Half external cavity linear polarization interference light source based on valley value power locking and design method thereof

Abstract

The invention discloses a half external cavity linear polarization interference light source based on valley value power locking and a design method thereof. The device comprises a laser power supply, a laser, a heating wire, heat preservation glue, an optical splitter BS, a PD photocell and a laser frequency stabilization system. The design method is characterized in that the laser is a three-longitudinal-mode half-external-cavity line partial helium-neon laser, a Brewster window is arranged in the laser, key parameters of the laser are obtained through calculation of a longitudinal mode light intensity coupling model, two side modes can be completely restrained, a high-proportion valley bottom in a light intensity tuning curve is formed, and the high-proportion valley bottom in the light intensity tuning curve is formed. And central longitudinal mode frequency locking can be realized through frequency stabilization algorithm identification. According to the method, the output power of the laser can reach 2.4 mW or above, the defect that the power of a traditional double-longitudinal-mode frequency stabilization interference light source is limited is overcome, and the single-mode power conversion efficiency is improved; a novel frequency stabilization technology and a novel interference light source which are complete in single-mode conversion, high in output power and good in wavelength stability are provided for various fields such as rapidly-developed ultra-precision machining and measuring technologies.

Description

Technical Field

[0001] The invention belongs to the field of laser precision measurement, and in particular relates to a semi-external cavity linear polarization interference light source based on valley power locking and a design method thereof. Background Art

[0002] Laser interferometer is a common optical measurement tool used for high-precision distance, angle, speed and other parameter measurements. It has become the best solution and research hotspot for the pursuit of high-resolution, high-precision and ultra-precision measurement. Stabilized HeNe laser is the most commonly used interference light source, which can provide hundreds of meters of coherence length and reliable high-stability wavelength. However, the disadvantages of mainstream commercial stabilized HeNe lasers are low output power, single-mode output requires external polarization devices, single-mode conversion is incomplete and low efficiency, usually not higher than 1mW. This leads to a long camera exposure time during interferometric measurement sampling, especially in the case of environmental vibration and air turbulence, which greatly affects the signal-to-noise ratio level of the interference fringes. Interferometers sometimes use multi-longitudinal mode lasers with higher output power that are not stabilized, but the instability of the mode during the measurement process also causes great trouble for the accuracy of the measurement results. Therefore, in order to improve the accuracy of measurement results and improve the anti-interference ability of the interference system, the industry urgently needs a more stable and higher output power interference light source.

[0003] The mainstream commercial frequency-stabilized He-Ne laser currently used in interferometers uses full-cavity random polarization dual-longitudinal-mode frequency stabilization technology. The dual-longitudinal-mode frequency-stabilized laser based on electrothermal devices has a relative frequency accuracy of 10 -8 The above methods have the advantages of simple structure, short preheating time, good vibration resistance, etc., and have been applied in commercial laser interferometry systems. For example, the British Reinishaw Company proposed a thermal frequency stabilization method for dual longitudinal mode lasers using electric heating wires as actuators (Pre-heat Control System for a Laser, International Patent: WO8801798; Frequency Stabilized Laser and Control System Therefor, International Patent: WO8801799).

[0004] Domestically, Sichuan University and Harbin Institute of Technology have proposed a dual longitudinal mode laser frequency stabilization method based on electromagnetic induction heating (Chinese patent CN100367579: Dual longitudinal mode laser electromagnetic induction heating frequency stabilization device and frequency stabilization method) and a dual longitudinal mode laser frequency stabilization method based on thermoelectric cooler (Chinese patent CN100382398: Dual longitudinal mode laser frequency stabilization method and device based on thermoelectric cooler) in recent years. Beijing Aerospace Metrology and Testing Technology Research Institute has stabilized the frequency of dual longitudinal mode lasers at asymmetric positions in order to improve the single mode conversion efficiency to 80% and the single mode output power to 0.8mW. (Chen Zhengchao, Li Huafeng, Zhu Guoqin. Research on thermal frequency stabilization technology of dual longitudinal mode He-Ne lasers [J]. Measurement Technology, 2014, 34(01): 31-34).

