Fiber optical parametric amplifier with doublepass pump configuration Kwok Shien Yeo,1 Faisal Rafiq Mahamd Adikan,2 Makhfudzah Mokhtar,1 Salasiah Hitam,1 and Mohd Adzir Mahdi1,* 1
Wireless and Photonics Networks Research Centre, Faculty of Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia 2 Department of Electrical Engineering, Faculty of Engineering, University of Malaya, Malaysia * [email protected]
Abstract: Characteristics of fiber optical parametric amplifier (FOPA) with double-pass pump configuration are experimentally investigated. The double-pass pump FOPA exhibits more than two-fold steeper gain slope in comparison to the conventional FOPA due to elongation of effective fiber length. In the L-band amplification band, a secondary idler is generated and used as the transmission signal in lieu of the original L-band signal. Gain measurement and bit error rate experiments are performed on the secondary idler and the results prove the usability of secondary idler, which is potentially useful for distribution networks. ©2013 Optical Society of America OCIS codes: (190.4970) Parametric oscillators and amplifiers; (060.4370) Nonlinear optics, fibers;
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
M. E. Marhic, Fiber Optical Parametric Amplifiers, Oscillators and Related Devices (Cambridge University, 2008). T. Torounidis, P. A. Andrekson, and B.-E. Olsson, “Fiber-optical parametric amplifier with 70-dB gain,” IEEE Photon. Technol. Lett. 18(10), 1194–1196 (2006). J. D. Marconi, J. M. C. Boggio, and H. L. Fragnito, “Nearly 100 nm bandwidth of flat gain with a doublepumped fiber optic parametric amplifier,” in Conference on Optical Fiber Communication and the National Fiber Optic Engineers Conference, 2007. OFC/NFOEC 2007 (2007), pp. 1–3. T. Torounidis and P. Andrekson, “Broadband single-pumped fiber-optic parametric amplifiers,” IEEE Photon. Technol. Lett. 19(9), 650–652 (2007). Z. Tong, C. Lundström, P. A. Andrekson, M. Karlsson, and A. Bogris, “Ultralow noise, broadband phasesensitive optical amplifiers, and their applications,” IEEE J. Sel. Top. Quantum Electron. 18(2), 1016–1032 (2012). M. E. Marhic, G. Kalogerakis, and L. G. Kazovsky, “Gain reciprocity in fibre optical parametric amplifiers,” Electron. Lett. 42(9), 519–520 (2006). Z. Tong, C. Lundström, M. Karlsson, and P. A. Andrekson, “Noise figure non-reciprocity in fiber optical parametric amplifiers with zero-dispersion-wavelength variations,” in 2010 36th European Conference and Exhibition on Optical Communication (ECOC) (2010), pp. 1–3. Z. Tong, A. Bogris, M. Karlsson, and P. A. Andrekson, “Full characterization of the signal and idler noise figure spectra in single-pumped fiber optical parametric amplifiers,” Opt. Express 18(3), 2884–2893 (2010). K. K. Y. Wong, M. E. Marhic, K. Uesaka, and L. G. Kazovsky, “Polarization-independent one-pump fiberoptical parametric amplifier,” IEEE Photon. Technol. Lett. 14(11), 1506–1508 (2002). G. Kalogerakis, M. E. Marhic, K. Uesaka, K. Shimizu, K. K. Y. Wong, and L. G. Kazovsky, “Methods for full utilization of the bandwidth of fiber optical parametric amplifiers and wavelength converters,” J. Lightwave Technol. 24, 3683–3690 (2006). T. Torounidis, B. E. Olsson, H. Sunnerud, M. Karlsson, and P. A. Andrekson, “Fiber-optic parametric amplifier in a loop mirror configuration,” IEEE Photon. Technol. Lett. 17(2), 321–323 (2005). K. S. Yeo, F. R. M. Adikan, M. Mokhtar, S. Hitam, and M. A. Mahdi, “Continuous wave tunable fiber optical parametric oscillator with double-pass pump configuration,” Appl. Phys. B 110(3), 353–357 (2013). K. S. Yeo, F. R. Mahamd Adikan, M. Mokhtar, S. Hitam, and M. A. Mahdi, “Gain smoothening filter in twosegment fiber-optical parametric amplifier,” Opt. Commun. 286, 353–356 (2013). G. Rustad, G. Arisholm, and Ø. Farsund, “Effect of idler absorption in pulsed optical parametric oscillators,” Opt. Express 19(3), 2815–2830 (2011). C. Jauregui, D. Nodop, J. Limpert, and A. Tünnermann, “Improved parametric generation of light in optical fibers,” in Lasers and Electro-Optics Europe (CLEO EUROPE/EQEC), 2011 Conference on and 12th European Quantum Electronics Conference (2011), p. 1.
#197476 - $15.00 USD Received 10 Sep 2013; revised 23 Nov 2013; accepted 4 Dec 2013; published 13 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031623 | OPTICS EXPRESS 31623
16. M. Karlsson, “Four-wave mixing in fibers with randomly varying zero-dispersion wavelength,” J. Opt. Soc. Am. B 15(8), 2269–2275 (1998). 17. S. Watanabe, T. Naito, and T. Chikama, “Compensation of chromatic dispersion in a single-mode fiber by optical phase conjugation,” IEEE Photon. Technol. Lett. 5(1), 92–95 (1993). 18. J. Hansryd, P. A. Andrekson, M. Westlund, J. Li, and P. O. Hedekvist, “Fiber-based optical parametric amplifiers and their applications,” IEEE J. Sel. Top. Quantum Electron. 8(3), 506–520 (2002).
