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A simple AMI-RZ transmitter based on single-arm intensity modulator and optical delay interferometer
Optics Communications 255 (2005) 35–40 www.elsevier.com/locate/optcom
A simple AMI-RZ transmitter based on single-arm intensity modulator and optical delay interferometer Guo-Wei Lu *, Lian-Kuan Chen, Chun-Kit Chan Department of Information Engineering, The Chinese University of Hong Kong, Room 827, HSH Building, CUHK, Shatin, N.T., Hong Kong, SAR Received 2 October 2004; received in revised form 8 April 2005; accepted 31 May 2005
Abstract In the paper, we propose and demonstrate a simple alternate-mark inversion return-to-zero (AMI-RZ) transmitter upgraded from existing plain RZ-OOK transmitter by employing an additional Mach–Zehnder delay interferometer (MZDI), eliminating the use of high-speed RF circuit and dual-drive Mach–Zehnder modulator. A delay-and-subtract operation is performed by the MZDI on the RZ-OOK signal to achieve alternating phases in successive mark pulses. The nonlinearity tolerance of the generated AMI-RZ signal was characterized by experiment at 10 Gb s 1 and simulation at 40 Gb s 1. The ghost-pulse, due to intra-channel four-wave-mixing (IFWM), with generated AMI-RZ and RZOOK formats was also investigated respectively at 40 Gb s 1. The results showed that the generated AMI-RZ exhibited higher nonlinear tolerance over RZ-OOK, especially the tolerance against IFWM in the 40-Gb s 1 optical transmission system. This AMI-RZ transmitter scheme is simple, potentially low cost and can be integrated easily with the existing RZ-OOK transmitters for upgrading to AMI-RZ transmitters. Ó 2005 Elsevier B.V. All rights reserved. PACS: 42.79.S Keywords: Alternate-mark inversion return-to-zero; Modulation format; Intra-channel four-wave-mixing; Modified duobinary returnto-zero; Duobinary
1. Introduction
*
Corresponding author. Tel.: +852 2609 8479; fax: +852 2603 5032. E-mail address: [email protected] (G.-W. Lu).
Alternate-mark inversion return-to-zero (AMIRZ) modulation format has attracted much attention as further advantages can be offered compared with regular RZ, duobinary RZ, and alternatingphase RZ signals. With alternating phases in the
0030-4018/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2005.05.039
36
G.-W. Lu et al. / Optics Communications 255 (2005) 35–40
successive mark pulses, AMI-RZ signal has higher tolerance against various detrimental effects, including self-phase modulation (SPM) in single channel systems, cross-phase modulation (XPM) in wavelength-division-multiplexing (WDM) transmission systems, intra-channel four-wave-mixing (IFWM) in high-speed optical systems [1]. It also helps to increase the threshold for the stimulated Brillouin scattering (SBS) [2]. Conventionally, in addition to the differential precoder needed for radio frequency (RF) driving signal, the generation of AMI signals required dual-drive Mach–Zehnder (MZ) modulator biased at zero transmission, with electrical three-level shaping filter [1], high-speed RF logical circuit [3], or high-speed RF driving circuit with well-controlled timing-delay [4]. Recently, a simplified duobinary transmitter based on a single-arm x-cut MZ modulator and a three-level shaping filter has been proposed to further reduce the complexity and cost [5], which also can be used to generate AMI-RZ with modification of the driver. However, these electrical methods suffer from one or more of the following potential drawbacks, (i) performance dependency on the word length due to shaping filter induced distortion, (ii) strict symmetric requirement for the two drivers in dual-drive MZ modulators, (iii) high complexity and cost, especially for high-speed systems, and (iv) high driving voltage required (2Vp). An optical method based on a piece of polarization-maintaining (PM) fiber and a polarizer to generate pulsewidth tuneable AMI-RZ has been proposed, but is less practical due to its polarization sensitivity [6]. Recently, it was demonstrated that optical differential phase shift keying (DPSK) signals after direct detection could be considered as a duobinary or AMI signals [7–9]. Based on this idea, in this paper, we propose and demonstrate a simple AMI optical transmitter based on the delay-and-subtract operation on the more commonly used RZ-OOK or NRZ-OOK signal with conventional intensity modulators. Compared with the typical RZ-OOK transmitter, only an additional Mach–Zehnder delay interferometer and a differential precoder are needed. This simple approach facilitates the upgrade of the existing RZ-OOK transmitters into the optical AMI-RZ transmitters. The transmission performance of the generated AMI-RZ in
the nonlinear operation regime with different signal launch power was investigated by experiment at 10 Gb s 1 and simulation at 40 Gb s 1. Compared with corresponding RZ-OOK system, it showed a 1-dB input power dynamic range (IPDR) improvement in 10 Gb s 1 and 1.7-dB improvement in 40-Gb s 1 AMI-RZ system at 1-dB penalty level.
