A method and device for measuring the ionization breakdown threshold of laser-liquid interaction

Abstract

The present invention belongs to the field of breakdown threshold measurement, and specifically discloses a method and device for measuring the ionization breakdown threshold of laser-liquid interaction, which includes the following steps: focusing an ultrashort pulse laser in a transparent liquid to be measured, and then adjusting the incident average power of the ultrashort pulse laser from small to large; at the same time, measuring the output power of the ultrashort pulse laser after focusing in the transparent liquid to be measured; drawing a scatter plot with the incident average power as the horizontal axis and the output power as the vertical axis, in which a sudden drop point of the output power will appear as the incident average power increases, and the incident average power corresponding to the sudden drop point is the ionization threshold of the ultrashort pulse laser breaking through the transparent liquid. The present invention can conveniently measure the ionization threshold of the short pulse laser breaking through the transparent liquid, which is of great significance for experimentally exploring the ionization mechanism of the ultrashort pulse laser in the transparent liquid and the plasma formation mechanism.

Description

Technical Field

[0001] The present invention belongs to the field of breakdown threshold measurement, and more specifically, relates to a method and device for measuring the ionization breakdown threshold of laser-liquid interaction. Background Art

[0002] The interaction between light and matter is widely used in many fields such as laser processing. The generation of ultrashort pulses has a profound impact on the transmission of lasers. When high-power laser pulses are focused into a transparent medium, free electrons are formed at high density as long as a certain irradiation threshold is exceeded. When the density of free electrons exceeds 10 18 -10 20 / cm 3 When the absorption coefficient of the medium suddenly rises, it becomes opaque to the laser radiation. This is due to the formation of a dense, optically absorbing plasma, a process known as optical breakdown or laser-induced plasma formation.

[0003] Laser-induced plasma formation has many important applications, such as stimulated Raman scattering, plasma waveguides, high-harmonic generation, plasma accelerators, glass micromachining, and oceanographic acoustic wave generation. In addition, it has also been widely used in medicine, examples of which are ophthalmic microsurgery, laser lithotripsy, and angioplasty.

[0004] In the experiment, the optical breakdown threshold is defined as the pulse input energy corresponding to the generation of plasma light in a laser pulse. The discussion and determination of this threshold is of great significance for studying the mechanism of laser-induced plasma generation. However, since the plasma formed when optical breakdown is just reached is mostly very small and difficult to observe visually. At present, it is difficult to find a suitable experimental method to determine this threshold, which is extremely unfavorable to the advancement of subsequent technologies and the development of related disciplines. Summary of the invention

[0005] In view of the above defects or improvement needs of the prior art, the present invention provides a method and device for measuring the ionization breakdown threshold of laser-liquid interaction, the purpose of which is to achieve the measurement of the ionization threshold and solve the problem that when optical breakdown occurs, the plasma formed is mostly very small and difficult to observe visually.

[0006] To achieve the above object, according to one aspect of the present invention, a method for measuring the ionization breakdown threshold of laser-liquid interaction is proposed, comprising the following steps:

[0007] Focus the pulsed laser into the transparent liquid to be tested, and then adjust the incident average power of the pulsed laser from small to large; at the same time, measure the corresponding output power after the pulsed laser is focused in the transparent liquid to be tested; because the transparent liquid is not broken down when the incident average power is small, the absorption coefficient of the transparent liquid to the pulsed laser remains unchanged, and the output power and the incident average power increase synchronously; when the pulsed laser increases to the ionization threshold, with the generation of plasma, the absorption coefficient mutates, and the output power drops suddenly, and the incident average power corresponding to the sudden drop point is the ionization threshold of the pulsed laser breaking through the transparent liquid.

[0008] As a further preferred embodiment, when adjusting the incident average power of the pulsed laser, the power value increased each time is 0.02W to 0.1W.

[0009] As a further preferred embodiment, when adjusting the incident average power of the pulsed laser, a smaller power increment is maintained in the early stage, and a larger power increment is used after the sudden drop point.

[0010] As further preferred, the pulse laser is an ultrashort pulse laser.

[0011] As a further preference, the output power is measured directly behind the focal point of the pulsed laser, and the measuring position and angle are kept unchanged during the whole process.

[0012] As a further preferred embodiment, a scatter plot is drawn with the incident average power as the horizontal axis and the output power as the vertical axis. In the scatter plot, a sudden drop point of the output power will appear as the incident average power increases. The incident average power corresponding to the sudden drop point is the ionization threshold of the pulsed laser penetrating the transparent liquid.

[0013] According to another aspect of the present invention, there is provided an ionization breakdown threshold measurement device for implementing the above-mentioned ionization breakdown threshold measurement method, comprising a laser, a focusing lens and a power meter, wherein:

[0014] The laser, focusing lens, transparent liquid to be tested and power meter are coaxially arranged in sequence; the laser is used to emit pulsed laser, the focusing lens is used to focus the pulsed laser into the transparent liquid to be tested, and the power meter is used to measure the output power of the pulsed laser after being focused in the transparent liquid to be tested.

