1. Introduction

Fiber-optic attenuators are passive or active optical components that are frequently used to reduce the optical power propagating in fiber-optic telecommunication networks. Actively controllable optical attenuators are ideal for dynamically characterizing and optimizing the optoelectronic responses of high-speed receivers in which the detection response depends upon the average optical power incident on the photodiode [1,2]. Commercially available and actively controlled fiber-optic attenuators are typically fabricated by severing step index guiding fiber and depositing a thin-film absorption filter on the cutting area [3]. Dynamically controlled attenuation is then achieved by mechanically rotating or sliding the filter, which alters the optical path length within the absorptive materials.

Recent side-polished fiber (SPF) research [4] demonstrates the practical use of SPF in the optical devices, such as fiber sensors [5–11], tunable filters [12–17], fiber lasers [18], couplers [19–21], polarizers [22,23] and fiber-optic attenuators or modulators [24–26]. In details, passive attenuators have been achieved by using intercore cladding layers of various thicknesses and polishing areas overlaid with high refractive index (RI) materials. This approach effectively transforms the guided fiber mode into a leaky mode, reduces the coupling of the evanescent field in the side-polished area, and increases attenuation [4]. An actively controllable SPF attenuator is possible if the RI of the overlaid materials can be changed by an external field [23]. Materials with birefringence or dispersion characteristics are especially useful in modulating the optical properties of a fiber-based photonic device. Notably, liquid crystals (LC) [27] with a large and tunable birefringence that allows easy RI modulation by an external field have been applied in photonic devices [28] such as linear and non-linear couplers, polarizers, filters, and attenuators.

This paper demonstrates the first optically controllable SPF attenuator operating near the telecommunication wavelength range of 1.5 μm. The side-polishing area is overlaid with a photoresponsive LC [29], in which an applied optical field can change the optical properties of the fiber. This optically tunable SPF attenuator is based upon a change in the coupled evanescent field in the polished region due to the photoinduced phase transition of the doped azobenzene LC in the overlaid photoresponsive LC mixture. A temperature response curve was generated to examine the thermally induced attenuation properties of the photoresponsive LC-overlaid SPF. The optically tunable capability of the device was dependent on the environmental temperature, in which 45 °C gave the largest tuning of 15 dB with an applied external optical field of 20 mW. The optically controllable attenuation was switchable and the response time was less than 5 s.

2. Experimental

A SPF with a 10 mm polishing area and 56.5 μm depth was fabricated by the wheel-polishing method [30] using a single mode fiber (SMF). The photoresponsive LC was 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, Taiwan branch), 10 wt% azobenzene LC (4-butyl-4`-methyl-azobenzene, BMAB), and 10 wt% chiral dopant (ZLI 811, Merck). No pre- or post-aligning treatment was applied to the overlaid LC mixture. The resulting photoresponsive LC-overlaid SPF was heated on a 100 °C hot plate until the LC mixture reached a transparent phase state, and then cooled to room temperature. This process produced a light-scattering state of the photoresponsive LC overlay. Figure 1 shows the experimental setup for characterizing the thermally and optically controlled attenuation properties of the photoresponsive LC-overlaid SPF. The device was placed on a hot plate and an IR thermal detector was used to detect the controlled ambient temperature. A distributed-feedback laser with a telecommunication wavelength of 1547 nm served as the probe beam. The output power was characterized by an optical spectrum analyzer (OSA). In the case of unpolarized light as the probe beam, the laser was connected directly to the SPF, which was mechanically spliced to single mode fiber (SMF). In the case of polarized light as the probe beam, a fiber polarization controller and a polarization beam splitter were used. Mechanical rotation of the polarization controller altered the polarization of the probe laser light. The polarization controller was positioned such that the output power in the x-direction was minimum while the output power in the y-direction was maximum. Pumped 458 nm laser light emitted from an argon ion laser initiated the photoisomerization of the azobenzene LC. A cylindrical lens expanded the pumped laser light to a cross-section of 10 mm, which is the same width of the polished area on the SPF device. The response time was captured by a Matlab-controlled OSA when the probe light was manually chopped.

