LED Superluminescenti (SLD)e LED per comunicazioni ottiche Docente: Mauro Mosca (www.dieet.unipa.it/tfl) A.A. 2013-14 Ricevimento: alla fine della lezione o per appuntamento Università di Palermo – Facoltà di Ingegneria (DEIM)
LED nelle comunicazioni ottiche • Free-space communication • 2. Optical fiber communication The emission wavelength of GaAs and GaInAs LEDs is limited to wavelengths in the IR ranging from 870 nm (for GaAs active regions) to about 950 nm (for GaInAs active regions) Transmission distances of less than 100 m (transmission medium (air) can be considered, to a good approximation, to be lossless and dispersionless) The light-emitting spot should be smaller than the core diameter of the optical fiber. Diameters of emission regions of 20–50 µm for devices used with multimode fibers. Silica multimode fibers have typical core diameters of 50–100 µm). Plastic optical fibers, on the other hand, can have core diameters as large as 1 mm. Accordingly, LEDs with larger light-emitting areas can be used with plastic fibers. The maximum bit rates attainable with LEDs are limited to rates ≤1 Gbit/s (spontaneous emission lifetime of about 1 ns in highly excited semiconductors) Applications include the remote control of appliances such as television sets and stereos, and data communication between a computer and peripheral devices High coupling efficiency of the light emanating from the LED to the fiber is essential. Only the light emanating from one surface can be coupled into the fiber The total light power is an important figure of merit The power emitted per unit area is a useful figure of merit (in contrast to free-space communication LEDs where the total power emitted by the LED is the appropriate figure of merit)
LED di Burrus chemically assisted mechanical polishing to about 150 µm thickness followed by a wet chemical etch reduce light absorption in the substrate The lateral size of the active region is determined by the p-type ohmic contact size of the LED The lateral extent of the active region is smaller than the core diameter of the optical fiber to maximize coupling efficiency
LED per telecomunicazioni a 1,5 mm The wavelength 1500 nm is of interest for long-distance silica fiber communication. Long-distance communication fibers must have a small core diameter to be single mode and avoid modal dispersion. Consequently, LEDs emitting at 1500 nm are not used as sources for silica fiber communication.
LED per telecomunicazioni a 650 nm the wafer is taken out of the growth system for patterning and etching, then is re-introduced for resumption of epitaxial growth current flows only through the Zn-diffused layer
LED per telecomunicazioni a 650 nm higher intensity due to the more directed emission pattern reducing chromatic dispersion High-speed transmission of 250 Mbit/s over plastic optical fibers
Struttura di un SLD Since the light is guided by the waveguide, the light intensity emitted by the device linearly increases with the length of the waveguide edge-emitting LED waveguide However, the electrical current required to drive the LED also increases with the stripe length
Caratteristiche di un SLD - Superluminescent diodes are edge-emitting LEDs that are pumped at such high current levels that stimulated emission occurs. - Thus SLDs have greater coherence compared with LEDs. - Spontaneous emission towards the top surface of the LED is reduced and emission into waveguide modes is enhanced. - Difference with semiconductor laser: SLDs lack the optical feedback provided by the reflectors of a semiconductor laser
Tipi di SLD lossy region (not pumped by the injection current) reflective back-side reflector facet AR coating expensive no feedback occurs if the length of the lossy region is much longer than the absorption length of the core region length of lossy region >> a -1 a~ 104 cm-1
Confronto tra LED, laser e SLD broad spontaneous emission spectrum Fabry–Perot cavity enhancement The spectral width of SLDs is narrower than that of LEDs due to increased coherence caused by stimulated emission.
Caratteristiche L-I due to stimulated emission spontaneous emission regime current overflow In the stimulated emission regime, an increasing number of photons are guided by the waveguide. The number of photons emitted into waveguide modes increases with injection current as stimulated emission becomes dominant. superlinear, but more distinct threshold