Polarization Rotator Based on Soft Glass Photonic Crystal Fiber With Liquid Crystal Core

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 29, NO. 18, SEPTEMBER 15, 2011 2725

Polarization Rotator Based on Soft Glass PhotonicCrystal Fiber With Liquid Crystal Core

Mohamed Farhat O. Hameed and S. S. A. Obayya

Abstract—A rigorous study of polarization rotation in a noveldesign of high-tunable polarization rotator based on soft glassphotonic crystal fiber (PCF) is introduced and analyzed. Thesuggested PCF has a central hole infiltrated with nematic liquidcrystal. At a wavelength of 1.55 m, nearly 100% polarizationconversion ratio is obtained, with a device length of 558 m. Thesimulation results are evaluated using the full-vectorial finite-dif-ference method along with the full-vectorial finite-difference beampropagation method.

Index Terms—Beam propagation method (BPM), finite-dif-ference method (FDM), nematic liquid crystal (NLC), photoniccrystal fibers (PCFs), polarization rotation (PR), soft glass.

I. INTRODUCTION

P olarization rotators (PRs) play an important role inmodern optoelectronic and communication systems.

Therefore, PRs have attracted the interest of many researchersin recent years. The PRs are used to control the polarizationstates in the communication systems such as polarizationmodulators [1] and polarization switches [2]. Initially, polar-ization rotation in multisections asymmetric periodic loadedrib waveguides was proposed in [3]. The multisections PRsuffers from its longer device length (several millimeters)and large transition losses at the interface between alternatingsections (several decibels). However, Obayya et al. [4] reporteda complete polarization rotation at a moderate device lengtharound 0.74 mm, and with minimal radiation loss as low as 0.13dB by a careful adjustment of the waveguide width and/or thewaveguide materials. Another PR based on cascaded configu-ration of curved waveguide sections each with an alternativecurvature direction was reported in [5]. In this configuration,the polarization conversion builds up as the wave propagatesalong that cascaded arrangement of the curved waveguides.Due to the radiation losses, great efforts have been directed

toward the design of a single-section polarization converter thatcontains no transition losses with shorter device length. Thesingle-section PR examples included slanted sidewall waveg-uides [5], [6] and curved optical waveguides [7]. However, PRswith slanted sidewall require a complex fabrication process in-cluding dry andwet etching techniques [8]. In addition, the com-pact PRs based on the curved optical waveguides rely on a very

Manuscript received May 06, 2011; revised July 05, 2011; accepted July 20,2011. Date of publication July 29, 2011; date of current version August 24, 2011.M. F. O. Hameed is with the Department of Mathematics and Engineering

Physics, Faculty of Engineering, Mansoura University, Mansoura 35516, Egypt(e-mail: [email protected]).S. S. A. Obayya is with Electronics and Communications Engineering

Department, Faculty of Engineering, Mansoura University, Mansoura 35516,Egypt (e-mail: [email protected]).Digital Object Identifier 10.1109/JLT.2011.2163297

small radius of curvature which will not be easy for fabrication.The photonic bandgap silica photonic crystal fiber (PCF) filledwith liquid crystal (LC) has been experimentally shown to havepotential for polarization conversion [9], [10]. Scolari et al. [9]demonstrated a PR by using a silica core PCF infiltrated with adual frequency LC while in [10] the PCF was filled with a nega-tive LC. Recently, the authors presented high-tunable PR basedon soft glass PCF whose cladding holes have been infiltratedwith nematic liquid crystal (NLCPCF) [11]. The NLCPCF PR[11] has no central hole and provides a strong polarization con-version ratio of 99.81% with a device length of 1072 m.In this paper, a novel design of high-tunable PR based on

