Poled fiber devices
2005 (English)Doctoral thesis, comprehensive summary (Other scientific)
The topic of this thesis is the development of devices for telecom applications based on poled optical fibers. The focus is on a specific function, optical switching/modulation.
Some of the most important results are summarized below. Optical switching at telecom wavelengths (1.55 μm) is demonstrated in an all-fiber switch based on a fiber with internal electrodes. The fiber is made electro-optically active with a thermal poling process in which a strong electric field is recorded in the glass at a temperature of 255 °C. After poling, the fiber is put in one arm of a Mach-Zehnder interferometer and by applying a voltage across the two electrodes the refractive index is modulated and the optical signal switched from one output port to the other. A switching voltage of 190 V at 1550 nm was achieved, which to the best of our knowledge is the lowest value reported. By carefully matching the lengths of the fibers in the two arms of the interferometer the optical bandwidth could be made as large as 20 nm. The extinction ratio, determined by the power ratio in the two arms, was 30 dB and the highest modulation frequency was 30 MHz. Poled fibers were packaged to increase the thermal and mechanical stability and to make handling easier. 40 Gb/s transmission test through the device showed no bit-error-rate performance degradation. Protection switching of a 10 Gb/s signal is also demonstrated.
The depletion region in a poled fiber was found to be wedge-shaped and very wide, 13 μm and completely overlapped with the core. In a time-resolved poling experiment the recorded electric field was measured. The sign of the field changed after ~20 min, when the depletion region passed through the core, which led to the conclusion that an electric field is present also outside of the depletion region.
A ring laser was constructed with an erbium doped fiber as the gain medium. A fiber modulator was placed inside the cavity and when a small RF signal, with a frequency matched to the cavity ground frequency, was applied to the modulator the laser was modelocked. The output pulse train contained pulses of sub ns duration and is the first demonstration of mode-locking using poled fibers.
A sampled grating with 16 channels spaced by 50 GHz was inserted into the cavity. The fiber modulator had optical bandwidth of 7 nm with center wavelength that depends on the applied voltage. By applying of 10 – 210 V to the modulator it was possible to tune the laser to 11 of the 16 channels for a total tuning range of over 4 nm.
A scheme to deposit 1 μm thin silver electrodes inside the holes of an optical fiber is demonstrated together with a new method of creating periodic electrodes by periodically ablating the silver film electrodes. The periodic electrodes are used to create a quasi-phase matched (QPM) nonlinearity in a fibers which is showed in a proof of principle experiment.
Place, publisher, year, edition, pages
Stockholm: KTH , 2005. , 77 p.
Trita-FYS, ISSN 0280-316X ; 2005:22
Physics, optical physics, Poling, twin-hole fiber, fiber electrodes, silver film electrodes, silver diffusion, quasi-phase matching, optical switching, frequency conversion, optical modulation
IdentifiersURN: urn:nbn:se:kth:diva-262ISBN: 91-7178-053-XOAI: oai:DiVA.org:kth-262DiVA: diva2:8273
2005-06-10, Sal FD5, AlbaNova, Roslagstullsbacken 21, Stockholm, 10:00
QC 201010152005-06-072005-06-072015-03-30Bibliographically approved
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