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Abstract
Flexible pressure sensors provide a promising platform for artificial smart skins, and photonic devices provide a new technique to fabricate pressure sensors. Here, we present a flexible waveguide-based optical pressure sensor based on a microring structure. The waveguide-based optical pressure sensor is based on a five-cascade microring array structure with a size of 1500 µm × 500 µm and uses the change in output power to linearly characterize the change in pressure acting on the device. The results show that the device has a sensing range of 0–60 kPa with a sensitivity of 23.14 µW/kPa, as well as the ability to detect pulse signals, swallowing, hand gestures, etc. The waveguide-based pressure sensors offer the advantages of good output linearity, high integration density and easy-to-build arrays.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Many physiological vital signs, such as pulse, blood pressure, swallowing, and respiration, can be monitored by wearable pressure sensors [1–6]. Electrical pressure sensors present the disadvantage of susceptibility to electromagnetic interference, and the emergence of optical pressure sensors provides a new option for wearable pressure sensors. Early optical pressure sensors were made of fibers [7–9]. Currently, optical fiber sensor development has been moving toward a high density of integration [10–12], but their densities continue to lag behind those of photonic integrated devices. The emergence of silicon-based optical waveguide-based pressure sensors, such as Mach–Zehnder modulators and waveguide Bragg gratings [13–15], has greatly improved the integration of optical pressure sensors. However, due to the limitation of silicon materials (high Young’s modulus), it is difficult to obtain high pressure sensitivity. Motivated by the extent of optical sensor usage and the importance of physiological signals, we investigated whether flexible polymer waveguide-based optical devices can serve as efficient pressure sensors to detect and process physiological signals.
In this paper, a fully polymer optical waveguide-based pressure sensor based on a microring structure is presented. The pressure sensor has a sensing range of 0–60 kPa with a sensitivity of 23.14 µW/kPa, and the pressure sensor has the ability to detect pulse signals, swallowing and hand gestures, which may have great applications in healthcare monitoring, human–machine interactions and soft robots.
2. Structure and simulation of the microring
2.1 Microring structure
The sensitivity of the pressure sensors based on the microring structure depends on the strain of the microring structure, so the materials of the microring structure must be flexible. Our microring structure adopts the three-layer waveguide structure of the lower cladding, core layer and upper cladding. Polydimethylsiloxane (PDMS) was used as the upper and lower cladding material of the microring structure, and polymethyl methacrylate (PMMA) was used as the core material of the microring structure. The Young’s moduli of PDMS and PMMA are 2.2 MPa and 3.16 GPa, respectively, which indicate the flexibility of the materials. In this article, the polymer waveguide has a width and height of 1 µm. The refractive indices were determined using an ellipsometer (J. A. Woollam, M-2000), and the refractive indices of PDMS and PMMA were 1.410 and 1.488, respectively.
Based on the core refractive index of 1.488 and cladding refractive index of 1.410, the relationship between waveguide width and effective refractive index for different modes is calculated, as shown in Fig. 1(a). It can be seen that the width of the single-mode cutoff waveguide is approximately below 1.8 µm. When the waveguide width and height are both 1 µm, only TE0 and TM0 modes exist in the polymer waveguide. Therefore, this work selects a waveguide width of 1 µm, a core layer thickness of 1 µm, and a cladding layer thickness of 10 µm. The mode field profile of the optical waveguide section with a height of 1 µm and a width of 1 µm obtained through the FDTD (Finite-Difference Time-Domain) method is shown in Fig. 1(b). It can be observed that the mode field is a single-mode field and that the light is confined within the core layer waveguide.
Fig. 1. Polymer waveguide. (a) The relationship between waveguide width and effective refractive index for different modes. (b) The mode field of the polymer waveguide.
The optical waveguide coupling effect is a key phenomenon involved in the sensor's operation. The essence of energy exchange and transmission in optical waveguides is the coupling between waveguide modes, which includes the transfer of energy from one waveguide to another and the entry of spatial optical waves into the waveguide. In a dual-channel directional coupler waveguide, at a certain interaction length, energy will alternately transfer from one waveguide to the other.
