Electro-optic high-speed optical beam shifting based on a lithium niobate tapered waveguide.

We propose an electro-optic on-chip beam shifting device based on gradient microstructured electrodes and an optical tapered waveguide fabricated using lithium niobate (LN). The distribution of refractive index variations of the optical waveguide can be electro-optically defined and tailored by the designed gradient microstructured electrodes, which directs the beam propagation and shifting. The length of the beam shifting device is 18 mm and the width of the waveguide is gradually increased from 8 µm to 80 µm. The functionality of the beam shifting device is experimentally demonstrated, and it is observed that it has an electro-optic tunability of 0.41 µm/V, and a high-speed response time of 19 ns (λ=1310 nm). This study can provide potential applications in optical switching and modulation, beam scanning and ranging, optical spatial communications, etc.

Ultrasensitive temperature sensor and mode converter based on a modal interferometer in a two-mode fiber.

This paper presents an ultrasensitive temperature sensor and tunable mode converter based on an isopropanol-sealed modal interferometer in a two-mode fiber. The modal interferometer consists of a tapered two-mode fiber (TTMF) sandwiched between two single-mode fibers. The sensor provides high-sensitivity temperature sensing by taking advantages of TTMF, isopropanol and the Vernier-like effect. The TTMF provides a uniform modal interferometer with LP01 and LP11 modes as well as strong evanescent field on its surface. The temperature sensitivity of the sensor can be improved due to the high thermo-optic coefficient of isopropanol. The Vernier-like effect based on the overlap of two interference spectra is applied to magnify the sensing capabilities with a sensitivity magnification factor of 58.5. The temperature sensor is implemented by inserting the modal interferometer into an isopropanol-sealed capillary. The experimental and calculated results show the transmission spectrum exhibit blue shift with increasing ambient temperature. Experimental results show that the isopropanol-sealed modal interferometer provides a temperature sensitivity up to -140.5 nm/°C. The interference spectrum has multiple dips at which the input LP01 mode is converted to the LP11 mode. This modal interferometer acts as a tunable multi-channel mode converter. The mode converter that can be tuned by varying temperature and mode switch is realized.

Broadband mode-selective couplers based on tapered side-polished fibers.

We propose the broadband mode-selective coupler (MSC) formed with a side-polished six mode fiber (6MF) and a tapered side-polished small core single-mode fiber (SC-SMF) or an SMF. The MSCs are designed to allow the LP01 mode in the SC-SMF and SMF to completely couple to the LP01, LP11, LP21, LP02, LP31, LP12 modes in the 6MF over a broadband wavelength range. The phase-matching conditions of the MSCs are satisfied by tapering the SC-SMF and SMF to specific diameters. The tapered fibers are side-polished to designed residual fiber thickness using the wheel polishing technique. The effective indices of the side-polished fibers are measured with the prism coupling method. The MSCs provide high coupling ratio and high mode purity. High coupling efficiencies in excess of 81% for all the higher-order modes are obtained in the wavelength range 1530-1600 nm. For the LP01, LP11, LP21, LP02, LP31, LP12 MSCs at 1550 nm, the coupling ratios are 96.2%, 99.8%, 89.5%, 85.0%, 90.9%, 96.1%, respectively, and the mode purity of the MSCs is higher than 88.0%. The loss of the MSCs is lower than 1.8 dB in the wavelength range 1530-1600 nm. This device can be applied in broadband mode-division multiplexing transmission systems.

Highly efficient second harmonic generation of thin film lithium niobate nanograting near bound states in the continuum.

Bound states in the continuum (BICs) are ubiquitous physical phenomena where such states occur due to strong coupling between leaky modes in side lossy systems. BICs in meta-optics and nanophotonics enable optical mode confinement to strengthen local field enhancement in nonlinear optics. In this study, we numerically investigate second-harmonic generation (SHG) in the vicinity of BICs with a photonic structure comprising one-dimensional nanogratings and a slab waveguide made of lithium niobate (LiNbO3, LN). By breaking the symmetry of LN nanogratings, BICs transition to quasi-BICs, which enable strong local field confinement inside LN slab waveguide to be supported, thereby resulting in improving SHG conversion with lower pump power of fundamental frequency (FW). With a peak intensity of 1.33 GW cm-2at the FW, our structure features a second-harmonic conversion efficiency up to 8.13 × 10-5at quasi-BICs. We believe that our results will facilitate the application of LN in integrated nonlinear nanophotonic.

