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arXiv:1303.7277 · Optics Letters

Multiple side-band generation for two-frequency components injected into a tapered amplifier

Hua Luo, Kai Li, Dongfang Zhang, Tianyou Gao, Kaijun Jiang

Published Mar 28, 2013• 12 citations
arXivHTMLPDF

Abstract

We have experimentally studied multiple side-band generation for two-frequency components injected into a tapered amplifier (TA) and demonstrated its effects on atomic laser cooling. A heterodyne frequency-beat measurement and a Fabry-Perot interferometer have been applied to analyze the side-band generation with different experimental parameters, such as frequency difference, injection laser power, and TA current. In laser-cooling potassium40 and potassium41 with hyperfine splitting of 1.3 GHz and 254 MHz, respectively, the side-band generation with a small frequency difference has a significant effect on the number of trapped atoms.

DOI
10.1364/ol.38.001161

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Acta Physica Sinica•Jan 1, 2006•7 citations
We have experimentally studied multiple side-band generation for two-frequency components injected into a tapered amplifier (TA) and demonstrated its effects on atomic laser cooling. A heterodyne frequency-beat measurement and a Fabry-Perot interferometer have been applied to analyze the side-band generation with different experimental parameters, such as frequency difference, injection laser power, and TA current. In laser-cooling potassium40 and potassium41 with hyperfine splitting of 1.3 GHz and 254 MHz, respectively, the side-band generation with a small frequency difference has a significant effect on the number of trapped atoms.
Simultaneous two-frequency amplification is highly desirable in cold atom experiments. The nonlinear response would appear in the two-frequency amplification with a semiconductor tapered amplifier (TA) and has a direct influence on the experimental result. We investigated in detail the effects of frequency difference, total power, and power ratio of two seeding lasers on the output components based on a simplified theoretical model. The simulation results showed that the multiple sideband generation in the amplifier due to self-phase and amplitude modulation could be suppressed and the TA tended to linearly amplify the power ratio between two-frequency components, when the two seeding lasers had a large frequency difference. This was verified experimentally in the output power ratio measurement via a calibrated Fabry–Perot interferometer method with a good linearity and an uncertainty of 1%. We also discussed the consequences of power ratio responses in the amplification in light of cold atom experiments, especially in the ac Stark shift related phase error of Raman-type atom interferometers (AIs). It was shown that the fluctuation of intensity ratio of Raman beams may induce significant systematic errors for an AI gyroscope.
Stimulated Raman transitions are often used in an atom interferometer (AI) for wave packet manipulation. Normally, two lasers with different frequencies contained in a Raman beam are combined first and then amplified by a single tapered amplifier (TA). This configuration can simplify the laser system of the AI, however, additional sidebands will be generated by the TA because of the nonlinear effect in the TA. In this work, the impact of additional sidebands generated with a single TA on the AI is studied. We first observe the additional sidebands in a Raman laser by a Fabry-Pérot interferometer (FPI), and the additional sidebands will be greatly suppressed by reducing the injection laser power of the TA. This is also confirmed by observing the position-dependent Raman transitions induced by additional sidebands at different injection power in an AI. However, the phase shifts induced by additional sidebands are not reduced obviously when the injection power of the TA is reduced. Therefore, it is necessary to separately amplify two lasers contained in the Raman laser by two TAs for a high precision AI. The spectroscopy of Raman laser generated by two TAs is also measured by the FPI, and the impact of additional sidebands on the AI is eliminated. This work has guiding significance for the design of the laser system in a high-precision AI.
Laser cooling of atoms usually necessitates several laser frequencies. Alkaline atoms, for example, are cooled by two lasers with a frequency difference in the gigahertz range. This gap cannot be closed with simple shifting techniques. Here, we present a method of generating sidebands at 6.6 GHz by modulating the current of a tapered amplifier, which is seeded by an unmodulated master laser. The sidebands enable trapping of 1.1 × 10(9) (87)Rb atoms in a chip-based magneto-optical trap. Compared to the direct modulation of the master laser, this method allows for an easy implementation, a fast adjustment over a wide frequency range, and the simultaneous extraction of unmodulated light for manipulation and detection. The low power consumption, small size, and applicability for multiple frequencies benefit a wide range of applications reaching from atom-based mobile sensors to the laser cooling of molecules.
We present the investigation on the frequency-dependent amplification (FDA) of a tapered amplifier (TA) and the corresponding influence on Raman-type atom interferometers. In our interferometer, the output of two phase-locked diode lasers is injected into a TA to generate Raman beams. The frequency of one laser is chirped during the interfering process, which induces a variance of the Raman lasers power as a result of the FDA of the TA. The corresponding power ratio variation of the Raman lasers is measured by beat note method, which shows a linear dependence with a slope of -0.087(4)/GHz when the laser frequency changes over 2 GHz at 780 nm. The corresponding error related to AC Stark effect due to this frequency-dependent variation is estimated for our atom interferometer. The investigation presented here may provide hints for other experiments involving TAs.
The output from two grating-stabilized external-cavity diode lasers were injected into a single-mode diode laser. Operating at a wavelength of 780 nm, this laser produced ~50 mW of power with two main frequency components of the same spectral characteristics of the seed lasers. The power ratio of the amplified components was freely adjustable due to gain saturation, and amplification was observed for frequency differences of the two seed lasers in the range from 73 MHz to 6.6 GHz. This system was used to realize a dual isotope magneto-optic trap (MOT) for rubidium (85Rb and 87Rb). The resulting position and cloud size of the dual isotope MOT was the same as that of the single species MOTs to within "±10 and "±20 μm, respectively. We also characterized the additional spectral components produced by four wave mixing (FWM) in the diode laser amplifier and utilized a particular FWM sideband to realize hyperfine pumping and subsequent laser trapping of 85Rb in the absence of a "repump" laser dedicated to hyperfine pumping.
We have developed a semiconductor laser system used in 40K-87Rb trapping and cooling experiment. The laser diode system is composed of three external cavity diode lasers and four injection diode lasers and a tapered amplifier. Four laser beams used for 40K and 87Rb cooling and repump lights are obtained by using acoustic optic modulators to offset the frequencies from the three external cavity diode lasers. Four light beams with different frequencies are injected into four slave lasers for injection locking. Then Rb cooling light, K cooling light and K repump light are injected into a semiconductor tapered amplifier in a master oscillator-power configuration. This configuration is flexible and stable and can simultaneously produce the cooling and repump light for 40K and 87Rb trapping and cooling experiment.