Lab Report 4: Diode Pumped Nd

Lab Report 4: Diode Pumped NdYAG Laser
LIN PEI-YING, BAIG JOVERIA
Introduction
Neodymium Yttrium Aluminum Garnet lasers (Nd:YAG) have numerous applications
in the world today. These include the medical and scientific fields for processes such
as laser spectroscopy and surgery. A Nd:YAG laser can be optically pumped using
lasers of several different wavelengths. This report will discuss some of the basic
characteristics of the Nd:YAG laser such as the efficiency, atomic levels, optical pump
source and modes of operation.
Background theory
Pulse Laser
Since the invention of the laser, the delivered peak power has increased steadily (see
Figure 1). To date, powers up to a few Peta Watt (1 PW = 1015 W) are generated in a
few facilities around the world with state-of-the-art technology. Clearly, such high
powers cannot be sustained for a long period of time, therefore high peak powers
are obtained by means of pulsed laser operation. In fact, extremely intense laser
sources can emit light only for a fraction of a picosecond (1ps =10-12 s).
Fig.1. Progress of laser peak power in the last 5 decades. (from [Mourou]).
Characteristics of optical pumping:
Optical pumping is a process in which light is radiated into a specimen under
investigation and the effect of the light on the specimen is examined. It was in this
way that the strange physical phenomenon was observed of atoms only being able
to accept or release energy in well-defined quantities. This observation led to the
conclusion that atoms only have discrete energy states or energy levels. When light
is absorbed or emitted, a transfer takes place between the energy levels.
Nd:YAG laser system
Fig.2. Energy level structure and common pump and laser transitions of the
trivalent neodymium ion in Nd:YAG. (from [RP Photonics Encyclopedia])
Nd:YAG is an abbreviation of Neodymium-doped YAG (Nd:YAG). It is a kind of solidstate lasers. It can be used for everything from low-power continuous-wave lasers to
high-power Q-switched (pulsed) lasers with power levels measured in the kilowatts.
Most Nd:YAG lasers produce infrared light at a wavelength of 1064 nm. Light at this
wavelength is rather dangerous to vision, since it can be focused by the
eye's lens onto the retina, but the light is invisible and does not trigger the blink
reflex. Nd:YAG lasers can also be used with frequency doubling or frequency
tripling crystals, to produce green light with a wavelength of 532 nm or ultraviolet
light at 355 nm. The dopant concentration in commonly-used Nd:YAG crystals
usually varies between 0.5 and 1.4 molar percent. Higher dopant concentration is
used for pulsed lasers; lower concentration is suitable for continuous-wave lasers.
Nd:YAG is pinkish-purple, with lighter-doped rods being less intensely colored than
heavier-doped ones. Since its absorption spectrum is narrow, the hue depends on
the light under which it is observed.
Q-switching mode operation:
Fig.3. A scheme of principal of Q-switching
The quality of a resonator is the quotient of the resonance frequency and the half
width of the resonance curve. The definition is the same as for oscillator circuits
known from electronics. A low Q figure for a resonator signifies high losses and vice
versa. In a laser resonator with low Q, a high inversion can be produced without
laser oscillation, because the threshold is high. If the resonator Q is then suddenly
switched to a higher value, a high photon density is formed and a large part of the
inversion stored in the laser-active material transfers into the photon field.
Intra-cavity frequency doubling of the laser wavelength:
In this step we observe that the 1064nm Infrared laser convert to green light with
wavelengths of 532nm. That is because of the nonlinear property of KDP crystal.
When KDP crystal satisfies the phase matching condition, the amplification for the
second harmonic can be obtained. Frequency doubling is also a kind of nonlinear
optical effect. Certainly we can use these effects to achieve the conversion of
different light.
Experiment :
Fig.4. Experimental setup (A = pump laser diode; B = anamorphic collimator;
C = focusing lens; D = Nd:YAG crystal; G = power meter)
Preliminary Alignment:
The experimental setup used is shown above. The beam is first aligned very carefully
by adjusting the collimation and anamorphic system such that it is parallel to the
bench. The mirrors are also aligned to make the back reflection collinear with the
forward pump beam.
Optimization:
The temperature of the laser diode also has an impact on the absorption peak of the
Nd:YAG, hence it is important to set the temperature of the laser diode to 22°C at
808 nm to optimize it according to figure 5.
