Ultrafast laser multi-pulse sequence preparation device and method

Abstract

The invention belongs to the technical field of laser pulse modulation, and particularly relates to an ultrafast laser multi-pulse sequence preparation device and method. According to the invention, ultrafast pulse laser is divided into a transmission light path and a reflection light path through a beam splitter, a spatial light modulator is utilized to perform programming control on a target pulse in the transmission light path, and amplitude and pulse width parameters can be controlled while multiple pulse strings are generated; and the light path of the reflection light path is controlled through the high-precision one-dimensional displacement table, so that the time delay of a reflection light path pulse relative to a transmission light path pulse is controlled. And after the reflected light path and the transmission path subjected to pulse shaping are combined again, a periodic pulse string sequence with controllable time delay, pulse number, pulse width parameter and pulse intensity is realized. The problem that high-precision multi-pulse dynamic programming cannot be achieved through a traditional adjusting mode is solved, and the problems that in the prior art, the number of excitation pulses and pulse width parameters cannot be controlled at the same time, and an ordered periodic pulse sequence cannot be generated are solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of laser pulse modulation, and particularly relates to an apparatus and method for preparing an ultrafast laser multi-pulse sequence. Background Art

[0002] With its characteristics of high peak power, ultrashort action time, and significant nonlinear effects, the ultrafast laser technology demonstrates unique advantages in the fields of precision microfabrication, nonlinear optical imaging, ultrafast spectroscopy analysis, and quantum precision measurement. In terms of the control of double-pulse (multi-pulse) sequences, the spatio-temporal selective control of ultrafast lasers can be achieved by precisely controlling the pulse interval, energy ratio, and phase relationship.

[0003] Currently, three main methods are adopted for the preparation technology of ultrafast laser multi-pulse sequences: (1) The passive beam splitting method based on a spectroscopic optical path, that is, a single pulse is split into multiple pulses by a polarization beam splitter or a wavelength division multiplexer and then the time sequence is reorganized. Although this method can achieve pulse splitting, there are problems such as low energy utilization efficiency (typical loss > 30%) and limited time sequence adjustment accuracy (adjustment step > 100 fs); (2) The electro-optic / acousto-optic modulation method based on active modulation, which uses the change of the crystal refractive index to achieve pulse splitting. Although GHz-level repetition frequency modulation can be achieved, it is limited by the modulator bandwidth (usually < 40 GHz) and the group velocity mismatch effect, and waveform distortion is easily caused when regulating femtosecond-level pulses (typical pulse width broadening > 20%); (3) The pulse splitting technology based on nonlinear effects, such as self-phase modulation-induced spectral splitting combined with dispersion compensation. Although sub-cycle pulse intervals can be obtained, the system stability is significantly affected by environmental disturbances (temperature drift sensitivity reaches 0.5 fs / °C), and the energy distribution uniformity is poor (fluctuation amplitude > 15%).

[0004] There are generally three major bottlenecks in the existing technologies: First, the flexibility of pulse time sequence control is insufficient, and the traditional mechanical adjustment method is difficult to achieve an interval accuracy of < 10 fs and multi-pulse dynamic programming; second, the controllability of multi-pulse energy ratio is poor, and the energy non-uniformity of the existing spectroscopic schemes exceeds ±25% when N ≥ 3 pulses; third, the system complexity and cost increase exponentially. The multi-channel independent control scheme requires multiple synchronization modules, resulting in a large device volume and high maintenance costs. These defects severely restrict the industrial application of ultrafast multi-pulse technologies in emerging fields such as quantum precision measurement, precision manufacturing and micro-nano processing, biomedicine and cell engineering, new energy and semiconductor materials. Summary of the Invention

