K(alpha) x-ray emission characterization of 100 Hz, 15 mJ femtosecond laser system with high contrast ratio

Abstract

We report K(alpha) x-ray production with a high energy (110 mJ per pulse at 800 nm before compression/15 mJ at 400 nm after compression), high repetition rate (100 Hz), and high pulse contrast (better than 10(-9) at 400 nm) laser system. To develop laser-based x-ray sources for biomedical imaging requires to use high-energy and high-power ultra-fast laser system where compression is achieved under vacuum. Using this type of laser system, we demonstrate long-term stability of the x-ray yield, conversion efficiency higher than 1.5 x 10(-5) with a Mo target, and the x-ray spot size close to the optical focal spot. This high-repetition K(alpha) x-ray source can be very useful for x-ray phase-contrast imaging.

Fig. 1

Schematic experimental setup. The 400 nm laser beam is represented by the turquoise lines. OAP, off-axis parabola; TCC, target chamber center. The x-ray diagnostics disposed outside the target chamber and filtered by a Be window (BW) are shown: the two symmetrically disposed PMT detectors (PMT A and PMT B); AS, CdZnTe detector for spectrum measurement; SCX, imaging x-ray CCD camera for source size measurement. The Z-axis shows the imaging direction of this camera. We also show the configuration of the imaging system (not at real scale) with: L, f = +10 cm lens; MO, ×10 microscope objective; CCD, far-field monitor CCD

Fig. 2

Measurement of the beam shape with the imaging system. The lens image the plane corresponding to the target chamber center (TCC). a Focal spot after alignment at air and low energy. b Focal spot under vacuum and 10% energy. c Beam shape evolution after 100 s at 100% energy. d Focal spot obtained after moving the off axis parabola along the laser axis to compensate for the thermal-induced distortion

Fig. 3

CdZnTe detector spectra. a Obtained with 110 mJ before compression and no correction for the thermally induced effects (laser beam focal spot is align at air and low energy). b Obtained with 90 mJ before compression and no correction for the thermally induced effect. c Obtained with 90 mJ before compression and with compensation for the thermally induced effect (beam is propagated through the experimental system until a steady state for the thermal loading is reached, then the focal spot is corrected by moving the off axis parabola)

Fig. 4

Summary of the current x-ray experimental results (solid points). The solid diamond points represent the results obtained with laser pulses with no corrections applied to compensate the laser beam distortion due to thermal effects. The solid square point is the result obtained when the laser focal spot was compensated for thermal effects. The open squares represent other measurements obtained with high-contrast femtosecond laser pulses. Each number inside the square denotes the reference number

Fig. 5

X-ray imaging of a Tungsten edge. The edge is positioned at 25 cm from the x-ray source. The SCX CCD is positioned 175 cm from the x-ray source. a Picture showing the lateral edge. b Differentiation of the lateral edge profile and Gaussian fit. c Differentiation of the vertical edge profile and Gaussian fit