When the frequency of a weak radio wave striking the nuclei with an unpaired proton in the
material contained in the mine corresponds to the Larmor
frequency, energy is absorbed from the radio wave. This selective absorption
called resonance, is produced by selectively tuning the natural precession frequency of
the nuclear magnets, which is determined by a steady magnetic field applied to the
material, to the frequency of a weak radio wave. Alternatively a fixed frequency radio
wave is applied to the material and the applied magnetic field is varied in value until
the precession frequency of the nuclear magnets corresponds to the frequency of the radio
Protons have a nuclear spin and a magnetic moment and when a magnetic
field is applied to a single-detector bottle filled with a liquid containing protons they
align themselves along this field. When the magnetic field applied to the liquid sample is
very much greater than the earth's magnetic field and is at right angles to the
direction of the earth's magnetic field the magnetic moment of the protons will be
aligned with the larger field. When the applied field is quickly removed the resultant
magnetic vector due to protons will not follow the decreasing magnetic field but will
relax to its original value and direction, by precessing around the earth's magnetic
field analogously to a gyroscope precessing around the earth's gravitational field,
at a Larmor frequency given by the product of the
gyromagnetic constant and the value of the earth's magnetic field. By measuring the
frequency of precession the total value of the earth's field can be evaluated.
An electron has angular mometum and a charge giving it a magnetic
moment which causes it to precess in a steady magnetic field with an angular frequency
called the Larmor frequency, equal to the
gyromagnetic constant times the magnitude of the steady magnetic field. If an alternating
field is applied at this frequency then magnetic resonance conditions are obtained and
energy is absorbed from the system. For an unpaired electron there are two possible energy
levels in the presence of the steady field magnetic field. The lower energy state is when
the magnetic moment is parallel with the magnetic field and the higher energy state is
when the magnetic moment is anti-parallel with the magnetic field. The separation of these
two energy levels for the single unpaired electron corresponds to 2.8 MHz per Oersted of
magnetic field applied to the sample. In some paramagnetic crystals two unpaired electrons
contribute to the magnetic moment and three unequally spaced energy levels are present
rather than two for the single unpaired electron. Appropiate separation of the three
energy levels is achieved by adjusting the value of the magnetic field. The number of
electrons in the three energy levels are n1, n2, and
n3 where n1 > n2 > n3. and the population n1 corresponds to the number of electrons in
the ground state.
.Microwave energy at a frequency corresponding to the
transition n1 n3(pumping
power)is applied to equalise the populations in these two energy
levels, which causes the population levels in the other two levels to be inverted ie. (n3
is now greater than n2) and when microwave energy applied to the sample placed in a microwave cavity resonating at the frequency
corresponding to this transition, stimulated emission is produced which is the maser
action required for amplifying weak radio signals.
In a certain type of paramagnetic crystal the electron spin energy
levels available for maser action is produced by an unpaired electron with two energy
states. The lower energy state corresponds to the electron spin parallel to the magnetic
field and the higher energy one with the spin anti-parallel to the magnetic field. The
electron spin population is inverted by adiabatic fast passage and when a weak
microwave pulse of energy is applied to the sample placed in a cavity resonant at the
frequency corresponding to the transition between these two energy states, two-level maser
action is obtained.
One of a new type of amplifiers and oscillators that utilise the interaction between
an electromagnetic field and uncharged particles. When a beam of ammonia molecules is
passed through an electrically charged enclosure two discrete energy levels of the
molecule are produced. The upper energy state undergoes a transition to a lower energy
state when it is subjected to an alternating electromagnetic field in a microwave cavity
resonant at 24,000MHz.
When an unpaired electron is placed in a steady magnetic field, due its magnetic
moment and angular momentum it will precess at an angular frequency called the Larmor frequency given by the product of the
gyromagnetic ratio and the value of the magnetic field. When microwave radiation is
incident on the sample in a microwave cavity resonanting at this frequency, electron spin
magnetic resonance is obtained.
Nuclei in which at least one proton is unpaired behave like nuclear magnets and
because they also have angular momentum, when subjected to a strong magnetic field they
will precess with an angular frequency called the Larmor frequency in the way that
gyroscopes do in the earth's gravitational field, with a value given by the product
of the gyromagnetic ratio constant and the value of the magnetic field.
An electron spin can have different energy states when situated in a magnetic field.
The population of spins in the lower energy states exceed that in the higher energy states
When a microwave signal of sufficient power is applied at a frequency corresponding to the
transition between these energy states then it is possible to achieve a population
In certain materials the electrical resistance becomes zero when the temperature is
reduced to liquid helium temperatures.
