Contents I. Introduction
With the demand for the increased channel number and narrower channel spacing in the dense wavelength-division-multiplexing (DWDM) transmission system, an internal wavelength locker to be integrated in a laser module is a good solution for the low cost, compact, and reliable system design. Schematic side view of the laser module. From the enhanced research results in widely tunable laser [1]– [3] and wide-band wavelength-selectable light source [4], multichannel capability over the wide-band wavelength range has been an inevitable requirement and the wavelength locker based on the etalon is the most attractive one among the various wavelength monitoring techniques by its capabilities of wide-band coverage, low-cost, and compactness. Recently, remarkable research activities have been reported on the precision wavelength monitoring and the locking capability over a large temperature range. The reported schemes are a second thermoelectric cooler (TEC) for etalon [3]–[5], firmware compensation for etalon with temperature sensing [6], and a temperature-insensitive etalon with a novel material [7]. As another considerable factor in the etalon-based technologies, a sensitive angle dependence of the etalon affects the manufacturing feasibility as well as the performance of the locker because the wavelength discrimination for the ITU grid is initially decided by the initial angle of the etalon and the divergence angle of the collimated laser beam in the module assembly. As a useful approach, an additional TEC for etalon [3], [5] has been introduced to make easy not only the packaging process but also the wavelength discrimination by a thermal adjustment of the etalon wavelength. This approach, however, may be improved by minimizing the cost, the space, and the power consumption brought in by using the second TEC.
