Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-02-03
2002-07-09
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200, C359S199200, C359S199200, C359S199200, C359S199200, C455S501000, C455S422100, C455S426100, C455S517000
Reexamination Certificate
active
06417942
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an optical communication system such as an optical CATV, optical ITV, or mobile communication system, which connects one main station and a plurality of sub-stations via an optical fiber and, more particularly, to an optical communication system which uses an optical wavelength multiplex technique in upstream signals from a sub-station to a main station, and a method of controlling the optical communication system.
Mobile communications represented by cellular phones, PHSs (personal handyphone system), and the like use radio, and have prevailed in recent years. However, since mobile communications use radio, they are often disrupted in a given area such as an underground mall, tunnel, dead zone of a base-station antenna in the shade of a building, and the like, where a radio wave is hard to reach.
In order to cover such uncommunicatable areas, sub-stations that are small-output stations are placed at various locations corresponding to the dead zones of a radio wave, thus providing communicatable areas to assure convenience for the users.
In this manner, a large number of sub-stations (base stations) that support the mobile communication system are placed in, e.g., an underground mall, tunnel, shade of a building, and the like where a radio wave is hard to reach. It is most preferable in terms of cost to feed a radio signal to such areas via optical fibers, and to use simple sub-stations (base stations) each having an antenna port module alone, as described in the article “The New Generation of Wireless Communications Based on Fiber-Radio Technologies” (IEICE Transaction on Communications vol. E76-B, no. 9, September 1993). Sub-stations connected via optical fibers are receiving lot of attention in an optical ITV (Industrial Televisions), CATV, and the like based on cable transmission, since a large number of sub-stations can be placed at various locations.
An optical network technique that accommodates a plurality of distributed sub-stations in a main station still suffers problems. One serious problem is beat noise produced upon interference of light sources of a plurality of sub-stations. The beat noise will be explained below.
Assume that optical signal A originating from a given sub-station is located at a wavelength position separated &Dgr;&lgr; from optical signal B originating from another sub-station, as shown in FIG.
1
A. When these optical signals are simultaneously received by a single receiver, beat noise due to optical signals A and B is produced at a higher frequency position, &Dgr;&lgr;, than the information signal band, as shown in FIG.
1
B.
At this time, if the wavelengths of optical signals A and B are sufficiently separated from each other, i.e., if &Dgr;&lgr; is small, beat noise falls within the information signal band, i.e., beat noise is produced within the information signal band, thus deteriorating reception sensitivity. In the worst case, these signals cannot be received at all.
Hence, in order to suppress beat noise, a wavelength multiplex technique that assures a given wavelength spacing between sub-stations is required.
Note that in the required wavelength multiplex technique, wavelengths need not be assigned at high density and their spacing need only be controlled to prevent beat noise from falling within the signal band, unlike in a technique used for a trunk system for long-distance transmission.
However, since the wavelengths may change due to changes in atmospheric temperature, and beat noise may influence the information signal, a means for detecting beat noise and means for controlling the wavelengths are required. Furthermore, which of a plurality of sub-stations has caused beat noise must be specified.
As the above-mentioned wavelength multiplex transmission system, for example, Jpn. Pat. Appln. KOKAI Publication No. 9-83434 proposed a system in which a main station comprises a beat noise detector. This invention has an arrangement shown in
FIG. 2
, and the main station has a beat detector. More specifically, referring to
FIG. 2
, reference numeral
1
denotes a main station;
2
-
1
,
2
-
2
,
2
-n, . . . , sub-stations;
3
a
, an optical fiber for transmitting an upstream optical signal; and
3
b
, an optical fiber for transmitting a downstream optical signal. The optical fiber
3
a
for transmitting an upstream optical signal forms a transmission path from the sub-stations
2
-
1
,
2
-
2
,
2
-n, . . . toward the main station
1
, and the optical fiber
3
b
for transmitting a downstream optical signal forms a transmission path from the main station
1
toward the
2
-
1
,
2
-
2
,
2
-n, . . . .
Reference numeral
4
denotes an E/O (electro-optical) converter for a sub-station;
6
, a branch optical fiber;
7
, a photocoupler;
8
, a sub-station controller;
9
, a sub-station modulator;
10
, an O/E (opto-electric) converter for a sub-station; and
11
, a sub-station demodulator. Each of the sub-stations
2
-
1
,
2
-
2
,
2
-n, . . . incorporates these devices.
Each of the sub-stations
2
-
1
,
2
-
2
,
2
-n, . . . is connected to the optical fiber
3
a
for transmitting an upstream optical signal by its E/O converter
4
via the branch optical fiber
6
. A plurality of photocouplers
7
are connected to the optical fiber
3
a
. When the distal ends of the branch optical fibers
6
are connected to these photocouplers
7
, they are optically connected to each other.
Also, another plurality of photocouplers
7
are connected to the optical fiber
3
b
. When the distal ends of other branch optical fibers
6
connected to the O/E converters
10
are connected to these photocouplers
7
, the optical fiber
3
b
and sub-stations
2
-
1
,
2
-
2
,
2
-n, . . . are optically connected to each other.
Hence, each sub-station branches information optically transmitted from the main station
1
by the photocoupler
7
and inputs the information to the O/E converter
10
. The O/E converter
10
photoelectrically converts the information into an electric signal. The electric signal is demodulated by the sub-station demodulator
11
. Control information contained in the demodulated information is supplied to the E/O converter
4
via the sub-station controller
8
to control electrooptical conversion. On the other hand, information transmitted from the sub-station is modulated by the sub-station modulator
9
, and the modulated information is converted by the E/O converter
4
into an optical signal. The optical signal is sent onto the optical fiber
3
a
via the branch optical fiber and photocoupler
7
.
Furthermore, in
FIG. 2
, reference numeral
12
denotes a main station controller;
13
, a main station modulator;
14
, a main station E/O converter;
15
, a main station O/E converter;
16
, a main station demodulator; and
17
, a beat detector, which construct the main station
1
. In the main station
1
, an optical signal transmitted via the optical fiber
3
a
is received by the O/E converter
15
, and an electric signal obtained by photoelectric conversion is supplied to the beat detector
17
and main station demodulator
16
. The electric signal is demodulated by the main station demodulator
16
, and the demodulated signal is output. The beat detector
17
detects beat noise from the electric signal output from the O/E converter
15
after photoelectric conversion.
The main station controller
12
modulates a transmission signal using the main station modulator
13
while controlling the modulator
13
in accordance with the detection output from the beat detector
17
. The modulated signal is converted by the E/O converter from an electric signal into an optical signal. The optical signal is then output onto the optical fiber
3
b.
The beat detector
17
provided to the main station
1
detects beat noise. In the main station
1
, the beat detector
17
detects beat noise produced when the wavelength spacing between certain sub-stations becomes small, by monitoring the power of beat noise.
Upon detection of the beat noise, in the main station
1
, a control means (no
Ohshima Shigeru
Seto Ichiro
Tomioka Tazuko
Kabushiki Kaisha Toshiba
Pascal Leslie
Phan Hanh
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