Investigation of homodyne demodulation of RZ-BPSK signal based on an optical Costas loop

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Investigation of homodyne
demodulation of RZ-BPSK signal
based on an optical Costas loop
Haijun Zhou, Zunzhen Zhu, Weilin Xie, Yi Dong
Haijun Zhou, Zunzhen Zhu, Weilin Xie, Yi Dong, "Investigation of homodyne
demodulation of RZ-BPSK signal based on an optical Costas loop," Proc.
SPIE 10617, 2017 International Conference on Optical Instruments and
Technology: Optoelectronic Devices and Optical Signal Processing, 106170F
(10 January 2018); doi: 10.1117/12.2295534
Event: International Conference on Optical Instruments and Technology 2017,
2017, Beijing, China
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Investigation of homodyne demodulation of RZ-BPSK signal based on
an optical Costas loop
Haijun Zhoua, Zunzhen Zhua, Weilin Xieb, and Yi Dong*b
aState Key Laboratory of Advanced Optical Communication Systems and Networks, Shanghai
Jiaotong University, Shanghai 200240, China
bSchool of Optoelectronics, Beijing Institute of Technology, Beijing 100081, China
ABSTRACT
We demonstrate the coherent detection of 10 Gb/s return-to-zero (RZ) binary phase-shift keying (BPSK) signal based on
a homodyne Costas optical phase-locked loop (OPLL). It demonstrates time misalignment tolerance of +/- 10% of the
transmitted RZ-BPSK signal, i.e. -20 to +20 ps between the pulse carver and the phase modulator for 5 Gb/s RZ-BPSK
signal, -10 to +10 ps or 10 Gb/s RZ-BPSK signal. Besides, the Costas coherent receiver shows a 2.5 dB sensitivity
improvement over conventional 5 Gb/s NRZ-BPSK and a 1.4 dB over 10 Gb/s NRZ-BPSK only at the cost of slightly
higher residual phase error. Those merits of sufficient tolerance to misalignment, higher receiver sensitivity, and low
residual phase error of RZ-BPSK modulation are beneficial to be applied in free space optical (FSO) communication to
achieve higher link budget, longer transmission distance.
Keywords: Binary phase-shift keying (BPSK), NRZ and RZ modulation, Optical phase-locked loop (OPLL), Costas
loop, Homodyne coherent detection, Optical coherent communication, Free space optical communication
1. INTRODUCTION
Return-to-zero (RZ) modulation has attracted much attention in long-haul optical fiber communication or in free-space
optical (FSO) communication [1-4]. In comparison with conventional non-return-to-zero (NRZ) modulation, a pulsed
intensity on-off keyed (OOK) modulation or RZ-PSK is preferable for its higher receiver sensitivity, enhanced
transmission capacity, immunity to transmission-induced nonlinear effects, etc [2-4]. Especial in FSO communication
link, high sensitive receivers are strictly required. For the optical satellite link (OSL) between GEO (geosynchronous
earth orbit) satellites, the received signal optical power may be as weak as -40 dBm. Receiver with high sensitivity not
only reduces transmitted signal power, extends link distance, but provides additional power budget, etc [3-4]. In addition,
RZ coding is also promising to mitigate inter-symbol interference (ISI) due to atmospheric effects in terrestrial FSO links
[4], robust to nonlinear propagation distortions, etc [2].
Even the receiver bandwidth was kept the same, a sensitivity gain of 0.5−3dB is observed in intensity-modulated/direct
detection (IM/DD) using RZ coding over the NRZ coding [3-6]. N. Chand, et al realized a 1.5 dB sensitivity gain with
33% duty cycle RZ coding at 2.499 Gb/s OOK modulation [4]. At a low bit rate of 37 Mb/s, L. Boivin, et al reported a
highest sensitivity gain of 5.8 dB impulsive coded OOK[6]. M. Pauer, et al also predicted sensitivity improvement of
3.14 dB in IM/DD with the RZ coding OOK [5].
Recently, the RZ-PSK signal has also been frequently used to improve sensitivity in FSO communication [7-8], to
increase transmission capacity in optical time-division multiplexing (OTDM) [9], to reduce transmission distortion or ISI
in optical fiber links [10-11], etc. For the RZ-PSK signal, an optical pulse appears in each bit slot, upon which the binary
data is encoded as either a 0 or 1800 phase shift between adjacent bits. Due to higher peak power of the RZ coding, RZ-
DPSK also outperforms NRZ-DPSK by ~ 0.5−1 dB at a data rate of 10 Gb/s [11].
