Compact Optical Filter Research Articles

Silicon waveguide as virtual photonic bandgap structure array for realizing compact optical filters

ABSTRACT An engineered silicon rib waveguide on Silicon-On-Insulator platform which can act as virtual photonic bandgap structure is proposed. Selective periodic etching of materials from a rib waveguide is done to create a periodic rib-ridge structure. Each of the rib-ridge sections will therefore have different effective refractive indices and will act as virtual multilayered periodic structure or one-dimensional photonic bandgap structure. Their spatial positions and widths can be controlled to realize different type of photonic filters for integrated photonics applications. Realization of narrowband, multi-band and band-pass filter are shown here. These types of waveguide based monomaterial optical filters avoid the challenges generally faced while fabricating multilayer structures of different heterogeneous materials having different refractive indices. Moreover, as the device is waveguide based, its size is very small and will be useful for compact integrated photonics.

Engineering the spectral properties of non-uniform coupled micro-ring resonator optical filters using numerical optimization

We discuss a new approach for tailoring the spectral properties of coupled micro-ring resonator optical filters using numerical optimization methods. By allowing the micro-rings to have different radii, we obtain considerably lesser crosstalk in a wide frequency region, unlike the uniform ring case commonly found in literature, where the stop-band is limited by the periodic nature of the transfer function. We apply the coupled-mode approach for efficiently calculating the transfer function at each optimization step, scalable to an arbitrary number of rings. Using standard minimization methods we obtain transfer functions with relatively flat pass-band and small cross-talk depending on the number of rings considered in the design. Our technique may open up new paths for designing and implementing compact optical filters with tailored spectral properties.

Dispersion synthesis with multi-ordered metatronic filters.

We propose the synthesis of frequency dispersion of layered structures based on the design of multi-ordered optical filters using nanocircuit concepts. Following the well-known insertion loss method commonly employed in the design of electronic and microwave filters, here we theoretically show how we can tailor optical dispersion as we carry out the design of several low-pass, high-pass, band-pass and band-stop filters of different order with a (maximally flat) Butterworth response. We numerically demonstrate that these filters can be designed by combining metasurfaces made of one or two materials acting as optical lumped elements, and, hence, leading to simple, easy to apply, design rules. The theoretical results based on this circuital approach are validated with full-wave numerical simulations. The results presented here can be extended to virtually any frequency dispersion synthesis, filter design procedure and/or functionality, thus opening up exciting possibilities in the design of composite materials with on-demand dispersion and high-performance and compact optical filters using one or two materials.

Dual-Core Photonic Crystal Fiber for Use in Fiber Filters

An asymmetrical dual-core photonic crystal fiber (DC-PCF), which possesses all circular air holes, is proposed. By setting appropriate geometrical parameters, the wavelength-selective coupling property is realized, and a compact optical filter with a short length of 1.83 mm based on the DC-PCF is designed. The spectral transmission characteristics of the filter are investigated by the beam propagation method. The results demonstrate that the optical filter possesses a bandwidth of ~58 nm and small sidelobes. The proposed optical filter could be used in the integrated optical systems.

Box-like filter response using multimode single-ring microresonators.

In this work we suggest a new technique to form a box-like filter response using a single microring resonator. The new design is based on exploiting the multimode propagation in the ring. The technique is implemented in the case of a two-mode ring resonator for different designs. The obtained results show that the new designs allow getting 1 to 10dB spectral width shape factor better than 0.4, as verified by the FDTD simulation at different wavelengths and for different spectral widths. This new design methodology opens the door for more compact optical filters with controlled box-like response.

