A superluminescent diode is a high-power LED built around a PN junction embedded in an optical waveguide. When electrically biased in the forward direction, it exhibits optical gain and generates amplified spontaneous emission over a wide range of wavelengths. An SLD light source has an output power similar to a laser diode, the wide oscillation spectrum width of an LED (Light Emitting Diode), and low coherence.
Superluminescent diodes are a type of device that can bridge the gap between light-emitting diodes (LEDs) and laser diodes. A light generation method based on amplified spontaneous emission promotes the intense beam of the light source and the efficient use of light. Simultaneously, the light’s low time coherence reduces interference effects, which are undesirable in applications such as Optical Coherence Tomography.
In this blog, we explore the definition, features, and properties of the SLD along with its working method. Let’s start with what a superluminescent diode is.
What Is a Superluminescent Diode (SLD)?
A superluminescent diode is a superluminescent edge-emitting semiconductor light source. It is a type of device that bridges the gap between light-emitting diodes (LEDs) and laser diodes, combining features from both technologies.
Superluminescent diodes are designed to provide high single-pass amplification for spontaneous emission generated along the waveguide. However, unlike laser diodes, they do not possess sufficient feedback to perform the lasing act, which allows them to emit a broad spectrum of light with low coherence.
SLDs were first developed in the arsenide material system in the early 1970s, aimed at producing an emitter with low time coherence that could be coupled to optical fibers. At the time, the semiconductor emitters were designed as double-heterostructure devices, and the waveguide was equipped with an anti-reflection coating to prevent lasing.
The development of the first nitride-based SLDs, particularly in the blue-violet spectrum, sparked extensive research and rapid growth in the field. This breakthrough opened the door to a wide range of applications for blue-violet SLDs, from high-density optical storage to biomedical imaging.
Let us now move on to the section on superluminescent diode properties.
The Properties of Superluminescent Diodes
- Current-limiting device: The optical power emitted by an SLD is proportional to the driving current. Unlike laser diodes, which exhibit a sharp increase in intensity at the lasing threshold, SLDs show a gradual increase in intensity as the current increases, making their behavior more similar to that of LEDs.
- Residual spectral modulation: This characteristic arises due to non-zero reflections from the SLD’s facets, leading to parasitic Fabry-Perot modulation. While this modulation is usually minimal, it distinguishes SLDs from ideal incoherent light sources.
- Power and spectrum: SLDs combine the high output power and brightness typical of laser diodes with a broad, LED-like optical spectrum. This unique combination provides the advantages of both technologies in one device.
- High sensitivity to external feedback: SLDs exhibit high optical gain in their active region, making them particularly sensitive to external optical feedback. This can influence their stability and performance in various applications.
- Antireflection coatings (AR): Similar to laser diodes, SLDs often feature perfect AR coatings on their chip facets to suppress unwanted reflections and prevent lasing action, ensuring broad-spectrum emission.
- Stable optical power and wavelength: SLDs maintain optical power levels comparable to laser diodes, and their emitted wavelength remains stable even under extreme temperature variations or device aging, ensuring long-term reliability.
These six exceptional and distinct properties make SLDs ideal light sources for numerous applications. Let’s move on to the section on how superluminescent laser diodes work.
How Does a Superluminescent Diode Work?
Similar to laser diodes, superluminescent LEDs rely on an electric drive applied to a p-n junction that becomes optically active. When biased in the forward direction, this junction generates amplified emission over a broad wavelength range. The specific wavelength and intensity of the SLED’s emission are determined by the composition of the active material, as well as the level of the injection current. To achieve high output, single-pass amplification for spontaneous emission occurs along the waveguide, although SLEDs do not reach lasing action.
When an electrical forward voltage is applied, an injection current is created across the active region. The current flows from the p-section, through the active region, and into the n-section. During this process, light is generated by the spontaneous and random recombination of positive electrical carriers (holes) and negative carriers (electrons). As this light travels along the waveguide of the SLED, it is further amplified, contributing to the overall emission.
Let us now look at some of the most important features of a superluminescent laser diode.
Features of Superluminescent Diode
- Superluminescent Diodes are high-power, broadband light sources, making them ideal for use in various applications.
- SLDs offer a broad optical bandwidth, serving as a versatile optical source.
- Superluminescent laser diodes, with their high spatial coherence, can be efficiently coupled into single-mode optical fibers.
- With SLDs, achieving high output power in single-mode (SM) or polarization-maintaining (PM) fibers is straightforward.
So, these are some noticeable features and properties of the Superluminescent Diode with its working method.
In conclusion, superluminescent diodes represent one of the newest and most promising classes of nitride optoelectronic devices, with their development steadily gaining momentum in the optoelectronic market. Due to their unique properties, they are irreplaceable in specific applications requiring a combination of excellent beam quality and low coherence.
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