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With the development of high-frequency carriers and high-capacity communications, ultrahigh-speed spectrum analyzer based on all-optical methods has been demonstrated in many fields. Limited by current data processing algorithms, ultrahigh-speed spectrum analyzers based on time-lens, such as frequency-domain light intensity spectrum analyzer (f-LISA) and parametric spectro-temporal analyzer (PASTA), are not yet to automatically process acquired data, which hinders their practicality. In this paper, a frame segmentation algorithm based on time-domain waveform analysis is proposed to achieve the automatic data processing of these analyzers, which can directly generate the spectro-temporal image of the signal under test. In a single-soliton test based on the f-LISA platform, the algorithm achieves an accuracy of 93.28% when the soliton changes and 90.04% when the soliton is stable. And in this test, the single-frame processing time of the algorithm is 419us. We believe the proposed algorithm will facilitate the practical application of spectrum analyzers like f-LISA and PASTA and provide a reference for other real-time measurement schemes.

Pure Q-switching (self-pulsating) behaviour is shown for the first time in an optically injected two-section quantum-dot laser. Upon CW optical injection, it was possible to switch the operation regime from free-running mode-locking to locked Q-switching. Under this regime, the pulse repetition rate of around 1 GHz was tunable with injection power (by more than 100 MHz) and absorber reverse bias. Moreover, through injection-locking, the Q-switched output was spectrally tunable by around 7 nm, by tuning the master laser. The seamless switch between mode-locking and Q-switching with optical injection as well as the tunability and control afforded by injection-locking could open up a range of applications, such as in multi-modal imaging. On a more fundamental level, this investigation has also generated new insights into the dynamics of quantum-dot lasers and the optical injection process.

A method to provide bandwidth compressed time-mapped spectrogram analysis based on temporal Talbot effect and temporal magnification is proposed and numerically demonstrated in this paper. The signal under test (SUT) is firstly sampled by a periodic pulse train, then passes through a dispersive medium under the Talbot condition and the spectrogram of the SUT is mapped to the time domain, so-called time-mapped spectrogram (TM-SP). Subsequently, a temporal magnification technique is implemented to stretch the time axis and compress the spectrum bandwidth, so that the effect of detection bandwidth on the frequency resolution is alleviated. Though the TM-SP cannot be acquired in a continuous way, the SUT is analyzed with ultrahigh frame rate in the temporal aperture. Additionally, a method to improve acquisition frame rate is proposed. The proposed bandwidth compressed TM-SP analysis is compared with the dispersion-based Fourier transform, and the related tradeoffs are provided in detail. The bandwidth compressed TM-SP analysis with a frequency resolution of 0.1 GHz, a measurable frequency range of 4.85 GHz, an analysis period of 0.1 ns, and an observation period of 4 ns is achieved by simulation. [ABSTRACT FROM AUTHOR]

We present a numerical approach for analysing simultaneous amplification and compression of a chirped hyperbolic secant soliton using a tapered Yb-doped photonic crystal fiber (PCF) based self-similar compressor. Theoretical studies reveal the conservation of input soliton during the amplification process due to self-similar property, which in turn helps for high quality compression of amplified soliton. In order to satisfy the self-similar condition, we note that the Yb-doped fiber should exhibit a decreasing group velocity dispersion (GVD). Such a constraint is met using a tapered PCF in our work, which is modeled using a fully vectorial effective index method (FVEIM). Influence of spatial and spectral dependent gain, saturation energy, amplified spontaneous emission (ASE), dispersion and nonlinearity on pulse propagation are analyzed using the combined solution of dynamic rate equations (DRE) and the generalized nonlinear Schrödinger equation (GNLSE). Numerical outcomes upon solving the aforementioned equations show that a 1 ps pulse is compressed down to a pulsewidth of 55.4 fs with a peak power of 1.03 kW in a total fiber length of $L=2.5$ m and taper ratio of 2.34. Overall quality of the compression scheme is evidenced by a relatively high compression factor (Fc) = 18.12, and low percentage of pedestal energy ($PE$) = 10.95 % [ABSTRACT FROM AUTHOR]

Three-dimensional integrated circuits (3D-ICs) are becoming more significant in portable devices, autonomous vehicles, and data centers. As the demand for highly integrated and high-performance semiconductor devices grows, recent 3D integration technologies focus on lowering the size of the micro-structures on such devices for high density. In order to inspect the 3D semiconductor devices, it is critical to measure the heights, depths, and overall surface profiles of the micro-structures made with silicon materials. Here, we demonstrate precise surface imaging for silicon devices by using a femtosecond mode-locked laser centered at 785 nm wavelength and an electro-optic sampling-based time-of-flight detection method with sub-10-nanometer axial precision. We could successfully measure the surface profiles as well as the step heights of silicon wafer stacks and micro-scale structures on silicon substrates. [ABSTRACT FROM AUTHOR]

In this letter, we present a photoreceiver front-end circuit that consists of an InP p-i-n photodetector (PIN PD) on an InP transimpedance amplifier (TIA) with double HBTs (DHBTs), in which the key integrated fabrication route of a transfer-printing (TP) process is introduced. Prior to integration, PDs and TIAs are individually fabricated on their respective wafers. Then, a discrete PD with only active layers is transferred onto the destination on a TIA wafer, thereafter, an on-chip coplanar waveguide connection is integrated. The PIN PD and DHBT TIA exhibit excellent performance, characterized by a responsivity of 0.51 A/W at $1.55~\mu \text{m}$ with a bandwidth of 20 GHz and a transimpedance gain of 36 dB $\Omega $ with a bandwidth of 27 GHz, respectively. Finally, the monolithically integrated circuit demonstrates a better frequency response by maximally reducing the parasitic effect, with an open clear eye diagram under a 20 Gb/s nonreturn-to-zero pattern, which evidently manifests its ability for ultrahigh-rate data operation. The present work, by employing a TP process, overcomes the issues in integrating discrete optoelectronic devices and proposes a unique approach for monolithic optoelectronic integrated circuit fabrication. [ABSTRACT FROM AUTHOR]

