Technology and market analysis of PLC optical splitter

Fiber fusion tapering technology is the most mature technology for making 2´2 optical splitters, optical splitters are mainly based on all-fiber optical splitters, and its main characteristics are: mature technology, convenient connection with fiber, and small insertion loss. However, with the increase of the number of power shunt, such as the optical power shunt more than 1×8 has large volume, low efficiency, high cost, and poor spectral uniformity. In addition, the passband of fiber optic splitter based on melt-cone technology has great limitations.

 

PLC Integrated optical device based on planar technology. Different from the traditional discrete device, it uses a semiconductor process, which can integrate optical components with different functions into a chip, which is the basic process technology to achieve the integration, scale and miniaturization of optoelectronic devices. Compared with fused tapered technology, planar waveguide technology has the characteristics of stable performance, low cost and suitable for large-scale production. Therefore, the optical power splitter will not be used in the optical fiber to the home system in the future, and the planar waveguide provides an effective way for the production of high performance and low cost access network optical devices.

 

3.1 PLC chip technology

 

PLC chips are generally made on six materials, they are: lithium niobate (LiNbO3), the waveguide is by diffusing Ti ions on the lithium niobate crystal into a waveguide, the waveguide structure is diffused type; Ⅲ-Ⅴ semiconductor compounds, the waveguide with InP as the base and lower cladding, InGaAsP as the core layer, InP or InP/ air as the upper cladding, the waveguide structure is buried ridge or ridge shape; SOI(silicon-on-insulator, Silicon on Insulator), the waveguide is made on the SOI substrate, called the bottom, the lower cladding, the core layer and the upper cladding materials are Si, SiO2, Si and air, the waveguide structure is ridge shape and Polymer (Polymer), silica (SiO2), glass ion exchange, etc.

 

At present, there is no problem with fiber to the home (FTTH) network technology, but whether it can be rapidly developed and popularized in China, in addition to the policy side, the most important and the most critical point is to reduce the cost of all aspects of the network. PLC optical splitter is one of the core devices in FTTx network, low cost is its important technical development goal. From the technology, cost and optical waveguide material characteristics given in the above table, it can be seen that silica, polymer and glass are the most suitable for making PLC chips. The following briefly introduces three kinds of PLC chip technology with the lowest cost and the easiest industrialization:

 

(1) Polymer (spin coating - etching)

 

The Polymer waveguide is based on silicon wafer, polymer material with different doping concentration is the core layer, and the waveguide structure is a buried rectangle. Polymer waveguide and device manufacturing process is simple, cheap, if the photosensitive better, lower production cost (theoretical value), very promising development. The problem is the high cost of fluorinated materials; Aging doubts and losses will be relatively high; The stability of the product also needs to consider its impact. At present, only Shanghai NITTA company has optical splitter products made of this chip.

 

(2) Silica

 

The SiO2 waveguide has a silicon wafer as its base, and different doped sio2 materials as its core layer and cladding layer. The waveguide structure is a buried rectangle. Silica-based silicon dioxide light wave technology is a new technology developed in the 1990s, and has been relatively mature abroad. Its manufacturing processes include flame hydrolysis method (FHD), chemical vapor deposition method (PECVD, developed by NEC Company of Japan), plasma CVD method (developed by Lucent Company of the United States), porous silicon oxidation method and Sol-gel. This waveguide and loss is very small, about 0.05dB/cm below. The AWG with 60 and 132 channels has been developed abroad by using this kind of waveguide. At present, flame hydrolysis (FHD) and chemical vapor deposition (PECVD) are used to grow multi-layer silica materials, and dry etching technology is used to complete waveguide etching. Its advantages are that it has very good physical and chemical stability technology, high device integration and low cost. At the same time, it has good compatibility with optical fiber, low transmission loss, mature process (mainly dependent on the import of equipment), stable and reliable products, and can theoretically make AWG and other PLC devices. At present, this process technology is the mainstream technology of chip product manufacturing, and it is widely used in the world. The problems are high equipment investment, high maintenance costs, and high raw material requirements (all imported materials); There are only a few scientific research institutes and universities online experimental equipment and Wuhan Guangxun technology silica PLC process line, there is no industrial scale production equipment. This technology and manufacturing are basically monopolized by foreign manufacturers such as South Korea and Japan.

 

(3) Glass base (ion exchange)

 

The glass waveguide is formed by diffusing Ag ions on the glass material, and the waveguide structure is diffused type. The advantage is that the process is relatively simple, the equipment is relatively simple, the total investment is not large, and the product is more stable and reliable. The manufacturing process of glass optical waveguide is divided into five steps: 1) a layer of aluminum is sputtered on the glass substrate, which is used as the mask layer during ion exchange; 2) For photolithography, the required waveguide pattern is protected with photoresist; 3) Use chemical corrosion to remove the aluminum film on the upper part of the waveguide; 4) Put the glass substrate of the mask into the mixed solution containing Ag+-Na+ ions, and carry out ion exchange at the appropriate temperature. Ag+ ions improve the refractive index to obtain a channel-based optical waveguide; 5) An electric field is applied to the channel-type optical waveguide to drive Ag+ ions deep into the glass substrate, and a buried glass optical waveguide is obtained. The main problems of this technology: ① Will it become the mainstream technology in the future, some experts have doubts at present? Because there is no large-scale commercialization and industrial production, the actual process stability of the product needs to be verified.

