JPH03267829A - Distributed optical information transmission network - Google Patents

Abstract

PURPOSE:To lower an optical output required to an optical amplifier, to avoid a limit in a division number due to gain saturation and to obtain an output having a sufficient S/N by providing demultiplexers bisecting a signal light and optical amplifiers alternately. CONSTITUTION:Demultiplexers 6a-6c bisecting an optical signal are employed and optical amplifiers 7a-7f amplifying each of bisected optical signals are provided on the transmission network and the demultiplexers 6a-6c and the optical amplifiers 7a-7f are provided alternately. In this case, the gain of the rare-earth dope fibers 7a-7f with respect to a signal light is adjusted to compensate the distribution loss at the X demultiplexers 6a-6c. Since the optical output required to the optical amplifiers 7a-7f is low, the limit in the division number due to gain saturation is not generated. Since the average photon number is not largely decreased, the deterioration in the S/N of the output due to a quantization noise limit is not generated, as well.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention relates to a distributed optical information transmission network that uses optical fibers to distribute information from a single signal source to multiple receivers with high sensitivity. It is.

[Prior art] Figure 4 shows distributed optical information disclosed in ``Study of long span gigabit/second optical transmission system using fiber amplifier'' (IEICE technical research report 0CS89-3) by Kuramoto et al. This is an example of a transmission network configuration. Broadcast information networks such as CΔT■ often use distributed transmission networks, and as the amount of information handled increases, light is increasingly used as a medium.

FIG. 4 shows a transmission network used for such purposes. The optical signal sent from the transmitter 1 is input to the receiver 3 after being split by the optical branch 2, but it is important to note that the splitting reduces the received optical power and degrades the signal-to-noise ratio in the receiver 3. In order to prevent this, the optical signal is amplified by the booster amplifier 4 before being divided.

Various types of optical amplifiers have been proposed, but at present, methods using rare earth-doped fibers are considered to be superior in terms of low noise and compatibility with optical fibers. . This optical amplifier excites erbium ions by injecting excitation light into an optical fiber doped with a rare earth element such as erbium, and uses the stimulated emission to amplify an incoming signal light.

FIG. 5 is shown in the material by Hagiki et al. mentioned above. This is a measured value of the gain saturation characteristic of a rare earth-doped optical fiber type optical amplifier. The power that an amplifier can output is generally limited to a finite value or less, and this value is called the saturated output. The saturated optical output in FIG. 5 is approximately -5 dBm, and if an attempt is made to obtain an optical output close to this value, the gain of the amplifier will be significantly reduced. In the configuration shown in Figure 4, in which the light is amplified before being split, the optical amplifier must amplify a large optical signal, and due to the gain saturation characteristic described above, the number of splits and the The received optical power is limited.

FIG. 6 shows an example of the configuration of a distributed optical information transmission network to avoid the above-mentioned problems. The optical signal sent from the transmitter 1 is
After being split by the optical branch 2, it is amplified by the preamplifier 5 and input to the receiver 3. In the configuration shown in FIG. 6, since the preamplifier 5 amplifies the optical signal after being divided, the above-mentioned gain saturation phenomenon does not occur, and even when the number of divisions is large, sufficient optical power is input to the receiver 3. can be done.

Next, we will discuss the signal-to-noise ratio in an optical amplifier using stimulated emission.

'/, by Yamamoto, “No1se an
dErrorRatePerformance
of Sem1conductor l, ase
rAmplifiers in PCM-IM 0pL
ical Transmjssjon Systems”
(IEEE< Journal of Quantum
Elcctronicsvol, QE16. no,
10. According to the relational expression shown in pp. 1073-1081), in an optical amplifier using stimulated emission, the average number of input photons per unit time is <n,>.

If the gain is the degree of G2 population inversion is r, the number of modes of spontaneous emission light is mL, and the spectral width of spontaneous emission light is Δf, then
Signal-to-noise ratio (S/N) of optical amplifier output. 1 can be expressed by the following formula.