P. S. Cho, et al has experimented coherent detection of 12.5-Gb/s NRZ-BPSK or RZ-BPSK signal with a CW LO or
pulsed LO [12]. In their schemes of coherent detection, one fiber laser is used as the signal laser and the local laser (LO).
Even ultra-short fiber links or super-stabilized fibers are strictly used to reduce phase randomness or phase variation, the
2017 International Conference on Optical Instruments and Technology: Optoelectronic Devices and
Optical Signal Processing, edited by Y. Dong, J. Chen, F. Bretenaker, Proc. of SPIE Vol. 10617,
106170F · © 2018 SPIE · CCC code: 0277-786X/18/$18 · doi: 10.1117/12.2295534
Proc. of SPIE Vol. 10617 106170F-1
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CLK
Driver
Signal
Laser
PS PPG
Ri pulse Driver
AAAA
DATA
RZ -BPSK
AAA
0
EDF: OBPF
Local
Oscillaror
Al
90°
Optical
Hybrid
LPF
-I -1
-I -2
-Q -1
Phase
error
AMP
Q-Arm
phase or frequency of the LO is hardly guaranteed to be synchronized with the signal light. This not only leads to rather
high noise floor on BER curves but error-free sensitivity at the 10-9 is also hardly guaranteed.
In practical communication link, an independent LO along with an optical phase-locked loop (OPLL) at the receiver are
needed to realize the phase or frequency synchronization. Even D. J. Geisler, et al also experimented the coherent
detection of 9.94-Gb/s RZ-BPSK with a homodyne Costas loop [8], performances on sensitivity, phase-locking, and link
requirements were never reported. Differs from the NRZ-BPSK signal, the homodyne coherent receiver for RZ-BPSK
signal behaviors as a sampled optical phase-lock loop [13-17]. It could also be applied to the homodyne detection with a
pulsed LO, which takes the advantage of optical sampling [15]. However, the coherent detection or mixing of input
signal (NRZ-BPSK or RZ-BPSK) with a pulsed LO is rather complex than a continue-wave (CW) LO. It is caused by the
fact that a locally recovered clock (CLK) at the receiver side is required and the pulsed LO must coincides with the bit of
signal light within the pulse width of the pulsed LO [15].
In FSO communication, it’s more preferable to apply homodyne detection of the RZ-BPSK signal with a CW LO. The
time alignment between a pulsed light and a CW light is relieved at the transmitter side, where a monitoring and
adjusting model could be applied [18-19]. So as to explore the potential of sensitivity improvement of the RZ-BPSK
signal, it’s significant to investigate those performances of time alignment of RZ-BPSK at the transmitter side, phase-
locking of the Costas OPLL with pulsed RZ-BPSK signal, frequency-dependent RZ coding gain, etc [13-15].
In this paper, we conduct experiments on coherent detection of RZ- versus NRZ-BPSK signal by utilizing a homodyne
Costas optical phase-locked loop (OPLL). Once the LO is homodyne phase locked to the 5 Gb/s RZ-BPSK signal, the
Costas coherent receiver shows misalignment tolerance of -20 to +20 ps between the pulse carver and the phase
modulation, which amounts to -10% to +10% of the bit rate. In similar, the tolerance is from -10 ps to +10 ps for 10
Gbps RZ-BPSK. Different from conventional NRZ-BPSK signal, the Costas coherent receiver not only shows slightly
higher residual phase error, but the receiver sensitivity could be 2.5 dB improved for the 5 Gb/s RZ-BPSK signal, a 1.4
dB for 10 Gb/s RZ-BPSK signal. Those merits of higher receiver sensitivity and lower residual phase error of RZ-BPSK
are benefical to be applied in free space optical (FSO) communication to achieve higher link budget, longer transmission
distance.
2. EXPERIMENTAL
Fig.1. Schematic setup of the homodyne coherent detection based on a Costas loop. EDFA: Erbium Doped Fiber Amplifier, OBPF:
Optical Band Pass Filter, AOFS: Acoustic-Optic Frequency Shifter, VOA: variable Optical Attenuator, PPG: Pattern Pulse Generator,
BERT: Bit Error Rate Tester, OSC: Real-time Oscilloscope (40 GSam/s), SA: Spectrum Analyzer, VCO: Voltage Controlled
Oscillator, LPF: Low-Pass Filter.
