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Photonic Integrated Circuits (PIC) technology enable optical signal processing, optical communication, biophotonics, and sensing

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.Thus photnics based products are primarily deployed in the field of optical fiber & optical free space communications.


Just as 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. A typical IPC may consist of single photon sources, nonlinear photon processing circuits and photon detectors all integrated onto a solid-state chip. Photonic integrated circuits (PICs) have attracted considerable attention owing to their small footprint, scalability, reduced power consumption and enhanced processing stability.


Photonic integrated circuits are a combination of photonic sensors and other electronic components. They use photons (light) to perform various optical functions. In a photonic integrated circuit, signals are processed using a combination of visible and infrared radiation. Photonic sensors are used in PICs to convert the light into an electric signal. The high speed and high bandwidth provided by photonic integrated circuits have been the major drivers for the global photonic integrated circuit market over recent years. Along with their high speed and bandwidth, photonic integrated circuits also consume less energy than conventional integrated circuits, making them doubly attractive to end users. This is likely to remain the major driver for the global photonic integrated circuit market.


Photonic integrated circuits have become widely popular in various applications, including nanoelectronics, LIDAR, calorimetry, and various silicon-based technologies. The growing use of photonic integrated circuits in quantum computing is likely to be a major driver for the global photonic integrated circuit market. Quantum computing has become a must-have in various crucial applications in recent years due to its higher processing speed and ability to multi-function easily. Developments in quantum computing have also come on at a rapid rate in recent years due to the significant investment made by major tech players in quantum computing. This has increased the demand from the photonic integrated circuit market in recent years and is likely to be a major driver for the global photonic integrated circuit market.


Research is being done to overcome all these challenges  enabling the the integration of  photonic components such as  lasers, optical amplifiers  modulators,MUX/DEMUX components and photodiodes on chip would enable them to be used in  optical signal processing, optical communication, biophotonics, and sensing applications.


Classification of Photonic Integrated Circuits

According to function PICs can be calssified as Passive Photonic Integrated Circuits or  all-optical photonic integrated circuits. These include  all passive optical devices, such as optical filters, optical multiplexers/ demultiplexers and adjustable optical attenuators. Active Photonic Integrated Circuits or photoelectric photonic integrated circuits, integrate active optical devices such as lasers, modulators, PIN detectors and optical amplifiers. It can also integrate passive filters such as optical filters and adjustable attenuators. Active photonic integrated circuits often integrates optical devices of different materials, so it’s more difficult to implement. Until 2004, dozens of active and passive optical devices successfully integrated on one chip, making active photonic integrated circuits available for commercial use.


The base materials used in photonic integrated circuits mainly include silicon and silicon dioxide, lithium niobate (LiNbO3), gallium arsenide (GaAs), and indium phosphide (InP).


Integration Level

Small-scale Photonic Integrated Circuits: It usually refers to a photonic integrated circuit with less than 10 monolithic function components, such as a laser with an integrated modulator and the optical transceiver module. Small-scale PIC products have been available for widespread commercial use.

Medium-scale Photonic Integrated Circuits: The number of monolithic function components is 10-50. In addition to the integration of several functional components, it can also be the parallel integration of multiple channels. The medium-sized photonic integrated circuits commercial products are basically the integration of passive optical devices.

Large-scale Photonic Integrated Circuits: The number of monolithic functional components is greater than 50, which means that each wavelength integrates several functional modules, realizing the concurrent integration of multiple wavelengths at the same time, for example. The large-scale photonic integrated circuits that have been used commercially are only the Infinera PIC products. One of the famous ones is the 10×10Gbit/s optical transmission photonic integrated circuit. It integrates 10 wavelength channels, and each wavelength channel realizes the integration of the laser, modulator, adjustable attenuator and other devices. The large-scale photonic integrated circuits can achieve photonic integration to the greatest extent. In the long term, it is the development direction of photonic integration in the future.


WDM Operating Principle


Demands of Photonic Integrated Circuit Industry

Because of the overall upgrade of the network, telecom operators need to invest a lot of capital and manpower, and due to the continuous influx of data traffic on the network, operators have to upgrade the optical transport network(ONT) with less and less profit.