Subwavelength Grating Structure with High Aspect Ratio and Tapered Sidewall Profiles

ABSTRACTCMOS-compatible fabrication and etching processes are often used in subwavelength grating structures manufacturing, it normally generates tapered sidewall profile of the gratings. In this work, we have studied the impacts on resonance mode characteristics of subwavelength grating structures due to the tapered sidewall profile, as well as grating with high aspect ratio. Our simulation results have revealed that both of these two factors play important roles on the resonance mode behavior of subwavelength grating devices. We also discussed the mechanism between the guided mode resonance and the grating cavity mode resonance. Our study will provide guidance for a series of integrated photonics devices applications, such as compact optical filter, photonics amplifier, and lasers, while the realistic subwavelength grating structure is considered.

Impacts of Tapered Sidewall Profiles With High Aspect Ratio on Subwavelength Grating Structure

We have numerically demonstrated the significant impacts on the resonance mode characteristics of subwavelength grating structures due to tapered sidewall profile and high aspect ratio, which is normally obtained due to the practical CMOS-compatible fabrication and etching processes. Our simulation results have revealed that the tapered sidewall profile with high aspect ratio plays important roles on the resonance mode behavior of subwavelength grating photonic devices. The coupling mechanism between the guided mode resonance and the grating cavity is also emphasized. Our numerical studies can be utilized for a series of integrated photonic devices applications, such as compact optical filter, photonic amplifier, and lasers, while considering realistic subwavelength grating structures due to current nanoelectronics fabrication process.

Significant role of photonic crystal resonant cavities in WDM and DWDM communication tunable filters

A method of photonic crystal filter using resonant cavities in two dimensional structures suitable for WDM and DWDM communication applications is proposed. For the design of the structure, movement of defects as resonant cavities is utilized. Tuning the desired wavelengths with characteristics according to ITU-T standard is shown. By changing characteristics such as radius of defects, distance of them from each other and location of defects, a compact optical filter proper for optical integrated circuits is proposed. Also, the combined effect of introduction of variable radius of defects and distances of them from each other, resulted interesting characteristics.

Compact optical filter for dual-wavelength fluorescence-spectrometry based on enhanced transmission through metallic nano-slit array

A novel optical filter based on enhanced transmission through metallic nano-slit is proposed for dual-wavelength fluorescence-spectrometry. A special structure, sampled-period slit array, is utilized to meet the requirement of dual-wavelength transmission in this system. Structure parameters on the transmission property are analyzed by means of Fourier transformation. With the features both to enhance the fluorescence generation and to enhance light transmission, in addition with the feasibility for miniaturization, integration on one chip, and mass production, the proposed filters are promising for the realization of dual-wavelength fluorescence-spectrometry in micro-total-analysis-system.

Proposal for Compact Optical Filters Using Large Index Step Binary Supergratings

Compact optical filters are proposed using an aperiodic grating of fixed element size [i.e., a binary supergrating (BSG)] with a large refractive index step. These filters allow for almost arbitrary wavelength filtering, yet they are more compact than previous demonstrations of BSG. The filters are designed using a combination of Boolean particle swarm optimization (B-PSO) and a one-dimensional transfer matrix method. To demonstrate the compact device size, several 50-mum-long single-wavelength transmission filters are demonstrated theoretically, each having a different wavelength while using the same structural parameters. A multiwavelength filter is also proposed in an 80-mum-long structure to show the versatility of the large refractive index step BSG. A genetic algorithm is substituted for the B-PSO; however, B-PSO shows better performance here. This work may be applied to produce compact optical filters for intrachip optical networks and compact tunable lasers, while using existing single-step photolithography processes.

Increasing the free spectral range of silica waveguide rings for filter applications

Ring resonators are promising building blocks for developing compact optical filters with arbitrary functions; however, a major challenge for planar waveguide filter implementations is to overcome the limited free spectral range (FSR) for a given core-to-cladding refractive-index difference (D) while maintaining low loss and a large range of coupling ratios. The loss and coupling of rings operating in the whispering-gallery-mode regime were investigated by use of Ge-doped silica waveguides with a low refractive-index difference (D=0.75%) . A FSR of 12 GHz with a loss of 0.36 dB/cm was demonstrated with coupling ratios of 30% (41% optimum).