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We propose and experimentally demonstrate a novel scheme to generate optical frequency-locked multi-channel multi-carriers (MCMC), using a recirculating frequency shifter (RFS) loop based on multi-wavelength frequency shifting single side band (MWFS-SSB) modulation. In this scheme, optical subcarriers with multiple wavelengths can be generated each round. Furthermore, the generated MCMC are frequency- and phase-locked within each channel, and therefore can be effectively used for WDM superchannel. Dual-wavelength frequency shifting SSB modulation is carried out with dual-wavelength optical seed source in our experimental demonstration. Using this scheme, we successfully generate dual-channel multi-carriers, and one channel has 28 subcarriers while the other has 29 ones with 25-GHz subcarrier spacing. We also experimentally demonstrate that this kind of source can be used to carry 50-Gb/s optical polarization-division-multiplexing quadrature phase shift keying (PDM-QPSK) signal.
©2012 Optical Society of America
1. Introduction
Superchannel based on multiple optical frequency- and phase-locked carriers is a promising candidate for future high speed optical systems and networks [1–4]. The generation of optical coherent and frequency-locked multi-carriers is one of the key techniques for the realization of high speed superchannels [5–14]. Until now, there are mainly three different kinds of schemes for the generation of multi-carriers, including supercontinuum [1], cascaded phase and/or intensity modulators [4], and recirculating frequency shifter (RFS) [2, 3, 10–14]. Generally speaking, RFS is usually in combination with in-phase/quadrature (I/Q) modulator for single side band (SSB) modulation [11–14]. The schemes based on RFS, which have been investigated theoretically and experimentally [10–14], have the advantage of flexible controlling and accurate frequency spacing of generated optical subcarriers. However, to our knowledge, the previous experiments for multicarrier generation are only for one single channel, and no WDM channel experiment with phase- and frequency-locked subcarriers has been demonstrated. If we can adopt the RFS loop to generate optical subcarriers with multiple wavelengths each round, then the generated multi-channel multi-carriers (MCMC) can be effectively used for WDM superchannel. As a result, how to implement this kind of source is a really interesting research topic.
In this paper, we propose and experimentally demonstrate a novel scheme to generate optical MCMC, using a RFS loop based on multi-wavelength frequency shifting SSB (MWFS-SSB) modulation. Dual-wavelength frequency shifting SSB modulation is carried out with dual-wavelength optical seed source in our experimental demonstration. Using this scheme, we successfully generate dual-channel multi-carriers, and one channel has 28 subcarriers while the other has 29 ones with 25-GHz subcarrier spacing. We also experimentally demonstrate that this kind of source can be used to carry high-speed optical signal. The required optical signal-to-noise ratio (OSNR) at the bit-error ratio (BER) of 1 × 10−3 is 11.5 dB when 50-Gb/s optical polarization-division-multiplexing quadrature phase shift keying (PDM-QPSK) signal is coherent-detected, which shows that this scheme has great potential in the future optical wavelength-division-multiplexing (WDM) communications.
2. Principle of MCMC generation
Figure 1 shows the principle of optical coherent and frequency-locked MCMC generation, which adopts a RFS loop with an I/Q modulator for MWFS-SSB modulation. Inset (a) gives the schematic diagram of output optical spectra. The structure of the proposed scheme is mainly composed of two parts, i.e., a multi-wavelength optical seed source and a closed RFS loop. The former, in practice, can be N continuous-wavelength (CW) lasers or N subcarriers generated by a CW source. The latter includes an optical coupler (OC), an I/Q modulator for frequency shifting, an Erbium-doped fiber amplifier (EDFA) to compensate for the loop loss, a wavelength selective switch (WSS) to control the number of optical subcarriers within each channel, and a polarization controller (PC) to control the polarization state of the signal if the optical components in the loop are not polarization-maintaining.
