JP4793675B2 - Distance measuring device - Google Patents

Description

  The present invention relates to a distance measuring apparatus that irradiates a measurement target with a laser beam and measures the distance to the measurement target using reflected light from the measurement target.

  Conventionally, in a light wave distance measuring device, distance measuring light is intensity-modulated and emitted at a constant frequency, reflected distance measuring light reflected by a measurement object is received, and the received reflected distance measuring light and distance measuring light are received. Or, compare the light received through the reference optical path formed inside the distance measuring device (hereinafter referred to as reference light) and measure the distance from the phase difference between the reflected distance measuring light and the reference light. Yes.

  When measuring the distance from the reflected distance measuring light, the drift of the detection circuit inside the distance measuring device appears as a measurement error, so the internal error is measured by the reference light, and the accurate distance is calculated.

  The distance measurement in the distance measuring device utilizes the fact that the phase difference changes according to the distance measured. The phase difference is Δφ, the distance is D, the modulation frequency is f, and the speed of light is C. For example, the phase difference Δφ is expressed as Δφ = 4πfD / C (formula 1), and the distance D can be obtained by measuring the phase difference Δφ.

  The measurement distance ranges from a short distance of about 1.5 m, which is the shortest focusing distance, to a long distance from the distance limit. Since the phase difference is measured, the same value occurs at a distance within the wavelength of the intensity modulation and at a distance greater than the wavelength. For this purpose, a plurality of measurement lights having different wavelengths are prepared as the intensity-modulated measurement light. In order to measure the distance with high accuracy, a wavelength with a short intensity modulation is used.

  In practice, there are many configurations in which the measurement effective digits of the distance value are determined at three or more frequencies having different sizes.

  Next, the distance measuring apparatus will be described with reference to FIG.

  The signal of the reference oscillator 1 is input to the frequency dividing circuit 2 and is divided to a necessary frequency. The frequency selection circuit 3 selects a frequency for driving the light emitting element 5 and outputs it to the light emitting element driving circuit 4. The light emitting element driving circuit 4 drives the light emitting element 5 at the selected frequency to emit distance measuring light 6.

  The distance measuring light 6 passes through the first half mirror 16, passes through the objective lens 7, and is reflected by a reflecting mirror or the like that is the measurement object 8 placed at the measurement point. The reflected light again enters the objective lens 7 as reflected distance measuring light 9. The incident reflected distance measuring light 9 is transmitted through the second half mirror 17 and received by the light receiving element 11. The light receiving signal fd sent from the light receiving element 11 is amplified by the broadband amplifier 12 and then mixed with the frequency signal fc + fd from the frequency dividing circuit 2 by the mixer 13 to be beat down. The beat-down range-finding light beat-down signal fe is selectively amplified by the filter 14 and input to the phase difference measurement circuit 15.

  The internal reference light 10 emitted from the light emitting element 5 and divided and reflected by the first half mirror 16 into the internal optical path is reflected by the second half mirror 17 and received by the light receiving element 11. The light receiving signal fd transmitted from the light receiving element 11 is amplified by the broadband amplifier 12 and then beat down by the mixer 13, and selectively amplified by the filter 14 as a reference light beat down signal fe ', and the phase difference Input to the measurement circuit 15.

  The distance measuring light 6 and the internal reference light 10 are alternatively selected by an optical path switch 19, and the phase difference measuring circuit 15 selects the beat-down signal fe of the reflected distance measuring light 9 and the internal reference light 10. A phase difference is obtained from the beat down signal fe ′. Further, based on the signal from the phase difference measurement circuit 15, the calculation unit 18 calculates the distance in consideration of the internal optical path length. Then, a phase difference is obtained by combining a plurality of frequencies according to the distance, and converted into the distance by the above equation (1).

  A conventional distance measuring device uses an LED as a light-emitting light source, and uses a crystal oscillation module as a measurement reference that is a reference for obtaining intensity modulation or phase difference, ie, a clock signal.

  In recent years, distance measurement apparatuses using femtosecond lasers that perform accurate reference oscillation have been realized because of the demand for high measurement accuracy. Using a femtosecond laser with high oscillation accuracy enables distance measurement with higher accuracy. However, there is a problem that the accuracy of the femtosecond laser cannot be utilized because the accuracy of the crystal oscillator is inferior to the distance measurement accuracy using the femtosecond laser.

