Optically tunable chirped fiber Bragg grating

Zhen Li; Zhe Chen; V.K.S. Hsiao; Jie-Yuan Tang; Fuli Zhao; Shao-Ji Jiang

doi:10.1364/OE.20.010827

1. Introduction

Tunable CFBGs have been applied in many fields such as dispersion compensator [1, 2], optical time delay lines [3, 4], optical fiber filter [5, 6], sensors [7] and pulse shaping apparatus [8]. Various techniques have been demonstrated to obtain tunable CFBG with the application of mechanical [1, 6], thermal [9, 10], magnetic [11] and electrical methods [12, 13].

Due to the large birefringence and fluidity of LC, LC undergoes variable RI which can be manipulated conveniently. They have been used in a variety of photonic application to enable tunable responses, with stimuli including thermal [14], magnetic [15], electrical [16] and optical fields [17–19]. In our previous works, light-tunable fiber devices have been demonstrated by infiltrating photoresponsive LCs into a photonic crystal fiber [20], or overlaying photoresponsive LCs onto a side-polished fiber [21] and onto a side-polished fiber Bragg grating [22]. Now, an optically tunable CFBG is demonstrated by overlaying the photoresponsive LCs onto a side-polished fiber Bragg grating (SPFBG) whose thickness of the residual cladding layer (D_RC) varies with position along the length of the grating. By irradiation with 0.64mW/mm² UV light, the broadening in the CFBG reflection spectrum as well as the corresponding shift in the central wavelength is observed. The broadening and the shift are repeatable and reversible.

2. Experimental

The optically tunable CFBG consists of a SPFBG whose D_RC varies with position along the length of the grating (as shown in Fig. 1 ) and a photoresponsive LC layer. A SPFBG is fabricated by the wheel-polishing method using a single mode fiber (SMF) containing a section of a fiber Bragg grating (FBG) with a grating period of 526nm [23, 24]. The surface roughness of the polished area is about 0.1µm. The diameters of fiber core and cladding are 8µm and 125µm respectively. The photoresponsive LC is a homogeneous mixture of 80 wt% nematic LC (E-series MDA-00-3461, n_e = 1.772 and n_o = 1.514 at 20 °C and 589 nm, Merck), 10 wt% azobenzene LC (4-butyl-4`-methyl- azobenzene, BMAB), and 10 wt% chiral dopant (ZLI 811, Merck), of which the absorption band is around 365nm.

Fig. 1 (a) Schematic of a SPFBG whose D_RC varies with position along the length of the grating ; (b) the scanning electronics microscope image of the end part of the side-polished area; (c) the microscope image (Nikon Eclipse 80i × 100) of the middle part of the FBG.

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Figure 2 shows the experimental setup for characterizing the optically tuning of the CFBG. An amplified spontaneous emission light source (ASE, WRI ASE-C-N) is used as the input light source. The reflection spectrum is characterized by an optical spectrum analyzer (OSA, ANDO AQ6371C). The CFBG is mechanically spliced using single mode fibers (SMF) with a FC connector. A 365 nm wavelength UV light source (USHIO SP-7) is applied to the polished area to initiate the photoresponsive LC phase transition. The broadening in the CFBG reflection spectrum and corresponding shift in the central wavelength are analyzed by comparing reflection spectra recorded by the OSA with a resolution of 10 pm.

Fig. 2 Experimental setup for characterizing the optically tuning of the CFBG.

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3. Results and discussion

In the case, D_RC decreases from the large end (left end in Fig. 1(c)) to the small end (right end in Fig. 1(c)) and the waveguide properties of the fiber will alter along the fiber axis in the side-polished part which results in a variation in the effective refractive index(n_eff) along the fiber axis [25]. The n_eff variation along the length of the grating leads to form a grating that has chirped characteristics. The fiber outline (as shown in Fig. 1(c)) and reflection spectrum (as shown in Fig. 3 ) of the CFBG is as same as the FBGs with linear chirp profile made in etched tapers [26]. The Bragg wavelength distribution of the CFBG ( $λ_{Β} (z)$ ) can be given as [27]:

λ_{Β} (z) = λ_{Β} (0) + Δ λ_{Β} \cdot f (z)

where

λ_{Β} (0)

is the Bragg wavelength at z = 0,

Δ λ_{Β}

is the total chirp value and

f (z)

is the chirp function. Judged from the Fig. 1(c), D_RC decreases linearly along the fiber axis from left to right. The chirp function

f (z)

can be given as:

f (z) = z / L

Where

L

is the grating length. And

λ_{Β} (z)

and

Δ λ_{Β}

are given as:

Fig. 3 Reflection spectrum of the FBG before (red dashed) and after (black) UV light irradiation.

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λ_{Β} (z) = λ_{Β} (0) + Δ λ_{Β} \cdot z / L

Δ λ_{Β} = λ_{Β} (L) - λ_{Β} (0)

The optical loss of the FBG is about 20dB. Figure 3 shows the broadening in the CFBG reflection spectrum and corresponding shift in the central wavelength after the irradiation of UV light. The spectrum become stable in 2 seconds. The spectrum bandwidth threshold is set to reflectivity of 10%. Thus, the bandwidth is tuned from 0.78nm to 2.13nm and the central wavelength shifts towards higher wavelengths of 0.63nm. When the UV light is off, the CFBG reflection spectrum recovers back to its initial shape and position in a few seconds. The spectrum change is repeatable when the UV light is turned on/off alternatively.

