Data augmentation using a generative adversarial network for a high-precision instantaneous microwave frequency measurement system.

In this Letter, an unsupervised-learning platform-generative adversarial network (GAN)-is proposed for experimental data augmentation in a deep-learning assisted photonic-based instantaneous microwave frequency measurement (IFM) system. Only 75 sets of experimental data are required and the GAN can augment the small amount of data into 5000 sets of data for training the deep learning model. Furthermore, frequency measurement error of the estimated frequency has improved by an order of magnitude from 50 MHz to 5 MHz. The proposed use of GAN effectively reduces the amount of experimental data needed by 98.75% and reduces measurement error by 10 times.

Bio-inspired photonics - marine hatchetfish camouflage strategies for RF steganography.

Camouflage is a strategy that animals utilize for concealment in their habitat, making themselves invisible to their predators and preys. In RF systems, steganography or stealth transmission is the camouflage of information - a technology of hiding and transmitting secret messages in public media. Steganography conceals the secret message in publicly available media such that the eavesdropper or attacker will not be able to tell if there is a secret message to look for. Marine hatchetfish have two effective camouflage skills to help them hide from their predators - silvering and counterillumination. Silvering in marine hatchetfish uses its microstructured skin on its sides to achieve destructive interference at colors that could indicate the presence of the fish, while they also emit light at their bottom part to match its color and intensity to its surrounding, making it invisible from below, referred to as counterillumination. In this work, we borrow the two underwater camouflage strategies from marine hatchetfish, mimic them with photonic phenomena, and apply the camouflage strategies for physical stealth transmission of a 200 MBaud/s 16QAM OFDM secret signal at 5 GHz over a 25-km of optical fiber. The proposed bio-inspired steganography strategies successfully hid the secret signal in plain sight in temporal, RF spectral, and optical spectral domains, by blending in using counterillumination and turning invisible using silvering techniques. The stealth signal can only be retrieved with the precise and correct parameter for constructive interference at the secret signal frequency to unmask the silvering.

Hybrid wideband multipath self-interference cancellation with an LMS pre-adaptive filter for in-band full-duplex OFDM signal transmission.

A wideband multipath self-interference cancellation (SIC) system employing both dual-drive Mach-Zehnder modulator-based analog SIC and least mean square (LMS) algorithm-based pre-adaptive filter digital SIC is proposed and demonstrated for the cancellation of multipath self-interference (SI) and facilitation of in-band full-duplex (IBFD) orthogonal frequency-division multiplexing (OFDM) signal transmission. The multipath effect is an unavoidable challenge in SIC due to the dynamic and unpredictable properties in each path, as well as the need for separate matching components for compensating for each path. In this Letter, an LMS algorithm-based adaptive filter is used as a pre-equalizer to adapt and generate the matching signal to the closest approximate of the multipath SI signal. The adaptation is based on the minimization of the error signal generated from the matching signal and multipath SI signal in the LMS algorithm. With the introduction of the LMS adaptive filter to the analog SIC, an additional 9 dB cancellation improvement is obtained, resulting in a total of 32 dB cancellation depth over a cancellation bandwidth of 2.7 GHz at a center frequency of 1.65 GHz. To the best of our knowledge, the achieved performance is by far the widest cancellation bandwidth in a multipath SIC system, which is essential in a large bandwidth and high data rate transmission system. With the help of the proposed LMS adaptive filter digital SIC assisted analog SIC located at the remote node, power-efficient IBFD transmission of an OFDM signal through a 25 km fiber is experimentally demonstrated with a 6 dB bit error rate and 8% error vector magnitude improvements.

Twining plant inspired pneumatic soft robotic spiral gripper with a fiber optic twisting sensor.

