We demonstrate 100 Gb/s/ $\lambda $ duo-binary PAM-4 (DB-PAM-4) transmission over 50 km standard single-mode fiber (SMF) in the O-band using a 20G silicon-based transmitter and a 25G optically-amplified PIN+TIA-based receiver for 100G PON downstream applications. Receiver sensitivities of −15, −14.5 and −12 dBm are achieved at a bit error rate (BER) below the low-density parity-check forward error correction (LDPC-FEC) threshold (i.e., $1\times 10^{-2}$ ) in back-to-back (B2B), and after 20 and 50 km, respectively using a 131-tap linear feed-forward equalizer (FFE) at the receiver. According to the results, a power budget of 20 dB can be achieved for 20 km transmission. Furthermore, we report the performance of 99 Gb/s (33 Gbaud) PAM-8 transmission and show that DB-PAM-4 provides 5.3 dB better sensitivity compared to PAM-8 at the LDPC-FEC threshold.

We report an electro-wetting liquid lens with dual apertures which consists of internal channel with two variable interfaces and external channel with a plano-concave solid lens. And we use LC cell and Polarizers to achieve switching between internal and external channel. The proposed lens can achieve a large turning range of focal length, and the shortest positive and negative focal length are ~24 mm and ~−16 mm, respectively. Meanwhile, the experimental results show that the angular resolution of the proposed lens can reach to $\sim 29^{\prime \prime }$ . It has potential to be applied in cameras, microscopes and so on.

Most of the traditional scanning feedback imaging systems cannot break through the limitations of scanning speed. In this letter, a fast scanning system with changeable scanning sequences by projecting patterns alternatively on the digital micromirror device is proposed. Combined with the laser feedback techniques, the system exhibits high sensitivity to weak signals from targets with low reflectivity. In experiments, the images of handwriting ink patterns on a piece of glass are obtained in 6 seconds, and the resolution of the system is evaluated to be $124.5~\mu \text{m}$ with a 3.24 mm by 3.24 mm field of view. This scanning feedback imaging system can be combined potentially with single pixel imaging techniques to reconstruct phase objects more efficiently.

We experimentally demonstrate an optical phase conjugation (OPC) enhanced transmission system based on the Kramers–Kronig direct detection scheme. The single-sideband signal is modulated at 80-Gb/s 16-QAM with orthogonal frequency-division multiplexing. In the 180-km transmission experiment, the OPC surpasses digital chromatic dispersion compensation in Q factor, optimum launched optical power (LOP) and optical signal-to-noise ratio (OSNR) sensitivity. The improvement in Q factor, optimum LOP and OSNR sensitivity are 0.9 dB, 1.0 dB, and 1.7 dB respectively. The superiority of OPC over pure optical dispersion compensation is also analyzed by simulation. The results indicate that the OPC is an effective means to enhance the transmission performance of single-sideband signals.

We propose a novel digital signal processing (DSP) technique that encrypts multiple baseband signals by dividing them into several spectral slices, which are then shuffled. Afterwards, the shuffled signals modulate optical carriers and are propagated through transparent optical communication systems. Simulation results reveal that the reach of shuffled signals is ~ 90% of that of unshuffled ones. We also evaluate a scenario where the shuffled signals are transmitted through multiple routes to increase security and investigate the robustness of the technique against brute force and chosen plaintext attacks.

When a disturbance acts on a fiber it induces a change in the local refractive index that influences the fiber backscattering trace. If a chirped pulse $\phi $ -OTDR setup is used to interrogate the fiber, this refractive index change appears as a local shift of the received trace, linear to the acting perturbation. However, the refractive index change influences the round trip time of all the backscattering components generated by further fiber sections as well. Due to the high sensitivity of chirped pulse $\phi $ -OTDR, the change in the round trip time of the backscattering components, which is usually negligible, may appear as a virtual perturbation in certain conditions. In this letter we derive a mathematical model for the virtual perturbation induced by a disturbance acting on the fiber, when the measurement is performed by a chirped pulse $\phi $ -OTDR. We experimentally validate the model by inducing a temperature change on a known span of fiber while monitoring its effects in a further fiber section kept at rest. The experimental results are then analyzed and compared with the theoretical ones.

The Q-switching performance of graphitic carbon nitride (g-C 3 N 4 ) based on saturated absorption is demonstrated in 2 micrometer wavelength region for the first time. The g-C 3 N 4 saturable absorbers, processed by thermal oxidation etching, are employed for passive Q-switching in a Tm:CNNGG laser. With a 5 hours long thermal oxidation etched g-C 3 N 4 saturable absorber, the Q-switched Tm:CNNGG laser generates a shortest pulse duration of 132 ns with a repetition frequency of 16 kHz and a peak power of 98.5 W. The experimental results indicate that g-C 3 N 4 is an excellent saturable absorber for passive Q-switching in 2 micrometer wavelength region.

An ultra-compact mode multiplexer for TE 0 , TE 1 , TE 2 and TE 3 modes is proposed and demonstrated experimentally. The device is realized via pixelated waveguides with a footprint of only $5.4\times 6~\mu \,\,\text{m}^{2}$ . The measured insertion loss of the multiplexer is less than 1.5 dB. The inter-mode crosstalk is ~−20 dB at 1550 nm and below −14.6 dB from 1510 nm – 1570 nm. The fabrication and temperature tolerance of the device are investigated. The device can well tolerate the nanohole diameter variation of ±10 nm and the temperature variation of 100 K. The compactness of the proposed mode multiplexer enables potential applications in densely integrated multimode photonic circuits.

