JECET; September 2015- November 2015; Sec. C; Vol.4.No.4, 501-508.
E-ISSN: 2278–179X
Journal of Environmental Science, Computer Science and
Engineering & Technology
An International Peer Review E-3 Journal of Sciences and Technology
Available online at www.jecet.org
Section C: Engineering & Technology
Research Article
Experimental Investigation of Xenon Flash lamp Driving
Circuit for Nd:YAG Laser
Wasfi. H. Rashid, Bushra. R. Mhdi, Nahla A. Aljabar and Lubna. G. Abdulatif
Ministry of Science & Technology Iraq
Received: 28 July 2015; Revised: 27 August 2015; Accepted: 04 September 2015
Abstract: Experimental investigation in the flash lamp driving circuit parameters are
presented for Xenon flash lamp that was used as an optical pumping source for Nd:YAG
laser. This is accomplished by designing and construction of power supply for this
purpose. A simple circuit of pulse forming network (PFN) consists of (RLC) network
used to control the delivered energy to the flash lamp. By determining the energy E0 is to
be discharged, and the pulse duration tp, the values of capacitance C, inductance L, and
charging voltage V0, are specified to yield a critically damped pulse at 0.8 damping factor
values, at which maximum transfer of energy from the capacitor bank to the flash lamp is
accomplished. Flash lamp output intensities at various input voltage to the capacitance
bank and the flash lamp pulse duration were monitored.
Keywords: Flash lamp RLC circuit, Nd: YAG power supply
INTRODUCTION
The basic function of power system in a flash lamp pumped pulsed solid state laser is to extract energy
from the power grid, shape and time compress it for delivery to the flash lamp that in turn pump the laser
active medium. Energy is extracted from power grid and converted to capacitor voltage by a charging
circuit. After the capacitors are charged to the required voltage, a high voltage and sharp trigger pulse is
generated to discharge the stored energy on the capacitor bank into the flash lamp taking into account
controlling the flow of current during the pulse to control the pulse shape 1. To have an idea about which
light sources that could be used to pump a Nd:YAG laser we have to study the absorption-profile of the
501
September 2015- November 2015; Sec. C; Vol.4.No.4, 501-508.
Experimental…
Wasfi. H. Rashid et al.
crystal and compare it with the spectral emission of Xenon flash lamp. In order to have high output
efficiency the spectral emission of the source must be suitable to the spectral absorption bands of the
Nd:YAG crystal. Figure.1 2 shows a strong absorption band around 800 nm (12500 cm-1) in Nd:YAG
crystal as well the emission of xenon flash lamp. If we take a closer look at this part we notice that the
absorption profile consists of a number of peaks.
a
b
Fig.1 (a): The absorption spectrum of Nd:YAG crystal, (b): The emission spectrum of Xenon lamp
In Nd:YAG laser rod usually employed xenon flash lamp as a pumping source. The selection base on its
efficiency to convert electrical input energy to radiation in the range of 0.2 to 1.0 µm regions which
match with the absorption band of Nd:YAG 3.
Transfer of energy from capacitor bank to the flash lamp is accomplished using nearly critically damped
RLC circuit 4, 5. There are a number of possible circuit schemes for charging of the capacitor banks and
discharging them through flash lamps in controlled manner 4. These power supplies are pulsed systems.
The pump source in a pulsed laser is demanding a much higher current compared to a continuous laser.
502
September 2015- November 2015; Sec. C; Vol.4.No.4, 501-508.
Experimental…
Wasfi. H. Rashid et al.
During these circumstances the xenon-lamp is the most efficient choice. This is because the continuous
background from the xenon-lamp is stronger and that there is a strong re-absorption at 810nm in krypton
lamp. In high current density, pulsed laser applications, the spectral output of the lamp is dominated by
continuum radiation, and the line structure is seen as a relatively minor element. It also follows that a high
current density shifts the spectral output toward the shorter wavelengths.
Resistivity of the xenon gas is not a fixed value, but depends upon the degree of ionization of the gas and
thus on the current density, and so the low- and high-power optical output spectra are markedly different.
An increase in current density decreases the resistivity of the gas. Because current density in the lamp
varies during the electrical pulse, resistivity and total lamp resistance also vary 6.
First step towards the development of optimum flash lamp single-mesh LC (PFN) driving circuits is to
know the impedance of flashlamp used as pumping source.