Fig. 1 The principle of MCMC generation using RFS loop based on MWFS-SSB modulation. Inset (a) shows the schematic diagram of output optical spectra. OC: optical coupler, I/Q Mod: I/Q modulator, RFS: radio frequency signal, PS: phase shifter, EDFA: Erbium-doped fiber amplifier, PC: polarization controller, WSS: wavelength selective switch, OSA: optical spectrum analyzer, chN: channel N.
Here, we assume that the multi-wavelength optical seed source can generate N different optical subcarriers with equal power, which can be expressed as
Where fo is the optical frequency of the first optical subcarrier, fs is the frequency spacing, and E0 is the amplitude of each optical subcarrier. The I/Q modulator, used for MWFS-SSB modulation, is driven by two radio-frequency (RF) sinusoidal signals with the same frequencyΔf and a fixed phase difference of + π/2 or -π/2. Thus, after passing through WSS, N new optical subcarriers can be generated with frequency spacing of fs and frequency shifting ofΔf at one time, which can be expressed asThe new generated N optical subcarriers are split into two branches, one coupled out and the other recirculating into the input port of the optical IQ modulator along with the original N optical subcarriers. Then, the second N optical subcarriers can be generated from the first ones. Round after round, the RFS loop can generate more and more subcarriers. Assuming that the EDFA perfectly compensates for all the insertion and modulation losses, the final output after M rounds can be expressed asThe total number of optical subcarriers is mainly limited by the bandwidth B of WSS, the gain band and flatness of EDFA, the bias of the SSB modulator and so on. The number of rounds for the final output is M = B/Δf, and therefore the total number of finally generated optical subcarriers is NM. As discussed in the introduction, the general RFS loop in combination with I/Q modulator for SSB modulation can simply generate single-channel multi-carriers [11–14]. In our proposed MWFS-SSB scheme, however, optical subcarriers with N different wavelengths can be generated each round. Furthermore, the generated MCMC are frequency- and phase-locked within each channel, and therefore can be effectively used for WDM superchannel. Thus, our proposed scheme has great potential in the future optical WDM communications.
3. Experimental setup and results
Figure 2 shows the experimental setup for the MCMC generation, 50-Gb/s PDM-QPSK modulation and coherent detection based on our proposed MWFS-SSB scheme. Here, dual-wavelength frequency shifting SSB modulation is carried out with dual-wavelength optical seed source in our experimental demonstration.
Fig. 2 The experimental setup for the MCMC generation, 50-Gb/s PDM-QPSK modulation and coherent detection based on our proposed MWFS-SSB scheme. Inset (a) shows the detailed structure of the MCMC source. ECL: external cavity laser, OC: optical coupler, I/Q Mod: I/Q modulator, EDFA: Erbium-doped fiber amplifier, PC: polarization controller, WSS: wavelength selective switch, PS: phase shifter, TOF: tunable optical filter, Pol. Mux: polarization multiplexer, DL: delay line, ATT: attenuator, PBS: polarization beam splitter, PBC: polarization beam combiner, LO: local oscillator, PD: photo detector, ADC: analog-to-digital converter.
As shown in Fig. 2(a), two external cavity lasers (ECLs), with the linewidth less than 100 kHz and the output power of 14.5 dBm, generate two CW light-waves at 1555.11 and 1563.06 nm, respectively. The two light-waves are combined as the dual-wavelength seed source, which is then injected into the loop. The seed source has a frequency spacing of ~1 THz, which is also the channel spacing. The I/Q modulator, used for MWFS-SSB modulation, has an insertion loss of 9 dB and a bandwidth larger than 30 GHz. The bias of each arm for the I/Q modulator is fixed at the null point. The RF clock frequency is 25 GHz. One phase shifter is used before Q port for π/2 phase shifting. After passing through WSS, frequency-locked multi-carriers with 25-GHz frequency spacing are generated within each channel. The OC has an insertion loss of 3.1 dB and a coupling ratio of 50:50. The EDFA has an output power of 23 dBm. It’s noted that the WSS used in the RFS loop, with an insertion loss of 7 dB, has two passband tones with the bandwidth of 5.2 and 5.4 nm, respectively. It’s also noted that the WSS, doesn’t have the function of polarization-controlling, and therefore we use a PC before it.
