Cascaded ring resonator-based temperature sensor with simultaneously enhanced sensitivity and range

Silica-Polymer Heterogeneous Hybrid Integrated Mach-Zehnder Interferometer Optical Waveguide Temperature Sensor

In this paper, a temperature sensor based on a polymer-silica heterogeneous integrated Mach-Zehnder interferometer (MZI) structure is proposed. The MZI structure consists of a polymer waveguide arm and a doped silica waveguide arm. Due to the opposite thermal optical coefficients of polymers and silica, the hybrid integrated MZI structure enhances the temperature sensing characteristics. The direct coupling method and side coupling method are introduced to reduce the coupling loss of the device. The simulation results show that the side coupling structure has lower coupling loss and greater manufacturing tolerance compared to the direct coupling structure. The side coupling loss for PMMA material-based devices, NOA material-based devices, and SU-8 material-based devices is 0.104 dB, 0.294 dB, and 0.618 dB, respectively. The sensitivity (S) values of the three hybrid devices are -6.85 nm/K, -6.48 nm/K, and -2.30 nm/K, which are an order of magnitude higher than those of an all-polymer waveguide temperature sensor. We calculated the temperature responsivity (RT) (FSR→∞) of the three devices as 13.16 × 10-5 K, 32.20 × 10-5 K, and 20.20 × 10-5 K, suggesting that high thermo-optic coefficient polymer materials and the hybrid integration method have a promising application in the field of on-chip temperature sensing.

Measurement accuracy in silicon photonic ring resonator thermometers: identifying and mitigating intrinsic impairments

Silicon photonic ring resonator thermometers have been shown to provide temperature measurements with a 10 mK accuracy. In this work we identify and quantify the intrinsic on-chip impairments that may limit further improvement in temperature measurement accuracy. The impairments arise from optically induced changes in the waveguide effective index, and from back-reflections and scattering at defects and interfaces inside the ring cavity and along the path between light source and detector. These impairments are characterized for 220 × 500 nm Si waveguide rings by experimental measurement in a calibrated temperature bath and by phenomenological models of ring response. At different optical power levels both positive and negative light induced resonance shifts are observed. For a ring with L = 100 µm cavity length, the self-heating induced resonance red shift can alter the temperature reading by 200 mK at 1 mW incident power, while a small blue shift is observed below 100 µW. The effect of self-heating is shown to be effectively suppressed by choosing longer ring cavities. Scattering and back-reflections often produce split and distorted resonance line shapes. Although these distortions can vary with resonance order, they are almost completely invariant with temperature for a given resonance and do not lead to measurement errors in themselves. The effect of line shape distortions can largely be mitigated by tracking only selected resonance orders with negligible shape distortion, and by measuring the resonance minimum wavelength directly, rather than attempting to fit the entire resonance line shape. The results demonstrate the temperature error due to these impairments can be limited to below the 3 mK level through appropriate design choices and measurement procedures.

Polymer and Hybrid Optical Devices Manipulated by the Thermo-Optic Effect

The thermo-optic effect is a crucial driving mechanism for optical devices. The application of the thermo-optic effect in integrated photonics has received extensive investigation, with continuous progress in the performance and fabrication processes of thermo-optic devices. Due to the high thermo-optic coefficient, polymers have become an excellent candidate for the preparation of high-performance thermo-optic devices. Firstly, this review briefly introduces the principle of the thermo-optic effect and the materials commonly used. In the third section, a brief introduction to the waveguide structure of thermo-optic devices is provided. In addition, three kinds of thermo-optic devices based on polymers, including an optical switch, a variable optical attenuator, and a temperature sensor, are reviewed. In the fourth section, the typical fabrication processes for waveguide devices based on polymers are introduced. Finally, thermo-optic devices play important roles in various applications. Nevertheless, the large-scale integrated applications of polymer-based thermo-optic devices are still worth investigating. Therefore, we propose a future direction for the development of polymers.

