Dispersion measurements of fiber-optic components and applications of a novel tunable filter for optical communications

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Optical communications has experienced a rapid development during the last decade. More bandwidth can be acquired by decreasing the spacing of the optical channels or by increasing the data rate. Characterization of the optical components and active monitoring of the network calls for accurate measurement methods. The objective of this thesis is to investigate and develop measurement methods and instruments for measuring important parameters of the components used in optical communications. Chromatic dispersion of optical fibers and frequency chirp of the laser transmitters set limits for the data rate and transmission distance. Measurements of dispersion have traditionally been performed using a phase-shift method. When high modulation frequencies are applied to achieve high resolution an error could be introduced. In this thesis, the measurement accuracy of this method is analyzed in detail and a novel method for estimating the accuracy and correcting the measurement result is developed. A Fabry-Perot interferometer finds numerous applications in many fields of optics. In this thesis, tunable Fabry-Perot etalon filters made of silicon were developed and several applications for these devices are demonstrated. A new device for measurements of time-resolved frequency chirp of directly modulated laser diodes in real time is developed. Interaction between the dispersion and frequency chirp limits the use of directly modulated lasers in long-haul optical links. Another application is monitoring of the wavelength of optical transmitters. The wavelength of the laser diode may shift due to aging and active monitoring and controlling of the wavelength is required. In addition, the filter improves the performance of the directly modulated transmitter by temporal reshaping of the pulses. The filter is also applied in reducing the frequency chirp of gain-switched pulses generated with a diode laser. These pulses can then be made transform limited and can for example be used in generation of optical solitons. Finally, the etalon is employed in realization of a compact wavelength reference for calibration of the wavelength scale of the optical spectrum analyzers and wavemeters. The transmission spectrum of the etalon consists of equidistant fringes. Each of these fringes can be applied as an accurate reference over a large wavelength range once the temperature of the filter is stabilized. This reference was developed to be automatic and it has an adequate accuracy for performing calibrations of field instruments.
wavelength division multiplexing, optical communications, fiber-optic components, dispersion measurements, Fabry-Perot etalon, tunable optical filters
Other note
  • T. Niemi, M. Uusimaa, and H. Ludvigsen, Limitations of phase-shift method in measuring dense group delay ripple of fiber Bragg gratings, IEEE Photonics Technology Letters, Vol. 13, pp. 1334-1336, 2001. [article1.pdf] © 2001 IEEE. By permission.
  • G. Genty, T. Niemi, and H. Ludvigsen, New method to improve the accuracy of group-delay measurements using the phase-shift technique, Optics Communications, Vol. 204, pp. 119-126, 2002. [article2.pdf] © 2002 Elsevier Science. By permission.
  • T. Niemi, S. Tammela, T. Kajava, M. Kaivola, and H. Ludvigsen, Temperature-tunable silicon-wafer etalon for frequency chirp measurements, Microwave and Optical Technology Letters, Vol. 20, pp. 190-192, 1999.
  • T. Niemi, S. Tammela, and H. Ludvigsen, Device for frequency chirp measurements of optical transmitters in real time, Review of Scientific Instruments, Vol. 73, pp. 1103-1107, 2002. [article4.pdf] © 2002 American Institute of Physics. By permission.
  • T. Niemi, J-G. Zhang, and H. Ludvigsen, Effect of optical filtering on pulses generated with a gain-switched DFB laser, Optics Communications, Vol. 192, pp. 339-345, 2001. [article5.pdf] © 2001 Elsevier Science. By permission.
  • T. Niemi, M. Uusimaa, S. Tammela, P. Heimala, T. Kajava, M. Kaivola, and H. Ludvigsen, Tunable silicon etalon for simultaneous spectral filtering and wavelength monitoring of a DWDM transmitter, IEEE Photonics Technology Letters, Vol. 13, pp. 58-60, 2001. [article6.pdf] © 2001 IEEE. By permission.
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