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

Barry, Liam P.

Huynh, Tam N.

Nguyen, Lim

English

DigitalCommons@University of Nebraska

University of Nebraska - LincolnDigitalCommons@University of Nebraska - LincolnFaculty Publications in Computer & ElectronicsEngineering (to 2015) Electrical & Computer Engineering, Department of2012Novel coherent self-heterodyne receiver based onphase modulation detectionTam N. HuynhDublin City University, ngoc.huynh2@mail.dcu.ieLim NguyenUniversity of Nebraska-Lincoln, lnguyen1@unl.eduLiam P. BarryDublin City University, Liam.Barry@dcu.ieFollow this and additional works at: https://digitalcommons.unl.edu/computerelectronicfacpubThis Article is brought to you for free and open access by the Electrical & Computer Engineering, Department of at DigitalCommons@University ofNebraska - Lincoln. It has been accepted for inclusion in Faculty Publications in Computer & Electronics Engineering (to 2015) by an authorizedadministrator of DigitalCommons@University of Nebraska - Lincoln.Huynh, Tam N.; Nguyen, Lim; and Barry, Liam P., "Novel coherent self-heterodyne receiver based on phase modulation detection"(2012). Faculty Publications in Computer & Electronics Engineering (to 2015). 109.https://digitalcommons.unl.edu/computerelectronicfacpub/109Novel coherent self-heterodyne receiver based on phase modulation detection Tam N. Huynh,1,* Lim Nguyen,2 and Liam P. Barry1 1Rince Institute, School of Electronics Engineering, Dublin City University, Dublin 9, Ireland 2Department of Computer and Electronics Engineering, University of Nebraska-Lincoln, Omaha, Nebraska, USA *ngoc.huynh2@mail.dcu.ie 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 Optical Society of America OCIS codes: (060.1660) Coherent communications; (060.2840) Heterodyne; (060.5060) Phase modulation. 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). 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). 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). 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). 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). 10. L. Coldren, “Monolithic tunable diode lasers,” IEEE J. Sel. Top. Quantum Electron. 6(6), 988–999 (2000). 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 [1–3]. The state-of-the-art optical coherent receiver employs a 90° hybrid front end and a local oscillator (LO) laser for homodyne demodulation [1, 4]. 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, 5] 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] have reported the realization of a direct-detection multilevel transmission for 16-QAM incorporating phase pre-integration and advanced digital signal processing (DSP). In this paper, we propose a novel coherent receiver scheme based on the phase modulation (PM) detection method [6] that can recover the differential E-field from the harmonics of the #162208 - $15.00 USD Received 27 Jan 2012; revised 28 Feb 2012; accepted 28 Feb 2012; published 6 Mar 2012(C) 2012 OSA 12 March 2012 / Vol. 20,  No. 6 / OPTICS EXPRESS  6610modulating carrier without an LO laser. In previous work, we employed PM detection method for characterizing the laser phase noise [7]. Similar approaches to harmonic demodulations have been employed in near-field scanning optical microscopy [8] and Fourier-domain optical coherence tomography [9], but to our knowledge this is the first demonstration in coherent optical communications. In this work we demonstrate a self-coherent DQPSK system operating at 10 Gb/s by implementing our PM detection method at the receiver. The performance of the proposed technique is compared with other widely employed DQPSK receiver structures through simulations and experimental results. The proposed receiver configuration provides a cost effective solution for the front-end of optical coherent receivers, and has the potential of migrating to higher order modulation formats without a significant increase in complexity. 2. Analytical model In the following, we develop the analytical model for the proposed receiver scheme in Fig. 1(a). 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  ( )( ) ( ) ,oj tj tE t Pa t e e ωϕ=  (1) where oω  is the angular optical frequency, P is the optical output power of the laser, and ( )tφ and ( )a t respectively are the symbol phase modulation and the normalized symbol amplitude modulation such that ( )a 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  [ ]sin( )1( ) ( ) ( ) ,2c cj b ti sE t E t T E t eω ϕ+ = × − − ×   (2) where Ts is the symbol duration. The input signal to the phase modulator is sin( )c cb tω φ+ , where b is the PM index,cω andcφ are the angular frequency and phase of the modulating carrier, respectively. The output current of the photo-detector with responsivityℜ is proportional to the intensity of the incident field:  [ ]{ }2 2( ) ( ) ( )1 ( ) ( ) 2 ( ) ( ) cos ( ) ( ) sin( )4i is s o s s c ci t E t E tP a t a t T a t a t T T