A Design of a Novel Silicon Photonics Sensor with Ultra-Large Free Spectral Range Based on a Directional Coupler-Assisted Racetrack Resonator (DCARR)

A novel refractive index-based sensor implemented within a silicon photonic integrated circuit (PIC) is reported. The design is based on a double-directional coupler (DC) integrated with a racetrack-type resonator (RR) to enhance the optical response to changes in the near-surface refractive index via the optical Vernier effect. Although this approach can give rise to an extremely large 'envelope' free spectral range (FSR_Vernier), we restrict the design geometry to ensure this is within the traditional silicon PIC operating wavelength range of 1400-1700 nm. As a result, the exemplar double DC-assisted RR (DCARR) device demonstrated here, with FSR_Vernier = 246 nm, has a spectral sensitivity S_Vernier = 5 × 10⁴ nm/RIU.

A Review of Photonic Sensors Based on Ring Resonator Structures: Three Widely Used Platforms and Implications of Sensing Applications

Optical ring resonators (RRs) are a novel sensing device that has recently been developed for several sensing applications. In this review, RR structures based on three widely explored platforms, namely silicon-on-insulator (SOI), polymers, and plasmonics, are reviewed. The adaptability of these platforms allows for compatibility with different fabrication processes and integration with other photonic components, providing flexibility in designing and implementing various photonic devices and systems. Optical RRs are typically small, making them suitable for integration into compact photonic circuits. Their compactness allows for high device density and integration with other optical components, enabling complex and multifunctional photonic systems. RR devices realized on the plasmonic platform are highly attractive, as they offer extremely high sensitivity and a small footprint. However, the biggest challenge to overcome is the high fabrication demand related to such nanoscale devices, which limits their commercialization.

High-resolution silicon photonic sensor based on a narrowband microwave photonic filter

Microwave photonic sensors are promising for improving sensing resolution and speed of optical sensors. In this paper, a high-sensitivity, high-resolution temperature sensor based on microwave photonic filter (MPF) is proposed and demonstrated. A micro-ring resonator (MRR) based on silicon-on-insulator is used as the sensing probe to convert the wavelength shift caused by temperature change to microwave frequency variation via the MPF system. By analyzing the frequency shift with high-speed and high-resolution monitors, the temperature change can be detected. The MRR is designed with multi-mode ridge waveguides to reduce propagation loss and achieves an ultra-high Q factor of 1.01 × 10⁶. The proposed MPF has a single passband with a narrow bandwidth of 192 MHz. With clear peak-frequency shift, the sensitivity of the MPF-based temperature sensor is measured to be 10.22 GHz/°C. Due to higher sensitivity and ultra-narrow bandwidth of the MPF, the sensing resolution of the proposed temperature sensor is as high as 0.019 °C.

Embedded racetrack microring resonator sensor based on GeSbSe glasses

In this article, a compact racetrack double microring resonator (MRR) sensor based on Ge₂₈Sb₁₂Se₆₀ (GeSbSe) is investigated. The sensor device consists of a racetrack microring, an embedded small microring, and a strip waveguide. Electron beam lithography (EBL) and dry etching are used to fabricate the device. The compact racetrack double MRR device are obtained with Q-factor equal to 7.17 × 10⁴ and FSR of 24 nm by measuring the transmission spectrum. By measuring different concentrations of glucose solutions, a sensitivity of 297 nm/RIU by linear fitting and an intrinsic limit of detection (iLOD) of 7.40 × 10^-5 are obtained. It paves the way for the application of chalcogenide glasses in the field of biosensing.

Optical Temperature Sensor Based on Polysilicon Waveguides

Traditional temperature detection has limitations in terms of sensing accuracy and response time, while chip-level photoelectric sensors based on the thermo-optic effect can improve measurement sensitivity and reduce costs. This paper presents on-chip temperature sensors based on polysilicon (p-Si) waveguides. Dual-microring resonator (MRR) and asymmetric Mach-Zehnder interferometer (AMZI) sensors are demonstrated. The experimental results show that the sensitivities of the sensors based on AMZI and MRR are 86.6 pm/K and 85.7 pm/K, respectively. The temperature sensors proposed in this paper are compatible with the complementary metal-oxide-semiconductor (CMOS) fabrication technique. Benefitting from high sensitivity and a compact footprint, these sensors show great potential in the field of photonic-electronic applications.

