Keysight Technologies 81618A User Manual

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Keysight Technologies 81618A User Manual Manual pdf 33 page image

Keysight 81618A/9A and Keysight 81620B/3B/4B/6B/8B Optical Heads User’s Guide 33Specifications 3Relative uncertainty due to speckle noiseSpecifies the uncertainty of the power reading caused by opticalinterference (see the note) between different optical paths within thepower meters optical assembly when using a coherent source.Conditions: constant wavelength, constant power level, angled connectoras specified, linewidth of source as specified, temperature as specified.Measurement: In contrast to the spectral ripple definition the sourcewavelength isn’t stepped when measuring speckle noise.Relative uncertainty (spectral ripple) due to interferenceUncertainty of power reading when using a coherent source, due to awavelength-dependent periodic change of the power meter's responsivitycaused by optical interference [1] between reflective interfaces within thepower meter's optical assembly.NOTE Spectral ripple is measured by stepping the source wavelength over thewavelength range specified.NOTE Incoming light to the integrating sphere undergoes numerous internalreflections (as many as several hundred) prior to hitting the detector. Ifthe source is sufficiently coherent, a complicated interference pattern(speckle pattern) appears spread over the whole sphere volume. Slightchanges of the ambient conditions (temperature, vibration, shock) affectparticular optical paths which consequently changes the speckle patternand results in an instability of the head power reading (speckle noise).NOTE Changes in the source wavelength have a similar effect on speckle noiseas environmental changes, because they cause fluctuations in theinterference pattern. If the source coherence length is small compared tothe effective path length, the interference pattern disappears andrelative uncertainty due to speckle noise becomes negligible.