JP2784435B2 - Fourier spectrometer - Google Patents

Description

The present invention relates to an infrared or visible Fourier spectrometer,
In particular, the present invention relates to a Fourier spectrometer that quickly and accurately extracts spectral data in a limited range from a spectrum covering a wide wave number range.

[Prior art] In Fourier spectroscopy, interferogram data
Performs FFT (Fast Fourier Transform) to restore the original spectral data. Here, the N-point interferogram data is converted into N-point spectral data.

[Problems to be Solved by the Invention] However, when trying to improve the wavenumber resolution or expanding the observation wavenumber range, the value of N increases. Particularly in the visible region, the number of data points becomes very large.

This increase in the number of data points causes two problems.
One is an increase in FFT calculation time due to an increase in the number of data points, and the other is an increase in the dynamic range of the A / D converter when acquiring interferogram data.

In the case of a continuous spectrum, the interferogram causes a large center burst, and the number of bits of the A / D converter must be increased in accordance with the accuracy in order to improve the measurement accuracy of the restored spectrum. Eventually, the price of the A / D converter, the conversion speed, and the word length per data point increase, which also affects the overall data processing speed.

As a form of use of the Fourier interferometer, qualitative analysis is often performed using the entire reconstructed spectrum waveform, but information of only a specific region in a wide range of spectral data is often sufficient. This is especially true for quantitative analysis represented by analysis of impurities in semiconductors. This also applies to measurements such as FT Raman. Even in such a case, the spectrum must be restored by performing the FFT on all data points in the past, and thus the problems of the increase in the calculation time of the FFT and the increase in the dynamic range of the A / D converter occur. I was

As a data point reduction method, there is a pre-filtering method.

The pre-filtering method, when desired to obtain a spectrum of only the band σ ₁ ~σ ₂ of sample light for all the wavenumber range 0～Shiguma _M, sampling the interferogram data at 1 / (2σ _M) following intervals After performing the convolution operation with the sinc function on the interferogram plane corresponding to the filters of the bands σ _{1 to} σ ₂ , it is necessary to further sample (thin out) the data of the operation result.
Therefore, the calculation time cannot be reduced much.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a Fourier spectroscope that can shorten the calculation time without lowering the wave number resolution and narrow the dynamic range required by the A / D converter.

[Means for Solving the Problems] In order to achieve this object, a Fourier spectrometer according to the present invention includes an interferometer for acquiring an interferogram of a sample light, and a maximum wave number arranged in an optical path of the interferometer. an optical band-pass filter for transmitting light in a target wavenumber range (σ _{1 to} σ ₂ ) of σ ₂ = (1 + 1 / m) σ ₁ (m is a positive integer) out of σ _M sample light; Interferogram data obtained by detecting light transmitted through the optical bandpass filter is calculated as 1 / (2σ ₂ ) <h ′ ≦ 1 / ｛2 (σ ₂ −
σ ₁ )｝ sampling means for sampling at an interval h ′, and Fourier transform means for performing a Fourier transform on the sampled interferogram data to obtain spectrum data.

Embodiment (1) One Embodiment One embodiment of the present invention will be described with reference to the drawings.

FIG. 1 shows a main configuration of a Fourier spectrometer according to one embodiment of the present invention.

In this embodiment, for the sake of simplicity, the spectrum S
As for (σ), as shown in FIG. 2 (a), it is assumed that there are two bright lines in a linear base line. Furthermore, interested only in the emission lines of the two, and to be obtained only spectrum wavenumber range σ ₁ ~σ ₂ including emission lines of the two of the total wavenumber range 0~σ _M.

As shown in FIG. 1, this embodiment shows a case where a continuous wavelength variable interference filter 12 is installed in a normal Michelson interferometer 10.

The Michelson interferometer 10 is for temporally acquiring an interferogram of the sample light by scanning the moving mirror, and includes an aperture 14 and a collimating lens 16 for collimating the sample light from the aperture 14. , A beam splitter 18 for splitting the light beam from the collimating lens 16 into two
And a fixed mirror that reflects and reverses one of the split light beams
20, a movable mirror 22 that reflects the other of the split light beams and travels backward, an imaging lens 24 that forms an image of the interference light multiplexed by the beam splitter 18, and a light that is arranged at the image forming position. And a photodetector 26 for detecting the intensity.

The continuous wavelength variable interference filter 12 is arranged on an optical path between the imaging lens 24 and the photodetector 26. The continuous wavelength variable interference filter 12 has a disk shape, and the transmission wavelength changes continuously or stepwise along the circumferential direction. The transmission band in the portion on the optical path of the continuous wavelength variable interference filter 12 changes continuously as the disk is rotated by the drive pulse from the control unit 28.

The output signal of the photodetector 26 is amplified by the amplifier 30,
The digital conversion is performed by the D converter 32.

The control unit 28 supplies the drive pulse to the continuous wavelength variable interference filter 12 to set the transmission range of the interference light.
The wavenumber transmission area σ ₁ ~σ ₂ set by also moving mirror 22 is step by step moved linearly by supplying a driving pulse to the movable mirror driving device 33. Here, σ ₁ and σ ₂ are σ ₂ =
It is set so as to satisfy (1 + 1 / m) σ ₁ (m is a positive integer).

