JP2000146516A - Laser length measuring instrument - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the influence of the spectrum ray width of a laser diode. SOLUTION: The laser length measuring instrument which uses heterodyne interference is provided with a head 53 which outputs laser light and also outputs a reference signal and an interferometer 4 which outputs interference light by reflecting laser light of one frequency of the laser light by a reference mirror in the interferometer 4 and the laser light of the other frequency by a mirror 5 fitted to a body to be measured respectively. Further, this instrument is provided with a receiver 6 which detects interference light and outputs an interference signal, a signal processing circuit 7 which detects and integrated the phase difference between the interference signal and reference signal to measure the position of the body to be measured, and a low-pass filter circuit 18 which attenuates the high-frequency signal of the output signal 100 of the signal processing circuit 7.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser length measuring device using heterodyne interference, and more particularly to a laser length measuring device capable of suppressing the influence of the spectral line width of a laser diode.

[0002]

2. Description of the Related Art A conventional laser length measuring apparatus using heterodyne interference uses laser beams of two frequencies, and attaches a laser beam of one frequency to a reference mirror and a laser beam of the other frequency to an object to be measured. The light is reflected by the mirrors and interferes.

When the DUT moves, the frequency of the interference light when the DUT moves changes compared to the frequency of the interference light when the DUT stops due to the Doppler effect.

Therefore, by using the frequency of the difference between the two frequencies as a reference signal and detecting the phase difference between this reference signal and the interference light when the device under test moves, the position of the device under test from its initial state is detected. The change, in other words, the position of the measured object can be measured.

FIG. 4 is a structural block diagram showing an example of such a conventional laser length measuring device. In FIG. 4, 1 is a drive circuit, 2 is a laser diode, 3 is a reference signal generating means, 4 is an interferometer, 5 is a mirror attached to an object to be measured, 6 is a receiver, 7 is a signal processing circuit, and 100 is positional information. A signal, 101 is an interference signal, and 102 is a reference signal.
1 to 3 constitute a head 50.

The drive signal of the drive circuit 1 is connected to the drive terminal of the laser diode 2, and the output light of the laser diode 2 enters the interferometer 4 via the reference signal generating means 3.
The laser light of one frequency of the output light of the laser diode 2 is reflected by a reference mirror inside the interferometer 4 and the laser light of the other frequency is reflected by a mirror 5 attached to the object to be measured. Cause interference.

The interference light generated by the interferometer 4 is incident on the receiver 6, the interference signal 101 output from the receiver 6 is connected to the signal processing circuit 7, and the reference signal 102 output from the reference signal generating means 3 is also a signal. It is connected to the processing circuit 7. Then, the signal processing circuit 7 outputs the position information signal 100.

Here, the operation of the conventional example shown in FIG. 4 will be described. The laser diode 2 is driven by a drive signal from the drive circuit 1 and emits laser light of two frequencies from the head 50. The laser beams of these two frequencies are incident on the interferometer 4 via the reference signal generating means 3.

In the interferometer 4, the laser light of one frequency of the output light of the laser diode 2 is reflected by a reference mirror inside the interferometer 4 and the laser light of the other frequency is reflected by a mirror 5 attached to the object to be measured. The interference light is made to interfere in the interferometer 4 and the interference light is emitted to the receiver 6.

[0010] The receiver 6 converts the incident interference light into an electric signal and outputs it as an interference signal 101 to the signal processing circuit 7. On the other hand, the reference signal generating means 3 generates a reference signal 102 from the difference between the two frequencies of the laser light of the laser diode 2 and outputs the reference signal 102 to the signal processing circuit 7.

The signal processing circuit 7 outputs the reference signal 102
By detecting the phase difference between the interference light 101 and the movement of the device under test, the change from the initial state of the device under test, in other words, the position of the device under test can be measured.

