JP3304011B2 - Digital optical transmission equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital optical transmission apparatus capable of reducing a bit error rate characteristic caused by a received waveform distortion caused by stimulated Brillouin scattering (SBS) and enabling long span and large capacity transmission. .

[0002]

2. Description of the Related Art In order to increase the non-relay transmission distance, it is necessary to increase the optical transmission level and to increase the sensitivity to the optical reception level. As for the optical reception level, high sensitivity has been achieved up to a few dB close to the theoretical limit. On the other hand, the optical transmission level can be relatively easily increased by an optical fiber amplifier.

[0003]

However, in an optical transmission system using an ordinary optical fiber, when the input level to the optical fiber exceeds a predetermined threshold (hereinafter, referred to as "SBS threshold"), backward transmission due to stimulated Brillouin scattering occurs. As the scattered light increases sharply, the saturation of the light output level after transmission through the optical fiber starts. Such a phenomenon affects the transmission characteristics (code error rate characteristics) and is a factor that significantly deteriorates the receiving sensitivity.

FIG. 5 shows the relationship between the optical fiber input level, the intensity of backscattered light due to stimulated Brillouin scattering, and the optical output level after transmission through the optical fiber. The input light to the optical fiber was modulated by CPFSK (phase continuous frequency shift keying) and had a fixed pattern of 16 bits having a transmission rate of 2.5 Gbit / s. As shown in the figure, when the optical fiber input level exceeds +12 dBm,
The intensity of the backscattered light due to stimulated Brillouin scattering sharply increases and the light output level begins to saturate.

Next, the degradation of the bit error rate characteristic caused by the received waveform distortion caused by stimulated Brillouin scattering will be described using the above-mentioned input light (16-bit fixed pattern) as an example. FIG. 6 shows a measurement system of a bit error rate (BER) and a waveform distortion for each bit. In the figure, an output signal of a pulse pattern generator 61 is modulated by a CPFSK optical modulation circuit 62, and the optical signal is amplified by an optical booster amplifier 63 and input to a single mode optical fiber 64. The optical signal transmitted through the single-mode optical fiber 64 is converted into an electric signal by a heterodyne optical receiving circuit 65, further demodulated and identified, and then separated into signals for each bit by a 1:16 DEMUX circuit 66. The sampling oscilloscope 67 measures the waveform distortion. 1:16 DEMUX
Each output of the circuit 66 always indicates a predetermined voltage corresponding to the code "1" or "0" unless an error occurs. Therefore, the change of each output is counted by the code error detector 68, and each bit is output. Is measured.

Here, the bit error rate for each bit (BE
R) The distribution is shown in FIG. Is the optical fiber input level Pin
= + 10 dBm, which is below the SBS threshold. Is the case where Pin = + 17.2 dBm, which is a level much higher than the SBS threshold. The black mark and the white mark represent codes “1” and “0”. Further, each reception level Pre was adjusted so that the code error rate in the entire 16-bit fixed pattern was constant. This will be described later.

In the case of Pin = + 10 dBm, a bit (1: No.1, No.11, 0: No.5) indicating a code error rate of about 10 ^-7 for either the code "1" or "0". , No.10, No.13, No.1
6) (reception level Pre = -43.0 dBm, identification threshold Vth = -0.440 V). On the other hand, when Pin = + 17.2 dBm, the code error rate of the specific bit (No. 10) of code “0” is
This is as large as 10 ^-6 (reception level Pre = −37.0 dBm, identification threshold Vth = −0.440 V), which causes deterioration of the overall bit error rate characteristics.

FIG. 8 shows the relationship between the optical fiber input level and the demodulated waveform amplitude (average and dispersion) for No. 10 and No. 1 as representative bits of codes "0" and "1". . In the case of No. 10 (“0”), the demodulation waveform amplitude sharply decreases with an increase in the optical fiber input level exceeding the SBS threshold, and the bit error rate of this bit is greatly deteriorated as shown in FIG. I understand.

