JPH06201316A - Interferometer - Google Patents

Abstract

PURPOSE:To provide an interferometer for which the measurement accuracy of the optical path difference from two reflection parts, is improved. CONSTITUTION:An interferometer is provided with a semiconductor laser element 1 for radiating laser beam to two reflection parts 2a, 2b, and a phase difference measurement means P for measuring the phase difference of the reflection lights from the reflection parts 2a, 2b, and the optical path difference of the two reflection light is measured based on the measurement information of the phase difference measurement means P. In the interferometer, a frequency changing means F for changing the oscillating frequency by changing the temperature of the semiconductor laser element 1, is provided, and the amount of the change in the ratio of the laser beam of the optical path difference to the frequency by means of the change in the oscillating frequency, is determined based on the measurement information of the phase difference measurement means P. An approximate value of the optical path difference is determined based on the amount of change. The integer part of the ratio is determined based on the approximate value of the optical path difference, and the decimal parts of the ratio is determined based on the measurement information of the phase difference measurement means P, to determine the optical path difference.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser device for irradiating two reflecting portions with laser light, and a phase difference measuring means for measuring the phase difference between the two reflected lights from the two reflecting portions. And an interferometer for measuring the optical path difference between the two reflected lights based on the measurement information of the phase difference measuring means.

[0002]

2. Description of the Related Art An interferometer of this type is used to find the optical path difference between two reflected lights and to find the distance between two reflecting portions. The general method for obtaining the optical path difference is as follows. If the optical path difference to be obtained is D, the optical path difference D
Is represented by D = (c / ν) · (N + ε) (1) c: speed of light, ν: oscillation frequency of laser light emitted from the semiconductor laser device. In the above formula (1), N + ε means the ratio of the optical path difference D to the wavelength of the laser light, N is the integer part of the ratio,
ε is the fractional part of the ratio. Although ε can be obtained by the phase difference measuring means, the value of N cannot be directly determined.

Therefore, when the oscillation frequency of the laser light is changed by Δν, the above equation (1) can be expressed as D = (c / Δν) · (ΔN + Δε) (2). In the above equation (2), ΔN and Δε are
The change amounts of N and ε when the oscillation frequency of the laser light is changed by Δν are shown respectively. This ΔN + Δε can be obtained by measuring the phase difference between the two reflected lights when the oscillation frequency is changed by Δν. That is, the integer part of the amount of change in the phase difference between the two reflected lights divided by 2π is ΔN.
And the part after the decimal point corresponds to Δε.
When ΔN + Δε is obtained, the optical path difference D can be obtained by the above equation (2).

As an interferometer for obtaining an optical path difference by such a method, there has been an interferometer having a configuration for obtaining an optical path difference by changing the oscillation frequency of the semiconductor laser device by changing the drive current.

[0005]

However, in the interferometer of the above-mentioned conventional structure, the variation width of the oscillation frequency due to the variation of the driving current of the semiconductor laser element is as small as about 10 to 50 GHz, and therefore the above (2) Even if the optical path difference is calculated by the formula, the accuracy is about several μm to several mm, and further improvement in measurement accuracy has been desired. The present invention has been made in view of the above circumstances, and an object thereof is to provide an interferometer with improved measurement accuracy of optical path difference.

[0006]

An interferometer of the present invention measures a phase difference between a semiconductor laser device that irradiates two reflecting portions with laser light and two reflected light beams from the two reflecting portions. And a phase difference measuring unit for measuring the optical path difference between the two reflected lights based on the measurement information of the phase difference measuring unit, wherein the first characteristic configuration is to measure the temperature of the semiconductor laser device. The frequency difference changing means for changing the oscillation frequency by changing it, and the change amount of the ratio of the optical path difference to the wavelength of the laser light, which changes with the change of the oscillation frequency of the semiconductor laser element, are measured by the phase difference measurement. Change amount detection means obtained based on the measurement information of the means, an approximate value determination means for obtaining an approximate value of the optical path difference based on the change amount, and a laser beam of the optical path difference based on the approximate value of the optical path difference Wave of An integer part determining means for determining an integer part of the ratio, and, based on the measurement information of the phase difference measuring means, a fractional part determining means for obtaining the part after the decimal point of the ratio of the optical path difference to the wavelength of the laser light. The optical path difference is obtained based on the determination information of the integer part determining means and the fractional part determining means.

According to a second characteristic configuration, in the interferometer of the first characteristic configuration, a part of the emitted light of the semiconductor laser device is made incident on one or a plurality of Fabry-Perot etalons having different free spectral regions, and the Fabry-Perot is used. Based on the transmitted light of the etalon, or based on the transmitted light of the Fabry-Perot etalon and the information of the frequency change amount due to the mode jump of the semiconductor laser element that is previously obtained, the semiconductor laser that the frequency changing means changes The point is that a change width measuring means for measuring the change width of the oscillation frequency of the element is provided.

