Posted: October 6th, 2023

Laser – The main risk is your eyesight

1) SAFETY - YOURS:
Class 3b laser - The main risk is your eyesight.
 YOU MUST WEAR SAFETY GLASSES WHENEVER THE DIODE
LASER IS TURNED ON.
 DO NOT BEND DOWN SO THAT YOUR EYES ARE LEVEL WITH THE
LASER BEAM.
 DO NOT POINT THE LASER BEAM RANDOMLY AROUND THE
ROOM OR UP.

1
2) SAFETY - THE LASER’S:
 THE DIODE LASER IS VULNERABLE TO STATIC. IT MUST NOT BE
DISCONNECTED FROM THE POWER SUPPLY. WEAR WRIST
STRAPS AND ENSURE THAT THE ANTISTATIC MATS ARE USED.
 THE FIBRE COUPLED TO THE DIODE LASER IS NOT HELD FIRMLY.
DO NOT DISLODGE IT OR BREAK IT.
 DO NOT OPERATE THE Q-SWITCH AMPLIFIER WITHOUT A LOAD
(Q-SWITCH) CONNECTED.

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Introduction and Aims:

In this experiment, you explore the behaviour of an Nd:YAG laser. For the first part of the
experiment the laser operates CW, and in the second part, it is pulsed by an acousto-optic Qswitcher.

Nd crystal is inside a laser cavity comprising two mirrors. Also in the cavity is an
acousto-optic q-switch: this has a low intrinsic loss, but the loss becomes higher when an RF signal
is applied. The q-switch can be used to rapidly change the cavity losses, and achieve pulsed
operation.

To start you off, read the RP Photonics page on Q-Switched lasers, and watch the video:
https://www.rp-photonics.com/q_switching.html

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Apparatus:

Familiarise yourself with the apparatus, but do not disconnect any cables.
 Fibre coupled diode laser mounted on a heat sink.
The diode has a thermo electric cooler inside the mount which enables the diode
temperature to be directly controlled.
The fibre is a multimode fibre with a 50 m core and is simply butt-coupled to the diode
laser.
 Diode laser power supply.
Diode temperature is maintained by extra feedback electronics to the T.E. cooler. The
power supply provides constant current, low voltage power to the diode. Because of
deficiencies in the feedback loop you may have difficulty stabilising the diode laser
temperature at low currents.
 Nd:YAG laser.
Diode light from the fibre is focused by a 0.29 pitch GRIN lens onto a Nd:YAG crystal.
January 2009

2
This flat/flat 8 mm long crystal has a dielectric coating at one end to transmit 800 nm
light and reflect 1064 nm light. The other end has an AR (anti-reflection) coating at
1064 nm. The output coupler is 7.5 cm radius of curvature (concave) and is 2 %
transmitting at 1064 nm.
 Acousto optic Q-switcher and driver.
This remains in the laser cavity at all times for convenience and safety. It is turned off if
CW operation is required. It is a flint glass active element with AR coatings at 1064 nm.
The driver for this contains a small oscillator and amplifier circuit with a resonant
frequency of 80 MHz. This is turned off by a TTL pulse from the pulse generator. A
20 - 28 V dc power supply is required to power the RF amplifier. The RF power is ~ 2W.
 Detectors.
Power meter: Use the power meter provided with the apparatus as it has been calibrated
for  = 1064 nm
Photodiode: use with the 50 Ohm setting to determine pulsewidths on the oscilloscope.
 Safety glasses.
Required wearing if the diode laser is on.

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PART 1 CW operation

Effect of laser diode current and temperature on Nd:YAG output power.
1. Turn ON key to turn on diode power supply.
2. Press ON button at right of power supply.
3. Put on safety glasses and wait for setup indicator LED to stop flashing.
4. Investigate the items the power supply can display, including current limit, average current and
temperature value and set point.
January 2009
fibre
diode
GRIN
heatsink
NdYAG
QS
impedance
matching
adjustable
mount
RF
out
diode power
supply
1.06 m
output mirror
video
in
RF oscillator
and amplifier
Figure 1. Setup of diode-pumped NdYAG laser.
DC power
+ 20 V
TTL pulse
generator

3
5. Using the centre BIAS LEVEL knob and watching the average current display, slowly dial to
700 mA and check that the actual temperature settles to 23 C (20 C on cold days).
6. Check the NdYAG laser is operating with a phosphor card and measure its output power.
7. Record the laser output power for diode currents in the range 300 mA to 750 mA. (Check
temperature is constant). Plot the Nd:YAG output power vs optical diode pump power,
(USE THE DIODE CURRENT-to-POWER GRAPH SUPPLIED below) and determine the
threshold power and slope efficiency.
8. Record the laser threshold pump power for a range of diode temperatures from 11C to 23C.
Note error bars in readings. Why does the NdYAG output power depend on diode
temperature?
9. View the CW laser output on the photodiode and oscilloscope (50 Ohm load is fine). Note
the signal level and temporal characteristics. Calculate a rough calibration factor for the
measured power and photodiode output.

PART 2 Q-switched operation

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1. Check that the Q-switcher is connected to the RF input of the impendence matching box, and
the TTL pulse from the pulse generator is connected to the video in BNC on the amplifier. The
amplifier is also connected to the DC power supply.
2. Turn on the pulse generator and set up to give TTL (5V) square pulses at a pulse repetition rate
of 30 kHz.
3. Turn on DC power supply to 20 V DC.
4. Check with photodiode for pulsed output, using the oscilloscope.
Question: Explain how the acousto optic Q-switch operates to give a "giant" pulse from a CW
pump laser.

