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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Phase Noise Directely Measurement of Optical Second Harmonic Generation in MgO:PPLN Waveguide based on the 120-degree Phase Difference Interferometer

Fei Yang, Jiejun Zhao, Xi Zhang, Zitong Feng, Rui Wu, Haiwen Cai, Rong Wei, Dongjin Wang

Published Apr 1, 2019• 2 citations
Publisher

Abstract

A method based on the 120-degree phase difference interferometer for directly measuring the phase noise of 780 nm and 1560 nm laser is proposed. It can measure the phase noise as low as -150 dBc/Hz. The fundamental and double frequency laser with 1560 nm and 780 nm in MgO:PPLN waveguide are measured simultaneously. The results show that measured phase noise relationship between the 1560 nm and 780 nm laser without modulation is consistent with the SHG theoretical model. But if phase modulation exists in the laser, there would be additional noise in the SHG process. Especially when the phase modulation is strong, the modulation sidebands would generate the second, third even higher harmonic noise in the SHG process. These additional uncertain noise may affect the locking performance of Rb fountain atomic clock used the SHG laser source.

DOI
10.1109/fcs.2019.8856057

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A method based on the 120-degree phase difference interferometer for directly measuring the phase noise of 780 nm and 1560 nm laser is proposed. It can measure the phase noise as low as -150 dBc/Hz. The fundamental and double frequency laser with 1560 nm and 780 nm in MgO:PPLN waveguide are measured simultaneously. The results show that measured phase noise relationship between the 1560 nm and 780 nm laser without modulation is consistent with the SHG theoretical model. But if phase modulation exists in the laser, there would be additional noise in the SHG process. Especially when the phase modulation is strong, the modulation sidebands would generate the second, third even higher harmonic noise in the SHG process. These additional uncertain noise may affect the locking performance of Rb fountain atomic clock used the SHG laser source.
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.
An extremely conspicuous passive power noise stabilization is the first, to the best of our knowledge, discovered in a cavity-enhanced second-harmonic generation (SHG) process. Differing from the SHG as a buffer reservoir, the stronger strength of the nonlinear interaction pushes the power noise suppression level to a higher value and exhibits a broadband noise reduction performance due to the mechanism of dynamic pump suppression in the SHG process. The noise is suppressed to near shot noise limit (SNL) among the kHz to MHz frequency range, accompanied by a maximum noise reduction of 35 dB. A comprehensive demonstration indicates that the nonlinear interaction has no function on the phase noise of fundamental and harmonic waves. A theoretical model is also established that is consistent well with the experimental results. The methodology is beneficial to multiple optical metrology experiments.
We report a bound on the relative frequency fluctuations in nonlinear second harmonic generation. A 1,064 nm Nd:YAG laser is used to read out the phase of a Mach-Zehnder interferometer while PPKTP, a nonlinear crystal, is placed in each arm to generate second harmonic light. By comparing the arm length difference of the Mach Zehnder as read out by the fundamental 1064 nm light, and its second harmonic at 532 nm, we can bound the excess frequency noise introduced in the harmonic generation process. We report an amplitude spectral density of frequency noise with total RMS frequency deviation of 3 mHz and a minimum value of 20 μHz/Hz½ over 250 seconds with a measurement bandwidth of 128 Hz, corresponding to an Allan deviation of 10⁻¹⁹ at 20 seconds.
A compact and robust laser system is essential for mobile atom interferometers. Phase modulation can provide the two necessary phase-coherent frequencies without sophisticated phase-locking between two different lasers. However, the additional laser frequencies generated can perturb the atom interferometer. In this article, we report on a novel method to produce a single high-power laser beam composed of two phase-coherent sidebands without the perturbing carrier mode. Light from a diode laser is phase-modulated by using a fiber-coupled electro-optic modulator driven at 3.4 GHz and passes through a Fabry-Perot cavity with a 6.8 GHz free spectral range. The cavity filters the carrier mode to leave the two first-order sidebands for the two-photon Raman transition between the two hyperfine ground states of 87 Rb. The laser beam is then fed to a single tapered amplifier, and the two sidebands are both amplified without mode competition. The phase noise is lower than that of a state-of-the-art optically phase-locked external-cavity diode laser (−135 dBrad 2 /Hz at 10 kHz) at frequencies above 10 Hz. This technique can be used in all-fiber-based laser systems for future mobile atom interferometers.
To transfer and expand in the optical telecommunication range the accurate 5S1/2(F = 3) – 5D5/2 (F = 5) rubidium frequency reference at 778 nm (385.285 THz), we employ a second harmonic generation (SHG) process coupled to an optical frequency comb (OFC) both based on the use of a low power and narrow linewidth laser diode operating around 1556 nm. The Rb optical frequency reference is known with an accuracy of 2.6 × 10−12 and exhibits a frequency stability of 4 × 10−13τ−1/2, with a Flicker plateau of 2 × 10−14 for τ > 200 s. With 20 mW of laser power at 1556 nm, we perform a SHG process in a periodically poled lithium niobate (PPLN) crystal and we obtain 6.5 μW of harmonic radiation at 778 nm, used to phase lock the IR laser against a Rb optical frequency standard (OFS). This Rb-referenced fundamental radiation at 1556 nm is duplicated as 34 lines spaced by 10 GHz using an electro-optic phase modulator (EOM). This preliminary result is mainly limited by unexpected high optical losses in the Fabry Perot cavity, which comprises the EOM.
A new robust laser system for mobile atom interferometers is reported. Phase modulation of a single diode laser provide two phase-coherent frequencies necessary for the Raman transition in light-pulse atom interferometers. Additional laser frequencies generated by the phase modulation, which would disturb the phase of atom interferometers, is filtered by a Fabry-Perot cavity. The two sidebands of the phase-modulated light is tuned to the adjacent resonances of the cavity and the carrier mode and even-order higher sidebands are filtered. The laser beam is then fed to a single tapered amplifier, and the two sidebands are both amplified without mode competition. The phase noise is lower than that of a state-of-the-art optically phase-locked external-cavity diode laser (-135 dBrad2/Hz at 10 kHz). This technique can be used in all-fiber-based laser systems for future mobile atom interferometers. The laser system has advantage of low phase noise, high mechanical stability, low power consumption and compactness.