A novel all-optical modulation format conversion from non-return-to-zero on-off-keying (NRZ-OOK) to return-to-zero quadrature-phase-shift-keying (RZ-QPSK) is proposed and experimentally demonstrated. The proposed format conversion scheme is based on parallel Mach-Zehnder interferometric (MZI) OOK/binary-PSK (BPSK) converters, consisting of integrated semiconductor optical amplifiers (SOAs). We experimentally demonstrate that in both decoded channels of the converted RZ-QPSK signal bit error rate (BER) curves show almost the same receiver sensitivity at a symbol-rate of 10.7 Gsymbol/s. In addition, a reasonable dispersion tolerance of the converted signal up to +295 ps/nm is observed. The numerical simulation based upon carrier rate equation verifies the experimental results.
©2007 Optical Society of America
All-optical signal processing is a promising technology to remove optical-to-electric (O/E) and electric-to-optical (E/O) converters in future all-optical networks. A considerable number of studies have been conducted on all-optical signal regeneration, which helps overcoming accumulation of signal degradation , . To complete all-optical networks, there would be a need for a wide variety of all-optical signal processing techniques such as optical 3R repeater, wavelength conversion, optical label processing, and modulation format conversion.
Optical communication systems have been long employing primarily conventional on-off-keying (OOK) signals, which convey the information in the amplitude, in either non-return-to-zero (NRZ) or return-to-zero (RZ). Recently, advanced optical modulation formats have attracted an increased attention –. Duobinary, alternate mark inversion (AMI), chirped-RZ (CRZ), and carrier-suppressed-RZ (CSRZ) are included in OOK modulation formats. In contrast, phase-shift-keying (PSK) formats carry information in the phase of the optical carrier itself. Due to the difficulty of generating absolute phase reference in direct-detection, the phase of the preceding bit is used as a relative phase reference for demodulation. This results in differential phase-shift-keying (DPSK), which carries the information in the form of phase change between adjacent bits. DPSK can improve the receiver sensitivity by a factor of 3 dB. Moreover, it is robust against fiber nonlinear effects. Recent studies have revealed that DPSK preferably exhibits better performance than conventional OOK for long-haul transmission , . More advanced modulation format, differential quadrature phase-shift-keying (DQPSK), appears to be promising technique in order to exploit the better receiver sensitivity and to secure the compatibility with 50 GHz channel spacing in ultra-dense wavelength division multiplexed (DWDM) transmission . One of the most straightforward advantages of the DQPSK compared to DPSK is the increase in the tolerance to chromatic dispersion (CD) and polarization mode dispersion (PMD) by approximately a factor of 2 . These advanced DPSK and DQPSK formats will be adopted to long-haul transmission systems according to type of network.
In future optical networks, different modulation formats may be selectively used depending on the network size and the bit rate. For example, cost effective OOK formats fit in metropolitan area networks (MANs). On the other hand, advanced PSK formats are preferably adopted in wide area networks (WANs) as well as transoceanic undersea cable networks. Obviously, as shown in Fig. 1(a), all-optical modulation format conversion at the gateway node between MAN and WAN will become a key technology to transparently interface different types of modulation formats. So far, various kinds of all-optical format conversions have been investigated, e.g., RZ-OOK to NRZ-OOK using semiconductor optical amplifier (SOA) loop mirror  and SOA-based Mach-Zehnder interferometeric (MZI) wavelength converter ; between RZ-OOK and CSRZ-OOK using SOA loop mirror  and frequency-shift-keying (FSK)-to-PSK by using optical double-sideband modulation technique [13 ]. We have proposed an all-optical OOK to BPSK converter using SOA-MZI wavelength converter . All-optical OOK/BPSK conversion based on optical fiber nonlinearity have been also reported . To the author’s knowledge, however, there has not been any all-optical modulation format converter from OOK to QPSK, which is expected to be applied to the future all-optical networks with OOK demultiplexing as shown in Fig. 1(b).