The experimental setup of the homodyne detection of RZ-BPSK and NRZ-BPSK is shown in Fig.1. The CW signal laser
(1550.012 nm) is firstly carved into return-to-zero (RZ) pulses by a Mach-Zehnder modulator (MZM) driven by a
synchronized clock wave (CLK). The RZ pulse is then encoded by a LiNbO3 phase modulator (PM) to yield a RZ-BPSK
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EwCO
613
J
-5 -
-6 -
-a -
-9 -
-10 -
-11
10G-RZ-BPSK
5G-RZ-BPSK
1'1'1'1T1TI'1`1TTT1`T'T
-120 -100 -80 -60 -40 -20 020 40 60 80 100
Time-Misalignment (ps)
signal, within which the length of Pseudo Random Binary Sequences (PRBS) is 231-1. To control phase deviation (i.e.,
the relative delay time) between the optical RZ pulse and the electrical NRZ data stream, a phase shifter (PS) with 3.3 ps
adjusting precision is used. In addition, the MZM modulator is not used for NRZ-BPSK signal. The RZ-BPSK (or NRZ-
BPSK) signal is then amplified by a EDFA, filtered by a OBPF, and followed by a VOA to control the input power to a
900 optical hybrid.
The optical BPSK signal and the LO light are mixed on the 900 hybrid. The optical signal at the In-phase arm (I-arm) and
the Quadrature-arm (Q-arm) of the hybrid is detected by two pairs of 10 GHz balanced photodetectors (BPD1 and
BPD2). After being amplified (AMP) and reshaped by a limiting amplifier (LIA), the signal from the I-arm and the Q-
arm is then mixed to produce phase error signal [16-17]. The phase error is then filtered by the LPF1 and fed back to
control the phase and the frequency of the LO aided by a composited feedback loop, as previous reported in ref [16].
Thanks to the PZT and the AOFS, phase locking with a wide locking range and a broad loop bandwidth is realized [16].
3. RESULTS
Once the LO was phase locked to the signal light by the Costas loop, homodyne coherent detection of the BPSK signal is
realized. The data is demodulated at the I-arm and simultaneously analyzed by the BERT/OSC. For proper generation of
RZ-BPSK signal, it is essential to locate the peak of RZ pulse in the middle of the bit slot. If not, the NRZ streams will
not gate the shaped pulses properly. When the Costas OPLL is in lock, we firstly investigate the sensitivity penalty
induced by time misalignment for the RZ-BPSK signal.
The bit-error-rate (BER) curve is measured under different timing alignments, which is referred to receiver sensitivity
with perfect alignment at the BER of 10-9. As shown in Fig.2, when the timing misalignment exceeds 20 ps for 5 Gbps
RZ-BPSK or 10 ps for 10 Gbps RZ-BPSK, the BER is deteriorated rapidly. The misalignment tolerance range is around
10% of the bit-rate, which fits well with previous report [18-19]. As suggested, a monitoring module could be used in the
RZ-BPSK transmitter to avoid drifting of time misalignment.
Fig. 2 BER performance for different timing misalignments between the pulse caver and the data modulator.
As theoretically analysis [13-14], the phase-locking performance for pulsed RZ-BPSK signal is slightly different from
NRZ-BPSK signal. Even up to 10 Gb/s BPSK signals is successfully homodyne demodulated, the comparison is still
made at 5 Gb/s BPSK signals for simplification. In Fig.3, the loop bandwidth for the NRZ-BPSK is optimized around 2.0
MHz, which is mainly restricted by the acoustic-optical delay time (~76 ns) of the AOFS. Nevertheless, for the RZ-
BPSK signal, the loop bandwidth is reduced to 1.8 MHz with same parameters in the Costas OPLL.
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30
-40
-50
-60
-70
-80
-90
-100-5
50-NRZ-BPSK
50-RZ-BPSK
-4 -3 -2 -1 01
Frequency(Hz)
2 3 4 5
106
5G-NRZ-BPSK
5G-RZ-BPSK
LO=9.3dBm
-10 iI
-46 -45 -44 -43 -42
(a). BER curve Ps (dBm)
-41 f
-40 -39 5G -RZ -BPSK
(b). Eye diagram
This means that the residual loop phase error for RZ-BPSK is slightly higher than that for the NRZ-BPSK. Even not
shown, it also works for 10 Gb/s signals. For the RZ-BPSK signal, the Costas OPLL behaviors as a sampled OPLL [13-
14], within which the phase information is sampled at a rate determined by the bit-rate, duty cycle. The sample of phase
error signal from one bit is integrated, filtered, and used to track the phase of the following bit. To avoid higher driven
voltage, the duty cycle of the RZ-BPSK is 50% in our experiment.
Fig.3 : Typical loop bandwidth of the Costas coherent receiver. So as to make fair comparison, the LO power and parameters of the Costas loop
remains unchanged for both the RZ-BPSK and the NRZ-BPSK.
