JPH0789185B2 - Optical amplification method for long-distance optical communication system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Object of the invention [Industrial field of use] The present invention relates to a long-distance optical communication system using an optical fiber, and more particularly to an optical fiber of a long-distance optical communication system for directly amplifying an optical signal. It relates to an amplification method.

[Prior Art] An optical fiber communication system that has been put into practical use is a relay system in which an optical signal is once converted into an electrical signal, amplified and waveform-shaped in the state of the electrical signal, and the amplified electrical signal is used again. Optical / electrical / optical conversion is performed, which is to drive a semiconductor laser. In this method, since one regenerative repeater circuit is required for one transmission signal carrier, it is difficult to perform wavelength multiplexing or frequency multiplexing. on the other hand,
In the repeater system that uses optical amplification, which directly amplifies the optical signal as it is, compared to the optical-electrical-optical conversion repeater system, the repeater circuit is simpler and the wavelength-multiplexed signal at an arbitrary transmission rate and There is an advantage that the frequency-multiplexed signals can be collectively amplified by one optical amplifier. Further, since the amplification degree is constant regardless of the incident direction of light, it is also possible to collectively amplify bidirectional signal light by using one optical amplifier.

As a conventional optical amplification element, an optical amplifier mainly using a semiconductor laser, an optical amplifier using an optical fiber in which a normal earth optical fiber is doped with a rare earth element, and a light using the stimulated Raman scattering phenomenon generated in the optical fiber are used. There is a fiber Raman amplifier, and the amplification degree can be 30 dB to 40 dB. The optical amplifier using the semiconductor laser is a normal semiconductor laser,
It is based on the principle that a current equal to or lower than a threshold current is flowed, and when light is incident in that state, it is amplified by stimulated emission.

FIG. 4 is an amplification characteristic diagram when a conventional semiconductor laser is used as an optical amplification element (Electronics Letters 1987, V
OL.23, P218-219). In the figure, the horizontal axis represents the output signal light level (amplified output power) at which the signal light is amplified by the optical amplification element and emitted, and the vertical axis represents the amplification gain G of the optical amplification element.
From this characteristic diagram, it can be seen that a constant amplification degree is obtained up to an output signal light level of about OdBm. FIG. 4 is a characteristic diagram when the saturation amplification is 15, 20, 25 dB,
The saturation amplification can be continuously adjusted by controlling the injection current to the semiconductor laser up to about 30 dB.

When such an optical amplifier is used in an optical communication system, even in a transoceanic optical submarine cable communication system such as the Atlantic Ocean and the Pacific Ocean, an all-optical communication network without electric signals is constructed, and the optical submarine cable is used in the same manner as space. Will be able to handle. When this system was constructed, the adoption of a coherent optical communication system that uses the characteristics of light waves will enable the frequency division multiplex communication system between multiple points, which has already been performed in sanitary communication. As described above, the optical amplification element becomes an important technology for constructing an international optical network in the future. However, despite the usefulness of the optical amplification technology, although research on a single optical amplification element was made, the study of system design using the optical amplification element was
Little was done.

The most important issue in system design using optical amplifiers is the effect on the SN of spontaneous emission from the optical amplifiers. Generation of spontaneous emission light is essential to the optical amplification element, and its power spectrum is considerably wider than that of signal light, as shown in FIG. Therefore, in the optical communication system using the optical amplification element, it is necessary to suppress the influence of the spontaneous emission light on the signal light as much as possible.

FIG. 6 shows a conventional method that has been considered to suppress the influence of the signal light of the spontaneous emission light.

In the figure, the polarization plane of the signal light propagating through the optical fiber 1 is coupled to the optical amplifier 4 via the lens 3 which is compensated by the polarization compensator 2 so as to match the polarization axis of the optical amplifier 4. The signal light L amplified by the optical amplifier 4 is guided to the light receiving element 6 through the lens 5 and the optical filter 7. The spontaneous emission light having a wide spectrum width as shown in FIG. 5 was suppressed by the filter 7.

[Problems to be Solved by the Invention] However, in a long-distance optical communication system in which a large number of optical amplifiers 4 are connected in cascade such as a transoceanic optical submarine cable, spontaneous emission light generated from each optical amplifier 4 is used. However, there is a problem in that a long-distance optical communication system capable of communication cannot be constructed because the signal may be accumulated and the signal light level may be exceeded.

The present invention has been made to solve the above-mentioned problems of the conventional technology, and does not provide an optical amplification method for a long-distance optical communication system capable of performing communication by reducing spontaneous emission light generated from an optical amplifier. It is what

(2) Configuration of the Invention [Means for Solving the Problems] A feature of the present invention is that in an optical amplification system in which an optical signal incident on an optical amplifier is amplified as it is to obtain outgoing light, polarization of the optical signal Spontaneous emission generated from the optical amplifier, having polarization compensating means for setting the wavefront to a predetermined polarization state, and an analyzer having a polarization axis aligned with the polarization plane of the optical signal in the predetermined polarization state. It is configured to reduce light.

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

In the following description, the same components as those of the prior art will be designated by the same reference numerals, and duplicate description will be omitted.

(Embodiment 1) FIG. 1 is a block diagram of an optical amplification system according to a first embodiment of the present invention. In the figure, reference numeral 8 is an analyzer that is inserted and arranged between the optical filter 7 and the light receiving element 6 so that the polarization axis, which is a feature of the present invention, coincides with the polarization plane of the signal light.

(Operation 1) The signal light L propagating in the optical fiber 1 is the polarization compensator 2
Is compensated so as to match the polarization axis of the optical amplifier 4 and coupled to the optical amplifier 4 via the lens 3.