Two optical waveguides, 0 and 1, are represented by A0(z) and A1(z), respectively, where A0(z) and A1(z) denote the amplitudes of waveguide 0 and waveguide 1. The relationship between the two modes can be expressed as follows:
where k01 and k10 are the mutual coupling coefficients between the waveguides and β0 and β1 are the propagation parameters. When the two waveguide structures are identical, k01 = k10, and the propagation parameters are also the same, satisfying β0 =β1. Assuming that light enters waveguide 0 at z = 0, the boundary conditions are as follows: A0(0) = 1, A1(0) = 0. The solution to the Eq. is:The power flow within the waveguide alternates as the transmission distance z increases. The power distribution curve of two mutually coupled waveguides shows that when the power is completely transferred from waveguide 0 to waveguide 1, the coupling length for the interaction should satisfy:
According to the optical waveguide coupling effect, when the distance between two optical waveguides is sufficiently small, light couples from one waveguide to the other. For a traditional microring structure, the coupling efficiency between the straight waveguide and the microring waveguide is very sensitive to the gap of the coupling region. To reduce the control of the coupling gap, we adopted a relatively common runway-type microring structure (Fig. 2(a)). In this structure, a straight waveguide is added to the coupling region of the microring waveguide so that the coupling efficiency is related not only to the coupling gap but also to the coupling length. The sensing region is located at the center of the microring structure. The working principle of the device is as follows: the radius of the microring is modifiable through the shear stress displacement in the radius direction; therefore, the output power of the device changes under different pressures. The coupling region is constructed to simulate the coupling efficiency of the region in Rsoft. The optical field map and output optical power of the coupling region are shown in Fig. 2(b). After confirming the parameters of the coupling region, a microring optical waveguide device is constructed in Rsoft. Based on the optical waveguide coupling effect, a parametric sweep was carried out to obtain an appropriate coupling gap by Rsoft (Fig. 2(c)). The microring optical waveguide structure in this article aims to maximize the coupling efficiency, which means that L needs to satisfy Eq. (5). The appropriate coupling length L was determined by parametric sweep (Fig. 2(e)). Combined with the accuracy of the fabrication process, the final selected coupling gap is 1.26 µm, the final selected coupling length is 100 µm, and the coupling efficiency is 0.92. Taking a radius of 120 µm, the output optical intensity at the drop and through ends is calculated (Fig. 2(d)). It can be observed that the output optical intensity at the drop end is significantly higher than that at the through end, indicating that most of the light is output from the drop end.
Fig. 2. Design of a single microring structure. (a) Architecture and working principle of the microring structure. (b) The light field of the coupling region. (c) The light field of the whole single microring structure. (d) The parametric sweep of the coupling gap. (e) Parametric sweep of the coupling length.
An appropriate microring radius must be determined after designing the coupling region. Based on the determined coupling gap and coupling length, we conducted simulations to observe the light propagation in microring waveguides of varying radii (50, 100, 150, 200, 250 µm). The optical field maps are depicted in Fig. 3(a), while the corresponding output power from the drop end of the microring structure under different radii is illustrated in Fig. 3(b). To achieve a larger sensing area, it is preferable to employ a microring with a larger radius. However, an increase in radius also results in higher device loss within a certain range. When the radius becomes excessively large (e.g., 200 µm, 250 µm), the device loss escalates significantly, leading to a situation where light cannot be output from the drop end. Consequently, to strike a balance between a larger sensing area and lower device loss, a microring structure with a radius of 150 µm is ultimately selected. The simulation results of the output power of the microring structure with different radii are shown in Fig. 3(b). The final radius is 150 µm, and the output light intensity of the through and drop terminals is shown in Fig. 3(c) and (d). The loss of the microring structure (drop terminal) is 2.03 dB.
Fig. 3. Characterizations of the single microring structure. (a) Optical field of the microring structure. (b) The output optical power of the microring structure at different radii at the drop port. The monitored power of (c) the drop terminal and (d) the through terminal of the single microring structure under 1500 nm to 1600 nm.
2.2 Pressure sensor design and simulation
The mechanical properties of the single microring structure were analyzed. The thicknesses of the upper and lower cladding layers were 10 µm, and the thickness of the core layer was 1 µm. Since the cladding material content composed more than 95% of the overall device, the mechanical properties of the PDMS film with a thickness of 21 µm were analyzed. According to the size of the microring structure, a rectangular film of 500 µm × 500 µm was modeled in COMSOL Multiphysics (version 6.0) to study the relationship between applied pressures and strain. Strain causes the output power of the microring structure to change, which mainly affects the radius. The degree of strain can be deconstructed into three directional deformations, and the strain that mainly affects the output power of the microring structure is the strain parallel to the plane where the microring structure is located. That is, the horizontal strain is the effective strain (Fig. 4(a)). Finally, the relationship curve between the effective strain and pressure is shown in Fig. 4(b). The fitting curve is ε=0.06723P (ε is strain, P is pressure), which indicates that 1 kPa will cause a 67.23 nm strain in the horizontal direction.
