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Optically controllable variable fiber optical attenuator integrated in conventional optical fiber
Optik 125 (2014) 7085–7088
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Optically controllable variable fiber optical attenuator integrated in conventional optical fiber Ivan Martincek, Dusan Pudis ∗ Department of Physics, Faculty of Electrical Engineering, University of Zilina, Univerzitna 1, SK-010 26 Zilina, Slovakia
a r t i c l e
i n f o
Article history: Received 6 December 2013 Accepted 10 July 2014 Keywords: Variable fiber optical attenuator Optical fiber device
a b s t r a c t New type of optically controllable variable fiber optical attenuator based on thermo-optical effect in liquid cladding of optical fiber is described. The thermo-optical effect in liquid cladding optical fiber causes refractive index contrast changes in core–cladding interface, what enables to change the propagating optical signal power with temperature. The temperature change is achieved by fiber based heating element using laser radiation. Attenuation up to −12 dB was achieved in static dependence and dynamic response confirmed rise time up to 24 ms. © 2014 Elsevier GmbH. All rights reserved.
1. Introduction Variable optical attenuators are optical components, which are widely used not only in optical networks for control of optical power levels in network channels, but also in waveguide optics for photonic signal processing and sensing. The variable fiber optical attenuators (VFOA) cover special class of variable optical attenuators based on optical fibers. Recently, several different types of VFOA have been developed with a variety of technologies such as liquid crystal technology [1], acusto-optical technology [2], micro-electro-mechanical systems technology [3], microstructured optical fiber technology [4], and optofluidical technology [5]. These technologies have the potential to create all-optical networks, especially communication network that works completely in the optical domain. Optically controllable waveguide and fiber optic component thus play an important role in telecommunications technology in all-optical networks developing. During the last few years, progress of a new multidisciplinary research field of optofluidics waveguide started, which is essentially the integration of microfluidic science and technology with optical waveguide components and methods. Developing optofluidic technology allows using unique properties of fluids for design of novel optical devices [6]. One of the most attractive results of the optofluidic activity is the realization of tunable optofluidic devices [7] using a large variety of physical mechanisms.
∗ Corresponding author. Tel.: +421 41 513 23 42. E-mail address: [email protected] (D. Pudis). http://dx.doi.org/10.1016/j.ijleo.2014.08.097 0030-4026/© 2014 Elsevier GmbH. All rights reserved.
Liquid-cladding optical fibers (LCOF) form a group of optofluidic waveguides, which have usually a solid core of circular crosssection, which is surrounded by appropriate liquid forming the cladding of optical fiber. For LCOF based on liquid cladding with low refractive index, light guiding through the core is based on the total internal reflection at a core–cladding interface of the LCOF. Refractive index changes between core and cladding open wide application area in optofluidic technology, including sensor applications, optical switching and modulation [7]. In this paper, we describe new type of VFOA based on thermooptical effect in LCOF. The VFOA is integrated on a standard telecommunication step-index optical fiber, so that a part of the primary coating and cladding of the optical fiber is completely removed and replaced by appropriate liquid. Due to the thermooptical effect a refractive index contrast between core and liquid cladding is changed with temperature, which enables to change the propagating optical signal power through the core. The change of liquid cladding temperature of VFOA is achieved by liquid heating from thin metal layer deposited on front facet of fused silica fiber. Thin metal layer is then directly heated by absorption of laser radiation. 2. Theoretical approach The mechanism of optical attenuation in the single mode VFOA is based on waveguide properties of fundamental mode LP01 in stepindex optical fiber. Based on fundamental theoretical approaches, optical power propagating in core of step-index optical fiber by LP01 mode decreases with refractive index contrast n = nco − ncl , where nco is core refractive index and ncl is cladding refractive index [8].
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Fig. 2. Schema of VFOA integrated in COF with heating element and liquid cladding.
Fig. 1. Attenuation mechanism in integrated optical fiber device.
