Superluminescent LED for focus-free handheld projection

A superluminescent light-emitting diode (SLED) is an optoelectronic device with the properties of both a laser diode and an LED. It has been referred to as “the best of two worlds”¹ due its combination of a laser-like beam with the broad emission spectrum typical of an LED. This combination makes it an ideal light source for use in focus-free handheld projection (pico-projection), in which the projected image remains in focus regardless of the surface it is being projected onto. Focus-free pico-projection has application in consumer-level electronics, including smartphones and cameras.

Pico-projectors can be realized using so-called flying-spot technology, in which full-color and grayscale images are created by high-intensity mixing of red, green, and blue beams. The three beams from the laser diodes or SLEDs are focused on a microelectromechanical system (MEMS) mirror, which draws the image on a screen sequentially, pixel-by-pixel. In our device, a laser-like beam (the result of emission from a cleaved-facet laser) can be properly focussed, and the emitted light has a broad spectral bandwidth in the range of 5nm. A laser emits only a single wavelength, so this increased bandwidth improves the image quality for pico-projection. If laser light is incident on a screen or wall with a slightly rough surface, constructive and destructive interference occurs, causing the image to be superimposed with brightness fluctuations (speckle). Photographs of the image—see Figure 1—show that a blue indium gallium nitride (InGaN) SLED produces far less speckle than a blue InGaN laser diode does. The visibility of speckle has an inverse correlation with the spectral bandwidth, so the image quality can be improved greatly by using light sources with a broad emission spectrum.

The first GaN-based SLEDs were reported in 2009.^2,3 Since then, there have been rapid developments with a focus on increased output power^{4, 5} and longer wavelengths.⁶ The performance of blue SLEDs, however, has been shown to deteriorate, due to a degradation of the active region material quality.¹ There is an inherent difficulty in reaching high output powers using SLEDs, especially in the visible spectral range, because the longer the emitting wavelength, the lower the performance.

Fabrication of an SLED is similar to that of an edge-emitting laser diode containing an electrically driven p-n junction and an optical waveguide. To suppress lasing and obtain amplified spontaneous emission (superluminescence), the reflectivity of the out-couple facet must be drastically reduced. In most cases, the cleaved facet is coated with an anti-reflection dielectric mirror and tilted by a few degrees. This tilt angle strongly influences the facet reflectivity. In general, a tilted facet yields a constructive or destructive coupling of the reflected and incident waveguide mode. The coupling is never completely constructive, so a percentage of the reflected light is always lost, which is necessary to suppress lasing.

Figure 1. Magnified photographs of the far-field image of a direct blue indium gallium nitride (a) laser diode and (b) superluminescent LED (SLED), using a sheet of white paper as a screen. The SLED image exhibits much less speckle (brightness fluctuations).

We have designed a blue SLED with an output power greater than 100mW using a curved ridge waveguide with a high-reflection rear facet and a tilted out-couple facet: see Figure 2. A perpendicular high-reflection dielectric mirror ensures the reflectivity of the rear facet is 0.95. The out-couple facet is coated with an anti-reflection dielectric mirror with a reflectivity of 5×10⁻³. Furthermore, this facet is tilted by an angle of 3.5^°, which results in a facet reflectivity as low as 2×10⁻⁵. Consequently, the lasing is sufficiently suppressed. This curved waveguide also has the advantage of enabling a double-pass of light in the resonator via reflection, thereby obtaining a higher light amplification. Spontaneously generated light propagates along the resonator toward the rear facet. This light is reflected and amplified a second time, doubling the resonator length and enabling a distinctly higher output power.

Figure 2. Drawing of a curved waveguide with a high-reflection rear facet and a tilted out-couple facet. R: Reflectivity.

Figure 3. Emission spectra of a blue SLED between an output power of 1mW and 105mW in continuous-wave operation at 25^°C. arb.: Arbitrary.

We mounted a diode into a TO56 can for continuous-wave (CW) operation at a temperature of 25^°C. Figure 3 shows the emission spectra between an output power of 1mW and 105mW, corresponding to an operation current between 110mA and 370mA. The spectra are broadband and smooth. At an output power of 105mW, longitudinal resonator modes—which indicate lasing action—appear for the first time. The spectral bandwidth, an important parameter for an SLED, depends on both current and output power. Remarkably, bandwidth values ranging from 2.5nm (105mW) to >6nm (1mW) were reached, which is 5–10 times higher than the spectral bandwidth of a direct laser diode (0.5nm). However, the power conversion efficiency is low. Due to loss at the tilted facet, an efficiency of up to 4.5% has been reached, which is less than a third that of a comparable blue laser diode.

In summary, we have fabricated a blue SLED with a CW output power of above 100mW at 25^°C. The spectral bandwidth of the SLED is 2.5–6nm, which corresponds to a coherence length ranging from 80μm down to <30μm. The power conversion efficiency is 4.5%. The diode also features a far-field that is single-mode over the whole operational range. Our work has shown that SLEDs are good candidates for focus-free pico-projection without speckle. Our next step is to perform in-depth characterizations of the SLEDs, with a focus on their temporal behavior and the transition from superluminescence to lasing.