A nonlinear frequency conversion device of a laser

Abstract

This invention relates to the field of high-power laser nonlinear frequency conversion technology, specifically to a nonlinear frequency conversion device for lasers used to generate ultra-wideband white lasers with flat spectra in the ultraviolet, visible, and near-infrared bands at the mJ level. The device is characterized by comprising a pump source, a third-order nonlinear stretcher, and a frequency multiplier. The pump source is a Ti:sapphire femtosecond laser with a center wavelength of 800 nm. The third-order nonlinear stretcher is made of fused silica glass material that produces significant third-order nonlinear effects under high-power laser interaction. This invention effectively stretches the pump source by utilizing the significant third-order nonlinear effect of the third-order nonlinear stretcher under high-power laser action. It also utilizes a chirped polarized nonlinear crystal with several wideband reciprocal lattice bands to achieve efficient second-order nonlinear optical effects, enabling the generation of ultra-continuous, spectrally flat white lasers at the mJ level covering the ultraviolet-visible-near-infrared bands.

Description

Technical Field

[0001] This invention relates to the field of high-power laser nonlinear frequency conversion technology, and in particular to a nonlinear frequency conversion device for lasers. Background Technology

[0002] Ultra-wideband laser sources possess advantages such as high brightness, high power, and wide frequency coverage, making them widely applicable in basic science, information technology, medicine, and environmental monitoring. However, the bandwidth achievable by laser sources is typically very limited. Supercontinuum generation technology is a practical method for realizing ultra-wideband spectral lasers. Currently, it is widely used in various fields of modern science, such as optical frequency combs, precision frequency metrology, pulse compression, and optical coherence tomography. Currently, the commonly used method for generating supercontinuum lasers utilizes third-order nonlinear optical effects to broaden the pump laser frequency range. Supercontinuum laser spectra are generated using high peak power pump pulses from femtosecond or picosecond lasers and third-order nonlinear optical effects in materials, including four-wave mixing, self-phase modulation, and stimulated Raman scattering. However, spectra obtained solely through third-order nonlinear effects have certain limitations. First, the spectral intensity far from the center wavelength decreases significantly, especially for supercontinuum spanning more than one octave; the intensity of the edge spectrum may be only -50 dB, or even lower than the center wavelength, resulting in very low flatness of the supercontinuum. Secondly, its output spectral broadening range is limited to the visible or infrared spectral bands, often failing to reach the short-wavelength ultraviolet band, which also limits its applications. Thirdly, supercontinuum generation based on fiber lasers has a small mode area and a high pump repetition rate, making it difficult to achieve high pulse energies in the output supercontinuum spectrum. Furthermore, the energy of the supercontinuum laser source in this scheme is mainly concentrated around the pump light, and a bandwidth of 10 dB is typically difficult to exceed one octave. It can be said that while supercontinuum lasers covering very wide bandwidths, whether in the ultraviolet-mid-infrared or ultraviolet-vacuum ultraviolet ranges, have been successfully achieved, it remains a difficult and challenging task. This third-order nonlinear effect scheme is still far from realizing the dream of generating supercontinuum white lasers with two octave bandwidths and high pulse energies in the ultraviolet-visible-infrared spectral range.

