JP3536058B2 - Pulse width compression method for short pulse laser light - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of compressing a pulse width of a short pulse laser beam, and more specifically, a pulse width of about femtosecond ( ^10-15 seconds) to picosecond ( ^10-12 seconds). The present invention relates to a simple pulse width compression method for short pulse laser light which is effective in compressing the pulse width of a short pulse laser light of a table. For example, soft X-ray coherent light to be a light source of an X-ray microscope is generated. Lasers for excitation, which are required for high intensity and short pulse, or for processing difficult-to-process materials using ablation action, short-pulse laser light of various wavelengths, or ultrafast chemical reactions The present invention relates to a pulse width compression method for short pulse laser light, which is suitable for use in high-intensity short pulse laser light in creating new materials.

[0002]

2. Description of the Related Art Conventionally, in the process of generating and utilizing a high-intensity short-pulse laser beam such as a titanium-sapphire laser, a technique for compressing the pulse width has occupy an important position.

That is, a short pulse laser beam having a pulse width on the order of femtoseconds to picoseconds is required to have a wide spectrum width inversely proportional to the pulse width. Requires a wide band of the spectrum and a pulse compression technique for controlling the phase of each wavelength component of the wide spectrum.

Further, it is known that the pulse width of this short pulse laser light is elongated by wavelength dispersion when it is transmitted through or propagated in a substance.
In various applications of short-pulse laser light, it is necessary to recompress the pulse width thus extended.

Therefore, as described above, as a means for broadening the spectrum required for the generation of short-pulse laser light, and for compressing a pulse having a wide spectrum but a pulse width not sufficiently compressed. As a method of generating a short pulse laser light by doing so, by further recompressing the pulse width of the short pulse laser light that has been stretched while transmitting or propagating in the substance, Various techniques have been proposed as a method for returning the pulse width to, for example, laser light is incident on a hollow fiber filled with various gases,
The spectrum width of the laser light is expanded by the self-phase modulation effect that occurs during propagation, and the phase of the emitted laser light is controlled by using a prism pair consisting of two prisms or a diffraction grating pair consisting of two diffraction gratings. By doing so, it is known to compress the pulse width.

Here, in the extension of the spectrum by the hollow fiber, the beam diameter of the incident laser light must be precisely adjusted so that the laser light propagates in the lowest harmonic mode of the hollow fiber. There wasn't.

Further, in order to make the laser light incident on the hollow fiber having an inner diameter of about 100 μm, the laser light and the hollow fiber must be accurately aligned.

If these adjustments are inadequate, there is a problem that the loss when the laser light propagates through the hollow fiber becomes large, and the incident end face of the hollow fiber is easily damaged. There was a problem.

Further, in pulse width compression by dispersion compensation using a prism pair consisting of two prisms or a diffraction grating pair consisting of two diffraction gratings, the distance between the prism pairs and the amount of insertion, or the distance between the diffraction grating pairs. There is a problem that the angle must be adjusted precisely.

When a short pulse laser beam having a pulse width on the order of femtoseconds to picoseconds is generated, and while the short pulsed laser light is transmitted through or propagates through a substance. In the case of re-compressing the pulse width that has been elongated due to wavelength dispersion, it requires a means having a complicated mechanism as described above, which significantly impairs the versatility of short-pulse laser light in various application fields. There was a problem.

[0011]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned various problems of the prior art, and an object thereof is to generate a short pulse laser beam. In some cases, and in the case of recompressing the pulse width of a short pulse laser light that has been elongated while transmitting or propagating in a substance, a simple short mechanism that does not require a precise adjustment mechanism is used. pulse·
An object of the present invention is to provide a pulse width compression method for laser light.

[0012]

In order to achieve the above object, the present invention provides a simple method of condensing short pulse laser light such as femtosecond laser light or picosecond laser light in a nonlinear medium. The pulse width of the short pulse laser light can be compressed.

