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Main results of Year 2

After the second year, the project has successfully achieved its objectives and is in line with the initial planning

Thanks to excellent cooperation within the consortium, high level results were achieved, leading to more than 60 papers in journals/conferences in two years

An update of the state of the art report was delivered and can be downloaded from the Helios website.

III-V on silicon lasers

Following first year’s results, III-Vlab improved the metallization process of the devices and this resulted in laser operation. Both molecular bonded samples (delivered by CEA) and BCB bonded samples (delivered by imec) were processed. The device consists of a central gain section (light in III-V), a taper for coupling the light to the silicon (at both sides) and a short piece of silicon waveguide (at both sides). Some results are shown in the pictures below (CW operation). The minimum threshold was 50mA and the output varied from 1.5mW (for 500um long device) to 3mW (for 1100um long devices).

spectre laser

Optical spectrum of the laser driven at 55mA

Modulators

Thick modulator with PIPIN structure:

Some promising results have been demonstrated from the first unsuccessful modulator run. A contrast ratio up to 25dB has been obtained from the MZI together with a high electrical bandwidth (35GHz) for the electrodes indicating, successful design of the passive optical structure and the RF electrodes.The design of the 40Gbit/s modulator has been successfully done. Both RC limitation and electrical signal propagation all along the electrodes have been taken into account, to finally design a modulator showing an extinction ratio larger than 2dB at 40Gbit/s, with an insertion loss of 5 dB.

Thin modulator with PIN structure:

Characterisation of the first fabrication batch of thin modulators has been performed. A large static extinction ratio (>35dB) has been demonstrated, this allows for large DC extinction ratios with modest length-drive voltages, for example approximately 25dB has been obtained with 3V applied to a 3.5mm phase shifter. Data transmission has been observed in excess of 5Gbit/s. Optimisation of the design has been performed in order to reach the 40Gbit/s performance targets.

Passive circuitry

Fiber coupling circuits:

With respect to grating couplers, three techniques have been developed by imec to improve directionality and thus coupling efficiency: a bottom DBR mirror, a silicon overlay and non-uniform gratings. Out of these methods, the highest efficiency gratings that has been experimentally demonstrated showed 69% efficiency (-1.6dB loss). This has been the case for gratings with a bottom mirror and also for gratings with a silicon overlay. The low loss (<2dB) bandwidth is similar in both cases (~30nm).

With respect to inverted taper based couplers, coupling losses below 1dB for a broad bandwidth of 100nm were demonstrated for the inverted taper approach embedded in a SiOx rib waveguide. The coupler was optimised by LETI and CNRS for coupling to standard single mode fibers by increasing the cross-section of the SiOx rib waveguide. Coupling efficiency of 55 % (-2.6 dB) was demonstrated for a taper length of 200 µm.

MUX/DEMUX circuits:

Two generations of AWG based demultiplexers, with variations in channel spacings and number of channels have been fabricated and tested by imec. For the 8 channel 400GHz device, crosstalk better than 20dB and losses lower than 1.5dB was demonstrated. For the 16 channel 200GHz device, losses of around 4dB for the central channel and crosstalk better than 15dB was demonstrated.

A microring demultiplexer filter has been fabricated and characterised by UPVLC. Experimental results confirmed simulation results and demonstrated the target filtering performance of two optical carriers separated by 60GHz with extinction ratios above 20dB and insertion losses below 3dB.

Polarization diversity circuits:

For the grating couplers approach, polarization independent demultiplexing using AWGs and a novel approach to reduce the PDL below 1dB by incorporating a phase section in one of the arms of the circuit has been demonstrated by imec. 2D grating couplers with raised silicon overlay were developed but a higher than expected loss (5-6dB coupling efficiency) was measured.

UPVLC fabricated and characterized the compact polarization splitters based on MMI structure. Extinction ratios better than 20dB were obtained for a 13.5µm long device. imec also developed a polarization rotator based on using symmetry breaking of an almost square waveguide. A polarization conversion efficiency of -0.5dB over a wavelength range of 80nm around 1550nm was measured.

Photodetection

The InGaAs MSM-detectors integrated on silicon waveguides show high responsivity (1A/W) and low dark current (<5nA at room temperature, <80nA at 80°C). Vertical and lateral pin germanium photodetectors exhibit low dark current if the thermal annealing is carried out, high responsivity (between 0.8 and 1A/W) and high bandwidths: 42GHz under -4V for the vertical pin diode and higher than 130GHz for the lateral diode have been obtained. A 40GBit/s data transmission has been also demonstrated and 10GBit/s data transmission without bias voltage is demonstrated.

