© 2009 OSA/OFC/NFOEC 2009 a1449_1.pdf JThA29.pdf JThA29.pdf
3 b/s/Hz 1.2 Tb/s Packet Generation using Optical Arbitrary Waveform Generation Based Optical Transmitter David J. Geisler, Nicolas K. Fontaine, Ryan P. Scott, Tingting He, Katsu Okamoto, Jonathan P. Heritage, and S. J. Ben Yoo Department of Electrical and Computer Engineering, University of California, Davis, 95616 Email:
[email protected]
Abstract: 40-bit OOK, 80-bit QPSK and 120-bit 16-QAM packets are generated with spectral efficiencies of 1, 2 and 3 b/s/Hz, exploiting concepts from OAWG. Results demonstrate potential for a Tb/s OAWG based transmitter using GHz electronics. ©2009 Optical Society of America OCIS codes: (320.5540) Pulse Shaping; (060.1660) Coherent Communications; (320.7100) Ultrafast Measurements.
1. Introduction 100 Gb-Ethernet is about to become an established standard for single data channel transmissions to maximize optical fiber bandwidth utilization. Soon, spectrally efficient 500 Gb/s and 1000 Gb/s transmission will become necessary to further maximize capacity utilization and maintain continued Internet growth. The recently developed digital coherent receiver [1], real-imaginary (i.e., I-Q) modulators, fast real-time analog-digital conversion and digital signal processing are enabling 2M-symbol modulation formats such as quadrature phase-shift keying (QPSK), and quadrature amplitude modulation (QAM) to obtain the ultimate spectral efficiency of M bit/s/Hz. A promising alternate approach is to generate Tb/s data using an optical arbitrary waveform generation (OAWG) based transmitter [2]. The OAWG transmitter converts a coherent optical frequency comb (OFC) source into a THz bandwidth arbitrarily shaped waveform by independently applying intensity and phase modulation to individual comblines. The coherent and parallel combinations of GHz modulations, including adjacent comb line crosstalk, synthesize a fully controllable THz waveform. The waveforms can replicate data in any data modulation format with any amount of linear distortion (e.g., chromatic dispersion or polarization mode dispersion) precompensation. All electrical components within the OAWG transmitter are restricted to less than 10 GHz bandwidth, manageable with current technologies. In comparison, other multi-tone coherent optical transmission methodologies such as optical orthogonal frequency division multiplexing (O-OFDM) and coherent-WDM (coWDM) use the coherence of adjacent subcarriers to help reject adjacent channel crosstalk [1] rather than to collectively generate packets. Low spectral efficiency (0.27 b/s/Hz) has been achieved for 100 Gb/s 8-bit return-to-zero (RZ) on-off keying (OOK) packets [3]. Previously, our group has demonstrated the concept of an OAWG based optical transmitter for generation of modulation format insensitive data and for precompensation of chromatic dispersion [2]. The results included generation of 360 Gb/s 9-bit long non-return-to-zero (NRZ) OOK and differential phase-shift keying (DPSK) packets with a modest spectral efficiency of 0.5 b/s/Hz. Recovery of the packets was possible after 10 km propagation through single mode fiber. This paper extends the concept to generate repetitive 400 Gb/s 40-bit NRZ-OOK, 800 Gb/s 80-bit NRZ-QPSK packets and 120-bit NRZ-16-QAM packets with spectral efficiencies of 1 b/s/Hz, 2 b/s/Hz and 3 b/s/Hz, respectively. From a waveform shaping perspective, the packets generated are of maximal complexity compared to previous examples of OAWG. (a) Frequency Domain Frequency Domain (b) Optical Signal
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Fig. 1. (a) Outline of OAWG methodology assuming 10 GHz optical freequency comb source in which intensity and phase modulations of individual comb source spectral components determines a desired waveform. (b) Experimental setup.
