JPS63168502A - Control apparatus of interferometer - Google Patents

Abstract

PURPOSE:To enhance measuring accuracy by performing measurement in the optimum state by correcting the whole of an interferometer, by a method wherein test luminous fluxes are allowed to be incident to the interferometer to be allowed to interferometer to be allowed to interfere with each other and the state of interference is received to control an optical element. CONSTITUTION:The reflected luminous flux P1 from a fixed mirror Mf and the reflected luminous flux P2 from a moving mirror Mv are projected on a four-split light detector Df and interfere with each other on the light receiving surface of the detector Df. As a result, the outputs of elements Df1, Df2... perform the variation of one cycle during a time when the moving moving mirror Mv moves by 1/2 a light wavelength. When the luminous fluxes P1, P2 are superposed in a state slightly shifted each other, the luminous intensity distributions of the cross-sections of the luminous fluxes lower toward the outer peripheries thereof and, therefore, the variation amplitudes of the outputs of two elements do not coincide with each other. When the posture of Mf or Mv is controlled so that the amplitude becomes max. in one light receiving element and the amplitude becomes min. in the other element at a diagonal position, the luminous fluxes P1, P2 perfectly coincide on the detector Df and optical axes coincide with each other to perfectly adjust the optical elements and measurement can be carried out in the optimum state.

Description

DETAILED DESCRIPTION OF THE INVENTION A. Field of Industrial Application The present invention relates to an automatic adjustment device for temporal changes in the optical system of a three-beam interferometer such as a Fourier transform spectrophotometer.

Consider a Michelson interferometer as an example of a conventional three-beam interferometer. In FIG. 7, L is a measurement light source, Mf is a fixed mirror, Mv is a movable mirror, and B is a semi-transparent mirror. With this configuration, each optical element M
The mounting angle of f + M v and B is adjusted and fixed in advance, but the mounting angle changes over time due to temperature changes and the like. Further, as the movable mirror moves, the inclination of the movable mirror changes.

Such changes cause the interference pattern to move, causing the output of the light receiving element to change over time, causing measurement errors.

Such a change over time in the position and orientation of the optical element can be corrected by adjusting the orientation (inclination in two directions perpendicular to the optical axis) of either the fixed mirror Mf or the movable mirror Mv. If you simply want to correct the deviation of the interferometer over time, you can manually readjust the posture of the interferometer's optical elements from time to time, but the posture of each optical element changes while the interferometer is in use. In order to cope with this, corrections must be made from time to time. For this purpose, a fixed mirror is detected by detecting the mutual deviation of the optical axes of the two beams that are split into two and then overlapped again.
An automatic control device is required to provide feedback to the attitude adjustment mechanism of any of the movable mirrors.

A conventionally proposed automatic control device for adjusting the optical system of an interferometer has a configuration as shown in FIG.

In the figure, L is the measurement light source, H is the automatic adjustment light source,
A laser is used. The laser beam incident on the interferometer is made incident on a four-split photodetector Df by a mirror m.

A light shielding plate K is placed on the optical axis of the fixed mirror Mf[4, so that only the laser beam reflected by the movable mirror Mv enters the four-split photodetector Df. With this configuration, the attitude of the movable mirror Mv is controlled so that the laser beam spot projected onto the 4-split photodetector Df is evenly distributed to each light receiving element of the 4-split photodetector. The posture control is performed by applying a voltage to two of the four movable mirror supports as piezoelectric elements. That is, the attitude of the movable mirror Mv is controlled so that the output of each light receiving element of the 4-split photodetector Df is equal to each other, and in this way, the laser beam reflected by the movable mirror Mv is always correctly 4 This means that the light is projected onto the divided photodetector, and the moving mirror Mv is prevented from photographing the reflected optical axis.

C6 Problems to be Solved by the Invention In the conventional example described above, the movement! Although the deflection of the optical axis of the reflected light during use of Mv is prevented, the posture of the fixed mirror and semi-transparent mirror also changes over time, and the dynamic adjustment of the optical system of the interferometer as a whole is affected. No corrections have been made.

An object of the present invention is to perform automatic correction of fluctuations in the adjustment state of an interferometer more completely than the conventional example described above.