A third characteristic structure is that the interferometer of the first characteristic structure or the second characteristic structure is provided with a laser output adjusting means for adjusting the amount of light emitted from the semiconductor laser element to be constant.

[0009]

According to the first characteristic construction of the present invention, when the oscillation frequency of the semiconductor laser device is changed by the frequency changing means, the phase difference between the two reflected lights changes accordingly. This is measured by the phase difference measuring means, and the change amount measuring means
From the measurement result of the phase difference measuring means, the amount of change in the ratio of the optical path difference to the wavelength of the laser light, that is, the amount of change in the phase difference between the two reflected lights is determined. The approximate value determining means determines the approximate value of the optical path difference from the above equation (2) based on the amount of change thus obtained.

When the approximate value of the optical path difference is obtained in this way, the integer part determining means can determine the integer part of the ratio of the optical path difference to the wavelength of the laser light from the above equation (1). Further, the fractional part determining means can determine the part below the decimal point of the ratio of the optical path difference to the wavelength of the laser light from the measurement result of the phase difference measuring means. If the integer part and the part after the decimal point of the ratio of the optical path difference to the wavelength of the laser light can be determined, the optical path difference between the two reflected lights can be obtained from the above equation (1).

In determining the optical path difference as described above, in order to determine the integer part of the ratio of the optical path difference to the wavelength of the laser light from the approximate value of the optical path difference, the phase difference is calculated with high accuracy. Since it can be measured, the error between the approximate value and the true value of the optical path difference needs to be within 1/2 of the wavelength of the laser light. The accuracy of the approximate value of the optical path difference determined by the approximate value determining means largely depends on the magnitude of the change width of the oscillation frequency of the semiconductor laser element, but the oscillation frequency obtained by changing the temperature of the semiconductor laser element. The change width of the laser light is sufficiently larger than the change width of the oscillation frequency obtained by changing the drive current of the semiconductor laser device, and the accuracy of the approximate value of the optical path difference can be improved. The integer part of the ratio of to the wavelength can be determined.

As a result, as the measurement error of the optical path difference, only the error in the part after the decimal point of the ratio of the optical path difference to the wavelength of the laser beam, that is, the measurement error of the phase difference remains, and for example, oscillation of about 200 to 400 THz occurs. If a semiconductor laser device having a frequency is used, the measurement accuracy of the optical path difference can be set to about several nm.

According to the second characteristic configuration of the present invention, the change width measuring means measures the transmitted light of the Fabry-Perot etalon to measure the change width of the oscillation frequency of the semiconductor laser element changed by the frequency changing means. . The Fabry-Perot etalon has the property of transmitting only light of a specific frequency arranged at constant frequency intervals corresponding to the free spectral range. When they match, the laser light of the semiconductor laser element passes through the Fabry-Perot etalon. Therefore, the variation width of the oscillation frequency of the semiconductor laser element can be measured by counting the number of times the laser light passes through the Fabry-Perot etalon while the oscillation frequency of the semiconductor laser element changes.

Further, in the semiconductor laser device, when the oscillation frequency is changed by the temperature change, the oscillation frequency which has been continuously changed with the temperature change is suddenly and discontinuously changed, which is called a mode jump. It has the property. The change in the oscillation frequency due to this mode jump can be measured in advance, so that even if the oscillation frequency of the semiconductor laser element changes discontinuously due to the mode jump within a predetermined temperature change range, The value of the oscillation frequency after the jump can be estimated to measure the change width of the oscillation frequency.

When the estimated value of the oscillation frequency after the above mode jump has a certain error width, and the estimated value including the error width falls between the transmission frequency of the Fabry-Perot etalon and the adjacent transmission frequency. Can accurately measure the variation width of the oscillation frequency even if the mode jump, but if the estimated value including the error width includes the transmission frequency of the Fabry-Perot etalon, the oscillation frequency after the mode jump Is difficult to pinpoint.

When it is difficult to accurately specify the oscillation frequency after the mode jump, Fabry-Perot etalons having different free spectrum regions are used in parallel and similarly measured. Since the free spectral range is different, the estimated value including the error width can be set to fall between the transmission frequency of the Fabry-Perot etalon and the adjacent transmission frequency, and as a result, the variation width of the oscillation frequency can be accurately measured. It can be measured.

