Modelling of UV direct-write waveguides in single crystal lithium niobate

Abstract

1. Fibres with micron-scale structure
The development of core-clad silica glass optical fibers has revolutionized communications systems over the past 30 years. These 'conventional' optical fibers have also made a significant impact in areas as diverse as sensing, medical imaging, laser welding and machining, and the realization of new classes of lasers and amplifiers. All of these advances have been enabled by one key factor: the reduction of the fiber loss. Reducing loss was a topic of intensive research and development for two decades, and dramatic improvements in the transmission of silica-based fibers in the 1.5 micron telecommunications window were achieved as a result. The widely used Coming SMF-28 fiber has a loss of less than 0.2 dB/km at 1550nm.
In the early 1970s, when the fabrication processes for the manufacture of core-clad preforms had not yet reached maturity, Kaiser et al. proposed an alternative route to achieving low fibre losses. Kaiser’s concept was to confine light within a pure (undoped) silica core by surrounding it with air [1], [2]. The core was supported by a sub-wavelength strand of silica glass and then jacketed in a silica cladding for strength. Although this new class of fibers showed promise, the fabrication methods It has been shown that waveguide structures can be directly written into single crystal LiNbO₃ through the influence of a focussed C.W. laser source at a wavelength of 244nm scanned across the surface [1]. A possible cause for this effect is the thermally induced diffusion of Li ions away from the regions heated by thk laser beam leading to a local decrease in the refractive index which is dependant upon the Li concentration [2]. Figure 1 shows a schematic of this process.
The diffusion of heat within the crystal has been modelled initially by analytical methods which include a Kirchhoff integral transform to include the effect of a temperature dependant thermal diffusivity. Using this analytical model we have investigated the characteristics of the temperature distribution in the vicinity of the scanning beam with variation of exposure parameters such as beam spot size, incident optical power and scanning speed. It has been found that the temperature distribution in the reference fiame of the moving beam is independent of the scan speed for all practical speeds.
A finite difference model has also been constructed for a static beam and the results compare very closely with the analytical model. The finite difference model has allowed the inclusion of a temperature-dependant heat capacity as well as a temperature-dependant thermal diffusivity. It has been found however, that the temperature-dependant heat capacity has only a small additional effect and so this justifies the use of the analytical model where only the thermal diffusivity is temperature-dependant.
The equations describing diffusion of Li ions in a dynamic, spatially non-uniform, temperature distribution have been derived and used to extend the finite difference temperature model into a model of Li diffusion induced by the scanning beam. For an incident power of 80mW, scan speed of 8 x 10^-4ms^-1 and spot size of 3.25µm a peak Li concentration change was calculated to be about 1% in the preliminary results. Calculations of the refractive index changes based on these predicted Li concentration changes will be presented and compared with measured values achieved via direct waveguide writing.

More information

Identifiers

ORCID for R.W. Eason: orcid.org/0000-0001-9704-2204

ORCID for S. Mailis: orcid.org/0000-0001-8100-2670

Catalogue record

Export record