Question

In: Physics

Laser beams are said to have high "spatial coherence". This means that the beam is highly...

Laser beams are said to have high "spatial coherence". This means that the beam is highly concentrated even at long distances (low spread).

Can this be achieved with radio waves (much longer waves) or is it due to laser's stimulated emission?

Solutions

Expert Solution

Laser light is spatially and temporally coherent. The stimulated emission is mainly responsible for the temporal coherence.

So the answer is yes, you can create an electromagnetic beam that is spatially but not temporally coherent by placing a pinhole close to the source, and then another pinhole in the far field of the first pinhole. This beam will not spread out very much. (But also remember that laser light does spread out.)

Note that for RF frequencies, a "pinhole" is probably several meters in diameter. The far field distance is given by this inequality: , where is the distance, a is the diameter of the hole, and ? is the wavelength.

However, creating a RF pencil beam is probably not practical. The term "pencil beam" mentioned in the Wikipedia article is explained as being diffraction-limited. The size of a diffraction-limited beam gets larger with, I believe, the square root of the wavelength. It would be more like a gas-pipeline beam than a pencil beam.


Related Solutions

a photon beam is made from a target with a high impedance. electron beams of 6...
a photon beam is made from a target with a high impedance. electron beams of 6 and 16Mev are used in the process, shot at the target. the ratio of beam intensity is measured in the direction of the electron beam (0 degrees) and at 25 degrees from the angle of incidence.nFor what electron bean energy (and why) is the ratio of intensities (i0/i25) the largest?
Very short pulses of high-intensity laser beams are used to repair detached portions of the retina...
Very short pulses of high-intensity laser beams are used to repair detached portions of the retina of the eye. The brief pulses of energy absorbed by the retina weld the detached portion back into place. In one such procedure, a laser beam has a wavelength of 810 nm and delivers 255 mW of power spread over a circular spot 163 μm in diameter. The vitreous humor (the transparent fluid that fills most of the eye) has an index of refraction...
I have a pinhole 50 microns in size illuminated by a coherent laser beam. Now, I...
I have a pinhole 50 microns in size illuminated by a coherent laser beam. Now, I have a lens (75mm focal length) 1 focal length from this pinhole, what size will the beam be when it is incident on this 75mm lens?
1. Why is a cash account said to have a high inherent risk of possible fraud?...
1. Why is a cash account said to have a high inherent risk of possible fraud? Explain some of the controls related to cash accounts! 2. You are an auditor at a public accounting firm. You are conducting an audit for the financial year ending December 31, 2019. Your client has go public. This client is a property development company. Your client builds property in the form of apartment units, housing / real estate and also property investment products in...
High-quality earnings metrics are said to have confirmatory and predictive value. What does this mean? Discuss...
High-quality earnings metrics are said to have confirmatory and predictive value. What does this mean? Discuss in the context of proforma earnings?
ADVERTISEMENT
ADVERTISEMENT
ADVERTISEMENT