In this paper, we propose a novel all-optical modulation format conversion from NRZ-OOK to RZ-QPSK using parallel SOA-MZI OOK/BPSK converters. This paper is organized as follows: in Section 2, we explain the basic principle of the proposed modulation format conversion. In Section 3, we then demonstrate the feasibility of the proposed scheme by numerical simulations and experiments. We numerically calculate the output phase and waveform after format conversion to verify that the converted signal is a proper QPSK signal. In Section 4, similar BER curves in both decoded channels at 10.7 Gsymbol/s is experimentally demonstrated. Moreover, we demonstrate that the converter has a suitable dispersion tolerance for long-haul transmission systems.
2. Principle of operation
Figure 2 shows the schematic diagram of the proposed modulation format converter. The basic configuration consists of an SOA-MZI OOK/BPSK modulation format converter (SOA-MZI#1) on the upper arm and a phase shifter and another SOA-MZI#2 in tandem on the lower arm. The SOA-MZI OOK/BPSK converters convert an NRZ-OOK data signal to an RZ-BPSK data signal by using cross-phase modulation (XPM) in SOA . NRZ-OOK data signals 1 and 2 with the wavelength of λ 0 are launched into the upper arm of the MZI (port 1) and the lower arm of the MZI (port 3), as control pulses 1 and 2, respectively. RZ-clock pulse sequence with λ1 and CW light with λ2 are launched into the MZI (port 2) as a probe pulse and an assist light, respectively. According to the NRZ-OOK data ”1” or ”0”, the phase of the probe pulse after passing through SOA-MZI#1 shifts either by ”0” or ”π”, respectively. On the other hand, when the data signal 2 is either ”0” or ”1”, the probe pulse after passing through the SOA-MZI#2 has the phases of ”π/2” or ”-π/2” due to the π/2 phase shifter. After passing through SOA-MZI#1 and #2, the probe pulses have equal peak power and cause orthogonal interference in each combination of the control pulses 1 and 2. The probe pulse at the output has the same peak amplitude as the incoming signals due to the orthogonal interference and four different phases depending on the combination of the control pulses 1 and 2. Therefore, NRZ-OOK data signals can be converted to RZ-QPSK data signals. The assist light is launched to suppress the rapid change of carrier density which induces frequency chirp and amplitude fluctuations. With the help of the assist light, data pattern dependent pulse distortions can be avoided even in high bit-rate operation . In the proposed method, it is desirable to adopt coherent detection at the receiver in order to demodulate the converted RZ-QPSK signal to the original digital data. Instead of the coherent detector, the combination of delayed interferometers and a decoder which consists of simple electronic circuits can be also utilized as the demodulator. In the experiment shown below, two output patterns dependent on the phase shift in the delayed interferometer, which can be calculated from the combination of the data 1 and 2, were memorized beforehand in a BER tester (BERT) instead of the electronic decoder.
3. Numerical simulation
3.1. Simulation Model
For analyzing the SOA, we divide the SOA into M sections as shown in Fig. 3 and use following rate equation of the carrier density  to calculate the carrier density variation during the time corresponding to the propagation of fields in each of the divided sections of the SOA.
where N is the carrier density, J is the injection current density, d is the active layer thickness, τ is the carrier life time, gm is the material gain, I is the injected light intensity, E is the photon energy, St is the average amplified spontaneous emission, and q is the electron energy. Subscript i corresponds to the different sections and superscript x (= c, p and a) denotes the control, probe and assist light, respectively. In Eq. (1), the first term on the right-hand side represents the increase in carrier density by current injection, while the second, third and fourth terms represent the decreasing carrier density due to spontaneous emission, stimulated emission, and amplified spontaneous emission (ASE), respectively. From ,
where Iavi is the average incident light intensity into the section i, ΔL is the section length, and
is the net optical gain in section i; Γ is the confinement factor and α is the material loss. The gain spectrum is assumed to be parabolic and the material gain is thus approximated to be ,
where a 1 and a 2 are gain constants, N 0 is transparency carrier density, λ represents the wavelength, and λp is the wavelength at gain peak given by 
where a 3 is also a gain constant and Np is the carrier density at λp. The injected light I i+1(t) into the section (i + 1) is obtained from the light Ii(t -1) injected into the adjacent section i multiplied by the optical gain G(Ni,λ) of the section i at the preceding time t -1, as
The nonlinear phase change, arising from carrier density-induced changes in the refractive index, is given by 
where Δn̄N is the rate of change of refractive index in the active region with carrier concentration. The values for the parameters used in the calculations are listed in Table 1. The losses from input port 1, 2 and 3 to SOAs and from SOAs to output port, shown in Fig. 2, are 8.5 dB, 8.5dB, 12.0 dB and 12.0 dB, respectively. The wavelength of the control pulses, the probe pulse, and the assist light are 1545.3 nm, 1535.0 nm, and 1542.0 nm, respectively. The peak power of the probe pulse is 3.0 dBm and the power of the assist light is 10.0 dBm. The peak power of the control pulses is set to 8.9 dBm so that the phase shift of the probe pulse induced by XPM is π. The input pulse pattern of the control pulses is 10 Gb/s pseudo-random binary sequence (PRBS) of length 25-1.
