Lecture 38 Lasers Final Exam next week

LASER L ight A mplification by S timulated E mission of R adiation

Electronic Energy Levels Energy e-e- e-e- Fluorescence or ‘Spontaneous Emission’ e-e- e-e-

Stimulated Emission Energy e-e- e-e- The stimulated emission has the same frequency and phase as the incoming photon

3 level system e-e- ‘Short’ lifetime ‘long’ lifetime

Stimulated Emission Light Amplifier Pump (does not need to be light) Gain medium can be gas, liquid, or solid All light is in phase and has the same frequency

Laser Cavity Once the amount of gain is equal to the cavity losses, ‘lasing’ happens Losses can include output coupler, reflectivity, other optics in cavity

Cavity modes The mirrors force the amplitude of the wave to zero. Only integral number of half wavelengths can exist. Every wave exits in phase

Allowed frequencies It is a combination to the energy spread and the cavity modes

Laser pulse generation PC “Q-switching”, ns pulses “Mode-locking”, fs pulses Cavity length ~1.5m Gain medium Q= 2  *Energy stored Energy Lost per cycle

Transverse modes

Transverse Modes The 00 mode is the smallest….

Types of lasers Gas: HeNe Solid-state: Semi-conductor Diode, Crystals (sapphire), nano-particles Liquid: Dyes, Chemical laser: CO 2, Excimer