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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 16, Iss. 14 — Jul. 7, 2008
  • pp: 10421–10426
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Generation of optical carrier suppressed-differential phase shift keying (OCS-DPSK) format using one dual-parallel Mach-Zehnder modulator in radio over fiber systems

Qingjiang Chang, Tong Ye, and Yikai Su  »View Author Affiliations


Optics Express, Vol. 16, Issue 14, pp. 10421-10426 (2008)
http://dx.doi.org/10.1364/OE.16.010421


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Abstract

We present and experimentally demonstrate a new method to generate optical carrier suppressed-differential phase-shift keying (OCS-DPSK) modulation format in radio over fiber (RoF) systems. In our scheme, a single dual-parallel Mach-Zehnder modulator (DPMZM) is used to produce OCS-DPSK signal based on amplitude shift keying (ASK) to DPSK format conversion by suppressing the spectral tones of an OCS-ASK signal.

© 2008 Optical Society of America

1. Introduction

In this paper, we propose a simple method to generate OCS-DPSK signal using a single dual-parallel Mach-Zehnder modulator (DPMZM) [17

17. K. Higuma, S. Oikawa, Y. Hashimoto, H. Nagata, and M. Izutsu, “X-cut lithium niobate optical singl-esideband modulator,” Electron. Lett. 37, 515–516 (2001). [CrossRef]

]. Based on non-return-to-zero (NRZ)-ASK to NRZ-DPSK format conversion, the OCS-DPSK format is generated by suppressing the spectral tones of the OCS-ASK signal. Since the DPMZM is a commercial off-the-shelf device fabricated on a single chip, our scheme enables the transmitter to possess a compact structure, convenient alignment, and small insertion loss compared with conventional OCS-DPSK transmitters.

2. Theory and operation principle

Fig. 1. Schematic diagram of ASK to DPSK conversion.

The optical spectrum of the NRZ-ASK signal consists of continuous spectral components corresponding to the data pulse, and a discrete tone at the carrier wavelength. The spectral density PNRZ-ASK(f) can be described as [18

18. B. P. Lathi, “Modern digital and analog communication systems,” Oxford University Press, 3rd edition (1998).

]:

PNRZASK(f)=Ts16[sinπ(f+fc)Tsπ(f+fc)Ts2+sinπ(ffc)Tsπ(ffc)Ts2]+116[δ(f+fc)+δ(ffc)]
(1)

where fc is the optical carrier and Ts is the bit period. While for the DPSK signal, the optical power is constant and the optical field shifts between “1” and “-1”, thus the average optical field is zero and there is no carrier component in the optical spectrum of the DPSK signal [19

19. P. J. Winzer and R-J. Essiambre, “Advanced optical modulation formats,” Proceedings of the IEEE. 94, 952–985 (2006). [CrossRef]

]. The spectral density PNRZ-DPSK(f) is given by [18

18. B. P. Lathi, “Modern digital and analog communication systems,” Oxford University Press, 3rd edition (1998).

]:

PNRZDPSK(f)=Ts4[sinπ(f+fc)Tsπ(f+fc)Ts2+sinπ(ffc)Tsπ(ffc)Ts2]
(2)

Equation (1) and (2) show that the spectra of the two formats are identical except the discrete carrier tone 1/16[δ(f+fc)+δ(f-fc)]. Figure 1(a) illustrates such relation between the NRZ-ASK and the NRZ-DPSK formats in frequency domain, where the NRZ-DPSK is generated by suppressing the carrier tone of the NRZ-ASK. On the other hand, in time domain, if the optical NRZ-ASK signal subtracts a CW light with a half amplitude but the same frequency and phase, the “0” bit of the NRZ-ASK becomes “-1/2”, and the subtraction of the CW from the “1” bit results in “1/2”. Consequently, the amplitude of the resulting signal keeps identical and the phase alternates between 0 and π, as shown in Fig. 1(b). Thus, an NRZ-ASK signal is converted into an NRZ-DPSK format by eliminating the tone at the carrier of the NRZ-ASK signal [20

20. T. Ye, Q. Chang, J. Gao, and Y. Su, “An optical PSK-RF-signal transmitter based on ASK-to-PSK conversion and self-heterodyning,” in Proc. OFC 2008, paper JWA64.

].

Fig. 2. Schematic diagram of the OCS-ASK to OCS-DPSK conversion (a) and the architecture of the OCS-DPSK signal generation based on a single DPMZM (b).