Temperature-insensitive optical sensors based on two cascaded identical microring resonators

We demonstrate a novel, to the best of our knowledge, temperature-insensitive optical sensor based on two cascaded identical microring resonators (CIMRR) in this Letter. The structural parameters of the reference ring and sensing ring are designed to be identical. The upper cladding in the sensing windows of the two rings is removed. With different microfluidic channels, the reference ring and sensing ring are exposed to the reference solution and reagent sample, respectively. For wavelength interrogation experiments in the transmission spectrum contrast ratio and low-cost intensity interrogation experiments, the sensitivities of refractive index (RI) sensing are 3402.4 dB/RIU and 1087.3 dB/RIU, respectively, while the temperature sensitivities are as low as 0.023 dB/K and 0.0124 dB/K, respectively.

Design of a compact and highly sensitive modal interferometer using hybrid modes of a dielectric loaded plasmonic waveguide

We show the presence of hybridization between fundamental TE and first higher-order TM modes in a dielectric loaded plasmonic waveguide of appropriately chosen core dimensions. Furthermore, a critical hybridization point is achieved at which both modes have nearly equal fraction of the TE and TM polarizations. Exploiting the interference among such modes, we propose the design of a compact and highly sensitive modal interferometer. The bulk and surface sensitivities of the proposed sensor are found to be ∼3-10µm/RIU for refractive index (RI) ∼1.33-1.36 and ∼0.7nm/nm for an adsorbed layer of RI 1.45, respectively. The proposed sensor gives robust performance against fabrication imperfections and is stable against temperature fluctuations due to extremely low temperature cross-sensitivity (∼10-15pm/^∘C for a temperature change up to ∼100^∘C).

An On-Chip Silicon Photonics Thermometer with Milli-Kelvin Resolution

Photonic-based thermometers have been attracting intense research interest as a potential alternative to traditional electrical thermometers due to their physical and chemical stability and immunity to electromagnetic interference. However, due to the high requirements for the stability of the laser source, the existing studies on resolution are only theoretical predictions and do not include real-measured results. In this paper, we report on the fabrication and characterization of an on-chip silicon whispering-gallery-mode (WGM) ring resonator thermometer. The strip grating and the ring structure were fabricated on the silicon-on-insulator (SOI) substrate by two-step etching. The quality-factor (Q-factor), temperature sensitivity, and measurement range of the packaged device were 21,400, 42 pm/K, and 150 K, respectively. The real-measured temperature resolution of 2.9 mK was achieved by virtue of the power and polarization stabilization of the laser source.

High-sensitivity transverse-load and high-temperature sensor based on the cascaded Vernier effect

In this paper, we demonstrate a novel, to the best of our knowledge, transverse-load and high-temperature sensor based on the cascaded Vernier effect. Two Fabry-Perot interferometers fabricated by a piece of hollow-core fiber (HCF) and a piece of polarization-maintaining photonic crystal fiber (PM-PCF) are connected by a long part of single-mode fiber with a length of 1 m, and play the roles of transverse-load sensor and high-temperature sensor, respectively. The sensitivity of not only the transverse load but also that of temperature can be enhanced by the Vernier effect. The sensitivity of the transverse load is raised by 7.7 times to 5.84 nm/N, and the temperature sensitivities increased by 5.5 and 5.9 times to -0.0689nm/^∘C and -0.1038nm/^∘C within the temperature range of 50-400°C to 400-900°C. Moreover, both the HCF cavity and PM-PCF cavity can be split and combined flexibly. Hence, such a sensor could have great potential in sensing applications.

Ultra-sensitive silicon temperature sensor based on cascaded Mach-Zehnder interferometers

An ultra-sensitive temperature sensor without sacrificing detection range is demonstrated on the silicon-on-insulator (SOI) platform using cascaded Mach-Zehnder interferometers (MZIs). The sensitivity enhancement is achieved by tailoring the geometric parameters of the two MZIs to have similar free spectral ranges (FSRs) but quite different sensitivities. The proposed sensor only needs single lithography for the sensing unit, without introducing negative thermo-optic coefficient (TOC) materials. The measured sensitivity is 1753.7 pm/°C from 27°C to 67°C, which is higher than any reported results on a silicon platform and about 21.9 times larger than conventional all-silicon temperature sensors.

