- Email: [email protected]
Optical NRZ-to-RZ format conversion based on frequency chirp linearization and spectrum slicing
Optics Communications 356 (2015) 113–117
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Optical NRZ-to-RZ format conversion based on frequency chirp linearization and spectrum slicing Dong Wang, Li Huo n, Xin Chen, Xiangyu Jiang, Caiyun Lou Tsinghua National Laboratory for Information Science and Technology, State Key Laboratory of Integrated Optoelectronics, Department of Electronic Engineering, Tsinghua University, Beijing 100084, China
art ic l e i nf o
a b s t r a c t
Article history: Received 30 May 2015 Received in revised form 2 July 2015 Accepted 15 July 2015
A flexible optical NRZ-to-RZ format converter based on a time lens followed by optical filtering is proposed and demonstrated experimentally. After frequency chirp linearization, 9-tone ultra-flat optical frequency comb of 25-GHz frequency spacing within 1 dB power variation is obtained. By changing the shape of the following optical band-pass filter, 3.4-ps Nyquist-shaped RZ signal and 3.7-ps Gaussianshaped RZ signal are both achieved. The sensitivity improvements at a bit error rate of 10 9 are 3.3 dB and 1.7 dB, respectively. & 2015 Published by Elsevier B.V.
Keywords: Optical format conversion Chirping Electrooptic modulators
1. Introduction Non-return-to-zero (NRZ) and return-to-zero (RZ) data formats have been widely used in optical communication systems. While NRZ format is simple to generate and bandwidth-efficient, RZ format provides better long-distance transmission performance and can be potentially useful in the converging layer to aggregate several low bit-rate signals into a high bit-rate signal trunk with optical time-division multiplexing (OTDM) method. Optical NRZto-RZ format converter is one of the key elements for connecting optical networks with different formats. NRZ-to-RZ format converter has been demonstrated in many schemes using semiconductor optical amplifier (SOA) [1], high nonlinearity fiber (HNLF) [2], cascaded modulators [3], and optoelectronic oscillator [4]. Recently, Nyquist optical time-division multiplexing (NOTDM) is proposed and demonstrated for enhancing the tolerance to inter-symbol interference (ISI) and improving spectral efficiency in OTDM network [5]. The optical RZ signal used in the N-OTDM network has a Nyquist-shaped temporal waveform and a rectangular or raised-cosine spectral profile. The conventional NRZ-to-RZ format conversion schemes cannot generate the Nyquist-shaped RZ signal. Several approaches have been demonstrated for optical Nyquist pulse generation, such as electrical Nyquist filtering [6], temporal optical pulse-shaping [7] and cascaded Mach Zehnder modulators (MZMs) [8]. However, these existing Nyquist pulse generation methods cannot be simply adapted for the NRZ-to-RZ n
Corresponding author. E-mail address: [email protected] (L. Huo).
http://dx.doi.org/10.1016/j.optcom.2015.07.034 0030-4018/& 2015 Published by Elsevier B.V.
format conversion. The electrical Nyquist filtering scheme based on digital signal processing (DSP) has a high system complexity and a limited electronic bandwidth. The optical pulse-shaping approach cannot work on the narrow spectrum of the input NRZ signal. Besides the requirements of high signal quality and simple configuration, tunable duty cycle of the converted RZ signal is also very important for the conversion. However, the duty cycle of the converted RZ signal is limited based on cascaded MZMs. In our recent work, we demonstrated an optical Nyquist pulse generation technique based on a time lens followed by optical filtering, which can generate optical Nyquist pulses with a duty cycle of only 8.1% over C band [9]. In this study, we propose an optical NRZ-to-RZ format converter based on frequency chirp linearization and spectrum slicing. The format converter comprises a time lens which consists of a phase modulator (PM) and a MZM with a segment of standard single mode fiber (SMF) inserted between the two modulators, a tunable optical band-pass filter (OBPF) and a piece of dispersion compensating fiber (DCF). Compared with our earlier results of Nyquist pulse generation, the tunable OBPF plays an important role in the format converter. Thanks to the ultra-flat optical frequency comb (OFC) with quasi-linear chirp after the MZM, errorfree NRZ signal to Nyquist-shaped RZ signal conversion and NRZ signal to Gaussian-shaped RZ signal conversion are both achieved by adjusting the shape of OBPF. Nyquist-shaped RZ signal with 8.5% duty cycle and Gaussian-shaped RZ signal with 9.25% duty cycle are obtained as the modulation index of the PM is 2π. The timing jitters are both reduced from 3.3 ps to 0.2 ps. The sensitivity improvements by the two conversions at a bit error rate
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Fig. 1. Schematic of the proposed NRZ-to-RZ format converter. PPG: Pulse pattern generator; BERT: Bit error rate tester.
