19 Metrology And Remote Sensing Applications

METROLOGY AND REMOTE SENSING APPLICATIONS Dr. Na. Venkat Nathan

LIDAR LIDAR (Light Detection and Ranging) is an optical remote sensing technology. Measures properties of scattered light to find range and/or other information of a distant target. The prevalent method to determine distance of an object or surface using laser pulses. 

The

range to an object is determined by measuring the time delay between transmission of a pulse and detection of the reflected signal. LIDAR technology has application in archaeology, geography, geology, geomorphology, seismology, remote sensing and atmospheric physics.

The term laser radar is also in use

but is misleading because it uses laser light and not the radio waves that are the basis of conventional radar. The primary difference between LIDAR and radar is that with LIDAR, much shorter wavelengths of the electromagnetic spectrum are used. Typically in the ultraviolet, visible, or near infrared.

In general it is possible to image a

object only about the same size as the wavelength, or larger. Thus LIDAR is highly sensitive to aerosols and cloud particles. Has many applications in atmospheric research and meteorology. An object needs to produce a dielectric discontinuity in order to reflect the transmitted wave. At radar (microwave or radio)

However non-metallic objects,

such as rain and rocks produce weaker reflections. Some materials may produce no detectable reflection. It means some objects are effectively invisible at radar frequencies. This is especially true for very

BACK SCATTERING Lasers provide one solution to these

problems. The beam densities and coherency are excellent. Moreover the wavelengths are much smaller than can be achieved with radio systems. The range is from about 10 micrometers to the UV (0.250 nm). At such wavelengths, the waves are

Different Types of Scattering Different types of scattering are

used for different LIDAR applications. Most common are Rayleigh scattering, Mie scattering and Raman scattering as well as fluorescence. The wavelengths are ideal for making measurements of smoke and other airborne particles

A laser typically has a very narrow

beam which allows the mapping of physical features with very high resolution compared with radar. In addition, many chemical compounds interact more strongly at visible wavelengths than at microwaves, resulting in a stronger image of these materials. Suitable combinations of lasers can allow for remote mapping of

LIDAR has been used extensively for

atmospheric research and meteorology. With the deployment of the GPS in the 1980's precision positioning of aircraft became possible. GPS based surveying technology has made airborne surveying and mapping applications possible and practical. Many have been developed, using downward-looking lidar instruments mounted in aircraft or satellites. A recent example is the NASA

TYPES OF LIDARS

MICROPULSE LIDAR Use

considerably less energy in the laser. Typically on the order of one micro joule. They are safer to eye can be used without safety precautions.

HIGH ENERGY LIDAR Widely used for measuring many

atmospheric parameters: the height, layering and densities of clouds, cloud particle properties (extinction

coefficient, backscatter coefficient, depolarization), temperature, pressure, wind, humidity, trace gas concentration (ozone, methane, nitrous oxide, etc.).

MAJOR COMPONENTS OF LIDAR

LASER SOURCE Laser — 600-1000 nm lasers are most

common for non-scientific applications. They are inexpensive but since they can be focused and easily absorbed by the eye the maximum power is limited by the need to make them eye-safe. A common alternative 1550 nm lasers are eye-safe at much higher power levels since this wavelength is not focused by the eye. They are also used for military applications as 1550 nm is not visible in

Airborne

topographic mapping lidars generally use 1064 nm diode pumped YAG lasers. While bathymetric systems generally use 532 nm frequency doubled diode pumped YAG lasers because 532 nm penetrates water with much less attenuation. Better target resolution is achieved with shorter pulses, provided the Lidar receiver detectors and

Scanner and optics There

are several options to scan the azimuth and elevation. It includes dual oscillating plane mirrors, a combination with a polygon mirror, a dual axis scanner. Optic choices affect the angular resolution and range that can be detected. A hole mirror or a beam splitter

Photo-detector and receiver electronics

POSITION AND NAVIGATION SYSTEMS Lidar sensors that are mounted on mobile platforms such as airplanes or satellites. This require instrumentation to determine the absolute position and orientation of the sensor. Generally Global Positioning System receiver and an Inertial Measurement Unit (IMU) are

