Eye safe solid state lasers for remote sensing and coherent laser radar
Jesper Munch, Matthew Heintze, Murray Hamilton, Sean Manning,Y. Mao, Damien Mudge and Peter Veitch
Department of PhysicsThe University of Adelaide
Adelaide SA 5005Australia
How to make a laser sensor “Eye-safe”
Keep energy/power low
Low power laser,Large transmittedbeam
Low DutyCycle
Select wavelengthfor maximum allowedpulse energy
Content of talk
• Coherent eye-safe laser radar: Review of current work in Er:Yb:glass slab lasers
• Planned work in Er:Yb:YAG
• New composite slab laser design
• Eye-safe sensing at low power
Our chosen Eyesafe laser species is Erbium
• Erbium lases at 1.5 – 1.6 μm, where laser safety allows:• 10× the energy per pulse allowed at 2 μm• 100x the energy per pulse allowed at 10 μm
• Allows better spatial resolution (for otherwise similar conditions)
• Can make use of available telecommunications photonic components: eg Master fiber oscillator
• BUT: it is a 3-level laser, normally in a phosphate glass host
Er:Yb energy level diagram
8 ms
95-99%
(1130 nm)
(1100 nm)
2H11/24S3/2
4F9/2
4I9/2
4I11/2
4I13/2
4I15/2
Laser(1535 nm)
Erbium
500 sμ2F5/2
2 ms
976nm PUMP
Ytterbium
2F7/2
1% ThermalPopulation
Limiting Processes:Ytterbium Bleaching11/2 Upconversion13/2 Upconversion15/2 Depletion
Energy Transfer
Summary of Early work in Adelaide*
• Demonstrated first injection seeding of single frequency Er:glasslaser at 1.5μm
• Demonstrated successful transform limited coherent Doppler measurement at 1.5μm
• Initial wind sensing measurements
*A. McGrath, J. Munch, G. Smith, P. Veitch,Appl. Opt. 37 (29), 5706-5709 1998.
Coherent Laser Radar
Transmitted Pulse &Reflected Signal
SLAVE LASER
DETECTOR 2
DETECTOR 1: Transmitted frequencyDETECTOR 2: Received power/frequency ADC & Signal
Processing
First injection seeded Er:glass at 1.5μm
Er:glass rodintra-cavitytelescope
AOM
Masteroscillator outcoupler
Littrowgrating
heterodynedetector
Q-switch
Expanded output pulse
Local oscillator beam
Tx/Rx telescope
PBS
λ/2 plate
λ/4 plate
Faraday isolators
λ/2 plate
The injection seeded, Q-switched laser produced a transform limited linewidth
0
0.5
1
1.5
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2.5
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3.5
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time (µs)
Out
put m
onito
r sig
nal (
arb.
uni
ts)
0
5
10
15
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30
15 20 25 30 35 40 45 50 55
Frequency (MHz)
Pow
er (a
rb. u
nits
) FWHM = 1.5 MHz
We used the Er:Glass laser to make a Doppler velocity measurement of moving hard-targets
0
0.02
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Frequency (MHz)
Pow
er (a
rb. u
nits
)
00.020.040.060.08
0.10.120.140.160.18
15 20 25 30 35 40
Frequency (MHz)
Pow
er (a
rb. u
nits
)Frequency shift = -4.5MHzReceding velocity 3.5m/s
Frequency shift = -8.5MHzReceding velocity 6.5m/s
Second Generation Er:Yb:Glass Slab
• Robust laser design
• Folded, total internal reflection, zig-zag slab
• Diode laser side-pumping (Q-CW)
• Injection seeded, Q-switched ring
• Long output pulse, using new resonator design with efficient out-coupling via throttled Q-switch
Standing-wave Er:Yb:Glass slab laser
0
Collimating lens
0
Laser slab
Diode laser0
Output 2
Output 1
R = 97.8 %
R = 97.4 %Flat Mirror
Flat Mirror
0 100 200 300 400 500 600
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10
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60
Tot
al m
ultim
ode
outp
ut p
ower
(mJ)
Input pump power (mJ)
Setup Output power
Side-pumped laser head
Heat Sink
TEC
Diode Array
Collimating lens
Pumped regionof slab
Slab
Injection seeded ring resonator
0
0
0
Laseroutput pulse
Pockels cell(Q-switch)
Image relay lenses
Max R mirrors
PZT
Slab
Diode laser
Master laserin
