Sasa Ristic

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.

An Optical Phase-Locked Loop Photonic Integrated Circuit

We present the design, fabrication, and results from the first monolithically integrated optical phase-locked loop (OPLL) photonic integrated circuit (PIC) suitable for a variety of homodyne and offset phase locking applications. This InP-based PIC contains two sampled-grating distributed reflector (SG-DBR) lasers, semiconductor optical amplifiers (SOAs), phase modulators, balanced photodetectors, and multimode interference (MMI)-couplers and splitters. The SG-DBR lasers have more than 5 THz of frequency tuning range and can generate a coherent beat for a wide spectrum of frequencies. In addition, the SG-DBR lasers have large tuning sensitivities and do not exhibit any phase inversion over the frequency modulation bandwidths making them ideal for use as current controlled oscillators in feedback loops. These SG-DBR lasers have wide linewidths and require high feedback loop bandwidths in order to be used in OPLLs. This is made possible using photonic integration which provides low cost, easy to package compact loops with low feedback latencies. In this paper, we present two experiments to demonstrate proof-of-concept operation of the OPLL-PIC: homodyne locking and offset locking of the SG-DBR lasers.

High Linearity InP-Based Phase Modulators Using a Shallow Quantum-Well Design

Phase modulator nonlinearity is a major problem for implementing an optical phase-locked loop (OPLL) phase demodulator. In this letter, we report an improved InP phase modulator design that uses a detuned shallow multiquantum-well structure. The phase modulator shows high linearity and low optical loss. Its phase IP3 and optical loss per unit length are ~ 4 π/mm and ~ 0.9 dB/mm, respectively. This phase modulator design is thus suitable for implementing the OPLL linear phase demodulator.

3D integrated hybrid silicon laser

A laser is realized by flip-chip bonding an indium phosphide reflective semiconductor optical amplifier with a turning mirror to a silicon photonic circuit with a surface grating coupler. An external cavity laser is formed and single-mode CW lasing is demonstrated.

Integrated optical phase-locked loop

We demonstrate the first integrated optical phase-lock loop (OPLL) photonic IC, containing two SG-DBR lasers with ≫5 THz tuning range, a balanced detector pair and output modulators. A proof-of-concept homodyne OPLL demonstration has been performed.

Highly linear integrated coherent receivers for microwave photonic links

A coherent receiver with feedback was developed to linearly demodulate the phase of an analog signal. The receiver demonstrates a spur-free dynamic range of 125 dBmiddotPHz2/3 at a signal frequency of 300 MHz.

Demonstration of a linear ultra-compact integrated coherent receiver

We demonstrate the operation of an ultra-compact coherent receiver for linear optical phase demodulation. The receiver, based on a broadband optical phase-locked loop (OPLL) has a bandwidth of 1.5 GHz. Physical delay in the feedback path is dramatically reduced by incorporating novel photonic and electronic components. Using the receiver in an analog link experiment, a spurious free dynamic range of 122dBHz2/3 is measured at 300 MHz. Additionally, the link loss is −2dB at low frequencies.

High-Thermal Performance 3D Hybrid Silicon Lasers

A 3D integrated hybrid silicon laser was realized for high-thermal performance by integrating silicon photonic (SiPh) chips and indium phosphide (InP) chips. The optical gain is provided by the InP chip with a total internal reflection mirror for surface emission. The surface grating couplers on the SiPh chip couples light into a silicon waveguide. The InP chips were flip-chip bonded P-side down to metal pads on the silicon chips. Two lasers are reported. For laser A, the InP chip was bonded on the top cladding oxide of the silicon waveguide. For laser B, the InP chip was bonded to the silicon substrate in an etched recess. Both lasers demonstrate milliwatt-level light coupled into the silicon waveguide. Laser B demonstrated three times better thermal performance with a measured thermal impedance of 6.2 °C/W.

InP/InGaAsP-Based Integrated 3-dB Trench Couplers for Ultra-Compact Coherent Receivers

We present the design, fabrication, and test results for ultra-compact 3-dB frustrated total internal reflection-based trench couplers in an InP/InGaAsP monolithic integration platform. The trench coupler is integrated with phase modulators and a balanced photodiode (BPD) pair to enable a 180° hybrid ultra-compact coherent receiver. Several trench splitter designs exhibit near 3-dB splitting with a loss of 3 dB. The BPD pair is used to characterize coherent mixing of two input optical signals into the trench splitter, and coherence efficiency of 75% is achieved.

Ultra-compact integrated coherent receiver for high linearity RF photonic links

We demonstrate a novel photonic integrated circuit(PIC) that combines an ultra compact trench beam splitter with monolithically integrated photodetectors and modulators. A coherent receiver is realized by flip chip bonding of this PIC with an electronic integrated circuit (EIC). Preliminary system results yield a third-order intermodulation distortion suppression of 46 dB at a signal frequency of 300 MHz.