Laser device and method for improving contrast / signal-to-noise ratio of laser based on air nonlinear effect

Abstract

The invention discloses a laser device and method for improving the contrast / signal-to-noise ratio of a laser based on an air nonlinear effect, and the method employs femtosecond laser as a seed source, and widens the spectrum in a gas multi-pass cell through the self-phase modulation of air as a Kerr medium. A chirp mirror is used for compensating chirp introduced into the gas multi-pass cell, and the pulse is compressed to be close to the Fourier limit. The grating is used for diffraction light splitting, a 4-f lens system image is transmitted to a second grating for collimation, and extra dispersion is not introduced. A light barrier is placed at the confocal point of the two lenses to filter initial spectral components of the laser, and newly generated spectral components are reserved. According to the nonlinear effect principle, the time domain main peak can effectively broaden the spectrum, the pre-pulse and spontaneous radiation substrate intensity is insufficient to generate spectrum broaden, and the original spectrum is reserved, so that the main pulse can be effectively extracted by filtering out newly generated spectrum components. According to the invention, the problem that the gain is influenced by strong pre-pulse in the amplification process of the laser can be effectively solved, and high-contrast and high-gain laser output is realized.

Description

Technical Field

[0001] The present invention belongs to the field of ultra-intense laser technology, and specifically relates to a laser device and method for improving the contrast ratio / signal-to-noise ratio of a laser based on the air nonlinear effect. Background Art

[0002] The development and application research of ultra-intense and ultra-short laser technology are important frontiers in the current international laser technology field, with profound scientific and technological significance. In the development of national high-tech and interdisciplinary research, ultra-intense and ultra-short laser technology plays a key promoting role. It not only promotes the exploration and development of basic disciplines and frontier interdisciplinary disciplines, but also demonstrates an irreplaceable powerful driving force in the innovation of strategic high-tech fields such as high-brightness new-band coherent light sources, ultra-high-gradient high-energy particle accelerators, strong-field laser nuclear medicine, fusion energy, and precision measurement.

[0003] However, when using a traditional solid laser as a seed source for amplification to obtain ultra-intense and ultra-short laser, pre-pulses will appear before the main pulse. These pre-pulses will extract pump energy in advance, thus affecting the amplification efficiency and resulting in a decrease in the main peak gain. At the same time, the high gain of the pre-pulses will reduce the peak power of the overall pulse, seriously affecting the practical application of ultra-intense and ultra-short lasers.

[0004] To solve this problem, a scheme of injecting and amplifying a high-contrast seed source is usually adopted. Traditional methods for obtaining a high-contrast seed source include optical parametric amplification technology and cross-polarized wave generation technology, etc. However, the efficiency of these schemes is generally lower than 30%, and usually expensive crystals are required as nonlinear media. Summary of the Invention

[0005] To overcome the problems of the above-mentioned prior art, the present invention provides a laser device and method for improving the contrast ratio (signal-to-noise ratio) of a laser by using air. Through air nonlinear filtering technology, the seed source pulse is purified to generate a high-contrast 1053nm seed source, which can be amplified using a neodymium glass gain medium, solving the problems of low efficiency and high cost in traditional schemes, and providing reliable technical support for the amplification application of ultra-intense and ultra-short lasers.

[0006] The technical solution of the present invention is as follows:

[0007] A laser device for improving the contrast ratio / signal-to-noise ratio of a laser based on the air nonlinear effect, characterized in that it includes:

[0008] A femtosecond laser (1) for generating an initial femtosecond laser pulse;

[0009] A gas multi-pass cell (2) composed of two concave mirrors and filled with air as a Kerr medium, where the femtosecond laser pulse is reflected multiple times therein and the spectrum is broadened through the self-phase modulation effect;

[0010] A chirped mirror (3) for compensating the chirp introduced into the gas multipass cell and compressing the pulse to a near Fourier-limited pulse width;

[0011] A first grating (4) for diffractive spectroscopy of the compressed laser pulse;

[0012] A 4-f lens system including a first lens (5) and a second lens (7) for dispersion compensation image transfer of the diffractively spectroscopied light beam;

[0013] A second grating (8) with parameters matching those of the first grating for collimating the light beam transferred by the 4-f lens system;

[0014] A light baffle (6) disposed at the common focus of the first lens (5) and the second lens (7) of the 4-f lens system for filtering out the short-wavelength components of the initial laser spectrum and retaining the broadened new spectral components.

[0015] The gas multipass cell (2) has concave mirrors with a radius of curvature of 700 mm and a spacing of 1115 mm, and the femtosecond laser pulse is reflected at least 7 times within the gas multipass cell.

