Selection of a Laser Source for Magnetic Field Imaging Using Magneto-optical Films

Transkript

1 Selection of a Laser Source for Magnetic Field Imaging Using Magneto-optical Films Jennifer Talmadge ECE 355 Final Project Presentation November 28, 2001

2 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.

3 Overview Magneto-optical films and imaging Laser system requirements Review of possible systems and selection of best fit

4 Magneto-optical films Yttrium Iron Garnet (YIG) thin films Principle Faraday effect Thin film (10 µm) allows nearly planar B field imaging Multi-pixel image obtained from individual domains ( µm) in the film B

5 Faraday rotation Θ = V v B v dl Θ = 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 L

6 YIG Film Faraday rotation Note: Θ F V

7 Imaging in Magneto-optical films 2D Imaging possible: v Θ( x, y) = V B( x, y) v dl Application: subsurface crack detection in aging airplanes using eddy currents L I B

8 Experimental Setup M-O Film Laser Pol. Pol. CCD Camera Test Coil Mu metal shield Computer

9 Choosing a laser system - considerations Wavelength Power Stability (noise) Beam Quality Efficiency System Size Cost

10 Wavelength Maximize Faraday rotation angle without losing too much power to absorption. Absorption Wavelength (nm) Green ( nm) is the optimal spectral region.

11 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.

12 Other Factors High efficiency (to minimize power consumption) Size (a compact laser source is important in designing a prototype imaging system) Cost (obviously)

13 Possible Laser Systems HeNe (λ = nm) Green semiconductor diode Rare-earth-doped upconversion fiber lasers (e.g. Ho 3+, λ ~ 545 nm) The above three are low power, and the diodes and fiber lasers are not really commercially available. Argon ion laser (λ = 488/514 nm) Diode-pumped doubled Nd:YAG (λ = 532 nm)

14 Argon Ion Principle lines: λ = 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

15 Diode-pumped CW Nd:YAG λ = 532 nm (doubled from µm) 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)

16 Diode-pumped CW Nd:YAG cavity with intracavity doubling crystal IR Pump Diodes Doubling crystal Laser rod OC: R=96% at 532 nm

17 Diode-pumped CW Nd:YAG example from literature Electrical power used: 215 W Diode pump power: 55W Effective 1.06 µm output power: 6 W Actual 532 nm output power: 2.8 W Effective doubling η = 47% Optical to optical (807 to 532 nm) η = 5.1% Electrical to optical η = 1.3%

18 Diode-pumped CW Nd:YAG check for power level 2 Watt system, A ~ 1 cm 2 1 CCD pixel ~ 10 µm x 10 µm W 1cm I = 2 = 2µ W 0 cm pixels 2 pixel highest sensitivity is for a rotation of 10-6 radians I I = = I T film pw sin / θ F pixel = /

19 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 = J I = [ J / s pixel] 1/ [ photons / J ] I = photons / s pixel

20 Questions?