By Zhensheng (Steve) Jia, Ph.D., L. Alberto Campos, Ph.D., Mu Xu, Ph.D, Haipeng Zhang, Ph.D., Junwen Zhang, Ph.D., Chris Stengrim & Curtis Knittle, Ph.D., CableLabs
The market for coherent optical links to reach between 10 km and 120 km is emerging in many application scenarios, such as router-to-router and point-to-point data center interconnect, mobile xhaul and cable aggregation applications. These market opportunities have catalyzed huge investment and development of new power and footprint optimized pluggable products in optical industry. In the cable environment, access networks have been undergoing significant technology and architecture changes driven by the ever-increasing residential data service tiers and an increasing number of services types being supported, such as business services and cellular connectivity. Digital fiber technologies and distributed access architecture (DAA) for fiber deep strategies offer an infrastructure foundation for cable operators to deliver the best service quality to the end users in the years ahead.
CableLabs® has recognized the need of coherent optics in the access network and has been working on point-to-point 100G and 200G coherent optics specifications. On June 29th, 2018, CableLabs publicly unveiled for the first time two new specifications: P2P Coherent Optics Architecture Specification and P2P Coherent Optics Physical Layer v1.0 Specification. These two new specifications are the result of a focused effort by CableLabs, its members, and the manufacturer partners to develop Coherent Optics technology for the access network and to bring coherent optical technology to market quickly. On March 11, 2019, CableLabs announced another addition to its family of Point-to-Point Coherent Optics specifications: The P2P Coherent Optics Physical Layer v2.0 Specification. This new specification defines interoperable P2P coherent optics links running at 200 Gbps (200G) on a single wavelength.
Low-cost coherent transceiver design is of great interest in bringing coherent optics to access networks because the existing commercially available coherent opto-electronic subsystems are associated with a high degree of complexity and cost for long haul and metro applications. Among different components of the coherent transceiver, active continuous-wave (CW) laser is of critical importance performing both transmitter source and receiver local oscillator (LO) for such low-cost coherent systems. The laser used in current coherent system is an external cavity laser, or ECL, which generates a narrower line width for coherent system needs. The typical line width of ECLs is ~50-500 kHz level. It uses a reflector that creates the cavity outside of the gain chip and allows the cavity to be longer than if it was confined to the gain chip alone. However, it is high cost and complicated, which is not attractive in access applications.
Fabry-Perot (FP) lasers, or FP laser diodes (FP-LD), on the other hand, is simple and ultra-low cost.
Cable operators have deployed inexpensive FP lasers in the HFC upstream optical links for years.
Unfortunately, FP lasers are not applicable for coherent optics in its current use. In this paper, we propose low-cost injection-locked FP laser source for coherent access applications. Through injection locking, the child laser closely adopts the optical frequency and line width characteristics of the parent laser which can be shared between multiple child lasers. As a result, the cost of the coherent optical transceiver can be significantly reduced. In this paper the injection locking technique is investigated by applying the external parent laser to multiple child FP-LDs. The static and dynamic characteristics are studied in detail, which include injection locking and detuning condition, line width reduction feature, output power maximization, and optimized design for coherent system. In addition, this paper discusses applications of injection locked FP-LDs and optical frequency combs in coherent access system and presents an experimental comparison of transmission performance with and without injection locking.