FIBER OPTIC FREQUENTLY ASKED QUESTIONS (FAQS – PART 2)

Answering More Questions About Fiber Optics

Fiber Optic Technology – Learning More

What wavelengths are used with multimode fiber?

Multimode fiber is efficient at transmitting a wavelength at or around 850 nm, 1300 nm, or 1550 nm.  Due to the availability of low-cost semiconductor light sources and photodetectors, the most popular wavelengths are 850 nm and 1300 nm.

What wavelengths are used with single mode fiber?

The most common and popular wavelengths have been found between 1310 nm and 1550 nm.  Why? At 1310 nm, chromatic dispersion is near zero, and at 1550 nm, attenuation is near its minimum.  In OS1 singlemode fiber, wavelengths around 1390 nm should be avoided due to high attenuation caused by absorption. OS2 singlemode fiber has the ability of transmitting any wavelength above its cutoff wavelength, which is typically around 1250 nm.

What is the cutoff wavelength for singlemode fiber?

Extensive testing has found the cutoff wavelength for singlemode fiber is the minimum wavelength that supports one mode of propagation.  Subsequently, singlemode fiber propagates only one above the cutoff wavelength mode.  Whereas singlemode fiber propagates more than one mode, similar to multimode fiber, below the cutoff wavelength

How is an electrical AV signal converted into an optical AV signal?

An electrical AV signal is converted into an optical AV signal using an optical transmitter or an electrical-to-optical converter.  An optical transmitter uses a laser diode as the light source, varying the intensity of the laser light in accordance with the electrical signal.  For an analog signal, the intensity of the light source differs with the voltage or current of the electrical signal.  The light intensity is high or low, for digital signals, which represents logical ones or zeros.

How is an optical AV signal converted back into an electrical AV signal?

Using an optical or an optical-to-electrical converter, an optical signal can be converted into an electrical signal.  The optical receiver uses a photodetector to receive the optical signal, which it uses to convert it into an electrical signal.

What are the problems with the direct conversion of analog video signals into optical signals?

The direct conversion of an analog electrical signal to an optical signal and back has non-linear effects that distort analog video signals.  It is challenging to compensate for these distortions.  Fiber also has attenuation that results in the dimming of optical power over longer distances which causes low voltage levels when converted back.

To compensate for low voltage, Amplifiers can be used to increase signal noise, thereby reducing the signal-to-noise ratio.  Fiber optic AV equipment that uses analog techniques suffers from increased noise and distortion, which produces a low-quality video signal.

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