(S/N). . , = ......+11 In order to increase the signal-to-noise ratio, if the gain and population inversion can be made sufficiently large and the spontaneous emission spectrum width can be made sufficiently narrow, then G) 1. r=1゜Δf (1 is the th (])
Substitute into the formula, (S/N) ouL =<rz>/2 -----
---+21 is obtained. In the system shown in FIG.
Let the average number of photons emitted from 〈n. 〉When the number of 9 divisions is m, the average number of photons for 5 preamplifiers is 〈■1〉.

<nt>=<n0>/rn
...131, so the quantum noise limit (S/N) of the signal-to-noise ratio in the receiver 3 is (S/N) from equations (2) and (3).
N), ”< n, >/ 2 m ---
It becomes A41.

Therefore, in the case of the configuration shown in FIG. 6, when the number of divisions m is large, the signal-to-noise ratio of the output of the receiver 3 deteriorates due to the quantum noise limit.

[Problems to be Solved by the Invention] Conventional distributed optical information transmission networks have been configured as described above, but due to gain saturation characteristics of optical amplifiers or quantum noise limitations, it is difficult to receive sufficient signal pairs from many receivers. It was difficult to obtain an output with a noise-to-noise ratio.

This invention was made in order to solve the above-mentioned problems, and it is a distributed optical information transmission network that can obtain outputs with a sufficient signal-to-noise ratio from many receivers. )
It is considered fJ-like.

[Means for Solving the Problems] A distributed optical information transmission network according to the present invention uses an optical branch that splits an optical signal into two as an optical branch, and includes an optical amplifier that amplifies each of the split optical signals. , the above-mentioned optical branches and optical amplifiers are provided alternately.

In a distributed optical information transmission network configured as described above, the optical output required of the optical amplifier is low, so there is no restriction on the number of divisions due to gain saturation. Furthermore, since the average number of photons does not decrease significantly, the output A and J noise ratios do not deteriorate due to the quantum noise limit.

[Example] Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of the present invention. In the figure, 1 and 3 are transmitters and receivers similar to the conventional example, and 68 to 6c.
7a to 7f are rare earth doped fibers connected to each of the two outputs of each of the X branches 68 to 6c, 8a to 8
d is a filter located on the input side of each receiver 3 and transmits only the signal light component; 9a to 9c are excitation light sources connected to one of the two X branches 6a to 6c; A transmitter 1 is connected to the other input of the fiber 6a, and its two outputs are a rare earth doped fiber 7a.

7b to the other inputs of the next-stage X branches 6b and 6c, and the two outputs of each are connected to the receiver 3 via rare earth doped fibers 70 to 7f and filters 88 to 8d. Note that the rare earth doped fiber 78
7f and their excitation light sources 98 to 9c constitute the optical amplifier of the present application.

In the distributed optical information transmission network configured as described above, the pumping light output from the pumping light source 9a is split into two by the X branch 6a and pumps the rare earth doped fibers 7a and 7b. On the other hand, the signal light transmitted from the transmitter 1 is also split into two by the X branch 6a, and then enters the rare earth doped fibers 7a and 7b, respectively. Rare earth doped fiber 7
Since a and 7b are excited as described above, they have a gain with respect to the signal light, but the magnitude of the gain is different from that of the X branch 6.
The output of the excitation light source 9a is adjusted in advance so as to have a value that almost compensates for the distribution loss caused by a. That is, the magnitude of the signal light at the output of the rare earth doped fibers 7a and 7b is:
Almost equal to the output light intensity of the transmitter 1. Excitation light source 9b, X
The combination of branch 6b, rare earth doped fibers 7c and 7d, and the combination of excitation light source 9c, It has the same function as the X branch G
excitation light source 9b, so as to compensate for the distribution loss in b, 6c;
The output of 9c has been adjusted. The filters 8a to 8d are
Among the outputs of the rare earth doped fibers 70 to 7f, the fibers are set to have a characteristic of blocking the excitation light component and transmitting the signal light component.