2. A simple AMI-RZ transmitter The configuration of the simple AMI-RZ transmitter is shown in Fig. 1(a). It consists of a singlearm LiNbO3 modulator (LN-MOD) followed by a Mach–Zehnder delay interferometer (MZDI). The MZDI consists of optical fiber and two 50:50 fiber couplers. The MZDI splits the incoming signal intensity equally into two branches, one of which has an additional path delay. A thermoelectric cooler (TEC) is utilized to change and stabilize the phase difference between two arms. By finetuning the temperature, the destructive (or constructive) interference can be easily achieved at the output of MZDI with (2m + 1)p (or 2mp) phase difference between the two arms of MZDI. For the proposed AMI-RZ transmitter module, the delay-and-subtract operation can be performed on an NRZ-OOK or an RZ-OOK signal, which corresponds to using a CW source or using a pulse source as the input signal of the module. In the case with a CW source input, the pulse-width of the generated AMI-RZ signal is determined by the speed of the intensity modulation, i.e., modulation rise/fall times, or the relative time delay of MZDI, Dt, which should be shorter than one-bit time in this case [9]. In the latter case with a pulse source input, the relative time delay of MZDI is chosen as one-bit time, and the pulse-width of generated AMI-RZ is the same as that of the input pulse train. In order to maintain the same pulse width for RZ-OOK and AMI-RZ signals for performance comparison, a pulse source is chosen as the input signal here. The working principle of this AMI-RZ transmitter is illustrated in Fig. 1(b) with a pulse source as the input signal. After the single-arm intensity modulator, the pulse train is modulated into RZOOK signal. When the two arms of MZDI are
G.-W. Lu et al. / Optics Communications 255 (2005) 35–40
37
11101111
Differential encoder 10110101
Time Delay & Phase Shifter
∆t
b a
CW or Pulse Train
a
LN MOD
c
MZDI
1 0 1 1 0 1 0 1
Lower Arm: Phase=0
a 0 1 0 1 1 0 1 0
Upper Arm: Phase=π
b 1 1 1 0 1 1 1 1
b
c
Alternating Phase Shift between Mark Level
Fig. 1. Illustration of (a) the configuration and (b) the operation principle of the AMI-RZ transmitter. (White pulses are of phase 0; grey pulses are of phase p).
set to be out-of phase, the signals in both arms (lower arm: sequence a; upper arm: sequence b, in Fig. 1(b)) will experience destructive interference at the output of the MZDI. Meanwhile, the relative phase difference between the two arms is still preserved for the generated mark pulses, as shown in the sequence c in Fig. 1(b). Thus the generated RZ signal will have an alternating p phase shift between the successive mark bits, conforming to the phase requirement of a typical AMI-RZ signal. By employing a differential precoder at the RF driving signal of the intensity modulator, as shown in Fig. 1(a), the intensity of generated sequence c (see Fig. 1(b)) is consistent with the original data. Hence, via MZDI, the signal was successfully converted from RZ-OOK to AMI-RZ. The delay-andsubtract operation applied on the OOK signal is equivalent to that applied on the DPSK signal [9] to generate AMI signals.