That is, according to the first aspect of the present invention, the short pulse laser beam is condensed and incident on the nonlinear medium, and the short pulse laser beam is self-focused by the nonlinear medium. The pulse width of the short pulse laser light is compressed to a pulse width shorter than the original pulse width.

Here, the non-linear medium may be a gas having a predetermined pressure, as in the invention described in claim 2 of the present invention.

Further, the gas having the predetermined pressure can be air in the atmosphere as in the invention according to claim 3 of the present invention.

The non-linear medium may be a solid material as in the fourth aspect of the present invention.

Further, the solid material may be a quartz plate having a predetermined thickness placed in the atmosphere, as in the invention according to claim 5 of the present invention.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An example of an embodiment of a pulse width compression method for short pulse laser light according to the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 1 is a schematic structural explanatory view showing an example of the structure of a pulse width compression device for carrying out the pulse width compression method for short pulse laser light according to the present invention.

The pulse width compression device 10 includes a reflecting mirror 12 for reflecting a short pulse laser light (incident short pulse laser light) such as an incident femtosecond laser light or picosecond laser light, and a reflecting mirror 12. A lens 14 that collects the reflected short pulse laser light, a high pressure cell 16 on which the short pulse laser light collected by the lens 14 is incident, and a short pulse that is emitted from the high pressure cell 16
A lens 18 for collimating laser light, a beam splitter 20 for splitting parallel short-pulse laser light emitted from the lens 2 into two directions, and one short-pulse laser split by the beam splitter 20. And an autocorrelator 22 that measures the pulse width of the short pulse laser light by injecting light, and emits the other short pulse laser light split by the beam splitter 20 (emission short pulse laser It is designed to get as light).

Here, the high-pressure cell 16 has an entrance window 16a into which the short pulse laser light is incident, an exit window 16b from which the short pulse laser light is exited, and these entrance window 16a and the exit window. The cell main body 16c which is located between 16b and can be filled with various gases as a nonlinear medium at an arbitrary pressure and sealed.

Here, various gases such as air, rare gas, and molecular gas can be used as the gas filled and sealed in the cell body 16c, but a single kind of gas is filled. It is preferable to enclose it. .

Specifically, as the rare gas, for example, He, Ne, Ar, Kr, Xe can be used, and as the molecular gas, for example, H ₂ , D ₂ , N ₂ ,
O ₂ , F ₂ , Cl ₂ , CO, NO, N ₂ O, CO ₂ , N
H ₃ , CH ₄ , and SF ₆ can be used.

The lens 14 is provided with a focal length such that, for example, the short pulse laser light can be condensed within the cell body 16c of the high pressure cell 16, and in particular, the high pressure cell 16 has a focal length. It is preferable that the cell body 16c has a focal length that allows light to be condensed in the vicinity of the center thereof.

The lens 18 is adapted to the divergence angle of a component (described later) consisting of the vicinity of the peak of the pulse of the short pulse laser light emitted from the high pressure cell 16 and makes this component parallel light. It has a curvature.

In the above structure, the cell body 16c of the high pressure cell 16 is filled with various gases such as air, methane, krypton or argon as a non-linear medium and sealed. The type of gas filled and sealed in the cell body 16c is not limited to a single type, but may be a mixed gas in which a plurality of types of gas are mixed. However, it is preferable to avoid the combination of the combustible gas and the combustion supporting gas.

Then, the incident short pulse laser light is reflected by the reflecting mirror 12 and enters the lens 14, and the lens 1
4 is a high-pressure cell 16 that receives the incident short-pulse laser light
The light is emitted so as to be condensed in the cell body 16c.

Here, the cell body 16 of the high pressure cell 16
In the vicinity of the focal point in c, the short-pulse laser light incident in the cell body 16c causes self-convergence due to the transient change in the nonlinear refractive index of the gas (gas medium) enclosed as the nonlinear medium. Become. The refractive index n (r, z, t) at this time is calculated by the following Equation 1.

Here, the refractive index n (r, z, t) is r,
It is a function of z and t, where r is "the direction orthogonal to the traveling direction of the short pulse laser light", z is "the traveling direction of the short pulse laser light", and t is "time". .

