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Abstract
Wavelength-tunable laser diode with a wide tuning range is required for optical communication systems and optical sensing. External cavity laser diodes with silicon-photonic wire waveguides and ring resonators have small footprint because of high refractive index contrast between Si. However, power coupling efficiency κ of conventional straight directional coupler between ring and bus waveguides have large wavelength dependence, which lowers tunable range. In this study, we demonstrate a hybrid wavelength-tunable laser diode using curved directional couplers, whose wavelength dependence on κ is low. The wavelength-tunable range record of 120.9 nm has been achieved. In addition, curved directional couplers are tolerant of waveguide width fabrication error.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Wavelength-tunable laser diodes with compact and wide tuning range are essential for optical communications and sensing. In the high-capacity optical communication systems using wavelength division multiplexing (WDM), wavelength tunable laser diodes with wide tunable range lower maintenance costs by using shared devices for optical transmitters [1–3]. They are also useful for wavelength routing networks [1,4], or light sources for LiDAR with optical beam steering in the field of optical sensing [5–9]. External cavity lasers using silicon-photonic integrated circuits have small device footprint because of high refractive-index contrast between Si core and SiO2 cladding. Moreover, they are suitable for mass production using CMOS processing technologies [10,11]. Wavelength tunable laser diodes with double ring resonators enable single-mode lasing on wide wavelength range by Vernier effects [12]. Usually, straight directional couplers (DCs) which consist of two parallel straight waveguides are used to transfer light between ring resonators and bus waveguides. However, their power coupling efficiency κ is highly wavelength dependent. Very low values of κ increase the losses of ring resonators, very large values reduce wavelength selectivity, both of which lower wavelength tunable range. Wavelength tunable ranges of external-cavity laser diodes using Si channel waveguides were restricted to around 100 nm [3,12–22]. A laser diode using Si rib waveguides achieved 118 nm tunable range [23], but the device requires the use of a large ring resonator with radii of around 600 µm to avoid bending loss, and the device footprint is large because it uses three ring resonators to obtain sufficient wavelength selectivity. In this study, we designed a curved directional coupler [24–27] and demonstrated that a curved DC’s wavelength dependence on κ is low and has large tolerance for fabrication errors. In addition, we fabricated a wavelength filter for external-cavity laser diode including double ring resonators with curved DCs. Combining this filter and a quantum well semiconductor optical amplifier (QW-SOA), very wide tunable range of 120.9 nm with single mode lasing was obtained. In addition, the demonstrated double-ring laser diode is compact, with the footprint of the external cavity lower than 0.6 × 0.2 mm, because the external cavity does not include an additional Mach–Zehnder interferometer or large ring resonators.
2. Characteristics of a curved directional coupler
DCs use coupled modes between two adjacent waveguides [27,28]. By the interference of even and odd modes, its power coupling efficiency κ(λ) depends on coupler length LC indicated by
In our study, we have used racetrack ring resonators including straight DCs [12,13,16,18]. The oval ring resonators with circular designs are also used [15,17,20,22], but the racetrack ring resonators have advantage that we can easily tune the coupling efficiency κ of DCs by the coupling length LC. The max coupling efficiency κmax of a conventional straight DC is 1 because of its symmetric coupled mode. Therefore, as shown in Fig. 1(a), κ(λ) becomes too large around longer lasing wavelength in the straight DC. Too large κ(λ) causes multimode lasing due to decreased wavelength selectivity.
The curved DC, whose peak of κ was designed to be 0.4 around λ = 1550 nm, is shown in Fig. 1(b). The curved DC consists of an Si waveguide with a width of 0.44 µm and height of 0.20 µm. The Gap, which is the distance from the adjacent curved waveguide, is 0.22 µm. Curvature radius R = 32.34 µm and coupler length LC = 43.00 µm are defined using the center of this Gap. Figure 2(a) shows the horizontal distribution of electric field of even and odd modes of curved DCs at λ = 1550 nm. The adjacent curved waveguides of curved DC have different lengths because of their slightly different curvature; thus, the shape of coupled modes becomes asymmetric due to the difference in the effective propagation constants of each waveguide.
