Localized high resolution brillouin spectrum measurement of a photonic integrated circuit

Abstract

Stimulated Brillouin scattering (SBS) is a phonon-photon interaction in which the energy of an optical pump transfers to a Stokes wave through an acoustic wave. Chalcogenide glass (As2S3) photonic integrated circuits (PIC) are among the most efficient platforms for SBS and have been extensively developed for several applications, including microwave photonic filters, lasers and optical memory. In these applications, the acoustic properties of the waveguide are paramount to achieve high performance. A distributed SBS measurement with high spatial resolution reveals information about the local phonon-photon interactions along the waveguide in contrast to traditional techniques, which only capture an integrated SBS response. Among different SBS-based distributed measurement techniques, Brillouin optical correlation domain analysis (BOCDA) has achieved the highest spatial resolution, which is essential for scanning cm-scale PICs. Different implementations of BOCDA use frequency and phase modulation [1][2] or are based on amplified spontaneous emission (ASE) [3]. The latter requires only a simple ASE source, however the drawback is that this technique typically leads to a low signal-to-noise ratio (SNR). In this work, we use an ASE-based BOCDA system with a lock-in amplifier to achieve higher SNR by rejecting the excess noise from the ASE spectrum. We scan the local SBS response of a chalcogenide PIC with 2.5 mm spatial resolution. This approach provides critical information about acoustic properties of the waveguide. In our BOCDA system, we use a filtered ASE source with spectral bandwidth of 25 GHz as pump and probe arms, where the pump is modulated with 500 ns pulses. The dynamic SBS response of the counter-propagating pump and probe build up an acoustic wave at the position of the correlation peak, where the delay between the pump and probe arms is zero. At all other positions, there is no correlation between the pump and the probe due to the randomness of the ASE source [3]. The zero delay position can be scanned along the medium by changing the delay between the pump and the probe. In our experiment, we introduce a lock-in amplifier, which relies on an electronic phase sensitive detection to improve the SNR of the backscattered signal. The waveguide is a 23.4 cm As2S3 spiral with 2.2 μm width and 930 nm height, topped with a silica cladding. Fig.1a) shows the local response of a 12 cm long section of the waveguide with 0.3 cm steps and a zoom-in image of the waveguide entrance, with 0.6 mm steps over 6.5 mm (Fig. 1b). The beginning of the waveguide can be clearly distinguished with high spatial resolution. Figure 1c) shows a local Brillouin spectrum and its Lorentz fit at 3 cm from the waveguide facet. The Brillouin frequency shift and linewidth associated with each point of the waveguide are provided in Fig.1d) and e) by fitting a Lorentz function to the data. The data indicates that the waveguide is highly uniform and consistent in terms of acoustic properties within the range of the scan. Future work will provide insights into inhomogeneous structures and capturing very fine features of the waveguide such as μm scale bends of the spiral waveguide, with an improved spatial resolution. Show more