|The ABC’s of Optical Transceiver Test
By Greg LeCheminant, Agilent Technologies
Almost every communications technology from a PCB to a glass fiber needs higher data throughput. When designing a communications system for high data rates, you must consider many tradeoffs. Two critical issues are system cost and the distance between the transmitter and receiver. The transmission loss and high information capacity of optical fiber is far superior to that of PCB traces and cables. Historically, the relative cost of optical systems has been high. As fiber optic technology matures, the cost per bit pushes lower. Even cost sensitive consumer applications may now use optical transceivers.
To understand how to test optical communications components, you should approach it from a system-level perspective. When optical communications systems were first deployed, verifying the physical-layer performance was straightforward. The entire network was usually installed and owned by a single company. If the system worked, extensive testing of the subcomponents was unnecessary. Today, however, most optical networks use components that may come from a variety of suppliers. Thus, you need a specification and test strategy to meet system-level requirements regardless of the source of network components.
The bottom line for any digital communications system, whether it’s a telecommunications link spanning the ocean or an interface bus within a laptop computer, is that bits are transmitted to a receiver with few if any mistakes being made. When a transmitter is paired with a receiver (through a fiber) and the system doesn't achieve your desired bit-error-ratio (BER), you must determine if the fault lies with the transmitter or receiver. Perhaps both are faulty. A low-quality transmitter can compensate for by a low-quality receiver (and vice versa). A well-designed optical link will balance the cost/performance burden at both ends of the fiber. Specifications should guarantee that any receiver will interoperate with a worst-case transmitter, and any transmitter will provide a signal with sufficient quality such that it will interoperate with a worst-case receiver.
Precisely defining ‘worst case’ is often a complicated task. If a receiver needs a minimum level of power to achieve the system BER target, then that level will dictate the minimum allowed output power of the transmitter. If the receiver can only tolerate a certain level of jitter, this will be used to define the maximum acceptable jitter from the transmitter. Transmitter parameters may include wavelength and shape of the output waveform while the receiver may specify tolerance to jitter and bandwidth.
An eye diagram is the common method to view the transmitter waveform and provides a wealth of information about overall transmitter performance. In the eye diagram, all the combinations of data patterns are superimposed on each other over a common time axis, usually less than two bit periods in width. Figure 1 shows a signal with good signal amplitude and low jitter. You can envision how an eye diagram is constructed by drawing the eight possible sequences of a three bit waveform (000, 001,….110, 111) overlapped on a common time axis.
Figure 1. The eye diagram provides a good indication of the quality of a transmitter
Rather than make several measurements to determine the quality of the eye, you can use and eye-mask test. A mask consists of several polygons placed in and around the eye diagram, indicating areas where the waveform shouldn't exist. A “good” waveform will never intersect the mask and will pass the mask test while a “bad” waveform will cross or violate the mask and fail. Stepping back and taking a system level view, an eye that is open indicates that the receiver will easily distinguish a logic 1 level from a logic 0 level. If the eye closes, the likelihood of mistakes (bit errors) increases. Figure 2 shows a waveform that easily passes an eye-mask test.
Figure 2: Masks are placed in the eye diagram to quickly verify the eye is ‘open’. Margin testing is achieved by expanding the size of the mask.
Wide-bandwidth oscilloscopes let you perform optical eye-mask tests. The instrument go by several names, including digital communications analyzers. The oscilloscope can perform the test and determine if any waveform samples fall on the mask. Laser manufacturers are often concerned not only that they pass the mask test with no violations, they also look for measurements with significant margin. Expanding the mask dimensions as large as possible without incurring any mask hits provides the maximum margin.
Most industry standards define a transmitted signal as being non-compliant if any waveform samples violate the mask. This brings up an interesting problem with eye mask testing. Compared to a small number of waveforms, a larger population of waveform samples should provide a more accurate assessment of the transmitter performance. Unfortunately, every eye diagram has random characteristics in both amplitude (noise) and time (jitter). As an oscilloscope collects more data, the likelihood of mask violations increases. In theory, if enough samples are acquired, eventually almost any transmitter will fail a mask test. The probability for mask violations may be very low, but mask testing generally does not account for probabilities. It is an all or nothing test, pass or fail. This leads to the question: How many samples must you acquire to perform the test?
At a minimum, you need enough samples to let the oscilloscope have sufficient data to align the mask to the waveform. Typically, a small population is enough. How much more data should you acquire? If there is significant space (margin) between waveform samples and the mask, then collecting more data will change the result only slightly. The problem occurs when a device barely passes the mask test. Collecting more samples could lead to a failure.
Some recent industry standards, such as IEEE 802.3 ah (a short reach optical standard), IEEE 802.3aq (10 Gb/s transmission using lower quality installed multimode fiber), and IEEE 802.3ba (40 and 100 Gb/s Ethernet) recognize the problem and allow mask failures to occur. They do, however allow only a very small ratio of hits to samples (one failed sample for every 20,000 samples measured). This allows a large population to be collected, possibly yielding a more accurate measurement, without an increased likelihood of failure.
The sampling question leads to a modified version of the classic eye-mask test. An additional step takes place once you've acquired the desired number of samples. An automatic eye-mask margin test lets you determine how far you can expand the mask dimensions before the ratio of mask hits to total waveform samples exceeds the ratio allowed by the standard. In Figure 3, the mask dimensions are expanded until 332 ‘hits’ are encountered. (The hit ratio was set to 1:1000 for easy visibility, with 332000 samples between the crossing points of the eye which the mask is aligned). The mask can be expanded significantly from the standard dimensions, indicative of a very good transmitter.