Fig. 2. Simulation results: temporal amplitude (blue solid line) and chirp (red solid line) of the optical signal after (a) PM, (b) SMF, (c) MZM, respectively. Spectra at the output of (d) MZM and (e) OBPF, respectively. (f) Temporal amplitude (blue solid line) of the optical signal at the output of the converter. The red open dots represent the fitted sinc function curve. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
(BER) of 10 9 are 3.3 dB and 1.7 dB, respectively, which further confirm the high performance of the proposed converter.
2. Working principle The schematic of the proposed NRZ-to-RZ format converter is shown in Fig. 1. The converter consists of two parts: one is used for generating ultra-flat OFC with quasi-linear chirp and the other is used for tuning the shape of RZ signal. The original NRZ signal is first launched into a PM driven by a sinusoidal radio frequency (RF) signal. A periodic sinusoidal distribution of the instantaneous frequency is impressed on the incoming NRZ signal by adjusting an electrical phase shifter (PS1), as shown in Fig. 2(a). The linear
regions of the frequency chirp around the center of the input signal correspond to a time span of 22.3% in a modulation period. The following SMF stretches or compresses the sinusoidal distribution chirp by its chromatic dispersion. As shown in Fig. 2(b), the down-chirp is stretched and can be better fitted to a linear curve within a relatively wide time interval of over 60% period in the central region of the NRZ signal at the output of the SMF. In our simulation, the modulation index of the PM is 2π and the accumulated dispersion of SMF is 4.05 ps/nm. The chirped NRZ signal is subsequently sent into a MZM which acts as a flat-top pulse carver. The relative phase between the RF signals applied to PM and MZM is adjusted by another electrical PS (PS2) so that the signal associated with the down-chirp is retained while the upchirp part is cut off after the MZM, which is shown in Fig. 2(c). The
D. Wang et al. / Optics Communications 356 (2015) 113–117
Fig. 3. The FWHM of the converted Nyquist-shaped RZ signal as a function of PMI.
induced power loss is 5.2 dB. Fig. 2(d) shows the spectrum of the signal at the output of the MZM. 9-Tone OFC within 0.2 dB power variation is obtained. Then, the optical spectrum after the MZM is filtered by a quasi-rectangular OBPF. Fig. 2(e) shows the filtered spectrum with a quasi-rectangular shape. After the quasi-linear down-chirp is compensated by the following DCF, the converted RZ signal is well fitted to a sinc function, which is shown in Fig. 2 (f). Finally, chirp-free Nyquist-shaped RZ signal is achieved. In addition, chirp-free Gaussian-shaped RZ signal also can be obtained as the quasi-rectangular OBPF is replaced by a Gaussianshaped OBPF. It is noted that the phase modulation applied to the converter is crucial to the full width at half maximum (FWHM) of the converted RZ signal. The FWHM of the converted Nyquist-shaped RZ signal as a function of the phase modulation index (PMI) is numerically studied. As shown in Fig. 3, the FWHM of the converted signal is reduced from 7.1 ps to 1 ps as the PMI is increased from 1π to 6π, which corresponds to a duty cycle from 17.8% to 2.5%. The length of SMF is accordingly optimized to ensure that the power variation of the spectrum of the converted signal is less than 1 dB. The simulation results indicate that the FWHM of the converted signal can be significantly decreased by increasing the PMI.
