Temperature-Sensitive Photonic Liquid Crystal Fiber ... - IEEE Xplore

Temperature-Sensitive Photonic Liquid Crystal Fiber Modal Interferometer Volume 4, Number 5, October 2012 Karolina Milenko Tomasz R. Wolinski Perry Ping Shum Dora Juan Juan Hu

DOI: 10.1109/JPHOT.2012.2215583 1943-0655/$31.00 ©2012 IEEE

IEEE Photonics Journal

Temperature-Sensitive PLCF Interferometer

Temperature-Sensitive Photonic Liquid Crystal Fiber Modal Interferometer Karolina Milenko, 1;2;3 Tomasz R. Wolinski, 1 Perry Ping Shum, 2 and Dora Juan Juan Hu 3 1 Faculty of Physics, Warsaw University of Technology, 00-662 Warszawa, Poland School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 3 Department of RF and Optical, Institute for Infocomm Research, Agency for Science, Technology, and Research (A*STAR), Singapore 2

DOI: 10.1109/JPHOT.2012.2215583 1943-0655/$31.00 Ó2012 IEEE

Manuscript received June 21, 2012; revised August 16, 2012; accepted August 21, 2012. Date of publication September 7, 2012; date of current version September 25, 2012. This work was supported by the Foundation for Polish Science in the frame of BMaster Program.[ Corresponding author: K. Milenko (e-mail: [email protected]).

Abstract: In this paper, we present an experimental investigation of the photonic crystal fiber (PCF) and photonic liquid crystal fiber (PLCF) modal interferometers. The PLCF interferometer is fabricated by infiltrating a liquid crystal (LC) into the PCF and then splicing the effective PLCF between two single-mode fibers (SMF). By heating the PLCF interferometer, we present a possibility to tune optical properties of the propagating light. We also compare PCF- and PLCF-interferometer responses in the presence of changing external temperature conditions. Index Terms: Fiber optics, modal interferometers, nematic liquid crystal, optical fiber devices, optical fiber sensors, photonic crystal fibers, temperature sensing.

1. Introduction Photonic crystal fibers (PCFs) are a special type of optical fibers, whose cladding is composed of periodically arranged microsized air channels [1], [2]. Propagation of light in a PCF depends on the geometry and structural parameters of the fiber. PCFs gained interest in the last decade because of their broad range of applications in telecommunication devices, fiber lasers, nonlinear optics, and optical fiber sensors. They can also be used to fabricate compact in structure, small-sized, and highly sensitive PCF interferometers. PCF-based interferometers can be applied in sensing of various physical parameters, e.g., temperature [3], strain [4], pressure [5], humidity [6], and refractive index [7], [8]. PCF modal interferometer can be fabricated by splicing a section of a PCF between two standard single-mode fibers (SMF) [9]. Splicing parameters are chosen to obtain a small region of the PCF with fully collapsed air holes. Light guided within the SMF diffracts when it reaches the collapsed region of the PCF. This diffraction induces mode broadening, which allows for the excitation of the higher order modes (HOMs) in the PCF section. The excited HOMs have different phase velocities compared with that of the fundamental mode due to the difference in effective mode indices. Therefore, they have relative phase difference when compared with the fundamental mode, and the signal collected from the interferometer end exhibits an interference pattern of the fundamental mode and HOMs. The structure of the PCF allows for cladding air channels to be infiltrated with a liquid crystal (LC) material, and in this way, we obtain a new class of fibers called photonic LC fibers (PLCFs) [10]–[14]. PLCFs can be used to fabricate tunable optoelectronic devices, such as electrically tuned

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Fig. 1. (a) Microscopic picture of the employed PCF 070124 UMCS. (b) Indices as a function of temperature of the 6CHBT LC.

phase-shifter [15], thermally tuned optical filters, electrically controlled polarizer, and thermally tunable attenuator [16]. Another application of the PLCFs are sensors, by tracking the shifts of the photonic bandgap wavelengths induced by the external, temperature, voltage, or pressure [17]. LC is characterized by intrinsic birefringence defined as n ¼ ne no , where no is the ordinary refractive index and ne is the extraordinary refractive index. In general, the refractive indices and n strongly depend on external temperature, which provide us possibility to tune optical properties of the PLCF [18]. The PCF interferometer tuned by LCs was presented elsewhere [19]. The interferometer was fabricated by tapering the PCF to micron size and coating it with an LC. By changing external temperature, a wavelength shift of the light propagated in the interferometer was observed. In this method, LC placed outside the PCF limits the usability of the interferometer for sensing applications. We can overcome this drawback by infiltrating the LC inside the PCF air holes. Also, tapering of the fiber interferometer makes the device more fragile. In this paper, we present PCF and PLCF modal interferometers. The infiltration method of the LC material into the PCF and fabrication of modal interferometers is shown. We also present PLCF interferometer thermal response and compare it with PCF modal interferometer.