A novel method enabling to produce a flat-top band-rejection filter is firstly proposed and experimentally demonstrated, which is achieved by using a phase-modulated helical long-period fiber grating (HLPG). Unlike most of the long-period fiber grating (LPG)-based and the HLPG-based band-rejection filters reported so far, the proposed band-rejection filter is inherently polarization-independent. Moreover, the proposed HLPG does not need a complex apodization in the grating’s profile, which considerably facilitates the fabrication processes and makes the designed HLPG particularly suitable for fabrication with using the CO 2 laser-writing platform. As a typical example, a polarization-independent band-rejection filter with a bandwidth of ~10 nm@-20dB and a rejection depth of ~28 dB has been successfully achieved.

The synergy of photonic and electronic signal transmission in the near-infrared spectrum is an ideal solution for optoelectronic integrated circuits in high-speed communication systems. In this study, we fabricated high-quality germanium mesa on Si by rapid-melting growth technique for a PIN photodetector. The quality of Ge was investigated through standard Raman spectroscopy and electronic microscopy. Rather than a single-crystalline Ge mesa, a polycrystalline structure differs from those of our previous reports, and may be caused by the multiple vicinities with oxide and Si. Ge/Ge/Si PIN photodetector and a Si waveguide was fabricated and measured to study the photodetector’s I-V characteristics at near-infrared spectrum. The normalized power-dependent current enhancement and photoresponsivity were investigated to reveal the effect of Ge mesa quality. This study demonstrated that a high-quality Ge mesa can be employed for optoelectronic integrated circuit with high photoelectric conversion efficiency and responsivity by a Si-based integrated circuit fabrication process.

We report a passively mode-locked fiber oscillator based on thulium-doped large-mode-area (LMA) fiber. Reliably mode-locking soliton operation can be readily achieved after the suppression of the higher-order modes by using mode field adapters in combination with coiling fiber method. The laser delivers 600-fs level pulses at $2~\mu \text{m}$ with an average power of up to 243 mW and a repetition rate of ~53.7 MHz. The output corresponds to a maximum pulse energy of 4.5 nJ, >5 times larger than that of a similar homemade mode-locked Tm laser with the LMA fibers replaced by single mode fibers, coinciding with theoretical prediction. The successful demonstration of our soliton mode-locked LMA fiber laser at $2~\mu \text{m}$ offers another strategy for high performance ultrafast fiber sources at mid-infrared region.

We experimentally demonstrate a passively harmonic mode locked (PHML) Er-doped fiber laser with pump power efficiency up to 17 MHz/mW operating at L-band based on single walled carbon nanotubes polyvinyl alcohol (SWCNTs-PVA) film. Under 233 mW pump power, the stable pulse train at 1594.97 nm with 40.5 dB side mode suppression ratio (SMSR) and 742 fs pulse duration is obtained at a repetition rate of 1.923 GHz, corresponding to 170 th harmonic of the fundamental frequency. Under optimized intracavity conditions, the pulses frequency is able to scale up to 2.415 GHz with a high level of 40 dB SMSR, which to the best of our knowledge, is the highest value yet reported from a L-band PHML fiber laser incorporating SWCNTs as saturable absorber (SA). Such high repetition rate and stable fiber laser operating at L band may be desirable for various applications.

Silicon photonics has been regarded as a promising technology for future small-footprint, low-cost and low-power 400-Gbit/s datacenter interconnects (DCIs). In this work, for the first time, we report an experimental demonstration of a single-wavelength, single-polarization 160-Gbit/s four-level pulse-amplitude modulation (PAM-4) employing a single integrated silicon carrier-depletion micro-ring modulator (MRM). The measured bit-error rates (BERs) for the back-to-back (BTB) and after 1-km standard single-mode fiber (SSMF) transmission are 1.36E–3 and 2.12E–3, respectively, all below the hard-decision forward error correction (HD-FEC) coding limit with 7% overhead. A data rate of up to 170 Gbit/s with a BER lower than the HD-FEC limit is demonstrated for the BTB transmission.

Single-photon detectors (SPDs) are important devices in the development of quantum technologies, like quantum metrology, quantum computers and quantum communication. The simplest, cheapest and most commonly used SPD employs avalanche photodiode. After the photon absorption, a photocurrent is produced and it must be quenched before to damage the device. In the present work we show a new strategy for quenching the photocurrent of avalanche photodiodes remotely. Our scheme makes easier the synchronism between sender and receiver in a quantum communication setup. Some experimental results are presented.

We have fabricated DML/EML-based subassembly modules based on chip-to-chip optical butt-coupling with straight waveguides between a silica AWG chip and commercial directly modulated laser (DML) or electro-absorption modulated laser (EML) chips. By properly inducing external optical feedback on the DML chips, we experimentally demonstrate that 3-dB bandwidths of the DML-based subassembly module can be increased by 5 ~ 9.5 GHz compared with those of commercial 28-Gbaud DML chips. The EML-based subassembly module exhibits optical characteristics insensitive to the external optical reflection with a side mode suppression ratio (SMSR) of over 50 dB, revealing excellent optical eye patterns under 112-Gbps PAM4 operations.