As described above, since the demodulation waveform amplitude of a specific bit is greatly distorted and the bit error rate characteristic is remarkably deteriorated, in the conventional digital optical transmission device, it is necessary to lower the optical transmission level to a level that does not have the effect of stimulated Brillouin scattering. was there. This also became a factor limiting the transmission distance.

There is a method in which three or more identification thresholds are used, and it is determined whether a mark is erroneous as a space or a space is erroneous as a mark based on the result of each identification determination, and the identification threshold is controlled (Japanese Patent Application Laid-Open (JP-A) No. 2001-110630). No. 59-53300). However, in this method, since the identification determination and the identification threshold control are performed for each bit, a high-speed operation is required for a D / A converter and other large-scale integrated circuits that are indispensable for the configuration. Therefore, it has been extremely difficult to support transmission speeds of gigabit or more.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a digital optical transmission apparatus capable of reducing bit error rate characteristic deterioration due to stimulated Brillouin scattering even if the optical transmission level is increased to some extent.

[0012]

According to the present invention, there is provided a means for setting an identification threshold for minimizing a bit error rate with respect to the intensity of stimulated Brillouin scattering generated in an optical fiber, as an identification threshold of an identification circuit of an optical receiver. Is provided.

[0013] The optical transmitting apparatus further includes means for detecting the intensity of backscattered light generated by stimulated Brillouin scattering of the optical fiber, and means for writing the intensity information of the backscattered light into a transmission signal. Means for reading the intensity information of the backscattered light written in the received signal, and based on the intensity information of the read backscattered light, the code error rate is minimized with respect to the intensity of the backscattered light. Means for setting an identification threshold in the identification circuit.

The optical receiving apparatus further includes means for detecting the intensity of a beat component generated between the input light of the optical fiber and the stimulated Brillouin scattered light, and a stimulated Brillouin based on a ratio of the intensity of the received light to the intensity of the beat component. Means for detecting the intensity of the scattering, and means for setting an identification threshold at which the bit error rate with respect to the intensity of the stimulated Brillouin scattering is minimized in the identification circuit.

[0015]

According to the present invention, the intensity of stimulated Brillouin scattering is detected, and based on the detected intensity, the identification threshold of the identification circuit of the optical receiver is controlled to a value that minimizes the bit error rate. As a result, it is possible to reduce the degradation of the bit error rate characteristic caused by the received waveform distortion caused by stimulated Brillouin scattering. Note that the intensity of stimulated Brillouin scattering is detected from the intensity of the backscattered light on the optical transmitter side and transferred to the optical receiver side,
Optical fiber input light and stimulated Brillouin scattering on the optical receiver side
It can be detected from the intensity of the beat component generated between the light and the scattered light .

Here, how the bit error rate characteristic is improved by controlling the identification threshold will be described. FIG. 9 shows the relationship between the optical reception level and the bit error rate (BER). The input light to the optical fiber is modulated by CPFSK,
A 16-bit fixed pattern having a transmission rate of 2.5 Gbit / s was used. Is the optical fiber input level Pin = +
It is a code error rate characteristic (total) of 10 dBm and an identification threshold Vth = −0.440 V (optimum identification threshold when Pin = + 10 dBm). Is a code error rate characteristic (total) of Pin = + 17.2 dBm and Vth = −0.440 V. Is Pin = + 17.2dB
m, Vth = −0.495 V (optimum identification threshold when Pin = + 17.2 dBm) is a code error rate characteristic (total). In addition,
FIG. 7 shows the bit error rate (BER) distribution for each bit in the case of.

In the case of (1), it can be seen that the bit error rate is significantly deteriorated due to stimulated Brillouin scattering, and a floor is generated. On the other hand, if the threshold value is adjusted to Vth = −0.495 V as shown in FIG. 5, there is no floor, and the code error rate is 1 × 10 ^-9 and the receiving sensitivity of 5.5 dB is improved. As shown in FIG. 7, the bit error rate of the code “0” (No. 10 and others) is greatly improved, and the worst bit error rates of the codes “1” and “0” are almost the same. Has become.

[0018]

FIG. 1 shows the configuration of a first embodiment of a digital optical transmission apparatus according to the present invention (claim 2).