JP 2001-13245 A

Optical frequency comb-New light measure-(http://www.aist.go.jp/aist_j/museum/keisoku/komu/komu.html)

  In view of such circumstances, the present invention is intended to improve the measurement accuracy of a distance measuring device using a laser beam by improving the accuracy of a measurement standard.

  The present invention includes a light source that generates distance measuring light, a light receiving unit that irradiates the object to be measured with the distance measuring light and receives reflected light from the object to be measured, and an external reference time for acquiring an external reference time. An acquisition device, and based on the reference time acquired by the external reference time acquisition device, the light source is controlled to modulate the ranging light, and the modulated ranging light is received via the internal optical path A phase measuring apparatus that measures a phase and a phase at which reflected light reflected by the target reflector is received and calculates a distance based on the phase difference; a light source that generates distance measuring light; and the measurement A light receiving unit that irradiates a measurement object with light and receives reflected light from the measurement object, an external reference time acquisition device that acquires an external reference time, and a frequency measurement that measures the frequency of the distance measurement light The frequency measurement unit is captured by the external reference time acquisition device. The frequency of the ranging light is measured based on the time reference, and the phase of the modulated ranging light received through the internal optical path and the phase of the reflected light reflected back from the target reflector are measured. And a distance measuring device that calculates a distance based on the phase difference, a light source that generates distance measuring light, and the object to be measured is irradiated with the distance measuring light and the reflected light from the object to be measured is received. A light receiving unit, an external reference time acquisition device that acquires an external reference time, a frequency measurement unit that measures the frequency of the ranging light, a comparison circuit that compares the measured value of the frequency and the reference time, A frequency changing unit that changes a frequency of ranging light emitted from a light source, and relates to a distance measuring device that controls the frequency changing unit based on an output of the comparison circuit, and the external reference time acquisition device includes: External reference time based on signals from GPS satellites Relates to a distance measuring apparatus that acquires, further or a light source for emitting modulated light is according to the distance measuring device is a femtosecond laser light source.

  According to the present invention, a light source that generates distance measuring light, a light receiving unit that irradiates the object to be measured with the distance measuring light and receives reflected light from the object to be measured, and an external that acquires an external reference time. A reference time acquisition device, and based on the reference time acquired by the external reference time acquisition device, the light source is controlled to modulate the ranging light, and the modulated ranging light is transmitted via the internal optical path. The phase that received the light and the phase that received the reflected light reflected back from the target reflector is measured, and the distance is calculated based on the phase difference. Therefore, the reference time for calculating the distance by the distance measurement using the light wave is set. It is obtained from the outside of the apparatus, and a highly accurate reference time can be used, so that the ranging accuracy can be improved.

  Further, according to the present invention, the external reference time acquisition device acquires the external reference time based on a signal from a GPS satellite, so that it is possible to easily obtain an external high-precision external reference time. To demonstrate.

  The best mode for carrying out the present invention will be described below with reference to the drawings.

  FIG. 1 shows a first embodiment. This first embodiment shows a case where an external reference time is used instead of a clock signal used as a measurement reference instead of a clock signal from a crystal oscillator. Yes. As the external reference time, for example, a reference time (clock signal) superimposed on a GPS satellite signal can be used. In recent years, the performance of the receiving antenna has been improved and the size of the receiving antenna has been reduced, and a GPS receiver for receiving a GPS satellite signal can be mounted on the distance measuring device.

  The external reference time described below uses a clock signal extracted from a signal from a GPS satellite as a reference time.

  In FIG. 1, the same components as those shown in FIG.

  In the figure, reference numeral 5 denotes a light emitting element such as a laser diode (LD) or LED that emits the distance measuring light 6, and the light emitting element 5 is driven and controlled by the light emitting element driving circuit 4. Modulation is controlled. A first half mirror 16 that divides a part of the distance measuring light 6 is provided on the optical path of the distance measuring light 6, and the distance measuring light 6 that has passed through the first half mirror 16 passes through the objective lens 7. Then, the measurement object 8 is irradiated. The measurement object 8 only needs to have a surface property that reflects the amount of light necessary for measurement. For example, a reflection prism, a retroreflective plate, or a bright natural object surface is selected as the measurement object. .

  The distance measuring light 6 reflected by the measurement object 8 passes through the objective lens 7 as reflected distance measuring light 9 and includes a light receiving element 11 such as a photodetector (for example, MSM: Metal-Semiconductor Metal). Is received.

