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Abstract
An ultra-small integrated photonic current sensor based on a silicon micro-ring resonator (MRR) with a cladding layer of Fe3O4 superparamagnetic nanoparticles (SPNPs) is demonstrated. In the magnetic field generated by an alternating current, the Fe3O4 SPNPs lose energy and change the MRR temperature, which leads to a spectral shift in the MRR transmission. The sensor was demonstrated with good linearity in the frequency range 0–60 kHz and current amplitudes from 0 to 0.5 A. This work provides a basis for integrated micro-current sensors, and promotes the development of photoelectric sensors on silicon substrates.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Current measurements are critical to power systems, aerospace, the automotive industry, and medical applications. Large size, heavy weight and inconvenient installation are characteristics of traditional current transformers, which are composed of oriented silicon sheet steel, winding, and so on [1]. Compared with normal current sensors, optical sensors have resistance to electromagnetic interference, strong insulation, and small sizes. Optical current sensors can be divided into three main categories: Faraday magneto-optical effect [2–4], magnetostrictive effect [5–7], and thermal effect [8,9]. The first type exhibit problems of measurement accuracy and stability due to inherent linear birefringence in optical fibers and susceptibility to environmental effects. The second type is generally a combination of magnetostrictive materials with fiber Bragg gratings (FBG). The last type of sensor is advantageous because of its high sensitivity and simple operation, and is used in situations where the response time is not important. Reference [9] reported a current sensor realized by wrapping a microfiber onto a CrNi wire. In this sensor, the heat produced by the current was proportional to the square of the current amplitude, and its sensitivity was 220.65 nm/A2. Besides, a sensor based on a microfiber loop wrapped around a copper rod was reported in [10], and the spectral shift was attributed to the thermal expansion of the current-loaded copper rod, the sensitivity was 26.5 pm/A. YAN S et al. spirally wrapped a microfiber around a graphene sheet, and reported an ultra-high sensitivity of 67.297 µm/A2 [11]. Above sensors are suitable for direct current (DC) measurement. In application, however, the need for alternating current (AC) measurement is greater than that of DC measurement. It is hard to measure AC current by thermo effect, for the effective value of AC current is numerically equal to the magnitude of DC current in the resistor producing the same thermo effect. Meanwhile, strict requirements on the insulation are necessary for these sensors due to contact measurement, which will inevitably increase the volume and weight of the sensor.
With the development of advanced materials and process technology, miniaturization and intelligentization are the trends in the development of sensor systems. However, these current sensors were all based on optical fibers, their complexity and large sizes hinder miniaturization, integration, and low-cost manufacturing. Excellent performance in low-power consumption, compatibility with traditional complementary metal–oxide–semiconductor (CMOS) technology, and high integration are acquired by integrated sensors at present. Regarding integrated silicon photonics, a micro-ring resonator (MRR) on a silicon-on-insulator (SOI) platform features ultrahigh compactness, high mechanical stability, and has received a lot of attention in the field of integrated optics, such as sensors, filters, lasers, and light modulators [12–18]. Device size decrease and sensitivity improvement can be achieved with MRR structure, and the micro-sensor can be placed in a narrow space regardless of the size of the field. In addition, by using mechanical microelectronics technology, the micro-sensor, signal processor and data processing device could be packaged on the same chip. Meanwhile, an integrated AC current sensor based on a silicon MRR is rarely reported.
Here, a contactless current sensor was developed from an ultraminiature integrated photonic device based on a silicon MRR and a Fe3O4 superparamagnetic nanoparticles (SPNPs) cladding layer. In the magnetic field generated by the AC current, the Fe3O4 SPNPs convert electromagnetic energy into heat which changes the effective refractive index of the optical waveguide. Hence, the current is measured by monitoring the shift in the MRR resonance wavelength. Theoretical and experimental results show that the shift is directly proportional to both the squares of the AC current amplitudes and frequencies. The linear correlation coefficient is greater than 0.98 for the current frequency range 0–60 kHz and the current amplitude range 0–0.5 A. The miniaturized sensor is simple in structure, and features high insulation, and safe and reliable operation.
