The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
Aalto

Fiber Amplifiers, Directly Modulated Transmitters and a Ring Network Structure for Optical Communications

Simo Tammela

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Electrical and Communications Engineering for public examination and debate in Auditorium B at Helsinki University of Technology (Espoo, Finland) on the 21st of January, 2004, at 12 o'clock noon.

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Dissertation is also available in print (ISBN 951-22-6902-3)

Abstract

The three technologies that are considered the key elements in building a metropolitan area optical network are studied in this thesis. They are optical amplification, high-speed low cost transmitters and ring network structures. These studies concentrate on cost reduction of these three technologies thus enabling the use of optical networks in small customer base metropolitan areas.

The research on optical amplification concentrated first on the solution doping process, at present the most used method for producing erbium doped fiber. It was found that separationing the soot growth and the sintering improved the uniformity of the porous layer. This made the homogeneity of the doping concentration in the fiber core better. The effects of index profile variations that arise from the non-ideal solution doping process were also simulated. In the search for a better doping method a new nanoparticle glass-forming process, the direct nanoparticle deposition, was developed. In this process the doping is done simultaneously with glass formation. Utilizing this new process it was possible to improve the uniformity of the doping resulting in higher usable doping levels and shorter erbium doped fiber lengths in the amplifiers. There were fewer limitations in the amplifier caused by optical non-linearities and polarization mode dispersion since shorter fiber lengths were needed.

The double cladding fiber, which avoids the costly coupling of the pump laser into a single mode waveguide, was also studied. This pumping scheme was found to improve the inversion uniformity in the erbium doped fiber core thereby enhancing the power conversion efficiency for the long wavelength band amplifier.

In characterizing the erbium doped fiber amplifier the gain and noise figure was measured with a temporal filter setup. It was made of simple, low cost components but yielded accurate measurements since the noise originating from the amplified spontaneous emission was measured at the signal wavelength. In the study of fiber amplifier controlling schemes the input power of the fiber amplifier was successfully used to regulate the pump laser. This feed-forward control scheme provides a simple, low cost control and managment system for the erbium doped fiber amplifier in metropolitan area network applications that require flexible adding and dropping of wavelength channels.

The transmitter research focused on the DFB laser due to its simplicity and low cost structure. A solid state Fabry-Perot etalon made from double polished silicon chip was used as a frequency discriminator in the chirp analyser developed for the DFB lasers. This wavelength discriminator did not require repeated calibration or active stabilisation and was controled electrically enabling automatic measurements. The silicon Fabry-Perot etalon was also used for simultaneous spectral filtering and wavelength control of the laser. The usable dispersion limited transmission length was increased when the filter was used in conjunction with the directly modulated distributed feedback laser transmitter.

The combination of spatial multiplexing and dense wavelength division multiplexing in ring topology was investigated in the course of the research on the ring network as the feeder part of the metropolitan network. A new way to organize different wavelengths and fibers was developed. This ring network structure was simulated and an experimental ring network built. The results of the studies demonstrated that the same limitations effecting uni-directional ring structures also are the main limitations on the scalability of the spatial and wavelength division multiplexed ring networks based on bi-directional transmission when the node spacing is short. The developed ring network structure demonstrated major cost reductions when compared with the heavy use of wavelength division multiplexing. The node structure was also greatly simplified resulting in less need for different wavelength transmitters in each node. Furthermore the node generated only minor losses for the passing signals thus reducing the need for optical amplification.

This thesis consists of an overview and of the following 12 publications:

  1. Tammela S., Zhan X. and Kiiveri P., 1990. Comparison of gain dependence of different Er-doped fibre structures. First publication in: Proceedings of the Conference on Fiber Laser Sources and Amplifiers II, volume 1373, pages 103-110, 1990. Reprinted in: SPIE Milestone Series, MS 37, pages 189-195, 1992.
  2. Kiiveri P. and Tammela S., 2000. Design and fabrication of erbium-doped fibers for optical amplifiers. Optical Engineering 39, number 7, pages 1943-1950.
  3. Tammela S., Hotoleanu M., Kiiveri P., Valkonen H., Sarkilahti S. and Janka K., 2003. Very short Er-doped silica glass fiber for L-band amplifiers. Technical Digest of the Optical Fiber Communication Conference (OFC) 1, pages 376-377.
  4. Söderlund M., Tammela S., Pöyhönen P., Leppihalme M. and Peyghambarian N., 2001. Amplified spontaneous emission in cladding-pumped L-band erbium-doped fiber amplifiers. IEEE Photonics Technology Letters 13, number 1, pages 22-24.
  5. Kiiveri P. and Tammela S., 1995. Spectral gain and noise measurement system for fiber amplifiers. Optical Engineering 34, number 9, pages 2592-2595.
  6. Söderlund M. and Tammela S., 2000. Performance of a practical gain controlling approach for erbium-doped fiber amplifier. In: Proceedings of the Conference on Rare-Earth-Doped Materials and Devices IV, SPIE, volume 3942, pages 210-216.
  7. Tammela S., Ludvigsen H., Kajava T. and Kaivola M., 1997. Time-resolved frequency chirp measurement using a silicon-wafer etalon. IEEE Photonics Technology Letters 9, number 4, pages 475-477.
  8. Niemi T., Tammela S., Kajava T., Kaivola M. and Ludvigsen H., 1999. Temperature-tunable silicon-wafer etalon for frequency chirp measurements. Microwave and Optical Technology Letters 20, number 3, pages 190-192.
  9. Niemi T., Uusimaa M., Ludvigsen H., Tammela S., Heimala P., Kajava T. and Kaivola M., 2000. Simultaneous spectral filtering and wavelength monitoring of a DWDM transmitter using a tunable Fabry-Perot filter. Technical Digest of the Optical Fiber Communication Conference (OFC), Wednesday, pages 28-30.
  10. Tammela S., Aarnio J. and Tervonen A., 1998. Survivable WDM/SDM ring for metropolitan networks. Proceedings of the Conference on All-Optical Networking: Architecture, Control, and Management Issues, SPIE, volume 3531, pages 448-454.
  11. Ylä-Jarkko K., Tammela S. and Tervonen A., 1999. Bidirectional WDM multifiber ring network with shared-pump EDFAs. Proceedings of the 25th European Conference on Optical Communication (ECOC'99), volume 2, pages 46-47.
  12. Ylä-Jarkko K., Leppihalme M., Tammela S., Niemi T. and Tervonen A., 2001. Scalability of a metropolitan bidirectional multifiber WDM-ring network. Photonic Network Communications 3, number 4, pages 349-362.

Keywords: optical networks, metropolitan area, optical amplification, erbium doped fiber

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© 2004 Helsinki University of Technology

Last update 2011-05-26