A method for realizing remote controllable femtosecond laser filament ignition using a telescope focusing system

Abstract

The present invention proposes a method for realizing remote controllable femtosecond laser filament ignition by using a telescope focusing system, which belongs to the field of laser ignition technology. The light beam emitted by a femtosecond laser amplifier is used to generate femtosecond filaments in the far field through a telescope focusing system composed of a concave lens and a convex lens and act on the premixed gas. Then, the single pulse energy of the output laser beam is adjusted to a threshold value, so that the filament ignition can be realized. The present invention uses a telescope system to focus the femtosecond Gaussian beam, which can effectively extend the ignition distance and realize long-distance stable ignition of lean mixed gas. At the same time, under the condition that the laser pulse energy is in the millijoule level, an ignition distance greater than 0.5m can be achieved. For femtosecond laser amplifiers with different maximum output energies, the telescope system is equipped with a concave lens and a convex lens with an appropriate focal length, which can maximize the ignition distance of the system.

Description

Technical Field

[0001] The invention belongs to the technical field of laser ignition, and in particular relates to a method for realizing remote controllable femtosecond laser filament ignition by utilizing a telescope focusing system. Background Art

[0002] As a non-invasive ignition method, the ignition time of laser ignition is controllable, the ignition position is flexible and changeable, and multi-point ignition can be performed. Therefore, it is widely used in premixed burners, internal combustion engines, turbines, rocket engines, scramjet engines and other fields. It can achieve rapid and effective ignition of lean-burn mixed gas, and at the same time can effectively reduce exhaust emissions, reduce heat loss, and improve combustion stability.

[0003] At present, the light source of the laser ignition system mainly adopts nanosecond laser. The ignition system generates plasma sparks in the gas by focusing high-energy laser pulses to achieve ignition. However, the ignition distance of nanosecond laser ignition is usually in the range of several centimeters to more than ten centimeters. The reason is that the minimum pulse energy in the nanosecond laser ignition process has a quadratic relationship with the ignition distance. The minimum pulse energy of ignition will increase rapidly with the increase of the ignition distance. Therefore, long-distance ignition is often difficult to achieve, which limits the application of laser ignition in some actual burners (such as large-caliber gas engines). Summary of the invention

[0004] In view of the problem of short ignition distance in the existing laser ignition technology, the present invention provides a method for realizing remote controllable femtosecond laser filament ignition by using a telescope focusing system. The method uses a telescope as a focusing system, which can effectively extend the ignition distance and realize long-distance stable ignition of lean mixed fuel gas; the method selects the best telescope lens group according to the output energy of the femtosecond laser system to obtain the maximum ignition distance and the optimal ignition effect; the method can realize stable ignition with an ignition distance greater than 0.5m when the pulse energy is in the millijoule level; the method can realize effective regulation of the ignition distance under the condition of low pulse output energy, and has great application prospects in the field of remote controllable laser ignition.

[0005] In order to achieve the above object, the present invention is implemented by the following scheme:

[0006] In the first aspect, the present invention provides a method for realizing remote controllable ignition of femtosecond laser filaments by using a telescope focusing system, wherein the telescope focusing system comprises a femtosecond laser amplifier 1, an aperture 2, a half-wave plate 3, a polarizer 4, a high-reflection mirror 5, a concave lens 6, a convex lens 7, a slide rail 8 and a standard combustion system 9. The laser beam emitted by the femtosecond laser amplifier 1 sequentially passes through the aperture 2, the half-wave plate 3, the polarizer 4, the high-reflection mirror 5, the concave lens 6, and the convex lens 7 and is focused on the standard combustion system 9. The concave lens 6 and the convex lens 7 are both mounted on the slide rail 8 and can move relatively to adjust the position of the incident laser to form the filaments; the laser beam is initially linearly polarized, and the direction of the linear polarization of the laser beam can be continuously changed by rotating the half-wave plate 3, and the polarization component in the corresponding direction is reflected by the polarizer 4 to achieve the purpose of controlling the energy of the laser beam. When the energy reaches a threshold, the filaments formed by the laser beam being focused by the telescope system can be ignited.

