205-240 GHz free-space-to-fiber mode adapter with an 80% mode conversion efficiency.

In this work, we propose an integrated terahertz mode adapter that couples broadband terahertz radiation from free-space to hollow-core fiber with a high mode conversion efficiency (Gaussian beam-to-T E 01) of up to 80%. The adapter consists of a pyramidal horn antenna, a broadband mode converter, and a conical horn. The simulation results indicate that the T E 01 mode in the hollow-core fiber can be efficiently excited by the terahertz mode adapter. The terahertz mode adapter successfully achieved a wide operating bandwidth of more than 15.7% ranging from 205 to 240 GHz in our simulation.

All-fiber stable orbital angular momentum beam generation and propagation.

An all-fiber scheme for stable orbital angular momentum (OAM) beam generation and propagation is proposed and demonstrated. This scheme is based on a self-designed and manufactured graded-index few mode fiber (GI-FMF) and compatible mode selection coupler (MSC). The MSC, which consists of a conventional single mode fiber (SMF) and the GI-FMF, can effectively couple the fundamental mode in the SMF to the desired OAM mode in the GI-FMF based on phase matching condition. Meanwhile, the GI-FMF breaks the degeneracy between the desired eigenmodes and neighboring vector modes, thereby allowing the preservation and propagation of the selectively excited OAM modes. As a proof-of-principle, we have implemented an all-fiber device operating in stable OAM modes with |l| = 1. The experimental results show stable propagation of OAM beams with the mode purity of ∼95% and bandwidth of 100 nm. This all fiber device could be useful for further development of wide bandwidth OAM mode division multiplexed application.

Transverse mode switchable all-fiber Brillouin laser.

A transverse mode switchable all-fiber Brillouin laser architecture is proposed. This laser architecture consists mainly of an optical switch, a set of cascaded mode selection couplers (MSCs), and a few-mode fiber ring cavity. By switching the output signal of the optical switch, specific transverse modes generated by MSCs can be coupled into the ring cavity to excite the corresponding Brillouin scattering light. Based on the Brillouin nonlinear effect, the desired mode resonant amplification is guaranteed, and a transverse mode switchable laser beam is obtained. As a proof of principle, we have implemented an all-fiber Brillouin laser with switchable output of fundamental transverse (LP01) mode and second-order transverse (LP11) mode at the wavelength of 1550 nm. The slope efficiency and linewidth for LP01 and LP11 modes are 25.1% and 4.9 kHz, 20.9% and 4.96 kHz, respectively. Additionally, the orbital angular momentum laser beam with l=+1, -1, or 0 switchable output is also demonstrated from our all-fiber Brillouin laser system. Owing to the compact all-fiber architecture, this transverse mode switchable Brillouin fiber laser is reliable during long-term operation and thus promising for many practical applications.

Low-crosstalk orbital angular momentum fiber coupler design.

A fiber coupler for low-crosstalk orbital angular momentum mode beam splitter is proposed with the structure of two separate and parallel microfibers. By properly setting the center-to-center distance between microfibers, the crosstalk is less than -20 dB, which means that the purity of the needed OAM mode in output port is higher than 99%. For a fixed overlapping length, high coupling efficiency (>97%) is achieved in 1545-1560 nm. The operating wavelength is tuned to the whole C-band by using the thermosensitive liquid. So the designed coupler can achieve the tunable coupling ratio over the whole C-band, which is a prospective component for the further OAM fiber system.

High spatial resolution distributed fiber strain sensor based on phase-OFDR.

A novel method to realize high spatial resolution distributed strain measurement is proposed based on phase demodulation scheme of optical frequency domain reflectometry (OFDR). Strain information can be demodulated directly by analyzing the phase change of Rayleigh backscattered light. Strain location can be obtained with high spatial resolution by cross-correlation method using a wide scanning range of tunable laser source. Based on the above scheme, breakpoint detection with 0.1 mm spatial resolution has been demonstrated, static and dynamic strain up to 100 Hz could be distributedly measured with 10 cm spatial resolution over 200 m sensing fiber, and the minimum measurable strain is about 1 µÎµ.

Gamma radiation induced darkening in barium gallo-germanate glass.

Barium gallo-germanate (BGG) glass is an important glass matrix material used for mid-infrared transmission and mid-infrared fiber laser. In this study, we investigated the γ-ray irradiation induced darkening effect of BGG glass. Optical transmittance spectra, electron paramagnetic resonance (EPR) and thermoluminescence (TL) spectra were employed to investigate the γ-ray irradiation induced defects. Two kinds of Ge-related defects in the irradiated BGG glass, named Ge-related non-bridging oxygen hole center (Ge-NBOHC) and Ge-related electron centers (GEC), were verified. In addition, the absorption bands of the two defects have been separated and the peak absorptivity of Ge-NBOHC and GEC defects is at 375 nm and 315 nm, respectively.

Optical fiber design with orbital angular momentum light purity higher than 99.9.

The purity of the synthesized orbital-angular-momentum (OAM) light in the fiber is inversely proportional to channel crosstalk level in the OAM optical fiber communication system. Here the relationship between the fiber structure and the purity is firstly demonstrated in theory. The graded-index optical fiber is proposed and designed for the OAM light propagation with the purity higher than 99.9%. 16 fiber modes (10 OAM modes) have been supported by a specific designed graded-index optical fiber with dispersion less than 35 ps/(km∙nm). Such fiber design has suppressed the intrinsic crosstalk to be lower than -30 dB, and can be potentially used for the long distance OAM optical communication system.