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Multiband microwave photonic filters (MPFs) are extensively required in emerging multifunction RF systems and high-performance radar systems. In this article, we present a reconfigurable and tunable multiband MPF. The flexible amplitude and phase controls of the multiband MPF can be achieved via optical spectral slicing by way of a programmable sampling function, i.e., varying the number of samples or the duty cycle of each sample. We verify that the number of uniform passbands can be adjusted (we obtain up to six passbands) and demonstrate a frequency tuning range of up to 432 MHz for a uniform triple-passband MPF. We also show a highly chirped triple-passband MPF with respective in-band chirps up to 30.0 ns/GHz, 42.1 ns/GHz, and 22.8 ns/GHz. These latter results illustrate a potential way to implement highly chirped multiband MPFs for high bandwidth signal processing applications.

We propose and demonstrate a tunable microwave photonic (MWP) filter using a quantum dash mode-locked laser. By shaping the optical frequency comb spectrum, we can configure the MWP filter responses accordingly., 2021 IEEE Photonics Conference (IPC), October 18021, 2021, Vancouver, BC, Canada

We demonstrate broadband apodized phase-shifted waveguide Bragg gratings (PS-WBGs) in silicon-on-insulator. The PS-SWG WBG has a resonance with a bandwidth of 0.16 nm and corresponding Q-factor of ~10,000 within a stopband of~ 40 nm.

Optical frequency combs (OFCs) play a key role in a variety of applications and have been implemented primarily using mode-locked lasers, Kerr resonators, and electro-optic (EO) modulation. EO modulation-based OFC generation may not yield as many comb lines nor span as broad a bandwidth as mode-locked lasers or Kerr resonators; however, it offers a high degree of tunability in central frequency and comb spacing. Integrated solutions are of interest as they significantly reduce device footprint and enable large scale system integration. In this paper, we demonstrate flexible on-chip OFC generation using two cascaded EO Mach-Zehnder modulators in silicon photonics. We demonstrate quasi-rectangular OFCs with 9 lines and a comb spacing of up to 10 GHz with an amplitude variation (comb flatness) within 6.5 dB. The corresponding time-domain waveforms have a good fit with sinc-shaped Nyquist pulses having a full-width at half maximum duration as short as 11.4 ps.

Subwavelength grating (SWG) waveguides in silicon-on-insulator are emerging as an enabling technology for implementing compact, high-performance photonic integrated devices and circuits for signal processing and sensing applications. We provide an overview of recent work on developing wavelength selective SWG waveguide filters based on Bragg gratings, ring resonators, and contra-directional couplers, as well as optical delay lines for applications in optical communications and microwave photonics. These components increase the SWG waveguide component toolbox and can be used to realize more complex photonic integrated circuits with enhanced or new functionality.

Theoretical uncertainties associated with the measurement of quantum bits have made Quantum Key Distribution (QKD) an attractive research topic for secure communication systems. Recent research in this area has focused on using weak coherent laser pulses as the carrier instead of the traditional approach of using single photon carriers. This opens the possibility to implement QKD infrastructure that is compatible with the existing optical fiber communication network. In this work, we report a theoretical study of a new approach that utilizes the Quantum Public Key Envelope (QPKE) based on the randomized Glauber states in a classical coherent optical communication system to obtain a secure communication. We verify the efficacy of the system using numerical simulation. We also study the possible approaches for an eavesdropper to test the degree of security enhancement of the proposed system at bitrate of 56 Gb/s. Two eavesdropping techniques are proposed for security analysis. A set of security parameters are used to simulate the variety of eavesdropping schemes.

We demonstrate second harmonic generation in a temperature-controlled MgO:PPLN crystal to obtain an output at 510 nm from a high-power polarization maintaining (PM) mode-locked fiber laser at 1020 nm with 35% conversion efficiency.

