Time-delayed optoelectronic oscillators are at the center of a large body of scientific literature. The complex behavior of these nonlinear oscillators has been thoroughly explored both theoretically and experimentally, leading to a better understanding of their dynamical properties. Beyond fundamental research, these systems have also inspired a wide and diverse set of applications, such as optical chaos communications, pseudorandom number generation, optoelectronic machine learning based on reservoir computing, ultrapure microwave generation, optical pulse-train synthesis, and sensing. The aim of this review is to provide a comprehensive survey of this field, to outline the latest achievements, and discuss the main challenges ahead.

Optoelectronic oscillators with time-delayed feedback

The Dragonfly topology provides low-diameter connectivity for high-performanceleb computing with all-to-all global links at the inter-group level. Our traffic matrix characterization of various scientific applications shows consistent mismatch between the imbalanced group-to-group traffic and the uniform global bandwidth allocation of Dragonfly. Though adaptive routing has been proposed to utilize bandwidth of non-minimal paths, increased hops and cross-group interference lower efficiency. This work presents a photonic architecture, Flexfly, which "trades" global links among groups using low-radix Silicon photonic switches. With transparent optical switching, Flexfly reconfigures the inter-group topology based on traffic pattern, stealing additional direct bandwidth for communication-intensive group pairs. Simulations with applications such as GTC, Nekbone and LULESH show up to 1.8x speedup over Dragonfly paired with UGAL routing, along with halved hop count and latency for cross-group messages. We built a 32-node Flexfly prototype using a Silicon photonic switch connecting four groups and demonstrated 820 ns interconnect reconfiguration time.

Flexfly: enabling a reconfigurable dragonfly through silicon photonics

This paper presents a low-cost bidirectional (BiDi) wavelength-division-multiplexed passive optical network (WDM-PON) employing colorless uncooled BiDi transceivers (TRxs) and superluminescent diode (SLD)-based broadband light sources (BLSs). The C band is allocated for upstream and the E+ band for downstream in consideration of BiDi packaging, SLD development, and wavelength alignment of dual-window arrayed waveguide gratings (AWGs). The BiDi TRx integrates an uncooled Fabry-Perot laser diode (FP-LD), a p-i-n photodiode (PD), and a 45°-angled thin-film filter in a small-form-factor (SFF) package. The SLD-based BLSs provide 13-dBm amplified spontaneous emissions (ASEs) with spectral ripples of < 3 dB and polarization dependencies of < 1 dB. Colorless operations over 32 100-GHz-spaced channels are demonstrated from -20 to 80 °C in 155-Mb/s BiDi transmissions over 25 km.

Low-cost WDM-PON with colorless bidirectional transceivers

A novel microwave photonic link (MPL) with an improved spurious-free dynamic range (SFDR) based on a bidirectional use of a polarization modulator (PolM) in a Sagnac loop is proposed and demonstrated. The PolM in the loop functions, in conjunction with a polarization controller and a polarization beam combiner, as a Mach Zehnder modulator (MZM), which only modulates the incident light wave along the clockwise direction, leaving the counter-clockwise light wave unmodulated due to the velocity mismatch. Two clockwise intensity-modulated signals along two paths (Path 1 and Path 2) are generated, with one (Path 2) combined with the non-modulated light wave from the counter-clockwise direction to suppress part of the optical carrier. By controlling the power relationship between the two paths, the third-order intermodulation distortion (IMD3) can be fully suppressed, and thus an MPL with improved dynamic range is achieved. A theoretical analysis is presented, which is validated by an experiment. The IMD3 can be suppressed by 50 dB, giving an improvement in SFDR of 16 dB.

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Dynamic range improvement of a microwave photonic link based on bi-directional use of a polarization modulator in a Sagnac loop.

