Jakob Hayden*, Bernhard Lendl

Institute of Chemical Technologies and Analytics, Technische Universität Wien, Getreidemarkt 9, 1060 Wien, Austria* jakob .hayden@tuwien.ac.at

Mid-IR Cavity Enhanced Spectroscopy in a Linear Brewster Window Cavity

Why choosing a High Finesse Brewster Window Cavity

Experimental

Ÿ Optical Feedback Cavity Enhanced Spectroscopy with a DFB - diode laser at 680 nm [2]

Ÿ Cavity enhanced frequency modulation spectroscopy of molecular ions in a plasma with an optical parametric oscillator [4]

Ÿ Cavity-Enhanced Faraday Rotation Spectroscopy exploiting the polarization sensitive reflectivity of the Brewster window [5]

Example applications of intracavity Brewster windows

Light resonant with a cavity mode is partially backreflected to the laser from the front and rear surface of the Brewster window. This phase sensitive optical feedback forces the laser to emit at the re-injected light‘s wavelength and keeps the laser in resonance with the cavity mode.[3] Depending on the window‘s thickness d the two reflections from the Brewster window can overlap signifficantly and interfere, leading to a feedback rate that changes with the Brewster window‘s angle and the wavelength. The rear surface‘s back-reflection should therefore be blocked where possible.

Optical Feedback from the Brewster Window

[1] R. Damaschini, Opt. Commun., vol. 20, no. 3, pp. 441–442, 1976[2] V. Motto-Ros, J. Morville, and P. Rairoux, Appl. Phys. B Lasers Opt., vol. 87, no. 3, pp.531–538, 2007.[3] Morville, J., Kassi, S., Chenevier, M., Romanini, D., Appl. Phys. B 80(8), 1027–1038, 2005[4] Hodges, J. N., Perry, A. J., Jenkins Ii, P. A., Siller, B. M., Mccall, B. J., J. Chem. Phys. 139(139), 164201–164201 (2013).[5] Westberg, J., Brumfield, B., Wysocki, G., 11th International User Meeting and Summer School on Cavity Enhanced Spectroscopy, Boulder (2015)

References

This work was performed within the Competence Centre ASSIC - Austrian Smart Systems Research Center, co-funded by the Federal Ministries of Transport, Innovation and Technology (bmvit) and Science, Research and Economy (bmwfw) and the Federal Provinces of Carinthia and Styria within the COMET - Competence Centers for Excellent Technologies Programme.

Acknowledgements

Right: Typical transmission through a cavity mirror during consecutive current ramps.Left: Illustration of the two-fold optical feedback.

The cavity design with intracavity Brewster window was investigated using the following equipment:Ÿ DFB-QCL: AdTech optics HHL-14-15, 4.59 µm, single mode, room temperature,

40 mW CW at 280 mAŸ Mirrors: CRD - optics, R = 0.9995 at 4.59 µm, ROC ~ 6 mŸ Brewster Window: CaF , Crystran Ltd., d = 2 mm, angle of incidence ~ 51 °2

-4The reflectivity of the Brewster window was measured to be R = 9*10 . With the p

mirror reflectivity of 0.9995 and the cavity length of 0.25 m, this corresponds to a finesse of ~ 2000. Using the light transmitted through one of the mirrors an intracavity power buildup of ~ 250 was measured relative to the incidence power.

PZT

λ/2 - platePolarizer

HR - mirror

From QCL

Brewsterwindow

HR - mirror

A CaF window in Brewster‘s angle inside a linear optical cavity was investigated for coupling light from a Mid-2

Infrared Distributed Feedback Quantum Cascade Laser (DFB-QCL) into the high finesse cavity.[1,2] The Brewster - window cavity design offers several advantages over a two mirror linear design or a three mirror V-shaped cavity:Ÿ Simple cavity alignment and mode-matching: The low finesse s-polarized modes are spectrally broader than

the typical laser linewidth and allow fast and simple optimization. A lambda/2 wave-plate can be used to switch between the frequency degenerate low- and high finesse modes. (R = 10.7 %, F ~ 10)s,Brewster

Ÿ Optical feedback locks the laser to the cavity without high bandwidth electronic feedback loops. Only resonant light is reflected back to the laser.

Ÿ The Brewster window can be used to confine the gas cell. This allows reducing the sample volume and response time and, using a second window, can isolate the mirrors from aggressive gases.

Ÿ The reflectivity of the Brewster - window is easily adjustable via the window‘s tilt.Ÿ Low cost of the Brewster - window compared to additional folding mirror

Det

ecto

r Sig

nal /

V 0.1

0

-0.1

-0.2

Lase

r Cur

rent

/ m

A211.5

211

210.5

2100 0.5 1

Time / ms

Detector

α

β

d

Δlopt

QCL

The Brewster window‘s angle strongly influences the cavity‘s finesse and power buildup and the intensity of the optical feedback via the window‘s reflectivity relative to the mirrors‘ reflectivity. Depending on the application, the angle can be optimized for the desired feedback strength, finesse or power buildup. For very high Finesse

-4 -1cavities, absorptive losses (CaF @ 5 µm = 5*10 cm ) and scatter losses must be 2 , 10αtaken into account.

Angle of the Brewster Window

Spectral profiles (simulated) of the feedback ratio, defined as the intensity I reflected back to the laser relative to the incidence feedback

power I , (left) and of the intensity transmitted through each cavity 0

mirror (right) for five angles of incidence on the Brewster window. Note: θ = 54.5 °, R = 0.9995B mirrors

Feed

back

ratio

I/I

feed

back

0

0

0.04

0.08

0.12

0.16

Tran

smis

sion

thro

ugh

mirr

or /

I 00.12

0.09

0.06

0.03

0

Frequency / MHz Frequency / MHz

50 °52 °53 °53.5 °54 °

-2 -1 0 1 2 -2 -1 0 1 2

Feed

back

ratio

-3R

/ 10

in

Angle of incidence / °

3C

avity

Fin

esse

/ 100

0.51

50 52 540

2

4

6

0

0.05

0.1

0.15

Reflectivity (top panel), feedback ratio on resonance and cavity finesse (bottom panel) vs. angle of incidence on the Brewster window (all simulated)

λ/2 - plate

HR - mirror

Brewster window

Small volumegas cell

Gas cell

HR - mirror

Detector

QCL

Concept of a linear optical cavity with a Brewster window. The CaF window can 2

be used to couple light into the cavity and reflects resonant light back to the laser, yielding a positive optical feedback.

Δlopt= 2dn

cos β - tan β sin α [ [( ( ( ( ( (

Δφ=2πΔlopt

λ +π

for p-pol. and α < θB

Δφ=2πΔlopt

λ -π

for p-pol. and α > θB

and for s-pol and any α

n sin β = sin α ( ( ( (for α=θB: β=90°-α