LASER

This lecture is based on post-graduate students of Ophthalmology (DO, DCO, MCPS, FCPS, MS) and optical principle of LASER, construction of laser and laser tissue interaction has cover the lecture

1. Md Anisur Rahman (Anjum) Professor & Head Ophthalmology Dhaka Medical College, Dhaka. 5/17/2021 1 [email protected] LASER

2. Laser vs Light Laser Light Stimulated Emission Spontaneous emission Monochromatic Polychromatic Highly energized Poorly energized Parallelism Highly divergence Coherence Non Coherence Can be sharply focussed Can’t be sharply focussed 5/17/2021 2 [email protected]

3. History of Laser 1960: The first laser was built by Theodore Maiman using a ruby crystal medium. 1963: The first clinical ophthalmic use of Laser in human 1968: L Esperance developed the Argon Laser 1971: Neodymium Yttrium aluminum garnet (Nd:YAG) and Krypton Laser develop 1983: Torkel developed the Excimer Laser 5/17/2021 [email protected] 3

4. What is laser? Laser is the acronym of L: Light A: Amplification by S: Stimulated E: Emission of R: Radiation 5/17/2021 4 [email protected]

5. Laser physics Laser as electromagnetic waves emitting radiant energy in tiny package called quanta/photon. Each photon has a characteristic frequency and its energy is proportional to its frequency Three basic ways for photons and atoms to interact . Absorption Spontaneous Emission Stimulated Emission 5/17/2021 [email protected] 5

6. 5/17/2021 [email protected] 6 Properties of laser

7. Properties of laser Laser is monochromatic A particular laser has single wavelength This depends on the medium used It cannot be white It is always coloured, i,e green, blue-green etc It is coherent, i,e each wave (photon) is in the same phase as the next. 5/17/2021 7 [email protected]

8. Properties of laser 7) It is collimated, i, e rays (photon) are exactly parallel 8) Polarization: The photons vibrate in the same plane 9) It produces bright light 10) It produces intense heat & energy at short distance 11) Laser can burn, coagulate, evaporate & disrupt 12) It can be concentrated in a very small area (Ref: Manual of Optics & Refraction PM Mukherjee Page: 2.3.4) 5/17/2021 8 [email protected]

9. Properties of laser The light emitted from a laser is   monochromatic , that is, it is of one wavelength (color).  In contrast, ordinary white light is a combination of many different wavelengths (colors). 5/17/2021 [email protected] 9

10. Properties of laser Lasers emit light that is highly directional.    Laser light is emitted as a relatively narrow beam in a specific direction.  Ordinary light, such as coming from the sun, a light bulb, or a candle, is emitted in many directions away from the source. 5/17/2021 [email protected] 10

11. Properties of laser 5/17/2021 [email protected] 11 The light from a laser is said to be  coherent,  which means the wavelengths of the laser light are in phase in space and time

12. 5/17/2021 [email protected] 12 Different issues to know to understand laser

13. Different issues to know to understand laser In order to understand the basic principle of a laser, it is instructive to first consider a p assive resonator ("cavity "),such as an arrangement of  mirror that creates a closed path for a   light   beam. The simplest configuration is made with only two mirror, one being flat and one being curved. 5/17/2021 13 [email protected]

14. Different issues to know to understand laser However, its optical power will decay, as some energy is lost in every resonator round trip. A so-called gain medium can now be inserted that, when supplied with energy ("pumped "). If the gain  g  is  lower than the resonator losses  l , the power decay is only slowed down. For  g  =  l , the optical power stays constant; and for  g  >  l , the power rises with each round trip. 5/17/2021 14 [email protected]

15. Different issues to know to understand laser The latter condition can not be maintained forever; sooner or later, the high intra cavity intensity will saturate the gain. In the steady state, as reached after some time, the gain will be exactly sufficient to compensate for the resonator losses. We then have continuous-wave laser operation with constant optical power and g  =  l . 5/17/2021 15 [email protected]

16. For extracting a laser beam as a useful output of the device, the left mirror, for example, acts as an output coupler, transmitting some percentage (say 10%) of the intra cavity power. 5/17/2021 [email protected] 16