A solution-processed near-infrared (NIR) photodetector based on a mixture of PbS colloidal quantum dots (CQDs) and Poly(3-hexylthiophene) (P3HT) was presented. In a reverse field-effect transistor (FET) device configuration Au(S,D)/P3HT:PbS/PMMA/Al(G), uniform-sized and well-dispersed PbS CQDs were employed as NIR absorbing materials in the active layer. Meanwhile, the poly(methyl methacrylate) (PMMA) dielectric layer could be seen as an encapsulation to enhance the device stability. Herein, high “on/off” current ratio ( $I_{on}/I_{off})$ of 10 4 was obtained in dark, and the maximum photosensitivity ( $P$ ) of 947 was gotten under 200 mW/cm 2 980 nm illumination. When the irradiance reduced to 0.1 mW/cm 2 , the responsivity ( $R$ ) and detectivity ( $D^{\ast }$ ) of the NIR photodetector reached 9.4 mA/W and $2.5\times 10^{11}$ Jones, respectively. Therefore, the P3HT:PbS hybrid FET-based NIR photodetector had shown both relatively high electrical and detecting performance, which provided an experimental foundation and method for the next fabrication of medical infrared detectors and sensors.

We present a simple two step process for the fabrication of single crystal germanium core fibers. Germanium is deposited into a capillary in a highly polycrystalline state using the pressure assisted melt filling technique. This core material is then melted and recrystallized in single crystal form using a scanning CO 2 laser process. This combination of techniques is an efficient means of producing fibers of practical lengths and core sizes for optical and optoelectronic devices. We produce small core fibers of $\mathrm {1~ \mu \text {m} }$ radii and lengths in the centimeter regime. It is anticipated that this process can be optimized so that sub-micron cores can be produced to create fibers that have single-mode operation.

We characterize both nonlinear refraction and absorption in a vertical-external-cavity surface-emitting laser (VECSEL) as a function of pump irradiance over the whole range of lasing wavelengths. We observe an approximately linear decrease in magnitude for both nonlinear refraction and nonlinear absorption with optical pumping when the thermally induced shift of the lasing wavelength is considered. Our results are of particular significance for understanding self-modelocking of VECSELs which might be driven by nonlinear lensing.

A sapphire fiber high temperature sensor based on Fabry-Perot interference was studied in the paper. A sapphire wafer served as the Fabry-Perot cavity was fixed to the end face of the sapphire fiber. The sensor was encapsulated in an alumina ceramic tube-ceramic sleeve structure to improve structural stability. Experiment results showed the sensor had a wide temperature measurement range of 25 °C–1550 °C. The temperature sensitivity reached 32.5 pm/°C at 1550 °C. Furthermore, the temperature was demodulated from the interference spectrum through fast Fourier transform algorithm. It was concluded that the sapphire fiber sensor with advantages of small size, good stability and a wide range of temperature measurement is very promising for measuring high temperature.

We report a highly sensitive refractive index sensor based on a looped micro-deformed tapered fiber. The micro-deformation is induced by first shaping the taper into a loop, and then, a ball-shaped fiber end bends the taper waist. While the tapered fiber loop is bent, a heat pulse from a CO 2 -laser is applied for 2 seconds, just enough to avoid the fusion with the balled fiber end. In this way, a small section of the taper’s waist is permanently micro-deformed. The micro-deformation has a semicircular shape with a size determined by the diameter of the balled fiber end. The micro-deformation dramatically enhances the sensitivity of the tapered fiber loop to changes in the external refractive index. Refractive index sensitivities of 1052 nm/RIU and 1636 nm/RIU were obtained with micro-deformation diameters of $220~\mu \text{m}$ and $570~\mu \text{m}$ , in a refractive index range from 1.3 to 1.345, and 1.3 to 1.34, respectively. The high sensitivity of the proposed device in the above mentioned refractive index range is attractive for biological and chemical sensing in water-based solutions.

We demonstrate a real-time transverse-mode switching of ultrafast pulse via a cascading mode conversion. The cascading all-fiber mode converter is comprised of a mode-selective coupler (MSC) and an acoustically induced fiber grating (AIFG), which functions as a broadband and controllable beam shaper, and offers a real-time mode switching between LP 11 and LP 21 modes with $\sim ~190~\mu \text{s}$ switching time. Such a transverse mode-switchable all-fiber laser presents a pulse duration of 1.4 ps and flexible transverse-mode switching. We believe that this compact and robust all-fiber laser source would find promising applications in the field of ultrafast optics and structured light microscope.

A method to detect of the topological charges (TCs) for vortex beams by using gradually changing-period spiral spoke grating (GCPSSG) is designed and experimentally demonstrated. The value of TC can be measured due to the fact that the diffraction spot will appear a bright high intensity when the TC is equal to the diffractive grating spoke number, and otherwise the central intensity remains doughnut-like structure. Meanwhile, the rotated direction of the twist fringes is opposite with the grating, therefore the sign of TC can be distinguished. Moreover, in addition to discussing the light beam illuminates the geometric center of the grating, the beam misalignment condition is also considered. In this case, the magnitude and sign of TC can be simultaneously determined by the number and orientation of the bright spots, respectively. The detection of TCs up to ± 160 is realized with this scheme. The experimental results indicate that this scheme is robust and effective because the GCPSSG shows good tolerance to environment vibration and beam misalignment for the TC diagnostics, which is anticipated to be useful for optical communication.