[0005] The present invention provides an ultrafast laser multi-pulse sequence preparation device and method, which simultaneously have functions of multi-pulse generation, pulse width and amplitude control, and time delay control of the pulse sequence. The present invention performs periodic modulation on ultrafast laser pulses, and can generate a periodic pulse train sequence including any multi-pulses and single pulses. Parameters such as the pulse width and amplitude of the multi-pulses can be programmed and controlled by a reflective liquid crystal spatial light modulator, and the time delay between the multi-pulses and the single pulse can be controlled by a high-precision one-dimensional displacement stage.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] The present invention provides an ultrafast laser multi-pulse sequence preparation device, including a first beam splitter. After passing through the first beam splitter, the ultrafast pulsed laser is divided into two beams with equal intensity and different directions, namely a reflected beam and a transmitted beam. The transmitted beam sequentially passes through a first reflector, a second reflector, a grating, a concave mirror, a third reflector and reaches a reflective liquid crystal spatial light modulator. The reflective liquid crystal spatial light modulator completes the modulation of the ultrafast pulsed laser. The modulated beam passes through the third reflector, the concave mirror, the grating, the second reflector, the fourth reflector, the fifth reflector and reaches a second beam splitter. The reflected beam sequentially passes through a sixth reflector and a seventh reflector, then returns to the first beam splitter along the original path, passes through the first beam splitter and reaches the second beam splitter after passing through an eighth reflector. The seventh reflector is installed on a high-precision one-dimensional displacement stage.

[0008] Further, the transmitted beam realizes the control of various beam parameters through the reflective liquid crystal spatial light modulator. The reflected beam controls the optical path difference through the seventh reflector installed on the high-precision one-dimensional displacement stage, thereby controlling the pulse delay time. The total optical path between the reflected beam and the transmitted beam to the second beam splitter is basically equal, so that the same pulse in the beams separated by the first beam splitter is recombined at the second beam splitter to form a pulse sequence.

[0009] Further, the second reflector is a pair of reflectors closely placed one above the other, and the angles of this pair of reflectors are different. By controlling the pitch of the third reflector and the reflective liquid crystal spatial light modulator, the transmitted beam and the returned modulated beam are separated vertically in space. The incident unmodulated beam is adjusted by the lower reflector, and the returned modulated beam is adjusted by the upper reflector, realizing a non-return shaping optical path.

[0010] Further, after the transmitted beam is dispersed in space according to the wavelength by the grating and then processed by the concave mirror and converged onto the liquid crystal panel of the reflective liquid crystal spatial light modulator to form a long and narrow light spot, by applying different phase parameters to the adjustable region on the liquid crystal panel of the reflective liquid crystal spatial light modulator, the modulation of the number of laser pulses, pulse width and amplitude is completed.

[0011] Furthermore, the minimum displacement travel of the high-precision one-dimensional displacement stage is 0.05 μm, and the maximum moving travel is 30 cm. The precise control of the pulse time delay is achieved by adjusting the optical path change amount of the high-precision one-dimensional displacement stage, and the range of the pulse time delay is from 0.33 fs to 2 ns.

[0012] Furthermore, the first mirror, the second mirror, the third mirror, the fourth mirror, the fifth mirror, the sixth mirror, the seventh mirror and the eighth mirror are all femtosecond-optimized silver mirrors, the first beam splitter and the second beam splitter are both 5:5 beam splitters, and the focal length of the concave mirror is 250 mm.

[0013] The present invention also provides a method for preparing an ultrafast laser multi-pulse sequence, comprising the following steps:

[0014] Step 1, set up the ultrafast laser multi-pulse sequence preparation device, and after calibrating the ultrafast pulsed laser, inject it into the device;

[0015] Step 2, the transmitted light path after passing through the first beam splitter is spatially dispersed by a grating according to the wavelength to form a strip-shaped chromatic light, which is reflected by the concave mirror and mapped to the response area of the reflective liquid crystal spatial light modulator;

[0016] Step 3, an ultrafast pulsed laser is expressed as the sum of monochromatic light components, and the expression is:

[0017]

[0018] In the formula, ω k is the frequency of each monochromatic light, A k is the amplitude of the k-th order light, is the phase. Since the ultrafast laser pulse can be considered periodic, the value of the phase modulation can be shifted to between 0 and 2π;

