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Fast all-fiber polarization scrambling using re-entrant Lefèvre controller
Optics Communications 279 (2007) 50–52 www.elsevier.com/locate/optcom
Fast all-fiber polarization scrambling using re-entrant Lefe`vre controller Yannick Keith Lize´ a,c,d,*, Robert Gomma a, Raman Kashyap a, Leigh Palmer b, A.E. Willner c b
a ´ Ecole Polytechnique de Montre´al, Montre´al, Quebec, Canada H3T 1J4 ARC Special Research Centre for Ultra-Broadband Information Networks, University of Melbourne, Australia c University of Southern California, Los Angeles, CA 90089, United States d ITF Laboratories, 400 Montpellier, Montreal, Quebec, Canada
Received 17 March 2007; received in revised form 20 June 2007; accepted 21 June 2007
Abstract We demonstrate a low-cost mechanical polarization scrambler capable of near 100 kHz scrambling speed using a single modified Lefe`vre controller. By reusing the same controller, many sections can be implemented and the scalability promises higher scrambling rates. The design is wideband, polarization-independent and low-loss providing a degree of polarization of <5%. Ó 2007 Elsevier B.V. All rights reserved.
1. Introduction Polarization effects in optical components and equipment can often be mitigated by depolarizing the input light with polarization scramblers [1–6]. Polarization scramblers have also been employed in optically amplified communication systems to combat polarization hole burning (PHB) [7,8] in the erbium-doped fiber amplifiers (EDFA) and polarization dependent gain. Polarization scrambling is useful in eliminating the polarization sensitivity of certain measurements and in the mitigation and emulation of PMD [9]. Fast polarization scrambling is also useful when using sub-burst error distributed polarization scrambling to mitigate the effects of PMD [10]. Such fast scramblers are typically made of lithium niobate [4,5] or acoustooptic crystals [6] and can therefore be polarization sensitive, have high insertion loss and/or may be costly. Polarization scrambler designs have been demonstrated based on interferometric depolarization [1,2] but such methods tend
* Corresponding author. Address: ITF Laboratories, 400 Montpellier, Montreal, Quebec, Canada. E-mail addresses: yklize@itflabs.com (Y.K. Lize´), [email protected] (A.E. Willner).
0030-4018/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2007.06.050
to distort data signals and can only be used before modulation. In this paper we demonstrate a scalable polarization scrambler which uses a single customized Lefe`vre polarization controller (PC) [11] to implement dynamic scrambling of a modulated signal. We show through Monte Carlo simulations and experimental investigation that the design can perform truly random 100 kHz scrambling speed of a polarized data signal and a degree of polarization (DOP) of <5%. Because of its scalability, the design has the potential of even higher scrambling speed. The design is low-cost with virtually no insertion loss with 95% coverage of the Poincare´ sphere. Most importantly, unlike lithium niobate scramblers, the device is polarization insensitive. 2. Single PC polarization scrambler The single re-entrant technique has been demonstrated to be effective to emulate polarization mode dispersion (PMD) with accurate statistics for all-order PMD [12] and very low background auto-correlation [13]. The single PC scrambler, shown schematically in Fig. 1, consists of a long length of standard single mode fibers (SSMF) spooled on the enlarged paddles of a single customized Lefe`vre PC. The design of the paddles allows for multiple independent
Y.K. Lize´ et al. / Optics Communications 279 (2007) 50–52
51
Fig. 1. Design of the single polarization controller polarization scrambler. After being wound onto the paddles, the single sm fiber is reinserted at the input and wound again. It is repeated to obtain N sections.
