JPH0315686B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an automatic optical path difference zero point adjustment device for a Fourier transform spectrometer. A Fourier transform spectrometer records an interferogram of the light to be measured by moving a movable mirror using an interferometer to change the optical path difference between two divided beams, and then performs a Fourier transform on this interferogram. This is to find the spectral distribution of the light to be measured. In this type of spectrometer, in order to sample the interferogram signal of the light to be measured, monochromatic light of a known wavelength, such as He-Ne laser light, is incident on an interferometer, and the interferogram of the monochromatic light is detected. Conventionally, a configuration has been used in which this detection signal is used as a sampling signal of an interferogram signal of the light to be measured.

Figure 1 shows an example of a Fourier transform spectrometer. M1 is a fixed mirror, M2 is a moving mirror, and BS1 is a main beam splitter. These three components constitute a Michelson interferometer. is an interferometer for separating the light to be measured. MC uses a collimator mirror to convert the light to be measured emitted from the light source S into a parallel beam of light and input it into the main beam splitter BS1.
MS uses a condenser mirror to condense the light emitted from the interferometer onto the photodetector L1. With this configuration, the movable mirror M2 is moved in one direction as shown by the arrow in the figure, and the vertical axis is the output level of the detector L1, and the horizontal axis is the amount of movement of the movable mirror M2, or twice the optical path difference between the two beams. When recorded, an interferogram I(x) as shown in FIG. 2a is obtained. beam splitter BS
At the position of the movable mirror M2 where the optical path lengths of the two light beams divided by 1 are equal, all the wavelength components of the light to be measured in the two light beams overlap in the same phase, so that one highest peak P appears. Also, in Figure 1, M
3 is an auxiliary fixed mirror, BS2 is an auxiliary beam splitter, and these two and the movable mirror M2 constitute another Michelson interferometer, which is an auxiliary beam splitter for monochromatic light to obtain sampling signals. It is an interferometer. A parallel beam U of monochromatic light is made incident on the auxiliary beam splitter BS2.
L2 is a detector that measures the interferogram of this monochromatic light. FIG. 2b shows an interferogram obtained by the auxiliary interferometer when the movable mirror M2 is moved. The sampling signal uses the pulse rising or falling edge obtained by waveform shaping this monochromatic light interferogram signal, but what is the point at which the optical path difference of the main interferometer is 0 and this rising or falling point? Conventionally, the position of the auxiliary fixing mirror M3 was finely adjusted by manual operation, since they usually do not match due to the structural precision of the device. However, in this conventional method, as shown in Figure 3, if the sampling time is 1 to n, sampling is performed accurately at the apex position of the highest peak P of the interferogram I(x) of the measured light, that is, at the point where the optical path difference is zero. It is difficult to match the signals, and the resulting phase errors reduce measurement accuracy. In order to solve these problems, the present invention aims to automate the zero point adjustment of the optical path difference between the interferogram of the light to be measured and the sampling signal by using an electric circuit instead of mechanically. It is something to do.

Figure 4 shows an embodiment of the device of the present invention, in which monochromatic light of a known wavelength is made incident on an interferometer, and the interferometer signal is detected and used as a sampling signal of the interferogram signal of the light to be spectrally analyzed. In the configuration shown in FIG. 1, as shown in FIG. 4, a voltage-controlled variable resistance element VVR, a fixed capacitance capacitor C, and a one-shot multivibrator MV are provided as means for delaying the sampling signal, and the sampled interferogram is A storage circuit MEM that stores each level value D1...Dn (see FIG. 3) of the signal, and two level values Dm before and after the maximum value Dm among the stored level values D1...Dn.
Microprocessor CPU that selects -1 and Dm+1, compares both level values, and performs control operations such as increasing or decreasing the time constant of the delay means based on the result.
and a D/A converter DA. AD is an A/D converter as a sampling means, and BUS is a bus line.

