Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2001-04-17
2002-11-12
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S341310, C356S073100
Reexamination Certificate
active
06480318
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an optical amplifier evaluation method and an optical amplifier evaluation instrument for evaluating the gain and noise figure of an optical amplifier when the optical amplifier amplifies a wavelength multiplexed signal light beam (or frequency multiplex signal light beam) provided by multiplexing a plurality of signal light beams different in wavelength (namely, frequency) in an optical signal.
2. Description of the Related Art
First, the measuring principle and a measuring method of the gain and noise figure of an optical amplifier in an optical amplifier evaluation instrument having been used hitherto.
Gain [G
n
] and noise figure [NF
n
] when the optical amplifier amplifies a multiplex signal (WDM: Wavelength Division Multiplex) of signal light beam having one wavelength (namely, frequency) or signal light beams output by n light sources (wavelengths &lgr;
1
to &lgr;
m
) different in wavelength (frequency) having been introduced heavily commercially in recent years are found according to expressions (3) and (4) respectively.
G
n
=
P
out
_
n
-
P
ASE
_
n
P
in_
n
(
3
)
NF
n
=
P
ASE_
n
h
·
n
G
n
·
·
n
+
1
G
n
(
4
)
where n is 1 to m.
[P
in
—
n
] is light power of signal light beam input to the optical amplifier, [P
out
—
n
] is output light power of amplified signal light beam output from the optical amplifier, [P
ASE
—
n
] is amplified spontaneous emission power output from the optical amplifier in the wavelength that the signal light beam has, [&Dgr;&ngr;
n
] is an optical signal light passage band width of a light intensity measuring instrument for measuring the amplified spontaneous emission power [P
ASE
—
n
], [&ngr;
n
] is the frequency proper to the signal light beam input to the optical amplifier, and [h] is a Planck's constant.
The suffix “ASE” is an abbreviation for Amplified Spontaneous Emission and refers to amplification based on a so-called spontaneous emission process in which excited atoms spontaneously emit light independently of the external effect and make a transition to any other stationary energy state.
However, to find the noise figure [NF
n
] using the above-mentioned expression (4), it is difficult to directly find the noise figure [NF
n
] because generally the output light power of amplified signal light beam [P
out
—
n
] is superposed on the amplified spontaneous emission power [P
ASE
—
n
] for output. Then, in a related art, the noise figure [NF
n
] is measured according to the following method:
The measuring method of the noise figure [NF
n
] in the related art will be discussed in detail.
FIG. 15
is a block diagram to show the configuration of an optical amplifier evaluation instrument and an optical amplifier evaluation method in the related art. That is, it is a diagram to describe the method of measuring the gain and noise figure of an optical amplifier in the related art. In the figure, signal light beams from light sources
101
a
,
101
b
,
101
c
, . . . , and
101
n
different in wavelength (frequency) are combined by an optical combiner
102
, then the resultant signal light beam is pulse-intensity-modulated by a first optical modulator
103
and is input through an input optical terminal
108
to a measured optical amplifier
107
. The input signal light spectrum at this time is a wavelength multiplexed input signal light spectrum
110
shown in
FIG. 16
, and the first optical modulator
103
is controlled by a pulse signal [a] output from a modulation signal generation section
105
.
The input signal light beam is amplified and output from the measured optical amplifier
107
. Since the input signal light beam is pulse-modulated, the amplified signal light beam output undergoes a propagation delay of the measured optical amplifier
107
and is shifted in phase, but is produced in a pulse state in the same period. The above-mentioned amplified spontaneous emission is output regardless of the presence or absence of pulse. At this time, the pulse modulation period is a period sufficiently shorter than the atomic lifetime at upper level of the amplification medium of the measured optical amplifier
107
or the carrier lifetime, so that the amplified spontaneous emission (generally called “ASE”) becomes an almost constant light output level regardless of whether the input signal light beam is on or off. The output light spectrum of the measured optical amplifier
107
becomes a waveform like an output light spectrum
111
shown in FIG.
16
.
FIG. 16
is a drawing to show the wavelength multiplexed signal light beam amplification form of the measured optical amplifier in the related art.
Output light of the measured optical amplifier
107
is input through an output optical terminal
109
to a second optical modulator
104
. The second light modulator
104
is controlled by a pulse signal [b] output from the modulation signal generation section
105
. The first optical modulator
103
and the second optical modulator
104
are driven in the same period and the phase of the second optical modulator
104
can be arbitrarily set in the 360-degree range based on the modulation timing of the first optical modulator
103
.
First, the input optical terminal
108
and the output optical terminal
109
are previously connected directly as
108
′ and
109
′ and the light power [P
in
—
n
] for each frequency [&ngr;
n
] of signal light beam input to the measured optical amplifier
107
is measured by a light intensity measuring instrument
106
. The spectrum at this time is exactly as
112
in FIG.
17
.
FIG. 17
is a drawing to show each light power measured in the related art as spectrum display.
Next, the input optical terminal
108
and the output optical terminal
109
are connected to the measured optical amplifier
107
and the light power [P
out
—
n
] of the amplified signal light beam output from the measured optical amplifier
107
is measured for each frequency. The spectrum at this time is exactly as
113
in FIG.
17
. The phases of the first optical modulator
103
and the second optical modulator
104
are the relationship between A (modulation timing of first optical modulator) and C (timing of second optical modulator when post-amplified signal light beam [P
out
—
n
] is measured), and the propagation time of a waveguide of the measured optical amplifier
107
appears as a delay.
FIG. 18
is a drawing to show the relative phase relationships among the first modulator, pulse light output by the measured optical amplifier, and the second modulator in the related art.
Next, the phase of the second modulator
104
is shifted 180 degrees with respect to C (timing of second optical modulator when post-amplified signal light beam [P
out
—
n
] is measured) like the relationship between A (modulation timing of first optical modulator) and D (timing of second optical modulator when amplified spontaneous emission power [P
ASE
—
n
] is measured), and amplified spontaneous emission power [P
ASE
—
n
] output by the measured optical amplifier
107
is measured for each frequency. The amplified spontaneous emission spectrum at this time is exactly as waveform
114
in FIG.
17
.
The measurement values are assigned to the above-mentioned expressions (3) and (4), whereby the gain and noise figure of the measured optical amplifier
107
can be calculated and found.
FIG. 19
shows the characteristics of the gain and noise figure of the measured optical amplifier
107
at the wavelength multiplexed signal light amplification time, found by the above-mentioned calculation, namely, shows wavelength characteristic of the gain,
115
, and wavelength characteristic of the noise figure,
Fukushima Masaru
Mori Tohru
Ando Electric Co. Ltd.
Choi William
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