CN103872568A - Chirped pulse stretching compression amplification system for eliminating high-order dispersion - Google Patents

Abstract

A chirped pulse stretching compression amplification system that eliminates high-order dispersion, including a femtosecond pulse oscillation source, a main grating stretcher, a main amplifier, and a main grating compressor, and is characterized in that the system also includes an auxiliary stretcher and an auxiliary compressor , preamplifier, first mirror and second mirror. In the present invention, pulse stretching and compression are separated from system material dispersion compensation, so that the residual high-order dispersion of the final output optical pulse is reduced by several orders of magnitude, and may even be completely eliminated. The phase spectrum of the output laser pulse is flat, the pulse width close to the transformation limit can be obtained, the ultra-strong ultra-short pulse amplification with a width below 15fs is supported, and the contrast at the picosecond position before the output laser main pulse is improved.

Description

Chirped Pulse Stretching, Compression and Amplification System with Elimination of Higher Order Dispersion

technical field

The invention relates to the field of laser science, in particular to a chirped pulse time-domain stretching compression amplification system for eliminating high-order dispersion.

Background technique

Ultra-intensive and ultra-short laser science is an important frontier field of science, focusing on the development of ultra-intensive and ultra-short lasers, the interaction between ultra-intense and ultra-short lasers and matter, and cutting-edge basic research in interdisciplinary and related high-tech fields. . Among them, the miniaturized ultra-short and ultra-intense laser system that can output high-power pulses is the basic equipment for the research in the field of ultra-intensive and ultra-short laser science. The "ultra-strong and ultra-short" here refers to the extremely narrow time width of the laser pulse (generally less than 200 femtoseconds, femtoseconds are 10 ^-15 s, referred to as fs), and the peak power is extremely high (greater than 1 TW, that is, 10 ¹² W , referred to as TW). The time width of ultrashort pulses is restricted by the spectrum. The wider the spectrum, the narrower the pulse width that pulse compression can theoretically achieve, but in practice it will be affected by the uneven laser phase spectrum and ideal compression cannot be achieved.

The ultra-short and ultra-intense laser system adopts Chirped Pulse Amplification (CPA for short) as the basic technical route. A typical scheme such as the Ti:Sapphire laser system using CPA technology: the ultrashort pulse with weak intensity directly generated by the femtosecond laser oscillation source is first sent to the grating stretcher, and the grating introduces a strong chirp, and the pulse time width is thus determined. It is widened by a million times and becomes a long pulse in the order of nanoseconds (nanoseconds are 10 ^-9 s, referred to as ns). Such a long pulse obtains energy amplification in the Ti:Sapphire gain medium; finally, a grating compressor is used to eliminate the chirp in the laser pulse, so that the nanosecond long pulse is compressed back to a femtosecond ultrashort pulse, and finally achieves ultra-intense ultra-short laser pulse output. The laser pulse has the characteristics of a wide spectrum, and the Fourier transform limit of its spectrum determines the narrowest pulse width that can be compressed.

The output laser pulse of the ultra-intense and ultra-short laser system has high energy, which is easy to cause damage to the optical devices of the system itself. Therefore, the compressor is usually required to use reflective elements, the simpler the structure, the better, and the smaller the loss, the better. The simplest structure is a grating compressor composed of two pairs of parallel grating mirrors, which can produce negative dispersion. In addition to being related to the wavelength of the light itself, the amount of negative dispersion also depends on the density of the grating lines, the incident angle of the laser and the distance between parallel gratings. In the CPA system, the laser pulse has a wide spectral bandwidth of tens of nanometers, and the dispersion spectrum can be decomposed into first-order, second-order, third-order and higher-order dispersion at the central wavelength. Among them, the first-order dispersion represents the overall speed of the laser pulse, which has nothing to do with the research problem; the ratio of the third- and fourth-order dispersion to the second-order dispersion is only related to the incident angle of the laser; and when the incident angle of the laser is determined, the second-order dispersion Relevant only to grating spacing.