 

Fig. 1 Experimental setup for characterizing the optical tuning of the photoresponsive LC-overlaid SPF.

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3. Results and discussion

The thickness and lower RI of the cladding in an unpolished fiber guarantees that the step-index guiding the optical wave prevents loss as an evanescent wave. As the cladding of the fiber is polished to within several micrometers of the fiber core, the evanescent wave existing in the cladding begins to leak from the polished area. The leakage of the evanescent wave increases when the RI in the polishing area is greater than that in the core area. We have observed that the insertion loss of the proposed SPF was 3 dB and 22 dB, respectively, before and after overlaying the photoresponsive LC. The huge insertion loss from the photoresponsive LC-overlaid SPF is due to the dramatically different RI change from the air (n_air = 1) to the LC. The average RI of E-series LCs in telecommunication wavelengths is in the range of 1.6 to 1.63 [31], which is larger than the RI of the fused silica composing the core of fiber (n_silica = 1.44 at 1547 nm). Figure 2 shows the curve of the attenuation from the photoresponsive LC-overlaid SPF attenuator dependent on the environmental temperature. In general, the attenuation decreases with increasing temperature. However, in the case of using unpolarized light as the probe, an increase of attenuation is first observed in the temperature range of 40 to 45 °C. Since the phase transition temperature of trans-azobenzene LC is in the temperature range of 35 to 45 °C [32], the occurrence of this transition causes different behaviors in the attenuation of the device. A similar result was also observed for the case of using polarized laser light as the probe (Fig. 2b); however the increase of attenuation happens in the temperature range of 45 to 50 °C.

 

Fig. 2 The attenuation of the photoresponsive LC-overlaid SPF under different environmental temperatures with an unpolarized (a) and polarized (b) laser probe beam.

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Previous researchers have systematically studied the photoinduced phase transition behavior of photoresponsive LCs [33–35]. Under the proper irradiation of an optical field, the generation of cis-azobenzene LC disrupts the director of the nematic LC host and changes its optical properties. This guest-host (azobenzene LC-nematic LC) synergy is particularly useful in creating optically controllable elements. Temperature is an important parameter in the formation of different LC phases, because if the temperature is set below the phase transition temperature of the trans-azobenzene LC, no phase transition will occur even under proper irradiation of an optical field [32]. Figure 3 demonstrates the optical attenuation tunability of the photoresponsive LC-overlaid SPF attenuator at different environmental temperatures and irradiation conditions. The temperature of 45 °C, which falls in the transition temperature of trans-azobenzene LC, produces the greatest attenuation. As described above, attenuation depends on the overlaid material, which can passively or actively change the RI of the polished area. Here we use a photoresponsive LC as overlay material, allowing the RI to be changed upon heating or irradiation by an optical field. From the previous study of passively changing the RI in the polishing area, the RI of overlaid optical oil has to be in the range of 1.453 to 1.47 [30] in order to get more than 10 dB. This attenuation gradually decreases as the RI of the overlaid optical oil increases. For example, less than 10 dB attenuation was observed when the RI of optical oil is around 1.6. However, the index oil dropping experiment does not agree with our experiment in that more than 20 dB losses were observed after overlaying the photoresponsive LC on the polished area (average RI of E-series LCs in telecommunication wavelengths is in the range of 1.6 to 1.63) [31]. Since the light-scattering, attenuating state is obtained during sample preparation, we believe that phase transition of the photoresponsive LC contributed to the attenuation of the SPF.

 

Fig. 3 The attenuation (a) and tunability (b) of a photoresponsive LC-overlaid SPF attenuator at different pumped optical fields and environmental temperatures.

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Since the environmental temperature of 45°C yields the largest attenuation, we then tried to characterize the optically controllable behavior of the SPF attenuator using a polarized laser beam probe, as shown in Fig. 4 . The attenuation of the probe laser polarized in the x-direction decreases as the optical field increases, a result similar to the effect observed by temperature (Fig. 3b); however, the attenuation increases as optical field increases for the probe laser polarized in the y-direction. The different behavior may be attributed to the generation of cis-azobenzene LC at environmental temperature of 45°C where the phase transition is dependent on the polarization of light upon irradiation by the optical field. Therefore, at a temperature of 45°C, the optically controllable SPF attenuator could also be utilized as a polarization modulator to change the output power of polarized light.