Lidar used for study the nature of Aerosol

Working The radiation that is scattered by

various particles present in the atmosphere is picked up by the receiver. The background sunlight is removed by using filters. This scattered light gives information regarding the particles present in the atmosphere . The sensitivity that is much more

One

usually measures the time dependence of intensity of the backscattered laser light using a photo detector. The time variation can be easily converted into the height from which the laser beam has been backscattered. With the Lidar one can also study the concentration and size of various particles in the

THE BACKSCATTERED LASER RADIANCE

THE BACKSCATTERED LASER RADIANCE

The atmosphere which has no aerosols and the laser is backscattered by pure molecular gases such as N2 , O2 , Ar, etc.

THE BACKSCATTERED LASER RADIANCE

THE BACKSCATTERED LASER RADIANCE

If the atmosphere contain aerosols, then the time dependence of the backscattered laser radiance would roughly be of the

The kinks that appear in the

curve at the points marked X and Y. These are due to the fact that between the heights h1 and h2 there are aerosols . Which are responsible for a greater intensity of the backscattered laser light.

It may be seen that corresponding to

the height ‘h2’ the intensity is roughly the same as that from a pure molecular atmosphere. Thus beyond the height h2 one does not expect the presence of any aerosols. With the Lidar one can also study the concentration and size of various particles in the atmosphere.

Small particles are difficult to

detect with the microwave radar. The microwave radar can detect the presence of rain, hail, or snow in the atmosphere. This difference arises essentially due to the large amount of scattering that

In addition a Lidar can also be

used to study, the visibility of the atmosphere, the diffusion of the particulate materials in the atmosphere, to study the presence of clouds, fog etc. The study of turbulence and

Remote Sensing and Lasers

Remote sensing is any technique

for measuring, observing or monitoring a process or object without physically touching the object under observation. Avoid hazardous or difficult to reach regions, such as inside nuclear or chemical reactors, in biological hot spots, behind obstacles, inside smoke stacks, on the freeway, in the ocean depths,

Measure a process without disturbance,

such as monitoring flow around an aircraft model in a wind tunnel or measuring temperature during an experiment. Probe large volumes economically and quickly, such as providing global measurements of aerosols, air pollution, agriculture, human impact on the environment, ocean surface roughness, and large scale geographic features.

Laser remote sensing advances

traditional radar technology with benefits of shorter wavelengths, less beam divergence and wavelength selectivity. Optical remote sensing from airborne operations can be either active or  passive, by

Active monitoring of environmental

changes, terrain profile, water abundance,  gas temperature and concentration for various molecules, clouds and other  parameters of special relevance to forest studies, have been carried out using  various laser techniques. Helped by field studies, both techniques can be used to classify

LASER INDUCED FLUORESCENCE (LIF) Lasers

induced fluorescence in plants, can be used to monitor  plant species, through its signatures in multiple wavelengths. A fluorescence technique could be used to  determine the concentration of chlorophyll with phytoplankton. The fluorescence time is short, close to a nanosecond.

Chlorophyll molecule P680 is detected

between 650 -685nm and its return fluorescence can be  used to measure the efficiency of photosynthesis. Plants also fluoresce in the presence of UV wavelengths (300-380nm) from pigments other than chlorophyll Bands from 530nm-575nm can be used to locate paved surfaces and minerals such as iron in rocks and soil.  Bands of 770nm-810nm show cellular arrangement and water content.

LASERS USED Solid-state lasers such as the

Nd:YAG (532nm-3µm) pulsed from 100 - 200 Hz are used. The 355nm is sufficiently short to be dominated by  Rayleigh scattering. The 1064 nm wavelength is sufficiently long to be dominated by aerosol particle

The 532nm wavelength is

suitable for  both aerosol and molecular scattering having good detector characteristics.

