Abstract:We demonstrate a novel mechanism for low power optical detection and modulation in a slotted waveguide geometry filled with nonlinear electro-optic polymers. The nanoscale confinement of the optical mode, combined with its close proximity to electrical contacts, enables the direct conversion of optical energy to electrical energy, without external bias, via optical rectification, and also enhances electro-optic modulation. We demonstrate this process for power levels in the sub-milliwatt regime, as compared to the kilowatt regime in which optical nonlinear effects are typically observed at short length scales. Our results suggest that a new class of detectors based on nonlinear optics may be practical.

Although gigahertz-scale free-carrier modulators have been demonstrated in silicon, intensity modulators operating at terahertz speeds have not been reported because of silicon's weak ultrafast nonlinearity. We have demonstrated intensity modulation of light with light in a silicon-polymer waveguide device, based on the all-optical Kerr effect-the ultrafast effect used in four-wave mixing. Direct measurements of time-domain intensity modulation are made at speeds of 10 GHz. We showed experimentally that the mechanism of this modulation is ultrafast through spectral measurements, and that intensity modulation at frequencies in excess of 1 THz can be obtained. By integrating optical polymers through evanescent coupling to silicon waveguides, we greatly increase the effective nonlinearity of the waveguide, allowing operation at continuous-wave power levels compatible with telecommunication systems. These devices are a first step in the development of large-scale integrated ultrafast optical logic in silicon, and are two orders of magnitude faster than previously reported silicon devices.

We have demonstrated electrical tuning in ring resonators fabricated from silicon-on-insulator wafers by incorporating nematic liquid crystals ͑NLCs͒ as the waveguide top and side cladding material. Photolithographically defined electrodes aligned around the ring resonator were used to control the orientation of the NLCs to modulate the cladding refractive index and, hence, the resonant wavelengths of the ring resonator. © 2003 American Institute of Physics. ͓DOI: 10.1063/1.1630370͔Microring resonators, fabricated with conventional semiconductor processing methods in silicon, offer significant advantages over the existing telecommunication filter technology and may be the foundation of future dense-wavelengthdivision-multiplexing ͑DWDM͒ filters. 1-5The high refractive index ͑RI͒ contrast available in silicon-oninsulator ͑SOI͒ ring resonators enables low loss and high-Q filters fabricated with radii down to a few microns.6,7 Such resonators can be designed as notch filters for adding or dropping individual channels in the telecommunication bands and can be densely integrated in photonic networks. For reconfigurable DWDM systems, and to compensate for temperature changes, it is desirable to tune the precise channel frequency dropped by such resonator add/drop multiplexers.Two primary methods exist to control the optical path length of a ring resonator and thus tune its resonant frequency. To statically tune a ring resonator one can either adjust the physical dimensions ͑in particular its circumference͒ or the refractive indices of the constituent materials of the resonator. Dynamically tunable resonators provide another level of functionality over statically tuned resonators and are most practically obtained by controlling the refractive indices of the constituent materials. Dynamic tuning is commonly achieved by thermally changing the RI, traditionally by introducing a heater close to the resonator. 8 However, power dissipation may provide a serious problem in such tunable ring resonator designs, especially when many resonators have to be integrated in a DWDM multiplexing system. In this letter we demonstrate the dynamic tuning of a ring resonator by changing the RI of its cladding via the orientation of the nematic liquid crystals ͑NLC͒.The resonator system under study, as shown in Fig. 1, was fabricated from a SOI wafer with silicon thickness of 205 nm and oxide thickness of 1 m, ring radius of 5 m, and ring and waveguide widths of 500 nm. The resonator was coupled to one waveguide, which served as both the input and output port and was separated from the resonator by a 100 nm gap. Modulation electrodes were then photolithographically defined and deposited using standard lift-off processing. The left and right electrodes were approximately 4.0 m wide and were spaced about 400 and 300 nm from the resonator, respectively ͑Fig. 1͒. The modulation electrodes were designed to preferentially orient the directors of the NLC molecules parallel ͑azimuthally oriented͒ to the resonator. To minimize their electrostatic ene...