According to the third characteristic configuration of the present invention, the semiconductor laser device has a property that the amount of emitted light changes with a change in temperature under a constant drive current. During the measurement of the difference, the amount of light emitted from the semiconductor laser device is kept constant.

[0018]

According to the first characteristic structure, as described above, it is possible to measure the optical path difference between two reflected lights with high accuracy, and to provide an interferometer with improved optical path difference measurement accuracy. It has arrived. According to the second characteristic configuration, the first
In addition to the effect of the characteristic configuration, it is possible to accurately measure the variation width of the oscillation frequency of the semiconductor laser device which is changed by the frequency changing means. According to the third characteristic configuration, in addition to the first characteristic configuration or the second characteristic configuration, the emitted light amount of the semiconductor laser device is kept constant, so that the accuracy of measuring the phase difference by the phase difference measuring means is improved. be able to.

[0019]

Embodiments of the present invention will be described below with reference to the drawings. In the schematic configuration diagram of the interferometer shown in FIG. 1, light emitted from the semiconductor laser device 1 is irradiated on two reflecting portions, that is, an upper surface 2a and a lower surface 2b of a sample 2 having a step, and the light is emitted from each of them. The reflected light enters the photodetectors 3 and 4, respectively, and the phase difference between the signal components included in the photodetector 3 and the photodetector 4 is measured by the phase meter 5. The output signal of the phase meter 5 is input to the control device 6 and used to measure the optical path difference between the laser light reflected by the upper surface 2a of the sample 2 and the laser light reflected by the lower surface 2b of the sample 2. To be done.

The specific structure of the interferometer will be described in detail below. The semiconductor laser device 1 is arranged on a Peltier device 10, and the Peltier device 10 is controlled by a temperature controller 11. Further, the semiconductor laser device 1 is supplied with a drive current required for laser oscillation from the LD current source 12. When a driving current is supplied from the LD current source 12 and the semiconductor laser element 1 oscillates, the emitted laser light is collimated by the collimator lens 13 and enters the first polarization beam splitter 14. The first polarization beam splitter 14 is arranged so as to split an incident laser beam into two beams whose polarization directions are orthogonal to each other. One of the split laser beams goes straight and the other splits.

The laser light traveling straight through the first polarization beam splitter 14 is incident on the first acousto-optic element 15 and has a frequency of several tens of MH.
It is subject to frequency deviation of about z. The laser light that has passed through the first acousto-optic element 15 enters the first beam expander 16, the beam diameter of which is expanded, and enters the second polarization beam splitter 17. The second polarization beam splitter 17 is arranged so as to cause the laser light, which has traveled straight through the first polarization beam splitter 14, to travel straight, and split the laser light having a polarization direction perpendicular to the polarization direction of the laser light. The laser light that enters the polarization beam splitter 17 and travels straight further passes through the quarter-wave plate 18 and is irradiated on the sample 2. The laser light applied to the sample 2 is reflected by the upper surface 2 a and the lower surface 2 b of the sample 2, respectively, and passes through the quarter-wave plate 18 again. Therefore, the laser light is 1 /
Since the light passes through the four-wave plate 18 twice, the polarization direction is rotated by 90 degrees and is split by the second polarization beam splitter 17.

The laser light reflected by the sample 2 and split by the second polarization beam splitter 17 passes through the half-wave plate 19 and has its polarization direction rotated by 90 degrees. As a result, the polarization direction of the laser light goes straight through the first polarization beam splitter 14 and returns to the polarization direction at the time point. The laser light that has passed through the half-wave plate 19 enters the third polarization beam splitter 20. On the other hand, the laser light split by the first polarization beam splitter 14 has its optical path bent by the reflecting mirror 21,
It is incident on the second acousto-optic element 22, and the first acousto-optic element 15
Due to the frequency deviation of several tens of kHz. The laser beam that has passed through the second acousto-optic element 22 is reflected by the beam splitter 23 into a second beam expander 24.
It is separated into the one that goes to the Fabry-Perot etalon 25.

The laser light that has passed straight through the beam splitter 23 and has entered the second beam expander 24 has its beam diameter enlarged and enters the third polarization beam splitter 20. The third polarization polarization beam splitter 20 is arranged so that the laser light incident from the side of the half-wave plate 19 is made to go straight and the laser light incident from the side of the second beam expander 24 is branched. Laser light entering from these two directions exits the third polarization beam splitter 20 in the same direction.

