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Annual Meeting Optical Society of America

Amplitude squeezing in second-harmonic generation

D. P. Bowler, J. M. Liu

Published Jan 1, 1990• 0 citations
Publisher

Abstract

A new method of amplitude squeezing a second-harmonic signal with negative feedback will be discussed. Negative feedback is known to produce amplitude squeezing in diode lasers.1 Previous studies of second-harmonic squeezing that did not include negative feedback predicted no squeezing to the sixth order in the low-conversion-efficiency limit2 and 3 dB squeezing in the limit of 100- conversion efficiency.3 By employing negative feedback, substantial squeezing is possible in the low-conversion-efficiency regime. Amplitude noise reductions in excess of 10 dB are possible. This approach does not require a resonant cavity to enhance amplitude squeezing. The squeezing dependence on key system parameters, such as second- harmonic photon number, will be discussed.

DOI
10.1364/oam.1990.fbb7

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Physical Review A•Oct 1, 1990•35 citations
A new method of amplitude squeezing a second-harmonic signal with negative feedback will be discussed. Negative feedback is known to produce amplitude squeezing in diode lasers.1 Previous studies of second-harmonic squeezing that did not include negative feedback predicted no squeezing to the sixth order in the low-conversion-efficiency limit2 and 3 dB squeezing in the limit of 100- conversion efficiency.3 By employing negative feedback, substantial squeezing is possible in the low-conversion-efficiency regime. Amplitude noise reductions in excess of 10 dB are possible. This approach does not require a resonant cavity to enhance amplitude squeezing. The squeezing dependence on key system parameters, such as second- harmonic photon number, will be discussed.
Second-harmonic generation, in a cavity resonant only at the fundamental, produces amplitude squeezing in both the fundamental and the second harmonic, with optimal squeezing of -1/3 and -8/9, respectively, at low frequency. It also produces nonclassical correlations between the two output beams. We show that these correlations can be used to enhance the squeezing in either beam by constructing a feedback loop. By using the second-harmonic photocurrent to control the amplitude of the pump, the optimal fundamental squeezing is enhanced by 50% to -1/2. The limit of -8/9 for the seocnd harmonic (for infinite nonlinearity strength) cannot be improved by feeding back the fundamental. However, for any fixed finite nonlinearity, such feedback can produce significant enhancement, with a maximum of 50%. Even with the losses characteristic of a practical experiment included, the enchancement offered by the feedback is still significant, being greater than 20% in either mode.
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.
This dissertation describes a series of measurements that confirmed the existence of amplitude squeezing in a commercially available type of laser diode and makes the critical observation that the primary requirement to obtain squeezing, in addition to sub-Poissonian pumping, is the suppression of longitudinal side modes. Side-mode suppression was achieved using optical feedback from a grating in an external cavity configuration or by injection locking to a single-wavelength master laser. The external cavity setup allowed as much as 1.8 dB of amplitude squeezing to be generated with the laser diode operating at room temperature, and provided a means for tuning the output wavelength over a range in excess of 20 nm. The injection locking approach achieved more than 3.0 dB of squeezing at cryogenic temperatures and made it possible to maintain squeezing in the presence of drive-current modulation signals, facilitating potential applications in precision measurement systems and communications. By identifying some of the noise mechanisms which prevent the observation of amplitude squeezing and establishing that squeezing is a much more robust phenomenon than previous work had indicated, this research has shown that there may be important practical uses for these sources as diode laser technology evolves.
A new scheme for generating an amplitude-squeezed state is proposed. The photon-flux fluctuation of a semiconductor-laser output wave is measured with a quantum nondemolition (QND) detector, and is negatively fed back to the laser pumping current. The operator Langevin equations are derived by combining the quantum-mechanical analyses on the laser internal-external field fluctuations, the quantum nondemolition detector based on an optical Kerr effect, and a negative-feedback circuit. The output wave features a reduced photon number noise below the standard quantum limit, 〈(\ensuremath{\Delta}n^)2{)}^{2})2〉^〉, and an enhanced phase noise above that, 〈(\ensuremath{\Delta}\ensuremath{\psi}^)2{)}^{2})2〉>(1/4)〈n^${〉}^{\mathrm{\ensuremath{-}}1}$, while the minimum uncertainty product is still preserved. The observed photoelectron statistics in a negative-feedback GaAs laser diode using a conventional p-i-n photodiode (not a QND detector) are shown to exhibit sub-Poissonian statistics with the variance 〈(\ensuremath{\Delta}n^)2{)}^{2})2〉apeq20.26〈n^〉. The measured photocurrent fluctuation spectral density is also indicated to be below the standard quantum limit by a factor of 0.2 (=-7 dB). The experimental results also confirm that such amplitude squeezing is only observed inside the feedback loop and cannot be extracted from the loop unless a quantum nondemolition detector is used.
We present the generation of nonclassical state using an amplified diode laser as a light source. The intensity noise of an amplified diode laser was significantly suppressed and reached the shot noise limit at 15 MHz using both a filter cavity and resonant optical feedback. Frequency doubling efficiency of 66% and up to 120 mW output power of green has been achieved in cw second-harmonic generation from 1080 nm to 540 nm. Bright two-mode amplitude-squeezed state was generated from a type-II nondegenerate optical parametric amplifier pumped by generated green light. The measured noise reduction is 2.1+/-0.2 dB below the shot-noise level.
We consider lasers with intracavity nonlinear elements, e.g., two-photon absorption, second-harmonic generation, as a possible means to reduce the amplitude fluctuations in a laser. While amplitude squeezing up to 37% may be obtained for the internal field, only a modest amount of noise reduction (10% for two-photon absorption, effectively zero for second-harmonic generation) is found in the output field at the laser frequency. In the second-harmonic field an amplitude-noise reduction of 50% below the shot-noise level may be achieved.