Robert S. Guzzon

High Performance InP-Based Photonic ICs—A Tutorial

The performance of relatively complex photonic integrated circuits (PICs) is now reaching such high levels that the long sought goal of realizing low-cost, -size, -weight, and -power chips to replace hybrid solutions seems to have been achieved for some applications. This tutorial traces some of the evolution of this technology that has led to an array of high-functionality InP-based PICs useful in optical sensing and communication applications. Examples of recent high-performance PICs that have arisen out of these developments are presented. Fundamental to much of this work was the development of integration strategies to compatibly combine a variety of components in a relatively simple fabrication process. For the UCSB work, this was initially based upon the creation of a single-chip widely tunable semiconductor laser that required the integration of gain, reflector, phase-tuning and absorber sections. As it provided most of the elements needed for many more complex PICs, their creation followed somewhat naturally by adding more of these same elements outside of the laser cavity using the same processing steps. Of course, additional elements were needed for some of the PICs to be discussed, but in most cases, these have been added without adding significant processing complexity. Generally, the integration philosophy has been to avoid patterned epitaxial growths, to use post-growth processing, such as quantum-well intermixing to provide multiple bandgaps, rather than multiple epitaxial regrowths, and to focus on processes that could be performed with vendor growth and implant facilities so that only basic clean room processing facilities are required.

Programmable Photonic Microwave Filters Monolithically Integrated in InP–InGaAsP

We demonstrate an integrated programmable photonic filter structure capable of producing bandpass filters with both tunable passband bandwidth and center frequency. Such filters could provide dynamic pre-filtering of very wide bandwidth analog microwave signals, essential to the next generation RF-front ends. The photonic filter is constructed from an array of uncoupled identical filter stages, each reconfigurable as a zero or a pole using an asymmetrical Mach-Zenhder Interferometer (MZI) structure with feedback. Integrated on a standard InP-InGaAsP material platform, semiconductor optical amplifiers (SOAs) and current injected phase modulators (PMs) are used to rapidly adjust the individual pole and zero locations, thereby reconfiguring the overall filter function. In this paper, we demonstrate cascaded filter structures with up to four filter stages, capable of producing a variety of higher order filters. Demonstrated filters have a free spectral range (FSR) of 23.5 or 47 GHz. A center frequency tunability over 28 GHz is demonstrated for a 2nd order bandpass filter, and a passband tunability of 1.9-5.4 GHz with stopband rejection >; 32 dB using 3rd and 4th order filters. Finally, the linearity of our active filters is investigated; a preliminary spurious-free dynamic range (SFDR) of 86.3 dB* Hz2/3 is obtained. However, we believe this number can be improved significantly by optimizing the design.

Integrated InP-InGaAsP tunable coupled ring optical bandpass filters with zero insertion loss

Second and third-order monolithically integrated coupled ring bandpass filters are demonstrated in the InP-InGaAsP material system with active semiconductor optical amplifiers (SOAs) and current injection phase modulators (PMs). Such integration achieves a high level of tunability and precise generation of optical filters in the RF domain at telecom wavelengths while simultaneously compensating for device insertion loss. Passband bandwidth tunability of 3.9 GHz to 7.1 GHz and stopband extinction up to 40 dB are shown for third-order filters. Center frequency tunability over a full free spectral range (FSR) is demonstrated, allowing for the placement of a filter anywhere in the telecom C-band. A Z-transform representation of coupled resonator filters is derived and compared with experimental results. A theoretical description of filter tunability is presented.

High verticality InP/InGaAsP etching in Cl2/H2/Ar inductively coupled plasma for photonic integrated circuits

High verticality and reduced sidewall deterioration of InP/InGaAsP in Cl2/H2/Ar inductively coupled plasma etching is demonstrated for a hydrogen dominant gas mixture. Selectivity >20:1, an etch rate of 24 nm/s, and a sidewall slope angle of >89° have been measured for etch depths >7 μm. The Ar flow is minimized to reduce surface etch damage while increased Cl2 and H2 gas flow is shown to increase etch rate and selectivity. The high chamber pressure required for plasma ignition causes isotropic etching at the start and creates an undercut beneath the masking layer. A novel ignition scheme using a hydrogen gas “flood” is suggested and results are presented.

Programmable photonic filters fabricated with deeply etched waveguides

Novel monolithic programmable optical filters are proposed and demonstrated. Deeply-etched waveguides are used throughout. Unit cells, incorporating a ring resonator in one arm of a Mach-Zehnder, have given programmable poles and zeros; cascaded unit cells have yielded flat-topped band-pass filter characteristics.