1. Introduction Fiber optical parametric amplifier (FOPA) is an all-optical fiber amplifier which operates principally on a nonlinear effect termed as four-wave mixing. Amplification based on such fiber nonlinear effect offers a great deal of design flexibility, e.g. arbitrary gain region [1], large gain [2], wide gain bandwidth [3,4] and ultra low noise figure in phase sensitive mode [5]. FOPA is a unidirectional device and only amplifies signals that co-propagate with the pump [1]. Interestingly, Marhic et al. in [6] reported that the FOPA gain spectrum is reciprocal, i.e., the gain spectra of a FOPA are similar in either amplification direction of fiber in the linear regime. Therefore the potential of FOPA is only half-exploited when it is operated conventionally (unidirectional amplification). In order to make use of both amplification directions of FOPA, the fiber medium must be pumped bidirectionally. However, the electrical noise figure (NF) of a FOPA is not reciprocal due to unavoidable Raman phonon noise [7,8]. A few FOPA experiments reported recently have made use of gain reciprocity of parametric amplification, most notably polarization-independent FOPA [9], full bandwidth FOPA [10] and loop-mirror-based FOPA [11]. However the gain characteristics achieved in these experiments are less attractive as compared to conventional FOPA. In the polarizationindependent FOPA experiment, a polarization beam splitter is used to distribute power equally to both polarization axes of a fiber loop which consists of a nonlinear fiber medium. The intrinsic gain loss is claimed to be at least 6 dB in this experiment, but good polarization independent gain has been achieved with low signal gain in the range of 7.4 – 9.3 dB. A 10 Gbps bit error rate (BER) experiment recorded a power penalty between FOPA with/without polarization-diversify technique of 0.9 dB at error level 10−9, with pump power of 25 dBm and signal power of −3.8 dBm [9]. Several full bandwidth FOPA systems have been proposed to omit the spectral occupancy of idler components in order to increase the bandwidth utilization of FOPA [10]. In a particular one-filter one-bidirectional FOPA system, a pump source was split by a coupler to bidirectionally pump a nonlinear fiber spool. The full bandwidth utilization of FOPA was obtained by using interleaver as the idlers filter. Gain values between 7.9 and 12 dB were achieved at different channel wavelengths while the pump power was fixed at 25 dBm in either direction (total combined pump power 0.63 W) of the nonlinear fiber medium. In a loop-mirror-based FOPA demonstrated in [11], both signal and pump were split equally by a 3-dB coupler and entered a 500 m highly nonlinear fiber (HNLF) from both directions, and later recombine via the same coupler. The best on-off gain achieved in this FOPA was 24 dB at 2.8 W pump power. The loop-mirror-based FOPA noise performance was confirmed with 10 Gbps transmission experiment and result showed the power penalty was 5 dB higher in comparison to a conventional erbium-doped fiber amplifier (EDFA). In contrast to the polarization-independent FOPA which uses relatively low pump power, the loop-mirror-based FOPA suffers higher power penalty because higher pump power is used. Toronidis et al. claimed that this is due to unsuppressed stimulated Brillouin scattering (SBS) in the loop, but Raman phonon noise could also be a contributing factor due to the use of high pump power. In this work, we propose a double-pass pump FOPA that provides amplification in C- and L-band with 29 dB gain at 1.05 W pump power, achieving 10 dB parametric gain improvement compared to the conventional FOPA. Due to the spectral characteristic of idler removal filter used in the design, an idler which is secondary-translated in L-band is used as the transmission signal in this experiment. To the best of our knowledge, this is the first time
#197476 - $15.00 USD Received 10 Sep 2013; revised 23 Nov 2013; accepted 4 Dec 2013; published 13 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031623 | OPTICS EXPRESS 31624
that the performance between the bidirectionally-pumped and conventional FOPAs is quantified. In addition, we demonstrate a transmission of secondary idler generated in FOPA. 2. Double-pass pump FOPA structure The experimental setup of FOPA with double-pass pump configuration is depicted in Fig. 1. A tunable laser source (TLS1) is used as a parametric pump and is phase modulated by 100, 312, 923 and 2730 MHz sinusoidal RF tones to mitigate SBS effect. The pump power is boosted by an EDFA with maximum output power up to 2 W. A tunable bandpass filter (TBF) with 1 nm bandwidth is used to filter out the amplified spontaneous emission (ASE) from the EDFA. TLS2 is used as a signal source and its power is controlled with a variable optical attenuator (VOA). A circulator (CIR) and an optical power meter (OPM) are connected to the signal path for SBS monitoring purposes. Another OPM is used at the 10% output port of 90/10 coupler to monitor the total parametric pump power. Polarization controllers (PCs) are placed at both pump and signal arms to ensure optimal polarization alignment when they are coupled with a 90/10 coupler. The dashed box in the figure shows the double-pass pump configuration. It consists of a circulator, a 500 m HNLF (insertion loss = 0.77 dB, nonlinear coefficient = 11.5 /W/km, zero dispersion wavelength (ZDW) = 1556.5 nm, dispersion slope = 0.015 ps/nm2/km), a C/L-wavelength division multiplexer (WDM) and a wideband mirror. Here, it is very crucial to include a C/L-WDM in this section because it functions as an idler (L-band) removal filter [12] to avoid wavelength-dependent gain modulation [13]. The mirror is used to reflect C-band light (1500-1564 nm) to realize bidirectional parametric process in the HNLF. Finally, an optical spectrum analyzer (OSA) is connected at the output of a 95/5 coupler. Simply note that, by replacing the dashed box section in the experimental setup with a section of HNLF, one gets conventional FOPA setup configuration.
Fig. 1. Experimental setup of the proposed double-pass pump FOPA.
The double-pass section in the dashed-box has a round-trip insertion loss of 4.3 dB in the passband (including HNLF loss) while suppressing 35 dB in the stopband. Throughout the experiment, the pump wavelength is fixed at 1560.6 nm, which falls inside the passband. With that, the pump propagates through the HNLF twice thus realizing the double-pass pump condition. There is no problem for a C-band signal to co-propagate with the pump in the HNLF because C-band signal is also within the passband of C/L-WDM, which allows the Cband signal to be amplified twice in the HNLF. It is worth to mention that the optimum pump wavelength for the HNLF used in this experiment is around 1565 nm but shorter pump wavelength is preferred in this experiment. This is because, in addition to contribution from the extra passive components, double-pass pump configuration essentially elongates the effective length of the fiber, thus making the design prone to pump depletion, gain saturation and back energy conversion [14,15]. By tuning the pump wavelength closer to the average ZDW of the fiber, the FOPA gain profile will be flatter and wider but at the cost of lower gain resulting from averaging effect of the longitudinal ZDW fluctuation within the fiber medium [16]. In this case, the averaging effect caused by ZDW fluctuation is used to limit the gain of #197476 - $15.00 USD Received 10 Sep 2013; revised 23 Nov 2013; accepted 4 Dec 2013; published 13 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031623 | OPTICS EXPRESS 31625
the FOPA and achieve linear and undepleted pump operation in the double-pass design. Figure 2 shows the gain measured as the input signal power is varied while the pump power is fixed at 1.05 W. For the case of conventional FOPA as depicted in Fig. 2(a), the gain is not saturated for signal power range from −36 dBm to 10 dBm. However in double-pass FOPA configuration, the gain roll off rapidly for signal power greater than −20 dBm, while for signal power smaller than −20 dBm the gain is rolling off in a much slower pace. Thus, choosing signal power below −20 dBm would keep the gain saturation phenomena to a minimum level. Although shortening the HNLF could also reduce the effect of gain saturation, we are not equipped with a suitable splicer to produce a low loss splice between standard single mode fiber and HNLF. (b) 35
30
30
25
25
20 15
Gain (dB)
Gain (dB)
(a) 35
20
1542 nm 1544 nm 1546 nm 1575 nm 1577 nm 1579 nm
15
10
10
5
5
0 -40 -30 -20 -10 0 10 Signal Power (dBm)
0 -40 -30 -20 10 0 10 Signal Power (dBm)
Fig. 2. Gain versus signal power for signal wavelength range from 1542 nm to 1579 nm for case (a) conventional FOPA and (b) double-pass FOPA.
