Effect of spectral width on short laser pulses propagation through upper layers of human skin: Monte Carlo simulations

Ultrashort laser pulses are widely used now for biomedical diagnostics in order to reveal various tissue abnormities, such as tumors. In particular, the use of these pulses is preferable to investigate relatively thin objects (e.g., layers of human skin). In this case, femtosecond rather than longer pulses should be used due to more pronounced broadening of their temporal profiles for forward detected radiation. However ultrashort pulses are characterized by continuous spectra covering a range of wavelengths. As the optical properties of the probed medium depend on the wavelength, this may cause problems in deconvoluting the signal. In this paper, an attempt is made to investigate this problem. A two-layer model of the object mimicking the upper layers of skin (epidermis and dermis) are considered. Laser pulses of 3.9 to 62.5-fs duration are simulated to impinge onto a plane two-layered medium. The layers differ by optical parameters. The laser beam radius is 0.1 mm. Central wavelengths of spectra of the incident pulses lie in UV, blue, or red spectral regions. Absorption and scattering coefficients of the medium correspond to the real skin; the thicknesses of the successive layers are 0.1 and 0.2 mm respectively. It is shown that the δ-function spectrum approximation may be applied to laser pulses, which central wavelengths lie within the red spectrum region for all tested pulse durations. If the central wavelengths lie within the blue or UV regions, the spectrum width should be taken into account for pulses shorter than 60 fs.