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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 28 — Dec. 31, 2012
  • pp: 29599–29604
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Improved multi-channel multi-carrier generation using gain-independent multi-channel frequency shifting recirculating loop

Junwen Zhang, Jianjun Yu, Nan Chi, Ze Dong, Xinying Li, and G. K. Chang  »View Author Affiliations


Optics Express, Vol. 20, Issue 28, pp. 29599-29604 (2012)
http://dx.doi.org/10.1364/OE.20.029599


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Abstract

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

1. Introduction

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)
Fig. 1 The principle of multi-channel optical frequency-locked multi-carrier source generation based on a gain-independent multi-channel recirculating frequency shifter loop.
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 [12

12. R. Wu, V. R. Supradeepa, C. M. Long, D. E. Leaird, and A. M. Weiner, “Generation of very flat optical frequency combs from continuous-wave lasers using cascaded intensity and phase modulators driven by tailored radio frequency waveforms,” Opt. Lett. 35(19), 3234–3236 (2010). [CrossRef] [PubMed]

]. 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
Eout(t)=EI(t)+jβEQ(t)EinJ1(π2R)exp(j2πfst)
(1)
Here, Ein is input optical signal, β is the unbalanced factor between the two arms in I/Q modulator, and R is the modulation index. We can see that, after I/Q modulator, the optical frequency of subcarriers in each channel is shifted by fs, which is the RF frequency. Considering the RFS loop, the generated N new subcarriers of N channels are split to two branches, one coupled out and the other recirculating back into the loop with the original N subcarriers. Round by round, the RFS loop can generate more subcarriers. The output signal after the optical coupler is coupled in and recirculating back into each EDFA to compensate for the losses in the closed loop. The channel DEMUX and MUX are used for controlling the number and filtering the other bands to avoid interference in each branch. The transfer function considering EDFAN and SSB modulation for each round trip can be express as
Fn(t)=grnexp(arn)J1(π2R)exp(j2πfst)exp(jθn)
(2)
Where grn is the gain of EDFAN, θn is the phase delay for each round of path n, and exp(-arn) represents the total insertion losses in the loop for each round trip of path n. Assuming all the insertion losses and the modulation loss are compensated by EDFA, the final output of RFS loop based on RFS-SSB after stable running is
Eout_M(t)=m=0Mn=1NEoexp[j2π(fn+mfs)t+ϕn]exp(jmθn)
(3)
Where M is the effective round trip time for stable output and limited by the bandwidth B of band-pass filter. θn is the phase shift each round for the nth channel. The number of round trip for total output is M = B/fS and the total number of subcarriers of N channels is MN.

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)
Fig. 2 (a) The experimental setup for the MCMC generation based on a gain-independent dual-carrier injection. (b) The optical spectrum after I/Q modulator when the RFS loop is open; (c) The transfer function of two channels with TOFs and WSS when the ECLs are turned off without SSB modulation.
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

12. R. Wu, V. R. Supradeepa, C. M. Long, D. E. Leaird, and A. M. Weiner, “Generation of very flat optical frequency combs from continuous-wave lasers using cascaded intensity and phase modulators driven by tailored radio frequency waveforms,” Opt. Lett. 35(19), 3234–3236 (2010). [CrossRef] [PubMed]

15

15. J. Li, X. Li, X. Zhang, F. Tian, and L. Xi, “Analysis of the stability and optimizing operation of the single-side-band modulator based on re-circulating frequency shifter used for the T-bit/s optical communication transmission,” Opt. Express 18(17), 17597–17609 (2010). [CrossRef] [PubMed]

]. 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)
Fig. 3 (a) The output of the RFS loop without seed source when there are only DEMUX filters; (b) The output of the RFS loop without seed source when both DEMUX and MUX filters are used.
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
Fig. 4 The optical spectra for the case with only DEMUX and without MUX filter (a) only Channel 1, (b) only Channel 2 as well as (c) both Channel 1 and Channel 2, respectively (all at 0.02-nm resolution).
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
Fig. 5 The optical spectra for the case with both DEMUX and MUX filters (a) only Channel 1, (b) only Channel 2 as well as (c) both Channel 1 and 2, respectively (all at 0.02-nm resolution).
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 [17

17. X. Li, J. Yu, Z. Dong, J. Zhang, Y. Shao, and N. Chi, “Multi-channel multi-carrier generation using multi-wavelength frequency shifting recirculating loop,” Opt. Express 20(20), 21833–21839 (2012). [CrossRef] [PubMed]

], 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.

