Optical Phase-Sensitive Sampling to Capture Optical Waveforms
by Peter A. Andrekson, Ph.D.; Mathias Westlund, Ph.D.; Henrik Sunnerud, Ph.D.; Mats Sköld, Ph.D.; Gregory W. Schinn, Ph.D.; and Francois Robitaille, EXFO
The advent of high-symbol-rate optical signals, encoded using advanced multilevel modulation techniques, has driven a corresponding requirement to accurately measure the resulting waveforms and constellation diagrams. A notable example of such an advanced modulation scheme is the dual-polarization quadrature phase-shift keying (QPSK) format, which is compatible with installed fiber infrastructure and allows implementation of 100-Gb Ethernet with high spectral efficiency.
More complicated formats also are expected to emerge, including quadrature amplitude modulation (QAM) signaling using both amplitude and phase modulation. While these concepts are well known in wireless communications, the challenges in optical communications arise due to the very high symbol rates being considered—28 Gbaud and perhaps as high as 100 Gbaud in the future. This imposes severe hardware-limited constraints on any system design.
As a result, it is necessary to ensure that there is an appropriate set of advanced test and measurement tools available to follow this trend toward sophisticated optical modulation at ever-increasing baud. The capture of waveforms by sampling is a convenient and powerful way to collect most of the relevant information about a signal. If this sampling is undertaken all-optically rather than electronically, the bandwidth bottleneck and ringing artifacts associated with impedance-mismatched electronic systems can be entirely alleviated. This facilitates very high-resolution, distortion-free replication of the signal-under-test, ensuring that the testing instrumentation does not itself impact the observed signal quality. The optical sampling technique can further be rendered sensitive to the optical phase to allow capture of the full optical field such as amplitude and phase by adopting the well-known method of coherent detection, which in practice means adding a local oscillator to the signal-under-test before detection.
All-Optical Sampling
Figure 1 illustrates the basic difference between all-optical sampling and traditional electrical sampling of optical waveforms. As can be seen, electrical sampling requires a high-speed optoelectronic component, the photodiode, whereas for the optical sampling case, no high-speed electronics are necessary.
Since fast detectors are not needed for the case of optical sampling, avalanche photodiodes (APDs) can be used to further improve the sensitivity. This is not true for high-speed electrical sampling since useful APDs having bandwidths much greater than 10 GHz do not exist.
Figure 1. Principles of Traditional Electrical Sampling (left)
and All-Optical Sampling of Optical Waveforms (right)
Figure 2 shows the implementation of a sampling system based on nonlinear four-wave mixing (FWM) in an optical fiber.1,2 A mode-locked fiber laser provides short sampling pulses, each of which temporarily generates an amplified idler wave at optical frequency
The idler wave contains an amount of optical power proportional to the signal power sampled during the same short time duration. This idler wave can subsequently be spectrally filtered to isolate it from residual pump and signal light, and the detected photo current then will consist of a string of spikes at a repetition rate determined by the pulse source, such as 100 MHz.
Figure 2. Implementation of FWM-Based All-Optical Sampling (top)
and a Measured 640-Gb/s RZ Data Pattern (bottom)
We analyzed the sensitivity vs. the temporal resolution trade-off in this type of optical sampling system.3 It is useful to introduce a figure-of-merit
This allows for a fair comparison among different systems, including those using electrical sampling. By varying the temporal resolution in the 1-ps to 18-ps range, we found a resolution-independent FOM = 2 ps-1 mW-1. In contrast, for the case where the high-speed photodiode was connected directly to a 50-GHz electrical sampling oscilloscope, the FOM was about 10 times smaller.
In Reference 4, we explored the sampling scheme in a real-time sampling setup involving four separate sampling gates with a Nyquist-limited 50-GHz bandwidth. The concept also was extended to the phase-sensitive mode by adding a local oscillator for coherent detection, allowing constellation diagram analysis at 40 Gbaud and beyond.5
Figure 3 illustrates real-time and phase-sensitive sampled optical data. You can see loops in the transitions from 0 to π in the sampled constellation diagram, likely caused by timing misalignment in the two Mach-Zehnder modulator arms. Figure 4 is a schematic setup for this phase-sensitive sampling system.
