Ravi Pant

Inducing and harnessing stimulated Brillouin scattering in photonic integrated circuits

We review recent progress in inducing and harnessing stimulated Brillouin scattering (SBS) in integrated photonic circuits. Exciting SBS in a chip-scale device is challenging due to the stringent requirements on materials and device geometry. We discuss these requirements, which include material parameters, such as optical refractive index and acoustic velocity, and device properties, such as acousto-optic confinement. Recent work on SBS in nano-photonic waveguides and micro-resonators is presented, with special attention paid to photonic integration of applications such as narrow-linewidth lasers, slow- and fast-light, microwave signal processing, Brillouin dynamic gratings, and nonreciprocal devices.

Photonic chip based tunable and reconfigurable narrowband microwave photonic filter using stimulated Brillouin scattering

We report the first demonstration of a photonic chip based dynamically reconfigurable, widely tunable, narrow pass-band, high Q microwave photonic filter (MPF). We exploit stimulated Brillouin scattering (SBS) in a 6.5 cm long chalcogenide (As2S3) photonic chip to demonstrate a MPF that exhibited a high quality factor of ~520 and narrow bandwidth and was dynamically reconfigurable and widely tunable. It maintained a stable 3 dB bandwidth of 23 ± 2MHz and amplitude of 20 ± 2 dB over a large frequency tuning range of 2-12 GHz. By tailoring the pump spectrum, we reconfigured the 3 dB bandwidth of the MPF from ~20 MHz to ~40 MHz and tuned the shape factor from 3.5 to 2 resulting in a nearly flat-topped filter profile. This demonstration represents a significant advance in integrated microwave photonics with potential applications in on-chip microwave signal processing for RADAR and analogue communications.

Optimal pump profile designs for broadband SBS slow-light systems

We describe a methodology for designing the optimal gain profiles for gain-based, tunable, broadband, slow-light pulse delay devices based on stimulated Brillouin scattering. Optimal gain profiles are obtained under system constraints such as distortion, total pump power, and maximum gain. The delay performance of three candidate systems: Gaussian noise pump broadened (GNPB), optimal gain-only, and optimal gain+absorption are studied using Gaussian and super-Gaussian pulses. For the same pulse bandwidth, we find that the optimal gain+absorption medium improves the delay performance by 2.1 times the GNPB medium delay and 1.3 times the optimal gain-only medium delay for Gaussian pulses. For the super-Gaussian pulses the optimal gain-only medium provides a fractional pulse delay 1.8 times the GNPB medium delay.

Design for broadband on-chip isolator using stimulated Brillouin scattering in dispersion-engineered chalcogenide waveguides

We propose a scheme for on-chip isolation in chalcogenide (As₂S₃) rib waveguides, in which Stimulated Brillouin Scattering is used to induce non-reciprocal mode conversion within a multi-moded waveguide. The design exploits the idea that a chalcogenide rib buried in a silica matrix acts as waveguide for both light and sound, and can also be designed to be multi-moded for both optical and acoustic waves. The enhanced opto-acoustic coupling allows significant isolation (> 20 dB) within a chip-scale (cm-long) device (< 10 cm). We also show that the bandwidth of this device can be dramatically increased by tuning the dispersion of the waveguide to match the group velocity between optical modes: we find that 20 dB isolation can be extended over a bandwidth of 25 nm.

Design of a tunable time-delay element using multiple gain lines for increased fractional delay with high data fidelity

A slow-light medium based on multiple, closely spaced gain lines is studied. The spacings and relative strengths of the gain lines are optimized by using the criteria of gain penalty and eye-opening penalty to maximize the fractional delay defined in terms of the best decision time for random pulse trains. Both numerical calculations and experiments show that an optimal design of a triple-gain-line medium can achieve a maximal fractional delay about twice that which can be obtained with a single-gain-line medium, at three times higher modulation bandwidth, while high data fidelity is still maintained.

Frequency agile microwave photonic notch filter with anomalously high stopband rejection.

We report a novel class microwave photonic (MWP) notch filter with a very narrow isolation bandwidth (10 MHz), an ultrahigh stopband rejection (>60 dB), a wide frequency tuning (1-30 GHz), and flexible bandwidth reconfigurability (10-65 MHz). This performance is enabled by a new concept of sideband amplitude and phase controls using an electro-optic modulator and an optical filter. This concept enables energy efficient operation in active MWP notch filters, and opens up a pathway toward enabling low-power nanophotonic devices as high-performance RF filters.

Si3N4 ring resonator-based microwave photonic notch filter with an ultrahigh peak rejection

We report a simple technique in microwave photonic (MWP) signal processing that allows the use of an optical filter with a shallow notch to exhibit a microwave notch filter with anomalously high rejection level. We implement this technique using a low-loss, tunable Si₃N₄ optical ring resonator as the optical filter, and achieved an MWP notch filter with an ultra-high peak rejection > 60 dB, a tunable high resolution bandwidth of 247-840 MHz, and notch frequency tuning of 2-8 GHz. To our knowledge, this is a record combined peak rejection and resolution for an integrated MWP filter.

Narrow linewidth Brillouin laser based on chalcogenide photonic chip

We present, to the best of our knowledge, the first demonstration of a narrow linewidth, waveguide-based Brillouin laser that is enabled by large Brillouin gain of a chalcogenide chip. The waveguides are equipped with vertical tapers for low-loss coupling. Due to optical feedback for the Stokes wave, the lasing threshold is reduced to 360 mW, which is five times lower than the calculated single-pass Brillouin threshold for the same waveguide. The slope efficiency of the laser is found to be 30%, and the linewidth of 100 kHz is measured using a self-heterodyne method.

Photonic chip-based all-optical XOR gate for 40 and 160 Gbit/s DPSK signals

We demonstrate a photonic chip-based all-optical exclusive-OR (XOR) gate for phase-encoded optical signals via four-wave mixing in a highly nonlinear, dispersion-engineered chalcogenide (As2S3) planar waveguide. We achieve error-free, XOR operation for 40 Gbit/s differential phase shift keying (DPSK) optical signals with no power penalty. The effectiveness and broad bandwidth operation of our approach is highlighted by implementing an XOR gate for 160 Gbit/s DPSK signals.

Maximizing the opening of eye diagrams for slow-light systems

We present a data-fidelity metric for quantifying distortion in slow-light optical pulse delay devices. We demonstrate the utility of this metric by applying it to the performance optimization of gain-based slow-light delay systems for Gaussian and super-Gaussian pulses. Symmetric Lorentzian double-line and triple-line gain systems are optimized and achieve maximum delay of 1.5 and 1.7 times the single-line gain system delay, respectively. The resulting double-line gain system design is qualitatively similar to the double-line gain system designed with a previous metric, but is tuned specifically to constrain data fidelity.