Low-power, chip-based stimulated Brillouin scattering microwave photonic filter with ultrahigh selectivity

Abstract

Highly selective and reconfigurable microwave filters are of great importance in radiofrequency signal processing. Microwave photonic (MWP) filters are of particular interest, as they offer flexible reconfiguration and an order of magnitude higher frequency tuning range than electronic filters. However, all MWP filters to date have been limited by trade-offs between key parameters such as tuning range, resolution, and suppression. This problem is exacerbated in the case of integrated MWP filters, blocking the path to compact, high performance filters. Here we show the first chip-based MWP band-stop filter with ultra-high suppression, high resolution in the MHz range, and 0-30 GHz frequency tuning. This record performance was achieved using an ultra-low Brillouin gain from a compact photonic chip and a novel approach of optical resonance-assisted RF signal cancellation. The results point to new ways of creating energy-efficient and reconfigurable integrated MWP signal processors for wireless communications and defence applications.The explosive growth in mobile communications demands radio-frequency (RF) technologies with exceptional spectral efficiency such as cognitive radios, which can adapt their frequencies to exploit the available spectrum in real-time [1,2]. Such frequency-agile systems will benefit hugely from RF filters that can be tuned over many gigahertz whilst keeping high MHz-scale resolution and high selectivity to prevent severe interference due to spectrumsharing. While this is difficult to achieve with all-electronic filters [3][4][5][6][7], integrated microwave photonic (IMWP) filters [8] can readily achieve multi-gigahertz tuning range without significant degradation in their frequency response. However, these filters typically exhibit limited resolution (GHz instead of MHz linewidths) and are plagued by trade-offs between key parameters, such as between the frequency tuning range and the resolution for multi-tap filters [9][10][11][12][13]; or between the peak rejection and the resolution for resonator-based filters [14][15][16][17][18].Stimulated Brillouin scattering (SBS) [19][20][21][22] offers a route to MHz-resolution IMWP filters. Although SBS has been widely studied in optical fibers, recently there has been a growing interest in harnessing SBS in nanophotonic waveguides [22][23][24][25][26][27]. The ability to control the coherent interaction of photons and acoustic phonons in chip-sized devices (as opposed to in optical fibers many kilometres long) promises not only fascinating new physical insights, but also opens the path to realising key technologies on-chip including slow light [28,29]; narrow linewidth lasers [30]; optical frequency combs [31,32]; RF signal processing [33][34][35] and filtering [36][37][38][39][40]. In particular, SBS filters can exhibit linewidths of the order of 10-100 MHz. Such a high resolution is unmatched by most on-chip devices because it requires extremely low material losses and impractically-large devices [41].Although IMWP filters exploiting SBS on ch...

Large Brillouin amplification in silicon

“…With careful design, such Brillouin nonlinearities overtake all other nonlinear processes in silicon [6,7]; these same Brillouin interactions are remarkably tailorable, permitting a range of hybrid photonic-phononic signal processing operations that have no analog in all-optical signal processing [8][9][10][11][12]. Using this physics, the rapidly growing field of silicon-based Brillouin-photonics has produced new frequency agile RFphotonic notch filters [8,10,13,14] and multi-pole bandpass filters [12] as the basis for radio-frequency photonic (RF-photonic) signal processing. Beyond these specific examples, the potential impact of such Brillouin interactions is immense; frequency combs [13,15,16], ultra-low phasenoise lasers [17][18][19], sensors [9,12,20], optical isolation [21][22][23][24], and an array of signal processing technologies [8,[12][13][14][25][26][27]] may be possible in silicon with further progress.…”

“…Using this physics, the rapidly growing field of silicon-based Brillouin-photonics has produced new frequency agile RFphotonic notch filters [8,10,13,14] and multi-pole bandpass filters [12] as the basis for radio-frequency photonic (RF-photonic) signal processing. Beyond these specific examples, the potential impact of such Brillouin interactions is immense; frequency combs [13,15,16], ultra-low phasenoise lasers [17][18][19], sensors [9,12,20], optical isolation [21][22][23][24], and an array of signal processing technologies [8,[12][13][14][25][26][27]] may be possible in silicon with further progress.However, strong Brillouin amplification-essential to many new Brillouin-based technologies-has yet to be realized in silicon photonics. Despite the creation of strong Brillouin nonlinearities in a range of new structures [6,7], nonlinear losses and free carrier effects have stifled attempts to demonstrate net optical amplification.…”

Programmable optical processor chips: toward photonic RF filters with DSP-level flexibility and MHz-band selectivity

“…Some salient works demonstrated remarkable filter features that outrun state-of-the-art allelectronics implementations, such as terahertz bandwidth [40], multi-octave tuning [41], nanosecond switching speed [42], microwatt control [43], and power extinction exceeding 60 dB [44]. Recent work of ours proposed and experimentally demonstrated a full-integration-ready approach of such filters with DSP-like flexibility by means of introducing software-defined circuit programmability into the processing core chips [45].…”

“…Utilizing the SBS effect in a 6.5-cm-long As2S3 waveguide, Morrison et al [109], Byrnes et al [110], and Choudhary et al [111] demonstrated respectively an RF bandstop filter and narrow bandpass filter with tunable bandwidth and center frequency as shown in Figure 26B. In a later work Marpaung et al [44], associated such SBS effect with a particular modulation spectrum comprising uneven sidebands and therewith demonstrated a highly selective RF notch filter with a very deep notch of 60 dB and a large frequency tuning range of 30 GHz as shown in Figure 27A. Recently, aimed to facilitate the integration of the SBS effect with other processing functions on a chip, Casas-Bedoya et al [112] demonstrated a similar RF filter function using a silicon waveguide that has its substrate particularly treated to provide high SBS efficiency as shown in Figure 27B.…”

“…For the light source, coherent and incoherent light have been demonstrated, which include a single or multiple continuous wave (CW) lasers [39], frequency comb sources [46], and amplified spontaneous emission from erbium doped fiber amplifiers [47]. The recent advances of electro-optical modulators not only allow RF frequencies up to 100 GHz to be modulated on a lightwave, but also enable flexible configurations of modulated optical spectra such as double sidebands (DSB), asymmetric sidebands (ASB) [44], single sideband (SSB), optical carrier suppression (SC), dual polarizations, and simultaneous generation of two complementary outputs with opposite modulation phases. All these can be combined with a following optical signal processing stage to manipulate the amplitude and phase of different frequency components of the RF modulation sidebands in the optical domain.…”