To understand just how vital a part vibration isolators play in today’s technological advances one need only scour the articles that go along with the high-tech industry’s latest headlines. In the past month alone, an examination of no less than three such articles revealed references to them. And these mentions weren’t just passing asides either. In all cases, the vibration isolators proved instrumental in putting the technologies being discussed in the spotlight.

The first up among these recent news reports involved a high speed multi-axis optical micrometer. As Automation World pointed out, this device not only “sets a new standard for high speed and high precision diameter inspection” but also “eliminates errors that were previously endemic to optical micrometers.” And yep, you guessed it, there are vibration isolators behind all those capabilities. The publication went on to praise the micrometer for possessing no moving parts, which dramatically increases its durability. Delving deeper into the specifics, Automation World stresses, “Incorporating a custom designed vibration isolation system within the head, the [micrometer] is able to resist damage from vibration and impact.”

Up next was a review of the 2014 Honda Acura MDX, whose advanced engineering has driven the SUV to the top of its vehicle class. Pointing to the sport utility vehicle’s ability to give consumers the fuel efficiency and affordability they crave without sacrificing “panache and human comfort accouterments,” the Honda Acura was recognized by Kiplinger’s Report and U.S. News and World report as both “2014 Best Value in a full-size luxury SUV” and “Best car for the money.”

So how do vibration isolators play into all of this, you ask. The reviewers closed their praise of the SUV with this praise: “We especially appreciate superior body roll stability and road vibration isolation via electronic sway control, fluid filled body mounts and exceptional sound deadening materials installed beneath seamless carpeting and in the rear wheel wells.”

Third but not least among the news nods came from TVTechnology.com. The tech website specifically looked at portable storage, marveling over its uncanny ability to increase in power and capability despite shrinking in size. One of the storage devices it lauded is the Sonnet Fusion F3 for “managing high-speed storage when in the field.” This portable storage device facilitates high-speed storage when on location and has a case that “is designed to withstand rough handling. Moreover, “each of the F3 drives is mounted on its own multi-axis shock vibration isolation sled to try and eliminate vibration problems.”
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Light can be reflected back and forth. This is also true in fiber optic communication networks. But in fiber optic networks, most of the reflections are harmful to the stability of the system which is especially true for lasers.

Laser is essentially a resonant cavity between two semi-transparent mirrors. The lasing process happens between these two mirrors. The lasing process is very delicate and can be easily interfered. If back-reflected and scattered light enters into the laser, the lasing process will fluctuate and the output power of the laser will fluctuate.

So that is where fiber optic isolator comes to play. Optical isolators are devices that transmit light only in one direction. They play a vital role in fiber optic systems by stopping back-reflection and scattered light from reaching sensitive components, particularly lasers.

How do optical isolators work?

The inside workings of optical isolators depend on polarization. An isolator is composed of a pair of linear polarizers and a Faraday rotator.

The two linear polarizers are oriented so the planes in which they polarize light are 45¡ã apart. The Faraday rotator sits between these two polarizers. The Faraday rotator rotates the plane of the polarization of light by 45¡ã in a single direction no matter the light traveling direction, may it be from the first polarizer(left) or the second polarizer(right).

So if the light goes from the first polarizer to the second polarizer (from left to right). The Faraday rotator will rotate the polarized light from the first polarizer by 45¡ã which exactly matches the polarization plane of the second polarizer. So the light will go through with minimum loss.

But if the light goes from the second polarizer to the first polarizer (from right to left). The Faraday rotator will rotate the polarized light from the second polarizer also by 45¡ã. But since it rotates the light as the same direction as from left to right, this time when the rotated light gets to the first polarizer, the polarization planes of the polarized light and the first polarizer are 90¡ã cross. So all light are blocked and no light will go through.

From above mentioned principles, you see that fiber optic isolators transmit light only in one direction and they work like a one way street.

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Light waves consist of oscillating electric and magnetic fields. These two fields are perpendicular to each other and to the direction of light traveling. We call the electric field plane as the light’s polarization.

Sunlight and many other ordinary light is made of many light waves, each with their electric and magnetic fields oriented randomly. If all waves’ electric fields were aligned parallel to each other, we call this light linearly polarized.

* Polarization’s effect on optical fibers

Polarization does not really matter in multimode fibers but it can be crucial in single mode fibers especially for long distance and high speed rate fiber communications. Why? Let me explain it below.

Technically speaking, single mode fibers actually have two modes traveling in it. These two modes have orthogonal polarization and perfect single mode fibers cannot differentiate between them. These two modes are functionally identical and light energy can shift easily between these two polarization modes.

However, fiber’s geometry is not perfect. As a result, these two modes actually travel along the fiber at slightly different speeds. The effect is called PMD (polarization mode dispersion). The slight speed difference can cause problems in high speed fiber optic links such as 10Gbit/s and 40Gbit/s.

* Polarization maintaining fibers come to the rescue!

Polarization maintaining fibers are special kinds of single mode fibers, they are also commonly called PM fibers or Panda PM fibers.

PM fibers have built-in asymmetry which is also called birefringence. The refractive index of PM fiber differs for the two polarizations and this effect prevent light energy from coupling between two polarizations.

PM fibers can transmit light in a single polarization if the input light polarization is aligned to one of its two birefringence axes. And that is why they are called polarization maintaining fibers.

* The applications of polarization maintaining fibers

PM fibers are rarely used for long distance transmission because of their expensive price and higher attenuation than single mode fiber. They are commonly used for telecommunication applications, fiber optic sensing and interferometry.

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Light loss in fiber cable is a serious issue because it can hamper or completely stop data transmission. With so many communications networks dependent on fiber optic cabling to transmit data, having a way to quickly diagnose problems means repairs can occur much more rapidly leading to less network downtime.

An OTDR works by injecting a series of optical pulses through the fiber which are reflected back to the injection point and measured for deviations or aberrations in the light wave. Based on the measurement of the deviation the trouble spot can usually be precisely located.

Some critics of OTDR systems point out that they can produce inaccurate measurements if two faults are located close to each other. There is also the potential issue of faulty measurements which can sometimes occur if the optical pulse has to travel a great distance. However with the proper training a technician can compensate for these issues and interpret the readings correctly.

All things considered when used correctly an optical technician will find an optical time domain reflectometer to be an invaluable instrument in the daily course of troubleshooting, repairing and maintaining fiber optic networks..
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1. Polarization mode and polarization mode dispersion (PMD)

In single mode fibers, light pulses are actually composed of two distinct polarization modes. The electric field vector of the two modes are perpendicular to one another, or called orthogonal. Normally the two polarization modes behave just the same in the fiber which means they can not be distinguished.

But that is only the theory with a perfect symmetrical fiber and no outside force on the fiber. Since the world is not perfect and neither is the fiber, these two polarization modes do behave differently in real world fibers.

Stresses within the fiber, and outside forces applied to the fiber cause the refractive index of glass to differ slightly for these two polarization modes. This phenomenon is called birefringence.

Birefringence makes these two polarization modes travel at slightly different speed. This speed difference broadens lightwave signal just as other dispersions and this fact is called Polarization Mode Dispersion (PMD).

2. PMD and its impact on single mode fiber optic systems

The potential effects of polarization mode dispersion became significant only a few years ago when high speed fiber optic digital communication systems came to play, such as the 40Gbit/s systems.

Polarization mode dispersion (PMD) is smaller in magnitude than other types of dispersions, but it is more difficult to compensate for, at least until now. PMD becomes a problem in systems with data rates higher than 2.5Gbit/s. PMD makes more challenge to sending higher data rates over long distance..
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