Novel large mode area photonic crystal fibers with selectively material-filled structure

Optics & Laser Technology 48 (2013) 375–380

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Novel large mode area photonic crystal fibers with selectively material-filled structure Jianhua Li a,n, Jingyuan Wang a, Yan Cheng b, Rong Wang a, Baofu Zhang a, Huali Wang a a b

Institute of Communications Engineering, PLA University of Science and Technology, Nanjing 210007, China Department of Computer Science and Engineering, East China University of Political Science and Law, Shanghai, China

a r t i c l e i n f o

abstract

Article history: Received 7 September 2012 Received in revised form 9 October 2012 Accepted 25 October 2012 Available online 7 December 2012

A novel large mode area (LMA) photonic crystal fibers (PCFs) are proposed in this paper. LMA is obtained by selectively material-filled structural PCFs in which six air-holes near the center are filled with high refractive index material. It provides a new method to design and realize LMA PCFs. The transmission characteristics such as effective mode areas, chromatic dispersion, and confinement losses are investigated and also the influences of structure parameters are numerically analyzed by employing the full vectorial finite element method (FEM). Numerical results demonstrate that effective mode area can improve, small and flat dispersion is achieved, and confinement losses are still acceptable by applying our novel selectively material-filled technology. Compared with the traditional method achieving LMA, it has perfect performances and it is simple and easy to fabricate. & 2012 Elsevier Ltd. All rights reserved.

Keywords: Photonic crystal fibers (PCFs) Large mode area Material-filled PCFs

1. Introduction Recently photonic crystal fibers (PCFs) [1,2] were still the research hotspot because of their unique features such as endlessly single mode, dispersion tailoring, high birefringence, etc [1–9]. Compared to conversional fibers, they can be designed with flexible structure and then excellent performances can be achieved. Among these features large mode area (LMA) is widely researched because LMA PCFs can support high power transmission in fiber laser [10], amplifier [11], and other applications [12–14]. It is easy for PCFs to obtain LMA by designed with flexible structure. Traditional method to realize LMA is using large fiber core or small differences of refractive index between core and cladding. Early in 1998 Knight et al. [15] proposed a kind of LMA PCFs with small air-holes and large core. Gates et al. [16] studied the propagation of modes of LMA holey fibers. The crosssection and propagation of modes of LMA holey fibers are measured by using near-field scanning optical microscopy. Mortensen [17] investigated effective mode area of PCFs with a triangular air-hole lattice in the cladding. Abdelaziz et al. [18] designed a new PCF structure configuration with an effective mode area as high as 3000 mm2. The proposed PCF structures consist of five air-hole rings, where the air hole diameters are different from one ring to another. In the second ring six air holes

n

Corresponding author. Tel.: þ86 25 80828449; fax: þ 86 25 80828448. E-mail address: [email protected] (J. Li).

0030-3992/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlastec.2012.10.032

are alternatively removed. There are still other novel LMA PCF structures to be reported [19–21]. For realizing novel and flexible features, different materials were selectively filled in the PCF structure [7–9]. For example, ¨ Huttunen and Torm a¨ [22] proposed a PCF structure with large dispersion and mode area based on the high refractive index material filled in the core. And also Yang et al. [23] realized theoretical study and experimental fabrication of this kind of high negative dispersion PCF with large area mode field. Recently Ademgil and Haxha [24] proposed a novel technique to design LMA PCFs. It used different doping levels in core area which had seven missing air holes. In order to increase the effective mode area further and reduce the difficulty to fiber fabrication, a novel kind of LMA PCFs are proposed in this paper. In the proposed structure six air-holes near the center are filled with high refractive index material. LMA is obtained from this kind of selectively material-filled structures. The guiding properties such as effective mode areas, chromatic dispersion, and confinement losses are investigated respectively by employing the full vectorial finite element method (FEM). Compared to PCFs with seven missing air holes in the core and other novel structure reported in literature [24], numerical results demonstrate that effective mode area of fundamental mode is improved, flat dispersion is achieved, and confinement losses are still acceptable by applying our novel selectively material-filled technology. Finally, the influences of structure parameters such as air hole diameters, filling hole diameters, and refractive index of filling materials are numerically analyzed respectively. It is helpful for the design and fabrication of our proposed LMA PCFs.