10.61-GHz sinusoidal wave, was employed to generate a 10.61-GHz optical RZ pulse train with 28ps full-width at half-maximum (FWHM) pulse width. Then the optical RZ pulse train at 1550 nm was fed into the AMI-RZ transmitter, which consisted of a single-arm LN-MOD and an MZDI with a relative delay of 94 ps between the two arms. A 10.61-Gb s 1 pseudorandom bit sequence (PRBS) with length of 231 1 was applied to the LN-MOD. In our experiment, a differential precoder was not employed since the differentially encoded PRBS is simply a timedelayed copy of the PRBS. However, a differential decoder would be needed in practical applications.
High Power EDFA
Clock 231-1 PRBS
RX EA MOD MZ-MOD
MZDI
VOA
40 km 8 km SMF DCF EDFA OBF
RZ-OOK AMI-RZ
3. Experiment and simulation results The experiment setup was shown in Fig. 2. An electro-absorption (EA) modulator, driven by a
Fig. 2. The experimental setup, MOD: Modulator, VOA: Variable optical attenuator, EDFA: Erbium-doped fiber amplifier, OBF: Optical Bandpass filter, MZDI: Mach–Zehnder Interferometer.
G.-W. Lu et al. / Optics Communications 255 (2005) 35–40
The output modulated signal from the transmitter was further amplified by a high-power Erbium doped fiber amplifier (EDFA), and then passed through a variable optical attenuator (VOA) so that the signal power could be varied from 0 to 19 dB m. The large dynamic range of 19 dB was used to emulate the nonlinearity scenario. The amplified signal was then fed into the transmission link consisting of a piece of 40-km standard signalmode fiber (SMF) with D = 17 ps/(nm Æ km)@ 1550 nm and a piece of 8-km dispersion compensating fiber (DCF) with D = 85 ps/(nm Æ km)@ 1550 nm for dispersion compensation, where the dispersion was fully compensated. Before being launched into the optical receiver, the signal was pre-amplified by an EDFA and extracted by a 1-nm FWHM optical bandpass filter (OBF). At the receiving end, the AMI-RZ signal was directly detected. To compare the nonlinear tolerance, a control experiment with 10-Gb s 1 RZ-OOK signal was also performed without the MZDI at the transmitter side. Fig. 3(a) showed the optical power spectrum of the AMI-RZ signal generated by the proposed transmitter. Power nulls were shown at the frequencies corresponding to the carrier clock tones of a typical RZ-OOK signal (see Fig. 3(b)). It means that the all discrete frequency tones that appear in the conventional RZ-OOK signal are suppressed in AMI-RZ, which contributes to the higher tolerance of AMI-RZ over RZ-OOK against IFWM [1]. The measured BER back-to-back performance of 10-Gb s 1 RZ-OOK and generated AMI-RZ signals was shown in Fig. 4. The receiver sensitivities (@BER = 10 9) of the RZ-OOK and AMI-RZ signals were 20.8 and 20.4 dB m, respectively. This indicated that for the generated AMI-RZ signal, there was a small power penalty of 0.4 dB, which could be attributed to the phase-to-intensity noise conversion through the MZDI due to the phase noise from the laser or the chirp from the modulator. The imperfect MZDI due to the unbalanced coupling ratio of couplers may further exacerbate the signal performance as it reduces the eye-opening. In order to evaluate the nonlinear tolerance of the AMI-RZ signal, we measured the receiver sensitivities at a BER of 10 9 against different launch
Fig. 3. Optical power spectra of (a) AMI-RZ and (b) RZOOK. Inset: the corresponding eye diagrams.
1e-2 10Gbps RZ-OOK 10Gbps AMI-RZ
1e-3 1e-4 1e-5
BER
38
1e-6 1e-7 1e-8 1e-9
~0.4 dB
1e-10 -24
-23
-22
-21
-20
Received Power (dBm)
Fig. 4. Measured BER curves for back-to-back and transmission of RZ-OOK and AMI-RZ signals.
