A method and device for generating high temporal contrast femtosecond laser pulses

Abstract

The present invention discloses a device for generating high temporal contrast femtosecond laser pulses and a method for generating high temporal contrast femtosecond laser pulses using the device. The device comprises a femtosecond laser oscillator, a laser pump source, a regenerative amplifier, an electro-optical switch, a stretcher, a contrast enhancement module, and a compressor. The low-energy pulse output by the femtosecond oscillator is directly sent to a regenerative amplifier for pre-amplification, and then the pre-amplified pulse is sent to the time contrast enhancement module to enhance the time contrast, and the pulse is stretched by a pulse stretcher, and then re-sent to the aforementioned regenerative amplifier to be amplified to a saturated output, and then controlled by a single-channel electro-optical switch to enter a pulse compressor, and finally generate a high temporal contrast and high-energy femtosecond laser pulse. This invention realizes the generation of high temporal contrast laser pulses with only a single amplifier, and at the same time has the characteristics of simple architecture, compact structure, high stability, and good spot quality.

Description

Technical Field

[0001] The invention belongs to the field of ultrafast laser amplifiers, and in particular relates to a method for generating high time contrast femtosecond laser pulses and a device for realizing the method. Background Art

[0002] Thanks to the invention of chirped pulse amplification (CPA) technology, high-power laser systems with ultrafast and ultra-intense pulses have made unprecedented progress in the past few decades. The interaction between ultrafast and ultra-intense pulses and materials can accelerate particles (electrons, protons, ions) or produce light sources of various wavelengths (extreme ultraviolet, X-rays, rays, etc.), shrinking the light source that can only be produced by traditional large accelerators to the size of a desktop.

[0003] At present, femtosecond laser systems based on CPA technology can reach a peak power of 10PW. As the peak power of the laser increases, the power of the pre-pulse generated by the amplified spontaneous emission (ASE) of the pump light before the main pulse (50 picoseconds to 1 nanosecond) will also increase (e.g. Figure 1 The intensity is sufficient to generate plasma on the material and destroy the main pulse or the properties of the material itself, thereby affecting the accelerated particle or ray source. Therefore, the temporal contrast of the pulse is defined as Figure 1 The ratio of P0 to P1 must be large enough to ensure that the intensity of the pre-pulse does not exceed the ionization threshold of the material.

[0004] In high-power femtosecond laser systems, the temporal contrast of the oscillator is about 10 8-9 However, due to the small pulse energy (nJ level) generated by the oscillator, the ASE generated in the subsequent amplification process overlaps in time and spectrum after the seed pulse is broadened. Therefore, it is difficult to suppress the ASE within the pulse width of the main pulse, resulting in a poor time contrast after entering the regenerative amplifier. For example, the time contrast of a typical regenerative amplifier is 10 6-7 Such temporal contrast is far from sufficient for many applications in laser systems with peak powers of more than 100 TW.

[0005] At present, the commonly used method for generating high temporal contrast femtosecond laser pulses is to use double chirped pulse amplification technology (DCPA, Double Chirped-Pulse-Amplification) (see prior art 1: "Double chirped-pulse-amplification laser: a way to clean pulses temporally.", MP Kalashnikov, E. Risse, H. and W.Sandner, Opt.Lett.30,923(2005)), and combined with the use of cross-polarized wave generation (XPW) or saturated absorber (SA) to enhance the temporal contrast. By combining these technologies, the laser system can produce a temporal contrast of 10 9 The above pulses. The cross polarization wave technology (XPW) can be found in the prior art 2:10 -10 temporal contrast for femtosecond ultraintense lasers by cross-polarized wave generation”, Aurélie Jullien, Olivier Albert, Frédéric Burgy, Guy Hamoniaux, Jean-Philippe Rousseau, Jean-Paul Chambaret, Frédérika Augé-Rochereau, Gilles Chériaux, Jean Etchepare, Nikolay Minkovski, and Solomon M. Saltiel, Opt. Lett. 30, 920 (2005); for the use of saturated absorber technology (SA), see prior art 3: “Suppression of the amplified spontaneous emission inchirped-pulse-amplification lasers by clean high-energy seed-pulse injection”, J Itatani, J Faure, M Nantel, G Mourou, S Watanabeb, Opt. Commun. 148, 70 (1998).