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In the structure 1 (see as Fig. 1 (a)), seven air holes in the core are missing. The diameter of air holes is d, the air-hole pitch isL, and the refractive index of air holes and background Silica are nair and nSi. While in the structure 2 (see as Fig. 1 (b)), the core area with seven missing air holes is doped with different levels and its refractive index is nf. The radii of the inner ring and the outer ring are ra and rc respectively. In our proposed structure 3 (see as Fig.1 (c)), six air-holes near the center core are filled with high refractive index material. Its refractive index is nf and its diameter is d2. The diameter of air holes in the cladding is d1, and the diameter of the center air core is d3. From the designed structure, it can be seen that the complex filling process is not needed in our proposed structure. Especially

Fig. 1. Cross-sections of three kinds of LMA PCFs. (a) Seven missing air holes in the core, (b) structure reported in literature [24], and (c) our proposed structure.

2. Structure and principle Cross-sections of three structural PCFs are shown in Fig. 1 (a)–(c). They are called as structure 1, 2, and 3 respectively in our discussion. The structure 1 has seven missing air holes in the core, the structure 2 is a novel LMA structure reported in literature [24], and the structure 3 is our proposed novel LMA PCFs.

Fig. 2. Mode fields of three kinds of LMA PCFs. (a) Seven missing air holes in the core, (b) structure reported in literature [24], and (c) our proposed structure.

J. Li et al. / Optics & Laser Technology 48 (2013) 375–380

where E is the amplitude of the transverse electric field propagating inside the fiber. LMA will result in a lower power density, can transmit very high laser powers without damage, and can minimize the occurrence of nonlinear effects. Chromatic dispersion is also one of the important factors in designing PCFs. The chromatic dispersion profile can be easily controlled by varying hole diameters and hole-pitch of PCFs. Its total dispersion includes waveguide dispersion and material dispersion. The controllable waveguide dispersion Dg ðlÞ is relative to the structural parameters and can be adjusted according to the design of PCFs. It can be calculated using [18] Dg ðlÞ ¼ 

l d2 Reðneff Þ 2 c dl

ð2Þ

where c is the velocity of light and Re(neff) is the real part of the effective refractive index neff. Confinement loss usually results from small amounts of power leakage between and through the holes of PCFs. It can be calculated by [21] L¼

  40p Imðneff Þ dB=m lnð10Þl

ð3Þ

where Im(neff) is the imaginary part of the effective refractive index neff.

1150 structure1 structure2 structure3

Effective Mode Area (um2)

1100 1050 1000 950 900 850 800 1.2

1.3

1.4

1.5

1.6

1.7

1.8

Wavelength (um) Fig. 3. Effective mode area of different structural PCFs.

2

Chromatic Dispersion (ps/nm*km)

compared to the structure reported in literature [24], the filling of six air holes is easier than that of the ring area. Hence, our proposed structure is easier and can be fabricated more conveniently. Using the FEM method fundamental mode fields of these three kinds of structures can be calculated. They are shown in Fig. 2 (a)–(c). Effective mode area is one of the important parameters in designing PCFs. It can denote how tightly the mode is confined to the core. So it is relative to the core size and doping levels. LMA PCFs are very important in the applications that required generation of high power optical beams. The effective mode area Aeff can be calculated using [24] RR   2  2 E dx dy Aeff ¼ RR 4 ð1Þ jEj dx dy

377

1.5

1

0.5

structure1 structure2 structure3

0

-0.5 1.2

1.3

1.4

1.5

1.6

1.7

1.8

Wavelength (um) Fig. 4. Chromatic dispersion of different structural PCFs.