Ultrafast optical imaging techniques for light propagation are useful in many fields, such as physics, chemistry, and biomedicine. In this letter, we propose an imaging technique for obtaining dual motion pictures of propagating light pulses with an ultrashort time difference. To realize the proposed technique, we employed a polarization imaging camera to record dual motion pictures of propagating light pulses without superposition. We also constructed and introduced an optical delay setup for generating light pulses with ultrashort time differences. In addition, the capturing of two propagating light pulses with ultrashort time differences given by the optical delay setup was experimentally demonstrated. Specifically, we recorded the first and second light propagations for 302 fs with an 890 fs time difference. [ABSTRACT FROM AUTHOR]

Recently, fully optical-driving neuromorphic devices have attracted growing interest. However, the current device faces great challenges due to the lack of negative photoconductivity materials, which seriously hinders the further development of the visual perceptual applications using photosensitive device. Here, we propose an ionotronic neuromorphic InGaZnO4 phototransistor to establish a fully optical-driving artificial neural network. Optical neuronal paired-pulse facilitation can be switched to optical depression characteristics by tuning the gate bias more negatively through the ion-coupling bioelectrolyte. Moreover, the biological high-pass, low-pass, and band-stop photofilter behaviors can be successfully mimicked in such an all-in-one phototransistor. Finally, a fully optical-driving artificial neural network is constructed to perform artificial visual perception with an accuracy of ~90%. This device may open new avenues for the fascinating applications, such as artificial visual system, intelligent bionic robots, and smart photoelectric devices. [ABSTRACT FROM AUTHOR]

The direct generation of mid-infrared light is of significant interest for various applications including spectroscopy, detection system, optical sensing and biophotonics. Ultrashort-pulse nonlinear interaction can be viewed as four-wave mixing process, which can generate new spectral components under appropriate conditions. Here, we numerically demonstrate a fully, yet effective, spectral conversion scheme where the effective portion and components of femtosecond pulses can access the mid-infrared region directly by four-wave mixing process in silicon nitride integrated photonics waveguide. Wideband conversions occur mainly towards the mid-infrared region around the traditional red-shift phase-matching point without the adjustment of waveguide geometry. The corresponding conversion mechanisms can be attributed to nonlinear interaction of optical waves, resulting in the dispersive wave generation, the soliton-probe collision and the phase-sensitive scattering process. Our results provide an alternative route to realize wideband tunable mid-infrared frequency conversion in silicon nitride system on a chip. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive review of optical fiber sensors (OFSs), including FBG, distributed optical fiber sensor and Fabry-Perot interferometer, and their applications within harsh environments, which include extremely high temperatures (from 275 °C to 1750 °C) and low temperatures (from −271.15 °C to −40 °C, namely cryogenic conditions: from 2 K to 233.15 K), and high levels of ionizing radiation (with a maximum gamma dose up to 2 GGy, and a maximum neutron fluence of approximately $5 \times 10^{19}$ n/cm2). After a brief introduction of the principles of OFSs and mechanisms of interrogation, this paper focuses on the existing works for the above three operating environments. Attention have been paid to material selection for fabricating fibers, effects of doping with rare earth elements, femtosecond laser engraving, pre-processing and post-processing (i.e., annealing) that are employed to overcome issues faced byOFSs in extreme temperatures and radiation environments. Application examples and practical test cases are also presented. Through these examples, the limitations in the current state-of-the-art are acknowledged and the key problems are identified. Potential solutions to some of these problems are also elucidated. A feature of this paper is the amalgamation of many research methodologies and outcomes in three seemingly distinct environmental conditions in one place so that different solution techniques can be integrated to advance OFS technologies, especially for extreme environment applications. [ABSTRACT FROM AUTHOR]

Femtosecond optical parametric oscillators (OPOs) have numerous fundamental applications due to their high conversion efficiency and extensive spectral availability. As a premium alternative, aperiodic quasi-phase-matching crystal can extend the phase-matching bandwidth with its numerous poling periods. In OPO, it can considerably simplify the schematic design and generate broadband spectral coverage. In this study, using an aperiodically poled lithium niobate, we reported a widely tunable broadband OPO generating signal radiation across 1.4– $1.8 \mu \text{m}$ and idler radiation across 4.0– $2.4 \mu \text{m}$. We also investigated the generated harmonics from the violet to near-infrared region, where up to 4th order harmonics (390 nm) upon cascaded quadratic nonlinearity was observed. This scheme demonstrates the potential of aperiodic QPM crystal for broadband OPO sources and provides a promising method for the future exploitation of broadband quasi-phase-matching phenomenon. [ABSTRACT FROM AUTHOR]

We report on the experimental observation of vector staircase noise-like pulses (NLPs) produced by an Er/Yb-codoped fiber laser with a nonlinear optical loop mirror (NOLM) as the saturable absorber (SA). The staircase NLPs consist of two rectangular pulses with different energies and durations, such that the two pulse widths both increase linearly with increasing pump power with an almost constant slope while the pulse amplitude remains constant, similar to the case with Dissipative Soliton Resonance pulses. The polarization rotation state of the staircase NLPs has also been investigated along with multiple polarization dynamic patterns, including group velocity locked and polarization locked states. We have also observed harmonic mode-locked (HML) staircase NLPs by carefully adjusting the orientation of the intra-cavity polarization at fixed pump power. It was found that the HML staircase NLPs were identical to the fundamental staircase NLPs as regards the vector character and the changes in pulse duration. The experimental results are expected to enrich understanding of the vector nature of staircase NLPs and may lead to a new laser source for material processing. [ABSTRACT FROM AUTHOR]

