JPH08204634A - Optical transmission system with non-relay - Google Patents

Abstract

PURPOSE: To obtain the system used to send an optical signal with high quality without relaying suitable for submarine communication or the like. CONSTITUTION: The transmitter side is provided with an optical transmitter 1 conducting direct modulation and a post amplifier 2, an the receiver side is provided with a preamplifier 4, a dispersion compensating optical fiber 6 and an optical receiver 7, and the optical signal is sent through a non-dispersive shift optical fiber 3 installed between the post-amplifier 2 and the preamplifier 4 without any repeater. Then the signal level received by the dispersion compensating optical fiber 6 is adjusted to be within a prescribed level. More specifically the signal level received by the dispersion compensating optical fiber 6 is monitored and the level is adjusted by an attenuator 5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a repeaterless high quality optical signal transmission system suitable for undersea communications.

[0002]

2. Description of the Related Art Recently, a long-distance / non-repeater optical transmission system represented by a submarine line has been studied using an Er-doped fiber type optical amplifier as a post-amplifier and a preamplifier. For example, there is a method in which the emitted laser light is subjected to external modulation such as electric field absorption to form signal light in which chirping can be ignored, and the signal light is transmitted through a non-dispersion shift optical fiber to reduce the influence of dispersion ( ECOC '9
3, WeC 10.1, 1993). Further, as shown in FIG. 4, the optical transmitter 1 in which the laser diode is directly modulated does not require a modulator and is formed only by the laser diode, so that the handling is easy, but the chirping of the signal light does not occur. Since it becomes large, there is a method of arranging the dispersion compensating optical fiber 6 after the preamplifier 4 to cancel the dispersion generated when propagating through the non-dispersion shift optical fiber 3 (OAA ').
94, WB 2, 1994).

[0003]

When such an external modulation method is used, the configuration for combining the laser light source and the external modulator in the optical transmitter becomes complicated, and the polarization adjustment of the signal light and There is a problem to suppress the stimulated Brillouin scattering. On the other hand, in the system using the direct modulation method, the configuration of the optical transmitter is simple and the handling is easy, but the chirping is large, and therefore a dispersion compensating optical fiber is required. There is a problem that the self-phase modulation effect occurs and the power penalty increases. Therefore, as a result of intensive studies, the present inventors have developed a method in which a self-phase modulation effect does not occur even in a system that employs direct modulation.Therefore, it is necessary to provide a repeaterless optical transmission system to which this method is applied. To aim.

[0004]

A repeaterless optical transmission system according to the present invention comprises a directly modulated optical transmitter and a postamplifier on the transmitting side, a preamplifier, a dispersion compensating optical fiber and an optical receiver on the receiving side. A system for transmitting an optical signal without a repeater for a non-dispersion shift optical fiber installed between the optical fiber and a signal level input to the dispersion compensating optical fiber is adjusted to fall within a predetermined range. Characteristically, the signal level input to the dispersion compensating optical fiber is monitored, and the level is adjusted by an attenuator provided between the preamplifier and the dispersion compensating optical fiber or by the gain of the preamplifier.

In the repeaterless optical transmission system, the range of the signal level input to the dispersion compensating optical fiber is the upper limit (due to the waveform distortion caused by the signal light propagating through the non-dispersion shift optical fiber and the dispersion compensating optical fiber. The power penalty exceeds a level of 1 dB) and the lower limit (the level of the minimum receiving sensitivity of the receiver, the total amount of dispersion / total loss in the dispersion compensating optical fiber, and the margin determined by the noise generated in the preamplifier). It is characterized by being in between.

[0006]

In the repeaterless optical transmission system according to the present invention, the signal light which is attenuated due to the propagation through the non-dispersion shifted optical fiber and has the waveform distortion due to the group velocity dispersion is to be restored by the preamplifier and the dispersion compensating optical fiber. In particular, it is characterized in that the signal level input to the dispersion compensating optical fiber is adjusted within a predetermined range. That is, the lower limit of the signal level input to the dispersion compensating optical fiber is a value obtained by adding the minimum receiving sensitivity of the optical receiver, the total amount of dispersion / the total amount of loss in the dispersion compensating optical fiber, and the margin determined by the preamplifier noise. is there. On the other hand, as for the upper limit, as shown in FIG. 3, the power penalty increases due to the waveform distortion caused while the signal light propagates through the non-dispersion shift optical fiber and the dispersion compensating optical fiber, but this value does not exceed 1 dB. Is to adjust. The inventors have found that if the input signal level is made higher than this upper limit, the dispersion cannot be compensated and the power penalty is increased.