20 Means for Solving Problems In a three-beam interferometer, a multi-segmented photodetector that is divided into multiple parts in the circumferential direction is arranged at a position where the interference light beam is received, and the cross-section of the light beam incident on the photodetector is A monochromatic light source is arranged to make a light beam that is about the same level or slightly larger than the light receiving surface of the photodetector enter the interferometer parallel to the optical axis of the main incident light beam of the interferometer, and the movable mirror of the interferometer is moved. The amplitude of the alternating current component of the output waveform of one light receiving element of the above-mentioned multi-split photodetector in the middle is the maximum, and the amplitude of the alternating current component of the output waveform of the other light receiving element placed diagonally therewith is the minimum. A feedback system was installed to control the posture of the fixed mirror or movable mirror of the interferometer.

E1 action In Fig. 3, Df is a multi-division photodetector, Pl, P2
indicates the luminous flux projected onto the photodetector Df, and Pl is the first
In the figure, the reflected light beam from the fixed mirror Mf, P2 is the reflected light beam from the movable mirror Mv, and both light beams interfere on the light receiving surface of Df.

As a result, when the movable mirror is moved, each light receiving element Df1°Df2. The output of ... fluctuates in one cycle while the movable mirror moves a distance of 1/2 the wavelength of the light.

In Figure 3A, the two light beams PI and P2 overlap each other with a slight deviation from each other. In such a state, the light intensity distribution in the cross section of the light beams is center symmetric and decreases toward the outer periphery, so the overlapping portion of the light beams The intensity ratio of the two beams at each point is not 1,
It differs depending on the location, the interference state also differs, and the amplitudes of fluctuations in the outputs of the light receiving elements Dfl, Df2°, . . . do not match each other.

If we control the posture of either the movable mirror or the fixed mirror so that this amplitude is maximum at one light-receiving element and minimum at the light-receiving element located diagonally opposite to it, two beams of light are generated as shown in Figure 3B. PL and P2 perfectly match on the photodetector Df. That is, the optical axes of the two light beams PL and P2 emitted from the interferometer coincide. Therefore, a complete adjustment has been made as an adjustment of the optical system of the interferometer, and such adjustment is performed while the movable mirror is moving.

Embodiment FIG. 1 shows an embodiment of the present invention. In this embodiment, the present invention is applied to an interferometer for a Fourier transform spectrophotometer. Mf is a fixed mirror, Mv is a movable mirror, and B is a semi-transparent mirror. The movable mirror Mv is held by a linear air bearing 1, and is driven left and right in the figure by a linear motor 2. The fixed mirror Mf is held on the fixed base 4 by the center leg 3 of an elastic rod, and as shown in FIG. compression splash 5.5
are interposed therebetween, and voltage elements 61 and 62 are interposed between the fixing base and the other two apex positions. Piezoelectric element 6
When a voltage is applied to 1.62, these piezoelectric elements expand and contract, and the fixed mirror Mf is therefore swung left and right and in the viewing direction about the attachment point of the center leg 3, that is, the center of the fixed mirror as a fulcrum. L is a measurement light source, and its emitted light is made into a parallel light beam by a collimator mirror 7 and is made to enter the interferometer, and is then emitted from the interferometer and condensed onto the main light detector 8 for measurement, where 2 The two divided beams of light interfere.

H is a monochromatic light source for automatic adjustment of the optical system, and a He-Ne laser is used, and its emitted light beam enters the interferometer through a small hole in the center of the collimator mirror 7 in parallel to the optical axis of the interferometer's incident optical path. The light is reflected by a small mirror m on the output optical path of the interferometer and projected onto a four-split photodetector Df. The light source He-Ne laser uses TEM00 oscillation mode,
The cross-sectional radius of the emitted light beam is 0.4 mm at the laser exit
The light beam spreads to a little over 1 mm after 1200 mm, and the light intensity distribution in the beam cross section is a centrosymmetric Gaussian distribution. This 1200 mm is approximately equal to the optical path length from the light source I to the photodetector Df,
The photodetector Df has a light-receiving surface diameter of about 1 mm and is divided into four parts as shown in FIG.

Four light receiving elements Dfl, Df2, Df of the 4-split photodetector
3. Df4 are amplifiers A1 to A, respectively, as shown in FIG.
4, the AC component of the output is sequentially sampled by the multiplexer MPX, AD converted, taken into the signal processing circuit 10, and fed back from the signal processing circuit 10 to the piezoelectric elements 61 and 62 via the piezoelectric element drive circuit 12. be done. The signal processing circuit 10 operates as follows.

