Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-01-12
2003-06-24
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S341200, C359S337130, C359S341410, C359S341420
Reexamination Certificate
active
06583909
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an optical communication system for transmitting digital data such as audio data, image data, and text data through an optical transmission path.
FIG. 6
is an entire block diagram showing an example of a conventional optical communication system. Referring to
FIG. 6
, the optical communication system is constituted by optical terminal station devices
1
and
2
each having the same arrangement and a beam-type repeater
3
connected to the optical terminal station devices
1
and
2
through optical fibers OF
1
to OF
4
, and two optical communication paths for transmitting a main signal (signal transmitted through optical terminal station devices) between the optical terminal station devices
1
and
2
in both the directions.
In an optical transmitter
4
of the optical terminal station device
1
, a transmission electric signal serving as a main signal is converted into an optical signal by using an electric/optical converter (E/O)
5
, and the optical signal is amplified to a predetermined level by an optical post-amplifier
6
. Thereafter, the optical transmitter
4
transmits the main signal to the transmission path (optical fiber OF
1
). In the repeater
3
, the main signal attenuated by loss of the transmission path is amplified by an optical inline amplifier
7
to a predetermined level, and the main signal is transmitted to the transmission path (optical fiber OF
2
) again. In an optical receiver
8
of the optical terminal station device
2
, the main signal attenuated by loss of the transmission path is amplified by an optical pre-amplifier
9
, and the main signal is converted into a reception electric signal by an optical/electric converter (O/E)
10
to output the reception electric signal.
As described above, the optical inline amplifier
7
of the repeater
3
does not convert the main signal into an electric signal. For this reason, an arrangement in which an electric signal according to the main signal is detected from the optical inline amplifier
7
and an arrangement in which the optical inline amplifier
7
is supervised and controlled on the basis of the detected electric signal cannot be employed. Therefore, in order to supervise and control the optical inline amplifier
7
, an optical signal for supervisory and control (optical supervisory transmission signal (OSC: optical supervised channel)) is used.
In the example shown in
FIG. 6
, the optical terminal station devices
1
and
2
and the repeater
3
have supervisory transmission signal processing units
11
and
12
. The supervisory transmission signal processing unit
11
transmits an OSC having an optical wavelength different from that of the main signal. The OSC includes supervisory/control information of the optical inline amplifier
7
. The OSC is optically wavelength-multiplexed with the main signal by an optical system
13
, transmitted to the repeater
3
, and input to the supervisory transmission signal processing unit
12
through an optical system
14
. The supervisory transmission signal processing unit
12
supervises and controls the operation of the optical inline amplifier
7
on the basis of the supervisory/control information included in the OSC. On the other hand, the supervisory transmission signal processing unit
12
outputs an OSC including information of the state, operation, and the like of the optical inline amplifier
7
. This OSC is input to the supervisory transmission signal processing unit
11
of the optical terminal station device
1
through optical systems
15
a
and
16
. The supervisory transmission signal processing unit
11
supervises and controls the optical inline amplifier
7
on the basis of information related to the optical inline amplifier
7
included in the OSC. As another method of transmitting the OSC to the repeater
3
, a method of superposing the OSC on the main signal to transmit the resultant signal to the repeater
3
in an optical region may be taken.
FIG. 7
is a block diagram of an example of arrangement of the optical inline amplifier
7
shown in FIG.
6
. Referring to
FIG. 7
, the optical inline amplifier
7
is constituted by an optical pre-amplifier (AGC unit)
19
, a distribution compensation fiber (DCF)
20
, and an optical post-amplifier (ALC unit)
21
which are connected in series with each other. The optical pre-amplifier
19
low-noise-amplifies the main signal by gain constant control (AGC: Auto Gain Control). The DCF
20
compensates for only waveform distortion caused by light distribution in the transmission path. The optical post-amplifier
21
amplifies the main signal amplified by the optical pre-amplifier
19
to a predetermined level by output level constant control (ALC: Auto Level Control).
FIG. 8
is a block diagram of an example of arrangement of the optical post-amplifier
21
shown in FIG.
7
. The optical post-amplifier
21
makes an average level of the main signal output from the optical post-amplifier
21
constant by the ALC. The optical post-amplifier
21
is constituted by an optical level controller (optical amplifier)
22
, an optical system
23
, an O/E
24
, a low-pass filter (LPF)
25
, and a comparator
26
.
The optical level controller
22
amplifies a main signal output from the DCF
20
. The optical system
23
partially branches the main signal output from the optical level controller
22
to input the signal to the O/E
24
. The O/E
24
optical/electric-converts the optical output signal from the optical system
23
. The LPF
25
detects an average level signal of the main signal output from the O/E
24
. The comparator
26
compares an average level signal output from the LPF
25
with a reference signal to output the error signal. On the basis of the error signal, the gain of the optical level controller
22
is controlled. The ALC is executed by the above control loop.
The response speed of the ALC is determined by a portion having the lowest response speed in the control loop. This response speed is generally determined by the LPF from the viewpoint of circuit stabilization. In general, a fixed value depending on the level of the main signal output from the optical level controller
22
is set as the reference signal.
To be compared with the ALC, an example of arrangement of the optical pre-amplifier
19
is shown in FIG.
9
. As shown in
FIG. 9
, the optical pre-amplifier
19
controls the gain of an optical level controller
27
by the AGC to be constant. For this reason, the input average level of the main signal is monitored by an optical system
28
, an O/E
29
, and an LPF
30
. The output average level of the main signal is monitored by an optical system
31
, an O/E
32
, and the LPF
33
. The results obtained by the monitoring operations are compared with each other by the comparator
34
. The gain of the optical level controller
27
is controlled such that the difference between the levels is constant.
The repeater
3
is arranged to amplify the main signal attenuated by loss of the transmission path to a predetermined level such that the main signal has a waveform being so close to an original waveform as possible. This means that the amplitude of the original main signal is set at the predetermined level, and does not mean that the average level of the main signal is kept constant. Therefore, the ALC is desirably controlled on the basis of the peak detection result of the main signal.
In order to detect the peak of the main signal, a circuit having a speed sufficiently higher than that of the main signal is required. However, the high-speed circuit hinders the superiority of the optical amplifier which has a simple circuit arrangement obtained by amplifying an optical signal and is free from a bit rate. In addition, in a system for optical wavelength division multiplexing (OWDM) transmission in which a plurality of optical signals are multiplexed to amplify the optical signals, peak detection and ALC must be performed in each channel. For this reason, the great advantage of the OWDM in which a plurality of opti
Pascal Leslie
Sedighian M. R.
Staas & Halsey , LLP
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