A compact in-fiber refractive index (RI) sensor based on a step index multimode polymer optical fiber with a micro-hole drilled by a miniature numerical control machine is presented. A good linear relationship between the transmission and RI over a large operating range from 1.335 to 1.475 and a sensitivity of 36 071.43 mV/RIU (RI unit) are found. The relationship between the transmission and the RI of the hole depends on the micro-hole’s diameter and depth. The RI sensor developed in this letter is low-cost, easily fabricated, and capable of continuous measurement.

A robust cantilever-based push–pull 3-component (3-C) optical fiber accelerometer is proposed and experimentally demonstrated. Sensitivity and resonance frequency can be enhanced simultaneously by increasing the number of turns of an optical fiber without increasing the accelerometer size at the mass of a certain value. The calibration results show that axis sensitivity is 45 dB (re: 0 dB = 1 rad/g), with a fluctuation less than 0.9 dB in a frequency bandwidth of 10–450 Hz. The cross sensitivity is approximately 15 dB, with a fluctuation less than 1.2 dB in a frequency bandwidth of 10–450 Hz. The crosstalk reaches up to 30 dB. Fluctuation of the responses of the acceleration sensitivity of different components is less than 0.7 dB over a frequency bandwidth of 10 –450 Hz, which proves the good consistency of the 3-C optical fiber accelerometer. By usingan all-metal structure is expected to improve the reliability of the designed accelerometer for long-term use in harsh environments. These desirable features show that the proposed 3-C optical fiber accelerometer is satisfactory for seismic wave monitoringin oil and gas exploration.

The effects of optical losses on a directly-modulated radio-over-fiber (RoF) system used for distributed antenna networks are determined. The results show that with a properly designed bidirectional amplifier, the RoF link can tolerate over 20 and 16 dB of optical losses for down- and up-links, respectively. Simulation results are also consistent with the experimental data. These findings can contribute to the design of RoF distributed antenna systems with different topologies.

A novel photonic-assisted approach to microwave frequency measurement is proposed and experimentally demonstrated. The proposed scheme is based on the frequency-to-power mapping with different transmission responses. A polarizer is used in one output branch of a phase modulator to simultaneously implement phase modulation and intensity modulation. Owing to the complementary nature of the transmission responses and the Mach-Zehnder interferometers (MZIs), this scheme theoretically provides high resolution and tunable measurement range. The measurement errors in the experimental results can be kept within 0.2 GHz over a frequency range from 0.1 to 5.3 GHz.

We propose a high sensitivity sensor based on a mode number-encoded multi-longitudinal mode fiber laser. The fiber laser incorporates a uniform fiber Bragg grating (FBG) and a fiber Fabry-Perot interferometer (FFPI) as sensitive components in the cavity. The sensor counts the number of longitudinal modes (NLM) of fiber laser, which is caused by the mismatch between the reflection band of FBG and the transmission band of FFPI resulting from the application of external perturbation to the FBG. An electrical spectrum analyzer is adopted to analyze the NLM. The strain sensor is experimentally demonstrated to have sensitivity of as high as 0.02 \mu \varepsilon/mode.

Few-mode fiber Bragg grating (FM-FBG) has wide applications in the field of multi-wavelength fiber lasers and fiber-optic sensing. In this letter, FBG is successfully written in a novel type of FM fiber (FMF) based on 248-nm excimer laser and a uniform phase mask. A low-loss coupling between the FMF to a singlemode fiber is realized by a graded-index multi-mode fiber with a length of about a quarter pitch. The sensing properties of the FMF grating are further investigated. The experimental results indicate that the novel FM-FBG has sensitivities of temperature, strain, and bending of 11.2 pm/oC, 1.3 pm/\mu \varepsilon, and 636.9 pm/m-1, respectively.

We propose a scheme for mitigating Rayleigh backscattering noise and demodulating differential phase-shift keying (DPSK) signals in wavelength-division-multiplexed passive optical networks (WDM-PONs) with injection-locked Fabry-Perot laser diodes (FP-LDs). Signal demodulation and wavelength conversion are simultaneously realized on the basis of the frequency deviation and red shift of longitude modes in the FP-LDs. Experimental results demonstrate that the demodulation and wavelength conversion of 2.5-Gb/s DPSK signals are achieved. A power penalty of about 1.6 dB at a bit error rate of 10-9 is measured after transmission over 25-km single mode fiber.

We experimentally demonstrate a 4-Gb/s radio-over-fiber (RoF) system with 40-km fiber and 2-m wireless distance downstream at 100-GHz carrier. To the best of our knowledge, this is for the first time in China to realize optical wireless link at 100 GHz. In this letter, simple intensity modulator with direct detector (IM-DD) modulation is employed and optical power penalty after 40-km single mode fiber (SMF)-28 and 2-m air link is 3.2 dB with bit-error-rate (BER) at 1×10-9.

