Low noise optical multi-carrier generation using ... - OSA Publishing

Low noise optical multi-carrier generation using optical-FIR filter for ASE noise suppression in re-circulating frequency shifter loop Jiachuan Lin,1 Lixia Xi,1 Jianrui Li,1 Xiaoguang Zhang,1,* Xia Zhang,1,2 and Shahab Ahmad Niazi3 1

2

State Key Laboratory of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications, Beijing 100876, China The Key Laboratory of Optical Communications Science & Technology in Shandong Province, Liaocheng University, Liaocheng 252000, China 3 Comsats Institute of Information and Technology, Attock 43600, Pakistan * [email protected]

Abstract: In this paper, an improved multi-carrier generation scheme based on single-side-band recirculating frequency shifter with optical finite impulse response (FIR) filter for amplified spontaneous emission (ASE) noise suppression is proposed and experimentally demonstrated. The carrier-to-noise-ratio (CNR) instead of tone-to-noise-ratio (TNR) is introduced to more reasonably and exactly evaluate the signal-to-noise-ratio of a multi-carrier source with non-flat noise floor. We have experimentally attain the worst case CNR of 22.5dB and 19.1dB for generated 50 and 69 flat low noise carriers, which has shown significant improvement than the previous cited works based on recirculating frequency shifter. ©2014 Optical Society of America OCIS codes: (060.0060) Fiber optics and optical communications; (060.2630) Frequency modulation.

References and links 1.

F. Tian, X. Zhang, L. Xi, A. Stark, S. E. Ralph, and G. K. Chang, “Experiment of 2.56-Tb/s, polarization division multiplexing return-to-zero 16-ary quadrature amplitude modulation, 25 GHz grid coherent optical wavelength division multiplexing, 800 km transmission based on optical comb in standard single-mode fiber,” Opt. Eng. 52(11), 116103 (2013). 2. D. Hillerkuss, R. Schmogrow, M. Meyer, S. Wolf, M. Jordan, P. Kleinow, and J. Leuthold, “Single-laser 32.5 Tbit/s Nyquist WDM transmission,” J. Opt. Commun. Netw. 4(10), 715–723 (2012). 3. Y. Ma, Q. Yang, Y. Tang, S. Chen, and W. Shieh, “1-Tb/s single-channel coherent optical OFDM transmission over 600-km SSMF fiber with subwavelength bandwidth access,” Opt. Express 17(11), 9421–9427 (2009). 4. S. Chandrasekhar, X. Liu, B. Zhu, and D. W. Peckham, “Transmission of a 1.2-Tb/s 24-carrier no-guard-interval coherent OFDM superchannel over 7200-km of ultra-large-area fiber,” European Conference on Optical Communications, paper PD2.6, Vienna, Austria (2009). 5. X. Liu, S. Chandrasekhar, X. Chen, P. J. Winzer, Y. Pan, T. F. Taunay, B. Zhu, M. Fishteyn, M. F. Yan, J. M. Fini, E. M. Monberg, and F. V. Dimarcello, “1.12-Tb/s 32-QAM-OFDM superchannel with 8.6-b/s/Hz intrachannel spectral efficiency and space-division multiplexed transmission with 60-b/s/Hz aggregate spectral efficiency,” Opt. Express 19(26), B958–B964 (2011). 6. G. Bosco, V. Curri, A. Carena, P. Poggiolini, and F. Forghieri, “On the performance of Nyquist-WDM terabit superchannels based on PM-BPSK, PM-QPSK, PM-8QAM or PM-16QAM subcarriers,” J. Lightwave Technol. 29(1), 53–61 (2011). 7. N. K. Fontaine, “Spectrally-sliced coherent receivers for THz bandwidth optical communications.” European Conference on Optical Communications, paper Mo.3.C.1, London, UK (2012). 8. M. A. Mirza and G. Stewart, “Multi-wavelength operation of erbiumdoped fiber lasers by periodic filtering and phase modulation,” J. Lightwave Technol. 27(8), 1034–1044 (2009). 9. J. Zhang, J. Yu, N. Chi, Z. Dong, X. Li, Y. Shao, J. Yu, and L. Tao, “Flattened comb generation using only phase modulators driven by fundamental frequency sinusoidal sources with small frequency offset,” Opt. Lett. 38(4), 552–554 (2013). 10. X. Zhou, X. Zheng, H. Wen, H. Zhang, and B. Zhou, “Generation of broadband optical frequency comb with rectangular envelope using cascaded intensity and dual-parallel modulators,” Opt. Commun. 313, 356–359

