Fiber loop reflector as a versatile all-fiber component

Fiber loop reflector as a versatile all-fiber component B.P. Pal 1, * G. Thursby, * Naveen Kumar, ** and M.R. Shenoy ** * Department of Electronic and Electrical Engineering University of Strathclyde, Glasgow G1 1XW, U.K. ** Department of Physics, Indian Institute of Technology Delhi New Delhi 110016, India ABSTRCT All-fiber loop reflectors are finding many applications in diverse areas in linear as well as nonlinear fiber optics. The inherent birefringence in the fiber loop due to the presence of bends and twists can change the reflectance/transmittance of the fiber loop mirror configuration. A simulation study of fiber loop mirror realized with an over-coupled fiber coupler, for application to gain flattening of erbium-doped fiber amplifier, is carried out. Based on this study, an overcoupled coupler with free spectral range (FSR) = 140 nm is fabricated, and a loop mirror is physically realized. We introduced an appropriate magnitude of birefringence in the loop, and flattened the amplified spontaneous emission spectrum of the erbium-doped fiber. By appropriately adjusting the orientation of two birefringent elements in the loop, flatness within ± 0.5 db is achieved over a wavelength range of 30 nm in the C- band. 1. INTRODUCTION Fiber loop mirror (FLM) as an all-fiber component has been shown to find extensive applications in configuring highspeed switches, optical time division multiplexer, and fiber lasers etc. [1]. More specifically, it has been exploited in soliton switching with a fiber nonlinear mirror [2], in ultrafast all-optical demultiplexing [3], in all optical demultiplexing of TDM data at 250 Gb/s [4], as nonlinear amplifying loop mirror [5], as an all-optical loop mirror switch employing an asymmetric amplifier/attenuator configuration [6], and so on. An FLM is formed through a fused fiber coupler, in which throughput and coupled ports are joined so that essentially a fiber loop having two ports is formed. It is easy to show that the loop along with the coupler together function like a 100% reflector if the coupler is a perfect 3-dB splitter/combiner, i.e. when a light beam is injected into one of its two ports, the entire light would return to the same port [7]. With the emergence of dense wavelength division multiplexing (DWDM), information carrying capacity of a fiber optic communication link could be greatly increased. A modern DWDM system operates in the low-loss window around 1550 nm with erbium-doped fiber amplifiers (EDFA) forming an integral part of it. The ASE (and accordingly gain) spectrum of a typical EDFA [8] (see Fig. 1) shows a peak around 1530 nm, which reduces the available flat gain-band. This often imposes a serious limitation on the number of channels that could be accommodated within the gain bandwidth of an EDFA due to inherent problem with regard to optical SNR. In general, the gain profile of an EDFA can be flattened by modifying the material composition in the erbium-doped fiber [9], or by the use of optical filters [10-12] to compensate for the variations in the gain spectrum. Various kinds of optical filters have been demonstrated for this application, including long-period fiber gratings [10], fiber Bragg gratings [11], and acousto-optic tunable filters [12]. Recently, use of high birefringent fiber loop mirror (Hi-Bi FLM) has been proposed [13] for gain flattening of EDFA. But, the involvement of Hi-Bi fiber leads to higher termination loss and makes the technique less attractive. In this talk, after a brief review of the utility of FLM as a switch, resonator, etc., we would report simulation and experimental results of gain flattening of EDFA using all-fiber loop reflector, realised with an over-coupled fiber directional coupler, formed with a conventional single-mode fiber SMF-28. 1 On leave of absence from IIT Delhi as an Academic Visitor