Side-polished few-mode fiber based surface plasmon resonance biosensor.

The fiber geometry, fiber parameters and mode-guiding properties are crucial for realizing high-performance fiber-based sensors. In this work, we propose and demonstrate a few-mode fiber (FMF)-based surface plasmon resonance (SPR) biosensor. The FMF-SPR sensor was fabricated via side-polishing a few-mode fiber and coating a thin layer of gold film, on the basis of the optimization of fiber geometry, thickness of the gold film and mode selection, which were performed with the finite element method. The refractive index (RI) sensing performance of three such sensors with different residual fiber thicknesses were investigated. In the RI range from 1.333 to 1.404, the highest sensitivity up to 4903 nm/RIU and a figure of merit of 46.1 RIU-1 are achieved. For testing the bovine serum albumin (BSA) solution, an averaged BSA RI sensitivity of 6328 nm/RIU and an averaged BSA concentration sensitivity of 1.17 nm/(mg/ml) are realized. Benefiting from only a few modes supported in the FMF, a smaller line-width of the SPR spectrum is obtained, which further results in a higher figure of merit (FOM). Moreover, when combined with the superiority of the mode-multiplexing technology brought by the FMF, the FMF-SPR sensors may find applications in biochemical analysis with high performance and high throughputs.

Electron-plasmon interaction on lithium niobate with gold nanolayer and its field distribution dependent modulation.

Surface plasmon resonance (SPR) enables strong field confinement, opening thereby new avenues for device miniaturization and reducing energy consumption. Plasmonic devices with electrical tunability attract tremendous interest for various applications. Most of the current researches achieved SPR modulation with relatively large driving voltages, or by other relatively low-speed tuning approaches, such as thermo-optic, magneto-optic, acousto-optic etc. In this paper, we propose and demonstrate an efficiently electrical SPR modulation based on lithium niobate (LN) with gold nanolayer (~81 nm) via electron-plasmon interaction. Efficient intensity modulation and wavelength shift (in visible band) of ~5.7 dB/V and ~36.3 nm/V are respectively obtained with low DC current. More importantly, modulation phenomenon of field distribution dependent is also observed and experimentally unveiled. Further performance is analyzed in terms of AC modulation and polarization characteristics. This key achievement opens up opportunities for applications such as optical interconnection, electric field sensing, electrically plasmonic modulation, etc.

Broadband all-light-control with WS2 coated microfibers.

All-optical light amplitude tuning functionality is demonstrated in a layered tungsten disulfide (WS2) nanosheets coated microfiber (MF) structure. Due to the strong light-matter interactions between WS2 nanosheets and the evanescent field around the MF, a large variation in the transmitted power can be observed under both external and internal pump light excitations over a broadband spectrum (~100 nm). A power variation rate of ~0.3744 dB/mW is obtained under external violet pump light excitation, whereas the power variation rate of similar devices in the state of the art are usually less than 0.3 dB/mW. In terms of the response time, a moderate rise/fall time of ∼20.5/19.6 ms is achieved, which is mainly limited by the employed structure fabrication methods. These results indicate that the optical transmitted power of the WS2 coated MF can be modulated by different pump light with the power in the order of mW, thus the proposed device might have potential applications in all optical controllable devices and sensors, etc.

Upper-limited angular Goos-Hänchen shifts of Laguerre-Gaussian beams.

The angular Goos-Hänchen shift of vortex beam is investigated theoretically when a Laguerre-Gaussian (LG) beam is reflected by an air-metamaterial interface. The upper limit of the angular GH shift is found to be half of the divergence angle of the incident beam, i.e., |Θup| = (|ℓ| + 1)1/2/k0w0, with ℓ, k0, and w0 being the vortex charge, wavenumber in vacuum, and beam waist, respectively. Interestingly, the upper limited angular GH shift is accompanied by the upper-limited spatial IF shift. A parameter F is introduced to compare the total beam shift with the beam size. F varies with the vortex charge ℓ and the propagation distance zr. The values of F at zr = ∞ plane can approach 0.5, which are always larger than those at zr = 0 plane. These findings provide a deeper insight into optical beam shifts, and they may have potential application in precision metrology.

