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

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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?

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• 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?

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What’s the tuning range?

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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?

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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?

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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?

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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