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

Abstract

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.

Quantum theory of continuum optomechanics

“…The situation differs significantly from standard cavity optomechanics, if only forward-scattering is considered. The crucial asymmetry between Stokes and anti-Stokes processes is absent here in forwardscattering, where phonons of wavenumber q can be emitted and absorbed equally likely, scattering laser photons into a comb [4,32,34,35] of sidebands ω L ±nΩ with Ω=Ω(q). As a consequence, basic phenomena in cavity optomechanics, like cooling or state transfer, do not translate to forward scattering with a single optical branch.…”

“…. Estimated values for (a) [35], (b) [44], (c) stimulated intermodal scattering (SIMS) in silicon (room temp.) [58], (d) considers same system [58] at 1 K wherein Brillouin active phonon mode at 1 GHz is used and Q factors increase by a factor of 100 at low temperatures.…”

Brillouin cooling in a linear waveguide

“…The integrals in equation (26) can be integrated analytically, however, the closed form expression is unwieldy and the complete expression provides little physical insight. An important parameter that characterizes the solution is given by…”

Electro-opto-mechanical radio-frequency oscillator driven by guided acoustic waves in standard single-mode fiber

“…[11][12][13][14][15][16][17][18][19][20][21] The acoustic modes may be optically stimulated through electro-strictive forces and can scatter guided light waves via photo-elasticity. 12,13 The phenomena are referred to as guided acoustic waves Brillouin scattering (GAWBS), 12 or as forward stimulated Raman-like, inter-polarization or inter-modal scattering.…”

“…12,13 The phenomena are referred to as guided acoustic waves Brillouin scattering (GAWBS), 12 or as forward stimulated Raman-like, inter-polarization or inter-modal scattering. 15 Photonic crystal fibers (PCFs) [14][15][16][17] and micro-structured fibers [18][19][20] applied in a series of papers by Russell and co-workers. [14][15][16][17][18][19][20] Remarkable demonstrations include the generation of radio-frequency combs, without feedback, over very short fiber segments, 19,20 and the ultra-stable mode-locking of soliton fiber lasers, which led to long-term bit storage.…”