CN106199623B - A kind of femtosecond laser intermode beat frequency method range-measurement system - Google Patents

Abstract

The invention relates to a femtosecond laser intermode beat frequency method ranging system, characterized in that the distance measuring system includes a femtosecond laser frequency comb, an acousto-optic modulator, a first beam splitting prism, a second beam splitting prism, a first mechanical A shutter, a second mechanical shutter, a first mirror, a first fiber coupling mirror, a second fiber coupling mirror, a beam expander, a corner mirror, a first photodetector, a second photodetector and a signal processing system. The present invention uses the acousto-optic modulator to realize the synthetic wavelength, expands the range of the femtosecond laser intermode beat frequency method, and does not need to change the length of the resonant cavity to adjust the femtosecond laser repetition frequency during the measurement process, because the acousto-optic modulator is an electronic control device , compared with the mechanical delay line that adjusts the length of the laser resonator, the AOM controller has a simple structure, is easy to operate, and has a short response time of the device, so it can greatly reduce the time required for a single measurement, simplify the measurement steps, and realize convenient and fast Measurement.

Description

A femtosecond laser ranging system based on beat frequency method between modes

technical field

The invention relates to a femtosecond laser absolute distance measurement system, in particular to a femtosecond laser intermode beat frequency method distance measurement system.

Background technique

Traditional laser ranging methods include time-of-flight, interferometry, and single-wavelength phase methods. The time-of-flight method measures the distance of the target by measuring the round-trip flight time of light between the light source and the target. Due to the extremely fast speed of light, the round-trip flight time of light is short and the measurement accuracy is limited, resulting in low ranging accuracy of this method; the interferometric method can Realize the measurement accuracy of sub-optical wavelength level, but the unambiguous distance is also at the wavelength level. During the measurement process, guide rails must be set up for uninterrupted incremental measurement, and absolute distance measurement cannot be realized; The amplitude modulation is used to calculate the distance by measuring the phase delay generated by the modulated wave during light propagation. The longer the modulation wavelength, the larger the range, but the lower the distance measurement accuracy. On the contrary, the shorter the modulation wavelength, the higher the accuracy, but the smaller the range. , so this method cannot take both large range and high precision into consideration. In the ranging occasions required for large-scale mechanical equipment, large aircraft, high-speed railways, civil engineering and satellite formations, there is a great demand for ranging technology with a range of microns and a range of kilometers. However, the above three traditional laser ranging methods are difficult to meet the requirements of range and accuracy at the same time.

With the development of laser technology, the emergence of femtosecond laser frequency comb light source has brought a breakthrough for large-scale high-precision absolute distance measurement. Femtosecond laser is an ultrashort pulse laser with a pulse width of femtosecond level. In the frequency domain, it is a series of stable longitudinal mode lines distributed at equal intervals. The interval between adjacent longitudinal modes is called repetition frequency, or repetition frequency for short. , represented by _frep , the repetition frequency is generally in the radio frequency band of tens to hundreds of MHz. The longitudinal modes of the femtosecond laser can beat each other to form an inter-mode beat signal. The frequency components contained in the beat signal are positive integer multiples of the repetition frequency. Using these different frequency components, the multi-wavelength phase method can be used for distance measurement. Low-frequency long-wavelength signals It can be used for large-scale rough measurement. Based on the rough measurement results, high-frequency short-wavelength signals can be used to further improve the measurement accuracy, thereby achieving high precision and large range at the same time. In 2000, the Japanese research team first reported the femtosecond laser frequency comb multi-wavelength interferometry ranging system and experimental results. Since the repetition frequency of the femtosecond laser is tens to hundreds of MHz, the component with the lowest frequency in the intermode beat frequency signal, namely f _rep , corresponds to the wavelength, that is, the measured unambiguous distance is only a few meters to tens of meters. If there is no prior The data is used as a reference, and the range of this method can only reach tens of meters, which still cannot meet the demand for a large range of kilometers in many engineering applications.

