Semiconductor laser with a monolithic intracavity volume Bragg grating for generating a continuous-wave single-frequency laser radiation

Abstract

The application discloses a kind of based on valley power lockout semi-out cavity linear polarization interference light source and its design method.The device includes laser power supply, laser, heating wire, heat preservation glue, beam splitter BS, PD photocell and laser frequency stabilization system;Design method is crucial in that: the laser is three longitudinal mode semi-out cavity linear polarization helium-neon laser, laser built-in Brewster window, and the key parameters of the laser are calculated by longitudinal mode light intensity coupling model, two side modes can be completely suppressed, form the high proportion valley bottom in light intensity harmonic curve, to facilitate frequency stabilization algorithm identification to realize central longitudinal mode frequency locking.The method of the application can make the output power of the laser reach 2.4mW or more, overcome the shortcomings of traditional double longitudinal mode frequency stabilization interference light source power limitation, improve single mode power conversion efficiency, provide a new type of frequency stabilization technology and new type of interference light source for the rapid development of ultra-precision machining and measurement technology and various fields such as single mode conversion complete, high output power, good wavelength stability.

Description

Technical Field

[0001] This invention belongs to the field of laser precision measurement, specifically relating to a semi-external cavity linear polarization interference light source based on valley power locking and its design method. Background Technology

[0002] Laser interferometers are common optical measurement tools used for high-precision determination of parameters such as distance, angle, and velocity, making them the best solution and a research hotspot for high-resolution, high-precision, and ultra-precision measurements. Frequency-stabilized HeNe lasers are the most commonly used interferometric light sources, providing coherence lengths of hundreds of meters and reliable, highly stable wavelengths. However, the main drawbacks of mainstream commercially available frequency-stabilized HeNe lasers are low output power, the need for external polarization devices for single-mode output, incomplete single-mode conversion, and low efficiency (typically no higher than 1mW). This results in long camera exposure times during interferometric sampling, especially under environmental vibration and air turbulence conditions, significantly affecting the signal-to-noise ratio of the interference fringes. Interferometers sometimes employ unstabilized, higher-power multi-longitudinal-mode lasers, but mode instability during measurement also greatly complicates the accuracy of the results. Therefore, to improve the accuracy of measurement results and enhance the anti-interference capability of interferometric systems, the industry urgently needs a more stable interferometric light source with higher output power.

[0003] Currently, the mainstream commercially available frequency-stabilized helium-neon lasers used in interferometers employ full-cavity random polarization dual-longitudinal-mode frequency stabilization technology. Dual-longitudinal-mode frequency-stabilized lasers based on electrothermal devices achieve a relative frequency accuracy of 10-1. -8 The above-mentioned methods, along with advantages such as simple structure, short preheating time, and good vibration resistance, have already been applied in commercial laser interferometry systems. For example, Reinishaw in the UK proposed a thermal frequency stabilization method for dual longitudinal mode lasers using an electrothermal wire as the actuating element (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 recently proposed a frequency stabilization method for dual-longitudinal-mode lasers based on electromagnetic induction heating (Chinese Patent CN100367579: Frequency Stabilization Device and Method for Dual-Longitudinal-Mode Lasers Based on Electromagnetic Induction Heating) and a frequency stabilization method for dual-longitudinal-mode lasers based on thermoelectric coolers (Chinese Patent CN100382398: Frequency Stabilization Method and Device for Dual-Longitudinal-Mode Lasers Based on Thermoelectric Coolers). The Beijing Aerospace Metrology and Testing Technology Research Institute, addressing the low single-mode conversion efficiency of the dual-longitudinal-mode thermal frequency stabilization method, stabilized the frequencies of the dual-longitudinal-mode lasers at asymmetric positions, increasing the single-mode conversion efficiency to 80% and achieving a single-mode output power of 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-mentioned dual-longitudinal-mode frequency-stabilized lasers with full internal cavity cannot output higher-power single-mode lasers due to the limitation of the number of longitudinal modes. In order to meet the needs of interferometric measurement, a three-longitudinal-mode laser can be used for frequency stabilization, and the intermediate film with a larger resonant cavity gain can be selected as the working mode.

