First, from a functional perspective, PICs can be divided into the following two categories.
① Passive PIC , also known as all-optical PIC , integrates all passive optical devices, such as optical filters, optical multiplexers / demultiplexers , and tunable optical attenuators. It generally adopts planar lightwave circuit ( PLC ) technology, which is relatively mature.
② Active PIC , also known as optoelectronic PIC , can integrate active optical devices such as lasers, modulators, PIN detectors, and optical amplifiers, as well as passive optical devices such as optical filters and adjustable attenuators. Because active PICs often involve the integration of optical devices made of different materials, their implementation is quite challenging. Therefore, a major breakthrough in active PICs was not achieved until 2004 , when dozens of active and passive optical devices were integrated onto a single chip, leading to successful commercialization.
Secondly, based on the integration level of PICs, they can be divided into the following three categories.
① Small-scale PIC : This typically refers to a PIC with fewer than 10 integrated functional components , such as a laser with an integrated modulator or an optical transceiver module. Small-scale PIC products are mature and widely used in the market. Major industry manufacturers such as JDSU and Bookham have mature related products.
② Medium-scale PIC : The number of functional components integrated on a single chip ranges from 10 to 50. Besides integrating several functional components, it can also be a parallel integration of multiple channels. Commercial medium-scale PIC products primarily integrate passive optical devices; there are no reports of commercial medium-scale active PICs .
③ Large-scale PIC : These PICs integrate more than 50 functional components on a single chip , typically meaning several functional modules are integrated for each wavelength, achieving parallel integration of multiple wavelengths. For example, each PIC may integrate 10 or more wavelength channels. Currently, only Infinera 's PIC products are commercially available as mature large-scale PICs . Figure 8-19 shows the structure and physical diagram of a 10 × 10 Gbit/s optical transmission PIC . It integrates 10 wavelength channels, each integrating components such as a laser, modulator, and adjustable attenuator. Large-scale PICs maximize photonic integration, and from a long-term perspective, they represent the future direction of photonic integration.
PICs can be classified according to their substrate materials. Current PICs primarily use indium phosphide ( InP ), gallium arsenide ( GaAs ), lithium niobate ( LiNbO3 ), silicon, and silicon dioxide. Firstly, silicon / silicon dioxide is a fundamental material used in the production of electronic integrated circuits. It is inexpensive, has stable performance, simple and mature processing technology, and high yield, making it very suitable for mass production. However, silicon-based materials have three fatal drawbacks when used in PICs : firstly, the laser emission efficiency is very low, making silicon-based lasers extremely difficult to implement; secondly, silicon-based materials cannot detect light at wavelengths of 1310nm and 1550nm , which are precisely the wavelengths used in optical communication; and thirdly, due to the inherent limitations of silicon-based materials, electro-optic modulation cannot be achieved. Although many research institutions have been dedicated to the research of silicon-based active optical devices and striving for breakthroughs, and research institutions such as Intel have achieved remarkable research results, silicon-based materials are mainly used in passive PICs (such as arrayed waveguide gratings ( AWGs )). Significant progress has been made in the hybrid integration of large-scale PICs using silicon-based materials . Secondly, lithium niobate crystals are widely used, primarily for fabricating high-performance electro-optic modulators due to their high modulation bandwidth and good modulation linearity. However, lithium niobate crystals cannot be used for laser emission or as photodetectors, and their processing is extremely complex. Therefore, they have no practical application value for large-scale photoelectric sensors (PICs ). Although active optoelectronic devices can be implemented using gallium arsenide (GaAs), GaAs's intrinsic bandgap limits its operation to a wavelength range of 850 nm . Therefore, active optical devices using GaAs are only suitable for local area networks (LANs), significantly restricting their application in long-distance, high-capacity WDM transmission systems. Only indium phosphide (IPS) can integrate both active and passive optical devices while ensuring their operating wavelengths are within the widely used 1310 nm and 1550 nm bands in optical communication . Furthermore, it can be mass-produced using mature, standardized semiconductor manufacturing processes, facilitating cost savings. Indium phosphide (IP) can be used to simultaneously achieve laser emission, detection, optical amplification, and electro-optic modulation. It can also enable wavelength multiplexing / demultiplexing, tunable optical attenuators, optical switches, and dispersion compensation. Therefore, it is possible to realize large-scale monolithic photocells ( PICs) using IPT . The commercially available Infinera large-scale PICs utilize IPT.
Finally, similar to classifying electronic integrated circuits, PICs can also be divided into monolithic integrated PICs and hybrid integrated PICs . Hybrid integrated PICs refer to the integration of different individual optical devices into a single device. Many common optical devices utilize hybrid integration technology; however, due to the inherent properties of materials, the optimal materials used for active and passive optical devices are not the same, and their physical properties (such as coefficient of thermal expansion) and packaging requirements also differ. This makes integrating multiple discrete components and ensuring device performance extremely complex, especially when implementing large-scale PICs . In contrast, monolithic integrated PICs use a single material to implement various active and passive optical devices, thus avoiding the compatibility issues between different materials. Monolithic PICs have significant advantages over hybrid PICs in terms of both energy efficiency and reliability .
The analysis above also shows that the key technologies of PIC mainly include the following aspects: First, what materials and processes are used to realize PIC ; second, different optical devices often use different substrate materials, how to integrate optical devices made of different materials, or how to realize all the functions of optical devices on the same substrate material; third, how to improve the mass production capacity of PIC , which will be the key to reducing PIC costs and realizing large-scale applications.
