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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 20 — Sep. 24, 2012
  • pp: 22079–22086
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Characterization of passive optical components with ultra-fast speed and high-resolution based on DD-OFDM

Banghong Guo, Tao Gui, Zhaohui Li, Yuan Bao, Xingwen Yi, Jianping Li, Xinhuan Feng, and Songhao Liu  »View Author Affiliations


Optics Express, Vol. 20, Issue 20, pp. 22079-22086 (2012)
http://dx.doi.org/10.1364/OE.20.022079


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Abstract

The passive optical components with very fine structures in wavelength domain are very sensitive to the mechanical vibrations or thermal fluctuations. If the measurement speed is lower than the temperature and mechanical fluctuation, we cannot measure the dynamic characteristics of the optical components. In this paper, we propose and demonstrate a novel method with ultra-fast measurement speed and high-resolution based on optical channel estimation using direct-detected orthogonal frequency division multiplexing (DD-OFDM) signal, which can be used to measure the dynamic characteristics and fine structure of the passive optical components. In experiment, by using fast Fourier transform (FFT) and a low-cost electro-absorption modulated laser (EML), we can achieve the transfer function characteristics with 3.9MHz resolution. Compared with the optical channel estimation using coherent OFDM signal reported before, the proposed measurement technique is cost-effective.

© 2012 OSA

1. Introduction

Some special passive optical components with very fine structures in wavelength domain tend to be unstable due to the mechanical vibrations or thermal fluctuations. These passive optical components sensitive to the environment, such as Mach-zehnder interferometer or fiber Bragg gratings (FBGs), can be used for sensor application. But in some sensor application with the requirement on the speed and resolution, such as ultra-fast mechanical vibration or ultrasonic sensor, it is difficult to know the response if it is going to continue to change rapidly. Conventional measurement techniques with slow speed, e.g., optical spectrum analyzer (OSA) or a laser scanning system, cannot meet these demands [1

1. T. Niemi, M. Uusimaa, and H. Ludvigsen, “Limitations of phase-shift method in measuring dense group delay ripple of fiber Bragg gratings,” IEEE Photon. Technol. Lett. 13(12), 1334–1336 (2001). [CrossRef]

4

4. D. J. Krause, J. C. Cartledge, L. Jakober, and K. Roberts, “Measurement of passive optical components using a carrier and single sideband,” in Optical Fiber Communication Conference 2006, Paper OFN5.

]. Hence, it is very important to propose a characterization method with high-resolution and ultra-fast measurement speed. To achieve these requirements, we have recently demonstrated a method based on coherent detection technique and optical orthogonal frequency division multiplexing (OFDM) signal [5

5. X. Yi, Z. Li, Y. Bao, and K. Qiu, “Characterization of passive optical components by DSP-based optical channel estimation,” IEEE Photon. Technol. Lett. 24(6), 443–445 (2012). [CrossRef]

]. Unfortunately, coherent optical detection technique needs several expensive devices, including the narrow line-width laser sources and optical coherent receiver [6

6. W. Peng, B. Zhang, K.-M. Feng, X. Wu, A. E. Willner, and S. Chi, “Spectrally efficient direct-detected OFDM transmission incorporating a tunable frequency gap and an iterative detection techniques,” J. Lightwave Technol. 27(24), 5723–5735 (2009). [CrossRef]

]. In contrast, direct-detection OFDM (DD-OFDM) signal can be detected without optical coherent receiver. In addition, for DD-OFDM signal, a low-cost electro-absorption modulated laser (EML) with several MHz line-width instead of the high-cost external cavity laser (ECL) is enough for the system requirement [7

7. Z. Zan, M. Premaratne, and A. J. Lowery, “Laser RIN and linewidth re-quirements for direct detection optical OFDM,” in CLEO 2008, paper CWN2.

]. Therefore, the high-speed and high-resolution measurement technique using DD-OFDM signal can be implemented cost-effectively.