[0019] Apply different phase parameters at different positions on the response area of the reflective liquid crystal spatial light modulator, so that the ultrafast pulsed laser is divided into multiple sub-pulses. The pulse width of each sub-pulse depends on the phase loading value of the corresponding modulation area, and the amplitude of each sub-pulse is achieved by changing the size of the modulation area of the reflective liquid crystal spatial light modulator corresponding to the sub-pulse;

[0020] It is also necessary to correct the set time interval, and the corrected linear phase modulation is written as:

[0021]

[0022] In the formula, △t is the time interval between the sub-pulse and the original pulse, and τ is the time shift parameter generated by the dispersion when canceling the original pulse;

[0023] The pulse train generated by the reflective liquid crystal spatial light modulator returns to the second beam splitter;

[0024] Step 4: The reflected light beam after passing through the first beam splitter is reflected by the mirror placed on the high-precision one-dimensional displacement stage, and the optical path is controlled by the high-precision one-dimensional displacement stage, thereby changing the time delay of the reflected light beam relative to the transmitted light beam;

[0025] Step 5: The reflected light beam and the transmitted light beam modulated by the reflective liquid crystal spatial light modulator are combined at the second beam splitter to form a pulse train sequence.

[0026] Furthermore, since the envelope shape of the ultrafast laser pulse generally follows a Gaussian distribution, the ultrafast pulsed laser is expressed as:

[0027] E(t) = exp{-mt 2 + i[2πω0t + (2πω0nt) 2}

[0028] In the formula, ω0 is the spectral center frequency, and m and n are the parameters of the time dispersion, pulse width, and spectral amplitude of the ultrafast pulsed laser.

[0029] Compared with the prior art, the present invention has the following advantages:

[0030] 1. Compared with the traditional mechanical adjustment method, the present invention can achieve high-precision multi-pulse dynamic programming control based on the reflective liquid crystal spatial light modulator, improving the controllability of laser energy distribution.

[0031] 2. Compared with the traditional method of splitting pulses by using the refractive index of crystals, the device and method of the present invention mostly adopt a reflective optical path, which can effectively avoid waveform distortions such as pulse broadening.

[0032] 3. Compared with the pulse splitting technology based on nonlinear effects, the device and method of the present invention are less affected by the environment and have a high energy distribution uniformity.

[0033] 4. The ultrafast laser multi-pulse sequence preparation device and method of the present invention can control the pulse width parameters of the laser while generating the excitation pulse train, and control the time interval between the transmitted light beam pulse and the reflected light beam pulse through the high-precision one-dimensional displacement stage, and arbitrarily regulate the pulse sequence. Description of the Drawings

[0034] Figure 1 It is a schematic diagram of an ultrafast laser pulse modulation device;

[0035] Figure 2 It is a schematic diagram of the second mirror;

[0036] Figure 3Schematic diagram of a light spot converged by a concave lens onto the panel of a reflective liquid crystal spatial light modulator;

[0037] Figure 4 An example of a phase diagram loaded on the liquid crystal panel of a reflective liquid crystal spatial light modulator. The expression of the phase diagram is 2π×sin[500(ω - ω0)], which has the function of adjusting the peak position of a single pulse by changing ω0, and the modulation depth is 10 bit;

[0038] Figure 5 Example diagram of the laser pulse width broadening effect;

[0039] Figure 6 Example diagram of the laser pulse width compression effect;

[0040] Figure 7 Example diagram of the effect of generating multi - pulse laser; Detailed implementation manners

[0041] To further elaborate on the technical solution of the present invention, the present invention will be further described below through embodiments.

[0042] As Figure 1 shown, an ultrafast laser multi - pulse sequence preparation device in this embodiment is used to generate a pulse train sequence with adjustable pulse delay, pulse width, pulse amplitude, and pulse number. It includes a first beam splitter 1. The ultrafast pulsed laser is split into two beams with equal intensity and different directions by the first beam splitter 1, namely a reflected beam and a transmitted beam. The transmitted beam sequentially passes through a first mirror 5, a second mirror 6, a grating 7, a concave mirror 8, a third mirror 9 and reaches a reflective liquid crystal spatial light modulator 10. The reflective liquid crystal spatial light modulator 10 completes the modulation of the ultrafast pulsed laser. The modulated beam passes through the third mirror 9, the concave mirror 8, the grating 7, the second mirror 6, a fourth mirror 11, a fifth mirror 12 and reaches a second beam splitter 14. The reflected beam sequentially passes through a sixth mirror 2 and a seventh mirror 4 and then returns along the original path to the first beam splitter 1. After passing through the first beam splitter 1, it reaches the second beam splitter 14 through an eighth mirror 13. The seventh mirror 4 is installed on a high - precision one - dimensional displacement stage 3.

