CN110806306A - Device and method for measuring temperature change of cavity of resonant cavity of multi-longitudinal-mode laser - Google Patents

Device and method for measuring temperature change of cavity of resonant cavity of multi-longitudinal-mode laser Download PDF

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CN110806306A
CN110806306A CN201911113726.5A CN201911113726A CN110806306A CN 110806306 A CN110806306 A CN 110806306A CN 201911113726 A CN201911113726 A CN 201911113726A CN 110806306 A CN110806306 A CN 110806306A
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longitudinal
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self
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CN110806306B (en
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吕亮
周俊峰
赵元坤
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Anhui University
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
    • G01M11/00—Testing of optical apparatus; Testing structures by optical methods not otherwise provided for
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01K—MEASURING TEMPERATURE; MEASURING QUANTITY OF HEAT; THERMALLY-SENSITIVE ELEMENTS NOT OTHERWISE PROVIDED FOR
    • G01K11/00—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
    • G01M11/00—Testing of optical apparatus; Testing structures by optical methods not otherwise provided for
    • G01M11/02—Testing optical properties
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
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Abstract

The application relates to the technical field of lasers, in particular to a device and a method for measuring temperature change of a resonant cavity of a multi-longitudinal-mode laser, wherein the measuring device comprises the multi-longitudinal-mode laser, a vibration target, a sliding block, a sliding rail, a beam splitter, an optical attenuator, a photoelectric detector, a signal preprocessing unit and a signal processing unit, a reflecting film or a reflecting plane mirror is attached to a vibration surface of the vibration target, the multi-longitudinal-mode laser emits laser to the vibration target, the vibration target receives the emitted laser and feeds back the laser to the resonant cavity of the laser, the bottom of the vibration target is fixed on the sliding block, the sliding block is arranged on the sliding rail, the beam splitter splits a self-mixing signal to the photoelectric detector, and the output end of the photoelectric detector is sequentially connected with the signal preprocessing unit. The split application has the advantages of simple structure, easy realization and low manufacturing cost, and can realize the real-time non-contact high-precision measurement of the temperature change of the cavity of the resonant cavity of the multi-longitudinal-mode laser.

Description

Device and method for measuring temperature change of cavity of resonant cavity of multi-longitudinal-mode laser
The application is divisional application with application number 201810326278.6, application date 2018, 4, 12 and invention title "measuring device and measuring method for temperature change of resonant cavity of multiple longitudinal mode laser and resonant cavity
Technical Field
The invention relates to the technical field of lasers, in particular to a device and a method for measuring temperature change of a resonant cavity of a laser resonant cavity of a multi-longitudinal-mode laser based on laser self-mixing signals.
Background
Lasers are widely used in the fields of medical treatment, communication, industry, national defense and the like because of their inherent excellent characteristics of high brightness, high monochromaticity, high directivity, high coherence and the like. The laser resonant cavity is used as a core component of the laser, and the health monitoring of the laser resonant cavity is an important link for maintaining the good operation of the laser. The indexes influencing the health degree of the laser resonant cavity mainly comprise the free spectral range (FSR for short) of the laser resonant cavity and the cavity temperature of the laser resonant cavity, so that the monitoring of the FSR of the laser resonant cavity and the cavity temperature of the laser resonant cavity is particularly important in the running process of the laser.
The conventional methods for measuring the laser resonator FSR mainly include the following two methods:
1. the method for directly observing the longitudinal mode spacing by using a spectrometer is limited by the wavelength resolution of the spectrometer, and has the problems of low measurement resolution and sensitivity, low measurement precision, high price and the like;
2. the method for measuring the FSR by combining the scanning FP and MZ interferometers and the spectrometer is not only required to be combined with large-scale instruments and equipment, but also is easily limited by PD bandwidth, and the measuring system has a complex structure and higher cost, and is not beneficial to popularization and application.
The traditional method for measuring the cavity temperature of the laser resonant cavity mainly comprises a PN junction characteristic temperature measurement method, a phase difference temperature measurement method, a fiber grating temperature measurement method and the like. The PN junction characteristic temperature measurement method and the phase difference temperature measurement method are both indirect temperature measurement methods, thermal balance (between a shell and the inside) needs to be achieved between a temperature measurement element and a laser tube core, so that the temperature measurement result has the problems of long response time, large measurement error and the like, and meanwhile, the PN junction characteristic temperature measurement method is only limited to a semiconductor laser and cannot be applied to other types of lasers. The fiber grating thermometry generally needs to be combined with an expensive spectrometer or a complex demodulation technology, and the measurement resolution and sensitivity are relatively low, so that great difficulty exists in practical application.
Disclosure of Invention
Aiming at the problems in the prior art, the invention provides a measuring device which is suitable for a multi-longitudinal-mode laser and can be used for measuring the FSR (frequency shift register) of a laser resonant cavity and measuring the temperature change of the laser resonant cavity.