17. Different issues to know to understand laser 1.Spontaneous absorption- electron will move from low energy level to high energy level by absorbing photon 2.Spontaneous radiation- electron will move from high energy level to low one by releasing photon 5/17/2021 17 [email protected]

18. 5/17/2021 [email protected] 18 Fig. 15.1. Energy levels of a simple laser . (ELKINGTON: 216)

19. 5/17/2021 [email protected] 19 Construction of Laser

20. Construction of Laser 5/17/2021 20 [email protected]

21. 5/17/2021 [email protected] 21 Construction of Laser Fig. 15.2 Laser tube (ELKINGTON 217)

22. Three basic components of Laser A Laser medium e,g solid, liquid or gas Exciting method Light or electricity Optical cavity (Laser tube) Around the medium acts as a resonator 5/17/2021 [email protected] 22

23. Construction of Laser Laser consists of a cylinder that may be solid or hollow; latter is filled with gas, liquid or a combination. These substances should have ability to absorb energy in one form and emit a new type of more useful energy. The energy can be thermal, mechanical, light or electrical. The process of conversion is called lasing. 5/17/2021 23 [email protected]

24. Construction of Laser A cavity of the cylinder has two concave mirrors at each end. One of them is fully reflective. The mirrors are coated with thin film of dielectric that reflects light close to the wavelength of the laser light. The other mirror is located on the other of the tube. 5/17/2021 24 [email protected]

25. The focal length of each mirror almost coincides with the centre of the tube The second mirror is partially reflective and is considered to be leaky . 5/17/2021 [email protected] 25

26. Construction of Laser There are two slanting windows that close each end of the tube. The cavity or the rod is surrounded by source of energy that raises the energy level of the atoms within the cavity to a high level in a very unstable state. This is called population inversion. 5/17/2021 26 [email protected]

27. The next step is spontaneous decay of the energized atom to a lower energy level. This phenomena is the basis behind the release of high energy in the form of light that is converted to suitable wavelength . 5/17/2021 [email protected] 27

28. Construction of Laser Thus, to summarized, there are 2 steps: Population inversion in active medium Amplification of appropriate wavelength. 5/17/2021 28 [email protected]

29. The energy stored in the laser material, i, e, gas, liquid or solid, is released in a narrow beam of monochromatic light. This light is a source of high thermal energy, which is used in ophthalmology for various purposes . 5/17/2021 [email protected] 29

30. Previously, we discuss that one mirror is partially transparent, some of the light is allowed to leave the tube. 5/17/2021 30 [email protected]

31. This light will be coherent (the wave fronts in phase), monochromatic (one wave length) and collimated (all the rays parallel). Light is produced continuously, and such a laser is said to be operating in continuous-wave (CW) mode . 5/17/2021 [email protected] 31

32. Laser mode Laser light is generally regarded as being coherent, as a practical level not all the light waves are preciously parallel as they resonate between the two mirrors of the Laser tube. Cross-section of laser beam at different points along its path reveals that it is very slightly divergent, and that it is more intense at certain points (called transverse electromagnetic modes) 5/17/2021 [email protected] 32

33. Transverse mode are not so important when energy is d elivered diffusely (retinal photocoagulation) But for photo disruption (YAG) it is important to have precisely focused energy a greater disruptive effect and, consequently, the effects of transverse modes need to be considered. 5/17/2021 [email protected] 33

34. Units of wavelength Unit Symbol Length Centimeter cm 10 -2 meter Angostrom 10 -8 meter Nanometer nm 10 -9 meter Micrometer μ m 10 -6 meter

35. 5/17/2021 [email protected] 35 Effects of laser energy on tissue

36. The effects of laser energy on ocular tissues depend upon the: Wavelength. pulse duration of laser light and the absorption characteristic of the tissue in questions (largely determined by the pigments contained within it). 5/17/2021 36 [email protected]

37. Effects of laser energy on tissue The effects can be Thermal Photochemical Ionizing effect 5/17/2021 37 [email protected]

38. Thermal effect Light energy is converted into heat energy if the wavelength coincides with the absorption spectrum of the tissue pigment on which it falls and if the pulse duration is between a few microsecond and 10 s Melanin in the retina absorb most of the visible spectrum & xanthophyll strongly absorb blue light, and hemoglobin absorb blue, green and yellow wavelength. 5/17/2021 38 [email protected]

