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Abstract
To advance the development of a compact and highly integrated fiber Bragg grating (FBG) interrogation system, to the best of our knowledge, this paper is the first to present the design and fabrication of a monolithic integration chip based on silicon-on-insulator (SOI), which is specifically intended for application in fiber grating sensing interrogation systems. By considering the impact of coupling structure dimensions on coupling efficiency as well as the effect of the photodetector (PD) parameters on the optical absorption efficiency of the device, we refine the structure of the monolithic integrated chip for arrayed waveguide grating (AWG) and PD. The test results reveal that the coupling loss between AWG and PD is -2.4 dB. The monolithic integrated interrogation chip achieves an interrogation accuracy of approximately 6.79 pm within a dynamic range of 1.56 nm, accompanied by a wavelength resolution of 1 pm. This exceptional performance highlights the potential of the monolithic integrated chip to enhance the integration of AWG-based fiber grating interrogation systems.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Fiber Bragg grating (FBG) [1–3] sensing interrogation system can perform real-time detection of external temperature, strain, noise, and other physical quantities by monitoring the wavelength shift of the FBG resonance center which has been widely employed in the energy, chemical industries, civil engineering, health monitoring, construction, rail transportation, and other areas. Tunable narrowband light source detection method [4], tunable laser detection method [5], unbalanced Mach-Zehnder interferometer method [6], and tunable Fabry-Perot filter detection method [7] are examples of common fiber grating interrogation techniques. The advancement of these interrogation techniques has progressed to a relatively mature stage, yet challenges remain in attaining miniaturization and high-speed interrogation for FBG sensing systems. When integrated into the FBG sensing interrogation system [8–13], the arrayed waveguide grating (AWG) interrogation method offers significant advantages, including reduced size, rapid interrogation, and multi-sensor interrogation. To meet the high-performance requirements for FBG interrogation system testing in specific environments, a highly integrated, high-precision, and compact FBG interrogation system can be realized when the AWG interrogation method is combined with photonic integrated circuit (PIC) technology [14–18].
The core layer Si (with a refractive index of n = 3.4757) and the cladding layer SiO2 (with a refractive index of n = 1.444) on the silicon-on-insulator (SOI) materials [19–21] exhibit a significant difference in refractive index. Consequently, the decoupling distance between waveguides is shorter and the bending radius of curved waveguides is smaller, both of which significantly reduce the footprint of AWG. Therefore, SOI stands out as the most promising and highly potential material platform. In 2014, the University of Ghent in Belgium [22] developed a miniaturized AWG interrogator based on SOI materials, capable of simultaneous interrogating 8 strain sensors. Li Hongqiang et al. achieved the hybrid integration AWG with lasers and detectors on an SOI chip, marking the first SOI-based hybrid integrated interrogation chip in 2017 [23]. With the relentless advancement of PIC technology, the integration of discrete optoelectronic components continues to evolve and improve.
To further investigate this issue, we present an interrogation chip featuring monolithic integration of AWG and photodetector (PD) for FBG interrogation systems for the first time. We fabricated a 1 × 9 SOI-AWG for the FBG interrogation system, simulating and testing its transmission spectrum. PIN-type germanium (Ge) waveguide detectors were integrated on each output waveguide of the AWG, with subsequent simulation and improvement of the coupling structure and the PD waveguide parameters. Additionally, the interrogation performance of the AWG monolithic integrated chip for application in the FBG interrogation system was also discussed.
