Optical: systems and elements – Optical amplifier – Optical fiber
Reexamination Certificate
1999-10-12
2001-04-03
Hellner, Mark (Department: 3662)
Optical: systems and elements
Optical amplifier
Optical fiber
C356S073100
Reexamination Certificate
active
06212003
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an optical amplifier evaluating method and optical amplifier evaluating apparatus for evaluating the characteristics of an optical fiber amplifier and, more particularly, to an optical amplifier evaluating method and optical amplifier evaluating apparatus for simply evaluating the gain and noise figure as the wavelength characteristics of an optical fiber amplifier by combining a pulse method using optical modulators and a probe method.
BACKGROUND ART
As is well known, in wavelength division multiplexing (WDM) optical communications used in recent large-capacity long-distance optical transmission systems, level deviations between channels lead to deterioration of signals.
Correspondingly, optical amplifiers used in long-distance transmission such as submarine optical cables must have flat and wide-band gain wavelength characteristics in addition to conventional low noise and high efficiency.
This makes evaluation of the wavelength characteristics of an optical fiber amplifier important.
An optical fiber amplifier is of course a kind of an amplifier. Therefore, it is necessary to measure a gain G indicated by the ratio of a light intensity PIN of an input optical signal to a light intensity POUT of an output optical signal.
As is well known, owing to its light amplification mechanism, an optical fiber amplifier produces spontaneous emission, even when no optical signal is input to its input terminal, and this spontaneous emission is amplified and output to its output terminal.
This amplified spontaneous emission (ASE) acts as noise with respect to an amplified optical signal.
It is, therefore, important to measure the light intensity PASE of this amplified spontaneous emission (ASE).
As an index indicating the noise resistance of an optical fiber amplifier, a noise figure NF indicated by equation (1) below which includes the measured gain G and light intensity PASE is generally employed:
NF=f
(
G, PASE
, &ngr;, &Dgr;&ngr;) (1)
where
&ngr;: the optical frequency of an input optical signal
G: gain
&Dgr;&ngr;: the measurement frequency resolution width (measurement frequency width) of a light intensity measurement apparatus
Hence, the characteristics of an optical fiber amplifier are evaluated in terms of the gain G and the noise figure NF.
Conventionally, to evaluate the characteristics of an optical fiber amplifier, in an arrangement as shown in
FIG. 10
, a laser light source
101
and an optical fiber amplifier
5
are connected to an optical spectrum analyzer
103
via an optical path switch
102
.
First, the optical path switch
102
is closed to the laser light source
101
. The optical spectrum analyzer
103
obtains the light intensity PIN, shown in
FIG. 11
, with respect to an optical wavelength &lgr; of an input optical signal to the optical fiber amplifier
5
.
Next, the optical path switch
102
is closed to the optical fiber amplifier
5
. The optical spectrum analyzer
103
obtains the light intensity POUT, shown in
FIG. 11
, at the optical wavelength &lgr; of an output optical signal from the optical fiber amplifier
5
.
Accordingly, the gain G is calculated by
G
=POUT/PIN (2)
As shown in
FIG. 11
, however, the light intensity PASE of amplified spontaneous emission (ASE) is buried in the light intensity POUT of the amplified output optical signal. This makes the light intensity PASE of the amplified spontaneous emission (ASE) difficult to directly measure.
As a method of measuring the light intensity PASE of this amplified spontaneous emission (ASE), a level interpolation method, polarization nulling method, and pulse method have been proposed.
(Explanation of Pulse Method)
Of these three methods, the pulse method (e.g., Jpn. Pat. Appln. KOKAI Publication Nos. 6-224492 and 9-18391) uses the fact that the recovery time to the ground state of rare earth element light of metastable erbium doped in the core of an optical fiber of an optical fiber amplifier is relatively long. In this method, an input optical signal to an optical fiber amplifier is turned on and off at periods shorter than this recovery time, the light intensity POUT of an output optical signal is measured during the ON period, and the light intensity PASE of the amplified spontaneous emission (ASE) is measured during the OFF period.
