Raman Pulse Research Articles

Sensitivity of a Point-Source-Interferometry-Based Inertial Measurement Unit Employing Large Momentum Transfer and Launched Atoms

We analyze theoretically the sensitivity of accelerometry and rotation sensing with a point source interferometer employing large momentum transfer (LMT) and present a design of an inertial measurement unit (IMU) that can measure rotation around and acceleration along each of the three axes. In this design, the launching technique is used to realize the LMT process without the need to physically change directions of the Raman pulses, thus significantly simplifying the apparatus. We also describe an explicit scheme for such an IMU.

Efficient tunable cascaded Raman source with all-silica fibers based on 2-µm DSR pulse pumping.

We present an efficient tunable all-silica-fiber 2nd-order cascaded Raman pulse laser utilizing 2-µm dissipative-soliton-resonance (DSR) rectangular pulses for pumping and highly GeO2-doped silica fiber as Raman gain medium. When pumped at 1966.5 nm, the maximum 1st-order Raman optical conversion efficiency is up to 64.4% at 2153 nm, with 92.4% spectral purity and 0.39-W average power. The maximum 2nd-order Raman optical conversion efficiency is 19.3% at 2370 nm, with 39.2% spectral purity and 0.25-W average power. To our knowledge, these conversion efficiencies and spectral purities represent the highest levels achieved in a mid-infrared all-silica-fiber cascaded pulsed Raman laser. Additionally, by adjusting the central wavelength of the DSR seed pulse, the 2nd-order Raman light can be tuned within a range of 41 nm (2354∼2395 nm). Our system provides a simple and easy-to-implement solution for realizing efficient tunable cascaded pulsed Raman lasers in the 2.4-µm band.

Raman-scattering-assisted noise-like pulse generation in an all-PM NOLM-based mode-locked fiber laser core-pumped by a 978 nm fiber laser

Stable Raman-scattering-assisted noise-like pulse is experimentally demonstrated in an all polarization-maintaining ytterbium-doped ‘nine-shaped’ mode-locked fiber laser based on nonlinear optical loop mirror. The pump source is a self-made single mode 978 nm fiber laser with a maximum output power of 8.45 W. Different pulse states are investigated through dispersive Fourier transform technology. First and second order Raman pulses are generated effectively with only 7.2 m length large-mode-area fiber. Numerical simulation is conducted to investigate the effect of gain fiber length on output laser wavelength. The output spectrum ranges from 980 nm to 1175 nm, the short-wavelength edge almost extends to the 978 nm pump wavelength, which is benefitted from the core-pumping scheme providing extremely high population inversion for gain fiber, as well as the short length YDF of 0.2 m suppressing reabsorption of radiation laser. The polarization extinction ratio is experimentally measured by a spectral subtraction method to be larger than 28 dB at each wavelength between 1010 nm and 1120 nm.

Sequential large momentum transfer exploiting rectangular Raman pulses

Sequential large momentum transfer exploiting rectangular Raman pulses

Coherent pulse generation based on noise-like-pulse pumped Raman amplification

The coherent Raman pulse generation based on the noise-like-pulse (NLP) pumped Raman amplification is numerically investigated in the cases with weak and strong walk-off effects between the pump and Stokes pulses. In the former case, the amplified Stokes pulse suffers from the temporal and spectral distortions because the random time-varying intensity profile of the NLP can be transferred to the Stokes pulse through combined effects of cross-phase modulation (XPM), stimulated Raman scattering (SRS), and spontaneous Raman scattering. In the latter case, the heavily germanium-doped fiber with large normal dispersion is firstly used as the Raman gain medium to improve the walk-off effect and Raman gain. As a result, the Stokes pulse can be amplified undistorted and possess linear frequency chirp and conversion efficiency of up to 82.4 %. Further, it is important to note that the NLP-pumped Raman fiber laser based on the large normal dispersion fiber can generate the Stokes pulse with linear frequency chirp, narrow temporal duration, and dissipative-soliton-like spectrum with steep edges, even though the saturable absorber is not introduced into the Raman fiber cavity.

