Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-03-02
2002-06-04
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C385S014000
Reexamination Certificate
active
06400483
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an optical signal transmission system and, more particularly, to an optical signal transmission system comprising an optical transmission line with at least one optical wave guide switch and an optical level adjuster.
Switching matrices are required in fiber optic systems, especially information transmission system, for switching between respective input and output wave guides. The optical cross-connect that combines the switching matrix with optical amplifiers for level regulation and/or transponders for wavelength conversion and/or WDM-components for wavelength separation is an especially important application for this type of switching circuit. In a complex cross-connect of this type level regulation means, which comprise a power measuring device arranged in the vicinity of the output optical fiber (by diverting a fixed part of the light power conducted in the output fiber to a photodetector) and one or more level adjusting devices, is an absolute necessity. The required quality, for example a guaranteed maximum error rate, of the data flow over several cascaded transmission paths and through several network junctions is only guaranteed by this type of level regulation in the larger photonic networks.
The level adjusting device should be controllable with electrical parameters (a voltage or a current level) and has at least one adjusting speed corresponding to the switching speed of the switching matrix. Its minimum insertion attenuation should be small, the adjusting range should be sufficient for compensation of different input levels and different insertion attenuation of the components in the switched paths, typically from 10 to 20 dB being required.
In the following the state of the art for the optical switch and then for the optical level adjuster or adjusting device is described.
Switches that can switch the path of light conducted in wave guide/glass fibers are required in fiber optic systems for special communication systems. This type of switch typically has one or more input wave guides and one or more output wave guides that are connected with each other by a switching matrix. The switching matrix causes a coupling of an input with a particular output wave guide (Proceedings ECIO'97, Stockholm, pp. 10/JWA4-1 to JWA4-6/15, Mats Gustavsson, et al: “Network Requirements on Optical Switching Devices”).
Primarily it is desired that as much of the light from the input wave guide is coupled as completely as possible into the selected output wave guide (high transmission=reduced insertion attenuation) and all other output wave guides receive a greatly reduced proportion of the light in the input wave guide (reduced cross talk=high cross talk damping). That should be possible in the wide optical wave length range used for telecommunications purposes, typically from 1.2 to 1.6 &mgr;m. Conventional specifications for an optical switching matrix used in telecommunications networks include insertion attenuation of less than 5 to less than 10 dB and cross talk attenuation of greater than 30 to greater than 60 dB.
Optical switches are embodied in a variety of technologies and have different switching principles.
Relay type switches shift a glass fiber in front of an array of opposing glass fibers so that a coupling between two fibers occurs with a precise alignment. Insertion attenuation and cross talk attenuation are outstanding, however the switching time is comparatively long and the manufacturing expense is great. The same goes for those switches that broaden the light from each glass fiber by collimated optics and then make a changeable optical coupling between different fibers with rotated or shifted mirrors, prisms and/or movable lenses.
Several switching principles are known for integrated optic circuits that operate entirely with wave guides.
Either the light of an input fiber is divided into several paths with a passive power divider and a switchable optical amplifier which has a high damping in the switched-out case and compensates for the loss in light distribution (active) in the switched-on case is inserted in each path. This amplifier together with a passive power divider can be manufactured in a semiconductor base (Semiconductor Laser Amplifier, SLA). The amplification process (based on induced emission) is generally a narrow bandwidth amplification and thus limited to a narrow wavelength range. Furthermore since the optical amplifier noise causes interference at high input powers, which is polarization dependent in its properties for the typical wave guide in a semiconductor substrate, the embodiments for the interesting wavelength range (1.2 to 1.6 &mgr;m) must be constructed using the very expensive InP semiconductors.
In contrast a purely passive integrated optic wave guide switch operates by modification of the index of refraction distribution in the vicinity of the coupling points of wave wave guides. Either interferometric devices, for example a Mach-Zehnder interferometer (MZI) with two inputs and two outputs in which the coupling between inputs and outputs can be changed by changing the index of refraction (transit time, phase) in one or both arms, or X- or Y-branching devices with reduced opening angle, can be used for the switch so that the light is conducted into one of two outputs. The latter type of device has the advantage that the switching operation is guaranteed over a wide range of refraction changes (the so-called “digital optical switch”, DOS), while the index of refraction must be very precisely changed in the MZI switch.
A temperature change can be used as a mechanism for index of refraction changes (Proc. 21
st
Eur. Conf. On Comm. (ECOC'95-Brussels, Th.L.3.5,pp. 1063 to 1066,R. Moosburger, et al: “A novel polymer digital optical switch with high temperature stability”). The wave guide material must have a high thermooptic coefficient (TO); typically organic materials (polymers, among others plastic materials) can be used. The switching time should be in the range of 1 ms, and the switching properties should be independent of light polarization. Instead of that the electrooptic effect (EO) which causes a index of refraction change under strong electric field in certain materials can be used (Y. Silberberg, et al, “Digital Optical Switch”, Appl. Phys. Lett. 51 (16) Oct. 19, 1987, pp. 1230 to 1232); here the switching time is very short (ps time scale), however the effect is different according to the light polarization. Furthermore materials with a high electrooptic effect, for example LiNbO
3
are expensive.
Another switching principle is used in thermooptic digital optical switches (TO-DOS). According to
FIG. 1
of the accompanying drawing wave guides
6
are in the polymer layers
3
,
4
,
5
provided in the silicon substrate
2
. One known embodiment is the strip wave guide in a three layer system; however other optical configurations are conceivable. Heating conductors
7
that increase the temperature in the vicinity of the wave guides
6
and thus lower the index of index of refraction in that material are provided in the vicinity of the wave guides
6
over the wave guides
6
, which means opposite to the silicon substrate
2
serving as heat sink. Typical index of refraction differences for the wave guide core-substrate are 5×10
−1
, while the thermooptic coefficient is typically about 10
−4
K
−1
. Thus a temperature increase of 50 K in the wave guide core is sufficient to cut off the light conduction. A reduced temperature difference between both wave guide outlets of about 20 K is enough for switching in a gradually opening Y branch device. Thus the desired coupling over of the light into a particular one of the outputs is the more complete, the less the opening angle of the Y branching device is; because of that the component length of the wave guide switch increases. Since the plastic materials used have high thermooptic coefficients, however also even with reduced optical absorption, an increased component length le
Mayer Klaus-Michael
Muehlnikel Gerd
Mueller-Fiedler Roland
Rech Wolf-Henning
Leung Christina Y.
Pascal Leslie
Robert & Bosch GmbH
Striker Michael J.
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