#205486 - $15.00 USD Received 27 Jan 2014; revised 16 Mar 2014; accepted 17 Mar 2014; published 27 Mar 2014 (C) 2014 OSA 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.007852 | OPTICS EXPRESS 7852

(2014). 11. J. Zhang, N. Chi, J. Yu, Y. Shao, J. Zhu, B. Huang, and L. Tao, “Generation of coherent and frequency-lock multi-carriers using cascaded phase modulators and recirculating frequency shifter for Tb/s optical communication,” Opt. Express 19(14), 12891–12902 (2011). 12. J. Li, X. Zhang, F. Tian, and L. Xi, “Theoretical and experimental study on generation of stable and high-quality multi-carrier source based on re-circulating frequency shifter used for Tb/s optical transmission,” Opt. Express 19(2), 848–860 (2011). 13. F. Tian, X. Zhang, J. Li, and L. Xi, “Generation of 50 stable frequency-locked optical carriers for Tb/s multicarrier optical transmission using a recirculating frequency shifter,” J. Lightwave Technol. 29(8), 1085–1091 (2011). 14. J. Zhang, J. Yu, N. Chi, Z. Dong, Y. Shao, L. Tao, and X. Li, “Theoretical and experimental study on improved frequency-locked multi-carrier generation by using recirculating loop based on multi-frequency shifting single-side band modulation,” IEEE Photon. J. 4(6), 2249–2261 (2012). 15. J. Zhang, J. Yu, N. Chi, Y. Shao, L. Tao, J. Zhu, and Y. Wang, “Stable Optical Frequency-Locked Multicarriers Generation by Double Recirculating Frequency Shifter Loops for Tb/s Communication,” J. Lightwave Technol. 30(24), 3938–3945 (2012). 16. J. Li and Z. Li, “Frequency-locked multicarrier generator based on a complementary frequency shifter with double recirculating frequency-shifting loops,” Opt. Lett. 38(3), 359–361 (2013). 17. J. Li, C. Yu, and Z. Li, “Complementary frequency shifter based on polarization modulator used for generation of a high-quality frequency-locked multicarrier,” Opt. Lett. 39(6), 1513–1516 (2014). 18. G. P. Agrawal, Applications of Nonlinear Fiber Optics (Academic Press, 2001), Chap. 4.

1. Introduction Multi-carrier source generation is crucial for achieving higher data rates optical communication systems with multi-carrier modulation formats, such as coherent dense wavelength division multiplexing (Co-DWDM) [1,2], coherent optical orthogonal frequency division multiplexing (Co-OFDM) [3–5] and Nyquist-WDM [6] based superchannel systems. Optical multi-carrier sources with high quality and large number of carriers are good candidates to serve as a laser array at transmitter end [1–5] or a local oscillator array at receiver end [7] for ultra-high speed and spectrum efficiency (SE) optical communication systems. On the other hand, these frequency locked carriers are very important to super-channel systems as small frequency drift of laser source would immediately lead to inter-channel crosstalk. Many endeavors have been made on generation of optical multi-carrier source such as multi-wavelength erbium-dropped fiber laser [8], cascaded modulators [5,9–11] and re-circulating frequency shifter (RFS) [3,4,11–13]. Among these technologies, single-side-band (SSB) modulation based RFS loop has attracted much attention due several advantages of relative simple structure, flexibility on frequency spacing control, low driving voltages, less sensitive to phase noise and ability to generate large number of flat carriers. Whereas, in most of SSB-RFS multi-carrier source applied terabits long reach transmission experiments, only 20~40 carriers (having a worst TNR of 20~25dB) are generated for signal loading. One of the reason is the noise nature of SSB-RFS method that ASE noise accumulates round by round, resulting limited number of available carriers, especially for cases of large desired carrier number (>50) and multi-EDFAs deployed in the loop [1,3,4]. Therefore, further investigations are required for low noise RFS based multi-carrier generation schemes. There are many improved RFS implementations techniques have been proposed to achieve better performance by halving required circulating times, such as multi-frequency shifting (MFS) method [14], double RFS structure [15] and complementary frequency shifter (CFS) loop [16,17], while increasing the complicity of the structures, doubling optical components, and limited noise characteristic improvements. In this paper, we propose and experimentally demonstrate an improved SSB-RFS optical multi-carrier generation configuration with an ASE noise suppression scheme using an optical FIR filter. By applying this scheme, notable carrier-to-noise ratio (CNR) performance improvement is achieved. With proposed optical FIR ASE noise suppression scheme deployed in SSB-RFS loop, 50 and 69 stable and flat carriers with high CNR are generated.


