The SLED’s peak wavelength and intensity are influenced by the active material’s composition as well as the degree of injection current. SLEDs, unlike laser diodes, are made to have strong single pass amplification for spontaneous emission produced along the waveguide but inadequate feedback to produce lasing activity.
One sort of technology that can fill the void between light-emitting diodes (LEDs) and laser diodes is superluminescent diodes. The concentrated beam of the light source and effective usage of light is promoted by a technique of producing light based on enhanced spontaneous emission.
In this blog, we’ll explore how SLEDs differ from LDs and LEDs.
What is a Superluminescent Diode?
A superluminescent diode (SLD) is a semiconductor device that emits light in the superluminescent edge state. Unlike laser diodes, superluminescent diodes do not have enough feedback to sustain lasing, yet they are designed with strong single-pass amplification for spontaneous emission generated along the waveguide.
SLDs were first developed in the 1970s in the selenide material system, aimed at producing an emitter with low temporal coherence that could easily couple into an optical fiber. At the time, the wavelength of the SLD was controlled using anti-reflective coatings, and the semiconductor emitters were built as double-heterostructure types.
Let’s move on to the differences and similarities between LED, LED, and SLED.
The Distinctions and Similarities between LD, LED, and SLED
The distinctions and similarities between superluminescent light-emitting diodes (SLEDs), laser diodes (LDs), and light-emitting diodes (LEDs) reveal key characteristics of broadband semiconductor devices. SLEDs are closely related to their better-known counterparts, with all three devices utilizing electrical current injection to emit light. Structurally, they all consist of positively (p)-doped and negatively (n)-doped regions, which contribute to their operation.
Superluminescent emitting diodes are a semiconductor type that employs electrically charged injections to produce broad-spectrum light. They generate bandpass light with a sharp laser beam, much like LDs, while also emitting broadband light rays, similar to LEDs. As such, SLEDs can be viewed as broadband semiconductor lasers that produce a beam-like output, combining features of both LDs and LEDs.
The term “broadband” refers to the wide range of wavelengths or variable frequencies in the light produced by SLEDs. This characteristic can be analyzed through the Fourier series, where the unique aspects of SLEDs are connected to their frequency response. In the context of feature space, a light source with a broad frequency response is often narrowband or exhibits a short coherence length. In this way, SLEDs can be considered incoherent laser diodes.
However, light from one source can only interact with light from another, or from the same source, within its coherence length. When the path differences between interfering light waves are smaller than the light’s cut-off wavelength, the light waves reflect off surfaces that are not perfectly flat, creating speckles. These random dark and white interference patterns are regarded as noise.
In contrast, the long coherence lengths of narrowband laser diodes make low speckle noise a common occurrence. SLEDs, however, significantly reduce speckle or interference noise in imaging applications due to their short coherence length. Their productivity coherence and enhancements make them the preferred light source for various uses, particularly in applications where speckle reduction is crucial. For example, broadband SLEDs, with coherence lengths measured in microns, are employed in fields like multi-beam photography, where they provide the necessary axial resolution for imaging instruments.
Quick Comparison of SLEDs, LEDs, and LDs
| Features | LED | SLED | LD |
|---|---|---|---|
| Principal of Light Generation | Spontaneous Emission | Amplified Spontaneous Emission | Stimulated Emission |
| Optical Spectrum | Broadband | Broadband | Narrowlated or Multiple Fabry-Perot Modes |
| Total Optical Output Power | Medium | Medium | High |
| Optical Power Density | Low | Medium | High |
| Light Emittance | No | Yes | Yes |
| Optical Waveguide | All Directions | Divergence-Limited | Diverence-Limited |
| Spatial Coherence | Low | High | High |
| Coupling into Single-Mode Fibers | Poor | Efficient | Efficient |
| Temporal Coherence | Low | Low | High |
| Generation of Speckle Noise | Low | Low | High |
| Polarization State | Random | Linear | Linear |
These are the differences and similarities between LD, LED, and SLED. Due to SLED’s special qualities, they are essential for several applications where a combination of excellent beam quality and low coherence is required. SLEDs offer unique advantages in fields such as optical coherence tomography, fiber optic sensing, and various imaging techniques, making them highly versatile across multiple industries. We hope that reading this blog has enhanced your understanding of LD, LED, and SLED, particularly in terms of their applications and functionality.
Inphenix is a laser and light source manufacturer based in the United States, specializing in designing and producing advanced optical components. Our products are widely used across a variety of industries, from telecommunications to biomedical imaging. Please contact us for more detailed information about our high-quality laser and light source offerings.