In order to solve the above problems, the prior art proposes a method of expanding the measurement range by combining wavelengths. In this method, by changing the length of the laser resonator, the repetition frequency of the femtosecond laser is slightly changed, and the repetition frequency before and after the change forms a synthesis wavelength. The phase method is roughly measured, and then the inter-mode beat frequency method is used for multi-wavelength ranging to obtain high precision. Since the synthesized wavelength can reach the order of several kilometers to tens of kilometers under the experimental accuracy, the range of this scheme can also be extended to the order of kilometers, that is, a large range and high precision can be realized at the same time. However, this method requires manual or electronic adjustment of the delay line length of the femtosecond laser resonator, which involves mechanical adjustment. The structure of the adjustment system is relatively complicated, and the adjustment takes a long time, resulting in a long measurement time, which is not conducive to fast real-time measurement; and the adjustment process It may affect the stability of the laser. In addition, some commercial femtosecond laser frequency comb light sources do not have the function of adjusting the laser resonator in a wide range, so it is impossible to realize the synthetic wavelength method to expand the range of the femtosecond laser intermode beat frequency method.

Contents of the invention

In view of the above problems, the purpose of the present invention is to provide a femtosecond laser without changing the repetition rate, without adjusting the length of the mechanical delay line in the laser resonator, the measurement scheme is simple, has little impact on the stability of the laser, and can realize fast measurement. Femtosecond laser distance measurement system between modes beat frequency method.

In order to achieve the above object, the present invention adopts the following technical solutions: a femtosecond laser intermode beating frequency ranging system, characterized in that the ranging system includes a femtosecond laser frequency comb, an acousto-optic modulator, a first beam splitting prism , second dichroic prism, first mechanical shutter, second mechanical shutter, first mirror, first fiber-coupled mirror, second fiber-coupled mirror, beam expander, corner mirror, first photodetector, second photoelectric Detector and signal processing system; the femtosecond laser frequency comb emits the frequency comb to the acousto-optic modulator; the 0th-order diffracted light that is not modulated by the acousto-optic modulator is reflected by several mirrors and then emitted To the first beam splitting prism, the frequency of the 0th-order diffracted light is the same as the optical frequency of the femtosecond laser frequency comb; the 1st-order diffracted light modulated by the acousto-optic modulator is reflected by a mirror at a certain modulation angle To the first dichroic prism, the 0th-order diffracted light and the 1st-order diffracted light are combined at the first dichroic prism and then split into two beams of light. , one of the two beams of light is transmitted to the first fiber coupling mirror as a fixed-length reference arm optical path, and the first fiber coupling mirror transmits the received reference light to the first photodetector through an optical fiber The other beam of light in the two beams is sent to the second dichroic prism, and a beam of light emitted by the second dichroic prism is sent to the first reflector as a fixed length through the first mechanical shutter The light path of the monitoring arm, the monitoring light reflected by the first mirror returns to the second beam splitting prism according to the original light path; another beam of light emitted by the second beam splitting prism passes through the second mechanical shutter and beam expander The measuring light reflected by the corner reflector returns to the second dichroic prism according to the original optical path, and the second beam splitting prism The beam splitting prism sends the monitoring light or measuring light to the second photodetector through the second fiber coupling mirror through an optical fiber; the first photodetector and the second photodetector send the detected signals to the the signal processing system.

Preferably, the signal processing system includes two signal sources, four low-pass filters, two gain amplifiers, a lock-in amplifier, a computer and a shutter controller; the first photodetector and the second photodetector The output terminals of the mixers are respectively connected to the input terminals of a mixer, the other input terminals of the two mixers are connected to the first signal source, the output terminals of the first signal source are connected to the computer, and the two mixers The output end of the frequency converter is connected in parallel with the input end of the lock-in amplifier through the first low-pass filter, the gain amplifier and the second low-pass filter respectively, and the output end of the lock-in amplifier is connected to the described lock-in amplifier through a data acquisition device. A computer, the computer also controls the opening and closing of the first mechanical shutter and the second mechanical shutter through the shutter controller; the input end of the second signal source is connected to the computer, and the second signal source The output terminal is connected to the acousto-optic modulator.

