UNIVERSITY OF TORONTO FACULTY OF APPLIED SCIENCE AND ENGINEERING. FINAL EXAMINATION, April 2017 DURATION: 2.5 hours

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1 UNIVERSITY OF TORONTO FACULTY OF APPLIED SCIENCE AND ENGINEERING ECE S - FINAL EXAMINATION, April 2017 DURATION: 2.5 hours Optical Communication and Networks Calculator Type: 2 Exam Type: X Examiner: J. K. S. Poon Instructions: Please write your work in the space provided and in the boxes if available. The exam has 4 parts (Short answers + 3 problems). Each part is out of 20 points. For instructor use only Q Total Name (last name, first name): Student Number: Page 1 of 11

2 1. (20 points) Short answers (4 points) For each type of optical communication link, select the fiber type that is the most commonly used today. Link Type Fiber Type (select one for each type) Optical USB or Thunderbolt Single-mode Multi-mode Polarization-maintaining I ntra-datacenter Single-mode Multi-mode Polarization-maintaining Metro area network Single-mode Multi-mode Polarization-maintaining Long haul network Single-mode Multi-mode Polarization-maintaining (2 points) Why doesn't the signal-to-noise ratio (and hence channel capacity) increase indefinitely with increasing optical fiber launched into an optical fiber? (4 points) List and briefly explain 2 ways to increase the spectral efficiency of an optical communication link. Page 2 o 11

3 (d) (4 points) What is group velocity dispersion? Give two causes of group velocity dispersion, and explain why dispersion limits the bit rate of a fiber optic link. (e) (6 points) You have 4 fiber Bragg gratings for 4 different wavelengths (?i,?2, 4?4), one 3-dB coupler, two 1550nm/980nm WDMs, one AWG, one section of erbium doped fiber, and a pump laser diode emitting a wavelength of 980 nm. Sketch the setup using these parts to construct a multi-wavelength laser that outputs the four wavelengths (Xi, 22, X3,?4) into 4 separate output fibers. Propose one application for this laser. Page 3 of 11

4 2. (20 points) An Optical Transmitter You are asked to assemble a 1550 nm optical transmitter module consisting of a single-mode edge-emitting laser coupled to a 40 GHz lithium niobate optical modulator. The modulator is in the single-arm drive configuration. The modulator driver electronics provides an AC-coupled peak-to-peak voltage swing of 3Vpp, and an independent DC bias. The modulator operation is biased at 50% transmission point (relative to the maximum transmission). You require an average output power of 5 mw from the transmitter module. Laser threshold current: 20 ma Laser slope efficiency: 0.2 mw/ma Modulator insertion loss: 3.2 db Modulator V: 5 V (a) (6 points) What is the extinction ratio in db of the transmitter? (a) Extinction ratio (db) Page 4 of 11

5 (b) (4 points) What percentage of the laser output power is transmitted at the '1' level and '0' level through the modulator? (b) % Transmission '1 level: '0' level: (C) (6 points) What should be the drive current to the laser in ma? (c) Laser drive current (ma) Page 5 of 11

6 (d) (4 points) Sketch the expected eye pattern of the transmitter for an RZ modulation format at 25 Gb/s. Label the expected power levels and the temporal duration of the bit slot. Page 6 of 11

7 3. (20 points) Lightpaths (a) (10 points) Consider a bi-directional ring network with N nodes, where N is an odd number. At each node, all the wavelengths are received, read, and either dropped (if the node should receive those wavelengths) or re-generated and transmitted to the adjacent node. All lightpaths are single-hop between adjacent nodes in the ring. Let t be the traffic from one node to all other nodes, and let each wavelength carry 1 unit of traffic. Show that the number of wavelengths needed, W, is t N2-1 W_[ Nl1 8 Clearly explain your steps. Page 7 of 11

8 (b) (10 points) Consider 5 lp routers connected in a unidirectional ring network illustrated below. Take the traffic to be in the counter-clockwise direction only. The following connections need to be made: A to B, D to C, C to E, and B to A. Each connection has to carry 40 Gb/s of traffic and each wavelength supports 10 Gb/s. Sketch the lightpaths that need to be setup in the network. How many wavelengths are minimally required? 3(b) Number of wavelengths Page 8 of 11

9 4. (20 points) Design an Optical Network You are planning a wavelength division multiplexed passive optical network (WDM- PON) between a central office and houses on a street. The network has the topology illustrated below. It should support upstream and downstream traffic from/to each home at B = 10 Gb/s. You have 32 wavelengths to connect to 32 homes. A1 House Central LX1, A2 Office AWG MUX+DEMUX House House 32 The distance between the central office and the AWG MUX+DEMUX is 5 km, and the distance between the AWG MUX+DEMUX and each home varies between 50 m and 800 m. The insertion loss through the AWG is 5 db. The fiber connector coupling loss at each home is 2 db. This network operates in the C-band, around a wavelength of 1550 nm. The fiber dispersion is 20 ps/(km-nm). The fiber propagation loss is 0.2 db/km. (a) (5 points) Assume the transmitter and receiver bandwidths are equal to each other and NRZ format is used. Determine the 10%-to-90% rise time required for the transmitter and receiver, and the bandwidth needed for the transmitter and receiver. Page 9 of 11

10 4(a) Rise time (ps) Receiver/Transmitter Bandwidth: (b) (5 points) Determine the worst case reduction in the peak power in db due to the spreading of a pulse if the chirp parameter is C = -2. You may assume the shortest pulse is Gaussian with a full-width at half-maximum intensity of 1/(2B) = 50 ps. 4(b) Reduction in peak power (db) Page 10of 11

11 (c) (10 points) Find the minimum average optical power in mw for each wavelength that should be launched in the central office to achieve a BER < 10 for every home and allowing for a system margin of 5 db. The transmitters for the 32 wavelengths output the same optical power. The photoreceiver at each home has a responsivity of 0.8 A/W, a dark current of 2 na, a resistance of 50 Q, and an electronic noise figure of 3 db. 4(c) Mm. launch power (mw) Page 11 of 11