Reduction Of Coupling Loss Research Articles

Performance enhancement of strip-loaded electro-optic modulator using photobleaching-assisted method

A low-cost hybrid electro-optic (EO) material with a high EO coefficient and excellent poling stability is used as the basis for a conventional Mach–Zehnder interferometer modulator with a strip-loaded waveguide structure, and thorough modeling, design and fabrication processes are proposed. To improve the performance of this strip-loaded EO modulator, a simple photobleaching technique is introduced into the fabrication process. Using this design, the confinement is increased by a factor of 1.71, and coupling loss reduction of 2.3 dB per facet is obtained at the same time. The device exhibits an insertion loss of 8.9 dB and an EO coefficient of 20.9 pm/V at 1550 nm with stable operation. This waveguide structure and the fabrication process are shown to be valuable for EO modulators and switches application.

Computation of self-field hysteresis losses in conductors with helicoidal structure using a 2D finite element method

It is well known that twisting current-carrying conductors helps to reduce their coupling losses. However, the impact of twisting on self-field hysteresis losses has not been as extensively investigated as that on the reduction of coupling losses. This is mostly because the reduction of coupling losses has been an important issue to tackle in the past, and it is not possible to consider twisting within the classical two-dimensional (2D) approaches for the computation of self-field hysteresis losses. Recently, numerical codes considering the effect of twisting in continuous symmetries have appeared. For general three-dimensional (3D) simulations, one issue is that no robust, widely accepted and easy to obtain model for expressing the relationship between the current density and the electric field is available. On the other hand, we can consider that in these helicoidal structures currents flow only along the helicoidal trajectories. This approach allows one to use the scalar power-law for superconductor resistivity and makes the eddy current approach to a solution of a hysteresis loss problem feasible. In this paper we use the finite element method to solve the eddy current model in helicoidal structures in 2D domains utilizing the helicoidal symmetry. The developed tool uses the full 3D geometry but allows discretization which takes advantage of the helicoidal symmetry to reduce the computational domain to a 2D one. We utilize in this tool the non-linear power law for modelling the resistivity in the superconducting regions and study how the self-field losses are influenced by the twisting of a 10-filament wire. Additionally, in the case of high aspect ratio tapes, we compare the results computed with the new tool and a one-dimensional program based on the integral equation method and developed for simulating single layer power cables made of ReBCO coated conductors. Finally, we discuss modelling issues and present open questions related to helicoidal structures and AC-loss computations in three dimensions.

Electrooptic Polymer Modulator With Single-Mode to Multimode Waveguide Transitions

Grayscale lithography has been used to create refractive index (RI) tapers in an electrooptic (EO) polymer modulator. These RI tapers are used to convert a passive single-mode photobleached polymer waveguide to the fundamental mode of an unbleached, active multimode polymer waveguide. The benefits of this modulator design include a coupling loss reduction of 2.6 dB per interface and an increased confinement factor of 0.83. Using this design, an EO polymer modulator was fabricated with a half-wave voltage of 1.9 V, modulation extinction ratio of 15 dB, and a fiber to lens insertion loss of 11.6 dB.

Low coupling loss core-strengthened Bi:2212/Ag Rutherford cables

In a comprehensive "vertically integrated" program multifilamentary (MF) high temperature superconducting (HTSC) Bi:2212/Ag strand was fabricated using the powder-in-tube process and heat treated in oxygen by a modified standard procedure. The reaction-heat-treatment (HT) was adjusted to maximize critical current (density), I/sub c/ (J/sub c/), as measured in various magnetic fields, B. A series of Rutherford cables was designed, each of which included a metallic (Nichrome-80) core for strengthening and reduction of coupling loss. Prior to cable winding a series of tests examined the possibility of strand "poisoning" by the core during HT. Small model Rutherford cables were wound, and after HT were prepared for I/sub c/(B) measurement and calorimetric measurement of AC loss and hence interstrand contact resistance I/sub c/(B). It was deduced that, if in direct contact with the strand during HT, the core material can degrade the I/sub c/ of the cable; but steps can be taken to eliminate this problem. Otherwise the core is unequivocally beneficial: the cored Bi:2212/Ag cable is mechanically strong and easy to handle and, from AC-loss results, yields an ICR that when extrapolated to Large Hadron Collider (LHC) inner-winding specifications predicts a cable with a fairly satisfactory combination of low coupling loss and stability.

Reduction of coupling loss in a one-to-many collimating system for a wavelength division (de)multiplexer

In a 1 x N wavelength division (de)multiplexer, N receiving (Rx-) gradient-index-rod lenses (GRIN's) are connected to a common transmitting (Tx-) GRIN. All GRIN's are a little longer (DZ(0) for the Tx- and DZ(i) with i = 1, 2, ..., N for the Rx-GRIN's) than the quarter-pitch. To reduce the average coupling loss and the deviations, DZ(0) and DZ(i) are optimized independently (unequally) or equally by computer programming for small N, such as N = 4 and 8. For a larger N (e.g., 16), a relay GRIN is required, which is a little (DZ(r)) longer than the half-pitch. The best position of the relay GRIN is located between the seventh and the eighth Rx-GRIN's. Other parameters including DZ(0), DZ(i), and DZ(r) are all optimized. As a result the (de)multiplexer has lower losses.

Reduction of coupling loss in many-to-many collimating system for optomechanical matrix switch

In a many-to-many collimating system using gradient-index- rod lenses (GRINs) for an optomechanical matrix switch, there are multiple cross-connections having different optical path lengths so that excess coupling losses arise. The total coupling loss is greatly reduced by properly choosing the GRIN lengths. Two schemes can do so. In the equal ?Z scheme, all GRINs have the same excess length ?Z0 than the quarter pitch, which is optimized at 13, 25, and 49 µm for the three focusing parameters of 0.237, 0.326, and 0.417 mm-1, respectively. The total coupling loss is further reduced in the unequal ?Z scheme, in which the ?Zµ of each GRIN is independently determined and optimized by numerical computation. The results are preferred rather than the zero ?Z scheme where just quarter-pitch GRINs are used.