[0015] As a further preference, the power of the laser is adjustable and readable.

[0016] As a further preference, the measuring device is enclosed in a box, and the box includes a plurality of light shielding panels assembled together.

[0017] In general, the above technical solution conceived by the present invention has the following technical advantages compared with the prior art:

[0018] 1. The present invention obtains the incident power of the pulsed laser and the corresponding output power after focusing in the transparent liquid. When the energy of the pulsed laser is sufficient to penetrate the transparent liquid and generate plasma, the absorption coefficient of the transparent liquid for the pulsed laser will increase rapidly due to the absorption effect of the plasma itself on the laser. This increase process is manifested in the scatter diagram as a sudden drop point of the output power. The sudden drop point is the ionization threshold of the pulsed laser penetrating the transparent liquid, so the ionization threshold of the pulsed laser penetrating the transparent liquid can be conveniently measured. This method is of great significance for experimentally exploring the ionization mechanism of the pulsed laser in the transparent liquid and the plasma formation mechanism.

[0019] 2. The traditional method of detecting the optical breakdown threshold is to detect whether plasma is generated. This method requires complex equipment and microscopic observation. The present invention provides a method for measuring the ionization threshold at the macroscopic level, which is more convenient, simple and intuitive.

[0020] 3. When adjusting the incident average power of the pulsed laser from small to large, the smaller the interval of the power value, the more the measurement result can be locked in a narrower range, and the accuracy of the ionization threshold is correspondingly improved. Taking into account the measurement difficulty and efficiency, the power value increased each time is determined to be 0.02W~0.1W. At the same time, keep a small power increment in the early stage of the measurement process to improve the accuracy; after the mutation point, the power increment can be appropriately expanded to improve the measurement efficiency.

[0021] 4. The present invention adopts pulsed laser, which is characterized by high peak power. Compared with continuous laser, it can achieve higher power density with lower energy, and can realize the measurement of ionization breakdown threshold; and further preferably adopts ultrashort pulse laser, whose pulse width ranges from nanosecond to femtosecond. Ultrashort pulse laser has the characteristics of high peak power and low thermal effect, so that the laser-induced ionization process can be accurately and conveniently studied.

[0022] 5. The power meter for measuring the output power of the present invention is located directly behind the focal point of the pulsed laser, and the position and angle are kept unchanged during the experiment, so as to eliminate the misjudgment caused by the measurement error of the power meter itself and ensure that the sudden drop point of the output power is captured.

[0023] 6. The pulsed laser light source provided in the present invention has adjustable power and can be read, so as to facilitate the drawing of the scatter diagram later; in addition, the entire measuring device is enclosed in a box composed of a shading plate to ensure that it is not disturbed by the surrounding space light during the power measurement process. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a structural schematic diagram of an ionization breakdown threshold measurement device according to an embodiment of the present invention;

[0025] Figure 2 It is a scatter plot drawn according to the incident average power and the output power according to an embodiment of the present invention.

[0026] In all the drawings, the same reference numerals are used to denote the same elements or structures, wherein: 1 - focusing lens, 2 - laser, 3 - pulse laser, 4 - transparent liquid to be measured, 5 - power meter, 6 - housing. DETAILED DESCRIPTION

[0027] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0028] An embodiment of the present invention provides a device for measuring the ionization breakdown threshold of laser-liquid interaction, such as Figure 1 As shown, it includes a laser 2, a focusing lens 1 and a power meter 5, wherein:

[0029] The laser 2, focusing lens 1, transparent liquid to be tested 4 and power meter 5 are coaxially arranged in sequence; the laser 2 is used to emit pulsed laser 3, the focusing lens 1 is used to focus the pulsed laser into the transparent liquid to be tested 4, and the power meter 5 is used to measure the output power of the pulsed laser after being focused in the transparent liquid to be tested 4.

[0030] The ionization breakdown threshold measurement is performed by the above-mentioned measuring device, comprising the following steps:

[0031] The pulse laser 3 emitted by the laser 2 is focused into the transparent liquid 4 to be tested through the focusing lens 1, and then the incident average power of the pulse laser 3 is adjusted from small to large, and the output power of the pulse laser 3 after being focused in the transparent liquid 4 to be tested is measured with the power meter 5. A scatter plot is drawn with the incident average power as the horizontal axis and the output power as the vertical axis. When the energy of the pulse laser 3 is sufficient to penetrate the transparent liquid 4 to be tested and generate plasma, the absorption coefficient of the transparent liquid 4 to be tested for the pulse laser will increase rapidly due to the absorption effect of the plasma itself on the laser. This increase process is manifested in the scatter plot as a sudden drop point of the output power, which is the ionization threshold of the pulse laser 3 penetrating the transparent liquid 4 to be tested.

[0032] Furthermore, the power of the laser 2 is adjustable and readable, so as to facilitate the drawing of the scatter plot later; the pulse laser 3 is preferably an ultrashort pulse laser.

