Coherent reception of 80 GBd QPSK using integrated spectral slice optical arbitrary waveform measurement

Abstract

We coherently receive a 160-Gb/s QPSK waveform across 80-GHz bandwidth in two 40-GHz-wide spectral slices using a silica planar lightwave circuit capable of 640-GHz measurements which includes two arrayed-waveguide gratings and 16 optical hybrids.

Full text

Coherent reception of 80 GBd QPSK using integrated
spectral slice optical arbitrary waveform measurement
Nicolas K. Fontaine1,2, Takahide Sakamoto2,3, David J. Geisler2, Ryan P. Scott2, Binbin Guan2,
and S. J. B. Yoo2
1Bell Laboratories, Alcatel-Lucent, 791 Holmdel Rd. Holmdel, NJ 07733, USA
2Dept. of Electrical and Computer Engineering, University of California, Davis, One Shields Ave, Davis, CA 95616 USA
3 National Institute of Information and Communications Technology, 4-2-1 Nukui-kitamachi, Koganei-shi, Tokyo 184-8795, Japan
nicolas.fontaine@alcatel-lucent.com
Abstract: We coherently receive a 160-Gb/s QPSK waveform across 80-GHz bandwidth in two
40-GHz-wide spectral slices using a silica planar lightwave circuit capable of 640-GHz
measurements which includes two arrayed-waveguide gratings and 16 optical hybrids.
OCIS codes: (060.1660) Coherent communications; (230.3120) Integrated optics devices.
In optical communication systems, the line rate of the supporting electronics limits the baud rate of a single optical
channel. Optical techniques such as WDM and optical time division multiplexing (OTDM) expand link capacity by
combining many channels together and separating them with either spectral and time guard bands. Using a coherent
receiver and digital signal processing (DSP) it is possible to reduce or remove the guard bands in a WDM system.
For example, optical orthogonal frequency division multiplexing (O-OFDM), coherent WDM (CoWDM), and
orthogonal time-frequency domain multiplexing (OTFDM) formats ensure orthogonality between channels by
setting the frequency spacing equal to the single channel baud-rate [1–3]. Alternatively, one could increase the
single optical channel measurement bandwidth using parallelization techniques such as parallel optical sampling [4].
We propose to increase the single optical channel measurement and generation capability without increasing the
electronic bandwidth by using spectral-slice optical arbitrary waveform generation (OAWG) and waveform
measurement (OAWM) techniques [5,6]. Previously, we demonstrated 60 GHz waveform generation [7] and
160-GHz waveform measurement capability and expect to eventually scale to terahertz bandwidths [5].
Additionally, the inherent bandwidth scalability of these techniques and their ability to continuously transmit/receive
full-field waveforms allows for communications systems that support flexible bandwidth allocation and modulation
format independence [7]. Here, we demonstrate an integrated OAWM device as a coherent communications receiver
with 80 GHz of instantaneous single channel bandwidth to measure an 80 GBd quadrature phase-shift keying
(QPSK) waveform.
Fig. 1(b,c) shows how the OAWM technique enables receiving a waveform with bandwidth B by dividing it into
N slices and then coherently measuring each slice with an optical hybrid using a paired line from an optical
frequency comb (OFC). Fig. 1(e) shows the passbands for a gapless spectral demultiplexer for slicing the signal
waveform, and the high-isolation demultiplexer for separating the comb lines. DSP removes distortions (filtering)
from the electrical and optical components and combines the slices into a contiguous spectrum yielding the complete
measured waveform. The use of a single integrated photonic chip enables maintaining coherence between the slices.
Fig. 1. (a) QPSK transmitter. (b) 80 GHz silica PLC spectral-slice OAWM receiver. (c) Signal recovery DSP. (d) Image of PLC. (e) Ideal signal
and reference spectral demultiplexers.
Fig. 1 shows the experimental arrangement, which consisted of 8!10 GBd time-multiplexed QPSK transmitter
(OTFDM) and a two-slice OAWM receiver. An 8-line ! 10-GHz spacing OFC with chirped pulses are modulated
using an I/Q modulator driven by 210"1 pseudo-random bit sequences (PRBS) from the complimentary output ports
S1
R1
j
90° Optical
Hybrids
20 GHz
BPDs
50 GS/s
ADCs
Silica OAWM PLC
Signal
Slices
Isolated
Reference
Comblines
S2
R2
AWG
AWG
1×8
S(t)
40 GHz
Reference OFC
Frequency (40 GHz/div) (40 GHz/div)
40 GHz Gapless
Signal AWG
Signal
Ref.
40 GHz High-Isolation
Reference AWG
AWG
AWG
Comb
Source
1×8
τ1
τ8
50 km SMF
I/Q Modulator
(a) 160 Gb/s QPSK Transmitter (b) Spectral-Slice OAWM Receiver
Data 10 Gb/s PRBS
Data
j
Apply
Post-emphasis
Combine
Slices
Frequency
Frequency
Time
Signal
Waveform (c) Digital Signal Processing
(e)
(d)
Real
Imag
+
+
j
j
16 Optical
Hybrids
Signal
AWG
Ref.
AWG
40 GHz
80 GHz
Outputs