Dual-frequency optoelectronic oscillators (OEOs) have potential applications in dual-band wireless networking and dual-parameter sensing systems. We propose a dual-frequency OEO incorporating a multiband microwave photonic filter (MPF). In particular, the two microwave signals are generated simultaneously in a single OEO cavity. By simply varying the parameters of optical spectral slicing and sampling (e.g., with a programmable optical filter) used to implement the MPF, we can readily achieve simultaneous tuning of the dual-frequency output, as well as alternate switching between single-frequency and dual-frequency output. The multi-passband nature of the MPF, enabled via optical spectral slicing, opens a path to multi-frequency OEO operation by scaling our scheme in the future. Such a structure provides a flexible way to generate simultaneously tunable and reconfigurable multi-frequency microwave signals.

We realize an index-variable optical true time delay line based on an array of 40 subwavelength grating waveguides in silicon-on-insulator (SOI). Using the measured time delays, we simulate and demonstrate the application for antenna beamforming.

A multiband microwave photonic filter (MPF) based on optical spectral slicing is proposed. The optical spectral slicing is implemented by a programmable sampling function, and the tunability and reconfigurability of the multiband MPF are achieved in both simulations and experiments, which shows good agreement.

We present a compact optical add-drop multiplexer in silicon-on-insulator based on combining mode selection with Bragg reflection. The add-drop channel has a 3-dB bandwidth of 2 nm, an extinction ratio of 30 dB, and an interferometric crosstalk of less than –42 dB. We characterize the inter-channel and interferometric crosstalk performance using BER measurements for 10-Gb/s NRZ data and obtain crosstalk induced power penalties less than 3.5 dB.

We demonstrate a simultaneous generation of wavelength-division-multiplexed chirped microwave waveforms using an optical spectral shaping followed by a wavelength-to-time mapping. We design two types of integrated spectral shapers based on parallel distributed Fabry–Perot cavities or an arrayed waveguide grating Sagnac interferometer incorporating linearly chirped waveguide Bragg gratings. The generated waveforms have different central RF frequencies and the opposite signs of RF chirp.

We review the recent work on developing integrated microwave photonic subsystems in silicon photonics for performing radio-frequency spectral analysis, generating microwave waveforms, and filtering microwave signals.

Wavelength-division multiplexing over optical links provides an effective solution for the bandwidth challenge of off-chip and on-chip communications. We demonstrate a compact silicon photonic four-channel optical add-drop multiplexer enabled by subwavelength-grating-based contradirectional couplers. Passbands of the device show on-chip insertion losses below 1.8 dB with wide 3 dB bandwidth of ~6.7 nm suitable for coarse wavelength-division Multiplexing in short-reach optical interconnect applications. Transmission of 10 Gb/s data stream through different channels of the multiplexer results in negligible power penalties, whereas interferometric crosstalk-induced power penalties are below 2.8 dB.

Radio frequency (RF) spectrum analysis is an essential technique for monitoring signal quality in ultrahigh-speed optical transmission, broad bandwidth photonic RF arbitrary waveform generation, and for characterization of high repetition rate optical pulse sources. Whereas the intrinsic bandwidth of electronics limits its use for RF spectrum measurement of high-speed signals, a photonic approach based on ultrafast Kerr nonlinear effects can readily measure the RF spectrum of optical signals having a bandwidth beyond 1 THz. The majority of fiber-based or integrated photonic RF spectrum analyzers reported to date can monitor only one channel at a time. We demonstrate how to harness mode-selective excitation of nonlinear optical effects to perform simultaneous multichannel RF spectrum analysis using a single integrated silicon photonic device. This approach, which can be scaled to a higher number of channels, opens up a new degree of freedom for realizing multichannel signal characterization using a single integrated photonic device.

We demonstrate a passively Q-switched, dual-band Tm 3+ :ZBLAN fiber laser operating simultaneously around 1895 and 2315 nm using bidirectional pumping at 795 nm and a graphene saturable absorber. The Q-switched pulse trains at 1895 and 2315 nm have a synchronized repetition rate from 10.8 to 25.2 kHz with pulse durations as short as 4.5 and 4.9 μs, respectively.