Figure 3. 40 Gb/s Signal Captured in Real Time at 100 GS/s (left)
and Constellation Diagram of a 40 Gb/s BPSK Signal (right)
Figure 4. Schematic for the Phase-Sensitive Sampling Scheme Used in Reference 5
Sampling of Differentially Phase-Encoded Data
For differentially encoded data, it is not necessary to use a local oscillator-based coherent detection approach as shown in Figure 4. We have implemented an all-optical balanced detection system that is conceptually similar to a balanced receiver;6 however, the sampling is performed all-optically (Figure 5). A delay interferometer is used to convert the phase to intensity, and the outputs of the two ports are sampled independently. Figure 6 shows a measurement example of a 170-Gb/s RZ-DPSK signal.
Figure 5. All-Optical Balanced Detection System
Figure 6. An RZ-DPSK Signal at 170 Gb/s Analyzed
With the All-Optical Balanced Detection System in Figure 5
Measurement of Advanced Optical Modulation Format Signals
Recently, we have extended the equivalent-time phase-sensitive sampling capability to allow capture of polarization-multiplexed signals with complex formats such as QPSK, 8PSK, and 16-QAM. Examples of such signals are found in Figure 7.
Figure 7. Measured Constellation Diagrams: QPSK Signal at 40
Gbaud (top left), 8PSK Signal at 40 Gbaud (top right),
and 16-QAM Signal at 28 Gbaud (bottom)
The QPSK and 8PSK symbol rate here is 40 GSymbols/s while that of the 16-QAM is 28 GSymbols/s. The gray dots represent the averaged transitions while the black ones indicate the nonaveraged symbols. The constellations represent the optical field without any intersymbol interference (ISI) mitigation via DSP. For that reason, our results are simpler to interpret in a direct way, making, for example, comparative studies of different transmitter solutions more straightforward.
Conclusions
For the characterization of advanced-modulation-format signals, operating high baud, all-optical sampling is an extremely promising approach. It offers distinct advantages over equivalent-time electrical sampling due to the substantially relaxed bandwidth requirements on the optical detectors and its immunity to potential impedance mismatch problems in the optoelectronic detection. Also, when repetitive signals are being analyzed, it permits a significantly better effective measurement bandwidth than real-time electrical measurement systems.
We can expect optical sampling test and measurement instrumentation to become standard tools in advanced optical telecommunications development labs, qualification processes, and advanced subsystem manufacturing.
References
1. Andrekson, P.A., “Picosecond optical sampling using four-wave mixing in fiber,” El. Lett., v. 27, pp. 1440-1441, 1991.
2. Westlund, M., et al., “High-Performance Optical-Fiber-Nonlinearity-Based Optical Waveform Monitoring,” Journal of Lightwave Technology, v. 23, pp. 2012-2022, 2005.
3. Sunnerud, H., Westlund, M., and Andrekson, P.A., “High Sensitivity All-Optical Sampling for 40 Gb/s Signals,” Optical Fiber Communication Conference, Paper OWN3, March 2006.
4. Sköld, M., Westlund, M., Sunnerud, H., and Andrekson, P.A., “100 GSample/s Optical Real-Time Sampling System With Nyquist-Limited Bandwidth,” European Conference on Optical Communication, PDP1.1, September 2007.
5. Westlund, M., Sköld, M., and Andrekson, P.A., “All-Optical Phase-Sensitive Waveform Sampling at 40 GSymbol/s,” Optical Fiber Communication Conference, San Diego, USA, PDP12, February 2008.
6. Sunnerud, H., Westlund, M., Sköld, M., and Andrekson, P.A., “All-Optical Balanced Detection System With Sub-ps Resolution,” Optical Fiber Communication Conference, March 2009.
About the Authors
Peter A. Andrekson, Ph.D., is director of EXFO Sweden. He received his doctorate from Chalmers University of Technology. e-mail: peter.andrekson@EXFO.com
Mathias Westlund, Ph.D., is technical leader at EXFO Sweden. He graduated from Chalmers University of Technology with a doctorate degree. e-mail: mathias.westlund@EXFO.com
Henrik Sunnerud, Ph.D., serves as the principal research scientist at EXFO Sweden. His doctorate is from Chalmers University of Technology. e-mail: henrik.sunnerud@EXFO.com
Mats Sköld, Ph.D., is an EXFO Sweden researcher who earned his doctorate from Chalmers University of Technology. e-mail: mats.skold@EXFO.com
Gregory Schinn, Ph.D., holds the position of R&D director— Research at EXFO. He was awarded a doctorate from JILA/University of Colorado. e-mail: greg.schinn@EXFO.com
François Robitaille is the senior product manager for the Optical Business Unit at EXFO. He received an M.Sc. degree from Laval University. e-mail: francois.robitaille@EXFO.com