Lasers in Laryngology Part 1
Mr. Vasant Oswal, MB, MS, FRCS(Eng), FRCS(Ed), DLO, DORL
The introduction of the surgical laser in the eighth decade of the last century revolutionised surgical instrumentation. It not only turned out to be an efficient knife, but also, intraoperatively, it sealed off blood vessels, lymphatics and nerve endings, making surgery nearly bloodless. The CO2 laser quickly became the workhorse for ENT since it is maximally absorbed by water, the main chromophore of the mucosa. Initially the laser machine itself was mounted on the horizontal arm of the microscope which made it very bulky. Later on an articulated arm was coupled to the microscope, the design which survived the onslaught of time. The beam exited coaxially during laryngeal microlaryngoscopy, thus retaining the advantage of magnification. Last but not the least; the beam was used as a free beam to the target, without a carrying handle. This resulted in an unobstructed view of the laryngeal tissues.
I acquired a CO2 laser in 1982 in the UK with funds raised by public appeal. I had to teach the technology to myself, which I did with several bench experiments and video recordings. I developed several suction- based dedicated instruments. The original experiments and the videos of the first few CO2 surgical procedures are on YouTube1. I authored and edited three books on Lasers, the last one in 2014, entitled “Principles and Practice of Lasers in Otorhinolaryngology and Head and Neck Surgery”2.
In the formative years of the laser, there were some, albeit not insurmountable, but still important concerns. The main and potentially hazardous consequence was the anaesthetic tube ignition leading to a surgical fire due to the hot beam in the airway. The consequences were particularly disastrous due to the inevitable high concentration of oxygen in the airway. During the early days of laser technology, reports of laser surgical fire with fatality kept appearing in the world media. To protect the rubber tube, we wrapped it in aluminium foil. We also quickly took the initiative to address the dreadful risk of the laser technology in the airway. I and my anaesthetic colleague John Hunton developed a fireproof metal anaesthetic tube which was marketed as the ‘Oswal-Hunton flexometallic anaesthetic tube’3. It was widely used until silicon coated ‘laser-safe’ tubes were commercially available. Norton et al4 in the US had also reported a similar design to ours.
Laser technology was a novel method, never experienced before. While with the knife one can increase the depth of cut by increasing the pressure, such was not the case with the laser technology. Activating the laser with foot pedal was an all or none phenomenon. It was not like the accelerator pedal of a car where increased pressure increased speed. In order to deepen the cut, the laser needed more power which also spread into the surrounding tissues, causing an unwanted collateral damage.
The cutting effect of the cold steel instrument was limited to the tissue in contact with the sharp edge of the knife. The laser cutting however was not limited to the point at which the laser struck, it was conducted beyond the target. This unwanted effect is undesirable during laser removal of a nodule or a polyp from the vocal fold; it means inevitable spread of energy in to the underlying normal tissue of the vocal, fold leading to necrosis and scarring. This collateral effect could not be seen in the solid tissue, its existence and extent could only be apparent on histological sections.
To demonstrate the collateral damage, I designed a transparent egg white model. Egg white being transparent, the collateral damage could be seen clearly, varying with a range of power settings: an experience, once seen-never forgotten. The egg white examples are included in the text at appropriate places on the Laser Physics Platform below. Minimising collateral damage with maximising cutting effect was earnestly pursued by the industry, which soon marketed superpulse and later, AcuBlade©, which successfully addressed this issue.
Laser plume was another unwanted by-product of heating tissue at 100°C. The plume contained char and many products of combustion hitherto not experienced in the operating suites. The central suction units soon got choked with carbon particles, since they were designed only for liquid waste such as blood and water. Dedicated laser suction units soon appeared in the market, with filtering efficiency down to 0.1µ.
Another hazard was exposure of eyes to the laser beam. Of course no one would aim the beam at anybody’s eye deliberately, but there was a real danger of reflected beam from the shiny instruments. Just blackening the instruments would still reflect the harmful beam in the same way as shiny instruments. The reflected beam must be dispersed. Dispersion was achieved by sandblasting the instruments. Laser-specific protective eye-wear had to be worn by anyone entering the laser zone.