2. Sensing principle
A schematic of the SOI-based current sensor is shown in Fig. 1. The MRR consists of bus and ring waveguides. To perform current sensing via the magnetocaloric effect, a cladding layer of Fe3O4 SPNPs is coated via optical adhesive on the surface of the ring waveguide, which is in the magnetic field generated by the current near the sensor.
Commonly used Fe3O4 magnetic nanoparticles (MNPs) have diameters ranging over 10–100 nm. In the alternating magnetic field (H) generated by the AC current (I) to be measured, MNPs absorb a large amount of electromagnetic energy and convert it into thermal energy via magnetic losses due to Neel and Brownian relaxation. According to the theory reported by Rosensweig [19], the power loss (P) of the MNPs in an alternating magnetic field is:
where µ0 is the permeability of free space, χ0 is the equilibrium susceptibility, f is the frequency of the current to be measured, and τ is the total relaxation time of the system. When fτ << 1 [20], Eq. (1) can be simplified to: where α is the conversion rate of current to magnetic field.In the early stage of heating, the temperature change (ΔT) of MNPs is:
where Δt is the duration of the magnetic field, and c is the MNP specific heat.Silicon has a good thermal optical coefficient, over the temperature range 300–600 K and near 1550 nm [21], the refractive index (n) of silicon varies almost linearly with temperature. Light in the MRR is sensitive to changes in the evanescent field. When the refractive index of silicon is changed by the magnetocaloric effect, a change in the effective refractive index (neff) of the mode propagating in the MRR also occur, and the resonance wavelength (λ) shifts accordingly. At last, based on above analysis, the relationship between the resonance wavelength shift and the current yields can be expressed as the following:
The sensor was designed on SOI wafer with a 220 nm Si layer on a 2 µm SiO2 insulator layer. The height and width of the waveguide were set to 220 nm and 500 nm, respectively, so that the MRRs were single mode for transverse-electric (TE) polarized light at a wavelength of 1550 nm. In order to determine the over-coupled regime and the optical power coupling efficiency, an array of such structure was simulated with different radius of the MRR by varying the gap between the ring and the bus waveguide. Finally, the radius of the MRR was set to 15 µm to ensure a low bending loss and a suitable free spectral range (FSR). In order to achieve high optical power coupling efficiency between the MRR and the bus waveguide, the gap between the bus and the MRR waveguides was chosen to be 100 nm.
3. Fabricated sample
When the MNP particle size is smaller than the critical size of its super-paramagnetism, the particles enter a superparamagnetic state without coercivity and remanence [22]. Furthermore, the SPNPs have an extremely high magnetic susceptibility, and can change instantaneously in a changing field. Here, the Fe3O4 MNP diameter in this work is 18 nm, the apparent density of the Fe3O4 MNP is 0.84 g/cm3, and the real density is 5.1 g/cm3. Furthermore, the apparent density of powder decreases with the decrease of particle size, the increase of non-spherical coefficient and the increase of surface roughness. In addition, the hysteresis loop of the Fe3O4 samples is shown in Fig. 2(a), which indicates that the samples are SPNPs at room temperature. Moreover, the saturation magnetization was 66.57 emu/g at room temperature, which was higher than that in the following experiments. Hence, the magnetization of the Fe3O4 SPNPs in the magnetic field was unsaturated.
Fig. 2. (a) Magnetic hysteresis loops for the Fe3O4 MNPs. (b) SEM of the MRR with a radius of 15 µm. (c) SEM of the vertical-coupling grating. (d) A micrograph of the MRR with Fe3O4 SPNPs cladding layer.
For SOI materials, the thermal conductivity of silicon dioxide is very low relative to that of silicon [23]. It can thus reduce heat conduction from the waveguide layer to the substrate and reduce the tuning power consumption. Meanwhile, the thermal response rate is at the sub-micron level, which approaches the tuning rate of the optical network to the device.
