Search

We demonstrate a fully tunable microwave photonic bandpass filter fabricated with Si3N4/SiO2 waveguides on a chip. The filter is based on coupled resonator optical waveguide architecture comprising a chain of eight coupled ring resonators, each with an optical roundtrip length of 21 cm (free spectral range of 1.4 GHz), with an adjustable resonance frequency, and with adjustable power coupling. The passband shows a 3-dB bandwidth of 72 MHz with at least 51-dB out-of-band rejection and features simultaneously a flat-top, a shape factor of 1.9, and a group delay variation smaller than 2 ns.

We demonstrate a polarization rotator-splitter (PRS) design on standard 220 nm silicon-on-insulator (SOI) wafers with all rib waveguides and 2 μm silicon dioxide (SiO2) claddings. The design is fully compatible with the imec iSiPP50G silicon photonics platform. We show TM0-TE0 converting loss < 0.5 dB and all polarization crosstalk < -10 dB in the wavelength range of 1500- 1600 nm.

LIDAR on a silicon chip holds strong potentials for LIDAR system solutions featuring low cost, small size, and high robustness. In line with this effort, on-chip circulators are of great interest as they bring significant benefit for system complexity reduction and SNR improvement by enabling the LIDAR transmitter and receiver to share a single common aperture. Here, we present our recent study on passive silicon photonics nonlinear switches as conditional circulators for LIDAR applications. We propose a device implementation to address the nonlinear switch working principle by controlling waveguide nonlinear coefficient using sub-wavelength gratings. This implementation is foundry-compatible using only regular passive silicon waveguide components and are fully demonstrated in the experiment. In addition, we propose a sub-splitting coupler-based switch potentially can achieve a better fabrication tolerance than sub-wavelength grating-based switch. This work builds up signal processing functions in silicon photonics technology for optical communication and sensing applications. In particular, for LIDAR applications, this work contributes to the critical components of important use, and the easy integration with other existing functions such as optical phased arrays and spectral filters pronounces the potential for LIDAR on a silicon chip.

We propose a bonding interface fabrication tolerant silicon nitride capped thin film lithium niobate on silicon modulator that operates with RF bandwidths >200 GHz in C- and O-bands and VπL 4V-cm and 3V-cm respectively.

Ever-increasing complexity of communication systems demands the co-integration of electronics and photonics. But there are still some challenges associated with the integration of thin film lithium niobate (TFLN) electro-optic modulators with the standard and well-established silicon photonics. Current TFLN platforms are mostly not compatible with the silicon photonics foundry process due to the choice of substrate or complicated fabrication requirements, including silicon substrate removal and formation of radio-frequency (RF) electrodes on the top of the TFLN. Here, we report on a platform where all the optical and RF waveguiding structures are fabricated first, and then the TFLN is bonded on top of the silicon photonic chip as the only additional step. Hence, the need for substrate removal is eliminated, and except for the last step of TFLN bonding, its fabrication process is silicon foundry compatible and much more straightforward compared to other fabrication methods.

Both long haul and short distance network interconnects for conventional data networks and intra-/interchip data links continue to scale in complexity and bandwidth. This has signalled the emergence of the optical interconnect, with silicon photonics being the leading candidate due to its unique combination of low fabrication costs and performance enhancements. Electronic-photonic integration compatibility with CMOS technology, makes integrated photonic circuits more appealing. Hence, it is very critical to make the fabrication of integrated photonic devices as foundry compatible as possible to fully utilize CMOS foundry capabilities. We have proposed a fabrication method for thin film lithium niobate (LN)-based electro-optic modulators which has excellent compatibility with silicon photonic foundry processes and minimizes the back end of the line processes. This paves the way for large scale production of the hybrid Silicon-lithium niobate devices.

