The optical transmission properties of photonic crystal fibers (PCFs) can be manipulated by modifying the pattern arrangement of the air channels within them. This paper presents a novel MEMS-based technique for modifying the optical transmission properties of commercial photonic-crystal fiber (PCF) by selectively filling the voids within the fiber structure with liquid crystals. In the proposed approach, an un-cured SU-8 ring pattern with a thickness of 5 μm is fabricated using a novel stamping method. The PCF is then brought into contact with the SU-8 pattern and an infra-red (IR) laser beam is passed through the fiber in order to soften the SU-8 surface; thereby selectively sealing some of the air channels with molten SU-8. Liquid crystals (LCs) are then infiltrated into the un-sealed holes in the PCF via capillary effects in order to modify the transmission properties of the PCF. Two selectively-filled PCFs are fabricated, namely an inner-ring LC-PCF and a single-line LC-PCF, respectively. It is shown that the two LC-PCFs exhibit significantly different optical behaviors. The practical applicability of the proposed selective-filling approach is demonstrated by fabricating an electric field sensor. The experimental results show that the sensor has the ability to measure electric fields with an intensity of up to 40 kV/cm.
© 2011 OSA
Photonic-crystal fibers (PCFs) are a relatively new form of optical fiber based on the principle of photonic crystals. The cross-section of a PCF contains a specifically-oriented pattern of voids within the cladding area, and thus PCFs are sometimes referred to as micro-structured fibers. The voids cause the cladding of the PCF to have a lower reflective index than the core. As a result, the cladding acts as a conventional fiber index guiding scheme in confining the transmission of the light energy to the solid core. In contrast to conventional optical fibers, however, the optical transmission properties of PCFs can be tuned by controlling the arrangement, size, space, shape or properties of the voids. For example, by filling the voids with a material with a higher reflective index than the core, photonic-bandgap (PBG) guiding takes the place of index guiding in determining the transmission properties of the fiber, and only certain wavelengths can be propagated along its length.
The optical properties of PCF fibers can be tuned by inserting gases , liquids , and even solid metals  into the fiber cavities. The tunable properties of PCFs render them suitable candidates for a wide variety of applications, including switches , filters , and laser sources . In addition, PCFs have been widely used for chemical , refractive index , electric field , temperature  and other  sensing applications. Amongst the various materials used to modify the structure of PCFs, liquid crystal (LCs) have attracted particular attention due to it is an electrical-field sensitive material such that the optical properties of the LCs can be tuned by applying different electric field strength. This unique property of LCs has made it potentially for various applications like E-field sensing or temperature sensing.
Many studies have shown that the optical properties of PCFs can be significantly changed by filling only certain of the holes in the fiber cladding. However, the size and pitch of these holes are on the scale of a few micrometers, and thus selectively filling specific holes presents a significant challenge. Nonetheless, the literature contains various selective-filling techniques, including UV glue sealing , arc fusion deformation , capillary forces , and others . Of these various techniques, sealing the holes using UV glue is the most straightforward. The technique involves using a micro tip stained with UV glue to touch and seal the desired holes directly. In practical, any specific void can be easily sealed with this technique . In the arc fusion deformation method, an electrical arc is used to melt and deform the fiber tip; causing the collapse of the outer air holes. However, only the outer layers of the air holes can be sealed using this method. Other techniques, e.g., capillary force differential selective filling, cannot achieve selective filling of PCF channels while the channels in the PCF having the same size or providing similar capillary force.
Given the limitations of the various methods described above, it is clear that there exists an outstanding requirement for a rapid, straightforward and efficient means of selectively filling the holes in commercial PCFs. Accordingly, this study presents a novel MEMS-based technique for modifying the optical properties of commercial PCF by selectively filling certain of the voids with molten SU-8 photoresist and then infiltrating liquid crystals into the remaining voids by means of capillary forces. Two different types of selective-filling pattern are demonstrated, namely an inner-circle configuration and a straight-line configuration. Finally, the proposed technique is used to fabricate an electric field (E-field) sensor.
