JPH09139713A - Noise suppression method for optical amplification transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress noise in single wavelength transmission without remarkable system revision such as gain and output of an optical amplification repeater, and a circuit of optical fiber by multiplexing a noise suppression light onto a signal light of a single wavelength and transmitting the multiplexed light. SOLUTION: An NRZ signal is given to an optical transmitter 2 from a generator 1 and a modulation light by a laser is generated. A signal light is fed to an optical fiber coupler 6 via an optical attenuator 3. A non modulation light different in a wavelength from this signal is generated from a generator 4 to be a noise suppression light. The noise suppression light is fed to the coupler 6 via the optical attenuator 5 and the signal light and the noise suppression light with two different wavelengths are synthesized in the couler 6. Then the generated optical wavelength multiplex signal is sent to a dispersion shift fiber cable 7-1 from th coupler 6.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a noise suppression method for noise caused by a nonlinear optical effect in an optical amplification transmission system.

[0002]

2. Description of the Related Art In an optical communication system, when performing long-distance optical communication in a trunk system composed of optical fiber lines, the deterioration of the signal-to-noise ratio (S / N) due to the attenuation characteristics of the optical fiber lines is compensated. Therefore, by arranging the optical amplification repeaters on the optical fiber line at regular intervals, deterioration of S / N during transmission is reduced.

Conventionally, as such an optical amplification transmission system, transmission by a multistage optical amplification transmission system using a single wavelength has been known. From the characteristics of the system length, the repeater, and the optical fiber, the signal wavelength to be used. The optimum design can be performed for. On the other hand, a wavelength division multiplexing transmission system using a plurality of wavelengths has been considered in order to increase the transmission capacity. In this wavelength division multiplex transmission system, since the number of signal wavelengths to be used is multiple, system specifications different from the system designed for single wavelength transmission are required, and the optimum design corresponding to the wavelength multiplex transmission is performed. .

[0004]

An example of the structure of a transmission line of an optical amplification transmission system in which optical amplification repeaters are installed in multiple stages is shown in FIG.
(A). In the optical amplification transmission system shown in this figure, a dispersion shift optical fiber (DSF) 101-1 and an erbium-doped fiber amplifier (EDF) constituting an optical amplification repeater are used.
A) 102-1, DSF 101-2, EDFA 102-
2, cut-off shift fiber (EF) 103-1,
.. are serially connected to form an optical fiber transmission line. Dispersion shifted optical fiber (DSF) 101-1
Numerals 101-k are optical fibers occupying most of the transmission line, and an erbium-doped fiber amplifier (EDFA) 102-1 is provided every about 33 km of this dispersion shift optical fiber.
102-n are installed. This erbium-doped fiber amplifier is an optical amplifier for compensating for signal power that deteriorates in the transmission line. The cut-off shift fibers (EF) 103-1 to 103-m are optical fibers for compensating the chromatic dispersion of the dispersion shift optical fibers (DSF) 101-1 to 101-k.

Here, chromatic dispersion is a phenomenon caused by a difference in the refractive index depending on the wavelength of light even in a uniform medium, and means that the propagation speed varies depending on the wavelength. For example, if chromatic dispersion occurs in an optical fiber in which a pulsed optical signal is propagated, the pulse waveform is deteriorated and the width thereof is widened along with the propagation of the pulsed optical signal. That is, intersymbol interference occurs. In order to prevent this, cutoff shift fibers (EF) 103-1 to 103-m are inserted at predetermined intervals as equalizing fibers. This EF103-1 to 103-m
Is a chromatic dispersion value having a sign opposite to that of the chromatic dispersion values of the DSFs 101-1 to 101-k, and has a length such that the accumulated chromatic dispersion of the transmission line is reset to zero.