FIG. 5 is a time chart for explaining the operation. In FIG. 5, A is the output AC component of the light receiving element Dfl. This output is differentiated by the differential amplifier circuit 11 shown in FIG. 4, resulting in a waveform Ad shown in FIG. The signal processing circuit 10 controls the multiplexer MPX by mq using this differential pulse signal,
At the falling edge of the same pulse signal, the light receiving elements Dfl to Df4
The output AC signals A, B, C, and D are sampled. The black circles in Figure 5 indicate this sampling point, and if the output amplitudes of the four light receiving elements are shifted from the two light beams PL and P2 in Figure 3, the intensity ratio of the two light beams on each light receiving element will be different. Therefore, the interference conditions are different, so they are not equal. Signal processing circuit 1
0 is two light beams P1.0 in the order shown in FIG. The piezoelectric elements 61 and 62 are controlled so as to overlap P2. FIG. 6A shows the initial state, and black circles P1 and P2 indicate the centers of the luminous fluxes PL and P2 in FIG. 3. Of the luminous fluxes PL and P2, the one that can be adjusted is the fixed mirror reflected luminous flux Pl, and P2 cannot be moved.

The signal processing circuit 10 first moves Pl so that the AC outputs of the light receiving elements Dfl and Df3 match, and also so that the AC outputs of Df2 and Df4 match. In this way, Pl moves to a position symmetrical to P2 with respect to the center of the photodetector Df, as shown in B in FIG. Next, as shown in FIG. 6C, Pl is moved to a position symmetrical to P2 with respect to the dividing line X of the photodetector Df. This operation means that the AC outputs of Df3 and Df2 match Dfl and Df4.
By controlling the piezoelectric elements 61 and 62 so that the AC outputs of All you have to do is control it. Finally, move Pl so that it overlaps P2 as shown in the sixth diagram. This is light receiving element D
This is to make the alternating current output of f3 reach the maximum of the four light receiving elements.If the y-axis in Fig. 6 is vertical, the fixed mirror Mf is moved in the direction of worship, and the piezoelectric element in Fig. 2 is 6
It is only necessary to control 1. Actually, P2 is the light receiving element Df
Although it is unclear on which light receiving element of l-Df4 the light receiving element is located, the operation may be performed in the above order.

The embodiment shown in FIG. 1 is a Fourier transform spectrophotometer that collects interferogram data while moving a movable mirror from left to right. In this case, the interference signal obtained by the light source H is obtained from the output of the light receiving element Dfl as shown in FIG. It is used for. Rather, the He-Ne laser for that purpose is utilized for the purpose of the present invention.

Since the signal processing circuit 10 controls the speed of the movable mirror and samples interferogram data while the movable mirror Mv moves from left to right, the above-mentioned fine adjustment of the fixed mirror can be performed from the right to the left of the movable mirror. This is done on the return trip from left to right, and in the measurement trip from left to right, the adjustment state from the previous return trip is memorized.

G. Effects According to the present invention, a test light beam is made incident on an interferometer to cause interference, and the interference state is received to adjust the optical element. Therefore, the adjustment is simply a correction of the optical axis of the moving mirror. Instead, corrections are made for errors in the adjustment state of the interferometer as a whole, and adjustments can be made even while the movable mirror is moving, so the interferometer can always be used in the best condition, improving measurement accuracy. Improvement can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a plan view of an optical system according to an embodiment of the present invention, FIG. 2 is a front view showing a fixed mirror holding structure of the same embodiment, FIG. 3 is a diagram explaining the present invention in detail, and FIG. FIG. 5 is a waveform diagram for explaining the operation of the circuit; FIG. 6 is a diagram for explaining the adjustment operation in the present invention;
FIG. 7 is a plan view of a conventional optical system. L...Light source for measurement, H...Light source for adjustment, Mf...
Fixed mirror, Mv... Movable mirror, B... Semi-transparent mirror, Df.
...Multi-segment photodetector, 3...Center leg, 5...Spring,
61.62...Piezoelectric element.

Claims (1)

[Claims] A multi-segment photodetector, which is divided into multiple sections in the circumferential direction, is placed at a position where the interference light beam is received, and the cross-section of the light beam incident on the photodetector is approximately the same or slightly larger than the light receiving surface of the photodetector. A monochromatic light source is arranged to make a light beam that is incident on the interferometer parallel to the optical axis of the main incident light beam of the interferometer, and an electric light source is used to finely adjust the attitude of either the fixed mirror or the movable mirror of the interferometer. A driving means is provided, and the fine adjustment means is controlled so that the AC output of one light receiving element in the multi-segment photodetector is maximum and the AC output of the other light receiving element located diagonally thereto is minimum. An interferometer adjustment device characterized by being provided with a signal processing circuit.