The papers in this special section focus on microwave photonics (MWP). MWP has experienced significant growth, fueled in part by technological advances enabled by photonic integration; the use of photonics to enhance instrumentation, sensing, and radar applications; and the use of microwave/millimeter-wave photonics for 5G applications and beyond. This special issue is intended to provide a “snapshot” of some of the latest results and key findings in the field.

We review recent work on developing integrated optical delay lines based on subwavelength grating waveguide devices in silicon photonics for microwave photonic signal processing. © 2020 The Author(s)

We use cascaded microring modulators in silicon photonics for electro-optic comb generation and achieve 7 comb lines with a spacing of 5 GHz, and 5 comb lines with a spacing of up to 10 GHz.

Self-reflection and reflective writing are often used to promote self-regulated learning amongst students (Nilson, 2013). A number of engineering programs are incorporating greater opportunities for student reflection (Turns et al., 2014); at the same time, there is a growing need for additional research on the impact of selfreflection and reflective exercises in engineering education (Clark and Dickerson, 2019). We describe the implementation and examine the impact of two types of reflective writing exercisesan exam wrapper and selfevaluation in two Electrical and Computer Engineering courses, a fundamental first year course on signals and systems and a final year technical elective course on photonics.

We demonstrate on-chip frequency comb generation using cascaded electro-optic Mach-Zehnder modulators in silicon photonics. Our on-chip frequency comb produces 9 lines with a frequency spacing of 6.67 GHz and an amplitude variation $\lt3$ dB.

We demonstrate a silicon microring modulator incorporating a compact multimode interference (MMI) as the coupling element. The resonance wavelength of the microcavity is controlled independently with two electrical signals, which can potentially be used as PAM-4 transmitters, directed logic gates, electro-optic switches, etc.

A method for measuring picosecond pulse width by using only fiber components and optical power meters is presented. We have shown that the output power splitting ratio of a non-linear fiber loop mirror can be used to extract the full-width half maximum of the optical pulse, assuming a known slowly varying envelope shape and internal phase structure. Theoretical evaluation was carried out using both self-phase and cross-phase modulation approaches, with the latter showing a twofold sensitivity increase, as expected. In the experimental validation, pulses from an actively fiber mode-locked laser at the repetition rate of 10 GHz were incrementally temporally dispersed by using SMF–28 fiber, and then successfully measured over a pulse width range of 2–10 ps, with a resolution of 0.25 ps. This range can be easily extended from 0.25 to 40 ps by selecting different physical setup parameters.

A comprehensive theoretical investigation of a dual wavelength, cascade cavity bismuth-doped fiber (BDF) laser operating in the wavelength range of 1.7μm is presented. The fiber laser model is based on parameters extracted from experimental characterization of the BDF. The BDF serves as an active medium with optical gain in the wavelength region from 1.65μm to 1.8μm. The laser cavity is defined by a 90% mirror on one end of the BDF and two fiber Bragg gratings (FBG1 and FBG2) separated with the BDF on the other end of the laser cavity. One of the gratings FBG1 with the peak reflectivity 95% is centered at 1.725μm. The second one FBG2 with peak reflectivity of 90% is centered at 1.729μm. Both FBGs have a 3-dB bandwidth of ~0.5nm. It is shown that the cascade laser can operate at two 1.725μm and 1.729μm wavelengths with different powers depending on the parameters of the structure.