 

Fig. 4 The attenuation of the photoresponsive LC-overlaid SPF attenuator is dependent upon the optical field of the pumping laser for a polarized laser beam probe at the environmental temperature of 45°C.

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To verify our prediction that the light scattering state contributes to the attenuation, we tried to characterize the phase transition of photoresponsive LC upon the irradiation of external energy. Figure 5 shows the optical micrographs and schematic diagrams of thermal and photoinduced phase transition between the chiral-nematic (cholesteric) phase and the compensated nenatic phase. At beginning, a light scattering state was generated in the chiral-nematic phase with a focal conic texture mixed with a finger print texture (left in Fig. 5) by adding low concentration of chiral molecules into the nematic host [36]. Upon the irradiation of thermal and optical field, a light transparent state was created in the compensated nematic phase with a homeotropic texture (right in Fig. 5) by the trans-cis photoisomerization of azobenzene LC [37]. The trans-azobenzene stabilizes the phase structure of the host LC such that before applying the external field, the photoresponsive LC mixture exhibits a light-scattering state due to the randomly oriented focal conic and finger print texture. Under the applied external field, the trans to cis isomerization of the dopant azobenzene LC results in phase transition from cholesteric to nematic and eliminates the light-scattering property. The disappearance of this light-scattering state reduces the optical loss in the photoresponsive LC-overlaid SPF attenuator when the environmental temperature or the optical field increases (Fig. 2 and 3).

 

Fig. 5 Optical micrographs and schematic representation of photoinduced phase transition between a cholesteric phase (light scattering) and a compensated nematic phase (light transparent).

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Figure 6a depicts optical spectrum of the probe laser before and after attenuation from the photoresponsive LC-overlaid SPF attenuator. Optical control of the SPF attenuator is reversible (Fig. 6b), and optical tuning time was estimated to be less than 5 s when modulating the pumped power between zero and 15 mW. Since we switched the light by chopping it manually, the measured values of response time here do not reflect the intrinsic speed of the photochromic reaction induced by azobenzene molecules [32]. The response time of the photoresponsive LC can be improved by several factors, including LC alignment, topology of substrate, the amount of azobenzene molecules, working temperature, and power of the pump light. For example, if a pulsed laser with a pulse width of picoseconds (ps) used as a pumping light source for the photochemical phase transition of guest/host system, the trasn-cis photoisomerization could be completed in the ps time scale [32]. Future work will use pulsed laser as pumping light source to achieve the intrinsic response time of the photoresponsive LC and to approach the ability of dynamically characterizing and optimizing the optoelectronic responses of high-speed receivers used in the telecommunication system.

 

Fig. 6 (a) The optical spectrum of a probe laser from the photoresponsive LC-overalid SPF attenuator before (dash line) and after (solid line) irradiating the optical field of 15 mW and (b) reversible modulation at the output power of the photoresponsive LC-overlaid SPF.

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4. Conclusion

This study presents the first optically controllable SPF attenuator operating at telecommunication wavelengths near 1.5 μm by overlaying photoresponsive LC in the polished area. The attenuation is caused by a photochemical-induced phase transition generated by the trans-cis photoisomerization of a doped azobenzene LC in a photoresponsive LC mixture. Before applying the pumped optical field, the photoresponsive LC is in the light-scattering state and the attenuation is high. During pumped light irradiation, the formation of a cis-azobenzene LCs disrupts the nematic host and generates the light-transparent state, decreasing the attenuation of photoresponsive LC-overlaid SPF attenuator. A maximum optical tuning range of 15 dB was obtained with a photoirradiation of 20 mW at a controlled ambient temperature of 45 °C, the phase transition temperature of a doped azobenzene LC. The all-optical design of this photoresponsive LC-overlaid SPF attenuator allows repeatable and reversible control, and the response time is less than 5 s. The proposed all-optical controllable attenuator has potential for use as an optical signal modulator in an all-fiber telecommunication system.