Coherent Lightwave Signal Analyzer Datasheet

Features & Benefits

Unparalleled Flexibility

• Coherent Lightwave Signal Analyzer Architecture Compatible with both Real-time and Equivalent-time Oscilloscopes*¹
• Complete Coherent Signal Analysis System for Polarization-multiplexed QPSK, Offset QPSK, QAM, Differential BPSK/QPSK, and Other Advanced Modulation Formats
• Displays Constellation Diagrams, Phase Eye Diagrams, Q-factor, Q-plot, Spectral Plots, Poincaré Sphere, Signal vs. Time, Laser Phase Characteristics, BER, with Additional Plots and Analyses Available through the MATLAB Interface
• Measures Polarization Mode Dispersion (PMD) of Arbitrary Order with Most Polarization Multiplexed Signals

Precise Optical Receiver

• Precise Coherent Receiver Hardware provides Minimal Variation over Temperature and Time for a High Degree of Accuracy and High-stability, Polarization-diverse, Optical Field Detection
• Highly Linear Photo Detection allows Operation at High Local Oscillator and Signal Power Levels to Eliminate Electrical Amplification
• An Integrated Pair of ECDL Tunable Lasers for Use as a Local Oscillator and Another for Self-test. Both Lasers have Industry-best Linewidth and Tuning Range for Any Wavelength within the Band
• Coherent Lightwave Signal Analyzer Software Tolerates >5 MHz Instantaneous Signal Laser Linewidth – Compatible with Standard Network Tunable Sources such as DBR and DFB Lasers
• No Laser Phase or Frequency Locking Required
• Smart Polarization Separation Follows Signal Polarization

User-defined Extensibility

• User Access to Internal Functions with a Direct MATLAB*² Interface
• The OM4000 can be Controlled through Ethernet for Remote Access
• Superior User Interface offers Comprehensive Visualization for Ease-of-Use Combined with the Power of MATLAB
• Coherent Lightwave Signal Analyzer Software included with OM1106 and OM4000 Series Products

400G and 1Tb/s Superchannel Support

• Multi-carrier software option allows user-definable superchannel setup
• Superchannel configuration allows user to define number of channels, channel frequency, and channel modulation format
• Test automation acquires complete measurements at each channel
• Integtrated measurement results allow easy channel-to-channel comparisons

 

*¹ Certain features may be available only when used with Tektronix oscilloscopes.

*² MATLAB is a registered trademark of MathWorks.

Introduction

The OM4000 Coherent Lightwave Signal Analyzer (CLSA) is a 1550 nm (C- and L-band) fiber-optic test system for visualization and measurement of complex modulated signals, offering a complete solution to testing both coherent and direct-detected transmission systems. The CLSA consists of a polarization- and phase-diverse receiver and analysis software enabling simultaneous measurement of modulation formats important to advanced fiber communications, including polarization-multiplexed (PM-) QPSK. The CLSA software performs all calibration and processing functions to enable real-time burst-mode constellation diagram display, eye-diagram display, Poincaré sphere, and bit-error detection.

OM4000 Series Instrument Flexibility

The OM4000 is unique in the industry in that it works with both real-time and equivalent-time oscilloscopes. This unprecedented architecture allows the user to get the benefits of either acquisition format all with a single CLSA. For customers whose analysis requires a high sample rate, using the CLSA with a real-time oscilloscope, such as the DPO73304D, may be optimal. For customers whose analysis requires high vertical resolution – such as modulator characterization – an equivalent-time oscilloscope may be the most beneficial. Working with a Tektronix oscilloscope solution of sufficient bandwidth, bit rates exceeding 240 Gb/s can be analyzed.

OM4000 Series User Interface (OUI)

Figure 1 – OM4000 User Interface (OUI) showing color-grade graphics options. Symbols can also be colored to a key indicating prior state. Data shown is 112 Gb/s PM-QPSK.


Figure 2 – OM4000 User Interface (OUI) showing display of select equivalent-time measurements.