Figure 5: Wavelength vs Temperature
The power characteristic of the pump diode are plotted as a function of pump
current right after the focalization lens. The result is shown in Figure 6. As expected,
the characteristic shows that above the threshold value of about 300 mA, the power
increases approximately linearly with current as was expected.
To analyze how much of this power
is absorbed by the pump diode, the
power transmitted from the pump
diode is also recorded. The graph
of absorbed power versus the
pump power is shown in Figure 7.
It shows a fairly constant
transmission over all pump powers,
hence showing a linear relationship
between absorbed power and
pump power.
pump power (mW)
700
Figure 6
600
500
pump power
(mW)
400
300
200
100
0
0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Current (A)
Absorbed power (mW)
The next part of the experiment involves the use of a function generator to provide a
supply square signal. The RG 1000 filter is then inserted in to the setup. This filter is
used to exclude the wave that
Absorbed Power vs Pump Power
maybe passing through the
150
system from the incidence.
Hence, this filter acts as a low
100
pass filter and its behavior is
observed on the oscilloscope by
50
adjusting the load resistance.
This result is shown in Figure 8.
0
0
200
400
600
800
Since this circuit behaves like an
-50
RC resonant circuit, the
Pump power (mW)
sensitivity is seen to increase
Figure 7: Pump Power vs Absorbed Power
with the load resistance as more
voltage is tapped and the
bandwidth is inversely proportional to the resistance as the resonance frequency is
determined by the inverse of the product of resistance and capacitance. Hence, this
load resistance is
chosen to be 1 MΩ
so that the lower
frequencies are
allowed to pass
through and
blocking the
contribution from
the 808nm
incidence light.
Figure 8: Oscilloscope showing the behavior of RG 1000
Using the method of slope at
the origin, the photon
lifetime was measured to be
0.243 which is found to be
very close to the tabulated
value of 0.230 found in the
datasheet of the Nd:YAG.
Outpu Laser Power (mW)
The next setup involved
analysis of the lasing effect by
creating a cavity. For this
purpose, the setup was
Figure 9: Setup for lasing
modified to that shown in
Figure 9.
The cavity is aligned to obtain the TM0 mode. This is a tricky alignment and in order
to create lasing in the
cavity, two effects must be
Output Laser Power vs Pump
considered. The diffraction
Power
of the beam will result in a
150.00
beam divergence and it
must be insured that the
100.00
length of the cavity is kept
shorter than the radius of
50.00
curvature of the lens.
Additionally in order for
0.00
must be ensured that
0
200
400
600
800
mirror is placed close
Pump power (mW)
Figure 10
enough that the light
intensity reaches it since the light diffracts and has a beam divergence associated
with it.
Current/charge of 1 electron
The laser power output power is measured and plotted against the pump power
taking in to account the transmission of RG 1000. The plot is as shown in Figure 10.
The graph shows
small deviations
Quantum efficiency
from a linear fit. At
0.6
this stage, it is
0.5
interesting to
quantify the
0.4
quantum efficiency
0.3
of the system since
this determines
0.2
how much of the
0.1
incident photons
are finally emitted
0
in lasing. This result
0
2
4
6
8
10
Figure 11
output Power/Energy of 1 photon
is displayed in Figure 11.
Frequency Doubling:
The experiment is then modified slightly to observe frequency doubling as shown in
Figure 12. A highly reflective output mirror is used for this purpose since the
frequency doubling phenomenon is based on a non linear effect.
The cavity is realigned with this mirror and a KTP crystal is introduced in to the cavity.
This is a fairly tricky alignment. Hence, initially, the beam is made to pass through
the KTP crystal and power is monitored while placing it out of the cavity in order to
align the beam parallel to the bench. Then this crystal is introduced in to the cavity.
With the correct alignment, the set up begins to perform lasing at a wavelength of
532 nm. A BG18 filter is used to exclude the power from the wavelength of 1064 nm
to interfere with the useful required component. The results are shown in Figure 13.
A power of 132 µW was observed.
Conclusion:
The lab report focused on the demonstration of lasing in a stable cavity created with
the help of Nd:YAG and highly reflective mirrors. The laser output power was
determined and quantum efficiency was calculated for this system. Finally, intra
cavity frequency doubling was achieved by the use of KTP crystal.

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