fibers to be spooled and the paddle diameters are such that a relative phase retardation between the polarizations of p/ 6 is induced per turn at 1550 nm when using SSMF. This value is calculated from the bent-induced birefringence in single silica fibers [14]. The fiber is spooled on the first paddle a specific number of times to obtain the proper phase retardation, then onto the second paddle and the third paddle before being looped back to the first paddle for the second section. In this manner we reuse the same three paddles for each section instead of a different PC. The PC paddles are motorized and rotated continuously using generic electrical motors over 280°. The speed of each motor is set to be constant but slightly different from one another to ensure that the periodicity of the angles is on the order of several seconds in the experiment. In the experimental setup periodicity is further minimized because of the used low-cost electrical motor with noisy motion. To confirm this we attempted to artificially create periodicity by making all three motors move at the same speed. It turned out to be very difficult to achieve which gives us good confidence in the low chance of periodicity. This design is inspired from the single PC PMD emulator [12,13] design. In the PMD emulator, a piece of highly birefringent (HiBi) fiber is inserted between each section. The scalability allows for unprecedented statistical replication of all-order PMD as well as record-low background auto-correlation function of PMD similar to what is encountered in a long haul fiber optic link. Without the HiBi fiber, the design becomes a polarization scrambler and the increase in the number of section translates into a faster rate of change of the polarization. 3. Results To determine the rate of change of our polarization scrambler we used the method involving a RF spectrum analyzer to properly analyze the time behaviour [13]. Fig. 2 illustrates the trace on an oscilloscope for a cw light going through a 10-section and a 60-section, (i.e. number times through the three paddles) single PC polarization scrambler which then goes through a polarizer and into a detector. Simulation and experimental traces are in good agreement and show that as the number of sections is increased, the polarization rate of change also increases. It appears that the experimental results do not have as much higher frequency components as the simulation
Fig. 2. Simulated and measured optical intensity at the output of a polarizer from a CW laser going through a single PC polarization scrambler with (a) 10-sections and (b) 60-sections.
Fig. 3. Simulated electrical spectra for the 10-section and 60-section scrambler illustrating that the scalability allows for greater scrambling speed. The broad spectrum indicates that the scrambler is truly pseudo random.
results. We attribute this small discrepancy in the mechanical movement of the paddles which did not provide the same polarization transformation as the simulation since the fibers tended to move during rotation. Fig. 3 illustrates the simulated electrical spectra for the 10-section and 60-section scrambler. The broad spectrum indicates that the scrambler is truly pseudo random and that no periodicity is observed. Some scramblers may produce single or a pair of spectral peaks corresponding to repetitive, well-defined predictable movements on the Poincare´ sphere which is not the case in our emulator due to the low periodicity of our paddles angles. We measure the bandwidth for a 60-section scrambler to be around 100 kHz. It is limited by the speed at which the paddles can be mechanically rotated. Since the single PC design is scalable, many sections can be implemented for faster polarization scrambling. It can be noted that the re-entrant technique can also be applied to other types of PC such piezo-electric polarization scrambler. The mechanical scrambling of the Lefe`vre PC limits the scrambling speed whereas piezo-electric polarization scrambler could achieve speed beyond the MHz range. Fig. 4 illustrates the Monte Carlo simulations results of the distribution on the Poincare´ sphere for a 60-section scrambler after one and 10 s. Very uniform coverage of the sphere is observed which was confirmed by the RF
52
Y.K. Lize´ et al. / Optics Communications 279 (2007) 50–52
the design has the potential of even higher scrambling speed. The design is low-cost with virtually no insertion loss with uniform coverage of the Poincare´ sphere. The device is also polarization insensitive. Acknowledgments
Fig. 4. Simulated traces on the Poincare´ sphere after 1 s (left) and 10 s (right) for a 60-section single PC scrambler. Uniform coverage can be obtained with the scalable scrambler.
analysis. We measured experimentally DOP of <5% using a commercial polarization analyzer which is comparable to other types of polarization scramblers. Insertion losses in our experimental demonstration were 4 dB which are much higher than expected. We attribute this high loss to improper splicing between SM fibers. A typical quality splice should provide <0.01 dB of loss so we are confident that this loss can be overcome with better splicing.
