Santur's tunable lasers make more of MEMS
The device is based on an array of DFB lasers on a indium-phosphide chip. Previously, such arrays required a passive combiner to select the output wavelength. Unfortunately, these combiners exhibit a large loss, and this loss increases with the number of lasers. As a result, it is impossible to combine enough lasers to cover the entire C-band at a high enough power.
Integrating 12 lasers in this way, for example, results in a maximum transmission of about 8%. "Typically the transmission is much lower than that because of imperfections in the combiner, absorption loss, and coupling losses," explained Bardia Pezeshki, Santur s vice-president of engineering. "The chip is also much larger and the fabrication is more complicated."
To overcome these problems, Pezeshki and co-workers designed a simple, low-loss coupling scheme. A collimating lens collects light from the DFB array chip and a microelectromechanical systems (MEMS) tilt mirror placed in the focal plane of the lens selects the appropriate laser. The mirror, which is gold-coated for maximum reflectivity at 1550 nm, outputs almost the full power of the chip.
Once the laser is selected, the wavelength is fine-tuned via temperature control; varying the temperature between 12 and 40 ºC corresponds to a 3 nm shift. "Our laser is no more complex than a non-tunable DFB laser, since the same temperature-controlling elements are used," Pezeshki explained. "The electronics adjusts the temperature automatically to tune to a few channels per DFB."
Santur recently unveiled its first commercial product based on this technology. The TL1010-C source contains 12 DFB lasers with a wavelength spacing of 3 nm, providing 10 mW of tunability across the C-band. The 51 x 95 x 13 mm device contains an integrated wavelength locker to optimize the fiber-coupled power and ensure that there is wavelength stability.
"Our module compares in size to other tunable lasers, and the heart of the product is in a butterfly package just like a single wavelength DFB laser," said Pezeshki. "Soon we will introduce 20 mW and L-band products, and later in the year we will have smaller, directly modulated sources for metro systems."
Author
Tami Freeman is technology editor on FibreSystems Europe magazine.