[0006] However, this DCPA architecture requires that the low-energy pulse output by the femtosecond laser oscillator be first widened by a stretcher, then amplified by the first amplifier, compressed by the first pulse compressor, and then sent to the time contrast enhancement module to enhance the time contrast, then amplified by another pulse stretcher and another amplifier, and finally sent to the second pulse compressor for compression (such as Figure 2 This architecture is highly complex and difficult to adjust. Due to the long optical path and large system size, it also affects the quality and stability of the laser output beam and is not conducive to maintenance. Summary of the invention

[0007] The present invention proposes a method for generating high temporal contrast femtosecond laser pulses: the low energy pulse output by the femtosecond oscillator is directly sent to a regenerative amplifier for pre-amplification, and then the pre-amplified pulse is sent to a temporal contrast enhancement module to enhance the temporal contrast, and the pulse is stretched by a pulse stretcher, and then sent back to the aforementioned regenerative amplifier to be amplified to saturated output, and then controlled by a single-channel electro-optical switch to enter a pulse compressor, and finally a high temporal contrast femtosecond laser pulse is generated. (such as Figure 3 As shown). This method can improve the time contrast of traditional regenerative amplifiers by 2 to 4 orders of magnitude, and overcome the shortcomings of the complex architecture of double-chirped pulse amplification technology (DCPA). The pulses that are directly pre-amplified without broadening can directly replace the millijoule-level ultrashort pulses that need to be broadened, amplified, and compressed in DCPA. In addition, this method has the following advantages: a single-stage amplifier avoids the use of two pump sources, and the use of a regenerative amplifier has the advantages of good beam quality and simple adjustment; only one stretcher and compressor are needed, which shortens the overall optical path, reduces the nonlinear effect of the beam during propagation, and makes the overall dispersion management of the system easier.

[0008] In addition, the present invention also provides a regenerative amplifier device for implementing the above method, which is composed of a regenerative amplifier, a single-channel electro-optical switch, a time contrast enhancement module, a pulse stretcher and a pulse compressor.

[0009] The regenerative amplifier comprises a gain crystal, a plane mirror, a curved mirror, a polarizing plate, and a multi-channel electro-optical switch component.

[0010] The stretcher uses a grating as a dispersion component to stretch the pulse in the time domain.

[0011] The temporal contrast enhancement module may be a SA or XPW module and is placed in the amplifier.

[0012] The compressor uses a grating pair as a dispersion compensation component to compress the pulse in the time domain.

[0013] The present invention has the following advantages:

[0014] 1. Applicable to all time contrast enhancement modules, whether SA module or XPW module, the time contrast is greatly improved.

[0015] 2. Simple architecture, compact structure, and high time contrast can be achieved with a single amplifier.

[0016] 3. The use of regenerative amplifiers has the advantages of good beam quality, high energy stability, and simple adjustment. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] 1. Figure 1Schematic diagram of the time contrast between the femtosecond laser main pulse and ASE.

[0018] 2. Figure 2 Schematic diagram of double chirped pulse amplification technology (DCPA).

[0019] 3. Figure 3 Schematic diagram of the method for generating high temporal contrast femtosecond laser pulses according to the present invention.

[0020] 4. Figure 4 This is a schematic structural diagram of a high time contrast femtosecond laser regeneration and amplification device according to the present invention.