Y.K. Lize´ and R. Kashyap would like to acknowledge support from Technology Exploitation and Networking (TEN) program of the Canadian Institute for Photonic Innovations (CIPI) and the Canada Research Chair program. Y.K. Lize´ would like to acknowledge the support of ITF Laboratories in Montreal. References [1] [2] [3] [4] [5] [6] [7] [8]
4. Conclusion We presented a scalable polarization scrambler which uses a single customized Lefe`vre polarization controller to implement dynamic scrambling of a modulated signal. We show through Monte Carlo simulations and experimental investigation that the design can perform 100 kHz scrambling of a polarized data signal and provide a degree of polarization (DOP) of <5%. Because of its scalability,
[9] [10] [11] [12] [13] [14]
K. Bohm et al., J. Lightwave Technol. LT-I (1983) 71. W.K. Bums et al., Opt. Lett. 16 (1991) 381. R. Noe´ et al., Electron. Lett. 30 (1994) 1500. A.D. Kersey, A. Dandridge, Electron. Lett. 23 (1987) 634. F. Heismann, J. Lightwave Technol. 14 (8) (1996) 1801. F. Heismann, R.W. Smith, IEEE J. Selec. Top. Quantum Electron. 2 (2) (1996) 311. M.G. Taylor, IEEE Photon. Technol. Lett. 5 (1993) 1244. N.S. Bergano, in: Conference Opt. Fiber Commun., Ser. OSA, Technical Digest Series Washington, DC, Opt. Soc. Am., vol. 4, Paper FF4, 1994, p. 305. A.E. Willner et al., J. Lightwave Technol. 22 (1) (2004) 106. X. Liu, in: Technical Digest OFC, Los Angeles, CA, February 2004, Paper WE2. H.C. Lefevre, Electron. Lett. 15 (20) (1980) 778. Y.K. Lize´, L. Palmer, N. Godbout, S. Lacroix, R. Kashyap, IEEE Photon. Technol. Lett. 17 (11) (2005) 2451. Y.K. Lize´, L. Palmer, M. Aube´, N. Godbout, S. Lacroix, R. Kashyap, IEEE Photon. Technol. Lett. 18 (1) (2006) 217. S.J. Garth, J. Lightwave Technol. 6 (3) (1988) 445.
Fast all-fiber polarization scrambling using re-entrant Lefe`vre controller Yannick Keith Lize´ a,c,d,*, Robert Gomma a, Raman Kashyap a, Leigh Palmer b, A.E. Willner c b
a ´ Ecole Polytechnique de Montre´al, Montre´al, Quebec, Canada H3T 1J4 ARC Special Research Centre for Ultra-Broadband Information Networks, University of Melbourne, Australia c University of Southern California, Los Angeles, CA 90089, United States d ITF Laboratories, 400 Montpellier, Montreal, Quebec, Canada
Received 17 March 2007; received in revised form 20 June 2007; accepted 21 June 2007
Abstract We demonstrate a low-cost mechanical polarization scrambler capable of near 100 kHz scrambling speed using a single modified Lefe`vre controller. By reusing the same controller, many sections can be implemented and the scalability promises higher scrambling rates. The design is wideband, polarization-independent and low-loss providing a degree of polarization of <5%. Ó 2007 Elsevier B.V. All rights reserved.
1. Introduction Polarization effects in optical components and equipment can often be mitigated by depolarizing the input light with polarization scramblers [1–6]. Polarization scramblers have also been employed in optically amplified communication systems to combat polarization hole burning (PHB) [7,8] in the erbium-doped fiber amplifiers (EDFA) and polarization dependent gain. Polarization scrambling is useful in eliminating the polarization sensitivity of certain measurements and in the mitigation and emulation of PMD [9]. Fast polarization scrambling is also useful when using sub-burst error distributed polarization scrambling to mitigate the effects of PMD [10]. Such fast scramblers are typically made of lithium niobate [4,5] or acoustooptic crystals [6] and can therefore be polarization sensitive, have high insertion loss and/or may be costly. Polarization scrambler designs have been demonstrated based on interferometric depolarization [1,2] but such methods tend
* Corresponding author. Address: ITF Laboratories, 400 Montpellier, Montreal, Quebec, Canada. E-mail addresses: yklize@itflabs.com (Y.K. Lize´), [email protected] (A.E. Willner).