[0005] The above all-intracavity dual-longitudinal-mode frequency-stabilized laser cannot output higher-power single-mode laser due to the limitation of the number of longitudinal modes. In order to meet the needs of interferometry, we can try to use a three-longitudinal-mode laser for frequency stabilization and select the intermediate film with a larger resonant cavity gain as the working mode.

[0006] Based on the method of stabilizing the power of the all-intracavity randomly polarized three-longitudinal mode laser, ZYGO has developed a high-power interference light source for its instantaneous interferometry system ( www.zygo.com.cn ), a high-power laser source can shorten the camera exposure time and effectively reduce the impact of vibration and air turbulence. However, due to the filling of the non-working mode, an external polarization device is required to filter it, and its single-mode power conversion efficiency can only reach 75%. And because the longitudinal mode of the randomly polarized laser may change its polarization direction due to environmental influences, its laser cannot completely output single mode. The optical feedback introduced by the external polarization splitter will also affect the stability of the resonant cavity, resulting in potential mode hopping risks. The above defects not only cause great trouble to its power stability, but also greatly affect the accuracy of the final result of the interferometric measurement. Summary of the invention

[0007] In view of the lack of high-power single-mode frequency-stabilized lasers in existing frequency-stabilization technologies, the present invention proposes a semi-external cavity linearly polarized interferometer light source based on valley power locking and its design method, which solves the problems of low output single-mode conversion efficiency, incomplete single-mode conversion, insufficient overall output power, and thus limited comprehensive measurement methods, anti-seismic performance, and measurement length and accuracy of the interferometer caused by the full-inner cavity random polarization laser plus PBS in the current laser interferometer measurement system. A new frequency-stabilization technology and a new frequency-stabilized laser light source with complete single-mode conversion, high output power, and good wavelength stability are provided for various fields such as the rapidly developing ultra-precision machining and measurement technology.

[0008] The technical solution to realize the present invention is: a semi-external cavity linear polarization interference light source based on valley power locking, including a laser power supply, a laser, a BS spectrometer, a PD photocell, an input signal amplifier, an A / D converter, an MCU microprocessor, a thermistor, a PWM controller, an electric heater, and a thermal insulation adhesive layer; the laser and the BS spectrometer are arranged in sequence on a common optical axis, the laser emitted by the laser is vertically incident on the BS spectrometer, and is divided by the BS spectrometer into an output light emitted in the direction of the optical axis and a feedback light vertical to the optical axis. The reflected light enters the PD photocell as a feedback light signal and is converted into an electrical signal; the laser power supply is connected to the laser, the PD photocell, the input signal amplifier, the A / D converter, the MCU microprocessor, the thermistor, the PWM controller, the electric heater, and the ... the output light and the feedback light vertical to the optical axis are vertically incident on the BS spectrometer. The input signal amplifier, A / D converter, MCU microprocessor, PWM controller, and electric heater are connected in sequence, the thermistor is attached to the laser tube of the laser, the thermistor is connected to the MCU microprocessor, the electric heater is wrapped around the laser tube of the laser, and the thermal insulation layer wraps the laser tube, thermistor and electric heater of the laser; the feedback optical signal enters the MCU microprocessor for frequency stabilization after passing through the PD photocell, input signal amplifier, and A / D converter, and the PWM controller and electric heater are used to control the temperature of the laser tube, thereby changing the length of the laser tube cavity to achieve longitudinal mode light intensity harmonics; the valley area of ​​the laser longitudinal mode light intensity harmonics curve accounts for 20%.

[0009] A method for designing a semi-external cavity linear polarization interferometer light source based on valley power locking, the steps are as follows:

[0010] Step 1: In order to make the power valley of the laser reach 20%, the laser uses a three-longitudinal-mode half-external-cavity linearly polarized He-Ne laser, and a Brewster window is built into the laser to output linearly polarized laser;

[0011] Step 2: Calculate the key parameters of the laser (L r ,l,φ,L,Δυ L ,R), to ensure that under the obtained resonant cavity loss conditions, in the case of mode symmetry, the two side modes can be completely suppressed, forming a valley in the optical intensity harmonic curve. At the valley bottom, only the central single longitudinal mode can oscillate and output, realizing the frequency locking of the central longitudinal mode. Among them, L r represents the resonant cavity loss, l represents the capillary length, φ represents the capillary diameter, L represents the resonant cavity length, Δυ L represents the uniformly broadened linewidth of the gain medium, and R represents the element Ne 20 with Ne 22 Abundance ratio.