The field of soft robotics has been significantly advanced with the recent developments of pneumatic techniques, soft materials, and high-precision motion control. While comprehensive motions can be achieved by sophisticated soft robots, multiple coordinated pneumatic controls are usually required to achieve even the simplest motions. Furthermore, most soft robotics are lacking the ability to sense the environment and provide feedback to the pneumatic control system. In this work, we design a twining plant inspired soft-robotic spiral gripper that requires only one single pneumatic control to perform the twining motion and to firmly hold onto a target object. The soft-robotic spiral gripper has an embedded high-birefringence fiber optic twisting sensor to provide critical information, including twining angle, presence of external perturbation, and physical parameter of the target object. Furthermore, finite element analyses (FEA) in parametric studies of the spiral gripper are performed for module design optimization. The unique single pneumatic channel design enables easy manipulation of the soft spiral gripper with a maximum of 540° twining angle and allows a firm grip of a target object as small as 1-mm in diameter. The embedded fiber optic sensor provides useful information of the target object as well as the twining angle of the soft robotic spiral gripper with high twining angle sensitivity of 0.03nm. The unique fiber-optic sensor embedded single-channel pneumatic spiral gripper that is made from non-toxic silicone rubber allows parallel and soft gripping of elongated objects located in a confined area, which is an essential building block for twining and twisting motions in soft robot.

Adaptive photonic RF spectral shaper.

The radio frequency spectral shaper is an essential component in emerging multi-service mobile communications, multiband satellite and radar systems, and future 5G/6G radio frequency systems for equalizing spectral unevenness, removing out-of-band noise and interference, and manipulating multi-band signal simultaneously. While it is easy to achieve simple spectral functions using either conventional microwave photonic filters or the optical spectrum to microwave spectra mapping techniques, it is challenging to enable complex spectral shaping functions over tens of GHz bandwidth as well as to achieve point-by-point shaping capability to fulfill the needs in dynamic wireless communications. In this paper, we proposed and demonstrated a novel spectral shaping system, which utilizes a two-section algorithm to automatically decompose the target RF response into a series of Gaussian functions and to reconstruct the desired RF response by microwave photonic techniques. The devised spectral shaping system is capable of manipulating the spectral function in various bands (S, C, and X) simultaneously with step resolution of as fine as tens of MHz. The resolution limitation in optical spectral processing is mitigated using the discrete convolution technique. Over 10 dynamic and independently adjustable spectral control points are experimentally achieved based on the proposed spectral shaper.

Movement Detection in Soft Robotic Gripper using Sinusoidally Embedded Fiber Optic Sensor.

Soft robotics is an emerging field, since it offers distinct opportunities in areas where conventional rigid robots are not a feasible solution. However, due to the complex motions of soft robots and the stretchable nature of soft building materials, conventional electronic and fiber optic sensors cannot be used in soft robots, thus, hindering the soft robots' ability to sense and respond to their surroundings. Fiber Bragg grating (FBG)-based sensors are very popular among various fiber optic sensors, but their stiff nature makes it challenging to be used in soft robotics. In this study, a soft robotic gripper with a sinusoidally embedded stretchable FBG-based fiber optic sensor is demonstrated. Unlike a straight FBG embedding configuration, this unique sinusoidal configuration prevents sensor dislocation, supports stretchability and improves sensitivity by seven times when compared to a straight configuration. Furthermore, the sinusoidally embedded FBG facilitates the detection of various movements and events occurring at the soft robotic gripper, such as (de)actuation, object holding and external perturbation. The combination of a soft robot and stretchable fiber optic sensor is a novel approach to enable a soft robot to sense and response to its surroundings, as well as to provide its operation status to the controller.

Adaptive over-the-air RF self-interference cancellation using a signal-of-interest driven regular triangle algorithm.

An optically-enabled radio frquency (RF) self-interference cancellation system is demonstrated for over-the-air in-band full duplex transmission, based on a signal-of-interest (SOI) driven regular triangle algorithm. Since the goal of a self-interference cancellation system is to retrieve the SOI that is masked by the in-band interference signal, using the SOI quality as the driven parameter for optimizing the self-interference cancellation performance is a natural and effective way to allow the system to adapt to changes and obtain the best cancellation performance. Since regular triangle algorithm has short iteration time, bursts of pseudo-random binary sequence would be used between real data transmission for optimizing the self-interference cancellation performance. The adaptive regular triangle algorithm optimizes the cancellation setting such that the in-band interference can be cancelled to a minimum, i.e., down to the noise floor. During the over-the-air experiment, 22 dB of cancellation depth is obtained over a 300 MHz bandwidth at 18.35 GHz without the need of digital self-interference cancellation.

