JP2004135018A - Optical communication system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical communication system which prevents the transmission characteristics of an optical amplifier from deteriorating owing to ASE without using any optical filter. <P>SOLUTION: This optical communication system is equipped with an optical transmitter 1 which emits signal light, an optical receiver 4 which receives the signal light, and an optical amplifier 2 which is arranged between the optical transmitter 1 and optical receiver 4 connected with optical fibers 5a and 5b forming a transmission line and amplifies the power of the signal light and further equipped with a polarizer 3 which is arranged on the input side of the optical receiver 4 and detects the plane of polarization of the signal light to automatically adjust its polarization direction to the polarization direction of the signal light. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical communication system for transmitting an optical signal.
[0002]
[Prior art]
In recent years, with the spread of the Internet, demand for high-speed data communication has spread not only to large-scale businesses but also to small businesses and ordinary households. In response to such demands, the introduction of optical communication systems has been progressing not only in trunk networks but also in optical access networks.
[0003]
In an optical access network, a Fabry-Perot laser (FP-LD) that oscillates in multiple modes with a wide spectrum width may be used as a signal light source to simplify the system and reduce costs. In addition, even when a distributed feedback laser (DFB-LD) that oscillates in a single mode is used as a signal light source, a system is constructed so that the laser can be used if the oscillation wavelength is in the range of several nm to several tens of nm.
[0004]
[Non-patent document 1]
Ono et al., "Development of Optical Fiber Amplifier for 1.3 μm Band Optical Access Network", IEICE CS2001-25, 2001.
[0005]
[Problems to be solved by the invention]
In the optical access network, similarly to the trunk network, an optical amplifier is used when the transmission distance is long or the current system is extended.
[0006]
However, in the above-described optical access network, since the oscillation wavelength of the signal light is not specified, it is difficult to provide the optical amplifier with a narrow-band optical filter. When an optical amplifier without an optical filter is used as an optical repeater, spontaneous emission amplified light (ASE) having a wide spectrum width is mixed with the output of the optical amplifier in addition to the amplified signal light. , The output power of the ASE is not small. If signal light is received by an optical receiver in the presence of a strong ASE, transmission characteristics are significantly degraded by noise due to interference between ASEs (beat noise between ASEs). In particular, when the intensity of the optical signal input to the optical amplifier becomes small due to the loss of the optical fiber forming the transmission line, the ASE intensity becomes relatively large as compared with the intensity of the amplified signal light. Is a cause of deterioration.
[0007]
The present invention has been made in view of the above problems, and has as its object to provide an optical communication system that suppresses deterioration of transmission characteristics due to ASE of an optical amplifier without using an optical filter.
[0008]
[Means for Solving the Problems]
An optical communication system according to claim 1 of the present invention that solves the above-mentioned problems includes an optical transmitter that generates signal light, an optical receiver that receives the signal light, and the optical transmitter that is connected by an optical fiber. An optical amplifier disposed between the optical receiver and amplifying the power of the signal light, further detecting a polarization plane of the signal light, and adjusting a polarization direction to a polarization direction of the signal light. A polarizer is provided.
[0009]
An optical communication system according to a second aspect of the present invention that solves the above problem is characterized in that the polarizer is arranged on the input side of the optical receiver.
[0010]
An optical communication system according to a third aspect of the present invention for solving the above-mentioned problems is characterized in that the optical amplifier is an optical repeater or a preamplifier.
[0011]
(Action)
In the above invention, a polarizer is provided on the optical receiving end side of an optical communication system transmission line having an optical amplifier, and the polarizer detects the polarization plane of the signal light, and determines the polarization direction of the signal light and the polarization in the polarizer. By automatically adjusting the wave direction, the ASE whose polarization direction is orthogonal to the signal light cannot pass through the polarizer and is not transmitted to the optical receiver.
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
The present invention is characterized in that, in an optical communication system, when weak transmitted signal light is amplified by an optical amplifier (optical amplifier), ASE, which is noise, is removed by a polarizer instead of an optical filter. . As a removing method, the polarization direction of the signal light and the polarization direction of the polarizer are automatically adjusted to exclude ASE in which the polarization direction is orthogonal to the signal light. This specific embodiment will be described below.
[0013]
FIG. 1 is a block diagram of an optical communication system showing an example of an embodiment according to the present invention.
[0014]
An optical communication system according to the present invention includes an optical transmitter 1 for generating signal light, an optical receiver 4 for receiving signal light, and an optical transmitter 1 connected by optical fibers 5a and 5b forming a transmission path. An optical amplifier 2 is provided between the optical receiver 4 and amplifies the power of the signal light. Further, a polarizer 3 is provided on the input side of the optical receiver 4 and detects the plane of polarization of the signal light and automatically adjusts the polarization direction to the polarization direction of the signal light. In the optical communication system shown in FIG. 1, the optical amplifier 2 is used as an optical repeater.