[0012] Step 3, continue to select other components: use BS splitter with 5% splitting ratio, PD photocell, input signal amplifier, A / D converter to provide optical feedback signal for MCU microprocessor. Through the frequency stabilization algorithm in MCU microprocessor, the signal output by PWM controller is wrapped around the laser tube through electric heater to realize temperature control, and then change the laser tube cavity length to realize longitudinal mode tuning.

[0013] Step 4: Place the thermistor close to the laser tube to ensure the working temperature.

[0014] Compared with the prior art, the present invention has the following significant advantages:

[0015] (1) This invention proposes for the first time a method for designing a semi-external cavity linear polarization interference light source based on valley power locking. The longitudinal mode light intensity coupling model is established using the hole burning effect and the cross relaxation effect, and the key process parameters of the semi-inner cavity laser are simulated and calculated to ensure that under the calculated resonant cavity loss, the two side modes of the three longitudinal modes in the mode symmetry condition can be completely suppressed, creating a high-precision valley in the light intensity harmonic curve. At the valley position, only the central single longitudinal mode can oscillate and output, which is convenient for the frequency stabilization algorithm to identify and achieve the central longitudinal mode frequency locking.

[0016] (2) The present invention uses a semi-cavity laser with a built-in Brewster window to output linearly polarized laser, eliminating the step of external PBS filtering single-mode output of ordinary commercial full-cavity random polarization stabilized laser, avoiding the problems of incomplete single-mode filtering and easy mode jumping caused by PBS, and can achieve complete conversion of single-mode output and reduce the error of interferometer measurement system.

[0017] (3) The present invention greatly improves the single-mode output power of the frequency-stabilized laser, overcomes the difficulty of the three-longitudinal-mode frequency-stabilization technology, and has a high single-mode stable output power, which is more than twice the output power of the dual-longitudinal-mode frequency-stabilized helium-neon laser. It greatly contributes to the improvement of the anti-seismic performance, measurement length, and measurement accuracy of the time phase-shifting interferometer, and can be widely used in the field of interferometry.

[0018] (4) The present invention optimizes the division accuracy of the valley bottom area of ​​the longitudinal mode symmetrical working point of the three-longitudinal mode half-cavity laser, effectively improves the discrimination of the frequency-stabilized signal, and the relative frequency stability is better than 0.05pm / h, which is better than the existing multi-longitudinal mode interference light source.

[0019] (5) The process of the present invention has a simple structure, low price, short preheating time (15 minutes), strong vibration resistance, and can be applied to a wide range of interferometric measurement scenarios. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a schematic diagram of the structure of the device of the present invention.

[0021] Figure 2It is a schematic diagram of the principle of the device of the present invention.

[0022] Figure 3 Schematic diagram of hole burning distribution on the laser gain curve, where (a) represents the single longitudinal mode hole burning distribution, and (b) represents the triple longitudinal mode hole burning distribution.

[0023] Figure 4 Schematic diagram of the principle of laser hole burning overlapping to suppress side mode oscillation.

[0024] Figure 5 This is a simulation diagram of the longitudinal mode light intensity harmonic curve of the laser.

[0025] Figure 6 This is the distribution diagram of the longitudinal mode as the temperature of the laser tube changes.

[0026] Figure 7 This is a schematic diagram of closed-loop control of the frequency stabilization algorithm of the present invention.

[0027] Figure 8 This is a diagram of the laser frequency drift of the present invention.

[0028] Fig. 9 This is a graph of laser power drift according to the present invention. DETAILED DESCRIPTION

[0029] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0030] It should be noted that all directional indications in the embodiments of the present invention (such as up, down, left, right, front, back, etc.) are only used to explain the relative position relationship, movement status, etc. between the components under a certain specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication will also change accordingly.