In order to achieve the technical purpose, the technical scheme of the invention is as follows:
a multi-longitudinal-mode laser resonant cavity FSR measuring device comprises a multi-longitudinal-mode laser, a vibration target, a sliding block, a sliding rail, a beam splitter, an optical attenuator, a photoelectric detector, a signal preprocessing unit and a signal processing unit, wherein the vibration target can vibrate, and a reflecting film or a reflecting plane mirror is attached to a vibration surface;
the multi-longitudinal-mode laser is a laser light source to be detected and emits laser to a vibration target;
the vibration target receives laser emitted by the multi-longitudinal-mode laser and feeds back the laser into a resonant cavity of the multi-longitudinal-mode laser through a reflecting film or a reflecting plane mirror to form a self-mixing signal;
the bottom of the vibration target is fixed on the sliding block;
the sliding block is arranged on the sliding rail and can horizontally move along the sliding rail, and the sliding rail and the emergent laser are positioned on the same straight line;
a beam splitter and an optical attenuator are sequentially arranged on an optical path between the multi-longitudinal-mode laser and the vibration target;
the beam splitter is used for splitting the self-mixing signal onto the photoelectric detector;
the photoelectric detector is used for converting the received laser signal into an electric signal and then sending the electric signal to the signal preprocessing unit;
the signal preprocessing unit is used for preprocessing the received electric signals, and the preprocessing at least comprises shaping and filtering;
and the signal processing unit is used for analyzing and processing the preprocessed electric signals to obtain the FSR measurement result of the laser resonant cavity.
Preferably, the vibration target is a speaker or a piezoelectric ceramic driven by a signal generator.
As an improvement, when the multi-longitudinal-mode laser adopts a semiconductor laser, the photoelectric detector is integrated in the multi-longitudinal-mode laser.
Preferably, the signal processing unit is a computer, an oscilloscope or a spectrometer.
The laser resonant cavity FSR measuring method based on the multi-longitudinal-mode laser resonant cavity FSR measuring device specifically comprises the following steps: the vibration target vibrates, the multi-longitudinal-mode laser is used as a laser light source to be detected, outgoing laser reaches the vibration target, the outgoing laser is reflected by a reflecting film or a reflecting plane and then fed back to the resonant cavity of the multi-longitudinal-mode laser to form a self-mixing signal, the sliding block is slightly moved along the sliding rail in the process so as to change the distance between the vibration target and the multi-longitudinal-mode laser, therefore, laser self-mixing signals under different laser external cavity lengths are formed, meanwhile, the intensity of feedback light is adjusted by the optical attenuator, the laser self-mixing signals under different laser external cavity lengths are collected by the photoelectric detector, then, the laser self-mixing signals are preprocessed by the signal preprocessing unit, and finally, the preprocessed laser self-mixing signals are analyzed by the signal processing unit, so that the FSR of the resonant cavity of the laser can be obtained, and the specific analysis method:
based on the laser self-mixing signal of the multi-longitudinal-mode laser, different longitudinal modes of the laser are considered to be only interfered with the self-mode, so that the finally formed laser self-mixing signal can be considered as the intensity superposition of the laser self-mixing signal formed by the respective longitudinal modes, and according to a related interference mixing theory model, the intensity of the self-mixing signal of the multi-longitudinal-mode laser can be further obtained as follows:
Figure BDA0002273481660000031
in formula (2): i is0Representing initial light intensity, β is the total number of oscillation starting modes in the multi-longitudinal mode laser, j represents the jth longitudinal mode in the multi-longitudinal mode laser, and delta IjAmplitude of variation of light intensity of j-mode laser, k0jNumber of waves, n, representing the j mode in vacuum0Denotes the refractive index of the external cavity, Lext(t) represents the real-time external cavity length;
when refractive index of external cavity n0When the number is equal to 1, the alloy is put into a container,
Figure BDA0002273481660000032
in formula (3): omega0Representing the angular frequency of the laser, c representing the speed of light in vacuum, and FSR representing the free spectral range of the laser cavity;
thus, it is possible to obtain
Figure BDA0002273481660000033
If the self-mixing signals of the laser in different modes are overlapped without waveform splitting or separation, the independent waveforms of the modes are required to keep the same phase or the phase delay is integral multiple of 2 pi, and the length of the external cavity needs to meet the condition:
Figure BDA0002273481660000034
in the formula (5), m is the external cavity mode order of the laser, is a positive integer, and LextIndicating the external cavity length; therefore, the multi-longitudinal-mode laser has a series of special position points, which ensure that the laser self-mixing signals of each longitudinal mode of the laser always keep the same phase or have the phase delay of 2 pi integral multiple, the superimposed laser self-mixing signals are not split or separated, the special position points are called equal phase points, and the relationship between the variation of the external cavity distance between adjacent stages of equal phase points and the free spectral region is as follows from equation (5):
delta L in the formula (6)extThe difference of the external cavity distances of the phase points of adjacent levels is shown;
therefore, the position of the phase point Δ L can be measured by measuring the adjacent levelextAnd the size of the laser resonant cavity FSR can be obtained.
As can be seen from the above description, the above technical solution has the following advantages:
1. the measuring device has simple structure, easy realization and low manufacturing cost;
2. based on the laser interference theory, the real-time non-contact high-precision measurement is realized by utilizing the laser self-mixing signal.
A temperature change measuring device for a resonant cavity of a multi-longitudinal-mode laser comprises the multi-longitudinal-mode laser, a vibration target, a sliding block, a sliding rail, a beam splitter, an optical attenuator, a photoelectric detector, a signal preprocessing unit and a signal processing unit, wherein the vibration target can vibrate, and a reflecting film or a reflecting plane mirror is attached to a vibration surface;
the multi-longitudinal-mode laser is a laser light source to be detected and emits laser to a vibration target;
the vibration target can vibrate, receives laser emitted by the multi-longitudinal-mode laser and feeds the laser back to the resonant cavity of the multi-longitudinal-mode laser through a reflecting film or a reflecting plane mirror to form a self-mixing signal;
the bottom of the vibration target is fixed on the sliding block;
the sliding block is arranged on the sliding rail and can horizontally move along the sliding rail, and the sliding rail and the emergent laser are positioned on the same straight line;
a beam splitter and an optical attenuator are sequentially arranged on an optical path between the multi-longitudinal-mode laser and the vibration target;
the beam splitter is used for splitting the self-mixing signal onto the photoelectric detector;
the photoelectric detector is used for converting the received laser signal into an electric signal and then sending the electric signal to the signal preprocessing unit;
the signal preprocessing unit is used for preprocessing the received electric signals, and the preprocessing at least comprises shaping and filtering;
and the signal processing unit is used for analyzing and processing the preprocessed electric signals to obtain a temperature change measurement result of the cavity of the resonant cavity of the laser.
