Fully automated, high precision, cost-effective assembly technology for photonic packages remains one of the main challenges in photonic component manufacturing. Next to the cost aspect the most demanding assembly task for multiport photonic integrated circuits (PICs) is the high-precision (±0.1 μm) alignment and fixing required for optical I/O in InP PICs, even with waveguide spot size conversion. In a European research initiative – PHASTFlex - we develop and investigate an innovative, novel assembly concept, in which the waveguides in a matching TriPleX interposer PIC are released during fabrication to make them movable. After assembly of both chips by flip-chip bonding on a common carrier, TriPleX based actuators and clamping functions position and fix the flexible waveguides with the required accuracy.
This paper proposes and tests a design of electro-thermal bimorph actuators for alignment of flexible photonic waveguides fabricated in 16 µm thick SiO2. The actuators are for use in a novel alignment concept for multi-port photonic integrated circuits (PICs), in which the fine alignment is taken care of by positioning of suspended, mechanically flexible waveguide beams on one or more of the PICs. The design parameters of the bimorph actuator allow to tune both the initial relative position of the waveguide end-facets, and the motion range of the actuators. Bimorph actuators have been fabricated and characterized. The maximum out-of-plane deflection of the bimorph actuator (with 720 μm-long poly-Si) can reach 18:5 μm with 126:42mW, sufficient for the proposed application.
Spiral-waveguide amplifiers in erbium-doped amorphous aluminum oxide are fabricated by RF reactive co-sputtering of
1-μm-thick layers onto a thermally-oxidized silicon wafer and chlorine-based reactive ion etching. The samples are
overgrown by a SiO2 cladding. Spirals with several lengths ranging from 13 cm to 42 cm and four different erbium
concentrations between 0.5−3.0×1020 cm-3 are experimentally characterized. A maximum internal net gain of 20 dB in
the small-signal-gain regime is measured at the peak emission wavelength of 1532 nm for two sample configurations
with waveguide lengths of 13 cm and 24 cm and erbium concentrations of 2×1020 cm-3 and 1×1020 cm-3, respectively.
The obtained gain improves previous results by van den Hoven et al. in this host material by a factor of 9. Gain
saturation as a result of increasing signal power is investigated. Positive net gain is measured in the saturated-gain
regime up to ~100 μW of signal power, but extension to the mW regime seems feasible. The experimental results are
compared to a rate-equation model that takes into account migration-accelerated energy-transfer upconversion (ETU)
and a fast quenching process affecting a fraction of the erbium ions. Without these two detrimental processes, several
tens of dB/cm of internal net gain per unit length would be achievable. Whereas ETU limits the gain per unit length to 8
dB/cm, the fast quenching process further reduces it to 2 dB/cm. The fast quenching process strongly deteriorates the
amplifier performance of the Al2O3:Er3+ waveguide amplifiers. This effect is accentuated for concentrations higher than
2×1020 cm-3.
We report the fabrication and optical characterization of long, spiral-shaped erbium-doped aluminum oxide (Al2O3:Er3+) channel waveguides for achieving high overall signal amplification on a small footprint. Al2O3:Er3+ films with Er3+ concentrations in the range between 0.44−3.1×1020 cm-3 were deposited by reactive co-sputtering onto standard, thermally oxidized silicon substrates. Spiral-shaped waveguides were designed and structured into the films by chlorinebased reactive ion etching. In the current design, each spiral waveguide occupies an area of 1 cm2. Typical background propagation losses near 1500 nm are (0.2±0.1) dB/cm. A commercially available, pigtailed diode laser at 976 nm was employed as the pump source. The erbium-doped waveguide amplifiers were characterized in the small-signal-gain regime at the peak-gain wavelength (λ = 1532 nm) of Al2O3:Er3+. A maximum of 20 dB of internal net gain was measured for a 24.5-cm-long spiral waveguide with an Er3+ concentration of 0.95×1020 cm-3. Similar results were obtained for a shorter spiral with an Er3+ concentration about twice as high. Samples with lower concentration exhibited lower gain because of insufficient pump absorption, while samples with higher concentration showed less gain because of migration-accelerated energy transfer up-conversion and, more importantly, a fast quenching process.
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.
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.