3. Numerical results Applying for the FEM method, the effective mode area, chromatic dispersion and confinement loss of these three kinds of PCFs are investigated. For simplifying the calculated process and being convenient for comparison to these three kinds of structural PCFs, their structural parameters are set as the same value. It means that the diameter of air holes in cladding is d¼d1 ¼2 mm, all the holepitch L are 12 mm, and the refractive indices of air holes nair and silicon nSi are set as 1.0 and 1.45, respectively. For structure 2 the refractive index of the filling material nf is set as 1.4515, and the radii of inner ring ra and outer ring rc are set as 2 mm and 20 mm, respectively. For the structure 3, nf is also set as 1.4515, the diameter of six filling holes d2 is 4 mm, and the diameter of the center air hole d3 is 4 mm (It equals to 2ra and it means the inner core is same for the structure 2 and 3). Fig. 3 shows the effective mode area of these three different structural PCFs at the given parameters. It can be seen that the effective mode area Aeff of the structure 1 has the smallest value in these three kinds of structures when their structural parameters are set as the same value mentioned above. It means that the effective mode area can be enhanced by the novel filling technology at the given parameters. Furthermore, compared to

the structure 2, the effective mode area Aeff of our proposed structure can be increased further at large wavelength range. Note that the mode area is also relative to the propagating wavelength. It can also be seen from Fig. 3. It means that contrary results might be achieved in the other wavelength for the two kinds of material-filled PCFs. In fact, the intersection of effective mode area to the different structures at specific wavelength has given us the information on the propagating wavelength in the PCF structure. In order to achieve the expected results in the other conditions, the structural parameters of PCFs need to be optimized and adjusted. Chromatic dispersion is also an important factor in designing of PCFs. Relative small and flat dispersion is usually needed in optical communications. Based on Eq. (2) and numerical method, the chromatic dispersion curves of these three different structures are shown in Fig. 4. From the calculated results, it can be seen that they are all relative small and flat near the zero dispersion. Note that the same structural parameter are used in the calculation and the last two kinds of materials-filled PCFs (the structure 2 and 3) can be controlled more easily by modifying structural parameters and refractive index of the filling materials at the same time. While high refractive index materials are filled in the structure of PCFs, confinement loss may change accordingly. Based on

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1150 1100

101

10

0

10-1

structure1 structure2 structure3

10-2

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Confinement loss (dB/km)

102

1050

d2=4um d2=6um d2=8um

1000 950 900 850 800 750

1.2

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1.7

1.8

700 1.2

Wavelength (um)

1.3

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Fig. 5. Confinement loss of three structural PCFs.

1200

d3=4um d3=6um d3=8um

Effective Mode Area (um2)

1150 1100 1050 1000 950 900 850 1.2

1.3

1.4

1.5

1.6

1.7

1.8

Wavelength (um)

nf=1.4515 nf=1.4530 nf=1.4545

1200

Effective Mode Area (um2)

Eq. (3) and the numerical method, confinement losses of these three different structures are shown in Fig. 5. It can be seen that structure 2 and 3 have the lower confinement losses than that of the structure 1. And also the confinement loss of our designed PCFs increases compared to that of the structure 2. However, it is still acceptable for the optical communication and other applications. From the whole numerical results and analyses mentioned above, our proposed novel structural PCFs which high refractive index materials are filled in the six air holes of the first ring have perfect performances. Compared the existing structural PCFs, the effective mode area increases, the chromatic dispersion is still small and flat, and the confinement loss is acceptable. In order to verify the tunable features of our designed structure, the influences of parameters such as diameter d2, d3 and refractive index of filled material nf to effective mode area, chromatic dispersion, and confinement loss, are studied and analyzed respectively. Especially, the refractive index of the filled material can be adjusted by the external parameters and then the features of the PCFs can be designed and adjusted conveniently in the fabrication process or later. It gives us a new method and more chances to adjust the features of the designed PCFs. Firstly, the effective mode area of our proposed LMA PCFs is shown when d2, d3 and nf are modified. The results are shown in Fig. 6 (a)–(c). From the results it can be seen that the effective mode area increases as d2 decreases, d3 increases, and nf decreases. It means that the performance can be adjusted by diversified parameters in our proposed structure. It provides more ways to realize the LMA PCFs. Furthermore, all the effective mode area increases as the wavelength increases at different parameters. It is because longer wavelength optical beams are inclined to the cladding area and then large mode area can be achieved. Secondly, the chromatic dispersion profiles of our designed LMA PCFs are shown in Fig.7 (a)–(c) when the structural parameters d2, d3 and nf are adjusted. From the numerical results it can be seen that almost all the chromatic dispersion profiles of our designed PCFs are relative small and flat, and also they can be controlled easily by the structural parameters and the refractive index of filling materials. Finally the confinement losses of our proposed LMA PCFs are shown in Fig. 8 (a)–(c) as d2, d3 and nf are modified respectively. From the calculated results it can be seen that the influences of parameters to confinement loss are same as those to the effective