3. Experimental results and discussion The experiments based on the configuration shown in Fig. 1 are carried out to demonstrate the proposed scheme. A continuouswave (CW) light is emitted from a distributed feedback laser (DFB) at the central wavelength of 1546.06 nm with an output power of
115
8 dBm and subsequently sent into a MZM driven by a 25-Gbit/s pseudo-random binary sequence (PRBS) with a word length of 231 1. The eye-diagram and the spectrum of the generated 25Gbit/s NRZ signal are shown in Fig. 4(a) and (b), respectively. The timing jitter of the NRZ signal is measured to be 3.3 ps. The eyediagrams of the optical signals are observed by a 500-GHz optical sampling oscilloscope (OSO) (EXFO PSO-102) and the spectrum is captured with an optical spectrum analyzer (OSA) (Ando AQ6317). The NRZ signal is then boosted by an Erbium-doped fiber amplifier (EDFA) with an output power of 7 dBm and launched into the format converter. In the converter, the PM and MZM are modulated with a 25-GHz sinusoidal RF signal. The modulation index of the PM is 2π and the MZM is biased at the half-maximum transmission point. The spectrum at the output of the PM is shown in Fig. 5(a). The PM induced sinusoidal distribution chirp is stretched by the inserted 250-m SMF with a total dispersion of 4.05 ps/ nm between the modulators. Fig. 5(b) shows that a flat OFC of 9 tones within 1 dB power variation is achieved at the output of the MZM. Then, the side lobes are sliced by a quasi-rectangleshaped OBPF with a flat-top bandwidth of 1.6 nm and the downchirp is compensated by 20 ps/nm DCF. The obtained RZ signal is well fitted to a sinc function with a FWHM of 3.4 ps and a roll-off factor α of 0.15, which is shown in Fig. 5(c). The time bandwidth product (TBP) of the converted Nyquist-shaped RZ signal is 0.85, which is very close to the transform limited TBP of 0.836. The timing jitter of the converted Nyquist-shaped RZ signal is reduced to 0.2 ps. The power loss of the proposed converter is 18 dB. The converted RZ signal is subsequently amplified by anther EDFA and launched into a bit error rate analyzer. Fig. 5(d) shows the BER performance of the format conversion. The receiver sensitivity of the original 25-Gb/s NRZ signal and the converted Nyquist-shaped RZ signal at a BER of 10 9 are measured to be 26 dBm and 29.3 dBm, respectively. Error-free NRZ to Nyquist-shaped RZ format conversion and the negative power penalty of 3.3 dB confirm the high performance of the proposed converter. The converted RZ signal is also transmitted over 40-km SMF followed by a matched dispersion compensation module (DCM). The power penalty of transmission for converted signal is only 0.4 dB, which further confirms the high quality of the signal after format conversion. The influence of the filter bandwidth on the FWHM of the converted signal is further investigated to demonstrate the feasibility of tunable duty cycle in the scheme. By adjusting the bandwidth of the quasi-rectangular OBPF, the FWHM of the converted Nyquist-shaped RZ signal is increased from 3.4 ps to 6.1 ps as the bandwidth of the OBPF is decreased from 1.6 nm to 0.8 nm, which is shown in Fig. 6. The corresponding duty cycle is from 8.5% to 15.3%. NRZ signal to conventional Gaussian-shaped RZ signal conversion is subsequently performed. Thanks to the spectrum with flattop envelope and quasi-linear chirp at the output of MZM,
Fig. 4. (a) Eye-diagram and (b) spectrum of the incident 25-Gbit/s NRZ signal.
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Fig. 5. (a) Spectrum at the output of the PM; (b) spectrum at the output of the MZM; (c) eye-diagram of the converted 25-Gbit/s Nyquist-shaped RZ signal (the blue dotted line represents the fitted sinc function curve); (d) BER performance of the NRZ to Nyquist-shaped RZ format conversion. B2B: back-to-back. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. The FWHM of the converted Nyquist-shaped RZ signal as a function of the filter bandwidth.
Gaussian-shaped RZ signal can be obtained by changing the shape of OBPF in the converter. In the experiment, the quasi-rectangular OBPF is replaced by a Gaussian-shaped OBPF. As shown in Fig. 7(a), the converted RZ signal is well fitted to a Gaussian function with a FWHM of 3.7 ps. The corresponding TBP is 0.463 and the transform limited TBP is 0.441, indicating that the converted Gaussianshaped RZ signal is nearly chirp-free. The timing jitter of the converted Gaussian-shaped RZ signal is 0.2 ps. Fig. 7(b) shows the BER measurement for the incident 25-Gbit/s NRZ signal, the converted Gaussian-shaped RZ signal, the converted Gaussianshaped RZ signal transmitted over the 40-km fiber link, and a reference RZ signal. The reference RZ signal generated by cascaded dual-parallel Mach–Zehnder modulator (DPMZM) and PM has an identical pulse width with the converted signal. Error-free NRZ to Gaussian-shaped RZ format conversion is achieved with 1.7 dB power penalty. Compared to the reference RZ signal, the converted signal has a 0.2-dB power penalty at a BER of 10 9. The power penalty for transmission is 0.4 dB.
