Core-cladding Index Contrast Research Articles

OAM-SDM Solution Toward a Submarine Cable

Determining how to add more signal channels without enlarging the cable volume is continually an issue in managing submarine cables. To improve the cost per bit and the power efficiency, space division multiplexing (SDM) is regarded as a promising solution that could be used to address optical density and cable capacity challenges regarding submarine cables. In this study, we focus on angular momentum (OAM) modes supporting few-mode ring-core fiber (RCF) solutions for submarine cables. Considering the inhibition of the hydrogen loss for the submarine cable, we propose a center-region-lifted 3-OAM-mode-group supporting RCF (abbreviated to 3-OAM-MG RCF) with a core-cladding index contrast of 0.6 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\%}$</tex-math></inline-formula> , and meanwhile we also design and fabricate a matched 3-OAM-MG erbium-doped fiber (EDF). To confirm the short-haul transmission ability, we conduct a wavelength/mode/polarization division multiplexing (WDM-MDM-PDM) transmission test through a 25.3-km 3-OAM-MG RCF, where the coupled modes inside MG are demultiplexed by using 2 × 2 multiple-input multiple-out (MIMO) equalization. The transmission capacity is approximately 35 Tbit/s and the bit error rates (BERs) of all the channels are less than 2.4 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\times 10^{-2}$</tex-math></inline-formula> . For the application scenario of long-haul submarine communication, a cascade WDM-MDM transmission system consisting of 3-OAM-MG RCF and 3-OAM-MG EDF is set up, which makes the BERs of all the mode and wavelength channels below the 20 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\%}$</tex-math></inline-formula> soft-decision forward error correction (SD-FEC) threshold. It indicates that data-carrying OAM mode multiplexing transmission has potential application in a long-haul submarine cable system.

Spiral Waveguides on Germanium-on-Silicon Nitride Platform for Mid-IR Sensing Applications

Spiral waveguides on a new germanium-on-silicon nitride (GON) platform with a wide transparency and a large core-clad index contrast for mid-infrared (mid-IR) sensing applications are demonstrated. Spiral waveguide sensors with a low bending loss on this platform enable compact sensors for mid-IR absorption spectroscopy. A minimum volumetric concentration of 5% isopropanol (IPA) in an IPA-acetone mixture is measured. This detection limit is three times lower than the counterpart waveguide, fabricated on the regular germanium-on-silicon platform with similar propagation loss at 3.73 μm wavelength. This silicon-compatible GON sensor is promising for applications such as environmental studies, industrial leak detection, process control, medical breath analysis, and many more.

Germanium-on-silicon nitride waveguides for mid-infrared integrated photonics

A germanium-based platform with a large core-clad index contrast, germanium-on-silicon nitride waveguide, is demonstrated at mid-infrared wavelength. Simulations are performed to verify the feasibility of this structure. This structure is realized by first bonding a silicon-nitride-deposited germanium-on-silicon donor wafer onto a silicon substrate wafer, followed by the layer transfer approach to obtain germanium-on-silicon nitride structure, which is scalable to all wafer sizes. The misfit dislocations which initially form along the interface between germanium/silicon can be removed by chemical mechanical polishing after layer transfer process resulting in a high-quality germanium layer. At the mid-infrared wavelength of 3.8 μm, the germanium-on-silicon nitride waveguide has a propagation loss of 3.35 ± 0.5 dB/cm and a bend loss of 0.14 ± 0.01 dB/bend for a radius of 5 μm for the transverse-electric mode.

Guided-wave infrared/visible coupler and its incorporation into a miniature beam transmitter

A guided-wave coupler based on silica waveguides, which enables an infrared (IR) light beam at λ=905nm to be integrated with a visible beam at λ=650nm, was designed and realized, where both the IR and visible guided modes are simultaneously available from the same port. The coupling length was set to 1240μm, in order to guarantee the nearly complete transfer of the visible beam power to the IR channel, for a channel waveguide in silica, with a core-cladding index contrast of 0.34% and a dimension of 5×5μm2. A miniature light beam transmitter was then built by taking advantage of the IR/visible coupler, which was fiber-pigtailed to the corresponding laser diodes, in conjunction with a collimating lens. The presented transmitter is predicted to potentially feature flexible beam shaping, a compact size, and cost effective fabrication. For the embodied IR/visible transmitter, the collimated IR beam acts as a long-range projectile, while the visible beam is useful for tracking the IR beam. The generated IR beam exhibited Gaussian-like profiles with a divergence angle of 0.2°, and was precisely aligned with the visible monitoring beam with an angular accuracy of 0.0002°. Finally, the effective trajectory was observed by the receiver over a distance of 600m along the propagation direction.

Large mode area silicon microstructured fiber with robust dual mode guidance

A silicon microstructured fiber has been designed and fabricated using a pure silica photonic bandgap guiding fiber as a 3D template for materials deposition. The resulting silicon fiber has a micron sized core but with a small core-cladding index contrast so that it only supports two guided modes. It will be shown that by using the microstructured template this fiber exhibits a number of similar guiding properties to the more traditional index guiding air-silica structures. The large mode areas and low optical losses measured for the silicon microstructured fiber demonstrate its potential to be integrated with existing fiber infrastructures.