 

The above technology process is currently only produced by French Teemphotonics company and Israeli Colorchip company, it is said that the original E-TeK company has this technology, but the specific details are unknown and cannot be verified. The research group of Professor Wang Minghua of the Department of Information and Telecommunications of Zhejiang University in China began to cooperate with communication enterprises to develop splitters based on glass ion-exchange optical waveguides several years ago, and achieved certain results. Their advantage is that the key technology is fully mature, all the raw materials do not need to be imported, the domestic can fully meet, and developed a performance index to reach the level of similar foreign products, and intellectual property rights are all their own. At present, commercial and industrial technology has not fully passed, but also to conduct intermediate tests, and further improve the technical parameters. Therefore, from the perspective of domestic industrialization, PLC chip manufacturing technology has a considerable gap with foreign countries, and the technology practical and industrialization process still needs a long way to go.

 

3.2 PLC optical fiber array technology

 

The output end of the PLC splitter is coupled with each output optical waveguide in the PLC by the array fiber band (ribbon). Each fiber in the fiber strip is positioned in a V-shaped slot to ensure that all optical waveguides can be automatically aligned with the fiber strip at one time. V-groove substrate can be made of monocrystalline silicon sheet by selective wet etching process, or can be made of quartz glass plate by precision machining.

 

Because the Fiber Array is made of V-shaped slots, a special bonding process is used to achieve precise fiber positioning and high reliability to meet different needs. The package design with matched thermal expansion coefficient guarantees the fiber array plate stress-free, high reliability and no fiber displacement at high temperatures. Therefore, high-precision V-groove and high-reliability UV glue are the key technologies in the production of optical fiber arrays.

 

The matrix material of the optical fiber array product is quartz glass and heat-resistant glass, but the quartz glass plate is generally made by precision machining, and the quartz glass is better from the point of view of reliability, and it is not easy to crack when grinding. The end polishing Angle can also be customized according to customer requirements, such as: 90 degrees, 98 degrees, 82 degrees, etc., the color and length of the fiber arrangement can also be customized according to customer needs.

 

As the optical fiber array is a labor-intensive product, many foreign manufacturers will transfer the production link to China, the current domestic can research and development and production of enterprises are Bochuang Technology, Fuchuang Optoelectronics, Okang Optoelectronic devices, Dongguan Dongyuan and Zhejiang Tongxing, etc., foreign countries are mainly Hataken Japan and AIDI companies. In terms of technology research and development and independent innovation, the development of domestic fiber array technology has also made breakthroughs, such as: high-precision U-shaped slot, the use of independent intellectual property rights of the etching manufacturing process, this technology but a breakthrough and into practical, will greatly reduce the cost of fiber array. There are also square capillary arrays, which have good characteristics in AWG and single channel arrays.

 

High reliability UV glue is another key technology in the fabrication of optical fiber arrays. The optical splitter manufacturing process has very high requirements for optical fiber array, in addition to requiring V-shaped groove with high precision, but also requires UV glue should have high temperature and humidity resistance and sufficient hardness and other characteristics. At present, the series of composite products developed and produced by NTT-AT company in Japan are the most advanced in technology, and there is still a blank in this technology in China.

 

3.3 PLC coupling and packaging technology

 

In addition to the chip and optical fiber array, another key technology is the coupling and packaging between the chip and the optical fiber, which involves the six-dimensional tight alignment of the optical fiber array and the optical waveguide.

 

The packaging of PLC splitter refers to the technology of aligning each light guide path (that is, waveguide path) on the planar waveguide splitter with the fiber in the fiber array one by one, and then bonding it together with a specific adhesive (such as epoxy adhesive). The alignment accuracy of PLC splitter and optical fiber array is the key of this technology. The packaging process includes coupling alignment and bonding. The coupling alignment of the PLC splitter chip and the optical fiber array has two kinds of manual and automatic, they rely on the hardware mainly includes six-dimensional precision trimmer, light source, power meter, microscopic observation system, etc., and the most commonly used is the automatic alignment, which forms a closed-loop control through the optical power feedback, so the docking accuracy and coupling efficiency of the docking are high. At present, the most advanced coupling alignment equipment suppliers in foreign countries are: Suruga precision machine (Suruga), Kuge (Kuge), Newport and so on; There are also domestic development, but the six-dimensional accuracy can not meet the requirements of coupling.

 

With the large increase in FTTH demand for optical shillers, the domestic PLC device packaging industry has developed rapidly, and the key representative enterprises with scale and research and development strength are Bochuang Technology, Fuchuang Optoelectronics, light fast technology, Rihai Communication, Wuxi Aiwofu, Fuchunjiang Optoelectronics, Chengdu Feiyang, Datang Communication (Kunshan), Zhongshan Aokang AgileCom, Shanghai Shangcheng, etc. There are more optical device companies and some optical fiber and cable companies have been on and are on the PLC optical splitter package project. Because the packaging technology of the optical splitter is relatively simple and the investment is not large (about 200,000 yuan for a manual alignment system), the project is easy to complete. At present, there are many high value-added PLC device products, packaging technology has special process technology and proprietary technology, domestic packaging manufacturers also need to increase research and development and investment in this area.