added comment on group_delay

9af53fe5 · Peter Jansweijer · 641b11a7 · 9af53fe5
Commit 9af53fe5 authored Jul 08, 2019 by Peter Jansweijer
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Showing with 7 additions and 2 deletions

analyze_sellmeier.py sw/insitu_alpha/analyze_sellmeier.py +7 -2

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--- a/sw/insitu_alpha/analyze_sellmeier.py
+++ b/sw/insitu_alpha/analyze_sellmeier.py
@@ -450,10 +450,15 @@ def group_delay(l, A, B, C, D, E):
  delay             -- <float> group delay value based on selm5 and l
  """

-  # l = lambda is in [nm]. convert to [m] before executing sellmeier fit
+  # l = lambda is in [nm]. convert to [m] before executing sellmeier fit.
+  # Note that Sellmeier is an empirical relationship
+  #(https://en.wikipedia.org/wiki/Sellmeier_equation) where dispersion is 
+  # expressed in ps/nm.km so converting l to [m] is a divide by 10e9 in the
+  # numerator as well as the denominator. The result has to be converted back
+  # to [ps] by multiplying numerator as well as the denominator by 10e9.
  l = l/1e9
  sellmeier_5 = A + B * l**2 + C * l**-2 + D * l**4 + E * l**-4
-  # and convert fitted value back to [nm]
+  # and convert fitted value back to [ps]
  return sellmeier_5 * 1e9
  
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