The common thread through the Coherent Lightwave Signal Analyzer product line is the OUI which governs the operation and display of data. This OUI can also be ordered separately without the OM4000 for analysis purposes with another coherent receiver system. The data-capture and analysis only version of the OUI software is called the OM1106. Color-grade, persistence, and color-key options are available to help you visualize the data. In Figure 1, the horizontal transitions are more rare than the vertical transitions due to the relative timing of the IQ data sequence (upper middle of Figure 1). The other polarization constellation is shown in color grade with only the symbol points (lower middle). Color grade is also available for the eye diagram (bottom right).

Interaction between OM4000 Series User Interface (OUI) and MATLAB

Figure 3 – Illustration of data flow under control of the OUI.

The OUI takes information about the signal provided by the user together with acquisition data from the oscilloscope and passes them to the MATLAB workspace, shown in Figure 3. A series of MATLAB scripts are then called to process the data and produce the resulting field variables. The OUI then retrieves these variables and plots them. Automated tests can be accomplished by connecting to the OUI or by connecting directly to the MATLAB workspace. The user does not need any familiarity with MATLAB; the OUI can manage all MATLAB interactions. However, advanced users can access internal functions through the MATLAB interface. This can be used to create user-defined demodulators and algorithms, or for custom analysis visualization.

Signal Processing Approach

Figure 4 – Data flow through the “Core Processing” engine.

For real-time sampled systems, the first step after data acquisition is to recover the clock and re-time the data at 1 sample per symbol at the symbol center for the polarization separation and following algorithms (shown as upper path in Figure 4). The data is also re-sampled at 10X the baud rate (user settable) to define the traces that interconnect the symbols in the eye diagram or constellation (shown as the lower path). The clock recovery approach depends on the chosen signal type. Laser phase is then recovered based on the symbol-center samples. Once the laser phase is recovered, the modulation portion of the field is available for alignment to the expected data for each tributary. At this point bit errors may be counted by looking for the difference between the actual and expected data after accounting for all possible ambiguities in data polarity. The polarity with the lowest BER is chosen. Once the actual data is known, a second phase estimate may be performed to remove errors that may result from a laser phase jump. Once the field variables are calculated, they are available for retrieval and display by the OUI. At each step the best algorithms are chosen for the specified data type, requiring no user intervention unless desired.

Get Up and Running Fast with the Easy-to-Use OUI

Figure 5 – QAM Measurements on the OM4000 User Interface (OUI).

The user interface for the Coherent Lightwave Signal Analyzer is called the OUI. The OUI allows you to easily configure and display your measurements and also provides a means of software control for 3rd-party applications using WCF or .NET communication. It can also be controlled from MATLAB or LabVIEW. A QAM measurement setup is shown in Figure 5. The plots can be moved, docked, or resized. You can close or create plots to display just the information you need.

Figure 6 – Annotated measurement table from OM4000 User Interface (OUI).

In addition to the numerical measurements provided on the plots, the measurements are also summarized on the Measurements window where statistics are also displayed. An example of some of these measurements is shown in Figure 6.

Make Adjustments Faster

The OUI is designed to collect data from the oscilloscope and move it into the MATLAB workspace with extreme speed to provide the maximum data refresh rate. The data is then processed in MATLAB and the resulting variables are extracted for display.

Take Control with Tight MATLAB Integration

Since 100% of the data processing is done in MATLAB, it is easy for test engineers to probe into the processing to understand each step along the way. R&D labs can also take advantage of the tight MATLAB integration by writing their own MATLAB algorithms for new techniques under development.

Use the Optimum Algorithm

Don’t worry about what algorithm to use. When you select a signal type in our OUI, for example, PM-QPSK, the optimal algorithm is applied to the data for that signal type. Each signal type has a specially designed signal processing approach that is best for the application. This means that you can get results right away.

Don’t Get Stymied by Laser Phase Noise

Signal processing algorithms designed for electrical wireless signals don’t always work well with the much noisier sources used for complex optical modulation signals. Our robust signal processing methods tolerate enough phase noise to even make it possible to test signals which would traditionally be measured by differential or direct detection such as DQPSK.

Find the Right BER

Q-plot.

Our Q-plots are a great way to get a handle on your data signal quality. Numerous BER measurements vs. decision threshold are made on the signal after each data acquisition. Plotting BER vs. decision threshold shows the noise properties of the signal. Gaussian noise will produce a straight line on the Q-plot. The optimum decision threshold and extrapolated BER are also calculated. This gives you two BER values: the actual counted errors divided by the number of bits counted, as well as the extrapolated BER for use when the BER is too low to measure quickly.