This power decay is caused by increasing of mode field diameter of LP01 mode with refractive index contrast decrease and then higher part is propagating by evanescent field. In special case, if nco ≤ ncl , LP01 mode transforms to leaky modes, where z component of the wave vector is complex and causes an attenuation in the direction of propagation [9]. It is well known that on-axis bonding of two fibers with different mode-field diameter (MFD) causes lossy modes in the fiber with smaller MFD if fundamental mode is propagating from the fiber with higher MFD. Here, only part of propagating power in the fiber with smaller MFD is transformed into propagating fundamental mode and the rest contributes to lossy mode [8]. Then, one can expect power decrease at the output of fiber with smaller MFD. It is well known that the MFD is a function of n at core–cladding interface. If we create special fiber device, where the n can be changed by different physical mechanism, the MFD of guided modes is modified. For single mode optical fiber, decrease of n causes MFD increase for LP01 mode and for n ≤ 0 the propagating LP01 mode is transformed to leaky mode. Such device can be integrated into stepindex optical fiber with refractive index contrast n1 . We suppose a mode LP01 propagating in optical fiber. This mode is transformed to LP01d in integrated part if n = n1 and at next interface again to LP01 in optical fiber. No losses are expected in such arrangement (Fig. 1a). However, if the integrated part has n < n1 , the LP01 mode will be transformed to LP01d with higher MFD or to leaky mode if n < 0 (Fig. 1b). The LP01d mode will be transformed at second interface to LP01 mode. But, the LP01d mode shows higher MFD than LP01 , or is leaky. Then in optical fiber with n1 a formation of lossy modes is expected. Optical power propagating by means of LP01 mode at output part of device is then reduced. The modification of n in integrated part is the way to control optical power propagating by means of LP01 mode. Quantity of optical power LP01d mode coupled into fundamental mode LP01 is dependent on several parameters of integrated fiber device as length, refractive index contrast n and also on fluctuation of core diameter of integrated fiber device, inhomogeneity of refractive index in core and cladding, etc. Then it is difficult to theoretically predict the quantity of optical power propagating by means of fundamental mode. However, we document simple experimental technique, which allows measure optical power propagating across such integrated fiber device. This theoretical principle documents the ability of optical fibers to control the optical power propagating through the core by means of modified refractive index contrast n of core and cladding of optical fiber. A thermo-optical effect can be used for achieving of appropriate refractive index contrast n, if the core and cladding of
optical fiber consist of materials with different thermal coefficients. We found an appropriate material combination for the integrated device consisting of fused silica used as a core and mineral oil (Cargille liquid) as a cladding. Generally, the material combination of fused silica and mineral oil allows realization of different optical devices based on thermo-optical effect, especially temperature optical fiber sensors, switches and VFOA. The refractive index for fused silica of used conventional optical fiber (COF) and Cargille liquid code 50350 in the temperature range 15–35 ◦ C shows thermal coefficients 1 × 10−5 K−1 and −3.86 × 10−4 K−1 , respectively [10,11]. Such substantial difference of thermal coefficients is suitable for achieving of sufficient refractive index contrast at core–cladding interface with increasing temperature, which is useful for integration of this principle in new VFOA based on thermo-optical effect in optical fiber with liquid cladding. 3. Experimental The attenuation in designed VFOA is due to the thermo-optical effect, which influences the guiding properties of LCOF. In this experiment, the VFOA thermal changes were realized by local heating using laser source. The designed VFOA consists of 2 m long conventional optical fiber with core and cladding diameter 10 m and 125 m, respectively. The cladding is coated with an acrylate primary coating of outer diameter 250 m. In the middle part of the COF, the primary coating was stripped. In this part, the cladding was then removed in app. 150 m long section using paraffin mask and etched in hydrogen fluoride acid. After cladding removing, it was replaced by Cargille fused silica matching liquid code 50350. Thermal changes of VFOA were achieved using specially prepared small heating element prepared from optical fibers. This heating element consists of two multimode optical fibers with core diameter of 50 m and cladding of 125 m, where the ends were placed close to COF core with d = 20 m (40 m) on opposite sites (Fig. 2). Front face of one was evaporated by 1 m thick copper layer. It works as an absorber of impacted light from the second fiber, which was coupled to diode laser with operation wavelength at 405 nm. Such radiation is app. 50% absorbed in deposited copper layer. Liquid cladding of VFOA was kept by capillary forces between etched core and heating element. Absorbed light in copper layer heats this layer and the surrounding cladding of LCOF. Thereafter, LCOF heating causes an increase of refraction index contrast n at core–cladding interface of LCOF, which improves guiding properties. LCOF temperature can be simply controlled by laser power in heating element. Such fully optical device based on thermo-optical effect can be used as VFOA and modulator controlled by laser light. Fig. 3 shows static dependence of propagating optical signal through LCOF on laser power of heating optical fiber if the heating fibers were placed in d = 20 m from each other at room
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Fig. 4. Dynamic characteristics measured at different laser power in heating element.
