Purnima Sethi

All-Optical Reversible Logic Gates with Optically Controlled Bacteriorhodopsin Protein-Coated Microresonators

We present designs of all-optical reversible gates, namely, Feynman, Toffoli, Peres, and Feynman double gates, with optically controlled microresonators. To demonstrate the applicability, a bacteriorhodopsin protein-coated silica microcavity in contact between two tapered single-mode fibers has been used as an all-optical switch. Low-power control signals (<200 μW) at 532 nm and at 405 nm control the conformational states of the protein to switch a near infrared signal laser beam at 1310 or 1550 nm. This configuration has been used as a template to design four-port tunable resonant coupler logic gates. The proposed designs are general and can be implemented in both fiber-optic and integrated-optic formats and with any other coated photosensitive material. Advantages of directed logic, high Q-factor, tunability, compactness, low-power control signals, high fan-out, and flexibility of cascading switches in 2D/3D architectures to form circuits make the designs promising for practical applications.

All-Optical Ultrafast Switching in 2 × 2 Silicon Microring Resonators and its Application to Reconfigurable DEMUX/MUX and Reversible Logic Gates

We present a theoretical model to analyze all-optical switching by two-photon absorption induced free-carrier injection in silicon 2 × 2 add-drop microring resonators. The theoretical simulations are in good agreement with experimental results. The results have been used to design all-optical ultrafast (i) reconfigurable De-multiplexer/Multiplexer logic circuits using three microring resonator switches and (ii) universal, conservative and reversible Fredkin and Toffoli logic gates with only one and two microring resonator switches respectively. Switching has been optimized for low-power (25 mW) ultrafast (25 ps) operation with high modulation depth (85%) to enable logic operations at 40 Gb/s. The combined advantages of high Q-factor, tunability, compactness, cascadibility, reversibility and reconfigurability make the designs favorable for practical applications. The proposed designs provide a new paradigm for ultrafast CMOS-compatible all-optical reversible computing circuits in silicon.

Ultrafast All-Optical Flip-Flops, Simultaneous Comparator-Decoder and Reconfigurable Logic Unit With Silicon Microring Resonator Switches

We present designs of all-optical SR, clocked-SR, D and T flip-flops, simultaneous single-bit comparator-decoder and reconfigurable logic unit based on all-optical switching by two-photon absorption induced free-carrier injection in silicon 2 × 2 add-drop microring resonators. The proposed circuits have been theoretically analyzed using time-domain coupled-mode theory and all-optical switching has been optimized for ultrafast (~25 ps), low-power operation (~25 mW) and high modulation (> 85%), enabling logic operations at 40 Gb/s. The designs are attractive due to advantages of high Q-factor, tunability, compactness, cascadibility, scalability, reconfigurability, simplicity and minimal number of switches and inputs for realization of the desired logic.

All-optical ultrafast XOR/XNOR logic gates, binary counter, and double-bit comparator with silicon microring resonators.

We present designs of all-optical ultrafast YES/NOT, XOR/XNOR logic gates, binary counter, and double-bit comparator based on all-optical switching by two-photon absorption induced free-carrier injection in silicon 2 × 2 add-drop microring resonators. The proposed circuits have been theoretically analyzed using time-domain coupled-mode theory based on reported experimental values to realize low power (∼ 28 mW) ultrafast (∼ 22 ps) operation with high modulation (80%) and bit rate (45 Gb/s). The designs are complementary metal-oxide semiconductor compatible and provide advantages of high Q-factor, tunability, compactness, cascadibility, scalability, reconfigurability, simplicity, and minimal number of switches and inputs for realization of the desired logic. Although a two-bit counter has been shown, the scheme can easily be extended to N-bit counter through cascading.

Ultra-Compact Low-loss Broadband Waveguide Taper in Silicon-on-Insulator

A novel design of large bandwidth, fabrication tolerant, CMOS-compatible compact tapers (15 um) have been proposed and experimentally demonstrated in silicon-on-insulator. The proposed taper along with linear grating couplers for spot-size conversion exhibits no degradation in the coupling efficiency compared to a standard focusing grating in 1550 nm band. A single taper design has a broadband operation over 600 nm that can be used in O, C and L-band. The proposed compact taper is highly tolerant to fabrication variations; 80 nm change in the taper width and 200 nm in end waveguide width varies the taper transmission by <0.4 dB. The footprint of the device i.e. taper along with the linear gratings is ~ 250 {\mu}m2; this is 20X smaller than the adiabatic taper and 2X smaller than the focusing grating coupler.

Ultrafast All-Optical Reversible Peres and Feynman-Double Logic Gates with Silicon Microring Resonators

We present designs of reversible Peres logic gate and Feynman-Double logic gate based on all-optical switching by two-photon absorption induced free-carrier injection in silicon add-drop microring resonators. The logic gates have been theoretically analyzed using time-domain coupled-mode theory and all-optical switching has been optimized for low-power (25 mW) ultrafast (25 ps) operation with high modulation depth (85 %) to enable logic operations at 40 Gb/s. The advantages of high Q-factor, tunability, compactness, cascadibility, reversibility and reconfigurability make the designs favorable for practical applications.

All-Optical Ultrafast Adder/Subtractor and MUX/DEMUX Circuits with Silicon Microring Resonators

We present designs of all-optical ultrafast simultaneous NOR logic gate/Half-Adder/Subtractor, Full-Adder/Subtractor and Multiplexer/De-Multiplexer circuits using add-drop silicon microring resonators. The proposed circuits require less number of switches and inputs for realization of the desired logic compared to earlier reported designs. Multiplexer/De-Multiplexer operations can be realized from the same circuit by simply interchanging the inputs and outputs. Lower energy consumption and delays along with reconfigurability and compactness make them attractive for practical applications.

All-optical reversible logic gates with microresonators

We present designs of all-optical reversible logic gates, namely, Feynman, Toffoli, Peres and Feynman Double gates, based on switching of a near-IR (1310/1550 nm) signal by low-power control signals at 532 nm and 405 nm, in optically controlled bacteriorhodopsin protein-coated silica microcavities coupled between two tapered single-mode fibers.

Compact broadband taper for low-loss coupling to a silicon nitride photonic wire

We demonstrate a compact efficient waveguide taper in Silicon Nitride platform. The proposed taper provides a coupling-efficiency of 95% at a length of 19.5 μm in comparison to the standard linear taper of length 50 μm that connects a 10 μm wide waveguide to a 1 μm wide photonic wire. The taper has a spectral response > 75% spanning over 800 nm (spanning O, C & L band) and robustness to fabrication variations; ±200 nm change in taper and end waveguide width varies transmission by <5%. We experimentally report a taper insertion loss of <0.1 dB/transition and reduction in the footprint of the photonic device by 50.8% for the proposed compact taper in comparison to the traditional adiabatic taper. To the best of our knowledge, the proposed taper is the shortest waveguide taper ever reported in Silicon Nitride.

Compact tapers for silicon grating fibre-chip couplers in O, C and L band

Grating fiber-chip coupler (GC) is a versatile scheme to couple light into submicron waveguides, particularly in SOI platform [1]. Typical GC is defined within a footprint of 10 × 10 μm to match the fiber mode field. The 10 μm wide waveguide is then adiabatically tapered (length ∼ 500 μm) down to a 400/500 nm single-mode waveguide. To reduce the taper length, focusing type grating was introduced and widely used [2]. One of the major issues with the focusing GC is the alignment tolerance and bandwidth limitation compared to linear GC. In this paper, we propose and demonstrate an ultra-compact taper structure with coupling efficiency of ∼92% for 1550 nm and 89% for 1310 nm and broad band operation.