Preferably, the vibration target is a speaker or a piezoelectric ceramic driven by a signal generator.
As an improvement, when the multi-longitudinal-mode laser adopts a semiconductor laser, the photoelectric detector is integrated in the multi-longitudinal-mode laser.
Preferably, the signal processing unit is a computer, an oscilloscope or a spectrometer.
The method for measuring the temperature change of the cavity of the resonant cavity of the laser based on the device for measuring the temperature change of the cavity of the resonant cavity of the multi-longitudinal-mode laser comprises the following specific steps: the vibration target vibrates, the multi-longitudinal mode laser is used as a laser source to be measured, laser is emitted to the vibration target, the emitted laser is fed back to the resonant cavity of the multi-longitudinal mode laser to form a self-mixing signal after being reflected by the reflecting film or the reflecting plane, in the process, the sliding block moves slightly along the sliding rail to change the distance between the vibration target and the multi-longitudinal-mode laser, thereby forming laser self-mixing signals under different laser external cavity lengths, simultaneously utilizing the optical attenuator to adjust the intensity of feedback light, utilizing the photoelectric detector to collect the laser self-mixing signals under different laser external cavity lengths, then, the signal preprocessing unit is used for preprocessing the laser self-mixing signal, and finally, the signal processing unit is used for analyzing the preprocessed laser self-mixing signal, so that the temperature change of the cavity of the resonant cavity of the laser can be obtained, wherein the specific analysis method comprises the following steps:
based on the laser self-mixing signal of the multi-longitudinal-mode laser, different longitudinal modes of the laser are considered to be only interfered with the self-mode, so that the finally formed laser self-mixing signal can be considered as the intensity superposition of the laser self-mixing signal formed by the respective longitudinal modes, and according to a related interference mixing theory model, the intensity of the self-mixing signal of the multi-longitudinal-mode laser can be further obtained as follows:
Figure BDA0002273481660000051
in formula (2): i is0Representing initial light intensity, β is the total number of oscillation starting modes in the multi-longitudinal mode laser, j represents the jth longitudinal mode in the multi-longitudinal mode laser, and delta IjAmplitude of variation of light intensity of j-mode laser, k0jNumber of waves, n, representing the j mode in vacuum0Denotes the refractive index of the external cavity, Lext(t) represents the real-time external cavity length;
when refractive index of external cavity n0When the number is equal to 1, the alloy is put into a container,
Figure BDA0002273481660000052
in formula (3): omega0Representing the angular frequency of the laser, c representing the speed of light in vacuum, and FSR representing the free spectral range of the laser cavity;
thus, it is possible to obtain
Figure BDA0002273481660000061
If the self-mixing signals of the laser in different modes are overlapped without waveform splitting or separation, the independent waveforms of the modes are required to keep the same phase or the phase delay is integral multiple of 2 pi, and the length of the external cavity needs to meet the condition:
in the formula (5), m is the external cavity mode order of the laser, is a positive integer, and LextIndicating the external cavity length; therefore, the laser has a series of special position points, the laser self-mixing signals of each longitudinal mode of the laser are ensured to always keep the same phase or the phase delay is integral multiple of 2 pi, the superposed laser self-mixing signals are not split or separated, and the special position points are called equal phase points;
at the position of the m-level equal phase point, the length of the outer cavity is finely adjusted by finely adjusting the position of the slide block, and the temperature change is trackedStabilizing the waveform by measuring the change of the length of the external cavity, delta LextAnd then according to the position change delta L of the m-level equal phase pointextmObtaining the change value of the cavity temperature of the laser resonant cavity by a relational expression of the change delta T of the cavity temperature of the resonant cavity, wherein the relational expression of the position change of the m-level equiphase phase point and the cavity temperature change of the resonant cavity is as follows:
in formula (7):
Figure BDA0002273481660000064
which represents the temperature coefficient of the refractive index,
Figure BDA0002273481660000065
which is indicative of the refractive index dispersion coefficient,
Figure BDA0002273481660000066
the temperature coefficient representing the wavelength of the light,
Figure BDA0002273481660000067
represents a linear expansion coefficient, LinIndicating the laser cavity length, Lin0Denotes the initial cavity length, n, of the laser resonatorgRepresenting the group refractive index, n, of the medium in the laser cavityg0The initial group refractive index of the medium in the laser resonant cavity is represented, and the value m is obtained through calculation of an expression (5);
and (3) obtaining the real-time temperature change of the laser by combining the correlation coefficient of the material, the cavity length of the laser resonant cavity at the initial temperature, the refractive index of the laser initial group and the value m.
As can be seen from the above description, the above technical solution has the following advantages:
1. the measuring device has simple structure, easy realization and low manufacturing cost;
2. based on the laser interference theory, the real-time non-contact high-precision measurement is realized by utilizing the laser self-mixing signal.
