JPH04169817A - Optical type displacement meter - Google Patents

Abstract

PURPOSE:To stabilize a wavelength of a laser light emitted from a light source and to improve precision by taking out directly a fluctuation in the wavelength and by controlling an injection current into the light source by a standardization signal on the basis of an intensity signal of the light emitted actually. CONSTITUTION:An interference means 10 for a wavelength fluctuation control has a surface 10' composed of a total reflector element 10'a and a semitransparent mirror element 10'b and a surface 10'' formed of a total reflector and a gap between the surfaces 10' and 10'' is fixed beforehand at a prescribed dimension (d). A fluctuation in the wavelength of a laser light emitted currently from a semiconductor laser 1 is taken out directly by the means 10, and a standardization signal based on an intensity signal of the light emitted actually is obtained by an arithmetic processing of signals from the means 10 and an interference means 13 for displacement detection. The standardization signal thus obtained being used, an injection current is controlled by an injection current control means 20 so that the wavelength of the laser 1 be fixed, and moreover detection of displacement and discrimination of the direction of the displacement are conducted. According to this constitution, a control including all factors of the fluctuation in the wavelength is conducted and stability of the wavelength and an improvement in precision can be attained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Summary] The present invention relates to an optical displacement meter that measures the displacement of a surface to be inspected using a light interference phenomenon. Conventionally, in order to improve measurement accuracy, temperature control and output control have been performed to stabilize the wavelength of light emitted from a light source, but there has been uncertainty in the control method.

In the present invention, an interference means for wavelength fluctuation control is provided in contrast to the conventional example, and the wavelength fluctuation of the laser light currently emitted from the light source is directly extracted and normalized based on the intensity signal of the light actually emitted. The signal controls the current injected into the light source and controls the output of the light source. Therefore, it becomes possible to perform control that includes all wavelength fluctuation factors other than temperature, which improves wavelength stability and measurement accuracy.

[Industrial application field]

The present invention relates to an optical displacement meter that measures displacement of a surface to be inspected using light interference phenomena, and is particularly applicable to fields such as X-Y stages where relatively large displacements need to be measured with high resolution. Ru.

[Conventional technology]

In order to improve the measurement accuracy of displacement meters that utilize light interference phenomena, we aim to stabilize the wavelength of the light emitted from the light source.
An optical displacement meter has been known that uses a Fizeau type interferometer having a configuration as shown in FIG. 6 as a displacement detection means.

As shown in the figure, the laser beam emitted from the semiconductor laser 1 is converted into parallel light by the lens 8 and transmitted through the half mirror 11, a part of which is reflected by the reference surface 13, and the other part is reflected by the reference surface 13. and is reflected by the surface to be inspected 14. In this way, each laser beam reflected by the reference surface 13 and the test surface 14 is refracted by 90 degrees by the half mirror 11, follows the same path and interferes, and the interference signal is detected by the photodetector element 15. The light intensity signal IC1I is detected and sent to the signal processing circuit 17.
d is obtained. The light intensity signal■. , Ia change by one cycle each time the surface to be measured 14 is displaced by λ/2 (λ is the oscillation wavelength of the semiconductor laser) (see FIG. 3), depending on the strength of the interference signal. For example, the signal processing circuit 17 By performing processing such as counting the number of periods of the output signal and multiplying this count number n by λ/2, the displacement ΔX of the surface to be measured 14 can be measured. Further, when determining the displacement direction, for example, by providing a step 13'p on the reference surface 13, two light beams Lc having a phase difference of 90 degrees are generated.

L, and these are respectively photodetecting elements 15c and 1.
'5d, the signal processing circuit 17 receives the light intensity signal IC,
Obtained as Ia. Then, as shown in Fig. 4, two light intensity signals ■. , Id have a phase difference of 90° from each other, and since the relationship between signal delay and advance is determined according to the direction of the displacement ΔX, the direction can be determined from this relationship. Then, the result of the displacement amount is displayed on the display 21.

In this way, in order to measure the displacement ΔX of the test surface 14, the wavelength λ of the laser beam is used as a reference. However, when a temperature change occurs in the semiconductor laser 1, the oscillation wavelength λ also changes accordingly (see FIG. 5).

When the oscillation wavelength changes, the amount of change results in a measurement error. Therefore, the oscillation wavelength is stabilized by performing feedback control so that the temperature of the semiconductor laser 1 is always constant. That is, the temperature of the semiconductor laser 1 is detected by a temperature detection element 2 such as a thermistor, and based on this detection signal, a calculation circuit 3a calculates a temperature variation, and a drive circuit 3b is activated to compensate for the temperature variation. Cooling or heating is performed by manipulating the current flowing through the Peltier element 4, which is a temperature actuator, and the temperature of the package of the semiconductor laser 1 in the semiconductor laser device 5 including the temperature detection element 2 and the Peltier element 4 is controlled to be constant. It is.