Most of the practical WDM(Wavelength Division Multiplex) transmission systems use separate components. In order to make use of the optical fiber resources to achieve long-distance transmission of 40G and 100G, 40G and 100G systems need to use a more complex modulation format. As a result, the structure of the optical transceiver is extremely complicated, and the cost is greatly increased. For example, a telecom operator with 10% of the Internet transmission capacity will need 4000 DWDM transponders every day, and the number of technicians should be increased by 200 times. How to reduce unit costs while upgrading the network has become a major problem for operators and equipment vendors. To solve this problem, photonic integrated circuits will play a vital role.


Structure of an Optical Transceiver


By integrating many optical components into a single chip, the large-scale single-chip photonic integrated circuits greatly improves the size, power consumption, and reliability of the system and greatly reducing the cost.


First, the application of photonic integrated circuit greatly reduces the number of independent optical devices required for the transmission system, and the number of packaging times. Generally, the cost of the optical device packaging and related assembly process accounts for more than half of the entire cost. For complex optical devices, packaging and assembly costs can even be as high as 80%. Therefore, integrating dozens of optical devices into a single chip and then packaging it can greatly reduce the cost of the system.


Besides, the integrated photonic integrated circuit eliminates the fiber connection between different optical devices, thereby avoiding the need for high-precision fiber coupling and reducing the coupling cost. At the same time, each fiber coupling is a possible failure point. Under the influence of mechanical dither, temperature changes and vibration, the fiber coupling is prone to fail. Therefore, the use of fiber coupling will reduce the reliability of the system.


For this reason, 70% of optical communication equipment failures are caused by fiber coupling failures. After adopting the photonic integrated circuit, the reliability of the system will be significantly improved. Moreover, when we upgrade an optical transmission system with a photonic integrated circuit, all-optical devices inside the photonic integrated circuits need only be upgraded once, which greatly saves the upgrade cost. Therefore, photonic integrated circuits can meet the needs of current network upgrades, and can greatly increase the transmission capacity while reducing unit costs.


Development Status of PIC technology

The development of small and medium-scale integration technologies is relatively mature now. Common products include passive photonic integrated circuits (such as AWG, ROADM, etc.) and active photonic integrated circuits (DFB + EA laser, DFB laser array, etc.). Some optical device companies are also working on the integration of tuned lasers and Mach-Zehnder modulator products to achieve tunability on XFP transceivers.

Schematic View of a Mach-Zehnder Modulator

Schematic View of a Mach-Zehnder Modulator

From the current perspective, when there is no substantial breakthrough in silicon-based photonic integrated circuits, monolithic integration with indium phosphide material is an effective solution to achieve large-scale photonic integrated circuits. However, the development of PIC technology is very slow since it’s introduced to us 40 years ago, and even large-scale single-chip photonic integrated circuits have only achieved a breakthrough in recent years. How to integrate active and passive optical devices on a single chip has always been an inherent technical problem.


The bursting of the Internet bubble in 2001 also had a great impact on the development of PIC technology. Many manufacturers chose to withdraw from the R&D of PIC technology. In 2004, PIC technology made an important breakthrough. The commercial 100Gbit/s light-emitting and the light-receiving PIC chips using indium phosphide materials were launched, integrating more than 50 optical components and 6 different functional units.



Photon Sources

First, it is necessary to be able to create a stream of single photons and control their direction. Photons and electrons behave very differently at the quantum level. A quantum is the smallest unit in the atomic world and photons are the basic units of light and electrons of electrical current. Electrons are so-called fermions and can easily flow individually, while photons are bosons that prefer to clump together. But because information for quantum communication based on photonics lies in the individual photon, it is necessary to be able to send them one at a time.


“The development of a reliable single photon source is a first step towards a future where photonic quantum technology could be used for quantum simulators, unbreakable codes or perhaps even a quantum internet. In the coming years, we hope to collaborate with other researchers to develop solutions based on photon technology – solutions which together can revolutionise the way we use information technology,” explains Søren Stobbe, associate professor in Quatnum Photonic and CEO of Sparrow Quantum.