N (r, z, t) = n ₀ + n ₂ I (r, z, t) Equation 1 In the above Equation 1, n ₀ is a gas medium at the wavelength of the short pulse laser light. Is the refractive index of.

N ₂ is the nonlinear refractive index of the gas medium at the wavelength of the short pulse laser light.

I (r, z, t): Short pulse laser beam intensity. This I (r, z, t) is a function of the direction r orthogonal to the traveling direction of the short pulse laser light, the traveling direction z of the short pulse laser light, and the time t.

As described above, the refractive index n (r,
z, t) is also given along with I (r, z, t) as a function of the direction r orthogonal to the traveling direction of the short pulse laser light, the traveling direction z of the short pulse laser light, and the time t.

By the way, the intensity distribution of the short pulse laser light decreases on the optical axis, that is, from "r = 0" toward the periphery with the increase of r, so that the refractive index n (r, z, t )
Has a similar distribution, and apparently the short-pulse laser light is converged as when passing through the lens. That is, the short pulse laser light causes self-focusing,
The actual focus is provided before the focus of the lens 14.

Due to the self-focusing action of the short pulse laser light, the pulse width of the short pulse laser is compressed to a pulse width shorter than the original pulse width.

In the present specification, the effect of compressing to a pulse width shorter than the original pulse width by the above-mentioned self-focusing action is referred to as "self-pulse compression effect".

The action of self-focusing described above exhibits a very fast time response that can follow the time change of the intensity of the short pulse laser light.

The strongest self-convergence occurs near the peak of the short pulse laser light on the time axis, that is, near the peak of the pulse, and this component has the actual focal point closest to the focal point of the lens 14, On the other hand, the self-convergence is relatively weak at the rising portion and the falling portion of the pulse, and it is considered that this component has the actual focus closest to the focus of the lens 14.

Therefore, the size of the beam and the divergence angle of the beam are different between the vicinity of the peak of the pulse of the short pulse laser light and the rising portion and the falling portion of the pulse. When passing through the exit window portion 16b of the cell 16 and exiting from the high pressure cell 16 to the outside, it is considered that they will appear as separate beams.

Therefore, of the components appearing as separate beams as described above, if only the component near the peak of the pulse of the short pulse laser light is extracted, it is compressed to a pulse width shorter than the original pulse width. It will be possible to obtain short pulsed laser light.

Therefore, in this embodiment, among the components appearing as separate beams as described above,
Only the component near the peak of the pulse of the short pulse laser light is configured to be extracted and emitted as parallel light by the lens 18.

That is, the short pulse laser light compressed to the shortest pulse width than the original pulse width is emitted from the lens 18 as parallel light.

Further, in this embodiment, the parallel short pulse laser light emitted from the lens 18 is split into two directions by the beam splitter 20, and one short pulse split by the beam splitter 20. The laser beam is made incident on the autocorrelation meter 22 to measure the pulse width of the short pulse laser beam, and the other short pulse laser beam split by the beam splitter 20 is emitted. It is designed so that it can be used as emitted light (emitted short pulse laser light).

Here, FIG. 2 shows experimental results when the cell body 16c of the high-pressure cell 16 was filled with methane (CH ₄ ) and sealed by using the pulse width compression device shown in FIG. ing.

In this experiment, as the incident short pulse laser light, the emitted light of a titanium sapphire laser having a wavelength of 745 nm is used.

The incident short pulse laser light is
The intensity (laser energy) was 1 mJ, the pulse width was 100 fs, the beam diameter was 4 mm, and the spectrum width was 10 nm.

Further, the lens 14 has a focal length f
Of 30 cm was used.

Under the above-mentioned experimental conditions, the beam splitter 20 was changed while changing the pressure of the methane filled and sealed in the cell body 16c of the high pressure cell 16.
When one of the short pulse laser beams divided by was incident on the autocorrelator 22 and the pulse width of the short pulse laser beam was measured, the experimental result shown in the graph of FIG. 2 was obtained.

