R.M. de Ridder

Ultra-narrow-linewidth, single-frequency distributed feedback waveguide laser in Al2O3:Er3+ on silicon.

We report the realization and performance of a distributed feedback channel waveguide laser in erbium-doped aluminum oxide on a standard thermally oxidized silicon substrate. The diode-pumped continuous-wave laser demonstrated a threshold of 2.2 mW absorbed pump power and a maximum output power of more than 3 mW with a slope efficiency of 41.3% versus absorbed pump power. Single-longitudinal-mode and single-polarization operation was achieved with an emission linewidth of 1.70+/-0.58 kHz (corresponding to a Q factor of 1.14 x 10(11)), which was centered at a wavelength of 1545.2 nm.

Quasi-one-dimensional photonic crystal as a compact building-block for refractometric optical sensors

We report the fabrication and the characterization of the refractometric and thermo-optical properties of a quasi-one-dimensional waveguide photonic crystal-a strong, 76-/spl mu/m-long Bragg grating. The transmission spectra (around 660 nm) of the structure have been measured as a function of both the cladding refractive index and the temperature. The transmission stopband was found to shift by 0.8-nm wavelength for either a cladding refractive index change of 0.05 or a temperature change of 120 K. The steep stopband edges provide a sensitive detection method for this band shift, by monitoring the transmitted output power.

Numerical studies of 2D photonic crystals: Waveguides, coupling between waveguides and filters

In photonic crystals, light propagation is forbidden in a certain wavelength range, the bandgap. In a two-dimensional crystal composed of parallel high-refractive index rods in a low-index background a line defect can be formed by removing a row of these rods, which can act as a waveguide for frequencies in the bandgap of the crystal. In order to get more insight into the main features of such waveguides we have studied a number of properties, using simulation tools based on the finite difference time domain method and a finite element Helmholtz solver. We show conceptually simple methods for determining the bandgap of the crystal as well as the dispersion of a waveguide for wavelengths in this bandgap. For practical applications, it is also important to know how much light can be coupled into the waveguide. Therefore, the coupling of light from a dielectric slab waveguide into the photonic crystal waveguide has been examined, showing that a coupling efficiency of up to 83% can be obtained between a silicon oxide slab and a waveguide in a crystal of silicon rods. Finally, calculations on an ultra-compact filter based on reflectively terminated side-branches of waveguides (similar to tuned stubs in microwave engineering) are shown and discussed.

Bragg waveguide grating as a 1D photonic band gap structure: COST 268 modelling task

Modal reflection, transmission and loss of deeply etched Bragg waveguide gratings were modelled by six European laboratories using independently developed two-dimensional (2D) numerical codes based on four different methods, with very good mutual agreement. It was found that (rather weak) material dispersion of the SiO2/Si3N4 system does not significantly affect the results. The existence of lossless Floquet-Bloch modes in deeply etched gratings was confirmed. Based on reliable numerical results, the physical origin of out-of-plane losses of 1D or 2D photonic band gap structures in slab waveguides is briefly discussed.

Nano-mechanical tuning and imaging of a photonic crystal micro-cavity resonance

We show that nano-mechanical interaction using atomic force microscopy (AFM) can be used to map out mode-patterns of an optical micro-resonator with high spatial accuracy. Furthermore we demonstrate how the Q-factor and center wavelength of such resonances can be sensitively modified by both horizontal and vertical displacement of an AFM tip consisting of either Si(3)N(4) or Si material. With a silicon tip we are able to tune the resonance wavelength by 2.3 nm, and to set Q between values of 615 and zero, by expedient positioning of the AFM tip. We find full on/off switching for less than 100 nm vertical, and for 500 nm lateral displacement at the strongest resonance antinode locations.

Photonic generation of stable microwave signals from a dual-wavelength

We report the fabrication and characterization of a dual-wavelength distributed-feedback channel waveguide laser in ytterbium-doped aluminum oxide. Operation of the device is based on the optical resonances that are induced by two local phase shifts in the distributed-feedback structure. A stable microwave signal at ~15 GHz with a -3 dB width of 9 kHz was subsequently created via the heterodyne photodetection of the two laser wavelengths. The long-term frequency stability of the microwave signal produced by the free-running laser is better than ±2.5 MHz, while the power of the microwave signal is stable within ±0.35 dB.

Al_2O_3:Yb^{3+}

We report the fabrication and performance of a highly efficient, monolithic distributed-Bragg-reflector channel waveguide laser in ytterbium-doped aluminum oxide. The 1 cm long device was fabricated on a standard thermally oxidized silicon substrate and was optically pumped with a 976 nm laser diode. Single-longitudinal-mode and single-polarization operation was achieved at a wavelength of 1021.2 nm. Continuous-wave output powers of up to 47 mW and a launched pump power threshold of 10 mW resulted in a slope efficiency of 67%.

distributed-feedback waveguide laser

An integrated arrayed-waveguide grating fabricated in silicon-oxynitride technology is applied to Raman spectroscopy. After its validation by reproducing the well-known spectrum of cyclohexane, polarized Raman spectra are measured of extracted human teeth containing localized initial carious lesions. Excellent agreement is obtained between the spectra of healthy and carious tooth enamel measured with our integrated device and spectra recorded using a conventional Raman spectrometer. Our results represent a step toward the realization of compact, hand-held, integrated spectrometers, e.g. for the detection of dental caries at an early stage.

Highly efficient, low-threshold monolithic distributed-Bragg-reflector channel waveguide laser in Al

We present experimental results of a spectral-domain optical coherence tomography system based on an integrated optical spectrometer. A 195-channel arrayed-waveguide-grating (AWG) spectrometer with 0.4-nm channel spacing centered at 1300 nm and a 125-channel AWG with 0.16-nm channel spacing centered at 800 nm have been fabricated in silicon oxynitride waveguide technology. Interferometric distance measurements have been performed by launching light from a broadband source into a free-space Michelson interferometer, with its output coupled into the AWG. A maximum imaging depth of 1 mm and axial resolution of 25 and 20 μm in air are demonstrated for the 800- and 1300-nm ranges, respectively.

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Optical coherence tomography (OCT) has enabled clinical applications that revolutionized in vivo medical diagnostics. Nevertheless, its current limitations owing to cost, size, complexity, and the need for accurate alignment must be overcome by radically novel approaches. Exploiting integrated optics, we assemble the central components of a spectral-domain OCT system on a silicon chip. The spectrometer comprises an arrayed-waveguide grating with 136-nm free spectral range and 0.21-nm wavelength resolution. The beam splitter is realized by a non-uniform adiabatic coupler with its 3-dB splitting ratio being nearly constant over 150 nm. With this device whose overall volume is 0.36 cm(3) we demonstrate high-quality in vivo imaging in human skin with 1.4-mm penetration depth, 7.5-µm axial resolution, and a signal-to-noise ratio of 74 dB. Considering the reasonable performance of this early OCT on-a-chip system and the anticipated improvements in this technology, a completely different range of devices and new fields of applications may become feasible.