2. Proposed scheme and noise characteristic analysis 2.1 Proposed ASE noise suppression scheme The proposed low noise SSB-RFS multi-carrier generator with ASE noise suppression scheme is illustrated in Fig. 1(a). This configuration includes a basic SSB-RFS loop and an optical FIR filter for noise suppression. In the basic SSB-RFS loop [3,4,12,13], the seed carrier of frequency f0 is provided by CW laser, and an I/Q modulator driven by two RF signals of frequency fs is used to implement carrier frequency shift. The exact polarization alignments are ensured by the polarization controllers (PCs) and the number of generated carriers is controlled by a band pass filter (BPF). The role of EDFA is to compensate the total loss suffered in one round trip (RT) with inevitable ASE noise accumulation that could result in a great system performance degradation. In our proposed scheme, an N-tap optical FIR structured notch filter is placed after EDFA to further reduce accumulated ASE noise, which ensures a significant improvement in overall system performance.

Fig. 1. (a) Schematic of proposed low noise SSB-RFS multi-carrier generation scheme; (b) parallel implementation of optical FIR filter for ASE noise suppression; (c) serial implementation of optical FIR filter for ASE noise suppression. PC: polarization controller; BPF: band pass filter; EDFA: erbium doped fiber amplifier; OSA: optical spectrum analyzer; RF: radio frequency; PS: phase shifter; EA: electrical amplifier.

In this work, two different structures of optical FIR filter implementations are taken into consideration, one is N-tap parallel structured FIR (direct implementation) and the other is N cascaded 2-tap one as illustrated in Figs. 1(b) and 1(c). The theoretical analysis of noise accumulation and reduction will be given out in the next part, and then the system performance improvement and experiment results. 2.2 Analysis of noise accumulation and CNR definition It is convenient to treat the output of SSB-RFS loop in frequency domain in the presence of ASE noise, and a recursive expression can be represented by Eq. (1). E1 ( f ) = E0 ( f ) +  g1 ⋅ l ⋅ T ( f ) ⊗ E0 ( f ) + n1 ( f )  H BPF ( f ) E2 ( f ) = E0 ( f ) +  g 2 ⋅ l ⋅ T ( f ) ⊗ E1 ( f ) + n2 ( f ) H BPF ( f ) , 

(1)

En ( f ) = E0 ( f ) +  g n ⋅ l ⋅ T ( f ) ⊗ En −1 ( f ) + nn ( f )  H BPF ( f )

where E0 ( f ) is the seed laser centered at f 0 , En ( f ) is the output of nth (n = 1,2,…N) RT. l is the total loop loss including modulation loss and insertion loss, and g n is EDFA gain in


nth RT. A stable and flat output is always ensured by EDFA condition of g n ⋅ l ≈ 1 .The transfer functions of IQ modulator is denoted by T ( f ) ≈ FFT ( e j 2π f s t ) = δ ( f − f s ) .

However, 3rd-order crosstalk is neglected for simplicity as this work mainly focus on processes of ASE noise accumulation and reduction. ⊗ denotes convolution operator and the BPF transfer function H BPF ( f ) is assumed ideal rectangle window here and represented by Eq. (2). 1 H rect ( f ) =  0 nn ( f

)

f 0 -f s /2