Fig. 2. (a): Coupled modes of the curved DC. (b): Incident light and completely coupled light calculated by linear combination of coupled modes in (a).
By calculating the linear combination of those modes, we can calculate electric field of incident light on the left waveguide and completely coupled light (Fig. 2(b)), and then the value of κmax can be estimated from peak height of the right waveguide in the completely coupled condition. The power coupling efficiency κ(λ) of the curved DC is affected by the wavelength dependence on κmax(λ) and L(λ). Optimizing values of the R, LC, and Gap, the wavelength dependence of κ is decreased because the fluctuation of κmax(λ) and L(λ) is cancelled. Calculated coupling efficiency κ of curved DC using expression (1) is shown in Fig. 3, which is maintained over 0.2 around the C-L band.
Another way to reduce the wavelength dependence of κ is by using a width-asymmetric straight DC whose adjacent waveguides have slightly different width [29,30]. The width-asymmetric straight DC with a waveguide width of 0.440 µm and 0.422 µm, and coupler length LC = 17.00 µm has a peak of κ at 0.4. Subsequently, we compared the fabrication error tolerance of the curved DC and the width-asymmetric straight DC. Figure 4 shows wavelength dependencies of their coupling efficiency κ when the width fluctuation Δw of the waveguides are −5, 0, and +5 nm. The fluctuations in κ of the curved and width-asymmetric straight DCs at a wavelength of 1550 nm are at most 0.0058 and 0.030, respectively. The fluctuation in κ of the curved DC was suppressed because the difference in the effective refractive indices between the adjacent waveguides is defined by the difference in the lengths of the waveguides. The fluctuation in complete coupling length L(λ) calculated by expression (2) of the curved DC is at most 0.18%, even though that of width-asymmetric straight DC is at most 4.29%. Therefore, we can conclude that curved DC is more tolerant to fabrication error than the width-asymmetric straight DC.
Fig. 4. Fabrication error dependence of coupling efficiency κ (a): curved DC, (b): width-asymmetric straight DC.
3. Characteristics of the hybrid wavelength-tunable laser
The structure of the wavelength tunable laser is depicted in Fig. 5. The left facet of quantum-well SOA with a length of 1.0 mm has a reflectivity of 6%. The SOA is cooled to 20 °C by the thermo-electric cooler. The gain spectrum of the SOA is shown in Fig. 6. The injection current was 200 mA. The SOA has broad gain bandwidth centered on the wavelength at 1520 nm. The SOA and silicon-photonic wavelength filter are butt coupled. The spot size converter with inverse-taper structure is tilted to avoid reflection at the chip facet. Wavelength filter consists of two ring resonators with curved DCs and a loop mirror. The circumference of ring 1 and ring 2 are 114.1 and 119.7 µm, respectively, corresponding to the designed free spectral range of 4.9 and 4.7 nm.
The measured power coupling efficiency κ of curved DC using transmission spectrum of a ring resonator is shown in Fig. 7. The measured κ shows good agreement with the calculation and has reduced wavelength dependence. Figure 8(a) shows round-trip transmittance difference and loss calculated from coupling efficiency of the DC. To calculate loss of ring resonators, we assumed 0.3 dB/mm waveguide propagation loss. The transmittance difference is defined by the ratio of transmittance between main and side mode in double-ring wavelength filter. The transmittance difference depends mainly on Q-factor of the ring resonator. Very large values of κ result in a low Q-factor of the resonant peak and reduce the transmittance difference. If κ is too low, loss of the ring resonator at resonant wavelength becomes high. This wavelength filter has a transmittance difference of over 1.51 dB and a loss below 0.80 dB at the wavelength range of 1450 to 1600 nm, which enables single-mode lasing around the gain peak of the SOA. The calculated typical transmittance spectrum of Vernier ring filter is shown in Fig. 8(b). An enlarged view of the Vernier spectrum is shown in the inset of Fig. 8(b) to show the transmittance difference at the lasing wavelength of 1550 nm, where wavelength selectivity is the smallest due to maximum coupling efficiency in bending DC as shown in Fig. 3.
