Ruby laser
1. Overview — First Laser (Maiman 1960)
Ruby laser = solid-state, three-level, pulsed laser with active medium Cr³+ in Al₂O₃ (ruby crystal, pink, ~0.05% Cr). Lasing wavelength 694.3 nm (red, doublet 692.9 nm weaker), pulsed via optical pumping by flashlamp.
Cylindrical ruby rod (≈10 cm long, 1 cm dia) with ends polished plane-parallel and silvered (M1 100%, M2 ~90% or external mirrors)
Helical xenon flashlamp surrounds rod inside reflecting cavity (elliptical)
Cooling (water/air) and power supply for flash.
4. Operation
Flashlamp fires ~ms pulse → pump to E3 → rapid decay to E2 → inversion → spontaneous photons along axis amplified between mirrors → gain > loss ⇒ stimulated avalanche → pulse of coherent red light out of partial mirror.
Pulsed: flashlamp cannot sustain CW (heat + ground-level inversion hard). Q-switch can give ns giant pulses (~MW).
5. Characteristics
- Wavelength 694 nm, pulsed (µs–ms, Q-switched ns), energy ~J per pulse (early ~1 J), linewidth ~0.5 nm (multimode), beam divergence ~mrad.
- Efficiency low (<1% optical to optical).
6. Applications
Holography, rangefinding (early), tattoo removal (Q-switched ruby), pumping dye lasers.
7. Exam Points
- Sketch construction and three-level diagram — 7 marks, very frequent (almost certain).
- Explain why three-level needs high pump, pulsed — 5 marks, very frequent.
- Wavelength 694 nm and transition — 3 marks.
- Drawback (three-level) — 3 marks.