2. Materials and method
The E-field sensor was fabricated using commercially-available PCF (LMA-10, NKT, USA) with an outer diameter of 125 μm and a core diameter of 10 μm. As shown in Fig. 1(A) , the PCF cladding contained four layers of voids arranged in the form of a hexagon. Each void had a diameter of 2.6 μm. Moreover, the void pitch was equal to 6.3 μm. As shown in Fig. 1(B), the basic principle in fabricating the E-field sensor was to seal certain of the holes in the fiber cladding with SU-8 “tape”. The low glass transition temperature of un-cured SU-8 photoresist (PR) (Tg < 60°C) renders it an ideal material for producing a patterned tape structure. However, SU-8 is a negative tone PR, a hardened structure is formed after the UV exposure process, and therefore it is difficult to generate the desired un-cured pattern. Accordingly, in the present study, the un-cured SU-8 tape structure was fabricated using a novel stamping method. The PCF was then brought into contact with the uncured SU-8 structure and an infra-red (IR) laser beam was passed through the fiber in order to soften the SU-8 tape, thereby causing the air holes covered by the SU-8 tape to become sealed. Finally, LCs were infiltrated into the un-sealed voids under the effects of capillary forces.
3. Fabrication process
Figure 2 presents a schematic overview of the fabrication process used to realize the liquid-crystal-filled PCF (LC-PCF). A clean glass substrate was coated with a 200-μm thick layer of SU-8 PR by means of a constant volume injection process (Fig. 2(A)). The upper surface of the SU-8 coating was then cured by UV light (exposure dose: 1000 mJ/cm2) to generate the stamp required to produce the desired un-cured SU-8 pattern (Fig. 2(B)). Note that any un-cured SU-8 pattern can be designed in the mask fabrication process. (The detailed procedures involved in fabricating the stamp structure are described in a previous study .)
For demonstration purposes, a LC-PCF was fabricated in which the three outermost layers of voids were sealed and LCs were infiltrated into the cavities within the innermost layer. Accordingly, a hexagonal open-centered SU-8 pattern was designed with dimensions corresponding to those of the three outmost layers of voids in the PCF cladding. A 100-nm thick Ti film was sputtered onto another bare glass substrate to serve as an IR absorption layer (Fig. 2(C)), and the hexagonal SU-8 stamp was then attached to the Ti-coated substrate by applying a pressure of 10 psi and a temperature of 80°C for 3 min. The un-cured SU-8 tape was then formed by vertically removing the stamp (Fig. 2(D)). The un-cured SU-8 tape and PCF were carefully aligned using a high-precision 6-axis stage under the assistance of a stereo microscope (SZX9, Olympus, Japan) (Fig. 2(E)). The aligned PCF was then brought into contact with the SU-8 pattern, and an IR laser beam with a wavelength of 976 nm was passed through the PCF to heat the Ti layer to a temperature higher than 60°C (Fig. 2(F)). Due to localized heating effects, a slight melting of the SU-8 pattern occurred, causing the cavities in the three outermost layers of the PCF to become sealed. The cavities within the innermost layer were then filled with LCs (E7, Merck, Germany) under the effect of capillarity forces. To enhance the LC infiltration efficiency, the filling process was conducted under vacuum conditions so as to extract any residual air from the voids (Fig. 2(G)). The filled length of the LC-PCF was controlled to approximately 5 mm. Finally, the LC-PCF was attached to two single mode fibers via an arc fusion splicing process to serve as light guides.
4. Results and discussion
In this work, we utilize a novel laser heating method to locally melt an un-cured SU-8 pattern. In order to feed the IR light source into the PCF, the PCF was connected with a single mode fiber by arc fusion splicing. The IR laser power emitted from the PCF core with the diameter of 10 μm can in fact be considered as a point heat source. Figure 3(A) shows the relationship between the input laser power and the Ti surface temperature. It can be seen that the surface temperature varies linearly with the applied laser power. Moreover, it is observed that a surface temperature in excess of 200°C can be achieved by applying a laser power of around 200 mW. The inset in Fig. 3(A) shows the absorption spectrum of the sputtered Ti layer. The results show that the Ti layer yields a uniform absorption of light at all wavelengths in the IR-to-visible range. Note that the Ti surface was deliberately heated using a laser beam with an IR wavelength (976 nm) such that the visible light observations during the experiments can be done without alternating by the heating light.
The viscosity of un-cured SU-8 can be substantial, depending on the temperature. Furthermore, the Gaussian-distributed nature of the thermal energy produced by the point laser heat source may cause an overheating of the central part of the un-cured SU-8 structure; thereby preventing an effective sealing of the voids. Accordingly, a splicing-shift was introduced between the single mode fiber feeding the IR laser and the PCF section in order to generate a more uniform thermal field. Figures 3(B-E) present far field images of the light emitted from the PCF given splicing-shifts of 0 μm, 4 μm, 12 μm and 16 μm, respectively. The insets beyond the far field images in Fig. 3 show the center-aligned and shift-spliced connection of the single mode fiber and the PCF. Figure 3 (B-E) indicates the measured results of different splicing shift distances. Furthermore, given a splicing-shift of 16 μm, the light field separates into six discrete spots. Note that the splicing-shift method results in a more uniform temperature distribution on the Ti surface, but leads to a greater energy coupling loss. As the result, the input IR strength must be well calibrated, the power for heated the SU-8 tape to 80°C must be ramp to 200 mW.
