Lasers and Photonics
Lasers and Photonics
This domain tests laser gain, oscillation conditions, beam characterization, pulsed operation, and photonic detection. You should connect atomic transitions to measurable beam properties.
Core rules
- Population inversion is required for net stimulated emission; threshold depends on gain, loss, and resonator feedback.
- Resonator modes are set by cavity length and geometry; longitudinal mode spacing is approximately (c/(2nL)).
- Gaussian beam waist and Rayleigh range: (z_R=\pi w_0^2/\lambda); beam area doubles over one Rayleigh range.
- (M^2) compares a real beam to an ideal Gaussian; (M^2=1) is diffraction-limited.
- Q-switching stores energy and releases short high-peak-power pulses; mode-locking produces a pulse train with repetition rate tied to cavity round-trip time.
Worked example
Find the photon energy of a 532 nm green laser. Use (E=hc/\lambda), or the convenient form (E(\text{eV})=1.240/\lambda(\mu m)). Here (\lambda=0.532,\mu m), so (E=1.240/0.532\approx2.33,\text{eV}). A 1 W continuous beam at this wavelength delivers about (2.8\times10^{18}) photons per second, since each photon carries (2.33\times1.602\times10^{-19}\approx3.73\times10^{-19}) J.
Traps
Average power and peak power are not interchangeable for pulsed lasers. Wavelength must be in micrometres for the 1.240 eV·µm shortcut. Do not apply Gaussian-beam formulas to highly multimode beams without (M^2). Threshold is not the same as slope efficiency.
Exam focus
Check whether the question asks for energy, power, fluence, or irradiance before calculating.