t t T b tω φ φ ω φ∗= ℜ×= ×ℜ + − − − + − − + +  (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:  [ ]1( ) ( ) ( ) cos ( ) ( ) sin( )2 s o s s c ci t Pa t a t T T t t T b tω φ φ ω φ= − ×ℜ − + − − + +  (4)  [ ] [ ] [ ] [ ]{ }1( ) ( ) ( )2cos ( ) ( ) cos sin( ) sin ( ) ( ) sin sin( )so s s c c o s s c ci t Pa t a t TT t t T b t T t t T b tω φ φ ω φ ω φ φ ω φ= − ℜ − ×+ − − + − + − − +  (5) Using the Bessel coefficient expansions:  [ ] [ ]cos sin( ) ( ) 2 ( )cos ( ) ,c c o k c ck evenb t J b J b k tω φ ω φ−+ = + +∑  (6) #162208 - $15.00 USD Received 27 Jan 2012; revised 28 Feb 2012; accepted 28 Feb 2012; published 6 Mar 2012(C) 2012 OSA 12 March 2012 / Vol. 20,  No. 6 / OPTICS EXPRESS  6611 [ ] ( )sin sin( ) 2 ( )sin ,c c k c ck oddb t J b k tω φ ω φ−+ = +  ∑  (7) here ( )kJ 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, [ ( ) ( )]( ) ( ) sj t t Tsa t a t T e ϕ ϕ− −− , 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:  [ ]0 11( ) ( ) ( ) 2 ( ) ( )sin( ) ,2 c ci t P J b I t J b Q t tω φ= ℜ − + +  (8) where [ ]( ) ( ) ( )cos ( ) ( )s s o sI t a t a t T t t T Tφ φ ω= − − − + and[ ]( ) ( ) ( ) sin ( ) ( )s s o sQ t a t a t T t t T Tφ φ ω= − − − + . Figure 2(a) shows the block diagram of the off-line DSP for the captured photo-detector current. A simple carrier phase recovery algorithm for cφ  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 Figure 1(a) shows the experimental setup for a back-to-back 5 Gbaud/s DQPSK link using the coherent PM detection method. The transmitter employed a sample grating distributed Bragg reflector (SGDBR) laser [10] that has a linewidth of 5 MHz at 1547.6 nm when biasing only the gain section at 100 mA. The I and Q arms of a dual parallel Mach-Zehnder modulator (biased close to the null points) was driven at 5 Gb/s by the complementary PRBS outputs of an Anritsu pulsed pattern generator (PPG), which were decorrelated by 59 bits using an electrical delay. The received power of the DQPSK signal was monitored after the first variable optical attenuator (VOA). The first low noise EDFA served as the pre-amplifier for the receiver front-end. The output signal was further amplified by a second EDFA. Each EDFA was followed by 2 nm optical filters to reduce the ASE noise, and the second VOA maintained the optical power falling on the photo-detector. 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] 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] 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). #162208 - $15.00 USD Received 27 Jan 2012; revised 28 Feb 2012; accepted 28 Feb 2012; published 6 Mar 2012(C) 2012 OSA 12 March 2012 / Vol. 20,  No. 6 / OPTICS EXPRESS  6612 Fig. 1. Experiment setup for self-heterodyne receiver with PM detection (a) and self-homodyne receiver with 90° hybrid (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, 5], 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) 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. #162208 - $15.00 USD Received 27 Jan 2012; revised 28 Feb 2012; accepted 28 Feb 2012; published 6 Mar 2012(C) 2012 OSA 12 March 2012 / Vol. 20,  No. 6 / OPTICS EXPRESS  6613  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). 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 ( 1)spm G n− (where spn is 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]. 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)s sssp spGN Nm G n mnγ = ≈− (9) where sN  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). 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. sin(ωct+φc) #162208 - $15.00 USD Received 27 Jan 2012; revised 28 Feb 2012; accepted 28 Feb 2012; published 6 Mar 2012(C) 2012 OSA 12 March 2012 / Vol. 20,  No. 6 / OPTICS EXPRESS  6614 Fig. 3. EVM versus received power from experimental results (a) and simulation results (b).  Fig. 4. Example power spectral densities of the photo-detector outputs with PM detection (a) and self-homodyne methods (b). 5. Conclusion We have proposed and demonstrated for the first time a coherent self-heterodyne receiver based on PM detection for DQPSK modulation through analytical model, simulations and experiments. The DQPSK signal from a back-to-back 10 Gb/s link was successfully demodulated using the proposed method. Although there is a 3dB penalty associated with heterodyne demodulation under ASE-limited condition, the coherent PM detection receiver not only offers the potential for a low-cost integrated solution but can also significantly simplify the front end of optical coherent receivers. Moreover, this receiver configuration could be feasibly migrated toward higher order modulation formats such as differential m-PSK and differential m-QAM without a significant increase in hardware requirements. 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. #162208 - $15.00 USD Received 27 Jan 2012; revised 28 Feb 2012; accepted 28 Feb 2012; published 6 Mar 2012(C) 2012 OSA 12 March 2012 / Vol. 20,  No. 6 / OPTICS EXPRESS  6615

