In this paper a generalized approach for modeling and design of photonic sensors based on Vernier effect is presented. Design criteria of integrated optical architectures based on multiple ring resonators are found, customized as a function of a specific gas to be monitored. Very efficient performance has been demonstrated in mid-IR operative wavelength range. Sensitivities of the order of 105 nm/RIU and limits of detection as low as 10^−5 RIU allow detecting very small traces of methane and ethane in air.

Fiber optic sensors are typically used with expensive tunable lasers or optical spectrum analyzers for wavelength interrogation. We propose to replace the tunable laser by a broadband optical source incorporated with novel thin linewidth acousto-optic tunable filter. It utilizes optical beam expanders constituted by photonic crystal rows of air holes in LiNbO3 waveguide. A new design is numerically studied for a short structure (with 32 photonic crystal rows) by 2D FDTD method. Extrapolation of these results to larger structure sizes (about 1 cm) demonstrates the possibility to develop compact interrogators with 0.4 pm wavelength resolution and 40 nm tunable range around 1550 nm.

This review presents the state-of-the-art of Bio-Chemical nanosensors based on silicon Photonics. SOI (Silicon on Insulator) technology offers a number of guiding structures such as slot, rib, strip and silicon wire waveguides. We discuss many sensing principles employed in optical detection, such as absorbance, reflectance, fluorescence, chemiluminescence, bioluminescence and refractive index (RI) measurement. The integration of ultra high sensitive photonic waveguides in interferometer architectures (e.g. Mach Zehnder interferometer) and resonant architectures (e.g. ring resonator, Fabry Perot cavity), allow the detection of ultra low traces of chemical/biochemical analytes and gases. Sensing performance of photonic nanosensors based on silicon Photonics are very attractive, exhibiting ultra high sensitivity and low limit of detection (i.e. pg/mL, pmol/L). Intensive research in this field is motivated by broad applications of photonic sensors in healthcare, environmental monitoring, homeland security, food industry, pharmaceuticals, which require sensitive and rapid analytical tools.

Recently, the Vernier effect has been proved to be very efficient for significantly improving the sensitivity and the limit of detection (LOD) of chemical, biochemical and gas photonic sensors. In this paper a review of compact and efficient photonic sensors based on the Vernier effect is presented. The most relevant results of several theoretical and experimental works are reported, and the theoretical model of the typical Vernier effect-based sensor is discussed as well. In particular, sensitivity up to 460 mu m/RIU has been experimentally reported, while ultra-high sensitivity of 2,500 mu m/RIU and ultra-low LOD of 8.79 x 10(-8) RIU have been theoretically demonstrated, employing a Mach-Zehnder Interferometer (MZI) as sensing device instead of an add drop ring resonator.

Nowadays, optical devices and circuits are becoming fundamental components in several application fields such as medicine, biotechnology, automotive, aerospace, food quality control, chemistry, to name a few. In this context, we propose a complete review on integrated photonic sensors, with specific attention to material, technologies, architectures and optical sensing principles. To this aim, sensing principles commonly used in optical detection are presented, focusing on sensor performance such as sensitivity, selectivity and rangeability. Since photonic sensors allow substantial benefits regarding compatibility with CMOS technology and integration on chip characterized by micrometric footprints, design and optimization strategies of photonic devices are widely discussed for sensing applications. In addition, several numerical methods employed in photonic circuits and devices, simulations and design are presented, focusing on their advantages and drawbacks. Finally, recent developments in the field of photonic sensing are reviewed, considering advanced photonic sensor architectures based on linear and non-linear optical effects and to be employed in chemical/biochemical sensing, angular velocity and electric field detection.

In this paper, an investigation of silicon nanocrystals-based sandwiched slot waveguides which are dispersion engineered for exciting optical solitons inside very short structures (only 3 mm long) is proposed. The possibility of using these structures for efficient surface sensing devices has been explored for the first time in cases of either air or water solution cover.

In this paper the wavelength interrogation of optical sensors by novel thermo-optic tunable filters with multiple partially reflected slanted mirrors in silicon-oninsulator (SOI) technology is presented. The tunable filter is used both for tunable band selection of broadband light source interrogating the optical sensors, as well as for precise detection of interrogation optical wavelength. New interrogator design is numerically studied by 2D FDTD and FEM for a compact structure. The investigation demonstrates the performance of an interrogator on SOI, 1 cm long, having about 1 pm wavelength resolution and 1 ms scanning over 40 nm range at 1550 nm, with an operation power of about 16 mW.

In this paper, a resonant sensor formed by a silicon-on-insulator waveguiding Bragg grating ring resonator working in linear and non-linear regime is proposed. In linear regime, the device shows a spectral response characterized by a photonic band gap (PBG). Very close to the band gap edges, it exhibits split resonant modes having a splitting magnitude equal to the PBG spectral extension, which is almost insensitive to the fabrication tolerances. When the device operates in nonlinear regime, exactly in that spectral region showing the split resonant mode structure, the sensing performance is strongly improved. This improvement has been demonstrated through a detailed model based on a set of full-vectorial equations taking into account not only all non-linear effects excited in the integrated silicon structure (i.e. Two Photon Absorption (TPA), TPA-induced Free Carrier Absorption, plasma dispersion, Self-Phase-Modulation and Cross-Phase-Modulation effects as induced by Kerr nonlinearity), but also the deleterious thermal and stress effects affecting the sensor performance.