Characterization of optical phase-locked two distributed-feedback fiber lasers for 87Rb atom interferometry

We report the optical phase-locking of two 1560 nm distributed-feedback (DFB) fiber lasers converted to 780 nm with a frequency difference of 6.834 GHz for 87Rb. The fiber laser exhibits a narrow linewidth and low-intensity noise, which are advantageous for a Raman pulse laser in an atomic gravimeter. However, the piezoelectric actuator of the DFB laser causes a low feedback rate in an optical phase-locking loop (OPLL). To overcome this limitation, we used an acousto-optic modulator as an additional actuator to increase the modulation bandwidth of the OPLL and achieved a residual phase noise of approximately -127 dBrad2/Hz in the offset frequency range from 10 Hz to 1 kHz. The sensitivity limit estimated using the sensitivity function and experimental parameters of 87Rb atomic gravimeter shows that our OPLL system can be used in state-of-the-art gravimeters.

Efficient Raman pulse fiber laser pumped by a dissipative soliton resonance pulse near 2 µm.

A high-efficiency Raman conversion from 1.987 µm to 2.177 µm is demonstrated experimentally in 45 m GeO2-doped silica fiber, adopting a dissipative soliton resonance (DSR) rectangular pulse as the pump. Over the entire spectral distribution, the spectral purity of the first-order Raman pulse is up to 96.8%, suggesting a nearly complete pump depletion before the onset of cascaded Raman shifts. The corresponding pump-to-Raman conversion efficiency of 67.4% is the highest up to date in this spectral region. Meanwhile, a large Raman pulse energy of 1.03 µJ was obtained at the repetition rate near MHz level, corresponding to 0.893 W average power. In the total output, the Raman-dominated spike has a Full Width Half Maximum (FWHM) of 1.18 ns far narrower than DSR's pulse duration of 10.25 ns. The results indicate that DSR is a promising candidate for developing efficient Raman nanosecond pulse fiber laser in mid-infrared (MIR) region.

Improving the fringe contrast in an atomic gravimeter by optimizing the Raman laser intensity

As the expansion of the atomic cloud’s diameter relative to the Gaussian Raman beam in an atom interferometer, the interaction between them would be reduced. Such an effect can decrease the effective Rabi frequency and lower the state transfer efficiency of the Raman laser, which finally reduces the fringe contrast and the sensitivity of the atom interferometer. In this work, we demonstrate experimentally that the fringe contrast can be improved by optimizing the laser intensities of three Raman pulses, and find that the criterion for the optimized Raman laser intensities should be the Raman pulse efficiency rather than the Rabi frequency. In the case of an atomic gravimeter, our method obtains an obvious enhancement of the fringe contrast when T > 60 ms and the maximum contrast enhancement is ∼5.1% (from 25.3% to 26.6%) at T = 120 ms. These results could be beneficial for atom-interferometer-based inertial sensors, including but not limited to the atomic gravimeter, gravity gradiometer and gyroscope.

Optimization and control of cold atom interference phase shift based on laser double-sideband suppression

Using the electro-optical modulation method to generate Raman beams for cold atom interference is one of the better methods for constructing a more compact and robust laser system. But this way will generate some residual sidebands resulting in the additional interference phase shift, which can affect the measurement accuracy of cold atom interferometer. In order to weaken the effect of laser modulation sidebands on the phase shift of cold atom interference, a double-sideband suppressed-carrier modulation laser system for cold atom interference is constructed. Based on the designed laser system, the principle of double-sideband generation and suppression is analyzed in detail, and some residual sidebands are adjusted and controlled. Moreover, some important optical parameters that affect the phase shift of cold atomic interference, such as the initial distance between the Raman retro-reflection mirror and the atomic cloud, the interrogation time between two adjacent Raman pulses, the laser modulation depth and the initial velocity of the atomic cloud, are discussed and optimized. By optimizing these relevant parameters, the influence of residual modulation sidebands on the phase shift of cold atomic interference is weakened drastically. The research results indicate, making use of the method of double-sideband suppression, the phase shift of cold atomic interference can be optimized to 0.7 mrad when the initial distance between the Raman retro-reflection mirror and the atomic cloud is 105 mm, and the interrogation time between two adjacent Raman pulses is 82 ms. More importantly, this work can provide a method for weakening the influence of Raman sideband effect on the phase shift of cold atom interferometer, and the corresponding laser system can be applied to other inertial sensors such as atomic gravimeter or atomic gravity gradiometer.