The MCMC source, with a structure illustrated in detail in Fig. 2(a), can generate 57 optical subcarriers in our experiment. The tunable optical filter (TOF), with the 3-dB bandwidth of 0.1 nm, is used to choose the measured optical subcarrier. The second I/Q modulator, driven by two 12.5-Gb/s pseudo-random binary sequence (PRBS) signals with a word length of (215-1) × 4, is used to modulate the selected optical subcarrier. The two parallel Mach-Zehnder modulators (MZMs) in the second I/Q modulator are both biased at the null point and driven at the full swing to achieve zero-chirp 0- and π-phase modulation. The phase difference between the upper and the lower branches of the second I/Q modulator is controlled at π/2. The subsequent polarization multiplexing is realized by the polarization multiplexer, comprising a polarization beam splitter (PBS) to halve the signal into two branches, an optical delay line (DL) to provide a delay of 150 symbols, an optical attenuator to balance the power of the two branches and a polarization beam combiner (PBC) to recombine the signal. The 50-Gb/s PMD-QPSK signal is finally generated after polarization multiplexing.
At the receiver, an ECL with a linewidth less than 100 kHz is used as the local oscillator (LO) source. A polarization-diversity 90-degree hybrid is used to realize the polarization- and phase-diversity coherent detection of the LO source and the received optical signal before balanced detection. The analog-to-digital conversion is realized in the digital scope with the sampling rate of 80 GSa/s and the electrical bandwidth of 30 GHz.
For the digital signal processing (DSP), the electrical polarization recovery is achieved using a three-stage blind equalization scheme. Firstly, the clock is extracted using the “square and filter” method, and then the digital signal is re-sampled at twice of the baud rate based on the recovered clock. Secondly, a T/2-spaced time-domain finite impulse response (FIR) filter is used for chromatic dispersion (CD) compensation, where the filter coefficients are calculated from the known fiber CD transfer function using the frequency-domain truncation method. Thirdly, two complex-valued, 13-tap, T/2-spaced adaptive FIR filters, based on the classic constant modulus algorithm (CMA), are used to retrieve the modulus of the PDM-QPSK signal and realize polarization de-multiplexing. The subsequent step is carrier recovery, which includes frequency-offset estimation and carrier-phase estimation. The former is based on fast Fourier transform (FFT) method while the latter fourth-power Viterbi-Viterbi algorithm. Finally, differential decoding is used to eliminate the π/2 phase ambiguity before BER counting. In this experiment, the BER is counted over 12 × 106 bits (12 data sets, and each data set contains 106 bits).
Figure 3 shows the optical spectra after MWFS-SSB modulation when the RFS loop is open and when both ECLs are turned off, respectively. As shown in Fig. 3(a), within each channel, the seed source and the generated subcarrier with 25-GHz frequency shifting are marked out, respectively. Some undesired harmonic tones are generated along with the desired subcarriers, due to the imbalance characteristics of the I/Q modulator used in the RFS loop. We can reduce the imbalance effect by accurately adjusting the amplitude of RF signals as well as the phase difference between them, in order to achieve optimal SSB output [13, 14]. The power ratio of the generated SSB tone to the undesired ones is larger than 30 dB. From Fig. 3(b), we can see that there are two passbands with the bandwidth of 5.2 and 5.4 nm, that is, Channel 1 around 1555.11 nm and Channel 2 around 1563.06 nm, respectively. It is worth noting that the output will be only amplified spontaneous emission (ASE) noise from the EDFA when the seed source is turned off. It demonstrates that this is not a multi-wavelength fiber ring laser.
Fig. 3 The optical spectra after MWFS-SSB modulation (a) when the loop is open (at the 0.1-nm resolution) and (b) when both ECLs are turned off (at the 0.02-nm resolution), respectively.