We present a widely tunable laser design based on three intra-cavity Mach-Zehnder interferometers. The laser has been fabricated in a multi-project wafer run and does not require high precision lithography, as needed for SGDBR lasers. We demonstrate 20 nm of tuning range with output powers of approximately 1 mW in fiber. With an excess of 30 dB in side-mode suppression ratio, the laser can find applications in gas spectroscopy, which is also supported by an absorption measurement in an HCN-filled gas cell. Tuning has been demonstrated solely relying on electro-optical phase modulators, providing a potentially faster tuning scheme than temperature or current based tuning mechanisms.

A dual-stage algorithm structure is proposed to improve estimation accuracy and reliability for low-margin elastic optical network. At the first-stage, a multitask learning-based artificial neural network (MTL-ANN) is proposed to estimate multiple parameters simultaneously. At the second-stage, a threshold-based decision module is deployed to divide the estimation results into reliable results and doubtful results. As to the doubtful results, we investigate the deviation range and underestimate the results to allocate adequate system margin. The algorithm structure is experimentally demonstrated for optical signal-to-noise ratio (OSNR) monitoring and modulation format identification (MFI) in a polarization division multiplexing (PDM) coherent optical system. Signals’ amplitude histograms (AHs) of circular constellation diagrams are selected as the input features. The results show that the MFI accuracy of nine M-QAM formats under consideration is 100%. With 93.6% OSNR estimation accuracy at first-stage, OSNR estimation with accuracy higher than 99% is achieved for the reliable results. In addition, the confidence level of doubtful results within 3 dB deviation is 0.96.

We report on a novel starting design structure of highly-dispersive mirror (HDM) which combines quarter-wavelength structure and multi-cavities, introducing more wavelength-dependent storage times of incident radiation and group delay (GD). Based on such structure, a HDM with new advanced group delay dispersion (GDD) of −20 000 fs 2 in the wavelength range of 1035–1045 nm is designed, fabricated and characterized. Our proposed initial multilayer structure is instructive in designing HDM aiming at introducing large GDD. Our HDM further pushes the maximum negative GDD allowed by the design’s total physical thickness to its limitation. It offers an ideal trade-off between GDD and total mirror thickness. The HDM has been applied in a fiber-chirped system, providing a considerable compensation of GDD reaching −200 000 fs 2 with only 10 reflections between two mirrors, and laser pulse compression from 2.8 ps to 213 fs is realized.

Hybrid organic-inorganic methylammonium lead halide (CH 3 NH 3 PbI 3 ) perovskite with a direct band gap has emerged as a material in the development of optoelectronic devices. Here, for the first time, at $1.34~\mu \text{m}$ , we fabricated CH 3 NH 3 PbI 3 perovskite film saturable absorbers by spin coating methods for passively Q-switched Nd:YVO 4 laser. The CH 3 NH 3 PbI 3 -SA shows a damage threshold of 276.6 mJ/cm 2 , a modulation depth of 18.2%, and a considerably low saturation intensity of 140.7 KW/cm 2 . By using CH 3 NH 3 PbI 3 -SA, a stable Q-switched laser with a maximum average output power of 182 mW and a pulse width of 181.6 ns can be obtained. These results reveal the promising application of CH 3 NH 3 PbI 3 in 1.34 $\mu \text{m}$ -lasers, optical switches or optical modulators.

The nonlinear polarization evolution-based passively mode-locked fiber laser is attracting increasing attention that can be attributed to its variety of operating regimes. However, the precise polarization tuning required to achieve these different regimes and the extreme vulnerability of these lasers to environmental disturbances have substantially hindered their widespread applications. Here, we experimentally demonstrate the first genetic algorithm-based real-time automatic mode-locked fiber laser, in which the fitness functions for the different regimes are based on temporal information only and where a modified genetic algorithm is proposed to accelerate the mode-locking time. The laser demonstrates an outstanding time-consumption performance, particularly when searching the second-order harmonic mode-locking regime and the Q-switching regime.

We consider an indoor visible light communication (VLC) system comprising of a transmitter (Alice) and a receiver (Bob). The communication from Alice to Bob is prone to active attacks by a malicious node (Eve) installed nearby. Specifically, when the VLC channel contains idle slots, Eve launches impersonation attack—thus, the VLC channel becomes orthogonal multiple access (OMA). On the other hand, when the VLC channel remains fully occupied by Alice, Eve is left with no choice other than to transmit simultaneously and this constitutes a jamming attack—thus, the VLC channel becomes non-orthogonal multiple access (NOMA). To thwart the impersonation attack, Bob authenticates the received packets via binary hypothesis testing (BHT) by utilizing the channel gain as the transmit device fingerprint. As for the jamming attack, Bob treats Eve’s interference as Gaussian noise to recover the data sent by Alice. We further study two different physical scenarios: a room and a corridor, respectively. In the former, Bob remains static, while in the latter, Bob is mobile. Thus, for the corridor scenario, Bob tracks Alice’s channel via a linear Kalman filter whose prediction is then fed to the BHT as the ground truth. Simulation results show that: i) for OMA VLC, authentication becomes more effective as Eve’s distance to Bob becomes more dissimilar to Alice’s distance to Bob; ii) for NOMA VLC, the average decoding error probability increases approximately logarithmically with the increase in the interference power of Eve.

