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 Back-reflection introduces –Intensity fluctuations –Frequency instability High Finesse Cavity Mode-matching optics AOM QCL “Collimating” Optics Development of (EC)-QCL in mid-IR Acquire high-resolution spectrum of C 60 ~8.5μm

Potential Solution QCL “Collimating” Optics Employ Optical Isolator Isolator High Finesse Cavity Mode-matching optics AOM

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) FR P1 P2 45° 0°0° 90° Gap in commercially available Faraday Rotators from 3.5 to 10 μm!

Faraday Effect In n-InSb Interband Effect CO 2 Lasers: n-InSb Dominates when: High n-doping: N e > 1x10 16 cm -3 N e (cm -3 ) free charge carrier electron concentration Independent of N e Dependent on high B Free Carrier Effect Dominates when: B field >10 kG N e < 1x10 16 cm -3 Melngailis et. al. J. Quantum Electron. 1996, 84, 227. Advantages: Table top design Disadvantage: Increased optical insertion loss Advantages: Low optical insertion loss Disadvantage: Need very strong magnets >15 kG

Testing Set-Up FR P1 P2 HgZnCdTe HgCdTe Lock-in PC Beam Chopper Wire Grid Polarizers ~400:1

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 P2 FR P1 M2 M1

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.

P4 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 FR P1 P2 P3

Acknowledgements NASA Laboratory Astrophysics Dreyfus UIUC Brian Siller