JP6836048B2 - Distance measuring device - Google Patents

Description

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

Optical measurement is suitable as a method for measuring the distance to the object to be measured in a non-contact manner, and an interferometer or a range finder is used. The interferometer is an instrument that obtains the distance from the interference fringes when the light passing through different optical paths (reference optical path and measurement optical path) is superimposed. A range finder is an instrument that calculates the distance from the time difference, phase difference, frequency difference change, etc. between the output wave and the reflected wave from the object.

In recent years, a distance measuring 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 pulsed laser (ultrashort pulsed laser). Since the shape of the spectrum having a large number of frequency components (longitudinal mode) arranged at equal intervals resembles a comb, it is called an optical frequency comb.

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

In addition, in a two-color pulse interferometer (optical comb interferometer) that uses the fundamental wave and the second harmonic of the optical comb, a method of changing the optical path length by changing the repetition frequency frep of the optical comb has been proposed. (See, for example, Non-Patent Document 1).

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

Japanese Unexamined Patent Publication No. 2006-184181 (Patent No. 4617434) Japanese Unexamined Patent Publication No. 2006-300753 (Patent No. 4793675)

“High-accuracy self-correction of refractive index of air usingtwo-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 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 and air fluctuation.

On the other hand, when the repeating frequency of the optical comb is changed to widen the variable range of the optical path length and the ranging range, the variable frequency range is limited.

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

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

As one configuration example of a distance measuring device, the present invention can be applied to an inter-pulse interferometer. In order to extend the variable range of the optical path length with the inter-pulse interferometer, the optical path length difference between the reference light pulse and the measurement light pulse that interfere with each other is increased so that a number of distant pulses interfere with each other. This method is effective for measuring long distances, but has limitations in applying it for short distances. On the other hand, in order to enable precise measurement over a short distance, the optical path length of the reference light is set long. When increasing the optical path length of the reference light, 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 measuring device according to one aspect of the present invention is
A pulsed light source that generates longitudinal modes at regular intervals in the frequency domain,
A frequency control unit that changes the repetition frequency of the pulse light source,
A splitter that divides the pulsed light output from the pulsed light source into reference light and measurement light,
A photodetector that detects the reference light and the measurement light reflected by the measurement surface of the object to be measured.
A signal processing unit that calculates the distance from the detected light to the measurement surface, and
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 is characterized in that the distance to the measurement surface is calculated based on the effectively changed optical path length difference.

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

It is a schematic block diagram of the inter-pulse 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 f _{rep of an optical comb is changed.} It is a figure which shows the development example of the inter-pulse interferometer of FIG. It is a figure which shows the configuration example which stabilizes a reference optical path in the configuration of FIG. It is a figure which shows another configuration example which stabilizes a reference optical path in the configuration of FIG. It is a schematic block diagram of an optical comb range finder 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 relationship _{between the change Δf rep} of the mode interval of an optical comb, and the phase difference φ.

<Expansion of variable optical path length range>
FIG. 1 shows a schematic configuration of an inter-pulse interferometer 10 as an example of the distance measuring device of the embodiment. The inter-pulse interference meter 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, and a beam splitter 13 that divides a pulse train from the optical comb light source 11 into reference light and probe light. It includes an optical detector 17 and a signal processing unit 21.

The optical comb light source 11 is composed of, for example, a mode-locked femtosecond pulse laser. The optical comb light source 11 composed of an ultrashort pulse laser such as a femtosecond pulse laser generates a plurality of longitudinal modes appearing at regular intervals in the frequency domain, as will be described later. 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, reflected by the beam splitter 13, and incident on the photodetector 17. The other pulse train component is reflected by the measurement surface 20a of the measurement object 20 through the probe optical path 16, passes through the beam splitter 13, and is incident on 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 the 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 fringe is generated by the superposition of the pulse, and the carrier wave 2 (electric field vibration) is included in the envelope 3 of the pulse.

In the case of the inter-pulse interferometer 10 using the optical frequency comb, 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 due to the spatial localization of the ultra-short pulse. Interferometric fringes occur only at the above position. The amplitude of the interference fringes is maximized when the peaks of the pulses propagating in the two optical paths completely overlap. In order to obtain the distance L1 to the object to be measured 20 using the pulse repetition interval as a measure of length, it is necessary to adjust the position of the reference surface 14 to the position where the interference fringes occur.

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

FIG. 2 is a diagram showing the relationship between the femtosecond laser pulse and the optical comb. 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 repeat interval T _rep is 20 nanoseconds (ns).

