The Michelson-Wavemeter

These corner-cube reflectors guarantee that the incoming light beam is always reflected exactly parallel to its indicent direction, irrespective of slight misalignments or movements of the travelling reflector. The two partial beams (BS1-T1-M3-M4-T2-BS2 and BS1-P-T3-P-BS2) for the reference laser interfere at the detector PDl, and the two beams BS2-T2-M4-M3-T1-BS1 and BS2-P-T3-P-BS1 from the unknown laser interfere at the 

The signal lines 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 tj when PD2 has reached the preset number Nq. From 

The unknown fractional number e 2 takes into account that the trigger signals from PD 1 may not exactly coincide with the start and stop times to and tj. 

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 at the time t(, and closed at tj, when both trigger signals from PD2 and PDl coincide within a 

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