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Abstract
The integrated optical isolator is an essential building block in photonic integrated chips. However, the performance of on-chip isolators based on the magneto-optic (MO) effect has been limited due to the magnetization requirement of permanent magnets or metal microstrips on MO materials. Here, an MZI optical isolator built on a silicon-on-insulator (SOI) without any external magnetic field is proposed. A multi-loop graphene microstrip operating as an integrated electromagnet above the waveguide, instead of the traditional metal microstrip, generates the saturated magnetic fields required for the nonreciprocal effect. Subsequently, the optical transmission can be tuned by varying the intensity of currents applied on the graphene microstrip. Compared with gold microstrip, the power consumption is reduced by 70.8%, and temperature fluctuation is reduced by 69.5% while preserving the isolation ratio of 29.44 dB and the insertion loss of 2.99 dB at1550 nm.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Silicon photonic integrated circuits (Si-PICs) have enabled chip-scale optical systems with high integration capacity and low power consumption, which has been considered as a promising platform for optical communication and microwave photonics technologies owing to its compatibility with mature complementary-metal-oxide-semiconductor (CMOS) fabrication. Meanwhile, as the rapid increase of global data volume, high-performance Si-PICs are in a growing demand [1–3]. A large number of optical components are integrated on a silicon-on-insulator (SOI) platform, including lasers, photodetectors, modulators, etc. [4–6]. The optical isolator with an asymmetric scattering matrix possesses the ability of breaking symmetry in the propagation of light and allows for unidirectional light transmission, which is an essential building block to control optical signal routing, multiplexing, and protecting lasers from reflections in PICs [7]. However, the high-performance integrated optical isolator remains a missing link in PICs.
Numerous approaches have been proposed to integrate optical isolators on the silicon-on-insulator (SOI) platform, such as nonlinear optics [8,9] and spatiotemporal modulation [10,11] and magneto-optical (MO) effect, etc. Among them, integrated MO materials with large Faraday rotation to obtain an asymmetric index tensor and break Lorentz reciprocity has been the primary approach. Besides, CMOS-compatible MO materials have been successfully achieved via heterogeneous wafer bonding techniques or depositing, such as cerium-substituted yttrium iron garnet (Ce:YIG) with high Faraday rotation up to −5900 deg/cm at 1550 nm wavelength [12]. And in turn, by integrating a MO material with a silicon waveguide and placing it in a magnetic field, the resulting nonreciprocal phase shift can be utilized in an Mach–Zehnder interferometer (MZI) or microring to realize optical isolators [13–18]. However, the static magnetic field required for realizing the MO effect is traditionally generated by adding external magnetic fields or permanent magnets, which impedes the density of integration and packaging [19].
Obviously, integrated electromagnets, directly generating magnetic fields above MO materials without the need of external magnetic fields, are much more attractive in high-performance Si-PICs. The method of magnetizing MO material by applying currents on a metal microstrip was proposed [19]. However, owing to the thermal effect of silicon and the large absorption of metals, a thick isolation layer (∼$\mu $m) must be added between a silicon waveguide and a metal microstrip to reduce crosstalk and excess loss [18]. Due to the existence of the thick isolation layer, a much larger current was required to produce the same saturation magnetic field, which in turn increased the power consumption and the temperature fluctuation of the device. Graphene, two-dimensional (2D) sheet, possesses outstanding performance whose underlying physical mechanism has been studied in a proof-of-principle devices. Its extraordinary properties, such as broadband absorption of ∼2.3% per layer for vertically incident light [20], a carrier mobility as high as 200,000 cm2/V·s at room temperature (RT) [21], a high conductivity, and excellent mechanical stability [22]. Therefore, graphene as a microstrip can help to tackle the problem of absorption loss and power consumption of metal microstrip.
Here, an MZI optical isolator with the multi-loop graphene electromagnetic microstrip has been proposed. The integrated magnetic field required for magnetizing Ce:YIG is generated by applying a current on the graphene microstrip, while the resulting nonreciprocal phase shift (NRPS) is utilized in an unbalanced Mach-Zehnder interferometer (MZI) to achieve broadband optical isolation. The microstrip can be directly contacted on the Ce:YIG layer without the need of the thick insulator layer required for traditional metal electromagnetic microstrip, therefore, the power consumption and temperature fluctuation can be further reduced while improving the density of integration.