The essence of the grating stretcher of the CPA system is to use a 1:1 telescopic optical imaging system to form an image grating, and the object grating and the image grating are parallel to each other and have a "negative" distance. The dispersion formula of the grating compressor can be used in the ideal stretcher completely, and the only modification required is to change the grating pitch into a negative value. That is to say, in theory, the dispersion of the ideal stretcher is opposite to that of the compressor, and they can achieve fidelity compensation for each other.

In the actual CPA system, the laser must pass through various optical components in the system, such as laser amplification medium, nonlinear crystal, lens and so on. These optical elements with positive material dispersion change the chirp amount of the transmitted laser light. Therefore, for the actual CPA system, the parameters between the stretcher and the compressor will be different.

In the prior art, the femtosecond laser oscillation source 1 generates ultrashort pulses, which are first sent to the grating stretcher 2, and the width becomes a long pulse on the order of nanoseconds (nanoseconds, or 10 ^-9 s). Such a long pulse passes through the multi-stage amplifier 3 to obtain energy amplification; the amplified pulse passes through the grating compressor 4, so that the nanosecond long pulse is compressed back to the femtosecond ultrashort pulse, and finally realizes the output of ultra-intense ultra-short laser pulses, such as image 3. Among them, the dispersion compensation of the CPA system follows the following method: by adjusting the parallel grating distance of the compressor, the amount of second-order dispersion can be adjusted to compensate for the second-order dispersion of the stretcher and system materials; by adjusting the incident angle of the grating of the compressor, to compensate Third-order dispersion of stretcher and system materials.

Under the above technical solution, the fourth-order or even higher-order dispersion cannot be compensated. The amount of residual high-order dispersion is very large, mainly provided by the dispersion compressor, compared to which, the amount of high-order dispersion inherent in the system material is negligible. The greater the stretching ability of the grating stretcher (that is, the wider the long pulse after stretching), the greater the amount of residual high-order dispersion. High-order dispersion will affect the width of the compressed optical pulse. The larger the high-order dispersion, the narrower the flat area of the laser phase spectrum, and the wider the compressed optical pulse. Figure 2 shows the results of computer simulation calculations, in which the initial light pulse has a Gaussian shape, the initial width is 15fs, the full width at half maximum of the spectrum is 60nm, and the second and third order dispersions are 0. As shown in Figure 2a, if the residual fourth-order dispersion is 10 ⁶ fs ⁴ , the width of the phase spectrum flat region (less than 1 radian) is 50nm, and the Gaussian pulse with a full width at half maximum of about 60nm (corresponding to a pulse width of 15fs) can only be compressed to 49fs; as shown in the figure 2b, when the residual fourth-order dispersion is 10 ⁵ fs ⁴ , the width of the phase spectrum flat region (less than 1 radian) is 80nm, and the pulse width can be compressed to 28fs. In practice, the residual dispersion of a high-energy ultra-intense ultra-short laser system usually exceeds 10 ⁵ fs ⁴ , and the wide-spectrum output pulse can only support an output of 30 fs. In order to increase the peak power of the output laser pulse, it is necessary to eliminate the higher-order dispersion, or at least reduce it to an acceptable level for the system.

Contents of the invention

The purpose of the present invention is to overcome the above-mentioned deficiencies in the prior art, and provide a chirped pulse time-domain stretching compression amplification system that eliminates high-order dispersion, so as to expand the range of the flat region of the laser output pulse phase spectrum and further reduce the output pulse width. , to increase the laser peak power, so as to support the amplification of ultra-strong ultra-short pulses with a width below 15 fs; and because of the flat phase spectrum, it provides practical significance for increasing the spectral bandwidth of the CPA system; in addition, it will also increase the output of the main pulse. The contrast of the second position meets the requirements of related physical experiments.

