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A Multi-format receiver for both non-return-to-zero binary-phase-shift-keyed (NRZ-BPSK) signal and non-return-to-zero amplitude-shift-keyed (NRZ-ASK) signal is demonstrated. Multi-format signal detection is based on incoherent BPSK demodulation and ASK-BPSK format conversion. Incoherent BPSK demodulation is realized by a Mach-Zehnder delay interferometer (MZDI) and a feedback decoder. Transmission experiments validate the feasibility of multi-format receiver. This receiver has potential to serve as a useful terminal block for all-optical wavelength division-multiplexed (WDM) networks.
©2009 Optical Society of America
1. Introduction
Future optical networks will employ different modulation formats according to different network scales and applications. For example, amplitude-shift-keyed (ASK) signal and phase-shift-keyed (PSK) signal may be simultaneously used in networks, because the former is cost-effective for metro area networks while the latter shows improved receiver sensitivity and enhanced tolerance to fiber nonlinearity in long-haul transmission systems [1–3]. Some format conversion methods from ASK to binary-phase-shift-keying (BPSK) have been proposed to solve the format mismatch between metro networks and long-haul backbone networks.
Format conversion from ASK to BPSK has been realized by cross-phase modulation (XPM) or self-phase modulation (SPM) in either a semiconductor optical amplifier (SOA) [4–6] or an optical fiber [7]. Nonlinear polarization rotation (NPR) effect in semiconductor optical amplifiers (SOAs) [8] and nonlinear phase shift in cascaded microring resonators (CMRR) [9] are also used for this format conversion. In all these schemes, logical “1” in ASK signal is converted into π-phase in BPSK signal, while logical “0” leads to a 0-phase.
In wavelength division-multiplexed (WDM) networks integrated with format conversion, the converters at edge nodes are expected to implement format conversion of different wavelength channels simultaneously [10]. In the case of multi-wavelength operation, most nonlinear optical signal processing schemes will inevitably suffer from crosstalk produced by the undesirable nonlinear effect such as XPM and four-wave mixing (FWM), which cannot be removed at the output [11]. One method to realize multi-wavelength format conversion is to demultiplex the wavelengths, implement format conversion for each wavelength separately, and then multiplex the converted signals of different wavelengths. This technique requires one convertor for each wavelength and increases the costs remarkably.
Fig. 1. WDM networks with different signal format. Multi-format transmitters and receivers are used in the aggregation nodes. ASK signal is used in access and metro networks. Signal transmitted in core networks is BPSK format.
A possible solution to format mismatch in WDM networks is to use multi-format transceiver. The network architecture and multi-format transmission scheme are shown in Fig. 1. Optical networks can be divided into core networks, metro networks, and access networks [12]. Aggregation nodes link metro networks and access networks, while edge nodes link metro networks and core networks. Multi-format transmitters and receivers at aggregation nodes are able to transmit and detect both ASK and BPSK signals. Transmitters choose signal format for transmission according to the distance between two terminals. Multi-format receivers detect the incoming signals regardless of signal format. In metro and access networks with transmission distances varying from a few meters to a few hundred kilometers, the low-cost ASK format is preferred. In long haul transmission, the aggregation nodes transmit signals in BPSK format. Since multi-format transmission is realized by multi-format transceivers, format converters at edge nodes are no longer required. The elimination of format converters can increase network efficiency and make the WDM networks more cost-effective.
This paper presents a multi-format receiver for non-return-to-zero BPSK (NRZ-BPSK) and non-return-to-zero ASK (NRZ-ASK) signals. The signal demodulation is based on incoherent BPSK demodulation and NRZ-ASK to NRZ-BPSK format conversion. Incoherent BPSK demodulation is realized by using a Mach-Zehnder delay interferometer (MZDI) and feedback decoder. NRZ-ASK to NRZ-BPSK format conversion is realized by using saturated semiconductor optical amplifier (SOA). Mach-Zehnder modulator (MZM) is used as multi-format transmitter to generate both ASK and BPSK signals.