In the graph shown in FIG. 2, the vertical axis represents the pulse width (Pulsewidt) measured by the autocorrelator 22.
h), and the methane pressure (CH ₄ Pressu) on the horizontal axis.
re).

Here, when the spectrum width of the short pulse laser light measured by the autocorrelator 22 is measured,
It spread from 10 nm to 20 nm.

The Fourier transform limit width (Fourier Transform) having a spectrum width of 20 nm is obtained.
Limit) is 30 fs, but as shown in the experimental result shown in FIG. 2, when the method of the present invention is used, the self-pulse compression effect acts and the methane pressure is approximately 1 atm to 1.5 at.
In the region of m and the region of approximately 4 atm to 6 atm, the pulse width could be compressed to a value close to the Fourier transform limit width.

Further, FIG. 3 shows conditions under which the self-pulse compression effect by the method of the present invention acts, specifically, "laser energy", "wavelength of short pulse laser light", and "wavelength of short pulse laser light". Pulse width of short pulse laser light ",
Conditions relating to “focusing conditions of short pulse laser light” and “type and pressure of gas as nonlinear medium” are shown respectively, but by appropriately combining these, self-pulse compression by the method of the present invention The effect can be activated.

Next, FIG. 4 shows a short pulse according to the present invention.
The schematic structure explanatory drawing of the other example of a structure of the pulse width compression apparatus for implementing the pulse width compression method of a laser beam is shown.

In the pulse width compression apparatus shown in FIG. 4, the same or corresponding components as those of the pulse width compression apparatus shown in FIG. 1 are denoted by the same reference numerals as those used in FIG. Detailed description of the configuration and operation will be omitted.

The pulse width compression device 20 shown in FIG.
This is different from the pulse width compression apparatus shown in FIG. 1 in that air in the atmosphere is used as a nonlinear medium without using the high pressure cell 16.

Using the pulse width compression device 20 described above,
In order to exert the self-pulse compression effect by the method of the present invention to compress the pulse width of the short pulse laser light, for example, as shown in FIG. 5, the wavelength of the incident short pulse laser light is set to 720 nm to 860 nm. , The intensity of the incident short pulse laser light (laser energy) is 2 mJ-1
The pulse width of the incident short pulse laser light is 3 m
The focusing condition may be 0 fs to 300 fs, and the lens 14 may have a focal length f of 30 cm to 150 cm.

In this way, the self-pulse compression effect according to the method of the present invention is actuated, and the pulse width of the incident short pulse laser light can be compressed to, for example, near the Fourier transform limit width.

Moreover, when the pulse width compressing device 20 shown in FIG. 4 is used, the pulse width of the short pulse laser light can be compressed in the atmosphere, so that the short pulse The pulse width of laser light can be compressed.

Next, FIG. 6 shows a short pulse according to the present invention.
The schematic structure explanatory drawing of the further another example of a structure of the pulse width compression apparatus for implementing the pulse width compression method of a laser beam is shown.

In the pulse width compression apparatus shown in FIG. 6, the same or corresponding components as those of the pulse width compression apparatus shown in FIG. 1 are indicated by the same reference numerals as those used in FIG. Detailed description of the configuration and operation will be omitted.

The pulse width compression device 30 shown in FIG.
In terms of using the quartz plate 32, which is a solid material, as the nonlinear medium, instead of using the gas as the nonlinear medium,
This is different from the pulse width compression device shown in FIG. The quartz plate 32 may be arranged in the atmosphere, and the thickness t thereof may be, for example, 1 mm to 2 mm, and the density is preferably constant.

In this pulse width compressing device 30, the lens 14 uses, for example, a short pulse laser beam to generate the quartz plate 3.
2 is provided with a focal length capable of condensing light immediately before the quartz plate 32, and particularly, has a focal length capable of condensing light immediately before the quartz plate 32. It is preferable.

Even when the pulse width compressing device 30 described above is used, the self-pulse compression effect by the method of the present invention is exerted to change the pulse width of the incident short pulse laser light to, for example,
It becomes possible to compress near the Fourier transform limit width.

