The Difference and Similarity Between LD/LED/SLED

Superluminescent Emitting Diodes (SLEDs, SLDs) are semiconductor devices that emit broadband light through electrical current injection. SLEDs are a hybrid between LEDs, which emit broadband light in all directions, and semiconductor laser diodes, which emit narrowband light with a well-defined laser beam. In this sense, SLEDs can be understood as broadband laser diodes with a beam-like output.

Broadband means that SLEDs emit an optical spectrum that is broad in the wavelength or frequency domain. The spatial domain is correlated to the frequency domain through the Fourier transform. A light source that is broadband in the frequency domain is therefore narrowband in the spatial domain, meaning it is having a short coherence length. In this sense, SLEDs can be understood as incoherent laser diodes.

Light from any light source can interfere with light from the same or another light source, but only within its coherence length. Reflections from surfaces, which are never perfectly flat and even, cause so-called “speckles” when the path differences between the interfering light waves are smaller than the coherence length of the light. Speckles are random dark and white interference patterns that are perceived as noise. Narrowband laser diodes typically generate speckle noise because of their large coherence length. In this sense, SLEDs can be understood as speckle-free laser diodes.

Broadband Light Generation in SLEDs

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. Electrical 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.

Comparison to LEDs and Laser Diodes

A broad range of possible energy transitions is something that also applies to a semiconductor laser, for example a Fabry-Perot (FP) laser diode (LD), or to a light-emitting diode, a LED.

FP laser diodes use the broadband gain of the semiconductor material, an optical waveguide and reflecting facets of the LD chip to build a laser cavity (resonator). Starting from spontaneous emission that is amplified along the waveguide of the LD, light travels back and forth (multiple roundtrips) inside the laser cavity, resulting in constructive amplification of cavity modes through stimulated emission. Consequently, the optical spectrum of a FP laser diode exhibits a comb of distinct longitudinal modes of the laser cavity.

SLEDs exploit the process of amplified spontaneous emission (ASE) where light of spontaneous emission is amplified only in a single pass through an optical waveguide as compared to amplification over multiple roundtrips in an LD. For this reason SLEDs typically have lower output powers for the same electrical drive current as compared to LDs, which also means that their wall-plug efficiency is lower. Furthermore, any kind of optical cavity is avoided in a SLED, resulting in a broadband spectrum with no or little amplitude modulation (spectral ripple).

LEDs do not feature any optical waveguide and are therefore not edge-emitting devices with a “laser beam” like LDs or SLEDs. Instead, LEDs emit in all directions light from spontaneous emission only (as compared to ASE). The total output power of LEDs can be considerably high, however due to the wide emission angles the optical power density (intensity) is lower as compared to SLEDs or laser diodes. For the same reason, coupling into single-mode optical fibers is rather impractical with LEDs and coupling efficiencies are very poor. LEDs produce light with an optical spectrum that might be even wider than that of a SLED because the process of amplified spontaneous emission along the waveguide results in spectral narrowing.

Summary

The following table summarizes and compares the optical properties of LEDs, SLEDs and LDs.