Yuanhua Feng

Scanning-free BOTDA based on ultra-fine digital optical frequency comb.

We realize a scanning-free Brillouin optical time domain analyzer (BOTDA) based on an ultra-fine digital optical frequency comb (DOFC) with 1.95MHz frequency spacing and 2GHz bandwidth. The DOFC can be used to reconstruct the Brillouin gain spectrum (BGS) and locate the Brillouin frequency shift (BFS) without frequency scanning and thus can improve the measurement speed about 100 times compared with the conventional BOTDA. This scanning-free BOTDA scheme has also been demonstrated experimentally with 51.2m spatial resolution over 10km standard single mode fiber (SSMF) and with resolution of 1.5°C for temperature and 43.3με for strain measurement respectively.

120 Gbit/s 2 × 2 Vector-Modes-Division- Multiplexing DD-OFDM-32QAM Free-Space Transmission

We demonstrate a 120-Gbit/s mode-division-multiplexing (MDM) system based on two typical vector modes of TE01 and TM01 with direct detection orthogonal frequency division multiplexing (DD-OFDM) and 32-quadrature-amplitude-modulation (32-QAM) signal. The vector mode conversion is achieved by the key q-plate and the mode crosstalk between the converted two vector modes are both less than -20 dB. This crosstalk can be further minimized by high-quality q-plate. In this demonstration, error-free transmission has been realized with the power penalties less than 2 dB. The experimental results show that the scheme proposed in this paper can be a good candidate in large-capacity short-reach optical interconnect.

Thermally triggered fiber lasers based on secondary-type-In Bragg gratings

The secondary-type-In grating formed in a small-core photosensitivity active fiber is discovered and investigated. Due to the different grating types, the transmission dip of a secondary grating structure chases and integrates with the type-In grating structure as the temperature increases, which strengthens the reflectivity of the grating. By use of these secondary-type-In gratings as Bragg reflectors, a thermally activated distributed Bragg reflector (DBR) fiber laser is proposed, which can be potentially used in high-temperature alarms and sensors.

High-Temperature Sensor Based on Fabry-Perot Interferometer in Microfiber Tip

A miniaturized tip Fabry-Perot interferometer (tip-FPI) is proposed for high-temperature sensing. It is simply fabricated for the first time by splicing a short length of microfiber (MF) to the cleaved end of a standard single mode fiber (SMF) with precise control of the relative cross section position. Such a MF acts as a Fabry-Perot (FP) cavity and serves as a tip sensor. A change in temperature modifies the length and refractive index of the FP cavity, and then a corresponding change in the reflected interference spectrum can be observed. High temperatures of up to 1000 °C are measured in the experiments, and a high sensitivity of 13.6 pm/°C is achieved. This compact sensor, with tip diameter and length both of tens of microns, is suitable for localized detection, especially in harsh environments.

Optical digital coherent detection technology enabled flexible and ultra-fast quantitative phase imaging.

Quantitative phase imaging has been an important labeling-free microscopy modality for many biomedical and material science applications. In which, ultra-fast quantitative phase imaging is indispensable for dynamic or transient characteristics analysis. Conventional wide field optical interferometry is a common scheme for quantitative phase imaging, while its data acquisition rate is usually hindered by the frame rate of arrayed detector. By utilizing novel balanced-photo-detector based digital optics coherent detection techniques, we report on a method of constructing ultra-fast quantitative phase microscopy at the line-scan rate of 100 MHz with ~2 μm spatial resolution.

Mode Multiplexing and High Efficient Switching in Few-Mode Fiber Based on Modeled Blazed Grating

We propose a novel approach based on modeled blazed grating (MBG) to realize the mode conversion, multiplexing, demultiplexing, and switching in few-mode transmission fibers. The proposed grating can distribute different modes to different diffraction orders. We demonstrate the generation, multiplexing, demultiplexing, and high efficient conversion and switching of independent coaxial modes in a few-mode transmission fiber using MBG experimentally.

Ultrasensitive Mach-Zehnder Interferometric Temperature Sensor Based on Liquid-Filled D-Shaped Fiber Cavity

A liquid-filled D-shaped fiber (DF) cavity serving as an in-fiber Mach–Zehnder interferometer (MZI) has been proposed and experimentally demonstrated for temperature sensing with ultrahigh sensitivity. The miniature MZI is constructed by splicing a segment of DF between two single-mode fibers (SMFs) to form a microcavity (MC) for filling and replacement of various refractive index (RI) liquids. By adjusting the effective RI difference between the DF and MC (the two interference arms), experimental and calculated results indicate that the interference spectra show different degrees of temperature dependence. As the effective RI of the liquid-filled MC approaches that of the DF, temperature sensitivity up to −84.72 nm/°C with a linear correlation coefficient of 0.9953 has been experimentally achieved for a device with the MC length of 456 μm, filled with liquid RI of 1.482. Apart from ultrahigh sensitivity, the proposed MCMZI device possesses additional advantages of its miniature size and simple configuration; these features make it promising and competitive in various temperature sensing applications, such as consumer electronics, biological treatments, and medical diagnosis.

All-fiber second-order optical vortex generation based on strong modulated long-period grating in a four-mode fiber

We propose an effective all-fiber method to generate a high-order optical vortex (OV) via twisting a strong modulated long-period fiber grating (LPFG) written in a four-mode fiber (4MF). With a special design and optimization of the procedures of CO2-laser irradiation, an LPFG with strong period deformation is achieved in the 4MF. Based on this LPFG, we can directly convert the linear polarization (LP) fiber fundamental mode (LP01) to the high-order LP core mode (LP21) with efficiency of 99.7% and then transform the LP21 mode into a high-order OV mode (±2 order). This is the first time, to the best of our knowledge, that ±2-order OV modes have been experimentally generated with just one fiber grating in an all-fiber-system.

Micrometer-resolution in-fiber OCT probe with tunable working distance.

Optical coherence tomography (OCT) is an attractive modality in biomedical imaging systems due to its non-invasive imaging character. Since the image quality of OCT may be limited by the decrease of transverse resolution away from the focus spot, working distance tunable probe can be a strategy to overcome such limitation and maintain high transverse resolution at different imaging depths. In this paper, a miniature, working distance-tunable in-fiber OCT probe is demonstrated. The influences of the graded index fiber (GIF) length as well as the air cavity length on the working distance and the transverse resolution are simulated and discussed. Experimental results prove that the working distance can be tuned freely from 337.31 μm to 22.28 μm, producing the transverse resolution from 13.86 μm to 3.6 μm, which are in good agreement with the simulated results. The application of the probe in an OCT system for imagining a standard USAF resolution target is investigated in detail. The best resolutions for the standard USAF resolution target imaging are 4.9 μm and 6.9 μm in horizontal and vertical direction, respectively.