KR101448831B1 - Distance measuring apparatus using phase-locked synthetic wavelength interferometer based on femtosecond laser - Google Patents

Abstract

The present invention relates to a femtosecond laser based distance measurement device. More specifically, the present invention relates to a distance measurement device which uses a femtosecond laser based phase-locked composite wave interferometer to maintain constant phases between composite waves and enable distance measurement by measuring repetition rate of femtosecond laser in order to achieve high resolving power equal to or less than 1 μm and remove nonlinear error due to phase. According to the present invention, no limitation exists in a distance to be measured, high resolving power equal to or less than 1 μm is realized by a locked phase, and periodic nonlinear error is not caused.

Description

BACKGROUND OF THE INVENTION Field of the Invention [0001] The present invention relates to a distance measuring apparatus using a phase locked composite wave interferometer based on a femtosecond laser and a phase locked synthetic interferometer based on a femtosecond laser,

The present invention relates to a femtosecond laser-based distance measuring apparatus, and more particularly, to a femtosecond laser-based phase locked composite wave interferometer capable of maintaining the phase of a synthetic wave constantly and measuring the distance by measuring the repetition rate of a femtosecond laser. And eliminates nonlinear errors depending on the phase and a high resolution of 1 mu m or less.

A synthetic wavelength interferometer is an absolute distance interferometer that uses long wavelength signals due to interference between lights of different frequencies for distance measurement.

This absolute distance interferometer can measure the distance difference between the reference arm and the measurement arm through a single measurement process without 2π phase ambiguity.

In addition, since errors that may occur during the measurement process are not accumulated and measurement information is not lost when the beam path is subjected to external interference, various disadvantages of the relative displacement interferometer Can be solved.

When a femtosecond laser is introduced as a light source of such a composite wave interferometer, a composite wave of a plurality of frequencies can be used for measurement by interference between a plurality of frequency modes of an optical comb of a femtosecond laser.

In this case, it is advantageous to measure the long distance by the resolution of the shortest wavelength synthetic wave using the composite wave of the longest wavelength and the shortest detectable wave.

In general, the measurement range of the synthetic wave interferometer is limited by the repetition rate of the femtosecond laser comb and the bandwidth of the electronic circuit. The measurement resolution is the difference between the reference wave and the measurement wave Is limited to the performance of a phase meter that detects phase difference.

However, in general, nonlinear errors of several to several tens of micrometers exist even when the resolution of the phase detector is limited to the resolution of synthesized waves.

This nonlinear error is caused by the complexity of the experimental equipment due to the use of electrical cross-talk between the reference signal and the measurement signal, optical cross-talk due to unnecessary frequency components, external modulator, ), Optical multipath between the reference arm and the measuring arm, and non-linearity of the phase detector.

However, in the case of the post-processing method, it is difficult to obtain error-free distance information at the same time as the real-time compensation is impossible.

A conventional phase locked synthetic wave interferometer using a femtosecond laser is composed of an acousto-optic modulator (AOM) or an electro-optic modulator (EOM) external to the laser resonator, The frequency of the wave is controlled. In addition to the elements added in the case of using such an active element, a high-voltage circuit for controlling the element is necessary, so that the system is complicated and reliability is lowered. In addition, the frequency shifted optical frequency modes generated by the optical modulator and the optical modulator are generally low in efficiency, and the signal-to-noise ratio of the composite wave for phase locking is lowered.

1. Korean Patent Publication No. 10-2008-0032821 2. Korean Patent No. 10-1109001 3. Korean Patent Publication No. 10-2012-0058275

The present invention has been proposed in order to solve the problem caused by the above background art, and it is possible to perform distance measurement by measuring the repetition rate of a femtosecond laser by making the phase between synthetic waves constant, The present invention provides a distance measuring apparatus using a phase locked synthetic wave interferometer based on a femtosecond laser.

It is another object of the present invention to provide a distance measuring apparatus using a femtosecond laser-based phase locked synthetic wave interferometer capable of real-time distance measurement through rapid repetition rate adjustment without complicated post-processing.