Fig. 8. (a): Transmittance difference and loss of double ring wavelength filter. (b): Typical transmittance spectrum of Vernier ring.
The L-I curve of the hybrid wavelength tunable laser diode is shown in Fig. 9. Using the heaters of the ring resonators and the loop mirror, the lasing wavelength was adjusted to 1551.2 ± 0.09 nm. A threshold current of 18.5 mA and maximum power of 9.15 mW with 200 mA injection current were obtained. Figure 10 shows the wavelength tuning spectra with 200 mA injection current. Wavelength tunable range of 120.9 nm was achieved, which is largest value as silicon-photonic external cavity laser diodes, as far as we know. Wavelength dependence of side-mode suppression-ratio (SMSR) is shown in Fig. 11. The SMSR was over 35 dB at every lasing wavelength. At lasing wavelengths longer than 1490 nm, the SMSR is 40 dB or higher, whereas at shorter wavelengths, the SMSR decreases. At shorter wavelengths, κ is smaller than 0.2 as shown in Fig. 7, resulting in an increase in optical intensity within the ring resonator. This increased optical intensity leads to two-photon absorption within the silicon waveguide, this nonlinear optical effect causes a decrease in lasing output power and a fluctuation in the refractive index of the silicon waveguide, resulting in a decrease in SMSR [12]. The heaters of two ring resonators were heated one after the other as shown in Fig. 10, and the heater in the loop mirror was tuned in such a way that the power of emitted light is the highest. The wavelength tuning efficiency of heaters of ring 1 and 2 were −1.91 nm/mW and 2.02 nm/mW, respectively. The tunable range was restricted by decrease in the threshold gain around the longer wavelength, whereas there was increase in the loss of ring resonators around shorter wavelength. Because of the wavelength mismatch between the peak gain and the peak of κ, the laser oscillation was not observed at the wavelength lower than 1473.3 nm, where the κ is lower than 0.2. Therefore, the wavelength tunable range will be increased by 150 nm by shifting the peak of κ to a shorter wavelength.
Fig. 10. Wavelength tuning spectra and corresponding heater power of double ring. Injection current of SOA was 200 mA.
4. Summary
We demonstrated that the curved DC has lower wavelength dependence on power coupling efficiency than that of straight DC. In addition, curved DC has large tolerance for fabrication errors than that of width-asymmetric straight DC. The hybrid wavelength tunable laser diode, that consists of wavelength filter using double ring resonators with curved DC, shows wide wavelength-tunable range of 120.9 nm. The tunable range can be expanded by improving the coupling efficiency of curved DCs. The wavelength tunable laser diode we reported is expected to play a key role in high-capacity optical communication systems or optical sensing.
Funding
Japan Society for the Promotion of Science (23H01472); National Institute of Information and Communications Technology (#1301); Ministry of Internal Affairs and Communications (JP235003005).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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. W. H. Tranter, D. P. Taylor, R. E. Ziemer, N. F. Maxemchuk, and J. W. Mark, “Dense wavelength division multiplexing networks: Principles and applications,” The Best of the Best: Fifty Years of Communications and Networking Research (IEEE Publications, 2007), pp. 73–89.
