Towards Arbitrary Time-frequency Mode Squeezing with Self-conjugated Mode Squeezing in Fiber

TL;DR

The paper shows that self-conjugated time-frequency modes enable arbitrary time-frequency mode squeezing in fiber when driven by a cw pump with broadband phase matching. It provides a theoretical framework for SC modes and heralded squeezing, and demonstrates an all-fiber implementation achieving 7.50 dB squeezing on a bichromatic SC mode, plus measurable squeezing on randomly shaped SC modes (4.38 dB and 0.88 dB) despite Raman and other noise sources. The approach relaxes modal constraints of OPAs, enabling dynamic, high-fidelity squeezing across customizable time-frequency modes with potential for sensing and quantum information applications. The work highlights practical benefits of all-fiber, low-loss integration and outlines pathways to further improve bandwidth and squeezing by exploring alternative materials and configurations.

Abstract

Optical parametric amplification generates squeezed light in device-specific sets of time-frequency eigenmodes, and it has been widely accepted that detection and utilization of squeezing must comply with this modal constraint. We show that this constraint can be considerably relaxed under the continuous-wave pump and broadband phase-matching approximation, where the modal decomposition is non-unique. Specifically, any time-frequency mode with "self-conjugated" spectral symmetry can approximate a squeezing eigenmode, and partial homodyne detection can herald squeezing in arbitrary time-frequency modes. We demonstrate this using a high-efficiency, low-loss all-fiber source, measuring 4.38 +- 0.11dB and 0.88 +- 0.09 dB squeezing on partially coherent and chaotic self-conjugated modes. Using a bichromatic self-conjugated mode with reduced local-oscillator noise, we achieve 7.50 +- 0.12dB squeezing, which represents the highest level reported for fully guided-wave squeezing sources based on chi(2) and chi(3) nonlinearities.

Figures (7)

• Figure 1: (a) Generation of SC mode squeezing through SFWM or spontaneous parametric down-conversion (SPDC). The corresponding LO light can be generated through FWM or parametric down-conversion process (PDC), respectively. (b) The spectral intensity of LO light (solid fill), squeezed vacuum (SV) on the SC mode (dashed curve), and the frequency-dependent squeezing parameter $|\xi(\omega)|$ (green background). Red and green colors denote the signal and idler frequency branches, respectively. (c) The procedure of converting squeezing on the mode into a displaced squeezed vacuum (DSV) on the signal mode through partial homodyne detection on the idler mode. (d) Examples of different signal seed light spectrum (red curve) and its corresponding idler mode spectrum (blue curve) as well as the corresponding SC mode squeezing and heralded signal mode squeezing (for 1.5 km single mode fiber as the nonlinear medium and the peak gain detuning is $2\pi\times50.62$ GHz from the pump frequency). Both SC mode squeezing and heralded squeezing decrease as the spectral amplitude of the SC mode extend beyond the spectral range where $|\xi(\omega)|$ is close to its maximum.
• Figure 2: The schematic of the squeezed light system setup consists of four different sections (green: the pump and signal seed light source, blue: the SFWM arm, yellow: the FWM arm, red: the BHD section). PG: pulse generator; AM1-2: electro-optical amplitude modulators; EDFA: erbium-doped fiber amplifier; PC1-3: inline fiber polarization controllers; BS1: a 50:50 fiber coupler for distributing pump light into the SFWM and FWM arms; BS2: a 10:90 fiber coupler that mixes pump and signal seed light before FWM; VOA: variable optical attenuator; BPF: tunable band-pass filter; PM: electro-optical phase modulator; PID: proportional-integral-derivative controller with peripheral circuits; BS3: the BHD coupler; FBG1-2: fiber Bragg gratings for pump light filtering; PD1-2: inline optical power monitors; TIA: transimpedance amplifier; BPD: balanced photodetector pair.
• Figure 3: The spectra of (a) filtered ASE light and (b) noise-modulated laser (with 1 GHz sine modulation) light and (c)coherent laser light. The frequency axes are normalized relative to their center frequencies (1549.55 nm, 1549.65 nm, and 1549.65 nm), respectively. The spacing of frequency and amplitude grid lines is equal for all three plots.
• Figure 4: The squeezed noise level (red), shot noise level (blue), and dark noise level (black) as a function of (a) electrical frequency and (b) time (zero span at 1.05 MHz) when coherent laser light is used as signal seed light. (c) the dependence of maximal squeezing (red) and antisqueezing (purple) relative to the shot noise level and their uncertainties as a function of SFWM pump average power.
• Figure 5: The spectra of squeezed noise level (red), apparent shot noise level (blue), true shot noise level (green), and dark noise level (black) with (a) noise-modulated laser (with 1 GHz sine wave frequency modulation) and (b) filtered ASE light are used as the signal seed light. The labeled value of squeezing in (a) and (b) is defined as the difference between the squeezed noise level and the true shot noise level, both measured in separate zero-span ESA measurements at the specified frequencies. (c) the dependency of measured squeezing (at 1.15 MHz) on the sine modulation frequency. Blue and Red error bars represent the measured apparent shot noise and squeezing noise level. The green horizontal line represents the true shot noise level.