2. Device structure and simulation results
The architecture of the AWG-based FBG wavelength interrogation system, as shown in Fig. 1(a), consists of an amplified spontaneous emission (ASE) broad-spectrum light source, an optical circulator, an array of FBGs, an AWG, a PD array, and data processing circuits. The circulator transfers light inputted at port 1 to exit solely through port 2, while light entering port 2 is directed exclusively towards port 3, effectively ensuring the transmission and isolation of optical signals. The WDM function of the AWG is utilized to filter the narrowband Gauss light of the FBG which is transferred from the circulator. The output optical signal is then transformed into an electrical signal by the PD array. Subsequently, the electrical signals are processed by the data processing circuits. Theoretically, the relative intensity interrogation (RII) technique is not constrained by the output power fluctuations of the light source and the FBG bandwidth and has high stability. Utilizing the linear relationship existing between the adjacent channel light intensity ratio logarithm of the AWG and the FBG central wavelength, the interrogation of the fiber grating wavelength is achieved, as displayed in Fig. 1(b). The theoretical formula of the RII method is as follows:
The output power of AWG channels m and $m + 1$ is denoted by ${P_m}$ and ${P_{m + 1}}$. $\Delta \lambda $ is the offset from the center wavelength. The full width at half maximum (FWHM) of the FBG is denoted by $\Delta {\lambda _{FBG}}$ while its center wavelength is represented by ${\lambda _{FBG}}$. ${\mathrm{\lambda }_\textrm{m}}$ and ${\mathrm{\lambda }_{\textrm{m} + 1}}$ are the center wavelengths of AWG channels m and $m + 1$. $\Delta {\lambda _m}$ is the FWHM of AWG channel m. The FBG can be interrogated by using the linear relationship between the center wavelength of the FBG and the logarithm of the ratio of the output optical power of adjacent channels in the AWG.
Fig. 1. AWG-based FBG interrogation system. (a) Diagram of interrogation system. (b) Schematic of RII method.
Using photonic integration technology an on-chip AWG monolithic integrated interrogator is proposed, as displayed in Fig. 2(a). A PD is integrated after each output waveguide of the AWG. Figure 2(b) depicts the architecture of the PD. Ge is an optimal material to realize SOI substrate-integrated PDs due to its high absorption coefficient in the infrared band, which allows it to efficiently control the electric field distribution in the absorption region [24,25]. Through vertical coupling, the Ge-PD receives light from the silicon (Si) waveguide. Integrating the vertically connected PDs with other optical waveguide devices is favorable as it reduces coupling loss and significantly enhances light absorption efficiency.
Fig. 2. Device structure diagram. (a) Schematic of on-chip integration of AWG and PDs in a monolithic chip. (b) Vertical coupling Ge-PD.
In this work, we have designed a 1 × 9 SOI-based AWG, employing a rectangular waveguide structure with a 220 $\textrm{nm}$ thick top silicon layer and a 2 ${\mathrm{\mu} \mathrm{m}}$ thick buried oxide layer. The key design parameters of the AWG are listed in Table 1. Utilizing Lumerical software, simulations were conducted on the output transmission spectrum of the AWG. The results are displayed in Fig. 3. According to the simulation, the minimum insertion loss of the central channel is -1.37 dB, with the crosstalk level is -25.65 dB.
Table 1. AWG design parameters
To ensure high coupling efficiency between the optical fiber and the input waveguide, a grating coupler is employed. As illustrated in Fig. 4(a), the grating coupler is comprised of optical waveguides and gratings on the optical waveguides. The optical signal propagates on the optical waveguides through the grating. We use the finite difference time domain (FDTD) method to simulate, on Lumerical software, the coupling of the optical fiber and the grating coupler, as depicted in Fig. 4(b). The simulation results are displayed in Fig. 4(c), in which the minimum coupling loss is -2.12 dB and the optical signal is effectively transmitted into the waveguide.
Fig. 4. Simulation of input grating coupler. (a) Schematic cross-section of the grating coupler. (b) Modeling of the fiber-grating coupler coupling using the FDTD method. (c) Input grating coupling efficiency with respect to wavelength (Insert) Simulated optical field diagram of the grating coupler.
