This paper demonstrates light-induced tuning of the optical spectrum by a microfiber-knot resonator overlaid with a photoresponsive liquid crystal (LC) mixture containing photosensitive diluents (non-mesogenic azobenzene molecules), a chiral dopant and a nematic LC. The high-quality resonator is made by drawing a single mode fiber to a micro-size diameter and causing the microfiber to self-twist into a knot. A thin layer of a photosensitive mixture was placed on the overlap (knot) area and gentle heating was used to obtain a uniform thin film which coated the fiber’s surface. Upon irradiation with UV light, noticeable changes to the peak resonance wavelengths were observed which we associate with a local change in the refractive index (RI) in the fiber’s tapering area. Repeatable and reversible spectral shifting (0.15 nm) of the resonance wavelength is demonstrated by irradiation with 50 mW/cm2 UV light.
©2011 Optical Society of America
Microelectronics, optoelectronics and photonics are the building blocks of modern telecommunication systems. The persistent need for enhanced performance, including faster response, miniaturization, higher sensitivity and low power consumption, drives the development of micro- or nanometer-scale optical components or devices. Micro- or nanofibers (MNFs), low-loss silica waveguides of subwavelength diameters in the shape of loops , knots , or coils [3,4], have been demonstrated in a variety of components including add-drop filters , lasing systems [6–8], nonlinear optical systems , and sensing devices [10,11]. Due to the small dimensions of MNFs, evanescent wave leakage from the tapered area is sensitive to changes in the refractive index (RI) of the surrounding medium. A tunable all-fiber filter based on a MNF has been demonstrated by coiling the microfiber onto a cylindrical rod of tunable radius [12,13]. Applying the voltage on the rod tunes the spectral signal resonating from the microfiber ring resonator.
Liquid crystals possess an easily variable RI due to their large birefringence and fluidity. They have been used in a variety of photonic applications with an eye towards enabling tunable optical responses, with stimuli including thermal [14,15], electrical [16–18], magnetic [19,20] and optical fields [21–26]. Our previous work demonstrated light-tunable fiber devices by infiltrating photoresponsive LCs into a photonic crystal fiber  or overlaying photoresponsive LCs onto a side-polished fiber . In the present work, a light-tunable, all-fiber resonator is demonstrated by overlaying similar photoresponsive LCs onto an MNF with a knot structure. Repeatable and reversible spectral shifting (0.15 nm) of the resonance wavelength is demonstrated by irradiation with 50 mW/cm2 UV light.
Figure 1(a) shows the microscopic image of the fabricated microfiber-knot resonator. The tapering of the microfiber from a standard single-mode fiber was accomplished using the flame-heated taper-drawing technique . An alcohol burner was used to provide a high enough temperature for fiber stretching. The frame-heated fiber was elongated with desired length and diameter by the axial tension from the pulling force at the two sides of fiber . A tunable laser (ANDO AQ4321DTSL, wavelength range 1520nm-1620nm) was used as the light source. The output signal was recorded by an optical spectrum analyzer (YOKOGAWA QA6317COSA, minimum resolution 0.01 nm). The experiment used a photoresponsive LC consisting of a homogeneous mixture of 85 wt% nematic LC (MDA 3461, no = 1.514 and ne = 1.771 at 589 nm, Merck), 5 wt% azobenzene LC (4-butyl-4′-methyl-azobenzene, BMAB) and 10 wt% chiral dopant (ZLI 811, Merck). The coated fiber (20 dB loss after overlaying the photoresponsive LC) was exposed to a UV light source (365 nm, USHIO SP7-250DB). 20 μL of the photoresponsive LC was placed on the surface of the microfiber and covered all tapered areas (10 mm) using capillary force. Gentle heating was used to ensure a uniform thickness. Figure 1(b) shows the UV light source irradiating the microfiber-knot resonator.