[0031] In the present invention, unless otherwise clearly specified and limited, the terms "connection", "fixation" and the like should be understood in a broad sense. For example, "fixation" can be a fixed connection, a detachable connection, or an integral connection; "connection" can be a mechanical connection or an electrical connection. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0032] In addition, the technical solutions between the various embodiments of the present invention can be combined with each other, but it must be based on the fact that ordinary technicians in the field can implement it. When the combination of technical solutions is contradictory or cannot be implemented, it should be deemed that such combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0033] The following will further introduce the specific implementation method, as well as the technical difficulties and inventive points of this invention in combination with this design example.

[0034] Combination Figure 1 The semi-external cavity linear polarization interference light source based on valley power locking described in the present invention includes: a laser power supply 1, a laser 2, a BS spectrometer 3, a PD photocell 4, an input signal amplifier 5, an A / D converter 6, an MCU microprocessor 7, a thermistor 8, a PWM controller 9, an electric heater 10, and a thermal insulation glue layer 11.

[0035] The laser 2 and the BS spectrometer 3 are arranged in sequence on a common optical axis. The laser emitted by the laser 2 is vertically incident on the BS spectrometer 3, and is divided by the BS spectrometer 3 into output light emitted in the direction of the optical axis and feedback light vertical to the optical axis. The reflected light enters the PD photocell 4 as a feedback light signal and is converted into an electrical signal; the laser power supply 1 is connected to the laser 2, and the PD photocell 4, the input signal amplifier 5, the A / D converter 6, the MCU microprocessor 7, the PWM controller 9, and the electric heater 10 are connected in sequence. The thermistor 8 is attached to the laser tube of the laser 2, the thermistor 8 is connected to the MCU microprocessor 7, the electric heater 10 is wrapped around the laser tube of the laser 2, and the thermal insulation glue layer 11 wraps the laser tube of the laser 2, the thermistor 8 and the electric heater 10.

[0036] Combination Figure 2 The feedback optical signal of the laser 2 is sent to the MCU microprocessor 7 for frequency stabilization after passing through the PD photocell 4, the input signal amplifier 5, and the A / D converter 6. The PWM controller 9 and the electric heater 10 are used to control the temperature of the laser tube, thereby changing the length of the laser tube cavity to achieve the longitudinal mode light intensity tuning. The valley area of ​​the laser longitudinal mode light intensity tuning curve accounts for 20%.

[0037] Laser 2 uses a triple longitudinal mode polarized helium-neon laser.

[0038] A method for designing a semi-external cavity linear polarization interferometer light source based on valley power locking, the steps are as follows:

[0039] Step 1: In order to make the power valley ratio of laser 2 reach 20%, laser 2 uses a three-longitudinal-mode half-external-cavity linearly polarized He-Ne laser, and a Brewster window is built into laser 2 to output linearly polarized laser.

[0040] Step 2: Calculate the key parameters of laser 2 (Lr ,l,φ,L,Δυ L ,R), to ensure that under the obtained resonant cavity loss, the two side modes of the three longitudinal modes in the mode symmetry can be completely suppressed, forming the valley bottom in the light intensity harmonic curve. At the valley bottom, only the central single longitudinal mode can oscillate and output, realizing the frequency locking of the central longitudinal mode. Among them, L r represents the resonant cavity loss, l represents the capillary length, φ represents the capillary diameter, L represents the resonant cavity length, Δυ L represents the uniformly broadened linewidth of the gain medium, and R represents the element Ne 20 with Ne 22 Abundance ratio.

[0041] The key parameters of the laser 2 (L r ,l,φ,L,Δυ L ,R) is calculated by the hole burning theory and the longitudinal mode intensity coupling model of the cross relaxation effect. Figure 3 As shown in Figure (a), the output power of the laser is affected by the hole burning effect, which can be represented by the hole burning area, which represents the number of stimulated emission photons. Figure 3 In Figure (b), when the cavity length of laser 2 increases, there are three resonant longitudinal modes within the envelope of the gain curve. The conditions for the steady-state operation of the three longitudinal modes under the standing wave condition are:

[0042]

[0043] Where G(υ) is the frequency-dependent signal gain, L r is the resonant cavity loss, d is the hole burning depth, is a 3*3 symmetric matrix, where each element The expression is as follows:

[0044]

[0045] υ i represents the frequency of the i-th resonant longitudinal mode, υ j represents the frequency of the jth resonant longitudinal mode, i and j∈[1,2,3], υ0 represents the center frequency.