FIG. 5 is a block diagram illustrating the configuration of the receiver 6 and the signal processing circuit 7 in detail. 5, reference numerals 100 to 102 denote the same reference numerals as in FIG. 4, 8 denotes a photodetector using a photodiode or the like, 9 and 12 denote amplifiers, 10 and 13 denote filter circuits, 11 and 14 denote comparators, Reference numeral 15 denotes a PLL circuit, 16 denotes a period measuring circuit, 17 denotes an integrating circuit, 103 denotes interference light, and 104 denotes a clock signal. 8 to 11 constitute a receiver 51, and 12 to 17 constitute a signal processing circuit 52, respectively.

The interference light 103 enters the photodetector 8, and the output of the photodetector 8 is connected to the amplifier 9. The output of the amplifier 9 is connected to a comparator 11 via a filter circuit 10,
The output of the comparator 11 is connected to the period measurement circuit 16 as an interference signal 101.

On the other hand, the reference signal 102 is connected to the amplifier 12, and the output of the amplifier 12 is connected to the comparator 14 via the filter circuit 13. The output of the comparator 14 is connected to a PLL circuit 15, and the output of the PLL circuit 15 is connected as a clock signal 104 to a period measurement circuit 16. The output of the cycle measuring circuit 16 is connected to the integrating circuit 17.

Here, the operation of the conventional example shown in FIG. 5 will be briefly described. The interference light 103 is converted into an electric signal by the photodetector 8 and then appropriately amplified by the amplifier 9. The output of the amplifier 9 is input to a comparator 11 via a filter circuit 10 and output as an interference signal 101 which is a binary digital signal.

Similarly, the reference signal 102 is appropriately amplified by the amplifier 12, and the output of the amplifier 12 is input to the comparator 14 via the filter circuit 13 to be binarized and converted into a digital signal. It is multiplied and output as a clock signal 104.

The period measuring circuit 16 counts the period of the interference signal 101 output from the receiver 51 based on the clock signal 104 and, based on the counted value, the reference signal 10.
The phase difference is obtained by subtracting the count value corresponding to the period of 2 and output. The integrating circuit 17 is provided with a period measuring circuit 16.
Are sequentially integrated.

Since this phase difference means the distance that the device under test moves in one cycle of the reference signal, the relative distance from the initial state of the device under test is calculated by successively integrating the movement distances in one cycle. It becomes possible to obtain the position.

As a result, the position of the object to be measured can be obtained by measuring the period of the interference signal 101, obtaining the phase difference from the obtained period, and integrating it.

[0020]

However, in the conventional example shown in FIG. 4, the frequency of the laser light emitted from the laser diode 2 is not constant but is distributed with a certain probability density around one frequency as shown in FIG. are doing.

FIG. 6 is a characteristic curve diagram showing an example of the frequency distribution of the output light of the laser diode 2. As shown in FIG. 6, the frequency of the output light of the laser diode fluctuates with such a probability distribution. become. Also, in FIG.
In the distribution characteristic indicated by LP01 ", the light power is the maximum at the center frequency" f ", and the power is" 1/2 (-3d
The frequency range of "B)" is called a spectral line width, and is as shown by "SW01" in FIG.

In general, the spectral line width of the laser diode 2 is more than 10,000 times larger than the spectral line width of a helium neon gas laser. Therefore, in a laser length measuring apparatus using a laser diode, the influence of the spectral line width appears as jitter. There was a problem.

For this reason, conventionally, a high spectral purity function is added to the driving circuit 1 to reduce the spectral line width to realize output light with a narrow spectral line width as shown in FIG.

FIG. 7 is a characteristic curve diagram showing an example of the frequency distribution of the output light of the laser diode 2 whose spectral line width has been reduced by the drive circuit 1 having a high spectral purification function. As a method of reducing the spectral line width as shown by “SW02” in FIG. 7 in the frequency distribution of “LP02” in FIG. 7, for example, a part of the power light may be a material serving as a frequency standard having a specific old main spectrum. There is a method in which the frequency is discriminated by transmitting the light through an absorption cell in which a laser diode is enclosed, and is detected by a detector such as a photodiode, and the driving current of the laser diode is used to control the frequency without negative feedback.

