CN103809166B - A kind of Michelson interference type spectral filter resonant frequency locking device and method - Google Patents

Abstract

The invention discloses a resonant frequency locking device and method of a Michelson interference type spectral filter. The invention includes a laser beam splitting system, a Michelson interference filter system and a photoelectric detection system; the laser beam splitting system includes a laser, a collimating beam expander, a first beam splitter, a first reflector, a second reflector, a second beam splitter mirror, the third reflector; the Michelson interference filter system includes a cubic beam splitter, the fourth reflector, and the fifth reflector; the photodetection system includes a lens, a first photomultiplier tube, a second photomultiplier tube, and a third photomultiplier Tube, differential amplifier, oscilloscope; Concrete steps: 1. Calculate the incident angle of two probe beams; 2. Regulate the incident angle of two probe beams; 3. Check the oscilloscope to judge the frequency-locked state; The present invention is simple to implement and can It avoids the complex requirements of the traditional frequency locking method on the circuit and optical path, so it has strong system stability and robustness.

Description

Device and method for locking resonant frequency of Michelson interference type spectral filter

technical field

The invention belongs to the technical field of laser radar, and in particular relates to a resonant frequency locking device and method of a Michelson interference type spectral filter.

Background technique

Due to the use of spectral filtering technology, high spectral resolution lidar solves the problem that traditional backscatter lidar needs many prior assumptions to invert atmospheric parameters, thus improving the accuracy of atmospheric remote sensing. In high spectral resolution lidar, the use of spectral filters is an extremely critical technology. Through the high spectral resolution capability of the spectral filter, the components scattered by atmospheric aerosols and components scattered by atmospheric molecules in the atmospheric backscattering spectrum can be separated, so that more details of the atmospheric backscattering spectrum can be obtained. Combined with relevant remote sensing principles, atmospheric optical properties such as atmospheric backscattering coefficient and extinction coefficient can be retrieved more accurately.

At present, the iodine molecular absorption filter has been used in many high spectral resolution lidars due to its high filtering rate of atmospheric aerosol scattering signals, high stability of spectral absorption characteristics, and independence from mechanical alignment with incident light. . However, since the absorption peak (absorption resonance peak) of this type of filter is determined by the natural absorption mechanism of molecules, its use band cannot be changed arbitrarily, which limits the spectral expansion of lidar. In order to solve this shortcoming, Michelson interferometric spectral filters have attracted more and more attention and are gradually applied to lidar. Due to the use of the interference principle of light, the resonance frequency of the Michelson interference spectral filter can be set at any laser wavelength of interest, which greatly broadens the spectral application field of lidar.

However, the resonance frequency of the Michelson interference type spectral filter is far less stable than that of the iodine molecular absorption filter. Temperature, external stress, etc. will cause the drift of the resonant frequency. How to lock the resonant frequency of the Michelson interferometric spectral filter to the desired laser center frequency is a major technical problem for its use in high spectral resolution lidar. In the literatures that have been reported, the technology of frequency modulation and frequency locking is adopted. This technology generates sideband frequency signals that are symmetrically distributed on both sides of the original laser frequency after the laser beam used for frequency locking is modulated by an electro-optic modulator or an acousto-optic modulator. After the sideband signal and the original laser signal pass through the interferometric spectral filter to be locked, an amplitude modulated voltage signal will be obtained on the photodetector. Finally, a voltage signal synchronous with the signal driving the modulator is needed to demodulate the amplitude modulation signal, so as to obtain the error signal when the frequency is out of lock. By feeding this error signal back to the frequency tuning device of the interferometric spectral filter, such as a piezoelectric transducer (PZT), it is possible to retune the filter out of lock to the laser frequency used. Although this technology is widely used, the disadvantage is that the required equipment is very complicated. For example, at least two electro-optic frequency modulators are needed to achieve a better modulation effect; in order to keep the synchronization of the demodulated signal and the modulator driving signal, a phase-locked loop circuit is often required; since the modulation frequency is usually in the order of MHz, the detector A high frequency response is required to detect the desired photoelectric signal. All of these increase the complexity of the technology's circuitry and optics.

