Abstract

We demonstrate a fiber-optic ultrafast tunable source for harmonic generation microscopy (HGM) in human skin. The source is based on a 31-MHz Er-fiber laser followed by self-phase modulation enabled spectral selection (SESS). The resulting pulses are tunable between 1.15 and 1.35 μm with up to >10-nJ pulse energy and ∼100-fs pulse duration. We employ this source to drive a scanning microscope for HGM imaging of ex vivo human skin. A systematic investigation on imaging depth versus excitation wavelength reveals that excitation wavelengths in the 1.15–1.25 μm range exhibit low optical attenuation within the tissue and allows larger imaging depth for HGM in human skin. HGM driven by fiber-based SESS sources constitutes an enabling tool for noninvasive virtual skin biopsy in clinical applications.

Highlights

  • H ARMONIC generation microscopy (HGM) is one of the most important modalities for optical virtual skin biopsy [1]–[7]

  • We demonstrate a wavelength widely tunable femtosecond source employing self-phase modulation enabled spectral selection (SESS) for HGM imaging in human skin

  • Besides 1.25 μm, our result suggests a ∼100-nm wide sub-window between 1.15 μm and 1.25 μm suitable for conducting HGM in human skin

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Summary

INTRODUCTION

H ARMONIC generation microscopy (HGM) is one of the most important modalities for optical virtual skin biopsy [1]–[7]. The corresponding THG (233–333 nm) falls within ultraviolet (UV) region and suffers from strong water/protein absorption, low objective transmittance, and low sensitivity of the photomultiplier tube (PMT) These disadvantages have stimulated research efforts in developing femtosecond driving sources at longer wavelengths. We improve our Er-fiber laser based SESS source to obtain femtosecond pulses tunable between 1.15 μm and 1.35 μm with pulse energy of 7.2-11.7 nJ This source is integrated with a scanning microscope for HGM imaging of ex vivo human skin tissue with a focus on investigating imaging performance versus excitation wavelength. Considering both the sensitivity and gain of the detector, our experimental results show that besides 1.25-μm (Cr:forsterite laser wavelength) excitation, wavelengths at 1.15 μm and 1.2 μm are suitable for conducting HGM in human skin

Fiber-Based SESS Source
Scanning Microscope
EXPERIMENTAL RESULTS
Effect of Excitation Wavelength
CONCLUSION
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