In the figure, an optical transmitting apparatus includes a section overhead processing circuit 11, an optical modulation circuit 12a, an optical booster amplifier 13a, an optical directional coupler 14, an optical / electrical converter (O / E) 15, and an A / D converter. It is constituted by a vessel 16. The optical receiver includes an optical / electrical converter (O / E) 21a,
It comprises a demodulation and shaping circuit 22a, a timing extraction circuit 23a, an identification circuit 24a, a section overhead processing circuit 25, a digital signal processing circuit (DSP) 26a, and a D / A converter 27a.

The output signal of the pulse pattern generator 61 is
The optical signal is input to the optical modulation circuit 12a via the section overhead processing circuit 11 of the optical transmission device, where the modulated optical signal is amplified by the optical booster amplifier 13a, and the single mode optical fiber 64 is transmitted via the optical directional coupler 14. Sent to On the other hand, the backscattered light corresponding to the stimulated Brillouin scattering of the single mode optical fiber 64 is input from the optical directional coupler 14 to the optical / electrical converter 15 and is converted into an electric signal, and the intensity information is A / D. The signal is supplied to the section overhead processing circuit 11 via the converter 16. The section overhead processing circuit 11 writes the intensity information of the stimulated Brillouin scattering into the transmission signal. Note that the intensity fluctuation of the backscattered light in response to stimulated Brillouin scattering, especially a burst-like fluctuation, is less than 10 kHz (Takushima and Okoshi, "Instab
ilities of Light Intensity inan Optical Fiber in t
he Presence of Stimulated Brillouin Scattering ", OF
C'92), and the aging of the output of the optical fiber amplifier is much slower than this, so that the problem due to the response speed of the A / D converter 16 and the like does not occur.

The optical signal transmitted through the single-mode optical fiber 64 is converted into an electric signal by the optical / electrical converter 21a of the optical receiver, demodulated and shaped by the demodulation and shaping circuit 22a, and then converted into a timing extracting circuit 23a. Is identified by the identification circuit 24a on the basis of the timing extracted in step (1). The identification signal is sent to the code error detector 68 via the section overhead processing circuit 25. Further, the section overhead processing circuit 25 reads out the intensity information of the stimulated Brillouin scattering written on the optical transmission device side from the identification signal, and supplies it to the digital signal processing circuit 26a. The digital signal processing circuit 26a determines an optimum identification threshold value of the identification circuit 24a corresponding to the intensity information, and the D / A converter 2
7a to the identification circuit 24a.

In the processing in the digital signal processing circuit 26a, the intensity information of stimulated Brillouin scattering and the identification circuit 24
The relationship between a and the optimal identification threshold is held in a memory, and the identification threshold is controlled by referring to this memory during service.
In the digital signal processing circuit 26a, the intensity of the backscattered light according to the stimulated Brillouin scattering is changed by changing the optical transmission level of the optical transmitter stepwise in advance during the test of this system, and the identification is performed for each intensity information. The threshold is swept, and the optimum identification threshold at which the code error rate is minimized is measured from the detection result of the code error detector 68. Therefore, according to the configuration of the present embodiment, even when the optical transmission level of the optical transmission device changes over time, the optimum identification threshold is always set based on the change in the intensity of stimulated Brillouin scattering (the intensity of backscattered light). Thus, the bit error rate can be minimized.

Here, a method of transferring the intensity information of stimulated Brillouin scattering from the optical transmitter to the optical receiver will be described. In the synchronous digital hierarchy (SDH) system, an international standard is defined by CCITT (current ITU) (G.709), and as shown in FIG. 2, a transmission signal is an STM-N frame (N = 16 at 2.48832 Gbit / s). Configuration.
The first N × 9 bytes are for sending various monitoring signals and control signals called section overhead (SOH). The next N × 261 bytes are a part for transmitting a service signal called a payload. Hereinafter, each section overhead and payload up to the ninth line are repeated to form one frame. The bits in the frame excluding the section overhead (N × 9 bytes) in the first row are scrambled by a primitive polynomial (1 × X ⁶ × X ⁷ ). The intensity information of the stimulated Brillouin scattering (the intensity information of the backscattered light) is written into a part of the section overhead by the section overhead processing circuit 11 of the optical transmission device, and is read by the section overhead processing circuit 25 of the optical reception device. Transfer between transmission and reception is performed.

