Electromagnetically-Induced-Absorption-Mediated Ultrahigh-Rejection Microwave Photonic Notch Filter Using Single-Sideband Modulation

The inherent narrow bandwidth and wavelength-transparent nature of the stimulated Brillouin scattering (SBS) process make it a potential candidate for high-resolution wideband microwave photonic notch filters (MPNFs). The simplest way to realize a SBS-based MPNF is to use the narrowband Brillouin loss resonance with single-sideband modulation, where the modulation sideband is at the anti-Stokes frequency. However, using Brillouin loss with single-sideband modulation is highly inefficient because a large notch extinction requires large Brillouin loss, and thus, a large pump power. Here, we exploit an analogue of electromagnetically induced absorption (EIA) in the microwave domain to enhance MPNF rejection, at a fixed Brillouin pump power, by more than 6 orders of magnitude. We use this analogue of EIA to realize a MPNF with $>75$-dB rejection up to 40 GHz, using a Brillouin loss of about 15 dB. We harness a coherent Brillouin interaction between the orthogonally polarized components of a probe, which is comprised of a laser carrier and a single-modulation sideband. Stimulated Brillouin scattering and other components used in our demonstration have recently been demonstrated on integrated platforms. The proposed scheme, therefore, has the potential for realizing efficient integrated microwave photonic notch filters with ultrahigh rejection.

Figure 1

Schematic representation of the concept of EIA-based MPNF. Node a, modulation of laser carrier of frequency ${\omega }_c$ by ${\mathrm{\Omega }}_{\mathrm{rf}}$; node b, upper and lower sidebands generated at intensity modulator of ${\omega }_c-{\mathrm{\Omega }}_{\mathrm{rf}}$ and ${\omega }_c+{\mathrm{\Omega }}_{\mathrm{rf}}$, respectively; node c, optical filtering of upper sideband to create a probe consisting of a lower sideband and carrier with different power distributions in orthogonal polarization components; node d, optical pump centered at frequency ${\omega }_c-{\mathrm{\Omega }}_{\mathrm{rf}}-{\mathrm{\Omega }}_B$ with different power distributions in orthogonal polarization components; node e, back-scattered probe on interaction with pump; amplitude of orthogonal polarization components of probe signal are equal at desired rf frequency (${\mathrm{\Omega }}_{\mathrm{rf}}$); node f, photodetection of different losses witnessed by orthogonal polarization components of lower sideband.

Figure 2

Simulation results for concept of MPNF. (a) Normalized rf response ($P_{\mathrm{rf}}$) of $x$- (red dashed-dotted line) and $y$-polarized (black solid line) probe components. Both plots are normalized with respect to $y$-polarization power. (b) The rf-phase responses of individual polarization components. (c) Normalized rf response ($S_{\mathrm{rf}}$) of the MPNF, demonstrating Brillouin loss enhancement due to the analogue of EIA. (d) Simulated rf-phase response of the MPNF.

Figure 3

Experimental setup for the demonstration of EIA-based MPNF. NLL, narrow-linewidth laser; TNL, tunable narrow-linewidth laser; EDFA, erbium-doped fiber amplifier; SMF, single-mode fiber; PC, polarization controller; VNA, vector network analyzer; PD, photodetector; RFSA, rf spectrum analyzer; OSA, optical spectrum analyzer; C1 and C2 are circulators 1 and 2, respectively.

Figure 4

Measured concept of EIA-based MPNF. (a) Normalized rf responses of $x$ (red) and $y$ (black) polarizations. (b) The rf phase responses of individual polarizations. Inset plots a magnified version of the rf phase for the two polarizations, showing that a $\pi$ shift is achieved at the MPNF frequency. (c) Normalized rf response of the MPNF. (d) The rf phase response of the MPNF.

Figure 5

Wideband study of MPNF. (a)–(i) Measured MPNF responses ($S_{21}$) up to 40 GHz. Throughout the tuning range, the MPNF achieves a record notch depth of $>75\mathrm{dB}$, while keeping the rf loss at $(22.9\pm 5.4)\mathrm{dB}$.

Figure 6

Measured rf loss (red) and rejection depth (blue) for different notch frequencies.

Figure 7

Filtering of a strong unwanted interferer placed 70 MHz apart from desired signal. (a) System when the pump is off, showing that the unwanted interferer is 25.4 dB stronger than the desired rf signal. (b) System when the pump is on, where the unwanted interferer is suppressed by 50.8 dB, while the signal remains unaffected.