This study proposed and demonstrated the potential use of LED phosphors as a simple light source for ultrahigh-resolution spectral-domain optical coherence tomography (SD-OCT) in the visible regime. Excited by a 405 nm diode laser, broadband spontaneous emission from three different LED phosphors was generated. The best axial resolution was 1.7 µm in air, and finally, corresponding three-dimensional (3D) ultrahigh-resolution SD-OCT imaging was performed with the proposed broadband light source.

The development of soft X-ray lasers sources having reduced size and cost and higher efficiency is required for applications. We have proposed for this purpose the use of highly ionized capillary plasmas created by a fast discharge [l]. Several inherent characteristics of capillary discharges make them attractive for extreme ultraviolet and soft X-ray amplification: high power density deposition, large length-to-diameter ratio and the possibility of creating small diameter plasma columns. In addition, for the implementation of recombination schemes the possibility of creating plasma columns which are almost entirely surrounded by walls provides enhanced cooling of the plasma by electron heat conduction from the hot plasma to the capillary walls. Besides the interest in plasma recombination lasers, the experimental results reported herein for fast discharges excitation of capillaries filled with pre-ionized gas show that conditions adequate to explore collisionally excited lasers in neon-like and nickel-like systems are achieved.We report experiments conducted with a fast (1 1 ns 10-90% risetime), high voltage pulse generator at currents up to 100 kA. Time resolved soft X-ray spectra from polyethylene, polyacetal and teflon capillary discharges 0.5-1.5 mm diameter were obtained to study the possible generation of population inversions by collisional recombination in hydrogen-like carbon and in lithium-like fluorine respectively. Spectra displaying an anomalously large ratio between the intensities of the 3-2 and 4-2 transitions of CVI have been obtained in polyethylene capillaries, but no exponential increase of the 182 A line has yet been obtained.To obtain increased control of the temperature, density and composition of the plasma column, we are studying a new variation to conventional discharges in evacuated capillaries. We are conducting experiments in which the plasma column is generated by fast excitation of pre-ionized material injected into the capillary. In this scheme the rapid detachment of the plasma column from the capillary walls by the rapidly increasing current pulse produces hotter plasma channels. Initial experiments were conducted injecting argon into a 2.5 mm diameter capillary utilizing a fast pulsed valve. Time resolved soft X-ray spectra obtained for 23 kA discharge pulses (Figure la) are observed to be dominated by fluorine-like argon (Arx) transitions. In these discharges it was possible also to select the charge state of the dominant ion specie by adjusting the discharge current. Figure lb, which corresponds to a 58 kA discharge, shows emission from Arm1 lines and a decrease in the intensity of the Arx lines. Hotter plasmas, with emission from Arwv lines, were generated in 1.5 mm diameter capillaries. Experiments designed to explore gain in collisionally excited neon-like and nickel-like ions in these discharge excited plasma columns will be discussed.

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