Selection of a Laser Source for Magnetic Field Imaging Using Magneto optical Films Jennifer Talmadge ECE 355 Final Project Presentation November 28 2001 Motivation Potential for non invasive imaging of biomagnetic signals especially the heart Magnetocardiography MCG holds great promise for early detection of cardiovascular disease and other heart conditions Overview Magneto optical films and imaging Laser system requirements Review of possible systems and selection of best fit Magneto optical films B Yttrium Iron Garnet YIG thin films Principle Faraday effect Thin film 10 m allows nearly planar B field imaging Multi pixel image obtained from individual domains 10 100 m in the film Faraday rotation v v V B dl L Faraday rotation angle follows the direction of the B field V Verdet constant of the medium B applied magnetic field L thickness of the medium YIG Film Faraday rotation Note F V Imaging in Magneto optical films 2D Imaging possible v v x y V B x y dl Application subsurface crack detection in aging airplanes using eddy currents L B I Experimental Setup M O Film CCD Camera Laser Pol Pol Test Coil Mu metal shield Computer Choosing a laser system considerations Wavelength Power Stability noise Beam Quality Efficiency System Size Cost Wavelength Maximize Faraday rotation angle without losing too much power to absorption 1 Absorption 1 2 0 8 0 6 0 4 0 2 0 400 425 450 475 500 525 550 575 600 625 650 Wavelength nm Green 500 550 nm is the optimal spectral region Power Stability and Beam Quality For now assume we need as much power as possible a few Watts will be sufficient Power stability and low noise from the laser are essential fluctuations in intensity onto the CCD will be interpreted as changes in the external magnetic field The beam needs to be uniform and circular for high quality images Other Factors High efficiency to minimize power consumption Size a compact laser source is important in designing a prototype imaging system Cost obviously Possible Laser Systems HeNe 543 5 nm Green semiconductor diode Rare earth doped upconversion fiber lasers e g Ho3 545 nm The above three are low power and the diodes and fiber lasers are not really commercially available Argon ion laser 488 514 nm Diode pumped doubled Nd YAG 532 nm Argon Ion Principle lines 488 514 nm Available power up to 10 s of Watts Amplitude noise 2 peak to peak 0 1 rms at low frequency Efficiency wall plug 0 03 Size 1 m long plus bulky power supply due to large power consumption Diode pumped CW Nd YAG 532 nm doubled from 1 064 m Available power up to 10W typically Amplitude noise 0 04 rms Efficiency wall plug 1 2 Size 4 x 5 x 13 inches plus a compact power supply Diode pumped CW Nd YAG cavity with intracavity doubling crystal IR Pump Diodes Laser rod Doubling crystal OC R 96 at 532 nm Diode pumped CW Nd YAG example from literature Electrical power used 215 W Diode pump power 55W Effective 1 06 m output power 6 W Actual 532 nm output power 2 8 W Effective doubling 47 Optical to optical 807 to 532 nm 5 1 Electrical to optical 1 3 Diode pumped CW Nd YAG check for power level 2 Watt system A 1 cm2 1 CCD pixel 10 m x 10 m 2 W 1cm I0 2 2 6 2 W pixel cm 10 pixels highest sensitivity is for a rotation of 10 6 radians I I 0 T film sin F 2 10 0 25 10 6 I 0 5 pW pixel 6 Diode pumped CW Nd YAG check for power level cont In terms of photons s A photon at 532 nm has an energy E 2 33 eV 3 73 10 19 J 12 I 0 5 10 J s pixel 19 1 3 73 10 photons J I 1 34 10 photons s pixel 6 Questions