3.2. Results of Simulation
Figure 4(a) and (b) respectively show the eye diagrams of waveform and phase of the probe pulse after passing through SOA-MZI#1. We observe a stable operation of format conversion from NRZ-OOK to RZ-BPSK. The waveform appears as a periodic clear pulse train since all pulses have almost the same peak power. The phase change is stable at the peak of the pulse and the information is modulated in its phase of “0” or “π”. Figure 5(a) and (b) respectively show the eye diagrams of waveform and phase of the probe pulse after passing through NRZ-OOK/RZ-QPSK modulation format converter. A periodic pulse train can be observed. However, the waveform has a slight intensity fluctuation since the orthogonal interference emphasizes the pattern effect. On the other hand, the phase change has almost the same stability to that observed before the orthogonal interference and the information is modulated in its phase of “0”, “π/2”, “π” or “3π/2”. The output phase fluctuations as a function of the input phase and intensity fluctuations show more gradual change compared with the output intensity fluctuations. Because of these results, this scheme seems a suitable candidate to perform the modulation format conversion from NRZ-OOK to RZ-QPSK.
4. Experimental demonstration
4.1. Experimental Setup
Figure 6 shows the experimental setup for the proposed modulation format conversion. We used double SOA-MZI wavelength converters  in which 2300 μm-length SOAs, whose active layer has a tensile-strain bulk structure with gain peak wavelength of 1550nm, is monolithically integrated on each arm of the MZIs. The NRZ-OOK data signals 1 and 2 were generated by modulating a CW light at 1535.0 nm in a lithium niobate (LiNbO3) modulator with 10.7 Gb/s PRBS of length 27-1. The average power of the data signals 1 and 2 launched into the port 1 and port 3 of the modulation format converter were +6.4 dBm and +4.6 dBm, respectively. The data signal 2 has a delay of 36 bit from the data signal 1 at each input ports. Each input polarization was optimized by a polarization controller (PC). An RZ clock pulse was generated by modulating a CW light at 1542.0 nm in an LN modulator by using the clock recovered from NRZ-OOK data 2. We used the electrical clock recovery including photo detector and electrical circuits. However, these electrical components can be replaced by one of reported all-optical clock recovery techniques ,. The RZ clock pulse was coupled with a CW assist light at 1545.3 nm. The average power of the RZ clock pulse and the assist light launched into port 2 of the modulation format converter were -0.6 dBm and +6.2 dBm, respectively. The converted signal was detected by a balanced receiver after passing through a 1-bit delay interferometer. We define the output signals after the 1-bit delay interferometer as channel 1 and channel 2 when the phase shifts of 1-bit delay interferometer are π/4 and -π/4, respectively. In the BER measurement, we entered output patterns, which was calculated from the combination of data signal 1 and 2, into a BERT instead of inserting decoder.
