III. Types of

1. CLASSIFICATIONS AND PROPERTIES OF Lasers come in many shapes and sizes. They are classified by various criteria:  medium is solid, liquid or  Wavelength is in the infrared, visible or ultraviolet spectral region  Mode of operation is continuous or pulsed  End or side pumping processes  Wavelength is fixed or tunable. The present state of the art includes:  Peak power > 1012 W;  Pulses shorter than 10−15 s;  Cheap, efficient diode lasers available at blue (400 nm), red (620–670nm), and near-infrared wavelengths (700–1600nm;(

Laser in Medicine Dr. Mohamed Sabry Laser wavelength Ranges  Infrared:

CO2 (10.6 μm), (1.55 μm), Nd:YAG (1.064 μm), Nd: (1.054 μm)  Visible: ruby (693nm), Kr+ (676, 647 nm), HeNe (633 nm), Cu (578 nm), Ar+ (514, 488 nm), HeCd (442 nm)  Ultraviolet: Ar+ (364, 351 nm), tripled Nd:YAG (355 nm),

N2 (337 nm) HeCd (325 nm), quadrupled Nd:YAG (266 nm), excimer (308, 248, 193, 150 nm  Tuneable lasers: Is a laser whose wavelength of operation can be altered in a controlled manner • Dye (range ~ 100 nm, dyes available from UV to near infrared) • Ti: (700–1000 nm, doubled: 350–500nm) • Free electron (far infrared to ultraviolet).

Laser in Medicine Dr. Mohamed Sabry Properties of Laser light 1. Monochromatic. Laser light is monochromatic; it consists of one color or a narrow range of colors. Ordinary light has a much wider range of wavelengths of colors. 2. Coherent Laser light is highest coherent (spatially and temporally) light present. 3. Directionality. laser light is emitted as a narrow beam and in a specific direction. This contrasts with light bulbs and discharge lamps, in which the light is emitted in all directions. The directionality is a consequence of the cavity. 4. Brightness Light is emitted in a well-defined beam means that the power per unit area is very high, even though the total amount of spectrum of the active atomic transition. This means that the spectral brightness (i.e. the intensity in the beam divided by the width of the emission line) is even higher in comparison with a white light source like a light bulb. For example, the spectral brightness of a 1 mW laser beam could easily be millions of time greater than that of a 100 W light bulb.

Laser in Medicine Dr. Mohamed Sabry Continuous and Pulsed Lasers Lasers can be made to operate continuously or in pulses. So far we have only considered continuous lasers, but many lasers in fact operate in a pulsed mode. Powerful pulsed flash lamps can give rise to very large pumping rates, with correspondingly large output pulse energies, especially when using a trick called Q-switching.

In , the losses in the cavity are kept artificially high by some external method. This prevents lasing and allows the build up of very large densities. If the losses are suddenly reduced, a very powerful pulse will build up because of the very high gain in the cavity.

Laser in Medicine Dr. Mohamed Sabry End and Side Pumping In addition to the high beam quality, end pumping also makes it possible to achieve a high efficiency (usually higher than achieved with side pumping). For these reasons, most diode-pumped solid-state lasers, particularly those with lower output powers, are end-pumped. Disadvantages of end-pumped laser designs are that pump light can be injected only from only two directions, that the optical intensity and crystal temperature vary along the beam direction, and that this approach leads to constraints on the beam quality of the pump source. Therefore, end pumping often cannot be used for high-power lasers In side pumping, high laser output is produced. This is because of the high area of the input photons to excite atoms, allowing more atoms to be excited and accordingly share in the lasing process. Another advantage is that the absorbed pump power can be smoothly distributed in the longitudinal direction.

Laser in Medicine Dr. Mohamed Sabry 2. Multi-level lasers Three-Level Lasers In a three-level laser, consider a group of N atoms, randomly exist in any of three energy states, levels

E1

in the ground state, i.e., N1 ≈ N, N2 ≈ N3 ≈ 0. If the atoms are excited by light or electric discharge of a frequency 1 휐 = 퐸 − 퐸 13 푕 3 1 will excite (pump) the atoms from the ground state to level 3, such that N3 > 0. In a medium suitable for laser operation, we require these excited atoms to quickly decay to level 2. The energy released in this transition may be emitted as a photon (spontaneous emission).

