Direct-laser-written integrated mid-IR directional couplers in a BGG glass

Category : Publications | Date : Mar 5, 2021

Arthur Le Camus, Yannick Petit, Jean-Philippe Bérubé, Matthieu Bellec, Lionel Canioni, and Réal Vallée


Optics Express Vol. 29, Issue 6, pp. 8531-8541 (2021)


The development of coherent sources and other optical components for the mid-infrared has been hampered by the lack of sturdy materials that can withstand high power radiation or exposition to harsh environment. BGG glasses are robust materials transmitting over the 2.5–5 μm region. We report here the direct femtosecond laser fabrication of efficient directional couplers integrated in a BGG glass chip. The photonic components are characterized from 2.1 to 4.2 μm and compared to similar structures inscribed in silica glass samples. At 2.85 μm, a 99% relative cross transmission is reported in BGG glass. The experimental measurements are in good agreement with the coupled mode theory for wavelengths up to 3.5 μm.

Fig. 1. (a) Sketch of the direct-laser writing technique used to fabricate the mid-IR integrated devices. A multi-pass approach allows to write smooth and homogeneous waveguides with a larger cross-section (typically 20 𝜇m wide). (b) Lateral pattern of the overlapped laser tracks. The gray zone corresponds to size of the modified area after a single-pass for BGG (8 𝜇m × 12 𝜇m) and silica (10 𝜇m × 10 𝜇m). The crosses indicate the position of each consecutive pass. The pitch for BGG is 1.6 𝜇m horizontally and 3.5 𝜇m vertically (0.6 𝜇m and 0.6 𝜇m respectively for silica). (c-d). Transverse (left panels) and longitudinal (central panels) bright field microscope images of waveguides written in BGG (c) and silica (d) glass samples. The right panels are the corresponding optical phase difference (OPD) images. The red crosses indicate the position where OPD is 0 nm. The white bars is 20 𝜇m. For BGG (resp. silica), the writing laser emits at 1030 nm (resp. 800 nm) and delivers pulses of duration 380 fs (resp. 85 fs) at energy 0.5 𝜇J (resp. 0.8 𝜇J) with a repetition rate of 250 kHz. The translation stages move at constant velocity 1 mm/s (resp. 5 mm/s).