In recent years, along with the rapid proliferation of the Internet, there has been a demand for an increase in capacity of a communication traffic. In response to this demand, an increase in transmission speed of a single channel per system and an expansion of the number of channels in accordance with the wavelength division multiplexing (WDM) are progressed. In the WDM, plural optical signals assigned to different carrier wavelengths (channels) can be transmitted simultaneously and a communication capacity can be increased according to the number of channels. Further, in the WDM, the respective channel wavelengths are sufficiently separated from one another. For example, the communication capacity reaches 1 terabit/sec by transmitting 100 channels using one common optical fiber with the modulation of 10 gigabit/sec per channel.
In the middle and long-distance optical communications in these years, a C band (1530 to 1570 nm) that can be amplified by an optical fiber amplifier (EDFA: erbium-doped fiber amplifier) is used widely. In the WDM, for example, with respect to 100 channels used in optical communications, 100 types of laser devices corresponding to the respective channels are required. Therefore, costs for inventory control, inventory count, and the like of the laser devices are increased. In order to solve this problem, in the middle and long-distance communications, there is a demand for a wavelength tunable laser device that can cover the entire C band singly.
An example of the light source of the wavelength tunable laser device includes a light source including a semiconductor optical amplifier (SOA), a wavelength tunable filter, and a reflector (external resonator type wavelength tunable laser or the like). The wavelength tunable filter controls the oscillation wavelength of the laser by changing the refractive index of a filter portion (the refractive index of a material of a filter portion). Examples of the publicly known document in which a wavelength tunable laser device provided with such a light source is described include Patent Documents 1 to 4.