Generated Stokes Fields Research Articles

Modeling and characterization of high-power single frequency free-space Brillouin lasers.

Free-space Brillouin lasers (BLs) are capable of generating high-power, narrow-linewidth laser outputs at specific wavelengths. Although there have been impressive experimental demonstrations of these lasers, there is an absence of a corresponding theory that describes the dynamic processes that occur within them. This paper presents a time-independent analytical model that describes the generation of the first-order Stokes field within free-space BLs. This model is based on the cavity resonance enhancement theory and coupled wave equations that govern the processes of stimulated Brillouin scattering (SBS). This model is validated using an experimental diamond BL to numerically simulate the influence of the cavity design parameters on the SBS threshold, pump enhancement characteristics, and power of the generated Stokes field. Specifically, the model is used to determine the SBS cavity coupler reflectance to yield the maximum Stokes field output power and efficiency, which is also a function of the pump power and other cavity design parameters. This analysis shows that the appropriate choice of Brillouin cavity coupler reflectance maximizes the Stokes field output power for a given pump power. Furthermore, the onset of higher-order Stokes fields that are undesirable in the context of single-frequency laser operation were inhibited. This study aids in understanding the relationship between the cavity parameters and resultant laser characteristics for the design and optimization of laser systems.

Efficient transfer of spatial intensity and phase information of arbitrary modes via four-wave mixing in an atomic vapor

We propose a scheme to transfer the spatial intensity as well as phase information encoded initially in the spatial profile of a weak probe field onto a newly generated Stokes field in a nonlinear process of four-wave mixing (FWM). The FWM process is explored in a gaseous medium consisting of atoms modeled as a three-level system in the $\mathrm{\ensuremath{\Lambda}}$ configuration. We found that different orders of Hermite-Gaussian and Laguerre-Gaussian modes carried by a probe beam are effectively transferred to a Stokes beam. Interestingly, the transferred intensity of the Stokes beam is a clone of the probe beam, whereas the phase profile is conjugate to the probe. The phase distribution of the transmitted modes at the exit of the medium is explored by superimposing them on a copropagating plane wave. Moreover, the parametric amplification due to the nonlinear process of FWM prevents any information loss due to linear absorption and permits us to work at high optical depths. We found a structural similarity between transferred images of about $99%$, which provides clear evidence for the successful transfer of information in the FWM process.

Polarization properties of noise-initiated stimulated Brillouin scattering in linear birefringent fibers

The transient theory of stimulated Brillouin scattering (SBS) is developed for optical fibers with constant modal birefringence along its length and arbitrary polarization of the pump and Stokes fields. SBS is initiated by thermally excited acoustic waves distributed along the length of the optical fiber, and the generated Stokes field fluctuates in time. It is shown that when a single-frequency pump field is launched such that the fiber is nearly equally excited along the slow and fast axes, the generated Stokes field is partially polarized for fiber lengths longer than its polarization beat length, due to temporal fluctuations in power and polarization. The Stokes field degree of polarization is reduced as the birefringence of the fiber is increased. For launched pump polarizations resulting in nonequal excitation along the slow and fast axes of the fiber, the generated Stokes field is preferentially linearly polarized along the principal axis that exhibits the highest Brillouin gain for fibers with birefringence such that its length is greater than twice its polarization beat length. As the fiber birefringence is reduced, the generated Stokes field state of polarization moves towards the launched pump polarization state, with the opposite sense of rotation. Numerical results are presented to quantify these effects as a function of single-pass Brillouin gain, fiber birefringence, and launched pump polarization state.

Optical angular momentum manipulations in a four-wave mixing process

We investigate the spatial and quantum intensity correlations between the probe and Stokes optical fields produced via four-wave mixing in a double-Λ configuration, when both incoming probe and control fields carry non-zero optical orbital angular momentum (OAM). We observed that the topological charge of the generated Stokes field obeyed the OAM conservation law. However, the maximum values and optimal conditions for the intensity squeezing between the probe and Stokes fields were largely independent of the angular momenta of the beams, even when these two fields had significantly different OAM charges. We also investigated the case of a composite-vortex pump field, containing two closely positioned optical vortices, and showed that the generated Stokes field carried the OAM corresponding to the total topological charge of the pump field, further expanding the range of possible OAM manipulation techniques.