[0043] The transmitted beam realizes the control of various beam parameters through the reflective liquid crystal spatial light modulator 10. The reflected beam controls the optical path difference through the seventh mirror 4 installed on the high - precision one - dimensional displacement stage 3, thereby controlling the pulse delay time. The total optical path between the reflected beam and the transmitted beam to the second beam splitter 14 is basically equal, so that the same pulse in the beams split by the first beam splitter 1 is recombined at the second beam splitter 14 to form a pulse sequence.

[0044] As Figure 2As shown, the second mirror 6 is a pair of mirrors closely placed one above the other, and the angles of this pair of mirrors are different. By controlling the pitch of the third mirror 9 and the reflective liquid crystal spatial light modulator 10, the transmitted light beam and the returned modulated light beam are separated vertically in space, where the incident unmodulated light beam is adjusted by the lower mirror and the returned modulated light beam is adjusted by the upper mirror.

[0045] The pulse shaping of the transmitted light beam is achieved by the reflective liquid crystal spatial light modulator. After the transmitted light beam passes through the grating 7 and is spatially dispersed by wavelength, it is then processed by the concave mirror 8 and converges onto the liquid crystal panel of the reflective liquid crystal spatial light modulator 10 to form a long and narrow light spot (as Figure 3 shown). The laser is separated by wavelength and evenly distributed on the liquid crystal panel of the spatial light modulator. By applying different phase parameters to the controllable area on the liquid crystal panel of the reflective liquid crystal spatial light modulator 10, the modulation of the number of laser pulses, pulse width, and amplitude is completed, thereby realizing the function of pulse shaping.

[0046] The minimum displacement travel of the high-precision one-dimensional displacement stage 3 in this embodiment is 0.05 μm, and the maximum moving travel is 30 cm. The precise control of the pulse time delay is achieved by adjusting the optical path change amount of the high-precision one-dimensional displacement stage 3, and the range of the pulse time delay is from 0.33 fs to 2 ns.

[0047] The first mirror 5, the second mirror 6, the third mirror 9, the fourth mirror 11, the fifth mirror 12, the sixth mirror 2, the seventh mirror 4, and the eighth mirror 13 in this embodiment are all femtosecond-optimized silver mirrors. The first beam splitter 1 and the second beam splitter 14 are both 5:5 beam splitters, and the focal length of the concave mirror 8 is 250 mm.

[0048] Based on the above device, a method for preparing an ultrafast laser multi-pulse sequence in this embodiment includes the following steps:

[0049] Step 1: After calibrating the ultrafast pulsed laser, inject it into the device;

[0050] Step 2: The transmitted optical path after passing through the first beam splitter 1 is spatially dispersed by wavelength through the grating 7 to form a strip-shaped colored light, which is reflected by the concave mirror 8 and mapped to the response area of the reflective liquid crystal spatial light modulator 10;

[0051] Step 3: An ultrafast pulsed laser is expressed as the sum of monochromatic light components, and the expression is:

[0052]

[0053] where ω k is the frequency of each monochromatic light, A k is the amplitude of the k-th order light, is the phase, which shifts the value of the phase modulation to between 0 and 2π;

[0054] Since the envelope shape of the ultrafast laser pulse generally follows a Gaussian distribution, the ultrafast pulsed laser is expressed as:

[0055] E(t) = exp{-mt 2 + i[2πω0t + (2πω0nt) 2}

[0056] In the formula, ω0 is the spectral center frequency, and m and n are the parameters of the time dispersion of the ultrafast pulsed laser, as well as the pulse width and spectral amplitude.

