Multimode emission of fluorinated ethylene propylene clad large diameter liquid-core lasers

TL;DR

The paper addresses a portable liquid-core dye laser architecture by embedding a Rhodamine B/glycerol gain medium in a FEP-clad tube (LiCo laser). It systematically varies dye concentration and core diameter, using 532 nm pulsed pumping and spectral analysis to assess ASE and lasing behavior. The results identify $0.1$ wt% and $0.3$ wt% concentrations as favorable for low-threshold emission, with $0.3$ wt% in 1/32'' tubing exhibiting a distinct laser mode near $630$ nm and a mode spacing of about $2.0 pm 0.5$ nm, not described by simple FP cavity theory due to transverse mode coupling and internal reflections. The work demonstrates practical advantages for rugged, misalignment-tolerant operation while acknowledging spectral broadening and parasitic feedback as key limitations, and suggests improvements via optimized output coupling and lower-scatter waveguide materials for enhanced performance and tunability.

Abstract

A liquid-core (LiCo) dye laser was demonstrated using Rhodamine B (RhB) dissolved in glycerol as the gain medium and fluorinated ethylene propylene (FEP) tubing as the waveguide. Photoluminescence and amplified spontaneous emission (ASE) studies identified optimal RhB concentrations of 0.1 wt.% and 0.3 wt.% for low-threshold laser operation. Laser emission was achieved in LiCo rods with 1/16 inch and 1/32 inch inner diameter FEP tubing, with narrower tubing providing enhanced mode confinement and spectral narrowing. The addition of cavity mirrors improved emission coherence, revealing a distinct laser mode at low pump energies with mode spacing inconsistent with a simple Fabry-Pérot cavity, indicating complex mode coupling and internal reflections. Limitations include spectral broadening and scattering-induced parasitic feedback, which suggest avenues for further optimization in waveguide materials and output coupling.

Figures (8)

• Figure 1: (a) An illustration of an FEP tube being filled with dye/solvent using a syringe. (b) A diagram of the experimental setup. (c) An image of the line-focused beam used to pump the liquid-core laser rod.
• Figure 2: The photoluminescence emitted from the end of a mirrorless liquid-core rod. The redshift of the peak emission and changes in the spectral profile occur as the intensity is increased.
• Figure 3: An example of a bimodal emission spectrum occurring at RhB concentration of $0.3\,$wt.% in glycerol. The dashed lines are Gaussian fit functions while the red line and thicker black line are the respective cumulative fit and experimental data.
• Figure 4: Emission spectra showing laser emission lines for different pump energies. The inset shows the emission spectrum for the lowest and highest pump energies tested.
• Figure 5: (a) A 10-peak Gaussian fit to an emission spectrum collected from one end of $1/32$" inner diameter LiCo laser filled with $0.3\,$wt.% RhB/glycerol. (b) The spectral width of the primary laser mode and its power dependence. The primary laser mode's peak wavelength as a function of pump power is shown in the set.