Light-emitting diode practical electronic components in 1962, the earliest LEDs emitted low-intensity infrared light. Infrared LEDs are still frequently used as transmitting elements in remote-control circuits, such as those in remote controls for a wide variety of consumer electronics. The first visible-light LEDs were of low intensity and limited to red. Modern LEDs are available across the visible, ultraviolet, and infrared wavelengths, with very high brightness. Early LEDs were often used as indicator lamps for electronic devices, replacing small incandescent bulbs. They were soon packaged into numeric readouts in the form of seven-segment displays and were commonly seen in digital clocks. Recent
Laser diode frequency doubling of near-IR VECSEL emitters to produce blue and green light. External-cavity diode lasers are tunable lasers which use mainly double heterostructures diodes of the AlGaAs type. The first external-cavity diode lasers used intracavity etalons and simple tuning Littrow gratings. Other designs include gratings in grazing-incidence configuration and multiple-prism grating configurations. Laser diodes have the same reliability and failure issues as light emitting diodes. In addition they are subject to "catastrophic optical damage" (COD) when operated at higher power. Many of the advances in reliability of diode lasers in the last 20 years remain proprietary to their developers. The