Last, but not the least, laser surgical skill was a totally new expertise which required dedicated hands-on teaching. I chaired the First British Conference on the CO2 laser in Otolaryngology H&N surgery in 1983, attended by over a hundred international delegates. The first Cleveland International Laser Course was run concurrently. It lasted a mere three and a half hours and was not much more than learning hand and foot coordination without feedback from the business end, and manipulating the beam with a joy stick to tattoo the operator’s name on an apple! Stars Wars experience indeed!
The course became an annual event, providing hand-on teaching for a maximum of 36 participants. The Cleveland course has a distinction of being one of the very first ones in the world where hundreds of surgeons were taught surgery in a simulated environment on the pig larynx and tongue. A wrought iron box housed the pig larynx. A tube protruded from one of its sides to mimic the laryngoscope5. The course had a long run of 28 years, unusual for specialised courses!
It is my pleasure and privilege to share with the readers of the E-Learning programme on the Asia Pacific Laryngology Association Website, thirty years of pioneering experience in using lasers and, teaching laser technology to hundreds of our colleagues from across the globe.
2. Principles and Practice of Lasers in Otorhinolaryngology and Head and Neck Surgery4 (2014) by V. Oswal (Editor), M. Remacle (Editor), Hardcover: 947 pages, Publisher: Kugler Pubns; ISBN-10: 9062992323, ISBN-13: 978-9062992324.
3. Hunton J, Oswal VH. Metal tube anaesthesia for ear, nose and throat carbon dioxide laser surgery. Anaesthesia 1985;40:1210-2.
5. The laryngeal box: an aid to laser microlaryngeal surgery. Hampal S, Oswal VH. J Laryngol Otol. 1991 Nov;105(11):946.
LASERS IN LARYNGOLGY
Although lasers in laryngology have been used since the late seventies and early eighties, their use got a foothold only in the past couple of decades, mainly due to technological refinements. Advances in robotics made it possible to undertake total laryngectomy transorally, as was presented in the inaugural APLA conference in Singapore in November 2019 by Georges Lawson from Belgium
The following coverage of the lasers in laryngology is presented in four parts:
Part 1: Laser technology –core knowledge
Part 2: Laser tissue interaction
Part 3: Laser surgical techniques
Part 4: Safety with lasers
The material is based on presentations during The International Cleveland Laser course in the UK, which started in 1983 and ran for 28 years.
PART 1: LASER TECHNOLOGY
Mr. Vasant Oswal, MB, MS, FRCS(Eng), FRCS(Ed), DLO, DORL
Mr. Liam Flood, FRCS, FRCSI
1) What is laser?
Slide 1: Laser light
Laser is an acronym of Light Amplification of Stimulated Emission of Radiation. It was coined by Gordon Gould.
Atoms consist of a nucleus which is made up of protons, neutrons, and electrons which circle the nucleus in orbits at various energy levels. An external energy such as a flash pump is used to energise the electrons which jump to a higher energy level, and become unstable. When enough electrons are excited, the material reaches a state called population inversion. From the unstable state, the electron falls to the original stable state.
A photon is produced whenever an electron in a higher-than-normal orbit falls back to its normal orbit. During the fall from high energy to normal energy, the electron emits a photon, a packet of electromagnetic energy with very specific characteristics. A photon has zero mass, zero rest energy and zero electric charge. Photons only exist as moving particles. It is the basic unit that makes up all light.
An example of a photon is what is created when the sun converts particles into both heat and light.
The excited photons are then stimulated to drop back to their original stable level. During this process, more photons are emitted, they are similar in wavelengths and phase. A continuous process of stimulation results in more photons to be emitted. The surge in light output produces LASER, an acronym of Light Amplification by Stimulated Emission of Radiation.
Laser is an intense beam of light produced by a device that uses special gases or crystals. The construction and the process involved in the production of the laser beam is described later.
