Traditional optical modules mostly use InP materials. In order to reduce the cost of product upgrade iterations, silicon-based optoelectronic products must be compatible with InP optical modules.
However, due to the characteristics of the silicon material itself, such as the fact that silicon-based optoelectronic chips cannot emit light, there is no high-efficiency first-order electro-optic effect, and the effective refractive index of silicon waveguides is sensitive to temperature, etc., these limit the application scenarios of silicon-based optoelectronic modules. To expand the application range and market size of silicon-based photonics technology, these performance challenges must be faced.
Silicon-based optoelectronic technology has the advantage of high integration. At the same time, higher integration also puts forward higher requirements for packaging technology. The packaging of silicon-based optoelectronic chips requires high precision and high technical difficulties.
At this stage, the packaging cost of silicon-based optoelectronic chips even accounts for the total cost of silicon-based optoelectronic modules. About 10% of the total. The development of silicon-based optoelectronic chip packaging technology with low cost and high reliability is one of the challenges facing the large-scale industrialization of silicon-based optoelectronics.
In the R&D and manufacturing of silicon-based optoelectronic chips, the current design of silicon-based optoelectronic chips mostly uses manual layout and wiring. The accuracy from schematic diagram to layout depends on the designer’s design ability, which is prone to errors. Special optical waveguide routing requirements can be time-consuming compared to electrical leads.
With the development of silicon-based optoelectronic technology, the system-on-chip is becoming more and more complex, especially in the single-chip integration application scenario where silicon-based optoelectronic chips and complementary metal-oxide-semiconductor (CMOS) chips are integrated into the same chip, and manual layout and layout are performed. – Schematic (LVS) verification is becoming less and less feasible, and the need for automation software for silicon-based optoelectronic chips is becoming more and more urgent.
To sum up, the main challenges in the development of silicon-based optoelectronics focus on issues such as device performance, packaging, and automated design.
Some device technical problems
On-chip light source
Although academia has adopted various methods to try to realize silicon-based light sources that can be monolithically integrated [51-52], most of the current silicon-based optoelectronic technology industry still adopts hybrid integration solutions. The laser chip is made of InP material and is independently manufactured with the silicon-based optoelectronic chip on their own platforms, and then mounted.
The light source on the silicon substrate has not been really solved, and it has become one of the bottlenecks restricting the development of silicon-based optoelectronics. The indirect bandgap characteristics of silicon bring great difficulties to the realization of high-efficiency silicon-based light sources. The practical on-chip silicon-based laser is A long-term goal of academia and industry.
Modulator
Silicon-based optoelectronic chips used in the field of optical communication require silicon-based optoelectronic modulators with small sizes, low power consumption, and large bandwidth. Due to its insensitivity to temperature, the Mach-Zehnder modulator (MZM) is generally the first choice in silicon-based optoelectronic chips for communication, as shown in Figure 4.
At this stage, the bandwidth of MZM has been greatly improved, and a broadband MZM with a bandwidth of up to 55 GHz has been realized, but further development is still needed in terms of size and power consumption.
MZM is a larger unit device on a silicon-based optoelectronic communication chip, and its size is generally at the millimeter level. Large unit device size reduces wafer yield and increases microwave signal loss. The development of small-sized silicon-based electro-optic modulators can not only reduce the cost but also improve the modulator performance.
Since silicon materials have no first-order electro-optic effect, silicon-based electro-optic modulators generally realize electro-optic modulation based on the plasmonic dispersion effect. The modulation efficiency of the plasmonic dispersion effect is low, and the power consumption required for electro-optic modulation is relatively large; at the same time, in order to obtain higher bandwidth, the high-speed silicon-based MZM usually uses a depletion traveling wave electrode structure for modulation.
The DC bias voltage required by the depletion modulation method will form a DC loop with the matching resistance of the traveling wave electrode terminal, and this DC power consumption will further increase the power consumption of the high-speed silicon-based MZM.
In order to reduce the loss of high-speed modulators, researchers have taken many measures, such as using modulators with special doped region structures, using segmented lumped electrodes instead of traveling wave electrodes, reducing the loss of microwave signals through hollow substrates, and using The energy consumption problem of silicon-based optoelectronic chips has been alleviated to a certain extent by using CMOS circuits for monolithic integration and advanced packaging technology to reduce losses.
WDM
Wavelength division multiplexing devices usually require that the waveguide has a relatively stable effective refractive index, so as to stably realize the splitting/combining of optical signals of different wavelengths.
Silicon is a material with a thermo-optic effect, and its thermo-optic coefficient is about 1.85×10−4. Ordinary silicon-based optoelectronic wavelength division multiplexing devices need additional control means to stabilize the ambient temperature when the device works, otherwise serious work will occur.
Wavelength drift or channel crosstalk, but temperature control equipment will increase the overall power consumption of the optical transceivers. Temperature-insensitive wavelength division multiplexing devices are key devices to realizing robust wavelength division multiplexing.
The industry has adopted various methods to realize silicon-based integrated wavelength division multiplexing devices, such as introducing reflective echelle grating structures into silicon-based optoelectronic chips In the middle, the cascaded Mach-Zehnder interferometer (MZI) to achieve stable working wavelength through special design, etc.
At present, silicon-based optoelectronic chips tend to use cascaded MZI solutions to realize thermally stable wavelength division multiplexing devices, but wavelength division multiplexing devices based on cascaded MZIs are difficult to comply with local area network wavelength division multiplexing (LWDM) and medium wavelength division multiplexing. (MWDM) and other channel intervals are relatively dense wavelength division multiplexing scheme standard requirements. Silicon-based integrated temperature-insensitive WDM devices still need breakthroughs.