[0003] Another more popular approach to extending the laser spectral range is to utilize various second-order nonlinear optical effects, including second harmonics, sum-frequency, difference-frequency, optical parametric oscillations, and amplification processes—a series of nonlinear frequency conversion processes. The core challenge in generating laser output at the desired frequency using second-order nonlinear frequency conversion technology is solving the phase-matching problem in the nonlinear process. However, due to dispersion in nonlinear materials, phase matching is often not automatically achieved. Currently, quasi-phase-matching technology is an effective way to achieve nonlinear frequency conversion. By making the nonlinear coefficients periodic, quasi-periodic, aperiodic, or chirped periodic, additional reciprocal lattice vectors can be introduced into the nonlinear frequency conversion process to compensate for phase mismatch, thereby achieving high-performance laser frequency conversion and extension. It is worth noting that chirped periodically polarized lithium niobate nonlinear crystals, through effective design and control of the crystal structure, can possess a series of discrete reciprocal lattice vectors with large effective nonlinear coefficients. This series of discrete reciprocal lattice vectors can not only satisfy broadband quasi-phase-matched second-harmonic conversion, but also be used for the simultaneous generation of broadband second and third harmonics, and even the simultaneous generation of supercontinuum higher harmonics, thereby realizing the output of ultra-wideband visible-near-infrared supercontinuum white lasers. Unfortunately, the conversion efficiency of these schemes is still relatively low. Meanwhile, this ultra-wideband quasi-phase-matched scheme also allows for the synergistic effect of second- and third-order nonlinear effects to generate higher energy, wider bandwidth, flatter spectral profiles, and widely tunable chromaticity. However, in previous studies, using cascaded second and third harmonics to achieve higher harmonics in the short-wavelength region may result in lower conversion efficiency of the generated higher harmonics compared to schemes using only the lowest-order second harmonic. It is foreseeable that if second- and third-order nonlinear optical effects can be effectively combined, while using only the second-harmonic conversion process of the nonlinear laser crystal, a wider bandwidth and higher conversion efficiency supercontinuum laser can be generated more effectively. Such a synergistic effect is rarely reported in the literature. Summary of the Invention

[0004] To address the aforementioned issues, this invention provides a nonlinear frequency conversion device for lasers that enables centralized connection of batteries in new energy vehicles, saves space, meets the power requirements of on-board systems, and has a reliable structure.

[0005] The technical solution adopted in this invention is: a nonlinear frequency conversion device for lasers, used to generate ultrawideband white lasers with flat spectra in the ultraviolet, visible, and near-infrared bands at the mJ level, including a pump source, a third-order nonlinear stretcher, and a frequency doubler. The pump source is a Ti:sapphire femtosecond laser with a center wavelength of 800nm, an output repetition frequency of 1kHz, an average power greater than 3W, a single pulse energy of more than 3mJ, and a pulse width of 50 fs. The third-order nonlinear stretcher is made of fused silica glass material that can produce significant third-order nonlinear effects under strong laser interaction. The frequency doubler is a 5% MgO-doped chirped periodically polarized lithium niobate crystal. The 5% MgO-doped chirped periodically polarized lithium niobate crystal includes multiple cells, and the length of the multiple cells in the light propagation direction changes according to the continuous chirping change in the light propagation direction.

[0006] A further improvement to the above scheme is that the third-order nonlinear stretcher is a fused silica glass material that can produce a significant third-order nonlinear effect under strong laser interaction, and its upper and lower surfaces are parallel and polished.

[0007] A further improvement to the above scheme is that the third-order nonlinear stretcher is cylindrical in shape.

[0008] A further improvement to the above scheme is that the diameter of the third-order nonlinear stretcher is 20~30mm and the thickness is 5~12mm.

[0009] A further improvement to the above scheme is that the length of each cell in the z-direction is determined by the following formula:

[0010] Where z represents the position coordinate of a cell along the z-direction, the position coordinate being the coordinate of the cell's starting point, and the z-direction being the direction of light propagation. The polarization period required for the frequency doubling process corresponding to the center wavelength of the pump laser source. Indicates the degree of chirping.

[0011] A further improvement to the above scheme is that the numerical combination of the polarization period, chirp, and nonlinear crystal length L causes the reciprocal lattice vector of the nonlinear laser crystal to appear as several reciprocal lattice vector bands distributed at different positions.

[0012] A further improvement to the above scheme is that the several reciprocal lattice vectors correspond to the nonlinear frequency conversion process involving broadband supercontinuum lasers of different bands, and each reciprocal lattice vector can effectively compensate for the nonlinear frequency conversion process involving broadband supercontinuum lasers with continuous spectrum distribution.

[0013] A further improvement to the above scheme is that the several reciprocal lattice vectors correspond to the nonlinear frequency conversion of the second harmonic of different pump light bands, providing effective phase compensation for these nonlinear frequency conversions, exciting effective broadband second harmonic nonlinear frequency conversions, and realizing the generation of supercontinuous white light spectrum covering the ultraviolet, visible, and near-infrared bands.