Novel coherent self-heterodyne receiver based on
phase modulation detection

UNL | Libraries

University of Nebraska - Lincoln
DigitalCommons@University of Nebraska - Lincoln
Faculty Publications in Computer & Electronics
Engineering (to 2015) Electrical & Computer Engineering, Department of
2012
Novel coherent self-heterodyne receiver based on
phase modulation detection
Tam N. Huynh
Dublin City University, ngoc.huynh2@mail.dcu.ie
Lim Nguyen
University of Nebraska-Lincoln, lnguyen1@unl.edu
Liam P. Barry
Dublin City University, Liam.Barry@dcu.ie
Follow this and additional works at: https://digitalcommons.unl.edu/computerelectronicfacpub
This Article is brought to you for free and open access by the Electrical & Computer Engineering, Department of at DigitalCommons@University of
Nebraska - Lincoln. It has been accepted for inclusion in Faculty Publications in Computer & Electronics Engineering (to 2015) by an authorized
administrator of DigitalCommons@University of Nebraska - Lincoln.
Huynh, Tam N.; Nguyen, Lim; and Barry, Liam P., "Novel coherent self-heterodyne receiver based on phase modulation detection"
(2012). Faculty Publications in Computer & Electronics Engineering (to 2015). 109.
https://digitalcommons.unl.edu/computerelectronicfacpub/109
Novel coherent self-heterodyne receiver based on 
phase modulation detection 
Tam N. Huynh,1,* Lim Nguyen,2 and Liam P. Barry1 
1Rince Institute, School of Electronics Engineering, Dublin City University, Dublin 9, Ireland 
2Department of Computer and Electronics Engineering, University of Nebraska-Lincoln, Omaha, Nebraska, USA 
*ngoc.huynh2@mail.dcu.ie 
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 Optical Society of America 
OCIS codes: (060.1660) Coherent communications; (060.2840) Heterodyne; (060.5060) Phase 
modulation. 
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). 
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). 
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). 
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). 
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). 
10. L. Coldren, “Monolithic tunable diode lasers,” IEEE J. Sel. Top. Quantum Electron. 6(6), 988–999 (2000). 
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 [1–3]. The state-of-the-art optical coherent 
receiver employs a 90° hybrid front end and a local oscillator (LO) laser for homodyne 
demodulation [1, 4]. 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, 5] 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] have reported 
the realization of a direct-detection multilevel transmission for 16-QAM incorporating phase 
pre-integration and advanced digital signal processing (DSP). 
In this paper, we propose a novel coherent receiver scheme based on the phase modulation 
(PM) detection method [6] that can recover the differential E-field from the harmonics of the 
#162208 - $15.00 USD Received 27 Jan 2012; revised 28 Feb 2012; accepted 28 Feb 2012; published 6 Mar 2012
(C) 2012 OSA 12 March 2012 / Vol. 20,  No. 6 / OPTICS EXPRESS  6610
modulating carrier without an LO laser. In previous work, we employed PM detection method 
for characterizing the laser phase noise [7]. Similar approaches to harmonic demodulations 
have been employed in near-field scanning optical microscopy [8] and Fourier-domain optical 
coherence tomography [9], but to our knowledge this is the first demonstration in coherent 
optical communications. In this work we demonstrate a self-coherent DQPSK system 
operating at 10 Gb/s by implementing our PM detection method at the receiver. The 
performance of the proposed technique is compared with other widely employed DQPSK 
receiver structures through simulations and experimental results. The proposed receiver 
configuration provides a cost effective solution for the front-end of optical coherent receivers, 
and has the potential of migrating to higher order modulation formats without a significant 
increase in complexity. 
2. Analytical model 
In the following, we develop the analytical model for the proposed receiver scheme in 
Fig. 1(a). 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 
 