Steps in the current sensor fabrication were as follows. The bus and ring waveguides were fabricated in the silicon layer via spin-coating photoresist, electron-beam lithography, inductively coupled plasma reactive-ion etching, and photoresist stripping. In addition, inhomogeneous gratings were etched at both ends of the bus waveguide. Then, the stainless steel thin film with sub-100-nm aperture was placed 1mm above the SOI wafer, and the narrow aperture was aligned with the MRR under a microscope, then the Fe3O4 SPNPs was coated on the surface of the MRR through the aperture. Finally, the SPNPs were covered with optical adhesive (Norland NOA 71). Scanning electron microscope (SEM) images of the MRR and a vertical-coupling grating are shown in Figs. 2(b) and 2(c). And Fig. 2(d) is an optical image of the Fe3O4 SPNPs upper cladding layer on the MRR, which shows that the SPNPs were coated evenly on the surface of the MRR.
4. Experimental setup and results
To achieve non-contact current measurement by detecting the magnetic field, the sensing characteristics were determined by placing the sensor in the center of two Helmholtz coils, as shown in Fig. 3(a). The coils produced uniform and stable magnetic fields, and also had a convenient open structure. The inner and outer coil diameters were 10 cm and 14 cm, respectively, the distance between the two coils was 8 cm, and the number (N) of single coil turns was 300. The magnetic field along the axis of the Helmholtz coil was the sum of the fields generated by each coil [24], as given by:
Fig. 3. (a) Schematic diagram of the current sensing platform. (b) Schematic diagram of the experimental setup. ASE: amplified spontaneous emission source, OSA: optical spectrum analyzer.
A schematic of the experimental arrangement is depicted in Fig. 3(b). An amplified spontaneous emission (ASE) light source (1525–1565 nm) was connected via a polarization controller through a single-mode fiber, and then was vertically coupled into the waveguide of the MRR through a tapered single-mode lens fiber and a grating coupler. An optical spectrum analyzer (OSA, Yokogawa AQ6370D) was connected to the other end of the MRR to observe the output transmission spectrum. A signal generator (Tektronix AFG 3011) and a power amplifier (AE Techron 7548) were combined to form a current source with an adjustable arbitrary waveform, which was connected to the Helmholtz coil to generate a magnetic field parallel to the MRR surface. The actual input current was measured with a Rogowski coil (Pearson Model 110A) and output to an oscilloscope (Tektronix TDS 2024C).
The transmission spectrum of the MRR without the Fe3O4 SPNPs cladding layer is shown in Fig. 4(a), the FSR was 4.2 nm, the Q factor was 5970 while the extinction ratio was 11.12 dB. Using Lumerical FDTD solutions, a three-dimensional model was utilized to simulate this MRR structure. After the numerical simulation, and the FSR near 1550 nm was 6.0 nm for a 15-µm MRR radius, the corresponding Q factor was 1.57 × 104, and the extinction ratio was 20.7 dB, as shown in Fig. 4(a), and the bus-ring power coupling coefficient was 0.0375. In addition, the experimental results slightly deviated from the simulation, and the Q factor was smaller. A possible explanation is that the fabrication tolerance may have produced larger energy losses when the optical signal was transmitted in the waveguide.
Fig. 4. (a) Modeled and experimental transmission spectra of the MRR without the Fe3O4 SPNPs cladding layer. (b) Variation of transmission spectrum of the sensor with the change of the frequency.
The effects of temperature and relative humidity on the MRR performance were examined under constant temperature-humidity conditions. Initially, the input current amplitude was 0.4 A, and the frequency was controlled with the signal generator. To ensure stability, spectra were recorded 20 s after the current was applied. Over this short time, the ambient temperature could be assumed constant.
