Pockels cell driver: Optimization of output energy of Q-switched Nd:YAG laser based on switching time

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Nov 2, 2013 (4 years and 9 days ago)

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Persidangan Fizik Kebangsaan

(

2009
)
,
Avillion Legacy Hotel, Malacca, Malaysia7
th

-

9
th


December 2009

Pockels cell driver:
Optimization of
output

energy of

Q
-
switched
Nd:YAG laser

based on

switching time


*
ABD RAHMAN

Tamuri,
YAACOB

Mat Daud &
NORIAH

Bidin


Laser Technology Laboratory,

Physics Department, Faculty of Science,

Universiti Teknologi Malaysia, 8
1310, Skudai,

Johor Bharu, Johor, Malaysia
.


Tel:
+607
-
5534096
Fax
: +6075566162
Email
:

rahman_t@hotmail.com



Abstract:

This

paper
reports the optimization of output Q
-
switch Nd:YAG
.
A free running Nd:YAG laser
was employed as source

of light
.
A
K
D*P crystal was utilized as a
Pockels

cell.
Avalanche

transistor pulser
was design
ed

to switch
a

high voltage power supply
.

The switching time was conducted via a control unit
based PIC16F84A

microcontroller. The pulser was able to
switch the voltage withi
n 3 ns
. The optimum
switching time of Q
-
switching is obtained
at

182.34 µs
.

T
he
corresponding
laser output
is
40 mJ with pulse
duration of 25 ns.


Keyword:
Pockels cell, KD*P crystal, Q
-
switch,
control unit
, high voltage switching, switching

time
.



1.0

Intr
oduction


Q
-
switched laser can be produced by using electro
-
optic mechanism.
Pockels

cell
effect is used to modulate or compress the beam.
The loss in modulation

beam

will be
manipulated to produce powerful Q
-
switched laser
.

In
a

Q
-
switch
laser
system, the

time
delay between flashlamp trigger and Pockels Cell (PC) switching is very important.
Research related in this project had been reported by
Dharmadhikari

et al
.
,

(1998)

[1]

,
Koechner
, (2006)

[2]

, Chadderton,

(1996)

[3]

, Fulkerson

& Box, (1994)

[4]

,
Alton &
Sundararajan
, (2004)

[5]

,
Fulkerson
et at
., (1997)
[6]

,
Molina

et al
.,

(2001)

[7]

,
Salvestrini

et al
.,
(2004)
[8]

. Similar works also reported in other texts
[9
,

10
, & 11
]
.
Sequential time for
generation of a Q
-
switched pulse laser is an importa
nt parameter.
Theoretically
Q
-
switched occurs
t
owards the end of flashlamp pulse, when the inversion has reached its
peaks. At this point, a photon flux starts to build in the cavity and a Q
-
switched pulsed i
s
emitted. The emission of the Q
-
switched laser
does not occur until after an appreciable
delay between flashlamp pulse and Q
-
switched

is achieved
.
In this present paper the
optimization of
Q
-
switched Nd:YAG laser output is discussed based on the performance
of switching
time
.





2
.0

Experimental setu
p


A free running pulsed Nd:YAG laser was employed as a source of light to be
modulated. The pulse duration of the free running laser is 120

s.
The
optical pumping
flashlamp was powered by capacitor charging voltage of
650 V with single pulse
Persidangan Fizik Kebangsaan

(

2009
)
,
Avillion Legacy Hotel, Malacca, Malaysia7
th

-

9
th


December 2009

operation
.
T
he laser head comprises of
a
plano
-
concave
optical

resonator with 50 cm
length.

A n
onlinear

optic KD*P crystal was employed as Pockel
s cell. The dimension of
the P
ockel
s

cell
was

10


10


10 mm
3
. The cell was provided with adjustable DC high
voltage powe
r supply with maximum voltage of 4.5 kV. The quarter

wave voltage of the
crystal is
normally
greater than 3.3 kV
[2]

.