All-optical silicon-photonics-based LiDAR systems allow for desirable features in scanning resolution and speed, as well as leverage other advantages such as size, weight, and cost. Implementing optical circulators in silicon photonics enables bidirectional use of the light path for both transmitters and receivers, which simplifies the system configuration and thereby promises low system cost. In this work, to the best of our knowledge, we present the first experimental verification of all-passive silicon photonics conditional circulators for monostatic LiDAR systems using a nonlinear switch. The proposed silicon nonlinear interferometer is realized by controlling signal power distribution with power-splitting circuits, allowing the LiDAR transmitter and receiver to share the same optical path. Unlike the traditional concept requiring a permanent magnet, the present device is implemented by using common silicon photonic waveguides and a standard foundry-compatible fabrication process. With several additional phase shifters, the demonstrated device exhibits considerable flexibility using a single chip, which can be more attractive for integration with photodetector arrays in LiDAR systems.

We present a 10-GHz granularity reconfigurable WDM superchannel filter in a photonic integrated circuit. The filter uses a ring-resonator-assisted tapped-delay-line topology. An example configuration with two ring resonators and four tapped delay lines provides a spectral filter with a flat-top passband, a roll-off factor of 8%, a tunable central wavelength, and a selectable free spectral range, i.e., 2× or 4× the 3-dB passband bandwidth. The device was fabricated in a silicon nitride (Si3N4/SiO2) waveguide with a chip area of 0.36 cm2 and has a fiber-to-fiber insertion loss of 5 dB. We experimentally demonstrate the filter for selecting a subchannel from a WDM superchannel comprising 10-GHz subchannels with 10% and zero guard bands. Owing to the infinite impulse responses of ring resonators, this filter requires a significantly smaller area and fewer tuning elements compared with designs based on only tapped-delay-line finite-impulse-response filters. This paper contributes to the creation of chip-scale all-optical WDM superchannel transceivers and reconfigurable optical add/drop multiplexers (ROADMs).

An overview of the most recent developments and improvements to the low-loss TriPleX Si3N4 waveguide technology is presented in this paper. The TriPleX platform provides a suite of waveguide geometries (box, double stripe, symmetric single stripe, and asymmetric double stripe) that can be combined to design complex functional circuits, but more important are manufactured in a single monolithic process flow to create a compact photonic integrated circuit. All functionalities of the integrated circuit are constructed using standard basic building blocks, namely straight and bent waveguides, splitters/combiners and couplers, spot size converters, and phase tuning elements. The basic functionalities that have been realized are: ring resonators and Mach–Zehnder interferometer filters, tunable delay elements, and waveguide switches. Combination of these basic functionalities evolves into more complex functions such as higher order filters, beamforming networks, and fully programmable architectures. Introduction of the active InP chip platform in a combination with the TriPleX will introduce light generation, modulation, and detection to the low-loss platform. This hybrid integration strategy enables fabrication of tunable lasers, fully integrated filters, and optical beamforming networks.

Increasing the power and spectral efficiency in intensity-modulated direct-detection short-haul fiber-optic links enables higher data rates in power- and bandwidth-limited optical communication systems. Augmented spectral efficiency discrete multi-tone (ASE-DMT) can improve the spectral efficiency of pulse-amplitude-modulated DMT while maintaining its power advantage over dc-biased DMT, whose transmitter requires only one inverse fast Fourier transform (IFFT) with Hermitian symmetric inputs. Although the ASE-DMT transmitter requires multiple IFFTs, we show how these can be mapped onto a single IFFT, by using both the real and imaginary outputs of the IFFT and by extracting some signals from within the IFFT's structure. Using only one IFFT, we first demonstrate a real-time PAM4-encoded optical ASE-DMT transmitter with a net data rate of 18.4 Gb/s. When implemented in a FPGA, using a single IFFT saves 30% of logic resources, compared with a four-IFFT ASE-DMT transmitter. Finally, a 1550-nm directly modulated laser is used to evaluate its optical transmission performance with offline signal processing in the receiver. Without using any optical amplifiers, the ASE-DMT signal can be successfully transmitted over 10-km standard single-mode fiber (SSMF), but fails over 20-km SSMF due to the influence of fiber dispersion and laser chirp.

The thirty-two papers in this special issue focus on the field of programmable photonics, one of the fastest growing fields in photonics with applications ranging from optical signal processing and computing to quantum photonics.

Asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) is theoretically more optically power efficient than dc-biased OFDM (DCO-OFDM); unfortunately, its spectral efficiency is halved to accommodate the clipping distortion. Recently, layered/enhanced ACO-OFDM has been developed to mostly regain the lost spectral efficiency, and theoretically compared with other modulation formats. In this letter, we experimentally demonstrate L/E-ACO-OFDM for the first time over a fiber link. We transmit over a 19.8-km single mode fiber at 4.375 Gb/s, and show a 2-dB improvement in signal quality for the same laser bias current and received optical power over DCO-OFDM.

We investigate the performance of a chip-enhanced wavelength selective switch (EWSS) in multipass add/drop experiments. The demonstrated EWSS device uses a ring-assisted Mach–Zehnder interferometer chip as a wavelength interleaver, to preprocess the incoming super channel before launch into a commercial wavelength selective switch. We show that a 4% guardband, quadrature-phase-shift-keying encoded Nyquist wavelength division multiplexing super channel can successfully pass through seven EWSS nodes. We further investigate the number of reachable nodes with varied guardbands, showing that a modest increase in guardband can translate to a significant increase in the number of reachable EWSS nodes.

Nyquist wavelength-division-multiplexed coherent optical superchannel systems closely pack a set of rectangular-shaped channels to realize high-spectral-efficiency transmission. In long-haul fiber links, dividing each of these channels into multiple subbands, each a few-gigahertz wide, can increase tolerance to fiber nonlinearity. However, these low-bandwidth subbands will be sensitive to laser drift, which could cause them to overlap, creating interchannel interference (ICI). In this paper, we propose the application of pairwise precoding across the middle and edge subbands to improve their ICI tolerance. The proposed scheme is verified in superchannel experiments, and shows about 1.5-dB Q 2 improvement after 3840-km transmission for QPSK modulation and 1-dB Q 2 gain with 16-QAM modulation after 800-km transmission.

We present a Nyquist-filtering optical (de)multiplexer using a ring resonator-assisted interferometer circuit. It features a near-rectangular passband spectrum, scalable port count, and can be designed with sub-GHz spectral resolution. The circuit can be constructed using ordinary passive photonic-integrated circuit building blocks. In contrast to its counterparts using tapped-delay-line circuit topologies, this circuit has a chip area two-orders-of-magnitude smaller. The results of this study show the potential for realizing high-spectral-efficiency multicarrier transceivers and reconfigurable optical add-drop multiplexers in a fully integrated form.

Radio frequency (RF) switches are essential for implementing routing of RF signals. However, the increasing demand for RF signal frequency and bandwidth is posing a challenge of switching speed to the conventional solutions, i.e., the capability of operating at a sub-nanosecond speed or faster. In addition, signal frequency reconfigurability is also a desirable feature to facilitate new innovations of flexible system functions. Utilizing microwave photonics as an alternative path, we present here a photonic implementation of an RF switch providing not only the capability of switching at a sub-nanosecond speed but also options of frequency doubling of the input RF signals, allowing for flexible output waveforms. The core device is a traveling-wave silicon modulator with a device size of 0.2 mm × 1.8 mm and a modulation bandwidth of 10 GHz. Using microwave frequencies, i.e., 15 GHz and 20 GHz, as two simultaneous RF input signals, we experimentally demonstrated their amplitude and frequency switching as well as that of the doubled frequencies, i.e., 30 GHz and 40 GHz, at a switching frequency of 5 GHz. The results of this work point to a solution for creating high-speed RF switches with high compactness and flexibility.

Electro-Photonics combines the best of both electronics and optics to tackle the challenges of next generation optical communication networks in terms of capacity, flexibility, and energy efficiency. The optimal sharing of the processing work load between the electrical and optical domain brings considerable benefits to communication system complexity, performance, and construction as well as operational cost. As a key technology, photonic integrated circuits (PICs) play a critical role for the implementation of signal processing in the optical domain, serving for signal generation, switching, and acquisition. PICs point to advancing of network devices such as transmitters, receivers, and switches with desirable features of large bandwidth, high stability, precise control, easy tuning, and potential for low-cost fabrication by enabling miniaturization of complex optical systems on a chip scale. In this paper, we review the PIC research progress of the Monash Electro-Photonics Laboratory and its major contributions to the fields of optical communications and microwave photonics. We focus on applications in optical orthogonal frequency division multiplexing (O-OFDM), Nyquist wavelength division multiplexing (N-WDM), optical pulse manipulation, and programmable optical processors. The results of these works underline not only hardware fabrication capability and performance improvement for electrophotonics, but, more importantly, a new engineering paradigm that stimulates innovations in information and communication technologies.

