Technical Insight
Nortel integrates SOA with gain-coupled tunable DFB laser (Research Review)
Tunable lasers are crucial to the development of adaptable optical networks. DWDM systems will need to utilize tunable sources in order to be rapidly reconfigurable, and to offer flexibility for channel assignment, and cost savings through reduced component inventories. Researchers at Nortel Networks have reported a three-section, gain-coupled distributed feedback (GC-DFB) laser, monolithically integrated with a semiconductor optical amplifier (SOA), with over 40 mW of frequency-locked fiber-coupled power for 34 consecutive International Telecommunications Union (ITU) channels (Elec. Lett. 2001 37(11) 691). The SOA not only provides high output power, but can also adjust amplitude to compensate for laser aging, equalize channel amplitudes, and attenuate the signal during network insertions or reconfigurations. The tunable laser consists of a 1.5 mm-long ridge waveguide, angled SOA output section, and three separately addressable GC-DFB lasers (). Fabrication of the monolithic chip requires only two MOCVD growth steps and so lends itself to high-volume production. The active chip and wavelength-locker optics are co-packaged within a fiber-pigtailed, 26 pin butterfly package measuring 12.7 30 mm. The grating pitches in the DFB sections are chosen to produce emission at increasing wavelength in steps of 4.8 nm. A particular ITU frequency is accessed by selecting the DFB section having an emission wavelength nearest the ITU frequency of interest. Fine-tuning onto that frequency is done by adjusting the chip heatsink temperature. When operating DFB sections two or three, the preceding DFB section or sections are biased to transparency. Heatsink temperature is variable from 5 to +50C, meaning each DFB section can access 5 nm continuously. The total tuning range is therefore 3 5 nm giving 15 nm, minus the two 0.2 nm spans over which adjacent DFB sections overlap. Output power is adjusted by selecting an SOA bias current. At low bias current or voltage, the SOA can heavily attenuate. 40 mW of fiber-coupled power is achieved with an SOA bias of 250500 mA. The superposition of power against wavelength at 40 mW of fiber-coupled power is shown for all 34 ITU channels in . Side mode suppression ratio (SMSR) is below 37 dB on all channels, and linewidth is below 10 MHz for a 75 mA DFB section bias, decreasing to 5 mA for 90 mA bias. The relative intensity noise spectrum collected at 40 mW for all channels was better than 118 and 141 dB/Hz at frequencies below and above 200 MHz, respectively. DFB and SOA sections were aged separately. SOA reliability was tested by fabricating FabryPerot lasers from the SOA material. A burn in period gave less than 4% reduction in efficiency. A further 2200 hours of aging at 100C with a current density equivalent to an SOA operating at 500 mA, gave no further changes. The DFB-laser sections were aged for 2200 hours at 100C at a bias of 150 mA. The estimated chip lifetime for a module power of 40 mW with SMSR of > 34 dB is over 24 years when operating at a frequency corresponding to a heatsink temperature of 50C.