The rest of the paper is organized as follows. In Section II, we introduce the principle of multi-format detection, especially the principle of incoherent BPSK demodulation. Then the experimental setup is illustrated in Section III. Results and discussion are provided in the final section.
Fig. 2. (a) Structure of multi-format receiver, (b) Illustration of the demodulation of NRZ-ASK signal using multi-format receiver.
2. Principle
The principle of multi-format detection is explained in this section. The multi-format receiver consists of a saturated SOA, a MZDI, a photodetector, and a feedback decoder. Fig. 2(a) shows the structure of the proposed multi-format receiver and Fig. 2(b) shows the principle of NRZ-ASK signal demodulation by using multi-format receiver. In NRZ-ASK signal demodulation, the incoming NRZ-ASK format signal is firstly converted into NRZ-BPSK format by a SOA. When the SOA is operated in the saturation regime, the “0” bit achieve higher power gain than the “1” bit. By adjusting the input power to proper values, an NRZ-ASK signal with finite extinction ratio (ER) can be converted to NRZ-BPSK format with approximately constant amplitude and a phase difference of π between “0”s and “1”s. [5] After format conversion, the NRZ-BPSK signal is demodulated incoherently.
Let a(k), b(k), c(k), and d(k) represent the data logic at A, B, C, and D positions in Fig. 2, respectively. The saturated SOA converts the incoming intensity modulated signal to phase modulated signal and leaves phase modulated signal unchanged. SOA does not change the data logic, therefore:
As shown in Fig. 2(a), incoherent BPSK detection is realized by using a MZDI and a feedback decoder. Since the MZDI implements opposite logic operation at different ports, the data restoration circuit should be designed according to the chosen output port of the MZDI. The outputs at destructive port and constructive port are given by:
When destructive port is chosen, XOR gate should be used in the feedback decoder to restore data. The output of the feedback decoder can be deduced as follow:
where d(k-1) can be expressed as:
Substituting Eq. (4) into Eq. (3) yields:
After iterative computation, the output of feedback decoder can be represented as:
Therefore when d(0) = a(0), we have d(k) = a(k) at position D.
Similarly, when constructive port is used, XNOR gate is needed to recover the data. The output of the feedback decoder is:
and d(k)=b(k) can be obtained only when d(0) = ā(0).
Equation (6) and Eq. (7) indicate that the data restoration depends on the relationship between d(0) and a(0). d(0) represents initial logic state of the output of the feedback decoder and a(0) represents the first bit of the incoming data. For instance, in a XOR based feedback decoder, when the initial state at D in Fig. 2 is the same as the first bit of transmitted data, the restored data is positive. Otherwise the restored data is reversed. This kind of receiver is also known as data-aided incoherent demodulator [13]. It needs partial knowledge (in this case the first bit) of the transmitted signal.
Fig. 3. Experimental setup. MZDI is 1-bit-delay Mach-Zehnder delay interferometer; PC is polarizer controller; OBPF is optical bandpass filter; MZM is Mach–Zehnder modulator; EDFA is erbium-doped fiber amplifier; VOA is variable optical attenuator; PD is photodetector; con is constructive port; des is destructive port; SOA is semiconductor optical amplifier; DCF is dispersion compensation fiber
3. Realization of multi-format receiver
Figure 3 shows the block diagram of experimental setup and the configuration of the proposed multi-format receiver. Both NRZ-ASK and NRZ-BPSK signals at 1551.83 nm are generated by a LiNbO3 MZM driven by a 4-Gb/s pseudorandom bit sequence (PRBS). For both NRZ-BPSK and NRZ-ASK signals, the signal power fed into the single mode fiber (SMF) is 0 dBm. BPSK signal is transmitted over 100-km SMF and 17-km dispersion compensation fiber (DCF). NRZ-ASK signal is transmitted over 25-km SMF. The 100-km SMF and 17-km DCF are used to emulate the core network, and the 25-km SMF is used to emulate the metro or access network. After transmission, an erbium-doped fiber amplifier (EDFA) amplifies the signals to 10 dBm. An optical band pass filter (OBPF) with a bandwidth of 0.4 nm suppresses amplified spontaneous emission (ASE) noise, and a variable optical attenuator (VOA) is used to adjust the power to proper level. The SOA used in the multi-format receiver is biased at 240 mA with a gain recovery time of approximately 100 ps and a saturation power of 6 dBm. Eye diagrams of NRZ-BPSK and NRZ-ASK demodulation at positions A, B, C, and D (see in Fig. 3) are shown in Fig. 4.
