Shankar Pidishety

All-Fiber Fused Coupler for Stable Generation of Radially and Azimuthally Polarized Beams

The stable generation of TE01 and TM01 beams is demonstrated through a novel all-fiber fused coupler fabricated from a standard single mode fiber (SMF) and a custom air-core fiber. The fundamental mode in the SMF is directly coupled to the TM01 or TE01 mode by appropriately phase matching the modes in the fibers resulting in efficiency of ~67% and ~85%, and polarization purity of 70% and 82%, respectively. The phase matching is achieved through pre-tapering the SMF with precise control of the tapering ratio. The air-core fiber ensures selective excitation of the desired modes, thereby improving propagation stability and polarization quality of the TE01 and TM01 beams.

Multimode-pumped Raman amplification of a higher order mode in a large mode area fiber

We report the first demonstration of Raman amplification in a fiber of a single Bessel-like higher order mode using a multimode pump source. We amplify the LP08-mode with a 559-µm2 effective mode area at a signal wavelength of 1115 nm in a pure-silica-core step-index fiber. A maximum of 18 dB average power gain is achieved in a 9-m long gain fiber, with output pulse energy of 115 µJ. The Raman pump source comprises a pulsed 1060 nm ytterbium-doped fiber amplifier with V-value ~30, which is matched to the Raman gain fiber. The pump depletion as averaged over the signal pulses reaches 36.7%. The conversion of power from the multimode pump into the signal mode demonstrates the potential for efficient brightness enhancement with low amplification-induced signal mode purity degradation.

Orbital angular momentum beam excitation using an all-fiber weakly fused mode selective coupler

Orbital angular momentum beam excitation through direct phase-matched coupling is experimentally demonstrated using an all-fiber weakly fused mode selective coupler consisting of a single-mode fiber and a ring-core fiber. Experimental results showing the excited OAM mode purity of up to 75% measured through the standard ring technique not only demonstrate the proof of concept but also provide a baseline for further improvement.

OAM generation in optical fibre and free space devices

Orbital angular momentum (OAM) beam generation has been investigated using all-fibre and free space configurations. In the first approach, the composite fused coupler is based on a single mode fibre (SMF) and an air-core fibre. The second approach exploits geometrical phase introduced by nanostructuring of silica glass. Both approaches are demonstrated to achieve power coupling efficiencies in excess of 80% at telecom wavelengths.

Investigation of scalability of all-fiber fused mode selective coupler for generating multiple OAM states

We present the scalability of all-fiber fused couplers for generating multiple OAM states (charge l=±5, ±6 and ±7) by mode’s phase matching in the SMF and air-core fiber coupler configuration carried out through the simulations.

A self-referencing surface plasmon resonance sensor

A zero cross sensitivity (CS) deliverable self-referencing (S-R) surface plasmon resonance sensor (SPR) experimental scheme is presented based on the retroreflecting prism configuration with single spectroscopic read out. The relationship between external and internal incident angle in retroreflection configuration results in the excitation of two spectroscopically distinct surface plasmons (SPs) modes corresponding to each side of the prism. The characteristics of two SPs modes are independent and thus used in differential refractive index (RI) sensing via S-R modes technique. By applying SR sensing theory, it is shown that the CS can be minimized to zero through the demonstrated configuration. This configuration is flexible for using in wavelength interrogation measurement method as well. The excited SPs modes are also tunable to acquire either identical or non-identical sensitivity by selecting corresponding metal film thickness on the either sides of the prism. From the preliminary experimentation on SR configuration and theoretical investigation on deliverability of zero CS, it can be expected that the presented scheme is a better SR configuration for high accurate differential RI sensing applications.

Studies on the propagation of orbital angular momentum beams through optical fibers with different index profiles

We report our studies on the propagation of light beams possessing orbital angular momentum (OAM) through optical fibers having different index profiles with the emphasis on stable propagation. Simulation results on propagation characteristics of OAM beam in different index fibers are presented. We also present preliminary experimental results validating the same. Presented study is expected to be useful in configuring an all-fiber cavity for a fiber laser with OAM beam output that is desired for communication and high power fiber laser applications.