Calibration interval for Optical Spectrum Analyzer

Do I need to send my Optical Spectrum Analyzers back to Yokogawa for calibration, and how often? Could I simply use the built-in source to perform the calibration?

If you do NOT physically move the OSA a lot - you should be OK. If you have an accurate wavelength meter, you can use it to check the accuracy. If you are testing to a national or worldwide standard, like a CSA specification, then you should consider calibrating your OSA once per year.