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Multicore optical fibers and ribbons based on fiber arrays allow for massively parallel transmission of signals via spatially separated channels, thereby offering attractive bandwidth scaling with linearly increasing technical effort. However, low-loss coupling of light between fiber arrays or multicore fibers and standard linear arrays of vertical-cavity surface-emitting lasers (VCSEL) or photodiodes (PD) still represents a challenge. In this paper, we demonstrate that 3D-printed facet-attached microlenses (FaML) offer an attractive path for connecting multimode fiber arrays as well as individual cores of multimode multicore fibers to standard arrays of VCSEL or PD. The freeform coupling elements are printed in situ with high precision on the device and fiber facets by high-resolution multi-photon lithography. We demonstrate coupling losses down to 0.35 dB along with lateral 1 dB alignment tolerances in excess of 10 $\mu$m, allowing to leverage fast passive assembly techniques that rely on industry-standard machine vision. To the best of our knowledge, our experiments represent the first demonstration of a coupling interface that connects individual cores of a multicore fiber to VCSEL or PD arranged in a standard linear array without the need for additional fiber-based or waveguide-based fan-out structures. Using this approach, we build a 3 x 25 Gbit/s transceiver assembly which fits into a small form-factor pluggable module and which fulfills many performance metrics specified in the IEEE 802.3 standard.

Electro-optic (EO) modulators rely on interaction of optical and electrical signals with second-order nonlinear media. For the optical signal, this interaction can be strongly enhanced by using dielectric slot-waveguide structures that exploit a field discontinuity at the interface between a high-index waveguide core and the low-index EO cladding. In contrast to this, the electrical signal is usually applied through conductive regions in the direct vicinity of the optical waveguide. To avoid excessive optical loss, the conductivity of these regions is maintained at a moderate level, thus leading to inherent RC-limitations of the modulation bandwidth. In this paper, we show that these limitations can be overcome by extending the slot-waveguide concept to the modulating radio-frequency (RF) signal. Our device combines an RF slotline that relies on BaTiO3 as a high-k dielectric material with a conventional silicon photonic slot waveguide and a highly efficient organic EO cladding material. In a proof-of-concept experiment, we demonstrate a 1 mm-long Mach-Zehnder modulator that offers a 3 dB-bandwidth of 76 GHz and a 6 dB-bandwidth of 110 GHz along with a small {\pi}-voltage of 1.3 V (U{\pi}L = 1.3 V mm). To the best of our knowledge, this represents the largest EO bandwidth so far achieved with a silicon photonic modulator based on dielectric waveguides. We further demonstrate the viability of the device in a data transmission experiment using four-state pulse-amplitude modulation (PAM4) at line rates up to 200 Gbit/s. Our first-generation devices leave vast room for further improvement and may open an attractive route towards highly efficient silicon photonic modulators that combine sub-1 mm device lengths with sub-1 V drive voltages and modulation bandwidths of more than 100 GHz., Comment: 8 pages main paper, 7 pages supplementary info, 11 figures

Complex photonic integrated circuits (PIC) may have strongly non-planar topologies that require waveguide crossings (WGX) when realized in single-layer integration platforms. The number of WGX increases rapidly with the complexity of the circuit, in particular when it comes to highly interconnected optical switch topologies. Here, we present a concept for WGX-free PIC that rely on 3D-printed freeform waveguide overpasses (WOP). We experimentally demonstrate the viability of our approach using the example of a $4 \times 4$ switch-and-select (SAS) circuit realized on the silicon photonic platform. We further present a comprehensive graph-theoretical analysis of different $n \times n$ SAS circuit topologies. We find that for increasing port counts $n$ of the SAS circuit, the number of WGX increases with $n^4$, whereas the number of WOP increases only in proportion to $n^2$., Comment: 22 pages, 8 figures

Silicon-organic hybrid (SOH) electro-optic (EO) modulators combine small footprint with low operating voltage and hence low power dissipation, thus lending themselves to on-chip integration of large-scale device arrays. Here we demonstrate an electrical packaging concept that enables high-density radio-frequency (RF) interfaces between on-chip SOH devices and external circuits. The concept combines high-resolution $\mathrm{Al_2O_3}$ printed-circuit boards with technically simple metal wire bonds and is amenable to packaging of device arrays with small on-chip bond pad pitches. In a set of experiments, we characterize the performance of the underlying RF building blocks and we demonstrate the viability of the overall concept by generation of high-speed optical communication signals. Achieving line rates (symbols rates) of 128 Gbit/s (64 GBd) using quadrature-phase-shiftkeying (QPSK) modulation and of 160 Gbit/s (40 GBd) using 16-state quadrature-amplitudemodulation (16QAM), we believe that our demonstration represents an important step in bringing SOH modulators from proof-of-concept experiments to deployment in commercial environments.

We report on the first demonstration of long-term thermally stable silicon-organic hybrid (SOH) modulators in accordance with Telcordia standards of high-temperature storage. The devices rely on an organic electro-optic sidechain polymer with a high glass transition temperature of 172 {\deg}C. In our high-temperature storage experiments at 85 {\deg}C, we find that the electro-optic activity converges to a constant long-term stable level after an initial decay. If we consider a burn-in time of 300 h, the {\pi}-voltage of the modulators increases on average by less than 15 % if we store the devices for additional 2400 h. The performance of the devices is demonstrated by generating high-quality 40 Gbit/s OOK signals both after the burn-in period and after extended high-temperature storage.

