D-shaped photonic crystal fiber sensor based on the surface plasmon resonance effect for refractive index detection.

In this paper, a photonic crystal fiber (PCF) sensor based on the surface plasmon resonance (SPR) effect for refractive index (RI) detection is proposed. We design a D-shaped polished PCF structure consisting of air holes arranged in a hexagonal lattice. The silver film is coated on the middle channel of the polished surface of the PCF. The finite element method is used to analyze the propagation characteristics of the proposed D-shaped SPR-PCF sensor. Simulation results show that the proposed D-shaped SPR-PCF sensor has a maximum wavelength sensitivity of 30,000 nm/RIU, an average wavelength sensitivity of 6785.71 nm/RIU, and a maximum resolution of 3.33×10-6 R I U in the RI range of 1.22-1.36. Owing to the high wavelength sensitivity in the considered RI range, the proposed D-shaped SPR-PCF sensor is suitable for applications in water contamination detection, liquid concentration measurement, food safety monitoring, etc.

Binocular holographic display based on the holographic optical element.

Due to the limited pixel pitch of the spatial light modulator (SLM), the field of view (FOV) is insufficient to meet binocular observation needs. Here, an optimized controlling light method of a binocular holographic three-dimensional (3D) display system based on the holographic optical element (HOE) is proposed. The synthetic phase-only hologram uploaded onto the SLM is generated with the layer-based angular spectrum diffraction theory, and two different reference waves are introduced to separate the left view and the right view of the 3D scene. The HOE with directional controlling light parameters is employed to guide binocular information into the left-eye and the right-eye viewing zones simultaneously. Optical experiments verify that the proposed system can achieve binocular holographic augmented reality 3D effect successfully with real physical depth, which can eliminate the accommodation-vergence conflict and visual fatigue problem. For each perspective, the FOV is 8.7° when the focal length of the HOE is 10 cm. The width of the viewing zone is 2.3 cm when the viewing distance is 25 cm.

Highly coherent and multi-octave mid-infrared supercontinuum generations in a reverse-strip AlGaAs waveguide with three zero-dispersion wavelengths.

In this paper, a reverse-strip AlGaAs waveguide with three zero-dispersion wavelengths (ZDWs) is designed. The corresponding three ZDWs are located at 3.74, 6.56, and 8.89 µm. The nonlinearity coefficient of the proposed reverse-strip AlGaAs waveguide is calculated as 2.09W-1m-1 at wavelength 4.9 µm. The effects of pump pulse parameters, waveguide length, and noise coefficient on the nonlinear dynamics of supercontinuum (SC) generation are investigated. When the hyperbolic secant pump pulse with a wavelength of 4.9 µm, peak power of 900 W, and duration of 100 fs is launched into the proposed waveguide and propagated after a 3 mm length, highly coherent and multi-octave mid-infrared (MIR) SC spanning from 2.2 to 14.5 µm (more than 2.7 octaves, at -40dB level) is generated. Finally, a possible fabrication process of the reverse-strip AlGaAs waveguide is introduced. Our research results have important applications in MIR photonics, MIR spectroscopy, optical precision measurement, and more.

A Novel Gold Film-Coated V-Shape Dual-Core Photonic Crystal Fiber Polarization Beam Splitter Covering the E + S + C + L + U Band.

In this paper, a novel gold film-coated V-shape dual-core photonic crystal fiber (V-DC-PCF) polarization beam splitter (PBS) based on surface plasmon resonance effect is proposed. The coupling lengths of the X-polarization (X-pol) and Y-polarization (Y-pol) and the corresponding coupling length ratio of the proposed V-DC-PCF PBS without gold film and with gold film are compared. The fiber structure parameters and thickness of the gold film are optimized through investigating their effects on the coupling lengths and coupling length ratio. As the propagation length increases, the normalized output powers of the X-pol and Y-pol of the proposed V-DC-PCF PBS at the three wavelengths 1.610, 1.631, and 1.650 µm are demonstrated. The relationships between the extinction ratio (ER), insertion loss (IL) and wavelength for the three splitting lengths (SLs) 188, 185, and 182 µm are investigated. Finally, it is demonstrated that for the proposed V-DC-PCF PBS, the optimal SL is 188 µm, the ILs of the X-pol and Y-pol are less than 0.22 dB, and the splitting bandwidth (SB) can cover the E + S + C + L + U band. The proposed V-DC-PCF PBS has the ultra-short SL, ultra-wide SB, and ultra-low IL, so it is expected to have important applications in the laser, sensing, and dense wavelength division multiplexing systems.

