Author name used in this publication: Ming-Leung Vincent TseRefereed conference paper2013-2014 > Academic research: refereed > Refereed conference paperVersion of RecordPublishe

Birks

Chen

Guan

Ortigosa-Blanch

Liu, Z

Wu, C

Tse, V

Lu, C

Tam, HY

PolyU Institutional Repository

W3.4.pdf Workshop on Specialty Optical Fibers and their Applications© OSA 2013Ultrahigh birefringence index-guiding photonic crystal fiber  Zhengyong Liu,1 Chuang Wu,1,2 Ming-Leung Vincent Tse,1 Chao Lu,3 and Hwa-Yaw Tam1,* 1Photonics Research Center, Department of Electrical Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong 2Institute of Photonics Technology, Jinan University, Guangzhou 510632, China 3Photonics Research Center, Department of Electronic and Information Engineering, The Hong Kong Polytechnic University,  Kowloon, Hong Kong *Author e-mail address: (hwa-yaw.tam@polyu.edu.hk)  Abstract: We reported an ultrahigh birefringence PCF with a germanium-doped elliptical core, which is fabricated in our lab using the stack-and-draw method, and an ultrahigh birefringence of 1.1×10-2 is obtained experimentally via FBG and Sagnac loop. OCIS codes: (060.2280) Fiber design and fabrication; (060.5295) Photonic crystal fibers; (060.3735) Fiber Bragg gratings  1. Introduction Photonic crystal fibers (PCFs) have been attracting great interest in the research community since the first endlessly single-mode PCF was reported in 1997 [1]. To realize single polarization device with zero polarization mode dispersion, polarization-maintaining fibers (PMFs) with high birefringence are needed. The PCF technology is far more superior in producing PMFs. High birefringence can be achieved simply by introducing two large air holes close to the core of conventional single-mode PCF. Most PMFs exhibit birefringence in the order of 10-4 to 10-3. For the commonly used commercial PM-1550-01™ fiber, the birefringence is ~3.9×10-4. The highest birefringence reported for fabricated index-guiding PCFs was in the order of 10-3 [2,3]. It should be noted that the highest group birefringence reported were found in a photonic bandgap fiber (0.025 at the wavelength of 1550nm), achieved via elliptical air-core [4]. However, such fiber does not allow the inscription of fiber gratings in its core. There are also some other designs of PCF to obtain high birefringence that can reach up to 10-2, such as the elliptical-like holes structure [5]. However, the structure is difficult to fabricate compared to the two large holes PCF and no fabricated elliptical-like holes fiber was reported. In this paper, we report on an ultrahigh birefringence index-guiding PCF (with two large holes) with measured phase modal birefringence of ~1.1×10-2 and group birefringence of ~1.25×10-2. The phase modal birefringence was measured by inscribing FBG in Ge-doped core, resulting in two distinct Bragg peaks separated by over 12 nm. A Sagnac loop with a very short length was set up to measure the group birefringence. The experimental value of birefringence was confirmed by simulation as well, giving the values of 1.4×10-2 for Bp and ~1.5×10-2 for Bg. 2. Fabrication of Hi-Bi PCF The Hi-Bi PCF was fabricated in-house using the stack-and-draw technique on a fiber drawing tower (Nextrom OFC20). The fabrication process is similar to the fabrication method conducted by us previously for a twin-core PCF [6]. Capillaries with diameter of 1.6 mm were firstly drawn from a silica tube that has an outer diameter of 44 mm and inner diameter of 17.6 mm. Meanwhile, pure silica rods with different diameters were drawn for stacking. During stacking, a germanium-doped rod (1 mm in diameter) was used to replace the center capillary and will eventually become the core of the Hi-Bi PCF. Two large capillaries, with a diameter of 1.6mm and air fraction of 0.76, were inserted adjacent to the core. The stacked preform was then drawn into canes with a diameter of 1.6 mm. One cane was chosen and inserted into a jacket tube of 7 mm in diameter, which was stretched from a silica tube with inner and outer diameters of 4 mm and 12 mm, respectively. This ensemble was then drawn into fiber at the temperature of 1870 ˚C. Lower temperature and vacuum control were used to enlarge the air holes, especially the two big holes so that an elliptical core can be formed to obtain high birefringence.  