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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 6 — Mar. 12, 2012
  • pp: 6610–6615
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Novel coherent self-heterodyne receiver based on phase modulation detection

Tam N. Huynh, Lim Nguyen, and Liam P. Barry  »View Author Affiliations


Optics Express, Vol. 20, Issue 6, pp. 6610-6615 (2012)
http://dx.doi.org/10.1364/OE.20.006610


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Abstract

We propose a novel coherent self-heterodyne receiver structure based on phase modulation detection that potentially simplifies the front-end of a coherent optical receiver. The scheme has been demonstrated via simulations and experimentally for a 10 Gb/s DQPSK transmission system.

© 2012 OSA

1. Introduction

In recent years the exponential growth of Internet data traffics has resulted in a steady increase of the transmission capacity of Dense Wavelength Division Multiplexed (DWDM) optical networks. Advanced optical modulation formats that offer high spectral efficiency are being increasingly deployed to cope with this trend [1

1. M. Nakazawa, K. Kikuchi, and T. Miyazaki, High Spectral Density Optical Communication Technology (Springer, 2010).

3

3. P. J. Winzer, G. Raybon, H. Song, A. Adamiecki, S. Corteselli, A. H. Gnauck, D. A. Fishman, C. R. Doerr, S. Chandrasekhar, L. L. Buhl, T. J. Xia, G. Wellbrock, W. Lee, B. Basch, T. Kawanishi, K. Higuma, and Y. Painchaud, “100-Gb/s DQPSK Transmission: from laboratory experiments to field trials,” J. Lightwave Technol. 26(20), 3388–3402 (2008). [CrossRef]

]. The state-of-the-art optical coherent receiver employs a 90° hybrid front end and a local oscillator (LO) laser for homodyne demodulation [1

1. M. Nakazawa, K. Kikuchi, and T. Miyazaki, High Spectral Density Optical Communication Technology (Springer, 2010).

, 4

4. M. Seimetz, High-order Modulation for Optical Fiber Transmission (Springer, 2009).

]. The integration of the hybrid remains a technical challenge and the laser phase noise requirements on the LO can be difficult to achieve, especially with higher order modulation formats. Differential modulation techniques, such as DQPSK, that employ self-homodyne direct detection using delay line interferometers (DLIs) [3

3. P. J. Winzer, G. Raybon, H. Song, A. Adamiecki, S. Corteselli, A. H. Gnauck, D. A. Fishman, C. R. Doerr, S. Chandrasekhar, L. L. Buhl, T. J. Xia, G. Wellbrock, W. Lee, B. Basch, T. Kawanishi, K. Higuma, and Y. Painchaud, “100-Gb/s DQPSK Transmission: from laboratory experiments to field trials,” J. Lightwave Technol. 26(20), 3388–3402 (2008). [CrossRef]

, 5

5. R. A. Griffin and A. C. Carter, “Optical differential quadrature phase-shift key (oDQPSK) for high capacity optical transmission,” WX6, OFC 2002.

] could potentially reduce the requirements on the laser linewidth. However, the DLI approach would require significantly more complex hardware implementation to migrate to higher order modulation formats such as differential m-PSK and differential m-QAM. The authors in [2

2. N. Kikuchi and S. Sasaki, “Highly sensitive optical multilevel transmission of arbitrary quadrature-amplitude modulation (QAM) signals with direct detection,” J. Lightwave Technol. 28(1), 123–130 (2010). [CrossRef]

] have reported the realization of a direct-detection multilevel transmission for 16-QAM incorporating phase pre-integration and advanced digital signal processing (DSP).

2. Analytical model

In the following, we develop the analytical model for the proposed receiver scheme in Fig. 1(a)
Fig. 1 Experiment setup for self-heterodyne receiver with PM detection (a) and self-homodyne receiver with 90° hybrid (b).
. The E-field of the modulated optical output from an IQ modulator can be expressed in complex notation (for brevity we have omitted the laser phase and intensity noises) as
E(t)=Pa(t)ejφ(t)ejωot,
(1)
where ωo is the angular optical frequency, P is the optical output power of the laser, and ϕ(t)anda(t)respectively are the symbol phase modulation and the normalized symbol amplitude modulation such thata(t) = 1 for PSK modulation format. The upper arm of the interferometer in Fig. 1(a) has a one-symbol delay and the lower arm is phase modulated by a sinusoidal waveform. The incident E-field falling on the photo-detector can be written as
Ei(t)=12×[E(tTs)E(t)×ej[bsin(ωct+φc)]],
(2)
where Ts is the symbol duration. The input signal to the phase modulator is bsin(ωct+ϕc), where bis the PM index,ωcandϕcare the angular frequency and phase of the modulating carrier, respectively. The output current of the photo-detector with responsivityis proportional to the intensity of the incident field:

i(t)=×Ei(t)Ei(t)=14×P{a2(t)+a2(tTs)2a(t)a(tTs)cos[ωoTs+ϕ(t)ϕ(tTs)+bsin(ωct+ϕc)]}
(3)

