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An Intense Continuous Source of Broadband Down-Converted Light

Avi Pe’er, Barak Dayan, Asher A. Friesem, Yaron R. Silberberg

Published Oct 27, 2005• 0 citations
OpenAlex

Abstract

The unique coherence and squeezing properties of high-power broadband down-converted light are very attractive for basic research in quantum optics as well as for applications. However, efficient sources did not exist until now, because of the need to stimulate the down-conversion process in a cavity, where mode competition narrows the spectrum drastically. Here we present the design of a low threshold optical parametric oscillator source that oscillates over the entire bandwidth allowed by phase matching. Two-photon absorption is exploited as a selective loss mechanism to suppress mode competition and achieve a broadband oscillation. The main theme of the design is to perform pairwise mode-locking of many frequency pairs, in direct equivalence to passive mode-locking of ultrashort pulsed lasers. We demonstrate the underlying principles of our design in an experiment with an oscillator cavity.

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The unique coherence and squeezing properties of high-power broadband down-converted light are very attractive for basic research in quantum optics as well as for applications. However, efficient sources did not exist until now, because of the need to stimulate the down-conversion process in a cavity, where mode competition narrows the spectrum drastically. Here we present the design of a low threshold optical parametric oscillator source that oscillates over the entire bandwidth allowed by phase matching. Two-photon absorption is exploited as a selective loss mechanism to suppress mode competition and achieve a broadband oscillation. The main theme of the design is to perform pairwise mode-locking of many frequency pairs, in direct equivalence to passive mode-locking of ultrashort pulsed lasers. We demonstrate the underlying principles of our design in an experiment with an oscillator cavity.
Broadband quantum light is a vital resource for quantum metrology and spectroscopy applications such as quantum optical coherence tomography or entangled two photon absorption. For entangled two photon absorption in particular, very high photon flux combined with high time-frequency entanglement is crucial for observing a signal. So far these conditions could be met by using high power lasers driving degenerate, type 0 bulk-crystal spontaneous parametric down conversion (SPDC) sources. This naturally limits the available wavelength ranges and precludes deterministic splitting of the generated output photons. In this work we demonstrate an integrated two-colour SPDC source utilising a group-velocity matched lithium niobate waveguide, reaching both exceptional brightness 1.52⋅10 6 pairssmWGHz and large bandwidth (7.8 THz FWHM) while pumped with a few mW of continuous wave (CW) laser light. By converting a narrow band pump to broadband pulses the created photon pairs show correlation times of Δ τ ≈ 120 fs while maintaining the narrow bandwidth Δ ω p ≪ 1 MHz of the CW pump light, yielding strong time-frequency entanglement. Furthermore our process can be adapted to a wide range of central wavelengths.
The coherence properties of broadband down converted light are investigated theoretically and experimentally in both the classical and quantum mechanical frameworks. Although broadband down converted light is one-photon incoherent noise, it possesses unique two-photon coherence. Thus, for performing two-photon processes, down-converted light is equivalent to coherent ultrashort pulses in some aspects and to coherent CW laser radiation in others. This equivalence is theoretically established and experimentally demonstrated in the classical, high-power regime. Two applications that take advantage of multi-photon coherence are then described in detail; one for optical spread spectrum communication and the other for sub-diffraction limit optical lithography. A key part of this research is the design and implementation of an efficient source that generates high-power broadband down conversion in a cavity. A broadband oscillation in the cavity is ensured by a mechanism very similar to passive mode-locking of ultrashort pulse laser sources, which we term as pair-wise mode locking, thus extending the equivalence to ultrashort pulses to the light generating sources also. Finally, the quantum mechanical properties of broadband down-conversion are explored experimentally, by performing a two-photon process (sum frequency generation) at the low power level of entangled photon pairs. The non-classical nature of the light is expressed by a linear intensity dependence of the non-linear process. Temporal shaping with femtosecond resolution of the entangled photon pairs is demonstrated using this non-linear interaction.
We experimentally demonstrate two-photon absorption with broadband down-converted light (squeezed vacuum). Although incoherent and exhibiting the statistics of a thermal noise, broadband down-converted light can induce two-photon absorption with the same sharp temporal behavior as femtosecond pulses, while exhibiting the high spectral resolution of the narrow band pump laser. Using pulse-shaping methods, we coherently control two-photon absorption in rubidium, demonstrating spectral and temporal resolutions that are 3-5 orders of magnitude below the actual bandwidth and temporal duration of the light itself. Such properties can be exploited in various applications such as spread-spectrum optical communications, tomography, and nonlinear microscopy.
The theory of the optical parametric oscillator is examined and compared to the experiment in the regime of far below threshold. It is found that the output state has no difference from spontaneous parametric down-conversion except that the bandwidth of down-conversion is reduced to that of the resonator and the conversion rate is enhanced by cavity resonance. The reduction of the bandwidth of the down-converted fields makes it possible for a direct measurement of the time interval distribution between two down-converted photons. The observed distributions are well explained by the theory. Such a narrow-band two-photon source will find wide applications in quantum information processing.