At a 0.4-A input current, the transmission spectra of the MRR sensor for 0 kHz, 30 kHz, 50 kHz and 60 kHz current frequencies are shown in Fig. 4(b). Comparing the experimental transmission spectra of the MRR shown in Figs. 4(a) and 4(b), it can be noted that the extinction ratio in Fig. 4(b) is 2.22 dB lower than that in Fig. 4(a), and the Q factor is also reduced. This is possibly because when the particle size is less than 20nm, the particles own light-absorbing property, resulting in higher loss in transmission and lower extinction ratio. Furthermore, the resonant wavelength shifts to longer wavelengths with the increase in current frequency. When the MMR was tested without the Fe3O4 SPNPs cladding layer under the same conditions, changes in current amplitude and frequency produced negligible shifts in the resonance spectrum. Hence, only the SPNPs affect the MRR spectra, because only they converted electromagnetic energy into heat, which shifted the resonance wavelength.
Figure 5 plots the shift in resonance wavelength vs. the square of the frequency over the range 0–60 kHz when the current amplitude was 0.4 A. The wavelength shift was essentially linear with the square of the current; the correlation coefficient of the fit was 0.9985. The sensitivity was 2.043 × 10−4 nm kHz-2. According to Eq. (4), and the experimental phenomena, when the current amplitude is 0.4 A, theoretically the maximum AC frequency of the proposed sensor is about 143 kHz.
Similarly, Fig. 6 plots the shift in resonance wavelength vs. the square of the current amplitude for frequencies 20 kHz, 40 kHz, and 60 kHz. Over the current range of 0–0.5 A, the shift in resonance wavelength was linearly proportional to the square of the current amplitude. Besides, the sensitivities under AC magnetic field were obtained, which were 0.677 nm A-2, 2.257 nm A-2, and 5.046 nm A-2, respectively, for 20 kHz, 40 kHz, and 60 kHz. Furthermore, the Rogowski coil was utilized to calibrate the measuring model parameters and analyze the measurement linearity of the sensor in the validation test. And the results showed that the linearity of the sensor was good, with the linear fitting correlation coefficients greater than 0.98.
The performance of the sensor was limited mainly by the Q factor, the FSR of the MRR, and the SPNPs heating characteristics of the SPNPs. These could be improved by changing the parameters and structure of the waveguide, and by using SPNPs with higher heating rates and heating power [25,26]. If the Q factor is increased, the resonant dip will be sharpened, making it easier to determine the location of the resonance peak, to reduce the loss caused by the SPNPs, and to improve the signal-to-noise ratio and the resolution of the sensor. In addition, the measurement range of the sensor was determined by the FSR of the MRR, and thus a reduction in the MRR radius could increase the current measurement range.
5. Conclusion
In summary, a current sensor based on a silicon MRR and SPNPs was demonstrated. The functional element was Fe3O4 SPNPs coated on the MRR, which converted AC current electromagnetic energy into heat energy in the magnetic field. The heat changed the effective refractive index of the optical waveguide, which shifted the resonant wavelength. When the current frequency ranged over 0–60 kHz, and when the current amplitude ranged over 0–0.5 A, the MRR transmission spectral shift was linearly proportional to the squares of the AC current frequencies and amplitudes, respectively. Additionally, the sensor could use a Mach–Zehnder structure, or integrated fiber Bragg gratings, to compensate for temperature changes in the surrounding environment. Micron-sized sensing units and integration based on silicon photonics contribute to the development of lab-on-a-chip platform for current measurement. In practical sensing scenarios, the current sensors need to acquire current values in multiple lines, and it could be solved by the method of magnetic field decoupling. Furthermore, if the on-chip sensor is packaged using a UV curable low-index polymer, one could realize miniaturized and long-term stable packaging eventually, offering significant potential for application in the future.
Funding
National Natural Science Foundation of China (51477018); Foundation for Innovative Research Groups of the National Natural Science Foundation of China (51321063).
Disclosures
The authors declare no conflicts of interest.
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