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This communication primarily deals with utilizing organic electro-optic (OEO) materials for the fabrication of active wavelength division multiplexing (WDM) transmitter/receiver systems and reconfigurable optical add/drop multiplexers (ROADMs), including the fabrication of hybrid OEO/silicon photonic devices. Fabrication is carried out by a variety of techniques including soft and nanoimprint lithography. The production of conformal and flexible ring microresonator devices is also discussed. The fabrication of passive devices is also briefly reviewed. Critical to the realization of improved performance for devices fabricated from OEO materials has been the improvement of electro-optic activity to values of 300 pm/V (or greater) at telecommunication wavelengths. This improvement in materials has been realized exploiting a theoretically-inspired (quantum and statistical mechanics) paradigm for the design of chromophores with dramatically improved molecular first hyperpolarizability and that exhibit intermolecular electrostatic interactions that promote self-assembly, under the influence of an electric poling field, into noncentrosymmetric macroscopic lattices. New design paradigms have also been developed for improving the glass transition of these materials, which is critical for thermal and photochemical stability and for optimizing processing protocols such as nanoimprint lithography. Ring microresonator devices discussed in this communication were initially fabricated using chromophore guest/polymer host materials characterized by electro-optic coefficients on the order of 50 pm/V (at telecommunication wavelengths). Voltage-controlled optical tuning of the pass band of these ring microresonators was experimental determined to lie in the range 1-10 GHz/V or all-organic and for OEO/silicon photonic devices. With new materials, values approaching 50 GHz/V should be possible. Values as high as 300 GHz/V may ultimately be achievable.

SummaryThe changes in light emitting diode current necessary to maintain a constant level of light incident upon a photodetector were measured in 20 volunteers at the two wavelengths employed by pulse oximeters. Three states of finger blood content were assessed; exsanguinated, hyperaemic. and normal. The changes in light emitting diode current with changes in finger blood content were small and are not thought to represent a significant source of error in saturation as measured by pulse oximetry. Key wordsEquipment; pulse oximeter. Haemoglobin; anaemia.Light emitting diodes (LEDs) are used as light sources in pulse oximeters as they produce a narrow band of wavelengths. The stability of the emitted wavelengths is critical to the accuracy of pulse oximeters as the absorption spectra of reduced and oxygenated haemoglobin exhibit large changes for small changes in wavelength. A change in LED wavelength during the operation of a pulse oximeter may thus result in inaccurate oximeter readings. For example, if the wavelength of the red LED is decreased, the absorption of red light by reduced haemoglobin increases, resulting in an apparent fall in saturation as measured by the oximeter.Severinghaus and Koh have recently shown that pulse oximeters under-read at low saturations [ 11 and speculated that at low saturations the relatively high absorption of red light causes a reduction in the amount of red light incident upon the detector (photodiode). Pulse oximeters alter the intensity of their LEDs according to the thickness and optical density of the finger or earlobe in order to maintain the intensity of light reaching the photodiode in a narrow range. A reduction in red light transmission thus leads to an increase in current at the red LED. These authors suggest that this heats the diode and results in a reduction of the peak wavelength of light emitted by the LED. This in turn causes the oximeter to under-read at low saturations. Paradoxically, this is worse in anaemic subjects.This explanation for pulse oximeters under-reading at low saturations seems unlikely to be correct for two reasons. Firstly, blood represents only a small proportion of the tissue of the finger or ear. Unless blood is the major light absorbing component of tissue, it is unlikely that changes in saturation will alter the total light absorption of the finger or ear sufficient to cause major changes in LED current. Secondly, increasing LED output from 10% to 100% of the manufacturer's maximum specified current has been shown to increase, not decrease, the peak wavelength. This increase in wavelength amounted to only 8 nm when LED current was increased ninefold [2].The contribution of blood to the total optical density of the finger has not been evaluated. The following study was undertaken to assess our hypothesis that the level of haemoglobin in the finger has no effect on LED current and is thus unlikely to affect pulse oximeter accuracy. MethodsChanges in light absorption were measured using a locally made two-wavelength photometer designed to...

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