ISE Magazine, August 2019
AUGUST 2019 WWW ISEMAG COM 77 FIBER EXPERT DWDM systems during commissioning and troubleshooting OSAs provide a visual display of optical power versus wavelength by scanning the spectrum of an input signal which allows OSNR measurements to be made in minutes OSAs can measure rise and fall times of sources and detectors and they can also measure spectral width of sources and amplifier characteristics They can also predict if a BER test will pass or fail allowing technicians to troubleshoot problems before BER tests are made Minimum OSNR values are specified by equipment manufacturers to assure their equipment functions properly These values are normally related to a BER value For 10 Gigabit systems greater than 15 dB or 18 dB is normal at the receiver And for 100 Gigabit systems the values are greater than 12 dB or 15 dB at the receiver In addition a 3 dB safety margin is normally designed into the system above the minimum OSNR value This helps address amplifier aging problems which can cause a degradation of the OSNR value For instance if the min OSNR value at Rx is 13 dB the system should be designed with a value of 16 dB at the Rx One thing to keep in mind is that BER testing will not reveal if the OSNR safety margin is met or not Todays systems continue to grow in complexity With advanced modulation formats higher speeds and increased number of wavelengths with tighter spacings the benefits of OSNR testing increases network uptime OSNR testing also helps with preventive maintenance of Figure 2 Different types of systems require different OSNR test methods Image courtesy of EXFO systems by reducing troubleshooting time for noise issues and identifying defective ROADMs and amplifiers Finally OSNR testing can predict the upgrade potential for 100 G networks to higher speeds to confirm whether they can achieve the required OSNR thresholds required These measurements help confirm that the safety margin is met and therefore that the risk of network failure is reduced while also saving expenses in SLA penalties The Net Net of OSNR The need to understand OSNR is critical for future system designs Especially those migrating to higher speeds and tighter channel spacings It is also key when testing and troubleshooting the systems and elements that can impact signal quality For those attending ISE EXPO 2019 be sure to attend a special session on Tuesday afternoon September 24th titled Advanced Fiber Optic Technologies for the Coming Decade It addresses OSNR DWDM Advanced Modulation Formats and Fiber Characterization n
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