5.1 Conclusion
In this thesis work, we successfully demonstrate Si-cored fibers with single crystalline state, lower transmission loss in telecommunication band, meter-long and uniform dimension in core and cladding by using a combined technique of rapid-drawing and powder-in-tube. Moreover, a novel method has been presented to fabricate a Si microsphere resonator from a Si-cored fiber by using CO2 laser reformation process.
WGM in a Si microsphere resonator has been obtained with Q as high as 4 x 105. To our best knowledge, such Q from a Si spherical resonator by using a silica tapered fiber for coupling is the highest as compared to the existing literature. We believe this Si resonator with high Q may find useful applications in sensing and nonlinear optics.
In addition to Si microsphere demonstration, a potential work about Si-cored tapered fibers is also highlighted in this thesis. As introduced in Chapter 3, Si-cored tapered fibers were fabricated from cored fibers by using a miniaturized fiber drawing tower. A Si-cored fiber of diameter 20 μm is heated by a hydro-oxygen flame and then rapidly drawn downward in order to form taper shape, resulting the core diameter as small as ~ 2.6 μm.
The resultant Si-cored tapered fiber has a total device loss ~ 30 dB, which is attributed to interface reflection at both end, transmission loss of smaller core diameter, end-face surface scattering loss and non-adiabatic taper transition.
5.2 Future Work
According to the previous report [38], the optical nonlinearities like Kerr effect can be greatly enhanced after the shrinkage in cross-section area of waveguide. Furthermore, the number of guided modes in the Si-core could be effectively reduced the Si-cored tapered fiber with smaller dimension. This fabrication method of tapered fiber can be used to provide a possibility for demonstrating single-mode transmission in Si-cored fiber, implying the application of nonlinear optics using Si-cored tapered fibers is promising in future work.
We believe that the Si WGM resonators with high Q’s have an opportunity of being good active elements according to the past reports on Raman effect in Si material. For example, a demonstration of all-Si Raman lasing experiment involving Si as the gain medium using stimulated Raman scattering has been reported [61]. Based on the high nonlinearities in Si and WGMs resonance with high-Q, Si WGMs resonators serve good candidates as amplifier or laser.
Next, let’s focus on sensing. Based on early reports, the detection limit is proportional to the quality factor [62]. The higher quality factor of WGM resonators, the sharper dips of spectra at resonant wavelength and the smaller amount of shift of resonant wavelength can be distinguished. So only if the quality factor of resonator is high enough, the low detection limit can be achievable. The good transmission spectral feature of Si at mid-infrared region is promising for bio-sensing and chemical sensing. Some reports on Si devices for bio-sensing and chemical sensing at mid-infrared region have been published [63][64].
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