Lasers

Build a HeNe laser

A helium–neon discharge amplifies red light a little on each pass. Put it between two mirrors that send the light back and forth, and if the gain beats the losses the light builds up into a laser beam. The lab finds the cavity, its mode and its losses, and decides whether it lases.

What you are looking at

A curved high reflector on the left, the tube, and a flat output coupler that lets 1% through. The power meter catches the output.

How the bench is set up

Spherical mirror R = 600 mm, a 280 mm HeNe tube with Brewster windows, a 99% output coupler 400 mm from the first mirror.

The theory

A two-mirror cavity of length $L$ is stable, so a beam stays inside it, when

0 < g_1 g_2 < 1, \qquad g_i = 1 - \frac{L}{R_i}.

It lases when the round-trip gain beats the losses, $e^{2g_0\ell}(1-T)(1-L_i) > 1$ . Above threshold the gain saturates until it just balances them, and the output is the coupler’s share of the circulating power.

What to measure

Lasing inside 0 < g₁g₂ < 1 and above threshold

Read the output power, about 2 mW.
Move the output coupler beyond 600 mm from the high reflector: the cavity is unstable and the laser stops.
Tip the high reflector by 100 µrad steps until it stops lasing: real HeNe alignment is that critical.

What the model leaves out

Gain and saturation follow a simple model; the discharge’s physics is not simulated.

Things to try

Change the output coupler’s reflectance and find the setting that gives the most power.
Lower the tube current until the laser drops below threshold.

References

A. E. Siegman (1986), Lasers, University Science Books.
A. Javan, W. R. Bennett and D. R. Herriott (1961), Population inversion and continuous optical maser oscillation in a gas discharge containing a He–Ne mixture, Physical Review Letters 6, 106.

More experiments, and how the lab works, are in the laser lab’s overview.