Optics: measuring and testing – For optical fiber or waveguide inspection
Reexamination Certificate
1999-01-08
2001-05-22
Font, Frank G. (Department: 2877)
Optics: measuring and testing
For optical fiber or waveguide inspection
Reexamination Certificate
active
06236452
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an optical amplifier evaluation method and apparatus for evaluating the characteristics of an optical fiber amplifier and, more particularly, to an optical amplifier evaluation method and apparatus for evaluating the gain and noise figure of the optical fiber amplifier using a pulse method.
BACKGROUND ART
As is well known, in recent years, optical cables such as optical submarine cables are often installed over long distances along with rapid advance of the optical communication network.
In the optical communication network, an optical signal attenuates during transmission through the optical cable to decrease the S/N of the optical signal. Such a decrease in S/N of the optical signal is prevented by installing repeaters every predetermined distance.
More specifically, each repeater installed at a predetermined distance converts an optical signal received from each optical fiber into an electrical signal at the terminal of an optical cable in a given section, amplifies the electrical signal, converts the amplified electrical signal into an optical signal, and transmits the optical signal to each optical fiber of an optical cable in the next section, thereby preventing a decrease in S/N of the optical signal.
Recently, optical fiber amplifiers for directly amplifying an optical signal have been developed.
This optical fiber amplifier amplifies communication signal light such that an optical fiber with a core doped with a rare-earth element such as erbium is excited by light having a shorter wavelength than that of the communication signal light.
This optical fiber amplifier can be inserted in each optical fiber of the optical cable to easily prevent a decrease in S/N of the optical signal.
It is important to evaluate the characteristics of the optical fiber amplifier when a new optical communication network is constructed or in periodic maintenance and inspection.
In evaluating the characteristics of the optical fiber amplifier, the gain G given by the ratio of the light intensity P
IN
of an input optical signal to the light intensity P
OUT
of an output optical signal must be measured because the optical fiber amplifier is a kind of amplifier.
As is well known, in the optical amplifier, even if no optical signal is input to the input terminal of the optical fiber amplifier, its optical amplification mechanism causes spontaneous emission, and the spontaneous emission is amplified and output to the output terminal of the optical fiber amplifier.
The amplified spontaneous emission (ASE) acts as noise to an amplified optical signal.
In, therefore, evaluating the characteristics of the optical fiber amplifier, the light intensity P
ASE
of the spontaneous emission (ASE) must be measured.
Evaluation of the characteristics of the optical fiber amplifier generally employs a noise figure NF given by equation (1) including the measured gain G and light intensity P
ASE
as indices representing the noise resistance performance:
NF=P
ASE
/(h·&ngr;·G·&Dgr;&ngr;) (1)
where h: Planck's constant
&ngr;: light frequency of input optical signal
G: gain
&Dgr;&ngr;: measurement frequency resolving power width (measurement frequency width) of light intensity measurement device.
The characteristics of the optical fiber amplifier can be evaluated by the gain G and noise figure NF.
To evaluate the characteristics of the optical fiber amplifier, a laser beam source
1
and an optical fiber amplifier
2
are conventionally connected to an optical spectrum analyzer
4
via an optical switch
3
, as shown in FIG.
14
.
The optical switch
3
is first switched to the laser beam source
1
side to cause the optical spectrum analyzer
4
to obtain the light intensity PIN as a function of the light wavelength &lgr; of an optical signal input to the optical fiber amplifier
2
(lower curve shown in FIG.
15
).
The optical switch
3
is switched to the optical fiber amplifier
2
to cause the optical spectrum analyzer
4
to obtain the light intensity P
OUT
as a function of the light wavelength &lgr; of an optical signal output from the optical fiber amplifier
2
(upper curve shown in FIG.
15
).
As a result, the gain G of the optical fiber amplifier is given by equation (2) including the input light intensity P
IN
and the output light intensity P
OUT
:
G=P
OUT
/P
IN
(2)
As shown in
FIG. 15
, the light intensity P
ASE
of spontaneous emission (ASE) is buried in the light intensity P
OUT
of the amplified output optical signal. For this reason, the light intensity P
ASE
of spontaneous emission (ASE) is difficult to directly measure.
As a method of measuring the light intensity P
ASE
of spontaneous emission (ASE), a level interpolation method, a polarization nulling method, and a pulse method are proposed.
Of the three methods, the pulse method utilizes a relatively long recovery time required to recover to a ground state for light of a metastable rare-earth element such as erbium doped in the core of the optical fiber of the optical fiber amplifier.
That is, in the pulse method, an optical signal input to the optical fiber amplifier is enabled/disabled in a cycle shorter than the recovery time. The light intensity P
OUT
of an output optical signal is measured in the ON period, and the light intensity P
ASE
of spontaneous emission (ASE) is measured in the OFF period.
FIG. 16
is a block diagram of an optical amplifier evaluation apparatus adopting this pulse method.
Light with a wavelength &lgr; emitted from the laser beam source
1
is incident on a first optical switch
7
via an input terminal
6
of an optical modulation unit
5
.
The first optical switch
7
switches the incident light to a second optical switch
8
or a first optical modulator
9
on the basis of an instruction from a controller
14
.
As shown in
FIGS. 17A
to
17
E, the first optical modulator
9
modulates the incident light into a rectangular optical signal which is enabled/disabled in a predetermined cycle T
0
of, e.g., 5 &mgr;s shorter than the above-mentioned recovery time, and outputs the optical signal to the input terminal of the optical fiber amplifier
2
via an output terminal
10
.
The amplified optical signal output from the output terminal of the optical fiber amplifier
2
is input to a second optical modulator
12
via an input terminal
11
.
The second optical modulator
12
functions to pass the optical signal only during a partial period T
S
of the ON period or a partial period T
A
of the OFF period of the optical signal output from the optical fiber amplifier
2
.
Either of the periods T
S
and T
A
is employed in accordance with an instruction from the external controller
14
.
The optical signal output from the second optical modulator
12
is input to the second optical switch
8
.
The second optical switch
8
selects the optical signal from the first optical switch
7
or the optical signal from the second optical modulator
12
on the basis of an instruction from the controller
14
, and inputs the selected one to the optical spectrum analyzer
4
.
The optical spectrum analyzer
4
analyzes the spectrum of the input optical signal to obtain the light intensity P for the wavelength &lgr; or light frequency &ngr;.
In the optical amplifier evaluation apparatus having this arrangement, the optical switches
7
and
8
are first switched to the partner sides.
Light incident on the optical modulation unit
5
from the laser beam source
1
passes through the optical switches
7
and
8
to directly enter the optical spectrum analyzer
4
.
The optical spectrum analyzer
4
regards the incident light as light incident on the optical fiber amplifier
2
, and measures the light intensity P
IN
. The optical switches
7
and
8
are respectively switched to the optical modulators
9
and
12
. In the second optical modulator
12
, the partial period T
S
of the ON period is set.
In this state, light is incident on the optical spectrum analyzer
4
during the partial period T
S
of the ON period of the optical signal output
Goto Hiroshi
Sonobe Youji
Tsuda Yukio
Anritsu Corporation
Font Frank G.
Frishauf, Holtz Goodman, Langer & Chick, P.C.
Nguyen Tu T.
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