Review of Last Class

1. Review of Last Class • Mode-locking is to “lock” all longitude modes into same phase • Mode-locking is the preferred method to generate ultra-short laser pulses

2. Review of Last Class • The pulse width in a mode-locked laser is determined by the band-width • The periodicity of the laser is determined by the cavity length T = 2L / c

3. Review of Last Class • Mode-locking condition is achieved by create cavity loss for un-pulsed lasing condition • Special case: Kerr lens effect is very convenient for select the mode-locking condition

4. Review of Last Class • The pulse traveling speed is called group velocity, which depends on the medium the light is traveling in • Vg is usually smaller than c, but can be larger than c or negative in “anomalous dispersion”

5. Review of Last Class • Group velocity can be different for different wavelength. This is called group velocity dispersion (GVD). GVD causes “chirp” in laser pulse that broadens it. • GVD is usually positive.

6. w w Lsep Review of Last Class • A prism pair can generate negative GVD and be used to compensate the normal positive GVD of glass medium.

7. Review of Last Class • Nonlinear optical response is due to the non-linear relationship between polarization and electrical field. P = (1) E + (2)E2 + (3)E3 + 

8. Measuring Pulsed Laser Optical detector can measure a time-averaged second power of the electric field from the light. Signal ~ Autocorrelation signal:

9. Optical Autocorrelation

10. Interferometric autocorrelation