The Helium-Neon Laser

A HeNe supply runs at 1.2 to 2 kV open-circuit with a ballast resistor in series with the tube. Treat it as lethal. Wire it with the mains OFF, keep the anode lead short and insulated, and never adjust the tube with the supply live. Mount the tube on the optical bench with the beam at a fixed height, aimed at a card 2 m away and well below eye level. Wear the safety glasses. Even a 1 mW beam is collimated: the whole milliwatt lands on one retinal spot. Switch on. The bore should glow orange-pink along its length and a red spot should appear on the card within a few seconds.

WAVELENGTH. Put the diffraction grating sheet in the beam and measure the first-order spot separation on a wall at a measured distance. With 1000 lines/mm, d = 1 micrometre and lambda = d sin(theta). You should land within a nanometre or two of 632.8 nm. POLARISATION. Rotate the polarising filter sheet in the beam while watching the spot. A tube with Brewster-angle windows is strongly linearly polarised and the spot will extinguish almost completely; an internal-mirror tube will only flicker as modes hop. Record which kind you have. DIVERGENCE. Measure the spot diameter at 1 m and at 5 m with the caliper against a card. Full-angle divergence is the difference over the distance. Expect roughly 1 milliradian, which is the diffraction limit for a beam a millimetre across.

Put the polarising filter in the beam, then the photodiode behind it, and connect the photodiode to the oscilloscope on a very slow timebase or to the multimeter logging every ten seconds. Switch the tube on cold and record for fifteen minutes. The power will rise, then oscillate up and down over tens of seconds, then settle. That oscillation is the cavity comb sliding under the gain curve as the glass expands: modes leave one side and appear on the other. In an internal-mirror tube adjacent modes are usually orthogonally polarised, so the polariser turns each hop into an amplitude change. Count the cycles. That count, against the temperature rise, measures the tube's thermal expansion through nothing but its own light.

HOW THE ENERGY ACTUALLY GETS TO NEON. The discharge is mostly helium — typically five to ten parts helium to one of neon, at a total pressure of a couple of torr. Electrons excite helium into its 2 3S and 2 1S metastable states, which cannot decay radiatively and so survive long enough to collide. Those two states sit within a few thousandths of an electronvolt of neon's 2s and 3s manifolds, so a collision transfers the energy resonantly. Neon is the laser; helium is the delivery service. WHY IT RUNS CONTINUOUSLY. The 632.8 nm transition ends on the 2p manifold, which is empty to begin with and drains to the 1s levels in tens of nanoseconds. There is no ground state to clear. That is the four-level advantage, and it is the entire reason this laser sips power while ruby gulps it. BREWSTER WINDOWS. A window tilted so the beam hits it at Brewster's angle transmits one polarisation with essentially no reflection loss and rejects a few percent of the other. A few percent per pass, hundreds of passes: the rejected polarisation never reaches threshold. That is why a Brewster-window tube is polarised, and it is a direct application of the polarisation blueprint this one links to. THE SIBLING TABLE. Ruby: three-level, solid, flashlamp-pumped, joules per pulse, megawatt peaks, cannot run continuously, but STORES energy. Helium-neon: four-level, gas, discharge-pumped, milliwatts continuous, superb beam and coherence, but stores nothing at all. Same cavity physics, opposite economics. Which one you want depends entirely on whether you need joules or a straight line. WHY IT CANNOT BE MADE ON A BENCH. Sealing mirrors to glass at Brewster's angle, pumping to high vacuum, baking out the tube, filling to a couple of torr with a precise He:Ne ratio and getting a life of thousands of hours needs a glass lathe and a vacuum station. The vacuum-pump blueprint linked here is the first step of that road, not the whole of it.