Drawings
FIG. 1 is a schematic structural view of a measuring apparatus according to embodiments 1 and 2 of the present invention;
FIG. 2 is a schematic diagram of laser self-mixing interference;
FIG. 3 is a diagram showing simulation results of embodiment 1 of the present invention;
FIG. 4 is a diagram showing simulation results of embodiment 1 of the present invention;
FIG. 5 is a diagram showing simulation results of embodiment 1 of the present invention;
FIG. 6 is a diagram showing simulation results of embodiment 1 of the present invention;
FIG. 7 is a diagram showing simulation results of embodiment 2 of the present invention;
FIG. 8 is a diagram showing simulation results of embodiment 2 of the present invention;
FIG. 9 is a diagram showing simulation results of embodiment 2 of the present invention;
FIG. 10 shows the sensitivity S of the external cavity variation in example 2 of the present inventionLextmAnd adjacent temperature difference DeltaTmGraph of relationship with m.
Detailed Description
Embodiment 1 of the present invention is described in detail with reference to fig. 1 to 6, but the present invention is not limited to the claims.
Lasers are generally composed of an optical resonant cavity, a gain medium, and an excitation source. The free spectral region of the laser resonant cavity is defined in accordance with the free spectral region of the FP cavity etalon, and the FSR is generally expressed by Deltav. The FSR expression of the laser cavity is:
FSR＝Δν＝c/2ngLin(1)
in the formula (1), LinIs the cavity length, n, of the laser resonant cavitygIs the group index of refraction of the medium in the cavity, and c is the speed of light in vacuum.
As shown in fig. 1, a multi-longitudinal-mode laser resonant cavity FSR measuring device is established, which includes a multi-longitudinal-mode laser 1, a vibration target 2, a slider 3, a slide rail 4, a beam splitter 6, an optical attenuator 7, a photodetector 8, a signal preprocessing unit 9 and a signal processing unit 10; the vibration target 2 can vibrate, a reflecting film or a reflecting plane mirror 11 is attached to a vibration surface, the multi-longitudinal-mode laser 1 is a laser light source to be detected, laser is emitted to the vibration target 2, the vibration target 2 receives the laser emitted by the multi-longitudinal-mode laser 1 and feeds the laser back to a resonant cavity of the multi-longitudinal-mode laser through the reflecting film or the reflecting plane mirror to form a self-mixing signal, and the bottom of the vibration target 2 is fixed on the sliding block 3; the sliding block 3 is arranged on the sliding rail 4 and can horizontally move along the sliding rail, the sliding rail 4 and the emergent laser are in the same straight line, a beam splitter 6 and an optical attenuator 7 are sequentially arranged on a light path between the multi-longitudinal-mode laser 1 and the vibration target 2, the beam splitter 6 is used for splitting a self-mixing signal into a photoelectric detector 8, the photoelectric detector 8 is used for converting a received laser signal into an electric signal and then sending the electric signal to a signal preprocessing unit 9, the signal preprocessing unit 9 is used for preprocessing (at least including shaping and filtering) the received electric signal, and the signal processing unit 10 is used for analyzing and processing the preprocessed electric signal to obtain a laser resonant cavity FSR measurement result.
Wherein:
(1) the vibration target 2 may be selected from a speaker or a piezoelectric ceramic driven by the signal generator 5.
(2) When the laser 1 to be measured is a multi-longitudinal-mode semiconductor laser, the multi-longitudinal-mode semiconductor laser integrated with the PD can be selected, that is, the photodetector in the device is integrated in the multi-longitudinal-mode laser, and at this time, the beam splitter can be cancelled.
(3) The optical attenuator may be a displacement type attenuator or an attenuation sheet type attenuator.
(4) The signal processing unit may be selected from a computer, an oscilloscope or a spectrometer.
Based on a laser self-mixing theoretical model (as shown in fig. 2), the measuring device is used for measuring the FSR of the multi-longitudinal mode laser resonant cavity, and the specific method is as follows:
the signal generator is used for driving the loudspeaker or the piezoelectric ceramics to vibrate, the multi-longitudinal-mode laser is used as a laser light source to be tested, laser is emitted to the loudspeaker or the piezoelectric ceramics, the emitted laser is reflected by the reflecting film or the reflecting plane mirror and then fed back to the resonant cavity of the multi-longitudinal-mode laser to form a self-mixing signal, in the process, the slide block is simultaneously slightly moved along the slide rail to change the distance between the loudspeaker or the piezoelectric ceramics and the multi-longitudinal-mode laser, so that laser self-mixing signals under different laser external cavity lengths are formed, meanwhile, the intensity of the feedback light is adjusted by the optical attenuator, the laser self-mixing signals under different laser external cavity lengths are collected by the photoelectric detector, then the signal preprocessing unit is used for preprocessing the laser self-mixing signals, finally, the signal processing unit is used for analyzing the preprocessed laser self-mixing signals, and the FSR of the resonant, the specific analysis method is as follows:
for the laser self-mixing signal of the multi-longitudinal-mode laser, different longitudinal modes of the laser only interfere with the self-mode, so that the finally formed laser self-mixing signal can be regarded as the intensity superposition of the laser self-mixing signals formed by the respective longitudinal modes. According to the relevant interference mixing theory model, the self-mixing signal intensity of the multi-longitudinal-mode laser can be further obtained as follows (no speckle influence is considered):
Figure BDA0002273481660000091
in formula (2): i is0Representing initial light intensity, β is the total number of oscillation starting modes in the multi-longitudinal mode laser, j represents the jth longitudinal mode in the multi-longitudinal mode laser, and delta IjAmplitude of variation of light intensity of j-mode laser, k0jNumber of waves, n, representing the j mode in vacuum0Denotes the refractive index of the external cavity, Lext(t) represents the real-time external cavity length;
