Applied Science and Innovative Research ISSN 2474-4972 (Print) ISSN 2474-4980 (Online) Vol. 5, No. 1, 2021 www.scholink.org/ojs/index.php/asir 56 Original Paper High-reliability, High-performance 25 Gb/s Directly Modulated Uncooled Lasers for 5G Wireless Communications Jack Jia-Sheng Huang1*, S.C. Huang1, NiYeh Wu1, Deo Yu1, C.K. Wang1, Ansel Chen1, David Klotzkin1, Yu-Heng Jan1, ChunKo Chen1, H.S. Chen1 & Emin Chou1 1 Source Photonics, No.46, Park Avenue 2nd Rd., Science Park, Hsinchu, Taiwan * Jack Jia-Sheng Huang, E-mail: jshuang6@yahoo.com Received: December 30, 2020 Accepted: January 15, 2021 Online Published: February 19, 2021 doi:10.22158/asir.v5n1p56 URL: http://doi.org/10.22158/asir.v5n1p56 Abstract Semiconductor laser diodes are important components for fifth-generation wireless technologies. To meet 5G wireless specifications, ever increasing performance and reliability requirements of each component become necessary to guarantee uptime air service. In this paper, we present highly reliable 25G DFB uncooled lasers that exhibit low threshold current, high single-mode, high bandwidth, and excellent eye pattern for uncooled operations of -40 to 85C. Ultra-high component reliability is demonstrated to ensure stable operations for 5G mobile communications. Keywords Reliability, directly modulated lasers, DML, 25G DFB, 5G wireless, mobile network, safety, national security, cellular tower 1. Introduction One of the key issues in the United States-China trade war involves technologies (Ellyatt, 2019; Reardon, 2019; Harrell, 2019). Among various technology segments, wireless communication is perhaps the area that draws most attentions and negotiations. The U.S. and China are locked in a race to dominate the next wave of wireless communications, namely fifth-generation, 5G (Lee, 2020; Soon, 2019; Tcheyan & Bresnick, 2020). The 5G wireless networks promise to empower mobile phones, self-driving vehicles, artificial intelligence (AI), internet of things (IoT), military, and space applications (Huang, Jan, Yu, Chang, Chang, Shiu, Ren, Wang, & Chou, 2018; Nordrum, 2017; https://en.wikipedia.org/wiki/5G). www.scholink.org/ojs/index.php/asir Applied Science and Innovative Research Vol. 5, No. 1, 2021 57 Published by SCHOLINK INC. Figure 1. 5G Wireless Applications Including 5G Mobile Phones, Autonomous Vehicles, IoT, Robots, Drones, Satellites, Aerospace, etc. Unlike 4G and other previous generations, 5G wireless is the network that communicates with all elements. The 5G mobile network requires ultra-high reliability to guarantee uptime air service for several reasons (shown in Table 1). The first is principally for safety (Nelson, 2017; Cohen, 2019). For example, one cannot have a self-driving car that doesn’t transmit data properly. Physicians, especially in remote areas, need to access healthcare data timely and accurately to provide the best possible medical treatments to their patients. The second regards national security (Hoehn & Sayler, 2020; Vincent, 2020; Barnett, 2020; Overview of risks introduced by 5G adoption in the United States, 2019). At the present, U.S. forces extensively use mid-band (1-6 GHz) and low band (<1 GHz) frequencies. Careful evaluation and caution need to be exercised before finalizing the plan to sharing some of the spectrum with commercial usage. The other national security concerns include the use of foreign equipment in the deployment of 5G wide-area network, cyberattacks, and perpetration. The third is concerning repairment cost and tower climber’s well-being (Keith, 2019; Rambo, 2019). For example, fronthaul wireless network involves connection between cellular towers, macro cells, metro cells, and base stations. High reliability at the component level for each wireless tower is required to guarantee uptime