Progress in photonics by monolithic integration for higher functional density, performance and reduced cost faces challenging hurdles due to technological and functional heterogeneities. Advanced local material growth techniques are enabling concepts towards high-density photonic integration, unprecedented performance and multi-functionality and ultimately optical systems-on-a-chip.
For example mode-locked laser diodes (MLLDs) are key devices for ultra-short pulse generation for all-optical Tbit/s communication networks. MLLDs suffer from material compromises and will benefit from the possibility to design the gain, absorber and passive-waveguiding sections independently. We have proposed and demonstrated the integration of a saturable absorber with a fast absorption recovery time based on an InP/InGaAsP uni-traveling-carrier structure (UTC) to achieve pulses below 1 ps with repetition rates up to 40 GHz. The use of the UTC absorber instead of the commonly employed reverse-biased gain material requires however the heterogeneous growth of multiple layer stacks on the same chip with the butt-coupled regrowth technique.Critical for the MLLD performance are the reflections and the optical coupling between the different monolithic integrated layer structures of passive, absorbing and amplifying sections. 2D FDTD simulations of the optical waveguides demonstrate that to minimize reflections an angled interface between the different structures is preferable and can lead to reflection coefficients as low as 10^-6. To obtain an angled interface we used a wet chemical etching process sequence of selective and non-selective etchants, which is sensitive to crystal orientation and yields a 55° tilted interface. In addition we can conclude from our simulations that in order to minimize both, insertion loss and reflections, a bending of the light guiding layers has to be prevented. Bendings can lead to measured losses of 5-7 dB per interface whereas correctly aligned light guiding layers results in losses of 1.5 dB and intensity reflections below 10^-5 per interface. The bendings originate from different growth rates near and far away from masked areas during regrowth due to reactants diffusion on the SiO2 mask. The bending can be minimized by optimizing the mask under etch of the SiO2 mask and low pressure MOVPE growth. We demonstrate operation of mode-locked laser diodes with an integrated UTC absorber and pulse durations below 1 ps.
Monolithic photonic integration offers unsurpassed perspectives for higher functional density, new functions, high per-formance, and reduced cost for the telecommunication. Advanced local material growth techniques and the emerging photonic crystal (PhC) technology are enabling concepts towards high-density photonic integration, unprecedented per-formance, multi-functionality, and ultimately optical systems-on-a-chip. In this paper, we present our achievements in photonic integration applied to the fabrication of InP-based mode-locked laser diodes capable of generating optical pulses with sub-ps duration using the heterogeneous growth of a new uni-traveling carrier ultrafast absorber. The results are compared to simulations performed using a distributed model including intra-cavity reflections at the sections inter-faces and hybrid mode-locking. We also discuss our work on InP-based photonic crystals (PhCs) for dense photonic integration. A combination of two-dimensional modeling for functional optimization and three-dimensional simulation for real-world verification is used. The fabricated structures feature more than 3.5μm deep holes as well as excellent pattern-transfer accuracy using electron-beam lithography and advanced proximity-effects correction. Passive devices such as waveguides, 60° bends and power splitters are characterized by means of the end-fire technique. The devices are also investigated using scanning-near field optical microscopy. The PhC activity is extended to the investigation of TM bandgaps for all-optical switches relying on intersubband transitions at 1.55μm in AlAsSb/InGaAs quantum wells.
The mode coupling of organic distributed feedback lasers is enhanced by using a distributed feedback grating that is etched into a thin layer of titanium dioxide (TiO2). The use of TiO2 increases the index contrast in the grating and the confinement in the waveguide. The enhanced mode coupling results in larger feedback given to the lasing modes, which lowers the laser threshold and allows smaller devices to be built. The lasing threshold of the TiO2-enhanced devices is twice as low as that of conventional devices whose grating is etched directly into the quartz.
We report on the investigation of planar photonic crystal waveguide
transitions with a scanning near-field optical microscope (SNOM) in
collection mode. An abrupt and a gradual taper design intended to
couple light from a W3 (three missing rows of holes) to a W1 waveguide
were fabricated in a InGaAsP slab waveguide. SNOM measurements reveal
that a taper design can efficiently funnel light into the W1
waveguide. For both designs a suppressed coupling of light into the W1
waveguide is measured for a frequency which corresponds to a mode
crossing which we determined by 3D plane wave simulations.
We have studied tapers that couple light from a conventional ridge
waveguide into a planar photonic crystal (PhC) waveguide. Tapering
is achieved by changing the PhC waveguide width either in steps or
gradually. Lag effects in fabrication provide an additional
tapering due to the fact that the hole depths scale with the
corresponding hole diameter. Our analysis deals with the
out-of-plane loss that arises within such taper sections. The PhC
consists of a triangular lattice of air holes introduced into an
InGaAsP/InP slab structure. For conceptual studies we use the 2D
multiple multipole method (MMP) in conjunction with an extended
phenomenological model. This model covers the out-of-plane
scattering providing a loss parameter and an effective index
correction for the holes under consideration. This realistic 2D
model is retrieved from full-wave 3D FDTD simulations and
The temporal coupled mode theory is applied on the design of
filters and waveguide crossings that feature a resonator with
a high quality factor. To determine the transmission properties of the device we calculate the decay rate of the resonator. The analysis using the decay rates requires far less computational effort than conventional FDTD transmission calculations and therefore the
optimum device properties can be determined quickly.