JP3081889B2 - Laser pulse width compression method and apparatus - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser pulse width compression method and apparatus for shortening a laser beam having a relatively long pulse width to a single pulse and increasing the intensity.

[0002]

2. Description of the Related Art In various fields of measurement and processing, it is often desired to greatly reduce the pulse width and increase the intensity of laser pulse light. On the other hand, in the related art, a technique for generating forward stimulated Raman scattered light whose purpose is limited to wavelength conversion has already been proposed and put to practical use. Also, the backward stimulated Raman scattering phenomenon that causes the compression of the pulse width and the enhancement of the intensity has been already known.

FIG. 2A shows a schematic configuration of a conventional apparatus for the latter, and FIGS. 2B and 2C show a laser beam Lb incident on the apparatus and a laser pulse train obtained from the apparatus. A waveform example of Is is shown. In order to explain, a convex lens (condensing lens) 13 is simply used as a condenser lens system in contrast to the Raman cell 11 which is already supplied as a product in the market, and a relatively long lens as shown in FIG. Laser beam Lb with pulse width
When the light is made incident while the aperture is stopped down, the backward stimulated Raman scattered light generated by the presence of the Raman medium 12, such as methane gas filled in the Raman cell 11, has a period corresponding to the type of the Raman medium 12 as shown in FIG. Thus, each becomes a laser pulse train Is having a relatively short pulse width.

That is, in the Raman cell 11 filled with the Raman medium 12, a natural Stokes light grows from the noise level at the laser energy concentrated portion near the focal point of the convex lens 13, and the laser light Lb The backward stimulated Raman scattered light (backward Stokes light) traveling in the direction grows while consuming the energy of the incident laser light Lb over a distance of about the gain length, and the peak power increases. The width becomes shorter and the pulse becomes shorter. And
When the shortened Stokes light passes through the gain region, natural Stokes light is generated again by the incident laser light,
It goes through the same amplification and short pulse process. As a result, the rear Stokes light Is output backward from the Raman cell 11 and selectively separated and extracted by the beam splitter 14 is shown in FIG.
As shown in (C), a pulse train Is having a period according to the type of the Raman medium 12 is obtained.

[0005]

As described above, the laser pulse light Is shortened as a pulse train by utilizing the power multiplication and the self-compression of the pulse width by the backward Raman amplification process. I was successful in getting it. However, there is also a high demand for obtaining a single-pulse laser beam Ip with high intensity as shown schematically in FIG. 2D instead of a pulse train. The present invention has basically been made to meet this demand, and while being simple in terms of apparatus, a method and apparatus capable of shortening and increasing the intensity of various laser pulse lights as single pulses in principle. Is not proposed.

[0006]

In order to achieve the above object, the present invention proposes to fill a Raman cell with two kinds of Raman media. In this way, based on the difference between the rise times of the vibrational levels of the two types of Raman media,
The first backward Stokes light obtained as a result of Raman scattering by a rapidly rising Raman medium undergoes power multiplication in the backward Raman scattering process, causes self-compression of the pulse width, and is output backward from the Raman cell. The second and subsequent pulses in the pulse train of the Raman medium are suppressed by Raman scattered light from the slowly rising Raman medium, and eventually only a single short pulse with increased intensity is output.

As is well known in this field, the Raman medium refers to a medium that can cause stimulated Raman scattering, and many media have been reported. Are different from each other, so that the combination of the mixed Raman medium itself is not particularly limited in satisfying the basic configuration of the present invention. However, in consideration of ease of handling or availability, for example, mention may be made of combinations of gas media such as methane and hydrogen, carbon dioxide and nitrogen, deuterium and hydrogen, oxygen and nitrogen, and ethylene and propylene. it can. It is also conceivable to fill the Raman cell with three or more types of Raman media. Since laser pulse light can be obtained, two types of filling medium are sufficient unless there is some other reason.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1A shows an outline of an experimental apparatus used to demonstrate the present invention. However,
This experimental device itself is a known existing device, and there are no members specifically modified for the present invention. Therefore, components denoted by the same reference numerals as those of the conventional device described with reference to FIG. 2A indicate the same or corresponding components.

What is different from the prior art by applying the present invention is that the Raman medium 21 in the Raman cell 11 is a mixed Raman medium 21 composed of two types of media.
As mentioned earlier, the combination of these two types of Raman media has considerable flexibility, but in order to gain an understanding of the present invention by giving specific examples, the experimental examples using methane and hydrogen are described in detail. I do.

The laser source device 10 has a wavelength of 248 nm and a pulse width of 20
ns, which outputs a KrF laser pulse light Lb with an output of 100 mJ. This laser pulse light Lb is a beam splitter 14
Then, the light is converged by a condenser lens system represented by a convex lens (condenser lens) 13 and enters the Raman cell 11. The Raman cell 11 is also known per se, has a length of 3000 mm, and the focal length of the condenser lens 13 is 1500 mm. The mixed Raman medium 21 filled in the Raman cell 11 is methane and hydrogen as described above, and the gas pressure of methane is 3 atm and hydrogen is 4a.
tm.

