Subharmonic synchronously intracavity pumped picosecond optical parametric oscillator for intracavity phase interferometry

The laser system suitable for precise intracavity phase interferometry is presented. The system is based on an intracavity pumped PPLN linear optical parametrical oscillator (OPO). For synchronous pumping of OPO a SESAM-mode-locked, picosecond, diode-pumped Nd:YVO₄ linear oscillator, operating at 1.06 µm was used. The OPO cavity was set to be twice as long as the pumping Nd:YVO₄ laser cavity. The pumping laser was set in such a manner that the parametric gain inside the PPLN overcame the OPO threshold only for one direction of pumping pulse propagation. This leads to the generation of two independent trains of pulses at the 1.5 µm spectral range. To verify the system performance, a LiNbO₃ electro-optic phase modulator was placed inside the OPO. The RF-signal derived from the pumping pulse train, detected by a fast photodiode and divided by two, was applied on the modulator. A stable beat-note signal between the two OPO trains was successfully measured for the first time from such a compact, all-diode-pumped laser system. For RF-signal amplitude from 100 up to 700 mV beat-note frequency varied from 232 up to 1847 Hz which corresponded to detected phase-shift 36–250 µrad. The bandwidth of beat-note was less than 1 Hz (FWHM) resulting in phase-shift measurement error 1.5  ×  10⁻⁷ rad. (letter)