4. Conclusion

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).

References and links

1.

M. Fujiwara, M. Teshima, J. Kani, H. Suzuki, N. Takachio, and K. Iwatsuki, “Optical carrier supply module using flattened optical multicarrier generation based on sinusoidal amplitude and phase hybrid modulation,” J. Lightwave Technol. 21(11), 2705–2714 (2003). [CrossRef]

2.

J. Yu, Z. Dong, and N. Chi, “1.96 Tb/s (21x100 Gb/s) OFDM optical signal generation and transmission over 3200-km fiber,” IEEE Photon. Technol. Lett. 23(15), 1061–1063 (2011). [CrossRef]

3.

N. Takachio, H. Suzuki, M. Fujiwara, J. Kani, K. Iwatsuki, H. Yamada, T. Shibata, and T. Kitoh, “Wide area gigabit access network based on 12.5 GHz spaced 256 channel super-dense WDM technologies,” Electron. Lett. 37(5), 309–311 (2001). [CrossRef]

4.

N. Takachio, H. Suzuki, M. Fujiwara, J. Kani, T. Kitoh, M. Teshima, and K. Iwatsuki, “12.5 GHz-spaced super-dense WDM ring network handling 256 wavelengths with tapped-type OADM’s,” in Proc. OFC’2002, ww2, 2002.

5.

X. Liu, S. Chandrasekhar, B. Zhu, and D. Peckham, “Efficient Digital Coherent Detection of A 1.2-Tb/s 24carrier no-guard-interval CO-OFDM signal by simultaneously detecting multiple carriers per sampling,” OFC. OWO2, (2010).

6.

Y. Ma, Q. Yang, Y. Tang, S. Chen, and W. Shieh, “1-Tb/s single-channel coherent optical OFDM transmission over 600-km SSMF fiber with subwavelength bandwidth access,” Opt. Express 17(11), 9421–9427 (2009). [CrossRef] [PubMed]

7.

K. Shimano, Y. Takigawa, M. Koga, and K. Sato, “Development of photonic MPLS router,” in Proc. ECOC’01, Tu.F.22, 2001.

8.

A. Clarke, D. Williams, M. Roelens, and B. Eggleton, “Reconfigurable optical pulse generator employing a Fourier-domain programmable optical processor,” IEEE Photon. Technol. Lett. 28, 97–103 (2010).

9.

P. J. Delfyett, S. Gee, H. Myoung-Taek Choi, Izadpanah, S. Wangkuen Lee, F. Ozharar, Quinlan, and T. Yilmaz, “Optical frequency combs from semiconductor lasers and applications in ultrawideband signal processing and communications,” J. Lightwave Technol. 24(7), 2701–2719 (2006). [CrossRef]

10.

K. Tamura, H. Kubota, and M. Nakazawa, “Fundamentals of stable continuum generation at high repetition rates,” J. Lightwave Technol. 36, 773–779 (2000).

11.

F. Parmigiani, C. Finot, K. Mukasa, M. Ibsen, M. A. Roelens, P. Petropoulos, and D. J. Richardson, “Ultra-flat SPM-broadened spectra in a highly nonlinear fiber using parabolic pulses formed in a fiber Bragg grating,” Opt. Express 14(17), 7617–7622 (2006). [CrossRef] [PubMed]

12.

