Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Reexamination Certificate
2000-10-10
2003-06-24
Evans, F. L. (Department: 2877)
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
C356S334000
Reexamination Certificate
active
06583874
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a spectrometer which is used to measure a spectrum of light supplied from a light source, in particular, a spectrometer having higher resolving power without enlarging a size of the apparatus.
2. Description of a Related Art
A spectrometer disclosed in Japanese patent application publication JP-A-11-132848 is known as a conventional optical apparatus which is used to measure a spectrum of light supplied from a light source.
FIG. 31
shows a constitution of the spectrometer.
As shown in
FIG. 31
, spectrometer
200
has slit board
201
, collimator lens
202
, beam splitter
203
, diffraction grating
204
, mirror
205
, magnifier lens
206
, and line sensor
207
.
In spectrometer
200
, light supplied from a light source passes through slit board
201
and is changed into parallel light by collimator lens
202
. The parallel light passes through beam splitter
203
and is incident into diffraction grating
204
. The parallel light incident into diffraction grating
204
is diffracted toward beam splitter
203
as a first diffraction light (a single pass).
A part of the single pass is reflected by beam splitter
203
toward a direction shifted by a small angle from the optical axis of collimator lens
202
. On the other hand, the rest of the single pass passes through beam splitter
203
, collimator lens
202
, mirror
205
and magnifier lens
206
, and then focuses on a first range of channels of line sensor
207
to form a spectrum image.
The single pass reflected by beam splitter
203
is incident again into diffraction grating
204
and then diffracted toward beam splitter
203
as a second diffraction light (a double pass).
A part of the double pass is reflected by beam splitter
203
toward a direction shifted by a small angle from the optical axis of collimator lens
202
. On the other hand, the rest of the double pass passes through beam splitter
203
, collimator lens
202
, mirror
205
and magnifier lens
206
, and then focuses on a second range of channels of line sensor
207
, which is different from the first range, to form a spectrum image.
Although the above-mentioned document disclose no concrete example of diffraction grating
204
for realizing a large diffraction angle as shown in
FIG. 31
, an Echelle grating is suitable for realizing such a large diffraction angle.
However, there are some problems in the case where the Echelle grating is used. That is, a number of groove lines or density of groove lines needs to be increased in order to make a diffraction grating which has a large diffraction angle and which can be used in a Littrow arrangement, however it is difficult to produce an Echelle grating having a large number of groove lines. On this account, diffraction light of a high diffraction order needs to be used in order to make the diffraction angle larger with a small number of groove lines. As a result, a difference between a wavelength of diffraction light of a certain diffraction order and a wavelength of diffraction light of the adjacent diffraction order, in other words, an FSR (free spectral range) becomes small.
Assuming that a wavelength of light incident into a diffraction grating is &lgr;
1
and a diffraction order of the incident light is m
1
, and a wavelength of diffraction light is &lgr;
2
and a diffraction order of the diffraction light is m
2
, every diffraction light is diffracted into the same direction when the following expression (1) is satisfied.
m
1
·&lgr;
1
=
m
2
·&lgr;
2
(1)
When expression (1) is satisfied, a wavelength difference &Dgr;&lgr; between a wavelength &lgr;
1
and a wavelength &lgr;
2
, that is, an FSR is expressed by expression (2).
&Dgr;&lgr;=&lgr;
1
/(
m
1+1) (2)
By the way, if incident light is diffracted by using an Echelle grating and the diffraction order m
1
is a very large number, for example,
100
, every wavelength &lgr;
2
satisfying m
1
·&lgr;
1
=m
2
·&lgr;
2
(m
2
: an integer) is observed at the same position as that of wavelength &lgr;
1
. For this reason, when plural spectrum lines exist in a wavelength range near to the wavelength &lgr;
1
, discrimination of those spectrum lines becomes difficult.
Concretely, for example, when light from an iron (Fe) hollow-cathode lamp is incident into an Echelle grating having a groove line number of 85.34 lines/mm, the diffraction angle of the 93rd order diffraction light having a wavelength &lgr;
1
=248.3271 nm of this lamp is different only by 0.018 degrees from the diffraction angle of the 55th order diffraction light having a wavelength &mgr;
2
=419.9098 nm as shown in FIG.
32
.
Thus, in a spectrometer in which the above-mentioned FSR is small, a difference of diffraction angles of diffraction lights of adjacent two diffraction orders is very small. In the case where the light source has plural emitting spectral lines as in the iron (Fe) hollow-cathode lamp, spectrums of respective diffraction lights are overlapped to each other and those spectrums can not be separated. As a result, it is difficult to identify the correspondence relation between the spectrums.
For example, even if a real spectrum having different wavelengths is distributed as shown in FIG.
33
(
a
), the spectrum observed by the spectrometer having a small FSR has wavelengths as shown in FIG.
33
(
b
) because a difference between diffraction angles of diffraction lights of diffraction orders adjacent to each other is very small in this spectrometer.
Further, in the spectrometer having a small FSR, there is a problem that a baseline portion of each spectrum can not be measured precisely because plural spectrums of distant orders are observed close to each other. As shown in
FIG. 34
, spectrum S
1
exists in the neighborhood of spectrum S
2
to be measured and a shape of spectrum S
1
overlaps with a shape of spectrum S
2
to be measured.
Although there are the above-mentioned problems when light having plural emission spectrum lines is incident into an Echelle grating, when light having only one emission spectrum line is incident into an Echelle grating, an output of the light can be detected.
In order to solve the above-mentioned problems, there is proposed the constitution as shown in
FIG. 35
including a preliminary dispersion element, which makes plural emission spectrum lines into one emission spectrum line, before the light is incident into spectrometer
1
as disclosed in JP-A-11-132848. The preliminary dispersion element corresponds to means for making one line, in which prism
4
is arranged between two lenses
2
and
3
.
This preliminary dispersion element makes the light that passed through lens
3
to be dispersed by prism
4
and then pass the dispersed light through lens
2
so that one-lined light is incident into slit
1
a
of spectrometer
1
.
However, one prism alone can not completely remove the overlapped spectrum as mentioned above because the resolving power is not enough, that is to say, the performance for dividing neighboring spectrums is not enough. Therefore, it is considered to use plural prisms for improvement of the resolving power. However, There is a problem that the prism constitution becomes larger.
Besides, since the light passing through the prism generates some thermal change (temperature change), a refractive index of the prism also changes along with the temperature change. Accordingly, the light that passed through the prism at a position once fixed may become not to pass through slit
1
a
. In other words, the spectrum may become not to be measured.
Alternatively, preliminary spectrometer
5
as shown in
FIG. 36
without using a prism may be used as means for making a single line. The preliminary spectrometer
5
is designed such that the light that passed through slit
6
is reflected by concave lens M
1
into diffraction grating
7
, the light diffracted by diffraction grating
7
is reflected by concave lens M
2
and further reflected by reflecting mirror
8
, thus the single-lined light is
Suzuki Toru
Wakabayashi Osamu
Evans F. L.
Geisel Kara
Komatsu Ltd.
Stevens Davis Miller & Mosher LLP
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