[0006] Based on the power stabilization method of a fully internal cavity randomly polarized three-longitudinal-mode laser, ZYGO has developed a high-power interferometric light source for its instantaneous interferometry system. www.zygo.com.cn High-power laser sources can shorten camera exposure time and effectively reduce the effects of vibration and air turbulence. However, because they fill non-operating modes, external polarization devices are needed to filter them, limiting their single-mode power conversion efficiency to only 75%. Furthermore, the longitudinal modes of randomly polarized lasers may change polarization direction due to environmental influences, meaning the laser output cannot be entirely single-mode. Optical feedback introduced by external polarization beam splitters can also affect resonant cavity stability, leading to potential mode-hopping risks. These drawbacks not only significantly impact power stability but also greatly affect the accuracy of the final interferometric measurement results. Summary of the Invention

[0007] To address the lack of high-power single-mode frequency-stabilized lasers in existing frequency stabilization technologies, this invention proposes a semi-external cavity linear polarization interferometric light source based on valley power locking and its design method. This solves the problems in current laser interferometry systems where the addition of a polarization-controlled beam splitter (PBS) to a fully internal cavity random polarization laser results in low single-mode conversion efficiency, incomplete single-mode conversion, and insufficient overall output power, thus limiting the overall measurement methods, vibration resistance, measurement length, and accuracy of the interferometer. This provides a novel frequency stabilization technology and a novel frequency-stabilized laser light source with complete single-mode conversion, high output power, and good wavelength stability for various fields such as rapidly developing ultra-precision machining and measurement technologies.

[0008] The technical solution for achieving this invention is as follows: a semi-external cavity linear polarization interference light source based on valley power locking, comprising a laser power supply, a laser, a BS beam splitter, a PD photovoltaic cell, an input signal amplifier, an A / D converter, an MCU microprocessor, a thermistor, a PWM controller, a heater, and an insulating adhesive layer; the laser and the BS beam splitter are arranged sequentially along the optical axis, and the laser emitted by the laser is incident perpendicularly to the BS beam splitter, which splits it into output light emitted along the optical axis and feedback light perpendicular to the optical axis. The reflected light is used as a feedback light signal and enters the PD photovoltaic cell to be converted into an electrical signal; the laser power supply is connected to the laser, the PD photovoltaic cell, and the input signal amplifier, A / D converter, MCU microprocessor, thermistor, PWM controller, heater, and insulating adhesive layer; The input signal amplifier, A / D converter, MCU microprocessor, PWM controller, and heater are connected in sequence. The thermistor is attached to the laser tube of the laser and connected to the MCU microprocessor. The heater is wrapped around the laser tube of the laser. The laser tube, thermistor, and heater are wrapped with insulating adhesive. The feedback optical signal passes through the PD photocell, input signal amplifier, and A / D converter before entering the MCU microprocessor for frequency stabilization. The PWM controller and heater are used to control the temperature of the laser tube, thereby changing the laser tube cavity length to achieve longitudinal mode optical intensity tuning. The valley region of the laser's longitudinal mode optical intensity tuning curve accounts for 20%.

[0009] A design method for a semi-external cavity linear polarization interferometer light source based on valley power locking, comprising the following steps:

[0010] Step 1: In order to achieve a power valley ratio of 20% for the laser, a three-mode half-external cavity linearly polarized helium-neon laser is selected, and a Brewster window is built into the laser to output linearly polarized laser.

[0011] Step 2: Using the hole-burning theory and the longitudinal mode optical intensity coupling model of cross-relaxation effect, calculate the key parameters (L) of the laser. r ,l,φ,L,Δυ L To ensure that, under the condition of the obtained resonant cavity loss, in the mode-symmetric case, the two side modes can be completely suppressed, forming the valley in the light intensity harmonic curve, at the valley position only the central single longitudinal mode can oscillate and output, realizing the frequency locking of the central longitudinal mode, where L r The resonant cavity loss is represented by l, the capillary length by φ, the capillary diameter by L, and the resonant cavity length by Δυ. L R represents the uniform broadened linewidth of the gain medium, and R represents the isotope Ne. 20 with Ne 22 Abundance ratio.

[0012] Step 3: Continue to select other components: Use a BS beam splitter with a 5% splitting ratio, PD photovoltaic cells, input signal amplifier, and A / D converter to provide optical feedback signals to the MCU microprocessor. Through the frequency stabilization algorithm in the MCU microprocessor, the signal output by the PWM controller is used to achieve temperature control by winding the laser tube with an electric heater, thereby changing the laser tube cavity length to achieve longitudinal mode tuning.

