High-reliability, High-performance 25 Gb/s Directly Modulated Uncooled Lasers for 5G Wireless Communications

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.

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).
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  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.

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.

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.    The high SMSR was attributed to the non-uniform grating associated with the mixed-pitch design.     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 (Jimé nez, 2003;Hempel et al., 2013;Fukuda, Okayasu, Temmyo, & Nakand, 1994;Tomm, Ziegler, & Elsaesser,, 2011;Huang, 2014Huang, , 2005Hausler, Zeimer, Sumpf, Erbert, & Trankle, 2008;Chuang, Nakayama, Ishibashi, Taniguchi, & Nakano, 1998;Huang, Nguyen, Hsin, Aeby, Ceballo, & Krogen, 2005;Ott, 1997).

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.