Stimulated Stokes and Antistokes Raman Scattering in Microspherical Whispering Gallery Mode Resonators

Summary

Efficient generation of nonlinear phenomena related to third order optical non-linear susceptibility Χ⁽³⁾ interactions in triply resonant silica microspheres is presented in this paper. The interactions here reported are: Stimulated Raman Scattering (SRS), and four wave mixing processes comprising Stimulated Anti-stokes Raman Scattering (SARS).

Abstract

Dielectric microspheres can confine light and sound for a length of time through high quality factor whispering gallery modes (WGM). Glass microspheres can be thought as a store of energy with a huge variety of applications: compact laser sources, highly sensitive biochemical sensors and nonlinear phenomena. A protocol for the fabrication of both the microspheres and coupling system is given. The couplers described here are tapered fibers. Efficient generation of nonlinear phenomena related to third order optical non-linear susceptibility Χ⁽³⁾ interactions in triply resonant silica microspheres is presented in this paper. The interactions here reported are: Stimulated Raman Scattering (SRS), and four wave mixing processes comprising Stimulated Anti-stokes Raman Scattering (SARS). A proof of the cavity-enhanced phenomenon is given by the lack of correlation among the pump, signal and idler: a resonant mode has to exist in order to obtain the pair of signal and idler. In the case of hyperparametric oscillations (four wave mixing and stimulated anti-stokes Raman scattering), the modes must fulfill the energy and momentum conservation and, last but not least, have a good spatial overlap.

Introduction

Whispering gallery mode resonators (WGMR) show two unique properties, a long photon lifetime and small mode volume that allow the reduction of the threshold of nonlinear phenomena^1-3. Whispering gallery modes are optical modes that are confined at the dielectric air interface by total internal reflection. The small mode volume is due to the high spatial confinement whereas the temporal confinement is related to the quality factor Q of the cavity. WGMR can have different geometries and there are different fabrication techniques suitable for obtaining high Q resonators^4-6 Surface tension cavities such as silica microspheres exhibit near atomic scal....

Protocol

1. Fabrication of Ultrahigh Factor of Quality Microspheres

1. Strip about 1-2 cm of a standard single-mode (SMF) silica fiber off its acrylic coating using an optical stripper.
2. Clean the stripped part with acetone and cleave it.
3. Introduce the cleaved tip in one arm of a fusion splicer and produce a series of electric arc discharges using the splicer controller. Select "manual operation" from the splicer controller menu, set the values for arc power level and arc duration to 60 and 800 msec, respectively; select "arc" and push the bottom "+".
4. Once a sphere is taking shape, stop, rotate the fiber by 90°....

Results

The Q factors of the microspheres fabricated following the protocol described above are in excess of 10⁸ (Figure 5) for large diameters (>200 μm) and in excess of 10⁶ for small diameters (< 50 μm). Resonance contrast above 95% (close to critical coupling) can be easily observed. For high circulating intensities, the following nonlinear effects in the infrared region can be observed: stimulated Raman scattering (SRS), cascaded SRS²¹, .......

Discussion

Microspheres are compact and efficient nonlinear oscillators and they are very easy to fabricate and handle. Tapered fibers can be used for coupling and extracting the light in/from the resonator. Resonance contrast up to 95% and Q factors of about 3 x 10⁸ can be obtained.

The main limitation of these fabrication techniques is mass production and integration. Cleanliness of the fibers is critical to both microspheres and tapers, and so is humidity. Both devices must be kept in dry e.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Optical Fiber	Corning	SMF28
Fiber coating stripper	Thorlabs	T06S13	Available from other vendors as well
Fiber cleaver	Fitel	S325A	Available from other vendors as well
Fusion splicer	Furakawa	S177A-1R	Available from other vendors as well
Butane and Oxygen Gas	n/a		any vendor
Microscope tube	Navitar	Zoom 6000	Modular Kit
CCD camera	n/a	N/A	any will fit
Monitor	n/a	N/A	any monitor is valid
3-Axis Stage	PI Instruments, Thorlabs, Melles
Assorted posts and mounts	Thorlabs		Available from other vendors as well
Polarization control	Thorlabs	FPC030	Available from other vendors as well
Attenuator	Throlabs	VOA50
Photodiode	Thorlabs	PDA400	discontinued, replaced by PDA10CS-EC
Oscilloscope	Tektronix	DPO7104
Optical spectrum analyzer	Ando	AQ6317B
Erbium Doped Fiber Amplifier	IPG Photonics	EAD-2K-C
Tunable Laser	Yenista	TUNICS