The
Pockels

cell
was

interposed in the laser
resonator associated with a thin film polarizer.

The
optical and electrical
experimental se
tup is shown in Figure
1
.
The voltage
supplied into the cell was measured via a digital oscilloscope Tektronix 3054B with
bandwidth of 500 MHz and

sampling rate of 1 GHz/ Sample
. The operation of both
Pockels cell driver and FLD was controlled
via

a
contro
l unit.
A silicon photodiode was
employed to detect the optical signals and coupled to the oscilloscope. A beam splitter
was conducted to split the beam into two; 90% of the beam goes to
the
Melles Griot
power meter for energy measurement, and 10% goes to
the
photodiode for signal
detection
. The control unit and HV switcher were also connected to the oscilloscope. The
electrical signals from both units were used to determine the time delay or switching
time.





Figure
1
:
Experi
mental setup of Pockels cell driver for solid state laser system


3.0



Results and Discussion


Typical result obtained from this experiment is shown in Figure 2.
Figure
2

shows
the
sequential
signal
s

obtained during
Q
-
switch in solid state laser

operation
.
T
hree
signals
are

displayed

simultaneously
.
Initially, the flashlamp signal is appeared
immediately after been triggered by control unit. As a result the first signal been detected
by the photodiode is the signal of flashlamp. A square waveform
with
50


s

i
s displayed
at the channel 4 on the screen of the oscilloscope. The middle signal in Figure 2 is the
flashlamp square waveform. The top signal
of

channel 2 is the signal
for

DC high voltage
Control

Unit

Pockel
Cell

Avalanche
transistor

HV DC Power
Supply

Photo
-
detector

Power


Meter

Output
coupler

Trigger signal from
flashlamp driver

Tektronix
Oscilloscope

Rear mirror

Polarizer

Laser
chambe
r

FLD

Unit

Pockel
s cell driver


HV probe

Persidangan Fizik Kebangsaan

(

2009
)
,
Avillion Legacy Hotel, Malacca, Malaysia7
th

-

9
th


December 2009

drop to ground level.
The voltage drop
s

to ground level, within 3
ns.
The signal
,

then rise
up within 90

s time. The bottom signal
of

channel 3 is displayed the signal from
Nd:YAG laser output. The signal appears
coincide

with the voltage drop or electric field
is off from the KDP crystal. At this moment, the light
is a
llowed to be

amplified in the
resonator.
After enough energy, some of them will be
transmitted
through

the output
coupler as a laser beam.
The delay time

or the switching time

is

the deduction
between
the trigger signal of FLD and the high voltage switch.
By adjusting the control unit, the
delay time between
both signals

can
be
easily manipulated.

Figure
3

shows the output energy
Q
-
switched laser
distribution for delay time
between 100 µs to 230 µs.
T
his time delay
is
called as
an

“optimum time delay”
.

An

a
ppropriate
time
delay for Q
-
switching is

expected to be
within 5 % of the optimum
delay

[1]
.

In this particular experiment
the
time delay
to optimize the production

of
Q
-
switch
ed laser beam is realized to be
in the range between 175 µs
to

195 µs.
The
maxim
um output energy occurs at

a

time delay of 184 µs.









The maximum output energy of Q
-
switched Nd:
YAG laser pulse

for this
particular laser is obtained as
40 mJ. The pulse duration of Q
-
switched laser was
measured using a silicon photodiod
e. From the experiment the pulse duration
of Q
-
switched beam is

measured to be

25 ns.
The pulse duration is greater than the usually Q
-
switched that is 10 ns. The reason for such
length
y

duration is
possibly due to the piezo
-
electric effect in the KD*P cry
stal
, the speed of
switching
voltage and
un
-
appropriate

ha
lf
-
wave or
quarter
-
wave voltage
supplied to the crystal.


