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Quantum-dot diodes provide sources for optical coherence tomography

Developing low-cost broadband semiconductor IR sources that could enhance the performance and resolution of OCT systems.
1 April 2006, SPIE Newsroom. DOI: 10.1117/2.1200602.0094

Optical coherence tomography (OCT) is an important application of ultra-broad-bandwidth light sources. Essentially, OCT is based on the well-known low-coherence interferometry technique sketched in Figure 1. A broadband source illuminates a fiber-optic Michelson interferometer. When the sample and reference path lengths match to within the coherence length of the source, then an interference pattern is detected. With appropriate scanning, ultra-high-resolution two- or three-dimensional cross-sectional images of tissue may be obtained non-invasively and in situ with axial resolution around 2 to 3μm (or better).1 Tissue morphology (see Figure 2 and Figure 3) as well as functional information such as blood flow, can be obtained. Today, commercial OCT systems are routinely used in ophthalmic clinics for examinations. In the near future, OCT could become an equally important tool for clinical applications in dermatology.

Figure 1. Optical coherence tomography (OCT) is based on a Michaelson interferometer, as shown. The inset shows the detected interference signal from (left) a single reflection site and (right) a full-depth scan, labeled a-scan (envelope).The light source is a quantum-dot superluminescent light-emitting diode (QD-SLED).

Figure 2. Three-dimensional images of part of a normal human eye obtained using OCT. The images (a)–(e) show both the topography and morphology of the entire foveal depression, and (f) shows the pattern of the retinal vascular net. (Figure courtesy of W. Drexler, Medical University, Vienna, Austria, 2005).

Figure 3. A 4 × 2mm cross-section of skin from the left thumb of a healthy volunteer. The image shows the stratum corneum, epidermis and dermis. This OCT image was obtained using a light source with a 1300nm center wavelength.

We are developing quantum-dot-based super-luminescent diodes to provide high-bandwidth high-power light sources as part of the European-Union Framework-6 program NANO-UB-SOURCES,2 which includes eight partners from seven nations. These devices consist of self-assembled indium-arsenide quantum dots within indium-gallium-arsenide quantum wells, with barrier layers made of gallium arsenide. We call this a dot-in-well (DWELL) structure.

The emission wavelength of DWELL structures is a strong function of the indium composition of the well layer.3 We use this fact to control the emission spectrum. We have adopted a multi-layer structure in which each well has a different indium composition, resulting in a different peak emission wavelength from each layer. Furthermore, we can gain another advantage by making use of the atom-like nature of quantum dots: by overlapping the emission of the ground state and excited states of different DWELL layers, we eliminate the unwanted multi-Gaussian emission observed by other researchers in this area.4We term these structures dots in compositionally-modulated wells (DCMWELLs).

When making these using molecular beam epitaxy, the use of two indium cells is crucial to realize the DCMWELL layer structure: it allows the indium composition of the quantum well to be varied independently of the quantum dot deposition. Figure 4(a) shows the light-current response for 6mm-long superluminescent diodes utilizing the DWELL and DCMWELL structures. A slight reduction in output power is observed for the DCMWELL structure, consistent with the broader gain spectrum of these devices. Figure 4(b) shows emission spectra obtained for these devices. For the DWELL structure, the light emission is dominated by the ground state of the quantum dots, and as the current density increases we see a slight narrowing of the emission linewidth. This is due to increased stimulated emission. For the DCMWELL devices, increasing the current density results in a considerable broadening of the emission line-width, consistent with state-filling within the quantum-dot structures and the overlap of ground and excited states of the different active layers. For the DCMWELL devices, an emission bandwidth of 85nm, and continuous output powers of 2.5mW are obtained with this method. We continue to work on enhancing the output power. Preliminary results utilizing p-doping of the active layers and optimized growth parameters indicate continuous-wave powers in excess of 40mW are possible for a DWELL device. We believe this approach, or a variant, could be optimized to provide spectral widths in excess of 150nm and emission powers of more than 10mW.

Figure 4. (a) As the current density increases, the pulsed light output increases for both DWELL and DCMWELL superluminescent LEDs at room temperature.(b) The emission spectrum for DWELL devices narrows with higher current densities, but the spectrum for DCMWELL devices broadens.

Quantum-dot-based super-luminescent light-emitting diodes provide small low-cost high-power, high-bandwidth sources for application optical coherence tomography. We have described recent work on engineering the emission bandwidth of these devices using a multilayer structure comprised of quantum dots in quantum wells of different indium compositions. Quantum-dot-based super-luminescent diodes are poised to make a large impact on the performance and cost of optical coherence tomography systems.

Richard Hogg
EPSRC National Centre for III-V Technologies, Department of Electronic and Electrical Engineering, University of Sheffield
Sheffield, UK
Peter Andersen
Optics and Plasma Research Department, Risø National Laboratory
Roskilde, Denmark