CN114069369A

CN114069369A - Method and device for improving femtosecond laser pulse spontaneous radiation time contrast

Info

Publication number: CN114069369A
Application number: CN202110840743.XA
Authority: CN
Inventors: 不公告发明人
Original assignee: Henan Qifeng Newlight Source Optoelectronic Technology Co ltd
Current assignee: Henan Qifeng Newlight Source Optoelectronic Technology Co ltd
Priority date: 2021-07-25
Filing date: 2021-07-25
Publication date: 2022-02-18

Abstract

The invention discloses a method and a device for improving the spontaneous radiation time contrast of femtosecond laser pulses. The device comprises a titanium gem femtosecond laser oscillator, a laser pumping source, a stretcher, a regenerative amplifier, an electro-optical switch, a compressor and the like. The low-energy pulse output by the femtosecond laser oscillator is subjected to time domain broadening of the laser pulse after passing through the stretcher, and then is sent to the regenerative amplifier for amplification, and at the moment, the transmissivity of the laser pulse with different wavelengths in the ring cavity is adjusted by adjusting the incident angle of the laser in the ring cavity entering the spectral shaping filter plate, so that the spontaneous radiation in the cavity is inhibited, and the spontaneous radiation time contrast of amplified light is improved. Then the laser pulse enters a pulse compressor through electro-optical switch control, and finally a high-energy high-time-contrast femtosecond laser pulse is generated. The invention improves the spontaneous radiation time contrast of the femtosecond laser pulse by spectral shaping and filtering in the annular cavity of the amplifier, and can effectively inhibit the spectral red shift and gain narrowing of amplified light and output amplified light with wider spectrum.

Description

Method and device for improving femtosecond laser pulse spontaneous radiation time contrast

Technical Field

The invention belongs to the field of ultrafast laser amplifiers, particularly relates to a method for improving the spontaneous radiation time contrast of femtosecond laser pulses and a device for realizing the method, and belongs to the field of ultrafast laser amplifiers.

Background

The chirp-Pulse Amplification technology (CPA) is provided, the bottleneck problem that the peak power of the laser cannot break through is solved, and an effective way is provided for the development of the ultra-short and ultra-strong laser. The development of ultrashort and ultrastrong laser greatly promotes the development of extreme physical experiments such as X-ray, inertial confinement nuclear fusion, laser ion accelerator and the like.

In a general CPA laser system, due to the self-generated Spontaneous Emission (ASE) and insufficient extinction ratio of a polarization element, background noise exists in an Amplified laser pulse inevitably, and the noise sources mainly include ASE and a pre-pulse of a laser leading edge. The ratio of the intensity between the pre-pulse, ASE and other background noise of these laser fronts and the main pulse is the so-called temporal contrast of the laser pulse, as shown in figure 1. In experiments in which high peak power laser pulses interact with materials, the focused peak power density of the laser pulses tends to be greater than 10¹⁸W/cm²The regenerative amplifier designed based on CPA technique can generate laser pulse time contrast ratio of only 10⁶At this time, the peak power density of ASE reached 10¹¹W/cm²If the ionization threshold of most materials is exceeded, the target material is ionized in advance, so that the interaction between the main pulse and the substance is influenced, and the analysis of the experimental result is seriously influenced. Therefore, in the strong field physical experiment, it is very important to improve the ASE time contrast of the laser pulse.

In the prior art, on one hand, the technique for improving the ASE time contrast of the laser pulse is to improve the seed light energy (see prior art 1: "Amplified specific emission contrast of CPA laser", Xu Y, Leng Y X, Lin L H, Wang W Y, Huang Y S, Li R X, Xu Z, chi, opt, lett. 8, 123-charge 125(2010)), but under the premise of not changing the optical setup, it is very difficult to improve the output energy of the oscillator, so the technique cannot effectively improve the ASE time contrast of the laser pulse. On the other hand, the ASE temporal Contrast of the laser pulse is improved 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 J, Phung V L J, and Suk H, Opti. express 25, 14158-. However, this technique is not only complicated in operation but also unable to compensate for spectral red shift and gain narrowing of the amplified light.

