This Review covers the state-of-the-art technologies on photonics-based terahertz communications, which are compared with competing technologies based on electronics and free-space optical communications. Future prospects and challenges are also discussed. Almost 15 years have passed since the initial demonstrations of terahertz (THz) wireless communications were made using both pulsed and continuous waves. THz technologies are attracting great interest and are expected to meet the ever-increasing demand for high-capacity wireless communications. Here, we review the latest trends in THz communications research, focusing on how photonics technologies have played a key role in the development of first-age THz communication systems. We also provide a comparison with other competitive technologies, such as THz transceivers enabled by electronic devices as well as free-space lightwave communications.

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Advances in terahertz communications accelerated by photonics

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.

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An introduction to InP-based generic integration technology

A review of the complexity development of InP-based Photonic ICs is given. Similarities and differences between photonic and microelectronic integration technology are discussed and a vision of the development of photonic integration in the coming decade is given.

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Moore's law in photonics

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Moore's law for photonics

Heterogeneous integration of III-V semiconductor materials on a silicon-on-insulator (SOI) platform has recently emerged as one of the most promising methods for the fabrication of active photonic devices in silicon photonics. For this integration, it is essential to have a reliable and robust bonding procedure, which also provides a uniform and ultra-thin bonding layer for an effective optical coupling between III-V active layers and SOI waveguides. A new process for bonding of III-V dies to processed silicon-on-insulator waveguide circuits using divinylsiloxane-bis-benzocyclobutene (DVS-BCB) was developed using a commercial wafer bonder. This “cold bonding” method significantly simplifies the bonding preparation for machine-based bonding both for die and wafer-scale bonding. High-quality bonding, with ultra-thin bonding layers (<50 nm) is demonstrated, which is suitable for the fabrication of heterogeneously integrated photonic devices, specifically hybrid III-V/Si lasers.

https://pure.tue.nl/ws/files/3828805/388352173937488.pdf

Ultra-thin DVS-BCB adhesive bonding of III-V wafers, dies and multiple dies to a patterned silicon-on-insulator substrate

Similarities and differences between photonic and microelectronic integration technology are discussed and a vision of the development of InP-based photonic integration in the coming decade is given.

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Generic foundry model for InP-based photonics

A new photonic integration technique is presented, which enables the use of indium-phosphide-based membranes on top of silicon chips. This can provide the electronic chips (complementary metal-oxide semiconductor (CMOS)) with an added optical layer (indium-phosphide membrane on silicon (IMOS)) for resolving the communication bottleneck. Very small passive devices have been realised, with performances comparable to other membrane devices (propagation loss 7 dB/cm, negligible bending loss for micron size radii, 3 dB splitter with 0.6 dB excess loss, resonator with Q-factor of 15.500). Also, a new passive device is introduced, a 4.12 micron long polarisation converter which in simulations promises broadband performance and tolerant fabrication. Finally, an active/passive regrowth technique is investigated for submicron active regions within an otherwise mostly passive membrane. A good morphology is obtained around the interfaces between the active and passive regions. The processing involved did not damage the materials severely, so that light emission in micro-PL measurements was found. However, an increasing blue shift with decreasing size occurred, due to quantum well intermixing. Optimising the design and the processing can take care of this. Taken together, the results presented here show that it is feasible to realise extremely small passive and active devices in a photonic circuit in an InP membrane.

Photonic integration in indium-phosphide membranes on silicon

A new photonic integration technique is presented, based on the use of an indium phosphide membrane on top of a silicon chip. This can provide electronic chips (CMOS) with an added optical layer (IMOS) for resolving the communication bottleneck. A major advantage of InP is the possibility to integrate passive and active components (SOAs, lasers) in a single membrane. In this paper we describe progress achieved in both the passive and active components. For the passive part of the circuit we succeeded to bring the propagation loss of our circuits close to the values obtained with silicon; we achieved propagation loss as low as 3.3 dB/cm through optimization of the lithography and the introduction of C60 (fullerene) in an electro resist. Further we report the smallest polarisation converter reported for membrane waveguides ( 95% polarisation conversion efficiency over the whole C-band and tolerant fabrication. We also demonstrate an InP-membrane wavelength demultiplexer with a loss of 2.8 dB, a crosstalk level of better than 18 dB and a uniformity over the 8 channels of better than 1.2 dB. For the integration of active components we are testing a twin guide integration scheme. We present our design based on optical and electrical simulations and the fabrication techniques. © (2014) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Photonic integration in indium-phosphide membranes on silicon (IMOS)

We report the experimental demonstration of two coupled laser cavities via self-imaging interference in a multimode waveguide. The coupling is optimized by considering images formed by two coherent phase-delayed signals at the input of a 3×3 splitter. As a result, the complex transfer coefficients of the coupling element can be chosen to increase the mode selectivity of the coupled system. A demonstration is given by the successful fabrication of a tunable laser with a side-mode suppression ratio (SMSR) up to 40 dB and a 6.5 nm tuning range. The laser delivers milliwatts of output power to a lensed fiber and is fully compatible with processes supporting vertically-etched sidewalls.

Coupled cavity laser based on anti-resonant imaging via multimode interference.

We investigate a new class of integrated mirrors, so called MMI reflectors. In addition to one-port full reflectors, we introduce two-port MMI reflectors, capable of both reflection and transmission. The reflection to transmission ratio in these devices can be set freely by changing their geometry. This is deinonstrated through numerical simulations as well as through a set ofworking devices realized on an indium phosphide layer stack.

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Hpmm Huub Ambrosius

Papers

Generic foundry model for InP-based photonics

Photonic integration in indium-phosphide membranes on silicon

Photonic integration in indium-phosphide membranes on silicon (IMOS)

Coupled cavity laser based on anti-resonant imaging via multimode interference.

MMI reflectors with free selection of reflection to transmission ratio