[0016] The chirped mirror (3) provides a dispersion compensation amount of -3200 fs 2 ~-3000 fs 2 , and the pulse width after compression is less than 50 fs.

[0017] Both the first grating (4) and the second grating (8) are transmission gratings with a ruling density of 1700 - 1800 lines / mm and an incident angle of 65° - 67°.

[0018] The first lens (5) and the second lens (7) in the 4-f lens system have a focal length of 90 - 110 mm, the light spot at the common focus of the two lenses is elongated, and the spectral components are different at different positions.

[0019] The light baffle (6) has a filtering range of 1030 - 1045 nm and a light transmission range of 1045 - 1080 nm, and the central wavelength of the new spectral components matches the gain peak of the neodymium glass gain medium.

[0020] On the other hand, the present invention also provides a method for improving the contrast / signal-to-noise ratio of a laser using air, characterized by including the following steps:

[0021] Introducing the initial pulse generated by a femtosecond laser into a gas multipass cell and broadening the spectrum to the range of 980 - 1080 nm through the self-phase modulation effect of the air Kerr medium;

[0022] The chirp introduced during the broadening process is compensated by a chirped mirror, and the pulse is compressed to a near Fourier-limited pulse width;

[0023] The compressed pulse is diffracted and spectrally dispersed by a grating pair, and is transmitted to the second grating for collimation by a 4-f lens system;

[0024] A light blocking plate is set at the confocal point of the 4-f lens system to filter out the short-wave components of the initial spectrum and retain the broadened spectral components in the range of 1045 - 1080 nm;

[0025] The filtered spectral components are output to a neodymium glass gain medium for amplification.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0027] The present invention utilizes the self-phase modulation (SPM) effect of an air Kerr medium to greatly broaden the spectrum of the main pulse (980 - 1080 nm). However, due to insufficient intensity, the prepulse and the spontaneous emission background cannot undergo significant spectral broadening and remain concentrated near the initial wavelength (1030 - 1045 nm). By precisely filtering out the short-wave components (below 1045 nm) with a light blocking plate, only the broadened long-wave spectrum (1045 - 1080 nm) is retained, thereby realizing the physical separation of the main pulse and the prepulse in the time domain.

[0028] The present invention uses air as the Kerr medium, without additional material costs. By optimizing the structure (7 reflections) and parameters (mirror curvature 700 mm, spacing 1115 mm) of the gas multi-pass cell, the self-phase modulation effect is maximized to achieve efficient spectral broadening. By adjusting the injection energy (450 μJ) and initial dispersion (negative dispersion) of the femtosecond laser, the long-wave center wavelength of the broadened spectrum can be precisely controlled.

[0029] The present invention can effectively solve the problem of prepulse loss of amplification gain during the amplification process of the light beam. It effectively realizes the output in the 1053 nm band with a high signal-to-noise ratio. Using the nonlinear filtering method, it has the advantages of adjustable wavelength, high pulse contrast, simple device, low cost, and high efficiency. Brief Description of the Drawings

[0030] Figure 1 It is a schematic diagram of a laser device based on air nonlinear effects to improve the contrast / signal-to-noise ratio of a laser according to the present invention.

[0031] Figure 2 It is the spectrum diagram after broadening. Detailed Embodiments

[0032] In order to make the objectives, technical solutions and advantages of the present invention more clear and understandable, the present invention will be further described in detail below with reference to 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 used to limit the present invention.

[0033] Embodiment:

[0034] Select a femtosecond laser with a central wavelength of 1030 nm and a long-wavelength cut-off of 1045 nm, a repetition frequency of 1 kHz, an adjustable energy output of 500 μJ, a pulse width of less than 200 fs, and an output spot of 4 mm.

[0035] As Figure 1 shown, inject the beam output by the femtosecond laser 1 into the gas multipass cell 2. The gas multipass cell is composed of two concave mirrors with a radius of curvature of 700 mm. The two concave mirrors are 1115 mm apart, and the cell is filled with air as the Kerr medium. The beam is reflected 7 times in the gas multipass cell 2, and the spectrum is broadened through the self-phase modulation effect (SPM). The broadened spectral range is 980 nm - 1080 nm, where the long-wavelength part after 1045 nm accounts for more than 70% of the overall spectrum, as Figure 2 shown. After the broadened beam is collimated by a lens (not shown in the figure), dispersion compensation is performed through the chirped mirror 3, and the compensation amount is -3200 fs 2 , so that the pulse is compressed to the near Fourier limit pulse width. The compressed beam enters the first transmission grating 4 at an incident angle of 66°. The grating line density of the first transmission grating 4 is 1740 lines / mm, realizing diffraction spectroscopy. After the spectroscopied beam is subjected to image transfer through a 4-f lens system, the beam is collimated to a Gaussian distribution by the second transmission grating 8. The grating line density of the second transmission grating 8 is the same as that of the first transmission grating 4, both being 1740 lines / mm, and the incident angles are both 66°.