When the ultra-short pulse train on the time axis is Fourier transformed and observed on the frequency axis, a large number of longitudinal modes arranged at _{the repeating frequency f rep} corresponding to the reciprocal of _{T rep are observed as shown in the right figure of FIG.} In this sense, the repetition frequency f _rep may be referred to as a "mode interval". In this example, the repetition frequency f _rep is 50 MHz.

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

L = m × c / (n × f _rep ) (1)
Here, m is the number of pulses separating the interfering pulses (hereinafter, referred to as "the 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, the repetition frequency f rep} can be changed 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 to the place where the interference fringes are observed, the same effect as changing the optical path length by mechanically moving the position of the reference surface 14 can be obtained.

However, there is a limit to the variable range of the repetition frequency f _rep. 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 and the pulse repetition interval T _rep are correlated, when the repetition frequency f _rep is _{changed by Δf rep} , the refractive index of air is set to 1, and the optical path length of the interference between pulses separated by m. The amount of change in 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 to obtain interference between two pulses separated from each other. Here, the number of pulses m is determined by detecting the phase change of the interference fringes or measuring the change of the peak position where the interference fringes appear when _{the known Δf rep is changed from the equation (2).} It is obtained by calculating the optical path length difference.

FIG. 3 is a diagram showing the envelope strength of the interference fringes appearing when _{the repetition frequency f rep is changed.} In the inter-pulse interferometer 10, as described above, interference fringes are generated only at a position 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. In addition, it is important to determine the peak position of the envelope of the interference fringes 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 the folding, so the f _rep required to detect the interference fringe signal and to find the maximum amplitude position. The maximum amount of change Δf _rep is f _rep / 4 (Δf _rep ≤ f _rep / 4). This is because the maximum amplitude position of the envelope of the interference fringe can be swept by changing _{the 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 fringe can be found without changing the geometrical length of the optical system.

For example, in the example of FIG. 2, f _rep = 50 MHz. f _rep the can be changed up to 900 kHz (Delta] f _rep = 900 kHz), also the number of pulses m is 10 (ten away interference pulses between). In this case, it is equivalent to changing the optical path length by 10.8 m from the equation (2).
<Application to short measurement distances>
FIG. 4 shows an inter-pulse interferometer 30 as a deployment example of the inter-pulse 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 separated. The variable range of the optical path length can be increased by interfering between the pulses (increasing the number of pulses m).

On the other hand, when the distance L1 to the object to be measured 20 is small, it becomes difficult to obtain interference between distant pulses. If the number of pulses m is small, the effect of expanding the optical path length difference by changing _{f rep cannot be sufficiently obtained.}

Therefore, in FIG. 4, the number of pulses m is increased by setting the length of the optical path 15 of the reference light to be 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.

If the distance L2 to the reference surface 14 is increased, the influence of disturbance becomes large, which may hinder high-precision measurement. Therefore, in the example of FIG. 4, the optical path of the reference light is configured by the optical fiber 31. Thereby, the optical path length difference can be stabilized and controlled. Further, by using the optical fiber 31, the inter-pulse interferometer 30 can be compactly configured 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 more than the pulse of the probe light propagating in the air. This makes it possible to effectively increase the change in the optical path length difference due to the change _{in f rep.}

5 and 6 show a configuration example for stabilizing the optical path 15 of the extended reference light. If there is a risk that the strength of the interference fringe signal will fluctuate due to fluctuations in the optical path length due to temperature changes, phase noise, etc. by increasing the length of the optical fiber 31, for example, fiber noise cancellation used for optical fiber transmission. By applying the method of, the reference optical path can be stabilized. In the inter-pulse interferometer 30A of FIG. 5, the optical separators 41 and 42, the phase comparison element 43, the signal processing unit 44, and the optical path length change compensation element 45 are inserted into the optical path 15 of the reference light. 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. The optical separator 42 branches a part of the reference light emitted from the optical fiber 31 and connects it to the other input of the phase comparison element 43. 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 on the exit side, and outputs the comparison result to the signal processing unit 44. The signal processing unit 44 converts the phase difference into an electric signal representing the compensation amount of the optical path length change, and outputs the electric signal to the optical path length change compensating element 45. The optical path length change compensating element 45 compensates for a change in the optical path length based on the signal from the signal processing unit 44. As a result, the extended reference optical path can be stabilized and fluctuations in the intensity of the detected interference fringes can be suppressed.

