The design of evanescent-field-coupled waveguide-mode sensors

An evanescent-field-coupled waveguide-mode sensor with a multilayer structure consisting of a dielectric waveguide, a thin reflecting layer, and a glass substrate illuminated under the Kretschmann configuration operates as a sensor that is capable of detecting modifications in the dielectric environment near the waveguide surface with superior sensitivity by measuring the change in reflectivity. The sensitivity of the sensor is strongly dependent on the optical constants of the reflecting layer. Numerical simulations show that a sensor having a reflecting layer with a small value of the real part of the complex refractive index shows a good sensitivity for both S-?and P-polarized light. Materials with values of the real and imaginary parts of the complex refractive index of >4 and ～0.5, respectively, are suitable for use as reflecting layers when S-polarized light excites only the lowest order waveguide mode. The simulations were experimentally confirmed using sensors with Au, Cu, Cr, W, a-Si, or Ge reflecting layers deposited by radiofrequency magnetron sputtering by observation of specific adsorption of streptavidin on biotinyl groups using an S-polarized laser beam with a wavelength of 632.8?nm. From the results, guidelines are given for the fabrication of preferred sensor configurations.     

Discrete mode lasers for communication applications

The authors present a novel, low cost laser transmitter for telecommunication systems. This device, called discrete mode (DM) laser, is basically a ridge waveguide Fabry?Pe′ rot (FP) laser, whose wavelength spectra has been modified to obtain a single mode operation. This is achieved by perturbing the effective refractive index of the guided mode along very small sections of the laser cavity, by etching features into the ridge waveguide. Suitable positioning of these interfaces allows the mirror loss spectrum of an FP laser to be manipulated in order to achieve single longitudinal mode emission. The waveguide structure requires only a single growth stage and uses optical lithography to realise the ridge. In addition, the fabrication process is re-growth free. Despite this simple and low cost fabrication process, the DM lasers portray many advantages over the distributed feedback and distributed Bragg reflector lasers, such as very high side mode suppression ratio, stable operation over a large temperature range, narrow linewidth and low sensitivity to optical feedback.     

Multimode distributed feedback laser emission in a dye-doped optically pumped polymer thin-film

We report on particular features of thin film distributed feedback (DFB) lasers. Devices are optically pumped using a Lloyd-mirror interferometer. For a given DFB grating period, the number of lasing modes is film thickness dependent. Spectral content of the devices is analysed using planar waveguide theory. An excellent agreement between the theoretical transverse electric mode structure and the laser emission spectrum is found.     

Dynamic-single-mode Lasers

Recent progress in the dynamic-single-mode (DSM) semiconductor laser for long wavelength optical fibre communications is reviewed, and applications to wide-band optical fibre communication in the lowest loss wavelength region of 1·5–1·65 μm are studied. A DSM laser consists of a mode selective resonator and a transverse-mode-controlled waveguide, such as the narrow-striped distributed Bragg reflector (DBR) laser, to maintain a fixed axial mode under rapid direct modulation. The technology of monolithic integration for optical circuits is applied to realize some DSM lasers. The structures and properties of DSM lasers are reviewed.     

Fiber-optic chemical sensing with langmuir-blodgett overlay waveguides

Fiber-optic chemical sensing has been demonstrated with a side-polished single-mode optical fiber, evanescently coupled to chemically sensitive Langmuir-Blodgett (LB) overlay waveguides. The sensors exhibit a channel-dropping response centered on a wavelength that is dependent on the thickness and the refractive index of the overlay waveguide. It has been shown that pH-sensitive organic dyes proved to be suitable materials for the formation of an overlay waveguide whereas LB deposition provides the required thickness control. A theoretical model of the sensor response, based on the Kramers-Kronig relations and phase matching of the guided modes within the optical fiber and overlay waveguide, shows good agreement with experimental results.     