Optical couplers with different power splitting ratios are essential building blocks in photonic integrated circuits. We theoretically and experimentally demonstrate a broadband and fabrication-tolerant $1\times 2$ integrated optical coupler with an arbitrary splitting ratio, based on a mode converter (MC). The MC converts the input mode into a targeted output mode with a desired power distribution, which determines the power splitting ratio at outputs. The geometry of the MC is engineered and optimized by using the particle swarm optimization algorithm combined with the finite-difference time-domain method. Couplers with 10/90, 20/80, 30/70, 40/60, and 50/50 splitting ratios are designed and fabricated on the silicon-on-insulator platform as representatives, with footprints smaller than $2.75~\mu $ m $\times 18.41~\mu $ m. The experimental results show that the ±1 dB bandwidths of these devices are greater than 42 nm around 1550 nm, and the average insertion losses are less than 0.65 dB.

We demonstrate long-wavelength InAs/GaSb superlattice photodiodes grown on InAs substrates in a production-scale metal-organic chemical vapor deposition system. An aluminum-free heterojunction scheme was adopted where mid-wavelength superlattices served as electron barriers alongside an ${n}$ -type long-wavelength absorber. Both electrical modeling and experimental characteristics suggest that suppressed dark current and unimpeded transport of photo-generated carriers can be both achieved with engineered barrier layers. Under 77 K and a reverse bias of −0.1 V, the optimized device has shown a cut-off wavelength around 12 $\mu \text{m}$ , a blackbody responsivity of 1.01 A/W, a dark-current density of 5 $\times \,\,10^{-4}$ A/cm 2 and a dark-current limited specific detectivity over 1 $\times \,\,10^{11}$ cm $\cdot \sqrt {\textrm {Hz}}$ /W.

Optical receiver modules play an important role in receiving signals from the Fiber Bragg Grating (FBG) sensors used for monitoring in civil structures, power facilities, and other sensing applications. When multiple sensors are used, more photo diodes receive optical signals; thin-film beam splitters with polarization characteristics are therefore used to reduce the size of the optical receiver module. Using a reflective semiconductor optical amplifier (R-SOA) with polarization characteristics in a sensing system with an FBG sensor, a structure that is less sensitive to polarization was developed by packaging a quad optical receiver module. A stabilization of ±4.67% was achieved using the dual optical receiver module in polarization and of ±1.20% with the quad optical receiver module in polarization.

This letter presents the design, fabrication and testing of an all-optical transducer based on a photoacoustic principle. The transducer consists of a fiber optic acoustic generator and a Fabry–Perot (FP) sensor. A lens fabricated with UV glue and Gold Nanocomposites coating were mounted on the distal end of a multimode optical fiber with 400/ $425~\mu \text{m}$ core/cladding to fabricate the generator. The lens was designed with a certain offset degree and a concaved surface to module the propagation direction and focal length of acoustic waves emitted by the generator. The FP sensor was made of PDMS between two gold layers using a single mode fiber. The validation test demonstrated promising applications in miniature space, harsh environments and even vivo tissue.

A low-order wavefront aberration self-compensation high power Nd:YAG slab laser module is developed and demonstrated. By numerical simulating the thermal distribution and transmission wavefront of the slab laser medium, a thermally-compensated heat removal geometry is developed to reduce the wavefront distortion. This method allows the non-uniform cooling to flatten the thermal gradient distribution in the width direction of the slab. By using the optimized gain module as an amplifier, the laser output power of 7.8 kW is achieved. Compared to the traditional module, the output power is 7.3 kW, and the peak-to-valley (PV) value of the wavefront distortion is reduced from 17.21 to $2.05~\mu \text{m}$ under full power, the beam quality factor $\beta $ is improved from 12.7 to 3.5 times diffraction limited while without additional correction system.

This paper proposes multilevel linear polarization shift keying (MLPolSK) modulation, based on a high polarization-dependent gain optical amplifier (PDG-OA), for spectral efficient free-space optical (FSO) communication. At the transmitter end, multiple linear states of polarization (SOPs) are adopted, and different SOPs are mapped for MLPolSK modulation according to the PDG-OA. First, in the receiver, scintillation is effectively mitigated by a polarization-independent semiconductor optical amplifier (SOA) with gain saturation. Next, various SOPs are transformed into different signal intensity levels by an OA with high PDG characteristics. Finally, transformed signal is directly detected by a single photodiode (PD) with multilevel-thresholds decision. The feasibility of the proposed technique is evaluated experimentally using a reflective semiconductor optical amplifier (RSOA), with a high PDG-up to 20 dB. A Mach–Zehnder modulator (MZM)-based fading simulator is used to accommodate lognormal distribution simulated turbulence effects into experiments. Experimental results illustrate that multilevel linear SOPs can be distinguished effectively by the proposed technique. Consequently, the MLPolSK detection is simplified, and the spectral efficiency (SE) is improved by up to 2 bit/s/Hz with effective scintillation mitigation in FSO.

We present a novel continuous-time microwave photonic link that translates eight contiguous folding bandwidths (FBW) to a single intermediate frequency (IF) bandwidth, with real-time discrimination of signals that are imaged or folded upon one another. A remote phase modulator imparts the source bandwidth of interest (BOI) onto the optical carrier, and one modulator of a dual-parallel Mach-Zehnder modulator (DPMZM) at the receiver imparts two local oscillator (LO) tones in parallel with the BOI. Third-order intermodulation distortion (IMD) between the LOs generates two more LOs, giving four LOs with eight FBWs downconverting to the IF band. A Hartley-type image rejection scheme discriminates between the upper and lower sideband images around each LO. Analysis shows the effects of different bias combinations or errors upon image rejection. Experimental results show up to at least 35dB of folded image rejection across 4GHz of original spectrum and the system successfully demodulated 64QAM signals, with almost no degradation, in the presence of a folded-image interferer.