temperature (25 ◦ C). COF was coupled to light emitting diode (1310 nm) as a source of optical signal and output optical signal was detected using InGaAs detector PDA 10CS-EC. The core of used COF was prepared by modified chemical vapor deposition technique. This fabrication technology increases the refractive index profile of fused silica by dopation. Also refractive index profile of such fibers show central dip and it is difficult to reliably determine the character of mode at 25 ◦ C if it is propagating or leaky. Anyway, the transmitted power propagating through the integrated fiber device was attenuated by −12 dB. Fig. 3 documents an increase of propagating signal through VFOA as the laser power in heating element increases in all measured range of laser power. At room temperature, the LCOF mode is created, which part is transformed in lossy modes in COF. The lossy modes are outcoupled from optical fiber by acrylate primary coating, which works as an absorption layer of the lossy modes. Next increase of laser power causes heating up of Cargille liquid cladding from copper layer. Due to the thermo-optical effect and appropriate thermal coefficients of core and cladding, thermal increase improves guiding properties and in measured static dependence it shows an increase of optical signal. At laser power higher than 2.0 mW the minimal attenuation was achieved. In this region, the ratio of output/input power was attenuated by 0.5 dB, which corresponds to insertion losses of prepared VFOA in static mode. In all measured range of optical power the maximal static attenuation shows −12 dB at temperature 25 ◦ C. Modulation and a dynamic response time of the VFOA were measured by the Agilent DSO-X 2012A (100 MHz) oscilloscope. We applied a square-wave driving voltage to modulate the heating laser light with pulse duration of 50 ms and delay time of 250 ms. The optical response of the VFOA was then measured. Fig. 4 shows
Fig. 5. Comparison of dynamic characteristics of small-volume VFOA construction (up, d = 20 m) and large-volume cladding VFOA (middle, d = 40 m). Applied square wave pulse is also shown (bottom).
the measured response of the VFOA at different laser power of heating element. The rise time (from 10% to 90%) and fall time is 24 ms and 159 ms for 2.75 mW of heating power. In the case of low power heating (0.74 mW) the rise time is 30 ms and fall time 147 ms. Dynamic characteristics document the effect of increasing laser power on the rise time reduction. High laser power shows shorter rise time but the response time is longer. Also higher laser power shows shift of measured signal to lower attenuation. The shift is caused by overall temperature increasing if the higher laser power is used. Factors which affect the dynamic response are related to thermal processes in whole VFOA system given by materials and their volumes used in the VFOA arrangement. Using this construction, we expected different behavior of VFOA depending on liquid volume creating LCOF cladding placed between facets of heating element. For the verification of volume liquid effect, also large volume variant of VFOA was prepared and measured. Large volume liquid was placed on the side between core and copper layer of heating element at d = 40 m. Such arrangement uses larger liquid volume, where we suppose worse dynamic response. Dynamic characteristic in Fig. 5 shows comparison of two arrangements with different d = 20 and 40 m for the same laser power in heating element of 1.29 mW. As is evident from Fig. 5, the VFOA with large-volume liquid cladding shows longer dynamic response times. The large-volume rise time is 40 ms and fall time 165 ms, while the small-volume arrangement shows only 29 ms rise time and 149 ms. Largevolume liquid cladding shows much slower heat exchange than small-volume arrangement, which finally degrades the dynamic response. However, there is one drawback in using of small-volume liquid cladding, where optical signal is attenuated in short time only, what affects lower attenuation level. Also other parameters such as VFOA length and heating element arrangement should induce the final static and dynamic properties. Definitely, final construction of liquid volume of VFOA can be optimized according to an application demand. 4. Conclusion In this paper, we describe new type of optically controlled VFOA and modulator based on thermo-optical effect. The VFOA is integrated on a single-mode step-index telecommunication optical fiber operating at 1310 nm. One advantage of described VFOA is direct integration on COF by simple adaptation. The transmission properties can be determined by arrangement of heating element and cladding parameters. As a limitation the dynamic response was observed in the range from 24 ms to 40 ms depending on VFOA construction.