Fig. 4. Characterization of the single microring structure pressure sensor. (a) Mechanical displacement field of the device. (b) The relationship curve of displacement and pressure. (c) The monitoring power of the drop end under different radii. (d) The sensitivity of the microring structure pressure sensor at 1550 nm.
It is necessary to combine the mechanical properties and optical properties to analyze the influence of the output power by pressure and obtain the sensitivity of the single microring structure pressure sensor. The output power of the drop terminal at different pressures is shown in Fig. 4(c), and the relationship between the monitor power and pressures at 1550 nm is shown in Fig. 4(d), which indicates that the sensitivity of the microring structure pressure sensor is 40 µW/kPa at 1550 nm (when the input optical power is 10 mW).
The sensing region is located at the center of the microring structure. The single microring has a smaller sensing region compared to the five-cascade microring array and it will be difficult to use. Moreover, a larger sensing area can generate greater deformation, which will also enhance the performance of the sensor. To achieve a higher sensitivity of the pressure senor, the single microring structure can be increased to a microring array structure. We thus designed a five-cascade microring array structure to realize a greater performance of the pressure sensor. The coupling gap between the microrings is still 1.26 µm, and a schematic diagram of the microring array structure is shown in Fig. 5(a). According to the size of the microring array structure, a rectangular PDMS film of 1500 µm × 500 µm was constructed in COMSOL to study the relationship between applied pressures and strain (Fig. 5(b)), and the relationship curve between pressures and strain is shown in Fig. 5(c). The resulting sensitivity of the microring array structure at 1550 nm is 150 µW/kPa (Fig. 5(d) and (e)).
Fig. 5. Characterization of the five-cascade microring structure array pressure sensor. (a) Schematic diagram of the five-cascade microring array structure. (b) Mechanical displacement field of the microring array structure. (c) The relationship curve of displacement and pressure. (d) The monitoring power of the drop end under different radii. (e) The sensitivity of the microring array structure pressure sensor at 1550 nm.
3. Device fabrication
The microring structure uses PMMA material to prepare the core layer and PDMS material to prepare the upper and lower cladding layers. The overall fabrication process of the microring structure is shown in Fig. 6(a). The substrate material used was ITO (Indium-Tin-Oxide) conductive glass (ITO, RDMICRO). A layer of LOR photoresist (LOR, Microchem) was spin-coated onto the substrate prior to spinning PDMS to facilitate the final stripping of the substrate. Subsequently, a PDMS film with a thickness of 10 µm was spin-coated on an ITO conductive glass sheet. During production, PDMS is hydrophobic after film formation. Thus, chemical modification of the PDMS surface must be executed. O2 plasma was used for PDMS chemical modification, and a mixed solution of the silylation reagent γ-(2,3 epoxypropoxy) propyltrimethoxysilane (GPTMS) and absolute ethanol with a volume fraction of 2% was used for a soaking treatment of 1 h to enhance the PDMS surface chemical modification effect. The fabrication process of the microring structure includes various techniques, such as spin coating, ultraviolet lithography and dry etching. The core layer pattern is patterned by an inductively coupled plasma (ICP) etching machine, and its minimum line width is 0.5 µm, which satisfies the design requirements. After the completion of spin-coating for the top layer, a developer solution was used to wash away the LOR photoresist, thereby separating the sensor from the substrate. After fabrication, the microring structure was bonded with fiber arrays (FAs) for easy testing. After the input/output ports of the microring pressure sensor are coupled with a fiber array (FA), one of the FAs is connected to a tunable laser, and the other FA is connected to an optical power meter. By inputting light at a wavelength of 1550 nm from the tunable laser (Santec, TSL-510), the output optical power was monitored with an optical power meter. When no stress is applied, the relationship between the output optical power and the input optical power is shown in Fig. 6(f), and the calculated device loss is shown in Fig. 6(e) (Loss = 10lg(Monitor power/Input power)). The loss of the microring structure was measured as 3.798 dB (Fig. 6(e) and (f)).
Fig. 6. Fabrication and characteristics of the microring structure. (a) Fabrication process of the microring structure. SEM photographs of the (b) five-cascade microring array structure, (c) single microring structure and (d) coupling region of the microring structure. (e) Loss of the five-cascade microring array structure. (f) The relationship between the input power and output power.