As the cladding is polished within several micrometers to the fiber core, the n_eff is closely related to n_out (the RI of surrounding material), which can be taken as a method to modulate the Bragg wavelength. The drop-liquid method [28] is applied to testify the relationship between $λ_{Β}$ and n_out. When liquids of different RI (Cargille Labs, USA) are overlaid onto the polished area of SPFBG whose D_RC is constant with position along the length of the grating, the Bragg wavelength red-shifts as the RI of the surrounding material increases as shown in Fig. 4(a) . It proves that the n_out increases and so does the n_eff [29].

Fig. 4 (a) The relation between Bragg wavelength shift of SPFBG and surrounding material RI with different residual cladding thickness (the circle dot line: 1.5µm and the triangle dot line: 0.2µm); (b) The relation between total chirp value and n_out in a designed CFBG.

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In a designed CFBG, D_RC is 1.5µm at the large end and is 0.2µm at the small end. According to the Eq. (3), is calculated as:

Δ λ_{B} = λ_{B}_{(1.5 u m)} - λ_{B}_{(1.5 u m)}

where is the reflective Bragg wavelength at the small end and is the reflective Bragg wavelength at the large end. The relation between and n_out is shown in Fig. 4(b). It is obvious that increases nonlinearly as n_out increases in the CFBG.

Figure 5 illustrates the RI modulation generated by the photoinduced phase transition of the photoresponsive LC overlay in the side-polished area of the CFBG. Since the wheel-polishing process is used to fabricate the SPFBG, many microgrooves parallel with the fiber axis appeared on the polished surface. The LC molecules near the polished surface align along the microgrooves in a direction that is in effect the same as using a ribbed substrate [30].

Fig. 5 Schematic of the photoresponsive LC-overlaid CFBG and the mechanism of photoinduced tune of chirped reflection spectrum. (a) An initial nematic phase generated in the polished area and (b) nematic-isotropic phase transition under the irradiation of UV light.

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Before UV light irradiation, the azobenzene molecules are stable and in the trans-form, rod-like shape as shown in Fig. 5(a). The nematic phase of the photoresponsive LC is preferred since the side-polished area is grooved in the same direction. In the fiber guiding mode, the RI difference between the core and the cladding is very small; therefore, the angle between the fiber axis and light propagation vectors has to be less than 0.087 rad in order to satisfy the conditions of total internal reflection (TIR). During ordinary light propagation, the n_out of the photoresponsive LC-overlaid area should be similar to the RI (n_o) [31] of the photoresponsive LC. The D_RC variation with position along the length of the grating leads to the chirped reflection spectrum, represented by the red-dashed line in Fig. 3.

Upon UV light irradiation, the trans-cis photoisomerization of the azobenzene moieties in the photoresponsive LC is induced [32]. The cis-form azobenzene molecule is bent and tends to destabilize the LC phase from nematic to isotropic. With respect to the propagating light, the RI of photoresponsive LC changes from n_o to n [30], where n is the RI of the LC in the isotropic phase, as illustrated in Fig. 5(b). Since the order of RI of the LC is n_e> n> n_o [33], the RI increase of the photoresponsive LC overlay results in the increase of the CFBG reflection spectrum (black line in Fig. 3) and the center wavelength red-shift.When the UV light is off, the back isomerization process of azobenzene moieties results the LC molecules reorienting to align along the fiber axis and the photoresponsive LC returns to the nematic phase. The LC RI decreases from n to n_o, and the chirped reflection spectrum shifts back to the initial position.

4. Conclusion

This study presents firstly the optically tunable CFBG overlaid by the photoresponsive LC. The D_RC variation with position along the length of the grating leads to the chirped reflection spectrum. The tuning is caused by a photoinduced phase transition generated by the trans-cis photoisomerization of an azobenzene-doped LC. Without the optical field stimulus, the photoresponsive LC is in the nematic phase and the RI equals n_o. During the light irradiation, the formation of a cis-azobenzene LCs disrupts the nematic host and generates the isotropic phase of the photoresponsive LC. The RI of overlaid LC layer increases from n_o to n. The center wavelength of the CFBG reflect spectrum is red shifted to longer wavelength and the total chirp value increases. When the light turn off, the photoresponsive LC molecules reorient into the nematic phase and the RI decrease to n_o. The spectrum withdraws to the initial shape and position. The center wavelength shift and broadening of chirped reflection spectrum are repeatable and reversible. The devices include several practical features, such as cheap, simple fabrication, compact size. The LC-overlaid CFBG has potential for use as an active filter in an all optical telecommunication system.

Acknowledgments

This work is supported by the National Natural Science Foundation of China (NNSFC) grants 10874250, 10674183, 10574165,60877044 and 10776009 (NSAF), National 973 Project of China grant 2004CB719804, the National Science Council of Taiwan grant 98-2221-E-260-001, Ph.D. Degrees Foundation of Ministry of Education of China grant 20060558068 and Natural Science Foundation of Guangdong province 8151027501000017 and Guangdong science research project 2009B011000017).

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Optically tunable chirped fiber Bragg grating

Abstract

1. Introduction

2. Experimental

3. Results and discussion

4. Conclusion

Acknowledgments

References and links

Cited By

Figures (5)

Equations (4)

Optics Express