Impedance of flashlamp, Ko depends on its geometry and several flashlamp parameter. It depends on the
arc length, l and bore diameter d of flashlamp.Other parameters are gas type and gas fill pressure, p. The
estimation of the xenon flash lamp impedance is given by equation 3, 7, 8 :
(1)
The dynamic behavior of the discharge circuit, which consists of the flash lamp impedance defined in (1),
a capacitance C, and an inductance L, is described by a nonlinear differential equation. The impedance Zo
and the time constant tLC of the circuit is given by:
and
(2)
In addition, a damping factor α is defined, which determines the pulse shape of the current pulse. It is:
(3)
Where Vo is the initial voltage of the capacitor. Solutions of the nonlinear differential equation reveal that
the current waveform is critically damped for a value of α = 0.8 3. The current pulse duration tp at the 10%
points is approximately 3tLC; during this time about 97% of the energy has been delivered. If we substitute
and
(4)
for a critically damped pulse into (2), (3), then we can determine for a given lamp type the relationship
between energy input, pulse duration, pulse shape, inductance, capacitance, and operating voltage. The
energy initially stored in the capacitor is:
(5)
Equation (5) is used to eliminate Vo from (3), from which follows the value of the capacitor
(6)
503
September 2015- November 2015; Sec. C; Vol.4.No.4, 501-508.
Experimental…
Wasfi. H. Rashid et al.
The inductance can be calculated from (4)
(7)
We now have a set of three equations (5)–(7) that, given the specifications of the flash lamp parameter Ko,
the desired input energy Ein, and pulse width tp, provide explicit values of C, L, and V0, The circuit voltage
(8)
The peak current for a critically damped current pulse is
(9)
The rise time to reach this peak value is
(10)
EXPERIMENTAL WORK
First step in designing power supply for pumping Q-switching Nd: YAG laser is to determine the required
energy for laser action in Nd:YAG rod.
Our rod has size of (50*4) mm and the required energy was theoretically estimated depending on the rod
parameters to be 4.232 Joule.
To design pulsed power circuitry for flash lamps, it is necessary to understand the lamp parameters and
their effect on the peak current, the delivered energy, and the temporal shape of the pulse. Other factors
such as explosion limits and lamp life also must be taken into consideration.
The flash lamp operates in an RLC circuit(PFN), and the shape of the current pulse depends on the values
of R, L, and C The resistance, R, is provided by the flash lamp; the inductance, L, by the series
inductance; and the capacitance, C, by the discharge capacitor. The influence of the triggering circuit
components is minimal and can be ignored.
The single mesh RLC circuit arrangement is shown in Figure (2).
In the figure, the charging supply is a classical DC supply which converts AC line to DC. A step up
transformer and a full bridge rectifier is used. This supply charges the capacitors which have an
equivalent capacitance of 40μF.
Since we are operate in single shot small a current limiting resistor is connected in series with flash lamp
to protect circuit components, which is equal to 5 K ohm.
504
September 2015- November 2015; Sec. C; Vol.4.No.4, 501-508.
Experimental…
Wasfi. H. Rashid et al.
When the discharge lamp is switched off, the capacitor (C ~ 40 F) is charged to the same voltage as the
DC power supply. When the capacitor is charged a trigger pulse is sent from the trigger unit to the trigger
transformer. High-voltage external trigger signal is applied directly to a wire wrapped around the tube
envelope creates an ionized spark streamer between the electrodes. This makes the voltage over the
discharge lamp high enough during a short time to ignite it and this lowers the resistance. The main
advantage of external triggering is that the energy-discharge circuit is independent of the trigger.
The capacitor is discharged through the inductor (L ~ 27 H) and the discharge lamp. The inductor
reduces the current and makes the discharge lamp pulse appropriately long. The capacitor, the inductor
and the discharge lamp form a strongly damped electrical system (RLC-circuit). The exact oscillation
time is hard to calculate due to the varying resistance.
RESULTS AND DISCUSSION
Given that the electric energy Eo is to be discharged which depend on required optical energy to pump the
Nd:YAG laser rod, lamp impedance Zo and the pulse duration, tp, is specified as the time between the
points on the leading and trailing edges of the pulse at 10% of the peak amplitude.
With these inputs, one desires to know the values of capacitance C, inductance L, and charging voltage
Vo, to yield a critically damped pulse with these characteristics. These values may be obtained from Eq
(5,6,7,). Table.1 represents the numerical value of the flash lamp circuit parameters that was dependent in
our research.
505
September 2015- November 2015; Sec. C; Vol.4.No.4, 501-508.
Experimental…
Wasfi. H. Rashid et al.
