Optical waveguides – Temporal optical modulation within an optical waveguide – Electro-optic
Reexamination Certificate
1998-09-22
2001-01-16
Sanghavi, Hemang (Department: 2874)
Optical waveguides
Temporal optical modulation within an optical waveguide
Electro-optic
C385S016000, C385S018000, C385S034000, C359S245000, C359S247000
Reexamination Certificate
active
06175667
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to optical amplitude modulators for use with fiber-optic cable waveguides. More particularly, the modulator uses an electro-optic material to controllably change the optical phase of two parts of a transmitted and reflected light beam, irrespective of polarization, causing interference and, hence, modulation when the two parts recombine.
2. Background
Great efforts have been made to obtain polarization-insensitive optical devices in fiber-optic communication systems, which include modulators, switches, attenuators, etc. Most of these need a set of polarization pre-process devices, such as TE-TM mode converters, polarization selective couplers, or polarization rotators, to adjust the polarization of the light beams to obtain polarization insensitive devices. For example, one type describes a polarization-insensitive switch which comprises polarization-dependent components advantageously arranged. The polarization insensitive switch is achieved by splitting incoming arbitrarily polarized light waves into two paths, a light wave with a TE radiation component and one with a TM radiation component. The light wave with the TE component is then converted to a light wave with a TM component. Both light waves with TM components are then switched in a polarization-dependent photonic switching device. The initial polarization state is recovered by converting the appropriate switched TM component to one having a TE component. The converted TE component is then combined with the remaining switched TM component in a polarization combiner. These may be difficult to keep aligned.
Another example uses a liquid crystal switch/attenuator using two birefringent polarizers. The first polarizer separates an arbitrarily polarized optical signal into two polarization modes, an ordinary ray and an extraordinary ray. The liquid crystal controls the rotation of the optical signals with respect to the optical axis of the first polarizer with respect to the optical axis of the second polarizer. The second polarizer analyzes the optical signals and sends them to a combiner. The main drawback to this device is that liquid crystal response times may not be fast enough for some telecommunications applications.
Another type of polarization-insensitive optical device is taken from the species of optical reflection modulators with movable mirror surfaces. In this, an end of an input fiber is near the focal point of a lens so that emerging light expands and is collimated to impinge on two mirrors, each covering about half of the expanded beam. By moving the mirrors with respect to each other by a fraction of the light wavelength, the path length in the two halves can be changed so that when the reflected light is imagined onto an output fiber, the two halves of the beam can be made to interfere constructively or destructively. Although the design is elegantly simple, since it uses piezoelectric or electrostatic mirror drivers, it is unlikely to reach the kind of speeds desirable in fiber-optic communication systems. Another problem is that, since the mirrors must be free to move, the structure must be constructed with an air gap so that maintaining alignment with the reflecting surface may be difficult.
SUMMARY OF THE INVENTION
Accordingly, the main objects of the invention are to produce a polarization insensitive solid-state optical amplitude modulator (attenuator) for use in fiber-optic systems which can operate at high modulation frequencies. Further objectives are to make one that is rugged in design, easy to keep in alignment, and cost effective.
In one version, a fiber-optic cable used as the input is located on one side of a lens designed to collimating light from the input and focus any back-reflections to a point equi-distant from its optical axis as the input (termed a “conjugate point”) where the output fiber-optic cable is located. Two planar optical phase retarders (which may be formed from or on a single plate) are placed on the side of the collimator/focuser opposite the input/output side covering the entire collimated beam from the input fiber, but having each phase retarder cover substantially half the beam. Generally, this means that the phase retarders will be separated by a linear gap which is located on the optical axis of the collimator/focuser. The phase retarders are made with an electro-optic material covered with separate transparent electrodes on the side facing the collimator and conducting reflector electrodes on the side away.
With no voltage applied to the phase retarder electrodes, light from the input beam will be collimated, travel through the phase retarder, be reflected back through the phase retarder, and focused onto the end of the output fiber, undiminished in amplitude. However, when a voltage is applied to one of the phase retarders, the half of the beam traversing it will, to a greater or lesser extent, be out of phase with the other half of the beam as they impinge on the output fiber. They will then destructively interfere with each other and reduce the amount of light that enters the output fiber. The reduction becomes total when the optical phase difference is 180°. If the phase retarder is made with an electro-optic layer that is optically isotropic for directions perpendicular to the beam travel, i.e., in the plane of the layer, then the effects will be insensitive to the polarization of the light. The highest modulation speed depends on the electro-optic material. For lead lanthanum zirconate titanate (PLZT), the speed is in the nanosecond range. Losses are caused primarily by reflections at various interfaces, but there is no fundamental reason why these can't be low with the proper use of anti-reflection coatings. For PLZT, except for the small effects of fringing fields, the change in optical phase is insensitive to polarization because it satisfies the above condition.
In the above version, two passes were made through the phase retarder and the resulting phase lags are additive. In order to reduce the control voltage further, more phase retarders can be used. In this case, a transparent electrode would be substituted for the reflective one in the first pair and a similar pair of the phase retarders would be located next to the first pair opposite the collimator/focuser. The second pair would use a reflective electrode as the first pair had in the first version. Depending on the electro-optic material, the layers may share common electrodes or may require transparent insulating layer between them. In both versions separate mirrors could be used.
When the phase retarder pairs are made using a single layer of electro-optic material such as a ceramic plate, there is a fringing field along the gap between the transparent electrodes in the material under the gap. This field has components which are parallel to the layer and primarily perpendicular to the gap. Since some part of the beam goes through the gap and the effects on this part will now depend on the direction of polarization, the device will not be ideal. However, the problem can be substantially eliminated by placing a quarter-wave plate between the phase retarders and the mirror. After traveling through the quarter-wave plate and back out, all polarizations will be rotated 90°, i.e., if the original polarization were perpendicular to the gap on its way to the quarter-wave plat, it would be parallel on the way out and conversely. Thus both polarizations will be affected by the phase retarders the same, either on the way through or the way back.
Use of a GRIN lens as the collimator/focuser makes it possible to cement input and output fibers to one end and the phase retarders to the other, resulting in a rugged compact unit which should stay in alignment. These and all other required components (for the preferred versions) are commercially available, relatively low in cost, and the process to make the electrodes and mirrors are straightforward and not expensive.
REFERENCES:
patent: 4474435 (1984-10-01), Carlsen et al.
paten
Jiang Hua
Melman Paul
Wang Feiling
Zou Yingyin
Hamilton Brook Smith & Reynolds P.C.
NZ Applied Technologies Corporation
Sanghavi Hemang
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