The optical amplifier 4 generates spontaneous emission light in addition to amplifying the signal light L as described above. The spontaneous emission light having the wide power spectrum is removed by the optical filter 7 having a narrow spectral width through which the signal light L passes, and then enters the analyzer 8 together with the signal light L. Here, the polarization compensator 2 is controlled so that the signal light L from the optical amplifier 4 is directly polarized, and the polarization axis of the analyzer 8 is arranged so as to match the polarization plane of the signal light L. If left, the analyzer 8 becomes transparent to the signal light L with no attenuation. On the other hand, the spontaneous emission light generated from the optical amplifier 4 is non-polarized light having the same power for all polarization planes. Therefore, the total power of the spontaneous emission light after passing through the analyzer 8 is attenuated by half.

As described above, the use of the present invention makes it possible to reduce the level of spontaneous emission light by 3 dB as compared with the conventional technique.

(Embodiment 2) FIG. 2 is a second embodiment of the present invention and is a configuration diagram of an optical amplification system for relaying in a long distance optical communication system. The difference from the first embodiment is that an optical fiber 9 for transmission is used instead of the light receiving element 6, and an analyzer 8 is inserted between the polarization compensator 2 and the lens 3.

(Operation 2) In this embodiment, the spontaneous emission light generated by the optical amplifier (not shown) in the previous stage is removed before being amplified by the optical amplifier 4 in the next stage, and the natural light generated by the previous optical amplifier is removed. The emitted light is analyzed by an analyzer 8 arranged in front of the next optical amplifier (not shown).
It is configured to be sequentially removed by. Therefore, such a configuration is effective only in an optical repeater system in which the optical amplifiers 4 are cascade-connected in a plurality of stages.
In the second embodiment, the polarization compensator 2 and the analyzer 8 may be arranged before and after the optical amplifier 4 as in the first embodiment.

(Embodiment 3) In the above description, a semiconductor laser is used as the optical amplifier 4, but the optical amplifier using the rare earth-doped optical fiber and the optical fiber using the optical fiber Raman described above have been described. The present invention can be applied to an amplifier.

FIG. 3 shows the third embodiment according to the present invention, which is a configuration diagram of an optical amplification system using a Raman optical amplifier.

In the figure, 10 is a pump light source for amplification, 11 is an optical coupler for guiding pump light to the main line system, and pump light source
The Raman optical amplifier 4a is composed of 10 and the optical coupler 11, and the other structures are the same as those of the first embodiment.

(Operation 3) In the third embodiment, the signal light L propagating through the optical fiber 1 is combined by the optical coupler 11 and amplified by the stimulated Raman effect, and then the polarization compensator 2 for setting the predetermined polarization state is provided. Except for the operation of the inserted Raman optical amplifier 4a, the operation is the same as that of the first embodiment. That is, when the Raman optical amplifier 4a is used, in order to enhance the amplification effect, the polarization compensator 2
Should be inserted after the Raman optical amplifier 4a.

(3) Effects of the Invention As described above, the present invention has at least the polarization compensator 2
Since the non-polarized spontaneous emission light generated from the optical amplifier 4 can be reduced by combining the optical analyzer 4 and the analyzer 8, a long-distance optical communication system can be constructed.

Further, by arranging the polarization compensator 2 which is the polarization compensating means on the incident side of the optical amplifier 4 and the analyzer 8 on the emitting side of the optical amplifier, the receiving end immediately receives an electric signal via the light receiving element 6. Can be converted to. By arranging the polarization compensator 2 and the analyzer 8 on the incident side of the optical amplifier 4, it is possible to apply the optical amplifier 4 to an optical repeater system which is used as an optical repeater. Furthermore, by arranging the polarization compensator 2 and the analyzer 8 on the emission side of the optical amplifier 4, the Raman optical amplifier 4a
It can also be applied to. Therefore, the optical amplification method of the present invention can be widely applied to the optical communication system using the optical amplifier 4, and its effect is great.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an optical amplification system according to a first embodiment of the present invention, and FIG. 2 is a configuration diagram of a relay optical amplification system in a long-distance optical communication system according to a second embodiment of the present invention.
FIG. 5 is a block diagram of an optical amplification system using a Raman optical amplifier according to a third embodiment of the present invention, FIG. 4 is an amplification characteristic diagram when a conventional semiconductor laser is used as an optical amplification element, and FIG. Is a power spectrum diagram of spontaneous emission light generated from an optical amplifier, and FIG. 6 is a configuration diagram of a conventional optical amplification system. 1,9 …… Optical fiber, 2 …… Polarization compensator 3,5 …… Lens, 4 …… Optical amplifier 4a …… Raman optical amplifier, 6 …… Light receiving element 7 …… Optical filter, 8 …… Analyzer 10 ...... Pump light source, 11 …… Optical coupler

Claims (4)

[Claims] 1. A polarization compensating means for setting a polarization plane of a polarization plane of an optical signal in a predetermined polarization state in an optical amplification system for amplifying an optical signal incident on an optical amplifier as it is to obtain outgoing light.
A long distance characterized by having an analyzer having a polarization axis aligned with a polarization plane of the optical signal in the predetermined polarization state, and configured to reduce spontaneous emission light generated from the optical amplifier. Optical amplification method for optical communication system 2. The length according to claim 1, wherein the polarization compensating means is arranged on the incident side of the optical amplifier, and the analyzer is arranged on the emitting side of the optical amplifier. Optical amplifier system for distance optical communication system 3. The optical amplification system for a long-distance optical communication system according to claim 1, wherein the polarization compensating means and the analyzer are arranged on the incident side of the optical amplifier. 4. The optical amplification system for a long-distance optical communication system according to claim 1, wherein the polarization compensating means and the analyzer are arranged on the emission side of the optical amplifier.