Optical frequency combs have emerged as an important tool enabling diverse applications from test-and-measurement, including spectroscopy, metrology, precision distance measurement, sensing, as well as optical and microwave waveform synthesis, signal processing, and communications. Several techniques exist to generate optical frequency combs, such as mode-locked lasers, Kerr micro-resonators, and electro-optic modulation. Important characteristics of optical frequency combs include the number of comb lines, their spacing, spectral shape and/or flatness, and intensity noise. While mode-locked lasers and Kerr micro-resonators can be used to obtain a large number of comb lines compared to electro-optic modulation, the latter provides increased flexibility in tuning the comb spacing. For some applications in optical communications and microwave photonics, a high degree of integration may be more desirable over a very large number of comb lines. In this paper, we review recent progress on integrated electro-optic frequency comb generators, including those based on indium phosphide, lithium niobate, and silicon photonics.

We propose and demonstrate experimentally tilted subwavelength grating (SWG) waveguide Bragg gratings (WBGs). By tilting the SWG segments and optimizing the duty cycle, we can achieve polarization-dependent tuning of the spectral response of the SWG WBG, namely, the spectral response of the fundamental transverse electric (TE) mode shifts toward shorter wavelengths, while that for the transverse magnetic (TM) mode remains almost unchanged. In particular, for tilting angles of 5° and 30°, we can obtain a blueshift in the Bragg wavelength of 7 and 35 nm for the TE mode, while the Bragg wavelength for the TM mode remains within 0.5 nm. The proposed tilted SWG WBGs provide a novel method to manage polarization and/or obtain polarization-dependent wavelength selectivity with integrated WBG devices.

[EN] An optical true time delay line (OTTDL) is a fundamental building block for signal processing applications in microwave photonics and optical communications. Here, we experimentally demonstrate an index-variable OTTDL based on an array of 40 subwavelength grating (SWG) waveguides in silicon-on-insulator. Each SWG waveguide in the array is 34 mm long and arranged in a serpentine manner; the average incremental delay between waveguides is about 4.7 ps, and the total delay between the first and last waveguides is approximately 181.9 ps. The waveguide array occupies a chip area of similar to 6.5mm x 8.7mm = 56.55mm(2). The proposedOTTDLs bring potential advantages in terms of compactness as well as operation versatility to a variety of microwave signal processing applications., Natural Sciences and Engineering Research Council of Canada; China Scholarship Council; European Research Council (Consolidator Grant Project #724663).

We provide an overview of our recent work on developing subwavelength grating (SWG) waveguide devices as an enabling technology for integrated microwave photonics. First, we describe wavelength-selective SWG waveguide filters, including ring resonators, Bragg gratings, and contradirectional couplers. Second, we discuss the development of an index variable optical true time delay line that exploits spatial diversity in an equal-length waveguide array. These SWG waveguide components are fundamental building blocks for realizing more complex structures for advanced microwave photonic signal processing.

Flexible conversion between multi-level modulation formats is one of the key processing functions to realize adaptive modulation techniques for flexible networking aimed at high spectral efficiency in optical fiber transmission. The authors have proposed an all-optical format conversion systems from binary phase-shift keying (BPSK) to quadrature PSK (QPSK) and its reverse conversion from QPSK to BPSK. The latter had an advantage of wavelength-shift-free conversion from an incident QPSK to simultaneous two BPSK outputs without loss of the transmitting data. However, it was limited only for a single polarization signal. In this paper, we propose a novel method of wavelength preserved conversion for polarization division multiplexed QPSK signal with arbitrary polarization rotation angle to the x-axis on the x-y polarization plane which is orthogonal to the propagation axis. The method is based on the orthogonal dual-pump four-wave-mixing (FWM) in the highly nonlinear fiber with a nonlinear optical loop mirror configuration, which has advantages that it separately outputs the original signal and the phase conjugate signal and has independent FWM efficiency of the signal polarization angle. We show the system performances such as bit-error-rate and optical signal-to-noise ratio penalty evaluated by numerical simulations.

We provide an overview of photonic generation of chirped microwave and millimeter wave pulses based on optical spectral shaping followed by wavelength-to-time mapping. We summarize results obtained using bulk optic/benchtop and all-fiber spectral shapers, and discuss recent developments on integrated versions in silicon photonics. In particular, we describe devices based on microring resonators and present new results obtained using integrated spectral shapers incorporating chirped Bragg gratings.