The following explains the evolution of the L-band signal in the double-pass pump configuration. In the upward direction, the signal co-propagates with the pump in the HNLF and generates a corresponding (firstly-translated) C-band idler. The C/L-WDM suppresses the original L-band signal (1570-1610 nm) while the firstly-translated C-band idler survives. In the downward direction, the firstly-translated C-band idler co-propagates with the pump and generates a secondary-translated L-band idler, which is located at the exact wavelength as the original input L-band signal. Although this secondary L-band idler is phase conjugated twice, the transmission of secondary L-band idler would be transparent to phase modulation format since phase conjugation is a linear conversion process [17], and definitely compatible with amplitude modulation format. The performance comparison is made between the amplified Cband signal and secondary L-band idler. This is to justify the effectiveness of the newly created wave to duplicate data from its original signal. Figure 3(a) shows the amplified spectrum when the signal wavelength is tuned to 1578 nm (L-band) and Fig. 3(b) depicts the gain profiles comparison between double-pass and conventional FOPAs. In both figures, the input signal is fixed at −30 dBm and pump power is set at 1.05 W. We can see that there is a 10 dB parametric gain improvement resulting from the double-pass pump FOPA configuration. As mentioned earlier, the insertion loss of the passive components in the double-pass FOPA setup exaggerates the gain saturation problem. With the pump wavelength nearer to the ZDW of the HNLF, excessive pump depletion is reduced on the first pass, leaving more pump power in the second pass to overcome component losses and pump depletion process. By this method, practically similar parametric gain spectrum can be maintained. Figure 4 illustrates that the gain slope value measured around the peak gain wavelength region for the double-pass pump FOPA is 46.1 dB/W and #197476 - $15.00 USD Received 10 Sep 2013; revised 23 Nov 2013; accepted 4 Dec 2013; published 13 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031623 | OPTICS EXPRESS 31626
conventional FOPA is 19.6 dB/W. However, due to larger components insertion loss in the double-pass pump FOPA (round-trip loss of 4.3 dB including HNLF loss), its gain is lower than that of the conventional FOPA (insertion loss of 0.77 dB) at pump power approximately smaller than 0.7 W. As the pump power increases beyond 0.7 W, the double-pass gain becomes larger and exceeds that of conventional FOPA gain because the total parametric gain overcomes the insertion losses of passive components. (a)
0
35
-10
30
-20
25
-30
15
-50
10
-60
5
1540
1560 1580 Wavelength (nm)
DP Conv
20
-40
-70
(b)
40
Gain (dB)
Power (dBm)
10
0
1540
1560 1580 Wavelength (nm)
Fig. 3. (a) Amplified spectrum when signal is fixed at 1578 nm, and (b) gain profiles comparison of double-pass FOPA (DP) and conventional FOPA (Conv).
50
Gain (dB)
40 30
Conv 1546 nm Conv 1575 nm DP 1546 nm DP 1575 nm linear fit
46.1 dB/W 19.6 dB/W
20 10 0 -10 0
0.5 1 Pump Power (W)
1.5
Fig. 4. Gain slope comparison for double-pass pump FOPA and conventional FOPA.
Figures 5 and 6 show the amplified C-band (1543 nm) and L-band (1579 nm) signal spectra with conventional FOPA and double-pass pump FOPA respectively in order to provide qualitative information of linewidth broadening of these signals. From the experimental results, the amplified signal linewidth is not broadened in conventional FOPA [18]. On the other hand, it is significantly broadened in the double-pass pump FOPA configuration. In double-pass pump FOPA, when the signal is travelling in the upward (or downward) direction, the corresponding opposite pump wave imposes phase modulation on the signal through backward cross-phase modulation (XPM), which matches findings reported in [9,10].
#197476 - $15.00 USD Received 10 Sep 2013; revised 23 Nov 2013; accepted 4 Dec 2013; published 13 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031623 | OPTICS EXPRESS 31627
(b) -10
-20
-20
-30
-30 Power (dBm)
Power (dBm)
(a) -10
-40 -50 -60
-40 -50 -60
-70
-70
-80
-80
-90 1542.5
1543 Wavelength (nm)
-90 1578.5
1543.5
1.12 W 1.05 W 0.87 W 0.68 W 0.54 W 0.44 W pump off
1579 Wavelength (nm)
1579.5
Fig. 5. Amplified spectra of conventional FOPA at signal wavelength (a) 1543 nm and (b) 1579 nm.
(b) 0
-10
-10
-20
-20
-30
-30
Power (dBm)
Power (dBm)
(a) 0
-40 -50 -60
-40 -50 -60
-70
-70
-80
-80
-90 1541
1542 1543 1544 Wavelength (nm)
1545
1.12 W 1.05 W 0.87 W 0.68 W 0.54 W 0.44 W pump off
-90 1577 1578 1579 1580 1581 Wavelength (nm)
Fig. 6. Amplified spectra of double-pass FOPA at signal wavelength (a) 1543 nm and (b) 1579 nm.