[0014] A further improvement to the above scheme is that the quasi-phase-matching condition satisfied by the 5% MgO-doped chirped periodically polarized lithium niobate crystal requires that the femtosecond pulse laser be incident perpendicularly into the nonlinear crystal, the chirped periodically polarized lithium niobate crystal be z-cut, and the incident light be e-polarized.

[0015] A further improvement to the above scheme is that the length of the 5% MgO-doped chirped periodically polarized lithium niobate crystal is 10~30 mm and the thickness is 0.5~4.0 mm.

[0016] The beneficial effects of this invention are:

[0017] 1. The present invention utilizes the significant third-order nonlinear effect of a third-order nonlinear stretcher under strong laser irradiation to effectively stretch the pump source, while using a chirped polarized nonlinear crystal with several broadband reciprocal lattice bands to achieve efficient second-order nonlinear optical effects, which can generate mJ-level supercontinuum, spectrally flat white laser covering the ultraviolet-visible-near-infrared bands.

[0018] 2. The crystal of the present invention has the advantages of controllable structure, easy preparation and flexible design.

[0019] 3. The nonlinear frequency converter of the present invention has the advantages of small device size, simplified optical path, easy tuning, strong portability and wide adaptability.

[0020] 4. The nonlinear frequency conversion device of the present invention is suitable for the generation of high-energy, ultra-wideband, spectrally flat white lasers. It has high energy conversion efficiency and the output laser spectrum has the advantages of high pulse energy (>1 mJ), high peak power, high spatiotemporal coherence, good spectral collimation, wide spectral coverage (385-1080 nm), high spectral flatness (extremely wide 3 dB bandwidth, about 700 nm) and good uniformity. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of a nonlinear frequency converter in one embodiment of the present invention;

[0022] Figure 2 This is a schematic diagram of the design of a chirped polarized lithium niobate crystal in one embodiment of the present invention;

[0023] Figure 3This is a reciprocal lattice vector distribution diagram of a chirped periodically polarized lithium niobate crystal in an embodiment of the present invention, where the vertical axis represents the effective nonlinear coefficients and the horizontal axis represents the numerical values ​​of the reciprocal lattice vectors.

[0024] Figure 4 The images show the spectral distribution of a pumped near-infrared femtosecond pulsed laser in one embodiment of the present invention, as well as the spectral distribution after broadening using the third-order nonlinear optical effect of fused silica glass.

[0025] Figure 5 This is a supercontinuum white light spectral distribution diagram generated by the nonlinear frequency converter in one embodiment of the present invention using the synergistic effect of second-order and third-order nonlinear effects. Detailed Implementation

[0026] To facilitate understanding of the present invention, a more complete description will be given below with reference to the accompanying drawings. Preferred embodiments of the invention are shown in the drawings. However, the invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the invention.

[0027] It should be noted that when a component is said to be "fixed to" another component, it can be directly attached to the other component or there may be an intervening component. When a component is said to be "connected to" another component, it can be directly connected to the other component or there may be an intervening component.

[0028] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0029] like Figures 1-5 As shown, a nonlinear frequency conversion device for lasers is provided for generating ultra-wideband, spectrally flat white laser light in the mJ range. The device is characterized by comprising a pump source, a third-order nonlinear stretcher, and a frequency multiplier. The pump source is a Ti:sapphire femtosecond laser with a center wavelength of 800 nm, an output repetition rate of 1 kHz, an average power greater than 3 W, a single pulse energy greater than 3 mJ, and a pulse width of 50 fs. The third-order nonlinear stretcher is made of fused silica glass material that can produce significant third-order nonlinear effects under strong laser interaction. The frequency multiplier is a 5% MgO-doped chirped periodically polarized lithium niobate crystal. The 5% MgO-doped chirped periodically polarized lithium niobate crystal comprises multiple cells, the length of which changes according to continuous chirping along the light propagation direction. Figure 1This illustration shows the principle of generating tunable, fully coherent mJ-level supercontinuum white laser light in the ultraviolet, visible, and near-infrared bands through the synergistic action of three modules: a pump source, a third-order nonlinear stretcher, and a second-order nonlinear frequency multiplier.

