Understanding femtosecond pulse propagation in biological media has become increasingly relevant with the widespread use of femtosecond laser systems for imaging and diagnostic applications. Intense, femtosecond pulses are prone to undergo nonlinear effects in media. The ability to accurately simulate the nonlinear processes femtosecond pulses undergo in biological media is critical for designing new diagnostic techniques and determining maximum permissible exposure (MPE) limits for laser safety standards such as ANSI Z136.1. The combination of strong absorption, broad bandwidth, and dispersive effects makes standard nonlinear simulation methods based on the slowly varying envelope approximation unsuitable for the study of near-infrared (near-IR) pulses in water and biological tissues. Building off an existing linear ultrafast pulse propagation model we present preliminary work simulating supercontinuum broadening in water without using an envelope approximation. Using a one-dimensional simulation of self-phase modulation, we explain the infrared continuum broadening observed in a previous experiment in water using 35 fs near-IR pulses, but fail to explain the visible continuum, suggesting that the continuum is further broadened by self-focusing. We then extend the model to simulate the propagation of near-IR pulses in the human eye at the 100 fs ANSI MPE limit for pulse durations from 10 fs to 1 ps. Using this simulation, we explore the implications of supercontinuum generation on the ANSI MPE limits.

Date Published: 21 March 2019
PDF: 17 pages
Proc. SPIE 10902, Nonlinear Frequency Generation and Conversion: Materials and Devices XVIII, 109020V (21 March 2019); doi: 10.1117/12.2511829

Published in SPIE Proceedings Vol. 10902:
Nonlinear Frequency Generation and Conversion: Materials and Devices XVIII
Peter G. Schunemann; Kenneth L. Schepler, Editor(s)