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Lecture forum InnovationPoint Magneto Optics Application in Faraday isolators Markus Fegelein Product Manager Business Unit Crystal Technology LINOS Photonics GmbH & Co. KG Author: Markus Fegelein, Product Manager Crystal Technology │ LASER 2009, Munich │ Slide 1 Lecture forum Faraday Isolator: Principle Of Operation Faraday Isolators are optical components, which allow light to travel in only one direction. In principle the function of an optical isolator is analogous to that of an electrical diode. However the principle is completely different to that of a semi-transparent mirror. Laser Faraday Isolator Laser Faraday Isolator Quelle 5 Author: Markus Fegelein, Product Manager Crystal Technology │ LASER 2009, Munich │ Slide 2 Lecture forum Physical Principle – The Faraday Effect Magneto optic effect Michael Faraday discovered the MO effect in 1845. Rotation of polarization by magnetic field The plane of polarization of light is rotated when passing through a transparent material which is subjected to a strong magnetic field parallel to the direction of propagation. Source 1 Circular birefringence The Faraday effect causes waves to be decomposed into two circularly polarized rays which propagate at different speeds. β = V ⋅B ⋅d V = Verdet constant B = Magnetic field d = length of the faraday media Author: Markus Fegelein, Product Manager Crystal Technology │ LASER 2009, Munich │ Slide 3 Lecture forum Physical Principle – The Faraday Effect Parallel propagation For light propagating parallel to the magnetic field, a positive Verdet constant corresponds to a counter clockwise rotation. Non-reciprocal Unlike optical activity in crystals (i.e. wave retarders) this effect is non-reciprocal. Anti-parallel propagation For light propagating anti parallel to the magnetic field, a positive Verdet constant corresponds to a clockwise rotation. Accumulation of rotation angle If light is passed through the Faraday media and reflected back through it, the rotation doubles. Source 2 Author: Markus Fegelein, Product Manager Crystal Technology │ LASER 2009, Munich │ Slide 4 Lecture forum Faraday Isolator – Principle Of Construction Correct dimensioning of all components Choose crystal material, crystal length and magnetic field to give the polarization a 45°rotation. Adjustment of output polarizer Polarizer B (tilted by 45°) enables the light with rotated polarization to pass. Polarizing refelcted light Reflected light passing coming in through the “output polarizer” becomes polarized at 45°. Accumulation of rotation angle The Faraday Rotator again rotates the polarization by 45°. Blocking back travelling light back travelling light is polarized horizontally when leaving the faraday rotator and blocked by “input polarizer A”. Source 2 Author: Markus Fegelein, Product Manager Crystal Technology │ LASER 2009, Munich │ Slide 5 Lecture forum Faraday Isolators – Technical Realization The faraday media (mostly TGG) is placed in a rare earth ring magnet. Inside the magnet the magnetic field is very strong and homogeneous. Thus the crystals can be kept short and the rotation is uniform over the whole aperture. Polarizing beam splitting cubes deflect the blocked light. This ensures thermal stable operation even at higher laser power levels. Blocked light can be used for other applications: i.e. feedback control circuits. polarizer compact design design compact small insertion insertion loss loss small S N polarizer polarizer maximum maximum extinction extinction maximum maximum all t transmission a t s transmission ys -ccrry S G S GG TG T N N Faraday-rotator Faraday-rotator large aperture aperture large high isolation isolation high Author: Markus Fegelein, Product Manager Crystal Technology │ LASER 2009, Munich │ Slide 6 Lecture forum Faraday Isolators - Applications Protection of solid state laser resonators from + feedback + damage Protection of diode lasers from + feedback + damage (low damage threshold of diodes!) Prevention of parasitic oscillation in multistage solid state amplifiers + Decoupling master laser from amplifier + Incoupler for injection lock-in Author: Markus Fegelein, Product Manager Crystal Technology │ LASER 2009, Munich │ Slide 7 Lecture forum Faraday Isolators – Engineering Challanges Verdet Constant Materials with high Verdet constant keep isolators small and enable good optical properties. TGG offers highest Verdet constant for most laser lines. Verdet constant decreases with increasing wavelength. IR-Isolators need long crystals and large magnets. V [°/(T*mm)] TGG Transmission Spectrum (dmr: 5mm, L: 20mm) 380 580 100 90 transmission [a.u.] 80 980 1180 The wavelength range of TGG prevents isolators for far IR, UV light, and @490nm. Substitutes can be magneto optical glasses and B.I.G. crystals films. Thermal stability requires low absorption, good thermal conductivity and short optical path lengths. 70 60 50 40 30 20 10 0 300 780 wavelength [nm] 500 700 900 1100 1300 1500 1700 wavelength [nm] Author: Markus Fegelein, Product Manager Crystal Technology │ LASER 2009, Munich │ Slide 8 Lecture forum InnovationPoint Thank you for listening! Markus Fegelein Product Manager Business Unit Crystal Technology LINOS Photonics GmbH & Co. KG Sources of pictures: Source 1: www.wikipedia.com SourceMarkus 2: „Fundamentals photonics“ by Saleh+Teich, Wiley-Interscience Author: Fegelein, Product of Manager Crystal Technology │ LASER 2009, Munich │ Slide 9