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We believe that proposed VFOA based on LCOF could find application in the optical fibers, where the attenuation level is adjusted to be external optical source and in utilization of optical fibers in sensor applications. Acknowledgements This work was partly supported by Slovak national Grant Agency Nos. VEGA 1/0528/12 and 1/0491/14 and the Slovak Research and Development Agency under the project No. APVV-0395-12. The authors wish to thank the support from the R&D Operational Program Centre of Eexcellence of Power Electronics Systems and Materials for their Components II. No. OPVaV-2009/2.1/02-SORO, ITMS 26220120046 funded by European Regional Development Fund. References [1] M. Sunish, G. Farrell, Y. Semenova, Experimental demonstration of an all-fiber variable optical attenuator based on liquid crystal infiltrated photonic crystal fiber, Microwave Opt. Technol. Lett. 53 (2011) 539–543.
[2] Q. Li, A.A. Au, Ch.-H. Lin, E.R. Lyons, H.P. Lee, An efficient all-fiber variable optical attenuator via acoustooptic mode coupling, IEEE Photon. Technol. Lett. 14 (2002) 1563–1565. [3] N.A. Riza, S.A. Reza, Broadband all-digital variable fiber-optic attenuator using digital micromirror device, IEEE Photon. Technol. Lett. 19 (2007) 1705– 1707. [4] C. Kerbage, A. Hale, A. Yablon, R.S. Windeler, B.J. Eggleton, Integrated all-fiber variable attenuator based on hybrid microstructure fiber, Appl. Phys. Lett. 79 (2001) 3191–3193. [5] D.P. Martincek, Variable liquid-core fiber optical attenuator based on thermo-optical effect, J. Lightwave Technol. 29 (2011) 2647–2650. [6] D. Psaltis, S.R. Quake, Ch. Yang, Developing optofluidic technology through the fusion of microfluidics and optics, Nature 442 (2006) 381–386. [7] U. Levy, R. Shamai, Tunable optofluidic devices, Microfluid Nanofluid 4 (2008) 97–105. [8] A.W. Snyder, J.D. Love, Optical Waveguide Theory, Chapman and Hall, London, 1983. [9] A.W. Snyder, Leaky-ray theory of optical waveguides of circular cross section, Appl. Phys. 4 (1974) 273–298. [10] M.J. Weber, Handbook of Optical Materials, CRC Press LLC, 2003. [11] Cargille Laboratories, Technical Bulletin, Cargille fused silica matching liquid code 50350, 2002.
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Optically controllable variable fiber optical attenuator integrated in conventional optical fiber Ivan Martincek, Dusan Pudis ∗ Department of Physics, Faculty of Electrical Engineering, University of Zilina, Univerzitna 1, SK-010 26 Zilina, Slovakia
a r t i c l e
i n f o
Article history: Received 6 December 2013 Accepted 10 July 2014 Keywords: Variable fiber optical attenuator Optical fiber device
a b s t r a c t New type of optically controllable variable fiber optical attenuator based on thermo-optical effect in liquid cladding of optical fiber is described. The thermo-optical effect in liquid cladding optical fiber causes refractive index contrast changes in core–cladding interface, what enables to change the propagating optical signal power with temperature. The temperature change is achieved by fiber based heating element using laser radiation. Attenuation up to −12 dB was achieved in static dependence and dynamic response confirmed rise time up to 24 ms. © 2014 Elsevier GmbH. All rights reserved.
1. Introduction Variable optical attenuators are optical components, which are widely used not only in optical networks for control of optical power levels in network channels, but also in waveguide optics for photonic signal processing and sensing. The variable fiber optical attenuators (VFOA) cover special class of variable optical attenuators based on optical fibers. Recently, several different types of VFOA have been developed with a variety of technologies such as liquid crystal technology [1], acusto-optical technology [2], micro-electro-mechanical systems technology [3], microstructured optical fiber technology [4], and optofluidical technology [5]. These technologies have the potential to create all-optical networks, especially communication network that works completely in the optical domain. Optically controllable waveguide and fiber optic component thus play an important role in telecommunications technology in all-optical networks developing. During the last few years, progress of a new multidisciplinary research field of optofluidics waveguide started, which is essentially the integration of microfluidic science and technology with optical waveguide components and methods. Developing optofluidic technology allows using unique properties of fluids for design of novel optical devices [6]. One of the most attractive results of the optofluidic activity is the realization of tunable optofluidic devices [7] using a large variety of physical mechanisms.
∗ Corresponding author. Tel.: +421 41 513 23 42. E-mail address: [email protected] (D. Pudis). http://dx.doi.org/10.1016/j.ijleo.2014.08.097 0030-4026/© 2014 Elsevier GmbH. All rights reserved.