1100 1000 900 800 700 600 500 1.2

1.3

1.4

1.5

1.6

1.7

1.8

Wavelength (um) Fig. 6. Effective mode area with different parameters. (a) Effective mode area with different d2, (b) effective mode area with different d3, and (c) effective mode area with different nf.

mode area. It means that the confinement loss increases as d2 decreases, d3 increases, and nf decreases. It is because the confinement loss increases as the effective mode area increases.

J. Li et al. / Optics & Laser Technology 48 (2013) 375–380

102

d2=4um d2=6um d2=8um

2 1.5

Confinement loss (dB/km)

Chromatic Dispersion (ps/nm*km)

2.5

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1 0.5 0 -0.5 -1

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Wavelength (um)

1.7

100 10-1 10-2 10-3

nf=1.4515 nf=1.4530 nf=1.4545

10-4 1.8

Wavelength (um) Fig. 7. Chromatic dispersion with different structural parameters. (a) Chromatic dispersion with different d2, (b) Chromatic dispersion with different d3, and (c) chromatic dispersion with different nf.

10-5 1.2

1.3

1.4

1.5

1.6

1.7

1.8

Wavelength (um) Fig. 8. Confinement loss with different parameters. (a) Confinement loss with different d2, (b) confinement loss with different d3, and (c) confinement loss with different nf.

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Luckily almost all the confinement losses of our designed PCFs are still acceptable for the most applications.

4. Conclusion In this paper a novel LMA PCFs are proposed. LMA is achieved from a new kind of PCFs which are based on selectively materialfilled technology. In the proposed structure six air holes near the center core are filled with high refractive index material. It is simple and easy to fabricate compared with the traditional method achieving large effective mode areas. The transmission characteristics such as effective mode areas, chromatic dispersion, and confinement losses are investigated respectively, and also the influences of structure parameters are numerically analyzed by employing the FEM method. Numerical results demonstrate that effective mode area can improve, small and flat dispersion is achieved, and confinement loss is still acceptable by applying our novel selectively material-filled technology. It provides a new method for designing LMA PCFs with a novel structure. The biggest challenge for this novel LMA PCF is how to find the appropriate material filled in the PCFs and how to fill this material into the structure of PCFs. After it is overcome, we believe this kind of filling technology and designing method will offer large research insight because of its simple working principle and flexible structure. Liquid crystal, soft glass, CS2, toluene, chloroform, and even water are referred to be used as the filled materials in the early works. It provides the probability to achieve diverse PCFs and PCF-based devices with wonderful features.

Acknowledgments This work was supported by Natural Science Foundation of Jiangsu Province, China (Nos. BK2012509 and BK2011114), National Natural Science Foundation of China (Grant No. 61271354), National Science and Technology support Program of Jiangsu Province, China (No. BE2011177) and Research Foundation of PLA University of Science and Technology (No. KYTYZLXY1207). References [1] Knight JC, Birks TA, Russell PSJ, Atkin DM. All-silica single-mode optical fiber with photonic crystal cladding. Optics Letters 1996;21(19):1547–9. [2] Knight JC, Broeng J, Birks TA, Russell PSJ. Photonic band gap guidance in optical fibers. Science 1998;282:1476–8. [3] Wang JY, Jiang C, Hu WS, Gao M. Properties of index-guided PCF with aircore. Optics and Laser Technology 2007;39(2):317–21.