[0007] Furthermore, the working wavelength of the femtosecond laser amplifier 1 is 800nm, the pulse width is 40fs-60fs, the maximum repetition frequency is 1KHz, the single pulse energy of the femtosecond laser is less than 2.6mJ, and the beam diameter is 10mm.

[0008] Furthermore, the focal length of the concave lens 6 can be determined according to the single pulse energy of the femtosecond laser amplifier and the diameter of the laser beam. In the present device, the focal length of the concave lens 6 is -20 cm.

[0009] Furthermore, the focal length of the convex lens 7 can be determined according to the single pulse energy of the femtosecond laser amplifier and the focal length of the concave lens 6. In the present device, the focal length of the convex lens 7 is 20 cm or 25 cm.

[0010] Furthermore, the slide rail 8 is provided with a scale with a minimum accuracy of 1 mm, so that the distance between the concave lens 6 and the convex lens 7 can be precisely adjusted.

[0011] Furthermore, the method for realizing remote controllable femtosecond laser filament ignition using a telescope focusing system specifically comprises the following steps:

[0012] Step 1: After the standard combustion system 9 is filled with mixed gas, it is moved to a fixed position, at which time the position of the standard combustion system can still be adjusted;

[0013] Step 2: Adjust the repetition frequency of the femtosecond laser amplifier 1 to 1 kHz to obtain clearly visible light filaments;

[0014] Step 3: Adjust the distance between the concave lens 6 and the convex lens 7 on the slide rail 8 to a certain value so that the light filaments are focused into the combustion space of the standard combustion system 9;

[0015] Step 4: Rotate the half-wave plate 3 and use a power meter (Newport) to measure the laser power to achieve regulation of the energy of a single femtosecond laser pulse, which can be continuously adjusted from complete extinction (0 mJ) to 2.6 mJ. When the output energy reaches the threshold, the mixed gas can be ignited.

[0016] Step 5: Adjust the repetition frequency of the femtosecond laser amplifier 1 to 10 Hz, and a clear and continuous single-pulse laser ignition phenomenon can be observed.

[0017] Furthermore, the standard combustion system 9 in step 1 is moved away from the telescope system, and the distance between the two lenses in the telescope system in step (3) is adjusted to move the position of the light filament to the combustion space of the combustion system. Then, the half-wave plate 3 is rotated to appropriately adjust the output energy of the laser pulse (usually the energy difference is less than 1 mJ) to ignite the mixed gas, thereby achieving the purpose of extending the ignition distance. The telescope system is continued to be moved in the same direction, and the distance between the two lenses and the output energy of the laser pulse are adjusted at the same time, so as to obtain the maximum ignition distance that can be achieved by the lens group.

[0018] Compared with the prior art, the advantages of the present invention are as follows:

[0019] 1. The present invention uses a telescope system to focus femtosecond laser pulses, can flexibly adjust the position of laser filamentation, and can continuously complete laser ignition at different distances;

[0020] 2. The present invention uses a telescope system to focus femtosecond laser pulses, which can form light filaments with shortened length but higher internal plasma density, effectively breaking through the ignition limit distance achieved by a single lens;

[0021] 3. The present invention uses a telescope system to focus the femtosecond laser pulse, and effectively extends the ignition distance under the condition that the laser pulse energy is in the millijoule level;

[0022] 4. The present invention can select the best telescope lens group for different femtosecond laser amplification systems, providing a solution for optimizing the ignition effect of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following is a brief introduction to the drawings required for the specific embodiments or the description of the prior art. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn according to the actual scale.

[0024] Figure 1The present invention is a schematic diagram of a device for realizing remote controllable ignition of femtosecond laser filaments by using a telescope focusing system; in the figure, a femtosecond laser amplifier 1, an aperture 2, a half-wave plate 3, a polarizer 4, a high-reflection mirror 5, a concave lens 6, a convex lens 7, a slide rail 8, and a standard combustion system 9 are provided.

[0025] Figure 2 This is a schematic diagram of the scene of remote controllable femtosecond laser filament ignition of the present invention. DETAILED DESCRIPTION

[0026] The embodiments of the technical solution of the present invention will be described in detail below in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention and are therefore only used as examples and cannot be used to limit the protection scope of the present invention.

