A new bound on excess frequency noise in second harmonic generation in PPKTP at the 10⁻¹⁹ level

We report a bound on the relative frequency fluctuations in nonlinear second harmonic generation. A 1,064 nm Nd:YAG laser is used to read out the phase of a Mach-Zehnder interferometer while PPKTP, a nonlinear crystal, is placed in each arm to generate second harmonic light. By comparing the arm length difference of the Mach Zehnder as read out by the fundamental 1064 nm light, and its second harmonic at 532 nm, we can bound the excess frequency noise introduced in the harmonic generation process. We report an amplitude spectral density of frequency noise with total RMS frequency deviation of 3 mHz and a minimum value of 20 μHz/Hz½ over 250 seconds with a measurement bandwidth of 128 Hz, corresponding to an Allan deviation of 10⁻¹⁹ at 20 seconds.