Likewise, in order to ensure high coupling efficiency between the output waveguide and the waveguide detector, the coupling structure is constructed in accordance with the optical properties of waveguide. In the vertical coupling PDs, the output optical signal of the output waveguides of the AWG is transmitted in the Si waveguide to the Ge waveguide layer through the coupling structure, which is schematically shown in Fig. 5(a).
Fig. 5. Optimization design and field distribution of coupling Structure. (a) Schematic diagram of the coupling structure. (b) Overhead and elevation views of the Si-Ge waveguide without tapered structure. (c) Overhead and elevation views of the Si-Ge with tapered structure. (d) Optical field distribution of the Si-Ge waveguide without tapered structure. (e) Optical field distribution of the Si-Ge waveguide with tapered structure.
The process of light absorption is influenced by PDs featuring different coupling structures. Figure 5(b) is a schematic diagram without a tapered structure, and the simulated optical field distribution of the silicon-germanium (Si-Ge) waveguide is shown in Fig. 5(d). It is apparent that the structure without tapered waveguide fails to efficiently restrict the optical field energy, resulting in leakage loss caused by the scattered optical field. The schematic diagram of the tapered coupling structure is shown in Fig. 5(c), while the simulated optical field distribution of the Si-Ge waveguide is depicted in Fig. 5(e). The simulation outcomes demonstrate that a Si-Ge waveguide employing a tapered waveguide structure can significantly mitigate coupling loss and facilitate efficient optical energy transfer. Therefore, adopting a tapered waveguide structure as the input structure for the detector accomplishes high-efficiency coupling.
The length and width of the tapered waveguide construction have an impact on the mode conversion efficiency as well as the transmission properties of the optical signal inside the waveguide. The length and width of the tapered waveguide structure are simulated independently to examine the impacts of changes in the coupling efficiency of AWG and PD array, with the goal of determining the optimal size of the structure. Figure 6(a) presents the simulation results of the coupling efficiency as a function of the tapered structure length. The results indicate that as the length of the tapered structure increases, the coupling efficiency progressively improves as well. When the length exceeds 30 $\mathrm{\mu}\textrm{m}$, the coupling efficiency stabilizes, achieving an absorption of over 95% of the light. The simulation results illustrating the impact of changes in the width of the tapered structure on the coupling efficiency are displayed in Fig. 6(b). The results indicate that the increasing width gradually raises coupling efficiency, although the magnitude of the change is small. Therefore, the impact of width variation on coupling efficiency is relatively minor. Based on these results, the design parameters for the tapered waveguide are determined, as shown in Fig. 6(e), with a length of 50 $\mathrm{\mu}\textrm{m}$ and an width of 8 $\mathrm{\mu}\textrm{m}$.
Fig. 6. Dimensional simulation of tapered waveguide structure. (a) Variation curve of the coupling efficiency with tapered waveguide length. (b) Variation curve of the coupling efficiency with tapered waveguide width. (c) Schematic diagram of the dimensions of the tapered waveguide structure.
