Hongdan Wan

An Injection-Locked Single-Longitudinal-Mode Fiber Ring Laser With Cylindrical Vector Beam Emission

We demonstrate a fiber ring laser with narrow bandwidth single-longitudinal-mode cylindrical vector beam (CVB) output at C-band wavelength range for the first time to the best of our knowledge. A step index two-mode fiber Bragg grating is used as a transverse mode selector for CVB generation, while both the injection-locking technique and narrow bandwidth of the fiber Bragg grating lead to single-longitudinal-mode operation. The 3-dB bandwidth of the laser output is measured to be <0.01 nm near a single wavelength of 1551.4 nm with a signal-to-background ratio of >60 dB. Mode distribution and optical spectra of few-mode fibers with periodic modulated refractive index profile, namely the few-mode fiber Bragg gratings with bent radius, are investigated theoretically and experimentally, which provide a comprehensive exploration of CVBs generation.

High efficiency mode-locked, cylindrical vector beam fiber laser based on a mode selective coupler

We propose and demonstrate an all-fiber passively mode-locked laser with a figure-8 cavity, which generates pulsed cylindrical vector beam output based on a mode selective coupler (MSC). The MSC made of a two mode fiber and a standard single mode fiber is used as both the intracavity transverse mode converter and mode splitter with a low insertion loss of about 0.65 dB. The slope efficiency of the fiber laser is > 3%. Through adjusting the polarization state in the laser cavity, both radially and azimuthally polarized beams have been obtained with high mode purity which are measured to be > 94%. The laser operates at 1556.3 nm with a spectral bandwidth of 3.2 nm. The mode-locked pulses have duration of 17 ns and a repetition rate of 0.66 MHz.

Switchable Dual-Wavelength Cylindrical Vector Beam Generation From a Passively Mode-Locked Fiber Laser Based on Carbon Nanotubes

Cylindrical vector beams (CVBs) with axial symmetry in both polarization and field intensity have attracted much attention because of their unique optical properties. Conventional methods to obtain CVBs including direct modulation of light beams in free space and high-order mode excitation by offset splicing single-mode fiber with few-mode fiber usually works at single wavelength with rather narrow bandwidth. Here, for the first time to the best of our knowledge, we demonstrate switchable dual-wavelength CVB generation from a passively mode-locked fiber laser using carbon nanotubes as saturable absorber for mode-locking and a home-made mode-selective coupler as both mode converter and birefringence filter. In experiments, the mode-locked fiber laser delivers CVB pulses of dual-wavelength (1532.5 nm and 1555.5 nm) and corresponding single wavelength with duration of hundreds of femtosecond, respectively. Moreover, the output polarization status is switchable between radially and azimuthally polarized states. The mode-locked CVBs with wavelength and polarization flexibility may have potential applications in mode-division multiplexing optical fiber communication, nanoparticle manipulation, material processing, nonlinear optics, and so on.

Solid Optical Fiber With Tunable Bandgaps Based on Curable Polymer Infiltrated Photonic Crystal Fiber

We demonstrated the realization and characterization of a solid photonic bandgap fiber (SPBF) with a compact size of about 10 mm and a high wavelength sensitivity of up to -4.034 nm/°C by means of fully infiltrating an ultraviolet curable polymer with a high refractive index of 1.515 into air holes of a photonic crystal fiber (PCF). To the best of our knowledge, it was the first time that the SPBF with tunable bandgaps was fabricated in the conventional index-guiding PCF. Compared with conventional fluid filled PBFs, the proposed SPBF can be stable to temperature and other environmental effects and maintain a large extinction ratio of more than 30 dB within a broad wavelength. The splicing between the SPBF and single mode fibers has been solved. Moreover, it is observed that the bandwidth of bandgap (G2) gradually broadens with the increase in temperature.

Focus modulation of cylindrical vector beams by using 1D photonic crystal lens with negative refraction effect.

Sub-wavelength focusing of cylindrical vector beams (CVBs) has attracted great attention due to the specific physical effects and the applications in many areas. More powerful, flexible and effective ways to modulate the focus transversally and also longitudinally are always being pursued. In this paper, cylindrically symmetric lens composed of negative-index one-dimensional photonic crystal is proposed to make a breakthrough. By revealing the relationship between focal length and the exit surface shape of the lens, a quite simple and effective principle of designing the lens structure is presented to realize specific focus modulation. Plano-concave lenses are parameterized to modulate the focal length and the number of focuses. An axicon constructed by one-dimensional photonic crystal is proposed for the first time to obtain a large depth of focus and an optical needle focal field with almost a theoretical minimum FWHM of 0.362λ is achieved under radially polarized incident light. Because of the almost identical negative refractive index for TE and TM polarization states, all the modulation methods can be applied for any arbitrary polarized CVBs. This work offers a promising methodology for designing negative-index lenses in related application areas.