FIG. 12
illustrates an optical fiber amplifier evaluating apparatus of the prior application using this pulse method.
That is, an optical modulation unit
21
shown in
FIG. 12
is proposed in an international application (PCT/JP98/02015) filed by this international applicant.
As depicted in
FIG. 12
, a light source
201
a
for outputting a wavelength &lgr;1 is connected to an optical attenuator
202
a
, a light source
201
b
for outputting a wavelength &lgr;2 is connected to an optical attenuator
202
b
, . . . , a light source
201
n
for optically outputting a wavelength &lgr;n is connected to an optical attenuator
202
n.
An optical multiplexer
203
multiplexes, as will be described later, light components from these optical attenuators
202
a
,
202
b
, . . . ,
202
n.
The optical signal multiplexed by this optical multiplexer
203
is input to an optical fiber amplifier
5
via the optical modulation unit
21
.
The output optical signal from this optical fiber amplifier
5
is again input to an optical spectrum analyzer
207
via the optical modulation unit
21
.
(Measurement of Light Intensity PIN)
A controller
208
switches, as indicated by the dotted lines in
FIG. 12
, a first optical path switch
28
and a second optical path switch
33
in the optical modulation unit
21
. The controller
208
also sends a light intensity measurement command to the optical spectrum analyzer
207
.
In this state, as shown in
FIG. 13
, a first optical modulator
23
in the optical modulation unit
21
modulates the light, that is emitted by the light sources
201
a
,
201
b
, . . . ,
201
n
and so wavelength-multiplexed as to have a plurality of wavelengths &lgr;1, &lgr;2, &lgr;3, . . . , &lgr;n−1, &lgr;n, . . . by the optical multiplexer
203
, into a rectangular optical signal which is turned on and off at a predetermined period T
0
(FIG.
2
A).
The optical signal modulated by this first optical modulator
23
is fed into the optical spectrum analyzer
207
via the first optical path switch
28
and the second optical path switch
33
.
The optical spectrum analyzer
207
analyzes the spectrum of this incoming light and obtains the light intensity PIN (&lgr;=&lgr;1, &lgr;2, &lgr;3, . . . , &lgr;n−1, &lgr;n, . . . ) at each wavelength &lgr;.
The optical spectrum analyzer
207
sends the measured light intensity PIN(&lgr;) to the controller
208
.
(Measurement of Light Intensity POUT)
As shown in
FIG. 12
, the controller
208
sets the first optical path switch
28
in the steady state indicated by the solid lines and the second optical path switch
33
in switched state indicated by the dotted lines, and sends a light intensity measurement command to the optical spectrum analyzer
207
.
In this state, as shown in
FIG. 13
, the first optical modulator
23
in the optical modulation unit
21
modulates the light, that is emitted by the light sources
201
a
,
201
b
, . . . ,
201
n
and so wavelength-multiplexed as to have the wavelengths &lgr;1, &lgr;2, &lgr;3, . . . , &lgr;n−1, &lgr;n, . . . by the optical multiplexer
203
, into a rectangular optical signal which is turned on and off at the predetermined period T
0
.
The optical signal modulated by this first optical modulator
23
is fed into the optical fiber amplifier
5
as an object to be measured and is optically amplified.
The amplified optical signal output from this optical fiber amplifier
5
is directly fed into the optical spectrum analyzer
207
via the first optical path switch
28
and the second optical path switch
33
in the optical modulation unit
21
.
The optical spectrum analyzer
207
analyzes the spectrum of this incident light and obtains the light intensity POUT (&lgr;=&lgr
Komazawa Hiroshi
Tsuda Yukio
Anritsu Corporation
Frishauf, Holtz Goodman, Langer & Chick, P.C.
Hellner Mark
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