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

[0014] Compared with the prior art, the significant advantages of this invention are:

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

[0016] (2) This invention uses a semi-cavity laser with a built-in Brewster window to output linearly polarized laser, which eliminates the step of external PBS filtering for single-mode output in ordinary commercial full-cavity random polarization frequency-stabilized lasers. This avoids problems such as incomplete single-mode filtering and easy mode switching caused by PBS, and can achieve complete conversion of single-mode output, reducing the error of the interferometer measurement system.

[0017] (3) This invention greatly improves the single-mode output power of the frequency-stabilized laser, overcomes the difficulties 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 facilitates the improvement of the anti-vibration performance, measurement length and measurement accuracy of the time-phase interferometer, and can be widely used in the field of interferometric measurement.

[0018] (4) This invention optimizes the division accuracy of the valley region of the longitudinal mode symmetrical operating point of the three longitudinal mode semi-cavity laser, effectively improves the discrimination of the stable frequency 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 structure of the present invention is simple, the price is low, the preheating time is short (15 minutes), and the vibration resistance is strong. It can be applied to a wide range of interferometric measurement scenarios. Attached Figure Description

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

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

[0022] Figure 3 The diagram shows the hole distribution on the laser gain curve, where (a) represents the single-mode hole distribution and (b) represents the three-mode hole distribution.

[0023] Figure 4 This is a schematic diagram illustrating the principle of laser hole burning overlap to suppress edge mode oscillation.

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

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

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

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

[0028] Figure 9 This is a power drift diagram of the laser in this invention. Detailed Implementation

[0029] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

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

[0031] In this invention, unless otherwise explicitly specified and limited, the terms "connection," "fixing," etc., should be interpreted broadly. For example, "fixing" can mean a fixed connection, a detachable connection, or an integral part; "connection" can mean a mechanical connection or an electrical connection. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0032] Furthermore, the technical solutions of the various embodiments of the present invention can be combined with each other, but only if they are feasible to those skilled in the art. If the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such combination of technical solutions does not exist and is not within the scope of protection claimed by the present invention.

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

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

[0035] Laser 2 and BS beam splitter 3 are arranged in sequence along the optical axis. The laser emitted by laser 2 is incident perpendicularly on BS beam splitter 3 and split into output light emitted along the optical axis and feedback light perpendicular to the optical axis. The reflected light is used as feedback light signal and enters PD photocell 4 to be converted into an electrical signal. Laser power supply 1 is connected to laser 2. PD photocell 4, input signal amplifier 5, A / D converter 6, MCU microprocessor 7, PWM controller 9, and heater 10 are connected in sequence. Thermistor 8 is attached to the laser tube of laser 2 and connected to MCU microprocessor 7. Heater 10 is wrapped around the laser tube of laser 2. Insulation adhesive layer 11 wraps the laser tube of laser 2, thermistor 8 and heater 10.

[0036] Combination Figure 2 The feedback optical signal from the laser 2 passes through the PD photocell 4, input signal amplifier 5, and A / D converter 6 before entering the MCU microprocessor 7 for frequency stabilization. A PWM controller 9 and a heater 10 control the laser tube temperature, thereby changing the laser tube cavity length to achieve longitudinal mode intensity tuning. The valley region of the laser's longitudinal mode intensity tuning curve accounts for 20%.

[0037] Laser 2 is a three-longitudinal-mode polarized helium-neon laser.

[0038] A design method for a semi-external cavity linear polarization interferometer light source based on valley power locking, comprising the following steps:

[0039] Step 1: In order to achieve a power valley ratio of 20% for laser 2, a three-mode half-external cavity linearly polarized helium-neon laser is selected for laser 2, and a Brewster window is built into laser 2 to output linearly polarized laser.

[0040] Step 2: Using the hole-burning theory and the longitudinal mode optical intensity coupling model of cross-relaxation effect, calculate the key parameters (L) of laser 2.r ,l,φ,L,Δυ L To ensure that, under the obtained resonant cavity loss, two of the side modes in the three longitudinal modes in the mode-symmetric case can be completely suppressed, forming the valley in the light intensity harmonic curve, only the central single longitudinal mode can oscillate and output at the valley position, thus achieving central longitudinal mode frequency locking, where L r The resonant cavity loss is represented by l, the capillary length by φ, the capillary diameter by L, and the resonant cavity length by Δυ. L R represents the uniform broadened linewidth of the gain medium, and R represents the isotope Ne. 20 with Ne 22 Abundance ratio.

