JPS63168522A - Adjuscting apparatus for interferometer - Google Patents

Abstract

PURPOSE:To well improve the error of an adjusted state, by a method wherein the light from a measuring light source is reflected from a translucent mirror when the change of the optical system of a two luminous flux type interferometer with the elapse of time is automatically controlled to be allowed to be incident to a main light detector and, at the same time, a fixed mirror and a moving mirror are used to allow the light from a monochromatic light source to be also incident thereto. CONSTITUTION:The light from a measuring light source L is reflected from the reflecting surface of a collimator mirror 7 having a small hole provided to the central par thereof and the reflected light is further reflected from a translucent mirror B to be allowed to be incident to a main light detector 8. In order to avoid the change of the two-luminous flux type interferometer thus constituted such as a Fourier transformation type spectrophotometer with the elapse of time, the light from a monochromatic light source H is allowed to be incident to the main light detector 8 through the translucent mirror along with measuring light apart from the measuring light source using a fixed Mf and a moving mirror Mv and this incident beam is detected by a four-split light detector Df and the moving mirror Mv is moved so that the ratio of the max. and min. values of each light receiving element constituting said detector Df becomes equal to always monitor whether the incident position of the measuring light is accurate.

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 two-beam interferometer such as a Fourier transform spectrophotometric needle.

Consider a Michelson interferometer as an example of a conventional three-beam interferometer. In FIG. 6, 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 Mf
, Mv, and B have their mounting angles adjusted and fixed in advance, but the mounting angles vary 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.

Correction for changes in the position and orientation of the optical element over time can be made by adjusting the orientation (tilt in two directions perpendicular to the optical axis) of either the fixed mirror Mf or the movable mirror Mv. be. 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 on the side of the fixed mirror Mf, 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 It is projected onto a split photodetector,
The deflection of the optical axis of the reflected optical axis due to the movable mirror Mv is prevented.

C1 Problems to be Solved by the Invention In the conventional example described above, the deflection of the optical axis of the reflected light while the movable mirror Mv is in use is prevented, but the posture of the fixed mirror or semi-transparent mirror also changes over time. However, there is no dynamic correction of misalignment of the interferometer's optical system as a whole.

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.

21 Means for Solving Problems In a three-beam interferometer, a multi-segmented photodetector that is multi-divided in the circumferential direction is arranged at the 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. Feedback for controlling the attitude of the fixed mirror or movable mirror of the interferometer so that the ratio between the maximum and minimum difference in output of each light receiving element of the multi-split photodetector and the output average is equal for each light receiving element. A system was established.

E9 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
The reflected light flux from the fixed mirror Mf in the figure, P2, is moving! The reflected light beams from tMv interfere with each other on the light receiving surface of Df. As a result, when the movable mirror is moved, each light receiving element Dfl of the photodetector Df.

Df2. The output of ... fluctuates in one cycle while the movable mirror moves a distance of 1/2 the wavelength of the light. In Fig. 3A, two light beams PL and P2 are overlapped with 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 light beams at each point is not 1, but differs depending on the location, and the interference state also differs, and the light receiving elements Dfl, D
The ratios of the amplitudes of the fluctuations in the outputs of f2° and the average value do not match each other. On the other hand, as shown in FIG. 3B, when the two light beams PI and P2 completely match on the photodetector Df, the values of the ratios are equal to each other for each light receiving element. That is, the optical axes of the two beams Pi 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 l, 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 spring 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 the TEM00 oscillation mode, and the cross-sectional radius of the emitted beam is 0.4 at the laser exit.
It spreads out to just over 1 mm at a point 1200 mm further from there, and the light intensity distribution in the beam cross section is a centrosymmetric Gaussian distribution. This 1200 mm is equal to the optical path length from the light source H to the photodetector Df by V, and 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 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 MPXefilJ by this differential pulse signal, and at the rising and falling edges of the pulse signal, the light receiving element Dfl
The output signals A, B, C, and D of ~Df4 are sampled. The black circles in Figure 5 indicate this sampling point, and the amplitude of the output fluctuations and the average value of the output of the four light receiving elements are the third
When the two light beams PL and P2 in the figure are out of alignment, the intensity ratios of the two light beams on each light receiving element are different, so the interference states are different and they are not equal. The signal processing circuit 10 calculates (maximum value - minimum value)/(maximum value + minimum value) for each light receiving element output using the above-mentioned sampling data.

The denominator in the above equation is twice the average.

The value of the ratio in the above equation is that when the two beams PL and P2 in Fig. 3 perfectly match, the intensities of both beams are the same at any point on the light receiving surface of the detector, so perfect interference is established. (Maximum+Minimum) becomes approximately 0, (Maximum-Minimum) becomes maximum, and the value of the ratio becomes maximum. Therefore, the control circuit 10 uses the piezoelectric element 61.
62 to drive one of the light receiving elements Dfl to Df4.
For example, the value of the ratio is maximized for one of the light receiving elements, for example, Dfl, and then the other piezoelectric element is driven so that the ratio is maximized, and then the value of the ratio is set for the other light receiving elements. It is checked whether or not is equal to the value of the quantity ratio to Dfl. If they are equal, the adjustment ends there; if they are not equal, the same operation as described above is performed for one of the remaining Df2 to Df4.

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.

The signal processing circuit 10 controls the speed of the movable mirror Mv while it moves from left to right, samples interferogram data, etc., so the above-mentioned fine adjustment of the fixed mirror can be performed from right to 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 matter of correcting the deflection of the optical axis of the movable mirror. (This means that the adjustment state of the interferometer as a whole is corrected, and adjustment can be made even while the movable mirror is moving. Therefore, the interferometer can always be used in the best condition, improving measurement accuracy. Furthermore, since control is performed using the ratio between the maximum and minimum differences and the average of the outputs of each light-receiving element of the multi-segment photodetector, even if there are variations in sensitivity between each light-receiving element, , has the advantage that it does not affect the adjustment results.

[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. 5 is a circuit diagram of a main part of a control circuit according to an embodiment of the present invention, FIG. 5 is a waveform diagram for explaining the operation of the circuit, and FIG. 6 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. Agent: Hiroshi Agata, Patent Attorney Figure I4

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 ratio of the difference between the maximum value and the minimum value and the average value is calculated for the output of each light receiving element of the multi-division photodetector, respectively,
An interferometer adjustment device characterized in that a signal processing circuit is provided for controlling the fine adjustment means so that the value of this ratio is equal for each light receiving element.