What is a Photonic Integrated Circuit?

A photonic integrated circuit (PIC), also referred to as a planar lightwave circuit or an integrated optical circuit, is a microchip consisting of two or more connected components, creating a circuit that generates, transports, modifies, or measures light. Whereas an electronic integrated circuit uses electrons, PICs use photons. Information on PICs is created, modified, and measured as an optical signal of light with wavelengths in the visible or near-infrared spectrum.

Advances in the manufacturing of PICs and component design within PICs have moved the application of photonic devices beyond communication to biomedical instruments, signal processing, quantum computing, and a wide variety of sensors. Design, simulation, and manufacturing advances continue to push the technology to smaller sizes, more capability per chip, and increased speed and accuracy. 

Where electronic integrated circuits (ICs) use silicon as the dominant material, most photonic chips are made with a combination of electro-optical crystals, including silicon nitride (SiN), lithium niobate, gallium arsenide (GaAs), and indium phosphide (InP). Silicon-based optical components can be created using standard complementary metal-oxide-semiconductor (CMOS) manufacturing, referred to as silicon photonics (SiPh), using silicon-on-insulator materials. 

The circuitry that makes up a PIC varies greatly, depending on the tasks needed by a given device. Engineers arrange photonic components to modify and measure the light signals traveling through the circuit. Photonic integration on a single chip delivers high-performance solutions in a small, power-efficient package. The value of a PIC increases with the number of functions in a single chip and, therefore, the circuit's complexity and the components' density also increase. 

Each component can be generally classified as a source, a signal carrier, an amplifier, a modulator, or a detector. They can also be grouped as passive or active components. Passive components have no electrical inputs or outputs, whereas active components have electrical inputs or outputs for modulating or sensing photons, respectively. Many advanced components are made by combining simpler components. 

Photonic I/O Components

There are several ways for engineers to get light into a PIC and introduced into the circuit. A grating coupler is used to introduce light perpendicular to the chip. Grating couplers consist of a periodic structure, similar to a diffraction grating, which is etched into the photonic chip. They do not require precise alignment and can be used to introduce light from various angles relative to the chip. Light can alternatively be directly inserted into a waveguide by attaching an optical fiber cable directly to the chip, usually an edge through an edge or direct coupler. This approach requires precise alignment and a robust bonding between the light source and the PIC.

Laser

Laser light sources can be introduced as active components in a PIC. Indium phosphide-based PICs can support the addition of laser components in the photonics circuit in the form of a laser diode. 

Waveguide

Waveguides are the interconnect between components in a photonic circuit. They are low-loss components that carry the optical signal through the optic network. Waveguides can be planar, a ridge, or a slot. Waveguides can support optical signals in a large spectrum range.

Phase Modulator/Shifter

A common function in a photonic circuit is to modulate or shift an optical signal's phase by changing the component material's refractive index using an electrical signal. Modulation in silicon is most commonly achieved by the plasma dispersion effect, where free carrier density changes by electrical input can induce changes in the refractive index and modulate the light. 

Coupler and Splitter

Signals in a photonic circuit are combined in a coupler that takes two or more input waveguides and combines the signal into a single output or a multiplexed signal can be split into separate waveguides, often based on wavelength.

One common example of separating a multiplexed signal is an arrayed waveguide grating (AWG) device. It uses gratings of different lengths arranged in an array as a wavelength division multiplexer, breaking an incoming multiplexed signal into individual wavelengths.

Filter

Interferometer structures, such as the Mach-Zehnder interferometer or micro-ring resonator, can be used to block or pass desired wavelengths. Filters can be classified as bandpass or notch filters. 

Optical Amplifier

A common need in a PIC is to amplify an optical signal without changing it. Various amplifier components, some electro-optic and some purely optical, are used to increase the amplitude of the light. 

Photo Detector

At some point in a circuit design, the optical signal transmitted and modified in a PIC must be measured. A photodetector converts the photon's energy into an electrical signal based on the photoelectric effect.  

