JP3218706B2 - Multicolor and ultrashort pulse laser device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention is used in basic research fields such as ultrafast optical phenomena and non-linear optical effects, and in various industrial fields such as optical communications and nuclear fusion where a momentarily high density of light is required. The present invention relates to a pulse laser device suitable for performing the above.

[0002]

2. Description of the Related Art There is an uncertainty principle expressed by the following equation (1) between an optical pulse width Δt and an energy width, that is, a spectrum width Δε. Δt · Δε ≧ h / 2π (1) where h is Planck's constant. In order to generate light of a short light pulse (Δt), the light energy width (Δ
ε) needs to be increased. For this reason, for short pulse light generation, a YAG laser with a wide spectrum width, a dye laser,
A titanium sapphire laser or the like is used. In order to generate ultrashort pulsed light, it is necessary not only to have a wide spectrum width but also to align the phases of the longitudinal modes of the laser oscillation light, that is, to synchronize the phases. For phase synchronization, it is common to insert optical elements that periodically cause losses in the laser resonator. By this method, a light pulse of about 30 fs (fs = 10 ⁻¹⁵ s) is currently obtained (see Opt. Lett., 10, 131 (1985)). In order to generate shorter optical pulses, an optical compression device including an optical fiber and a diffraction grating or a prism is used. With such a method, an optical pulse of 6 fs is currently obtained (see Opt. Lett., 12, 483 (1987)).

[0003]

As long as a solid or liquid is used as a laser medium or an optical compression device, group velocity dispersion (a phenomenon in which the speed at which light travels depending on the wavelength) occurs.
The width of the light pulse is widened. Normally, this is corrected by a diffraction grating or prism to generate a short light pulse.
The above 6f is corrected by correcting up to the third order term of the wavelength dependence of the refractive index.
s light pulses are obtained. In order to further shorten this pulse, it is necessary to correct up to higher-order terms, but it is technically difficult to achieve this, and even if it can be achieved, it is not possible to significantly reduce the pulse width. Can't expect. In addition, since a large number of optical elements are used in that method, if the intense laser light is focused, the optical elements will be damaged. Therefore, it is not suitable for shortening the pulse of a high-power laser.

[0004] On the other hand, there is a method of generating short pulse light using gas as a laser medium. However, since a gas laser generally emits monochromatic light, it is not expected to shorten the pulse much. For example, an excimer (gas) laser is known to have a relatively wide energy width (spectrum width), but only an optical pulse of about 100 fs is obtained. Therefore,
It is difficult to generate an optical pulse shorter than the current one by extension of the conventional technology. An object of the present invention is to use a gas as a Raman medium and generate polychromatic and ultrashort pulsed light exceeding the conventional limit by the two-color stimulated Raman effect.

[0005]

According to the present invention, a two-wavelength oscillating laser beam of a Fourier limit pulse is introduced into a gas that causes the Raman effect, and a large number of rotating Raman beams are simultaneously generated. (Forced phase synchronization) generates an ultrashort pulse laser beam. In the present invention, a laser beam having a high intensity that causes a nonlinear optical effect is introduced into a gas that causes the Raman effect, a large number of rotating Raman lights are generated at the same time, and the phase synchronization (self-phase Synchronization) to generate an ultrashort pulse laser beam.

[0006]

A laser using gas as a medium does not increase the pulse width due to group velocity dispersion, but generally emits monochromatic light, and is not suitable for generating ultrashort pulse light. However, when two-wavelength oscillation laser beams having different frequencies by the amount of the rotation energy are condensed on the Raman active material, a large number of rotation Raman beams can be generated simultaneously. For example, since in the case of using the ortho hydrogen as a Raman material, each oscillation line is 10 or more oscillation line at intervals of (354cm ^-1 in the case of parahydrogen) 587cm ^-1 is obtained, a laser in a wide energy range An oscillation line can be obtained. By synchronizing the phases of these oscillation lines, an ultrashort pulse light can be obtained.

[0007]

FIG. 1 schematically shows a laser apparatus according to one embodiment. A dye laser 2 is provided as a light source that emits laser light of a single wavelength oscillation, and an excimer laser 4 is provided to pump the dye laser 2. A part of the laser light from the excimer laser 4 is reflected by the quartz plate 6 and guided to the dye laser 2. The laser light from the dye laser 2 is bent by a prism 7 and a semi-resonator is arranged on the optical axis via a semi-transparent mirror 8. The half-resonator includes a dye cell 10, a quarter-wave plate 12, a condenser lens 14,
Raman cell 16 filled with pressurized hydrogen, condenser lens 18
Further, a reflecting mirror 20 provided perpendicular to the optical axis is arranged along the optical axis.