3. Secondary idler transmission In order to verify the usability of double-pass pump FOPA in modern optical networks, on-off keying transmission experiment is carried out using the setup as shown in Fig. 7. The signal now is externally modulated with 10 Gbps, 27-1 pseudo random bit sequence (PRBS) using a Mach-Zehnder modulator (MZM). The signal power is fixed at −30 dBm, while its wavelengths are tuned at 1548 nm and 1573 nm to represent C-band and L-band transmissions respectively. In this experiment, the pump power is fixed at 0.68 W, thus both double-pass and conventional configurations would have about the same signal gain: the onoff gain is around 13.5 dB for the conventional configuration and 14.8 dB for the double-pass pump configuration. It is important to use a relatively low pump power and low signal power in driving the double-pass FOPA. This is to minimize the generation of Raman phonon noise, backward XPM and also avoid gain saturation problem in order to obtain measureable BER. The output port of the FOPA is routed into two sections depending on the signal wavelength. The upper route consists of a C-band EDFA to amplify the signal power to meet the minimum sensitivity level of the 60 GHz photodetector (PD). The notch filter with a central wavelength of 1560.61 nm is deployed to suppress the parametric pump and a TBF1 is utilized as the bandpass filter for the amplified C-band signal while removing the L-band idler
#197476 - $15.00 USD Received 10 Sep 2013; revised 23 Nov 2013; accepted 4 Dec 2013; published 13 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031623 | OPTICS EXPRESS 31628
signal. Another TBF2 after the EDFA is used to suppress bulk of ASE. On the other hand, the lower route consists of a double-stage L-band EDFA, serving the same purpose as the C-band EDFA. A C/L-WDM is used to suppress the parametric pump and the C-band idler. The TBF3 in the mid-stage is used as the bandpass filter of the secondary L-band idler while removing the unwanted ASE. In this experiment, the EDFA is necessary here to boost up the signal as the use of low pump power and low signal power in the FOPAs resulted in low output signal power (around −15 dBm), which does not fall within the optimum detection range of the 60 GHz photodetector (−5 to 7 dBm) used in the experiment. The VOA before PD is adjusted accordingly in order to plot the BER performance with respect to the received power. After amplifying the received signal, its quality is measured using the high-speed oscilloscope. In order to reduce the discrepancy among the BER performance of the back-toback, conventional and double-pass FOPA caused by the EDFA, its drive current is made constant while the BER measurements are performed at one particular wavelength for all cases. Therefore, the EDFA noise is consistent for all BER curves obtained thus justifying our findings on the performance of conventional and double-pass FOPAs.
Fig. 7. Experimental setup to measure the transmission performance at 10 Gbps.
Figures 8(a) and 8(b) depict the back-to-back eye diagrams at signal wavelength of 1548 nm and 1573 nm respectively. In addition, Figs. 8(c) and 8(d) illustrate the widely open eye diagrams for the output signal of the amplified C-band signal at 1548 nm and the secondary idler at 1573 nm respectively.
#197476 - $15.00 USD Received 10 Sep 2013; revised 23 Nov 2013; accepted 4 Dec 2013; published 13 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031623 | OPTICS EXPRESS 31629
Fig. 8. 10 Gbps eye diagrams back-to-back signal at (a) 1548 nm and (b) 1573 nm, double-pass pump amplified signal at (c) 1548 nm and (d) 1573 nm.
Figures 9(a) and 9(b) show the BER curve comparison for back-to-back, conventional FOPA and double-pass pump FOPA evaluated at signal wavelength 1548 nm and 1573 nm respectively. It is insightful to evaluate the BER of these two wavelengths; by evaluating the BER at signal wavelength of 1548 nm (C-band), we obtain BER performance of the ‘original’ double-pass signal. On the other hand, by measuring BER at signal wavelength of 1573 nm, we obtain the BER performance of the secondary idler. Due to the broad linewidth and marginally degraded OSNR of the signal, the double-pass signal BER could not be better than 10−8, thus BER evaluation at 10−6 is chosen. For reference purposes, the power penalty of conventional FOPA at BER of 10−6 is 1.4 dB and 2.2 dB for wavelength 1548 nm and 1573 nm respectively. For the double-pass amplified C-band signal, the power penalty at BER of 10−6 is 4.1 dB and the power penalty for the secondary L-band idler is 4.7 dB. From here, we can see that the transmission performance of the secondary idler in double-pass pump configuration is acceptable. In double-pass FOPA, forward and backward propagating pumps co-exist inside the nonlinear fiber medium. When a signal is propagating inside the fiber medium, it experiences XPM induced particularly by the counter-propagating pump (relative to the signal propagation direction). Due to the fact that the signal propagates bidirectionally in this experiment (thus achieve the objective elongation of fiber effective length), the backward XPM impact on the signal is virtually doubled. As shown earlier in Fig. 6, the output signal linewidth of a double-pass FOPA is significantly broader than the product of a conventional FOPA. It is also noticed that the OSNR of the signal is lower in double-pass FOPA. This is due to the strong Raman phonon noise (as a result of the elongation of effective fiber length) imposed on the signal. Besides that, the non-reciprocity nature of noise figure and low gain saturation threshold of the double-pass FOPA are also limiting factors. In order to improve the design, a short and low-ZDW-fluctuation HNLF should be used to increase the gain saturation threshold, minimize the Raman phonon noise and reduce noise figure nonreciprocity. To operate the double-pass FOPA in high pump power regime without significant XPM, a two-pump scheme could possibly be adopted to attain the same pump power but with lower intensity.
#197476 - $15.00 USD Received 10 Sep 2013; revised 23 Nov 2013; accepted 4 Dec 2013; published 13 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031623 | OPTICS EXPRESS 31630
(a)
0
-2
-2
-4
-4 log10(BER)
log10(BER)
0
-6
-8
-10
-10
-8 -6 -4 -2 0 2 Received Power (dBm)
B2B Conv DP
-6
-8
-12
(b)
-12
-8 -6 -4 -2 0 2 Received Power (dBm)
Fig. 9. Bit error rate curves comparison (a) at signal 1548 nm and (b) 1573 nm; back-to-back (B2B), conventional FOPA (Conv) and double-pass FOPA (DP).
4. Conclusion In conclusion, we have demonstrated a double-pass pump FOPA that makes full use of the bidirectional parametric gain within the HNLF section. Due to elongation of the effective fiber length, the gain slope in the double-pass pump configuration has improved by more than two fold. For the L-band amplification band, the translated secondary idler is adopted and it is proven with BER experiment. The demonstration of the secondary idler transmission could be very useful in fiber optical passive distribution network, where large duplication of signals is generally required to cover more users. Since the double-pass pump FOPA configuration could duplicate data at the same wavelength with inherent gain, this would greatly extend the distribution network coverage. However, due to the bidirectionally propagating pump, the double-pass signal and idler linewidth are broadened significantly as a result of XPM [9,10], which should be addressed in future research. Acknowledgment This work is partly supported by the Ministry of Higher Education, High Impact Research #A000007-50001 and the Ministry of Science, Technology and Innovation (National Science Fellowship).