3. Results and Discussion
Figure 2(a) shows the recorded optical spectrum from a typical microfiber-knot resonator fabricated by the flame-heating technique. Figure 2(b) is an image of He-Ne laser light passing through the microfiber-knot resonator. The important characteristic of a resonator is its Q-factor which determines the ratio of the radiation wavelength in free space, λ, to the full width at half maximum (FWHM), Δλ, of the resonance at that wavelength (Q = λ/Δλ) . The Q-factor of the fabricated microfiber-knot resonator was calculated at 17111. The finesse (7.5) of a resonator was calculated from the ratio of its free spectral range (FSR) of 0.68 nm to the resonance FWHM (0.09 nm between 1540 and 1541 nm). The achieved extinction ratio of transmission power oscillations is around 20 dB. These characteristics prove that our fabricated microfiber-knot resonator is suitable for add-drop filters and laser devices .
To demonstrate the effect of all-optical modulation on the characteristics of a microfiber-knot resonator, the output spectrum was measured when the photoresponsive LC-overlaid microfiber-knot resonator was under the UV light irradiation (Fig. 3(a) ). The measured photo-induced spectral shifting is 0.15 nm upon the UV light of 50 mWcm−2 which, to date, is the first achievement of an all-fiber and optically-tunable resonator. Figure 3(b) shows the transmission spectrum of the microfiber-knot resonator overlaid with only nematic LCs. Obviously, the UV light irradiation has no effect on the wavelength shifting. Since the photoresponsive LC contains an azobenzene dye in which the trans-cis photoisomerization of azobenzene molecules generates the RI modulation of photoresponsive LC , under UV light irradiation no RI modulation occurred in the only nematic LC coated micro-fiber knot resonator. Compared with the spectral shifting generated by the electro-optically tunable micro-ring resonator using lithium niobate (0.94 nm/300 Volts)  and liquid crystal (0.22 nm/20 Volts) , or the electrically tunable microfiber-loop resonator using a copper rod (50 pm/1.6 Amps) , the spectral shifting present here is suitable for application in telecommunications filtering technology and may be used in all-optically-tunable dense-wavelength-division-multiplexing (DWDM) filters.
Because there is an addition phase shift π/2 after the light travels a round trip in the ring and then coupled back to knot-ring, the resonance condition for microfiber knot-ring resonator is determined by modified ring resonance formula in :Eq. (1):Fig. 3(a). For the same order of resonance wavelength shifting, assuming the RI change all comes from the photoresponsive LC, and using a peak resonance wavelength of 1531.4 nm and a resonance wavelength shift of 0.15 nm (Fig. 3(a)), we obtained Fig. 4 which represents the photo-induced RI change in the microfiber-knot resonator dependent on the RI of the photoresponsive LC, nLC. Based on Fig. 4 and assuming nLC = 1.59 at a wavelength of 1.55 μm (the ne and no of the nematic LC used in our experiment is similar to that from ref. 28), we obtained that ΔnLC is only 1.55×10−4, possibly due to the anchoring effects reducing the LC birefringence at telecommunication wavelengths . Increasing the RI of the phtoresponsive LC (nLC) may result in a higher ΔnLC in the microfiber-knot resonator.
Once the UV irradiation source is removed, the peak resonance wavelength reverts to its original position, as shown in Fig. 5 . We also observed that the extinction ratio of the transmission power oscillations decreases with exposure. We speculate that, upon photosiomerization, significant trans-cis photoisomerization of the LC mixture changes the value of nclad, which is not the same following UV light irradiation.
This work presents what is, to the best of our knowledge, the first demonstration of a photo-induced spectral shifting of microfiber-knot resonator by overlaying the photoresonsive LC onto the microfiber. The wavelength shift is caused by the effective RI change generated by the photoisomerization of azobenzene within the photoresponsive LC under UV irradiation. The spectral shift of the LC-overlaid microfiber ring resonator is repeatable and reversible. The demonstrated device has potential use as an actively controlled add-drop filter in an all-fiber telecommunications system.
This work is supported by the National Natural Science Foundation of China (No. 60877044, No. 61027010, No. 11004086), the National Science Council, Taiwan (NSC 99-2221-E-260-024), and the Fundamental Research Funds for the Central Universities (No. 21609508; No. 21609421). The authors thank Dr. Timothy J. Bunning for his assistance with the preparation of this paper.
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