Solid-state lasers, the TEA

CO2 laser at up to 11µm can be operated at PRFs of up to 300-400Hz with pulse

PRECISION LENGTH MEASUREMENT

For

precision length measurement using interferometric techniques the laser light should have, The large coherence length High output intensity

The method essentially consists of

dividing the beam from the laser by a beam splitter into two portions. Then making them interfere after traversing two different paths. One of the beams emerging from the beam splitter is reflected by a fixed reflector. The other usually by a retro reflector mounted on the surface

Thus a reflecting surface is moved, one

would obtain alternatively constructive and destructive interference,. It can be detected using a photo detector. Accuracies up to 0.1µm can be obtained by using such a technique. The conventional cadmium light source can be used only over path differences of about 20cm. The lasers can make very accurate

This technique is being

used, for accurate positioning of aircraft components on a machine tool, for calibration and testing of machine tools, for comparison with

The most common type of

laser used in such applications is the helium neon laser. Since the distance measurement is being made in terms of wavelength. In these measurements, high wavelength stability of the

THERMONUCLEAR FUSION

In laser induced fusion reactor,

deuterium-tritium pellet in the form of cryogenic solid The particle densities are ≈ 4 ×1022 cm-3 The laser lights are focused from all direction. Within a very short time, the outer surface of the pellet is heated considerably.

So that the pellet gets converted

into a very hot plasma. The hot ablated layer expands into vacuum As reaction gives a push to the rest of the pellet in the opposite direction (Inward, towards centre). Since the spherical pellet is irradiated from all sides, the spherical implosion front

With a incident intensity of 1017

W/cm2, the inward pressure will be ≈1012 atmospheres. When the implosion front accelerates towards the center, it sets up a sequence of shock waves travelling inwards. These shock wave leads to a very high compression densities along with the high

In

order to obtain high compression densities, the time variation of laser pulse has to be adjusted. The successive shock wave do not meet until they reach the centre of the pellet . The time variation of the laser power should roughly be (t0-t)-2 When t=t0, at that time all the

Laser Spectroscopy Absorption

spectroscopy usually involves with a tunable frequency source. Producing a plot of absorption is a function of frequency. This was not feasible with lasers until the advent of the dye lasers . Dye lasers can be tuned over a nearly continuous range of

Laser spectroscopy has led to

advances in the precision with which spectral line frequencies can be measured. This has fundamental significance in understanding the basic atomic processes. The precision has been obtained by passing two laser beams through the absorption

This

has selectively triggered absorption only in those atoms that have a zero velocity component in the direction of the beams. This effectively eliminates the Doppler broadening of spectral lines from the distribution of atomic

Laser Induced Breakdown Spectroscopy (LIBS) A kind of spark, initiated by intense

laser light. for sufficiently high electric field strengths in an insulating medium (e.g. air or glass) a breakdown can occur. The medium becomes electrically conducting. The mechanism behind this effect is based on the acceleration of free

This

starts an avalanche process, during which appreciable densities of free carriers can be built up within a short time. A plasma is formed, which can have a significant electrical conductivity. The plasma can be maintained by further current flow, which

Breakdown

in air and in other transparent media can be initiated by intense light. The electromagnetic waves with frequencies of hundreds of terahertz. The high optical intensities required can be reached in pulses as generated e.g. in a Q-switched laser (with nanosecond durations) or in a mode-locked laser and amplified in a

The intensity required for optical

breakdown depends on the pulse duration. For example, for 1-ps pulses an optical intensity of ∼ 2 × 1013  W/cm2 is required. In solid media, breakdown can lead to a modification of the material properties. Due to material damage or even to

Physical Mechanisms Breakdown with optical pulses are

different from those for static electric fields. Also breakdown of optical pulses depend on the pulse duration. For femto second pulses, multiphoton ionization can efficiently generate free carriers in the initial phase of the pulse. It is followed by strong absorption by the generated plasma. 