Fig. 7. (a) Eye-diagram of the converted 25-Gbit/s Gaussian-shaped RZ signal (the blue dotted line represents the fitted Gaussian function curve); (b) BER performance of the NRZ to Gaussian-shaped RZ format conversion. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
D. Wang et al. / Optics Communications 356 (2015) 113–117
4. Conclusion We proposed and demonstrated an approach for optical NRZto-RZ format conversion based on frequency chirp linearization and spectrum slicing. Nyquist-shaped RZ signal and Gaussianshaped RZ signal with low timing jitter and small duty cycle are obtained. The negative power penalty of error-free format conversion further confirms the high performance of the proposed converter. The influences of phase modulation index and the filter bandwidth on the duty cycle of the converted RZ signal are also studied.
[2]
[3]
[4]
[5]
[6]
Acknowledgments This work is supported by “973” Major State Basic Research Development Program of China (No. 2011CB301703), the National Natural Science Foundation of China (No. 61275032) and Tsinghua University Initiative Scientific Research Program.
[7]
[8]
[9]
References [1] H.N. Tan, M. Matsuura, N. Kishi, Transmission performance of a wavelength and
117
NRZ-to-RZ format conversion with pulsewidth tunability by combination of SOA- and fiber-based switches, Opt. Express 16 (23) (2008) 19063–19071. C.H. Kwok, C. Lin, Polarization-insensitive all-optical NRZ-to-RZ format conversion by spectral filtering of a cross phase modulation broadened signal spectrum, IEEE J. Sel. Topic Quantum Electron. 12 (3) (2006) 451–458. X. Zhao, C. Lou, H. Zhou, D. Lu, L. Huo, Optical regenerative NRZ to RZ format conversion based on cascaded lithium niobate modulators, Opt. Express 18 (23) (2010) 23657–23663. S. Pan, J. Yao, Multichannel optical signal processing in NRZ systems based on a frequency-doubling optoelectronic oscillator, IEEE J. Sel. Topic Quantum Electron. 16 (5) (2010) 1460–1468. M. Nakazawa, T. Hirooka, P. Ruan, P. Guan, Ultrahigh-speed “orthogonal” TDM transmission with an optical Nyquist pulse train, Opt. Express 20 (2) (2012) 1129–1140. R.M. Schmogrow, M. Meyer, P.C. Schindler, A. Josten, S. Ben-Ezra, C. Koos, W. Freude, J. Leuthold, 252 Gbit/s real-time Nyquist pulse generation by reducing the oversampling factor to 1.33, in: Proceedings of OFC Conference 2013, March, Paper OTu2I. 1. K. Harako, D. Seya, T. Hirooka, M. Nakazawa, 640 Gbaud (1.28 Tbit/s/ch) optical Nyquist pulse transmission over 525 km with substantial PMD tolerance, Opt. Express 21 (18) (2013) 21062–21075. M.A. Soto, M. Alem, M.A. Shoaie, A. Vedadi, C.S. Brès, L. Thévenaz, T. Schneider, Generation of Nyquist sinc pulses using intensity modulators, in: Proceedings of CLEO Conference 2013, June, Paper CM4G. 3. D. Wang, L. Huo, Y. Xing, X. Jiang, C. Lou, Optical Nyquist pulse generation using a time lens with spectral slicing, Opt. Express 23 (4) (2015) 4329–4339.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Optical NRZ-to-RZ format conversion based on frequency chirp linearization and spectrum slicing Dong Wang, Li Huo n, Xin Chen, Xiangyu Jiang, Caiyun Lou Tsinghua National Laboratory for Information Science and Technology, State Key Laboratory of Integrated Optoelectronics, Department of Electronic Engineering, Tsinghua University, Beijing 100084, China
art ic l e i nf o
a b s t r a c t
Article history: Received 30 May 2015 Received in revised form 2 July 2015 Accepted 15 July 2015
A flexible optical NRZ-to-RZ format converter based on a time lens followed by optical filtering is proposed and demonstrated experimentally. After frequency chirp linearization, 9-tone ultra-flat optical frequency comb of 25-GHz frequency spacing within 1 dB power variation is obtained. By changing the shape of the following optical band-pass filter, 3.4-ps Nyquist-shaped RZ signal and 3.7-ps Gaussianshaped RZ signal are both achieved. The sensitivity improvements at a bit error rate of 10 9 are 3.3 dB and 1.7 dB, respectively. & 2015 Published by Elsevier B.V.
