Superluminescent diode
[4][3] By 1986 Dr. Gerard A. Alphonse at RCA Laboratories (now SRI International), invented a novel design enabling high power superluminescent diodes.
[5] 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.
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.
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 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.
Unlike laser diodes, the output intensity does not exhibit a sharp threshold but it gradually increases with current.
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.
It can be detected using high-resolution optical spectrum analyzers and can be ascribed to the residual reflectivity of the chip facets and of the coupling fibre.
An excessive level of optical back-reflection can cause unexpected irregularities of the spectral distribution of SLEDs that have not to be confused with the ripple.
As described above, superluminescent light emitting diodes are based on the generation and on the amplification of spontaneous emission in a semiconductor waveguide.
SLEDs operating in the wavelength range of 1300 and 1400 nm are mostly based on a bulk material and a chip structure both characterized by a low polarization dependence of the gain.
On the contrary, devices operating in the 1550 and 1620 nm range make mostly use of a quantum well (QW) active region that has a strong polarization-dependent gain.
The optical field emitted by the SLED chips, being a combination of unpolarized spontaneous emission and amplified radiation, has therefore a certain degree of polarization (DOP).
The optical power emitted by semiconductor active devices is always affected by fluctuations (intensity noise) that are induced by the spontaneous emission.
The frequency dependence of RIN is thought to be related to spatial correlation effects induced by the gain saturation.
In that they differ from both lasers, that have a very narrow spectrum, and white light sources, that exhibit a much larger spectral width.
This characteristic mainly reflects itself in a low temporal coherence of the source (which is the limited capability of the emitted light wave to maintain the phase over time).
SLEDs may however exhibit a high degree of spatial coherence, meaning that they can be efficiently coupled into single-mode optical fibers.
Some applications take advantage of the low temporal coherence of SLEDs sources to achieve high spatial resolution in imaging techniques.
It is related to the path difference between the two arms of an optical interferometer over which the light wave is still capable to generate an interference pattern.
From a practical point of view a definition independent on the spectral distribution (non-Gaussian spectrum) of the source is more suitable.
SLEDs exhibit a large spectral width even at the highest power levels so that corresponding FWHM values of the visibility less than 20 μm are easily achieved.
On the one hand SLEDs are semiconductor devices that are optimized to generate a large amount of amplified spontaneous emission (ASE).
However, SLEDs from certain component manufacturers are on the market featuring intrinsically safe designs with high robustness against optical back reflections.
By means of the above-mentioned optimized optical cavity design the SLEDs exhibit high output power, large bandwidth and low residual spectral ripple, making them an ideal light source for a number of applications.
Based on the application's requirements and specifications, SLED devices are available in various packages or form factors covering a broad range of wavelengths and power levels.
The packages include cooled 14-pin dual-in-line (DIL) and butterfly (BTF) modules or low-cost uncooled TOSA and TO-56 devices.
Usage of gallium nitride (GaN) based designs is breaking ground for SLEDs in the ultraviolet and blue spectral range.
SLEDs are commercially available from a number of suppliers, e.g. Denselight (Singapore), EXALOS (Switzerland), InPhenix (US), Superlum (Ireland), or Thorlabs Quantum Electronics (US).
SLEDs find application in situations demanding high intensity and spatial coherence but where a need for a broad, smooth optical output spectrum makes laser diodes unsuitable.