Photonic polymeric structures and electrodynamics simulation method based on a coupled oscillator finite-difference time-domain (O-FDTD) approach

We use femtosecond laser-based two-photon polymerization (TPP) to fabricate a 2.5D micropillar array. Using an angular detection setup, we characterize the structure's scattering properties and compare the results against simulation results obtained from a novel electrodynamics simulation method. The algorithm employs a modified formulation of the Lorentz Oscillator Model and a leapfrog time differentiation to define a 2D coupled Oscillator Finite-Difference Time-Domain (O-FDTD). We validate the model by presenting several simulation examples that cover a wide range of photonic components, such as multi-mode interference splitters, photonic crystals, ring resonators, and Mach-Zehnder interferometers.

Silicon Photonic Polarization Multiplexing Sensor with Both Large Range and High Resolution

A silicon photonic polarization multiplexing (PM) sensor featuring both a large range and a high resolution is proposed and experimentally demonstrated. The sensor includes a Fabry-Pérot (FP) resonator and a microring resonator (MRR) functioning as the sensing parts. With PM technology, the FP resonator only works on the transverse-electric mode while the MRR only on the transverse-magnetic mode. Thus, the proposed sensor can simultaneously achieve a large range with a short FP resonator and a high resolution with a high-Q MRR. Measured results show a range of 113 °C and a resolution of 0.06 °C for temperature sensing, and a range of 0.58 RIU (refractive index unit) with the resolution of 0.002 RIU for analyte refractive index sensing.

Highly sensitive silicon photonic temperature sensor based on liquid crystal filled slot waveguide directional coupler

A highly sensitive silicon photonic temperature sensor based on silicon-on-insulator (SOI) platform has been proposed and demonstrated. A two-mode nano-slot waveguide device structure cladded with a nematic liquid crystal (LC), E7, was adopted to facilitate strong light-matter interaction and achieve high sensitivity. The fabricated sensor was characterized by measuring the optical transmission spectra at different ambient temperatures. The extracted temperature sensitivities of the E7-filled device are 0.810 nm/°C around room temperature and 1.619 nm/°C near 50°C, which match well with simulation results based on a theoretical analysis. The results obtained represent the highest experimentally demonstrated temperature sensitivity for a silicon-waveguide temperature sensor on SOI platform. The slot waveguide directional coupler device configuration provides submicron one-dimensional spatial resolution and flexible selection in LC materials for designing temperature sensitivity and operational temperature range required by specific applications.

Thermo-Optical Tuning Cascaded Double Ring Sensor with Large Measurement Range

In this paper, a thermo-optic tuning optical waveguide sensor system based on a cascaded double micro-ring resonator is investigated. The system consists of a micro-ring resonator with the microheater as a reference ring and a micro-ring resonator with removing the upper cladding layers as a sensing ring, combined with a microfluidic control. The refractive index change of the sample is measured by the electric power change of the microheater. The experimental results show that the sensitivity of the thermo-optic tuning is 34.231 W/RIU (refractive index units), and the measurement range is 4.325 × 10^-3 RIU, almost eight times larger than that of the cascaded double micro-ring resonator without thermo-optic tuning for the intensity interrogation.

Releasing the light field in subwavelength grating slot microring resonators for athermal and sensing applications

Silicon-on-insulator (SOI) platforms have attracted increasing interest for photonic integrated devices with an ultra-small footprint. The distinct feature is the strong light confinement in the silicon region due to a high refractive-index-contrast. In contrast, releasing the light field out of the silicon region is also of great significance for providing a useful supplement to existing light guiding mechanisms and for facilitating versatile applications. Here, subwavelength grating slot (SWGS) microring resonators, which can effectively release light out of the silicon region for athermal and sensing applications, are proposed and demonstrated. The mechanism of releasing light relies on the combination of a surface enhanced supermode in a slot waveguide and a Bloch mode in a subwavelength grating waveguide. Four types of racetrack microring resonators (strip, slot, strip-SWGS, and slot-SWGS) were fabricated for comparison. The slot-SWGS microring resonator shows the best performance for athermal and sensing applications. The demonstrations may be useful for new releasing-light-enabled devices and applications.