Research demonstrates that a Fresnel hologram can be generated and simultaneously encrypted numerically based on a secret symmetric key formed by the maximal length sequence (M-sequence). The method can be directly extended to encrypt a video holographic clip in a frame-by-frame manner. However, given the limited combination of signals in the family of M-sequence, hacking the secret key through trial and error can be time consuming but not difficult. In this letter, we propose a method that is difficult to crack with brute force for encrypting a holographic video sequence. An M-sequence is first randomly assigned to encrypt each frame of the holographic video signal. Subsequently, the index of the selected M-sequence, which is necessary to decrypt the hologram, is encrypted with the RSA algorithm before transmitting to the receiving end. At the receiving end, the decoder is provided with a private key to recover the index for each frame, and the corresponding M-sequence is used to decrypt the encoded hologram.

A compact line scanning quasi-confocal ophthalmoscope (LSO) is presented in this letter. Compared with a conventional scanning laser ophthalmoscope (SLO), the bench-top LSO significantly reduces the size, complexity, and cost of SLO utility with routine use. The LSO uses one moving part to produce high-contrast and high-resolution quasi-confocal images with nearly the same performance as a SLO. The LSO has a moderate field of view (~10o), which enables images of the macula, the optic nerve head, and other targets to be obtained more quickly and efficiently. An image of the optic nerve head is taken in a preliminary investigation on human subjects. Individual nerve fiber bundles and vessels are resolved at a shallow depth, with a lateral resolution of nearly 10 \mu m.

Specular and strong reflections are the main problems encountered during part image defect inspection of shiny or highly reflective surfaces. In this letter, we propose an improved illumination method for defect inspection. A diffuse light source is designed based on the physics analysis of light reflection. The distribution of intensity is simulated according to a known model to verify the illumination uniformity of the source. Experiments show that defect expressivity when using the proposed illumination method has a better performance. The optical model is not only suitable for the defect detection of metal balls but also for the defect detection of planes and cylinders.

Coherent diffraction imaging (CDI) and ptychography techniques bypass the difficulty of having highquality optics in X-ray microscopy by using a numerical reconstruction of the image that is obtained by inverting the diffracted intensity recorded by a charge-coupled device array. However, the reconstruction of the image from the intensity data obtained from a weakly diffracting specimen is known to be difficult because of the obvious reduction in signal-to-noise ratio (SNR). In this case, the specimen only slightly modifies the probe diffraction pattern, resulting in difficulty in the identification of the detailed structure of the specimen from the reconstructed image because of the poor contrast and sharpness of the image. To address this situation, a modification in the image retrieval algorithms used in the iterative reconstruction of the image is suggested. This modification should double the presence of high spatial frequencies in the diffraction pattern to enhance the contrast and edge detection in existing imaging techniques.

A colored object encoding scheme in a ghost imaging (GI) system using orbital angular momentum is investigated. A colored object is decomposed into three components and then each component is obtained in the idler arm using a multiple grayscale encoding scheme. Afterward, we synthesize the three reconstructed components into a colored image. The scheme is conducted and then presented through numerical simulations and experiments. The simulation result shows that the average peak signal-to-noise ratio (PSNR) is at 21.636 for the reconstructed color of the "Lena" image with 255 gray scales. The experiment also shows that the PSNR is 8.082 for the reconstructed color of the "NUPT" characters. The successful imaging of colored objects extends the further use of the GI technique.

An alternative technique for infrasound detection based on the self-mixing (SM) interference of a laser diode is described. The principle involved is the dependence of the power emitted by the laser diode on infrasound-induced membrane vibration. The Fourier transform and fringe-counting methods are used to analyze the self-mixing signal. Infrasound signals are experimentally measured from 2 to 20 Hz with a resolution of 0.25, and the results well agree with the theoretical ones.

A compact moving optical-wedge interferometer (CMOWI) is presented. This device consists of a moving optical wedge (MOW), a fixed optical wedge (FOW), a fixed compensating plate, and a beam-splitting cube. The optical path difference (OPD) is calculated and analyzed. The factor between the OPD and the displacement of the MOW is less than 1 if the refractive index and wedge angle of the MOW and FOW are chosen properly. Therefore, the CMOWI is insensitive to scanning speed variations compared with the traditional Michelson interferometer. The CMOWI is compact, small-sized, and suitable for low-resolution Fourier transform spectroscopy.

The phase noises of two narrow-linewidth fiber laser and laser diode are measured by using unbalanced Michelson interferometers with various optical path differences (OPDs). The measured results indicate that the phase noises of the two lasers do not change linearly with the OPD over the range from 1 to 100 m. The laser diode exhibits phase noise levels higher than that of the fiber laser at OPDs longer than 10 m. However, the laser diode outperforms the fiber laser at OPDs shorter than 10 m. The results obtained can assess laser performance and determine the suitable laser for use in a particular application.

