As operators roll out high-speed optical transmission systems, they’ll need capable optical spectrum analyzers to measure critical parameters such as OSNR, and they might have to test an alphabet soup of optical devices, including DFB-LDs, EDFAs, FP-LDs, LEDs, and VCSELs (see Glossary).
Wolfgang Moench, senior product line manager for fiber optics at JDSU, said, “xWDM is the enabler for high-speed optical transmission systems. An OSA is the key instrument to measure power, wavelength, and OSNR. A fast OSA measurement of the OSNR is used to predict the bit error rate of the system.” He added, “NEMs and operators use the OSA during installation, turn-up, maintenance, and troubleshooting of DWDM systems to check channel power, channel wavelength, channel spacing, OSNR, gain tilt, and flatness.”
Francis Audet, advisor in the CTO office at EXFO, cited several trends driving the need for innovation in optical telecom test. “The most important one, applicable to more than just OSAs, is onboard intelligence,” he said. “Technology and networks are getting more and more complicated while human resources are scarce and less trained than a few years back. This is a combination that spells disaster. This is why test instruments that can set up themselves and give complete diagnostics instead of only a value are crucial for the industry. This is true for the simple fiber inspection probe to the popular OTDR and all the way up to advanced OSAs.”
EXFO, he said, “…also is tapping into size, cost, and multitasking requirements that have become important trends in the industry: Customers are requiring smaller, more powerful, more versatile, and smarter units.”
400G at our doorsteps
Of course, transmission speeds remain critical. 400G, Audet said, is at our doorsteps, and it will be transmitted using super-channels and a flexible channel grid. “These are two new attributes that totally change how, for example, OSAs will function,” he said. “Innovation is required to have a solution address this future challenge.”
Moench at JDSU explained, “High-speed coherent networks at 100G and higher are using the polarization multiplexing technique, which presents special challenges for measuring the OSNR.” Traditional IEC and polarization-nulling methods do not work (as outlined in the JDSU document “JDSU On/Off Method for Pol Mux Signals”), and the on/off test method is only applicable for out-of-service OSNR tests, whereas, he said, “All manufacturers and operators are asking for an in-service, in-band OSNR method for testing 100G coherent systems.”
Further, Moench said, “NG networks going toward super-channel 400G or even 1T transmission using Nyquist channels need new OSAs having a much better resolution to measure the characteristics of the super-channels. OSAs with a resolution bandwidth 10 to 20 times better than current free space optics OSAs are needed.”
To meet emerging needs, JDSU offers the OSA-610, which Moench described as “a new design based on a coherent detector” that meets the measurement needs of new 400G+ super-channel systems. In addition, he said, JDSU has a full range of optical spectrum analyzers ranging from compact to high performance. Specific instruments include the compact OSA-110M and OSA-110H; the high-performance OSA-500 (Figure 1), OSA-501, and OSA-500M; and the in-band/ROADM OSA-110R and OSA-500RS.
Figure 1. MTS-8000 platform with OSA-500 module
Courtesy of JDSU
JDSU, Moench said, was the first to offer a channel isolation function, and the company’s OSA-500x portfolio also includes a built-in physical-constant wavelength calibrator to ensure wavelength accuracy over the instrument’s life. “In addition,” he said, “JDSU was first to offer an OSA measuring the ‘true’ OSNR in ROADM networks—in-band OSNR based on polarization nulling”—in the OSA-110R and OSA-500RS. He added, “JDSU also can measure in-band OSNR in coherent systems with polarization multiplexing transmission signals such as 100G” (using the on/off method).
Michael Kwok, product marketing manager for optical test and measurement at Yokogawa Corp. of America, said Yokogawa has two models of a telecom band OSA with a 600-nm to 1,700-nm range. “The high-performance AQ6370D-20 (Figure 2) adds better wavelength accuracy and dynamic range to the specs of our AQ6370D-10 model,” he said.
Kwok added, that in addition to introducing eight new models in the past eight years, Yokogawa offers a unique noncontact free-space optical input port that allows the use of both multimode and single-mode fibers and flat and angled connectors, giving users “…the peace of mind of knowing that there is no internal input connector to clean or damage.”
Kwok said, “Our OSAs are designed to address the demands for both R&D and manufacturing by providing 0.02-nm resolution, C-band wavelength accuracy of 0.01 nm, a wavelength range from 600 nm to 1,700 nm, and very fast sweeps—especially with the 2x speed mode.”
Figure 2. AQ6370D-20 optical spectrum analyzer
Courtesy of Yokogawa
Yokogawa provides 12 built-in analysis routines commonly used for telecom testing, Kwok said, including automated routines for evaluating WDM (OSNR), spectral width/notch, SMSR, PMD, and optical power. The routines enable go/no-go analysis and target EDFA, DFB-LD, FP-LD (VCSEL), optical-filter, and LED devices. “We also offer optional remote-viewer software to provide a duplicate GUI interface on a PC screen to control the OSA using a PC remotely,” he said.