powers, for systems using the 10-Gb s 1 AMI-RZ and RZ-OOK modulation formats, respectively. The corresponding power penalties, obtained with
G.-W. Lu et al. / Optics Communications 255 (2005) 35–40
Power Penalty @ BER=10-9 (dB)
reference to the measured back-to-back receiver sensitivities, were shown in Fig. 5. When the signal launch power exceeded 14 dB m, the signal began to suffer from fiber nonlinearities including SBS and intra-channel nonlinearities, i.e., intrachannel XPM (IXPM) and IFWM. These nonlinear effects caused a drastic increase of the power penalty when the signal launch power was further increased. At 1-dB power penalty level, it was shown that the AMI-RZ format offered a 1-dB enhancement of the IPDR over the RZ-OOK modulation format. This could be attributed to the higher tolerance of AMI-RZ against fiber nonlinearities, such as IXPM and SBS, over RZ-OOK format in 10-Gb s 1 transmission systems [10]. To assess the suppression of IFWM with AMIRZ format, we also performed a 40-Gb s 1 transmission numerical simulation for both RZ-OOK and AMI-RZ, and compared ghost-pulse levels with these two formats in the nonlinear operation regime. The simulation configuration was similar to that of the experiment except that the 10-Gb s 1 transmitter was replaced by a 40-Gb s 1 one, and the relative time delay of MZDI was changed as 45 ps. Fig. 6(a) showed the power penalty of RZOOK and AMI-RZ under different launch power. A 1.7-dB enhancement of IPDR of AMI-RZ over RZ-OOK was achieved at 1-dB power penalty level, which showed AMI-RZs robustness against IFWM. As discussed in [11], in high-speed RZ sys-
39
tems with high launch power, the ghost-pulse may appear in isolated ‘‘zeros’’ with the maximum number of neighbouring ‘‘ones’’. Thus we employed input pattern ‘‘0111110111110’’ with the launch power of 15 dB m to evaluate the ghostpulse levels with RZ-OOK and AMI-RZ formats, respectively. The output pattern was shown in Fig. 6(b) with the ghost pulse position indicated by arrows. It demonstrated clearly that ghost-pulse generation at the center bit slot was effectively suppressed, and less pulse distortion was obtained in AMI-RZ format.
10Gbps RZ-OOK 10Gbps AMI-RZ
6
4
2
0 2
4
6
8
10
12
14
16
18
20
Signal Launch Power (dBm)
Fig. 5. Experimental result: Power penalty versus the signal launch power for 10-Gb s 1 AMI-RZ and RZ-OOK signals.
Fig. 6. Simulation result: (a) power penalty versus the signal launch power for 40-Gb s 1 AMI-RZ and RZ-OOK signals; (b) portion of output bit sequence of 40-Gb s 1 data with the input sequence of ‘‘0111110111110’’ at 15-dB m launch power for (i) AMI-RZ and (ii) RZ-OOK systems.
40
G.-W. Lu et al. / Optics Communications 255 (2005) 35–40
4. Summary
References
We proposed and experimentally demonstrated a simple optical AMI-RZ transmitter based on a conventional single-arm LiNbO3 intensity modulator and an MZDI. Compared with the typical RZ-OOK transmitter, only a Mach–Zehnder delay interferometer and a differential precoder are additionally utilized. This simple transmitter facilitates the upgrade of the existing RZ-OOK transmitters to optical AMIRZ transmitters that provides better nonlinearity tolerance. Transmission performance in the nonlinear operation regime was evaluated by experiment at 10 Gb s 1 and simulation at 40 Gb s 1. The ghost-pulse levels with the generated AMIRZ and RZ-OOK formats were also investigated at 40 Gb s 1. The results showed higher nonlinear tolerance of the generated AMI-RZ over RZ-OOK, especially the tolerance against IFWM at 40-Gb s 1 optical transmission system.
