Fig. 5-1 Concept of the generation of the coherent X-ray laser The spatially high-coherent X-ray beam is generated using two gain media. A part of the X-ray laser generated in the first medium is used as a seed X-ray laser, and this is amplified in the second medium.

Fig. 5-2 Beam pattern of the X-ray laser This figure shows the spatial profile of the output energy of the fully coherent X-ray laser.The beam pattern is close to a Gaussian profile, and the beam divergence is 0.2 mrad, which is quite close to the diffraction-limited divergence.

Fig. 5-3 Interference patterns formed with the X-ray laser This figure shows the interference patterns formed by Young's experiment. The contrast of the interference fringes is high even though separation of the double slits is as large as the beam size. This X-ray laser is said to be fully coherent.

We have succeeded in generating a spatial fully coherent X-ray laser having a wavelength of 14 nm. The gain medium of the X-ray laser is the plasma, which is generated by irradiating a silver target by intense ps laser pulses. This method can produce a high-gain X-ray laser, which only requires low pumping energy. However, the beam divergence of this X-ray laser is large (~ 10 mrad) and the spatial coherence is inadequate. There are two reasons for the large beam divergence of the X-ray laser. First, plasma of a short length is used as the gain medium, because the amplification of the X-ray laser saturates within a short gain medium. Thus, the beam divergence is limited geometrically. The second reason is the refraction of the X-ray beam due to the large density gradient in the gain medium plasma. Two gain medium plasmas are used to improve the spatial coherence of the X-ray laser. A part of the X-ray laser generated in the first medium is used as a seed X-ray laser pulse. To do so, it is injected into the second medium, which is used as an amplifier (Fig. 5-1). The separation of the two media is determined so as geometrically calculated beam divergence becomes 0.2 mrad.This value is equal to the theoretically limited divergence due to diffraction of the X-ray laser (diffraction limit). The refraction influence is minimized by adjusting the injection timing of the seed pulse. We think the gain region moves to the low-density area, where the density gradient is small. Using this adjusting method, we succeeded in generating the fully coherent X-ray laser having diffraction limited beam divergence (Fig. 5-2) and high spatial coherence over the whole beam spot (Fig. 5-3).

Reference M. Tanaka et al., X-ray Laser Beam with Diffraction-Limited Divergence Generated with Two Gain Media, Opt. Lett., 28, 1680 (2003).

Select a topic in left column

Persistent Quest Research Activities 2004 Copyright (C) Japan Atomic Energy Research Institute