when refractive index of external cavity n0When the number is equal to 1, the alloy is put into a container,
Figure BDA0002273481660000092
in formula (3): omega0Representing the angular frequency of the laser, c representing the speed of light in vacuum, and FSR representing the free spectral range of the laser cavity;
accordingly, equation (1) can be varied as:
Figure BDA0002273481660000093
if the self-mixing signals of the laser in different modes are overlapped without waveform splitting or separation, the independent waveforms of the modes are required to keep the same phase or the phase delay is integral multiple of 2 pi, and the length of the external cavity needs to meet the condition:
Figure BDA0002273481660000094
in the formula (5), m is the external cavity mode order of the laser, is a positive integer, and LextThe external cavity length is expressed, so that the laser has a series of special position points, the laser self-mixing signals of each longitudinal mode of the laser are ensured to be always kept in the same phase or have phase delay of integral multiple of 2 pi, the superposed laser self-mixing signals are not split or separated, the special position points are called equal phase points, and the relation between the external cavity distance change between adjacent stages of equal phase points and the free spectral region is shown in an equation (5):
Figure BDA0002273481660000101
delta L in the formula (6)extThe difference of the external cavity distances of the phase points of adjacent levels is shown, therefore, the position DeltaL of the phase points of the adjacent levels can be measuredextAnd obtaining the size of the FSR of the multi-longitudinal mode laser resonant cavity.
And establishing an experimental model and carrying out analog simulation on the measuring method. When a dual longitudinal mode laser is selected, m is 10, and FSR is 104.17GHz, the simulation results of the superimposed laser self-mixed signal are shown in fig. 3 to fig. 6.
As can be seen from fig. 3 to 6: corresponding to different FSRs when the length of the external cavity is
Figure BDA0002273481660000102
At integer multiples of (2 pi), the phase delay is integer multiples of (2 pi), and the phases of the self-mixing signals generated by the two modes are consistent, so that the waveform superposition between the two modes cannot cause the waveform change, namely the situations shown in fig. 3 and 6; whileWhen the external cavity length is not an integral multiple of the resonant cavity length, the positions of the laser self-mixing signal waveforms generated by different modes respectively on the time domain are different because the phase delay is not an integral multiple of 2 pi, and the waveforms are split or separated after the waveforms are superimposed, namely, the situations shown in fig. 4 and 5 are shown. Therefore, the variation of the length of the external cavity is 1.44mm, which can be obtained by the positions of the equiphase point where the adjacent-stage waveforms are not split or separated, as shown in fig. 3 and fig. 6, and the FSR is 104.17GHz by substituting formula (5) for calculation, which is consistent with the FSR value set by experiments, and the result is reliable.
As can be seen from the above description, the present embodiment has the following advantages:
1. the measuring device has simple structure, easy realization and low manufacturing cost;
2. based on the laser interference theory, the real-time non-contact high-precision measurement is realized by utilizing the laser self-mixing signal.
Embodiment 2 of the present invention is described in detail with reference to fig. 1, fig. 2, and fig. 7 to fig. 10, but the present invention is not limited in any way by the claims.
Lasers are generally composed of an optical resonant cavity, a gain medium, and an excitation source. The free spectral region of the laser resonant cavity is defined in accordance with the free spectral region of the FP cavity etalon, and the FSR is generally expressed by Deltav. The FSR expression of the laser cavity is:
FSR＝Δν＝c/2ngLin(1)
in the formula (1), LinIs the cavity length, n, of the laser resonant cavitygIs the group index of refraction of the medium in the cavity, and c is the speed of light in vacuum.
As shown in fig. 1, a device for measuring temperature change of a resonant cavity of a multi-longitudinal mode laser is established, which comprises a multi-longitudinal mode laser 1, a vibration target 2, a slide block 3, a slide rail 4, a beam splitter 6, an optical attenuator 7, a photoelectric detector 8, a signal preprocessing unit 9 and a signal processing unit 10; the vibration target 2 can vibrate, a reflection film or a reflection plane mirror 11 is attached to a vibration surface, the multi-longitudinal-mode laser 1 is a laser light source to be detected, laser is emitted to the vibration target 2, the vibration target 2 receives the laser emitted by the multi-longitudinal-mode laser and feeds back the laser to a resonant cavity of the multi-longitudinal-mode laser through the reflection film or the reflection plane mirror 11 to form a self-mixing signal, the bottom of the vibration target 2 is fixed on a sliding block 3, the sliding block 3 is arranged on a sliding rail 4 and can horizontally move along the sliding rail 4, the sliding rail 4 and the emitted laser are in the same straight line, a signal generator 5 is used for driving the vibration target 2 to vibrate, a beam splitter 6 and an optical attenuator 7 are sequentially arranged on a light path between the multi-longitudinal-mode laser 1 and the vibration target 2, the beam splitter 6 is used for splitting the self-mixing signal to a photoelectric detector 8, the photoelectric detector 8 is used for converting the, the signal preprocessing unit 9 is used for preprocessing (at least including shaping and filtering) the received electrical signal; the signal processing unit 10 is configured to analyze and process the preprocessed electrical signal to obtain a measurement result of a change in the temperature of the cavity of the resonant cavity of the laser.