A dye cell 22, a condenser lens 24, a Raman cell 26 filled with pressurized hydrogen, a condenser lens 28, and a condenser lens 28 are arranged on the optical axis of the light reflected by the semi-transparent mirror 8. Reflecting mirror 30 is disposed along the optical axis.
A part of the laser light from the excimer laser 4 passes through the quartz plate 6, and a part of the laser light enters the dye cell 10 through the cylindrical lens 32, and the rest of the laser light transmitted through the quartz plate 6 is reflected by the reflecting mirror 34. The reflected light is incident on the dye cell 22 via the cylindrical lens 36.

The laser light extracted by the reflecting mirror 30 is incident on a spectroscope 40 via a reflecting mirror 38 to be split and detected by a photomultiplier 42. The detection signal of the photomultiplier 42 is measured by a boxcar integrator 44. In order to operate the excimer laser 4 and the boxcar integrator 44 in synchronization, a trigger signal is output from the pulse generator 46.
48 is a recorder for recording the measurement result of the boxcar integrator 44. The excimer laser 4 as a light source oscillates a dye laser of a Fourier limit pulse (an optical pulse in which both the energy width and the pulse width are narrowed to the limit determined by the uncertainty principle). To extract a single light pulse, the pulse width must be 1
It is desirably about 00 fs or less.

Next, the operation of this embodiment will be described.
The laser light of single wavelength oscillation from the dye laser 2 is injected into the semi-resonator via the prism 7 and the semi-transparent mirror 8. In the semi-resonator, the light is amplified by the dye cell 10 and becomes circularly polarized light through the quarter-wave plate 12, and then condensed by hydrogen in the Raman cell 16 to generate rotational Raman light. The fundamental wave and the rotating Raman light are reflected by the reflecting mirror 20 and condensed on the hydrogen of the Raman cell 16 again to increase the efficiency of the rotating Raman light. Raman cell 16
The reflected light that has passed through becomes a linearly polarized light through the quarter-wave plate 12 and is further amplified in the dye cell 10. Dye cell 10
The linearly-polarized laser light amplified by the laser beam is partially reflected by the semi-transmissive mirror 8 and is incident on the dye cell 22, and is further amplified through the dye cell 22.
It is focused on the hydrogen inside. In the Raman cell 26, a large number of rotating Raman lights are generated, and a polychromatic laser light is obtained. At this time, when the phase of the fundamental wave coincides with the phase of the rotating Raman light, the phase of the polychromatic laser light is fixed, and the laser light becomes an ultrashort pulse laser light. The generated polychromatic laser light is incident on the spectroscope 40 via the condenser lens 28 and the reflecting mirrors 30 and 38, and the photomultiplier 4
2 is detected.

In the embodiment, the laser light having a single oscillation wavelength is 1 /
The light becomes circularly polarized light through the four-wavelength plate 12, and passes the pressurized hydrogen of the Raman cell 16 to generate one rotating Raman light. The light is reflected by the reflecting mirror 20 and is again a quarter-wave plate 1
After returning to the linearly polarized light through 2, the two-wavelength oscillation laser light of the fundamental wave and the rotating Raman light is again focused on the pressurized hydrogen of the Raman cell 26, thereby generating a large number of rotation-induced Raman lights. The reason for selectively using the circularly polarized light and the linearly polarized light in this way is to efficiently cause a rotational Raman phenomenon and a four-wave mixing phenomenon (a phenomenon that generates a large number of rotational Raman lines). Phase synchronization is performed in the Raman cell 26. Here, phase synchronization (forced phase synchronization) is performed by introducing a two-wavelength oscillation laser beam, but direct phase synchronization (self-phase synchronization) may be performed by a nonlinear optical effect.

FIG. 2 shows the spectrum of the polychromatic laser light generated by this embodiment. In this spectrum, 11 or more rotating Raman lights are obtained at the same time, and the energy width extends to 5870 cm ^-1 .

FIG. 3 shows a result of Fourier transform of this spectrum, in which the vertical axis represents light emission intensity and the horizontal axis represents time. By this, it is possible to know what width of the light pulse can be obtained. In this result, a light pulse of about 6 fs is obtained.

In the embodiment, the multicolor laser light generator uses a dye laser as a light source. This is to change the wavelength freely, and a fixed wavelength laser such as an excimer laser or a glass laser is used as a light source. Is also good. However, in that case, the oscillation wavelength cannot be finely changed.
When the intensity of the excitation light is high, it is not necessary to selectively use circularly polarized light and linearly polarized light as in the embodiment to increase the efficiency. Therefore, a large number of rotating Raman lights may be directly generated from the beginning using elliptically polarized light.