  On the optical path of the reflected distance measuring light 9 between the objective lens 7 and the light receiving element 11, a second half mirror 17 that transmits the reflected distance measuring light 9 is disposed, and the reflected distance measuring light 9 is The light is received by the light receiving element 11 through the second half mirror 17.

  Part of the distance measuring light 6 is reflected by the first half mirror 16 as internal reference light 10, and the internal reference light 10 is further reflected by the second half mirror 17 and enters the light receiving element 11. It has become. The light reception signal of the light receiving element 11 is amplified by the broadband amplifier 12 and sent to the mixer 13.

  An optical path switcher (chopper) 19 is provided for switching the optical path across the optical path of the distance measuring light 6 transmitted through the first half mirror 16 and the optical path of the internal reference light 10 reflected by the first half mirror 16. It has been. The distance measuring light 6 and the internal reference light 10 are alternatively selected by the optical path switch 19, and the reflected distance measuring light 9 and the internal reference light 10 are alternately incident on the light receiving element 11.

  The clock signal from the external reference time acquisition device 54 is input to the frequency dividing circuit 2 and is divided to a necessary frequency. The frequency selection circuit 3 selects a frequency for driving the light emitting element 5 and outputs it to the light emitting element driving circuit 4. The light emitting element driving circuit 4 drives the light emitting element 5 at the selected frequency to emit the distance measuring light 6. The distance measuring light 6 passes through the first half mirror 16, passes through the objective lens 7, and is reflected by a reflecting mirror or the like that is a measurement object 8 placed at a measurement point. The reflected light enters the objective lens 7 again as the reflected distance measuring light 9 through the objective lens 7. The incident reflected distance measuring light 9 passes through the second half mirror 17 and is received by the light receiving element 11. The light receiving signal transmitted from the light receiving element 11 is amplified by the broadband amplifier 12 and then mixed with the frequency signal from the frequency dividing circuit 2 by the mixer 13 to be beat down. The range-finding light beat-down signal that has been beat-down is selectively amplified by the filter 14 and input to the phase difference measurement circuit 15.

  The internal reference light 10 emitted from the light emitting element 5 and divided and reflected by the first half mirror 16 into the internal optical path is reflected by the second half mirror 17 and received by the light receiving element 11. The light receiving signal transmitted from the light receiving element 11 is amplified by the broadband amplifier 12, beat-down by the mixer 13, and selectively amplified by the filter 14 as a reference light beat-down signal, and the phase difference measuring circuit 15 Is input.

  The distance measuring light 6 and the internal reference light 10 are alternatively selected by the optical path switch 19 and input to the phase difference measuring circuit 15.

  The phase difference measuring circuit 15 obtains an accurate phase difference from the beat-down signal of the reflected distance measuring light 9 and the beat-down signal of the internal reference light 10 based on the reference signal from the frequency dividing circuit 2. Further, based on the signal from the phase difference measurement circuit 15, the calculation unit 18 calculates the distance in consideration of the internal optical path length. Then, a phase difference is obtained by combining a plurality of frequencies according to the distance, and is converted into a distance by the equation (1).

  Next, a second embodiment in which an external reference time is used for a distance measuring device using a femtosecond laser will be described.

  First, a femtosecond mode-locked pulse laser will be described.

  In recent years, a compact and stable femtosecond mode-locked pulse laser (ultra-short pulse light) has been obtained, and its application to high-function and high-accuracy measurement is rapidly expanding. Unlike conventional pulse rangefinders, femtosecond frequency comb rangefinders do not use a short time span of femtoseconds. The femtosecond frequency comb rangefinder is an optical modulation rangefinder that uses a number of stable beat components between optical frequency modes of a femtosecond mode-locked pulse laser.

  The femtosecond mode-locked pulse laser has a very narrow time width (pulse width) of femtosecond, and each pulse is generated at a constant interval (repetition frequency) depending on the resonator length. As shown in FIG. 2A, one pulse has a wide spectrum width, and has a spectrum of several THz having a number of longitudinal modes in the optical frequency domain. The longitudinal mode interval is repeatedly generated at a constant interval. It is known that the accuracy of this interval is very high.

  It is called an optical frequency comb because there are many modes with precise intervals like comb teeth. When this light spectrum is photoelectrically detected by a photodetector, only the beat frequency between the modes is detected as shown in FIG. These are called beats between modes.