Pulsed laser deposited ZnO film on side-polished fiber as a gas sensing element

A simple sensor element consisting of a side-polished single-mode fiber and a planar metal oxide waveguide is described. The thin ZnO planar waveguide was produced on the polished fiber surface by pulsed laser deposition at optimized processing parameters. A measurement scheme for in situ control of the film thickness during the deposition process was developed and used. X-ray diffraction measurements and scanning electron microscopy were used to characterize the structure and the surface morphology of the planar waveguide, respectively. The numerical evaluation of the sensor sensitivity predicts the possibility to detect refractive index changes of less than 10(-4). Furthermore, preliminary gas sensor tests were performed by using a mixture of 1.5% butane diluted in N(2) and pure butane. A shift of the spectral position of the resonance points was observed from 3 to 5 s after gas exposure, which corresponds to refractive index changes of 3 x 10(-5) and 1.2 x 10(-3) for 1.5% butane and for pure butane, respectively.     

Does photosensitivity pave the way towards the fabrication of miniature coherent light sources in inorganic glass waveguides?

Photosensitivity designates the ability to permanently change the refractive index of a glass by optical processing. The phenomenon allows the fabrication of numerous phase structures, the simplest of which is the Bragg grating obtained by photoimprinting a periodic index modulation within the material. Bragg gratings have changed the way in which optical fibre (or planar waveguide) lasers are now used. Distributed Bragg Reflector (DBR) or Distributed Feed Back (DFB) lasers, when intracore Bragg gratings are used for linear cavity feedback, are commonly fabricated in rare earth doped optical silica-based fibres. On the other hand, photosensitivity can also result in effects which can prove to be detrimental to the fabrication of miniature coherent light sources. The paper will cover some of the advances that have been made in improving the photosensitivity of inorganic glasses, in searching new photosensitive materials and in characterizing Bragg grating properties relevant to laser applications.     

Distributed feedback leaky laser emission from dye doped gel-glass dispersed liquid crystal thin film patterned by soft lithography

Gel-glass dispersed liquid crystal (GDLC) thin films doped with organic laser dye Rhodamine 6G (R6G) were prepared via a sol-gel procedure of tetraethyl orthosilicate (TEOS) and ethyl triethoxisilane (ETES). As characterized by scanning electronic microscope (SEM), surface-relief structures were successfully patterned on lower refractive index GDLC thin films by soft lithographic technology, which support distributed feedback (DFB) laser emission based on leaky mode propagation. The performance of the DFB laser emission was investigated and the spectral narrowing of the emitted radiation and the fine structure pattern were found to be controlled by the doping concentration of liquid crystal (LC) 4-cyano-4′-pentylbiphenyl (5CB). We also showed the synchronous excitation of a DFB lasing with random lasing mediated by light scattering inside the same GDLC leaky waveguide.     

Plasmonic green nanolaser based on a metal-oxide-semiconductor structure

Realization of smaller and faster coherent light sources is critically important for the emerging applications in nanophotonics and information technology. Semiconductor lasers are arguably the most suitable candidate for such purposes. However, the minimum size of conventional semiconductor lasers utilizing dielectric optical cavities for sustaining laser oscillation is ultimately governed by the diffraction limit (～(λ/2n)(3) for three-dimensional (3D) cavities, where λ is the free-space wavelength and n is the refractive index). Here, we demonstrate the 3D subdiffraction-limited laser operation in the green spectral region based on a metal-oxide-semiconductor (MOS) structure, comprising a bundle of green-emitting InGaN/GaN nanorods strongly coupled to a gold plate through a SiO(2) dielectric nanogap layer. In this plasmonic nanocavity structure, the analogue of MOS-type "nanocapacitor" in nanoelectronics leads to the confinement of the plasmonic field into a 3D mode volume of 8.0 × 10(-4) μm(3) (～0.14(λ/2n)(3)).     