[0057] At different positions on the response region of the reflective liquid crystal spatial light modulator 10, different phase parameters are applied, so that the ultrafast pulsed laser is divided into multiple sub-pulses. The pulse width of each sub-pulse depends on the phase loading value of the corresponding modulation region, and the amplitude of each sub-pulse is achieved by changing the size of the modulation region of the reflective liquid crystal spatial light modulator 10 corresponding to the sub-pulse;

[0058] Modify the set time interval, and the modified linear phase modulation is written as:

[0059]

[0060] In the formula, △t is the time interval between the sub-pulse and the original pulse, and τ is the time shift parameter generated by the dispersion when canceling the original pulse;

[0061] The pulse train generated by the reflective liquid crystal spatial light modulator 10 returns to the second beam splitter 14;

[0062] Step 4: The reflected beam after passing through the first beam splitter 1 is reflected by the mirror 4 placed on the high-precision one-dimensional displacement stage 3, and the optical path is controlled by the high-precision one-dimensional displacement stage 3, thereby changing the time delay of the reflected beam relative to the transmitted beam;

[0063] Step 5: The reflected beam and the transmitted beam modulated by the reflective liquid crystal spatial light modulator 10 are combined at the second beam splitter 14 to form a pulse train sequence.

[0064] An example of the phase diagram loaded on the liquid crystal panel is as Figure 4 shown. The expression of the phase diagram is 2π×sin[500(ω - ω0)], which has the function of adjusting the single-pulse peak position by changing ω0, and the modulation depth is 10bit. By controlling the reflective liquid crystal spatial light modulator 4 to load the corresponding phase diagram, the functions of pulse broadening, pulse compression, and generating multi-pulse sequences as shown in Figures 5 to 7 can be achieved.

[0065] The foregoing has shown and described the main features and advantages of the present invention. For those skilled in the art, it is obvious that the present invention is not limited to the details of the above-described exemplary embodiments, and without departing from the spirit or basic characteristics of the present invention, the present invention can be implemented in other specific forms. Therefore, from any point of view, the embodiments should be regarded as exemplary and non-limiting. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be embraced by the present invention.

[0066] In addition, it should be understood that although this specification is described according to embodiments, not every embodiment only contains an independent technical solution. This narrative way of the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. An ultrafast laser multi-pulse sequence preparation device, characterized in that: The invention comprises a first beam splitter (1). After passing through the first beam splitter (1), an ultrafast pulse laser is split into two beams of light with equal intensity and different directions, namely a reflected beam and a transmitted beam. The transmitted beam sequentially passes through a first reflector (5), a second reflector (6), a grating (7), a concave mirror (8) and a third reflector (9) to reach a reflective liquid crystal spatial light modulator (10). The reflective liquid crystal spatial light modulator (10) completes the modulation of the ultrafast pulse laser. The modulated beam passes through a third reflector (9), a concave mirror (8), a grating (7), a second reflector (6), a fourth reflector (11) and a fifth reflector (12) to reach a second beam splitter (14). The reflected beam sequentially passes through a sixth reflector (2) and a seventh reflector (4) and then returns to the first beam splitter (1) along the original path. After passing through the first beam splitter (1), the reflected beam passes through an eighth reflector (13) and then reaches the second beam splitter (14). The seventh reflector (4) is mounted on a high-precision one-dimensional displacement stage (3).

2. The ultrafast laser multi-pulse sequence preparation device according to claim 1, characterized in that: The transmitted light beam controls various light beam parameters through a reflective liquid crystal spatial light modulator (10); the reflected light beam controls the optical path difference through a seventh reflector (4) mounted on a high-precision one-dimensional displacement stage (3) to control the pulse delay time; the total optical path between the reflected light beam and the transmitted light beam to the second beam splitter (14) is substantially equal, so that the light beams of the same pulse separated by the first beam splitter (1) are combined again by the second beam splitter (14) to form a pulse sequence.

