Stimulated Emission and How a Laser Works · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

How a laser builds its beam

Science · Waves, Light & Sound · ages 21-22
Name ______________________   Date ____________

  1. Which statement describes spontaneous emission?

    • Emission needs an incoming beam to schedule each decay.
    • Emission needs no incoming beam; each decay has probabilistic timing.
    • Emission needs a pump to set the phase of each photon before it can leave.

  2. For a coherent incident field, which set of properties does light added by stimulated emission match?

    • The frequency, direction, polarization and field phase all match.
    • The frequency and direction match, but each photon has twice the energy.
    • The direction and phase match, but polarization reverses.

  3. A laser can have a narrow but nonzero frequency spread and finite coherence.

    Circle one:   True   False

  4. In the three-level scheme taught here, which path lets atoms build up in the upper laser level?

    • Pump higher, decay fast to a long-lived upper laser level, then emit to ground.
    • Pump higher, decay fast to ground, then stop pumping so all atoms remain in the lower laser level.
    • Pump higher, keep all atoms in the pump level, then emit while bypassing the upper laser level.

  5. For two laser levels with equal statistical weights and 100 atoms in total, which distribution gives net stimulated gain before other losses?

    • 40 atoms in the upper level; 60 in the lower
    • 60 atoms in the upper level; 40 in the lower
    • 50 atoms in the upper level; 50 in the lower

  6. What is the role of the mirrors in the two-mirror laser described here?

    • Supply pump energy to the gain medium and schedule each atom's emission.
    • Double the light on every pass and eliminate all losses inside the gain medium.
    • Provide feedback, favor resonant modes and transmit some output light.

  7. A bright ordinary lamp and a dim laser are compared. Which property tests coherence, rather than intensity?

    • Stability of their light-field phase relationships.
    • Optical energy delivered to each unit of area during each second.
    • Electrical energy consumed by each source in each second.

  8. At startup, an inverted medium gives an intensity gain factor of 1.20 per round trip. Other losses retain 0.75 of that intensity. How does a small circulating signal change?

    • The intensity becomes 1.60 times its previous value each round trip.
    • The intensity becomes 0.90 times its previous value each round trip.
    • The intensity becomes 1.00 times its previous value each round trip.
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Answer key

For grown-ups. Fold this page away before handing over the rest.

How a laser builds its beam W1-mt_xgjD0mzEAV-s1

  1. Emission needs no incoming beam; each decay has probabilistic timing. · An excited atom can emit without an incoming beam. The time of a particular spontaneous decay is probabilistic.
  2. The frequency, direction, polarization and field phase all match. · Stimulated emission adds light to the occupied optical mode. For the coherent field described here, the added light also has a matching field phase.
  3. True · Laser output need not have exactly one frequency or unlimited phase stability. A narrow frequency spread and finite coherence are compatible with lasing.
  4. Pump higher, decay fast to a long-lived upper laser level, then emit to ground. · Fast transfer from the pump level feeds a longer-lived upper laser level. Atoms can accumulate there, and the laser transition returns them to ground.
  5. 60 atoms in the upper level; 40 in the lower · With equal statistical weights, compare the populations directly. The 60 upper-level atoms outnumber the 40 lower-level atoms, giving net stimulated gain.
  6. Provide feedback, favor resonant modes and transmit some output light. · The mirrors return light through the gain medium and favor resonant cavity modes. The output coupler transmits part of the circulating light.
  7. Stability of their light-field phase relationships. · Coherence concerns predictable field phase relationships. Optical intensity measures energy arriving per unit area per second; electrical consumption measures the source's input.
  8. The intensity becomes 0.90 times its previous value each round trip. · Multiply gain by retention: 1.20 × 0.75 = 0.90. A net factor below 1 makes the small circulating signal fade despite population inversion.
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