We demonstrate the all-optical generation of a 3.8-THz wide superchannel, using a photonic-chip-based filter for sub-channel definition. The photonic chip is able to shape and aggregate 304× NRZ-32-QAM sub-channels, carrying 10-Gbd data, with an effective data-rate of 24.79 Tb/s.

The computational load for ASE-DMT can be reduced to that of DCO-OFDM by mapping of the inverse fast Fourier transform's (IFFTs) inputs and extraction of signals from within the IFFT. A realtime transmitter enables 9.2 Gbit/s over 20-km SSMF.

Layered/enhanced ACO-OFDM is a promising candidate for intensity modulation and direct-detection based short-haul fiber-optic links due to its both power and spectral efficiency. In this paper, we firstly demonstrate a hardware-efficient real-time 9.375 Gb/s QPSK-encoded layered/enhanced asymmetrical clipped optical OFDM (L/E-ACO-OFDM) transmitter using a Virtex-6 FPGA. This L/E-ACO-OFDM signal is successfully transmitted over 20-km uncompensated standard single-mode fiber (S-SMF) using a directly modulated laser. Several methods are explored to reduce the FPGA’s logic resource utilization by taking advantage of the L/E-ACO-OFDM’s signal characteristics. We show that the logic resource occupation of L/E-ACO-OFDM transmitter is almost the same as that of DC-biased OFDM transmitter when they achieve the same spectral efficiency, proving its great potential to be used in a real-time short-haul optical transmission link.

Programmable optical chips are a key technology for integrated microwave photonics. They enable flexible and stable signal processing operations on miniaturized chips with ultimate control precision and potential for low-cost fabrication. Integrated microwave photonic filters are an important function for photonic-assisted RF front-ends which opens a path to overcome the bandwidth limitation of the current digital electronics. Programmable optical chips enable such filters with high function flexibility, continuous tunability, and sharp frequency selectivity, which facilitate innovations of a wide range of new applications. Here, we review the recent works in this area that pave the path towards realizing microwave photonic filters with DSP-level flexibility, MHz-level frequency selectivity, and very importantly, full function integration on a chip scale.

Active photonic circuits use combinations of active semiconductor devices and passive elements to support many applications. We present four functions on a single active photonic integrated circuit (4.5 mm × 4 mm).

Optical delay lines implemented in photonic integrated circuits (PICs) are essential for creating robust and low-cost optical signal processors on miniaturized chips. In particular, tunable delay lines enable a key feature of programmability for the on-chip processing functions. However, the previously investigated tunable delay lines are plagued by a severe drawback of delay-dependent loss due to the propagation loss in the constituent waveguides. In principle, a serial-connected amplifier can be used to compensate such losses or perform additional amplitude manipulation. However, this solution is generally unpractical as it introduces additional burden on chip area and power consumption, particularly for large-scale integrated PICs. Here, we report an integrated tunable delay line that overcomes the delay-dependent loss, and simultaneously allows for independent manipulation of group delay and amplitude responses. It uses a ring resonator with a tunable coupler and a semiconductor optical amplifier in the feedback path. A proof-of-concept device with a free spectral range of 11.5 GHz and a delay bandwidth in the order of 200 MHz is discussed in the context of microwave photonics and is experimentally demonstrated to be able to provide a lossless delay up to 1.1 to a 5 ns Gaussian pulse. The proposed device can be designed for different frequency scales with potential for applications across many other areas such as telecommunications, LIDAR, and spectroscopy, serving as a novel building block for creating chip-scale programmable optical signal processors.

