JPS6376532A - Optical-soliton time division multiplex transmission system - Google Patents

Abstract

PURPOSE:To transmit highly-dense optical solitons, by successively transmitting optical solitons produced from plural optical-soliton exciters through one optical fiber so that the amplitude ratio between adjacent optical solitons can be set at >=1.2. CONSTITUTION:Optical attenuators ATT1-ATTn are adjusted so that the amplitude of an optical soliton propagated through an optical fiber F can be adjusted in such a way that the amplitude ratio between adjacent optical solitons can be set at >=1.2. In other words, the ratio of the amplitudes of the optical solitons produced from oddnumbered optical-soliton exciters L1, L3, ..., to those of the optical solitons produced from even-numbered optical-soliton exciters L2, L4, ...is set at >=1.2. Therefore, each optical-soliton exciter excites optical solitons of a fixed amplitude, but the interaction of the optical solitons in the optical fiber becomes sufficiently small and high-density optical soliton transmission can be realized.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an optical soliton time division multiplexing transmission system that performs high-speed optical pulse transmission using optical solitons having a pulse width of several picoseconds. It is.

[Prior art] When a light pulse propagates through an optical fiber, the pulse width widens to -1'IQ, and when it propagates over a certain distance ril1,
Two adjacent pulses begin to overlap, making it difficult to distinguish between individual optical pulses. Furthermore, even if the propagation distance is kept constant and the repetition rate of the optical pulse is increased,
Above a certain speed, the received light pulses can no longer be identified individually.

Pulse broadening of light propagating in a single moat fiber is caused by the material and structural dispersion of the optical fiber. Here, material dispersion is caused by the fact that the refractive index of glass differs depending on the wavelength of light, and structural dispersion is caused by the fact that the group velocity of the propagation mode varies depending on the wavelength of light. These two factors are collectively called chromatic dispersion. If an ideal light source that oscillated at a single wavelength existed, the pulse broadening described above would not occur, but when transmitting a signal, the light from this light source must be modulated in some way. As a result, sidebands appear and pulse broadening occurs.

By the way, the existence of optical pulses in which optical pulse broadening does not exist in principle has been confirmed both theoretically and experimentally, and is called an optical soliton. This principle works by propagating light under conditions in which pulse compression due to self-phase modulation caused by optical nonlinear effects of the light propagation medium and pulse broadening due to chromatic dispersion are exactly balanced, resulting in an optical pulse with no broadening. This means that it can be propagated.

The self-phase modulation effect is based on the optical power-effect in which the refractive index of the glass changes with the intensity of the propagating light. In other words, as the optical pulse propagates, the refractive index changes over time,
As a result, a change in optical frequency appears that is proportional to the time differential value of the light intensity. Therefore, the form of this change is such that the wavelength of the light becomes longer than the original wavelength at the part where the intensity changes at the rising edge of the optical pulse. This is a change in which the wavelength becomes shorter than the original wavelength.

On the other hand, if the wavelength of the light source is in the anomalous dispersion region, that is, in a wavelength region where the longer the wavelength, the slower the propagation speed.
The aforementioned two parts with steep intensity changes approach each other as the signal propagates. That is, it causes the effect of pulse compression.

When the pulse waveform is a second high public type (sech), the pulse compression effect caused by the change in optical wavelength caused by self-phase modulation and the pulse broadening due to the dispersion inherent in the optical fiber are balanced, and the pulse waveform propagates. It is known that it does not change due to

When a normal single mode optical fiber is used, the pulse width of an optical soliton that can be propagated is 10 ps or less, so if a digital signal is transmitted using this optical soliton, high-speed signal transmission is possible.

[Problems to be Solved by the Invention] However, when adjacent optical solitons have the same amplitude, the mutual interaction between the optical solitons is limited, for example, if the interval between the optical solitons is not more than 10 times the pulse width of the optical solitons. There was a drawback that the optical solitons would coalesce while propagating for several kilometers due to this action.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an optical soliton time division multiplex transmission system that prevents adjacent optical solitons from merging and transmits digital signals at high speed using optical solitons.

