Driven by optical communication, optical interconnection, integrated microwave photonics and other application fields, the photonic integrated loop based on silicon-on-insulator can overcome the electronic delay bottleneck of traditional copper interconnect and provide optical solutions with large bandwidth, high speed, low power consumption and low latency. The silicon photonics platform is characterized by compatibility with complementary metal-oxide-semiconductor processes and is manufactured in batches.
This is a huge advantage in terms of cost reduction. Moreover, the silicon photonics platform can take advantage of the advanced packaging process to achieve compact, low-power, high-bandwidth density optoelectronic integrated chips.
The key to the photonic integration loop
As one of the core devices of silicon-based photonic integrated circuits, the function of electro-optical modulator is to modulate external electrical signals to optical signals for transmission and processing in the optical domain. High modulation efficiency.
Silicon-based modulators with low insertion loss and large bandwidth are the key to achieving photonic integration loops with large capacitance, low power consumption, and large dynamic range.
Therefore, in view of the compromise relationship between efficiency, loss and speed of silicon-based traveling BoMach-Zehnder modulators, high-efficiency, high-linearity and high-speed silicon-based modulators have been studied from the aspects of physical mechanism, theory and process preparation.
On this basis, focusing on the application of silicon-based electro-optical modulators in the two hot fields of microwave photonics, optical communication and optical interconnection, an optical single-sideband modulation integrated chip with high sideband rejection ratio and a four-stage pulse amplitude modulation optical emission integrated chip with high energy efficiency and large capacity were developed. Since the first transistor was successfully prepared in 1947, mankind has gradually entered the "age of microelectronics".
After decades of development, people have summed up an important empirical law, that is, the number of transistors integrated into an integrated circuit chip doubles every 18 months, which is the famous "Moore's Law". With the increase in transistor integration density and the application of more advanced complementary metal-oxide-semiconductor process nodes, the overflight frequency of transistors also increases exponentially.
However, it was not until the application of the 45nm CMOS process node that this growth trend began to stop after 2005. The main reason for this phenomenon is the physical bottleneck limitations of the silicon material itself and the quantum tunneling that occurs at the process node of deep nanometers. In addition to this, low power consumption is the highest priority for individual transistor performance.
However, the inherent limitations of electrical interconnects also lead to an increase in chip heat, which shifts the priority to reducing the heat dissipation of the chip at a reasonable cost. Moreover, as the integration of integrated circuits increases, electrical interconnection will also lead to a series of problems such as crosstalk and latency, which will bring significant challenges to applications such as high-performance computing and high-speed data transmission.
In the face of the physical bottlenecks and technical challenges encountered in the development of microelectronic technology, people have begun to use photons as information carriers to achieve photonic integration loops with larger bandwidth, higher speed, lower power consumption and lower latency. An important goal of the photonic integration loop is to realize the integration of functional devices such as light generation, coupling, modulation, filtering, transmission, and detection.
The initial driving force of the photonic integration loop came from data communication, and then it has been greatly developed in the fields of microwave photonics, quantum information processing, nonlinear optics, sensors, lidar, and so on. Based on the physical properties of different materials, the current photonic integration platforms are: silicon on insulators. The platform relies on mature technology and high-quality ecological environment.
It can realize high-volume, low-cost manufacturing of silicon photonic chips. In terms of material properties, silicon is an indirect bandgap material, and its band gap width is 112EV, not suitable for lasers. However, the waveguide of the system has a large difference in refractive index and strong mode field limiting ability, so it is possible to prepare a compact, low-loss and high-consistency optical waveguide. By way of waveguide doping.
Using the plasma dispersion effect, an electro-optical modulator with a bandwidth of 50 GHz can be fabricated. Silicon materials have wavelengths greater than 1Infrared light of 1 m is transparent, so in the communication band, pure silicon is not suitable for photodetectors. However, the silicon germanium photodetector is compatible in the manufacturing process, the bandwidth can reach more than 67GHz, and the responsivity can also reach 1A W.
Use photons as information carriers
The SOI platform is highly scalable, enabling monolithic integration of electronic and photonic devices through the bipolar complementary metal-oxide-semiconductor process. Due to the small effective area of the mode field and the high optical power density of the silicon waveguide, when the optical power in the waveguide is strong, in addition to causing large two-photon absorption loss, it may also cause irreversible physical damage.
