In the world of photonics, where light-based technologies are rapidly advancing, Photonic Integrated Circuits (PICs) have emerged as a cornerstone of numerous applications. These compact and efficient devices integrate various optical components onto a single chip, enabling functionalities such as optical communication, sensing, and signal processing. However, to harness the full potential of PICs, accurate calibration and meticulous control are essential. In this article, we delve into the intricacies of calibrating and controlling PICs, exploring the importance of these processes and the methodologies involved.
What are Photonic integrated circuits (PIC) or Integrated Photonic circuits (IPC)
Photonics is the analog of electronics in that it describes the technology in which photons instead of electrons are used to acquire, store, transmit, and process information. Photonics is a breakthrough technology as it uses photons (smallest unit of light) as the data carrier instead of electrons (smallest unit of electricity) used in electronic ICs. As light travels at very high speeds, photonics is widely used to transfer huge amounts of data at a very high speed.
Photonics, or the use of light particles to store and transmit information, is a burgeoning field, supporting our need to create faster, better, more efficient and more sustainable technology. Whether it’s downloading movies or keeping a satellite on the course, photonics is radically changing the way we live, revolutionising the processing capability of large-scale equipment onto a chip the size of a human fingernail.
Just as an Integrated circuit (IC) is a microelectronic device that houses multiple electric circuits on a chip, a photonic integrated circuit (PIC) or Integrated Photonic circuits (IPC) are devices that integrate multiple photonic functions on a chip. Programmable photonic integrated circuits (PICs), offer diverse signal processing functions within a single chip, and present promising solutions for applications ranging from optical communications to artificial intelligence.
Photonic Integrated Circuits (PIC) are slowly growing beyond the field of telecommunication and data communication that has pushed most technological developments over the past decades. Photonic integrated circuits have become widely popular in various applications, including nanoelectronics, LIDAR, calorimetry, and various silicon-based technologies. Nowadays, integrated photonics technologies are envisioned as fundamental for applications such as optical communications, optical interconnects, bio-sensing, 5G networks and quantum photonics.
Among all the technological platforms that can be used to realize PIC, two semiconductors technologies have been emerging in the last decade: Indium Phosphide and Silicon. Thanks to the high index contrast offered by these photonic platforms, it is possible to integrated a large number of devices on the same chip and to implement complex functionalities for the generation, manipulation and detection of light. Indium phosphide offers the possibility of monolithically integrating on-chip waveguides, detectors, modulators and light sources, while silicon allows an unprecedented number of integrated components.
To reach the objective of replacing electronic circuits with low-power consumption photonic circuits, especially for telecom/datacom applications, it is necessary to handle more and more complex functions in the optical domain. To realize such complex functions, photonic circuits must reach a higher level of complexity, interconnecting many photonic devices on the same chip.
The fabrication techniques are similar to those used in electronic integrated circuits in which photolithography is used to pattern wafers for etching and material deposition. Unlike electronics where the primary device is the transistor, there is no single dominant device. The range of devices required on a chip includes low loss interconnect waveguides, power splitters, optical amplifiers, optical modulators, filters, lasers and detectors. These devices require a variety of different materials and fabrication techniques making it difficult to realize all of them on a single chip.
The extreme device miniaturization reached by state-of-the art photonic technologies now enables the realization of hundreds or even thousands of photonic elements in a footprint of less than 1 mm2. Although many building blocks potentially provide the required degrees of freedom to realize flexible and arbitrarily complex photonic architectures, reconfigurable optical circuits aggregating many different functionalities are still encountering strong difficulties emerging. The urgent need for adaptability and programmability is to enable the realization of arbitrary, reconfigurable, complex circuits thus shifting the paradigm from a device-level to a “system-on-a-chip” one.
Because they are generic and programmable, programmable PICs can change the way PICs are used in the development of new applications and products. This can be compared with programmable electronics: the widespread availability of programmable electronics (FPGAs, microprocessors, digital signal processors, …) makes it possible to implement new functions in a matter of days, without the need to fabricate custom silicon.
The reason is that in photonics, similarly to electronics, device miniaturization is not synonymous with large scale of integration, and some keys still need to be found to make photonics step up from the current device level to complex, adaptive and reconfigurable integrated circuits.
In other side, PICs are evolving towards on-chip re-configurable architectures and general-purpose programmable photonic processors, enabling the implementation of many different functionalities on-demand. These schemes rely on the use of a large number of optical interferometers, such as MZI and MRRs, whose individual working point is inherently related to the phase delay between the interfering optical beams. Therefore, any kind of phase perturbation may substantially affect the overall behavior of the PIC.
To reach these goals, feedback control is mandatory to steer and hold the entire system to the desired functionality, and make it immune to fabrication tolerances, functional and environmental drifts, and mutual crosstalk effects. In fact sensitivity to temperature fluctuations is one of the strongest limiting factors to the exploitation of integrated optical devices. This effect is particularly relevant in Silicon on Insulator (SOI), where the large thermo-optic coefficient (TOC) of silicon (1.8∗10−4K−1 at 300 K ) is responsible for a wavelength shift of any interference-based device of about 10GHz⋅K−1.
In interferometric devices, a waveguide width deviation of only 1 nm can produce a frequency shift of about 100 GHz in the spectral response. Due to this sensitivity, and to the tolerances of current fabrication technologies, the response of fabricated PIC hardly matches the design performance; moreover, it is possible that reconfiguration capabilities will be needed to adapt in real time the circuit to new requirements (i.e. dynamic switching and routing, channels add/drop and so on). Due to functional drifts and components ageing over time, is not possible to have an accurate and robust control only relying on lookup tables; hence, there is a need of an automatic feedback loop to set the new working point automatically and to control real time the PIC to counteracts unwanted drifts.
