Cascaded Brillouin lasing in monolithic barium fluoride whispering gallery mode resonators

Abstract

We report the observation of stimulated Brillouin scattering and lasing at 1550 nm in barium fluoride (BaF 2 ) crystal. Brillouin lasing was achieved with ultra-high quality (Q) factor monolithic whispering gallery mode (WGM) mm-size disk resonators. Overmoded resonators were specifically used to provide cavity resonances for both the pump and all Brillouin Stokes waves. Single and multiple Brillouin Stokes radiations with frequency shift ranging from 8.2 GHz up to 49 GHz have been generated through cascaded Brillouin lasing. BaF 2 resonator-based Brillouin lasing can find potential applications for high-coherence lasers and microwave photonics.Stimulated Brillouin scattering (SBS) is a nonlinear optical process resulting from the coherent interaction of light and acoustic waves. It is usually related to the effect of electrostriction and gives rise to inelastic light backscattering with a Doppler downshift related to the acoustic phonon frequency. Over the past years, SBS has been extensively studied in numerous optical waveguides such as optical fibers 1-3 , photonics crystal fibers 4,5 , and on-chip photonic integrated circuits [6][7][8] . A variety of nonlinear materials including silica, chalcogenide or silicon have been investigated. Enhanced SBS has been recently predicted and demonstrated in nanoscale silicon photonic waveguides 9,10 , where the radiation pressure combines with electrostriction to greatly improve the Brillouin gain, thus bridging the gap between SBS and optomechanics. Enhanced and cascaded SBS can also be easily achieved in nonlinear optical cavities, leading to narrow-linewidth and efficient SBS lasing 11,12 . Among optical resonators, whispering gallery mode (WGM) resonators have a number of qualities that make them very attractive for investigating SBS. Their advantages include a strong light confinement due to small mode volumes and ultra-high Q factors 13 . Moreover, crystalline WGM resonators are very interesting because of their broad transparency window ranging from the ultraviolet to the mid-infrared region [14][15][16] . These photonic platforms thus appear as alternative and promising solutions for nonlinear applications, and thus opens an approach to harness and enhance the interaction between photons and acoustic phonons. For instance, Brillouin lasing with microwatt threshold power has recently been observed in ultra-high Q-factor CaF 2 WGM resonators 17 . A narrow-linewidth Brillouin microcavity laser and an ultra-low-phase-noise microwave synthetizer have also been demonstrated using chemically etched ultrahigh-Q silica-on-silicon wedge resonators 18,19 . Brillouin scattering from surface acoustic waves has also been reported in MgF 2 WGM resonators and in silica microspheres [20][21][22] .

Kerr optical frequency combs: theory, applications and perspectives

“…The issue of dispersion engineering is also critical, as it directly defines the spectral span of the comb [103]. Other nonlinear effects that might compete with the Kerr nonlinearity also have to be accounted for, such as Brillouin scattering [104] or even thermal effects [105]. From the application standpoint, Kerr combs can also be combined with other traditional techniques to provide a versatile set of carriers for coherent optical telecommunications [106], while the versatility of primary combs (which can permit to generate combs with an intermodal frequency higher than a terahertz) could be exploited for the purpose of terahertz generation [50].…”

Nonlinear and Quantum Optics with Whispering Gallery Resonators

“…In table 4 we summarize the key parameters for backward [462,476,477] and forward [478][479][480] scattering experiments in resonators ranging from approximately 100 μm [476,479,480] to 5-6 mm [462,477] in diameter. Many of these experiments also show cascaded Brillouin scattering.…”

Opto-acoustic phenomena in whispering gallery mode resonators

“…Nevertheless, the multimode structure of WGMs as shown in Figure 2(a) can facilitate the realization of this condition. In a centimeter-scale WGM resonator, Brillouin lasing can be observed in several modes pumped by a mW level CW laser within one FSR [38] .…”

“…The ultrahigh Q factors and multiresonance conditions fulfilled in such a centimeter scale resonator lead to the realization of Cascaded SBS. Single SBS can also be realized with selected pumping when these conditions are not fulfilled for the Second Stokes [38] . It is worth noting that the 5th Stokes component is missing in the backward direction as shown in Figure 4(b).…”

“…The material choices for such resonators include lithium niobate [28,29] , barium borate [30,31] , fluoride crystals [32][33][34][35] , diamond [36] and so on. At this date, SBS has been reported in crystalline WGM resonators made of calcium fluoride [37] and barium fluoride [38] . Compared with other fluoride crystals, barium fluoride is an interesting scintillation crystal for high energy particle detection [39] .…”