Fig. 4. Experimental results of multi-format detection. (a), (b), and (c) Optical eye diagrams at positions A, B, and C in Fig. 3 for NRZ-BPSK demodulation. (d) Electrical eye diagram of the demodulated NZR-BPSK signal; (e), (f), and (g) Optical eye diagrams at positions A, B, and C in Fig. 3 for NRZ-ASK demodulation. (h) Electrical eye diagram of the demodulated NRZ-ASK signal.
Fig. 5. Waveforms of a typical pattern. (a), (b), and (c) Waveforms at positions A, B, and C in Fig.3 for NRZ-BPSK demodulation. (d) Waveform of the demodulated NRZ-BPSK signal; (e), (f), and (g) Waveforms at positions A, B, and C in Fig.3 for NRZ-ASK demodulation. (h) Waveform of the demodulated NRZ-ASK signal. A.U.: arbitrary unit.
3.1 NRZ-BPSK signal detection
NRZ-BPSK signal is generated by using a MZM biased at its transmission null. Since the optical power of BPSK signal is constant, the phase shift caused by SOA is almost the same. The information is maintained because the phase difference between adjacent bits does not vary. The eye diagrams and waveforms of NRZ-BPSK signals before and after the SOA are shown as (a), (b) in Fig. 4, and (a), (b) in Fig. 5, respectively. The following MZDI with a 1-bit delay converts the incoming BPSK signal into intensity-modulated signals at its two output ports. The output at destructive port is illustrated as (c) in Fig. 5. This signal is then detected by a photodetector and restored to the original data through feedback decoding. The functionality of the feedback decoder is established by connecting the output of the XOR gate to another input port after 1-bit delay. The original data can be restored by doing logical XOR operation between the output data and the present data. The restored data is shown as (d) in Fig. 5.
3.2 NRZ-ASK signal detection
The MZM is biased at quadrature point in its transmission curve for NRZ-ASK modulation. In the multi-format receiver, the input NRZ-ASK signal is firstly converted into NRZ-BPSK format. The quality of the converted NRZ-BPSK signal depends on the ER of the input NRZ-ASK signal. NRZ-ASK signal with input power of 2.2 dBm and ER of 6.8 dB is used to achieve exact ‘π’ phase difference between “1”s and “0”s. After the SOA, “1”s and “0”s of NRZ-ASK signal are amplified to almost the same level according to different gains. The waveforms of original NRZ-ASK binary data and its converted NRZ-BPSK signal are shown as (e) and (f) in Fig. 5. The demodulation in the rest part of the receiver is the same as the NRZ-BPSK demodulation illustrated in section 3.1. Waveforms at the positions C and D are shown as (g) and (h) in Fig. 5.
Fig. 6. (a) BER results for two different PRBS pattern lengths, (b) BER results before and after transmissions.
4. Results and discussion
The experimental demonstration utilizes different PRBS pattern lengths to evaluate the performance of the multi-format receiver. Fig. 6(a) shows the bit error rate (BER) measurement results for back-to-back (B2B), using PRBS pattern sequence with the lengths 27-1 and 231-1. When the pattern length is 27-1, the receiver sensitivities for NRZ-BPSK signal and NRZ-ASK signal at BER of 10-9 are -20.1 dBm and -20.8 dBm, respectively. When the pattern length is 231-1, the sensitivities decrease to -19.8 dBm and -20.3 dBm, respectively. When PRBS sequence length changing from 27-1 to 231-1, the pattern dependence in terms of receiver sensitivities at BER 10-9 are 0.5 dB for NRZ-ASK signal and 0.3 dB for NRZ-BPSK signal, respectively. Fig. 6(b) shows BER performance of NRZ-ASK and NRZ-BPSK signals using a 231-1 PRBS pattern for back-to-back and after transmissions. First we measured the BER performance for NRZ-ASK signal. After transmitted over 25-km SMF, the power penalty is 0.9 dB at BER of 10-9. Then we connected the 100-km transmission fiber and its matching DCF. It is shown that the power penalty for BPSK signal is 1.3 dB after 117-km transmission.