2. Device structure and principle
A schematic diagram of the proposed MZI optical isolator built on 220nm-SOI platform is shown in Fig. 1(a). It consists of two 3 dB 1 × 2 couplers, nonreciprocal phase shift (NRPS) waveguides with the multi-loop graphene electromagnetic microstrip, and asymmetric reciprocal phase shift (RPS) waveguides. Considering the fabrication tolerance, the dimension of the silicon waveguide is set as 500 nm × 220 nm. The cross-section of MO waveguide, as shown in Fig. 1(b). A 200nm-Ce:YIG layer and a 31nm-YIG layer are stacked on top of the silicon waveguide, while a multi-loop graphene layer operating as an electromagnetic microstrip sitting on the Ce:YIG layer. The width of graphene loop is Wm and the gap width Wg between each loop is half the width of Wm.
Fig. 1. (a) Sketch of the proposed MZI magneto-optical isolator on SOI; (b) The cross-section of the NRPS waveguide, the magnetic field profile generated by a multi-loop graphene electromagnetic microstrip at a given current; (c) The magnetic field distribution in the MO waveguide; (d) Propagation constants of opposite directions in MO waveguides when MO is magnetized in the + z direction; (e) Propagation constants in opposite directions in MO waveguides when MO is magnetized in the −z direction.
A magnetic field Hz in the positive + z axis is generated to magnetize the Ce:YIG layer when a current Ix on the graphene microstrip along x axis is applied, as shown in Fig. 1 (c). And the reversely magnetic field −Hz when a current is applied in an opposite direction, as shown in Fig. 1(d). Its asymmetric dielectric constant tensor can be expressed as:
The nonreciprocal phase shifter provides a phase difference ΔφNRPS=Δβ×LNRPS =${\mp} $π/2 for ${\pm} $x-axis direction derived from MO phase shift Δβ induced by the MO effect. While the reciprocal phase shifter (RPS) provides phase shift of ΔφRPS = β×ΔLRPS = π/2 caused by the length difference between two arms, as shown in Fig. 2. When the light propagates forward (x-direction), the total phase difference is $\mathrm{\Delta }\varphi = $0. The input light induces constructive interference and transmits through the device as shown in Fig. 2(a). When the light propagates backward, the total phase difference is $\mathrm{\Delta }\varphi = $π, destructive interference, as shown in Fig. 2(b). That is, optical isolation is achieved.
Fig. 2. Working principle of MZI magneto-optical isolator; (a) Forward propagation; (b) Backward propagation.
3. Results and discussion
3.1 Calculation of NRPS
As shown in Fig. 3 (a), both NRPS and transmission loss deteriorate with the increase of the thickness of YIG seed layer, however, the YIG layer should be thick enough to ensure a high-quality crystallization of Ce: YIG. Therefore, a trade-off is made and the thickness of YIG seed layer is chosen to be 31 nm [23]. For a fixed thickness of the YIG seed layer, NRPS varies proportional to the thickness of Ce: YIG layer, while the transmission loss of NRPS waveguide changes reversely, as shown in Fig. 3(c) (d). The coupling efficiency decreases with the silicon width, leading to the reduction of NRPS and the increase of loss. Therefore, a 200nm-thickness Ce: YIG layer and a 500nm-width waveguide are chosen, resulting in an NRPS of 7298 rad/m and the loss of 1.87 dB.
Fig. 3. (a) Relationship between the thickness of YIG seed layer and NRPS and loss; (b) TM mode distribution in the NRPS waveguide; (c) NRPS versus the waveguide width and the Ce: YIG material thickness; (d) The loss as a function of the waveguide width and the Ce: YIG thickness.