[Means for Solving the Problems] In order to achieve such an object, the present invention has a plurality of optical soliton exciters that periodically generate optical solitons, and a plurality of optical soliton exciters is controlled by the signal to be transmitted, the optical soliton digitally modulated by the signal to be transmitted is made to enter from one end of the optical soliton line, and the optical soliton that has propagated through the optical soliton line is taken out from the other end of the optical soliton line. In the optical soliton transmission method, optical solitons are generated from multiple optical soliton exciters in a time-sequential manner in a time-division manner, and then the multiple optical solitons are multiplexed by an optical multiplexer. When the combined optical soliton propagates through the optical soliton line, the amplitude ratio of temporally adjacent optical solitons at the input end of the optical soliton line is 1.2 or more.

[Operation] In the present invention, optical solitons emitted from a plurality of optical soliton exciters that generate optical solitons are multiplexed and sequentially input into a single optical fiber to become a single unit, and adjacent optical solitons Since the ratio of the amplitudes of is set to be 1, 2 or more, each optical soliton exciter excites optical solitons with a constant amplitude, but the interaction of optical solitons in the optical fiber is sufficiently small. Can transmit high-density optical solitons.

[Example code] The present invention will be described in detail below with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of the present invention, in which n digital signal input terminals, B1, . . .
Bn is n digital signal output terminal, Ll...Ln
is an optical soliton exciter, M is an optical soliton exciter, and M is a delay of n parallel digital signals from input terminals A, . and send the pulse to the exciter L1...L
These exciters are sequentially used as drive timing signals of n,
. . . It is a transmission signal distributor that sends out to Ln.

ATTl...ATTn are optical attenuators for adjusting the amplitudes of optical solitons from the optical soliton exciters L1...l, n, respectively; C is an optical attenuator ATT,...ATT
It is a multiplexing device for inputting n optical solitons extracted through n into a single optical fiber F, and %M+...M
n is a half mirror constituting the multiplexer C. The multiplexer C can also be configured using an optical fiber type optical coupler.

F is an optical fiber that exhibits anomalous dispersion in the wavelength of the optical soliton emitted from the optical soliton exciter Lbi...Ln. propagate.

R is a photodetector that detects the propagating light from the optical fiber F, D
is a received signal distributor that sequentially switches the electric pulse signals taken out from the photodetector R to the output terminals B1...Bn and sends them out.

FIG. 2(A) shows an electric pulse train, where the horizontal axis represents time, and PIJ means the j-th pulse waveform of the electric pulse supplied to the input terminal A1.

In the example of FIG. 2 (8), up to j-3 is shown, but in reality, the required number of pulses exist. In the example in Figure 2 (A), the 8 terminal has a signal of 1, 0.0, the A2 terminal has a 1, 1.0 signal, and the 8 terminal has a signal of 1, 1.1. , a signal of 0 or 1.0 will be applied to the terminal.

Such an electrical signal is converted by a transmission signal distributor M into a pulse Slj shown in FIG. 2(B). That is,
Pulse Slj is shifted so that it rises only a certain time later than pulse 5l-1j. When the time slot width of the electric signal applied to the digital signal input terminals Δ1 . . . -An is T, τ≦− must be satisfied. Further, the pulse SIJ does not necessarily have to be a narrow pulse width, but it is important that the rise time is shifted.

Providing such a shift can be easily accomplished by digital signal processing techniques.

The optical solitons emitted from the optical soliton exciter 1 using the pulse SIJ as a trigger pulse are multiplexed by a half mirror Ml and enter one optical fiber F. In this way, the electric pulses incident on the input terminals An are time-division multiplexed in the form of optical solitons and propagate through the optical fiber F. The optical soliton exciters L1...Ln can be constructed by combining a high-output light source and an optical fiber.

Here, the pulse width of the optical soliton must be much smaller than . The amplitude of the optical soliton propagating through the optical fiber F is determined by adjusting the optical attenuators ATT, . . . , ATTn so that the ratio of the amplitudes of adjacent optical solitons is 1.2 or more. The simplest method is the odd-numbered optical soliton exciter,
, L3. ... and the amplitude of the optical soliton emitted from the even-numbered optical soliton exciter L2. This condition can be satisfied by setting the ratio to the amplitude of the optical soliton emitted from L4° to 1.2 or more.

The reason why it is sufficient that the amplitude ratio of the optical soliton is 1.2 or more will be described below.

The equation describing the optical soliton is , and it is known that the simplest solution is given by a second hyperpublic function with one peak. Here, U and ξ respectively indicate the envelope function of the dimensionless optical pulse at 3 positions and 5 times.

Since the equation describing the optical soliton is a nonlinear equation, it is difficult to analytically obtain a solution that implies a large number of pulse trains, but it is known that a solution with two peaks is given by (References P, L.

