmitted through the optical fiber. Based on the intensity of the signal light measured by the measuring means, a branching means for branching a part of the light and a phase modulator are inserted, and a measuring means for measuring the intensity of the signal light branched by the branching means is provided. The optical fiber transmission system is characterized in that the phase modulator is driven so as to apply the phase modulation whose magnitude is equal to and the opposite direction to the integrated phase amount of the self-phase modulation effect to the signal light propagating through the transmission line. . 10. An optical fiber transmission system in which a transmitter and a receiver are connected to both ends of a single-mode optical fiber forming a transmission line, and phase-modulated signal light is transmitted through the optical fiber. A branching means for branching a part of the light and an intensity modulator are inserted, and a measuring means for measuring the intensity of the signal light branched by the branching means is provided, and based on the intensity of the signal light measured by the measuring means. The optical fiber transmission system is characterized in that the intensity modulator is driven so that the output intensity of the modulator is constant.