JP4617434B2 - Distance measuring device - Google Patents

Description

  The present invention relates to a distance measuring device using an optical frequency comb.

  As a distance measuring device, there is a light wave distance measuring device that irradiates a measuring object with a laser beam and measures the distance to the measuring object using reflected light from the measuring object.

  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, it compares the light received through the reference optical path formed inside the distance measuring device (hereinafter referred to as reference light), and the distance is calculated from the phase difference of the intensity modulation between the reflected distance measuring light and the reference light. Measuring.

  When measuring the distance from the distance measuring light and the reflected distance measuring light, the drift of the detection circuit inside the distance measuring device appears as a measurement error, so refer to the phase difference between the distance measuring light and the reflected distance measuring light. In general, the phase difference between the light and the distance measuring light is obtained, and both phase differences are subtracted to perform accurate distance measurement from the distance measuring light and the reference light.

  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. Actually, two or more modulated lights at different frequencies are used for measurement, and the measurement effective digit of the distance value is determined according to the resolution.

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

  The signal of the reference oscillator 1 is input to the frequency divider 2 and divided to a necessary frequency. The frequency selector 3 selects the frequency k 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 a frequency k to emit distance measuring light 6.

  The distance measuring light 6 passes through the objective lens 7 and is reflected as reflected distance measuring light 9 by the reflecting mirror 8 which is a measurement object placed at the measurement point. The reflected distance measuring light 9 passes through the objective lens 7. Incident. The incident reflected distance measuring light 9 is received by the light receiving element 11, and the light receiving signal fd transmitted from the light receiving element 11 is amplified by the broadband amplification filter 12 and then mixed with the frequency signal fc + fd from the frequency divider 2. And beat down. The beat-down range-finding light beatdown signal fe is amplified by the amplification band filter 14 and input to the phase difference measuring device 15.

  The reference light 17 emitted from the light emitting element 5, reflected by the half mirror 16 and passed through the internal optical path is received by the light receiving element 11, and the received light signal fd transmitted from the light receiving element 11 is the broadband amplification filter 12. After being amplified in step (b), the signal is beat-down by mixing, amplified as the reference light beat-down signal fe ′ by the amplification band filter 14, and input to the phase difference measuring device 15.

  The reflected distance measuring light 9 and the reference light 17 are alternatively selected by an optical path switch (not shown), and the phase difference measuring device 15 selects the beat-down signal fe of the reflected distance measuring light 9 and the reference light 17. A phase difference is obtained from the beat down signal fe ′. Further, based on the signal from the phase difference measuring device 15, the calculation unit 18 obtains a phase difference by combining a plurality of frequencies according to the distance, and converts it to the distance by the above equation (1). The display 19 displays the determined distance.

  In the conventional distance measuring apparatus described above, the light emitting element driving circuit 4 drives the light emitting element 5 based on the frequency divided by the frequency divider 2. In order to improve the accuracy of the distance meter that measures by the phase difference, the modulation frequency must be increased. However, in the laser diode that is currently widely used as the light source of the distance measuring device, the upper limit is about several GHz, and more High frequency cannot be expected. Further, the light emitting element driving circuit 4 of the light emitting element 5 is likely to be a source of noise and may cause a measurement error.

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 enables highly accurate and stable measurement, omits the light emitting element driving circuit, simplifies the circuit configuration, suppresses the generation of noise from the circuit, and improves the measurement accuracy and reliability. Provided is a distance measuring device that improves the above.

