New DWDM networks will require fast tunable lasers to allow rapid reconfiguration. While such lasers are available, manufacturers are still struggling with methods of making sure that enough power is available for every channel while minimizing additional cost and size to the devices. Researchers at Nortel Networks (Ottawa, Canada) recently reported a tunable laser integrated with a wavelength locker and amplifier that can deliver 40 mW of fiber-coupled power on 34 ITU channels.1
The researchers at Nortel made a continuously tunable laser, monolithically integrated with a semiconductor optical amplifier (SOA). The chip and wavelength locker optics are packaged together in a hermetically sealed fiber-pigtailed 26-pin butterfly package, with a footprint of 12.7 x 30 mm.
The laser has three inline gain-coupled DFB sections, each with separate contacts. They are designed to be capable of producing wavelengths in steps of 4.8 nm. A particular wavelength on the ITU grid is provided by choosing the nearest wavelength available, then temperature-tuning the chip at temperatures ranging from -5°C to +50°C.
The SOA is a 1.5-mm-long ridge waveguided angled section near the output end of the chip. The SOA does far more than just provide higher powers: it also can be used to adjust the output power to compensate for aging, to equalize channel amplitudes, and to attenuate during network insertions or reconfigurations. With low current or voltage biases, the SOA can attenuate the signal by more than 35 dB, which is desirable during tuning or transmitter installation. The SOA works over the full range of temperatures. When the SOA is biased with a current of 250 to 500 mA, the chip emits 40 mW of fiber-coupled power.
The linewidth of the channels was less than 10 MHz on all 34 channels. For a 35 mA DFB section bias, and less than 5 MHz at a 90 mA bias.
The gain-coupled grating structure is quite easy to manufacture. The researchers report that it provides a high yield of lasers that oscillate in a stable way in a single longitudinal mode. Also, the chip fabrication requires only two MOCVD growth steps, enabling volume production with high chip reliability.
The researchers also tested the device for reliability. The DFB and SOA sections were aged separately. Assuming a heat-sink temperature of 50°C and output power of more than 40 mW, the researchers estimate a module would last for more than 24 years.
- D. M. Adams et. al., Elect. Lett 37(11), 691 (MAY 24, 2001).