Figure 3: The mask is automatically magnified from its original dimensions to verify a compliant hit ratio. Mask hits (shown in red) are allowed, but at a very small percentage of the total number of samples.
An oscilloscope's frequency response can strongly impact the shape of the eye-diagram. To achieve consistent results throughout the industry, many instruments use a reference receiver. Eye-mask tests are performed using a test system that has a frequency response that follows a fourth-order Bessel filter response. The 3 dB bandwidth is at 75% of the data rate. For example, a 10 Gb/s reference receiver would have a 7.5 GHz bandwidth. Compared to other filter designs, the Bessel response achieves minimal time domain waveform distortion. The 75% bandwidth is the lowest bandwidth that does not cause vertical closure of the eye. This specific design is commonly called a Bessel-Thomson reference receiver. Industry standards typically define the exact response required to perform transmitter tests. Digital communications analyzers integrate these reference receivers as part of the complete test system.
In addition to measuring the shape of a transmitter's waveform, you should measure signal strength. While an average-power measurement is useful, it does not provide much information about how well a transmitter is carrying information. The average power of a laser that carries no information (constant optical power level due to electrical data signal driving the laser disabled) is virtually the same as a laser that is highly modulated. Extinction ratio is a parameter that indicates how efficiently laser power is converted to information power. Extinction ratio is the ratio of the power level of the logic ‘1’ to the power level of the logic ‘0’. Consider a laser that is turned almost all the way off to transmit a ‘0’ and turned on to transmit a ‘1’. The ratio of ‘1’ to ‘0’ and hence extinction ratio will be high. On the other hand, if significant power is used to transmit a ‘0’, extinction ratio will be low indicating that some laser power is wasted. Extinction ratio is derived from the eye diagram. An oscilloscope can construct histograms in the central 20% of the eye to determine the mean value of the composite ‘1’ bits and the mean value of the composite ‘0’ bits (Figure 4).
Figure 4: Extinction ratio is derived from the eye diagram.
OMA (optical-modulation amplitude) is similar to extinction ratio, except that rather than being the ratio of the ‘1’ to ‘0’ levels, it is the difference. Essentially it is a measurement of the modulation power of the laser. While it can also be derived from the eye diagram, many industry standards extract this parameter from a special square wave pattern that is five ‘0’s followed by five ‘1’s, with the middle ‘0’ and middle ‘1’ being used for the computation.
Which is more important, extinction ratio or OMA? They both indicate modulation power, but from different perspectives. Long-haul transmission systems that employ optical amplifiers shouldn't waste available power while transmitting a ‘0’. A high extinction ratio signal minimizes power loss. Short-haul systems generally minimize the high cost associated with tuning a laser for a high extinction ratio. But, they need to guarantee a minimum separation between ‘1’ and ‘0’ levels. In those cases, OMA has priority.
Testing the receiver side of a transceiver boils down to achieving an acceptably low BER when the receiver is presented with the worst-case signal allowed from the fiber at the end of the network. As systems became more complex, faster, and even cheaper, it became necessary to test receivers using non-ideal signals with different types of impairments. This is commonly referred to as a ‘stressed’ signal. The first types of stressed signals that gained wide acceptance in high speed fiber optic communication were for long-haul applications where the use of Erbium Doped Fiber Amplifiers (EDFA’s) was common. The ASE (amplified spontaneous emission) noise could cause excess errors on receivers that otherwise performed well when tested under typical ideal conditions. Vendors of long-haul network equipment required test signals intentionally degraded by ASE noise. These tests are still in wide use today.
Shorter span systems such as Ethernet and Fibre Channel also use the stressed eye concept. A well-defined combination of impairments is added to an ideal signal and is presented to the receiver under test. However, the signal degradations are dominated by different mechanisms compared to long haul systems. The degradations emulate the worst case noise, attenuation, jitter and ISI (intersymbol interference) that might occur in a system.
Figure 5: A ‘clean’ eye can be precisely degraded to create a stressed eye with modern BERT equipment
Receiver BER measurements are straightforward. Producing a distorted signal is also easy. However, producing a precision distorted signal can be difficult. If the signal distortion is too small, a bad receiver may appear good. If the distortion is too severe, a good receiver may appear bad. Modern BER testers, often called BERTs, have pattern generators that produce calibrated levels of distortion or stress. A stressed signal will typically have the ability to produce a data signal that has precise amounts of timing jitter, bandwidth limiting (inter-symbol interference), and attenuation. Figure 5 shows the difference between a clean eye and a stressed eye.
Most optical receivers employ some form of clock recovery to extract a timing reference for the decision circuit. Clock recovery systems generally have a limit on how fast jitter can be and still be ‘tracked’ and tolerated by the receiver. To verify proper operation of the clock recovery system, standards often require that receivers tolerate both low-frequency and high-frequency jitter. A jitter tolerance template, consisting of specific jitter frequency/amplitude pairs is applied to every receiver. While the standards require receivers to be compliant for all jitter amplitude/frequency pairs on the jitter tolerance curve, the typical manufacturing compliance test focuses on the highest frequency case, as receivers tend to easily tolerate low frequency jitter. A receiver test process that emulates real system conditions helps ensure the interoperability of network components.
Testing transceivers has evolved to accommodate the changes in system architecture and performance. As data rates increase, the basic eye-mask test is still viable, but important changes have shown up in recent standards, are gaining in popularity, and likely will be used in future standards. Receiver testing, using complex ‘stress’ signals are also becoming more common.