JP2016048188A - Distance measuring apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve high accurate measurement by extending the variable range of an optical path length difference, in distance measurement using an optical frequency com.SOLUTION: A distance measuring apparatus includes a pulse light source which generates a longitudinal mode of a fixed interval in a frequency region, a frequency control part which changes the repetition frequency of the pulse light source, a splitter which divides pulse light outputted from the pulse light source into reference light and measurement light, a photodetector which detects the reference light and the measurement light reflected by the measuring surface of a measuring object, and a signal processing part which calculates a distance from the detected light and the measuring surface. The frequency control part changes the repetition frequency of the pulse light source, to effectively change the optical path length difference between the reference light and the measurement light. The signal processing part calculates the distance up to the measuring surface in the optical path length difference effectively changed.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a distance measurement technique using an optical frequency comb.

  As a method for measuring the distance to the measurement object in a non-contact manner, optical measurement is suitable, and an interferometer or a distance meter is used. The interferometer is an instrument that obtains a distance from interference fringes when light beams that have passed through different optical paths (reference optical path and measurement optical path) are superimposed. The distance meter is a meter that obtains a distance from a time difference, a phase difference, a frequency difference change, and the like between an output wave and a reflected wave from an object.

  In recent years, a ranging technique using a light source called an optical comb or an optical frequency comb has been developed. The optical frequency comb is a spectrum obtained from a mode-locked pulse laser (ultra-short pulse laser). Since the shape of a spectrum having a number of frequency components (longitudinal modes) arranged at equal intervals resembles a comb, it is called an optical frequency comb (optical comb).

  As a distance meter using an optical comb, a method for obtaining a distance to an object based on a phase difference between a beat signal of reference light and a beat signal of distance measuring light reflected by the measurement object is known (for example, (See Patent Documents 1 and 2). In these documents, it is proposed that the measurable distance range is widened by changing the repetition frequency of the pulses output from the laser resonator, changing the interval of the optical comb, and changing the beat frequency.

  In addition, in a two-color interpulse interferometer (optical comb interferometer) using the fundamental wave and the second harmonic of the optical comb, a method for changing the optical path length by changing the repetition frequency frep of the optical comb is proposed. (For example, refer nonpatent literature 1).

  In an optical comb interferometer, in order to obtain the distance from the occurrence position of interference fringes between different pulses, it is necessary to provide an optical path length difference between the reference optical path and the probe optical path so that different pulses overlap each other. In order to accurately determine the interference order, means for changing the optical path length difference over a wide range is required.

JP 2006-184181 A (Patent No. 4617434) JP 2006-300753 A (Patent No. 4793675)

“High-accuracy self-correction of refractive index of air using two-color interferometry of optical frequency combs”, K. Minoshima, et al., Optics Express, vol. 19, 26095, 2011

  In the method of changing the optical path length by using the moving stage, a large moving stage is required to give a large optical path length, and the measurement signal fluctuates due to mechanical fluctuation or air fluctuation.

  On the other hand, when the variable range of optical path length and the distance measurement range are expanded by changing the repetition frequency of the optical comb, there is a limit to the frequency variable range.

  Therefore, it is an object to provide a distance measurement technique that enables highly accurate and stable measurement at an arbitrary distance by extending the variable range of the optical path length.

  In order to change the optical path length without using mechanical moving means, the effective optical path length difference is changed by changing the repetition frequency of the optical comb light source.

  As an example of the configuration of the distance measuring device, the present invention can be applied to an interpulse interferometer. In order to extend the variable range of the optical path length with the interpulse interferometer, the optical path length difference between the reference optical pulse and the measurement optical pulse that interfere with each other is increased to cause interference between a number of distant pulses. This method is effective for measuring long distances, but has limitations in applying to short distances. On the other hand, in order to enable precise measurement at a short distance, the optical path length of the reference light is set to be long. When the optical path length of the reference light is increased, it is desirable to use the reference optical path of the optical fiber in order to suppress the influence of disturbance. As another configuration example of the distance measuring device, the present invention can be applied to a distance meter using an optical comb.

Specifically, the optical measurement device which is one embodiment of the present invention includes:
A pulsed light source that generates a longitudinal mode at regular intervals in the frequency domain;
A frequency controller for changing the repetition frequency of the pulsed light source;
A splitter that divides the pulsed light output from the pulsed light source into reference light and measurement light;
A photodetector for detecting the reference light and the measurement light reflected by the measurement surface of the measurement object;
A signal processing unit that calculates a distance from the detected light to the measurement surface;
Have
The frequency control unit effectively changes the optical path length difference between the reference light and the measurement light by changing the repetition frequency,
The signal processing unit calculates a distance to the measurement surface based on the effectively changed optical path length difference.