soft glass PCF with air holes and nematic liquid crystal core(SGLC-PCF) is introduced and analyzed. The suggested de-sign depends on using soft glass of type SF57 (lead silica) anda central hole infiltrated with nematic liquid crystal (NLC) oftype E7. The refractive index of the SF57 material is greaterthan the ordinary and extraordinary refractive indices ofthe E7 material. In addition, the effective index contrast be-tween the core and the cladding regions ensures index guidingthrough the suggested SGLC-PCF. Moreover, arc-fusion tech-niques have been successfully implemented for the infiltrationof central defect cores [12]–[16]. Therefore, the suggested de-sign is easier for fabrication than the NLCPCF PR [11] withfilling all the cladding holes. The effect of the structure geomet-rical parameters, temperature, and operating wavelength on thedevice length and polarization efficiency is investigated thor-oughly. The SGLC-PCF PR offers polarization conversion ratioof 99.67% with low crosstalk (CT) of dB and a devicelength of 558 m. Therefore, the suggested PR is shorter thanthe NLCPCF PR of device length 1072 m [11]. It is also ex-pected that over the 1.53–1.6 m wavelength range, polariza-tion conversion of the suggested PR would be more than 99%and polarization CT would be better than dB. In the nextsection, the numerical methods are described briefly. Followingthat, in the results section, the design and simulated results ob-tained from a novel design of single-section PR are presented,following which conclusions are drawn.

II. NUMERICAL APPROACHES

Modal solutions are very useful for the characterizationand design of PRs. To calculate accurately vector modal fieldprofiles of the input waveguide, a full-vectorial modal solu-tion approach is necessary. Many modal solution techniqueshave been proposed in the past few years for modal anal-ysis of PCFs such as the finite element method (FEM) [17],finite-element-based imaginary distance beam propagationmethod (BPM)[18], finite-difference method (FDM) [19],and multipole method [20]. In this study, the full-vectorialfinite-difference method (FVFDM) [19] with perfect matched

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2726 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 29, NO. 18, SEPTEMBER 15, 2011

Fig. 1. Cross section of the SGLC-PCF sandwiched between two electrodesand surrounded by silicone oil [26].

layer boundary conditions [21] is used to calculate the full-vec-torial quasi-TE and quasi-TM modes for the input waveguide.Through all simulations, the grid sizes and in x- andy-directions, respectively, are taken as 0.05 m.To study the propagation through the PRwaveguide section, a

more suitable approach such as BPM [22], [23] is required. TheBPM has been considered in the last two decades as one of themost popular methods used for the simulation of wave propa-gation in various photonic devices. A full-vectorial approach isparticularly necessary to calculate the polarization conversionin the optical guided wave devices or systems. Due to its nu-merical efficiency and versatility, some full-vectorial BPM al-gorithms have been formulated based on the FEM [22]. In ad-dition, many full-vectorial BPM approaches based on the pop-ular FDM have been reported [23]. In this study, the full-vec-torial finite-difference BPM (FVFD-BPM) [23] is applied tostudy the polarization conversion based on the reported struc-ture. Through all simulations, the transverse step sizes are alsofixed to m while the longitudinal step size

is taken as 1 m. In addition, the reference index thatis used to satisfy the slowly varying envelope approximation ofthe FVFD-BPM [23] is taken as the effective index of the funda-mental mode launched at the input waveguide. Moreover, ischosen within the range, , at which the FVFD-BPM[23] is unconditionally stable. The parameter is responsiblefor controlling the scheme that is used to solve the finite differ-ence equations.

III. DESIGN AND NUMERICAL RESULTS

Fig. 1 shows cross section of the suggested triangular lat-tice SGLC-PCF with air holes in the cladding regions. All thecladding holes have the same diameter d and are arranged witha hole pitch . The central hole of diameter is infiltrated withan NLC of type E7. The reported structure is different from theNLCPCF PR that was introduced in [11]. All the cladding holesof the NLCPCF have been infiltrated by the NLC with solid softglass core. Therefore, the new design has the advantage of easefor fabrication. The NLCs used in the proposed structure areanisotropic materials consisting of rod-like molecules that arecharacterized by ordinary index and extraordinary index .The following Cauchy models can be used to calculate the or-dinary and extraordinary refractive indices of the E7 ma-terial [24]