Here, since the waveform distortion caused by propagating in the non-dispersion shifted optical fiber is to be restored by the dispersion compensating optical fiber, in principle, the dispersion of the dispersion compensating optical fiber is suppressed in the signal wavelength band. It must be zero when the total amount and the total amount of dispersion of the non-dispersion shifted optical fiber are added. However, the dispersion of the non-dispersion shifted optical fiber depends mainly on the chirping of the optical transmitter, and in practice, it is -1000 to 500 ps / n.
It has been confirmed that there is no problem in the range of m. Therefore, within this range, it is defined to be substantially zero.

[0008]

Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a schematic diagram showing a configuration of a repeaterless optical transmission system according to the present embodiment, in which a directly modulated optical transmitter 1 and a postamplifier 2 are provided on a transmitting side, a preamplifier 4 is provided on a receiving side, and a dispersion compensating optical fiber. 6 and an optical receiver 7, and a non-dispersion optical signal transmission system for a non-dispersion shift optical fiber 3 composed of a 1.3 μm band zero dispersion single mode fiber or a pure silica core fiber installed between them. Then, the signal level input to the dispersion compensating optical fiber 6 is adjusted to fall within a predetermined range.

As a method of adjusting the signal level input to the dispersion compensating optical fiber 6, a beam splitter 8 for extracting a part of the signal light, a photodiode 9 for converting the extracted signal light into an electric current, and this value is predetermined. This is done via a feedback circuit consisting of an attenuator 5 or a device 10 for manipulating the gain of the preamplifier 4 (in the case of FIG. 2) to fall within the range.

The optical transmitter 1 has an emission wavelength of 1552 nm.
Is a distributed feedback laser diode directly modulated with a 2.4 Gb / s-NRZ signal, and has an α parameter of 7.5 with a poor chirp characteristic. The postamplifier 2 and the preamplifier 4 are usually fiber type optical amplifiers doped with a rare earth element such as Er, and have a wavelength of 1.55 μm.
Amplifies the band signal light. The non-dispersion shifted optical fiber 3 is not limited, but a single mode optical fiber made of pure silica is used as the core, and the attenuation is 0.175 d in the wavelength band of 1.55 μm.
B / km, group velocity dispersion is 18 ps / nm / km.
The dispersion compensating optical fiber 6 has an attenuation of 0.345 dB / km and a group velocity dispersion of −79.1 ps / in the wavelength band of 1.55 μm.
nm / km, figure of merit (FOM): total amount of dispersion / total amount of loss of dispersion-compensating optical fiber = 230 ps / nm / dB.

Using this apparatus, the relationship between the power penalty and the input level to the dispersion compensating optical fiber 6 was examined for the lengths of the non-dispersion shifted optical fiber 3 of 300, 200 and 100 km. From a practical point of view, the allowable value of the power penalty was set to 1 dB. As a result, as shown in FIG. 3, the shorter the length of the non-dispersion shifted optical fiber 3, the smaller the dispersion, and the smaller the intersymbol interference, the larger the upper limit of the input level to the dispersion compensating optical fiber 6. By the way, 300
The upper limit in the case of km is 4.8 dBm.

[Experimental example] In the above-mentioned repeaterless optical transmission system, the length of the non-dispersion shifted optical fiber 3 is 306 km.
(Dispersion: 5440 ps / nm, Loss: 52.3 dB),
The output of the postamplifier 2 is 17.2 dBm, the minimum receiving sensitivity of the optical receiver 7 is -32.5 dBm, and the total dispersion is -5590.
ps / nm, loss 27.1 dB (average FOM = 20
A dispersion-compensating optical fiber of 6.3 ps / nm / dB) was used.