#197476 - $15.00 USD Received 10 Sep 2013; revised 23 Nov 2013; accepted 4 Dec 2013; published 13 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031623 | OPTICS EXPRESS 31631
Wireless and Photonics Networks Research Centre, Faculty of Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia 2 Department of Electrical Engineering, Faculty of Engineering, University of Malaya, Malaysia * [email protected]
Abstract: Characteristics of fiber optical parametric amplifier (FOPA) with double-pass pump configuration are experimentally investigated. The double-pass pump FOPA exhibits more than two-fold steeper gain slope in comparison to the conventional FOPA due to elongation of effective fiber length. In the L-band amplification band, a secondary idler is generated and used as the transmission signal in lieu of the original L-band signal. Gain measurement and bit error rate experiments are performed on the secondary idler and the results prove the usability of secondary idler, which is potentially useful for distribution networks. ©2013 Optical Society of America OCIS codes: (190.4970) Parametric oscillators and amplifiers; (060.4370) Nonlinear optics, fibers;
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
M. E. Marhic, Fiber Optical Parametric Amplifiers, Oscillators and Related Devices (Cambridge University, 2008). T. Torounidis, P. A. Andrekson, and B.-E. Olsson, “Fiber-optical parametric amplifier with 70-dB gain,” IEEE Photon. Technol. Lett. 18(10), 1194–1196 (2006). J. D. Marconi, J. M. C. Boggio, and H. L. Fragnito, “Nearly 100 nm bandwidth of flat gain with a doublepumped fiber optic parametric amplifier,” in Conference on Optical Fiber Communication and the National Fiber Optic Engineers Conference, 2007. OFC/NFOEC 2007 (2007), pp. 1–3. T. Torounidis and P. Andrekson, “Broadband single-pumped fiber-optic parametric amplifiers,” IEEE Photon. Technol. Lett. 19(9), 650–652 (2007). Z. Tong, C. Lundström, P. A. Andrekson, M. Karlsson, and A. Bogris, “Ultralow noise, broadband phasesensitive optical amplifiers, and their applications,” IEEE J. Sel. Top. Quantum Electron. 18(2), 1016–1032 (2012). M. E. Marhic, G. Kalogerakis, and L. G. Kazovsky, “Gain reciprocity in fibre optical parametric amplifiers,” Electron. Lett. 42(9), 519–520 (2006). Z. Tong, C. Lundström, M. Karlsson, and P. A. Andrekson, “Noise figure non-reciprocity in fiber optical parametric amplifiers with zero-dispersion-wavelength variations,” in 2010 36th European Conference and Exhibition on Optical Communication (ECOC) (2010), pp. 1–3. Z. Tong, A. Bogris, M. Karlsson, and P. A. Andrekson, “Full characterization of the signal and idler noise figure spectra in single-pumped fiber optical parametric amplifiers,” Opt. Express 18(3), 2884–2893 (2010). K. K. Y. Wong, M. E. Marhic, K. Uesaka, and L. G. Kazovsky, “Polarization-independent one-pump fiberoptical parametric amplifier,” IEEE Photon. Technol. Lett. 14(11), 1506–1508 (2002). G. Kalogerakis, M. E. Marhic, K. Uesaka, K. Shimizu, K. K. Y. Wong, and L. G. Kazovsky, “Methods for full utilization of the bandwidth of fiber optical parametric amplifiers and wavelength converters,” J. Lightwave Technol. 24, 3683–3690 (2006). T. Torounidis, B. E. Olsson, H. Sunnerud, M. Karlsson, and P. A. Andrekson, “Fiber-optic parametric amplifier in a loop mirror configuration,” IEEE Photon. Technol. Lett. 17(2), 321–323 (2005). K. S. Yeo, F. R. M. Adikan, M. Mokhtar, S. Hitam, and M. A. Mahdi, “Continuous wave tunable fiber optical parametric oscillator with double-pass pump configuration,” Appl. Phys. B 110(3), 353–357 (2013). K. S. Yeo, F. R. Mahamd Adikan, M. Mokhtar, S. Hitam, and M. A. Mahdi, “Gain smoothening filter in twosegment fiber-optical parametric amplifier,” Opt. Commun. 286, 353–356 (2013). G. Rustad, G. Arisholm, and Ø. Farsund, “Effect of idler absorption in pulsed optical parametric oscillators,” Opt. Express 19(3), 2815–2830 (2011). C. Jauregui, D. Nodop, J. Limpert, and A. Tünnermann, “Improved parametric generation of light in optical fibers,” in Lasers and Electro-Optics Europe (CLEO EUROPE/EQEC), 2011 Conference on and 12th European Quantum Electronics Conference (2011), p. 1.
#197476 - $15.00 USD Received 10 Sep 2013; revised 23 Nov 2013; accepted 4 Dec 2013; published 13 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031623 | OPTICS EXPRESS 31623
16. M. Karlsson, “Four-wave mixing in fibers with randomly varying zero-dispersion wavelength,” J. Opt. Soc. Am. B 15(8), 2269–2275 (1998). 17. S. Watanabe, T. Naito, and T. Chikama, “Compensation of chromatic dispersion in a single-mode fiber by optical phase conjugation,” IEEE Photon. Technol. Lett. 5(1), 92–95 (1993). 18. J. Hansryd, P. A. Andrekson, M. Westlund, J. Li, and P. O. Hedekvist, “Fiber-based optical parametric amplifiers and their applications,” IEEE J. Sel. Top. Quantum Electron. 8(3), 506–520 (2002).