This can be applied to achieve the conversion from OCS-ASK to OCS-DPSK format, as illustrated in Fig. 2(a), where the OCS-DPSK signal is generated by suppressing the two tones of the OCS-ASK signal, which can be implemented using one DPMZM. The DPMZM [17

17. K. Higuma, S. Oikawa, Y. Hashimoto, H. Nagata, and M. Izutsu, “X-cut lithium niobate optical singl-esideband modulator,” Electron. Lett. 37, 515–516 (2001). [CrossRef]

] consists of a pair of x-cut LiNbO3 MZMs (MZMA, MZMB) embedded in the two arms of a main MZM structure. The two sub-MZMs have the same architecture and performance, and the main MZM combines the outputs of the two sub-MZMs. The DPMZM have three bias ports, which belong to the two sub-MZMs and the main modulator, respectively. Fig. 2(b) plots the architecture of the OCS-DPSK signal generation based on the DPMZM. An electrical sub-carrier multiplexed (SCM) signal is obtained by mixing a baseband data with a local oscillator (LO) signal. The sub-MZMA is biased at its transmission null and driven by the SCM signal to generate an OCS-ASK format [5

5. C. Lin, W. Peng, and P. Peng, “Simultaneous generation of baseband and radio signals using only one single-electrode Mach-Zehnder modulator with enhanced linearity,” IEEE Photon. Technol. Lett. 18, 2481–2483 (2006). [CrossRef]

], which equals to two NRZ-ASK components with a spacing of twice the LO frequency. The sub-MZMB is also biased at the null and driven by a same LO signal to generate an optical clock signal, which compromises two CW lights with the same frequency spacing as the OCS-ASK signal. The two optical signals from the two sub-MZMs are destructively combined by adjusting the bias of the main MZM. It should be noted that the two optical signals need to have a proper intensity ratio and phase matching, which can be adjusted by changing the amplitudes of the two electrical driving signals and controlling an electrical phase shifter, respectively. Then, the two tones of the optical OCS-ASK signal are completely counteracted by the optical clock and thus result in an OCS-DPSK signal. Therefore, the generation of the OCS-DPSK format is dependent on the intensity ratio and the phase matching of the two modulating signals. The mismatch of these parameters results in residual tones in the generated OCS-DPSK, causing certain power fluctuations and thus degrading the signal performance. However, once the optimized parameters are set, they do not drift much and can maintain good stability in operation.

3. Experimental results

Fig. 3. Experimental setup for the generation of OCS-DPSK based on the DPMZM. Spectrum resolution: 0.07 nm; Start wavelength: 1548.86nm; Stop wavelength: 1550.86nm; The X axis scale is 0.2 nm/div, the Y axis scale is 5 dB/div. (i) The optical eye diagram of the OCS-ASK signal, (ii) The optical spectrum of the OCS-ASK signal, (iii) The optical eye diagram of the optical clock signal, (iv) The optical spectrum of the optical clock signal, (v) The optical eye diagram of the OCS-DPSK signal, (vi) The optical spectrum of the OCS-DPSK signal, (vii) The optical eye diagram of the filtered sideband, (viii) The optical eye diagram of the filtered sideband after MZDI demodulation, (ix) The optical eye diagram of the OCS-DPSK signal after MZDI demodulation.