In order to obtain a single light pulse, a Fourier limit pulse of about 100 fs is used as an excitation light source, but a laser having a wider pulse width may be used as a light source.
In that case, a pulse (pulse train) is generated periodically. In the embodiment, the generation of the rotating Raman light is used, but this is because it is easy to obtain the Fourier limit pulse required as the excitation light, and the vibration Raman light may be used. In that case, the condition of the Fourier limit pulse is more strictly required, but a shorter optical pulse can be generated. In that case,
In order to obtain a single pulse light, it is necessary to previously narrow an incident laser pulse to be used to about 10 fs. In addition, if the Raman effect using the electron transition in the soft X-ray region and the four-wave mixing are used, at second (as; 1as = 10 ⁻¹⁸ s)
Can be generated.

As the laser beam to be introduced, a two-wavelength oscillation laser beam having an energy difference corresponding to the rotation or oscillation Raman transition of the Raman substance is used. May be used as a light source. In that case, the interval between the pulse trains is increased by an integral multiple, so that the pulse compression ratio increases. In the embodiment, hydrogen is used as the Raman medium because the efficiency is highest. Other Raman substances may be used for the purpose of adjusting the interval between the oscillation line and the pulse train. Also,
As for hydrogen, para-hydrogen may be used in addition to normal ortho-hydrogen. In that case, the interval between the pulse trains becomes wider, which is advantageous in extracting a single optical pulse. However, a low-temperature device for producing para-hydrogen is required.

[0017]

According to the laser apparatus of the present invention, since gas is used as a laser substance, there is no group velocity dispersion, and there is no need to correct the pulse spread. Therefore, the limit of short pulse light generation in the solid-state laser can be exceeded. Since the energy width, that is, the number of rotating Raman lights is determined by the intensity of the excitation light, it is possible to generate shorter pulsed light by increasing the intensity of the excitation light. Therefore, an optical pulse of 1 fs or less can be generated. Since the wavelength region of the ultrashort pulse light is determined by the laser to be introduced first, an ultrashort pulse light of an arbitrary wavelength can be obtained.
In particular, when hydrogen is used as the Raman substance, ultrashort pulsed light can be generated in a vacuum ultraviolet region. In order to increase the energy width, it is necessary to increase the light energy itself. If vacuum ultraviolet light is used, a shorter light pulse can be generated. When a multi-wavelength oscillation laser is used, the compression ratio of an optical pulse can be increased. Therefore, if a laser beam having a sufficiently large output is used, it is possible to compress a 1 ns laser pulse beam into a 1 fs laser pulse beam by a factor of 1 million at a stroke. No optical element is used when generating ultrashort pulse light. Therefore, there is no need to worry about optical damage to the optical components. Further, when the laser output is increased, the laser beam diameter may be simply increased, so that the invention can be applied to a giant laser for nuclear fusion.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing one embodiment.

FIG. 2 is a diagram showing a spectrum of a polychromatic laser beam generated according to one embodiment.

FIG. 3 is a diagram showing a result of Fourier transform of the spectrum of FIG. 2;

[Explanation of symbols]

 2 Dye laser 4 Excimer laser 10,22 Dye cell 12 Quarter wave plate 14,18,24,28 Condenser lens 16,26 Raman cell 20,30 Reflector

──────────────────────────────────────────────────続 き Continued on front page (56) References 53th Analytical Chemistry Symposium Abstracts 1B13 p. 100-101 Japanese Journal of Applied Physics Vol. 30 No. 2B (1991) pp. 283-285 Spectroscopic Research Vol. 40, No. 5, (1991) p. 291-292 (58) Field surveyed (Int.Cl. ⁷ , DB name) H01S 3/108 H01S 3/10 H01S 3/30

Claims (6)

(57) [Claims] 1. A two-wavelength oscillation laser beam of a Fourier limit pulse is introduced into a gas that causes the Raman effect, a large number of rotating Raman beams are simultaneously generated, and an ultrashort pulse laser beam is generated by phase synchronization of the generated oscillation beams. Device to make. 2. A pulse width of said two-wavelength oscillation laser light is 1
2. The device according to claim 1, wherein the difference is about 00 fs or less. 3. The apparatus according to claim 1, wherein the two-wavelength oscillation laser light is generated by a half resonator. 4. A laser beam having a high intensity for generating a nonlinear optical effect is introduced into a gas causing the Raman effect, a large number of rotating Raman lights are simultaneously generated, and an ultrashort pulse is generated by phase synchronization of each oscillation light by the nonlinear optical effect. A device that generates laser light. 5. The apparatus according to claim 4, wherein a pulse width of the laser beam having a high intensity is about 100 fs or less. 6. The apparatus according to claim 4, wherein the oscillating light for phase synchronization is a fundamental wave and rotational Raman light.