An optical parametric chirped pulse amplification method based on field mapping pump beam shaping

Abstract

The application discloses an optical parametric chirped pulse amplification method based on field mapping pump beam shaping, wherein a Gaussian light beam output by a pump source is input into a refractive index shaping system to perform flat-top light beam shaping, and a flat-top pump light beam is obtained; a stretcher is used to perform time expansion on signal light, and expanded signal light is obtained; the flat-top pump light beam and the expanded signal light are simultaneously incident on a same position of a nonlinear crystal to perform optical parametric amplification, and amplified signal light is obtained; the pump light beam is shaped from a Gaussian type into a flat-top type through the refractive index shaping system, so that the signal light obtains uniform gain at different spatial positions, and conversion efficiency of the pump light to the signal light is improved; in addition, energy of output light obtained through the refractive index shaping system is almost not lost relative to input light, and the technical problems of low optical parametric chirped pulse amplification efficiency and low pulse space-time quality of the pump light beam in the prior art are solved.

Description

Technical Field

[0001] The invention belongs to the field of ultrafast optics and relates to a pulse amplification method, in particular to an optical parametric chirped pulse amplification method based on field mapping pump beam shaping. Background Art

[0002] Near-mid-infrared, short-cycle, high-power femtosecond pulses are an excellent driving source and have important applications in many ultrafast dynamics experiments, such as laser wake acceleration, high-harmonic generation, and coherent diffraction imaging.

[0003] Optical parametric chirped pulse amplification (OPC) is an effective approach for generating these high-power femtosecond pulses. Nonlinear crystals used for OPCCA typically provide ultra-broadband gain to support the generation of short-cycle pulses. Furthermore, the amplification process is independent of thermal effects, allowing the crystal to withstand high-intensity pump light. The OPA process involves energy conversion between the pump and signal beams at a specific point in space and time. The spatiotemporal shape of the pump beam directly influences the overall amplification process. Common pump source outputs have a Gaussian shape in both time and space. Due to the varying amplification at different locations during the OPC process, the gain of the signal beam is higher in the center and lower at the edges, significantly impacting the conversion efficiency of the pump beam to the signal beam and introducing spatiotemporal coupling effects to the amplified signal beam. A spatially flat-top pump beam not only uniformizes the gain of the signal beam, but also mitigates the adverse effects of spatial walk-off effects and reduces spatiotemporal coupling. Therefore, the spatiotemporal shape of the pump beam, especially a flat-top pump beam, is crucial for improving the conversion efficiency and spatiotemporal quality of the OPA process.

[0004] At present, pump beam shaping is mainly achieved by the following methods: (1) using a liquid crystal spatial modulator to control the phase of the incident beam through the birefringence of the liquid crystal. However, due to the low damage threshold of the liquid crystal material, this method is only suitable for pulses with low peak power; (2) using a sawtooth aperture cutter, this method is suitable for high-energy, low-repetition-rate pulses, but its pulse utilization rate is low (about 10%); (3) using a frequency doubling method to consume the strongest part of the beam intensity distribution, and the remaining beam is roughly flat-top, but limited by the conversion efficiency of the second harmonic, the beam utilization rate is only about 50%; (4) using a diffraction phase plate method, the energy utilization efficiency of the shaping system can reach 95%, but because the shape of the flat-top beam after shaping varies greatly with the propagation distance, it can only remain flat-top within a small propagation range, which to a certain extent limits the design of the optical system. On the other hand, the light intensity modulation during the shaping process can easily cause damage to the components. Summary of the Invention

[0005] In view of the shortcomings of the existing technology, the purpose of the present invention is to provide an optical parametric chirped pulse amplification method based on field mapping pump beam shaping to solve the technical problems of low optical parametric chirped pulse amplification efficiency and low spatiotemporal quality of pulses in the existing technology.

[0006] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0007] A method for optical parametric chirped pulse amplification based on field mapping pump beam shaping, the method comprising the following steps:

[0008] Step 1: Input the Gaussian beam output by the pump source into the refractive index shaping system for flat-top beam shaping to obtain a flat-top pump beam;

[0009] The refractive index shaping system includes a first aspheric mirror and a second aspheric mirror arranged in sequence;

[0010] The surface shape function of the first aspheric mirror and the surface shape function of the second aspheric mirror are shown in formula (1) and formula (2):

[0011]

[0012]

[0013]

[0014]

[0015] in:

[0016] z(r) represents the surface function of the first aspheric mirror;

[0017] Z(R) represents the surface function of the second aspheric mirror;

[0018] r represents the input beam height;

[0019] R represents the output beam height;

[0020] d represents the distance between the two aspheric mirror vertices;

[0021] n represents the refractive index of the aspheric mirror material;

[0022] h(x) represents the light mapping function related to the shape and size of the input beam;