Ring Oscillator Q-switch results
Pump
OutputVolta
ge o
n Ph
otod
iode
outp
ut p
ulse
(V)
Volta
ge o
n P
hoto
diod
epu
mp
puls
e (V
)
Time (sec)
-4
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0.00 0.02 0.04 0.06 0.08 0.10-1
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Q-switched pulse (E=3mJ/pulse) Q-switch pulse – expandedscale: ms
Gain switched lasing(E=8mJ/pulse)
0 2 4 6 8 10-0.5
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-2
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Time (sec)
Volta
ge o
n Ph
otod
iode
outp
ut p
ulse
(V)
Volta
ge o
n P
hoto
diod
epu
mp
puls
e (V
)
Pump
Output
0 2 4 6 8 10-4
-2
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Time (sec)
Volta
ge o
n P
hoto
diod
eou
tput
pul
se (V
)
Volta
ge o
n P
hoto
diod
epu
mp
puls
e (V
)
Pump
Output
Current results with Er:Glass
• Good long pulse energy in standing-wave oscillator, near TEMoo(50mJ)
• Q-switched ring oscillator demonstrated
• Injection seeding demonstrated
However :However :
Problems with current Er:Glass slab laser
• Energy output limited by Er bleaching (measured)• High intra-cavity losses in ring oscillator (Pockels cell)• Serious thermal lensing limitations • Optical damage of glass host• Currently max energy per pulse Q-switched is 10mJ/pulse,
but need 20-50mJ/pulse raw laser output for scalable systems (eg: larger aperture, system losses)
• Pulse repetition rate will be limited by thermal effects• Pumping limited by frequency chirp in diode-lasers used
Continuing effort in Erbium
Two parallel approaches:
1. Improve and optimize Er:Yb:glass subject to its inherent thermal limitations.• Experiments using different Er, Yb concentrations for optimum
pumping• Reduce resonator losses• Complete injection seeding characterization as laser radar
2. Investigate third generation Er:Yb:YAG
Third generation: Er:Yb:YAG at 1.645μm
• Greatly improved thermal properties of YAG host• Better control of thermal lens• Better efficiency (lower level has 2% population)• Scalable to higher power, rep. rate• Manufacture as ceramic YAG material• Permits use of our new end-pumped composite slab geometry• Experience from our successful Nd:YAG designs directly
relevant• But requires a new, single frequency master oscillator• Recently demonstrated in bulk Er:Yb:YAG*
* Georgiou & Kiriakidi, Opt. Eng., 44 Jun. 200580mJ output, pumped by 4.7J
High pump intensities and necessary cooling of the gain medium leads to strong thermal gradients which cause undesirable effects.
Issues• strong thermal lensing
- change from top/bottom cooling to side cooling
• thermally induced birefringence- use specialized pump distribution
Scaling to higher power slabs
Effect of pump profile on depolarization loss in Nd:YAG
-4.0
-2.0
0.0
2.0
4.0
1.0 0.9 0.8 0.7 0.6
Tophat pump profileGaussian pump profile
Intensity Transmission
x [m
m]
-4.0
-2.0
0.0
2.0
4.0
1.0 0.9 0.8 0.7 0.6
Tophat pump profile Gaussian pump profile
Intensity Transmission
x [m
m] Pump
region
(Birefringence modeling: M. Ostermeyer)
Effect of pump profile on depolarization loss in Nd:YAG
-4.0
-2.0
0.0
2.0
4.0
1.0 0.9 0.8 0.7 0.6
Tophat pump profile Gaussian pump profile
Intensity Transmission
x [m
m]
-4.0
-2.0
0.0
2.0
4.0
1.0 0.9 0.8 0.7 0.6
Tophat pump profile Gaussian pump profile
Intensity Transmission
x [m
m] Pump
region
(Birefringence modeling: M. Ostermeyer)
Zones ofstrong depolarization
Composite end-pumped, side-cooled folded zigzag Nd:YAG slab
Silicon Dioxide
Brewsterangledwindow
Laser mode
TOP VIEW
SIDE VIEW
GlassUndoped:YAG
Nd:YAG (0.6 at.%)
Undoped:YAGGlass
Silicon Dioxide
PumpingCooling
AR 0.808 mHR 1.064 m
μμ
Off-axis zigzag pumping
• Rectilinear zigzag duct allows pumping at normal incidence andmixes pump light prior to slab entry
• Can pump using fibers by collimated bar-stack-array, and use non-imaging lens duct
