BASICS OF LASERS AND LASER LIGHT Jayant K. Jogi Asst. Professor in Physics, L. E. College, MORBI.

The word LASER is an acronym for L ight A mplification by S timulated E mission of R adiation

Types of LASER Depending upon the active medium used there are FOUR types of LASER. 1. Solid LASER 2. Liquid LASER 3. Gas LASER 4. Free Electron LASER

Laser Fundamentals The light emitted from a laser is monochromatic, that is, it is of one color/wavelength. In contrast, ordinary white light is a combination of many colors (or wavelengths) of light. Lasers emit light that is highly directional, that is, laser light is emitted as a relatively narrow beam in a specific direction. Ordinary light, such as from a light bulb, is emitted in many directions away from the source. The light from a laser is said to be coherent, which means that the wavelengths of the laser light are in phase in space and time. Ordinary light can be a mixture of many wavelengths. These three properties of laser light are what can make it more hazardous than ordinary light. Laser light can deposit a lot of energy within a small area.

Incandescent v/s Laser Light Many wavelengths Multidirectional Incoherent Monochromatic Directional Coherent

Common Components of all Lasers Active Medium The active medium may be solid crystals such as ruby or Nd:YAG, liquid dyes, gases like CO2 or He - Ne, or semiconductors such as GaAs. Active mediums contain atoms whose electrons may be excited to a metastable energy level by an energy source. 2. Excitation Mechanism Excitation mechanisms pump energy into the active medium by one or more of three basic methods; optical, electrical or chemical. High Reflectance Mirror A mirror which reflects essentially 100% of the laser light. Partially Leaky Mirror A mirror which reflects less than 100% of the laser light and transmits the remainder.

Partially Reflecting) Laser Components (Totally Reflecting) (Partially Leaky/ Partially Reflecting) Gas lasers consist of a gas filled tube placed in the laser cavity. A voltage (the external pump source) is applied to the tube to excite the atoms in the gas to a population inversion. The light emitted from this type of laser is normally continuous wave (CW).

Before discussing the action of LASER below mentioned three processes are important to discuss. 1. Absorption 2. Spontaneous Emission 3. Stimulated Emission

Absorption of a Photon by an Atom Before After EX - E0 = hf EX EX E0 E0 Here, E0 and EX are energy of the Ground State and Excited State of an atom respectively.

Difference between Spontaneous Emission and Stimulated Emission

Spontaneous Emission Stimulated Emission Before After EX EX EX - E0 = hf E0 E0 Stimulated Emission Before After EX - E0 = hf EX - E0 = hf EX EX E0 E0 Stimulating Photon These Photons are identical in every way

To obtain continuous LASER beam Metastable State and Population Inversion are required. Normally, the mean life of excited atom before spontaneous emission occurs is about 10-8 sec. However, for some excited states, this mean life is perhaps as much as 105 times longer (i.e.,10-8 sec). These states are called Metastable States. Population Inversion Population inversion is a state in which more number of atoms are in excited state compared to the ground state. To achieve population inversion Optical Pumping is required.

Population Inversion and Normal Population are shown below. Population Inversion Normal Population Optical Pumping The process of exciting electrons from the ground to the metastable state is called Optical Pumping. EX EX E0 E0

Lasing Action Energy is applied to a medium raising electrons to an unstable energy level. These atoms spontaneously decay to a relatively long-lived, lower energy, metastable state. A population inversion is achieved when the majority of atoms have reached this metastable state. Lasing action occurs when an electron spontaneously returns to its ground state and produces a photon. If the energy from this photon is of the precise wavelength, it will stimulate the production of another photon of the same wavelength and resulting in a cascading effect. The highly reflective mirror and partially reflective mirror continue the reaction by directing photons back through the medium along the long axis of the laser. The partially reflective mirror allows the transmission of a small amount of coherent radiation that we observe as the “beam”. Laser radiation will continue as long as energy is applied to the lasing medium.