Hollow-Core Negative Curvature Fiber with High Birefringence for Low Refractive Index Sensing Based on Surface Plasmon Resonance Effect.

In this paper, a hollow-core negative curvature fiber (HC-NCF) with high birefringence is proposed for low refractive index (RI) sensing based on surface plasmon resonance effect. In the design, the cladding region of the HC-NCF is composed of only one ring of eight silica tubes, and two of them are selectively filled with the gold wires. The influences of the gold wires-filled HC-NCF structure parameters on the propagation characteristic are investigated by the finite element method. Moreover, the sensing performances in the low RI range of 1.20-1.34 are evaluated by the traditional confinement loss method and novel birefringence analysis method, respectively. The simulation results show that for the confinement loss method, the obtained maximum sensitivity, resolution, and figure of merit of the gold wires-filled HC-NCF-based sensor are -5700 nm/RIU, 2.63 × 10-5 RIU, and 317 RIU-1, respectively. For the birefringence analysis method, the obtained maximum sensitivity, resolution, and birefringence of the gold wires-filled HC-NCF-based sensor are -6100 nm/RIU, 2.56 × 10-5 RIU, and 1.72 × 10-3, respectively. It is believed that the proposed gold wires-filled HC-NCF-based low RI sensor has important applications in the fields of biochemistry and medicine.

Viewing-angle and viewing-resolution enhanced integral imaging based on time-multiplexed lens stitching.

A method for the viewing angle and viewing resolution enhancement of integral imaging (InIm) based on time-multiplexed lens stitching is demonstrated using the directional time-sequential backlight (DTS-BL) and the compound lens-array. In order to increase the lens-pitch of the compound lens-array for enlarging the viewing angle of InIm, DTS-BL is used to continuously stitch the adjacent elemental lenses in the time-multiplexed way. Through the compound lens-array with two pieces of lens in each lens unit, the parallel light beams from the DTS-BL converge and form a uniformly distributed dense point light source array (PLSA). Light rays emitting from the PLSA are modulated by the liquid crystal display (LCD) panel and then integrated as volumetric pixels of the reconstructed three-dimensional (3D) image. Meanwhile, time-multiplexed generation of the point light sources (PLSs) in the array is realized by time-multiplexed lens stitching implemented with the DTS-BL. As a result, the number of the PLSs, as the pixels of the perceived 3D image, is increased and then the viewing resolution of the 3D image is obviously enhanced. Additionally, joint optical optimization for the DTS-BL and the compound lens-array is used for suppressing the aberrations, and the imaging distortion can be decreased to 0.23% from 5.80%. In the experiment, a floating full-parallax 3D light-field image can be perceived with 4 times the viewing resolution enhancement in the viewing angle of 50°, where 7056 viewpoints are presented.

A flipping-free 3D integral imaging display using a twice-imaging lens array.

Integral imaging is a promising 3D visualization technique for reconstructing 3D medical scenes to enhance medical analysis and diagnosis. However, the use of lens arrays inevitably introduces flipped images beyond the field of view, which cannot reproduce the correct parallax relation. To avoid the flipping effect in optical reconstruction, a twice-imaging lens array based integral display is presented. The proposed lens arrangement, which consists of a light-controlling lens array, a field lens array and an imaging lens array, allows the light rays from each elemental image only pass through its corresponding lens unit. The lens arrangement is optimized with geometrical optics method, and the proposed display system is experimentally demonstrated. A full-parallax 3D medical scene showing continuous viewpoint information without flipping is reconstructed in 45° field of view.

Highly coherent supercontinuum generation in a polarization-maintaining CS2-core photonic crystal fiber.