When stimulated Brillouin scattering occurs, stimulated Brillouin scattered light having a frequency shift of about 11 GHz with respect to the input light of the optical fiber is included in the output light of the optical fiber, and a beat is generated between the light and the input light. Due to this beat, a spectrum is observed around 11 GHz in the spectrum after the optical / electrical conversion of the received signal, as shown in FIG. 3 (2). FIG. 3A shows the optical fiber input level Pin.
= + 12 dBm when stimulated Brillouin scattering does not occur, and FIG. 3B shows a case where stimulated Brillouin scattering occurs when the optical fiber input level Pin = + 17 dBm.

Therefore, the intensity of stimulated Brillouin scattering can be detected even on the receiving device side by observing the beat component around 11 GHz and calculating the ratio to the total power. In this case, unlike the first embodiment, it is not necessary to detect the intensity of stimulated Brillouin scattering from the intensity of the backscattered light on the optical transmitter side and transfer the intensity information to the optical receiver side. Hereinafter, a second embodiment will be described.

FIG. 4 shows the configuration of a second embodiment of the digital optical transmission apparatus according to the present invention (claim 3). In the figure, the optical transmission device includes an optical modulation circuit 12b and an optical booster amplifier 13b. The optical receiving device includes an optical / electrical converter (O / E) 21b, a demodulation and shaping circuit 22b, a timing extracting circuit 23b, a discriminating circuit 24b, a band-pass filter (BPF) 28, power detectors 29 and 30, and a divider 3.
1, A / D converter 32, digital signal processing circuit (DS
P) 26b and a D / A converter 27b.

The output signal of the pulse pattern generator 61 is
The optical signal is input to the optical modulation circuit 12b of the optical transmission device, where the modulated optical signal is amplified by the optical booster amplifier 13b and transmitted to the single mode optical fiber 64.

The optical signal transmitted through the single-mode optical fiber 64 is converted into an electric signal by the optical / electrical converter 21b of the optical receiver, demodulated and shaped by the demodulation and shaping circuit 22b, and then converted by the timing extracting circuit 23b. Are identified by the identification circuit 24b based on the timing extracted in the step (1), and sent to the code error detector 68. On the other hand, a band component around 11 GHz is extracted from the received signal by the band-pass filter 28, and the intensity is detected by the power detector 29. The power detector 30 detects the intensity of the entire received signal, and the divider 31 calculates the ratio between the two. Here, the time constants of the power detectors 29 and 30 are shorter than the intensity fluctuation period due to stimulated Brillouin scattering,
Scramble period (7 stages in SDH transmission system,
The period is set to a value longer than (2 ⁷ -1) / B, B: bit rate. This makes it possible to cancel the loss fluctuation of the optical fiber and detect only the fluctuation in the intensity of the beat component caused by stimulated Brillouin scattering.

The output of the divider 31 is supplied to a digital signal processing circuit 26b via an A / D converter 32. The digital signal processing circuit 26b determines an optimum discrimination threshold of the discrimination circuit 24b corresponding to the intensity of the beat component caused by stimulated Brillouin scattering, and supplies it to the discrimination circuit 24b via the D / A converter 27b. Note that the identification threshold control in the digital signal processing circuit 26b is the same as in the first embodiment.

In the description of the first and second embodiments, the intensity modulation / direct detection method has been described as an example. However, the present invention can be applied to a coherent homodyne method, a coherent heterodyne method, and other optical transmission methods. Is possible.

[0031]

As described above, the digital optical transmission apparatus according to the present invention sets an identification threshold value for minimizing the bit error rate in the identification circuit based on the intensity of stimulated Brillouin scattering, thereby enabling the digital Brillouin scattering. It is possible to reduce bit error rate characteristic deterioration due to the received waveform distortion. As a result, the optical transmission level can be set higher, and long span and large capacity transmission can be realized.

