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Fiber Optics Notes

Dispersion

Modal
Present in multimode fibers, this occurs when different internal reflection paths have different path lengths. Not a problem in single-mode fibers, which have much higher bandwidth capacity.
Units: ps/nm-km
Chromatic
Change in refractive index with wavelength.
Units: ps/nm-km
Waveguide
Due to refractive index profile in single-mode fiber. Can cancel chromatic dispersion, but going through zero dispersion causes 4 wave mixing.
Units: ps/nm-km
Polarization mode
In systems above 2.5GHz, consider different velocities for different polarizations. A problem around discontinuities, such as splices.
Units: ps/sqrt(km)

Losses

Typical losses:

Devices

Fiber
Dispersion-shifted fiber typically 3ps/nm-km.
Fibers can preserve polarization or select for polarization.
There is lots of step-index single mode fiber already installed.
Fiber is made of silica, SiO2. It is doped with germania, GeO2, to raise the refractive index. Silica + Flourine lowers (depresses) the refractive index.
LED
Broad spectrum, low wavelength-power density. Slow.
Laser
To make a laser you need stimulated emission, a population inversion, and light confinement. Stimulated emission occurs when a photon of just the right energy triggers the release of an identical photon by knocking down an electron that is at just the right energy. Population inversion happens when more electrons are at higher energy levels than lower energy levels. This prevents lower energy electrons from absorbing the light that the higher energy electrons are giving off. Light confinement is done with mirrors at the ends of the laser and with a high-index material along the length, to confine the light just like a fiber.
Integrated Optics
Most promising for sources, since it is hard to launch wave into integrated optics. Can make a laser followed by a modulator and avoid the dispersion associated with bias changes in the laser.
Fiber Gratings
Make a filter by changes in refractive index along a length of fiber.
Optical isolator
Optical isolator uses a polarizer then a 45 degree phase shifter called a Faraday Rotator, then another polarizer shifted 45 degrees to the orginal.
Fiber Types
Fiber Attenuation
There are three material loss mechanisms. These are: Bends also cause loss, when light stries cladding boundary at an angle greater than the critical angle so some leaks out.
Nonlinear Fiber Effects
Lasers
The laser is an amplifier with coatings set up to oscillate. Can pull the frequency of the laser with a diffraction grating or Bragg grating in order to pull the frequency.
Distributed Feedback Lasers
A bandpass filter inside of a laser selects a single spectral line for lasing.
Modulators
Using an external modulator can reduce the wavelength spreading and therefore jitter that occurs when semiconductor lasers are turned on an off with bias current. The modulator can be integrated on the same substrate as the laser. It is essentially an amplifier whose bias circuit is turned on and off.
Erbium-Doped Amplifiers
These operate at 1500 to 1600 nm, which is also where the attenuation of fiber is lowest. Uses a pump beam, and couplers and isolators to keep the pump beam out of the rest of the system. The pump wavelength is shorter than the beam being amplified.
Couplers
Coupled optical transmission lines. Tradeoff between loss in path and amount of signal coupled out.

Simple Optical Formulas

c = freq * wavelength
E = plank's constant * wavelength
Snell's Law
ni*sin(I)=nr*sin(R)
Dispersion Budget
dispersion =sqrt(sum of squares of component dispersions)
Fresnel Reflection
For light going through a discontinuity at a right angle, it is:
10*log10(1-((n1-n0)/(n1+n0))**2)
Air-glass loss is typically 0.3dB, with an index matching gel it goes to about .01dB. The problem with gel is that it gets dirty, so instead use polished glass that mates directly without scratching.
Numerical Aperture
sin(Acceptance Angle of a fiber)= sqrt(n0**2-n1**2) = NA

Power Budget

Source Power
Power level, 0dBm
Receiver Power range
Losses

8/2/2006

By toma