A superluminescent diode (SLED or SLD) is an edge-emitting semiconductor light source based on superluminescence. It combines the high power and brightness of laser diodes with the low coherence of conventional light-emitting diodes. Its emission optical bandwidth, also described as full-width at half maximum, can range from 5 up to 750 nm.
History The superluminescent diode was reported for the first time by Kurbatov et al. (1971) and Lee, Burrus, and Miller (1973). By 1986 Dr. Gerard A. Alphonse at RCA Laboratories (now SRI International), invented a novel design enabling high power superluminescent diodes. This light source was developed as a key component in the next generations of fibre optic gyroscopes, low coherence tomography for medical imaging, and external cavity tunable lasers with applications to fiber-optic communications. In 1989 the technology was transferred to GE-RCA in Canada, which became a division of EG&G. Superluminescent light emitting diodes are also called sometimes superluminescent diodes, superluminescence diodes or superluminescent LEDs.
Principles of operation A superluminescent light emitting diode is, similar to a laser diode, based on an electrically driven p-n junction that, when biased in forward direction, becomes optically active and generates amplified spontaneous emission over a wide range of wavelengths. The peak wavelength and the intensity of the SLED depend on the active material composition and on the injection current level. SLEDs are designed to have high single pass amplification for the spontaneous emission generated along the waveguide but, unlike laser diodes, insufficient feedback to achieve lasing action. This is achieved through the joint action of a tilted waveguide and anti-reflection coated (ARC) facets.
When an electrical forward voltage is applied, an injection current across the active region of the SLED is generated. Like most semiconductor devices, a SLED consists of a positive (p-doped) section and a negative (n-doped) section. Electric current will flow from the p-section to the n-section and across the active region that is sandwiched in between the p- and n-section. During this process, light is generated through spontaneous and random recombination of positive (holes) and negative (electrons) electrical carriers and then amplified when travelling along the waveguide of a SLED. The pn-junction of the semiconductor material of a SLED is designed in such a way that electrons and holes feature a multitude of possible states (energy bands) with different energies. Therefore, the recombination of electron and holes generates light with a broad range of optical frequencies, i.e. broadband light. The output power performance of an ideal SLED can be described with a simple model, not taking spectral effects into account and considering both a uniform distribution of carrier densities and zero reflections from the facets.
P o u t = h c ⋅ ν ⋅ Π ⋅ R s p exp [ ( g − α ) L ] − 1 g − α {\displaystyle P_{out}={\frac {h}{c}}\cdot \nu \cdot \Pi \cdot R_{sp}{\frac {\exp[(g-\alpha )L]-1}{g-\alpha }}}
Where h is the Planck constant, ν the optical frequency, Π the size of the optical mode, Rsp the spontaneous emission rate into the guided mode, g the modal gain, α the non-resonant optical losses, L the length of the active channel and c the velocity of light. So the output power depends linearly on the spontaneous emission rate and exponentially on the optical gain. Obviously a high modal gain is required to obtain high optical output power.
Main characteristics
Dependence of power on current
The total optical power emitted by an SLED depends on the drive current. Unlike laser diodes, the output intensity does not exhibit a sharp threshold but it gradually increases with current. A soft knee in the power vs. current curve defines a transition between a regime dominated by spontaneous emission (SE), typical for surface emitting LEDs, and one that is dominated by amplified spontaneous emission (ASE), i.e. superluminescence. Even if the output power is based on spontaneous emission, the amplification mechanism affects the polarization state of the emitted radiation in a way which is related to the SLED structure and on the operating conditions. The maximum value of the current that allows a safe operation of the device depends on the model and ranges between 70 mA (for low power SLED) and 500 mA for the most powerful devices.
Centre wavelength and optical bandwidth
The optical power emitted by SLEDs is distributed over a wide spectral range. Two useful parameters that are related to the power density distribution at different wavelengths are the optical bandwidth (BW) and the peak wavelength, λ {\displaystyle \lambda } peak. The first is defined as the full width at half maximum (FWHM) of the power density vs. wavelength curve at the nominal operating conditions while the latter corresponds to the wavelength having the highest intensity. The centre wavelength, λ {\displaystyle \lambda } centre is defined as the central point between the two FWHM points of the spectral curve; it can be different from the peak wavelength since it is related to the spectrum asymmetry. Typical values for SLED modules are for the BW between 5 nm and 100 nm with central wavelengths covering the range between 400 nm and 1700 nm. A trade off between maximum output power and bandwidth exists, however, the latter being larger for devices with lower output power.
Spectral ripple
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