The laser light emitted from the polarization beam splitter 20 is a mixture of the laser light from the half-wave plate 19 side and the laser light from the second beam expander 24 side in which the polarization directions are different by 90 degrees. However, when it passes through the polarizer 26, the polarizer 26 is arranged so as to transmit a component of the polarization direction of a direction that makes an angle of 45 degrees with each polarization direction of the mixed laser light. Therefore, the laser beams emitted from the deflector 26 have the same polarization direction and thus interfere with each other. As a result, the polarizer 26
Of the laser light emitted from the sample 2, a photodetector 3 that receives a portion of the reflected light from the upper surface 2a of the sample 2 and a photodetector 4 that receives a portion of the reflected light from the surface 2b to the lower side. Outputs a so-called heterodyne signal which is a signal having a frequency difference between the frequency deviation caused by the first acousto-optic element 15 and the frequency deviation caused by the second acousto-optic element 22.

The phase difference between the heterodyne signals of the photodetector 3 and the photodetector 4 measured by the phase meter 5 is the reflected light from the upper surface 2a of the sample 2 and the lower surface 2b of the surface.
It directly reflects the phase difference from the reflected light from. still,
The frequency difference between the frequency deviation due to the first acousto-optic element 15 and the frequency deviation due to the second acousto-optic element 22 is several tens kHz.
This is a degree and can be easily detected by using, for example, an APD or the like as the photodetectors 3 and 4.

The laser beam branched to the Fabry-Perot etalon 25 side by the beam splitter 23 passes through the Fabry-Perot etalon 25 and enters the photodetector 27. The Fabry-Perot etalon 25 has a characteristic of transmitting only light of a specific frequency arranged at constant frequency intervals corresponding to the free spectral range, and only when the frequency of the laser light coincides with the specific frequency, The detector 27 receives the laser. The output signal of the photodetector 27 is input to the control device 6 and is used to measure the change in the oscillation frequency of the semiconductor laser device 1, and the drive current control circuit 2 is used.
8 is input. The drive current control circuit 28 includes the photodetector 2
LD current source 12 so that the output signal of 7 maintains a constant value.
Control the output current of. As a result, the semiconductor laser device 1
The amount of emitted light is kept substantially constant.

The operation of the interferometer having the above structure will be described below. The temperature of the semiconductor laser device 1 is controlled by the temperature controller 11 to drive the Peltier device 10 to change it in a fixed direction of rising or falling. Along with this, the oscillation frequency of the semiconductor laser element 1 changes, and the wavelength of the laser light having an optical path difference between the laser light reflected on the upper surface 2a of the sample 2 and the laser light reflected on the lower surface 2b of the sample 2. The ratio to The amount of change in the ratio is obtained by the control device 6 from the amount of change in the phase difference detected by the phase meter 5. That is, the amount of change in the phase difference divided by 2π is the amount of change in the ratio of the optical path difference to the wavelength of the laser light.

The control device 6 obtains the semiconductor laser device obtained by counting the amount of change in the ratio thus obtained and the pulsed signal output from the photodetector 27 which detects the transmitted light of the Fabry-Perot etalon 25. An approximate value of the optical path difference is obtained from the change width of the oscillation frequency of 1. At this time, the oscillation frequency of the semiconductor laser device 1 may change discontinuously due to so-called mode jump, but in this case, the phase difference measured by the phase meter 5 also changes discontinuously, and It is not possible to obtain the variation width of the oscillation frequency from the count value of the pulsed signal output from the photodetector 27. Therefore, it is necessary to correct the change in the oscillation frequency due to the mode jump, but this correction is performed by obtaining the change in the oscillation frequency due to the mode jump in advance.

With respect to the discontinuous change in the phase difference, the ratio of the change amount of the phase difference to the change amount of the oscillation frequency in the portion where the oscillation frequency changes continuously and the phase difference after the mode jump From this, the amount of change in the phase difference due to the mode jump is calculated and corrected. Further, the variation width of the oscillation frequency is corrected by converting the variation of the oscillation frequency due to the mode jump into the count value of the pulsed signal of the photodetector 27 and adding it to the measured count value. .

Next, the control device 6 determines the integer part of the ratio of the optical path difference to the wavelength of the laser light by using the approximate value of the optical path difference obtained as described above. That is, the fractional part of the ratio of the optical path difference to the wavelength of the laser light is
Since it is found by dividing the phase difference detected by the phase meter 5 by 2π, the integer part can be determined if the error between the approximate value and the true value of the optical path difference is within 1/2 of the wavelength of the laser light. Of.

As a result, the control device 6 accurately obtains the ratio of the optical path difference to the wavelength of the laser light. From the product of this value and the wavelength of the laser light emitted from the semiconductor laser element 1, the upper stage of the sample 2 is determined. The optical path difference between the laser light reflected on the side surface 2a and the laser light reflected on the lower side surface 2b can be accurately obtained.

