Packaged microbubble resonator optofluidic flow rate sensor based on Bernoulli Effect

Multimode sensing based on optical microcavities

Optical microcavities have the ability to confine photons in small mode volumes for long periods of time, greatly enhancing light-matter interactions, and have become one of the research hotspots in international academia. In recent years, sensing applications in complex environments have inspired the development of multimode optical microcavity sensors. These multimode sensors can be used not only for multi-parameter detection but also to improve measurement precision. In this review, we introduce multimode sensing methods based on optical microcavities and present an overview of the multimode single/multi-parameter optical microcavities sensors. Expected further research activities are also put forward.

Simultaneous temperature and pressure sensing based on a single optical resonator

We propose a dual-parameter sensor for the simultaneous detection of temperature and pressure based on a single packaged microbubble resonator (PMBR). The ultrahigh-quality (∼10⁷) PMBR sensor exhibits long-term stability with the maximum wavelength shift about 0.2056 pm. Here, two resonant modes with different sensing performance are selected to implement the parallel detection of temperature and pressure. The temperature and pressure sensitivities of resonant Mode-1 are -10.59 pm/°C and 0.1059 pm/kPa, while the sensitivities of Mode-2 are -7.69 pm/°C and 0.1250 pm/kPa, respectively. By adopting a sensing matrix, the two parameters are precisely decoupled and the root mean square error of measurement are ∼ 0.12 °C and ∼ 6.48 kPa, respectively. This work promises the potential for the multi-parameters sensing in a single optical device.

Label-free detection of cardiac troponin-I with packaged thin-walled microbubble resonators

The measurement of cardiac troponin-I (cTnI) is widely used for diagnosing acute myocardial infarction (AMI) diseases because of its myocardial specificity. Packaged microbubble resonators with thin wall are utilized for label-free and specific detection of cTnI based on whispering gallery mode (WGM). This packaged structure can provide a good protection for the biosensor, improve the anti-interference ability of the sensor and reduce the system noise. The theoretical detection limit of the biosensors for cTnI in phosphate buffer saline (PBS) is 0.4 ag mL^-1 (0.02 a_M ). Furthermore, we demonstrated that the biosensors can be used to detect cTnI molecules in simulated serum and the theoretical detection limit is also 0.4 ag mL^-1 (0.02 a_M ). These results are much far below the clinical cut-off value and show a huge application potential for the detection of cardiac biomarkers of AMI.

Optical Whispering-Gallery-Mode Microbubble Sensors

Whispering-gallery-mode (WGM) microbubble resonators are ideal optical sensors due to their high quality factor, small mode volume, high optical energy density, and geometry/design/structure (i.e., hollow microfluidic channels). When used in combination with microfluidic technologies, WGM microbubble resonators can be applied in chemical and biological sensing due to strong light–matter interactions. The detection of ultra-low concentrations over a large dynamic range is possible due to their high sensitivity, which has significance for environmental monitoring and applications in life-science. Furthermore, WGM microbubble resonators have also been widely used for physical sensing, such as to detect changes in temperature, stress, pressure, flow rate, magnetic field and ultrasound. In this article, we systematically review and summarize the sensing mechanisms, fabrication and packing methods, and various applications of optofluidic WGM microbubble resonators. The challenges of rapid production and practical applications of WGM microbubble resonators are also discussed.

Operando monitoring transition dynamics of responsive polymer using optofluidic microcavities

Optical microcavities have become an attractive platform for precision measurement with merits of ultrahigh sensitivity, miniature footprint and fast response. Despite the achievements of ultrasensitive detection, optical microcavities still face significant challenges in the measurement of biochemical and physical processes with complex dynamics, especially when multiple effects are present. Here we demonstrate operando monitoring of the transition dynamics of a phase-change material via a self-referencing optofluidic microcavity. We use a pair of cavity modes to precisely decouple the refractive index and temperature information of the analyte during the phase-transition process. Through real-time measurements, we reveal the detailed hysteresis behaviors of refractive index during the irreversible phase transitions between hydrophilic and hydrophobic states. We further extract the phase-transition threshold by analyzing the steady-state refractive index change at various power levels. Our technology could be further extended to other materials and provide great opportunities for exploring on-demand dynamic biochemical processes.

Optical flow sensor based on the thermal time-of-flight measurement

This paper presents a dielectric, all-optical thermal time-of-flight fluid flow velocity sensor. The proposed sensor utilizes a sequence of three short sections of optical fibers, which are positioned in a direction perpendicular to the measured fluid flow. One of these three fiber sections is highly doped with vanadium and acts as an optically controlled heater, while the other two fiber sections contain fiber Bragg gratings (FBG) that act as dynamic temperature sensors. The vanadium-doped fiber is heated periodically by a laser source, while observing temperature variations within the fluid flow downstream by the two fiber sections with inscribed Bragg gratings. The time delay in temperature variations recorded at both FBG sensors correlates directly with the flow rate of the fluid. When the sensor was placed within the glass capillary with inner diameter of 650 µm, it enabled a flow rate measurement range between 1 ml/h and 1200 ml/h. The sensor thus provides a broad flow-rate dynamic range and is insensitive to changes in losses in the lead optical fibers or optical heating source power fluctuations. Furthermore, the thermal properties of the measured liquid, for example, the liquid's thermal conductivity and heat capacity, have mostly limited effects on the measurement results, which allows for thermal-principle-based flow velocity measurements in cases of liquids with variable or poorly defined compositions.

Optical whispering-gallery mode barcodes for high-precision and wide-range temperature measurements

Temperature is one of the most fundamental physical properties to characterize various physical, chemical, and biological processes. Even a slight change in temperature could have an impact on the status or dynamics of a system. Thus, there is a great need for high-precision and large-dynamic-range temperature measurements. Conventional temperature sensors encounter difficulties in high-precision thermal sensing on the submicron scale. Recently, optical whispering-gallery mode (WGM) sensors have shown promise for many sensing applications, such as thermal sensing, magnetic detection, and biosensing. However, despite their superior sensitivity, the conventional sensing method for WGM resonators relies on tracking the changes in a single mode, which limits the dynamic range constrained by the laser source that has to be fine-tuned in a timely manner to follow the selected mode during the measurement. Moreover, we cannot derive the actual temperature from the spectrum directly but rather derive a relative temperature change. Here, we demonstrate an optical WGM barcode technique involving simultaneous monitoring of the patterns of multiple modes that can provide a direct temperature readout from the spectrum. The measurement relies on the patterns of multiple modes in the WGM spectrum instead of the changes of a particular mode. It can provide us with more information than the single-mode spectrum, such as the precise measurement of actual temperatures. Leveraging the high sensitivity of WGMs and eliminating the need to monitor particular modes, this work lays the foundation for developing a high-performance temperature sensor with not only superior sensitivity but also a broad dynamic range.