Microwave photonic multiband filter with independently tunable passband spectral properties.

Multiband RF filters with independently controllable passbands are an essential component in dynamic multiband RF communications. Unfortunately, even a fixed multiband RF filter without the capability to adjust the passband properties individually is very difficult to achieve using either RF electronics or microwave photonic technologies. In microwave photonic approaches, the critical limitation is the close relationship between passbands-the tuning of one passband leads to a change in another, hindering the ability to independently control each passband. In this Letter, a programmable microwave photonic multiband filter with full control of amplitude, frequency, bandwidth, group delay slope, and the spectral shape of each passband has been experimentally demonstrated. A multiband filter design algorithm has also been developed that considers each RF passband as an individual, then uses inverse Fourier transform and filter design rule to determine the corresponding optical parameters and combines a series of shaped cosine functions to achieve the desired RF properties.

Stretchable fiber-Bragg-grating-based sensor.

A fiber-Bragg-grating-(FBG)-based sensor is a very popular fiber optic sensor due to its simplicity, mature fabrication technology, and sensitivity to a number of physical stimuli. However, due to the stiff nature of optical fiber, it is impossible to heavily stretch a FBG sensor. A stretchable sensor is highly desired in soft robotics, human motion detection, and biomedical applications, which could involve motions like tension, bending, and twisting. In this Letter, we demonstrate a stretchable FBG-based fiber optic sensor by embedding a FBG-written optic fiber sinusoidally in a soft silicone film at an off-center position. This unique structure enables 30% of elongation in length that facilitates tension, bending, and twisting measurement.

Biomimetic photonics: jamming avoidance system in Eigenmannia.

Biomimetic photonics extract the good design of nature and mimic it with photonics. The weakly electric fish genus, Eigenmannia, has a unique neural algorithm - jamming avoidance response, to facilitate their survival in the deep dark ocean, by automatically adjusts the local transmitter carrier frequency to move away from the jamming frequency when it is within the jamming spectral range. Examining our own wireless microwave systems, the situation of inadvertent jamming is very similar as that in Eigenmannia. In this article, a biomimetic photonic approach inspired by the jamming avoidance response in a weakly electric fish genus, Eigenmannia, is naturally adopted to experimentally tackle signal jamming in wireless systems. Mimicking the system with photonics enables the proposed scheme to work for frequencies from hundreds of MHz to tens of GHz.

Dual-layer orthogonal fiber Bragg grating mesh based soft sensor for 3-dimensional shape sensing.

A soft shape sensor for 3-dimensional object shape measurement is demonstrated. The proposed sensor is based on dual-layer fiber Bragg grating arrays with an orthogonal mesh structure, which enables multi-point bi-directional shape sensing. The 3D shape reconstruction is based on a bi-directional curvature measurement at each sensing point, which is achieved through measuring the direction and amount of wavelength shift of each off-center embedded FBG. The conversion coefficient between the wavelength shift and bending curvatures is acquired and is used to convert the change in FBG to the corresponding bending curvatures and bending direction. The measurement error on the bending radius of each sensing point is about 2.7%. A 3D shape of the object surface is reconstructed with the help of a curve fitting method based on the curvature information across the whole FBG mesh. This design successfully achieved visualized 3D shape sensing, which has great practical value in soft robotics and biomedical applications.

A Two-Phase Coverage-Enhancing Algorithm for Hybrid Wireless Sensor Networks.

Providing field coverage is a key task in many sensor network applications. In certain scenarios, the sensor field may have coverage holes due to random initial deployment of sensors; thus, the desired level of coverage cannot be achieved. A hybrid wireless sensor network is a cost-effective solution to this problem, which is achieved by repositioning a portion of the mobile sensors in the network to meet the network coverage requirement. This paper investigates how to redeploy mobile sensor nodes to improve network coverage in hybrid wireless sensor networks. We propose a two-phase coverage-enhancing algorithm for hybrid wireless sensor networks. In phase one, we use a differential evolution algorithm to compute the candidate's target positions in the mobile sensor nodes that could potentially improve coverage. In the second phase, we use an optimization scheme on the candidate's target positions calculated from phase one to reduce the accumulated potential moving distance of mobile sensors, such that the exact mobile sensor nodes that need to be moved as well as their final target positions can be determined. Experimental results show that the proposed algorithm provided significant improvement in terms of area coverage rate, average moving distance, area coverage-distance rate and the number of moved mobile sensors, when compare with other approaches.

