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Who would have guessed that Cisco had released a book on the subject in 2003. If you can get your hand on “WDM Network Design” there is a chapter that explains what is OSNR and why it is important to measure it.  The page I have linked to explains the relationship of OSNR to the Q-factor.

Excellent whitepaper on OSNR and ROADM.  The “shoulder method” is explained in an easy way.

Several things are very clear about ROADM-based networks regarding the ability to measure the
OSNR:
• The typical signal shape is absent on a ROADM-based network.
• The OSNR measurement of one filter differs from that of a cascade of multiple ROADMs.
• A deployed ROADM-based network (typically 15 to 20 ROADMs cascaded) offers a wide
variation of filter shapes and signals.
• Choosing an OSA to install, troubleshoot, and maintain a ROADM-based network requires
getting the right answer.

http://www.jdsuniphasecorp.com/product-literature/osnrroadm_wp_lab_tm_ae.pdf

The optical signal to noise ratio is a key parameter for each channel of a Dense Wavelength Division Multiplexing (DWDM) optical network.

To optimize the use of existing fiber and network equipment, service providers increase the modulation frequency from 10 Gbit/s to 40 Gbit/s. They may also reduce the channel spacing from 100 GHz to 50 GHz.

Network configuration evolved to Reconfigurable Optical Add Drop Multiplexer (ROADM) which led to a different noise level within each individual channel.

The In-band OSNR as opposed to the Out-of –band OSNR requires to be able to measure accurately the noise energy level buried under the total signal + noise energy.

I-OSNR measurement solution is mainly related to the fact that the signal is essentially polarized and the noise is highly unpolarized. Test equipments using a polarization nulling technique with appropriate signal processing and analysis capability are available for lab or field deployment.

Only a few companies develop equipment capable of meeting this challenge.

The product selection criteria will be mainly based on the following, in no particular order:

• Accuracy
• Speed
• Repeatability
• Cost

The primary task in evaluating any I-OSNR test equipment product is to carefully control the noise and signal source and having the test configuration reflecting as close as possible the actual signal & noise characteristics.

You may want to go beyond the manufacturer proposed test configuration and come up with your own adapted to your needs.

To assist you in this task, a series of links are listed below to provide better understanding of the different measurement techniques related to the product available.

» Read more: In-band Optical Signal to Noise Ratio (I-OSNR)

Movie showing the basis of the OSNR measurement solution with an AQ6370 Optical Spectrum Analyzer.