We propose and experimentally demonstrate an improved scheme to generate optical frequency-locked multi-channel multi-carriers (MCMC), using a gain-independent multi-channel recirculating frequency shifter (MC-RFS) loop based on single sideband (SSB) modulation. We re-build the RFS structure with better performance. By using MC-RFS loop, we can generate N-channel subcarriers each round trip without interference. These subcarriers of each channel are stable and frequency-locked, which can be used for multi-channel WDM source. The dual-channel RFS loop is carried out for demonstration in our experiment with dual-carrier source. Using this scheme, we successfully generate 62 frequency-locked subcarriers with 25-GHz frequency spacing in 2 channels and each channel has 31 tones.
© 2012 OSA
Optical frequency-locked multi-carrier or comb generation has attracted much research interest in recent years because it can offer several attractive applications such as wideband sources in optical communications [1–7], reconfigurable optical pulse generator , and optical frequency reference for measurement and signal processing . Especially for wavelength-division-multiplexing (WDM) system, wide-area access networks such as a metro ring network proposed [1–6], and the photonic multi-protocol label switching (MPLS) router based on multi-wavelength , increasing the number of channels or wavelengths is the same as increasing the number of accommodated users of access networks or increasing the number of accommodated paths for core and metro networks and also the system transmission capacity . However, how to generate such a compact, stable and frequency-locked multi-channel multi-carrier (MCMC) source is one of the key issues of WDM systems and networks. Generally, there are several methods to generate multi-carrier optical source including the optical supercontinuum technique based on nonlinear effects [8, 10, 11], the cascaded phase modulator and intensity modulator [2, 12, 13], recirculating frequency shifter (RFS) loop based on single sideband (SSB) modulation with in-phase /quadrature (I/Q) modulator [14–18]. Of all the technologies mentioned above, the last one based on RFS loop, which has been investigated a lot theoretically and experimentally [5, 6, 14–16], has the advantages of flexible controlling and accurate frequency spacing with lower driving voltage. In addition, these carriers are exactly frequency-locked and far less sensitive to phase noise and nonlinear propagation effects. However, for regular RFS loop reported by [5, 6, 14–16], only one subcarrier is generated each time within each channel. In our previous work , we adopt the RFS loop to generate optical subcarriers with multiple wavelengths each round and obtain MCMC, which can be effectively used for WDM superchannel. However, as the MCMC share the same optical amplifier, the amplitude difference between each carrier is large and the amplitude of each carrier is not easily controlled due to gain competition in optical amplifier. In another work of ours in , we use double EDFAs for the two channels. However, the two channels are not totally independent due to power sharing and part of optical power after EDFA is lost after blocked by the channel filter. Thus, independent gain of each channel is required for a more stable output. As a result, how to implement this kind of source with new RFS structure is a really interesting research topic.
In this paper, we propose and experimentally demonstrate an improved scheme to generate optical frequency-locked MCMC, using a gain-independent multi-channel recirculating frequency shifter (MC-RFS) loop based on SSB modulation. We improve the RFS structure with independent optical amplifier for each channel. By using MC-RFS loop, we can generate N-channel subcarriers each round trip without interference. These subcarriers of each channel are stable and frequency-locked, and can be used for multi-channel WDM source. The dual-channel RFS loop is carried out for demonstration in our experiment with dual-carrier source. Using this scheme, we successfully generate 62 frequency-locked subcarriers with 25-GHz frequency spacing in two channels and each channel has 31 tones.
2. Principle of MCMC generation
Figure 1(a) shows the structure of optical frequency-locked MCMC source, using a gain-independent MC-RFS loop based on SSB modulation. The structure of the proposed scheme is mainly composed of a multi-wavelength optical seed source and an improved RFS loop. The former, in practice, can be N continuous-wavelength (CW) lasers or N subcarriers generated by a CW source. The latter includes a polarization-maintaining optical coupler (PM-OC) for loop output, an I/Q modulator for frequency shifting, a N-channel de-multiplexer (DEMUX), N independent Erbium-doped fiber amplifiers (EDFAs) to compensate for the loop loss and a N-channel multiplexer (MUX). Each Mach-Zehnder modulator (MZM) in the I/Q modulator is DC-biased at its null points with the half-wave voltage for SSB modulation . The loop is kept polarization maintaining. We can use the polarization controllers (PCs) to control the polarization state of the signal if any optical component in the loop is not polarization-maintaining. After I/Q modulator, the RFS loop is split to N branches with independent EDFAs and different pass band for each channel.