Disclosure of Invention

The invention provides a method for improving the ASE time contrast of femtosecond laser pulses, which comprises the following steps: the low-energy laser pulse output by a femtosecond oscillator is sent to a stretcher, the stretcher stretches the laser pulse to hundreds of picoseconds in a time domain, then the stretched laser pulse is sent to a regenerative amplifier to be amplified to saturation output, a spectrum Shaping Filter (SSF for short) is placed in an annular cavity of the amplifier, the full width at half maximum of an amplified optical spectrum is adjusted to dozens of nanometers by adjusting the incident angle of laser entering the SSF, and the center wavelength of the spectrum is near 800 nm. The transmission curve of the SSF selected by the present invention is shown in FIG. 2. At this time, the central wavelength of the ASE spectrum in the ring cavity is between 780-785nm, and the spectrum with the lowest SSF transmittance is also close to 780-785nm, as shown in FIG. 3. It can be seen that the intracavity placement of SSF can suppress the generation of ASE in the ring cavity more effectively than the amplified light in the ring cavity, thereby improving the ASE time contrast of the laser pulse output from the amplifier. The amplified light is then directed through a mirror into a pulse compressor, ultimately producing high temporal contrast femtosecond laser pulses, as shown in fig. 4. The method not only can effectively inhibit the generation of ASE in the cavity and improve the ASE time contrast of the laser pulse output by the regenerative amplifier, but also can effectively inhibit the spectral red shift and gain narrowing of the amplified light and realize the wider spectrum of the amplified light output. In addition, the method also has the following advantages: only one SSF is placed in the annular cavity of the amplifier, so that the structure is simple and compact; the regenerative amplifier has the advantages of good beam quality, simple adjustment and the like.

In addition, the invention also provides a regenerative amplification device for realizing the method, and the device consists of a femtosecond laser oscillator, a regenerative amplifier, a pumping source, a pulse stretcher and a pulse compressor.

The regenerative amplifier comprises a gain crystal, a curved cavity mirror, a polarization beam splitter, an electro-optical switch and an SSF.

The gain crystal is a titanium-doped sapphire crystal.

The SSF is supplied by ARO and placed in an amplifier.

The stretcher adopts a transmission type grating as a dispersion component to stretch the pulse in the time domain.

The compressor adopts a reflection-type grating pair as a dispersion compensation component to compress the pulse in the time domain.

The invention has the following advantages:

1. the regenerative amplifier is suitable for all ring cavities and the gain crystal is titanium sapphire.

2. The spectral red shift and gain narrowing in the amplification process can be effectively inhibited.

3. The femtosecond laser pulse with high ASE time contrast can be achieved by using only one SSF, and the structure is simple and compact.

4. The regenerative amplifier has the advantages of good beam quality, high energy stability, simple mode adjustment and the like.

Drawings

1. FIG. 1 is a schematic diagram showing the time contrast between the femtosecond laser main pulse and the ASE.

2. FIG. 2 is a graph of transmission using SSF according to the present invention (from Alpine Research Optics).

3. FIG. 3 shows an SSF transmission curve (thin solid line), a spectrum (dotted line) of ASE in a ring cavity, and a spectrum (thick solid line) of amplified light generated from a regeneration device for enhancing the temporal contrast of ASE according to the present invention.

4. FIG. 4 is a schematic diagram of the method for improving the time contrast of the ASE of the femtosecond laser pulse according to the present invention.

5. FIG. 5 is a schematic structural diagram of an amplifying device for improving the ASE time contrast of femtosecond laser pulses according to the present invention.

6. FIG. 6 is a comparison of the ASE time contrast data of the ring cavity plus SSF and the amplifying device for enhancing the ASE time contrast of femtosecond laser pulses according to the present invention.

Detailed Description

The following describes an embodiment of the present invention with reference to the drawings. FIG. 5 is a schematic view showing the structure of the apparatus for enhancing the contrast of ASE time regeneration of femtosecond laser pulses according to the present invention. The parts corresponding to the reference numbers of the parts are as follows: the laser comprises a 1-femtosecond laser oscillator, a 2-stretcher, a 3-532nm pump laser, a 4-plane mirror, a 5-plane mirror, a 6-regenerative amplifier, a 7-gain crystal (titanium sapphire doped), an 8-curved mirror, a 9-curved mirror, a 10-SSF, an 11-electro-optical switch, a 12-polarization beam splitter, a 13-curved mirror, a 14-curved mirror, a 15-plane mirror and a 16-compressor.

The method for generating the laser pulse with high ASE time contrast by the regenerative amplification device is as follows:

1. the seed laser output from the femtosecond laser oscillator 1 is sent to the stretcher 2 to stretch the laser pulse width from tens of femtoseconds to hundreds of picoseconds.

2. The stretched laser pulses enter the amplifier 6 via the

mirrors

4, 5.

3. And adjusting a ring cavity consisting of the titanium jewel 7, the

cavity mirrors

8, 9, 13 and 14, the electro-optical switch 11 and the polarization beam splitter 12 in the amplifier 6 to amplify the energy of the seed laser pulse. The energy of the regenerative amplifier 6 is provided by the laser pump source 3.

