LEDs emitting at 650 nm are useful for plastic optical fiber communication. These fibers have a loss minimum and relatively low dispersion at 650 nm. Communication LEDs emitting in the 600-650 nm range are based on the (AlGa)05In05P material system just as for 650 nm visible - spectrum LED lamps.

Typical LED structures used for 650 nm plastic fiber communication applications are shown in Figs. 23.5 (a) and (b). Both LEDs are top-emitting devices due to the opaqueness of the GaAs substrate. The LEDs employ current-blocking layers that guide the current to the active region. Light emitted from this region is not obstructed by the metallic ring contact.

The figure shows two types of current-blocking regions. Figure 23.5 (a) shows a current - blocking p-n-p structure in the transparent top layer. A Zn-diffused region in the center region of the LED overcompensates donors in the current-blocking layer making it p-type. As a result, current flows only through the Zn-diffused layer. Figure 23.5 (b) shows a current-blocking layer fabricated by epitaxial regrowth. After growth of an n-type blocking layer on top of the p-type confinement layer, the wafer is taken out of the growth system for patterning and etching. Subsequently, the wafer is re-introduced into the growth system for resumption of epitaxial growth. Epitaxial regrowth is a more expensive process and usually the device yield decreases for structures employing regrowth. The transparent window layers can consist of AlGaAs, as shown in Fig. 23.5, GaP, or another transparent semiconductor.

Resonant-cavity LEDs (RCLEDs) have several advantages over conventional LEDs including high brightness and a narrow spectral width (Schubert et al., 1992, 1994; Schubert and Hunt, 1999). An RCLED emitting at 650 nm is shown in Fig. 23.6 (Streubel and Stevens, 1998; Streubel et al., 1998; Whitaker, 1999; Mitel, 2000). As in the two previous structures, the top contact is ring-shaped and the current is guided to the center opening of the ring. An ion - implanted region is used for current blocking. Hydrogen and oxygen have been employed to render the implanted region insulating. Oxygen implants are more stable than hydrogen implants since small hydrogen atoms tend to easily diffuse out of the semiconductor at moderate annealing temperatures.

The emission spectra of an RCLED and a conventional LED are shown in Fig. 23.7. The spectra shown are the spectra of the fiber-coupled light. Inspection of the spectra reveals two features. Firstly, the RCLED exhibits a higher spectral purity thereby reducing chromatic dispersion. Secondly, the fiber-coupled intensity of the RCLED is higher due to the more directed emission pattern. High-speed transmission of 250 Mbit/s over plastic optical fibers has been demonstrated with 650 nm RCLEDs (Streubel and Stevens, 1998).