Superluminescent diodes (SLD) have great potential for application as light sources including optical gyroscopes and sensors, optical time domain reflectometers (OTDR) and wavelength-division multiplexing (WDM) system testing. High output power and large spectral bandwidth are key features for SLD. High output power has been attempted by antireflection coating of the facet and utilizing the tapered active region. In addition, as the spectral width broadened, the coherence length is reduced. The short coherence length can reduce the Rayleigh backscattering in fiber gyroscope systems and can improve the spatial resolution in OTDR application. Therefore, broadening spectral width of SLD offers an advantage for obtaining the ultimate sensitivity in these applications.