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Conference on Lasers and Electro-Optics

Spectrally asymmetric mode correlation in amplitude-squeezed laser diodes

Christoph Becher, Klaus J. Boller

Published Nov 27, 2002• 0 citations
Publisher

Abstract

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.

DOI
10.1109/iqec.1998.680272

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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.
We report on the experimental observation of a spectrally asymmetric quantum correlation between the intensities of the main mode and a large number of weakly excited side modes in a pump-noise-suppressed laser diode. The total intensity noise of the laser diode results from a generally incomplete cancellation process between these mode intensities, which influences the generation of amplitude squeezed light. Nonlinear gain due to cross mode gain saturation in the semiconductor lasing medium is found to be the origin of the asymmetric intensity noise distribution between short- and long-wavelength side modes. The experimental results are in good agreement with the predictions of a multimode Langevin rate equation model including asymmetric nonlinear gain saturation.
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.
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.
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.
Amplitude-squeezed light has been generated by semiconductor quantum well lasers in numerous optical configurations, at both room and cryogenic temperatures. Various intensity noise sources associated with nonideal laser operation, in particular mode partition and polarization-dependent noise, can introduce intensity fluctuations that limit or remove amplitude squeezing. We discuss amplitude noise reduction in low-temperature free-running and injection-locking laser configurations and room-temperature injection locking and external-cavity laser configurations. Drive-current modulation with a nonclassical SNR improvement is achieved with an injection-locked laser.