Liquid-cladding optical fibers (LCOF) form a group of optofluidic waveguides, which have usually a solid core of circular crosssection, which is surrounded by appropriate liquid forming the cladding of optical fiber. For LCOF based on liquid cladding with low refractive index, light guiding through the core is based on the total internal reflection at a core–cladding interface of the LCOF. Refractive index changes between core and cladding open wide application area in optofluidic technology, including sensor applications, optical switching and modulation [7]. In this paper, we describe new type of VFOA based on thermooptical effect in LCOF. The VFOA is integrated on a standard telecommunication step-index optical fiber, so that a part of the primary coating and cladding of the optical fiber is completely removed and replaced by appropriate liquid. Due to the thermooptical effect a refractive index contrast between core and liquid cladding is changed with temperature, which enables to change the propagating optical signal power through the core. The change of liquid cladding temperature of VFOA is achieved by liquid heating from thin metal layer deposited on front facet of fused silica fiber. Thin metal layer is then directly heated by absorption of laser radiation. 2. Theoretical approach The mechanism of optical attenuation in the single mode VFOA is based on waveguide properties of fundamental mode LP01 in stepindex optical fiber. Based on fundamental theoretical approaches, optical power propagating in core of step-index optical fiber by LP01 mode decreases with refractive index contrast n = nco − ncl , where nco is core refractive index and ncl is cladding refractive index [8].
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Fig. 2. Schema of VFOA integrated in COF with heating element and liquid cladding.
Fig. 1. Attenuation mechanism in integrated optical fiber device.
This power decay is caused by increasing of mode field diameter of LP01 mode with refractive index contrast decrease and then higher part is propagating by evanescent field. In special case, if nco ≤ ncl , LP01 mode transforms to leaky modes, where z component of the wave vector is complex and causes an attenuation in the direction of propagation [9]. It is well known that on-axis bonding of two fibers with different mode-field diameter (MFD) causes lossy modes in the fiber with smaller MFD if fundamental mode is propagating from the fiber with higher MFD. Here, only part of propagating power in the fiber with smaller MFD is transformed into propagating fundamental mode and the rest contributes to lossy mode [8]. Then, one can expect power decrease at the output of fiber with smaller MFD. It is well known that the MFD is a function of n at core–cladding interface. If we create special fiber device, where the n can be changed by different physical mechanism, the MFD of guided modes is modified. For single mode optical fiber, decrease of n causes MFD increase for LP01 mode and for n ≤ 0 the propagating LP01 mode is transformed to leaky mode. Such device can be integrated into stepindex optical fiber with refractive index contrast n1 . We suppose a mode LP01 propagating in optical fiber. This mode is transformed to LP01d in integrated part if n = n1 and at next interface again to LP01 in optical fiber. No losses are expected in such arrangement (Fig. 1a). However, if the integrated part has n < n1 , the LP01 mode will be transformed to LP01d with higher MFD or to leaky mode if n < 0 (Fig. 1b). The LP01d mode will be transformed at second interface to LP01 mode. But, the LP01d mode shows higher MFD than LP01 , or is leaky. Then in optical fiber with n1 a formation of lossy modes is expected. Optical power propagating by means of LP01 mode at output part of device is then reduced. The modification of n in integrated part is the way to control optical power propagating by means of LP01 mode. Quantity of optical power LP01d mode coupled into fundamental mode LP01 is dependent on several parameters of integrated fiber device as length, refractive index contrast n and also on fluctuation of core diameter of integrated fiber device, inhomogeneity of refractive index in core and cladding, etc. Then it is difficult to theoretically predict the quantity of optical power propagating by means of fundamental mode. However, we document simple experimental technique, which allows measure optical power propagating across such integrated fiber device. This theoretical principle documents the ability of optical fibers to control the optical power propagating through the core by means of modified refractive index contrast n of core and cladding of optical fiber. A thermo-optical effect can be used for achieving of appropriate refractive index contrast n, if the core and cladding of
