Some of the section dealing with the physics of lasers was written with the help of an article by David Tilbrook, published in the July 1980 issue of Electronics Today International (ETI). That magazine regrettably went out of print some years ago. David's fascinating article was one of the things that sparked my interested in electronics and I am grateful to him.
The universe contains two fundamental quantity types:
Many quantities in molecular, atomic and sub-atomic physics are quantised. 'Classical Physics' assumed these were continuous and only after the advent of Quantum Physics (c1900) were working models of atomic structure and electromagnetic radiation possible.
QUANTUM PHYSICS AND LIGHT. 1.
Most light sources are heated solids or gases (e.g. electrically heated tungsten filaments) emitting a spectrum of wavelengths, the relative intensities of which are dependent on the temperature. Classical physicists found there were too many variables to analyse so to simplify things somebody invented the 'cavity radiator', simply a hollow solid with a hole in the side. When heated to a particular temperature the light from the hole is brighter than that from the outside. It is also largely independent of the material used to make the radiator and varies more simply with the temperature. Nevertheless, classical physicists using the wave theory of light still could not explain the relative intensities of the various wavelengths of this cavity or 'black body' radiation.
In 1900 Max Planck realised that the atoms in the cavity were behaving as electromagnetic oscillators which could emit light into and absorb light from the cavity but only at characteristic frequencies. He concluded that the atoms could not have a continuum of different energies but only energies given by the equation:
E = nhv
Also the atoms could not radiate continuously but only in 'quanta' and only when the atom jumped from a high energy state to a lower one. Planck, however, still thought of light as a wave.
QUANTUM PHYSICS AND LIGHT. 2.
If light is shone on a clean metal surface some electrons are liberated from the metal - the photo-electric effect. If the metal is put in a vacuum tube with an external circuit a measurable current will flow. By then connecting this to a power supply the voltage which just stops the photo-electric current can be measured and used to calculate the energy of the photo-electrons. Several things become apparent:
Fig.1. The Photo-electric Experiment
Classical physicists tried unsuccessfully to explain these phenomena using wave theory.
In 1905 Albert Einstein applied Planck's theories to the problem and was able to explain the observations. He postulated that light was not continuous waves but quanta he called photons, each with energy 'E' given by Planck's equation. Any single photon can interact with a single electron so the energy imparted to this electron will depend only on the energy of the photon and therefore its frequency. Increasing the intensity of the light will increase the number of photo-electrons released and hence the current. Emission will start instantaneously since all the energy required for a photo-electron release is contained in a single photon.
SPONTANEOUS AND STIMULATED EMISSION.
When a photon interacts with a bound electron it may not have enough energy to overcome the binding forces. In this case the photon is absorbed by the electron but the electron is not liberated from the atom. Instead, it jumps to a higher energy level, forcing the atom from its 'ground state' with energy 'E1' to a higher energy level, say 'E2'. If the photons absorbed have energy E = hv then the increase in electron energy will also be hv i.e. E2 - E1 = hv.
After about 10-8 seconds the electron will drop back to the ground state, re-emitting a photon, again with energy hv.
The excited atom has no preferred direction in space and so the photon can be radiated in any direction while the atom recoils in the opposite direction. This process is called spontaneous emission. If a group of atoms are excited in this way they will radiate photons in all directions randomly as the atoms return to their ground states.
If however a photon with energy hv interacts with another electron at energy level E2 the electron is forced down to its ground state with the emission of a second photon. This is called stimulated emission and is the basis for laser activity.
The most important point about stimulated emission is that both photons leave the atom with the same phase, direction and wavelength as the incoming photon. If they did otherwise they would interfere and cancel thus violating the law of energy conservation. The two photons are said to be coherent. If a group of atoms are excited in this way the initial beam of photons will be augmented by the additional photons and so is 'amplified'.
