Michael R. Watts

Large-scale nanophotonic phased array

Electromagnetic phased arrays at radio frequencies are well known and have enabled applications ranging from communications to radar, broadcasting and astronomy. The ability to generate arbitrary radiation patterns with large-scale phased arrays has long been pursued. Although it is extremely expensive and cumbersome to deploy large-scale radiofrequency phased arrays, optical phased arrays have a unique advantage in that the much shorter optical wavelength holds promise for large-scale integration. However, the short optical wavelength also imposes stringent requirements on fabrication. As a consequence, although optical phased arrays have been studied with various platforms and recently with chip-scale nanophotonics, all of the demonstrations so far are restricted to one-dimensional or small-scale two-dimensional arrays. Here we report the demonstration of a large-scale two-dimensional nanophotonic phased array (NPA), in which 64 × 64 (4,096) optical nanoantennas are densely integrated on a silicon chip within a footprint of 576 μm × 576 μm with all of the nanoantennas precisely balanced in power and aligned in phase to generate a designed, sophisticated radiation pattern in the far field. We also show that active phase tunability can be realized in the proposed NPA by demonstrating dynamic beam steering and shaping with an 8 × 8 array. This work demonstrates that a robust design, together with state-of-the-art complementary metal-oxide–semiconductor technology, allows large-scale NPAs to be implemented on compact and inexpensive nanophotonic chips. In turn, this enables arbitrary radiation pattern generation using NPAs and therefore extends the functionalities of phased arrays beyond conventional beam focusing and steering, opening up possibilities for large-scale deployment in applications such as communication, laser detection and ranging, three-dimensional holography and biomedical sciences, to name just a few.

Ultra compact 45 GHz CMOS compatible Germanium waveguide photodiode with low dark current

We present a compact 1.3 × 4 μm2 Germanium waveguide photodiode, integrated in a CMOS compatible silicon photonics process flow. This photodiode has a best-in-class 3 dB cutoff frequency of 45 GHz, responsivity of 0.8 A/W and dark current of 3 nA. The low intrinsic capacitance of this device may enable the elimination of transimpedance amplifiers in future optical data communication receivers, creating ultra low power consumption optical communications.

Fabrication and analysis of add-drop filters based on microring resonators in SiN

Add-drop filters based on microring resonators were fabricated in silicon-rich silicon nitride. Third-order microring filters showed an 80 GHz bandwidth, a 4 dB loss from input to drop, and a 24 nm free spectral-range.

Integrated mode-evolution-based polarization rotators

For the first time to our knowledge, designs for mode-evolution-based integrated polarization rotators requiring only a pair of waveguide core layers are presented. Finite-difference time-domain and eigenmode expansion simulations demonstrate the near-ideal performance of the approach. In contrast with approaches based on mode coupling, no significant wavelength sensitivity is observed.

Multistage high-order microring-resonator add-drop filters

We propose and demonstrate a multistage design for microphotonic add-drop filters that provides reduced drop-port loss and relaxed tolerances for achieving high in-band extinction. As a result, the first microring-resonator filters with a rectangular notch stopband in the through port (to our knowledge) are shown, with extinctions exceeding 50 dB. Reaching 30 dB beyond previous results, without postfabrication trimming, such extinction levels open the door to microphotonic notch circuits for spectroscopy, wavelength conversion, and quantum cryptography applications. Combined with a low-loss, high-index-contrast electromagnetic design in SiN and frequency-matched microring resonators, this approach led to the first demonstration of flattop microphotonic filters meeting the stringent criteria for high-spectral-efficiency integrated add-drop multiplexers. The 40 GHz wide filters show a 20 nm free spectral range, 2 dB drop loss, and suppression of adjacent channels by over 30 dB.