[0023] h -1 (x) represents the light mapping function related to the output beam shape and size;

[0024] R FL Indicates the half-width of the output beam;

[0025] ω0 represents the radius of the input beam when the light intensity drops to 1 / e2;

[0026] q represents the order of the output beam shape parameter;

[0027] Step 2: Using a stretcher to temporally stretch the signal light to obtain a stretched signal light; the stretching amount required for the signal light matches the pulse time of the flat-top pump beam;

[0028] Step 3: The flat-top pump beam and the broadened signal light are simultaneously incident on the same position of the nonlinear crystal to perform optical parametric amplification to obtain the amplified signal light;

[0029] The angle between the flat-top pump beam and the broadened signal light is an acute angle α;

[0030] Step 4: Use a compressor to compress the amplified signal light in time to obtain a pulse.

[0031] The present invention also includes the following technical features:

[0032] The acute angle α has a value range of 0 to 3°.

[0033] The stretcher is made of a block material, a grating or a prism with dispersion properties.

[0034] A plurality of reflectors are arranged between the refractive index shaping system and the nonlinear crystal.

[0035] Compared with the prior art, the present invention has the following beneficial technical effects:

[0036] (I) In the present invention, a refractive index shaping system composed of two aspheric mirrors is used to shape the pump beam from a Gaussian type to a flat-top type, so that during the optical parametric amplification process, the signal light obtains uniform gain at different spatial positions, thereby improving the conversion efficiency of pump light to signal light and greatly suppressing the spatiotemporal coupling effect. In addition, the energy of the output beam obtained by the refractive index shaping system is almost lossless relative to the input beam, and the output beam is collimated, with small wavefront distortion and pulse front distortion, thus solving the technical problems of low optical parametric chirped pulse amplification efficiency and low spatiotemporal quality of pulses of the pump beam in the prior art.

[0037] (II) The compressor in the present invention compensates for the dispersion caused by the entire system, such as the stretcher amplification process. Accurate dispersion compensation is beneficial to obtaining shorter pulses.

[0038] (III) The progress of the present invention in improving the conversion efficiency of the optical parametric chirped pulse amplification process and the spatiotemporal quality of the amplified signal light is of great significance and practicality for the development of high-power optical parametric chirped pulse amplification in the near-infrared and mid-infrared bands. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 Schematic diagram of the device structure of the present invention;

[0040] Figure 2 Schematic diagram of the evolution of the signal light energy after optical parametric chirped pulse amplification along with the crystal;

[0041] Figure 3 Schematic diagram of the spatiotemporal distribution of the compressed signal light, where the left side is Gaussian beam pumping and the right side is flat-top beam pumping.

[0042] The specific contents of the present invention are further explained in detail below with reference to the embodiments. DETAILED DESCRIPTION

[0043] It should be noted that, unless otherwise specified, all components in the present invention are components known in the art.

[0044] Specific embodiments of the present invention are given below. It should be noted that the present invention is not limited to the following specific embodiments, and all equivalent modifications made on the basis of the technical solution of this application fall within the protection scope of the present invention.

[0045] A method for optical parametric chirped pulse amplification based on field mapping pump beam shaping, the method comprising the following steps:

[0046] Step 1: Input the Gaussian beam output by the pump source into the refractive index shaping system for flat-top beam shaping to obtain a flat-top pump beam;

[0047] The refractive index shaping system includes a first aspheric mirror and a second aspheric mirror arranged in sequence;

[0048] The surface shape function of the first aspheric mirror and the surface shape function of the second aspheric mirror are shown in formula (1) and formula (2):

[0049]

[0050]

[0051]

[0052]

[0053] in:

[0054] z(r) represents the surface function of the first aspheric mirror;

[0055] Z(R) represents the surface function of the second aspheric mirror;

[0056] r represents the input beam height;

[0057] R represents the output beam height;

[0058] d represents the distance between the two aspheric mirror vertices;

[0059] n represents the refractive index of the aspheric mirror material;

[0060] h(x) represents the light mapping function related to the shape and size of the input beam;

[0061] h -1 (x) represents the light mapping function related to the output beam shape and size;

[0062] R FL Indicates the half-width of the output beam;

[0063] ω0 represents the radius of the input beam when the light intensity drops to 1 / e2;

[0064] q represents the order of the output beam shape parameter;

[0065] Step 2: Using a stretcher to temporally stretch the signal light to obtain stretched signal light;

[0066] The required broadening amount of the signal light is matched to the pulse duration of the flat-top pump beam;

[0067] Step 3: The flat-top pump beam and the broadened signal light are simultaneously incident on the same position of the nonlinear crystal to perform optical parametric amplification to obtain the amplified signal light;

[0068] The angle between the flat-top pump beam and the broadened signal light is an acute angle α;

[0069] Step 4: Use a compressor to compress the amplified signal light in time to obtain a pulse.