In addition, the present invention performs phase locking through frequency control of a synthetic wave generated in a femtosecond laser, which is possible by precisely changing the length of the laser resonator itself. This enables phase locking without the need for additional active elements and additional circuitry to control it, such as conventional acoustic optical modulators (AOMs) or electro-optical modulators (EOMs), and does not use low efficiency devices, Acquisition is possible.

In order to achieve the above-mentioned object, in order to achieve the above-mentioned object, a phase-locked synthetic wave interferometer based on a femtosecond laser is used to maintain the phase between synthetic waves constantly and to perform distance measurement by measuring the repetition rate of a femtosecond laser, And a distance measuring device for eliminating non-linear errors according to the phase.

The distance measuring apparatus includes:

A distance measuring apparatus using a femtosecond laser based phase locked synthetic wave interferometer,

Femtosecond laser light source;

A beam splitter for separating the femtosecond laser light emitted from the femtosecond laser source into a measurement light reflected from the measurement target and a reference light;

A reference light detector for detecting the reference light;

A measurement photodetector for detecting the reflected measurement light;

A high frequency circuit for generating and amplifying a reference composite wave and a measurement composite wave from the detected reference light and measurement light;

A phase lock unit for locking the phase of the amplified synthetic waves and controlling the femtosecond laser source so that the phase of the synthesized wave is constant; And

 A frequency measurement unit for measuring the repetition rate of the controlled femtosecond laser light back to the time and frequency standards; And

A distance transform unit for transforming the distance information between the measurement target and the beam splitter using the measured repetition rate;

And a control unit.

Here, the high-frequency circuit may include: a function generator for generating a high-frequency signal; A first mixer for synthesizing the generated high frequency signal and the detected reference light; A second mixer for combining the generated high frequency signal with the detected measurement light; A first low pass filter for filtering the synthesized reference light in a low frequency band; A second low pass filter for filtering the synthesized measurement light in a low frequency band; A first amplifier for amplifying the filtered reference synthesized light; And a second amplifier for amplifying the filtered measured synthetic light; And a control unit.

The phase lock unit may further include: a third mixer for synthesizing the amplified reference composite wave and the measured composite wave; A third low pass filter for filtering the synthesized two synthetic waves in a low frequency band; And a controller for using a point at which the phase difference of the two filtered synthetic waves is zero as a phase lock.

Also, the phase locked synthetic wave interferometer may be a Michelson interferometer.

The femtosecond laser source may be an optical fiber femtosecond laser having a center wavelength of 1550 nm, a bandwidth of 60 nm, and a pulse width of 150 fs.

Alternatively, the femtosecond laser source is titanium having a center wavelength, a bandwidth of 100㎚, 100f _s pulse width of 800㎚: may be characterized in that the sapphire femtosecond laser.

(ΔL은 빔스플리터(170)와 측정 타겟(190)의 거리차, c는 빛의 속도, f_r은 광원의 반복률, i는 사용한 합성파의 인덱스에 해당하는 정수부, N은 공기의 군굴절률, m은 장거리측정시 나타나는 모호성에 해당하는 부분으로 정수를 나타낸다)에 의해 산출되는 것을 특징으로 할 수 있다.Further, the distance information is calculated by the following equation,

(Where L is the distance difference between the beam splitter 170 and the measurement target 190, c is the speed of light, f _r is the repetition rate of the light source, i is the integer part corresponding to the index of the composite wave used, N is the index of refraction of the air, and m is an integer corresponding to ambiguity in long distance measurement).

(여기서, φ₁ 과φ₂는 각각 반복률 조절 전과 반복률 조절 후의 사용한 합성파의 파장이고, Λ₁ 과Λ₂는 각각 반복률 조절 전과 조절 후에 측정된 위상을 나타낸다)에 의해 산출되는 것을 특징으로 할 수 있다.Further, the above-mentioned m is expressed by the following equation,

Can be characterized in that calculated by (wherein, φ ₁ and φ ₂ is the composite wave wavelength used after each repetition rate adjustment before repetition rate adjustment, Λ ₁ and Λ ₂ represents the phase measurement after the repetition rate adjustment before adjusting, respectively) have.

According to the present invention, there is no limitation on the measurement distance, and there is an advantage of realizing a high resolution of 1 mu m or less through phase locking and periodically non-linearity error.

