A method and apparatus for improving the spontaneous emission time contrast of femtosecond laser pulses

Abstract

The application discloses a method and device for improving the spontaneous emission time contrast of femtosecond laser pulses. The device comprises a titanium sapphire femtosecond laser oscillator, a laser pump source, a stretcher, a regenerative amplifier, an electro-optical switch, a compressor and the like. The low-energy pulses output by the femtosecond laser oscillator are time-domain stretched by the stretcher, and then are sent into the regenerative amplifier for amplification. At this time, the transmittance of laser pulses of different wavelengths in the ring cavity is adjusted by adjusting the incident angle of the laser into the spectrum shaping filter in the ring cavity, so as to suppress the spontaneous emission in the cavity and improve the spontaneous emission time contrast of the amplified light. Subsequently, the electro-optical switch is controlled to make the amplified light enter the pulse compressor, and finally the femtosecond laser pulses with high energy and high time contrast are generated. The application improves the spontaneous emission time contrast of femtosecond laser pulses by spectrum shaping filtering in the ring cavity of the amplifier, and can also effectively suppress the spectral red shift and gain narrowing of the amplified light, and output the amplified light with a wide spectrum.

Description

Technical Field

[0001] This invention belongs to the field of ultrafast laser amplifiers, specifically relating to a method and apparatus for improving the time contrast of spontaneous emission of femtosecond laser pulses. Background Technology

[0002] The introduction of chirped-pulse amplification (CPA) technology has solved the bottleneck problem of the inability to break through the peak power of lasers, providing an effective approach for the development of ultrashort and ultra-intense lasers. The development of ultrashort and ultra-intense lasers has greatly promoted the progress of extreme physics experiments such as X-ray, inertial confinement fusion, and laser-ion accelerators.

[0003] In typical CPA laser systems, due to inherent spontaneous emission (ASE) and insufficient extinction ratio of polarization elements, background noise is unavoidable in the amplified laser pulse. These noise sources primarily include ASE and the pre-pulse at the laser leading edge. The intensity ratio between these pre-pulses, ASE, and other background noise and the main pulse is called the laser pulse temporal contrast. Figure 1 As shown. In experiments involving the interaction of high peak power laser pulses with materials, the focused peak power density of the laser pulse is often greater than 10. 18 W / cm 2 The laser pulse time contrast generated by regenerative amplifiers designed based on CPA technology is generally only 10. 6 At this point, the peak power density of ASE reached 10. 11 W / cm 2 If the ionization threshold of most materials is exceeded, the target material will be pre-ionized, thus affecting the interaction between the main pulse and the matter and having a significant adverse impact on the analysis of experimental results. Therefore, improving the ASE time contrast of the laser pulse is very important in strong-field physics experiments.

[0004] In existing technologies, techniques to improve the ASE time contrast of laser pulses can be categorized in two ways. One approach is to increase the seed light energy (see Prior Art 1: “Amplified spontaneous emission contrast of CPA laser”, Xu Y, Leng YX, Lin LH, Wang WY, Huang YS, Li RX, Xu ZZ, Chin. Opt. Lett. 8, 123-125 (2010)). However, increasing the oscillator output energy without altering the optical setup is extremely difficult, thus this technique cannot effectively improve the ASE time contrast of the laser pulse. Another approach is to improve the ASE time contrast of the laser pulse by matching the spectra of the seed light and the ASE (see Prior Art 2: “Contrast ratio enhancement by spectral matching of a seed laser pulse and ASE in a Ti:sapphire laser system”, Kim M, Kim JJ, Phung VLJ, and Suk H, Opt. express 25, 14158-14163 (2017)). However, this technique is not only complex to operate but also cannot compensate for the spectral redshift and gain narrowing of the amplified light. Summary of the Invention