1. Introduction Fiber optical parametric amplifier (FOPA) is an all-optical fiber amplifier which operates principally on a nonlinear effect termed as four-wave mixing. Amplification based on such fiber nonlinear effect offers a great deal of design flexibility, e.g. arbitrary gain region [1], large gain [2], wide gain bandwidth [3,4] and ultra low noise figure in phase sensitive mode [5]. FOPA is a unidirectional device and only amplifies signals that co-propagate with the pump [1]. Interestingly, Marhic et al. in [6] reported that the FOPA gain spectrum is reciprocal, i.e., the gain spectra of a FOPA are similar in either amplification direction of fiber in the linear regime. Therefore the potential of FOPA is only half-exploited when it is operated conventionally (unidirectional amplification). In order to make use of both amplification directions of FOPA, the fiber medium must be pumped bidirectionally. However, the electrical noise figure (NF) of a FOPA is not reciprocal due to unavoidable Raman phonon noise [7,8]. A few FOPA experiments reported recently have made use of gain reciprocity of parametric amplification, most notably polarization-independent FOPA [9], full bandwidth FOPA [10] and loop-mirror-based FOPA [11]. However the gain characteristics achieved in these experiments are less attractive as compared to conventional FOPA. In the polarizationindependent FOPA experiment, a polarization beam splitter is used to distribute power equally to both polarization axes of a fiber loop which consists of a nonlinear fiber medium. The intrinsic gain loss is claimed to be at least 6 dB in this experiment, but good polarization independent gain has been achieved with low signal gain in the range of 7.4 – 9.3 dB. A 10 Gbps bit error rate (BER) experiment recorded a power penalty between FOPA with/without polarization-diversify technique of 0.9 dB at error level 10−9, with pump power of 25 dBm and signal power of −3.8 dBm [9]. Several full bandwidth FOPA systems have been proposed to omit the spectral occupancy of idler components in order to increase the bandwidth utilization of FOPA [10]. In a particular one-filter one-bidirectional FOPA system, a pump source was split by a coupler to bidirectionally pump a nonlinear fiber spool. The full bandwidth utilization of FOPA was obtained by using interleaver as the idlers filter. Gain values between 7.9 and 12 dB were achieved at different channel wavelengths while the pump power was fixed at 25 dBm in either direction (total combined pump power 0.63 W) of the nonlinear fiber medium. In a loop-mirror-based FOPA demonstrated in [11], both signal and pump were split equally by a 3-dB coupler and entered a 500 m highly nonlinear fiber (HNLF) from both directions, and later recombine via the same coupler. The best on-off gain achieved in this FOPA was 24 dB at 2.8 W pump power. The loop-mirror-based FOPA noise performance was confirmed with 10 Gbps transmission experiment and result showed the power penalty was 5 dB higher in comparison to a conventional erbium-doped fiber amplifier (EDFA). In contrast to the polarization-independent FOPA which uses relatively low pump power, the loop-mirror-based FOPA suffers higher power penalty because higher pump power is used. Toronidis et al. claimed that this is due to unsuppressed stimulated Brillouin scattering (SBS) in the loop, but Raman phonon noise could also be a contributing factor due to the use of high pump power. In this work, we propose a double-pass pump FOPA that provides amplification in C- and L-band with 29 dB gain at 1.05 W pump power, achieving 10 dB parametric gain improvement compared to the conventional FOPA. Due to the spectral characteristic of idler removal filter used in the design, an idler which is secondary-translated in L-band is used as the transmission signal in this experiment. To the best of our knowledge, this is the first time
#197476 - $15.00 USD Received 10 Sep 2013; revised 23 Nov 2013; accepted 4 Dec 2013; published 13 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031623 | OPTICS EXPRESS 31624
that the performance between the bidirectionally-pumped and conventional FOPAs is quantified. In addition, we demonstrate a transmission of secondary idler generated in FOPA. 2. Double-pass pump FOPA structure The experimental setup of FOPA with double-pass pump configuration is depicted in Fig. 1. A tunable laser source (TLS1) is used as a parametric pump and is phase modulated by 100, 312, 923 and 2730 MHz sinusoidal RF tones to mitigate SBS effect. The pump power is boosted by an EDFA with maximum output power up to 2 W. A tunable bandpass filter (TBF) with 1 nm bandwidth is used to filter out the amplified spontaneous emission (ASE) from the EDFA. TLS2 is used as a signal source and its power is controlled with a variable optical attenuator (VOA). A circulator (CIR) and an optical power meter (OPM) are connected to the signal path for SBS monitoring purposes. Another OPM is used at the 10% output port of 90/10 coupler to monitor the total parametric pump power. Polarization controllers (PCs) are placed at both pump and signal arms to ensure optimal polarization alignment when they are coupled with a 90/10 coupler. The dashed box in the figure shows the double-pass pump configuration. It consists of a circulator, a 500 m HNLF (insertion loss = 0.77 dB, nonlinear coefficient = 11.5 /W/km, zero dispersion wavelength (ZDW) = 1556.5 nm, dispersion slope = 0.015 ps/nm2/km), a C/L-wavelength division multiplexer (WDM) and a wideband mirror. Here, it is very crucial to include a C/L-WDM in this section because it functions as an idler (L-band) removal filter [12] to avoid wavelength-dependent gain modulation [13]. The mirror is used to reflect C-band light (1500-1564 nm) to realize bidirectional parametric process in the HNLF. Finally, an optical spectrum analyzer (OSA) is connected at the output of a 95/5 coupler. Simply note that, by replacing the dashed box section in the experimental setup with a section of HNLF, one gets conventional FOPA setup configuration.
Fig. 1. Experimental setup of the proposed double-pass pump FOPA.
The double-pass section in the dashed-box has a round-trip insertion loss of 4.3 dB in the passband (including HNLF loss) while suppressing 35 dB in the stopband. Throughout the experiment, the pump wavelength is fixed at 1560.6 nm, which falls inside the passband. With that, the pump propagates through the HNLF twice thus realizing the double-pass pump condition. There is no problem for a C-band signal to co-propagate with the pump in the HNLF because C-band signal is also within the passband of C/L-WDM, which allows the Cband signal to be amplified twice in the HNLF. It is worth to mention that the optimum pump wavelength for the HNLF used in this experiment is around 1565 nm but shorter pump wavelength is preferred in this experiment. This is because, in addition to contribution from the extra passive components, double-pass pump configuration essentially elongates the effective length of the fiber, thus making the design prone to pump depletion, gain saturation and back energy conversion [14,15]. By tuning the pump wavelength closer to the average ZDW of the fiber, the FOPA gain profile will be flatter and wider but at the cost of lower gain resulting from averaging effect of the longitudinal ZDW fluctuation within the fiber medium [16]. In this case, the averaging effect caused by ZDW fluctuation is used to limit the gain of #197476 - $15.00 USD Received 10 Sep 2013; revised 23 Nov 2013; accepted 4 Dec 2013; published 13 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031623 | OPTICS EXPRESS 31625
the FOPA and achieve linear and undepleted pump operation in the double-pass design. Figure 2 shows the gain measured as the input signal power is varied while the pump power is fixed at 1.05 W. For the case of conventional FOPA as depicted in Fig. 2(a), the gain is not saturated for signal power range from −36 dBm to 10 dBm. However in double-pass FOPA configuration, the gain roll off rapidly for signal power greater than −20 dBm, while for signal power smaller than −20 dBm the gain is rolling off in a much slower pace. Thus, choosing signal power below −20 dBm would keep the gain saturation phenomena to a minimum level. Although shortening the HNLF could also reduce the effect of gain saturation, we are not equipped with a suitable splicer to produce a low loss splice between standard single mode fiber and HNLF. (b) 35
30
30
25
25
20 15
Gain (dB)
Gain (dB)
(a) 35
20
1542 nm 1544 nm 1546 nm 1575 nm 1577 nm 1579 nm
15
10
10
5
5
0 -40 -30 -20 -10 0 10 Signal Power (dBm)
0 -40 -30 -20 10 0 10 Signal Power (dBm)
Fig. 2. Gain versus signal power for signal wavelength range from 1542 nm to 1579 nm for case (a) conventional FOPA and (b) double-pass FOPA.