2. G. Sarlet, G. Morthier, and R. Baets, “Control of Widely Tunable SSG-DBR Lasers for Dense Wavelength Division Multiplexing,” J. Lightwave Technol. 18(8), 1 (2000). [CrossRef]
3. T. Chu, N. Fujioka, and M. Ishizaka, “Compact, lower-power-consumption wavelength tunable laser fabricated with silicon photonic-wire waveguide micro-ring resonators,” Opt. Express 17(16), 14063–14068 (2009). [CrossRef]
4. S.-J. Ben Yoo, “Optical packet and burst switching technologies for the future photonic Internet,” J. Lightwave Technol. 24(12), 1 (2006). [CrossRef]
5. Y. Misugi, H. Okayama, and T. Kita, “Demonstration of 2D beam steering using large-scale passive optical phased array enabled by multimode waveguides with reduced phase error,” Appl. Phys. Express 15(10), 102002 (2022). [CrossRef]
6. Y. Misugi, H. Okayama, and T. Kita, “Compact and low power-consumption solid-state two-dimensional beam scanner integrating a passive optical phased array and hybrid wavelength-tunable laser diode,” J. Lightwave Technol. 41(11), 3505–3512 (2023). [CrossRef]
7. J. C. Hulme, J. K. Doylend, M. J. R. Heck, J. D. Peters, M. L. Davenport, J. T. Bovington, L. A. Coldren, and J. E. Bowers, “Fully integrated hybrid silicon two dimensional beam scanner,” Opt. Express 23(5), 5861–5874 (2015). [CrossRef]
8. T. Baba, T. Tamanuki, H. Ito, M. Kamata, R. Tetsuya, S. Suyama, H. Abe, and R. Kurahashi, “Silicon photonics FMCW LiDAR chip with a slow-light grating beam scanner,” IEEE J. Select. Topics Quantum Electron. 28(5: Lidars and Photonic Radars), 1–8 (2022). [CrossRef]
9. W. Xu, Y. Guo, Xinhang Li, Chuxin Liu, L. Lu, J. Chen, and L. Zhou, “Fully integrated solid-state LiDAR transmitter on a multi-layer silicon-nitride-on-silicon photonic platform,” J. Lightwave Technol. 41(3), 832–840 (2023). [CrossRef]
10. R. Soref, “The past, present, and future of silicon photonics,” IEEE J. Select. Topics Quantum Electron. 12(6), 1678–1687 (2006). [CrossRef]
11. C. Yang, L. Liang, L. Qin, H. Tang, Y. Lei, P. Jia, Y. Chen, Y. Wang, Y. Song, C. Qiu, C. Zheng, H. Zhao, X. Li, D. Li, and L. Wang, “Advances in silicon-based, integrated tunable semiconductor lasers,” Nanophotonics 12(2), 197–217 (2023). [CrossRef]
12. T. Kita, R. Tang, and H. Yamada, “Narrow spectral linewidth silicon photonic wavelength tunable laser diode for digital coherent communication system,” IEEE J. Select. Topics Quantum Electron. 22(6), 23–34 (2016). [CrossRef]
13. T. Kita, N. Yamamoto, T. Kawanishi, and H. Yamada, “Ultra-compact wavelength tunable quantum dot laser with silicon photonic external cavity,” Conference on Lasers and Electro-Optics (CLEO), San Jose, CA, United States, 2015, 2015, 1–2.
14. J. C. Hulme, J. K. Doylend, and J. E. Bowers, “Widely tunable Vernier ring laser on hybrid silicon,” Opt. Express 21(17), 19718–19722 (2013). [CrossRef]
15. H. Guan, A. Novack, T. Galfsky, Yangjin Ma, S. Fathololoumi, A. Horth, T. N. Huynh, J. Roman, R. Shi, M. Caverley, Y. Liu, T. Baehr-Jones, K. Bergman, and M. Hochberg, “Widely tunable, narrow-linewidth III-V/silicon hybrid external-cavity laser for coherent communication,” Opt. Express 26(7), 7920–7933 (2018). [CrossRef]
16. T. Kita, K. Nemoto, and H. Yamada, “Long external cavity Si photonic wavelength tunable laser diode,” Jpn. J. Appl. Phys. 53(4S), 1 (2014). [CrossRef]
17. N. Kobayashi, K. Sato, M. Namiwaka, K. Yamamoto, S. Watanabe, T. Kita, H. Yamada, and H. Yamazaki, “Silicon photonic hybrid ring-filter external cavity wavelength tunable lasers,” J. Lightwave Technol. 33(6), 1241–1246 (2015). [CrossRef]
18. T. Kita, R. Tang, and H. Yamada, “Compact silicon photonic wavelength-tunable laser diode with ultra-wide wavelength tuning range,” Appl. Phys. Lett. 106(11), 111104 (2015). [CrossRef]
19. K. Iwanaga, W. Masuda, A. Matsumoto, N. Yamamoto, and T. Kita, “Quantum Dot SiPh Hybrid Wavelength Tunable Laser Diode with 100 nm Tunable Range,” Proceedings of International Semiconductor Laser Conference (ISLC2022)1-2, WB-05 (2022).