optical fiber consist of materials with different thermal coefficients. We found an appropriate material combination for the integrated device consisting of fused silica used as a core and mineral oil (Cargille liquid) as a cladding. Generally, the material combination of fused silica and mineral oil allows realization of different optical devices based on thermo-optical effect, especially temperature optical fiber sensors, switches and VFOA. The refractive index for fused silica of used conventional optical fiber (COF) and Cargille liquid code 50350 in the temperature range 15–35 ◦ C shows thermal coefficients 1 × 10−5 K−1 and −3.86 × 10−4 K−1 , respectively [10,11]. Such substantial difference of thermal coefficients is suitable for achieving of sufficient refractive index contrast at core–cladding interface with increasing temperature, which is useful for integration of this principle in new VFOA based on thermo-optical effect in optical fiber with liquid cladding. 3. Experimental The attenuation in designed VFOA is due to the thermo-optical effect, which influences the guiding properties of LCOF. In this experiment, the VFOA thermal changes were realized by local heating using laser source. The designed VFOA consists of 2 m long conventional optical fiber with core and cladding diameter 10 m and 125 m, respectively. The cladding is coated with an acrylate primary coating of outer diameter 250 m. In the middle part of the COF, the primary coating was stripped. In this part, the cladding was then removed in app. 150 m long section using paraffin mask and etched in hydrogen fluoride acid. After cladding removing, it was replaced by Cargille fused silica matching liquid code 50350. Thermal changes of VFOA were achieved using specially prepared small heating element prepared from optical fibers. This heating element consists of two multimode optical fibers with core diameter of 50 m and cladding of 125 m, where the ends were placed close to COF core with d = 20 m (40 m) on opposite sites (Fig. 2). Front face of one was evaporated by 1 m thick copper layer. It works as an absorber of impacted light from the second fiber, which was coupled to diode laser with operation wavelength at 405 nm. Such radiation is app. 50% absorbed in deposited copper layer. Liquid cladding of VFOA was kept by capillary forces between etched core and heating element. Absorbed light in copper layer heats this layer and the surrounding cladding of LCOF. Thereafter, LCOF heating causes an increase of refraction index contrast n at core–cladding interface of LCOF, which improves guiding properties. LCOF temperature can be simply controlled by laser power in heating element. Such fully optical device based on thermo-optical effect can be used as VFOA and modulator controlled by laser light. Fig. 3 shows static dependence of propagating optical signal through LCOF on laser power of heating optical fiber if the heating fibers were placed in d = 20 m from each other at room
Optical Signal (a.u.)
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temperature (25 ◦ C). COF was coupled to light emitting diode (1310 nm) as a source of optical signal and output optical signal was detected using InGaAs detector PDA 10CS-EC. The core of used COF was prepared by modified chemical vapor deposition technique. This fabrication technology increases the refractive index profile of fused silica by dopation. Also refractive index profile of such fibers show central dip and it is difficult to reliably determine the character of mode at 25 ◦ C if it is propagating or leaky. Anyway, the transmitted power propagating through the integrated fiber device was attenuated by −12 dB. Fig. 3 documents an increase of propagating signal through VFOA as the laser power in heating element increases in all measured range of laser power. At room temperature, the LCOF mode is created, which part is transformed in lossy modes in COF. The lossy modes are outcoupled from optical fiber by acrylate primary coating, which works as an absorption layer of the lossy modes. Next increase of laser power causes heating up of Cargille liquid cladding from copper layer. Due to the thermo-optical effect and appropriate thermal coefficients of core and cladding, thermal increase improves guiding properties and in measured static dependence it shows an increase of optical signal. At laser power higher than 2.0 mW the minimal attenuation was achieved. In this region, the ratio of output/input power was attenuated by 0.5 dB, which corresponds to insertion losses of prepared VFOA in static mode. In all measured range of optical power the maximal static attenuation shows −12 dB at temperature 25 ◦ C. Modulation and a dynamic response time of the VFOA were measured by the Agilent DSO-X 2012A (100 MHz) oscilloscope. We applied a square-wave driving voltage to modulate the heating laser light with pulse duration of 50 ms and delay time of 250 ms. The optical response of the VFOA was then measured. Fig. 4 shows
Fig. 5. Comparison of dynamic characteristics of small-volume VFOA construction (up, d = 20 m) and large-volume cladding VFOA (middle, d = 40 m). Applied square wave pulse is also shown (bottom).