[0046] Hole burning depth modulation coefficient F(δυ c ) is:

[0047]

[0048] In the formula, δυ c is the frequency difference between the original hole burning or image hole burning center frequency and the longitudinal mode resonant frequency, Δυ L is the uniformly broadened linewidth of the gain medium.

[0049] When the center frequency of hole burning is equal to the resonant frequency of the longitudinal mode, the modulation coefficient has a maximum value of 1.

[0050] Combination Figure 4 , during the frequency tuning process, the original burned hole moves in the same direction as the resonant frequency, and the image burned hole moves in the opposite direction. When the three longitudinal modes are tuned to the vicinity of the symmetrical distribution about the center frequency of the curve, the original burned hole of the low-frequency side mode overlaps with the image burned hole of the high-frequency side mode. At the same time, the image burned hole of the former overlaps with the original burned hole of the latter, and their total burned hole area decreases as the frequency difference between the intermediate mode frequency υ2 and the frequency υ0 decreases. In the mode symmetry region, the two side modes will experience intensity attenuation due to competition for gain particles in the overlapping burned hole region. The burned hole area of ​​the intermediate mode also decreases relatively with the overlap of its own original burned hole and image burned hole during the aforementioned frequency tuning process, which means that the intensity of the two side modes and the intensity of the intermediate mode decay simultaneously. Through calculation, the side modes can be completely attenuated, and at the same time, a high-precision valley power curve with obvious characteristics can be shaped under the condition of total power output.

[0051] The gain medium of the semi-external cavity linear polarization interference light source is Ne 20 with Ne 22 A mixed gas of two isotopes, each neon isotope medium will produce an independent gain curve. When the three longitudinal modes are tuned to the vicinity of the frequency symmetrical distribution, hole burning overlap occurs between the side mode and the middle mode. The middle mode, which is a strong mode, can always compete for more gain particles in the mode competition, and the disadvantaged side mode is further suppressed. Appropriately shortening the laser cavity length to increase the longitudinal mode spacing can make the hole burning of the side mode closer to the fluorescence edge of the gain curve under the condition of mode symmetry, thereby further reducing the effective gain of the side mode, or even completely suppressing the oscillation of the side mode.

[0052] In the case of a di-neon isotope gain medium, the conditions for steady-state operation of the three longitudinal modes under standing wave conditions are:

[0053]

[0054] Where k = 1, 2 represent the index of the function related to Ne20 and Ne22 respectively, d 1(k) ,d 2(k) With d 3(k) are the hole burning depths of different longitudinal modes on the corresponding neon gain curves, It is a 3*3 symmetric matrix, and the elements in the matrix are The expression is as follows:

[0055]

[0056] G k (υ) is the signal gain related to the frequency of the neon gain curve, and its expression is:

[0057]

[0058] In the formula, Δυ Dk is the Doppler linewidth of the neon gain curve at the center frequency υ 0k The signal gain at mk is the neon gain curve at the center frequency υ 0k The signal gain at

[0059] Signal gain G at different center frequencies m1 And signal gain G m2 as follows:

[0060]

[0061] In the formula, G m is the total unsaturated amplification factor, and its expression is:

[0062] G m =3×10 -4 l / φ

[0063] Where l is the capillary length, φ is the capillary diameter; R represents the position Ne 20 with Ne 22 abundance ratio;

[0064] Output power P of HeNe laser with double neon isotope in three longitudinal modes t It is formed by the superposition of the light intensity of each longitudinal mode. The longitudinal mode light intensity coupling model expression is:

[0065]

[0066] Where C0 is the proportional constant, which is 2×10 4 , G1(υ i ) and G2(υ i ) is the signal gain of the neon gain curve at each center frequency related to the three longitudinal mode frequencies.

