Tunable-laser technologies vs. optical-networking requirements

Figure 6. A representative output spectrum of a DRIE (deep reactive ion etching) MEMS (micro-electromechanical system) ECL (external-cavity laser) locked to the 100-GHz ITU wavelength grid with about 10-mW constant output power across a 13-nm tuning range and >50-dB side-mode suppression.

ECLs with continuous tuning have been traditionally used in optical test and measurement equipment since they provide high power, large tuning range, and narrow line widths with high stability and low noise. Furthermore, they provide continuous tuning through the entire spectrum of the gain medium, where other common laser technologies (like DBRs) exhibit mode hops between stable points in the spectrum. However, ECLs were generally too large, costly, and sensitive to shock and other environmental influences to be used in telecommunications components.

Recent technological advances, however, have brought ECLs to the forefront of optical-networking component technology. In particular, the application of MEMS to optical-component designs produces high-performance micro-optics that readily fit on standard transmitter cards and can be manufactured at competitive costs in the optical-networking industry.

One key breakthrough in the development of MEMS-based ECLs is the use of deep reactive ion etching (DRIE) techniques to fabricate the MEMS actuators. DRIE techniques allow the cost-effective and reliable production of rigid mechanical drive structures that provide suitable force for high-speed and high-precision movement of optical elements over large linear and angular deflections. Further more, a low-cost, precision servo-control system can provide real-time error compensation, making the MEMS actuators quite accurate and insensitive to effects from shock, vibration, temperature changes, or creep.

DRIE MEMS actuators, externally fabricated optics, and high-precision servo-control systems are ideal building blocks for the creation of new optical-networking components. MEMS actuators can be readily combined with externally fabricated optics (diffraction-limited lenses, high-reflectivity mirrors, wavelength-selective coatings) using precision servo-control systems to rapidly develop solutions for critical telecom-component requirements. For instance, these building blocks could be repackaged to produce tunable receivers, polarization controllers, optical monitors, variable attenuators, optical switches, and tunable filters.

The DRIE MEMS ECL performs very well. Figure 6 shows a representative output spectrum of a MEMS ECL locked to the 100-GHz ITU wavelength grid with about 10-mW constant output power across a 13-nm tuning range. This design is capable of providing high-power output (products soon will be available at 20 mW) and can continuously tune across a 40-nm tuning range. Furthermore, the device exhibits narrow line width, low RIN, and excellent side-mode suppression-all while meeting the market requirements for small-module footprint.

In general, tunable ECLs provide many key advantages for DWDM systems and perform quite well against the technical requirements for optical-networking components.