## Eye diagram

The eye diagram allows one to estimate the overall performance of an optical communication system. The eye diagram is the receiver signal of a randomly generated digital signal. An eye diagram of an LED operating at 622 Mbit/s is shown in Fig. 24.6. A data rate of 622 Mbit/s is used in the well-known “synchronous […]

## 3 dB frequency

The 3 dB frequency of an LED can be determined by measuring the frequency at which the detector signal decreases to one-half of the low-frequency value. The frequency response of the detector needs to be taken into account in the measurement. A 3 dB frequency of about 500 MHz was determined for the LED for […]

## Current shaping

A common method to reduce the rise and the fall time is current shaping (Lee, 1975; Zucker, 1978; Saul et al., 1985). The diagram of a current-shaping circuit is shown in Fig. 24.5 (a). The current-shaping circuit is essentially a capacitor and a resistor in series with the LED. The capacitor creates a current transient […]

## Voltage dependence of the rise and fall times

The measurement of the rise time and fall time of an LED is shown in Fig. 24.2 (Schubert et al., 1996). As indicated in the figure, the rise and fall times are measured from the 10-90% values of the optical signal. The photocurrent of a p-n junction photodetector is used in the measurement. It must […]

## Rise and fall times, 3 dB frequency, and bandwidth in linear circuit theory

A simple RC circuit is shown in Fig. 24.1 (a). When subjected to a step-function input pulse, the output voltage increases according to Vout(t) = V0[1 — exp(-1/T1)] (24.1) where t1 = RC is the time constant of the RC circuit. When the input voltage returns to zero, the output voltage decreases according to Vout(t) […]

## LED modulation characteristics

LEDs are the most commonly used light source for local-area communication systems operating from very short (< 1 m) to medium distances (5 km). Typical bit rates are tens of Mbit/s up to about 1 Gbit/s. LEDs are non-linear devices and as such the series resistance, shunt resistance, and capacitance depend strongly on the applied […]

## Edge-emitting superluminescent diodes (SLDs)

Edge-emitting LEDs are motivated by the need for high-brightness LEDs that allow for high- efficiency coupling to optical fibers. Edge-emitting LEDs comprise an optical waveguide region that guides light emitted along the waveguide by total internal reflection. Superluminescent light-emitting diodes or superluminescent diodes (SLDs) are broad-band high-intensity emission sources that emit incoherent light. Incoherent light […]

## Communication LEDs emitting at 650 nm

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 […]

## Surface-emitting communication LEDs emitting at 1300 nm

Communication LEDs emitting at 1300 nm, when used with graded-index silica fibers, are suited for high-speed data transmission. A communication LED structure emitting at 1300 nm is shown in Fig. 23.2 (Saul et al., 1985). The light is emitted through the InP substrate, which is transparent at 1300 nm. Accordingly, the device is mounted epi-side […]

## Surface-emitting Burrus-type communication LEDs emitting at 870 nm

One of the first LED structures suitable for optical fiber communication was developed by Charles Burrus of AT&T Bell Laboratories (Burrus and Miller, 1971; Saul et al., 1985). The Burrus-type LED is shown in Fig. 23.1 (a) and consists of a double heterostructure with a GaAs active region grown lattice-matched on a GaAs substrate. The […]