performance, avoid time consuming repair, and minimize tower climbing. The failure rate requirement for 5G mobile is typically <0.1% versus 1-2% for CATV or telecommunication applications. www.scholink.org/ojs/index.php/asir Applied Science and Innovative Research Vol. 5, No. 1, 2021 58 Published by SCHOLINK INC. Table 1. Reasons for Ultra-high Reliability for 5G Wireless Applications Aspects of high reliability 5G applications Reasons for high reliability Safety - Self-driving car - Healthcare - Autonomous cars need to transmit data properly all time to maintain safe street driving - Physicians need to access patients’ data timely and accurately National security - Military - Space - Command and control (C2) need 5G signals to reduce latency from satellites - Spacecraft needs highly accurate data for launch, space travel, and landing Repair - Fronthaul wireless - Wireless cellular towers need high reliability to minimize repair cost and tower climbing Reliability is ultimately the principal denominator to determine the success of the 5G wireless components. Due to the high reliability requirement in 5G, component suppliers need to guarantee the chip reliability to the ever increasingly stringent level. One of the most challenging reliability issues is to assure continuous uptime operation for source lasers. This is because the laser diode needs to meet both high-speed (25 Gb/s) and high reliability where engineering tradeoff may occur. For example, high-speed laser typically requires a design of short cavity that would lead to high current density and accelerated degradation rate. In this paper, we have demonstrated highly reliable 25 Gb/s directly modulated lasers for 5G mobile applications. We show low drive current, high modulation bandwidth, and clear eye opening for the modulation of 25 Gb/s. Long-term reliability is established to meet the ultra-high reliability requirement for 5G. 2. Experimental First, the n-type indium phosphide (InP) epitaxial layer was grown on S-doped n-type InP substrate as the buffer using metal organic chemical vapor deposition (MOCVD). The active region of InGaAlAs multi-quantum well (MQW) was grown to achieve high optical confinement for ridge waveguide (RWG) structure, as shown in Figure 2(a). The grating layer of InGaAsP was then grown and patterned by holographic technique to form distributed feedback (DFB) laser (Huang & Jan, 2017). To improve the single-mode DFB performance, non-uniform grating involving mixed pitch was employed as illustrated in Figure 2(b). The p-InP cladding was overgrown directly on top of the grating layer. Finally, the contact layer was formed with the heavily-doped p+-InGaAs to make Ohmic contact with Ti/Pt/Au p-metal. www.scholink.org/ojs/index.php/asir Applied Science and Innovative Research Vol. 5, No. 1, 2021 59 Published by SCHOLINK INC. (a) (b) Figure 2. (a) Schematics of 25 Gb/s DFB Laser Showing the Ridge Waveguide Structure and (b) Cross-Sectional View of the Laser Structure For 25 Gb/s DFB laser, the wafer was cleaved into 160 m bars and coated with anti-reflection (AR) and high-reflection (HR) films. The mirror coating reflectivities of AR and HR were around <1% and 80%, respectively. The laser chip was mounted on transistor outline (TO) header. Gold wire was bonded to connect the p-metal contact of the laser chip to the lead of the TO header. Finally, the laser TO was assembled to high-speed optical module for 25 Gb/s modulation eye diagram test. The burn-in and reliability tests were conducted on TO samples. The laser was screened by burn-in based on the condition of 110C, 85 mA for 24 hours. The long-term reliability aging was tested at 90C with a constant stress current of 65 mA. The failure criterion for aging test was defined as 50% increase in the initial threshold current. 