This paper presents a technology of hybrid integration vertical cavity surface emitting lasers (VCSELs) directly on silicon photonics chip. By controlling the reflow of the solder balls used for electrical and mechanical bonding, the VCSELs were bonded at 10 degree to achieve the optimum angle-of-incidence to the planar grating coupler through vision based flip-chip techniques. The 1 dB discrepancy between optical loss values of flip-chip passive assembly and active alignment confirmed that the general purpose of the flip-chip design concept is achieved. This hybrid approach of integrating a miniaturized light source on chip opens the possibly of highly compact sensor system, which enable future portable and wearable diagnostics devices.
This paper summarizes the results of an EU project called ACTION: ACTive Implant for Optoacoustic Natural sound
enhancement. The project is based on a recent discovery that relatively low levels of pulsed infrared laser light are capable
of triggering activity in hair cells of the partially hearing (hearing impaired) cochlea and vestibule. The aim here is the
development of a self-contained, smart, highly miniaturized system to provide optoacoustic stimuli directly from an array
of miniature light sources in the cochlea. Optoacoustic compound action potentials (oaCAP) are generated by the light
source fully inserted into the unmodified cochlea. Previously, the same could only be achieved with external light sources
connected to a fiber optic light guide. This feat is achieved by integrating custom made VCSEL arrays at a wavelength of
about 1550 nm onto small flexible substrates. The laser light is collimated by a specially designed silicon-based ultra-thin
lens (165 um thick) to get the energy density required for the generation of oaCAP signals. A dramatic miniaturization of
the packaging technology is also required. A long term biocompatible and hermetic sapphire housing with a size of less
than a 1 cubic millimeter and miniature Pt/PtIr feedthroughs is developed, using a low temperature laser assisted process
for sealing. A biofouling thin film protection layer is developed to avoid fibrinogen and cell growth on the system.
A novel method for fabricating a single mode optical interconnection platform is presented. The method comprises the miniaturized assembly of optoelectronic single dies, the scalable fabrication of polymer single mode waveguides and the coupling to glass fiber arrays providing the I/O’s. The low cost approach for the polymer waveguide fabrication is based on the nano-imprinting of a spin-coated waveguide core layer. The assembly of VCSELs and photodiodes is performed before waveguide layers are applied. By embedding these components in deep reactive ion etched pockets in the silicon substrate, the planarity of the substrate for subsequent layer processing is guaranteed and the thermal path of chip-to-substrate is minimized. Optical coupling of the embedded devices to the nano-imprinted waveguides is performed by laser ablating 45 degree trenches which act as optical mirror for 90 degree deviation of the light from VCSEL to waveguide. Laser ablation is also implemented for removing parts of the polymer stack in order to mount a custom fabricated connector containing glass fiber arrays. A demonstration device was built to show the proof of principle of the novel fabrication, packaging and optical coupling principles as described above, combined with a set of sub-demonstrators showing the functionality of the different techniques separately. The paper represents a significant part of the electro-photonic integration accomplishments in the European 7th Framework project “Firefly” and not only discusses the development of the different assembly processes described above, but the efforts on the complete integration of all process approaches into the single device demonstrator.
Widely tunable vertical cavity surface emitting lasers (VCSEL) are of high interest for optical communications,
gas spectroscopy and fiber-Bragg-grating measurements. In this paper we present tunable VCSEL operating at
wavelength around 850 nm and 1550 nm with tuning ranges up to 20 nm and 76 nm respectively. The first versions
of VCSEL operating at 1550 nm with 76 nm tuning range and an output power of 1.3mW were not designed for
high speed modulation, but for applications where only stable continious tuning is essential (e.g. gas sensing).
The next step was the design of non tunable VCSEL showing high speed modulation frequencies of 10 GHz with
side mode supression ratios beyond 50 dB. The latest version of these devices show record output powers of
6.7mW at 20 °C and 3mW at 80 °C. The emphasis of our present and future work lies on the combination of
both technologies. The tunable VCSEL operating in the 850 nm-region reaches a modulation
bandwidth of 5.5GHz with an output power of 0.8mW.
An investigation into the carrier and spectral dynamics of a 1.55 μm Buried Tunnel Junction (BTJ) VCSEL
was carried out by examining the emission spectra under high resolution and the voltage across the junction
as polarisation resolved light from a tunable laser source was injected into the cavity. The VCSEL combines
an epitaxial InGaAlAs distributed Bragg reflector with a Si/ZnS dielectric reflector and an oval shaped BTJ
leading to a predominantly single transverse polarisation mode and laser linewidths as low as 20 MHz. Around
lasing threshold and injecting into the primary mode, the voltage required to maintain the current drops due to
stimulated emission and a consequent reduction in the carrier density. Locking behaviour associated with this
characteristic is measured with increased input power. Voltage drops as large as 6 mV are measured. Above
threshold, injection locking is measured in addition to features associated with the relaxation oscillations of the