Optical: systems and elements – Optical modulator – Light wave temporal modulation
Patent
1992-08-21
1993-06-08
Arnold, Bruce Y.
Optical: systems and elements
Optical modulator
Light wave temporal modulation
359254, 359276, 385 2, G02F 103, G02F 101, G02F 1035
Patent
active
052184684
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to electro-optic devices and in particular but not exclusively to lithium niobate electro-optic waveguide devices.
BACKGROUND OF THE INVENTION
With lithium niobate (LNB) devices such as directional couplers and Mach Zehnder (MZ) interferometers, there exists the problem that in order to achieve a given level of extinction, switching or modulation, continually greater electrode potentials are required throughout the operating life of the device. This phenomenon is known as voltage induced drift. Ultimately a limit is reached where the driving electronics are supplying their maximum potential and it is no longer possible to achieve the desired performance. Alternatively the device may fail catastrophically with electrode breakdown caused by the high applied potential. Moreover, there is a further disadvantage of using high electrode potentials with directional couplers, in that the extinction ratio is impaired relative to that attainable with lower electrode potentials.
Clearly devices subject to such drift are unsuitable for any long term systems applications such as telecommunications or optical signal processing, because there is no long term certainty that any particular device output corresponds to a certain applied potential.
One method which has been suggested as a means of overcoming the problem of voltage induced drift in electro-optic devices for long term systems use is to divert part of the device's optical output to a detector in order to monitor the drift so that the bias voltage can be varied to trace it. The disadvantages of this hypothetical arrangement are twofold: first, the difficulty of monitoring part of the optical output and deriving the required bias level: second, the drift tends to continue at a near linear rate, consequently higher and higher bias levels are still required.
SUMMARY OF THE INVENTION
The present invention seeks to provide a means of controlling voltage induced drift.
According to a first aspect, the present invention provides a method of minimizing voltage induced drift in electro-optic devices the optical state of which is controllable by the application of an electrical potential between first and second electrodes of the device, the method comprising controlling the electrical potentials applied to the first and second electrodes such that in use the average potential difference between the first and second electrodes is substantially zero.
According to a second aspect, the present invention provides a method of controlling an electro-optic device the optical state of which is controllable by the application of an electrical potential between first and second electrodes of the device, the method comprising the steps of applying a first electrical potential to the first electrode, and applying a second potential to the second electrode, characterised in that one or both of the first and second electrical potentials is/are adjusted so that, in use, the average potential difference between the first and second electrodes tends to zero.
According to a third aspect, the present invention provides a driving arrangement for electro-optic devices, arrangement comprises electrode driving means to supply both positive and negative drive voltages, averaging means to generate an average signal indicative of the average electrode voltage, and control means for controlling said electrode driving means in response to said average signal, which control means causes said electrode driving means to supply positive or negative drive voltages as necessary to maintain, in use, a substantially zero average electrode voltage.
According to a fourth aspect, the present invention provides a driving arrangement for electro-optic devices, arrangement comprises a data input to receive an input data stream of known disparity, and control means responsive to the input data stream for controlling said electrode driving means to supply positive or negative drive voltages as necessary to maintain, in use, a substantially zero average ele
REFERENCES:
patent: 4253734 (1981-03-01), Komurasaki et al.
Patent Abstracts of Japan, vol. 7, No. 32, (P-174)(1177), Feb. 8, 1983; & JP A, 57185418 (Nippon Denki K.K.) Nov. 15, 1982.
Patent Abstracts of Japan, vol. 7, No. 180, (P-215)(1325), Aug. 9, 1983; & JP,A. 5882223 (Matsushita Denki Snagyo K.K.) May 17, 1983.
Applied Physics Letters, vol. 35, No. 1, Jul. 1, 1979, "Efficient LiNbO.sub.3 balanced bridge modular/switch with an ion-etched slot", Minakata, pp. 40-42.
Arnold Bruce Y.
British Telecommunications public limited company
Lester Evelyn A.
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