Hiroshi Goto, business development manager at Anritsu, said, “Engineers designing and developing optical devices and networks now require a flexible instrument providing a broad range of settings that enables accurate OSNR measurements.”
The company’s MS9740A, he said, with its flexible platform and variety of options, meets this need by providing a variety of features and settings to meet specific customer requirements. The MS9740A (Figure 3), which covers 600 nm to 1,750 nm, addresses the need for more accurate OSNR measurements of WDM signals and LD chips and modules. It has a signal integration function, he said, “that incorporates a proprietary algorithm so users can more accurately estimate noise levels, even if the noise is not flat and the spectrum edges appear to overlap at adjacent channels. This function allows the normalization of noise bandwidth to be determined by the user—either by each WDM channel or other user-specified parameter—thereby allowing for accurate and repeatable OSNR measurements to be made, compared to traditional polarization nulling techniques.
Figure 3. MS9740A benchtop optical spectrum analyzer
Courtesy of Anritsu
Goto added that the MS9740A has wavelength sweeping time of <0.2 s/5 nm to help reduce measurement and inspection times for improved production efficiency. “This lowers cost-of-test and increases production throughput,” he said, “which are key factors for manufacturers of active optical devices.” He also said that the MS9740A is the lightest benchtop OSA on the market (15 kg), and it consumes less than 75 VA.
The MS9740A, Goto said, supports multiple application measurement modes (for a device such as a DFB-LD, FP-LD, LED, or multichannel optical amplifier and to make PMD and WDM measurements, for example).
Speed also is an issue. “Fast evaluation of optical devices requires short inspection times using high-efficiency measuring equipment,” Goto said. “The MS9740A features a WDM filter analysis function supporting group display for optical bandpass filters, such as WSS and WDM filter devices. The WDM filter analysis function allows efficient evaluation of optical bandpass filter transmittance characteristics including signal level, peak signal number, signal wavelength, spacing, pass band, and ripple.” He said the MS9740A Trace Mode function supports measurements using optical switches to measure DUT insertion loss through waveform difference comparison. The instrument can save the results in one file and display up to 10 waveforms simultaneously on one screen.
“When evaluating and measuring optical devices, it is important to suppress the effect of reflections at the optical input section,” Goto added. “The MS9740A achieves maximum reflection attenuation (return loss) of 35 dB using a fiber input structure for high-accuracy spectrum measurements.”
An array of OSAs
Audet at EXFO said, “We offer an array of OSAs as there are many possible applications. For access networks and mobile backhaul networks, the wavelength density is less and often transmitted by CWDM. These networks have short distances and therefore often are not amplified. These two characteristics decrease the need of high-end OSAs as the wavelength resolution and optical rejection ratio do not need to be of very high performance.” In addition, he said, because of the absence of amplification, little noise is present in the network (because amplifiers are the most common source of noise). Consequently, OSNR measurement is not often required. If it is required, only limited performance is necessary.
“On the other end of the application spectrum,” Audet continued, “we have very high-density, high-speed, ultra-long-haul networks. These require superior wavelength resolution, optical rejection ratio, and OSNR measurement capabilities.” OSNR, he said, is the most important measurement these OSAs can perform. “What’s more,” he added, “the size, portability, and field ruggedness cannot be neglected.” An example of a portable instrument is the company’s FTB-2 Pro (Figure 4). He added, “All of our OSAs have a multitude of intelligent post-processing optional applications.”
Figure 4. FTB-2 Pro optical spectrum analyzer
Courtesy of EXFO
Audet cited some key features of the company’s OSAs. “First, our OSAs offer a level of intelligence called WDM Investigator,” he said. “This is a dashboard of all the potential issues affecting the quality of transmission. Power, wavelength, and OSNR may or may not be in-spec. If they are, there might be something else going on in a network that a normal OSA will not detect. If they are not in-spec, you want to know how to fix the issue. WDM Investigator goes beyond the numerical values and gives a detailed diagnostic per channel, informing users of a potential presence or threat of PMD, nonlinear effects, crosstalk, and much more.”
Second, he said, today’s high-speed transmissions (including most 40G and all 100G transmissions) are performed not as on-off keying, as is the case with slower transmissions, but with DPQPSK. “Our solutions are the only field OSAs today capable of measuring OSNR on such a transmission scheme, making them fully compatible with all networks being currently deployed,” he said.
Manufacturers, NEMs, and operators will continue to make demands on test equipment makers as optical transmission speeds rise. Kwok of Yokogawa concluded, “In addition to increasing data rates for 100G and beyond, we are seeing demands for in-band OSNR, varying modulation formats, recirculating loops, and multimode and multicore fiber testing.”