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Journal of Shanxi University

Experiment of Laser Phase Lock with Source Locking CW Microwave Frequency Counter

Jing Zhang

Published Jan 1, 2012• 0 citations
OpenAlex

Abstract

Technology of achieving stable laser frequency plays an important role in the application of high precision measurement and high fine spectrum.In this paper,we phase-locked a laser for 3 to 4 hours to the other laser by the signal of beat frequency between two lasers,based on the instrument:Source locking CW microwave frequency counter.This work enables us to investigate the electromagnetically induced transparency spectroscopy,Raman spectroscopy and four-wave-mixing in atomic system.

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Review of Scientific Instruments•Jan 1, 2008•60 citations
Technology of achieving stable laser frequency plays an important role in the application of high precision measurement and high fine spectrum.In this paper,we phase-locked a laser for 3 to 4 hours to the other laser by the signal of beat frequency between two lasers,based on the instrument:Source locking CW microwave frequency counter.This work enables us to investigate the electromagnetically induced transparency spectroscopy,Raman spectroscopy and four-wave-mixing in atomic system.
The difference frequency between two single-mode CW lasers was stabilized to a microwave (MW) frequency reference by using an optical phase-lock loop (PLL). The long-term frequency jittering of the beat spectrum was much suppressed by this method. The difference frequency was able to be continuously scanned from 0.1 to 18 GHz by changing the microwave reference frequency. It was also possible to scan the two lasers synchronously keeping the difference frequency constant. This stable and flexible light source would be an ideal light source for optical-optical double resonance (OODR) spectroscopy and resonance laser Raman experiments. For the demonstration, this new system was applied to an OODR experiment using a Rubidium atom. The hyperfine structures in the D2line were observed in the sub-Doppler condition.
The authors have used an electromagnetically induced transparency resonance in rubidium as a dispersive reference to lock the relative frequency of two lasers to the atomic ground-state hyperfine splitting. The beat frequency between the two lasers directly generates a microwave signal at 3.036GHz (Rb85) or 6.835GHz (Rb87). High bandwidth (600kHz) feedback was achieved with only low-frequency (10MHz) electronics using the frequency modulation sideband method. The spectral width of the microwave beat frequency was reduced to less than 1kHz. The technique offers a convenient and low-cost method suitable for many topical two-frequency experiments, including coherent population trapping, slow light, lasing without inversion, and Raman sideband cooling.
The technology of generating Raman laser is not only an important research content in the field of quantum precision measurement, but also a core technology of quantum inertial sensors such as cold atom gravimeter, gyroscope. For 87Rb atoms, two 780-nm lasers with a frequency difference of 6.834 GHz and a stable phase are needed to generate Raman light. Raman lasers can be generated by optical phase-locked loops of two 780-nm narrow linewidth external cavity tunable semiconductor lasers (ECDL). But the system thus developed is complicated in structure and very poor in environmental adaptability. The other method to generate Raman laser is based on intracavity 1560-nm laser with frequency doubling and electro-optic modulation technology. This system is simple in structure and strong in environmental adaptability, but it will introduce sideband effects and cannot achieve phase lock due to the limit by the linewidth and feedback bandwidth performance of the laser. In view of this, based on two new 1560-nm external cavity lasers and a home-made phase-locked circuit, in this paper the phase lock of the laser is achieved, and a Raman laser with low phase noise is obtained. The phase noise of beat note signal is as low as –95 dBc/Hz at the Fourier frequency in a range from 1 kHz to 10 kHz. A comparison of this system with the phase-locked performance of the 780-nm dual laser and the hybrid dual laser shows that this scheme has a slight advantage. In addition, the effect of the phase-locking performance on the phase noise of the cold atom interferometer through the method of piecewise integration is analyzed in this work. The experimental results given in this work provide a scheme for developing a miniaturized Raman optical system suitable for external fields.
In this Letter, we present a widely tunable laser frequency offset locking technique based on four-wave mixing (FWM). The Raman-amplified probe light and newly generated conjugate light exhibit high signal amplitude and ultra-narrow spectral characteristics, which contribute to robust laser frequency stabilization. The laser frequency can be selectively locked to the Stokes or anti-Stokes frequency of the four-wave mixing spectrum, maintaining a fixed frequency difference relative to the pump light that corresponds to the atomic ground state hyperfine splitting. By adjusting the detuning of the pump light frequency, a wide tuning range of several GHz can be achieved. Compared to the frequency modulation spectroscopy method, the modulation transfer technique in a double-lambda atomic system provides an atomic coherence-enhanced error signal with a larger peak-to-peak amplitude and a steeper zero-crossing gradient, resulting in significantly improved laser frequency stabilization performance. This method can enhance the performance of atomic sensors, such as improving the stability of the Raman light for atomic interferometers and increasing the laser stability of highly sensitive atomic magnetometers.
We produced a pair of coherent laser beams with a 3-GHz frequency difference by optically phase locking two modes from a single, multimode extended-cavity diode laser. This method is complementary to either a direct modulation or an optical phase locking of two independent lasers. A large differential frequency shift between the two modes of the laser allows efficient phase locking. We developed a simple theory to account for the large differential frequency shift. Allan deviation of the beat frequency when the two modes are phase-locked drops as an inverse of the measurement time and it reaches 10(-14) when the time is 1 h. Coherent population trapping spectroscopy of Rb atoms using the phase-locked beams resulted in a spectrum as narrow as that of the case using direct modulation by a stable frequency synthesizer.
We describe a phase-coherent laser system designed for use in experiments involving coherently prepared atomic media. We implement a simple technique based on a sample-and-hold circuit together with a reset of the integrating electronics that makes it possible to scan continuously the relative frequency between the lasers of over tens of gigahertz while keeping them phase locked. The system consists of three external-cavity diode lasers operating around 795 nm. A low-power laser serves as a frequency reference for two high-power lasers which are phased locked with an optical phase-locked loop. We measured the residual phase noise of the system to be less than 0.04 rad(2). In order to show the application of the system towards atomic coherence experiments, we used it to implement electromagnetically induced transparency in a rubidium vapor cell and obtained a reduction in the absorption coefficient of 92%.