In this paper, we demonstrate an optical channel estimation technique using DD-OFDM signal to characterize the passive optical components. In experiment, by using FFT, EML and photo detector (PD), we can realize sub-MHz resolution with a measurement time bounded by the theoretical limit. In order to compare the measurement performance using DD-OFDM and CO-OFDM signals [5

5. X. Yi, Z. Li, Y. Bao, and K. Qiu, “Characterization of passive optical components by DSP-based optical channel estimation,” IEEE Photon. Technol. Lett. 24(6), 443–445 (2012). [CrossRef]

], we measure a fiber Bragg grating (FBG) and a home-built delay interferometer (DI) based on two techniques. The experimental results using DD-OFDM signal agree well with that using CO-OFDM signal.

2. Technical principle

In optical coherent detection system, the coherent optical receiver down-converts the whole optical signal modulated with RF signals linearly to an electrical signal by means of heterodyne or homodyne detection. The combination of coherent detection and digital signal processing techniques provide us the capability to obtain both the phase and intensity response of the passive optical components directly based on double sideband (DSB) or single sideband (SSB) modulated optical signal [8

8. K. Kikuchi, “Coherent transmission systems,” in Proc. ECOC, paper no. Th.2.A.1, 21–25, Belgium, 2008.

10

10. W. Shieh, X. Yi, Y. Ma, and Q. Yang, “Coherent optical OFDM: has its time come?” J. Opt. Netw. 7(3), 234–255 (2008). [CrossRef]

].

In contrast, for the DD-OFDM detection system, it is difficult to obtain the phase information from the passive optical components only based on DSB modulated optical signal directly, since the lower sideband and upper sideband optical signals after the direct detection at PD have the same amplitude but opposite phase and exactly cancel each other. Therefore, we adopt optical single-sideband (OSSB) modulation technique for DD-OFDM detection system, which can be described as “self-coherent” detection. With the help of self-coherent detection technique, the phase or delay information can also be recovered from OSSB modulated optical signal [11

11. M. Sieben, J. Conradi, and D. E. Dodds, “Optical single sideband transmission at 10 Gb/s using only electrical dispersion compensation,” J. Lightwave Technol. 17(10), 1742–1749 (1999). [CrossRef]

13

13. J. Leibrich, A. Ali, and W. Rosenkranz, “Optical OFDM as a promising technique for bandwidth efficient high-speed data transmission over optical fiber,” in Int. OFDM-Workshop 2007, Hamburg, Germany.

].

Therefore, we use the OSSB-DD-OFDM signal to measure the property of passive optical components. In general, the OSSB-DD-OFDM can be described as follows [14

14. A. Lowery, L. Du, and J. Armstrong, “Orthogonal frequency division multiplexing for adaptive dispersion compensation in long haul WDM systems,” in Optical Fiber Communication Conference 2006, paper PDP39.

,15

15. B. Schmidt, A. Lowery, and J. Armstrong, “Experimental demonstrations of 20 Gbit/s direct-detection optical OFDM and 12 Gbit/s with a colorless transmitter,” in Optical Fiber Communication Conference 2007, paper PDP18.