Technical solution of the present invention is as follows:

A chirped pulse stretching compression amplification system that eliminates high-order dispersion, including a femtosecond pulse oscillation source, a main grating stretcher, a main amplifier, and a main grating compressor, and is characterized in that the system also includes an auxiliary stretcher and an auxiliary compressor , a preamplifier, a first reflector and a second reflector, the relative positions of the components are as follows:

The femtosecond-level ultrashort pulse generated by the femtosecond pulse oscillation source first enters the auxiliary grating stretcher and is stretched into a short pulse. The short pulse enters the preamplifier, receives energy under the gain of the preamplifier, and realizes preliminary amplification. The pre-amplified short pulses enter the auxiliary compressor and are partially compressed back to hundreds of femtosecond pulses. Under the guidance of the first mirror, the hundreds of femtosecond pulses enter the main grating stretcher to be stretched to long pulses, and then enter the main amplifier under the guidance of the second mirror, and receive energy under the gain of the main amplifier to realize energy amplification. The energy-amplified long pulse enters the main grating compressor and is compressed into a femtosecond-level ultrashort pulse.

The femtosecond pulse oscillation source is a titanium sapphire femtosecond mode-locked pulse laser, a dye laser or a fiber mode-locked laser.

The auxiliary stretcher is composed of a grating stretcher, a fiber stretcher, and a prism equivalent dispersion element, which provides a relatively small amount of chirp for the optical pulse, and stretches the femtosecond optical pulse to the order of picoseconds to hundreds of picoseconds.

The auxiliary compressor is composed of a grating compressor, or an optical fiber compressor, or a prism equivalent dispersion element, which is used to compensate the dispersion caused by the auxiliary stretcher and all materials of the system, without introducing the main grating stretcher and the main grating In the case of a compressor, short pulses are compressed and stretched back to femtosecond-scale ultrashort pulses at the end of the main amplifier.

The preamplifier is a laser amplification technology based on a laser medium, such as titanium sapphire, or an optical parametric amplification technology based on a nonlinear crystal, such as a BBO crystal.

The main grating stretcher is based on a grating and has a large stretching ability. The main grating stretcher can stretch the optical pulse to nanosecond level.

The main amplifier is a laser amplification technology based on a laser medium, such as titanium sapphire, or an optical parametric amplification technology based on a nonlinear crystal, such as a BBO crystal.

The main grating compressor is based on the grating and has a large compression capability. The main grating compressor is used to compensate the pulse chirp caused by the main stretcher, and can compress the nanosecond level optical pulse back to the limit pulse width of the femtosecond level.

The present invention has the following innovative points:

The chirped pulse stretching compression amplification system for eliminating high-order dispersion in the present invention innovatively adopts two sets of stretching compressors: one set of main stretching compressors is only used to stretch femtosecond ultrashort pulses into nanosecond long pulses, and After the pulse energy amplification is completed, the nanosecond-level long pulse is compressed back to the femtosecond ultrashort pulse with fidelity. The main stretching compressor is not responsible for the compensation of material dispersion in the system; another set of auxiliary stretching compressor is mainly used for material dispersion in the system. Compensation for chromatic dispersion, in addition, can also be used in non-linear pulse cleanup techniques where pulse cleanup is required to improve pulse contrast.

The present invention has the following technical effects:

The innovative feature of the separation of pulse stretching and compression from system material dispersion compensation of the present invention reduces residual high-order dispersion by several orders of magnitude, and may even completely eliminate it. The phase spectrum of the laser pulse output by the CPA is flat, and the pulse width close to the transformation limit can be obtained, which can support the amplification of ultrashort and ultra-intense pulses with a width below 15fs, and improve the contrast of the picosecond position before the output laser main pulse. The ultra-short and ultra-intense laser system with 30fs level output of the present invention can obtain further improvement of the output peak power on the basis of the existing output capability.

Description of drawings

Figure 1 is a schematic diagram of the common structure of an ultra-intense and ultra-short laser amplification system

Figure 2 shows the effect of fourth-order dispersion on laser pulse compression

Fig. 3 is a structural schematic diagram of a chirped pulse stretching compression amplification system for eliminating high-order dispersion according to the present invention.