As described above, the cutoff shift fiber (E
F) 103-1 to 103-m shows the state of cumulative chromatic dispersion when inserted at predetermined intervals.
The cumulative chromatic dispersion value gradually increases in the negative direction as it propagates through 01-1, 101-2, .... Then, the EF 103 having a predetermined length in which the chromatic dispersion value has the opposite sign.
By propagating the part where -1 is inserted, the cumulative chromatic dispersion value is gradually reset to zero, and the chromatic dispersion characteristic is compensated. Therefore, the cumulative chromatic dispersion characteristic of the transmission line shown in FIG. 9A becomes the cumulative chromatic dispersion characteristic in which the sawtooth-like cumulative chromatic dispersion change repeatedly appears. Note that this cumulative chromatic dispersion characteristic has, for example, 368 optical amplification repeaters (EDFAs) and an optical fiber length of about 1
The characteristics are 2,000 km, the optical amplification repeater (EDFA) interval is about 33 km, the optical amplification repeater output is about +3 dBm, and the optical amplification repeater (EDFA) gain is about 7 dB.

A peculiar problem in the optical amplification transmission system is that transmission noise occurs due to the nonlinear optical effect, and the influence of this noise is intensified as the transmission distance increases. The nonlinear optical effect is related to almost all design parameters such as the wavelength dispersion characteristic of the optical fiber, the nonlinear characteristic, the output characteristic of the optical amplification repeater, and the system length. Therefore, in order to optimize the transmission characteristics, whether these factors (parameters) are an optical amplification transmission system using a single wavelength or a wavelength multiplexing optical amplification transmission system in which multiple wavelengths are multiplexed is used. Therefore, it is necessary to determine the optimum value for the signal wavelength used.

In this case, the optical amplification transmission system is constructed so that the determined optimum parameters can be obtained. However, it is technically and costly to obtain the optimum parameters strictly. It is difficult, and the optical amplification transmission system actually has parameters within the allowable range of the optimum parameters. For example, FIG. 7 shows an optical spectrum before transmission in the optical amplification transmission system in which a parameter is determined assuming that a single wavelength is used, and FIG. 8 shows an optical spectrum after transmission. As shown in FIG. 7, when signal light of a single wavelength propagates through the optical fiber line, the receiving side has a spectrum with widened tail as shown in FIG. 8 due to the above-mentioned nonlinear optical effect. Bit error rate (Bit Error
Ratio (BER) is about 2 × 10 ⁻¹¹ , and the S / N Q conversion value is about 16.4 dB. The value of this transmission characteristic is a measured value when the optical fiber length is set to 12,000 km, but the optical fiber length is designed to be 9,000 km.

However, such BER and S
The Q-converted value of / N cannot be said to be a sufficient value, and it will deteriorate with age. If a sufficient characteristic value cannot be obtained, it is necessary to change the specification to change the system, but this is physically and economically difficult. For example, in the case of an already installed submarine cable system, etc., it may be necessary to adjust the output characteristics of the repeater, replace the erbium-doped fiber, etc., or replace the optical fiber used in the transmission line. It is necessary to collect and re-lay the optical amplification repeater and the submarine cable accompanying the replacement, and the work amount and cost equal to or more than those when newly laying are required.

Therefore, according to the present invention, the noise of the optical amplification transmission system capable of suppressing the noise in the single wavelength transmission without making a drastic system change such as the gain and output of the optical amplification repeater and the characteristics of the optical fiber. It is intended to provide a method of suppression.

[0011]

In order to achieve the above object, a method of suppressing noise in an optical amplification transmission system according to the present invention is an optical amplification transmission system in which an optical amplification repeater is installed at predetermined intervals of an optical fiber. In order to adjust the transmission characteristics of the optical fiber and the optical amplification repeater by multiplexing noise suppression light having a wavelength offset by a predetermined wavelength from the wavelength of the transmitted signal light and transmitting the multiplexed signal light. Instead, the transmission noise due to the nonlinear optical effect is suppressed.