]:
s(t)=ej2πf0t+αej2π(f0+Δf)tsB(t)
(1)
where s(t) is the optical OFDM signal, f0 is the optical carrier frequency, Δf is the guard band between the optical carrier and the OFDM band, and α is the scaling coefficient indicating the relative amplitude between the OFDM signal and optical carrier. sB(t) is the baseband OFDM signal given by Eq. (2):
sB(t)=k=12Nsc+112Nscckej2πfkt
(2)
where ck represents the mapped 4-QAM symbols on the kth subcarrier, fk and Nsc represents the frequency of the kth subcarrier and the total number of subcarriers respectively. After the signal passing through the device under test (DUT), its impulse response function can be described as h(t) and thus the OFDM signal can be approximated as Eq. (3):
r(t)=s(tτ)h(τ)dτ=[ej2πf0(tτ)+αej2π(f0+Δf)(tτ)k=12Nsc+112Nscckej2πfk(tτ)]h(τ)dτ=ej2πf0th(τ)ej2πf0τdτ+αej2π(f0+Δf)tk=12Nsc+112Nscckej2πfkth(τ)ej2π(f0+Δf+fk)τdτ
(3)
Since the channel response function H(f) represents the Fourier transform of the impulse response function, i.e.
H(f)=h(τ)ej2πfτdτ
(4)
Equation (3) can be simplified as
r(t)=H(f0)ej2πf0t+αej2π(f0+Δf)tk=12Nsc+112NscH(f0+Δf+fk)ckej2πfkt
(5)
At the receiver, the photo-detector can be modeled as the square law detector and the detected photocurrent is represented by Eq. (6):
I(t)|r(t)|2=|H(f0)|2+2αRe{ej2πΔftk=12Nsc+112Nsc[|H(f0+Δf+fk)|ejϕ(fk)ck]receiveddataej2πfkt}+|α2|k1=12Nsc+112Nsck2=12Nsc+112Nscck2*ck1ej(2π(fk1fk2)t)H(f0+Δf+fk1)H(f0+Δf+fk2)
(6)
In Eq. (6), the first term is the DC component that can be filtered out. The second term consists of the recovered linear OFDM signal, which contains both the phase and intensity response information. So, the second term can be used for optical channel estimation. Let dk represents the received data on the kth subcarrier, the complex channel response can be obtained by H(f0+Δf+fk)=dk/ck. The intensity response is the absolute value of H(f0+Δf+fk) and the phase response is the angle value of H(f0+Δf+fk). The third term is the intermodulation term in the low frequency, which should be removed [16

16. W. Peng, X. Wu, A. Arbab, B. Shamee, L. Christen, J. Ynag, K. Feng, A. Willner, and S. Chi, “Experimental demonstration of a coherently modulated and directly detected optical OFDM system using an RF-tone insertion,” in Optical Fiber Communication Conference 2008, paper OMU2.

]. We know that αis small, and |α2| should be much smaller, so the power of the third term is much smaller than the second term, and can be ignored.

For optical channels estimation using DD-OFDM signal, there are some possible measurement error induced by the sensitivity of PD, noise of EDFA, non-ideal of IQ-mixer and intermodulation products (i.e. the third term of Eq. (6) etc.)

In order to minimize the effect of the noise from optical amplifiers and PD, averaging and low pass filtering techniques are necessary for the detected electric signals, which will influence the measurement speed. Even so, this method is still much faster than conventional methods, such as OSA or laser scanning technique.

Unlike the optical coherent detection, the direct detection is insensitive to polarization state of the optical signal. Polarization insensitivity is very important and convenient for the fast and fine measurement system since we need not to keep the polarization stability.

3. Experimental setup

Figure 1
Fig. 1 Experimental setup and DUT (device under test) (S/P: serial to parallel; P/S: parallel to serial; DDC: digital down-conversion; CP: cyclic prefix ratio; DUT: device under test.)
shows the experimental setup of ultra-fast speed and high-resolution measurement system based on DD-OFDM signal. An OFDM baseband signal was generated in a computer and uploaded into a Tektronix AWG 7122B arbitrary waveform generator (AWG). The waveforms generated by AWG were continuously output at 8GS/s. 4-QAM signal was used to map the bit stream data to every OFDM sub-carrier. The measurement frequency resolution is determined by AWG sampling rate divided by FFT points. The longer of the OFDM symbol, which means the more points FFT, leads to a higher resolution. So a tunable frequency resolution can be achieved only by varying the length of the OFDM symbol. In the experiment, we use 2048 points FFT to achieve the frequency resolution about 3.9MHz. If using more points FFT, we can realize sub-MHz resolution with a measurement time bounded by the theoretical limit. However, too many points of FFT will increase the calculation complexity but also decrease the measurement speed. In addition, the signal will become more sensitive to the phase noise if the OFDM symbol is too long. Therefore, the length of OFDM symbol should be optimized for different applications.