We have got 100 µsec flash lamp optical pulse duration as shown In Fig.3, and remains constant at
various value of input voltage to the capacitance except the change in intensity. The signal were taken by
200Mhz OSC from ATTEN instrument with SFH2030 photo detector from Siemens Co having 5nsec rise
time and 0.6 A/W spectral sensitivity at 850 nm.
Fig.3: Optical flash lamp pulse with 100 µsec duration
Figure 4 represents the comparison of the flash lamp optical intensities at various input voltage to the
capacitor bank as they were taken by the (UV-NIR) spectrometer from Ocean Optics Company. It’s clear
that the optical intensity increases as the input voltage increases and on the right side at the top of the
figure the shape of the Nd:YAG pulses at various input voltage.
Also the optical intensities of flash lamp at various input electrical energy were shown individually in
Fig(5) staring at 450 volt were no laser action appear at 1064 nm then at 460 volt where the threshold
energy for the laser action represent by a small line at 1064nm .The voltage 460 volte represents the
minimum voltage applied to the capacitor in order to get the laser action which equivalent to 4.232 joule
506
September 2015- November 2015; Sec. C; Vol.4.No.4, 501-508.
Experimental…
Wasfi. H. Rashid et al.
electrical input energy and in these figures as the voltage increases the peak intensity at 1064nm laser
action also increase.
Fig.5: Flashlamp optical intensities at various voltage (a {450V},b {460V}c{500V},d{550V},e{600v}.)
507
September 2015- November 2015; Sec. C; Vol.4.No.4, 501-508.
Experimental…
Wasfi. H. Rashid et al.
CONCLUSION
Optimum parameters for single-mesh flash lamp driving circuit was successfully calculated and applied in
designing and construction of our power supply. To get maximum transfer of energy at critically damping
situation, we have got good approximation between the theoretical approach and the experimental one in
predicating the minimum threshold energy to get laser action and the appearance of 1.06 µm laser action
in the emission spectra improves the success and the appropriate parameters value that were inserted in
this design. Also it should be noted that in order to shorten the flashlamp duration, the inductance must
be reduced as much as possible to match the life time of Nd:YAG laser upper state.In single shot laser
action the current intensity approximately low since the time available to charge the capacitor is relatively
long which is unlike high pulse-repetition rates, also high current density shifts the spectral flashlamp
output toward the shorter wavelengths which is mismatch the absorption spectra of laser rod.The start
point in designing the power supply is to Calculate the threshold energy required for pumping the laser
rod, pulse duration
and the flashlamp impedance .The threshold electrical pumping energy for laser
action appears at 4.232 joule. It clears that from the intensity distribution, the flashlamp optical pumping
output increases as the input voltage to the capacitor bank increases which leads at certain lamp intensity
to get laser action and thereafter the higher laser power while this situation have no effect on laser pulse
duration.
REFERENCES
1. M. S. Ansari, M S Bhatia,” Investigation of Electromagnetic Interference from a Pulsed
Solid State Laser Power Supply”, 2013, (IJERA) ISSN: 2248-9622, Vol. 3, Issue 1,
pp.1577-1581.
2. E. Eryilmaz,” Design and Construction of a CW Mode Nd: YAG Laser rototype”, A
Master Thesis, 2004, Middle East technical University.
3. W. Koechner, M. Bass” Solid-State Lasers” Springer-Verlag New York, Inc., 2003.
4. W. Gagnon, G. Albrecht, J. Trenholme, M.Newton,” Pulsed Power for Solid- State
Lasers”, 2008.
5. E. Khaled, I. Hisham. Design and Construction of Q-Switched Nd: YAG Laser System
for LIBS Measurements, Optics & Laser Technology, 2012, 44 130–135.
6. J. H. Goncz,”Resistivity in Xenon Plasma”. Journal of Applied Physics, 1965, Vol.36,
Part 3, p. 742.
7. D.C. Brown, T.N. Nee. Design of Single Mesh Flashlamp Driving Circuits with Resistive
Losses. IEEE Transactions on Electron Devices, 1977, 24(11):1285-1287.
8. M. F. Maulud,” Study of Single-Mesh LC Flashlamp Driving Circuit for Xenon
Flashlamp”, J. Fiz. UTM., 2008, Vol. 3. 95-98.
Corresponding Author: Bushra. R. Mhdi;
Ministry of Science & Technology, Iraq
508
September 2015- November 2015; Sec. C; Vol.4.No.4, 501-508.