2. Materials 2.1. PCF The PCF that was used to create the interferometers was fabricated in Maria Curie-Skaodowska University (UMCS), Lublin, Poland. Fig. 1(a) shows a microscopic picture of the 070124 UMCS fiber cross section; it is an air–silica isotropic PCF with a solid core and three rings of hexagonally distributed air holes in the cladding. Fiber cladding diameter is 125 m, hole diameter and hole spacing are 3.9 m and 6.5 m, respectively, and core diameter is 9 m.

2.2. LC Material LC material used in experiments was 4-(trans-40 -n-hexylcyclohexyl)Visothiocyanatobenzene (6CHBT), and it is a nematic LC with medium material birefringence. 6CHBT was fabricated at the Military University of Technology (MUT), Warsaw, Poland. Thermal characteristics of the refractive indices for the 6CHBT LC are presented in Fig. 1(b), where clearing temperature Tc is 43 C. Both ordinary and extraordinary refractive indices of the 6CHBT LC are higher than the refractive index of the employed PCF, which is made from silica glass with n ¼ 1:46 [see Fig. 1(b)].

3. Experimental Study Two sets of experiments were carried out: in the first one, the PCF interferometer was fabricated and its temperature sensitivity was measured. Next, the PCF was infiltrated with the 6CHBT LC


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Fig. 2. Schematic representation of the PCF interferometer.

Fig. 3. (a) PCF interferometer measured power spectrum. (b) Fundamental and second order mode profile.

forming the PLCF. Then, the PLCF interferometer was manufactured and tested in the same way as the PCF.

3.1. PCF Modal Interferometer Fabrication PCF modal interferometer was fabricated by using only a conventional fiber cleaver and a fusion splicer. Both ends of the PCF were spliced to a normal telecommunication SMF. Splicing parameters were chosen in the way that this process formed a small part of the PCF with completely collapsed air holes at both ends (see Fig. 2). The length of the spliced PCF was 17 cm. This collapsing of air holes created a high strength of the splice, which provided durability of the interferometer. The first collapsed region of the PCF caused an extension of the light propagating out of the SMF, and this was responsible for the higher modes excitation. The fundamental and second order modes are the strongest and have the relative phase difference. The second splice between the PCF and SMF recombines the modes, and we observe stable interference pattern [see Fig. 3(a)] in the measured power spectrum. When the power fundamental mode and excited second-order mode are comparable, the extinction ratio of the spectrum is bigger.

3.2. Theoretical Analysis of the PCF Interferometer Response The fabricated device power spectrum shows an interference pattern; this can be analyzed by tracking the period at which we observe picks positions, and it is called the free spectral range (FSR). FSR depends on the wavelength of the propagating light ðÞ, the distance covered by the modes, i.e., the length of the PCF ðLÞ, and the difference between the effective refractive indices ðneff ¼ n1 n2 Þ of the fundamental ðn1 Þ and second-order ðn2 Þ modes FSR ¼

2 : neff L

(1)

To analyze light propagation properties and calculate modes profiles propagating in the employed PCF, we used final-element method (FEM). For the FEM simulations, RSOFT software was used [20]. To confirm that the interference pattern comes from the first- and second-order modes, we calculated their mode profiles that are presented in Fig. 5. The calculated effective refractive index difference between these modes is neff ¼ 0:0044, and the FSR for the wavelength ¼ 1252 nm is FSR ¼ 2:1, while the measured FSR is FSR ¼ 2:3. By comparing the measured and theoretically calculated FSRs, we can see that they agree.


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Fig. 4. (a) Liquid crystal infiltration by using capillary forces. (b) pushing infiltrated LC by using highpressure air. (c) Schematic representation of the LC molecules alignment in the PCF air channels.