To realize the OSSB modulation and avoid the second-order intermodulation distortion (IMD) due to the square law detection at PD, the OFDM signal should be up-converted from the optical baseband onto the radio frequency (RF) signal [17

17. A. Ali, H. Paul, J. Leibrich, W. Rosenkranz, and K. Kammeyer, “Optical biasing in direct detection optical-OFDM for improving receiver sensitivity,” in Optical Fiber Communication Conference 2010, paper JThA12.

]. In this experiment, an analog IQ-mixer with 5GHz bandwidth and 8-12GHz local oscillator frequency range is used to up-converted the OFDM signal from the baseband to a 8.5GHz RF signal. After amplified by the electric driver, OFDM signals are modulated to an optical signal through an EML. As shown in Fig. 1, OSSB OFDM signal is realized by using an optical filter to remove the lower sideband optical signal.

Figure 2(a)
Fig. 2 Optical OFDM spectrum after EAM modulation. (a) DSB modulated optical signal; (b) SSB modulated optical signal
and Fig. 2(b) show the optical spectrum of DSB and SSB optical OFDM signal respectively. An erbium doped fiber amplifier (EDFA) is used to compensate for the loss of DUT. A FBG and a home-built delay interferometer with 3m long relative arm difference are used as DUT in the experiment.

After passing through the DUT, OSSB OFDM signal is detected at PD. A real-time oscilloscope operated at 25GS/s is used to digitalize the detected electric OFDM signal. The OFDM symbols are converted to multiple subcarriers in frequency domain through FFT in computer. Digital signal processing techniques are used to estimate the phase and intensity response of the OFDM signal induced by passive optical components [18

18. A. J. Lowery and J. Armstrong, “10 Gbit/s multimode fiber link using power-efficient orthogonal frequency-division-multiplexing,” Opt. Express 13(25), 10003–10009 (2005). [CrossRef] [PubMed]

].

4. Experimental results and discussions

According to the above analysis, the frequency resolution is determined by FFT length. By setting the FFT length to 2,048 and the CP length to 128, measurement frequency resolution is approximately 3.9MHz theoretically. Obviously, it is difficult for OSA or a laser scanning system to achieve such a high resolution and fast measurement speed. The duration of each OFDM symbol is 0.272μs with the AWG sampling rate of 8GS/s. The effective bandwidth of the OFDM signal is about 6GHz.

We first measure the properties of DI using DD-OFDM and CO-OFDM signals and then compare the measurement results [5

5. X. Yi, Z. Li, Y. Bao, and K. Qiu, “Characterization of passive optical components by DSP-based optical channel estimation,” IEEE Photon. Technol. Lett. 24(6), 443–445 (2012). [CrossRef]

]. Figure 3(a)
Fig. 3 Measured transfer function of a DI with 66.7MHz FSR. The resolution is 3.9MHz. (a) DD-OFDM method within 6GHz span, (b) CO-OFDM method within 6GHz span, (c) DD-OFDM method within 1GHz span in detail, (d) CO-OFDM method within 1GHz span in detail.
and Fig. 3(b) illustrates the measured transfer function by using the DD and CO-OFDM signal respectively. Figure 3(c) and Fig. 3(d) shows the measurement results in detail within 1GHz span. The experimental results are averaged by using 22 OFDM symbols. The measured extinction ratios of intensity response are larger than 20dB and the measured free spectral range (FSR) is 66.7-MHz.

The discrete phase level with a difference of π and the periodic phase jumps demonstrate that we have accurately measured the phase response of the DI [19

19. J. Li, K. Worms, R. Maestle, D. Hillerkuss, W. Freude, and J. Leuthold, “Free-space optical delay interferometer with tunable delay and phase,” Opt. Express 19(12), 11654–11666 (2011). [CrossRef] [PubMed]

,20

20. J. Li, K. Worms, D. Hillerkuss, B. Richter, R. Maestle, W. Freude, and J. Leuthold, “Tunable free space optical delay interferometer for demodulation of differential phase shift keying signals,” in Optical Fiber Communication Conference 2010, paper JWA24.

]. The experimental result based on DD-OFDM signal agree well with those measured using the CO-OFDM-based technique. Although there are some phase spikes using both techniques, they can be neglected since they only occurred at the edge of every phase jump.

