Laser Light is characterized by monochromaticity, coherence and collimation. These properties make it the brightest existing light.
Production of LASER Beam
In the Laser system atomic environments of various types are stimulated to produce Laser light. A laser system consists of a transparent crystal rod or a gas or liquid filled cavity constructed with a fully reflective mirror at one end and a partially reflective mirror at the other. Surrounding the rod or cavity is an optical or electrical source of energy that will raise the energy level of the atoms within the cavity or rod to a high and unstable level. This Phenomenon is called population inversion. From this level, the atoms spontaneously decay back to a lower energy level, releasing the excess energy in the forum of light which is amplified to an appropriate wavelength. Thus, laser is created mainly by two means: population inversion in inactive medium and amplification of appropriate wavelengths of light.
TYPES OF LASERS
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Type of Laser
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Atomic Environment Used
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Effects Produced
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Argon
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Argon Gas
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Photocoagulation
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Krypton
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Krypton gas
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Photocoagulation
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Diode
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Diode crystal
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Photocoagulation
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Diode Pumped frequency Doubled Nd: YAG
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Diode and Nd: YAG crystals
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Photocoagulation
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Nd :YAG
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A liquid dry or a solid compound of yttrium-aluminum garnet and neodymium
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Photo disruption
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Excimer
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Helium and Fluorine gas
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Photo ablation
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1. Photocoagulation
The Principal lasers used in ophthalmic therapy are the thermal lasers, which depend upon absorption of the laser light by tissue pigments. The absorbed light is converted in to heat, thus raising the temperature of the target tissue high enough to coagulate and denature cellular elements. Argon diode, krypton and diode pump frequency doubled Nd- YAG lasers are based on this mechanism.
Modes of action. Photocoagulation is effective in treating ocular diseases by production of a scar, occlusion of vessels, tissue atrophy, and tissue contraction.
Therapeutic Applications based on photocoagulation are as follows:-
1. Eyelid Lesions such as Hemangioma.
2. Corneal Conditions e.g. reduction of postoperative astigmatism from cataract sutures-by argon laser suturotomy and treatment of corneal neovascularisation.
3. Laser for Glaucoma. Procedures employed include laser iridotomy for narrow angle glaucoma, argon laser trabeculoplasty (ALT) for open angle glaucoma, laser goniopunctures for developmental glaucoma, prophylactic pan-retinal photocoagulation to prevent neovascular glaucoma in patients with retinal hypoxic states (e.g., central retinal vein occlusion and cyclophotocoagulation for absolute or near absolute glaucoma.
4. Lesions of the iris. These include laser coreoplasty for updrawn pupil, photomydriasis, for pathologic miotic pupil, laser sphincterotomy and laser shrinkage of iris cyst.
5. Lesions of the retina and choroid. These form the most important indications, Common conditions are :- Diabetic retinopathy in which pan-retinal photocoagulation (PRP) is carried out for proliferative retinopathy and focal or grid-photocoagulation for exudative maculopathy.
- Peripheral retinal vascular abnormalities such as Easles’ disease, proliferative sickle cell disease, Coats’ disease and retinopathy of prematurity.
- Intraocular tumours such as retinoblastoma, malignant melanoma and choroidal haemangioma.
- Macular diseases, such as central serous retinopathy, and age related macular degeneration(ARMD)
- For sealing holes in retinal detachment.Complications of Laser photocoagulation.- Accidental foveal burns
- Macular edema
- Macular pucker
- Pre-retinal fibrosis
- Hemorrhage from retina and choroid
- Retinal hole formation
- Ischaemic papillitis
- Localized opacification of lens
- Accidental corneal burns.
2. Photodisruption.
Laser based on this mechanism ionize the electrons of the target tissue producing physical state called plasma. This Plasma expands with momentary pressures as high as 10 kilo bars, exerting a cutting/incising effect upon the tissues. Nd: YAG laser is based on this mechanism.
Therapeutic applications of Nd: YAG laser
These include capsulotomy for thickened posterior capsule and membranectomy for papillary membranes. Recently it has also been tried for phacolysis (laser phacosurgery) in phacoemulsification technique of cataract extraction.
3. Photoablation.
Lasers based on this mechanism produce UV light of very short wavelength which breaks chemical bonds of biologic materials, converting them in to small molecules that diffuse away. These lasers are collectively called excimer (excimer dimmer) lasers. These act by tissue modeling.
Therapeutic Applications:
- Photorefractive Keratectomy (PRK)
- Laser assisted in-situ keratomileusis (LASIK)
- For correction of refractive errors
- Phototherapeutic Keratectomy (PTK) for corneal diseases such as band-shaped keratopathy.