0030-4018/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2007.06.050
to distort data signals and can only be used before modulation. In this paper we demonstrate a scalable polarization scrambler which uses a single customized Lefe`vre polarization controller (PC) [11] to implement dynamic scrambling of a modulated signal. We show through Monte Carlo simulations and experimental investigation that the design can perform truly random 100 kHz scrambling speed of a polarized data signal and a degree of polarization (DOP) of <5%. Because of its scalability, the design has the potential of even higher scrambling speed. The design is low-cost with virtually no insertion loss with 95% coverage of the Poincare´ sphere. Most importantly, unlike lithium niobate scramblers, the device is polarization insensitive. 2. Single PC polarization scrambler The single re-entrant technique has been demonstrated to be effective to emulate polarization mode dispersion (PMD) with accurate statistics for all-order PMD [12] and very low background auto-correlation [13]. The single PC scrambler, shown schematically in Fig. 1, consists of a long length of standard single mode fibers (SSMF) spooled on the enlarged paddles of a single customized Lefe`vre PC. The design of the paddles allows for multiple independent
Y.K. Lize´ et al. / Optics Communications 279 (2007) 50–52
51
Fig. 1. Design of the single polarization controller polarization scrambler. After being wound onto the paddles, the single sm fiber is reinserted at the input and wound again. It is repeated to obtain N sections.
fibers to be spooled and the paddle diameters are such that a relative phase retardation between the polarizations of p/ 6 is induced per turn at 1550 nm when using SSMF. This value is calculated from the bent-induced birefringence in single silica fibers [14]. The fiber is spooled on the first paddle a specific number of times to obtain the proper phase retardation, then onto the second paddle and the third paddle before being looped back to the first paddle for the second section. In this manner we reuse the same three paddles for each section instead of a different PC. The PC paddles are motorized and rotated continuously using generic electrical motors over 280°. The speed of each motor is set to be constant but slightly different from one another to ensure that the periodicity of the angles is on the order of several seconds in the experiment. In the experimental setup periodicity is further minimized because of the used low-cost electrical motor with noisy motion. To confirm this we attempted to artificially create periodicity by making all three motors move at the same speed. It turned out to be very difficult to achieve which gives us good confidence in the low chance of periodicity. This design is inspired from the single PC PMD emulator [12,13] design. In the PMD emulator, a piece of highly birefringent (HiBi) fiber is inserted between each section. The scalability allows for unprecedented statistical replication of all-order PMD as well as record-low background auto-correlation function of PMD similar to what is encountered in a long haul fiber optic link. Without the HiBi fiber, the design becomes a polarization scrambler and the increase in the number of section translates into a faster rate of change of the polarization. 3. Results To determine the rate of change of our polarization scrambler we used the method involving a RF spectrum analyzer to properly analyze the time behaviour [13]. Fig. 2 illustrates the trace on an oscilloscope for a cw light going through a 10-section and a 60-section, (i.e. number times through the three paddles) single PC polarization scrambler which then goes through a polarizer and into a detector. Simulation and experimental traces are in good agreement and show that as the number of sections is increased, the polarization rate of change also increases. It appears that the experimental results do not have as much higher frequency components as the simulation
Fig. 2. Simulated and measured optical intensity at the output of a polarizer from a CW laser going through a single PC polarization scrambler with (a) 10-sections and (b) 60-sections.