R. Wu, V. R. Supradeepa, C. M. Long, D. E. Leaird, and A. M. Weiner, “Generation of very flat optical frequency combs from continuous-wave lasers using cascaded intensity and phase modulators driven by tailored radio frequency waveforms,” Opt. Lett. 35(19), 3234–3236 (2010). [CrossRef] [PubMed]

13.

T. Yamamoto, T. Komukai, K. Suzuki, and A. Takada, “Multicarrier light source with flattened spectrum using phase modulators and dispersion medium,” J. Lightwave Technol. 27(19), 4297–4305 (2009). [CrossRef]

14.

T. Kawanishi, S. Oikawa, K. Higuma, and M. Izutsu, “Electrically tunable delay line using an optical single-side-band modulator,” IEE Photon. Technol. Lett. 14(10), 1454–1456 (2002). [CrossRef]

15.

J. Li, X. Li, X. Zhang, F. Tian, and L. Xi, “Analysis of the stability and optimizing operation of the single-side-band modulator based on re-circulating frequency shifter used for the T-bit/s optical communication transmission,” Opt. Express 18(17), 17597–17609 (2010). [CrossRef] [PubMed]

16.

F. Tian, X. Zhang, J. Li, and L. Xi, “Generation of 50 stable frequency-locked optical carriers for Tb/s multicarrier optical transmission using a recirculating frequency shifter,” J. Lightwave Technol. 29(8), 1085–1091 (2011). [CrossRef]

17.

X. Li, J. Yu, Z. Dong, J. Zhang, Y. Shao, and N. Chi, “Multi-channel multi-carrier generation using multi-wavelength frequency shifting recirculating loop,” Opt. Express 20(20), 21833–21839 (2012). [CrossRef] [PubMed]

18.

J. Zhang, J. Yu, N. Chi, Z. Dong, X. Li, Y. Shao, and L. Tao, “Multichannel optical frequency-locked multicarrier source generation based on multichannel recirculation frequency shifter loop,” Opt. Lett. 37(22), 4714–4716 (2012). [PubMed]

OCIS Codes
(060.2320) Fiber optics and optical communications : Fiber optics amplifiers and oscillators
(060.2330) Fiber optics and optical communications : Fiber optics communications
(060.2630) Fiber optics and optical communications : Frequency modulation

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: October 23, 2012
Revised Manuscript: December 5, 2012
Manuscript Accepted: December 11, 2012
Published: December 20, 2012

Citation
Junwen Zhang, Jianjun Yu, Nan Chi, Ze Dong, Xinying Li, and G. K. Chang, "Improved multi-channel multi-carrier generation using gain-independent multi-channel frequency shifting recirculating loop," Opt. Express 20, 29599-29604 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-28-29599