  The variable range of the optical path length can be expanded to enable highly accurate and stable distance measurement at any distance.

It is a schematic block diagram of the interpulse interferometer as an example of a distance measuring device. It is a figure which shows the relationship between a femtosecond laser pulse and an optical comb. It is a figure which shows the interference fringe which appears when the repetition frequency _frep of an optical comb is changed. It is a figure which shows the development example of the interpulse interferometer of FIG. FIG. 5 is a diagram illustrating a configuration example for stabilizing a reference optical path in the configuration of FIG. 4. It is a figure which shows another structural example which stabilizes a reference optical path with the structure of FIG. It is a schematic block diagram of the optical comb rangefinder as an example of a distance measuring device. It is a figure explaining the distance measurement by an optical comb. It is a figure which shows the change (DELTA) _frep of mode space _| interval of an optical comb, and the phase difference (phi).

<Expansion of variable optical path length>
FIG. 1 shows a schematic configuration of an interpulse interferometer 10 as an example of a distance measuring apparatus according to an embodiment. The interpulse interferometer 10 includes an optical comb light source 11, a frequency control unit 12 that changes the repetition frequency of the optical comb light source 11, a beam splitter 13 that splits a pulse train from the optical comb light source 11 into reference light and probe light, A photodetector 17 and a signal processing unit 21 are included.

  The optical comb light source 11 is composed of, for example, a femtosecond pulse laser that is mode-locked (locked). As will be described later, the optical comb light source 11 configured by an ultrashort pulse laser such as a femtosecond pulse laser generates a plurality of longitudinal modes that appear at regular intervals in the frequency domain. The pulsed light (pulse train) output from the optical comb light source 11 is split by the beam splitter 13. One pulse train component is reflected by the reference surface 14 such as a mirror through the reference optical path 15, is reflected by the beam splitter 13, and enters the photodetector 17. The other pulse train component passes through the probe optical path 16, is reflected by the measurement surface 20 a of the measurement target 20, passes through the beam splitter 13, and enters the photodetector 17. Both the reference surface 14 and the measurement surface 20a may be fixed.

  The output of the photodetector 17 is input to the signal processing unit 21. The signal processing device calculates the distance to the measurement object 20 based on interference fringes between the pulse train passing through the reference optical path 15 and the pulse train passing through the probe optical path 16. The interference fringes are generated by superposition of the pulses, and the carrier wave 2 (electric field oscillation) is included in the envelope 3 of the pulse.

  In the case of the interpulse interferometer 10 using the optical frequency comb, due to the spatial localization of the ultrashort pulse, the difference between the optical path length of the reference optical path 15 and the optical path length of the probe optical path 16 is an integral multiple of the pulse repetition interval. Interference fringes are generated only at the positions. The amplitude of the interference fringes is maximized when the peaks of the pulses propagating through the two optical paths completely overlap. In order to obtain the distance L1 to the measurement object 20 using the pulse repetition interval as a measure of length, it is necessary to adjust the position of the reference surface 14 to a position where an interference fringe is generated.

  In the conventional method, the distance L2 to the reference surface 14 is adjusted by mechanically changing the position of the reference surface 14 with a motor stage or the like. On the other hand, in the interpulse interferometer 10 of the embodiment, the effective optical path length between the reference surface 14 and the measurement surface 20a is changed by changing the repetition frequency of the optical comb light source 11, that is, the pulse repetition interval, by the frequency control unit 12. The difference is adjusted to a position where interference fringes are observed. As a result, mechanical vibration and drift of the servo mechanism are suppressed, the stability of the entire optical system is maintained, and high-precision measurement is possible.

FIG. 2 is a diagram showing the relationship between femtosecond laser pulses and optical combs. The pulse width output from the optical comb light source 11 is on the order of femtoseconds, for example, 100 femtoseconds (fs). This ultrashort pulse is output at a constant repetition interval T _rep on the time axis. In this example, the repetition interval T _rep is 20 nanoseconds (ns).

When an ultrashort pulse train on the time axis is Fourier-transformed and observed on the frequency axis, as shown in the right diagram of FIG. 2, a number of longitudinal modes arranged at a repetition frequency f _rep corresponding to the reciprocal of T _rep are observed. In this sense, the repetition frequency f _rep may be called a “mode interval”. In this example, the repetition frequency f _rep is 50 MHz.

  The condition for generating interference fringes in the interpulse interference is when the optical path length difference L between the two optical paths is expressed by equation (1).