[0021] 5. Figure 5 This is the time contrast comparison data of the high time contrast femtosecond laser regeneration device of the present invention and DCPA. DETAILED DESCRIPTION

[0022] The specific implementation modes of the present invention are described below with reference to the accompanying drawings. Figure 4 The figure shows a schematic diagram of the structure of the high temporal contrast regeneration amplifier of the present invention. The components corresponding to the numbers of the parts are: 1-femtosecond laser oscillator, 2-laser pump source, 3-regeneration amplifier, 4-single-channel electro-optical switch, 5-polarizer, 6-temporal contrast enhancement module, 7-pulse stretcher, 8-half-wave plate, 9-pulse compressor.

[0023] The method for generating high time contrast laser pulses by the regenerative amplifier is as follows:

[0024] 1. The seed pulse laser output from the femtosecond laser oscillator 1 is directly sent to the regenerative amplifier 3 for pulse pre-amplification. The energy of the regenerative amplifier 3 is provided by the laser pump source 2.

[0025] 2. After the seed pulse is cyclically amplified to a certain energy in the regenerative amplifier, the polarization is changed through the multi-channel electro-optical switch in the regenerative amplifier, and the pre-amplified laser is reflected out of the amplifier through the polarizer in the amplifier.

[0026] 3. The pre-amplified laser pulse after exiting the amplifier will pass through a single-channel electro-optical switch 4 without rotating its polarization state, and then be reflected by a polarizer 5 into the time contrast enhancement module 6.

[0027] 4. If the temporal contrast enhancement module is an SA module, the laser pulse will first be focused to a saturated absorber through a lens, and then adjusted to parallel light output through a lens with the same focal length as the front lens, and then rotated to horizontal polarization through a half-wave plate.

[0028] 5. If the temporal contrast enhancement module is an XPW module, the laser pulse will first pass through a dispersion compensator, a horizontal polarizer, a lens to focus on the nonlinear crystal and adjust the laser to parallel light through a lens of the same focal length, and finally pass through a vertical polarizer and a half-wave plate to rotate the polarization to horizontal polarization again.

[0029] 6. After the time contrast is enhanced, the laser pulse enters the pulse stretcher 7 for pulse stretching.

[0030] 7. After the broadening, the laser pulse passes through a half-wave plate 8 to rotate the polarization to vertical, and then is reflected back to the regenerator amplifier through the polarizer, and is selected by a multi-channel electro-optical switch for a second amplification.

[0031] 8. After the laser pulse is amplified to saturation in the regenerative amplifier, the amplified pulse is reflected out of the amplifier by the multi-channel electro-optical switch and polarizer in the amplifier. Then the pulse passes through the single-channel electro-optical switch 4 again and rotates the polarization to horizontal polarization, and then penetrates the polarizer 5.

[0032] 9. The laser pulse enters the pulse compressor 9 for pulse width compression.

[0033] Experimental process and data:

[0034] The device can output energy greater than 3mJ, pulse width less than 45fs, and time contrast greater than 10 10 pulse.

[0035] First, the pulse with energy of 2nJ output by the femtosecond oscillator is directly sent to a regenerative amplifier for pre-amplification. After the energy is pre-amplified to 200 microjoules, the pre-amplified pulse is then sent to the time contrast enhancement module to enhance the contrast. At this time, the pre-amplified pulse will be focused on two saturated absorbers with a thickness of 2mm for contrast enhancement. The pulse will then pass through a stretcher to stretch the pulse to 200ps. After stretching, it will be sent back to the aforementioned regenerative amplifier to be amplified to saturation, and then the pulse will be output through an electro-optical switch control. The amplified pulse energy is 4mJ. This pulse then enters the compressor to compress the pulse width, and the pulse width is finally compressed to 40fs, with an energy of 3.2mJ. We use a spectroscope to separate 500 microjoules of energy from the compressed pulse and send it to the third-order autocorrelator Sequoia of Amplitude Technologies of France to measure the time contrast. Figure 5 shown.

