Cylindrical vector beam generation in fiber with mode selectivity and wavelength tunability over broadband by acoustic flexural wave

Fiber Vector Light-Field-Based Tip-Enhanced Raman Spectroscopy

Tip-enhanced Raman spectroscopy (TERS) has been extensively employed to investigate the light-matter interaction at the nanoscale. However, the current TERS strategies lack the ability to excite the low-background inhomogeneous electromagnetic field with significant enhancement of electric field, electric field gradient, and optomagnetic field, simultaneously. To overcome this, we developed a fiber vector light-field-based TERS strategy aimed at exploring the multipole Raman scattering processes of molecules. By modulating the excitation power, we have observed for the first time the Stark effect associated with Raman-forbidden transitions, revealing a strong electric-field gradient and optomagnetic effect within the plasmon cavity. Furthermore, by manipulating the plasmon tip to minimize the nanogap, we demonstrate that splitting occurs in the dipole Raman spectrum, indicating that the plasmon cavity enters a strong coupling regime. This fiber vector light-field-based TERS approach offers a unique opportunity to investigate weak matter responses with potential applications in single-molecule spectroscopy, sensors, and catalysis monitoring.

Experimental demonstration of acoustically induced polarization-dependent fiber optical vortex inversion

A recently proposed theoretical model of acousto-optic interaction in optical fibers with a traveling flexural acoustic wave of the fundamental order [M.A. Yavorsky, et al., Opt. Lett.44, 598 (2019)10.1364/OL.44.000598] is experimentally examined. We show the effect of inversion of topological charge of optical vortices, which is governed by the direction of incident linear polarization. This vector effect of a coupling of polarization and orbital degrees of freedom proves the inconsistency of the conventional microbending-based model and confirms the recently suggested approach of the description of acousto-optic interaction that is based on the actual displacement vector. In addition, the obtained results demonstrate the realization of a controlled-NOT gate for orbital angular momentum (OAM) states.

Acousto-optic reconfigurable filter based on vector mode fusion in dispersion-compensating fiber

An acousto-optic reconfigurable filter (AORF) is proposed and demonstrated based on vector mode fusion in dispersion-compensating fiber (DCF). With multiple acoustic driving frequencies, the resonance peaks of different vector modes in the same scalar mode group can be effectively fused into a single peak, which is utilized to obtain arbitrary reconfiguration of the proposed filter. In the experiment, the bandwidth of the AORF can be electrically tuned from 5 nm to 18 nm with superposition of different driving frequencies. The multi-wavelength filtering is further demonstrated by increasing the interval of the multiple driving frequencies. The bandpass/band-rejection can also be electrically reconfigured by setting the combination of driving frequencies. The proposed AORF gains the feature of reconfigurable filtering types, fast and wide tunability, and zero frequency shift, which is advantageous for high-speed optical communication networks, tunable lasers, fast optical spectrum analyzing and microwave photonics signal processing.

Self-starting high-order mode oscillation fiber laser

In this paper, we proposed and demonstrated two kinds of all few-mode fiber lasers with self-starting high-order mode (HOM) oscillation. The fundamental mode can be completely suppressed by using a bandpass filter with a few-mode fiber pigtail. In the continuous-wave (CW) regime, the fiber laser directly oscillates in HOM with a signal-to-noise ratio as high as 70 dB, and the slope efficiency is up to 46%. The self-starting HOM mode-locked pulse can be easily achieved by employing a saturable absorber. The HOM oscillation pulsed fiber laser stably operates at 1063.72 nm with 3dB of 0.05 nm, which can deliver cylindrical vector beams with a high mode purity of over 98%. To our knowledge, this is the first demonstration for self-starting HOM direct oscillation in stable CW and pulsed operation states without additional adjustment. This compact and stable HOM fiber laser with a simple structure can have important applications in materials processing, optical trapping, and spatiotemporal nonlinear optics. Moreover, this work may offer a promising approach to realizing high-power fiber laser with arbitrary HOMs stable output.

Super-resolution optical microscopy using cylindrical vector beams

Super-resolution optical microscopy, which gives access to finer details of objects, is highly desired for fields of nanomaterial, nanobiology, nanophotonics, etc. Many efforts, including tip optimization and illumination optimization etc., have been made in both near-field and far-field super-resolution microscopy to achieve a spatial resolution beyond the diffraction limit. The development of vector light fields opens up a new avenue for super-resolution optical microscopy via special illumination modes. Cylindrical vector beam (CVB) has been verified to enable resolution improvement in tip-scanning imaging, nonlinear imaging, stimulated emission depletion (STED) microscopy, subtraction imaging, superoscillation imaging, etc. This paper reviews recent advances in CVB-based super-resolution imaging. We start with an introduction of the fundamentals and properties of CVB. Next, strategies for CVB based super-resolution imaging are discussed, which are mainly implemented by tight focusing, depletion effect, plasmonic nanofocusing, and polarization matching. Then, the roadmap of super-resolution imaging with CVB illumination in the past two decades is summarized. The typical CVB-based imaging techniques in fields of both near-field and far-field microscopy are introduced, including tip-scanning imaging, nonlinear imaging, STED, subtraction imaging, and superoscillation imaging. Finally, challenges and future directions of CVB-illuminated super-resolution imaging techniques are discussed.