Observing phase jumps of solitons in Bose-Einstein condensates

The phase difference of the macroscopic wave function is a unique structure of the soliton in an atomic Bose--Einstein condensate (BEC). However, experiments on ultracold atoms so far have observed the valley of the density profile to study the dynamics of solitons. We propose a method to observe the phase difference of a soliton in a BEC by using an interference technique with Raman and rf pulses. We introduce a phase jump factor, which is an indicator to measure the phase difference between two points. It is demonstrated by using the projected Gross--Pitaevskii equation that an interference density ratio, the density ratio of two-component BECs after the Raman and rf pulses, reproduces the phase jump factor well. This technique will become an alternative method to study the decay and breakdown of a phase imprinted soliton in atomic BECs.

Numerical simulation of nonlinear optical gain modulation in a Raman fiber amplifier.

Nonlinear optical gain modulation (NOGM) in a Raman fiber amplifier is numerically simulated with the generalized nonlinear Schrödinger equation. In the NOGM setup, a single frequency continuous wave seed laser is gain modulated into femtosecond pulses by an ultrafast pump, which can induce strong stimulated Raman scattering in a piece of single mode optical fiber. Different parameters regarding seed, pump and nonlinear gain medium (Raman fiber) are investigated in detail to find the best condition for higher Raman conversion efficiency. Simulated results reveal that the walk-off between pump and Raman pulses due to dispersion is one of the most important factors affecting the NOGM pulse's performance. Only when the speed of walk-off matches with the one of Raman conversion process can the conversion efficiency be optimized. This work offers a guild-line for the design of a fiber-based NOGM laser, which is able to produce wavelength-agile, femtosecond laser pulses with µJ-level pulse energy under more than 85% efficiency.

Pulse-Profile-Maintaining Characteristics of Gain-Switched Nanosecond Raman Fiber Laser and Amplifier

This paper investigates pulse-profile-maintaining characteristics of a gain-switched nanosecond Raman fiber laser (RFL) and its Raman fiber amplifier (RFA). It is found that the pulse profiles can be precisely maintained after the gain-switched Raman conversion, only if the pump pulse repetition frequency (PRF) is an integer multiple of, a unit fraction of, or multiple unit fractions of the fundamental cavity-defined PRF. The produced Raman pulses are then amplified in an RFA, and the pulse profiles can also be precisely maintained. However, it is further noticed that the detuning-induced defects can be amplified if they are with the pulse itself. All these observed phenomena should originate from the transient property of Raman gain, not involving any notable delaying or saturation effects that are typically seen with rare-earth-doped gain fibers. Our results demonstrate that, in the nanosecond scale, the Raman-converted pulse can well maintain its profile and shows strong resistance to detune between PRFs of the pump and Raman pulses. Benefiting from that, we can reasonably neglect the pump depletion effect due to walk-off effect typically seen in various short pulse regimes, which can largely simplify the design of nanosecond pulsed pumping Raman sources.

The Effect of Imperfect Raman Pulses on the Atomic Interference Signal

The Effect of Imperfect Raman Pulses on the Atomic Interference Signal

Interference fringe fitting of atom gravimeter based on fitness particle swarm optimization

In the atom gravimeter, three Raman pulses are utilized to realize the interference of atom matter waves, and atom interference fringes are obtained by scanning the chirp rate of the Raman laser during the interference time. Previously, fringe data analysis methods used LS (Least Squares) to fit the cosine function of each interference fringe data to minimize the standard deviation between the estimated value and the observed value of each group of fringe data or the EKF (Extended Kalman Filter) method to obtain the estimation of the gravity value. In this paper, we propose a new method applied to the interference fringe fitting of the atom gravimeter, namely, through the FPSO (Fitness Particle Swarm Optimization) method to estimate the parameters of the interference fringe atom and then estimate the gravity value. First, the theoretical analysis and proof are carried out by using simulation data. On this basis, we carried out a gravity measurement experiment in the ship-mounted mooring state, which further verified the feasibility and effectiveness of the algorithm. The simulation and experimental results show that, compared with LS and EKF methods, the FPSO method can search the relatively optimal fitting parameters of atom interference fringes quickly and accurately and improve the accuracy and stability of the atom gravimeter measurement. It is feasible and effective to apply the FPSO method to fitting atom interference fringes. The FPSO method proposed in this paper can be used as a new method for fitting atom interference fringes, which provides a new idea and choice for accurate gravity measurement in a dynamic environment.

