Laser Diodes and LEDs 13 Example 1.1 Consider an LED emitting at l ¼ 1550 nm wavelength window. The inter- nal quantum efficiency is 70%, the external efficiency is 2%, the carrier lifetime is t ¼ 20 ns, and the injection current is 20 mA. Find: 1. The output optical power of the LED 2. The 3 dB optical bandwidth and the required driver electrical bandwidth Solution: 1. Output optical power is: P opt ¼ q ext hc 6:63 Â 10 À34 Â 3 Â 10 8 Â 20 Â 10 À3 ¼ 0:225 mW I ¼ 0:7 Â 0:02 Â lq 1550 Â 10 À9 Â 1:6 Â 10 À19 2. To find the 3 dB optical bandwidth, we use: P opt ð 0 Þ P opt ð o Þ ¼ p ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þ o 2 t 2 À Á ' & P opt o opt ¼ À3 dB, that is: For 3dB optical bandwidth 10 log