Introduction. Increasing the single channel capacity requires either increasing its baud rate (bandwidth), or increasing its spectral efficiency (bits per symbol).
Dynamic optical arbitrary waveform generation and measurement for telecommunications Nicolas K. Fontaine1, David J. Geisler2, Ryan P. Scott2, and S.J. Ben Yoo2
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1 Bell Laboratories, Alcatel-Lucent, Holmdel, NJ 07733 USA Department of Electrical and Computer Engineering, University of California, Davis, CA 95616 USA
Abstract: Spectral slice optical arbitrary waveform generation and measurement techniques synthesize and characterize wideband waveforms in many spectral slices. We show 160-GHz bandwidth measurements using 4 slices and 60-GHz bandwidth waveform generation in 6 slices. OCIS codes: (060.1660) Coherent communications; (230.3120) Integrated optics devices.
1. Introduction Increasing the single channel capacity requires either increasing its baud rate (bandwidth), or increasing its spectral efficiency (bits per symbol). With the fiber bandwidth nearly exhausted [1] both the spectral efficiency, through advanced modulation formats, and bandwidth must increase simultaneously. In many links employing advanced modulation formats, electrical digital-to-analog converters (DACs) create the modulator driving signals and coherent receivers with high-fidelity analog-to-digital converters (ADCs) record the full field and recover the transmitted symbols. Simultaneously obtaining both high-fidelity, commonly quantified as the effective number of bits (ENOB), and high bandwidth while relying on electrical ADCs and DACs is difficult because their fidelity decreases rapidly with bandwidth. Continued scaling of capacity will benefit from high-fidelity THz-bandwidth optical waveform generation and measurement capability. Spectral slice dynamic optical arbitrary waveform generation (OAWG) techniques can synthesize optical waveforms that have bandwidths and fidelity larger than those obtained by relying on electronics ADCs and DACs alone [2-3]. OAWG uses integrated optical elements to generate a large bandwidth signal in narrower spectral slices with bandwidths accessible by current electronics. Similar to OAWG, spectral slice optical arbitrary waveform measurement (OAWM) measures wideband waveforms in many narrower spectral slices. Additionally, spectralslice dynamic OAWG and OAWM directly extends the transmitter and the receiver bandwidth towards several terahertz without introducing losses as the bandwidth scales or requiring channelization of the spectrum [2–7]. 2. Spectral slice dynamic optical arbitrary waveform generation In OAWG, shown in Fig. 1(a), the spectral slices are generated by separately modulating each line of an optical frequency comb (OFC) using an in-phase and quadrature-phase modulator (I/Q) and then coherently combining the slices to produce a waveform. A spectral multiplexer with isolating transmission separates each comb line from the reference OFC, and a spectral multiplexer with gapless transmission combines the slices [Fig. 2(c)]. Digital signal
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30 -30 -10 10 -20 0 20 40 ps 66.7 ps Frequency (GHz) Time Fig. 1. (a) Spectral-slice dynamic optical arbitrary waveform generation. (b,c,d) 60-GHz bandwidth generation of a waveform consisting of three BPSK sub-carriers with different baud rates and no spectral gaps. (b) Time-domain, (c) spectrum and (d) eye diagrams.
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Fig. 2. (a) Spectral-slice dynamic OAWM. (b) Signal reconstruction DSP. (c) Ideal multiplexer transmission for the signal waveform and the reference OFC. (d,e) 160-GHz measurement example obtained using a silica planar lightwave circuit and four spectral slices.