Single-photon cannon integrated on an optical chip

Researchers at the Niels Bohr Institute have developed a kind of single-photon cannon integrated on an optical chip. The optical chip consists of an extremely small photonic crystal that is 10 microns wide (1 micron is a thousandth of a millimeter) and 160 nanometers thick (1 nanometer is a thousandth of micron.) Embedded in the centre of the chip is a light source, a so-called quantum dot.


“When a laser light falls on a light source, called quantum dot, embedded in the centre of the chip it excites the electrons, thereby emitting one photon at a time. Normally, light is scattered in all directions, but we have designed the photonic chip so that all of the photons are sent through only one channel,” explains Søren Stobbe, Associate Professor of the Quantum Photonic research group at the Niels Bohr Institute.


On a semiconductor chip, light confinement can be achieved by an array of holes in a “photonic crystal” that create a “forbidden” region for light at certain frequencies to enter. A thin material strip sandwiched between two photonic crystal regions can act as a waveguide structure. Researchers could control the photons and send them in the desired direction with a 98.4 percent success rate.


The two researchers, Peter Lodahl and Søren Stobbe are in the process of patenting several parts of their work, with a specific goal of developing a prototype high-efficiency single-photon source, which could be used for encryption or for calculations of complex quantum mechanical problems and in general, is an essential building block for future quantum technologies.


Core Modulator Technology Shrinks to Chip Scale

Electro-optic modulators, which convert electronic data to optical signals, are key components of long-haul telecom networks—and, for decades, lithium niobate (LN) modulators have constituted the workhorse technology. But these bulky, power-hungry devices have proved resistant to chip-scale integration.


Now, a research team led by OSA Fellows Marko Lončar of Harvard University and Peter Winzer of Nokia Bell Labs has devised a way to radically shrink both the size and the driving voltage of LN modulators (Nature, doi: 10.1038/s41586-018-0551-y). The result is a device 100 times smaller and 20 times more efficient than existing modulators—and one that’s poised for on-chip integration. The researchers envision a vast application space for the new modulators, both in high-bandwidth data communications and in reconfigurable optical circuits for other areas, ranging from microwave signal processing to photonic neural-network computing.


Because of fundamental challenges in etching waveguides into lithium niobate, current-generation NB modulators must rely on waveguides with relatively large mode sizes and poor light confinement. That shortcoming, which in turn imposes limitations on other design details, forces the modulators to operate at drive voltages of 3 to 5 V—well beyond the roughly 1-V levels required to play well with typical CMOS circuitry.


The modulator required a driving voltage of only 1.4 V, within the range that it could be directly driven by a CMOS circuit, without bulky amplifiers. And the devices can support data transmission rates of up to 210 Gbit/s—with rates as high as a blistering 1 Tbit/s a distinct possibility with more advanced modulator designs. “It’s like Antman,” co-lead author Wang said in a press release. “Smaller, faster and better.”


A key advantage of the new modulator, according to Peter Winzer, is that it will speed up progress toward moving optics and electronics closer on a single chip—“paving the way toward future fiber-in, fiber-out opto-electronic processing engines,” he said. The result could be a variety of fast, low-loss photonic circuits and applications.


In a particularly intriguing note, the study concludes that the device’s advantages of low optical losses, good electro-optical response, integration and scalability could combine to help create “a new generation of active integrated optoelectronic circuits that can be reconfigured on a picosecond timescale using attojoules of electrical energy.” The team believes those circuits could find use in microwave photonics, quantum networks, topological photonic circuits and photonic neural networks, among other areas.


A quantum optical circulator

A circulator is a passive three- or four-port device that routes signals according to a simple protocol: If the ports are numbered in ascending order, a signal that enters the circulator through port 1, 2, 3, or 4 exits it through port 2, 3, 4, or 1, respectively.


Scheucher et al. demonstrate a fiber-integrated  optical circulator that operates by using the internal quantum state of a single atom. Moreover, the routing can be reversed by flipping the atomic spin. “We demonstrate that the internal quantum state of the atom controls the operation direction of the circulator and that it features a strongly nonlinear response at the single-photon level.,” write researchers. Such an integrated optical device may be important for routing and processing quantum information in scalable integrated optical circuits.