  If the beat between modes in this region is considered in the time domain, it is an electric wave having a frequency that is an integral multiple of the repetition frequency. That is, the femtosecond mode-locked pulse laser can be regarded as a collection of many intensity-modulated waves. In the femtosecond frequency comb rangefinder, the distance measurement is performed by measuring the phase of the electric wave in the same manner as the phase measurement of the modulated wave in the optical modulation rangefinder.

Assuming that the accuracy of this femto distance measuring device is 15 μm, there is an accuracy of 15 × 10 ⁻⁶ / 500 = 3 × 10 ⁻⁸ at a reach distance of 500 m. As described above, since the accuracy of the temperature-compensated crystal oscillator is about 1 × 10 ⁻⁶ , frequency accuracy is insufficient for high-precision distance measurement.

Currently, the cesium oscillator, which is an atomic clock used as a time standard, has an accuracy of about 10 ⁻¹¹ , and the relatively inexpensive rubidium oscillator has an accuracy of about 10 ^−10. is there. Therefore, in the second embodiment, an accurate frequency is obtained from the outside from the cesium oscillator or the rubidium oscillator.

  Next, the generation principle of the femtosecond mode-locked pulse laser will be described.

  In a general laser, the phase in the resonator varies depending on each mode and is not synchronized in time. However, in a resonator of a femtosecond mode-locked pulse laser, pulses are generated by temporally synchronizing the phases between the modes and making the phase difference between the modes constant. This method is called mode synchronization.

  When a phase synchronization condition (mode synchronization) is established between the modes, pulse peaks appear in the resonator that interfere with each other and strengthen each other. The maximum intensity of the pulsed light is larger than that when the mode synchronization is not achieved.

  The time interval between the generated pulse peaks is equal to the time for the pulsed light to reciprocate in the resonator. That is, in the resonator, a large number of lights overlap and reciprocate in the resonator as pulsed light. Further, the pulse generated by mode synchronization has a wider spectrum width as the pulse time width is shorter.

  To actually generate an optical frequency comb using ultrashort optical pulses, a resonator with a laser gain medium with a wide gain spectrum is used, and excitation is performed to generate more longitudinal modes at a constant phase interval. is required. There are two types of mode synchronization methods, active (forced) mode synchronization and passive (self) mode synchronization.

  The active mode synchronization is a method in which a modulator is inserted in a resonator, the resonator loss is periodically changed at the modulation frequency, and the mode interval is synchronized with the modulation frequency.

  Further, the passive mode locking has a saturable absorber function in the resonator. For example, a typical example is a saturable dye. It consists of a material with an absorption transition with a very large absorption cross section for the laser frequency. Therefore, when light passes through the saturable absorber, light below the saturation intensity is absorbed, and only light above the saturation intensity is amplified. Therefore, the pulse width of the pulsed light reciprocating in the resonator gradually narrows the pulse width. At present, a resonator in which a solid-state laser crystal has a saturable absorber function is the mainstream. In general, passive mode locking is used to generate pulsed light of 100 fs or less.

  Note that a laser device that generates an optical frequency comb can also be realized by incorporating an electro-optic crystal in an optical resonator, injecting a CW laser into the optical resonator, and applying optical modulation with the electro-optic crystal.

  FIG. 3 shows a second embodiment using a femtosecond laser and an external reference time.

  In FIG. 3, 21 is an ultrashort pulse light emission source, that is, a femtosecond mode-locked laser device.

  A splitter 23 that divides a part of the distance measuring light 31 is provided on the optical path of the ultrashort pulse light emitted as the distance measuring light 31 from the femtosecond mode-locked laser device 21, and the light transmitted through the splitter 23 is transmitted through the splitter 23. The distance measuring light 31 passes through the objective lens 24 and irradiates the measurement object 25. The measuring object 25 only needs to have a surface property that reflects the amount of light necessary for measurement. For example, a reflecting prism, a retroreflective plate, or a bright natural object surface is selected as the measuring object 25. Is done.

  The distance measuring light 31 reflected by the measurement object 25 passes through the light receiving lens 26 as reflected distance measuring light 32 and is provided with a first light receiving element 27 such as a photodetector (for example, MSM: Metal-Semiconductor Metal). The light is received by the unit. The objective lens 24 may also serve as the light receiving lens 26.