Although the laser is light energy, it is different from everyday light produced by sources such as an incandescent bulb. Laser processes emit a beam which has extremely high energy content. It is also a non-divergent beam, thus capable of travelling vast distances without losing its energy. Its emission is in a narrow frequency range depending on the lasing medium used.
A laser with a specific wavelength will have its specific target chromophore (a substance that can absorb its energy). As an example, visible light falling on water passes straight through it without affecting it in any way. Water is therefore not a suitable chromophore for visible light. However, if a CO2 laser wavelength emitting at 10,600 nm strikes the surface of water, it cannot pass through since water is an opaque medium for 10,600 nm wavelength. All of the energy is immediately absorbed by the water which heats up with production of vapour at its boiling point of 100 degree C.
Slide 2: Laser light vs everyday light
The energy in the laser beam is intense. Lasers could heat materials to temperatures hotter than the centre of the Sun in only 20 quadrillionths of a second, according to new research.
2) How did the concept of the Laser come into existence?
Everyday light is due to spontaneous emission. It is generated in the following way.
Slide 3: Spontaneous Emission
2.1 Spontaneous emission
Examples of such spontaneous emissions are fluorescence and thermal emissions.
- When an electron is excited, it gains energy and moves from a lower energy level to a higher energy level where it becomes unstable
- It decays to a lower stable energy state, emitting its energy in the form of a photon
- The photon is a massless, stable particle described by its wavelength λ, and its direction of propagation. The ‘size’ of the photon is basically the width of its wavelength. The wavelength of green light is about 500 nanometres, or two thousandths of a millimetre. The typical wavelength of a microwave oven is about 12 centimetres
- When decay takes place without external influence and results in emission of a photon, it is called ‘spontaneous emission’
- In spontaneous emission, photons have no common phase, they emanate in random directions
2.2 Stimulated emission: Laser light
Stimulated emission was a theoretical discovery by Einstein in 1917.
Atoms consist of a nucleus which is made up of protons and neutrons, and electrons which circle the nucleus in orbits at various energy levels. When an external energy such as flashpump is used to energise the electrons, they jump to a higher energy level. When enough electrons are excited, the material reaches a state called ‘population inversion’, which is an unstable state.
There are now more electrons in the higher energy state than in the lower energy state. If this happens in a medium known as gain medium, the energy of the emitted photon transfers to the electromagnetic field, creating a new photon. The new photon is exactly identical to the emitted photon and has the same phase, frequency, polarization, and direction of travel.
A photon is a bundle of electromagnetic energy. It is the basic unit that makes up all light. A photon has zero mass, zero rest energy and zero electric charge.
Photons only exist as moving particles transmitting light. In other words, light is carried over space by photons.
A continuous process results in more photons which are of a similar wavelength and phase are emitted. The surge in photons produces L A S E R - Light Amplification by Stimulated Emission of Radiation.
Slide 4: Stimulated Emission
Wavelengths of the various light emissions
The shortest wavelengths, from 10 to 400 nanometres (nm), produce ultraviolet (UV) light. Intermediate wavelengths, from 380 to 740 nm, produce visible light from violet to red. The longest wavelengths, from 700 nm to 1 mm, produce infrared (IR) light which, like UV, is invisible to the human eye.
The energy level in a wavelength
Wavelength of a particular laser and its energy level is specific to the lasing material used. The energy level is inversely proportional to the wavelength. Thus, shorter wavelengths have higher energy levels. The power source and the beam parameters also influence the ultimate energy level at the point of strike on the tissue
3) What are the components of the laser?
The basic components are:
1. Optical resonator
2. Gain medium
5. Laser aperture
Slide 5: Bench model of a laser
Slide 6: Nd:YAG laser
Slide 7: Ruby laser
Components of the laser and their functions
3.1 Optical resonator
Optical resonator is a cavity consisting of a tube which holds the gain medium and a set of reflecting mirrors.
3.2 Gain medium
The active laser medium (also called gain medium or lasing medium) is the source of optical gain within a laser. The gain medium is the material in to which the external energy is pumped to excite the electrons of that medium. Therefore, the properties of the gain medium influence the properties of the laser output (i.e., the wavelength).
