In this paper, we report a fiber-optic pressure sensor fabricated by three-dimensional (3D) printing of glass using direct laser melting method. An all-glass fiber-housing structure is 3D printed on top of a fused silica substrate, which also serves as the pressure sensing diaphragm. And an optical fiber can be inserted inside the fiber housing structure and brought in close proximity to the diaphragm to form a Fabry-Perot interferometer. The theoretical analysis and experimental verification of the pressure sensing capability are presented.

Benefitting from the advancement of digital electronics in the 20th century, sensors have also gradually evolved from analog to digital. As semiconductor components are required in digital sensors, a power supply is still needed for the sensor. To eliminate the need for a power supply on the sensor, a concept of passive digital sensing is proposed. Different from the semiconductor-based electrical digital sensing method, the parameter to be measured is first converted to a mechanical signal, then the mechanical signal is encoded to multiple digital bits by a passive digitizer, where each bit is identified as open or short state that is detectable by general IO ports. To demonstrate the proposed sensing concept, a passive RFID temperature sensor is implemented using a bimetallic coil as the temperature sensing unit. The 6-bit binary codes are acquired and transmitted by a passive RFID that is powered by an energy harvester. The test results show that the temperature sensor has a measurement sensitivity of 1.32 °C/bit and a working distance over 10 meters.

In this paper, we propose and demonstrate a novel corrosion-resistant, embeddable open-end coaxial cable soil moisture sensor. This microwave resonator is constructed using two reflectors along the coaxial line. The first reflector is a metal post at the signal input end, short-circuiting the inner conductor to the outer conductor. The second reflector comprises a welded metal plate parallel to the open-end of the coaxial line, maintaining a fixed gap. A moisture-sensitive Polyvinyl Alcohol (PVA) film is inserted into this gap. The resonance frequency of the open-end coaxial cable resonator is highly dependent on the fringe capacitance, which varies with soil moisture levels. As such, tracking resonance frequency changes allows for correlation with soil moisture fluctuations. We provide a detailed discussion of the embeddable open-end microwave coaxial cable resonator (EOE-MCCR) mathematical model and a proof of concept for soil moisture measurement. The demonstration experiments investigate soil moisture content ranging from 4% to 24%. The prototype device exhibits a soil moisture measurement sensitivity of 0.76MHz/% for soil moisture between 4% and 10%, and 1.44MHz/% for soil moisture between 10% and 24%. The soil moisture sensor presented here is robust, easy to manufacture, chemically resistant, low-cost, and suitable for long-term applications and potential industrial uses. This innovative sensor is ideal for sensing applications in harsh environments, advancing the field of chemical trace sensing.

A microwave photonics method has been developed for measuring distributed acoustic signals. This method uses microwave-modulated low coherence light as a probe to interrogate distributed in-fiber interferometers, which are used to measure acoustic-induced strain. By sweeping the microwave frequency at a constant rate, the acoustic signals are encoded into the complex microwave spectrum. The microwave spectrum is transformed into the joint time–frequency domain and further processed to obtain the distributed acoustic signals. The method is first evaluated using an intrinsic Fabry Perot interferometer (IFPI). Acoustic signals of frequency up to 15.6 kHz were detected. The method was further demonstrated using an array of in-fiber weak reflectors and an external Michelson interferometer. Two piezoceramic cylinders (PCCs) driven at frequencies of 1700 Hz and 3430 Hz were used as acoustic sources. The experiment results show that the sensing system can locate multiple acoustic sources. The system resolves 20 nε when the spatial resolution is 5 cm. The recovered acoustic signals match the excitation signals in frequency, amplitude, and phase, indicating an excellent potential for distributed acoustic sensing (DAS).

A noncontact liquid level sensor using a cylindrical coplanar waveguide (CCPW) is proposed. The concept is demonstrated using a prototype device made by directly spraying conductive paint on the outer surface of a glass cylinder. The position of the air-liquid interface is measured using the time domain reflection signal from the CCPW. The geometric parameters of the prototype sensor were optimized using a full-wave simulation tool. The experimental results show that the prototype CCPW-based liquid level sensor has a resolution of 29.8 μm over a range of 210 mm at the sweeping frequency bandwidth of 500 MHz - 4 GHz. The tradeoff between the resolution and the measurement range was discussed. The proposed sensor provides a low-cost, easy-to-fabricate, and noncontact method for liquid level measurement, which has potential applications in fuel storage, chemical production, and wastewater treatment.

This paper presents a fiber optic rotary encoder-based pressure sensor for downhole applications. The rotary encoder, which is composed of an encoding pad and an all-glass optical fiber sensing head, converts the rotation angles of a pressure transducer to digital codes. The optical fiber only serves as the telemetry channel to directly transmit the data in digital format, such that the environmental temperature influence and the long-time drift are minimized. The glass additive and subtractive manufacturing techniques are used to embed the multi-channel optical fibers into a bulk-fused silica glass substrate. Because the fused silica glass shares a similar material property to the optical fiber (e.g., thermal expansion coefficient), the optical fiber sensing head is well sealed and fixed even at an elevated temperature. The picosecond laser blackening technique is used to fabricate the high reflectivity contrast encoding pad with high accuracy. The proposed pressure sensor was manufactured and experimentally verified to have a signal-to-noise ratio of 23 dB and a linear pressure response with a root-mean-square error of 0.37. During the 400-hour 250°C long-term stability test, there was no digital code variation. In addition, a mathematical model to study the relationships between the sensor’s performances and design parameters were established. According to the model, a 12-bit encoder (0.02% pressure sensitivity) can be achieved with a radius of 20 mm.