What are the PLC (optical splitter) technologies and manufacturing processes?

PLC is more widely known in the field of electronic technology, it is the abbreviation of Programmable Logic Controller (Programmable Logic Controller). In the field of optical communication technology, PLC is the abbreviation of Planar Lightwave Circuit. It is a variety of optical waveguide structures prepared based on integrated optical technology. Technically, the functional devices that can be realized include directional coupler DC, Y branch Multimode interference coupler MMI, arrayed waveguide grating AWG, optical comb filter ITL, Mach-Zender MZ electro-optic modulator, thermo-optic adjustable attenuator TO-VOA, thermo-optic switch TO-SW, etc.

In the optical communication industry, widely used PLC devices mainly include optical splitters, AWGs, MZ electro-optic modulators, TO-VOA, etc., among which optical splitters are optical splitters based on Y branches connected in series and parallel, such as a 1 An optical splitter with ×16 ports requires 15 Y splitters. AWG is a 1×N port device that can split dozens of input wavelengths into different output ports. The MZ modulator based on lithium niobate optical waveguide is currently the most mainstream modulator solution; while silicon optical modulator technology has matured and has become the first choice for high-speed modulators above 50G. The combination of TO-VOA and AWG based on PLC technology constitutes a wavelength division multiplexer/demultiplexer VMUX module with channel equalization function.

There are many optical communication devices that are widely used based on PLC technology, but in the industry, PLC usually refers to an optical splitter, which is an optical passive device that is most widely used in FTTH networks. After the Internet bubble in 2000, the optical communication industry entered a period of depression; around 2004, before the application scenario appeared, Japan took the lead in investing in FTTH as infrastructure; after 2008, with China’s accession, FTTH Construction peaked around 2012. FTTH usually adopts passive optical network PON, and its core is PLC optical splitter, which is widely deployed in various commercial buildings and residences. In life experience, the closest to us is the “cat” tail of the home modem, which is upgraded from the early twisted pair to the current fiber jumper, which is a port from the PLC optical splitter. It can support a network speed of 100-200M, which is much higher than the 4M transmission rate that cables can support.

Optical Splitter (PLC) Manufacturing Process Encyclopedia

In the field of optical communication, PLC is the abbreviation of planar optical path, which is based on various optical waveguide structures prepared by integrated optical technology to realize certain functional devices. There are four main fabrication techniques for optical waveguides: ion exchange, ion implantation, chemical vapor deposition and flame hydrolysis.

1) Ion exchange

The principle of the ion exchange process is to soak the glass material containing A+ ions in the solution containing B+ ions, and use the property that the ions will diffuse from the high-concentration area to the low-concentration area to replace the A+ ions in the glass with the B+ ions in the solution. swap out. Since the glass material containing A+ ions has a higher refractive index than the glass material containing B+ ions, a high refractive index is obtained in the region where the ion exchange occurs, which acts as the core layer of the optical waveguide, and the region where the ion exchange does not occur acts as the core layer of the optical waveguide. cladding, to obtain the desired optical waveguide structure.

The general process flow of optical waveguide preparation by ion exchange is shown in Figure 1:

1) Cover a mask layer on the glass substrate by evaporation or sputtering process;

2) Open a waveguide structure window in the mask layer through photolithography and etching process;

3) Soak the glass material with the prepared mask layer and open the window in the solution for ion exchange;

4) Driven by an electric field, the exchanged ions distributed on the surface are driven to a certain depth to form a waveguide structure.

In the actual process, in order to better ensure the ion exchange effect, the above two steps 3-4 need to be carried out at the same time, which depends on the specific process design.

figure 1. Process Flow of Optical Waveguide Prepared by Ion Exchange

In order to improve the ion exchange efficiency and obtain good optical waveguide characteristics, it is necessary to properly select the two kinds of ions for mutual exchange, optimize the glass formulation, control the concentration and temperature of the solution, and properly apply the electric field.

2) Ion implantation

Ion implantation is a material surface modification technology, which belongs to a standard processing technology in the semiconductor industry. Ion implantation into the optical waveguide is to accelerate the ions to a high energy of tens of thousands to hundreds of thousands of electron volts through an ion accelerator, bombard the surface of the substrate material, and cause damage or defects on the surface of the material through the interaction between atoms or molecules, changing Refractive index, forming an optical waveguide structure.

A typical ion implantation fabrication process for optical waveguides is shown in Figure 2. An ion implanter usually consists of an ion source, ion extraction and pre-acceleration, a magnetic analyzer, a back-end accelerator, an electronic scanning system, an ion implantation chamber, and a vacuum system. In the cavity of the ion source, the ions generated by gas discharge are exported by the electrodes in the ion extractor and pre-accelerated; the magnetic analyzer controls the quality of the ion beam to obtain a better directional ion beam; it is accelerated through the back channel The final ion beam, under the control of an electronic deflector, is injected into the sample in the cavity.

figure 2. Ion Implantation Fabrication Process of Optical Waveguide

The substrate material placed in the ion implantation cavity needs to be pretreated, and the mask layer is prepared according to the optical waveguide pattern. After the ion implantation, post-processing is required, such as annealing process, to reduce the impact of material defects caused by implantation on loss .

3) Chemical vapor deposition

The chemical vapor deposition CVD process is also a standard process in the semiconductor industry. The process of preparing optical waveguides by CVD process is shown in Figure 3. It is to successively deposit optical fibers with different doped layers on a silicon substrate (or quartz substrate). The waveguide layer, such as the core layer, increases the refractive index by doping phosphorus and boron, and the cladding layer reduces the refractive index by doping germanium. After depositing the core layer and before depositing the upper cladding layer, a mask layer needs to be prepared by a photolithography process to define an optical waveguide pattern. After each layer is deposited, an annealing hardening process is required to enhance the density and uniformity of the deposited layer and reduce stress.

image 3. Process flow of optical waveguide prepared by chemical vapor deposition

4) flame hydrolysis method

The process flow of optical waveguide prepared by flame hydrolysis FHD is similar to that of chemical vapor deposition CVD, the difference is only in the process conditions for forming thin film layers. CVD is to pass various simple substances and compounds containing film elements into the cavity, and chemical reactions occur at a certain temperature, thereby depositing the required film layer on the surface of the substrate. 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 the gas burner, and chemically react with water in a high-temperature flame , to generate a silicon dioxide film layer doped with various impurity elements.

5) Process comparison

Ion exchange and ion implantation processes can produce low-cost optical waveguides, but the control of the cross-sectional shape of the waveguides is slightly poor. They are mainly used to make optical splitters, and the production efficiency of ion implantation processes is much higher than that of ion exchange. CVD and FHD have much better control over the cross-sectional shape of the waveguide, and can be used to make high-end optical waveguide devices such as arrayed waveguide gratings AWG, among which FHD is more conducive to the preparation of thick films than CVD.