Wherein:
(1) the vibration target 2 may be selected from a speaker or a piezoelectric ceramic driven by the signal generator 5.
(2) When the laser to be measured is a multi-longitudinal-mode semiconductor laser, the multi-longitudinal-mode semiconductor laser integrated with the PD can be selected, namely, a photoelectric detector in the device is integrated in the multi-longitudinal-mode laser, and at the moment, the beam splitter can be cancelled.
(3) The optical attenuator may be a displacement type attenuator or an attenuation sheet type attenuator.
(4) The signal processing unit may be selected from a computer, an oscilloscope or a spectrometer.
Based on a laser self-mixing theoretical model (as shown in fig. 2), the measuring device is utilized to measure the change of the resonant cavity temperature of the multi-longitudinal mode laser, and the specific method is as follows:
the signal generator is used for driving the loudspeaker or the piezoelectric ceramics to vibrate, the multi-longitudinal-mode laser is used as a laser light source to be detected, laser is emitted to the loudspeaker or the piezoelectric ceramics, the emitted laser is reflected by the reflecting film or the reflecting plane mirror and then fed back to the multi-longitudinal-mode laser resonant cavity to form a self-mixing signal, in the process, the slide block is simultaneously slightly moved along the slide rail to change the distance between the loudspeaker or the piezoelectric ceramics and the multi-longitudinal-mode laser, so that laser self-mixing signals under different laser external cavity lengths are formed, meanwhile, the intensity of the feedback light is adjusted by the optical attenuator, the laser self-mixing signals under different laser external cavity lengths are collected by the photoelectric detector, then the laser self-mixing signals are preprocessed by the signal preprocessing unit, finally, the preprocessed laser self-mixing signals are analyzed by the signal processing unit, and the temperature change of the laser resonant cavity can be obtained, the specific analysis method is as follows:
for the laser self-mixing signal of the multi-longitudinal-mode laser, different longitudinal modes of the laser only interfere with the self-mode, so that the finally formed laser self-mixing signal can be regarded as the intensity superposition of the laser self-mixing signals formed by the respective longitudinal modes. According to the relevant interference mixing theory model, the self-mixing signal intensity of the multi-longitudinal-mode laser can be further obtained as follows (no speckle influence is considered):
Figure BDA0002273481660000121
in formula (2): i is0Representing initial light intensity, β is the total number of oscillation starting modes in the multi-longitudinal mode laser, j represents the jth longitudinal mode in the multi-longitudinal mode laser, and delta IjAmplitude of variation of light intensity of j-mode laser, k0jNumber of waves, n, representing the j mode in vacuum0Denotes the refractive index of the external cavity, Lext(t) represents the real-time external cavity length;
when refractive index of external cavity n0When the number is equal to 1, the alloy is put into a container,
Figure BDA0002273481660000122
in formula (3): omega0Representing the angular frequency of the laser, c representing the speed of light in vacuum, and FSR representing the free spectral range of the laser cavity;
accordingly, equation (1) can be varied as:
if the self-mixing signals of the laser in different modes are overlapped without waveform splitting or separation, the independent waveforms of the modes are required to keep the same phase or the phase delay is integral multiple of 2 pi, and the length of the external cavity needs to meet the condition:
Figure BDA0002273481660000124
in the formula (5), m is the external cavity mode order of the laser, is a positive integer, and LextThe external cavity length is shown, so that the laser has a series of special position points, the laser self-mixing signals of each longitudinal mode of the laser are ensured to be always kept in the same phase or the phase delay is integral multiple of 2 pi, the superposed laser self-mixing signals are not split or separated, and the special position points are called equal phase points.
At the position of the m-level equiphase point, the length of the outer cavity is finely adjusted by finely adjusting the position of the slide block, the temperature change is tracked, the waveform is kept stable, and the change of the length of the outer cavity is measured and is delta L according to the position change of the m-level equiphase pointextmObtaining a change value of the temperature of the cavity of the laser resonator (since the initial temperature of the cavity of the laser resonator is known, the absolute value of the temperature of the cavity of the laser resonator after the temperature change can be calculated) by a relational expression of the change value delta T of the temperature of the cavity of the laser resonator, wherein the relational expression of the position change of the m-level equiphase phase points and the temperature change of the cavity of the laser resonator is as follows:
Figure BDA0002273481660000131
in formula (7):
Figure BDA0002273481660000132
which represents the temperature coefficient of the refractive index,
Figure BDA0002273481660000133
which is indicative of the refractive index dispersion coefficient,
Figure BDA0002273481660000134
indicating wavelengthThe temperature coefficient of (a) is,
Figure BDA0002273481660000135
represents a linear expansion coefficient, LinIndicating the laser cavity length, Lin0Denotes the initial cavity length, n, of the laser resonatorgRepresenting the group refractive index, n, of the medium in the laser cavityg0The initial group refractive index of the medium in the laser cavity is shown, and the value of m can be obtained by calculating the formula (5).
And (3) obtaining the real-time temperature change of the laser by combining the correlation coefficient of the material, the cavity length of the laser resonant cavity at the initial temperature, the refractive index of the laser initial group and the value m.
In this embodiment, if the laser cavity FSR is unknown, the laser cavity FSR can be measured according to the method of embodiment 1 and then substituted into equation (5) to calculate the value of m.
