Method for generating sub-cycle pulses based on cascaded positive and negative nonlinear phase shift modulation

Abstract

The present invention relates to a method for generating periodic pulses based on cascaded positive and negative nonlinear phase shift modulation. A negative phase shift plate utilizes a cascaded second-order nonlinear process to produce a redshift in the spectrum, while a positive second-order phase shift plate utilizes a femtosecond filament plasma to produce a blueshift in the spectrum. A first stage of positive and negative phase shift plates, both of which exhibit second-order nonlinear phase shifts, is constructed. Multiple stages are cascaded sequentially. By modulating each stage with equal phase shifts but opposite signs, the laser light passing through the multiple cascaded positive and negative phase shift plates broadens the spectrum to a supercontinuum exceeding 100 nm, with a linear chirp. This method achieves broadband coherent radiation, which is difficult to achieve with traditional methods, and can achieve a spectral bandwidth exceeding 100 nm, providing an effective approach for achieving periodic pulses of 1030 nm.

Description

Technical Field

[0001] The present invention relates to an attosecond pulse generation technology, and in particular to a method for generating periodic pulses based on cascaded positive and negative nonlinear phase shift modulation. Background Art

[0002] Periodic ultrashort laser pulses, characterized by high energy, short pulse width, and an ultra-wide spectral range, are also characterized by phase stability and a very large spectral bandwidth. They are used to generate attosecond pulses (one attosecond is 10 to the negative eighteenth power of a second), playing a crucial role in ultrafast spectroscopy and attosecond optics, and have numerous applications in processes such as high-order harmonic generation. The emergence of periodic pulses enables typical quantum systems, such as atoms, to withstand ultra-intense light field radiation without being damaged by ionization, thereby stimulating novel nonlinear processes.

[0003] The generation of period-scale femtosecond or even attosecond pulses is a critical research topic. Existing methods for compressing optical pulses include grating-pair time-domain compression, filamentation time-domain compression, and second-order nonlinear time-domain compression. However, grating-pair compression methods incur significant diffraction losses. In filamentation techniques, the output beam is affected by plasma, inevitably undergoing a blueshift (the output center wavelength shifts toward shorter wavelengths, shortening the wavelength and increasing the center wavelength frequency). This blueshift ultimately leads to uncompensated higher-order dispersion. A 2014 article on filamentation compression published in Optica [optica, 1, 400 (2014)], while deliberately reducing input energy to avoid plasma generation, also prevented the ability to achieve spectral ranges exceeding 100 nanometers at high energies, resulting in pulse widths below 5 fs. Second-order nonlinear time-domain compression, however, suffers from phase mismatch, which causes a spectral redshift during energy recirculation (the output center wavelength shifts toward the red, lengthening the wavelength and decreasing the center wavelength frequency). Therefore, ultrashort pulses below 5 fs cannot be achieved. Summary of the Invention

[0004] In response to the problems existing in the current compressed optical pulse technology in generating periodic ultrashort pulses, a method based on cascaded positive and negative nonlinear phase shift modulation to generate periodic pulses is proposed. This method effectively utilizes the parts that previous technologies deliberately avoided, and makes use of the spectral blue shift produced by plasma filaments and the spectral red shift in the cascade second-order process, so that the two compensate each other to form a linearly chirped ultra-wideband spectrum, which can ultimately achieve periodic laser pulses.

[0005] The technical solution of the present invention is: a method for generating periodic pulses based on cascaded positive and negative nonlinear phase shift modulation, wherein a negative phase shift sheet utilizes a cascaded second-order nonlinear process to generate a spectral red shift, and a positive second-order phase shift sheet utilizes a femtosecond filament plasma to generate a spectral blue shift, a first-level positive and negative phase shift sheet is composed of positive and negative phase shift sheets, both of which are second-order nonlinear phase shifts, and multiple levels are cascaded in sequence. By modulating the phase shift amount of each level to be equal and the sign to be opposite, the laser passes through the multiple-level cascaded positive and negative phase shift sheets, and the spectrum is broadened to a supercontinuum spectrum of more than 100nm, and the chirp is a linear chirp.

