Laser beam focusing method and system for coupling water beam optical fiber

Abstract

The invention discloses a laser beam focusing method and system for coupling water beam optical fiber. A phase plate (6) is provided. After the light beam is collimated and expanded, it is subjected to phase modulation by the phase plate (6) to generate a non-diffraction light beam, thereby reducing the side lobe effect of the non-diffraction light beam and improving the energy density of the central light spot. A positive / negative axicon combination unit (7) is provided to generate a non-diffraction light beam with a small central light spot and a long collimation area. An annealing algorithm is used to analyze and optimize the parameters of the phase plate (6). In combination with the amplitude transmittance function of the phase plate (6), the amplitude transmittance function of the positive / negative axicon combination unit (7), an evaluation function, and a water beam-light beam coupling condition, the laser beam collimation and expansion unit (5), the phase plate (6), and the positive / negative axicon combination unit (7) are further analyzed and optimized to achieve the best focusing effect, improve the quality of the light beam and the water beam-light beam coupling efficiency, and reduce the difficulty of water beam-light beam coupling.

Description

Technical Field

[0001] The invention relates to the technical field of water-conducting laser processing, and in particular to a laser beam focusing method and system coupled with a water beam optical fiber. Background Art

[0002] Water-guided laser is a technology that uses water beam optical fiber to conduct laser beams. Since the refractive index of water is greater than that of air, when the focused beam meets the critical condition of total reflection at the interface between the air and liquid in the water beam, the formed water beam optical fiber can confine the focused beam that undergoes total reflection inside it to the water beam. Due to total reflection inside the water beam, the focused beam propagates along the water beam optical fiber to the surface of the workpiece to be processed. While the laser ablates and melts the surface of the workpiece, the fine water beam impacts the workpiece to remove material and cool the workpiece, thereby completing the processing of the workpiece.

[0003] Since the water-guided laser processing technology has the problems of high component processing cost and great difficulty in water beam-beam coupling, the existing technology uses a single positive axis prism to generate a diffraction-free beam to solve the problem. Since the diffraction-free beam is close to the extremely small divergence angle of the parallel beam, the focal depth range is large, and the central spot is small, it can overcome the difficulty of focusing and aberration problems, thereby reducing the difficulty of water beam-beam coupling. However, the cone angle of the single-axis prism is usually 1°-5°, and the processing accuracy requirements are high. The processing error can easily cause the non-diffraction beam to be distorted. In addition, during the installation process, the single-axis prism with a smaller taper is difficult to focus, and the cone angle wear can easily affect the beam quality. At the same time, the sidelobe effect of the diffraction-free beam leads to low energy utilization.

[0004] Therefore, it is necessary to propose a laser beam focusing method and system for coupling water beam optical fiber, which can not only improve the beam quality and the water beam-beam coupling efficiency, but also reduce the difficulty of water beam-beam coupling. Summary of the invention

[0005] The present invention provides a laser beam focusing method and system for coupling water beam optical fiber, in order to solve the problems existing in the prior art of "poor beam quality, low water beam-to-beam coupling efficiency, and great difficulty in water beam-to-beam coupling".

[0006] The present invention solves the technical problem through the following technical solutions:

[0007] A laser beam focusing method for coupling a water beam optical fiber comprises the following steps:

[0008] (1) Preset the initial collimation and beam expansion parameters;

[0009] (2) presetting phase plate parameters, wherein the phase plate parameters include shape, phase, total number of rings, and ring diameter; calculating the initial total number of rings and the corresponding initial ring diameter of the phase plate using an annealing algorithm; and calculating the amplitude transmittance function and ring depth of the phase plate;

[0010] (3) Calculate the parameters of the initial positive / negative axicon combination unit

[0011] 31) presetting the axicon angle of the positive / negative axicon combination unit parameters, and calculating the positive / negative axicon combination unit parameters, wherein the positive / negative axicon combination unit parameters include an equivalent cone angle, a maximum non-diffraction beam area, a non-diffraction beam center spot diameter, a ray angle, and an amplitude transmittance function of the positive / negative axicon combination unit;

[0012] 32) Calculate the total amplitude transmittance function;

[0013] 33) Calculating the total light field amplitude function and the amplitude function on the spatial light field axis using the light field amplitude function of the incident light beam of the positive / negative axicon combination unit and the total amplitude transmittance function, and then calculating the total light field intensity distribution and the intensity distribution on the spatial light field axis;

[0014] 34) Calculating the parameters of the initial positive / negative axicon combination unit using the total light field intensity distribution, the light intensity distribution on the spatial light field axis, the evaluation function, the preset contrast center intensity, the preset number of contrast main side lobes, and the ratio of the preset contrast center intensity to the main side lobes;

[0015] (4) Determine the optimal collimation beam expansion parameters, the optimal phase plate parameters, and the optimal positive / negative axis prism combination unit parameters using the initial collimation beam expansion parameters, the initial number of phase plate rings and the corresponding initial ring diameter, the initial parameters of the positive / negative axis prism combination unit, and the water beam-fiber coupling conditions.

[0016] Furthermore, in step (2), the initial number of rings and the corresponding initial ring diameter of the phase plate are calculated using an annealing algorithm, and the specific steps are:

[0017] 21) Preset initial temperature, total number of loops and search length;

[0018] 22) Preset the current ring diameter corresponding to the current ring number as the first random number; generate a random adjacent solution to obtain a second random number;

[0019] 23) Calculate the light field intensity of the system;

[0020] 24) Calculate the cost of the first random number and the second random number using the cost function; if the cost of the second random number is less than the cost of the first random number, execute step 25); otherwise, execute step 27);

[0021] 25) obtaining the minimum value of the difference between the random number of the next ring diameter and the random number of the current ring diameter as the ring diameter difference; when the ring diameter difference is less than the first preset value, removing the next ring diameter and the current ring diameter, covering the ring diameter difference, and replacing the first random number with the second random number as the initial ring diameter corresponding to the current ring number, and executing step 26);

[0022] 26) If the difference between the initial temperature and the decrement step is still less than the final temperature, the second random number is returned as the initial ring diameter corresponding to the current number of rings. Otherwise, the total number of rings is iterated by subtracting 2, and step 22) is continued to be executed to calculate the ring diameter of the next number of rings until the initial ring diameter calculation of all numbers of rings is completed;

[0023] 27) Calculate the probability associated with the first random number and the second random number. If the obtained probability is greater than the second preset value, execute step 25), otherwise execute step 26).

[0024] Furthermore, in step (2), the amplitude transmittance function of the phase plate is:

[0025] in, r represents the ring diameter of the phase plate, M represents the total number of rings, and r M Indicates the ring diameter corresponding to the Mth ring number, r m It represents the ring diameter corresponding to the mth ring number, m=1,2,...,M, and circ(.) is a circular function.

[0026] Further, in step 31), the amplitude transmittance function of the positive / negative axicon combination unit is t(l')=t1(l')·t2(l')=exp[-ik(n-1)γ1l']·exp[-ik(n-1)γ2l']=exp[-ik(n-1)γl', wherein t1(l') represents the amplitude transmittance function of the negative axicon, t2(l') represents the amplitude transmittance function of the positive axicon, l' represents the radial distance from the light beam incident on the positive / negative axicon combination unit to the optical axis, i represents a complex number, k represents the number of waves, n represents the refractive index of the negative axicon and the positive axicon, γ1 is the cone angle of the negative axicon in the positive / negative axicon combination unit, γ2 is the cone angle of the positive axicon in the positive / negative axicon combination unit, the equivalent cone angle of the positive / negative axicon combination unit is γ=γ2-γ1, and λ is the wavelength of the laser beam.

[0027] Further, in step 32), the total amplitude transmittance function is T Z (l',r) = t(l')·T(r), wherein t(l') is the amplitude transmittance function of the positive / negative axicon combination unit, and T(r) is the amplitude transmittance function of the phase plate in step (2).

[0028] Further, in step 34), the evaluation function is L is the actual search length of the annealing algorithm, I c I1(z) is the peak intensity of the central lobe of the undiffracted beam, and I1(z) is the peak intensity of the main sidelobe of the undiffracted beam.

[0029] Furthermore, in step (4), the water beam-light beam coupling condition is Among them, n w is the refractive index of water, n a is the refractive index of air, θ a is the incident angle of the light entering the water beam fiber, d f is the focus spot diameter, d j The diameter of the fiber forming the water beam in the coupling cavity is d n is the nozzle aperture in the coupling cavity, NA j is the diameter of the water beam fiber.

[0030] A system based on the laser beam focusing method of the above-mentioned coupled water beam optical fiber comprises a controller, a laser emitter, a reflector, a laser beam collimating and expanding unit, a phase plate, and a positive / negative axicon combination unit; the laser beam collimating and expanding unit, the phase plate, and the positive / negative axicon combination unit are coaxially arranged;

[0031] The laser emitter emits a laser beam toward the reflector under the control of the controller, and the laser beam is collimated and expanded by the laser beam collimation and expansion unit and then incident on the phase plate for phase modulation;

[0032] The positive / negative axicon combination unit is composed of a negative axicon and a positive axicon. The light beam after phase modulation by the phase plate passes through the negative axicon and the positive axicon and then couples with the external water beam optical fiber.

[0033] Furthermore, the negative axicon and the positive axicon have the same refractive index; the negative axicon and the positive axicon are fitted and installed and are coaxially arranged.

[0034] Furthermore, the phase plate is a ring structure, and the phase of the phase plate is a binary phase 0 / π.

[0035] Compared with the existing technology, it has the following characteristics:

[0036] 1. A phase plate is set on the hardware. After the laser beam is reflected, collimated and expanded, it is phase modulated on the phase plate to generate a non-diffraction beam, which can reduce the sidelobe effect of the non-diffraction beam and increase the energy density of the central spot of the beam, which not only improves the quality of the beam but also improves the coupling efficiency of the water beam-beam. A positive / negative axicon combination unit is also set to generate a non-diffraction laser beam with a small central spot and a long collimation zone from the phase-modulated beam, reducing the difficulty of coupling with the water beam fiber in the subsequent stage;

[0037] In terms of the method, the parameters of the laser beam collimating and expanding unit, the parameters of the phase plate and the parameters of the positive / negative axicon combination unit are first preset, and then the parameters of the phase plate are analyzed and optimized using the annealing algorithm. The amplitude transmittance function of the phase plate, the transmittance function of the positive / negative axicon combination unit, the evaluation function, and the water beam-beam coupling condition are combined to further analyze and optimize the collimating and expanding unit, the phase plate, and the positive / negative axicon combination unit to obtain the optimal parameter value, so as to optimize the focusing effect, improve the quality of the beam and the water beam-beam coupling efficiency, and reduce the difficulty of water beam-beam coupling.

[0038] 2. Using a negative axicon and a positive axicon that are fitted, have the same refractive index, and are coaxially set to obtain a non-diffraction laser beam with a small central spot and a long collimation zone is more reliable than using only a single positive axicon to obtain a non-diffraction beam. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is a structural principle diagram of the present invention.

[0040] Figure 2 It is a structural parameter diagram of the positive / negative axicon combination unit.

[0041] Figure 3 The structure and phase characteristics of the phase plate.

[0042] The numbers in the figure are:

[0043] 1. Controller; 2. Laser emitter; 3. Laser beam; 4. Reflector; 5. Laser beam collimation and expansion unit; 5-1. First collimation and expansion mirror; 5-2. Second collimation and expansion mirror; 6. Phase plate; 7. Positive / negative axis prism combination unit; 7-1. Negative axis prism; 7-2. Positive axis prism. DETAILED DESCRIPTION

[0044] The present invention will be further described below with reference to examples, but the present invention is not limited to these examples.

[0045] A laser beam focusing system coupled with a water beam optical fiber comprises a controller 1, a laser emitter 2, a reflector 4, a laser beam collimating and expanding unit 5, a phase plate 6 and a positive / negative axis prism combination unit 7; the laser beam collimating and expanding unit 5, the phase plate 6 and the positive / negative axis prism combination unit 7 are coaxially arranged; the laser emitter 2 emits a laser beam 3 toward the reflector 4 under the control of the controller 1, and the laser beam 3 is collimated and expanded by the laser beam collimating and expanding unit 5 and then incident on the phase plate 6 for phase modulation; the positive / negative axis prism combination unit 7 is composed of a negative axis prism 7-1 and a positive axis prism 7-2, and the light beam after phase modulation by the phase plate 6 is coupled with an external water beam optical fiber after passing through the negative axis prism 7-1 and the positive axis prism 7-2. The structural principle diagram of the present invention is shown in FIG. Figure 1 shown.

[0046] The controller 1 is the core component of the hardware structure, which is used to control the laser emitter 2 and set the parameters of the laser beam 3, so that the laser emitter 2 emits the laser beam 3 that meets the parameter requirements. The laser emitter 2 is controlled by the controller 1 and emits the laser beam 3 that meets the parameter requirements according to the control instructions of the controller 1.

[0047] The wavelength of the laser beam 3 is 532nm or 1064nm. The laser beam 3 is a pulsed laser with a pulse width of 5ns-50ns, a power of 50W-500W, and a frequency range of 20kHz-200kHz.

[0048] The reflector 4 is arranged in front of the laser emitter 2, and the laser beam 3 emitted by the laser emitter 2 is at an angle of 45° to the axis of the reflector 4. The mirror surface of the reflector 4 faces the laser emitter 2, and a reflective film is coated on the mirror surface of the reflector 4 to increase the reflectivity of the laser beam 3 and reduce energy loss.

[0049] The laser collimating and beam expanding unit 5 is composed of a first collimating and beam expanding mirror 5-1 and a second collimating and beam expanding mirror 5-2; the first collimating and beam expanding mirror 5-1 and the second collimating and beam expanding mirror 5-2 form an inverted telescope structure and are virtually confocal. The first collimating and beam expanding mirror 5-1 and the second collimating and beam expanding mirror 5-2 can complete the collimating and beam expanding function as long as the parameters match. Usually, the first collimating and beam expanding mirror 5-1 is set as a plano-concave lens, and the second collimating and beam expanding mirror 5-2 is set as a plano-convex lens. The first collimating and beam expanding mirror 5-1 and the second collimating and beam expanding mirror 5-2 can also be set as convex lenses, but the setting method of the two is not limited to the above description.

[0050] In the present invention, the first collimating beam expander 5-1 is an input negative lens, and the second collimating beam expander 5-2 is an output positive lens. The input negative lens and the output positive lens are in a virtual confocal structure. The input negative lens transmits a virtual focus light beam to the output positive lens, and the output positive lens emits a collimated beam expander to complete the collimating beam expander function.

[0051] Beam expansion multiple Among them, f2 is the focal length of the output positive lens, and f1 is the focal length of the input negative lens. Among them, r 21 is the front curvature radius of the output positive lens, r 22 is the back curvature radius of the output positive lens; Among them, r 11 is the front curvature radius of the input negative lens, r 12 is the back curvature radius of the input negative lens. The parallel laser beam 3 is incident on the laser collimation and beam expansion unit 5. After being expanded by the input negative lens and the output positive lens, the laser beam 3 is emitted in parallel, the diameter of the laser beam 3 becomes larger, and the divergence angle becomes smaller, thereby improving the collimation of the laser beam 3. If the diameter of the laser beam 3 is 4 mm, and the beam expansion factor K of the laser collimation and beam expansion unit 5 is 5, the diameter of the beam incident on the phase plate 6 is 20 mm.

[0052] The phase plate 6 is used to perform phase modulation on the collimated and expanded light beam, so that the positive / negative axicon combination unit 7 generates a non-diffraction light beam with a small central spot and a long collimation zone, and the generated non-diffraction light beam has the characteristics of small sidelobe effect and high central spot energy density. The phase plate 6 is an annular structure, and the phase of the phase plate 6 is set to a binary phase of 0 / π by coating or groove, that is, the phase of the collimated and expanded light beam is advanced or delayed by a phase of π in different regions of the phase plate 6. The structure and phase characteristics of the phase plate 6 are shown in the figure below. Figure 3 As shown. Figure 3 In the figure, r1 represents the ring diameter of the first ring of the phase plate 6, r2 represents the ring diameter of the second ring of the phase plate 6, r3 represents the ring diameter of the third ring of the phase plate 6, r4 represents the ring diameter of the fourth ring of the phase plate 6, and r5 represents the ring diameter of the fifth ring of the phase plate 6. The parameters of the phase plate 6 include the number of rings, the ring diameter, the phase, the ring depth, and the phase modulation function formed. In addition, the outer diameter of the phase plate 6 is greater than the diameter of the output beam of the laser beam collimating and expanding unit 5.

[0053] If the present invention only uses the positive / negative axicon combination unit 7 to generate a non-diffracting light beam, the sidelobe effect is serious and the energy density of the central light spot is low. Therefore, a phase plate 6 is added to solve the above problems. The phase plate 6 uses a binary phase 0 / π to perform wavefront phase modulation on the collimated and expanded light beam, and the amplitude transmittance function of the non-diffracting light beam is subsequently modulated, and the amplitude transmittance function is used to modulate the light field with a transmittance of 0 to suppress the sidelobe, and the light beam with a non-zero transmittance is interfered and superimposed to improve the energy density distribution of the non-diffracting light beam. It can be seen that the setting mode of the binary phase 0 / π of the phase plate 6 reduces the sidelobe effect of the non-diffracting light beam, improves the quality of the light beam, and also improves the coupling efficiency of the water beam-light beam.

[0054] In the present invention, the refractive indexes of the negative axicon 7-1 and the positive axicon 7-2 may be the same or different. In order to obtain a better cone angle matching effect, the negative axicon 7-1 and the positive axicon 7-2 use the same refractive index. The negative axicon 7-1 and the positive axicon 7-2 are fitted together or integrated with geometric grinding and photolithography, and are coaxially arranged. The positive / negative axicon combination unit 7 may be made of BK7 or K9 material.

[0055] Theoretical calculation of equivalent cone angle of positive / negative axicon combination unit 7 Wherein, n is the refractive index of the negative axicon 7-1 and the positive axicon 7-2, R1 is the diameter of the output light beam of the negative axicon 7-1, R2 is the diameter of the incident light beam of the positive axicon 7-2, R1≈R2, γ1 is the cone angle of the negative axicon 7-1, γ2 is the cone angle of the positive axicon 7-2, and γ=γ2-γ1 is the equivalent cone angle of the positive / negative axicon combination unit 7. The cone angle of the positive axicon 7-2 is greater than that of the negative axicon 7-1, and the cone angles of the negative axicon 7-1 and the positive axicon 7-2 are matched when combined.

[0056] Analyze the light beam propagating along the Z direction: if the wavelength of the laser beam 3 is 532nm, the radius of the light beam incident on the positive / negative axicon mirror combination unit 7 is R=10mm, the cone angle range of the positive axicon mirror 7-2 is 5°-15°, and the cone angle range of the negative axicon mirror 7-1 is 1°-15°. Set the cone angle of the positive axicon mirror 7-2 to γ2=10°, and the cone angle of the negative axicon mirror 7-1 to γ1=9°, then the equivalent cone angle of the positive / negative axicon mirror combination unit 7 is γ=1°, that is, 10°-9°=1°. If the refractive index n of the negative axicon mirror 7-1 and the positive axicon mirror 7-2 are both 1.5, then Calculate the maximum non-diffracted beam area Z max ≈1146mm, by The diameter of the center spot of the non-diffracting beam is calculated to be d0≈46.7μm, r0 represents the radius of the center spot of the non-diffracting beam, and the focusing light angle θ=0.5° is calculated by θ=(n-1)γ. The diameter range of the coupled water beam fiber in the water-guided laser is 50μm-200μm, so the fiber incident angle of the light that meets the critical condition of total reflection of the water beam fiber incident on the entrance end face of the water beam fiber is Among them, n a is the refractive index of air, n w is the refractive index of water. From the above analysis, we can see that θ<<41.25°, indicating that the reflection angle formed at the total reflection interface is greater than the total reflection critical angle 48.75° (90°-41.25°). It can be seen that the water beam-light beam coupling has a large tolerance and the difficulty of water beam-light beam coupling is very low.

[0057] In the prior art, only a single positive axis prism is used to generate a non-diffracting beam, which has the advantages of a very small divergence angle of the non-diffracting beam close to a parallel beam, a maximum non-diffracting beam area length of tens of millimeters or even hundreds of millimeters, and a small focal depth range and central spot, which overcomes the difficulty of focusing and aberration problems, and reduces the difficulty of water beam-beam coupling in water-guided lasers. However, since the cone angle of the single-axis prism is very small, usually 1°-5°, the processing accuracy requirements are high, and the processing error is easy to cause the non-diffracting beam to be distorted. In addition, it is difficult to adjust the single-axis prism with a smaller taper during the installation process, and the cone angle wear is easy to affect the beam transmission characteristics. The sidelobe effect of the non-diffracting beam leads to low energy utilization. The present invention adopts the positive / negative axicon combination unit 7, which not only reduces the difficulty of water beam-light beam coupling in water-guided laser, but also reduces the processing accuracy requirements of the single-axis axicon, reduces the influence of processing errors on the distortion of the non-diffracting light beam, reduces the difficulty of adjusting the single-axis axicon and the influence of cone angle wear on the light beam transmission characteristics during operation, and well solves the problem of low energy utilization of the non-diffracting light beam due to the sidelobe effect, thereby improving the quality of the non-diffracting light beam and the reliability of the entire system.

[0058] A focusing method of a laser beam focusing system based on the above-mentioned coupled water beam optical fiber comprises the following steps:

[0059] (1) Preset the initial collimation and beam expansion parameters;

[0060] (2) Presetting the parameters of the phase plate 6, wherein the parameters of the phase plate 6 include shape, phase, total number of rings, and ring diameter; calculating the initial total number of rings and the corresponding initial ring diameter of the phase plate 6 by using an annealing algorithm; and calculating the amplitude transmittance function and the ring depth of the phase plate 6;

[0061] (3) Calculate the parameters of the initial positive / negative axicon combination unit 7

[0062] 31) Presetting the axicon angle of the positive / negative axicon combination unit 7 parameters, and calculating the positive / negative axicon combination unit 7 parameters, wherein the positive / negative axicon combination unit 7 parameters include an equivalent cone angle, a maximum non-diffraction beam area, a non-diffraction beam center spot diameter, a light angle, and an amplitude transmittance function of the positive / negative axicon combination unit 7;

[0063] 32) Calculate the total amplitude transmittance function;

[0064] 33) Calculate the total light field amplitude function and the amplitude function on the spatial light field axis using the light field amplitude function of the incident light beam of the positive / negative axicon combination unit 7 and the total amplitude transmittance function, and then calculate the total light field intensity distribution and the intensity distribution on the spatial light field axis;

[0065] 34) Calculating the initial parameters of the positive / negative axicon combination unit 7 using the total light field intensity distribution, the light intensity distribution on the spatial light field axis, the evaluation function, the preset contrast center intensity, the preset number of contrast main side lobes, and the ratio of the preset contrast center intensity to the main side lobes;

[0066] (4) Determine the optimal collimation and beam expansion parameters, the optimal phase plate 6 parameters, and the optimal positive / negative axis prism combination unit 7 parameters using the initial collimation and beam expansion parameters, the initial number of phase plate rings and the corresponding initial ring diameter, the initial parameters of the positive / negative axis prism combination unit 7, and the water beam-fiber coupling conditions.

[0067] In step (1), the collimation and expansion parameters of the laser beam collimation and expansion unit 5 are preset, including the wavelength λ of the laser beam 3, the diameter d1 of the laser beam 3, and the beam expansion factor K. The K value is continuously adjusted until the output beam collimation of the laser beam collimation and expansion unit 5 is achieved. The wavelength λ of the laser beam 3, the diameter d1 of the laser beam 3, and the beam expansion factor K when the beam collimation is achieved are the initial collimation and expansion parameters.

[0068] In step (2), the phase plate 6 is preset to be in the shape of a ring, with a phase of a binary phase of 0 / π, a total number of rings of M, and a ring diameter of r1, r2, r3...r M , use the annealing algorithm to calculate the initial number of rings and the corresponding initial ring diameter of the phase plate 6, the specific steps are:

[0069] 21) Preset initial temperature, total number of loops and search length;

[0070] 22) Preset the current ring diameter corresponding to the current ring number as the first random number; generate a random adjacent solution to obtain a second random number;

[0071] 23) Calculate the light field intensity of the system;

[0072] 24) Calculate the cost of the first random number and the second random number using the cost function; if the cost of the second random number is less than the cost of the first random number, execute step 25); otherwise, execute step 27);

[0073] 25) using the min function to obtain the minimum value of the difference between the random number of the next ring diameter and the random number of the current ring diameter as the ring diameter difference; when the ring diameter difference is less than the first preset value, remove the next ring diameter and the current ring diameter, cover the ring diameter difference, and replace the first random number with the second random number as the initial ring diameter corresponding to the current ring number, and execute step 26);

[0074] 26) If the difference between the initial temperature and the decrement step is still less than the final temperature, the second random number is returned as the initial ring diameter corresponding to the current number of rings. Otherwise, the total number of rings is iterated by subtracting 2, and step 22) is continued to be executed to calculate the ring diameter of the next number of rings until the initial ring diameter calculation of all numbers of rings is completed;

[0075] 27) Calculate the probability associated with the first random number and the second random number. If the probability obtained is greater than the second preset value, execute step 25), otherwise execute step 26).

[0076] In step 21), the initial temperature is preset to be T, the total number of loops is M=7, and the initial search length is l=50 μm.

[0077] In step 22), the ring diameter of the mth ring is preset to be r m = random(0~2mm), where m=1,2,...,M; the first random number is Q0={r m}; The second random number is Q1 = {r m}.

[0078] The method for calculating the light field intensity of the system in step 23) is the same as that in step 33), which is described in detail in step 33).

[0079] In step 24), the cost function is an existing technology and can be directly called. The costs involved include computer running time, storage space, etc. The cost of the first random number is F(Q0), and the cost of the second random number is F(Q1). If F(Q1)<F(Q0), execute step 25), otherwise execute step 26).

[0080] Step 25) The ring diameter difference is Δr = min{r m+1 -r m}, min function is the minimum function value, the first preset value is 1μm, if Δr<1μm, then remove r m+1 and r m , and covers Δr, and replaces the first random number with the second random number as the initial ring diameter corresponding to the current ring number, and executes step 26).

[0081] In step 26), if (T-ΔT) < T f , then return to the second random number in step 25) as the initial ring diameter corresponding to the current ring number, otherwise, M-2 replaces the original M value for iteration, and continues to execute step 22), and the corresponding initial ring diameter calculation is performed for the next ring number until the initial ring diameter calculation of all ring numbers is completed, where ΔT is the decrement step length (cooling law), T f is the final temperature (according to Metropolis standard).

[0082] ΔT can be set according to the initial temperature T and the number of iterations. If the initial temperature and the final temperature are set too low or too high, the search time will be prolonged. If the cooling step is too fast, the global optimal point will often be missed, causing the algorithm to converge to the local optimal point. If the cooling step is too slow, the calculation time for searching the global optimal point will be greatly prolonged, and the actual application effect will be poor. T can be understood as a control parameter. If the annealing initial temperature T is set to 1000℃, ΔT can be appropriately set to 10℃. T f Determined by the Metropolis criterion, after multiple iterations, T f will approach zero degrees.

[0083] In step 27), P(ΔF) is the Maxwell-Boltzmann probability function, P(ΔF)=exp{[F(R0)-F(R1)] / ξT}, ξ is the Boltzmann constant, and the second preset value is random(0~1). If P(ΔF)>random(0~1), execute step 25), otherwise execute step 26).

[0084] In step (2), the amplitude transmittance function and the ring depth of the phase plate 6 are calculated:

[0085] The amplitude transmittance function of the phase plate 6 is: in, r represents the ring diameter of the phase plate 6, M represents the total number of rings, and r M Indicates the ring diameter corresponding to the Mth ring number, r m represents the ring diameter corresponding to the mth ring number, m = 1, 2, ..., M, circ(.) is a circular function;

[0086] The ring depth of the phase plate 6, that is, the ring depth to achieve a phase shift of π radians, is expressed as Wherein, λ is the wavelength of the laser beam 3 , and N is the refractive index of the phase plate 6 material at the corresponding wavelength.

[0087] In step 31), the cone angle of the negative axicon 7-1 in the positive / negative axicon combination unit 7 is preset to be γ1, and the cone angle of the positive axicon 7-2 is preset to be γ2. Then, the equivalent cone angle of the positive / negative axicon combination unit 7 is γ=γ2-γ1, and the maximum non-diffraction beam area is The central spot diameter of the non-diffracting beam is The focusing light angle is θ=(n-1)γ, where R represents the radius of the light beam incident on the positive / negative axis prism combination unit 7, n represents the refractive index of the negative axis prism 7-1 and the positive axis prism 7-2, r0 represents the radius of the central spot of the non-diffraction light beam, and λ is the wavelength of the laser beam 3.

[0088] The amplitude transmittance function of the positive / negative axicon combination unit 7 is:

[0089] t(l')=t1(l')·t2(l')=exp[-ik(n-1)γ1l']·exp[-ik(n-1)γ2l']=exp[-ik(n-1)γl'

[0090] , where t1(l') represents the amplitude transmittance function of the negative axicon 7-1, t2(l') represents the amplitude transmittance function of the positive axicon 7-2, l' represents the radial distance from the light beam incident on the positive / negative axicon combination unit 7 to the optical axis, i represents a complex number, k represents the number of waves, λ is the wavelength of the laser beam 3 .

[0091] In step 32), the total amplitude transmittance function is the product of the amplitude transmittance function of the phase plate 6 and the amplitude transmittance function of the positive / negative axicon combination unit 7, and its function expression is: Z (l',r) = t(l')·T(r), wherein t(l') is the amplitude transmittance function of the positive / negative axicon combination unit 7, and T(r) is the amplitude transmittance function of the phase plate 6 in step (2).

[0092] In step 33), the incident light beam of the positive / negative axicon combination unit 7 is analyzed as a plane wave, and the process of calculating the total light field amplitude function and the total light field intensity function is as follows:

[0093] 331) Calculate the total light field amplitude function and the amplitude function on the spatial light field axis

[0094] The amplitude of the incident light beam of the positive / negative axicon combination unit 7 is E1. When the incident light beam propagates a distance z in the Z direction, the optical transmission matrix is ​​obtained: Substitute the transmission matrix elements A = 1, B = z, D = 1 into the generalized Huygens-Fresnel diffraction integral formula in cylindrical coordinates, and then combine the amplitude transmittance function of the positive / negative axicon combination unit 7

[0095] t(l') = t1(l') · t2(l') = exp[-ik(n-1)γl'], we can get the total light field amplitude function as,

[0096]

[0097] The amplitude function on the spatial light field axis is

[0098]

[0099] In the above formula, i represents a complex number, k represents the number of waves, n is the refractive index of the negative axis prism 7-1 and the positive axis prism 7-2, λ is the wavelength of the laser beam 3, ρ1 is the radial distance from the light incident on the positive / negative axis prism combination unit 7 to the optical axis, ρ2 is the radial distance from the light beam to the optical axis when the laser beam 3 propagates z in the Z direction, R is the radius of the light beam incident on the positive / negative axis prism combination unit 7, and J0 is the zero-order Bessel function. In the formula, i represents a complex number, α represents the radial wave vector, θ=(n-1)γ, θ is the light angle, n is the refractive index of the negative axicon 7-1 and the positive axicon 7-2, γ is the equivalent cone angle of the positive / negative axicon combination unit 7, and x and y represent the coordinate values ​​in the cylindrical coordinate system.

[0100] 332) Calculate the total light field intensity distribution and the light intensity distribution on the spatial light field axis

[0101] The total intensity distribution expression of the generated non-diffraction beam is I(ρ2,z)=|E(ρ2,z)| 2 , the light intensity distribution expression on the spatial light field axis is I(0,z)=|E(0,z) 2 .

[0102] In step 34), the evaluation function is L is the actual search length formed during the iterative optimization process of the annealing algorithm, I c (z) is the peak intensity of the central lobe of the undiffracted beam, I1(z) is the peak intensity of the main side lobe of the undiffracted beam, and I c (z) and I1(z) are obtained from the total intensity distribution I(ρ2,z) of the undiffracted beam; the preset contrast center lobe peak intensity I c , compare the main sidelobe number Q (for example, 5), compare the ratio of the center lobe peak intensity to the main sidelobe number (for example, 30). When the actual center lobe peak intensity, the actual main sidelobe number, and the ratio of the actual comparison center lobe peak intensity to the main sidelobe number obtained by the evaluation function are all greater than the preset values, the axis prism angle, equivalent cone angle, maximum non-diffraction beam area, non-diffraction beam center spot diameter, light angle and amplitude transmittance function of the positive / negative axis prism combination unit 7 parameters corresponding to the positive / negative axis prism combination unit 7 in this case are used as the initial parameters of the positive / negative axis prism combination unit 7.

[0103] In step (4), the optical properties of the entire system are analyzed using the initial collimation and beam expansion parameters in step (1), the amplitude transmittance function and the ring depth of the phase plate 6 in step (2), and the parameters of the initial positive / negative axis cone combination unit 7 in step (3). When each initial value satisfies the water beam-light beam coupling condition, it can be determined that each initial value is the optimal solution, and the optimal solution is used as the final parameter value accordingly.

[0104] The water beam-light coupling condition is Among them, n w is the refractive index of water, n a is the refractive index of air, θ a is the incident angle of the light entering the water beam fiber (i.e. the focusing angle of the laser beam 3), d f is the focus spot diameter, d j The diameter of the fiber forming the water beam in the coupling cavity is d n is the nozzle aperture in the coupling cavity, NA j is the diameter of the water beam fiber. The critical angle θ for total reflection of the laser beam 3 at the water beam fiber interface is c =arcsin(1 / n w )≈48.75°, then the ray angle of the focused laser beam 3 coupled to the water beam fiber needs to satisfy θ i <π / 2-θ c ≈41.25°.

Claims

1. A laser beam focusing method for coupling water beam optical fiber, characterized in that: The steps include: (1) Preset the initial collimation and beam expansion parameters; (2) presetting phase plate (6) parameters, wherein the phase plate (6) parameters include shape, phase, total number of rings, and ring diameter; Calculating the initial total number of rings and the corresponding initial ring diameter of the phase plate (6) by using an annealing algorithm; calculating the amplitude transmittance function and the ring depth of the phase plate (6); (3) Calculate the parameters of the initial positive / negative axicon combination unit (7) 31) presetting the axicon angle of the positive / negative axicon combination unit (7) parameters, and calculating the positive / negative axicon combination unit (7) parameters, wherein the positive / negative axicon combination unit (7) parameters include an equivalent cone angle, a maximum non-diffraction beam area, a non-diffraction beam center spot diameter, a light angle, and an amplitude transmittance function of the positive / negative axicon combination unit (7); 32) Calculate the total amplitude transmittance function; 33) using the light field amplitude function of the incident light beam of the positive / negative axicon combination unit (7) and the total amplitude transmittance function to calculate the total light field amplitude function and the amplitude function on the spatial light field axis, and then calculating the total light field intensity distribution and the intensity distribution on the spatial light field axis; 34) Calculating the parameters of the initial positive / negative axicon combination unit (7) using the total light field intensity distribution, the light intensity distribution on the spatial light field axis, the evaluation function, the preset contrast center light intensity, the preset contrast main side lobe number, and the preset ratio of the contrast center light intensity to the main side lobe; (4) Determine the optimal collimation beam expansion parameters, the optimal phase plate (6) parameters, and the optimal positive / negative axicon combination unit (7) parameters using the initial collimation beam expansion parameters, the initial number of phase plate (6) rings and the corresponding initial ring diameter, the initial parameters of the positive / negative axicon combination unit (7), and the water beam-fiber coupling conditions. In step (2), the initial number of rings and the corresponding initial ring diameter of the phase plate (6) are calculated using an annealing algorithm. The specific steps are: 21) Preset initial temperature, total number of loops and search length; 22) Preset the current ring diameter corresponding to the current ring number as the first random number; generate a random adjacent solution to obtain a second random number; 23) Calculate the light field intensity of the system; 24) Calculate the cost of the first random number and the second random number using the cost function; if the cost of the second random number is less than the cost of the first random number, execute step 25); otherwise, execute step 27); 25) Obtain the minimum value of the difference between the random number of the next ring diameter and the random number of the current ring diameter as the ring diameter difference; When the ring diameter difference is less than the first preset value, remove the next ring diameter and the current ring diameter, cover the ring diameter difference, and replace the first random number with the second random number as the initial ring diameter corresponding to the current ring number, and execute step 26); 26) If the difference between the initial temperature and the decrement step is still less than the final temperature, the second random number is returned as the initial ring diameter corresponding to the current number of rings. Otherwise, the total number of rings is iterated by subtracting 2, and step 22) is continued to be executed to calculate the ring diameter of the next number of rings until the initial ring diameter calculation of all numbers of rings is completed; 27) Calculate the probability associated with the first random number and the second random number, if the obtained probability is greater than the second preset value, execute step 25), otherwise execute step 26); In step (2), the amplitude transmittance function of the phase plate (6) is: in, r represents the ring diameter of the phase plate (6), M represents the total number of rings, and r M Indicates the ring diameter corresponding to the Mth ring number, r m represents the ring diameter corresponding to the mth ring number, m = 1, 2, ..., M, circ(.) is a circular function; In step 34), the evaluation function is L is the actual search length of the annealing algorithm, I c (z) is the peak intensity of the central lobe of the undiffracted beam, and I1(z) is the peak intensity of the main side lobe of the undiffracted beam; In step (4), the water beam-light beam coupling condition is Among them, n w is the refractive index of water, n a is the refractive index of air, θ a is the incident angle of the light entering the water beam fiber, d f is the focus spot diameter, d j The diameter of the fiber forming the water beam in the nozzle of the coupling cavity, d n is the nozzle aperture in the coupling cavity, NA j is the diameter of the water beam fiber.

2. The laser beam focusing method of a coupled water beam optical fiber according to claim 1, characterized in that: In step 31), the amplitude transmittance function of the positive / negative axicon combination unit (7) is t(l')=t1(l')·t2(l')=exp[-ik(n-1)γ1l']·exp[-ik(n-1)γ2l']=exp[-ik(n-1)γl', wherein t1(l') represents the amplitude transmittance function of the negative axicon (7-1), t2(l') represents the amplitude transmittance function of the positive axicon (7-2), l' represents the radial distance from the light beam incident on the positive / negative axicon combination unit (7) to the optical axis, i represents a complex number, k represents the number of waves, n represents the refractive index of the negative axicon (7-1) and the positive axicon (7-2), γ1 is the cone angle of the negative axicon (7-1) in the positive / negative axicon combination unit (7), γ2 is the cone angle of the positive axicon (7-2) in the positive / negative axicon combination unit (7), the equivalent cone angle of the positive / negative axicon combination unit 7 is γ=γ2-γ1, and λ is the wavelength of the laser beam (3).

3. The laser beam focusing method of a coupled water beam optical fiber according to claim 2, characterized in that: In step 32), the total amplitude transmittance function is T Z (l',r) = t(l') · T(r), wherein t(l') is the amplitude transmittance function of the positive / negative axis conical lens combination unit (7), and T(r) is the amplitude transmittance function of the phase plate (6) in step (2).

4. A system based on the laser beam focusing method of coupled water beam optical fiber according to any one of claims 1 to 3, characterized in that: It comprises a controller (1), a laser emitter (2), a reflector (4), a laser beam collimating and expanding unit (5), a phase plate (6) and a positive / negative axicon mirror combination unit (7); the laser beam collimating and expanding unit (5), the phase plate (6) and the positive / negative axicon mirror combination unit (7) are coaxially arranged; The laser emitter (2) emits a laser beam (3) toward a reflector (4) under the control of a controller (1); the laser beam (3) is collimated and expanded by a laser beam collimating and expanding unit (5) and then incident on a phase plate (6) for phase modulation; The positive / negative axicon combination unit (7) is composed of a negative axicon (7-1) and a positive axicon (7-2); the light beam after phase modulation by the phase plate (6) passes through the negative axicon (7-1) and the positive axicon (7-2) and then couples with an external water beam optical fiber; the negative axicon (7-1) and the positive axicon (7-2) have the same refractive index; the negative axicon (7-1) and the positive axicon (7-2) are fitted and installed and coaxially arranged; the phase plate (6) is a ring structure, and the phase of the phase plate (6) is a binary phase 0 / π.

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