JPH0219701A - Light interference measuring instrument - Google Patents

Abstract

PURPOSE:To exactly execute the length measurement by irradiating plural light reflecting means by a laser light, allowing its reflected light beam to interfere and forming an interference fringe. CONSTITUTION:A divergent light beam emitted from an LD element 3 is made incident on a beam splitter 5, and divided into two beams. A beam reflected by the beam splitter 5 is received by a light receiving element 7, a light intensity variation signal of a laser light source is obtained, and this signal 9 is inputted to a dividing circuit 17. On the other hand, a laser beam which has transmitted through the beam splitter 5 is led into an interferometer 19, and this laser beam is emitted in the direction being orthogonal to the incident direction. The laser beam which has been emitted from the interferometer 19 is received by a light receiving element 14, the signal is amplified by an amplifier 15, and a fringe variation signal 16 which is formed in accordance with prescribed optical path length of a fixed mirror 11 and a movable mirror 12 is obtained. Two signals which have been obtained in such a way are inputted to the circuit 17, the signal 16 is divided by the signal 9, and by inputting its output to a computer 18, an analysis of the signal is executed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical interference measuring device, and particularly to an optical interference measuring device that measures length by observing interference fringes obtained using a semiconductor laser as a light source.

[Prior Art] Semiconductor lasers (hereinafter referred to as LDs) are simpler, cheaper, smaller, and lighter in device configuration than gas lasers, etc., and are widely used as light sources for optical communication, audio, and video optical discs, etc. . In addition, applications to light sources for optical interferometers have recently been actively researched.

In particular, interferometric length measurement devices that measure the optical path length to two reflecting members through observation of interference fringes are known, which utilize the oscillation wavelength characteristics that depend on the injection current or device temperature of a semiconductor laser device. .

In this type of device, the oscillation light of a semiconductor laser is incident on an interferometer with a certain optical path difference to form interference fringes, and wavelength scanning is performed by controlling the injection current or element temperature. Time change signal (hereinafter referred to as fringe change signal)
is detected, and the optical path difference of the interferometer is determined from the amount of phase change or frequency of this signal.

[Problem to be solved by the invention] The number of cycles of the sinusoidal milling signal obtained with the conventional structure as described above is too small, and it is difficult to obtain sufficient measurement accuracy when using a fringe counting method or the like. Ta.

In addition, in the above configuration, wavelength scanning is performed by changing the current injected into the semiconductor laser element, but since the output light intensity changes along with the oscillation wavelength due to the change in the injection current, the detected polishing signal does not reflect the interference pattern. It contains information regarding the change in the intensity of the light source, as well as information about the intensity of the output light from the light source.

For example, when the injection current i of the semiconductor laser device is changed in the form of a triangular wave with a period T as shown in FIG. It will look like Figure 6. When the DC offset of the signal changes in this way, it is impossible to assume a constant zero level, so frequency measurement using the zero-crossing method is impossible. It is also possible to use an electrical filter to remove the DC offset, but
In this case, there are problems in that a signal phase shift occurs and the cutoff frequency of the filter must be constantly adjusted in accordance with the frequency of the signal under test, which is not practical.

The object of the present invention is to solve the above problems, correct the influence of light intensity accompanying wavelength scanning in interferometric length measurement using a semiconductor laser element, and accurately measure phase change by performing frequency measurement using zero official law. It is to be able to carry out long-term care.

[Means for Solving the Problems] In order to solve the above problems, in the present invention, a plurality of light reflecting means are irradiated with laser light generated by a semiconductor laser element, and the reflected light from these reflecting means is Interference is caused to form interference fringes, and the wavelength of the laser light is scanned by periodically changing the injection current of the semiconductor laser element, and the frequency of the interference fringe intensity change signal that changes with time of the interference fringes as the wavelength of the laser light changes. In an optical interference measurement device that measures the optical path difference between the reflecting means based on the frequency, the optical interference measuring device includes a means for dividing a part of the luminous flux of the laser beam, and a time of the intensity of the luminous flux divided by the dividing means. means for measuring the change in intensity; means for dividing the interference fringe intensity change signal by the light intensity change signal obtained by the measuring means; and measuring the frequency of the output signal of the dividing means through zero-crossing interval detection. A configuration is adopted in which a control means is provided for calculating the optical path difference based on the detected frequency.