Gain medium may consist of different materials with linear spectra or wide spectra. The output of the wide spectrum material can be tuned to produce different laser frequencies as required.
The laser medium can be a solid, gas, liquid or semiconductor. Lasers are commonly designated by the type of lasing material employed:
Some of the examples of different gain media and their uses in medical science:
The gain medium is liquid, e.g., methanol, to which chemical dyes such as rhodamine and fluorescein are added. Dye lasers are used in dermatology, cosmetic, cardiology, laser treatment of vascular lesions, laser cancer phototherapy etc. These are pumped by flashlamp.
Commonly used gases are carbon dioxide, argon, krypton and mixtures such as helium–neon. These lasers are pumped by electrical discharge. Carbon Dioxide (CO2) laser was one of the earliest gas lasers invented by Kumar Patel of Bell Labs in 1964. Its high absorption coefficient for water is most useful in oral and laryngeal surgery – deservedly known as workhorse laser. The CO2 laser produces a beam of infrared light with the principal wavelength bands centring on 9.4 and 10.6 micrometres (μm).
Material used is in the form of a crystal or glass. E.g., in Nd:YAG laser, typical host material is YAG (yttrium aluminium garnet). It is doped with Neodymium. Nd:YAG laser wavelength has a deep penetration property in the tissue and is thus unsuitable for ENT.
A KTP laser is another solid-state laser gaining popularity in ENT. It uses potassium titanyl phosphate (KTP) crystal which is engaged by a beam generated by Nd: YAG. This doubles its frequency, readily absorbed by red pigment (of blood). Coagulating blood supply to a tumour results in avascular necrosis.
Diode laser is a semiconductor laser with a current-carrying p-n junction as the gain medium. They are the most important type of electrically pumped lasers. To understand how it generates laser, the reader is advised to read further on the net! Semiconductor lasers are typically very small, and can be pumped with a simple electric current, enabling them to be used in consumer devices such as compact disc players.
A supply of energy is necessary to amplify light. The pump source provides energy to the laser system. There are a variety of pumping sources; electrical discharges, flash lamps, arc lamps, light from another laser, chemical reactions etc. The type of pump source used depends on the gain medium.
In the CO2 laser, internal mirrors are used to generate, maintain and amplify the laser beam by forming a reflective “resonator” around the excited CO2 gas mixture. Internal mirrors are sometimes called resonator or cavity mirrors.
External mirrors are used to deliver, manipulate, split and focus the laser beam. Most mirrors have flat reflective surfaces, but some have curved surfaces designed to reduce beam divergence.
3.5 Laser aperture
One end of the optical resonator is only partially reflective, and it is through this end that the beam exits the chamber. The beam is composed of photons travelling parallel to one another, at the same wavelength, and in phase with one another. The result is a highly concentrated, highly directed light beam that can cut, coagulate, or vaporize a patient's tissue.
That is how the laser is constructed; the beam is generated and delivered for clinical use!
4) How the laser light is different from ordinary light?
The two light sources are different in three parameters:
|Incoherent: All photons are out of phase with each other and spread randomly
||Coherent: All photons are travelling in the same direction with very little divergence
|Divergent: Spreading in all directions, losing intensity rapidly as it leaves the source
||Collimated: Has parallel rays, and therefore will spread minimally as it propogates
|Emission in all visible wavelengths (400 - 700 nm)
||Emission is monochromatic (in single wavelength) and very pure in colour
Slide 8: Ordinary light (incoherent, random, a mixture of all visible light)
Slide 9: Laser light
Slide 10: Coherent (in phase)
Slide 11: Collimated (directional)
Slide 12: Monochromatic (of one wavelength)
5) What is an Electromagnetic spectrum?
Slide 13: Electromagnetic spectrum
The electromagnetic spectrum1
- A spread of all electromagnetic waves is arranged according to frequency and wavelength
- The energy passes through the space at the speed of light in the form of sinusoidal waves which have oscillating electric and magnetic fields
- The entire electromagnetic spectrum, from the lowest (longest wavelength) to the highest (shortest wavelength) includes (from left to right):
- All radio waves (e.g., commercial radio and television, microwaves, radar),
- Invisible infrared radiation,
- Visible light,
- Ultraviolet radiation,
- X-rays, and Gamma rays.