3. The ultrafast laser pulse modulation device according to claim 1, characterized in that: The second reflector (6) is a pair of reflectors closely placed up and down, and the angles of the pair of reflectors are different. By controlling the pitch of the third reflector (9) and the reflective liquid crystal spatial light modulator (10), the transmitted light beam and the returned modulated light beam are separated up and down in space.

4. The ultrafast laser pulse modulation device according to claim 1, characterized in that: The transmitted light beam is dispersed spatially according to wavelength by a grating (7), and then converged on a liquid crystal panel of a reflective liquid crystal spatial light modulator (10) after being processed by a concave mirror (8) to form a narrow light spot. By applying different phase parameters to an adjustable area on the liquid crystal panel of the reflective liquid crystal spatial light modulator (10), the number of laser pulses and the pulse width and amplitude are modulated.

5. The ultrafast laser pulse modulation device according to claim 1, characterized in that: The minimum displacement stroke of the high-precision one-dimensional translation stage (3) is 0.05 μm, and the maximum displacement stroke is 30 cm. Accurate control of pulse time delay is achieved by adjusting the optical path variation of the high-precision one-dimensional translation stage (3), and the range of the pulse time delay is 0.33 fs to 2 ns.

6. An ultrafast laser pulse modulation device according to any one of claims 1 to 5, characterized in that: The first reflector (5), the second reflector (6), the third reflector (9), the fourth reflector (11), the fifth reflector (12), the sixth reflector (2), the seventh reflector (4) and the eighth reflector (13) are all femtosecond optimized silver reflectors, the first beam splitter (1) and the second beam splitter (14) are both 5:5 beam splitters, and the focal length of the concave mirror (8) is 250 mm.

7. A method for preparing an ultrafast laser multi-pulse sequence, characterized in that: The following steps are involved: Step 1, constructing an ultrafast laser multi-pulse sequence preparation device as described in any one of claims 1 to 6, and injecting an ultrafast pulse laser into the device after calibration; Step 2: the transmission light path after passing through the first beam splitter (1) is spatially dispersed by wavelength through a grating (7) to form a strip-shaped dispersed light, which is reflected by a concave mirror (8) and mapped to a response area of ​​a reflective liquid crystal spatial light modulator (10); Step 3: An ultrafast pulse laser is represented as the sum of monochromatic light components, expressed as: In the formula, ω k is the frequency of each monochromatic light, A k is the amplitude of the k-th order light, is the phase, shifting the phase modulation value to between 0 and 2π; Different phase parameters are applied at different positions on the response area of ​​the reflective liquid crystal spatial light modulator (10) to divide the ultrafast pulse laser into a plurality of sub-pulses, wherein the pulse width of each sub-pulse depends on the phase loading value of the corresponding modulation area, and the amplitude of each sub-pulse is achieved by changing the size of the modulation area of ​​the reflective liquid crystal spatial light modulator (10) corresponding to the sub-pulse; Corrected set time interval, corrected linear phase modulation Written as: Where △t is the time interval between the sub-pulse and the original pulse, and τ is the time shift parameter generated when offsetting the dispersion of the original pulse; The pulse group generated by the reflective liquid crystal spatial light modulator (10) is returned to the second beam splitter (14); Step 4, the reflected light beam after passing through the first beam splitter (1) is reflected by a reflector (4) placed on a high-precision one-dimensional translation stage (3), and the optical path is controlled by the high-precision one-dimensional translation stage (3), thereby changing the time delay of the reflected light beam relative to the transmitted light beam; Step 5: The reflected light beam and the transmitted light beam modulated by the reflective liquid crystal spatial light modulator (10) are combined at the second beam splitter (14) to form a pulse train sequence.

8. The method for preparing an ultrafast laser multi-pulse sequence according to claim 7, characterized in that: Since the envelope shape of the ultrafast laser pulse generally obeys Gaussian distribution, the ultrafast pulse laser is expressed as: E(t)=exp{-mt 2 +i[2πω0t+(2πω0nt) 2 ]} Where ω0 is the center frequency of the spectrum, and m and n are the parameters of the temporal dispersion, pulse width, and spectral amplitude of the ultrafast pulse laser.

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