We have studied the generation of low-noise ultrashort multibound solitons in the telecommunication spectral window in an erbium-doped all-fiber ring laser with a highly-nonlinear resonator mode-locked by a nonlinear polarization evolution effect. The multibound soliton generation is obtained with more than 20 bound dechirped pulses with a duration of ~ 240 fs at a repetition rate of ~ 11.3 MHz (with a signal-to-noise ratio of ~ 73.3 dB), the relative intensity noise is <−140 dBc/Hz, and the Allan deviation of the repetition frequency does not exceed $\sim 1.3\cdot 10^{\mathbf {-8}}$ with a time averaging window of ~ 100 s.

A fine tunable parity-time symmetric optoelectronic oscillator (PT-OEO) based on laser wavelength tuning is proposed and experimentally demonstrated. In the PT-OEO, an intensity modulator (IM) and a dispersion compensating fiber (DCF) are merged into the loop. Tuning the laser wavelength leads to a reconfigurable dispersion-induced power fading, serving as rough tunable oscillation mode selection. Meanwhile, to implement a fine-mode selection, parity-time symmetry breaking is introduced. Consequently, a fine tunable PT-OEO is achieved without using a high quality-factor (Q-factor) electrical filter. Experiment shows that the frequency of the generated signal can be tuned from 20.34 to 19.49 GHz with a step of 203 MHz, when the laser wavelength is manipulated from 1545 to 1565 nm. The phase noise is evaluated as −116.1 dBc/Hz @ 10-kHz offset frequency.

A method for estimating the limiting optical pulse parameters, which can be measured by uncooled bolometers, is proposed. Calculations show that modern microbolometers with a $10~\mu \text{s}$ time constant can provide a response time approximately 10 −12 s at the measuring pJ pulse energy with SNR > 500. An algorithm based on analyzing the time dependence of the voltage drop across the resistive element in the cooling period is proposed and described for measuring the average per pulse power of optical radiation with known pulse duration and period equal to several thermal constants of the bolometer. This algorithm has high-accuracy in measuring the pulse power using the low-cost detection system due to the high efficiency and the minimum noise bandwidth of the ADC chip. The results of the experimental studies of the NiTi-bolometer confirmed the model estimates.

Ultra-wideband spoof surface plasmon polaritons (SSPPs) based on compact balanced coplanar stripline (CPS) waveguides are proposed. Compared with the conventional CPS-based terahertz (THz) SSPP unit cell, the proposed one has achieved a size reduction of 55.2% under the condition of the same asymptotic frequency. The propagation and attenuation characteristics of the proposed SSPP waveguide can be easily manipulated by adjusting the geometry dimensions of the SSPP unit cell. It indicates that the cut-off frequency of the SSPP waveguide has its tunable flexibility thereby facilitating the filter design. To further validate the proposed idea, a similar topology in microwave regime is designed and measured, where the microwave feeding can be easily realized by embedding a balun structure from unbalanced microstrip line to balanced CPS. The measurement of the microwave filter prototype illustrates ultra-wideband bandpass filtering characteristics with return losses of over 10 dB and average insertion losses of 3.2 dB in the passband of 1.9-14 GHz. The presented work may have significant potentials to develop the miniaturization of various planar plasmonic devices and integrated circuits in microwave and THz regimes.

In this work, we present the design and performance of a monolithic master oscillator tilted tapered power amplifier emitting at 1060 nm. The device consists of three sections: a 2 mm long ridge waveguide (RW) master oscillator (MO) section with two surface distributed Bragg reflection (DBR) mirrors, a 0.5 mm long bent RW control (CON) section and a 3.5 mm long tapered power amplifier (TTPA) section tilted by 4°. With this design, unwanted back reflection from the front facet of the device towards the MO can be suppressed and the injection of the emitted light from the MO in the TTPA can be controlled by the applied current to the CON section. The device reaches an optical output power of up to 9.5 W. The emitted spectra are mode jump free over a wide power range having a spectrally narrow emission of $\Delta \lambda < 20$ pm. By adjusting the control current an optimal driving condition of the device can be found with high output power, narrow spectral bandwidth and a nearly diffraction-limited beam quality factor (1/e 2 ) $M^{2} < 1.5$ .

Experimental data collection methods and a corresponding numerical model are presented to investigate the quality of waveguide fabrication in nonlinear optics. The method utilises white light interferometry and standard image recognition techniques to calculate a waveguide propagation constant function. This enables comparison of a numerical second harmonic spectrum in quasi-phasematched materials, such as periodically poled lithium niobate, with the waveguide’s experimental phasematching spectrum. Using the presented method, a 3rd order polynomial fit to waveguide ridge width is demonstrated to be in good agreement with experimental phasematching spectra. The presented technique provides a nondestructive route to discriminate between issues in fabrication steps in nonlinear waveguide design.

