Introduction
Metrology is the science of measurements. The major contribution of lasers in metrology is in the length measurement field. However, lasers have also provided new techniques for measuring various physical variables such as angular position, angular speed, velocity of fluids, traverse displacement, etc.
Using lasers beam accurate measurements can be performed. The three different basic methods that are used for optical distance measurements are :
(1) Interferometric technique (with laser beam upto = 50 m in free air)
(2) Telemetry with modulated beams (with laser beam from I m = 100 m to 50 km )
( 3 ) Optical radars ( with laser beam above 10 km )
All the above three methods are based on the measurement of the propagation time of an electromagnetic wave over the examined distance . Major improvements were possible in all the three basic techniques of optical distance measurements using lasers. This is due to the fact that laser radiation is an em wave traveling at the speed of light. Hence, very accurate measurements can be performed. Also, the above methods could be used for larger distances as compared to conventional light sources.
LASER AS PRIMARY STANDARD OF LENGTH
Oscillators based on stimulated emission may provide standards of length and time. From the metrological point of view, the main features required in a new length standard are:
(i) Better reproducibility than the existing ones.
(ii) Very high stability (on long and short terms).
(iii) Ease of comparison with other standards. For example, in the calibration of secondary standards.
Lasers are known to have a comparable reproducibility with the present Kr length standard. With regards to stability, optical interferometers provide reference frequencies with a very good short term and a good long term stability if operated at constant pressure and temperature in a controlled environment. With respect to the third point, it is generally agreed that frequency stabilized laser sources permit an accurate and fairly convenient comparison with other sources by interferometric techniques. Hence, it is expected that in the near future, a frequency stabilized laser will be defined as the new primary standard of length. The introduction of a more accurate length standard will in turn allow a more accurate determination of the speed of light and other basic physics experiments.
INTERFEROMETRIC TECHNIQUE USING LASER
The distance to be measured is compared with the wavelength of a reference source. This source can be a spectral lamp or a frequency stabilized laser. Interferometry makes use of the principle of superposition to combine separate waves together. It is done in such a way that will cause the result of their combination to have some meaningful property that will tell us about the original state of the waves. When the two waves with the same frequency combine, the resulting pattern is determined by the phase difference between the two waves. The waves that are in-phase will undergo constructive interference. On the other hand, the waves that are out-of-phase will undergo destructive interference. The conditions for constructive and destructive interference between two beams, which come from common origin, is:
Destructive Interference: When the path difference between the two beams in an odd multiplication of half a wavelength, the result is dark bands in these regions. Michelson invented the interferometer which is based on the interference of twe interferometer waves coming from the same origin. That is why it is called Michelson on the interference of two waves coming from the same origin. That is why it is called Michelson interferometer. Aterferometer is one of the most accurate measuring instrument in use. Michelson terferometer is an instrument which uses interference between two beams of ectromagnetic radiation, that are created by a common origin. In Michelson erferometer, a parallel beam of light is split by a beam splitter into two beams.
From the figure we observe that :
१- The two beams travel in direction at 90° to each other.
२- Each beam hits a mirror and is reflected back to the beam splitter. ३-From the beam splitter the two beams arrive at the screen in a common path.
On the screen an interference pattern is seen, if the optical path difference between the two beams is less than the coherence length of the laser. In interferometric techniques the property of temporal coherence of lasers plays a very important role. Due to it the interferometric technique can be used over distances which are larger by several orders of magnitude than those of conventional light sources. Most of the spectral lamps have coherence time in the nanosecond range. Therefore, the coherence length is in the order of 30 cm. Interference effects cannot be obtained with an optical path difference larger than the coherence length. Frequency stabilized lasers have a coherence length in the range of a thousand kilometers. The practical limit is laser illuminated interferometers is set not by the coherence length but by perturbations associated with propagation (e.g., atmospheric propagation). This is the reason why we use interferometric techniques using laser upto 50 m in free space only,
For interferometric length measurement using laser, the Michelson interferometer set-up will be the same. The beam from the laser is sent to a beam expanding telescope which reduces the beam divergence as the result of a wavefront curvature correction. To obtain a uniform and symmetrical intensity distribution in the output beam, a circular aperture is usually inserted in the focal plane which performs spatial filtering of the input beam.
LASER RADAR
The technique used in laser radar is similar to that used in the electronic radar. In the conventional radar, the electromagnetic pulse is transmitted and the reflected echo is recorded. Similarly, in the laser radar, light energy is sent out in pulses and the reflected light echo is collected. Range information can be obtained by measuring the propagation time of a light pulse from the source to the target and back to the receiver. The target can be either a diffuse object or a cooperative reflector such as a Cat's eye system or a corner cube.
Light is diffused by the targets. So, we require high peak power with a suitable pulse repetition rate. Solid state lasers in Q-switched operation typically deliver pulses with peak power in the kW range. Semiconductor laser radar is preferred for range up On the other hand, semiconductor laser arrays emit pulses of a few n sec in duration with a time duration of the order of 20 n sec and peak power in the 10 to 100 MW few kilometers due to its compactness, light weight and efficiency of the semiconduct laser sources.
When ranges in the order of 100 km upto 1000 km or more (typical distances et satellites orbiting the earth) are to be measured, the use of an array of corner cu reflectors is necessary to obtain an echo pulse of sufficient intensity. A number of artificial satellites and even the moon have been equipped with a set of commerce reflectors. One of the known precise measurements with a laser was measuring the distance from earth to the moon. The astronauts who landed on the surface of the moon left there a corner cube (a system of three perpendicular mirrors that reflect light in the same direction from where it came). A pulsed laser beam was sent from earth to the moon and was reflected from this corner cube back to earth. The travel time of the pule was recorded. From the known speed of light (c) the distance was calculated, with an accuracy of tens of centimetres. The advantages of laser radar as compared to ordinary radar are:
(1) The laser radar uses a short wavelength. This gives a much better resolutions that even tiny objects can be detected.
(2) Laser diode operates without getting influenced by the natural light and is not affected by presence or absence of daylight.
(3) The laser radar cannot be jammed like the conventional radar.
(4) Very small and compact laser radar devices can be built. For example, we use laser radar in fire control system replacing the bulky conventional radar.
The only disadvantage of laser radar is that the range is badly limited by the atmospheric conditions of fog and rain.
