What is Laser cavity?

Example 23.11 (The helium–neon laser)

A glass tube $30\,\mathrm{cm}$ long, bore $1\,\mathrm{mm}$, holding helium and neon at about a thousandth of an atmosphere, crossed by a discharge of a few milliamperes. Electrons excite helium to a metastable level at $20.6\,\mathrm{eV}$, which hands its energy by collision to a neon level at almost exactly the same height; from there neon decays to a lower level (emptying fast to the ground state — four-level) by emitting at $632.8\,\mathrm{nm}$. The small-signal gain is tiny, a few per cent per pass, so the mirrors must be excellent ($R_1 = 0.999$, $R_2 = 0.99$) and the tube clean; the gain line is Doppler-broadened to $\Delta\nu \approx 1.5\,\mathrm{GHz}$, the modes are $c/2L = 500\,\mathrm{MHz}$ apart, so two or three modes oscillate at once. Output $1\,\mathrm{mW}$ for several watts of discharge: efficiency $10^{-4}$; but a wavelength defined to $10^{-6}$ and a beam that stays $1\,\mathrm{mm}$ wide across the laboratory.

Example 23.14 (Cutting, reading, pointing)

A $1\,\mathrm{kW}$ carbon-dioxide laser at $10.6\,\text{µ}\mathrm{m}$, beam radius $10\,\mathrm{mm}$, focused by $f = 100\,\mathrm{mm}$: $w_0' = 34\,\text{µ}\mathrm{m}$, peak irradiance $2P/\pi w_0'^2 = 5 \times 10^{11}\,\mathrm{W}/\mathrm{m}^{2}$ — steel boils. A $1\,\mathrm{mW}$ pointer, $w_0 = 0.5\,\mathrm{mm}$ at $633\,\mathrm{nm}$: $\theta = 0.4\,\mathrm{mrad}$, a $4\,\mathrm{cm}$ spot at $50\,\mathrm{m}$, an irradiance of $2.5\,\mathrm{kW}/\mathrm{m}^{2}$ at the exit — above sunlight, which is why even a milliwatt must never enter an eye: the eye’s lens would focus it to a $10\,\text{µ}\mathrm{m}$ spot on the retina at millions of watts per square metre. A disc reader at $650\,\mathrm{nm}$ with a lens of numerical aperture $0.6$ focuses to about $\lambda/2\mathrm{NA} \approx 0.5\,\text{µ}\mathrm{m}$, the size of a pit.