[0030] In this embodiment, a chirped nonlinear lithium niobate crystal is used as the nonlinear photonic crystal for receiving and broadening the pump light through nonlinear frequency conversion, and outputting ultrawideband, supercontinuous, spectrally flat white laser light covering a bandwidth of nearly 700 nm in the ultraviolet-visible-near-infrared band. The chirped nonlinear lithium niobate crystal is based on a periodic nonlinear photonic crystal, with a suitable variation rule applied to the polarization period; that is, the length of the polarization period is continuously chirped along the light propagation direction. In this embodiment, the chirped periodically polarized lithium niobate crystal is composed of a series of cells, wherein the lengths of the negative and positive domains of each cell change simultaneously so that the length of the series of cells satisfies the chirped variation rule. Figure 2 A schematic diagram of the design of the chirped nonlinear photonic crystal in this embodiment is shown.

[0031] In this embodiment, based on practical needs, the frequency doubling process corresponding to a suitable pump light wavelength is first selected to determine the required crystal polarization period. Find suitable chirp and sample length L. According to the formula... To determine the length of each unit cell in the z-direction of the crystal Where z is the starting position coordinate of the corresponding domain structure in the z-direction. For chirping degree.

[0032] In this embodiment, the chirp degree of the chirped nonlinear lithium niobate crystal is... 6 It has a rectangular parallelepiped shape, a crystal polarization period of 38-22 mm, and a crystal thickness of 2 mm. For example... Figure 3 As shown, the reciprocal lattice vector distribution of the chirped nonlinear photonic crystal with the aforementioned structure and parameters consists of several reciprocal lattice vector bands. Each reciprocal lattice vector band can provide effective phase compensation for the second harmonic nonlinear frequency conversion process of a broadband supercontinuum pulse laser with a continuous spectrum distribution. The several reciprocal lattice vector bands can respectively correspond to the original frequency bandwidth range of the supercontinuum pulse laser, as well as the supercontinuum nonlinear frequency conversion process within the extended frequency bandwidth range.

[0033] In this embodiment, the pump light has a center wavelength range of 800 nm (output wavelength range of 750-850 nm), and outputs a near-infrared femtosecond pulsed laser with a repetition frequency of 1 kHz, an average power greater than 3 W, a single pulse energy greater than 3 mJ, and a pulse width of 50 fs. It features short pulse width, high energy, and good spot quality. The third-order nonlinear stretcher is a fused silica glass material that exhibits a significant third-order nonlinear effect under strong laser irradiation. In this example, under the strong focusing effect of the pump source, a strong third-order nonlinear effect is excited, effectively broadening the bandwidth of the pump source and enabling richer frequency components to participate in the nonlinear frequency conversion process of the frequency multiplier. Figure 4 The spectral distribution of the Ti:sapphire femtosecond pulsed laser in this embodiment is shown, as well as the broadband supercontinuum spectral distribution after passing through the third-order nonlinear stretcher used in this example. The high-intensity pump laser produces a significant third-order nonlinear optical effect broadening in the fused silica glass material used, extending the spectral width of the pump source from nearly 50 nm (3 dB bandwidth) to approximately 400-500 nm (extremely wide 3 dB bandwidth). Therefore, the spectral broadening effect of the third-order nonlinear stretcher provides richer frequency components for the second-order nonlinear effect of the frequency doubler, making the generation of broadband supercontinuum lasers possible.

[0034] Figure 5 The diagram illustrates the supercontinuous white laser output spectrum of the chirped polarized lithium niobate crystal in this embodiment under the excitation of a broadened broadband pump source. The spectral range of the output laser can cover the ultraviolet-visible to near-infrared waves (385-1080 nm), while exhibiting extremely high spectral flatness (extremely wide 3 dB bandwidth, approximately 700 nm). That is, by utilizing third-order nonlinear broadening technology and second-order nonlinear frequency conversion, a fully coherent, high pulse energy (>1 mJ), high peak power, high spatiotemporal coherence, good spectral collimation, and ultra-wideband white laser with wide spectral coverage and high flatness (385-1080 nm) can be generated in a chirped periodically polarized lithium niobate crystal.

