Laser Diodes

Quick Answer

A laser diode starts as the same kind of light-emitting junction as an LED, with an optical cavity built around it. Below a threshold current it emits like an ordinary LED. Above that current, light in the cavity provokes more light identical to itself, and the output turns into a narrow coherent beam that climbs steeply with drive.

An LED makes a photon each time a carrier crosses the junction and settles, and every one leaves on its own account: its own moment, its own direction, its own phase. Put two mirrors facing each other across the junction and they stop being independent. A photon passing a carrier that has not yet settled can provoke it into settling now, and the one that comes out matches the one that caused it in wavelength, direction and phase. That is stimulated emission, and the mirrors are what give each photon enough passes to make it happen. In a laser diode they are usually the crystal's own cleaved end faces, and the layer between them guides light along the junction, so the structure carrying the current is the cavity too.

Everything then turns on one number. Once round the cavity, light is amplified by the carriers it meets and lost to absorption, to scattering, and to the fraction leaving through the end faces. At low current the loss wins, the junction emits spontaneously, and what you have is an expensive LED. Raise the current until a round trip's gain equals its loss and the balance tips: further current goes into stimulated light, and the output climbs on a far steeper slope. That current is the threshold, the first number to know about a part.

The knee there makes the drive a design problem rather than a detail. Take a part reaching threshold at 30 mA and rated to 60 mA, figures invented for this entry rather than read off a datasheet: the useful range is thirty milliamps wide, and an error that would be trivial on an indicator LED is most of it. Threshold also moves, climbing as the part warms, so a laser on fixed current fades as it heats and one set up on a warm bench overdrives on a cold morning. The drive is therefore neither a voltage nor, usually, a plain fixed current. A monitor photodiode in the same package watches the back facet, and a loop trims the current to hold the light steady while threshold wanders underneath.

The beam differs in kind too. The emitting region is tiny and a small aperture diffracts, so light from a bare die spreads quickly and unevenly, faster across the thin guiding layer than along it. What is genuinely narrow is the spectrum and the phase, and that coherence is what an LED cannot supply at any price.