The following explains the evolution of the L-band signal in the double-pass pump configuration. In the upward direction, the signal co-propagates with the pump in the HNLF and generates a corresponding (firstly-translated) C-band idler. The C/L-WDM suppresses the original L-band signal (1570-1610 nm) while the firstly-translated C-band idler survives. In the downward direction, the firstly-translated C-band idler co-propagates with the pump and generates a secondary-translated L-band idler, which is located at the exact wavelength as the original input L-band signal. Although this secondary L-band idler is phase conjugated twice, the transmission of secondary L-band idler would be transparent to phase modulation format since phase conjugation is a linear conversion process [17], and definitely compatible with amplitude modulation format. The performance comparison is made between the amplified Cband signal and secondary L-band idler. This is to justify the effectiveness of the newly created wave to duplicate data from its original signal. Figure 3(a) shows the amplified spectrum when the signal wavelength is tuned to 1578 nm (L-band) and Fig. 3(b) depicts the gain profiles comparison between double-pass and conventional FOPAs. In both figures, the input signal is fixed at −30 dBm and pump power is set at 1.05 W. We can see that there is a 10 dB parametric gain improvement resulting from the double-pass pump FOPA configuration. As mentioned earlier, the insertion loss of the passive components in the double-pass FOPA setup exaggerates the gain saturation problem. With the pump wavelength nearer to the ZDW of the HNLF, excessive pump depletion is reduced on the first pass, leaving more pump power in the second pass to overcome component losses and pump depletion process. By this method, practically similar parametric gain spectrum can be maintained. Figure 4 illustrates that the gain slope value measured around the peak gain wavelength region for the double-pass pump FOPA is 46.1 dB/W and #197476 - $15.00 USD Received 10 Sep 2013; revised 23 Nov 2013; accepted 4 Dec 2013; published 13 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031623 | OPTICS EXPRESS 31626
conventional FOPA is 19.6 dB/W. However, due to larger components insertion loss in the double-pass pump FOPA (round-trip loss of 4.3 dB including HNLF loss), its gain is lower than that of the conventional FOPA (insertion loss of 0.77 dB) at pump power approximately smaller than 0.7 W. As the pump power increases beyond 0.7 W, the double-pass gain becomes larger and exceeds that of conventional FOPA gain because the total parametric gain overcomes the insertion losses of passive components. (a)
0
35
-10
30
-20
25
-30
15
-50
10
-60
5
1540
1560 1580 Wavelength (nm)
DP Conv
20
-40
-70
(b)
40
Gain (dB)
Power (dBm)
10
0
1540
1560 1580 Wavelength (nm)
Fig. 3. (a) Amplified spectrum when signal is fixed at 1578 nm, and (b) gain profiles comparison of double-pass FOPA (DP) and conventional FOPA (Conv).
50
Gain (dB)
40 30
Conv 1546 nm Conv 1575 nm DP 1546 nm DP 1575 nm linear fit
46.1 dB/W 19.6 dB/W
20 10 0 -10 0
0.5 1 Pump Power (W)
1.5
Fig. 4. Gain slope comparison for double-pass pump FOPA and conventional FOPA.
Figures 5 and 6 show the amplified C-band (1543 nm) and L-band (1579 nm) signal spectra with conventional FOPA and double-pass pump FOPA respectively in order to provide qualitative information of linewidth broadening of these signals. From the experimental results, the amplified signal linewidth is not broadened in conventional FOPA [18]. On the other hand, it is significantly broadened in the double-pass pump FOPA configuration. In double-pass pump FOPA, when the signal is travelling in the upward (or downward) direction, the corresponding opposite pump wave imposes phase modulation on the signal through backward cross-phase modulation (XPM), which matches findings reported in [9,10].
#197476 - $15.00 USD Received 10 Sep 2013; revised 23 Nov 2013; accepted 4 Dec 2013; published 13 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031623 | OPTICS EXPRESS 31627
(b) -10
-20
-20
-30
-30 Power (dBm)
Power (dBm)
(a) -10
-40 -50 -60
-40 -50 -60
-70
-70
-80
-80
-90 1542.5
1543 Wavelength (nm)
-90 1578.5
1543.5
1.12 W 1.05 W 0.87 W 0.68 W 0.54 W 0.44 W pump off
1579 Wavelength (nm)
1579.5
Fig. 5. Amplified spectra of conventional FOPA at signal wavelength (a) 1543 nm and (b) 1579 nm.
(b) 0
-10
-10
-20
-20
-30
-30
Power (dBm)
Power (dBm)
(a) 0
-40 -50 -60
-40 -50 -60
-70
-70
-80
-80
-90 1541
1542 1543 1544 Wavelength (nm)
1545
1.12 W 1.05 W 0.87 W 0.68 W 0.54 W 0.44 W pump off
-90 1577 1578 1579 1580 1581 Wavelength (nm)
Fig. 6. Amplified spectra of double-pass FOPA at signal wavelength (a) 1543 nm and (b) 1579 nm.
