At this time, by collaborating our research with Tohoku Electric Power Co., Inc. we have developed an underground transmission line surveillance system using a passive optical network. It has been installed on Tohoku Electric's - Hakusan-Honcho underground transmission line (Niigata city, Niigata prefecture). This is the first underground transmission line surveillance system using a passive optical network in the world.

With this underground transmission line surveillance system using a passive optical network the only devices installed in the underground transmission line are: an optical sensor and an optical communication line (optical fiber cable and an optical splice box). This is different from the usual underground transmission line surveillance systems, as because a wavelength division multiplexing device for transmitting a variety of sensor information and power is unnecessary, it is possible to make big cuts on construction and maintenance costs.

Electric power supply for cities is mainly by underground transmission cable and in order to ensure its reliability, maintenance management of the cable itself and the underground transmission line is important.
Usual underground transmission line surveillance systems detect water level, the opening and closing of doors, gas, fire and temperature etc. by a variety of sensors which use electricity. To avoid the effects of electromagnetic induction, after the electrical signal is changed into an optical signal by wavelength division multiplexing devices installed at 100m intervals inside the underground transmission line, it is transmitted through an optical fiber with a surveillance device system.

However, regarding the usual system:

1.	As it uses many extremely expensive wavelength division multiplexing devices: costs are high.
2.	Because wavelength division multiplexing devices have very complicated built in electronic circuits, the high temperatures and humidity inside the underground transmission line makes it is necessary to build a waterproof case to ensure their reliability.
3.	As it is necessary to make the device installation spaces secure etc., in a narrow area, these lead to problems with maintenance and high construction costs.

The underground transmission line surveillance system using a passive optical network that has now been developed, consists of a 'surveillance device' which is installed in a sub-station, then a single 'optical fiber' installed in the underground transmission line and a 'coupler' which splits it, each of these are connected to 'optical sensor'. The optical signal which is transmitted from the surveillance device is reflected by the optical sensor and returns to the surveillance device, but because of the sensor's operation - the strength of the returned optical signal is changed. This means that it is no longer necessary to have the wavelength division multiplexing devices and electrical power that marked the down side of the old system.

Also the sensors listed below which are used by this system are newly developed.

Optical Digital Sensor on Faraday Effect

With this Faraday Effect sensor: when a magnetic field approaches, Faraday Rotator revolve around theoptical polarization , resulting in the cessation of the optical flow. This is used, for example in an opening/closing door mechanism; when the magnet placed in the mechanism is pulled away when the door is opened, there is an optical flow and therefore it can detect the opening and closing of the door. In this new system, it is being used as a detection sensor to establish if a door is open or closed.

Optical Digital Sensor using Electromagnet

As this works as an electromagnetic shading plate it is a system sensor which detects abnormalities when there is no optical flow. For example, when there is abnormal oxygen density, because of the electric flow there is electromagnetism and the shading plate moves resulting in the stopping of the optical flow, and the detection of the abnormal oxygen density. In this new system, it is being used as: an oxygen sensor, a combustible gas sensor, and a hydrogen sulfide gas sensor.

On 29th November 1999, the underground transmission line surveillance system using a passive optical network for the Hakusan-Honcho underground transmission line, was delivered to Tohoku Electric Power Co., Inc and tests were safely completed. This system is up and running, and we are continuing to meet Tohoku Electric's underground transmission line needs by supplying systems for their 'Gokou line' underground transmission line (Sendai city, Miyagi prefecture). The installation will be delivered from February 2000 to March 2001.