Linearization technologies that can be used for linearizing RoF transmission are reviewed. Three main linearization methods, i.e. electrical analog linearization, optical linearization, and electrical digital linearization are presented and compared. Analog linearization can be achieved using analog predistortion circuits, and can be used for suppression of odd order nonlinear distortion components, such as third and fifth order. Optical linearization includes mixed-polarization, dual-wavelength, optical channelization and the others, implemented in optical domain, to suppress both even and odd order nonlinear distortion components, such as second and third order. Digital predistortion has been a widely used linearization method for RF power amplifiers. However, digital linearization that requires analog to digital converter is severely limited to hundreds of MHz bandwidth. Instead, analog and optical linearization provide broadband linearization with up to tens of GHz. Therefore, for broadband radio over fiber transmission that can be used for future broadband cloud radio access networks, analog and optical linearization are more appropriate than digital linearization. Generally speaking, both analog and optical linearization are able to improve spur-free dynamic range greater than 10 dB over tens of GHz. In order for current digital linearization to be used for broadband radio over fiber transmission, the reduced linearization complexity and increased linearization bandwidth are required. Moreover, some digital linearization methods in which the complexity can be reduced, such as Hammerstein type, may be more promising and require further investigation.

Linearization Technologies for Broadband Radio-Over-Fiber Transmission Systems

A high spectral efficiency coherent microwave photonic link (MPL) supporting amplitude and phase modulation incorporating a digital phase noise cancellation is proposed and experimentally demonstrated. At the transmitter, a continuous-wave light wave is amplitude- and phase-modulated by two microwave vector signals carried by a microwave carrier at an identical frequency. The modulated optical signal is polarization multiplexed with an unmodulated optical carrier and transmitted over a length of a single-mode fiber (SMF). At the receiver, the optical signal is detected coherently by a coherent receiver to which a local oscillator (LO) laser source is also applied. Through advanced digital signal processing, the microwave vector signals are recovered, and the phase noise introduced by both the transmitter laser source and LO laser source is cancelled. An experiment is performed. The transmission of a 2.5-Gb/s 16-QAM and a 1.25-Gb/s QPSK microwave vector signals both at 2.5 GHz over a 25-km SMF is implemented. The total bit rate of the MPL is 3.75 Gb/s. The transmission performance of the MPL in terms of error vector magnitudes and bit error rates is evaluated.

A High Spectral Efficiency Coherent Microwave Photonic Link Employing Both Amplitude and Phase Modulation With Digital Phase Noise Cancellation

A microwave photonic link (MPL) using a single polarization modulator (PolM) with an improved spurious free dynamic range (SFDR) is proposed and experimentally demonstrated. The proposed MPL is designed based on the destructive combination of the distortion signals in the electrical domain, which is realized using a PolM with its output split into two channels. In the upper channel, the PolM, a polarization controller (PC), and a polarization beam splitter (PBS) form an equivalent Mach-Zehnder modulator (MZM) that is biased at the quadrature point. The optical carrier is suppressed using a phase-shifted fiber Bragg grating (PS-FBG). In the lower channel, the PolM, another PC, and the PBS form a second equivalent MZM that is also biased at the quadrature point but at the opposite slope of the transfer function. The signals from the two channels are sent to a photodetector (PD). If the optical powers into the two channels are identical, the third-order intermodulation terms are completely suppressed. An experiment is performed. An enhancement in SFDR of 10 dB is achieved. The insertion loss of the proposed MPL is also estimated, which is 9.5 dB smaller than a phase modulator based MPL.

