Juliana Müller

Low V π Silicon photonics modulators with highly linear epitaxially grown phase shifters

We report on the design of Silicon Mach-Zehnder carrier depletion modulators relying on epitaxially grown vertical junction diodes. Unprecedented spatial control over doping profiles resulting from combining local ion implantation with epitaxial overgrowth enables highly linear phase shifters with high modulation efficiency and comparatively low insertion losses. A high average phase shifter efficiency of VπL = 0.74 V⋅cm is reached between 0 V and 2 V reverse bias, while maintaining optical losses at 4.2 dB/mm and the intrinsic RC cutoff frequency at 48 GHz (both at 1 V reverse bias). The fabrication process, the sensitivity to fabrication tolerances, the phase shifter performance and examples of lumped element and travelling wave modulators are modeled in detail. Device linearity is shown to be sufficient to support complex modulation formats such as 16-QAM.

Advances in silicon photonics segmented electrode Mach-Zehnder modulators and peaking enhanced resonant devices

We report recent progress made in our laboratory on travelling wave Mach-Zehnder Interferometer based Silicon Photonics modulators with segmented transmission lines, as well as on resonant ring modulators and add-drop multiplexers with peaking enhanced bandwidth extended beyond the photon lifetime limit. In our segmented transmission lines, microstructuring of the electrodes results in radio-frequency modes significantly deviating from the transverse electromagnetic (TEM) condition and allows for additional design freedom to jointly achieve phase matching, impedance matching and minimizing resistive losses. This technique was found to be particularly useful to achieve the aforementioned objectives in simple back-end processes with one or two metallization layers. Peaking results from intrinsic time dynamics in ring resonator based modulators and add-drop multiplexers and allows extending the bandwidth of the devices beyond the limit predicted from the photon lifetime. Simple closed form expressions allow incorporating peaking into system level modeling.

Silicon photonics WDM transmitter with single section semiconductor mode-locked laser

Abstract We demonstrate a wavelength domain-multiplexed (WDM) optical link relying on a single section semiconductor mode-locked laser (SS-MLL) with quantum dash (Q-Dash) gain material to generate 25 optical carriers spaced by 60.8 GHz, as well as silicon photonics (SiP) resonant ring modulators (RRMs) to modulate individual optical channels. The link requires optical reamplification provided by an erbium-doped fiber amplifier (EDFA) in the system experiments reported here. Open eye diagrams with signal quality factors (Q-factors) above 7 are measured with a commercial receiver (Rx). For higher compactness and cost effectiveness, reamplification of the modulated channels with a semiconductor optical amplifier (SOA) operated in the linear regime is highly desirable. System and device characterization indicate compatibility with the latter. While we expect channel counts to be primarily limited by the saturation output power level of the SOA, we estimate a single SOA to support more than eight channels. Prior to describing the system experiments, component design and detailed characterization results are reported including design and characterization of RRMs, ring-based resonant optical add-drop multiplexers (RR-OADMs) and thermal tuners, S-parameters resulting from the interoperation of RRMs and RR-OADMs, and characterization of Q-Dash SS-MLLs reamplified with a commercial SOA. Particular emphasis is placed on peaking effects in the transfer functions of RRMs and RR-OADMs resulting from transient effects in the optical domain, as well as on the characterization of SS-MLLs in regard to relative intensity noise (RIN), stability of the modes of operation, and excess noise after reamplification.

Modification of Level Dependent ASE-Signal Beat Noise by Optical and Electrical Filtering in Optically Preamplified Direct Detection Receivers

We derive compact equations describing the modification of amplified spontaneous emission signal beat noise arising from optical and electrical filtering in optically preamplified direct detection receivers. In particular, we show that this modification typically results in a further decrease of the signal quality factor. This is particularly pronounced in the presence of electrical filters with steep transfer functions such as, e.g., occurring when feeding the signal through an antialiasing filter prior to analog-to-digital conversion or in a real-time oscilloscope, in the latter case leading to counter-intuitive dependencies of the measured signal quality on the characteristics of the test setup. Predictions are exemplified in concrete system models and verified with experiments. While the modeling assumptions and the accuracy of the predictions are in line with models previously reported in the literature, derived expressions allow straightforwardly tying the modification of the level dependent noise to signal levels, baud rate, signal spectrum, and filter transfer functions.We derive an analytical model describing the effect of filtering on amplified spontaneous emission noise during or after opto-electronic conversion. In particular, we show that electrical filtering results in a further reduction of the signal quality factor associated with an effective increase of the noise levels and can lead to counter-intuitive dependencies of the measured signal quality on the characteristics of the test setup. Closed form equations are compared with numerical models and experiments, showing excellent agreement.

High-speed resonantly enhanced silicon photonics modulator with a large operating temperature range

We present a novel resonant Mach-Zehnder modulator whose arms are each loaded with five identical resonators. Size and power consumption are aggressively reduced compared to conventional modulators based on linear phase shifters. At the same time, a large optical bandwidth of 3.8 nm is maintained. We experimentally demonstrate open eye diagrams at 30 Gbps with a signal Q-factor remaining within a factor of 2 of its peak value in an operational temperature range spanning 55°C.