Further, as another effect of the present invention, there is no phase ambiguity in long distance measurement. A change in light intensity of the light source used and insensitivity to noise.

Yet another advantage of the present invention is that real-time distance measurement is possible through rapid repetition rate adjustment without complicated post-processing.

1 is a schematic block diagram of a distance measuring apparatus 100 using a femtosecond laser based phase locked synthetic wave interferometer according to an embodiment of the present invention.
2 is a circuit block diagram for explaining a phase lock unit 120 performing a phase lock function among the distance measuring apparatus 100 shown in FIG.
FIG. 3 is a conceptual diagram in which the nonlinear error is eliminated in the distance measuring apparatus 100 shown in FIG.
4 is a graph illustrating a process of eliminating phase ambiguity in long distance measurement according to an embodiment of the present invention.

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It is to be understood, however, that the invention is not to be limited to the specific embodiments, but includes all modifications, equivalents, and alternatives falling within the spirit and scope of the invention.

Like reference numerals are used for similar elements in describing each drawing.

The terms first, second, etc. may be used to describe various components, but the components should not be limited by the terms. The terms are used only for the purpose of distinguishing one component from another.

For example, without departing from the scope of the present invention, the first component may be referred to as a second component, and similarly, the second component may also be referred to as a first component. The term "and / or" includes any combination of a plurality of related listed items or any of a plurality of related listed items.

Unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

Terms such as those defined in commonly used dictionaries are to be interpreted as having a meaning consistent with the contextual meaning of the related art and are to be interpreted as either ideal or overly formal in the sense of the present application Should not.

Hereinafter, a femtosecond laser-based distance measuring apparatus using a phase locked synthetic wave interferometer according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings.

1 is a schematic block diagram of a distance measuring apparatus 100 using a femtosecond laser based phase locked synthetic wave interferometer according to an embodiment of the present invention. 1, the distance measuring apparatus 100 includes a femtosecond laser light source 110; A beam splitter 170 separating the femtosecond laser light emitted from the femtosecond laser source 110 into measurement light reflected from the measurement target 190; A reference light detector 180-1 for detecting the reference light; A measurement photodetector 180-2 for detecting the reflected measurement light; A high-frequency circuit (130) for generating and amplifying a reference composite wave and a measurement composite wave from the detected reference light and measurement light; A phase locking unit 120 for locking the phase of the amplified synthetic waves and controlling the femtosecond laser light source so that the phase of the synthesized wave is constant; And a frequency measurement unit 140 for measuring the repetition rate of the femtosecond laser light for which control has been completed retroactively to time and frequency standards; And a distance transform unit 150 for transforming the distance information between the measurement target 190 and the beam splitter 170 using the measured repetition rate.

The distance difference between the beam splitter 170 and the measurement target 190 can be calculated from the measurement of the repetition rate of the light source.

Here,? L is the distance difference between the beam splitter 170 and the measurement target 190, c is the speed of light, f _r is the repetition rate of the light source, i is an integer part corresponding to the index of the synthetic wave used, N is the refractive index , m is an integer that corresponds to the ambiguity in long distance measurement.

In addition, the femtosecond laser light source 110 may include an optical fiber femtosecond laser having a central wavelength of 1550 nm, a bandwidth of 60 nm, and a pulse width of 150 fs, or a titanium: A sapphire femtosecond laser, or the like may be used. Therefore, there is an advantage that the deterioration of pulse width performance due to dispersion in air is not affected.

The femtosecond laser light emitted from the femtosecond laser light source 110 through a beam splitter 170 is incident on the reference light detector 180 -1) with the measurement light directed toward the measurement target 190 and reflected therefrom.

When the overlap amount of the pulses is controlled to be constant, the repetition rate of the femtosecond laser is precisely measured through the frequency measurement unit 140 which is retrofitted to the time / frequency standard, and the measured values are measured by the distance conversion unit 140, .

The time measurement of the frequency measuring unit 140 is performed through the provided reference clock 141, thereby enabling precise measurement.