The present invention has the following advantages due to the adoption of the above technical scheme: 1. The present invention adopts the frequency shifting of the acousto-optic modulator to realize the synthetic wavelength measurement, without changing the femtosecond laser repetition rate and adjusting the length of the mechanical delay line in the laser resonator, the optical path And the circuit structure is simple. The femtosecond laser in the system only needs to lock the repetition frequency, and does not need to lock the carrier envelope offset frequency. The control of the optical frequency comb is not difficult, and fast measurement can be realized. The time required for a single measurement can reach within 1s . 2. The present invention realizes the synthetic wavelength through the acousto-optic modulator, expands the range of the femtosecond laser intermode beat frequency method, and does not need to change the length of the resonant cavity to adjust the repetition frequency of the laser during the measurement process; since the acousto-optic modulator is an electronically controlled device , compared with the mechanical delay line that adjusts the length of the laser resonator, the AOM controller has a simple structure, is easy to operate, and has a short response time of the device, so it can greatly reduce the time required for a single measurement, simplify the measurement steps, and achieve convenient and fast Measurement. 3. Since the asymmetric drift of the circuit system will cause the drift of the phase measurement result, the present invention uses the optical path of the monitoring arm and the mechanical shutter to alternately switch the measurement mode to eliminate the drift of the phase measurement result, which can realize high-precision phase measurement and improve the measurement accuracy. 4. The present invention utilizes the electric heterodyne method to reduce the high-frequency oscillating signal to a low-frequency signal, and measures the phase of the high-frequency oscillating signal with high precision, so as to realize the distance measurement by the phase method; in addition, the synthetic wavelength method is used to obtain a large ranging range, and the range reaches Absolute distance measurement at the kilometer level with an accuracy of micron level. The invention can be widely used in femtosecond laser absolute distance measurement.

Description of drawings

Fig. 1 is a schematic structural diagram of the ranging system of the present invention;

Fig. 2 is a schematic diagram of the principle of the signal processing system of the present invention.

Detailed ways

The present invention will be described in detail below in conjunction with the accompanying drawings. However, it should be understood that the accompanying drawings are provided only for better understanding of the present invention, and they should not be construed as limiting the present invention.

As shown in Figure 1, the femtosecond laser intermode beat frequency method distance measuring system provided by the present invention includes a femtosecond laser frequency comb L, a collimating lens N, an acousto-optic modulator AOM, a first beam splitting prism BS ₁ , a second Dichroic prism BS ₂ , first mechanical shutter S ₁ , second mechanical shutter S ₂ , mirror M ₁ , first fiber coupling mirror D ₁ , second fiber coupling mirror D ₂ , beam expander K, corner mirror C, The first photodetector PD ₁ , the second photodetector PD ₂ and the signal processing system.

The femtosecond laser frequency comb L emits a frequency comb with a central wavelength of 1560nm and a repetition rate of 56.27MHz (this is an example, not limited to this), and transmits it to the collimating lens N via an optical fiber, and then transmits it to the acousto-optic modulator AOM;

The 0th-order diffracted light not modulated by the acousto-optic modulator AOM is reflected by several mirrors (not shown in the figure) and then emitted to the first beam splitting prism BS ₁ , the frequency of the 0th-order diffracted light is the same as that of the femtosecond laser frequency comb the same frequency;

The first-order diffracted light modulated by the acousto-optic modulator AOM is reflected to the first beam-splitting prism BS ₁ by a mirror at a certain modulation angle, and the 0-order diffracted light and the first-order diffracted light are combined at the first beam-splitting prism BS ₁ After that, it is divided into two beams of light (it is necessary to ensure that the optical path of the 0th-order diffracted light and the 1st-order diffracted light before the combination of light are the same, and the specific use process can be adjusted by using several mirrors to adjust the optical path, so I won’t go into details here), one of which A beam of light passes through the first fiber coupling mirror D ₁ as a fixed-length reference arm optical path. The first fiber coupling mirror D ₁ transmits the reference light to the first photodetector PD ₁ through an optical fiber, and the other beam to the second beam splitter Prism BS ₂ , a beam of light emitted by the second dichroic prism BS ₂ is sent to the mirror M ₁ through the first mechanical shutter S ₁ as a fixed-length monitoring arm optical path, and the monitoring light reflected by the mirror M ₁ returns according to the original optical path The second dichroic prism BS ₂ ; another beam of light emitted by the second dichroic prism BS ₂ is sent to the beam expander K through the second mechanical shutter S ₂ , and the light expanded by the beam expander K is sent to the fixedly arranged The corner reflector C on the measured object is used as the measuring arm for the distance to be measured. The measuring light reflected by the corner reflector C returns to the second beam splitting prism BS ₂ according to the original optical path, and the second beam splitting prism BS ₂ passes the monitoring light or measuring light through the second beam splitting prism. The second optical fiber coupling mirror D ₂ transmits to the second photodetector PD ₂ through the optical fiber, and the first photodetector PD ₁ and the second photodetector PD ₂ send the detected signals to the signal processing system.