Nanometre range is in the UV and visible part of the electromagnetic spectrum (EMS). Micrometre ranges are in the infrared part of the EMS. The longest waves thus have low frequency and the shortest waves have high frequency.
The whole frequency range is divided into separate bands, and the electromagnetic waves within each frequency band have different names. At the low frequency (long wavelength) end of the spectrum are radio waves, microwaves, infrared, and at the mid to high ranges (short wavelengths) are the visible light, ultraviolet, X-rays, and gamma rays.
5.1 Ionizing and Non-ionising radiation
Gamma rays, X-rays, and high ultraviolet are classified as ionizing radiation as their photons have enough energy to ionize atoms, causing chemical reactions.
It is clear that laser is a form of radiation, but the energy in the photons of none of the lasers is high enough to ionise atoms and therefore, they are ‘non-ionizing’ radiations. None the less, most surgical lasers have high power due to the collimated nature of the beam, which retains its power over a long distance and therefore presents a potential hazard. They fall under the control of the Radiation Protection Officer2 who must ensure that operation of the lasers takes place in the controlled designated zone.
6) In what region in the EMS do the commonly used lasers emit?
Conventionally, the EMS wavelengths are divided into:
High-end Non laser
Low-end Non laser
||TV and Radio
Slide 14: Laser Emission Spectrum
Slide 15: Laser Emission Spectrum
6.1 Frequency and the length of the wave
The length of each ‘wave’ increases from UV to Far Infrared. Consequently, the frequency of each region decreases from UV to Infrared.
6.2 Characteristic of the wave
The wave energy is described as photon towards the UV region and wave towards the far infrared region (e.g., microwave)
6.3 Ionising radiation (See slide 12)
Wavelengths in the cosmic rays, x-rays and high-end ultraviolet region are ionising radiation and thus cannot be used to produce laser. Boundary between high-end and Low-end ultraviolet is not well defined.
6.4 The primary wavelengths of laser radiation
The primary wavelengths of laser radiation include the ultraviolet, visible, and infrared regions of the spectrum.
6.5 Mechanism of tissue effects
Absorption of the energy results in photochemical effect in the UV region. In the infrared region, the action is primarily thermal; and in the visible region, both effects are present.
7) What are the Visible Laser Emissions?
The visible region consists of radiation with wavelengths between 400 and 700 nm. The visible wavelength region has clinically useful wavelengths such as Argon, KTP, Pulse dye and Ruby lasers. The absorption of energy results in both photochemical and thermal effects.
The wavelengths in the ‘visible’ region are no different than those in other regions. What makes them visible is the characteristic of our retina which is sensitive to the wavelengths ranging from violet at 400 nm to red at 700 nm, and the other colours of the visible spectrum in between. Not all vertebrates can see the colour that we see. For example, dogs’ retinae are not sensitive to our range, and from our viewpoint, they are ‘colour blind’.
Furthermore, the ‘visible’ wave energy itself is really ‘invisible’ as it travels in space. It becomes visible only when it strikes an object to which a particular wavelength is opaque and thus reflected back into the space. E.g., the colour red of the haemoglobin is red because only the 700nm is reflected back in to the space.
Any wavelength that passes through the cornea and the vitreous of the eye to reach the retina can be used to treat retinal conditions. But equally, when the intensity of the radiation is sufficiently high, the retina can be irreversibly damaged with corresponding reduction in visual acuity.
Lasers in visible zone are: Argon, KTP, Pulse dye He-Ne and Ruby lasers
Slide 16: Visible Laser Emissions
By definition, ‘visible’ must mean that you can see the laser beam. The retina is a complex part of the eye, and its job is to turn light into signals about images that the brain can understand. ‘Seeing’ is thus done by the retina. It follows that a visible laser beam can either be therapeutically used for the treatment of retinal condition, or it can accidentally strike the retina, resulting in reversible or irreversible damage.
