R&D Information
Successful Preparation of KTN Crystals with theHighest Reported Electro-optic Effect and thePotential for Providing a Great Improvement in
Optical Device Performance

NTT has successfully prepared KTN crystals(KTa1–xNbxO3) with the highest electro-optic effect*1
ever reported. In addition, NTT has developed film
growth and lithographic techniques for KTN crystals
that can be used to fabricate an optical waveguidewith excellent optical transmission performance. An
optical switch using these KTN waveguides exhibited a low driving voltage less than 1/10 that of conventional optical switches. This result clearly showsthe excellent potential of KTN crystal in terms ofoptical device performance.
By using these KTN crystals, it should be possible
to fabricate optical modulators and optical switcheswith a driving voltage one order of magnitude lower
than that of conventional optical devices. This will
provide a way to develop the next generation of highly functional devices.
KTN is a transparent optical crystal composed ofpotassium, tantalum, niobium, and oxygen. The first
KTN crystal was synthesized in the 1950s and it iswell known as a crystal with a very large quadraticelectro-optic effect (Kerr effect), which determines
device performance. However, KTN crystal has been
considered an impractical material because it was difficult to grow.
NTT Photonics Laboratories found that precise
temperature control is essential to grow a large crystal, and they successfully prepared 40-mm-square
KTN crystals, which is a practical size. The electrooptic coefficient of these KTN crystals is about 600
pm/V, or 20 times larger than that of conventional
LiNbO3*2. This coefficient means that an improvement in device performance of at least one order canbe expected.
BackgroundIn recent years, the explosive growth in trunk line
capacity has largely ended and the goal is now large56

capacity and highly functional metropolitan or accessnetworks. In order to achieve this goal, the size andpower consumption of optical devices must bereduced and their functionality increased. Of thesedevices, optical modulators and optical switches
made of conventional LiNbO3 materials are especially important for the networks. However, these devices
are large and have relatively high driving voltages asa result of the small electro-optic effect of LiNbO3.
Therefore, attention has been drawn to the development of novel materials with a larger electro-opticeffect with a view to overcoming these problems. In
addition, in the near future, highly functional materials are expected to be used to make high-speed andhighly integrated optical signal processing devices
for network innovation.
Technological points(1) Technology for growing large and high-quality
KTN crystals
NTT successfully prepared the largest KTN
crystals ever grown (40 mm square × 30 mmlong) by optimizing the growth conditions andachieving precise temperature control (Fig. 1).
This crystal is at least 20 times larger than a conventional LiNbO3 crystal.
(2) Waveguide fabrication technology
NTT has established KTN crystal films on substrates prepared from bulk crystals by using liq*1 Electro-optic effect: A refractive index change is produced when
an electrical field is applied to some materials. If the relationshipbetween this refractive index change and the applied electricalfield is linear, it is called the first-order electro-optic effect (Pockels effect). A quadratic effect is called the second-order electrooptic effect (Kerr effect).
*2 LiNbO3 (Lithium niobate) has the highest electro-optic coefficientamong practical materials and is widely used in optical modulators
for telecom applications.

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R&D Information

Crystal growth

Rotation

CrystalPt crucible
Seed crystal

10 mmHeaterFig. 1. Synthesized KTN crystal.
Procedure
Cross-sectional view
KTN substrate
LPE
Undercladding
LPE
Core: 6 µmCoreCladding: 18 µm∆n = 0.5%
Dry etching
KTNsubstrateCore ridgeLPEOvercladdingCore

Refractive index difference: 0.01

Cladding

Fig. 2. Waveguide fabrication.

uid phase epitaxy (LPE)*3. High-quality KTN

films with a controlled thickness of 5–10 µm forwaveguide fabrication can be prepared by controlling the growth rate and suppressing any temperature fluctuation on the growth surface. This
makes it possible to fabricate low-loss waveguides with a loss of 0.5 dB/cm*4 by dry etching*5. Complex optical circuits can be fabricatedwith these technologies (Fig. 2).

Vol. 1 No. 9 Dec. 2003

*3 Liquid phase epitaxy is a crystal film growth technology wheresubstrates are soaked in a solution to prepare specific crystal films
on the substrate surface. LPE can grow high-quality crystal filmwith a high growth rate. .
*4 dB/cm is a unit that indicates the transmission loss of an opticalwaveguide. 0.5 dB/cm indicates about a 10% loss of transparency
per centimeter.
*5 Dry etching is a lithographic technique using highly reactive
species such as ions and radicals formed in a plasma (discharge).
This technique is widely used for the lithography of crystals andglasses.

57

R&D Information

Low driving voltage: only 1.3 V
Polarization independent operation

Output 1
Output 2

Electrode

Control signal
Silica waveguide

KTN crystal waveguide

Silica waveguide
Input

Fig. 3. KTN optical switch.

(3) Demonstration of optical switching
A Mach-Zehnder interferometer*6 was constructed using KTN crystal waveguides and a silica glass waveguide. An optical switch is operated by applying a voltage to electrodes preparedon the KTN waveguide surface. Optical switching at a very low voltage of 1.3 V was observed.
Polarization-independent operation was also confirmed*7 and this is indispensable for optical*6 Mach-Zehnder interferometer: Interference is a phenomenon
whereby two optical waves overlap thus intensifying or diminishing their amplitudes. A Mach-Zehnder interferometer is a device
in which one optical wave is first split into two waves and thenrecombined into one wave to induce optical interference. MachZehnder interferometers are widely used for telecom applications
such as optical switches and optical modulators.
*7 Polarization-independent operation: Light is composed of twoorthogonal vibrations that are perpendicular to the propagation
direction. In general, the optical properties parallel and perpendicular to a waveguide substrate are slightly different. The optical
performance thus changes depending on the polarization. In particular, electro-optic crystals have a large optical birefringencethat depends on the optical axis. Therefore, these polarizationdependent properties are serious problems that must be overcome.

58switches. NTT has therefore made an opticalswitch with the lowest driving voltage yet reported and polarization-independent operation (Fig.
3).
Future plans
In order to develop highly functional optical signal
processing devices, NTT plans to proceed with large
crystal growth and large-scale optical circuit fabrication with a view to demonstrating the very highpotential of KTN crystals.
For further information, please contact
NTT Science and Core Technology Laboratory
GroupAtsugi-shi, 243-0198 Japan
E-mail: st-josen@tamail.rdc.ntt.co.jp

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