A. Butsch

CW-pumped single-pass frequency comb generation by resonant optomechanical nonlinearity in dual-nanoweb fiber

Recent experiments in the field of strong optomechanical interactions have focused on either structures that are simultaneously optically and mechanically resonant, or photonic crystal fibers pumped by a laser intensity modulated at a mechanical resonant frequency of the glass core. Here, we report continuous-wave (CW) pumped self-oscillations of a fiber nanostructure that is only mechanically resonant. Since the mechanism has close similarities to stimulated Raman scattering by molecules, it has been named stimulated Raman-like scattering. The structure consists of two submicrometer thick glass membranes (nanowebs), spaced by a few hundred nanometers and supported inside a 12-cm-long capillary fiber. It is driven into oscillation by a CW pump laser at powers as low as a few milliwatts. As the pump power is increased above threshold, a comb of Stokes and anti-Stokes lines is generated, spaced by the oscillator frequency of ∼6  MHz. An unprecedentedly high Raman-like gain of ∼4×106  m−1 W−1 is inferred after analysis of the experimental data. Resonant frequencies as high as a few hundred megahertz are possible through the use of thicker and less-wide webs, suggesting that the structure can find application in passive mode-locking of fiber lasers, optical frequency metrology, and spectroscopy.

Optomechanical Self-Channeling of Light in a Suspended Planar Dual-Nanoweb Waveguide

It is shown that optomechanical forces can cause nonlinear self-channeling of light in a planar dual-slab waveguide. A system of two parallel silica nanowebs, spaced ~100 nm and supported inside a fiber capillary, is studied theoretically and an iterative scheme developed to analyze its nonlinear optomechanical properties. Steady-state field distributions and mechanical deformation profiles are obtained, demonstrating that self-channeling is possible in realistic structures at launched powers as low as a few mW. The differential optical nonlinearity of the self-channeled mode can be as much as 10×10(6) times higher than the corresponding electronic Kerr nonlinearity. It is also intrinsically broadband, does not utilize resonant effects, can be viewed as a consequence of the extreme nonlocality of the mechanical response, and in fact is a notable example of a so-called accessible soliton.

Resolving the mystery of milliwatt-threshold opto-mechanical self-oscillation in dual-nanoweb fiber

It is interesting to pose the question: How best to design an optomechanical device, with no electronics, optical cavity, or laser gain, that will self-oscillate when pumped in a single pass with only a few mW of single-frequency laser power? One might begin with a mechanically resonant and highly compliant system offering very high optomechanical gain. Such a system, when pumped by single-frequency light, might self-oscillate at its resonant frequency. It is well-known, however, that this will occur only if the group velocity dispersion of the light is high enough so that phonons causing pump-to-Stokes conversion are sufficiently dissimilar to those causing pump-to-anti-Stokes conversion. Recently it was reported that two light-guiding membranes 20 μm wide, ∼500 nm thick and spaced by ∼500 nm, suspended inside a glass fiber capillary, oscillated spontaneously at its mechanical resonant frequency (∼6 MHz) when pumped with only a few mW of single-frequency light. This was surprising, since perfect Raman gain suppression would be expected. In detailed measurements, using an interferometric side-probing technique capable of resolving nanoweb movements as small as 10 pm, we map out the vibrations along the fiber and show that stimulated intermodal scattering to a higher-order optical mode frustrates gain suppression, permitting the structure to self-oscillate. A detailed theoretical analysis confirms this picture. This novel mechanism makes possible the design of single-pass optomechanical oscillators that require only a few mW of optical power, no electronics nor any optical resonator. The design could also be implemented in silicon or any other suitable material.

Effects of squeezed-film damping on the optomechanical nonlinearity in dual-nanoweb fiber

The freely-suspended glass membranes in a dual-nanoweb fiber, driven at resonance by intensity-modulated light, exhibit a giant optomechanical nonlinearity. We experimentally investigate the effect of squeezed-film damping by exploring the pressure dependence of resonant frequency and mechanical quality factor. As a consequence of the unusually narrow slot between the nanowebs (22 μm by 550 nm), the gas-spring effect causes a pressure-dependent frequency shift that is ∼15 times greater than typically measured in micro-electro-mechanical devices. When evacuated, the dual-nanoweb fiber yields a quality factor of ∼3 600 and a resonant optomechanical nonlinear coefficient that is ∼60 000 times larger than the Kerr effect.

Optoacoustic isolators in photonic crystal fibre

Coherent optically-driven GHz acoustic waves, tightly guided in the micron-sized core of photonic crystal fibre, enable reconfigurable dynamic optical isolation and switching, providing new functionality that is useful in various types of all-optical fibre systems.

Pressure-tuning of the optomechanical nonlinearity in dual-nanoweb fibre

Optical driving of evacuated dual-nanoweb fibre at an acoustic resonance yields an optomechanical nonlinear coefficient ~90,000× greater than the Kerr-effect. Squeezed-film damping caused the resonant frequency to fall, and Q-factor to rise, with decreasing pressure.

Optomechanical and optoacoustic phenomena in microstructured silica fibres

Recent results on optomechanical and optoacoustic nonlinearities in optical fibres are reported. In a new type of a microstructured silica fibre, comprising two ultra-thin closely spaced glass waveguides, an extremely high and optically broadband optomechanical nonlinearity is shown to occur. This nonlinearity originates from the optical gradient forces between coupled waveguides, can exceed the Kerr effect by many orders of magnitude and allows the formation of stable self-trapped optical modes that represent a novel kind of optical soliton. Furthermore, optoacoustic interaction via electrostriction in the micron-sized core of a photonic crystal fibre is studied. It is demonstrated, that coherent optically-driven acoustic waves, tightly guided in the core, can facilitate in-fibre dynamic optical isolation and all-optical switching.

Recent progress in nonlinear optomechanics in microstructured optical fibers

We will present our recent progress in nonlinear optomechanics in a platform of microstructured optical fibers. By tightly confining transverse mechanical motion and guided laser light simultaneously in a micron-sized glass fiber core, we have successfully demonstrated novel types of nonlinear optomechanical effects at modest optical powers, thanks to the strongly enhanced optomechanical interactions. We will describe the principles, characteristics and potential applications of the effects.

Enhanced optomechanical nonlinearity in evacuated dual-nanoweb fiber

The non-resonant optomechanical nonlinearity is enhanced in dual-nanoweb fibers by improved design. A 10-fold increase in Q-factor, resulting in a resonant nonlinearity 200,000 times larger than the Kerr effect, is observed after evacuating the fiber.

Switching characteristics of forward SIPS in photonic crystal fiber

Forward stimulated light scattering by GHz acoustic resonances (ARs) guided in a µm-sized photonic crystal fiber (PCF) core gives rise to two classes of scattering: forward stimulated Raman-like scattering (SRLS) between pump and Stokes waves in the same optical mode [1], and forward stimulated inter-polarization scattering (SIPS) between orthogonally polarized pump and Stokes waves [2]. Here we study for the first time the transient switching dynamics of forward SIPS, building on previous studies that were restricted to the steady-state regime [2].