Ignoring the first two terms in Eq. (3) (which are constant for PSK and can be cancelled using balanced photo-detectors, or a single photo-detector with a DC-block) and expanding the third term:

i(t)=12×Pa(t)a(tTs)cos[ωoTs+ϕ(t)ϕ(tTs)+bsin(ωct+ϕc)]
(4)
i(t)=12Pa(t)a(tTs)×{cos[ωoTs+ϕ(t)ϕ(tTs)]cos[bsin(ωct+ϕc)]sin[ωoTs+ϕ(t)ϕ(tTs)]sin[bsin(ωct+ϕc)]}
(5)

Using the Bessel coefficient expansions:
cos[bsin(ωct+ϕc)]=Jo(b)+2kevenJk(b)cos[k(ωct+ϕc)],
(6)
sin[bsin(ωct+ϕc)]=2koddJk(b)sin[k(ωct+ϕc)],
(7)
here Jk(b)is the Bessel function of the first kind with order k. It is clear from Eqs. (5), (6) and (7) that the I and Q components of the complex differential optical modulation envelope,a(t)a(tTs)ej[φ(t)φ(tTs)], can be found at the even and odd harmonics of i(t). For bandwidth efficient demodulation, we can also obtain the I and Q components from the baseband and the first harmonic, respectively. The output current of the photo-detector in terms of baseband and first harmonic could be expressed as:
i(t)=12P[J0(b)I(t)+2J1(b)Q(t)sin(ωct+ϕc)],
(8)
Where I(t)=a(t)a(tTs)cos[ϕ(t)ϕ(tTs)+ωoTs]andQ(t)=a(t)a(tTs)sin[ϕ(t)ϕ(tTs)+ωoTs]. Figure 2(a)
Fig. 2 Off-line DSP and DQPSK constellation of self-heterodyne receiver with PM detection method (a), (c), and self-homodyne receiver with 90° hybrid method (b), (d).
shows the block diagram of the off-line DSP for the captured photo-detector current. A simple carrier phase recovery algorithm for ϕc was applied prior to demodulation. The least mean-square (LMS) algorithm compensates for the differential drift in the interferometer arms due to the random fluctuation, for example, in temperature. For balanced I/Q outputs the PM index b is adjusted so that Jo(b) = 2J1(b) ~0.81, or b ~0.90, as verified on the electrical spectrum analyzer.

3. Experiment setup

The one-symbol delay in one arm of the interferometer was set to Ts = 200 ps at 5 Gbaud/s. An EOspace Z-cut lithium niobate phase modulator in the other arm was driven by a 5 GHz signal generator. The signal generator output was amplified to provide sufficient drive (modulation index b) to the phase modulator in order to achieve balanced power at baseband and the first harmonic as observed on an RF spectrum analyzer. The light from the delayed and phase modulated arms were recombined via a coupler whose output was detected by an 11 GHz photo-detector with an integrated transimpedance amplifier (TIA). Two polarization controllers (one on each arm of the interferometer) were employed to manually optimize the beat signal at the TIA output and minimize the polarization mode dispersion effect of the modulator. An Agilent real-time scope captured the output signal at 40 GSamples/s that was then fed into a computer for off-line DSP processing as illustrated in Fig. 2(a). Matched filtering [4

4. M. Seimetz, High-order Modulation for Optical Fiber Transmission (Springer, 2009).

] was applied to the baseband I-component and the demodulated Q-component.

In order to evaluate the PM detection technique, we also implemented a self-homodyne, optical coherent receiver [4

4. M. Seimetz, High-order Modulation for Optical Fiber Transmission (Springer, 2009).

] as shown in Fig. 1(b). The two arms of the delay interferometer were now fed into an optical 90° hybrid with the output signals detected by a pair of 43 Gb/s photo-detectors with integrated TIAs. The I and Q output signals from the photo-detectors were also captured by the Agilent real-time scope at 40 GSamples/s for post-processing as shown in Fig. 2(b).

