development of an optical isolator for a fp cw qcl at 8 5 m using an experimental faraday rotator l.
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Development Of An Optical Isolator For A FP-CW-QCL At 8.5 μ m 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

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development of an optical isolator for a fp cw qcl at 8 5 m using an experimental faraday rotator

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
Motivation/Problem
  • Development of (EC)-QCL in mid-IR
  • Acquire high-resolution spectrum of C60~8.5μm
  • Back-reflection introduces
    • Intensity fluctuations
    • Frequency instability

“Collimating” Optics

Mode-matching optics

High Finesse Cavity

AOM

QCL

potential solution

AOM

High Finesse Cavity

Mode-matching optics

Isolator

Potential Solution
  • Employ Optical Isolator

“Collimating” Optics

QCL

the faraday effect
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
Essentials of An Optical Isolator
  • Gap in commercially available Faraday Rotators from 3.5 to 10 μm!
  • Three components
    • Pair of polarizers
    • Faraday rotator (FR)

P1

P2

FR

45°

45°

90°

faraday effect in n insb
Faraday Effect In n-InSb

CO2 Lasers: n-InSb

Interband Effect

Free Carrier Effect

  • Independent of Ne
  • Dependent on high B
  • Ne (cm-3) free charge carrier electron concentration
  • Dominates when:
    • B field >10 kG
    • Ne < 1x1016 cm-3
  • Dominates when:
    • High n-doping:
    • Ne > 1x1016 cm-3

Melngailis et. al. J. Quantum Electron. 1996, 84, 227.

  • Advantages:
  • Low optical insertion loss
  • Disadvantage:
  • Need very strong magnets >15 kG
  • Advantages:
  • Table top design
  • Disadvantage:
  • Increased optical insertion loss
testing set up
Testing Set-Up

PC

Lock-in

HgZnCdTe

Beam Chopper

HgCdTe

P2

P1

FR

Wire Grid Polarizers ~400:1

single pass analysis
Single-Pass Analysis
  • Transmission curve recorded 10° increments
  • Fit to:
  • Measured 6 ± 1° rotation
triple pass
Triple-Pass
  • Single-pass rotation provided undesirable ~6 ± 1°
    • Why?
  • Can increase power throughput by multi-passing
  • Z-Pass Configuration

M1

P1

P2

FR

M2

triple pass results comparisons
Triple-Pass Results/Comparisons
  • Rotation Low
    • High temperature
    • Thickness
    • Wavelength
  • Insertion Loss High
    • Reduction of transmission due to P2 setting
    • Triple Pass

100 West 18th

Tomasetta et. al. J Quantum Electron. 1979, QE-15, 266.

future directions

P4

P3

P2

P1

FR

Future Directions
  • Short Term
    • Test for adequate isolation
    • Inadequate isolation-additional WG polarizers
    • Acquisition of highly doped n-InSb
  • Long Term
    • Optimization of n-InSb material
acknowledgements
Acknowledgements

Brian Siller

NASA Laboratory

Astrophysics

Dreyfus

UIUC