According to the present invention as described above, when the noise-suppressed light is multiplexed with the signal light of a single wavelength and transmitted, the amount of transmission noise due to the nonlinear optical effect is reduced, and the S of the received signal light is reduced.
/ N and BER can be improved.
This is because the energy of the noise component of each repeater is converted into the amplification energy of the noise suppression light by multiplexing the light source (noise suppression light) different from the signal light, and the noise in the main signal light is converted. Is considered to be reduced. Due to this action, the code error rate and the S / N Q of the optical amplification transmission method can be achieved without drastically changing the system.
The converted value can be improved and sufficient transmission characteristics can be obtained. Further, the present invention can be applied to the existing optical amplification transmission system, and can improve the transmission characteristics of the optical amplification transmission system which has deteriorated over time.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an example of the configuration of a multistage optical amplification repeater transmission system adopting the noise suppression method of the optical amplification transmission system of the present invention. However, FIG. 1 shows only the one-way configuration (only the upstream or downstream configuration).
The multistage optical amplification repeater transmission system shown in this figure comprises an optical fiber cable and an optical amplification repeater inserted at predetermined intervals of the optical fiber cable. The optical amplification repeater is an erbium-doped fiber amplifier (EDFA) 8- 1-8
-N, and the optical fiber cable is 1.
5 μm zero-dispersion wavelength dispersion-shifted fiber cable (DS
F) 7-1 to 7-k, and 1. for chromatic dispersion compensation.
Cutoff shift fiber cables (EF) 9-1 to 9-n having a 3 μm zero dispersion wavelength are used.

In this case, for example, the number of optical amplification repeaters is 368.
The length of the optical fiber cable is about 12,000 km, the distance between the optical amplification repeaters is about 33 km, the output of the optical amplification repeaters is +3 dBm, and the gain of the optical amplification repeaters is about 7 dB. In addition, DSF5-1 to 5-k and EF6-1 to 6-n
Cumulative chromatic dispersion characteristics in combination with
It becomes as shown in. On the transmitter side, the optical transmitter (O
S) 2 is applied with a 5.3 Gbit / s NRZ signal (Non-Return Zero) generated by a pulse pattern generator (PPG) 1 to modulate the laser light, and the modulated signal light is OS 2 Is generated from. In this case, the wavelength of the generated signal light is 1558.8 nm, for example.

This signal light is supplied to an optical fiber coupler (CPL) 6 via an optical attenuator (ATT) 3. Further, the noise-suppressed light generator (NS) 4 generates unmodulated (CW) light having a wavelength of 1556.8 nm, which is different from the signal light by 2 nm, but this unmodulated light is regarded as noise-suppressed light. The noise-suppressed light from NS4 passes through CPL6 via optical attenuator (ATT) 5.
Are supplied to the CPL 6 and are input to the CPL 6, and the lights of two different wavelengths, that is, the signal light and the noise suppression light are multiplexed. Then, the generated optical wavelength division multiplexed signal is transmitted from CPL6 to DSF7.
-1.

The optical attenuators (ATT) 3, 5 are arranged so that the input level of the first EDFA 8-1 becomes a standard value of -4 dBm.
The level of the signal light and the level of the noise suppression light are respectively adjusted by. In this case, the levels of the signal light and the noise suppression light are adjusted to be almost equal. In this way DSF7-
The optical wavelength division multiplexed signal sent to No. 1 is the DSF 7-1, ED
FA8-1, DSF7-2, EDFA8-2, EF9-
1, ..., DSF7-k, EDFA8-n, EF9-
Propagate m to reach the receiving side.

On the receiving side, the received optical wavelength division multiplexed signal is sent to the optical filter (OB) by the optical switch (SW) 10.
PF) 11 or optical spectrum analyzer (OS
A) One of 15 When the optical wavelength division multiplexed signal is guided to the optical filter 11 by the SW 10, the optical filter 1
Only the main signal light is separated from the optical WDM signal by 1 and is input to the optical receiver (OR) 12 to regenerate the main signal light. The signal reproduced by the OR 12 is input to the error detector (ED) 13 and its code error rate (BER)
Is calculated by the computer (PC) 14, the transmission characteristics are calculated. In addition, by SW10 O
The optical spectrum of the optical wavelength division multiplexed signal guided to SA15 is displayed on the display unit of OSA15. The ED 13, PC 14, and OSA 15 are provided only when measuring transmission characteristics.

