prof. van erps sure does take his time during exam interviews, they've even been at it with floris for around 45min. and since it's a closed book exam i had time to rewrite the questions for lack of anything else to do while waiting for my turn.
for my alone time. with my crush. aw. hahaha.
Q1. you were hired as a telecom operator to design a trans-oceanic link spanning 4000km operating at a bitrate of 800Gb/s. your employer has proposed the following components to be used in the link:
- directly modulated DFB laser operating at 1550nm
- gradient index multimode fiber
- semiconductor optical amplifier
- silicon PIN photodetector
give a comprehensive assessment on whether or not these devices are suitable for the link. propose a solution otherwise. provide figures, equations, and a discussion of the underlying physical phenomena.
A1. (just a summary because i'm lazy and..how do you write down equations on blogspot?)
- use an externally modulated laser because a DM laser would generate chirp. at very high bitrate, like 800Gb/s, inter-symbol interference would be so profound you can no longer differentiate between signals/data. DFB laser is a good choice because a narrow linewidth of ~0.001nm is achievable which would reduce the effect of intramodal dispersion in the fiber. how to code 800Gb/s? WDM. the operating wavelength of 1550 nm is also a good choice. it falls in the middle of the 3rd transmission window and has the lowest loss among the three transmission windows. attenuation of 0.2dB/km is easily achievable at 1550 nm.
- use a single mode fiber for long haul transmission to avoid intermodal dispersion.
- use an optical fiber amplifier instead of a SOA because it has lower loss and is data transparent. EDFA is used for amplification at 1550nm.
- depending on the receiver sensitivity and power budget, PIN detector may or may not be a good choice. should the receiver sensitivity fall below the required value, this could be increased by either adding a pre-amplifier, using an APD instead of a PIN detector, or doing coherent detection instead of direct detection. the material also determines at which wavelength you can detect photons. for 1550 nm, a suitable choice would be InGaAsP as silicon is transparent at that wavelength.
Q2. give and derive the dispersion criterion for chromatic intramodal dispersion. what can you learn from it?
A2. dispersion criterion: B・L・|D|・dlambda < 1. basically tells you the relationship between different factors affecting transmission. for example, in order to transmit at longer lengths, L, the laser linewidth, dlambda, should be significantly small..etc.
Q3. which nonlinear effects play a role in optical telecom systems? how do you avoid them?
A3. nonlinear effect occurs because of the intensity dependence of the refractive index. generally, to avoid these effects, modulate the input power and amplifier gain.
SPM - use a power source with as narrow a linewidth as possible.
XPM - nonlinearity occurs in MMF. use non-zero dispersion shifted fiber.
FWM - use non-zero dispersion shifted fiber.
Q4. what is the cut-off wavelength of a fiber? where does it play a role?
A4. (see notes)
Q5. give an example of how WDM can be used to facilitate communication in a local area network.
A5. (see notes. what i did, though, was give different LAN topologies and explained where multiplexing and demultiplexing can take place as well as what kind of lasers are best suited at each node.)
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