Laser in Medicine Dr. Mohamed Sabry • An atom in level 2 may decay by spontaneous emission to the ground state,

releasing a photon of frequency ν12 (given by E2 – E1 = hν12), which is shown as the transition L, called the laser transition in the diagram.

• If the lifetime of the transition, 2 → 1 τ21 is much longer than the lifetime of the 3 → 2 transition τ32 (τ21 ≫ τ32), the population of the E3 will be essentially zero (N3 ≈ 0) and a population of excited state atoms will accumulate in level 2 (N2 > 0). • If over half the N atoms can be accumulated in this state, this will exceed the

population of the ground state N1. A population inversion (N2 > N1 ) has thus been achieved between level 1 and 2, and optical amplification at the

frequency ν21 can be obtained.

Because at least half the population of atoms must be excited from the ground state to obtain a population inversion, the laser medium must be very strongly pumped. This makes three-level lasers rather inefficient, despite being the first type of laser to be discovered (based on a medium, by Theodore Maiman in 1960). In practice, most lasers are four-level lasers, described below.

Laser in Medicine Dr. Mohamed Sabry Four-Level Lasers In this system, there are four energy

levels, E1

Since the lifetime of the laser transition L is long compared to that of Ra (τ32 ≫ τ43), a population accumulates in level 3 (the upper laser level), which may relax by spontaneous or into level 2 (the lower laser level). This level likewise has a fast decay Rb into the ground state.

In a four-level system, any atom in the lower laser level E2 quickly de-excite, leading to a negligible population in that state (N2 ≈ 0). This is important, since any appreciable population accumulating in level 3, the upper laser level, will

form a population inversion with respect to level 2. That is, as long as N3 > 0, then N3 > N2 and a population inversion is achieved. Thus optical amplification, and laser operation, can take place at a frequency of ν32 (E3-E2 = hν32).

Laser in Medicine Dr. Mohamed Sabry JAVA Applet for LASER

Laser in Medicine Dr. Mohamed Sabry 3. Gas lasers He-Ne Laser A – laser or He-Ne laser, is a type of whose gain medium consists of a mixture of helium and neon inside of a small bore capillary tube, usually excited by a DC electrical discharge. The best known and most widely used He-Ne laser operates at a wavelength of 632.8 nm in the red part of the visible spectrum.

The gain medium of the laser, as suggested by its name, is a mixture of helium and neon , in approximately a 10:1 ratio, contained at low pressure in a glass envelope.

The gas mixture is mostly helium, so helium atoms can be excited. The excited helium atoms collide with neon atoms, exciting some of them to the state that radiates 632.8 nm. A neon laser with no helium can be constructed but it is more difficult. The energy source of the laser is provided by a high voltage electrical discharge pass through the gas between anode and cathode.

Laser in Medicine Dr. Mohamed Sabry A DC current of 20 mA is required for CW operation. The of the laser consists of two concave mirrors or one plane and one concave mirror. When electrical discharge pass through the gas, electrons accelerates through the tube and collide with helium and neon atoms and excite them to higher energy levels.

The helium atoms are excited to levels F2 and F3. Since the levels E4 and E6 of neon atoms have almost the same energy as F2 and F3, excited helium atoms colliding with neon atoms in the ground state can excite the neon atoms to E4 and E6. Since the He atoms are 10 times Ne atoms, then population inversion occurs in the Ne atoms and lasing action happens by transition of Ne electrons in

• E6 to E5 with wavelength 3.391 µm IR • E6 to E3 with wavelength 633 nm Red • E4 to E3 with wavelength 1.152 µm IR

Laser in Medicine Dr. Mohamed Sabry Usage of He-Ne Laser • Interferometry, holography and • barcode scanning, • Scientific and optical systems alignment, • optical demonstrations. • internal against polyps and other growths • Stimulating hair growth • cutaneous applications (related to skin medical treatments)

Related Lasers: Helium The population inversion scheme in He-Cd is similar to that in He-Ne except that the active medium is Cd+ . The laser transitions occur in the blue and the ultraviolet at 442, 354 and 325 nm. The UV lines are useful for high precision printing on photosensitive materials. Examples include lithography of electronic circuitry and making master copies of compact disks. (Ar+) Lasers Population inversion is achieved in a two-step process. First, the electrons in the tube collide with argon atoms and ionize them. The Ar+ ground state has a long lifetime and some of the Ar+ ions are able to collide with more electrons before recombining with slow electrons. This puts them into the excited states. Due to fine structure (spin-orbit coupling) this is actually a doublet. The two emission lines are at 488 nm (blue) and 514.5 nm (green). Several other visible transitions are also possible, making Ar+ lasers very good for colorful laser light shows.

