Microwave signal processing on a photonic chip

Processing microwave signals at frequencies in the tens of GHz is becoming more and more important as it allows for higher bandwidth wireless communication and advanced radar systems. However, traditional RF micro-electronic systems are facing major challenges at these high frequencies due to an increase in losses and even MEMS based signal processors are struggling to maintain their high Q-factor at these frequencies.

Here we are using the unique properties of stimulated Brillouin scattering (SBS) to achieve high-quality microwave signal processing capabilities. SBS is a nonlinear effect that resonantly couples two optical waves via a traveling acoustic wave. The frequency difference of the two optical waves is in the GHz range, whereas the spectral width over which the coupling occurs is very narrow, with only a width of tens of MHz.

In our group, we developed integrated photonic circuits that are optimised to exhibit strong coupling between the optical and acoustic wave. This platform, made out of chalcogenide glass surrounded by a silica cladding and substrate, not only allows small-footprint integration but also enables improved functionalities with a higher energy efficiency, as the Brillouin interaction strength in these engineered circuits exceeds the strength in an optical fiber by two orders of magnitude.

In our group we published several seminal demonstrations of microwave signal processing on a chip based on SBS. We demonstrated widely tunable, 1 – 40 GHz RF notch [1] and bandpass filters [2], signal delay schemes [3], phase shifter [4], and pure microwave sources [5]. An overview of the achieved schemes and a photo of one of our filter prototypes is given in figure 1.

The power of SBS based RF signal processing lies in the ability to achieve ultrawide tunability without degradation in the device performance. The SBS response is highly tailorable as the resonance is induced by the optical pump wave as opposed to microwave photonics schemes based on structural resonances. This tailorability allows to broaden or narrow the width of the SBS resonance.

Future directions of the project aim at further increasing the bandwidth of the achieved microwave devices, pushing to higher RF frequencies and further improving the overall performance. We explore new platforms in which we can harness SBS, such as silicon. The world-class facilities provided in the SNH allow us to design future circuits that allow processing of high-frequency microwave signals within a small footprint.

Selected publications

1. D. Marpaung, B. Morrison, M. Pagani, R. Pant, D.-Y. Choi, B. Luther-Davies, S. J. Madden, and B. J. Eggleton, “Low-power, chip-based stimulated Brillouin scattering microwave photonic filter with ultrahigh selectivity,” Optica, vol. 2, no. 2, p. 76, 2015.
2. A. Choudhary, I. Aryanfar, S. Shahnia, B. Morrison, K. Vu, S. Madden, B. Luther-Davies, D. Marpaung, and B. J. Eggleton, “Tailoring of the Brillouin gain for on-chip widely tunable and reconfigurable broadband microwave photonic filters,” Opt. Lett., vol. 41, no. 3, pp. 436–439, 2016.
3. A. Choudhary, Y. Liu, B. Morrison, K. Vu, D.-Y. Choi, P. Ma, S. Madden, D. Marpaung, and B. J. Eggleton, “High-resolution, on-chip RF photonic signal processor using Brillouin gain shaping and RF interference,” Sci. Rep., vol. 7, no. 1, p. 5932, 2017.
4. M. Pagani, D. Marpaung, D.-Y. Choi, S. J. Madden, B. Luther-Davies, and B. J. Eggleton, “Tunable wideband microwave photonic phase shifter using on-chip stimulated Brillouin scattering,” Opt. Express, vol. 22, no. 23, pp. 28810–28818, 2014.
5. M. Merklein, B. Stiller, I. V. Kabakova, U. S. Mutugala, K. Vu, S. J. Madden, B. J. Eggleton, and R. Slavík, “Widely tunable, low phase noise microwave source based on a photonic chip,” Opt. Lett., vol. 41, no. 20, p. 4633, Oct. 2016.

Research capabilities

The chalcogenide-based integrated circuits are fabricated at the Australian National University in Canberra.

Development of silicon-based circuits is undertaken in the Sydney Nanoscience Hub. Fabrication of these circuits is based on e-beam lithography followed by dry end wet etching steps.