Generation of cylindrical vector beams in a linear cavity mode-locked fiber laser based on nonlinear multimode interference

In this paper, a linear cavity mode-locked pulsed fiber laser generating cylindrical vector beams (CVBs) is proposed and demonstrated based on a nonlinear multimode interference. A homemade long-period fiber grating with a broad bandwidth of 121 nm is used as a mode converter inside the cavity. The saturable absorber was formed by single-mode fiber-graded index multimode fiber-single mode fiber (SMF-GIMF-SMF) structure. By controlling the pump power, the operation states are switchable among continuous-wave, Q-switched mode-locked (QML), and mode-locked regimes. The repetition rate of the QML CVB pulse envelope varies from 57.4 kHz to 102.7 kHz at the pump range of 118 to 285 mW. When increasing pump power to 380 mW, mode-locked CVB pulse repetition rate of 3.592 MHz, and pulse duration of 4.62 ns are achieved. In addition, the maximum single-pulse envelope energy can reach 510 nJ, and 142 mW average-power CVBs with a slope efficiency of as high as 20.2% can be obtained. Moreover, azimuthally and radially polarized beams can be obtained with mode purity over 95% in different operating regimes. The proposed fiber laser has a simple structure, and the operation is controllable in both temporal and spatial domains, which presents a flexible pulsed CVB source for application of laser processing, time or mode division multiplexing system, and spatiotemporal nonlinear optics.

Dynamic vortex mode-switchable erbium-doped Brillouin laser pumped by high-order mode

We experimentally demonstrated that the transversal vortex modes of an all-fiber erbium-doped Brillouin laser can be dynamically switched by using the high-order mode (HOM) of Brillouin pump (BP), which is used to achieve the oscillation of HOM inside the ring cavity. Core-mode conversion in a few-mode fiber (FMF) between the fundamental mode and HOM is obtained by cascading an acoustically induced fiber grating (AIFG) and a mode selection coupler (MSC) operating at the same wavelength region. Through frequency shift keying (FSK) modulation of the AIFG signal, the output transversal modes can be switched dynamically between LP₀₁ and vortex modes, and the measured purities of output HOM are more than 82%. Moreover, the output Brillouin wavelength can also be tuned via altering the input wavelength of BP and the resonant response of AIFG. We have achieved HOM Brillouin-shifted laser output within the wavelength band from 1545-1560 nm. The output linewidth of the proposed Brillouin laser is less than 4 kHz.

Recent progress in all-fiber ultrafast high-order mode lasers

Ultrafast high-order mode (HOM) lasers are a relatively new class of ultrafast optics. They play a significant role in the fieldsof scientific research and industrial applications due to the high peak power and unique properties of spatial intensity and polarization distribution. Generation of ultrafast HOM beams in all-fiber systems has become an important research direction. In this paper, all-fiber mode conversion techniques, pulsed HOM laser strategies, and few-mode/multi-mode fiber (FMF/MMF) lasers are reviewed. The main motivation of this review is to highlight recent advances in the field of all-fiber ultrafast HOM lasers, for example, generating different HOM pulses based on fiber mode converters and mode-locking in the FMF/MMF lasers. These results suggest that mode selective coupler can be used as a broad bandwidth mode converter with fast response and HOM can be directly oscillated in the FMF/MMF laser cavity with high stability. In addition, spatiotemporal mode-locking in the FMF/MMF is also involved. It is believed that the development of all-fiber ultrafast HOM lasers will continue to deepen, thus laying a good foundation for future applications.

Segmented cylindrical vector beams for massively-encoded optical data storage

The possibility to achieve unprecedented multiplexing of light-matter interaction in nanoscale is of virtue importance from both fundamental science and practical application points of view. Cylindrical vector beams (CVBs) manifested as polarization vortices represent a robust and emerging degree of freedom for information multiplexing with increased capacities. Here, we propose and demonstrate massively-encoded optical data storage (ODS) by harnessing spatially variant electric fields mediated by segmented CVBs. By tight focusing polychromatic segmented CVBs to plasmonic nanoparticle aggregates, record-high multiplexing channels of ODS through different combinations of polarization states and wavelengths have been experimentally demonstrated with a low error rate. Our result not only casts new perceptions for tailoring light-matter interactions utilizing structured light but also enables a new prospective for ultra-high capacity optical memory with minimalist system complexity by combining CVB's compatibility with fiber optics.