• Scalable by increasing pump power, height of doped and undopedregion (mode volume)
Opticalfibres(2D array)
rectilinearzigzag duct
Optic axis of pump sourceOptic axis +θOptic axis -θ
• Tophat pump distribution – minimum birefringence
• Good absorption efficiency due to quasi end-pumping
• More uniform power loading within slab due to double-clad structure transporting pump light along slab before absorption
• No hard-edged apertures in vertical direction
• Large pump input aperture and acceptance angle accommodates real divergent pump sources
• Insensitive to pump beam-quality due to mixing of pump light in slab
• Undoped YAG layers produce reduced thermally induced stress
• Conduction-cooled
Composite slab advantages
End view of conduction-cooled laser head
Coolant
TECHeatsink
Heatsink
Cu
Cu
Indiumcontact
Nd:YAG
YAG
YAG
40 60 80 100 120 140 160 18010
15
20
25
30
35
40
45
Slope efficiency 29.5%
Out
put P
ower
[W]
Pump Power [W]
Initial Laser Performance in Nd:YAG
Approximately 90% pump light absorption in end-pumped slab
Composite slab design for Er:Yb:YAG
• Ceramic
• Doped and undoped Er:Yb:YAG
• Doping concentrations easily changed
• Slab configuration based on success with Nd:YAG
Er:Yb:YAG laser radar system
• New master oscillator under development– NPRO (non-planar monolithic ring oscillator)– Ceramic Er:Yb:YAG– To be developed in collaboration with Innolight
• Injection seeded slave ring oscillator
• Ceramic composite slab slave as described
The DIAL program(DIAL = differential absorption lidar)
• Aim: Low-Cost profiling of water vapour up to top of boundary layer
• Provide water vapour concentrations for
– Quantitative precipitation forecasting, Bushfire danger assessment, fog prediction
– current technique - radiosondes, high recurrent cost, infrequent data
• 830nm GaAs diode lasers (mature technology)
– Single mode limited to ca 0.5W (Average power ca 0.5mW - eyesafe!)
– Detector technology well developed (low-noise single photon)
• Wavelength control
– On-line laser (master oscillator) stabilised to peak of water resonance
– Off-line/ On-line difference frequency stabilised to 15GHz
– Water resonance ~ 6GHz width @ sea level ~ 1GHz width @ 4km altitude
– Freq. stability of ~ 20MHz adequate
Setup for DIAL
Spectral properties of amplifier
Wavelength control of master lasers
On-line laser stabilisation– BLUE LOOP
Wavelength difference stabilisation- GREEN LOOP
Water resonances near 829nm
• accessible for diode lasers
• appropriate line intensity (10-23cm-1)
• sufficiently isolated from other resonances
• other lines at 832nm
Stabilization to water resonance (832nm)- error signal at lock-in output
V
wavelength
Conclusion
• Er:glass at 1.53 μm is a useful approach for a simple, low average power eye-safe coherent laser radar, but is limited by thermal effects and damage in glass.
• Er:Yb:YAG is a promising new, preferred option at 1.6μmDesign experience form Nd:YAG directly transferable
• Low cost alternatives to eye-safe incoherent sensing for short range (<4km) applications using shorter wavelengths are feasible.
Producing a tophat pump distribution
• How? – Use a composite slab (doped & undoped YAG layers)– End-pumped for good efficiency– Side-cooled zigzag slab
Pump absorption is a tophat profile, thus minimizing thermally induced birefringence loss (even though diode-laser pump profiles typically produce Gaussian transverse profiles)
Thermal lensing minimized by using a zigzag mode-path in the plane of cooling, and by controlling the heat flow in the orthogonal plane
Small-signal gain measurement proves bleaching of Erbium
Gai
n
Proportional to Pump Energy (mJ)
Gai
n
Proportional to Pump Energy (mJ)
1
1.2
0.8
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