[1] K.S. Cheng, J. Conradi, IEEE Photon. Technol. Lett. 14 (2002) 98. [2] T. Franck, T.N. Nielsen, A. Stentz, in: European Conference on Optical Communication (ECOC), 1997, p. 22. [3] K. Yonenaga, S. Kuwano, S. Norimatsu, N. Shibata, Electron. Lett. 31 (1995) 302. [4] J. Yu, IEEE Photon. Technol. Lett. 15 (2003) 1455. [5] W. Kaiser, T. Wuth, M. Wichers, W. Rosenkranz, IEEE Photon. Technol. Lett. 13 (2001) 884. [6] S.B. Jun, K.J. Park, H. Kim, H.S. Chung, J.H. Lee, Y.C. Chung, in: Optical Fiber Communications Conference (OFC), 2004, Paper ThN1. [7] D. Penninckx, H. Bissessur, P. Brindel, E. Gohin, F. Bakhti, in: European Conference on Optical Communication (ECOC), 2001, p. 456. [8] X. Wei, X. Liu, S, Chandrasekhar, A.H. Gnauck, G. Raybon, J. Leuthold, P. Winzer, in: European Conference on Optical Communication (ECOC), 2002, Paper 9.6.3. [9] P.J. Winzer, A.H. Gnauk, G. Raybon, S. Chandrasekhar, Y. Su, J. Leuthold, IEEE Photon. Technol. Lett. 15 (2003) 766. [10] Chongjin Xie et al., J. Lightwave Technol. 22 (2004) 806. [11] R.-J. Essiambre et al., Electron. Lett. 35 (1999) 1576.
A simple AMI-RZ transmitter based on single-arm intensity modulator and optical delay interferometer Guo-Wei Lu *, Lian-Kuan Chen, Chun-Kit Chan Department of Information Engineering, The Chinese University of Hong Kong, Room 827, HSH Building, CUHK, Shatin, N.T., Hong Kong, SAR Received 2 October 2004; received in revised form 8 April 2005; accepted 31 May 2005
Abstract In the paper, we propose and demonstrate a simple alternate-mark inversion return-to-zero (AMI-RZ) transmitter upgraded from existing plain RZ-OOK transmitter by employing an additional Mach–Zehnder delay interferometer (MZDI), eliminating the use of high-speed RF circuit and dual-drive Mach–Zehnder modulator. A delay-and-subtract operation is performed by the MZDI on the RZ-OOK signal to achieve alternating phases in successive mark pulses. The nonlinearity tolerance of the generated AMI-RZ signal was characterized by experiment at 10 Gb s 1 and simulation at 40 Gb s 1. The ghost-pulse, due to intra-channel four-wave-mixing (IFWM), with generated AMI-RZ and RZOOK formats was also investigated respectively at 40 Gb s 1. The results showed that the generated AMI-RZ exhibited higher nonlinear tolerance over RZ-OOK, especially the tolerance against IFWM in the 40-Gb s 1 optical transmission system. This AMI-RZ transmitter scheme is simple, potentially low cost and can be integrated easily with the existing RZ-OOK transmitters for upgrading to AMI-RZ transmitters. Ó 2005 Elsevier B.V. All rights reserved. PACS: 42.79.S Keywords: Alternate-mark inversion return-to-zero; Modulation format; Intra-channel four-wave-mixing; Modified duobinary returnto-zero; Duobinary
1. Introduction
*
Corresponding author. Tel.: +852 2609 8479; fax: +852 2603 5032. E-mail address: [email protected] (G.-W. Lu).
Alternate-mark inversion return-to-zero (AMIRZ) modulation format has attracted much attention as further advantages can be offered compared with regular RZ, duobinary RZ, and alternatingphase RZ signals. With alternating phases in the
0030-4018/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2005.05.039
36
G.-W. Lu et al. / Optics Communications 255 (2005) 35–40
successive mark pulses, AMI-RZ signal has higher tolerance against various detrimental effects, including self-phase modulation (SPM) in single channel systems, cross-phase modulation (XPM) in wavelength-division-multiplexing (WDM) transmission systems, intra-channel four-wave-mixing (IFWM) in high-speed optical systems [1]. It also helps to increase the threshold for the stimulated Brillouin scattering (SBS) [2]. Conventionally, in addition to the differential precoder needed for radio frequency (RF) driving signal, the generation of AMI signals required dual-drive Mach–Zehnder (MZ) modulator biased at zero transmission, with electrical three-level shaping filter [1], high-speed RF logical circuit [3], or high-speed RF driving circuit with well-controlled timing-delay [4]. Recently, a simplified duobinary transmitter based on a single-arm x-cut MZ modulator and a three-level shaping filter has been proposed to further reduce the complexity and cost [5], which also can be used to generate AMI-RZ with modification of the driver. However, these electrical methods suffer from one or more of the following potential drawbacks, (i) performance dependency on the word length due to shaping filter induced distortion, (ii) strict symmetric requirement for the two drivers in dual-drive