Advanced Materials Research Vols. 123-125 (2010) pp 419-422 Online available since 2010/Aug/11 at www.scientific.net (2010) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/amr.123-125.419 Effective Cutoff Wavelength Measurement of Bend-insensitive Fiber by Longitudinal Misalignment Loss Method Lin Htein 1, a, Weiwei Fan 2, b, Youngwoong Kim 1,c, Pramod R. Watekar 2, d and 1, 2, e Won-Taek Han 1 Graduate Program of Photonics and Applied Physics, 2 Department of Information and Communications, Gwangju Institute of Science and Technology (GIST), 1 Oryong-dong, Buk-gu, Gwangju 500-712, South Korea a htein77@gist.ac.kr, b fww@gist.ac.kr, c kyw@gist.ac.kr, d pramod@gist.ac.kr, e wthan@gist.ac.kr Keywords: cutoff wavelength, bend-insensitive fiber, misalignment loss Abstract: We propose a simple experimental set-up to determine the effective cutoff wavelength of bend-insensitive optical fibers by applying the longitudinal misalignment loss with spectral transmission. Introduction The cutoff wavelength is the most important parameter for optical fiber because it determines the wavelength at which the light propagates only in a single mode. At wavelengths longer than the effective cutoff wavelength, high-order modes power is below a given level. A standard wellknown to measure an effective cutoff wavelength of an optical fiber is a bend reference technique. In this, the spectral power variation of a 2 m long straight fiber is measured and compared with spectral output power of the fiber with a loop of 60 mm diameter. The effective cutoff wavelength is defined as the wavelength where the long wavelength edge of the bend induced loss is greater than the long wavelength baseline by 0.1 db [1]. Bend-insensitive optical fiber (BIF) has been an important issue in optical-fiber design when fiber-to-the-home becomes significant feature in optical communication system. Because of its superior low bending loss, there is neither standard nor experimental instruments available to determine the cutoff wavelength of the BIF fibers. Many techniques have already been proposed to determine the cutoff wavelength of optical fibers with improved bending [2, 3]. For example, in a simple step index single mode optical fiber, the loss due to longitudinal misalignment of identical fibers is given by using the Gaussian approximation [1, 4], where, in which D is the distance between two fiber ends, is the refractive index of the medium between the fiber ends (for our case, ), is the free space wavelength and is the spot size. In the current communication, we describe a new for evaluating the effective cutoff wavelength of the BIF fibers by measuring the wavelength dependence of longitudinal misalignment loss. Experiments We used various fibers for measurements: a single mode fiber (SMF, ) [5], a commercial bend-insensitive fiber () [6] and two bend-insensitive optical fibers fabricated in our laboratory, (Fiber-3 and ). Fiber-3 [7] with double-trenches and [8] with a singletrench for BIF application were manufactured by using the MCVD and the high temperature drawing processes. The optical parameters of these fibers are listed in Table 1 To measure the longitudinal misalignment loss, we placed two fiber ends at a perfectly aligned position and obtained the spectral variation power. Then, one of the fibers was moved longitudinally to opposite direction and again measured spectral output power. The transmitted power difference between and was calculated as splice loss: All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 203.237.42.50-20/08/10,03:33:37)
420 Multi-Functional Materials and Structures III Table1. Measured parameters of the bend-insensitive optical fibers, Fiber-3[5] and [6]. n max * Δn Trench1 * Δn Trench2 * Core radius ( m) MFD ( m) Bending loss (db/loop) Fiber-3 0.0055-0.005-0.0046 4.45 10.4 0.0095 1.456-0.002 3.05 7.7 0.0485 * n max is the maximum value of core refractive index, Δn Trench1 is inner trench index and Δn Trench2 is outer trench index. Bending loss was measured to be at 1550 nm for 10 mm of the bending radius and MFD was also measured at 1550 nm. The broadband white light source was used as an input source and the optical output power was measured with an optical spectrum analyzer. Each fiber sample had the length of 0.5 m and a few drops of index matching oil were used to leak cladding modes. We also measured the cutoff wavelength of the BIF fibers by using the near-field and farfield transverse misalignment loss to know cutoff wavelength at various splice loss conditions. The transverse offset technique which is well known for measuring mode field diameter of optical fiber is based upon the loss of a misaligned fiber splice. The power transmission coefficient that is a Gaussian function of the transverse offset can be described as, where in which and are spot sizes (separation of two fiber ends at ) of the launched and received fibers, respectively (in our experiments,, for the same fiber), is the far-field spot size of the fiber at separation z, u is the transverse misalignment, z is separation of two fibers and is free space wavelength [1,4]. and the splice loss in decibels can simply express as. To measure transverse misalignment loss, two fiber ends were separated and one of the fibers was moved transversely [9]. The separation distances were maintained less than 5 μm for near-field pattern and 1 mm for far-field pattern. All experimental set-up was the same as that with the longitudinal misalignment loss measurement. Results and Discussion Measurements showing the longitudinal splice loss with various separations for the single mode optical fiber () are illustrated in Fig. 1. The curve shows an abrupt decrement of splice loss between the multimode and single mode regions. Effective cutoff wavelength was approximately identified as a mean value of the upper end and lower end of the transition as shown in Fig.2. Spectral variation of the longitudinal splice loss of various fibers ( to ) is shown in Fig. 3. Transverse movements of the near-field transverse misalignment loss for the SMF () are shown in Fig. 4. The figure exhibits an abrupt increase at the transition between the multimode and the single mode region. The attenuation along the short wavelength edge is due to leakage of the LP 11 mode field. The effective cutoff wavelength was defined as the wavelength at the middle of the lowest and the highest transition levels. Near-field scanning was difficult to measure with accuracy without an excellent end face. Comparing with the other two s, it is also a tedious work to move the fiber in μm range. Far-field scanning is the popular in mode field diameter measurement because it is not stringent about end face cutting. But, this is not applicable for determining the cutoff wavelength without high power broadband light source or highly sensitive detector. It took almost an hour to scan the whole wavelengths with high resolution in the present experiment.