In the multistage optical amplification repeater transmission system configured as above, the CPL on the transmission side when the wavelength of the main signal light is 1558.8 nm and the wavelength of the noise suppression light is 1556.8 nm
2 shows the optical spectrum of the optical wavelength division multiplexed signal sent from 6 and FIG. 3 shows the optical spectrum of the optical wavelength division multiplexed signal inputted to SW10 on the receiving side. The optical spectrum on the receiving side is shown by the broken line in FIG. It is the optical spectrum measured by OSA15 connected to SW10 shown. The optical spectrum before transmission shown in FIG. 2 is wavelength-multiplexed with optical S / N that is good in signal light and noise suppression light. When such wavelength-multiplexed light propagates through the multistage optical repeater amplification system shown in FIG. 1, the optical spectrum after transmission shown in FIG. 3 is obtained.

At this time, the optical S / N and BER (bit
The error ratio) will be described. As for the optical S / N, the signal level (S) is the peak value of the optical spectrum and the noise level (N) is the background noise of the optical spectrum as shown in the conceptual diagram of FIG. To be done. Further, the BER is an error detector (ED) 13 and a computer (PC) as shown by a broken line in FIG.
14 is measured after being connected to the optical receiver (OR) 12 subsequently. According to the noise suppression method of the optical amplification transmission system of the present invention, the BER which was about 2 × 10 ^-11 when conventionally transmitted through an optical fiber of 12,000 km was 6 × 10 ^-15.
Was improved by 3 digits or more, and similarly, the S / N Q conversion value, which was about 16.4 dB in the past, is 17.75 dB and about 1.4 dB.
Be improved.

Although the reason for the above improvement has not been clarified exactly, a non-linear effect, etc., of each optical amplification repeater is caused by the combination of a light source (noise suppression light) having a wavelength different from that of the main signal light. It is considered that the noise of the main signal light is reduced by converting the energy of the noise component due to the noise energy into the amplification energy of the noise suppression light. In other words, the wavelength of the noise-suppressing light must be a wavelength that can be separated from the wavelength of the main signal light on the receiving side without improving the transmission characteristics of the main signal light unless it is within a wavelength range that can be amplified by the optical amplification repeater. is there. Therefore, a wavelength range that can be amplified by the optical amplification repeater and that can be separated from the wavelength of the main signal light at the receiving side is selected as the wavelength of the noise suppression light.

The S / N Q conversion can be performed by the following equation. Q = 20log {√S (1) −√S (0)} / {√N (1) +
√N (0)} where S (1) and S (0) are the powers of the respective signals when the signals are "1" and "0", and N (1) and N (0) are the signals It is the power of each noise at "1" and "0".

Next, Q when the wavelength of the noise suppressing light is changed
The value of ² is shown in FIG. However, in this figure, when the wavelength of the main signal light is set to 0, the offset wavelength of the noise suppression light [n
m] is the horizontal axis and the vertical axis is Q ² [dB].
The reference value indicated by the broken line is the value in Q ² of the main signal light when the noise suppressing light is not added. The black circles shown in this figure indicate the Q ² of the main signal light when noise suppression light is added as the offset wavelength at the point where the vertical line from the black circle intersects the horizontal axis.
Are shown. That is, if there is a black circle above the reference value indicated by the broken line, it indicates that the value of Q ² has been improved, and if it is above it, the degree of improvement is large.

Q with respect to changes in the wavelength of the noise-suppressed light shown in FIG.
Observing the degree of improvement of the value of ² , when the wavelength of the noise suppression light is close to the wavelength of the main signal light, the degree of improvement is small. This is because the characteristic of the optical filter that separates the main signal light is tailed, so that noise-suppressed light is slightly included as noise in the separated signal light. Further, when the offset wavelength of the noise suppression light becomes large, the degree of improvement also decreases. This is because the bandwidth that can be amplified by the optical amplification repeaters installed at predetermined intervals in the optical fiber line is limited. Therefore, the larger the offset wavelength of the noise suppression light, the more the optical amplification repeater has it. It is considered that the energy of the noise component due to the non-linear effect is not converted into the amplified energy of the noise suppression light.