Contents of the invention

The purpose of the present invention is to overcome the above-mentioned deficiencies in the prior art, reduce the device complexity of Michelson interference type spectral filter resonance frequency locking, and propose a kind of Michelson interference type spectral filter resonance frequency locking method.

The present invention cleverly utilizes the dependence of the resonant frequency of the Michelson interference type spectral filter on the incident angle of the incident laser, and detects the resonant frequency of the Michelson interference filter in real time through two probe beams with matching incident angles. If lock is lost, an error signal can be generated and fed back to the filter's frequency tuning device to retune it to frequency lock. Since high-frequency modulation and synchronous demodulation are not required, the complexity of circuits and optical paths is greatly reduced.

A Michelson interference type spectral filter resonant frequency locking device, including a laser beam splitting system, a Michelson interference type filter system and a photoelectric detection system;

The laser beam splitting system includes a laser, a collimating beam expander, a first beam splitter, a first mirror, a second mirror, a second beam splitter, and a third mirror; the Michelson interference filter system includes a cubic beam splitter, a Four reflecting mirrors and the fifth reflecting mirror, wherein the fifth reflecting mirror is mechanically connected with the frequency tuning device to adjust the resonant frequency; the photodetection system includes a lens, a first photomultiplier tube, a second photomultiplier tube, a third photomultiplier tube, a differential Amplifiers, oscilloscopes;

The laser beam emitted by the laser is expanded into a wide-beam parallel light by a collimator beam expander; the wide-beam parallel light is divided into two paths through the first beam splitter, one of which is transmitted through the first beam splitter and directly enters the Michelson laser beam to be frequency-locked The interference filtering system is used as the monitoring beam; the other path passes through the first reflector and the second reflector in turn, and then is divided into two paths by the second beam splitter, of which the first path passes through the second beam splitter and then passes through the third reflector The reflection enters the Michelson interference filter system to be frequency-locked at an angle θ ₂ as a probe beam; the second path directly enters the Michelson interference filter system to be frequency-locked at an angle θ ₁ after being reflected by the second beam splitter as a probe beam; When the two probe beams and the monitoring beam pass through the Michelson interference filter system to be frequency-locked, the two probe beams and the monitoring beam are divided into two by the cubic beam splitter in the Michelson interference filter system, and one is transmitted through the cubic beam splitter Then it is reflected back to the cube beamsplitter by the fourth reflector, and then reflected into the lens through the cube beamsplitter; the other way is reflected by the cube beamsplitter, then reflected back to the cube beamsplitter by the fifth reflector, and transmitted into the lens through the cube beamsplitter ; Several beams are focused on different positions of the focal plane by the lens and interfere respectively, and the interference signals are respectively received and converted into electrical signals by the first photomultiplier tube, the second photomultiplier tube, and the third photomultiplier tube; The electrical signals output by the photomultiplier tube and the second photomultiplier tube are input to the differential amplifier, and the output signal of the differential amplifier is fed back to the frequency tuning device; while the output electrical signal of the third photomultiplier tube is input to the oscilloscope as a frequency locking state monitoring signal.

A method for locking the resonant frequency of a Michelson interference type spectral filter, comprising the steps of:

Step 1. Calculate the incident angles of the two probe beams;

Step 2. Adjust the incident angles of the two probe beams;

Step 3. Check the oscilloscope to judge the frequency lock status;

The incident angles of the two probe beams described in step 1 include θ ₁ and θ ₂ ; θ ₁ and θ ₂ need to meet the following matching requirements:

The selection of θ ₁ needs to make the difference ΔOPD(θ ₁ ) between the optical path difference of the probe beam incident at this angle and the optical path difference at 0 degree angle incident by the Michelson interference filter be (n ₁ +1/4)λ ₀ ,Right now

ΔOPD(θ ₁ )=(n ₁ +1/4)λ ₀ (1)

Among them, λ ₀ is the center wavelength of the laser, n ₁ is an integer to be selected (the recommended value is 0-10); the calculation method of ΔOPD(θ ₁ ) is related to the structural parameters of the Michelson interference filter used; if the Michelson interference The lengths of the two interference arms of the filter are d ₁ and d ₂ respectively, and the refractive indices are ρ ₁ and ρ ₂ respectively, then ΔOPD(θ ₁ ) can be calculated according to the following formulas

