We present an improved optodynamic (OD) method which enables measurement of the distance between the OD source on the ablated surface and a piezoelectric sensor above it, with a relative error of about 1%. The method is based on the point explosion model and allows determination of the distance to the OD source and the released energy for each detected OD signal. We estimate the distance and released energy on the basis of two measured OD signal characteristics: the time of flight and the duration of the compressive phase. We show that the finite aperture of the sensor needs to be taken into account to improve measurement accuracy. We present experimental validation of the method using an Er:YAG laser and water as a tissue phantom. We observe an excellent agreement between the measured and theoretical OD signals and between the measured and estimated distances. The method opens the way to practicable implementations of on-line OD monitoring of laser ablation in surgery and medicine.
A computational model, which describes EM field formation in a pulsed laser from a randomly generated initial
spontaneous field inside the laser cavity has been developed. The model is based on a two-dimensional fast
Fourier transform and describes a real laser system taking into account a lensing and a diaphragm effect of the
laser rod. The laser cavity is described by five effective planes, which represent different laser cavity elements-the back and the front mirror, the Q-switch element and the laser rod. At each plane the EM field is calculated in
real space and propagation between the planes is achieved in Fourier space by multiplication with an appropriate
phase factor. The computational time needed for simulation of a realistic pulse formation is in order of minutes.
The model can predict the shape and the integral energy of the pulse, its transverse profile at different distances
from the front mirror (including near and far field) and beam divergence. The results of the model were found
to be in good agreement with measured parameters for a Q-switched ruby laser system running in stable as well
as unstable cavity configurations. The temporal shape of a laser pulse was measured and calculated not only for
the ruby laser, but also for a Nd:YAG laser. It was found that FWHM of a pulse produced by ruby laser is three
times longer than FWHM of a pulse produced by Nd:YAG laser.
Because of their unique properties with regard to the absorption in organic tissue, pulsed Er:YAG lasers are of interest for various applications in medicine, such as dentistry, dermatology, and cosmetic surgery. The relatively low thermal side effects, and surgical precision of erbium medical lasers have been attributed to the micro-explosive nature of their interaction with organic tissue. In this paper, we report on preliminary results of our study of the thresholds for tissue ablation, using an opto-acoustic technique. Two laser energy thresholds for the interaction are observed. The lower energy threshold is attributed to surface water vaporization, and the higher energy threshold to explosive ablation of thin tissue layers.
Transmission properties of a plastical hollow waveguide at wavelength of 2.94 micrometers have been studied. The measured transmitted beam profiles for different waveguide bending curvatures are compared with theoretical transmissions, and profiles of ablation holes in hard dental tissue.
Measured population dynamics of erbium metastable level in a Yb:Er:phosphate glass following the excitation with a Nd:glass laser is reproduced by a theoretical model based on rate equations. To our knowledge, this is the first model to include frequency hole burning of inhomogeneously broadened pumping transition of ytterbium ion. The model explains also the previously observed dependence of pumping efficiency on Nd:glass laser pulse length.
A FFT based method is used to calculate the development of electromagnetic (EM) laser field inside a rotating mirror Q-switch laser. The method is applied to study the influence of pumping on the laser pulse temporal evolution and on the near and far field fluence distributions in an "eye-safe" Er: glass laser. Iterative computation of the lowest order modes for the misaligned plano--plano empty resonator shows that for small tilt angles resonator losses depend linearly on the tilt angle. Using linear Q-switching function an analytical expression based on rate equations can be developed which relates the maximum attainable single pulse output energy to the laser parameters. Theoretical results are compared with the characteristics of an "eye-safe" Er: glass laser.
Measurements of energy storage and heat deposition in flash-lamp pumped Nd:Yb:Er: glass, and Cr:Nd:Yb:Er:
glass lasers are reported. Energy storage is determined indirectly by measuring output vs. input energy
characteristics of erbium lasers operating in a free oscillation mode. A thermal camera is used to measure
temperature increase following isolated flash-lamp pulses. Contributions of different sensitizers are distinguished
by spectrally filtering flash-lamp pump radiation. It is determined that in Nd:Yb:Er: glass ytterbium sensitized
pumping contributes 85% to the total erbium inverted population energy storage. Long flash-lamp pulse
durations are therefore required for efficient Nd:Yb:Er: glass laser operation. Direct erbium, and neodimium
sensitized pumping is determined to be relatively inefficient, although the corresponding visible part of the pump
spectrum contributes in Nd:Yb:Er: glass as much as 65% of the total deposited heat. Chromium co-doping is
observed to significantly increase energy conversion efficiency of pumping in the visible, allowing shorter flashlamp
pulse durations in Cr:Nd:Yb:Er: glass lasers. In addition, analysis shows the ratio of the total heat
generated per unit stored energy to be in Cr:Nd:Yb:Er: glass lower than in Nd:Yb:Er: glass.