Surface Plasmon Resonance-Based Temperature Sensor with Outer Surface Metal Coating on Multi-Core Photonic Crystal Fibre

We report an innovative design of a multi-core photonic crystal fibre-based surface plasmon resonance temperature sensor using ethanol and benzene as temperature-sensitive materials with a segmented outer-surface metal coating scheme. A stable sensing performance for a detection range of 10–80 ∘ C was found while using ethanol as the temperature-sensitive material; while using benzene both blue and red frequency shifts were observed. The maximum temperature sensitivities obtained from this proposed temperature sensor were 360 pm/ ∘ C and 23.3 nm/ ∘ C with resolutions of 2.78 × 10 − 1 ∘ C and 4.29 × 10 − 3 ∘ C, respectively, when using ethanol or benzene as the sensing medium.

Photonic temperature and wavelength metrology by spectral pattern recognition

Spectral pattern recognition is used to measure temperature and generate calibrated wavelength/frequency combs using a single silicon waveguide ring resonator. The ring generates two incommensurate interleaving TE and TM spectral combs that shift independently with temperature to create a spectral pattern that is unique at every temperature. Following an initial calibration, the ring temperature can be determined by recognizing the spectral resonance pattern, and as a consequence, the wavelength of every resonance is also known. Two methods of pattern-based temperature retrieval are presented. In the first method, the ring is locked to a previously determined temperature set-point defined by the coincidence of only two specific TE and TM cavity modes. Based on a prior calibration at the set-point, the ring temperature and hence all resonance wavelengths are then known and the resulting comb can be used as a wavelength calibration reference. In this configuration, all reference comb wavelengths have been reproduced within a 5 pm accuracy across an 80 nm range by using an on-chip micro-heater to tune the ring. For more general photonic thermometry, a spectral correlation algorithm is developed to recognize a resonance pattern across a 30 nm wide spectral window and thereby determine ring temperature continuously to 50 mK accuracy. The correlation method is extended to simultaneously determine temperature and to identify and correct for wavelength calibration errors in the interrogating light source. The temperature and comb wavelength accuracy is limited primarily by the linewidth of the ring resonances, with accuracy and resolution scaling with the ring quality factor.

Design Rule of Mach-Zehnder Interferometer Sensors for Ultra-High Sensitivity

A design rule for a Mach-Zehnder interferometer (MZI) sensor is presented, allowing tunable sensitivity by appropriately choosing the MZI arm lengths according to the formula given in this paper. The present MZI sensor designed by this method can achieve an ultra-high sensitivity, which is much higher than any other traditional MZI sensors. An example is given with silicon-on-insulator (SOI) nanowires and the device sensitivity is as high as 10⁶ nm/refractive-index -unit (or even higher), by choosing the MZI arms appropriately. This makes it possible for one to realize a low-cost optical sensing system with a detection limit as high as 10⁻⁶ refractive-index-unit, even when a cheap optical spectrum analyzer with low-resolution (e.g., 1 nm) is used for the wavelength-shift measurement.

Progress of infrared guided-wave nanophotonic sensors and devices

Nanophotonics, manipulating light-matter interactions at the nanoscale, is an appealing technology for diversified biochemical and physical sensing applications. Guided-wave nanophotonics paves the way to miniaturize the sensors and realize on-chip integration of various photonic components, so as to realize chip-scale sensing systems for the future realization of the Internet of Things which requires the deployment of numerous sensor nodes. Starting from the popular CMOS-compatible silicon nanophotonics in the infrared, many infrared guided-wave nanophotonic sensors have been developed, showing the advantages of high sensitivity, low limit of detection, low crosstalk, strong detection multiplexing capability, immunity to electromagnetic interference, small footprint and low cost. In this review, we provide an overview of the recent progress of research on infrared guided-wave nanophotonic sensors. The sensor configurations, sensing mechanisms, sensing performances, performance improvement strategies, and system integrations are described. Future development directions are also proposed to overcome current technological obstacles toward industrialization.