In this paper, we design a polarization-maintaining CS2-core photonic crystal fiber (PM-CCPCF). The two air holes in the x direction are infiltrated with C2H5OH in order to introduce birefringence. By optimizing the structure parameters of the PM-CCPCF, it is demonstrated that the x-polarization fundamental mode has an all-normal dispersion profile and the corresponding y-polarization fundamental mode has an anomalous dispersion profile for a pump wavelength of 1.76 µm. Then, we investigate the supercontinuum (SC) generations when different fiber lengths, pump peak powers, and pump pulse widths are chosen, respectively. Simulation results show that for the x-polarization and y-polarization fundamental modes, highly coherent SCs can be generated by appropriately choosing the fiber length and pump pulse parameters. Finally, nonlinear propagation dynamics are analysed when the optimized fiber length and pump pulse parameters are used. The bandwidth of the SCs generated for the x-polarization and y-polarization fundamental modes can be up to 0.82 and 1.26 octave, respectively.

Multi-octave mid-infrared supercontinuum and frequency comb generation in a suspended As2Se3 ridge waveguide.

In this paper, we numerically investigate the mid-infrared supercontinuum (SC) generation in a suspended ${{\rm As}_2}{{\rm Se}_3}$As2Se3 ridge waveguide, which is designed with the two zero-dispersion wavelengths. Simulation results show that when the pump pulses at wavelength 3.3 µm with width of 100 fs and peak power of 900 W are launched into the anomalous dispersion region of the designed waveguide with a length of 0.87 mm, the SC can be generated in the wavelength range from 1.76 to 14.42 µm (more than three octaves), extending deep into the "fingerprint" region. The stability of the generated SC is confirmed by the first-order coherence. Moreover, we demonstrate the performance of the SC-based frequency comb by assuming a 50 pulse pump source at a repetition rate of 100 MHz.

A crosstalk-suppressed dense multi-view light-field display based on real-time light-field pickup and reconstruction.

A crosstalk-suppressed dense multi-view light-field display based on real-time light-field pickup and reconstruction is demonstrated, which is capable of realizing high view density in the horizontal direction with low crosstalk between micro-pitch viewing zones. The micro-pinhole unit array and the vertically-collimated backlight are specially developed and used, instead of refraction-based optical components like lenticular lens, to avoid aberrations and to suppress crosstalk for accurately projecting multiple view perspectives into each eye pupil of the viewer. Additionally, the spatial information entropy is defined and investigated to improve 3D image perception for balancing resolution, which can be generally applicable to better-reconstructed 3D images with the limited number of resolution pixels. To enlarge the viewing angle of 3D images with smooth motion parallax, the novel high-efficient light-field pickup and reconstruction method based on the real-time position of the viewer's pupils is implemented with an eye tracker to scan 750 view perspectives with the correct geometric occlusion in real time at the frame rate of 40 fps. In the experiment, a floating horizontal-parallax 3D light-field image with the view density of 0.75 mm-1 and the micro-pitch crosstalk of less than 7% can be perceived with the clear floating focus depth of 10 cm and the high resolution of 1920 × 1080 in the viewing angle of 70°.

Wavefront aberration correction for integral imaging with the pre-filtering function array.

In integral imaging, the quality of a reconstructed image degrades with increasing viewing angle due to the wavefront aberrations introduced by the lens-array. A wavefront aberration correction method is proposed to enhance the image quality with a pre-filtering function array (PFA). To derive the PFA for an integral imaging display, the wavefront aberration characteristic of the lens-array is analyzed and the intensity distribution of the reconstructed image is calculated based on the wave optics theory. The minimum mean square error method is applied to manipulate the elemental image array (EIA) with a PFA. The validity of the proposed method is confirmed through simulations as well as optical experiments. A 45-degree viewing angle integral imaging display with enhanced image quality is achieved.

Deterministic generation of single soliton Kerr frequency comb in microresonators by a single shot pulsed trigger.

Kerr soliton frequency comb generation in monolithic microresonators recently attracted great interests as it enables chip-scale few-cycle pulse generation at microwave rates with smooth octave-spanning spectra for self-referencing. Such versatile platform finds significant applications in dual-comb spectroscopy, low-noise optical frequency synthesis, coherent communication systems, etc. However, it still remains challenging to straightforwardly and deterministically generate and sustain the single-soliton state in microresonators. In this paper, we propose and theoretically demonstrate the excitation of single-soliton Kerr frequency comb by seeding the continuous-wave driven nonlinear microcavity with a pulsed trigger. Unlike the mostly adopted frequency tuning scheme reported so far, we show that an energetic single shot pulse can trigger the single-soliton state deterministically without experiencing any unstable or chaotic states. Neither the pump frequency nor the cavity resonance is required to be tuned. The generated mode-locked single-soliton Kerr comb is robust and insensitive to perturbations. Even when the thermal effect induced by the absorption of the intracavity light is taken into account, the proposed single pulse trigger approach remains valid without requiring any thermal compensation means.

