Transcript Review of Last Class - University of Connecticut Health Center

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
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
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
Review of Last Class
• The pulse traveling speed is called group
velocity, which depends on the medium
the light is traveling in
v g   dk / d  
1
• Vg is usually smaller than c, but can be
larger than c or negative in “anomalous
dispersion”
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.
03 d 2 n
GVD  k (0 ) 
2 c02 d 02
• GVD is usually positive.
Review of Last Class
• A prism pair can generate negative GVD
and be used to compensate the normal
positive GVD of glass medium.

Lsep

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 + 
Measuring Pulsed Laser
Optical detector can measure a timeaveraged second power of the electric
field from the light.
Signal ~
 E (t )
Autocorrelation signal:
2
dt
Optical Autocorrelation
Interferometric autocorrelation