Carrier-envelope phase (CEP) detection of ultrashort optical pulses and low-energy waveform field sampling have recently been demonstrated using direct time-domain methods that exploit optical-field photoemission from plasmonic nanoantennas. These devices are compact and integratable solid-state detectors operating at optical frequencies under ambient conditions using low pulse energies (picojoule-level). Potential applications include frequency-comb stabilization, optical time-domain spectroscopy, compact tools for attosecond science and metrology, and petahertz-scale information processing. However, to date these devices have been driven by free-space optical waveforms and their implementation within integrated photonic platforms has yet to be demonstrated. In this work, we design and simulate fully-integrated plasmonic nanoantennas coupled to a Si $_{3}\text{N}_4$ -core waveguide for CEP detection. We find that when coupled to realistic on-chip, few-cycle supercontinuum sources, these devices are suitable for direct time-domain CEP detection within integrated photonic platforms. We estimate a signal-to-noise ratio of 30 dB at 50 kHz resolution bandwidth, and address technical details, such as the tuning of the nanoantennas plasmonic resonance, CEP slippage in the waveguide, optical losses, and sensitivity to driving pulse characteristics such as energy and duration. Our results provide the basis for future design and fabrication of time-domain CEP detectors and allow for the development of fully-integrated attosecond science applications, frequency-comb stabilization and light-wave-based PHz electronics. [ABSTRACT FROM AUTHOR]

Two-dimensional graphene (2-D graphene) has limited application potential in optoelectronics because of its low light absorptivity and gapless property. This work presents photodetectors with increased light absorption efficiency, which couple 3-D graphene with the optical cavity structure of germanium-on-insulator (GeOI). Multiple reflections are generated at the 3-D graphene/GeOI heterojunction due to the unique plasmonic nanoresonator and optical cavity structures. The photodetectors exhibit excellent responsivity (${R}\,{=}\,0.9\,{\times }\,10^{4}$ mAW−1) and specific detectivity (${D}^{\ast }\,{=}\, 3.96\,{\times }\,10^{10}$ cmHz $^{1/2}\text{W}$ −1) under 1550-nm illumination with fast response rates, excellent reproducibility, and long-term stability. The technique paves the way for creating novel high-performance graphene-based photodetectors with hybrid architectures. [ABSTRACT FROM AUTHOR]

Pure Q-switching (self-pulsating) behaviour is shown for the first time in an optically injected two-section quantum-dot laser. Upon CW optical injection, it was possible to switch the operation regime from free-running mode-locking to locked Q-switching. Under this regime, the pulse repetition rate of around 1 GHz was tunable with injection power (by more than 100 MHz) and absorber reverse bias. Moreover, through injection-locking, the Q-switched output was spectrally tunable by around 7 nm, by tuning the master laser. The seamless switch between mode-locking and Q-switching with optical injection as well as the tunability and control afforded by injection-locking could open up a range of applications, such as in multi-modal imaging. On a more fundamental level, this investigation has also generated new insights into the dynamics of quantum-dot lasers and the optical injection process. [ABSTRACT FROM AUTHOR]

We experimentally investigated the fabrication and characterization of long-period fiber grating in few-mode fiber (FMF-LPFGs) by using the femtosecond laser direct writing technique. Thanks to the high flexibility and repeatability of this technique, stable FMF-LPFGs fabrication technology has been developed. In addition, the properties of FMF-LPFGs have also been characterized. The coupling of LP01- LP11 and LP01- cladding mode LP13 were investigated by both the transmission spectrum and the mode patterns. The temperature, strain and twist sensitivities of the resonance dips were calibrated by experiments, and the sensitivity difference between the dips was explained. By using their disparate responses to external temperature and strain, a cross coefficient matrix was established and dual-parameter discrimination determination is realized. This work provides a thoroughly theoretical and experimental investigation on the fabrication and characterization of FMF-LPFGs based on femtosecond laser direct writing technique. It is believed that the flexible and efficient LPFGs fabrication technique will benefit a lot in the future for developing various functional specialty optical fiber devices and sensors. [ABSTRACT FROM AUTHOR]

We develop a method based on the cross-spectrum of an intensity-modulated CW laser, which can extract a signal from an extremely noisy environment and image objects hidden in scattering media. We theoretically analyzed our scheme and performed the experiment by scanning the object placed in between two ground glass diffusers. The image of the object is retrieved by collecting the amplitudes at the modulation frequency of all the cross-spectra. Our method is non-invasive, easy-to-implement, and can work for both static and dynamic media. [ABSTRACT FROM AUTHOR]

Highly localized fiber Bragg gratings (HLFBGs) with strong cladding mode have been successfully inscribed in hydrogen loaded single mode fiber by low intensity femtosecond laser and phase mask. The HLFBGs possess strong cladding mode resonances which are deep than 20 dB and wide than 100 nm in the transmission spectrum. Experimental results reveal that localized refractive index (RI) modulation in fiber core leads to the emergence of strong cladding mode and higher localization leads to stronger cladding mode resonances. The HLFBGs can withstand high temperature up to 800 °C, and the strength of cladding mode still remains about 8 dB after thermal annealing. With surrounding RI varying from 1.3330 to 1.4583, the cut-off wavelength of cladding mode shows a good linear shift with sensitivity of 527.43 nm/RIU. The HLFBGs are expected for using in chemical and biological detections. [ABSTRACT FROM AUTHOR]

Lithium niobate (LiNbO3, LN) on insulator (LNOI) is a promising material platform for integrated photonics to realize high-capacity and high-speed information communication. Here, we developed the 515 nm femtosecond laser micro-machining technique with a 316 nm processing accuracy and demonstrated the fabrication of monolithically integrated waveguide-coupled micro-resonators with the quality factors (Q) of $1.8\times 10^{5}$ using 515 nm femtosecond laser ablation assisted ions beam etching (IBE) process. The chemical–mechanical polishing (CMP) is adopted to smoothing the etched edge of the hard mask patterned by femtosecond laser ablation. These fabrication techniques are compatible with high efficiency and high precision wafer-scale production, without counting on the UV/E-beam lithography technique, which is essential for LN photonic devices moving towards industrial applications. [ABSTRACT FROM AUTHOR]

