Optical communications system

Details Optical communications system Optical communications system

1997-09-05

2001-05-15

Negash, Kinfe-Michael (Department: 2733)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200

Reexamination Certificate

active

06233076

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical communications system applicable to long-distance communications such as underseas cable communications, etc.
2. Description of the Related Art
Recently, optical communications systems have been widely developed to realize large-capacity and high-speed communications systems. Especially when a large volume of information is to be simultaneously transmitted, an optical wave-length multiplexing system is highly evaluated and is studied for practical use in the near future. In the optical wave-length multiplexing system, an optical signal which carries information and have a plurality of wavelengths is wavelength-multiplexed for transmission. An optical signal of each wave length corresponds to at least one communications channel. In the optical wavelength multiplexing system applicable to the long-distance communications such as underseas cable communications, an optical add-drop system is under development in which an optical signal having a specific wavelength or an optical signal along a specific channel among optical signals wavelength-multiplexed in the communications line is branched to transmit an optical signal along a channel branched to a terminal station, and the optical signal transmitted from the terminal station with the same wavelength as the branched channel is combined again to the optical signal transmitted through the original transmission line for transmission to the terminating station.
FIGS. 1A through 1F
show the conventional optical add-drop system and the problem with the system.
FIG. 1A
is a block diagram showing the entire configuration of the optical add-drop system. The basic configuration in the optical add-drop system has a terminal station A as a transmitting station for transmitting an optical-wavelength multiplexed optical signal, a terminal station C as a receiving station for receiving a signal from the terminal station A, a branching unit
1100
for branching or combining an optical signal of a specific wavelength in the optical signals from the terminal station A, and. a terminal station B for receiving the optical signal branched by the branching unit
1100
, and transmitting new information with an optical signal having the same wavelength as the received optical signal. Normally in the underseas cable communications, the branching unit
1100
is mounted underseas to transmit optical signals to, for example, the terminal stations A, B, and C provided in different nations. Typically, the distance between the terminal stations A and C is approximately 3,000 km, and the branching unit
1100
is provided around the central point between these stations. Since the intensity of an optical signal is attenuated when the optical signal is transmitted for a long distance, the transmission lines between the terminal station A and the branching unit
1100
, between the terminal station B and the branching unit
1100
, and between the terminal station C and the branching unit
1100
have a plurality of optical amplifiers
1101
,
1102
, and
1103
respectively.
FIG. 1A
shows the optical amplifiers
1101
,
1102
, and
1103
apiece for respective transmission lines for a simple illustration, but there are actually much more optical amplifiers for each transmission line. Normally, each of the optical amplifiers
1101
,
1102
, and
1103
has an automatic output level control circuit (ALC circuit) to keep the output level of each of the optical amplifiers
1101
,
1102
, and
1103
constant so that the optical signal can be constantly amplified to a specific output level.
FIG. 1A
shows the transmission line for one-way communications. Actually, the circuit is designed to establish two-way communications, that is, up-line and down-line communications.
FIGS. 1B through 1F
show an optical signal and its problem in each transmission line.
FIG. 1B
shows the optical signal at point A in FIG.
1
A. In the case shown in
FIG. 1B
, optical signals having four different wavelengths are wavelength-multiplexed and transmitted from the terminal station A. The mound under each optical signal is called an amplified spontaneous emission (ASE) noise. It is produced when a noise superposed to an optical signal is amplified with the optical signal by an optical amplifier. The characteristics of the operations of the optical communications system depend on the S/N ratio of the optical signal to the ASE.
In the branching unit
1100
, the optical signal having a wavelength &lgr;
1
is branched and transmitted to the terminal station B, and an optical signal having the wavelength &lgr;
1
is transmitted from the terminal station B to the terminal station C.
An optical signal having a wavelength other than wavelength &lgr;
1
in the signal (
FIG. 1B
) transmitted from the terminal station A is not branched by the branching unit
1100
, but is transmitted as is to the terminal station C. The terminal station B receives the optical signal having wavelength &lgr;
1
and transmits an optical signal having the same wavelength &lgr;
1
.
FIG. 1C
shows the state at point B of the signal transmitted from the terminal station B and amplified by the optical amplifier
1102
. The branching unit
1100
combines the optical signal having wavelength &lgr;
1
transmitted from the terminal station B with the light having wavelength &lgr;
2
through &lgr;
4
, and transmits the result to the terminal station C.
FIG. 1D
shows the state at point C of the optical signal from the terminal station B which is combined by the branching unit
1100
and amplified by an optical amplifier
1103
.
FIGS. 1C and 1D
show the case where the power level of an optical signal is equal to that of each other when the optical signal from the terminal station B is combined with the optical signal from the terminal station A. In this case, an optical signal having any wavelength indicates the same S/N ratio to the ASE noise as shown in FIG.
1
D.
FIG. 1E
also shows the state of the optical signal at point B. In this case, the power level of the optical signal from the terminal station B is high. When the power level of the optical signal from the terminal station B is high, the state of the optical signal at point C after being combined by the branching unit
1100
and being amplified by the optical amplifier
1103
, becomes as shown in FIG.
1
F. Therefore, although the S/N ratio of wavelength &lgr;
1
is high, because the operation characteristics of the optical communications system are based on the lower S/N ratio, when the S/N ratios of the other wavelengths are low, the system is recognized as poor in operation characteristics.
FIGS. 2A
,
2
B,
3
A, and
3
B show the operation of the optical amplifier and the S/N ratio.
In this example, the two optical signals having different wavelengths are multiplexed, and an optical signal of a total of 0 dBm power is input to the optical amplifier. The optical amplifier includes an automatic output level control circuit having a gain of 10 dB and an optical output is limited to 10 dBm. The state of the optical signal at the input terminal is −3 dBm each for the power of the optical signals of two wavelengths, a total of 0 dBm as shown in FIG.
2
A.
FIG. 2B
shows the output when such optical signals are input to the optical amplifier. That is, the optical signal of each wavelength is amplified, and the power of each optical signal is +7 dBm with a total power of the output light indicating +10 dBm. On the other hand, the ASE noise is also amplified, and the S/N ratio to the ASE noise of each optical signal is 30 dB. Therefore, the operation characteristic of the optical amplifier indicates the S/N ratio of 30 dB.
FIGS. 3A and 3B
show the case where an input optical signal is multiplexed with an optical signal having a different power level. The characteristic of the optical amplifier is the same as that of the optical amplifier shown in
FIGS. 2A and 2B
. However, as shown in
FIG. 3A
, a total power of the optical signals having two different wavel

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