Optical: systems and elements – Optical amplifier – Optical fiber
Reexamination Certificate
2000-05-18
2001-07-17
Hellner, Mark (Department: 3662)
Optical: systems and elements
Optical amplifier
Optical fiber
C359S199200
Reexamination Certificate
active
06262835
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to optical transmission networks, optical communication systems or optical transmission systems, various optical transmission devices which include the optical amplifier systems used in those systems, and methods of controlling the systems and devices. More particularly, the present invention relates to an optical amplifier unit controlling method, an optical amplifier system, and a system which uses the method and system.
It is necessary to suppress light surges to the utmost in general optical amplifier systems. The “light surge” referred to herein points out an optical signal with an extremely high gain which is outputted from an optical amplifier system when the optical signal input to the optical amplifier system increases momentarily. The light surge is generated on the following reasons. It is necessary to expand the power of pumping light and to increase the amplification degree of an optical amplifier unit to obtain a desired optical signal output when the inputted optical signal decreases. Thus, in that case, large amplified optical signal energy is accumulated potentially in the optical amplifier unit. In such a state, if the optical signal input increases, the optical signal receives the energy accumulated so far and is outputted with a very high gain from the amplifier. If a light surge is generated, destruction of a photodetector in the optical communication end and melting of an end face of an connector concerned would be invited but also human (sight) injury would be caused. Therefore, it is necessary to suppress the generation of the light surge to the utmost. Especially, when optical amplifier systems are arranged in a multi-stage connection, the situation would be further serious. The reason for this is as follows: a light surge generated once is amplified one after another in the respective subsequent optical amplifier systems. As a result, the optical parts which compose each of those optical amplifier systems might be fatally destroyed with the respective increasing surges.
Current examples of measures against optical surges are described in the paper “Discussion of Light Surge in Multistageous Connection of Optical Amplifiers” (Spring Meeting B-941, Institute of Electronics, Information and Communication Engineers of Japan, 1993). The composition of an experimental system in the example of the measures is shown in FIG.
41
A. The optical output level of each of the multistage-connected optical amplifiers is shown is FIG.
41
B. As shown in
FIG. 41A
, an optical signal with available risetime can be generated from an laser diode (LD) (an LD module of the DFB (Distribution Feed Back) type having a center wavelength of 1.55 &mgr;m) as the optical signal source by driving the LD with a current. Optical signals from that LD are passed sequentially via the amplifiers AMP
1
-AMP
5
, which are erbium doped optical fiber amplifiers which are pumped by a 1.48 &mgr;m wavelength pump laser) with optical attenuators (ATTs) arranged before the corresponding amplifiers, and provided as an optical signal output. The waveforms of the respective optical signals outputted from those optical amplifiers are observed by corresponding photodiodes (PDs) via ATTs. As will be seen from
FIG. 41B
, the surge is suppressed in proportion to an increase in the risetime of the optical signal from the LD. Especially, when the risetime is set at the order of several milliseconds, light surges are hardly generated.
The amplifier composition of JP-A-6-45682 is shown in FIG.
42
. As shown in
FIG. 42
, the optical signal multiplexed by an optical multiplexer
52
and pumping light from a laser diode
53
pass forwardly through the optical isolator
54
and enter a doped fiber
55
. Then, the pumping light and the rare earth elements doped in the waveguide area causes induced emissions, and the optical signal is amplified. The amplified optical signal and the pumping light which remains unconsumed enter an optical bandpass filter
56
. In the bandpass filter
56
, the pumping light and spontaneous emission light which will be elements of noise are removed. The amplified optical signal alone passes an optical bandpass filter
56
. Thereafter, a part of the optical signal is separated by an optical splitter
57
, and the separated signal part is received by a photodetector
58
. A bias control circuit
59
compares a direct current voltage from the photodetector
58
with a reference voltage Vref
1
and controls a bias current flowing in the laser diode
53
so that an error between the direct current voltage and the reference voltage may become zero. Reference numeral
60
denotes a 4-port optical circulator having ports
60
A,
60
B,
60
C and
60
D. The light supplied to the port
60
A is outputted only from the port
60
B, the light supplied to the port
60
B is outputted only from the port
60
C, the light supplied to the port
60
C is outputted only from port
60
D, and the light supplied to the port
60
D is outputted only from the port
60
A. The control light from the laser diode
61
is supplied to the port
60
A. The port
60
B is connected with a port
57
B of the optical splitter
57
, the port
60
C is connected with an output optical transmission path (not shown), and the port
60
D is made a dead end. The control light from the laser diode
61
is introduced into the doped fiber
55
by passing the optical circulator
60
, optical splitter
57
, and optical band pass filter
56
in this order. Simultaneously, a bias control circuit
62
controls a bias current flowing through the laser diode
61
to thereby control the power of the control light from the laser diode
61
so that the error between a direct current voltage from the photodetector
58
and a reference voltage Vref
4
may be zero.
According to the prior art JP-A-45682, the wavelength of the control light is set in a wavelength band where induced emission occurs in the doped fiber
55
, for example at substantially the wavelength of the optical signal. When the power of the input signal changes comparatively slowly, the photodetector
58
receives a part of the optical signal which has passed through the optical bandpass filter
56
. A bias control circuit
59
controls the power of the pumping light from the laser diode
54
. When the power of the input optical signal changes rapidly, a bias control circuit
62
controls the power of the control light supplied by the laser diode
61
. As a result, even if the input signal changes rapidly, the power of the output signal is kept constant.
In addition, the composition of the prior art JP-A-8-18138 is shown in FIG.
43
. As shown in
FIG. 43
, in this composition, Two optical amplifiers AMP
1
and AMP
2
are connected in cascade. The first optical amplifier AMP
1
is provided with a first pump source
102
composed of a first EDF, an LD, etc., a first multiplexer
104
, and a first isolator
106
. An optical signal input is applied to the first optical amplifier AMP
1
via an optical isolator ISO connected with one end of the input side optical fiber. The first EDF
100
is pumped by the first pump source
102
via the first multiplexer
104
. The optical signal which has passed the first EDF
100
is inputted to the second optical amplifier AMP
2
through the first optical isolator
106
.
The second optical amplifier has a second EDF
108
, delay fiber
110
, second pumping source
112
, second multiplexer
114
, second optical isolator
116
, third EDF
118
, attenuator
120
, third optical isolator
122
, first splitting coupler
124
, second splitting coupler
126
and photodetector
128
. The first splitting coupler
124
splits the light from the first isolator
106
into two light portions at a predetermined ratio. The first split light portion enters the second EDF
108
through the delay fiber
110
. The second split light portion enters the third EDF
118
through the attenuator
120
. The second EDF
108
is connected with the splitting coupler
126
. The third EDF
118
is connected with the seco
Kosaka Junya
Sakano Shinji
Antonelli Terry Stout & Kraus LLP
Hellner Mark
Hitachi Ltd
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