[0005] This invention proposes a method to improve the ASE (Extreme Acoustic Contrast) time contrast of femtosecond laser pulses: A low-energy laser pulse output from a femtosecond oscillator is fed into a stretcher, which stretches it to several hundred picoseconds in the time domain. The stretched laser pulse is then fed into a regenerative amplifier to amplify it to saturation output. A Spectral Shaping Filter (SSF) is placed inside the ring cavity of the amplifier. By adjusting the incident angle of the laser entering the SSF, the full width at half maximum (FWHM) of the amplified light spectrum is adjusted to tens of nanometers, with the center wavelength around 800 nm. The transmission curve of the SSF used in this invention is shown below. Figure 2 As shown. At this time, the center wavelength of the ASE spectrum in the ring cavity is between 780-785 nm, and the spectrum when the SSF transmittance is lowest is also close to 780-785 nm, as shown. Figure 3 As shown in the figure, placing an SSF inside the cavity can more effectively suppress the generation of ASE within the cavity compared to amplifying light in the ring cavity, thereby improving the ASE time contrast of the amplifier's output laser pulse. Subsequently, the amplified light is guided into the pulse compressor through a mirror, ultimately generating a femtosecond laser pulse with high time contrast, as shown in the figure. Figure 4As shown, this method can not only effectively suppress the generation of intracavity ASE and improve the ASE time contrast of the output laser pulse of the regenerative amplifier, but also effectively suppress the spectral redshift and gain narrowing of the amplified light, achieving a wider spectrum of amplified light output. In addition, this method has the following advantages: only one SSF is placed inside the ring cavity of the amplifier, resulting in a simple and compact architecture; using a regenerative amplifier offers advantages such as good beam quality and simple tuning.

[0006] In addition, the present invention also provides a regenerative amplification apparatus for implementing the above method, which consists of a femtosecond laser oscillator, a regenerative amplifier, a pump source, a pulse stretcher, and a pulse compressor.

[0007] The regenerative amplifier includes a gain crystal, a curved cavity mirror, a polarization beam splitter, an electro-optic switch, and an SSF.

[0008] The gain crystal is a titanium-doped sapphire crystal.

[0009] The SSF was provided by ARO and placed in the amplifier.

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

[0011] The compressor uses a reflective grating pair as a dispersion compensation component to compress the pulse in the time domain.

[0012] The present invention has the following advantages:

[0013] 1. Applicable to all ring cavities, the gain crystal is a titanium-doped sapphire regenerative amplifier.

[0014] 2. It can effectively suppress spectral redshift and gain narrowing during the amplification process.

[0015] 3. A femtosecond laser pulse with high ASE time contrast can be achieved using only one SSF, with a simple and compact architecture.

[0016] 4. Using a regenerative amplifier has advantages such as good beam quality, high energy stability, and simple tuning. Attached Figure Description

[0017] 1. Figure 1 This is a schematic diagram showing the time contrast between the femtosecond laser main pulse and the ASE.

[0018] 2. Figure 2 The transmission curve of SSF used in this invention (from Alpine Research Optics).

[0019] 3. Figure 3The SSF transmission curve (thin solid line), the spectrum of the ASE in the ring cavity (dashed line), and the spectrum of the amplified light (thick solid line) generated by the regeneration device for improving the temporal contrast of the femtosecond laser pulse ASE according to the present invention are shown.

[0020] 4. Figure 4 This is a schematic diagram of the method for improving the ASE temporal contrast of femtosecond laser pulses according to the present invention.

[0021] 5. Figure 5 This is a schematic diagram of the amplification device for improving the temporal contrast of femtosecond laser pulses (ASE) according to the present invention.

[0022] 6. Figure 6 This invention provides a comparison of the amplification device for improving the ASE time contrast of femtosecond laser pulses and the ASE time contrast of an external SSF in a ring cavity. Detailed Implementation

[0023] The specific implementation method of the present invention will be described below with reference to the accompanying drawings. Figure 5 The diagram shows the structure of the regenerative amplification device for improving the ASE time contrast of femtosecond laser pulses according to the present invention. The components corresponding to the labels are: 1-femtosecond laser oscillator, 2-stretcher, 3-532nm pump laser, 4-plane mirror, 5-plane mirror, 6-regenerative amplifier, 7-gain crystal (titanium-doped sapphire), 8-curved cavity mirror, 9-curved cavity mirror, 10-SSF, 11-electro-optic switch, 12-polarization beam splitter, 13-curved cavity mirror, 14-curved cavity mirror, 15-plane mirror, 16-compressor.

[0024] The method for generating high ASE time-contrast laser pulses using the regenerative amplification device is as follows:

[0025] 1. The seed laser output from the femtosecond laser oscillator 1 is fed into the stretcher 2 to stretch the laser pulse width from tens of femtoseconds to hundreds of picoseconds.

[0026] 2. The broadened laser pulse passes through mirrors 4 and 5 and enters amplifier 6.

[0027] 3. Adjust the ring cavity formed by the Ti:sapphire crystal 7, cavity mirrors 8, 9, 13, and 14, electro-optic switch 11, and polarization beam splitter 12 in amplifier 6 to amplify the energy of the seed laser pulse. The energy of regenerative amplifier 6 is provided by laser pump source 3.

[0028] 4. During laser amplification, by adjusting the angle of SSF10, the full width at half maximum (FWHM) of the amplified light spectrum is adjusted to tens of nanometers, and the center wavelength of the spectrum is around 800 nm.