Detailed ways

FIG. 3 is a schematic structural diagram of a specific embodiment of the chirped pulse stretching compression amplification system for eliminating high-order dispersion according to the present invention. It can be seen from the figure that the chirped pulse stretching compression amplification system for eliminating high-order dispersion of the present invention includes a femtosecond pulse oscillation source 5, an auxiliary stretcher 6, a preamplifier 7, an auxiliary compressor 8, a first mirror 9, and a main grating stretcher A device 10, a second mirror 11, a main amplifier 12, and a main grating compressor 13 are formed.

Among them, the optical pulse output by the femtosecond pulse oscillation source 5 has the characteristics of extremely narrow pulse width and wide spectral width. In the present invention, the femtosecond pulse oscillation source 5 is used as a seed source, which can be a mode-locked pulse laser such as a Ti:Sapphire femtosecond mode-locked pulse laser, a dye laser, or a fiber mode-locked laser. In this embodiment, the femtosecond pulse oscillation source 5 is a titanium sapphire femtosecond mode-locked pulse laser, the central wavelength of the optical pulse is 800 nm, the spectral bandwidth is 100 nm, and the pulse width is 10 fs.

Among them, the feature of the auxiliary stretcher 6 is that it can stretch the femtosecond light pulse to the order of picoseconds to hundreds of picoseconds. In the present invention, it may be composed of a grating stretcher, a fiber stretcher and prism equivalent dispersion elements. In this embodiment, the auxiliary stretcher 6 is a grating stretcher with a grating of 1200 lines/mm, an incident angle of light pulses of 43°, and a total distance of 80 cm between object and image gratings in the direction of diffracted light at the central wavelength. The radius of the concave mirror is 100cm, the radius of the convex mirror is 50cm, and the bandpass is 100nm.

Wherein the characteristic of the preamplifier 7 is to provide preliminary amplification for the pulse. In the present invention, it may be a laser amplification technology based on a laser medium, such as titanium sapphire, or an optical parametric amplification technology based on a nonlinear crystal, such as a BBO crystal. In this embodiment, the preamplifier 7 adopts Ti:Sapphire laser amplification technology.

The feature of the auxiliary compressor 8 is that it can compensate the dispersion caused by the auxiliary stretcher and all materials of the system. In the present invention, it may be composed of a grating compressor, or a fiber compressor, or a prism equivalent dispersion element. In this embodiment, the auxiliary compressor 8 adopts a grating compressor, the grating is 1200 lines/mm, the incident angle of the light pulse is about 45°, and the total distance between the parallel gratings is about 81 cm in the direction of the central wavelength diffracted light. Bandpass 100nm.

Wherein, the characteristic of the first reflector 9 and the second reflector 11 is to guide the light beam into the main stretcher. In this embodiment, both the first mirror 9 and the second mirror 11 are silver mirrors, which can support a bandwidth of more than 100 nm.

Among them, the main grating stretcher 10 is characterized by a large stretching capability, which can stretch the light pulse to the nanosecond level. In this embodiment, the main grating stretcher 10 is a grating with 1400 lines/mm, the incident angle of the light pulse is 50°, and the total pitch of the object-image grating is 160 cm in the direction of the central wavelength diffracted light. . The radius of the concave mirror is 120cm, the radius of the convex mirror is 60cm, and the bandpass is 100nm. Capable of stretching to 2ns

The characteristic of the main amplifier 12 is to provide energy amplification for the pulse. In the present invention, it may be a laser amplification technology based on a laser medium, such as titanium sapphire, or an optical parametric amplification technology based on a nonlinear crystal, such as a BBO crystal. In this embodiment, the energy amplifier 12 adopts Ti:Sapphire laser amplification technology.

