Beat-note detection in subharmonic synchronously intracavity pumped OPO

The goal of the presented work is the development of a laser system suitable for intracavity phase interferome-try (IPI), allowing to detect a phase shifts in the order of 0.1 µrad, based on beat-note detection. The key part of a such system is a laser oscillator in which two intracavity pulses circulate independently but with the same repe-tition rate. Our solution is based on an intracavity pum-ped PPLN linear optical parametrical oscillator (OPO), operating at the wavelength tunable in the range from 1.4 up to 1.6 µm. It was used a SESAM-modelocked, picosecond, diode pumped Nd:YVO₄ linear oscillator, operating at 1.06 µm for synchronous pumping of OPO. To obtain a long-term stable generation of two indepen-dent pulse trains inside the OPO, the OPO cavity was set to be twice longer than the pumping Nd:YVO₄ la-ser cavity. Simultaneously, the pumping laser system, containing the PPLN crystal, was set in such manner that the parametric gain inside the PPLN overcame the OPO threshold only for one direction of pumping pulses propagation. Since the both pulse trains inside the OPO were pumped by the same spatial mode of the pump laser, the possible pointing instability of the pump laser does not affect the system performance and the beat-note bandwidth. The OPO output beams were derived be semireflected mirror, placed close to the OPO cros-sing point of pulse trains. After proper delay of one pulse train both trains interfered on a slow InGaAs detector, where the beat note was detected. In order to verify the system performance a LiNbO₃ electro-optic phase mo-dulator was placed inside the OPO. To modulate one of the pulse trains, RF-signal obtained from pumping pulse train detected by fast InGaAs, divided by two, and am-plified to desired value (maximum 10 V), was applied on the modulator. The beat-note signal with frequency from 1 up to 20 kHz was successfully measured in de-pendency on RF-signal amplitude.