As described above, the degree of improvement of the value of Q ² of the main optical signal with respect to the wavelength of the noise-suppressed light has a peak at a certain offset wavelength, so that the noise suppression is performed so that the degree of improvement becomes a peak. The wavelength of light may be determined. Also, the wavelength of the noise-suppressed light at which the degree of improvement reaches its peak changes due to the optical amplification repeater, optical filter, etc.
The wavelength of the noise suppression light is determined using these characteristics as parameters. In the above description, the noise-suppressed light is non-modulated light, but when modulated at a frequency that is sufficiently lower than the modulation frequency of the main signal light, the transmission characteristics can be improved as described above, so noise-suppressed light is sufficient. It may be modulated at a low frequency.

Although it is conceivable to transmit single wavelength light to a transmission system in which each parameter is designed as an optical amplification transmission method for transmitting wavelength-division multiplexed light, the present invention is applied to such a case. When the noise-suppressed light is multiplexed and then transmitted, the transmission characteristics represented by the code error rate and the Q conversion value can be improved.

[0026]

Since the present invention is configured as described above, when the noise suppression light is multiplexed with the signal light of a single wavelength and transmitted, the amount of transmission noise due to the non-linear optical effect is reduced and the S of the signal light is reduced. / N and BER can be improved. Further, when the transmission characteristics of the optical amplification transmission system are evaluated using the Q conversion value and the bit error rate (BER), the Q conversion value can be improved by about 1.4 dB from the conventional value.
Can be improved by three digits or more compared to the conventional one.

With this operation, the code error rate and the S / N Q of the optical amplification transmission system can be achieved without making a drastic system change.
The converted value can be improved and sufficient transmission characteristics can be obtained. Further, the present invention can be applied to the existing optical amplification transmission system, and can improve the transmission characteristics of the optical amplification transmission system which has deteriorated over time.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of the configuration of a multistage optical amplification repeater transmission system for implementing a noise suppression method of an optical amplification transmission system of the present invention.

FIG. 2 is a diagram showing a signal light spectrum before transmission in the noise suppression method of the optical amplification transmission system of the present invention.

FIG. 3 is a diagram showing a signal light spectrum after transmission in the noise suppression method of the optical amplification transmission system of the present invention.

FIG. 4 is a diagram showing a Q conversion value with respect to a wavelength of noise suppression light in the noise suppression method of the optical amplification transmission system of the present invention.

FIG. 5 is a diagram showing a concept of optical S / N.

FIG. 6 is a diagram showing a configuration of an optical fiber line of a conventional optical amplification transmission system and a cumulative chromatic dispersion characteristic of the optical fiber line.

FIG. 7 is a diagram showing an optical spectrum of signal light before transmission in the conventional optical amplification transmission system.

FIG. 8 is a diagram showing an optical spectrum of signal light after transmission in a conventional optical amplification transmission system.

[Explanation of symbols]

1 pulse pattern generator 2 optical transmitter 3, 5 optical attenuator 4 random suppression optical generator 6 optical fiber coupler 7-1 to 7-k dispersion shift fiber 8-1 to 8-n erbium-doped fiber amplifier 9-1 to 1 9-m equalizing fiber (cut-off shift fiber) 10 optical switch 11 optical filter 12 optical receiver 13 error detector 14 computer 15 optical spectrum analyzer

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Reference number in the agency FI Technical display location H04B 10/16 (72) Inventor Haruo Abe 2-3-2 Nishishinjuku, Shinjuku-ku, Tokyo International Telegraph Telephone Co., Ltd. (72) Inventor Masato Tanaka, 2-3-2 Nishishinjuku, Shinjuku-ku, Tokyo International Telegraph and Telephone Corporation

Claims (1)

[Claims] 1. An optical amplification transmission system in which optical amplifiers are installed at predetermined intervals of an optical fiber, and noise suppression light having a wavelength offset by a predetermined wavelength from the wavelength of the transmitted signal light is multiplexed on the signal light. A noise suppression method of an optical amplification transmission method, wherein transmission noise due to a non-linear optical effect is suppressed without adjusting transmission characteristics of the optical fiber and the optical amplifier.