of a pattern generator [2]. A 1!8 time multiplexer with 12.5 ps incremental delays, decorrelation delays, and the
ability to block channels produces either 20 Gb/s, 40 Gb/s, 80 Gb/s, or 160 Gb/s QPSK waveforms with an 80-GHz
bandwidth. The waveform was amplified and transmitted over a 50 km fiber link. The OAWM receiver consists of a
two-line ! 40-GHz spacing OFC, a silica planar lightwave circuit (PLC) with two arrayed-waveguide grating
(AWG) demultiplexers and an array of 16 optical hybrids. Off-chip, four sets of balanced photodiodes (BPDs) and a
four-channel real-time oscilloscope (50 GS/s) detected and digitized the hybrid outputs. The device supports a
maximum of 640 GHz of measurement bandwidth using a 16-line ! 40-GHz spacing OFC and 32 BPDs and
digitizers. The waveform recovery DSP, shown in Fig. 1(c), corrects for optical hybrid and front-end errors,
equalizes each slice for electrical and optical filtering, and combines the post-emphasized slices. Further details of
the OAWM device and the DSP algorithm is in [5].
Fig. 2. (a) Typical signal spectrum, and noise floor of OAWM receiver. Eye diagrams for (b) 20 Gb/s back-to-back and (c) 160 Gb/s after 50 km.
50 km transmission (d) BER performance after 50-km link and (e) constellations at OSNR of 20 dB.
Fig. 2 shows the transmission results. Fig. 2(a) shows the received spectrum (80 GHz wide without spectral
gaps), the OAWM noise floor (average SNR is 12 dB), and the locations of the two spectral slices. Bit-error rate
(BER) results were obtained through offline processing of the full-field with the following steps: dispersion
compensation, equalization to remove the chirp from each pulse, temporal sampling, phase correction, thresholding
and BER measurement. Fig. 2(b) shows the temporal amplitude eye for the 20 Gb/s (1 channel) case to show the
OFC pulse shape which spans the full 80 GHz spectrum and the 100 ps time window. After equalizing for linear
distortions (i.e., flattening the spectral phase and amplitude) the temporal waveform has a sinc shape. Fig. 2(c)
shows the measured eye after the 50-km link for the 160 Gb/s waveform (8 channels) prior to and after the
electronic dispersion compensation and equalization. After equalization, each channel compresses to a sinc shape
and there are eight clear eye openings. Fig. 2(d) shows 50-km BER results plotted vs. OSNR per channel to
normalize the peak-power to the temporal noise level to emphasize the penalties introduced from inter-symbol
interference and optical noise. At high-BER there is a 2-3 dB OSNR penalty between the 20 Gb/s and 160 Gb/s case
which is most likely due to inter-symbol-interference or inter-channel-interference. An error floor occurs at an
OSNR per channel of ~19 dB which is consistent with the noise floor of the OAWM system and all curves have
BER below the forward-error correction limit of 3.8!10-3. The noise floor is relatively high due to a damaged input
to the chip, excess losses due to coupling and design errors (14 dB), and required 13 dB electrical pre-emphasis at
the edge of each spectral slice due to the oscilloscope’s 16-GHz bandwidth ("3 dB). For example, future designs
using a real-time scope with 20 GHz bandwidth, will improve the measurement SNR by 5 dB and reduce the 160
Gb/s error floor. OAWM is a bandwidth scalable and power efficient technique because no excess losses are
introduced by wavelength splitting. With improvements in OAWM PLC design and the availability of digitizers, we
expect to receive telecommunication waveforms with single-channel bandwidths of a terahertz.
1. A.D. Ellis et al., "Spectral density enhancement using coherent WDM," Photonics Technology Letters, IEEE 17, 504-506 (2005).
2. T. Sakamoto et al., "160-Gb/s orthogonal time-frequency domain multiplexed QPSK for ultra-high-spectral-efficient transmission," in
European Conference and Exhibition on Optical Communication (2011)
3. X. Liu et al., "448-Gb/s Reduced-Guard-Interval CO-OFDM Transmission Over 2000 km of Ultra-Large-Area Fiber and Five 80-GHz-Grid
ROADMs," J. Lightwave Technol. 29, 483-490.
4. J. K. Fischer et al., "High-Speed Digital Coherent Receiver Based on Parallel Optical Sampling," J. Lightwave Technol. 29, 378-385.
5. N. K. Fontaine et al., "Real-time full-field arbitrary optical waveform measurement," Nature Photonics 4, 248-254.
6. Chao Zhang et al., "Ultrafast digital coherent receiver based on parallel processing of decomposed frequency subbands," in European
Conference and Exhibition on Optical Communication (2010).
7. D. J. Geisler et al., "Demonstration of a Flexible Bandwidth Optical Transmitter/Receiver System Scalable to Terahertz Bandwidths," IEEE
Photonics Journal 3, 1013-1022.
This work was supported by in part DARPA and SPAWAR under OAWG contract HR0011-05-C-0155 and under NSF ECCS grant 1028729 and
by Grant-in-Aid for Young Scientists (A), 22686039, Japan Society for the Promotion of Science (JSPS). We thank Jonathan Heritage and
Robert Tkach for their discussions and support and Pat Tier for assistance.
Time (ps)
Amplitude (au)
025 50 75 Time (ps)
025 50 75
1 Channel (B2B) 8 Channel (50 km)
No Equal.
−40 −20 0 20 40
Power (5 dB/div)
Frequency (GHz)
No Equal.
Equalized
Equalized
Slice 2
Slice 1
Signal
Noise Floor
(a) (b) (c) (d)
2 Ch.
8 Ch.
4 Ch.
2 Ch.
1 Ch.
(e)
OSNR@0.1 nm/Channel (dB)
8 Ch.
4 Ch.
1 Ch.
15 dB