[0067] In addition to the hole burning effect, the cross-relaxation effect also affects the shape of the power-frequency tuning curve. Atoms of different energy levels transfer energy through elastic collision, resonant absorption capture, and excitation exchange, and some excited atoms are redistributed on the gain curve, which is called the cross-relaxation effect. This effect causes the stimulated radiation of the longitudinal mode within its hole burning range to produce a certain degree of saturation on the entire gain curve, thereby affecting the laser output power. When considering the cross-relaxation effect, the modified longitudinal mode light intensity coupling model is:

[0068]

[0069] Where: K0 is a constant corresponding to uniform broadening, which is 0.35.

[0070] Combination Figure 5 According to the calculation of the longitudinal mode light intensity coupling model based on the hole burning theory and the cross relaxation effect, we can derive the design parameters that enable laser 2 to achieve high-power single longitudinal mode output under the condition of mode symmetry. This parameter also ensures that the single-mode power range has a significant resolution characteristic valley bottom ratio of 20% on the power tuning curve, which is convenient for the frequency stabilization algorithm to identify, thereby achieving frequency stabilization of the linearly polarized helium-neon laser. The resonant cavity loss L is calculated to be r =0.027, capillary length l = 180 mm, capillary diameter φ = 1.30 mm, resonant cavity length L = 270 mm, uniformly broadened line width Δυ L =120MHz, isotope Ne 20 with Ne 22 Abundance ratio R=1.

[0071] Step 3, continue to select other components: use BS splitter 3 with 5% splitting ratio, PD photocell 4, input signal amplifier 5, A / D converter 6 to provide optical feedback signal for MCU microprocessor 7. Through the frequency stabilization algorithm in MCU microprocessor 7, the signal output by PWM controller 9 is wrapped around the laser tube via electric heater 10 to achieve temperature control, thereby changing the laser tube cavity length to achieve longitudinal mode tuning.

[0072] Combination Figure 6 The laser tube 2 manufactured using the parameters calculated by the above method has a clear relationship between the longitudinal mode and the temperature, which is convenient for the subsequent algorithm to stabilize the frequency.

[0073] Step 4: Thermistor 8 is placed close to the laser tube to ensure the working temperature.

[0074] Combination Figure 7 Through high-precision valley power feedback and PID frequency stabilization algorithm identification and calculation, the PWM control of the heating wire output voltage is carried out, thereby realizing the temperature tuning of the cavity length and finally outputting a stable frequency laser.

[0075] Combination Figure 8 and Fig. 9 The method of the present invention can make the stable output power of the laser reach more than 2.4mW, and the frequency stability is better than 0.05pm / h. It provides a new frequency stabilization technology and a new interference light source with complete single-mode conversion, high output power and good wavelength stability for various fields such as rapidly developing ultra-precision processing and measurement technology.

Claims

1. A semi-external cavity linear polarization interferometer light source based on valley power locking, characterized in that: The invention comprises a laser power supply (1), a laser (2), a BS optical splitter (3), a PD photocell (4), an input signal amplifier (5), an A / D converter (6), an MCU microprocessor (7), a thermistor (8), a PWM controller (9), an electric heater (10), and a thermal insulation adhesive layer (11); the laser (2) and the BS optical splitter (3) are arranged in sequence on a common optical axis; the laser light emitted by the laser (2) is vertically incident on the BS optical splitter (3), and is divided by the BS optical splitter (3) into output light emitted in the direction of the optical axis and feedback light vertical to the optical axis. The reflected light enters the PD photocell (4) as a feedback light signal and is converted into an electrical signal; the laser power supply (1) is connected to the laser (2), the PD photocell (4), the input signal amplifier (5), the A / D converter (6), the M / C microprocessor (7), the thermistor (8), the PWM controller (9), the electric heater (10), and a thermal insulation adhesive layer (11); the laser (2) and the BS optical splitter (3) are arranged in sequence on a common optical axis; the laser light emitted by the laser (2) is vertically incident on the BS optical splitter (3), and is divided by the BS optical splitter (3) into output light emitted in the direction of the optical axis and feedback light vertical to the optical axis. The CU microprocessor (7), the PWM controller (9), and the electric heater (10) are connected in sequence, the thermistor (8) is attached to the laser tube of the laser (2), the thermistor (8) is connected to the MCU microprocessor (7), the electric heater (10) is wound around the laser tube of the laser (2), and the thermal insulation adhesive layer (11) wraps the laser tube, the thermistor (8), and the electric heater (10) of the laser (2); the feedback optical signal enters the MCU microprocessor (7) for frequency stabilization processing after passing through the PD photocell (4), the input signal amplifier (5), and the A / D converter (6); the PWM controller (9) and the electric heater (10) are used to control the temperature of the laser tube, thereby changing the laser tube cavity length to realize the longitudinal mode light intensity harmonic; the valley bottom area of ​​the laser longitudinal mode light intensity harmonic curve accounts for 20%.

