Development of a Frequency-Stabilized Mid-Infrared External Cavity-QCL Cavity Ringdown Spectrometer
Bradley M. GibsonDepartment of Chemistry, University of Illinois at Urbana-ChampaignBenjamin J. McCall Departments of Chemistry and Astronomy, University of Illinois at Urbana-Champaign
Outline
{2}
1. Overview of EC-QCLs2. Initial Performance3. Output Frequency Stabilization
a) Passive stabilization• Vibrational isolation
b) Active stabilization• Side-of-fringe locking• Automated re-locking• Tilt tuning
Why use an EC-QCL?
{3}
• Emission in the mid-IR (4-10 μm)• Small minimum tuning steps, <15 MHz• Wide tuning range, ~100 cm-1
• Relatively high output power, ~10 mW• Laser chips can be swapped for additional tuning range
What is an EC-QCL?
Figure from: Wysocki et al., Appl. Phys. B: Lasers Opt. (2008), 92, 305. {4}
What is an EC-QCL?
{5}
What’s the tuning range?
{6}
8
6
4
2
Pow
er (m
W)
12201200118011601140
Frequency (cm-1
)
How can we control the output?
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Tuning elements:• QCL injection current / temperature• External cavity length (piezo)• Cavity grating angle (piezo)
1180.6
1180.5
1180.4
1180.3
1180.2
1180.1
1180.0
Freq
uenc
y (c
m-1
)
1086420
Laser current modulation voltage (V)
1180.5
1180.4
1180.3
1180.2
1180.1Freq
uenc
y (c
m-1
)
1086420
Translation stage piezo voltage (V)
How do we control the output?
{8}
1199.9
1199.8
1199.7
1199.6
1199.5
1199.4
1199.3
Freq
uenc
y (c
m-1
)
765432
Three-element modulation voltage (V)
How stable is the output?
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1178.700
1178.695
1178.690
1178.685
1178.680
1178.675
Freq
uenc
y (c
m-1
)
3.02.52.01.51.00.50.0
ControlStandard Deviation: 0.007549
(~226 MHz)
Time (m)
How can we stabilize the frequency?
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1. Isolate the laser from vibrations / acoustic noisea) Float the laser tableb) Decrease water chiller pulsingc) Acoustically isolate the laser
2. Improve injection current / thermal stability3. Actively lock the laser to an external reference
How can we isolate the laser?
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Water Chiller DampingVacuum Chamber / Table Isolation
How well did the isolation work?
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Standard deviation: 0.00437 cm-1
(~131 MHz)
1180.080
1180.075
1180.070
1180.065
Freq
uenc
y (c
m-1
)
250024002300220021002000
Time (s)
How can we actively lock the laser?
Figure from: RP Photonics Encyclopedia, Tilt Tuning of Etalons {13}
Germanium Etalon Side-of-Fringe Locking• Simple, low-cost• Limited by laser power fluctuation, etalon temperature stability
• In practice, both effects are small• Tuning is difficult; how can we make steps smaller than the FSR?
• Use tilt tuning!
Fringe Position with Varying Angle of Incidence
How can we actively lock the laser?
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Spectrometer QCL
Etalon
ControlElectronics
Detector
Piezo Mirror
LensBeamsplitter
How well did the locking work?
{15}
1180.080
1180.075
1180.070
1180.065
Freq
uenc
y (c
m-1
)
250024002300220021002000
Time (s)
Locked Frequency~27 MHz St.Dev.
Unlocked Frequency~131 MHz St.Dev.
How well did the tuning work?
{16}
1179.480
1179.478
1179.476
1179.474
1179.472
1179.470
1179.468
1179.466
Freq
uenc
y (c
m-1
)
1601401201008060
Tuning Step
St. Dev.: ~24 MHz
How well did the re-locking work?
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1179.52
1179.50
1179.48
1179.46
1179.44
Freq
uenc
y (c
m-1
)
2000150010005000
Time (s)
~20 MHz St.Dev.
~16 MHz St.Dev.
Wavemeter Readings During Automated Re-Locking
Can we do all-digital locking?
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2x Amp ADC
Beaglebone
DAC
Mirror
EC-QCL Length Piezo
EC-QCL Rot. Piezo
QCL Current Mod.
2.5x Amp
1.8x Amp
PC
• Single-frequency locking comparable to analog approach (15-30 MHz St. Dev.)
• Automatic re-locking generally fails• Maintaining lock / acquiring new lock competing for computational resources?
Acknowledgements
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• McCall Group- Jacob Stewart
• Gerard Wysocki