Double optomechanical transparency with direct mechanical interaction**Project supported by the National Natural Science Foundation of China (Grant Nos. 61178021, 11474118, and 11204099) and the National Basic Research Program of China (Grant No. 2012CB921604).

We present a mechanism for double transparency in an optomechanical system. This mechanism is based on the coupling of a moving cavity mirror to a second mechanical oscillator. Due to the purely mechanical coupling and the radiation pressure, three pathways are established for excitations of the probe photons into the cavity photons. Destructive interference occurs at two different frequencies, leading to double transparency to the probe field. It is the coupling strength between the mechanical oscillators that determines the locations of the transparency windows. Moreover, the normal splitting appears for the generated Stokes field and the four-wave mixing process is inhibited on resonance.

Double electromagnetically induced transparency and narrowing of probe absorption in a ring cavity with nanomechanical mirrors

We study the effect of a strong coupling field on the absorptive property of a ring cavity with two mirrors oscillating at slightly different frequencies to a weak probe field. We observe double electromagnetically induced transparency windows separated by an absorption peak at line centre in the output probe field under the action of a strong coupling field. We find that increasing driving power can broaden the two transparency windows, which results in narrowing of the central absorption peak. At high driving power, the linewidth of the sharp central absorption peak is approximately equal to the mechanical linewidth. We show the normal mode splitting in both the output probe field and the generated Stokes field. We also find that the suppression of the four-wave mixing process can be achieved on resonance.

Synchronous control of dual-channel all-optical multistate switching

We have experimentally observed optical multistabilities (OMs) simultaneously on both the signal and generated Stokes fields in an optical ring cavity with a coherently prepared multilevel atomic medium. The two observed OMs, which are governed by different physical processes, are coupled via the multilevel atomic medium and exhibit similar threshold behaviors. By modulating the cavity input (signal) field with positive or negative pulses, dual-channel all-optical multistate switching has been realized and synchronously controlled, which can be useful for increasing communication and computation capacities.

Observation of temporal beating in first- and second-order intensity measurement between independent Raman Stokes fields in atomic vapor

By using spontaneous Raman processes in the high-gain regime, we produce two independent Raman Stokes fields from an atomic ensemble. Temporal beating is observed between the two directly generated Stokes fields in a single realization. The beat frequency is found to be a result of an ac Stark frequency shift effect. However, due to the spontaneous nature of the process, the phases of the two Stokes fields change from one realization to another so that the beat signal disappears after averaging over many realizations. On the other hand, the beat signal is recovered in an intensity correlation measurement, showing a two-photon interference effect. The beat signal in intensity correlation enables us to obtain dephasing information in the Raman process. The dephasing effect is found to depend on the temperature of the atomic medium.

Group velocity study in hot 87Rb vapour with buffer gas

We study the behaviour of the group velocity of light under conditions of electromagnetically induced transparency (EIT) in a Doppler broadened medium. Specifically, we show how the group delay (or group velocity) of probe and generated Stokes fields depends on the one-photon detuning of drive and probe fields. We find that for atoms in a buffer gas the group velocity decreases with positive one-photon detuning of the drive fields, and increases when the fields are red detuned. This dependence is counterintuitive to what would be expected if the one-photon detuning resulted in an interaction of the light with the resonant velocity subgroup.

Group velocity study in hot 87 Rb vapour with buffer gas

We study the behavior of the group velocity of light under conditions of electromagnetically induced transparency (EIT) in a Doppler broadened medium. Specifically, we show how the group delay (or group velocity) of probe and generated Stokes fields depends on the one-photon detuning of drive and probe fields. We find that for atoms in a buffer gas the group velocity decreases with positive one-photon detuning of the drive fields, and increases when the fields are red detuned. This dependence is counter-intuitive to what would be expected if the one-photon detuning resulted in an interaction of the light with the resonant velocity subgroup.