Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-12-01
2002-05-28
Mullen, Thomas (Department: 2632)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200
Reexamination Certificate
active
06396601
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical signal quality monitoring system for monitoring the signal-to-noise ratio (S/N) of digital optical signals having different bit rates, transmitted in an optical fiber transmission network.
This application is based on Patent Applications Nos. Hei 9-330553 and Hei 10-229659 filed in Japan, the contents of which are incorporated herein by reference.
2. Description of the Related Art
In the network hierarchical structure SDH (Synchronous Digital Hierarchy) which was internationally standardized in the 1990s, a parity check called “bit interleave parity (BIP)” is executed between repeaters (this case is called “BIP-8” and is explained later) and also between line terminal multiplexers (this case is called “BIP-N×24”), thereby identifying an erroneous section and obtaining a signal for switching and activating operations. Here, “N” (an integer) relates to the level of the multiplex. With the basic symbol “STM-
1
” indicating 156 Mbit/s, “STM-N” means the level obtained by multiplying the above level by N. The cases of N=1, 4, 16, and 64 are internationally standardized. In addition, “BIP-M” indicates a parity check for every M bits, and a set of M bits for checking are obtained. At the transmission side, the parity check of parallel M bits of a signal included in a frame is executed, and the checked bits are stored in the next frame and are transmitted with the main signal. At the receiving side, a similar parity check is executed, and a transmission error is detected by collating the checked bits with the above checked bits stored in a specified area in the next frame.
FIG. 18
shows an example of conventional systems for measuring a (bit) error rate. In the figure, a portion of an optical signal through a transmission path is separated and extracted by optical coupler
51
-
1
. The extracted portion is amplified by optical amplifier
52
and is further separated into two portions by optical coupler
51
-
2
. One portion of these two portions is input into clock-extracting circuit
53
so that a clock signal of frequency f
0
(i.e., “clock f
0
”) is extracted. The other portion separated by optical coupler
51
-
2
is input into receiving circuit
54
, an output thereof further input into error-rate detecting circuit
55
which consists of a frame detecting circuit, parity checking circuit, and collation circuit. The receiving circuit
54
and error-rate detecting circuit
55
are operated in accordance with the above clock f
0
extracted by clock-extracting circuit
53
, and the error rate of the optical signal is measured. Here, the clock-extracting circuit
53
, receiving circuit
54
, and error-rate detecting circuit
55
must have specific structures corresponding to the bit rate of the target optical signal. That is, in order to perform error-rate detection corresponding to plural kinds of bit rates, plural circuits which respectively correspond to the different bit rates are necessary, and thus the error-rate detection cannot be executed using a single circuit based on the conventional technique.
Generally, the error rate of a signal is directly measured in order to evaluate a transmission system. However, if the error rate is very low in this method, a long measuring time is necessary and thus the measuring efficiency is low.
Therefore, regarding a transmission system for receiving binary digital signals, a method for estimating an error rate was presented, in which according to a tendency of error rates obtained when the threshold for a decision circuit is shifted, the error rate at the optimal operating point is estimated (refer to Reference 1: N. S. Bergano, et al., “Margin Measurements in Optical Amplifier Systems”, IEEE Photonics Technology Letters, Vol. 5, No. 3, pp. 304-306, March, 1993).
FIG. 19
shows a relevant eye diagram of an optical signal and an amplitude histogram indicating light intensity. The threshold of the decision circuit is changed at the time to when the eye-diagram opening is maximum (i.e., the decision point), thereby discriminating between the “High” (or “1” or “MARK”) level and the “Low” (or “0” or “SPACE”) level in the binary data transmission, and measuring each error rate.
In practice, a measuring system as shown in
FIG. 20
is constructed, which consists of clock-extracting circuit
53
, photoelectric converter
56
, and electrical signal processing means
57
, and the Q-factor (as an evaluation index) corresponding to the S/N is calculated based on dependency of the error rate on the threshold. In more detail, a portion of an optical signal extracted from a transmission path is converted into an electrical signal by the photoelectric converter
56
, and this electrical signal and a clock (signal) extracted by the clock-extracting circuit
53
are input into electrical signal processing means
57
such as a sampling oscilloscope, so that an eye diagram and an amplitude histogram as shown in
FIG. 19
are obtained.
At time t
0
when the eye-diagram opening is maximized, with signal amplitude (such as a voltage) &mgr;(t
0
), standard deviation &sgr;
1
(t
0
) of noises at the “MARK” level, and standard deviation &sgr;
0
(t
0
) of noises at the “SPACE” level, the Q(t
0
) (i.e., the Q-factor at t
0
) is represented as follows:
Q(t
0
)=&mgr;(t
0
)/(&sgr;
1
(t
0
)+&sgr;
0
(t
0
)) (1)
On the assumption of a Gaussian distribution of the amplitude of noises, in a low error-rate range, the following relationship between error rate P and the Q-factor is obtained:
P=(1/(Q(2&pgr;)
½
)) exp (−Q
2
/2) (2)
Therefore, if the Q-factor can be determined, the error rate can be estimated.
However, in the conventional Q-factor measuring system, an optical signal is converted into an electrical signal, and the waveform of the converted signal is sampled so as to determine the Q-factor. Therefore, the possible bit rate of the optical signal is limited to approximately 40 Gbit/s, depending on the range or processing speed of the photoelectric converter and the electrical signal processing circuit.
In addition, the Q-factor at the time when the eye-diagram opening is maximum is measured; thus, the system cannot monitor plural digital optical signals having different bit rates.
Furthermore, in the conventional Q-factor measuring system, a portion of an optical signal to be monitored must be extracted from the transmission path. Therefore, power loss due to the separation of the optical signal transmitted in the transmission path is generated, thereby degrading the S/N.
SUMMARY OF THE INVENTION
Accordingly, the present invention has an objective to provide a system for monitoring the quality of optical signals which are transmitted in an optical fiber transmission network and which have different bit rates, where the S/N of each optical signal can be monitored.
The present invention has another objective to provide a system for monitoring the quality of optical signals having bit rates of a few dozen Gbit/s or more.
The present invention has another objective to provide a system for monitoring the quality of optical signals, by which the effect on the S/N of each optical signal transmitted in a transmission path can be reduced.
Therefore, the present invention provides an optical signal quality monitoring system comprising:
sampling means for sampling an optical signal having a bit rate N·f
0
, that is, N times as much as the basic clock frequency f
0
where N is a natural number, by using a pulse repetition frequency f
0
1
−&Dgr;f or f
0
1
+&Dgr;f where n
1
is a predetermined natural number and the pulse repetition frequency slightly differs from f
0
1
by &Dgr;f; and
electrical signal processing means for determining an amplitude histogram of the light intensity of the optical signal based on the results of the sampling, and regarding the sampling points which constitute the histogram, the processing means extracting a set of higher-level points and a set of lower-level points and
Shake Ippei
Takara Hidehiko
Yamabayashi Yoshiaki
Mullen Thomas
Nippon Telegraph and Telephone Corporation
Pennie & Edmonds LLP
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