Since the distributed optical information transmission network shown in FIG. , which is sufficiently large. In addition, the signal-to-noise ratio caused by splitting and amplifying the light is as follows.5 Now, in a typical state, the X branches 68 to 6c equally distribute the signal light, and the gain of the rare earth doped fibers 78 to 7f Consider that is set to 2 to compensate for this distribution loss. Further, assuming that the passband widths of the filters 8a to 8d can be made sufficiently narrow and the excitation of the rare earth doped fibers 78 to 7f is performed seven times larger, then G=2. Δf(
By substituting 1°r=1 into the above equation (11), (
S/N). -L = 2<nt>/3
-・---(51 is obtained. In the first stage optical amplifier, <n r >=<n 0>/2 ・
・Group・Since (6), the signal-to-noise ratio (S
/N). From equations (5) and (6), i+tl is (S/N). ,,t+=<no>/3
------- (obtain 71. Also, the signal-to-noise ratio (S/N) of the Nth stage amplifier output. uLN is (S / N ). -N = < n. > / 3 N
. . . -181 is easily derived. 1st
As in the configuration shown in the figure, if all X branches are connected to the outputs of the X branches near transmitter 1 and the number of branches is gradually increased, then m = 2''
・It becomes (9). When comparing the signal-to-noise ratio in the conventional system shown by equation (4) and the signal-to-noise ratio in the system according to the present invention shown by equation (8) using the relationship shown in equation (9) , N>1, that is, m>2, the system according to the present invention has a larger signal-to-noise ratio, and the larger N is, the larger the difference becomes.

In a distributed optical information transmission network, the total number of receivers 3 is
As shown in equation 9), in many cases it is not a power of 2. FIG. 2 shows an example of the configuration of a distributed optical information transmission network according to the present invention when the total number of receivers is not a power of 2. . In the case of this embodiment, the X branch 6c immediately before the receiver 3 in FIG. 1 is eliminated, and the receiver 3c is directly connected to the X branch 6a one step upstream, making the total number of receivers 3. It has become. From equation (8), the signal-to-noise ratio in FIG. 2 is calculated by the receivers 3a and 3b. 〉/6, the receiver 3c is (
n,>/3, which is the same as or larger than the value <n,>/6 in the conventional system shown in equation (4). FIG. 3 shows the results of similarly determining the signal-to-noise ratio when the total number of receivers is two or more. As shown in the figure, in the distributed optical information transmission network according to the present invention, even in the worst-case receiver, a signal-to-noise ratio comparable to or higher than that in the conventional case can be expected.

[Effects of the Invention] As described above, in the distributed optical information transmission network according to the present invention, by alternately providing optical branches that split signal light into two and optical amplifiers, the optical output required for the optical amplifiers can be reduced. Therefore, there is no restriction on the number of divisions due to gain saturation, and since the average number of photons does not decrease sharply, there is no deterioration of the output signal-to-noise ratio due to the Keiko noise limit. Therefore, outputs with a signal-to-noise ratio of 1-minute can be obtained from many receivers.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a configuration example of a distributed optical information transmission network according to the present invention, and FIG. 2 is a configuration example of a distributed optical information transmission network according to the present invention when the total number of receivers is not a power of 2. FIG. 3 is a block diagram showing a comparison of the signal-to-noise ratio with respect to the number of receivers between the conventional example and the configuration example according to the present invention.
The figure is a block diagram showing a conventional example in which an optical amplifier is provided before an optical branch, Figure 5 is a diagram showing the gain saturation characteristics of the amplifier, and Figure 6 is a block diagram showing a conventional example in which an optical amplifier is provided after an optical branch. It is a diagram. ■ is the transmitter, 3, 38-3c is the receiver, 6a-6c is X
Branches 7a to 7f and 9a to 9c are rare earth doped fibers and excitation light sources (optical amplifiers), and 88 to 8d are filters.

Claims (1)

[Claims] In a distributed optical information transmission network in which an optical signal sent from a single transmitter to an optical fiber is split by an optical branch and inputted to a plurality of receivers, the optical signal is split into two as the optical branch. In addition to using optical branching to
A distributed optical information transmission network characterized in that the above-mentioned optical branches and optical amplifiers are provided alternately.