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Main results of Year 3

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

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

III-V/Si lasers

InP on Si hybrid lasers full characterisation of different building blocks for verifying origin of problems in earlier processing runs was conducted. Improved bonding processes (BCB-bonding and molecular bonding) have been obtained with planarized SOI wafers with separation in the order of 100nm between the silicon waveguide and the InP part. The InP on Si process has been optimised, now reaching deep ridge structures with 400nm taper tips. FP-type hybrid lasers showing <30mA threshold current at RT, have reached one of the main specifications for the laser.

Modulators

Modulators with 10 Gbit/s operations were demonstrated in different configurations. With thick Si designs (PIPIN diode) integrated in a Mach Zehnder interferometer with 1.8 mm long phase shifter, lead to 8 dB extinction ratio with optical loss of only 6 dB. This structure was also integrated in a 50 µm radius ring resonator, showing low optical loss and a 4.4 dB extinction ratio. In addition a new electrical structure to obtain intensity modulation has been demonstrated, based on interdigitated PN diodes. A ring modulator showing 4 dB extinction ratio at 10Gbit/s has been obtained. With thin Si design (PN diode), processing of thicker electrodes has resulted in a reduction in the electrode RF loss and the demonstration of 10Gbit/s modulation from devices of the first fabrication batch. Devices from the second fabrication batch have been characterised. 40Gbit/s modulation with a 10dB extinction ratio has been demonstrated from a 3.5mm long MZI although with a normalised optical loss of 15dB. 40Gbit/s modulation has also been demonstrated from a 1mm MZI with a modulation depth of 3.5dB and normalised optical loss of 5dB, these results exceed the current state of the art. A new engineered slow-wave corrugated structure was proposed and experimentally demonstrated. The proposed slow wave structure consists of a deep-etched laterally corrugated waveguide with circular holes patterned onto its wide section. The addition of this perturbation in the periodic structure enables tailoring the dispersion relation in order to obtain a nearly flat band, i.e., a region inside the Brillouin zone where the group index is constant over a determined frequency range. Nearly constant group index as high as 13.5 over a wavelength range of ~14 nm was experimentally demonstrated in a 50 µm long waveguide. A slow wave based thin modulator exhibited a Vπ.Lπ ~1.27V·cm for a group index of 11 and as low as ~0.45V·cm when the group index increased up to ~22. Slow light enhancement was exploited for scaling the modulator length down to 500 μm (325 µm2 footprint) while demonstrating error free modulation up to 20 Gbit/s at a moderate group index of only 11 due to the enhanced modulation efficiency.

Packaging

The direct butt coupling between a lensed fibre inserted in a V-groove etched in a silicon submount including a special notch allowing free space and epoxy free focusing of light from the fibre onto the inverted taper of a silicon wire waveguide was first demonstrated. Fibre to fibre insertion loss is in the range of –7dB. Thermal cycling (10x) between –40°C and +85°C gave 1dB of insertion loss variations.

Design environment

On design environment, it was obtained the fleshing out of component libraries for photonic and electrical components, the exploration of tools for phase-independent photonic routing using electrical-side place-and-route tools, and the development of phase-matched photonic routing using photonic-side design tool

Photonic/electronic integration

  • For electronic-photonic integration, work on the assembly on the integration of a Ge or InGaAs receiver on top of ams wafer containing TIA was carried on. The Ge diode has a dark current of about 2 µA at a bias voltage of -2 V while the IMEC III/V diode has a dark current of 200 nA at -6 V bias voltage. In the next step the AC response and the eye diagram of both detectors were measured at a wavelength of 1550 nm. The 3 dB-cut-off frequency was found at 46 GHz for the Ge detector, respectively 13.5 GHz for the InGaAs detector. A 16 channels Ge receiver was designed and fabricated. Before bonding, tests revealed the operation of all the channels up to 20GHz
eye diagram detector

Eye diagram IMEC III/V detector @ 10 Gbps

 

Ge PD + AWG

Ge Photo detector & AWG

  • A 16x10GBit/s receiver has been designed which consists of a 1 x 16 AWG demultiplexer in a polarization diversity configuration. III-V PIN or Ge photodetectors are waveguide coupled to each output of the AWG. On a first demonstration, III-V photodetectors, were integrated on a 1 x 8 AWG, fabricated in a standard waveguide process. The response shown below includes the losses of the grating couplers at the input and below the detector
III-V PD + AWG_gif

(a) III-V photodetector on silicon grating. (b) Response of photodetectors integrated on top of 1x8 AWG, including losses from input grating coupler.