If a material is in thermal equilibrium at a temperature T, the distribution of atoms in a lower energy state to those in higher states is normally heavily accented towards the lower state. If N1 is the density of atoms in the lower state and N2 the density of more excited atoms, then the ratio of N2 to N1 is given by:
N2/N1 = e(kT/hv)
If the material is at 103K then N2/N1 = 10-5 so only one atom in 105 is in the excited state!
The condition in which the number of excited atoms exceeds those in the ground state is a non-equilibrium condition called population inversion. This condition is necessary to maintain laser activity. If the vast majority of atoms are in the ground state then only absorption and spontaneous emission can occur. If a population inversion can be maintained then stimulated emission will occur leading to photon multiplication. The process used to maintain population inversion is called pumping.
THE ACTUAL PHYSICS OF A LASER.
So how do you turn all this quantum physics into a laser? This is probably best illustrated by example. Let's look at the helium-neon laser which has a tube containing about 90% He and 10% Ne at a pressure of one to three Torr, through which high voltage dc can be passed.
Helium has energy levels at 20.61 and 19.82 electron volts (eV) which are metastable. Once at a metastable energy level an atom cannot move to a lower state by emission of a photon. It can only be de-excited by some other process e.g. transfer of energy to another atom. So, once an atom is excited to a metastable level it will stay there for a long time, about 10-3 seconds, hence large metastable populations can exist.
Two of neon's energy levels, 20.66 and 19.78eV, closely coincide with the metastable levels of helium. Large amounts of metastable He are produced by the electric discharge. Energy transfer will take place between metastable He and ground state Ne producing large populations of excited Ne atoms at 20.66 and 19.78eV. The population of excited Ne atoms produced in this way vastly exceeds that achievable by direct excitation by the electric discharge. Below these two highly populated energy levels are two lower Ne energy levels (20.3 and 18.7eV) that are only populated by direct excitation and therefore have much lower populations. We have a population inversion!
There are three possible transitions:
As Ne and He atoms jump between energy levels photons are emitted randomly in all directions. Since there are many Ne atoms at the 20.66 and 19.78eV energy levels, any photon with one of the above three wavelengths has a high probability of causing stimulated emission of a second identical photon. Those photons travelling parallel to the axis of the tube are reflected back and forth by mirrors at each end of the tube giving rise to more and more identical photons by stimulated emission. A limit is finally reached when the rate of production of Ne atoms at the higher energy levels equals the rate of stimulated emission.
One of the end mirrors is made a few percent transparent and a portion of the coherent radiation can escape from the tube to become the laser output. Although laser stands for light amplification by stimulated emission of radiation the HeNe laser is not really an amplifier but an oscillator generating coherent electromagnetic radiation at three distinct frequencies.
WHAT SORTS OF LASER ARE THERE?
Solid State Lasers. In laser physics 'solid state' does not refer to semiconductors but to a breed of laser having a medium that is formed by 'doping' a crystalline or glass material with an impurity which produces the laser action when pumped. The most common of these is the ruby laser which consists of a central, cylindrical synthetic ruby crystal made from aluminium oxide as a base material doped with chromium as the impurity. The crystal is mounted with mirrors at each end (the front one slightly transparent) and is surrounded by a xenon flash tube which is the 'pump'. Optical pumping is a requirement of all solid state lasers. The Nd-YAG laser falls into this group as well. Its crystal is a garnet made from yttrium-aluminium oxide doped with neodymium.
Fig.2. A Ruby Laser
Semiconductor Lasers. These are relatives of the LED, the most common being the gallium arsenide laser consisting of a semiconductor diode junction formed by GaAs doped with two different impurities to form the p and n materials. When forward bias is applied a large number of electrons and 'holes' move toward the junction and recombine to generate the laser light. Power output is low generally and efficiency is high. The output is easily modulated making these very useful for optical communication.
