E Emil Kleijn

An introduction to InP-based generic integration technology

Photonic integrated circuits (PICs) are considered as the way to make photonic systems or subsystems cheap and ubiquitous. PICs still are several orders of magnitude more expensive than their microelectronic counterparts, which has restricted their application to a few niche markets. Recently, a novel approach in photonic integration is emerging which will reduce the R&D and prototyping costs and the throughput time of PICs by more than an order of magnitude. It will bring the application of PICs that integrate complex and advanced photonic functionality on a single chip within reach for a large number of small and larger companies and initiate a breakthrough in the application of Photonic ICs. The paper explains the concept of generic photonic integration technology using the technology developed by the COBRA research institute of TU Eindhoven as an example, and it describes the current status and prospects of generic InP-based integration technology.

Multimode Interference Reflectors: A New Class of Components for Photonic Integrated Circuits

Multimode interference devices are very versatile components that are often used as components for power splitting or combining in larger circuits. In this paper, we present a new class of multimode interference couplers that are designed to reflect all or part of the light. The components can act as on-chip mirrors with partial out-coupling, or they can act as compact mirrors with full reflection. We present an overview of such devices, the basic theory and practical design guidelines. Their simple layout makes these devices easy to fabricate and tolerant to fabrication errors. We fabricated devices of several types and present the experimental results.

Multimode Interference Couplers With Reduced Parasitic Reflections

Parasitic reflections can deteriorate the performance of a photonic integrated circuit. This is especially true in circuits containing amplifiers, but even in passive circuits, small reflections can already have a strong influence on circuit performance. It is known that strong reflections can be present when using a 2 × 1 multimode interference coupler (MMI) as a combiner. We investigate methods for reducing these spurious reflections in a generic integration technology. We present a novel MMI shape whose measurements show reduced reflections by 17.5 dB.

New Analytical Arrayed Waveguide Grating Model

An analytical model of star couplers in arrayed waveguide gratings (AWG) is derived. By retaining the real 1-D mode shapes, the model is able to calculate the star coupler response to fundamental modes, as well as higher order modes. This is desirable for modeling passband flattened AWGs. The model can calculate the response of an AWG very fast, because no numerical root finding or integral calculation is involved. This allows it to be used in circuit level simulations.

Sidelobes in the response of arrayed waveguide gratings caused by polarization rotation

Earlier it was observed that polarization rotation in an AWG built from birefringent waveguides can result in sidelobes in its response. This effect was measured in a polarization sensitive AWG with an orthogonal layout. Now we investigate through detailed simulation whether this effect also exists in polarization desensitised AWGs. It is shown that a dispersion compensated AWG does not suffer from a polarization sidelobe. Alternatively, the AWG can be designed to minimize polarization rotation to suppress the sidelobe.

A dense spot size converter array fabricated in a generic process on InP

Integrated spot size converters (SSCs) are key components for efficient coupling between Photonic Integrated Circuits (PICs) and fibre-arrays. We report a compact SSC which is suitable for integration into dense arrays with a pitch down to 25 μm and compatible with our generic InP-based platform technology, which supports integration of SOAs and Electro Optical Modulators with a range of passive components. The small pitch supports coupling tens of on-chip optical waveguide ports to fiber arrays via a low-loss dielectric interposer chip. The density allows the design of a customized optical bus between the InP PIC and the interposer chip. The dielectric chip may simply expand to the pitch of a fiber array but also contain low-loss passive circuitry like delay-lines, high Q-filters and multiplexers. The latter enables the formation of a hybrid integration platform with our InP-based technology. Efficient coupling is obtained by adiabatically transforming the sub-micron modes of the InP waveguides to the 3 μm diameter mode of the interposer. We tested our SSCs by coupling to a lensed fibre with a mode field diameter of 2.5 μm. Coupling losses were found to be as low as 0.6 dB per fiber chip coupling for device lengths of a few 100 μm. We also measured the crosstalk from one input port to output ports adjacent to the targeted output port. We present simple design rules for reducing the crosstalk to neighbouring output ports below -50 dB. The quality and uniformity of the SSCs is demonstrated by fabrication of an 8 x 8 AWG demultiplexer between two SSC arrays placed at input and output ports. We measured an insertion loss between fibres of 4 dB for the central channel of the AWG, which is record low for an InP-based device.

High-Resolution AWG-based fiber bragg grating interrogator

A novel InP-based monolithically integrated multi-wavelength detector is used as the central part of a fiber Bragg grating interrogator unit. We have designed the arrayed waveguide grating in the device such that at least two detectors have a significant signal at any wavelength being measured. Consequently, the measurement resolution is increased while keeping the area of device small. We demonstrate the resolution of the measurement to be 0.32 pm over a working range of 10 nm. The corresponding relative resolution of 0.003% is to our knowledge the best reported to date in an integrated interrogator.

Analysis of parasitic effects in PICs using circuit simulation

An understanding of parasitic effects is essential to maximize the performance of a Photonic Integrated Circuit (PIC). Using a circuit simulator, we are able to model mode conversion at the interface between straight and bent waveguides, parasitic reflections in multi-mode interference couplers (MMIs), interference between multiple modes, residual facet reflections, and reflections at junctions between components. Even though these effects are usually low in intensity, around -20 dB to -30 dB from the main signal level, they can still have a strong influence on the circuit performance. This is because the mentioned parasitic effects are coherent with the desired signal and interference between them is therefore a field effect. By analyzing three different circuits, and comparing the results to measurements, we show that these effects need to be carefully managed in order to ensure circuit performance. The circuits we investigate are a Fabry-Perot cavity, a Mach- Zehnder interferometric structure, and a Michelson interferometer. Especially residual reflections coming from angled facets and back-reflections in MMIs are shown to be the main parasitic effects in the investigated circuits.

Novel lasers using multimode interference reflector

We present a novel laser using multimode interference (MMI) reflectors. By using the MMI reflectors to form the laser cavity, the laser light is available on the chip, which makes the lasers very promising building blocks in photonic integrated circuits (PIC). These lasers are one of the first InP PIC devices fabricated in a generic multi-project wafer run.

Sidelobes caused by polarization rotation in arrayed waveguide gratings

Polarization rotation within an Arrayed Waveguide Grating (AWG) is found to cause sidelobes in the response of the AWG. The devices in which this effect was observed are briefly discussed. Their measured responses are compared to a detailed simulation.