However, in order to reduce the spectral line width by the driving circuit 1, it is necessary to add a high spectral purification function to the driving circuit 1, which complicates the configuration, increases the circuit scale and increases the cost. There was a problem that said. In addition, there is a problem that it is difficult to realize a spectral line width of “１ ／ to ３” by adding a high spectral purification function to the drive circuit 1. Therefore, an object of the present invention is to realize a laser length measuring device capable of suppressing the influence of the spectral line width of a laser diode.

[0026]

In order to achieve the above object, according to a first aspect of the present invention, there is provided a laser measuring apparatus using heterodyne interference, which outputs a laser beam and outputs a reference signal. And a reference mirror, the laser light of one frequency of the laser light,
An interferometer that reflects the laser light of the other frequency with mirrors attached to the object to be measured and outputs interference light,
A receiver that detects the interference light and outputs an interference signal,
A signal processing circuit that measures the position of the device under test by detecting and integrating a phase difference between the interference signal and the reference signal, and a low-pass filter circuit that attenuates a high-frequency component of an output signal of the signal processing circuit. With this arrangement, it is possible to reduce the fluctuation caused by the jitter. In addition, since it is not necessary to add a high spectral purification function to the driving circuit, the configuration of the driving circuit is simplified.

According to a second aspect of the present invention, in the laser measuring apparatus according to the first aspect, the low-pass filter circuit has a characteristic of attenuating a frequency component of jitter caused by a spectral line width of the laser light. With this, it is possible to reduce the fluctuation due to the jitter. Further, since it is not necessary to add a high-spectrum purifying function to the driving circuit, the configuration of the driving circuit is simplified.

According to a third aspect of the present invention, in the laser length measuring apparatus according to the second aspect of the present invention, the low-pass filter circuit has a characteristic that a delay time in a measurement frequency region is constant, so that the measured signal is measured. It is possible to keep the time from sampling the position of the object to processing the data and outputting the output signal constant.

According to a fourth aspect of the present invention, in the laser measuring device according to the second aspect, the low-pass filter circuit is a Butterworth filter circuit.
Since a low-pass characteristic having a steep cutoff characteristic is obtained, fluctuation due to jitter can be further reduced.

According to a fifth aspect of the present invention, in the laser measuring device according to the second aspect of the present invention, since the low-pass filter circuit is a Bessel filter circuit, the delay characteristic becomes flat and the waveform is distorted. Since it can be prevented, the time until the output signal is output can be made constant.

[0031]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings. FIG. 1 is a configuration block diagram showing an embodiment of a laser length measuring apparatus according to the present invention.

In FIG. 1, reference numerals 2 to 7 and 100 to 102 denote the same reference numerals as in FIG. 4, 1a denotes a drive circuit, 18 denotes a low-pass filter circuit, and 105 denotes an output signal. 1a, 2 and 3 constitute the head 53.

The driving signal of the driving circuit 1 a is connected to the driving terminal of the laser diode 2, and the output light of the laser diode 2 enters the interferometer 4 via the reference signal generating means 3. The laser light of one frequency of the output light of the head 53 is reflected by the reference mirror inside the interferometer 4 and the laser light of the other frequency is reflected by the mirror 5 attached to the object to be measured, and interferes in the interferometer 4. Let it.

The interference light generated by the interferometer 4 is incident on the receiver 6, the interference signal 101 output from the receiver 6 is connected to the signal processing circuit 7, and the reference signal 102 output from the reference signal generator 3 is also a signal. It is connected to the processing circuit 7. Then, the position information signal 100 output from the signal processing circuit 7 is connected to the low-pass filter circuit 18, and the low-pass filter circuit 18 outputs an output signal 105.