Fig. 3. Simulated electrical spectra for the 10-section and 60-section scrambler illustrating that the scalability allows for greater scrambling speed. The broad spectrum indicates that the scrambler is truly pseudo random.
results. We attribute this small discrepancy in the mechanical movement of the paddles which did not provide the same polarization transformation as the simulation since the fibers tended to move during rotation. Fig. 3 illustrates the simulated electrical spectra for the 10-section and 60-section scrambler. The broad spectrum indicates that the scrambler is truly pseudo random and that no periodicity is observed. Some scramblers may produce single or a pair of spectral peaks corresponding to repetitive, well-defined predictable movements on the Poincare´ sphere which is not the case in our emulator due to the low periodicity of our paddles angles. We measure the bandwidth for a 60-section scrambler to be around 100 kHz. It is limited by the speed at which the paddles can be mechanically rotated. Since the single PC design is scalable, many sections can be implemented for faster polarization scrambling. It can be noted that the re-entrant technique can also be applied to other types of PC such piezo-electric polarization scrambler. The mechanical scrambling of the Lefe`vre PC limits the scrambling speed whereas piezo-electric polarization scrambler could achieve speed beyond the MHz range. Fig. 4 illustrates the Monte Carlo simulations results of the distribution on the Poincare´ sphere for a 60-section scrambler after one and 10 s. Very uniform coverage of the sphere is observed which was confirmed by the RF
52
Y.K. Lize´ et al. / Optics Communications 279 (2007) 50–52
the design has the potential of even higher scrambling speed. The design is low-cost with virtually no insertion loss with uniform coverage of the Poincare´ sphere. The device is also polarization insensitive. Acknowledgments
Fig. 4. Simulated traces on the Poincare´ sphere after 1 s (left) and 10 s (right) for a 60-section single PC scrambler. Uniform coverage can be obtained with the scalable scrambler.
analysis. We measured experimentally DOP of <5% using a commercial polarization analyzer which is comparable to other types of polarization scramblers. Insertion losses in our experimental demonstration were 4 dB which are much higher than expected. We attribute this high loss to improper splicing between SM fibers. A typical quality splice should provide <0.01 dB of loss so we are confident that this loss can be overcome with better splicing.
Y.K. Lize´ and R. Kashyap would like to acknowledge support from Technology Exploitation and Networking (TEN) program of the Canadian Institute for Photonic Innovations (CIPI) and the Canada Research Chair program. Y.K. Lize´ would like to acknowledge the support of ITF Laboratories in Montreal. References [1] [2] [3] [4] [5] [6] [7] [8]
4. Conclusion We presented a scalable polarization scrambler which uses a single customized Lefe`vre polarization controller to implement dynamic scrambling of a modulated signal. We show through Monte Carlo simulations and experimental investigation that the design can perform 100 kHz scrambling of a polarized data signal and provide a degree of polarization (DOP) of <5%. Because of its scalability,
[9] [10] [11] [12] [13] [14]
K. Bohm et al., J. Lightwave Technol. LT-I (1983) 71. W.K. Bums et al., Opt. Lett. 16 (1991) 381. R. Noe´ et al., Electron. Lett. 30 (1994) 1500. A.D. Kersey, A. Dandridge, Electron. Lett. 23 (1987) 634. F. Heismann, J. Lightwave Technol. 14 (8) (1996) 1801. F. Heismann, R.W. Smith, IEEE J. Selec. Top. Quantum Electron. 2 (2) (1996) 311. M.G. Taylor, IEEE Photon. Technol. Lett. 5 (1993) 1244. N.S. Bergano, in: Conference Opt. Fiber Commun., Ser. OSA, Technical Digest Series Washington, DC, Opt. Soc. Am., vol. 4, Paper FF4, 1994, p. 305. A.E. Willner et al., J. Lightwave Technol. 22 (1) (2004) 106. X. Liu, in: Technical Digest OFC, Los Angeles, CA, February 2004, Paper WE2. H.C. Lefevre, Electron. Lett. 15 (20) (1980) 778. Y.K. Lize´, L. Palmer, N. Godbout, S. Lacroix, R. Kashyap, IEEE Photon. Technol. Lett. 17 (11) (2005) 2451. Y.K. Lize´, L. Palmer, M. Aube´, N. Godbout, S. Lacroix, R. Kashyap, IEEE Photon. Technol. Lett. 18 (1) (2006) 217. S.J. Garth, J. Lightwave Technol. 6 (3) (1988) 445.