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All-optically reconfigurable and tunable fiber surface grating for in-fiber devices: a wideband tunable filter

Jianhui Yu, Yuqi Han, Hankai Huang, Haozi Li, Vincent K. S. Hsiao, Weiping Liu, Jieyuan Tang, Huihui Lu, Jun Zhang, Yunhan Luo, Yongchun Zhong, Zhigang Zang, and Zhe Chen

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Abstract

A fiber surface grating (FSG) formed from a photosensitive liquid crystal hybrid (PLCH) film overlaid on a side-polished fiber (SPF) is studied and has been experimentally shown to be able to function as an all-optically reconfigurable and tunable fiber device. The device is all-optically configured to be a short period fiber surface grating (SPFSG) when a phase mask is used, and then reconfigured to be a long period FSG (LPFSG) when an amplitude mask is used. Experimental results show that both the short and long period FSGs can function as an optically tunable band-rejection filter and have different performances with different pump power and different configured period of the FSG. When configured as a SPFSG, the device can achieve a high extinction ratio (ER) of 21.5dB and a wideband tunability of 31nm are achieved. When configured as a LPFSG, the device can achieve an even higher ER of 23.4dB and a wider tunable bandwidth of 60nm. Besides these tunable performances of the device, its full width at half maximum (FWHM) can also be optically tuned. The reconfigurability and tunability of the fiber device open up possibilities for other all-optically programmable and tunable fiber devices.

© 2014 Optical Society of America

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Figures (8)
Fig. 1
Fig. 1 Optical micrographs of three regions of a fabricated SPF: (a) left transitional region, (b) flat region and (c) right transitional region. Positions of the three regions are indicated by the blue arrows in Fig. 1(d), which is a plot of residual cladding thickness along the SPF. Figure 1(e) is the SEM micrograph of the surface of the SPF flat region, showing 1~2μm roughness of the surface.

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Fig. 2
Fig. 2 Microscopic top views of the SPF flat region: (a) before coating the PLCH film; (b) after coating the PLCH film; (c) with the PLCH films pre-illuminated by a 405nm wavelength laser for 2 minutes and settled down for half an hour.

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Fig. 3
Fig. 3 (a) Schematic diagram of experimental setup for testing the performance of an optically reconfigurable and tunable SPF-based surface grating; (b) microscopic graph of a phase mask under a 60X objective lens, showing the period is measured to be of 528nm; (c) microscopic graph of an amplitude mask under a 4X objective lens showing the mask period is measured to be of 500μm.

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Fig. 4
Fig. 4 Schematic diagrams showing formation of surface gratings on a SPF. (a) Photosensitive mechanism of PLCH; (b) Formation of a short period FSG on SPF with a phase mask; (c) Formation of a long period FSG on SPF with an amplitude mask; (d) Micrograph showing the interference pattern of ~528nm period, which is formed by the interference between −1st,0th and 1st order diffractions beams after 405nm laser beam passed through the phase mask as illustrated in (b); (e) Micrograph showing the long period grating of 500μm period in a ~30μm thick PLCH film, which is produced by 405nm wavelength pump light illumination through the amplitude mask shown in (c).

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Fig. 5
Fig. 5 Normalized transmission (a) and reflection (b) spectrums of the PLCH-coated SPF. The transmission spectrums under different pump powers are shown in (a), where the power of the pump light is changed from 0mW(black solid), to 9.6mW(red dash), 14.4mW(blue dot), 26.3mW(dark cyan dash dot), 33mW(magenta dash dot dot),45.3mW(dark yellow short dash), 62.9mW(navy short dot), and back to 0mW(brown short dash dot). The spectrums of reflection power shown in (b) are measured without (black solid) and with (red dash) illumination of the pump light at 40.1mW, and the spectrum of normalized reflection associated with the right vertical axis in (b) is shown by blue solid line in (b).

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Fig. 6
Fig. 6 Variations of resonant wavelength (RW) in (a), and extinction ratio (ER) and full width at half maximum (FHWM) in (b) with increase of the pump power illuminating on the PLCH-coated SPF.

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Fig. 7
Fig. 7 (a) Transmission spectrums of PLCH-coated SPF upon illumination from pump light through an amplitude mask with different pump powers. The pump power is successively 0mW(black solid), 2.8mW(red dash), 8.7mW(blue dot), 11.5mW(dark cyan dash dot), 15.5mW(meganta dash dot dot), 21.1mW (dark yellow short dash), and 0mW(navy short dot). (b) Reflection spectrums without (black solid) and with pump light power at 8.7mW (red dash), and the spectrum of normalized reflection (blue dash) calculated from the above two spectrums of reflection power.

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Fig. 8
Fig. 8 Variation of resonant wavelength in (a), and variations of extinction ratio and FWHM in (b) of the long period FSG with the pump power increasing from 2.7mW to 21.1mW.

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Equations (1)

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λ r = Λ ( n P L C H m − n c o r e )
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