Negative-index gratings formed by femtosecond laser overexposure and thermal regeneration.

Jun He,Xiaocong Yuan,Ying Wang,Changrui Liao,Guo Ping Wang,Shen Liu,Chao Wang,Yiping Wang,Kaiming Yang,Wenjing Zhang

doi:10.1038/srep23379

Jun He, Xiaocong Yuan + Show 8 more

Open Access

https://doi.org/10.1038/srep23379

Copy DOI

Abstract

We demonstrate a method for the preparation of negative-index fibre Bragg gratings (FBGs) using 800 nm femtosecond laser overexposure and thermal regeneration. A positive-index type I-IR FBG was first inscribed in H2-free single-mode fibre using a femtosecond laser directed through a phase mask, and then a highly polarization dependant phase-shifted FBG (P-PSFBG) was fabricated from the type I-IR FBG by overexposure to the femtosecond laser. Subsequently, the P-PSFBG was thermally annealed at 800 °C for 12 hours. Grating regeneration was observed during thermal annealing, and a negative-index FBG was finally obtained with a high reflectivity of 99.22%, an ultra-low insertion loss of 0.08 dB, a blueshift of 0.83 nm in the Bragg wavelength, and an operating temperature of up to 1000 °C for more than 10 hours. Further annealing tests showed that the thermal stability of the negative-index FBG was lower than that of a type II-IR FBG, but much higher than that of a type I-IR FBG. Moreover, the formation of such a negative-index grating may result from thermally regenerated type IIA photosensitivity.

Highlights

Of up to 98.42% by using an ultra-strong seed Fibre Bragg gratings (FBGs) with a transmission depth of more than 50 dB and a grating length of 50 mm[23]
The researchers in Hong Kong Polytechnic University reported that the out-of-band insertion loss of type I-IR gratings was below 0.1 dB, whereas the insertion loss of type II-IR gratings was ranged from 0.6 dB to a few dB30
The P-PSFBG was annealed at 800 °C for 12 hours

Summary

Discussion

In the case of the original type I-IR FBG formed in step 1, as shown in Figs 1 and 5, a positive index modulation was induced by low-intensity NIR femtosecond laser pulses without overexposure, and erased by thermal annealing at high temperatures. The formation of such a type I-IR FBG is related to multiphoton nonlinear absorption process[25]. It could be inferred from our analysis that the formation of such a negative-index grating is most probably due to thermally regenerated type IIA photosensitivity

Conclusions

Findings

Author Contributions

Additional Information