JPH05231939A - Step scan fourier transferm infrared spectral apparatus - Google Patents

Abstract

PURPOSE:To obtain highly-precise equal-interval interferogram data by a method wherein an optical path difference between a fixed mirror and a moving mirror at step stop positions is measured with precision of the wavelength of laser light or below and extrapolation is executed on the basis of the measure optical path difference when intervals of the stop positions are not equal. CONSTITUTION:After modulation by a modulator 2, an infrared light source 1 is passed via a concave mirror 3, a beam splitter 4, a moving mirror 5 and a fixed mirror 6, transmitted through a sample 7, converged by a concave mirror 8 and detected by an infrared detector 9. Only a modulated frequency component of an output of this detector is amplified by a lock-in amplifier 20 and inputted to a data processing device 22 via an A/D converter 21. Meanwhile, a laser light of a frequency stabilization laser 11 is transmitted through an infrared optical path by a mirror 12, made to fall on the splitter 4 and the mirrors 5 and 6, taken out from a mirror 13 via the splitter 4, divided in two by a dividing mirror 14 and then detected by detectors 15 and 16. Outputs of these detectors are amplified by a preamplifier 17, subjected to phase division by a dividing circuit 18 thereafter, interference fringes are counted by a counter 19 and thereby an optical path difference is obtained. An interferogram is obtained from data at each position of movement.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Fourier transform infrared spectroscope, and particularly to a step scan effective for improving the S / N ratio when highly sensitive measurement of a weak infrared spectrum is performed by a modulation means or when time-resolved measurement is performed. In the method, the present invention relates to a spectroscopic device capable of measuring an infrared spectrum with high wave number accuracy by measuring the optical path difference between a fixed mirror and a moving mirror with high accuracy.

[0002]

2. Description of the Related Art In a conventional step-scan Fourier transform infrared spectrometer, Applied Spectroscopy 42, No. 4 (1988), pages 546 to 555 (Applie
d Spectroscopy 42 (4), (1988) PP546-5
55), the interference intensity of the laser light transmitted coaxially with the infrared light is monitored, and the movable mirror is moved in a stepwise manner at a constant distance. By using microvibration with a frequency sufficiently higher than the moving speed and applying feedback so that the laser interference intensity becomes zero, the optical path difference between the fixed mirror and the moving mirror is held high at a position where it is a multiple of the laser wavelength. I was getting wave number accuracy.

[0003]

However, this method requires a complicated feedback circuit and mechanism, and further requires measurement at a modulation frequency equal to or higher than the frequency of microvibration or a time interval represented by the reciprocal of the microvibration frequency. There is a drawback that measurement with short time resolution is impossible.

[0004]

This drawback can be solved by using a highly precise mechanical moving mirror feed. However, it is very difficult to realize mechanical moving mirror feed with a high precision of 0.5 micrometer or less, which is the wavelength of visible laser, and variations in step feed position precision due to processing precision and feed mechanism backlash. Occurs. Since the Fourier transform is premised on data sampling at equidistant positions, this variation deteriorates the wave number accuracy on the horizontal axis of the spectrum and also adversely affects the spectrum intensity. This is to measure the optical path difference between the fixed mirror and the movable mirror at each step-like stop position with an accuracy of not more than the wavelength of the laser light.If the step-like stop position intervals are not equal, extrapolate from the measured optical path difference. This makes it possible to obtain highly accurate equidistant interferogram data.

[0005]

The infrared light emitted from the infrared light source is split by the beam splitter and enters the fixed mirror and the movable mirror. The movable mirror is moved for a predetermined distance by a driving device for moving the movable mirror in a stepwise manner and then stopped for a predetermined time. The infrared lights re-superimposed by the beam splitter interfere with each other according to the optical path difference between the beam splitter and the fixed mirror and the movable mirror at this stop position, and the intensity changes. This infrared light passes through the sample or is reflected to absorb the wave number component light based on the intrinsic molecular vibration of the sample, and then enters the infrared light detector. At this time, a frequency-stabilized laser is passed coaxially with the infrared light,
The beam is split by a beam splitter, made incident on and reflected by a fixed mirror and a movable mirror, respectively, and after overlapping again, laser light is extracted, and the light is split into two and detected by two detectors. After amplifying the outputs of the two detectors, a sine wave output based on interference is phase-divided by a division circuit, and the number of interference fringes is counted by a counter to obtain an optical path difference. As a result, the optical path difference can be accurately determined even in the optical path difference in which the interference intensity does not become maximum or minimum, and the pair of optical path difference and the infrared light interference intensity at the step stop position can be obtained. A predetermined interferogram can be obtained by repeating this for the required number of times. Since the Fourier transform requires equidistant data, if the actually measured optical path difference data are not equidistant, the equidistant data can be obtained by interpolating from the measured data.