Fig. 1: Typical amplified spontaneous emission (ASE) spectrum of an EDFA. 2. THEORY In an FLM configuration, polarization plays an important role because the fiber loop acts as a wave plate [14] due to the presence of bends and twist etc. The amount of birefringence introduced by the fiber loop is equivalent to that of a wave plate [14-15] with retardation φ and oriented at an angle θ, and depends on the orientation of the plane of the fiber loop. This birefringence modulates the phases of the fields traveling round the loop in opposite direction. The input light is totally or partially reflected and transmitted, depending on the nature of interference between the clockwise and counterclockwise propagating fields through the loop. Thus, rotating the wave plate can alter the reflectance and the transmittance of the FLM. Mortimore [14] and Morishita [15] reported detailed mathematical formulation to study the effect of rotating the plane of loop on the reflectance/ transmittance of the FLM configuration, and they could explain the experimental results successfully. But, the birefringence present is not sufficient to produce a wavelength filtering action if the FLM is constructed using conventional single-mode fiber. To accomplish this, the concept of Hi-Bi FLM was introduced [16], and by inserting multi-sections of Hi-Bi fibers inside the loop, fan-solc filters [16] and Lyot-type [17] filters were realized. In this paper, we demonstrate use of an over-coupled coupler, fabricated with SMF 28 fiber, to form an all-fiber loop, in which introduction of bend-induced birefringence along with the angular displacement of the FLM w.r.t. the coupler enables flattening of the amplified spontaneous emission (ASE) spectrum of EDFA. It is known that the free spectral range (FSR) in the wavelength response of a fused taper over-coupled coupler can be tuned with relatively low loss by twisting the fused region of the coupler [15]. This wavelength tunable feature could be exploited for tuning the wavelength characteristics of fiber lasers, fiber filters, and nonlinear fiber loop mirrors. The FSR of the transmission/reflection spectrum of the loop reflector, formed with an over coupled coupler, is one half of the FSR of the coupler (which forms the fiber loop). Thus, the required FSR of the coupler-based FLM, to be used as a filter for gain flattening of the ASE spectrum of an EDFA over a wavelength range of 35 nm (from 1525 1560 nm) is = 70 nm. We simulated the transmittance of an all-fiber loop reflector, formed with a over-coupled coupler with an FSR of 70 nm and a transmission dip at a wavelength of approximate 1530 nm, for different retardation and orientation. The corresponding wavelength response is shown in Fig. 2.

0 Transmitted Power (db) -2-4 -6-8 -10-12 θ = π /4 φ = 43.88π rad 16.26π rad 14.37π rad -14 1520 1530 1540 1550 1560 1570 1580 Wavelength (nm) Fig. 2: Simulated spectral response at the transmitted port of an FLM with FSR = 35 nm for different values of birefringence (φ) inside the fiber loop. 3. EXPERIMENT AND RESULTS We fabricated the over-coupled fiber coupler by using the in-house developed fused fiber coupler fabrication rig comprising of an oxy-butane microflame burner. The FSR of an over-coupled coupler was seen to decreases exponentially [15] with increase in the number of coupling cycles observed during the fabrication process. But the number of cycles required for achieving a particular FSR depends on the degree of fusion in the fused region; this may vary from one rig to another. In our experiment, the number of coupling cycles was about 24. The wavelength spectrum of the coupler thus fabricated, and the transmittance of the loop mirror realized with this coupler are shown in Fig. 3. The FSR of the coupler is about 140 nm and the excess loss of the coupler is 2.8 db. Fig. 3: Wavelength spectrum of the over-coupled fiber coupler at the coupled- and throughput ports, and that of the loop mirror at its transmitted port. The coupler used to form the FLM was an approximately 3 db coupler at the wavelength 1532 nm (see Fig.3), which suggests a dip at the same wavelength in the spectral response of the loop mirror at the transmitted port. It can be seen

that the FSR of the loop at the transmitted port is one half of the FSR of the coupler, as expected. A schematic of the experimental setup is shown in Fig. 4. Here by PC 1 and PC 2, we imply two polarization controllers, which were essentially two cylindrical rods around which small lengths of the fiber (constituting the loop of the FLM) were wound so as to introduce bend-induced birefringence. The rods were mounted on frames that can be rotated. Er doped SFS Circulator Coupler PC 1 PC 2 Output Port 2 Fig. 4: Schematic of the experimental set-up used in gain flattening of the ASE spectrum of EDFA. By appropriately adjusting the orientation of the polarization controller (PC 1 ), we obtained a flat pass band of 15 nm from 1541 nm to 1556 nm; the flatness was within ± 0.2 db (see Fig. 5). Fig. 5: Typical gain flattened spectra of EDFA for different orientations of only one polarization controller (PC 1 ) with the loop mirror layout. Subsequently a second polarization controller PC 2, in addition to PC 1, was introduced within the layout of the loop. By appropriately adjusting the orientation of both the polarization controllers (PC 1 and PC 2 ), flatness within ± 0.5 db could be achieved over a range of 30 nm in the C-band. The wavelength response at the transmitted port, for different orientation of PC1 and PC 2 is shown in Fig. 6. Precise flatness in the gain spectra was experimentally found to be realizable through adjusting the number of fiber turns in PC 1 and PC 2.