Reconfigurable structured light generation in a multicore fibre amplifier

Structured light, with spatially varying phase or polarization distributions, has given rise to many novel applications in fields ranging from optical communication to laser-based material processing. However the efficient and flexible generation of such beams from a compact laser source at practical output powers still remains a great challenge. Here we describe an approach capable of addressing this need based on the coherent combination of multiple tailored Gaussian beams emitted from a multicore fibre (MCF) amplifier. We report a proof-of-concept structured light generation experiment, using a cladding-pumped 7-core MCF amplifier as an integrated parallel amplifier array and a spatial light modulator (SLM) to actively control the amplitude, polarization and phase of the signal light input to each fibre core. We report the successful generation of various structured light beams including high-order linearly polarized spatial fibre modes, cylindrical vector (CV) beams and helical phase front optical vortex (OV) beams.

Lab on D-shaped fiber excited via azimuthally polarized vector beam for surface-enhanced Raman spectroscopy

We present a method for Raman examination using a silver-nanoparticles (Ag-NPs) coated D-shaped fiber (DSF) internally excited via an in-fiber azimuthally polarized beam (APB) generated by an acoustically induced fiber grating. Simulation results show that an electric-field intensity enhancement factor can be effectively improved under APB excitation compared with the linear polarization beam (LPB) excitation, because the strong gap-mode is uniformly generated between two adjacent Ag NPs on the surface of the DSF planar side. Experimental results show that the Raman signal intensity of the methylene blue (MB) detected by DSF in the case of APB excitation is ∼4.5 times as strong as that of LPB excitation, and the Raman detection sensitivity is ∼10^-9 M. The time stability of this method is also tested to be guaranteed.

Experimental study of an in-fiber acousto-optic tunable bandpass filter for single- and dual-wavelength operation in a thulium-doped fiber laser

A tunable single- and dual-wavelength thulium-doped all-fiber laser is demonstrated based on the implementation of an in-fiber acousto-optic tunable bandpass filter (AOTBF). The AOTBF is fabricated to be operated in the 1.9 µm region, and takes advantage of the intermodal coupling effect produced by traveling flexural acoustic waves in an optical fiber. It exhibits a 3-dB bandwidth of 2.04 nm with an insertion loss of 4.75 dB. The tuning properties of the AO device allows a continuous-wave operation with characteristics of wide tuning range (211.5 nm), narrow linewidth (50 pm) and high signal-to-noise ratio (60 dB). In the dual-wavelength regime, the laser is capable of independent tuning of each of the laser lines, achieving a tunable dual-wavelength emission that extends from 1802.67 to 1932.75 nm. A controllable wavelength spacing with minimum and maximum separations of 1.04 and 130.08 nm is obtained.

Tip-Enhanced Raman Spectroscopy with High-Order Fiber Vector Beam Excitation

We investigated tip-enhanced Raman spectra excited by high-order fiber vector beams. Theoretical analysis shows that the high-order fiber vector beams have stronger longitudinal electric field components than linearly polarized light under tight focusing conditions. By introducing the high-order fiber vector beams and the linearly polarized beam from a fiber vector beam generator based on an electrically-controlled acoustically-induced fiber grating into a top-illumination tip-enhanced Raman spectroscopy (TERS) setup, the tip-enhanced Raman signal produced by the high-order fiber vector beams was 1.6 times as strong as that produced by the linearly polarized light. This result suggests a new type of efficient excitation light beams for TERS.

Acousto-optic tunable bandpass filter based on acoustic-flexural-wave-induced fiber birefringence

An acousto-optic tunable bandpass filter was proposed and fabricated based on acoustic-flexural-wave-induced single-mode fiber birefringence via coupling the core mode to a single-cladding vector mode. In the experiment, the resonant wavelength and insertion loss could be electrically tuned with a span of nearly 100 nm and a lowest insertion loss of -1.7  dB. The structure gains advantages of a large tuning range, fast speed, easy fabrication, low insertion loss, and zero frequency shift.