(1)

where and are the coefficients of theCauchy model. The Cauchy coefficients at C are

m mm , and m . The relative permit-

tivity tensor of the E7 material is taken as [25]

(2)

where is the rotation angle of the director of the NLCas shown in Fig. 1. The proposed in-plane alignment of theNLC can be exhibited under the influence of an appropriatehomeotropic anchoring conditions [15], [25]. In addition,Haakestad et al. [26] demonstrated experimentally that inthe strong field limit, the NLC of type E7 is aligned in planein capillaries of diameter 5 m. Moreover, Alkeskjold andBjarklev [27] presented experimentally in-plane alignment ofthe E7 material in PCF capillaries of diameter 3 m with threedifferent rotation angles, 0 , 45 , and 90 using two sets ofelectrodes.The fiber is placed between two pairs of electrodes allowing

for the arbitrary control of the alignment of the NLC directorvia an external voltage, as schematically shown in Fig. 1. Inaddition, two silica rods with appropriate diameter are used tocontrol the spacing between the electrodes and the fiber is sur-rounded by silicone oil, which has higher dielectric strength thanair [26]. Therefore, the external electric field will be uniformacross the fiber cross section which results in good alignmentof the director of the NLC with a constant rotation angle .Moreover, the nonuniform electric field region will only be atthe edges, far away from the core region where the light will bepropagating. As a result, the proposed PR overall performancewill not be affected by the nonuniform field distribution at theedges. In addition, Wei et al. [28] proved that by using sets ofelectrodes and controlling them independently, the direction ofthe electrical field is rotatable under effective driving voltage of

.The background material of the SGLC-PCF is a soft glass of

type SF57 (lead silica) that has the following Sellmeier equation[29]:

(3)

where is the refractive index of the SF57 material,m m

m m , andm [29]. The soft glass SF57 mate-

rial has good mechanical and thermal stability and its Sellmeiercoefficients are temperature independent in the studied temper-ature range from 15 to 50 C [29].In the proposed structure, all the cladding air holes have the

same diameter d and are arranged with a hole pitch mand ratio of 0.6 while the central hole diameter is takenas 1.0 m. In addition, and of the E7 material are fixedto 1.5024 and 1.6970, respectively, at the operating wavelength

m and at a temperature of 25 C. The rotation angleof the director of the NLC is taken as 45 and is fixed to1.802 at m. The refractive index of the SF57 mate-rial is greater than and of the E7 material. In addition, the

HAMEED AND OBAYYA: POLARIZATION ROTATOR BASED ON SGLC-PCF 2727

effective index contrast between the core and the cladding re-gions ensures index guiding through the suggested SGLC-PCF.In this study, the hybridness is defined as

(4)

where u and v are x and y for quasi-TE mode while y and xfor quasi-TM mode, respectively. The minimum longitudinaldistance at which maximum polarization conversion occurs iscalled the conversion length or the half-beat length whichcan be calculated using

(5)

where and are the propagation constants of thequasi-TE and quasi-TM polarized modes, respectively. For asingle-section PR, the length of the SGLC-PCF with the hybridmode should be equal to the half-beat length to reverse thepolarization state.The degree to which the modes of the SGLC-PCF are hybrid

can be affected by the rotation angle of the NLC. Therefore, theeffect of the rotation angle on the modal hybridness is investi-gated thoroughly. Fig. 2 shows the variation of the hybridnessfor the quasi-TE and quasi-TMmodes, with the rotation angle .It is evident from this figure that as is increased from 0 to 45 ,the hybridness of the modes is increased to reach a maximumvalue of 0.98895 and 0.9988 for the quasi-TE and quasi-TMmodes, respectively, at a rotation angle of 45 . Therefore, com-plete polarization conversion can occur at . However,if is further increased, the modal hybridness is reduced fromits maximum value. Also, it is observed that the modal hybrid-ness for the quasi-TMmode is approximately equal to that of thequasi-TE mode. Moreover, the numerical results reveal that therotation angle has a slight effect on the conversion length. Ini-tially, the conversion length slightly decreases from 566 to 558m with increasing the rotation angle from 0 to 45 . Then theconversion length slightly increases to 560 m with increasingthe rotation angle to 90 .According to the coordinate system defined in Fig. 1, the fun-