3. Results and Discussions Figure 3 shows the light versus current (LI) curves at various temperatures. The 25 Gb/s DFB laser showed very low threshold current (Ith) of 9.5 mA at 25C. The threshold current remained low at high temperatures, about 15.0 and 18.0 mA at 70 and 85C, respectively. At low temperature, the Ith was about 7.5 mA at -40C. Figure 3. LI Curves of the 25 Gb/s DFB Laser Showing Over-temperature Characteristics in the Range of -40 to 85C www.scholink.org/ojs/index.php/asir Applied Science and Innovative Research Vol. 5, No. 1, 2021 60 Published by SCHOLINK INC. Figure 4 shows the optical spectra of the 25 Gb/s DFB laser measured at -40, 25 and 85C. The laser showed excellent single-mode performance with side-mode-suppression-ratio (SMSR) of about 45 dB. The high SMSR was attributed to the non-uniform grating associated with the mixed-pitch design. Figure 4. Optical Spectra of the 25 Gb/s DFB Laser Showing Single-mode Bragg Wavelength (a) -40C, (b) 25C and (c) 85C Figure 5 shows the small-signal modulation bandwidth of the 25 Gb/s DFB laser measured at various bias current ranging from 20 to 60 mA. At the bias current of 60 mA, the 3dB bandwidth reached 27.5 and 17.3 GHz at 25 and 85C, respectively. Figure 6 shows the relaxation oscillation frequency (fr) against the square root of modulation current at 25 and 85C. High resonance frequency slopes for uncooled operations have been achieved. The slopes of fr were estimated to be 3.0 and 1.9 GHz/mA1/2 for 25 and 85C, respectively. The values of the slopes were comparable to those reported in the InGaAlAs buried heterostructure (Takada, Matsuda, Okumura, Ekawa, & Yamamoto, 2006) and ridge waveguide lasers (Paoletti et al., 2009; Nakahar, Wakayama, Kitatani, Fukamachi, Sakuma, & Tanaka, 2014). (a) (b) Figure 5. Small Signal Modulation Bandwidth of the 25 Gb/s DFB Laser at Various Bias Current in the Range of 20-60 mA at (a) 25C and (b) 85C www.scholink.org/ojs/index.php/asir Applied Science and Innovative Research Vol. 5, No. 1, 2021 61 Published by SCHOLINK INC. Figure 6. Relaxation Oscillation Frequency Versus Bias Current of the 25 Gb/s DFB Laser at 25 and 85C. Interpolated Slopes Show 3.0 and 1.9 GHz/mA 1/2 for 25 and 85C, Respectively Figure 7(a)-(c) shows the eye diagrams of 25 Gb/s modulation measured at -40, 25 and 85C. The bias current and modulation current were set at 35.2 and 22.4 mA, respectively. Figure 8 shows the eye mask margin over the wide temperature range of -40 to 85C. The 25 Gb/s DFB laser achieved high mask margin (>35%) for the extinction ratio of 4.5 dB, substantially higher than the eye-opening requirement of 15%. Figure 7. Eye Diagram at 25Gb/s at -40, 25 and 85C Figure 8. Eye Mask Margin at 25 Gb/s at -40, 25 and 85C. The Lasers Show Excellent Mask Margin for Uncooled Operations from -40 to 85C www.scholink.org/ojs/index.php/asir Applied Science and Innovative Research Vol. 5, No. 1, 2021 62 Published by SCHOLINK INC. Figure 9 shows the aging plot of the 25G DFB laser that was subjected to the stress condition of 90C, 65 mA. The relative change of threshold current was monitored during aging test, and no failure occurred after 3300 hours. Based on the small change in threshold current, robust long-term reliability of the 25G DFB lasers has been demonstrated to meet 5G wireless requirement. We attribute the high reliability to the optimized design in the quantum well and optical coating. With the optimization of design and process, the common issues of InGaAlAs lasers such as sudden failure or catastrophic optical damage (COD) have been successfully eliminated (Jiménez, 2003; Hempel et al., 2013; Fukuda, Okayasu, Temmyo, & Nakand, 1994; Tomm, Ziegler, & Elsaesser,, 2011; Huang, 2014, 2005; Hausler, Zeimer, Sumpf, Erbert, & Trankle, 