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Abstract
Polymer materials have the advantages of a low Young’s modulus and low-cost preparation process. In this paper, a polymer-based optical waveguide pressure sensor based on a Bragg structure is proposed. The change in the Bragg wavelength in the output spectrum of the waveguide Bragg grating (WBG) is used to linearly characterize the change in pressure acting on the device. The polymer-based WBG was developed through a polymer film preparation process, and the experimental results show that the output signal of the device has a sensitivity of 1.275 nm/kPa with a measurement range of 0-12 kPa and an accuracy of 1 kPa. The experimental results indicate that the device already perfectly responds to a pulse signal. It has significant potential application value in medical diagnostics and health testing, such as blood pressure monitoring, sleep quality monitoring, and tactile sensing.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
A pressure sensor is a transducer that transforms measured pressure into detectable electrical or optical signals, which is often referred to as an electrical pressure sensor [1–3] or an optical pressure sensor [4–6]. Electrical pressure sensors have vast applications in mechanical, electrical and biomedical engineering [7]. However, electrical devices are sensitive to many factors, such as electromagnetic fields and humidity [8,9]. The majority of pressure sensors currently explored rely on capacitive, piezoelectric, and piezoresistive systems, which are vulnerable to electromagnetic interference. In addition, for multisample pressure measurement, these sensors require an array design, resulting in a complex fabrication procedure, which might lead to an unequal current distribution and certain safety hazards in practical implementations. As an alternative solution, optical pressure sensors have been rapidly developed because they have apparent merits such as immunity to electromagnetic interference, light weight, small device scale, high sensitivity, and ease of signal transmission [10]. Most optical pressure sensors are based on optical fibers [11–13]. However, optical fibers are difficult to integrate and there is still a long way to go compared to photonic integrated devices. Compared with the existing technology, the polymer-based waveguide Bragg grating (WBG) pressure sensor overcomes the shortcomings of traditional electrical technology, such as a weak anti-electromagnetic interference ability, and the shortcomings of a high Young's modulus and a low sensitivity of traditional silicon-based optical waveguide pressure sensor materials and can realize high-sensitivity and linear output pressure detection based on optical technology.
We propose a polymer-based pressure sensor based on a WBG structure with a core size of 2641 µm × 2.2 µm and a working center wavelength of 1550 nm. The optical waveguide pressure sensor has the advantages of a small size, a light weight, a high sensitivity, good output linearity and easy integration. After preparation and experiments, the sensitivity was measured to be approximately 1.275 nm/kPa with a range of 0-12 kPa.
2. Device design
2.1 WBG design
In the WBG, a periodic structure is added to a straight waveguide. The light field that could have been stably transmitted in the waveguide, with a specific frequency and spatial distribution, is destroyed, and the orthogonal relationship between the modes is transformed into mutual coupling, including between the radiation mode and guide modes (a guide mode is a light field that can be stably transmitted along the waveguide system without attenuation) and between guide modes [14–18]. Mode coupling in the Bragg grating occurs between the guide modes propagating in the opposite direction to the transmission direction. The optical waveguide has a structure with periodic changes in the refractive index as shown in Fig. 1(a), and periodic changes in the equivalent refractive index of the waveguide are realized throughout the thickness of the waveguide layer. Light reflection will be triggered at the locations of periodic changes, and light interference will occur between the reflected light waves. Periodic modulation of the incident light field enables light reflection of specific wavelengths. Only when the phase matching conditions of the Bragg grating coupling mode are met is reflected, which determines the specific wavelength reflected. When the operating wavelength and the period of the structure meet the conditions of the Bragg reflection equation (Eq. (1)), reflection can efficiently occur. A 3D mode field of the WBG waveguide (Rsoft) is used to determine the field distribution of the waveguide. A waveguide with a width of W + 2d = 2.2 µm and a height of h = 1 µm was selected. The corresponding mode field distribution of the waveguide is shown in Fig. 1(b).
where λB is the Bragg grating reflection wavelength, neff is the periodic equivalent refractive index, ${\varLambda}$ is the grating period and m is the grating diffraction order. The design parameters that affect the reflection spectrum of WBG are grating diffraction order (m = 5) and period (Λ=1550 nm) as shown in Fig. 1(c).
