Power Measurement and Laser Delivery Systems PDF

NMPCE Power Measurement and Laser Delivery Systems Medical Physics | INIR Specialism Fearnley Evison MPhys IRadP  Lecture Outline NMPCE Measurement tools Fibre optic delivery Articulated arm delivery Hollow wave-guide deliv...

NMPCE Power Measurement and Laser Delivery Systems Medical Physics | INIR Specialism Fearnley Evison MPhys IRadP  Lecture Outline NMPCE Measurement tools Fibre optic delivery Articulated arm delivery Hollow wave-guide delivery Hand-pieces HSE, L64: Safety signs and signals. Guidance on the Health and Safety Regulations 1996  Laser power meter NMPCE  All medical lasers have an integral display which estimates power based on the factors selected.  Power will need to be measured by an engineer when the unit is serviced or when novel techniques are employed.  Power will need to be measured more Thermopiles or photodiodes regularly for specialist procedures (such as PDT). are common types of measurement device.  Thermopile NMPCE Cold end heat exchanger  Bank of thermocouples (fan assisted?)  Seebeck Effect  Measures temperature  Indicates power  Two dissimilar metals  Joined together  Junctions heated or cooled  Voltage indicates temp www.eolsurplus.com  Cold end kept at ambient Sensor  Photodiode detectors NMPCE  Si photo diode  Measures current  Indicates power  Voltage indicates temp  Wavelength specific  Quick response time Sensor Limited to a few mW continuous  Questions relating to thermopiles NMPCE  Sensor big enough for laser beam?  Typically about 10 mm diameter  All light absorbed?  Typically reflection from matte black ~2%  Good wavelength response?  Typically a few percent 200 – 20,000 nm  Good power handling?  Typically 10 W maximum power  Typically 10 kWcm-2 max average power density  Typically 10 GWcm-2 damage threshold  Good response time?  Typically 1 – 2 seconds  Integrating Sphere NMPCE Multiple diffuse reflections from inner surface: power of laser (or broad beam source) measured at output free from influences of beam shape or geometry. http://oceanopticsfaq.com/apps/reflectance-2/integrating-spheres-for-reflectance-measurements/ http://www.bing.com/images/search? q=integrating+spheres+photo&view=detail&id=251F502BA61398DADC1D8A87C03EDEEA7094CBE2&first =1&FORM=IDFRIR http://www.bing.com/images/search?q=integrating+spheres+photos&view=detail&id=BE70EE0A74788589DAC715FAD9FDE59F06C706CB&first=1&FORM=IDFRIR  Spectro-radiometer NMPCE Since lasers have well known properties, a simple power output is normally sufficient. An unknown or novel broadband source (such as LED or Xenon flash lamp) may give inconsistent and unsatisfactory results. For precise work (such as PDT) the spectra need to be investigated using a spectro-radiometer. Fibre optics: total internal reflection NMPCE http://www.teknik.uu.se/ftf/education/ftf2/Optics_FresnelsEqns.pdf Fibre optics: total internal reflection NMPCE Reflection and refraction are caused when optical radiation encounters a boundary to higher or lower wave velocities. Refractive Index (n) is the ratio of velocity in a vacuum (air) to that of the medium e.g. water 1.33, glass ~ 1.5 For a normally incident beam from air into medium with refractive index n: Ireflected / Iincident = [(n-1)/(n+1)]2  Fibre optics: total internal reflection NMPCE At the interface Fresnel equations show behaviour of the component waves Incident, reflected and transmitted beam all within the same plane Equal angle of incidence and reflection Refraction obeys Snell’s Law http://www.goldastro.com/images/generalrefractreflect For further details see the following http://encyclopedia2.thefreedictionary.com/Fresnel+equati ons Fibre optics: total internal reflection NMPCE For smooth air-glass interface reflection increases from 4% as angle of incidence increases  Fibre optics: total internal reflection NMPCE For glass-air interface reflection increases from 4% until ϴi = 42° (Brewster’s Angle) when Ir ~ Ii, i.e. Total Internal Reflection Fibre optics: total internal reflection NMPCE An optical fibre is formed by surrounding a highly transparent core with a cladding of slightly reduced refractive index (n) For a fibre, core n1 and cladding n2 with air incident beam n0 (=1) then maximum angle of acceptance θ is such that www.rp-photonics.com/numerical_aperture.html n0sinθ = √ (n12- n2 2 ) n0sinθ is normally referred to as numerical aperture NA e.g. n1 = 1.56, n2 = 1.46, NA = 0.55, θ = 33°  Fibre optics: total internal reflection NMPCE For illumination in air Cone of acceptance but output to water has an apex angle (e.g. urology) NA defined by numerical remains the same aperture. For a plane but delivery cone fibre delivering into air, changes, e.g. NA = output cone equals encyclopedia2.thefreedictionary.com/Acceptance+angle 0.55, θ = 33° input, acceptance cone 25° output Modes for fibres NMPCE Most fibres used in surgery are multimode, allowing a range of harmonics which increases illumination. However fibres for communication are normally graded (glass index varied with radius which reduce signal noise) or single mode (long distance) www.thefoa.org/tech/ref/premises/fiber.html  Materials for fibres and lenses NMPCE Silica glass; used Plastic Optical Fibres extensively visible near IR; (POF) e.g. PMMA; cheap very low loss. but with high loss; normally not index gradable ZBLAN glass (Zirconium, Barium, Lanthanum, Aluminium, Sodium): e.g. CO2, etc: Suitability of Holmium and Erbium Lasers material for fibre use determined by other losses, cost and manufacturing difficulties  Hollow wave-guides NMPCE  Problem with far IR transmission even if core transmits that wavelength  Fresnel loss (reflective) within the core remains very high (>25%)  Hollow (air) core wave-guides now being developed  Cladding can be either n=