[0027] It should be noted that, unless otherwise specified, the technical terms or scientific terms used in this application should have the common meanings understood by those skilled in the art to which the present invention belongs.

[0028] Example 1

[0029] like Figure 1 As shown, this embodiment provides a method for realizing remote controllable ignition of femtosecond laser filaments by using a telescope focusing system. The telescope focusing system comprises a femtosecond laser amplifier 1, an aperture 2, a half-wave plate 3, a polarizer 4, a high-reflection mirror 5, a concave lens 6, a convex lens 7, a slide rail 8 and a standard combustion system 9. The laser beam emitted by the femtosecond laser amplifier 1 sequentially passes through the aperture 2, the half-wave plate 3, the polarizer 4, the high-reflection mirror 5, the concave lens 6, and the convex lens 7 and is focused on the standard combustion system 9. The concave lens 6 and the convex lens 7 are both mounted on the slide rail 8 and can move relatively to adjust the position of the incident laser to form the filament. The laser beam is initially linearly polarized. By rotating the half-wave plate 3, the direction of the linear polarization of the laser beam can be continuously changed, and the polarization component in the corresponding direction is reflected by the polarizer 4 to achieve the purpose of controlling the energy of the laser beam. When the energy reaches a threshold, the filament formed by the laser beam being focused by the telescope system can be ignited.

[0030] In this embodiment, the operating wavelength of the femtosecond laser amplifier 1 is 800nm, the pulse width is 40fs-60fs, the maximum repetition frequency is 1KHz, the maximum single pulse energy of the femtosecond laser is 2.6mJ, and the beam diameter is 10mm.

[0031] In this embodiment, the focal length of the concave lens 6 can be determined according to the single pulse energy of the femtosecond laser amplifier and the diameter of the laser beam. In this device, the focal length of the concave lens 6 is -20 cm.

[0032] In this embodiment, the focal length of the convex lens 7 can be determined according to the single-pulse energy of the femtosecond laser amplifier and the focal length of the concave lens 6. The focal length of the convex lens 7 in this device is 20 cm or 25 cm.

[0033] In this embodiment, the slide rail 8 is provided with scales with a minimum accuracy of 1 mm, which can accurately adjust the distance between the concave lens 6 and the convex lens 7.

[0034] In this embodiment, the standard combustion system 9 includes but is not limited to industrial combustion systems, standard combustion chambers, or standard premixed burners, etc., which can all achieve the adjustment of the equivalence ratio from the lean combustion zone to the rich combustion zone, and can also adjust the temperature and pressure according to requirements.

[0035] As Figure 2 shown, the distance between the concave lens 6 and the convex lens 7 is D, the ignition distance is L, and the standard combustion system used is the standard Mckenna burner 9; the lens distances at position one, position two, and position three are D1, D2, D3 (where D1 > D2 > D3), and the achievable ignition distances are L1, L2, L3 (where L1 < L2 < L3); the inner diameter of the nozzle of this standard burner is 10 mm, and different proportions of mixed gas can be introduced to achieve the adjustment of the equivalence ratio from the lean combustion zone to the rich combustion zone.

[0036] Example 2

[0037] For easy understanding of this embodiment, first, a method for realizing remotely controllable femtosecond laser filament ignition using a telescope focusing system proposed in this embodiment will be introduced in detail. The overall schematic diagram of the device used is as Figure 1 shown. First, introduce a certain proportion of mixed gas into the standard Mckenna burner 9 and adjust it to a fixed position. Adjust the distance between the two lenses to move the filament above the nozzle of the standard Mckenna burner 9, and then adjust the output energy of the laser pulse to reach the threshold to complete the laser ignition; adjust the standard burner to different positions and repeat the adjustment steps of the laser to achieve remotely controllable distance laser ignition.