The beam is selectively separated by the beam splitter 14,
The extracted backward Stokes light Ip is further incident on the prism 15, undergoes wavelength selection, and is split into two by the second beam splitter 16. The rear Stokes light Ip that passes straight through the second beam splitter 16 is incident on a 1KV biased photoelectric tube 17 of model number R1193U-2 manufactured by Hamamatsu Photonics, and the converted electric output signal of the photoelectric tube 17 is model number HP54542A manufactured by Hewlett-Packard Company. Sampling frequency 50
The signal is given to the 0 MHz oscilloscope 19, and the waveform is observed. Another rear Stokes light Ip split by the second beam splitter 16 is a model number C2 manufactured by Hamamatsu Photonics.
The light enters the streak camera 18 with a maximum time resolution of 20 ps of 950-01.

With such an experimental apparatus, when a KrF laser pulse light Lb having a relatively long pulse width is incident on the Raman cell 11 as shown in FIG. 1B, its waveform is as shown in FIG. 1C. As a result, a single short pulse Ip with a self-pulse width compressed by the mechanism described above was obtained. Also,
Pulse width of the obtained single Stokes light Ip (full width at half maximum: FW
HM) was measured to be 67 ps, which was reduced to about one-third of the pulse width of the incident laser pulse light Lb of 20 ns. Although not shown, model number ED-500 manufactured by GENTEC
When the energy was measured with a calorimeter, the energy of the single Stokes light Ip was estimated to be about 2 mJ.

FIG. 1D shows, for reference, a pulse train waveform Is obtained when the Raman cell 11 is filled with only methane gas.
, Which corresponds to the result of a conventionally performed method. From this, it is understood that a very large effect can be obtained in the present invention by a simple method of using two types of mixed Raman media.

The medium that causes Raman scattering with a delay (hydrogen in the above experimental example) has higher gain than the medium that rises earlier (methane in the above experimental example). The effect of suppressing the rear Stokes light is increased. Incidentally, in the range of several atm, such as 3 atm to 4 atm in the above experimental example, the gain of hydrogen is about ten times that of methane compared to that of methane. In terms of phonon lifetime, methane has a length of several ns, whereas methane has a length of several ns in the range of several atm.

As described above, the principle of the present invention is embodied not only in the above experimental example but also in the combination of other Raman media, but it is necessary to adjust the gas pressure of each medium to obtain a single laser pulse light. Is there. Obviously, the incident laser pulse light is not limited to the KrF laser pulse light in view of the principle of the present invention, and includes all laser pulse lights which are so-called energy beams.

[0016]

According to the present invention, the device itself can be obtained by a simple method of using two types of Raman medium without using any special device. It is possible to obtain a single laser pulse light with high intensity and short pulse, which is not available, and it can be expected to be widely applied in measurement and processing fields.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram relating to an apparatus used in a method of the present invention, a laser pulse light waveform of a main part, and a waveform of a reference pulse train.

FIG. 2 is an explanatory diagram of a laser pulse light waveform of a conventional apparatus and a main part thereof, and an expected single laser pulse light waveform.

[Explanation of symbols]

 10 Laser source device 11 Raman cell 13 Condenser lens 14 Beam splitter 16 Second beam splitter 17 Phototube 18 Streak camera 19 Oscilloscope 21 Mixed Raman medium

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Yoshiro Owadano 1-1-4 Umezono, Tsukuba, Ibaraki Pref., National Institute of Advanced Industrial Science and Technology (56) References JP-A-7-49512 (JP, A) Kohei 2-25269 (JP, B2)

Claims (6)

(57) [Claims] 1. A laser pulse width compression method for obtaining a backward stimulated Raman scattered light having a relatively short pulse width from a Raman cell by injecting a laser pulse light having a relatively long pulse width into the Raman cell; When a Raman cell is filled with two types of mixed Raman media with different rise times of vibrational levels ,
Lama with Raman medium that rises quickly based on the difference between
The first backward Stokes light resulting from the
Due to the power multiplication in the Mann scattering process, the pulse width self-pressure
Is output backward from the Raman cell due to compression and
The second and subsequent pulses in the pulse train due to the rising Raman medium
The pulse group is Raman scattering due to the slow rising Raman medium.
A method for compressing the backward stimulated Raman scattered light having a relatively short pulse width into a single pulse, which is suppressed by scattered light; 2. The method according to claim 1, wherein the laser pulse light is a KrF laser pulse light. 3. The method of claim 1, wherein said mixed Raman medium is comprised of methane and hydrogen. 4. A laser source device for emitting a laser pulse light having a relatively long pulse width; and a difference between rise times of two types of Raman media filled therein.
Raman scattering by Raman medium
The first backward Stokes light resulting from the backward Raman scattering
Due to power multiplication in the turbulent process, self-compression of pulse width is generated
Raman medium that is output backwards and rises quickly
The second and subsequent pulses in the pulse train depending on the quality
Suppressed by Raman scattered light from the rising Raman medium
A condensing lens system for condensing the laser pulse light emitted from the laser source device and guiding the laser pulse light into the Raman cell; and separating backward stimulated Raman scattering light emitted backward from the Raman cell from the laser pulse light. And a beam splitter for extraction. 5. The apparatus according to claim 4, wherein the laser source device is a device for emitting KrF laser pulse light. 6. The apparatus according to claim 4, wherein the mixed Raman medium filled in the Raman cell is composed of methane and hydrogen.