4. Optical and mechanical characterizations
As shown in Fig. 7, by utilizing a precision micropressure pump (Aileike, JDYFQ-600S), air pressure is precisely applied to the microring sensor. The precision micropressure pump offers a pressure range of 0-100 kPa with an accuracy of 1 Pa. To accommodate the dimensions of the microring sensor, a 1 mm diameter nozzle is chosen. The microring sensor chip is carefully positioned on a fiber-to-chip alignment system (PS-1000, Suruga Seiki), and the needle of the micropressure pump is securely fixed via a probe station. The airflow direction of the pump is perpendicular to the plane where the microring sensor is situated. Through manipulation of the applied air pressure, any corresponding changes in the output optical power can be effectively monitored.
The performance of the pressure sensors was thus measured. The output power of the pressure sensor under different pressures applied to the device is shown in Fig. 8(a). The relationship between the output power and the pressure for the pressure sensor was investigated and is summarized: The output power linearly increased until the pressure reached approximately 25 kPa, and then it gradually stabilized in the range of 25–60 kPa. The sensitivity regions are divided into three regions: high sensitivity region (0–25 kPa, 35.71 µW/kPa), moderate sensitivity region (25–60 kPa, 16.48 µW/kPa) and low sensitivity region (60–120 kPa, 1.38 µW/kPa). The sensing region of the sensor (0–25 kPa, 0–60 kPa, 0–120 kPa) was further recorded, as shown in Fig. 8(c)-(e). For the sensitive regions with higher sensitivity, such as 0-25 kPa and 0-60 kPa, we utilized a first-order linear fitting method to obtain the fitting curve. This choice was made because a first-order linear fitting can more effectively demodulate the detected stress level based on the output optical intensity. The fitting curve for the pressure-response curve between 0-25 kPa is represented by the Eq. y = 0.03571x + 3.0919, with an R2 value (coefficient of determination) of 0.9496 (Fig. 8(c)). Likewise, the fitting curve for the pressure-response curve between 0-60 kPa is described by the Eq. y = 0.02314x + 3.24725, with an R2 value of 0.9444 (Fig. 8(d)). However, when considering the entire sensing range of 0-120 kPa, employing a first-order linear fitting method would introduce significant errors. Consequently, we opted for a cubic fitting function to minimize these errors. The fitting curve for the pressure-response curve between 0-120 kPa is given by the Eq. y = 1.5969E-6x^3-4.6832E-4x^2 + 0.04585x + 3.05802, with an R2 value of 0.9951 (Fig. 8(e)). The final sensing range of the pressure sensor is 0-60 kPa with a sensitivity of 23.14 µW/kPa. The usual pressure range for physiological signs is 0-40 kPa, and the proposed pressure sensor has a pressure range of 0-60 kPa, which is sufficient for physiological sign detection [16].
Fig. 8. Experimental characteristics of the pressure sensor. (a) Pressure-response curve of the pressure sensor. (b) The response time of the pressure sensor. (c) Pressure-response curve between 0-25 kPa. (d) Pressure-response curve between 0-60 kPa. (e) Pressure-response curve between 0-120 kPa. (f) and (g) Stability tests for the pressure sensor.
In addition to the sensitivity, linearity and sensing range, measures such as response time and stability were also important for the practical utilization of the sensor. To receive the response time of the device, we design a circuit to record the output power. The circuit consists of a photodetector, transimpedance amplifier (TIA) and microcontroller unit (MCU). The PD (InGaAs PIN, Shiweitong Science and Technology) has a wavelength response range of 1530−1570 nm and an average responsivity that reaches 0.98 A/W. In addition, the dark current density of the PD array is 0.2 nA. The output end of the pressure sensor was connected to the photodetector, and the output power of the device was changed to an electrical signal by the TIA. Finally, the measured signal was sent to a computer by the MCU with a sampling frequency of 1kHz. As shown in Fig. 8(b), the response time and the recovery time of the sensor were determined to be approximately 33 ms (from 27-60 ms) and 36 ms (from 103-139 ms), respectively, indicating the fast response time of the sensor. In Fig. 8(f) and (g), the stability of the sensor was measured by performing repeatable loading and unloading tests from 1 kPa to 10 kPa. We applied air pressure to the microring pressure sensor using a digital inkjet device (Mozuku, MX5000XII) equipped with an automatic jetting function. We set the jetting interval to 4 s and recorded the changes in the sensor's output voltage. The upper and bottom values of the voltage barely changed after 1000 cycles. After performing repeatable loading and unloading tests, the pressure sensor could also work normally, indicating good electrical and mechanical properties.