Further, the control unit 28 supplies the A / D converter 32 with a conversion signal at intervals (a change in the optical path difference between the two light beams in the interferometer 10) h 'that satisfies the following condition, and outputs the interferogram data. Perform sampling and digital conversion.

h ≦ 1 / (σ ₂ + σ _M ) h ′ = 1 / {2 (σ ₂ −σ ₁ )} = hn n is a positive integer Data obtained sequentially from the A / D converter 32 is an interferogram memory 34 , And a discretized double-sided interferogram is created in the interferogram memory 34.

FIG. 2 (b) shows a modeled transmission characteristic F (σ) of the continuous wavelength variable interference filter 12, and FIG. 2 (c) shows S (σ) and a comb function (shah function). Convolution with (D) shows a filter function F (σ) and a comb function Convolution with (E) is the product of (c) and (d) Is shown. Here, _{1 / h} indicates a period. In general, the product and convolution on the spectrum surface correspond to the convolution and product on the interferogram surface, respectively.

If sampling is performed at the interval h (provided that the equal sign is satisfied), the interferogram memory 34 stores
A discretized interferogram obtained by Fourier-transforming (e) is written. If sampling is performed at h ′, a discretized interferogram obtained by Fourier-transforming (f) is written into the interferogram memory 34.

Next, the fast Fourier transformer 40 performs Fourier transform on the data in the interferogram memory 34 to obtain a spectrum in the wavenumber range 0 to (σ ₂ −σ ₁ ) shown in FIG. Supply to device 42. This spectrum data is visualized and extracted from the spectrum output device 42.

In this way, conventionally, in a normal measurement, 1 / (2σ _M )
Interferogram data, which had to be sampled at the following intervals, may be sampled at intervals of 1 / {2 (σ ₂ −σ ₁ )} or less, and the sampling interval is σ _M / ｛(σ ₂ −σ ₁ )｝, and therefore a high-batch Fourier transformer with a long operation time
Point N of interferogram data to be supplied to 40
Can be reduced to N (σ ₂ −σ ₁ ) / σ _M and FF
T calculation time can be greatly reduced.

Further, since the peak of the interferogram is dispersed to both sides by passing through the continuous wavelength variable interference filter 12, the necessary dynamic range of the A / D converter 32 is reduced by K
(Σ ₂ −σ ₁ ) / σ _M times. Here, K is K ≦ 1, and K = 1 when the sample light spectrum is perfect white light.

Further, by passing through the filter 12, there is no need to perform a convolution operation corresponding to the product of FIGS. 2A and 2B, and the total calculation time can be further reduced.

(2) Extension The present invention includes various other modified examples.

For example, in the above embodiment, the continuous wavelength tunable interference filter 12 is used as an optical bandpass filter.However, for example, an interference filter having a plurality of different transmitted wave number ranges is prepared, and these are set according to the target wave number range. It may be configured to switch.

It goes without saying that various types of interferometers for producing an interferogram, for example, those which form an interferogram spatially and obtain it can be used.

[Effects of the Invention] As described above, according to the Fourier spectrometer of the present invention, an optical band-pass filter is disposed in the optical path of the interferogram acquisition interferometer, and the sample light having the maximum wave number σ _M is obtained. The light in the target wavenumber range (σ ₁ −σ ₂ ) of σ ₂ = (1 + 1 / m) σ ₁ (m is a positive integer) is transmitted, and the interferogram data obtained by the interferometer is
Since sampling is performed at an interval h ′ of h ′ ≦ 1 / {2 (σ _{2 to} σ ₁ )} and Fourier transform is performed on the sampled interferogram data, the number of sampling points of the interferogram data can be reduced, and Since there is no need to perform the above-mentioned complicated convolution calculation, there is an excellent effect that the calculation time can be reduced, the dynamic range of the A / D converter can be reduced, and the wave number resolution does not decrease. .

[Brief description of the drawings]

1 and 2 relate to an embodiment of the present invention, FIG. 1 is a block diagram of a main part of a Fourier spectrometer, and FIG. 2 is a waveform diagram on a spectrum plane for explaining operation. In the figure, 10: Michelson interferometer 12: Continuous wavelength variable interference filter 16: Collimating lens 18: Beam splitter 20: Fixed mirror 22: Moving mirror 24: Imaging lens 26: Photodetector 34: Interferogram memory 40: Fast Fourier Transformer 42: Spectrum output device

Claims (1)

(57) [Claims] 1. An interferometer (10) for acquiring an interferogram of a sample light, and σ ₂ = (1 + 1 / m) σ ₁ (σ ₁ ) of the sample light having a maximum wave number σ _M arranged in an optical path of the interferometer. (m is a positive integer) an optical band-pass filter (12) that transmits light in a target wavenumber range (σ _{1 to} σ ₂ ), and light obtained by detecting light transmitted through the optical band-pass filter. The interferogram data obtained is expressed as 1 / (2σ ₂ ) <
sampling means (28) for sampling at intervals h 'of h'≤1 / {2 (σ ₂ -σ ₁ )}, and Fourier transform for obtaining spectrum data by performing Fourier transform on the sampled interferogram data A Fourier spectrometer comprising: means (40);