In this study, the bending losses of chalcogenide glass channel optical waveguides consisting of an As2Se3 core and an As2S3 lower cladding layer were numerically evaluated across the astronomical N-band, which is the mid-infrared spectral range between the 8 µm and 12 µm wavelengths. The results reveal the design rules for bent waveguides in mid-infrared astrophotonic devices.

Lightwave switching is discussed with a cascaded connection of optical couplers with light intensity control elements. By employing wavelength-selective amplifiers such as a waveguide-type Raman amplifier, all-optical wavelength-selective switching can be realized. We discuss analytically using coupled-mode theory that the lightwave switching is feasible by controlling the intensity of propagating lightwave. The switching operation is verified numerically using finite-difference beam-propagation method. As a result, the expected operation is realized and some characteristics involved with dependencies of wavelength and phase are also investigated. A preliminary experiment using attenuators, beam splitters and mirrors is also described to verify the switching operation with only light-intensity control in interferometers.

A waveguide-based surface plasmon resonance (SPR) sensor with an adsorbed layer is analyzed using the beam-propagation method. For two-dimensional (2-D) models, numerical results show that the change in thickness of the adsorbed layer placed on the metal leads to a significant shift of the maximum absorption wavelength. Through eigenmode analysis, the maximum absorption wavelength is found to be consistent with the cutoff wavelength of the second-order surface plasmon mode. The designed 2-D sensor shows an absorption wavelength shift from 0.595 to 0.603 µm, when the analyte refractive index is increased from 1.330 to 1.334. After a basic investigation using the 2-D models, we next study 3-D models. When the metal with the absorbed layer is wide enough to cover the core region, the 3-D results are similar to the 2-D results. However, as the metal width is reduced, the absorption wavelength shifts toward a shorter wavelength and the sensitivity to the refractive index change degrades gradually. The degradation of the sensitivity is considerable when the metal width is narrower than the core width. As a result, the metal width of the practical SPR sensor should be slightly wider than the core width so as to maintain the sensitivity corresponding to that obtained for the 2-D model.

Switching characteristics such as wavelength dependency and phase dependency are investigated for our proposed switch which consists of waveguide-type Raman amplifiers and 3-dB couplers. As a result, the available range of wavelength and phase shift due to nonlinear effect are estimated about 20 nm around 1.55 µm and about 10 degrees, respectively.

A bent-waveguide-based multimode interference (MMI) demultiplexer is designed for the operation at 0.85- and 1.55-µm wavelengths using the three-dimensional semi-vectorial beam-propagation method. First, it is shown that the use of a straight MMI waveguide results in a long coupler length of more than 1000µm for wavelength demulitplexing. To reduce the coupler length, we next introduce a bent MMI waveguide. Bending with a radius of 1500µm leads to a coupler length of less than 200µm. After designing two output waveguides connected to the MMI section, we finally choose a coupler length to be 175µm for efficient demultiplexing properties. Consequently, an output power of more than 90% can be obtained, leading to a low insertion loss of 0.34dB at both 0.85- and 1.55-µm wavelengths. The demultiplexer achieves small polarization dependence, i.e., less than 2dB difference in contrast and 0.02dB difference in insertion loss.

The finite-difference beam-propagation method is applied to the analysis of a bent step-index slab optical waveguide. The results obtained in the rectangular coordinates with a modified index profile are compared with those in the cylindrical coordinates with a real index profile. It is found that the attenuation constant for TMo mode is larger than that for TEo mode. The polarization dependence of bending loss is negligible, provided the refractive index difference is less than 2%.

The finite-difference beam-propagation method is applied to the analysis of hollow slab waveguides (HSWs). The attenuation constants for the TE0 and TE1 modes are evaluated and compared with those obtained by the perturbation theory. The propagating field and differential power loss in the transition from a straight HSW to a bent HSW are revealed and discussed.