2. The semi-external cavity linear polarization interferometer light source based on valley power locking according to claim 1, characterized in that: The laser (2) adopts a three-longitudinal-mode polarized helium-neon laser.

3. A method for designing a semi-external cavity linear polarization interference light source based on valley power locking as claimed in any one of claims 1 to 2, characterized in that: Here are the steps: Step 1: In order to make the power valley ratio of the laser (2) reach 20%, the laser (2) uses a three-longitudinal-mode half-external-cavity linearly polarized helium-neon laser, and a Brewster window is built into the laser (2) to output linearly polarized laser light; Step 2: Using the hole burning theory and the longitudinal mode intensity coupling model of the cross relaxation effect, calculate the key parameters (L r ,l,φ,L,Δυ L ,R), to ensure that under the obtained resonant cavity loss conditions, in the case of mode symmetry, the two side modes can be completely suppressed, forming a valley in the optical intensity harmonic curve. At the valley bottom, only the central single longitudinal mode can oscillate and output, realizing the frequency locking of the central longitudinal mode. Among them, L r represents the resonant cavity loss, l represents the capillary length, φ represents the capillary diameter, L represents the resonant cavity length, Δυ L represents the uniformly broadened linewidth of the gain medium, and R represents the element Ne 20 with Ne 22 abundance ratio; Step 3, continue to select other components: use a BS splitter (3) with a splitting ratio of 5%, a PD photocell (4), an input signal amplifier (5), and an A / D converter (6) to provide an optical feedback signal for the MCU microprocessor (7). Through the frequency stabilization algorithm in the MCU microprocessor (7), the signal output by the PWM controller (9) is wound around the laser tube via an electric heater (10) to achieve temperature control, thereby changing the laser tube cavity length to achieve longitudinal mode tuning; Step 4: The thermistor (8) is placed close to the laser tube to ensure the working temperature.

4. The method for designing a semi-external cavity linear polarization interference light source based on valley power locking according to claim 3 is characterized in that: In step 2, the key parameters (L r ,l,φ,L,Δυ L ,R), as follows: The output power of the laser (2) is affected by the hole burning effect, which is represented by the hole burning area representing the number of stimulated radiation photons; during the frequency tuning process, the original hole burning moves in the same direction as the resonance frequency, and the image hole burning moves in the opposite direction; when the three longitudinal modes are tuned to a vicinity symmetrically distributed about the center frequency υ0 of the curve, the original hole burning of the low-frequency side mode overlaps with the image hole burning of the high-frequency side mode, and at the same time, the image hole burning of the low-frequency side mode overlaps with the original hole burning of the high-frequency side mode, and the total hole burning area decreases as the frequency difference between the intermediate mode frequency υ2 and the center frequency υ0 decreases; When the cavity length of the laser (2) increases, there are three resonant longitudinal modes within the envelope of the gain curve, namely, three longitudinal modes; The conditions for the steady-state operation of the three longitudinal modes under standing wave conditions are: Where G(υ) is the frequency-dependent signal gain, L r is the resonant cavity loss, d is the hole burning depth, is a 3*3 symmetric matrix, where each element The expression is as follows: υ i represents the frequency of the i-th resonant longitudinal mode, υ j represents the frequency of the jth resonant longitudinal mode, i and j∈[1,2,3]; Hole burning depth modulation coefficient F(δυ c ) is: In the formula, δυ c is the frequency difference between the original hole burning or image hole burning center frequency and the longitudinal mode resonant frequency, Δυ L is the uniformly broadened linewidth of the gain medium; When the center frequency of hole burning is equal to the resonant frequency of the longitudinal mode, the modulation coefficient has a maximum value of 1; In the mode symmetry region, the two side modes will experience intensity attenuation due to competition for gain particles in the overlapping hole-burning region; the hole-burning area of ​​the middle mode will also decrease relatively during the frequency tuning process as the original hole-burning and the image hole-burning overlap; this means that the intensity of the two side modes and the intensity of the middle mode decay simultaneously, and the side modes are completely attenuated through calculation, while shaping a valley power curve with obvious characteristics under the total power output.

