Kwangwoong Kim

Four-Channel 100-Gb/s Per Channel Discrete Multitone Modulation Using Silicon Photonic Integrated Circuits

The emerging applications for inter and intradata center communications demand low-cost and small-from-factor optical transceivers with 100G/400G capacity. Various techniques, such as wavelength-division multiplexing (WDM), multilevel pulse-amplitude modulation, and discrete multitone (DMT), are heavily investigated in this field. Among these techniques, DMT can offer 100G capacity with 10-20G optical devices using direct detection, which provides a very promising low-cost 100G/400G solution. In this paper, we report silicon photonic four-channel DMT integrated circuits, which demonstrate net channel rates of 70 and 100 Gb/s detected by integrated germanium receivers and commercial receivers, respectively. The silicon photonic chip integrates four silicon Mach-Zehnder modulators and WDM multiplexers using thermally tuned second-order microring filters. Besides the wavelength multiplexing functionality, we also demonstrate that these microrings can serve as vestigial sideband filters to enhance the transmission performance in 20-40 km ranges. These demonstrations indicate the promise of using low-cost and high-integrated silicon photonic circuits in high capacity 100G/400G applications.

128-Gb/s 100-km transmission with direct detection using silicon photonic Stokes vector receiver and I/Q modulator.

Recently, there is increasing interest in utilizing Stokes vector receiver, which is a direct-detection technique with the capability to digitally track the polarization changes in fibers and decode information in multiple dimensions. Here, we report a monolithically integrated silicon photonic Stokes vector receiver, which consists of one polarization beam splitter, two polarization rotators, one 90-degree optical hybrid, and six germanium photodetectors. Paired with a silicon in-phase/quadrature modulator incorporating a power-tunable carrier in the orthogonal polarization, transmission at 128-Gb/s over 100-km fiber is achieved with direct detection.

Simultaneous wavelength locking of microring modulator array with a single monitoring signal

A microring modulator array coupled to a common bus waveguide can be used to construct low power, compact and flexible wavelength-division-multiplexing (WDM) transmitters. However, due to extremely small working bandwidths of the rings, it is challenging to find the right resonant wavelength setting and locking the resonance to an external laser. In the paper, we propose a novel technique enabling simultaneous wavelength locking of a microring modulator array with a single monitor, together with automatically optimizing the wavelength setting. We experimentally demonstrate locking three rings over a temperature range >40 °C at 3x20 Gb/s on-off-keying (OOK) modulation and ~3x75 Gb/s discrete multi-tone (DMT) modulation.

Dual hybrid silicon-photonic laser with fast wavelength tuning

We designed and fabricated a fast-wavelength-tunable hybrid laser diode using two selectable silicon photonic Vernier ring-based cavities. Single mode operation (SMSR > 30 dB) over more than 35 nm and switching speed of 35 ns are demonstrated.

Four-channel vestigial sideband discrete multi-tone modulation using silicon photonic integrated circuits

We report four-channel discrete multi-tone modulation at 100G per channel using silicon photonic integrated circuits. Second-order rings are used for both wavelength multiplexing and vestigial sideband filtering to enhance the transmission performance in 20-40 kilometers.

Multi-plane light conversion of high spatial mode count

Multi-plane light conversion is a method of performing spatial basis transformations using cascaded phase plates separated by Fourier transforms or free-space propagation. In general, the number of phase plates required scales with the dimensionality (total number of modes) in the transformation. This is a practical limitation of the technique as it relates to scaling to large mode counts. Firstly, requiring many planes increases the complexity of the optical system itself making it difficult to implement, but also because even a very small loss per plane will grow exponentially as more and more planes are added, causing a theoretically lossless optical system, to be far from lossless in practice. Spatial basis transformations of particular interest are those which take a set of spatial modes which exist in the same or similar space, and transform them into an array of spatially separated spots. Analogous to the operation performed by a diffraction grating in the wavelength domain, or a polarizing beamsplitting in the polarization domain. Decomposing the Laguerre-Gaussian, Hermite-Gaussian or related bases to an array of spots are examples of this and are relevant to many areas of light propagation in free-space and optical fibre. In this paper we present our work on designing multi-plane light conversion devices capable or operating on large numbers of spatial modes in a scalable fashion.

Silicon photonics enabled hyper-wideband wireless communication link

We demonstrate the first silicon photonics enabled hyper-wideband wireless link with an instantaneous bandwidth of 12 GHz, which is 85% of the center frequency of 14 GHz. The silicon photonics based RF receiver consists of a four-channel optical phase encoder, an integrated hybrid-silicon mode-locked laser, and two silicon ring notch filters. The received CDMA RF wireless signal is correlated to baseband using coherent optical heterodyne at a data rate of 3 Gbps error-free with electronics bandwidth of only 3 GHz. Hyper-wideband RF transmission allows for data obfuscation and increased jamming resistance from narrowband interferers. The narrowband silicon photonic ring filters allow for further interference rejection of greater than 27 dB tunable over the full 20 GHz of RF spectrum.

Experimental study of electro-optic crosstalk in parallel silicon photonic Mach-Zehnder modulators

The electro-optic crosstalk between two parallel silicon Mach-Zehnder modulators is characterized. Up to 1.6 dB power penalty is observed for 36-Gb/s on-off-keying signals with a ∼-20 dB crosstalk, posing challenge to dense photonic integration.

Simultaneous four-channel thermal adaptation of polarization insensitive silicon photonics WDM receiver

We propose a novel approach to demonstrate simultaneous multi-wavelength locking during temperature changes in a silicon photonic polarization insensitive microring-based wavelength division multiplexing (WDM) receiver. The DC component of a single monitoring photodetector at the through port of the microring filter array is exploited as a feedback signal with no additional power consumption. This feedback signal is used in control circuitry to properly tune the microring filters using ohmic heating, thus creating a feedback loop for thermal adaptation. We describe the necessary information, specifically each microring filters room temperature resonant wavelength and tunability, which can be used to calibrate and achieve proper wavelength configurability and locking. In addition, we describe a simple control algorithm based on an adaptive gradient method often used in machine learning, allowing the receiver to endlessly demultiplex at different temperatures. We successfully achieve thermal adaptation over a temperature range >37°C and demultiplex a 4 × 25 Gb/s on-off-keying signal of 150 GHz channel spacing, all while the polarization is scrambling.

Experimental demonstration of quadrature phase-shift keying silicon ring modulator based on intensity modulation

We experimentally demonstrate 24-Gb/s carrier-suppressed quadrature phase-shift keying modulation based on intensity modulation of microring modulators using a silicon photonic integrated circuit. The penalty of optical signal-to-noise ratio is ~3 dB at a bit error ratio of 1×10-3.