[0041] The key parameters of the laser 2 (L) r ,l,φ,L,Δυ L R) was calculated using the hole-burning theory and the longitudinal mode optical intensity coupling model based on the cross-relaxation effect. Combined with... Figure 3 In Figure (a), the laser's output power is affected by the hole-burning effect, which can be represented by the hole area, characterizing the number of stimulated emission photons. Combined with... Figure 3 In Figure (b), as the cavity length of the laser increases, three resonant longitudinal modes exist within the envelope of the gain curve. The steady-state operation condition of the three longitudinal modes under standing wave conditions is:

[0042]

[0043] In the formula, G(υ) is the frequency-dependent signal gain, and L... r The resonant cavity loss is represented by d, which indicates the hole depth. It is a 3x3 symmetric matrix, where each element... The expression is as follows:

[0044]

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

[0046] The hole depth modulation coefficient F(δυ) c The expression for ) is:

[0047]

[0048] In the formula, δυ c Δυ is the frequency difference between the center frequency of the original or image-burned hole and the longitudinal mode resonant frequency. L To achieve uniform linewidth expansion for the gain medium.

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

[0050] Combination Figure 4 During frequency tuning, the original aperture moves in the same direction as the resonant frequency, while the image aperture moves in the opposite direction. When the three longitudinal modes are tuned to a region near the symmetrical distribution about the center frequency of the curve, the original aperture of the low-frequency side mode overlaps with the image aperture of the high-frequency side mode, and simultaneously, the image aperture of the former overlaps with the original aperture of the latter. Their total aperture area decreases as the frequency difference between the intermediate mode frequency υ2 and frequency υ0 decreases. In the mode-symmetric region, the two side modes experience intensity attenuation due to competition for gain particles within the overlapping aperture area. The aperture area of ​​the intermediate mode also decreases relatively during the aforementioned frequency tuning process due to the overlap of its own original aperture and image aperture, meaning that the intensity of both side modes and the intermediate mode attenuates simultaneously. Through calculation, the side modes can be completely attenuated, while simultaneously shaping a highly refined valley power curve with distinct characteristics under total power output conditions.

[0051] The gain medium of the semi-external cavity linear polarization interference light source is made of Ne 20 with Ne 22 Composed of a mixture of two isotopes, each neon isotope medium produces an independent gain curve. When the three longitudinal modes are tuned to a region near the frequency-symmetric distribution, hole overlap occurs between the edge modes and the intermediate modes. The intermediate modes, being stronger modes, always compete for more gain particles in mode competition, while the weaker edge modes are further suppressed. Appropriately shortening the laser cavity length to increase the longitudinal mode spacing can bring the hole burn-off of the edge modes closer to the fluorescence edge of the gain curve under mode symmetry, thereby further reducing the effective gain of the edge modes or even completely suppressing their oscillations.

[0052] In the case of a dual-neon isotope gain medium, the steady-state operating conditions of the three longitudinal modes under standing wave conditions are as follows:

[0053]

[0054] In the formula, k = 1, 2 represent the indices of the functions related to Ne20 and Ne22, respectively, and d 1(k) ,d 2(k) With d 3(k) These represent the hole depths for different longitudinal modes on the corresponding neon gain curves. Given a 3x3 symmetric matrix, the elements in the matrix are... The expression is as follows:

[0055]

[0056] G k (υ) represents the signal gain of the neon gain curve as a function of frequency, and its expression is:

[0057]

[0058] In the formula, Δυ Dk The Doppler linewidth of the neon gain curve at the center frequency υ 0k Signal gain at point G mk The neon gain curve at the center frequency υ 0k Signal gain at the location;

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

[0060]

[0061] In the formula, G m The total unsaturated gain is expressed as:

[0062] G m =3×10 -4 l / φ

[0063] In the formula, l is the capillary length, φ is the capillary diameter; R represents the isotope Ne. 20 with Ne 22 Abundance ratio;

[0064] The output power P of the dual-neon isotope helium-neon laser in three longitudinal modes is as follows: t The intensity of light is composed of the superposition of the intensity of each longitudinal mode. The expression for the longitudinal mode intensity coupling model is:

[0065]

[0066] In the formula, C0 is a proportionality constant with a value of 2 × 10⁻⁶. 4 ,G1(υ i ) and G2(υ i () represents the signal gain of the neon gain curve at its respective center frequency and the three-mode frequency.