All-fiber radially/azimuthally polarized lasers based on mode coupling of tapered fibers

We demonstrate a mode converter with an insertion loss of 0.36 dB based on mode coupling of tapered single-mode and two-mode fibers, and realize all-fiber flexible cylindrical vector lasers at 1550 nm. Attributing to the continuous distribution of a tangential electric field at taper boundaries, the laser is switchable between the radially and azimuthally polarized states by adjusting the input polarization. In the temporal domain, the operation is controllable among continuous-wave, Q-switched, and mode-locked statuses by changing the saturable absorber or pump strength. The duration of Q-switched radially/azimuthally polarized laser spans from 10.4/10.8 to 6/6.4 μs at the pump range of 38 to 58 mW, while that of the mode-locked pulse varies from 39.2/31.9 to 5.6/5.2 ps by controlling the laser bandwidth. The proposed laser combines the features of a cylindrical vector beam, a fiber laser, and an ultrafast pulse, providing a special and cost-effective source for practical applications.

Tunable dual-wavelength fiber laser with unique gain system based on in-fiber acousto-optic Mach-Zehnder interferometer

A fast tunable dual-wavelength laser based on in-fiber acousto-optic Mach-Zehnder interferometer (AO-MZI) with new fabrication process is proposed. Not only could the center wavelength of the output laser be optimized with enhanced tuning range about 30 nm by tuning the polarization and the driving frequency of the radio frequency (RF) signal accordingly, but also the spectral spacing between the two output wavelengths could be tuned from ~0 nm to 2.65 nm by controlling the power of the RF signal. The tuning mechanism was also discussed.

Multi-channel mode converter based on a modal interferometer in a two-mode fiber

In this Letter, we propose a multi-channel mode converter with the concept of a modal interferometer in a two-mode fiber (TMF). Two lateral stress points in a TMF function as in-line fiber mode couplers to construct the modal interferometer, and both transmission spectra and near-field patterns confirm that the LP₀₁ mode is successfully converted into an LP₁₁ mode at the multiple channels. The measured mode conversion efficiency almost completely follows the theoretical tendency. Finally, the mode conversion is realized at 20 channels in the C+L wavelength band with conversion efficiency up to 99.5% and insertion loss as low as 0.6 dB. Furthermore, the channel spacing can be freely tailored by adjusting the distance between two stress points.

On-demand tailored vector beams

We introduce an effective optical system to produce optical beams with arbitrary, inhomogeneous polarization states. Using our system, we are capable of generating vector beams with discretionarily chosen transverse complex fields in a straightforward way. We generate several different instances of well-known vector beams and the less common spirally polarized vector beams, as well as a full Poincaré beam. We visually show the continual transition between azimuthally and radially polarized beams via a collection of spirally polarized beams. We experimentally determine the polarization states of the generated beams and quantitatively assess the performance of our system. We find that the measured polarization distributions accurately coincide with the intended input polarization distributions.

Acousto-optic interaction in polyimide coated optical fibers with flexural waves

Acousto-optic coupling in polyimide-coated single-mode optical fibers using flexural elastic waves is demonstrated. The effect of the polyimide coating on the acousto-optic interaction process is analyzed in detailed. Theoretical and experimental results are in good agreement. Although the elastic attenuation is significant, we show that acousto-optic coupling can be produced with a reasonably good efficiency. To our knowledge, it is the first experimental demonstration of acousto-optic coupling in optical fibers with robust protective coating.

Generation of cylindrical vector beams and optical vortex by two acoustically induced fiber gratings with orthogonal vibration directions

Mode coupling from the fundamental vector mode (HE11x) to the cylindrical vector beams (CVBs) and optical vortex beams (OVBs) of a few-mode fiber excited by two acoustic flexural waves with orthogonal perturbations is achieved by using a composite acoustic transducer. The HE11x mode is converted to TM₀₁ and TE₀₁ modes, which have radial and azimuthal polarizations, by using the lowest-order acoustic flexural modes of F11x and F11y, respectively. Furthermore, HE11x mode can also be converted to the ± 1-order OVBs of HE21even±iHE21odd through the combined acoustic modes of F11x±iF11y. This technique provides a useful way of generating CVBs and OVBs in optical fiber with conveniently electrically-controlled mode conversion characteristics.

Optical vortex generation with wavelength tunability based on an acoustically-induced fiber grating

We presented a method to actualize the optical vortex generation with wavelength tunability via an acoustically-induced fiber grating (AIFG) driven by a radio frequency source. The circular polarization fundamental mode could be converted to the first-order optical vortex through the AIFG, and its topological charges were verified by the spiral pattern of coaxial interference between the first-order optical vortex and a Gaussian-reference beam. A spectral tuning range from 1540 nm to 1560 nm was demonstrated with a wavelength tunability slope of 4.65 nm/kHz. The mode conversion efficiency was 95% within the whole tuning spectral range.