4. In the laser amplification process, the full width at half maximum of the amplified light spectrum is adjusted to be tens of nanometers by adjusting the angle of the SSF10, and the center wavelength of the spectrum is around 800 nm.

5. After the laser pulse is amplified to saturation in the annular cavity, the amplified laser pulse is reflected out of the amplifier by an electro-optical switch and a polarization beam splitter in the cavity.

6. The laser pulses enter a pulse compressor 16 for pulse width compression.

Experimental procedures and data:

in order to study the influence of SSF placed in and out of an amplifier cavity on the ASE contrast of laser pulses, the laser pulses with the output energy of 2.2nJ of a femtosecond oscillator are firstly transmitted into an Offner type stretcher after passing through an optical isolator, the Offner type stretcher stretches seed light to 400ps in the time domain, and the output efficiency of the energy reaches more than 50%. At the moment, an SSF is placed in an annular cavity of an amplifier, the broadened laser pulse is sent to a regenerative amplifier for amplification, the energy of the amplified laser pulse is output to be 8.6mJ by adjusting the angle of SSF laser incident in the cavity of the amplifier, the spectral width is 45nm (FWHM), and the amplified light passes through an electro-optical switch for controlAnd outputting by the polarization beam splitter. The amplified light is compressed to 26fs and energy is 6.4mJ through the grating pair compressor. The compressed laser pulse passes through a proper amount of attenuation sheet to attenuate the energy to 400 micro-focus, and is sent to a third-order auto-correlator Sequoia of French Amplified Technologies to measure the time contrast, and when the SSF is added in the cavity, the ASE contrast of 400ps before the laser main pulse is measured to be 1.0 multiplied by 10^-8. Then, the SSF in the amplifier cavity is arranged behind the stretcher and in front of the amplifier, and the SSF and the energy of the pump light are adjusted to ensure that the output energy is the same while the spectral widths of the two groups of experimental amplified light are the widest. Using the same method, the ASE contrast 400ps before the laser main pulse was measured to be 5.0X 10 when the SSF was applied to the cavity^-8。

FIG. 6 shows the experimental results of the present invention that placing SSF inside and outside the cavity of the amplifier has an effect on the ASE contrast, and it can be seen that the ASE in the ring cavity is better suppressed by the SSF inside the cavity than by the SSF outside the cavity, and the ASE contrast of the output laser pulse can be improved by about 5 times. This shows that the present invention has simple structure, stable output, high beam quality and high ASE time contrast. The pulse before the main pulse in fig. 6 is a false signal, which is an artifact caused by reflection light of the intra-cavity SSF and the attenuation sheet in the third-order autocorrelator.

Claims

1. A method and a device for improving the spontaneous radiation time contrast of femtosecond laser pulses comprise the following steps:

(1) the low-energy femtosecond laser pulse generated by the titanium gem femtosecond laser oscillator enters the stretcher and stretches the laser pulse to hundreds of picoseconds in the time domain;

(2) the broadened laser pulse enters a regenerative amplifier to amplify the energy;

(3) adjusting the angle of a spectral shaping filter in an annular cavity of the amplifier, and adjusting the full width at half maximum of an amplified light spectrum to dozens of nanometers, wherein the center wavelength of the spectrum is near 800 nm;

(4) the amplified light enters a pulse compressor to compress the pulses.

2. The device for realizing the method of claim 1, which consists of a femtosecond laser oscillator, a laser pump source, a pulse stretcher, a regenerative amplifier for spectrum shaping and filtering in a ring cavity, an electro-optical switch and a pulse compressor.

3. The method of claim 1, wherein the regenerative amplifier ring cavity comprises a gain crystal, a curved cavity mirror, a polarization beam splitter, an electro-optic switch, and a spectral shaping filter.

4. The method of claim 1, wherein the spectral shaping filter has a transmittance for different wavelengths of laser light that varies with the angle of incidence and polarization of the laser light.

5. The method of claim 1, wherein a spectral shaping filter is placed in the annular cavity of the amplifier, and the spontaneous emission time contrast is enhanced by adjusting the incident angle of laser light entering the spectral shaping filter in the annular cavity to suppress the generation of spontaneous emission in the cavity.

6. The apparatus of claim 2 wherein titanium doped sapphire is used as the gain crystal in the regenerative amplifier.

7. The apparatus of claim 2, wherein the pulse stretcher employs a transmissive grating as the dispersive material.

8. The apparatus of claim 2, wherein the pulse compressor employs a reflective grating as the dispersive material.