Green light at 555 nanometres is the most visible colour to the human eye. (100%). Most green consumer laser pointers and handhelds emit at 532 nm light. This is perceived as being 88% as bright, compared with the potential maximum (555 nm light).
As a general rule, green lasers emitting at 532 nm are 5 – 7 times brighter than any other laser colour, at the same power. Whether blue, red, purple/violet, or a light colour like yellow, green is the best at strength for visibility. Any laser beam emitting in the visible part of the spectrum can cause retinal effects during unintended exposure. The following are some examples:
7.1 Eye injury from green laser pointers1
Laser pointers are low powered hand held lasers for use to highlight certain text or illustrations in lecture rooms. The output of laser pointers is limited to less than 1 milliwatt by the U.K. Health Protection Agency (but varies by country). In the U.S., regulatory authorities allow lasers up to 5 mW.
Even a laser of 5 mW can cause retinal damage if exposed for several seconds – a blink response avoids such a possibility. However, there are reports of afterimages, flash blindness and glare.
Laser pointers available for purchase online can be capable of significantly higher power output than the pointers typically available in stores.
7.2 Malicious use of green laser pointers
There are reports in the medical literature documenting permanent injury to the macula and the subsequently permanent loss of vision after laser light from laser pointer being shone to human's eyes.
When pointed at aircraft at night, laser pointers may dazzle and distract pilots, and increasingly strict laws have been passed to ban this. The eyes of pilots in an illuminated night-time cockpit are most sensitive to greenish-yellow light (of wavelength around 500–600 nanometres, peaking at 555 nm).
The main concerns of safety experts are focused on laser and bright light effects on pilots, especially when they are in a critical phase of flight: take off, approach, landing, and emergency manoeuvres. An unexpected laser or bright light could distract the pilot during a night-time landing or take off. There may be temporary flash blindness with night vision temporarily disrupted. There may be afterimages, like a bright camera flash leaving temporary spots2.
High power visible laser light could cause permanent eye injury, though it is unlikely.
Helicopter pilots, who fly at lower altitude and speed are at increased risk of more or less continuous malicious beam exposure.
8) What are the Invisible Laser Emissions?
The infrared region of the spectrum consists of radiation with wavelengths between 700 nm and 1 mm. The absorbing energy results in thermal effect.
Slide 17: Invisible Laser Emissions
Lasers in the infrared region are invisible to the human eye. It is therefore not possible to locate their position on the tissue.
8.1 Lasers in infrared zone: Nd:YAG, Ho:YAG, Er:YAG, CO2
Of course, it is necessary to identify its position of the tissue for precision surgery prior to its effect. Another laser, a low power laser which emits in the visible zone is superimposed on its path. He;Ne laser is used for this purpose. It emits in red colour. By positioning the red spot on the intended target, one can be certain that the target will be struck by the CO2 beam when activated, provided always that the superimposition is accurately maintained.
Slide 18: Aiming beam
The following slide demonstrates how to locate the invisible CO2 laser strike on the target.
Slide 19: Superimposition of visible HeNe beam
The laser beam is carried to the operating site by an articulated arm which contains a series of precision mirrors so that the beam accurately exits at the aperture. The articulated arm is highly engineered for precision delivery. It may be accidentally knocked while being moved. The CO2 laser beam and the HeNe beam may thus suffer malalignment as shown in the following slide.
Slide 20: Superimposition of visible HeNe beam
It is mandatory to test the alignment prior to every laser surgical session for alignment as shown in the following slide and a video clip. When the CO2 laser is used for stapedotomy, an alignment should be tested both at the beginning of the surgical session and also just prior to creating a perforation of the footplate.
Slide21: Testing for alignment on a wooden spatula.
Testing alignment just prior to one shot stapedotomy
As explained later, The CO2 beam cannot be transmitted via the optical fibre. However, other infrared lasers such as the Nd:YAG, Ho:YAG and Er; YAG can be transmitted via the optical fibre to the target and thus a guiding beam is not necessary.