Design environment

The overall result was the establishment of a photonics design flow modelled after the electronics design flow enabling (i) design using functional specifications, (ii) derivation of the physical implementation and (iii) accurate analysis of the functional behaviour, using hierarchical behavioural models. This flow was enabled through three specific contributions:

  • Implementation of the OpenAccess API enabling tool interoperability.
  • Hierarchical photonic device modelling strategy and implementation with Verilog-A.
  • Development of the design kit for the CMOS-photonics co-integration strategy.

Electronic/Photonic convergence

For the front end integration IHP worked on the integration of its high-performance BiCMOS technology on photonic SOI substrates (220nm SOI, 2000 nm buried oxide). Formation of quasi-bulk areas on SOI was achieved by selective silicon epitaxy through the buried oxide. The epitaxial re-growth leaves islands of SOI suitable for photonic devices surrounded by bulk-like areas.

A first BiCMOS test pattern was processed on a chess-board wafer. The analysis of electric in-line and off-line parameters showed that electric device specs were met to a large extend.

On wafers delivered by austriamicrosystems which were stopped after structuring of top metal, LETI has performed a pre-bonding process such as covering with thick SiO2 the top metal, planarization of the surface with CMP. With one CMOS wafer and with a trial photonic wafer, we performed successfully the wafer bonding and the substrate removal down the box. LETI performed the test of the bonded 1D surface grating coupler with one layer of amorphous silicon acting as a mirror. The coupling efficiency was found to be almost 50% (3dB) with TE polarization.

intégration réseau

Bonded 1D surface grating coupler with mirror on CMOS wafer from austriamicrosystems

austriamicrosystems delivered two multi product wafer lots with the TIA designed by TUW for 10G operation for InGaAs or Ge photodetectors. The design demonstrates successful 10 Gbps data transmission even at low input currents. With an estimated total photodiode responsivity of 0.5 A/W a sensitivity of around ‑23.1 dBm (BER = 10-9) can be reached.

Innovative proofs of concept

Amorphous Si modulator

Several multi-stack (N ´ a-Si:H/insulator) planar waveguides for characterizing the electric field-induced effect on optical absorption were produced. The best results have been achieved on a six bi-layers sample (a-Si:H/0.2mm, a-SiCN/30 nm). The modulation depth is higher for thinner waveguides.. As set by specifications, the fabricated waveguides perform <8 dB/cm losses. It has been possible to raise the free carrier concentration in the a-Si:H films from 109cm-3, in undoped samples, to ~5×1013cm-3 without reaching prohibitive optical losses (< 8 dB/cm).A clear electro-absorption effect has been observed and characterised in the fabricated multi-stacks. This effect is enhanced in thinner waveguides and is strongly affected by the number of a-Si:H/insulator couples. A very good agreement has been found between simulated and measured variations of the absorption coefficient a as a function of the applied electric field E.

Electro-optic coefficients in a-Si:H/insulator multi-layers have been measured in-guide for the first time. Absorption modulation induced by free carriers has been characterised in several structures. A maximum ∂α/∂E coefficient of 0.75 cm/V has been measured. A clear phase modulation has been also observed and characterised in simple Fabry-Perot cavities fabricated starting from a 1mm-thick waveguide composed of 3 a-Si:H/insulator couples with lightly doped a-Si:H films. From these measurements, we estimated that the ∂n/E coefficient is 6.1´10-9 cm/V.

All-optical modulation studies have been pursued to demonstrate the existence of high-speed carrier-related optical phenomena in a-Si:H. An in-guide pump-probe setup has been used (532 nm pump) through which rise-fall of the order of 10 ns have been observed for the probe beam, limited by the pump pulse duration.

Finally, extra-program measurements were performed on p-i-n waveguides in which a ∂n/∂E of 2.3´10-10 cm/V was estimated in reverse bias. Rise-fall times of the order of 100 ns have been observed in 2mm-thick wide-area planar Fabry-Perot cavities.

Silicon nanocrystals for light emission and amplification

LED Devices were fabricated: 0.1% of external quantum efficiency has been obtained. The LEDs physical principles are understood and modelled.

LED cross section

Schematic cross-section of the LED

LED top

Top-view image of emitting visible LED

Novel concepts of heterogeneous integration with 2.5D cavity)
PC laser

Exemplifying sketch of the 2.5D HELIOS structure for optical pumping

spectre PC laser

One of the first PL spectrum obtained during the preliminary analysis.To be noted the PL peak at 1550 nm

The first 2.5D devices for optical pumping showed quite promising results in terms of PL spectra, leaving enough room for obtaining lasing effect in such innovative structures. It can be noted that the optically-pumped active medium provides a rather good gain especially around the expected wavelength of the TE00 cavity mode.