3.2 Graphene electromagnetic microstrip design
Since the Ce: YIG in-plane configuration is the easy axis of magnetization, it takes only ∼50 Oe to saturate the Ce:YIG [19]. Obviously, in order to maintain the magnetic field intensity at 50Oe, the current in electromagnetic microstrip increases with the thickness of the isolation layer, thus increasing the power consumption and temperature fluctuation of the device. That is, the performance of the device degrades with the increase of the thickness of the isolation layer. For gold microstrip, ∼10$\mu $m thick isolation layer is required to avoid extra insertion loss, and a current of at least 180 mA is required to saturated magnetize Ce:YIG [18]. It results in a quite high-power consumption of P = I2R = 260 mW and a temperature variation of ΔT = 74.84 K, leading to significant thermo-optic effect. Compared with gold microstrip, graphene microstrip can be directly transferred on the MO waveguide owing to its transparency property, thus reducing the driving current. However, due to the atomic thickness of a monolayer graphene, the power consumption is large P = 1.8W as well as the temperature fluctuation for generating the same magnetic field (50 Oe) in the gold microstrip. That is because, although the resistivity of graphene is smaller than that of gold, the total resistance of graphene microstrips is much greater than that of gold microstrips for the same length.
In order to generate a magnetic field intensity (50 Oe) while reducing the temperature fluctuation and power consumption, a large current and small resistance is required for the microstrip. This issue can be solved by increasing the number of graphene microstrip loops. The structure of 5-loops graphene microstrip is shown in Fig. 4(a)(b). For H = 50Oe, the temperature distribution (in Celcius) of 5-loops graphene microstrip is shown in Fig. 4(c). Temperature fluctuation ΔT of 5-loops graphene microstrip is obviously smaller than that of monolayer loop graphene, reduced by nearly 1000 K.
Fig. 4. (a) Structure schematic of graphene microstrip with 5-loops and 1500 nm width; (b) The Cross-section schematic of the NRPS waveguide with 5-loops graphene microstrip; (c) The temperature distribution (in Celcius) of the device; (d) The temperature fluctuation $\Delta T$ varies with the applied currents.
The local temperature distribution generated by the electrical current using COMSOL Multiphysics simulation software is calculated and shown in Fig. 4(c). Owing to the thermal effect of silicon, the derivative ∂n/∂T, the additional phase shift or loss would be enlarged by the increase of the applied current. From Fig. 4(d), to ensure Ce: YIG saturation (50Oe), the required current and the temperature fluctuation are reduced as the number of loops increases. However, the temperature fluctuation changes slightly when increasing more than 15 loops, because the external loops do not contribute as much to the transverse magnetic field. Therefore, 15-loops of graphene microstrip is chosen. Meanwhile, optimizing the width of graphene microstrips can further reduce the power consumption and temperature fluctuation of devices, as shown in Fig. 5. A minimum temperature fluctuation ΔTmin = 17.04 K is obtained with 400 nm width of graphene microstrip (Wm = 400 nm), while the power is P = 119.23 mW. And the minimum power Pmin = 76 mW is obtained with Wm = 900 nm, while its temperature fluctuation is ΔT = 22.8 K. Therefore, a trade-off is made and the width of the loop is chosen as Wm = 900 nm. Compared with gold microstrips, the optical isolator using multi-loop graphene as the electromagnetic microstrip can significantly decrease the power consumption and the temperature fluctuation, which are reduced by 70.8% and 69.5%, respectively. Considering the thermo-optical coefficient of silicon 1.84e-4 /K, the refractive index change of silicon is ΔnSi ≈10−2 for the metal microstrip (gold), while ΔnSi≈10−4 for 15-loops graphene microstrip, which can be ignored for an MZI based optical isolator. Obviously, the multi-loops graphene can sufficiently reduce the thermal effect of silicon, that is, paving a way for improving the stability of the optical isolator.
Fig. 5. (a) Thermal field distribution of NRPS waveguide cross section using 1500 nm wide and 900 nm wide 15-loops graphene microstrip respectively; (b) The temperature fluctuation $\Delta T$ and power consumption vary with the width of graphene microstrip.
3.3 Device characterization
The insertion loss of the optical isolator consists of MMI loss, junction loss, RPS waveguide loss and NRPS waveguide loss. Each MMI loss can be less than 0.06 dB around 1550 nm [24]. The junction loss originates from the coupling loss between RPS and NRPS waveguides, related to the mode overlap. The loss at each connection is 0.25 dB. obtained by Eq. (3). There are four such coupling structures in the device, so the total junction loss is 1 dB.