[0035] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A nonlinear frequency conversion device for lasers, used to generate ultrawideband white lasers with flat spectra in the ultraviolet, visible, and near-infrared bands at the mJ level, characterized in that: Includes a pump source, a third-order nonlinear stretcher, and a frequency multiplier; The pump source is a Ti:sapphire femtosecond laser with a center wavelength of 800nm, an output repetition frequency of 1kHz, an average power greater than 3W, a single pulse energy of more than 3mJ, and a pulse width of 50 fs. The third-order nonlinear stretcher is an optical element made of fused silica glass with parallel and polished upper and lower surfaces. The frequency multiplier is a chirped periodically polarized lithium niobate crystal doped with 5% MgO; at the same time, a chirped periodically polarized lithium niobate crystal with several broadband reciprocated lattice bands is used to achieve a highly efficient second-order nonlinear optical effect. The 5% MgO-doped chirped periodically polarized lithium niobate crystal comprises multiple cells, the length of which changes along the light propagation direction according to a continuous chirping variation. The device broadens the spectrum of the femtosecond laser emitted by the pump source through the third-order nonlinear stretcher, and then doubles the frequency of the broadened broadband laser through the frequency multiplier to generate an ultra-wideband white laser with a flat spectrum covering the 385-1080nm band, a 3dB bandwidth of 700nm, and a single pulse energy greater than 1mJ.

2. The nonlinear frequency conversion device for lasers according to claim 1, characterized in that: The third-order nonlinear stretcher is cylindrical in shape.

3. The nonlinear frequency conversion device for lasers according to claim 1, characterized in that: The diameter of the third-order nonlinear stretcher is 20~30mm, and the thickness is 5~12mm.

4. The nonlinear frequency conversion device for lasers according to claim 1, characterized in that: The length of each cell in the z-direction is determined by the following formula: Where z represents the position coordinate of a cell along the z-direction, the position coordinate being the coordinate of the cell's starting point, and the z-direction being the direction of light propagation. The polarization period required for the frequency doubling process corresponding to the center wavelength of the pump laser source. Indicates the degree of chirping.

5. The nonlinear frequency conversion device for lasers according to claim 4, characterized in that: The numerical combination of the polarization period, chirp degree, and chirped period polarized lithium niobate crystal length L causes the reciprocal lattice vector of the nonlinear lithium niobate crystal to appear as several reciprocal lattice vector bands distributed at different positions.

6. The nonlinear frequency conversion device for lasers according to claim 5, characterized in that: The aforementioned reciprocal lattice vectors correspond to the nonlinear frequency conversion process involving broadband supercontinuum lasers in different bands, and each reciprocal lattice vector can effectively compensate for the nonlinear frequency conversion process involving broadband supercontinuum lasers with continuous spectrum distribution.

7. The nonlinear frequency conversion device for lasers according to claim 6, characterized in that: The aforementioned reciprocal lattice bands correspond to the nonlinear frequency conversion of the second harmonic of different pump light bands, providing effective phase compensation for these nonlinear frequency conversions, exciting effective broadband second harmonic nonlinear frequency conversions, and realizing the generation of supercontinuous white light spectrum covering the ultraviolet, visible, and near-infrared bands.

8. The nonlinear frequency conversion device for lasers according to claim 1, characterized in that: The quasi-phase-matching condition required for the 5% MgO-doped chirped periodically polarized lithium niobate crystal is that the femtosecond pulsed laser is incident perpendicularly into the chirped periodically polarized lithium niobate crystal, the chirped periodically polarized lithium niobate crystal is z-shaped, and the incident light is e-polarized.

9. The nonlinear frequency conversion device for lasers according to claim 1, characterized in that: The length of the 5% MgO-doped chirped periodically polarized lithium niobate crystal is 10~30 mm, and the thickness is 0.5~4.0 mm.

Images