Gigabit Ethernet signal transmission using asynchronous optical code division multiple access.

We propose and experimentally demonstrate a novel architecture for interfacing and transmitting a Gigabit Ethernet (GbE) signal using asynchronous incoherent optical code division multiple access (OCDMA). This is the first such asynchronous incoherent OCDMA system carrying GbE data being demonstrated to be working among multi-users where each user is operating with an independent clock/data rate and is granted random access to the network. Three major components, the GbE interface, the OCDMA transmitter, and the OCDMA receiver are discussed in detail. The performance of the system is studied and characterized through measuring eye diagrams, bit-error rate and packet loss rate in real-time file transfer. Our Letter also addresses the near-far problem and realizes asynchronous transmission and detection of signal.

Passband switchable microwave photonic multiband filter.

A reconfigurable microwave photonic (MWP) multiband filter with selectable and switchable passbands is proposed and experimentally demonstrated, with a maximum of 12 simultaneous passbands evenly distributed from 0 to 10 GHz. The scheme is based on the generation of tunable optical comb lines using a two-stage Lyot loop filter, such that various filter tap spacings and spectral combinations are obtained for the configuration of the MWP filter. Through polarization state adjustment inside the Lyot loop filter, an optical frequency comb with 12 different comb spacings is achieved, which corresponds to a MWP filter with 12 selectable passbands. Center frequencies of the filter passbands are switchable, while the number of simultaneous passbands is tunable from 1 to 12. Furthermore, the MWP multiband filter can either work as an all-block, single-band or multiband filter with various passband combinations, which provide exceptional operation flexibility. All the passbands have over 30 dB sidelobe suppression and 3-dB bandwidth of 200 MHz, providing good filter selectivity.

Ultra High-Speed Radio Frequency Switch Based on Photonics.

Microwave switches, or Radio Frequency (RF) switches have been intensively used in microwave systems for signal routing. Compared with the fast development of microwave and wireless systems, RF switches have been underdeveloped particularly in terms of switching speed and operating bandwidth. In this paper, we propose a photonics based RF switch that is capable of switching at tens of picoseconds speed, which is hundreds of times faster than any existing RF switch technologies. The high-speed switching property is achieved with the use of a rapidly tunable microwave photonic filter with tens of gigahertz frequency tuning speed, where the tuning mechanism is based on the ultra-fast electro-optics Pockels effect. The RF switch has a wide operation bandwidth of 12 GHz and can go up to 40 GHz, depending on the bandwidth of the modulator used in the scheme. The proposed RF switch can either work as an ON/OFF switch or a two-channel switch, tens of picoseconds switching speed is experimentally observed for both type of switches.

Bit rate transparent interferometric noise mitigation utilizing the nonlinear modulation curve of electro-absorption modulator.

we propose a bit-rate transparent interferometric noise mitigation scheme utilizing the nonlinear modulation curve of electro-absorption modulator (EAM). Both the zero-slope region and the linear modulation region of the nonlinear modulation curve are utilized to suppress interferometric noise and enlarge noise margin of degraded eye diagrams. Using amplitude suppression effect of the zero-slope region, interferometric noise at low frequency range is suppressed successfully. Under different signal to noise ratio (SNR), we measured the power penalties at bit error rate (BER) of 10<(-9) with and without EAM interferometric noise suppression. By using our proposed scheme, power penalty improvement of 8.5 dB is achieved in a signal with signal-to-noise ratio of 12.5 dB. BER results at various bit rates are analyzed, error floors for each BER curves are removed, significantly improvement in receiver sensitivity and widely opened eye diagrams are resulted.

Photonic implementation of a neuronal algorithm applicable towards angle of arrival detection and localization.

