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Quantum and Semiclassical Optics Journal of the European Optical Society Part B

Quantum noise of free-running and externally-stabilized laser diodes

T -C Zhang, J.-Ph. Poizat, Philippe Grelu, Jean-François Roch, Philippe Grangier, Francesco Marin, Alberto Bramati, V. Jost, M. D. Levenson, E. Giacobino

Published Aug 1, 1995• 78 citations
Publisher

Abstract

We have investigated the intensity and phase noise of single-mode laser diodes, either free-running or using different types of line narrowing techniques at room temperature, namely feedback from an external grating and injection locking. We have measured an intensity squeezing of 1.2 dB in the first case, and 1.4 dB in the second case (respectively, 1.6 dB and 2.3 dB inferred at the laser output). We have observed that the intensity noise of a free-running 'single-mode' laser diode actually results from a cancellation effect between large anticorrelated fluctuations of the main mode and of weak longitudinal side modes. It is also shown that free-running diodes exhibit very large excess phase noise. Typically more than 80 dB above shot noise at 10 MHz, which can be significantly reduced by the above-mentioned techniques.

DOI
10.1088/1355-5111/7/4/015

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We have investigated the intensity and phase noise of single-mode laser diodes, either free-running or using different types of line narrowing techniques at room temperature, namely feedback from an external grating and injection locking. We have measured an intensity squeezing of 1.2 dB in the first case, and 1.4 dB in the second case (respectively, 1.6 dB and 2.3 dB inferred at the laser output). We have observed that the intensity noise of a free-running 'single-mode' laser diode actually results from a cancellation effect between large anticorrelated fluctuations of the main mode and of weak longitudinal side modes. It is also shown that free-running diodes exhibit very large excess phase noise. Typically more than 80 dB above shot noise at 10 MHz, which can be significantly reduced by the above-mentioned techniques.
We demonstrate experimentally that the intensity noise of so-called "free-running single-mode" laser diodes results from a cancellation between very large anticorrelated fluctuations of the main mode, on one hand, and of many weak longitudinal side modes, on the other hand. When line narrowing techniques are used, intensity squeezing can be observed at room temperature, but this noise reduction is not always single-mode squeezing. These experimental results are in agreement with a simple phenomenological model using Langevin-type equations.
Intensity fluctuation and phase noise are key factors limiting the applications of the diode lasers is many fields. The intensity noise of a free-running diode laser was investigated experimentally. Tens of longitudinal modes from the diode laser were successfully separated and identified by a self-made monochromator with 0.01 nm resolution. The noise of either the individual mode or the jointed modes of main mode and the sideband modes was detected using a low-noise radio frequency photodetector. The properties of the intensity noise between either the main mode and the side modes, or the side modes themselves were investigated systematically. The anticorrelation between main mode and side modes was confirmed, and the periodical anticorrelations between small longitudinal modes were found. These results can help us to understand the mechanism of the intensity noise of the diode lasers as well as the noise suppression for intensity-squeezed light generation.
Summary form only given.Laser diodes with pump noise suppression are reliable sources for the generation of amplitude-squeezed light. Recently, it was shown that this amplitude squeezing is not always a single mode effect but can result from a cancellation among anticorrelated fluctuations between the main mode and competing side modes. To obtain more details on this correlation effect, we performed spectrally resolved intensity noise measurements with a pump noise suppressed SDL 5410-C (810 nm) diode laser in different configurations: free-running, injection-locked, and with an external Littrow grating feedback.
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.
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.