Raman-Free, Noble-Gas-Filled Photonic-Crystal Fiber Source for Ultrafast, Very Bright Twin-Beam Squeezed Vacuum

We report a novel source of twin beams based on modulational instability in high-pressure argon-filled hollow-core kagome-style photonic-crystal fiber. The source is Raman-free and manifests strong photon-number correlations for femtosecond pulses of squeezed vacuum with a record brightness of $\sim 2500$ photons per mode. The ultra-broadband ($\sim 50\mathrm{THz}$) twin beams are frequency tunable and contain one spatial and less than 5 frequency modes. The presented source outperforms all previously reported squeezed-vacuum twin-beam sources in terms of brightness and low mode content.

Figure 1

(a) Setup for twin beam generation and analysis: Bandpass (BP) filter, half-wave plate (HWP), polarizing beam splitter (PBS), notch filter (NF), long-pass (LP) filter, short-pass (SP) filter, scanning electron micrograph (SEM), flat-to-flat (FF) diameter. (b) Modulation instability spectrum at 75 bar and 250 nJ pulse energy measured with an optical spectrum analyzer (OSA/ANDO AQ 6315-E). The dotted curve shows the unfiltered spectrum, while the solid curve shows the spectrum after blocking the pump light with notch filters. The insets show near-field mode profiles, confirming that the light is in the fundamental mode.

Figure 2

(a) Measured pressure dependence of the MI sidebands for a constant pump pulse energy of 320 nJ. The spectra were measured with the pump blocked by notch filters. The dashed line marks the locus predicted for perfect phase matching. (b) Simulation of the pressure dependence of the nonclassical light spectra at a parametric gain of 4.2. The accessible spectral range (ASR) for measuring twin-beam squeezing is limited by the quantum efficiency (QE) of the silicon photodiode and by the width of the antireflection (AR) coating on the optical components.

Figure 3

(a) NRF plotted against the average total number of twin-beam photons, changed by varying the pump power, at 70, 72, and 76 bar. The difference in slope is caused by unequal frequency dependence of the detection efficiency in the two optical channels (the sideband frequencies shift when the pressure is changed). (b) Measured values of NRF plotted against attenuation in the optical channel performed with the HWP and the PBS. Each data point in the diagram corresponds to one million recorded pulses. Error bars represent the standard error of the measurement.