MZ modulators, (iii) high complexity and cost, especially for high-speed systems, and (iv) high driving voltage required (2Vp). An optical method based on a piece of polarization-maintaining (PM) fiber and a polarizer to generate pulsewidth tuneable AMI-RZ has been proposed, but is less practical due to its polarization sensitivity [6]. Recently, it was demonstrated that optical differential phase shift keying (DPSK) signals after direct detection could be considered as a duobinary or AMI signals [7–9]. Based on this idea, in this paper, we propose and demonstrate a simple AMI optical transmitter based on the delay-and-subtract operation on the more commonly used RZ-OOK or NRZ-OOK signal with conventional intensity modulators. Compared with the typical RZ-OOK transmitter, only an additional Mach–Zehnder delay interferometer and a differential precoder are needed. This simple approach facilitates the upgrade of the existing RZ-OOK transmitters into the optical AMI-RZ transmitters. The transmission performance of the generated AMI-RZ in
the nonlinear operation regime with different signal launch power was investigated by experiment at 10 Gb s 1 and simulation at 40 Gb s 1. Compared with corresponding RZ-OOK system, it showed a 1-dB input power dynamic range (IPDR) improvement in 10 Gb s 1 and 1.7-dB improvement in 40-Gb s 1 AMI-RZ system at 1-dB penalty level.
2. A simple AMI-RZ transmitter The configuration of the simple AMI-RZ transmitter is shown in Fig. 1(a). It consists of a singlearm LiNbO3 modulator (LN-MOD) followed by a Mach–Zehnder delay interferometer (MZDI). The MZDI consists of optical fiber and two 50:50 fiber couplers. The MZDI splits the incoming signal intensity equally into two branches, one of which has an additional path delay. A thermoelectric cooler (TEC) is utilized to change and stabilize the phase difference between two arms. By finetuning the temperature, the destructive (or constructive) interference can be easily achieved at the output of MZDI with (2m + 1)p (or 2mp) phase difference between the two arms of MZDI. For the proposed AMI-RZ transmitter module, the delay-and-subtract operation can be performed on an NRZ-OOK or an RZ-OOK signal, which corresponds to using a CW source or using a pulse source as the input signal of the module. In the case with a CW source input, the pulse-width of the generated AMI-RZ signal is determined by the speed of the intensity modulation, i.e., modulation rise/fall times, or the relative time delay of MZDI, Dt, which should be shorter than one-bit time in this case [9]. In the latter case with a pulse source input, the relative time delay of MZDI is chosen as one-bit time, and the pulse-width of generated AMI-RZ is the same as that of the input pulse train. In order to maintain the same pulse width for RZ-OOK and AMI-RZ signals for performance comparison, a pulse source is chosen as the input signal here. The working principle of this AMI-RZ transmitter is illustrated in Fig. 1(b) with a pulse source as the input signal. After the single-arm intensity modulator, the pulse train is modulated into RZOOK signal. When the two arms of MZDI are
G.-W. Lu et al. / Optics Communications 255 (2005) 35–40
37
11101111
Differential encoder 10110101
Time Delay & Phase Shifter
∆t
b a
CW or Pulse Train
a
LN MOD
c
MZDI
1 0 1 1 0 1 0 1
Lower Arm: Phase=0
a 0 1 0 1 1 0 1 0
Upper Arm: Phase=π
b 1 1 1 0 1 1 1 1
b
c
Alternating Phase Shift between Mark Level
Fig. 1. Illustration of (a) the configuration and (b) the operation principle of the AMI-RZ transmitter. (White pulses are of phase 0; grey pulses are of phase p).
set to be out-of phase, the signals in both arms (lower arm: sequence a; upper arm: sequence b, in Fig. 1(b)) will experience destructive interference at the output of the MZDI. Meanwhile, the relative phase difference between the two arms is still preserved for the generated mark pulses, as shown in the sequence c in Fig. 1(b). Thus the generated RZ signal will have an alternating p phase shift between the successive mark bits, conforming to the phase requirement of a typical AMI-RZ signal. By employing a differential precoder at the RF driving signal of the intensity modulator, as shown in Fig. 1(a), the intensity of generated sequence c (see Fig. 1(b)) is consistent with the original data. Hence, via MZDI, the signal was successfully converted from RZ-OOK to AMI-RZ. The delay-andsubtract operation applied on the OOK signal is equivalent to that applied on the DPSK signal [9] to generate AMI signals.