Subwavelength grating (SWG) waveguides in silicon-on-insulator are emerging as an enabling technology for implementing compact, high-performance photonic integrated devices and circuits for signal processing and sensing applications. We provide an overview of our recent work on developing wavelength selective SWG waveguide filters based on Bragg gratings and ring resonators, as well as optical delay lines. These components increase the SWG waveguide component toolbox and can be used to realize more complex photonic integrated circuits with enhanced or new functionality.

Multitude applications of photonic devices and technologies for the generation and manipulation of arbitrary and random microwave waveforms, at unprecedented processing speeds, have been proposed in the literature over the past three decades. This class of photonic applications for microwave engineering is known as microwave photonics (MWP). The vast capabilities of MWP have allowed the realization of key functionalities which are either highly complex or simply not possible in the microwave domain alone. Recently, this growing field has adopted the integrated photonics technologies to develop microwave photonic systems with enhanced robustness as well as with a significant reduction of size, cost, weight, and power consumption. In particular, silicon photonics technology is of great interest for this aim as it offers outstanding possibilities for integration of highly-complex active and passive photonic devices, permitting monolithic integration of MWP with high-speed silicon electronics. In this article, we present a review of recent work on MWP functions developed on the silicon platform. We particularly focus on newly reported designs for signal modulation, arbitrary waveform generation, filtering, true-time delay, phase shifting, beam steering, and frequency measurement.

We present a comprehensive theoretical and experimental investigation of a dual-wavelength, bismuth-doped fiber (BDF) laser operating near 1700 nm based on cascaded cavities. The BDF provides optical gain from 1650 nm to 1800 nm when pumped at 1550 nm. The linear laser cavity is defined by a 90% fiber mirror on one end and two fiber Bragg gratings (FBGs) with two lengths of BDF in cascade on the other. The laser can operate at either wavelength alone, or both wavelengths simultaneously by simple adjustment of the pump power. We develop a model based on rate equations and radiation transport equations for two level BDF lasers which we then extend to the case for cascaded cavities. Experimental results agree with simulations.

Reflective writing is a strategy used to promote self-regulated skills among students. We describe the implementation of reflective writing assignments in two compulsory undergraduate non-technical engineering courses on the engineering profession and professional practice. We discuss our observations on the content of the student papers and the challenges associated with the implementation of these reflective writing assignments

In this paper, we demonstrate a microwave photonic (MWP) approach for interrogating multiple linearly chirped fiber Bragg grating (LCFBG) temperature sensors. The MWP interrogation system is based on an arrayed waveguide grating Sagnac interferometer, incorporating multiple LCFBG sensors, that is used to generate multiple chirped microwave waveforms whose characteristics are uniquely related to those of the LCFBGs. Compressing the generated microwave signals allows monitoring simultaneously and independently the shifts associated with applying temperature changes to each LCFBG in real time.

We introduce and experimentally demonstrate a reconfigurable microwave photonic filter based on temporal Talbot effects. An optical pulse source is employed to sample the microwave signal through intensity modulation. The sampled signal is then propagated through the Talbot-based microwave photonic filter, involving temporal phase modulation and chromatic dispersion. The microwave photonic filter exhibits a periodic transfer function whose passband frequency and frequency periodicity (free spectral range) can be programmed electrically by adjusting the phase-modulation profile, e.g., using an arbitrary waveform generator, without the need for manual adjustment of the optical components and with the potential for fast tuning of the filter’s response. The bandwidth of the filter passband can also be easily customized by adjusting the sampling pulse width using an optical bandpass filter.

We describe on-chip devices in silicon photonics for optical and microwave signal processing. We describe a mode selective nonlinear device, an integrated nonlinear optical loop mirror, a programmable comb filter, and a cascaded intensity modulator.