Length of the RPS waveguide is ΔLRPS =π/(2β0)= 0.22 $\mu $m, where ${\beta _0} = 7.18 \times {10^6}\textrm{rad}/\textrm{m}$ is the propagation constant. According to Eq. (4), the transmission of forward and backward propagation around 1550 nm wavelength is calculated, as shown in Fig. 6.
4. Conclusion
In summary, an on-chip MZI magneto-optical isolator with the multi-loop graphene electromagnetic microstrip is proposed. Its isolation ratio is 29.44 dB at 1550 nm with 2.99 dB insertion loss. The saturation magnetic field for the nonreciprocal effect is generated by the graphene microstrip, eliminating the need for permanent magnets, therefore, greatly increase the density of integration and easing packaging. Meanwhile, in order to solve the problem of high-power consumption and temperature fluctuation caused by the high resistivity of monolayer graphene microstrip, multi-loop graphene is utilized. Especially, compared with the metal microstrip, both the power consumption and temperature fluctuation are reduced by 70.8% and 69.5%, respectively, in turn achieving a high stability of the optical isolator. It will benefit lots of applications in Si-PICs with tunable and integrated magnetic field functionality, such as optical neural networks and even in advanced integrated opto-electronic systems.
Funding
National Natural Science Foundation of China (62205164, 61974078, 61871244); Natural Science Foundation of Zhejiang Province (LQ21F040001, LY21F040002); State Key Laboratory of Transient Optics and Photonics (SKLST202007).
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.
References
1. D. Perez, I. Gasulla, L. Crudgington, D. J. Thomson, A. Z. Khokhar, K. Li, W. Cao, G. Z. Mashanovich, and J. Capmany, “Multipurpose silicon photonics signal processor core,” Nat. Commun. 8(1), 636 (2017). [CrossRef]
2. D. Marpaung, J. Yao, and J. Capmany, “Integrated microwave photonics,” Nat. Photonics 13(2), 80–90 (2019). [CrossRef]
3. W. Bogaerts, D. Perez, J. Capmany, D. A. B. Miller, J. Poon, D. Englund, F. Morichetti, and A. Melloni, “Programmable photonic circuits,” Nature 586(7828), 207–216 (2020). [CrossRef]
4. Y. Shoji, K. Miura, and T. Mizumoto, “Optical nonreciprocal devices based on magneto-optical phase shift in silicon photonics,” J. Opt. 18(1), 013001 (2016). [CrossRef]
5. D. Jalas, A. Petrov, M. Eich, W. Freude, S. Fan, Z. Yu, R. Baets, M. Popović, A. Melloni, J. D. Joannopoulos, M. Vanwolleghem, C. R. Doerr, and H. Renner, “What is — and what is not — an optical isolator,” Nat. Photonics 7(8), 579–582 (2013). [CrossRef]
6. S. Ghosh, S. Keyvaninia, Y. Shirato, T. Mizumoto, G. Roelkens, and R. Baets, “Optical Isolator for TE Polarized Light Realized by Adhesive Bonding of Ce:YIG on Silicon-on-Insulator Waveguide Circuits,” IEEE Photonics J. 5(3), 6601108 (2013). [CrossRef]
7. W. Yan, Y. Yang, W. Yang, J. Qin, L. Deng, and L. Bi, “On-Chip Nonreciprocal Photonic Devices Based on Hybrid Integration of Magneto-Optical Garnet Thin Films on Silicon,” IEEE J. Select. Topics Quantum Electron. 28(3), 1–15 (2022). [CrossRef]
8. L. Del Bino, J. M. Silver, M. T. M. Woodley, S. L. Stebbings, X. Zhao, and P. Del’Haye, “Microresonator isolators and circulators based on the intrinsic nonreciprocity of the Kerr effect,” Optica 5(3), 279–282 (2018). [CrossRef]