  The present invention includes a laser device that generates an optical frequency comb as a laser beam, a dividing unit that divides the laser beam from the laser device into a reference light and a ranging light, and a plurality of beat signals that are received by the reference light. A reference light receiving unit that outputs, a measurement light receiving unit that receives the ranging light and outputs a number of beat signals, a first electrical filter that extracts at least one beat signal from the measurement light receiving unit, and the reference light receiving unit A second electric filter for extracting at least one beat signal from the phase, and a phase obtained based on the beat signal output from the first electric filter and a beat signal output from the second electric filter. A phase difference is obtained based on a phase obtained based on the phase difference, and a distance measuring device that measures a distance based on the phase difference, further includes a third electrical filter, the third electrical filter, The beat signal from the reference light receiving unit is input to each of the two electrical filters, and the beat signal selected by the third electrical filter is slightly different from the beat signal selected by the first electrical filter, A self-beat signal is generated from beat signals selected by one electrical filter and a third electrical filter, and the second electrical filter selects a beat signal having the same frequency as the self-beat signal, and selects the self-beat signal. The present invention relates to a distance measuring device for measuring a distance based on a phase difference from a beat signal, and a resonator length changing means for changing a resonator length is provided in the laser device, and a measuring distance is changed by changing an oscillation frequency. The present invention relates to a distance measuring device, a distance measuring device including an electrical filter corresponding to the oscillation frequency of the laser device by the resonator length changing means, and The first electrical filter and the second electrical filter have an oscillator that selects a beat signal having the same frequency and generates a frequency signal having a frequency slightly different from that of the selected beat signal. A distance measuring device having a first mixer and a second mixer for taking out a beat signal based on the phase difference of the beat signals of the first mixer and the second mixer, and the oscillator Instead of this, the distance measuring device is provided with a fifth filter for extracting a frequency slightly different from the selected beat signal from the reference light receiving unit.

  According to the present invention, the present invention provides a laser device that generates an optical frequency comb as a laser beam, a splitting unit that divides the laser beam from the laser device into a reference light and a ranging light, and receives the reference light. A reference light receiving unit that outputs a large number of beat signals, a measurement light receiving unit that receives the distance measuring light and outputs a large number of beat signals, and a first electrical filter that extracts at least one beat signal from the measurement light receiving unit And a second electrical filter for extracting at least one beat signal from the reference light receiving unit, and a phase obtained based on the beat signal output from the first electrical filter and the second electrical filter The phase difference is calculated based on the phase obtained based on the output beat signal, the distance is measured based on the phase difference, and the optical frequency comb is very stable and can be emitted with high accuracy. It is possible, also the light emitting element driving circuit of the light emitting element is not required, generation of noise from the light emitting element driving circuit together with a circuit configuration is simplified eliminated.

  Further, according to the present invention, the laser device is provided with a resonator length changing means for changing the resonator length, and the measurement distance is changed by changing the oscillation frequency, so that distance measurement in various measurement ranges becomes possible. Exhibits excellent effects such as.

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

  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. 1A, one pulse has a wide spectrum width, and has a spectrum of several THz with 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.

  The stability of the inter-mode beat is the stability of the repetition frequency, which is very stable in the femtosecond mode-locked pulse laser.

  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.

  Here, 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.

  In order to generate an optical frequency comb using ultrashort optical pulses, a resonator with a built-in laser active medium with a wide gain spectrum is used to excite 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. This consists of a substance with an absorption transition having a very large absorption cross section with respect to 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.

  Next, the first embodiment will be described with reference to FIG.

  In FIG. 2, 21 is an ultrashort pulse light emission source, that is, a femtosecond mode-locked laser device, and 22 indicates an oscillation frequency changing means. The oscillation frequency changing means 22 changes the oscillation frequency 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. For example, a piezo element is used as the resonator length changing means.

  A splitter 23 for dividing 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 measurement that has passed through the splitter 23 is provided. The distance 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 surface of a bright natural object is selected as the measuring object. .

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

  A second half mirror 28 is provided on the optical path of the distance measuring light 31 between the splitter 23 and the objective lens 24, and the reflection distance measuring between the light receiving lens 26 and the first light receiving element 27. A third 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 passes through the third half mirror 29 and is received by the first light receiving element 27. Is done. A part of the distance measuring light 31 is reflected as internal reference light 33 by the second half mirror 28, and the internal reference light 33 is reflected by the third half mirror 29 and enters the first light receiving element 27. It has become. A light reception signal is sent from the first light receiving element 27 to the mixer 35 via the first filter 34.

  An optical path switch (chopper) 30 is provided across the optical path of the distance measuring light 31 transmitted through the second half mirror 28 and the optical path of the internal reference light 33 reflected by the second half mirror 28. Yes. The distance measuring light 31 and the internal reference light 33 are alternatively selected by the optical path switch 30, and the reflected distance measuring light 32 and the internal reference light 33 are alternately incident on the first light receiving element 27.

