Design of High Performance and Low-Cost Single Longitudinal Mode Laser Module for DWDM Application

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Over the past years, the internet core rapidly increases, transparent protocols become more available for DWDM system in the fiber to the home, fiber to the institutions service. In densely spaced wavelength-routed and large-capacity optical network systems, single-mode lasers with narrow linewidth and low temperature sensitivity are of particular importance as the light sources in transmitter modules. The efficient coupling of a laser diode to an optical fiber has been a problem of general concern since the advent of fiber-optic communication systems. This chapter will introduce to fabricate a fiber grating external cavity laser (FGECL) module with a low cost while still maintaining a good single longitudinal mode performance by using a low-cost AR(Anti-reflection)-coated(5×10-3) laser and a tapered hyperbolic-end fiber (THEF) microlens for strong coupling the fiber grating external cavity. Previously, high-performance FGECL modules have only been available by using a complicated AR-coated(1×10-5) laser process that leads to a high packaging cost. The FGECL consisted of a HR/AR-coated diode laser, an uncoated THEF microlens, and a fiber grating. The results showed that the FGECL module exhibits a side-mode suppression ratio (SMSR) higher than 44 dB, a higher output power of more than 2 mW, and a larger operation current range of over 50 mA. In addition, excellent wavelength stability and a low-penalty directly modulated 2.488 Gbit/s were also obtained for FGECL modules. The THEF microlens demonstrated up to 86% coupling efficiency for a laser with an aspect ratio of 1:1.5. A fiber microlens which provides an efficient coupling mechanism to match the spot size of the diode laser to the fiber is commonly used for optical alignment in FGECL modules. Low-cost FGECL modules with good performance were achieved primarily owing to the THEF microlens having a high coupling efficiency (typically 75%) which enhanced the feedback power from the fiber grating external cavity to the HR/AR-coated laser compared with the currently available hemispherical microlens which has low coupling efficiency (typically 50%). Therefore, the packaged FGECL module is suitable for use in low cost 2.5 and 10 Gbit/s lightwave transmission systems, such as gigabit passive optical networks (GPONs), metropolitan area networks (MANs), and fiber-to-the-home (FTTH) applications. This chapter is organized as follows: Part I: A tapered hyperbolic-shaped microlens has been improved for efficient the coupling of the output from a laser diode into an optical fiber. A tapered hemispherical-end