Q-switching is a technique used to produce high-power laser pulses. It involves preventing the laser from oscillating to allow the population inversion in the lasing medium to build up to a high level. Then, by suddenly allowing oscillation, all the stored energy is emitted in a single giant pulse with peak power much higher than during normal operation. The pulse duration is typically 10-7 to 10-8 seconds. Q-switching provides a means to drastically increase the laser power output through stimulated emission of a very large number of atoms in the active medium.

1. Q-SWITCHING FOR
GIANT PULSES
Submitted by,
KAAAVYA B
Holy Cross College,Trichy 2

2. Why Q- Switching ?
▪ For obtaining high power pulses from a laser, there are
different techniques available.
▪ They are Q-switching, cavity dumping and mode locking.
▪ Q-switching and cavity dumping techniques make large peak
powers available for a short time.
▪ Since a laser is an optical oscillator, its resonator cavity may be
characterized by the quality factor Q.
▪ Q = 2Π Energy stored in the resonator / Energy lost in a cycle.

3. ▪ Q-factor is a measure of the mirror losses.
▪ The power of a laser beam can be drastically increased, provided a very large
number of atoms in the active medium participates in stimulated emission.
▪ It can happen only when a very high population inversion density is
established in the active medium.
▪ Normally when the laser starts oscillations, high population inversion drops
back to the threshold value of the steady state condition.
▪ The laser can be prevented from oscillating if , for example , the parallelism of
the resonator mirrors is disturbed.
▪ If one of the end mirrors is misaligned, it cannot reflect incident photons
into the active medium and therefore, stimulated emission cannot
take place.

4. ▪ The pumping process can build up the population
inversion to a very high value in the medium.
▪ In effect, Q-factor of resonator is reduced and optical
losses are increased to a high value.
▪ If now the end mirror is aligned suddenly, it reflects
photons into the laser medium.
▪ The feed back of photons triggers a chain of
stimulated emissions and builds up rapidly a photon
avalanche.

5. ▪ Thus laser oscillations set in suddenly and the cavity Q
is increased abruptly.
▪ All the energy stored in the cavity is emitted in a
single giant pulse with peak power much higher than
the laser could produce normally.
▪ The pulse lasts for a short time and depopulates the
upper energy level quickly and lasing action stops.
▪ This method of controlling the laser output power is
called Q- switching method.

6. ▪The pulse duration depends on many factors
but usually is of the order of 10-7 to 10-8 sec.
▪Expression for the total number of photons φ
in the laser cavity and will find peak power.
▪Basic Laser Cavity Rate Equations.
▪Decay time constant tc = μl / c(l-R)
▪But tc = μl / c(αL – ln(R1R2)1/2 )

7. ▪The change in light intensity per unit
time within the amplifier is given by
▪dI / dt = dI/dz . dz/dt
▪I(z) = I eγz
▪dI/dt = γI
▪dz/dt = c/μ
▪dI / dt = γ (c/μ) (L/l) I

8. ▪The number of photons φ is proportional
to the light intensity I
▪dφ/ dt α dI /dt
▪The decrease in the number of photons
per unit time is given by φ /tc which is
due to the incidental resonator losses.

9. ▪Total change in the number of photons
within the cavity is
dφ /dt = φγcL /μl – φ /tc
dφ /dt = φ[γcL /μl – 1 /tc ]
where τ = 1/tc
dφ /dτ = φ[γcLtc /μl – 1 ]

10. ▪dφ /dτ = φ[γ / (μl / cLtc ) – 1 ]
▪= Φ [γ / γth – 1]
▪Where γth = μl / cLtc is the threshold gain
at which laser oscillations will be
sustained.
▪Since dφ / dτ =0 and γ = γth
▪dφ /dτ = Φ [n/ nth – 1]

11. ▪Where nth =N2 –N1 / V ---threshold
inversion.
▪Every time a photon is emitted in the
stimulated emission process, n changes
by a factor of 2.
▪dn/dτ = -2φ (n/nth)

12. ▪dφ/dn = ½ [nth/n -1]
▪On integration
▪φ-φ0 = ½ [ nthln (n/n0 ) – (n-n0 ) ]
▪Φ0 & n0 are initial values.
▪No photons initially φ0 = 0

13. ▪φ = ½ [ nthln (n/n0 ) – (n-n0 ) ]
▪For t>>tc (φ=0) -----no photons in cavity
▪Ln (nf /n0 ) = nf – n0 / nth
▪nf is the final value of population
inversion
▪nf /n0 = e[n
f
–no /n
th
]

14. ▪φ = ½ [ nthln (n/n0 ) – (n-n0 ) ]
▪For t>>tc (φ=0) -----no photons in cavity
▪Ln (nf /n0 ) = nf – n0 / nth
▪nf is the final value of population
inversion
▪nf /n0 = e[n
f
–no /n
th
]

22. Title and Content Layout with List
▪ Add your first bullet point here
▪ Add your second bullet point here
▪ Add your third bullet point here

23. Title and Content Layout with Chart
0
1
2
3
4
5
6
Category 1 Category 2 Category 3 Category 4
ChartTitle
Series 1 Series 2 Series 3

24. Two Content Layout with Table
▪ First bullet point here
▪ Second bullet point here
▪ Third bullet point here
Class Group A Group B
Class 1 82 95
Class 2 76 88
Class 3 84 90

25. Two Content Layout with SmartArt
Group A
• Task 1
• Task 2
Group B
• Task 1
• Task 2
Group C
• Task 1
▪ First bullet point here
▪ Second bullet point here
▪ Third bullet point here

26. Add a Slide Title - 1

27. Add a Slide Title - 2

28. Add a Slide Title - 3

30. Add a Slide Title - 4

31. Add a Slide Title - 5