This letter presents an alternative approach to achieve a high-bandwidth RF splitter functionality based on a microwave photonic system. The proposed approach has an arbitrary amplitude and a phase offset between the two outputs. In addition, it features a simple system architecture and easy tuning mechanism. In the experimental demonstration, an instantaneous RF band from 5 to 20 GHz was shown.

A novel, hardware-compressive architecture for broadband and continuously tunable integrated optical truetime- delay beamformers for phased array antennas is proposed and experimentally demonstrated. The novel idea consists in employing the frequency-periodic response of optical ring resonator (ORR) filters in conjunction with on-chip wavelength division multiplexing (WDM), in order to create multiple signal paths on an individual beamformer channel. This novel idea dramatically reduces the network complexity and, in turn, its footprint on the wafer. This allows the integration of an unprecedented number of delay channels on a single chip, ultimately overcoming the main limitation of integrated optical beamformers, that is, the difficulty to feed antenna arrays with many elements using a single integrated chip. A novel beamformer has been realized based on this technique, using the ultra-low-loss TriPleXTM waveguide platform with CMOScompatible fabrication equipment, and its functionality is demonstrated over an instantaneous bandwidth from 2 to 10 GHz. This result, at the best of our knowledge, represents at the same time the record instantaneous bandwidth (8 GHz) for an optical beamformer based on optical ring resonators (ORR), and the first demonstration of an integrated beamformer where the periodic response of ORRs is exploited to process signals from different antenna elements, simultaneously, using a single delay line.

In this paper, novel photonic delay lines (DLs) using Vernier/non-identical ring resonators (VRRs) are proposed and demonstrated, which are capable of simultaneous generation of multiple different delays at different wavelengths (frequencies). The simple device architectures and full reconfigurability allow the DLs to be integrated with other functional building blocks in photonic integrated circuits to realize on-chip, complex multi-λ microwave photonic signal processors with reduced system complexity. To prove the concept, DLs using VRRs in cascaded and coupled configurations have been fabricated in TriPleXTM waveguide technology, which enables a very low delay-induced loss of approximately 0.18 dB/100 ps. The fabricated DLs demonstrated simultaneous generation of four incremental delays, where a maximum incremental step of 550 ps and a corresponding top delay of 1650 ps were measured for a bandwidth up to 1 GHz. To our knowledge, this is the first report on VRRs for delay generation functionalities.

We present the first demonstration of a broadband and continuously tunable integrated optical beamforming network (IOBFN) capable of providing continuously tunable true-time-delay up to 236 ps over the entire DVB-S band (10.7–12.75 GHz), realized with a CMOS compatible process. The tunable delays are based on reconfigurable optical ring resonators in conjunction with a single optical sideband filter integrated on the same optical chip. The delays and filter responses are software programmable. Four tunable delay lines are integrated on a single chip and configured to feed a 16-element linear antenna array. The broadband beam steering capability of the proposed IOBFN is demonstrated by the squint-free antenna pattern generated from the measured RF amplitude and phase responses of the optical delay line.

We propose and demonstrate by simulations a novel Nyquist-WDM (N-WDM) superchannel transmitter based on an arrayed waveguide grating router (AWGR). This approach can generate Nyquist pulses at multiple wavelengths using a single AWGR. Results for a 3-channel 960-Gbit/s QPSK superchannel system show that a 10% guard band reduces the inter-channel interference (ICI) sufficiently. The design introduces less than 0.16-dB penalty when the waveguide loss is 2 dB/cm and 0.73-dB penalty when the standard deviation of phase error is 10°. Such Nyquist pulse shapers can be realised on a chip scale using photonic integrated circuits technology, and could be compactly integrated with other functional components to create single-chip N-WDM superchannel transmitters.

Integrated optical signal processors have many applications in photonic processing of microwave signals. They provides wideband and stable signal processing operations on miniaturized chips with ultimate control precision. Creating general-purpose processors by introducing programmability is a key to enable system function flexibility and therewith offer great potential for a wide range of applications. Here, we review the recent progress in this area with highlights of a number of key programmable chips and their promising applications.

We propose pairwise coding to enhance a superchannel's tolerance to non-idea ROADM filtering effect. Experimental results show that the number of ROADMs can be double in links with 4% guard-band superchannels.