9) How the laser energy is delivered to the target tissue?
One of the important aspects of the laser usage is the delivery of the energy to the target tissue. In cold steel instrument surgery, the business end, e.g. crocodile scissors has a shaft carrying it to the target tissue. Such is not the case with the laser, since, unlike the cold steel instrumentation, the laser energy is in the form of a beam of light. It is therefore come to be known as handle-less knife.
There are two principle ways of delivering the energy to the target issue:
- As free beam or
- Via an optical fibre
Slide 22: Free beam vs fibre delivery of the laser energy
9.1 As free beam
In free beam mode, the energy leaves the laser aperture and travels in air in a straight line. Thus the target in this mode must be in line of ‘vision’ of the aperture. As we have seen earlier, the beam is directional, with very little diversion. Most of the energy is thus available at the point where it strikes the target.
In laser microlaryngoscopy, the beam is delivered coaxially via a coupler. It exits from the objective of the microscope and then travels through the hollow laryngoscope as a free beam, to the target in the larynx.
9.2 Via hand piece
A hand piece is commonly used for surgery in the oral cavity. A guide at the end of the handpiece is placed on the target tissue. The emerging beam travels a short distance during which it remains in focus, retaining most of its power.
Slide 23: Free beam with hand piece
9.3 Reflected beam mode
Being light, the laser light has all the properties of light – the target tissue must be in line of sight. It can of course be reflected by putting a shiny surface, e.g. a mirror, in its path. The reflected light can then be directed at an angle. In cholesteatoma laser surgery, a small mirror is used in the middle ear. The incident energy is reflected. By positioning the mirror, the energy can be use to ablate the tissue, not in direct line of vision.
Another example of free beam delivery is the laser pointer used during lectures to point to the text of the screen.
9.4 Via an optical fibre
Optical fibres are made of silica. Within the fibre, there is a core of high refractive index glass surrounded by a thin cladding of low refractive index glass or polymer. The fibre transmission is easily achievable from the core-cladding boundary due to total internal reflection, as long as the angle between the light and the boundary is greater than the critical angle for wavelengths of between 250 nm and 2500 nm and for powers of up to 10 kW. In Laryngology, fibre transmissible KTP and Diode lasers are used for certain pathological conditions. However, since CO2 laser emits at 10.600 nm, it will not pass down the silica of the optical fibre.
Bending an optical fibre excessively may cause the optical signal to refract and escape through the cladding. The escaping laser beam is a fire hazard. It is a sound practice to inspect a re-usable fibre for any visible evidence of damage. If the energy of the emerging beam is less than the expected energy, a break in the cladding or coating might have occurred. Excessive bending during storage may cause permanent damage by creating micro cracks of the delicate glass fibres.
Slide 24: Fibre delivery of the laser energy
Typical silica-based fibres heavily absorb light with wavelengths above 2100 nm. HeNe laser, emitting at 633 nm is not absorbed by the silica in the glass and passes through a wine glass. However, CO2 laser beam emitting at 10,600 nm is not transmitted since silica in glass can only transmit wavelengths between 250 nm and 2500 nm. The result is a thermal reaction which causes some of the silica to vaporize with a creation of a crater. If strikes continue, the glass will shatter since it is a poor conductor of heat.
Slide 25: Effect of exposure of visible and invisible lasers on glass (silica)
9.5 Hollow Silica Waveguide
The waveguides consist of a fused silica capillary tube with an internal coating of optically reflective silver halide. The capillary tube is coated with an external jacket of acrylate, which improves the strength and flexibility of the waveguide. The CO2 energy is thus transmitted through a series of reflections. Waveguides thus should not be confused with optical fibres. They are available with core diameters of 300, 500, 750, and 1000 microns for transmission of mid to far infrared wavelength. However, they are not as flexible as the true optical fibre.
Slide 26: Effect of exposure of visible and invisible lasers on the eye
Slide 26 shows that a visible wavelength such as red HeNe laser used as a pointer will pass through the structure of the eye and reach the retina. If the energy of the beam is high enough, it will cause flares and even retinal damage described under 7.1 and 7.2.
