L10-11

Question 1

Methods of q switiching

Answer 1

Rotating mirrors
Electro optical switches : Pockel and Kerr cell
(Birefringence induce with applied voltage,
Polariser send Light into Kerr cell lin pol -> circ pol reflects back to Kerr becomes lin pol orth and cannot pass back through polarised )

Saturable absorberes

Question 2

Idea behind q switching

Answer 2

Use lossy cavity (low q)
- NO field built up due to stim emission even though N2 is pumped to a high level
GAIN CANNOT BE SATURATED BUT WE MAKE SMALL SIGNAL VALUE G0 VERY LARGE

High q cavity switch
-Permit laser oscillation
-g0&raquo_space; gt
- I grows rapidly large stim emission rate

Question 3

Oscill and num modes

Answer 3

Oscill occurs on many longitudinal modes
esp when the gain medium is pumped far about threshold
many modes under gain curve will then meet the threshold cond

Question 4

Gain curve

Answer 4

Only if the curve is above the loss line on y axis, can theat mode actually lase

so more modes above thres - > more output bands of laser (brpader?)

Question 5

MOde locking motivation

Answer 5

Want pulses shorter than q switching

Question 6

Phase locking

Answer 6

N identical HOsc with same aplitudes x- and phases phi0
separated by same distances

As N increased the spikes get shorter in time and sharper (I)

Light coming out is a train of pulses separated by fixed time, each pulse same length

Question 7

Mode locking

Question 8

What determines the number of longitudinal modes that can simultaneously lase

Answer 8

THe gain linewidth FWHP Delta V_g
freq separation c/2L between modes

Question 9

Shortest pulse duration we can acieve by mode locking

Answer 9

Approx reciprocal of the gain line width
1/DetlaV_g

Question 10

AM modelocking vs FM

Answer 10

am:Amplidue is periodically modulated
FM: Phase is ‘’

Question 11

FM locking wm

Answer 11

Carrier freq - Central band

Question 12

FM locking side bands
what is Omega

Answer 12

Generated at frew w = wm +- Omega
Fm = whole series of pairs of sidebands
Omega is the modulation frequency

This also happens in Am locking

Question 13

what happens with freq mode sep in FM locking

Answer 13

Omega = pi c/L so that sidebands are ub resonance with carrier frequencies of other modes

Results in stong ocupling of modes (cause them to lock togehter)
Produce mode locked train of pulses

Question 14

AM locking vs FM

Answer 14

Both result in locking / strong coupling

AM: Produce one side band either side of wm
Fm: “ whole series fo pairs of sidebands

Question 15

Short resonator

Answer 15

deltanu = c/2L spacing
mode spacing is very far apart since L is small
If gain width is narrow enough, you cn have single mode oscillation