Laser in Medicine Dr. Mohamed Sabry CO2 Laser The CO2 laser is one of the best examples of a molecular laser invented in 1964. lasers are the highest-power lasers that are currently available. The active laser medium is a gas discharge which is air-cooled (water-cooled in higher power applications). The filling gas within the discharge tube consists primarily of:

• Carbon dioxide (CO2) (around 10–20%) • (N2) (around 10–20%) • Hydrogen (H2) and/or (Xe) • Helium (He) (The remainder of the gas mixture)

CO2 laser uses the transitions occurring between different vibrational states of the carbon dioxide molecule. The carbon dioxide molecule consists of a central carbon atom with two atoms attached one on either side. Such a molecule can vibrate in the three independent modes of vibration shown. These correspond to the symmetric stretch, the bending, and the asymmetric stretch modes. Each of these modes is characterized by a definite frequency of vibration. These vibrational degrees of freedom are quantized.

Symmetric stretching Bending Asymmetric stretching

Laser in Medicine Dr. Mohamed Sabry Thus if we call ν1 the frequency corresponding to the symmetric stretch mode then the 1 molecule can have energies of 퐸1 = 푚 + 푕휈1 , m=0, 1 ,2 , . . 2 only when it vibrates in the symmetric stretch mode. Thus the degree of excitation is characterized by the integer m when the carbon dioxide molecule vibrates in the symmetric stretch mode. In general, since the carbon dioxide molecule can vibrate in a combination of the three modes the state of vibration can be described by three integers (mnq); the three integers correspond, respectively, to the degree of excitation in the symmetric stretch, bending, and asymmetric stretch modes.

When stimulated by an electric current, nitrogen molecules in the gas mixture become excited. Nitrogen is used because it can hold this excited state for long periods of time without discharging the energy. The high-energy of the nitrogen in turn excite the carbon dioxide molecules by collision. At this point, the laser achieves population inversion. For the laser to produce a beam of light, the CO2 atoms must lose their excited state by releasing energy in the form of photons.

Laser in Medicine Dr. Mohamed Sabry The CO2 laser possesses an extremely high efficiency of ∼30%. This is because of efficient pumping to the (001) level. Thus the atomic quantum efficiency which is the ratio of the energy difference corresponding to the laser transition to the energy difference of the pump transition, i.e., 퐸5 − 퐸4 휂 = 퐸5 − 퐸1

Output powers of several watts to several kilowatts can be obtained from CO2 lasers. High- power CO2 lasers find applications in materials processing, welding, hole drilling, cutting, etc., because of their very high output power. In addition, the atmospheric attenuation is low at 10.6 μm which leads to some applications of CO2 lasers in open air communications.

Laser in Medicine Dr. Mohamed Sabry 4. Solid-State lasers Ruby Laser A ruby laser is a solid-state laser that uses a synthetic ruby crystal as its gain medium. The first working laser was a ruby laser made by Maiman on 1960. • Ruby lasers produce pulses of visible light at a wavelength of 694.3 nm, with a very narrow linewidth of 0.53 nm, which is a deep red color. • Typical ruby laser pulse lengths are on the order of a millisecond. • The ruby laser is a three level solid state laser. The active laser medium (laser gain/amplification medium) is a synthetic ruby rod that is energized through optical pumping, typically by a xenon . • Ruby, which is the lasing medium, consists of a matrix of aluminum oxide in which some of the aluminum ions are replaced by ions around 0.05% of the crystal. It is the energy levels of the chromium ions which take part in the lasing action.

