Optical waveguides – Temporal optical modulation within an optical waveguide – Electro-optic
Patent
1991-03-19
1994-01-04
Bovernick, Rodney B.
Optical waveguides
Temporal optical modulation within an optical waveguide
Electro-optic
385 8, 385 11, G02B 610
Patent
active
052767442
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND
I. Field of the Invention
This invention relates to electro-optic waveguide devices and in particular to interferometers and directional couplers having travelling-wave electrodes.
II. Prior Art and Other Considerations
Electro-optic materials, such as lithium niobate (LNB) and KTP, have refractive indices which vary according to the magnitude and direction of applied electric field. Waveguide devices based on such materials are potentially useful for optical fibre communication and signal processing systems. Typically such devices are required to operate with light of wavelengths in the range 0.6 to 1.6 .mu.m, and in particular with light in the range 1.3 to 1.6 .mu.m.
There are two basic device types: directional couplers; and Mach-Zehnder (MZ) interferometers. The first of these utilizes the electro-optic effect to control the coupling between a pair of adjacent waveguides. By controlling their refractive indices it is possible to couple light from one waveguide to the other or vice versa. In an MZ interferometer an input waveguide is coupled to an output waveguide by a pair of waveguide arms. Each arm has an associated electrode by means of which it is possible to control the refractive indices of, and hence the velocity of propagation in, the two arms independently. It is therefore possible, by controlling the applied electric fields, to produce phase differences between signals travelling in the two arms resulting in constructive or destructive interference when they are combined. Thus it is possible to amplitude modulate input optical signals according to the voltage difference between the electrodes.
Unfortunately, materials such as LNB, which exhibit the electro-optic effect tend also to be pyroelectric: electric fields are produced within the material as the result of a temperature change. With some materials, notably Z-cut LNB, the pyroelectric effect is so strong that a temperature change of a degree or less may be sufficient to produce an electric field comparable to that applied to produce switching of states in a directional coupler or MZ interferometer made of the material. Such electric fields strongly affect the optical states of the devices. Consequently it is necessary, with materials such as Z-cut LNB which exhibit a strong pyroelectric effect, to provide very precise temperature control if reliable and repeatable performance is to be achieved from electro-optic waveguide devices based on such materials. However, even with good control of environmental temperature effects, thermally-induced instabilities may remain in devices in which there is power dissipation in the electrodes.
Examples of devices with power dissipating electrodes include directional couplers and MZ interferometers having travelling-wave electrodes. The use of travelling-wave electrodes potentially enables the production of devices capable of very high speed operation (typically switchable at gigabit rates). A further advantage of such devices is that they offer a very large bandwidth, typically from dc to 4 GHz.
Because the travelling-wave electrode is part of a transmission line and has finite resistance, non-zero signal levels cause power to be dissipated in the electrode, thus raising the temperature of the underlying waveguide. The stability of these devices is jeopardised if there is in the electrical signal applied to the electrodes a low frequency component having a period longer than the thermal response time of the electrodes (of the order of 0.1 second) as such components cause variations in power dissipation and hence temperature fluctuations. This power variation can shift the transfer characteristic by as much as 3 volts or more which, the switching voltage being in the range 3.5 to 4.0 volts, makes the device unusable.
One solution to this problem which has been proposed is to decouple the travelling-wave electrode by inserting a capacitor (e.g. 47 nF) between the travelling-wave electrode and the transmission line termination to remove the dc component of the switching voltage. The capacit
REFERENCES:
patent: 4253734 (1981-03-01), Komurasaki et al.
patent: 4703996 (1987-11-01), Glass et al.
patent: 4903029 (1990-02-01), Newberg et al.
patent: 4936644 (1990-06-01), Raskin et al.
patent: 4936645 (1990-06-01), Yoon et al.
patent: 5040865 (1991-08-01), Chen et al.
Applied Optics, vol. 22, No. 13, Jul. 1, 1983, "Traveling-wave electrooptic modulator", Gee et al.
Applied Physics Letters, vol. 44, No. 5, Mar. 1, 1984, "Microwave integrated optical modulator", Cross et al.
Applied Physics Letter, vol. 47, No. 3, Aug. 1, 1985, "Minimizing dc drift in LiNbO3 waveguide devices", Gee et al.
Applied Physics Letter, vol. 49, No. 19, Nov. 10, 1986, "Novel electro-static mechanism in the thermal instability of z-cut, etc." Skeath et al.
Quantum Electronics, Second Edition; Amnon Yariv, California Institute of Technology Electrooptic Amplitude Modulation; pp. 339-341; Jan. 1975.
Bovernick Rodney B.
BT&D Technologies Limited
Wise Robert E.
LandOfFree
Electro-optic device does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Electro-optic device, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Electro-optic device will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-313686