The Ge absorbing layer is placed on top of the Si waveguide in a waveguide-coupled Si-Ge PD, with the light incidence direction perpendicular to the carrier transport direction. The optical signal from the Si waveguide is coupled into the Ge absorption layer through evanescent field coupling, where the refractive index of the Ge layer exceeds that of Si, facilitating efficient light absorption and detection. The dimensions of the Ge detector play a crucial role in light transmission, coupling, and absorption. By optimizing the length, width, and thickness of the active region of the detector, the optical performance of the device can be enhanced. Employing the Si tapered waveguide structure described above as the input end, we analyze the optical absorption efficiency of Si-Ge detectors through modeling and simulation. The relationship curve between the length of the Ge layer and the light absorption efficiency derived from the simulation is displayed in Fig. 7(a). The length of the Ge detector corresponds to the direction of light absorption. It is evident that as the length of the Ge layer increases, the light absorption efficiency progressively rises and stabilizes. Conversely, the change in Ge layer width has a relatively minor impact on optical absorption efficiency by looking at the relationship between the change in Ge layer width and the percentage of optical power absorption in Fig. 7(b). The direction of carrier transport aligns with the thickness of the Ge layer. The simulated relationship curve between optical absorption efficiency and Ge layer thickness, as depicted in Fig. 7(c), shows that optical power absorption increases with thickness. At a thickness of 400 $\textrm{nm}$, the Ge layer can absorb almost 90% of the optical power. Consequently, based on the aforementioned simulation results, the designed dimensions of the Ge detector are a length of 55 $\mathrm{\mu}\textrm{m}$, a thickness of 500 $\textrm{nm}$, and a width of 8 $\mathrm{\mu}\textrm{m}$. The optical absorption efficiency of the Si-Ge PD is plotted against wavelength in Fig. 7(d). The results suggest that in this device, 86% of the optical energy can be absorbed by the Ge layer, converting into electrical field energy. The coupling efficiency between the Si waveguide and the Ge absorption layer is high, leading to a predominant direct coupling of the majority of light from the Si waveguide to the Ge layer under these conditions. The optical field distribution of the Si-Ge waveguide with a tapered construction is depicted in Fig. 7(e), while Fig. 7(f) displays the internal electric field energy distribution of the Ge detector under these circumstances.
Fig. 7. Structural simulation of Ge layer. (a) Variation curve of the absorption efficiency with Ge layer length. (b) Variation curve of the absorption efficiency with Ge layer width. (c) Variation curve of the absorption efficiency with Ge layer thickness. (d) Optical absorption efficiency of Si-Ge detectors with Si tapered structure. (e) Optical field distribution of Si–Ge waveguide with Si tapered structure. (f) Electric field distribution inside the Ge waveguide of the Si–Ge detector.
3. Fabrication and performance discussion
Manufactured on a SOI substrate material, the chip is produced using a 180 $\textrm{nm}$ production line in accordance with predefined parameters. The AWG fabrication process is illustrated in Fig. 8(a). The preprocessing of the Si wafer ensures the cleanliness and quality of the substrate. Firstly, the waveguide pattern is transferred onto the Si wafer using deep ultraviolet lithography (DUVL). Subsequently, inductively coupled plasma (ICP) technology is used to etch the Si waveguide to a depth of 150 $\textrm{nm}$. Upon completion of the etching process, the photoresist on the waveguide surface is removed. Through the aforementioned lithography and etching processes, the optical waveguide structure of the AWG is accomplished.
Fig. 8. Process preparation of devices. (a) The fabrication process of AWG. (b) The fabrication process of PD.
The subsequent processes are tailored for the electrical structure of the device, as shown in Fig. 8(b). Initially, lithography and etching are employed to expose ion implantation windows. Heavily doped P-type ohmic contact regions are formed following boron (B) ion implantation and a high-temperature annealing process. Subsequently, a SiO2 mask layer is deposited via plasma enhanced chemical vapor deposition (PECVD), exposing a growth window by DUVL and ICP for the selective epitaxial growth of a 500 $\textrm{nm}$ Ge layer. Lithography and etching processes are subsequently employed to create ion implantation windows. Phosphorus (P) ion implantation is conducted, followed by high-temperature annealing to form a heavily doped N-type ohmic contact region. Next, the surface oxide layer is removed, followed by cleaning, and a 1.2 $\mathrm{\mu}\textrm{m}$ passivation layer is deposited using PECVD. Finally, the contacts of the electrodes are exposed through lithography and etching, and metal evaporation is used to create the aluminum (Al) electrodes. The image of our monolithic integrated chip, viewed under an optical microscope with high magnification, is displayed in Fig. 9.
We fabricated an AWG with identical structure and parameters as the aforementioned AWG on the same chip, albeit without integrating PDs. Subsequently, this AWG chip was tested using a spectrum analyzer in a clean room with a steady temperature to assess its functionality, yielding the transmission spectrum depicted in Fig. 10. Our prepared AWG exhibits an insertion loss of -3.23 dB for the center channel and a crosstalk level of -20.31 dB. The test results meet our predetermined standards.
