Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
2003-02-10
2004-10-19
Hellner, Mark (Department: 3663)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
C356S073100
Reexamination Certificate
active
06807000
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a gain measurement device for an optical amplifier. More particularly, the invention relates to gain measurement device and method for an optical amplifier which can continuously measure gain versus wavelength characteristics of an optical amplifier to be measured at high speed and high precision.
2. Description of the Related Art
Conventionally, in an optical communication, such as a wavelength division multiplexing communication to be used for a large capacity and long distance optical transmission system, for example, level deviations between respective channels (wavelength) cause deterioration of signals. On the other hand, in the long distance transmission, characteristics of the optical amplifiers to be used at appropriate interval are important factor to cause deterioration of signals. Accordingly, in addition to low-noise characteristics and high efficiency are required for the optical amplifier, flattening and widening of band of a gain versus wavelength are required. For this purpose, evaluation of the gain versus wavelength characteristics of the foregoing optical amplifier has been heretofore important.
FIG. 6
is an illustration for explaining the conventional gain measuring system for the optical amplifier of this kind. Referring to
FIG. 6
, a multiple wavelength light source
10
is designed for outputting saturated lights having a plurality of predetermined wavelengths &lgr;1 to &lgr;n. A variable wavelength light source
112
is designed for outputting a fine probe light of variable wavelength. A polarization scrambler
113
is designed to make a light polarization surface for the variable wavelength fine (very little) probe light to output a polarized light to an optical coupler
11
. The optical coupler
11
multiplexes the output from the multiple wavelength light source
10
and the output from the polarization scrambler
113
. A variable light attenuator
14
is designed for performing level control of the output from the optical coupler
11
depending upon a control signal from a control portion
21
.
An optical switch
15
receives an output of the optical attenuator
14
at an input port
15
a
to selectively output through one of output ports
15
b
and
15
c
. A measurement objective optical amplifier
16
has an input
16
a
connected to one output port
15
c
of the optical switch
15
and an output
16
b
connected to one input port
17
b
of an optical switch
17
at a next stage. The other output port
15
b
of the optical switch
15
is connected to an input port
17
a
of the optical switch
17
. The optical switch
17
is designed to arbitrarily establish connection between two input ports
17
a
and
17
b
and two output ports
17
c
and
17
d.
The output port
17
c
of the optical switch
17
is connected to a light power meter
18
. The other output port
17
d
of the optical switch
17
is connected to an optical spectrum analyzer
19
. Outputs of the optical power meter
18
and the optical spectrum analyzer
19
are fed to a gain measuring portion
20
. An output of the optical power meter
18
is input a control portion
21
.
FIGS. 7 and 8
are operational flowchart for explaining a gain measurement process of the conventional optical amplifier shown in FIG.
6
. In
FIG. 7
, at first, control is performed so that only multiple wavelength lights (&lgr;1 to &lgr;n) from the multiple wavelength light source
10
are input to the measurement objective optical amplifier
16
. At this condition, the control portion
21
monitors an input power of the measurement objective optical amplifier
16
by the optical power meter
18
to control an attenuation amount (ATT amount) of the variable optical attenuator
14
so that the input power becomes a rated (nominal) value (Pnom) (step S
01
).
At this condition, by employing the optical spectrum analyzer
19
, an input spectrum Pin1(&lgr;) of the input for the measurement objective optical amplifier
16
is measured (step S
102
). The input spectrum at this time is shown on left side of FIG.
9
A. In the condition where only lights having wavelengths &lgr;1 to &lgr;n of the multiple wavelength light source are input, an output spectrum Pout1 of the measurement objective optical amplifier
16
is measured by means of the optical spectrum analyzer
19
(step S
103
). The output spectrum at this time is shown on right side of FIG.
9
A.
Here, as shown in
FIG. 10
, a relationship between an input and an output of the optical amplifier
16
is expressed by:
Pin×G+Pase=Pout
wherein G is a gain, Pase is a power of a spontaneous emission light.
Accordingly, when only lights having wavelengths &lgr;1 to &lgr;n of the multiple wavelength light source is input, the relationship between input and output of the optical amplifier
16
is expressed by an expression (1) of FIG.
9
A.
Thereafter, the light of the multiple wavelength light source
10
and the fine probe light by the variable wavelength light source
112
are superimposed to be input to the measurement objective optical amplifier
16
. Then, the control portion
21
monitors the input power of the amplifier by means of the optical power meter
18
for controlling attenuation amount of the variable optical attenuator
14
in order to maintain the input power at the rated value (step S
104
). At this time, a wavelength of the probe light emitted from the variable wavelength light source
112
is assumed to be set at &lgr;′1.
At this condition, an input spectrum Pin2 (&lgr;) of the measurement objective optical amplifier
16
is measured by means of the optical spectrum analyzer
19
(step S
105
). The input spectrum to the measurement objective optical amplifier
16
at this condition is shown on the left side (solid line) of FIG.
9
B. Then, an output spectrum Pout2 (&lgr;) of the measurement objective optical amplifier
16
is measured by means of the optical spectrum analyzer
19
(step S
106
). The output spectrum at this time is shown on right side of FIG.
9
B.
Next, the wavelength of the variable wavelength light source
112
is varied to &lgr;′2 (see
FIG. 9B
) to repeat the foregoing process through steps S
104
to S
106
(step S
107
). Similarly, for &lgr;′3 to &lgr;′m, the process through steps S
104
to S
106
is repeated respectively (steps S
108
to S
109
). Finally, the expression (2) of
FIG. 9B
can be obtained. A solution of the expressions (1) and (2) is obtained with respect to G(&lgr;) to establish an expression (3) as shown in
FIG. 9C
(step S
110
of FIG.
8
). G(&lgr;) obtained from the expression (3) is indicative of a gain G(&lgr;′1), . . . , G(&lgr;′m) of wavelength (&lgr;′1 to &lgr;′m) of the variable wavelength light source. On the other hand, gains G (&lgr;1), . . . , G (&lgr;n) of wavelength &lgr;1 to &lgr;n of the multiple wavelength light source obtained from the expression (1) of
FIG. 9A
obtained from the input and output spectrum in the process through steps S
101
to S
103
, namely, in the condition where only the light from the multiple wavelength light source
10
is input, are obtained arithmetically (step S
111
).
From these steps S
110
to S
111
, gain versus wavelength characteristics in full wavelength band of the measurement objective optical amplifier
16
can be derived (step S
112
). One example of the result of measurement is shown in FIG.
11
.
In the conventional gain measurement method of the foregoing optical amplifier, it becomes necessary to repeat processes for setting wavelength of the variable wavelength light source
112
, setting of the optical spectrum analyzer
19
and so forth per wavelength to be measured. Accordingly, when number of wavelengths to be measures is increased, the measurement period is proportionally expanded.
On the other hand, as shown in
FIG. 6
, the output of the variable wavelength light source
112
has to be non-polarized wave employing the polarized scrambler
113
.
The reason is that since output li
Hellner Mark
NEC Corporation
Ostrolenk Faber Gerb & Soffen, LLP
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