And establishing an experimental model and carrying out analog simulation on the measuring method. The simulation conditions are as follows: the initial wavelength of the laser is 670nm, and the initial temperature is T1Degree C, initial outer cavity length LextThe results of the simulated simulation of the superimposed laser self-mixing signals are shown in fig. 7 to 9, where m is 150 and δ T is 4 ℃ at 157.5 mm.
As can be seen from FIGS. 7 to 9, when the temperature is T1At the time of DEG C, the external cavity length L at this timeext157.5mm is
Figure BDA0002273481660000141
Integral multiple of the above, no splitting of self-mixing waveform, and temperature change caused by the increase of external cavity temperature by 4 deg.C
Figure BDA0002273481660000142
The self-mixing waveform will split when a small change occurs, and when we fine-tune the length of the external cavity, it is again
Figure BDA0002273481660000143
The self-mixing waveform is changed from split to coincidence, and then the corresponding temperature change can be obtained by measuring the change of the external cavity distance, so as to realize the purposeAnd measuring the temperature of the cavity of the resonant cavity of the laser.
As can be seen from the equation (7), the external cavity variation sensitivity S of the temperature measuring deviceLextmAnd adjacent temperature difference DeltaTmAre determined by the value of m, the refractive index of the laser medium material and the rate of change of the cavity length of the resonant cavity with temperature. Wherein the external cavity variation sensitivity SLextmIs the external cavity distance change and the adjacent temperature difference delta T caused by unit temperature changemIs referred to as the temperature T2(after change) induced external cavity equiphase point position (m +1) and temperature T1The maximum temperature difference value of the external cavity equiphase point position (m) without coincidence directly determines the maximum measurable temperature difference in two continuous measurement intervals. Sensitivity of external cavity variation SLextmAnd adjacent temperature difference DeltaTmThe expression is as follows:
Figure BDA0002273481660000144
Figure BDA0002273481660000145
FIG. 10 shows the sensitivity S of external cavity variation when using a multi-longitudinal mode semiconductor laserLextmAnd adjacent temperature difference DeltaTmGraph of relationship with m. In the calculation, the initial group refractive index n of the medium in the laser resonant cavityg0Is 3.5, the initial length L of the cavity of the resonant cavityin0300um, the coefficient of variation of the length of the resonant cavity material with the temperature is 5.73 multiplied by 10-6℃-1(i.e., coefficient of linear expansion), the coefficient of change of refractive index with temperature being 2.5X 10-4℃-1(i.e., temperature coefficient of refractive index) having a refractive index Abbe number of-2.298 um-1Temperature coefficient of wavelength is 0.3 nm/DEG C, and external cavity variation sensitivity SLextmAnd absolute value of temperature difference between adjacent stages | Δ TmAll will be influenced by the length L of the resonant cavityinRefractive index n of medium group in resonant cavitygThe length of the material and the coefficient of variation of the refractive index with temperature.
As can be seen from FIG. 10, the temperature difference between adjacent stages gradually decreases as the value of m increasesSensitivity of external cavity variation SLextmIt increases with the increase of the m value, so that the appropriate operating point (number of stages) needs to be selected according to the specific test requirements by comprehensively considering the intensity of the temperature change and the measurement sensitivity. For example, in the specific measurement of the temperature of the laser resonant cavity, the initial temperature change of the laser during starting is severe, a small m-value point is required to be selected for temperature measurement, when the laser runs for a period of time, the temperature tends to be stable, the temperature change is small, and a large m-value point can be selected for temperature measurement, so that high measurement sensitivity is achieved.
As can be seen from the above description, the present embodiment has the following advantages:
1. the measuring device has simple structure, easy realization and low manufacturing cost;
2. based on the laser interference theory, the real-time non-contact high-precision measurement is realized by utilizing the laser self-mixing signal.
3. By selecting the value of m, better measurement sensitivity can be obtained.
In summary, the invention has the following advantages:
1. the device structure of the multi-longitudinal-mode laser resonant cavity FSR measuring device and the device structure of the multi-longitudinal-mode laser resonant cavity temperature change measuring device are basically the same, the difference is only that the analysis processing and analysis methods of laser self-mixing signals are different, the same device structure can be applied to both the FSR measurement and the temperature change measurement of the laser resonant cavity, and the device has two purposes and wide application range;
2. the measuring device has the advantages of simple structure, easy realization and low manufacturing cost;
3. the measuring method is based on the laser interference theory, and realizes the real-time non-contact high-precision measurement of the FSR and the cavity temperature of the resonant cavity of the multi-longitudinal-mode laser by using the laser self-mixing signal.
It should be understood that the detailed description of the invention is merely illustrative of the invention and is not intended to limit the invention to the specific embodiments described. It will be appreciated by those skilled in the art that the present invention may be modified or substituted equally as well to achieve the same technical result; as long as the use requirements are met, the method is within the protection scope of the invention.

Claims (5)

1. The utility model provides a many longitudinal mode laser resonant cavity temperature variation measuring device which characterized in that: the device comprises a multi-longitudinal-mode laser, a vibration target, a sliding block, a sliding rail, a beam splitter, an optical attenuator, a photoelectric detector, a signal preprocessing unit and a signal processing unit, wherein the vibration target can vibrate, and a reflecting film or a reflecting plane mirror is attached to a vibration surface;
the multi-longitudinal-mode laser is a laser light source to be detected and emits laser to a vibration target;
the vibration target can vibrate, receives laser emitted by the multi-longitudinal-mode laser and feeds the laser back to the resonant cavity of the multi-longitudinal-mode laser through a reflecting film or a reflecting plane mirror to form a self-mixing signal;
the bottom of the vibration target is fixed on the sliding block;
the sliding block is arranged on the sliding rail and can horizontally move along the sliding rail, and the sliding rail and the emergent laser are positioned on the same straight line;
a beam splitter and an optical attenuator are sequentially arranged on an optical path between the multi-longitudinal-mode laser and the vibration target;
the beam splitter is used for splitting the self-mixing signal onto the photoelectric detector;
the photoelectric detector is used for converting the received laser signal into an electric signal and then sending the electric signal to the signal preprocessing unit;
the signal preprocessing unit is used for preprocessing the received electric signals, and the preprocessing at least comprises shaping and filtering;
and the signal processing unit is used for analyzing and processing the preprocessed electric signals to obtain a temperature change measurement result of the cavity of the resonant cavity of the laser.
