All-fiber CW cylindrical vector beam fiber laser based on few-mode fiber Bragg grating

An all-fiber CW cylindrical vector beam (CVB) fiber laser based on a few-mode fiber Bragg grating (FW-FBG) with switchable radially and azimuthally polarized beam generation has been demonstrated. The CVB fiber laser operates at a wavelength of 1053.95 nm with a 3 dB line width of 0.1 nm, a signal-to-background ratio of more than 50 dB. The CVB output power can reach 75 mW, and the mode purity is measured to be >95.5%. This compact CVB fiber laser has potential applications in many areas such as optical tweezers, optical trapping and optical sensing systems

Carbon Nanotube Mode-Locked Fiber Laser Generating Cylindrical Vector Beams with a Two-Mode Fiber Bragg Grating

We propose and demonstrate a compact all-fiber laser generating cylindrical vector beam (CVB) using carbon nanotubes as the saturable absorber for mode-locking and a two-mode fiber Bragg grating (TM-FBG) as the mode discriminator. Both radially and azimuthally polarized beams with a polarization purity of 90% were obtained by simply adjusting the polarization controllers. The CVB mode-locked fiber laser operates at 1552.9 nm with a 3-dB line width of less than 0.02 nm, generating ns CVB pulses. The all-fiber CVB laser may have potential applications from fundamental research to practical applications, such as particle capture, high-resolution measurement and material processing

Mode and wavelength-switchable pulsed fiber laser with few-mode fiber grating

We propose and demonstrate a mode and wavelength-switchable pulsed fiber laser using two-mode fiber Bragg grating (TM-FBG) as both transverse mode converter and wavelength selector. The mode-locking mechanism is based on saturable absorption of semiconductor saturable absorption mirror (SESAM). Due to the use of low polarization-dependent mode-locking method, polarization disturbation between mode-locking and transverse-mode selection has been effectively eliminated, thus enabling flexible transverse-mode selection with simultaneous mode-locking operation. Moreover, through the optimized offset launching technique, this laser can operate at the wavelengths of the fundamental mode (LP01) and second-order mode (LP11), and thus export cylindrical vector beams (CVBs) and fundamental mode beams at the TM-FBG output port, respectively. This all-fiber laser is a simple, low-cost and flexible source for mode-division multiplexing system and other applications

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 (SMF) with few-mode fiber (FMF) 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 (CNTs) as saturable absorber for mode-locking and a home-made mode-selective coupler (MSC) as both mode converter and birefringence filter. In experiments, the mode-locked fiber laser delivers CVB pulses of dual-wavelength (1532.5 nm, 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

Ultrasensitive sensing in air based on graphene-coated hollow core fibers

The mismatching between permittivities of guided mode and air limits the operation of accurately monitoring the change in the refractive index of the surrounding air. To solve it, we propose a platform using a hollow core fiber with the integration of graphene coating. Experimental results demonstrate that the anti-resonant reflecting guidance has been enhanced while it induces sharply and periodically lossy dips in the transmission spectrum. We conclude a sensitivity of -365.9 dB/RIU and a high detection limit of 2.73 × 10?6 RIU by means of interrogating the intensity of the lossy dips. We believe that this configuration opens a direction for highly sensitive sensing in researches of chemistry, medicine, and biology

Optical property of few-mode fiber with non-uniform refractive index for cylindrical vector beam generation

This paper investigates optical properties of few-mode fiber with non-uniform refractive index, namely: the few mode fiber with U-shape refractive index and the two-mode and four-mode few-mode fiber with bent radius. Finite element method is used to analyze the mode distributions based on their non-uniform refractive index. Effective mode control can be achieved through these few mode fibers to achieve vector beam generation. Finally, reflection spectra of a few-mode fiber Bragg grating are calculated theoretically and then measured under different bending conditions. Experimental results are in good accordance with the theoretical ones. These few mode fibers show potential applications in generation of cylindrical vector beam both for optical lasing and sensing systems

Orthogonally polarized bright–dark pulse pair generation in mode-locked fiber laser with a large-angle tilted fiber grating

We report on the generation of orthogonally polarized bright–dark pulse pair in a passively mode-locked fiber laser with a large-angle tilted fiber grating (LA-TFG). The unique polarization properties of the LA-TFG, i.e., polarization-dependent loss and polarization-mode splitting, enable dual-wavelength mode-locking operation. Besides dual-wavelength bright pulses with uniform polarization at two different wavelengths, the bright–dark pulse pair has also been achieved. It is found that the bright–dark pulse pair is formed due to the nonlinear couplings between lights with two orthogonal polarizations and two different wavelengths. Furthermore, harmonic mode-locking of bright–dark pulse pair has been observed. The obtained bright–dark pulse pair could find potential use in secure communication system. It also paves the way to manipulate the generation of dark pulse in terms of wavelength and polarization, using specially designed fiber grating for mode-locking

Dissipative soliton resonance Ytterbium-doped fiber laser with cylindrical vector beam generation

We experimentally demonstrate a dissipative soliton resonance (DSR) Ytterbium-doped fiber laser with high-purity cylindrical vector beam (CVB) generation, using a mode-selective coupler (MSC) as transverse mode converter and splitter. The all-normal-dispersion mode-locked Ytterbium-doped fiber laser operating at DSR regime without spectral filter can deliver CVB pulses with a pulse duration of ∼733 ps and a pulse energy as high as ∼0.439 nJ. The radially and azimuthally polarized beams can be switched by adjusting the polarization controller, with a high mode purity measured to be >94.5%. This compact DSR fiber laser could find potential applications in the field of material processing, nonlinear optics and so on

Microfiber-based polarization beam splitter and its application for passively mode-locked all-fiber laser

Nonlinear polarization evolution based on polarization beam splitter (PBS) is a classical technique for passive mode-locking of fiber lasers. Different from commonly used bulky PBS, in this paper all-fiber PBSs composed of two parallel coupled microfibers have been proposed and fabricated under the condition of appropriate microfiber diameter and coupling length. Using our fabricated microfiber PBSs, passively mode-locked all-fiber lasers have also been demonstrated. The results indicate that the microfiber-based PBS has advantages of simple fabrication, compact size, and most importantly, variable polarization extinction ratio and operation bandwidth. The all-fiber mode-locked lasers with the microfiber PBSs generating stable pulses at both 1.0 μm and 1.5 μm wavelength bands have comparable performance with their counterparts based on bulky PBSs. It may be a step towards true all-fiber mode-locked laser and other all-fiber systems

Ultrasensitive refractive index sensor based on graphene coated hollow core fiber

A high-quality nanolayer graphene (NLG) coated hollow core fiber (HCF) platform has been demonstrated for accurate monitoring of refractive index (RI) changes so far mainly operate in liquids but not in air. The NLG with high index is deposited on the outer surface of the HCF, and an enhanced anti-resonant reflecting guidance is formed, which induces sharp periodic lossy dips in the transmission spectrum. A cute experiment conducted interrogating the transmission intensity of the lossy dip demonstrates a high resolution of 2.73×10-6 RIU and a sensitivity of -365.9 dB/RIU, which is two or three times higher than that of intensity-modulated RI sensors reported previously. We believe that this configuration opens research directions for highly sensitive sensing in researches of chemistry, medicine, and biology