Figure 3 shows the experimental setup for the generation and transmission of the OCS-DPSK signal. At the central station (CS), a 10-GHz DPMZM (COVEGA Mach-10TM060, 5.8-dB insertion loss) is used to modulate a CW light. The Vπ of the two sub-MZMs is 5.6 V at direct current (DC) frequency and that of the main MZM is 5.2 V. An electrical SCM signal is obtained by mixing a 1.25-Gbps pseudo-random bit sequence (PRBS) baseband data of a 27-1 word length with a 10-GHz LO signal. Then the electrical SCM signal is amplified (~6.8-V peak-to-peak voltage) to drive the sub-MZMA, which is biased at the transmission null to obtain an OCS-ASK signal with a 20-GHz frequency spacing, the optical eye diagram and the optical spectrum of the OCS-ASK signal are provided in Fig. 3(i) and Fig. 3(ii), respectively. The sub-MZMB is also biased at the null and driven by an amplified 10-GHz LO signal (~5.5-V peak-to-peak voltage) to generate an optical clock with the same frequency spacing, the optical eye diagram and the optical spectrum are shown in Fig. 3(iii) and Fig. 3(iv), respectively. There is no obvious difference between the optical spectra of Fig. 3(ii) and (iv) due to the limited resolution of the optical spectrum analyzer (OSA) of 0.07 nm. The two optical signals from the two sub-MZMs are destructively added to produce OCS-DPSK format, which is then amplified by an erbium-doped fiber amplifier (EDFA). The amplified spontaneous emission (ASE) noise is suppressed by a tunable optical filter (TOF) with 3-dB bandwidth of 0.4 nm. The optical eye diagram and the optical spectrum are shown in insets (v) and (vi) of Fig. 3, respectively. For the particular device we used, the tones of the OCS-ASK signal are not completely suppressed and the generated OCS-DPSK contains residual tones, which show small intensity modulation and cause certain power fluctuation in the optical eye diagram (v). This issue can be resolved if a DPMZM with better performance is used. Currently, there are such commercially available modulators of choices with the advance in device technology. We use a fiber Bragg grating (FBG) to filter one sideband of the OCS-DPSK signal for detection, the eye diagrams of the filtered tone (in Fig. 3(vii)) and after the Mach-Zehnder delay interferometer (MZDI) demodulation (in Fig. 3(viii)) further verify that the generated signal is an OCS-DPSK format. After transmission over a 25-km standard single-mode fiber (SMF), the 1-bit MZDI is used to convert the OCS-DPSK signal into an intensity signal with the optical eye diagram after MZDI demodulation shown in Fig. 3(vii). The MZDI is made from two couplers, where the length difference between the two arms to realize 1-bit delay is ~15.96 cm for the 1.25-Gb/s DPSK signal. There are some ripples in the eye diagram due to the imperfect OCS-DPSK signal. In practice, a high-speed receiver is needed to convert the demodulated optical signal to a 20-GHz electrical wireless signal for broadcasting through an antenna. At the receiver, the 20-GHz wireless signal is down-converted to baseband by mixing with a 20-GHz LO signal [4

4. Z. Jia, J. Yu, and G. K. Chang, “A full-duplex radio-over fiber system based on optical carrier suppression and reuse,” IEEE Photon. Technol. Lett. 18, 1726–1728 (2006). [CrossRef]

] for BER detection. In this particular experiment, we focus on the OCS-DPSK signal generation and transmission, and the signal is detected using a method similar to that in [16

16. Q. Chang, H. Fu, and Y. Su, “Simultaneous generation and transmission multi-band signals and upstream data in a bidirectional radio over fiber system,” IEEE Photon. Technol. Lett. 20, 181–183 (2008). [CrossRef]

], where the bit-error-ratio (BER) measurements is performed by detecting the upper-sideband component employing a 2.5-GHz low-speed photo-detector (PD) due to the lack of high-speed electronics. The detected electrical eye diagram is provided in Fig. 4(a). The measured BER performance after transmission of the 25-km SMF is indicated in Fig. 4(b), where the receiver sensitivity is ~-11.5 dB. Compared with the back-to-back case, the power penalty is less than 1.2 dB, which can be attributed to the chromatic dispersion of the SMF in RF frequency band, and the residual components during the generation process of the OCS-DPSK signal. We compare the difference of the BER performances in the receivers between filtering one side-band and using a mixer for the down-conversion [4

4. Z. Jia, J. Yu, and G. K. Chang, “A full-duplex radio-over fiber system based on optical carrier suppression and reuse,” IEEE Photon. Technol. Lett. 18, 1726–1728 (2006). [CrossRef]

] by VPI simulation software. The recovered signals based on the two detection methods show similar quality, and the penalties for the signal through 25-km transmission with the two detection schemes are almost the same.

Fig. 4. Electrical eye diagram (a) and BER curves (b).

4. Conclusion

We have proposed a simple scheme to obtain the OCS-DPSK format using only one integrated DPMZM based on ASK to DPSK conversion technique with a linear processing. We experimentally demonstrated the generation and transmission of 1.25-Gb/s DPSK data at 20-GHz microwave frequency, the power penalty after the 25-km fiber transmission is less than 1.2 dB. The use of the single DPMZM enables the transmitter to possess a compact structure.

Acknowledgment

This work was supported by the 863 High-Tech program (2006AA01Z255), and the Fok Ying Tung Fund (101067).

References and links

1.

D. Wake, M. Webster, G. Wimpenny, K. Beacham, and L. Crawford, “Radio over fiber for mobile communications,” in. IEEE Int. Topical Meeting Microwave Photonics (MWP 2004).

2.

J. Yu, Z. Jia, L. Yi, Y. Su, G. K. Chang, and T. Wang, “Optical millimeterwave generation or upconversion using external modulators,” IEEE Photon. Technol. Lett. 18, 265–267 (2006). [CrossRef]

3.

G-K. Chang, J. Yu, Z. Jia, and J. Yu, “Novel optical-wireless access network architecture for simultaneously providing broadband wireless and wired services,” in Proc. OFC 2006, OFM1.