We review our recent results on integrating biomedical optical systems onto a silicon chip. Light collection by integrated
waveguides has been investigated. Confocal light delivery and collection by a combination of two arrayed-waveguide
gratings has been achieved. Using an arrayed-waveguide grating as an integrated spectrometer, Raman spectroscopy and
spectral-domain optical coherence tomography have been demonstrated.
The optical amplifier performance of Nd3+-doped polymer and amorphous Al2O3 channel waveguides with single-mode and multi-mode behavior around 880 nm is compared. Internal net gain in the wavelength range 865-930 nm is
investigated under continuous-wave excitation near 800 nm, for Nd3+ dopant concentrations typically in the range of 0.6-
1.0 × 1020 cm-3. A peak gain of 2.8 dB at 873 nm is obtained in a 1.9-cm-long polymer waveguide at a launched pump
power of 25 mW. The small-signal gain measured in a 1-cm-long sample is 2.0 dB/cm. In Al2O3, a peak gain of 1.57
dB/cm in a short and 3.0 dB in a 4.1-cm-long waveguide is obtained at 880 nm. Tapered multi-mode Nd3+-doped
amplifiers are embedded into an optical backplane and a maximum 0.21 dB net gain is demonstrated in a structure
consisting of an Al2O3:Nd3+ amplifier placed between two passive polymer waveguides on an optical backplane. The
gain can be further enhanced by increasing the pump power and improving the waveguide geometry, and the wavelength
of amplification can be adjusted by doping other rare-earth ions.
Integrated optical probes for detecting backscattered light in, e.g., Raman spectroscopy show desirable characteristics
compared to conventional optical fiber probes, although the latter ones may have better collection efficiency in many
cases. Major advantages of integrated probes include reduced size; reduced background noise due to scattering in the
probe because of reduced propagation length; potential for monolithic integration with filters and spectrometers; very
small collection volume, providing high spatial resolution; and polarization maintenance. We demonstrate that when
scattered light needs to be collected from a thin layer close to the probe surface, integrated probes can have better
collection efficiency than fiber probes do. We modeled a multimode integrated waveguide probe by adapting an
analytical model that had been developed for fiber probes. The model was extended in order to account for arbitrary
waveguide geometries and a low number of discrete waveguide modes compared to the quasi-continuum of modes in a
typical multimode fiber. Using this model we compared the collection efficiencies of integrated and fiber probes for a
thin scattering sample. We found that the integrated probe has a higher collection efficiency for scattering layer thickness
and probe-to-layer distance both smaller than ~100 μm.
Erbium-doped aluminum oxide channel waveguides were fabricated on silicon substrates and their characteristics were
investigated for Er concentrations ranging from 0.27 to 4.2 × 1020 cm-3. Background losses below 0.3 dB/cm at 1320 nm
were measured. For optimum Er concentrations in the range of 1 to 2 × 1020 cm-3, internal net gain was obtained over a
wavelength range of 80 nm (1500-1580 nm) and a peak gain of 2.0 dB/cm was measured at 1533 nm. Integrated
Al2O3:Er3+ channel waveguide ring lasers were realized based on such waveguides. Output powers of up to 9.5 μW and
slope efficiencies of up to 0.11 % were measured. Lasing was observed for a threshold diode-pump power as low as 6.4
mW. Wavelength selection in the range 1530 to 1557 nm was demonstrated by varying the length of the output coupler
from the ring.
Erbium-doped aluminum oxide channel waveguides were fabricated on silicon substrates and their characteristics were
investigated for Er concentrations ranging from 0.27 to 4.2 × 1020 cm-3. Background losses below 0.3 dB/cm at 1320 nm
were measured. For optimum Er concentrations in the range of 1 to 2 × 1020 cm-3, internal net gain was obtained over a
wavelength range of 80 nm (1500-1580 nm) and a peak gain of 2.0 dB/cm was measured at 1533 nm. 170 Gbit/s high-speed
data amplification was demonstrated in an Al2O3:Er3+ channel waveguide with open eye diagrams and without
penalty. A lossless 1×2 power splitter has been realized in Al2O3:Er3+ with net gain over a wavelength range of 40 nm
(1525-1565 nm) across the complete telecom C-band.