Fig. 1. Structure of the WBG and optical design. (a) Schematic diagram of the bilateral WBG structure. The waveguide width (W), etching depth (d) and cycle length (Λ) are labeled in the figure. (b) Mode field of the WBG waveguide. (c) The center wavelength of the WBG is 1550 nm, and the bandwidth is 0.9 nm.
2.2 Device deformation analysis
The polymer device deforms when subjected to stress, which can be broken down into the sum of deformations in multiple directions. According to the mechanical properties of the polymer material and the area of the prepared device, the change in the structure of the designed device when the device is stressed is analyzed, and the center wavelength shift of the device is obtained to achieve pressure sensing. The three-dimensional elastic formulas are as follows:
For the three-layer device shown in Fig. 2(a), the upper and lower cladding layers are formed by spin-coated polydimethylsiloxane (PDMS) with a thickness of 10 µm, a Young's modulus of 2.2 MPa and a Poisson ratio of 0.49, and the core layer is formed by spin-coated methyl methacrylate (PMMA) with a thickness of 1 µm, a Young's modulus of 3.16 GPa and a Poisson ratio of 0.21. The refractive indices were determined by using an ellipsometer (J. A. Woollam, M-2000) and the refractive indices of PDMS and PMMA were 1.410 and 1.488, respectively. By combining the three-dimensional elastic formulas with the polymer film parameters, the following stress and strain matrix can be obtained: where E is the Young's modulus, and v is the Poisson ratio.
Fig. 2. Schematic diagram of the polymer film and polymer film shape analysis. (a) Schematic diagram of the cross-sectional view of the device. The cycle length (Λ) is labeled in the figure. (b) Schematic diagram of device deformation. The cycle length (Λ) and the change in the cycle length (ΔΛ) are labeled in the figure. (c) Mode field of the WBG shape analysis. (d) Relationship curve between pressure and deformation.
As shown in Fig. 2(b), for a single material film, P is the pressure acting on the film, ${\varLambda}$ is the cycle length of the WBG, and the deformation variable Δ${\varLambda}$ is the deformation variable in the same direction as the WBG, which is also the main deformation variable affecting the WBG reflection spectrum. Mode field of the WBG shape when the pressure = 10 kPa, As shown in Fig. 2(c). The relationship between the final deformation variable (u) and pressure (P) is shown in Fig. 2(d), and the fitting curve function is:
2.3 Optical characterization and analysis
When the polymer device experiences stress, the device will deform, and the horizontal deformation in the direction of the WBG will change the grating period and the effective refractive index of the WBG. Therefore, the center wavelength λB of the WBG reflection spectrum will shift. Differentiating and reducing both sides of the Bragg grating reflection condition equation yields:
Since Δneff is approximately equal to 0, the shift in the center wavelength of the WBG ΔλB is mainly caused by periodic changes, that is, deformation of the polymer device. The device deformation results are combined with the WBG reflection spectrum to obtain the reflection spectrum at different pressures, as shown in Fig. 3(a). The wavelength shifted from 1543.32 to 1557.01 nm when the pressure increased from 0 to 10 kPa. The fitting curve is λ=3.065*P + 1543, as shown in Fig. 3(b).
Fig. 3. Relationship between pressure and reflectance spectrum. (a) WBG reflectance spectra obtained at different pressures. (b) Relationship curve between pressure and the center wavelength of the WBG reflectance spectrum.
Finally, the designed WBG structural parameters are as follows: the waveguide width is 1 µm, the etching depth is 600 nm, the cycle length is 2.641 µm, and the cycle number is 2000. The relationship between the center wavelength and pressure is λ=3.065*P + 1543, so the sensitivity of the pressure sensor is approximately 3.065 nm/kPa.