At this time, the upper limit to the dispersion compensating optical fiber 6 is 4.8 dBm from FIG. The lower limit is -32.5 + 2
7.1 + 5 (margin) = − 0.4 dBm. That is, the incident power on the dispersion compensating optical fiber 6 is -0.4d.
The attenuator was adjusted as shown in the figure so as to be in the range of Bm to 4.8 dBm. As a result of measuring the relationship between the incident power on the dispersion compensating optical fiber and the error rate, the signal light transmitted from the postamplifier is back-to-to within the above range.
-Compared to the back state, the level was almost the same.

[0014]

As described above, according to the present invention, since the direct modulation system is adopted as the optical transmitter, its structure is simple and its handling is easy. By adjusting the signal level input to the dispersion compensating fiber within a predetermined range, it is possible to realize a repeaterless optical transmission system with a small power penalty without the self-phase modulation effect.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a configuration of a repeaterless optical transmission system according to an embodiment.

FIG. 2 is a schematic diagram showing a configuration of a repeaterless optical transmission system according to another embodiment.

FIG. 3 is a diagram showing an example of a relationship between a power penalty and an input level to a dispersion compensating optical fiber.

FIG. 4 is a schematic diagram showing a configuration of a conventional repeaterless optical transmission system.

[Explanation of symbols]

 1: Optical transmitter 2: Postamplifier 3: Non-dispersion shift optical fiber 4: Preamplifier 5: Attenuator 6: Dispersion compensation optical fiber 7: Optical receiver 8: Beam splitter 9: Photodiode 10: Level adjuster

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Koji Nakazato 1 Taya-cho, Sakae-ku, Yokohama-shi, Kanagawa Sumitomo Electric Industries, Ltd. Yokohama Works (72) Inventor Chie Fukuda 1-taya, Sakae-ku, Yokohama, Kanagawa Sumitomo Electric Ki Industry Co., Ltd.Yokohama Works (72) Inventor Masashi Onishi 1 Taya-cho, Sakae-ku, Yokohama-shi, Kanagawa Sumitomo Electric Co., Ltd. Yokohama Works (72) Inventor Masayuki Nishimura 1 Taya-cho, Sakae-ku, Yokohama-shi, Kanagawa Sumitomo Electric Ki Industry Co., Ltd. Yokohama Works

Claims (8)

[Claims] 1. A direct modulation optical transmitter and postamplifier on the transmitting side, a preamplifier, a dispersion compensating optical fiber and an optical receiver on the receiving side, and a non-dispersion shift optical fiber laid between them is non-repeatered. A repeaterless optical transmission system for transmitting an optical signal by means of which the signal level input to a dispersion compensating optical fiber is adjusted to fall within a predetermined range. 2. The signal level input to the dispersion compensating optical fiber is monitored, and the level is adjusted by an attenuator provided between the preamplifier and the dispersion compensating optical fiber. Repeaterless optical transmission system. 3. The repeaterless optical transmission system according to claim 1, wherein the signal level input to the dispersion compensating optical fiber is monitored and the level is adjusted by the gain of the preamplifier. 4. The range of the signal level input to the dispersion compensating optical fiber is the upper limit (the power penalty is 1 dB due to the waveform distortion caused by the signal light propagating in the non-dispersion shift optical fiber and the dispersion compensating optical fiber. Between the upper limit and the lower limit (the minimum receiving sensitivity of the receiver, the total amount of dispersion / the total amount of loss in the dispersion compensating optical fiber, and the margin determined by the noise generated in the preamplifier). The repeaterless optical transmission system according to claim 1. 5. The signal wavelength band according to claim 1, wherein the sum of the total amount of dispersion of the dispersion compensating optical fiber and the total amount of dispersion of the non-dispersion shifted optical fiber is substantially zero. Repeaterless optical transmission system. 6. The repeaterless optical transmission system according to claim 1, wherein the post-amplifier and the pre-amplifier are optical amplifiers configured by supplying pumping light to a rare earth element-doped fiber. 7. The non-dispersion shifted optical fiber has a wavelength of 1.3 μm.
The repeaterless optical transmission system according to claim 1, which is a single-mode optical fiber having zero dispersion in the m band and anomalous dispersion in the 1.55 μm band. 8. The repeaterless optical transmission system according to claim 1, wherein the core of the non-dispersion shifted optical fiber is pure silica silica.