[Show abstract][Hide abstract] ABSTRACT: We demonstrate a fabrication procedure for the direct integration of micro-ball lenses on planar integrated optical channel waveguide chips with the aim to reduce the divergence of light that arises from the waveguide in both horizontal and vertical directions. Fabrication of the lenses is based on photoresist reflow which is a procedure that allows for the use of photolithography for careful alignment of the lenses with respect to the waveguides and enables mass production. We present in detail the design and fabrication procedures. Optical characterization of the fabricated micro-ball lenses demonstrates a good performance in terms of beam-size reduction and beam shape. The beam half divergence angle of 1544 nm light is reduced from 12.4 ° to 1.85 °.
[Show abstract][Hide abstract] ABSTRACT: Our recent work on distributed-feedback, narrow-linewidth channel waveguide lasers in Er3+- and Yb3+-doped Al2O3 layers deposited on silicon microchips is reviewed.
[Show abstract][Hide abstract] ABSTRACT: 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<sup>3</sup> 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.
[Show abstract][Hide abstract] ABSTRACT: An integrated intra-laser-cavity microparticle sensor based on a dual-wavelength distributed-feedback channel waveguide laser in ytterbium-doped amorphous aluminum oxide on a silicon substrate is demonstrated. Real-time detection and accurate size measurement of single micro-particles with diameters ranging between 1 µm and 20 µm are achieved, which represent the typical sizes of many fungal and bacterial pathogens as well as a large variety of human cells. A limit of detection of ∼500 nm is deduced. The sensing principle relies on measuring changes in the frequency difference between the two longitudinal laser modes as the evanescent field of the dual-wavelength laser interacts with micro-sized particles on the surface of the waveguide. Improvement in sensitivity far down to the nanometer range can be expected upon stabilizing the pump power, minimizing back reflections, and optimizing the grating geometry to increase the evanescent fraction of the guided modes.
[Show abstract][Hide abstract] ABSTRACT: Optical coherence tomography (OCT) is a widely used optical imaging technology . Current OCT systems contain a variety of fiber and free-space optical components, which add to the instrument size and cost. By utilizing a suitable mass-fabrication technology and a monolithic design, integrated optics can provide miniaturized OCT systems that offer dramatic cost and size reduction as well as more stable interferometric detection [2,3]. In this paper we present an important step toward a cheap, compact, and quasi-maintenance-free spectral-domain OCT (SD-OCT) system by integrating its central components, the beam splitter and spectrometer, on a silicon chip as schematically shown in Fig. 1 A. An arrayed waveguide grating (AWG)  operating at a center wavelength of 1250 nm with a large free spectral range of 136 nm and a high wavelength resolution of 0.21 nm forms the integrated spectrometer, and a non-uniform adiabatic coupler  forms the beam splitter in our device. The coupler and AWG were fabricated in silicon oxynitride (SiON) . Single-mode SiON channel waveguides with a 1.8-μm width, 1-μm height, 1.54 core refractive index, and 1.4485 cladding refractive index, enabling bending radii down to 0.5 mm, were fabricated.
[Show abstract][Hide abstract] ABSTRACT: In many applications, such as on-chip optical coherence tomography (OCT) or Raman spectroscopy, it is necessary to produce a collimated light beam from an optical waveguide, and collimation in one dimension has been demonstrated, see e.g. [1,2,3]. However, in order to increase the efficiency of chip-sample coupling, collimation in two dimensions is required [4,5]. In this work, we demonstrate an on-chip reflowed polymer microlens design to enable light collimation in both, horizontal and vertical directions, and the experimental results show reduction of the divergence angle by a factor of 25.
[Show abstract][Hide abstract] ABSTRACT: 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, the central components of a spectral-domain OCT (SD-OCT) system can be integrated on a chip. Arrayed-waveguide grating (AWG) spectrometers with their high spectral resolution and compactness are excellent candidates for on-chip SD-OCT systems. However, specific design-related issues of AWG spectrometers limit the performance of on-chip SD-OCT systems. Here we present advanced AWG designs which could overcome the limitations arising from free spectral range, polarization dependency, and curved focal plane of the AWG spectrometers. Using these advanced AWG designs in an SD-OCT system can provide not only better overall performance but also some unique aspects that a commercial system does not have. Additionally, a partially integrated OCT system comprising an AWG spectrometer and an integrated beam splitter, as well as the in vivo imaging using this system are demonstrated.