The maximum optical power that can be withstood in silicon waveguides cannot exceed 20 dBm Silicon nitride platform is a common material in the process, commonly used as an insulator and chemical barrier layer, generally prepared by low-pressure chemical vapor deposition at high temperature >700 or by plasma-enhanced chemical vapor deposition at low temperature <400. The difference in refractive index between the waveguide and the oxide cladding is only 38%. Therefore, in terms of propagation loss, it outperforms passive devices in terms of performance. In addition, the two-photon absorption loss of waveguides is also lower than that of waveguides, so they can be used to fabricate microrings with high-quality factors and power distribution networks in high-power application scenarios. The thermo-optical coefficient of a waveguide is about an order of magnitude smaller than that of a waveguide.
It has good thermal sensitivity and can be applied to the design of temperature-insensitive devices, such as multiplexers. Although the platform enables high-performance passives, it lacks monolithically integrated high-speed devices such as modulators and detectors. The fabrication of these high-speed devices relies on integration with other materials.
The silica platform SiO2 planar optical waveguide has low loss (15 2 dB m), large process tolerance, and good matching with single-mode fiber mode field, etc., it is widely used in power dividers of passive optical networks, wavelength division multiplexers of optical transceiver modules, etc. According to the differences in substrate materials, planar optical waveguides are divided into silicon substrates and quartz substrate devices.
Devices made of silicon substrates have excellent heat dissipation performance and are often used to make temperature-sensitive devices such as array waveguide gratings and thermo-optical switches. Devices made of quartz substrates have high consistency and high yield, but have poor heat dissipation performance, and are often used in wavelength-sensitive devices such as beam splitters. Indium phosphide platform.
In terms of material properties, INP is a direct bandgap material that can be used as a gain medium for lasers, and the INP-based photonic integration platform is the most suitable for active photonic integration loops that require optical gain, such as lasers or semiconductor optical amplifiers.
As a fully integrated photonic integration platform, INP has the most complete functions and the best performance. Optical transceiver chips based on the INP platform have achieved large-scale mass production in the field of optical communication.
However, the small wafer size of INP is only 1 4 inches, and the complex epitaxial process results in a very high unit cost. Lithium niobate platform. LN has excellent physical and chemical properties, such as large electro-optical coefficient (33PM V), large optically transparent window, high Curie temperature, etc., which makes the commercialization scale of LN platform very large.
Traditional LN modulators define optical waveguides by using proton exchange or ion diffusion techniques to cause small perturbations in the refractive index of amorphous wafers, but the disadvantages are large spot size, large electrode spacing, and weak electro-optical modulation efficiency. In addition, the bending radius of traditional LN waveguides is large, exceeding 1mm, which is not conducive to large-scale photonic integration.
The LN-on-Insulator platform inherits almost all of the material advantages of traditional lithium niobate and amplifies them with smaller size, greater bandwidth, and lower power consumption. It is expected that in the future, LNOI modulators will achieve both CMOS-level drive voltage (sub-1V) and bandwidth greater than 100GHz.
Less than 0On-chip loss of 5dB, optical power handling in the order of watts, linear electro-optical response, extinction ratio greater than 50dB, and excellent stability are unmatched by SOI and INP platforms. However, lithium niobate is only suitable for modulation devices, and it still needs to be mixed and integrated with other material systems in terms of photonic integration.
Advantages and disadvantages of photonic integration circuits based on different material systems. The III-V compound semiconductor platform, represented by INP, can realize almost all the devices required for photonic integration loops, such as lasers, modulators, detectors, amplifiers, etc. However, INP is a compound wafer, with small wafer size, complex epitaxial process, and low yield.
This results in a very high unit cost. The LN platform has very superior electro-optical modulation characteristics and a high degree of industrialization, but it can only rely on hybrid integration in the preparation of other active and passive components. The Si, Sin, and SiO2 platforms have the best performance in terms of passive components and can complement each other in terms of material properties.
However, the performance of active components such as modulators, the SI platform is not as good as that of the INP and LN platforms. However, the silicon photonics platform is characterized by large wafer size and process compatibility with CMOS, which has a great advantage in reducing costs. Moreover, the silicon photonics platform can use the advanced packaging process of CMOS to realize compact, low-power, and high-bandwidth density optoelectronic integrated chips.