4.

Z. Jia, J. Yu, and G. K. Chang, “A full-duplex radio-over fiber system based on optical carrier suppression and reuse,” IEEE Photon. Technol. Lett. 18, 1726–1728 (2006). [CrossRef]

5.

C. Lin, W. Peng, and P. Peng, “Simultaneous generation of baseband and radio signals using only one single-electrode Mach-Zehnder modulator with enhanced linearity,” IEEE Photon. Technol. Lett. 18, 2481–2483 (2006). [CrossRef]

6.

A. H. Gnauck and P. J. Winzer, “Optical phase-shift-keyed transmission,” J. Lightw. Technol. 23, 115–130 (2005). [CrossRef]

7.

X. Liu, X. Wei, Y-H. Kao, J. Leuthold, C. R. Doerr, and L. F. Mollenauer, “Quaternary differential-phase amplitude-shift-keying for DWDM transmission,” in Proc. ECOC 2003, Th2.6.5.

8.

M. Ohm and J. Speidel, “Quaternary optical ASK-DPSK and receivers with direct detection,” IEEE Photon. Technol. Lett. 15, 159–161 (2003). [CrossRef]

9.

S-S. Pun, C-K. Chan, and L-K. Chen, “Demonstration of a novel optical transmitter for high-speed differential phase-shift-keying/inverse-return-to-zero (DPSK/Inv-RZ) orthogonally modulated signals,” IEEE Photon. Technol. Lett. 17, 2763–2765 (2005). [CrossRef]

10.

T. Miyazaki and F. Kubota, “2-bit per symbol modulation/demodulation by DPSK over inverse-RZ optical pulses,” in Proc. CLEO 2004, CThBB2.

11.

M. Zitelli, “Optical phase and intensity modulation using dark pulses,” IEEE Photon. Technol. Lett. 16, 1972–1974 (2004). [CrossRef]

12.

J. Yu, O. Akanbi, Y. Luo, L. Zong, T. Wang, Z. Jia, and G-K. Chang, “Demonstration of a novel WDM passive optical network architecture with source-free optical network units,” IEEE Photon. Technol. Lett. 19, 571–573 (2007). [CrossRef]

13.

Q. Chang, Y. Tian, C. Yan, X. Xu, J. Gao, and Y. Su, “A PON system providing triple play service based on a single dual-parallel Mach-Zehnder modulator,” in Proc. ECOC 2007, 4.4.7.

14.

W. Hung, C-K. Chan, L-K. Chen, and C. Lin, “System characterization of a robust re-modulation scheme with DPSK downstream traffic in a WDM access network,” in Proc. ECOC 2003, We 3.4.5.

15.

Z. Jia, J. Yu, D. Boivin, M. Haris, and G-K. Chang, “Bidirectional ROF links using optically up-converted DPSK for downstream and remodulated OOK for upstream,” IEEE Photon. Technol. Lett. 19, 653–655 (2007). [CrossRef]

16.

Q. Chang, H. Fu, and Y. Su, “Simultaneous generation and transmission multi-band signals and upstream data in a bidirectional radio over fiber system,” IEEE Photon. Technol. Lett. 20, 181–183 (2008). [CrossRef]

17.

K. Higuma, S. Oikawa, Y. Hashimoto, H. Nagata, and M. Izutsu, “X-cut lithium niobate optical singl-esideband modulator,” Electron. Lett. 37, 515–516 (2001). [CrossRef]

18.

B. P. Lathi, “Modern digital and analog communication systems,” Oxford University Press, 3rd edition (1998).

19.

P. J. Winzer and R-J. Essiambre, “Advanced optical modulation formats,” Proceedings of the IEEE. 94, 952–985 (2006). [CrossRef]

20.

T. Ye, Q. Chang, J. Gao, and Y. Su, “An optical PSK-RF-signal transmitter based on ASK-to-PSK conversion and self-heterodyning,” in Proc. OFC 2008, paper JWA64.

