Japan Telecom Researchers Boost Kerr Effect
An optical switch using these KTN waveguides had a driving voltage less than 10% that of conventional optical switches. By using these KTN crystals, it should be possible to fabricate optical modulators and optical switches with a driving voltage an order of magnitude lower than that of conventional devices.
KTN is a transparent optical crystal composed of potassium, tantalum, niobium, and oxygen. The first KTN crystal was synthesised in the 1950s and is well known as a crystal with a very large quadratic electro-optic effect (Kerr effect), which determines device performance. However, KTN has been considered an impractical material for optical communication use because it was difficult to grow.
NTT's Photonics laboratory found that precise temperature control is essential in order to grow a large crystal. The scientists successfully prepared 40x40x30mm KTN crystals. The electro-optic coefficient of these KTN crystals is about 600pm/V, or 20 times larger than that of the conventional LiNbO3 used in optical modulators and switches. Further, the new KTN crystal is at least 20 times larger than a conventional LiNbO3 crystal.
The LiNbO3 devices have problems in terms of their large size and relatively high driving voltage resulting from the small electro-optic effect of the active material.
NTT has produced KTN crystal films on substrates prepared from bulk crystals by using liquid phase epitaxy (LPE). High quality KTN films with a controlled thickness of 5-10microns for waveguide fabrication can be prepared by controlling the growth rate and suppressing temperature fluctuations on the growth surface. This makes it possible to fabricate low-loss waveguides with a loss of 0.5dB/cm by using a dry etch technique. This allows complex optical circuits to be fabricated.
A Mach-Zehnder interferometer was constructed using KTN crystal waveguides and a silica glass waveguide. Optical switching was achieved through a voltage of 1.3V applied to the KTN surface. Polarisation independent operation was also confirmed - an indispensable property for optical switches.
NTT plans to proceed with large crystal growth and large-scale optical circuit fabrication with a view to demonstrating the potential of KTN crystals.
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