[Show abstract][Hide abstract] ABSTRACT: An integrated intra-laser-cavity microparticle sensor based on a dual-phase-shift, dual-wavelength distributed-feedback channel waveguide laser in Al2O3:Yb3+ is presented. Real-time detection and accurate size measurement of single microparticles with diameters ranging between 1 μm and 20 μm are achieved, which represent the typical sizes of many fungal and bacterial pathogens as well as a large variety of human cells. The sensing principle relies on measuring changes in the frequency difference between the two longitudinal laser modes, as the evanescent field of the dual-wavelength laser interacts with micro-sized particles on the surface of the waveguide.
[Show abstract][Hide abstract] ABSTRACT: We report on diode-pumped distributed-feedback (DFB) and
distributed-Bragg-reflector (DBR) channel waveguide lasers in Er-doped
and Yb-doped Al2O3 on standard thermally oxidized
silicon substrates. Uniform surface-relief Bragg gratings were patterned
by laser-interference lithography and etched into the SiO2
top cladding. The maximum grating reflectivity exceeded 99%. Monolithic
DFB and DBR cavities with Q-factors of up to 1.35×106
were realized. The Erdoped DFB laser delivered 3 mW of output power with
a slope efficiency of 41% versus absorbed pump power.
Singlelongitudinal- mode operation at a wavelength of 1545.2 nm was
achieved with an emission line width of 1.70 0.58 kHz, corresponding to
a laser Q-factor of 1.14×1011. Yb-doped DFB and DBR
lasers were demonstrated at wavelengths near 1020 nm with output powers
of 55 mW and a slope efficiency of 67% versus launched pump power. An
Yb-doped dualwavelength laser was achieved based on the optical
resonances induced by two local phase shifts in the DFB structure. A
stable microwave signal at ~15 GHz with a -3-dB width of 9 kHz and a
long-term frequency stability of +/- 2.5 MHz was created via the
heterodyne photo-detection of the two laser wavelengths. By measuring
changes in the microwave beat signal as the intra-cavity evanescent
laser field interacts with micro-particles on the waveguide surface, we
achieved real-time detection and accurate size measurement of single
micro-particles with diameters ranging between 1 μm and 20 μm,
which represents the typical size of many fungal and bacterial
pathogens. A limit of detection of ~500 nm was deduced.
No preview · Article · Mar 2013 · Proceedings of SPIE - The International Society for Optical Engineering
[Show abstract][Hide abstract] ABSTRACT: A theoretical and experimental evaluation is given of a principle of direct, label-free opto-chemical sensing. According to this principle, which is applicable in compact planar optical sensors, measurand-induced wavelength shifts of the sharp fringes in the transmission spectra near the stop band edges of a resonant grating-based cavity are monitored. Such fringes are the results of Fabry–Perot resonances of the Bloch modes propagating in the cavity. Two sensor configurations have been considered, a first one for measuring the concentration of a single compound dissolved in water in the vicinity of the grating (bulk sensing), and a second one for determining the concentration of a specific compound adsorbed at the grating surface from a watery mixture of many compounds (surface sensing). In the latter, a thin interface layer which contains receptors specific to the targeted analyte, the PepN enzyme, is applied on top of the grated waveguide section. Filling of the receptors can be effectively seen as growth of an adlayer. Experimentally resolutions of 6×10−66×10−6 refractive index unit and ∼4 pm adlayer growth have been obtained for bulk and surface sensing, respectively. With a statistical analysis the limitations to obtain lower resolutions with the current set-up are identified. The compact devices (footprint ∼200 μm × 15 μm) are well suited for multi-sensing in lab-on-a-chip systems and can easily be fabricated with standard micro-fluidic and CMOS technologies.
No preview · Article · Nov 2012 · Sensors and Actuators B Chemical
[Show abstract][Hide abstract] ABSTRACT: We present a new flat-focal-field arrayed-waveguide grating (AWG) design that utilizes an integrated field-flattening lens placed in the second star coupler. The effective index difference between slab and lens region is obtained by introducing a thin silicon nitride (SiN) layer to a silicon oxynitride environment. Depending upon the SiN layer position, two different lens designs are implemented. As a proof of concept two 81-channel AWGs, one with and one without the lens, are designed, fabricated, and characterized for each lens design. The measurements show that the adjacent crosstalk at the peripheral channels is improved by 2 dB, an improvement which is predicted to become more pronounced for AWGs with higher number of output waveguides (e.g., ∼16 dB for 200 output waveguides). Only 0.4 dB of extra excess loss is introduced by the lens.
