SUPERLUMINESCENT DIODES

Superluminescence occurs when the spontaneous emissions of an ELED experience gain due to higher injected currents and reflections from facets. Superluminescent diodes (SLDs) are differentiated from both conventional LEDs and LDs. Although the output is not fully coherent,

SLDs emit light that consists of amplified spontaneous emissions. The spectral width and beam angle of SLDs are narrower than that of conventional LEDs and wider than that of LDs.

An SLD is, in essence, a combination of a laser and an ELED. SLDs are similar in geometry to lasers but have no built-in optical feedback mechanism required by laser diodes for stimulated emission to achieve lasing. SLDs have structural features similar to those of ELEDs that suppress the lasing action by reducing the reflectivity of the facets. SLDs are essentially highly optimized ELEDs.

While SLDs operate like ELEDs at low current levels, their output power increases superlinearly and the spectral width narrows at high currents.

Optical gain resulting from the higher injection currents causes the superlinear power increase and narrowing of the spectral width.

The advantages of SLDs over conventional LEDs include higher coupled power, narrower spectral width, and greater bandwidths. The disadvantages include nonlinear power-current characteristics, higher temperature sensitivity, and lower reliability.

Q.11 What are the three basic LED types?
Q.12 Which types of LEDs are the preferred optical sources for short-distance, low-data-rate fiber optic systems?
Q.13 What are facets?
Q.14 What is lowest current at which stimulated emission exceeds spontaneous emission in a semiconductor laser called?
Q.15 Describe the output of a laser diode.
Q.16 Which type of optical source usually lacks reflective facets and in some cases are designed to suppress reflections back into the active region?
Q.17 Which type of optical source tends to operate at higher drive currents to produce light?
Q.18 Are the effects of temperature changes on LDs more or less significant than for LEDs?
Q.19 Specify the mechanism that SLDs lack that is required by laser diodes to achieve lasing.