damental quasi-TE mode refers to the fundamental ormodes, while the fundamental quasi-TMmode refers to the fun-damental or modes. Fig. 3(a) and (b) shows the dom-inant and nondominant field profiles of the quasi-TEmode of the SGLC-PCF obtained from the FVFDM. It is ob-served that the field profiles of the dominant and nondominantcomponents of the quasi-TE mode are very similar. The max-imum value of is 0.99, normalized to the maximum valueof the dominant component. This ensures that the proposedSGLC-PCF supports highly hybrid modes which is very usefulin designing polarization conversion devices. In addition, theexpected overlap between the vector field components of thequasi-TE and quasi-TM modes can improve the polarizationconversion through the SGLC-PCF.The dominant and nondominant field components of

the quasi-TE mode of a soft glass PCF with air holes are shownin Fig. 3(c) and (d), respectively. In this case, the central holeof diameter m is also filled with air and all thecladding holes have the same diameter d and are arranged witha hole pitch m, and ratio of 0.6. It is found thatthe maximum magnitude of is only 0.03, normalized to themaximum value of . The nondominant field profile of the

Fig. 2. Variation of the hybridness of the quasi-TE and quasi-TM modes withthe rotation angle of the director of the NLC.

Fig. 3. Contour plot of the dominant and nondominant field profiles ofthe fundamental quasi-TE mode for (a) and (b) the SGLC-PCF and for (c) and(d) the soft glass PCF with air holes.

quasi-TM mode is not shown here, but this profile is similar tothe nondominant field profile of the quasi-TE mode. Onlya small amount of mode conversion can take place in the softglass PCF with air holes because the dominant and nondom-inant field profiles of the two polarized modes are not of veryunequal amplitudes. This type of PCF with very little hybridiza-tion can be used as an input or output waveguide. In this study,the quasi-TE or quasi-TM mode of the soft glass PCF with airholes is launched into the SGLC-PCF and the FVFD-BPM [23]is used to study the wave propagation along the SGLC-PCF.As shown in the inset of Fig. 4, when a TE polarizedmode ob-

tained from the soft glass PCF with air holes is launched directlyinto the SGLC-PCF, the input power excites two hybrid modesalong the suggested PR waveguide. These two modes becomeout of phase at a distance equal to from the beginning of thePR section. Therefore, component will be cancelled whilethe component will be added which produces a nearly pureTM mode. The calculated using the FVFD-BPM is 558 mwhich is in an excellent agreement with 557 m calculated bythe FVFDM. The polarization power factors and are de-fined as the power carried by the and field components,respectively, over the PR waveguide cross section, normalizedto the total power. Fig. 4 shows the variations of the TM po-larized power for the TE input along the axial direction, atdifferent rotation angles of the director of the NLC. It can beobserved from this figure that for the TE excitation, initially


Fig. 4. Evolution of the TM powers along the propagation direction at dif-ferent rotation angles of the director of the NLC.

