Optical devices having improved temperature stability

Details Optical devices having improved temperature stability Optical devices having improved temperature stability

1999-08-19

2001-02-27

Epps, Georgia (Department: 2873)

Optical: systems and elements

Optical modulator

Light wave temporal modulation

C359S322000, C359S248000

Reexamination Certificate

active

06195191

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to optical devices using pyroelectric non-centric crystals.
DESCRIPTION OF THE RELATED ART
Non-centric crystals are of use in making electro-optic devices. Some non-centric crystals, such as, for example, lithium niobate, are also pyroelectric. Pyroelectricity is a property of crystals whereby the polarization of the crystal changes when its temperature changes. The change in polarization in turn produces a self-induced electric field. For example, a temperature change of 100° C. can result in an electric field at the surface of a lithium niobate crystal of about 10
7
volts/centimeter (V/cm). Such an electric field can interfere with the operation of optical devices, reducing their usefulness in applications where temperature fluctuations are expected.
The effect of temperature change on the crystals can be controlled by cooling, such as, for example, by the use of thermoelectric coolers. However, the use of such coolers can be limited, because optical crystals are also often piezoelectric. Piezoelectricity means that the application of a mechanical stress to an optical crystal can effect its polarization. Thus, assembling piezoelectric crystals with thermal electric coolers can create mechanical stresses which can aggravate or complicate the pyroelectric effect. Thermal expansion mismatches between the crystal, the cooler, and any material used to attach the crystal to the cooler, can increase the mechanical stresses placed on the crystal.
Another method for controlling the effect of temperature changes on a crystal in an optical device is by the application of a resistive field shield to both sides of the device, thereby uniformly terminating the self-induced electric field. A field shield is generally provided by a dissipation layer, which includes a film, of about 800 angstroms in thickness, of a moderately conducting material such as silicon titanium nitride or indium tin oxide. The film serves to remove pyroelectric charges from the surface of the crystal. A bias voltage is applied to maintain the operation of the crystal. However, the effectiveness of a resistive field shield can degrade over time on some optical devices, and the bias voltage required to maintain operation of the crystal varies with temperature. The magnitude of the change is dependent upon the time for which the bias voltage is being applied.
Improved methods for controlling the effect of temperature on the operation of pyroelectric non-centric crystals are desired.
SUMMARY OF THE INVENTION
One aspect of the present invention is a method for reducing the undesired effects of pyroelectricity on optical devices that include non-centric crystals. The method includes etching the surface of an optical crystal to a depth of less than about 300 angstroms. An optical device may include a waveguide, a dielectric buffer layer, a dielectric field shield on the buffer layer, and one or more metal electrodes. The buffer layer may contain a dopant material, such as indium oxide (In
2
O
3
), which may be contained in a matrix of silicon dioxide (SiO
2
).
In preferred embodiments, the surface of the crystal is etched to a depth of about 275 angstroms or less. Even more preferably, the depth of the etching is about 250 angstroms or less. Also in preferred embodiments, the etched surface of the crystal has a surface defect density of about 5×10
16
defects per square centimeter, more preferably less than about 1×10
6
defects per square centimeter.
The crystal may be of lithium niobate, barium titanate, lead titanate, potassium lithium niobate, or calcium niobate. Lithium niobate is preferred.
Another aspect of the present invention is an optical device comprising a pyroelectric non-centric crystal having an etched surface and a voltage source for applying a bias voltage to said device, wherein the depth of said etching into said surface is less than about 300 angstroms.
In preferred embodiments, the crystal has a surface defect density of less than about 5 ×10
6
defects per square centimeter. A bias voltage may be variably applied as required to maintain the operation of the optical device. Preferably, the bias voltage varies by less than about 1 volt corresponding to a temperature change of about 70 degrees C. In certain highly preferred embodiments, the bias voltage may vary by less than about 0.7 volt, or even less than about 0.5 volt.
The optical devices of the invention can have an optical modulator operable to modulate an input light wave, according to an input signal, at an operating point which is determined by a bias voltage. The optical devices may also include a detector that detects a deviation of the operating point from a selected optimal operating point, based on an output of the optical modulator. Also, the devices of the invention may include a bias generating means for generating said bias voltage within a predetermined voltage range of bias voltages so as to reduce deviation from the selected optimal operating point.
In some embodiments, the devices of the invention include a means for setting the bias voltage at a predetermined voltage associated with the determined operating point when the optical modulator is initialized. The optical modulator may modulate the input light wave according to the input signal as a predetermined voltage is set as the bias voltage. The predetermined voltage generally varies by less than about I volt corresponding to a temperature change of about 70 degrees C. In preferred embodiments, the optical device includes a lithium niobate crystal. Preferably, the crystal has a buffer layer that includes indium oxide. Also preferably, the device includes a charge dissipation layer made of a moderately conducting material such as silicon titanium nitride. The silicon titanium nitride can comprise silicon, titanium and nitride in varying ratios and its composition can be represented by the formula Si
2
Ti
X
N
(8/3)+x
wherein x is an integer. The device also preferably includes electrodes that can transmit an electrical signal to the device. The electrodes are made from a conducting metal such as, for example, copper or gold, and are preferably made from gold.
A further aspect of the invention is a method of manufacturing an optical device. The method includes the steps of:
providing a non-centric electro-optic crystal;
forming a waveguide in the non-centric electro-optic crystal;
etching the surface of the crystal to a depth of less than about 300 angstroms;
depositing a buffer layer onto the etched surface of the crystal;
forming a charge dissipation layer on the buffer layer; and
attaching one or more electrodes to the crystal.
In preferred embodiments, the etched surface of the crystal has a surface defect density of less than about 5×10
6
defects per square centimeter.
In some embodiments, such as, for example, when the electro-optic crystal is a X-cut lithium niobate crystal, a buffer layer may not be used. It is to be understood that such devices not having a buffer layer are within the scope of the present invention.


REFERENCES:
patent: 4093345 (1978-06-01), Logan et al.
patent: 5841568 (1998-11-01), Miyakawa
patent: 5958644 (1999-09-01), Ueda et al.
patent: 6014241 (2000-01-01), Winter et al.

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