[0006]

Embodiment An embodiment of the present invention will be described below with reference to FIG.
In FIG. 1, infrared light from an infrared light source 1 is modulated by a modulator 2 and then converted into parallel light by a concave mirror 3, and is made to interfere with an interferometer composed of a beam splitter 4, a moving mirror 5 and a fixed mirror 6, and a sample 7 is transmitted. After that, the Fourier transform infrared spectrometer collects the light with the concave mirror 8 and detects it with the infrared light detector 9. The movable mirror 5 is moved in steps by a predetermined distance by the driving device 10. The laser beam of the frequency stabilizing laser 11 is transmitted through the mirror 12 into the infrared light optical path, is incident on the beam splitter 4, the moving mirror 5, and the fixed mirror 6 in the same manner as the infrared light, and is emitted from the beam splitter 4. Is taken out by the mirror 13 and is detected by the detectors 15 and 16 after being divided by the split mirror 14. The outputs from the detectors 15 and 16 are phase-divided by the preamplifier 17 after amplification by the division circuit 18, and the counter 19 counts the number of interference fringes to output the optical path difference between the movable mirror and the fixed mirror. Only the modulation frequency component of the output from the infrared detector 9 is amplified by the lock-in amplifier 20, converted into digital data by the A / D converter 21, and input to the data processing device 22. At this time, the data of the optical path difference is also input to the data processing device 22 at the same time.
A pair of interferograms can be obtained by repeating the step movement of the movable mirror, measurement and acquisition of optical path difference data, and acquisition of infrared detector output for a predetermined number of times. When the values of the optical path difference on the horizontal axis of the interferogram are not evenly spaced, the Fourier transformable interferogram is obtained by obtaining data at predetermined intervals by interpolation from the measurement data. An infrared spectrum is obtained by Fourier transforming this interferogram.

[0007]

EFFECTS OF THE INVENTION According to the present invention, even if a moving mirror driving mechanism having poor position reproducibility is used, highly accurate measurement is possible, the S / N ratio of the spectrum is improved, and highly sensitive qualitative and quantitative determination is possible. Become.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Infrared light source, 2 ... Modulator, 3 ... Concave mirror, 4 ... Beam splitter, 5 ... Moving mirror, 6 ... Fixed mirror, 7 ... Sample, 8 ... Concave mirror, 9 ... Infrared detector, 10 ... Moving mirror drive Device, 11 ... Frequency stabilizing laser, 12, 13 ... Mirror, 14 ... Split mirror, 1
5, 16 ... Laser light detector, 17 ... Preamplifier, 18 ...
Dividing circuit, 19 ... Counter, 20 ... Lock-in amplifier,
21 ... A / D converter, 22 ... Data processing device.

Claims (2)

[Claims] 1. An infrared light source, a beam splitter, a fixed mirror, and a movable mirror. The movable mirror is moved in a stepwise manner by a predetermined distance and is stopped, and the beam splitter is radiated from the infrared light source at each stop position. In a step-scan type Fourier transform infrared spectroscopic device that obtains an interferogram by measuring the intensity of infrared light that passes through and interferes with a fixed mirror and a movable mirror, a stop position detector for each of the movable mirrors is provided and the detection is performed. The optical path difference between the fixed mirror and the movable mirror is obtained from the detection value of the optical device, and the interferogram at any equal-interval optical path difference is interpolated from the optical path difference at each stop position and the infrared light intensity value. Scan Fourier Transform Infrared Spectrometer. 2. A step-scan Fourier transform infrared spectroscopic apparatus according to claim 1, wherein a laser interferometer using a frequency-stabilized laser as a light source is used as an optical path difference measuring means between a fixed mirror and a movable mirror. .