1. Field
The following description relates to wired/wireless network technology, and more particularly, to wavelength division multiplexing.
2. Description of the Related Art
Recently, due to the introduction of portable multi-function devices, such as smart phones, smart TVs, etc., excessive traffic is generated in wired/wireless networks. In order to cope with such excessive traffic, studies into applying Wavelength Division Multiplexing (WDM) to a wired subscriber network or an integrated wired/wireless subscriber network are actively conducted. The WDM is a method of multiplexing multiple optical wavelengths and transporting them over a single optical fiber at the same time, so that the WDM can greatly reduce the cost of lines by the number of the optical wavelengths, as well as having many advantages in view of security, Quality of Service (QoS), and protocol transparency since each data channel is carried on its own unique wavelength.
In order to use the WDM, each subscriber device has to be allocated its own wavelength for communication with other parties. This requires optical sources with a number of unique wavelengths corresponding to the number of subscribers belonging to a wired subscriber network that is spread across remote nodes, or the number of separated-type base stations that exist in an integrated wired/wireless network. The need for optical sources with various unique wavelengths indicates that different specific kinds of optical sources should be further prepared, in view of fabrication, installation, and equipment management, in case failure occurs. This further requirement may be a considerable burden to providers. For these reasons, studies into development and commercialization of a wavelength-independent optical source are more actively conducted.
Wavelength-independent optical sources can be broadly classified into two types: one is a reflective optical source, such as a Reflective Semiconductor Optical Amplifier (RSOA) or a Fabry-Perot laser diode; and the other is a wavelength-tunable optical source whose lasing wavelength can be tuned. The transmission performance of the reflective optical source strongly depends on the power level of injected seed light. Therefore, the link would have some constraints, such as scalability and transmission distance. The wavelength-tunable optical source is considered as an attractive solution due to its flexibility. However, the output wavelength of the wavelength tunable optical source is variable; therefore, the wavelength initialization process is indispensably necessary before starting communication. A straightforward and simple way to achieve wavelength initialization is using the lookup table, usually predetermined and loaded in the tunable transmitter module. A lookup table has to be generated for each of the lasers because of the manufacturing variations. Moreover, the value of the control parameters in the lookup table need to be adjusted due to either laser aging or temperature changes. Although the time for generating the lookup table depends on the tuning mechanisms of the laser diodes, and there are some proposals to generate lookup tables in a short time, the overall generating process is exhaustive and requires a time-consuming scanning process. This can increase the devices' packaging cost.
Korean Patent Registration No. 10-0945423 discloses a tunable external cavity laser which tunes an output wavelength using the Littmann-Metcaff scheme, Korean Patent Registration No. 10-0945422 discloses a tunable external cavity laser which applies heat near a waveguide configuring Bragg gratings, and Korean Laid-Open Patent Application No. 10-2011-00732232 discloses a tunable laser module which tunes a wavelength by integrating a narrow-band wavelength tunable laser.