A 16 channel AWG 200 GHz receiver with lateral Ge PIN photodiodes was designed and fabricated. It consists of a 2D coupler, two AWGs with 16 channels, 200GHz channel spacing for TE mode, 16 Ge photodiodes with two inputs (one for the TE coming from the fiber, the other for the TM coming from the fiber). Figure below reports the characteristics of one channel with random polarisation. The response of the total receiver is reported also. First tests on bandwidth exhibited 20GHz operation, more than sufficient for 10G operations.

I(V) PD

Voltage-current curve of a photodiode with random polarization

 

I(lambda) AWG

Response of photodetectors integrated on top of 1x16 AWG, including losses from input grating coupler.

 

  • A DQPSK demodulator for 10GBps with optical output was fabricated using MZI micro-heater controlled switches to control the power ratio for the input and output of the delay-line MZI.
DQPSK demod

An optical image of the fabricated DQPSK demodulator.

 

Systematically the phase shifters were optimized to find the ideal response of the demodulator. The FSR correspond to 10.3Gbps that is within the specifications for the 10G that was the design. The total IL to the system was more than 40dB, mostly because of the butt-coupling of the external fibre which was estimated to 32dB.

response DPSK demod

The optical response of the DPSK demodulator for 10Gbps signal.

 

Innovative proofs of concept

Amorphous Si modulator

The first Mach-Zehnder interferometers (MZI) based on an a-Si:H p-i-n structure have been designed, fully in-house fabricated, and preliminarily tested, providing surprising results for low-temperature processed photonic devices (T<150°C), in particular very encouraging under the point of view of bandwidth. Switch-on and switch-off times of the order of 3 ns have been measured in fact, with a Vπ´Lπ figure of merit of ~20 V´cm. First measurements on the polarization-dependence of the modulation show that the effect is weaker for TM-polarised waves (-20%).

Silicon nanocrystals for light emission and amplification

Electrical and electroluminescence (EL) properties of Er-doped Si nanocrystals were studied in detail under both direct and alternating current excitation schemes in a broad range of applied voltages and driving frequencies. Dependence of the Si-nc LED external power efficiency on the AC driving frequency is worked out, see Appl. Phys. Lett. 98, 201103 (2011).

We found that silicon oxide matrix (SRO) is much better for Er EL than silicon nitride (SRN), which is in contradiction with a raising consensus that SRN is much better than SRO for lasing (see, e.g., Si Laser MURI or KAIST publications). The high current in SRN is mostly leakage through conducting paths via defects.

Impact ionization is responsible for a great part of EL emission and we know what material is the best to maximize it. It is necessary at least 4-5 MV/cm to make most samples to emit and this is the threshold for the Fowler-Nordheim injection. Furthermore, we are able to saturate excitable Er in SRO by electrical pumping and we have checked that emission is comparable (within a factor 2, see below) to Er optically active in stoichiometric SiO2. Even though results show that optically. active Er is only 10-20% (see the preceding point 3) of the total Er concentration, we have to admit that those measurements and calculations were done by optical means with a strong pumping at 980 nm or 1480 nm and the uncertainty is large. So, being a factor of 2 less than Er in SiO2 emission by electrical pumping is a very interesting result because it leaves great expectation on lasing in such system.

EL

EL intensity at 1.54 mm (left axis, blue symbols) at a fixed injected current of 1 mA as a function of average content of silicon excess for a series of multilayer LEDs (squares) and single-layer LEDs (stars) and corresponding driving voltages (right axis, red symbols).

Novel concepts of heterogeneous integration with 2.5D cavity

Optical spectra of optically pumped first-generation of PCM-VCSEL cavities have been measured using an upgraded temperature controlled set up. Quality factors ranging between some hundreds up to 2000 have been experimentally found. For the latter, the devices were close to reach laser threshold, which is a strong indication for the second generation of devices to reach readily laser action.

PCM Vcsel

Infrared camera images of µ-PL emitted from VCSEL’s top PCM at room temperature along with FDTD-computed far field patterns of TE00 fundamental mode. In each panel, the orientation and the architecture of the PCM is provided in the bottom left corner. PL emission is compressed along slits while experiences losses in the perpendicular direction (a), revealing an anisotropic light transport in PCMs. A photonic crystal heterostructure is introduced to enhance and tailor the light control in PCMs. Guiding (b) and antiguiding (c) barriers are shown in real devices.