High-power SLED delivers broad 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.
The novel emitter has a 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 centered 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 fiber gyroscope system and greatly improve the spatial resolution in optical coherence tomography (OCT)."
Zhang cites three factors as being crucial to this development. First, a high-density and highly inhomogeneous QD active region provides relatively high gain. Second, the combination of an unpumped absorber region and V-groove back facet effectively inhibits lasing. And finally, the tapered waveguide structure prevents gain saturation effects at the output.
High output power and broadband emission spectra are desirable features of SLEDs, but in practice they 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 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 inhomogeneity," 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 fiber-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."
Marie Freebody is a reporter for optics.org and Optics & Laser Europe magazine.