Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrophotometer
Reexamination Certificate
1999-05-21
2001-10-16
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrophotometer
C356S302000, C356S319000, C356S320000, C356S213000
Reexamination Certificate
active
06304324
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of calculating optical frequency spectrum for use in an optical-spectrum measuring apparatus for measuring optical spectrum characteristics of a light source.
2. Description of the Related Art
FIG. 4
is a block diagram showing the structure of a conventional optical-spectrum measuring apparatus.
Referring to
FIG. 4
, reference numeral
10
designates a light source containing a variety of wavelength components and capable of emitting light, the spectrum of which is to be measured. Reference numeral
12
designates an incident slit for limiting the intensity of light emitted from the light source
10
. Reference numeral
14
designates a concave mirror for converting light made incident on the concave mirror
14
through the incident slit
12
into parallel light beams.
Reference numeral
16
designates a diffraction grating having the surface provided with a multiplicity of grooves so as to, for each wavelength, spatially split the parallel light beams converted by the concave mirror
14
. The diffraction grating
16
is mounted on a stage
17
which is rotative in a direction indicated with symbol D
1
so as to be rotated in the direction indicated with the symbol D
1
when the stage
17
is rotated. Reference numeral
18
designates a concave mirror for focusing only a light beam made incident on the concave mirror
18
to the position of an emission slit
20
, the light beam being included in the light beams split by the diffraction grating
16
for each wavelength. Reference numeral
20
designates the emission slit for limiting bandwidth of the wavelengths of the light beam focused to the position of the slit by the concave mirror
18
.
The incident slit
12
, the concave mirror
14
, the diffraction grating
16
, the concave mirror
18
and the emission slit
20
constitute a Czerny-Turner spectroscope.
Reference numeral
22
designates a photodetector, such as a photodiode, to convert the intensity of light emitted from the emission slit
20
into an electric signal. Reference numeral
24
designates an amplifier for amplifying the electric signal output from the photodetector
22
. Reference numeral
26
designates an analog/digital converter (hereinafter referred to as an D/A converter) for converting the value amplified by the amplifier
24
into a digital signal.
In the drawing, reference numeral
28
designates a motor for rotating the stage
17
on which the diffraction grating
16
is mounted. When a rotating shaft
29
of the motor
28
is rotated in a direction indicated with symbol D
2
, the stage
17
and the diffraction grating
16
are rotated in the direction indicated with the symbol D
1
. Reference numeral
30
designates a motor rotating circuit for controlling the rotation of the rotating shaft
29
of the motor
28
in response to a control signal output from a CPU
34
to be described later.
Reference numeral
32
designates a slit-width control unit for changing the width of the emission slit
20
in response to a control signal output from the CPU
34
to be described later.
Reference numeral
36
designates a display unit, such as a CRT (Cathode Ray Tube) or a liquid crystal display unit. The CPU
34
is connected to the A/D converter
26
, the motor rotating circuit
30
, the slit-width control unit
32
and the display unit
36
through a bus B. The CPU
34
outputs a control signal for controlling each of the motor rotating circuit
30
and the slit-width control unit
32
. Moreover, the CPU
34
calculates the digital signal output from the A/D converter
26
so as to display, for example, the spectrum distribution on the display unit
36
.
In the foregoing structure, light emitted from the light source
10
is made incident on the incident slit
12
. Light allowed to pass through the incident slit
12
is converted into parallel light beams by the concave mirror
14
so as to be made incident on the diffraction grating
16
. The diffraction grating
16
is rotated by the motor
28
around a shaft which is in parallel with the many grooves formed thereon so as to make an arbitrary angle from the parallel light beams. The arbitrary angle is determined when the motor rotating circuit
30
controls the motor
28
in response to the control signal output from the CPU
34
.
The diffraction grating
16
spatially splits incident parallel light beams for each wavelength. Among the wavelengths spatially split by the diffraction grating
16
, only light having a wavelength determined by an angle made between a direction of transmission of the parallel light beams and the diffraction grating
16
is emitted to the concave mirror
18
. The concave mirror
18
focuses only light having the wavelength, which has been made incident on the concave mirror
18
, to the position of the slit of the emission slit
20
. Only a wavelength component within the width of the emission slit
20
is allowed to pass through the emission slit
20
. The slit-width control unit
32
sets the width of the emission slit
20
in response to the control signal output from the CPU
34
.
The photodetector
22
receives light allowed to pass through the emission slit
20
to convert the light into an electric signal proportional to the intensity of the light. The amplifier
24
amplifies an output from the photodetector
22
to a voltage suitable to be input to the A/D converter
26
. The A/D converter
26
converts an output from the amplifier
24
into a digital signal. The digital signal output from the A/D converter
26
is supplied to the CPU
34
. The CPU
34
calculates the digital signal. The CPU
34
outputs a result (for example, spectrum distribution) of a calculation to the display unit
36
through the bus B. The display unit
36
displays contents in accordance with the result of the calculation output from the CPU
34
.
The procedure of the measurement will now be described. The CPU
34
outputs a control signal to the slit-width control unit
32
so as to set the width of the emission slit
20
. Then, the CPU
34
issues a command to the motor rotating circuit
30
to change the angle of the diffraction grating
16
so as to set a wavelength which is allowed to pass through the emission slit
20
. Moreover, the CPU
34
fetches the intensity of light allowed to pass through the emission slit
20
from the output of the A/D converter
26
. The CPU
34
outputs a control signal to the motor rotating circuit
30
. Thus, the wavelength allowed to pass through the emission slit
20
is swept from a measurement-start wavelength to a measurement-completion wavelength. Characteristics about the relationship between the measuring wavelength and the intensity of light obtained repeatedly are displayed on the display unit
36
.
Recently, in the field of the optical communication, an optical spectrum is usually displayed as an optical frequency spectrum, that is, a characteristic of the relationship between an optical frequency and the intensity of light in place of display as a characteristic of the relationship between a wavelength and the intensity of light. In the above case, the measuring wavelength at each of the measuring points is converted into an optical frequency in accordance with the characteristics about the relationship between the measuring wavelength and the intensity of light which have repeatedly been obtained. The optical frequency is, as an optical frequency spectrum, displayed on the display unit
36
. Recently, some of optical spectrum measuring apparatuses put on the market have a function capable of selectively displaying an optical spectrum and an optical frequency spectrum.
Incidentally, the bandwidth RB (also called a wavelength resolution) allowed to pass through the spectrometer
5
of the czerny-Turner spectroscope type shown in
FIG. 4
is substantially expressed by the following equation (1). Note that the following expression is satisfied under the conditions that the focal distance of the concave mirror
14
is the same as that of the concave mir
Ando Electric Co. Ltd.
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Font Frank G.
Ratliff Reginald A.
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