$\mathrm{<span\; class="notranslate">\; OPD</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; abs</span>}<span\; class="notranslate">\; [</span>{\mathrm{<span\; class="notranslate">\; \&rho;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; d</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; sin</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}{{{\mathrm{<span\; class="notranslate">\; \&rho;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&rho;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; d</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; sin</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}{{{\mathrm{<span\; class="notranslate">\; \&rho;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; a</span>}<span\; class="notranslate">\; )</span>$

OPD(0)＝2·abs(ρ ₁ d ₁ -ρ ₂ d ₂ ) (2b)

ΔOPD(θ ₁ )=abs[OPD(θ ₁ )−OPD(0)] (2c)

Wherein, abs ( ) represents to take the absolute value, OPD (θ ₁ ) represents the optical path difference of Michelson interference filter when incident with angle θ ₁ , OPD (0) represents the optical path difference of Michelson interference filter when light is incident; As long as the integer n ₁ is selected, the simultaneous equations (1) and (2) can be solved for θ ₁ ;

To match θ ₁ , the incident angle θ ₂ must satisfy the change between the optical path difference of the Michelson interference filter for the probe beam and the optical path difference at normal incidence:

ΔOPD(θ ₂ )=(n ₂ -1/4)λ ₀ (3)

In formula (3), n ₂ is another integer to be selected, ranging from 0 to 50 and larger than the selected n _1. Similarly, ΔOPD(θ ₂ ) can be calculated according to the following formulas

$\mathrm{<span\; class="notranslate">\; OPD</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; abs</span>}<span\; class="notranslate">\; [</span>{\mathrm{<span\; class="notranslate">\; \&rho;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; d</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; sin</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{{\mathrm{<span\; class="notranslate">\; \&rho;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&rho;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; d</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; sin</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{{\mathrm{<span\; class="notranslate">\; \&rho;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}\mathrm{<span\; class="notranslate">\; a</span>}<span\; class="notranslate">\; )</span>$

ΔOPD(θ ₂ )=abs[OPD(θ ₂ )−OPD(0)] (4b)

Where OPD(θ ₂ ) represents the optical path difference of the Michelson interference filter when it is incident at an angle θ ₂ . As long as the integer n ₂ is selected, the simultaneous equations (3) and (4) can be solved for θ ₂ ;

Adjust the incident angle of the probe beam described in step 2, specifically as follows:

2-1. Before the adjustment of the two probe beams, manually adjust the output of the third photomultiplier tube to 0 through the frequency tuning device, that is, let the Michelson interferometer filter be initialized in the frequency locked state;

2-2. Adjust the second beam splitter and the third reflector so that the angles of the angles of incidence θ ₁ and θ ₂ of the probe beam are consistent with the angles of incidence θ ₁ and θ ₂ calculated in step 1;

The specific consistent judgment method is as follows:

When adjusting the actual optical path, first disconnect the differential amplifier and connect it to the feedback terminal of the frequency tuning device, use a precision moving turntable to adjust the incident angle of one of the probe beams to θ ₁ , and then adjust the angle of the other probe beam to Determine that it is near the theoretically calculated θ ₂ , and then fine-tune the precision moving turntable of the second probe beam until the output of the differential amplifier reaches 0; after the angle adjustment of the two probe beams is completed, connect the feedback terminal of the differential amplifier to Frequency tuning devices for Michelson interference filters;

Check the oscilloscope as described in step 3 to judge the frequency lock status, as follows:

If the Michelson interference filter is just locked at the center frequency of the laser, the output signal of the oscilloscope is 0; if the external environmental factors cause the frequency of the Michelson interferometer to lose lock, the differential amplifier will output an error signal and feed back to the frequency of the Michelson interference filter Tuning equipment, frequency tuning equipment automatically adjusts the resonant frequency of the interferometer driven by the error signal until it locks to the required laser center frequency; during this process, the output signal of the oscilloscope will gradually approach 0.

The first beam splitter is a beam splitter whose reflectivity is greater than the transmittance; the second beam splitter is a beam splitter with a splitting ratio of 50%:50%.

The ratio of reflectivity and transmittance of the first beam splitter is as follows: T:R=10%:90%.