L = m × c / (n × f _rep ) (1)
Here, m is the number of pulses separating the interfering pulses (hereinafter referred to as “number of pulses”, m is an integer), n is the refractive index of the propagation medium (air), and c is the speed of light.

Here, it is possible to change the repetition frequency f _rep by changing conditions such as the length of the laser resonator of the optical comb light source 11. By controlling the repetition frequency of the optical comb light source 11 so that interference fringes are observed, the same effect as changing the optical path length by mechanically moving the position of the reference plane 14 can be obtained.

However, the variable range of the repetition frequency f _rep has a limit. Therefore, in order to expand the variable range of the effective optical path length, it is conceivable to increase the number of pulses m.

Since the repetition frequency f _rep correlates with the pulse repetition interval T _rep , when the repetition frequency f _rep is changed by Δf _rep , when the refractive index of the air is 1, and the interference between the m separated pulses, the optical path length The change amount of the difference is multiplied by m as shown in the equation (2).

ΔnL = m × (c / f _rep ) × (Δf _rep / f _rep ) (2)
Therefore, the variable range of the effective optical path length difference can be increased by increasing m and obtaining interference between two pulses separated from each other. Here, when the number of pulses m is changed by the known Δf _rep from the equation (2), the phase change of the interference fringes is detected, or the change of the peak position where the interference fringes appear is measured, It is obtained by calculating the optical path length difference.

FIG. 3 is a diagram showing the envelope intensity of interference fringes that appear when the repetition frequency f _rep is changed. In the interpulse interferometer 10, as described above, interference fringes are generated only at positions where the difference between the optical path length of the reference optical path and the optical path length of the probe optical path is an integral multiple of the pulse repetition interval. It is also important to determine the peak position of the interference fringe envelope for distance measurement. Let f _peak be the value of f _rep when the envelope of the interference fringe has the maximum amplitude.

Considering the optical system folded by the reflector, the effect of the optical path length change is doubled by folding, so that the f _rep necessary for detecting the interference fringe signal and finding the maximum amplitude position is required. The amount of change Δf _rep is required to be f _rep / 4 at maximum (Δf _rep ≦ f _rep / 4). This is because the maximum amplitude position of the interference fringe envelope can be swept by changing f _rep regardless of the position of the arranged reference plane (mirror) 14. By utilizing this characteristic, the reference plane (mirror) 14 can be set at an arbitrary position, and the maximum amplitude position of the interference fringes can be found without changing the geometric length of the optical system.

For example, f _rep = 50 MHz in the example of FIG. f _rep can be changed up to 900 kHz (Δf _rep = 900 kHz), and the number of pulses m is 10 (interference between 10 pulses apart). In this case, the equation (2) is equivalent to changing the optical path length by 10.8 m.
<Application to short measurement distance>
FIG. 4 shows an interpulse interferometer 30 as a development example of the interpulse interferometer 10 of FIG. As shown in FIG. 1, when the distance L1 to the measurement object 20 is much larger than the distance L2 to the reference plane 14 (L1 >> L2), the repetition frequency of the optical comb light source 11 is changed and the distance is increased. By interfering between pulses (increasing the number of pulses m), the variable range of the optical path length can be increased.

On the other hand, when the distance L1 to the measuring object 20 is small, it becomes difficult to obtain interference between the separated pulses. When the number of pulses m is small, the effect of extending the optical path length difference by changing f _rep cannot be obtained sufficiently.

  Therefore, in FIG. 4, the number of pulses m is increased by setting the length of the optical path 15 of the reference light larger than the length of the optical path 16 of the probe light. By making the distance L2 to the reference surface 14 much larger than the distance L1 to the measurement surface 20a (L2 >> L1), the distance L1 of the measurement object 20 located nearby can be accurately measured.

Increasing the distance L2 to the reference surface 14 increases the influence of disturbance and may hinder high-accuracy measurement. Therefore, in the example of FIG. 4, the optical path of the reference light is configured by the optical fiber 31. Thereby, stabilization control of the optical path length difference can be performed. Moreover, by using the optical fiber 31, the interpulse interferometer 30 can be configured in a compact manner while increasing the optical path length of the reference light. Further, since the refractive index of the optical fiber 31 is larger than the refractive index of air, when the probe optical path is an optical path in the air, the reference light pulse is delayed from the pulse of the probe light propagating in the air. The change in the optical path length difference due to the change in f _rep can be effectively increased.