Raman-enhanced multicolor pulses in dissipative soliton mode-locked ring-cavity fiber laser based on GIMF-SA

The stable multicolor complex with dissipative solitons (DS) and Raman pulses (RP) is experimentally demonstrated in the mode-locked Yb-doped ring-cavity fiber laser based on a graded-index multimode fiber (GIMF) as a saturable absorber (SA). The central wavelength of DS and RP are 1032.01 nm and 1078.08 nm, respectively. By wavelength-dividing the output pulse, it is found that the pulse width and period of DS and RP are consistent with the total pulse, but the DS pulse intensity is larger than that of RP. The single-to-noise ratio (SNR) of DS is about 55 dB, which is large than that of the complex pulse. We further improve the stability of the mode-locked state in the cavity by separating the RP out of the cavity. The experimental results obtained by two methods of separating pulses indicates that the stimulated Raman scattering (SRS) effect destabilizes the DS mode-locked fiber laser and doesn’t disrupt the single pulse generation regime.

Feedback control of atom trajectories in a horizontal atom gravity gradiometer.

The coincidence between the atom trajectory and the Raman pulse sequence is very important for an intersection type atom interferometer. Here we present a feedback control technique for the atom trajectories in our horizontal gravity gradiometer, which improves the stabilities of the trajectories by about 2 orders of magnitude. Through the further study of the dependence of the interferometer contrasts on the atom trajectories, we lock the trajectories at optimal positions. And by this technique, the sensitivity of the gravity gradiometer is improved from 982 E/Hz1/2 to 763 E/Hz1/2, while the long-term stability is enhanced more significantly and reaches 8.9 E after an integration time of 6000 s. This work may provide hints to other experiments based on intersection type atom interferometers.

Cascaded nonlinear optical gain modulation for coherent femtosecond pulse generation.

Nonlinear optical gain modulation (NOGM) is a method to generate high performance ultrafast pulses with wavelength versatility. Here we demonstrate coherent femtosecond Raman pulse generation through cascaded NOGM process experimentally. Two single-frequency seed lasers (1121 and 1178 nm) are gain-modulated by 117 nJ 1064 nm picosecond pulses in a Raman fiber amplifier. Second-order (1178 nm) Stokes pulses are generated, which have a pulse energy of 76 nJ (corresponding to an optical conversion efficiency of 65%) with a pulse duration of 621 fs (after compression). Dynamic evolution of both pump and cascaded Stokes pulses within the Raman amplifier are investigated by numerical simulations. The influences of pump pulse duration and energy are studied in detail numerically. Moreover, the simulations reveal that NOGM pulses with higher energy and shorter pulse duration could be obtained by limiting the impact of walk-off effect between pump and Raman pulses. This approach can offer a high energy and wavelength-agile ultrafast source for various applications such as optical metrology and biomedical imagining.

Engineering infinite-range SU(n) interactions with spin-orbit-coupled fermions in an optical lattice

We study multilevel fermions in an optical lattice described by the Hubbard model with on-site $\mathrm{SU}(n)$-symmetric interactions. We show that in an appropriate parameter regime this system can be mapped onto a spin model with all-to-all $\mathrm{SU}(n)$-symmetric couplings. Raman pulses that address internal spin states modify the atomic dispersion relation and induce spin-orbit coupling, which can act as a synthetic inhomogeneous magnetic field that competes with the $\mathrm{SU}(n)$ exchange interactions. We investigate the mean-field dynamical phase diagram of the resulting model as a function of $n$ and different initial configurations that are accessible with Raman pulses. Consistent with previous studies for $n=2$, we find that for some initial states the spin model exhibits two distinct dynamical phases that obey simple scaling relations with $n$. Moreover, for $n>2$ we find that dynamical behavior can be highly sensitive to initial intraspin coherences. Our predictions are readily testable in current experiments with ultracold alkaline-earth-metal(-like) atoms.