Theoretical investigation of optical modulators based on graphene-coated side- polished fiber.

The effective mode index (EMI) of a graphene-coated side-polished fiber (GSPF) is calculated numerically. Whereby, the influences of graphene atom layer number, residual radius of SPF, light frequency, scattering rate of graphene, and temperature on the EMI are investigated comprehensively. Two types of mechanisms for the electro-optical absorption modulation are found for such GSPF-based modulator. One mechanism is Pauli blocking effect (PBE) and the other is plasmonic attenuation effect (PAE). With the optimal design parameters, a PBE-based modulator is theoretically predicted to have a 0.0072 dB/µm modulation depth, 2.92 V driving voltage swing, 6.35 nJ/bit power consumption, and 56.2 THz optical modulation bandwidth. It is also predicted that a PAE-based modulator could have a 0.0056 dB/µm modulation depth, 0.6 V driving voltage swing, 0.27 nJ/bit power consumption, and 2.5 THz optical modulation bandwidth. By further optimization, the modulator performance such as the relatively high power consumption and the narrow operation bandwidth can be improved. Owing to their seamless connection to optical fiber networks, the GSPF-based modulators have great potential to be used in fast and high-capacity optical communication systems.

Electro-optic deflection in a lithium niobate quasi-single mode waveguide with microstructured electrodes.

We propose an electro-optic mode deflection device based on an annealed proton exchange (APE) waveguide in lithium niobate, associated with isosceles-triangle-shaped array electrodes and a horn-shaped input waveguide. The input waveguide is tapered down to ensure that the output of the device has a good beam quality, i.e., a quasi-single mode in this case. This new device allows beam deflection at a relative low voltage and large deflection angle. At an APE-waveguide width of 80 µm, mode deflections of 0.265 and 0.240 µm/V are obtained for 1064 and 980 nm, respectively. This beam deflection device can be applied in high-speed optical switch, and beam smoothing of a high-power laser, etc.

High performance all-fiber temperature sensor based on coreless side-polished fiber wrapped with polydimethylsiloxane.

A novel fiber structure, coreless side-polished fiber (CSPF) that is wrapped by polydimethylsiloxane (PDMS), is demonstrated to be highly sensitive to temperature because of the high refractive index sensitivity of the CSPF and the large thermal optic coefficient of the PDMS. Our numerical and experimental results show that the several dips in the transmitted spectra of PDMSW-CSPF is originated from the multimode interference (MMI) in the CSPF and will blueshift with the increase of temperature. Furthermore, for such a PDMSW-CSPF, we investigate its temperature sensing characteristics and the influences of residual thickness (RT) and dip wavelength on the sensitivity both numerically and experimentally. In the temperature range of 30~85°C, the PDMSW-CSPF with RT = 43.26 µm exhibits a high temperature sensitivity of -0.4409 nm/°C, the high linearity of 0.9974, and the high stability with low standard deviation of 0.141 nm. Moreover, in the cycle experiments, where the environmental temperature was set to automatically increase and then decrease, the PDMSW-CSPF exhibits a low relative deviation of sensitivity (RSD) of down to ± 0.068%. Here, the RSD is defined as the ratio of sensitivity deviation to the average sensitivity measured in the heating/cooling cycle experiments. The lower RSD indicates that PDMSW-CSPF has better reversibility than other fiber structure. The investigations also show that the sensitivity of the PDMSW-CSPF could be enhanced further by reducing the residual thickness and choosing the dip at a longer wavelength.

Highly sensitive all-optical control of light in WS2 coated microfiber knot resonator.

