Polychromatic photonic Floquet-Bloch oscillations.

Photonic Floquet-Bloch oscillations (FBOs), a new type of Bloch-like oscillations in photonic Floquet lattices, have recently been observed as a typical discrete self-imaging effect. Here, we theoretically investigate the spectral range of approximate photonic Floquet-Bloch oscillations in arrays of evanescently coupled optical waveguides and show the adjustability of the spectral range. At an appropriate amplitude of the Floquet modulation, we have demonstrated approximate photonic FBOs over a broad spectral range, termed "polychromatic photonic Floquet-Bloch oscillations," which manifest as approximate self-imaging of polychromatic beams. Furthermore, by designing the functional form of the Floquet modulation, we can cascade two polychromatic photonic FBOs and further enhance the performance of polychromatic self-imaging. Our results provide a simple and novel mechanism for achieving polychromatic self-imaging in waveguide arrays and may find applications in polychromatic beam shaping and broadband optical signal processing.

Functionalized gold nanoparticle enhanced nanorod hyperbolic metamaterial biosensor for highly sensitive detection of carcinoembryonic antigen.

Hyperbolic metamaterial (HMM) biosensors based on metals have superior performance in comparison with conventional plasmonic biosensors in the detection of low concentrations of molecules. In this study, a nanorod HMM (NHMM) biosensor based on refractive index changes for carcinoembryonic antigen (CEA) detection is developed using secondary antibody modified gold nanoparticle (AuNP-Ab2) nanocomposites as signal amplification element for the first time. Numerical analysis based on finite element method is conducted to simulate the perturbation of the electric field of bulk plasmon polariton (BPP) supported by a NHMM in the presence of a AuNP. The simulation reveals an enhancement of the localized electric field, which arises from the resonant coupling of BPP to the localized surface plasmon resonance supported by AuNPs and is beneficial for the detection of changes of the refractive index. Furthermore, the AuNP-Ab2 nanocomposites-based NHMM (AuNP/Ab2-NHMM) biosensor enables CEA detection in the visible and near-infrared regions simultaneously. The highly sensitive detection of CEA with a wide linear range of 1-500 ng/mL is achieved in the near-infrared region. The detectable concentration of the AuNP/Ab2-NHMM biosensor has a 50-fold decrease in comparison with a NHMM biosensor. A low detection limit of 0.25 ng/mL (1.25 pM) is estimated when considering a noise level of 0.05 nm as the minimum detectable wavelength shift. The proposed method achieves high sensitivity and good reproducibility for CEA detection, which makes it a novel and viable approach for biomedical research and early clinical diagnostics.

Visual observation of photonic Floquet-Bloch oscillations.

Bloch oscillations (BOs), an important transport phenomenon, have been studied extensively in static systems but remain mysterious in Floquet systems. Here, by harnessing notions from photonic analogy, we propose a generalization of the existing BOs in photonic Floquet lattices, namely the "photonic Floquet-Bloch oscillations", which refer to rescaled photonic Bloch oscillations with a period of extended least common multiple of the modulation period and the Bloch oscillation period. Next, we report the first visual observation of such photonic Floquet-Bloch oscillations (FBOs) by employing waveguide fluorescence microscopy. Most significantly, the FBOs surpass the existing BOs in Floquet systems and exhibit exotic properties on their own, including fractal spectrum and fractional Floquet tunneling. This new transport mechanism offers an intriguing method of wave manipulation that may contribute to rapidly developing fields in photonics, condensed matter physics, and quantum physics.

Few-mode fiber Bragg grating-based simultaneous multichannel CSRZ to NRZ format conversion scheme for LP01 and LP11.