We demonstrate the generation of higher-order modulation formats using silicon-based inphase/quadrature (IQ) modulators at symbol rates of up to 100 GBd. Our devices exploit the advantages of silicon-organic hybrid (SOH) integration, which combines silicon-on-insulator waveguides with highly efficient organic electro-optic (EO) cladding materials to enable small drive voltages and sub-millimeter device lengths. In our experiments, we use an SOH IQ modulator with a {\pi}-voltage of 1.6 V to generate 100 GBd 16QAM signals. This is the first time that the 100 GBd mark is reached with an IQ modulator realized on a semiconductor substrate, leading to a single-polarization line rate of 400 Gbit/s. The peak-to-peak drive voltages amount to 1.5 Vpp, corresponding to an electrical energy dissipation in the modulator of only 25 fJ/bit.

Efficient electro-optic (EO) modulators crucially rely on advanced materials that exhibit strong electro-optic activity and that can be integrated into high-speed and efficient phase shifter structures. In this paper, we demonstrate ultra-high in-device EO figures of merit of up to n3r33 = 2300 pm/V achieved in a silicon-organic hybrid (SOH) Mach-Zehnder Modulator (MZM) using the EO chromophore JRD1. This is the highest material-related in-device EO figure of merit hitherto achieved in a high-speed modulator at any operating wavelength. The {\pi}-voltage of the 1.5 mm-long device amounts to 210 mV, leading to a voltage-length product of U{\pi}L = 320 V{\mu}m - the lowest value reported for MZM that are based on low-loss dielectric waveguides. The viability of the devices is demonstrated by generating high-quality on-off-keying (OOK) signals at 40 Gbit/s with Q factors in excess of 8 at a drive voltage as low as 140 mVpp. We expect that efficient high-speed EO modulators will not only have major impact in the field of optical communications, but will also open new avenues towards ultra-fast photonic-electronic signal processing., Comment: 9 pages, 4 figures

Electro-optic modulators for high-speed on-off keying (OOK) are key components of short- and mediumreach interconnects in data-center networks. Besides small footprint and cost-efficient large-scale production, small drive voltages and ultra-low power consumption are of paramount importance for such devices. Here we demonstrate that the concept of silicon-organic hybrid (SOH) integration is perfectly suited for meeting these challenges. The approach combines the unique processing advantages of large-scale silicon photonics with unrivalled electro-optic (EO) coefficients obtained by molecular engineering of organic materials. In our proof-of-concept experiments, we demonstrate generation and transmission of OOK signals with line rates of up to 100 Gbit/s using a 1.1 mm-long SOH Mach-Zehnder modulator (MZM) which features a {\pi}-voltage of only 0.9 V. This experiment represents not only the first demonstration of 100 Gbit/s OOK on the silicon photonic platform, but also leads to the lowest drive voltage and energy consumption ever demonstrated at this data rate for a semiconductor-based device. We support our experimental results by a theoretical analysis and show that the nonlinear transfer characteristic of the MZM can be exploited to overcome bandwidth limitations of the modulator and of the electric driver circuitry. The devices are fabricated in a commercial silicon photonics line and can hence be combined with the full portfolio of standard silicon photonic devices. We expect that high-speed power-efficient SOH modulators may have transformative impact on short-reach optical networks, enabling compact transceivers with unprecedented energy efficiency that will be at the heart of future Ethernet interfaces at Tbit/s data rates.

Structuring optical materials on a nanometer scale can lead to artificial effective media, or metamaterials, with strongly altered optical behavior. Metamaterials can provide a wide range of linear optical properties such as negative refractive index, hyperbolic dispersion, or magnetic behavior at optical frequencies. Nonlinear optical properties, however, have only been demonstrated for patterned metallic films which suffer from high optical losses. Here we show that second-order nonlinear metamaterials can also be obtained from non-metallic centrosymmetric constituents with inherently low optical absorption. In our proof-of-principle experiments, we have iterated atomic-layer deposition (ALD) of three different constituents, A = Al$_2$O$_3$, B = TiO$_2$ and C = HfO$_2$. The centrosymmetry of the resulting ABC stack is broken since the ABC and the inverted CBA sequences are not equivalent - a necessary condition for non-zero second-order nonlinearity. To the best of our knowledge, this is the first realization of a bulk nonlinear optical metamaterial., Comment: 3 figures, 11 pages; v2: corrected email

Optical frequency combs enable coherent data transmission on hundreds of wavelength channels and have the potential to revolutionize terabit communications. Generation of Kerr combs in nonlinear integrated microcavities represents a particularly promising option enabling line spacings of tens of GHz, compliant with wavelength-division multiplexing (WDM) grids. However, Kerr combs may exhibit strong phase noise and multiplet spectral lines, and this has made high-speed data transmission impossible up to now. Recent work has shown that systematic adjustment of pump conditions enables low phase-noise Kerr combs with singlet spectral lines. Here we demonstrate that Kerr combs are suited for coherent data transmission with advanced modulation formats that pose stringent requirements on the spectral purity of the optical source. In a first experiment, we encode a data stream of 392 Gbit/s on subsequent lines of a Kerr comb using quadrature phase shift keying (QPSK) and 16-state quadrature amplitude modulation (16QAM). A second experiment shows feedback-stabilization of a Kerr comb and transmission of a 1.44 Tbit/s data stream over a distance of up to 300 km. The results demonstrate that Kerr combs can meet the highly demanding requirements of multi-terabit/s coherent communications and thus offer a solution towards chip-scale terabit/s transceivers.