© 2009 OSA/OFC/NFOEC 2009 a1449_1.pdf JThA29.pdf JThA29.pdf
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2. Experiment Fig. 1(a) outlines the OAWG methodology, which includes generation of THz waveforms from GHz modulations. First, a spectral demultiplexer separates the OFC comblines to separate modulators. Next, each comb line is individually modulated in both intensity and phase before being spectrally multiplexed together to produce a shaped waveform. The OAWG based optical transmitter demonstrated in this paper uses a silica based loopback arrayedwaveguide grating (AWG) for both spectral demultiplexing and multiplexing [4]. Resistive heaters provide intensity and phase modulation, which can only support kHz modulations. As a result, the silica based OAWG transmitter can only apply relatively static intensity and phase shifts to the OFC. The Fourier transform of the shaped OFC defines the generated temporal waveform. This process is known as Fourier synthesis [5] and produces repetitive timedomain waveforms with a duration (period) equal to the inverse of the OFC spacing. The finest temporal features that can be shaped are proportional to the inverse of the OFC bandwidth. Fig. 1(b) shows the experimental setup for generation and measurement of optical waveforms. The generated OFC contains over 40 comb lines with 10 GHz spacing. It is produced by strong amplitude and phase modulation of a single-frequency laser using a dual-electrode Mach-Zehnder modulator and two phase modulators. The corresponding maximum duration of a shaped waveform is 100 ps with a minimum feature size of ~2.5 ps. The OFC is then sent through the OAWG transmitter for generating the data packets. The OAWG device intensity and phase modulators can obtain 20–30 dB of extinction and over 2π of phase shift, respectively. The specific values needed to drive the intensity and phase modulators used for target packet generation are determined by the waveform control (DSP) using the following algorithm. First, an ideal impulse train in the desired modulation format is created, with impulses spaced every 2.5 ps for 400 Gb/s OOK and 1.25 ps for 800 Gb/s QPSK. Next, the impulse train is passed through a raised cosine modulation filter with a β-rolloff factor of 0 for maximum spectral efficiency, though other Nyquist functions also work [6]. Nyquist pulses are advantageous modulation functions for this application due to their bandlimited nature and their unique property of zero intersymbol interference (ISI) at each impulse location. The shaped packet is measured using X-FROG [7], which uses a pre-characterized reference pulse to measure an unknown pulse. The high-spectral efficiency packets are maximally complex waveforms that occupy the entire time and spectral domains. Figs. 2(a,b) & 4(a,b) show rapid line-to-line changes in the spectral domain and rapid variations in time. Such waveforms are not easily described using traditional metrics such as time-bandwidth product. The X-FROG reference pulse FWHM is 3.5 ps and is generated by filtering a 0.8 nm transform limited portion of the OFC. The low bandwidth reference pulse helps to simplify the X-FROG trace by reducing the spectral resolution requirements and decreasing ambiguity in the X-FROG retrieval. Feedback is used from the X-FROG measurements to adjust the OAWG transmitter thereby generating the desired waveforms in approximately five iterations. (b) (a) (c)
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Fig. 2. 40-bit 400 Gb/s NRZ-OOK packet. (a) Spectral domain (blue) intensity and (red) phase targets indicated by ‘x’. (b) Time-domain optical field (blue) intensity and (red) phase. Target packet indicated by lighter shades. (c) Target and (d) measured eye diagrams.
Fig. 2(a) presents the target and measured optical spectrum for the 40-bit 400 Gb/s NRZ-OOK repetitive data packet (Fig. 2(b)). The spectrum is trimmed to exactly 400 GHz by attenuating all out-of-band modes as they pass through the OAWG transmitter. The resulting packet has a spectral efficiency of exactly 1 b/s/Hz. The generated packet matches well in intensity and phase to the target packet in both temporal and frequency domains. The differences result from waveform shaping errors due to crosstalk and errors in the X-FROG measurement. The time domain ringing of the NRZ-OOK packet is due to the tight spectral confinement of the 400 Gb/s packet to a 400 GHz bandwidth. Fig. 2(c) shows the eye diagram generated from the target field magnitude. The center of each symbol (eye center) is a point of zero (ISI) and the eye contains exactly two levels (1 bit/symbol). The time-domain ringing is apparent at the edges of the eye diagram. The ringing can be reduced and the eye opening increased by increasing the bandwidth at the expense of spectral efficiency. Fig. 2(d) shows the eye diagram generated from the measured field magnitude. There is noticeable spreading of the 0 and 1 levels, however, the eye still remains open.
