Keysight TechnologiesElectronic Synthesis of Optical Signals at Runtime – A New Approach for Coherent Transmission Stress Testing

Application Note Version 1.0

Introduction

The latest development in high-speed arbitrary waveform generators (AWG) offers sampling rates which enable to directly synthesize and generate complex modulated baseband signals with highest data rates, driving E/O converters and generating a true optical transmission channel with 256 Gb/s and beyond. This technique not only allows to flexibly switch data rates and transmission formats for coherent transmission testing but also allows to emulate optical signal properties and impairments which occur in a real optical fiber network, such as polarization rotation, polarization mode dispersion (PMD) or carrier phase noise.

In traditional coherent receiver stress testing using an optical fiber test bed, signal impairments are generated optically. Testing is subject to numerous stochastic processes where it is difficult to deterministically and repeatedly generate stress conditions1. However, with an AWG this can be accomplished and test coverage can be increased at reduced test time (see Figure 1).

Numerical models and simulation tools for optical fiber transmission characteristics and optical impairments are used to pre-calculate test signals off-line on a PC that are then downloaded to a waveform memory and played from an AWG2. Although this approach is convenient, it suffers from practical limitations and unwanted drawbacks when adding optical impairments and signal properties to the clean, undistorted optical signal.

Optical referencetransmitter

Optical signalsynthesizer

Fiber testbed setup:Stochastic testing

ReceiverError

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Optical signal synthesizer setup:Deterministic & repeatable testing

ReceiverError

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Figure 1. Comparison of traditional fiber testbed and AWG based optical signal synthesis

03 | Keysight | Electronic Synthesis of Optical Signals at Runtime - Application Note

Limitations of Current AWG Hardware Architectures and Simulation Tools

1. Due to the AWG’s limited waveform memory, slow optical effects and impairments cannot properly be emulated. For example in an AWG with 16 GSamples memory at 64 GSamples/s the waveforms emulating slow polarization effects repeat at least every 250ms.

2. Additionally due to the necessity of pre-calculating the whole optical signal, including all emulated impairments, the waveform’s bit stream is strongly cor-related to the emulated impairments and repeats exactly after each reiteration of the stored waveform and is limited by the waveform memory length.

3. The pre-calculated optical signal does not allow to quickly change any optical signal properties such as pulse shaping or emulated impairments but typically requires extensive numerical recalculation and transfer of large amounts of data which results in a poor waveform update rate, depending on the computer hardware and interface.

Novel Proposed Real-Time Synthesis Architecture

To overcome these drawbacks, a new real-time testing architecture is realized by adding digital signal processing (DSP) blocks into the M8195A AWG hardware3, which provide the means to generate the desired signal properties and impairments independently of the clean optical signal and at runtime. Figure 2 shows the basic block diagram of the realized architecture where the DSP block is embedded in an ASIC.

Each functional block contains its own set of coefficients, which can be set and altered independently from the continuously played clean signal. The coefficient memory holds up to 10k coefficient sets. These can be recalled at a defined rate thus continuously varying the DSP and signal properties for emulating dynamic stress patterns. The repeat rate can be slowed down in such a way that the pattern repeats only every 5 seconds.

The real-time DSP in the M8195A AWG realized the following processing blocks. The waveform is synthesized and equalized with the following 4 DSP blocks:

Figure 2. Architecture of an AWG with real-time signal property synthesis at the example of a Keysight M8195A prototype

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04 | Keysight | Electronic Synthesis of Optical Signals at Runtime - Application Note

PSK / QAM EncoderWhen using the real-time DSP feature, the AWG memory stores the data pattern symbols which are then encoded using the PSK/QAM encoder based on reconfigurable look-up tables (LUT), instead of storing the pre-calculated waveform samples of the desired signal. Integer (2, 4, 8-time) oversampling is realized. Thus with the AWG running at 64GSa/s, signals with baud rates of 32, 16, and 8 GBaud are supported.

Pulse Shaper FIRFor spectrally efficient transmission channels, the data symbols can be filtered with a general 16-tap FIR filter to flexibly vary the pulse shaping such as raised or root raised cosine (RRC) or the roll-off factor.

Non-linear equalizerThe non-linear equalizer applies a non-linear transfer function based on a broken line approximation algorithm to the data signal, allowing to compensate for example the transfer characteristic of an electro-optical modulator.

Linear equalizerThe frequency response of the DAC / output amplifier and connected cables & fixtures can be equalized with a 16-tap FIR filter. Skew between output channels can be further changed with the linear equalizer.

The synthesized and equalized waveform can now be varied for stress generation with the following DSP blocks:

IQ RotatorThe IQ Rotator consists of two identical blocks where an instantaneous rotation angle, controlled from a pre-programmed coefficient pattern memory, is applied to the complex IQ signal for both, the X and Y polarization, emulating e.g. carrier phase noise or a frequency offset.

Polarization ControllerThe polarization controller is realized as a one-tap butterfly filter and alters the state of polarization (SOP) by acting like a concatenation of a circular and a linear retarder. Rota-tion angle and retardation can be controlled from a pre-programmed pattern memory, which allows to synthesize complex SOP trajectories on the Poincaré sphere.

PMD EmulatorBased on the model of multi-segment concatenation of birefringent elements4 the PMD emulator block is realized as a complex butterfly filter structure made of sixteen 7-tap FIR filters in which its filter coefficients represent the optical channel’s PMD impulse response.

In combination with the 2 polarization controller blocks in front and after the PMD block realistic PMD stress tests like in actual fiber test beds can be performed.

