Tunable filters are indispensable elements for tunable laser diodes, which provide an electronic control of the emission
wavelength by exploiting the free-carrier plasma effect. A tunable filter consists of a combination of tuning region that
forms a waveguide core and a Bragg grating for the wavelength-selective feedback. By injection of carriers into the
tuning region, the refractive index and, thus, the Bragg wavelength can be decreased. Thereby, it is obvious that the
maximum tuning range depends on the achievable carrier density in the tuning region. In this paper, a type-II diode is
presented, which shows an improved carrier density-current characteristic. This is achieved by spatial separation of
electrons and holes, which in turn leads to a suppression of carrier recombination. Since high carrier densities can be
achieved at comparatively low injection currents, type-II superlattices are particularly well suited for application as
tuning region in tunable filters. They have the potential to double the tuning range that is presently achievable using
bulk tuning layers.
|