We also characterize and compare the reflective and transmission spectrum of an FBG using the conventional OSA and DD-OFDM-based techniques. Figure 4
Fig. 4 Reflective optical spectrum of FBG measurements by (a) Conventional laser scanning technique with 125MHz frequency resolution (b) DD-OFDM technique with 3.9 MHz frequency resolution of the red circle part.
and Fig. 5
Fig. 5 Transmission optical spectrum of FBG measurements by (a) Conventional laser scanning technique with 125MHz frequency resolution (b) DD-OFDM technique with 3.9 MHz frequency resolution of the red circle part.
illustrate the reflective and transmission optical spectrum of FBG respectively based on two measurement techniques respectively. Figure 4(a) and Fig. 5(a) show the intensity response of FBG using OSA. Figure 4(b) and Fig. 5(b) show the measured relative intensity and phase response of FBG based on the DD-OFDM method. Here, we only show part of the measured intensity spectrum indicated by the red circle in Fig. 5(b). The measured intensity response has a good agreement with that measured by using OSA. However, optical channel estimation using DD-OFDM signal has a much better frequency resolution about 3.9MHz. In addition, our method can also obtain the phase response information shown by the pink line in Fig. 4(b) and Fig. 5(b), which cannot be obtained by using OSA.

Conclusion

We have demonstrated a novel optical channel estimation technique using DD-OFDM signal to characterize the passive optical components. By using optical DD-OFDM signal, the intensity and phase response of the passive optical components can be measured with a 3.9MHz frequency resolution. The experimental results indicate that the experimental result using DD-OFDM signal has a good agreement with those using CO-OFDM signal.

Acknowledgments

The authors would like to acknowledge the support from National High Technology Research and Development Program of China (863 Program) (No. 2012AA040210), Key Program of Natural Science Foundation of Guangdong Province, China (No. 10251063101000001), National Natural Science Foundation of China (No. 61071097, No. 61107060).

References and links

1.

T. Niemi, M. Uusimaa, and H. Ludvigsen, “Limitations of phase-shift method in measuring dense group delay ripple of fiber Bragg gratings,” IEEE Photon. Technol. Lett. 13(12), 1334–1336 (2001). [CrossRef]

2.

M. Froggatt, E. Moore, and M. Wolfe, “Interferometric measurement of dispersion in optical components,” in Optical Fiber Communication Conference 2002, Paper WK1.

3.

T. Kawanishi, T. Sakamoto, and M. Izutsu, “Fast optical frequency sweep for ultra-fine real-time spectral domain measurement,” Electron. Lett. 42(17), 999–1000 (2006). [CrossRef]

4.

D. J. Krause, J. C. Cartledge, L. Jakober, and K. Roberts, “Measurement of passive optical components using a carrier and single sideband,” in Optical Fiber Communication Conference 2006, Paper OFN5.

5.

X. Yi, Z. Li, Y. Bao, and K. Qiu, “Characterization of passive optical components by DSP-based optical channel estimation,” IEEE Photon. Technol. Lett. 24(6), 443–445 (2012). [CrossRef]

6.

W. Peng, B. Zhang, K.-M. Feng, X. Wu, A. E. Willner, and S. Chi, “Spectrally efficient direct-detected OFDM transmission incorporating a tunable frequency gap and an iterative detection techniques,” J. Lightwave Technol. 27(24), 5723–5735 (2009). [CrossRef]

7.

Z. Zan, M. Premaratne, and A. J. Lowery, “Laser RIN and linewidth re-quirements for direct detection optical OFDM,” in CLEO 2008, paper CWN2.

8.

K. Kikuchi, “Coherent transmission systems,” in Proc. ECOC, paper no. Th.2.A.1, 21–25, Belgium, 2008.

9.

W. Shieh, H. Bao, and Y. Tang, “Coherent optical OFDM: theory and design,” Opt. Express 16(2), 841–859 (2008). [CrossRef] [PubMed]

10.