Therefore, under the trade-off of the industry, silicon-based photonics integration is the most cost-effective choice in the fields of high-speed optical transceiver chips and microwave photonics.
The core device of the silicon-based photonic integrated circuit
The core devices of silicon-based photonic integrated circuit are various optoelectronic devices with excellent performance, which are responsible for the tasks of light coupling, routing, beam splitting, modulation, filtering and detection in the photonic integrated circuit.
The combination design and co-optimization between them are the cornerstones of functionalized and differentiated photonic integrated chips. The core components of the silicon-based photonic integrated circuit include: optical waveguide, grating coupler, end face coupler, beam splitter, beam combiner, multiplexer, electro-optical modulator and photodetector. Since silicon is an indirect bandgap material, it cannot be used to make lasers and optical amplifiers.
Optical waveguides, optical waveguides, are commonly used for routing in silicon-based photonic integrated circuits, and the types include strip waveguides, ridged waveguides, and curved waveguides. The optical waveguide is divided into a core layer and an cladding, and at a wavelength of 1550nm, the refractive index of the core layer silicon material is 348。
The refractive index of cladding silica is 145. The high refractive index difference between the two materials makes the size of the silicon waveguide very compact on the one hand, and on the other hand, it also leads to the transmission loss of light in the waveguide is very sensitive to the surface roughness of the waveguide, and the transmission loss of the optical waveguide is mainly related to the lithography process. On the silicon photonic platform, for C-band 1530 1565nm and O-band 1260 1360nm, the bar waveguide for single-mode transmission has a height of 220 nm and a width of 410 500nm, and the waveguide prepared with 193 248nm lithography resolution has a transmission loss of 1 2dB cm. Currently, by high-resolution immersion lithography.
The method of hydrogen thermal annealing can reduce the propagation loss of the optical waveguide to 01db/cm。In photonic chips with high integration density, such as lidar and optical exchange, in addition to straight waveguides, dozens or hundreds of curved waveguides are required for interconnection between devices.
Therefore, the size and loss of the curved waveguide are also very important. In practical applications, the radius of the curved waveguide should not be less than 5 m due to factors such as scattering loss, radiation loss, mode mismatch loss, and material absorption loss.
For example, when the radius is 25 m, the loss of a curved waveguide is 002db 90°, while when the radius is reduced to 1 m, the loss increases to 0367db 90° with beizer bending.
Losses can be further reduced to 0117db/90°。
Grating couplers as well as face couplers. One of the challenges restricting the large-scale application of silicon photonic chips is the efficient coupling between single-mode optical fibers and optical waveguides. The mode field size in an optical waveguide is usually less than 1 m, but the mode field diameter in a single-mode fiber is typically 8 10 m.
The difference between the two mode field sizes results in lower coupling efficiency and larger coupling losses. The grating coupler couples the optical waveguide through the diffraction effect of the grating, which has the advantage of small size and large alignment tolerance, which can be placed anywhere on the chip, which is conducive to wafer-level testing.
The disadvantages are polarization sensitive, wavelength sensitive, and large insertion loss, which is not suitable for photonic integration, and the insertion loss of a typical transverse mode grating coupler is <4dB facet, and the 1dB bandwidth is 40nm. The coupling efficiency of the grating coupler and the energy of the upward diffraction of light in the grating are related to the ratio of the total diffraction energy, that is, the directionality of the grating, the better the directionality, the higher the coupling efficiency of the grating.
Directionality can be increased by adding a metal mirror to the bottom of the grating to increase the reflection of diffracted light from the substrate, or by depositing a layer of polysilicon on the grating structure to excite the Bloch wave of the grating.
Conclusion
An end-face coupler is a horizontal docking coupling scheme where the single-mode fiber and waveguide are on the same level when coupled. The end face coupler generally has a forward wedge, reverse wedge, and trident structure. At present, the commonly used structure is a reverse wedge, close to the single-mode waveguide side, the wedge width gradually increases, and close to the optical fiber side, the wedge width gradually decreases.
As the size of the waveguide decreases, the guide mode becomes less constrained, its effective cross-section increases, and the effective refractive index decreases, allowing light to be coupled from the waveguide to the fiber.
The coupling loss of the end-face coupler is determined by factors such as reflection at the chip interface, mode mismatch from fiber to tip, and mode transition loss along the tapered SOI waveguide. The benefits of face couplers are low coupling losses and large optical bandwidth.