Integrated Arbitrary Filter with Spiral Gratings: Design and Characterization

TL;DR

This paper presents the design and experimental validation of a high-performance integrated optical filter covering 1450-1640 nm, capable of suppressing specific sky emission lines for astronomical applications, using novel inverse scattering algorithms.

Contribution

It introduces a new on-chip filter design with superior spectral performance and compares two inverse scattering algorithms, proposing improvements for non-uniformity issues in grating reconstruction.

Findings

Achieved a 55-notch filter with 28 dB depth and 0.22 nm width.

Demonstrated low insertion loss of 2.5 dB and waveguide loss of 0.10 dB/cm.

Validated the effectiveness of the f-DLP algorithm for accurate grating design.

Abstract

We report the design and characterization of a high performance integrated arbitrary filter from 1450 nm to 1640 nm. The filter's target spectrum is chosen to suppress the night-sky OH emission lines, which is critical for ground-based astronomical telescopes. This type of filter is featured by its large spectral range, high rejection ratio and narrow notch width. Traditionally it is only successfully accomplished with fiber Bragg gratings. The technique we demonstrate here is proven to be very efficient for on-chip platforms, which can bring many benefits for device footprint, performance and cost. For the design part, two inverse scattering algorithms are compared, the frequency domain discrete layer-peeling (f-DLP) and the time domain discrete layer-peeling (t-DLP). f-DLP is found to be superior for the grating reconstruction in terms of accuracy and robustness. A method is proposed to resolve the non-uniformity issue caused by the non-zero layer size in the DLP algorithm. The designed 55-notch filter is 50-mm-long and implemented on a compact Si3N4/SiO2 spiral waveguide with a total length of 63 mm. Experimentally, we demonstrate that the device has a insertion loss as low as 2.5 dB, and that the waveguide propagation loss is as low as 0.10 dB/cm. We are also able to achieve uniform notch depths and 3-dB widths of about 28 dB and 0.22 nm, respectively.