  A first half mirror 28 is provided on the optical path of the ranging light 31 between the splitter 23 and the objective lens 24, and the reflection ranging between the light receiving lens 26 and the first light receiving element 27. A second half mirror 29 that transmits the reflected distance measuring light 32 is disposed on the optical path of the light 32, and the reflected distance measuring light 32 is transmitted through the second half mirror 29 to the first light receiving element 27. Received light. A part of the ranging light 31 is reflected by the first half mirror 28 as internal reference light 33, and the internal reference light 33 is reflected by the second half mirror 29 and enters the first light receiving element 27. It has become. The light reception signal of the first light receiving element 27 is amplified by the first amplifier 52 which is a high frequency amplifier, and is sent to the mixer 35 through the first filter 34.

  An optical path switcher (chopper) 30 for switching the optical path in the optical path of the distance measuring light 31 transmitted through the first half mirror 28 and the optical path of the internal reference light 33 reflected by the first half mirror 28 is provided. ing. The distance measuring light 31 and the internal reference light 33 are alternatively selected by the optical path changer 30.

  A part of the distance measuring light 31 divided by the splitter 23 is received as a reference light 36 by a second light receiving element 37 such as a photodetector (MSM: Metal-Semiconductor Metal). The light receiving signal from the second light receiving element 37 is amplified by the second amplifier 51 which is a high frequency amplifier, and then sent to the second filter 38 and the third filter 39, respectively, and is input to the third filter 39. Is sent to the mixer 35, where the signal from the first filter 34 and the signal from the third filter 39 are mixed and input to the phase difference measuring circuit 42 via the fourth filter 41. .

  Further, the signal input to the second filter 38 is input to the phase difference measuring circuit 42 and the frequency counter 53. In the phase difference measuring circuit 42, the phase difference between the signal from the fourth filter 41 and the signal from the second filter 38 is measured.

  The frequency counter 53 measures the period of the signal input from the second filter 38 during the phase difference measurement (period of the reference light 36) based on the highly accurate external reference time from the external reference time acquisition device 54. .

  Signals from the phase difference measurement circuit 42 and the frequency counter 53 are input to the calculation unit 43, and the calculation unit 43 is based on the phase difference measured by the phase difference measurement circuit 42 and the measurement of the frequency counter 53. The distance to the measurement object 25 is calculated.

  The operation will be described below.

  Accurate pulse light generated by the femtosecond mode-locked laser device 21 is periodically oscillated at, for example, 50 MHz, and is split into the distance measuring light 31 and the reference light 36 by the splitter 23. Each pulsed light to be oscillated is composed of a wide frequency band with an accurate frequency difference of 50 MHz due to the characteristics of the femtosecond mode-locked laser.

  The distance measuring light 31 that has passed through the splitter 23 is further divided into the distance measuring light 31 and the internal reference light 33 by the first half mirror 28. The internal reference light 33 and the distance measuring light 31 are alternatively selected by the optical path switcher (chopper) 30 and sequentially received by the first light receiving element 27. The internal reference beam 33 is used to correct an error specific to the measurement circuit. The distance measuring light 31 is reflected by the measurement object 25, and the reflected light enters the light receiving lens 26 as the reflected distance measuring light 32 and is received by the first light receiving element 27.

  The reference light 36 incident on the second light receiving element 37 is photoelectrically converted, and a number of inter-mode beat signals are output from the second light receiving element 37. The output signal is amplified by the second amplifier 51, and one beat signal is selected by the third filter 39. Similarly, a large number of inter-mode beat signals photoelectrically converted by the first light receiving element 27 are output. The output signal is amplified by the first amplifier 52 and then one beat signal is selected by the first filter 34.

  The beat signal from the first filter 34 and the beat signal from the third filter 39 are input to the mixer 35, and two beat signals having the frequency of the sum and difference of the frequencies of the two input beat signals are It is generated by the mixer 35. There is a slight frequency difference between the beat signal selected by the first filter 34 and the beat signal selected by the third filter 39. In this embodiment, there is a difference of 50 MHz, and the mixer 35 generates a 50 MHz self-beat signal.

  The fourth filter 41 selects the difference frequency between the beat signal selected by the third filter 39 and the beat signal selected by the first filter 34 (hereinafter, selected by the fourth filter 41). The beat signal is referred to as a self-beat signal).