The operation of the embodiment shown in FIG. 1 will be described with reference to FIGS. FIG. 2 is an explanatory diagram showing ideal waveforms and waveform fluctuations of output light from the laser diode 2, and FIG. 3 is a characteristic curve diagram showing characteristics of the low-pass filter circuit 18.

The laser diode 2 is driven by a drive signal from the drive circuit 1a, and emits laser beams of two frequencies from the head 53. However, the drive circuit 1a is not provided with the above-described high spectral purification function for reducing the spectral line width, and emits a laser beam having a wide spectral line width as shown in FIG. 6, for example. The laser light of the two frequencies of the laser diode 2 is
Through the interferometer 4.

In the interferometer 4, the laser light of one frequency of the output light of the laser diode 2 is reflected by a reference mirror inside the interferometer 4 and the laser light of the other frequency is reflected by a mirror 5 attached to the object to be measured. The interference light is made to interfere in the interferometer 4 and the interference light is emitted to the receiver 6.

The receiver 6 converts the input interference light into an electric signal and outputs the electric signal as an interference signal 101 to the signal processing circuit 7. On the other hand, the reference signal generating means 3 generates a reference signal 102 from the difference between the two frequencies of the laser light of the laser diode 2 and outputs the reference signal 102 to the signal processing circuit 7.

The signal processing circuit 7 outputs the reference signal 102
Then, the position information signal 100 of the device under test is output by detecting the phase difference between the device and the interference light 101 when the device under test moves. Finally, the low-pass filter circuit 18 attenuates the high-frequency component of the position information signal 100 and outputs the output signal 105
Output as

Here, a description will be given with reference to FIG. FIG.
2A shows an example of an ideal laser light waveform having no frequency fluctuation, and FIG. 2B shows an example of an actual laser light waveform of the laser diode 2. Although the actual waveform is a sine wave, it is represented by a rectangular wave for the sake of simplicity.

As shown in FIG. 2B, the waveform of the laser beam from the laser diode 2 fluctuates with a probability distribution as shown in FIG. 6, for example, as compared with the waveform shown in FIG. Such fluctuation appears in the form of jitter in time. For this reason, even if the object to be measured does not actually move, if the position is measured with reference to the waveform shown in FIG. 2B, the position information appears to fluctuate around a certain value. However, over a long period of time, the frequency of the laser beam is averaged to the center frequency, so that the position information itself does not drift.

On the other hand, the measurement frequency band necessary for measuring the position of the device under test is "10 kHz", and the main frequency band of the above-mentioned jitter is "several hundred kHz" or more. For this reason, by attenuating the frequency component of “several hundreds of kHz” or more in the position information signal 100 by the low-pass filter circuit 18, it is possible to reduce the fluctuation of the position information due to the jitter.

For example, in the case of a low-pass characteristic indicated by “L001” in FIG. 3, when the center frequency of the measurement frequency region is “fs” and the center frequency of the main frequency band of jitter is “fn”, The cut-off frequency “fc” of the filter circuit 18 is changed to “fs” and “f” as shown in FIG.
By setting it between n ″, the main frequency component of the jitter is attenuated, and the measured frequency component is output as it is.

As a result, the low-pass filter circuit 18 attenuates the frequency component of the jitter caused by the spectral line width from the position information signal 100, thereby making it possible to reduce the jitter caused by the jitter. Also, the driving circuit 1
Since it is not necessary to add a high spectral purification function to a, the configuration of the driving circuit is simplified.

By using the low-pass filter circuit 18 having a characteristic in which the delay time in the measurement frequency region is constant, the signal processing circuit 7 and the low-pass filter circuit 18 are sampled after sampling the position of the device under test. And the time until the output signal 105 is output can be kept constant.

When a Butterworth filter circuit is used as the low-pass filter circuit 18, a low-pass characteristic having a steep cutoff characteristic can be obtained, so that fluctuation due to jitter can be further reduced.