Our flexible pressure sensor offers great promise for monitoring pressure signals related to human activity. In Fig. 9(a), the pressure sensor was attached to the wrist of a subject. Note that two volunteers were selected as subjects for pulse signal monitoring. Distinct pulse signals were obtained by the pressure sensor, and the pulse rate of the first subject was determined to be 77 min-1 based on the collected data (Fig. 9(c)). Furthermore, after filtering, the waveform of the pulse, including the percussion wave (P1), tidal wave (P2), and diastolic wave (P3), was clearly identified, which offers potential applications in biomedical diagnostics. As shown in Fig. 9(d), the flexible sensor was attached to the neck of a subject, and clear signals were obtained when the subject was swallowing. Hand gestures have long been considered one of the most natural interactive methods for human–machine interfaces. Therefore, our pressure sensor was also employed to recognize such hand gestures. As displayed in Fig. 9(e), an increase and decrease in muscle tension was clearly observed when the subject made a fist or loosed a fist with the pressure sensor fixed on the forearm. Hence, the pressure sensor could clearly recognize hand gestures with good sensing capability.
Fig. 9. Real-time recordings of body physical signals. (a) Diagram of pulse measurement. (b) The waveform of the pulse after filtering. (c) The measurement results of the pluses. (d) The measurement results of swallowing. (e) The measurement results of made or released a fist.
5. Discussion
Our flexible waveguide-based pressure sensor offers high scalability for further performance improvements and functionality expansions. Although this work demonstrates the preliminary proof-of-concept of an optical pressure sensor based on a five-cascade microring array structure, it is necessary to improve the microring array structure and design a sensing network that comprises more microring arrays. By combining the output power of the microring array structure pressure sensors, the sensitivity and sensing region of the sensing network will be greatly improved.
According to the simulations, the sensitivity of the pressure sensor with 5 cascade microrings should have a sensitivity of 150 µW/kPa; however, the prototype has a sensitivity between 23 and 35 µW/kPa (depending on the pressure range). We attribute the primary cause to the manufacturing process. First, due to inherent limitations in precision during fabrication, errors may arise. Moreover, we observed incomplete removal of LOR photoresist on the final sensor after fabrication (The thickness is approximately 0-500 nm). We applied a layer of LOR photoresist before spin coating PDMS to facilitate better chip detachment. However, due to concerns about the impact of the developing solution on the core pattern, we reduced the soaking time of the LOR photoresist, resulting in a small amount of incomplete removal of the photoresist. The Young's modulus of the LOR photoresist is 1-2 GPa, which also affects the overall sensitivity of the sensor. Further improvements are needed in the fabrication process in the future.
Our waveguide-based pressure sensor recorded physiological signals of the human body by a combined transducer and signal processing circuit. In our ongoing work, the circuit will be integrated with the waveguide-based pressure sensor to realize a photoelectronic smart skin for pressure sensing.
The pressure sensor is not sufficiently refined in terms of calibration. The calibration of the sensor involves manually recording the output value when applying a stress of 0 kPa and using it as the initial value. In the next generation of pressure sensors, they will be more mature, for example, by integrating the light source, microring pressure sensor chip, PD, and signal processing circuit together. This integration can enable the signal processing circuit to set the output value corresponding to a stress of 0 kPa as the initial value for calibration.
6. Conclusion
This work has demonstrated a flexible optical pressure sensor based on a microring structure, which may offer great applications in healthcare monitoring, human–machine interaction and soft robots. The experimental results show that our pressure senor has a linear sensing range of 0–60 kPa with a sensitivity of 23.14 µW/kPa and a high sensitivity linear sensing range of 0–25 kPa with a sensitivity of 35.71 µW/kPa. We explored a couple of applications of the pressure sensor, and the results show great performance in healthcare monitoring. The pressure sensor offers the advantages of high sensitivity, good output linearity, high integration density and easy-to-build arrays. This work represents an advance in high-performance flexible waveguide-based pressure sensors and paves the way for the implementation of flexible waveguide-based pressure sensors in practical healthcare monitoring.
Funding
National Natural Science Foundation of China (61177078, 61675154, 61711530652).
Acknowledgments
Hongqiang Li acknowledges the support from the Fundamental Research Funds of Shaoxing Keqiao Research Institute of Tiangong University and the Tianjin Talent Special Support Program. Joan Daniel Prades acknowledges the support from the Serra Hunter Program, the ICREA Academia Program and the Tianjin Distinguished University Professor Program.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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