Equally, the invisible CO2 laser will not pass through cornea and cause immediate damage due to thermal reaction.
Prescription lenses made of glass (silica) will not provide protection to eye damage from both visible and invisible lasers. The eyes of all personnel in the laser area must be protected by wearing wavelength-specific eye ware.
Visible KTP laser is being used increasingly in laryngology as an angiolytic laser. It is fibre deliverable and thus can be used in office-based procedures.
The fibre must be used in contact mode or near contact mode, since the beam diverges by about 10 degrees as it leaves the fibre, and thus loses its power density within a short distance.
10) Variable tissue effects by changing the distance between the tip of the fibre and the target
Maximum laser energy is delivered when the tip of the fibre is in contact with the target. Since the emerging beam from the tip of the fibre is divergent by 15 degrees, a target away from the tip will receive reduced energy, resulting in coagulation (useful to control oozing from capillaries) rather than ablation.
Slide 27: shows coagulation of the egg white when the distance between the tip of the fibre and the target is 1/0 cm.
Slide 28: Instant tissue loss is achieved when tip of the fibre is closer to the target.
11) Classification of lasers
END of part 1/ authored on 19th Jan 2020
Class 1 laser products are safe under reasonably foreseeable conditions of operation, including long-term direct intra-beam viewing, even when using optical viewing instruments, for example eye loupes or binoculars. Laser printers and compact disc players belong to this class
Class 1C laser products are products which are designed explicitly for contact application to the skin or non-ocular tissue. Examples of such products include home use hair removal products.
Class 1M laser products produce beams with a large diameter. Therefore, only a small part of the whole laser beam can enter the eye.
Class 2 lasers are limited to a maximum output power of 1 milliwatt or one-thousandth of a watt (abbreviated to mW) and the beam must have a wavelength between 400 and 700 nm. A person receiving an eye exposure from a Class 2 laser beam, either accidentally or as a result of someone else’s deliberate action (misuse) will be protected from injury by their own natural aversion response. This is a natural involuntary response which causes the individual to blink and avert their head thereby terminating the eye exposure. Repeated, deliberate exposure to the laser beam may not be safe. Some laser pointers and barcode scanners are Class 2 laser products.
Class 2M laser products produce beams with a large diameter beam in the wavelength range 400 to 700 nm. Therefore, only a small part of the whole laser beam can enter the eye and this is limited to 1 mW, similar to a Class 2 laser product. However, these products can be harmful to the eye if the beam is viewed using magnifying optical instruments.
Class 3R laser products are higher powered devices than Class 1 and Class 2 and may have a maximum output power of 5 mW. The laser beams from these products exceed the Minimum Permissible Exposure (MPE) for accidental viewing and can potentially cause eye injuries, but practically the risk of injury in most cases is relatively low for short and unintentional exposure. The risk is limited because of natural aversion behaviour for exposure to bright light for the case of visible radiation and by the response to heating of the cornea for far infrared radiation.
Examples of Class 3R laser products include some laser pointers and some alignment products used for home improvement work.
Class 3B laser products may have an output power of up to 500 mW (half a watt). Class 3B laser products may have sufficient power to cause an eye injury, both from the direct beam and from reflections. The higher the radiant power of the device the greater the risk of injury. Class 3B laser products are therefore considered hazardous to the eye. However, the extent and severity of any eye injury arising from an exposure to the laser beam of a Class 3B laser product will depend upon several factors including the radiant power entering the eye and the duration of the exposure.
Class 3B laser products which approach the upper limit for the Class may produce minor skin injuries or even pose a risk of igniting flammable materials. Examples of Class 3B products include lasers used for physiotherapy treatments and many research lasers.
Class 4 laser products have an output power greater than 500 mW (half a watt). There is no upper restriction on output power. Class 4 laser products are capable of causing injury to both the eye and skin from direct exposure and reflections also may be hazardous. Class 4 laser beams also present a fire hazard. Lasers used for many laser displays, laser surgery and cutting metals may be Class 4 products.