• Ruby has very broad and powerful absorption bands in the visual spectrum, at 400 and 550 nm, and a very long lifetime of 3 milliseconds. This allows for very high energy pumping

Laser in Medicine Dr. Mohamed Sabry • One of the first applications for the ruby laser was in rangefinding. • The ruby laser is used to optically pump tunable dye lasers. • Ruby lasers are rarely used in industry, mainly due to low efficiency. One of the main industrial uses is drilling holes through diamond

Laser in Medicine Dr. Mohamed Sabry Nd-YAG and Nd-Glass Laser ions form the basis for a series of high power solid state lasers. In the two most common variants, the Nd3+ ions are doped into either (YAG) crystals or into a phosphate glass host. These two lasers are known as either Nd:YAG or Nd:Glass. The main laser transition is in the near infrared at 1.06 μm which are four-level lasers. This Laser can operate in either continuous or pulsed lasers. Nd:YAG lasers are used in ophthalmology in cataract surgery, and in patients with acute angle-closure glaucoma. Frequency-doubled Nd:YAG lasers (wavelength 532 nm) are used for pan-retinal photocoagulation in patients with diabetic retinopathy. Nd:YAG lasers emitting light at 1064 nm are used for laser-induced thermotherapy, in which benign or malignant lesions in various organs are ablated by the beam. In oncology, Nd:YAG lasers can be used to remove skin cancers. They are also used to reduce benign thyroid nodules, and to destroy primary and secondary malignant liver lesions.

Laser in Medicine Dr. Mohamed Sabry Dye Lasers One of the most widely used tunable lasers in the visible region is the organic . The dyes used in the lasers are organic substances which are dissolved in solvents such as water, ethyl alcohol, methanol, and ethylene glycol. These dyes exhibit strong and broad absorption and fluorescent spectra and because of this they can be made tunable. By choosing different dyes one can obtain tenability from 3000 Å to 1.2 μm. The levels acting in the absorption and lasing correspond to the various vibrational sublevels of different electronic states of the dye molecule. Typical energy level

diagram of a dye is shown in which S0 is the ground state, S 1 is the first excited state, and T1, T2 are the excited states of the dye molecule. Each state consists of a large number of closely spaced vibrational and rotational sublevels. Because of strong interaction with the solvent, the closely spaced sublevels are collision broadened to such an extent that they almost form a continuum

Laser in Medicine Dr. Mohamed Sabry Semiconductor Laser ( ) The laser diode is the most common type of laser produced. Laser diodes have a very wide range of uses that include, but are not limited to, fiber optic communications, barcode readers, laser pointers, CD/DVD/Blu-ray reading, , scanning, etc. A semiconductor is a material which has electrical conductivity to a degree between that of a conductor (such as copper) and that of an insulator (such as glass). A bandgap is an energy range in a solid where no electron states can exist. The band gap is the energy difference (in electron volts) between the top of the valence band and the bottom of the conduction band in insulators and semiconductors. Semiconductor conductivity increases with increasing temperature or by light because this energy gives electrons energy to move from valence to conduction band

Laser in Medicine Dr. Mohamed Sabry Properties of Semiconductor Laser • Variable conductivity: A pure semiconductor is a poor electrical conductor. Through doping, the semiconductor can be modified to have an excess of electrons (becoming an N-type semiconductor) or a deficiency of electrons (becoming a P-type semiconductor). In both cases, the semiconductor becomes much more conductive). • Depletion Layer: This layer forms when P- and N- regions are brought in contact. The two parts are then called P-N junction. If a potential difference is applied on the junction, electrons in the N- moves to the positive side, and P- moves to the negative side. The region between the P- and N- is called Depletion Layer. • Light emission: If energy is given to the P-N junction, electron are excited and could move across the depletion layer. These excited electrons can relax by emitting light instead of producing heat. If photon could bounce between the two reflecting surfaces as shown, Laser is produced from the Laser Diode

Laser in Medicine Dr. Mohamed Sabry Questions (Homework II) 1. Write down an equation by which we can calculate the population of level

2 at any time t, knowing the spontaneous lifetime of the electron tsp. 2. What do we mean by halfwidth of spectral line. 3. Describe the operating method of three level laser and give an example of a laser produced by such method. 4. Describe the operating method of four level laser and give an example of a laser produced by such method. 5. What are the properties of Laser 6. Describe the operating principle and wavelength emitted of each of the following Laser (answer only 3 from these questions) 1. Describe the operation theory of HeNe Laser

2. Describe the operation theory of Co2 Laser 3. Describe the operation theory of Ruby Laser 4. Describe the operation theory of Nd:YAG Laser 5. Describe the operation theory of Dye Laser 6. Describe the operation theory of Semiconductor Laser

Laser in Medicine Dr. Mohamed Sabry