Moreover, when the pulse width compression device 30 shown in FIG. 6 is used, the pulse width of the short pulse laser light can be compressed in the atmosphere, so that the short pulse The pulse width of laser light can be compressed.

In the pulse width compression device 30 shown in FIG. 6, the quartz plate 32 is used as a solid material as a nonlinear medium, but it is needless to say that the quartz plate 32 is not limited to this. As shown, LiF, Mg
F ₂ , CaF ₂ , BaF ₂ , NaCl, KCl, KB
r, SiO ₂ , Al ₂ O ₃ , BK7, various flint glasses, crown glasses, ZnS, ZnSe, etc. can be used.

[0066]

EFFECTS OF THE INVENTION Since the present invention is constructed as described above, when the short pulse laser light is generated, it is extended while being transmitted or propagated through the substance. In the case of re-compressing the pulse width of a short pulse laser beam that has been trapped, it is possible to provide a simple method for compressing the pulse width of a short pulse laser beam that does not require a precise adjustment mechanism.

[Brief description of drawings]

FIG. 1 is a schematic configuration explanatory diagram of an example of a configuration of a pulse width compression device for carrying out a pulse width compression method for short pulse laser light according to the present invention.

FIG. 2 is a graph showing an experimental result when methane (CH ₄ ) is filled and sealed in a cell body of a high pressure cell using the pulse width compression device shown in FIG.

FIG. 3 is a chart showing conditions under which a self-pulse compression effect according to the method of the present invention works.

FIG. 4 is a schematic configuration explanatory diagram of another example of the configuration of the pulse width compression device for implementing the pulse width compression method for short pulse laser light according to the present invention.

FIG. 5 is a chart showing conditions under which a self-pulse compression effect works in the atmosphere by the method of the present invention.

FIG. 6 is a schematic configuration explanatory diagram of still another example of the configuration of the pulse width compression device for implementing the pulse width compression method for short pulse laser light according to the present invention.

FIG. 7 is a chart showing solid materials exhibiting self-pulse compression effect.

[Explanation of symbols]

10, 20, 30 pulse width compressor 12 Reflector 14 lenses 16 high pressure cell 16a entrance window 16b exit window 16c cell body 18 lenses 20 beam splitter 22 Autocorrelator 32 quartz plate

─────────────────────────────────────────────────── ─── Continuation of the front page Patent Law Article 30 Clause 1 Application is applied RIKEN symposium 3rd coherent science-from light to physical system-Announcement with documents Patent Law Article 30 Clause 1 Application is applied CLEO / Paci Fic Rim '99 announced with a document Patent application of Article 30 (1) of the Patent Act Jpn. Appl. Phys. Vol. 38ppL978-l980 to the announcement published in the Patent Law Article 30 Paragraph 1 applied application there Nikkan Kogyo Shimbun, Ltd. 200 17 January (58) investigated the field (Int.Cl. ^7, DB name) G02F 1/35 - 1 / 39 IEEE Digital Library

Claims (5)

(57) [Claims] 1. A short pulse laser beam is focused and incident on a nonlinear medium, and the short pulse laser beam in the nonlinear medium is collected.
Due to the self-focusing action of the laser light, the short pulse
A pulse width compression method for short pulse laser light that compresses the pulse width of the laser light to a pulse width shorter than the original pulse width. 2. The pulse width compression method for short pulse laser light according to claim 1, wherein the nonlinear medium is a gas having a predetermined pressure.
Laser light pulse width compression method. 3. The pulse width compression method for short pulse laser light according to claim 2, wherein the gas having the predetermined pressure is air in the atmosphere. 4. The pulse width compression method for short pulse laser light according to claim 1, wherein the nonlinear medium is a solid material, and the pulse width compression method for short pulse laser light is used. 5. The pulse width compression method for short pulse laser light according to claim 4, wherein the solid material is a quartz plate having a predetermined thickness and placed in the atmosphere. Width compression method.