Photoacoustic microscopy with a tunable source based on cascaded stimulated Raman scattering in a large-mode area photonic crystal fiber

Abstract

Conventional tunable pulsed lasers for photoacoustic microscopy (PAM) are bulky and expensive. We have previously demonstrated a compact tunable source using cascaded stimulated Raman scattering (SRS) in an ordinary single-mode fiber. In this paper we report an improved tunable source with both higher pulse energy (over 200 nJ), repetition rate (30 kHz), and extended tuning range (532 - 610 nm). The key feature is a large mode area photonic crystal fiber (LMA-PCF). The large mode area, pure silica composition, and honeycomb cladding of the LMA-PCF produce a much higher optical damage threshold than ordinary fiber. We have found that cascaded SRS with very high Raman gain occurs in the LMA-PCF to produce both discrete and a continuum of wavelengths between 560 and 610 nm, offering the possibility of rapid tunability within this physiologically important wavelength range. Our Q-switched Nd:YAG laser produces 2 ns duration pulses at 532 nm with 10 uJ of energy at a 30 kHz repetition rate. The laser pulses are coupled into a 30 meter long LMA-PCF. The multi-color fiber output goes through a band pass filter (10 nm width), where the selected wavelength is sent to a 50 MHz photoacoustic microscopy system employing optical focusing. The individual pulse energy is 270, 360, 520, 530, and 400 nJ at wavelengths of 532, 546, 568, 589, and 600 nm, respectively. Test imaging experiments are performed on a silicone tube phantom containing red and blue ink. To our knowledge, this is the first demonstration of a tunable pulsed optical source using a LMA-PCF. The pulse energy and wavelength range of our system are suitable for oxygenation measurements. We believe these results can significantly benefit the development of functional PAM systems.