The beneficial effects of the present invention are as follows:

The invention is suitable for locking the resonant frequency of various Michelson interference type spectral filters, is simple to implement, and can avoid complex requirements on circuits and optical paths in traditional frequency locking methods, thereby having strong system stability and robustness.

Description of drawings

Fig. 1 is the optical path figure of device of the present invention;

Fig. 2 is an example of Michelson interferometric spectral filter optical path difference variation and incident angle relation among the present invention;

Fig. 3 is a relationship curve between the error signal output by the differential amplifier and the amount of frequency loss of locking when the Michelson interference type spectral filter is locked in the present invention.

In the figure, laser 1, collimator beam expander 2, first beam splitter 3, first mirror 4, second mirror 5, second beam splitter 6, third mirror 7, cubic beam splitter prism 8, fourth Mirror 9, fifth mirror 10, lens 11, first photomultiplier tube (PMT) 12, second photomultiplier tube 13, third photomultiplier tube 14, differential amplifier 15, frequency tuning device 16, oscilloscope 17.

detailed description

The present invention will be further described below in conjunction with accompanying drawing.

As shown in Figure 1, a Michelson interference type spectral filter resonance frequency locking device includes a laser beam splitting system a, a Michelson interference type filter system b and a photoelectric detection system c;

The laser beam splitting system a includes a laser 1, a collimator beam expander 2, a first beam splitter 3, a first mirror 4, a second mirror 5, a second beam splitter 6, and a third mirror 7; Michelson interference filter The reflector system b includes a cubic dichroic prism 8 , a fourth reflector 9 , and a fifth reflector 10 . The fifth reflector 10 is mechanically connected to the frequency tuning device 16 to adjust the resonant frequency; the photodetection system c includes a lens 11, a first photomultiplier tube (PMT) 12, a second photomultiplier tube 13, a third photomultiplier tube 14, Differential amplifier 15, oscilloscope 17.

The laser beam emitted by the laser 1 is expanded into a wide-beam parallel light by the collimator beam expander 2; the wide-beam parallel light is divided into two paths through the first beam splitter 3, and one of them is transmitted through the first beam splitter 3 and then directly injected into the waiting beam. The frequency-locked Michelson interference filter system b is used as the monitoring beam; the other path passes through the first reflector 4 and the second reflector 5 in turn, and then is divided into two paths by the second beam splitter 6, of which the first path passes through the second beam splitter 6 After transmission, it is reflected by the third reflector 7 and enters the Michelson interference filter system b to be frequency-locked at an angle of θ ₂ as a probe beam; the second path is directly reflected by the second beam splitter 6 and enters the frequency-locked Michelson interference filter system b at an angle of θ ₁ frequency Michelson interference filter system b as the probe beam. When the two probe beams and the monitoring beam pass through the Michelson interference filter system b to be frequency-locked, the two probe beams and the monitoring beam are divided into two by the cubic beamsplitter 8 in the Michelson interference filter system b, and one passes through the cubic After the beam splitting prism 8 is transmitted, it is reflected back to the cubic beam splitting prism 8 by the fourth reflector 9, and then reflected into the lens 11 through the cube beam splitting prism 8; the other path is reflected by the cube beam splitting prism 8 and then reflected back to the cube beam splitting lens by the fifth mirror 10 Prism 8, and transmits into lens 11 through cubic dichroic prism 8; Several beams are focused on different positions of its focal plane by lens 11 and interfere respectively, and the interference signals are respectively received by first photomultiplier tube 12, second photomultiplier tube 13 , The third photomultiplier tube 14 receives and converts the electric signal. The electrical signal output by the first photomultiplier tube 12 and the second photomultiplier tube 13 is input to the differential amplifier 15, and the output signal of the differential amplifier 15 is fed back to the frequency tuning device 16; and the output electrical signal of the third photomultiplier tube 14 is input to the oscilloscope 17 As a frequency lock status monitoring signal.

A Michelson interference type spectral filter resonant frequency locking method specifically comprises the following steps:

Step 1. Build a Michelson interference type spectral filter resonant frequency locking device;

Step 2. Calculate the incident angles of the two probe beams;

Step 3. Adjust the incident angles of the two probe beams;

Step 4. Check the oscilloscope to judge the frequency lock status.