( )( ) ( ) ,oj tj tE t Pa t e e ωϕ=  (1) 
where 
o
ω  is the angular optical frequency, P is the optical output power of the laser, and 
( )tφ and ( )a t respectively are the symbol phase modulation and the normalized symbol 
amplitude modulation such that ( )a 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 
 
[ ]sin( )1( ) ( ) ( ) ,
2
c cj b t
i sE t E t T E t e
ω ϕ+ = × − − ×   (2) 
where Ts is the symbol duration. The input signal to the phase modulator is sin( )c cb tω φ+ , 
where b is the PM index,
c
ω and
c
φ are the angular frequency and phase of the modulating 
carrier, respectively. The output current of the photo-detector with responsivityℜ is 
proportional to the intensity of the incident field: 
 
[ ]{ }2 2
( ) ( ) ( )
1 ( ) ( ) 2 ( ) ( ) cos ( ) ( ) sin( )
4
i i
s s o s s c c
i t E t E t
P a t a t T a t a t T T t t T b tω φ φ ω φ
∗= ℜ×
= ×ℜ + − − − + − − + +
  (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: 
 [ ]1( ) ( ) ( ) cos ( ) ( ) sin( )
2 s o s s c c
i t Pa t a t T T t t T b tω φ φ ω φ= − ×ℜ − + − − + +  (4) 
 
