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Abstract
A reflective quadrature interferometer was constructed by integrating polymeric optical waveguide components, to demonstrate an optical current sensor that could operate without bias feedback control. In order to obtain two interference signals with a phase difference of 90°, half-wave and quarter-wave plates were inserted in the polymeric optical waveguide chip, and a polarization-dependent birefringence modulator was used for the initialization of the optical sensor, including detector gain adjustment. During the bias-free operation of the sensor, the measurement error was less than ± 0.2%, and it was confirmed that the sensor output was stable for 15 h even if the operating point was not maintained.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
For stable power monitoring in large-scale power generation and distribution systems, it is effective to use optical sensors that measure the current and voltage using optical fibers [1,2]. The optical current sensor is made of insulator without any metal conductor, which facilitates safe operation, as there is no possibility of heat generation or explosions caused by surge currents [3]. The electric-current–induced magnetic field causes an instantaneous circular birefringence on the optical fiber, and the optical interferometric sensor can measure the rapidly changing fault current in real time [4]. Because the current-induced circular birefringence in optical fibers is small, the optical current sensor has a wide dynamic range without saturation [5]. Moreover, in a high-power generating facility, the optical current sensor can produce a noise-suppressed sensing signal, because of the optical signal’s immunity to electromagnetic interference. Optical current sensors are smaller and lighter than conventional electrical current transducers (CTs); hence, they are easier to install and operate with less maintenance cost. Unlike electrical CTs, the optical sensors are manufactured from insulating materials; therefore, they do not need gas or insulating oil [6].
A polarization-rotated reflection interferometry (PRRI) was proposed to improve the operating stability of the optical current sensor [7]. Two orthogonal polarization components were utilized to measure the minute circular birefringence in the optical fiber coil, which was proportional to the electrical current intensity. The PRRI structure provided operating stability by canceling the phase change caused by the influence of the ambient environment such as vibrations or temperature fluctuations [8,9]. However, PRRI based optical current transducers (OCTs) required several types of optical components, which complicated the sensor configuration and reduced the production yield. To solve this problem, photonic integrated circuits (PICs) that could reduce the complexity of PRRI by integrating various optical components on a single substrate were proposed [10–12].
The optical interferometer for measuring the phase difference between two waves can obtain a linear response with maximum sensitivity when the operating point is maintained at π/2. However, when the birefringence of the optical fiber changes, because of the influence of ambient vibrations or temperature changes, the operating point drifts with time. A phase modulator made of piezoelectric transducer (PZT) and LiNbO3 was utilized to compensate for such changes in the operating point [13]. The operating point was adjusted with a passive optical device such as a polarizer or a waveplate; however, precise alignment between the components was required, along with a low-birefringence optical waveguide [14,15].
To obtain a stable sensor signal, regardless of the bias point, a quadrature interferometer that used two interference signals with a phase difference of 90° was investigated. By combining four directional couplers, 90° phase-shifted signals were obtained in a Mach–Zehnder interferometer [16]. Two wavelengths were utilized in a Fabry–Perot interferometer because it had wavelength-dependent phase shifts [17]. When two orthogonal polarizations passed through several waveplates, a quadrature interference signal could be obtained [18,19]. A multimode-interference waveguide device was also incorporated to obtain the 90° phase-shifted output, in which the dimension of waveguide was critical to the phase difference [20].
In this work, we propose a novel quadrature interferometer with large fabrication tolerance, by considering the production efficiency of the OCT module. Two types of waveplates are inserted in the center of a cascaded Y-branch waveguide, and waveguide polarizers and a birefringence modulator are integrated. The birefringence modulator produces a phase difference between the orthogonal polarizations, and is used for initializing the sensor. By using the proposed method, we demonstrate linear output response and long-term stable operation for the PIC-OCT, without using any feedback control for the operating point.
2. Device configuration and operating principle
The reflective optical current sensor proposed in this work consists of two parts: a sensor head and a PIC, which consists of optical components made out of polymer waveguides, as shown in Fig. 1. The PIC transmits the input light from SLED to the optical fiber coil of the sensor head and serves as a path to transmit the reflected light to the photo detectors (PDs). The SLED source has a 3-dB bandwidth of 40 nm, and then the splitting ratio of directional coupler could deviate by ± 0.6 dB, which could affect the insertion loss slightly. In the middle of the waveguide, a half-wave plate (HWP) and quarter-wave plate (QWP) made of polyimide are inserted, with their optic axis directions inclined at 22.5° and 45°, respectively, to the substrate [21].