processing (DSP) determines the I and Q modulation signals for each slice that continuously generates the desired waveform. Using N slices, the bandwidth can increase by a factor of N over the current state-of-the art electronics. Fig. 1(b,c,d) shows a six-slice 60-GHz waveform generation example highlighting some of OAWGs capabilities. Details of the experiment can be found in [8] which includes examples of QPSK, 8-PSK, OFDM and of flexible bandwidth networking waveform generation. The OAWG transmitter simultaneously produces three binary phase shift keyed (BPSK) channels at 25-GBd, 20-GBd, and 15-GBd. Each channel has a rectangular spectral shape without a guard band, an important advantage of OAWG over other architectures. Fig. 1(c) shows the waveform’s spectrum where the different colored sections indicate the different channels and the bars and dot indicate the location of each spectral slice and comb line. Fig. 1(b) shows a 2-ns section comparison between the waveform (blue/red) and the target (grey). The aggregate waveform does not resemble data because it is the superposition of three different BPSK data signals at different center wavelengths. However, the measured eye diagrams are open [Fig. 1(d)] indicating the successful generation of the three subcarriers. 3. Spectral slice dynamic optical arbitrary waveform measurement Similar to OAWG, spectral slice OAWM measures a waveform in M spectral slices with spacing ΔfM against each comb line from an M-line OFC. Fig. 2(a) shows an OAWM device consisting of two spectral demultiplexers which pair a reference comb line with a slice and a digital coherent receiver (i.e., an optical 90 degree hybrid, two balanced photoreceivers and two high-speed ADCs) to measure the I and Q components of the slice and DSP [Fig. 2(b)] reproduces the waveform. Generally, electronic ADC technology outpaces DAC technology and therefore OAWM more easily obtains larger bandwidths than OAWG. As an example of OAWM performance, Fig. 2(d,e) show a 160GHz bandwidth measurement in four 40-GHz spectral slices using an integrated receiver of a complex waveform consisting of a transform limited pulse interleaved with a cubic spectral waveform in a pre-programmed pattern [5]. 4. Conclusions We have shown the capabilities of the dynamic OAWG and OAWM system to handle continuous waveforms with scalable bandwidths. In particular, an OAWG transmitter produced three BPSK waveforms with different baud rates without guard bands simultaneously. Future work will continue scaling the OAWG system to terahertz bandwidths. [1] [2] [3] [4] [5] [6] [7] [8]
A. Chraplyvy, “Plenary paper: The coming capacity crunch,” in ECOC 2009, p. 1–1. R. P. Scott, N. K. Fontaine, J. P. Heritage, and S. J. B. Yoo, “Dynamic optical arbitrary waveform generation and measurement,” Optics Express, vol. 18, no. 18, pp. 18655–18670, 2010. S. T. Cundiff and A. M. Weiner, “Optical arbitrary waveform generation,” Nature Photonics, vol. 4, no. 11, pp. 760–766, 2010. N. K. Fontaine, “Optical Arbitrary Waveform Generation and Measurement,” Univeristy of California, Davis, 2010. N. K. Fontaine, R. P. Scott, L. Zhou, F. Soares, J. P. Heritage, and S. J. B. Yoo, “Real-time full-field arbitrary optical waveform measurement,” Nature Photonics, vol. 4, pp. 248–254, 2010. N. K. Fontaine, R. P. Scott, and S. J. B. Yoo, “Dynamic optical arbitrary waveform generation and detection in InP photonic integrated circuits for Tb/s optical communications,” Optics Communications, vol. 284, no. 15, pp. 3693–3705, 2011. F. M. Soares, N. K. Fontaine, R. P. Scott, et al., “Monolithic InP 100-Channel X 10-GHz Device for Optical Arbitrary Waveform Generation,” Ieee Photonics Journal, vol. 3, no. 6, pp. 975–985, 2011. D. J. Geisler, N. K. Fontaine, R. P. Scott, and S. J. B. Yoo, “Demonstration of a Flexible Bandwidth Optical Transmitter / Receiver System Scalable to Terahertz Bandwidths,” IEEE Photon. Journal, vol. 3, no. 6, pp. 1013–1022, 2011.
The authors thank many former and current members of the Next Generation Networking Systems Group at UC Davis, in particular, Takahide Sakamoto, Francisco Soares, Jonghwa Baek, Tiehui Su, Stevan Djordjevic, Xinran Cai, Binbin Guan, collaborators at Multiplex, Royal Institute of Technology, Sweden, and Nistica, and Robert Tkach and Pat Tier at Bell Laboratories.