  5 and 6 show a configuration example for stabilizing the optical path 15 of the extended reference light. When the length of the optical fiber 31 is increased, there is a possibility that the intensity of the interference fringe signal may fluctuate due to fluctuations in the optical path length due to temperature changes, phase noise, or the like. For example, fiber noise cancellation used in optical fiber transmission By applying this method, the reference optical path can be stabilized. In the interpulse interferometer 30A of FIG. 5, optical separators 41 and 42, a phase comparison element 43, a signal processing unit 44, and an optical path length change compensation element 45 are inserted into the optical path 15 of the reference light. In the optical separator 41, a part of the reference light exiting the beam splitter 13 is branched and connected to one input of the phase comparison element 43. In the optical separator 42, a part of the reference light emitted from the optical fiber 31 is branched and connected to the other input of the phase comparison element 43. The phase comparison element 43 compares the phase of light on the incident side of the optical fiber 31 with the phase of light on the output side, and outputs the comparison result to the signal processing unit 44. The signal processing unit 44 converts the phase difference into an electrical signal representing the compensation amount of the optical path length change, and outputs the electrical signal to the optical path length change compensation element 45. The optical path length change compensation element 45 compensates for the change in the optical path length based on the signal from the signal processing unit 44. Thereby, the extended reference optical path can be stabilized, and fluctuations in the intensity of detected interference fringes can be suppressed.

FIG. 6 shows another example of stabilization of the optical path 15 of the reference light. In the interpulse interferometer 30 </ b> B of FIG. 6, the optical separator 41 branches a part of the reference light emitted from the beam splitter 13 and connects it to one input of the phase comparison element 43. Part of the light propagating through the optical fiber 31 is folded back by the folding mirror 47, and part of the return light propagating through the optical fiber 31 is branched by the optical separator 46 and connected to the other input of the phase comparison element 43. To do. The phase comparison element 43 compares the phase of the light on the incident side of the optical fiber 31 with the phase of the light reciprocating through the optical fiber 31, and outputs the comparison result to the signal processing unit 44. The signal processing unit 44 converts the phase difference into an electrical signal representing the compensation amount of the optical path length change, and outputs the electrical signal to the optical path length change compensation element 45. The optical path length change compensation element 45 compensates for the change in the optical path length based on the signal from the signal processing unit 44. Even with this configuration, it is possible to suppress fluctuations in the intensity of interference fringes detected by stabilizing the reference optical path.
<Application to optical comb rangefinder>
FIG. 7 shows a schematic configuration of an optical comb rangefinder 40 as another example of the distance measuring device. The output light output from the optical comb light source 11 is split into reference light and ranging light by the beam splitter 13. The reference light is detected by a photodetector 42 (not shown in FIG. 7) and input to the signal processing unit 41 as it is. The distance measuring light is guided to the measurement object 20, and the reflected light from the measurement object is detected by a photodetector (not shown in FIG. 7) and input to the signal processing unit 41. The signal processing unit 41 generates a large number of beat signals, of which frequency components used for measurement are selected, and the distance to the measurement object 20 based on the phase difference between the reference light and the reflected light in the beat frequency components. D is calculated.

FIG. 8 is a diagram for explaining distance calculation by the optical comb distance meter 40. When the output light output from the optical comb light source 11 is reflected by the measurement object 20 and received by the photodetector 42, a large number of beat signals corresponding to an integral multiple of the optical comb repetition frequency f _rep are generated. Is done. Among them, the beat frequency f used for measurement is selected by a filter or the like. The reference light is similarly detected and the phase difference between the two is electrically measured. At this time, the optical path length difference between them corresponds to the wavelength and fraction (phase difference φ) of N modulation waves corresponding to the beat frequency f. Therefore, the optical path length difference can be calculated from the phase difference measurement between the distance measuring light and the reference light. The phase difference φ is changed by changing the repetition frequency of the optical comb light source 11 by the frequency control unit 12. This means that the same effect as changing the optical path length to the measuring object 20 is obtained, and the optical path length can be effectively varied.

If the beat frequency is f = Mf _rep (M is a known integer), the air group refractive index is _ng , and the speed of light is c, the wavelength of the modulated wave is c / ( _ng × Mf _rep ).

When there are N pulses and a phase difference φ in a round trip, the distance D to the measurement object 20 is
_{D = (c / 2n g Mf} rep) × (N + φ / 2π) (3)
It is expressed. By specifying the integer N and the phase difference φ, the absolute distance can be measured.

When the repetition frequency f _rep of the optical comb light source 11 is changed little by little, the frequency f of the beat signal corresponding to an integral multiple thereof also changes, and the phase difference φ also changes, and a number of simultaneous equations are established for the equation (3). By solving this, the integer N and the phase difference φ, that is, the distance D can be obtained.

FIG. 9 is a graph showing the relationship between the change Δf _{rep in the} repetition frequency and the change in the phase difference φ. From equation (3), the phase difference φ can be expressed as a function of the repetition frequency f _rep .