[0036] Figure 5The time contrast comparison data of the high time contrast femtosecond laser regeneration device of the present invention and DCPA, wherein the time contrast of the high time contrast regeneration device of the present invention is equivalent to that of DCPA. This shows that in addition to the simple structure, stable output, good beam quality and low cost, the present invention can also obtain the same time contrast as DCPA. Figure 5 The pulses of -24ps, -48ps and -72ps are post-pulses, not pre-pulses.

Claims

1. A method for generating high temporal contrast femtosecond laser pulses, comprising the following steps: (1) The seed pulse laser output from the femtosecond laser oscillator 1 is directly sent to the regenerative amplifier 3 for pulse pre-amplification. The energy of the regenerative amplifier 3 is provided by the laser pump source 2; (2) After the seed pulse is cyclically amplified to a certain energy in the regenerative amplifier, the polarization is changed through the multi-channel electro-optical switch in the regenerative amplifier, and the pre-amplified laser is reflected out of the amplifier through the polarizer in the amplifier; (3) After exiting the amplifier, the pre-amplified laser pulse passes through a single-channel electro-optical switch 4 without rotating its polarization state, and then is reflected by a polarizer 5 and enters the time contrast enhancement module 6; (4) If the temporal contrast enhancement module is a saturated absorber module, the laser pulse will first pass through a lens to focus on the saturated absorber, and then pass through a lens with the same focal length as the front lens to adjust the laser to output parallel light, and pass through a half-wave plate to rotate the polarization to horizontal polarization; (5) If the temporal contrast enhancement module is a cross-polarized wave module, the laser pulse will first pass through a dispersion compensator, a horizontal polarizer, a lens to focus on the nonlinear crystal and adjust the laser to parallel light through a lens of the same focal length, and finally pass through a vertical polarizer and a half-wave plate to rotate the polarization to horizontal polarization again; (6) After the time contrast is enhanced, the laser pulse enters the pulse stretcher 7 for pulse stretching; (7) The broadened laser pulse passes through a half-wave plate 8 to rotate the polarization to vertical, and then is reflected back to the regenerator amplifier through the polarizer, and is selected by a multi-channel electro-optical switch for a second amplification; (8) After the laser pulse is amplified to saturation in the regenerative amplifier, the amplified pulse is reflected out of the amplifier by the multi-channel electro-optical switch and polarizer in the amplifier. Then, the pulse passes through the single-channel electro-optical switch 4 again and rotates the polarization to horizontal polarization, and then passes through the polarizer 5; (9) The laser pulse enters the pulse compressor 9 for pulse width compression.

2. The method according to claim 1, characterized in that The resonant cavity of the regenerative amplifier contains a gain crystal, a plane mirror, a curved mirror, a polarization beam splitting element, and a multi-channel electro-optical switch element.

3. The method according to claim 2, characterized in that The multi-channel photoelectric switch includes more than two channels and can be used to control pulses to enter and exit the regenerative amplifier multiple times.

4. The method according to claim 1, characterized in that The contrast enhancement module utilizes a saturated absorber or a cross-polarization wave technology to enhance the temporal contrast.

5. The method according to claim 1, characterized in that The pulse stretcher and the pulse compressor use gratings as dispersion materials.

6. A device for implementing the method of claim 1, which comprises a femtosecond laser oscillator 1, a regenerative amplifier, a single-channel electro-optical switch, a time contrast enhancement module, a polarizer 5, a half-wave plate 8, a pulse stretcher and a pulse compressor; The resonant cavity of the regenerative amplifier contains a gain crystal, a plane mirror, a curved mirror, a polarization beam splitter element, and a multi-channel electro-optical switch element; The contrast enhancement module utilizes a saturated absorber or a cross-polarization wave technology to enhance the temporal contrast.

7. The device according to claim 6, characterized in that The multi-channel photoelectric switch includes more than two channels and can be used to control pulses to enter and exit the regenerative amplifier multiple times.

8. The device according to claim 6, characterized in that The pulse stretcher and the pulse compressor use gratings as dispersion materials.

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