Fig. 1. Experimental setup. A fiber coupled CW telecom laser at 1560.44 nm is amplified (EDFA) and split into two by means of a 65:35 fiber beam splitter (f-BS). The less intense output (upper arm) serves as local oscillator (LO) while the brighter one (lower arm) is frequency doubled via SHG in a PPLN/RW and used to pump an on-chip squeezing generation stage (SPDC). The power of the beam at 780.22 nm is controlled with an in-line fibered attenuator (Pwr Ctrl) and its polarization is adjusted by means of a fiber polarization controller. At the output of the SPDC stage, squeezed light at 1560.44 nm is optically mixed with the LO beam inside the same chip in an integrated directional coupler realizing the interferometric part of the homodyne detector. At the chip output, after passing through a bulk lens followed by an optical filter suppressing residual pump at 780.22 nm, light is sent to two InGaAs photodiodes (PDs). The LO phase is scanned thanks to a home-made fiber-stretcher ().
Schematic of our photonic chip on lithium niobate. The chip includes an SPDC stage, consisting in a periodically poled waveguide (3 cm long with poling period
) for squeezing generation at 1560.44 nm, and an integrated directional coupler realizing the interferometric part of the homodyne squeezing detection. The whole chip length is 5 cm. All waveguides are obtained by soft proton exchange [29
] and have a width of 6 μm. The 127 μm spacing between the input (output) waveguides is compatible with of-the-shelf fiber-arrays. The homodyne photodiodes are outside the chip and are bulk commercial components.
Fig. 3. Normalized noise variances at 2 MHz obtained for a coupled pump power of 40 mW as a function of the LO phase (proportional to the time) and with a sweep time of 1 s. The electrical spectrum analyzer resolution and the video bandwidths are 100 kHz to 30 Hz, respectively.