We propose what we believe is a novel format conversion scheme using a few-mode fiber Bragg grating (FM-FBG) that can perform multichannel format conversion from carrier-suppressed return-to-zero (CSRZ) to non-return-to-zero (NRZ) for both LP01 and LP11. The multichannel spectral response of FM-FBG is designed according to the algebraic difference between the CSRZ and NRZ spectra outlines. Additionally, the FM-FBG response spectra of LP11 are designed to shift with that of LP01 by the WDM-MDM channel spacing for filtering both modes together. Numerical results demonstrate the successful conversion of both LP01 and LP11 channels, carrying four channels of 200-GHz-spaced CSRZ signals at 40 Gbit/s, into NRZ signals with a high Q-factor (exceeding 14 dB), and the converted NRZ signals exhibit clean and open eye diagrams. Furthermore, the performance analysis also shown that our proposed FM-FBG is robust to central wavelength detuning.

Configurable SNAP microresonators induced by axial pre-strain-assisted CO2 laser exposure.

Flexible engineering of the complex shapes of the surface nanoscale axial photonics (SNAP) bottle microresonators (SBMs) is challenging for future nanophotonic technology applications. Here, we experimentally propose a powerful approach for the one-step fabrication of SBMs with simultaneous negative and positive radius variations, exhibiting a distinctive "bump-well-bump" profile. It is executed by utilizing two focused and symmetrical CO2 laser beams exposed on the fiber surface for only several hundred milliseconds. The spectral characteristics of different eigenmodes are analyzed, providing deep insights into the complex physical processes during the CO2 laser exposure. The shapes of the SBMs can be flexibly adjusted by the exposure time, laser power, and applied pre-strains. As a proof of this technique, the developed approach enables the efficient production of a bat SBM, ensuring a uniform field amplitude of the bat mode over the length exceeding 120 µm with 7% deviation. Our proposed technique provides a powerful technique for the efficient fabrication of SBMs with predetermined shapes, laying the groundwork for its applications on microscale optical signal processing, quantum computing, and so on.

Mode-locked fiber laser based on a small-period long-period fiber grating inscribed by femtosecond laser.

We demonstrate stable mode-locked pulses in an erbium-doped fiber laser (EDFL) using a femtosecond laser-inscribed small-period long-period grating (SP-LPG). The SP-LPG has a period of 25 µm and a length of 2.5 mm. The polarization dependent loss (PDL) of the SP-LPG reaches 20 dB at the wavelength of 1556 nm and 25 dB at the wavelength of 1607 nm, which is sufficient to trigger the mode-locking mechanism. In addition, a mode-locked fiber laser (MLFL) based on the SP-LPG has been demonstrated to generate 1.58-ps pulses at 1577 nm with a bandwidth of 4 nm and a repetition rate of 1.54 MHz. The signal-to-noise ratio (SNR) of 50 dB shows the high stability of this system. This work indicates various potential applications of the SP-LPG in ultra-fast laser technologies due to its simple fabrication, compact structure, and high damage threshold.

Dual-step reconstruction algorithm to improve microscopy resolution by deep learning.

Deep learning plays an important role in the field of machine learning, which has been developed and used in a wide range of areas. Many deep-learning-based methods have been proposed to improve image resolution, most of which are based on image-to-image translation algorithms. The performance of neural networks used to achieve image translation always depends on the feature difference between input and output images. Therefore, these deep-learning-based methods sometimes do not have good performance when the feature differences between low-resolution and high-resolution images are too large. In this paper, we introduce a dual-step neural network algorithm to improve image resolution step by step. Compared with conventional deep-learning methods that use input and output images with huge differences for training, this algorithm learning from input and output images with fewer differences can improve the performance of neural networks. This method was used to reconstruct high-resolution images of fluorescence nanoparticles in cells.

High-performance vector torsion sensor based on high polarization-dependent in-fiber Mach-Zehnder interferometer.

We propose a high-performance vector torsion sensor based on an in-fiber Mach-Zehnder interferometer (MZI), which consists of a straight waveguide inscribed in the core-cladding boundary of the SMF by a femtosecond laser in only one step. The length of the in-fiber MZI is 5 mm, and the whole fabrication time does not exceed 1 min. The asymmetric structure makes the device have high polarization dependence, and the transmission spectrum shows a strong polarization-dependent dip. Since the polarization state of the input light entering the in-fiber MZI varies with the twist of the fiber, torsion sensing can be achieved by monitoring the polarization-dependent dip. Torsion can be demodulated by both the wavelength and intensity of the dip, and vector torsion sensing can be achieved by setting the appropriate polarization state of the incident light. The torsion sensitivity based on intensity modulation can reach 5763.96 dB/(rad/mm). The response of dip intensity to strain and temperature is weak. Furthermore, the in-fiber MZI retains the fiber coating, so it maintains the robustness of the complete fiber structure.