Advanced Materials Research Vols. 123-125 421 1.8 4.0 3.5 3.0 Longitudinal Separation 0.08 mm 1.6 1.4 Longitudinal Separation 0.04 mm 2.5 2.0 0.06 mm 1.2 0.5 0.04 mm 0.02 mm 0.0 600 Fig.1 Typical measurement of longitudinal misalignment loss of with various separations 0.8 c Fig.2 Effective cutoff wavelength determination of 2.4 2.1 1.8 Fiber-3 10 9 8 Transverse movement 8 m 7 1.2 6 6 m 0.9 5 0.6 4 4 m 0.3 3 0.0 2 2 m -0.3 1-0.6 800 900 1000 1100 1200 1300 1400 1500 1600 0 600 Fig. 3 Spectral variations of longitudinal misalignment loss from four different fibers. The separation was 0.04 mm. Fig.4 Typical measurement of transverse misalignment loss of with various separations 4.0 7 6 3.5 5 3.0 4 2.5 3 2.0 2 1 0 Fig. 5 Spectral variations of near-field transverse splice loss from three different fibers. Transverse movement was at 4 m. -1 Fig. 6 Spectral variations of far-field transverse splice loss from three different fibers. Transverse movement was at 0.03 mm. The spectral variations of near-field and far-field misalignment losses for different fibers are shown in Fig.5 and Fig.6, respectively. To estimate the effective cutoff wavelength of the BIFs, we repeated the experiment and the theoretical and the experimental cutoff wavelength values by using different techniques are listed in Table 2. All three s are capable of accurately determining
422 Multi-Functional Materials and Structures III the cutoff wavelength of but the longitudinal shows the better result. The result by the near-field are in good agreement with that by the bending loss for all fibers, while the far-field showed the smallest standard deviation errors for and Fiber-3. The longitudinal and the far-field s did not show good precisions for ; their cutoff wavelengths were shifted towards the shorter wavelength region comparing with the bending loss results. The effective cutoff wavelength of the simple step index single mode optical fiber could be determined within 10 nm range and the estimated cutoff wavelength value varied up to 20 nm for the complicated index profiles. Longitudinal misalignment splice losses are greatly tolerant comparing with transverse and angular misalignment splice losses. Thus high power source was not required and the power was still high enough even with the broadband white light source which we used to measure at longer distances. Note that determination of cutoff wavelength does not depend on peak power. Transmission losses of single mode fiber splices especially depend on the alignment accuracy and end face cutting condition. Unnecessary misalignment conditions and bad end face cutting can increase the standard deviation values to nearly double. Thus longitudinal misalignment loss we demonstrated here can be applied to determine the effective cutoff wavelength of the BIFs at any operating wavelength. Table 2. Cutoff wavelengths estimated by the matrix, standard bending loss, and different misalignment loss s. Theoretical value Bending loss Longitudinal Near-field Far-field * <1240 1195.9 * <1280 1183.2 Fiber-3 1205 1140.4 1350 1207.0 * unknown profile, * 1 Standard deviation error 1195.0 1188.6 1187.4 (4.33)* 1 (2.88)* 1 (7.99)* 1 1195.5 1188.2 1192.9 (5.46)* 1 (2.68)* 1 (0.99)* 1 1134.9 (6.57)* 1 - - 1187.5 1209.2 1174.9 (2.04)* 1 (2.77)* 1 (0.07)* 1 Acknowledgments This work was supported by the Brain Korea-21 Information Technology Project, Ministry of Education and Human Resources Development, and by the GIST Top Brand Project (Photonics 2020), Ministry of Science and Technology. References [1] A. Ghatak, K. Thyagarajan, Introduction to Fiber Optics, Cambridge University Press, USA (1998). [2] D.L. Franzen, Member, IEEE, J.Lightwave Technol., 3 (1), 128-134 (1985). [3] D. Pagnoux and et al., J.Lightwave Technol., 12 (3), 385-391 (1994). [4] D. Marcuse, Bell Syst.Tech. J. 56, 703-718 (1977). [5] Samsung single mode fiber data-sheets (2009). [6] Draka BendBright Single-Mode Fiber data-sheets (2009). [7] P.R. Watekar,S.Ju, and W.-T.Han, Optics Express 17 (22), 20155-20166 (2009). [8] P.R. Watekar,S.Ju, and W.-T.Han, Optics Express 17(12), 10350-10363 (2009). [9] P. R. Watekar, S. Ju, L. Htein and W. T. Han, Optics Express, (accepted, 2010).
Multi-Functional Materials and Structures III doi:10.4028/www.scientific.net/amr.123-125 Effective Cutoff Wavelength Measurement of Bend-Insensitive Fiber by Longitudinal Misalignment Loss Method doi:10.4028/www.scientific.net/amr.123-125.419