Nicolas Riesen

Design of mode-sorting asymmetric Y-junctions

The theory of mode-sorting in bimodal asymmetric Y-junctions is extended to multimode asymmetric Y-junctions with multiple output arms. This theory allows for the optimization of these mode-sorting planar structures. Asymmetric Y-junctions provide unique opportunities for spatial mode division multiplexing (MDM) of optical fiber. Spatial MDM is considered paramount to overcoming the bandwidth limitations of single-mode fiber. The design criteria presented in this paper facilitate their design.

Mode-selective couplers for few-mode optical fiber networks

Tapered mode-selective couplers are shown to allow for ultra-broadband mode-division multiplexing of few-mode optical fiber. Using appropriate three-core configurations, modes of arbitrary spatial-orientation can be demultiplexed. The successful fabrication of these wavelength-insensitive couplers would represent the realization of compact low-loss mode-multiplexers for use in high-bandwidth few-mode fiber networks.

Single-, Few-, and Multimode Y-Junctions

The theory of modal propagation through symmetric and asymmetric 2-D weakly guiding Y-junctions is extended to cover few-mode, multimode, and multiarm Y-junctions. A conceptual approach based on the evolution of modal effective indexes and composite supermodes is used to determine the qualitative functionality of these devices, with quantification being determined numerically using the beam propagation method.

Few-Mode Elliptical-Core Fiber Data Transmission

Spatial mode-division-multiplexing is seen as paramount to overcoming the bandwidth limitations of single-mode fiber. In this letter, spatial-multiplexing of polarization-maintaining, elliptical-core fiber is proposed using asymmetric Y-junctions. Asymmetric Y-junctions also allow for straightforward wavelength- and polarization-multiplexing. Numerical beam propagation method simulations are used to demonstrate the functioning of a three-mode elliptical-core fiber data link, which could easily be extended to more modes. The multiplexing of multiple spatial modes could potentially see multifold increases in fiber capacity.

Tapered Velocity Mode-Selective Couplers

In this paper tapered velocity mode-selective couplers are presented. These tapered couplers are approximately adiabatic devices which unlike standard mode-selective couplers, do not rely on precise phase conditions to be satisfied over an extended length, to achieve their mode-selective functionality. Therefore these mode couplers permit ultra-wideband mode-division multiplexing of few-mode waveguides. Moreover, their behavior is largely independent of parameters such as index contrast and the precise coupler dimensions. The mode-selective functionality is demonstrated in slab geometry, with the principles generalizing to planar or fiber-based devices.

Femtosecond direct-written integrated mode couplers.

We report the design and fabrication of three-dimensional integrated mode couplers operating in the C-band. These mode-selective couplers were inscribed into a boro-aluminosilicate photonic chip using the femtosecond laser direct-write technique. Horizontally and vertically written two-core couplers are shown to allow for the multiplexing of the LP11a and LP11b spatial modes of an optical fiber, respectively, with excellent mode extinction ratios (25-37 + dB) and low loss (~1 dB) between 1500 and 1580 nm. Furthermore, optimized fabrication parameters enable coupling ratios close to 100%. When written in sequence, the couplers allow for the multiplexing of all LP01, LP11a and LP11b modes. This is also shown to be possible using a single 3-dimensional three-core coupler. These integrated mode couplers have considerable potential to be used in mode-division multiplexing for increasing optical fiber capacity. The three-dimensional capability of the femtosecond direct-write technique provides the versatility to write linear cascades of such two- and three-core couplers into a single compact glass chip, with arbitrary routing of waveguides to ensure a small footprint. This technology could be used for high-performance, compact and cost-effective multiplexing of large numbers of modes of an optical fiber.

Ultra-Broadband Tapered Mode-Selective Couplers for Few-Mode Optical Fiber Networks

Tapered mode-selective couplers are shown to allow for ultra-broadband mode-division multiplexing of few-mode optical fiber. Using appropriate three-core configurations, modes of arbitrary spatial-orientation can be demultiplexed. The successful fabrication of these wavelength-insensitive couplers would represent the realization of compact low-loss mode-multiplexers for use in high-bandwidth few-mode fiber networks.

Polymer based whispering gallery mode laser for biosensing applications

Whispering gallery mode lasers are of interest for a wide range of applications and especially biological sensing, exploiting the dependence of the resonance wavelengths on the surrounding refractive index. Upon lasing, the Q factors of the resonances are greatly improved, enabling measurements of wavelength shifts with increased accuracy. A way forward to improve the performance of the refractive index sensing mechanism is to reduce the size of the optical resonator, as the refractive index sensitivity is inversely proportional to the resonator dimensions. However, as the lasing threshold is believed to depend on the Q factor among other parameters, and the reduction of the microresonator size results in lower Q, this poses additional challenges for reaching the lasing threshold. In this letter, we demonstrate lasing in 10 μm diameter dye doped polystyrene microspheres in aqueous solution, the smallest polystyrene microsphere lasers ever reported in these conditions. We also investigate the dependence of the lasing threshold on the Q factor by changing the refractive index surrounding the sphere, highlighting a much stronger dependency than initially reported.

Weakly-Guiding Mode-Selective Fiber Couplers

This paper presents analytical descriptions of the behavior of weakly-guiding/weakly-coupled mode-selective fiber couplers, based on coupled-mode theory. Mode-selective couplers rely on the phase-matching of a higher-order mode in one fiber with the fundamental mode of a second, closely-positioned fiber. Their behavior is shown to be highly-dependent on the principle and azimuthal mode numbers of the higher-order mode, as well as its spatial-orientation. Zero coupling is shown to be possible for an asymmetric higher-order mode even when there is perfect phase-matching. The theory is also numerically simulated, and could assist in the future design of efficient mode-selective couplers for a wide range of optical communications and sensor systems.

Few-Core Spatial-Mode Multiplexers/Demultiplexers Based on Evanescent Coupling

The multiplexing and demultiplexing of the spatialmodes of few-mode fiber is demonstrated using few-core couplers. The mode combination/separation is achieved through mode-selective evanescent coupling. In particular, multiplexers/demultiplexers for three fiber modes are simulated that use either circular (fiber) or rectangular (planar) cores. The demultiplexing can be made independent of the spatial-orientations of the fiber modes by judicious choice of core configuration.