5. Record the pulsewidth (FWHM), buildup time (time from the square trigger pulse to the start
of the laser pulse), and the average output power as a function of diode current (up to 750
mA). Plot these quantities as a function of pump power, and comment on the form each takes

Question: What pulsewidth would you expect from the theory of part 3?

Question: For your maximum average output power, calculate the energy of each pulse, and
so estimate the peak power during the laser pulse. (Hint: pulse energy is instantaneous power
integrated over the pulse shape.) How does this compare to the peak power during CW
operation?

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6. Change the repetition rate to 200 Hz, and look at the pulsed output on the photodiode, exploring
the full range of pump powers. At high pump powers, the laser does not work correctly – work
out what goes wrong and what causes the problem. Determine the highest pump power for
which the laser operates correctly at 200 Hz.

7. Confirm that the same problem does not occur at 10 kHz? Why not? You need to know that the
upper level lifetime (storage lifetime) of the laser material is
= 230 µs.

8. Using the highest pump power determined in part 6 for correct operation at 200 Hz, measure

2
the average output power as a function of repetition rate, between 200 Hz and 30 kHz. Plot the
average output power, and the calculated pulse energy, as a function of repetition rate.
24/07/2018. 4
Question: Discuss why the average output power, and the calculated pulse energy behave in
this way. Consider how the time between pulses
affects the stored energy in the two cases:
low rep rate with

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1. Turn off DC power supply for Q-switch by winding voltage down to zero.
2. Turn off TTL pulse generator.
3. Wind down diode current slowly to zero.
4. Turn off ON button on right of diode power supply.
5. Turn off key of diode power supply.

PART 3. Analysis of the laser cavity and Q-switched operation

The efficiency of the laser is governed by the available pump power and the overlap between the
pump beam and the laser cavity mode (as well as the cavity losses). In general, the pumped gain
volume is designed to be smaller in diameter than the laser cavity mode. This ensures that the gain
available is as high as possible, and that the lowest order transverse laser mode (TEM
00
) operates
preferentially.

The laser cavity is a simple hemispherical design, with a flat crystal surface forming one mirror
and a spherically-curved concave output coupler as the other. The radius of curvature of the output
coupler is 7.5 cm. This cavity is “stable” (ie can support a stable laser mode) if the output coupler
is placed within approximately 7.5 cm of the high reflector.
The cavity stability is determined by matching the curvature of the gaussian beam of the
wavefront to that of the end mirrors. (Thus the beam is reflected with a constant phase front
across its profile.) This is calculated in terms of the cavity g parameters.
g
1
= 1 - L/R
1
and
g
2
= 1 - L/R
2
where L is the optical path length of the cavity and R
1

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and R
2
are the mirror radii of curvature.
R
1
and R
2
are positive if the mirrors are concave toward the laser cavity. The laser crystal is 8
mm long and has a refractive index of 1.78. The Q-switch is 3 mm long with a refractive index
of approximately 1.6.

Real solutions for the cavity modes only exist when
0≤g
1
g
2
≤1.
These parameters can be used to estimate the cavity mode spotsizes w
1
and w
2
(diameters) at the
end mirrors using:
w
L g
1
2

 ( )
1


2
g g g
1 1 2
w
L g
2
2

 ( )
1


1
g g g
2 1 2

and
.

24/07/2018. 5
When the laser is Q-switched, it produces short, high-power pulses, at a repetition frequency
governed by the Q-switch “switching” rate. The pulse width from the laser is determined by the
pump power (the number of times above threshold that the laser is being pumped) and the cavity
lifetime 
c
. The pulsewidth 
p
may be approximated by:

 
r r
r r
( )
ln( )1

 
p
c

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where r is the initial inversion ratio (the number of times above threshold that the laser is pumped)
and (r) is the energy extraction efficiency as a function of inversion ratio. The energy extraction
efficiency is approximately 100 % for r >2. Thus 
p
is of the order of 
c .
The cavity lifetime is
given by:
where T
rt

is the cavity round trip time and 2
0
c
rt

T
p






1


2
ln
0
1 2
R R

p is the distributed cavity loss in one round trip. The
mirror reflectivities are R
1
and R
2
.

Question: Using the Melles Griot graphs supplied (see Resource Notes) determine the spotsize of
the pump beam after it is focused by the 0.29 pitch GRIN lens. Assume a distance of 0.5 mm
between the GRIN lens and the fibre. For best mode matching, the optimum laser alignment has
the pump focused on the back face of the laser crystal

Question: Calculate the laser cavity mode spotsizes at each end mirror. Assume the measured
distance between the two end mirrors is 6.5 cm. Show that this forms a stable cavity.

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Question: Assuming a low distributed cavity loss and an energy extraction efficiency of 100 %,
calculate the cavity lifetime and hence estimate the Q-switched pulsewidth.

Question: What could you do to reduce the pulsewidth?

References:
A. Siegman, Lasers, University Science Books, Mill Valley Ca, 1986.
C. Davis, Lasers and Electro-optics, Cambridge University Press, Cambridge, 1996.
Melles Griot catalogue on Gradient Index Lenses.
J. Quellette, The Diode Pumped Laser Revolution, AIP Industrial Physicist pp7-9 1996.
24/07/2018. 6
Diode current to power conversion data

24/07/2018. 7

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