Furthermore, even if the temperature of the semiconductor laser 1 is constant, the output may fluctuate due to factors other than temperature, and if the output intensity of the semiconductor laser 1 fluctuates, the oscillation wavelength also changes, so the measurement is performed based on that change. Errors will occur. Therefore,
In order to keep the output of the semiconductor laser 1 constant, the light intensity of the laser beam is detected by a photodetector element 6 consisting of a photodiode or the like built into the semiconductor laser 1, and based on the detection signal, a negative feedback circuit 7 detects the intensity of the laser beam. , semiconductor laser 1
This is to perform feedback control of the current injected into the semiconductor laser 1 to keep the output of the semiconductor laser 1 constant.

[Problem to be solved by the invention]

In the conventional optical displacement meter described above, temperature control of the package of semiconductor laser I and wavelength control of semiconductor laser 1 are performed in order to keep the oscillation wavelength constant. This is done based on the pre-acquisition of a constant wavelength, but since it is not the temperature of the laser itself, there remains uncertainty in keeping the wavelength constant. Also,
There is a time lag in temperature control of the semiconductor laser 1, and even if the temperature is kept constant by temperature control, the semiconductor laser 1
When the injection current is manipulated as feedback control to compensate for fluctuations in the output of There were problems in that it did not stabilize and the measurement accuracy did not improve.

In view of the above-mentioned conventional problems, the present invention stabilizes the oscillation wavelength and improves measurement accuracy by eliminating the influence of fluctuations in the oscillation wavelength of laser light caused not only by temperature changes in the light source but also by factors other than temperature changes. The purpose of the present invention is to provide an optical displacement meter that can realize improvements.

[Means to solve the problem]

The optical displacement meter of the present invention includes a light source having coherent properties and a single wavelength, an interference means for controlling wavelength fluctuations for directly measuring wavelength fluctuations of the light source, and an interference means for controlling wavelength fluctuations.
A light intensity signal is detected from the reference surface and a displacement detection interference means for obtaining reflected interference light from the test surface, the detected light intensity signal is normalized, and the test surface is detected based on the detected light intensity signal. and an injection current control means that controls the current injected into the light source based on the light intensity signal normalized by the calculation processing means.

[For production]

In the present invention, the wavelength variation control interference means directly extracts the wavelength variation of the laser light currently emitted from the light source, and the wavelength variation control interference means and the displacement detection interference means directly extract the wavelength variation of the laser light currently emitted from the light source. A normalized signal based on the intensity signal of the light is calculated by the arithmetic processing means, and the injection current is controlled by the injection current control means using the normalized signal so as to keep the wavelength of the semiconductor laser constant. Furthermore, the normalized signal is used to detect displacement and determine the direction of displacement.

Therefore, it is possible to perform control that includes all wavelength fluctuation factors, including temperature and other wavelength fluctuation factors, so that wavelength stability can be further improved.

〔Example〕

Hereinafter, embodiments of the present invention will be described with reference to the drawings, in which the same members as those in the conventional example are given the same reference numerals.

FIG. 1 is a block diagram of an embodiment of the present invention, which is constructed using a Fizeau type interferometer.

In the figure, 1 is a semiconductor laser that emits coherent light with a single wavelength and a uniform phase.The temperature is measured by a temperature detection element 2 such as a thermistor, and based on the detection signal, an arithmetic circuit 3a The temperature fluctuation is calculated, and the drive circuit 3b is operated to compensate for the temperature fluctuation, thereby adjusting the current flowing through the Peltier element 4, which is a temperature actuator, and performing a cooling or heating operation. The package temperature of the semiconductor laser 1 is controlled to be constant. The semiconductor laser 1, the temperature detection element 2. Peltier element 4 is provided within semiconductor laser device 5 . The operation of this configuration 2 is similar to that of the conventional example.

8 is a lens that emits the incident light as parallel light; 9 is a first half mirror that partially transmits the incident light and refracts the rest by 90 degrees; 110 is the distance between the reference surface and the test surface of the Fizeau interferometer; It is an interference means for wavelength fluctuation control having a fixed configuration, and has a first surface 10' consisting of a total reflection mirror section 10'a and a semi-transmission mirror section 10'b, and a second surface formed on the total reflection mirror. IO
The distance between the first surface 10' and the second surface lO'' is fixed to a predetermined dimension d in advance.

