| Broad spectral light sources are used in low coherent interferometry (LCI) sensing applications because the spectral bandwidth is inversely proportional the axial resolution. Optical Coherence Tomography (OCT), based on LCI, is an emerging non-invasive bio-imaging technology that produces high resolution images of large volumes using back-scattered near-infrared (NIR) light. In addition to the axial resolution, the signal quality and imaging depth of present OCT systems are limited by the temporal coherency and the optical power of the broadband light source. Superluminescent diodes (SLDs) which operate in the amplified spontaneous emission (ASE) regime are commonly used in OCT due to their compact, batch-fabricated nature and competitive optical characteristics. In this research, we investigate the monolithic integration of a multiple electrode SLD and a photon absorber (PA) with nanostructure based active regions aiming to increase the emission power and bandwidth, while maintaining low spectral ripple.;Three types of SLDs emitting at various NIR wavelengths have been investigated. Sets of (i) 850nm GaAs/AlGaAs quantum-well (QW), (ii) 1210nm InGaAs/GaAs quantum-dot (QD), and (iii) 1650nm InAs/InAlGaAs quantum-dash (Qdash) devices have been designed, fabricated and characterized. Emission bandwidths as broad as 80 nm, 135 nm and 100 nm with emission powers up to 10 mW, 2 mW, 600 mW have been obtained from the QW, QD and Qdash SLDs respectively. In addition, broadband stimulated emission observed from devices from the same wafer structures based on inhomogeneous self-assembled QDs and Qdash have also been characterized. This new class of broadband semiconductor lasers produce 21 nm and 22 nm bandwidth spectra, 600 mW and 400 mW of power centered at 1150 nm and 1650 nm wavelengths for QD and Qdash lasers, respectively.;A detailed investigation of the ASE modal gain and temporal coherence has also been performed. In this study, a new technique developed enables the gain measurement to show for the first time multiple energy level contribution in symmetric GaAs/AlGaAs QW structures, thus confirming the origin of the broad flat-top SLD emission. To verify the usefulness of our devices in OCT systems, the temporal coherences from InGaAs/GaAs and InAs/InAlGaAs devices have been measured using a new fiber-based Fourier domain technique. Coherence measurements show no significant side lodes and subpeaks which can otherwise decrease resolution and image integrity. The analysis of modal gain and temporal coherence through new measurement methods show the significance and applicability of our nanostructure based broadband light emitters for OCT. |
