Source and Detectors PDF

Source and Detectors Session 4  Coefficient Scattering Consider a collimated beam of light travelling in the z-direction through a thin, elastically scattering, non-absorbing medium. As the beam passes through the medium, some of the light will b...

Source and Detectors Session 4  Coefficient Scattering Consider a collimated beam of light travelling in the z-direction through a thin, elastically scattering, non-absorbing medium. As the beam passes through the medium, some of the light will be scattered out of the beam into different directions and the intensity of the beam will follow an exponential decay,  Coefficient Scattering the probability that a photon will be scattered while travelling the short distance between  Tissue in Propagation Light Transport theory is an area of statistical mechanics which describes the passage of particles (neutrons, photons, molecules, etc) through ascattering medium.  Tissue in Propagation Light It is only possible to give some statistics about a large population of particles. Tissue in Propagation Light  Light of Detectors and Sources Incoherent Light Sources (LightEmitting & Superluminescent Diodes) Lasers (Coherent Light Sources)  Diodes Superluminescent & Emitting Light Light emitting diodes, or LEDs, are semiconductor p-n junctions that spontaneously emit light when electrons and holes recombine at the junction. LEDs are not narrowband sourcesto the extent that lasers are, but they have a peak wavelength. LEDs are cheap, very reliable, and can be modulated at high frequencies. Diodes Superluminescent & Emitting Light SLED) or diode (SLD superluminescent high power sources of broadband light. generate light at p-n junctions,although they differ from LEDs in that the light undergoes some amplification due to stimulated emission as it passes through the device. of Detectorsand Sources Light Lasers  Lasers Medical laser dioxide Carbon Carbon dioxide laser (λ = 10.6 µm) The laser levels are vibrational levels of the CO2 molecule, although the active medium also contains He and N₂ to improve the laser efficiency.  Lasers Medical The IR light is absorbed very strongly by water, so it can be used to vaporize tissue (which has significant water content). For this reason, it used today as laser scalpels and for laser skin resurfacing. The power of clinical systems is typically 10-50W  Lasers Medical laser dioxide Carbon  Lasers Medical laser Nd:YAG Nd:YAG laser (λ = 1064 nm) A four-level solid state laser in which the active medium is an yttrium-aluminum-garnett crystal (YAG, Y3Al5O12) doped with neodymium Nd3+ ions  Lasers Medical laser Nd:YAG Pulsed Nd:YAGs can generate pulses of tens to a few hundred mJ. Clinically, Nd:YAG lasers are used for photodisruption in posterior capsulotomy, photoacoustic imaging, and coagulation Lasers Medical laser Nd:YAG  Lasers Medical laser ArF excimer ArF excimer laser (λ = 193 nm) Excimer lasers are gas lasers that can emit at UV wavelengths. ‘Excimer’ is a contraction of excited-dimer. The active medium is a gas of diatomic molecules they are bound only when in an excited state, on return to the ground state the atoms immediately separate.  Lasers Medical laser ArF excimer Ionic bonds are formed between the Ar and K atoms through collisions with electrons sent through the gas.  Lasers Medical laser ArF excimer Because in the ground state the molecule immediately dissociates, the lower laser level is empty. Pulsed excimer lasers typically produce ~10ns pulses with 0.2-1 J energy.  Lasers Medical laser ArF excimer Clinically, the ArF excimer laser is used for corrective laser eye surgery (LASIK), because its short wavelength allows photoablation of corneal tissue without thermal damage  Lasers Medical Diode Laser Laser diodes ( λ=650-2000nm) are semiconductor lasers whose operation is akin to superluminescent diodes except that they use a resonant cavity to form the laser.  Lasers Medical Diode Laser Very compact, Inexpensive, Long lifetimes. Available in a wide range of red-NIR wavelengths, Output power can be as high as 50W for NIR diode arrays.  