12 is a second half mirror 113 that transmits the light transmitted from the first half mirror 9, is reflected by a displacement detection interference means 13 (described later), and refracts the reflected light 90 degrees and interferes with the reflected light that follows the same path. A reference surface 13' is a semi-transmissive mirror that is provided with a step 13'p to give a 90° phase difference to a part of the incident light and has portions 13'c and 13'd of different thickness formed therein. and the test surface 13, which is a total reflection mirror.
This is a displacement detection interference means having # and has a similar configuration to the displacement detection section of a Fizeau type interferometer.

Reference numeral 16 denotes a photodetecting element consisting of four elements 16a, 16b, 16c, and 16d, and the element 16a detects a light beam that is refracted by 90 degrees by the first half mirror 9 and totally reflected by the reference surface 10'a. The element 16b receives the light, and the element 16b becomes the first half mirror.
9, the reference surface 10'b and the test surface 10''
A beam of light that is reflected from the
16c receives the light beam Lc that is reflected by the reference surface 13'C and the test surface 13'', travels along the same path, is wavefront-combined, and is refracted by 90 degrees by the second half mirror 12, and 16d
receives a light beam L4 that is reflected by the reference surface 13'd and the test surface 13#, travels along the same path, is wavefront-combined, and is refracted by 90 degrees by the second half mirror 12, and
, 16c, and 16d are output to an arithmetic processing means 18, which will be described later. Here, the luminous flux L□ does not receive any interference, so it remains the same as the luminous flux emitted from the laser beam, and the luminous flux LC (!: Ld is provided with a step 13'P and a portion 13'c of the reference surface with different thickness. 13'd
They have a phase difference of 90°.

18 is an arithmetic processing means to which the signal detected by the photodetecting element 16 is input, and from the detected signal, a corresponding light intensity signal is obtained.
Ib, I. , the signal processing circuit 18a that calculates Id and the normalization of the light intensity signal, that is, 1. /1. .

Ie/1. ,I,/■, and from 1,/I□, which is one of the normalized signals, interferometer 1 for wavelength fluctuation control
Fixed distance d between the reference surface 10' and the test surface 10'' at 0
A signal calculation circuit 18b is provided which calculates the wavelength fluctuation of the currently emitted laser beam and also performs displacement detection and direction determination from the normalized signals Ie/L and I+/L.

20 is the normalized signal 1 calculated by the arithmetic processing means 18;
This is an injection current control circuit for stabilizing the wavelength of laser light by controlling the injection current to the semiconductor laser 1 based on the wavelength fluctuation of the semiconductor laser 1 determined from h/I-.

21 is the test surface 13 calculated by the arithmetic processing means 18;
This is an indicator for displaying the amount of displacement and its direction.

Next, the operation based on the above configuration of the present invention will be explained.

The laser beam emitted from the semiconductor laser 1 is converted into parallel light by a lens 8, and a part of it is converted into a parallel beam by a first half mirror 9.
The light is refracted and guided to the interference means 10 for wavelength fluctuation control. The light flux reflected by the reference surface 10' and the test surface 10'',
are received by the photodetecting elements 16a and 16b, respectively, and the detection signals are processed by the signal processing circuit 18a to obtain a light intensity signal of 11°. Here, the light intensity signal 11 is the intensity of the light currently emitted from the light source, since the light beam L1 is not subjected to any interference. First half mirror 9
The light that has passed through passes through the second half mirror 12 and is guided to the displacement detection interference means 13. It is reflected by the reference surface 13' and the test surface 13'' and is reflected by the second half mirror 12 at 90
'ff Light flux Lc that is bent, proceeds along the same path, and undergoes wavefront synthesis
, La are received by the photodetecting elements 16c and 16d, and the detection signals are processed by the signal processing circuit 18a, resulting in a light intensity signal (2). , Ia are obtained. Since the luminous fluxes Lc and La have a phase difference of 90' as described above, the light intensity signal (2) is obtained.

and I have a phase difference of 90°. The above four light intensity signals obtained in this way 1. From I to I, the signal Ib/T, , IC/I is standardized by the signal calculation circuit 18b.
, , Ia/T, and based on the normalized signal, the variation in the wavelength of the laser light currently emitted by the semiconductor laser 1 is calculated, the displacement ΔX and the displacement direction of the test surface 13'' are determined, and the displacement ΔX and the displacement direction are displayed on the display 21.

In the present invention, a wavelength variation control interference means 10 is provided in the conventional optical displacement meter, and the wavelength variation of the semiconductor laser 1 is directly monitored, and the wavelength variation of the semiconductor laser 1 is controlled according to the directly monitored wavelength variation. By controlling the injection current, the wavelength of the emitted laser light is made constant.