If a Bessel filter circuit is used as the low-pass filter circuit 18, the delay characteristic can be flattened and the waveform can be prevented from being distorted.
The time until 5 is output can be made constant.

[0048]

As is apparent from the above description,
According to the present invention, the following effects can be obtained. According to the first and second aspects of the present invention, the low-pass filter circuit attenuates the frequency component of the jitter caused by the spectral line width from the position information signal, thereby making it possible to reduce the jitter caused by the jitter. . Further, since it is not necessary to add a high-spectrum purifying function to the driving circuit, the configuration of the driving circuit is simplified.

According to the third aspect of the present invention, data is processed after sampling the position of the device under test by making the low-pass filter circuit have such characteristics that the delay time in the measurement frequency region is constant. Thus, the time until the output signal is output can be kept constant.

According to the fourth aspect of the present invention, since the low-pass filter circuit is realized by the Butterworth filter circuit, a low-pass characteristic having a steep cutoff characteristic can be obtained, so that fluctuation due to jitter can be further reduced.

According to the fifth aspect of the present invention, since the low-pass filter circuit is realized by the Bessel filter circuit, the delay characteristic can be flattened and the waveform can be prevented from being distorted, so that an output signal is output. The time until is constant.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of an embodiment of a laser length measuring apparatus according to the present invention.

FIG. 2 is an explanatory diagram showing an ideal waveform and a waveform variation of output light of a laser diode.

FIG. 3 is a characteristic curve diagram showing characteristics of a low-pass filter circuit.

FIG. 4 is a configuration block diagram showing an example of a conventional laser length measuring device.

FIG. 5 is a configuration block diagram illustrating details of a receiver and a signal processing circuit.

FIG. 6 is a characteristic curve diagram showing an example of a frequency distribution of output light of a laser diode.

FIG. 7 is a characteristic curve diagram showing an example of a frequency distribution of output light of a laser diode having a reduced spectral line width.

[Explanation of symbols]

 1, 1a drive circuit 2 laser diode 3 reference signal generation means 4 interferometer 5 mirror 6 receiver 7 signal processing circuit 8 photodetector 9, 12 amplifier 10, 13 filter circuit 11, 14 comparator 15 PLL circuit 16 period measurement circuit 17 Integrating circuit 18 low-pass filter circuit 50, 53 head 51 receiver 52 signal processing circuit 100 position information signal 101 interference signal 102 reference signal 103 interference light 104 clock signal 105 output signal

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Akira Oya 2-9-132 Nakamachi, Musashino-shi, Tokyo F-term in Yokogawa Electric Corporation (reference) 2F064 AA01 CC01 EE01 FF01 GG12 2F065 AA02 DD00 DD04 FF13 FF52 FF55 GG06 LL12 QQ00

Claims (5)

[Claims] 1. A laser length measuring apparatus using heterodyne interference, comprising: a head for outputting a laser beam and outputting a reference signal; a laser beam having one frequency of the laser beam, a reference mirror, and a laser beam having the other frequency. An interferometer that outputs the interference light by reflecting the laser light with mirrors attached to the device under test, a receiver that detects the interference light and outputs an interference signal, and a position of the interference signal and the reference signal. A laser processing device comprising: a signal processing circuit that measures the position of the device under test by detecting and integrating a phase difference; and a low-pass filter circuit that attenuates a high-frequency component of an output signal of the signal processing circuit. Long equipment. 2. The laser length measuring apparatus according to claim 1, wherein the low-pass filter circuit has a characteristic of attenuating a frequency component of jitter caused by a spectral line width of the laser light. 3. A laser length measuring apparatus according to claim 2, wherein said low-pass filter circuit has a characteristic that a delay time in a measurement frequency region is constant. 4. The laser length measuring apparatus according to claim 2, wherein said low-pass filter circuit is a Butterworth filter circuit. 5. The circuit according to claim 2, wherein said low-pass filter circuit is a Bessel filter circuit.
The laser measuring device as described in the above.