image: Figure 1| Forward Brillouin Scattering in Few-Mode Fibers. (a) Guided ultrasonic, mechanical modes of standard fibers. The images show the calculated profiles of material displacement in the radial direction. Left: radially symmetric mode. Center: first-order azimuthal symmetry. Right: Two-fold azimuthal symmetry. Only the radially symmetric modes and the two-fold symmetric ones are accessible through Brillouin scattering processes in single-mode fibers, whereas first-order symmetric modes are not. (b) Measured spectrum of forwards Brillouin scattering through high order mode LP02 of a few-mode fiber. The spectrum extends up to 1.8 GHz, about three times higher than in single-mode fibers. (c) Measured and calculated spectral of acoustic modes of first-order azimuthal symmetry, observed for the first time in the few-mode fiber. (d) Frequency detuning of stimulated acoustic modes above their cut-off values in a few-mode fiber. In single-mode fibers, these modes may only be generated exactly on cut-off.
Credit: Elad Layosh et al.
Optical fibers are built to guide light over long distances. Their ability to do so has enabled the internet and changed human society profoundly. In addition to light waves, however, the structure of an optical fiber is also a waveguide for ultrasound. This is an inherent, built-in property of the fiber structure which is separate from its optical purpose. Moreover, light and ultrasonic mechanical waves within the fiber structure are not entirely independent. While we are accustomed to thinking of light and ultrasound as distinct and unrelated phenomena, this is not the case: the propagation of optical waves in a fiber may generate and launch ultrasound. At the same time, ultrasonic waves can scatter and modulate guided light. These physical interactions are known as Brillouin scattering. The effect has been studied in optical fibers for over fifty years. On top of its fundamental physics significance, Brillouin scattering is very useful towards ultra-narrow laser sources and advanced sensing systems.
The vast majority of fibers are designed for the single-mode regime: Guided light may only propagate with a single, well-defined velocity and a single spatial profile. However, the capacity of data communication in the single guided mode has largely run out. To carry more data on a single fiber, recent years have witnessed the rapid development of few-mode optical fibers. As their name suggests, several distinct wavefronts can propagate in parallel along such fibers. Each waveform is capable of carrying an additional, independent data payload, thereby multiplying the transmission capacity of the fiber beyond the single-mode bottleneck.
Research and applications of Brillouin scattering in fibers have also focused primarily on the single mode regime. The emergence of few-mode fibers, however, raises a new question: What would Brillouin scattering processes look like in these new fibers? Would they be similar to corresponding phenomena in the single-mode, or perhaps different? Works trying to address this new opportunity have thus far been few and far between. In a new paper published in Light: Science & Applications, a team of scientists led by Professor Avi Zadok and graduate student Elad Layosh from Bar-Ilan University and the Technion in Israel provide in-depth analysis of Brillouin scattering in few-mode fibers.
"We have found," says Prof. Zadok, "that interactions between light and ultrasound in the few-mode fiber are fundamentally different from the well-known, single-mode regime. They are far richer and more interesting. For example, the range of ultrasound frequencies we could reach in the few-mode fiber was much higher: up to 1.8 GHz, about three times higher than in single-mode fibers. The higher frequencies can serve to make better fiber sensors, and oscillator sources of both microwaves and laser light."
"Another difference has to do with symmetries," Prof. Zadok continues. "Ultrasound propagates along fibers in discrete modes, just like light. These modes exhibit many orders and forms of symmetry with respect to the azimuthal angle. However, in the standard single-mode fibers we are only able to access specific classes of ultrasound wavefronts: ones that are purely radial, and others that maintain two-fold symmetry (See Fig. 1). In the few-mode fiber, this restriction is lifted. Our calculations and measurements already showed the excitation of ultrasonic waves with symmetry orders 1 and 4. In theory, any order should be possible. This extension is not merely a scientific curiosity. The azimuthal symmetries signify the angular momentum of guided waves. In few-mode fibers, we may freely exchange angular momentum between light and ultrasound. On top of the basic principles, this degree of freedom could be highly useful for data communication, signal processing, and even quantum technologies"
"Lastly, the ultrasound modes of the fibers have a cut-off frequency, below which they no longer propagate. In standard single-mode fibers, we may only excite the ultrasound waves right at their cut-off. This is yet another limitation that is lifted by in the few-mode fibers case. In our work, we have shown the stimulation of mechanical ultrasound mode appreciably above cut-off. Here too, the result goes beyond basic research. Ultrasound waves above cut-off remove the inherent reciprocity of light waves propagating in the fiber. All of a sudden, we may switch light waves from one mode to another, but only in one direction of propagation. In the opposite direction, nothing happens. Such removal of reciprocity is rather illusive in optics and often mandates magnetic media. This capability is extremely important for isolators and lasers applications."
"To conclude," says Prof. Zadok, "the study of Brillouin scattering in few-mode fibers already extends our understanding of the fundamental phenomenon. It opens a new and exciting engineering playground for new and better laser sources and sensor systems in future works."
Article Title
Forward Brillouin Scattering in Few-Mode Fibers