III. Types of lasers 1. CLASSIFICATIONS AND PROPERTIES OF LASER Lasers come in many shapes and sizes. They are classified by various criteria: Gain medium is solid, liquid or gas 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), erbium (1.55 μm), Nd:YAG (1.064 μm), Nd:glass (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: sapphire (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 pulsed laser, the losses in the cavity are kept artificially high by some external method. This prevents lasing and allows the build up of very large population inversion 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<E2<E3, with populations N1, N2, and N3, respectively. At thermal equilibrium, the majority of the atoms will be 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 ruby laser 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<E2<E3<E4, and populations N1, N2, N3, N4, respectively. The pumping transition P excites the atoms in the ground state (level 1) into the pump band (level 4). From level 4, the atoms again decay by a fast transition Ra into the level 3. 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 stimulated emission 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 helium–neon laser or He-Ne laser, is a type of gas laser 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 gases, 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 optical cavity 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 spectroscopy • barcode scanning, • Scientific and optical systems alignment, • optical demonstrations. • internal laser surgery against polyps and other growths • Stimulating hair growth • cutaneous applications (related to skin medical treatments) Related Lasers: Helium cadmium The population inversion scheme in He-Cd is similar to that in He-Ne except that the active medium is Cd+ ions.