The intensity of stimulated Brillouin scattering can be easily detected on the optical transmitter side from the intensity of backscattered light. Also, the input light of the optical fiber and the guide
It can be easily detected from the intensity of the beat component generated between the light and the Lilouin scattered light .

Further, in the digital optical transmission device of the present invention, a corresponding optimum identification threshold can be directly obtained from the intensity of stimulated Brillouin scattering of an optical fiber. We can respond enough.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a first embodiment of a digital optical transmission device according to the present invention.

FIG. 2 is a diagram showing an STM-N frame configuration.

FIG. 3 is a diagram showing a received signal spectrum according to whether or not stimulated Brillouin scattering has occurred;

FIG. 4 is a block diagram showing a configuration of a digital optical transmission apparatus according to a second embodiment of the present invention.

FIG. 5 is a diagram showing a relationship between an optical fiber input level, an intensity of backscattered light due to stimulated Brillouin scattering, and an optical output level after transmission of the optical fiber.

FIG. 6 is a block diagram showing a measurement system of a bit error rate (BER) and a waveform distortion for each bit.

FIG. 7 is a diagram showing a bit error rate (BER) distribution for each bit.

FIG. 8 is a diagram illustrating a relationship between an optical fiber input level and a demodulated waveform amplitude in a representative bit.

FIG. 9 is a diagram showing a relationship between an optical reception level and a bit error rate (BER).

[Explanation of symbols]

 11, 25 Section overhead processing circuit 12 Optical modulation circuit 13 Optical booster amplifier 14 Optical directional coupler 15, 21 Optical / electrical converter (O / E) 16, 32 A / D converter 22 Demodulation and shaping circuit 23 Timing extraction circuit 24 Identification Circuit 26 Digital Signal Processing Circuit (DSP) 27 D / A Converter 28 Band Pass Filter (BPF) 29, 30 Power Detector 31 Divider 61 Pulse Pattern Generator 62 CPFSK Optical Modulation Circuit 63 Optical Booster Amplifier 64 Single Mode optical fiber 65 Heterodyne optical receiving circuit 66 1:16 DEMUX circuit 67 Sampling oscilloscope 68 Code error detector

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H04B 10/00-10/28 H04J 14/00-14/08 JICST file (JOIS)

Claims (3)

(57) [Claims] In a digital optical transmission device in which an optical transmitting device and an optical receiving device are connected via an optical fiber, the intensity of stimulated Brillouin scattering generated in the optical fiber is determined.
Stimulated Brillouin scattering intensity detection means for detecting, an optical transmission level of the optical transmitter and identification of the optical receiver
By changing the identification threshold of the circuit, the stimulated Brillouin scattering
The bit error rate of the signal received by the optical receiver is
The minimum identification threshold is measured in advance and used as measurement information.
Means for storing Te, stimulated Brillouin scattering which is detected after storage of the measurement information
The bit error rate is minimized based on the strength of the
Identification threshold setting means for setting an identification threshold to the identification circuit
Digital optical transmission apparatus characterized by comprising and. 2. A digital optical transmission device according to claim 1.
In the optical transmission device, as the stimulated Brillouin scattering intensity detection means, means for detecting the intensity of backscattered light generated by stimulated Brillouin scattering of the optical fiber, and intensity information of the backscattered light into a transmission signal and means for writing, in the light receiving apparatus, prior to the intensity information of the backscattered light that has been written to the receive signal
A digital optical transmission device comprising means for reading out the intensity of stimulated Brillouin scattering . 3. A digital optical transmission device according to claim 1.
In induction, the said optical receiving apparatus, as the stimulated Brillouin scattering intensity detecting means, and means for detecting the intensity of the received light, the input light of the optical fiber
Means for detecting the intensity of a beat component generated between the Brillouin scattered light and means for detecting the intensity of stimulated Brillouin scattering from the ratio of the intensity of the received light to the intensity of the beat component. Digital optical transmission equipment.