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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Subshot noise FM spectroscopy with an amplitude-squeezed diode laser at room temperature

Steven Kasapi, Seema Lathi, Y. Yamamoto

Published Apr 26, 1995• 1 citation
Publisher

Abstract

We have developed a sub-shot-noise diode laser system and used it to perform sub-shot-noise FM spectroscopy. By inducing FM sidebands on a direct-current modulated external-cavity diode laser and injection locking a slave laser, we have suppressed the residual AM in the master by 50 dB and achieved a noise level of 0.8 dB below the shot-noise level. We expect that such techniques will provide useful and practical tools for future precision measurements and novel atom-photon interaction experiments.

DOI
10.1117/12.208224

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Apr 26, 1995•3 citations
We have developed a sub-shot-noise diode laser system and used it to perform sub-shot-noise FM spectroscopy. By inducing FM sidebands on a direct-current modulated external-cavity diode laser and injection locking a slave laser, we have suppressed the residual AM in the master by 50 dB and achieved a noise level of 0.8 dB below the shot-noise level. We expect that such techniques will provide useful and practical tools for future precision measurements and novel atom-photon interaction experiments.
We have developed a frequency-modulated, tunable, amplitude-squeezed, diode-laser-based source and used it to perform FM spectroscopy on rubidium. The setup consists of a free-running diode laser injection locked by a frequency-stabilized, current-modulated diode laser. The injection-locked slave laser beam adopted the frequency spectrum of the master laser beam while rejecting residual AM in the master laser beam by more than 50 dB. Injection locking also enhanced amplitude squeezing in the slave laser beam by suppressing uncorrelated longitudinal sidemodes. The noise floor of the measurement was 0.8 dB below the shot-noise level.
We have demonstrated sub-shot-noise FM spectroscopy on thermal cesium atoms, using amplitude-squeezed light from a semiconductor laser. The light source used in our experiment was a custom-made GaAs transverse junction stripe semiconductor laser featuring broadband amplitude squeezing and a large FM bandwidth. We have also developed a new technique by which a semiconductor laser can be frequency modulated with infinitesimally small residual AM and with complete preservation of the squeezed-amplitude fluctuations.
We demonstrate that the large FM noise of a semiconductor laser can be combined with a squeezed AM noise spectrum to make a sub-shot-noise, FM-noise-based spectroscopic measurement. In particular, we probe the spectrum of magneto-optically trapped and cooled rubidium atoms using the FM noise on squeezed AM background noise. The natural absorption line as well as a stimulated Raman transition are clearly visible in the FM-to-AM converted noise even though the signal is below the classical shot-noise level.
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.
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.
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.