4. Simulation and experimental results

The first VOA in the experimental setup adjusted the back-to-back optical power in order to compare the receiver sensitivity of the PM detection method with the self-homodyne optical receiver. We also performed simulations of both experimental systems in Fig. 1, and of a direct detection DQPSK receiver with two DLIs [3

3. P. J. Winzer, G. Raybon, H. Song, A. Adamiecki, S. Corteselli, A. H. Gnauck, D. A. Fishman, C. R. Doerr, S. Chandrasekhar, L. L. Buhl, T. J. Xia, G. Wellbrock, W. Lee, B. Basch, T. Kawanishi, K. Higuma, and Y. Painchaud, “100-Gb/s DQPSK Transmission: from laboratory experiments to field trials,” J. Lightwave Technol. 26(20), 3388–3402 (2008). [CrossRef]

, 5

5. R. A. Griffin and A. C. Carter, “Optical differential quadrature phase-shift key (oDQPSK) for high capacity optical transmission,” WX6, OFC 2002.

], using VPIphotonics software. The results were compared in terms of the error vector magnitudes (EVM).

Figures 2(c) and 2(d) show the example experimental symbol constellations at −30 dBm received power for the self-heterodyne receiver with PM detection and the self-homodyne receiver with 90° hybrid, respectively. The experimental results show that the recovered DQPSK symbols have an EVM of 18% with PM detection and 14.7% with the 90° hybrid. Figure 3(a)
Fig. 3 EVM versus received power from experimental results (a) and simulation results (b).
compares the receiver sensitivity of the two methods. The plots show that PM detection has a power penalty that is about 3 dB (at 21% EVM) relative to the self-homodyne receiver with 90° hybrid. The simulation results in Fig. 3(b) are in general agreement with the experimental results and confirmed the sensitivity reduction between PM detection method and self-homodyne receivers.

The 3dB penalty of PM detection method was primarily caused by the additional ASE beat noise from the EDFAs due to heterodyne demodulation. The accumulated number of noise photons after the EDFAs chain was m(G1)nsp(where nspis the spontaneous emission factor, m is the number of EDFAs, and G is the amplifier gain), which is much greater than the number of photons from shot-noise (1 photon) [1

1. M. Nakazawa, K. Kikuchi, and T. Miyazaki, High Spectral Density Optical Communication Technology (Springer, 2010).

].

The carrier-to-noise ratio after the EDFAs chain was largely determined by ASE noise, as the shot-noise-limit is negligible in such a system [1

1. M. Nakazawa, K. Kikuchi, and T. Miyazaki, High Spectral Density Optical Communication Technology (Springer, 2010).

]:
γs=GNsm(G1)nspNsmnsp,
(9)
where Ns is the number of photons per symbol. For the self-homodyne receiver, the signal bandwidth was equal to the baud rate at 5 GHz, while the bandwidth for the self-heterodyne receiver with PM detection was twice at 10 GHz (if we used the baseband and first harmonic for I and Q). This can be seen by comparing the example power spectral densities of the photo-detector outputs for the two receiver methods in Fig. (4)
Fig. 4 Example power spectral densities of the photo-detector outputs with PM detection (a) and self-homodyne methods (b).
. Consequently, the in-band ASE noise of the self-heterodyne receiver was two times that of the self-homodyne receiver. This leads to a 3 dB penalty in the receiver sensitivity for PM detection.

5. Conclusion

Acknowledgments

This work was supported in part by the Irish Research Council for Science, Engineering and Technology, by Science Foundation Ireland through the Investigator Programme, by the Fulbright Commission and by the Faculty Development Fellowship of the University of Nebraska-Lincoln.

References and links

1.

M. Nakazawa, K. Kikuchi, and T. Miyazaki, High Spectral Density Optical Communication Technology (Springer, 2010).

2.

N. Kikuchi and S. Sasaki, “Highly sensitive optical multilevel transmission of arbitrary quadrature-amplitude modulation (QAM) signals with direct detection,” J. Lightwave Technol. 28(1), 123–130 (2010). [CrossRef]

3.

P. J. Winzer, G. Raybon, H. Song, A. Adamiecki, S. Corteselli, A. H. Gnauck, D. A. Fishman, C. R. Doerr, S. Chandrasekhar, L. L. Buhl, T. J. Xia, G. Wellbrock, W. Lee, B. Basch, T. Kawanishi, K. Higuma, and Y. Painchaud, “100-Gb/s DQPSK Transmission: from laboratory experiments to field trials,” J. Lightwave Technol. 26(20), 3388–3402 (2008). [CrossRef]

4.