20. Ruijun Wang, Stephan Sprengel, Anton Vasiliev, Gerhard Boehm, Joris Van Campenhout, Guy Lepage, Peter Verheyen, Roel Baets, Markus-Christian Amann, and Gunther Roelkens, “Widely tunable 2.3 µm III-V-on-silicon Vernier lasers for broadband spectroscopic sensing,” Photonics Res. 6(9), 858–866 (2018). [CrossRef]
21. Ruijun Wang, Aditya Malik, Ieva Šimonytė, Augustinas Vizbaras, Kristijonas Vizbaras, and Gunther Roelkens, “Compact GaSb/silicon-on-insulator 2.0x µm widely tunable external cavity lasers,” Opt. Express 24(25), 28977–28986 (2016). [CrossRef]
22. Jia Xu Brian Sia, Xiang Li, Wanjun Wang, Zhongliang Qiao, Xin Guo, Jin Zhou, Callum G. Littlejohns, Chongyang Liu, Graham T. Reed, and Hong Wang, “Sub-kHz linewidth, hybrid III-V/silicon wavelength-tunable laser diode operating at the application-rich 1647-1690 nm,” Opt. Express 28(17), 25215–25224 (2020). [CrossRef]
23. P. A. Morton, C. Xiang, J. B. Khurgin, C. D. Morton, M. Tran, J. Peters, J. Guo, M. J. Morton, and J. E. Bowers, “Integrated coherent tunable laser (ICTL) with ultra-wideband wavelength tuning and sub-100 Hz Lorentzian linewidth,” J. Lightwave Technol. 40(6), 1802–1809 (2022). [CrossRef]
24. H. Morino, T. Maruyama, and K. Iiyama, “Reduction of wavelength dependence of coupling characteristics using Si optical waveguide curved directional coupler,” J. Lightwave Technol. 32(12), 2188–2192 (2014). [CrossRef]
25. C. R. Doerr, M. Cappuzzo, E. Chen, A. Wong-Foy, L. Gomez, A. Griffin, and L. Buhl, “Bending of a planar Lightwave circuit 2×2 coupler to desensitize it to wavelength, polarization, and fabrication changes,” IEEE Photonics Technol. Lett. 17(6), 1211–1213 (2005). [CrossRef]
26. Y. Ito, M. Mendez-Astudillo, and T. Kita, “Wavelength Tunable filter with Curved Directional Coupler,” Proceedings of 24th Microoptics Conference (MOC2019), P-5, 106–107 (2019).
27. H. A. Haus and W. Huang, “Coupled-mode theory,” Proc. IEEE 79(10), 1505–1518 (1991). [CrossRef]
28. A. Yariv, “Coupled-mode theory for guided-wave optics,” IEEE J. Quantum Electron. 9(9), 919–933 (1973). [CrossRef]
29. A. Takagi, K. Jinguji, and M. Kawachi, “Wavelength characteristics of (2*2) optical channel-type directional couplers with symmetric or nonsymmetric coupling structures,” J. Lightwave Technol. 10(6), 735–746 (1992). [CrossRef]
30. H. Yanagawa, S. Nakamura, I. Ohyama, and K. Ueki, “Broad-band high-silica optical waveguide star coupler with asymmetric directional couplers,” J. Lightwave Technol. 8(9), 1292–1297 (1990). [CrossRef]