Figure 1(b) shows the principle of MCMC generation. Here, each channel bandwidth is limited and controlled by the N-channel DEMUX and MUX. We assume that the multi-wavelength optical seed source is with N different optical carriers with equal power and fn is the optical frequency of each channel seed source from N CW lasers. The I/Q modulator is used for SSB modulation and frequency shifting, which is driven by two same frequency radio-frequency (RF) sinusoidal signals with a fixed phase difference of + π/2 or -π/2 through I and Q port. The transfer function of I/Q modulator as SSB modulation is
Figure 1(c) shows the principle of DEMUX and MUX used in our improved scheme. To make the gain of each channel totally independent, we use both channel DEMUX and MUX before and after each EDFA. The N-channel DEMUX is the combination of N-channel selective band-pass filters. In practice, we can use an N-port wavelength selective switch (WSS) or N band-pass filters with optical coupler to separate different channels. In this way, different channels of multi-carrier travel different paths with independent EDFAs. Although, after DEMUX band-pass filters, the channels are separated and can be amplified separately, the amplified spontaneous emission (ASE) noise caused by each EDFA covers all the bands of N channels. Thus, we need another band-pass filter to block the noise spreading from other channels. After the N branches of EDFAs, the signal is combined together with the N-channel MUX, which is also the combination of N-channel selective band-pass filters. The pass-band is the same to that of N-channel DEMUX. These MUX filters are used to block the noise. In this way, we can obtain stable and frequency-locked multi-carrier in multi-channels without interference. It is worth noting that there should be a tradeoff between the benefit of the large numbers of subcarrier generation with different channels and the complexity of the structure. The total number of subcarriers is limited by the maximum input power of I/Q modulator. The generated MCMC are frequency-locked within each channel, and they are totally independent. Therefore these MCMC can be effectively used for WDM systems and networks.
3. Experimental setup and results
Figure 2(a) shows the experimental setup for a gain-independent two-channel frequency-locked multi-carrier generation with dual-carrier injection. Here, two external cavity lasers (ECLs), with the linewidth less than 100 kHz and the output power of 12 dBm are used as the source carriers at 1541.40 and 1549.40 nm. The seed source has a frequency spacing of ~1 THz, which is also the channel spacing. The I/Q modulators with a 9-dB insertion loss used here for SSB modulation and frequency shifting is driven by 25-GHz RF signal. One phase shifter is used before Q port for π/2 phase shifting. The PM-OCused here are with an insertion loss of 3.1 dB. Figure 2(b) shows the optical spectrum of SSB modulation after I/Q modulator when the RFS loop is open. Within each channel, the seed source and the generated subcarrier are marked out with 25-GHz frequency shifting. As shown in Fig. 2(b), some undesired harmonic tones are generated along with the desired subcarrier, due to the imbalance characteristics of the I/Q modulator. We can reduce the effect of imbalance by accurately adjusting the amplitude of RF signals as well as the phase difference between them, in order to achieve optimal SSB output [12–15]. The power ratio of the generated SSB tone to the undesired ones is about 30 dB. After I/Q modulator, we use the two tunable optical filters (TOFs) with the PM-OC as the channel DEMUX. In this way, the two channels are separated by the two TOFs before injected the EDFAs. The bandwidth of the TOF and WSS is described in the transfer function of two channels as shown in Fig. 2(c). The 3-dB bandwidth (BW) of the combined profile of the TOF and WSS for channel 1 is 6.18 nm, and the 20-dB BW is 6.44 nm; while the 3-dB and 20-dB BW for channel 2 is 6.15 and 6.5 nm, respectively. The combined insertion loss of TOFs (7 dB) and WSS (7 dB) is 14 dB for each channel. The polarization-maintaining EDFA works in Automatic Current Control (ACC) mode for stable output and the output power is 23 dBm. Since the gain of the EDFA is different or adaptive for different input power, the gain of EDFA is around the total loss in the loop to generate flattened subcarriers. After EDFAs, the two channels of subcarriers in two branches are combined together by a two-port WSS, which is used as the channel MUX. The WSS is also used as two band-pass TOFs. Figure 2(c) shows the transfer function of two channels with the two TOFs and WSSs. It is worth noting that the output will be only ASE noise from the EDFAs. Four extra PCs are used in the setup because the TOF 2 and WSS are not polarization maintaining.