OCIS Codes
(060.4510) Fiber optics and optical communications : Optical communications
(060.5060) Fiber optics and optical communications : Phase modulation
(230.2090) Optical devices : Electro-optical devices
(060.5625) Fiber optics and optical communications : Radio frequency photonics

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: March 5, 2008
Revised Manuscript: April 25, 2008
Manuscript Accepted: May 19, 2008
Published: June 27, 2008

Citation
Qingjiang Chang, Tong Ye, and Yikai Su, "Generation of optical carrier suppressed-differential phase shift keying (OCS-DPSK) format using one dual-parallel Mach-Zehnder modulator in radio over fiber systems," Opt. Express 16, 10421-10426 (2008)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-14-10421


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References

  1. D. Wake, M. Webster, G. Wimpenny, K. Beacham, and L. Crawford, "Radio over fiber for mobile communications," in. IEEE Int. Topical Meeting Microwave Photonics (MWP 2004).
  2. J. Yu, Z. Jia, L. Yi, Y. Su, G. K. Chang, and T. Wang, "Optical millimeterwave generation or up-conversion using external modulators," IEEE Photon. Technol. Lett. 18, 265-267 (2006). [CrossRef]
  3. G-K. Chang, J. Yu, Z. Jia, and J. Yu, "Novel optical-wireless access network architecture for simultaneously providing broadband wireless and wired services," in Proc. OFC2006, OFM1.
  4. Z. Jia, J. Yu, and G. K. Chang, "A full-duplex radio-over fiber system based on optical carrier suppression and reuse," IEEE Photon. Technol. Lett. 18, 1726-1728 (2006). [CrossRef]
  5. C. Lin, W. Peng, and P. Peng, "Simultaneous generation of baseband and radio signals using only one single-electrode Mach-Zehnder modulator with enhanced linearity," IEEE Photon. Technol. Lett. 18,2481-2483 (2006). [CrossRef]
  6. A. H. Gnauck and P. J. Winzer, "Optical phase-shift-keyed transmission," J. Lightwave Technol. 23, 115-130 (2005). [CrossRef]
  7. X. Liu, X. Wei, Y-H. Kao, J. Leuthold, C. R. Doerr, and L. F. Mollenauer, "Quaternary differential-phase amplitude-shift-keying for DWDM transmission," in Proc. ECOC 2003, Th2.6.5.
  8. M. Ohm and J. Speidel, "Quaternary optical ASK-DPSK and receivers with direct detection," IEEE Photon. Technol. Lett. 15, 159-161 (2003). [CrossRef]
  9. S-S. Pun, C-K. Chan, and L-K. Chen, "Demonstration of a novel optical transmitter for high-speed differential phase-shift-keying/inverse-return-to-zero (DPSK/Inv-RZ) orthogonally modulated signals," IEEE Photon. Technol. Lett. 17, 2763-2765 (2005). [CrossRef]
  10. T. Miyazaki and F. Kubota, "2-bit per symbol modulation/demodulation by DPSK over inverse-RZ optical pulses," in Proc. CLEO 2004, CThBB2.
  11. M. Zitelli, "Optical phase and intensity modulation using dark pulses," IEEE Photon. Technol. Lett. 16, 1972-1974 (2004). [CrossRef]
  12. J. Yu, O. Akanbi, Y. Luo, L. Zong, T. Wang, Z. Jia, and G-K. Chang, "Demonstration of a novel WDM passive optical network architecture with source-free optical network units," IEEE Photon. Technol. Lett. 19, 571-573 (2007). [CrossRef]
  13. Q. Chang, Y. Tian, C. Yan, X. Xu, J. Gao, and Y. Su, "A PON system providing triple play service based on a single dual-parallel Mach-Zehnder modulator," in Proc. ECOC 2007, 4.4.7.
  14. W. Hung, C-K. Chan, L-K. Chen, and C. Lin, "System characterization of a robust re-modulation scheme with DPSK downstream traffic in a WDM access network," in Proc. ECOC 2003, We 3.4.5.
  15. Z. Jia, J. Yu, D. Boivin, M. Haris, and G-K. Chang, "Bidirectional ROF links using optically up-converted DPSK for downstream and remodulated OOK for upstream," IEEE Photon. Technol. Lett. 19, 653-655 (2007). [CrossRef]
  16. Q. Chang, H. Fu, and Y. Su, "Simultaneous generation and transmission multi-band signals and upstream data in a bidirectional radio over fiber system," IEEE Photon. Technol. Lett. 20, 181-183 (2008). [CrossRef]
  17. K. Higuma, S. Oikawa, Y. Hashimoto, H. Nagata, and M. Izutsu, "X-cut lithium niobate optical single-sideband modulator," Electron. Lett. 37, 515-516 (2001). [CrossRef]
  18. B. P. Lathi, "Modern digital and analog communication systems," Oxford University Press, 3rd edition (1998).
  19. P. J. Winzer, and R-J. Essiambre, "Advanced optical modulation formats," Proceedings of the IEEE. 94, 952-985 (2006). [CrossRef]
  20. T. Ye, Q. Chang, J. Gao, and Y. Su, "An optical PSK-RF-signal transmitter based on ASK-to-PSK conversion and self-heterodyning," in Proc. OFC 2008, paper JWA64.

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