A Photonic Temporal Integrator With an Ultra-Long Integration Time Window Based on an InP-InGaAsP Integrated Ring Resonator

A photonic temporal integrator with an ultra-wide integration time window implemented based on a photonic integrated circuit (PIC) in an InP-InGaAsP material system consisting of semiconductor optical amplifiers (SOAs) and current-injection phase modulators (PMs) is proposed and experimentally demonstrated. The proposed photonic integrated integrator employs a ring structure coupled with two bypass waveguides. The tunable coupling between the ring and the waveguides is realized by a multi-mode interference (MMI) Mach-Zehnder interferometer coupler. Within the ring, two SOAs are incorporated to compensate for the insertion loss. In addition, there is a current injection PM in the ring for wavelength tuning. The use of the device provides a photonic temporal integrator with an ultra-wide integration time window and a tunable operation wavelength in a single PIC. The proposed integrator is fabricated and experimentally verified. The integration time window as wide as 6331 ps is achieved, which is an order of magnitude longer than that provided by the previously reported photonic integrators.

Indium Phosphide Photonic Integrated Circuits for Coherent Optical Links

We demonstrate photonic circuits monolithically integrated on an InP-based platform for use in coherent communication links. We describe a technology platform that allows for the integration of numerous circuit elements. We show examples of an integrated transmitter which offers an on-chip wavelength-division-multiplexing source with a flat gain profile across a 2 THz band and a new device design to provide a flatted gain over a 5 THz band. We show coherent receivers incorporating an integrated widely tunable local oscillator as well as an optical PLL. Finally, we demonstrate a tunable optical bandpass filter for use in analog coherent radio frequency links with a measured spurious-free dynamic range of 86.3 dB-Hz2/3 as well as an improved design to exceed 117 dB-Hz2/3.

An integrated parity-time symmetric wavelength-tunable single-mode microring laser

Mode control in a laser cavity is critical for a stable single-mode operation of a ring laser. In this study we propose and experimentally demonstrate an electrically pumped parity-time (PT)-symmetric microring laser with precise mode control, to achieve wavelength-tunable single-mode lasing with an improved mode suppression ratio. The proposed PT-symmetric laser is implemented based on a photonic integrated circuit consisting of two mutually coupled active microring resonators. By incorporating multiple semiconductor optical amplifiers in the microring resonators, the PT-symmetry condition can be achieved by a precise manipulation of the interplay between the gain and loss in the two microring resonators, and the incorporation of phase modulators in the microring resonators enables continuous wavelength tuning. Single-mode lasing at 1,554.148 nm with a sidemode suppression ratio exceeding 36 dB is demonstrated and the lasing wavelength is continuously tunable from 1,553.800 to 1,554.020 nm.

30-Gb/s Optical Link Combining Heterogeneously Integrated III–V/Si Photonics With 32-nm CMOS Circuits

We present a silicon photonics optical link utilizing heterogeneously integrated photonic devices driven by low-power advanced 32-nm CMOS integrated circuits. The photonic components include a quantum-confined Stark effect electroabsorption modulator and an edge-coupled waveguide photodetector, both made of III-V material wafer bonded on silicon-on-insulator wafers. The photonic devices are wire bonded to the CMOS chips and mounted on a custom PCB card for testing. We demonstrate an error-free operation at data rates up to 30 Gb/s and transmission over 10 km at 25 Gb/s with no measured sensitivity penalty and a timing margin penalty of 0.2 UI.

Theory and Design of THz Intracavity Gain-Flattened Filters for Monolithically Integrated Mode-Locked Lasers

We present the theory and design of a tunable gain-flattening filter for integrated mode-locked lasers (MLLs). The filter provides the inverse of the semiconductor spectral gain profile and produces a broad flattened net gain. This improves the performance of MLLs by allowing more modes to lase simultaneously. We demonstrate a gain-flattened MLL with a record 10 dB bandwidth of 2.08 THz, the widest frequency comb span for an integrated quantum-well-based laser at 1.55 μm. Gain-flattening theory is used to extend the integrated comb span to 40 nm. We use scattering matrices to investigate feed-forward filters based on asymmetric Mach-Zehnder interferometers (MZIs). We compare MZI filters designed for a fixed coupling value to those that use an active gain arm to adjust the extinction ratio. Tunable zero placement of these filters is achieved using a passive phase tuning arm. The optimized gain-flattening filter has a 5 dB extinction ratio and a 70 nm free-spectral-range. When the filter is incorporated into a ring MLL, simulations predict a 40 nm, i.e., 5 THz, comb span with a power variation <; 3.5 dB.