[0015]
The optical transmitter 1 uses an appropriate signal light source such as a Fabry-Perot laser (FP-LD) or a distributed feedback laser (DFB-LD), modulates this signal light source into a signal light, and transmits the signal light to the optical fiber 5a. I do.
[0016]
The optical receiver 4 receives the signal light transmitted to the optical fibers 5a and 5b using a light receiver, converts the signal light into an electric signal, and outputs the electric signal as a data signal and a clock signal.
[0017]
As the optical amplifier 2, an optical amplifier that amplifies the output of an optical signal as it is without converting it into an electric signal is used. Generally, there are a semiconductor amplifier, a fiber Raman scattering amplifier, a fiber Brillouin amplifier, a rare earth doped fiber amplifier and the like. In particular, erbium-doped fiber (EDF) amplifiers, which are one of the rare-earth-doped fiber amplifiers, are widely used because they have no polarization dependence and can be pumped by a semiconductor laser. Distance transmission is possible.
[0018]
As a method of amplifying an optical signal, for example, in an EDF amplifier, erbium (Er) is added to an optical fiber which is a medium for transmitting an optical signal, and the erbium-doped fiber (EDF) itself functions as an optical amplifier. I have. Specifically, an optical signal is amplified by appropriately supplying pumping light of a specific wavelength band to the EDF by a semiconductor laser or the like and further inputting an optical signal to the EDF.
[0019]
In such an optical amplifier, light called spontaneous emission amplified light (ASE) is output not only in the signal light but also in a wide wavelength range around the wavelength of the signal light during the amplification operation. This ASE becomes a factor of deteriorating the transmission characteristics of the signal light.
[0020]
Therefore, the optical communication system according to the present invention includes the polarizer 3 on the input side of the optical receiver 4 so as to remove ASE. The polarizer 3 has a function of detecting the plane of polarization of the optical signal and automatically matching the polarization direction of the signal light with the polarization direction of the polarizer 3. As an example of a specific configuration of the polarizer 3, a polarization rotator arranged on the input side of the signal light to change (rotate) the polarization direction of the signal light, and propagation of the signal light from the polarization rotator A polarizer that is arranged in the direction and transmits the signal light in a predetermined polarization direction, a coupler that branches the signal light that has passed through the polarizer to the output side and the photodetector side, and the intensity of the signal light that is branched by the coupler And a control circuit for controlling the polarization rotator based on a signal from the photodetector. Note that a polarizer having another configuration may be used as long as the polarizer has the same function.
[0021]
The polarization rotation element rotates the polarization of the signal light by an angle specified by the control signal according to the control signal. The polarization rotator includes, for example, a combination of a plurality of wavelength plates (a half-wave plate, a quarter-wave plate, etc.) and a device composed of a Faraday rotator, but those having the same function. Others may be used. The polarizer extracts a component in a predetermined polarization direction from the signal light from the polarization rotation element. The signal light from the polarizer is input to the coupler, and most of the signal light is output as output light, and a part of the signal light is supplied to the photodetector. The photodetector converts a part of the signal light from the coupler into an electric signal. The output of the photodetector is proportional to the power of the signal light from the polarizer. Based on the output of the photodetector, the control circuit controls the polarization of the polarization rotation element so that the output of the photodetector becomes maximum. Control the amount of rotation.
[0022]
By performing such control, the polarizer 3 can automatically output the polarization direction of the signal light and the polarization direction of the polarizer 3 automatically regardless of the polarization direction of the signal light. Therefore, in the polarizer 3, the signal light is transmitted, and the ASE whose polarization direction is orthogonal to the signal light cannot be transmitted and is removed. The power loss of the signal light in the polarizer 3 is only a loss in the polarization rotator, the polarizer, and the coupler, and is extremely small. Even if the polarizer 3 is arranged immediately before the optical receiver 4, the optical loss Has little effect on the receiver.
[0023]
That is, the optical communication system according to the present invention is different from the conventional optical communication system in that the optical communication system includes the above-described polarizer 3 before the signal light is received by the optical receiver 4. . That is, the polarizer 3 detects the plane of polarization of the signal light, and operates so that the polarization direction is automatically adjusted to the polarization direction of the signal light. Of the polarization components of the ASE light generated by the optical amplifier 2, the component orthogonal to the signal light polarization plane can be removed. Thereby, at the time of receiving the signal light, the beat noise between ASEs can be reduced, and the transmission characteristics can be improved.
[0024]
In the optical communication system according to the present invention, the optical transmitter 1 transmits a signal light having a wavelength of 1300 nm, a signal light power of 0 dBm, and a bit rate of 156 Mbit / s. The signal light power attenuates while propagating through the optical fiber 5a, and the input signal light power to the optical amplifier 2 becomes -25 dBm. The optical amplifier 2 does not have an optical filter for removing ASE. The optical amplifier is used as an optical repeater and amplifies the signal light power to +10 dBm. The receiving sensitivity of the optical receiver 4 is −38.9 dBm (error rate 10 ⁻⁹ ). Under such conditions, the transmission characteristics of signal light were compared with those of a conventional optical communication system.