The results of the characteristic test of the PD using a semiconductor analyzer are presented in Table 2. The dark current measures 0.34 $\mathrm{\mu}\textrm{A}$ at the reverse bias voltage of -1 V, accompanied by a responsivity of 0.95 A/W and a quantum efficiency of 0.76.
Table 2. Performance parameters of the PD
We established an interrogation performance testing system, as depicted in Fig. 11(a), to assess the interrogation performance of the prepared Si-based AWG monolithic integrated chip. The system consists of a Supercontinuum laser (NKT photonics-SuperK Extremer), a circulator, an FBG, the Si-based AWG monolithic integrated chip, and a semiconductor analyzer (Keysight B1500A). The reflected light of the FBG is fed into the AWG via a circulator, where the PD converts it into a current signal for output. Variations in the photocurrent, recorded by the semiconductor analyzer, are analyzed to assess the performance of the interrogation system. The center wavelength of the FBG is adjusted by applying strain to the FBG mounted on the surface of an equi-intensity cantilever beam. The minimum drift change of the FBG is 10 $\textrm{pm}$.
Fig. 11. The test of the AWG-based FBG interrogation system. (a) Testing system for interrogation performance. (b) Test results.
Figure 11(b) displays the results of the interrogation system test. The wavelength interrogation accuracy can be quantified by the root mean square error (RMSE) derived from linear fitting. The RMSE value obtained in our test is 9.86, indicating an interrogation accuracy of 9.86 $\textrm{pm}$. The change in ln(I2/I1) is detectable with a significant shift when the FBG wavelength changes by 10 $\textrm{pm}$, yielding the resolution of 10 $\textrm{pm}$. However, it is worth noting that the wavelength resolution is constrained by the minimum tuning step of 10 $\textrm{pm}$ for the FBG. In actuality, the integrated chip exhibits an even better resolution.
To ascertain the interrogation range and optimal interrogation resolution of the integrated chip, a tunable laser is employed to simulate the Gauss wavelength reflected by the FBG sensor during the interrogation test shown in Fig. 12(a). The photocurrent output from a 1 × 9 AWG integrated PD array is shown in Fig. 12(b), in which the photocurrent distribution across different channels is non-uniform. This might be attributed to the coupling loss between the AWG and PD, as well as the influence of the responsivity of the PD, as the phenomenon does not show up in the spectrum test of the AWG. The average responsivity of the monolithic integrated chip obtained from the test is 0.13 A/W. The coupling loss between the AWG and PD, denoted by $I{L_{AWG\& PD}}$, can be expressed as follows:
$I{L_{Total}}$ is the overall loss of the silicon-based AWG integrated chip. $I{L_{AWG}}$ is the transmission loss of the AWG itself. $I{L_{AWG\& Fiber}}$ is the coupling loss between the AWG and the input fiber. The coupling loss between the AWG and PD is -2.4 dB.
Fig. 12. The test of the AWG-based FBG interrogation system. (a) Testing system for interrogation performance. (b) Output photocurrent.
The RMSE resulting from the linear fitting of the curve and experimental data is determined to be 10.64, which represents an interrogation accuracy of 10.64 $\textrm{pm}$. A notable variation in ln(I2/I1) occurs with a 1 $\textrm{pm}$ variation in the tunable laser wavelength displayed in Fig. 13(a). Consequently, the system achieves a wavelength resolution of 1 $\textrm{pm}$. The figure also illustrates the logarithm of the ratio of output optical power between CH1 and CH2 as a function of the input wavelength, yielding the FBG center wavelength interrogation formula:
Taking the logarithm of the ratio of the photocurrent outputs from CH1 and CH2 of the AWG integrated chip and substituting it into Eq. (3), we obtain the center wavelength value of the FBG interrogated by the chip. Comparing this value with the actual input wavelength, we generate the wavelength interrogation error plot which is shown in Fig. 13(b).