Fig. 6: Typical gain flattened spectra of EDFA for different orientations of two polarization controllers in the loop mirror configuration. 4. DISCUSSION From Fig. 6, it is clear that we were able to flatten the gain profile of an EDFA using the fabricated over-coupled coupler with an FSR = 140 nm. However, simulation revealed that optimum FSR for flattening the gain spectrum of an EDFA should be about 70 nm. Any large FSR would incur insertion losses even though the filter might have the desired notch at 1530 nm. By incorporating two or more polarization controllers in an FLM layout, the degree of freedom for controlling gain flatness was increased. However, there could be other combinations of the orientations of the two polarization controllers, which might as well flatten the gain spectrum as much or still better. One has to tackle the issue experimentally. The insertion losses that incurred during the experiment were mostly due to the excess loss of the coupler and in this case the excess loss of the coupler was 2.8 db. The fused fiber coupler fabrication technique is well developed and couplers with FSR less than 50 nm, having excess losses 0.2 db, have been reported [15]. The method of using an over-coupled coupler to replace the Hi-Bi fiber sections in the loop [13] for gain flattening of the ASE spectrum of EDFA, in our opinion, is a better approach. In this technique, we utilized only the communication grade SMF-28 fiber and the losses incurred are also less. It may not always be easy to fabricate an LPG with a dip of particular depth at a particular wavelength. We believe that this technique could be exploited to tune the loss characteristics of an LPG. For example, an LPG could be placed within the layout of the loop mirror and its loss characteristics could be modulated through the polarization controllers. 5. CONCLUSION All-fiber loop reflector is a versatile component and some of its applications in the field of linear and nonlinear fiber optics have been observed. We simulated the fiber loop mirror configuration, realized with an over-coupled fiber coupler, for application to gain flattening of erbium-doped fiber amplifier. Based on this study, an over-coupled coupler with free spectral range (FSR) of 140 nm was fabricated, and a loop mirror was physically realized. We introduced an appropriate bend-induced birefringence inside the loop layout and flattened the ASE spectrum of an EDFA. By adjusting the orientation of the two polarization controllers (PC 1 and PC 2 ), flatness within ± 0.5 db has been achieved over a range of 30 nm in the C-band. REFERENCES 1. N. J. Doran and D. Wood, Opt. Lett., 13,56, (1988) 2. M. N. Islam, E. R. Sunderman, R. H. Stolen, W. Pleibal, and J. R. Simpson,Opt. Lett., 15, 811, (1989) 3. K. J. Blow, N. J. Doran, and B. P. Nelson, Electron. Lett., 26, 262, (1990) 4. I. Glask, J. P. Sokoloff, and P. R. Prucnal, Electron. Lett., 30, 339, (1990)

5. M. E. Fermann, F. Haberl, M. Hofer, and H. Hochreiter, Opt. Lett., 15, 752, (1990) 6. A. W. Oneill, and R. P. Woff, Electron. Lett., 26,2008, (1990) 7. B. P. Pal, All-fiber waveguide components, in Electromagnetic fields in unconventional structures and materials, O. N. Singh and A. Lakhtakia, eds. (John Wiley, New York, 2000), pp. 359-432. 8. J. Kaur, S. Sinha Roy, K. Thyagarajan, B. P. Pal, Opt. Fib. Tech., 5, 390-402 (1999). 9. M. Yamada, T. Kanamori, Y. Terunuma, K. Oikawa, M. Shimizu, S. Sudo, and K. Sagawa, IEEE Photon. Technol. Lett., vol. 8, pp.882-884, June1996 10. P. F. Wysocki, J. B. Judkins, R. P. Espindola, M. Andrejco, and A. M. Vengsarkar, IEEE Photon. Technol. Lett., vol. 9, pp. 1343-1345, Oct. 1997 11. S. K. Liaw, K. P. Ho, and S. Chi, IEEE Photon. Technol. Lett., vol. 11, pp.797-799, July1999 12. R. Feced, C. Algeria, M. N. Zervas, and R. I. Laming, IEEE J. Select. Top. Quantum. Electron., vol. 5, no. 3, pp.1278-1288, 1999 13. S. Li., K. S. Chiang, and W. A. Gambling, IEEE Photon. Technol. Lett., vol. 13, pp.942-944, Sept. 2001 14. D. B. Mortimore, J. Lightwave Technol., vol. 6, pp. 1217-1224, July 1988 15. K. Morishita, and K. Shimanto, J. Lightwave Technol., vol. 13, pp. 2276-2281, Nov. 1995 16. X. Fang, and R. O. Claus, Opt. Lett., vol. 20, pp. 2146-2148, Oct. 1995 17. X. Fang, H. Ji, C. T. Allen, K. Demarest, and L. Pelz, IEEE Photon. Technol. Lett., vol. 9, pp.458-460, April 1997 Corresponding author s e-mail: bppal@physics.iitd.ernet.in