We demonstrate simultaneous interrogation of multiple fiber Bragg grating temperature sensors based on chirped microwave pulse generation using an arrayed waveguide grating Sagnac interferometer. Results show that the system provides a resolution of 16.3 ps/°C with an error of +/− 1 °C.

We demonstrate a thermally tunable optical filter based on periodic heating of a uniform Bragg grating structure on silicon-on-insulator. Five metal heater blocks are used to heat the grating to introduce multiple phase shifts and realize tunability of the reflection spectra.

We demonstrate a compact silicon photonic 4-channel optical add-drop wavelength multiplexer enabled by subwavelength-grating-based contra-directional couplers. Pass-bands of the device show on-chip insertion losses below 1.3 dB with wide 3 dB bandwidth of ∼ 6.7 nm suitable for coarse wavelength division multiplexing (CWDM) in short-reach optical interconnect applications.

We review recent progress on developing passive subwavelength grating (SWG) waveguide devices in silicon photonics (SiP), including optical filters based on Bragg structures, ring resonators, and contra-directional couplers. We also discuss how to use SWG structures to implement an index variable optical true time delay line based on spatial diversity with an equal length waveguide array. These components expand significantly the SiP SWG waveguide ‘toolbox’ thereby enabling the design and implementation of more complex devices for advanced signal processing and filtering.

We demonstrate an integrated spectral shaper based on a Sagnac loop incorporating a chirped Bragg grating in silicon photonics for the photonic generation of chirped microwave pulses. The technique is based on optical spectral shaping combined with linear frequency-to-time mapping. By tuning the central wavelength of the input optical pulse, we obtain chirped microwave pulses with central frequencies ranging from $\sim 10$ GHz to $\sim 30$ GHz and an RF chirp rate of $\sim 20$ GHz/ns with both positive and negative signs.

We demonstrate alternate wavelength switching in a widely tunable, dual-wavelength Tm $^{3+} $ -doped fiber laser operating at 1900 nm. A comb filter based on a Sagnac loop incorporating a length of high-birefringence fiber is used to define the lasing wavelengths. By simple variation of a polarization controller within the laser cavity, alternate and sequential switching between single- and dual-wavelength output, as well as tunable operation over a range of 70 nm, is obtained.

We demonstrate 1-to-6 wavelength multicasting of a 22-GBaud 16-QAM single polarization ( $6\times 88$ Gb/s) signal based on four-wave mixing in a dispersion engineered silicon nanowire. All six multicast signals perform below the bit error rate forward error correction threshold of $3.8\times 10^{-3}$ , with a worst case power penalty of 8 dB. We compare our wavelength multicasting performance against the theoretical power transfer during a three-wave mixing process and show that the average power of degenerate idlers agrees with the predicted 6 dB difference with respect to that of nondegenerate idlers. When distinguishing the nonlinear conversion efficiency from the absolute idlers power at the output of the silicon nanowire, we also show the importance of the output grating coupler’s loss and how it impacts the multicasting performance, in conjunction with the four-wave mixing efficiency.

We demonstrate photonic generation of chirped microwave pulses (CMPs) using an integrated distributed Fabry–Perot cavity (DFPC) in silicon-on-insulator as a spectral shaper. The DPFC is based on a pair of spatially offset chirped Bragg gratings and has a response with varying free spectral range. The spectral shaper is used to filter broadband pulses from a mode-locked fiber laser, and CMPs are obtained at the output of a photodetector following a wavelength-to-time mapping process in a dispersive medium. We generate CMPs with a central frequency of up to 20 GHz, a radio-frequency (RF) chirp of 0.012 GHz/ps, and a time–bandwidth product of 14.5.

We demonstrate an integrated multi-channel spectral shaper in silicon photonics for generating two chirped microwave waveforms simultaneously with a central frequency of ~ 45 GHz and RF chirps of −273.3 GHz/ns and +233.1 GHz/ns and time bandwidth products of 17.8 and 26.9.