Fig. 5. Experimental setup: SLD light source, OSA, Peltier module.

3.3. PLCF Modal Interferometer Fabrication To form the PLCF interferometer, the PCF was infiltrated with the 6CHBT LC by placing one end of the PCF into a container with the filling LC [see Fig. 4(a)]. The LC flows into the PCF air channels by the capillary forces. The length of the PCF was 17 cm, and the part infiltrated with the LC was 2 cm; the infiltration time was 20 minutes. Next, the LC was pushed into the middle of the PCF with the use of high-pressure air, where one end of the fiber is placed in a hermetically sealed container, which is supplied with high-pressure air [see Fig. 4(b)]. Both filling and pushing processes of the LC molecules were controlled by launching the red light source into one end of the PCF and tracking the scattered light on the side of the fiber as an indicator of the amount and location of the LC. The LC infiltration process influences the alignment of the LCs molecules inside the fiber air holes. It was presented elsewhere [21], [22] that this process provides flow-induced planar alignment [see Fig. 4(c)]. After the infiltration process had been completed, the PLCF modal interferometer was fabricated by splicing PLCF between two SMFs in the same way as the PCF described in Section III-A. In order to compare the PCF and PLCF interferometers, both devices were fabricated with the same fiber length. The process of pushing the LC in the middle of the PCF ensures that the LC molecules were not destroyed by the high temperature arising during fusion the splicing procedure.

3.4. Influence of Changes in External Temperature The experimental setup is presented in Fig. 5. One end of the fabricated interferometer was connected to the fiber-coupled superluminescent diode (SLD) light source; power of light was detected at the other end by the optical spectrum analyzer (OSA). The Peltier module was used to control the external temperature changes. The temperatures generated in the experiment ranged from 24 C to 33 C. The interferometers spectra are presented within a 10-nm wavelength range, and we observed stable and regular interference pattern [see Fig. 6(a) and (b)]. Fig. 6(a) shows the spectrum of the PCF modal interferometer, along with its thermal response for two different temperatures. We can see that, although there are some variations in the device response, changes in the external temperature do not result in wavelength shift or intensity change. In Fig. 6(b), we present an interference spectrum of the PLCF interferometer. When we compare it with Fig. 6(a), we can see


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Fig. 6. (a) Transmitted spectrum of the PCF interferometer for two different temperatures. (b) Transmitted spectrum of the PLCF interferometer for three different temperatures.

that LC infiltration does not change the FSR and does not increase losses in the output power of the interferometer. The thermal response of the PLCF interferometer shows that, by increasing external temperature, we observe a wavelength shift into the lower wavelengths and reduction of the intensity. The temperature-induced wavelength shift is 1.3 [nm] for the temperature change of 6.3 C. As presented in Fig. 1(b), the refractive index of the LC is highly sensitive to changes in the external temperature. Because of the planar alignment of the LC molecules, in the PLCF, the propagating light exhibits ordinary refractive index no of the LC. When we increased the external temperature, both refractive indices of the LC were changing, and changed temperature decreases initial planar alignment of the LC molecules. Because of this, the refractive index of the fiber air holes was changed, and the light propagating through the fiber exhibited different effective refractive index, being the combination of the ordinary and extraordinary refractive indices. This phenomenon is responsible for the wavelength shift in the PLCF interferometer spectrum.

4. Conclusion We have presented a simple fabrication process of the PCF modal interferometer. The fundamental part of the fabrication process is to collapse a small part of the PCF during fusion splicing, in order to excite the HOMs. By infiltrating the PCF with a nematic LC and splicing it between two SMFs, we have presented PLCF modal interferometer. The LC infiltration does not enlarge the losses in the interferometer output. The advantage of the PLCF interferometer is that, by tuning the external temperature, we change the refractive index and the orientation of the LC molecules; therefore, we obtain the wavelength shift in the interferometer response.

Acknowledgment The authors would like to thank Prof. R. Dabrowski and Dr. E. Nowinowski-Kruszelnicki of Military University of Technology, Warsaw, Poland for supplying the liquid crystal material; and Dr. P. Mergo from the Maria Curie-Skaodowska University, Lublin, Poland for supplying the photonic crystal fiber.

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