2008; Chuang, Nakayama, Ishibashi, Taniguchi, & Nakano, 1998; Huang, Nguyen, Hsin, Aeby, Ceballo, & Krogen, 2005; Ott, 1997). Figure 9. Aging Plot of 25 Gb/s DFB Laser Based on the Stress Condition of 90C, 65 mA 4. Conclusion We have demonstrated stable single-mode performance of 25 Gb/s DML for uncooled operations (-40 to 85C). The 25 Gb/s DFB lasers show low drive current and excellent single-mode performance. The lasers also show high bandwidth with 3dB readpoint of 17.3 GHz at 85C, 60 mA. At 25 Gb/s modulation, the eye opening was excellent for uncooled conditions (-40 to 85C), with good mask margin (>35%) over the 15% requirement. With optimized design in the quantum well and optical coating, we also achieve robust reliability performance with the aging data over 3300 hours. Acknowledgment The authors would like to thank Shannon Huang (UCLA, Los Angeles, CA, USA) for proofreading. References Ellyatt, H. (2019). China US trade talk is creating a new tech landscape. https://www.cnbc.com/2019/07/02/china-us-trade-war-is-creating-a-new-tech-landscape.html https://ieeexplore.ieee.org/author/37268873200 www.scholink.org/ojs/index.php/asir Applied Science and Innovative Research Vol. 5, No. 1, 2021 63 Published by SCHOLINK INC. Chuang, S. L., Nakayama, N., Ishibashi, A., Taniguchi, S., & Nakano, K. (1998). Degradation of II-VI blue-green semiconductor lasers. IEEE J. Quantum Electron., 34(5), 851-857. https://doi.org/10.1109/3.668773 Cohen, J. K. (2019). Why 5G matters for healthcare. Modern Healthcare. Retrieved from https://www.modernhealthcare.com/technology/why-5g-matters-healthcare Nordrum, A. (2017). Everything you need to know about 5G. Retrieved from https://slt.co/Downloads/News/1102/5G%20-%20IEEE%20with%20Health%20comments.pdf Fukuda, M., Okayasu, M., Temmyo, J., & Nakand, J. (1994). Degradation behavior of 0.98m strained quantum well InGaAs/AlGaAs lasers under high-power operation. IEEE J. Quantum Electronics, 30(2), 471-476. https://doi.org/10.1109/3.283796 Hausler, K., Zeimer, U., Sumpf, B., Erbert, G., & Trankle, G. (2008). Degradation model analysis of laser diodes. J. Mater. Sci.: Mater. Electron., 19, 160-164. https://doi.org/10.1007/s10854-007-9534-8 Hempel, M., Tomm, J. W., Mattina, F. L., Ratschinski, I., Schade, M., Shorubalko, I., … Elsaesser, T. (2013). Microscopic Origins of Catastrophic Optical Damage in Diode Lasers. IEEE J. Selected Topics Quantum Electron., 19(4), 1500508. https://doi.org/10.1109/JSTQE.2012.2236303 Hoehn, J. R., & Sayler, K. M. (2020). National Security Implications of Fifth Generation (5G) Mobile Technologies. Congressional Research Service (June 4, 2020). Reardon, M. (2019). How 5G got tied up in a trade war between Trump and China. Retrieved from https://www.cnet.com/news/how-5g-got-tied-up-in-a-trade-war-between-trump-and-china/ https://www.en.wikipedia.org/wiki/5G Huang, J. S. (2005). Temperature and current dependences of reliability degradation of buried heterostructure semiconductor lasers. IEEE Trans. Device Mater. Reliab., 5(1), 150-154. https://doi.org/10.1109/TDMR.2005.843834 Huang, J. S. (2015). Reliability of Optoelectronics. Woodhead Publishing Series in Electronic and Optical Materials: Number 74 (Cambridge, United Kingdom, Edited by Jonathan Swinger), Chapter 6, pp. 83-114. https://doi.org/10.1016/B978-1-78242-221-1.00006-X Huang, J. S., & Jan, Y. H. (2017). Environmental engineering of photonic and electronic reliabilities: From technology and energy efficiency perspectives. Edited Marco Braga (Scholars’ Press, Saarbrücken, Germany). Huang, J. S., Jan, Y. H., Yu, D., Chang, R., Chang, J., Shiu, G., Ren, D., Wang, K., & Chou, E. (2018). Manufacturing excellence and future challenges of wireless laser components for 4G/5G optical mobile fronthaul networks. IEEE 27th Wireless Optical Communications Conference (WOCC, Taiwan) (pp. 166-167). https://doi.org/10.1109/WOCC.2018.8372709 Huang, J. S., Nguyen, T., Hsin, W., Aeby, I., Ceballo, R., & Krogen, J. (2005). Reliability of etched mesa buried heterostructure semiconductor lasers. IEEE Transactions Device and Materials Reliability, 5(4), 665-674. https://doi.org/10.1109/TDMR.2005.860562 https://www.modernhealthcare.com/technology/why-5g-matters-healthcare https://ieeexplore.ieee.org/author/37268873200 https://ieeexplore.ieee.org/author/37268873200 https://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=3 https://ieeexplore.ieee.org/xpl/tocresult.jsp?isnumber=7019 www.scholink.org/ojs/index.php/asir Applied Science and Innovative Research Vol. 5, No. 1, 2021 64 Published by SCHOLINK INC. Jiménez, J. (2003). Laser diode reliability: Crystal defects and degradation modes. C. R. Physique, 4, 663-673. https://doi.org/10.1016/S1631-0705(03)00097-5 Keith, R. (2019). 5G Network Reliability Explained. Retrieved from https://www.a10networks.com/blog/5g-network-reliability-explained/ Lee, N. T. (2020). Navigating the US-China 5G competition. Retrieved from https://www.brookings.edu/research/navigating-the-us-china-5g-competition/ Nakahar, K., Wakayama, Y., Kitatani, T., Fukamachi, T., Sakuma, Y., & Tanaka, S. (2014). 1.3 m InGaAlAs asymmetric corrugation-pitch-modulated DFB lasers with high mask margin at 28 Gbit/s. Electron. Lett., 50(13), 947-948. https://doi.org/10.1049/el.2014.0797 Vincent, B. (2020). National strategy to secure 5G. White House. Nelson, P. (2017). Reliability, not principally speed, will drive 5G. Retrieved from https://networkworld.com/article/3157041/reliability-not-principally-speed-will-drive-5g.html Ott, M. (1997). Capabilities and reliability of LEDs and laser diodes. NASA memo. Overview of risks introduced by 5G adoption in the United States. Cybersecurity and Infrastructure Security Agency (2019, July 31). Paoletti, R., Agresti, M., Bertone, D., Bruschi, C., Codato, s., Coriasso, C., … Meliga, M. (2009). Uncooled 20 Gb/s direct modulation of high yield, highly reliable 1300 nm InGaAlAs ridge DFB lasers. Optical Fiber Conf. (OFC, San Diego, CA). Barnett, J. (2020). Pentagon’s plan to help the nation stay ahead on 5G includes aggressive support for U.S. telecommunications. Retrieved from https://www.militaryaerospace.com/communications/ article/14177558/5g-military-telecommunications Rambo, S. (2019). The Growing Challenges Of 5G Reliability. Semiconductor Engineering. Soon, S. (2019). Here is how the US can beat China in in the race for dominance in next generation networks. Retrieved from https://www.cnbc.com/2019/11/26/5g-race-how-the-us-can-beat-china- in-the-competition-for-dominance.html Takada, K., Matsuda, M., Okumura, S., Ekawa, M., & Yamamoto, T. (2006). Low-drive-current 10-Gb/s operation of AlGaInAs buried-heterostructure /4-shifted DFB lasers. European Conf. Optical Comm. (ECOC, Cannes, France). Tcheyan, L., & Bresnick, S. (2020). How the US can compete in 5G. Retrieved from https://www.thediplomat.com/2020/03/how-the-us-can-compete-in-5g/ 1/8 Tomm, J. W., Ziegler, M., & Elsaesser, M. H. T. (2011). Mechanisms and fast kinetics of the catastrophic optical damage (COD) in GaAs‐based diode lasers. Laser & Photonics Review, 5(3), 422-441. https://doi.org/10.1002/lpor.201000023 Harrell, P. E. (2019). The U.S.-Chinese Trade War Just Entered Phase 2. Retrieved from https://www.foreignpolicy.com/2019/12/27/united-states-china-trade-war-fought-export-import-co ntrols-investment-restrictions-sanctions/ https://www.a10networks.com/blog/5g-network-reliability-explained/ https://networkworld.com/article/3157041/reliability-not-principally-speed-will-drive-5g.html https://www.militaryaerospace.com/communications/ https://www.cnbc.com/2019/11/26/5g-race-how-the-us-can-beat-china-