XY(a) (b)  Fig. 1 The SEM photos of the cross-section of the Hi-Bi PCF Fig.1 shows the SEM photos of the fabricated fiber. x and y denotes the fast and slow axis respectively. The outer diameter of the fiber is ~125µm. The small holes have diameter of ~2.35µm and pitch of ~3.04µm. The two W3.4.pdf Workshop on Specialty Optical Fibers and their Applications© OSA 2013holes adjacent to the core were enlarged with size of 4.3µm×3.81µm. Consequently, the core became elliptical and has a dimension of ~3µm×1µm. The measured propagation loss of the PCF is ~0.2dB/m. And the splicing loss of the fiber to SMF28 is ~2.1dB. 3. The measurement of birefringence Simulation of the Hi-Bi PCF was performed using finite element method. The refractive index for core is set as 1.47 and 1.444 for the silica. Fig. 2(a) illustrates the phase modal birefringence (Bp) as a function of different wavelength. It can be seen that the birefringence is in the order of 10-2 for wavelengths beyond ~1300nm. The effective refractive indices for fast and slow axis at 1550nm are 1.370937 and 1.384772, respectively, showing a phase birefringence (Bg =Δn) of 1.4×10-2. High birefringence was confirmed by writing FBG in the fiber using the phase mask technique. Two phase masks were used separately in two measurements with pitches of 1072.21 nm and 1110.3 nm. Fig. 2(b) shows the reflection spectra of two FBGs. The two reflective peaks, corresponding to LP01-x and LP01-y are separated by more than 12nm, much larger than that of FBG inscribed in commercial PM-PCF (<1nm) [7]. The large peak separation indicates ultrahigh Bp of 1.1×10-2, which is close to the simulation value. Phase birefringence of Hi-Bi PCF vs wavelengthFit of Phase birefringence vs wavelengthQuadratic fitting:Δnp=6.4×10-9λ2-5.5×10-9λ-7.08×10-4Phase birefringence0.0050.0100.015Wavelength (nm)800 900 1000 1100 1200 1300 1400 1500 1600LP01-x LP01-yΛp=1072.21nm, L=12mmΔλ=10.796nm,  Δn=1.007×10-21472.712nm1483.508nmΛp=1110.3nm, L=12mmΔλ=12.072nm,  Δn=1.087×10-21522.816nm 1534.888nmReflectioin (dBm)−25−20Reflection (dBm)−35−30Wavelength (nm)1470 1475 1480 14851520 1525 1530 1535(a) (b)  Fig. 2 (a) The phase modal birefringence of Hi-Bi PCF as a function of wavelength, insets show the intensity  profile of LP01-x and LP01-y (b) Reflection spectra of FBGs written on the fiber Fig. 3(a) illustrates the group birefringence (Bg) as a function of wavelength and the inset shows the relationship of Bp and Bg. The curve shows that the Hi-Bi PCF also has ultrahigh Bg, larger than 10-2 beyond the wavelength of 1200nm. The Bg at 1550nm is calculated to be 1.5×10-2. To confirm this, a Sagnac interferometer was constructed using the Hi-Bi PCF to determine Bg. Due to the ultrahigh group birefringence, PCF with a short length of 5.5cm is sufficient to obtain a reasonable fringe spacing. The transmission of Sagnac loop is shown in Fig. 3(b), with a spacing of ~3.48nm between two adjacent minimums. Therefore, the measured group birefringence is 1.255×10-2, which is reasonably close to the simulation value. Group birefringence of Hi-Bi PCF vs wavelengthFit of Group birefringence vs wavelengthCubic fitting:Δng=-7.07×10-12λ3+2.57×10-8λ2                 -1.54×10-5λ+3.9×10-3Group birefringence0.0040.0060.0080.0100.0120.0140.016Wavelength (nm)800 1000 1200 1400 1600Fringe spacing at ~1550nm: ~3.48nmGroup birefringence: Δng=λ2/(Δλ·L)=1.255×10-2Transmission (dB)−30−25−20−15−10Wavelength (nm)1530 1540 1550 1560 1570(a) (b)Bp(λ) = nx − nyBg(λ) = ng,x − ng,y = Bp(λ)−λdBp(λ)dλ Fig. 3 (a) The group birefringence of Hi-Bi PCF as a function of wavelength and  (b) the transmission spectrum of Sagnac interferometer using Hi-Bi PCF. References  [1] T. a Birks, J. C. Knight, and P. S. Russell, "Endlessly single-mode photonic crystal fiber.," Optics Letters 22, 961–3 (1997). [2] A. Ortigosa-Blanch, J. Knight, and W. Wadsworth, "Highly birefringent photonic crystal fibers," Optics Letters 25, 1325–1327 (2000). [3] A. Ortigosa-Blanch, "Ultrahigh birefringent nonlinear microstructured fiber," IEEE Photonics Technology Letters 16, 1667–1669 (2004). [4] X. Chen, M.-J. Li, N. Venkataraman, M. Gallagher, W. Wood, A. Crowley, J. Carberry, L. Zenteno, and K. Koch, "Highly birefringent hollow-core photonic bandgap fiber.," Optics Express 12, 3888–93 (2004). [5] D. Chen and G. Wu, "Highly birefringent photonic crystal fiber based on a double-hole unit," Appl. Opt. 49, 1682–1686 (2010). [6] Z. Liu, M.-L. V. Tse, C. Wu, D. Chen, C. Lu, and H.-Y. Tam, "Intermodal coupling of supermodes in a twin-core photonic crystal fiber and its application as a pressure sensor," Optics Express 20, 21749–21757 (2012). [7] B. Guan, D. Chen, and Y. Zhang, "Bragg gratings in pure-silica polarization-maintaining photonic crystal fiber," IEEE Photonics Technology Letters, 20, 1980–1982 (2008).  

Ultrahigh birefringence index-guiding photonic crystal fiber

Zhengyong Liu

Chuang Wu

Ming-Leung Vincent Tse

Chao Lu

Hwa-Yaw Tam

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We reported an ultrahigh birefringence PCF with a germanium-doped elliptical core, which is fabricated in our lab using the stack-and-draw method, and an ultrahigh birefringence of 1.1×10-2 is obtained experimentally via FBG and Sagnac loop.Department of Electrical EngineeringDepartment of Electronic and Information EngineeringAuthor name used in this publication: Ming-Leung Vincent TseRefereed conference pape

The Hong Kong Polytechnic University Pao Yue-kong Library

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Ultrahigh birefringence index-guiding photonic crystal fiber

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