Tunable large free spectral range microring resonators in lithium niobate on insulator

Microring resonators are critical photonic components used in filtering, sensing and nonlinear applications. To date, the development of high performance microring resonators in LNOI has been limited by the sidewall angle, roughness and etch depth of fabricated rib waveguides. We present large free spectral range microring resonators patterned via electron beam lithography in high-index contrast Z-cut LNOI. Our microring resonators achieve an FSR greater than 5 nm for ring radius of 30 μm and a large 3 dB resonance bandwidth. We demonstrate 3 pm/V electro-optic tuning of a 70 μm-radius ring. This work will enable efficient on-chip filtering in LNOI and precede future, more complex, microring resonator networks and nonlinear field enhancement applications.

Ultra-sensitive polymeric waveguide temperature sensor based on asymmetric Mach-Zehnder interferometer

We proposed and designed an ultra-sensitive polymeric waveguide temperature sensor based on an asymmetric Mach-Zehnder interferometer that has different widths in the two interferometer arms. A polymer with a larger thermo-optic coefficient (TOC) was employed to enhance the sensitivity of the waveguide temperature sensor. The influence of the width difference between the two arms and the cladding materials with different TOCs on the sensitivity of the sensor was studied and experimentally demonstrated. The devices were fabricated by using the standard photolithography and simple all-wet etching process. When the cladding material Norland optical adhesive 73 (NOA 73) and the width difference of 6.5 μm were selected, the sensitivity of the waveguide temperature sensor was measured to be 30.8 nm/°C. Moreover, the minimum temperature resolution was about 0.97×10^-3°C. The proposed sensor has the distinct advantages of high sensitivity, high resolution, easy fabrication, low cost, and biological compatibility, which make it have potential applications in temperature detection of organisms, molecular analysis, and biotechnology.

Temperature-insensitive waveguide sensor using a ring cascaded with a Mach-Zehnder interferometer

We demonstrate a temperature-insensitive waveguide sensor based on a silicon-on-insulator platform. The sensor consists of a ring resonator and a Mach-Zehnder interferometer (MZI). A free spectral range of the sensing ring is designed to be slightly different from that of the MZI; hence, the Vernier effect can be employed to improve sensitivity. By optimizing structural parameters of the MZI, the envelope peak position of a cascaded transmission spectrum can be immune to the temperature variation, and only dependent on analyte change in the sensing area. The experimental results show that bulk refractive index (RI) sensitivity of the proposed sensor is 3552 nm/RI unit, while its temperature sensitivity is less than 4 pm/K, which is two orders of magnitude smaller than the conventional cascaded sensor structure without temperature compensation. The proposed temperature-insensitive waveguide sensor does not need polymer cladding or extra thermal stabilization, making it more robust in practical applications.

A Tellurium Oxide Microcavity Resonator Sensor Integrated On-Chip with a Silicon Waveguide

We report on thermal and evanescent field sensing from a tellurium oxide optical microcavity resonator on a silicon photonics platform. The on-chip resonator structure is fabricated using silicon-photonics-compatible processing steps and consists of a silicon-on-insulator waveguide next to a circular trench that is coated in a tellurium oxide film. We characterize the device’s sensitivity by both changing the temperature and coating water over the chip and measuring the corresponding shift in the cavity resonance wavelength for different tellurium oxide film thicknesses. We obtain a thermal sensitivity of up to 47 pm/°C and a limit of detection of 2.2 × 10⁻³ RIU for a device with an evanescent field sensitivity of 10.6 nm/RIU. These results demonstrate a promising approach to integrating tellurium oxide and other novel microcavity materials into silicon microphotonic circuits for new sensing applications.