Constellation Diagrams

Constellation Diagram.

Once the laser phase and frequency fluctuations are removed, the resulting electric field can be plotted in the complex plane. When only the values at the symbol centers are plotted, this is called a Constellation Diagram. When continuous traces are also shown in the complex plane, this is often called a Phase Diagram. Since the continuous traces can be turned on or off, we refer to both as the Constellation Diagram. The scatter of the symbol points indicates how close the modulation is to ideal. The symbol points spread out due to additive noise, transmitter eye closure, or fiber impairments. The scatter can be measured by symbol standard deviation, error vector magnitude, or mask violations.

Measurements made on constellation diagrams are available on the “fly-out” panel associated with each graphic window. The measurements available for constellations are described below.

<?Pub _bookmark Command="[pmark]" ?>Constellation Measurements

Measurement	Description
Elongation	The ratio of the Q modulation amplitude to the I modulation amplitude is a measure of how well balanced the modulation is for the I and Q branches of a particular polarization’s signal
Real Bias	Expressed as a percent, this says how much the constellation is shifted left or right. Real (In-phase) bias other than zero is usually a sign that the In-phase Tributary of the transmitter modulator is not being driven symmetrically at eye center
Imag Bias	Expressed as a percent, this says how much the constellation is shifted up or down. Imaginary (Quadrature) bias other than zero is usually a sign that the Quadrature Tributary of the transmitter modulator is not being driven symmetrically at eye center
Magnitude	The mean value of the magnitude of all symbols with units given on the plot. This can be used to find the relative sizes of the two Polarization Signals
Phase Angle	The transmitter I-Q phase bias. It should normally be 90 degrees
StdDev by Quadrant	The standard deviation of symbol point distance from the mean symbol in units given on the plot. This is displayed for BPSK and QPSK
EVM (%)	The RMS distance of each symbol point from the ideal symbol point divided by the magnitude of the ideal symbol expressed as a percent
EVM Tab	The separate EVM tab shown in the right figure provides the EVM% by constellation group. The numbers are arranged to correspond to the symbol arrangement. This is ideal for setting Transmitter modulator bias. For example, if the left side groups have higher EVM than the right side, this usually means that the In-phase Transmitter modulator bias needs to be adjusted to drive the negative rail harder
Mask Tab	The separate Mask tab shown in the right figure provides the number of mask violations by constellation group. The numbers are arranged to correspond to the symbol arrangement. The mask threshold is set in the Engine window and can be used for pass/fail transmitter testing

Color Features

Color Grade Constellation.


Color Grade with fine traces.

The Color Grade feature provides an infinite persistence plot where the frequency of occurrence of a point on the plot is indicated by its color. This mode helps reveal patterns not readily apparent in monochrome. Note that the lower constellation groups of the example below have higher EVM than the top groups. In most cases this indicates that the quadrature modulator bias was too far toward the positive rail. This is not evident from the crossing points which are approximately correct. In this case an improperly biased modulator is concealing an improperly biased driver amp.

Color Key Constellation – If the prior symbol was in Quadrant 1 (upper right) then the current symbol is colored Yellow. If the prior symbol was in Quadrant 2 (upper left) then the current symbol is colored Magenta. If the prior symbol was in Quadrant 3 (lower left) then the current symbol is colored Light Blue (Cyan). If the prior symbol was in Quadrant 4 (lower right) then the current symbol is colored Solid Blue.

Color Key Constellation Points is a special feature that works when not in Color Grade. In this case the symbol color is determined by the value of the previous symbol. This helps reveal pattern dependence. Here it shows that pattern dependence is to blame for the poor EVM on the other groups. The modulator nonlinearity would normally mask this type of pattern dependence due to RF cable loss, but here the improper modulator bias is allowing that to be transferred to the optical signal.

Eye Diagrams

Field Eye Diagram.