[0038] A method for realizing remotely controllable femtosecond laser filament ignition using a telescope focusing system is as follows:

[0039] Step (1): Use a mass flow controller to adjust the intake ratio of fuel and air so that the equivalence ratio of the premixed gas ejected from the nozzle of the standard Mckenna burner is 0.75, and adjust the burner to a fixed position;

[0040] Step (2): Adjust the repetition frequency of the femtosecond laser amplifier to 1 KHz so that the obtained filament is clearly visible;

[0041] Step (3): select a concave lens 6 with a focal length of -20 cm and a convex lens 7 with a focal length of 20 cm, adjust the distance D1 between the concave lens 6 and the convex lens 7 on the slide rail 8 to 25 cm, and move the light filament to the top of the nozzle of the burner 9, with a height of 10 mm from the nozzle;

[0042] Step (4): Rotate the half-wave plate 3 and use a power meter (Newport) to test the laser power, and obtain a laser single pulse energy of 1.9 mJ, which is the minimum energy required for ignition at this distance, and achieves laser ignition; at this time, the ignition distance L1 measured is 37.2 cm;

[0043] Step (5): Adjust the position of the standard Mckenna burner 9 to keep it away from the telescope system;

[0044] Step (6): adjust the distance D2 between the concave lens 6 and the convex lens 7 on the slide rail 8 to 20 cm, and move the light filament to the top of the burner nozzle at a height of 10 mm;

[0045] Step (7): Rotate the half-wave plate 3 and use a power meter (Newport) to test the laser power, and obtain a laser single pulse energy of 1.8 mJ, which is the minimum energy required for ignition at this distance, and achieves laser ignition; at this time, the ignition distance L2 measured is 41.5 cm;

[0046] Step (8): Adjust the position of the standard Mckenna burner 9 so that it is away from the telescope system again;

[0047] Step (9): adjust the distance D3 between the two lenses on the slide rail 8 to 15 cm, and move the light filament to above the nozzle of the burner 9 at a height of 10 mm;

[0048] Step (10): Rotate the half-wave plate 3 and use a power meter (Newport) to test the laser power, and obtain a laser single pulse energy of 1.9 mJ, which is the minimum energy required for ignition at this distance, and achieves laser ignition; at this time, the ignition distance L3 is 48 cm, which is close to the maximum ignition distance that can be achieved by the lens group;

[0049] Step (11): Keep the focal length of the concave lens 6 at -20 cm, select the focal length of the convex lens 7 to be 25 cm, and adjust the position of the standard McKenna burner 9;

[0050] Step (12): adjust the distance D1 between the concave lens 6 and the convex lens 7 on the slide rail 8 to 35 cm, and move the light filament to above the nozzle of the burner 9 at a height of 10 mm;

[0051] Step (13): Rotate the half-wave plate 3 and use a power meter (Newport) to test the laser power, and obtain a laser single pulse energy of 2.3 mJ, which is the minimum energy required for ignition at this distance, and achieve laser ignition; at this time, the ignition distance L1 is 47.5 cm;

[0052] Step (14): Continue to adjust the position of the standard Mckenna burner 9 to keep it away from the telescope system;

[0053] Step (15): adjust the distance D2 between the concave lens 6 and the convex lens 7 on the slide rail 8 to 30 cm, and move the light filament to above the nozzle of the burner 9 at a height of 10 mm;

[0054] Step (16): Rotate the half-wave plate 3 and use a power meter (Newport) to test the laser power, and obtain a laser single pulse energy of 2.2 mJ, which is the minimum energy required for ignition at this distance, and achieve laser ignition; at this time, the ignition distance L2 is 52 cm;

[0055] Step (17): Continue to adjust the position of the standard Mckenna burner 9 so that it is away from the telescope system again;

[0056] Step (18): adjust the distance D3 between the concave lens 6 and the convex lens 7 on the slide rail 8 to 25 cm, and move the light filament to above the nozzle of the burner 9 at a height of 10 mm;

[0057] Step (19): Rotate the half-wave plate 3 and use a power meter (Newport) to test the laser power, and obtain a laser single pulse energy of 2.4 mJ, which is the minimum energy required for ignition at this distance, and achieves laser ignition; at this time, the ignition distance L3 is 58.5 cm, which is close to the maximum ignition distance that can be achieved by the lens group;

[0058] Comparative Example

[0059] A convex lens with a focal length of 40 cm was used to replace the telescope system to focus the laser beam. The light filament was moved to 10 mm above the nozzle of the standard burner. The half-wave plate 3 was rotated and the laser power was tested using a power meter (Newport). The laser single pulse energy was 2.6 mJ, which is the maximum single pulse output energy of the laser system. The mixed gas could not be ignited. This shows that this distance has reached the ignition limit distance of single lens focusing, and using a telescope system for focusing can effectively break through this boundary value.

