PLC (Planar Lightwave Circuit)

The 4 th blog is featured for PLC (Planar Lightwave Circuit). NTT Electronics (NEL) has produced PLC chip and modules products that support optical communication networks. In this blog, we will give an overview of our PLC technology then will introduce the current R&D activities in our PLC development team.

Overview of PLC technology

Everyone who is looking at this page is, of course, using the Internet to display this page. So how is the Internet realized?
Figure 1 shows network architecture of the Internet. As the name implies, the Internet is a collection of networks, but it is roughly divided into three layers of networks called core, metro and access. The core network is an intercity / interregional network that exceeds 100 km, the metro network is an intranet network that is less than 100 km, and the access network is several kilometers to several tens of kilometers from your home to the telecommunications carrier building.

Figure 1

At each node in optical communication network, a variety of handling optical signal is required. To consider handling very high transmission speed signals and lower power consumption in a node, it is advantageous to handle optical signals at the optical layer.
For example ...

-In the core / metro network, the wavelength division multiplexing (WDM) transmission method^* are used to handle multiple communication pathes over network. To receive a signal, the mechanism of selecting a wavelength is required. Also, to send a signal, we aggregate several different wavelength path into a single physical network. In the access network, since so many endpoints should be connected efficiently. A typical method is that one station-side device processes communication with a plurality of (up to 32,) users in a time-division manner, an optical splitter that splits an optical signal into 1: N is required.

* A method of multiplex transmission of multiple wavelength channels with a single optical fiber.

PLC (Planer Lightwave Circuit) is one of key devices to realize the Internet. PLC implement pathes for optical communication on silicon or quartz substrate. A path is so called "Optical waveguide" that is composed by core that has a higher refractive index than the surroundings on the substrate. These structure is fabricated using the microfabrication technology used in LSI manufacturing process such as photolithography and dry etching. Pathes are merged / splitted. Once you select a wavelength (a color), you can choose one of possible pathes in a PLC device. This is purely optical and very low power for selecting a path.

PLCs are used in various applications as key components of optical networks because they can be implemented in small factor form and have high reliability.

Figure 2 shows typical PLC components used in optical communication networks. AWG is used as an optical duplexer for WDM in core / metro networks, and optical splitters are widely used in access networks.
In addition to AWG and splitters, there are various components using PLCs such as DPOH (Dual Polarization Optical Hybrid) and MCS (Multi Cast Switch) depending on the purpose for use. THese products are used in the world to implement optical communication systems.

Figure 2

*  DPOH : single PBS , single polarization rotator and two 90-degree optical hybrid are integrated into one PLC chip , and used for the optical receiver front-end modules. Highly accurate and stable optical phase / interference control can be realized, and a digital coherent receiver can be configured by combining with DSP.

*  MCS : Optical switch to realize CDC (Colorless, Directionless and Contentionless) function that improves the operability of multi-degree ROADM (Reconfigurable Optical Add / Drop Multiplexer).

History of PLC

NEL's PLC activities has a deep connection with NTT Laboratories. PLC is originally has been studied in the NTT laboratory. For example, 20 years ago, the AWG had the function of demultiplexing 16ch signals at 100GHz spacing, and the size was about 35mm x 35mm. In order to reduce the size of the chip, it is necessary to increase the refractive index of the core film deposited on the silicon substrate and control it with high precision. NTT / NEL has advanced the precision of its manufacturing technology. Currently we can realize 50GHz 96channels AWG with 20% reduced chip size compared with 20 years ago. This miniaturization contributed to more than 15 times higher transmission efficiency. In this way, NEL can contribute to the development of communication systems around the world by developing products using the cutting-edge technology cultivated at NTT Laboratories and sending them out to the world.

Recently we are exploiting other possible area of utilizing PLC technology as follows :

• PLC for visible light: PLC for visible light for around 0.5µm wavelength band. It is expected to be applied to smart glasses and so on.
• PLC for LiDAR ( Light Detection and Ranging ): LiDAR is an object detection device that uses light. It is expected to be installed in self-driving cars, but it is still expensive at present. Price reduction by PLC is expected.
• PLC for quantum cryptography: Quantum cryptography is a new encryption system that breaks away from classical cryptography. No matter how much computational resources the attacker has, it will never be decrypted. In order to realize a quantum cryptography system, it is necessary to send and receive a single photon, and noise reduction by a PLC optical interferometer is expected.

In addition, we improve the efficiency of our manufacturing system of PLC.

• automatically and highly precision of the measurement: There is a limit to the amount of manufacturing data that can be acquired by human operation. Also, the accuracy may vary from person to person. For this reason, we need equipment that can perform measurements fully automatically and with high accuracy, and we are developing various equipment in collaboration with domestic and overseas manufacturers and in-house production.
• visualization of measurement data: The above will increase the amount of data that can be obtained. It takes time and effort to process the data numerically manually. Utilizing BI tools and databases, we are not only acquiring data but also making effective use of it.

As we shown here, "PLC" has been invented in NTT and has been making effort to improve its capability and provide many products to support optical communication in the world. We are continuously working on exploiting PLC technology and its application.