As shown in Figure 2, the signal processing system includes two signal sources, two mixers, four low-pass filters, two gain amplifiers, a lock-in amplifier, a computer and a shutter controller; the first photodetector PD ₁ and the output end of the second photodetector PD ₂ are respectively connected to an input end of a mixer HP, and the other input ends of the two mixers HP are connected to the first signal source SN1, and the output end of the first signal source SN1 is connected to Computer, the frequency of the output signal of the first signal source SN1 is controlled by the computer, and the output ends of the two mixers HP are respectively connected to the lock-in amplifier in parallel through the first low-pass filter LB1, the gain amplifier FD and the second low-pass filter LB2 The input end of RS and the output end of the lock-in amplifier RS are connected to a computer through a data acquisition device, and the computer performs data acquisition and parameter control on the lock-in amplifier RS. The computer can also control the opening and closing of the first mechanical shutter _S1 and the second mechanical shutter _S2 respectively through the shutter controller; the input end of the second signal source SN2 is connected to the computer, and the output end of the second signal source SN2 is connected to the acousto-optic modulation AOM, the computer controls the output of the second signal source SN2 to turn on or turn off the acousto-optic modulator AOM.

During the distance measurement process of the femtosecond laser intermode beating frequency method distance measurement system of the present invention, the first mechanical shutter _S1 and the second mechanical shutter _S2 are quickly and alternately opened to measure the phase difference between the monitoring arm and the reference arm respectively Phase difference between measuring arm and reference arm Since both the reference arm and the monitoring arm are of fixed length, and the measurement arm and the monitoring arm share the measurement circuit, A change in can reflect a drift or jitter in the phase difference measurement due to the first photodetector PD ₁ and the second photodetector PD ₂ and the circuits connected to them. Since the drift speed of the circuit is much lower than the switching speed of the mechanical shutter, it can be considered that and The measurement of the circuit drift is simultaneous, that is, take As a phase difference measurement result, to eliminate the influence of circuit drift or jitter error on the ranging result.

The process of measuring the distance to be measured using the femtosecond laser intermode beat frequency method ranging system of the present invention can be divided into two steps:

1. The computer starts the second signal source SN2 to drive the acousto-optic modulator AOM, the 0th-order diffracted light and the 1st-order diffracted photosynthetic light, and the electrical signals output by the first photodetector PD ₁ and the second photodetector PD ₂ are divided into mf _rep mode In addition to the interbeat frequency components, it also includes the components ±f _AOM +mf _rep obtained by the difference between the two diffracted lights. When choosing a suitable f _AOM , there will be a low frequency component f _s much smaller than f _rep among these components, such as f When _rep = 56.27MHz, take f _AOM = 168.61MHz, then there is a low-frequency component of f _s = 0.2MHz in the output signals of the first photodetector PD ₁ and the second photodetector PD ₂ , and the corresponding wavelength of this frequency component is much larger than the optical frequency Comb the fundamental wavelength. After the low-frequency component signal is filtered out by the low-pass filter, the phase difference is detected by the lock-in amplifier, and the obtained distance measurement result is used as a rough measurement result.

2. The computer controls to turn off the second signal source SN2, and the acousto-optic modulator AOM does not generate first-order diffracted light. At this time, the electrical signals output by the two photodetectors include the fundamental wave of the laser frequency comb intermode beat signal and several higher harmonic components mf _rep , and the upper limit of the frequency spectrum is limited by the bandwidth of the photodetectors. Femtosecond laser intermode beat signal frequency is high, the frequency interval is small, it is difficult to directly filter out a certain component through a low-pass filter and measure the phase difference, so it is necessary to use a heterodyne measurement method to measure the phase difference of the harmonic component of mf _rep When using a computer to control the first signal source to generate a sinusoidal signal with a frequency of mf _rep +Δf, which is mixed with the output signals of the two photodetectors respectively, the mixed output signal will contain Δf low-frequency components. When the input signal phase of the first signal source to the two mixers is equal, the output signal of the two mixers retains the phase difference information of the two photodetector signals, which can be filtered out by a low-pass filter and detected by a lock-in amplifier. Difference.