05 | Keysight | Electronic Synthesis of Optical Signals at Runtime - Application Note

Stress Test Setup

For a practical stress test setup, the AWG and real-time blocks in the M8195A AWG first need to be configured (see also Fig. 2) via the 81195A optical modulation genera-tor software. A PRBS 211 - 1 is loaded into the symbol memory of the M8195A AWG, the encoder is set to a dual polarization 32 GBaud QPSK or 16-QAM modulation at a sampling rate of 64 GSamples/s. The pulse-shaping block applies a RRC filter with a roll-off factor of 0.5. The real-time blocks are configured in different experiments to model polarization rotation, PMD and phase noise. The experimental setup is depicted in Fig. 3. The electrical outputs of the signal property generator are connected to a dual-polarization IQ modulator (ID Photonics Optical Multi-Format Transmitter). The optical signal is given to the N4391A optical modulation analyzer (OMA) for analysis and verification of the generated optical signal properties. For the experiment, a homodyne setup configuration is chosen where the OMA local oscillator is used as signal laser for lowest intrinsic phase noise.

Figure 3. Experimental setup for verification of real-time signal property generation

Figure 4. (a) Real-time generation of polarization rotation for a signal and (b) the measured results

AWG with real-timesignal property

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Measurement Results

Polarization Rotation: Fig. 4a shows a synthesized polarization trajectory with a SOP change rate of up to several Mrad/s and its corresponding measured polarization trajectory (Fig. 4b). The model and the measurement results are in good alignment.

06 | Keysight | Electronic Synthesis of Optical Signals at Runtime - Application Note

Figure 5. Real-time generation of DGD over wavelength, the mean value of the model and the measured mean value

Figure 6. (a) PMD setup

Figure 6. (b) 1st and 2nd order PMD measurement with N4391A OMA

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Polarization Mode Dispersion: In a second measurement the polarization controller block is set to a fixed position and the PMD block generates differential group delay (DGD) and 2nd order PMD which according to the underlying model results in a DGD curve as shown in Fig. 5 (dotted curve). We compare the mean value of the model (straight line) with the 1st order PMD measured by the N4391A optical modulation analyzer (dashed line) which are in very good agreement. In a different setting 1st and 2nd order PMD are imposed on a 32 GB 16-QAM signal (see Fig. 6a,b).

07 | Keysight | Electronic Synthesis of Optical Signals at Runtime - Application Note

Conclusion

In this application note we presented a new test concept for coherent receiver dynamic stress testing using an AWG hardware architecture comprising a real-time DSP block, allowing to add and repeatedly and deterministically alter dynamic optical signal proper-ties and impairments at run-time. Exemplary results have been given with a prototype unit of a Keysight M8195A AWG and analyzed with the Keysight N4391A OMA.

Figure 7. Phase Noise Power Spectrum Density (PSD) according to the model and measured

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Phase Noise: We also emulated the large linewidth of signal lasers by configuring the IQ rotator blocks with the phase noise model5 of a laser source with 1 MHz linewidth. This linewidth is in the range of good DFB or monolithic DBR lasers. For verification we compare the power spectrum density (PSD) of the model with the OMA measured PSD as shown in Fig. 6 and 7.

08 | Keysight | Electronic Synthesis of Optical Signals at Runtime - Application Note

Speed Comparison

With the M8195A AWG in combination with the 81195A optical modulation generator software, a new approach for coherent receiver stress testing is realized where optical signals beyond 256Gb/s are generated in real time and signal stress conditions such as polarization rotation, polarization mode dispersion and carrier phase noise are electroni-cally synthesized and altered at runtime.

Being able to change the signal properties at run time speeds up multiple scenario test-ing by factors of 100 and above. As an example consider that recalculation and upload of waveforms containing a PRBS23 pattern with RRC pulse shaping and optical stress patterns such as polarization rotation with a PC take in the order of > 100s. However the coefficients of the DSP can be changed on the fly within less than 1 second.

References1. L.E. Nelson, et, al., “A Robust Real-Time 100G Transceiver With Soft-Decision

Forward Error Correction”, J. Optical Communications and Networking, Vol.4, No.11, 2012

2. A. Ragheb et al., “384Gb/s per Carrier Quad-DAC based Dual-Polarization Software defined Optical Transmitter”, High-capacity optical networks and emerging/enabling technologies (HONET), 2014

3. J.C. Rasmusssen et al., “Digital Coherent Receiver Technology for 100-Gb/s Optical Transport Systems”, FUJITSU Sci. Tech. J., Vol.46, No.1, pp.63-71 (2010)

4. H. Kogelnik et al., “Emulation and Inversion of Polarization-Mode Dispersion”, Journal of Lightwave Technology, Vol.21, No.2, 2003 S. Camatel, “Narrow Linewidth CW Laser Phase Noise Characterization Method for Coherent Transmission System Applications”, Journal of Lightwave Technology, Vol. 26,

Configuration

Software runs on a PC stand-alone or in conjunction with an M8195A or M8196A arbitrary waveform generator

Product

81195A Optical Modulation Generator Software

81195A-OSP Optical Signal Properties

81195A-RSP Real-Time Signal Processing Only for M8195A

Related Literature

Publication title Publication number

81195A Optical Modulation Generator Software Data Sheet 5992-0972EN

M8195A 65 GSa/s Arbitrary Waveform Generator Data Sheet 5992-0014EN

M8196A 92 GSa/s Arbitrary Waveform Generator Data Sheet 5992-0971EN

N4391A Optical Modulation Analyzer Data Sheet 5990-3509EN

09 | Keysight | Electronic Synthesis of Optical Signals at Runtime - Application Note

This information is subject to change without notice.© Keysight Technologies, 2017Published in USA, December 1, 20175992-1294ENwww.keysight.com

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