Lasing Action Diagram Excited State Spontaneous Emission Energy Introduction Metastable State Stimulated Emission of Radiation Ground State

ND:YAG Laser Construction:

Working:

HELIUM-NEON GAS LASER Gas lasers use gas atoms in a tube as an active medium. The excitation mechanism is usually an electric current through the gas. The current excites gas atoms to the correct energy level for lasing to occur. Mirrors on each end of the tube are aligned to reflect the laser beam through the active medium. About 2% of the light passes through the output coupler at the lower left. This photo is a typical 5 mW HeNe laser. This was the most common type of laser until the mid 1980s when reliable, low-cost diode lasers became available. HeNe lasers are still the second most common lasers. They provide higher beam quality than most diode lasers. They are widely used in scientific applications where low power, high quality beams are needed. There are many other types of gas lasers. Argon lasers produce powerful blue beams and are used in scientific research, medical applications, and laser light shows. Carbon dioxide lasers produce beams with powers of thousands of watts and are used for cutting and welding metals. Other gas lasers find a wide range of applications.

ND:YAG Laser Class 3B lasers are hazardous for direct eye exposure to the laser beam, but diffuse reflections are not usually hazardous (unless the laser is near the class limit and the diffuse reflection is viewed from a close distance). The maximum average power for a CW or repetitive pulse class 3B laser is 0.5 W. The maximum pulse energy for a single pulse class 3B laser in the visible and near IR varies with the wavelength. For visible lasers the maximum pulse energy is 30 mJ. It increases to 150 mJ per pulse in the wavelength range of 1050-1400 nm. For the ultraviolet and the far IR the limit is 125 mJ. Class 3B lasers operating near the upper power or energy limit of the class may produce minor skin hazards. However, this is not usually a real concern. Most class 3B lasers do not produce diffuse reflection hazards. However, single pulse visible or near IR class 3B lasers with ultrashort pulses can produce diffuse reflection hazards of more than a meter. Your laser safety officer will perform a hazard analysis.

Ruby (Al2O3) Laser

ELECTROMAGNETIC SPECTRUM Blue Green Yellow Red Visible Radio Gamma Ray X-ray Ultraviolet Infrared Microwaves Radio Short Wavelength Long Wavelength The electromagnetic spectrum extends from gamma rays at the short wavelength end to radio waves at the long wavelength end. The visible spectrum is a narrow slice somewhere in the middle, with blue light at the short wavelength end and red light at the long wavelength end. The next shortest wavelength region from the visible is the ultraviolet. Ultraviolet light causes sunburn, skin cancer, and cataracts. The next longest wavelength region from the visible is the infrared. Infrared light is invisible to the eye but can be felt as heat. It can cause burns to the skin or eyes. Lasers operate in the ultraviolet, visible, and infrared regions of the spectrum. Lasers in each spectral region present unique safety issues. Lasers operate in the ultraviolet, visible, and infrared.

Some Applications It is used as a convenient drill to bore holes in diamond. It is used in long distance surveying where great precision is required. Light received from small laser (about 200 mW) is used in optical fibers for communications. Large lasers are used in nuclear fusion and for military and research purpose. Lasers are also used in bar code reading and in reading and writing CDs and DVDs. Lasers are used in garment industries in cutting cloths and also in automobile industries. In the field of medicine, laser is being used for the retina detachment surgery, blood vessel cut etc. Lasers are also used in various types of alarm systems. Lasers are used in LIDAR (Light Detection and Ranging).

Laser Scanners Class 2 lasers must be visible. The natural aversion response to bright light will cause a person to blink before a class 2 laser can produce an eye injury. The average time for a human aversion response to bright light is 190 ms. The maximum aversion time is always less than 0.25 s. The only protection you need from a class 2 laser is to know not to overcome the aversion response and stare directly into the beam. This has been done, and people have burned their retinas doing it.

Laser Pointers Class 3R lasers are “Marginally Unsafe.” This means that the aversion response is not adequate protection for a direct exposure of the eye to the laser beam, but the actual hazard level is low, and minimum precautions will result in safe use. The CDRH Standard (FLPPS) allows only visible lasers in class IIIa. The CW power is limited to 5 mW. If the laser has a small beam so that more than 1 mW can enter the pupil of the eye, it carries a DANGER label. If the beam is expanded to be large enough that only 1 mW can pass through the pupil, the laser carries a CAUTION label. (This category of expanded beam laser is in class 2M in the new classification scheme.) The ANSI Standard has the same limits for visible class 3R lasers as the old ANSI class 3a and CDRH IIIa. It also allows invisible lasers in this class. An invisible laser with 1 to 5 times the class 1 limit is a class 3R invisible laser under the ANSI Standard. The only precautions required for safe use of a class 3R laser are that the laser user must recognize the level of hazard and avoid direct eye exposure.

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