[0006] Furthermore, the cascaded positive and negative phase shift sheets are placed in the oscillation cavity to oscillate the incident laser back and forth to obtain an extended pulse width.

[0007] A device for generating periodic pulses based on cascaded positive and negative nonlinear phase shift modulation includes a light source section, an intracavity oscillator section, and a cascaded positive and negative second-order nonlinear modulation section. The cascaded positive and negative second-order nonlinear modulation section is formed by cascading multiple groups of positive and negative phase shift slices. The cascaded positive and negative second-order nonlinear modulation section is placed in the intracavity oscillator section. The light source section outputs ultrashort pulse laser with a pulse width less than 400fs. The ultrashort pulse laser enters the intracavity oscillator section. The multiple groups of positive and negative phase shift slices are cascaded to broaden the spectrum of the laser entering the intracavity oscillator section to a supercontinuum spectrum of more than 100nm, and the chirp is linear, thereby obtaining a periodic near-infrared pulse laser corresponding to 500as-10fs.

[0008] Preferably, the intra-cavity oscillation part includes a first concave mirror, a second concave mirror and a reflecting mirror. The focal lengths of the two concave mirrors are both 5 cm. The distance between the concave centers of the first concave mirror and the second concave mirror is 10 cm. The first concave mirror and the second concave mirror are not completely opposite each other. The angle between the extended lines of the focal points of the two concave lenses is 5 degrees. The two concave lenses form a quasi-confocal cavity structure. The laser is incident on the first concave mirror to enter the quasi-confocal cavity, and the reflecting mirror reflects the scattered light beam back into the cavity below the second concave mirror.

[0009] Preferably, the positive and negative phase shift thin slices are composed of BBO crystals and corresponding rare earth quantum dot doped fused quartz thin slices.

[0010] Preferably, the light source output is solid laser, gas laser or fiber laser.

[0011] The beneficial effects of the present invention are as follows: the present invention realizes broadband coherent radiation that is difficult to achieve with traditional means based on the method of generating periodic pulses by cascaded positive and negative nonlinear phase shift modulation, and can achieve a spectral bandwidth of more than 100nm, providing an effective way to achieve 1030nm periodic pulses. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1This is a schematic diagram of the structure of a device for generating periodic pulses based on cascaded positive and negative nonlinear phase shift modulation according to the present invention;

[0013] Figure 2 This is an optical path diagram for the method of generating periodic pulses based on cascaded positive and negative nonlinear phase shift modulation according to the present invention;

[0014] Figure 3 This is an ultra-wideband spectrum diagram obtained by the method of the present invention in the experiment. DETAILED DESCRIPTION

[0015] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0016] A method for generating periodic pulses based on cascaded positive and negative nonlinear phase shift modulation utilizes cascaded positive and negative second-order nonlinear modulation to generate periodic near-infrared laser pulses with pulse widths of 500as-10fs. The negative second-order nonlinear phase shift is generated by a negative phase-shifting thin plate using a cascaded second-order nonlinear process, resulting in a redshift in the spectrum; the positive nonlinear phase shift is generated by a positive second-order phase-shifting thin plate using a femtosecond filament plasma, resulting in a blueshift in the spectrum. Since both are second-order nonlinear phase shifts, they can be modulated to equalize the phase shifts with opposite signs. This results in symmetrical spectral broadening of the corresponding pulses to a supercontinuum spectrum, and the chirp is linear.

[0017] In practice, the spectrum of the laser can be broadened to a supercontinuum spectrum of more than 100 nm by cascading multiple positive and negative phase-shifting thin films, and the chirp is linear.

[0018] The following describes the cascaded positive and negative nonlinear phase shift modulation method for generating periodic pulses in combination with the design of the device.