As a key component in the engine system, silicon nitride igniter’s stable and reliable operation is a necessary condition for the proper functioning of the engine system. However, an online high temperature sensor of silicon nitride igniter inside the engine is absent. This paper presents and demonstrates a novel in-fiber integrated high temperature sensor array with high spatial resolution for online temperature field measurement on a silicon nitride igniter. The sensor array with a series of weak reflectors is written by a femtosecond laser in the core of single-mode fiber. The reflectivity of each reflector can be controlled from 10−5~10−8 by adjusting the written power of femtosecond laser. Two adjacent weak reflectors form a compact fiber sensor. The size of a single sensor can be as small as 3–5 mm, so it is able to measure the temperature fluctuation in a space down to several millimeters. The five-sensor-array, with the help of a white light interference demodulation system, can measure high temperature up to 1000°C. Due to the advantages of high reliability, compact construction and simple fabrication, this high temperature sensor array can be widely applied in high temperature sensing applications with space constrains. [ABSTRACT FROM AUTHOR]

Objective: The ocular vascular system plays an important role in preserving the visual function. Alterations in either anatomy or hemodynamics of the eye may have adverse effects on vision. Thus, an imaging approach that can monitor alterations of ocular blood flow of the deep eye vasculature ranging from capillary-level vessels to large supporting vessels would be advantageous for detection of early stage retinal and optic nerve diseases. Methods: We propose a super-resolution ultrasound localization microscopy (ULM) technique that can assess both the microvessel and flow velocity of the deep eye with high resolution. Ultrafast plane wave imaging was acquired using an L22-14v linear array on a high frequency Verasonics Vantage system. A robust microbubble localization and tracking technique was applied to reconstruct ULM images. The experiment was first performed on pre-designed flow phantoms in vitro and then tested on a New Zealand white rabbit eye in vivo calibrated to various intraocular pressures (IOP) – 10 mmHg, 30 mmHg and 50 mmHg. Results: We demonstrated that retinal/choroidal vessels, central retinal artery, posterior ciliary artery, and vortex vein were all visible at high resolution. In addition, reduction of vascular density and flow velocity were observed with elevated IOPs. Conclusion: These results indicate that super-resolution ULM is able to image the deep ocular tissue while maintaining high resolution that is comparable with optical coherence tomography angiography. Significance: Capability to detect subtle changes of blood flow may be clinically important in detecting and monitoring eye diseases such as glaucoma. [ABSTRACT FROM AUTHOR]

In this paper, an iterative algorithm is developed to combine time shifted traces in pulsed terahertz time domain spectroscopy (THz-TDS). This method intends to correct errors due to mechanical sampling in the trailing ends of the traces in systems that utilize a combination of a fast voice coil optical delay line and a slow long delay range one. The algorithm works by iteratively combining three basic modifications on the trace shape to match the overlapping areas of consecutive time windows. In order to validate the method, both simulated and experimental traces affected by ODL distortions are stitched. Proper stitching is shown to mitigate the increase of noise floor and the frequency shift of spectral resonances that arises with direct stitching, i.e. just compensating for the offset between traces. An enhancement in acquisition speed by a factor 7 in comparison to conventional lock-in detection based on a long range optical delay line is shown in tests with the experimental setup available. [ABSTRACT FROM AUTHOR]

High-speed photodetectors operating at mid-wave infrared are crucial for free-space optical communication and frequency comb spectroscopy. In this letter, we report a high-speed mid-wave infrared uni-traveling carrier photodiode based on InAs/InAsSb type-II superlattice at room temperature for the first time. The device exhibits a cut-off wavelength of around 5.5 $\mu \text{m}$ at room temperature. The responsivity of the device is about 0.6 A/W (at 4.5 $\mu \text{m}$) under −1 V at room temperature. The frequency response of the device is characterized by an optical parametric amplification system generating mid-infrared femtosecond pulses. A device with a 20 $\mu \text{m}$ diameter has a 3-dB bandwidth of 12.8 GHz at −4 V. These promising results suggest that the device could be potential candidates to be employed in the emerging high-speed MWIR applications. [ABSTRACT FROM AUTHOR]

In this work, we report the inscription of fiber Bragg grating through the over-clad in cyclic transparent fluoropolymer (CYTOP) optical fiber using a phase mask technique and a 400 nm femtosecond pulsed laser. 8 mm-long grating were obtained in less than 20 s with 500 $\mu$ W average beam power. To investigate the FBG properties and discriminate between the protective over-clad influence and the core-cladding effect, we inscribe FBGs in the fiber with and without the over-clad. We also show that the temperature sensitivity of CYTOP-FBGs with the over-clad depends on the relative humidity, whereas CYTOP-FBGs without the over-clad achieve almost zero sensitivity to humidity and sensitivity to temperature of 26.7 pm/ $^{\circ }$ C. [ABSTRACT FROM AUTHOR]

A whispering-gallery-mode (WGM) resonator with an L-shaped microcavity for humidity sensing is proposed and demonstrated. The L-shaped microcavity is fabricated by using femtosecond laser micromachining, fusion discharge and chemical etching methods. By placing two microspheres in the microcavity, of which one is used as the whispering gallery mode resonant cavity, and the other plays the role of changing the optical path to help activating whispering gallery mode resonance. The device has a linear response to humidity with the linearity of 99.1% and the sensitivity of −0.23 dB/%RH, within the relative humidity range of 30%–70%, under the room temperature of 25 °C. The response time for the humidity increasing is as short as 240 ms, and the recovery time is 453 ms. The device is simple in fabrication, robust and has good operation stability. [ABSTRACT FROM AUTHOR]

