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Journal of the Optical Society of America B

Method of characterizing the multicomponent spectrum of a VCSEL in devices based on the CPT effect

Daba A. Radnatarov, Sergey M Kobtsev, Valeriy Andryushkov

Published Oct 8, 2021• 3 citations
Publisher

Abstract

We propose and experimentally demonstrate a method of spectral measurement of multicomponent radiation emitted by a modulated vertical cavity surface-emitting laser (VCSEL) relying on peculiarities of absorption in alkali metal vapor. The method consists in determination of the radiation spectrum (which is formed due to RF modulation of the injection current of a diode laser) from the dependence of transmittance of rubidium vapor upon the radiation wavelength. We show that the proposed method allows fairly precise measurement of the spectrum of multicomponent radiation used in devices based on the coherent population trapping (CPT) effect when the frequency difference between the radiation components matches that between absorption lines of an alkali metal.

DOI
10.1364/josab.442253

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We propose and experimentally demonstrate a method of spectral measurement of multicomponent radiation emitted by a modulated vertical cavity surface-emitting laser (VCSEL) relying on peculiarities of absorption in alkali metal vapor. The method consists in determination of the radiation spectrum (which is formed due to RF modulation of the injection current of a diode laser) from the dependence of transmittance of rubidium vapor upon the radiation wavelength. We show that the proposed method allows fairly precise measurement of the spectrum of multicomponent radiation used in devices based on the coherent population trapping (CPT) effect when the frequency difference between the radiation components matches that between absorption lines of an alkali metal.
At present, to create quantum frequency standards, the phenomenon of coherent population trapping (CPT) is widely used, in which, when a certain intensity is reached and the frequency interval between the spectral components coincides with the frequency of the hyperfine transition in an atom (most often in rubidium or cesium), a narrow resonance is observed in the absorption spectrum and the transmission of a cell with alkali metal vapor increases sharply. Since the amplitude of the CPT resonance critically depends on the intensity and position of the spectral components of the pump radiation, it is necessary to control the parameters of these components, which are usually recorded in digital form. This paper describes a fundamentally new method for processing the laser emission spectrum recorded with a scanned Fabry – Perot interferometer (FPI), based on using the experimentally recorded transmission form of the scanned FPI as a reference spectral component. The paper presents examples of processing the experimentally recorded radiation spectra of a diode laser with an external resonator under microwave modulation of its injection current. The error of processing by the proposed method turned out to be approximately an order of magnitude smaller than when processing by the well-known Airy formula describing the transmission of the FPI. This is explained by the fact that the Airy formula is valid for light beams whose wave front curvature is consistent with the curvature of the IFP mirrors, which is often not true in the case of a real experiment. The method can be used to process the spectra of modulated radiation of various types of lasers, which is an important task in detecting CPT resonances and further creating quantum frequency standards based on them.
Previously, we have proposed a method to control the emission spectrum of the vertical-cavity surface-emitting laser (VCSEL) with the synchronized modulation of the injection current at single and doubled frequencies. In this work, the above method is used to improve the metrological characteristics of the coherent population trapping (CPT) resonance in 87 Rb. The dual-frequency (DF) modulation reduces the carrier power and suppresses the light shift of the resonance frequency, if it is unattainable with the single-frequency modulation. In addition, a higher resonance contrast is achieved by equalizing the powers of the resonant components of the spectrum.
We have performed a series of experiments demonstrating the use of a vertical-cavity surface-emitting laser for nonlinear spectroscopy on the cesium D2 transition at 852 nm wavelength. Due to the high modulation efficiency of the VCSEL sufficiently strong modulation sidebands at 9.2 GHz frequency can be produced by direct modulation of the laser injection current. Using, the carrier and one of the modulation sidebands coherent population trapping (CPT) resonances in a buffered cesium vapor can be prepared with linewidths of only 60 Hz. For application of these narrow resonances to precision magnetometry we find a sensitivity of 10 pT in 0.3 s integration time. For the use of our setup as a frequency reference a relative stability of 1 • 10 − 11 / τ is predicted. In both applications of CPT resonances the use of VCSELs as light sources helps to build compact and reliable devices.
Abstract The paper presents the results of studying the coherent population trapping (CPT) resonances observed in a cell with 87Rb vapor with addition of a buffer gas under multifrequency excitation by radiation from an external cavity diode laser (ECDL). The laser diode injection current is modulated simultaneously by microwave (MW) and very high-frequency (VHF) signals. It has been revealed that, for a certain band of VHF modulation frequencies, the amplitude of a CPT resonance is proportional to the product of the electrical field strengths of the spectral components contributing to resonance excitation. The proposed model of calculating the resonance amplitude is in good agreement with experimental data.
We investigate a versatile vertical cavity surface-emitting laser (VCSEL) with a slight optical feedback from a simple wedge positioned at a well determined distance from the VCSEL. We use such a laser in conjunction with a miniature Rb atomic cell to detect coherent population trapping (CPT) when the external cavity mode spacing frequency corresponds to half the hyperfine frequency of Rb. The feedback greatly enhances the rf modulation efficiency of the VCSEL and as a result we obtain high contrast CPT signals at an input of −16dBm of rf modulation power compared to +1.25dBm needed in the absence of the external cavity.