[0033] Furthermore, when adjusting the incident average power of the pulsed laser, the power value increased each time was 0.02W to 0.1W; a small power increment was maintained in the early stage of the measurement process to improve the accuracy, and the power increment could be appropriately expanded after the mutation point to improve the measurement efficiency.

[0034] Furthermore, the power meter 5 is located directly behind the focal point of the pulsed laser 3 and keeps the position and angle unchanged during the experiment, so as to eliminate the misjudgment caused by the measurement error of the power meter 5 itself and ensure that the sudden drop point of the output power is captured.

[0035] Furthermore, the measuring device is enclosed in a box 6 composed of a light shielding plate to ensure that the power is not disturbed by the surrounding space light during the measurement process.

[0036] To make the present invention clearer, the principle of the measuring method of the present invention is further described below:

[0037] The absorption coefficient α of laser propagation in liquid can be expressed by the following formula:

[0038]

[0039] Where e is the basic charge, τ is the average collision time of the electron, m is the mass of the electron, ε0 is the vacuum dielectric constant, c is the speed of light in a vacuum, n is the refractive index of the liquid, ω is the circular frequency of the laser, ρ(t) is the density of free electrons, and α w is the inherent absorption coefficient of the liquid.

[0040] In the range of small incident average power, since the transparent liquid is not broken down to produce plasma, the density of free electrons is small (the threshold of free electron density is generally regarded as 10 20 / cm 3 ), at this time, only the inherent absorption of the liquid affects the laser, and the absorption coefficient of the transparent liquid to the pulsed laser remains unchanged, which is reflected in the scatter diagram as the output power is proportional to the incident power; when the pulsed laser reaches the ionization threshold, with the generation of plasma, the density of free electrons increases, the absorption coefficient suddenly increases, and the incident power and the output power no longer change linearly, which is reflected in the scatter diagram as the sudden drop point of the output power.

[0041] The following are specific embodiments:

[0042] The wavelength of the ultrashort pulse laser emitted by the laser is 1064nm, the pulse width is 16.8ps, the repetition frequency is 100kHz, the transparent liquid used is distilled water, and the output power is measured every 0.02W to 0.1W. The scatter plot based on the incident average power and the output power is shown in the figure below. Figure 2As shown, it can be seen that: since the transparent liquid is not broken down to produce plasma within the range of small incident power, the absorption coefficient of the transparent liquid to the pulsed laser remains unchanged, which is reflected in the scatter diagram as the output power is proportional to the incident power; when the pulsed laser reaches the ionization threshold, with the generation of plasma, the absorption coefficient suddenly changes, which is reflected in the scatter diagram as the sudden drop point of the output power.

[0043] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. An ionization breakdown threshold measurement device, characterized in that: The invention comprises a laser (2), a focusing lens (1) and a power meter (5), wherein: The laser (2), the focusing lens (1), the transparent liquid to be measured (4) and the power meter (5) are coaxially arranged in sequence; the laser (2) is used to emit ultrashort pulse laser light, and the power of the laser (2) is adjustable and readable; the focusing lens (1) is used to focus the pulse laser light into the transparent liquid to be measured (4), and the power meter (5) is used to measure the output power of the pulse laser light after being focused in the transparent liquid to be measured (4); the power meter (5) is located directly behind the focus point of the pulse laser light, and the measurement position and angle are kept unchanged during the whole process; the measuring device is enclosed in a box (6), and the box (6) includes a plurality of light shielding plates assembled together; When the measuring device is working, the pulse laser is focused in the transparent liquid to be measured, and then the incident average power of the pulse laser is adjusted from small to large; at the same time, the corresponding output power of the pulse laser after being focused in the transparent liquid to be measured is measured; because the transparent liquid is not broken down when the incident average power is small, the absorption coefficient of the transparent liquid to the pulse laser remains unchanged, and the output power and the incident average power increase synchronously; when the pulse laser increases to the ionization threshold, with the generation of plasma, the absorption coefficient suddenly changes, and the output power suddenly drops, and the incident average power corresponding to the sudden drop point is the ionization threshold of the pulse laser breaking through the transparent liquid.

2. The ionization breakdown threshold measurement device according to claim 1, characterized in that: When adjusting the incident average power of the pulsed laser, the power value increased each time is 0.02W to 0.1W.

3. The ionization breakdown threshold measurement device according to claim 1, characterized in that: When adjusting the incident average power of the pulsed laser, a smaller power increment is maintained in the early stage, and a larger power increment is used after the sudden drop point.

4. The ionization breakdown threshold measurement device according to any one of claims 1 to 3, characterized in that: A scatter plot is drawn with the incident average power as the horizontal axis and the output power as the vertical axis. In the scatter plot, a sudden drop point of the output power will appear as the incident average power increases. The incident average power corresponding to the sudden drop point is the ionization threshold of the pulsed laser penetrating the transparent liquid.

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