2 is a circuit block diagram for explaining a phase lock unit 120 performing a phase lock function among the distance measuring apparatus 100 shown in FIG. 2, the high-frequency circuit 130 includes a function generator 211 for generating a high-frequency signal, a first mixer 210-1 for synthesizing the generated high-frequency signal and the detected reference light, A first low pass filter 220-1 for filtering the synthesized reference light in a low frequency band, and a second low pass filter 220-1 for filtering the synthesized measurement light in a low frequency band A second low pass filter 220-2, a first amplifier 230-1 for amplifying the filtered reference synthesized light, a second amplifier 230-2 for amplifying the filtered measured synthesized light, and the like .

The phase lock unit 120 includes a third mixer 240 for synthesizing the amplified reference composite wave and the measured composite wave, a third low-pass filter 250 for filtering the synthesized two synthetic waves in a low frequency band, And a controller 260 that uses a point at which the phase difference of two filtered synthetic waves is zero as a phase lock.

As shown in FIG. 2, the reference signal detected by the two detectors 180-1 and 180-2 and the signal generated by the function generator 211 for extracting a synthetic wave of several hundreds of MHz to several hundreds of GHz for use in measurement in the measurement signal The high frequency signal and the signals detected by the detectors 180-1 and 180-2 are input to the first and second mixers 210-1 and 210-2, respectively, and the signals output from the first and second low- Pass filters 220-1 and 220-2.

Through this, it is possible to extract only the synthetic wave which is used for the distance measurement among hundreds to thousands of synthetic waves, and the phase of the synthetic wave having the distance information is maintained and the frequency of the synthetic wave is reduced to several kHz to facilitate the signal processing .

Generally, since the amplitude of the signal is low as the frequency of the composite wave is high, the extracted reference composite wave and the measured composite wave are amplified and then input to the third mixer 240 and the third low-pass filter 250, 260).

Second, the signal passed through the third mixer 240 and the low-pass filter 250 has a voltage that varies according to the phase difference between the reference composite wave and the measurement composite wave. When the phase difference of the two composite waves becomes zero, The voltage of the signal to be output becomes zero. Therefore, the control unit 260 uses this point for the phase lock.

In order to perform such phase locking, the control unit 260 includes a microprocessor, a memory, and the like.

FIG. 3 is a conceptual diagram in which the nonlinear error is eliminated in the distance measuring apparatus 100 shown in FIG. Generally, a nonlinear error occurs in a distance measuring system using a phase measurement, which is caused by electrical cross-talk between a reference signal and a measurement signal, optical cross-talk due to unnecessary frequency components, ), The complexity of the experimental instrument with the use of optical multipaths and / or phase meters between the reference arm and the measuring arm (in particular using a Michelson interferometer) And non-linearity of the nonlinearity.

FIG. 3 shows a form in which a rough nonlinear error occurs. When nonlinearity occurs periodically according to the wavelength used or causes various nonlinear errors, more complex nonlinear errors occur.

In one embodiment of the present invention, the phase is locked to zero to eliminate the nonlinear error in phase and measure the repetition rate of the light source with very high resolution, thereby enabling a high resolution distance measurement.

4 is a graph illustrating a process of eliminating phase ambiguity in long distance measurement according to an embodiment of the present invention. Depending on the repetition rate of the light source used, distance ambiguity occurs when measuring distances of several meters or more, which can be solved through determination of the m value of the food. This can be determined by adjusting the phase shift of the composite wave with the longest wavelength depending on the measurement distance when adjusting the repetition rate for pulse overlap.

Here, φ ₁ and φ ₂ are the wavelengths of the synthetic wave used before the repetition rate adjustment and after the repetition rate adjustment respectively, and Λ ₁ and Λ ₂ represent the phases measured before and after the repetition rate adjustment, respectively.

Therefore, as shown in FIG. 4, it is possible to determine the distance value (c) and the m value corresponding to the unknown number through the two measurement processes (a) and (b).

The distance measuring apparatus according to an embodiment of the present invention configured as described above eliminates the non-linear error by using the phase locking principle and enables a distance measuring resolution of 1 μm or less. In addition, it is possible to perform long distance measurement by eliminating the phase ambiguity and insensitivity to noise and / or variation in the amount of light of the reference light and the measurement light, and the dispersion in the air is not affected because the ultra-short pulse is used in the frequency domain.