Wideband multi-stage CROW filters with relaxed fabrication tolerances

We present wideband and large free spectral range optical filters with steep passband edges for the selection of adjacent WDM communication channels that can be reliably fabricated with mainstream silicon photonics technology. The devices are based on three cascaded stages of coupled resonator optical waveguides loaded on a common bus waveguide. These stages differ in the number of resonators but are implemented with exactly identical unit cells, comprised of a matched racetrack resonator layout and a uniform spacing between cells. The different number of resonators in each stage allows a high rejection in the through port response enabled by the interleaved distribution of zeros. Furthermore, the exact replication of a unique cell avoids the passband ripple and high lobes in the stopband that typically arise in apodized coupled resonator optical waveguide based filters due to fabrication and coupling induced variations in the effective path length of each resonator. Silicon photonics filters designed for the selection of 9 adjacent optical carriers generated by a 100 GHz free spectral range comb laser have been successfully fabricated with 248 nm DUV lithography, achieving an out-of-band rejection above 11 dB and an insertion loss of less than 0.5 dB for the worst channels.

High speed WDM interconnect using silicon photonics ring modulators and mode-locked laser

We demonstrate an 8 by 14 Gbps compatible WDM link based on a single-section semiconductor mode-locked laser, silicon photonics resonant ring modulators and joint channel reamplification with a semiconductor optical amplifier operated in the linear regime. Individual channels reach a data rate of 25 Gbps with signal quality-factors above 7.

Silicon photonics WDM interconnects based on resonant ring modulators and semiconductor mode locked laser

We demonstrate wavelength domain multiplexed (WDM) data transmission with a data rate of 14 Gbps based on optical carrier generation with a single-section semiconductor mode-locked laser (SS-MLL) and modulation with a Silicon Photonics (SiP) resonant ring modulator (RRM). 18 channels are sequentially measured, whereas the best recorded eye diagrams feature signal quality factors (Q-factors) above 7. While optical re-amplification was necessary to maintain the link budgets and therefore system measurements were performed with an erbium doped fiber amplifier (EDFA), preliminary characterization done with a semiconductor optical amplifier (SOA) indicates compatibility with the latter pending the integration of an additional optical filter to select a subset of carriers and prevent SOA saturation. A systematic analysis of the relative intensity noise (RIN) of isolated comb lines and of signal Q-factors indicates that the link is primarily limited by amplified spontaneous emission (ASE) from the EDFA rather than laser RIN. Measured RIN for single comb components is below -120 dBc/Hz in the range from 7 MHz to 4 GHz and drops to the shot noise level at higher frequencies.

Silicon Photonics Transmitter with SOA and Semiconductor Mode-Locked Laser

We experimentally investigate an optical link relying on silicon photonics transmitter and receiver components as well as a single section semiconductor mode-locked laser as a light source and a semiconductor optical amplifier for signal amplification. A transmitter based on a silicon photonics resonant ring modulator, an external single section mode-locked laser and an external semiconductor optical amplifier operated together with a standard receiver reliably supports 14 Gbps on-off keying signaling with a signal quality factor better than 7 for 8 consecutive comb lines, as well as 25 Gbps signaling with a signal quality factor better than 7 for one isolated comb line, both without forward error correction. Resonant ring modulators and Germanium waveguide photodetectors are further hybridly integrated with chip scale driver and receiver electronics, and their co-operability tested. These experiments will serve as the basis for assessing the feasibility of a silicon photonics wavelength division multiplexed link relying on a single section mode-locked laser as a multi-carrier light source.

Passively biased resonantly enhanced silicon photonics carrier depletion modulator with high optical bandwidth

Siliconphotonics modulators based on carrier depletion are a promising approach for optical interconnects due to their outstanding high-speed capabilities, fabrication with CMOS compatible processes and high reliability. The mainstream configurations are two-fold: ring resonator modulators (RRM) and Mach-Zehnder interferometer modulators (MZM). RRMs leverage the resonance enhancement to reach high modulation efficiencies. Moreover, RRMs are very compact and can be electrically driven as lumped elements. However, the resonant enhancement also limits their optical bandwidth so that RRMs have to be stabilized against temperature fluctuations. The power consumption associated to this control system is the limiting factor in regards to the power efficiency of the devices. On the other hand, MZMs need long phase shifters or high drive voltages to reach high extinction and have a higher RF power consumption. Furthermore, although a symmetric configuration enables a very large optical bandwidth, the compensation of fabrication asymmetries accumulating over the long MZM arms requires a corrective phase tuner that assures biasing at the 3 dB point of the interferometer. In this work we discuss a novel MZM modulator configuration that addresses these issues in order to aggressively reduce power consumption. First, the corrective phase tuner is replaced by an integration scheme in which the 3 dB point is passively set during attachment of the input fiber. The adjustment of the phase relation between MZM arms is enabled by positioning of the input fiber on top of a misalignment tolerant multimode grating coupler also used as the first splitter element of the interferometer. Second, the typically straight phase shifters are replaced by arrays of overcoupled resonators. The challenge there lies in highly overcoupling the resonators to achieve a high optical bandwidth, while at the same time obtaining a sufficiently high finesse to maintain resonant enhancement and minimizing the excess losses at the resonator to waveguide junctions so as not to overly burden insertion losses by the cascaded resonators. By optimizing the resonator with a novel design, we achieve a very compact cavity size and thus an acceptable finesse not withstanding the low quality factor of the highly overcoupled resonant phase shifter elements. These characteristics provide a large resonance bandwidth, which enables a wide thermal operation range of more than 25°C in the absence of dynamic compensation. Our design has been optimized for 28 Gbps and a drive voltage of 2 Vpp. The modulation enhancement of the phase shifter elements allows a lumped electrode driving scheme and a reduction of power consumption by a factor larger than seven relative to a conventional MZM relying on the same pn junction configuration.