10.61-GHz sinusoidal wave, was employed to generate a 10.61-GHz optical RZ pulse train with 28ps full-width at half-maximum (FWHM) pulse width. Then the optical RZ pulse train at 1550 nm was fed into the AMI-RZ transmitter, which consisted of a single-arm LN-MOD and an MZDI with a relative delay of 94 ps between the two arms. A 10.61-Gb s 1 pseudorandom bit sequence (PRBS) with length of 231 1 was applied to the LN-MOD. In our experiment, a differential precoder was not employed since the differentially encoded PRBS is simply a timedelayed copy of the PRBS. However, a differential decoder would be needed in practical applications.
High Power EDFA
Clock 231-1 PRBS
RX EA MOD MZ-MOD
MZDI
VOA
40 km 8 km SMF DCF EDFA OBF
RZ-OOK AMI-RZ
3. Experiment and simulation results The experiment setup was shown in Fig. 2. An electro-absorption (EA) modulator, driven by a
Fig. 2. The experimental setup, MOD: Modulator, VOA: Variable optical attenuator, EDFA: Erbium-doped fiber amplifier, OBF: Optical Bandpass filter, MZDI: Mach–Zehnder Interferometer.
G.-W. Lu et al. / Optics Communications 255 (2005) 35–40
The output modulated signal from the transmitter was further amplified by a high-power Erbium doped fiber amplifier (EDFA), and then passed through a variable optical attenuator (VOA) so that the signal power could be varied from 0 to 19 dB m. The large dynamic range of 19 dB was used to emulate the nonlinearity scenario. The amplified signal was then fed into the transmission link consisting of a piece of 40-km standard signalmode fiber (SMF) with D = 17 ps/(nm Æ km)@ 1550 nm and a piece of 8-km dispersion compensating fiber (DCF) with D = 85 ps/(nm Æ km)@ 1550 nm for dispersion compensation, where the dispersion was fully compensated. Before being launched into the optical receiver, the signal was pre-amplified by an EDFA and extracted by a 1-nm FWHM optical bandpass filter (OBF). At the receiving end, the AMI-RZ signal was directly detected. To compare the nonlinear tolerance, a control experiment with 10-Gb s 1 RZ-OOK signal was also performed without the MZDI at the transmitter side. Fig. 3(a) showed the optical power spectrum of the AMI-RZ signal generated by the proposed transmitter. Power nulls were shown at the frequencies corresponding to the carrier clock tones of a typical RZ-OOK signal (see Fig. 3(b)). It means that the all discrete frequency tones that appear in the conventional RZ-OOK signal are suppressed in AMI-RZ, which contributes to the higher tolerance of AMI-RZ over RZ-OOK against IFWM [1]. The measured BER back-to-back performance of 10-Gb s 1 RZ-OOK and generated AMI-RZ signals was shown in Fig. 4. The receiver sensitivities (@BER = 10 9) of the RZ-OOK and AMI-RZ signals were 20.8 and 20.4 dB m, respectively. This indicated that for the generated AMI-RZ signal, there was a small power penalty of 0.4 dB, which could be attributed to the phase-to-intensity noise conversion through the MZDI due to the phase noise from the laser or the chirp from the modulator. The imperfect MZDI due to the unbalanced coupling ratio of couplers may further exacerbate the signal performance as it reduces the eye-opening. In order to evaluate the nonlinear tolerance of the AMI-RZ signal, we measured the receiver sensitivities at a BER of 10 9 against different launch
Fig. 3. Optical power spectra of (a) AMI-RZ and (b) RZOOK. Inset: the corresponding eye diagrams.