[Function] According to the above configuration, when calculating an optical path difference, the influence of a change in light intensity due to wavelength scanning of a semiconductor laser element can be corrected by the above-mentioned division process.

[Example] Hereinafter, the present invention will be described in detail based on the example shown in the drawings.

FIG. 1 shows the configuration of an interference measuring device employing the present invention.

In Figure 1, the laser light source is a single longitudinal mode oscillation LD.
The element 3 receives temperature control from an ATM (temperature control circuit) 2. The ATM 2 controls the temperature of the LD element 3 to a desired constant value. The control temperature value is determined by computer 18.

Further, the drive current of the LD element 3 is controlled by the LD drive circuit 1, and the oscillation wavelength of the LD element 3 is adjusted by changing the drive current. In the LD element 3, the refractive index of the waveguide changes due to a change in the injection current, and the oscillation wavelength changes.

The diverging light emitted from the LD element 3 is transmitted through a coma lens 4.
The light beams are made parallel and incident on the beam splitter 5, where they are split into two beams.

The light reflected by the beam splitter 5 is received by a light receiving element 7 via a light amount adjustment filter 6, and the signal is amplified by a variable gain amplifier 8 to obtain a light intensity change signal 9 of the laser light source. This light intensity change signal 9 is input to a division circuit 17.

On the other hand, the laser beam transmitted through the beam splitter 5 is guided into the interferometer 19. Here, a Michelson type interferometer is illustrated as the interferometer 19.

The laser beam incident on the interferometer 19 passes through the beam splitter 10
The light is divided into two beams. The two light beams are reflected by a fixed mirror 11 and a movable mirror 12 with optical path differences, respectively.
The beams are combined into one again by the beam splitter 10, interfere with each other, and are emitted in a direction perpendicular to the direction of incidence.

The laser beam emitted from the interferometer passes through the light amount adjustment filter 13 and is received by the light receiving element 14, and the signal is amplified by the amplifier 15. Interference is formed according to the predetermined optical path length of the fixed mirror 11 and the movable mirror 12. A striped intensity change signal (refined signal) 16 is obtained.

The two signals obtained in this way are input to the division circuit 17, the refined signal 16 is divided by the light intensity change signal 9, and the output is input to the computer 18 for signal analysis.

The division circuit 17 is constructed from an analog circuit, and the computer 18 is constructed from a computer system comprising a microprocessor, memory, and the like.

Next, the operation of the above configuration will be explained in detail. First, length measurement on the interferometer 19 side will be explained.

The reflected light from the fixed mirror 11 and the reflected light from the movable mirror 12 obtained by inputting a laser beam of wavelength λ0 into the interferometer 19 are expressed by the following equations, respectively, where L is the optical path difference.

However, A and B are constants and φ. is the initial phase.The interference fringes obtained by interfering these two reflected lights are expressed by the following equation.

FIG. 2 shows a graph in which 11 is plotted on the vertical axis and L is plotted on the horizontal axis based on equation (3).

For example, here L is 0 to 4λ. If you change it up to 4
Interference fringes corresponding to the period can be obtained. This is N,=L/λ. ;
4λ0/λo=4.

Here, in FIG. 3, the wavelength is λ1 = 2λ. The graph below shows the result. In this case, even if L is changed from 0 to 4λ0, only two cycles of interference fringes can be obtained. This is shown as N,=L/λ1=2λ1/λ1=2.

As is clear from Figures 2 and 3, now L-4λ. When the wavelength of the laser beam is kept constant and changed from λ0 to λ1, the interference fringes change by two periods with n=N, -N, and =2. Since this fringe change n depends on the wavelength change and the optical path difference, the optical path difference can be determined by determining the fringe change and the wavelength change. These relationships are given by the following equation.

In the present invention, a single longitudinal mode oscillation semiconductor laser is used as a variable wavelength coherent light source. A typical injection current-oscillation wavelength characteristic of a single longitudinal mode oscillation semiconductor laser is as shown in FIG. Although the linear wavelength tuning range is limited by mode hops, there is a linear relationship between the injection current and the oscillation wavelength in the section between mode hops. In the processing described below, this straight line portion is preferably used.

FIG. 5 shows the waveform of the injection current injected into the semiconductor laser. The wavelength is scanned at a constant rate by changing the injection current at a constant rate. The wavelength change rate of the semiconductor laser is K(n
rn/mA), and the oscillation wavelength when the injection current is 10 is λ
. Then, when 10-10+Δi, λ. →λ. Δi to +
becomes.

