Optical waveguides – Temporal optical modulation within an optical waveguide
Reexamination Certificate
1999-09-16
2002-04-23
Epps, Georgia (Department: 2873)
Optical waveguides
Temporal optical modulation within an optical waveguide
C385S002000, C385S009000, C385S040000
Reexamination Certificate
active
06377716
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
“Not Applicable”
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
“Not Applicable”
REFERENCE TO A MICROFICHE APPENDIX
“Not Applicable”
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention pertains to an optical intensity modulator which comprises a waveguide containing a core adjacent to at least one cladding layer and exhibits a refractive index n(core) different from the refractive index n(cl) of the cladding layer.
Such optical intensity modulators are known from, for instance, Akkari et al.,
2. Description of the Related Art
Journal of Non
-
Crystalline Solids
187 (1995) 494-497.
This publication concerns a thermo-optic mode extinction modulator where mode extinction occurs due to the counteracting effect that arises from having a guiding polymer thin film with a negative thermo-optic coefficient as a core and a glass substrate with a positive thermo-optic coefficient. All discussed devices were made of polyurethane varnish on BK-7 glass substrates. A stripeheater located on top of a protective layer (PMMA) was put over the polyurethane core layer used to control the mode extinction via the thermo-optic effect. Akkari et al. achieved complete mode extinction at reported switching times from cut-off (full extinction) to total transmission in the order of 6.7 ms. However, the known devices leave some room for improvement. Firstly, due to the stacked layer structure of the guiding polymer thin film and the glass substrate, the optical intensity modulator described above is relatively large in size, which renders it less suitable for application in small and compact devices. Secondly, also due to the stacked layer configuration there is no control, over the direction of the generated radiation mode. Thirdly, in order to increase efficiency, it is desirable that comparatively small temperature changes result in an efficient coupling out of the propagating mode. Being able to apply low temperatures would have the additional advantage of avoiding degradation of the mostly polymeric material used for optical intensity modulators. It is noted that EP 0219069 also describes a rather complex light modulator that comprises a stack of a waveguide layer and an adjacent layer normally exhibiting a refractive index smaller than that of the waveguide layer, at least one of these layers being formed of a material whose refractive index changes by application of energy. An energy applicator is provided at least in one of these layers, and a dielectric grating is positioned at the surface of the adjacent layer over a section where energy is applied by the energy applicator. When a change in refractive index is caused to arise in the waveguide layer and/or the adjacent layer, the guided mode is radiated out of the stack. Like Akkari, also EP 0219069 only refers to stacked layer geometry, and radiation of the guided mode out of the waveguide layer is based on transversal differences in refractive indices. Although the interaction with the grating described in EP 0219069 renders the direction of the generated radiation mode more easily controllable and improves the extinction ratio of the modulated light, the items discussed in Akkari with respect to the geometry of the stack and the temperature to be applied also apply for EP 0219069.
BRIEF SUMMARY OF THE INVENTION
This invention has for its object to reduce or even obviate the said disadvantages. This is achieved by an optical intensity modulator as described in the first paragraph which waveguide is a bent channel waveguide and that contains means for altering the temperature at or near the bend and that at least at the bend, the core and the cladding layer exhibit different thermo-optic coefficients such that the contrast between n(core) and n(cl) can be changed by altering the temperature.
In contrast to the prior art devices, the optical intensity modulator of the present invention comprises a bent channel waveguide and thus the waveguide has a different structure. Such a structure, which is neither described nor taught by Akkari or in EP 0219069, reduces the size of the optical intensity modulator considerably compared to the stacked layer type and renders the device more compact. A particular advantage of a channel waveguide lies also in the fact that it renders the optical intensity modulator compatible to optical fibers which can be connected to that device.
DETAILED DESCRIPTION OF THE INVENTION
The basic principle of the device according to the present invention that renders it suitable as an intensity modulator and also improved compared to the devices of the prior can be outlined as follows.
In order to attain guiding of the propagating mode usually at a given temperature (which often is the room temperature) the refractive index of the core is higher than that of the adjacent cladding layer thus keeping the propagating mode in the core of the waveguide. To achieve attenuation, it is, however, desired to efficiently couple out this propagating mode. In the waveguide of the present invention, core and cladding layer additionally show different thermo-optic coefficients which leads to the fact that when changing the temperature the difference of refractive indices of core and cladding layer either increase or decrease. As a consequence, light that propagates in the core will either stay there or couple out of the core (hereinafter referred to as loss). As a matter of fact this depends on the materials used for core and cladding layer and the temperature change applied (cooling or heating), which together form the lateral refractive index contrast of the waveguide.
The term thermo-optic coefficient (toc) is defined by the change of the refractive index of a given material upon changing of the temperature. It can be expressed by the formula (1):
toc
(
a
)
=
ⅆ
n
(
a
)
ⅆ
T
(
1
)
with:
toc(a)=thermo-optic coefficient of material (a)
n(a)=refractive index of material (a)
T=temperature in Kelvin
The refractive index of a material showing a positive thermo-optic coefficient thus will increase upon temperature rise and decrease, if the material is cooled. As a consequence, the refractive index of a material showing a negative thermo-optic coefficient thus will decrease upon heating and increase, if the material is cooled. the thermo-optic coefficient is known to the person of ordinary skill in the art and need no further elucidation here. When two materials with both different refractive indices and thermo-optic coefficients are combined, whereas one is forming the core and the other the cladding layer of a waveguide, a plurality of effects are possible that can be used for intensity modulation.
If, e.g. the refractive index of the core n(core) is at room temperature higher than the refractive index of the cladding layer and further the toc(core) of the core is negative, whereas the toc(cl) of the cladding layer is positive (such a combination is possible, when using a polymeric material for the core and an inorganic material for the cladding layer), at room temperature the light will remain in the core, provided the contrast in refractive index of core and cladding is high enough. Upon heating of the waveguide the refractive index of the core decreases and that of the cladding layer increases. If the refractive index contrast is now lowered sufficiently, part of the light will be radiated out of the waveguide, the exact fraction depending on the value of that contrast. By adjusting the contrast value in such a way, the precise transmitted power can be controlled.
A similar effect can be achieved if both toc(core) and toc(cl) of the aforementioned waveguide are negative, provided that toc(core) has a greater absolute value, which leads to a faster decrease of n(core) than of n(cl) upon heating (such combination is achievable, when using e.g. a rubbery polymer for the core and a glassy polymer for the cladding layer).
In both cases outlined above the loss (i.e. coupling-out of the propagating mode of the core) is obtained upon rising th
Diemeer M.
Hoekstra T. H.
Lambeck P. V.
Veldhuis G. J.
Epps Georgia
Greene Kevin E.
JDS Uniphase Inc.
Lacasse Randy W.
Lacasse & Associates
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