Fig. 5. Evolution of the TM power for the TE excitation along the propaga-tion direction at different values of the central NLC infiltrated hole diameter .

is zero and it slowly increases to a maximum value atand if the PR section is not terminated at this position the op-tical power starts decreasing. It should be noted that nearly99.67% polarization conversion can be obtained at mwhen the rotation angle is equal to 45 . It is also evident fromFig. 4 that the conversion ratio increases with increasing the ro-tation angle from 0 to 45 until complete conversion occurs at

. Then the conversion ratio decreases with increasingthe rotation angle. In addition, the hybridness and, hence, theconversion ratios, when , 20 , and 30 , are approxi-mately equal to the hybridness and, hence, the conversion ra-tios at , 70 , and 60 , respectively, as shown in Figs. 2and 4.The effects of the structure geometrical parameters, rotation

angle of the NLC, temperature, and wavelength on the perfor-mance of the suggested PR are studied in detail. First, the influ-ence of the central hole diameter on the conversion length andthe conversion ratio is investigated. Fig. 5 shows the variationof the TM polarized powers for the TE excitation along thepropagation direction at different , 1.0, 1.2, and 1.4 m whileand ratio are fixed to 5 m and 0.7, respectively. In addi-

tion, the rotation angle of the director of the NLC, temperature,and operating wavelength are taken as 45 , 25 C, and 1.55 m,respectively. It is evident from this figure that the conversionlength decreases with increasing the diameter . The conver-sion lengths at , 1.0, 1.2, and 1.4 m, are equal to 442, 410,and 388 m, respectively. The numerical results also reveal thatthe conversion ratio decreases with increasing the central hole

Fig. 6. Evolution of the TM powers for the TE excitation along the propa-gation direction at different ratios.

Fig. 7. Evolution of the TM powers for the TE excitation along the propa-gation direction at different hole pitch values.

diameter . The crosstalk (CT) is a measure of the unwantedTE power, or , remaining at the end of the polarization con-verter. The CT at , and 1.4 m is equal to ,

, and dB, respectively.The effect of the ratio at constant hole pitch is also in-

vestigated. Fig. 6 shows the evolution of the TM powers forthe TE excitation along the propagation direction at different

ratios, 0.6, 0.7, and 0.8, while the hole pitch ,temperature, and the operating wavelength are taken as 5 m,1.0 m, 45 , 25 , and 1.55 m, respectively. It is revealedfrom this figure that the conversion length decreases with in-creasing the ratio at constant . The conversion lengthsat and 0.8 are found to be 558, 442, and 346m, respectively. The corresponding CTs are equal to ,

, and dB, respectively.The influence of the hole pitch at constant ratio is the

next parameter to be studied thoroughly. Fig. 7 shows the evolu-tion of the TM powers for the TE excitation along the propa-gation direction at different hole pitch , 4, 4.5, and 5 m, while

ratio, , temperature, and the operating wavelength aretaken as 1.0 m, 0.6, 45 , 25 , and 1.55 m, respectively. Itis revealed from this figure that the conversion length increaseswith increasing the hole pitch at constant ratio. The conver-sion lengths at , 4.5, and 5 m are found to be 315, 413,and 558 m, respectively. In addition, it is found that the CT de-creases with increasing the hole pitch. The corresponding CTsare equal to , , and dB, respectively. ThePR with m, m, C, and


Fig. 8. Variation of the converted power for the TE excitation and the CTat and m with the rotation angle .

offers high-polarization conversion ratio of 99.67%with a device length of 558 m. Thus, this design will be con-sidered as the appropriate device in the subsequent simulations.It should be noted that the infiltration of the NLC in a hole ofdiameter 1.0 m is experimentally achieved by Wolinski et al.[30] by the capillary effect. In addition, arc-fusion techniqueshave been successfully implemented for the infiltration of cen-tral defect cores [12]–[16]. The NLCPCF PR [11] with fillingall the cladding holes with NLC also offers polarization conver-sion ratio of more than 99% with a device length of 1072 m.Therefore, the presented SGLC-PCF PR is not only shorter thanthat reported in [11] but also is easier for fabrication.Hence, it is of great importance to consider the fabrication