All-optical light-control-light functionality is realized in a layered tungsten disulfide (WS2) nanosheet coated microfiber knot resonator (MKR) structure. Mainly due to the photon generated excitons induced refractive index variation in WS2 nanosheets, a large variation in the transmitted power (∆T) can be observed under external violet/red laser excitation. The ∆T variation rates can reach up to ~0.4 dB/mW under violet pump light excitation whereas the state of the art light-control-light structures usually has a variation rate of less than 0.25 dB/mW. In terms of the response time, the averaged rise/fall time is ~0.12/0.1 s. The demonstrated structure has the advantages of easy fabrication, low cost and high sensitivity, therefore, it might be a promising candidate for building future all-fiber-optics based functional devices and all-optical circuitry.

Sensitivity-enhanced surface plasmon sensor modified with MoSe2 overlayer.

In this work, a sensitivity-enhanced surface plasmon resonance (SPR) sensor, which is integrated with MoSe2 as modification overlayer, is proposed and investigated. The sensor is constructed by physically depositing MoSe2 onto the surface of a conventional SPR sensor based on Krestchman configuration. Thanks to the commendable properties of MoSe2 including high carrier mobility, high refractive index (RI), large surface area, and so forth, adding an overlayer within a certain thickness can effectively improve the RI sensitivity. Experimental results show that, with the increased number of deposition cycles-which positively correlates with the duty ratio and the MoSe2 overlayer's thickness-the sensitivity at first increases, and then declines. The highest sensitivity of 2524.8 nm/RIU is achieved experimentally, which corresponds to the 2 deposition cycles. This shows an improvement of 36.3%, compared with the case without the MoSe2 modification. The ease of fabrication, efficiency of performance enhancement, and great potentials (such as the large surface area of MoSe2 for linking abundant functional groups) allow the method presented in this paper to contribute to the development of performance-enhanced SPR sensors for the biological, chemical, and medical fields.

High-sensitivity vector magnetic field sensor based on side-polished fiber plasmon and ferrofluid.

A high-sensitivity vector magnetic field is proposed and demonstrated. The sensor consists of a side-polished-fiber (SPF)-based surface plasmon resonance (SPR) structure integrated with ferrofluid. Because of the high refractive index sensitivity of the SPR scheme and the outstanding magneto optical properties of ferrofluid, the sensor shows a high sensitivity (up to 598.7 pm/Oe) to magnetic field intensity. Moreover, owing to the non-circular-symmetric geometry of the SPF and non-uniform distribution of the ferrofluid around the SPF, the sensor exhibits a sensitivity of -5.63 nm/deg to the orientation of the magnetic field. The proposed vector magnetic field sensor, integrating over magnetic, plasmonic, and fiber-optic schemes, is highly sensitive to both the intensity and orientation of the magnetic field simultaneously, and holds potential in applications in many fields, such as medicine, industry, and the military.

Coreless side-polished fiber: a novel fiber structure for multimode interference and highly sensitive refractive index sensors.

A novel fiber structure, coreless side-polished fiber (CSPF), is proposed and investigated to implement multimode interference (MMI) and high sensitive refractive index (RI) sensors. For such CSPF, the part of the cladding and the core of a single-mode fiber are side-polished off so as to make the remained cladding a D-shaped multimode waveguide. The excitation and evolution of MMI in the CSPF are simulated numerically. The simulation results show that the high-order modes excited within the D-shaped multimode waveguide are mainly TE0,1 (TM0,1)~TE0,6 (TM0,6) modes. Moreover, the RI sensing characteristics and the influences of residual thickness and dip wavelength on the sensitivity are investigated both numerically and experimentally. The experimental results show that the CSPF with a residual thickness of 43.1 µm can reach an ultra-high sensitivity of 28000 nm/RIU in the RI range of 1.442~1.444. It is also found that the sensitivity can be further increased by reducing the residual thickness and choosing the dip at a longer wavelength. Thanks to the ultra-high RI sensitivity and the ease of fabrication, the CSPF could provide a cost-effective platform to build high-performance fiber devices of various functions.

Reduced graphene oxide wrapped on microfiber and its light-control-light characteristics.