This paper reports the seamless fabrication of an Al/MoS 2 /SiO 2 /Si/Ag structure metal-oxide-semiconductor (MOS) photosensitive capacitor using two-dimensional (2D) MoS 2 films. The ~2 nm MoO 3 thin film is transformed into a 2D MoS 2 film through sulfurization in the vapor phase reaction. The morphological, structural and photophysical properties of the transformed MoS 2 films are investigated using Raman spectroscopy, atomic force microscopy, energy-dispersive X-ray spectroscopy, UV-Vis spectroscopy, and X-ray photoelectron spectroscopy. It is found that the grown MoS 2 films are of good quality and continuous over the entire surface. The fabricated MoS 2 based MOS capacitor is electrically characterized through aluminum and silver contacts. When the MOS capacitor is exposed to light, its capacitance is observed to be increasing with the light intensity. The unbiased capacitance of 477 pF under dark condition is increased to 591 pF when exposed with light ( $\lambda \sim ~600$ nm) of 500 $\mu \text{W}$ /cm 2 .

Microbubble whispering-gallery resonators have shown great promise in fiber-optic communications because of their low confinement loss and hollow cores, which allow for facile stress-based tunability. Usually, the transmission spectrum of taper-coupled microbubbles contains closely spaced modes due to the relatively large radii and oblate geometry of microbubbles. In this letter, we develop an optical add-drop filter using a microbubble coupled to fiber taper and angle-polished fiber waveguides. Because of the extra degree of freedom in the angle of its polish surface, the angle-polished fiber can be used for the discriminatory excitation of certain radial-order modes in the optical microcavity, reducing the high modal density and enhancing add-drop selectivity. Our robust and tunable add-drop filter demonstrated a drop efficiency of 85.9% and quality factor of $2\times 10^{\mathbf {7}}$ , corresponding to a linewidth of 9.68 MHz. As a proof of concept, the drop frequency was tuned using internal aerostatic pressure at a rate of 7.3 ± 0.2 GHz/bar with no diminishing effects on the add-drop filter performance.

Presents the table of contents for this issue of this publication.

We are pleased to serve as the Guest Editors of this Special Issue on the topic of Optical Frequency Combs, published by the IEEE Photonics Technology Letters (PTL) journal. The science and technology of optical frequency combs (OFCs) have developed dramatically in the last few decades, and frequency combs have evolved to become a key enabling tool for a myriad of pertinent applications, from ultrahigh-precision metrology and spectroscopy to novel, exciting uses in high-speed telecommunications, advanced microwave signal generation and processing, astronomy observations, and even for quantum information processing and computing. The field of OFCs continues attracting a great deal of attention and keeps progressing in several important directions. OFC technologies based upon mode-locked lasers are very mature, so current research is mainly directed towards implementations in integrated formats, such as frequency combs generated in micro-resonator devices, or in alternative designs with enhanced capabilities, such as platforms based upon electro-optic modulation or frequency-shifted laser cavities. Intensive research is being also conducted toward the utilization of OFCs across the above-mentioned practical application areas and others.

Microcombs are powerful tools as sources of multiple wavelength channels for photonic radio frequency (RF) signal processing. They offer a compact device footprint, large numbers of wavelengths, and wide Nyquist bands. Here, we review recent progress on microcomb-based photonic RF signal processors, including true time delays, reconfigurable filters, Hilbert transformers, differentiators, and channelizers. The strong potential of optical micro-combs for RF photonics applications in terms of functions and integrability is also discussed.

Classical optical frequency combs have revolutionized a myriad of fields, from optical spectroscopy and optical clocks to arbitrary microwave synthesis and lightwave communication. Capitalizing on the inherent robustness and high dimensionality of this mature optical platform, their nonclassical counterparts, so-called “quantum frequency combs,” have recently begun to display significant promise for fiber-compatible quantum information processing (QIP) and quantum networks. In this review, the basic theory and experiments of frequency-bin QIP, as well as perspectives on opportunities for continued advances, will be covered. Particular emphasis is placed on the recent demonstration of the quantum frequency processor (QFP), a photonic device based on electro-optic modulation and Fourier-transform pulse shaping that is capable of realizing high-fidelity quantum frequency gates in a parallel, low-noise fashion.

The development of technologies for quantum information (QI) science demands the realization. and precise control of complex (multipartite and high dimensional) entangled systems on practical and scalable platforms. Quantum frequency combs (QFCs) represent a powerful tool towards this goal. They enable the generation of complex photon states within a single spatial mode as well as their manipulation using standard fiber-based telecommunication components. Here, we review recent progress in the development of QFCs, with a focus on results that highlight their importance for the realization of complex quantum states. In particular, we outline recent work on the use of integrated QFCs for the generation of high-dimensional multipartite optical cluster states – lying at the basis of measurement-based quantum computation. These results confirm that the QFC approach can provide a stable, practical, low-cost, and established platform for the development of quantum technologies, paving the way towards the advancement of QI science for out-of-the-lab applications, ranging from practical quantum computing to more secure communications.

Spectral purity of laser sources is typically investigated using phase or frequency noise measurements, which require extraction of the optical phase. This is a challenging task if the signal–to–noise–ratio (SNR) of the spectral line or the linewidth–to–noise–ratio (LNR) are not sufficiently high. In this letter, we present a statistically optimal method for optical phase noise measurement that relies on coherent detection and Bayesian filtering. The proposed method offers a record sensitivity, as the optical phase is measured at a signal power of −75 dBm (SNR of −11 dB in 1.1 GHz receiver bandwidth). Practically, this means that the phase noise measurements are, up to a high–degree, not limited by the measurement noise floor. This allows measurements down to −200 dB rad 2 /Hz and up to 10 GHz, which is useful when measuring the Schawlow–Townes (quantum noise limited) laser linewidth. Finally, the estimated optical phase is highly accurate allowing for quantum limited signal demodulation. The method thus holds the potential to become a reference measurement tool.

