1. Technical Field
This invention relates to a method of increasing the contrast of a high-intensity laser by using optical parametric chirped pulse amplification (OPCPA) in which energy is transferred in a nonlinear optical crystal from pump light to signal light and idler light. The method of the present invention for increasing the contrast of a high-intensity laser is such that signal light outputted from a laser oscillator, a preamplifier or the like is amplified by OPCPA which is excited by pump light having a short pulse width comparable to that of the signal light, thereby producing high-intensity laser light of an extremely high contrast.
2. Background Art
As FIGS. 1 and 2 show, when signal light 1 at a frequency of ωs and a wavenumber vector of ks and pump light 2 at a frequency of ωp and a wavenumber vector of kp are simultaneously launched into a nonlinear optical crystal 3 such as a beta-barium borate crystal, idler light is generated at a frequency of ωi and a wavenumber vector of ki. When pump light is launched into a nonlinear optical crystal, there occurs light oscillation at two wavelengths and the light at the shorter wavelength is called signal light whereas the longer wavelength of light is called idler light. Since the idler light is discarded in the present invention, it is not shown in FIGS. 1 and 2.
It is known that if the following equations hold between those parameters, the signal and the idler light are amplified by the energy they obtain from the pump light. This phenomenon is called an optical parametric effect in which energy is transferred in the nonlinear optical crystal from the pump light to the signal and the idler light; in the case where chirped pulsed light is used as the signal light, the phenomenon is specifically called optical parametric chirped pulse amplification (OPCPA). Chirped pulsed light is pulsed laser light that is temporally stretched by providing it with spectral dispersion (making the optical path shorter at the shorter wavelength than at the longer wavelength) by means of a pulse stretcher using a diffraction grating and the like.ωp=ωs+ωikp=ks+ki
In the case where weak signal light as outputted from a laser oscillator or the like is to be amplified with an optical parametric chirped pulse amplifier (OPCPA), pump light that has a comparatively long pulse width of about 10 ns and which finds extensive use in commercial products is used as excitation light. Since the signal light usually has a time duration of 1 ns or less, temporal overlap between the signal and the pump light is poor as shown in FIG. 1, in which they are indicated by 1 and 2, respectively. As shown in FIG. 2, the signal light 1 is amplified by obtaining energy from the pump light 2 so as to grow into a high-energy pulse (main pulse 5). However, due to the poor temporal overlap between the signal and the pump light, unwanted background light (pre-pulse 6) is amplified in the area where there is no overlap with the signal light. As a result, a contrast which is approximately eight orders of magnitude is obtained in the ns temporal region (meaning the intensity ratio between the pre-pulse and the main pulse is approximately eight orders of magnitude).
With the recent advances in the high-intensity laser generating technology, laser light has been realized that provides a focusing intensity of 1020 W/cm2. Hence, without a contrast of at least ten orders of magnitude, the main pulse with high intensity will break the target material before it interacts with the substance of interest and at the same time a preliminary plasma is formed to prevent interaction between the main pulse and a solid substance such as a thin metal film. Therefore, the contrast that can be achieved by this method has been inadequate. As another problem, the pump light in the area where it does not overlap the signal light makes no contribution to amplifying the main pulse, causing a disadvantage of low efficiency of energy conversion.
Known documents in the art include three patent documents and one non-patent document; the patent documents are one that relates to a method of improving the contrast of laser light and a laser apparatus that uses the method (JP 2006-229079 A), one that relates to a method and apparatus for controlling laser pulses, as well as a method and apparatus for generating X-rays (JP 2003-270551 A), and one that relates to a method and apparatus for compressing optical pulses (JP 2002-62553 A); the non-patent document is one that relates to “stabilization of optical parametric chirped pulse amplification” (Journal of the Japan Society of Mechanical Engineers, Vol. 109, No. 1056, November 2006). However, these documents have nothing to do with the present invention in which signal light outputted from a laser oscillator, a preamplifier or the like is amplified by OPCPA which is excited by pump light having a short pulse width comparable to that of the signal light, thereby producing high-intensity laser light of an extremely high contrast.