For

long pulses, multi-photon ionization is less important. The breakdown starts primarily from the few carriers which are already present before the pulse. For longer pulses the random occurrence of free carriers makes optical breakdown less deterministic . Whereas the breakdown threshold can be very well defined for femto

LIBS operates by focusing the laser

onto a small area at the surface of the specimen. When the laser is discharged, it ablates a very small amount of material, in the range of nanograms to Pico-grams. This generates a plasma plume with temperatures in excess of 100,000 K. During data collection, typically

At the high temperatures during

the early plasma, the ablated material breaks down into excited ionic and atomic species. During this time, the plasma emits a continuum of radiation But this radiation does not contain any useful information about the species present. Within a very small timeframe the

At

this point the characteristic atomic emission lines of the elements can be observed. The delay between the emission of continuum radiation and characteristic radiation is in the order of 10 µs. This is why it is necessary to temporally gate the detector. LIBS can often be referred to as its alternative name: laserinduced plasma spectroscopy

LIBS is technically very similar to a

number of other laser-based analytical techniques, sharing much of the same hardware. These techniques are the vibrational spectroscopic technique of Raman spectroscopy, and the fluorescence spectroscopic technique of laserinduced fluorescence (LIF). In fact devices are now being manufactured which combine these techniques in a single instrument, allowing the atomic, molecular and

LIBS Design A typical LIBS system consists of a

Nd:YAG solid-state laser and a spectrometer. With wide spectral range, a high sensitivity, fast response rate, time gated detector. This is coupled to a computer which can rapidly process and interpret the acquired data. The Nd:YAG laser generates energy in the near infrared region

The pulse duration is in the

region of 10 ns. The power density generated can exceed 1 GW·cm-2 at the focal point. Other lasers have been used for LIBS mainly Excimer Lasers. 

Spectrometer The scanning type spectrometer

consists of a monochromator and photomulitiplier. Non scanning type consists of a polychromator and CCD detector respectively. The spectrometer collects electromagnetic radiation over the widest wavelength range possible. This maximizes number of

Spectrometer

response is typically from 1100 nm (near infrared) to 170 nm (deep ultraviolet). The energy resolution of the spectrometer can also affect the quality of the LIBS measurement. Since high resolution systems can resolve spectral emission lines in close combination. Also reduces interference and

This

feature is particularly important in specimens which contains a large number of different elements. The spectrometer and detector is connected by a delay generator. The delay generator accurately gates the

Applications Spectroscopic investigation of the

light emitted from the plasma is used for determining what chemical elements exist in the sample. Such techniques are currently being developed for Mars exploration. Where laser-induced breakdown spectroscopy should allow the rapid analysis of the composition of Marsian rocks from a robotic

Anemometer An anemometer is a device that is

used for measuring wind speed, used in weather station. The term is derived from the Greek word “ANEMOS”, meaning wind. Anemometers can be divided into two classes. A) measure the wind's velocity, and B) measure the wind's pressure Since there is a close connection between the pressure and the velocity, an anemometer designed

Laser Doppler anemometers

Laser Doppler anemometers •The laser is emitted (1) through the front lens (6) of the anemometer and is backscattered off the air molecules (7). •The backscattered radiation (dots) re-enter the device and are reflected and directed into a detector (12)

Laser Doppler

anemometers use a beam of light from a laser that is split into two beams. One is propagated out of the anemometer. Particulates flowing along with air molecules near where the beam exits

The light back into a detector,

where it is measured relative to the original laser beam. When the particles are in great motion, they produce a Doppler shift for measuring wind speed in the laser light. Which is used to calculate the speed of the particles, and therefore the air around the

Laser Doppler velocimetry (LDV)

LDV is also known as laser Doppler

anemometry, or LDA). It is a technique for measuring the direction and speed of fluids like air and water. In its simplest form, LDV crosses two beams of collimated, monochromatic, and coherent laser light in the flow of the fluid being measured. The two beams are usually obtained by splitting a single beam,

The

two beams are made to intersect at the focal point of a laser beam. They interfere and generate a set of straight fringes. The sensor is then aligned to the flow such that the fringes are perpendicular to the flow direction. As particles pass through the fringes, they reflect light from the regions of constructive interference

Since

the fringe spacing d is known from calibration, the velocity can be calculated to be v = f x d Where ‘f’ is the frequency of the signal