Long-Period Fiber Grating Sensors for Chemical and Biomedical Applications.

Optical fiber biosensors (OFBS) are being increasingly proposed due to their intrinsic advantages over conventional sensors, including their compactness, potential remote control and immunity to electromagnetic interference. This review systematically introduces the advances of OFBS based on long-period fiber gratings (LPFGs) for chemical and biomedical applications from the perspective of design and functionalization. The sensitivity of such a sensor can be enhanced by designing the device working at or near the dispersion turning point, or working around the mode transition, or their combination. In addition, several common functionalization methods are summarized in detail, such as the covalent immobilization of 3-aminopropyltriethoxysilane (APTES) silanization and graphene oxide (GO) functionalization, and the noncovalent immobilization of the layer-by-layer assembly method. Moreover, reflective LPFG-based sensors with different configurations have also been introduced. This work aims to provide a comprehensive understanding of LPFG-based biosensors and to suggest some future directions for exploration.

Femtosecond laser-inscribed off-axis high-order mode long-period grating for independent sensing of curvature and temperature.

We propose and demonstrate a novel curvature and temperature sensor based on an off-axis small-period long-period fiber grating (SP-LPG) which is inscribed in a single mode fiber by a femtosecond laser in one step. The total length of the SP-LPG is only 2.1 mm. The period of the SP-LPG is 30 µm, which is smaller than that of conventional long period fiber gratings. Essentially, the SP-LPG is a high-order mode long period fiber grating. Due to the off-axis structure, the SP-LPG can be used for two-dimensional vector bending sensing. The curvature can be demodulated by the intensity variation of the dips in the transmission spectrum. When the incident light is polarized, the instantaneous curvature sensitivity of the SP-LPG can exceed 20 dB/m-1. Meanwhile, a series of Bragg resonant peaks can be observed in the reflection spectrum, which can be used to monitor the fluctuation of temperature. The transmission dip is insensitive to temperature and the reflection peak is insensitive to curvature, which allows the SP-LPG to measure curvature and temperature independently. The characteristics of high curvature sensitivity, two-dimensional bending direction identification, real-time temperature measurement, and compact structure make the device expected to be applied in the field of structural health monitoring and intelligent robotics.

Generation of different mode-locked states in a Yb-doped fiber laser based on nonlinear multimode interference.

We demonstrated an ultrafast Yb-doped fiber laser with a single mode fiber-graded index multimode fiber-single mode fiber (SMF-GIMF-SMF) structure based saturable absorber. The GIMF was placed in the groove of an in-line fiber polarization controller to adjust its birefringence, enabling the SMF-GIMF-SMF structure to realize efficient saturable absorption based on nonlinear multimode interference without strict length restriction. By adjusting two intra-cavity polarization controllers, stable dissipation solitons and noise-like pulses were achieved in the 1030 nm waveband with pulse durations of 10.67 ps and 276 fs, respectively. We also realized Q-switched mode-locked pulses in the same fiber laser cavity. By the dispersive Fourier transform method, the real-time spectral evolution in the buildup process of the Q-switched mode-locked state was captured, which showed that the continuous-wave in this laser could gradually evolved into the stable Q-switched mode-locked pulses through unstable self-pulsation, relaxation oscillation and rogue Q-switching stage. To the best of our knowledge, our work reveals the buildup dynamics of the Q-switched mode-locked operation in a fiber laser for the first time. And we also studied the real-time spectral evolution of the stable Q-switched mode-locked pulses, which exhibited periodic breathing property.

Microsecond-resolved smartphone time-gated luminescence spectroscopy.

