Optical Links

• Fibers
  • Multimode
  • GRIN
  • Single-Mode
• Light sources
  • Lasers
  • Diodes
• Light detectors
  • Photodiodes (PN)
  • PIN diodes
  • Avalanche photodetectors
• Performance of each: R * L for fibers, bandwidth of sources and detectors

Optic Fibers: Basics

• guide light along a path
• glass or plastic
• refractive index \eta
• speed of light c/\eta
• light can be infrared (1.3u m, 1.55u m) or visible
• attenuation of as little as 0.2 dB / km.

Attenuation

• 10 dB attenuation is a 90% loss; 20 dB attenuation is a 99% loss; 3 dB attenuation is approximately a 50% loss
• attenuation at length L is P(0) / P(L)
• with constant attenuation A dB / km, P(L) = P(0) * 10 ^ {-(AL/10)}
• example with 100 km of fiber and A = 0.2 dB / km, total attenuation is AL = 20 dB, so P(L) = P(0) / 100.

Multimode "Step" Fiber

• Core with refractive index \eta
• Cladding with refractive index \nu, \nu < \eta (e.g. nu = 0.99 \eta)
• All light hitting the boundary at angle less than \theta (where \cos (\theta) = \nu / \eta) undergo total reflection back into the core.
• therefore, light is guided along the core

Dispersion in Step Fibers

• The quickest light rays go straight, covering distance L.
• The slowest light rays travel at angle \theta, and therefore travel L' = L/\cos(\theta) > L
• Speed of light c/\eta
• Time T for reaching destination is T1 = L / (c / \eta) <= T <= T2 = L' / (c / \eta)
• Total dispersion DT = T2 - T1
• Dispersion/kilometer D = ((\eta / \nu) - 1)\eta/c =~ (\eta - \nu)/c
• with \eta - \nu = 0.01, D =~ 3 * 10^{-11}
• since R * L = 1/2D, R * L for step fiber is 10 Mbps * km.

Multimode Graded-Index Fiber

• GRIN
• refractive index decreases gradually from core
• decrease in refractive index causes wave to bend back into core
• "Outer" paths are faster because lower refractive index means faster speed of light
• result: less dispersion R * L = 1 Gbps * km.

Single-mode Fiber

• Light can only travel at restricted number of angles described by quantum physics
• for a narrow enough fiber (8u m), only a single mode is permitted
• no dispersion! (almost)
• still have slightly different refractive index at different wavelengths (frequencies)
• dispersion R * L = 250 Gbps * km for a very narrow-frequency source (laser diode).
• dispersion R * L = 80 Gbps * km for a less narrow-frequency source (LED).

LED

• diodes
• light proportional to current (for frequencies up to cutoff frequency)
• current is modulation signal
• frequency of light is carrier signal
• cutoff frequency can be as high as 100 MHz
• modulation signal frequency can be as high as 100 MHz

Laser Diode

• LED in an optical cavity: a space with two mirrors at the ends
• Light emission is stimulated by light in the optical cavity
• As a result, emitted light is coherent (all with the same phase) and has essentially the same frequency (narrow frequency spectrum)
• current is modulation signal
• cutoff frequency can be as high as 10 GHz
• modulation signal frequency can be as high as 10 GHz

Photo Diode

• Amplified photoelectric effect
• Photons "free" electrons
• Freed electrons/holes flow across junction giving current
• Maximum frequency around 1 MHz

P-I-N Diode

• Intrinsic (un-doped) layer sandwiched between P and N layers
• Intrinsic layer is isolator (SiO2), so can put large voltage across layer
• large voltage leads to faster response when photons free electrons
• Maximum frequency around 20 GHz

Avalanche Photodiode

• Same as PIN, but
• Electric field is so large that freed electrons knock loose more electrons
• More sensitive than PIN

Homework

1. This exercise may help you with your project:
   • telnet to port 80 of www.ics.hawaii.edu and type:

     GET /~esb/ HTTP/1.0<CR>
     <CR>
     

     (<CR> stands for a carriage return)
   • turn in the first 10 lines of the output
   • points will be deducted if you turn in more than 12 lines!
2. If my photodetector can reliably detect 1% of the light emitted by my light source, and my attenuation is 0.3 dB/{\rm km}, how much distance (in km) can I cover? Show your work.