Improved photosensitive response of doped silica to a 213 nm pulsed laser using a multi-pass writing approach

Abstract

Direct UV writing enables the fabrication of waveguides [1] and Bragg gratings in doped planar silica [2], achieving a change in the refractive index of 5.1×10^-3 [3]. This index change is smaller in comparison with etched devices; therefore, finding routes to improve the photosensitivity response in doped silica is essential. However, the photosensitive response of doped silica to pulsed 213 nm laser writing is more complex than with a 244 nm CW argon-ion laser, and can lead to damage when operating at higher peak powers [4]. Here we present a new writing approach where UV radiative fluence is deposited over multiple passes, reducing the peak fluence. The multi-pass writing approach significantly increases the induced change in the effective index and grating's strength.
We used small-spot direct UV writing (see Fig. 1(a and b)) to fabricate channel waveguides and gratings simultaneously in hydrogen-loaded doped FHD silica [2]. A total laser fluence of 1 kJ cm⁻² was used for all devices; however, these ranged from 1 pass at 1 kJ cm⁻² to 100 passes at 0.01 kJ cm⁻². Fig.1(c) illustrates the reflection spectra of the 4-mm long uniform grating written in a single pass and over-written in 20 and 40 passes; showing broadening and a significant red shift in the central wavelength of overwritten gratings compared to those written in a single pass. Fig. 1(d) plots the variation in n_eff and ∆_nac for the gratings written at a net fluence of 1 kJ cm⁻² in different passes. Increasing the number of writing passes from 1 to 100; we observe a 1.4×10⁻³ increase in the effective refractive index of the gratings compared to those written in a single pass. Similarly, we perceive a significant increase in the grating's strength by increasing the number of passes to 40; the achieved ∆_nac of gratings written in 40 passes is 1.6 times higher than those written in a single pass. A further increase in writing passes (beyond 40) leads to reduced grating strength, possibly due to the limitations associated with the dynamics of the UV writing interferometer and air-bearing stages. The multi-pass writing significantly improves the time limitations of fabrication by reducing the artefacts of hydrogen outgassing. We will present in-depth investigations on the photosensitive response of doped silica to multi-pass writing at different laser powers and fluences.

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Identifiers

ORCID for Salman Ahmed: orcid.org/0000-0003-0731-5390

ORCID for Paul C. Gow: orcid.org/0000-0002-3247-9082

ORCID for Christopher Holmes: orcid.org/0000-0001-9021-3760

ORCID for Corin Gawith: orcid.org/0000-0002-3502-3558

ORCID for Peter G R Smith: orcid.org/0000-0003-0319-718X

ORCID for James C. Gates: orcid.org/0000-0001-8671-5987

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