The Design Of L-Band High Speed Low Chirp Directly Modulated Lasers

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). This problem seriously hinders DFB directly modulated lasers' applications in downstream of 10G/40 G PONs.This paper aims at finding an effective way to reduce the parasitic frequency chirp for DFB directly modulated lasers through the optimization of the strained layer multiple quantum well active region and further investigating the possibility of its application in next generation PON.The contents and major achievements of this paper include:(1) Have established the material gain simulation model for SL-MQW active region in semiconductor lasers. The validity of the numerical model has been proved through comparing the published results in existing literatures.(2) Have studied the methods to reduce the parasitic chirp of the DML and the gain detuning method is applied to optimize the active region of In Al Ga As SL-MQW. More concretely, three major parameters, i.e. the band gap energy, the strain and the well width, are examined and optimized to achieve a low chirp design of the DML.(3) The L-band SL-MQW DML chips were fabricated according to the design and optimization results. We firstly tested their output power and single mode performance including L-I curve and the SMSR, which are shown to meet the design targets. After the standard uncooled transistor-outlet(TO) packaging, such devices were directly modulated under the bit rate of 10 Gbps, and the output optical signal was transmitted over the standard SMF for 20 km. The measured results show that the receiver bit-error-rate is better than the requirement, while the power penalty is not. The experimental results are finally analyzed and discussed.(4) Accelerated aging test has been conducted and is ongoing for the purpose of reliability study.