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Physical Review A

Squeezed light from a laser with an internal χ(2) -nonlinear element

Rüdiger Schack, Andreas Sizmann, Axel Schenzle

Published Jun 1, 1991• 20 citations
Publisher

Abstract

We present a theoretical investigation of the potential for squeezing of a doubly resonant cavity that contains a laser medium as well as a ${\mathrm{\ensuremath{\chi}}}^{(2)}$ nonlinearity. If the lasing mode is coupled to its second harmonic, i.e., for a self-frequency-doubling laser, we find more than 60% squeezing in the up-converted mode outside the cavity. In the opposite case of a ``self-down-converting laser,'' i.e., when the lasing mode is coupled to its subharmonic, a perfectly squeezed vacuum in the down-converted mode develops into a less squeezed state with a nonvanishing coherent amplitude as the pump rate is increased. Our analysis extends beyond the first stable regime of the classical dynamics and compares results for different laser models.

DOI
10.1103/physreva.43.6303

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Journal of the Optical Society of America B•Jul 1, 1996•18 citations
We present a theoretical investigation of the potential for squeezing of a doubly resonant cavity that contains a laser medium as well as a \ensuremathχ(2){\mathrm{\ensuremath{\chi}}}^{(2)}\ensuremathχ(2) nonlinearity. If the lasing mode is coupled to its second harmonic, i.e., for a self-frequency-doubling laser, we find more than 60% squeezing in the up-converted mode outside the cavity. In the opposite case of a ``self-down-converting laser,'' i.e., when the lasing mode is coupled to its subharmonic, a perfectly squeezed vacuum in the down-converted mode develops into a less squeezed state with a nonvanishing coherent amplitude as the pump rate is increased. Our analysis extends beyond the first stable regime of the classical dynamics and compares results for different laser models.
We present the analysis of a laser in which an intractivity χ (2) -nonlinear element couples the lasing mode to its second harmonic. The cavity is resonant for both modes. We investigate the potential for squeezing in this system by linearizing the quantum fluctuations around the steady states of the problem. In principle, more than 50% noise reduction below the vacuum level is possible in the second-harmonic field outside the cavity. The maximum squeezing, which is neither pure amplitude nor pure phase squeezing, occurs at a finite frequency-offset from the cavity resonance.
In this paper, we explore the squeezing effect generated by two coupled optical cavities. Each cavity contains a second-order nonlinear material and coherently pumped by a laser. Our results show that light intensity is strongly improved due to the presence of the nonlinearities and mainly depends on the detunings between external laser frequencies and cavity modes. More interestingly, the proposed scheme could enhance light squeezing for moderate coupling between cavities : the squeezing generated by one cavity is enhanced by the other one. For resonant interaction, highest squeezing effect is obtained near resonance. When fields are non resonant, squeezing increases near resonance of the considered cavity, but decreases for large detunings relative to the second cavity. Further, when the dissipation rate of the second cavity is smaller than the first, the squeezing could be improved, attaining nearly the perfect squeezing. While the temperature elevation has a negative impact overall on the nonclassical light, squeezing shows an appreciable resistance against thermal baths for appropriate parameter sets.
A quantum theory of self-convers of the lasing frequency in active nonlinear crystals placed in a cavity is developed. The generation of nonclassical (quadrature squeezed) light upon self-doubling of the laser radiation frequency is studied in detail, with no limitations on the relations between the relaxation times of the inverse population and the polarization of the medium and the lifetime of a photon in the cavity. The fluctuation spectra of the quadrature components of the fields of the laser radiation and its second harmonic are studied in dependence on the pumping parameters and the parameters of the cavity. Numerical calculations are performed for a Nd:Mg:LiNbO 3 crystal with a regular domain structure.
The characteristics of squeezed light generated by both passive (occurring in a cavity external to a laser) and active (occurring within a laser cavity) second-harmonic generation (SHG) are investigated and contrasted. Squeezing of both the fundamental and the second harmonic is discussed, and the issue of doubly versus singly resonant behavior is addressed. We examine passive squeezing by explicitly modeling the intrinsically noisy output of the pump laser and coupling it to a passive, multiport, lossy SHG cavity. Low-frequency degradation of the squeezing caused by the laser pump noise is predicted and provides a probable explanation for previous discrepancies between theory and experiment. Active squeezing is quantitatively modeled by a three-level laser model that retains all laser dynamics. Previously disparate predictions are reconciled. For one parameter set two regimes of squeezing are predicted: 50% squeezing at frequencies lower than the laser relaxation oscillation, and near-perfect squeezing at frequencies above. A particular problem of active squeezing is highlighted: The fast dephasing of the laser coherence introduces considerable excess noise likely to mask squeezing in experimental situations. We conclude that, although passive SHG is a practical source of squeezing, active SHG is unlikely to be so in the foreseeable future.