tolerances and their effects on the polarization converter perfor-mance. The variation of the rotation angle of the director of theNLC is the first parameter to be considered. To study the fabri-cation tolerances of the device length, the power conversion andthe corresponding CT for the TE excitation at specified longi-tudinal positions have been considered for various rotation an-gles while the hole pitch ratio, , and T are fixed to5.0 m, 0.6, 1.0 m, and 25 C, respectively. Fig. 8 shows thevariation of the TM power and the CT values, in decibel,with the rotation angle in the range from 10 to 80 at boththe exact value of and the specified device length of 558m. It is found that the behavior of the variation with the ro-tation angle is very compatible with the behavior of the modalhybridness, shown in Fig. 2. As it has been shown before thatthe hybridness is maximum at , the value of reachesits maximum value at and also the corresponding CTis a minimum value of dB. It is also evident from Fig. 8that changing yields large variations in the TM power, ,and the corresponding CT. In addition, the TM power, , andthe CT values, in decibel, at both the exact value of and thespecified device length of 558 m are approximately equal. Thisis compatible with the small effect of changing on the conver-sion length. It should be noted that when fabricating the devicewithin the angular range of the rotation angle from 42 to 48 ,the maximum power and the CT at the designed length willbe always better than 0.9848 and dB, respectively. How-ever, for ranges from 40 to 10 and from 50 to 80 , themaximum value of will decrease to 0.12 and the CT willdeteriorate to 8.7 dB. The control and tunability of the rotation

Fig. 9. Variation of the converted power for the TE excitation and the CTat and m with the central infiltrated NLC hole radius.

angle of the NLC can be achieved with a good accuracy in astrong field limit [26] with sets of electrodes as successfully ex-perimentally shown in [26], [27].The effects of the central hole diameter , shape, and posi-

tion are also studied. First, the fabrication tolerances of the de-vice length, the power conversion, and the corresponding CT atspecified longitudinal positions have been considered for var-ious central hole radii while the hole pitch and ratioare fixed to 5.0 m and 0.6, respectively. For this purpose, theinfiltrated NLC hole radius has been varied from 0.4 to 0.65,while the optimum device length has been fixed at 558 m, asit corresponds to the ideal in design conditions, m,

C, and . As shown in Fig. 9, thevalue of for the TE excitation at the corresponding valuesof decreases from 0.997 to 0.996 as the central hole radiusincreases from 0.4 to 0.65. On the other hand, if the power con-version is to be calculated at a fixed device length of 558 m,the values will be much lower than those at the exact valuesof , except at a hole radius of 0.5, where the exact value ofis itself 558 m. In addition, the CT measured at the specifieddevice length of 558 m decreases from dB to a valueof dB as the central hole radius is increased from 0.4 to0.5. Then the CT increases to a value of dB as the valueof central hole radius is increased to 0.65. Also, it is importantto mention that for the central hole radius range from 0.4 to 0.6,the CT value will be still less than dB.Next, the effect of the deformation of the central hole into el-

liptical hole on the PR performance is investigated. Here, theellipticity is defined as the ratio (a/b) where, a and b are theradii in x- and y-directions, respectively, of the elliptical hole.In this study, the radius b in y-direction is fixed to 0.5 m. Inaddition, the hole pitch, and ratio are taken as 5.0 m, and0.6, respectively. Moreover, the rotation angle of the director ofthe NLC, temperature, and the operating wavelength are fixedto 45 , 25 C, and 1.55 m, respectively. The numerical resultsshow that as the ellipticity ratio increases from 0.8 to 1.15, theconversion length decreases from 566 to 541 m. It is also re-vealed from the numerical results that fabricating the device fora tolerance of ellipticity ratio equals , the CT at thedesigned length will always be better than dB. The effectof the ellipticity (b/a) ratio on the PR performance is also con-sidered. It is found that the (b/a) ratio has the same effect of the(a/b) ratio on the PR performance.


Fig. 10. Variation of the converted power for the TE excitation and the CTat and m with the shift ratio in -direction.