[0070] In the above technical solution, a refractive index shaping system composed of two aspheric mirrors is used to shape the pump beam from a Gaussian type to a flat-top type, so that during the optical parametric amplification process, the signal light obtains uniform gain at different spatial positions, thereby improving the conversion efficiency of pump light to signal light and greatly suppressing the spatiotemporal coupling effect. In addition, the energy of the output beam obtained by the refractive index shaping system has almost no loss relative to the input beam, and the output beam is collimated, with small wavefront distortion and pulse pre-distortion, which solves the technical problems of low optical parametric chirped pulse amplification efficiency and low spatiotemporal quality of pulses of the pump beam in the prior art.

[0071] In addition, the compressor compensates for the dispersion caused by the entire system, such as the stretcher amplification process. Accurate dispersion compensation is beneficial for obtaining shorter pulses.

[0072] Specifically, the acute angle α ranges from 0 to 3°.

[0073] Specifically, the stretcher is a block material, a grating or a prism with dispersive properties.

[0074] Specifically, a plurality of reflectors are provided between the refractive index shaping system and the nonlinear crystal to adjust the angle between the flat-top pump beam and the broadened signal light.

[0075] Example:

[0076] This embodiment provides a method for optical parametric chirped pulse amplification based on field-mapped pump beam shaping. A Yb:YAG laser with a central wavelength of 1030 nm, 50 mJ, and 6 ps is used as the pump source. Its Gaussian output spot diameter is 20 mm (1 / e²). After passing through a refractive index mapping shaping system, a flat-top beam with a full-width at half maximum (FWHM) of 10 mm is obtained. After the signal light is stretched to time-match the pump light, the flat-top pump beam and the stretched signal light are simultaneously injected into the same position of an LN crystal for optical parametric amplification. The included angle between the two is 0.2°, with a phase matching angle of 45.1°, indicating first-class phase matching. The amplified signal light is compressed to produce high-efficiency pulses with high temporal and spatial quality.

[0077] In order to highlight the effect of spatial flat-top pump beam on optical parametric chirped pulse amplification, an unshaped Gaussian pump beam was also used for amplification under the same conditions as the flat-top pump beam mentioned above. Figure 2 and Figure 3 As shown in the figure, the conversion efficiency of optical parametric chirped pulse amplification after pump beam shaping is increased from 22% to 36%. In addition, according to currently available data, this conversion efficiency is the highest efficiency that can be achieved for 2μm optical parametric chirped pulse amplification. At the same time, the spatiotemporal distribution after compression is more uniform, and pulses with better spatiotemporal quality and shorter duration can be obtained.

Claims

1. A method for optical parametric chirped pulse amplification based on field mapping pump beam shaping, characterized in that: The method comprises the following steps: Step 1: Input the Gaussian beam output by the pump source into the refractive index shaping system for flat-top beam shaping to obtain a flat-top pump beam; The refractive index shaping system includes a first aspheric mirror and a second aspheric mirror arranged in sequence; The surface shape function of the first aspheric mirror and the surface shape function of the second aspheric mirror are shown in formula (1) and formula (2): in: z(r) represents the surface function of the first aspheric mirror; Z(R) represents the surface function of the second aspheric mirror; r represents the input beam height; R represents the output beam height; d represents the distance between the two aspheric mirror vertices; n represents the refractive index of the aspheric mirror material; h(x) represents the light mapping function related to the shape and size of the input beam; h -1 (x) represents the light mapping function related to the output beam shape and size; R FL Indicates the half-width of the output beam; ω0 represents the radius of the input beam when the light intensity drops to 1 / e2; q represents the order of the output beam shape parameter; Step 2: Using a stretcher to temporally stretch the signal light to obtain stretched signal light; The required broadening amount of the signal light matches the pulse time of the flat-top pump beam; Step 3: The flat-top pump beam and the broadened signal light are simultaneously incident on the same position of the nonlinear crystal to perform optical parametric amplification to obtain the amplified signal light; The angle between the flat-top pump beam and the broadened signal light is an acute angle α; Step 4: Use a compressor to compress the amplified signal light in time to obtain a pulse.

2. The optical parametric chirped pulse amplification method based on field mapping pump beam shaping according to claim 1, characterized in that: The acute angle α has a value range of 0 to 3°.

3. The optical parametric chirped pulse amplification method based on field mapping pump beam shaping according to claim 1, characterized in that: The stretcher is made of a block material, a grating or a prism with dispersion properties.

4. The optical parametric chirped pulse amplification method based on field mapping pump beam shaping according to claim 1, wherein: A plurality of reflectors are arranged between the refractive index shaping system and the nonlinear crystal.

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