Eye diagram plots can be selected for appropriate modulation formats. Supported eye formats include Field Eye, which is simply the real part of the phase trace in the complex plane, Power Eye which simulates the eye displayed with a Tektronix oscilloscope optical input, and Diff-Eye, which simulates the eye generated by using a 1-bit delay-line interferometer. As with the Constellation Plot you can right-click to choose color options as well. The Field Eye diagram provides the following measurements:

<?Pub _bookmark Command="[pmark]"?>Field Eye Measurements

Measurement	Description
Q (dB)	Computed from 20 × Log10 of the linear decision threshold Q-factor of the eye
Eye Height	The distance from the mean 1-level to the mean 0-level (units of plot)
Rail0 Std Dev	The standard deviation of the 0-level as determined from the decision threshold Q-factor measurement
Rail1 Std Dev	The standard deviation of the 1-level as determined from the decision threshold Q-factor measurement

In the case of multilevel signals, the above measurements will be listed in the order of the corresponding eye openings in the plot. The top row values correspond to the top-most eye opening.

The above functions involving Q-factor use the decision threshold method described in the paper by Bergano*⁴. When the number of bit errors in the measurement interval is small, as is often the case, the Q-factor derived from the bit error rate may not be an accurate measure of the signal quality. However, the decision threshold Q-factor is accurate because it is based on all the signal values, not just those that cross a defined boundary.

*⁴N.S. Bergano, F.W. Kerfoot, C.R. Davidson, “Margin measurements in optical amplifier systems,” IEEE Phot. Tech. Lett., 5, no. 3, pp. 304-306 (1993).

Additional Measurements Available for Non-offset Formats

Measurement	Description
Overshoot	The fractional overshoot of the signal. One value is reported for the tributary, and for a multilevel (QAM) signal it is the average of all the overshoots
Undershoot	The fractional undershoot of the signal (overshoot of the negative-going transition)
Risetime	The 10-90% rise time of the signal. One value is reported for the tributary, and for a multilevel (QAM) signal it is the average of all the rise times
Falltime	The 90-10% fall time of the signal
Skew	The time relative to the center of the power eye of the midpoint between the crossing points for a particular tributary
Crossing Point	The fractional vertical position at the crossing of the rising and falling edges

Measurements vs. Time

Errored symbol in Measurement vs. Time plot.

In addition to the eye diagram, it is often important to view signals versus time. For example, it is instructive to see what the field values were doing in the vicinity of a bit error. All of the plots which display symbol-center values will indicate if that symbol is errored by coloring the point red (assuming that the data is synchronized to the indicated pattern). The Measurement vs. Time plot is particularly useful in this way as it helps to distinguish errors due to noise, pattern dependence, or pattern errors.

3D Visualization Tools

Complex-modulation signals are inherently 3D since in-phase and quadrature components are being changed vs. time. The 3D Eye Diagram provides a helpful combination of the Constellation and Eye diagrams into a single 3D diagram. This helps to visualize how the complex quantity is changing through the bit period. The diagram can be rotated and scaled.

Also available in 3D is the Poincaré Sphere. The 3D view is helpful when viewing the polarization state of every symbol. The symbols tend to form clusters on the Poincaré Sphere which can be revealing to expert users. The non-normalized Stokes Vectors can also be plotted in this view.

Analysis Controls

The Analysis Controls window allows you to set parameters relevant to the system and its measurements.

<?Pub _bookmark Command="[pmark]" ?>Analysis Parameters

Parameter	Description
Frequency	Clock recovery is performed in software, so only a frequency range of expected clock frequencies is required
Signal Type	The signal type (such as PM-QPSK) determines what algorithms will be used to process the data
Data Patterns	Specifying the known PRBS or user pattern by physical tributary permits error counting, constellation orientation, and two-stage phase estimation

MATLAB window.

User patterns may be assigned in the MATLAB window shown here. The data pattern can be input into MATLAB or found directly through measurement of a high SNR signal.

Signal Spectra

Signal Spectrum window.


Laser Phase Spectrum window.

An FFT of the corrected electric field vs. time can reveal much about the data signal. Asymmetric or shifted spectra can indicate excessive laser frequency error. Periodicity in the spectrum shows correlation between data tributaries. The FFT of the laser phase vs. time data can be used to measure laser phase noise.

Poincaré Sphere

Poincaré Sphere window.