Determining the Three-Dimensional Position of an Atomic Cloud in an Atom Interferometer Using One Laser Beam

Accurately measuring the three-dimensional position of an atomic cloud is essential for improving the precision of atom interferometers. In the work presented here, the three-dimensional position of an atomic cloud in an atom interferometer is measured with only one laser beam, generated by a phase modulator. The vertical position is determined from the change in the additional-sideband effect with the time interval $T$ between adjacent Raman pulses in the atom interferometer. Based on this method, the vertical position of the atomic cloud is measured with an uncertainty of 0.2 mm; therefore, the uncertainty in a gravity measurement contributed by the gravity gradient is not limited by the uncertainty in the measurement of the vertical position at the level of $0.1\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{Gal}$. Furthermore, a method of measuring Rabi oscillations at different horizontal positions to determine the horizontal position of the atomic cloud is proposed and is demonstrated experimentally. With this method, the horizontal position can be determined with an uncertainty of less than 0.1 mm, and the horizontal velocity of the atomic cloud can also be deduced. The methods demonstrated in this paper are helpful for evaluating systematic errors in atom interferometers, and also useful in precision measurements based on an atomic fountain.

High-precision magnetic field locking system for cold atoms

In ultracold atomic experiments, evaporative cooling is usually achieved by using Feshbach resonance magnetic fields on the order of Gauss to hundreds of Gausses. The frequency of resonant transition induced by the optical field or radiofrequency is directly affected by the stability of the quantum axis. For example, the phase between two linearly independent vectors of a qubit is affected by the magnetic field noise. Based on the Feshbach resonance technique, magnetic field regulation has become a basic tool to control the interaction between atoms. Narrow Feshbach resonance shows unique advantages in high-temperature superconducting, superfluidity, neutron star state simulation, etc. However, since its resonance width and Fermi energy can be compared with each other, the scattering characteristics are greatly disturbed by the magnetic field. Therefore, a stable and uniform magnetic field is a prerequisite for studying the narrow Feshbach resonances. In experiment, Helmholtz coils are usually used to provide the magnetic field for cold atomic gas, and the magnetic field noise is generally determined by the coil current noise and other magnetic field noises of the environment. However, there are relatively few researches of the high-precision control of large magnetic fields above hundreds of Gausses. With a larger coil current required, the coil current noise contributes more to the magnetic field noise, thus high-precision control of large magnetic fields is still challenging. In this paper, a magnetic field locking system is used to realize a <inline-formula><tex-math id="Z-20220704103959">\begin{document}$2.27 \times 10^{-6} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20220399_Z-20220704103959.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20220399_Z-20220704103959.png"/></alternatives></inline-formula> level locking of the Feshbach magnetic field. A feedback locking system is used to achieve the stability by shunting the magnetic field coil current noise. Compared with the non-locked magnetic field, the low-frequency current noise is suppressed by more than 45 dB. To assess the stability of the actual magnetic field at the atoms, the Rabi oscillation is measured, the coherence time increases nearly 9.6 times, which effectively improves the stability of the ultracold atomic system. Furthermore, we measure the atom number fluctuation at the Gaussian inflection point of the loss spectrum under different Raman pulse widths to evaluate the noise of the magnetic field. Roman pulse duration up to a 24 μs is used to increase the sensitivity of atom number fluctuation in loss spectrum relative to magnetic field noise, of which the root mean square (RMS) noise is suppressed from 20.66 mGs to 1.2 mGs, a 16-fold reduction of the noise is obtained. Such a magnetic field locking system can provide an accurate and stable background magnetic field for ultracold atomic gases, which is of great significance for extending quantum storage time, precisely controlling atomic scattering, and simulating of condensed matter and other ultracold quantum gas in experiment.