In Fig. 4, the pulse width d of the sampling signal generated by the one-shot multivibrator MV
is determined by the product of the resistance value of VVR and the capacitance of capacitor C. Therefore, the waveform-shaped He-Ne fringe signal is the one-shot multivibrator MV.
When input to the trigger terminal of the He-Ne fringe signal, a sampling signal with a pulse width d is output from the rising edge of the He-Ne fringe signal as shown in Fig. 5, and the sampling signal of the A/D converter is A hold is performed, and the sampling timing is adjusted by controlling the width of the output pulse of the one-shot multivibrator MV.
EOC is an A/D conversion end signal.

FIG. 7 shows an example of a flowchart of the device of the present invention. First, in A, each level value D1...Dn of the interferogram signal is sampled, and this is stored in the memory circuit as shown in the memory map of FIG. Store in MEM. Next, in b, data Dm−1 and Dm+1 before and after the maximum value Dm
is calculated, and if the difference between both data is larger than the allowable error, the data output to the D/A converter DA is increased or decreased by a unit amount in c. D/A converter
The output voltage of the DA is applied to the voltage-controlled variable resistance element VVR to change the resistance of the element, and this resistance change changes the output pulse of the one-shot multivibrator MV to adjust the sampling timing. This operation is Dm−1 and Dm
This is repeated until the difference of +1 becomes less than the above tolerance. Now, the He−Ne fringe interval is 300μs,
If VVR is 50KΩ/V and C is 0.01μF, VVR
When the control input voltage is changed from 0 to 1V, the delay time can be changed from 0 to 250 μs. Also
VVR can be realized by the VSG-R characteristics of the FET.
Since the interferogram is symmetrical with respect to the point at which the optical path difference is 0, the zero point adjustment is performed with the required accuracy by the above-described operation.

The present invention is configured as described above, and since the sampling signal can be automatically adjusted to the zero point of optical path difference, the trouble of manual operation can be saved.
It has the advantage that adjustment can be completed instantly because it does not involve mechanical operations, and since it uses two pieces of data before and after the maximum level value, it utilizes the steep slope on both sides of the maximum peak in the interferogram signal. It has the advantage of being extremely accurate. Although the program used in this embodiment was designed to keep the difference between both data values within a certain error range, it is also possible to program it so that this difference is minimized. Furthermore, although a Michelson interferometer is used as the interferometer, the present invention is not limited to the Michelson interferometer.

[Brief explanation of the drawing]

FIG. 1 is a plan view of an apparatus according to an embodiment of the present invention, and FIG.
The figure is the interferogram obtained in the same way, the third
The figure is an explanatory diagram of the sampling state as above, FIG. 4 is a circuit diagram of the main part of the device of the present invention, FIG. 5 is a waveform diagram of each part of the same as above, FIG. 6 is a memory map of same as above, and FIG. 7 is an operation of same as above. It is a flowchart. BS1 is the main beam splitter, BS2 is the auxiliary beam splitter, M1 is the fixed mirror, M2 is the movable mirror, MC is the collimator mirror, MS is the condensing mirror, S is the light source of the light to be measured, L1 and L2 are the photodetectors , M3 is an auxiliary fixed mirror, VVR is a voltage-controlled variable resistance element, C is a fixed capacitance capacitor, MV is a one-shot multivibrator, DA is a D/A converter, AD is an A/D converter, CPU is a microprocessor,
MEM is a memory circuit, BUS is a bus line.

Claims (1)

[Claims] 1. In a configuration in which monochromatic light of a known wavelength is made incident on an interferometer and the peak of the interference signal is detected as a sampling signal of the interferogram signal of the light to be spectroscopy, a means for delaying the sampling signal, and a means for delaying the sampled signal. means for storing each level value of the interferogram signal; means for comparing two level values sampled before and after the maximum value of each stored level value; A Fourier transform spectrometer, comprising: means for increasing/decreasing the constant by a unit amount, and configured to repeat the sampling and the increase/decrease of the time constant until the difference between the two level values becomes sufficiently small. Optical path difference zero point automatic adjustment device.