The measurement process of the femtosecond laser mode beat frequency method ranging system of the present invention is described in detail below by specific embodiments:

1. Add a modulation signal with a frequency of f _AOM to the acousto-optic modulator AOM to generate a synthetic wavelength λ _s =c/nf _s , measure the phase difference between the reference light and the measurement light at this wavelength, and obtain a rough measurement result D', specifically The process is:

The computer controls and starts the second signal source SN2 to drive the acousto-optic modulator AOM. When the acousto-optic modulator AOM has a suitable frequency for f _AOM signal modulation, there is a low frequency far smaller than f _rep in the frequency components of the electrical signals of the two photodetectors The component f _s , the wavelength λ _s =c/nf _s corresponding to the frequency signal is called the synthetic wavelength, and the distance measurement range using the synthetic wavelength is D' _max =λ _s /2. Since f _AOM can be adjusted to make f _s << f _rep , this ranging range is large. Use a low-pass filter to filter out the f _s signal and use a lock-in amplifier to measure the phase difference. The distance to be measured is D' measured within the synthetic wavelength range, which is used as the rough measurement result of the distance. Theoretically, as long as f _s is close to zero, λ _s can be close to infinity, and a large ranging range can be obtained. However, in subsequent measurements, in order to correctly calculate the number N ₁ of integral periods of the fundamental wavelength included in the distance to be measured, the above-mentioned measurement deviation of D' should have an upper limit. The larger the λ _s used, the greater the measurement error of D', so limited to the measurement error, the range of D' _max = λ _s /2 cannot be infinitely expanded, but the general experimental accuracy is sufficient to reach the kilometer level.

2. Turn off the modulation signal of AOM on the acousto-optic modulator, so that only the 0th-order diffracted light exits, adjust the output of the first signal source SN1, so that the signal and the fundamental frequency component of f = f _rep generate a low-frequency heterodyne signal, and measure The phase difference between the reference light and the measurement light at the fundamental wavelength, and obtain the measurement result of the phase difference between the reference light and the measurement light at the fundamental wavelength and by D' and To determine the fundamental frequency ranging result D ₁ , the specific measurement principle and process are:

Using the component of frequency mf _rep (m is a positive integer) in the intermodal beat signal for interferometric distance measurement, the measured phase difference between the reference signal and the measurement signal is Then the absolute distance measurement result is:

In the formula, c is the speed of light in vacuum, n is the refractive index of air, which can be determined from the environmental parameters according to the relevant theory of air refractive index, and λ=c/nf _rep is the fundamental frequency wavelength of the laser frequency comb. Since the phase difference measured by the instrument is in the range of 0 to 2π, there is a fuzzy range in the ranging results. The natural number N _m in formula (1) represents the number of periods of the phase difference 2π that the instrument cannot determine.

After the repetition frequency is locked by the atomic clock, the laser frequency comb repetition frequency jitter δf _rep <1mHz, comprehensively considering the instrument error and measurement random error, the measurement error of the phase difference On the order of 0.01°, the relative error of the repetition frequency is much smaller than the relative error of the phase difference measurement, and the phase difference measurement is the main error source. According to formula (1), the distance measurement error can be obtained as That is, the higher the beating frequency between the laser frequency comb modes used for ranging, the smaller the ranging error and the higher the accuracy. Considering the range and accuracy comprehensively, first use the fundamental frequency component of f=f _rep to perform a large-scale rough measurement during the measurement. At this time, according to the rough measurement result D' of the synthesized wavelength and the phase difference measurement result at the fundamental frequency The measurements determine N ₁ , namely:

Then the fundamental frequency ranging result D ₁ can be expressed as:

3. Adjust the output of the first signal source SN1 so that the signal and a certain high-frequency component of f=mf _rep produce a low-frequency heterodyne signal, wherein the selection of the positive integer m can be determined according to the upper limit of the bandwidth of the photodetector used, and the measurement The phase difference between the reference light and the measurement light at this wavelength, and obtain the phase difference measurement result and by _D1 and The measurement result determines N _m , and then determines the distance measurement result D _m , which is the final measurement result. The specific process is:

Use the higher harmonics of f=mf _rep for accurate measurement, and the integer N _m is determined by the phase difference Measurement results and _D1 determined:

At this time, the measurement result with higher precision can be obtained from formula (1):

Substituting the measurement result of _D1 according to formula (3), the final distance measurement result can be obtained:

In the above-mentioned steps of measurement, the optical path of the monitoring arm and double-shutter switching can be used to eliminate the drift error of the circuit. The specific principle and data processing method have been mentioned above and will not be repeated here; The method of taking the average value of multiple measurements reduces the random error in the measurement.

The above-mentioned embodiments are only used to illustrate the present invention, wherein the structure, connection mode and manufacturing process of each component can be changed to some extent, and any equivalent transformation and improvement carried out on the basis of the technical solution of the present invention should not excluded from the protection scope of the present invention.

Claims (2)

1. A femtosecond laser mode intermode beat frequency method ranging system, characterized in that, the ranging system comprises a femtosecond laser frequency comb, an acousto-optic modulator, the first beam splitting prism, the second beam splitting prism, the first mechanical shutter , a second mechanical shutter, a first mirror, a first fiber coupling mirror, a second fiber coupling mirror, a beam expander, a corner mirror, a first photodetector, a second photodetector and a signal processing system; The femtosecond laser frequency comb transmits the emitted frequency comb to the acousto-optic modulator; the 0th-order diffracted light that is not modulated by the acousto-optic modulator is reflected by several mirrors and then sent to the first dichroic prism , the frequency of the 0th-order diffracted light is the same as the optical frequency of the femtosecond laser frequency comb; The first-order diffracted light modulated by the acousto-optic modulator is reflected to the first dichroic prism through a mirror at a certain modulation angle, and the 0-order diffracted light and the first-order diffracted light are combined at the first dichroic prism After being divided into two beams of light, the optical path of the 0th-order diffracted light and the 1st-order diffracted light before the light combination is the same, and one of the two beams of light is sent to the first fiber coupling mirror as a fixed-length reference arm optical path, The first fiber coupling mirror transmits the received reference light to the first photodetector through an optical fiber; the other beam of light in the two beams is transmitted to the second beam splitting prism, and passes through the second beam splitting prism A beam of light emitted by the first mechanical shutter is sent to the first mirror as a fixed-length monitoring arm optical path, and the monitoring light reflected by the first mirror returns to the second dichroic prism according to the original optical path; Another beam of light emitted by the second dichroic prism passes through the second mechanical shutter and the beam expander and then is emitted to the corner reflector fixed on the object to be measured as the measuring arm for the distance to be measured. The measurement light reflected by the corner reflector returns to the second beam splitting prism according to the original optical path, and the second beam splitting prism transmits the monitoring light or measurement light to the second photodetector through the second fiber coupling mirror through an optical fiber. the detector; the first photodetector and the second photodetector send the detected signal to the signal processing system. 2. a kind of femtosecond laser mode intermode beating frequency ranging system as claimed in claim 1, is characterized in that, described signal processing system comprises two signal sources, four low-pass filters, two gain amplifiers, a lock phase amplifier, a computer and a shutter controller; The output end of the first photodetector is connected to the input end of the first mixer, the output end of the second photodetector is connected to the input end of the second mixer, the first mixer, the The other input end of the second mixer is connected to the first signal source, and the input end of the first signal source is connected to the computer, and the output end of the first mixer is passed through the first low-pass filter, the first The gain amplifier and the second low-pass filter are connected to the input of the lock-in amplifier, and the output of the second mixer is connected to the third low-pass filter, the second gain amplifier and the fourth low-pass filter. The input end of the lock-in amplifier, the output end of the lock-in amplifier is connected to the computer through a data acquisition device, and the computer also controls the first mechanical shutter and the second mechanical shutter respectively through the shutter controller. turn on and off; the input end of the second signal source is connected to the computer, and the output end of the second signal source is connected to the acousto-optic modulator.