This paper reports the fabrication of parallel integrated sapphire fiber Bragg gratings (SFBGs) probe sensor by the femtosecond laser point by point (PbP) method. The SFBGs probe sensor transmits optical signals through quartz multimode fiber, and takes an 8.5 mm sapphire fiber as the sensing probe, which can work stably for a long time at 1200 °C. This method can not only quickly prepare and obtain SFBGs with small bandwidth, but also obtain gratings with high reflectivity, which is about 15%. The mode field distribution in sapphire fibers with different lengths is studied by the beam quality analyzer. High-order modes are easily excited as the length of sapphire fibers increases. In addition, a spherical lens is prepared at the end of sapphire fiber, which effectively improve the signal-to-noise ratio (SNR) to 22 dB. The good spectral quality can still be maintained after holding at 1200 °C for 24 hours, and the temperature sensitivity can reach 30.19 pm/°C in the high temperature range, which is more than 2 times higher than that of quartz FBGs sensors. This parallel integrated SFBGs probe sensor overcomes the problems of doped quartz fiber element diffusion at high temperature and the instability of microstructure caused by material softening, and can be developed for long-term stable operation at 1200 °C high temperature and harsh environments. [ABSTRACT FROM AUTHOR]

We present the first modelocked backside-cooled GaSb VECSEL (Vertical External Cavity Surface Emitting Laser) operating above $2 ~\mu \text{m}$. Using a two quantum well InGaSb SESAM (SEmiconductor Saturable Absorber Mirror) in a V-shaped cavity arrangement we obtain femtosecond modelocking at a center wavelength of 2061 nm, with pulses as short as 324 fs, an average output power as high as 65 mW at a repetition rate of 3 GHz. An operation in a picosecond regime is further demonstrated with an even higher average output power of up to 260 mW and 4.5-ps pulses. We perform a characterization in both the femto- and picosecond regimes with measurements of the pulse duration, the optical spectrum, and the pulse repetition rate (frequency and time domain analysis) at low and high power. In this context, the influence of intracavity group delay dispersion (GDD) is investigated. We find that GDD has a strong influence on the pulse duration (fs or ps regime) and thus on the performance of our SESAM-modelocked 2- $\mu \text{m}$ GaSb VECSEL. [ABSTRACT FROM AUTHOR]

We demonstrate an optical arbitrary waveform measurement (OAWM) system that exploits a bank of silicon photonic (SiP) frequency-tunable coupled-resonator optical waveguide (CROW) filters for gapless spectral slicing of broadband optical signals. The spectral slices are coherently detected using a frequency comb as a multi-wavelength local oscillator (LO) and stitched together by digital signal processing (DSP). For high-quality signal reconstruction, we have implemented a maximum-ratio combining (MRC) technique based on precise calibration of the complex-valued opto-electronic transfer functions of all detection paths. In a proof-of-concept experiment, we demonstrate the viability of the scheme by implementing a four-channel system that offers an overall detection bandwidth of 140 GHz. Exploiting a femtosecond laser with precisely known pulse shape for calibration along with dynamic amplitude and phase estimation, we reconstruct 100 GBd QPSK, 16QAM and 64QAM optical data signals. The reconstructed signals show improved quality compared to that obtained with a single high-speed intradyne receiver, while the electronic bandwidth requirements of the individual coherent receivers are greatly reduced. [ABSTRACT FROM AUTHOR]

In addition to the exploration of more spectrum at high-frequency bands, next-generation wireless networks will witness an intelligent convergence of radio frequency (RF) and non-RF links such as optical and visible light communication. Optical attocell (OAC) networks provide an additional layer to RF-based wireless networks with gigabit-per-second data transmission rate and centimeter-level location accuracy. However, the directionality, line-of-sight constraints, as well as strong sensitivity to the location and orientation of user terminals challenge the stringent requirements for throughput and latency. In this article, we consider mobile heterogeneous networks (HetNets) incorporating indoor OAC with femtocells and macrocells to provide a low-cost and energy-efficient solution. The HetNets solution satisfies diverse service requirements in terms of user-experienced data rate, mobility, latency, accuracy, and security in the Internet of Things. To support seamless connectivity and optimal resource allocation in the proposed HetNets with mobility awareness, handover in dynamic environments needs to be addressed efficiently. Incorporating rich environmental parameters into such a decision making problem facilitates the self-optimization process, but extensively expands the state space. To achieve a fast convergence speed, a deep reinforcement learning approach is proposed to optimize the handover parameters (e.g., time-to-trigger and hysteresis margin). This is a model-free and off-policy reinforcement setting that trains and employs a deep neural network to predict future rewards for successions of states and actions. Thus, the optimal parameters are obtained by selecting the best actions to take. Through numerical simulation and performance analysis, we discover the gain from enriching the state space and the adaptability of the system to dynamic environments. [ABSTRACT FROM AUTHOR]

A dispersion-managed, figure-of-9, mode-locked ultrafast Yb-doped fiber laser is presented in this letter. The laser cavity employs a chirped fiber Bragg grating with a proper anomalous dispersion for intra-cavity dispersion compensation. The laser with large normal net dispersion delivers an output with maximum average power of 20 mW and pulse duration of 5.50 ps at the repetition rate of 39.1 MHz, and the pulse can be dechirped to 228 fs. While net dispersion of cavity is very close to zero, the laser delivers an output with an average power of 12 mW at the repetition rate of 47.3 MHz, and the pulse duration is 1.86 ps, which can be dechirped to 175 fs. The whole cavity is very compact and environmentally stable, for its all-polarization-maintaining and all-fiber structure. [ABSTRACT FROM AUTHOR]

In this letter, indium phosphide (InP) nanowires (NWs) are fabricated by Au-nanoparticle assisted vapor-liquid-solid method and applied as a saturable absorber (SA) for continuous-wave (CW) mode-locked femtosecond Yb:KYW bulk laser. I-scan method was used to characterize the saturable absorption properties of the prepared InP NWs SA. Pulses as short as 394 fs with the repetition rate of 41.5 MHz and maximum average output power of 315 mW are achieved. To the best of our knowledge, this is the first demonstration of InP NWs working as SA for the fs-pulse generation in the solid-state bulk laser. The results indicate that InP NWs are a promising SA candidate for applications in ultrafast photonic devices. [ABSTRACT FROM AUTHOR]