Fig.3. The Semiconductor Laser
Liquid Lasers. Most liquid lasers use an organic dye as the laser medium and are optically pumped, sometimes by other lasers. Their big advantage is that they are 'tunable'. This is achieved by changing the dye for course tuning or by rotating a diffraction grating for fine tuning of up to 50nm. Our most familiar liquid laser is the Candela. Its 585nm light passes through dermal structures and is strongly absorbed by HbO2 resulting in thermocoagulation of the ectatic vessels in vascular skin lesions (Selective Photothermolysis).
Gas Lasers. The HeNe laser is the most common in this group and its workings have already been described in detail. Commonly used medically are the CO2 and Argon lasers.
Fig.4. A CO2 Laser
OTHER BITS AND PIECES.
Frequency Doublers. These can be used to convert laser light to a different, shorter wavelength. A beam of light passed through a crystal of potassium-titanyl-phosphate (KTP) will emerge with a mixture of the original wavelength and a wavelength one-half the original (twice the frequency). In medical lasers, KTP is most often used with Nd-YAG.
Light Guides direct the laser beam to the surgical site. Fibreoptic bundles can be used for visible and near-infrared wavelengths while wavelengths out of this range require either an articulated arm containing front-surface mirrors at each joint or, rarely, more exotic type fibreoptics. The light may then be focused to the site by an operating microscope aimed using a lower power (usually HeNe) beam, or it may be used to heat a contact probe (usually sapphire tipped).
ADVANTAGES OF LASER SURGERY.
Lasers have some unique advantages in surgery. They allow precise microsurgery, even in difficult locations, by using fibreoptic delivery for example. The types of surgery amenable to lasers therefore includes all surface lesions and those accessible by laparoscopy or endoscopy e.g. thoracic surgery, ophthalmology, gynaecology, plastics, ENT, urology and neurosurgery.
The ability to focus the beam on a tiny area concentrates the intensity enormously, producing heat at a rate of many thousands of degrees in some cases, allowing precise, rapid vaporisation of tissue.
Laser surgery is relatively dry, providing near instantaneous sealing of small vessels and lymphatics. There is also minimal damage to adjacent tissues resulting in less oedema, scarring and post operative pain.
LASER INTERACTION WITH TISSUE.
Living tissue is basically an aqueous solution of light absorbing molecules. When an atom interacts with a photon whose energy does not exactly match a possible electron transition the atom is made to vibrate and produce heat. The wavelength of the light determines its degree of absorption. Other factors include the power density and duration of the laser burst and the scatter, thermal conductivity and local circulation of the tissue.
Long infrared wavelengths like that from the CO2 laser (10,600nm) are completely absorbed by water in the first few layers of cells leading to explosive vaporisation of the tissue surface and little damage to underlying tissue. Excimer lasers in the ultraviolet range are even more strongly absorbed by water and have an even more superficial effect.
Near infrared from a Nd-YAG (1064nm) is much less absorbed by water and the beam is transmitted and scattered through hundreds of times more tissue than the CO2. This produces less vaporisation and more thermal coagulation to a much deeper level (i.e. a few mm).
Red light from a ruby laser (694nm) is poorly absorbed except by cells containing dark pigment. The green (488nm) and blue (514nm) light from an argon laser and similarly from the krypton laser (476, 521, and 568nm) is transmitted by water and intensely absorbed by haemoglobin, thus penetrating skin and ocular structures to coagulate vascular or pigmented lesions.
Infrared and visible lasers have exclusively thermal effects on tissue but the ultraviolet photons from the excimer are energetic enough to disrupt chemical bonds and cause ionisation potentially leading to mutagenesis.
HAZARDS OF LASER SURGERY. 1. Non Airway Surgery.
Atmospheric Contamination. Tissue vaporisation by laser or electrosurgery produces smoke plumes and fine particulates with mean size 0.3mm. These can deposit in alveoli potentially causing interstitial pneumonia, bronchiolitis and reduced mucociliary clearance. It may also be mutagenic, laser smoke supposedly having half the mutagenic potential of that produced during electrocautery. Inhaling the smoke from a gram of tissue has been equated with 3-6 cigarettes. The smoke may also carry viral DNA or bacterial spores but the infectivity of this is yet to be proven. Prevention is with smoke evacuators and high efficiency masks (normal masks are useless).
