JP4696297B2 - Ultrashort pulse laser generation method and apparatus - Google Patents

Description

  The present invention relates to a technique used for ultrafast spectroscopy, information communication, laser processing, and the like.

  So far, various methods for expanding the spectrum width have been proposed in order to shorten the pulse width of the laser pulse output from the laser apparatus.

  As one of them, a method of expanding the spectrum width by self-phase modulation of a high-intensity short pulse laser is shown (see Non-Patent Document 1).

  In order to solve the limitation due to the intensity of self-phase modulation, a method using induced phase modulation has been invented (see Patent Document 1 below).

In these spectral width expansion methods using a nonlinear refractive index that depends on the laser intensity, a high intensity of tens of GW / cm ² or more is generally required. In addition, since the phase in the spectrum does not have a uniform distribution, the ultrashort pulse obtained by compression becomes a pulse that does not have a well-formed shape with pre-pulses and post-pulses before and after time.

  On the other hand, as another method for expanding the spectral width, a method based on Raman scattering, particularly the impulsive Braman method has been shown (see Non-Patent Document 2). In this method, first, a high-intensity pulsed laser whose pulse width is shorter than the molecular vibration period is incident on the Raman medium to drive the molecular vibration first to excite the dielectric polarization wave, which is incident with a delay in time. The spectral width of the comparatively weak short pulse laser was expanded to obtain ultrashort pulses.

  Also in this method, the laser that drives molecular vibrations must use an ultra-high intensity and ultra-short pulse laser, so that the apparatus becomes large and complicated, and the intensity is limited by converting the Raman medium into plasma. In addition, there are problems in application such as being restricted to molecules having a long vibration period as a Raman medium.

  On the other hand, as another method for efficiently forming the above-mentioned dielectric polarization wave, a method by two-color light excitation having a frequency difference corresponding to the Raman transition has been shown (see Non-Patent Document 3).

Further, it has been shown that high-order Stokes groups are formed from excitation light by the two-wavelength parametric Raman scattering method, and that they form a pulse train of ultrashort pulses (see Patent Document 2). This method has a problem in application because the ultrashort pulse is obtained by a pulse train.
M. Nisoli, “A novel high energypulse compression system: generation of multi-gigawatt sub 5fs pulses”, Appl. Phys. B (1997) p 189 N. Zhavoronkov, and G. Korn, “Generation of Single Intense Short Optical Pulses by Ultrafast Molecular Phase Modulation”, Phys. Rev. Lett. 2002 Vol. 88, No. 20, 203901 Eimerl, RSHargrove, and JA Paisner, “Efficient Frequency Conversion by Stimulated Raman Scattering” Phys. Rev. Lett. (1981) Vol. 46, No. 10 p. 651 JP 2001-83558 A JP-A-6-21550

  As an ultrashort pulse laser source, a very compact and stable fiber laser has been attracting attention. However, since erbium or yttrium is used as the laser medium, the pulse width remains at about 100 fs. If the pulse width can be reduced to 10 fs or less, which is realized with a titanium sapphire laser, research on ultra-high-speed phenomena can be greatly advanced. Therefore, an object of the present invention is to develop an ultrashort pulse laser capable of realizing an easy and small system size for obtaining a well-formed femtosecond ultrashort pulse.

  The spectrum width can be expanded by efficiently exciting the Raman polarization wave using two-color lasers having a frequency difference corresponding to the Raman transition of the Raman medium and injecting the probe laser light as the affected light. To obtain ultrashort pulses after compression.

  Conventionally, in order to obtain a laser pulse shorter than the pulse width of a fiber laser, a large-sized laser light source such as a titanium sapphire laser and lacking stability has been required. According to the present invention, the fiber output can be directly converted to a few femto level by a small device, and a very compact and highly stable system can be supplied for applications such as spectroscopic measurement of ultra-high speed phenomena. it can.

  Below, it demonstrates in detail using drawing.

  As shown in FIG. 1, two colors of excitation laser light having wavelengths λ1 and λ2 having a frequency difference corresponding to the level difference of the Raman medium are output. Each wavelength laser is oscillated from a continuous laser whose frequency is stabilized, amplified to a required intensity, and then incident on a Raman medium. Here, deuterium is used as the Raman medium, but methane, nitrogen, sulfur hexafluoride, or the like may be used.

  As a specific example, a rotational Raman transition of KrF laser and deuterium (rotational quantum number J = 0 to 2) is used. The KrF laser amplifies the third harmonic of the output of two titanium sapphire lasers that continuously oscillate with a narrow spectral width. In the case of deuterium, the wave number Ω = 179 cm −1, and when a KrF laser with a wavelength of 248.0 nm is used, the laser light has two colors having a wavelength difference of about 1 nm. The laser beams of the two colors are incident on the Raman medium as laser beams propagating on the same optical axis by using a partial reflection mirror having a reflectance of 50%.

  The Raman medium is efficiently excited by this two-color light excitation, and has a large polarization wave in the medium. In the case of the above deuterium, the vibration reaches a steady state by excitation with two-color light having a pulse width of several nanoseconds or more.

  Here, probe laser light is incident. The probe light propagates together with the polarization wave and is subjected to the effect of expanding the spectrum width. Since the probe light is separated and extracted by the wavelength selection mirror 3, the wavelength λ3 of the probe light is different from the wavelengths λ1 and λ2 of the excitation light. Further, the wavelength selective mirror 3 is used in order to make it enter the Raman medium coaxially with the excitation light. In order to obtain a single pulse after compression, it is desirable that the pulse width of the probe light is equal to or less than the vibration period of the polarization wave.

  Under the above deuterium condition, for example, when a fiber laser is used as a probe laser and probe light having an output wavelength of 780 nm and a pulse width of 200 fs or less is incident, the spectrum width is expanded.

  FIG. 2 shows how the spectrum is expanded. The polarized wave excited by the two-color excitation laser corresponds to a refractive index that varies with time, and expands the spectrum of the probe light that propagates together at a phase velocity of approximately the speed of light so that the frequency changes with time.

  The probe pulse with an expanded spectrum width can be compressed by using an appropriate optical element such as a dispersion mirror after being output from the medium.

  In FIG. 2, as an example of a method that simplifies the system, the probe light is compressed in the medium by appropriately adjusting the dispersion of the medium and the direction of the temporal change (chirp) of the frequency received by the probe light. Shows the case.

  Since the direction of the chirp of the spectrum differs depending on the phase relationship between the probe pulse and the polarization wave, the relative phase relationship is kept constant by changing the position of the oscillator mirror etc. for stable ultrashort pulse generation. For this purpose, a method of adding a feedback system, or dividing the output from the probe short pulse oscillator and using the chirped pulse as two-color excitation laser light can be considered.

  The present invention can be used for diagnosis or analysis of an ultrafast phenomenon using a short pulse laser.

Conceptual diagram of an apparatus according to the present invention Functional explanatory diagram of the present invention

Claims (2)

An ultra-short pulse laser generation method that excites rotational Raman transition of a Raman medium composed of deuterium using two-color laser beams with different wavelengths amplified by a KrF laser. An ultrashort pulse laser generation method characterized in that the spectrum width of the third laser beam is expanded based on the above to form an ultrashort pulse. An ultrashort pulse laser generator, in which two-color pulse laser light amplified by a KrF laser is incident on a Raman medium composed of deuterium, and then has a pulse width shorter than that of the two-color pulse laser. An ultrashort pulse laser generator characterized by injecting pulsed laser light into the medium.

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