Alternative machine learning approaches that have extremely low latency and can work with only a small training dataset are needed for applications where the insatiable demands of deep learning methods for computing power and large training data cannot be met. Here we report a new optical accelerator for AI that exploits femtosecond pulses for both data acquisition and computing enabling classification at short time scales for fast optical imaging, sensing, and metrology without increasing data dimensions. Modulation of data onto the spectrum of femtosecond optical pulses followed by projection into a new space using nonlinear optics reduces the latency in the nonlinear classification of certain data by orders of magnitude. This Femtocomputing approach is validated by the classification of various datasets, including brain intracranial pressure, cancer cell imaging, spoken digit recognition, and the classic exclusive OR benchmark for nonlinear operation. The concept is demonstrated by seeding the nonlinear effect that is responsible for many fascinating natural phenomena, such as optical rogue waves. Stimulation of nonlinear optical interactions with spectrally modulated data transforms the data such that a computationally-light linear algorithm can learn a nonlinear decision boundary that separates the data into the correct classes. Since the optical kernel is not trained, its performance is inevitably data-dependent. Quantitative comparison with a popular numerical kernel offers insights into how this physical technique accelerates inference. Single-shot operation is demonstrated using time stretch data acquisition. [ABSTRACT FROM AUTHOR]

In this work, a tellurite microstructed optical fiber (TMOF) was designed and fabricated by 78TeO2-5ZnO-12LiCO3-5Bi2O3 (TZLB) glass, which provided an opportunity for investigating the stimulated Raman scattering (SRS) and four wave mixing (FWM). Using a nanosecond laser operated at 1545 nm as a pump signal, mid-infrared cascaded SRS was observed. At 60 mW, the continuous-wave (CW) signal gain was as high as 10.46 dB. During the FWM process, with the CW signal wavelengths changing from 1547 nm to 1560 nm, a maximum conversion efficiency (CE) of −27.05 dB was obtained. The SRS and FWM in the TMOF were numerically simulated, and the obtained results agreed well with the experiment. To the best of our knowledge, it was the first time to demonstrate simultaneously SRS and FWM in a TMOF. The proposed TMOF of extremely high nonlinearity can be exploited for the development of multifunctional multiplexing devices of Raman amplifiers and wavelength converters, and is expected to meet the increasing requirements of high-performance all-optical devices. [ABSTRACT FROM AUTHOR]

A two-dimensional tapered fiber core mimicking a commonly used tapered fiber coupler is proposed and experimentally demonstrated in a piece of single-mode fiber (SMF) for whispering gallery mode (WGM) excitation. Tapering the fiber core with femtosecond laser ablation can simultaneously enhance the evanescent field and decrease the effective refractive index of the core mode, which lowers the phase mismatch between the core mode and the WGM in a spherical cavity with a diameter less than the fiber cladding radius. Experimental results show that symmetric Lorentzian dip with a quality factor (Q-factor) of $3.6\times 10 ^{3}$ and asymmetric Fano-like lineshape can be achieved. Such a tapered fiber core provides an effective coupling method for a single WGM based photonic device or WGM array-based lab in-fiber due to its inherent compatibility with fiber optic hardware, high integration, and robustness. [ABSTRACT FROM AUTHOR]

In recent years, significant efforts have been made to develop ultrafast laser source around 2- $\mu \text{m}$ wavelength. Meanwhile, the characterization of the ultrashort pulses within ultracompact footprints has been attracting growing interests. Here relying on the transverse second-harmonic generation in a single CdTe microwire, we successfully demonstrate an optical autocorrelator for femtosecond pulse characterization at 2- $\mu \text{m}$ wavelength. With 220-fs laser pulses (38 pJ/pulse) as input light, the measured pulse width after calibration is 236 fs, showing a good accuracy of this microwire autocorrelator. Due to the low-loss transmission window and high nonlinearity of CdTe in the mid-infrared (MIR) region, the microwire-based autocorrelator shows great potential for MIR applications ranging from on-chip optical communication to ultracompact laser spectroscopy. [ABSTRACT FROM AUTHOR]

Compared to fiber-based equivalents, CMOS-compatible silicon-based nonlinear optical devices harness the superior features of lower operating powers, precise dispersion control and extensive transparency range. In this article, we propose a scheme of cascaded soliton compression on CMOS-compatible Silicon Nanophotonic Wire Waveguides in the mid-infrared region which includes the wavelength of 2.0 μm and 2.8 μm. Thanks to the cascaded pulse compression mechanism, we demonstrate the large compression factor on an integrated chip-scale waveguide. We can realize a high degree pulse compression that the pulse width decreases from 1 ps to 24.0 fs with a high compression factor of 41.7 and a low operating peak power of 1.56 W at 2.0 μm. In addition, even as the wavelength migrate to 2.8 μm, our scheme is equally effective. We achieve 31.9 compression factor of 1 ps chirp-free higher-order soliton pulse which can be compressed to 31.3 fs with a small initial peak power of 4.87 W. This research indicates the potential of dispersion engineering waveguides for mid-infrared low-power applications, such as ultrafast integrated lasers, chip-scale frequency-comb generation, minimally invasive skin surgeries and molecular spectroscopy. [ABSTRACT FROM AUTHOR]

Optical fiber diffraction gratings with periodic structure across the fiber section and fabricated by femtosecond laser are proposed and demonstrated. The diffraction patterns can be clearly observed and greatly enhanced by adopting multiple layer gratings. An optical fiber in-line Mach-Zehnder interferometer can be constructed by use of multiple layer diffraction grating pairs. The fiber diffraction gratings represent a new type of optical fiber device featured with high robustness, good operation stability and great potential for many photonics applications. [ABSTRACT FROM AUTHOR]