To explore sensitivity improvement of RZ coded signal, those measured BER curves for coherent demodulated RZ-
BPSK and NRZ-BPSK signals is demonstrated in Fig.4. The receiver sensitivity for 5 Gbps RZ-BPSK signal shows a 2.5
dB enhancement over conventional 5 Gbps NRZ-BPSK. Due to the limiting amplification, the eyediagram of the RZ-
BPSK behaviors like NRZ-BPSK. Even so, the eyediagram of the RZ-BPSK still open more clearly than NRZ-BPSK
signal.
Fig.4 : Performance comparison of the RZ-BPSK and the NRZ-BSPK under 5 Gb/s. (a) BER curves for different input signal powers (b) Eye diagrams
(50ns/div) is synchronously measured for BER = 10-9.
As RZ coding requires more bandwidth than NRZ signal, the sensitivity improvement versus different bit-rate is also
demonstrated in Fig.5. For low-speed RZ-BPSK, e.g., 1 Gb/s RZ-BPSK, the coherent receiver shows nearly a 3.8 dB
sensitivity improvement. However, the sensitivity improvement drops to a 2.5 dB for 5 Gb/s RZ-BPSK, bounces to a 2.8
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4.0 Sensitivity Improvement
3.5
.-.
mV.
= 3.0
0IN.
4;)
`-
i
22.5'-
p. .. ,
2.0
1n IIIIIIII I I
012345678 9 10
Bit -rate (G b/s)
dB for 7.5 Gb/s, and then reduces to merely a 1.4 dB for 10 Gb/s RZ-BPSK. The frequency-dependent improvement is
determined by two separate effects, which account the improvement limitation attainable for the RZ coding. The first
effect, yielding a maximum gain of 3 dB, is attributed the power-efficiency of the RZ coding over the NRZ coding.
Another effect is the higher response of a photodetector to RZ pulses [6[, [20], upon which the photodetector outputs a
higher output voltage swing compared with NRZ pulse. As the receiver noise (mainly due to shot noise) is independent
on pulse shape. Thus, the Costas coherent receiver shows higher signal-to-noise-ratio (SNR) for RZ-BPSK signals.
However, not only the RZ coding gain but the response of the photodetector strongly depends on the RC limitation or the
bandwidth of the photodetecor, the bandwidth of loop filter, etc [5], [12].
Fig.5 Sensitivity improvement of the RZ-BPSK vs. different bit-rate. For both RZ- and NRZ-BPSK signals, the bandwidth of
photodetector is kept at 10 GHz and parameters of the Costas OPLL are kept the same.
For those 10 GHz photodectors in our scheme, that’s the reason why the sensitivity gain is peaked (2.8 dB) for 7.5 Gb/s
RZ-BPSK signal. As RZ signal requires more bandwidth than NRZ signal, it could be deduced that the sensitivity gain of
RZ coding disappears or deteriorates once the bit-rate approaches or exceeds the bandwidth of the photodector, as
theoretical analysis [5-7]. Also as predicted, such a receiver is promising to reach a 2.5 dB sensitivity improvement
under 50% RZ coding, or a 3.8 dB improvement under 33% RZ coding. In addition, a maximum sensitivity gain is also
dependent upon the bandwidth of photodetector in different duty cycles. That’s the reason why the sensitivity gain could
be increased to a 3.8 dB at 1 Gb/s. For instance, the sensitivity of commercial photodetectors can be improved by 5 dB
by employing ultra-short duty cycle [6]. In our experiment, the duty cycle of the RZ-BPSK is chosen to be 50% because
higher driven-voltage on the MZM is required for 33% RZ coding, which is not power-efficiency in FSO communication.
4. CONCLUSION
In conclusion, coherent detection of 10 Gb/s RZ- and NRZ-BPSK is experimentally investigated utilizing a homodyne
optical Costas loop. For the RZ-BPSK signal, the Costas coherent receiver tolerates time misalignment of 10% of the bit-
rate, which could also be monitored and dynamically adjusted at the transmitter side. The phase-locking performance for
the RZ-BPSK signal is similar as the NRZ-BPSK signal only at the cost of a slightly higher loop bandwidth. In
comparison with NRZ-BPSK signal, the Costas coherent receiver demonstrates a sensitivity improvement of 1.4 dB for
10 Gbps RZ-BPSK , 2.8 dB for 7.5 Gbps RZ-BPSK , and 2.5 dB for 5 Gbps RZ-BPSK. Those merits of high tolerance to
time alignment, higher receiver sensitivity, similar residual phase error of RZ coding are beneficial for FSO
communication.
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