Substituting this into equation (4) yields the following equation.

The following equation is obtained.

... (5) Furthermore, λ. >>KΔi, so the following equation can be obtained by approximation.

However, ,K, is a constant The intensity of interference fringes formed from these is given by the following equation.

From equation (5) or equation (6), n is Δ11λ. By measuring , the optical path can be determined.

However, as described above, when the injection current of the LD element 3 is changed, the output light intensity changes along with the oscillation wavelength. In order to perform this correction, correction systems indicated by reference numerals 5 to 9.17 are provided.

Here, let the change in the output light intensity of the semiconductor laser light due to the injection current be T(i), the change in the oscillation wavelength be λ(i), and (
Rewriting equations 1) and (2), λ(j,) By dividing equation (9) by 72(i), the following equation is obtained.

From this, it can be seen that by dividing the polished signal 16, which is the change in the intensity of the interference fringes, by the output light intensity 9 of the light source, the influence of the change in the light source output light intensity can be removed from the polished signal 16.

Here, the waveforms of the milling signal 16 and the output light intensity change signal 9 are shown in FIGS. 6 and 7, respectively.

In addition, the dividing circuit 17 generates a milling signal 16 (FIG. 6).
When divided by the output light intensity change signal 9 (FIG. 7), the output signal waveform becomes as shown in FIG. 8.

As is clear from FIG. 8, although some changes in the envelope of the refined signal remain, triangular wave bias changes can be largely removed, making it possible to analyze the frequency using the zero-crossing method.

Here, the method for analyzing the signal frequency after correction shown in FIG. 8 will be explained with reference to the flowchart shown in FIG. 9. 9th
The illustrated procedure shows the processing procedure performed by the computer 18.

In step S1 of FIG. 9, the corrected signal (FIG. 8) obtained from the output of the division circuit 17 is taken into a memory or the like via A/D conversion.

Next, in step S2, an appropriate threshold value corresponding to the zero level is set, and in step S3, the sampling data in the memory is compared with the threshold value, the zero crossing point where the signal intersects is detected, and each adjacent zero crossing point is detected. Find the time interval between points (zero point interval). At this time, where discontinuous points occur due to turning points of the injected current, mode hopping, etc., data at the corresponding zero point interval is discarded.

Thereafter, in step S4, the average of the zero point intervals is calculated from the remaining data and used as a representative value in one period of the triangular wave change of the injected current.Furthermore, in step S5, the signal frequency fs is calculated backward from the period value obtained in step S4. Here, if the frequency of the triangular wave change of the injected current is fd, then (5
), the number n of cycles of interference fringe changes in equation (6) is expressed as n = f s / 2 f d - (12), so this number n of cycles is determined in step S6.

Next, in step S, the optical path difference can be obtained by substituting the period number n into equation (6) using the computer 18 (constants measured in advance are used for λ and K). The obtained optical path differences are sequentially stored in a memory or the like.

The above process is repeated in units of one period of injection current change.

In step S8, it is determined whether a time corresponding to a predetermined multiple of the period 1'' of the injection current change has elapsed or whether a corresponding number of data have been processed. That is,
When the injection current change is repeated for a predetermined number of cycles, the process moves to step S9, where the average value of the optical path differences obtained so far is taken and this average value is output as the final measured value.

As described above, the frequency of the interference fringe change signal can be measured by dividing the laser beam and measuring the frequency, and the length measurement calculation can be performed based on this frequency value.

Therefore, the influence of light intensity during wavelength scanning can be removed, and the measurement accuracy of the interferometer can be improved. In particular, in the above embodiment, since division is performed by an analog circuit, high-speed processing in real time is possible.

Additionally, since the zero-crossover method can be used, the processing system's hardware/software configuration is simpler than methods that use fast Fourier exchange, etc., making it possible to reduce costs or make the equipment smaller and lighter, and the processing time is shorter. It's done.

In addition, in the above, the method of taking the average of the optical path differences was shown at the end, but the optical path difference L, 7
7. Plot the data one cycle at a time between two objects;7
．． , 'l (It is also possible to measure the time change in road difference.