Keywords: Optical format conversion Chirping Electrooptic modulators
1. Introduction Non-return-to-zero (NRZ) and return-to-zero (RZ) data formats have been widely used in optical communication systems. While NRZ format is simple to generate and bandwidth-efficient, RZ format provides better long-distance transmission performance and can be potentially useful in the converging layer to aggregate several low bit-rate signals into a high bit-rate signal trunk with optical time-division multiplexing (OTDM) method. Optical NRZto-RZ format converter is one of the key elements for connecting optical networks with different formats. NRZ-to-RZ format converter has been demonstrated in many schemes using semiconductor optical amplifier (SOA) [1], high nonlinearity fiber (HNLF) [2], cascaded modulators [3], and optoelectronic oscillator [4]. Recently, Nyquist optical time-division multiplexing (NOTDM) is proposed and demonstrated for enhancing the tolerance to inter-symbol interference (ISI) and improving spectral efficiency in OTDM network [5]. The optical RZ signal used in the N-OTDM network has a Nyquist-shaped temporal waveform and a rectangular or raised-cosine spectral profile. The conventional NRZ-to-RZ format conversion schemes cannot generate the Nyquist-shaped RZ signal. Several approaches have been demonstrated for optical Nyquist pulse generation, such as electrical Nyquist filtering [6], temporal optical pulse-shaping [7] and cascaded Mach Zehnder modulators (MZMs) [8]. However, these existing Nyquist pulse generation methods cannot be simply adapted for the NRZ-to-RZ n
Corresponding author. E-mail address: [email protected] (L. Huo).
http://dx.doi.org/10.1016/j.optcom.2015.07.034 0030-4018/& 2015 Published by Elsevier B.V.
format conversion. The electrical Nyquist filtering scheme based on digital signal processing (DSP) has a high system complexity and a limited electronic bandwidth. The optical pulse-shaping approach cannot work on the narrow spectrum of the input NRZ signal. Besides the requirements of high signal quality and simple configuration, tunable duty cycle of the converted RZ signal is also very important for the conversion. However, the duty cycle of the converted RZ signal is limited based on cascaded MZMs. In our recent work, we demonstrated an optical Nyquist pulse generation technique based on a time lens followed by optical filtering, which can generate optical Nyquist pulses with a duty cycle of only 8.1% over C band [9]. In this study, we propose an optical NRZ-to-RZ format converter based on frequency chirp linearization and spectrum slicing. The format converter comprises a time lens which consists of a phase modulator (PM) and a MZM with a segment of standard single mode fiber (SMF) inserted between the two modulators, a tunable optical band-pass filter (OBPF) and a piece of dispersion compensating fiber (DCF). Compared with our earlier results of Nyquist pulse generation, the tunable OBPF plays an important role in the format converter. Thanks to the ultra-flat optical frequency comb (OFC) with quasi-linear chirp after the MZM, errorfree NRZ signal to Nyquist-shaped RZ signal conversion and NRZ signal to Gaussian-shaped RZ signal conversion are both achieved by adjusting the shape of OBPF. Nyquist-shaped RZ signal with 8.5% duty cycle and Gaussian-shaped RZ signal with 9.25% duty cycle are obtained as the modulation index of the PM is 2π. The timing jitters are both reduced from 3.3 ps to 0.2 ps. The sensitivity improvements by the two conversions at a bit error rate
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Fig. 1. Schematic of the proposed NRZ-to-RZ format converter. PPG: Pulse pattern generator; BERT: Bit error rate tester.
Fig. 2. Simulation results: temporal amplitude (blue solid line) and chirp (red solid line) of the optical signal after (a) PM, (b) SMF, (c) MZM, respectively. Spectra at the output of (d) MZM and (e) OBPF, respectively. (f) Temporal amplitude (blue solid line) of the optical signal at the output of the converter. The red open dots represent the fitted sinc function curve. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
(BER) of 10 9 are 3.3 dB and 1.7 dB, respectively, which further confirm the high performance of the proposed converter.
