Development of a Fiber Laser with Independently Adjustable Properties for Optical Resolution Photoacoustic Microscopy.

Esra Aytac-Kipergil,Sarper Salman,Nasire Uluc,Tunc Kayikcioglu,Seydi Yavas,Sohret Gorkem Karamuk,Mehmet Burcin Unlu,Aytac Demirkiran,Fatih Omer Ilday

doi:10.1038/srep38674

Abstract

Photoacoustic imaging is based on the detection of generated acoustic waves through thermal expansion of tissue illuminated by short laser pulses. Fiber lasers as an excitation source for photoacoustic imaging have recently been preferred for their high repetition frequencies. Here, we report a unique fiber laser developed specifically for multiwavelength photoacoustic microscopy system. The laser is custom-made for maximum flexibility in adjustment of its parameters; pulse duration (5–10 ns), pulse energy (up to 10 μJ) and repetition frequency (up to 1 MHz) independently from each other and covers a broad spectral region from 450 to 1100 nm and also can emit wavelengths of 532, 355, and 266 nm. The laser system consists of a master oscillator power amplifier, seeding two stages; supercontinuum and harmonic generation units. The laser is outstanding since the oscillator, amplifier and supercontinuum generation parts are all-fiber integrated with custom-developed electronics and software. To demonstrate the feasibility of the system, the images of several elements of standardized resolution test chart are acquired at multiple wavelengths. The lateral resolution of optical resolution photoacoustic microscopy system is determined as 2.68 μm. The developed system may pave the way for spectroscopic photoacoustic microscopy applications via widely tunable fiber laser technologies.

Highlights

Photoacoustic microscopy (PAM) is a promising imaging modality that combines optical and ultrasound imaging
Q-switched Nd:YAG pumped dye lasers, Ti:Sapphire lasers, and optical parametric oscillators (OPOs) are usually preferred for providing necessary wavelength tuning with high pulse energies (>1 mJ)[7,8,14,24,25,26,27,28,29,30,31,32]; yet, they have some major limitations of their practical applications such as having low pulse repetition rate, being bulky and expensive, and requiring external cooling units[35]
In order to evaluate the performance of our laser system, pulse energy, average power and repetition rate values are compared with the ones in existing systems within the literature including fiber components and independent of seeding laser type

Summary

Introduction

Photoacoustic microscopy (PAM) is a promising imaging modality that combines optical and ultrasound imaging. The laser system needs to produce short enough pulses, i.e., several nanoseconds, in order to generate photoacoustic signals efficiently and emit wavelengths in the visible range to cover absorption peaks of tissue chromophores in their spectra[4,13,14]. For the sake of enabling spectroscopic measurements, multiwavelength spectrum is obtained from a single wavelength emitting Q-switched Nd:YAG microchip laser, either through stimulated Raman scattering (SRS) or nonlinear broadening by coupling its output to a fiber[36,37,38,39,40,41,42,43,44,45]. Pulse energy per band increased dramatically and became comparable to the ones produced through SRS39,45

Methods

Results

Conclusion