If the wavelength of the laser light emitted from the semiconductor laser 1 is constant, the standardized signal 1b/1. becomes constant. Also, when the wavelength of the laser light changes due to temperature and other factors, the standardized signal I.

/ 1. also changes. Therefore, the standardized signal T.

/1. If the injection current to the semiconductor laser 1 is controlled by the injection current control means 20 so that the wavelength is constant, the wavelength of the laser light will be stabilized.

Here, in order to improve the accuracy of wavelength stabilization, it is advisable to set the interval d to a position where the slope of Ib/I- due to a change in d is maximum. In other words, as shown in FIG. 2, if we compare the point M where the slope is minimum and the point N where the slope is maximum, we find that for the same wavelength fluctuation @Il, Ib/1. The change in is large at point N. Therefore, the sensitivity to wavelength fluctuation is high at the N point,
You can choose this point.

In the present invention, the optical intensity signal Ib is normalized by (1) because when the current injected into the semiconductor laser 1 is changed, the optical intensity signal (2) emitted from the laser changes. Therefore, the light intensity signal 1. which is a component of the light intensity signal 1b. This is because the optical intensity signal Ih also changes due to the change in the optical intensity signal Ih, and it is not possible to extract the influence only related to the wavelength fluctuation using the optical intensity signal 1h alone.

Similarly, the light intensity signal from the displacement detection interference means 13 is
. , Id4:)1. and the standardized signal IC/M,
and Id/1. The arithmetic processing means 18 calculates the displacement and determines the direction of displacement using the . Since the determination of the displacement direction and the determination of the displacement direction are the same as in the conventional example, the description thereof will be omitted.

On the other hand, temperature detection element 2. The operation of configuration 1 of the arithmetic circuit 3a, drive circuit 3b, and Peltier element 4 is the same as that of the conventional example, and a description thereof will be omitted. The package temperature of the semiconductor laser 1 is controlled by the temperature control means constituted by these components.

The present embodiment is configured as described above, and in order to stabilize the wavelength of the laser beam that serves as a reference for length measurement, on the other hand, a temperature control means is used to roughly control the temperature of the package of the semiconductor laser 1 to be constant. and on the other hand, interference means 1 for wavelength fluctuation control.
0 directly monitors the variation in the wavelength of the laser light currently emitted from the light source, and uses a signal that is standardized based on the intensity signal of the actually emitted light, that is, includes correction for the variation, Since the injection current control means 20 performs "fine" control to keep the wavelength of the semiconductor laser 1 constant, it is possible to perform control that includes temperature and all wavelength fluctuation factors other than temperature. Therefore, wavelength stability can be further improved.

In addition, in the present invention, it is also possible to employ various laser light sources other than semiconductor lasers as the light source having coherence. Further, the optical system used in the above embodiment has a basic configuration, and it is also possible to add or replace other optical systems.

〔Effect of the invention〕

According to the present invention, the wavelength fluctuation of the light emitted from the light source is directly obtained from the actually emitted light, and the light source is determined by a normalized signal based on the light intensity signal obtained from the actually emitted light. Since the wavelength is controlled, it is possible to perform control that includes all wavelength fluctuation factors, and the stability of the wavelength can be further improved. Furthermore, in addition to temperature control, injection current control is performed using a normalized signal that also takes into account the wavelength fluctuation factors, so that control time lag can be reduced. Therefore, it is possible to improve measurement accuracy when measuring displacement.

[Brief explanation of drawings]

Fig. 1 is a configuration diagram of an embodiment of the present invention, Fig. 2 is a diagram explaining the sensitivity of the interference means for wavelength fluctuation control of the present invention, and Fig. 3 is a photodetector when the surface to be detected is displaced. Figure 4 shows the two output signals given the phase difference of the photodetector when the test surface is displaced. Figure 5 shows the temperature-oscillation wavelength characteristics of the semiconductor laser. FIG. 6 is a diagram showing an example of the configuration of a conventional optical displacement meter. ■... Semiconductor laser, 10... Interfering means for wavelength fluctuation control, 13... Interfering means for displacement detection, 18... Arithmetic processing means, 20... Injection current controlling means.

Claims (1)

[Scope of Claims] A light source having coherence and a single wavelength, an interference means for controlling wavelength fluctuation for directly measuring wavelength fluctuation of the light source, the interference means for controlling wavelength fluctuation, and a reference surface and an object. Detecting a light intensity signal from a displacement detection interference means for obtaining reflected interference light from the test surface, normalizing the detected light intensity signal, and calculating the displacement of the test surface based on the light intensity signal. and injection current control means for controlling the current injected into the light source based on the light intensity signal standardized by the calculation processing means, the wavelength variation control of the light emitted from the light source is provided. An optical displacement meter that is characterized by the ability to: