Method for mass flattening of hemispherical resonator using sequence of vertically polarized femtosecond laser pulses

Abstract

The present application relates to a kind of vertical polarization femtosecond laser pulse sequence realizes the method for mass leveling of hemispherical harmonic oscillator, belong to laser application technical field.The present application is to solve the problem of low precision and low efficiency of the mass leveling method of existing hemispherical harmonic oscillator, provide a kind of vertical polarization femtosecond laser pulse sequence realizes the method for mass leveling of hemispherical harmonic oscillator;The high precision of this method is reflected in that the ablation area can be effectively reduced by using pulse sequence, and at the same time, it is found for the first time that the influence of laser on the recast layer around the ablation area of fused quartz material can be reduced by using vertical polarization double pulse, which further improves the processing precision;High efficiency processing is realized by combining high repetition frequency and high scanning speed.

Description

TECHNICAL FIELD

[0001] The application relates to a method for realizing mass leveling of a hemispherical resonator by using a vertical polarization femtosecond laser pulse sequence, and belongs to the technical field of laser application. BACKGROUND

[0002] As an inertial sensing device, the hemispherical resonator gyroscope is widely applied to space fields such as aerospace due to small volume, low power consumption, high precision and high reliability. The mass balance of the hemispherical resonator directly affects the precision of the whole inertial sensing device, and further affects the performance of the spacecraft, so effective adjustment of the mass balance of the hemispherical resonator has become an urgent demand in the field of inertial devices and even the field of aerospace.

[0003] The mass removal precision of the milling and grinding machining method is limited by the tool and can only reach the milligram level; the focused ion beam has the disadvantages of low processing efficiency and high cost. The hemispherical resonator is composed of fused quartz, and the wide-band transparent material cannot effectively realize energy absorption of the laser with a long pulse width, and thus cannot realize ablation and material removal. The femtosecond laser can realize high-quality and high-precision mass removal of the wide-band material due to the multi-photon absorption effect, and can improve the mass removal efficiency by increasing the repetition frequency and scanning speed. The femtosecond laser has unique advantages for realizing mass removal of the hemispherical resonator.

[0004] The mass removal precision of the ordinary femtosecond laser processing fused quartz is still in the picogram level, and the surrounding recast layer is obvious, which is not conducive to the evaluation of the removed mass and cannot guarantee the precision of the mass removal. In recent years, with the development of the time-domain shaping technology and polarization control technology of the femtosecond laser, it is possible to improve the processing precision of the femtosecond laser and control the energy absorption process of the laser, which provides a new solution for realizing high-precision mass removal and applying it to the mass leveling of the hemispherical resonator. SUMMARY

[0005] The application aims to solve the problems of low precision and low efficiency of the existing mass leveling method of the hemispherical resonator, and provides a method for realizing mass leveling of the hemispherical resonator by using a vertical polarization femtosecond laser pulse sequence. The high precision is reflected in that the pulse sequence can effectively reduce the ablation area, and it is found that the vertical polarization double pulse can reduce the influence of the laser on the recast layer around the ablation area of the fused quartz material compared with the traditional double pulse, and further improve the processing precision. The high repetition frequency and high scanning speed are combined to realize efficient processing of the adjustment area.

[0006] The application is realized by the following scheme.

[0007] The method for realizing mass leveling of the hemispherical resonator by using a vertical polarization femtosecond laser pulse sequence comprises the following specific steps:

[0008] Step one, fix the hemispherical resonator which has been detected and calibrated the area to be removed on the rotating fixture;

[0009] Step two, install and rotate the birefringent crystal with different thickness, the incident polarized laser is decomposed into two lasers with orthogonal polarization direction along the fast axis and perpendicular to the fast axis, forming four sub-pulses; the energy distribution is adjusted by the rotation angle, and the pulse delay is adjusted by the thickness of the birefringent crystal;

[0010] Step three, by adjusting the installation angle, four sub-pulses with different energy distribution are generated, and different energy sub-pulses are used to process different topography on the sample, and then the material is quantitatively removed.

[0011] The number of birefringent crystals with different thicknesses in step two is two; the thickness is obtained according to the required pulse delay:

[0012] d1=(v1-v2)Δt1

[0013] d2=(v1-v2)Δt2

[0014] Wherein, v1, v2 are the propagation speeds of the laser with polarization direction parallel and perpendicular to the fast axis of the birefringent crystal in the birefringent crystal; Δt1 is the pulse delay after passing through the first birefringent crystal, Δt2 is the pulse delay after passing through the second birefringent crystal; d1 is the thickness of the first birefringent crystal, d2 is the thickness of the second birefringent crystal.

[0015] The specific implementation method of adjusting the energy distribution by the rotation angle in step two is:

[0016] P 11 =Pcosαcos(α-β)

[0017] P 12 =Pcosαsin(α-β)

[0018] P 21 =Psinαcos(α-β)

[0019] P 22 =Psinαsin(α-β)

[0020] Wherein, α is the installation angle of the first birefringent crystal, β is the installation angle of the second birefringent crystal; the installation angle is the angle between the fast axis of the birefringent crystal and the polarization direction of the incident laser; P is the energy of the incident laser P; the incident laser passes through the first birefringent crystal to obtain two sub-pulses P1 and P2, wherein the polarization direction of P1 is parallel to the fast axis of the first birefringent crystal, and the polarization direction of P2 is perpendicular to the fast axis of the first birefringent crystal; P1 passes through the second birefringent crystal to obtain two sub-pulses P 11 and P 12 , wherein P11 the polarization direction of P2 is parallel to the fast axis of the second birefringent crystal, P 12 the polarization direction of P2 is perpendicular to the fast axis of the second birefringent crystal; P2 passes through the second birefringent crystal to obtain two sub-pulses P 21 and P 22 , wherein the polarization direction of P2 is parallel to the fast axis of the second birefringent crystal, P 21 the polarization direction of P2 is perpendicular to the fast axis of the second birefringent crystal; P2 passes through the second birefringent crystal to obtain two sub-pulses P 22 , P 11 , P 12 , P 21 , P 22 are sub-pulses P 11 , P 12 , P 21 , P 22 respectively; the energy distribution among the four sub-pulses is achieved by adjusting the installation angle.

[0021] The device for implementing the above method comprises a femtosecond laser, an attenuator, a vertical polarization pulse sequence generator, a mirror, an imaging system, a computer, a plano-convex / 10X and 20X objective lens, a hemispherical resonator gyro fixture, a high-precision translation stage and a CCD monitoring imaging device.

[0022] Connection: the laser generated by the femtosecond laser passes through a mechanical switch and an attenuator, and then passes through a first birefringent crystal and a second birefringent crystal in the vertical polarization pulse sequence generator in sequence, is emitted by a mirror and introduced into a processing objective lens to act on a sample, the sample is fixed on a high-precision translation stage by a hemispherical resonator gyro fixture. The translation stage is moved and the mechanical switch is closed under the control of a computer, and the processing process is monitored in real time by a CCD monitoring imaging device.

[0023] The processing method of the device comprises the following steps:

[0024] Step one: the femtosecond laser generated by the laser enters the attenuator to adjust the energy, and the mechanical switch controlled by the computer controls the passage of light;

[0025] Step two: introduce the laser into the vertical polarization pulse sequence generator, pass through birefringent crystals with different thicknesses to generate femtosecond laser pulse sequences with different time delays and vertical polarization; rotate the installation angle of the birefringent crystal to adjust the energy distribution among the sub-pulses;

[0026] Step three: fix the hemispherical resonator sub fixed after detection and calibration of the processing area on the rotating fixture, connect the rotating fixture with the high-precision translation stage, introduce the shaped laser into the objective lens and act on the calibrated processing area, and realize single-point de-replication of different qualities by selecting processing objective lenses with different focusing capabilities and adjusting pulse energy, pulse delay and sub-pulse energy.

[0027] Step four, in order to improve the processing efficiency, the time of flight dot processing mode is adopted, by adjusting the relationship between the repetition frequency f and the scanning speed v, v / f=d, d is the distance between single point ablation pits, control it to be greater than the diameter of the ablation pit, and the scanning pitch is also controlled to be greater than the diameter of the ablation pit; for single point ablation pit removal quality 10pg ablation pit, the fastest can realize the efficiency of 10ng / s of the removal efficiency.

[0028] Advantages

[0029] 1. The femtosecond laser pulse sequence generated by the self-made vertical polarization pulse sequence generator is used for removing the calibration position on the hemispherical resonator, the ablation area is reduced, and the minimum removal quality of single point processing is 220fg;

[0030] 2. By selecting different processing objectives, changing pulse energy and distribution, pulse delay, the single point processing from femtogram to nanogram is realized;

[0031] 3. The polarization direction of the femtosecond laser pulse sequence is perpendicular to each other, compared with the traditional femtosecond laser pulse, the recasting layer of the processing position is effectively reduced, and the removal precision is ensured;

[0032] 4. For larger mass removal, the time of flight scanning mode can be used, the precision is ensured, the processing efficiency is greatly improved, and the removal efficiency reaches 10ng / s.

[0033] 5. Time domain shaping and polarization control can be realized only by birefringent crystal, which is simple and reliable. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 The method for realizing hemispherical resonator mass leveling by the proposed vertical polarization femtosecond laser pulse sequence is shown in the schematic diagram.

[0035] Figure 2 (a) is a schematic diagram of a hemispherical resonator in a hemispherical resonator gyroscope, which is composed of fused quartz; Figure 2 (b) is a schematic diagram and a physical diagram of a birefringent crystal stack of a vertical polarization pulse sequence generator; Figure 2 (c) shows a method for adjusting the energy of each pulse in the pulse sequence; Figure 2 (d) shows the comparison between the processing results by using this method and the processing results by using unshaped pulses.

[0036] Wherein, 1-femtosecond laser, 2-attenuation piece, 3-mechanical switch, 4-vertical polarization pulse sequence generator, 5-CCD monitoring imaging device, 6-imaging white light source, 7-dichroic mirror, 8-thin film mirror, 9-objective lens, 10-hemispherical resonator, 11-hemispherical resonator fixture, 12-translation stage, 13-computer control system, 14-first birefringent crystal, 15-second birefringent crystal. DETAILED DESCRIPTION

[0037] The application will be further described below in conjunction with the drawings and examples.

[0038] Example 1: The mass removal of a single ablation pit is less than 500fg, and the mass removal efficiency is greater than 100pg / s.

[0039] In order to ensure that the mass removal of a single ablation pit is less than 500fg, when the pulse delay is about 0.91ps, a 20X objective lens is used, the pulse energy ratio of the pulse sequence is 1:1:1:1, the laser flux is 1.5J / cm 2 , the mass removal of a single ablation pit can be 220fg, and there is no obvious recast layer.

[0040] The specific steps of this example are as follows:

[0041] (1) The optical path system used in the application is shown in Figure 1 . The laser output of femtosecond laser 1 is Gaussian laser with a pulse width of 50fs and a wavelength of 800nm, the laser polarization state is linear polarization, and the repetition frequency is adjustable from 1-1000Hz. The laser emitted by the laser is adjusted in energy by attenuation piece 2, and the passing of light is controlled by mechanical switch.

[0042] (2) The pulse delay and sub-pulse energy distribution process of the vertical polarization femtosecond laser pulse sequence is shown in Figure 2 (c). The vertical polarization pulse sequence generator 4 is composed of the first birefringent crystal 14 and the second birefringent crystal 15 of Figure 2 (b). The incident laser P polarization direction is shown by the arrow, and the laser energy arrow length represents that after passing through the first birefringent crystal, the laser is divided into pulse P1 with polarization direction parallel to the fast axis of the first birefringent crystal and pulse P2 perpendicular to the fast axis, and after passing through the second birefringent crystal, P1, P2 are decomposed into P 11 , P 21 , P 12 , P 22 . The laser with polarization direction parallel and perpendicular to the fast axis of the birefringent crystal propagates in the birefringent crystal at speeds v1 and v2, respectively, so the pulse delay of P1 and P2 is

[0043]

[0044] P 11 and P 21 , P 12 and P 22 The pulse delay of P

[0045]

[0046] Wherein, d1 and d2 are the thickness of the first birefringent crystal and the second birefringent crystal.

[0047] P 11 is the laser energy of the sub-pulse through the fast axis of the first birefringent crystal, the fast axis of the second birefringent crystal; P 12 is the laser energy of the sub-pulse through the fast axis of the first birefringent crystal, the slow axis of the second birefringent crystal; P 21 is the laser energy of the sub-pulse through the fast axis of the first birefringent crystal, the slow axis of the second birefringent crystal; P 22 is the laser energy of the sub-pulse through the fast axis of the first birefringent crystal, the slow axis of the second birefringent crystal; The energy between pulses is respectively

[0048] P 11 = Pcosαcosβ

[0049] P 12 = Pcosαsinβ

[0050] P 21 = Psinαcosβ

[0051] P 22 = Psinαsinβ

[0052] Wherein, α and β are the installation angles of the first birefringent crystal and the second birefringent crystal (the angle with the polarization direction of the incident laser P), by adjusting the installation angle, different energy distribution pulse sequences can be generated, different pulse sequences process different appearances and different material removal, and the installation angle and the birefringent crystal thickness are adjusted according to the processing requirements. When the pulse energy ratio is 1:1:1:1, α and β are respectively 0° and 45°.

[0053] (3) The shaped pulse sequence is introduced into the plano-convex / objective lens 9 through the mirror 8 and focused on the sample surface. The objective lens with different focusing ability can be determined according to the actual removal quality requirements. The imaging system provides illumination light through the white light source 6, which is reflected by the sample and enters the CCD monitoring imaging device through the beam splitter 7.

[0054] (4) Hemisphere harmonic oscillator 10 is fixed on the translation table 12 by special fixture 11, by adjusting the special rotating fixture 11 to be removed area, by computer 13 control translation table 12 movement and mechanical switch 3 open and close, realize in sample on single point processing.

[0055] (5) Figure 2 (d) shows the same laser flux, after single point processing of unshaped and shaped pulse contrast, can be seen that the unshaped pulse effect and sample formed around the ablation pit obvious protruding recast layer, and the vertical polarization pulse sequence processing size smaller at the same time, eliminate the recast layer around the ablation pit, the highest to achieve single point 220fg mass removal, results as Figure 2 (e) shown.

[0056] (1) in order to improve the processing efficiency, using time of flight dot processing method, the distance d between each ablation pit is controlled by adjusting the repetition frequency f and the scanning speed v, their relationship is v / f=d, at the same time should control it is greater than the ablation pit diameter. When the scanning speed is 1000μm / s, the laser repetition frequency is 500Hz, the removal efficiency reaches 110pg / s. Figure 2 (f) shows the time of flight scanning processing results in Figure 2 (e) single point removal 1.8pg.

[0057] The above specific description, the purpose of the invention, technical scheme and beneficial effects are further described in detail, should be understood is, the above described only for the specific embodiments of the present application, and not for limiting the protection scope of the present application, any modification, equivalent replacement, improvement, etc. within the spirit and principles of the present application, should be included in the protection scope of the present application.