Lasers Medical Diode Laser clinicalapplications, NIR spectroscopy, Optical imaging, Photodynamic therapy, Photocoagulation. Table 1 Summary of properties and applications of laser and LED lights in cosmetics and dermatology. Sources Wavelength(nm) Properties and applications Laser lights It emits a blue-green light;It is strongly 488-515 Argon laser absorbed by both hemoglobin and water,It is rarely applied in aesthetic or dermatologic treatments. Pulsed dye laser 577-585 It emits a yellow light and can be absorbed by hemoglobin;It is especially utilized in treating vascular lesions.  Sources Wavelength(nm) Properties and applications He-Ne laser 632.8 Employed as a therapeutic instrument in many clinical situations, including vitiligo management and repair of nerve injury and wound healing. 694 It emits red light;It can be strongly absorbed by melanin in Ruby laser skin and hair and by blue and black pigment;It is usually used for laser hair removal and treatment of pigmented lesions such as freckles, liver spots Alexandrite Laser 755 Emitting a deep red light and being absorbed by melanin;Permitting deeper penetration into skin;Application in hair reduction. Emitting near-infrared lights; Being absorbed by Diode laser 800-900 melanin; Used for hair removal. Sources Wavelength(nm) Properties and applications Emitting a near infrared beam and strongly absorbed by Nd:YAG laser 1064 hemoglobin; Used for photocoagulation such as in treating broken blood vessels. Emitting a mid-infrared beam and absorbed by water; Er: YAG laser 2940 Used to ablate tissue for cosmetic laser resurfacing. Emitting a mid-infrared beam and strongly absorbed CO₂ laser 10,600 by water, Used for deep laser resurfacing.  Sources Wavelength(nm) Properties and applications LED light Dark red to near 650-1000 Treating a variety of conditions such as: ageing of the skin; fine lines and wrinkles; dull, coarse sun-damaged infrared light skin. Red 600-650 Anti-aging, stimulates collagen, reduces fine lines, scar reduction and, rejuvenates skin Improves photo-aged skin and used for photo-rejuvenation of skin; tightens Yellow to orange 570-600 and tones, detoxifies and stimulates the sensory motor nervous system  Sources Wavelength(nm) Properties and applications Low penetration into tissue due to its shorter Blue to green 495-570 wavelengthBlue light fights off and kills bacteria, as utilized in the treatment and control of acne Violet to deep blue 380-495 Treatment of psoriasis, dermatitis and vitiligo Van Tran, V., Chae, M., Moon, J.Y. and Lee, Y.C., 2021. Light emitting diodes technology-based photobiomodulation therapy (PBMT) for.dermatology and aesthetics: Recent applications, challenges, and perspectives. Optics & Laser Technology, 135, p.106698 Van Tran, V., Chae, M., Moon, J.Y. and Lee, Y.C., 2021. Light emitting diodes technology-based photobiomodulation therapy (PBMT) for.dermatology and aesthetics: Recent applications, challenges, and perspectives. Optics & Laser Technology, 135, p.106698  Optics Fibre One of the significant advantages of using light, and particularly laser light, over other wavelengths of radiation, is that it can often be coupled into optical fibres, which can then be used to deliver the light easily and precisely to where it needs to be.  Photodiodes This is particularly convenient for endoscopy and minimally-invasive surgery, in which all the surgical tools must be able to fit into quite small holes  Photodiodes Total Internal Reflection Optical fibres exploit this effect in order to trap light beams so that they can be transmitted over long distances without significant losses and into otherwise inaccessible areas.  Photodiodes Aperture Numerical Step-index fibres have a core medium in which the light travels and a cladding medium around the outside  Photodiodes Therefractive index of the core medium is slightly higher than that of the cladding so that the light can undergo total internal reflection. Thecore diameter will be 8-10 µm in a singlemode fibre but as large as 50 µm for a multi-mode fibre.  Photodiodes where is called the numerical aperture. As well as governing the maximum acceptance angle, the numerical aperture is important as it dictates the divergence angle of the light as it exits the fibre  Photodiodes Detectors Light Many of theapplications of light clinically are therapeutic, eg. to destoy tumours, or reshape the cornea. In such applications it is not necessary to be able to detect the light, other than for dosimetry or calibration purposes. In imaging and sensing applications, however, it is essential.