Mode-locked fiber and solid state lasers have played an essential role in several scientific and technological developments. The integration of mode-locked lasers on chips could enable their use in a wide range of applications. The advancement of semiconductor mode-locked laser diodes has been going on for several decades, but has recently seen the development of novel devices based on generic InP and III-V-on-silicon photonic integration platforms. These photonic integration platforms enable the use of standardized components and low-loss waveguides within the laser cavity, allowing for the design of advanced extended cavities. In this manuscript we give a review of these novel devices and compare their performance.

Optical frequency combs have established their presence in many fields among which is microwave photonics. Here, we briefly introduce optical frequency comb generators, focusing on high repetition rate combs. Examples of comb-based signal processing are highlighted, including Brillouin mitigation, beamforming, RF-filtering, sub-sampling and RF-channelizers.

A recirculating loop that includes an acousto-optic frequency shifter (AOFS) constitutes a versatile procedure to generate an optical frequency comb (OFC) from a continuous wave (cw) laser. This scheme, implementable with conventional fiber equipment, is capable of producing spectra composed of at least hundreds of coherent lines using a single-stage system. In this letter, we summarize the method to generate these acousto-optic frequency combs and briefly describe a simple model to optimize their performance. We also overview their main reported applications, paying special attention to those based on the peculiarities exhibited by the optical field at the output of the fiber loop.

Microwave Photonics is promising for generation and synthesis of radio frequency, microwave as well as millimeter wave signals of outstanding spectral purity and arbitrary wave form. High optical quality factors lead to the low noise of the signals. Small size of the optical components results in small form factor of the photonic devices. As such, the photonic oscillators outperform the majority of high frequency microwave ones of purely electronic nature. In this letter we review the major recent advances and trends as well as discuss future steps in the development of the field.

We review tri-comb spectroscopy, a novel optical approach to nonlinear spectroscopy. This optical method that we have recently developed enables the measurement of comb resolution multidimensional coherent spectra in under one second. We show the improvement in resolution and acquisition speed by applying the method to Doppler broadened rubidium vapor. We also show how this method can be used for real-time chemical sensing applications.

Precision calibration of astronomical spectrographs is essential to the hunt for exoplanets, the study of cosmology and determining variation of the fundamental constants. Frequency combs (‘astrocombs’) can serve as real-time references, providing unprecedented accuracy and precision. Here we provide a brief overview over demonstrated astrocombs and recent advances.

Lithium niobate is an excellent photonic material with large $\chi ^{\!(2)}$ and $\chi ^{\!(3)}$ nonlinearities. Recent breakthroughs in nanofabrication technology have enabled ultralow-loss nanophotonic devices based on a lithium-niobate-on-insulator (LNOI) platform. Here we present an overview of recent developments in the LNOI platform for on-chip optical frequency comb generation. These devices could lead to new opportunities for integrated nonlinear photonic devices for classical and quantum photonic applications.

Precision measurements represent a cornerstone in fundamental science. The capability of observing and quantifying subtle phenomena and events allows new discoveries and it confirms or confutes the theories describing our understanding of nature. The optical frequency comb, providing hundreds of thousands of phase-locked and evenly spaced laser lines, is one of the most fascinating enabling optical technologies and it is the result of a continuous pursuit for precision. Within the two decades from its inception, it has become a key instrument in many laboratories and has revolutionized numerous fields, spanning from time, frequency and length metrology to attosecond physics, gas-sensing and molecular fingerprinting. In this letter we summarize some steps of an exciting journey started 20 years ago, with a certain focus on the authors’ contribution, finally leading to the demonstration of frequency measurements at the 20 th decimal digit, and we show some prospective developments.

Presents the table of contents for this issue of the publication.

A structure-modulated ultralong-period microfiber grating (SULPMG) was successfully designed for simultaneous measurement of microdisplacement and temperature. The SULPMG was fabricated based on the standard single-mode fiber using the arc discharge method followed by hydrogen-oxygen flame drawing technology. The resonance wavelength of the two notches shifts toward a shorter range when the microdisplacement varies from 0 $\mu \text{m}$ to 65 $\mu \text{m}$ . The results show that the wavelength sensitivity could reach a maximum value of −288.9 pm/ $\mu \text{m}$ with a total depth change of 2.95 dB and a high resolution of 69 nm. The temperature sensitivity of the proposed sensor could reach approximately −26.4 pm/°C over an ambient temperature range of 22–90 °C. The proposed sensor can be expected to be applied in fields pertaining to microfabrication and many industrial applications.

Nonlinear frequency division multiplexing (NFDM) systems have been showing remarkable progress in the past few years. However, the majority of the demonstrations have neglected the impact of laser phase noise by employing narrow-linewidth lasers and self-homodyne receivers. The impact of transmitter laser linewidth on NFDM transmission is here numerically and experimentally investigated for dual-polarization discrete NFDM systems. The scaling of linewidth tolerance is analyzed for different signal symbol rates and the results are validated experimentally at 250 MBd. Numerical and experimental analysis show a limited degradation due to laser phase noise with relevant penalty appearing only for linewidth-symbol duration products ( $\Delta \nu \times T_{s}$ ) above $10^{-3}$ . The 250-MBd experimental results indeed show limited penalty for laser linewidths up to 750-kHz and 100-kHz, for back-to-back and 2000-km transmission, respectively.