_{φ = (4πn g D / c} ) × Mf rep -2πN (4)
By taking a large number of data points by changing the repetition frequency f _rep, it can be obtained gradient (4πn _g DM / c) more accurately. Since this corresponds to obtaining an approximate value of the absolute distance, the integer N can be determined. At that time, as shown in the measurement example of FIG. 9, there is a limit to the measurement accuracy of the phase difference φ due to variations in phase measurement data, and a minute phase change amount less than the measurement resolution cannot be measured correctly. For this reason, it is important to ensure a large range of phase change. As a solution for that, it is possible to increase the optical path length difference between the reference optical path and the distance measuring optical path as in the case of the interpulse interferometer described above. That is, this corresponds to increasing the effective distance D in Formula (4) to increase the slope of the change, and the change in phase difference with respect to the same change in f _rep can be increased. In other words, since the number N of modulated waves can be effectively increased by increasing the optical path length difference to be measured, the effect of the modulation wavelength change due to the repetition frequency change Δf _rep is multiplied, and as a result Changes in the phase difference φ are also multiplied. That is, the same effect as increasing the number of pulses m in the interpulse interferometer can be obtained. Also in this case, as in the case of the interpulse interferometer, by increasing the reference optical path, a large optical path difference can be obtained even when measuring a short distance.

  As described above, according to the configuration and method of the embodiment, it is possible to obtain the same effect as changing the optical path length by changing the repetition frequency of the optical comb light source.

  Further, the variable range of the optical path length difference can be expanded by increasing the number of pulses m to cause interference between pulses that are separated from each other, or by increasing the measurement optical path difference and increasing the number N of modulated waves.

  Furthermore, by increasing the reference optical path or the reference optical path, the number of pulses m or the number N of modulated waves can be accurately determined from a large optical path length difference even when measuring a short distance.

  By configuring the reference optical path with an optical fiber, the measurement apparatus can be configured compactly and the optical path can be stabilized.

10, 30, 30A, 30B Interpulse interferometer (distance measuring device)
11 Optical Comb Light Source 12 Frequency Control Unit 12
13 Beam splitter 14 Reference surface 17 Photo detector 20 Measurement object 20a Measurement surface 21, 41 Signal processing unit 31 Optical fiber 40 Optical comb distance meter (distance measuring device)

Claims (6)

A pulsed light source that generates a longitudinal mode at regular intervals in the frequency domain;
A frequency controller for changing the repetition frequency of the pulsed light source;
A splitter that divides the pulsed light output from the pulsed light source into reference light and measurement light;
A photodetector for detecting the reference light and the measurement light reflected by the measurement surface of the measurement object;
A signal processing unit that calculates a distance from the detected light to the measurement surface;
Have
The frequency control unit effectively changes the optical path length difference between the reference light and the measurement light by changing the repetition frequency of the pulse light source,
The said signal processing part calculates the distance to the said measurement surface in the said optical path length difference changed effectively, The distance measuring apparatus characterized by the above-mentioned. A reference surface for reflecting the reference light;
Further comprising
The photodetector detects the first pulsed light reflected by the reference surface and the second pulsed light reflected by the measurement surface;
The frequency control unit effectively changes the optical path length difference by changing the repetition frequency of the pulsed light source to a position where interference between the first pulsed light and the second pulsed light is observed.
The signal processing unit calculates a distance to the measurement surface based on the number of pulses between the first pulsed light and the second pulsed light that interfere with each other in the effectively changed optical path length difference. A distance measuring device characterized by. The effective optical path length difference is L, the repetition frequency of the longitudinal mode is frep, the amount of change of the repetition frequency is Δfrep, the number of pulses between the first pulse light and the second pulse light is m (m is an integer), When the refractive index of the pulsed light propagation medium is n, the amount of change in the optical path length difference is
ΔnL = m × (c / f _rep ) × (Δf _rep / f _rep )
The distance measuring device according to claim 2, wherein   3. The distance measuring device according to claim 2, wherein L2 >> L1, where L1 is a distance from the splitter to the measurement surface and L2 is a distance from the splitter to the reference surface.   The distance measuring device according to claim 4, wherein an optical path from the splitter to the reference plane is configured by an optical fiber. The frequency control unit changes the phase difference between the reference light and the measurement light at a beat frequency by changing the effective optical path length difference by changing the repetition frequency of the pulse light source,
The signal processing unit calculates a distance to the measurement surface based on a correlation between the change amount of the repetition frequency and the change amount of the phase difference;
The distance measuring device according to claim 1.

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