Over the last 10 years the adoption of tunable based solutions has grown significantly. Particularly in the recent years, tunable transceivers are becoming more and more widely used in DWDM transmission systems. The most significant difference between tunable transceivers and ordinary transceivers are the laser. As we know, the early DWDM systems used fixed-wavelength lasers as light sources, which meant that many kinds of optical transceivers were needed for the wavelength channels. However, the use of tunable lasers, which can operate at any channel wavelength, means that only one kind of transceiver is needed, so the inventory cost can be reduced. As the core of tunable transceiver modules, tunable laser plays an very important role in tunable transceivers. Today, we are going to learn and further understand the tunable laser.