112 orbital angular momentum modes amplification based on a 7RC-EDF with low differential mode gain.

We demonstrate 112 orbital angular momentum (OAM) modes amplification based on a designed and fabricated 7-ring-core erbium-doped fiber (7RC-EDF). The differential mode gain (DMG) of all the intra-core OAM modes in 7RC-EDF is effectively reduced. The DMG of intra-core modes can be suppressed by the high overlap between the signal modes and pump fundamental mode resulting from the designed structures of the ring core assisted by a trench. The differential gain of the inter-core can be controlled by the power of the core-pump configuration as well. With an experimentally optimized scheme of the pump power of each core, a record low-DMG of 2.8 dB among the 112 OAM modes (16-OAM-mode per core in the 7-core) is achieved at a wavelength of 1550 nm. The obtained favorable performance of the 112 OAM modes based on 7RC-EDF indicates it may promote a long-haul high-density OAM modes multiplexed transmission.

Amplification of 20 orbital angular momentum modes based on a ring-core Yb-doped fiber.

An orbital angular momentum (OAM) fiber amplifier supporting 20 OAM modes based on a ring-core Yb-doped fiber (RC-YDF) is proposed and demonstrated. The RC-YDF we designed and fabricated has two successive Yb-doped annular layers in the ring-core and can support the amplification of OAM (|l|=1, 2, 3, 4, 5) modes at the wavelength of 1064 nm. With a core pump configuration, we characterize the amplification performance of the RC-YDF based amplifier by simulation and experiments. The amplification of each supported OAM mode is proved by the achieved gain of more than 8 dB and a low differential modal gain less than 1dB with an input signal power of about 5dBm. This is the first experimental demonstration, as far as we know, of the amplification of the OAM mode from 1- to 5-order in aYb-doped fiber.

Third-order orbital angular momentum pulse generation from a passively Q-switched fiber laser.

We propose and demonstrate an all-fiber passively Q-switched laser generating a third-order orbital angular momentum (OAM) pulse by introducing a few-mode long-period fiber grating (LPFG) into the laser cavity. The LPFG with asymmetric cross structure and strong refractive index modulation overcomes the coupling issue between the fundamental and the third-azimuthal-order (LP31 or OAM3) modes and realizes their direct conversion. A homemade graphene-based saturable absorber is used to realize Q-switched operation. The laser operates at a center wavelength of 1548.2nm, with a 3 dB spectral bandwidth of 0.4nm, and the OAM+3 and OAM-3 beams can achieve the purity of 90.0% and 90.2%, respectively. This all-fiber Q-switched laser has simple and compact structure and high purity of OAM±3 beams, which has potential applications in the fields of optical tweezers and material processing.

Third- and fourth-order orbital angular momentum multiplexed amplification with ultra-low differential mode gain.

In this Letter, a ring-core erbium-doped fiber (RC-EDF), with a two-layer erbium-doped structure, supporting up to the fourth-order orbital angular momentum (OAM) mode was designed and fabricated for the OAM mode multiplexed amplification. Using the RC-EDF, the amplification of the third- and fourth-order OAM modes with an ultra-low differential mode gain (DMG) has been demonstrated by observing both the modal intensity and phase distribution and measuring the modal gain under the fundamental mode core pumping. The measured average gain of four modes' (OAM±3,1 and OAM±4,1) multiplexed amplification is >19dB, covering the C band, and the DMG is <1dB. Additionally, the gains of two conjugate OAM modes are almost the same under different pump power, regardless of whether they are amplified simultaneously or separately.

Optical fiber bio-sensor for phospholipase using liquid crystal.

A cost-effective and label-free optical fiber sensor was proposed to detect phospholipase A2 (PLA2) in nM concentration. The sensor is made of an alkoxysilane-modified side-polished fiber (SPF) coated with 4'-pentyl-4-cyanobiphenyl (5CB) and self-assembled phospholipid (L-DLPC). It is found that the relative transmission optical power (RTOP) of the fiber sensor decreases due to the 5CB realignment and redistribution induced by the PLA2 hydrolysis of L-DLPC. The response-time at 5 dB RTOP variation exhibits an exponential dependence on PLA2 concentration, allowing us to detect the PLA2 by the 5 dB-response time. This detection method can reduce the detection time. Compare with the traditional copper-grid sensor, the proposed novel fiber sensor has a lower detection limit (<1 nM). Furthermore, the sensor has good repeat-ability and specificity.The sensor's RTOP variation for PLA2 detection at 1 nM is ~21 times higher than that for five other enzymes (trypsin, amylase, thrombin, glucose oxidase, pepsin) at 1000 nM and lipase at 50 nM. This confirms the sensor's excellent PLA2 specificity. The fiber sensor provides a potential way to be incorporated into micro-flow chips to quantitatively detect biological molecules in a real-time and online manner.