The Challenges and Opportunities of Silicon Photonics in the Application of Optical Modules
The status of silicon photonic technology in the application of optical module
With the maturity of waveguide theory and the production of a series of new design devices, the industry has proposed silicon photonic technology based on CMOS manufacturing processes. Silicon photonics utilizes the very mature silicon wafer processing technology of the traditional semiconductor industry. It can process large-scale waveguide devices on the silicon substrate by etching process. By using processes such as epitaxial growth, it is possible to prepare key devices such as modulators and receivers, and finally achieve high integration of modulators, receivers, and passive optical devices.
In addition to the laser, the silicon photonic technology can realize the integrated manufacturing of various devices in the optical module, covering most components inside the optical module, but does not include the laser chip.
Since silicon is an indirect band gap, the conduction band minimum (conducting band bottom) and the full band maximum are in different positions in k space. The electronic transition needs to change the potential energy and kinetic energy at the same time. The laser needs to be phonon-based, and the recombination of hole efficiency is very low and the luminous efficiency is extremely low.
At present, chip-level devices that have been matured based on silicon photonic processes mainly include optical waveguides, multiplexer devices, external modulation devices, APD receivers, etc.
However, the design and process routes of major manufacturers still have great differences, and there are many technical routes. From this perspective, it can also be seen that silicon photonic technology is still in the early stage of development, and the solution with the highest cost performance and technical stability has not yet been stand out from the crowd, silicon photonic technology still needs a period of precipitation and development, in order to focus on the final winning mainstream technology, and then to play a larger role in the scale effect of CMOS process, cost and yield can be continuously optimized.
At present, there are the three key points of silicon photonic technology as follows.
1. Silicon waveguide
The waveguide uses a different refractive index to form total reflection, which constrains the transmission path of the optical fiber. The refractive index of silicon is very large, the absolute refractive index is above 3.4, and the refractive index of silica material is about 1.5. The current common waveguide processing technology is SiOx/SiON overcladding or laser direct writing, by laser focusing, melts and changes the refractive index inside the silica.
The silicon waveguide is an important passive structure in the silicon optical module. The transmission control and coupling of the optical path are completed by the waveguide. The waveguide technology is one of the core technologies in the silicon photonic technologies, and a large number of passive components (splits, joints waves, ring resonators, etc.) can be implemented on-chip by silicon waveguide technology.
However, since the refractive index of silicon is too high, and the waveguide size is small, the spot size in the waveguide is small, and the optical module is finally connected to the optical fiber (In fact, the core layer of the optical fiber), and the mismatch between the silicon waveguide and the optical fiber mode field results in a high insertion loss. The mode field diameter (MFD) of the silicon waveguide is about 0.4–0.5μm, while the mode field diameter of the single-mode fiber is 9–10μm, and the coupling loss is extremely large (about -20 dB).
2. Silicon-based modulator
The optical modules modulation, there are internal modulation and external modulation.
The internal modulation applies different voltages to the laser chip through the driving chip, and adjusts the luminous power to realize optical signal modulation. As the transmission rate continues to increase, the internal modulation faces performance bottlenecks. At high rates, the extinction ratio and the chirp of the internal modulation optical chip are difficult to meet the requirements. Due to the dispersion of the optical fiber, the error rate of the transmission system is too high. The development of modulation is gradually accelerating. In the external modulation, the laser continues to emit photonic, and the intensity of the output optical signal is output through the modulator’s On-Off modulation.
The silicon material has strong electro-optical effect and thermo-optic effect. After applying external energy, the phase change can be realized, and the phase difference between the two beams can be controlled to realize the Mach-Zehnder interference (MZI) to realize the intensity modulation of the output optical signal. Therefore, based on silicon photonic technology, it is very suitable for the preparation of highly integrated MZI external modulators.
3. Silicon germanium detector
Matching with the transmitting end, the optical module also needs a receiving detector that receives the optical signal. Since Si has a band gap of 1.12eV and an absorption cutoff wavelength of 1.1μm, the silicon material cannot be absorbed for the communication band (1.31μm to 1.55μm, which mainly matches the low loss wavelength window of the fiber). For the detectors applied to the silicon-based receiving end, other materials must be used. The materials that can absorb the optical communication working band are mainly InP, InGaAs and Ge.
The absorption coefficients of the germanium materials at both the 1310 nm and 1550 nm wavelengths are Larger, commonly used receiver detectors are typically fabricated from this type of material.
The challenges of silicon photonics in the application of optical modules
The package based on silicon optical chips still difficult, and the process and yield still need to be improved.
The silicon optical chip technology is relatively mature, but there are still many technical difficulties in the packaging process from chip to optical module. The packaging yield and cost still need to be optimized.
1. Fiber coupling
In the two technical routes, the design of the end-coupled mode-field conversion structure needs to be improved, and the isolator is required to reduce the echo interference. The fixed processing of the fiber array in the V-groove is difficult to automate, and the overall processing cost is high. The grating coupling method has a low bandwidth and is sensitive to polarization. It needs to control the polarization square, and requires a proper fixing glue to provide sufficient strength. At the same time, the fiber is vertically led out, and the module with a standard shape needs to reduce the bending loss.
2. Laser integration
The placement accuracy of the placement is extremely high, the chip sealing process is complicated, and the yield is low.
3. Thermal management
The silicon optical chip is sensitive to temperature. If the grating structure is used, the temperature influence is more significant. More stringent simulation design and structural optimization are needed, and power consumption control needs further optimization.
The opportunities of silicon photonics in the application of optical modules
Currently, silicon optical module products mainly include short-distance data center optical modules and medium-long-distance telecom coherent optical modules.
1. Data center optical module
Due to the ideal working environment of constant temperature and humidity in the data center, optical module packaging requirements are relatively low. In the data center optical module market, due to the large demand within 2 km, coupled with the strong requirements of low cost, high speed, high density, etc., it is more suitable for a large number of applications of silicon light.
2. Medium-long-distance coherent optical module
The coherent optical modules are used for DCI connection in data centers or telecom metropolitan area backbone network long-distance communication, coherent modulation adds phase modulation in addition to amplitude modulation compared to traditional MZI external modulation. Currently mainstream technologies include 8QAM and 16QAM, and 64QAM is also under research and development.
Coherent modulation requires dividing the source signal into two channels of the same frequency with a phase difference of 90°, and then separately modulating the two channels of photonic to form interference fringes. After the 50/50 split directional coupler at the receiving end and the intrinsic source, the original signal at the transmitting end can be derived to eliminate noise in the transmission link. The highly integrated waveguide technology of silicon photonic and the phase modulation capability are very suitable for the preparation of coherent modulation devices. However, the coherent optical modules based on DSP processing technology of high-speed digital signal processing technology often encounter large power consumption problems.
In the research field of coherent optical communication, Gigalight independently developed a 100G CFP-DCO coherent optical transceiver. It customized R&D for metropolitan area network applications for data center interconnection. In the application scenario below 100km, the low-power silicon optical device and driver, optimized DSP algorithm, the overall power consumption is lower than 24W, according to the link environment one-key selection to meet ITU-T G.692 100km and IEEE (More than 10km), fully embodying the characteristics of usability, flexibility, low power consumption and low latency, representing the future development direction of high-speed optical communication devices.
At the same time, Gigalight can provide customized system solutions according to customer application scenarios.
In the next few years, Gigalight engineers will adopt new silicon photonic technology under the existing conditions to develop a lower power coherent optical communication module for 200G or even 400G optical transmission network for the construction of a new generation of optical communication networks.