[0019] Figure 1 It is a schematic diagram of the structure of the experimental device of the present invention. The incident pulse is emitted by the light source part 100. In the actual process, the light source is a laser system such as solid, gas, or optical fiber. The light source part outputs ultrashort pulse laser with a pulse width of less than 400fs. The ultrashort pulse laser enters the intracavity oscillation part 200, and the two large-sized concave mirrors 201 and 202 of the intracavity oscillation part 200 converge the light spot, and the reflector 203 makes the light beam go back and forth multiple times in the cavity. The cascaded positive and negative second-order nonlinear modulation part 300 arranged in the intracavity oscillation part 200 is composed of a plurality of groups of positive and negative phase-shifting thin films cascaded together, which broadens the spectrum of the laser entering the intracavity oscillation part 200 to a supercontinuum spectrum of more than 100nm, and the chirp is a linear chirp. This corresponds to a near-infrared pulse laser with a period of 500as-10fs. As Figure 2As shown, each stage of positive and negative phase shift sheets is composed of a positive phase shift sheet 301 and a negative phase shift sheet 302 placed at an angle, and multiple stages are cascaded in sequence. The angle can preferably be Brewster angle of incidence to purify the output polarization.

[0020] In this case, a preferred embodiment of the present invention, negative phase-shifting sheets 301 and 303 can be BBO frequency-doubling crystals, practically 50 microns thick. Positive phase-shifting sheets 302 and 304 can be rare-earth quantum dot-doped fused silica, with an effective bandgap between 1.5 times the energy of a single photon at 800 nm, and 50 microns thick. In the experiment, multiple BBO crystals and rare-earth quantum dot-doped fused silica sheets were cascaded and alternately placed together to serve as the spectrum-broadening medium.

[0021] For specific experimental optical path, see Figure 2 . Figure 2 In the embodiment shown, the focal lengths of concave mirror 201 and concave mirror 202 are both 5 cm. The distance between the centers of the concave surfaces of concave mirror 201 and concave mirror 202 is 10 cm. Concave mirror 201 and concave mirror 202 are not completely opposite each other, and the angle between the extended lines of the focal points of the two concave mirrors is 5 degrees. The cavity formed by concave mirror 201 and concave mirror 202, since the focal lengths of the two concave mirrors are 5 cm and the distance between them is 10 cm, constitutes a quasi-confocal cavity structure, with the beam waist in the cavity being at the midpoint of the cavity length (the cavity length refers to the line connecting the centers of the two concave surfaces). The corresponding input laser single pulse energy is 2 mJ, the pulse width is 200 fs, the central wavelength is 800 nm, and the spectral width is 50 nm. After an incident light beam strikes the concave surface of concave mirror 201 from below, it enters the cavity and begins to oscillate. After passing through multiple sets of positive and negative phase-shifting thin films, it strikes concave mirror 202 and is scattered. It is then reflected back into the cavity by reflector 203 below concave mirror 202. It then passes through a combination of positive and negative phase-shifting thin films composed of multiple BBO crystals and fused silica thin films doped with corresponding rare earth quantum dots, returning to concave mirror 201. After this reciprocating oscillation, the light is emitted from concave mirror 202 and onto reflector 203. The light is then reflected again by reflector 203, returning to the cavity to oscillate back and forth again. After several rounds of oscillation, the light is output from above the incident light. During the specific optical path construction process, because the two concave mirrors are not completely aligned, the angle of exit is approximately 1 degree greater than the angle of incidence. During the oscillation process, the output beam separates from the incident beam. After multiple oscillations, the output light can be guided out.

[0022] Since the spectral redshift and blueshift caused by the positive and negative phase-shifting sheets are both second-order nonlinear phase shifts, they can be modulated to make their phase shifts equal and opposite in sign, forming a linearly chirped ultra-wideband spectrum. Therefore, after passing through multiple sets of cascaded positive and negative phase-shifting sheets, a spectral bandwidth of more than 100nm can be achieved. Figure 3In this experiment, the above-mentioned technique was used to obtain an ultra-wideband spectrum covering the 400nm-1000nm range with a central wavelength of 1030nm. This spectrum was then compressed using a chirped mirror, ultimately achieving a 5fs period-scale pulsed laser, approximately three optical cycles. Ytterbium-doped laser crystals near 1030nm are subject to higher thermal damage than titanium sapphire. However, achieving broadband coherent radiation is difficult with conventional methods. The present method provides an effective approach for achieving 1030nm period-scale pulses.