Silicon oxynitride (SiON) is a highly attractive material for integrated optics, due to its excellent properties such as high
transparency, adjustable refractive index and good stability. In general, the growth of SiON layers by plasma enhanced
chemical vapor deposition (PECVD) is followed by a high temperature annealing step in order to remove hydrogen and
to achieve low propagation losses in the 1.5-μm wavelength window. The high annealing temperature (>1100°C)
required for sufficient hydrogen removal induces, however, side effects like significant inter-layer diffusion and micro-cracks
resulting in deterioration of the device performance.
In this paper compositional and optical properties of as-deposited and annealed boron (B) and phosphorous (P) doped
SiON layers were investigated. The doped layers have been fabricated by introducing PH3 and B2H6 gaseous precursors
into the PECVD process. Hydrogen contents of the samples have been studied by Fourier transform infrared (FTIR)
spectroscopy. Compared to undoped film, a 50% reduction of the hydrogen content was measured in as-deposited P-doped
SiON layers. Further reduction down to the FTIR detection limit was achieved upon annealing at temperatures as
low as 700°C.
Besides hydrogen reduction the reflow properties of B and P doped SiON are also highly relevant for the realization of
low-loss integrated optical circuits. Reactively ion etched channel waveguides have been reflown applying a temperature
of 900°C. Significant reduction of the sidewall roughness has been confirmed by scanning electron microscopy.
Amorphous Al2O3 is a promising host material for active integrated optical applications such as tunable rare-earth-ion-doped
laser and amplifier devices. The fabrication of slab and channel waveguides has been investigated and optimized
by exploiting reactive co-sputtering and ICP reactive ion etching, respectively. The Al2O3 layers are grown reliably and
reproducibly on thermally oxidized Si-wafers at deposition rates of 2-4 nm/min. Optical loss of as-deposited planar
waveguides as low as 0.11±0.05 dB/cm at 1.5-μm wavelength has been demonstrated. The channel waveguide
fabrication is based on BCl3/HBr chemistry in combination with standard photoresist and lithography processes. Upon
process optimization channel waveguides with up to 600-nm etch depth, smooth side walls and optical losses as low as
0.21±0.05 dB/cm have been realized. Rare-earth-ion doping has been investigated by co-sputtering from a metallic Er
target during Al2O3 layer growth. At the relevant dopant levels (~1020 cm-3) lifetimes of the 4I13/2 level as high as 7 ms
have been measured. Gain measurements have been carried out over 6.4-cm propagation length in a 700-nm-thick Er-doped
Al2O3 waveguide. Net optical gain has been obtained over a 35-nm-wide wavelength range (1525-1560 nm) with
a maximum of 4.9 dB.
Reactively co-sputtered amorphous Al2O3 waveguide layers with low propagation losses have been deposited. In order to define channel waveguides in such Al2O3 films, the etching behaviour of Al2O3 has been investigated using an inductively coupled reactive ion etch system. The etch rate of Al2O3 and possible mask materials was studied by applying various common process gases and combinations of these gases, including CF4/O2, BCl3, BCl3/HBr and Cl2.
Based on a comparison of the etch rates and patterning feasibility of the different mask materials, a BCl3/HBr plasma and
and standard resist mask were used to fabricate channel waveguide structures. The etched structures exhibit straight
sidewalls with minimal roughness and etch depths of up to 530 nm, sufficient for defining waveguides with strong
optical confinement and low bending losses. Low additional propagation losses were measured in single-mode Al2O3 ridge waveguides defined using the developed etch process. In initial investigations, Al2O3:Er layers fabricated using the same deposition method applied for the undoped layers show typical emission cross-sections, low green upconversion luminescence and lifetimes up to 7 ms.
When tackling the issue of low cost and enabled mass production for photonic circuits, the application of flip-chip technology creates huge expectations. We report on the results of a European project, in which it was the goal to demonstrate standard packaging technology in combination with specific integrated optics devices, entailing demands
and limitations different from IC technology. In integrated optics devices, mainly the fiber attachment, but also some special features as the accessibility of windows at the top-side of the chip (e.g. sensing devices) are prohibiting the positioning of the optical layer stack and the solder pads for the flip-chip processing at the same side of the silicon wafer. Therefore, a feed through technology for the electrical wiring had to be included. Compatibility issues in
combining the feed through technology with integrated optics processing have been solved. In this paper, we will demonstrate the successful feed-through metallization and flip-chip assembly in combination with an integrated optical sensor. The sensor, which has been designed for the measurement of relative humidity from 0-100 %RH, has been realized and packaged according to this technology. The feed-through metallization, solder type and chip carrier material have been chosen in a way that the demands of the demonstrator device are fulfilled according to a given specification.
