Photonics and Optical Communication Spring 2005

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1 Photonics and Optical Communication Spring 2005 Final Exam Instructor: Dr. Dietmar Knipp, Assistant Professor of Electrical Engineering Name: Mat. -Nr.: Guidelines: Duration of the Final Exam: 2 hour You can get 128 points for the final exam (100 points are necessary for the undergraduate students to get 100%, 110 points are necessary for the graduate students to get 100%). The smallest available unit for the answer of a question is 0.5 point. Try to provide short answers. The answer can be shorter than the question. Good Luck!

2 1 Introduction (2 points) The transition from the 3 rd to the 4 th Generation of Optical Communication Systems was initiated by what kind of invention? (2 points) 2 Review of optics (14 points) 2.1 A laser diode emits light at a wavelength of 1.55µm. The light is coupled in an optical fiber. Calculate the wavelength, the frequency and the speed of the light in the optical fiber, which has a refractive index of (4 points) 2.2 The reflection of a glass sheet is given in figure 1. Explain why the reflection of the glass sheet changes as a function of the thickness. Explicitly explain why the reflection changes between 0% and 16% (Mathematical explanation required.) The glass sheet has a refractive index of 1.5. (10 points) Fig. 1: Reflection of a glass sheet as a function of the thickness. The thickness of the glass sheet is normalized to the incident wavelength.

3 3 Waveguide Coupler (8 points) 3.1 Explain the general operating principle of the waveguide coupler shown in figure 2. (4 points) 3.2 Explain how the waveguide coupler in figure 2 can be used to split the incoming signal in two equal output signals A and B. (2 points) 3.3 Now the same waveguide coupler (question 3.2) should be used to merge two signals coupled in waveguide A and B. What are the consequences? (2 points)

4 in B A Fig. 2: Schematic sketch of a waveguide coupler. 4 Optical Fibers (22 points) 4.1 Explain the underlying physical principle of chromatic dispersion. (4 points) 4.2 List the most important sources of dispersion for a single mode fiber. (2 points)

5 4.3 Why are only single mode fibers used for high-speed optical communication systems? (2 points) 4.5 Explain the scattering mechanisms in single mode and multi mode fibers. Provide a detailed explanation. (4 points) 4.6 An optical power of 150µW is launched in an optical fiber. The optical fiber has an attenuation of 0.52dB/km. The minimum optical power, which can be detected by an optical detector on the receiver side, is -45dBm. No optical amplifiers are used to amplify and reshape the signal. Every 10km an optical fiber joint (optical fiber interconnect) is required with an attenuation of 1.5dB. Determine the maximum possible length of the optical fiber. (10 points)

6 5 Light emitting diodes (10 points) 5.1 Explain the operating principle of a light emitting diode (LED). Provide a detailed description. (10 points) 6 Laser diodes (18 points) 6.1 Explain stimulated emission in a lasing media. Simply assume that the lasing media has only 2 energy levels. Try to be as precise as possible with your explanation. (10 points) 6.2 In practice lasing media typically exhibit 4 energy levels. Explain why four level systems are used. (4 points)

7 6.3 What types of edge emitting laser diodes are used for long distance DWDM communication systems? Explain why these laser diodes are used. (4 points) 7 Optical Detectors (15 points) The photocurrent of a silicon pin diode is given by the following equation. e λ = P 0 exp h c ( 1 R) [ 1 ( α d )] I ph Explain the meaning of each term of the equation. (3 points) 7.2 Calculate the spectral responsivity of the pin diode. The refractive index of silicon is assumed to be 3.3. The absorption coefficient can be extracted from figure 3. The diode has a thickness of 10µm. Calculate the spectral sensitivity for a given wavelength of 900nm. (8 points) Constants: e=1.602 x As, h=6.626 x Js, c=3 x 10 8 m/s,

8 Fig. 3: Absorption coefficient of different materials. 7.3 The thickness of the diode is increased by 10µm. Calculate the new spectral responsivity for a given wavelength of 900nm. (4 points)

9 8 Erbium doped optical fiber amplifiers (5 points) 8.1a Explain the meaning of modulation and speed transparent. (3 points) 8.1b Why are these properties important (2 points) 9 Dense Wavelength Division Multiplex (6 points) 9.1 Sketch a point-to-point DWDM communication system and indicate all important components (6 points)

10 10. Dispersion Management (12 points) 10.1 What is the zero dispersion point? Provide a detailed explanation for a single mode fiber. (4 points) 10.2 Let s assume that a single mode fiber has a dispersion of 17ps/km/nm. The signals should be transmitted over a distance of 1000km. A compensated fiber is used to compensate for the dispersion of the single mode fiber. The dispersion compensated fiber has a dispersion coefficient of -100ps/km/nm. Design the fiber link in such a way that the total dispersion of the fiber gets zero. Explain your solution. (8 points) 11 Power budgeting (16 points) 11.1 Express the optical power levels of 1mW and 0.5µW in terms of db and dbm. (4points)

11 11.2 A DWDM optical fiber communication system operating at 1.55µm should be designed. The power launched into the fiber is 0.2mW and the optical fiber has an attenuation of 0.4dB/km. The attenuation caused by splices is 0.2dB/km. Ten equally spaced optical amplifiers with a gain of 12dB will be installed to amplify the signals. On the receiver side a minimum signal-to-noise ratio of 15dB is required. The optical detector on the receiver side has a noise level (dark current), which corresponds to 10pA. The spectral sensitivity of the optical detector is 0.5A/W. Determine the maximum distance between the transmitter and the receiver. (12 points)

Photonics and Optical Communication

Photonics and Optical Communication (Course Number 300352) Spring 2007 Dr. Dietmar Knipp Assistant Professor of Electrical Engineering http://www.faculty.iu-bremen.de/dknipp/ 1 Photonics and Optical Communication