The electron spin can have two energy states corresponding to the electron spin being
either parallel (low energy) or anti-parallel (high energy) with the magnetic field.
Transitions between these two energy states can be induced when microwave energy is
applied at the Larmor frequency determined by a
particular value of the magnetic field. If the microwave frequency is maintained constant
and the magnetic field traversed rapidly through the value required to cause transitions
between the high and low energy states, the spin population will be inverted with an
excess of spins in the higher energy state.
Energy is exchanged between the electron spin in an energy state,
either parallel (low energy) or anti-parallel(high enrgy) with the magnetic field and the phonons due to crystal lattice vibrations. In a direct energy
exchange the electron spin either absorbs or emits a phonon when it undergoes a
transition between the parallel and anti-parallel energy state. When the electron spin
population has been inverted by adiabatic fast
passage the time for
them to revert to their original lower energy state is determined by the number of phonons
available to provide the energy for this transition. Extensive measurements have shown
that when the temperature of a Phosphorous doped Silicon crystal is reduced to liquid
helium temperatures the time for electron spins to remain inverted can be many seconds.
When a crystal lattice vibrates due to thermal energy, the distance
between the ions in the crystal is modulated at the frequency of the lattice vibrations.
The ensuing motion of the neighbouring magnetic dipoles associated with the vibrating ions
produces an oscillatory magnetic field component. When the frequency of the oscillations
equals the Larmor frequency, the energy associated
with this lattice vibration, a phonon, induces a transition between the parallel and
anti-parallel states of the electron spin energy.
When the incident laser light is incident on a molcule it can either
be scattered elastically with no loss of energy and hence the frequency remains
unchanged, or it can be scattered inelastically, in which case it either gives up its
energy to the scattering system or takes energy from it. The quantum of energy that is
either taken up or released depends on the energy states in the molecule. The Raman
spectrum consists of the original laser light together with two spectral lines having
frequencies above and below the laser frequency. The higher energy one is known as the
Stokes line and the lower energy as the anti-Stokes line.
Laser action is obtained where the electron energy level system is
inverted by a powerful source of light energy causing transitions between the appropiate
energy levels in doped crystals or a gas mixture which emit quanta in the optical region.
The photons emitted by the spontaneous emission from a "pumped" upper energy
level to a lower energy level stimulates further emission from these excited states. The
photons produced by this stimulated emission travel through the volume of the crystal and
increase in intensity as they are reflected back and forth between plane reflecting
surfaces in a Fabry-Perot optical resonator cell. Oscillations of energy occur until the
inverted population is depleted. The process is repeated by applying pumping energy and
re-establishing the population inversion. The energy is coupled out using either a half
silvered mirror at the end of the cavity or a coupling hole in the centre of the
reflector. For Ruby and Neodymium doped solids the pumping energy required to maintain an
inverted population produces a substantial rise in temperature and unless cooled, the
efficiency of the lasing action is seriously impaired. Pulsed operation is usual for the
solid state laser and for continuous use a gas laser is used.
Rare-earth doped crystals of calcium and strontium fluoride
fluoresced in the visible region of the spectrum when irradiated by infra-red radiation.
The rare earth ions absorb radiation at one wavelength and when they are irradiated by a
second infra-red wavelength they emit visible light. By flooding the crystal with a local
source of infra-red, visible radiation was emitted when the crystal was irradiated by
infra-red radiation from the scene. With this mechanism image conversion from infra-red to
visible was obtained without intermediate electronics.
- When the laser light is passed through a crystal with suitable non-linear properties,
ammonium dihydrogen phosphate is one example, different frequencies can be generated with
various orientations of the crystal.
By changing the Q of the Fabry-Perot resonator from a very low to a
very high value by either mechanical (active) or electro-optical (passive) methods the
emission from a laser is released in a giant pulse rather than a series of oscillations.
One of the mechanical methods is to use a spinning prism or reflector and bleachable glass
and dye solutions can be used for the electro-optical method.
Information is conveyed in the form of variations in the envelope of a
carrier frequency. The change in the voltage of this wave is known as amplitude
In pulse-rate modulation the rate at which pulses of equal amplitude are generated varies
with the changes in the modulating signal. In pulse-width modulation the width (duration)
of the pulses increases (or decreases) with an increase in the amplitude of the modulating
voltage. In pulse-position modulation (ppm) the position of a pulse or series of pulses
with respect to a reference clock is varied in accordance with the modulating signal.