Local Effective Index Research Articles

High‐Performance Silicon Photonic Filter Using Subwavelength‐Structure Multimode Waveguide Gratings

AbstractOn‐chip photonic filters are one of the most important elements for spectrum manipulation in various optical systems, including wavelength‐division multiplexing and spectrum analysis. Currently, it is still very challenging to achieve scalable photonic filters with flexibly‐designed bandwidths, low excess losses, flat tops, and ultra‐wide free‐spectral ranges (FSRs). Here an unprecedented silicon photonic filter is proposed and demonstrated with high performances by using apodised antisymmetric multimode subwavelength gratings (MSWGs) structures. For the present silicon photonic filter, the MSWGs enable the index engineering in the Bragg lattice, which easily manipulates the coupling coefficient, the local effective index, and the mode profile flexibly. As a result, the strong side lobes of the grating filter are efficiently suppressed, while the bandwidth can be designed flexibly and the FSR is free. The fabricated silicon photonic filter is FSR‐free and shows a low excess loss of <1 dB and a high sidelobe suppression ratio of 20–40 dB.

Narrow-Band Add-Drop Filter Based on Phase-Modulated Grating-Assisted Contra-Directional Couplers

A narrow-band silicon photonic add-drop filter is demonstrated by using phase-modulated grating-assisted contra-directional couplers (contra-DCs) on the silicon-on-insulator platform. The apodization coupling is achieved by modulating the phase of one grating in a contra-DC, while keeping the other grating uniform. As the local effective index of the gratings remains almost unchanged, the designed apodized contra-DC has a large designing flexibility and a high sidelobe suppression ratio (SLSR). To obtain a narrow bandwidth, weak coupling is designed. Single-stage and dual-stage filtering are both studied. The experimental results show that single-stage filtering device has a 3-dB bandwidth of 0.9 nm and an SLSR of 18 dB. Dual-stage filtering device has a 3-dB bandwidth of 0.5 nm and an SLSR of 23 dB. Our devices are promising for on-chip applications that require narrow bandwidths such as wavelength-division-multiplexed channel filters.

Subwavelength plasmonics for graded-index optics on a chip

Planar plasmonic devices are becoming attractive for myriad applications, owing to their potential compatibility with standard microelectronics technology and the capability for densely integrating a large variety of plasmonic devices on a chip. Mitigating the challenges of using plasmonics in on-chip configurations requires precise control over the properties of plasmonic modes, in particular their shape and size. Here we achieve this goal by demonstrating a planar plasmonic graded-index lens focusing surface plasmons propagating along the device. The plasmonic mode is manipulated by carving subwavelength features into a dielectric layer positioned on top of a uniform metal film, allowing the local effective index of the plasmonic mode to be controlled using a single binary lithographic step. Focusing and divergence of surface plasmons is demonstrated experimentally. The demonstrated approach can be used for manipulating the propagation of surface plasmons, e.g., for beam steering, splitting, cloaking, mode matching, and beam shaping applications.

Internal spatial modes in glass microring resonators

We have used near-field scanning optical microscopy to measure the internal optical modes inside glass waveguides and microring resonators. The period of the observed standing modes provides a direct measure of the local effective index. The measured effective index and modal shape determines the values of all components of the wave vector. The observed standing modes inside the ring resonator are unexpected, but are caused by the standing modes in the coupling waveguide. Last, we describe a technique that can obtain detailed information about the locations of the dielectric interfaces.

Measurement of internal spatial modes and local propagation properties in optical waveguides

Internal spatial modes and local properties controlling optical wave propagation have been measured in glass/silica buried waveguides. The period of the observed standing modes provides a direct measure of the local effective index. The measured effective index and transverse model shape determines the values of all components of the wave vector. In addition, we describe a technique that can obtain detailed information about the locations of remote dielectric interfaces.

A simple variational analysis of diffused channel waveguides

The variational analysis of Ti:LiNbO3 strip waveguides is resolved into two related one-dimension problems, which provides a new iterative approach for the calculation of the mode parameters and the propagation constant. The analytical expressions of the local effective index profiles can be obtained from the variational analysis. The parameters of four trial solutions for the fundamental mode in a diffused-channel waveguide have been determined by using the present method. Comparison with results using the effective method shows that these four approximate expressions are accurate enough. It is also shown that this method converges very quickly and is suitable for both diffused channel waveguides and optical waveguides with rectangular cross-section.

Mode selection with a three-layer dielectric rib waveguide

Analysis of a riblike waveguide structure consisting of three dielectric films shows that properly chosen film thicknesses permit the lateral containment of any selected waveguide mode, with the unselected modes radiating laterally away from the rib structure. The waveguide consists of two high-index films separated by a low-index layer in which the fields are evanescent. A perturbation in the thickness of the low-index layer increases the local effective index for some planar waveguide modes while decreasing it for others. Hence a ridge or trough in the low-index layer can guide certain selected modes along its axis if the high-index-layer thicknesses are correctly chosen. Unlike mode-stripping structures based on preferential absorption of the unwanted modes, this all-dielectric structure introduces no loss to the selected mode other than the unavoidable scattering and residual absorption in the dielectric media. Also, unlike mode-selecting structures based on Bragg effects, no maintenance of phase-matched conditions over controlled propagation lengths is required.