The Michelson interferometric spectral filter resonant frequency locking device described in step 1 is the device shown in Figure 1;

The incident angles of the two probe beams described in step 2 include θ ₁ and θ ₂ ; θ ₁ and θ ₂ need to meet certain matching requirements, which are determined as follows:

The selection of θ ₁ needs to make the difference ΔOPD(θ ₁ ) between the optical path difference of the probe beam incident at this angle and the optical path difference at 0 degree angle incident by the Michelson interference filter be (n ₁ +1/4)λ ₀ ,Right now

ΔOPD(θ ₁ )=(n ₁ +1/4)λ ₀ (1)

Among them, λ ₀ is the center wavelength of the laser, n ₁ is an integer to be selected (the recommended value is 0-10); the calculation method of ΔOPD(θ ₁ ) is related to the structural parameters of the Michelson interference filter used; if the Michelson interference The lengths of the two interference arms of the filter are d ₁ and d ₂ respectively, and the refractive indices are ρ ₁ and ρ ₂ respectively, then ΔOPD(θ ₁ ) can be calculated according to the following formulas

$\mathrm{<span\; class="notranslate">\; OPD</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; abs</span>}<span\; class="notranslate">\; [</span>{\mathrm{<span\; class="notranslate">\; \&rho;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; d</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; sin</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}{{{\mathrm{<span\; class="notranslate">\; \&rho;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&rho;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; d</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; sin</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}{{{\mathrm{<span\; class="notranslate">\; \&rho;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; a</span>}<span\; class="notranslate">\; )</span>$

OPD(0)＝2·abs(ρ ₁ d ₁ -ρ ₂ d ₂ ) (2b)

ΔOPD(θ ₁ )=abs[OPD(θ ₁ )−OPD(0)] (2c)

Wherein, abs ( ) represents to take the absolute value, OPD (θ ₁ ) represents the optical path difference of Michelson interference filter when incident with angle θ ₁ , OPD (0) represents the optical path difference of Michelson interference filter when light is incident; As long as the integer n ₁ is selected, the simultaneous equations (1) and (2) can be solved for θ ₁ ;

To match θ ₁ , the incident angle θ ₂ must satisfy the change between the optical path difference of the Michelson interference filter for the probe beam and the optical path difference at normal incidence:

ΔOPD(θ ₂ )=(n ₂ -1/4)λ ₀ (3)

In formula (3), n ₂ is another integer to be selected, ranging from 0 to 50 and larger than the selected n _1. Similarly, ΔOPD(θ ₂ ) can be calculated according to the following formulas

$\mathrm{<span\; class="notranslate">\; OPD</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; abs</span>}<span\; class="notranslate">\; [</span>{\mathrm{<span\; class="notranslate">\; \&rho;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; d</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; sin</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{{\mathrm{<span\; class="notranslate">\; \&rho;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&rho;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; d</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; sin</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{{\mathrm{<span\; class="notranslate">\; \&rho;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}\mathrm{<span\; class="notranslate">\; a</span>}<span\; class="notranslate">\; )</span>$

ΔOPD(θ ₂ )=abs[OPD(θ ₂ )−OPD(0)] (4b)

Where OPD(θ ₂ ) represents the optical path difference of the Michelson interference filter when it is incident at an angle θ ₂ . As long as the integer n ₂ is selected, the simultaneous equations (3) and (4) can be solved for θ ₂ .

Adjust the incident angle of the probe beam as described in step 3, as follows:

3-1. Before the adjustment of the two-way probe beams, manually adjust the output of the third photomultiplier tube 14 to 0 through the frequency tuning device, that is, let the Michelson interferometer filter be initialized in the frequency locked state;

3-2. Adjust the second beam splitter 6 and the third mirror 7 so that the incident angles θ ₁ and θ ₂ of the probe beam are consistent with the incident angles θ ₁ and θ ₂ calculated in step 2.