[0060] The focal length of the lens group selected in this embodiment and the maximum ignition distance that can be achieved are determined according to the laser parameters of the femtosecond laser amplifier 1 in the device; by replacing the femtosecond laser amplification system with other laser parameters and selecting a suitable lens combination, the method according to the present invention can also achieve remote controllable femtosecond laser ignition, maximum ignition distance and optimal ignition effect; selecting a femtosecond laser amplification system with higher output pulse energy helps to extend the maximum ignition distance.

[0061] The preferred embodiments of the present invention are described in detail above in conjunction with the accompanying drawings. However, the present invention is not limited to the specific details in the above embodiments. Within the technical concept of the present invention, a variety of simple modifications can be made to the technical solution of the present invention, and these simple modifications all belong to the protection scope of the present invention.

[0062] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not further describe various possible combinations.

[0063] In addition, various implementation scenarios of the present invention, as long as they do not violate the concept of the present invention, should also be regarded as the contents disclosed by the present invention.

Claims

1. A method for remote controllable femtosecond laser filament ignition using a telescope focusing system, characterized in that: The telescope focusing system comprises a femtosecond laser amplifier (1), an aperture (2), a half-wave plate (3), a polarizing plate (4), a high-reflection mirror (5), a concave lens (6), a convex lens (7), a slide rail (8) and a standard combustion system (9). The laser beam emitted by the femtosecond laser amplifier (1) passes through the aperture (2), the half-wave plate (3), the polarizing plate (4), the high-reflection mirror (5), the concave lens (6) and the convex lens (7) in sequence and is then focused on the standard combustion system (9). The concave lens (6) and the convex lens (7) are both mounted on the slide rail (8) and can move relative to each other, thereby adjusting the position of the incident laser to form a light filament. The laser beam is initially linearly polarized. By rotating the half-wave plate (3), the direction of the linear polarization of the laser beam can be continuously changed, and the polarization component in the corresponding direction is reflected by the polarizing plate (4), so as to achieve the purpose of controlling the energy of the laser beam. When the energy reaches a threshold, the light filament formed by the laser beam being focused by the telescope system can be ignited. The femtosecond laser amplifier 1 has an operating wavelength of 800 nm, a pulse width of 40 fs-60 fs, a maximum repetition frequency of 1 KHz, a single pulse energy of the femtosecond laser of less than 2.6 mJ, and a beam diameter of 10 mm; The focal length of the concave lens 6 of the focusing system is -20 cm; The focal length of the convex lens (7) of the focusing system is 20 cm or 25 cm; The slide rail (8) is provided with a scale with a minimum accuracy of 1 mm, and the distance between the concave lens (6) and the convex lens (7) can be precisely adjusted.

2. A method for realizing remote controllable femtosecond laser filament ignition using a telescope focusing system as claimed in claim 1, characterized in that: The specific steps include: Step 1: After the standard combustion system (9) is filled with mixed gas, it is moved to a fixed position, at which point the position of the standard combustion system can still be adjusted; Step 2: adjusting the repetition frequency of the femtosecond laser amplifier (1) to 1 kHz so as to obtain clearly visible light filaments; Step 3: Adjust the distance between the concave lens (6) and the convex lens (7) on the slide rail (8) to a certain value so that the light filaments are focused into the combustion space of the standard combustion system (9); Step 4: Rotate the half-wave plate (3) and use a power meter to measure the laser power to achieve regulation of the energy of a single femtosecond laser pulse, and continuously adjust it within a range from complete extinction to 2.6 mJ; when the output energy reaches a threshold, the mixed gas can be ignited; Step 5: Adjust the repetition frequency of the femtosecond laser amplifier (1) to 10 Hz, and a clear and continuous single-pulse laser ignition phenomenon can be observed.

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