W. Shieh, X. Yi, Y. Ma, and Q. Yang, “Coherent optical OFDM: has its time come?” J. Opt. Netw. 7(3), 234–255 (2008). [CrossRef]

11.

M. Sieben, J. Conradi, and D. E. Dodds, “Optical single sideband transmission at 10 Gb/s using only electrical dispersion compensation,” J. Lightwave Technol. 17(10), 1742–1749 (1999). [CrossRef]

12.

R. Dischler and F. Buchali, “Experimental assessment of a direct detection optical OFDM system targeting 10Gb/s and beyond,” in Optical Fiber Communication Conference 2008, Paper OMI2.

13.

J. Leibrich, A. Ali, and W. Rosenkranz, “Optical OFDM as a promising technique for bandwidth efficient high-speed data transmission over optical fiber,” in Int. OFDM-Workshop 2007, Hamburg, Germany.

14.

A. Lowery, L. Du, and J. Armstrong, “Orthogonal frequency division multiplexing for adaptive dispersion compensation in long haul WDM systems,” in Optical Fiber Communication Conference 2006, paper PDP39.

15.

B. Schmidt, A. Lowery, and J. Armstrong, “Experimental demonstrations of 20 Gbit/s direct-detection optical OFDM and 12 Gbit/s with a colorless transmitter,” in Optical Fiber Communication Conference 2007, paper PDP18.

16.

W. Peng, X. Wu, A. Arbab, B. Shamee, L. Christen, J. Ynag, K. Feng, A. Willner, and S. Chi, “Experimental demonstration of a coherently modulated and directly detected optical OFDM system using an RF-tone insertion,” in Optical Fiber Communication Conference 2008, paper OMU2.

17.

A. Ali, H. Paul, J. Leibrich, W. Rosenkranz, and K. Kammeyer, “Optical biasing in direct detection optical-OFDM for improving receiver sensitivity,” in Optical Fiber Communication Conference 2010, paper JThA12.

18.

A. J. Lowery and J. Armstrong, “10 Gbit/s multimode fiber link using power-efficient orthogonal frequency-division-multiplexing,” Opt. Express 13(25), 10003–10009 (2005). [CrossRef] [PubMed]

19.

J. Li, K. Worms, R. Maestle, D. Hillerkuss, W. Freude, and J. Leuthold, “Free-space optical delay interferometer with tunable delay and phase,” Opt. Express 19(12), 11654–11666 (2011). [CrossRef] [PubMed]

20.

J. Li, K. Worms, D. Hillerkuss, B. Richter, R. Maestle, W. Freude, and J. Leuthold, “Tunable free space optical delay interferometer for demodulation of differential phase shift keying signals,” in Optical Fiber Communication Conference 2010, paper JWA24.

OCIS Codes
(120.5050) Instrumentation, measurement, and metrology : Phase measurement
(220.4840) Optical design and fabrication : Testing

ToC Category:
Instrumentation, Measurement, and Metrology

History
Original Manuscript: June 4, 2012
Revised Manuscript: August 18, 2012
Manuscript Accepted: August 24, 2012
Published: September 12, 2012

Citation
Banghong Guo, Tao Gui, Zhaohui Li, Yuan Bao, Xingwen Yi, Jianping Li, Xinhuan Feng, and Songhao Liu, "Characterization of passive optical components with ultra-fast speed and high-resolution based on DD-OFDM," Opt. Express 20, 22079-22086 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-20-22079


