PLC is more known in the field of electronic technology. It is the acronym of the terminology ‘programmable logic controller’. However, in the field of optical communication, PLC is the acronym of another terminology, i.e. ‘planar lightwave circuit’, which is variable optical waveguide structures fabricated with integrated optics technologies. PLC technologies can implement functional devices as directional coupler (DC), Y-branch splitter, multimode interferometer (MMI), arrayed waveguide grating (AWG), optical interleaver (ITL), Mach-Zehnder (MZ) electro-optical modulator, thermo-optical variable optical attenuator (TO-VOA), thermo-optical switch (TO-SW), etc.
In the optical communication industry, the widely deployed PLC devices include optical power splitter, AWG, MZ electro-optical modulator and TO-VOA, etc. optical power splitter is a device consisting of Y-branch splitters in cascade and parallel connection. For example, a 1×16 power needs 15 Y-branch splitters. AWG is a device with 1×N ports, which separates tens of wavelengths from the input to each output. MZ modulator based on lithium niobate optical waveguide is the main approach for optical modulators. Meanwhile, modulators based on silicon optics are commercialized and become the first choice for high-speed modulators >50G. TO-VOA based on PLC technology is deployed in combination with AWG, which compose a VMUX module with functions of multiplexing/demultiplexing and dynamic channel equalization.
Optical communication devices based on PLC technology and widely deployed are variable. However, PLC is usually specified as optical power splitters in the industry, which is a most widely deployed passive optical device in the FTTH network. After the internet bubble in 2000, optical communication industry entered a period of depression. Then in 2004, Japan first invested on FTTH construction as a new infrastructure, although the application scenario of FTTH had not emerged yet. In 2008, China added in the construction of FTTH and reached the peak in 2012. FTTH network usually adopts PON structure. The key device is optical power splitter, which is widely deployed in the business and residential buildings. In our lives, the most approach to an optical power splitter is the modem for internet access. The pigtail of the modem was upgraded from twist-pair electrical wires to an optical fiber jumper, which is led to one of the ports of an optical power splitter. The optical fiber access can support transmission speed of 100-200M, which is much higher than 4M through electrical wires.
In the field of optical communication, PLC is the abbreviation of plane optical path, which is based on the integrated optical technology to prepare various optical waveguide structures, in order to achieve some functional devices. There are four kinds of fabrication processes for optical waveguides: ion exchange, ion implantation, chemical vapor deposition and flame hydrolysis.
1) Ion exchange
The principle of ion exchange process is to soak the glass material containing A+ ions in the solution containing B+ ions, and exchange the A+ ions in the glass with the B+ ions in the solution by using the property that the ions will diffuse from the high concentration area to the low concentration area. Because the glass material containing A+ ions has higher refractive index than the glass material containing B+ ions, high refractive index can be obtained in the region where ion exchange occurs, which can be used as the core layer of the optical waveguide, and the region where ion exchange does not occur can be used as the cladding layer of the optical waveguide to obtain the required optical waveguide structure.
The general process flow of preparing optical waveguide by ion exchange is shown in Figure 1:
1) A mask layer is covered on the glass substrate by evaporation or sputtering process;
2) Through photolithography and etching process, a window of waveguide structure is opened in the mask layer;
3) The glass material with the mask layer prepared and the window opened is immersed in the solution for ion exchange;
4) Through the electric field driving, the exchange ions distributed in the surface layer are driven to a certain depth to form the waveguide structure.
In the actual process, in order to better ensure the ion exchange effect, the above 3-4 steps need to be carried out at the same time, which depends on the specific process design.
In order to improve the ion exchange efficiency and obtain good optical waveguide characteristics, it is necessary to select two kinds of exchanged ions, optimize the glass formula, control the concentration and temperature of the solution, and apply the electric field appropriately.
2) Ion implantation
Ion implantation is a kind of material surface modification technology, which belongs to a standard processing technology in semiconductor industry. Ion implantation optical waveguide is to accelerate ions to tens of thousands to hundreds of thousands of electron volts of high energy by ion accelerator, bombard the surface of substrate materials, cause damage or defects on the surface of materials through the interaction between atoms or molecules, change the refractive index, and form optical waveguide structure.
The typical process of preparing optical waveguide by ion implantation is shown in Fig.2. The ion implanter is usually composed of ion source, ion extraction and pre acceleration, magnetic analyzer, back channel accelerator, electron scanning system, ion implantation cavity and vacuum system. In the cavity of the ion source, ions are generated by gas discharge. The electrode in the ion extractor is led out and pre accelerated. The magnetic analyzer controls the quality of the ion beam and obtains the ion beam with good directivity. The ion beam accelerated by the back channel is injected into the sample in the cavity under the control of the electron deflector.
The substrate material placed in the ion implantation cavity needs to be pretreated. The mask layer is prepared according to the optical waveguide pattern. After ion implantation, post-processing, such as annealing, is also needed to reduce the influence of material defects on the loss.
3) Chemical vapor deposition
Chemical vapor deposition (CVD) process is also a standard process in the semiconductor industry. The process of preparing optical waveguide by CVD process is shown in Fig.3. It is to successively deposit optical waveguide layers with different doping layers on silicon substrate (or quartz substrate). For example, the core layer is doped with phosphorus and boron to improve the refractive index, and the cladding layer is doped with germanium to reduce the refractive index. After the core layer is deposited and before the upper cladding layer is deposited, the mask layer needs to be prepared by photolithography to define the optical waveguide pattern. After each layer is deposited, annealing hardening process is needed to enhance the density and uniformity of the deposited layer and reduce the stress.
4) Flame hydrolysis deposition
The fabrication process of optical waveguide by Flame hydrolysis deposition (FHD) is similar to that by CVD, but the difference is only in the process conditions. CVD is a process in which various elements and compounds containing film elements are introduced into the cavity, and chemical reaction occurs at a certain temperature, so as to deposit the required film on the substrate surface. FHD is to pass volatile halides containing film elements, such as silicon tetrachloride, and halides containing various doping elements, such as phosphorus, boron and germanium, into a gas burner, and react with water in a high temperature flame to form a silicon dioxide film layer doped with various impurity elements.
5) Process comparison
Ion exchange and ion implantation can produce low-cost optical waveguides, but the control of the cross-section shape of the waveguides is slightly poor. They are mainly used to make optical splitters. The production efficiency of ion implantation is much higher than that of ion exchange. CVD and FHD can be used to fabricate high-end optical waveguide devices, such as arrayed waveguide grating (AWG). Among them, FHD is more suitable for thick film fabrication than CVD.
Written by Zhujun Wan,Jianwei Feng HYC Co., Ltd