Transcript Development Of An Optical Isolator For A FP-CW

Development Of An Optical Isolator
For A FP-CW-QCL At 8.5μm Using
An Experimental Faraday Rotator
Brian E. Brumfield*
Scott Howard**
Claire Gmachl**
Donald K. Wilson†
Mark Percevault†
Benjamin McCall‡
*Department of Chemistry, University of Illinois, Urbana, IL
**Department of Electrical Engineering, Princeton University, Princeton Institute for the Science and Technology of Materials,
Princeton, NJ
†Optics For Research, Division of Thor Labs, Caldwell, NJ
‡Departments of Chemistry and Astronomy, University of Illinois, Urbana, IL
Motivation/Problem
• Development of (EC)-QCL in mid-IR
• Acquire high-resolution spectrum of C60
~8.5μm
“Collimating” Optics
AOM
QCL
• Back-reflection introduces
– Intensity fluctuations
– Frequency instability
Potential Solution
• Employ Optical Isolator
“Collimating” Optics
Isolator
QCL
The Faraday Effect
• Discovered 1845 by Michael Faraday
• Amount of rotation found equal to product of:
– V: Verdet constant (degrees/ G*cm)
– B: Magnetic field strength (G)
– L : Length of material traversed (cm)
Essentials of An Optical Isolator
• Three components
– Pair of polarizers
– Faraday rotator (FR)
• Gap in commercially available
Faraday Rotators from 3.5 to 10 μm!
P1
FR
P2
0°
45°
90°
45°
Faraday Effect In n-InSb
CO2 Lasers: n-InSb
Free Carrier Effect
Interband Effect
• Ne (cm-3) free charge carrier
electron concentration
• Independent of Ne
• Dependent on high B
• Dominates when:
• Dominates when:
• High n-doping:
Advantages:
Ne > 1x1016 cm-3
•Table top design
Disadvantage:
•Increased optical insertion loss
Melngailis et. al. J. Quantum Electron. 1996, 84, 227.
• B field >10 kG
Advantages:• N < 1x1016 cm-3
e
•Low optical insertion loss
Disadvantage:
•Need very strong magnets >15 kG
Testing Set-Up
PC
Lock-in
HgZnCdTe
Beam Chopper
P1
FR
P2
Wire Grid Polarizers ~400:1
HgCdTe
Single-Pass Analysis
• Transmission curve recorded 10° increments
• Fit to:
• Measured 6 ± 1° rotation
Triple-Pass
• Single-pass rotation provided undesirable ~6 ± 1°
– Why?
• Can increase power throughput by multi-passing
• Z-Pass Configuration
M1
P1
M2
FR
P2
Triple-Pass Results/Comparisons
100 West 18th
• Rotation Low
– High temperature
– Thickness
– Wavelength
• Insertion Loss High
– Reduction of transmission due to P2 setting
– Triple Pass
Tomasetta et. al. J Quantum Electron. 1979, QE-15, 266.
Future Directions
• Short Term
– Test for adequate isolation
– Inadequate isolation-additional WG polarizers
– Acquisition of highly doped n-InSb
P3 P1
• Long Term
– Optimization of n-InSb material
FR
P2 P4
Acknowledgements
Brian Siller
NASA Laboratory
Astrophysics
Dreyfus
UIUC