Polarization data signals typically start out well aligned to the PM-fiber axes. However, once in standard single mode fiber, the polarization states will start to drift. However, it is still possible to measure the polarization states and determine the polarization extinction ratio. The software locks onto each polarization signal. The polarization states of the two signals are displayed on a circular plot representing one face of the Poincaré sphere. States on the back side are indicated by coloring the marker blue. The degree of orthogonality can be visualized by inverting the rear face so that orthogonal signals always appear in the same location with different color. So, Blue means back side (negative value for that component of the Stokes vector), X means X-tributary, O means Y-tributary, and the Stokes vector is plotted so that left, down, blue are all negative on the sphere.

InvertedRearFace - Checking this box inverts the rear face of the Poincaré sphere display so that two orthogonal polarizations will always be on top of each other.

Impairment Measurement and Compensation

When studying transmission implementations, it is important to be able to compensate for the impairments created by long fiber runs or optical components. Chromatic Dispersion (CD), and Polarization Mode Dispersion (PMD) are two important linear impairments that can be measured or corrected by the OM4000 software. PMD measurement is based on comparison of the received signal to the back-to-back transmitter signal or to an ideal signal. This produces a direct measure of the PMD instead of estimating based on adaptive filter behavior. The user can specify the number of PMD orders to calculate. Accuracy for 1st-order PMD is ~1 ps at 10 Gbaud. There is no intrinsic limit to the CD compensation algorithm. It has been used successfully to compensate for many thousands of ps/nm.

Recording and Playback

Workspace Record and Playback.

You can record the workspace as a sequence of .MAT files using the Record button in the Offline ribbon. These will be recorded in a default directory, usually the MATLAB working directory, unless previously changed.

You can play back the workspace from a sequence of .MAT files by first using the Load button in the Offline Commands section of the Home ribbon. Load a sequence by marking the files you want to load using the Ctrl key and marking the filenames with the mouse. You can also load a contiguous series using the Shift key and marking the first and last filenames in the series with the mouse. Use the Run button in the Offline Commands section of the Home ribbon to cycle through the .MAT files you recorded. All filtering and processing you have implemented is done on the recorded files as they are replayed.

Multi-carrier Superchannel Support

Multi-carrier setup.


Multi-carrier measurements.


Superchannel spectrum.

Even as 100G coherent optical systems are being deployed, architectures for 400G and beyond are being proposed and developed. One architecture gaining prominence is the “superchannel.” The configurations of superchannels vary considerably. Some proposals call for 400G to be achieved by 2 carriers of DP-16QAM. Other proposals are for 500 Gb/s consisting of 10 or more carriers of DP-QPSK. Some of these carriers are arranged on a standard ITU carrier grid, while others support 12.5 GHz “grid-less” layouts. Clearly, flexible test tools are needed for such next-generation systems. Option MCS to the OM4106D and OM1106 offers the complete flexibility to carrier out such tests.

User-Definable Superchannels

For manufacturers getting a jump on superchannels, or researchers investigating alternatives, user-definable superchannel configurations are a must. Option MCS allows the user to set up as many carriers within the superchannel definition as necessary. Each carrier can have an arbitrary center frequency; no carrier grid spacing is imposed. The carrier center frequencies can be set as absolute values (in THz) or as relative values (in GHz). Typically, the OUI will re-tune the OM4106D local oscillator for each carrier. However, in cases where multiple carriers may fit within the scope bandwidth, multiple carriers can be demodulated in software from a common local oscillator frequency. The user is given the flexibility to specify the preferred local oscillator frequency for each carrier.

Automated Measurements

Once the superchannel has been configured, the system is capable of taking measurements at each channel without further intervention by the user. The OUI automatically tunes the OM4106D local oscillator, takes measurements at that channel, re-tunes to the next channel, and so forth until measurements of the entire superchannel have been taken. Results of each channel are displayed in real-time and persist after all measurements are made for easy comparison.

Integrated Measurement Results

All of the same measurement results that are made for single channels are also available for individual channels in a superchannel configuration. Additionally, multi-carrier measurement results are available side-by-side for comparison between channels. Visualizations such as eye diagrams, constellation diagrams, and optical spectrum plots can be viewed a single channel at a time, or with all channels superimposed for fast comparison. For separating channels in a multi-carrier group, several different filters can be applied, including raised cosine, Bessel, Butterworth, Nyquist, and user-defined filters. These filters can be any order or roll-off factor and track the signal frequency.