Figure 4(A) presents the hexagonal open-centered un-cured SU-8 tape designed to seal the three outermost layers of air channels. Figure 4(B) presents an image of the PCF following the selective hole-filling process. As shown, the air channels in the outermost three layers are covered by the SU-8 PR tape, leaving only those channels in the innermost layer open for LC infiltration. The effect of the selective-filling process on the transmission properties of the PCF was investigated by fabricating PCFs with two different LC filling configurations, namely an inner-ring configuration and a single-line configuration, respectively. In the former case, LCs were inserted into all of the air channels within the innermost layer of cavities, while in the latter case, LCs were inserted into all of the cavities along a straight line extending from one side of the PCF to the other. In both cases, the remaining cavities were sealed with un-cured SU-8 PR using the method illustrated in Fig. 2.
Figure 5(A) illustrates the experimental setup used to evaluate the transmission properties and sensing performance of the selectively-filled PCFs. The selectively-filled fiber was first cut into lengths of 20 mm and spliced between two single mode fibers. The LC-PCF section was then sandwiched between two electrodes, with the gap between the two electrodes being specifically chosen as 125 μm (i.e., equal to the outer diameter of the fiber) in order to create the maximum E-field strength utilizing the minimum applied voltage. A commercial white light source (AQ-430, YOKOGAWA, Japan) was coupled into one end of the LC-PCF, while the emitted light was coupled to an optical spectrum analyzer (AQ-6315E, YOKOGAWA, Japan). The fabricated LC-PCF for E-field sensing was evaluated by measuring the light transmission spectra at different wavelengths. The transmission spectra of the two LC-PCFs were then evaluated at different wavelengths and electric field intensities. Figure 5(B) shows the electric field sensing results obtained for an un-filled PCF. It can be seen that the transmission spectrum of the PCF is insensitive to the electric field; even for an E-field intensity of as much as 64 kV/cm.
Figures 6(A) and (B) present the transmission spectra of the inner-ring and straight-line LC-PCFs given an electric field intensity ranging from 0~40 kV/cm. The filled LC has an ordinary refractive index of 1.52 and an extraordinary refractive index of 1.75. The effective refractive index also changed with the applied E-field. In contrast to the un-filled PCF, both selectively-filled PCFs exhibit a significant PBG behavior. The behaviors of LC molecules in an applied electric field have been well investigated in some previous reports [17, 18]. Briefly, the LC molecules were re-orientated under the action of the electric field, which caused a loss in the uniformity of long range LC molecules alignment. The increasing applied electric field strength would again cause the formation of reverse tilted domains and also increase the loss of the LC long range orientation order . The transmitting wavelengths regarding the photonic bandgap condition are coupled into the cladding region, resulting in the intensity loss in the PCF fiber. Therefore, the transmitting wavelengths regarding to the photonic bandgap condition are coupled into the cladding region, resulting the intensity loss in the PCF fiber. In both cases, a higher electric field intensity reduces the intensity of the transmitted light and prompts a slight shift in the transmitted wavelength. However, the actual transmission spectra of the two PCFs are quite different; reflecting a difference in the LC filling patterns of the two fibers. Figure 7 plots the variation of the transmitted light intensity with the electric field strength given an incident wavelength of 700 nm. (Note that the results relate to the inner-ring selectively-filled PCF.) It can be seen that the transmission intensity reduces with an increasing field strength in accordance with the basic LC electrical response property. Overall, the results demonstrate that the inner-ring LC-PCF is capable of measuring electric fields with an intensity of up to 40 kV/cm.
This study developed a simple, novel and straight forward method to modify the optical properties of PCFs utilizing MEMS techniques. A 5-μm thick un-cure SU-8 ring pattern was firstly fabricated utilized a novel stamping method. Un-cured SU-8 PR with different patterns can be easily fabricated with this approach to seal the selected air holes in a PCF. A laser absorb heating method was developed to locally heat and soften the un-cured SU-8 structure. In addition a shifted fiber coupling technique was proposed to create a dispersed optical field to help the heating uniformity. The developed technique is capable of producing various sensors such as temperature, E-field, magnetic field or even pressure sensors by filling different liquids into the selected holes of the PCF.
The financial support from National Science Council of Taiwan is greatly acknowledged. (NSC 97-2221-B-110-018-MY3)
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