Rand Ismaeel

All-fiber fused directional coupler for highly efficient spatial mode conversion

We model and demonstrate a simple mode selective all-fiber coupler capable of exciting specific higher order modes in two- and few-mode fibres with high efficiency and purity. The coupler is based on inter-modally phase-matching the propagation constants in each arm of the asymmetric fused coupler, formed by dissimilar fibres. At a specific coupler diameter, the launched fundamental LP(01) mode is coupled into the higher order mode (LP(11), LP(21), LP(02)) in the other arm, over a broadband wave-length range around 1550 nm. Unlike other techniques, the demonstrated coupler is composed of a multimode fiber that is weakly fused with a phase matched conventional single mode telecom fiber (SMF-28). The beating between the supermodes at the coupler waist produces a periodic power transfer between the two arms, and therefore, by monitoring the beating while tapering, it is possible to obtain optimum selection for the desired mode. High coupling efficiencies in excess of 90% for all the higher order modes were recorded over 100 nm spectral range, while insertion losses remain as low as 0.5 dB. Coupling efficiency can be further enhanced by performing slow tapering at high temperature, in order to precisely control the coupler cross-section geometry.

Dual mode fused optical fiber couplers suitable for mode division multiplexed transmission

We experimentally demonstrate 2 × 2 and 3 × 3 fused fiber couplers made from dual mode fiber. A unique mode dependent power transfer characteristics as a function of pulling length is obtained that support various optical functionalities. Exploiting this we demonstrate several devices of interest for mode division multiplexed data transmission including LP11 mode filter, LP11 mode tap coupler, and 50:50 power splitter for both LP01 and LP11 modes.

Resonantly Enhanced Faraday Rotation in an Microcoil Current Sensor

A proof-of-principle experimental demonstration with theoretical modeling is presented for resonantly enhanced Faraday rotation in a microcoil current sensor. The recirculation of resonant light within the coil gives rise to an accumulated polarization rotation and thus improved responsivity. According to simulations, microcoil resonators with sharper resonances could offer significantly larger enhancements. This new type of current sensor has the potential to be ultrafast, compact, and low-cost.

Nonlinear microfiber loop resonators for resonantly enhanced third harmonic generation

We model and demonstrate resonantly enhanced third harmonic generation in microfiber loop resonators, in which the large pump field intensity is exploited to improve the conversion on resonance. Silica microfibers were fabricated with waist diameters near 0.76 μm to ensure intermodal phase matching. When pumped with λ=1.55  μm 4 ns pulses at 100 W peak power, the conversion efficiency is 3×10(-6) over an estimated interaction length of ∼1  mm near the waist. The resonator is then formed by manually translating and twisting the microfiber ends to produce loop diameters down to 6 mm and resonant enhancements up to 7.7 dB for the same pump parameters.

Mid-infrared Raman sources using spontaneous Raman scattering in germanium core optical fibers

Mid-infrared Raman sources based on spontaneous Raman scattering have been demonstrated in a 15-μm diameter glass-clad crystalline germanium optical fiber and in a germanium-wire. A quantum cascade laser was used as pump at a wavelength of 5.62 μm. Because of their ultra-high optical nonlinearities and extremely broad transparency window, germanium core fibers offer the possibility of fabricating compact and efficient mid-infrared sources.

Second harmonic generation and enhancement in microfibers and loop resonators

We model and experimentally investigate second harmonic generation in silica microfibers and loop resonators, in which the second order nonlinearity arises from the glass-air surface dipole and bulk multipole contributions. In the loop resonator, the recirculation of the pump light on resonance is used to increase the conversion. The effect of the loop parameters, such as coupling and loss, is theoretically studied to determine their influence on the resonance enhancement. Experimentally, microfibers were fabricated with diameters around 0.7 μm to generate the intermodally phase matched second harmonic with an efficiency up to 4.2 × 10−8 when pumped with 5 ns 1.55 μm pulses with a peak power of 90 W. After reconfiguring the microfiber into a 1 mm diameter loop, the efficiency was resonantly enhanced by 5.7 times.

Removing the directional degeneracy of LP11 mode in a fused-type mode selective coupler

The removal of polarization and modal degeneracy in the excitation of higher order modes is realized in all-fiber mode-selective weakly fused couplers by controlling the coupler geometry. Experiments carried out with short couplers showed strong polarization dependence, while long couplers with a slow varying cross section have shown to remove polarization and spatial degeneracy in the first high-order mode.

A self-coupling multi-port microcoil resonator

A new type of self-coupling multi-port microcoil resonator using a microfiber coupler is presented. The microresonators, a simple combination of a microfiber coupler and microcoil resonator, were fabricated by coiling a four port microfiber coupler around a low index support rod to induce optical resonance via coupling between adjacent turns. Light propagates along the coil whilst the beating between the supermodes of the coupler is still present, giving an increased extinction ratio and an output spectrum strongly dependent on the microfiber coupler diameter. The multiport microcoil resonator was embedded in a low refractive index polymer to improve its robustness and the polarization dependence was further analyzed.

Design and optimization of a microfiber coupler for biosensing

We design and optimize a simple, low-cost and compact biosensor based upon a microfiber coupler, with a predicted sensitivity and detection limit up to S~10^4 nm/RIU and DL~10^(-6) RIU respectively.

Sensitive optical Microfiber-based Biosensors for DNA Detection

We demonstrate an efficient method for DNA Sensing using simple devices, the microfiber coupler, and the Multi port Microcoil Resonator. These devices promise, high sensitivities, up to 200nm/RIU, and low detection limit of 10^-6 RIU .