[0067] Besides the hole-burning effect, the cross-relaxation effect also influences the shape of the power-frequency tuning curve. Atoms at different energy levels transfer energy through elastic collisions, resonant absorption trapping, and excitation exchange. Some excited atoms redistribute on the gain curve, a phenomenon known as the cross-relaxation effect. This effect causes stimulated emission of the longitudinal mode within its hole-burning range to saturate the entire gain curve to a certain extent, thus affecting the laser output power. Considering the cross-relaxation effect, the modified longitudinal mode optical intensity coupling model is as follows:

[0068]

[0069] In the formula: K0 is a constant corresponding to uniform broadening, with a value of 0.35.

[0070] Combination Figure 5 Based on the longitudinal mode optical intensity coupling model calculated using the hole-burning theory and cross-relaxation effect, we can derive the design parameters for laser 2 to achieve high-power single longitudinal mode output under mode symmetry. These parameters also ensure that the single-mode power range has a significant resolution characteristic valley ratio of 20% on the power tuning curve, facilitating frequency stabilization algorithm identification and thus achieving frequency stability of the linearly polarized helium-neon laser. The calculated resonant cavity loss L... r =0.027, capillary length l = 180mm, capillary diameter φ = 1.30mm, resonant cavity length L = 270mm, uniformly broadened linewidth Δυ L =120MHz, isotope Ne 20 with Ne 22 The abundance ratio R = 1.

[0071] Step 3: Continue to select other components: Use a BS beam splitter 3 with a 5% splitting ratio, a PD photovoltaic cell 4, an input signal amplifier 5, and an A / D converter 6 to provide optical feedback signals to 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 by the heater 10 to achieve temperature control, thereby changing the length of the laser tube cavity to achieve longitudinal mode tuning.

[0072] Combination Figure 6 The laser tube 2, fabricated using the parameters calculated by the above method, has a clear relationship between the longitudinal mode and temperature, which facilitates the subsequent frequency stabilization algorithm.

[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, the PID frequency stabilization algorithm identifies and calculates the output voltage of the heating wire to perform PWM control, thereby achieving temperature tuning of the cavity length and ultimately outputting a frequency-stabilized laser.

[0075] Combination Figure 8 and Figure 9 The method of this invention can enable the laser to achieve a stable output power of over 2.4mW and a frequency stability better than 0.05pm / h. It provides a novel frequency stabilization technology and a novel interference light source with complete single-mode conversion, high output power, and good wavelength stability for various fields such as rapidly developing ultra-precision machining and measurement technologies.

Claims

1. A semi-external cavity linear polarization interference light source based on valley power locking, characterized in that: The system includes a laser power supply (1), a laser (2), a BS beam splitter (3), a PD photovoltaic cell (4), an input signal amplifier (5), an A / D converter (6), an MCU microprocessor (7), a thermistor (8), a PWM controller (9), a heater (10), and an insulation layer (11). The laser (2) and the BS beam splitter (3) are arranged along the optical axis. The laser emitted by the laser (2) is incident perpendicularly onto the BS beam splitter (3), and is split by the BS beam splitter (3) into output light emitted along the optical axis and feedback light reflected perpendicular to the optical axis. The reflected light is used as feedback light signal and enters the PD photovoltaic cell (4) to be converted into an electrical signal. The laser power supply (1) is connected to the laser (2), the PD photovoltaic cell (4), the input signal amplifier (5), the A / D converter (6), and the MCU microprocessor (7), the thermistor (8), the PWM controller (9), the heater (10), and the insulation layer (11). The CU microprocessor (7), PWM controller (9), and heater (10) are connected in sequence. The thermistor (8) is attached to the laser tube of the laser (2) and connected to the MCU microprocessor (7). The heater (10) is wrapped around the laser tube of the laser (2). The heat insulation layer (11) wraps the laser tube, thermistor (8), and heater (10) of the laser (2). The feedback optical signal enters the MCU microprocessor (7) for frequency stabilization after passing through the PD photocell (4), input signal amplifier (5), and A / D converter (6). The PWM controller (9) and heater (10) are used to control the temperature of the laser tube, thereby changing the laser tube cavity length to achieve longitudinal mode optical intensity tuning. The valley area of ​​the longitudinal mode optical intensity tuning curve of the laser reaches 20%. The laser (2) is a three-longitudinal-mode polarized helium-neon laser; Based on the hole-burning theory and cross-relaxation effect, the design parameters of the semi-external cavity linearly polarized laser are obtained through the modified longitudinal mode optical intensity coupling model, so that the laser (2) can achieve only single longitudinal mode output under mode symmetry. The above design parameters also ensure that the power tuning curve has significant valley resolution characteristics under mode symmetry, thereby achieving frequency stability of the linearly polarized helium-neon laser. Considering the cross-relaxation effect, the modified longitudinal mode optical intensity coupling model is as follows: ， In the formula: It is a constant corresponding to uniform broadening, with a value of 0.35; To achieve the output power of a dual-neon isotope helium-neon laser operating in three longitudinal modes, It is a proportionality constant. and The signal gain is represented by the neon gain curve, which is related to the three-mode frequencies at their respective center frequencies. , and These represent the hole depths for different longitudinal modes on the corresponding neon gain curves. This indicates the function index related to Ne20.