FIG. 6 shows another example of stabilizing the optical path 15 of the reference light. In the inter-pulse interferometer 30B of FIG. 6, a part of the reference light emitted from the beam splitter 13 is branched by the optical separator 41 and connected to one input of the phase comparison element 43. A part of the light propagating through the optical fiber 31 and emitted is folded back by the folding mirror 47, and a part of the returning 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 in 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 electric signal representing the compensation amount of the optical path length change, and outputs the electric signal to the optical path length change compensating element 45. The optical path length change compensating element 45 compensates for a change in the optical path length based on the signal from the signal processing unit 44. Even with this configuration, it is possible to stabilize the reference optical path and suppress fluctuations in the intensity of the detected interference fringes.
<Application to optical frequency comb rangefinder>
FIG. 7 shows a schematic configuration of the optical comb distance meter 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 the photodetector 42 (not shown in FIG. 7) and is directly input to the signal processing unit 41. The ranging 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. In the signal processing unit 41, a large number of beat signals are generated, frequency components used for measurement are selected, and the distance to the measurement object 20 is based on the phase difference between the reference light and the reflected light in the beat frequency components. Calculate D.

FIG. 8 is a diagram illustrating 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 repetition frequency f rep of the optical comb are generated.} Will be done. Among them, the beat frequency f used for the measurement is selected by a filter or the like. The reference light is also detected in the same manner, and the phase difference between the two is electrically measured. At this time, the optical path length difference between the two corresponds to the wavelength and fraction (phase difference φ) of N modulated 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. By changing the repetition frequency of the optical comb light source 11 in the frequency control unit 12, the phase difference φ changes. This means that the same effect as changing the optical path length up to the measurement object 20 can be obtained, and the optical path length can be effectively changed.

Assuming that 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 phase difference φ in the round trip, the distance D to the object to be measured 20 is
_{D = (c / 2n g Mf} rep) × (N + φ / 2π) (3)
It is expressed as. The absolute distance can be measured by specifying the integer N and the phase difference φ.

_{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 of the frequency f also changes, the phase difference φ also changes, and a large 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 in the repetition frequency Δf _rep and the change in the phase difference φ. From equation (3), the phase difference φ can be expressed as a function of the _{repeating 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 finding an approximate value of the absolute distance, the integer N can be determined. At that time, as seen in the measurement example of FIG. 9, the measurement accuracy of the phase difference φ is limited due to variations in the phase measurement data and the like, and a minute phase change amount equal to or less than the measurement resolution cannot be measured correctly. Therefore, it is important to have a large range of phase change. As a solution for that, as in the case of the above-mentioned inter-pulse interferometer, there is a method of increasing the optical path length difference between the reference optical path and the distance measuring optical path. That is, it corresponds to increasing the effective distance D in the equation (4) to increase the slope of the change, and the change in the phase difference with respect _{to the same change in f rep can be made large.} In other words, by taking a large difference in the optical path length to be measured, the number N of the modulated waves can be effectively increased, so that _{the effect of the modulation wavelength change due to the repetition frequency change Δf rep} is multiplied, and as a result, the position is increased. The change in phase difference φ is also multiplied. That is, the same effect as increasing the number of pulses m in the inter-pulse interferometer can be obtained. Also at this time, by lengthening the reference optical path as in the case of the inter-pulse interferometer, a large optical path difference can be obtained even in a short distance measurement.

As described above, according to the configuration and method of the embodiment, the same effect as changing the optical path length can be obtained 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 pulse number m to cause interference between distant pulses, or by increasing the measured optical path difference and increasing the number N of the modulated waves.

Further, by lengthening 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 in a short distance measurement.

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

10, 30, 30A, 30B Interferometer between pulses (distance measuring device)
11 Optical comb light source 12 Frequency control unit 12
13 Beam splitter 14 Reference surface 17 Photodetector 20 Measurement target 20a Measurement surface 21, 41 Signal processing unit 31 Optical fiber 40 Optical comb range finder (distance measuring device)

Claims (1)

A pulsed light source that generates longitudinal modes at regular intervals in the frequency domain,
A frequency control unit that changes the repetition frequency of the pulse light source,
A splitter that divides the pulsed light output from the pulsed light source into reference light and measurement light,
A photodetector that detects the reference light and the measurement light reflected on the measurement surface of the measurement object and generates a plurality of beat signals.
A signal processing unit that calculates the distance from the beat signal to the measurement surface based on the phase difference between the reference light and the measurement light in the beat frequency component used for measurement.
Have,
Wherein the frequency control unit, by changing the repetition frequency of the previous SL pulsed light source by changing the phase difference,
The signal processing unit acquires the data of the phase difference when the repetition frequency is changed, optimizes the coefficient when the phase difference is expressed by a function of the amount of change of the repetition frequency, and of the repetition frequency. A distance measuring device characterized in that the distance to the measuring surface is calculated based on the correlation between the amount of change and the amount of change in the phase difference.

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