Reduced graphene oxide (rGO) sheet wrapped on the tapered region of microfiber is demonstrated to enhance the interaction between rGO and strong evanescent field of optical fiber. The 405 nm and 980 nm lasers are employed to illuminate the rGO to investigate the response characteristics of the optical transmitted power (λ = 1550 nm) in the MF. The transmitted optical power of the MF with rGO changes with ~1.7 dB relative variation when the violet light is ranging from 0 mW to 12 mW (~0.21dB/mW) in the outside-pumped experiment. And in the inside-pumped experiment, the change of the 980 nm laser power from 0 mW to 156.5 mW makes ~6 dB relative variation power of the transmitted optical powers of the MF with rGO. These results indicate the optical transmitted power of the MF with wrapped rGO can be manipulated by the 405 and 980 nm light (order of mW), which signifies the device can potentially be applied as all optically and versatilely controllable devices.

Interlinked add-drop filter with amplitude modulation routing a fiber-optic microring to a lithium niobate microwaveguide.

We propose and experimentally demonstrate a new electro-optically controllable add-drop filter based on light coupling between a microfiber knot ring (MKR) and a lithium niobate (LN) microwaveguide. In our design, the MKR works as a resonator and routes the resonant light into the LN microwaveguide. The LN microwaveguide, as an excellent intermediary between electronics and optics, is a robust platform that not only enables stable support and manipulation of the MKR but also provides amplitude tunability taking advantage of its electro-optic property. Two add-drop filters with different diameters of the MKR, 1.12 mm, and 560 µm respectively, are studied, and a maximum amplitude tunability of ∼0.139 dB/V is obtained. The results show that this design can be a solution to interconnect a microstructured optical fiber with a microstructured on-chip device and provide an effective method to realize the active on-chip integration of the conventional fiber system.

Guided resonances on lithium niobate for extremely small electric field detection investigated by accurate sensitivity analysis.

We present a theoretical study of guided resonances (GR) on a thin film lithium niobate rectangular lattice photonic crystal by band diagram calculations and 3D Finite Difference Time Domain (FDTD) transmission investigations which cover a broad range of parameters. A photonic crystal with an active zone as small as 13µm×13µm×0.7µm can be easily designed to obtain a resonance Q value in the order of 1000. These resonances are then employed in electric field (E-field) sensing applications exploiting the electro optic (EO) effect of lithium niobate. A local field factor that is calculated locally for each FDTD cell is proposed to accurately estimate the sensitivity of GR based E-field sensor. The local field factor allows well agreement between simulations and reported experimental data therefore providing a valuable method in optimizing the GR structure to obtain high sensitivities. When these resonances are associated with sub-picometer optical spectrum analyzer and high field enhancement antenna design, an E-field probe with a sensitivity of 50 µV/m could be achieved. The results of our simulations could be also exploited in other EO based applications such as EEG (Electroencephalography) or ECG (Electrocardiography) probe and E-field frequency detector with an 'invisible' probe to the field being detected etc.

Fano resonance-based highly sensitive, compact temperature sensor on thin film lithium niobate.

In this Letter, we report a Fano resonance-based highly sensitive and compact temperature sensor fabricated on thin film lithium niobate (TFLN) Suzuki phase lattice (SPL) photonic crystal. The experimental sensitivity is estimated to be 0.77 nm/°C with a photonic crystal size of only 25 µm × 24 µm. This sensitivity is 38 times larger than the intrinsic one of lithium niobate which is 0.02 nm/°C. The demonstrated sharp and high extinction ratio characteristics of the Fano lineshape resonance could be an excellent candidate in developing a high sensitivity temperature sensor, electric field sensor, etc.

Optical characterization of ultra-short Bragg grating on lithium niobate ridge waveguide.

In this Letter, we report a technique to etch giant aspect ratio nanostructures in lithium niobate. An 8 µm long Bragg grating on a Ti:LiNbO3 ridge waveguide was fabricated by combining optical-grade dicing and focused ion beam milling. The reflectivity was evaluated using an optical coherence tomography system: it is measured to be 53% for the TM wave and 47% for the TE wave. We study by 2D-FDTD the modeled behavior of the electromagnetic field when an angle exists between two consecutive sidewalls of the grating in order to understand the difference between ideal Bragg grating and experimental samples. These simulations allow us to optimize the parameters in order to increase the reflection of the grating up to 80%.