Simultaneous Vector Bend and Temperature Sensing Based on a Polymer and Silica Optical Fibre Grating Pair.

The bending response of polymer optical fibre Bragg grating (POFBG) and silica optical fibre Bragg grating (SOFBG) mounted on a brass beam have been systematically studied and compared. The results indicate that POFBG has higher (almost twice as much) bend sensitivity than SOFBG. Based on the difference between the bend and temperature sensitivity of POFBG and SOFBG, a new method of measuring vector bend and temperature simultaneously was proposed by using a hybrid sensor head with series connection of one POFBG and one SOFBG with different Bragg wavelengths. It provides high sensitivity and resolution for sensing bend and temperature changes simultaneously and independently. The proposed sensor can find some applications in the fields where high sensitivity for both bend and temperature measurements are required.

Image quality improvement of multi-projection 3D display through tone mapping based optimization.

An optical 3D screen usually shows a certain diffuse reflectivity or diffuse transmission, and the multi-projection 3D display suffers from decreased display local contrast due to the crosstalk of multi-projection contents. A tone mapping based optimizing method is innovatively proposed to suppress the crosstalk and improve the display contrast by minimizing the visible contrast distortions between the display light field and a targeted one with enhanced contrast. The contrast distortions are weighted according to the visibility predicted by the model of human visual system, and the distortions are minimized for the given multi-projection 3D display model that enforces constrains on the solution. Our proposed method can adjust parallax images or parallax video contents for the optimum 3D display image quality taking into account the display characteristics and ambient illumination. The validity of the method is evaluated and proved in experiments.

Experimental generation of discrete ultraviolet wavelength by cascaded intermodal four-wave mixing in a multimode photonic crystal fiber.

In this Letter, we demonstrate experimentally for the first time, to the best of our knowledge, discrete ultraviolet (UV) wavelength generation by cascaded intermodal FWM when femtosecond pump pulses at 800 nm are launched into the deeply normal dispersion region of the fundamental guided mode of a multimode photonic crystal fiber (MPCF). For pump pulses at average input powers of Pav=450, 550, and 650 mW, the first anti-Stokes waves are generated at the visible wavelength of 538.1 nm through intermodal phase matching between the fundamental and second-order guided mode of the MPCF. The first anti-Stokes waves generated then serve as the secondary pump for the next intermodal FWM process. The second anti-Stokes waves in the form of the third-order guided mode are generated at the UV wavelength of 375.8 nm. The maximum output power is above 10 mW for Pav=650 mW. We also confirm that the influences of fiber bending and intermodal walk-offs on the cascaded intermodal FWM-based frequency conversion process are negligible.

Polarization-dependent intermodal four-wave mixing in a birefringent multimode photonic crystal fiber.

In this Letter, polarization-dependent intermodal four-wave mixing (FWM) is demonstrated experimentally in a birefringent multimode photonic crystal fiber (BM-PCF) designed and fabricated in-house. Femtosecond pump pulses at wavelengths ∼800 nm polarized along one of the principal axes of the BM-PCF are coupled into a normal dispersion region away from the zero-dispersion wavelengths of the fundamental guided mode of the BM-PCF. Anti-Stokes and Stokes waves are generated in the 2nd guided mode at visible and near-infrared wavelengths, respectively. For pump pulses at an average input power of 500 mW polarized along the slow axis, the conversion efficiencies ηas and ηs of the anti-Stokes and Stokes waves generated at wavelengths 579.7 and 1290.4 nm are 19% and 14%, respectively. For pump pulses polarized along the fast axis, the corresponding ηas and ηs at 530.4 and 1627 nm are 23% and 18%, respectively. We also observed that fiber bending and intermodal walk-off have a small effect on the polarization-dependent intermodal FWM-based frequency conversion process.

High Sensitivity Refractometer Based on Reflective Smf-Small Diameter No Core Fiber Structure.