Experimental and theoretical analysis of core-to-core coupling on fiber bundle imaging

Flexible endoscopes commonly use coherent fiber bundles with high core density to facilitate in vivo imaging. Small, closely spaced cores are desired for achieving a high number of resolvable pixels in a small diameter fiber bundle. On the other hand, closely spaced cores potentially lead to strong core-to-core coupling. Based on numerical simulations, it was previously explained that image fiber bundles can successfully transmit images because of nonuniformities in the core size that reduce coupling. In this paper, we show numerically and experimentally that, due to the randomness of the structural nonuniformity, significant core-to-core coupling still exists in fiber bundles that are routinely used for imaging. The coupling is highly dependent on the illumination wavelength and polarization state. We further show that the resolution achievable by a fiber bundle depends not only on the core density, but also on the inter-core coupling strength. Finally, we propose that increasing the core-cladding index contrast is a promising approach to achieve a fiber bundle with low core coupling, high core density, and effectively single moded propagation in individual cores.

Liquid-core, liquid-cladding photonic crystal fibers

We experimentally demonstrate a simple and novel technique to simultaneously insert a liquid into the core of a hollow-core photonic crystal fiber (PCF) and a different liquid into its cladding. The result is a liquid-core, liquid-cladding waveguide in which the two liquids can be selected to yield specific guidance characteristics. As an example, we tuned the core-cladding index difference by proper choice of the inserted liquids to obtain control over the number of guided modes. Single-mode guidance was achieved for a particular choice of liquids. We also experimentally and theoretically investigated the nature of light confinement and observed the transition from photonic bandgap to total internal reflection guidance both with the core-cladding index contrast and with the PCF length.

Eliminating the birefringence in silicon-on-insulator ridge waveguides by use of cladding stress

We propose and demonstrate the use of the cladding stress-induced photoelastic effect to eliminate modal birefringence in silicon-on-insulator (SOI) ridge waveguides. Birefringence-free operation was achieved for waveguides with otherwise large birefringence by use of properly chosen thickness and stress of the upper cladding layer. With the stress levels typically found in cladding materials such as SiO2, the birefringence modification range can be as large as 10(-3). In arrayed waveguide grating demultiplexers that were fabricated in a SOI platform, we demonstrated the reduction of the birefringence from 1.2 x 10(-3) (without the upper cladding) to 4.5 x 10(-5) when a 0.8-microm oxide upper cladding with a stress of -320 MPa (compressive) was used. Because the index changes induced by the stress are orders of magnitude smaller than the waveguide core-cladding index contrast, the associated mode mismatch loss is negligible.

Optimal waveguide dimensions for nonlinear interactions

We investigate strong light confinement in high core-cladding index contrast waveguides with dimensions comparable to and smaller than the wavelength of incident light. We consider oval and rectangular cross sections and demonstrate that an optimal core size exists that maximizes the effective nonlinearity. We also determine that waveguides with asymmetrical cross sections provide the maximum possible nonlinearity, although only a small improvement over the symmetric case. Furthermore, we find that for a specified waveguide shape the largest nonlinearity occurs for nearly the same core area in all cases. Calculations of the dispersion for the optimal-size waveguide at a particular wavelength indicate that the group-velocity dispersion is normal. Ultimately, such designs could be used to develop low-power all-optical devices and to produce waveguides for ultra-low threshold nonlinear frequency generation such as supercontinuum generation.

Index contrast scaling for optical amplifiers

A scaling methodology for optically pumped waveguide amplifiers is presented as a function of their core-cladding index contrast. Increasing index contrast results in two crucial advantages: 1) an increase in gain efficiency and 2) a decrease in the areal footprint of a planar structure. Increasing index contrast is observed to have no effect on the output noise figure. A figure of merit summarizing these advantages demonstrates the powerful role of index contrast as an enabler for improving planar amplifier performance. Design rules are presented in the form of performance maps, allowing waveguide designers to optimize amplifier length and footprint. Using the Er-doped waveguide amplifier as a case study, we design an optical amplifier with >3-dB/cm gain within a 300/spl times/300-/spl mu/m/sup 2/ area, powered by a single 1-mW pump source. This work represents a design rule approach for making a scalable microphotonic optical amplifier.

Bending-induced phase shifts in dual-mode planar optical waveguides

We examine the manner in which the effective index for each of the two modes of a bent dual-mode planar waveguide changes with curvature. We find that the bending-induced changes in the effective indices depend strongly on core-cladding index contrast and the value of V. For waveguides with large contrast, the changes in effective indices are such that the change in the phase difference between the modes is positive (or negative) at large (or small) values of V. The change becomes zero at a V value that depends on the waveguide parameters and curvature. If the index contrast is small, the bending-induced phase difference may change sign with increase in curvature. This might help in ultimately explaining the bipolar phase shift seen in a recent experiment. The results of our study can be used to increase or decrease the bending sensitivity of dual-mode optical-waveguide sensors and devices.