Applications LDV is used in clinical research

as a mechanism to partially quantify blood flow in human tissues such as skin. However, within the clinical environment, LDV is referred to as laser Doppler flowmetry (LDF). It has gained popularity because it is simple to use,

In

principle a monochromatic laser beam is directed at the skin surface. Light that is reflected off stationary tissue undergoes no shift whilst light that is reflected off cells with velocity (like red blood cells) undergoes Doppler shift. The degree of Doppler shift is proportional to the velocity of the cell into which it collided. This light is randomly reflected back out of the tissue and onto a photo detector which calculates the average

VELOCITY MEASUREMENT It is well known that when a

light beam gets scattered by a moving object. The frequency of the scattered wave is different from that of the incident wave. The shift in the frequency depends on the velocity of

If

“ν” represents the light frequency, “v” represents the velocity of the moving object which is moving at an angle θ with respect to the incident and the reflected beams, then ∆ν/v= (2ν/c) cosθ Where ‘c’ represents the velocity of light in free space. The change in the frequency ‘∆ν’ is

This is known as Doppler shift. Thus, by measuring the change

in the frequency suffered by a beam when scattered by a moving object, one can determine the velocity of the object. This method has been successfully used for velocity determination of many types of materials from about 10mm/min

Applications Using the this principle, portable velocity-measuring meters have been fabricated Which measures speeds in the range of 10-80 miles/hr. These have been used by traffic police. Laser Doppler velocimeters

BASIC ARRANGEMENT FOR THE VELOCITY MEASUREMENTS

The

beam from a cw laser is split by a beam splitter. One of the components is reflected back from a fixed mirror. The other component

The two beams are then

combined and made to interfere as shown in fig. Because of the difference in

frequency between the two beams, beating occurs. The beat frequency is a direct measure of the

LASER TRACKING By Laser tracking, the determination of the trajectory of a moving object like an aircraft or a rocket. Also it can determine the daily positions of heavenly bodies or an artificial satellite. The basic principle of laser tracking is same as the one

This technique is to measure the

time taken to travel to and fro for a sharp laser pulse sent by the observer. Suitably modulated continuous wave lasers can also be used for tracking. The basic principle of laser tracking is same as the one used in microwave radar systems.

RETRO REFLECTOR In a retro reflector, the incident ray and reflected ray are parallel and travel in opposite directions. A cube corner is usually used to act as a retro reflector. For example the surface of the moon or on a satellite, a retro reflector can be kept to reflect

The size of the retro reflector

is much smaller than the corresponding microwave reflector. This is because of the smaller wavelength of the optical beam. Hence the reflector can be more conveniently mounted in

In a microwave radar system, one

has to incorporate corrections because of the presence of the ionosphere and also because of the presence of water vapour in the troposphere. These corrections are much easier to incorporate in the case of an optical beam.

On the other hand, there are some

disadvantages in using laser tracking system. For example, when fog and snow are present in the atmosphere, it is extremely difficult to work at optical frequencies. A portion of the pulse that is sent is collected and is made to start an electronic counter. The counter stops counting as soon as the reflected pulse is received

ISOTOPE SEPARATION Lasers have been successfully used

to separate the isotopic species of the elements present in an isotopic mixture. The technique of isotopic separation is of immense importance for nuclear power engineering. Natural uranium ore, used to fuel nuclear stations, contains mainly the isotope 238U and only 0.7% of 235U.

Principle The technique is based on the fact the different isotopes absorbs the light at different frequencies. A particular species is selectively excited by a highly monochromatic laser. Properly tuned to specific resonance, and then separated

For

example, fig shows two isotopes, A and B, their excitation and ionization levels and the isotopic shift. The isotope A is selectively excited by absorption of photon h 1, from a tunable laser. The excited atom is subsequently

ionized by another photon hν2 and

The frequency separation

between the absorption bands of 238U and 235 U is more than ≈ 5GHZ. The dye laser has a bandwidth of ≈0.1GHZ and, hence, it can be tuned to pump the required isotope.