A high sensitivity refractive index sensor based on a single mode-small diameter no core fiber structure is proposed. In this structure, a small diameter no core fiber (SDNCF) used as a sensor probe, was fusion spliced to the end face of a traditional single mode fiber (SMF) and the end face of the SDNCF was coated with a thin film of gold to provide reflective light. The influence of SDNCF diameter and length on the refractive index sensitivity of the sensor has been investigated by both simulations and experiments, where results show that the diameter of SDNCF has significant influence. However, SDNCF length has limited influence on the sensitivity. Experimental results show that a sensitivity of 327 nm/RIU (refractive index unit) has been achieved for refractive indices ranging from 1.33 to 1.38, which agrees well with the simulated results with a sensitivity of 349.5 nm/RIU at refractive indices ranging from 1.33 to 1.38.

High sensitivity sol-gel silica coated optical fiber sensor for detection of ammonia in water.

A high sensitivity ammonia sensor based on a tapered small core singlemode fiber (SCSMF) structure for measurement of ammonia concentration in water is reported. Two tapered SCSMF fiber structures with different waist diameters of 23 µm and 13.5 µm are fabricated by using a customized microheater brushing technique. The silica based material prepared by the sol-gel method is used as a coating applied to the surface of the tapered fiber structures. To investigate the influence of the coating thickness on the sensitivity to ammonia in water, silica coatings with different thicknesses (2-pass and 8-pass coatings) are deposited on the surface of the fiber sensor with a waist diameter of 23 µm. Experiments demonstrate that the sensor with a thicker (8-pass) silica coating shows better sensitivity of 0.131 nm/ppm to ammonia compared to that of 0.069 nm/ppm for the thinner silica coating (2-pass). To further improve the sensor sensitivity, the taper waist diameter is reduced. For an 8-pass coating (249nm at the taper waist section) applied to a tapered SCSMF structure based fiber sensor with a reduced waist diameter of 13.5 µm. Experimental results show that the sensitivity to ammonia is significantly improved to 2.47nm/ppm. The best measurement resolution for ammonia concentration in water is estimated to be 4 ppb while the response and recovery times are less than 2 and 5 minutes respectively. The proposed sensor also offers good performance in terms of repeatability and good selectivity for sensing ammonia compared to that of other common ions and organic molecules in water.

Spectrally-isolated violet to blue wavelength generation by cascaded degenerate four-wave mixing in a photonic crystal fiber.

Generation of spectrally-isolated wavelengths in the violet to blue region based on cascaded degenerate four-wave mixing (FWM) is experimentally demonstrated for the first time in a tailor-made photonic crystal fiber, which has two adjacent zero dispersion wavelengths (ZDWs) at 696 and 852 nm in the fundamental mode. The influences of the wavelength λp and the input average power Pav of the femtosecond pump pulses on the phase-matched frequency conversion process are studied. When femtosecond pump pulses at λp of 880, 870, and 860 nm and Pav of 500 mW are coupled into the normal dispersion region close to the second ZDW, the first anti-Stokes waves generated near the first ZDW act as a secondary pump for the next FWM process. The conversion efficiency ηas2 of the second anti-Stokes waves, which are generated at the violet to blue wavelengths of 430, 456, and 472 nm, are 4.8, 6.48, and 9.66%, for λp equalling 880, 870, and 860 nm, respectively.

Integrated label-free optical biochemical sensor with a large measurement range based on an angular grating-microring resonator.

We propose and design a photonic-integrated optical biochemical sensor, which comprises a microring resonator and angular gratings in a silicon-on-insulator waveguide. With the combination of the angular gratings, the measurement range of the angular grating-microring resonator-based sensor significantly increases without the restriction of a free spectral range. Optimization of the several key structural parameters is investigated to achieve favorable transmission properties. A high-quality factor of more than 1.03×105 can meet the requirements of high sensitivity and low detection limit. The simulation results on the biochemical bulk sensing show that a concentration sensitivity of more than 95.27 pm/% and detection limit of less than 0.329% can be obtained. A large measurement range of 50.2 nm is achieved by the combination of the angular gratings. The investigation on the combination of microring resonator and angular grating is a valuable exploration of the liquid and gas biomedical sensing for the ultra-large measurement range.