  The second filter 38 selects a beat signal having the same frequency as the self beat signal from the inter-mode beat signal photoelectrically converted by the second light receiving element 37. The selected signal is output to the phase difference measuring circuit 42 and the frequency counter 53.

  In the phase difference measurement circuit 42, the self beat signal of the reflected distance measuring light 32 and the internal reference light 33 selected by the fourth filter 41 on the basis of the beat signal selected by the second filter 38. The phase of the self-beat signal is measured.

  The frequency counter 53 measures the cycle of the input beat signal from the second filter 38 based on the highly accurate clock signal from the external reference time acquisition device 54 and outputs the measured value to the calculation unit 43. .

  The calculation unit 43 calculates the distance to the measurement object 25 in consideration of the internal optical path length from the phase difference measured by the phase difference measurement circuit 42 based on the measurement of the frequency counter 53.

  The femtosecond mode-locked laser device 21 which is a pulsed light emission source oscillates an accurate pulse. However, since it is affected by the device temperature and the outside air temperature, the distance is measured based on a highly accurate external reference time. By measuring the wavelength accurately, a highly accurate distance is measured from the phase difference. The external reference time used here is, for example, an atomic clock that is a GPS reference time that is far more accurate than a crystal oscillator used in a phase difference distance measuring device.

  Since the phase of the ranging light 31 selected by the first filter 34 is stored in the self beat signal, the phase difference between the self beat signal and the reference light 36 selected by the second filter 38 is The optical path distance (external optical path distance) of the distance measuring light 31 can be measured from the obtained phase difference. Similarly, the optical path distance (internal optical path distance) of the internal reference light 33 can be measured by the reference light 36 and the self-beat signal when the first light receiving element 27 receives the internal reference light 33.

  By measuring the internal optical path distance and comparing the internal optical path distance with the external optical path distance, it is possible to correct a phase change due to a circuit and measure a more accurate distance.

  Although the distance is measured at the frequency of the beat signal selected by the first filter 34, the beat signal between the modes is in a wide frequency band. Therefore, the beat signal selected by the first filter 34 and the third filter 39 is selected. By changing, the frequency used for distance measurement can be changed.

  For example, when 10 GHz is selected for the first filter 34, the distance that can be measured by the phase difference is 15 mm, and when 50 MHz is selected by the first filter 34, the distance that can be measured by the phase difference is 3 m. A technique for measuring the distance with high accuracy while expanding the measurement range using several types of frequencies is a technique generally used in surveying instruments. The first filter 34, the second filter 38, the third filter 39, and the fourth filter 41 may be electric elements or electric circuits.

  When a longer distance measurement is required, in a femtosecond mode-locked laser device in which the oscillation frequency depends on the resonator length, the frequency cannot be changed unless the resonator length is changed. For example, in this embodiment in which only an inter-mode beat signal having a frequency lower than 50 MHz can be obtained, a distance longer than 3 m cannot be measured. In that case, for example, when the resonator length is changed by a piezo element so as to move the reflecting mirror constituting the optical resonator, the frequency comb interval can be changed. As the resonator length changing means, moving means such as a moving stage can be used instead of the piezo element.

  FIG. 4 shows a third embodiment. In FIG. 4, the same components as those shown in FIG. 3 are denoted by the same reference numerals. Since the optical system has the same configuration, the following description is omitted.

  In the second embodiment, the distance is obtained with high accuracy by accurately measuring the frequency at the time of measuring the distance. In the third embodiment, a femtosecond mode-locked laser, which is a pulsed light emission source, is used. The oscillating frequency of the device 21 is controlled to measure an accurate distance. Similarly, the external reference time is used as the control reference.

  In FIG. 4, reference numeral 22 denotes oscillation frequency changing means, which oscillates by changing the distance between the resonator mirrors built in the resonator of the femtosecond mode-locked laser device 21 and changing the resonator length. The frequency is changed, and the resonator length can be changed by moving the resonator mirror. For example, a piezo element is used as the resonator length changing means.

  The ultrashort pulsed light emitted from the femtosecond mode-locked laser device 21 is divided into a distance measuring light 31 and a reference light 36 by a splitter 23, and the distance measuring light 31 is further internally referenced by a first half mirror 28. It is divided into 33. The distance measuring light 31 and the internal reference light 33 are alternatively received by the first light receiving element 27, and the reference light 36 is received by the second light receiving element 37.