  A part of the distance measuring light 31 divided by the splitter 23 is received as a reference light 36 by a light receiving unit including 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 sent to the second filter 38 and the third filter 39, respectively, and the signal inputted to the third filter 39 is sent to the mixer 35, and the mixer 35 The signal from the first filter 34 is mixed and input to the phase difference measurement circuit 42 via the fourth filter 41. The signal input to the second filter 38 is also input to the phase difference measurement circuit 42, and the signal from the fourth filter 41 and the signal from the second filter 38 are input to the phase difference measurement circuit 42. The calculation unit 43 calculates the distance to the measurement object 25 based on the phase difference calculated by the phase difference measurement circuit 42.

  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 divided into the distance measuring light 31 and the reference light 36 by the splitter 23. Each pulsed light to be oscillated consists of a wide frequency band with a frequency interval of 50 MHz, which has a precise and regular frequency difference 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 second half mirror 28. The internal reference light 33 and the distance measuring light 31 are alternatively selected by the 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 distance measuring light 32 enters from the light receiving lens 26 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 one beat signal selected by the third filter 39 from a large number of inter-mode beat signals output from the second light receiving element 37. One beat signal selected by the first filter 34 from a number of inter-mode beat signals that are photoelectrically converted and output by the first light receiving element 27 is input to the mixer 35, and two input beats are input. Two beat signals having the sum and difference frequencies of the signals are 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 phase difference measuring circuit 42 measures a phase difference from the beat signal selected by the second filter 38 and the self beat signal.

  Since the phase of the beat signal of the distance measuring light 31 selected by the first filter 34 is stored in the self beat signal, the beat of the reference light 36 selected by the self beat signal and the second filter 38 is stored. The phase difference with the signal is obtained, and 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 from the beat signal of the reference light 36 and the self-beat signal when the first light receiving element 27 receives the internal reference light 33. Become.

  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. In the present embodiment, since a frequency lower than 50 MHz cannot be obtained by the beat signal between modes, 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 may be used instead of the piezo element.

  Since the frequency comb interval corresponds to the measurable distance, the distance exceeding 3 m can be measured by changing the frequency.

  If the frequency change of the beat signal between modes due to the change of the oscillation wavelength is several MHz or less, the transmission characteristics of the first filter 34, the second filter 38, and the third filter 39 are selected so that the frequency change is passed in advance. do it. Furthermore, when the frequency change is large, a plurality of filters having different characteristics may be prepared, and the filters may be switched according to the selected frequency. The filter may be switched in correspondence with the change of the resonator length by the oscillation frequency changing means 22.

  FIG. 3 shows a second embodiment. In FIG. 3, the same components as those shown in FIG. 2 are denoted by the same reference numerals. Since the optical system has the same configuration, the following description is omitted.

  The light reception signal from the first light receiving element 27 is input to the first mixer 45 via the first filter 34, and the light reception signal from the second light receiving element 37 is input to the second mixer 46 via the second filter 38. The A frequency 48 for mixing generated by an oscillator 47 is input to the first mixer 45 and the second mixer 46.

  The beat signal sent from the first mixer 45 is sent to the phase difference measuring circuit 42 via the third filter 49, and the beat signal sent from the second mixer 46 is sent to the phase difference via the fourth filter 51. It is sent to the measurement circuit 42. The phase difference measurement circuit 42 calculates the phase between both beat signals, and the calculated phase is output to the calculation unit 43.

  Hereinafter, the operation in the second embodiment will be described.

  In the second embodiment, the oscillator 47 generates a frequency signal 48 having a slight frequency difference with respect to the same frequency selected from beat signals between modes obtained from the ranging light 31 and the reference light 36. It is a thing.

  The first filter 34 and the second filter 38 select an inter-mode beat signal having the same frequency and input it to the first mixer 45 and the second mixer 46. A frequency signal 48 having a frequency different from that of the selected inter-mode beat signal is input to the first mixer 45 and the second mixer 46 from the oscillator 47.

  The first mixer 45 outputs a beat signal that is the sum and difference of the frequency selected by the first filter 34 and the frequency of the frequency signal 48. The third filter 49 selects a beat signal having a frequency that is the difference between the two beat signals output from the first mixer 45.