This study presents the concept of high-damage-threshold chirped mirrors (HDTCMs) for ultrafast high-power laser systems. We describe a novel design method for the fabrication of HDTCMs to simultaneously achieve high reflectivity and high laser-induced damage threshold (LIDT) over a broad bandwidth. In the proposed novel structure, the squared electric field distribution oscillations of the laser pulse stagger each other, leading to a low average squared E-field distribution and effectively enhancing the LIDT. As a proof-of-concept study, an HDTCM is designed based on our simulation. The LIDT of the HDTCM increases by a factor of ~2.3. Moreover, a HDTCM pair is successfully applied in a cross-polarized-wave (XPW) filter, supporting a near Fourier transform limit (FLT) 17.7-fs pulse generation. This concept can be generalized to virtually any optical coating design, thus advancing high damage resistant mirror fabrication to a level well suited to high-power laser applications. [ABSTRACT FROM AUTHOR]

Ultrafast nonlinear dynamics driven by high-peak-power ultrashort mid-infrared (mid-IR) pulses gives rise to ultrabroadband radiation whose spectrum spans over multiple decades, stretching from the terahertz (THz) to the microwave range. We show that, despite its enormous, multidecade bandwidth, an accurate spectral and spatial-mode characterization of this radiation is possible via a suitable combination of mutually complementary signal-analysis methods borrowed from ultrafast optics, radioelectronics, THz photonics, and microwave engineering. This analysis reveals intense subgigahertz radiation emitted as a part of mid-IR-driven supercontinuum generation and shows that microwave-to-THz field waveforms can almost reach the diffraction limit in their beam focusability, yielding field strengths above 3 MV/cm. [ABSTRACT FROM AUTHOR]

Supercontinuum generation has been extensively studied in optical waveguides pumped with femtosecond pulses, while there are few reports using picosecond pump pulses with a small spectral power variation. In this paper, we numerically demonstrate the supercontinuum generation in a dispersion-engineered silicon nitride waveguide pumped with 10-ps pulses at 1550 nm wavelength. A supercontinuum from 656 to 3867 nm (2.5 octaves) is enabled by uniquely tailored dispersion. The spectrum has a power variation of < 15 dB and shows a very flat pedestal varying by < 5 dB. The spectra generated from different dispersion profiles are compared and analyzed, which verifies the unique role of dispersion tailoring in flattened supercontinuum generation. To the best of our knowledge, it is the first study on such broad and flat supercontinuum generation on a chip using picosecond pump pulses, which would be highly desirable for many applications. [ABSTRACT FROM AUTHOR]

An optical fiber Mach-Zehnder interferometer based on an inner air-cavity with an open micro-channel structure is presented. The device is fabricated by using femtosecond laser to inscribe a cavity structure in the optical fiber core, followed by discharging the cavity area with a fusion splicer to form an inner air-cavity in the fiber core, and finally create an open micro-channel with the help of hydrofluoric acid corrosion. The fiber in-line interferometric device is miniature, robust, and stable in operation and exhibits a high pressure sensitivity of ~3351 pm/MPa, with an extremely low temperature cross-sensitivity of 2.58 kPa/°C. [ABSTRACT FROM AUTHOR]

This paper introduces an under-sampling E-field measurement system involving three femtosecond pulse lasers, which can be used to measure a multi-frequency electric field (E-field) in a wide frequency range. Optical under-sampling technology utilizes femtosecond laser pulses to sample the electrical signals and convert the high-frequency signal down to the first pulse repetition frequency (RPF) intervals for measurement, i.e., tens to hundreds of MHz. However, previous work rarely studied the measurement of multi-frequency signals. Therefore, in this work, we use three RPFs and propose the Remainder Matching (RM) method of three RPFs to eliminate false-alarm-triggered frequencies. Moreover, to solve the challenging problem of grouping the down-conversion components of each measured frequency, we suggest the Eigenvalue Matching of Remainder’s Difference (EMRD) method to obtain the correct group accurately. Hence, this work combines the RM and EMRD methods to improve the system’s measurement accuracy. To evaluate the suggested measurement technique, detailed theoretical analysis and simulation are performed to demonstrate the universality of our method that can effectively guide the design of E-field measuring systems using optical under-sampling technology. The proposed scheme manages an error rate less than 0.1% without any measurement error of the down-converted components when six frequencies exist simultaneously and is less than 5% when the measured frequency deviation is ±25kHz. Finally, the proposed system is challenged on noise perturbations, with the results demonstrating that the suggested combined RM and RDEM scheme attains an appealing performance. [ABSTRACT FROM AUTHOR]

We inscribe and characterize a series of helical intermediate-period fiber gratings (HIPFGs) on single-mode fiber with pitches from 24 μm to 48 μm. The fabricated HIPFGs have low temperature sensitivity, low torsion sensitivity, and high refractive index sensitivity. A series of separated peaks with low insertion losses and high coupling strengths are observed in the transmission spectra of the HIPFGs. For the HIPFG with grating pitch of 36 μm (HIPFG-3), the peaks located at around 1596 nm and 1603 nm are chosen to evaluate the sensor performance of the fiber gratings. The temperature sensitivities are 6.86 pm/°C and 7.53 pm/°C for peaks around 1596 nm and 1603 nm, respectively. The corresponding values of the torsion sensitivities are 8.67 nm/(rad/mm) and 9.49 nm/(rad/mm) for those two peaks, respectively. In addition, the average refractive index sensitivity of the grating reaches 393.9 nm/RIU in the range of 1.310-1.408. The sensitivity of the HIPFG-3 at the RI of 1.408 is calculated to be 1291 nm/RIU. Therefore, we conclude that the HIPFGs proposed in this paper can be used as a high-sensitivity refractive index sensor which is not sensitive to temperature and torsion. [ABSTRACT FROM AUTHOR]