[4] Wang JY, Jiang C, Hu WS, Gao M. Dispersion and polarization properties of elliptical air-hole-containing photonic crystal fibers. Optics and Laser Technology 2007;39(5):913–7. [5] Wang JY, Jiang C, Hu WS, Gao M, Ren H. Modified design of photonic crystal fibers with flattened dispersion. Optics and Laser Technology 2006;38(3): 169–72. [6] Wang JY, Gao MY, Jiang C, Hu W. Design and parametric amplification analysis of dispersion-flat photonic crystal fibers. Chinese Optics Letters 2005;3(7):380–2. [7] Li JH, Wang R, Wang J, Liu Y. Highly birefringent photonic crystal fibers with selectively liquid-filled structure in cladding. Optics Engineering 2011;50(2): 025001–5. [8] Wen K, Peng H, Wang J, Wang R, Li JH. Finite element analysis of a novel weak-pressure sensor based on fiber Bragg grating in photonic crystal fibers. Optics Engineering 2009;48(3):1–5. [9] Li JH, Wang J, Wang R, Liu Y. A novel polarization splitter based on dual-core hybrid photonic crystal fibers. Optics and Laser Technology 2011;43(4): 795–800. [10] Zhang Y, Zhang C, Hu M, Song S, Chai L, Wang C. High-energy subpicosecond pulse generation from a mode-locked Yb-doped large-mode-area photonic crystal fiber laser with fiber facet output. IEEE Photonics Technology Letters 2010;22(5):350–2. [11] Varshney SK, Saitoh K, Koshiba M, Pal BP, Sinha RK. Design of S-band erbiumdoped concentric dual-core photonic crystal fiber amplifiers with ASE suppression. Journal of Lightwave Technology 2009;27(11):1725–33. [12] Mitrofanov AV, Ivanov AA, Alfimov MV, Zheltikov AM. Microjoule supercontinuum generation by stretched megawatt femtosecond laser pulses in a largemode-area photonic-crystal fiber. Optics Communications 2007;280:453–6. [13] Zsigri B, Peucheret C, Nielsen MD, Jeppesen P. Transmission over 5.6 km large effective area and low-loss (1.7 dB/km) photonic crystal fibre. Electronics Letters 2003;39(10):796–8. [14] Ritari T, Niemi T, Ludvigsen H, Wegmuller M, Gisin N, Folkenberg JR. Polarization-mode dispersion of large mode-area photonic crystal fibers. Optics Communications 2003;226:233–9. [15] Knight JC, Birks TA, Cregan RF, Russell PSJ, Sandro JP. Large mode area photonic crystal fibre. Electronics Letters 1998;34(13):1347–498. [16] Gates JC, Hillman CWJ, Baggett JC, Furusawa K, Monro TM, Brocklesby WS. Structure and propagation of modes of large mode area holey fibers. Optics Express 2004;12(5):847–52. [17] Mortensen NA. Effective area of photonic crystal fibers. Optics Express 2002;10(7):341–8. [18] Abdelaziz I, AbdelMalek F, Ademgil H, Haxha S, Gorman T, Bouchriha H. Enhanced effective area photonic crystal fiber with novel air hole design. Journal of Lightwave Technology 2010;28(19):2810–7. [19] Rostami A, Soofi H. Correspondence between effective mode area and dispersion variations in defected core photonic crystal fibers. Journal of Lightwave Technology 2011;29(2):234–41. [20] Matsui T, Zhou J, Nakajima K, Sankawa I. Dispersion-flattened photonic crystal fiber with large effective area and low confinement loss. Journal of Lightwave Technology 2005;23(12):4178–83. [21] Haxha S, Ademgil H. Novel design of photonic crystal fibres with low confinement losses, nearly zero ultra-flatted chromatic dispersion, negative chromatic dispersion and improved effective mode area. Optics Communications 2008;281:278–86. ¨ [22] Huttunen A, Torm a¨ P. Optimization of dual-core and microstructure fiber geometries for dispersion compensation and large mode area. Optics Express 2005;13(2):627–35. [23] Yang S, Zhang Y, Peng X, Lu Y, Xie S, Li J. Theoretical study and experimental fabrication of high negative dispersion photonic crystal fiber with large area mode field. Optics Express 2006;14(7):3015–23. [24] Ademgil H, Haxha S. Bending insensitive large mode area photonic crystal fiber. Optik—International Journal for Light and Electron Optics 2011;122(21): 1950–6.