9. N. Singh and F. X. Kärtner, “Nonlinear Mach-Zehnder interferometer isolator,” Opt. Express 30(4), 5973–5980 (2022). [CrossRef]
10. S. Bhandare, S. K. Ibrahim, D. Sandel, H. Zhang, F. Wust, and R. Noé, “Novel nonmagnetic 30-dB traveling-wave single-sideband optical isolator integrated in III/V material,” IEEE J. Select. Topics Quantum Electron. 11(2), 417–421 (2005). [CrossRef]
11. D. B. Sohn, D. S. Kim, and G. Bahl, “Time-reversal symmetry breaking with acoustic pumping of nanophotonic circuits,” Nat. Photonics 12(2), 91–97 (2018). [CrossRef]
12. Y. Zhang, C. T. Wang, X. Liang, B. Peng, H. P. Lu, P. H. Zhou, L. Zhang, J. X. Xie, L. J. Deng, M. Zahradnik, L. Beran, M. Kucera, M. Veis, C. A. Ross, and L. Bi, “Enhanced magneto-optical effect in Y1.5Ce1.5Fe5O12 thin films deposited on silicon by pulsed laser deposition,” J. Alloys Compd. 703, 591–599 (2017). [CrossRef]
13. X. Y. Sun, Q. Du, T. Goto, M. C. Onbasli, D. H. Kim, N. M. Aimon, J. Hu, and C. A. Ross, “Single-Step Deposition of Cerium-Substituted Yttrium Iron Garnet for Monolithic On-Chip Optical Isolation,” ACS Photonics 2(7), 856–863 (2015). [CrossRef]
14. Y. Shoji and T. Mizumoto, “Magneto-optical non-reciprocal devices in silicon photonics,” Sci. Technol. Adv. Mater. 15(1), 014602 (2014). [CrossRef]
15. K. Shui, L. Nie, Y. Zhang, B. Peng, J. Xie, L. Deng, and L. Bi, “Design of a compact waveguide optical isolator based on multimode interferometers using magneto-optical oxide thin films grown on silicon-on-insulator substrates,” Opt. Express 24(12), 12856–12867 (2016). [CrossRef]
16. D. Karki, V. Stenger, A. Pollick, and M. Levy, “Broadband Bias-Magnet-Free On-Chip Optical Isolators With Integrated Thin Film Polarizers,” J. Lightwave Technol. 38(4), 827–833 (2020). [CrossRef]
17. Y. Zhang, Q. Du, C. Wang, T. Fakhrul, S. Liu, L. Deng, D. Huang, P. Pintus, J. Bowers, C. A. Ross, J. Hu, and L. Bi, “Monolithic integration of broadband optical isolators for polarization-diverse silicon photonics,” Optica 6(4), 473–478 (2019). [CrossRef]
18. D. Huang, P. Pintus, Y. Shoji, P. Morton, T. Mizumoto, and J. E. Bowers, “Integrated broadband Ce:YIG/Si Mach-Zehnder optical isolators with over 100 nm tuning range,” Opt. Lett. 42(23), 4901–4904 (2017). [CrossRef]
19. D. Huang, P. Pintus, C. Zhang, Y. Shoji, T. Mizumoto, and J. E. Bowers, “Electrically Driven and Thermally Tunable Integrated Optical Isolators for Silicon Photonics,” IEEE J. Select. Topics Quantum Electron. 22(6), 271–278 (2016). [CrossRef]
20. F. Bonaccorso, Z. Sun, T. Hasan, and A. C. Ferrari, “Graphene photonics and optoelectronics,” Nat. Photonics 4(9), 611–622 (2010). [CrossRef]
21. L. Yu, Y. Yin, Y. Shi, D. Dai, and S. He, “Thermally tunable silicon photonic microdisk resonator with transparent graphene nanoheaters,” Optica 3(2), 159–166 (2016). [CrossRef]
22. Q. Bao, K. P. Loh, and Graphene Photonics, “Plasmonics, and Broadband Optoelectronic Devices,” ACS Nano 6(5), 3677–3694 (2012). [CrossRef]
23. T. Goto, M. C. Onbaşlı, and C. A. Ross, “Magneto-optical properties of cerium substituted yttrium iron garnet films with reduced thermal budget for monolithic photonic integrated circuits,” Opt. Express 20(27), 28507–28517 (2012). [CrossRef]
24. S. Zhen, W. Zhiqi, Q. Chao, L. Le, A. Pang, W. Aimin, W. Xi, Z. Shichang, and G. Fuwan, “A Compact and Low-Loss MMI Coupler Fabricated With CMOS Technology,” IEEE Photonics J. 4(6), 2272–2277 (2012). [CrossRef]