the measured response of the VFOA at different laser power of heating element. The rise time (from 10% to 90%) and fall time is 24 ms and 159 ms for 2.75 mW of heating power. In the case of low power heating (0.74 mW) the rise time is 30 ms and fall time 147 ms. Dynamic characteristics document the effect of increasing laser power on the rise time reduction. High laser power shows shorter rise time but the response time is longer. Also higher laser power shows shift of measured signal to lower attenuation. The shift is caused by overall temperature increasing if the higher laser power is used. Factors which affect the dynamic response are related to thermal processes in whole VFOA system given by materials and their volumes used in the VFOA arrangement. Using this construction, we expected different behavior of VFOA depending on liquid volume creating LCOF cladding placed between facets of heating element. For the verification of volume liquid effect, also large volume variant of VFOA was prepared and measured. Large volume liquid was placed on the side between core and copper layer of heating element at d = 40 m. Such arrangement uses larger liquid volume, where we suppose worse dynamic response. Dynamic characteristic in Fig. 5 shows comparison of two arrangements with different d = 20 and 40 m for the same laser power in heating element of 1.29 mW. As is evident from Fig. 5, the VFOA with large-volume liquid cladding shows longer dynamic response times. The large-volume rise time is 40 ms and fall time 165 ms, while the small-volume arrangement shows only 29 ms rise time and 149 ms. Largevolume liquid cladding shows much slower heat exchange than small-volume arrangement, which finally degrades the dynamic response. However, there is one drawback in using of small-volume liquid cladding, where optical signal is attenuated in short time only, what affects lower attenuation level. Also other parameters such as VFOA length and heating element arrangement should induce the final static and dynamic properties. Definitely, final construction of liquid volume of VFOA can be optimized according to an application demand. 4. Conclusion In this paper, we describe new type of optically controlled VFOA and modulator based on thermo-optical effect. The VFOA is integrated on a single-mode step-index telecommunication optical fiber operating at 1310 nm. One advantage of described VFOA is direct integration on COF by simple adaptation. The transmission properties can be determined by arrangement of heating element and cladding parameters. As a limitation the dynamic response was observed in the range from 24 ms to 40 ms depending on VFOA construction.
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I. Martincek, D. Pudis / Optik 125 (2014) 7085–7088
We believe that proposed VFOA based on LCOF could find application in the optical fibers, where the attenuation level is adjusted to be external optical source and in utilization of optical fibers in sensor applications. Acknowledgements This work was partly supported by Slovak national Grant Agency Nos. VEGA 1/0528/12 and 1/0491/14 and the Slovak Research and Development Agency under the project No. APVV-0395-12. The authors wish to thank the support from the R&D Operational Program Centre of Eexcellence of Power Electronics Systems and Materials for their Components II. No. OPVaV-2009/2.1/02-SORO, ITMS 26220120046 funded by European Regional Development Fund. References [1] M. Sunish, G. Farrell, Y. Semenova, Experimental demonstration of an all-fiber variable optical attenuator based on liquid crystal infiltrated photonic crystal fiber, Microwave Opt. Technol. Lett. 53 (2011) 539–543.
[2] Q. Li, A.A. Au, Ch.-H. Lin, E.R. Lyons, H.P. Lee, An efficient all-fiber variable optical attenuator via acoustooptic mode coupling, IEEE Photon. Technol. Lett. 14 (2002) 1563–1565. [3] N.A. Riza, S.A. Reza, Broadband all-digital variable fiber-optic attenuator using digital micromirror device, IEEE Photon. Technol. Lett. 19 (2007) 1705– 1707. [4] C. Kerbage, A. Hale, A. Yablon, R.S. Windeler, B.J. Eggleton, Integrated all-fiber variable attenuator based on hybrid microstructure fiber, Appl. Phys. Lett. 79 (2001) 3191–3193. [5] D.P. Martincek, Variable liquid-core fiber optical attenuator based on thermo-optical effect, J. Lightwave Technol. 29 (2011) 2647–2650. [6] D. Psaltis, S.R. Quake, Ch. Yang, Developing optofluidic technology through the fusion of microfluidics and optics, Nature 442 (2006) 381–386. [7] U. Levy, R. Shamai, Tunable optofluidic devices, Microfluid Nanofluid 4 (2008) 97–105. [8] A.W. Snyder, J.D. Love, Optical Waveguide Theory, Chapman and Hall, London, 1983. [9] A.W. Snyder, Leaky-ray theory of optical waveguides of circular cross section, Appl. Phys. 4 (1974) 273–298. [10] M.J. Weber, Handbook of Optical Materials, CRC Press LLC, 2003. [11] Cargille Laboratories, Technical Bulletin, Cargille fused silica matching liquid code 50350, 2002.