Claims

1. A method for mass flattening of a hemispherical resonator using a sequence of vertically polarized femtosecond laser pulses, characterized in that: The specific steps are as follows: Step one, fix the hemispherical resonator to be removed in the rotating fixture after detection and calibration; Step two, install and rotate the birefringent crystal of different thickness, the incident polarized laser is decomposed into two lasers with orthogonal polarization directions along the fast axis and the perpendicular fast axis, forming four sub-pulses; the energy distribution is adjusted by the rotation angle, and the pulse interval delay is adjusted by the thickness of the birefringent crystal; The specific implementation method of the energy distribution adjusted by the rotation angle is: P 11 = P cos α cos (α - β) P 12 = P cos α sin (α - β) P 21 = P sin α cos (α - β) P 22 = P sin α sin (α - β) wherein a is the installation angle of the first birefringent crystal, β is the installation angle of the second birefringent crystal; the installation angle is the angle between the fast axis of the birefringent crystal and the polarization direction of the incident laser; P is the energy of the incident laser P; the incident laser passes through the first birefringent crystal to obtain two sub-pulses P1 and P2, wherein the polarization direction of P1 is parallel to the fast axis of the first birefringent crystal, and the polarization direction of P2 is perpendicular to the fast axis of the first birefringent crystal; P1 passes through the second birefringent crystal to obtain two sub-pulses P 11 and P 12 , wherein the polarization direction of P 11 is parallel to the fast axis of the second birefringent crystal, and the polarization direction of P 12 is perpendicular to the fast axis of the second birefringent crystal; P2 passes through the second birefringent crystal to obtain two sub-pulses P 21 and P 22 , wherein the polarization direction of P 21 is parallel to the fast axis of the second birefringent crystal, and the polarization direction of P 22 is perpendicular to the fast axis of the second birefringent crystal; P 11 , P 12 , P 21 , P 22 are the energies of sub-pulses P 11 , P 12 , P 21 , P 22 respectively; the energy distribution among the four sub-pulses is realized by adjusting the installation angle. Step three, by adjusting the installation angle, four sub-pulses with different energy distributions are generated, and different energy sub-pulses are used to process different morphologies on the sample, thereby realizing quantitative mass removal of the material.

2. The method for realizing mass leveling of the hemispherical resonator by the vertical polarization femtosecond laser pulse sequence according to claim 1, characterized in that: The number of birefringent crystals of different thickness in step two is two; the thickness is obtained according to the required pulse delay: d1=(v1-v2)Δt1 d2=(v1-v2)Δt2 Wherein, v1 and v2 are the propagation speeds of the laser with polarization directions parallel and perpendicular to the fast axis of the birefringent crystal in the birefringent crystal; Δt1 is the pulse delay after passing through the first birefringent crystal, and Δt2 is the pulse delay after passing through the second birefringent crystal; d1 is the thickness of the first birefringent crystal, and d2 is the thickness of the second birefringent crystal.

3. Apparatus for implementing the method of claim 1 or 2, characterized in that: The device comprises a femtosecond laser, an attenuator, a vertical polarization pulse sequence generator, a mirror, an imaging system, a computer, a plano-convex / 10X and 20X objective lens, a hemispherical resonator gyro fixture, a high-precision translation stage, and a CCD monitoring imaging device; the connection relationship is that the laser generated by the femtosecond laser passes through the mechanical switch and the attenuator, and then passes through the first birefringent crystal and the second birefringent crystal in the vertical polarization pulse sequence generator, and then is emitted by the mirror to be introduced into the machining objective lens to act on the sample; the sample is fixed on the high-precision translation stage by the hemispherical resonator gyro fixture; the computer controls the movement of the translation stage and the closing of the mechanical switch, and the CCD monitoring imaging device monitors the machining process in real time.

4. The machining method of the device according to claim 3, characterized in that: Step one, the femtosecond laser generated by the laser enters the attenuator to adjust the energy, and the mechanical switch controlled by the computer controls the passing of the light; Step two, the laser is introduced into the vertical polarization pulse sequence generator, different delay and vertical polarization femtosecond laser pulse sequences are generated by installing birefringent crystals of different thickness, and the energy distribution between the sub-pulses is adjusted by rotating the installation angle of the birefringent crystal; Step three, fix the hemispherical resonator of the machining area after detection and calibration in the rotating fixture, connect the rotating fixture with the high-precision translation stage, introduce the shaped laser into the objective lens and act on the calibration machining area, and realize single-point mass removal with different mass by selecting machining objective lenses with different focusing abilities and adjusting the pulse energy, pulse delay, and sub-pulse energy. Step four, in order to improve the processing efficiency, the time of flight dot processing mode is adopted, by adjusting the relationship between the repetition frequency f and the scanning speed v, v / f=d, d is the distance between single point ablation pits, control it to be greater than the diameter of the ablation pit, at the same time control the scanning pitch to be greater than the diameter of the ablation pit; for the single point ablation pit removal quality is 10pg ablation pit, the fastest can realize the efficiency of 10ng / s of the removal efficiency.

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