Time-gated luminescence spectra are usually measured by laboratory instruments equipped with high-speed excitation sources and spectrometers, which are always bulky and expensive. To reduce the reliance on expensive laboratory instruments, we demonstrate the first, to the best of our knowledge, use of a smartphone for the detection of time-gated luminescence spectra. A mechanical chopper is used as the detection shutter and an optical switch is placed at the edge of the wheel to convert the chopping signal into a transistor-transistor logic (TTL) signal which is used to control the excitation source and achieve synchronization. The time-gated luminescence spectra at different delay times of Eu(TTA)3 powder and the solutions of Eu-tetracycline complex are successfully detected with a temporal resolution of tens of microseconds by the proposed approach. We believe our approach offers a route toward portable instruments for the measurement of luminescence spectra and lifetimes.

Multi-wavelength random fiber laser based on a tilted parallel inscribed apodized fiber Bragg grating array.

We propose and demonstrate a multi-wavelength random fiber laser (RFL) based on a novel, to the best of our knowledge, compact apodized fiber Bragg grating array (AFBGA). The AFBGA is fabricated by a femtosecond laser with the point-by-point tilted parallel inscription method. The characteristics of the AFBGA can be flexibly controlled in the inscription process. Hybrid erbium-Raman gain is used in the RFL and reduces the lasing threshold to sub-watt level. Stable emissions at two to six wavelengths are achieved with the corresponding AFBGAs, and more wavelengths are expected with higher pump power and AFBGAs containing more channels. A thermo-electric cooler is employed to improve the stability of the RFL, and the maximum wavelength and power fluctuations of a three-wavelength RFL are 64 pm and 0.35 dB, respectively. With flexible AFBGA fabrication and simple structure, the proposed RFL enriches the choice of multi-wavelength devices and has significant potential in practical applications.

Accurate fabrication of SNAP microresonators via a femtosecond laser with multidimensional optimized parameters.

Surface nanoscale axial photonics (SNAP) microresonators with nanoscale effective radius variations (ERVs) along the optical fiber axis can be fabricated by inscribing axially oriented lines inside the fiber with a femtosecond laser. Here, we propose the multi-dimensional fabrication parameter system for the femtosecond laser fabrication of SNAP devices and systematically investigate the relationships between the introduced ERV and the multidimensionally controllable fabrication parameters. Specifically, both the qualitative and quantitative processing principles are revealed. As a proof-of-principle, by multidimensionally optimizing the fabrication parameters, we realize a SNAP microresonator with the characteristics of both small axial size and maximal ERV. The achieved ERV is almost 5 times larger than the ERV achieved with the previous unoptimized method. Our work promotes the fs laser inscription technology to be a flexible and versatile approach for fabricating the SNAP devices with ultra-high precision, ultra-low loss and high robustness.

Random fiber laser based on a partial-reflection random fiber grating for high temperature sensing.

A stable single wavelength random fiber laser (RFL) with a partial-reflection random fiber grating (PR-RFG) for high temperature sensing is proposed and demonstrated for the first time, to the best of our knowledge. The PR-RFG is fabricated with the help of a femtosecond laser, with its highest reflection peak significantly higher than all other reflection peaks, which can ensure the stability of this filter-free RFL. Theoretical calculations also show that such a PR-RFG should be designed with reflectivity in the range of ∼30%-90% to obtain one reflection peak significantly higher than other peaks. The threshold of this laser is only 6.4 mW. In addition, the RFL realizes temperature sensing in the range from 25°C to 500°C and has an optical signal-to-noise ratio of up to 70 dB.

SNAP structures fabricated by profile design of in-fiber inscribed regions with a femtosecond laser.

Fabricating a surface nanoscale axial photonics (SNAP) microresonator with a specific profile is a challenging and important issue since its advent. We propose a powerful approach for the flexible fabrication of the SNAP structures with arbitrary profiles by a femtosecond laser. Our method is to design the profile of the length distribution of the inscribed lines to match the profile of the required SNAP microresonator, and to combine it with other fabrication parameters to precisely control the radius variation of the SNAP structure. In experiments, we demonstrate the design and fabrication of the SNAP structures with the parabolic, semi-parabolic, and bat profiles. The developed approach is expected to be universal for the fabrication of complex high Q-factor SNAP structures, which lays the groundwork for exploring the versatile performances of the SNAP devices.

