JPS581576B2 - Optical fiber submarine relay system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is used in the field of optical fiber submarine cable communication that uses low-loss optical fiber as a transmission medium. This relates to an optical fiber submarine relay system for communication.

Optical fiber submarine relay systems are expected to realize an economical long-distance submarine cable system due to the characteristics of optical fibers, such as low loss, small diameter, wide bandwidth, and light weight.

In submarine relay systems using optical fibers, consideration is being given to increasing the capacity of the system by taking advantage of the small diameter of optical fibers and accommodating a large number of optical fibers in one submarine cable.

This is based on the assumption that the price of optical fiber will decrease in the future, and the ratio of the price of optical fiber to the price per unit length of an optical fiber submarine cable will become smaller.

However, the system length is 7,000 km long and the optical fiber is laid in deep sea as much as 8,000 m.
It is questionable whether highly reliable optical fibers capable of providing stable communications over a long period of five years or more can be realized at a low cost.

Optical fibers installed in the deep sea require measures such as protection against high water pressure and waterproof protection against seawater, reducing the cost of each optical fiber in submarine optical fiber cables to the point where they can be ignored. It is considered difficult to do so.

For these reasons, it is desirable to minimize the number of optical fibers used in the optical fiber submarine relay system.

The present invention provides an optical fiber submarine relay system that can perform large-capacity bidirectional transmission using a single optical fiber.

Since the optical fiber submarine relay system does not have good linearity of light emitting elements and light receiving elements for optical communication, application of digital transmission such as PCM transmission is being considered.

The optical fiber used here is preferably a single-mode fiber that allows broadband transmission with one mode of propagating optical signals, and the wavelength of the optical signal is 1, which allows for low loss in the optical fiber. A long wavelength band of .2 to 1.6 μm is advantageous in that the relay interval can be increased.

As for bidirectional transmission using a single optical fiber, research is being conducted using a short wavelength band (0.8 μm).

There are two main methods for performing bidirectional transmission.

One type uses the same wavelength, and uses an optical branching/coupling circuit at the terminal station and repeater to separate the transmitted and received signals.This optical branching/coupling circuit increases the amount of suppression and reduces passing loss. would be a difficult and uneconomical system.

This method has the disadvantage that it cannot sufficiently suppress unnecessary signals of the same wavelength, and is particularly difficult to apply to long-distance systems with multiple repeaters.

Another method uses two different wavelengths for upstream and downstream signals.

This method uses an optical demultiplexer to separate the two wavelengths λ0 and λE, and it is possible to reduce the transmission loss to 1 dB or less.

The interval between the wavelengths λ0 and λE is required depending on the separation characteristics of the available optical demultiplexer.

When the relay interval is controlled by optical fiber loss, the loss characteristics of the optical fiber are roughly proportional to 1/λ4, so if the wavelengths λ0 and λE differ by 5%, the optical fiber loss difference will be about 10%. The relay interval is limited by wavelengths with high loss, making the system uneconomical.

As described above, with conventional methods, it has been difficult to realize an optical fiber submarine relay system that efficiently performs bidirectional communication by taking advantage of the low loss properties of optical fibers.

The present invention efficiently performs bidirectional transmission in a long wavelength range with low loss using a single mode fiber capable of widening the band.

In a long wavelength range, when the bit rate of a digital transmission signal is constant and the relay interval is determined using the wavelength as a parameter, the relay interval is the same for two specific different wavelengths for a certain bit rate.

The present invention focuses on this point and uses these two wavelengths to perform efficient bidirectional transmission.

Hereinafter, specific examples will be described.

FIG. 1 shows the configuration of an optical fiber submarine relay system according to the present invention.

In Figure 1, 1 indicates an optical fiber submarine cable using a single mode optical fiber, 2 indicates an optical submarine repeater, and 3 indicates a different 2
An optical demultiplexer for separating two wavelengths λ0 and λE, and 4 indicate an optical digital regenerative repeater including a light emitting element and a light receiving element.

