In this study, we investigate light diffraction from a circular phase step, which is a circular aperture created in a transparent plate. The distribution of the scattered light amplitude in a plane at an arbitrary distance is derived, and the change of the diffracted intensity profile with respect to propagation distance is investigated. The difference between this type of diffraction and the diffraction of light from an aperture in an opaque plate is also described. One of the important aspects of this problem is its relevance to particle sizing methods based on laser diffraction, which assume that particles are light-absorbing disks, while transparent particles have not yet been investigated. Furthermore, in optical imaging systems, Rayleigh's criterion is defined by using the results of light diffraction from absorbent bodies to determine the resolution limit. However, as far as we know, no study has been done to check the case of microscopic transparent bodies.

Introduction: Photo-acoustic that idiomatically is called optoacoustic or laser-ultrasound is a technique that uses optical energy for the generation of ultrasound waves in various media. The waveform of the generated ultrasound contains useful information about the optical properties of the absorbing media and is influenced by laser pulse specifications. The obtained pressure signal undergoes changes prior to reaching the transducer due to the propagation in the media. In this research, the effect of laser parameters, the properties of the absorbent media and acoustic diffraction in generated ultrasound wave, have been investigated.Materials and Methods: In this research, a frequency domain has been introduced for the generation of acoustic pressure as a result of laser energy reception transfer functions in two modes of forward and backward. By the use of this model, the effect of the pulse duration, the profile and wavelength of laser and also the absorption coefficient of the sample in the generated ultrasound properties have been investigated. A transfer function for the acoustic diffraction has been introduced and the effect of diffraction in ultrasound waveform has been simulated. Also the effect of the longitudinal and the radial distance of ultrasound transducer from the irradiated surface and also the ultrasonic source dimension that is influenced by laser beam diameter in ultrasound waveform, have been surveyed. All the simulations have been accomplished in MATLAB7 software platform. Results: Increasing laser pulse duration causes an increase in the amplitude of ultrasound and a decrease in its frequency content. The increase in the amplitude causes an increase in the absorption coefficient of the sample. The generated optoacoustic waves in response to lasers with Gaussian and asymmetric Gaussian profile are not significantly different, consequently Gaussian pulse can be used in modeling with good approximation. As a result of acoustic diffraction while propagating in media, a backward single pole wave changes to a bipolar one. An increase in ultrasonic source dimension (laser beam diameter) causes an increase in the amplitude and the width of the optoacoustic waves.Discussion and Conclusion: To design a laser-ultrasound system with maximum efficiency, one has to choose a laser wavelength in which tissue has a high absorption coefficient. The pulse duration and the beam diameter of the laser should be chosen correctly with respect to the ultrasonic transducer properties.

Background and Aim: Undesirable appearance of the scar tissue after aesthetic surgery can affect the outcomes and patient satisfaction. In recent years, early laser therapy had been encouraged to reduce the scar tissue formation. However, the available evidence regarding the outcomes of laser therapy utilizing combined lasers are limited. In the current study, the effects of combined laser therapy using pulsed dye laser (PDL ) and fractional CO2 laser on the appearance of the scar tissue after aesthetic surgeries were investigated. Methods: There were 15 patients enrolled in the current study including 13 mammoplasty and 2 abdominoplasty patients. In mammoplasty, each breast was randomly assigned to the laser therapy or control group. In abdominoplasty, split scar design was used. In laser therapy group, at the end of the third postoperative week, PDL 585 nm was radiated. Furthermore, at the end of the sixth week, fractional CO2 laser was radiated. The patients were followed for 6 months after the last laser therapy session. Visual analogue scale was used to rate the appearance of the scar tissues by 3 surgeons blinded to the treatment of scars. The appearance of the scars was determined as poor, fair, good or excellent. Results: VAS averaged 5. 5±, 0. 8 in laser therapy group and 4. 8±, 0. 8 in the control group. The difference was not statistically significant (P=0. 057 ). There was no scar with excellent appearance in the current study. All of the scars were fair and good in the laser therapy group. In the control group, 13 scars were fair. The difference was not significant (P=0. 227 ). Conclusion: Early combined laser therapy using PDL and fractional CO2 laser was associated with improved appearance of the scar tissue in short-term follow up, however, the effects were not statistically significant.