[0025]
FIG. 2 is a diagram showing a comparison of transmission characteristics (code error rate) between the optical communication system according to the present invention and a conventional optical communication system.
[0026]
The BB (Back to Back) line in the graph of FIG. 2 shows the transmission characteristics of the bit error rate when the optical transmitter 1 is directly connected to the optical receiver 4. Based on this, the transmission characteristics of the optical communication system according to the present invention and the conventional optical communication system will be compared. The error rate was measured while an optical variable attenuator was arranged at the optical power input end of the optical receiver 4 and the input optical power of the optical receiver 4 was changed. In the case of an error rate of 10 ⁻⁹ , the power penalty of the conventional optical communication system was 3.8 dB, whereas the power penalty of the optical communication system of the present invention was 1.8 dB and the transmission characteristic was improved by 2.0 dB. Was. The power penalty is an amount obtained by estimating the deterioration of the receiving sensitivity as a substantial loss of the receiving power.
[0027]
In this embodiment, the signal light wavelength is 1300 nm, but the signal light wavelength is not limited to this. In this embodiment, the polarizer 3 is arranged at the optical power input end of the optical receiver, but may be arranged at the output end of the optical amplifier 2.
[0028]
Further, when the optical amplifier was used as a preamplifier, the effect of reducing the power penalty was similarly obtained. FIG. 3 is an optical communication system using an optical amplifier as a preamplifier, and is a block diagram showing another example of the embodiment according to the present invention.
[0029]
The optical communication system according to the present invention shown in FIG. 3 has substantially the same configuration as that of the optical communication system of the above-described embodiment, and a duplicate description will be omitted. 4 is characterized in that an optical amplifier 2 as a preamplifier is arranged before the optical amplifier 4 and a polarizer 3 is arranged between the optical receiver 4 and the optical amplifier 2. In any of the embodiments, the polarizer 3 is arranged on the input side of the optical receiver 4 so as to eliminate the influence of ASE from the optical amplifier 3.
[0030]
Each of the optical amplifiers used in the optical communication systems of the embodiments shown in FIGS. 1 and 3 does not include an optical filter for removing ASE. However, when the spectrum of the optical signal generated by the optical transmitter 1 is in the range of several nm, a part of the ASE can be removed by providing the optical amplifier 2 with an optical filter having a larger half-value width, and the power penalty can be reduced. Becomes smaller. Specifically, when the optical amplifier 2 including the optical filter for ASE removal (half-width 5 nm) is used, the power penalty is improved by 0.8 dB.
[0031]
Conventionally, a narrow-band filter must be prepared according to the wavelength of the signal light, and when the wavelength of the signal light changes, a new narrow-band filter corresponding to the wavelength has to be prepared. However, as is apparent from the present embodiment, if the optical communication system of the present invention is used, the above filter is not necessary, and even in an optical communication system in which the wavelength of the signal light is different, ASE which is noise accompanying optical amplification is reduced. It could be easily and efficiently removed. Further, by arranging the polarization planes of a plurality of channels, the same effect can be obtained even in an optical communication system using a plurality of wavelengths simultaneously (wavelength multiplexing: WDM).
[0032]
【The invention's effect】
As described above, according to the present invention, the polarizer detects the polarization plane of the signal light and automatically adjusts the polarization direction to the polarization direction of the signal, thereby reducing the ASE light at the time of receiving the signal light. Thus, beat noise between ASEs can be suppressed, and when an optical amplifier is used as an optical repeater, deterioration of transmission characteristics can be suppressed without using an optical filter.
[Brief description of the drawings]
FIG. 1 is a block diagram of an optical communication system showing an example of an embodiment according to the present invention.
FIG. 2 is a diagram showing a comparison of transmission characteristics (code error rate) between the optical communication system according to the present invention and a conventional optical communication system.
FIG. 3 is a block diagram of an optical communication system showing another example of the embodiment according to the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Optical transmitter 2 Optical amplifier 3 Polarizer 4 Optical receiver 5 Optical fiber 5a Optical fiber 5b Optical fiber

Claims (3)

An optical transmitter that generates signal light, an optical receiver that receives the signal light, and an optical transmitter that is disposed between the optical transmitter and the optical receiver connected by an optical fiber and amplifies the power of the signal light. An optical communication system comprising an optical amplifier
An optical communication system comprising: a polarizer that detects a polarization plane of the signal light and adjusts a polarization direction to a polarization direction of the signal light. The optical communication system according to claim 1,
An optical communication system, wherein the polarizer is arranged on an input side of the optical receiver. In the optical communication system according to claim 1 or 2,
An optical communication system, wherein the optical amplifier is an optical repeater or a preamplifier.

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