Vertical junction silicon microdisk modulators and switches

Vertical junction resonant microdisk modulators and switches have been demonstrated with exceptionally low power consumption, low-voltage operation, high-speed, and compact size. This paper reviews the progress of vertical junction microdisk modulators, provides detailed design data, and compares vertical junction performance to lateral junction performance. The use of a vertical junction maximizes the overlap of the depletion region with the optical mode thereby minimizing both the drive voltage and power consumption of a depletion-mode modulator. Further, the vertical junction enables contact to be made from the interior of the resonator and therein a hard outer wall to be formed that minimizes radiation in small diameter resonators, further reducing the capacitance and drive power of the modulator. Initial simple vertical junction modulators using depletion-mode operation demonstrated the first sub-100 fJ/bit silicon modulators. With more intricate doping schemes and through the use of AC-coupled drive signals, 3.5 μm diameter vertical junction microdisk modulators have recently achieved a communications efficiency of 3 fJ/bit, making these modulators the smallest and lowest power modulators demonstrated to date, in any material system. Additionally, the demonstration was performed at 12.5 Gb/s, required a peak-to-peak signal level of only 1 V, and achieved bit-error-rates below 10(-12) without requiring signal pre-emphasis. As an additional benefit to the use of interior contacts, higher-order active filters can be constructed from multiple vertical-junction modulators without interference of the electrodes. Doing so, we demonstrated second-order active high-speed bandpass switches with ~2.5 ns switching speeds, and power penalties of only 0.4 dB. Through the use of vertical junctions in resonant modulators, we have achieved the lowest power consumption, lowest voltage, and smallest silicon modulators demonstrated to date.

Silicon photonics manufacturing.

Most demonstrations in silicon photonics are done with single devices that are targeted for use in future systems. One of the costs of operating multiple devices concurrently on a chip in a system application is the power needed to properly space resonant device frequencies on a systems frequency grid. We asses this power requirement by quantifying the source and impact of process induced resonant frequency variation for microdisk resonators across individual die, entire wafers and wafer lots for separate process runs. Additionally we introduce a new technique, utilizing the Transverse Electric (TE) and Transverse Magnetic (TM) modes in microdisks, to extract thickness and width variations across wafers and dice. Through our analysis we find that a standard six inch Silicon on Insulator (SOI) 0.35 μm process controls microdisk resonant frequencies for the TE fundamental resonances to within 1 THz across a wafer and 105 GHz within a single die. Based on demonstrated thermal tuner technology, a stable manufacturing process exhibiting this level of variation can limit the resonance trimming power per resonant device to 231 μW. Taken in conjunction with the power to compensate for thermal environmental variations, the expected power requirement to compensate for fabrication-induced non-uniformities is 17% of that total. This leads to the prediction that thermal tuning efficiency is likely to have the most dominant impact on the overall power budget of silicon photonics resonator technology.

Fabrication of add-drop filters based on frequency-matched microring resonators

Frequency mismatches between resonators significantly impact the spectral responses of coupled resonator filters, such as high-order microring filters. In this paper, techniques allowing fabrication of frequency-matched high-index-contrast resonators are proposed, demonstrated, and analyzed. The main approach consists of inducing small dimensional changes in the resonators through alteration of the electron-beam dose used to expose either the actual resonator on a wafer or its image on a lithographic mask to be later used in filter fabrication. Third-order microring filters fabricated in silicon-rich silicon nitride, with optical resonator frequencies matched to better than 1 GHz, are reported. To achieve this, the average ring-waveguide widths of the microrings are matched to within less than 26 pm of a desired relative width offset. Furthermore, optimization and calibration procedures allowing strict dimensional control and smooth sidewalls are presented. A 5-nm dimensional control is demonstrated, and the standard deviation of sidewall roughness is reduced to below 1.6 nm.

Coupling-induced resonance frequency shifts in coupled dielectric multi-cavity filters.

Coupling-induced resonance frequency shifts (CIFS) are theoretically described, and are found to be an important fundamental source of resonance frequency mismatch between coupled optical cavities that would be degenerate in isolation. Their deleterious effect on high-order resonant filter responses and complete correction by pre-distortion are described. Analysis of the physical effects contributing to CIFS shows that a positive index perturbation may bring about a resonance shift of either sign. Higherorder CIFS effects, the scaling of CIFS-caused impairment with finesse, FSR and index contrast, and the tolerability of frequency mismatch in telecom-grade filters are addressed. The results also suggest possible designs and applications for CIFS-free coupled-resonator systems.

Integrated mode-evolution-based polarization splitter

A mode-evolution-based polarization splitter suitable for high-index-contrast systems and directly integratable with a recently reported on-polarization rotator is described and its performance verified through both finite-difference time-domain and eigenmode expansion simulations. For a device length of 200 microm, greater than 22 dB of extinction is obtained across a 1.45-1.75-microm bandwidth.