4.1 Summary of results
We have successfully induced second-order nonlinearity in optical fibers by means of the thermal poling technique. From the SHG measurement, we observe that the SH power is proportional to the square of the fundamental pump power and the SH power generate from d31 is lower than the SH power generate from d33. These observations agree with the expectation from the principles of nonlinear optics. We also observe that the SH power is varied by tuning the angle of the half-wave plate. This is conform to the characteristics of the half-wave plate, for which if the polarization of the incident light makes an angle θ with the axis of the half-wave plate, the polarization of the laser light rotates by an angle -2θ. We thus
observe that the maximum and the minimum SH powers occur at angles that differ by 450. The rough estimation of the achieved d33 is about 0.023 pm/V. By the UV erasure and BOE solution etching, we can obtain the periodic pattern on the surface of thermally poled fused silica plate. It confirms that the UV erasure method should be applicable in fabricating QPM SHG devices in optical fibers. Finally, we observe that the second-order nonlinearity in thermally poled optical fiber can be totally erased by the UV erasure of 10 minutes.
4.2 Future work
The final goal of this study is to fabricate poled QPM fiber devices. Because of the low
dispersion and low group-velocity mismatch possibly achieved in optical fibers, the relatively low value of the nonlinear coefficient can be compensated by an increase in the length of the poled fiber and by the technique of QPM. Poled QPM optical fibers also can be used for the generation of entangled photon-pairs by parametric down-conversion, which can be useful for achieving all-fiber quantum information applications. Due to the incorrect grating period we use, we have not observed the QPM effects so far. In the future we will continue to actually fabricate poled QPM fiber devices and to explore their applications.
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