An optical fiber in-line Mach-Zehnder interferometer based on an inner air-cavity with long cavity length is demonstrated. The device is fabricated by using femtosecond laser to inscribe a waveguide structure in the optical fiber core, followed by discharging the waveguide area with a fusion splicer. The inner air-cavity structure is highly robust, and the relatively long cavity length corresponds to a small free spectral range, which makes it possible to implement accurate measurement. Such an inner air-cavity device fabricated in single mode fiber has good high temperature sustainability. The device is also “open” to the external environment when being fabricated in multimode fiber, thus supporting refractive index measurement. The proposed device is attractive in many optical fiber sensing applications. [ABSTRACT FROM AUTHOR]

Microwave camera is a passive focal plane microwave imaging system which is used for large-scale, wideband electromagnetic interference (EMI) imaging. Microwave camera images the EMIs with their amplitude and location by the reflector and sensor array, like taking a photograph. This camera, however, only receives the power density of the EMIs within a wideband and the frequency information is aliased and abandoned. To address this issue, in this article, we propose a frequency estimation method to recover the frequency of the wideband EMI from its amplitude image without a need to add new hardware. For the developed imaging system, the proposed method realizes the frequency estimation in the 3–6-GHz frequency range with a resolution of 100 kHz consistently in most of the central part of the field of vision (FOV). Our proposed method is also applicable to other passive imaging applications such as passive millimeter radar imaging and infrared imaging, to extract the related information of the detected objects. [ABSTRACT FROM AUTHOR]

In this article authors present a prototype of a dichroic scanning nonlinear optical microscope with a photodetecting polymer Scanner and a customized objective lens with an external back focal plane. Two types of two-axis electromagnetic actuated polymer scanners are manufactured by a standard printed circuit board (PCB) fabrication process. Both scanners share the same overall dimensions of 45 mm by 45 mm. The maximum optical scan angle of a type I scanner is 13 degrees at 420 Hz by the fast axis in resonance and 4 degrees by the slow axis in the linear mode. The type II scanner shows similar performance. A scan lens as well as a tube lens in a conventional nonlinear optical microscope are replaced by placing such a polymer scanner on the external back focal plane of a customized water immersion objective lens. The objective lens has the back working distance of 6 mm, the numerical aperture of 0.7, the front working distance of 1.16 mm, a field of view of 1.67 mm in the diameter and the angle of view of +/−15 degrees. Preliminary imaging results are also presented. [ABSTRACT FROM AUTHOR]

Achieving multi-wavelength synchronous ultra-short pulses with arbitrary wavelength intervals is a difficult problem for fiber lasers. Herein, a laser based on multi-stage cascaded Mamyshev regenerators is proposed as a solution. The regenerators are cascaded to form a single laser cavity, each at a different position within the cavity and corresponding to a different output wavelength. A single pulse oscillating in the cavity has its wavelength sequentially switched by the regenerators. Synchronous multi-wavelength pulsed lasers with 5 nm and 0.01 nm output center-wavelength intervals are realized through numerical simulation; the output pulses have 10 nm spectral widths, peak powers of ~0.7 kW, and durations of ~0.4 ps. Design principles, system stability, and the effects of the filter bandwidth, length of single mode fiber and wavelength ordering in the cavity are discussed.

Single-walled carbon nanotube encapsulated in boron nitrite nanotube (SWCNT@BNNT) is a novel nanomaterial with a one-dimensional van der Waals (1D-vdW) heterostructure. In this paper, we demonstrated that the SWCNT@BNNT has an enhanced optical power tolerance compared to that of the pristine SWCNT while preserving the ultrafast optical saturable absorption properties. The thermal damage threshold of SWCNT@BNNT is 2 dB higher than that of the pristine SWCNT, resulting in 9 times longer lifetime for 1 dB degradation and 214 times longer lifetime for 3 dB degradation under optical power intensity of 6.8 kW/cm2. The high optical power tolerance of SWCNT@BNNT is further confirmed in a short-cavity mode-locked laser with a high repetition rate of 1 GHz with a high optical power intensity. We have shown the potential of using 1D-vdW heterostructure fabrication technique to modify and enhance the optical properties of the encapsulated nanomaterials [CE: Please check aff. we have deleted addrline mater and table 1 missing in source file we have capture in pdf. Please need to proof reading.]. [ABSTRACT FROM AUTHOR]

Raman spectroscopy provides a non-invasive, chemically-specific optical imaging of biological objects without relying on endogenous labels. Nonlinear Raman spectroscopy allows non-invasive imaging at much faster speed with an improved spatial resolution and axial sectioning capability. In this report we propose a novel use of nonlinear Raman spectroscopy as a sensor of local nano-environment. Time-resolved coherent anti-Stokes Raman spectrograms are found to be sensitive to small variations of local structural changes, which are not normally observed using conventional Raman spectroscopy. [ABSTRACT FROM AUTHOR]

The resolution performance of space charge measurement method determines its application environment and measured information, which has put forward higher temporal and spatial resolution requirements for the charge evaluation of electronic equipment insulation. Based on the ellipsometry detection principle and the photoelastic effect of the sensor, a novel optical method for space charge measurement is proposed. The potentials of the method in terms of single measurement with high signal-to-noise ratio, terahertz bandwidth and nanometer spatial resolution are analyzed. By deriving the mathematical relationship of the signal transmission, it is indicated that the light intensity difference measured by the system can linearly present the elastic force generated from space charge. Based on the built ellipsometry detection system, it is found that the detected signal from the sensor can quickly track the imposed elastic waves with different amplitudes and frequencies, which verifies the feasibility of the crucial detection part and the fast measurement speed of the proposed method. Further simulation results indicate that the refractivity performance of the sensor under the effect of elastic waves can present the deformation variation with a spatial resolution of 2 nm. Therefore, the given explorative research can provide the key theoretical basis and experimental verification for the proposed method. [ABSTRACT FROM AUTHOR]