Venkat Veerasubramanian

Single rolled-up InGaAs/GaAs quantum dot microtubes integrated with silicon-on-insulator waveguides.

We report on single rolled-up microtubes integrated with silicon-on-insulator waveguides. Microtubes with diameters of ~7 μm, wall thicknesses of ~250 nm, and lengths greater than 100 μm are fabricated by selectively releasing a coherently strained InGaAs/GaAs quantum dot layer from the handling GaAs substrate. The microtubes are then transferred from their host substrate to silicon-on-insulator waveguides by an optical fiber abrupt taper. The Q-factor of the waveguide coupled microtube is measured to be 1.5×10(5), the highest recorded for a semiconductor microtube cavity to date. The insertion loss and extinction ratio of the microtube are 1 dB and 34 dB respectively. By pumping the microtube with a 635 nm laser, the resonance wavelength is shifted by 0.7 nm. The integration of InGaAs/GaAs microtubes with silicon-on-insulator waveguides provides a simple, low loss, high extinction passive filter solution in the C+L band communication regime.

Focusing-curved subwavelength grating couplers for ultra-broadband silicon photonics optical interfaces

We report on the design and characterization of focusing-curved subwavelength grating couplers for ultra-broadband silicon photonics optical interfaces. With implementation of waveguide dispersion engineered subwavelength structures, an ultra-wide 1-dB bandwidth of over 100 nm (largest reported to date) near 1550 nm is experimentally achieved for transverse-electric polarized light. By tapering the subwavelength structures, back reflection is effectively suppressed and grating coupling efficiency is increased to -4.7 dB. A compact device footprint of 40 µm × 20 µm is realized by curving the gratings in a focusing scheme.

Design, analysis, and transmission system performance of a 41 GHz silicon photonic modulator

The design and characterization of a slow-wave series push-pull traveling wave silicon photonic modulator is presented. At 2 V and 4 V reverse bias, the measured -3 dB electro-optic bandwidth of the modulator with an active length of 4 mm are 38 GHz and 41 GHz, respectively. Open eye diagrams are observed up to bitrates of 60 Gbps without any form of signal processing, and up to 70 Gbps with passive signal processing to compensate for the test equipment. With the use of multi-level amplitude modulation formats and digital-signal-processing, the modulator is shown to operate below a hard-decision forward error-correction threshold of 3.8×10-3 at bitrates up to 112 Gbps over 2 km of single mode optical fiber using PAM-4, and over 5 km of optical fiber with PAM-8. Energy consumed solely by the modulator is also estimated for different modulation cases.

A Low-Voltage 35-GHz Silicon Photonic Modulator-Enabled 112-Gb/s Transmission System

We present a silicon photonic traveling-wave Mach-Zehnder modulator operating near 1550 nm with a 3-dB bandwidth of 35 GHz. A detailed analysis of travelingwave electrode impedance, microwave loss, and phase velocity is presented. Small-and large-signal characterization of the device validates the design methodology. We further investigate the performance of the device in a short-reach transmission system. We report a successful 112-Gb/s transmission of four-level pulse amplitude modulation over 5 km of SMF using 2.2 Vp_p drive voltage. Digital signal processing is applied at the transmitter and receiver. 56-GBaud PAM-4 and 64-Gb/s PAM-2 transmission is demonstrated below a pre-FEC hard decision threshold of 4.4 x 10-3.

Silicon Photonic Segmented Modulator-Based Electro-Optic DAC for 100 Gb/s PAM-4 Generation

We report on the design and characterization of a silicon-on-insulator traveling-wave multi-electrode Mach-Zehnder modulator (MZM). The 2-bit electro-optic (EO) digital-to-analog converter is formed by dividing a series push-pull MZM into two segments, one for each bit. The EO bandwidth of the longer segment of the MZM is measured to be 48 GHz at 0 V reverse bias. We operate the device at speeds up to 50 GBd to create a four-level pulse amplitude modulation signal, and thus generating 100 Gb/s on a single wavelength without signal processing at the transmitter or the receiver. The pre-forward error correction (FEC) bit error rate is estimated to be lower than the hard-decision FEC threshold of 3.8 × 10-3 over 1 km of standard single-mode fiber, and thus leading to error-free transmission at 100 Gb/s.

