The following is a summary of basic laser studies.
USEFUL REFERENCE TEXT on basic laser principles: "An Introduction to Lasers and their Applications," by O'Shea, Callen and Rhodes; Addison-Wesley Publishing Company
INTRODUCTION :
LASER is a remarkable device,
1) A laser time standard is accurate to a tiny fraction of a second per year
2) A laser beam is highly convergent, can reflect from the moon and back to earth for easy detection.
3) A single atom can now be suspended in vacuum by means of radiation pressure from a laser
4) Laser based measuring system called range finders, so accurate that the altitude of the earth based sattelites can be measured within a few meters of uncertainty. The surface deformation of vibrating objects to within 0.05 nanometer (billionth of a meter). The same principle applied to vibrating glass windows and decoded for revealing what the conversation is inside the room for spying on people.
5) Focussed laser beam so intensely hot, it can trigger nuclear reactions to explode nuclear bombs.
6) State of the art research for Star wars
AS WE SPEAK, MORE TECHNOLOGICAL DEVELOPMENTS ARE TAKING PLACE WITH LASER
Short History: Albert Einstein (Time magazine's man of the century) showed in 1917 A.D. that a basic process of light amplification is possible just as it is in a laser today. This basic process is called "Stimulated or induced emission of radiation from atoms".
The parent device invented before the laser was by C.H. Townes at Columbia University in during 1950's. It is called MASER an acronym for microwave amplification for stimulated emission of radiation. Townes and Schawlow showed that the device can be extended to visible radiation in a joint publication. T.H. Maiman in Hughes Research Lab was the first to construct a working laser now known as Ruby Laser and soon after now famous HeNe gas laser was first made by Ali Javan of Bell labs.
LASER LIGHT IS EXTREMELY BRIGHT AND EXCEPTIONALLY NARROW
Light waves tell us about the wave nature of light. Light waves move in space and time. Ordinary light does not have a high degree of COHERENCE, POLARIZATION AND MONOCHROMATICITY.
MONOCHROMATICITY: Greek words, monos for single and chroma for color. Laser light has an extremely narrow band of frequencies which no other ordinary source can provide.
LINE WIDTH = Dl= -(c/Dn2)Dn BROAD BAND = about 300 nanometers
NARROW BAND = 0.01 nanometer
DIRECTIONALITY : Extremely narrow and concentrated
BEAM DIVERGENCE is very small at considerable distance from the source laser. FULL ANGLE BEAM DIVERGENCE
HUYGEN'S PRINCIPLE AND DIFFRACTION : Diffraction limitation produces minimum beam divergence of beam angle.
GAUSSIAN BEAM : 1/e of the maximum intensity is enclosed by narrow width of beam.
Beam waist is defined by the minimum Spotsize of radius wo.
LASER IS BRIGHTER THAN SUN
COHERENCE : Space or Spatial coherence
Partial coherence.
Longitudinal or temporal coherence
CORRELATION
Measuring coherence: Interferometer : Fringe visibility,
V = (maximum fringe irradiance - minimum fringe irradiance)/(maximum fringe irradiance + minimum fringe irradiance)
POLARIZATION
Types of polarization : linear, circular and elliptical
The PHASE DIFFERENCE
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BASIC LASER PRINCIPLES :
1. The Active Medium, the atomic basis for laser action, gas discharge spectral lamps, duscrete energy levels, quantum numbers, selection rules, transition probabilities, life time of atomic states;
ABSORPTION AND EMISSION :
Spontaneous emission
Induced or Stimulated emission
EINSTEIN'S RELATIONS : Einstein's coefficients
Spontaneous emission rate
Stimulated emision rate
Stimulated absorption rate
Absorption and Small-Signal Gain Coefficients
LASER PUMPING, Population Inversion
Optical Pumping, Three level and four level Schemes
Optical feedback, The laser Resonator, Optical Resonator,
Round Trip Power Gain AND the Threshold conditions
STRUCTURE OF LASER CAVITY :
Laser STABILITY, different geometry of laser cavities.
Radiation inside ther laser cavity is usually thought to be propagating back and forth between plane parallel mirrors in the form of well collimated beam. If after a large no. of reflections from end mirrors, the ray is observed to diverge fron the cavity resonator, it would mean high loss of internal laser beam and if found to converge towards the optic axis of resonator, it would mean low loss configuration of the laser cavity structure.
A stability criterion for laser cavity has been formulated in terms of so called "g-parameters" given in terms of the distance L between the two mirrors and radii r1 and r2 of the two mirrors.
g1 = [1-(L/r1)] and g2 = [1-(L/r2)] for the two mirrors.
And, 0 < g1 g2 < 1 is the stability criterion for laser cavity
TYPES : Planar, nearly planar convex, nearly planar concave, nearly confocal, confocal, concentric, nearly concentric and hemi-concentric etc. Telescopic resonator is another type.
LINE SHAPE : Gaussian and Lorentzian line profile
BROADENING : Doppler broadening, Collision broadening
LASER MODES
Axial modes of a laser : Total phase change
This means ln = 2L/n gives the longitudinal or axial modes of laser radiation.
Frequency,
n = c/l = nc/2L gives frequencies of axial modes.
COMMENTS: M.Azad Islam