Creating dual waveguides with femtosecond laser writing

Researchers from the Nuclear and Energy Research Institute (IPEN) in Brazil have demonstrated that ultrashort laser pulses can be used to produce double waveguides in glasses. When the glass is doped with rare earth ions, the waveguides can be used to achieve laser amplification with low loss for straight, curved and “Y” geometries. This could enable the development of new types of compact, portable photonic devices.

Due to their compactness and simplicity, waveguides are often used to guide light in small photonic devices. The researchers have now developed a way to create active and passive double waveguides in glasses doped with heavy metals oxides.

Wagner de Rossi, a researcher from IPEN, said the glasses are relatively inexpensive, with a high nonlinear refractive index and a wide transmission window from visible to infrared. “The simplicity of production, good optical properties of the material, low cost and its versatility make these guides suitable for use in optical communication amplifiers, beam splitters, optomicrofluidic systems, compact interferometers, etc.,” De Rossi said.

To create the waveguides, the researchers focused femtosecond laser pulses beneath the surface of germanate and tellurite glasses to form tracks of overlapped pulses. Due to spherical aberration and self-focusing effects, these tracks become thin, high walls that have a decreased refractive index, preventing guidance within them. Therefore, a pair of closely spaced walls was used to guide light between them.

The researchers created straight and curved guides and varied the energy per pulse and the number of overlapping pulses to optimise the guiding mode and reduce losses. The study showed that when a repetition rate of 10 kHz was used for laser writing, the best conditions occurred with a line spacing of 10 µm, energy of 30 µJ per pulse and a scan speed of 1 mm/s.

To achieve laser amplification, the researchers created double waveguides in germanate glass doped with neodymium and erbium/ytterbium. The neodymium-doped glass showed a 3.6 dB/cm gain at 1310 nm with 1.6 mW of pump power at 805 nm. The Er/Yb-doped glass exhibited a 7.5 dB/cm gain at 1550 nm with 105 mW of pump power at 980 nm.

The researchers found that incorporating silver nanoparticles into neodymium-doped germanate glasses increased the gain and guidance performance of the waveguides. The figure below shows a pair of double guides and the luminescence produced upon amplification in a guide produced in Er/Yb doped GeO2-PbO glass.

Image caption: Top view microscope images of double line waveguides in GPO:Er/Yb. Image captured by using a 200x magnification and polarised light showing two pairs of guides separated by 40 microns from each other (a). Single double line waveguide pumped at 980 nm, for amplification at 1550 nm (b). Image credit: Optica.

Top image credit: iStock.com/narong sutinkham