Figure 4 shows the optical spectra for the case of only Channel 1, only Channel 2 as well as both Channel 1 and Channel 2, respectively. As shown in Fig. 4(a) and Fig. 4(b), in Channel 1, 28 frequency-locked optical subcarriers with 25-GHz subcarrier spacing are generated with tone-to-noise ratio (TNR) larger than 20.0 dB, while 29 ones in Channel 2 with TNR larger than 23.0 dB. The reason Channel 1 and Channel 2 have a different number of subcarriers is that the WSS used in the RFS loop has two passband tones of different bandwidth, one with the bandwidth of 5.2 and the other 5.4 nm, just as shown in Fig. 3(b). As shown in Fig. 4(c), compared to the case of single channel, the power and TNR of high-order optical subcarriers are reduced in the case of WDM Channel. The highest-order optical subcarrier in Channel 1 and Channel 2 has a TNR reduction of 5 and 3 dB, respectively. It is because the phenomenon of so called ‘gain competition’ appears in the EDFA due to the adoption of multiple operating wavelengths in the WDM case. As a result, Channel 1 and Channel 2 affect each other, causing the power of each optical subcarrier reallocated. Furthermore, Channel 1 has a larger deterioration than Channel 2 in the case of WDM channel. It is because the optical subcarriers in Channel 2 are stronger than those in Channel 1, and due to gain competition, more power is reallocated into Channel 2. Power fluctuation is mainly due to the non-flatness characteristics of the EDFA gain spectrum.
Fig. 4 The optical spectra for the case of (a) only Channel 1, (b) only Channel 2 as well as (c) both Channel 1 and Channel 2, respectively (all at the 0.02-nm resolution).
Figure 5 shows the measured BER versus OSNR for the 50-Gb/s optical PMD-QPSK signal in the case of the single-laser source and WDM multi-carrier channel at 1553.02 nm, respectively. The insets (a) and (b) give the constellations for both cases, respectively. The WDM multi-carrier channel at 1553.02 nm is only one of the generated optical subcarriers in Channel 1, which is selected out by the TOF after the MWMC source, just as shown in Fig. 2.We can see that the BER performance for the WDM multi-carrier channel is very similar to that of the single channel carried by one single CW light-wave generated from one ECL, which demonstrates the optical subcarrier generated by our scheme has good performance. The required OSNR is 11.5 dB when the BER is 1 × 10−3 for both cases. The solid curve represents the straight-line fitting of the measured data points. We also measure and confirm that all the other optical subcarriers generated by our scheme exhibit the similar performance. It shows that our proposed scheme has great potential in the future optical WDM communications. In addition, if we consider an ideal case, where the OSNR is not reduced when it is used to carry 50-Gb/s PDM-QPSK signal, then the required OSNR for each tone is 11dB to get the BER less than the FEC limit of 3.8x10−3, just as shown in Fig. 5. In this case, all the tones in Fig. 4(c) can meet this OSNR requirement.
Fig. 5 The measured BER versus OSNR for 50-Gb/s optical PMD-QPSK signal in the case of the single-laser source and WDM multi-carrier channel, respectively. Inset (a) and (b) show the constellations for the single-laser source and WDM multi-carrier channel, respectively.
4. Conclusion
We propose and experimentally demonstrate a novel scheme to generate optical MCMC using a RFS loop based on MWFS-SSB modulation. Dual-wavelength frequency shifting SSB modulation is carried out with dual-wavelength optical seed source in our experimental demonstration. Using this scheme, we successfully generate dual-channel multi-carriers, and one channel has 28 subcarriers while the other has 29 ones with 25-GHz subcarrier spacing. The required OSNR at the BER of 1 × 10−3 is 11.5 dB when 50-Gb/s optical PMD-QPSK signal carried by one of the subcarriers is coherent-detected, which is similar to that carried by one single CW light-wave generated from one ECL. These results show that this scheme has great potential in the future optical WDM communications.
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