Fiber Source for Pumping a Fiber-Optical Parametric Generator at ∼800 nm

Abstract

Recently, methods of optical diagnostics that allow non-invasive, real-time monitoring of processes occurring in biological tissues and cells (bio-imaging) have been actively developed. It is worth noting coherent anti-Stokes Raman spectroscopy (CARS) techniques, which explore the molecular vibrational properties of organic compounds included in the composition of biological objects. CARS microscopy opens new perspectives in the study of interand intracellular interactions, cytoplasm, lipids, and nuclei. It requires coherent, time-synchronized optical pump and signal radiation with a difference of optical frequencies of 1000-3300 reverse centimeters, depending on the composition of the test substance. Usually the pump wavelength is taken around 800 nm and the thermal heating of the sample limits the pulse duration to <100 ps. Solid-state lasers based on bulk optical elements are typically used for CARS microscopy: a Ti-sapphire laser and optical parametric oscillators (OPO) based on nonlinear crystals. Providing a large tuning range and high peak and average powers, they are nevertheless expensive and complicated to maintain. On the other hand, fiber lasers which are characterized by compactness, stability, high beam quality, and relatively low cost, can find application in medicine. The currently existing fiber sources cannot replace solid-state lasers primarily because of a limited range of generation wavelengths (near-IR). Thereby, nonlinear frequency conversions in fibers of different kind are applied to obtain a wide tuning range. The most perspective nonlinear conversion technique is a parametric four wave mixing in photonic crystal fibers (PCFs). Using 50-100 ps pulses the group of A. Tünnemann demonstrated fiber systems for CARS where the pump and signal pulses were automatically overlapped at the output of the PCF [1]. The group of F Wise in [2] has shown that the scheme of a fiber optic parametric oscillator allows reducing the relative intensity noise. Though the subsequently developed fiber OPO (FOPO) schemes include elements of bulk optics [3], the researchers work on the all-fiber configuration of FOPO [4], which implementation and optimization is one of the main tasks at the moment. Here we present a new type of all-fiber source for pumping a FOPO at ∼800 nm. It consists of a master oscillator – Yb-doped fiber source of dissipative solitons [5] at 1030 nm with a standard fiber core diameter of 6 micron and repetition rate of 13 MHz. A special fiber Bragg grating cuts out a narrowband (40 pm) pulses of 85 ps duration [6] which are amplified in several cascades of Yb-doped fiber amplifier up to 400 W of peak power. Using this fiber source as a pump, we observed single-pass parametric generation at 793 nm in a 5-m-long LMA5-PM PCF. A fiber cavity is applied to enhance the parametric signal. The details of the study will be presented at the conference. Acknowledgements: The work is supported by Russian Science Foundation (RSF) grant 17-72-10129.