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Polarization

Updated Mary 11, 2009

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New for March 2009! This page is on polarization is under construction, please help us out with content/corrections/edits!

Polarization is a property of electromagnetic waves as they are radiated in space. EM waves in space are transverse, such that oscillations in E-fields and H-fields are perpendicular to the direction the wave travels, and at 90 degrees to each other. By comparison, sound waves are longitudinal, they oscillate along the direction that the sound is traveling.

History of polarizers

Etienne Malus discovered polarization in 1808. Malus's law gives the intensity of light after it has been polarized by an ideal polarizer as:

cos^2 theta

For incoherent light, half of the intensity is lost. It would take calculus to derive that, it is the average value under the cos^2 curve, integrated over all possible angles (2 pi radians or 360 degrees).

The origin of polarizers dates back to the 1850s, and involves passing quinine through a dog and treating the urine with iodine to create Herapathite crystals. We did NOT make this up. Aligning the crystals on a film, Edwin Land, the inventor/genius who started the Polaroid company, was the first engineer to exploit polarizers, in the 1920s. The crystals are needle-like, and when they are aligned on a sheet, a polarizer is the result. Light polarized parallel to the crystals is absorbed, and light polarized perpendicular to the crystals is transmitted, which is the opposite of how you might imagine how it works.

Types of polarization

Linear polarization

The E-fields all oscillate in one direction at a fixed place in space over time, or at a fixed time over distance. Linear polarization can be purely vertical, horizontal, or any angle in between. Most antennas produce or receive a single polarization.

The images below are some animated gifs you can find on the web site of Hungarian András Szilágyi, who gave us permission to post his work. Go there and review his tutorial, chances are you'll learn something!


Horizontal polarization


Vertical polarization

Circular polarization

EM waves at any polarization angle can be decomposed into two waves at 45 degrees to the original vector, such that the decomposed waves are 90 degrees apart (orthogonal). If one of the orthogonal components is phase shifted exactly 90 degrees, the composite vector will appear to rotate at a fixed time or distance.


Circular polarization (LCP and RCP)

Circular polarization can be left handed (LCP) or right handed (RCP), depending on whether the phase shift between the components is +90 or -90 degrees.

Why would you want CP? In a comm link then you don't have to worry about your receiver's angle with respect to the transmitter, the wave will just spin right in!

Elliptical polarization

Elliptical polarization results when one of the two linear components in CP is greater amplitude than the other. The ratio of the two voltage amplitudes is called the axial ratio. When axial ratio is one, you have CP.

One way to look at elliptical polarization is that the two components of circular polarization are not exactly 90 degrees apart.

Polarization in radar systems

Most radar systems operate with a single polarization. This is because the transmitter and receiver operate out of the same antenna, so whatever polarization goes out will come back strongly. The polarization that is identical to that transmitted is called "co-pol" and the opposite one is called "cross-pol".

Below is a demonstration of a radar detector interacting with a WalMart door opener. You can see why you don't want to mount your radar detector vertically! By the way, the microwave door opening is an example of microwave object detection, another way microwave engineers improve our lives.

What if you are a policeman and you are trying to catch more speeders than your buddies? Just turn your police radar gear 90 degrees and none of the lead-foot scofflaws with radar detectors will have a chance to jam on their brakes in time to avoid your trap! Bwa ha ha ha! Another public service announcement by Microwaves101.com!

Actually, there is another consideration. Police radar is vertically polarized, because there is more likely to be clutter and multipath when horizontal is used, because more objects happen to be horizontal. So the radar gun might have reduced range and accuracy when help sideways. Anyone out there have any experience with this?

Polarization in communication systems

Polarization requirements depend on how the system is used. For example, if you wanted to create a "comm-on-the-move" system that points to a satellite, the system would likely have to use circular polarization so that the antenna (and the vehicle it is mounted on) could rotate with respect to the transmitter. This is exactly what is used in XM radio and Sirius radio receivers. The penalty is that the satellite must supply twice the transmit power compared to a single pol system. That is one reason that XM satellites "Rock" and "Roll" were the largest commercial satellites ever launched! The other reason is that the tiny receive antenna has very low gain, it is almost omni.

Compare satellite radio to satellite television. Your dish must face the satellite exactly to grab the signal, and the angle must be near perfect, because the signals are linearly polarized. We said signals, because there are actually two, one is vertically polarized and one is horizontally polarized; this technique allows the network to provide twice as much content compared to a singular polarization, so you can get even more home shopping networks, reality shows and other mind numbing garbage. You'd think with all that bandwidth there might be a "masters degree channel" where you could learn how a remote control works instead of getting butter on one while you enjoy a bag of microwave popcorn. At least the microwave industry is there to help you out with warm and tasty snacks.

An orthomode transducer is a often used to separate orthogonally polarized signals in a communication system. We'll cover that topic as soon as someone asks about it!

Polarization purity is the ratio of the desired polarization component to the undesired component. 40 dB is a good figure of merit.

Polarization of visible light

Light waves are of course electromagnetic waves, but at much higher frequency than microwaves. The wavelengths of the visible spectrum span 4000 to 7000 Angstroms (which is 400 to 700 nanometers, thanks for the correction, Morten!) Put in the same units, X-band microwave radiation has a free-space wavelength of 300,000,000 Angstroms. Incoherent waves (all visible light with the exception of laser light is incoherent) contain energy distributed at all angles, perpendicular (transverse) to the direction of propagation.

Polarized sunglasses have a vertical polarizer, or polaroid. When light reflects from a flowing stream, it becomes partially polarized. Most often the polarization is horizontal, because objects that cause unwanted reflections are often horizontal, like bodies of water, or roadways. Polarized sunglasses or (filters on a camera) can remove unwanted reflections (blocking horizontal polarization) and provide a higher-quality image, but at reduced intensity. Reduced intensity is not usually a problem, cameras and eyes adjust automatically.


no filter

with filter

These pictures were used with permission from Professor Ching-Kuang Shene at Michigan Tech. Thank you sir!

One consumer niche application of polarizers is 3D glasses for viewing movies. The original 3D experience crudely separated the two images using color filters; later polarized glasses with polarizers at +45 and -45 degrees allowed full color for both eyes. However, if you tilted your head, you saw double images and the effect was lost and a headache was often the result. Today, RealD provides a new 3D experience. Their technology separates the two pictures into two circular polarizations, left hand and right hand. The glasses that they distribute filter out the unwanted images and work regardless of what angle your head happens to be.

In order to change linearly polarized light to circularly polarized, a quarter-wave retarder is used. This is a media that presents different indices of refraction to the light wave, such that one is retarded (phase shifted) by 90 degrees.

If you want to be a real nerd, bring home your RealD 3D glasses and play with them in front of a mirror. Put on the glasses and close one eye - the open eye remains shaded, but you can see the closed eye clearly! This is because you're looking OUT through one lens and IN through the other, reversing the polarization of the reflected wave (as the whole thing is reflected back at you through the mirror). If you have two mirrors positioned at a 90 degree angle you can re-reverse the image and you will find that your OPEN eye is the one that you see clearly in the reflection!

 


 
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