Microwave Photonic Link With Improved Dynamic Range Using a Polarization Modulator

Photonic generation of a phase-coded chirped microwave waveform with an increased time bandwidth product (TBWP) using a frequency-tunable optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. The frequency-tunable OEO is implemented using a tunable laser source (TLS), a phase modulator (PM), a phase-shifted fiber Bragg grating, and a photodetector (PD), with the frequency tuning realized by tuning the wavelength of the TLS. A frequency-tunable optical sideband with a frequency that is equal to that of the optical carrier plus the OEO oscillation frequency is generated by the OEO, which is then orthogonally polarization multiplexed with the optical carrier from the TLS at a polarization beam combiner, and applied to a polarization modulator, to which a binary phase-coded parabolic electrical signal is applied. By beating the two orthogonally polarized optical signals at a PD, a phase-coded chirped microwave waveform is generated. The TBWP is significantly increased due to the increase of the temporal duration of the microwave waveform. The proposed approach is experimentally demonstrated. Two phase-coded chirped microwave waveforms with TBWPs of 58.5 and 80 000 using two phase coding signals corresponding to a 13 Barker code and a 20480-bit pseudorandom sequence are generated.

Photonic Generation of a Phase-Coded Chirp Microwave Waveform With Increased TBWP

An intensity-modulation and coherent-detection (IM/CD) microwave photonic (MWP) link with digital signal processing (DSP)-based phase noise cancellation is proposed and experimentally demonstrated. At the transmitter, the optical carrier is intensity-modulated by a radio-frequency (RF) signal and sent to a coherent receiver over a single-mode fiber (SMF). At the receiver, the intensity-modulated optical signal is coherently detected using a local oscillator (LO) laser source. The phase noise introduced by both the transmitter laser source and the LO laser source is converted into an amplitude noise at the output of the coherent receiver, which would be added to the in-phase component and quadrature component from the coherent receiver. The noise can be completely cancelled by summing the squared magnitudes of the in-phase and quadrature components from the coherent receiver, which can be implemented using a DSP unit. Error-free transmission of a 1.25-Gb/s quadrature phase shift keying modulated RF signal with a center frequency of 1.6 GHz over 25-km SMF is experimentally demonstrated. For a signal with a bit rate of 834 Mb/s, the receiver sensitivity can reach ${-}{\rm 24.5}~{\rm dBm}$ which is 20 dB better than that based on an intensity-modulation and direct-detection MWP link.

Digital Phase Noise Cancellation for a Coherent-Detection Microwave Photonic Link

A symmetrical radio over a colorless wavelength-division multiplexing passive optical network with wavelength reuse based on polarization multiplexing and coherent detection incorporating digital phase noise cancellation is proposed and experimentally demonstrated. For the downlink, two optical signals are intensity modulated by two different 16-QAM downstream microwave vector signals that are polarization multiplexed at the optical line terminal (OLT). Then, the two orthogonally polarized optical signals are transmitted to the optical network unit (ONU). At the ONU, one of the two orthogonally polarized optical signals is selected to be reused for the uplink transmission. Then, the reused optical signal is split into two channels by a 3-dB coupler. One portion of the reused optical signal is phase modulated by a 16-QAM upstream microwave vector signal, while the other portion is intensity modulated by another 16-QAM upstream microwave vector signal. Again, the two portions of the optical signals are polarization multiplexed and transmitted back to the OLT. At the OLT, a coherent receiver with a digital signal processing (DSP) unit is used to detect the upstream optical signals. Two DSP algorithms are developed to recover the two 16-QAM upstream microwave vector signals. The transmission of two 2.5-Gb/s 16-QAM downstream microwave vector signals and two 2.5-Gb/s 16-QAM upstream microwave vector signals over a 10.5-km single-mode fiber is experimentally demonstrated. The error vector magnitudes for both the down and upstream transmissions are measured to be 7.3% and 8.65%, which are good enough to achieve error-free transmission with forward error correction.

Xiang Chen

Papers

A High Spectral Efficiency Coherent Microwave Photonic Link Employing Both Amplitude and Phase Modulation With Digital Phase Noise Cancellation

Microwave Photonic Link With Improved Dynamic Range Using a Polarization Modulator

Photonic Generation of a Phase-Coded Chirp Microwave Waveform With Increased TBWP

Digital Phase Noise Cancellation for a Coherent-Detection Microwave Photonic Link

Wavelength Reuse in a Symmetrical Radio Over WDM-PON Based on Polarization Multiplexing and Coherent Detection