Application of a thermal source in non-contact forming of sheet metal is known for some time. Replacement of this thermal source with a laser beam promises the much greater controllability of the process. To date, research focuses on dealing with rectangular plates, and only a few studies are presented for axis-symmetric geometries like circular plates. This study presents the work to get the dish or bowl shape by an initially flat circular plate. Two different scanning strategies circular and radial are attempted to get the desired dish shape. Following the unexpected distortion throughout the plate, a second series of experiments are conducted on a wide range of specimen geometries. An interesting phenomenon is observed. It is suggested that homogeneous dissemination of heat along with combined form of both of the scanning strategies, could have more potential to form dome shape.

As a new research area, laser surface wettability modification brings new applications for both laser and materials for industry. The picoseconds (10 ps) pulse laser surface micro-processing over alumina covered aluminium is researched. In the experiment, 10-ps laser pulse is employed and the energy threshold for different laser wavelength and repetition rate is measured. At the repetition rate of 5 kHz, the energy thresholds are: 1 064 nm: 4.0 \mu J/pulse, 532 nm: 2.8 \mu J/pulse, and 355 nm: 4.2 \mu J/pulse. Picoseconds pulse laser is demonstrated feasible in surface scanning at industrial level.

The ion substitution characteristics of Y3+-doped (Tb0.8Y0.2)3Al5O12 transparent ceramics synthesized by a solid-state reaction and vacuum sintering are investigated. The sample sintered at 1 680 oC exhibits the best optical properties, yielding a transmittance >75% from 900 to 1 600 nm. The Verdet constant of this sample at 632.8 nm is –108.79 rad.T-1.m-1. X-ray diffraction (XRD) results show that all of the samples have a pure garnet crystal structure without secondary phases. The microstructure of the samples reveals homogeneous grain sizes that averages <10 \mu m.

We present a method for the noninvasive measurement of blood glucose levels, which are determined by the ultrasound-modulated optical technique. The method is based on the optical scattering coefficient. A sensitivity analysis of the ultrasound-modulated light signals in a scattering medium is conducted. Glucose concentrations in intralipid and hemoglobin solutions are measured using the modulation depth of ultrasound-modulated scattered light. The effects of incident light intensity and sample temperature on the ultrasound-modulated signals are also estimated. Preliminary experimental results suggest that the proposed method is a promising technique for noninvasive blood glucose measurement.

Convolution kernel-based non-uniform fast Fourier transform (NUFFT) is an effective image reconstruction method for Fourier domain optical coherence tomography. By measuring the reconstruction error, a general method for finding the optimal parameters of the kernel function is investigated. Performances in terms of point spread function and computation time are evaluated. The NUFFT with optimal parameters yields signal sensitivity of over 40 dB, with a computation time that is decreased by 85% compared with the conventional oversampling NUFFT. In vivo images of finger tissue are efficiently reconstructed through the proposed reconstruction method.

Non-axisymmetric cavities are widely used as standard blackbody sources for radiation thermometry. The integrated effective emissivity is central to the blackbody design. Integrated effective emissivities are numerically calculated for non-isothermal, non-axisymmetric cavities. The average relative deviation is 0.087% when compared with Monte-Carlo results, indicating that this method can accurately calculate the integrated effective emissivities for non-isothermal, non-axisymmetric cavities. The effects of the wavelength, temperature uniformity, and bottom inclination angle are then analyzed.

We present linear conjugated combined aberration modes with a concentric pupil diameter of 4 mm. The combinations are according to the coupling relationship between Zernike modes over the concentric circle domain within the unit circle and the root mean square decreasing amplitude ratio of the corresponding aberration modes in the concentric pupil, in which the reconstruction pupil diameter is 6 mm. Each combined mode shares the characters of 2 radial orders apart, the same azimuth frequency, the same coefficient sign, and the prescribed amount, such as (C02 = 0.7\lambda, C04 = 0.3\lambda), (C-33 = 0.8\lambda, C-35 = 0.3\lambda), and so on. We also analyze the influence of the combined modes on optical quality. Simulations and experiments show improvement after combination; they also indicate that the influence of conjugated combination on optical quality has compensation and not superposition.

Surface-plasmon (SP) enhancement of amorphous-silicon-nitride (a-SiNx) light emission with single-layer gold (Au) waveguides is experimentally demonstrated through time-resolved photoluminescence measurement. The a-SiNx active layer with strong steady-state photoluminescence at 560 nm is prepared by plasma-enhanced chemical vapor deposition, and the Au waveguide on the top of the a-SiNx layer is fabricated by magnetron sputtering. The maximum Purcell factor value of ~3 is achieved with identified SP resonance of the Au waveguide at ~530 nm.

We report an experimental observation of the optical transparency enhancement resonances in Hanle-electromagnetically induced transparency (EIT) configuration with a microwave field excitation. In this experimental system, a strong control field and a weak probe field form three-level -type configurations of EIT. The fourth level is coupled by an additional microwave field. With the microwave field excitation, the probe field undergoes a process of absorption to transparency when the probe field decreases. Compared with the EIT effect, this result indicates optical transparency enhancement. A simple theoretical model and a numerical simulation are presented to explain the observed experimental results. The applications of these optical features are also discussed.