Todays Wavemeters

The different types of wavemeters for very accurate measurements of laser wavelengths are based on modifications of the Michelson interferometer [184], the Fizeau interferometer [185], or on a combination of several Fabry-Perot interferometers with different free spectral ranges [186-188]. The wavelength is measured either by monitoring the spatial distribution of the interference pattern with photodiode arrays, or by nsing traveling devices with electronic counting of the interference 

The signal Unes to both counters are simultaneously opened at the time to when the detector PD2 just delivers a trigger signal. Both counters are simultaneously stopped at the time h when PD2 has reached the preset number A/o. From 

The unknown fractional number e 2 takes into account that the trigger signals from PDl, which define the start and stop times to and h (Fig. 4.69), may not exactly coincide with the pulse rise times in channel 2. The two worst cases are shown in Fig. 4.70. For case a, the trigger pulse at to just misses the rise of the signal pulse, but the trigger at t just coincides with the rise of a signal pulse. This means that the 

For M = 100 the limitation of the accuracy by the counting error due to the unknown fractional number e is reduced by a factor of 100. 

Instead of the phase-locked loop a coincidence curcuit may be employed. Here the signal paths to both counters are opened and closed at selected times to and 0, when both trigger signals from PD2 and PDl coincide within a small time interval, say 10 s. Both techniques reduce the counting uncertainty to a value below 2 x 10- . 

Traveling Michelson interferometer for accurate measurements of wavelengths of single-mode cw lasers 

For a maximum optical path difference As = 4 m, the number of counts for A = 500 nm is 8 x 10, which allows a precision of about 10, if the counting error is not larger than 1. Provided the signal-to-noise ratio is sufficiently 

Signal sequences in the two detection channels of the traveling Michelson wavemeter. The grey signal pulses are not counted 

The corner-cube reflectors guarantee that the incoming light beam is always reflected exactly parallel to its indicent direction, irrespective of slight 

The maximum relative uncertainty of the absolute vacuum wavelength Xx can be written as a sum of five terms  

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