5. The method for designing a semi-external cavity linear polarization interference light source based on valley power locking according to claim 4, characterized in that: The gain medium of the laser (2) is Ne 20 with Ne 22 The gas mixture of two isotopes will produce an independent gain curve for each neon isotope medium. When the three longitudinal modes are tuned to the vicinity of the frequency symmetrical distribution, the hole burning overlap occurs between the side mode and the middle mode. The middle mode, which is a strong mode, can always compete for more gain particles in the mode competition, and the disadvantaged side mode is further suppressed. Shortening the cavity length of the laser to increase the longitudinal mode interval can make the hole burning of the side mode closer to the fluorescence edge of the gain curve under the condition of mode symmetry, thereby further reducing the effective gain of the side mode, and even completely suppressing the oscillation of the side mode. In the case of a di-neon isotope gain medium, the conditions for steady-state operation of the three longitudinal modes under standing wave conditions are: Where k = 1, 2 represent the index of the function related to Ne20 and Ne22 respectively, d 1(k) ,d 2(k) With d 3(k) are the hole burning depths of different longitudinal modes on the corresponding neon gain curves, It is a 3*3 symmetric matrix, and the elements in the matrix are The expression is as follows: G k (υ) is the signal gain related to the frequency of the neon gain curve, and its expression is: In the formula, Δυ Dk is the Doppler linewidth of the neon gain curve at the center frequency υ 0k The signal gain at mk is the neon gain curve at the center frequency υ 0k The signal gain at Signal gain G m1 And signal gain G m2 as follows: In the formula, G m is the total unsaturated amplification factor, and its expression is: G m =3×10 -4 l / f Where l is the capillary length, φ is the capillary diameter; R represents the position Ne 20 with Ne 22 abundance ratio; Output power P of HeNe laser with double neon isotope in three longitudinal modes t It is formed by the superposition of the light intensity of each longitudinal mode. The longitudinal mode light intensity coupling model expression is: Where C0 is the proportional constant, which is 2×10 4 , G1(υ i ) and G2(υ i ) is the signal gain of the neon gain curve at each center frequency related to the three longitudinal mode frequencies.

6. The method for designing a semi-external cavity linear polarization interference light source based on valley power locking according to claim 5, characterized in that: In addition to the hole burning effect, the cross-relaxation effect also affects the shape of the power-frequency tuning curve. Atoms at different energy levels transfer energy through elastic collision, resonant absorption capture, and excitation exchange, and some excited atoms are redistributed on the gain curve, which is called the cross-relaxation effect. This effect causes the stimulated radiation of the longitudinal mode within its hole burning range to produce a certain degree of saturation on the entire gain curve, thereby affecting the laser output power. Considering the cross-relaxation effect, the modified longitudinal mode intensity coupling model is: Where: K0 is a constant corresponding to uniform broadening, and its value is 0.35; Based on the hole burning theory and the cross-relaxation effect, the design parameters of the semi-external cavity linearly polarized laser are obtained through the modified longitudinal mode light intensity coupling model, so that the laser (2) can achieve only single longitudinal mode output under the condition of mode symmetry. The above design parameters also ensure that the power tuning curve has significant valley resolution characteristics under the condition of mode symmetry, thereby achieving frequency stability of the linearly polarized helium-neon laser.

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