2. The apparatus of claim 1, wherein: the vibration target is a speaker or a piezoelectric ceramic driven by a signal generator.
3. The apparatus of claim 1, wherein: when the multi-longitudinal-mode laser adopts a semiconductor laser, the photoelectric detector is integrated in the multi-longitudinal-mode laser.
4. The apparatus of claim 1, wherein: the signal processing unit is a computer, an oscilloscope or a frequency spectrograph.
5. The method for measuring the temperature change of the cavity of the resonant cavity of the laser based on the device for measuring the temperature change of the cavity of the resonant cavity of the multi-longitudinal-mode laser as claimed in any one of claims 1 to 4, wherein the method comprises the following steps: the vibration target vibrates, the multi-longitudinal mode laser is used as a laser source to be measured, laser is emitted to the vibration target, the emitted laser is fed back to the resonant cavity of the multi-longitudinal mode laser to form a self-mixing signal after being reflected by the reflecting film or the reflecting plane, in the process, the sliding block moves slightly along the sliding rail to change the distance between the vibration target and the multi-longitudinal-mode laser, thereby forming laser self-mixing signals under different laser external cavity lengths, simultaneously utilizing the optical attenuator to adjust the intensity of feedback light, utilizing the photoelectric detector to collect the laser self-mixing signals under different laser external cavity lengths, then, the signal preprocessing unit is used for preprocessing the laser self-mixing signal, and finally, the signal processing unit is used for analyzing the preprocessed laser self-mixing signal, so that the temperature change of the cavity of the resonant cavity of the laser can be obtained, wherein the specific analysis method comprises the following steps:
based on the laser self-mixing signal of the multi-longitudinal-mode laser, different longitudinal modes of the laser are considered to be only interfered with the self-mode, so that the finally formed laser self-mixing signal can be considered as the intensity superposition of the laser self-mixing signal formed by the respective longitudinal modes, and according to a related interference mixing theory model, the intensity of the self-mixing signal of the multi-longitudinal-mode laser can be further obtained as follows:
Figure FDA0002273481650000021
in formula (2): i is0Representing initial light intensity, β is the total number of oscillation starting modes in the multi-longitudinal mode laser, j represents the jth longitudinal mode in the multi-longitudinal mode laser, and delta IjAmplitude of variation of light intensity of j-mode laser, k0jNumber of waves, n, representing the j mode in vacuum0Denotes the refractive index of the external cavity, Lext(t) represents the real-time external cavity length;
when refractive index of external cavity n0When the number is equal to 1, the alloy is put into a container,
Figure FDA0002273481650000022
in formula (3): omega0Representing the angular frequency of the laser, c representing the speed of light in vacuum, and FSR representing the free spectral range of the laser cavity;
thus, it is possible to obtain
Figure FDA0002273481650000023
If the self-mixing signals of the laser in different modes are overlapped without waveform splitting or separation, the independent waveforms of the modes are required to keep the same phase or the phase delay is integral multiple of 2 pi, and the length of the external cavity needs to meet the condition:
Figure FDA0002273481650000024
in the formula (5), m is the external cavity mode order of the laser, is a positive integer, and LextIndicating the external cavity length; therefore, the laser has a series of special position points, the laser self-mixing signals of each longitudinal mode of the laser are ensured to always keep the same phase or the phase delay is integral multiple of 2 pi, the superposed laser self-mixing signals are not split or separated, and the special position points are called equal phase points;
the length of the outer cavity is finely adjusted by finely adjusting the position of the slide block at the position of the m-level equal phase point,the waveform is kept stable by tracking the temperature variation, and the variation delta L of the external cavity length is measuredextAnd then according to the position change delta L of the m-level equal phase pointextmObtaining the change value of the cavity temperature of the laser resonant cavity by a relational expression of the change delta T of the cavity temperature of the resonant cavity, wherein the relational expression of the position change of the m-level equiphase phase point and the cavity temperature change of the resonant cavity is as follows:
Figure FDA0002273481650000031
in formula (7):
Figure FDA0002273481650000032
which represents the temperature coefficient of the refractive index,
Figure FDA0002273481650000033
which is indicative of the refractive index dispersion coefficient,the temperature coefficient representing the wavelength of the light,
Figure FDA0002273481650000035
represents a linear expansion coefficient, LinIndicating the laser cavity length, Lin0Denotes the initial cavity length, n, of the laser resonatorgRepresenting the group refractive index, n, of the medium in the laser cavityg0The initial group refractive index of the medium in the laser resonant cavity is represented, and the value m is obtained through calculation of an expression (5);
therefore, the real-time temperature change of the laser can be obtained by combining the correlation coefficient of the material, the cavity length of the laser resonant cavity at the initial temperature, the refractive index of the laser initial group and the value m.
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