In addition, when calculating the zero point interval in one cycle, first find the average value of the zero point interval, delete the data with the zero point interval that exceeds the predetermined error range with respect to this average value, and then calculate the remaining data again. It is also possible to reduce the variation by repeating the process of taking the average value and deleting the data again using this newly determined average value, and then taking the average value of ΔP when there is no longer any data to be deleted.

Further, as the beam splitter 5.1o shown in FIG. 1, a cube beam splitter, a wedged half mirror, or the like is used. Particularly in a Fizeau interferometer or the like, a polarizing beam splitter may be used as the beam splitter 10, in which case a λ/4 plate is inserted between each reflecting mirror.

There is no light returning to the light source side, the oscillation wavelength of the LD element 3 is stabilized, and accurate measurement is possible.

[Effects of the Invention] As is clear from the above, according to the present invention, a plurality of light reflecting means are irradiated with laser light generated by a semiconductor laser element, and the reflected lights from these reflecting means are caused to interfere with each other to form interference fringes. The wavelength of the laser beam is scanned by periodically changing the injection current of the semiconductor laser element, and the frequency of the interference fringe intensity change signal that changes over time as the wavelength of the laser beam changes is measured. An optical interference measurement device that measures the optical path difference between the reflecting means based on frequency, comprising means for dividing a part of the luminous flux of the laser beam, and means for measuring temporal changes in the intensity of the luminous flux divided by the dividing means. and,
means for dividing the interference fringe intensity change signal by the light intensity change signal obtained by the measuring means; and a means for measuring the frequency of the output signal of the dividing means through zero-crossing interval detection, and based on the measured frequency, the optical path. Since the configuration is equipped with a control means for calculating the difference, when calculating the optical path difference, the effect of light intensity changes due to wavelength scanning of the semiconductor laser element can be corrected by the above-mentioned division process, allowing accurate measurement. become. In particular, it is possible to eliminate changes in the light intensity of the lightening signal due to atmospheric fluctuations in the optical path, unnecessary interference fringes in the optical system of the device, or temperature conditions of the semiconductor laser element.
Measurement accuracy is greatly improved.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of an optical interference measuring device employing the present invention, Figs. 2 and 3 are waveform diagrams showing the characteristics of interference fringes obtained with the device of Fig. 1, and Fig. 4 is a diagram of the LD element. A diagram showing the wavelength characteristics depending on the injection current. Figure 5 is a waveform diagram showing the injection current waveform of the LD element. Figure 6 is a waveform diagram of the pigeon conversion signal. Figure 7 is a diagram showing the waveform of the injection current of the LD element. Figure 8 is a waveform diagram after correction obtained by dividing the light intensity change by the light intensity change, and Figure 9 shows the frequency analysis processing of the signal in Figure 8. It is a flowchart figure. 1...LD drive circuit 2...ATM3...LD
Element 4... Collimating lens 5... Beam splitter 6... Light amount adjustment filter 7... Light receiving element 8... Variable gain amplifier 9...
...Light intensity change signal 10...Beam splitter 11...Fixed mirror 12...Movable mirror 13...
Light amount adjustment filter 14...light receiving element 15...amplifier 16...pigmentation signal 17.
...Division circuit 18...Computer 19...
Interferometer Figure 2 LD Jutsugo's cum &Takefusu's massage Figure 4 〉 〉

Claims (1)

[Claims] 1) Laser light generated by a semiconductor laser element is irradiated onto a plurality of light reflecting means, and the reflected lights from these reflecting means are caused to interfere with each other to form interference fringes, thereby reducing the current injected into the semiconductor laser element. The wavelength of the laser beam is scanned by changing it periodically, and the frequency of the interference fringe intensity change signal that changes with time of the interference fringes as the wavelength of the laser beam changes is measured. Based on this frequency, the optical path difference between the reflecting means is determined. An optical interference measurement device for measuring a part of the light beam of the laser beam, a means for dividing a part of the luminous flux of the laser beam, a means for measuring a temporal change in the intensity of the luminous flux divided by the dividing means, and a light beam obtained by the measuring means. means for dividing the interference fringe intensity variation signal by an intensity variation signal;
An optical interference measuring device comprising: a control means for measuring the frequency of the output signal of the dividing means through zero-crossing interval detection and calculating the optical path difference based on the measured frequency. 2) An optical interference measuring device characterized in that the optical path difference is measured every predetermined measurement cycle, and the optical path difference data is plotted one cycle at a time to measure the time change in the optical path difference between the reflecting means.