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Journal of the Optical Society of America B

Bloch oscillations in an optical lattice generated by a laser source based on a fiber amplifier: decoherence effects due to amplified spontaneous emission

M. Andia, Étienne Wodey, F. Biraben, Pierre Cladé, Saïda Guellati-Khélifa

Published May 4, 2015• 11 citations
Publisher

Abstract

We have developed a laser source generated by frequency doubling of a seed laser at 1560 nm using a periodically poled lithium niobate crystal. The seed laser passes through a 30 W fiber amplifier. The laser source at 780 nm is used to perform Bloch oscillations of ultra-cold atoms in an optical lattice. In this paper we investigate the decoherence effects induced by the amplified spontaneous emission (ASE) in the gain medium. We show that the ASE drastically reduces the efficiency of Bloch oscillations. We totally filter out the ASE around the D2 line using a heated rubidium vapor cell. We precisely determine the power spectral density of the ASE by measuring the fraction of atoms transferred from the hyperfine level F=1 to F=2, when the laser is switched on.

DOI
10.1364/josab.32.001038

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We have developed a laser source generated by frequency doubling of a seed laser at 1560 nm using a periodically poled lithium niobate crystal. The seed laser passes through a 30 W fiber amplifier. The laser source at 780 nm is used to perform Bloch oscillations of ultra-cold atoms in an optical lattice. In this paper we investigate the decoherence effects induced by the amplified spontaneous emission (ASE) in the gain medium. We show that the ASE drastically reduces the efficiency of Bloch oscillations. We totally filter out the ASE around the D2 line using a heated rubidium vapor cell. We precisely determine the power spectral density of the ASE by measuring the fraction of atoms transferred from the hyperfine level F=1 to F=2, when the laser is switched on.
Master thesis in physics at University of Torino. ABSTRACT :Optical lattice clocks occupy a very important position in metrological research. Diffused worldwide, they are a strong candidate for a future redefinition of the second of the International System of Units (SI) and thanks to their extremely small fractional uncertainty about 10−18 they are involved in fundamental physics experiments, for example in the field of general relativity. In a optical clock, the lattice is used to trap ultracold atoms and it is realized with a retro-reflected laser tuned on magic wavelength to reduce perturbations on the clock transition. An optical lattice is generated usually starting by a titanium:sapphire laser: its main qualities are high output power and narrow linewidth, but it is delicate and difficult to use so its application in new kinds of atomic clocks, such as transportable or spacial, is not strongly recommended. Replacing it with an amplified laser diode would be preferable because these instruments are better for cost, reliability, size and weight, so suitable for applications out of laboratory, but at the same time present the problem of spectral purity as they have a not negligible amplified spontaneous emission (ASE): this phenomenon produces a broad spectrum detuned from magic wavelength inducing a perturbation on the clock transition. In my thesis I investigated experimentally the possibility to generate an optical lattice starting from an amplified laser diode emitting at 759.35 nm, ytterbium magic wavelength, and I applied the laser system that I developed on the atomic clock IT-Yb1 at Istituto Nazionale di Ricerca Metrologica (INRiM) located in Torino. The target of this work is to achieve fractional uncertainty due to ASE below 10−18, making the performances of the two different lasers sources equivalent. First, I calculated the theoretical model of the light shift induced by the lattice on the clock transition between ytterbium atomic states 1S0 →3 P0, comparing it with experimental results and I applied it to estimate ASE contribution to lattice shift. In the experimental part of this work I realized an optical system to study amplified laser diode emission and manipulate laser beam to obtain a good lattice from this laser source, filtering ASE with a volume Bragg grating and controlling power with an acousto optic modulator. Moreover I studied ASE spectral distribution in different conditions and its interaction with optical elements. Next studies foresee to measure experimentally with IT-Yb1 the shift induced by the laser system that I realized to calculate its uncertainty contribution and compare this result with the one obtained with a titanium:sapphire laser. This work will be useful to the development of compact or transportable atomic clocks based exclusively on semiconductor lasers with applications in the generation of optical timescales, travelling optical standards and the measurements of Earth gravitational redshift.
We report the characteristics of GHz bandwidth amplified spontaneous emission (ASE) from a hot Cs atom vapor cell, where the optical feedback was inhibited. When pumped by an 852 nm laser, both forward and backward ASE output near 894 nm showed a nonlinear increase in its power without a pump power threshold. A continual decrease in spectral width down to 4.7 GHz was experimentally observed as the ASE output power increased. Using the same vapor cell, we injected a 1mW signal to configure a single-pass optical amplifier, and we monitored the forward output both in temporal and spectral domains. We found the signal laser efficiently suppressed the ASE and obtained a large amplification factor over 700 at the pump power of 1.2 W.
Paraxial Maxwell-Bloch theory is used to describe amplified spontaneous emission (ASE) in strongly inverted media. Counterpropagation of waves coupled through gain saturation and diffraction, and pumping by a laser beam which propagates transverse to the amplifier are included in the treatment. The reactive part of the medium polarization is calculated by population-rate equations in the limit of rapid dephasing of the atomic dipoles. The spontaneous emission noise source needed for the numerical simulation of Maxwell's equations is derived from microscopic theory by coarse-graining, thereby establishing the compatibility of the spherically symmetric microscopic source with paraxial propagation theory. The three coupled Maxwell equations for the counterpropagating ASE waves and the transversely propagating pump wave are solved numerically in time and two spatial dimensions. This simulation is used to calculate the longitudinal profile of the transverse fluorescence, which measures the saturation of the gain, the longitudinal profile of the transmitted pump wave, which measures the saturation of pump absorption, and the transverse coherence function of the radiation emitted from the ends of the amplifier.
One of the prominent platforms for quantum technologies, cold atoms require reliable laser systems. We present the design, implementation, and characterization of a simple, compact, and economical laser system at 780 nm, entirely based on fiber components. Two semiconductor lasers at 1560 nm are amplified in a single erbium-doped fiber amplifier and frequency-doubled in a periodically-poled lithium niobate crystal. We characterize the amplitude noise and the linewidth of the lasers, as well as the SHG efficiency. With an RMS relative amplitude noise of 3 × 10−4 at 1 s and linewidths below 1 MHz, our system is suitable for cooling and trapping of Rb atoms.
Experiments involving optical traps often require careful control of the ac Stark shifts induced by strong confining light fields. By carefully balancing light shifts between two atomic states of interest, optical traps at the magic wavelength have been especially effective at suppressing deleterious effects stemming from such shifts. Highlighting the power of this technique, optical clocks today exploit Lamb-Dicke confinement in magic-wavelength optical traps, in some cases realizing shift cancelation at the ten parts per billion level. Theory and empirical measurements can be used at varying levels of precision to determine the magic wavelength where shift cancelation occurs. However, lasers exhibit background spectra from amplified spontaneous emission or other lasing modes that can easily contaminate measurement of the magic wavelength and its reproducibility in other experiments or conditions. Indeed, residual light shifts from laser background have plagued optical lattice clock measurements for years. In this work, we develop a simple theoretical model allowing prediction of light shifts from measured background spectra. We demonstrate good agreement between this model and measurements of the background light shift from an amplified diode laser in a Yb optical lattice clock. Additionally, we model and experimentally characterize the filtering effect of a volume Bragg grating bandpass filter, demonstrating that application of the filter can reduce background light shifts from amplified spontaneous emission well below the ${10}^{\ensuremath{-}18}$ fractional clock frequency level. This demonstration is corroborated by direct clock comparisons between a filtered amplified diode laser and a filtered titanium:sapphire laser.