A photonic system exemplifying the neurobiological learning algorithm, spike timing dependent plasticity (STDP), is experimentally demonstrated using the cooperative effects of cross gain modulation and nonlinear polarization rotation within an SOA. Furthermore, an STDP-based photonic approach towards the measurement of the angle of arrival (AOA) of a microwave signal is developed, and a three-dimensional AOA localization scheme is explored. Measurement accuracies on the order of tens of centimeters, rivaling that of complex positioning systems that utilize a large distribution of measuring units, are achieved for larger distances and with a simpler setup using just three STDP-based AOA units.

High-speed tunable microwave photonic notch filter based on phase modulator incorporated Lyot filter.

A high-speed tunable microwave photonic notch filter with ultrahigh rejection ratio is presented, which is achieved by semiconductor optical amplifier (SOA)-based single-sideband modulation and optical spectral filtering with a phase modulator-incorporated Lyot (PM-Lyot) filter. By varying the birefringence of the phase modulator through electro-optic effect, electrically tuning of the microwave photonic notch filter is experimentally achieved at tens of gigahertz speed. The use of SOA-polarizer based single-sideband modulation scheme provides good sideband suppression over a wide frequency range, resulting in an ultrahigh rejection ratio of the microwave photonic notch filter. Stable filter spectrum with bandstop rejection ratio over 60 dB is observed over a frequency tuning range from 1.8 to 10 GHz. Compare with standard interferometric notch filter, narrower bandwidth and sharper notch profile are achieved with the unique PM-Lyot filter, resulting in better filter selectivity. Moreover, bandwidth tuning is also achieved through polarization adjustment inside the PM-Lyot filter, that the 10-dB filter bandwidth is tuned from 0.81 to 1.85 GHz.

Wideband co-site interference cancellation based on hybrid electrical and optical techniques.

A wideband co-site co-channel interference cancellation system (ICS), based on hybrid electrical and optical techniques, is proposed and is experimentally demonstrated. The demonstrated cancellation system subtracts the in-band wideband interfering signal from the received signal, such that the weak signal of interest (SOI) can be recovered. Our system utilizes a broadband radio frequency (RF) Balun transformer to invert the phase of the interfering signal, while electro-absorption modulated lasers are used for converting the RF signals into the optical domain to enable fine adjustment with the hybrid ICS. We experimentally achieve 45 dB of cancellation over a 220 MHz bandwidth, and over 57 dB of cancellation for a 10 MHz bandwidth, at a center frequency of 900 MHz. The proposed system also experimentally shows good cancellation (30 dB) over an enormously wide bandwidth of 5.5 GHz. To the best of our knowledge, this is the first demonstration of such a wide bandwidth cancellation with good cancellation depth. This property is extremely useful when there are multiple interference signals at various frequency bands. The proposed hybrid ICS has a spurious-free dynamic range of 93.2 dBm/Hz(2/3). Moreover, a 10 Gb/s SOI is recovered from a strong interfering signal, sweeping over 3 GHz bandwidth. A widely open eye diagram, as well as error-free bit-error rate measurements, is experimentally achieved, with the use of the hybrid ICS. The approach also works well for various frequency bands that are within the bandwidth of the Balun transformer and electro-absorption modulated lasers.

Rayleigh backscattering noise suppression based on real-time heterodyne receiver for loop-back WDM-PON.

In this paper, we propose a Rayleigh backscattering (RB) noise mitigation scheme based on the use of real-time heterodyne receiver for loop-back wavelength division multiplexing passive optical network (WDM-PON). Heterodyne detection has been utilized to increase the upstream receiver sensitivity, while an electro-absorption modulator (EAM) is used to simultaneously turn heterodyning bipolar signal into single polar signal and mitigate accumulated carrier RB noise. With the help of the nonlinear negative-slope transfer function of EAM, low frequency interference noise is suppressed successfully. RB noise mitigation performance is studied over 45-km single mode fiber (SMF) transmission, and the optical-signal-to-Rayleigh-noise-ratio (OSRNR) is reduced to 15.6 dB, when bias voltage of EAM is at -4 V. Through utilizing this real-time heterodyne receiver in single fiber loop-back structure, upstream error free transmission is realized with receiver sensitivity of -25 dBm.