4
.0


Conclusion


A free running Nd:YAG laser
was

modulated to produce Q
-
switched laser.
Adjustable
h
igh voltage power supply
was

provided
into
Pockel
s

cell with maximum
voltage of 4.5 kV. Av
a
lanche transistor pulser
was

designed to switch the high voltage
within 3 ns. The time delay between the FLD and the voltage switch
was

manipulated to
achieve the optimum output.
The optimum time delay
was

obtained at

184 µs with the
HV switch

Flashlamp
trigger

Laser output

Delay


tim
e

Figure 2:

The sequential events during Q
-
switch
ing

process.

Figure 3:

Distribution of output
Q
-
switched
laser based
on time delay

Persidangan Fizik Kebangsaan

(

2009
)
,
Avillion Legacy Hotel, Malacca, Malaysia7
th

-

9
th


December 2009

corresponding
maximum energy of 40 mJ. The
pulse

duration of Q
-
switched Nd:YAG
laser
was

25 ns.



5.0



Acknowledgement


The authors would like to thank to government of Malaysia
through IRPA grant
vot
e 74531
for
the
financial support and to Universiti Teknologi Malaysia

through RMC

for
management and performance of

th
e

project.


6
.0


References

[1]

J.A. Dharmadhikari, A.K. Dharmadhikari, N.Y. Mehendale & R. C. Aiyer,

(1998)

Technical Note:
Low cost
Pockels cell driver for pulsed solid state lasers,
Optics & Laser Technology

30
,
447
-
450,

[2]

W. Koechner,
(
2006
)
, “
Solid State Laser Engineering
”, 6th Edition, United State of America,
Springer Science
-
Business Media, pg 514
-
516
.

[3]

N. Chadderton,
(
1996
)
, “
The ZTX 415 Avalanche Mode Transistor: An Introduction to
Characteristics, Performance and Application
”, Application Note, Issue 2

[4]

E. S. Fulkerson & R. Box,
(
1994
)


Design of reliable high voltage avalanche transistor pulsers
”,
Lawrence Livermore Na
tional Laboratory,
Power

Modulator Symposium 1994, Conference Record
of the 1994 Twenty
-
First International, p 101
-
103

[5]

C. Alton & R. Sundararajan, (2004), “Simple MOSFET
-
Based High
-
Voltage Nanosecond Pulse
Circuit”,
IEEE Transactions on Plasma Science
,

Vol. 32
(
5
)
, pp. 1919
-
1924
.

[6]

E. S. Fulkerson, D. C. Norman & R. Booth, (1997), “Driving Pockels Cell Using Avalanche
Transistor Pulsers”, 11
th

IEEE International Pulse Power Conference
, Baltimore, Maryland.

Vol.2

1341
-

1346

[7]

Molina, L.L., Mar, A. Z
utavern, F.J. Loubriel, G.M. O'Malley, M.W, (2001),

Sub
-
nanosecond
avalanche transistor drivers for low impedance pulsed power applications,
Pulsed Power Plasma
Science, 2001
,

PPPS
-
2001. Digest of Technical Papers
, Vol
.

1
:

178
-

181
.


[8]

J.P Salvestrini, M

Abarkan, M D Fontana, (2004),
C
omparative study of nonlinear optical crystals
for electro
-
optic Q
-
switching of laser resonators.

Opt. Mater. 26, 449
-
458.

[9]

A. R. Tamuri
,

N. Bidin, and Y. M. Daud
,

(2008),


Quality of the Beam Produced a Pulsed Nd:YAG
La
se
r

”,
Laser Physics,

Vol. 18,
(1)
,
p
18

21.

[10]

Abd Rahman Tamuri, Noriah Bidin & Yaacob Ma
t

Daud, (2009), Nanoseconds switching for high
voltage circuit using Avalanche transistors, Applied Physics Research,
Vol.
1
(2), p
25
-
2
9.

[
11]

Noriah Bidin,
Abd Ra
hman Tamuri
, Yaacob Mat Daud,(2008),
High Voltage Avalanche Transistor
Pulser System
, Pat App. in Malaysia No
20085164