Tunable nanophotonic delay lines using linearly chirped contradirectional couplers with uniform Bragg gratings

We demonstrate an integrated tunable optical delay line in grating-assisted contradirectional couplers using a CMOS-compatible photonic technology. The input signal is delayed through dispersive Bragg gratings and distributedly coupled to the drop port of the coupler without backreflections. This add-drop design enables monolithic integration of grating-based delay lines without using optical circulators. The gratings are formed by slab perturbations in rib waveguides, with the index chirping realized by linearly tapering the rib widths. Both the pitch and size of the gratings are constant through the entire coupler, for a higher tolerance to fabrication errors. Continuous tuning of the optical group delay of up to 96 ps has been obtained, with a low insertion loss of less than 2 dB and a negative chromatic dispersion of -11 ps/nm that allows for bit rates of up to almost 100 Gb/s at the maximal delay. The device has a small footprint of 0.015 mm2, and can be used for on-chip optical buffering, dispersion compensation, and pulse compression.

Selective polarization mode excitation in InGaAs/GaAs microtubes

We report on selective polarization mode excitation in InGaAs/GaAs rolled-up microtubes. The microtubes are fabricated by selectively releasing a coherently strained InGaAs/GaAs quantum dot layer from its host GaAs substrate. An optical fiber abrupt taper is used to pick up the microtube, while an adiabatically tapered optical fiber is used to couple light into the resonant optical modes of the microtube. By varying the polarization of the light in the adiabatically tapered fiber both transverse electric and transverse magnetic modes are observed in the microtube. We also show that the microtube can be used as a red (0.6 μm) to infrared light (1.5 μm) optical-optical modulator taking advantage of the thermal-optical effect.

Digital Signal Processing for Dual-Polarization Intensity and Interpolarization Phase Modulation Formats Using Stokes Detection

We study and compare two digital signal processing (DSP) approaches to recover the intensity on two orthogonal polarizations and the interpolarization phase modulation using a Stokes-vector direct detection receiver. We focus on higher order modulation of each of the three degrees of freedom allowed in Stokes-vector detection. 2 bits are encoded on each intensity of the two orthogonal polarizations, and 2 bits are encoded in the phase difference between the two polarizations, giving a 6 bits per symbol format. In this study, we propose a novel three-stage DSP algorithm and we compare this new algorithm with our earlier two-stage algorithm using the following metrics: the computational complexity and the bit error rate (BER) performance. Using the three stage approach, waveform filtering and derotation are applied in series rather than in parallel as was done in the two stage algorithm. We show that the three-stage approach exhibits equal BER performance, while significantly reducing the total required number of real additions and real multiplications. Moreover, tracking the state of polarization using the proposed method is m -times more efficient, where m is the number of taps in the first filtering stage.

Thermally controlled coupling of a rolled-up microtube integrated with a waveguide on a silicon electronic-photonic integrated circuit

We report on the first experimental demonstration of the thermal control of coupling strength between a rolled-up microtube and a waveguide on a silicon electronic-photonic integrated circuit. The microtubes are fabricated by selectively releasing a coherently strained GaAs/InGaAs heterostructure bilayer. The fabricated microtubes are then integrated with silicon waveguides using an abruptly tapered fiber probe. By tuning the gap between the microtube and the waveguide using localized heaters, the microtube-waveguide evanescent coupling is effectively controlled. With heating, the extinction ratio of a microtube whispering-gallery mode changes over an 18 dB range, while the resonant wavelength remains approximately unchanged. Utilizing this dynamic thermal tuning effect, we realize coupling modulation of the microtube integrated with the silicon waveguide at 2 kHz with a heater voltage swing of 0-6 V.

High-speed compact silicon photonic Michelson interferometric modulator

We present the detailed analysis and characterization of a silicon Michelson modulator with short 500 μm phase shifters and a low VπLπ of 0.72 V-cm under reverse bias. We investigate optical modulation of reverse biased p-n and forward biased p-i-n junctions. We demonstrate for the first time that error-free operation up to 40 Gbps is possible with lumped silicon interferometric modulators. For reverse bias operation, we show that even greater bandwidth can be obtained with lower impedance drivers. Forward bias operation with pre-emphasized signals is shown to have clean eye diagrams up to 40 Gbps, however, error counting reveals a strong dependence on test patterns and that error-free operation is achievable for short pattern lengths.