[ ] [ ] [ ] [ ]{ }
1( ) ( ) ( )
2
cos ( ) ( ) cos sin( ) sin ( ) ( ) sin sin( )
s
o s s c c o s s c c
i t Pa t a t T
T t t T b t T t t T b tω φ φ ω φ ω φ φ ω φ
= − ℜ − ×
+ − − + − + − − +
  (5) 
Using the Bessel coefficient expansions: 
 [ ] [ ]cos sin( ) ( ) 2 ( )cos ( ) ,c c o k c c
k even
b t J b J b k tω φ ω φ
−
+ = + +∑  (6) 
#162208 - $15.00 USD Received 27 Jan 2012; revised 28 Feb 2012; accepted 28 Feb 2012; published 6 Mar 2012
(C) 2012 OSA 12 March 2012 / Vol. 20,  No. 6 / OPTICS EXPRESS  6611
 [ ] ( )sin sin( ) 2 ( )sin ,c c k c c
k odd
b t J b k tω φ ω φ
−
+ = +  ∑  (7) 
here ( )kJ 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, [ ( ) ( )]( ) ( ) sj t t T
s
a t a t T e ϕ ϕ− −− , 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: 
 [ ]0 1
1( ) ( ) ( ) 2 ( ) ( )sin( ) ,
2 c c
i t P J b I t J b Q t tω φ= ℜ − + +  (8) 
where 
[ ]( ) ( ) ( )cos ( ) ( )s s o sI t a t a t T t t T Tφ φ ω= − − − + and
[ ]( ) ( ) ( ) sin ( ) ( )s s o sQ t a t a t T t t T Tφ φ ω= − − − + . Figure 2(a) shows the block diagram of the 
off-line DSP for the captured photo-detector current. A simple carrier phase recovery 
algorithm for 
c
φ  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 
Figure 1(a) shows the experimental setup for a back-to-back 5 Gbaud/s DQPSK link using the 
coherent PM detection method. The transmitter employed a sample grating distributed Bragg 
reflector (SGDBR) laser [10] that has a linewidth of 5 MHz at 1547.6 nm when biasing only 
the gain section at 100 mA. The I and Q arms of a dual parallel Mach-Zehnder modulator 
(biased close to the null points) was driven at 5 Gb/s by the complementary PRBS outputs of 
an Anritsu pulsed pattern generator (PPG), which were decorrelated by 59 bits using an 
electrical delay. The received power of the DQPSK signal was monitored after the first 
variable optical attenuator (VOA). The first low noise EDFA served as the pre-amplifier for 
the receiver front-end. The output signal was further amplified by a second EDFA. Each 
EDFA was followed by 2 nm optical filters to reduce the ASE noise, and the second VOA 
maintained the optical power falling on the photo-detector. 
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] 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] 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). 
#162208 - $15.00 USD Received 27 Jan 2012; revised 28 Feb 2012; accepted 28 Feb 2012; published 6 Mar 2012
(C) 2012 OSA 12 March 2012 / Vol. 20,  No. 6 / OPTICS EXPRESS  6612
 Fig. 1. Experiment setup for self-heterodyne receiver with PM detection (a) and self-homodyne 
receiver with 90° hybrid (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, 5], 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) 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. 
#162208 - $15.00 USD Received 27 Jan 2012; revised 28 Feb 2012; accepted 28 Feb 2012; published 6 Mar 2012
(C) 2012 OSA 12 March 2012 / Vol. 20,  No. 6 / OPTICS EXPRESS  6613
  
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). 
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 ( 1)
spm G n− (where spn is 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]. 
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)
s s
s
sp sp
GN N
m G n mn
γ = ≈
−
 (9) 
where 
s
N  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). 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. 
sin(ωct+φ
c) 
#162208 - $15.00 USD Received 27 Jan 2012; revised 28 Feb 2012; accepted 28 Feb 2012; published 6 Mar 2012
(C) 2012 OSA 12 March 2012 / Vol. 20,  No. 6 / OPTICS EXPRESS  6614
 Fig. 3. EVM versus received power from experimental results (a) and simulation results (b). 
 
Fig. 4. Example power spectral densities of the photo-detector outputs with PM detection (a) 
and self-homodyne methods (b). 
5. Conclusion 
We have proposed and demonstrated for the first time a coherent self-heterodyne receiver 
based on PM detection for DQPSK modulation through analytical model, simulations and 
experiments. The DQPSK signal from a back-to-back 10 Gb/s link was successfully 
demodulated using the proposed method. Although there is a 3dB penalty associated with 
heterodyne demodulation under ASE-limited condition, the coherent PM detection receiver 
not only offers the potential for a low-cost integrated solution but can also significantly 
simplify the front end of optical coherent receivers. Moreover, this receiver configuration 
could be feasibly migrated toward higher order modulation formats such as differential m-
PSK and differential m-QAM without a significant increase in hardware requirements. 
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. 
#162208 - $15.00 USD Received 27 Jan 2012; revised 28 Feb 2012; accepted 28 Feb 2012; published 6 Mar 2012
(C) 2012 OSA 12 March 2012 / Vol. 20,  No. 6 / OPTICS EXPRESS  6615


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Novel coherent self-heterodyne receiver based on phase modulation detection

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