Kyocera Realizes Crystal Filter By Using Atomic Diffusion Bonding

Dec 19, 2012
Hiroki Yomogida, Nikkei Electronics
The "temperature characteristic free" etalon filter
The "temperature characteristic free" etalon filter
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Kyocera Crystal Device Corp developed a crystal part by using atomic diffusion bonding and will start to ship samples of the part in January 2013.

It is a filter designed for large-capacity optical communication systems and expected to reduce sizes of modules and devices.

Specifically, Kyocera Crystal Device developed an "etalon filter," which is used for the tunable laser modules of WDM transmission systems. Etalon filters are used inside laser modules to check if the oscillation wavelength of a laser light source is stable.

When the temperatures of traditional etalon filters change, their light transmission rates also change, making it impossible to accurately monitor the change in wavelength. Therefore, to place an etalon filter in a module, it is necessary to control temperature with Peltier devices, which are used to keep temperature constant but increase the footprint and power consumption of the laser module.

Because the temperature characteristic of the new etalon filter is stable, it is not necessary to control temperature with, for example, Peltier devices. Kyocera Crystal Device said that the filter is "temperature characteristic free."

The company's old etalon filter has a wavelength change property (in response to temperature change) of 5.4pm/°C. But, this time, it drastically reduced the property to "±0.15pm/°C.

"With the new filter, the size of tunable laser module can be reduced so that its package size will be reduced to about 1/3," Kyocera Crystal Device said.

One of the reasons why Kyocera Crystal Device succeeded in drastically improving temperature characteristic is that it used atomic diffusion bonding. Specifically, the company bonded a crystal having a "positive temperature characteristic," which shifts wavelength characteristic to the positive side in response to temperature change, to a crystalline material having a "negative temperature characteristic," which shifts wavelength characteristic to the negative side in response to temperature change, by using atomic diffusion bonding.

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In the past, even though Kyocera Crystal Device was considering combining a crystal having a positive temperature characteristic with a crystal having a negative temperature characteristic to realize an etalon filter that does not require temperature control, it could not commercialize such a filter because of problems related to bonding technologies.

This time, by using atomic diffusion bonding, the company realized a high bonding strength without affecting the properties of the filter, it said. The bonding strength of the new filter is more than five times higher than that of the "optical contact" technology, which the company used in the past.

There is another method for realizing a temperature characteristic free filter, and it is called "air gap type." But, with this method, the size of etalon filter becomes large.

Atomic diffusion bonding is a bonding technology that does not require any organic adhesive. And it was developed based on a research conducted by Takehito Shimatsu, professor at Tohoku University. Kyocera has been engaged in a research on atomic diffusion bonding in cooperation with the university. And it made an announcement about basic technologies for applying atomic diffusion bonding to crystal devices in May 2010.

This will the first time that a crystal device product developed based on atomic diffusion bonding has been actually released, Kyocera Crystal Device said. And the company plans to apply the bonding technology to other crystal parts in the future.