M. Seimetz, High-order Modulation for Optical Fiber Transmission (Springer, 2009).

5.

R. A. Griffin and A. C. Carter, “Optical differential quadrature phase-shift key (oDQPSK) for high capacity optical transmission,” WX6, OFC 2002.

6.

T. N. Huynh, L. Nguyen, and L. P. Barry, “Coherent optical receiver using phase modulation detection,” ThL5, IEEE IPC 2011.

7.

T. N. Huynh, L. Nguyen, and L. P. Barry, “Delayed self-heterodyne phase noise measurements with coherent phase modulation detection,” IEEE Photon. Technol. Lett. 24(4), 249–251 (2012). [CrossRef]

8.

M. Vaez-Iravani and R. Toledo-Crow, “Pure linear polarization imaging in near field scanning optical microscopy,” Appl. Phys. Lett. 63(2), 138–140 (1993). [CrossRef]

9.

K. Wang, Z. Ding, Y. Zeng, J. Meng, and M. Chen, “Sinusoidal B-M method based spectral domain optical coherence tomography for the elimination of complex-conjugate artifact,” Opt. Express 17(19), 16820–16833 (2009). [CrossRef] [PubMed]

10.

L. Coldren, “Monolithic tunable diode lasers,” IEEE J. Sel. Top. Quantum Electron. 6(6), 988–999 (2000). [CrossRef]

OCIS Codes
(060.1660) Fiber optics and optical communications : Coherent communications
(060.5060) Fiber optics and optical communications : Phase modulation
(060.2840) Fiber optics and optical communications : Heterodyne

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: January 27, 2012
Revised Manuscript: February 28, 2012
Manuscript Accepted: February 28, 2012
Published: March 6, 2012

Citation
Tam N. Huynh, Lim Nguyen, and Liam P. Barry, "Novel coherent self-heterodyne receiver based on phase modulation detection," Opt. Express 20, 6610-6615 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-6-6610


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References

  1. M. Nakazawa, K. Kikuchi, and T. Miyazaki, High Spectral Density Optical Communication Technology (Springer, 2010).
  2. N. Kikuchi and S. Sasaki, “Highly sensitive optical multilevel transmission of arbitrary quadrature-amplitude modulation (QAM) signals with direct detection,” J. Lightwave Technol.28(1), 123–130 (2010). [CrossRef]
  3. P. J. Winzer, G. Raybon, H. Song, A. Adamiecki, S. Corteselli, A. H. Gnauck, D. A. Fishman, C. R. Doerr, S. Chandrasekhar, L. L. Buhl, T. J. Xia, G. Wellbrock, W. Lee, B. Basch, T. Kawanishi, K. Higuma, and Y. Painchaud, “100-Gb/s DQPSK Transmission: from laboratory experiments to field trials,” J. Lightwave Technol.26(20), 3388–3402 (2008). [CrossRef]
  4. M. Seimetz, High-order Modulation for Optical Fiber Transmission (Springer, 2009).
  5. R. A. Griffin and A. C. Carter, “Optical differential quadrature phase-shift key (oDQPSK) for high capacity optical transmission,” WX6, OFC 2002.
  6. T. N. Huynh, L. Nguyen, and L. P. Barry, “Coherent optical receiver using phase modulation detection,” ThL5, IEEE IPC 2011.
  7. T. N. Huynh, L. Nguyen, and L. P. Barry, “Delayed self-heterodyne phase noise measurements with coherent phase modulation detection,” IEEE Photon. Technol. Lett.24(4), 249–251 (2012). [CrossRef]
  8. M. Vaez-Iravani and R. Toledo-Crow, “Pure linear polarization imaging in near field scanning optical microscopy,” Appl. Phys. Lett.63(2), 138–140 (1993). [CrossRef]
  9. K. Wang, Z. Ding, Y. Zeng, J. Meng, and M. Chen, “Sinusoidal B-M method based spectral domain optical coherence tomography for the elimination of complex-conjugate artifact,” Opt. Express17(19), 16820–16833 (2009). [CrossRef] [PubMed]
  10. L. Coldren, “Monolithic tunable diode lasers,” IEEE J. Sel. Top. Quantum Electron.6(6), 988–999 (2000). [CrossRef]

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