  The second mixer 46 outputs two beat signals, the sum and difference of the frequency selected by the second filter 38 and the frequency of the frequency signal 48. The fourth filter 51 selects a beat signal having an internal difference frequency between the two beat signals output from the second mixer 46.

  In the phase difference measuring circuit 42, a phase difference between beat signals selected by the third filter 49 and the fourth filter 51 is calculated, and the calculation unit 43 calculates a distance based on the calculated phase difference. .

  Note that the internal optical path distance is obtained based on the internal reference light and the reference light, and the external optical path distance is corrected as in the first embodiment.

  FIG. 4 shows a third embodiment. In the third embodiment, a fifth filter 50 is provided instead of the oscillator 47 shown in FIG. The fifth filter 50 selects the frequency signal 48 from the inter-mode beat signal obtained from the second light receiving element 37 and inputs it to the first mixer 45 and the second mixer 46. Since the operation is the same as that of the second embodiment, the description thereof is omitted.

(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 block diagram which shows the 1st Embodiment of this invention. It is a block diagram which shows the 2nd Embodiment of this invention. It is a block diagram which shows the 3rd Embodiment of this invention. It is a block diagram which shows a prior art example.

Explanation of symbols

21 Femtosecond mode-locked laser device 22 Oscillation frequency changing means 23 Splitter 27 First light receiving element 31 Distance measuring light 32 Reflected distance measuring light 33 Internal reference light 34 First filter 35 Mixer 37 Second light receiving element 38 Second filter 39 Third filter Filter 41 Fourth filter 42 Phase difference measurement circuit 45 First mixer 46 Second mixer 48 Frequency signal 49 Third filter 50 Fifth filter 51 Fourth filter

Claims (6)

A laser device that generates an optical frequency comb that is a laser beam formed by making the phase difference between the modes constant , and a splitting unit that splits the laser beam from the laser device into a reference light and a ranging light; A reference light receiving unit that receives the reference light and outputs a number of beat signals, a measurement light receiving unit that receives the distance measurement light and outputs a number of beat signals, and at least one beat signal from the measurement light receiving unit. A first electrical filter to be taken out, and a second electrical filter to take out at least one beat signal from the reference light receiving unit, and a phase obtained based on a beat signal output from the first electrical filter; A distance measuring device characterized in that a phase difference is obtained based on a phase obtained based on a beat signal output from a second electrical filter, and a distance is measured based on the phase difference. A laser device that generates an optical frequency comb that is a laser beam formed by making the phase difference between the modes constant, and a splitting unit that splits the laser beam from the laser device into a reference light and a ranging light; A reference light receiving unit that receives the reference light and outputs a number of beat signals, a measurement light receiving unit that receives the distance measurement light and outputs a number of beat signals, and at least one beat signal from the measurement light receiving unit. It possesses a first electrical filter is taken out, and a second electrical filter for taking out at least one beat signal from the reference light receiving unit, and a third electrical filter,
Beat signals from the reference light receiving unit are respectively input to the third electrical filter and the second electrical filter, and a beat signal selected by the third electrical filter is a beat signal selected by the first electrical filter. The self-beat signal is generated from the beat signal selected by the first electric filter and the third electric filter, and the second electric filter generates a beat signal having the same frequency as the self-beat signal. A distance measuring apparatus that selects and measures a distance based on a phase difference between the self beat signal and the selected beat signal.   2. The distance measuring device according to claim 1, wherein the laser device is provided with a resonator length changing means for changing a resonator length, and the measuring distance is changed by changing an oscillation frequency.   4. The distance measuring device according to claim 3, further comprising an electrical filter corresponding to the oscillation frequency of the laser device by the resonator length changing means.   The first electric filter and the second electric filter have an oscillator that selects a beat signal having the same frequency and emits a frequency signal slightly different in frequency from the selected beat signal, and the beat signal and the frequency signal 2. A distance measuring apparatus according to claim 1, further comprising a first mixer and a second mixer for taking out beat signals based on each of the first and second mixers, and measuring a distance based on a phase difference between beat signals of the first mixer and the second mixer.   6. The distance measuring device according to claim 5, further comprising a fifth filter that takes out a frequency slightly different from the selected beat signal from the reference light receiving unit, instead of the oscillator.

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