Therefore, the photodetectors 3 and 4 and the phase meter 5 are
The Peltier element 10 and the temperature controller 11 function as phase difference measuring means P for measuring the phase difference between the laser light reflected on the upper surface 2a of the sample 2 and the laser light reflected on the lower surface 2b. Functions as frequency changing means F for changing the oscillation frequency of the semiconductor laser device 1, and the controller 6
Is a change amount detecting means D for detecting a change amount of the ratio of the optical path difference to the wavelength of the laser light, a rough value determining means A for obtaining a rough value of the optical path difference, and an integer of the ratio of the optical path difference to the wavelength of the laser light. It functions as an integer part determining means I for determining a part and a fractional part determining means U for determining a part below the decimal point of the ratio of the optical path difference to the wavelength of the laser light. In addition, Fabry-Perot etalon 25, photodetector 27,
Further, the control device 6 functions as a change width measuring unit W that measures the change width of the oscillation frequency of the semiconductor laser element 1, and the drive current control circuit 28 and the LD current source 12 function as a laser output adjusting unit O.

[Other Embodiments] Other embodiments will be listed below. In the above embodiment, the phase difference between the laser light reflected on the upper surface 2a of the sample 2 and the laser light reflected on the lower surface 2b of the sample 2 is obtained by the so-called heterodyne method. The phase difference may be obtained by directly interfering the laser light reflected by the side surface 2a with the laser light reflected by the lower surface 2b and directly observing the interference pattern.

In the above embodiment, the semiconductor laser device 1
If the oscillation frequency of changes discontinuously due to the mode jump, it is corrected by measuring the variation width of the oscillation frequency due to the mode jump in advance.
To identify the jump destination frequency from the variation width of the oscillation frequency due to the mode jump, and to cope with the case where the jump destination frequency is close to the transmission frequency of the Fabry-Perot etalon 25 and the jump destination cannot be accurately identified. It is also possible to provide a plurality of Fabry-Perot etalons 25 having different free spectral regions, perform the same measurement, and specify the jump destination accurately.

In the above embodiment, the drive current control circuit 2
Reference numeral 8 controls the output current of the LD current source 12 so that the output signal of the photodetector 27 maintains a constant value. A transmission amount variable filter capable of driving a motor is installed in the optical path to adjust the light amount. You may.

It should be noted that reference numerals are given in the claims for convenience of comparison with the drawings, but the present invention is not limited to the structures of the accompanying drawings by the entry.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of an interferometer according to an embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Semiconductor laser element 2a, 2b Reflecting part A Approximate value determining means D Change amount detecting means F Frequency changing means I Integer part determining means P Phase difference measuring means U Fraction part determining means

Continued Front Page (72) Inventor Masami Yamamoto 1-1-1 Hama, Amagasaki City, Hyogo Prefecture Kubota Technology Development Laboratory

Claims (3)

[Claims] 1. A semiconductor laser device (1) for irradiating two reflection parts (2a, 2b) with laser light, and a phase difference between two reflection lights from the two reflection parts (2a, 2b). An interferometer for measuring the optical path difference between the two reflected lights based on the measurement information of the phase difference measuring means (P), the semiconductor laser device comprising: Frequency changing means (F) for changing the oscillation frequency of the semiconductor laser element (1) by changing the temperature of 1), and the wavelength of the laser light of the optical path difference that changes with the change of the oscillation frequency of the semiconductor laser element (1). Change amount detecting means (D) for obtaining the change amount of the ratio based on the measurement information of the phase difference measuring means (P), and an approximate value determining means (for obtaining an approximate value of the optical path difference based on the change amount). A) and the approximate value of the optical path difference Then, based on the measurement information of the integer part determining means (I) for determining the integer part of the ratio of the optical path difference to the wavelength of the laser light, and the measurement information of the phase difference measuring means (P), A fractional part determining means (U) for obtaining a fractional part of the ratio to the wavelength, and for obtaining the optical path difference based on the determination information of the integer part determining means (I) and the fractional part determining means (U). An interferometer configured to. 2. A part of the emitted light of the semiconductor laser device (1) is made incident on one or a plurality of Fabry-Perot etalons having different free spectrum regions, and based on the transmitted light of the Fabry-Perot etalon, or Based on the transmitted light of the Fabry-Perot etalon and the information of the amount of frequency change due to the mode jump of the semiconductor laser device (1) which is obtained in advance, the frequency changing means (F) changes the semiconductor laser device (1). The interferometer according to claim 1, further comprising change width measuring means (W) for measuring a change width of the oscillation frequency. 3. The interferometer according to claim 1, further comprising laser output adjusting means (O) for adjusting the amount of light emitted from the semiconductor laser element (1) to be constant.