[0023] In traditional filamentation technology, the emitted light beam is affected by the plasma, inevitably producing a blue shift. This blue shift ultimately leads to uncompensated high-order dispersion, so the pulse width cannot reach below 5 fs (for 800 nm light). Second-order nonlinear time-domain compression technology, however, cannot achieve ultrashort pulses below 5 fs (for 800 nm light) due to phase mismatch, which causes a spectral red shift during energy reflux. This invention allows the two to compensate for each other, forming a linearly chirped ultra-wideband spectrum, ultimately achieving period-scale laser pulses.

[0024] It should be emphasized that although the components used in the present invention are a combination of current technologies, it effectively utilizes a combination of thinking and structure that previous technologies deliberately avoided. Both the spectral blue shift generated by plasma and the spectral red shift in the cascade second-order process are avoided by previous technologies. However, the reverse thinking of the present invention allows the two to compensate for each other, forming a linearly chirped ultra-wideband spectrum, and ultimately achieving cycle-level laser pulses. For 800nm near-infrared light, it is expected to achieve single-light cycle compression (2.66fs); for 200nm ultraviolet light, it is expected to achieve a pulse width of 660as.

[0025] like Figure 2 As shown, each set of positive and negative phase-shifting flakes is evenly and symmetrically arranged between concave mirror 201 and concave mirror 202. During the compensation process, the multi-order phase shifts contained within the incident pulse are first measured, and the shift amounts are incorporated into the material's high-order nonlinear equations. For example, when using barium metaborate crystals as cascaded negative phase-shifting flakes, the actual parameters are substituted into the following equation:

[0026]

[0027] For the above formula is the value of the cascaded second-order nonlinear shift. n1 and n2 are the linear and nonlinear refractive indices of the material respectively. E is the electric field intensity of the phase shift plate in the cavity at this time, c is the speed of light, d eff is the effective frequency conversion coefficient in the actual process. In the calculation process, let the second-order nonlinearity shift to Equal to the phase shift to be compensated in the previous step, the parameters are substituted into the calculated crystal length L. L is the required length of the phase shift plate in the cavity at this point. The phase compensation of each stage of the phase shift plate in each direction is then calculated sequentially to ensure that linear chirp is added to the spectrum.

[0028] The above-described embodiments merely illustrate several implementations of the present invention, and while their descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be determined by the appended claims.

Claims

1. A device for generating periodic pulses based on cascaded positive and negative nonlinear phase shift modulation, characterized in that: It includes a light source part, an intracavity oscillator part and a cascaded positive and negative second-order nonlinear modulation part. The cascaded positive and negative second-order nonlinear modulation part is composed of a plurality of groups of positive and negative phase-shifting thin slices cascaded together. The cascaded positive and negative second-order nonlinear modulation part is placed in the intracavity oscillator part. The light source part outputs an ultrashort pulse laser with a pulse width less than 400fs. The ultrashort pulse laser enters the intracavity oscillator part. The cascaded plurality of groups of positive and negative phase-shifting thin slices will broaden the spectrum of the laser entering the intracavity oscillator part to a supercontinuum spectrum of more than 100nm, and the chirp is a linear chirp, thereby obtaining a near-infrared pulse laser corresponding to a period of 500as-10fs.

2. The device for generating periodic pulses based on cascaded positive and negative nonlinear phase shift modulation according to claim 1, characterized in that: The intracavity oscillation part includes a first concave mirror, a second concave mirror and a reflecting mirror. The focal lengths of the two concave mirrors are both 5 cm. The distance between the concave centers of the first concave mirror and the second concave mirror is 10 cm. The first concave mirror and the second concave mirror are not completely opposite each other. The angle between the extended lines of the focal points of the two concave lenses is 5 degrees. The two concave lenses form a quasi-confocal cavity structure. The laser is incident on the first concave mirror to enter the quasi-confocal cavity, and the reflecting mirror below the second concave mirror reflects the scattered light beam back into the cavity.

3. The device for generating periodic pulses based on cascaded positive and negative nonlinear phase shift modulation according to claim 1 or 2, characterized in that: The positive and negative phase shift thin slices are composed of BBO crystals and corresponding rare earth quantum dot doped fused quartz thin slices.

4. The device for generating periodic pulses based on cascaded positive and negative nonlinear phase shift modulation according to claim 3, characterized in that: The light source outputs solid laser, gas laser or fiber laser.

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