The specific method for judging the consistency is as follows: when adjusting the actual optical path, first disconnect the differential amplifier 15 and connect it to the feedback terminal of the frequency tuning device 16, and adjust the incident angle of one of the probe beams to θ ₁ with a precision moving turntable. Determine the angle of another probe beam near the theoretically calculated θ2, then fine-tune the precision moving turntable of the _second probe beam until the output of the differential amplifier 15 reaches 0; the angle adjustment of the two probe beams is completed Finally, the feedback terminal of the differential amplifier is connected to the frequency tuning device 16 of the Michelson interference filter.

Check the oscilloscope as described in step 4 to judge the frequency lock status, as follows:

If the Michelson interference filter is just locked at the center frequency of the laser, the output signal of the oscilloscope 17 is 0; if the external environment and other factors cause the frequency of the Michelson interferometer to lose lock, the differential amplifier will output an error signal and feed it back to the Michelson interference filter Driven by the error signal, the frequency tuning device automatically adjusts the resonant frequency of the interferometer until it locks to the required laser center frequency. During this process, the output signal of the oscilloscope 17 will gradually approach 0.

The first beam splitter 3 is a beam splitter whose reflectivity is much greater than the transmittance, such as T:R=10%:90%; the second beam splitter 6 is a beam splitter with a splitting ratio of 50%:50%. In order to facilitate angle adjustment, the second beam splitter 6 and the third reflector 7 can be placed on a precision rotating platform.

Example

In Figure 1, the laser 1 and the lidar transmitter are shared by splitting light, and its frequency is the reference frequency that needs to be locked;

The above-mentioned collimating beam expander 2 can be an ordinary beam expander, such as the GCO-141602 beam expander of Beijing Daheng Company, which has a 6-fold beam expander;

The above-mentioned first beam splitter 3 adopts a beam splitter of T: R=10%:90%, such as GCC-411215 of Beijing Daheng Company; the second beam splitter 6 uses a common T:R=50%:50% beam splitter Yes, such as GCC-411102 of Beijing Daheng Company;

Above: the first reflector 4, the second reflector 5, and the third reflector 7 can be ordinary reflectors, such as GCC-101102 of Beijing Daheng Company, with a diameter of 25.4mm;

Lens 11 is a long focal length lens, such as Beijing Daheng Company GCL-010214, focal length 400mm;

The first photomultiplier tube (PMT) 12, the second photomultiplier tube 13, and the third photomultiplier tube 14 can use the R6358 photomultiplier tube of Hamamatsu Corporation of Japan;

The differential amplifier 15 can be selected from the chip INA126 produced by Texas Instruments (TI);

The oscilloscope 17 can be an ordinary oscilloscope, such as YB4320/20A/40.

Michelson interference filters can be homemade or purchased integrated devices. If it is self-made, its frequency tuning device 16 generally uses a piezoelectric sensor PZT, such as the PZ150E of PI Company, and the cubic beam splitter 8 is 50%: 50% of the common cubic beam splitter, such as Beijing Daheng Company GCC-401012, the fourth reflector The mirror 9 and the fifth mirror 10 are common mirrors, such as GCC-101102 of Beijing Daheng Company; if they are purchased integrated products, they will be accompanied by a frequency tuning device.

The frequency locking method is further described below in conjunction with specific Michelson interference filter parameters.

First set up the device according to the optical path shown in Figure 1, and then calculate the incident angles of the two probe beams according to the method described in step 2.

Assume that the length and refractive index of the two interference arms of the Michelson interference filter to be frequency locked are d ₁ =87.578mm, d ₂ =59.318mm, ρ ₁ =1.4765, ρ ₂ =1.00027. In order to determine the incident angles θ ₁ and θ ₂ of the two probe beams by formula (1-4), it is also necessary to select two integers n ₁ and n ₂ arbitrarily. Usually, the n ₁ can be selected arbitrarily, and the recommended value is 0-10, which can not only ensure that the calculated incident angle will not be too small, but also ensure that the OPD change of the Michelson interference filter for the probe beam incident at this angle will not be too small. too big. Similarly, the selection of n ₂ should not only ensure that the difference between the calculated θ ₁ and θ ₂ is not too small, but also ensure that the OPD change of the Michelson interference filter for the probe beam is at the wavelength level as much as possible. The recommended value is 0 to 50 And smaller than the selected n ₁ .

