  As sensitivities improve and more detectors are added to the global network
of gravitational wave observatories, calibration accuracy and precision are
becoming increasingly important. Photon calibrators, relying on power-modulated
auxiliary laser beams reflecting from suspended interferometer optics, enable
continuous calibration by generating displacement fiducials proportional to the
modulated laser power. Developments in the propagation of laser power
calibration via transfer standards to on-line power sensors monitoring the
modulated laser power have enabled generation of length fiducials with improved
accuracy. Estimated uncertainties are almost a factor of two smaller than the
lowest values previously reported. This is partly due to improvements in
methodology that have increased confidence in the results reported. Referencing
the laser power calibration standards for each observatory to a single transfer
standard enables reducing relative calibration errors between elements of the
detector network. Efforts within the national metrology institute community to
realize improved laser power sensor calibration accuracy are ongoing.