A polarization independent optical waveguide structure suited for operation in the third communication window has been developed and optimized towards minimized dependence on deviations in the processing parameters and very low processing complexity. The tolerance analysis and optimization have been based on the thin film parameters of the widely applied silicon oxynitride technology. The silicon oxynitride layers have typically a material birefringence (nTMnTE) between 1-2 x 10-3 and can be deposited within a uniformity and reproducibility of 1% in thickness (d), 5x10-4 in refractive index (n) and 100 nm in channel width (w). The optimized waveguide structure meets the criterion of a channel birefringence (Δneff,TM-TE) within 5x10-5 taking the processing tolerance into account. Moreover, it was found
that the channel birefringence is thickness independent (within the 10-5 criterion) over a range of up to 200 nm
(δΔneff,TM-TE ,/ δd = 0). Furthermore, the optimized waveguide is fulfilling the remaining demands of the application
aimed at, such as monomodality, low fiber to chip coupling loss (< 0.5 dB/facet) and low loss bends with a radius down to 600 nm. This waveguiding structure has been applied for the realization of a passband flattened add-drop multiplexing device (or interleaver) with 0.4 nm free spectral range and 0.03 nm TE-TM shift. Based on this shift, a polarization dependence of 3 x 10-5 was calculated for the optical waveguides.
Silicon oxynitride (SiON)- technology has been widely accepted for realizing integrated optical devices for application in optical telecommunication. Some of the severe requirements put in this field to devices and hence to technology are more relaxed in sensing applications, but other ones pop up in this area. These differences are explained from the general requirements put on the performance of integrated optical sensors performance and they are analyzed with respect to their consequences for applying SiON technology. Data about the technology are given. Application of the technology is illustrated on some chemo-optical sensors, a Mach-Zehnder interferometer, a polarimeter and a bend sensor, which have been developed in the MESA+-institute.
In this paper, we demonstrate a thermo-optically tunable periodic wavelength filter (interleaver) with a 50 GHz free spectral range (FSR). It has an almost rectangular wavelength response and consists of an asymmetric Mach-Zehnder Interferometer (MZI) consisting of two tunable 3dB couplers interconnected by two waveguide channels of unequal length, with a ring resonator coupled to one of the branches of the MZI. The filter is fabricated in silicon oxynitride (SiON) waveguide technology. The bar and cross transmission spectra and chromatic dispersion of the filter have been measured and passband flattening and stopband broadening was observed in good agreement with the simulation. The isolation was 15 dB and 12 dB for TM and TE polarized light respectively, which was lower than the designed 29 dB. The main cause of lower isolation is an inaccuracy of the realization of the power coupling coefficient to the ring (59% instead of the designed 82%). The measured dispersion of the filter varies from 0 ps/nm at the center to 1660 ps/nm at the edge of the passband.
Integrated optical waveguides, as well as splitters and switches utilizing the principle of multimode interference have been realized in PECVD SiON/SiO2 technology. The waveguide design has been optimized to meet a number of requirements, such as small device size, polarization- independent operation, and low-power active operation. The design, fabrication, and initial experimental results for these devices are discussed.
As a novel application of silicon-based integrated optics, the results of a compact Mach-Zehnder interferometer are presented. The deposition of a ZnO thin-film transducer on the reference arm of the interferometer transforms this optically passive device under an active sinusoidal phase modulation. This device will be used as an in situ biosensor.
The thickness non-uniformity and refractive index in- homogeneity of silicon oxynitride thin films, grown by low pressure chemical vapor deposition, have been optimized. The present work was especially motivated by the application of these thin films as well defined waveguides in phase-matched second harmonic generating devices, which are well known for their extremely high requirements to uniformity and homogeneity. However, other demanding integrated optical components like gratings, sensor systems, telecommunication devices, etc., also strongly benefit from highly uniform waveguides.
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