Broad-spectrum light emitting diodes for fiber-optic sensor systems

Light emitting diodes (LEOs) with a broad emission spectrum (e.g., 750 to 900 nm) are a desirable optical source for a wavelength-division-multiplexing (WDM) based fiber-optic sensor network system. The number of sensors or channels served by such a broadband light source depends on the resolution of the WDM and the spectral width of the LED. The conventional LEOs with a spectral width of 30 to 40 nm are insufficient for most of systems requiring more than 10 channels for a common WDM resolution of 5 nm. In this paper, we report novel quantum-well LED structures capable of a 100 nm spectral width and high coupling efficiency into a multimode fiber. We have studied the generation of broadband light using emission from multiple states in an active layer consisting of either a single quantum well or two asymmetric quantum wells. Similaf concepts have been used for superluminescence diodes (SLDs) and wavelength switching, respectively. Basically, we have used the gain saturation effect in a high loss quantum well cavity structure to squelch lasing and force the device to simultaneously obtain gain from either the higher energy states in the same well or the higher energy wells. The loss has been introduced by reducing the mode confinement factor and adding an unpumped section to the cavity. With these designs, we have achieved a spectral width of 90 to 110 nm which, to our knowledge, the largest reported for this wavelength range. The quantum-well LED structure used in this work is shown.