Symbols A and B indicate terminal stations installed at the amphibious landing station connected to the optical fiber submarine cable.

In Figure 1, the wavelength λ0 is transmitted from the A terminal station to the B terminal station, and from B to A
The signal is transmitted at wavelength λE.

Wavelength λ transmitted by terminal station A.

The optical signal is coupled to the optical fiber 1 by the optical demultiplexer 3 in the terminal station and transmitted to the optical repeater 2.

The signal of wavelength λ0 attenuated by the optical fiber transmission line is separated by a demultiplexer 3 in the repeater, regenerated and amplified by an optical digital regenerative repeater 4, and connected to the optical fiber transmission line through the optical demultiplexer 3 again. .

Figure 2 shows an example of the loss characteristics of a single mode fiber used in an optical fiber submarine relay system.

The dotted line indicates the theoretical loss limit of the optical fiber.

The solid line is an actual measurement example.

The loss of the optical fiber is low in the long wavelength range of 1.2 to 1.6 μm.

By appropriately selecting the core diameter (2a), cladding refractive index (n), and core-cladding refractive index difference (4n), single mode fiber can achieve a normalized frequency of 2πa at a wavelength of 1.3 to 1.5 μm. /λ・ is possible, and ■ is 1.6 to 2.4
It has been experimentally confirmed that if it is possible to set this, an optical fiber with single mode propagation and less loss due to bending can be realized.

In Figure 2, the increase in loss near 1.38 μm is due to the OH group, and the fundamental vibration of the OH group is 2.7 μm.
This corresponds to a wavelength of approximately 1/2 of 3 μm.

Although it is possible to remove a considerable amount of OH groups in fiber manufacturing, it is considered difficult to remove them completely, and it is not desirable to have a transmission band around 1.38 μm.

Therefore, the transmission band is 1.3μ, which reduces loss.
It is conceivable to use around m and around 1.5 μm.

In order to perform bidirectional transmission using a single optical fiber, it is desirable to use two wavelengths that are as close as possible without significantly changing the transmission characteristics of the optical fiber. Depends on the characteristics.

Figure 3 shows an example of the characteristics of an optical demultiplexer. Various types of optical demultiplexers can be considered, such as a prism type, an interference film type, and a grating type.

However, these demultiplexing characteristics depend on the wavelength, and as the wavelength becomes longer, the characteristics deteriorate, and in order to secure a certain amount of suppression, it is necessary to widen the interval between the wavelengths used.

Furthermore, since the demultiplexing characteristics are affected by the spread of the spectrum of the light source, from this point of view as well, it is desirable to widen the wavelength interval to be demultiplexed as much as possible.

In digital transmission using a single mode optical fiber as a transmission medium, the repeating interval is determined primarily by the loss of the optical fiber on the relatively short wavelength side (λ0) within the long wavelength range.

On the other hand, on the long wavelength side (λE), as the bit rate of the digital signal is increased, the relay interval is limited by the influence of dispersion, and the relay interval rapidly decreases.

This situation is shown in FIG.

FIG. 4 is a diagram showing how the relay interval changes depending on the wavelength when transmitting a signal at a certain bit rate for two wavelengths λ0 and λE in the long wavelength range.

In the vicinity of 13 μm, where the material dispersion and structural dispersion of the optical fiber cancel each other out, broadband properties are exhibited, and even if the bit rate increases, the relay interval does not become much shorter.

From this, a certain λ. ,λ. When determining , there is a point where the relay interval becomes the same at a certain bit rate.

These are the eight points in Figure 4. The present invention focuses on this point, and performs efficient bidirectional transmission using two different wavelengths with the same relay interval for a certain bit rate.

FIG. 5 shows another embodiment using the present invention.

Figure 5 shows the configuration of an optical submarine repeater.

The optical demultiplexer 3 separates the transmitted optical signal, the light receiving element 5 that receives the wavelength λ0 converts the optical signal into an electrical signal, and the regenerative repeater 6 electrically amplifies and regenerates the signal. Wavelength λE
The light emitting element 7 converts the light into an optical signal, which is coupled to the optical fiber 1 through the optical demultiplexer 3.