1e-2 10Gbps RZ-OOK 10Gbps AMI-RZ
1e-3 1e-4 1e-5
BER
38
1e-6 1e-7 1e-8 1e-9
~0.4 dB
1e-10 -24
-23
-22
-21
-20
Received Power (dBm)
Fig. 4. Measured BER curves for back-to-back and transmission of RZ-OOK and AMI-RZ signals.
powers, for systems using the 10-Gb s 1 AMI-RZ and RZ-OOK modulation formats, respectively. The corresponding power penalties, obtained with
G.-W. Lu et al. / Optics Communications 255 (2005) 35–40
Power Penalty @ BER=10-9 (dB)
reference to the measured back-to-back receiver sensitivities, were shown in Fig. 5. When the signal launch power exceeded 14 dB m, the signal began to suffer from fiber nonlinearities including SBS and intra-channel nonlinearities, i.e., intrachannel XPM (IXPM) and IFWM. These nonlinear effects caused a drastic increase of the power penalty when the signal launch power was further increased. At 1-dB power penalty level, it was shown that the AMI-RZ format offered a 1-dB enhancement of the IPDR over the RZ-OOK modulation format. This could be attributed to the higher tolerance of AMI-RZ against fiber nonlinearities, such as IXPM and SBS, over RZ-OOK format in 10-Gb s 1 transmission systems [10]. To assess the suppression of IFWM with AMIRZ format, we also performed a 40-Gb s 1 transmission numerical simulation for both RZ-OOK and AMI-RZ, and compared ghost-pulse levels with these two formats in the nonlinear operation regime. The simulation configuration was similar to that of the experiment except that the 10-Gb s 1 transmitter was replaced by a 40-Gb s 1 one, and the relative time delay of MZDI was changed as 45 ps. Fig. 6(a) showed the power penalty of RZOOK and AMI-RZ under different launch power. A 1.7-dB enhancement of IPDR of AMI-RZ over RZ-OOK was achieved at 1-dB power penalty level, which showed AMI-RZs robustness against IFWM. As discussed in [11], in high-speed RZ sys-
39
tems with high launch power, the ghost-pulse may appear in isolated ‘‘zeros’’ with the maximum number of neighbouring ‘‘ones’’. Thus we employed input pattern ‘‘0111110111110’’ with the launch power of 15 dB m to evaluate the ghostpulse levels with RZ-OOK and AMI-RZ formats, respectively. The output pattern was shown in Fig. 6(b) with the ghost pulse position indicated by arrows. It demonstrated clearly that ghost-pulse generation at the center bit slot was effectively suppressed, and less pulse distortion was obtained in AMI-RZ format.
10Gbps RZ-OOK 10Gbps AMI-RZ
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Fig. 5. Experimental result: Power penalty versus the signal launch power for 10-Gb s 1 AMI-RZ and RZ-OOK signals.
Fig. 6. Simulation result: (a) power penalty versus the signal launch power for 40-Gb s 1 AMI-RZ and RZ-OOK signals; (b) portion of output bit sequence of 40-Gb s 1 data with the input sequence of ‘‘0111110111110’’ at 15-dB m launch power for (i) AMI-RZ and (ii) RZ-OOK systems.
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G.-W. Lu et al. / Optics Communications 255 (2005) 35–40
4. Summary
References
We proposed and experimentally demonstrated a simple optical AMI-RZ transmitter based on a conventional single-arm LiNbO3 intensity modulator and an MZDI. Compared with the typical RZ-OOK transmitter, only a Mach–Zehnder delay interferometer and a differential precoder are additionally utilized. This simple transmitter facilitates the upgrade of the existing RZ-OOK transmitters to optical AMIRZ transmitters that provides better nonlinearity tolerance. Transmission performance in the nonlinear operation regime was evaluated by experiment at 10 Gb s 1 and simulation at 40 Gb s 1. The ghost-pulse levels with the generated AMIRZ and RZ-OOK formats were also investigated at 40 Gb s 1. The results showed higher nonlinear tolerance of the generated AMI-RZ over RZ-OOK, especially the tolerance against IFWM at 40-Gb s 1 optical transmission system.
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