Theoretical And Experimental Study Of Slow-light Effect In Microfiber Knot Resonator

Slow light(slowing down the group velocity of the light) has been attracting an extensive interest for researchers owning to its potential applications, such as data synchronization, variable optical delay lines, and optical switching in all-optical signal processing systems. By now, there have been two principles to produce slow-light effect——Based on the material dispersion and based on the structural dispersion. Microfiber knot resonator(MKR) fabricated with a microfiber due to its special characteristics, such as large evanescent field, tight optical confinement, and low propagating and curving loss, has some extensive applications in micro-/nano-optical filtering, sensing, tuning, nonlinear and laser, etc. Its special structure determines that it has a periodic modulation for the light field propagating in it, which can produce the strong structural dispersion. Therefore, this resonator can produce slow-light effect. However, there hasn‘t been a systematic and deep report about the study of the slow-light technology in this kind of resonators yet. Under the support of the National Natural Science Foundation of China, we have developed the theoretical and experimental study on the light propagation and slow-light effect and technology in the MKRs with different structures in this dissertation. The main researching contents and innovative achievements can be concluded in the following:(1) An investigation about the theoretical analysis models of the light propagation and the spectral modulation in the single-ring MKR, the MKR with a two-ring parallel structure and the MKR with a two-ring serial structure is made. Based on them, the theoretical relationships of the MKR with a multi-ring(n-ring) parallel structure and the MKR with a multi-ring(n-ring) serial structure are deduced. Then, a simple, polymer-microfiber-assisted approach to fabricate the MKR with different structures is proposed. Using the method, we have successfully fabricated the single-ring MKR, the MKR with a longer intertwisted overlap at the contact area, the MKR with a two-ring parallel structure or a two-ring serial structure. Comparing with the traditional methods, this technique we proposed is quite simple and is easy to fabricate much more complicated multi-ring MKR.(2) The changing rules of the transmission spectra of the MKR with a two-ring parallel structure and the MKR with a two-ring serial structure are investigated theoretically under the different diameter ratios of their two rings. Numerical simulation indicates that the number of the transmission peaks of the MKR with a two-ring serial structure under the envelope of its transmission spectrum is equal to the diameter ratio of its two rings, the number of the transmission peaks of the MKR with a two-ring parallel structure increases with the increasing of the diameter ratio of its two rings and there is a narrower transmission peak per equal spacing peaks of the diameter ratio. Experimental results demonstrate that the theoretical simulation is correct. Corresponding explanations about the changing rules of them are given. Then, the group delays in the single-ring MKR, the two-ring MKR and the three-ring MKR are calculated numerically and a comparison about them is made. The results indicate that a large group time delay with very narrow bandwidth can be obtained in the MKR with a n-ring parallel structure and a broadband group time delay can be achieved in the MKR with a n-ring serial structure. In experiment, we fabricate the single-ring MKR and the MKR with a two-ring parallel structure and build the experimental setups used to measure the slow-light delays in them. The group delays of about 57 ps and 75 ps are achieved in them, respectively.(3) Numerical simulations indicate that one can change the transmission strength and the transmission bandwidth of the EIT-like spectrum via changing the coupling coefficients of the two knot rings in the MKR with a two-ring parallel structure. Based on the effect of the EIT-like, we study the slow-light characteristics in the resonator. A group time delay of about 72.4 ps with a flat wavelength bandwidth of about 82.7 pm at the induced transparency window is achieved theoretically, the corresponding full width at half-maximum(FWHM) is about 228 pm. And the group delay dispersion at the same induced transparency window nearly reaches to zero with a wavelength bandwidth of about 82.7 pm. In experiment, we fabricate the MKR with a two-ring parallel structure. An average pulse delay of about 69.4 ps with a flat-top wavelength bandwidth of about 190 pm is obtained in the resonator by modulating some corresponding parameters(such as coupling coefficients, diameters of two rings). Based on this structure, we proposed and fabricated the microfiber double-knot resonator with a Sagnac loop reflector. The theoretical and experimental study indicates that a larger group delay can be achieved in it.