The beam of uranium atom

emerging from a heated furnace is pumped by a continuous dye laser tuned to a desired wavelength, viz. λ=5915.4 Å. Atoms are then ionized by exciting them be ultraviolet

The

isotopes can also be separated by photos-deflection methods. Absorbing a photon of energy hν requires a momentum hν/c in the direction of the propagation of the photon. When it emits a photon, it acquires a same amount of

If it is made to absorb and

emit the photon repeatedly a large number of times. It may acquire a momentum sufficient to deflect away from the main stream. The atoms, physically deflected in this way, are

Various Types of Separation Hydrogen and boron were separated by photo dissociation. Boron and chlorine by photochemical method. Calcium by two-step photo ionization. Barium and calcium by photo

The laser system sends properly

tuned and timed pulses into a vacuum chamber. Which contain a both uranium vapour and electromagnetic or plasma type of ion separator to remove U235 ion from the neutral U238 background vapours. The module is surrounded by a 100-200 gauss magnetic field

Which

focuses a high energy electron beam along a narrow line on the surface of the molten uranium contained in a water-cooled crucible. The electron beams heats the uranium to 3000 ˚K, producing a vapour which is allowed to expand radially to speed comparable to that of sound.

The

resulting electric field produces electron current within the vapour. With the magnetic field, deflects ions out of the main stream onto the product collection surfaces. For a high ionization probability, an average power

Communication by Laser The four techniques commonly used for transmitting large amount of message over a long distance, are: Coaxial cable system Microwave radio- relay Wave-guide

For

modulation- the act of transferring a signal from one frequency band to another- to be effective without any interference to the signal. The system requires an oscillator capable of producing a carrier wave with a narrow spectral width. The unique properties of laser light viz. coherence and

Even when a highly directed laser

beam is used, factors such as rain, snow, fog can cause heavy power losses. Laser transmission can be shielded from the atmosphere but putting lenses in an air tight pipe spaced 300 feet or more apart. However to allow the beam to follow a curved path, a large

This

difficulty could be overcome by using a “gas lens”. A gas, for example carbon dioxide is passes through a heated tube. Because the gas travels faster in the centre of the

Establishing

the refractive index gradient which makes the gas to act as a converging lens. In this lens the losses are limited only to the slight scattering by the gas molecule. Laser communication lines based on the fiber optics have been extremely useful in the

LASERS IN INDUSTRY The precision properties of laser

light have been of immense help in industry. Particularly in testing the quality of the optical components: lens, prism, etc. Accuracy in the measurement of the sizes of the physical quantity is considerably increased. 

The ability of the laser beam to

concentrate large-power in a small volume is easily utilized for drilling of small holes and for the welding of small metal parts. Lasers are found to be very effective in cutting different types of material. A CO2laser of 100 W continuous

It can prepare cut-species

for about 50 suits per hours. Laser cutting technology is widely used in the fabrication of space graft. A CO2 laser of 3 KW continuous output cuts titanium sheets of 50mm

Lasers have been used as light source

for telephoto pictures. Pulsed Q-switched lasers are suitable for technical motion picture photography. It is possible to obtain pictures of bubble formation at the rate of 200,000 frames per seconds with a Qswitched ruby laser. The spiky output from the normal ruby laser has been found to be useful in

The intensity of individual spikes is

high enough for the light reflected from the moving object to be recorded on the photographic plate. The successive travelling of a bullet at a speed of 20,000 cm sec-1 has been photographed using ruby laser. One obvious use of high-power

For

example, one could use such source to clean the exhaust gases from combustion by selectively decomposing noxious substances. Similarly one could purify the feed stocks for the chemical process by selective destruction of contaminants. It has been observed that the finger print can be detected under laser light where the normal method of obtaining finger print trough dusting powder is

It

is likely people carrying some kind of fluorescent contaminants on hands, picked from minute traces of ink, paints, etc. If the blue light of an argonion laser is directed at a suspected finger prints. Latent finger prints emit the

The laser technique can also

be used for detection and analysis of older finger prints on documents, current bills, cloths, etc. This is possible because finger print contains, ultra minute quantities o f stable amino acids, which when treated with