[Show abstract][Hide abstract] ABSTRACT: In this work, we report on utilization and optimization of the focused-ion-beam technique for
the fabrication of nanostructures on Al2O3 waveguides for applications in integrated photonic
devices. In particular, the investigation of the effects of parameters such as ion-beam current,
dwell time, and scanning strategy is addressed. As a result of optimizing these parameters,
excellent quality gratings with smooth and uniform sidewalls are reported. The effects of
redeposition are minimized and good control of the nanostructuring process is reported.
The effect of Ga+ ion implantation during the milling process on the optical performance of
the devices is discussed.
Full-text · Article · Oct 2012 · Journal of Micromechanics and Microengineering
[Show abstract][Hide abstract] ABSTRACT: Photonic generation of microwave carriers by using a dual-frequency distributed feedback waveguide laser in ytterbium-doped aluminum oxide is demonstrated. A high-performance optical frequency locked loop is implemented to stabilize the microwave carrier. This approach results in a microwave frequency at ~14 GHz with a phase noise of -75 dBc/Hz at 1 MHz offset from the center frequency. The frequency stability of the photonic microwave carrier has an Allan deviation of better than 1×10-10 for an averaging time of 1000 s with a loop settling time of 15 μs.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate the photonic generation of microwave signals by using a dual-frequency distributed feedback waveguide laser in ytterbium-doped aluminum oxide (Al2O3:Yb3+). An optical frequency locked loop (OFLL) was implemented to stabilize the center frequency of the microwave signal. This approach resulted in a microwave frequency at ~14 GHz with a phase noise of -75 dBc/Hz at 1 MHz offset from the center frequency. The frequency stability of the photonic microwave signal has an Allan deviation of more than 1 × 10-10 for an averaging time of 1000 s. The combination of the dual-frequency laser and the OFLL scheme holds great potential for the photonic generation and distribution of highly stable microwave or millimeter-wave signals.
No preview · Article · Aug 2012 · IEEE Photonics Technology Letters
[Show abstract][Hide abstract] ABSTRACT: We present a new synchronized design for flattening the passband of an arrayed-waveguide grating (AWG) over a broad wavelength range of 90 nm. A wavelength-insensitive 3-dB balanced coupler is designed to be used in duplicate in a Mach-Zehnder interferometer (MZI); the phase deviation created by one of the balanced couplers is cancelled by flipping the other coupler around. This MZI is arranged in tandem with the AWG such that the output signal of the MZI is the input signal of the AWG. We demonstrate a 5-channel, 18-nm-spacing AWG with a 0.5-dB bandwidth of 12 nm over a 90-nm spectral range. A low-loss cascaded AWG system is demonstrated by using the MZI-synchronized flat-top AWG as a primary filter.
[Show abstract][Hide abstract] ABSTRACT: The fabrication and characterization of waveguide Bragg gratings integrated with aluminum oxide channel waveguides are reported. Passive and lasing Bragg-grating-based cavities with Q-factors exceeding 1.5 × 106 and 1.1 × 1011, respectively, are demonstrated.
[Show abstract][Hide abstract] ABSTRACT: A micro-bimorph cantilever with self-aligned nanotips is monolithically integrated with a photonic crystal based device using optical and deep UV lithography techniques. Upon electrostatic actuation, the dielectric nanotips perturb the optical field, providing electromechano-optical modulation of light. Static tuning of the optical transmission spectra by more than 600 pm is measured with a sub-1 V drive voltage, resulting in a modulation as high as 21 dB. The observed strong electromechano-optical effect may find application in power efficient devices for optical communication networks, such as wavelength routing elements.
[Show abstract][Hide abstract] ABSTRACT: The performance of an arrayed-waveguide grating (AWG) as an integrated spectrometer in spectral-domain optical low-coherence reflectometry (SD-OLCR) is significantly improved. By removing the output waveguides of the AWG, the depth range is enhanced from 1 to 3.3 mm at 800 nm and 4.6 mm at 1300 nm. Periodic signal fading, that was previously observed in the sensitivity roll-off curve in depth ranging measurements, is shown to be evoked by beat-frequency generation between the two polarizations of partially polarized signal light in a birefringent AWG. By carefully controlling the polarization state-of-light, the signal fading is eliminated. As a permanent solution to this problem, a polarization-independent AWG is demonstrated, which can reduce the size and cost of OCLR and optical coherence tomography systems further by eliminating the components for polarization control.
No preview · Article · May 2012 · IEEE Photonics Technology Letters