Closed loop control of PIC. The signals from detectors placed in strategic positions throughout the circuits is used by a controller to estimate the current working point. A controller reads the control signals and estimates the working point of the PIC based on the information provided; the controller then drives the actuators to steer the working point accordingly to the algorithm implemented in the controller itself. The actuators rely on physical effects to modify the working point of the PIC. Conveniently, control systems should be low cost, energy efficient, insensitive to fluctuations of the optical power, applicable to both passive and active devices, and should not require additional photonic structures.
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The Significance of Calibration
Calibration plays a vital role in ensuring the optimal performance of a Photonic Integrated Circuit. It involves the adjustment of various components and parameters within the circuit to achieve desired functionalities and enhance overall efficiency. Calibration is necessary to mitigate manufacturing imperfections, align optical paths, optimize power transfer, and reduce unwanted signal interference. Additionally, it compensates for environmental variations, such as temperature fluctuations, to maintain consistent and reliable operation.
- Power and Energy Calibration: To ensure accurate power and energy measurements within a PIC, calibration of input and output optical power levels is essential. Techniques such as power meter calibration, calibrated reference standards, and calibrated photodetectors help establish reliable power measurement setups.
- Spectral Calibration: The spectral response of PIC components, such as waveguides and filters, needs calibration to ensure accurate wavelength control. Spectral calibration techniques involve the use of calibrated wavelength references, such as tunable lasers and calibrated wavelength meters, to establish precise spectral measurements.
- Phase Calibration: Phase calibration is crucial for applications involving coherent light, such as optical communications and quantum computing. Phase calibration techniques enable the synchronization and alignment of different optical paths within a PIC. Interferometric methods, such as Mach-Zehnder interferometers, are commonly employed for precise phase calibration.
The advanced photonic chip calibration technology
Researchers from Monash University, RMIT, and the University of Adelaide have developed an advanced method for calibrating optical circuits on photonic integrated circuits (PICs). The team’s work builds upon their previous development of the world’s first self-calibrated photonic chip. The new method involves adding a common reference path to the chip, allowing for stable and accurate measurements of phase, time delays, and losses. By incorporating the reference path on the chip itself, the measurements are immune to phase errors caused by external connections. The researchers utilized a fractional delay method that requires less optical power for calibration while maintaining accuracy.
This advancement in calibration technology enables precise programming of the chip for various applications, such as pattern recognition and optimizing optical communications networks. The research team will continue their work in exploring the use of photonic chips for ultrafast information processing and machine intelligence. The breakthrough in calibration offers a solution to the increasing complexity of PICs, ensuring their robustness and applicability in advanced applications.
New self-calibrating photonic chip shines a light on ultrafast tech of the future
Researchers at Monash University and RMIT have developed a self-calibrating photonic chip that has the potential to revolutionize photonics technology. Publishing their findings in Nature Photonics, the team believes the efficiency of their new photonic chip can help advance research into artificial intelligence as well as in driverless cars, language processing, and data transfer.
This chip, building upon a previous optical microcomb chip, enables faster and more efficient data transfer, making it suitable for applications in artificial intelligence, driverless cars, language processing, and more. The chip incorporates a signal processing core and an integrated reference path for self-calibration, allowing for real-world usability.
The ability to self-calibrate makes tunable photonic integrated circuits practical for various applications, including optical communication systems, scientific instrumentation, and astronomy. With the increasing demand for high-speed data transmission in technologies like AI and self-driving cars, this breakthrough in photonics paves the way for achieving faster and more reliable data transfer.
The chip’s on-chip reference system enables complex systems to be integrated into a single chip, addressing internet bottlenecks and facilitating rapid reconfiguration of optical networks. By calibrating the chips after manufacturing, precise tuning is achieved, overcoming the challenges of nanoscale manufacturing precision. This advancement in photonics technology showcases the potential for compact and efficient devices to support the future of high-speed data transmission.
Control of Photonic Integrated Circuits
Once calibrated, achieving effective control over a Photonic Integrated Circuit is essential for its proper operation. Control refers to the ability to manipulate and dynamically adjust the behavior of the circuit to fulfill specific requirements. The following aspects are vital for successful control of PICs:
- Actuators and Feedback Mechanisms: PICs rely on actuators to actively control various optical components. These actuators can adjust parameters such as phase, polarization, and wavelength. Feedback mechanisms, such as integrated photodetectors and optical monitoring systems, provide real-time information about the circuit’s performance, enabling dynamic adjustments for optimal operation.
- Digital and Analog Control: The control of PICs can be achieved through digital or analog control schemes. Digital control involves using microcontrollers or field-programmable gate arrays (FPGAs) to send digital commands to the PIC, allowing precise adjustments of optical components. Analog control, on the other hand, relies on analog electronic circuits to manipulate the properties of the optical signals.
- Closed-Loop Control: Closed-loop control systems utilize feedback from the PIC’s output to dynamically adjust the input and optimize the circuit’s performance. By continuously monitoring the circuit’s response and comparing it to the desired output, closed-loop control ensures stability, accuracy, and adaptability in real-time.
Calibration and control are essential steps in maximizing the performance and functionality of Photonic Integrated Circuits. Calibration enables precise adjustment of various components, compensating for manufacturing imperfections and environmental variations. Meanwhile, control mechanisms empower users to manipulate the circuit’s behavior to meet specific requirements. The continuous advancement of calibration techniques and control methodologies allows researchers and engineers to unlock the full potential of PICs and drive innovation in fields such as telecommunications, quantum computing, and biosensing. As the demand for efficient and reliable photonic devices continues to grow, the significance ofcalibration and control in the realm of Photonic Integrated Circuits cannot be overstated. With further advancements and ongoing research, we can expect even greater achievements and groundbreaking applications in the world of photonics.