Monolithic mode-selective few-mode multicore fiber multiplexers.

With the capacity limits of standard single-mode optical fiber fast approaching, new technologies such as space-division multiplexing are required to avoid an Internet capacity crunch. Few-mode multicore fiber (FM-MCF) could allow for a two orders of magnitude increase in capacity by using the individual spatial modes in the different cores as unique data channels. We report the realization of a monolithic mode-selective few-mode multicore fiber multiplexer capable of addressing the individual modes of such a fiber. These compact multiplexers operate across the S + C + L telecommunications bands and were inscribed into a photonic chip using ultrafast laser inscription. They allow for the simultaneous multiplexing of the LP01, LP11a and LP11b modes of all cores in a 3-mode, 4-core fiber with excellent mode extinction ratios and low insertion losses. The devices are scalable to more modes and cores and therefore could represent an enabling technology for practical ultra-high capacity dense space-division multiplexing.

Holey fiber mode-selective couplers.

Directional mode coupling in an asymmetric holey fiber coupler is demonstrated both numerically and experimentally for the first time. The holey fiber mode couplers have interesting spectral characteristics and are also found to exhibit increased dimensional tolerances. Following a design based on numerical investigations, a dual-core polymer holey fiber coupler for LP(01) and LP(11) mode multiplexing was fabricated via a drilling and drawing technique. The measurements are compared with the simulation results.

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.

Mode-selective couplers for few-mode optical fiber networks.

The excitation and separation of individual modes in a few-mode optical fiber network can be realized using mode-selective couplers. For excitation at the beginning of the fiber, two-core mode-selective couplers can be used, while at the end of the fiber, either two- or three-core mode-selective couplers are required for demultiplexing of the field symmetric or field asymmetric modes, respectively. Both analytical and numerical solutions are presented to quantify the mode-selective functionality.

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.

Multiple-cladding fibers with reduced bend loss.

We demonstrate that a highly bend-resistant fiber can be realized. It is shown theoretically that, by introducing both depressed and elevated rings into the cladding, bending loss can be reduced significantly. A fiber based on this design has been fabricated and characterized as a first step toward achieving this goal. The results show that a multiple-cladding fiber is highly bend resistant when compared with the standard telecom single-mode fiber.

Waveguide analysis of heat-drawn and chemically etched probe tips for scanning near-field optical microscopy.

We analyze two basic aspects of a scanning near-field optical microscope (SNOM) probe's operation: (i) spot-size evolution of the electric field along the probe with and without a metal layer, and (ii) a modal analysis of the SNOM probe, particularly in close proximity to the aperture. A slab waveguide model is utilized to minimize the analytical complexity, yet provides useful quantitative results--including losses associated with the metal coating--which can then be used as design rules.

Self-aligning method of fiber-to-waveguide pigtailing.

An innovative self-aligning technique for the pigtailing of optical fibers to buried channel planar waveguides is presented, based on selective etching. This technique utilizes a plug-and-socket mechanism that is intrinsically self-aligning and mechanically stable. The processes involved have been specifically designed to facilitate the bulk manufacture of pigtailed single or multiple fibers and waveguides. An optimized alignment geometry for the physical connection of fibers to waveguides is presented.

Symmetry-selective reflection gratings.

We show that gratings can be designed to be symmetry selective, that is, reflecting modes with a particular symmetry. The idea behind a symmetry-selective grating is to replace a grating written over the entire core cross section of a waveguide with a grating that is written only over a part of the core. This new kind of grating exhibits high-order reflectivity and selectivity in comparison with standard gratings and enables the design of more effective and compact wavelength add/drop devices.

Controlled modification and direct characterization of multimode-fiber refractive-index profiles.

A combination of controlled annealing and characterization by scanning probe microscopy (SPM) is used to demonstrate that the refractive-index proffle of a commercially available silica-based optical fiber can be accurately reconfigured for use as an evanescent field sensor. The process relies on the controlled relocation of the silica glass dopants across the fiber cross section through heat treatment and the accurate measurement of the resulting dopant redistribution with SPM and differential etching techniques. The effect of variable annealing along a length of fiber is to produce a mode transformer to couple light from a laser source into the sensing region of the fiber.