The measured sensitivity of NRZ-ASK signal in Fig. 6 is superior to the NRZ-BPSK signal. That is because the NRZ-BPSK signal generated by MZM has some residual amplitude modulation at the transition of two bits, which results in intensity dips. And there is also some amplitude variation that results from the drive-waveform overshoots and limited rise times of drive-signal [14]. After the SO A, the residual amplitude and variation lead to phase noise for NRZ-BPSK signal, and the phase difference between “0” and “1” is no longer exactly π [15]. However, in the NRZ-ASK signal detection, we set the ER of the NRZ-ASK signal to 6.8 dB and adjusted the input power to 2.2 dBm in order to achieve an exact differential phase of π between “0” and “1”. Therefore after the MZDI, the NRZ-ASK signal has better performance than NRZ-BPSK signal.
To compare the effectiveness of the multi-format receiver with traditional receivers, the back-to-back BER curves of NRZ-ASK and NRZ-BPSK signals using traditional detection methods are measured. As shown in Fig. 7, triangle symbols show the BER results of NRZ-ASK signal using single photodetector. The diamond symbols show the BER results of NRZ-BPSK signal using MZDI and single photodetector. The data sequences used in the measurements are PRBS with the length of 231-1. Since differential encoding of a PRBS pattern only delays the pattern, the differential encoder is eliminated in our BPSK measurement. The photodetectors used in traditional detection methods are the same as the one used in the multi-format receiver. Compared with NRZ-ASK signal detected by single photodetector, a -3.3 dB power penalty of NRZ-ASK signal detection using multi-format receiver is measured. This sensitivity improvement is due to the ER enhancement after MZDI in the multi-format receiver. An extra 3dB sensitivity improvement could also be expected by using balanced photodetector. [1] Compared with detection method using interferometric demodulation and direct detection for NRZ-BPSK signal, there is a 0.9 dB sensitivity degradation for NRZ-BPSK signal detection using the multi-format receiver. This sensitivity degradation is caused by spontaneous noise emitted by SOA.
In the multi-format receiver, the performance of the receiver is mainly limited by the recovery time of the SOA. In our experiment, the data rate is limited by the slow recovery time of the used SOA. SOA with shorter response time would achieve better quality and higher data rate for NRZ-ASK detection. In [5], 8 Gbit/s intensity modulation to phase modulation conversion is demonstrated using SOA with gain recovery time of 45 ps. Commercial SOA with gain recovery time of 25 ps is also available.
In NRZ-ASK detection, the ER of the input NRZ-ASK signal is limited to 6.8 dB to achieve converted NRZ-BPSK signal of high quality from the SOA. NRZ-ASK signal with finite ER can be used in access networks, optical Ethernets, and metro networks [16–17].
5. Conclusion
We have presented a practical multi-format receiver, which is successfully implemented for both NRZ-ASK and NRZ-BPSK signals in our experiments. In NRZ-ASK detection, the proposed receiver provides 3.3 dB better performance than traditional receiver using single photodetector. Moreover, compared with NRZ-BPSK signal detection using interferometric demodulation and direct detection, a 0.9 dB power penalty is obtained with the use of multi-format receiver. The multi-format receiver enables detection of signals in different formats by means of the same device. It is able to offer good flexibility for various format-related application scenarios.
Acknowledgments
This work is jointly supported by the National Nature Science Fund of China (No. 60632010 and No. 60572029) and the National “863” Hi-tech Project of China.
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