Photoacoustic microscopy of lipids using a graded-index multimode fiber amplifier

Photoacoustic microscopy (PAM) of lipid-rich tissue has applications in identifying atherosclerotic plaques and monitoring peripheral neuropathy. Many lipids have an optical absorption peak in the 1200 nm spectral range, which is typically produced with very expensive optical parametric oscillator (OPO) lasers. We have previously demonstrated that an inexpensive 1064 nm laser and nonlinear fiber optics can produce pulses in the 1200 nm spectral range. In this paper, we demonstrate an improved system producing a 10-fold increase in pulse energy at a wavelength (1215 nm) that is closer to the optical absorption peak of lipids. The key factors are: (1) a higher energy laser at a slightly shorter wavelength (2) and a graded-index multimode fiber (GIMF). Our system is based on a 1047 nm Q-switched Nd:YLF laser producing 14 ns pulses with 150 uJ of energy at a 1 kHz repetition rate. The laser pulses are coupled into a 95 meter long GIMF (0.2 NA, core diameter = 50 um). Stimulated Raman scattering produces four new spectral lines at 1098, 1150, 1215, and 1275 nm. The GIMF output pulse energies are measured to be 60, 12, 9, 7, and 3 uJ, respectively. Despite the multimode nature of the GIMF, beam profile measurements suggest that the new wavelengths occupy the fundamental mode of the fiber. PAM experiments are performed on a lipid phantom consisting of a small strip of butter placed between approximately 1.2 mm thick pieces of chicken breast meat. The butter strip is only visible at 1215 nm, which is consistent with the optical absorption peak of lipids. We believe the graded-index multimode fiber amplifier is a promising cost-effective method to produce microjoule-level pulses near 1210 nm for high-speed spectroscopic photoacoustic microscopy of lipid-rich tissue.