3. Preparation process
For the designed polymer WBG, CLD-1/PMMA is used to prepare the core layer, and PDMS is used to prepare the cladding. The overall preparation process of the device mainly includes spin coating, etching, and PDMS surface modification, in which the core layer pattern is patterned by a step-by-step ultraviolet lithography machine, and its minimum line width is 1 µm, which meets the design requirements. The overall preparation process flow of the device is shown in Fig. 4.
The specific preparation process is as follows:
- (1) Substrate cleaning: Ultrasonic cleaning with polytetrafluoroethylene, ethanol, acetone and deionized water in turn is performed.
- (2) Preparation of the lower cladding: First, spin coating of the PDMS lower cladding is carried out with a homogenizer, and the spin coating parameters are 500 r/min for 5 s and 4700 r/min for 100 s at high speed. Then, the spin-coated PDMS substrate is placed on a heating table for curing with curing parameters of 80 °C for 5 min and 110 °C for 25 min.
- (3) Surface modification of the PDMS film: First, the PDMS film is treated with O2 plasma by a plasma cleaner, and the process parameters are 500 W and 5 min. Then, the film is soaked in the 2% volume fraction silanylation reagent 3-glycidoxypropyltrimethoxysilane (GPTMS) for 1 h.
- (4) Preparation of the PMMA core layer: First, spin coating of the PMMA core layer is performed with a homogenizer, and the spin coating parameters are 500 r/min for 5 s and a high speed of 3000 r/min for 60 s. The PMMA film is then cured by means of a heating table with curing parameters of 65 °C for 10 min.
- (5) Etching of the core layer: The core layer pattern is prepared by a dry etching process, and the etching parameters of the PMMA layer are an RF power of 200 W, SF4:O2 = 10:50 sccm, and a rate of approximately 140 nm/min.
- (6) Surface modification of the PMMA film: The sample that has the completed core layer is modified by a plasma cleaner; only O2 plasma is used, and the process parameters are 500 W and 30 s.
- (7) Preparation of the upper cladding: The upper cladding is also made of PDMS, and the preparation process parameters are the same as those for the lower cladding.
4. Experimental results
The device was characterized by scanning electron microscopy (SEM), as shown in Fig. 5. The pressure sensor based on the WBG structure was tested by using a fiber-to-chip alignment system (PS-1000, SURUGA SEIKI). The broadband source had a 14 mW output power over the spectrum from 1530 nm to 1570 nm. The light was input from one end of a 2 × 1 multimode interferometer (MMI) and monitored from the other end using an optical spectrum analyzer (AP2052A, Apex Technologies).
Fig. 5. SEM photos of the device. (a) Top view of the WBG. The waveguide width (W) of the WBG is approximately 1 µm, the etching depth (d) of the WBG is approximately 600 nm, and the cycle length (Λ) of the WBG is approximately 2.641 µm. (b) Side view of the WBG. The thickness (h) of the WBG is approximately 1 µm.
The output spectra of the nano-optomechanical pressure sensor with different pressures applied to the device are shown in Fig. 6(b). The range of applied pressure is 0-20 kPa, and the step size is 1 kPa. The center wavelength of the device redshifts from 1554.011 nm to 1571.128 nm when the pressure on the device increases from 0 kPa to 20 kPa. The linear sensing region of the device is approximately 0-12 kPa, and the fitting function is shown in Fig. 6(a) and Fig. 6(b). In the low-pressure range, a linear response is typically observed, where the sensor output is directly proportional to the applied pressure. However, in the high-pressure range, as the sensor approaches its maximum sensing capability, a nonlinear response occurs due to physical factors such as nonuniform deformation of the sensor. The fitting curve for the linear region is λ=1.275*P + 1554.011, as shown in Fig. 6(c). The difference between the measured pressures and actual pressures is shown in Fig. 6(d), and the error is approximately 0.2 kPa.