[0036] The 4-f lens system consists of a first lens 5 and a second lens 7 with focal lengths of 100 mm each. A light blocking plate 6 is placed at the common focus of the first lens 5 and the second lens 7 of the 4-f lens system to cut off the short-wavelength components (below 1045 nm) of the initial spectrum, and only allow the new spectral components of 1045 nm - 1080 nm to pass through. After filtering out the initial spectral components, the remaining energy is 236 μJ, and the overall energy conversion efficiency reaches 52.4%. The main peak of the filtered spectrum is located near 1053 nm, which matches the gain peak of the neodymium glass gain medium and can be directly used for subsequent amplification. By adjusting the injection energy (450 μJ) and the initial dispersion of the femtosecond laser, the central wavelength of the long-wavelength components of the broadened spectrum can be finely adjusted to ensure its stable alignment with 1053 nm.

[0037] With the device of this embodiment, the interference of the prepulse on the gain during the laser amplification process is successfully suppressed, and the main pulse contrast is significantly improved. More than 70% of the output spectrum is concentrated in the range of 1045 nm to 1080 nm, and the energy conversion efficiency reaches 52.4%, meeting the requirements of high signal-to-noise ratio and high-gain laser output.

[0038] Compared with the traditional technology, the core innovation points and effects of the present invention are compared as follows:

[0039]

[0040]

[0041] It is easy for those skilled in the art to understand that the above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A laser device for improving the contrast ratio / signal-to-noise ratio of a laser based on the non-linear effect of air, characterized in that Comprising: A femtosecond laser (1) for generating an initial femtosecond laser pulse; A gas multi-pass cell (2) composed of two concave mirrors and filled with air as the Kerr medium inside, where the femtosecond laser pulse is reflected multiple times therein and the spectrum is broadened through the self-phase modulation effect; A chirped mirror (3) for compensating the chirp introduced in the gas multi-pass cell and compressing the pulse to a near Fourier-limited pulse width; A first grating (4) for diffractive spectroscopy of the compressed laser pulse; A 4-f lens system including a first lens (5) and a second lens (7) for dispersion compensation image transfer of the diffractively spectroscopied light beam; A second grating (8) with parameters matching those of the first grating for collimating the light beam transferred by the 4-f lens system; A light blocking plate (6) disposed at the common focus of the first lens (5) and the second lens (7) of the 4-f lens system for filtering out the short-wave components of the initial laser spectrum and retaining the broadened new spectrum components.

2. The laser device according to claim 1, characterized in that, The gas multi-pass cell (2) is composed of two concave mirrors with a radius of curvature of 700 mm and a spacing of 1115 mm, and the femtosecond laser pulse is reflected at least 7 times inside the gas multi-pass cell.

3. The laser device according to claim 1, characterized in that, The dispersion compensation amount provided by the chirped mirror (3) is -3200 fs 2 ~ -3000 fs 2 , and the pulse width after pulse compression is less than 50 fs.

4. The laser device according to claim 1, characterized in that, Both the first grating (4) and the second grating (8) are transmission gratings with a grating line density of 1700 - 1800 lines / mm and an incident angle of 65° - 67°.

5. The laser device according to claim 1, characterized in that, The focal lengths of the first lens (5) and the second lens (7) in the 4-f lens system are 90 - 110 mm, the light spot at the common focus of the two lenses is strip-shaped, and the spectral components are different at different positions.

6. The laser device according to claim 1, wherein, The light filtering range of the light blocking plate (6) is 1030 - 1045 nm, the light transmission range is 1045 - 1080 nm, and the central wavelength of the new spectrum components matches the gain peak of the neodymium glass gain medium.

7. A method for improving the contrast ratio / signal-to-noise ratio of a laser using air, characterized in that, Including the following steps: Introducing the initial pulse generated by the femtosecond laser into the gas multi-pass cell and broadening the spectrum to the range of 980 - 1080 nm through the self-phase modulation effect of the air Kerr medium; Compensating the chirp introduced during the broadening process using the chirped mirror and compressing the pulse to a near Fourier-limited pulse width; Performing diffractive spectroscopy on the compressed pulse through the grating and transferring it to the second grating for collimation using the 4-f lens system; Setting a light blocking plate at the common focus of the 4-f lens system to filter out the short-wave components of the initial spectrum and retaining the broadened spectrum components of 1045 - 1080 nm; Outputting the filtered spectral components to the neodymium glass gain medium for amplification.

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