Ultra-narrow-band optical filter based on whispery-gallery-mode hybrid-microsphere-cavity

We demonstrate an ultra-narrow-band mode-selection method based on a hybrid-microsphere-cavity which consists of a coated silica microsphere. Optical field distribution and narrow-band transmission spectrum of the whispering gallery modes (WGM) are investigated by finite-difference time-domain method. WGM transmission spectra are measured for microsphere and tapered fibers with different diameters. A high refractive index layer coated on the microsphere-cavity make the Q factor increased, the transmission spectrum bandwidth compressed and the side-mode suppression ratio increased. Parameters of the hybrid-microsphere-cavity, namely, the coated shell thickness and its refractive index are optimized under different excitation light source as to investigate the whispering-gallery-modes’ transmission spectrum. The 3dB bandwidth of the proposed filter can be less than MHz which will have great potential for applications in all-optical sensing and communication systems.

Novel wireless sensor system based on power-over-fiber technique

In this paper, we develop a novel wireless sensor system based on power-over-fiber technique, to realize a flexible distributed sensing over a middle distance, especially under extreme environments like high voltage, strong magnetic field, flammable and explosive occasion. In this system, the optical energy out of a high power laser in the base-station is transmitted via a fiber and then converted into the electrical energy by a photovoltaic power converter in the remote unit. This optically power-supplied remote unit operates as the coordinator in the wireless sensor network (WSN) at the remote site, and exchanges the sensing information with the base station via another fiber. In this way, the sensing information can be collected by WSN at the remote site, and be transmitted over a middle distance with the fiber. In order to improve the power supply ability of PPC, a maximum power point tracking technique is applied and more than 80% of PPCs maximum output power can be extracted. Moreover, to reduce the power consumption of the remote unit and sensor nodes, a simple and stable low-power communication protocol is designed. This novel sensor system can collect, transmit, and process the sensing information over 2 kms distance.

Optically powered active sensing system for Internet Of Things

Internet Of Things (IOT) drives a significant increase in the extent and type of sensing technology and equipment. Sensors, instrumentation, control electronics, data logging and transmission units comprising such sensing systems will all require to be powered. Conventionally, electrical powering is supplied by batteries or/and electric power cables. The power supply by batteries usually has a limited lifetime, while the electric power cables are susceptible to electromagnetic interference. In fact, the electromagnetic interference is the key issue limiting the power supply in the strong electromagnetic radiation area and other extreme environments. The novel alternative method of power supply is power over fiber (PoF) technique. As fibers are used as power supply lines instead, the delivery of the power is inherently immune to electromagnetic radiation, and avoids cumbersome shielding of power lines. Such a safer power supply mode would be a promising candidate for applications in IOT. In this work, we built up optically powered active sensing system, supplying uninterrupted power for the remote active sensors and communication modules. Also, we proposed a novel maximum power point tracking technique for photovoltaic power convertors. In our system, the actual output efficiency greater than 40% within 1W laser power. After 1km fiber transmission and opto-electric power conversion, a stable electric power of 210mW was obtained, which is sufficient for operating an active sensing system.

High-power cylindrical vector beam fiber laser based on few-mode fiber Bragg grating

An all-fiber CVB laser based on a few-mode FBG with switchable radially and azimuthally polarized beam generation has been demonstrated. The CVB fiber laser operates at single wavelength of 1054 nm with a 3 dB line width of less than 0.02 nm, signal-to-background ratio of more than 55 dB. And the output power can reach 75 mW, which is the highest CVB power from all-fiber laser.

Single-Longitudinal-Mode Fiber Ring Lasers With Taper-Coupled Double-Microsphere-Cavities

This letter proposes and demonstrates a fiber ring laser using taper-coupled double-microsphere-cavities (DMCs) to achieve single-longitudinal-mode operation. Whispering-gallery-mode (WGM) intensity distributions and transmission spectra of the DMC with different microsphere diameters are investigated both theoretically and experimentally. Due to the Vernier effect, the DMC can produce WGM spectra with a higher extinction ratio, a higher side-mode-suppression ratio, a larger free spectral range, and a narrower bandwidth, as compared with a single-microsphere cavity. A single-longitudinal-mode fiber ring laser operating near