2. A design method for a semi-external cavity linear polarization interference light source based on valley power locking as described in claim 1, characterized in that, The steps are as follows: Step 1: In order to make the power valley ratio of the laser (2) reach 20%, the laser (2) is a three-longitudinal-mode half-external cavity linearly polarized helium-neon laser, and the Brewster window is built into the laser (2) to output linearly polarized laser. Step 2: Using the hole-burning theory and the longitudinal mode optical intensity coupling model of cross-relaxation effect, calculate the key parameters of laser (2). , , , , , To ensure that, under the obtained resonant cavity loss conditions, in the mode-symmetric case, both side modes can be completely suppressed, forming the valley in the optical intensity harmonic curve, only the central single longitudinal mode can oscillate and output at the valley position, thus achieving central longitudinal mode frequency locking. Indicates the resonant cavity loss. Indicates the length of the capillary tube. Indicates the capillary diameter. Indicates the length of the resonant cavity. The uniform broadened linewidth of the gain medium. Indicates the isotope Ne 20 with Ne 22 Abundance ratio; Step 3: Continue to select other components: Use a BS splitter (3) with a 5% splitting ratio, a PD photovoltaic cell (4), an input signal amplifier (5), and an A / D converter (6) to provide optical feedback signals to the MCU microprocessor (7). Through the frequency stabilization algorithm in the MCU microprocessor (7), the signal output by the PWM controller (9) is used to achieve temperature control by winding the laser tube with a heater (10), thereby changing the length of the laser tube cavity to achieve longitudinal mode tuning. Step 4: Thermistor (8) is placed close to the laser tube to ensure the working temperature.

3. The design method for a semi-external cavity linear polarization interference light source based on valley power locking according to claim 2, characterized in that: In step 2, the key parameters of the laser (2) are calculated using the hole-burning theory and the longitudinal mode optical intensity coupling model based on the cross-relaxation effect. , , , , , The details are as follows: The output power of the laser (2) is affected by the hole-burning effect, which is represented by the hole area, which characterizes the number of stimulated emission photons. During frequency tuning, the original hole moves in the same direction as the resonant frequency, while the image hole moves in the opposite direction. When the three longitudinal modes are tuned to the frequency about the center of the curve... In the vicinity of symmetrically distributed areas, the original burned-in apertures of the low-frequency side modes overlap with the image burned-in apertures of the high-frequency side modes. Simultaneously, the image burned-in apertures of the low-frequency side modes also overlap with the original burned-in apertures of the high-frequency side modes. The total burned-in aperture area increases with the intermediate mode frequency. With center frequency The frequency difference between them decreases, and the frequency decreases accordingly; As the cavity length of the laser (2) increases, there are three resonant longitudinal modes within the envelope of the gain curve, i.e., three longitudinal modes; The steady-state operating conditions of the three longitudinal modes under standing wave conditions are as follows: In the formula, For frequency-dependent signal gain, For resonant cavity loss, Indicates the depth of the burn-in hole. It is a 3x3 symmetric matrix, where each element... The expression is as follows: ， This represents the frequency of the i-th resonant longitudinal mode. This represents the frequency of the j-th resonant longitudinal mode. and ; The center frequency under a single-gain curve is... The original hole burning pair resonant frequency is The modulation coefficient of the longitudinal die hole depth. The center frequency under a single-gain curve is... The resonant frequency of the hole-burning pair is The modulation coefficient of the longitudinal die hole depth; Hole depth modulation coefficient The expression is: ； In the formula, The difference between the center frequency of the original or image-based hole burn-in and the longitudinal mode resonant frequency. To achieve uniform linewidth expansion for the gain medium; When the center frequency of the hole burning is equal to the resonant frequency of the longitudinal mode, the modulation coefficient has a maximum value of 1; In the mode-symmetric region, the two side modes will experience intensity attenuation due to competition for gain particles in the overlapping burn-in area; the burn-in area of ​​the middle mode will also decrease relatively during frequency tuning as its original burn-in area overlaps with the image burn-in area; this means that the intensity of the two side modes and the middle mode attenuate simultaneously. By calculation, the side modes are completely attenuated, and a valley power curve with obvious characteristics is shaped under the total power output.

