L-band directly modulated laser for 10G PONs
Among a variety of techniques for broadband access, the infrastructure based on Passive Optical Network (PON) is one of the most viable solutions. In 10- and 40-Gigabit capable PON, the downstream wavelength in optical line terminals (OLTs) has been set in L-band. The electroabsorption modulated laser (EML) is a popular lasing source in such PONs due to its low chirp and high modulation speed. However, its complicated epi-growth technology of active layers poses a serious reliability issue and causes a substantial increase of the fabrication cost. The conventional DFB laser has advantages of compact structure, reliable operation, low cost and power consumption. It has been well recognized that the major obstacle of implementing the L-band DFB laser in such system is its relatively large parasitic frequency chirp when directly modulating the injection current. This chirp broadens the linewidth of the output spectrum, aggravating the dispersion power penalty seriously, especially when the operation wavelength is far away from the zero-dispersion wavelength for the standard single mode fiber (SMF). Gain detuning is an effective method to reduce the linewidth enhancement factor (LEF). Based on this idea, this paper aims at investigating the potential design to reduce the LEF of the L-band InAlGaAs DFB laser through the optimization of the strained layer multiple quantum well active region, including the layer thickness, strain and band-gap energy, etc. The band structure is obtained by solving the Schrodinger's equation where the valence band structure is described by 6 × 6 Luttinger-Kohn Hamiltonian with spin-orbit coupling taken into consideration. The optical material gain of a quantum well derived from Fermi's golden rule is applied. The differential gain and the refractive index change are then calculated, followed by the LEF. The calculated change of LEF with the lasing wavelength detuning is shown in Fig. 1, in which each dot presents a design achieved by a certain combination of active region parameters when they are scanned within their respective reasonable range of variation. The laser chip with the optimized design is then fabricated and tested in a system shown in Fig. 2. The lasing wavelength is approximately 1589 nm. After the standard uncooled transistor-outlet (TO) packaging, such device is directly modulated under 10 Gbps, and the output optical signal is transmitted over the standard SMF for 20 km. The eye-diagram after the transmission is shown in Fig. 3, which suggests that the optimized design could be a promising candidate in PON system where the L-band lasing source is needed.