Modulation and networking Apr. 24, 2018

• Fiber circulator (sec 3.2): similar to isolator, based on Faraday effect. It directs signal sequentially from one port to the next, L=1dB, I=25dB.
• Optical cross-connects: hybrid approach - cross connecting electronically between EO/OE converters.
All optical switching - a table lookup for a device with N inputs and N outputs. The table will be a square matrix and has only one nonzero element in each row and column to maintain 1-1 correspondent.
• Device implementations: WGR - a$\text{ }N×N$ ~ generalized MZ interferometer.
Solid state cross-connects (Fig. 3.58) - speed dep. on material, e.g. directional coupler. Use multiple layers of couplers to implement a device with N inputs.
Result in higher loss and bigger size.
Microelectromechanical systems (MEMS) (sec 3.7.2) - Use standard chip fabrication technology to make rotating micromirror (0.1mm$\text{ }×$ 0.1mm).
Rotation angle controlled by another signal.
Use mirror array to make up$\text{ }N×N$ device.
Speed ms to$\text{ }\mu s$
• Switching speed: MEMS and$\text{ }Si{O}_{2}$ (ms to$\text{ }\mu s$ ),$\text{ }LiNb{O}_{3}$ (ns), polymer based electro-optic devices (ps) which has strong absorption at 1.3-1.5$\text{ }\mu m$
Optical add-drop multiplexers (filters) (OADM) (sec 7.3): A device enables adding or extracting a channel wavelength from a data stream.
By inserting an optical switch between multiplexer and demultiplexer, we make an OADM.
Fixed wavelength selectable - pre-selected wavelength, e.g. using fiber Bragg gratings (Fig. 3.14) and circulators, ring resonators.
Dynamically wavelength selectable - add/drop wavelength can be dynamically chosen.
• Coherent detection (sec 4.4.7): similar to heterodyne and homodyne detection in radio. Require local oscillator with narrow spectral width that is comparable to the laser source in the incoming signal.
It is more complicated than intensity modulation / direct detection (IM/DD) using incoherent detection.
• Coding: RZ, NRZ, unipolar, bipolar signals. Bipolar signal is impossible optically since there is not negative photon.
4B/5B - 4 bit to 16 predetermine 5-bit codes
8B/10B - 8 bit to 256 predetermined 10-bit codes; avoid consecutive zeros, e.g. for sync. purpose.
• Modulation format: ASK - amplitude shift (on-off) keying, incoherent detection.
PSK - phase shift keying, require coherent detection, e.g. "0" corresponds to zero phase and "1" correspond to$\text{ }\pi$ phase.
Phase changes are encoded by a phase modulator,$\text{ }\Delta \phi \text{\hspace{0.17em}}=\text{\hspace{0.17em}}2\pi \Delta n{L}_{m}/\lambda$ , which can be changed by modulating$\text{ }\Delta n$ with electro-optic material.
FSK - frequency shift keying, usually require coherent detection, e.g. "1" corresponds to$\text{ }f\text{\hspace{0.17em}}+\text{\hspace{0.17em}}\Delta f$ and "0" corresponds to$\text{ }f\text{\hspace{0.17em}}-\text{\hspace{0.17em}}\Delta f$
Total bandwidth =$\text{ }2\left(\Delta f\text{\hspace{0.17em}}+\text{\hspace{0.17em}}B\right)$ where$\text{ }B$ is the bit rate.

$\text{ }\Delta f\text{\hspace{0.17em}}\gg \text{\hspace{0.17em}}B$

- wideband FSK,$\text{ }\Delta f\text{\hspace{0.17em}}\ll \text{\hspace{0.17em}}B$ - narrowband FSK.
Modulation index$\text{ }{\beta }_{FM}\text{\hspace{0.17em}}=\text{\hspace{0.17em}}\Delta f/B$
FSK can be realized in DFB lasers. A small change in current (1mA) causes freq shift (1GHz).
• Coherent detection: Advantages - better SNR, smaller channel spacing
Disadvantages - req. local oscillator with low noise and narrow bandwidth, oscillator and amplitude selection, req. coupler, .i.e. complicated and attenuation.
• Demodulation: Requirements for long distance: low coupling loss between receiver and signal, dispersion compensated, high sensitivity.
PSK - signal is mixed coherently with a local oscillator, resulting in destructive or constructive interference.
FSK - received signal is split into two branches. Each branch has an optical filter with peak at average$\text{ }\Delta \omega$
Also demodulated incoherently with a tunable filter.
Digital/ analog receivers: Both require detectors, amplifiers, filters. Digital receiver (sec 4.4.8) needs a timing circuit which is a square-wave oscillator. It tells the regenerator when to sample, to ensure that the output is clocked consistently and timing jitter is removed.
Analog receiver has a demodulator to separate signal from the carrier.
• Point-to-point networking terms: In telephony, signaling sets up calls. In data communication, virtual circuit (a logical connection) is set up for a session. Data are sent in packet.
• Information exchange: Application data processing, transmitted data processing, network access, traffic switching.
Data communication: 7 layers (Fig. 1.7) - application, presentation (put data in a standardized format), session (set up a communication session), transport (partition large data into small packets), network (switch and route packets), data-link (ensure accurate transmission), physical (hardware).
Network layer does header processing and checks transmission errors. Routing of packets is determined by access control protocol called medium access control (MAC). MAC is specified in data-link layer.
• MAC: determines how a transmission node sends data to a shared medium.
multiplexing - multiplexer between transmission nodes and shared medium, telephone.
multiple access - all nodes directly connected to the shared medium, e.g. ethernet.