2. Working principle The schematic of the proposed NRZ-to-RZ format converter is shown in Fig. 1. The converter consists of two parts: one is used for generating ultra-flat OFC with quasi-linear chirp and the other is used for tuning the shape of RZ signal. The original NRZ signal is first launched into a PM driven by a sinusoidal radio frequency (RF) signal. A periodic sinusoidal distribution of the instantaneous frequency is impressed on the incoming NRZ signal by adjusting an electrical phase shifter (PS1), as shown in Fig. 2(a). The linear
regions of the frequency chirp around the center of the input signal correspond to a time span of 22.3% in a modulation period. The following SMF stretches or compresses the sinusoidal distribution chirp by its chromatic dispersion. As shown in Fig. 2(b), the down-chirp is stretched and can be better fitted to a linear curve within a relatively wide time interval of over 60% period in the central region of the NRZ signal at the output of the SMF. In our simulation, the modulation index of the PM is 2π and the accumulated dispersion of SMF is 4.05 ps/nm. The chirped NRZ signal is subsequently sent into a MZM which acts as a flat-top pulse carver. The relative phase between the RF signals applied to PM and MZM is adjusted by another electrical PS (PS2) so that the signal associated with the down-chirp is retained while the upchirp part is cut off after the MZM, which is shown in Fig. 2(c). The
D. Wang et al. / Optics Communications 356 (2015) 113–117
Fig. 3. The FWHM of the converted Nyquist-shaped RZ signal as a function of PMI.
induced power loss is 5.2 dB. Fig. 2(d) shows the spectrum of the signal at the output of the MZM. 9-Tone OFC within 0.2 dB power variation is obtained. Then, the optical spectrum after the MZM is filtered by a quasi-rectangular OBPF. Fig. 2(e) shows the filtered spectrum with a quasi-rectangular shape. After the quasi-linear down-chirp is compensated by the following DCF, the converted RZ signal is well fitted to a sinc function, which is shown in Fig. 2 (f). Finally, chirp-free Nyquist-shaped RZ signal is achieved. In addition, chirp-free Gaussian-shaped RZ signal also can be obtained as the quasi-rectangular OBPF is replaced by a Gaussianshaped OBPF. It is noted that the phase modulation applied to the converter is crucial to the full width at half maximum (FWHM) of the converted RZ signal. The FWHM of the converted Nyquist-shaped RZ signal as a function of the phase modulation index (PMI) is numerically studied. As shown in Fig. 3, the FWHM of the converted signal is reduced from 7.1 ps to 1 ps as the PMI is increased from 1π to 6π, which corresponds to a duty cycle from 17.8% to 2.5%. The length of SMF is accordingly optimized to ensure that the power variation of the spectrum of the converted signal is less than 1 dB. The simulation results indicate that the FWHM of the converted signal can be significantly decreased by increasing the PMI.
3. Experimental results and discussion The experiments based on the configuration shown in Fig. 1 are carried out to demonstrate the proposed scheme. A continuouswave (CW) light is emitted from a distributed feedback laser (DFB) at the central wavelength of 1546.06 nm with an output power of
115
8 dBm and subsequently sent into a MZM driven by a 25-Gbit/s pseudo-random binary sequence (PRBS) with a word length of 231 1. The eye-diagram and the spectrum of the generated 25Gbit/s NRZ signal are shown in Fig. 4(a) and (b), respectively. The timing jitter of the NRZ signal is measured to be 3.3 ps. The eyediagrams of the optical signals are observed by a 500-GHz optical sampling oscilloscope (OSO) (EXFO PSO-102) and the spectrum is captured with an optical spectrum analyzer (OSA) (Ando AQ6317). The NRZ signal is then boosted by an Erbium-doped fiber amplifier (EDFA) with an output power of 7 dBm and launched into the format converter. In the converter, the PM and MZM are modulated with a 25-GHz sinusoidal RF signal. The modulation index of the PM is 2π and the MZM is biased at the half-maximum transmission point. The spectrum at the output of the PM is shown in Fig. 5(a). The PM induced sinusoidal distribution chirp is stretched by the inserted 250-m SMF with a total dispersion of 4.05 ps/ nm between the modulators. Fig. 5(b) shows that a flat OFC of 9 tones within 1 dB power variation is achieved at the output of the MZM. Then, the side lobes are sliced by a quasi-rectangleshaped OBPF with a flat-top bandwidth of 1.6 nm and the downchirp is compensated by 20 ps/nm DCF. The obtained RZ signal is well fitted to a sinc function with a FWHM of 3.4 ps and a roll-off factor α of 0.15, which is