By processing uplink and downlink signals in the same way and transmitting them alternately using λ0 and λE for one transmission direction in each relay section, timing jitter caused by the optical fiber transmission line and light emitting/receiving elements can be reduced. This has the effect of averaging the addition over the entire system, and it is possible to provide a multi-relay system with good transmission quality.

FIG. 6 shows a repeater configuration in the case of increasing the capacity using the present invention.

This method performs wavelength division multiplexing by using an optical wavelength division filter to separate signals with almost no level difference after separating transmitting and receiving signals with a large level difference using a demultiplexer, as shown in Figure 6. 8 indicates a wavelength division filter placed on the input side of the repeater, and 9 indicates a wavelength division filter on the output side.

The wavelength division filter used here separates signals of the same level, so it does not require a large amount of suppression and is easy to implement.

Here, the wavelength division filter 9 can be omitted if the emission spectrum of the light emitting element 7 becomes small enough to not affect adjacent transmission wavelengths.

In this way, the wavelength band of λ0 is changed to λo1, λo2~λQ
By setting the wavelength band of λE to λE1, λE2 to λEn for n and using subdivision wavelengths, it becomes possible to achieve an extremely large capacity using one optical fiber.

As described above, the effects of the present invention are as follows.

(1) In the optical fiber submarine relay system that uses single-mode optical fiber and long wavelength range, we focused on the fact that the relay interval is loss-limited at certain wavelengths and dispersion-limited at other wavelengths, and is fixed. An efficient and economical optical fiber submarine relay system that performs bidirectional optical transmission through a single optical fiber using two different wavelength bands with the same repeating interval depending on wavelength selection when transmitting a bit rate of can be realized.

(2) When performing bidirectional transmission using two different wavelengths, by alternately using two different wavelengths in each relay section for one transmission direction, digital signals based on optical fiber transmission lines and optical elements can be A high-quality submarine relay system can be realized by averaging the additive phase jitter in the system.

(3) After separating two different wavelength bands using an optical demultiplexer, the optical signals are multiplexed using an optical wavelength division filter, making it possible to realize an ultra-high capacity submarine repeater system using a single optical fiber.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIGS. 2, 3 and 4 are characteristic diagrams for explaining the principle of the present invention, and FIG.
and FIG. 6 are block diagrams showing embodiments of the present invention. 1... Optical fiber submarine cable, 2...
・Optical submarine repeater, 3... Optical demultiplexer, 4...
...Optical digital regenerative repeater, 5... Light receiving element, 6... Regenerative repeater, 7... Light emitting element, 8,9... Wavelength division filter.

Claims (1)

[Claims] 1. Maintaining the required transmission quality when transmitting digital signals through optical fibers in an optical fiber submarine relay system that uses a single mode optical fiber as a transmission medium and uses transmission signals in a long wavelength range. When transmitting a digital signal with a constant bit rate, the relay interval is dominated by optical fiber loss in a certain wavelength range, and by waveform distortion due to optical fiber dispersion in other wavelengths. An optical fiber submarine relay system that performs bidirectional digital transmission through a single optical fiber using two different wavelengths with the same wavelength. 2. In the optical fiber submarine relay system that uses single mode optical fiber as the transmission medium and transmits signals in the long wavelength range, when transmitting digital signals using optical fiber, the relay interval is set to maintain the required transmission quality. Because some wavelengths are dominated by optical fiber loss and other wavelengths are dominated by waveform distortion due to optical fiber dispersion, the repeating interval remains the same when transmitting a digital signal with a constant bit rate. Two wavelengths are used to perform bidirectional digital transmission over a single optical fiber, and two different wavelengths are alternately used in each repeating section for one transmission direction. An optical fiber submarine relay system that is characterized by system-averaging the additive timing jitter of digital signals based on elements.