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Acta Physica Sinica

INTENSITY NOISE SQUEEZING OF LASER DIODE WITH INPHASE EXTERNAL WEAK FEEDBACK BY HALF WAVE PLATE

Ruifang Dong, Junxiang Zhang, Tiancai Zhang, Jing Zhang, Changde Xie, Kunchi Peng, 山西大学光电研究所,教育部量子光学重点实验室,太原030006

Published Jan 1, 2001• 1 citation
Publisher

Abstract

Due to the birefringence effect of the quantum well laser and the collimating lens in a low temperature cryostat, the polarization of the output light was changed, therefore the photon number fluctuation of the output individual linearly polarized mode was increased. By compensating the birefringence with a quarter wave plate, and utilizing an inphase external weak feedback to laser diode laser by means of residuary reflection from the two surfaces of a half wave plate, we obtained the output light of 24mW with the photon number fluctuation 2.8dB below the Standard Quantum Limit at the pump current of 34.65mA.

DOI
10.7498/aps.50.462

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Influence of birefringence induced at low temperature on balanced detection of polarization-dependent photon-number squeezing and its optical compensation

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The polarization-dependent photon-number squeezing of 1.5 dB from a grating feedback locking quantum-well laser in a cryostat has been experimentally detected with a balanced homodyne detector including an imperfect nonpolarized-beam splitter. The great influence of the birefringence induced at low temperature on the determination of the shot-noise-limit and on the detection of photon-number squeezing is discussed in detail. Birefringence is compensated for by insertion of a half-wave plate of adjustable inclination into the output beam before the polarization components of the optical system.

Journal of the Optical Society of America B•Jul 1, 2001•1 citation

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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^${)}^{2}$〉^〉, and an enhanced phase noise above that, 〈(\ensuremath{\Delta}\ensuremath{\psi}^${)}^{2}$〉>(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^${)}^{2}$〉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.

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•May 8, 1994•0 citations

OBSERVATION OF INTENSITY SQUEEZING AND LARGE PHASE NOISE REDUCTION IN SELF-LOCKED SEMICONDUCTOR LASERS

Tiancai Zhang, 李天宇 Li Tianyu, D. van Effenterre +3 more

Both intensity squeezing and dramatic phase noise reduction have been obtained with a confocal cavity used to feedback the beam from the quantum well diode laser.We have measured an intensity squeezing of 19.3%(37.8% inferred at the laser output) around 35MHz and achieved phase noise reduction of 40—25dB from 9kHz to 100MHz.Quasi-minimum uncertainty state have been demonstrated.

Acta Physica Sinica•Jan 1, 1998•3 citations
Due to the birefringence effect of the quantum well laser and the collimating lens in a low temperature cryostat, the polarization of the output light was changed, therefore the photon number fluctuation of the output individual linearly polarized mode was increased. By compensating the birefringence with a quarter wave plate, and utilizing an inphase external weak feedback to laser diode laser by means of residuary reflection from the two surfaces of a half wave plate, we obtained the output light of 24mW with the photon number fluctuation 2.8dB below the Standard Quantum Limit at the pump current of 34.65mA.
The polarization-dependent photon-number squeezing of 1.5 dB from a grating feedback locking quantum-well laser in a cryostat has been experimentally detected with a balanced homodyne detector including an imperfect nonpolarized-beam splitter. The great influence of the birefringence induced at low temperature on the determination of the shot-noise-limit and on the detection of photon-number squeezing is discussed in detail. Birefringence is compensated for by insertion of a half-wave plate of adjustable inclination into the output beam before the polarization components of the optical system.
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.
This thesis describes the phase and amplitude noise properties of semiconductor lasers subjected to weak, dispersive optical feedback. In the first half, experiments demonstrating reductions in the laser frequency noise power spectrum and spectral linewidth by several orders of magnitude are presented. Weak optical feedback is applied to the laser by an external cavity containing an atomic vapor. The presence of the vapor adds to the dispersion of the cavity and simultaneously locks the laser to a fixed frequency reference. The role of 1/f frequency noise in limiting the effectiveness of this linewidth reduction technique is investigated and 1/f noise is found to be the dominant contribution to the linewidth under strong optical feedback conditions. An electronic feedback scheme utilizing FM sideband locking is then implemented alongside the optical feedback, and an additional reduction in the low frequency 1/f frequency noise power spectrum by over two orders of magnitude is obtained. With both systems operating simultaneously, the spectral linewidth is narrowed from its free-running value of about 20 MHz to 1.4 kHz. Excellent absolute frequency stability is also achieved. In the second half, the effects of optical feedback on the quantum mechanical amplitude noise properties of the laser are examined. A fully quantum mechanical theory of amplitude and phase noise for a semiconductor laser with weak optical feedback is developed, and the nature and limits of the noise reduction using this technique are established. Particular attention is given to the feedback-induced enhancement of the amplitude squeezing which can be obtained in a pump-suppressed semiconductor laser: an improvement in the squeezing by 3 dB is predicted under moderate pumping. Somewhat larger noise reductions are expected when the laser is operating closer to threshold. Measurements performed on a laser biased close to threshold are then described and a reduction in the low frequency amplitude noise power spectrum by 7 dB is obtained. An experimental investigation of the effects of optical feedback on the amplitude squeezing in a semiconductor laser is then discussed. The low frequency squeezing in a room temperature device is increased from 3% below the standard quantum limit (SQL) under free-running conditions to 19% below the SQL with optimal feedback. The experimental results are found to agree poorly with the single-mode model and a multi-mode model including the effects of asymmetrical cross-mode non-linear gain is developed to explain the discrepancy. Finally, further experimental investigation into the generation of amplitude squeezed light is presented using a commercial laser with no intentional external modifications. Squeezing as large as 29% below the SQL is measured using a balanced homodyne detector with the laser operating near room temperature, corresponding to 41% below the SQL at the output facet of the laser.
Amplitude noise spectra from the nonclassical output of an injection-locked low-temperature quantum-well laser analyzed along orthogonal polarization axes reveals correlated polarization-dependent noise. Using polarization-preserving balanced homodyne detection to suppress these effects, we demonstrate photon-number fluctuations 4.5 dB below the semiclassical shot-noise limit (within 95% of the expected squeezing based on the device efficiency). Polarization mixing through birefringent components and, more significantly, correlated fluctuations originating in the laser gain medium are shown to account for the observations.
The generation of light with amplitude-noise levels below the standard quantum limit (SQL) by constant-current operation of light-emitting and semiconductor laser diodes has been well established.1 Nearly all of this work, however, has been conducted with the light source cooled to cryogenic temperatures. At room temperature, the highest observed squeezing levels were 0.6 dB for light-emitting diodes and 0.2 dB for laser diodes.2 We presently report up to 1.8 dB of squeezing from a commercially available index-guided quantum-well laser (SDL-5410-C) operated at room temperature. Although cross-gain saturation can lead to amplitude squeezing of the total number of photons emitted by a multimode semiconductor laser in the limit of fast intraband scattering and the absence of wavelength-dependent loss,3 we found that it was necessary to suppress the >100 near-threshold longitudinal side modes of the laser to obtain squeezing at room temperature. This was accomplished either by using optical feedback from a grating in an external cavity configuration or by injection-locking the laser to a single-frequency laser. Both of these techniques allow the output wavelength of the laser to be named and thus the squeezing level as a function of wavelength to be studied.