39. Thermal effect In the retina, heat is transferred to the adjacent layers of the retina to cause a 10-20 degree rise in tissue temp. The result is photocoagulation and a localized burn. When visible or infrared light raises the tissue temp to 100 deg water vaporizes and causes tissue disruption. Example: Carbon di oxide. Argon laser. 5/17/2021 39 [email protected]

40. Photochemical effect When a pulse duration of 10 s or more is required to cause damage, the mechanism is the formation of free radical ions which are highly reactive and toxic to cells. Shorter wavelengths ( blue & UV) causes damage at lower levels of irradiance and are therefore more harmful. 5/17/2021 40 [email protected]

41. Ionisation Photon energy delivered in a nanosecond or less may be sufficient to strip electrons from molecules to form a collection of ions and electrons called a plasma. A plasma has a very high temperature and rapidly expands to cause a mechanical shock wave sufficient to displace tissue. Energy released as photons may produce a flush. Example: Nd-YAG & Argon-fluoride excimer laser 5/17/2021 41 [email protected]

42. When the laser energy exceeds the threshold for causing tissue damage, the mechanism of any damage depends largely upon the duration of exposure . 5/17/2021 [email protected] 42

43. Laser tissue interaction Laser Tissue 5/17/2021 [email protected] 43 Thermal effect Photocoagulation Photodisruption Photovaporization Photochemical Photo radiation Photoablation Ionizing Effect

44. Photocoagulation 5/17/2021 [email protected] 44 Target Tissue Generate heat Denatures Protein (Coagulation) LASER LIGHT

45. Photodisruption (Mechanical effect) 5/17/2021 [email protected] 45 Laser Light Miniature lighting bolt Optical breakdown Vapor Tissue damage Quickly collapses Acoustic shockwave Thunder clap

46. Photoablation Breaks the chemical bonds that hold tissue together essentially vaporizing the tissue e,g, photorefractive keratectomy. Argon-Fluoride (ArF) excimer Laser. Usually Visible wavelength: Photocoagulation Ultraviolet: Photo ablation Infra red: Photodisruption & Photocoagulation 5/17/2021 [email protected] 46

47. Photo ablation Vaporization of tissue to CO 2 and water occurs when it’s temp rise 60 – 100 deg or greater. Commonly used CO 2 Absorbed by water of cells Visible vapor (vaporization) Heat Cell disintegration Cauterization Incision 5/17/2021 [email protected] 47

48. Photochemical effect Photo radiation Also called photo dynamic Therapy (PDT) Photochemical reaction following visible/infrared light particularly after administration of exogenous chromophore Commonly used photosensitizer: Hematoporphyrin Benzaporphyrin derivatives 5/17/2021 [email protected] 48

49. Uses of Laser in ophthalmology Mode Lesion Tissue treated Photocoagulation Thermal burn Retina & TM Photoablation Breakdown of chemical bonds without thermal change Cornea Photodisruption Breakdown of form plasma resulting in disruption of tissue PCO Photovaporization Vaporization of fluid from the tissue to cut Small tumor 5/17/2021 49 [email protected]

50. Commonly used Laser in Ophthalmology Laser Wave length Effect Argon Laser Green Argon Laser Blue 514 nm 488 nm Photocoagulation Photocoagulation Nd YAG single frequency Nd YAG double frequency 1064 nm 532 nm Photodisruption Photocoagulation Diode Laser 810 nm Photocoagulation Excimer Laser 193 nm Photoablation Ruby Laser 550 nm Photocoagulation Krypton Laser Red Krypton Laser Yellow 647 nm 568 nm Photocoagulation Photocoagulation 5/17/2021 50 [email protected]

51. Modes of Laser Operation Continuous Wave Laser: It deliver their energy in a continuous stream of photons Pulse Laser: Produce energy pulses of a few ten of micro to few mili second Q Switches Laser: Deliver energy pulses of extremely shorter duration (nanosecond) A mode locked Laser: Emits a train of short duration pulses (picosecond) 5/17/2021 [email protected] 51