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References

  1. T. Niemi, M. Uusimaa, and H. Ludvigsen, “Limitations of phase-shift method in measuring dense group delay ripple of fiber Bragg gratings,” IEEE Photon. Technol. Lett.13(12), 1334–1336 (2001). [CrossRef]
  2. M. Froggatt, E. Moore, and M. Wolfe, “Interferometric measurement of dispersion in optical components,” in Optical Fiber Communication Conference 2002, Paper WK1.
  3. T. Kawanishi, T. Sakamoto, and M. Izutsu, “Fast optical frequency sweep for ultra-fine real-time spectral domain measurement,” Electron. Lett.42(17), 999–1000 (2006). [CrossRef]
  4. D. J. Krause, J. C. Cartledge, L. Jakober, and K. Roberts, “Measurement of passive optical components using a carrier and single sideband,” in Optical Fiber Communication Conference 2006, Paper OFN5.
  5. X. Yi, Z. Li, Y. Bao, and K. Qiu, “Characterization of passive optical components by DSP-based optical channel estimation,” IEEE Photon. Technol. Lett.24(6), 443–445 (2012). [CrossRef]
  6. W. Peng, B. Zhang, K.-M. Feng, X. Wu, A. E. Willner, and S. Chi, “Spectrally efficient direct-detected OFDM transmission incorporating a tunable frequency gap and an iterative detection techniques,” J. Lightwave Technol.27(24), 5723–5735 (2009). [CrossRef]
  7. Z. Zan, M. Premaratne, and A. J. Lowery, “Laser RIN and linewidth re-quirements for direct detection optical OFDM,” in CLEO 2008, paper CWN2.
  8. K. Kikuchi, “Coherent transmission systems,” in Proc. ECOC, paper no. Th.2.A.1, 21–25, Belgium, 2008.
  9. W. Shieh, H. Bao, and Y. Tang, “Coherent optical OFDM: theory and design,” Opt. Express16(2), 841–859 (2008). [CrossRef] [PubMed]
  10. W. Shieh, X. Yi, Y. Ma, and Q. Yang, “Coherent optical OFDM: has its time come?” J. Opt. Netw.7(3), 234–255 (2008). [CrossRef]
  11. M. Sieben, J. Conradi, and D. E. Dodds, “Optical single sideband transmission at 10 Gb/s using only electrical dispersion compensation,” J. Lightwave Technol.17(10), 1742–1749 (1999). [CrossRef]
  12. R. Dischler and F. Buchali, “Experimental assessment of a direct detection optical OFDM system targeting 10Gb/s and beyond,” in Optical Fiber Communication Conference 2008, Paper OMI2.
  13. J. Leibrich, A. Ali, and W. Rosenkranz, “Optical OFDM as a promising technique for bandwidth efficient high-speed data transmission over optical fiber,” in Int. OFDM-Workshop 2007, Hamburg, Germany.
  14. A. Lowery, L. Du, and J. Armstrong, “Orthogonal frequency division multiplexing for adaptive dispersion compensation in long haul WDM systems,” in Optical Fiber Communication Conference 2006, paper PDP39.
  15. B. Schmidt, A. Lowery, and J. Armstrong, “Experimental demonstrations of 20 Gbit/s direct-detection optical OFDM and 12 Gbit/s with a colorless transmitter,” in Optical Fiber Communication Conference 2007, paper PDP18.
  16. W. Peng, X. Wu, A. Arbab, B. Shamee, L. Christen, J. Ynag, K. Feng, A. Willner, and S. Chi, “Experimental demonstration of a coherently modulated and directly detected optical OFDM system using an RF-tone insertion,” in Optical Fiber Communication Conference 2008, paper OMU2.
  17. A. Ali, H. Paul, J. Leibrich, W. Rosenkranz, and K. Kammeyer, “Optical biasing in direct detection optical-OFDM for improving receiver sensitivity,” in Optical Fiber Communication Conference 2010, paper JThA12.
  18. A. J. Lowery and J. Armstrong, “10 Gbit/s multimode fiber link using power-efficient orthogonal frequency-division-multiplexing,” Opt. Express13(25), 10003–10009 (2005). [CrossRef] [PubMed]
  19. J. Li, K. Worms, R. Maestle, D. Hillerkuss, W. Freude, and J. Leuthold, “Free-space optical delay interferometer with tunable delay and phase,” Opt. Express19(12), 11654–11666 (2011). [CrossRef] [PubMed]
  20. J. Li, K. Worms, D. Hillerkuss, B. Richter, R. Maestle, W. Freude, and J. Leuthold, “Tunable free space optical delay interferometer for demodulation of differential phase shift keying signals,” in Optical Fiber Communication Conference 2010, paper JWA24.

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