Single-Mode Tunable Laser Backs up Optical Networks

Kathleen G. Tatterson

KISTA, Sweden -- Researchers have designed a semiconductor laser tunable in the erbium range that could replace more than 40 conventional fixed-wavelength lasers in dense wavelength division multiplex (WDM) systems.
The new laser, a grating coupler sampled reflector from Altitun AB, will allow dense WDM equipment to back up multiple fixed-wavelength lasers with a single tunable laser, thereby offering high-channel-count systems that are cost-
effective and reliable.
"This solves the problem of how to ensure network reliability without backing up every fixed-wavelength laser by a spare laser of precisely the same wavelength," said Rob Plastow, director of marketing and sales.

Matches Er-doped bandwidth

The semiconductor laser boasts tunability between 1525 and 1565 nm -- matching the gain bandwidth of commercially available erbium-doped fibers -- compared with the approximately 0.1-nm tuning range for fixed-wavelength lasers and 3- to 5-nm range for existing tunable lasers.
The design of the 2-mm-long laser is based on a three-section distributed Bragg
reflector in that it comprises an InGaAsP gain section to generate optical power, a phase section for fine tuning and a Bragg grating section that reflects one wavelength. Altitun redesigned the grating to reflect a combination of 10 or more wavelengths, thus increasing the operating range by an order of magnitude.
Then, developers added a vertical coupler filter between the gain and the rest of the laser, which can tune more than 100 nm. The coupler selects one wavelength from the combination of reflected wavelengths. The device works like a distributed Bragg reflector, except that once it reaches the end of the tuning range, the vertical coupler can select a new peak several nanometers away, repeating the process. "The end result is like having 10 or more distributed Bragg reflector lasers in a single chip, all overlapping in tuning range," Plastow said.
Altitun has packaged the single-chip, integrated design in an industry standard telecommunications butterfly package and expects to launch it as a product at the end of February. The cost of the system, with polarization-maintaining fiber, electronic control board and software, is $18,500 in small volume.