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References

  1. M. Fujiwara, M. Teshima, J. Kani, H. Suzuki, N. Takachio, and K. Iwatsuki, “Optical carrier supply module using flattened optical multicarrier generation based on sinusoidal amplitude and phase hybrid modulation,” J. Lightwave Technol.21(11), 2705–2714 (2003). [CrossRef]
  2. J. Yu, Z. Dong, and N. Chi, “1.96 Tb/s (21x100 Gb/s) OFDM optical signal generation and transmission over 3200-km fiber,” IEEE Photon. Technol. Lett.23(15), 1061–1063 (2011). [CrossRef]
  3. N. Takachio, H. Suzuki, M. Fujiwara, J. Kani, K. Iwatsuki, H. Yamada, T. Shibata, and T. Kitoh, “Wide area gigabit access network based on 12.5 GHz spaced 256 channel super-dense WDM technologies,” Electron. Lett.37(5), 309–311 (2001). [CrossRef]
  4. N. Takachio, H. Suzuki, M. Fujiwara, J. Kani, T. Kitoh, M. Teshima, and K. Iwatsuki, “12.5 GHz-spaced super-dense WDM ring network handling 256 wavelengths with tapped-type OADM’s,” in Proc. OFC’2002, ww2, 2002.
  5. X. Liu, S. Chandrasekhar, B. Zhu, and D. Peckham, “Efficient Digital Coherent Detection of A 1.2-Tb/s 24carrier no-guard-interval CO-OFDM signal by simultaneously detecting multiple carriers per sampling,” OFC. OWO2, (2010).
  6. Y. Ma, Q. Yang, Y. Tang, S. Chen, and W. Shieh, “1-Tb/s single-channel coherent optical OFDM transmission over 600-km SSMF fiber with subwavelength bandwidth access,” Opt. Express17(11), 9421–9427 (2009). [CrossRef] [PubMed]
  7. K. Shimano, Y. Takigawa, M. Koga, and K. Sato, “Development of photonic MPLS router,” in Proc. ECOC’01, Tu.F.22, 2001.
  8. A. Clarke, D. Williams, M. Roelens, and B. Eggleton, “Reconfigurable optical pulse generator employing a Fourier-domain programmable optical processor,” IEEE Photon. Technol. Lett.28, 97–103 (2010).
  9. P. J. Delfyett, S. Gee, H. Myoung-Taek Choi, Izadpanah, S. Wangkuen Lee, F. Ozharar, Quinlan, and T. Yilmaz, “Optical frequency combs from semiconductor lasers and applications in ultrawideband signal processing and communications,” J. Lightwave Technol.24(7), 2701–2719 (2006). [CrossRef]
  10. K. Tamura, H. Kubota, and M. Nakazawa, “Fundamentals of stable continuum generation at high repetition rates,” J. Lightwave Technol.36, 773–779 (2000).
  11. F. Parmigiani, C. Finot, K. Mukasa, M. Ibsen, M. A. Roelens, P. Petropoulos, and D. J. Richardson, “Ultra-flat SPM-broadened spectra in a highly nonlinear fiber using parabolic pulses formed in a fiber Bragg grating,” Opt. Express14(17), 7617–7622 (2006). [CrossRef] [PubMed]
  12. R. Wu, V. R. Supradeepa, C. M. Long, D. E. Leaird, and A. M. Weiner, “Generation of very flat optical frequency combs from continuous-wave lasers using cascaded intensity and phase modulators driven by tailored radio frequency waveforms,” Opt. Lett.35(19), 3234–3236 (2010). [CrossRef] [PubMed]
  13. T. Yamamoto, T. Komukai, K. Suzuki, and A. Takada, “Multicarrier light source with flattened spectrum using phase modulators and dispersion medium,” J. Lightwave Technol.27(19), 4297–4305 (2009). [CrossRef]
  14. T. Kawanishi, S. Oikawa, K. Higuma, and M. Izutsu, “Electrically tunable delay line using an optical single-side-band modulator,” IEE Photon. Technol. Lett.14(10), 1454–1456 (2002). [CrossRef]
  15. J. Li, X. Li, X. Zhang, F. Tian, and L. Xi, “Analysis of the stability and optimizing operation of the single-side-band modulator based on re-circulating frequency shifter used for the T-bit/s optical communication transmission,” Opt. Express18(17), 17597–17609 (2010). [CrossRef] [PubMed]
  16. F. Tian, X. Zhang, J. Li, and L. Xi, “Generation of 50 stable frequency-locked optical carriers for Tb/s multicarrier optical transmission using a recirculating frequency shifter,” J. Lightwave Technol.29(8), 1085–1091 (2011). [CrossRef]
  17. X. Li, J. Yu, Z. Dong, J. Zhang, Y. Shao, and N. Chi, “Multi-channel multi-carrier generation using multi-wavelength frequency shifting recirculating loop,” Opt. Express20(20), 21833–21839 (2012). [CrossRef] [PubMed]
  18. J. Zhang, J. Yu, N. Chi, Z. Dong, X. Li, Y. Shao, and L. Tao, “Multichannel optical frequency-locked multicarrier source generation based on multichannel recirculation frequency shifter loop,” Opt. Lett.37(22), 4714–4716 (2012). [PubMed]

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