Versatile waveguide-coupled optofluidic devices based on liquid core optical ring resonators

A versatile waveguide-coupled optofluidic device using the liquid core optical ring resonator (LCORR) that can be operated with liquid of any refractive index (RI) is theoretically analyzed and experimentally demonstrated. The results confirm the confinement of resonant modes for all sample RIs, and reveal that confined modes in a high-RI core are excited by an external waveguide by resonant tunneling through the LCORR wall. It is further found that a thin wall must be used for effective interaction between the core mode and the waveguide. The results have important applications in optofluidic devices, including sensors, microfluidic lasers, and nonlinear optics.     

In situ evaluation of density, viscosity, and thickness of adsorbed soft layers by combined surface acoustic wave and surface plasmon resonance

We show the theoretical and experimental combination of acoustic and optical methods for the in situ quantitative evaluation of the density, the viscosity, and the thickness of soft layers adsorbed on chemically tailored metal surfaces. For the highest sensitivity and an operation in liquids, a Love mode surface acoustic wave (SAW) sensor with a hydrophobized gold-coated sensing area is the acoustic method, while surface plasmon resonance (SPR) on the same gold surface as the optical method is monitored simultaneously in a single setup for the real-time and label-free measurement of the parameters of adsorbed soft layers, which means for layers with a predominant viscous behavior. A general mathematical modeling in equivalent viscoelastic transmission lines is presented to determine the correlation between experimental SAW signal shifts and the waveguide structure including the presence of the adsorbed layer and the supporting liquid from which it segregates. A methodology is presented to identify from SAW and SPR simulations the parameters representatives of the soft layer. During the absorption of a soft layer, thickness or viscosity changes are observed in the experimental ratio of the SAW signal attenuation to the SAW signal phase and are correlated with the theoretical model. As application example, the simulation method is applied to study the thermal behavior of physisorbed PNIPAAm, a polymer whose conformation is sensitive to temperature, under a cycling variation of temperature between 20 and 40 degrees C. Under the assumption of the bulk density and the bulk refractive index of PNIPAAm, thickness and viscosity of the film are obtained from simulations; the viscosity is correlated to the solvent content of the physisorbed layer.     

Magneto-optical non-reciprocal devices in silicon photonics

Silicon waveguide optical non-reciprocal devices based on the magneto-optical effect are reviewed. The non-reciprocal phase shift caused by the first-order magneto-optical effect is effective in realizing optical non-reciprocal devices in silicon waveguide platforms. In a silicon-on-insulator waveguide, the low refractive index of the buried oxide layer enhances the magneto-optical phase shift, which reduces the device footprints. A surface activated direct bonding technique was developed to integrate a magneto-optical garnet crystal on the silicon waveguides. A silicon waveguide optical isolator based on the magneto-optical phase shift was demonstrated with an optical isolation of 30 dB and insertion loss of 13 dB at a wavelength of 1548 nm. Furthermore, a four port optical circulator was demonstrated with maximum isolations of 15.3 and 9.3 dB in cross and bar ports, respectively, at a wavelength of 1531 nm.     

Fabrication of a distributed-feedback dye laser with a grating structure in its plastic waveguide

Two approaches of fabricating grating structures for waveguided plastic dye lasers are described and compared for lasing performance. Rhodamine6G-doped poly(methyl methacrylate) (PMMA) film on a PMMA substrate was used for the waveguide, and a distributed-feedback (DFB) laser operation with a single-propagation mode was demonstrated. The performances of both types of permanent grating structured DFB dye laser were better than those of a DFB dye laser on a plain waveguide with a dynamic grating formed by the interference of two pump beams. Wide tuning range is expected by use of a multistripe DFB laser with different grating pitches.     