© 2009 OSA/OFC/NFOEC 2009 a1449_1.pdf JThA29.pdf JThA29.pdf
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Fig. 3(a) shows the 80-bit 800 Gb/s NRZ-QPSK packet with 2 bit/s/Hz spectral efficiency. The symbol duration is 2.5 ps and the spectral width is exactly 400 GHz. It is difficult to see the four QPSK symbols directly from the time-domain packet. The constellation diagram is generated by selecting only a 0.5 ps window around the center of each symbol, and then plotting the real and imaginary components of the gated packet on an X-Y plot. Fig. 3(c,f) compare the target constellation to the measured constellation. Clearly, the measured and the target constellation contain four symbols equally spaced around the unit circle. Fig. 3(d,e,g,h) show eye diagrams of the target and measured packets. There are two eye-diagrams for the data; one is from the real component of the packet and other from the imaginary component of the packet. The center of the eyes is a point of zero ISI and the 2 amplitude levels are clearly visible. Fig. 3(g,h) shows that measured real and imaginary eyes are clearly open. (c) (d) Real (e) Imag (a)
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Fig. 3. (a) 80-bit 800 Gb/s NRZ-QPSK packet optical field (blue) intensity and (red) phase. Target packet indicated by lighter shades. Target (c) constellation diagram and (d,e) eye diagrams of real and imaginary field components. Measured (f) constellation diagram and (g,h) eye diagrams of real and imaginary field components.
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3. Outlook to Tb/s data modulation
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Fig. 4. 120 bit 1200 Gb/s NRZ-QAM packet (a) spectral intensity (blue) and phase (red), and (b) optical field (blue) intensity, (red) phase. Target indicated by lighter shades and ‘x’. (c) Target and (d) measured constellation diagrams.
Fig. 4 presents 120-bit 1.2 Tb/s 16-QAM generated packet with a spectral efficiency of 3 b/s/Hz. The generated packet matches the target packet in many of the temporal slices, but errors are evident. These mismatches cause the 16 symbols on the measured constellation diagram to shift or blur. However, the Tb/s packet shows potential for a future ultra-high speed transmitter. 4. Conclusion We have demonstrated an OAWG based transmitter with high spectral efficiencies of 1 b/s/Hz and 2 b/s/Hz for 400 Gb/s 40-bit OOK and 800 Gb/s 80-bit QPSK packets, respectively. The potential for a future ultra-high speed 1.2 Tb/s transmitter based on 16-QAM with a spectral efficiency of 3 b/s/Hz was also presented. References [1] E. Ip, et al., “Coherent detection in optical fiber systems,” Optics Express, 16, 753-791 (2008). [2] D. J. Geisler, et al., “360 Gb/s Data Modulation With Dispersion Precompensation Using Optical Arbitrary Waveform Generation,” in Lasers & Electro-Optics Society Annual Meeting, 2008). [3] R. Kobe, et al., “Generation of 100-Gbps optical packets with 8-bit RZ pulse patterns using an optical pulse synthesizer,” in Conference on Lasers and Electro-Optics - Pacific Rim, 2007. CLEO/Pacific Rim 2007. [4] N. K. Fontaine, et al., “Compact 10 GHz loopback arrayed-waveguide grating for high-fidelity optical arbitrary waveform generation,” Optics Letters, 33, 1714-1716 (2008). [5] A. M. Weiner, “Femtosecond Optical Pulse Shaping and Processing,” Prog. Quant. Electr., 19, 161-235 (1995). [6] A. Assalini, et al., “Improved Nyquist Pulses,” IEEE Communications Letters, 8, 87-89 (2004). [7] R. P. Scott, et al., “High-fidelity line-by-line optical waveform generation and complete characterization using FROG,” Optics Express, 15, 9977-9988 (2007). This work was supported in part by the DARPA DSO and SPAWAR under OAWG contract HR0011-05-C-0155.
