This invention relates to a laser technique for use in laser processing, in laser spectroscopy, in cluster generation, and in laser production of thin film.
To generate ultrashort excimer laser pulses of pulsewidths in the picosecond or femtosecond range, generally, a mode-locking oscillator, an amplifier, and wavelength conversion have been used by being combined with one another. This mode locking method is a technique of generating a train of ultrashort pulses by causing longitudinal modes of a laser resonator through the use of means, such as a supersaturated dye and Kerr effect, to be coherent in phase. Moreover, because an output of the oscillator itself is small, practically, it is necessary to increase the output thereof by using an amplifier. Furthermore, because the operating wavelength of such an oscillator is in the range from an infrared region to a visible region, the conversion of the operating wavelength to an ultraviolet excimer laser wavelength is needed.
The oscillator requires a continuously oscillating strong pump laser. Chirped-pulse amplification is employed in an amplification system. Additionally, higher-order wavelength conversion is needed. Thus, inevitably, the system has become complex and costly.
Attempts have been made to make the excimer laser itself perform a mode locking operation, and to an output of a long-pulse excimer laser by stimulated-scattering. At any of the attempts, only the generation of long pulses ranging from several tens ps to several hundreds ps has been achieved (see Patent Documents 1 to 3 and Non-Patent Documents 1 to 3 described hereinbelow).
An object of the invention is to perform the generation of ultrashort excimer laser pulses without using a costly mode-locked laser when the generation thereof is performed.
According to the invention, the wavelength of a laser pulse is once converted to another wavelength by using a nonlinear optical action. A ratio of the intensity at a temporal peak of a pulse to that at a front part thereof is increased. The converted laser light is reconverted by using a nonlinear optical action in such a way as to have an initial wavelength again. Thus, simultaneously with extremely enhancing the contrast at the front part of the pulse, the amplification thereof by a used laser amplifier is enabled. Saturation amplification is then performed on the pulse to thereby form ultrashort pulses.
First, regarding the preparing portion, a laser pulse (see the waveform of a pulse shown in
To realize an efficient conversion utilizing stimulated Raman scattering, which corresponds to the wavelength conversion to be first performed according to the invention, laser light having a pulse width of 5 nanoseconds, which is outputted by the oscillator, is preliminarily pulse-compressed by stimulated Brillouin scattering into a pulse having a pulse width of about 300 picoseconds. The previously amplified laser pulse is made to be incident upon the stimulated Brillouin cell by being simultaneously converged thereto. In the apparatus, a (λ/4)-wavelength plate and a polarizer are preliminarily provided so that this stimulated Brillouin scattering light and the incident laser light can be separated from each other. The stimulated Brillouin cell is filled with carbon-fluorine-based liquid. Scattering light generated in the vicinity of a focal point grows up while propagating through an optical path, upon which the incident light has been previously incident, in an opposed manner. Pulse-compression effect acts upon the scattering laser light pulse, which has propagated therethrough in an opposed manner, so that the width of the scattering laser light pulse becomes shorter than the width of the incident laser light. The waveform of the pulse, which is measured by using a photo multiplier tube, is shown in (b) of
Laser light outputted from the aforementioned preparing portion is provided as a pulse, which is improved in the focusing property and the monochromaticity thereof and has a duration of about 300 picoseconds. The main portion for carrying out the invention improves the contrast of a front part of this pulse and saturation-amplifies and forms ultrashort pulses. The necessity for such a preparing portion depends upon the width of necessary final output laser pulses and the characteristics of individual lasers. Thus, the portion may have various configurations. For example, the aforementioned pulse can be obtained by using an electro-optic device.
Generally, the wavelength conversion uses a nonlinear response of a medium with which laser light rays interact. Consequently, the ratio of the intensity of the peak to the intensity of the front or rear part of a laser pulse, that is, the contrast thereof can be increased, and thus improved. In this embodiment, input laser light is converted into laser light having a longer wavelength by performing a nonlinear stimulated backward Raman scattering process based on the third-order nonlinearity of the medium. The scattering laser light formed by the stimulated Raman scattering is the scattering light due to the polarization of molecules. The wavelength thereof is largely shifted from that of the incident laser light. In this embodiment, the wavelength of an output of the oscillator is 248 nm. This output is made by using the lens to be converged and incident on the cell, which is filled with a methane medium. Consequently, longer-wavelength laser light having a wavelength of 268 nm is generated by the stimulated Raman scattering so that this laser light propagates therethrough in an opposed manner. The incident laser light and the generated Raman scattering light are separated by a wavelength selection mirror from each other. Further, similarly, the contrast can be improved by generating second harmonic laser light using second-order nonlinearity.
The waveform of laser light converted by this stimulated Raman scattering in such a way as to have a longer wavelength is shown in (b) of
Generally, the wavelength of the wavelength-converted light having high contrast is outside the range of wavelengths of light that the laser oscillator can amplify. The reconversion of converting this light to light having the initial wavelength by utilizing the response of the nonlinear medium is performed. In this embodiment, the scattered laser light having a wavelength of 268 nm is converted back to light having a wavelength of 248 nm by employing the four-wave mixing process using the same methane medium as that used for the aforementioned stimulated Raman scattering. Practically, the scattered laser light is incident on the cell by being converged thereon. The laser light having a wavelength of 268 nm and light, which is generated therefrom and having a wavelength of 291 nm, parametrically interact with the nonlinear Raman medium to thereby form light having a wavelength of 248 nm. This process is also based on the third-order nonlinearity of the medium and thus contributes to the improved contrast. The spectra of output light of this embodiment are indicated by a solid curve in
When the laser light having a very high contrast, which is formed in this way, is saturation-amplified by a laser amplifier, a laser pulse having a short pulse width is formed at a rise portion thereof. This saturation amplification is defined as the amplification of a laser pulse having an energy density that is higher than a saturated energy density inherent in each of individual lasers.
Although a KrF excimer laser is used therein, the laser according to the invention is not limited to the excimer laser. As long as a laser has a damage threshold value for high laser power, which enables the saturation amplification, such a laser can be used in the method and the apparatus of the invention.
When the generation of ultrashort excimer laser pulses is performed, the generation thereof can be performed without using a costly mode-locked laser.
Number | Date | Country | Kind |
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2002-286116 | Sep 2002 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP03/12524 | 9/30/2003 | WO | 3/30/2005 |