Waveguide properties of optical structures fabricated by oxidation of porous silicon

Results of an investigation of the optical properties of channel waveguides fabricated by oxidation of porous silicon are described. The waveguide parameters are estimated and the existence of optical anisotropy is established. The effective refractive index of the dominant quasi-TM waveguide mode is measured. The results suggest that a buffer layer exists between the waveguide and the silicon substrate. It is hypothesized that a second refractive index peak exists within this layer. Pis’ma Zh. Tekh. Fiz. 23, 86–89 (May 26, 1997)     

Refractive indices and thicknesses of optical waveguides fabricated by silicon ion implantation into silica glass

Planar optical waveguides formed by Si ion implantation into PECVD SiO₂ have been characterized by the dark mode spectroscopy method at a wavelength of 0.6328 μm. The measured effective index values of the guided modes have been used to investigate the optical properties of the core layers of the waveguides after different pre-implantation treatments. It was found that annealing the specimens before implantation, affected both the refractive index and thickness of the core layers. In the annealed specimens a thicker core layer and a larger relative refractive index difference between the core and the buffer layer resulted.     

Inside Front Cover: An Ultraviolet Organic Thin‐Film Solid‐State Laser for Biomarker Applications (Adv. Mater. 1/2005)

Thin‐film organic solid‐state lasers operating in the ultraviolet wavelength region are fabricated using a novel spiro‐linked material as active organic layer in an optically pumped distributed feedback (DFB) structure in work reported by Riedl and co‐workers on p. 31. The laser wavelength is tunable between 377.7 nm and 395 nm, the shortest laser wavelength reported so far for thin‐film organic solid‐state lasers. The lasers' suitability for spectroscopic applications was tested by use as an excitation source for solutions containing the common fluorescent dyes Coumarin 6, Coumarin 152, and Rhodamine 6G.     

Optical properties and sensing applications of lithium iron phosphate thin films

A composite optical waveguide sensor, consisting of lithium iron phosphate (LiFePO₄, LFP) as the sensing material, was constructed and utilized for the detection of volatile organic compound gases. Nano-LFP powder was prepared via the hydrothermal method and was subsequently utilized in a dip-coating procedure for the fabrication of LFP thin films. The effect of heat treating temperature on the refractive index of the thin films was studied. A glass optical waveguide gas sensor was fabricated by coating an LFP thin film on the surface of single-mode tin-diffused glass optical waveguide. The sensor was found to exhibit a linear response to xylene in the range of 50-1000 ppm, with response times of less than 5 s.     

Optical response of laser-doped silicon carbide for an uncooled midwave infrared detector

An uncooled mid-wave infrared (MWIR) detector is developed by doping an n-type 4H-SiC with Ga using a laser doping technique. 4H-SiC is one of the polytypes of crystalline silicon carbide and a wide bandgap semiconductor. The dopant creates an energy level of 0.30 eV, which was confirmed by optical spectroscopy of the doped sample. This energy level corresponds to the MWIR wavelength of 4.21 μm. The detection mechanism is based on the photoexcitation of electrons by the photons of this wavelength absorbed in the semiconductor. This process modifies the electron density, which changes the refractive index, and, therefore, the reflectance of the semiconductor is also changed. The change in the reflectance, which is the optical response of the detector, can be measured remotely with a laser beam, such as a He-Ne laser. This capability of measuring the detector response remotely makes it a wireless detector. The variation of refractive index was calculated as a function of absorbed irradiance based on the reflectance data for the as-received and doped samples. A distinct change was observed for the refractive index of the doped sample, indicating that the detector is suitable for applications at the 4.21 μm wavelength.     

Bessel-beam-pumped tunable distributed-feedback laser

A distributed-feedback (DFB) dye laser that is pumped by a standing Bessel-beam wave is constructed. Because of the long line focus of the Bessel beam, the laser medium is pumped in only a very thin filament (a few micrometers) along the optical axis. At the same time, longitudinal-mode selection is achieved because of the DFB effect. It is demonstrated that when the effective wavelength of the Bessel pump beam is varied, the Bragg wavelength for DFB is altered, and as a result the output wavelength can be tuned.