Among them, the main grating compressor 13 is characterized by a grating-based compressor with a large compression capacity, which can compensate the pulse chirp caused by the main stretcher, and compress the nanosecond-level optical pulse back to the femtosecond-level limit pulse width. In this embodiment, the auxiliary compressor 8 adopts a grating compressor, the grating is 1480 lines/mm, the incident angle of the light pulse is 50°, the total spacing of the parallel gratings is 160 cm in the direction of the center wavelength diffracted light, and the bandpass is 100 nm.

The working process of this embodiment is as follows:

(1) The femtosecond pulse oscillation source 5 generates an ultrashort mode-locked pulse beam with a single pulse width of 10fs in the 800nm band;

(2) The ultrashort pulse beam enters the auxiliary stretcher 6 and is stretched to 20 ps, and the stretched short pulse enters the preamplifier 7 . The preamplifier 7 operates, and after the energy is amplified to the microjoule level, the light pulse enters the auxiliary compressor 8 .

(3) The light pulse enters the main amplifier 12 directly after passing through the auxiliary compressor 8 . The optical pulse width is measured after the main amplifier 12 (which does not need to be running). Based on the measurement results, the incident angle and grating spacing of the auxiliary compressor 8 are adjusted to achieve the shortest pulse width and realize system dispersion compensation.

(4) Insert the first mirror 9 into the optical path before the main amplifier 12 to guide the light beam to the main grating stretcher 10 . Under the action of the grating stretcher 10, the pulse is stretched to 2ns. The 2ns long pulse laser is led back to the main amplifier 12 by the second mirror 11 .

(5) The main amplifier 12 operates, and the energy of the light pulse is amplified to an energy greater than 1 Joule.

(6) The amplified pulse enters the main compressor 13. The optical pulse width is measured after the main compressor 13 . Based on the measurement result, the incident angle and the grating spacing of the main compressor 13 are adjusted to achieve the shortest pulse width and realize the dispersion compensation of the main stretcher.

After careful debugging, such a device can obtain a pulse width output close to the transformation limit, and can generate ultra-intense ultra-short optical pulses below 15 fs.

Claims (6)

1. A chirped pulse stretching compression amplification system for eliminating high-order dispersion, comprising a femtosecond pulse oscillation source, a main grating stretcher, a main amplifier and a main grating compressor, characterized in that the system also includes an auxiliary stretcher, an auxiliary compression device, preamplifier, first reflector and second reflector, the relative positional relationship of the components is as follows: The femtosecond-level ultrashort pulse generated by the femtosecond pulse oscillation source is stretched into a short pulse by the auxiliary grating stretcher, and the short pulse enters the pre-amplifier and the pre-amplified short pulse enters the auxiliary compressor to be Partially compressed back to a hundred femtosecond pulse, the hundred femtosecond pulse enters the main grating stretcher under the guidance of the first reflector to expand to a long pulse, and enters the main amplifier under the guidance of the second reflector to realize Energy amplification, the energy-amplified long pulse enters the main grating compressor, and is compressed into a femtosecond-level ultrashort pulse. 2. The chirped pulse stretching compression amplification system for eliminating high-order dispersion according to claim 1, wherein said femtosecond pulse oscillation source is a titanium sapphire femtosecond mode-locked pulse laser, a dye laser or a fiber mode-locked laser wait. 3. The chirped pulse stretching compression amplification system for eliminating high-order dispersion according to claim 1, characterized in that said auxiliary stretcher and main grating stretcher are composed of grating stretcher, fiber stretcher and prism pair dispersion element . 4. The chirped pulse stretching compression amplification system for eliminating high-order dispersion according to claim 1, wherein the auxiliary compressor is composed of a grating compressor, an optical fiber compressor, or a prism equivalent dispersion element. 5. The chirped pulse stretching compression amplification system for eliminating high-order dispersion according to claim 1, wherein said preamplifier and main amplifier are laser amplifiers based on laser media, or optical amplifiers based on nonlinear crystals. parametric amplifier. 6. The chirped pulse stretching compression amplification system for eliminating high-order dispersion according to claim 1, characterized in that said main grating compressor is based on a grating and has a large compression capability.

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