emitting photons in all directions in the process. Those photons are in essence “noise”
while the stimulated emission photons are “the signal.” The spontaneous emission
occurs after about 10í^8 seconds, ten thousand or so times faster than it occurs with
metastable states.
This rapid transition is important. It means that not many neon atoms linger in the EC
state. They quickly return to the E 0 state where collisions with helium atoms can excite
them back to EB, and the lasing process can begin anew. And there is another reason
why it is important that few atoms have energy EC: If there were too many of these
around, then they would absorb photons and return to energy EB, which would thwart
the lasing process. (Neon atoms at E 0 cannot absorb photons and rise to either EB or
EC, since the required energy differences do not match the energy of the photons being
used.)
He-Ne lasers are inexpensive and common. The laser tubes can be purchased for less
than $100. These may yield a power output of 1.0 mW when connected to a DC power
input of 10 W. This means they are about 0.01% efficient. Such lasers have been used
for purposes ranging from supermarket scanners to laser printers, but semiconductor
lasers, which are cheaper to fabricate, are rapidly superceding them.
Laser process: part 2
Photons enter laser medium
Stimulated emission occursNeon returns to initial state
·light is amplified
36.21 - Sample problem: ruby laser
Theodore Maimann built the first operational laser in 1960, a flash-pumped ruby laser. Maimann placed a cylindrical ruby rod inside a flash
pump. The flash pump excited the chromium atoms in the ruby to a metastable state using an intense burst of light. Then the ions fluoresced
back to the ground state, releasing light of wavelength 694.3 nm. Carefully aligned mirrors were placed at either end of the ruby to produce a
coherent laser oscillation.
Variables
What is the strategy?
- Calculate the energy of a photon of the light emitted from a ruby laser.
- Use the energy of the light to calculate the energy level difference between E 2 and E 1.
Physics principles and equations
Energy of a photon
E = hf
Energy level difference
ǻE = EfíEi
The light emitted by a ruby laser has
wavelength 694.3 nm, and ruby emits
this light when electrons drop from
energy level E 2 to E 1 in the ruby’s
chromium atoms.
What is the energy level difference
betweenE 2 and E 1 in a chromium
atom?
energy E
wavelength λ =694.3 nm