The polarizer absorbs TM polarization through the surface plasmon absorption. The thermo-optic (TO) birefringence modulator produces a relative phase difference between the TE and TM polarizations because it has a slightly different TO effect for each polarization [22]. When TE polarized light is incident on the PIC, the 22.5° inclined HWP rotates the polarization angle to produce TE and TM polarizations in equal amounts. These polarization components pass through the directional coupler of the PIC and are coupled into the fast- and slow-axis components of the PM fiber, which connects the PIC to the sensor head. The sensor head consists of 20 turns of spun Hi-Bi optical fiber, mirror, and QWP made of polarization-maintaining (PM) fiber. They are again converted into two circularly polarized lights, which are the eigenmodes in Faraday rotation, by the fiber-optic QWP. In the coil, the current-induced magnetic field produces a phase difference between the two circularly polarized lights. These lights are reflected by the mirror at the end of the coil. Subsequently, in the return path, the QWP converts the reflected lights into linearly polarized lights again. As the light returns to the PIC, the directional coupler splits the light into a component that propagates to the monitoring PD (MPD) and other components that propagate to PD1 and PD2. These components pass through the HWP and QWP, respectively, and then the waveguide polarizers. In this process, the interference signals detected by PD1 and PD2 will possess a phase difference of π/2. The MPD output is used to normalize the PD1 and PD2 signals by monitoring the returned power.
The interference signals produced by a quadrature phase-shifted interferometer with two types of waveplates are derived using the Jones matrix. The Jones matrix for each optical component included in the PIC is summarized in Table 1. By considering the optical devices included in the PIC and the optical components inserted in the sensor head, the electric fields of the waves finally detected at PD1 and PD2 can be expressed as follows:
From Eq. (1) and (2), the optical intensities measured at PD1 and PD2 are expressed as Then, the final Faraday rotation angle can be obtained asThe birefringence modulator integrated in the PIC is utilized to change the operating point of the sensor during the initialization step. The polarization dependence of the TO modulator is useful for producing θF and normalizing the gains of PDs [23,24].
Table 1. Jones matrix representation of the optical components included in the optical integrated circuits for PRRI
3. Fabrication of the polymer waveguide device and sensor head part
The refractive indices of the polymer materials used in the core and cladding were 1.4375 and 1.4300, respectively, and the polymers were obtained from ChemOptics, Co. Based on the effective index method calculation, a single-mode waveguide was designed with a core width of 6.0 μm and thickness 5.0 μm. The fabrication procedures of the PIC are shown in Fig. 2.
Fig. 2 Fabrication procedure of the PIC for optical current sensors. (a) Coated buffer layer on silicon substrate. (b) Patterned metal thin film on the buffer layer for birefringence modulator and polarizer. (c) Optical waveguide core pattern. (d) Grooving process for inserting waveplates. (e) Inserted waveplates in the middle of waveguide
The ZPU-430 polymer was spin-coated on a silicon wafer, cured by UV light, and baked at 160 °C to form a heat-insulating buffer layer with a thickness of 15 μm. Then, Cr and Au were deposited to a thickness of 10 nm and 100 nm, respectively, and patterned using photolithography to form an electrode. The lower cladding layer was coated to a thickness of 7 μm and UV cured. To produce a TE-pass polarizer based on surface plasmon absorption, a Cr–Au metal thin film was deposited to a thickness of 10–100 nm and a polarizer pattern was formed by wet-etching. Then, a 6.5 μm thick cladding layer was coated, which was subjected to photolithography to form a core pattern. It was then etched with O2 plasma. The core polymer was coated to fill the etched channel, and the thickness of the core was adjusted to 5 μm by etching the whole surface of the substrate. The cladding polymer was coated to a thickness of 7 μm over the core. After completing the optical waveguide structure, the contact pad of the birefringence modulator covered by the polymer layers was exposed to the outside by plasma etching.
A groove line with a width of 30 μm was engraved on the finished waveguide using a dicing saw, for inserting the waveplates. The HWP and QWP were inserted into the groove line one by one, and fixed using an epoxy resin. During the waveplate-insertion process, the output polarization states were monitored carefully to adjust the optic axis angle of the waveplates. A fiber optic array was attached to the waveguide and the electric wire was connected to the birefringence modulator. The PIC device with complete packaging is shown in Fig. 3.