Applications of Photonic Crystal Nanobeam Cavities for Sensing

In recent years, there has been growing interest in optical sensors based on microcavities due to their advantages of size reduction and enhanced sensing capability. In this paper, we aim to give a comprehensive review of the field of photonic crystal nanobeam cavity-based sensors. The sensing principles and development of applications, such as refractive index sensing, nanoparticle sensing, optomechanical sensing, and temperature sensing, are summarized and highlighted. From the studies reported, it is demonstrated that photonic crystal nanobeam cavities, which provide excellent light confinement capability, ultra-small size, flexible on-chip design, and easy integration, offer promising platforms for a range of sensing applications.

Temperature sensor with enhanced sensitivity based on silicon Mach-Zehnder interferometer with waveguide group index engineering

We propose a highly-sensitive temperature sensor employing a Mach-Zehnder interferometer (MZI) based on silicon-on-insulator (SOI) platform. The waveguide widths in the two MZI arms are tailored to have different temperature sensitivities but nearly the same group refractive indices. A temperature sensor with an enhanced sensitivity of larger than 438pm/°C is experimentally demonstrated, which is over seven times larger than that of conventional silicon optical temperature sensor (about 60pm/°C for quasi-TM mode). Moreover, the sensor is easy to fabricate, only by a single mask, and no need of any polymer cladding, which makes it more robust, and can be used in lab-on-chip systems as a temperature monitor.

Assessing Radiation Hardness of Silicon Photonic Sensors

In recent years, silicon photonic platforms have undergone rapid maturation enabling not only optical communication but complex scientific experiments ranging from sensors applications to fundamental physics investigations. There is considerable interest in deploying photonics-based communication and science instruments in harsh environments such as outer space, where radiation damage is a significant concern. In this study, we have examined the impact of cobalt-60 γ-ray radiation up to 1 megagray (MGy) absorbed dose on silicon photonic devices. We do not find any systematic impact of radiation on passivated devices, indicating the durability of passivated silicon devices under harsh conditions.

Silicon-on-insulator microring resonator sensor based on an amplitude comparison sensing function

A novel, highly sensitive integrated sensor based on a silicon-on-insulator microring resonator is proposed and experimentally demonstrated. To achieve a fast-response and cost-effective sensing system, the new structure establishes a linear amplitude comparison sensing function (ACSF) by monitoring the optical powers from both the through port and drop port of an add-drop microring resonator simultaneously, where the contrast of the two ports eliminates the effect of unexpected power fluctuation of the input laser on sensor performance. A highly enhanced linear relationship between the resonant wavelength shift and the ACSF value is achieved with an R-squared value over 0.99. A proof-of-concept experiment for temperature sensing demonstrates an almost constant ACSF with only ±0.9% discrepancy, while the laser power is varied between 0 dBm and -7  dBm.

Towards Replacing Resistance Thermometry with Photonic Thermometry

Resistance thermometry provides a time-tested method for taking temperature measurements that has been painstakingly developed over the last century. However, fundamental limits to resistance-based approaches along with a desire to reduce the cost of sensor ownership and increase sensor stability has produced considerable interest in developing photonic temperature sensors. Here we demonstrate that silicon photonic crystal cavity-based thermometers can measure temperature with uncertainities of 175 mK (k = 1), where uncertainties are dominated by ageing effects originating from the hysteresis in the device packaging materials. Our results, a ≈ 4-fold improvement over recent developments, clearly demonstate the rapid progress of silicon photonic sensors in replacing legacy devices.

Sensitivity enhanced fiber sensor based on a fiber ring microwave photonic filter with the Vernier effect

A temperature sensor employing the Vernier effect generated from a cascaded fiber rings based microwave photonic filter (MPF) is proposed and experimentally demonstrated. The structure of the fiber ring is used as a sensing element as well as the sampling and delaying component of the MPF in our proposed sensing scheme. The sensing characteristics of both single ring and cascaded fiber rings based sensors have been studied and compared. By employing two cascaded fiber rings of slightly different length, the Vernier effect can be generated in the frequency response of the MPF. The sensing interrogation of the cascaded fiber rings based sensor is conducted by detecting the frequency shift of the upper envelope of the measured frequency response curve. The experimental results show that the sensitivity of the cascaded fiber rings based sensor can be improved about 30 times compared with the single fiber ring based temperature sensor.

