INDEX Contrasts

ULTRACOMPACT OPTICAL SENSORS BASED ON HIGH INDEX-CONTRAST PHOTONIC STRUCTURES... 

BASIC PROPERTIES OF HIGH INDEX-CONTRAST WAVEGUIDING STRUCTURES... 

Special attention has to be given to low-loss bends, which for decreasing bend radius ask for increasing index contrast An. The minimum bend radius... 

In the foregoing the description of microresonators was limited to the index guiding type. Resonators with the same functionality can be implemented in PBG structures5. In these cases, with the high index contrast used, the cavity volume is even more reduced resulting in FSRs of 10 - 100 nm. 

Fig. 2.4 BPM simulation results for polymer microring resonators (a) Round trip attenuation vs. the core cladding index contrast and radius of curvature, (b) Coupling induced attenuation vs. the length and gap of the coupler. The width and height of the waveguide is 2 pm. The wavelength used in the calculations is 1.55 pm. Reprinted from Ref. 15 with permission. 2008 Institute of Electrical and Electronics Engineers...

Del Villar et al. studied the phenomenon of the attenuation bands fading during the modal transition, which is not predicted by the numerical analysis presented here. They sophisticated the model first introducing a lossy overlay36 and successively the hybrid modes instead of the LP modes37. They also found in this last case a double step transition as opposed to the one step transition seen in this section. The double step transition is better observed when there is high index contrast between the cladding and the overlay and if low radial order modes are considered. 

To evaluate its capability for refractive index measurement, the fiber FPI device was tested using various liquids including methanol, acetone, and 2-propanol at room temperature. The interference spectra of the device immersed in various liquids are shown in Fig. 7.12 for comparison. The signal intensity dropped when the device was immersed in liquids as a result of the reduced refractive index contrast and thus lowered Fresnel reflections from the cavity endfaces. However, the interference fringes maintained a similar visibility. The spectral distance between the two adjacent valleys also decreased, indicating the increase of refractive index of the medium inside the cavity. Using (7.4), the refractive indices of the liquids were calculated to be nmethanoi = 1 -3283, acetone = 1 -3577, and n2-propanoi = 1.3739, which was close to the commonly accepted values. 

The principle of operation of this sensor is based on the fact that, as the fundamental optical mode travels through the MNF, its shape is modified depending on the index contrast between the solution in the channel and the polymer. Consequently, the change of the fundamental mode results in variation of the MNF... 

The dependence of PS porosity on doping density has been exploited to fabricate optical waveguides. Such structures require only a single variation of refractive index. If a p-type substrate with implanted p+ lines is anodized and partially oxidized, mesoporous lines of high refractive index are embedded in a low refractive index material. An obtained refractive index contrast between core and clad of 30% is promising for future applications [Tal2]. 

The most common examples of high index contrast waveguides are silicon-on-insulator (SOI) waveguides, and silicon oxynitride... 

Table 1. Comparison of normal and high index contrast material systems...

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