An auxiliary method for selecting n ₁ and n ₂ is to make a schematic diagram of the relationship between the incident angle of the Michelson interference filter and the change in OPD according to formula (2) or formula (4), as shown in Figure 2. It can be seen from Fig. 2 that the OPD of the Michelson interference filter does not change significantly with the incident angle, so smaller n ₁ and n ₂ can be selected. For example, n ₁ =0, n ₂ =1 can be selected, and θ ₁ =4.024°, θ ₂ =5.086° can be solved by substituting into formula (1-4).

After calculating the incident angles of the two probe beams, adjust the incident angles of the two probe beams according to step 3. From step 4, the state of resonant frequency locking can be observed.

In order to illustrate the feasibility of the scheme, Fig. 3 shows the relationship curve of the error signal with frequency out of lock. It can be seen that this technical solution can detect the direction of frequency loss locking: when the resonance frequency of the Michelson interference filter is greater than the center frequency of the laser, the differential amplifier outputs a positive error signal, and when the resonance frequency of the Michelson interference filter is less than the center frequency of the laser frequency, the differential amplifier outputs a negative error signal. The system can automatically adjust the sign and magnitude of the error signal according to the direction and magnitude of the frequency loss lock to drive the Michelson interferometer frequency tuning device to move in the correct direction, so that the interferometer returns to the frequency lock state. Moreover, the error signal changes steeply, indicating high frequency locking sensitivity.

Claims (4)

