Optics: measuring and testing – By dispersed light spectroscopy
Reexamination Certificate
2002-02-21
2003-11-04
Evans, F. L. (Department: 2877)
Optics: measuring and testing
By dispersed light spectroscopy
C356S307000, C356S328000, C356S334000
Reexamination Certificate
active
06643011
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an SNR (signal-to-noise ratio) calculation method used with an optical spectrum measurement apparatus for measuring the optical spectrum characteristic of a light source.
2. Description of the Related Art
FIG. 1
is a drawing to show the configuration of an optical spectrum measurement apparatus in a related art.
In
FIG. 1
, numeral
1
denotes a light source containing various wavelength components for emitting light on which spectrum measurement is to be conducted.
Numeral
2
denotes an incidence slit for limiting the intensity of light emitted from the light source
1
.
Numeral
3
denotes a concave mirror for converting light incident thereon through the incidence slit
2
into collimated light.
Numeral
4
denotes a diffraction grating being formed on a surface with a large number of grooves for spatially separating the collimated light provided by the concave mirror
3
for each wavelength.
The diffraction grating
4
is placed on a rotation stage
5
and rotates with rotation of the rotation stage
5
.
Numeral
6
denotes a concave mirror for forming only the light of the spatially separated light by the diffraction grating, incident on the concave mirror
6
at a slit position of an emission slit
7
.
Numeral
7
denotes the just-mentioned emission slit for limiting the wavelength range of the light formed at the slit position by the concave mirror
6
.
The incidence slit
2
, the concave mirror
3
, the diffraction grating
4
, the concave mirror
6
, and the emission slit
7
make up a monochromator called Zerni Turner type dispersion monochromator.
Numeral
8
denotes a photodetector such as a photodiode for converting the intensity of the light emitted through the emission slit
7
into an electric signal.
Numeral
9
denotes an amplifier for amplifying the electric signal output from the photodetector
8
.
Numeral
10
denotes an analog-digital converter (A/D converter) for converting the electric signal amplified by the amplifier
9
into a digital signal.
Numeral
11
denotes a motor for rotating the rotation stage
5
on which the diffraction grating
4
is placed; the motor
11
rotates the rotation stage
5
and the diffraction grating
4
.
Numeral
12
denotes a drive circuit for controlling the turning operation of the motor
11
in response to a control signal output from a CPU
13
described later.
Numeral
14
denotes a display, such as a CRT (cathode-ray tube) display or a liquid crystal display.
The CPU
13
, which is connected to the A/D converter
10
, the drive circuit
12
, and the display
14
by a bus B, outputs the control signal for controlling the drive circuit
12
and performs arithmetic processing on the digital signal output from the A/D converter
10
and then displays a spectrum distribution, for example, on the display
14
.
In the described configuration, when light is emitted from the light source
1
, the emitted light is incident on the incidence slit
2
.
The light passed through the incidence slit
2
is converted into collimated light by the concave mirror
3
and the collimated light is incident on the diffraction grating
4
.
The diffraction grating
4
is rotated on the axis parallel to a large number of grooves formed on the surface by the motor
11
, forming an arbitrary angle with the collimated light.
This arbitrary angle is determined by the drive circuit
12
which controls the motor
11
in response to the control signal output from the CPU
13
.
The diffraction grating
4
spatially separates the incident collimated light for each wavelength. Only the light of the wavelength determined by the angle between the collimated light and the diffraction grating
4
, etc., of the wavelengths provided by spatially separating the light through the diffraction grating
4
is emitted to the concave mirror
6
.
The concave mirror
6
allows only the incident light of the wavelength to be formed at the slit position of the emission slit
7
.
Only the wavelength component within the range of the slit width of the emission slit
7
is allowed to pass through the emission slit
7
.
The photodetector
8
receives the light passed through the emission slit
7
and converts the light into an electric signal proportional to the intensity of the passed-through light.
The amplifier
9
amplifies the output of the photodetector
8
to a voltage appropriate for input of the A/D converter
10
.
The A/D converter
10
converts the output of the amplifier
9
into a digital signal. The digital signal output by the A/D converter
10
is input to the CPU
13
, which then performs arithmetic processing on the digital signal.
The CPU
13
outputs the result of the arithmetic processing (for example, a spectrum distribution) to the display
14
via the bus B.
The display
14
displays the display contents responsive to the arithmetic result output by the CPU
13
.
Next, the measurement procedure will be discussed.
To begin with, the CPU
13
commands the drive circuit
12
to vary the angle of the diffraction grating
4
, thereby setting the wavelength passing through the emission slit
7
and inputs the intensity of the light passed through the emission slit
7
from the output of the A/D converter
10
.
The CPU
13
outputs a control signal to the drive circuit
12
to sweep the wavelength passing through the emission slit
7
from the measurement start wavelength to the measurement end wavelength, and displays the measurement wavelength vs light intensity characteristic repeatedly provided on the display
14
as an optical spectrum.
When the optical spectrum measurement apparatus shown in
FIG. 1
measures the light source having a signal of only a single wavelength component, a spectrum as shown in
FIG. 2
is provided.
In the spectrum shown in
FIG. 2
, the amount representing the degree of skirting of the waveform of both side portions of the signal wavelength is called dynamic range of the optical spectrum measurement apparatus and is represented as the ratio between the light intensity at the signal wavelength and the light intensity at a distance of X nm from the signal wavelength.
In the optical spectrum measurement apparatus, the skirting of the waveform is caused to occur by the stray light components produced by the optical parts, etc., in the monochromator, and the value of the dynamic range is used as an important reference to determine the performance of the optical spectrum measurement apparatus.
As the light source on which spectrum measurement is to be conducted, a light source having the configuration shown in
FIG. 3
is possible. In
FIG. 3
, numerals
15
to
18
denote light sources different in signal wavelength.
Numeral
19
denotes an optical multiplexer for multiplexing light of the light source
15
to light of the light source
18
different in signal wavelength into one optical fiber.
Numeral
20
denotes an optical fiber amplifier for amplifying and outputting an input optical signal.
The system of transmitting information using the light source as shown in
FIG. 3
is called wavelength division multiplexing transmission, on which attention is focused as a next-generation information transmission system.
When the output light in
FIG. 3
is measured by the optical spectrum measurement apparatus, a spectrum as shown in
FIG. 4
is provided and various signal analyses are performed.
Among them, an analysis of an SNR (signal-to-noise ratio) represented as the ratio between the optical signal level and the noise level is one of the important analysis items to maintain the transmission quality on a transmission line.
Here, the SNR analysis method will be discussed using the spectrum in FIG.
4
.
The SNR is measured as the ratio between an optical signal optical level Pn and a noise level Nn.
As for the spectrum as shown in
FIG. 4
, generally the noise level Nn is calculated using the level between signal wavelengths as follows:
Nn=Pns
+(
Pnl−Pns
) (&lgr;
n&lgr;ns
)/(&lgr;
nl−&lgr;ns
)(
dBm
) (1)
Therefore,
Ando Electric Co. Ltd.
Evans F. L.
Fish & Richardson P.C.
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