The present invention relates to a variable-wavelength waveguide type optical branching filter capable of switching the propagation paths for an optical signal of a desired wavelength.
A wavelength division multiplexing (WDM) optical fiber transmission system is being investigated very actively because it will not only afford a substantial increase in the capacity of a basic transmission line but also its flexibility can offer various services at different bands such as telephone, high-speed data and video information in a subscriber's system, in particular, in a local area network (LAN). The WDM system is intended primarily for multiplexing by propagating optical signals of plural wavelengths over one transmission line, and the development of an optical branching filter of low loss and small crosstalk is indispensable to this system since it is necessary to join or branch wavelengths carrying desired signals in the transmission line at a desired place or places. In conventional systems the number of services rendered is very small and only several wavelengths are used. Accordingly, respective channels are spaced more than 200 .ANG. apart in wavelength and conventional optical branching filters are mainly those mode of the interference film which are relatively wide in band.
With the expansion of service contents, the number of channels needed will be increased in the future and the channel spacing will have to be reduced accordingly. This will call for the use of many optical branching filters of narrow band. A waveguide type optical branching filter is suitable for efficient joining and branching many channels and for providing the device in compact form. Furthermore, the waveguide type optical branching filter, if formed using a semiconductor, could be integrated with a light source such as a semiconductor laser, or an optical active element such as a photodetector. In addition, if the center wavelength of the optical branching filter can be varied by an external control signal, it will be possible not only to enhance the performance of the element but also to implement a novel function.
In order to control the center wavelength of the optical branching filter, prior art employs the surface acoustic wave. However, prior art does not provide high efficiency diffraction, adversely affects the reliability of the device and presents a very low response speed.