19 Jun 2008
A superluminescent light emitting diode has for the first time been engineered to exhibit high output power as well as a broadband emission spectrum.
By carefully controlling the annealing process parameters, a team from the UK and China has successfully fabricated a quantum-dot (QD) based SLED that has the sought-after combination of high output power and broadband emission spectra, and has an emission bandwidth of 146 nm and a continuous-wave output power of 15 mW at room temperature (Optics Letters 33 1210).
"Although similar bandwidths have been reported at longer wavelengths, we have achieved a broad emission spectrum centred at 980 nm," Ziyang Zhang, a researcher at the University of Sheffield in the UK, told optics.org. "This corresponds to a very small coherence length of 6.6 µm, which can significantly reduce Rayleigh backscattering in a fibre gyroscope system and greatly improve the spatial resolution in OCT."
Zhang cites three factors as being crucial to this development. Firstly, a high density and highly inhomogeneous QD active region provides relatively high gain. Secondly, the combination of an unpumped absorber region and V-groove back facet effectively inhibits lasing and thirdly, the tapered waveguide structure prevents gain saturation effects at the output.
High output power and broadband emission spectra are key features of SLEDs, but are difficult to achieve simultaneously. Various approaches have been explored, such as using chirped quantum well (QW) structures or introducing higher order transitions from QWs. However, the resulting emission spectrum is usually irregular and sensitive to injection-current.
Most recently, self-assembled QD structures have attracted considerable interest for their naturally broad inhomogeneously broadened emission spectrum. However, the corresponding output powers of such devices are only around 1 mW or less.
Zhang and his colleagues adopted a QD SLED structure based on a typical p-i-n configuration. The active region of the QD SLED consists of five layers of InAs QD sandwiched by 1 µm AlGaAs cladding layers. According to Zhang, careful production of high density QDs within the active layer helps to achieve a higher power output and broader emission spectra.
"QDs are very sensitive to high annealing temperatures. By using a relatively low growth temperature during the formation of the QDs, we obtained a high QD density (~1011/cm2 per quantum layer) with large size inhomogenity," he explained. "The highly dense and inhomogeneous QD active region provides relatively high gain and broader emission."
The next steps for the team are to further improve the performance of QD SLEDs by broadening the emission spectrum and obtaining high powers in fibre-coupled devices. "There are a number of challenges in epitaxy for the realization of high, broad gain spectra and high spontaneous emission efficiency," concluded Zhang. "We also need to use a number of strategies to inhibit lasing in these devices and minimize thermal effects."