Ultrahigh-sensitive temperature sensor based on modal interference in a metal-under-clad ridge waveguide with a polymer upper cladding

We propose a highly sensitive temperature sensor based on modal interference in a metal-under-clad ridge waveguide (MUCRW) with polydimethylsiloxane as the upper cladding. The proposed sensor exploits the interference between the fundamental and the first higher order TE modes of the MUCRW. The increased fractional modal power in the ambient medium due to the metal under-cladding along with the high thermo-optic coefficient of the upper cladding results in a very significant change in the modal characteristics of the two interfering modes with temperature variation. Moreover, the effect of temperature change is more pronounced for the higher order mode compared with the fundamental mode, resulting in an ultrahigh sensitivity of the modal interference to the ambient temperature. The sensitivity of the proposed sensor structure is found to be as high as 8.35  nm/°C, which, to the best of our knowledge, is the highest reported sensitivity in any integrated optic waveguide-based temperature sensor.

High-speed optical sensor interrogator with a silicon-ring-resonator-based thermally tunable filter

On-chip optical interrogators employing a silicon-ring-resonator-based thermally tunable filter (SRRTF) offer a promising solution for realizing portable, compact optical sensing systems. However, the slow interrogation speed of conventional SRRTF-based interrogators (less than a few Hz) hinders real-time sensing of dynamic parameters. In this Letter, we report a 100 kHz of high-speed optical interrogation system based on the SRRTF. The speed enhancement is achieved by using the nonlinear transient response of the SRRTF to a square-wave input voltage. The entire spectral range of the SRRTF is scanned twice during its thermal response cycle. The time-domain sensor output signal, which is obtained by scanning the SRRTF over the sensor spectrum, is converted into spectrum domain based on the experimentally characterized, time-dependent resonance wavelength shifts of the SRRTF. With this system, we demonstrate high-speed interrogation of a fiber Bragg grating sensor under dynamic temperature change (200 Hz).

High-sensitivity and wide-range optical sensor based on three cascaded ring resonators

We demonstrate a three cascaded micro-ring resonators based refractive index optical sensor with a high sensitivity of 5866 nm/RIU, the measurement range of which is significantly improved 24.7 times comparing with the traditional two cascaded micro-ring resonators.

High sensitivity temperature sensor based on cascaded silicon photonic crystal nanobeam cavities

We present the design, fabrication and characterization of a high sensitivity temperature sensor based on cascaded silicon photonic crystal (PhC) nanobeam cavities. Two PhC nanobeam cavities, one with stack width modulated structure and the other one with parabolic-beam structure are utilized to increase the sensitivity. Most of the light is designed to be confined in the cladding and the core for these two cavities, respectively. Due to the positive thermo-optic (TO) coefficient of silicon and the negative TO coefficient of SU-8 cladding, the wavelength responses red shift for parabolic-beam cavity and blue shift for stack width modulated cavity as the increase of the ambient temperature, respectively. Thus, the sensitivity for the temperature sensor can be improved greatly since the difference in resonant wavelength shifts is detected for the temperature sensing. The experimental results show that the sensitivity of the temperature sensor is about 162.9 pm/°C, which is almost twice as high as that of the conventional silicon based resonator sensors.

Optical temperature sensor with enhanced sensitivity by employing hybrid waveguides in a silicon Mach-Zehnder interferometer

We report on a novel design of an on-chip optical temperature sensor based on a Mach-Zehnder interferometer configuration where the two arms consist of hybrid waveguides providing opposite temperature-dependent phase changes to enhance the temperature sensitivity of the sensor. The sensitivity of the fabricated sensor with silicon/polymer hybrid waveguides is measured to be 172 pm/°C, which is two times larger than a conventional all-silicon optical temperature sensor (~80 pm/°C). Moreover, a design with silicon/titanium dioxide hybrid waveguides is by calculation expected to have a sensitivity as high as 775 pm/°C. The proposed design is found to be design-flexible and robust to fabrication errors.