The channel DEMUX and MUX filters are used for channel separation and ASE noise blocking. To make this structure stable and two channels totally independent, both DEMUX and MUX are needed. Figure 3(a) shows the output of RFS loop without seed source injection when there are only DEMUX filters, while Fig. 3(b) shows the output when both DEMUX and MUX filters are used. We can see that, the ASE noise after EDFAs spreads the whole channel band due to the wide gain spectrum of EDFA. However, as shown in Fig. 3(b), the noise crosstalk from other channel can be avoided by using the channel MUX filters. We can also find the noise accumulation after the output as shown in Fig. 3. It demonstrates that this is not a multi-wavelength fiber ring laser because no laser can be observed.
In order to investigate the effect of DEMUX and MUX filters, we first use only DEMUX filters in our experiment. In this case, the two channels are directly combined together by a PM-OC. 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, 31 frequency-locked optical subcarriers with 25-GHz subcarrier spacing are generated with tone-to-noise ratio (TNR) larger than 22.0 dB, while 31 ones in Channel 2 with TNR larger than 18.0 dB. Figure 4(c) shows the result of 2 WDM channels. We can see that, the power profile of each WDM channel shows no change compared with single channel thanks to the channel separation by DEMUX filters. However, the power and TNR of high-order optical subcarriers are decreased in the case of WDM Channel. The highest-order optical subcarrier in Channel 1 and Channel 2 has a TNR reduction of 7 and 4 dB, respectively. It is because of the noise crosstalk from other channel. As a result, Channel 1 and 2 affect each other, causing the TNR reduction. Figure 5 shows the output result when we use both DEMUX and MUX filters. We can see that, both the power profile and the TNR of each WDM channel show no change compared with single-channel case. As a result, we can obtain 31 frequency-locked subcarriers with TNR larger than 22.0 dB in channel 1 and 31 ones in Channel 2 with TNR larger than 18.0 dB. The ON or OFF of the other channel shows no impact on the performance of the channel output. Thus, these two channels are totally independent. It is worth noting that the TNR has great impact on the signal optical signal-to-noise ratio (OSNR) in the transmission system , which directly determines the bit-error-ratio (BER) performance for most transmission systems. Thus, a reduced TNR means a reduced OSNR for transmission system and a worse BER performance for this subcarrier. Therefore, the subcarriers generated with both DEMUX and MUX filters in Fig. 5 have better system performance compared with Fig. 4.
We propose and experimentally demonstrate a better scheme to generate optical frequency-locked MCMC, using a gain-independent -MC-RFS loop based on SSB modulation. We rebuild the RFS structure with independent optical amplifier for each channel. By using MC-RFS loop, we can generate N channel subcarriers each round trip without interference. The dual-channel RFS loop is carried out for demonstration in our experiment with dual-carrier source. Using this scheme, we successfully generate 62 frequency-locked subcarriers with 25-GHz frequency spacing in 2 channels and each channel has 31 tones. This work was partially supported by NNSF of China (No. 61107064, No. 61177071, No. 60837004,No.61250018), NHTRDP (863 Program) of China (2011AA010302, 2012AA011302), the NKTR&DP of China (2012BAH18B00), ICPSSTA of Shanghai (12510705600), the Creative Talent Project Foundation for Key Disciplines of Fudan University, and the China Scholarship Council (201206100076).
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