3. Secondary idler transmission In order to verify the usability of double-pass pump FOPA in modern optical networks, on-off keying transmission experiment is carried out using the setup as shown in Fig. 7. The signal now is externally modulated with 10 Gbps, 27-1 pseudo random bit sequence (PRBS) using a Mach-Zehnder modulator (MZM). The signal power is fixed at −30 dBm, while its wavelengths are tuned at 1548 nm and 1573 nm to represent C-band and L-band transmissions respectively. In this experiment, the pump power is fixed at 0.68 W, thus both double-pass and conventional configurations would have about the same signal gain: the onoff gain is around 13.5 dB for the conventional configuration and 14.8 dB for the double-pass pump configuration. It is important to use a relatively low pump power and low signal power in driving the double-pass FOPA. This is to minimize the generation of Raman phonon noise, backward XPM and also avoid gain saturation problem in order to obtain measureable BER. The output port of the FOPA is routed into two sections depending on the signal wavelength. The upper route consists of a C-band EDFA to amplify the signal power to meet the minimum sensitivity level of the 60 GHz photodetector (PD). The notch filter with a central wavelength of 1560.61 nm is deployed to suppress the parametric pump and a TBF1 is utilized as the bandpass filter for the amplified C-band signal while removing the L-band idler
#197476 - $15.00 USD Received 10 Sep 2013; revised 23 Nov 2013; accepted 4 Dec 2013; published 13 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031623 | OPTICS EXPRESS 31628
signal. Another TBF2 after the EDFA is used to suppress bulk of ASE. On the other hand, the lower route consists of a double-stage L-band EDFA, serving the same purpose as the C-band EDFA. A C/L-WDM is used to suppress the parametric pump and the C-band idler. The TBF3 in the mid-stage is used as the bandpass filter of the secondary L-band idler while removing the unwanted ASE. In this experiment, the EDFA is necessary here to boost up the signal as the use of low pump power and low signal power in the FOPAs resulted in low output signal power (around −15 dBm), which does not fall within the optimum detection range of the 60 GHz photodetector (−5 to 7 dBm) used in the experiment. The VOA before PD is adjusted accordingly in order to plot the BER performance with respect to the received power. After amplifying the received signal, its quality is measured using the high-speed oscilloscope. In order to reduce the discrepancy among the BER performance of the back-toback, conventional and double-pass FOPA caused by the EDFA, its drive current is made constant while the BER measurements are performed at one particular wavelength for all cases. Therefore, the EDFA noise is consistent for all BER curves obtained thus justifying our findings on the performance of conventional and double-pass FOPAs.
Fig. 7. Experimental setup to measure the transmission performance at 10 Gbps.
Figures 8(a) and 8(b) depict the back-to-back eye diagrams at signal wavelength of 1548 nm and 1573 nm respectively. In addition, Figs. 8(c) and 8(d) illustrate the widely open eye diagrams for the output signal of the amplified C-band signal at 1548 nm and the secondary idler at 1573 nm respectively.
#197476 - $15.00 USD Received 10 Sep 2013; revised 23 Nov 2013; accepted 4 Dec 2013; published 13 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031623 | OPTICS EXPRESS 31629
Fig. 8. 10 Gbps eye diagrams back-to-back signal at (a) 1548 nm and (b) 1573 nm, double-pass pump amplified signal at (c) 1548 nm and (d) 1573 nm.
Figures 9(a) and 9(b) show the BER curve comparison for back-to-back, conventional FOPA and double-pass pump FOPA evaluated at signal wavelength 1548 nm and 1573 nm respectively. It is insightful to evaluate the BER of these two wavelengths; by evaluating the BER at signal wavelength of 1548 nm (C-band), we obtain BER performance of the ‘original’ double-pass signal. On the other hand, by measuring BER at signal wavelength of 1573 nm, we obtain the BER performance of the secondary idler. Due to the broad linewidth and marginally degraded OSNR of the signal, the double-pass signal BER could not be better than 10−8, thus BER evaluation at 10−6 is chosen. For reference purposes, the power penalty of conventional FOPA at BER of 10−6 is 1.4 dB and 2.2 dB for wavelength 1548 nm and 1573 nm respectively. For the double-pass amplified C-band signal, the power penalty at BER of 10−6 is 4.1 dB and the power penalty for the secondary L-band idler is 4.7 dB. From here, we can see that the transmission performance of the secondary idler in double-pass pump configuration is acceptable. In double-pass FOPA, forward and backward propagating pumps co-exist inside the nonlinear fiber medium. When a signal is propagating inside the fiber medium, it experiences XPM induced particularly by the counter-propagating pump (relative to the signal propagation direction). Due to the fact that the signal propagates bidirectionally in this experiment (thus achieve the objective elongation of fiber effective length), the backward XPM impact on the signal is virtually doubled. As shown earlier in Fig. 6, the output signal linewidth of a double-pass FOPA is significantly broader than the product of a conventional FOPA. It is also noticed that the OSNR of the signal is lower in double-pass FOPA. This is due to the strong Raman phonon noise (as a result of the elongation of effective fiber length) imposed on the signal. Besides that, the non-reciprocity nature of noise figure and low gain saturation threshold of the double-pass FOPA are also limiting factors. In order to improve the design, a short and low-ZDW-fluctuation HNLF should be used to increase the gain saturation threshold, minimize the Raman phonon noise and reduce noise figure nonreciprocity. To operate the double-pass FOPA in high pump power regime without significant XPM, a two-pump scheme could possibly be adopted to attain the same pump power but with lower intensity.
#197476 - $15.00 USD Received 10 Sep 2013; revised 23 Nov 2013; accepted 4 Dec 2013; published 13 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031623 | OPTICS EXPRESS 31630
(a)
0
-2
-2
-4
-4 log10(BER)
log10(BER)
0
-6
-8
-10
-10
-8 -6 -4 -2 0 2 Received Power (dBm)
B2B Conv DP
-6
-8
-12
(b)
-12
-8 -6 -4 -2 0 2 Received Power (dBm)
Fig. 9. Bit error rate curves comparison (a) at signal 1548 nm and (b) 1573 nm; back-to-back (B2B), conventional FOPA (Conv) and double-pass FOPA (DP).
4. Conclusion In conclusion, we have demonstrated a double-pass pump FOPA that makes full use of the bidirectional parametric gain within the HNLF section. Due to elongation of the effective fiber length, the gain slope in the double-pass pump configuration has improved by more than two fold. For the L-band amplification band, the translated secondary idler is adopted and it is proven with BER experiment. The demonstration of the secondary idler transmission could be very useful in fiber optical passive distribution network, where large duplication of signals is generally required to cover more users. Since the double-pass pump FOPA configuration could duplicate data at the same wavelength with inherent gain, this would greatly extend the distribution network coverage. However, due to the bidirectionally propagating pump, the double-pass signal and idler linewidth are broadened significantly as a result of XPM [9,10], which should be addressed in future research. Acknowledgment This work is partly supported by the Ministry of Higher Education, High Impact Research #A000007-50001 and the Ministry of Science, Technology and Innovation (National Science Fellowship).
#197476 - $15.00 USD Received 10 Sep 2013; revised 23 Nov 2013; accepted 4 Dec 2013; published 13 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031623 | OPTICS EXPRESS 31631