Fig. 13. Interrogation performance analysis. (a) Linear fitting of interrogation test results. (b) Relationship of interrogation test wavelength error derived using linear fitting. (c) Gauss fitting of interrogation test results. (d) Relationship of interrogation test wavelength error derived using Gauss fitting.
Upon comparing simulation results with actual testing, deviations from Gauss profiles were observed in the AWG transmission spectra, resulting in limited interrogation accuracy when linear fitting was applied. Consequently, a separate study was conducted utilizing Gauss functions for fitting, as depicted in Fig. 13(c). Within a dynamic range of 1.56 $\textrm{nm}$, Gauss fitting achieved an interrogation accuracy of 6.79 $\textrm{nm}$, with the resulting wavelength interrogation error plot illustrated in Fig. 13(d). Different fitting functions yielded varying results for interrogation curves. Linear fitting is suitable for linear relationships due to its straightforward, intuitive, and computationally efficient nature. Conversely, nonlinear fitting offers enhanced capability in fitting complex curve shapes, despite its higher computational complexity and susceptibility to overfitting. In practical experiments, selecting the appropriate fitting function based on the specific circumstances is crucial to optimize interrogation performance. Based on the test results, within the dynamic range of 1556.32∼1557.88 $\textrm{nm}$, our developed AWG monolithic integrated interrogation chip has a measured interrogation accuracy of roughly 6.79 $\textrm{pm}$ and a wavelength resolution of 1 $\textrm{pm}$.
We compared the designed interrogation system with those of recent studies, as summarized in Table 3. In the table, “C” refers to the number of output channels from AWG necessary for interrogating an FBG. The on-chip interrogation system proposed by Yuan Zhuang et al. employs the center of gravity algorithm, boasting high interrogation accuracy. Nevertheless, this approach mandates 4 AWG output channels to interrogate a single FBG, consequently limiting the multiplexing capability of the system, as well as the resolution of the system is constrained by the spacing between AWG channels. Consequently, these studies compromise the multiplexing capability of FBGs and interrogation resolution in favor of achieving high interrogation accuracy. In contrast, the interrogation chip proposed in this study demonstrates favorable performance in terms of resolution and exhibits a relatively higher multiplexing capability, utilizing fewer channels. Notably, we have introduced, for the first time, an on-chip monolithic integrated interrogation chip based on SOI material for FBG interrogation systems. This effectively enhances the integration level of the interrogation system, providing a novel solution for the application of PIC technology in the field of interrogation and fostering further development and application of interrogation systems.
Table 3. Comparison of recent studies
4. Conclusion
For an AWG-based fiber grating sensing interrogation system, we propose a monolithic integrated Si optical interrogation chip with SOI-based photonic integration. We designed a low-loss, low-crosstalk AWG and optimized the coupling structure between AWG and PDs, as well as the dimension of the Ge detector, achieving their monolithic integration. Through experimental tests, the coupling loss between AWG and PD is -2.4 dB. The monolithic integrated Si photonics chip demonstrates an interrogation accuracy of approximately 6.79 $\textrm{pm}$ within a dynamic range of 1.56 $\textrm{nm}$, with a wavelength resolution of 1 $\textrm{pm}$. The interrogation chip developed in this study exhibits excellent performance, effectively enhancing the integration of the interrogation system. This research holds significant importance in the exploration of Si photonic integrated chips, laying a fundamental framework for further investigations into an all-Si-based optoelectronic device integrated FBG sensor system.
Funding
Beijing Municipal Education Commission (KM202211232019); National Natural Science Foundation of China (No. 62205030).
Acknowledgments
The authors acknowledge the support from the National Natural Science Foundation of China and R&D Program of Beijing Municipal Education Commission.
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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