4.2. Demonstration of format conversion
Figure 7(a) and (b)show the eye diagram and the spectra for 10.7 Gsymbol/s converted signal before the 1-bit delay interferometer, respectively. We can observe an RZ pulse train, which has a slight intensity fluctuation corresponding to the waveform shown in Section 3. The carrier suppressed spectrum, which implies the generation of PSK signal, was observed. The asymmetric spectrum may be caused by the imbalance between two SOA-MZI arms, imperfect adjustment of current into the SOAs, and/or wavelength dependent gain characteristics of the SOAs and EDFAs. However, it does not induce so critical effect on a BER measurement and a transmission experiment shown below. Figure 8(a) to (d) show the eye diagrams of the converted signal after 1-bit delay interferometer. The clearly open eye diagrams of both constructive (a) and destructive (b) output for the channel 1 exhibit that the converted signal is stablely modulated in its phase with QPSK format. We observed almost the same eye diagrams for the channel 2 as shown in (c) and (d). This indicates that the phase of the converted signal is properly modulated as QPSK signal by interference in MZI. Figure 9(a) and (b) show a clear eye openings after the balanced receiver in both channel 1 and 2, respectively.
Figure 10 shows the obtained BER of the converted signal in both channel 1 and 2. Two BER curves were almost the same and both signals nearly had the same receiver sensitivity. The above mentioned results reveal that double NRZ-OOK signals can be converted into the RZ-QPSK signal using the proposed method, as predicted by the simulation in Section 3. If the phase relation between the 2 BPSK signals is not quadrature or the timing between the 2 NRZ-OOK signals is not synchronized, the tributary dependent degradation would be observed.
4.3. Dispersion Tolerance
We also investigated the dispersion tolerance by transmitting the converted RZ-QPSK signal through standard single mode fibers (SMFs). We measured the eye diagrams and the power penalty at BER = 10-9 as a function of residual dispersion ranging from 0 to +640 ps/nm. Figure 11(a) to (c) show the eye diagrams of the received signals for channel 1 observed at the input of the balanced receiver after transmission through the SMFs with a length of 20, 30 and 50 km. The power penalties as a function of the imposed dispersion are plotted in Fig. 12. The power penalty at the residual dispersion of +295 ps/nm is 1 dB. In the case of a dispersion of +310 ps/nm, corresponding to transmission in the SMFs of 20 km, we observed the eye opening with a slight degradation, which induces the power penalty of 1.1 dB. In the case of a dispersion of +465 ps/nm, corresponding to transmission in the SMFs of 30 km, the eye diagram became critically narrow. After transmission in the SMF of 50 km, the clock recovery was not possible due to a serious degradation as shown in Fig.11(c). However, a reasonable dispersion tolerance of +295 ps/nm for 50%-duty cycle RZ-QPSK signal was observed, which is comparable with the 500 ps/nm torelance for 66%-duty cycle CSRZ-OOK , . Therefore, we can see that the converted QPSK signal is suitable for long-haul transmission.
4.4. Tolerance to Timing Mismatch
We investigated the tolerance to the timing mismatch between input NRZ-OOK data 1 and 2 by measuring the power penalty at BER = 10-9 with forced mismatch. We assumed that the input timing of NRZ-OOK data 2 was matched to the input timing of RZ pulse because the RZ pulse was generated by using the recovered clock from NRZ-OOK data 2. The input timing of NRZ-OOK data 1 was delayed by a tunable delay line (TDL). Figure 13 shows the measured power penalty as a function of the timing mismatch between input NRZ-OOK 1 and 2. The power penalty is less than 1.0 dB in the range of timing mismatch of about ±10 ps. As the timing mismatch is increased, the power penalty grows exponentially, not linearly. This results indicates that timing mismatch induces serious degradation of the converted signal. However, the range of ±10 ps is not narrow to adjust the timing when a TDL controlled by feedback system is used.
In this paper, we have proposed and experimentally demonstrated an all-optical modulation format conversion scheme from NRZ-OOK to RZ-QPSK using parallel SOA-MZI OOK/BPSK converters. The simulation predicted that RZ-QPSK mldulated signal can be generated by proper phase modulation through XPM in the SOA and orthogonal interference.
In experiments, we have observed clear eye opening of the converted signal and proper optical spectra of the QPSK signal, showing almost the same BER curves in both channels at 10.7 Gsymbol/s. The proposed modulation format converter will become powerful for higher bit rate of 40 Gb/s system or beyond, because phase modulation formats have greater advantages at such a high bit rate. Even though the further study on the conversion penalty should be done before applying to the real system, this modulation format conversion will become a key technique at the gateway node interfacing MAN and WAN in the future all-optical networks.
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