Multi-wavelength random fiber laser with switchable wavelength interval.

A random fiber laser with flexible wavelength interval switching is proposed and demonstrated through two switching methods. One is to change the effective structure of the laser cavity by controlling the switches of 980 nm pump laser diodes (LDs) for erbium-doped fibers (EDFs), which can achieve the switching of the wavelength interval from a single Brillouin frequency shift (BFS) of 0.088 nm to a double BFS of 0.176 nm. Another method is to manipulate the gain provided by the two EDF amplifiers by controlling the power of the three 980 nm LDs, thereby realizing the optical switching of the wavelength interval. This kind of wavelength interval switchable random fiber laser increases the flexibility and functionality of multi-wavelength light sources, and further expands the application range of the random fiber lasers. Furthermore, the alternative wavelength interval switching mechanisms with simple structure enable it to meet the application requirements of various occasions.

Auto-Phase-Locked Time-Resolved Luminescence Detection: Principles, Applications, and Prospects.

Time-resolved luminescence measurement is a useful technique which can eliminate the background signals from scattering and short-lived autofluorescence. However, the relative instruments always require pulsed excitation sources and high-speed detectors. Moreover, the excitation and detecting shutter should be precisely synchronized by electronic phase matching circuitry, leading to expensiveness and high-complexity. To make time-resolved luminescence instruments simple and cheap, the automatic synchronization method was developed by using a mechanical chopper acted as both of the pulse generator and detection shutter. Therefore, the excitation and detection can be synchronized and locked automatically as the optical paths fixed. In this paper, we first introduced the time-resolved luminescence measurements and review the progress and current state of this field. Then, we discussed low-cost time-resolved techniques, especially chopper-based time-resolved luminescence detections. After that, we focused on auto-phase-locked method and some of its meaningful applications, such as time-gated luminescence imaging, spectrometer, and luminescence lifetime detection. Finally, we concluded with a brief outlook for auto-phase-locked time-resolved luminescence detection systems.

Dual interference effects in a line-by-line inscribed fiber Bragg grating.

Fiber Bragg grating (FBG) usually can be seen as a stack of Fabry-Perot (FP) cavities, which result in strong Bragg resonance through multi-cavity FP interference. In this Letter, we report surprising and interesting dual interference effects in a line-by-line (LBL) inscribed FBG with a femtosecond laser. Besides the well-known FP effect, the equivalent Mach-Zehnder interference (MZI) effect caused by mode interference can also be observed in the LBL FBG simultaneously. The experimental results of the comparison between the LBL FBGs and the point-by-point inscribed FBGs show that the dual interference effects are merely observed in the LBL FBGs. Meanwhile, the achieved MZI exhibits a strong polarization dependence. Sharing the merits of the FBG and MZI simultaneously, the achieved optical fiber device may find potential applications in optical fiber communication, fiber lasers, and multi-parameter sensor systems.

Mode-locked and Q-switched mode-locked fiber laser based on a ferroferric-oxide nanoparticles saturable absorber.

We demonstrated an ultrafast erbium-doped fiber laser (EDFL) based on ferroferric-oxide (Fe3O4) nanoparticles as a saturable absorber (SA). The investigated SA was based on magnetic fluid deposited on the end face of a fiber ferrule connector. When the SA was inserted into an EDFL cavity, a stable 2.93 ps mode-locked pulse can be achieved by adjusting the intra-cavity polarization controller. The pulse had a central wavelength of 1572.39 nm and a 3 dB bandwidth of 1.39 nm. We also obtained Q-switched mode-locked pulses at 1593.4 nm. The repetition frequency and the temporal width of the Q-switched pulse envelope varied with the pump power. When the pump power reached 225 mW, the maximum average output power and the pulse envelope energy were up to 4.51 mW and 235.5 nJ. To the best of our knowledge, this is the first time that mode-locked and Q-switched mode-locked pulses have been obtained in a fiber laser based on Fe3O4 nanoparticles.