4. The design method for a semi-external cavity linear polarization interference light source based on valley power locking according to claim 3, characterized in that: The gain medium of the laser (2) is made of Ne 20 with Ne 22 Composed of a mixture of two isotopes, each neon isotope medium produces an independent gain curve; when the three longitudinal modes are tuned to a region near the frequency-symmetric distribution, the burn-off overlap occurs between the side modes and the middle mode. The middle mode, which is a strong mode, can always compete for more gain particles in mode competition, while the disadvantaged side modes are further suppressed; shortening the cavity length of the laser to increase the longitudinal mode spacing can make the burn-off of the side modes closer to the fluorescence edge of the gain curve under mode symmetry, thereby further reducing the effective gain of the side modes, or even completely suppressing the oscillation of the side modes; In the case of a dual-neon isotope gain medium, the steady-state operating conditions of the three longitudinal modes under standing wave conditions are as follows: ； In the formula, These represent the indices of functions related to Ne20 and Ne22, respectively. , and These represent the hole depths for different longitudinal modes on the corresponding neon gain curves. Given a 3x3 symmetric matrix, the elements in the matrix are... The expression is as follows: ； The center frequency under the double gain curve is... The original hole burning pair resonant frequency is The modulation coefficient of the longitudinal die hole depth. The center frequency under the double gain curve is... The resonant frequency of the hole-burning pair is The modulation coefficient of the longitudinal die hole depth; The expression for the frequency-dependent signal gain of the neon gain curve is as follows: In the formula, The Doppler linewidth of the neon gain curve at the center frequency Signal gain at that location, The neon gain curve at the center frequency Signal gain at the location; Signal gain and signal gain as follows: ； In the formula, The total unsaturated gain is expressed as: In the formula, The length of the capillary tube. The diameter is the capillary tube diameter. Indicates the isotope Ne 20 with Ne 22 Abundance ratio; Output power of dual-neon isotope helium-neon laser in three longitudinal modes The intensity of light is composed of the superposition of the intensity of each longitudinal mode. The expression for the longitudinal mode intensity coupling model is: In the formula, It is a proportionality constant with a magnitude of , and The signal gain is the correlation between the neon gain curve and the three-mode frequency at their respective center frequencies.

5. The design method for a semi-external cavity linear polarization interference light source based on valley power locking according to claim 4, 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 collisions, resonant absorption capture, and excitation exchange. Some of the stimulated atoms are redistributed on the gain curve, which is called the cross-relaxation effect. This effect causes the stimulated emission of the longitudinal mode within its hole-burning range to have a certain degree of saturation effect on the entire gain curve, thereby affecting the laser output power. Considering the cross-relaxation effect, the modified longitudinal mode optical intensity coupling model is as follows: ； In the formula: It is a constant corresponding to uniform broadening, with a value of 0.35; Based on the hole-burning theory and cross-relaxation effect, the design parameters of the semi-external cavity linearly polarized laser are obtained through the modified longitudinal mode optical intensity coupling model, so that the laser (2) can achieve only single longitudinal mode output under mode symmetry. The above design parameters also ensure that the power tuning curve has significant valley resolution characteristics under mode symmetry, thereby achieving frequency stability of the linearly polarized helium-neon laser.

Images