Fig. 3 Photographs of (a) the fabricated PIC after inserting waveplates and (b) the packaged PIC with pigtailed optical fibers
4. Characteristics of the integrated optical current transducers
The insertion loss of the fabricated device was measured using a distributed feedback (DFB) laser with a center wavelength of 1550 nm. The insertion loss to the PM fiber output was 9.7 dB, which could be divided into a fiber-coupling loss of 2.4 dB (1.2 dB/facet), waveguide propagation loss of 1.5 dB for a length of 33 mm, groove loss of 1.1 dB, polarizer loss of 1.2 dB, and 3-dB coupler loss of 3.5 dB. The electrode of birefringence modulator had a width of 40 μm and length of 10 mm, and the resistance was measured as 90 Ω. The birefringence modulation characteristics were measured with 45° linearly polarized light input. The power required for a phase change of 2π, between the TE and TM polarizations, was 300 mW [22].
SLED light source was connected to the sensor, then an initialization process was carried out before measuring the current. The birefringence modulator was operated by a 4 Hz, 310 mW sinusoidal signal, and the output interference signals were measured as shown in Fig. 4(a). The signal amplitude of PD1 was approximately double that of PD2 because of the additional Y-branch located in front of PD2. The MPD was used to monitor the total returned power, regardless of the polarization. By comparing the output signals, the gains of PD1 and PD2 were adjusted to produce the same modulation amplitude. Then, a Lissajous diagram was obtained, as shown in Fig. 4(b), where an 88° phase difference was observed between the two output signals, with an eccentricity of 0.255, which was a much improved from our previous work [20]. The phase difference could be 90° if we adjust the angle of waveplate perfectly during the active alignment process. The current phase difference of 88° was caused by the angular misalignment by 0.95°.
Fig. 4 (a) Interference signals detected at PDs when voltage was applied to the birefringence modulator, (b) Lissajous diagrams drawn using the two measured signal (black) and ideal case (red) for comparison, and (c) Deviation of Lissajous pattern by turning on the birefringence modulator (BM).
After initialization, the birefringence modulator was turned off and the sensor output signal was monitored by flowing a current of 1.0 kArms through the sensor coil. At this time, the Lissajous pattern was deviating slightly from the Lissajous circle measured during the initialization procedure, as shown in Fig. 4(c). This was because the heat generated by the birefringence modulator affected the birefringence of the PIC device and the initialized gains of the PDs. Hence, the birefringence modulator was turned on during the sensor operation by applying the same waveform used in the initialization step. It did not give any effect on the quadrature interferometric sensor.
During the sensor operation, PD signals were measured with a 16-bit A/D converter in 50 kHz sampling rate. To find the amplitude of 60 Hz current signal, phase conversion and root mean square (rms) calculation were done for every 0.1s. The position of sensor coil inside the conducting wire was not affecting the output because the line integral of the magnetic field over a closed path (along the optical fiber) is a constant regardless of the integral path.
The sensor head was packaged by using plastic case and consisted of 20 turns of spun highly-birefringence fiber. To amplify the intensity of the magnetic field applied to the optical fiber by 10 times, a toroid-type current loop was used, as shown in Fig. 5. The current applied to the toroid was increased gradually from 0.3 to 0.5 kArms (5 kArms effectively on the coiled fiber), and then decreased, to verify the linearity of the sensor.
Fig. 5 (a) Photograph of the current toroid setup utilized to amplify the intensity of the magnetic field transferred to the sensing fiber coil 10 times (b) Schematic of the toroid setup exhibiting the wires under the support plate.
The measured current signal was compared to the output of a conventional electrical CT, as shown in Fig. 6. The relative error was within ± 0.2% over the entire current range, and the stability of the sensor was confirmed by performing long-term current measurement. When the sensor was operated for 15 h in a laboratory with no temperature control, the signal error fluctuation was found to be within ± 0.5%, as shown in Fig. 7.
Fig. 6 Linearity of the sensor output measured when the applied current varied from 0.3 to 5.0 kArms and relative error compared with the electrical CT.
Fig. 7 Result of the long-term current measurement without operating point feedback bias control, the error data (blue) was calculated with measured data from OCT (black) and applied current signal (red).
5. Conclusion
A reflective quadrature interferometer with two types of waveplates inserted into a polymeric waveguide device was demonstrated. The PIC included a 3-dB coupler, Y-branches, TE-pass polarizers, and a birefringence modulator for initializing the sensor. By virtue of the quadrature interferometer, current sensing was demonstrated successfully, regardless of the operating-point drift influenced by the ambient temperature change. The birefringence modulator was used to produce a phase shift between the TE and TM modes, which were used for sensor initialization. Linearity and long-term stability of the sensor, without using any bias feedback control, were confirmed. An error of less than 0.2% was obtained when measuring a 5.0 kArms, 60 Hz AC current signal. During long-term operation, the sensor produced a stable output, irrespective of the operating-point deviation. The optical current sensor incorporating a PIC has the advantages of high yield and excellent stability.
Funding
National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (2017R1A2A1A17069702)
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