1. A Michelson interference type spectral filter resonant frequency locking device is characterized in that comprising a laser beam splitter system, a Michelson interference type filter system and a photoelectric detection system; The laser beam splitting system includes a laser, a collimating beam expander, a first beam splitter, a first mirror, a second mirror, a second beam splitter, and a third mirror; the Michelson interference filter system includes a cubic beam splitter, a Four reflecting mirrors and the fifth reflecting mirror, wherein the fifth reflecting mirror is mechanically connected with the frequency tuning device to adjust the resonant frequency; the photodetection system includes a lens, a first photomultiplier tube, a second photomultiplier tube, a third photomultiplier tube, a differential Amplifiers, oscilloscopes; The laser beam emitted by the laser is expanded into wide-beam parallel light by the collimator beam-expanding system; the wide-beam parallel light is divided into two paths through the first beam splitter, one of which is transmitted through the first beam splitter and directly enters the Michelson to be frequency-locked The interference filtering system is used as the monitoring beam; the other path passes through the first reflector and the second reflector in turn, and then is divided into two paths by the second beam splitter, of which the first path passes through the second beam splitter and then passes through the third reflector The reflection enters the Michelson interference filter system to be frequency-locked at an angle θ ₂ as a probe beam; the second path directly enters the Michelson interference filter system to be frequency-locked at an angle θ ₁ after being reflected by the second beam splitter as a probe beam; When the two probe beams and the monitoring beam pass through the Michelson interference filter system to be frequency-locked, the two probe beams and the monitoring beam are divided into two by the cubic beam splitter in the Michelson interference filter system, and one is transmitted through the cubic beam splitter Then it is reflected back to the cube beamsplitter by the fourth reflector, and then reflected into the lens through the cube beamsplitter; the other way is reflected by the cube beamsplitter, then reflected back to the cube beamsplitter by the fifth reflector, and transmitted into the lens through the cube beamsplitter ; Several beams are focused on different positions of the focal plane by the lens and interfere respectively, and the interference signals are respectively received and converted into electrical signals by the first photomultiplier tube, the second photomultiplier tube, and the third photomultiplier tube; The electrical signals output by the photomultiplier tube and the second photomultiplier tube are input to the differential amplifier, and the output signal of the differential amplifier is fed back to the frequency tuning device; while the output electrical signal of the third photomultiplier tube is input to the oscilloscope as a frequency locking state monitoring signal. 2. use the method for a kind of Michelson interference type spectral filter resonant frequency locking device as claimed in claim 1, it is characterized in that comprising the steps: Step 1. Calculate the incident angles of the two probe beams; Step 2. Adjust the incident angles of the two probe beams; Step 3. Check the oscilloscope to judge the frequency lock status; The incident angles of the two probe beams described in step 1 include θ ₁ and θ ₂ ; θ ₁ and θ ₂ need to meet the following matching requirements: The selection of θ ₁ needs to make the difference △OPD(θ ₁ ) between the optical path difference of the probe beam incident at this angle and the optical path difference at 0 degree angle incident by the Michelson interference filter be (n ₁ +1/4)λ ₀ , that is △OPD(θ ₁ )=(n ₁ +1/4)λ ₀ (1) Among them, λ ₀ is the central wavelength of the laser, n ₁ is an integer to be selected, and its value ranges from 0 to 10; the calculation method of △OPD(θ ₁ ) is related to the structural parameters of the Michelson interference filter used; if Michelson The lengths of the two interference arms of the interference filter are d ₁ and d ₂ respectively, and the refractive indices are ρ ₁ and ρ ₂ respectively, then △OPD(θ ₁ ) can be calculated according to the following formulas OPD(0)＝2□abs(ρ ₁ d ₁ -ρ ₂ d ₂ ) (2b) △OPD(θ ₁ )=abs[OPD(θ ₁ )-OPD(0)] (2c) Among them, abs ( ) means to take the absolute value, OPD (θ ₁ ) means the optical path difference of the Michelson interference filter when the angle θ ₁ is incident, and OPD (0) means the optical path difference of the Michelson interference filter when the light is incident; As long as the integer n ₁ is selected, the simultaneous equations (1) and (2) can be solved for θ ₁ ; To match θ ₁ , the incident angle θ ₂ must satisfy the change between the optical path difference of the Michelson interference filter for the probe beam and the optical path difference at normal incidence: △OPD(θ ₂ )=(n ₂ -1/4)λ ₀ (3) In formula (3), n ₂ is another integer to be selected, ranging from 0 to 50 and larger than the selected n _1. Similarly, △OPD(θ ₂ ) can be calculated according to the following formulas △OPD(θ ₂ )=abs[OPD(θ ₂ )-OPD(0)] (4b) Among them, OPD(θ ₂ ) represents the optical path difference of the Michelson interference filter when the angle θ ₂ is incident; as long as the integer n ₂ is selected, the simultaneous equations (3) and (4) can be solved for θ ₂ ; Adjust the incident angle of the probe beam described in step 2, specifically as follows: 2-1. Before the adjustment of the two probe beams, manually adjust the output of the third photomultiplier tube to 0 through the frequency tuning device, that is, let the Michelson interferometer filter initialization be in the frequency locked state; 2-2. Adjust the second beam splitter and the third reflector so that the angles of the angles of incidence θ ₁ and θ ₂ of the probe beam are consistent with the angles of incidence θ ₁ and θ ₂ calculated in step 1; The specific consistent judgment method is as follows: When adjusting the actual optical path, first disconnect the differential amplifier and connect it to the feedback terminal of the frequency tuning device, use a precision moving turntable to adjust the incident angle of one of the probe beams to θ ₁ , and then adjust the angle of the other probe beam to Determine that it is near the theoretically calculated θ ₂ , and then fine-tune the precision moving turntable of the second probe beam until the output of the differential amplifier reaches 0; after the angle adjustment of the two probe beams is completed, connect the feedback terminal of the differential amplifier to Frequency tuning devices for Michelson interference filters; Check the oscilloscope as described in step 3 to judge the frequency lock status, as follows: If the Michelson interference filter is just locked at the center frequency of the laser, the output signal of the oscilloscope is 0; if the external environmental factors cause the frequency of the Michelson interferometer to lose lock, the differential amplifier will output an error signal and feed back to the frequency of the Michelson interference filter Tuning equipment, frequency tuning equipment automatically adjusts the resonant frequency of the interferometer driven by the error signal until it locks to the required laser center frequency; during this process, the output signal of the oscilloscope will gradually approach 0. 3. the method for a kind of Michelson interference type spectral filter resonant frequency locking as claimed in claim 2 is characterized in that described first beam splitter is the beam splitter that reflectivity is greater than transmittance; The second beam splitter is 50 %: Beamsplitter with 50% splitting ratio. 4. the method for a kind of Michelson interference type spectral filter resonant frequency locking as claimed in claim 3, is characterized in that the reflectance of described first spectroscopic mirror and transmittance ratio are as follows: T:R=10%: 90%.