shown in Fig. 5(c). The time bandwidth product (TBP) of the converted Nyquist-shaped RZ signal is 0.85, which is very close to the transform limited TBP of 0.836. The timing jitter of the converted Nyquist-shaped RZ signal is reduced to 0.2 ps. The power loss of the proposed converter is 18 dB. The converted RZ signal is subsequently amplified by anther EDFA and launched into a bit error rate analyzer. Fig. 5(d) shows the BER performance of the format conversion. The receiver sensitivity of the original 25-Gb/s NRZ signal and the converted Nyquist-shaped RZ signal at a BER of 10 9 are measured to be 26 dBm and 29.3 dBm, respectively. Error-free NRZ to Nyquist-shaped RZ format conversion and the negative power penalty of 3.3 dB confirm the high performance of the proposed converter. The converted RZ signal is also transmitted over 40-km SMF followed by a matched dispersion compensation module (DCM). The power penalty of transmission for converted signal is only 0.4 dB, which further confirms the high quality of the signal after format conversion. The influence of the filter bandwidth on the FWHM of the converted signal is further investigated to demonstrate the feasibility of tunable duty cycle in the scheme. By adjusting the bandwidth of the quasi-rectangular OBPF, the FWHM of the converted Nyquist-shaped RZ signal is increased from 3.4 ps to 6.1 ps as the bandwidth of the OBPF is decreased from 1.6 nm to 0.8 nm, which is shown in Fig. 6. The corresponding duty cycle is from 8.5% to 15.3%. NRZ signal to conventional Gaussian-shaped RZ signal conversion is subsequently performed. Thanks to the spectrum with flattop envelope and quasi-linear chirp at the output of MZM,
Fig. 4. (a) Eye-diagram and (b) spectrum of the incident 25-Gbit/s NRZ signal.
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Fig. 5. (a) Spectrum at the output of the PM; (b) spectrum at the output of the MZM; (c) eye-diagram of the converted 25-Gbit/s Nyquist-shaped RZ signal (the blue dotted line represents the fitted sinc function curve); (d) BER performance of the NRZ to Nyquist-shaped RZ format conversion. B2B: back-to-back. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. The FWHM of the converted Nyquist-shaped RZ signal as a function of the filter bandwidth.
Gaussian-shaped RZ signal can be obtained by changing the shape of OBPF in the converter. In the experiment, the quasi-rectangular OBPF is replaced by a Gaussian-shaped OBPF. As shown in Fig. 7(a), the converted RZ signal is well fitted to a Gaussian function with a FWHM of 3.7 ps. The corresponding TBP is 0.463 and the transform limited TBP is 0.441, indicating that the converted Gaussianshaped RZ signal is nearly chirp-free. The timing jitter of the converted Gaussian-shaped RZ signal is 0.2 ps. Fig. 7(b) shows the BER measurement for the incident 25-Gbit/s NRZ signal, the converted Gaussian-shaped RZ signal, the converted Gaussianshaped RZ signal transmitted over the 40-km fiber link, and a reference RZ signal. The reference RZ signal generated by cascaded dual-parallel Mach–Zehnder modulator (DPMZM) and PM has an identical pulse width with the converted signal. Error-free NRZ to Gaussian-shaped RZ format conversion is achieved with 1.7 dB power penalty. Compared to the reference RZ signal, the converted signal has a 0.2-dB power penalty at a BER of 10 9. The power penalty for transmission is 0.4 dB.
Fig. 7. (a) Eye-diagram of the converted 25-Gbit/s Gaussian-shaped RZ signal (the blue dotted line represents the fitted Gaussian function curve); (b) BER performance of the NRZ to Gaussian-shaped RZ format conversion. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
D. Wang et al. / Optics Communications 356 (2015) 113–117
4. Conclusion We proposed and demonstrated an approach for optical NRZto-RZ format conversion based on frequency chirp linearization and spectrum slicing. Nyquist-shaped RZ signal and Gaussianshaped RZ signal with low timing jitter and small duty cycle are obtained. The negative power penalty of error-free format conversion further confirms the high performance of the proposed converter. The influences of phase modulation index and the filter bandwidth on the duty cycle of the converted RZ signal are also studied.
[2]
[3]
[4]
[5]
[6]
Acknowledgments This work is supported by “973” Major State Basic Research Development Program of China (No. 2011CB301703), the National Natural Science Foundation of China (No. 61275032) and Tsinghua University Initiative Scientific Research Program.
[7]
[8]
[9]
References [1] H.N. Tan, M. Matsuura, N. Kishi, Transmission performance of a wavelength and
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