Pulse stretcher with variable pulse length for excimer laser applications

Pulse stretcher with two beamsplitting elements for excimer laser pulses

A pulse stretcher with two beamsplitting elements and an optical ring cavity is proposed. With a two-beamsplitting-element configuration, the temporal profile of the stretched output pulse is effectively improved, and the transient peak power is significantly reduced. By optimizing the radius of curvature of the concave mirrors used in the ring cavity, the spatial profile of the stretched pulse is kept nearly unchanged. The performance of the pulse stretcher is experimentally investigated with a 193 nm excimer laser. By optimizing the splitting ratios of the two beamsplitting elements, the pulse width is extended from 15.5 ns to 46.2 ns with a stretching ratio of 2.89; the transient peak power of the stretched pulse is reduced to 27% of that of the input laser pulse; an energy transmission efficiency of 87.8% is achieved for the proposed pulse stretcher with two beamsplitting elements.

Application of laser pulse stretching scheme for efficiently delivering laser energy in photoacoustic imaging

High-energy and short-duration laser pulses are desirable to improve the photoacoustic image quality when imaging deeply seated lesions. In many clinical applications, the high-energy pulses are coupled to tissue using optical fibers. These pulses can damage fibers if the damage threshold is exceeded. While keeping the total energy under the Food and Drug Administration limit for avoiding tissue damage, it is necessary to reduce the peak intensity and increase the pulse duration for minimizing fiber damage and delivering sufficient light for imaging. We use laser-pulse-stretching to address this problem. An initial 17-ns pulse was stretched to 27 and 37 ns by a ring-cavity laser-pulse-stretching system. The peak power of the 37-ns stretched pulse reduced to 42% of the original, while the fiber damage threshold was increased by 1.5-fold. Three ultrasound transducers centered at 1.3-, 3.5-, and 6-MHz frequencies were simulated, and the results showed that the photoacoustic signal of a 0.5-mm-diameter target obtained with 37-ns pulse was about 98, 91, and 80%, respectively, using the same energy as the 17-ns pulse. Simulations were validated using a broadband hydrophone. Quantitative comparisons of photoacoustic images obtained with three corresponding transducers showed that the image quality was not affected by stretching the pulse.