Integrated nonreciprocal optical components, which have an inherent asymmetry between their forward and backward propagation direction, are key for routing signals in photonic circuits. The nonreciprocal behavior of the circulator arises from a chiral interaction between the atom and the transversally confined light. The circulator’s nonlinear response at the single-photon level enables photon number-dependent routing.


This enables, for example, photon number–dependent routing and novel quantum simulation protocols. Furthermore, such a circulator can in principle be prepared in a coherent superposition of its operational states and may become a key element for quantum information processing in scalable integrated optical circuits.


Integrated chip-mounted arrays of light detectors with single-photon sensitivity

For the detector function Superconducting nanowire single-photon detectors (SNSPDs) are particularly attractive because of high detection efficiency, sub-50-ps jitter and nanosecond-scale reset time.The SNSPD consists of a thin (≈ 5 nm) and narrow (≈ 100 nm) superconducting nanowire typically  hundreds of micrometers in length, and the nanowire is patterned in a compact meander geometry to create a square or circular pixel with high detection efficiency. The nanowire is cooled well below its superconducting critical temperature and biased with a DC current that is close to but less than the superconducting critical current of the nanowire.


A photon incident on the nanowire breaks Cooper pairs and reduces the local critical current below that of the bias current. This results in the formation of a localized non-superconducting region, or hotspot, with finite electrical resistance. This resistance is typically larger than the 50 ohm input impedance of the readout amplifier, and hence most of the bias current is shunted to the amplifier. This produces a measurable voltage pulse that is approximately equal to the bias current multiplied by 50 ohms


However, to date there has been no scalable approach for integrating SNSPDs into PICs: while single, isolated waveguide-integrated SNSPDs have been demonstrated. The system detection efficiency of multiple SNSPDs in one photonic circuit—required for scalable quantum photonic circuits—has been limited to < 0.2%. The central challenge when building systems with multiple SNSPDs remains the low fabrication yield, which is limited by defects at the nanoscale. This yield problem is exacerbated when such detectors are integrated onto photonic chips, which can require tens of additional fabrication steps of their own.


Researchers from MIT, Columbia University and California Institute of Technology have developed a technique for creating integrated chip-mounted arrays of light detectors with single-photon sensitivity. Moreover, these sensors can be mounted on regular old silicon computer chips using regular old manufacturing processes, opening door toward practical quantum computing.


It involves fabricating and testing the detectors separately and then transferring those that work to an optical chip built using standard manufacturing processes. They were able to increase the system detection efficiency from 0.2 to as high as 20 percent. That’s still a long way from the 90 percent or more required for a practical quantum circuit, but it’s a big step in the right direction.


“This work is a technical tour de force,” says Robert Hadfield, a professor of photonics at the University of Glasgow who was not involved in the research. “There is potential for scale-up to large circuits requiring hundreds of detectors using commercial pick-and-place technology.”


Photonic Packaging and Test Technologies

PhotonicLEAP, a European Horizon 2020 collaborative research project based in Ireland, has been awarded over €5 million Euros in funding from the European Commission to develop disruptive technologies that will drive down the cost of integrated photonic packaging and test processes.


Photonic Integrated Circuit (PIC) technologies are the light-based equivalent of electronic circuits – a technology that is becoming increasingly important for existing markets in communications, medical devices, sensors and for emerging markets such as quantum computing and security. However, existing PIC manufacturing processes, in particular packaging and test processes, are difficult to automate, with limited manufacturing capacity, and are costly, with packaging and testing typically accounting for over 75% of the total manufacturing cost. As a result, existing PIC manufacturing processes greatly impede the uptake of PIC technologies across many mass markets.


To address this challenge, PhotonicLEAP will develop disruptive wafer-level PIC module integration, packaging and test technologies which will reduce the costs of PIC production by over 10 times, revolutionising existing applications and creating completely new markets. PhotonicLEAP will use these disruptive technologies to produce a revolutionary Surface Mount Technology (SMT) PIC package, which for the first time incorporates multiple optical and electrical connections. The project will validate these technologies through state-of-the-art demonstrators, including a high-speed optical communication module and a medical device for cardio-vascular diagnostics. Furthermore, the technologies will be implemented by the flagship European PIC Packaging Pilot Line, PIXAPP, for future commercialisation. PIXAPP has an extensive and growing user-base across multiple markets.



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