100: Distance measuring device
110: femtosecond laser light source
120: phase locking part
130: high frequency circuit
140: Frequency measurement unit
141: Reference clock
150: Distance conversion unit
160: lens
170: beam splitter
180-1: Reference light detector 180-2: Measurement light detector
190: Measurement target
210-1: first mixer 210-2: second mixer
211: Function Generator
220-1: first low-pass filter 220-2: second low-pass filter
230-1: first amplifier 230-2: second amplifier
240: Third mixer
250: third low-pass filter
260:

Claims (8)

A distance measuring apparatus using a femtosecond laser based phase locked synthetic wave interferometer,
A femtosecond laser light source 110;
A beam splitter 170 separating the femtosecond laser light emitted from the femtosecond laser source 110 into measurement light reflected from the measurement target 190;
A reference light detector 180-1 for detecting the reference light;
A measurement photodetector 180-2 for detecting the reflected measurement light;
A high frequency circuit 130 for synthesizing the detected reference light and the measurement light respectively with the high frequency signal, extracting and amplifying the reference synthesized wave and the measured synthesized wave respectively by passing the synthesized reference light and the measurement light through the low frequency pass filter;
A phase locking unit 120 for locking the phase of the amplified synthetic waves and controlling the femtosecond laser light source so that the phase of the synthesized wave is constant; And
A frequency measuring unit 140 for measuring the repetition rate of the controlled femtosecond laser light back to the time and frequency standards; And
A distance transform unit 150 for transforming the distance information between the measurement target 190 and the beam splitter 170 using the measured repetition rate;
And a phase locked composite wave interferometer based on the femtosecond laser.
The method according to claim 1,
The high-frequency circuit (130)
A function generator 211 for generating a high frequency signal;
A first mixer 210-1 for combining the generated high frequency signal and the detected reference light;
A second mixer 210-2 for combining the generated high frequency signal and the detected measurement light;
A first low pass filter 220-1 for filtering the synthesized reference light in a low frequency band;
A second low pass filter 220-2 for filtering the combined measurement light into a low frequency band;
A first amplifier 230-1 for amplifying a reference composite wave; And
A second amplifier 230-2 for amplifying the measured composite wave; And a phase locked composite wave interferometer based on the femtosecond laser.
The method according to claim 1,
The phase locking unit 120 includes:
A third mixer 240 for synthesizing the amplified reference composite wave and the measured composite wave;
A third low-pass filter 250 for filtering the synthesized waves in a low-frequency band; And
And a control unit (260) for using a point at which a phase difference of two filtered synthetic waves is zero as a phase lock. The distance measurement apparatus using a phase locked synthetic wave interferometer based on a femtosecond laser.
The method according to claim 1,
Wherein the phase locked synthetic wave interferometer is a Michelson interferometer. ≪ RTI ID = 0.0 > [10] < / RTI >
The method according to claim 1,
Wherein the femtosecond laser source 110 is an optical fiber femtosecond laser having a center wavelength of 1550 nm, a bandwidth of 60 nm, and a pulse width of 150 fs.
The method according to claim 1,
Wherein the femtosecond laser source 110 is a titanium: sapphire femtosecond laser having a center wavelength of 800 nm, a bandwidth of 100 nm, and a pulse width of 100 fs.
The method according to claim 1,
The distance information is calculated by the following equation,

(Where L is the distance difference between the beam splitter 170 and the measurement target 190, c is the speed of light, f _r is the repetition rate of the light source, i is the integer part corresponding to the index of the composite wave used, N is the index of refraction of the air, and m is an integer corresponding to the ambiguity in long distance measurement.) The distance measurement apparatus using the femtosecond laser based phase locked synthetic wave interferometer.
8. The method of claim 7,
M is expressed by the following equation,

Femtosecond, characterized in that calculated by (wherein, φ is ₁ and φ ₂ is the composite wave wavelength used after each repetition rate adjustment before repetition rate adjustment, Λ ₁ and Λ ₂ represents the phase measurement after the repetition rate adjustment before adjusting, respectively) A distance measuring device using a laser - based phase - locked composite wave interferometer.

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