In this paper a detailed investigation of novel photonic sensors based on slot waveguides has been carried out. Appropriate alloys of group IV materials, such as germanium (Ge), silicon (Si), carbon (C) and tin (Sn), are applied in silicon-on-insulator (SOI) technology for homogeneous optical sensing at 2.883 µm and 3.39 μm. Electronic and optical properties of group IV alloys have been investigated. In addition, we have designed novel group IV vertical slot waveguides in order to achieve ultra-high sensitivities, as well as good fabrication tolerances. All these features have been compared with well-known SOI slot waveguides for optical label-free homogeneous sensing at 1.55 µm. In conclusion, theoretical investigation of ring resonators based on these novel slot waveguides has revealed very good results in terms of ultra high sensing performance of methane gas, i.e., limit of detection ~ 3.6×10-5 RIU and wavelength sensitivity > 2×103 nm/RIU.

Advances in silicon photonics have resulted in rapidly increasing complexity in integrated circuits. New methods that allow the direct characterization of individual optical components in situ, without the need for additional fabrication steps or test structures, are desirable. Here, we present a device-level method for the characterization of photonic chips based on a highly localized modulation in the device using pulsed laser excitation. Optical pumping perturbs the refractive index of silicon, providing a spatially and temporally localized modulation in the transmitted light, enabling time- and frequencyresolved imaging. We demonstrate the versatility of this all-optical modulation technique in imaging and in the quantitative characterization of a range of properties of silicon photonic devices, from group indices in waveguides, to quality factors of a ring resonator, and to the mode structure of a multimode interference device. Ultrafast photomodulation spectroscopy provides important information on devices of complex design, and is easily applicable for testing at the device level.

Slot waveguides are becoming more and more attractive optical components, especially for chemical and bio-chemical sensing. In this paper an accurate analysis of slot waveguides fabrication tolerances is carried out, in order to find optimum design criteria for either homogeneous or absorption sensing mechanisms, in cases of low and high aspect ratio slot waveguides. In particular, we have focused on Silicon On Insulator (SOI) technology, representing the most popular technology for this kind of devices, simultaneously achieving high integration capabilities, small dimensions and low cost. An accurate analysis of single mode behavior for high aspect ratio slot waveguide has been also performed, in order to provide geometric limits for waveguide design purposes. Finally, the problem of coupling into a slot waveguide is addressed and a very compact and efficient slot coupler is proposed, whose geometry has been optimized to give a strip-slot-strip coupling efficiency close to 100%.

The mid-infrared wavelength region offers a plethora of possible applications ranging from sensing, medical diagnostics and free space communications, to thermal imaging and IR countermeasures. Hence group IV mid-infrared photonics is attracting more research interest lately. Sensing is an especially attractive area as fundamental vibrations of many important gases are found in the 3 to 14 μm spectral region. To realise group IV photonic mid-infrared sensors several serious challenges need to be overcome. The first challenge is to find suitable material platforms for the mid-infrared. In this paper we present experimental results for passive mid-infrared photonic devices realised in silicon-on-insulator (SOI), silicon-on-sapphire (SOS), and silicon on porous silicon (SiPSi). Although silicon dioxide is lossy in most parts of the mid-infrared, we have shown that it has potential to be used in the 3-4 μm region. We have characterized SOI waveguides with < 1 dB/cm propagation loss. We have also designed and fabricated SOI passive devices such as MMIs and ring resonators. For longer wavelengths SOS or SiPSi structures could be used. An important active device for long wavelength group IV photonics will be an optical modulator. We present relationships for the free-carrier induced electro-refraction and electro-absorption in silicon in the mid-infrared wavelength range. Electro-absorption modulation is calculated from impurity-doping spectra taken from the literature, and a Kramers-Kronig analysis of these spectra is used to predict electro-refraction modulation. We have examined the wavelength dependence of electro-refraction and electro-absorption, and found that the predictions suggest longer-wave modulator designs will in many cases be different than those used in the telecom range.

In this paper an ultra high sensitivity chemical photonic sensor is proposed, employing a Mach-Zehnder Interferometer (MZI)-Enhanced Vernier Effect. The proposed sensor is demonstrated to reach ultra high overall sensitivity (> 1000 μm/RIU) and very low limit of detection (LOD < 10 -6 RIU), with a compact, standard SOI chip. Furthermore, two very efficient sensors are designed. The first one is a CO2 sensor for health safety purpose, able to detect gas concentration as low as 5,000 ppm. The second one is an ammonia sensor in aqueous solution, able to detect ammonia concentrations down to 2 ppm. The designed sensors are proposed in Silicon On Insulator (SOI) technology due to its compatibility with standard CMOS technology, but the Enhanced Vernier Effect could be applied to each technology allowing to simultaneously fabricate ring resonators and MZIs.