Photonic integrated circuits have similar advantages to electronic integrated circuits. They offer the ability to combine multiple discrete photonic components onto a single chip, improving size, efficiency, and performance while reducing the cost and enabling mass production. Some of the most important advantages are:

• Lower power consumption: Photonic circuits are inherently more efficient than electronic circuits. The resistance in electrical circuits results in greater heat generation and more signal strength loss than in PICs. 
• Reduced cost: PICs can leverage the cost advantages of mass production using standard semiconductor manufacturing technologies, including the shared resources of foundries. 
• Decreased size: When compared to discrete photonic technology, PICs afford a significantly smaller footprint for photonic devices. This has advantages in reducing the cost and footprint of data communications equipment. Using PICs as sensors results in tiny devices easily mounted into equipment or embedded in wearables. 
• Integration: The ability to mix photonic integrated circuits with silicon-based microelectronics is one of their more compelling advantages. Electronic and photonic circuits can be combined in the same monolithic microchip or connected within advanced optoelectronic packages using advanced semiconductor packaging technology. 
• Reliability: As compared to discrete photonic technology, potential failure modes are reduced or completely eliminated when the components are integrated into a single chip. Also, the precision of semiconductor manufacturing can control material and dimensional variability. 

Engineers are constantly developing new applications for photonic integrated circuits. Their ability to generate, modify, and read data as light, coupled with their small size, makes them ideal for a wide range of industries, including communication, computing, sensing, and data processing. 

Some of the most common applications include:

Optical Communications

The most common use of PICs is for various types of communication. PIC-based transceivers connect computers in data centers, cell towers, or even multiple vehicles through light fidelity (Li-Fi.). PICs are used as amplifiers and multiplexers for data transmission through high-speed fiber-optic networks or to connect processors in high-performance computing applications. 

Lidar

Light detection and ranging (lidar) is a sensor technology that uses pulses of laser light to map the location of physical objects. PICs are critical to producing the specific light pulses sent out by a lidar sensor and accurately measuring the returning light signatures. The growth of autonomous vehicles has seen the large-scale adoption of lidar technology. 

Property Measurement

Light-sensing devices that contain PICs can measure temperature, chemical composition, location, velocity, acceleration, pressure, vibrations, and surface finish with extreme accuracy. Some sensors use light to measure physical characteristics, and others use PICs with spectrometers. 

Lab on a Chip

The integration of optical components on a chip can be leveraged in medical sensing to miniaturize a chemical laboratory in a single, tiny package containing electronic and photonic integrated circuits. Light is used to take measurements of fluid samples from a patient directly at the point of care instead of sending the sample to a lab for processing through multiple diagnostic devices. 

Quantum Computing

Quantum computing uses the quantum behavior of photons, and PICs are a key enabling technology for this quickly developing technology. Photonic circuits are required to control and measure photonic quantum states. They are also used in quantum networking between multiple quantum computers or to connect them to digital computers. 

Artificial Intelligence and Machine Learning (AI/ML)

Photonics integrated circuits are also playing an important role in the explosive growth of AI applications. The primary current application is optical communications within or between computers. Researchers are also finding that certain AI algorithms, especially neural networks, are well suited for PICs. They are also using AI/ML to design PICs for use in AI/ML applications, creating a technological virtuous circle. 

Photonic integrated circuits are complex devices requiring rigorous and detailed engineering to design. The behavior of photons; the interaction of photons with materials; and the modification of a light signal’s frequency, magnitude, and phase involve complex physics. This makes photonic circuit design a perfect application for simulation to help engineers make design decisions and optimize the performance and robustness of their design. 

Engineers can start with a circuit-level simulation using a tool like Ansys Lumerical INTERCONNECT software. Engineers can lay out both classical and quantum photonic integrated circuits as components, run simulations with input signals, and look at the signal at any point in the model. INTERCONNECT software is compatible with the device libraries offered by major foundries. It works with major electronic design automation (EDA) tools and workflows, and its parametric nature makes statistical studies easy to conduct.

Each component in the circuit can also be simulated and optimized using tools like Ansys Lumerical FDTD software and Ansys Lumerical MODE software. FDTD software is an electromagnetic solver that accurately models the photonics of components as 3D objects. MODE software is used to look at the detailed behavior of waveguides and couplers. Once a design is finalized, the results can be converted into compact model library representations in industry-standard formats for use in system-level tools like INTERCONNECT software. 

Engineers also need to take the impact of heat generation and charge transport on photonic components into consideration. They can use a tool like Ansys Lumerical Multiphysics software to measure how the changes of temperature and charge distributions cause material index change in the structure, and therefore, impact the photonic performance.

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