[0029] 5. After the laser pulse is amplified to saturation in the ring cavity, the amplified laser pulse is reflected out of the amplifier by the electro-optic switch and polarization beam splitter inside the cavity.

[0030] 6. The laser pulse enters the pulse compressor 16 for pulse width compression.

[0031] Experimental procedure and data:

[0032] To investigate the effect of placing a seed laser (SSF) inside and outside the amplifier cavity on the ASE contrast of a laser pulse, a 2.2 nJ laser pulse from a femtosecond oscillator was first passed through an optical isolator and then into an Offner-type stretcher. The Offner-type stretcher stretched the seed light to 400 ps in the time domain with an output energy efficiency exceeding 50%. An SSF was then placed inside the amplifier's annular cavity. The stretched laser pulse was then amplified by a regenerative amplifier. By adjusting the angle of the incident SSF laser within the amplifier cavity, the output amplified laser pulse had an energy of 8.6 mJ and a spectral width of 45 nm (FWHM). Subsequently, the amplified light was output through a polarization beam splitter controlled by an electro-optic switch. The amplified light was then compressed by a grating compressor to a pulse width of 26 fs with an energy of 6.4 mJ. We passed the compressed laser pulse through an appropriate attenuator to reduce the energy to 400 microjoules, and then sent it to a Sequoia third-order autocorrelation instrument from Amplitude Technologies, France, to measure the time contrast. The measured ASE contrast for the first 400 ps of the main laser pulse with an intracavity SSF was 1.0 × 10⁻⁶. -8 Then, the SSF inside the amplifier cavity was placed after the stretcher and before the amplifier. By adjusting the SSF and pump light energy, the spectral width of the amplified light in both sets of experiments was maximized while maintaining the same output energy. Using the same method, the ASE contrast ratio in the first 400 ps of the laser main pulse was measured to be 5.0 × 10⁻⁶ when an SSF was applied outside the cavity. -8 .

[0033] Figure 6 The figure shows the experimental results of placing SSFs inside and outside the amplifier cavity on the ASE contrast. As can be seen from the figure, compared with adding an SSF outside the cavity, adding an SSF inside the cavity has a better suppression effect on ASE in the ring cavity, which can improve the ASE contrast of the output laser pulse by about 5 times. This shows that the present invention not only has a simple structure, stable output, and good beam quality, but also achieves high ASE time contrast. Figure 6 The pulse before the main pulse is a false signal, which is an artifact produced by the intracavity SSF and the reflected light from the attenuator in the third-order autocorrelation instrument.

Claims

1. A method for improving the time contrast of spontaneous emission of femtosecond laser pulses, comprising the following steps: (1) The low-energy femtosecond laser pulse generated by the Ti:sapphire femtosecond laser oscillator enters the stretcher, which stretches it to several hundred picoseconds in the time domain; (2) The broadened laser pulse enters the regenerative amplifier to amplify the energy; (3) Adjust the angle of the spectrum shaping filter in the amplifier ring cavity to adjust the full width at half maximum (FWHM) of the amplified light spectrum to tens of nanometers, with the center wavelength of the spectrum around 800 nm. (4) The amplified light enters the pulse compressor to compress the pulse; The regenerative amplifier ring cavity includes a gain crystal, a curved cavity mirror, a polarization beam splitter, an electro-optic switch, and a spectrum shaping filter. The transmittance of the spectrum-shaping filter for different wavelengths of laser light changes with the laser's entry angle and polarization. A spectrum-shaping filter is placed inside the annular cavity of the amplifier. By adjusting the incident angle of the laser entering the spectrum-shaping filter inside the annular cavity, the generation of spontaneous emission within the cavity is suppressed, thereby improving the spontaneous emission time contrast. While suppressing spontaneous emission within the cavity, the spectrum-shaping filter can also effectively suppress the spectral redshift and gain narrowing of the amplified light, resulting in a wider output spectrum of amplified light.

2. An apparatus for implementing the method of claim 1, comprising a femtosecond laser oscillator, a laser pump source, a pulse stretcher, a regenerative amplifier with intra-cavity spectral shaping and filtering, an electro-optic switch, and a pulse compressor.

3. The apparatus as described in claim 2, characterized in that, The regenerative amplifier uses titanium-doped sapphire as the gain crystal.

4. The apparatus as claimed in claim 2, characterized in that, The pulse stretcher uses a transmission grating as the dispersive material.

5. The apparatus as described in claim 2, characterized in that, The pulse compressor uses a reflective grating as the dispersive material.

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