The Ruby Laser

Work in a darkened room. Mount the synthetic ruby rod on an optical bench carrier and aim a 405 nm violet module at its side, not its end. Wear the safety glasses: 405 nm is poorly visible and the eye's blink reflex does not protect against it. The rod will glow deep red. That is not scattered violet light — it is Cr3+ ions absorbing in the violet and blue-green bands, dropping non-radiatively into the metastable level, and emitting at 694.3 nm. Move the pump around the rod and note that the red glow follows the illuminated volume. In a laser this volume is the whole rod, which is why Maiman's flashlamp was a helix wrapped around it rather than a lamp pointed at it.

Drive the 405 nm module from the function generator with a 20 Hz square wave so it switches hard off. Put the photodiode against the rod behind a piece of red filter or exposed film that blocks the violet, and watch the decay on the oscilloscope. Set the timebase to 2 ms per division. The fluorescence does not vanish when the pump stops — it decays over roughly ten milliseconds. Capture the trace and read off values every 0.2 ms for the fit in the next step. This decay IS the invention. A dye that fluoresces for nanoseconds cannot store anything; the pump energy is gone before the next ion is excited. Three milliseconds lets a millisecond flash accumulate.

WHAT MAIMAN ACTUALLY USED. A synthetic ruby rod roughly 1 cm across and 2 cm long, both ends ground flat and parallel and silvered — the cavity was evaporated onto the rod itself, with one end left partly transmitting. Around it a General Electric FT-506 helical xenon photographic flashlamp. Behind that a polished aluminium cylinder to send stray light back in. The output was a burst of 694.3 nm light lasting under a millisecond. MODERN EQUIVALENTS, CLEARLY LABELLED AS DERIVED. Verneuil-grown boules and Czochralski pulling both produce laser-grade ruby today; the growth blueprint this one links to is the same process family. Dielectric mirror coatings replaced evaporated silver and cut the round-trip loss by more than an order of magnitude. The flashlamp would now be driven by a pulse-forming network rather than a bare capacitor. THE HIGH-VOLTAGE REALITY, STATED PLAINLY. A ruby laser flash needs of the order of a kilojoule at over a kilovolt. A charged bank of that size stays lethal after the supply is switched off and must be shorted through a bleeder before anyone touches it. This is not a scale-it-down-and-try-it project; it is supervised-laboratory equipment. The measurements in steps 1 to 4 give you the physics honestly and safely. WHY IT MATTERED ANYWAY. Ruby is a bad continuous laser and a superb energy store. That same three-level structure that makes it hard to start makes it hold a large inversion, and holding an inversion is precisely what the next-but-one blueprint, Q-switching, exists to exploit.