Erik J. Norberg

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.

Widely Tunable Narrow-Linewidth Monolithically Integrated External-Cavity Semiconductor Lasers

We theoretically analyze, design, and measure the performance of a semiconductor laser with a monolithically integrated external cavity. A ~4 cm long on-chip cavity is made possible by a low-loss silicon waveguide platform. We show tuning in excess of 54 nm in the O-band as well as significant reduction in laser linewidth due to controlled feedback from the external cavity. The measured linewidth in full tuning range is below 100 kHz and the best results are around 50 kHz. Approaches to further improve the performance of such laser architectures are described.

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.

An optical-frequency synthesizer using integrated photonics.

Integrated-photonics microchips now enable a range of advanced functionalities for high-coherence applications such as data transmission, highly optimized physical sensors, and harnessing quantum states, but with cost, efficiency, and portability much beyond tabletop experiments. Through high-volume semiconductor processing built around advanced materials there exists an opportunity for integrated devices to impact applications cutting across disciplines of basic science and technology. Here we show how to synthesize the absolute frequency of a lightwave signal, using integrated photonics to implement lasers, system interconnects, and nonlinear frequency comb generation. The laser frequency output of our synthesizer is programmed by a microwave clock across 4 THz near 1550 nm with 1 Hz resolution and traceability to the SI second. This is accomplished with a heterogeneously integrated III/V-Si tunable laser, which is guided by dual dissipative-Kerr-soliton frequency combs fabricated on silicon chips. Through out-of-loop measurements of the phase-coherent, microwave-to-optical link, we verify that the fractional-frequency instability of the integrated photonics synthesizer matches the 7.0x10^(−13) reference-clock instability for a 1 second acquisition, and constrain any synthesis error to 7.7x10^(−15) while stepping the synthesizer across the telecommunication C band. Any application of an optical frequency source would be enabled by the precision optical synthesis presented here. Building on the ubiquitous capability in the microwave domain, our results demonstrate a first path to synthesis with integrated photonics, leveraging low-cost, low-power, and compact features that will be critical for its widespread use.Optical-frequency synthesizers, which generate frequency-stable light from a single microwave-frequency reference, are revolutionizing ultrafast science and metrology, but their size, power requirement and cost need to be reduced if they are to be more widely used. Integrated-photonics microchips can be used in high-coherence applications, such as data transmission1, highly optimized physical sensors2 and harnessing quantum states3, to lower cost and increase efficiency and portability. Here we describe a method for synthesizing the absolute frequency of a lightwave signal, using integrated photonics to create a phase-coherent microwave-to-optical link. We use a heterogeneously integrated III–V/silicon tunable laser, which is guided by nonlinear frequency combs fabricated on separate silicon chips and pumped by off-chip lasers. The laser frequency output of our optical-frequency synthesizer can be programmed by a microwave clock across 4 terahertz near 1,550 nanometres (the telecommunications C-band) with 1 hertz resolution. Our measurements verify that the output of the synthesizer is exceptionally stable across this region (synthesis error of 7.7 × 10−15 or below). Any application of an optical-frequency source could benefit from the high-precision optical synthesis presented here. Leveraging high-volume semiconductor processing built around advanced materials could allow such low-cost, low-power and compact integrated-photonics devices to be widely used.An optical-frequency synthesizer based on stabilized frequency combs has been developed utilizing chip-scale devices as key components, in a move towards using integrated photonics technology for ultrafast science and metrology.

Harmonically Mode-Locked Hybrid Silicon Laser With Intra-Cavity Filter to Suppress Supermode Noise

We present results from two hybrid silicon mode-locked lasers, each with a 2 GHz cavity and one with an intra-cavity ring resonator filter. We compare the performance of the two lasers with respect to the harmonic mode-locking behavior at 20 GHz, i.e., the tenth harmonic. The filter based laser design passively mode-locked at 20 GHz with an electrical spur mode suppression >45 dB. Furthermore, an optical supermode suppression of 55 dB and an RF linewidth of 52 kHz was also observed. Hybrid mode-locking the laser without the filter required 10 dBm input microwave power for 25 dB (electrical) spur mode suppression as opposed to the design with the filter showing >45 dB suppression at 0 dBm input power.

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.