Dual frequency modulation

This paper reports a sensitive optical interferometric technique dual frequency modulation (DFM) for measuring and stabilizing a laser frequency by comparison, in a single step, to a radio frequency (rf) standard. Conversely, a low-noise rf source can be stabilized by a laser frequency reference. A prototype has demonstrated a resolution of 2 parts in 10 , but devices currently under development should have a resolution of 10 and an absolute accuracy of 10" . The method may be competitive with the optical frequency synthesis chain in accuracy and its simplicity suggests its convenient use in metrology, high-precision optical spectroscopy, and gravity wave detection. 

The DFM technique overcomes this problem by using two phase modulators, driven at frequencies wj and (02 respectively, where 2/217 is nonresonant with c/2L. Dual frequency modulation creates, in lowest order, sidebands at ... 

ITO electrode and then flattened by a polymer film. For phase-only modulation, homogeneous LC alignment should be used. Thus, the polymer surface and the bottom ITO glass substrate with a thin polyimide alignment layer are rubbed in anti-parallel directions. The cell gap is controlled at 18 pm by Mylar spacers and hermetically sealed using ultraviolet-curable glue. To achieve a fast response time, a dual-frequency liquid crystal (DFLC) is used. The physical properties of the DFLC mixture are summarized as follows crossover frequency /<, 5 kHz, An 0.285 (at X=633nmand T=22°C), and dielectric anisotropy Ae = 4.73 at/= 1 kHz, and Ae = -3.93 at 50 kHz. 

Kirby, Andrew K., and Gordon D. Love. 2004. Fast, large and controllable phase modulation using dual frequency liquid crystals. Opt Express. 12 1470-1475. 

Based on the interplay between the complexing ability of crown-ethers or cry tands and the dual fluorescence (monomer/excimer) of aromatic rings (especially anthracene), new supramolecular receptors were designed and synthesized. Modulation of the fluorescrace (wavelengA, quantum yield, excimer/monomer ratio) of different photoactive systems is observed as a specific response to the complexation of metal (Na ", K" ", Ag" ", Tl", Rb" ") or molecular Ql3N -(CH2)n- NH3) cations. These properties could be used for light frequency conversion (anthraceno-coronand), detection and estimation of cations at low concentration (craterophanes, tonnelet) and evaluation of the solvent polarity (semaphorene). 

Instrumentation. Commercial atomic force microscopes Explorer from Topometrix Inc., Nanoscope Illa Digital Instruments Co. Ltd.) which are based on the laser beam deflection detection scheme were used in conjunction with digital oscilloscopes of very stable low frequency (1-20 Hz) trigger system for lateral force (friction) measurements, and dual-phase lock-in amplifiers and function generators for force modulation measurements. Various triangular silicon nitride cantilevers were used. The lateral spring constants were determined with the "blind torsional calibration method discussed in more detail in the Appendix. 

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