The influence of shifting the position of the central hole onthe PR performance will be next studied. Here, the shift ratio isdefined as (S/ ) where S is the shift distance in - or -di-rection. In this study, the other parameters are kept constant at

m, m, C, ,and m. The numerical results reveal that the con-version length increases with increasing the shift ratio in both- and -directions. It is also found that the shift in -di-

rection has the same effect of that in -direction. As the shiftpercentage increases to 5% in positive or negative x, the con-version length increases from 558 to 583 m. In order to inves-tigate the tolerance in the position of the central hole over thepolarization rotation, the converted TM power for the TEexcitation, at and m and the CT are obtainedwithin the range of shift ratio percentage from 0 to 5% in posi-tive and negative x-directions as shown in Fig. 10. The positiveand negative shift ratio indicates the shift in positive and neg-ative x-direction, respectively. It is revealed from Fig. 10 thatas the shift percentage increases from 0 to 5%, the convertedpower at m does not show much deviation withthe shift in -direction from the curve at exact , becausechanges only by 25 m along the whole shift range consid-

ered. The effect of the shift in -direction on the performanceof the PR has also been performed and it is found that the shiftin -direction has the same effect of the shift in -direction.It is worth noting that fabricating the device for a tolerance ofshift percentage of 5% in - or -direction, the CT at thedesigned length will always be better than dB.The ordinary and extraordinary refractive indices of

the NLC are temperature dependent [11], [24] which affects theperformance of the proposed PR. As T increases from 15 to50 C, the of the E7material decreases from 1.7096 to 1.6438at the operating wavelength m. However, the ofthe E7 material decreases slightly from 1.5034 to 1.5017 whenthe temperature changes from 15 to 35 C at m. Thenincreases from 1.5017 to 1.5089 when T increases from 35 to

50 C at m. Therefore, the effect of the temperatureT variation will be the next parameter to be considered. In thisstudy, the other parameters are still invariant at m,

m, m, and . In orderto investigate the tolerance in the temperature difference overthe polarization rotation, the converted TM power for the TE

Fig. 11. Variation of the converted power for the TE excitation and the CTat and m with the temperature.

Fig. 12. Variation of the converted power for the TE excitation and the CTat and m with the operating wavelength.

excitation, at and m, and the CT are obtainedwithin the T range from 15 to 50 C as shown in Fig. 11. Itis observed from this figure that the CT at exact decreasesfrom to dB with increasing the temperature from15 to 50 C. However, the CT at m decreases from

to dB as T increases from 15 to 25 C and thenthe CT increases to dB as the temperature increases to50 C. In addition, there is a large difference between at

and at m due to the significant effect of thetemperature on the conversion length. The conversion lengthincreases from 494 to 994 m as the temperature increases from15 to 50 C, respectively. It can also be seen from Fig. 11 thatfabricating the device for a tolerance of 25 C C, the CTat the designed length will always be better than dB. Thetemperature can be controlled by using thermo-electric moduleas experimentally described by Wolinski et al. [31] allowingfor temperature control in the 10–120 C range with 0.1 Clong-term stability and electric field regulation in the 0–1000 Vrange with frequencies from 50 Hz to 2 kHz.Also, for this single-section PR, the effect of varying the op-

erating wavelength on the performance of the PR is studiedin the range from 1.53 to 1.63 m. In this study, the change ofthe refractive index of the composing materials with the wave-length is taken into account. Fig. 12 shows the variation of theTM power, for the TE excitation, and the corresponding CTat the exact values of and the design length of 558 m. At the


given device length of 558 m, if the wavelength varies in therange of 1.55 m m, the CT is better than dB.

IV. CONCLUSION

A novel design of single-section high-tunable PR based onindex guiding SGLC-PCF has been introduced. The proposedpolarization converter design is based on the infiltration of thecentral hole with NLC that is believed to be better, in the senseof easier fabrication, than that based on the infiltration of allcladding air holes. The numerical results indicate that more than99% polarization conversion could be achieved with a devicelength of 558 m and low CT of dB. The results shownhere demonstrate a reasonably stable performance with slightvariations in the fabrication parameters or the operating wave-length as might be expected.

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Authors’ biographies not included at authors request due to space con-straints.

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