Fig. 6. Measurement results of the wavelength and pressure based on the WBG. (a) Relationship between pressure and center wavelength. The range of applied pressure is 0-12 kPa, the step size is 1 kPa, and the wavelength shifts from left to right. (b) Relationship between pressure and center wavelength. The linear region is approximately 0-12 kPa, and the nonlinear region is approximately 12-20 kPa. (c) Fitting curve for the linear region of the pressure sensor. (d) Difference between measured pressure and actual pressure.
After the photonic device was coupled with a fiber array (FA), a fiber grating interrogator was connected to continuously monitor the center wavelength change of the WBG to achieve continuous stress monitoring. The photonic chip was attached to the wrist artery to monitor the pulse signal. The attachment location is shown in Fig. 7(a), and the monitored pulse signal is shown in Fig. 7(c). The monitored signal has obvious pulse wave characteristic peaks, including the main peak and side peak of the pulse wave, and the signal obtained after filtering by the filtering algorithm is shown in Fig. 7(b). Click signals were monitored by the fiber grating interrogator, as shown in Fig. 7(d) and (e), which illustrates the response of the sensor to various finger press times. The wavelength of the sensor sharply increases and then recovers when the sensor is touched by the finger. In addition, a gentle touch and a hard press can be distinguished from the relative values of the signals. Attributed to the interesting characteristics of the flexible pressure sensor, it could be further used to fabricate flexible keyboards and flexible switches and for other applications. In addition to monitoring the pressure in the low-pressure range (<10 kPa) and the middle-pressure range (10-20 kPa), the pressure sensor with a wide range could detect the pressure in the high-pressure range of over 20 kPa. Planar pressure is very common in the daily life of humans. Acquiring many human physiological parameters by analyzing the variation in planar pressure is an important technique.
Fig. 7. Output spectra of the WBG for different signals. (a) Relationship between pressure and center wavelength. (b) The monitored signal has obvious pulse wave characteristic peaks, including the main peak and side peak of the pulse wave. (c) Relationship between pressure and center wavelength. The electrical signal of the pulse and its waveform are captured with the pressure sensor. (d) The response of the sensor to various finger press times. (e) Monitored click signals detected by the pressure sensor.
5. Discussion
Although we have completed a series of pressure sensing experiments and studied the effects of the diffraction order, the waveguide structure, and different pressures on the output wavelength of polymer-based WBG devices, the spectrum of the device hardly shifts under small motion, whereas the spectrum will have weak shifts under severe motion. Therefore, analyzing the results of a single WBG sensor detecting human pulses is not representative, and we will consider a demodulation scheme combining the WBG array structure pressure sensors. The sensitivity and sensing region of the sensing network will be greatly improved. By combining an arrayed waveguide grating (AWG) in a further design, the pulse can be more accurately measured. In addition, polymer materials have the disadvantages of poor temperature stability and poor matching of the refractive index of the optical waveguide. Future work will focus on improving this field and overcoming the above challenges, which will help develop next-generation polymer-based WBG devices for wearable technology.
6. Conclusion
This paper proposed a polymer-based highly sensitive pressure sensor based on a WBG structure. The change in the Bragg wavelength in the output spectrum is used to linearly characterize the change in pressure acting on the device. The results show that the output signal of the new device has a sensitivity of 1.275 nm/kPa with a measurement range of 0-12 kPa and an accuracy of 1 kPa. The experimental results show that the device promotes the application potential of polymer-based WBG photonic sensors in wearable technology, such as blood pressure monitoring, sleep quality monitoring, and tactile sensing. The proposed sensor has significant potential application value in medical diagnostics and health testing.
Funding
National Natural Science Foundation of China (61177078, 61675154, 61711530652); Tianjin Municipal Science and Technology Program (18KPXMSF00050); Science Popularizing Research Fund for the Graduate Students of China Association for Science and Technology (KXYJS2022091).
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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