  The light reception signal from the second light receiving element 37 is amplified by the second amplifier 51 which is a high frequency amplifier, and then output to the second filter 38 and the third filter 39, respectively, and the signal input to the third filter 39. Is sent to the mixer 35. The signal is mixed with the signal from the first filter 34 by the mixer 35 and input to the phase difference measuring circuit 42 via the fourth filter 41. The signal input to the second filter 38 is output to the phase difference measurement circuit 42 and the phase lock circuit 55.

  The phase difference measuring circuit 42 uses the beat signal selected by the second filter 38 as a reference, and the self beat signal of the reflected distance measuring light 32 and the internal reference light 33 selected by the fourth filter 41. The phase with the self beat signal is measured.

  The calculation unit 43 calculates the distance to the measurement object 25 from the phase difference measured by the phase difference measurement circuit 42 in consideration of the internal optical path length.

  The phase lock circuit 55 compares the phase of the signal based on the external reference time of the external time acquisition device 54 and the signal selected by the second filter 38 and outputs a signal corresponding to the difference. The oscillation frequency changing means 22 controls the output pulse of the femtosecond mode-locked laser device 21 based on the output signal from the phase lock circuit 55.

It is a schematic block diagram which shows the 1st Embodiment of this invention. (A) is explanatory drawing of the spectrum of the optical frequency comb utilized by this invention, (B) is explanatory drawing of the light reception signal at the time of light-receiving a light frequency comb with a light receiving element. It is a schematic block diagram which shows the 2nd Embodiment of this invention. It is a schematic block diagram which shows the 3rd Embodiment of this invention. It is a schematic block diagram which shows a prior art example.

Explanation of symbols

2 Frequency Divider 4 Light Emitting Element Drive Circuit 5 Light Emitting Element 8 Measurement Object 9 Reflected Distance Measuring Light 11 Light Receiving Element 13 Mixer 15 Phase Difference Measuring Circuit 18 Arithmetic Unit 21 Femtosecond Mode Synchronized Laser Device 22 Oscillation Frequency Changing Means 27 First Light Receiving Unit Element 34 First filter 37 Second light receiving element 38 Second filter 39 Third filter 42 Phase difference measurement circuit 43 Calculation unit 53 Frequency counter 54 External reference time acquisition device

Claims (5)

A laser device that generates an optical frequency comb as a laser beam;
Splitting means for splitting the laser beam from the laser device into reference light and ranging light;
A reference light receiving unit that receives the reference light and outputs a number of beat signals;
A light receiving unit that receives the distance measuring light and outputs a number of beat signals;
A first electrical filter for extracting at least one beat signal from the measurement light receiving unit;
A second electrical filter for extracting at least one beat signal from the reference light receiving unit;
An external reference time acquisition device that issues an external reference time,
It obtains a phase difference based on the phase obtained based on the beat signals output from the phase and the second electrical filter obtained based on the beat signals output from the first electrical filter, the phase difference and the external A distance measuring device that measures a distance based on the reference time obtained from a reference time acquisition device. Before Symbol further comprising a frequency measuring section for measuring the frequency of the distance measuring light, the frequency measurement unit measures the frequency of the distance measuring light based on time reference acquired by the external reference time acquiring unit, and the frequency The distance measuring device according to claim 1, wherein the distance is calculated based on the phase difference. A frequency measurement unit for measuring the frequency before Symbol distance measuring light, a comparator circuit for comparing the reference time and the measured value of the frequency, and a frequency changing means for changing the frequency of the distance measuring light which the light source emits light more The distance measuring device according to claim 1, further comprising: controlling the frequency changing unit based on an output of the comparison circuit.   The distance measuring device according to claim 1, wherein the external reference time acquisition device acquires an external reference time based on a signal from a GPS satellite.   A beat signal from the reference light receiving unit is input to the third electrical filter and the second electrical filter, respectively, and the beat signal selected by the third electrical filter is the The beat signal selected by the first electrical filter is slightly different from the beat signal selected by the first electrical filter and the third electrical filter, and a self beat signal is generated from the beat signal selected by the first electrical filter. The distance measuring device according to claim 1, wherein a beat signal having the same frequency as the self beat signal is selected, and a distance is measured based on a phase difference between the self beat signal and the beat signal selected by the second electric filter.

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