Venous Gas Embolism. This is a consideration with laparoscopic laser surgery and also with laser probes that use coaxial gas cooling systems. Saline coolant has been recommended over gas but then there is the potential for TURP-like syndromes. Embolism has also been reported during laser resection of tracheal tumours.
Organ or Vessel Perforation. Vessels greater than 5mm diameter will not be coagulated by laser. Pneumothorax has occurred with laryngeal procedures. Burn depth is difficult to assess with Nd-YAG lasers and perforation can occur days after the surgery when oedema and necrosis peak.
Inappropriate Heat Transfer. Laser energy can potentially strike anything in the theatre causing fires and damage to patients, staff and equipment. Lasers should not be fired in any direction except at the lesion. The light may be reflected from any polished surfaces so dull or matt instruments should be used. If the light is not in the visible range then another coloured one must accompany the beam. Cover patients eyes with moistened pads or metal guards. Theatre doors and windows must be covered and warning signs posted. Moisten drapes to prevent ignition and have a bucket of water in the theatre. Staff should wear goggles of appropriate colour with side guards (clear plastic or glass for C02, green for Nd-YAG, orange for argon).
HAZARDS OF LASER SURGERY. 2. Airway Surgery.
All the above hazards apply with the additional problems of shared airways, potentially obstructing lesions, poor respiratory reserve in many cases, and the potential for airway fires. All the usual anaesthetic considerations for ENT surgery apply and will not be discussed here.
The high energy of laser beams can perforate or ignite endotracheal tubes, and anaesthetic gases vigorously support combustion potentially causing 'blowtorch' fires. The incidence of this is between 0.5 and 1.5% in the USA.
Relative Flammability. The data about the relative flammability of various types of ETT is conflicting. Many other factors are important such as the amount of blood and mucous present and the type of laser. The composition of the anaesthetic gases is also important. PVC tubes produce more toxic substances than red rubber and while silicone is harder to ignite it produces silica ash when it burns which may be blown far down into the lungs.
Gas Composition. The minimum FiO2 should be used to maintain oxygenation and N2O must be avoided. Other diluents should be used such as air or He, the lower density of which may be of advantage in airway obstruction, and its higher thermal conductivity may help prevent fires. The volatile agents are not flammable in clinical concentrations but can't be used in venturi systems.
Protective Taping. Muslin saturated with water and wrapped around the ETT has been used but it catches fire when dry so epidural catheters have been taped on as well to keep the muslin wet. More popular these days are foil tapes of aluminium, copper or metallised plastic. Copper appears to be best and the plastic ones are generally useless. A single length of 1/4 inch wide adhesive backed tape (tapes specially made for this purpose are available) should be wrapped spirally and overlapping from the cuff to the pilot tube entry point. This method of tube protection is cheap and generally effective but risks kinking, airway trauma, and aspiration of tape fragments. The cuff remains unprotected of course and should be filled with methylene blue stained saline so that perforation may be quickly recognised.
Special ETT's. Some special tubes have been made to deal with the problems of flammability and cuff rupture. From the literature it doesn't appear that these are much better than a taped tube but I'll describe them anyway.
Airway Fire Protocol.
Techniques Without ETT's. Spontaneous ventilation via ventilating bronchoscopes using 100% O2, halothane and topical anaesthesia (most important in paediatrics) is one alternative but the vocal cords are not immobilised and there are problems with air dilution and scavenging. Venturi jet ventilation via bronchoscope, Carden tube or Benjet tube is another option. Its advantages are the good conditions for laryngeal microsurgery and reduced fire risk. It does however require i.v. maintenance and muscle relaxation and potential problems are barotrauma, stomach inflation, unprotected airway, respiratory obstruction and mucosal dehydration.