Chu and C, Pesem', “Mutual
1interaction between solitary
ons of unique amplitudes
in optical fiber”, Elect
ronics Letters, 21st Nove
mber1985, Vol, 21. No, 24P,
1133).

η1. η2. γ. is a constant determined by the initial condition at ξ=0, and η1. η2 corresponds to the strength of the two solitons. Since the denominator of q (τ, ξ) mentioned above includes the variable ξ of distance as a cosine function, it can be seen that q fluctuates periodically with respect to direct.

FIG. 3 shows how the interval between optical pulses changes.

In Figure 3, the horizontal axis is the time axis, the vertical axis is the pulse height, and the depth is the propagation distance.Here, it can be seen that the interval between two optical pulses changes periodically with respect to the propagation distance. Recognize.

In FIG. 3, To indicates the pulse width of the optical pulse with the higher pulse height. This pulse width means the width of time (full width at half maximum) in which the amplitude is greater than or equal to half the maximum amplitude of the pulse.

Figure 4 shows the relationship between the minimum time difference between two peaks at the initial position of an optical soliton propagating through a single optical fiber and an adjacent optical soliton, and the time difference between the two peaks at ξ=0, using the above-mentioned q (τ.

This is the result obtained by numerically calculating the functional form of ξ). Tatashi,
η, #1. η2 Cape k (amplitude ratio), and the horizontal axis is T the interval between the peaks of the optical pulses at ξ=0.

The vertical axis shows the value normalized by To, and the vertical axis shows the value normalized by To.

Here, curve (1) is = 1.1', curve (2) is =
12, curve (3) is shown for =1.3.

It can be seen that as the amplitude ratio k increases, the pulse interval does not change much compared to the pulse interval when ξ=0.

Assuming that the pulse interval of ξ-0 is 4, the minimum pulse interval is 3.8 or less when = 1.1, but is 3.9 or more when k -1,2, so k≧1.2 By doing so, it becomes possible to perform pulse transmission that can be regarded as a substantially constant pulse interval.

[Effects of the Invention] As explained above, the present invention transmits optical solitons emitted from a plurality of optical soliton exciters sequentially through a wooden optical fiber, so that the amplitude ratio of adjacent optical solitons is 1. .2 or more, so each optical soliton exciter excites optical solitons with a constant amplitude, but the interaction of optical solitons in the optical fiber is sufficiently small and the advantage is that high-density optical solitons can be transmitted. be.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing one embodiment of the present invention, Fig. 2 is a diagram explaining the operation of the transmission signal splitter, and Fig. 3 shows how light pulses approach and separate while propagating through an optical fiber. Figure 4 shows the time difference between the peaks at the initial position of an optical soliton propagating through a single optical fiber and an optical soliton in contact with Va, and the time difference when the optical solitons come closest during propagation. It is a characteristic diagram showing a relationship. A1...An...Digital signal input terminal, B1...
...Bn...Digital signal output terminal, Ll...L
n... Optical soliton exciter, ATT,...ATTn.
... Optical attenuator, C... Multiplexing device for inputting the optical soliton into one optical fiber, Ml... Mn... Half mirror-1 F... Optical fiber, M... A2...Transmission signal distributor that delays the human pulse of An and makes it a series signal on the time axis, R...Photodetector, D...Reception signal distributor, PIJ...Input terminal Human input digital signal from before, Sij...Electrical signal sent to optical soliton exciter 1, τ...51-1 Time difference between rise of j and SIJ. The last wind, the current plague, no Q5) a) side figure 3

Claims (1)

[Claims] It has a plurality of optical soliton exciters that periodically generate optical solitons, the plurality of optical soliton exciters are controlled by a signal to be transmitted, and the optical soliton is digitally modulated by the signal to be transmitted. In an optical soliton transmission method in which an optical soliton is made incident on one end of an optical soliton line and an optical soliton propagated through the optical soliton line is taken out and received from the other end of the optical soliton line, the optical soliton is emitted from the plurality of optical soliton exciters. A plurality of time-sequential optical solitons are generated in a time-sequential manner in the form of division, and are multiplexed by an optical multiplexing device. When the multiplexed optical solitons propagate through the optical soliton line, the optical soliton is incident on the optical soliton line. An optical soliton time division multiplex transmission system characterized in that the amplitude ratio of temporally adjacent optical solitons at an edge is 1.2 or more.