Tunable Laser Array Features Micromirror
Santur Corp. of Fremont, Calif., has developed a tunable laser module in which an external microelectromechanical tilt mirror, rather than a relatively lossy combiner, selects the distributed feedback. The mirror enables control of optical coupling through an electronic feedback loop and offers a passively aligned package, potentially reducing costs. Furthermore, because no semiconductor optical amplifier is needed to make up for the combiner, it allows production of a simpler chip.
A novel tunable diode laser system (top) uses a microelectromechanical mirror to determine distributed feedback. The 20-mW laser array covers the C-band from 1530.7 to 1563.9 nm (bottom).
Tunable lasers are useful in optical networking, both for reconfiguring networks and systems and for reducing inventory costs. Tuning usually has been accomplished with an array of different-wavelength lasers integrated with a passive combiner on an InP chip. When only one wavelength is selected at a time and temperature is used for fine tuning, the distributed feedback reliability and spectral characteristics that are associated with this method are comparable to those associated with fixed-wavelength sources. However, the use of an on-chip combiner leads to a trade-off between power and tuning range.
The new distributed feedback array chip, which Santur produces in-house, integrates 12 lasers with a wavelength spacing of 2.8 nm. The lasers are set at a 10-µm pitch, and the tilt mirror, which is positioned at the focal plane of a collimating lens, corrects for the offset of the beams. Because the lasers are closely spaced, the mirror requires a deflection of only ±1.5°, but it was designed to tilt ±2° in-plane and ±0.5° in the other axis to provide coarse adjustment of the components. Overall, the module offers 20 mW of power at C-band ITU channels 17 to 58.5 (1530.7- to 1563.9-nm wavelengths).
Other companies have developed solutions to the same problem, including external-cavity lasers assembled around gain chips and monolithic sampled grating distributed Bragg reflector lasers. Both of these display optical properties that are quite different from conventional distributed feedback lasers. In contrast, said Santur researcher Bardia Pezeshki, the new laser looks and feels like a distributed feedback laser and therefore is easier to use.
Sampled grating distributed Bragg reflector lasers offer a very fast tuning time, however. The new laser has a typical tuning time of two seconds and, as a result, is not as applicable to reconfigurable networks, which require lasers that can change channels very quickly.
The firm currently offers lasers that provide 10 or 20 mW of power over the 36-nm C-band for use in optical networking systems.
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